panel report on new materials published under the …
TRANSCRIPT
PANEL REPORT
ON
NEW MATERIALS
Published Under the Auspices
of the
COUNCIL ON MATERIALS SCIENCE
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for the
DIVISION OF MATERIALS SCIENCE
U. S. DEPARTMENT OF ENERGY
DECEMBER 1979
INTRODUCTION
This report is one of a series resulting from studies conducted under the auspices of the Department of Energy (DOE) Council on Materials Science. The Council was established to provide the DOE Division of Materials Sciences with in-depth studies of selected important areas of materials research and was an outgrowth of a series of overview workshops held in the Spring of 1977. The workshops were held to lay the foundation for basic materials research needs and opportunities for the then impending new energy department. Reports of these workshops were subsequently published. Professor Robert Maurer of the University of Illinois is the current Council Director.
The Council sponsored two panel studies in 1978 on High Temperature Ceramics (chaired by Professor Kent Bowen of M.I.T.) and on the Theory of Surfaces (chaired by Dr. Donald Haraann of Bell Telephone Laboratories). The panel studies of 1979 were on New Materials (chaired by Professor T. H. Geballe of Stanford University) and on Corrosion (co-chaired by Professors Robert Rapp and Digby McDonald of Ohio State University).
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Preface ^H ig h l ig h ts v iChapter 1. INORGANIC SYNTHESIS 1
In t ro du c t io n 1Oppo r tun i t ie s f o r research 4Research needs 6
Nat ional e f f o r t 6Nat ional needs: Use o f s ta te -o f -a r t - e qu ipm en t 7Use o f theory 7Data c o l l e c t i o n and eva lua t ion 8
Table I : Examples o f compounds where an unusual s t r u c t u re o runusual p rope r ty s igna ls a research oppo r tu n i t y 10
Table I I ; Some ino rgan ic syntheses lead ing to landmark deve l opments in s o l i d - s t a t e science 11
References 12
Chapter 2. BASIC RESEARCH ON NEW POLYMERIC MATERIALS 14In t roduc t ion 14High temperature polymers 17High s t reng th - high modulus ma te r ia ls 18Radiat ion s e n s i t i v e / i n s e n s i t i v e polymers 20B a r r i e r f i lm s - Semipermeable membranes - water so lub le
polymers 22Conduct ing polymers 23Research op po r tu n i t i e s 24
New polymers 24Polymer c h a r a c te r i s t i c s 25Technological p o s s i b i l i t i e s 26P ie zoe le c t r i c polymers 26
References 2 0
TABLE OF CONTENTS
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Chapter 3: TNTERMETALLIC COMPOUNDSO ppo r tu n i t i e s , bas ic research 29
Classes o f ma te r ia ls which a f f o r d an o p po r tu n i t y f o r systema t i c in v e s t i g a t i o n o f var ious physica l phenomena 31
A-15 compounds 31Layered and quasi-one dimensional compounds 32Ternary molybdenum chalcogenides (Chevrel phases) 32Ternary ra re ea r th rhodium bor ides 35Other c l u s t e r compounds 36In termedia te valence rare ear th compounds 37
Innova t ive synthesis 38Metastable and k in e fc ica l l y -1 im i te d e q u i l i b r i u m phases by vapor phase condensat ion 38S ing le c r y s t a l s and new phases by molten s a l t e l e c t r o depos i t ion 40Synthesis o f new ma te r ia ls and growth o f s in g le c r y s t a l s from low me l t ing l i q u i d metals 4 1Composite ma te r ia ls and techno log ica l c o n f i g u ra t io n s 4 2
Bas ic research needs 4 5E x p e r im e n ta l methods o f determin ing de fec ts s t r u c t u re s 4 5Higher re so lu t io n e lec t ron s ta te spect roscopies 4 5
Technology needs 4 6References 5 0
Chapter 4: AMORPHOUS SOLIDS 5 2Glass f o rm a b i l i t y and s t a b i l i t y 5 2Basic research oppo r tun i t ie s 53
Cha rac te r i za t ion and measurement o f c r y s t a l n u c lea t io nra tes in a l l o y glass formers 53Extension o f composit ion ranges o f glass f o rm a b i l i t yby more rap id quenches 54Development o f understanding o f the r o l e o f im p u r i t i e s in amorphous s o l i d metal fo rmat ion 55
Theory f o r dependence o f T on a l l o y composit ionCha rac te r iza t ion o f theory f o r e f f e c t o f im pu r i t y admix ture cn amorphous sol id fo rmat ion in melt quenching o r condensat ion
Chapter 4 • ^ 5 -S t ruc tu re and de fec ts 55
Research o ppo r tu n i t i e s 55S t ru c tu ra l s tud ies 55Comparative s tud ies 56Radia t ion e f f e c t s 56Surface s t r u c t u r e 57
Atomic t r a n s p o r t and t rans fo rma t ion behav ior 57Basic research o p po r tu n i t i e s 58
Cha rac te r iza t ion o f atomic t r a n spo r t c o e f f i c i e n t s ina l l o y g lass formers 58Fur the r e x p lo ra t io n f o r and c h a ra c te r i z a t i o n o f l i q u i d - l i q u i d phase separa t ion in a l l o y glasses 59
Superconduct ing ma te r ia ls 59E le c t ron ic p ro pe r t i e s and magnetism 60Corros ion , defo rmat ion behav io r , hardness and coa t ings 61
Research o ppo r tu n i t i e s 61Development o f a theory o f pass iva t ion 61Mic roscop ic theory f o r mechanical s t reng th and d u c t i l i t y o f amorphous a l l o y s 62High hardness, wear res is tance 62
Technolog ica l needs 63Amorphous ox ide and chalcogenide ma te r ia ls 63
Research o p p o r tu n i t i e s 63Technolog ica l o p po r tu n i t i e s 64Nuclear waste storage o f f i s s i o n products 64
Summary o f o p p o r tu n i t i e s in research 65Summary o f p o te n t i a l impact on techno log ies 66References 67
Chapter 5: THIN FILMS 69In t ro du c t io n 69Thin f i lm p repa ra t ion 70
Evaporat ion 70Spu t te r ing 70Chemical vapor depos i t ion 71L iqu id phase ep i ta xy 7]Organometal1i c vapor depos i t ion 71E le c t r o p la t i n g 71
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A comment on techniques 72Oppo r tun i t ie s f o r research 72
Thin f i lm growth 7 2Surface m od i f i c a t io n 74Ma te r ia ls p repa ra t ion 75
Use o f phase spread 75Metastable phases 76
Surfaces and in te r f a ce s 77Surface roughness 77E p i t a x ia l growth 78In te r fa c e compounds and m u l t i l a y e r s 70Other 78 .
Novel geometr ica l methods 78D ie l e c t r i c s 79Cermet, nonhomogeneous and g ranu la r f i lm s 80Fi lm c ha ra c te r i z a t i o n 81Depos i t ion method c h a ra c te r i z a t i o n 81Theory 82
Research needs 82Research oppor tun i ' . i es f o r technology 83
S p e c t r a l l y s e le c t i v e s o la r absorbers 83Pho to vo l ta ic * 83In te r f a c e s t a b i l i t y 83Surfaces f o r s p e c i f i c environments 84High Tc superconductors 84M ic ro s t ru c tu re s 85
References 86
Chapter 6: SOLID STATE IONICS 88In t r o d u c t io n 88O ppo r tu n i t i e s , basic research 88
I n t e r c a l a t i o n compounds ■ 91Basic research needs 95Technology need-; 97
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Chapter 5,Thin Films (continued)
Chapter 6, So l id S ta te Ion ics (con t inued)Ba t te r ie s
The Anode The Cathode The So l id e l e c t r o l y t e Separators
Ca ta ly s is Meta l lu rgy Semiconductors References
Chapter 7: CATALYSIS
Chapter 8: SEMICONDUCTORSIn t roduc t io nStatus o f semiconductor techno logy , emphasizing s o l a r c e l l sOppo r tun i t ies f o r basic research
Prepara t ion and understand ing o f p e r fe c t c r y s t a l sI n t e r a c t i o n between defec tsRe la t ion o f bulk to sur face p rope r t ie sE le c t ron ic s t r u c t u r e and o p t i c a l p rope r t ie s o f po ly c r y s - t a l l i n e semiconductorsNucleat ion and growthAmorphous semi conductorsIn t e r a c t i o n o f energy beams w i t h semiconductors
Needs f o r bas ic research Exp lo ra to ry p repa ra t ion o f new semiconductors Refrac to ry ma te r ia lsC l a s s i f i c a t i o n o f p repa ra t ion techniquesTheories f o r mass and heat t r a n s f e r du r ing s o l i d i f i c a t i o n and condensat ionMethods f o r simultaneous s t r u c t u r a l , e l e c t r i c a l , o r chemical ana lys isTheories o f deep le v e l * i n t e r f a c e s , and .o f c a r r i e r recomb ina t ionPho toexc i ta t ion and charge separa t ion Technology needs
Mate r ia ls needs: Semiconductors; A n c i l l a r y m a te r ia l sGrowth and processing
References
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PREFACE
The present r e p o r t on research o p p o r t u n i t i e s i n new m a te r ia l s i s the product o f a d ive rse panel o f p h y s i c i s t s , chem is ts , and m a t e r i a l s s c i e n t i s t s . At the bes t , i t w i l l p rov ide some wisdom and guidance. A t the very l e a s t , the panel members themselves gained f rom the o p p o r t u n i t y to exchange ideas and p r i o r i t i e s .
The Panel was organized a t the request o f the Council o f M a te r i a l s Sciences o f the U.S. Department o f Energy 's M a te r ia l s Sciences D i v i s i o n to generate a report on needs and opportunities in basic research on new
materials w i th the i n t e n t i o n to p rov ide adv ice which " t h e D iv i s io n o f M a te r ia l s Science can use in p lann ing f u t u r e research p rograms.” F u r th e r , i t was made c le a r t h a t there should be a non-tenuous l i n k between bas ic research o p po r tu n i t i e s and ene rgy - re la ted problems f a c i n g techno logy . In o th e r words, the Panel was given the l i c e n se to take an u n in h ib i t e d " f a r - o u t " lo ok , but to bear in mind the mission o f the Department o f Energy. Any r e po r t by committee hammered toge the r i n one work ing sess ion has, by i t s very na tu re , to present the consensus which p r e v a i l s a t t h a t moment i n t ime by thosie present . The boundary c o n d i t i o n s , imposed by the Counci l o f Ma te r ia ls Science, ensure t h a t the panel r e p o r t i s t im e l y i f not s c h o la r l y . The r epo r t should be taken in the pe r spe c t i v e through which i t was prepared: a “ bes t e f f o r t " o f those present a t Pajaro Dunes, C a l i
f o r n i a , f rom Sunday n i g h t , August 19, to Fr iday noon, August 24, 1579.
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The Panel was a c t i v a te d in the Spr ing o f 1979. Members were chosen w i th e xpe r t i s e in d i f f e r e n t areas. Before meet ing, we prepared a l i s t o f p o te n t i a l t op ic s c l a s s i f i e d by types o f m a te r i a l s , phenomena, techn ique or f u n c t i o n . In doing so, i t was recogn ized t h a t a b u i l t - i n redundancy would r e s u l t : i . e . , a g iven research o p p o r tu n i t y m ight be d iscussed as a
type o f m a t e r i a l , o r a p roduc t o f a deve lop ing techn ique , o r i t s f u n c t i o n . Those who read the f i n a l r e p o r t w i l l recogn ise t h a t some redundancy has remained. For th i s ,w e o f f e r no apo logy ; in f a c t , the c o n t r a r y : i t i s a
p lus when some research appears a t t r a c t i v e from d i f f e r e n t p o in t s o f v iew.
A la rge m a t r i x was prepared p r i o r to the meet ing , l i s t i n g a l l the top ics on one ax is and the panel members on the o th e r . A t the i n i t i a l meeting on Sunday n ig h t , each panel member checked the t o p i c s o f i n t e r e s t to him. The top ics were then conso l ida ted i n t o subgroups which subsequen t ly became the chapte rs o f t h i s r e p o r t , and a coo rd in a to r was chosen f o r eachgroup as noted by the supe rs c r ip t s on p. i v . A t l e a s t one t o p i c , ce ram ics ,
was om i t ted because o f the comprehensive r e p o r t submit ted by th e Bowen
Panel l a s t year on t h a t s ub je c t . Morning and evening meet ings were scheduled
and a f te rnoons were l e f t f r e e f o r in fo rma l i n t e r a c t i o n s as i n the Gordon Conferences. Group meet ings were he ld in se r ie s and p a r a l l e l du r ing the f i v e days. There were a lso per iods where the e n t i r e panel met t o hear and reac tto the group th i n k in g a t severa l stages o f development.
An a t tempt was made by each group to s t r u c t u r e i t s c hap te r along the f o l l o w in g l i n e s :
• An in fo rm a t iv e d is cuss ion o f the presen t s t a t e o f the sc ience .
* S p e c i f i c areas where research o p p o r t u n i t i e s l i e .
• Needs o f the s c i e n t i f i c community which, i f met, would he lp app rec iab ly in addressing the research o p p o r tu n i t i e s .
• Needs and oppo r tu n i t i e s o f technology which stand to gain i f research o ppo r tu n i t i e s are rea l i z ed .
Each subgroup repo r t evolved in d i f f e r e n t ways, so th a t i n the f i n a l r epo r t a common format i s sca rce ly recogn izab le . This i s not s u rp r i s i n g s ince each f i e l d has unique fea tu res .
Each c oo rd ina to r took r e s p o n s i b i l i t y f o r c onso l id a t in g and e d i t i n g the rough d r a f t s prepared by the members working on h is o v e r a l l t o p i c a t the meet ing. The e n t i r e panel gave inpu ts du r ing the meet ing and dur ing the subsequent f i n a l phases o f p repa ra t ion .
We hope readers accept t h i s r epo r t in the s p i r i t in which i t was prepared: a snapshot taken in August 1979 o f promising researchoppo r tu n i t i e s in new ma te r ia ls apparent to the c o l l e c t i v e wisdom o f panel members.
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NEEDS AND OPPORTUNITIES IN BASIC RESEARCH ON NEW MATERIALSAugust 19 - 24, 1979
Council on Materials Science Panel on
Pro f . Nei l B a r t l e t t Pro f . J.D. C o rbe t t1
Dr. James Econorny Dr. Frank Fradin Dr. Fred R. Gamble Pro f . T.H. Gebal le*
Dr. John K. Hulm
Pro f . W i l l i am Johnson1*
P r o f . M. Br ian Maple3Pro f . Gerd M. Rosenb lat tDr. John Rowel l5Dr. A r thu r S l e i g h t 7
Dr. G. Br ian S t r e e t2
Pro f . David Turnbu l l
Dr. Sigurd Wagner8
Dr. Jack H. Wernick
Dr. M. Stan ley Whitt ingham*
Chemistry Dep t . , U n iv e r s i t y o f C a l i f o r n i a , Berke ley
Ames Laboratory & Dept, o f Chemist ry, Iowa S ta te U n iv e r s i t y , Ames
IBM Research Center , San Jose, C a l i f o r n i a
Argonne Nat iona l Labo ra to r ie s , Argonne, I l l i n o i s
EvX0N Research and Eng ineer ing , Linden, New Jersey
Dept, o f App l ied Physics & Center f o r M a te r i a l s ■ Research, Stan ford U n i v e r s i t y , S tan fo rd , C a l i f o r n i a
Westinghouse Research & Development, P i t t s b u r g h , Pennsylvania
Ma te r ia ls Science Dept . , C a l i f o r n i a I n s t i t u t e o f Technology, Pasadena, C a l i f o r n i a
Physics Dep t . , U n ive r s i t y o f C a l i f o r n i a , San Diego
Chemistry Dept . , Pennsylvania S ta te U n i v e r s i t yBe l l L a bo ra to r ie s , Murray H i l l , New Jersey
DuPont Centra l Research, Wilm ing ton , DelawareIBM Research Center , San J o s e , .C a l i f o r n i a
Div. Engrg & Appl ied Phys ics , Harvard U n i v e r s i t y
So la r Energy Research I n s t i t u t e , Golden, Colorado
Be l l Labo ra to r ie s , Murray H i l l , New Jersey
EXXON Research & Engrg., L inden, New Jersey
Superscript numbers identify Coordinator of corresponding chapter. *Chair
F o r S t a n f o r d U n iv e r s i t y :
R. G i roua rd , A dm in is t r a to r Gail Hamaker, P ro je c t Secre ta ry
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Speaker In v i t e d to Address the Panel:R.S. Fe ige lson , S tanfo rd U n iv e r s i t y Consu l tan t on E l e c t r o c r y s t a l 1 i z a t i o n
Counci l on Ma te r ia ls Science:
R.J. Maurer, U n iv e r s i t y o f I l l i n o i s , D i r e c t o r , CMS.
P.W. Anderson, Be l l Telephone Labora to r ies R.W. B a l l u f f i , M . I .T .W.D. Compton, Ford Motor Company B.R.T. Frost-, Argonne Nat iona l Laboratory W.D. K ingery , M . I .T .
D.A. S h i r l e y , U n iv e r s i t y o f C a l i f o r n i a , Berke ley J.C. Wheatley, U n iv e r s i t y o f C a l i f o r n i a , San Diego.
Department o f Energy:
W.L. C l i n t o n , DMo & Georgetown U n ive rs i t y0. W«rren, DMS & Los AlamosL. C. I a n n i e l l o , Ch ie f >■ Metal lu rgy & Ceramics Branch, DMS Donald K. Stevens, D i r e c t o r , DMS
REPORT HIGHLIGHTS
The Panel was g iven a broad (and d i f f u s e ) c h a r t e r and responded by producing e i g h t d i f f e r e n t c hap te rs , any one o f which cou ld e a s i l y be expanded i n t o a f u l l r e p o r t . The chapte rs are summaries o f oppo r
t u n i t i e s in t h e i r r e spec t iv e a reas . The reader i s r e f e r r e d d i r e c t l y
to the chap te r o f i n t e r e s t to ga in the pe rspec t i v e developed by the
panel. Here, we presen t h i g h l i g h t s o f general i n t e r e s t .
1. A la rg e number o f new phases can be made w i t h o u t a massive
e f f o r t , s imp ly by im ag in a t i v e a p p l i c a t i o n o f a v a r i e t y o f s t a t e - o f - t h e - a r t t e c hn iq ue s .New paths, beyond the t r a d i t i o n a l rou tes o f syn thes is , which
have the p o t e n t i a l o f v a s t l y in c re a s in g the number o f known compounds are d iscussed in Chapters 1, 3 and 5. T ranspo r t o f the re a c ta n ts through 1iq u id , v a p o r , o r vacuum can be h ig h l y c o n t r o l l e d w i t h modern i n s t r u m e n t a t i o n and feedback techn iques . F i lm syntheses-and c h a r a c t e r i z a t i o n make i t p o s s ib le to survey phase space ove r a w id e r range o f tempera tures and compos i t ion and i n much f i n e r d e t a i l than he re to fo re .
2• C lose -coup l ing o f s yn thes is and cha r a c t e r i z a t i o n e f f o r t s w i t h i n v e s t i g a t i o n s o f phys ica l and chemi c al p r o p e r t i e s i s h i gh ly d e s i r a b le .
The ideas which f l o w around a c losed loop cv- i n t e g r a t e d research i n v o l v i n g syn thes is - c h a r a c t e r i z a t i o n — i n v e s t i g a t i o n o f p r o p e r t i e s — m o d i f i c a t i o n o f s y n t h e s i s - . . . g e n e r a t e a s t im u l a t i n g and v igo rous env i ronmen t , conducive f o r c r e a t i v e re sea rch . An example i s d iscussed in Chapte r 3 where unusual magnet ic and supe rconduc t ing responses to chemica l s u b s t i t u t i o n on c e r t a i n s i t e s a re found i n t e r n a r y
phases. Progress in u n an t i c ip a te d d i r e c t i o n s — a ha l lmark o f bas ic research - can be a n t i c i p a t e d . The d is cove ry o f cathodes f o r h igh energy -dens i ty b a t t e r i e s r e s u l t i n g from research on the supe rconduc t ing p rope r t ie s o f i n t e r c a l a t e d laye red metal d icha lcogen ides i s an example discussed in Chapter 6. The use o f model systems to lea rn about r e l a t i o n
ships between s t r u c t u r e , compos i t ion and p ro p e r t i e s i s advocated f o r polymers (Chapter 2 ) , c a t a l y s i s (Chapter 7) and semiconductors (Chap te r 8 ) .
3. Amorphous m a t e r i a l s (Chapter 4 ) , and t h i n f i lm s (Chapter 5) of f e r a wide range o f systems which can increase bas ic under
s tand ing and o f f e r promise o f s o lu t io n s to t e c h n ic a l p rob lems . Basic concepts need to be e s tab l i s hed : answers to such
quest ions as - what i s a d e fe c t in an amorphous m a t e r i a l ; what r o l e does i t p lay in p l a s t i c de fo rm a t io n , d i f f u s i o n , atomic t r a n s p o r t , o r io n ic conduct ion? What i s the o r i g i n o f the extreme co r ro s io n r e s is ta n ce o f c e r t a in amorphous a l l o y s ? How does one descr ibe e le c t r o n s t r u c t u r e s and t r a n s p o r t phenomena i n amorphous a l lo ys?
4. Most i n n o va t i v e ino rgan ic syn thes is has been done in Europe w i th the United S ta tes p la y ing a minor and perhaps d im i n i s h ing r o l e . Un ive r s i t y -b a sed research e f f o r t s in the Un i ted Sta tes should be encouraged.
U n t i l r e c e n t l y , Europe has provided the U.S. w i t h w e l l - t r a i n e d s o l i d - s t a t e chem is ts , and has been a source o f new compounds. Tab le I I (Chapter I ) , re fe rences the f i r s t syn thes is o f some p ro to type " landmark compounds." The r e l a t i v e l y smal l c o n t r i b u t i o n from the Un ited S ta te s i s ev iden t . The long t ime i n t e r v a l s between the f i r s t s yn thes is o f t h e pro to type compound and r e c o g n i t i o n o f spec ia l p rope r t i e s i s a consequence o f the f a c t t h a t s yn thes is i n Europe was done f o r i t s own sake and n o t c lo s e l y coupled to p ro p e r t y measurements, phenomena o r th eo ry . Recent
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t rends in Europe (and Japan) show more i n t e r e s t and e f f o r t in e va lu a t in g newly synthesized m a te r ia l s . The needs o f the U.S. f o r s o l i d - s t a t e s c i e n t i s t s , s k i l l e d in syn thes is and f o r sources o f compounds to he lp in overcoming m a te r ia ls problems in ene rgy - re la ted techno logy w i l l r e q u i r e more un i ve rs i t y - ba sed research in our coun t ry along the l i n e s d iscussed in ( 1 ) , ( 2 ) , and (3) above. I t i s a cha l lenge f o r the fund ing agenc ies in Washington to encourage the c l ima te f o r doing so. Updated c r i t i c a l reviews o f in fo rm a t ion sca t te red throughout l i t e r a t u r e (Chapter 1) would he lp in b r in g in g new research to the f r o n t i e r s .
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INORGANIC SYNTHESIS
INTRODUCTIONI t would no t be poss ib le to c a l c u la t e the ground s ta te o f
2210 condensed atoms, l e t alone a simple p rope r ty such as t h e i r m e l t in g po in t , even i f one were given the atomic wave fu n c t i o n s , the l a r g e s t d i g i t a l computers, the most ta len ted s c i e n t i s t s a v a i l a b l e , and t ime s ince the beg inn ing o f the un ive rse . On the o th e r hand, a p r o p e r l y prepared system can be an analogue computer, o r , as i t i s more commonly pu t , a rea c t io n chamber and can p rov ide answers i n an e f f i c i e n t , economical way th a t sometimes leads to the unexpected. M a te r ia l s s yn thes is i s not l i k e l y to become an outmoded d i s c i p l i n e in the fo rseeab le f u t u r e .
A l l conce ivab le energy techno log ies are l im i t e d by ma te r ia l p e r
formance. I t is an obvious but perhaps over looked t ru i sm th a t to f i n d b e t t e r ma te r ia ls one f i r s t has to produce the m a te r ia l s . Some needs w i l l be met by purer o r b e t t e r forms o f known m a te r ia l s o r by a l l o y a d d i t i o n s and con t ro l o f morphology, but o thers w i l l r e qu i re the syn thes is o f t o t a l l y new chemical s t r u c t u re s .
Among the undiscovered compounds some w i l l be e a s i l y syn thes ized but have not y e t been sought. A g rea te r number re q u i re c r i t i c a l s y n th e s is c o n d i t i o n s , not on ly as regards the usual v a r ia b le s , bu t a lso as t o the c on ta in e r , p u r i t y , t r a n s p o r t i n g agents , "m in e r a l i z e r s , " e t c . The number
CHAPTER 1
V
o f remarkable new phases which are being discovered even in supposedly
simple systems (vide infva) a t t e s t s to the frequency w i t h which these
cond i t ions have not been met, even in p r i o r i n v e s t i g a t i o n s which m igh t
have been judged as thorough. In a d d i t i o n , a presumably la rge group o f1)mate r ia ls remain undiscovered ' e i t h e r because
• the ra te o f fo rmat ion under any se t o f c o n d i t i o n s i s ex t remely slow, or
• an upper temperature l i m i t f o r synthes is which i s f i x e d by some decomposit ion reac t ion is s t i l l too low f o r a reasonable r a t e fo rmat ion .
A good example is MOgSg, a coric' .ptual predecessor o f many Chevrel phases which can be made on ly by i n d i r e c t rou tes ; the analogous WgSQ and W6Seg probably e x i s t but have never been made, presumably because o f low de
composit ion temperatures ( l a t t e r case above). One can on ly imagine th a t some un t r ied so lven t , t r a n s p o r t i n g agent, o r vapor d epos i t io n rou te w i l l make these access ib le . Because o f these c i rcumstances, i t can be seen t h a t the boundaries o f what is poss ib le in the s o l i d s ta te are p o o r l y def ined and, co r respond ing ly , so is one 's v i s io n i n s o f a r as a p p l i c a t i o n s and processes.
Some degree o f synthes is by design can a lso be env is ioned : newphases which have the combined a t t r i b u t e s or any combinat ion o f van der Waals bonding o r open s t r u c tu re s ( f o r easy chemical s u b s t i t u t i o n and r ap id io n i c d i f f u s i o n ) , m e t a l l i c bonding ( f o r e l e c t r i c a l , thermal p rope r
t i e s and d u c t i l i t y ) and io n ic and cova len t bonding ( f o r s t reng th and hardness) .
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I t is use fu l to recognize t h a t the i n t e l l e c t u a l inpu t o f the s yn the t i c component to research can vary g r e a t l y . We recogn ize fo u r categories*.
1. Synthesis o f known ma te r ia ls in a s p e c i f i e d q u a l i t y , q u a n t i t y , c r y s t a l 1 i n i t y , shape, o rde r , e tc . This e f f o r t almost always has a se rv ice component; however, i t may a lso requ i re c re a t i v e e f f o r t s in the development o f techniques and apparatus.
2. Synthes is in o rde r to study a s p e c i f i c p rope r ty . The
e f f o r t may range from development to basic research. The phenomenological o r i e n t a t i o n may o f ten dominate, w i th synthes is pe r ae making a narrow c o n t r i b u t i o n . Product ion o f new s t ru c tu re s from such syn thes is e f f o r t s
is ra re .
3. Synthesis w i t h in a s t r u c t u r a l or r e la te d c lass f o r the
purposes o f e x p lo ra t io n , ex tens ion , o r e x t r a p o la t io n o f p ro pe r t ie s . Systematic measurements o f re la ted physica l p rope r t ie s are f r e q u e n t l y
the basis f o r ex tend ing p r e d i c t i v e c a p a b i l i t i e s and f o r t e s t i n g models.
In ves t ig a t io n s o f magnetic and e le c t r o n i c p rope r t ie s are examples. Th is work over laps w i t h type 2 (above), but the r o le o f the syn thes is e f f o r t i s more c r e a t i v e here.
4. Synthesis o f e n t i r e l y new types o f compounds. Research o f
t h i s cha rac te r f r e quen t l y precedes those o f types 1-3 above. Success provides new s o l i d - s t a t e s t r u c t u re s , and w i th these, the promise o f novel and new p r o p e r t i e s , new phenomena and new so lu t io n s f o r m a te r ia l s problems. These r e s u l t s a lso shape, r ede f ine , and broaden our understand ing and concept o f the s o l i d s ta te . Na tu ra l ly , the p r e d i c t . a b i l i t y o f any r e s u l tin t h i s ca tegory may be very smal1.
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OPPORTUNITIES FOR RESEARCHThe s yn th e t i c e f f o r t i s enhanced when there 1s a c losed loop between
the s yn thes is , c h a r a c te r i z a t i o n , and p rope r ty eva lua t ion , J o i n t e f f o r t s across academic departments in u n i v e r s i t i e s are to be encouraged i n order to b e n e f i t f rom i n t e r d i s c i p l i n a r y inpu ts as is done in the l a r g e r indus
t r i a l l a b o r a to r i e s . More e f f e c t i v e understand ing o f the r e l a t i o n s h i p between s t r u c tu r e and va r ious phenomena very much needs to be developed.
A novel s t r u c tu re i s o f t e n the f i r s t i n d i c a t i o n o f p o t e n t i a l l y i n t e r e s t i n g p ro pe r t i e s , and converse ly , measurement o f an unusual p rope r ty s ig na ls an unusual s t r u c tu re .
The search f o r t o t a l l y new ma te r ia ls n a t u r a l l y has assoc ia ted w i t h i t the h ighes t r i s k , and the l a r g e s t reward as w e l l . The i n i t i a l proposal
•must p rov ide some good evidence or basis f o r b e l ie v in g t h a t a p a r t i c u l a r system o r type o f reac t ion w i l l produce a p a r t i c u l a r kind o f p roduc t , bonding type or s t r u c t u r e , al though ac tua l r e s u l t s o f ten demonstrate the resea rche r 's f a l l i b i l i t y in p r e d i c t i n g the unknown. A broad pe rspec t ive in d e s c r i p t i v e and s t r u c t u r a l chemis t ry and in s o l i d - s t a t e physics i s necessary as we l l as the a b i l i t y to recogn ize and i d e n t i f y s i g n i f i c a n t new reac t ions and products . Synthesis can a lso g re a t l y b e n e f i t f rom t heo r ies which a id i n t u i t i o n , make p re d i c t i o n s , and are y e t r igo rous enough to go beyond emp ir ic ism in a id in g the understanding o f s t r u c t u r e s , s t a b i l i t y , and p r o p e r t i e s , e s p e c ia l l y o f e n t i r e l y new types o f compounds. The natu re o f hydrogen in in te rme ta l 1ics and o the r ino rgan ic phases is an example.
A number o f novel m a te r ia ls can poss ib ly be prepared by -'high temperat u r e " rou tes where one takes advantage o f the unusua l, o f ten complex, mo lecu la r species which form in h igh- temperatu re gaseous systems as we l l as in h igh- temperature l i q u i d s where k i n e t i c b a r r ie r s are a lso min im ized. Amorphous
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products from quenching experiments in the l i q u i d systems are discussed in Chapter' 4. New and usefu l t r a n s po r t reac t ions u t i l i z i n g complex e q u i l i b r i a may be l im i t e d by the i n v e s t i g a t o r ' s understanding o f these systems and the imag ina t ion in dev is ing t r a n s p o r t i n g agen ts . A proper choice o f p recursor species f o r syn thes is may be in v a lu a b le . The u l t im a te approach in s yn th e t i c e f f o r t s , us ing e p i t a x i a l or mo lecu la r beam techn iques to lay down reac tan ts a few laye rs a t a t ime , is discussed in Chapter 5. M o l t e n - s a l t so lven t systems as a s yn th e t i c means have rece ived r e l a t i v e l y l i t t l e a t t e n t i o n , save f o r unusual compounds which may be f r e q uen t l y separated from b ina ry systems. E lec t rochemica l techn iques f o r s yn th e s is , discussed in Chapter 3, have been on ly rough ly exp lo red . The choice o f s p e c i f i c systems in which to seek new m a te r ia l s i s the most d i f f i c u l t as
pect and may o f ten depend on s l im leads, chemical i n t u i t i o n o r j u s t e xp lo ra t io n o f the unknown.
On the o the r hand, Category 3 (pg. 3) synthes is i s more p r e d i c t a b le .I t can lead to e x c i t i n g research, p a r t i c u l a r l y when e xp lo r in g systems w i t h unusual s t r u c tu re s or p ro p e r t i e s . I t i s a lmost a p r e r e q u i s i t e to have the syn thes is and p rope r ty in v e s t i g a t i o n s and theory c l o s e l y coup led. (By p rope r ty here we mean any o f a wide v a r i e t y o f phys ica l o r chemical phenomena. Examples might be e l a s t i c i t y , e l e c t r i c a l c o n d u c t i v i t y , r e a c t i v i t y ,
e t c . ) The cha l lenge is to e s tab l i s h the connect ion between compos i t ion and/or s t r u c t u r e and the p r o p e r t y ( i e s ) .
T r a d i t i o n a l methods o f support o f u n iv e rs i t y -ba sed research have not a c t i v e l y prov ided a c l im a te in which the c lo se -coup l in g would t h r i v e , in c on t r a s t to the o rgan iza t io n o f i n d u s t r i a l and na t iona l l a b o r a t o r i e s . Thought should be given w i t h i n the D iv i s io n o f M a te r ia l s Science to ways in which be t t e r Category 3 and 4 ( p g .3) type research can be made to t h r i v e in u n i v e r s i t i e s .
A c o l l e c t i o n o f compounds w i t h unusual s t r u c t u r e s o r p r o p e r t i e s , which o f f e r o p p o r t u n i t i e s f o r f u r t h e r resea rch , a re g iven in Table I . Noteworthy is the la rg e number o f h ig h l y reduced m a te r i a l s more o r l e s s along the i n t e r f a c e between meta ls and conven t iona l s a l t s . Many o f these have been found in supposedly "s imp le " b ina ry systems, g iven tes t imony f o r the e a r l i e r a s se r t io n t h a t the number o f t r u l y unusual compounds to be d iscovered is la rge indeed.
RESEARCH NEEDS
1. Nat iona l e f f o r tA broad and pervas ive need f o r new and b e t t e r m a t e r i a l s r e q u i r e s t h a t
inc reased e f f o r t s be devoted to t h e i r c r e a t i o n •— b e t t e r examples o f known m a te r ia l s in impor tan t a p p l i c a t i o n s , broad ranges o f m a te r i a l s to a i d and t e s t ou r unders tand ing o f s o l i d - s t a t e s t r u c t u r e and p r o p e r t i e s , and t o t a l l y new compounds f o r which a pp l i c a t i o n s and phenomena have no t y e t been con
ce ived . The long-range fu t u r e depends p a r t i c u l a r l y on the l a s t . The h i s t o r y o f major advances in s o l i d - s t a t e sc ience and o f s i g n i f i c a n t a p p l i
c a t io n s and dev ices there f rom in d ic a te s t h a t in n e a r l y a l l cases th e o r i g i n a l p ro to type compound was syn thes ized w i t h o u t des ign f o r a p p l i c a t i o n o r p ro p e r t y , a f t e r which subsequent i n v e s t i g a t i o n s and development l e d to the advance o r a p p l i c a t i o n . An a dm i t te d ly incomp le te , bu t perhaps rep resen
t a t i v e , l i s t o f such landmark advances in syn thes is - u s u a l l y unrecognized a t the t ime f o r t h e i r s i g n i f i c a n c e — is g iven in Tab le I I .
A con t inu ing na t iona l problem rega rd ing the o r i g i n o f these landmark . m a te r i a l s seems apparent; by a l l measures the amount and breadth o f s o l i d - s t a t e ino rgan ic syn thes is i n the United Sta tes i s r e l a t i v e l y sm a l l , i n c o n t r a s t w i t h the general s i t u a t i o n in Europe and Japan. In Germany, the re a re 25 u n i v e r s i t i e s where a t l e a s t one p ro fe sso r w i t h i n o rg an ic I n t e r e s t s d i r e c t s research in s o l i d - s t a t e syn thes is and c h a r a c t e r i z a t i o n , p lu s th ree w e l l - I n t e g r a t e d syn thes is programs a t the M a x -P la n c k - I n s t i t u t e 1n S t u t t g a r t .
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The U.S. ou tpu t o f novel s yn thes is , a t l e a s t as desc r ibed in the s c i e n t i f i c l i t e r a t u r e , r e f l e c t s the f a c t th a t s tuden ts i n chem is t r y are o n ly r a r o l y ex
posed to the cha l lenges o f the s o l i d s t a t e . Government and i n d u s t r i a l l a b o ra to r ie s f i n d i t d i f f i c u l t to f i n d U .S . - t r a i n e d personnel in s o l i d - s ta te syn thes is and c h a r a c t e r i z a t i o n . As po in ted ou t i n Chapter 6, European- trained s o l i d - s t a t e s c i e n t i s t s have been an impo r tan t source which seems to be d r y in g up a t p resen t . A recen t survey o f p u b l i c a t i o n s in the Journal of Solid State Chemistry shows th a t c o n t r i b u t i o n s from the United Sta tes have f a l l e n from a lmost 50% a t the beg inn ing o f t h i s decade to less than 20% in 1979. ( See l e t t e r f rom M.J. Sienko a t tached as an appendix to t h i s c h a p te r . )
2, Nat ional Needs: Use o f s t a t e - o f - a r t equipment
On the o the r hand, some opt im ism can be der ived from the o p p o r t u n i t i e s i f the subs tan t ia l ga ins in i n s t r um en ta t io n and techn iques can be r e a l i z e d . These new c a p a b i l i t i e s a re expens ive , and means should be found to in c rease t h e i r u t i l i z a t i o n in l a b o r a t o r i e s . Apparatus and to o l s which have become a v a i l a b le o r have been app l ie d to s yn thes is problems o n l y i n re cen t yea rs in c lude :
• Super io r con ta ine rs and means f o r t h e i r f a b r i c a t i o n ,• E x ce l le n t i n e r t atmosphere f a c i l i t i e s ,• High r e s o lu t i o n x - r a y powder techn iques ; au tomat ic d i f f r a c t o m e t e r s ,• Improved means o f a na ly s i s (m ic rop robe , ESCA, e le c t r o n m ic roscopy ,
e t c . ) .
3. Use o f theory
Both i o n i c d i f f u s i o n and phase s t a b i l i t y are o f pr ime impor tance to syn thes is . Many s o l i d - s t a t e s yn thes is r e a c t io n s r e q u i r e good i o n i c d i f
fu s io n a t temperature i n o rde r to succeed*, the need f o r a much b e t t e r under
s tand ing o f t h i s process i s d iscussed more f u l l y in Chapter 6. The prob lem
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o f phase s t a b i l i t y pervades many pa r ts o f t h i s r e p o r t i m p l i c i t l y and e x p l i c i t l y . A l though the re e x i s t a number o f e m p i r i c a l l y based p r e d i c t i v e t h eo r ie s o f phase s t a b i l i t y o f s imple in te rme ta l 1i c phases, i . e . , o f Engel - B r e w e r , ^ and o f M iedema,^^ these have had o n l y l im i t e d success and no a p p l i c a b i l i t y to a wide range o f o the r m a te r ia l s which are no t i n t e r - m e t a l l i c . One a n t i c i p a t e s t h a t new, complex, s t rong ly -bonded m a te r ia l s
w i l l occur w i th a proper combinat ion o f t r a n s i t i o n meta ls and non-metals as regards s i z e , e l e c t r o n e g a t i v i t y , e le c t ron coun t , adm ix tu re o f c o va le n t and m e t a l l i c bonding, and q u a l i t a t i v e band fea tu re s . The search f o r new m a te r ia l s f o r superconduct ing a p p l i c a t i o n s , hydrogen s to rage , and many o the r energy-based uses very much needs advanced t h e o r e t i c a l i n s i g h t f o r guidance. Increased speed and accuracy in the c a l c u l a t i o n o f e l e c t r o n i c band s t r u c t u r e s g ives some hope t h a t such c a l c u la t i o n s may be use fu l f o r p r e d i c t i v e as we l l as a n a l y t i c a l purposes. Both em p i r i c a l approx imat ions and more r igo rous c a l c u la t i o n s are badly needed to gu ide t h i n k i n g on s yn th e t i c o r o je c t s on a week-to-week bas is.
4. Data c o l l e c t i o n and eva lua t ion
Problems o f phase s t a b i l i t y are a lso c en t ra l i n d i s c u s s io n s . i n l a t e r chap te rs . The search f o r new compounds w i th unusual p r o p e r t i e s depends
upon ready access ' ib i l 1 ty to p r e v io u s l y obtained da ta on the thermodynamic
■and phys ica l p r o p e r t i e s o f p o t e n t i a l candidates and arid t h e i r ana logues,
"keady a c c e s s i b i l i t y ' 1 in t h i s sense requ i res con t inuous e f f o r t s to p ro v ide update comp i la t io n s o f c r i t i c a l l y evaluated data o f va r ious types .
j K w S k y l a r l y use fu l has been the c r i t i c a l e va lua t io n o f b in a r y phasefn /b l ished in volumes by Hansen and Anderko (1958 )17 and supp le -
iby £ ! 1 i o t t , ^ and S h u n k . ^ For many purposes , the phase diagram
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i n fo rm a t ion generated in the ten to f i f t e e n years s ince Shunk's review l i e s b u r ie d , and unused by those who need i t . The la c k o f c r i t i c a l l y eva lua ted data i s f e l t so seve re ly t h a t i t has lead one i n d u s t r i a l l i b r a r y ^ to d i s t r i b u t e c u r r e n t phase diagrams i n a l o o se lea f fo rma t? 1 which i s use fu l but l im i t e d by la ck o f completeness and c r i t i c a l i t y . The same is t r u e o f many o th e r data needed by m a t e r i a l s s c i e n t i s t s , such as the a l l o y thermodynamic data compi led f o r an extended pe r iod by Hu l tg ren and coworkers o f the Un ive rs i ty o f Cal i f o r n i a , and o the r thermodynamic data fo rme r ly computed by K.K. K e l le y o f the U.S. 8ureau o f Mines. An example o f a usefu l ongoing p r o j e c t i s Roberts c o n t in u in g n o n - c r i t i c a l , but complete c om p i la t i o n o f supe r conduct ing da ta .
The Panel i s aware t h a t the need f o r da ta o r g a n i z a t i o n and e va lu a t io n e x i s t s over a broad range o f phys ica l sc ience ,and t h a t t h i s problem has been s tud ied by a recen t Nat iona l Acaderny o f Sciences committee ( th e Committee on Data Needs) and commented upon by o t h e r Academy committees ( the Committee on Chemical Sciences and the Committee on High Temperature Science and Technology) . Those committees proposed t h a t 0.1 - 0 .2% o f the Federal research and development budget be s p e c i f i c a l l y earmarked f o r c r i t i c a l e va lua t io n and c om p i l a t i o n o f data in a l l f i e l d s o f sc ience.
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TABLE I:
COMPOUND
Examples o f compounds where an unusual s t r u c t u r e o r an unusual p rope r t y s ig na ls a research o p p o r t u n i t y .
More d e t a i l e d d is c u s s io n in l i t e r a t u r eUNUSUAL FEATURE
Nb5Se4
C s ^ 0 3 , CSyO
I n f i n i t e chains o f MgXg-type c l u s t e r s shar ing faces
Nonmetal s t r u c t u r e in a metal a r ra y ; photoemiss ion
Ref. 2
Ref. 3
T V S t rong ly bonded columns o f metal polyhedra
Ref. 4
NaMo^O^, ScgClg I n f i n i t e chains o f metal oc tahedra , high s t a b i l i t y
Ref. 5, 6
Z rC l , ZrBr I n f i n i t e , s t r o n g l y bound double metal sheet s t r u c t u r e
Ref. 7,
SmBg, SmS
(RE)Mo6Se8 Nb3X X=A£,Ge,Si
(RE)Rh4B4
M(Ch)2
L i 3N
Te2C l2 , B i l ,
Hg3-xAsF6
Mixed va lency (5d, 4 f )
C lu s te r compound/high c r i t i c a l f i e l d High c r i t i c a l c u r r e n t
Coexis tence o f s upe rc ondu c t i v i t y and magnetism
Layered cha lcogenide charge den s i t y waves,
High i o n i c c o n d u c t i v i t y Unusual metal chains
I n f i n i t e 1 i n e a r . s t r i n g s o f Hg
C a ^ B i ^ , Na3Hg2 » Heavy meta1 c l u s t e r s in in te rm e ta l 1 i cKSn(Ag608 )AgN03
phases.Superconduct ing , i o n i c c l a t h r a t e
PdSi (amorphous) So f t magnet ic m a te r i a l s , co r ro s ionand d e r i v a t i v e s re s is ta n ce
Chapter 3
Chapter 3 Chapter 3
Chap te r 3
Chapters 3, 6
Ref. 8 Ref. g, 10
Ref. 11 Ref. 12
Ref. 13 Ref. 14
PVF P ie z o e l e c t r i c polymer Chapter 2
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TABLE I I : Some ino rgan ic syntheses le ad in g to landmark developmentsin s o l i d - s t a t e science.
PROTOTYPE COMPOUNDFIRST REPORTED
BY COUNTRY YEARSUBSEQUENTOEVELOPMENT
K0 .5^°^2 ’LiMoSp *0 .6 NH3
Rudor f f Germany 1959 Ba t te r y cathodes (L1xT1S2 )
COC_> Fredenhagen Germany 1926 M e t a l l i c g r a p h i t e ,
h igh -ene rgy b a t t e r i e s
0 -a lumina (Mg, Ca,) (Na)
RankinS t i l l w e l l
U.S.A.U.S.A.
19161926
So l id e l e c t r o l y t e
Zr0 2 (Y2 03 ) Nernst Germany 1900 0 2 sensorZr02 (Ca0) Ruf f Germany 1929 0 2sensor
V3Si Wallbaum Germany 1939 A15 h igh - tempe ra tu re superconducto rs
SnMogSg(Mo6 Se8 ) MH Mo6 ChB
EspelundChevrel
NorwayFrance
19671971
Superconducto rs - very h ig h f i e l d s
K2 [P t(CN) 4 ]X0 > 3 Krogmann Germany 1968 One-dimens iona l metals by o x id a t i o n
B i 2 (Mo04 ) 3 Zambonini I t a l y 1915 O x id a t io n c a t a l y s tHxM° ° 3 Glemser Germany 1951 E le c t ro ch rom ic
d i s p l a y s , p ro ton m o b i l i t i e s in s o l i d s
Ba T i O3 Tammann Germany 1925 F e r r o e l e c t r i cLiNb03 Slie France 1937 p i e z o e l e c t r i c , i.on-
1 i n ea r o p t i o s •
BaFe12°19 Ade lsko ld , Schrewel ius
Sweden 1938 F e r r i t e s , memories
InP Th ie l Germany 1910 I I I - V semiconductors( RE) N i 5 Klemm Germany 1943 Strong magnets
Syn the t ic z e o l i t e Bar re r England 1948 Many c a t a l y s t s
Amorphous Si Konig Germany 1944 So la r c e l l s
- 12 -
REFERENCES:
1. L. Brewer, J. Chem. Ed. , 3_5, 153 (1958) .
2. K. Se l te and A. Kjekshus, Acta Chem. Scand., 1_7, 25.60 (1963) .
3. A. Simon, 1. anorg. a l l g . Chem., 301 (1973)
4. H.F. Franzen and J.G. Smeggi l , Acta C r y s t . , 825, 1736 (1969).
5. C. To ra rd i and R.E. McCarley, 0. Am. Chem. Soc . , 101, 3963 O 979)-
6. K.R. Poeppelmeier and J.D. C o rb e t t , ibid., 100, 5039 (1978) .
7. R . L . Daake and 0 .0 . C o rbe t t , Ino rg . Chem., 16, 2029 (1977).
8. U. v. A lpen , «3. So l . S ta te Chem., 29, 379 (1979).9. R. Kniep, D. Mootz and A. Rabenau, Z. anorg. a l l g . Chem., 422, 17 (1976) .10. H.G. von Schnc r ing , H. von Benda, and C.Kalveram, Z. anorg. a l l g .
Chem., 438, 37 (1978) .
11.. 1.0 . Brown, B.D. C u t f o r t h , C.G. Davies, R.J . G i l l e s p i e , P.R. I r e l a n d ,and O.E. V e k r i s , Can. 0. Chem., 52, 791 (1974).
12. Reviewed by H. Scha fe r , B. Eisenmann, and W. M u l l e r , Angew Chem., 85,742 (1973) . .
13. M. B. Robins, K. Andres, T.H. Geba l le , N.A. Kuebler, and D.B. MclJlman,Phys. Rev. L e t t . 17, 917 (1967) .
14. W.Klement, R.H. W i l l e n s , and P. Duwe-z, Nature 187, 869 (1977) .
15. L. Brewer in Phase S t a b i l i t y i n Metals and A l l o y s , eds. p .S , Rudman,J. S t renge r and R . I . J a f f e , McGraw-H i l l , New York, N.Y.> 1967, p. 39.
16. A.R. Miedema, F.R. deBoer, and P.F. deChate l , J . Phys. F: Metal Phys . ,3, 1558 (1973).
17. M. Hansen and K. And i rko , C o n s t i t u t i o n o f B ina ry A l l o y s , 2nd e d . ,McGraw H i l l , New Yo rFTT958TT ~
18. R.P. E l l i o t , C o n s t i t u t i o n o f B ina ry A l l oys , F i r s t Supplement,McGraw Hit ' 1, ~New~York (1965)1
19. F.A. Shunk, C o n s t i t u t i o n o f B ina ry A l l o y s , Second Supplement,McGraw Hi 11, 'New York "(1969)
20. W.G. M o f f a t t , The'Handbook o f B ina ry Phase Diagrams, GeneralE l e c t r i c T f 9 7 6 } .
21. — The Handbook o f B ina ry Phase Diagrams, Vo ls . 1, 2 , 3 ; GeneralG e n e r a T E l e c t r i c (1976).
EXHIBIT 1. as r e f e r r e d t o on pg . 7 o f C h a p t e r 1."•s
jQ v J o u r n a l o f S o l id S t a t e C h e m i s t r y
I - ' ' / A n I n t e r n a t i o n a !. J o u r n a l
O ’ Oc tobe r 16, 19 79
Prof . T . H , Geba l le Depa r tmen t o f App l ied Phys ics Stan fo rd U n i v e r s i t y S tan fo rd , C a l i f o r n i a 94305
Dea r Ted:
We have j u s t comp le ted an a na ly s i s o f the papers p ub l i s h ed in the Journa l o f So l id State Chem is t r y in connec t ion w i t h p la n n in g for 1980 and 1981. The resu l t s c le a r l y show tha t there has been a de c l i n e in the percen tage o f papers o r i g in a t i n g in the U .S . The appa ren t ga iners are F rance, Germany, and Japan. Here are the r e la t i v e pe rcen tages , compar ing the 3 - y r per iods 1970-72 and 1976- 78. A lso in c lu ded are the re su l t s fo r the f i r s t 9 months o f 1979 .
1970-72 1976-78 19 79 (9 mos .)Aus t ra l ia 8 .4% 3.3% 4 .4%England 2 .8% 5.9% 3.7%France 15.6% 25 .7% 25 .7%Germany 3 .1% 5.1% 9.6%Japan 1 .1% 7.4% 7.4%Nethe r lands 7.8% 4.2% 8 .1%U .S . 47 .2% 25 .6% 19.1%a l l o thers 14 .0% 22 .8% 22 .0%to ta l number o f
papers pub l ished 358 544 136
Acco rd ing to the French members o f my Ed i to r ia l Adv iso ry Board , the enhanced p ub l i c a t i o n rate o f the French labo ra to r ies is a d i r e c t r e su l t o f in c reased government suppor t fo r s o l i d s ta te chem is t r y re sea rch in F rance . The ac tua l e xp lo s io n o f Trench p ub l i c a t io n in t h i s aroa is even b igge r than our f igu res in d ic a te s ince we need to ta ke i n t o accoun t t h a t the French have s ta r ted two ma te r ia ls jou rna ls in the French language s in ce 1972.
S in ce re ly y ou rs ,
M ,J. S ienko , Edi to r Pro fessor o f C hem is t r y
M JS /by
Editor:Profenor M. J. SienVo Deportment of ChemiUry Cornell Unfvertity llhoco, New YorJf 14853 (607) 256-3303
Publisher!* ACADEMIC PMSS INC., J11 Fifth A»«nM«, N«w York, N«w York 10003 (214) 741-6»00
CHAPTER 2
BASIC RESEARCH ON NEW POLYMERIC MATERIALS
INTRODUCTION
The importance o f polymer ma te r ia ls to the major energy techno log ies
is c l e a r l y i l l u s t r a t e d by the many programs c u r r e n t l y be ing funded by the
Department o f Energy. Most o f these programs are o f a sho r t - range na tu re and depend on the use o f , o r m od i f i c a t io n o f , c u r ren t s t a t e - o f - t h e - a r t polymer m a t e r i a l s P I t i s the purpose o f t h i s sec t ion to de f ine va r ious areas o f polymer research which could lead to new m a te r i a l s which combine p ro pe r t i e s c r i t i c a l to f u t u r e DOE needs. This i s not in tended as a comprehensive overv iew f o r polymer research; bu t r a t h e r , f i v e areasenumerated below have been se lected f o r p a r t i c u l a r c o n s ide ra t io n because o f the s p e c i f i c backgrounds o f the "a u th o rs . " These in c lu d e :
• High temperature polymers• High strength-modulus polymers and composites
• Rad ia t ion s e n s i t i v e / s t a b le polymers• Imperv ious/semipermeable /so lub le polymers
• E l e c t r i c a l p rope r t ie s o f polymers.In cons ide r ing these f i v e to p i c s , there are several themes t h a t seemed to be common to a l l o f the areas. Hence, these are d iscussed in these opening paragraphs.
We i d e n t i f i e d as a very impor tan t requ irement the need to c l o s e l y v/ed resea rch on fundamental polymer phenomena to s trong s y n t h e t i c programs.
1
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In a l l o f the proposed programs the need to b e t t e r understand s t r u c t u r e - p rope r t y r e l a t i o n s h ip s was c l e a r l y ev iden t . For example, in conduct ing polymers, s t r u c t u r a l requ irements assoc ia ted w i t h the development o f conduct ing e le c t ron s i s not y e t understood. Conceptual approaches t o
combine the unique e l e c t r i c a l c o n d u c t i v i t y fea tu res w i t h des ired Theolog
i c a l p ro pe r t ie s have as y e t not been de f ined . With h igh- tempera tu re polymers, s t r u c t u r a l f ea tu re s deemed necessary f o r thermal s t a b i l i t y are s t i l l no t c l e a r l y understood, l e t alone the complex mechanisms o f degradat ion observed under d i f f e r e n t environments. In the case o f h igh strength-modulus polymers and composites there is v i r t u a l l y no under
s tand ing o f the rheo log ica l and morpholog ical c h a r a c t e r i s t i c s o f thermo
t r o p i c ( r i g i d , r o d - l i k e ) polymers which would p rov ide a basis f o r understanding t h e i r unusual behav io r. Work on deve lop ing new h ighe r strength-modul us r e i n f o r c i n g agents f o r use in composites i s a t a s t a n d s t i l l because o f the lack o f i n s i g h t as to the s t r e n g t h - l im i t i n g fea tu res i n f i l amen ts w i th g lassy , p o ly c r y s ta l 1 ine o r r i b b o n l i k e morpho logies . In the area o f r a d ia t i o n s tab le ( s e n s i t i v e ) polymers t h i s f i e l d remains more an a r t than a sc ience. Fundamental unders tand ing o f mechanisms o f re a c t io n s o f polymers when exposed to d i f f e r e n t types o f r a d i a t i o n i n c lu d ing neutron, e l e c t r o n beam, x - r a y , deep I)V and near UV i s despe ra te ly needed, s ince polymer s c i e n t i s t s work ing in these areas a re fo rced to operate on an emp i r ica l bas is . With respec t to pe rm eab i l i t y o f gases through f i lm s , some understanding as to the mechanisms has been developed but as y e t l i t t l e progress made on d e v i s in g new, more imperv ious f i lm s o r semi-permeable membranes based on these concepts.
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One very impor tant concern i d e n t i f i e d by t h i s s tudy group, which in pa r t may exp la in the slow ra te o f progress on deve lop ing new polymers, i s the almost complete absence o f strong polymer s y n th e t i c c a p a b i l i t i e s in the u n i v e r s i t i e s . Thus, wh i le most polymer groups in u n i v e r s i t i e s have brought toge ther s c i e n t i s t s w i th ou ts tand ing s k i l l s in polymer c h a r a c t e r i za t ion f o r study o f s t r u c t u r e - p r o p e r t y r e l a t i o n s h ip s , the complementary e f f o r t in synthes is i s la c k in g . H i s t o r i c a l l y , t h i s balance was main ta ined p r im a r i l y through the very strong syn the t ic e f f o r t s in i n d u s t r y ; however, dur ing the past decade the chemical in d u s t r y has s h i f t e d i t s focus from developing new polymers to improving e x i s t i n g systems and processes.
Hence, a la rge void now e x is t s in the area o f new polymer syn thes is .
Another p o in t which is common to a l l o f the in d ic a te d programs is the need to design in t o polymer s t r u c tu re s those fe a tu re s which permit ease o f rap id melt processing in to the myriad o f forms such as i n j e c t i o n molded pa r t s , extruded f i lm s and f i b e r s c oa t ings , e t c . Those polymers which are processed from so lven ts are c l e a r l y less d e s i r a b le , not on ly because o f the energy requ i red to evaporate the so lven t but a lso because o f the need to e l im ina te so lven t wastes from e f f l u e n t s . F i n a l l y , i t i s apparent to t h i s group tha t the po te n t ia l to develop new and unusual polymers which combine unique p rope r t ie s t a i l o r e d to s p e c i f i c goals i s j u s t in i t s in fancy . This v e r s a t i l i t y is a n ' a t t r a c t ! v e fea tu re o f polymer ma te r ia ls and the need f o r h ig h ly t ra ined polymer s yn th e t i c sc i .en t i s ts w i l l increase sharp ly in the coming years.
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With respec t to basic s tud ies d i re c ted a t high temperature po lymers , there is cons iderab le in fo rma t ion o f an emp ir ica l natu re which has b u i l t up over the past 20-25 years on t h i s s u b je c t . 2 U n fo r tu na te ly , there i s considerab le confus ion as to the l im i t a t i o n s o f present m a te r ia l s , and, in p a r t i c u l a r t h e i r mechanisms o f degradat ion. Thus, e lastomers such as s i l i c o n e s , polyphosphazenes and carborane polymers are thought to degrade i n the range o f 200°C. A program aimed a t d e f i n in g the exact l im i t a t i o n s can be designed around using a v a i la b le ma te r ia ls . A second approach to high temperature elastomers invo lves synthes is o f new p e r f l u o ro polymer s t r u c tu re s which can be re a d i l y c ro ss - l in ked to y i e l d usefu l e las tomer ic p ro pe r t i e s . The optimum design f o r such s t ru c tu re s as we l l as t h e i r mechanisms o f degradat ion would appear to be o f cons iderab le s c i e n t i f i c i n t e r e s t . F i n a l l y , another approach which may be o f va lue is to take advantage o f the high temperature polymer technology developed over the past 25 years . Thus, by modify ing these var ious s t r u c tu re s to reduce t h e i r Tq below ^ 150°C could y i e l d a mater ia l which behaves as an e las tomer a t temperatures above 175°C.
The design o f high- temperature polymers which inco rpo ra te the des ired melt rheology fea tu res combined w i th ex tremely high s t re ng th - s t i f f n e s s fea tu res represents a major chal lenge to polymer s c i e n t i s t s .
Basic s tud ies on morphology, rheology and new syn th e t i c approaches must be pursued to r e a l i z e th i s goal.
The s ig n i f i c a n ce o f the above programs to long-range needs i s c l e a r l y i l l u s t r a t e d by cons ider ing the fo l l ow in g a p p l i c a t i o n s . Thus i n
HIGH TEMPERATURE POLYMERS
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geothermal energy requirements* there is a need f o r new e lastomers as gaskets and sea lants which are s tab le to 275°C and r e s i s t a n t to b r in e and H2S. S im i l a r l y , po lymer-concre te composites designed as c o r r o s i o n - r e s i s t a n t s t r u c t u r a l ma te r ia ls w i l l requ i re development o f polymers s ta b le to 400°C. Eas i l y fa b r ic a ted h igh- temperatu re polymers w i t h s i g n i f i c a n t l y supe r io r p ro pe r t ie s over l i g h t and r e f r a c t o r y metals are needed f o r many areas concerned w i t h energy conserva t ion . Other programs p e r t i n e n t to energy conserva t ion inc lude h igh temperature s o l i d l u b r i c a n t s and lam ina t ing re s in s f o r high performance composites.
HIGH STRENGTH- HIGH MODULUS MATERIALS
In t h i s sec t ion s tud ies on both r i g i d , r o d - l i k e polymers w i l l be considered along w i th needs f o r improved r e i n f o r c i n g agents . Work on r i g i d r o d - l i k e polymers has focussed almost comple te ly on the development o f l y o t r o p i c systems t h a t are spun from so lu t ions in to f i l amen ts ( e . g . , K e v l a r ) . ^ As such, a cons ide rab le body o f knowledge has developed on such ma te r ia ls w i t h respec t to t h e i r rheo log ica l and morpho log ica l c ha r a c t e r i s t i c s . On the o the r hand, thermot rop ic polymers they can
be processed from the me lt ) are j u s t now being descr ibed in the l i t e r a t u r e ^ and there i s e s s e n t i a l l y no bas ic understanding as to t h e i r morphology and rheology. Thus there is a g rea t need to develop a f a r b e t t e r knowledge base as to the fundamental behav io r o f these melts c o n s i s t i n g o f r i g i d r o d - l i k e s t r u c t u r e s , s ince they o f f e r a f a r g rea te r degree o f v e r s a t i l i t y in terms o f f a b r i c a t i n g a wide v a r i e t y o f forms w i t h o u t s ta n d in g ly h igh mechanical p r o p e r t i e s . To da te , on ly c e r ta in aromat ic po lyes te rs
- 19 -
have shown these unique fea tu res and i t i s c r i t i c a l t h a t the key s t r u c t u r a l fea tu res assoc ia ted w i th the rmo t rop ic behav io r be i d e n t i f i e d p e rm i t t i n g op t im iz a t i o n o f the aromat ic po lyes te rs and development o f new systems.
The to p i c o f new o r improved r e i n f o r c i n g agents i s cons idered here because o f i t s importance to high performance composites. I t i s e s p e c ia l l y c r i t i c a l in l i g n t o f the almost t o t a l lack o f research in t h i s area s ince the l a t e 1960's, a t which t ime the A i r Force and NASA s h i f t e d the focus
c\o f research on new r e i n f o r c i n g agents to p ro to type s tu d ie s . ' One o f the most impor tan t quest ions i s to determine the s treng th l i m i t i n g fea tu res in carbon f i l am en ts w i th the goal o f inc reas ing the s t reng th by three to f o u r t imes. Th is kind o f research should be d i r e c te d to a bas ic understanding o f the in t e rn a l s tresses in carbon f i b e r s which a r ise from d i f f e r e n ce s in c r y s t a l l i n e o r i e n t a t i o n across the f i b e r diameter. A s im i l a r problem may e x i s t in high s trength-modulus BN f i lam en ts and bas ic s tud ies on t h i s mate r ia l would be h ig h ly des i ra b le . Studies to develop ceramic composites r e in fo r c e d w i th high strength-modul us f i lamentsand designed to w i ths tand temperatures f a r in excess o f 1000°C represent a major cha l lenge f o r the ma te r ia ls s c i e n t i s t . Candidate systems t h a t might f u l f i l l these r e q u i r e
ments in c lude B^C o r SiC f i lam en ts embedded i n t o a SiC o r Si^N^ m a t r i x . Research s tud ies to cha rac te r ize the high temperature mechanical p r o p e r t i e s o f the cand idate f i lamen ts and e s pe c ia l l y the i n t e r f a c e w i t h the m a t r i x mate r ia l would appear bas ic to progress in t h i s area. As one looks t o the next genera t ion o f r e i n f o r c i n g m a te r i a l s , s tud ies shou ld be d i r e c t e d to development of-new p lana r r e i n f o r c i n g m a te r ia l s , based on s ing le c r y s t a l f l a kes w i th aspect r a t i o s o f 100/1. E xp lo ra to ry s tud ies in d i c a te t h a t
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l i g h tw e ig h t composites w i th p lanar modulus values o f 30-40 Msi can be f a b r i c a te d , which represents a two to th ree times increase over values ob ta inab le w i th f i b e r r e in fo rced s t r u c tu re s .
The above programs would appear to present a number o f impor tan t advantages to the Department o f Energy, p a r t i c u l a r l y in the areas o f energy conserva t ion and energy storage. Thus r i g i d , r o d - l i k e polymers would prov ide a h igh- tempera tu re h igh -s t reng th ma te r ia l s i g n i f i c a n t l y improved over l i g h t metals such as aluminum with the add i t io na l advantage o f be ing f a b r i c a te d by i n j e c t i o n molding and poss ib ly ex truded in t o f i lm s . Flywheels f a b r i c a te d from new h igher s treng th g raph i te f i b e r s would prov ide a th ree t imes increase in the f i g u re o f me r i t f o r f l ywhee ls . High temperature composites re in fo r ced w i th f i lamen ts s tab le to 1300°C would prov ide a meaningful a l t e r n a t i v e to the in he ren t l y b r i t t l e ceramic which are c u r r e n t l y being developed f o r use as h igh- temperatu re tu rb in e b lades.
RADIATION SENSITIVE/INSENSITIVE POLYMERS
Systemat ic s tud ies to understand the nature o f the i n t e r a c t i o n s between polymer s t ru c tu res and d i f f e r e n t types o f r a d ia t i o n sources has y e t to be c a r r ie d ou t. A la rge body o f emp i r ica l knowledge has been generated on photodegradat ion o f polymers r e s u l t i n g in p a r t i a l s o lu t i o n s through use o f s t a b i l i z e r s . 6 C le a r l y more basic s tud ies are needed to develop a p r e d i c t i v e c a p a b i l i t y w i th respect to behav ior o f polymers in neu t ron , e le c t r o n beam, x - r a y , Deep UV and Near UV r a d ia t i o n . Programs which undertake to e lu c id a te the degradat ion mechanisms in polymers on exposure t o these va r ious types o f r a d ia t io n should be c lo se ly t i e d to cor respond ing
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research on the e f f e c t s o f r a d ia t i o n on the o p t i c a l and mechanical p r o p e r
t i e s o f f i lm s and coa t ings . With t h i s k ind o f understanding i t would then be poss ib le to design s t a b i l i z e r s t a i l o r e d to screen aga in s t those wave
leng ths to which a given polymer i s p a r t i c u l a r l y s u scep t ib le . Ano ther area o f p a r t i c u l a r i n t e r e s t are those polymers which d isp lay r e v e r s i b le responses on exposure to s p e c i f i c wavelengths in the UV. Thu?, c ro s s l i n k e d po lyme r ic t r ibenzopyran or d s -b ip heny la zo s t r u c t u re s in the form o f f i lm s can d is p la y o ne - to - fo u r -pe rcen t expansion and con t rac t ion i n the plane o f the f i l m . ^ This unusal phenomenon appears to depend on r e v e r s i b l e r i n g open ing /c los ing reac t ions o r on c i s - t r a n s isomer iza t ion which are induced photochemica l ly . Basic research to b e t t e r understand t h i s phenomenon and to syn thes ize new systems which can d is p la y la rg e r volume changes w i t h a high le ve l o f r e v e r s i b i l i t y would seem h igh ly des i ra b le . Of a f a r more specu la t ive nature is basic research d i r e c te d a t development o f new polymers which mimic the fea tu res o f semiconductors f o r use i n p h o to v o l
t a i c systems.
The above programs, e s pe c ia l l y on photodegradat ion r e a c t io n s i n polymers, i s o f very high p r i o r i t y to both the s o la r thermal energy c o n ve r
sion and pho to vo l ta ic energy conversion programs, p a r t i c u l a r l y w i t h respect: t o the g laz ings , concen t ra to rs and re ce ive rs . A bas ic unders tand ing o f the mechanisms o f i n t e r a c t i o n o f polymers w i t h var ious forms o f r a d i a t i o n ( p a r t i c u l a r l y e le c t ron beam, x - ray and Deep UV) i s c r i t i c a l to deve lop ing f u t u r e l i t h o g ra p h ic processes requ i red f o r one -m ic ron -and -sma l le r c i r c u i t r y f o r h igh dens I t y m ic ro e le c t ro n ic dev ices . At p resen t , research i n these areas i s pursued p r im a r i l y by the i n t u i t i o n s o f the exper imenter w i t h very
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l i t t l e fundamental knowledge as a guide. Development o f polymers r e s i s t a n t to neutron f l u xes would have major impact in the development o f c ryogen ic i n s u l a t i o n f o r fus ion reac to rs .
BARRIER FILMS - SEMIPERMEABLE MEMBRANES - WATER SOLUBLE POLYMERS
In the work on c h a ra c te r i z i n g pe rm eab i l i t y and d i f f u s i v f t y o f polymers to rtioisure and co r ros ive gases much progress has been made in d e f i n i n g the r o l e o f f ree volume, degree o f c r y s t a l l i n i t y and na tu re o f polymer f u n c t i o n a l i t y . More recen t s tud ies have begun to e lu c id a te the e f f e c t o f s t resses on pe rmeab i l i t y and the tendencies f o r c e r t a in mixed gases t o f a c i l i t a t e d i f f u s i o n . With t h i s r e l a t i v e l y broad knowledge base ’. t would seem h ig h l y des i rab le to i n i t i a t e bas ic research programs d i r e c te d a t deve lop ing new polymer f i lm s which inco rpo ra te the fea tu re s o f c r y s t a l l i n i t y w i th minimal free volume and low f u n c t i o n a l i t y . With respec t to semiper- me&ble membranes, basic s tud ies to b e t t e r cha rac te r ize the mechanisms o f hydrogen ion t r a n spo r t through f i lm s o f sul fonated pe r f luo ropo lymers would appear h ig h l y des i ra b le , p a r t i c u l a r l y in the search f o r improved systems.
A s im i l a r argument cou ld be made f o r develop ing new high mo lecu la r w e ig h t , wate r s o lu b le , polymers w i t h rheo log ica l and f u n c t i o n a l i t y fea tu res t a i l o r e d t o min imize in t e r r e a c t i o n s w i th d i f f e r e n t i o n i c env ironments; however, the bas ic research aspects are no t as c le a r since these s tud ie s are f a r more m ean in g fu l l y ca r r ied ou t in smal l scale f i e l d te s t s .
Development o f f i lm s and coa t ings w i t h improved b a r r i e r p r o p e r t i e s would have g rea t consequences not on ly to most o f the s o l a r techno log ies but i n many areas o f commercial importance. By i n h i b i t i n g t r a n s f e r o f c o r r o s i v e
- 23 -
gases through polymer coa t ings , a major step would be made in reducing the degradat ion o f the po lymer-subs t ra te i n t e r f a c e s and g re a t l y extend ing l i f e t im e s . The development o f a su l fona ted pe r f luo ropo lymer known as "Na f ion" has prov ided a major advance in the e l e c t r o l y t i c process f o r hydrogen p roduc t ion from h y d ro e le c t r i c sources. Thus e l e c t r o l y z e r s w i t h e f f i c i e n c i e s o f 85-90% have been achieved us ing t h i s polymer as a s o l i d polymer e l e c t r o l y t e compared to commercial e le c t r o l y z e r s which f u n c t i o n a t an e f f i c i e n c y o f 65%. With respec t to water so lub le polymers f o r use in enhanced o i l recovery , the urgency f o r pushing research in t h i s area der i v e s from the p o te n t ia l o f recover ing 280 b i l l i o n s ba r re ls o f o i l which were p re v io u s ly thought to be unrecoverab le .
CONDUCTING POLYMERS
Since the d iscovery o f m e t a l l i c and superconduct ing p r o p e r t i e s i n8)the in o rgan ic polymer (SN) i t has been found that, s im i l a r p r o p e r t i e s
AQ \ .
cha rac te r ize several o rgan ic polymers based on po lyace ty lene , ' '7 p o l y p y r - r o l e ^ and p o l y p a ra p h e n y le n e J ^ Unl ike (SN) o r brominated (SN) these
A A
organ ic polymers are not i n t r i n s i c a l l y m e t a l l i c nor do they e x h i b i t supe r
c o n d u c t i v i t y ; they are in f a c t c losed she l l i n s u la to r s which become conduct ing to the degree they can be o x id ized o r reduced. Po lya ce ty le ne , f o r in s tance , i s an i n s u l a t o r whose e l e c t r i c a l c o n d u c t i v i t y can be v a r i e d from ^ 10"9 to 103 ft-1 cm"1 by trea tment w i th o x id i z i n g agents such as AsFg, I 2> Br2 ,Ag+ , e t c . o r reducing agents such as the a l k a l i me ta ls . S im i l a r p rope r t ie s cha rac te r i ze the polyparaphenylene based polymers. The e l e c t r o - chem ica l l y prepared po ly p y r r o le t e t r a f l u o r o - b o r a t e is somewhat d i f f e r e n t .
In t h i s case o x id a t io n i s ach ieved e le c t r o chem ica l l y du r ing the s yn thes is
- 24. -
r a t h e r than by "doping" e x i s t i n g ma te r ia l as i s the case f o r po lyace ty lene
powder o r molded p e l l e t s . The c o n d u c t i v i t y o f the p o l y p y r r o le f i lm s can-3 2 -1 -1p re sen t l y be va r ied between 10 and 10 ' ft cm by in c o rp o ra t i o n o f
c o n t r o l l e d amounts o f N-methyl p y r r o le i n t o the f i lm s .
The progress du r ing the l a s t fou r years r e f l e c t s the f a c t t h a t the area has a t t r a c t e d and sus ta ined the coope ra t i ve i n t e r d i s c i p l i n a r y e f f o r t s o f the chemists and phys ic is t ' s both c r u c ia l to meet ing the cha l lenges o f t h i s complex f i e l d . Today the f i e l d remains e x c i t i n g s c i e n t i f i c a l l y and con t inues to show promise t e c h n o lo g i c a l l y . Though c u r r e n t successes have led to the s t r e s s in g o f the e l e c t r i c a l p r o p e r t i e s o f po lymers , e f f o r t s should a lso be d i r e c t e d toward developing polymers which show assoc ia ted p rope r t i e s such as high thermal c o n d u c t i v i t y and magnet ic behav io r .
RESEARCH OPPORTUNITIES' .
As i s to be expected in a r e l a t i v e l y new, p r im a r i l y ma te r ia ls le d f i e l d , the research o p p o r t u n i t i e s and needs are numerous in o rde r t o
achieve a bas ic understand ing o f conduct ing polymers comparable to the c l a s s i c a l metals and semiconductors .
1. New Polymers
In the area o f new polymers s i g n i f i c a n t te chno log ic a l b e n e f i t s would d e r ive from polymers which besides d i s p la y i n g conduct ing p rope r t ie s a lso possess the a t t r a c t i v e phys ica l p ro p e r t i e s o f p l a s t i c s , e . g . , processr a b i l i t y and s t a b i l i t y . Fu r the r at tempts shou ld be pursued to prepare new conduc t ing po lymers , from monomers, where the po lym e r i za t ion i s i n i t i a t e d by the Lewis a c id o r the Lewis base requ i red t o induce c o n d u c t i v i t y . The r o l e o f e le c t r o c h em is t r y in the f i e l d needs more emphasis — no t on ly 1-
- 25 -
produc ing new polymers bu t a lso in he lp in g de f ine o x id a t i o n and r e d u c t i o n p o t e n t i a l s o f e x i s t i n g polymers. In t h i s l a t t e r con tex t , i t i s o f p a r
t i c u l a r importance to f i n d new polymers capable o f be ing doped n - t y pe i f the j u n c t i o n p ro pe r t i e s o f po lymeric semiconductors are to be c a p i t a l i z e d on. Aga in , in deve lop ing gu ide l ines f o r new polymer systems, i t i s impo r
ta n t to de f ine the importance o f polymer geometry. The search f o r o t h e r p r o p e r t i e s in polymers should be sought, e . g . , thermal c o n d u c t i v i t y and magnet ism.
2. Polymer c h a r a c t e r i s t i c s
Polymers in gene ra l , and the p resen t i n s o lu b le , poo r ly c r y s t a l
l i n e conduct ing polymers in p a r t i c u l a r , p resen t s i g n i f i c a n t b a m e r s t o t h e i r c h a r a c t e r i z a t i o n . I t i s impo r tan t to lea rn how to adapt c u r r e n t techniques to these ma te r ia ls and a lso to develop new techn iques . A second approach i s to develop new polymers which are so lub le o r w e l l c r y s t a l l i n e which would be more amenable to s tandard c h a r a c te r i z a t i o n techn iques .
Al though cons ide rab le progress has been made in i n t e r p r e t i n g the semiconducto r ideas , i t i s necessary to le a rn more about the phys ics and chem is t ry o f de fec ts and traps in these m a te r ia l s and t h e i r i n f l u e n c e on the c o n d u c t i v i t y needs to be evaluated,, In p a r t i c u l a r , the search f o r exper imenta l evidence f o r s o l i tons in these m a te r ia l s should be ex tended.
I t i s a l s o c r i t i c a l t h a t we learn more about the natu re and d i s t r i b u t i o n o f the dopant spec ies , p a r t i c u l a r l y in the semiconduct ing re g ion .
- 26 -
I t i s too e a r l y to r e a l i s t i c a l l y assess-the t e ch n o lo g i c a l im p l i c a t i o n o f conduct ing po lymers . For many a pp l i c a t i o n s the te ch n o lo g i c a l aspec ts would be s t r o n g l y p o s i t i v e l y impacted by the demons tra t ion t h a t these conduct ing p r o p e r t i e s cou ld be ob ta ined i n polymers possess ing the a t t r a c
t i v e p ro p e r t i e s o f p l a s t i c s , e . g . , p l a s t i c i t y , e l a s t i c i t y , low d e n s i t y chemical and therma l s t a b i l i t y . However, in view o f the p rogress i n t h i s area i t i s no t unreasonable to specu la te a commercial f u t u r e f o r o rg a n ic conductors i n the same way o rgan ic photoconductors have begun to compete w i th t h e i r i n o r g a n ic analogs.
4. P i e z o e l e c t r i c polymers
P o l y v in y l i d e n e f l u o r i d e (PVF2 ) is a s e m i c r y s t a l l i n e po lymer which i s c u r r e n t l y o f g rea t i n t e r e s t i n a v a r i e t y o f research areas because o f i t s high p i e z o e l e c t r i c and p y r o e l e c t r i c c o e f f i c i e n t s , and because o f i t s la rge n o n l i n e a r o p t i c a l c o e f f i c i e n t and high d i e l e c t r i c breakdown s t r e n g th . The polymer cha in composed o f CH2 CF2 monomer u n i t s e x h i b i t s a s t r o ng t r a n s
verse d ip o le moment which i s respons ib le f o r most o f i t s i n t e r e s t i n g p rope r t ies . , However, charge i n j e c t i o n and t r a p p in g a lso o c cu r , and the re i s c u r r e n t l y some con t rove rsy as to which e f f e c t i s the dominant mechanism. Fundamental ques t ions such as the r o l e o f the amorphous reg ion in charge t r a p p in g , charge conduc t ion , and d i e l e c t r i c loss have no t been addressed.
The o p p o r t u n i t y to answer these ques t ions e x i s t s by v a r i a t i o n s i n s yn th e s is and p rocess ing . Several avenues are c u r r e n t l y be ing s t u d i e d , i n c l u d in g c o po lym e r i z a t io n w i t h nonpola r species such as t e t r a f l u o r e t h y l e n e (C^F^j), and low tempera ture homopolymerizat ion which reduces cha in de fc c ts
3c Technological possibilities
- 27 -
such as head-to-head l inkages . The c rys ta l ! ine - to -amo rphous r a t i o in the l a t t e r ma te r ia l i s cons ide rab ly enhanced over commercial ma te r ia l due to i t s lower de fec t den s i t y , and some v a r i a t i o n i n p rope r t i e s has been achieved.
The bu lk o f the s tud ies have been o r ien ted toward c h a r a c t e r i
za t ion and a p p l i c a t i o n o f commercia l ly a va i la b le f i lm . T he i r most se r ious handicaps are t h a t , a t the present t ime , there is no domestic source o f e lec t roded and po led f i lm s having a wide range o f p r o p e r t i e s . One ve ry impo r tan t a p p l i c a t i o n , the nondes t ruc t ive te s t i n g o f m a te r ia l s and s t r u c t u re s .by acous t ic imaging using conformable PVF2 t ransduce rs , has been i d e n t i f i e d . This a p p l i c a t i o n would b e n e f i t g r e a t l y from f i lm s having increased p ie z o e l e c t r i c modulus and reduced loss tangent .
- 28 -
1■ Proceedings on Polymeric Mat e r i a l s Basic Research Needs f o r EnergyAppl ica t , ionsT (CONF-780643 UC-25, Cleveland- , O h i o f Case Western U n iv e r s i t y , June 27-29, 1979) pp. 5-68.
2. R.A. Mark le , Appl . Pol . S c i . , ACS, Chapter 30, pp. 560-584 (1975) .
3. P.W. Morgan, Macromolecules, 10, No. 6, p. 1381 (1977).
4. W.J. Jackson, J r . , N.F. Kuhfuss, J. Pol . S c i . , Chem. Ed. 14,p. 2043 (1976).
5. J. Economy, SAMPE J.» 12, No. 6, p. 5 (Nov./Dec. 1976).6. W. Moreau, N. Viswanathan, Pol. P r p r t s . , J_8, No. 1, p. 18 (M a r . , 1977).
7. G. Smeto, 26th IVPAC Congres. Abs t ra c t , Ser. 3, 5EI , p. 11 (1977) .8. (a) R . L . Greene and G.B. S t r e e t , Chemistry and Phys ics o f One -D imen
s iona l Metal s , ed. H.J. K e l l e r , (New York:* Plenum Press, r977).
(b) H.P. Gersor ich and L. P in tschov ious , Advances in S o l id S ta te Phys ics , 16, 65 (1976).
(c ) M.M. Labes, P. Love., and L.F. N icho ls , Chem. Rev. 79, 1 (1979).\
(d) G.B. S t ree t and W. G i l l , Molecu lar Me ta ls , e d . , W.E. H a t f i e l d ,(New York: Plenum Press, 1979T
9. (a) C.K. Chiang, A .J . Heeger, and A.G. MacDiarmid, Ber. Bunsenges.Phys. Chem., 83, p. 407 (1979).
(b) G.B. S t ree t and T.C. C la rke , Advances in Chem is t r y .Se r ie s ,A.C.S. (1979).
10. Kanazawa e t a l . , J .C .S . , Chem. Commun. ( t o be pub l ished ) 1979.
11. D.M. I v o r y , G.G. M i l l e r , J.M. Sousa, L. W. Shack le t te , R . R . Chance,and R.A. Baughman, J. Chem. Phys. 71 , ( 3 ) , p. 1506 (1979).
REFERENCES:
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CHAPTER 3
IN TERMETALLI C COMPOUNDS
INTRODUCTION
Free energ ies o f fo rmat ion o f in te rmed ia te phase compounds o f te n are small - bu t t h e i r phys ica l p rope r t ie s — s t r e n g th , hardness, co r ros ion re s is ta n ce , hydrogen storage c a p a c i t y , superconduct i ng p r o p e r t i e s , magnet ic p r o p e r t i e s , e t c . , can a l l be v a s t l y d i f f e r e n t , and in fa vo rab le cases can be app l ied to so lve te c h n o lo g i c a l l y impo r tan t prob lems.
A g rea t deal has been learned about b ina ry phase diagrams in the past us ing the te c hn iq ue s 'o f thermal a n a l y s i s , powder m e ta l l u rg y , o p t i c a l microscopy, and x - ray d i f f r a c t i o n . However, there remains an even r i c h e r f i e l d o f in te rmed ia te phases w i th th ree o r more types o f atoms per u n i t c e l l which are sometimes c a l le d te rna ry phases. Under fa vo rab le c o n d i t i o n s , as a l ready po inted ou t in Chapter i , these phases can have the combined
a t t r i b u t e s o f p a r t i a l van der Waals bonding o r open channels ( f o r easychemical s u b s t i t u t i o n ) , m e t a l l i c bonding ( f o r e l e c t r i c a l p r o p e r t i e s , d u c t i l i t y ) , and i o n i c and cova len t bonding ( f o r s t r e n g th , hardness).There f u r t h e r remains an uncharted f i e l d o f in te rmed ia te phases which have not been reached by t r a d i t i o n a l methods o f p re pa ra t io n because o f
n .b . : l;F r m T n temetallic has come by general useage to inc lude compoundsbetween metals and me ta l lo id s as wel l as between metals o n l y , wh ich al i t e r a l meaning o f the term imp l ie s . In the r e p o r t we go a long w i t hgeneral useage. MBM/THG
- 30 -
kinetic limitations at the relatively low temperatures at which they
become stable. Vapor or liquid phase quen c h i n g as well as new solvent
systems under the appropriate conditions have been used to circumvent
the need for macroscopic diffusion and thus to prepare n ew phases.
Once a new phase has been properly prepared there can, in fa v o r
able cases, follow a splurge of new science, and o c c a s i o n a l l y of technology.
Active fields in solid-state science which depend h e a v i l y upon synthesis
include the study of metal-insulator transitions, mi x e d valency, su p e r
conductivity, and magnetism. A good example of how t e c h n o l o g y benefits
is provided by the rare earth-cobalt permanent m a g n e t s , w h i c h were only
synthesized for the first time two decades ago, and mak e it possible to
design lightweight motors.
In the section immediately below, we discuss f o u r examples, all
superconductors, which have and are providing new insights into the
properties and possibilities of intermetallic compounds. Superconduc
tivity is of interest for the technological reasons m e n t i o n e d in the
section on technology needs (below), but also because it is a very
sensitive probe of structure, composition, order, and morphology, which
in turn relate to. many other important technological problems. A case
in point is how investigations of the superconducting p r o p e r t i e s of the
intercalated transition metal dichalcogenides led to n e w battery cathode?
( discu s s e d i n Chapter 6 on Solid State Ionic Conductors).
- 31 -
1. Classes of materials which afford an o ppo rtunity for s y s t e m a tic
investigation of various physica l phenoinena
a. A15 compounds
The highest transition temperature superconductors form
in the A l 5 phase. The compound Nb^Sn which is stable and easily formed
from the elements by diffusion is likely to serve as the "copper" and the
"silicon" of superconducting Iarge-scale-technology. More remains to be
learned about its superconductivity: the nature of defects and their role
in flux pinning, the nature of surface barriers to the entry and exit o f
flux, and the superconducting and mechanical performance when incorporated
in the composite structures needed to produce high magnetic fields for
rotating machinery and fusion containment magnets. The o p p o r tunity exists
for studying the effect of well defined physical and chemical changes upon
its macroscopic superconducting properties such as critical current and
critical fields. There is a need to develop quantitative theories. This
approach will directly bear upon the technological need for higher p e r f o r
mance superconducting magnets.
The compound N b ^ S i , which may turn out to be the highest Tc A15 phase
superconductor, lias been prepared as a metastable A l 5 phase only by vapor
phase quenching, and then only as Nb-rich as discussed later in this chapter.
Its synthesis is a more extreme challenge than the synthesis of the p r esently
known three highest-temperature-superconductors, Nb^A*., Nb^Ge, and Nb-jGa,
none of which has been properly prepared as a pure ordered phase. U n d e r s t a n d
ing the conditions which favor the stabilization and strong electron - p h o n o n
coupling may lead to the synthesis of even-less-stable pseudo-binary ordered
A15 phases, none of which is presently known.
OPPORTUNITIES, BASIC RESEARCH
- 32 -
The marked susceptibility of A15 compounds to radiation
damage not only needs to be well understood for technological reasons
but affords an opportunity to generalize the results and extend them
to different classes of intermetal 1 ic compounds which combine partially
covalent, metallic and ionic bonds. The severe degradation of the
superconducting properties is easily removed by simple annealing. Some
analyses show that there is a large redistribution of charge and atomic
motion within the unit cell upon being irradiated which may be understood
qualitatively at least in terms of the formation of covalent bonds. An
equally important and probably related problem is the strong dep e n d e n c e
of the physical properties upon stoichiometry. Tunnel junctions have
recently been prepared with adequate characteristics to permit the
application of the powerful technique of electron tunnelling s p e ctroscopy
to unravel the microscopic p a r a m e t e r s . ^
b. Layered and quasi-one-dimensional compounds
Much progress has been made in the past several years in
understanding the relation between the electronic and lattice d e g r e e s of
freedom, particularly in low dimensional phases such as Ta$2> Nb S e ^ and
TTF-TCNQ. Van der Waals forces hold sheets or chains together g i v i n g
the structures a quasi one- or two-dimensional character which is r e f l e c
ted in the physical properties. The ability to synthesize compounds
where composition can be varied in a well-defined way permits properties
to be related back to band structure. For instance, lattice instability
in T a S 2 varies with substitution of Ti for Ta in a way which c l e a r l y
establishes the driving force as a charge density wave which d e r ives from
21a particular wave vector spanning the Fermi surface. ' The interaction
- 33 -
and coexistence of charge density waves with superconductivity provides
valuable experimental data for understanding both p h e n o m e n a .
When two-dimensional superconducting crystals based upon
TaS^ are intercalated with organic molecules the resulting s u p e r c o n d u c t i n g
transition temperatures are above 3K and upper critical fields with
dH o ,H parallel to the crystal layers have slopes, ( ) > 200 kOeK ,
several times higher than any other superconductor. These inter c a l a t i o n
compounds are extreme examples of ternary compounds and have some features
in common with the Chevrel and rare earth rhodium boride phases discussed
below. The conduction band is well localized within the m e t a l l i c layers
of the unit cell. Metallocene radicals incorporating 3d elements with
uncompensated spins (see Chapter 6 on Solid State Ionics, p. 9 3 ) . can be
introduced with little apparent effect on the superconducting properties
indicating an as yet uninvestigated example of the coexistence of m a g n e t
ism and superconductivity.
c. Ternary molybdenum ohaloogenidea (Chevrel phases)
The ternary molybdenum chalcogenides, or Chevrel phases,
M xM°6 ^8 *iave bsen and remain promising candidates for investigations o f
superconductivity, magnetism, and the interplay o f these two phenomena.
The component M ranges throMghout the periodic table of the elements
(alkali metals, alkaline earths, transition metals, s-p metals, rare
earths and actinides), the X atoms can be either S, Se or Te, or, in part,
Br or I, and the value of the index x generally lies within the range
1 < x < 4. The compounds have rhombohedral symmetry and c o n s i s t o f M o ^ X g
units or "clusters" and channels into which the M atoms can be inserted.
Some of the Chevrel phase compounds exhibit superior supercon d u c t i v e
- 34 -
properties with critical temperatures T as high as ^ 15 K ^ and upper
critical fields H c 2 approaching 70 T . 5,6 Although it is generally
believed that the extremely high values of Hc 2 (the highest known for
any material) can be traced to the "cluster" character of these compounds,
a fundamental understanding of the origin of the high H c 2 values is
lacking and systematic studies are clearly called for. In addition,
experiments yielding knowledge of the electronic band structure and phonon
characteristics would be very useful. The Chevrel phases may represent a
successful means of avoiding the electron-phonon instability, i . e . , by
allowing very anharmonic behavior associated with the M element, w h i l e
containing these degrees of freedom by rather rigid MogXg cages. Information
about the electronic structure of these materials may accrue fro m the
exploration of mixed ternary systems and may possibly lead to increases of
T c and H ^ above their present limits. The preparation of more dense
polycrvstalline and, in some instances, single crystal specimens, has
recently been accomplished, and should be extended in o r d e r to make c o n t a c t
with theoretical studies of the electronic band structure and lattice
dynamics. In particular, it should be possible to address the l o n g - s t a n d
ing questions concerning the coexistence of superconductivity and various
kinds of magnetic order. Efforts directed towards understanding critical
phenomena involving coupled order parameters should be pursued. Apparently,
the reason that the ternary rare earth (RE) molybdenum c h alcogenide c l u s t e r .
compounds, and other ternary RE cluster compounds, are so well suited to
this type of investigation is twofold:
• The periodic disposition of RE ions with partially-filled 4 f
electron shells and corresponding magnetic m o m ents leads to
- 35 -
long-range magnetic o r d e r i n g via the RKKY interaction. As a
result, the magnetic o r d e r i n g temperature is w e l l - d e f i n e d
and the features in the physical properties at are sharp
(e.g., A-type anomaly in the specific heat at TM ).
• The clusters that are a p p a r e n t l y responsible for some of the
remarkable s u p e r c o n d u c t i n g properties are r e l a t i v e l y isolated
from the rare earth ions. Consequently, the o v e rlap b e t w e e n the
conduction electron and localized 4f electron w a v e f u n c t i o n s is
rather small, leading to a small value of the c o n duction e l e c t r o n
spin-RE magnetic m o m e n t exchange interaction p a r a m e t e r J o f the
order of 0.01 eV, n e arly an order of m a g n i t u d e smaller than in
typical rare earth m e t a l l i c compounds.
Recent experiments on various REx M 0 g S 8 and REx M o 6SeQ c o m p o u n d s have y i e l d e d
several examples in which superco n d u c t i v i t y and anti ferro m a g n e t i c o r d e r
c o e x i s t ; ^ ’ and one example, Ho^ 2 ^°6 S8 ’ 1n s u p erconductivi ty is
destroyed at a second lower critical temperature by the o n s e t o f f e r r o
magnetic order."*^
Further systematic studies o f superconductivity a nd m a g n e t i s m in
these compounds will also shed light on the electronic s t r u c t u r e of a
class of complex intermetallic com p o u n d s that m ay be p r o t otypes for
catalysts in important chemical reactions.
d. Ternary rare earth rhodium borides
The ternary rare earth (RE) rhodium b o r i d e s c o n s t i t u t e
a n o t h e r class of cluster compounds which exhibit a rich i n terplay b e
tween superconductivity and magnetism. In these mat e r i a l s , the free
energies of the supercon d u c t i n g a nd m a gnetically o r d e r states a c h ieve
a delicate balance as reflected in the original report by Matthias et
- 36 -
that some of them were superconducting (those with RE Nd, Sm, Er, Tm
and Lu), while the others were ferromagnetic (those with RE are Gd, Tb,
12)Dy and Ho). In E r R h ^ , superconductivity is reentrant ' due to the
onset of ferromagnetic o r d e r j 3 while in the NdRh^B^ ^ and S m R h ^ J 5
s uperconductivity and magnetic order coexist. In these compounds, the
ro]e of crystalline electric fields appears to be central and needs to
be elucidated. Since these materials have tetragonal symmetry, single
crystal specimens would be particularly valuable in a c hieving a d e t a i l e d
understanding of their remarkable behavior. Some pseudoternary RE r h o d i u m
boride systems have been investigated. Systematic studies may lead to
other systems of ternary rare earth cluster compounds.
Radiation damage studies have been initiated on ErRh^B^ and
(Er,Ho)Rh^B^ pseudoternaries 1 and have revealed some surprising results.
This appears to be a promising technique that may prove as useful in
investigating cluster compounds and superconducting-magnetic interactions
as it is in the A15 compounds.
These compounds represent a challenge to solid-state theory
since a description of the ground state of the coupled superconducting and
magnetic system goes beyond the Bardeen, Cooper, and S c h r ieffer theory
of superconductivity. Further theoretical and experimental work is n e e d e d
to clearly identify the important features of these compounds.
e. Other1 cluster1 compounds
In view of the str-iking superconductive properties e x h i b i t e d by
the ternary m o l y b d e n u m chal^.ogenides and rare earth r h o d i u m borides, so m e
of w h i c h arise from the interplay between superconductivity and lo n g - r a n g e
- 37 -
magnetic order, it seems worthwhile to look for other systems of c l u ster
compounds. It can reasonably be anticipated that in addition to s u p e r
conductivity and magnetism, new cluster compounds will be discovered that
have other unusual and useful properties such as mechanical strength,
resistance to c o r r o s i o n , 'catalytic activity, phase transitions, s u p e r
ionic conduction, etc., and conceivably, new phenomena (see Chapter 1).
f. Intermediate valence rave earth compounds
During the past several years, there has been a great deal
of interest in a certain class of metallic rare earth (RE) compounds
which exhibit an intermediate or nonintegral valence of the RE ion c o n
stituent. In such compounds, it is generally believed that the 4f electron
shell fluctuates between two configurations 4 f n and 4fn - \ a c c o m p a n i e d by
the emission and absorption of an electron by the RE 4f shell. The RE ions
that have been found to exhibit this type of behavior include Ce, Sm,
Eu, Tm and Yb, and examples of materials of this type include a-Ce,
CePd^, SmBg, SmS in its collapsed metallic phase, EuCi^Si^* TmSe, YbCuAJ,,
etc. The materials have striking and unusual properties such as n o n m a g
netic behavior of the magnetic susceptibility below a characteri:tic
otemperature TQ ^ 10 K which is related to the valence or interconflgur-
-13ation fluctuation lifetime t ^ 10 s; other characteristics include a
large electronic specific heat coefficient, a peak in the t hermoelectric
power near T , and various types of anomalies in the electrical re s i s t i v i t y
near T . This class of materials has also been studied by means of
Mb'ssbauer and XPS techniques which support the basic picture o f temporal
fluctuations between two 4f configurations. In view of the unique ele c t r o n i c
structure of these materials, more experimental work to document and
- 38 -
classify their striking physical properties would seem to bo in order.
Moreover, a completely adequate theoretical description, particularly
one that incorporates the role of the phonons, has yet to emerge. C o m
parison with the behavior of 4f~related compounds should prove i l l umin
ating. These materials may also have other interesting properties that
are applicable to catalysis.
2. Innovative synthesis
a. Metastable and kinetically-limited equilibrium phases
by vapor phase condensation
Intimate mixtures of species can be obtained by c o n d e n
sation from the vapor. The reaction product which follows is not limited
by kinetic factors and diffusion. Equilibrium, or m e t a stable equilibrium
states, are readily achieved. While the above ideas are not new, the
hardware and techniques which have been developed in the past few ye a r s
make for a possible major increase in the number of known compounds.
In the simplest application, the co-deposition technique can
be compared to powder metallurgy where the size of the powder has been
reduced to a single atom or molecule. Solid-state reactions, which at
low temperatures require months or longer, can occur in seconds. Thus,
portions of phase diagrams involving a refractory metal as one component
can be explored below, say, 1000°C. Likewise, materials with low or very
incongruent melting points can be prepared with facility well below these
points, as can phases which become highly disordered o r defect-laden at
temperatures necessary for classical synthesis.
In more sophisticated applications, the number of d i fferent
species, their sequence and rate of arrival at the substrate, their kinetic
- 39 -
and internal energy, the interface energy and epitaxy with the substrate,
the substrate temperature, the presence of trace gases which can affect,
surface mobility during growth are all controllable d e p o sition parameters
which can be used to obtain stable and metastable phases that normally c a n
not be isolated. The superstructures of GaAs-GaA£As, which have been grown
by MBE techniques and described in Chapter 5 on Thin Films, represent an
extreme example which is perhaps only the tip of the iceberg.
Perhaps a more representative example is the case of Nb^Si.
The elements react at elevated temperatures to form a stable compound with
the Ti^P structure. It has been found that films deposited below 850°C
form in the A15 structure, the phase desirable because of potentially strong
superconducting p r o p e r t i e s . ^ Classical metallurgical techniques a l o n e
would never be able to isolate the A15 phase of Nb^Si, but once the
necessary conditions for obtaining it have been established from a m o n g
the multi-infinity of possible trajectories in parameter space, o t h e r
methods of fabrication may become possible.
The use of two, three, or even more independently c o n t r o l l e d
sources permits the deposition to be carried out over a range of composition.
This Tiakes it possible to investigate the compositional dependence and
to synthesize new phases in a systematic way. The rapid d e v e l o p m e n t of
new sputtering sources, evaporation and v a c u u m techniques makes the
synthesis of new compounds from the vapor phase a promising and o p e n - e n d e d
approach.
Vapor transport is an established technique for the g r o w t h
of single crystals and often makes use o f volatile halides of the c o m p o n e n t
- 40 -
elements, if they exist, or if they form in requisite amounts during the
growth process. The flfa^Sn single crystals have been grown by chemical
vapor deposition and served as the source of many important investigations.
The use of o rganometallic compounds for growth from the vapor phase o f f e r s
another degree of choice and it is an area that has not been looked into
for vapor phase growth of intermetallics.
Vapor phase condensation (or electrodeposition) offers the
possibility of fabricating layered composites. With a technique such as
Rutherford back scattering such composites can be used as dif f u s i o n couples
in which the diffusion of A into B can then be measured, and compound
formation can be detected at much lower temperatures and in shorter times
tnan conventional techniques permit. Ion implantation followed by p ulsed
annealing offers the possibility of producing new compounds at surfaces
which can be selected to have desirable properties for wear, cor r o s i o n
resistance, etc.
b. Sincle crystals and new phases by molten ealt
eleczrodeposition
Techniques have been developed for the growth of s i n g l e
crystals of L a B ^ ^ and PrB^ by the electrolysis of suitable m olten sa l t
solutions in the 700-800°C range. Large centimeter-size crystals were
2produced at low current densities (approximately 20 mA/cm ). Examples
o f other compounds synthesized by these electrolysis techniques include
^ r3 ^ 4’ 2 ’ ^ 2 * f'J1'2G e’ ^ n5 Ge 3 5 C ^ G e , ^ 4 ancl a r ange of Cr-B
compounds including Cr^B^, CrB^, Cr^B^, Cr^B, Cr^B^, and Cr^B. One can
recognize that the range of possibilities for preparing new and unusual
c o m positions by this method is extremely broad.
- 41 -
c. Synthesis of new materials and growth of single
crystals from low melting liquid metals
Although the flux growth of high melting oxide
crystals (e.g., garnets) and the growth of semiconductor single c r y s
tal alloy layers from liquid metal solutions for semic o n d u c t o r d e v i c e s
are well established laboratory and technological processes, little
has been done on the synthesis and growth of intermetal 1 ic compounds
from low melting metallic or salt solutions.
Synthesis from low melting solutions offers several
advantages over conventional methods including the po s s i b i l i t y of
single crystal growth. For example, intermetal1 ic comp o u n d s of b a s i c
and technological interest generally contain transition elem e n t s and
exhibit such high melting points that inert crucibles are not a v a i l
able. The int e r m e t a l 1 ic compound may contain one c o m p o n e n t that is
volatile and thus requires a low temperature synthesis technique.
Compounds of interest may form incongruently from the m e l t and the
formation of a single phase material may only be made via growth f r o m
solution. Finally, synthesis and crystal growth from a novel solvent
system offers the possibility of obtaining a more pure ph a s e if the
impurities in the solutes segregate into the host liquid d u r i n g growth.
Recently, the rare earth (RF.) borides (REB^ and R E B & )
19)were synthesized from liquid A I. ' There are, of course, ot h e r low
20)me l t i n g elements and many eutectic alloys and AH-Ga m i x t u r e s ' and
this opens an opportunity for the synthesis of many new three and f o u r
com p o n e n t int e r m e t a l 1 i c s , particularly those that might c o n t a i n s t o i
chiometric amounts of the flux components. However, y ou need an e t c h
to remove the solidified flux from grown crystals, and this m a y limit the
fluxes used. The solution growth process offers the o p p o r t u n i t y for g r owth
of large single crystals of important materials for fundamental studies of
intrinsic properties. Synthesis from molten salt systems (other than the
parent binary melts) has been little explored.
d. C o m p o s i t e m a t e r i a l s a n d T e c h n o l o g i c a l c o n f i g u r a t i o n s
Fundamental research on materials systems is usually
carried out on samples which are, as far as possible, h omogeneous and
defect-free across the bulk of the material. Materials technology,
however, frequently necessitates the deliberate introduction of a con t r o l l e d
variation of either composition or imperfections on various scales at the
surface or in the interior of materials. A few outsta n d i n g examples of
this include the semiconductor p-n junction, precipitation hardened a lloys
for great mechanical strength, fiber-reinforced plastics for lightweight
structures and fine filamentary superconductors for high field magnets and
and electrical machines. The last-mentioned example represents a r a p i d l y
developing technology of intermetal 1ic compounds. This will be discussed
in more detail in order to illustrate the great potential of composite
int e r m e t a l 1 ic c o m p o u n d s .
High field, high current density supercon d u c t o r s were
discovered about twenty years ago. It was immediately a p p a r e n t that th e s e
materials could be used to construct zero loss e l e c t r o m a g n e t s of great
value to an important group of energy technologies including electric
power, MHD and fusion generators. The actual development o f s u p e r c o n
d u c t i n g magnets then revealed that the superconducting w i r e could not be
used in bul k form. This was due to magnetic instabilities o r "flux j u m p s "
- 42 -
- 43 -
which occurred during any changes of current or field within the magnet.
These instabilities caused an uncontrolled normalization of the m a g n e t s
during excitation.
To overcome the magnetic instability proilem, composite
conductors were prepared in which the superconductor consisted of a
large number of fine filaments embedded in a good normal conductor
such as copper. This procedure reduced the large flux jumps to a lot
of small ones which released less energy per unit volume. Further
advances included further segmentation of the copper with alloy zones
to reduce electromagnetic losses.
At present, fine filamentary, composite superconductors a r e
produced by a multistep, repetitive process of bundling copper and
superconductor bars together and drawing down to small sizes. This is
an expensive process. It lends itself very well to certain alloy
superconductors, for example, niobium-titanium alloys. These alloys
have rather low critical temperature ( ^ 10°K) and m a x i m u m upper critical
fields around 10 T. Magnet designers would prefer to use other m a t e r i a l s
in particular the A15 intermetal1 ic compounds, for which T c and H c? a r e
roughly double the values of niobium-titanium. Unfortunately, the Al 5
compounds are brittle and cannot be drawn down in fine filamentary f o r m
in their native state.
One solution to this problem is to draw down one of the
components of the A15 compound, e . g . , niobium, within a m a trix of b r o n z e
and then to form the A15 intermetallic at the end of the dre.wing p r o c e s s
by a high temperature diffusion step. This has been quite successful, but
it has certain disadvantages. The most serious one is the high i m p u r i t y
level in the copper stabilizer.
- 44 -
Looking to the future applic a t i o n of superconducting materials,
it seems likely that i n t e r m e t a l ]ic compounds will provide the highest
superconducting critical temperatures and critical fields. Therefore,
we must find other novel methods of m a k i n g these materials in finely-
divided form combined with high c o n ductivity normal metals. A n u m b e r of
approaches have been suggested. For example, high speed quenching
techniques, such as are used to produce amorphous metals, can also p r o
duce finely divided crystalline systems. Alternatively, small grain
composites can be produced by chemical vapor decomposition carried out
in a plasma-arc reactor. Precipitation of dendrites or filaments f r o m
a solvent compound is a third approach.
These unconventional approaches may also be used to p r o d u c e
finely-divided forms of metastable in t e r m e t a l 1 ic compounds which c a n n o t
be attained by a diffusion reaction process. Metastable in t e r m e t a l 1 i c s ,
as discussed previously, offer promise of major improvements of the basic
superconducting parameters T , H -> and J .c c
In summary, we recommend that new emphasis be placed on
seeking out radically new methods of preparing intermetallic-metal-alloy
composite systems of controlled geometry and structures extending down
to the 100 A scale. Innovation in this area will give the materials d e
signer new opportunities to attain novel performance in the electrical,
magnetic, mechanical and physical performance of materials.
- 45 -
BASIC RESEARCH NEEDS
1. Experimental methods of determining defect structures
Although there has been a rapid advance in the development of
new tools for establishing deviations from ideal crystal structures of
complex intermetal1 ic compounds, the application of these advanced
techniques needs to become more widespread. This is particularly crucial
in the more complex intermetallic compounds since the physical properties
are dependent on small deviations from stoichiometry or long-range order.
Noteworthy among the new techniques are:
• High resolution electron microscopy including electron energy
loss analysis or lattice imaging
• Ion beam analysis techniques such as Rutherford scattering,
channeling, and nuclear resonance scattering
• Extended x-ray absorption fine structure (EXAFS) and diffuse
scattering
• Neutron techniques including diffuse scattering.
Advances in the above techniques for the study of defect structures are
expected as atomic resolution imaging becomes practical in scanning t r a n s
mission electron microscopy and with the availability of high intensity
synchrotron sources of x-rays and with intense pulsed neutron sources.
2« H i g h er resolution electron state spectroscopies
It would be very desirable to develop higher resolution s p e c
troscopies for characterizing electron states in narrow band systems.
Materials of current interest include A l 5 compounds, ternary cluster
compounds, and intermediate valence RE compounds. An energy resolution
approaching 0.01 eV would be particularly useful.
- 46 -
TECHNOLOGY NEEDS
Intermetal1 ic compounds already play a vital role in present-day
technology. Examples include their use as dispersion hardening agents
in steels, the very extensive use of the refractory carbides for grinding
and machine tools where extreme hardness is essential and permanent magnet
materials such as Alnico and the rare-earth-cobalt compounds wh i c h have
recently become prominent because of their very high magnetic energy
p r o d u c t s .
As we look to future applications of intermetallic compounds, it
appears that they will enter into almost every phase of advanced energy
technology, including production, conversion, storage, distribution, and
utilization of energy. It should be noted that from an electrical c o n
ductivity viewpoint the metallics cover an almost continuous spectrum
from pure metallic through semiconductor properties* with extension into
the low resistivity insulator region. Hence, much of our comment here is
equally applicable to semiconductors as to metallic properties. Vie see
the following major functions and roles to be played by i n t e r m e t a l 1 ics in
future energy technology:
• There will be an increasing demand for hard or refractory coatings
resistant to erosion, corrosion and ablation at a variety of
temperatures. Such protective coatings will be needed in coal
liquefaction and gasification plants, breeder reactors, magneto-
hydrodynamic power generators and in deep geothermal systems.
They will be put in place by plasma spraying, chemical vapor
deposition and probably ion implantation. Research on new
methods of surface coating is vital.
- 47 -
• There will be a need for new composite or multi-component
high strength, refractory material systems for turbine blades
in gas and steam turbines. These materials will enable higher
inlet operating temperatures and thus greater efficiency in
power plants.
• High temperature materials will be needed to w ithstand extreme
nuclear radiation environments in the fuel system of breeder
reactors and in the first wall of fusion reactors. These
materials must retain strength and be resistant to swelling
caused by intense neutron bombardment. Intermetal1 ic compounds
seen to offer the best candidates for these applications.
• New magnetic intermetal1 ic compounds are vitally needed for a p
plication to electrical machinery. In particular, ways must be
found of replacing cobalt in the rare-earth-cobalt systems because
of the scarcity and high cost of cobalt. If these systems can be
cost reduced or a higher B*H product attained, they will find wide
application in reducing losses in electrical m otors and generators.
Possibly, high saturation rare-earth compounds can also be applied
to magnetic cooling or heating systems for heat pumps.
• Superconducting technology is in the process of moving out o f the
laboratory to prototype manufacture in electrical power generators,
ship propulsion systems, and fusion magnets. In all cases, the
great attraction is greater efficiency or energy saving. To
accelerate the progress in this new technology, we need better
■superconducting properties (Tc , h q 2 , Jc ) combined with high inechan-
cal strength and electromagnetic stability. I n t e r m e t a l 1 ic
- 48 -
compounds of the A15 and related structures of f e r the best
avenue of progress to these goals and intensified research
on these superconductors is essential.
• The forthcoming decline in crude oil production and the s u b s t i
tution of alternative hydrocarbon resources to supply the fuel
and petrochemical needs of the United States will require new
chemical reactions which will, in turn, place severe demands on
the availability of suitable catalysts. Many compounds, p a r
ticularly the compounds of metals with unfilled d and f bands,
are of interest as potential catalysts in the future.
• Many compounds exhibit solid-state phase transitions in which
major changes in physical properties may occur over a narrow
range of temperature . Steep variations in electrical c o n d u c
tivity, dielectric constant, or magnetic susceptibilities will
often occur at these phase changes. Such property changes p r o
vide an excellent basis for new sensor development. Sensors are
required to provide critical temperature rise signals in power
equipment or in some cases to provide direct feedback to limit
electric currents or voltages to the device. Alternatively,
there is a need for new solid material sensors to d e t e c t the
concentration of various gases in combustion systems, e.g.,
02» C02> CO, S02 > etc. This type of sensor is vital to improving
the efficiency of combustion and for control o f undesirable
emission.
- 49 -
• For the future development of hydrogen as a fuel, possibly
coupled with its direct generation in solar p h o t o - e l e c t r o
chemical cells, there is a need for better means of storing
hydrogen gas to replace compressed gas cylinders. Many solids,
for example palladium, absorb hydrogen very well but c a n n o t be
considered as bulk storage materials because o f cost. There is
a need for low cost intermetallie compounds to perform this
function.
# Recent research on intercalated graphite compounds has sug g e s t e d
that it may be possible to synthesize conductors with electrical
conductivity at room temperature higher than that of metallic
copper. There are some difficulties connected with extreme a n i s
otropy in such materials, but the importance of a breakthrough
of this type to the electrical power industry is great e nough
to v/arrant further study.
- 50 -
1. R.H. Hammond, J. Vac. Sci. & Tech., 1_5, 382 (1978).
D.F. Moore, R.B. Zubeck, J.M. Rowell and M.R. Beasley, Phys. Rev. B20,
, (1979). (to be published)
2. J.A. Wilson, F. J. DiSalvo, S. Mahajan, Adv. Phys. 2^, 117 (1975).
3. See for example, R.M. White and T. Geballe, Solid State P h y s i c s ,
S u p p l . 15., ed., H. Ehrenreich, F. Seitz, and D. Turnbul'n (New York:
Academic Press, 1979).
4. B.T. Matthias, M. Marezio, E. Corenzwit, A.S. Cooper, and H. Barz,
Science 175, 1465 (1972).
5. 0. Fischer, R. Odermatt, G. Bongi, H. Jones, R. C h e v r e l , and M. Ser-
gent, Phys. Lett. 45A, 87 (1973);
R. Odermatt, 0. Fischer, H. Jones and G. Bongi, J. Phys. C7, L 13 (1974).
6. S. Foner, E.J. McNiff, and E.J. Alexander, Phys. Lett. 4 9 A , 269 (1974).
REFERENCES:
7. M. Ishikawa and 0. Fischer, Solid State Commun. _2£, 747 (1977).
8. See, for example, R.W. McCallum, D.C. Johnston, R.N. Shelton,
and M.B. Maple, Solid State Commun. 24* 391 (1977); and
R.W. McCallum, D.C. Johnson, R.N. Shelton, W.A. Fertig, and M.B.
Maple, Solid State Commun. 24, 501 (1977).
9. M. Ishikawa and 0. Fischer, Sol id State C o m m u n . , 23, 37 (1977).
10. J.W. Lynn, D.E. Moncton, W. Thomlinson, G. Shirane, and R.N. Shelton,
Solid State Commun., 26, 493 (1978).
11. B.T. Matthias, E.Corenzwit, J.M. Vandenberg, and H. Barz, Proc.
Nat. Acad. Sci., USA, 74, 1334 (1977).
12. W.A. Fertig, D.C. Johnston, L.E. OeLong, R.W. McCallum, M.B. Maple,
and B.T. Matthias, Phys. Rev. Lett. 38, 987 (1977).
13. D.E. Moncton, D.E. McWhan, J. Eckert, G. Shirane, and W. Th o m l i n s o n ,
Phys. Rev. Lett., 39, 1164 (1977).
14. H.C. Hamaker, L.D. Woolf, H.B. MacKay, Z. Fisk, and M.B. Maple,
Solid State Commun. 31, 139 (1979).
15. H.C Hamaker, L.D. Woolf, H.B. MacKay, Z. Fisk, a nd M.B. Maple,
Solid State Commun. (to be published) 1979.
- 51 -
References (Chap. 3)
cont'd.
16. J.M. Rowell, R.C. Dynes, and P.H. Schmidt, to be published.
17. R. Feldman, R.H. Hammond, and T.H. Geballe, Applied Phys. Lett.
(in press).
18. i.v. Zubeck, R.S. Feigelson, R.A. Huggins, P.A. Pettit, J. Cryst.
Growth, 34, 85 (1976).
19. Z. Fisk, A.S. Cooper, P.H. Schmidt, and R.N. Castellano, Mat.
Res. Bull. 7, 285 (1972).
20. Z. Fisk, P.H. Schmidt, and L.D. Longinotti, Mat. Res. Bull.
1019 (1976).
- 52 -
CHAPTER 4
A M O R P H O U S S O L I D S
GLASS FORMABILITY AND STABILITY
To discuss glass formation by melt quenching it will be convenient
to allude frequently to the following parameters:
T . - g - W W (T k c - V / T 9 > T N
w h e r e :
T rg is the reduced glass temperature
Tg is the actual glass temperature
T^ is the thermodynamic liquidus temperature
Tm is the average/atom of the thermodynamic crystallization
temperatures of the glass constituents when pure
T (<c the "kinetic crystallization t e m p e r a t u r e ," is the temperature
of rapid crystallization onset when the glass is reheated
T^ is the onset temperature at which the homophase crystal
nucleation rate reaches measurable values when a liquid is
undercooled.
Experience indicates, with no exception w e know of, that T rg < 1.0.
That is, all glasses, and more generally amorphous solids^ are less stable
- 53 -
than p o m e crystallized state of the material; this statement is based on
the proviso that chemical bonds may be broken and re-formed in the c r y s
tal 1ization p r o c e s s .
Those nonmetal lie liquids, e.g. silica and o-terphenyl, which can
be slow cooled in large continuous masses to glass form, all exhibit
2/3 < 1.0. Further, the frequency of h o mophase crystal nucleation
in these materials appears to be negligible; they dev i t r i f y only if seeded.
In contrast, for the metallic alloy glass f o r m e r s so far Id e n
t i f i e d 1 , 2 , 0.45 < T < 0.70.
1 2 4 5)Generally the liquidus of these glass formers is g r e a t l y depressed ’ ’ ’ '
relative to T . In particular, 0.40 < T/T < 0.79. The displacement of m m
1T. from I is usually small; it is sometimes negat i v e so that the KC 9
glass transition is obscured, and only a few a lloys have exhibited
T - T----£L greater than 0.10 to 0.15. No simple c o r r e l a t i o n for T of alloy
' gglass formers seems to have been found. For m a n y such alloys
T g / ? m ^ 0.39 ± 0.03 but there are some marked exceptions.
BASIC RESEARCH OPPORTUNITIES
1• C h a r a c terization and m e a s u r e m e n t of crystal, nucleation r a t es
in alloy glass formers.
• Is nucleation homophase or h e terophase?
• If homophase, how does the nuc l e a t i o n rate depend on u nder-
cooling and T .' y
There seems to be no proof for the o c c u r r e n c e (or non-occurrence) of
h omophase nucleation in undercooled alloy glass f o r ming melts. However,
there are scattered observations which suggest that (L./T ) < 0.80 (for
- 54 -
most pure metals (T^/T^ < 0.75 ] . From this behavior it would seem
that the frequency of homophase nucleation in those alloy glass formers
for which T rg > 0.60 ought to be negligibly small at all c o o l i n g ^ so
that they could, if freed of heterophase impurities, be s l o w cooled in
bulk to the glass state. The potential technological impact might be
processes for making useful alloy glasses in massive form.
Alternatively, if crystal nuclei are formed copiously at relatively
slow quench rates either by homophase nucleation or by s e condary nucleation
processes starting from heterophase impurities , it may be possible to
obtain by varying quench rates, in addition to a completely amorphous
structure, the following types of microcrystalline structures;
© High d e n sitv of microcrystallites dispersed in an amorphous matrix.
This structure would have high mechanical strength. If the amorphous
matrix were transformed by some means to a superconductor, the
microcrystallites would serve as flux pinning centers.
• A c o m p letely microcrystalline structure which, e.g< depending on
the composition, might exhibit outstanding mechanical strength or
high critical currents in s u p e r c o n d u c t i o n
We note that NASA and O NR are supporting some research in these areas
but the magnitude of the effort is small relative to that in o t h e r fields
of amorphous metal research.
2 . Extension of composition ranges o f glass f ormability by more
rapid quenches
12It appears that quench rates of 10 °C/sec or greater may be
attained in very thin, e.g. < O J micron, molten metal layers by picosecond
- 55 -
laser pulsing. This capability would seem to offer the o pportunity to
markedly extend the composition range of glass formation by m e l t q u enching
and so obtain glasses with T well below 0.45.
3. Development o f understanding of the role of impurities in
amorphous solid metal formation
a. T h e o r y f o r d e p e n d e n c e o f T o n a l l o y c o m p o s i t i o n ^
. There are some empirical correlations of T^ with various
parameters but none appear to be wholly successful. There is no s a t i s f a c
tory microscopic theory for T , If we had either a successful p h e n o m e n o
logical or a microscopic theory, we might be able to identify alloys with
T rg > and so ^ a v e a 9reater potential for bulk glass formation.
b. C h a r a c t e r i z a t i o n o f a n d t h e o r y f o r e f f e c t o f i m p u r i t y
a d m i x t u r e o n a m o r p h o u s s o l i d f o r m a t i o n i n m e l t q u e n c h i n g
o r c o n d e n s a t i o n .
How low in impurity content can we go in e i t h e r of these
processes and still obtain an amorphous solid? There seems to be no clear
answer to this question from either theory or experience.
STRUCTURE A ND DEFECTS:
1. Research o p p o r t u n i t i es
a. S t r u c t u r a l 's t u d i e s
The physical and electronic properites of amorphous materials
depend to a great extent o n the atomic scale structure o f these materials.
The nature and extent of chemical and topological short-range o r d e r are of
fundamental importance. X-ray, electron, and neutron di f f r a c t i o n methods
give useful information in the form of atomic pair correlation f u n c t i o n s . ^
- 56 -
There is a need to obtain more detailed information not available from
conventional diffraction studies. The use of NMR and Mrjssbauer s p e c t r o s
copy has not been fully exploited. Little use has been made of recently
developed EXAFS techniques1*^ which promise to y i e l d considerable i n f o r m a
tion on the atomic-scale chemistry of amorphous materials. Small angle
x-ray scattering (SAS) offers a rather unexploited means of studying defect
structures in these materials. Internal friction measurements and possibly
Raman spectroscopy may also be a useful means of detecting defect structures.
The concept of a defect in amorphous materials needs further
clarification. In amorphous semiconductors, defects can be identified by
their influence on electron transport and optical properties. In amorphous
metals, defects appear to govern the plastic deformation beha v i o r of the
materials. Other material properties are likely influenced by defect
structures and this area of research should be explored.
b. C o m p a r a t i v e s t u d i e s
Various methods are presently used to prepare amorphous materials.
These include vapor deposition, sputtering, electrodeposition, laser melting,
and melt quen c h i n g methods. The same material prepared by different methods
is frequently found to exhibit rather different properties. Again the
concept of defects may well prove to give useful insight into this problem.
Do materials prepared by different methods relax to a common structure upon
thermal aging? Can the various materials be described by their variation
from an ideal structure?
c. R a . d i c . t i o n e f f e c t s
Radiation damage provides a controlled means o f altering the
atomic scale structure of amorphous materials. Little has been done to
- 57 -
determine the nature and effect of neutron, heavy ion, and electron
damage on various properties. This is particularly important since
e xisting d a t a ^ ’ ^ suggest that many properties of amorphous metals
are highly insensitive to radiation damage. Many possible applications
o f amorphous materials in the area of fusion technology seem promising.
They include inner wall materials, structural materials, and supercon
d ucting magnets. Mor e extensive study of the effects c f irradiation is
clearly very desirable.
d. S u r f a c e s t r u c t u r e
Finally, little o r no attention has been paid to the structure,
composition, and short-range order of surface and interface regions of
amorphous materials. Since this is of importance for such areas as
corrosion, wear, and catalysis, it would seem that opportunities e x i s t to
utilize recently developed surface techniques on amorphous materials.
Nearly nothing has been done in this interesting and important area.
ATOMIC TRANSPORT A NO TRANSFORMATION BEHAVIOR
The following are the frequencies, reciprocal time constants, for
some of the important atomic transport processes in liquids and glasses:
k : frequency of flow or shear relaxation process; proportional
to 1/n, where n is the shear viscosity
kp : frequency of diffusive transport process; pruportio.nal to the
atomic mobility
k y : frequency of interfacial rearrangement process in crystal
growth; proportional to crystal growth rate
- 58 -
ky : frequency of volume relaxation process; proportional to
volume viscosity, nv *
In nonmetal lie glass forming liquids these constants often, but not
always, scale together. Such scaling is very useful and often permits
fairly reliable estimates of kD, ku and k , which are fairly difficult
to measure, from shear viscosities, which can be measured fairly easily
to very high accuracies. When these scaling relations are fulfilled, the
rates of crystal 1ization, phase separation and volume relaxation, as well
as of shear relaxation, become negligible at temperatures only a few
degrees below the glass temperature.
1 • B a s ic research o pportunities
a. C h a r a c - t e r i x a i i o n o f a t o m i c t r a n s p o r t c o e f f i c i e n t s i n a l l o y
g l a s s f o r r . e r s ^ ^
There is evidence that in some alloy glass formers, a p p reci
able crystal growth and impurity transport occur at temperatures as
low as 100 to 150® below T , Also there seem to be no reported measurements
of volume relaxation rates in metal glasses. There is opportunity as
well as great need to characterize these transport coefficients more fully
and definitively. Such measurements will permit tests o f the validity o f :
the scaling relationship for alloy glasses as well as information on the
stability of these glasses at T below T .y
The atomic diffusivities in alloy glasses are in the range
T O O I10 c m sec and lower. ' There is need for further development of
experimental techniques for measurement of such small diffusivities.
I
1
- 59 -
b. Further exploration for and characterization of liquid-
liquid phase separation in alloy glasses.
The occurrence of liquid-liquid phase separation upon reheating
to the glass transition range after melt quenching has been d e m o n
strated. However, the exploration for such separation as well as
the characterization of it has been quite limited. We might expect
that its occurrence in ternary glass forming alloys might be as w i d e
spread as in silicate systems provided crystal 1 ization were avoided.
The onset of crystallization can be minimized by careful selection of
annealing temperatures and times. Also it is evident that the larger
is (T^c - Tg)/Tg the easier it will be for I - I phase separation to
be exposed.
Liquid-Liquid phase separation can be carried out in a way such
that two phases are interdispersed on a spatial scale as small as 30 to
50 A . Such structures might be exploited, e.g. in the following ways:
• Formation of porous metal membranes by selective dissolution of
one of the phases,
* By crystallization to form a high density of flux-pinning centers
in a superconducting alloy.
SUPERCONDUCTING MATERIALS
Amorphous superconductors are a unique class of metastable materials
which exhibit high critical f i e l d s , ^ ’ ^ high strength, and ductility.
The microscopic origin of superconductivity in these materials and the
parameters which determine Tc are of interest. The systematics of T c in
- 60 -
amorphous transition metals have been shown to differ substantially
from those of the corresponding crystalline m a t e r i a l s . ^’ These
differences reflect the fundamental influence of disorder on the
electronic structure of d-band metals and alloys. Atomic-like p r o p e r
ties appear to have a far more dominant influence on superconductivity
in the amorphous state. A fundamental understanding of this beha v i o r
should enhance o ur understanding of the role of long-range order in
crystalline s u p e r c o n d u c t o r s .
In some sense, amorphous superconductors are the extreme example
of metastability. Starting from an amorphous material, a variety of
other metastable materials are obtainable by thermal treatment. These
include composite materials which contain both an amorphous and a
crystalline superconducting phase interspersed in a highly refined
18)morphology.. ' Such structures are promising as practical high field
magnet materials. In such composites, one can exploit both the
desirable mechanical properties of the amorphous phase together with
the high Tc and high H g of the crystalline phase. Finally, amorphous
superconductors offer a novel method of synthesizing high T c c r y s t a l
line superconductors with high refined grain size through complete
19)crystallization of the amorphous matrix. 1
ELECTRONIC PROPERTIES AND MAGNETISM
The understanding of the electronic properties of amorphous m etals
poses a unique challenge to solid-state theory. As yet, no comprehensive
theory of electronic transport in amorphous metals has e m e r g e d . ^ Among
- 61 -
the variety of phenomena observed are the anomalous temperature dependence
21 22) 23)of electrical resistivity, v ' Kondo-type anomalies, 'a positive
Hall coefficient in some amorphous alloys a negative magnetoresistance.
As yet little experimental information is available on ot h e r transport
properties such as the thermoelectric power.
Much effort has been devoted to the study of the 3D transition
metal alloys (Cr, Mn, Fe, Co, and Ni base materials). These alloys have
been fairly well characterized and are now being developed for technical
applications. Several important problems remain in spite o f considerable
research. Magnetostrictive effects in these materials are still poorly
understood, and pose a serious obstacle to practical use in transformer
cores. The effect of thermal relaxation on magnetic properties is
another technically important problem which disserves f u r t h e r study. M a g
netic alloys containing rare earth metals have only recently been
studied extensively. The nature of magnetic interactions among rare
earth ions in amorphous alloys is not well understood. The RKKY inter
action which mediates this interaction is presumably a t t e nuated by the
24)strong electron scattering 1 in the amorphous state as suggested by
25)deGennes. ‘ The effect of magnetic ordering on superconductivity in
the amorphous state has not been studied at all. Several opportunities
in basic solid-state physics exist in all of the above m e ntioned areas.
CORROSION, DEFORMATION'BEHAVIOR, HARDNESS AND COATINGS
1. R esearch opportunities
a. Development of a theory of passivation
Several investigations have now indicated that C e r tain Fe-Cr-
metalloid alloys are much more resistant to corrosion when in amorphous
as opposed crystalline form.2 6 It appears that this exceptional corrosion
resistance of amorphous alloys derives from their e x t r a o r d i n a r y c o m p o s i
tional and structural homogeneity on a spatial scale g r e a t e r than a few
angstroms, and thus the composition and structural s i n g u l a r i t i e s which
provide the preferred points of corrosive attack on passive films are
largely lacking.
The research op p o r t u n i t y is not restricted to the b e h a v i o r of passive
films on amorphous metals. Rather it is to understand the m e c h a n i s m of
passive resistance generally. This is an old problem but an a t t a c k on
it by highly qualified structural metallurgists and inorganic chemists
in concert might be fruitful. It appears that the d i s s o l u t i o n of the
passive film is an interface (film-solution) limited process and that
the problem is to characterize the dissolution kinetics and the role of
structural irregularities thereon.
b. Microscopic theory for mechanical strength and ductility
of amorphous alloys
The microscopic theory for the exceptional strength and c o n
siderable ductility of a large number of glassy metal a lloys is still far
27}from clear. ' However the problem has attracted w i d e s p r e a d attention,
and there is now a large effort to characterize and explain the mechanical
properties of alloy glasses.
c. High hardness, wear resistance
A number of amorphous metallic alloys have been found to exhibit
Vicker's Hardness in the range from 1000 to 2000 Kg/mm.2**’ The h o m o
geneous nature o f the amorphous phase, together with the high hardness and
- 62 -
- 63 -
strength, suggest possible use of these materials as protective w e a r - r e
sistant coatings. Hardness, together with high resistance to corrosion*
makes such a possibility extremely attractive.
2 • Technological needs
A wide variety of technologies, including deep-well oil drilling,
corrosion resistant high hardness bearings, and solar photovoltaic arrays,
will obviously benefit if reliable economic methods of producing a p p r o priate
amorphous coatings are developed.
AMORPHOUS OXIDE AND CHALCOGENIDE MATERIALS
1. Research opportunities
It has recently been shown that the crystalline octahedrally
30 31 'coordinated ferroelectric materials, LiNbO^, LiTaO-j, and BaTiO^, ’ ' can
be quenched from the melt to form amorphous dielectrics. A unique
unexplained feature of these materials is the fact that their dielectric
constants are much larger than their crystalline c o u n t e r p a r t s . Very
large dielectric anomalies also appear on heating prior to the onset of
crystallization, and the microscopic nature of this e ffect is also not
understood. Other properties, such as the pyroelectric effect, suggest
that amorphous LiNbO^ and LiTaO^ are amorphous ferroelectrics, but f e r r o
electric domains have not been observed.
Amorphous LiNbO^ and LiTaO^ exhibit room temperature ionic
-5 -1 -1conductivities of 10 Q c m , six orders of magnitude greater than their
crystalline counterparts.
- 64 -
There is a need to develop an understanding o f local atomic
structure and bonding levels for heavy metal ions in s i l i c a t e glasses.
Research should be devoted to understanding cation d i f f u s i o n m e c h a n i s m s
in the glasses. What types of defects are introduced by ioni z i n g r a d i a t i o n ?
2. Technological opportunities
There is a technological need to develop small all-sol id-state
high-energy d e n sity thin-film batteries for microcomputers. Films of
amorphous LiNbO^ and LiTaO^ and ot h e r octahedral c o o r d i n a t e glasses co u l d
serve as the basis of solid electrolytes for Li s e c o n d a r y batteries. S o l i d
electrolytes based on amorphous polyolefin alkali t h i o c y a n a t e s are being
•considered by the French.
The metallic layered c rystalline dichalcogenides, such as T i S 2
and V$2 > are being used as cathode materials in the d e v e l o p m e n t of high-
energy density Li/TiSg, nonaqueous batteries {as d i s c u s s e d in C h a p t e r 6
below). There is evid e n c e in the case o f MoS^ that the a m o r p h o u s e l e c t r o d e
can reversibly interc a l a t e significantly more Li than the c r y s t a l l i n e
electrode, thus increasing the capacity of the cell. Thi s result suggests
that the capacity o f o t h e r metallic cathode matrices, suc h as the v a n a d i u m
oxides, might be increased by making them amorphous.
3. Nuclear w a s t e storage of fission products (Many e l e m e n t s in the p e r i o d i c
t a b l e ).
D e v e l o p m e n t o f a basic understanding of i n c o r p o r a t i o n and r e s i s
tance to leaching in silicate glasses is a need. This w o r k is to be d o n e with
a v i e w toward d e v e l o p i n g n e w complex glasses for i n c o r p o r a t i n g r a d i o a c t i v e
e l e m e n t s wh i c h w o u l d be stable, resistant to leaching, and pro t e c t e d fr o m
b r e a k d o w n due to ionization.
- 65 -
1. Characterization and measurement of crystal nucleation rates in
alloy glass formers.
2. Extension of composition ranges of glass formability by more rapid
quenches.
3. . Development o f understanding of impurity role in amorphous solid
formation.
4. Experimental characterization of nature and extent o f short-range
order in amorphous solids.
5. Effect of irradiation on structure and properties of amorphous solids.
6. Comparison o f amorphous solids of the same composition prepared by
different techniques.
7. Use of new techniques to characterize the surface of amorphous
m a t e r i a l s .
8. Characterization of microscopic parameters which lead to s u p e r c o n
ductivity in amorphous alloys.
9. Characterization of atomic transport coefficients in glassy materials.
10. Further exploration of liquid-liquid phase separation in alloy glasses.
11. Studies of amorphous or amorphous-crystal! ine composite materials for
superconductivity and ot h e r areas.
!2. Develop a comprehensive theory of electronic transport in amorphous
solids.
13. Magnetic interactions in amorphous alloys containing rare earth
elements. Magnetostriction.
14. Development of a theory of chemical passivation of surfaces.
SUMMARY OF OPPORTUNITIES IN RESEARCH
- 66 -
15. Development of a microscopic theory of mechanical strength and
ductility of amorphous alloys.
16. Development of a theory for electronic behavior of amorphous
oxide glasses and other non-metal lie amorphous solids.
1'/. Development of a theory of atomic scale structure and bonding
levels of metal ions in amorphous oxides; also understand
ionic conduction, etc.
SUMMARY OF POTENTIAL IMPACT ON TECHNOLOGIES
1. Soft magnetic materials for electrical power applications are now
being commercially developed by General Electric, Allied Chemical,
and some foreign companies. Device scale applications in magnetic
memory are also under development.
2. Temperature stable resistors and other device components.
3. Materials for use in high radiation environments.
4. High strength fibrous and lamellar composite structural materials.
5. Corrosion and wear-resistant coatings for many specialized technologies.
6. High-strength formable conductors for superconducting magnets.
7. Semiconducting materials for photovoltaic device applications.
8. Porous metal membranes for filtration, osmosis and catalysis.
9. Solid-state thin-film batteries for microcomputers.
10. Containers for nuclear waste storage.
1
REFERENCES:
1. P. Chaudhari and D. Turnbull, Science 199, 11 (1978).
2. D.E. Polk and B.C. Giessen, Metallic G l a s s e s , ed., J.J. Gilman and
H.J. Leamy, (Metals Park, Ohio: A m e r . S o c . M e t . , 1978) pp. 1-35.
3. H.S. Chen and K.A. Jackson, i b i d . , pp. 74-96.
4. D. Turnbull, J. de Physique, 35, ( M , 1-9 (1974).
5. M.A. Marcus and D. Turnbull, Mats. Sci. & Engrg., 23, 211 (1976).
6. F. Spaepen and D. Turnbull, Proc.. 2nd International C o n f erence on Rapidly
Quenched M e t a ls, ed. N.J. Grant and B.C. G i e s s enTTCamb'ridge, Mass:
MIT Press, 1976) vol. 1, p. 205,
7. C-P. Peter Chou and F. Spaepen, Actr Met. 23, 609 (1975).
8. N. Bloembervjn, L a ser-Solid Interactions and Laser Pro c e s s i n g , ed. ,
S.D. Ferris, H.J. Leamy and J.M. Poate, pp. 1-10. A.I.P. Conf.
Proc. No. 50, A.I.P., New York, N.Y. (1979).
9. G.S. Cargill, Solid State Physics, 3_0, 227 (1975).
10. T.M. Hayes, J.W. Allen, J. Tauc, B.C. Giessen, and J.J. Hauser,
Phys. Rev. Lett., 40, 1282 (1978).
11. M.R. LeSueur, C.R. Acad. S c i ., 266, 1038 (1968).
12. A. Kramer, W.L. Johnson, and C. CVine, A p p l . Phys. Lett., 35,
(in press) (1979),
13. F. Spaepen and D. Turnbull, o p . c i t . , (2)> pp. 114-127.
14. C-P. Peter Chou and D, Turnbull, J. Non-Cryst. Solids, j_7, 169 (1975).
15. M.A. Marcus, J. Non-Cryst. Solids, 30, 317 (1979).
16. W.L.Johnson, j. Appl. Phys., 50 (3), 1557 (1979).
17. M.M. Collver and R.H. Hammond, Phys. Rev. Lett., 30, 92 (1973).
18. B.C. Clemens, W.L. Johnson, and J. Benott., J. Appl. Phys., 51_
(in press) (1979).
19. C.C. T s u e i , Appl. Phys. Lett., 33, 262 (1978).
20. H.J. Giintherodt and H.U. Kunzi, o p . c i t . , [ 2 ) , p. 247.
- 68 -
References {Chap. 4)
{ c o n t ' d .)
21. P.J. Cote, Solid State Cornrnun. 1_8, 1311 {1976);
P.J, Cote, A morphous M a g n e t i s m , ed., R.A. Levy and R. Hosegawa
(New York: Plenum Press, 1977).
22. T.E. Faber and J.M. Ziman, Phil. Mag. (8) 1J, 1 53 (1965).
23. B.V. Bouncher, J. Non-Cryst. Solids, 277 (1972);
R. Hasegawa, Phys. Lett., 3 8 A , 5 (1972).
24. S.J. Poon and J. Durand, Solid State Commun. 21, 999 (1977).
25. P.G. deGennes, Progress in Solid State C h e m i s t r y , (Oxford: Pergamon
Press, 1 9 6 6 V vol. 3.
26. T. Masumoto, K. Hashimoto and M. Naka, Rapidly Quenched Metals I_IJ_
ed., B. Cantor, (London: Metals Society 1978) v o l . 2, p. ~4~35.
27. L.A. Davis, o p a i t . (2), pp. 190-223.
28. S. Davis, M. Fischer, B.C. Giessen, and R.E. Polk, Proc e e d in g s ,
3rd International Conference on Rapidly Quenched M a t e r ia l s ,'
ed., B. Cantor (London: Chameleon Press, 1978).
29. W.L. Johnson and A.R. Williams, Phys. Rev. B 2 0 , 1 (1979).
30. N. Tsuya and K.I, Arai, Structure and Properties of Amorphous
Me ta1s , Reports of the Research Institute, Tohuku University, Sendai, Japan (1978), p. 43.
31. A.M. Glass, M.E. Pines, K. Nassau, and J.W. Shiever, Appl. Phys.
Letts., 31_, 249 (1977);
A.M. Glass, K. Nassau, and T.J. Negran, J. Appl. Phys., 49,4808 (1978).
- 69 -
CHAPTER 5
T H I N F I L M S
INTRODUCTION
1 2)The field of thin film research 5 1 has progressed extr e m e l y rapidly
in the past twenty years, largely due to the incentive of technological
applications, particularly circuits initially comprised of discrete devices
and now demonstrating great complexity of function on a single substrate.
It is only realistic to assume that the future of the field will again be
strongly influenced by the progress expected in semiconductor and magnetic
bubble integrated circuitry, with their associated techniques of pattern
generation by photo, e - b e a m and x-ray resist lithography.
Huwever, strides in thin film research have primarily occu r r e d in
materials, techniques and instrumentation strongly influenced by the s e m i
conductor industry; other aspects of the field remain at a stage where ample
opportunity exists for research at a fundamental level. An example is
amorphous and polycrystalline silicon (to be discussed in detail in Chapter 0
dealing with semiconductors) where it is clear that knowledge is rudimentary
compared to the artistry of an integrated circuit in single crystal silicon.
Preparative methods not commonly used by industry have been neglected, and
may be more interesting from a research point of view and more valuable for
other emerging applications. For these reasons, although we realize that the
greatest progress in the field of thin films in the future will probably be
associated with semiconductor circuitry, we will be brief when dealing with
that aspect of the field. A recent report3 outlines the research that will
be required to support the future of the electronics industry.
- 70 -
To help in later discussion, we briefly summarize the common methods
of t h i n - f i l m preparation, and note the peculiarities of each.
1. E v a p o ration
Using a single source heated resistively or w i t h an e-beam, this
*method is primarily useful only for elements. Using m u l t i p l e sources,
compounds and alloys can be prepared. Molecular beam e p i taxy ( M B E ) ^ is the
growth of single crystal material by multiple source evaporation. In
addition to evaporation of the elements comprising the compound, impurities
can also be introduced as d o p a n t s . ^ M u l tilayer structures.with each layer
being a few monolayers of a given compound,have been g r o w n . ^ Thus MBE can
be regarded as the most carefully controlled thin film growth to date.
As evaporation is a high vacuum process, all the tools developed
for surface science can be utilized and i n s i t u surface studies of the g r o w
ing film can be carried out, but in practice this is rarely done e xcept in
MBE.
2. S p utteri ng
By bombarding a target with energetic ions from a discharge, films
can be prepared whose composition generally matches that of the target.
Thus this method is ideal for alloys and compounds. A dc discharge is often
enhanced by confinement by a magnetic field (magnetron h e a d s ) ^ or by an
electron source (triodes), and insulators can be sputtered with r.f. However,
typical gas pressures are 1-100 ij , thus gas incorporation is very likely and
high v a c u u m surface tools cannot be used during deposition. The use of
7)focussed ion beams ' allows some improvement in the v a c u u m level. "Getter
8'sp uttering", in which a 1iquid-nitrogen cooled c ontainer is placed around
* Certain compounds deposit stoichiometrically when evaporated, e.g. CdS;
m o l e c u l a r solids also.
THIN FILM PREPARATION
- 71 -
the discharge target and substrate, reduces the partial pressure of active
impurity gases.
9)3• Chemical vapor deposition '
Chemical vapor deposition is carried out by reacting gases over a
substrate, usually in a heated tube. Different CVD processes such as the
halide or o r g a n o m e t a l 1 ic methods depend upon the composition of metal-bearing
gas. I n s i t u surface analysis is not possible, but growth rates are large
and thick ( > 10 y ) layers can be prepared.
4. Liquid p hase e p i t a x y ^^^
Liquid phase epitaxy is growth from a melt saturated with elements
of the material of interest. It has been used particularly for semiconductors
such as GaAs and GaA£As and doping of the individual layers can be carried out.
5. Organom e t a l 1 ic vapor deposition
It has recently been s h o w n ^ that the thermal decomposition of
organometallics to form solid solution, lattice matched submicron s e m i c o n
ductor layers will result in devices exhibiting properties comparable to
those produced by liquid phase epitaxy and chemical vapor deposition. The
use of organo m e t a l 1 ics should be applicable to the synthesis of thin films of
many materials, particularly i n t e rmetal1ic compounds. In addition, it has
recently been demonstrated that dissociation of organoinetal1 ic compounds can
be accomplished by laser-induced photodissociation and this offers another
12)possibility of accomplishing controlled growth of thin films. ' It m ay also
be possible to produce alloy films from two or more volatile compounds, e.g.
an alkyl compound in interaction with a chloride to produce the alloy with
elimination of alkyl chloride.
6. Electroplating
Electroplating can take place from aqueous solutions at room
temperature, or from molten salts at quite high temperatures, say % 600°C.
In general, evaporation and sputtering are the means rnost commonly used
to prepare thin metallic films; the other techniques above are more s p e c i f i
cally applied to certain materials such as semiconductors.
A COMMENT ON TECHNIQUES
As a means of material preparation, thin film methods are unusual in that
much of the required equipment, both for attaining vacuum and for film
deposition, can be purchased commercially, as can the surface analysis tools
that may be added to the same system. There appears to be a danger that
basic research in thin films will be partially dictated by what equipment
is most easily available, and thus the field will lack variety and novelty.
We suggest that unusual approaches to pumping systems, deposition methods
and film characterization should be encouraged whenever possible. As an
13)example we cite the work of Thompson, ' who cools an entire vacuum chamber
to liquid helium temperatures. The operating pressure is orders of m a g
nitude lower than in conventional systems at room temperature.
OPPORTUNITIES FOR RESEARCH
Compared to other methods of material preparation, thin film deposition
has some unique advantages and disadvantages which offer opportunities for
basic research. To utilize these fully, thin film growth itself should first
be understood in more detail.
1• Thin film growth
Although this problem has been considered in the past, the full
power of modern vacuum techniques, i n s i t u surface studies and c h a r a c t e r i z a
tion by, for example, high resolution electron microscopy, has not y e t been
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brought to hear except in isolated cases. For example, new electron
microscopes working at -v Iff9 Torr will allow i n o i t u structure
studies and energy loss spectroscopy of a growing film.' There is a need
for fundamental investigations of the detailed atomic scale mechanisms
by which films, both crystalline and amorphous, form and grow. Such
understanding will allow greater control over thin film composition,
morphology, microstructure and electronic properties, and could also
lead to new selective techniques to produce films of unusual chemical
composition and atomic structure. Research of this type is also closely
allied to other problems in modern surface physics and chemistry; crystal
growth, amorphous film formation from the vapor, epitaxy and coatings in
device production, corrosion and other gas-surface reactions. Interesting
possibilities for research include combining kinetic measurements with
detailed observation and characterization of incipient films during their
early growth stages. Such studies could be carried out as a function of
vapor chemical constituents, of surface temperature and composition and
of overall incident flux. Deliberate changes could be made in the vapor
molecular structure and effects on film growth studied (e.g. atomic As
versus As^). Lasers and other techniques (plasmas, energy transfer) allow
selective (optical) electronic and vibrational excitation of the condensing
molecules. Another rich area involves looking at the production of films
from low energy ions, from very complex, high temperature vapors where one
has unusual molecular aggregates, and under conditions where molecules are e x
cited (optically or perhaps by electron impact) while they are on the surface
(e.g., by focussing a laser on the growing film and comparing film formation
in the illuminated region with that outside). At high laser powers, this
process would become an i n a i t u annealing of the growing film (which might
allow improvements of crystallographic order at low overall substrate
temperatures). We note that there is a high level of current interest in
151laser annealing of semiconductov' films 1 after preparation. Irradiation
of various kinds during growth, as proposed above, would seem to offer
more opportunity for observation of unusual growth effects. Laser a n nealing
of completed films other than semiconductors should also be studied as a
method of producing metastable phases.
2. S urface modification
There are a number of ways in which the surface of a bulk (or
film) material can be modified to produce more desirable properties: for
example, resistance to corrosion, erosion and wear, or improvement in
conductivity or oxidation characteristics.
The formation of a surface compound by reaction of a dep o s i t e d
layer with the bulk is a relatively simple process (and is commonly used
to produce transition metal silicides on silicon). The mechanical p r o p
erties of such surface layers in particular should be investigated.
Ion implantation is a way of introducing impurity ions into the
surface of a material, the depth of implant extending to ^ 1 p and being
variable with ion energy. It is widely used to dope semiconductors and
compound formation can occur. It has produced metastab'le compounds (e.g.,
there is a strong increase in superconducting Tc of Pd with H implantation
at low temperatures). In addition to implantation, sputtering and damage
of the surface occurs. It has been r e p o r t e d ^ that this strongly changes
t h e c o r r o s i o n resistance and oxidation rates of the implanted/damaged part
of the surface. Much further work is required in this field to understand
the relative importance of the nature of the implanted ion, its energy,
compound formation,'sputtering and damage on these surface properties. This
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is an area where surface tools can be used to advantage, and i n s i t u
studies in high resolution electron microscopes, including energy loss
spectrometers, should be possible.
In some situations, for example in a fusion reactor first wall,
materials will be at elevated temperatures and subjected to intense
radiation. The interaction of this displacement producing radiation with
metallic alloys gives rise to the formation of nonequilibrium phases (or
phase distributions). The kinetics and morphology of this microstructural
evolution can now be studied using high resolution analytical microscopy.
171Since the process- is phenomemologically understood ' as the preferential
binding of one alloy component to one of the mobile radiation produced
defects (e.g. interstitial atom-silicon at o m binding in Ni-Si alloys)
that tend to annihilate preferentially at defect sinks (e.g. dislocation
loops, grain boundaries, and surfaces), this phenomena needs basic research
efforts in understanding defect-defect, defect-solute, and defect-sink
interactions.
3• Materials preparation
The conventional evaporation and sputter techniques can be utilized
in novel ways to prepare materials, or to rapidly search for new phases and
physical properties within a given alloy or compound system,
a. U s e o f P h a s e S p r e a d
By controlling the rate of two evaporation sources, and
placing substrates above and parallel to the line between them, a fraction
of the phase diagram of the two evaporating constituents can be produced.
Likewise a gradient in substrate temperature can be introduced. Amorphous
films can be prepared at lower temperatures. Thus the structural, mechanical
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and electrical properties of every part of the phase d i a g r a m can be rapidly
surveyed. This method has been used with success recently for A15 su p e r
conductors and structure, lattice constant, T c and the e nergy gap determined
18)as a function of composition. / It should be noted that in some cases the
phase diagram will differ from that of the bulk, in that film deposition can
produce metastable phases, Nb^Ge being one example.
Sputtering from multiple sources can be used similarly and, as
the substrates can often be placed closer to the sources, practically the
19)whole phase diagram can be produced in a single deposition. ' Use of
magnetron heads appears very desirable, in that their deposition rate is
high, rate can be set by the sputtering current,and unwanted heating of the
substrates is smal 1.
By using three sources, ternary phase diagrams can be produced.
20)This method was employed to search for magnetic films for bubble circuits. '
It is clear that this multiple source deposition method,
although demonstratedly powerful in the production of materials, is being
used only in isolated cases and for particular phenomena such as s u p e r
conductivity and magnetism. We feel it should become m u c h more widespread,
one example being in producing amorphous and polycrystalline semiconductor
films. Mechanical properties of films produced by this m e t h o d should be
studied; for example, it has been found by chance that films of Mo^Ru^ and
21)W^Ru*, being made for superconduct.ivity studies ' exhibited unusual hardness
2 2 )and corrosion resistance. '
b. M e t a s t a b l e p h a s e s
Film deposition is often regarded as a rapid quench process to
the temperature of the substrate. This has allowed the preparation of
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metastable phases which cannot be made by conventional bulk methods as in
the case of Nb^Ge where Tc in films has reached 23 K compared to an
onset of 17 K in bulk arc-melted, quenched and annealed samples. Also, by
the use of substrate temperature down to 4 K if necessary, amorphous
materials can be readily produced in a form ideal for in situ electrical
and structural characterization. With change in substrate temperature the
amorphous-crystal 1 ine transition can be readily studied in a controlled
manner.
4. Surfaces and interfaces
Full advantage should be taken of the opportunities offered by
thin film preparation, in that a fresh clean surface is produced (at least
momentarily) and that an interface, or multiple interfaces, can be readily
prepared.
a . Surface roughness
For one material, variation of substrate temperature, d e p o
sition rate and gas pressure can result in films with a surface which is
optically smooth*or so granular that it appears black. This variation in
growth should be better characterized, particularly in systems of interest
as solar absorbers. It is obviously related to the understanding o f thin
film nucleation and growth, as discussed above.
b. Epitaxial growth
Epitaxial growth of a film on a fresh surface of an u n d e r l y i n g
film- or on a substrate has been demonstrated in a limited number of cases,
M8E of GaAs/GaA£As layers being the primary example. In other cases, lattice
matching, but not single crystal growth, is achieved. By deposition of
Nb^Ge on the stable compound Nb^Ir it was shown that the A l 5 phase of
Nb^Ge could be extended beyond its equilibrium c o m p o s i t i o n . ^ An
amorphous silicon layer, deposited on a grooved silica substrate and
subsequently laser annealed, became single crystal with an o r i e n t a t i o n
2 5 )determined by the grooves. ' As the grooves were ^ 2 ^ wide, which is
huge on an atomic scale, this suggests that the initial nucleation and
growth of thin films might be controlled in a predictable way by s u i t a b l e
preparation of substrate surfaces.
c. I n t e r f a c e c o m p o u n d s a n d m u l t i l a y e r s
The use of "diffusion couples" to measure i nterdiffusion
and compound formation is well known. However, multiple source d e p o s i t i o n
allows the formation of films with alternating components where the
individual layers become very thin (a few atomic layers). These s t r u c t u r e s
would allow not only the study of very slow diffusion rates but, m o r e
importantly, the interface regions can become a large fraction of the total
film. Thus the chance for observing unusual compounds stabilized by
interfacial energies is optimized. Again, i n s i t u studies of such films
as a function of annealing temperature, by x-ray or electron s c a t t e r i n g
and Rutherford backscattering at low angles, should be encouraged.
d. O t h e r
The clean surface of a film is ideal for the subsequent
preparation of tunnel junctions, which can be a most useful probe of the
properties of superconductors and s e m i c o n d u c t o r s , * ^ and also for m a k i n g
Schottky barriers and metal-insulator semiconductor barriers now o f interest
as photovoltaic devices.
5• Novel geometrical m a t erials
The thin film geometry can be made as small as desired in o ne
dimension, although uniformity of the film may be hard to achieve. Wh e n
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.combined with modern lithographic techniques, it is possible to make two
or even all three of the dimensions o f a film small, i.e., ;> 1000A , and
thus approach the electronic wavelength, at least in semiconductors and
semirnetals. This offers the opportunity o f the most fundamental studies,
particularly of electronic properties of materials, and also of preparing
new materials b y '‘molecular engineering" (or at least "molecular layer
engineering"). The use of molecular beam epitaxy in the GaAs-GaAJ,As
system where repetition of monolayers or placement of dopant impurities
into specific layers has been achieved- can serve as an example of what
might also be done in many other systems given sufficient understanding
and control of the growth process. This monolayer engineering is not
restricted to semiconducting systems: already work on magnetic/non-
0~] \ O R \magnetic ' and on superconducting/normal metal ' multilayers has been
reported.
6• D i e l e c t rics
The success of silicon integrated circuitry relies heavily on
the insulating properties of silicon oxide. However, many other useful
semiconductors (such as GaAs) do not grow a thermal oxide of a quality
suitable for device use. When integrated circuitry utilizes linewidths
< 1 j j , oxide insulators will become thinner than at present. There is
at present little basic understanding of the limitations of evaporated
dielectrics as insulators. Research is required into the effects of
nonuniformity and defects, nonstoichiometry, ionic and electronic c o n
duction and interface reactions with metals and semiconductors. T h e s e
studies should not be restricted to inorganic compounds such as oxides,
fluorides, nitrides, etc., but polymeric thin films should also be c o n
sidered. The formation of single crystal polymer monolayers should be
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- 80 -
possible using techniques of epitaxial growth, plasma polymerization,
vapor deposition, liquid crystal orientation and spin coating. Basic
questions to be answered include the effect of the substrate on o r i e n t a
tion, the limits of film thickness to achieve insulating (and other)
properties, morphology and the nature of rnetal-polymer interactions and
interdiffusion. In addition to insulating properties, the corrosion
resistance, wear resistance, adhesion strength to metals and phot o s e n
sitivity and radiation sensitivity should be determined.
7. Cermet, nonhomogeneous and granular films
By codeposition of metals and insulators, or of a metal in oxygen,
films have been prepared which exhibit electrical behavior ranging from
metallic to i n s u l a t i n g . ^ Considerable work has been carried out on such
systems but recent interest in the general nature of localization in
1-, 2-, and 3-dimensions is already giving new insight into their behavior.
The optical properties of such materials are also of interest, in that
their absorption can be made to peak at different wavelengths depending
on the volume fraction of metal. A promising class of selective solar
absorbers derives its absorption characteristic from a two-phase metal/
oxide structure. Particle size and thickness of these "Dlack chrome" or
"black cobalt" films lie below optical wavelengths, but their structure
and arrangement are not known in detail. Therefore, the theoretical basis
is missing for directed efforts to prepare such films and to prevent their
degradation at operating temperatures.
8. Film character!zation
Thin film research has been, and still is, plagued by n o n r e p r o
d u c i bility of films made under "the same" conditions. This is due to the
many parameters affecting the film, some of which may not be controlled
or oven appreciated. Gas impurities can be incorporated from the source,
the vacuum system, or from the substrate; metallic impurities from the
source, from the chamber walls or the masks near the substrate. The film
structure is sensitive to substrate temperature, gas pressure (in sputtering
systems) and the nature of nucleation on the substrate. If full u n d e r s t a n d
ing of the materials is to be achieved, and complete utilization of thin
film methods in the preparation of materials such as amorphous silicon or
superconducting Nb^Ge, a much better characterization of the completed
films must be carried out and their properties related to deposition
parameters. Much more widespread use of i n s i t u surface analysis tools
would be a major step forward, in that growth parameters could be varied
and their effect observed immediately. At present, such studies are fairly
common only in semiconductor MBE. This either requires that traditional
thin film scientists learn and appreciate the use of available surface
tools, or that surface scientists become interested in a much wi d e r variety
of materials and interface systems than they have to date.
9. Deposition method characterization
Given the progress in thin film characterization and the r e l a t i o n
ship between film properties and deposition parameters outlined in (8) above,
an aim would be to characterize the limitations and advantages of each
deposition method so that for a given material property the method could
be chosen to optimize this characteristic. Two examples, both poorly u n d e r
stood at present, are that sputtering seems to yield Nb^Ge with a h i g h e r T c
than evaporated material, but sputtering of polycrystalline silicon y ields
material which is unsuitable for device fabrication.
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10. Jheory
Surface physics theory has in large part dealt with a very limited
number of solid-vacuum interfaces, particularly the crystal faces of silicon.
Thin fil m research deals with less ideal materials and interfaces. To
c o n t ribute to many of the research problems outlined in (1) - (9) above,
theory should begin to consider semiconductor-dielectric and metal-dielectric
interfaces, and the contact between dissimilar metals, particularly its
electrical resistance. In many cases there is no lattice matching in such
interfaces; perhaps an understanding of the defects and grain boundaries
generated in such cases will even lead to prediction of interface and s u r
face compounds.
RESEARCH NEEDS
In addition to the control of deposition process, and characterization
of the resulting film by existing surface and bulk methods, it is often of
interest to look at the electronic properties, impurity content and structure
at a given depth within a film. Many methods accomplish chemical analysis
by destruction of the film by ion milling. There is a need to identify and
develop nondestructive tools which accomplish this and give structural and
e l e c tronic information as well. Rutherford backscattering is of value
(but the depth resolution is generally ' v l O O A , unless low angles are used)
and specific nuclear reactions can be used to profile hydrogen and oxygen.
Electron microscopy has good lateral resolution but is not very specific in
depth. The scanning acoustic microscope should be valuable for film and
interface studies.
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RESEARCH OPPORTUNITIES FOR TECHNOLOGY
In a field which has been stimulated largely by technology, the
division between basic and technological research is somewhat arbitrary
and there is strong overlap with the recommendations made above. We have
attempted to arrange this section in order of decreasing importance to
energy needs.
1 * Spe c t rally selective solar a b sorbers
Selective absorbers are films and surfaces whose absorbance (a)
for solar radiation is high, but is low for their own thermal re-radiation
in the infrared (then, by Kirchoff's law, their omittance (e) in the i.r.
is also low). One group of selective absorbers are composites of an i.r.
reflecting metal substrate coated with a thin absorber layer (semiconductor,
metal-dielectric interference stack, granular metal-oxide film). The
second important group comprises simple metal films with high a/e, or metal
substrates covered with small particles of the same material. Particularly
311for this latter group, practical limits to a/e need yet to bo determined. ‘
2 . Photovo l taics
Photovoltaics will be discussed in Chapter 8. Although the semi-
conductor-semiconductor crystalline junction is most efficient at present,
other considerations (cost of materials, energy cost of production, etc.)
make it worthwhile to examine other heterojunctions. A l r e a d y the semi-
conductor-metal and semiconductor-oxide-metal junctions are receiving
attention. It would be worth determining, perhaps from carrier lifetime
estimations alone, whether the metal-oxide-metal junction can ever be a
useful photovoltaic cell.
3• Interface stability
In photovoltaic cells, and in all thin-film circuitry, interfaces
between different materials are subjected to electric field and current
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flow. The stability of these interfaces, in particular their compos i t i o n
profile and defect distribution, should be investigated. Again, unusual
compound formation at the interface might be possible.
4. Surfaces for specific environments
Given the number of preparative methods of films and surfaces of
alloys, intermetallic compounds and amorphous matervils mentioned so far, it
should be possible to produce surfaces having desirable properties for a
given application. Many films based on transition or rare earths m e t a l s
are very hard, exhibit corrosion, erosion and wear resistance, and some do
not oxidize to produce an insulating coating. However, interest in these
films is often for another characteristic, such as s u p e r c onductivity or
magnetism, and the mechanical properties are generally not measured or
published in any detail. Some way needs to be found to encourage those in
the thin film field to be more aware of the importance of such c h a r a c t e r
istics. Similarly, dielectrics, which have been mentioned above, should be
considered as passivating and protecting layers for photovoltaics.
A self-replenishing low 1 coating for a fusion reactor first
wall is required, as are corrosion resistant structural materials for
fission and fusion reactors.
5. High Tc superconductors
The highest Tc superconductor Nb^Ge is easily m a d e in thin fil m
form by evaporation, sputtering or CVD. Ways to use this compound, Nb^Afl.,
NbstAJlxGe-j_x ) and also the Chevrel phases (with even h i g h e r critical field)
in useful conductor form should be devised.
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1
- 85 -
^ • M i c rostructures
Although the physics and applications of microstructures will
probably be more important commercially than all the above* we comment only
briefly in view of an earlier report. However, mechanical properties o f
materials made on the scale of 1000 A, such as diffusion, electromigration,
and strength as well as interface stability, should be considered. It is
often a useful scientific approach to consider limiting behavior, and such
studies of extremely small structures might shed light on failure m e c h anisms/
which occur, but less frequently, in semiconductor devices and m e t a l l i
zations of more conventional size. Thermal boundary resistances between
different materials (and to liquids and gases) will also become important
as semiconductor circuitry shrinks in size, and in solar power applications.
- 86 -
1. K . L . Chopra, T h i n Film P h e n o m e n a , (New York: McGraw-Hill, 1969).
2. L ,I . Maissel and R. Glang, eds., Handbook of T h in Film T e c h n o l o g y ,
(New York: McGraw-Hill, 1970).
3. Microstructure Science, Technology and Eng i nee r i ng, National
"Academy of Sc fences (1979).
4. A.Y. Cho and J.R. Arthur, Progress in Solid State C h e m i s t r y ,
eds., G. Somorjai and 0. McCaldin7 (New Y o r k : Pergamon Press,
1975) vol. 10, p. 157.
5. R. Dingle, H.L. Stormer, A.C. Gossard and W. Wiegmann, Appl. Phys.
Lett., 33, 665 (1978).
6. R. Dingle, A.C. Gossard, and W. Wiegmann, Phys. Rev. Lett., 34,
1327 (1975).
7. J.L. Vos sen and W.Kern, eds. Thin Film P r o c e s s e s , (New York:
Academic Press, 1978).
8. H.C. Theuerer.and J .J . Hauser, J. Appl. Phys. 35_> 554 (1964).
9. C.F. Powell, J.H. 0x1ey and J.M. B1ocher, Jr., eds,, Vapor D e p o s i t i o n ,
(New York: Wiley, 1966).
10. M.B. Panish, Proc. IEEE 64, 1512 (1976).
11. N. Holonyak, Jr., R.M. Kolbos, E.A. Rezek, R. Chin, R.D. Dupuis,
and P.O. Dapkus, J. Appl. Phys. 49, 5392 (1970).
12. T.F. Deutsch, D.J. Ehrlich, and R.M. Osgood, Jr., Appl. Phys. Lett.,
35, 175 (1979).
13. W.A. Thompson, Bull, Am. Phys. Soc., 22, 255 (1977).
14. A.V. Crewe, M. Isaacson, and D. Johnson, Rev. Sci. Inst, 4£,
411 (1971).
15. S.D. Ferris, H.J. Leai^y, and J.M. Poate, eds., Laser-Solid I n t e r
actions and Laser P r o c e s s i n g , P r o c . A l P . ,~No. 50' O 9 7 9 ) 7 .
16. S.T. Picraux, E.P. EerNisse, and F.L. Volk, eds., A p p l i c a t i o ns
of Ion Beams to M e t a l s , (New York: Plenum Press, ‘1974) p.' 63.
17. D.I. Potter, P.R. Okamoto, H. Wiedersich, J.R. Wallace, and
A.W. McCormick, Acta Met. 27, 1175-1185 (1979);
J. Nuc. Matls. 83 (1979): entire volume, particularly pp, 2-23,
98-108, 208-213.
REFERENCES:
- 87 -
References (Chap 5)
( c o n t ' d ) .
10. R. H. Hammond, J. Vac. S c i . & Tech., TJ5, 382 (1978);
D.F. Moore, R.b. Zubeck, J.M. Rowell and M.R. Beasley, Phys. Rev.
20, (in press) 1979.
19. G.R, Johnson and D.H. Douglass, J. Low Temp. Phys. 14_, 575 (1974).
20. K. Lee and J.C. Suits, Conference Proceedings, A.i.P. 10th C o n f e r
ence on Magnetism (1972).
21. L.R. Testardi, J.H. Wernick, W.A. Royer, D.D. Bacon, and A.R. Storm,
J. A p p l . Phys. 45, 446 (1974).
22. L.R. Testardi, W.A. Royers, D.D. Bacon, A.R. Storm, and J . H . Wernick,
Met. Trans. 4, 2195 (1973).
23. J.R. Gavaler, Appl. Phys. Lett. 23!» 480 (1973).
24. A . H . Dayem, T.H. Geballe, R.B. Zubeck, A.F. Hal 1 a k , and G.W. Hull,Jr.,
J. Phys. Chem. S o l ids,39, 529 (1978).
25. M.W. Geis, D.C. Flanders, and H.I. Smith, Appl. Phys. Lett. 35,
. 7 1 , 1439 (1978).
26. EiL. Wolf, Rep. Prog. Phys. 41_, 1439 (1978).
27. P ,J . Thaler, J.B. Ketterson, and J.E. Hilliard, Phys. Rev. Lett.41, 336 (1978).
28. S. Ruggiero, T. Barbee, and M.R. Beasley, Bull. Am. Phys. Soc. 24,357 (1979).
29. B. Abeles, in Applied Solid State S c i e n c e , ed., Raymond Wolfe,
(New Y o r k : Academic P r e s s , 1976), v o l . 6.
30. V. Jipson and C.F. Quate, Appl. Phys. Lett. 32, 789 (1978).
31. Chapters by B.O. Seraphin and by A.J. Sievers in Solar Energy
Conversjon, ed., B.O. Seraphin, Topics in Applied Physics,
v o l . T T . T N e w York: Springer, 1979).
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CHAPTER 6
S O L I D S T A T E I O N I C S
INTRODUCTION
Solid-state ionics is the science related to how the ionic properties,
and in particular ionic motion, affect the overall properties of a solid.
Little is really known about how ions diffuse in solids ~ why some solids
show ionic conductivities orders o f magnitudes higher than others* w h a t is
the effective size of the ion as it moves through the lattice potential,
what can be predicted about diffusion in a solid even of known structure*
and how can one molecularly engineer a solid to give the desired level of
ionic mobility, be it high or zero? Ionic mobility is criticfil in a
large number of technologically important areas, including corrosion of
metals, hydrogen embrittlement, semiconductor construction, solar cell
junctions, hydrogen storage materials, oxidation catalysts, battery
electrodes- and electrolytes, electrochromic devices, sensors for
automobile pollution control, etc. Even slight improvements in our
knowledge in this one area could have major societal impact.
OPPORTUNITIES, BASIC RESEARCH
In this section the opportunities (and needs) for basic research
•will be briefly described, followed by an in-depth discussion of work
in one area, intercalation compounds, to exemplify where opportunities
might be found generally and the approach taken in this one case. This
approach, particularly as related to lithium incorporation in solids, is
- 89 -
directly applicable to tho understanding of reactions o c curring between
hydrogen and various solids. At the present time the scientists in these
two areas are not communicating, which is to the neglect o f the high
potential synergism.
In order to generate new materials exhibiting the desired ionic
motion (properties) there is a need for a broadly a p p l i c a b l e theory o f
ion transport at-the atomic level. Such a theory should be able to
explain which lattice defects are important for diffusion within a
solid of its c onstituent ions and also o f the probability o f incorporating
foreign ions/molecules (guest species) into the host lattice. Data are
now beginning to be collected on diffusion in solids in the bulk state,
on surfaces and at grain boundaries, so that the time is ripe for a
coordinated theory or model of.ion transport to be generated. Closely
related to such a model would be an understanding of the driving force
behind intercalation-type reactions: — what are the t h e r m o d y n a m i c s ,
v/hat is the bonding? For the latter, a few specific ques t i o n s come to
mind: what is the ionicity o f hydrogen in solids? It is clearly H +
in H vW0^, but only partially ionized in PdH. What is the ionicity inA
LaNigHg? And even more interesting, how does the e l e c t r o n i c nature
change as the ion diffuses? Related questions include: what happens
as the electronic properties of the host change (metals vs. insulator;
and what role is the bonding in molecular intercalates and how does that
affect diffusion; and what role does electron transfer p l a y in d e t e r
mining the electronic and optical properties of the c o m p o u n d formed
(i.e., how much chemical manipulation of these properties can one hope
to achieve)?
Sufficient compounds have now been studied that it should be
possible to elucidate the role of the crystal structure in determining
the magnitude o f the ionic mobility. Particularly, what is the relation
between the ionic size of the mobile species, the diffusion path size and
the ionic mobility, and what role does the dimensionality of the lattice
(1~, 2-, or 3-D) play? In addition, does electron delocalization in the
host lattice favor diffusion, i.e., reduce the potential wells, o v e r that
in an electronic insulator? An understanding here would allow in the
ideal case the synthesis of materials with pre-determined properties, and
in any event, give guidance to the synthetic chemist or the device
engineer. Diffusion is important in many preparative reactions besides
the obvious intercalation reactions (e.g., battery reactions); examples
include the formation of oxides and sulfides by reaction o f their metals
with oxygen or sulfur (also important in corrosion), where in many cases
the metal must diffuse through the product layer to react — in these
cases product morphology is controlled by the diffusion process,
e.g., Ti$2 , V$4 pipes"1); H 93-6A s F 6 1s als0 aPParGntl>' f ormed2 by
tho transportation of mercury along the mercury-atom chains in the
crystals; and the doping of many semiconductors is performed by diffusing
the foreign ion Into the host lattice. Diffusion resulting in the
formation of an intercalation compound may possibly be the precursor to
a whole new set of composites at the molecular level; an example might
involve the incorporation of a monomer into a structure followed by a
polymerization step. Growth of novel zeolites is also being accomplished
through construction of the inorganic lattice around long-chain molecules;
these molecules are subsequently removed leaving channels of predetermined
3 )size and with minimum defects."'
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In the next few pages some aspects of Intercalation compounds will
be discussed to exemplify the opportunities available for materials of
this type. Intercalation compounds may be defined as those compounds
formed reversibly between guest and host species in which the host
structure is essentially maintained; they may be 1~, 2 - , or 3-dimensional
Typical examples of these are shown in the table below:
Hosts Gu e s t s
Sulfides: e.g., T i S 2> V(i„x )Fe xS 2 >
Ni P S 3 , M o S 3
Metals, organic electron donors
Oxides: e.g., W 0 3 , MoO^, FeOCl
,, n Metals, 'oxygens', molecules
6 13
Metals: e.g., LaNi^ Hydrogen
Clays Organics (-)-ion exchange)
Graphite/Boron nitride Metals, halogens, Lewis Acids
1. Intercalation compounds
The intercalation compounds of graphites and of the transition
metal dichalcogenides — both alkali metal and m o lecular compounds ~
were first synthesized in Europe, as is the case with so many o f the
novel materials now the subject of research in the United States. T h e
4)alkali metal intercalates o f graphite have been known since 1926, '
5)whereas those of the dichalcogenides were not reported until 1959. '
Both classes of compounds were rapidly enlarged by U.S. efforts driven
by an interest in novel superconducting compounds and phase transitions
in two-dimensional materials. The initial hopes of finding higher
temperature superconductors or evidence for non-phonon mediated
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superconductivity were not realized. However, research on the simple
binary chalcogenides and their alkali metal intercalation compounds has
been extensive because of the discovery of charge density waves in the
unintercalated compounds and the discovery of highly energetic electro
chemical intercalation of lithium in TiS and its resulting potential
for high energy density* reversible, ambient temperature batteries.
The existence of independently variable, interpenetrating,
interacting sublattices offers not only clear technological oppor
tunities to discover new physical phenomena or to study well-known
phenomena in controllable systems. An example of this is found in the
compou ids Li^TiS (0 < x < 1) where the Li array acts as a two-dimen
sional lattice gas with a tendency to order at certain rational values
of X.6) An understanding of this phenomenon on a fundamental level
appears possible and is now the subject of research at a number of
universities. Such an understanding will be important beyond the
Li TiSp compounds and should be useful in identifying some of the keyr\ C,
features which control the freedom of ions in lonically conducting solids.
Perhaps the most significant implication of the Li/TiSg cell
discovery is that reversible, rapid, highly energetic redox chemistry is
possible at ambient temperature. Henceforth, a good deal of research
could profitably be directed toward the search for other examples of a
similar nature. This would be a search for compounds which undergo a
similar reaction} that is , for compounds containing transition metals in
a reasonably high oxidation state within a structure which can accommodate
alkali ions (Li, Na) without the necessity of breaking covalent bonds or
radically altering the host structure. Some progress has been made in
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in this area but no candidates for battery cathodes have been identified
which are significantly better than T i . A need exists for a clearly
enunciated theory or set of principles to guide this research. Recent
7)work ' suggests that a profitable turn in the search for high energy
systems might be found in amorphous transition metal sulfides like HoS .
Alternatively, graphite intercalates such as CgAsFg, CgAsFg, etc.
might be combined with suitable electrolytes to give a battery approxi
mating an alkali/halogen system. Although many such graphite intercalation
compounds are now known, only in a few instances are the guest species
well defined. Typically, there is confusion concerning the extent of
charge transfer between host and guest, lack of definition of the guest
species (e.g., whether it is AsF or l/3AsF3 + 2/3 AsFg), lack of
knowledge of the placement of the guest species with respect to the host
lattice and ignorance of the impact of the guest on the host structure.
An added attractive feature of many of these intercalates is that there
is very l i t t le change in crystalline structure during reaction, thus
minimizing swel1ing/contracting and disintegration of the electrode.
Since the early and mid-twenties, research on molecular inter
calation complexes has been conducted at a low level in a few places.
This is in part because progress in this area requires the more traditional
chemical skil ls sorely lacking in the materials science community in the
United States and, more recently, because the hoped-for effects on the
superconductivity of the layered host were not observed. Most recently,
work in Germany has made significant steps in elucidating the nature of
the bonding in certain of these systems, relating it to that of the simpler
ionic compounds. The technological opportunities for molecular
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intercalation complexes are not clear at present. They do represent
scientif ical ly interesting systems however, and the abil ity to alter
the electronic and other solid-state properties of these materials in
subtle and not so subtle ways by the applications of the traditional
notions of molecular manipulation argues well for an interesting future
for these compounds in the hands of imaginative chemists and physicians.
A particularly interesting subset of these materials is that
which results from the insertion of the metallocenes dicyclopentadienyl
cobalt and dicyclopentadienyl chromium between the layers of the dichal-— •{* -f-
cogenides. The resulting compounds, Ta$2 (Cp2Co) , and Ta$2(Cp2Cr) ,
are superconductors even though the chromium ions carry a local magnetic
moment. Such complexes may have interesting catalytic implications.
Another subclass nf molecular compounds of interest is those
in which a long chain amine is inserted between the layers. These yield
compounds containing a molecular bilayer specifically similar to those
in cell walls. These layers are undoubtedly liquid-like and undergo
"crystal!ization" as the temperature is decreased, Thus, sandwiched
between metallic layers 6 A thick, one has a solvent where solution
chemistry and diffusion should be of some interest. The liquid-like
character of this region should be enhanced by the introduction of
disorder via admixture of chains with double bonds and side groups.
All in a l l , such compounds appear to offer an interesting, unusual, and
Variable electrochemical environment which could prove fascinating to
explore, but as yet no known technological application.
Part of the diff iculty associated with the study of intercalation
is the failure of researchers to prepare crystals suitable for a single
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crystal structure work on any intercalation compound, be it of the layered
dichalcogenides, graphite, or other host. This has been of lesser concern
for the structurally simpler metallic intercalation compounds than it has
been for the molecular compounds where it tends to preclude progress in
elucidating the nature of the guest host bond. The greater application
of electron diffraction and EXAFS should help solve these problems.
BASIC RESEARCH NEEDS
As discussed earlier, one of the success stories in solid-state
science encompasses the class of materials called Intercalation Compounds.
Progress in this area came about through a basic science drive rather than
from a technology need. Such intercalation compounds, particularly of the
layered disulfides, were f irst reported in the German literature in 1959.
They were then studied to see how intercalation e.ffected their physical
9)properties, in particular superconducting. ' Out of a study to determine
the exact chemical composition of the highest superconductor, a potassium
compound, the high free energy of formation of these intercalates was
discovered which led to the battery application.^ In this battery
program, a synthesis effort was originated to prepare TiS economically.TO)
One technique found 1 here gave sulfides of high surface area which in
turn allowed a study of the catalytic properties of all the transition
metal sulfides. These catalytic studies, which involved predominantly
amorphous compounds, then led to the finding that amorphous materials were
active as battery cathodes.^ It is such an interdisciplinary approach to
a broad class of compounds that is roost likely to uncover new materials of
unusual properties that may in the future have important energy implications.
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What is needed are a few more broad synthesis/characterization studies
of this type.
More specific needs in this area include an expansion of synthesis
capabilities; for example, greater emphasis might be given to low-temper-
Lure reaction routes or to the application of electrochemical tools. To
meet this need, the academic community should place a greater emphasis on
synthesis. This will also increase the general synthesis expertise and
the resources available to the next generation. At the same time,
students get a training in the basics of structural characterization; it
is surprising how many times a piece of work is reported where the
simplest x-ray evidence for the phases discussed has not been obtained.
(The advanced structural tools are now becoming more broadly available,
and are not felt to be an impediment to progress in the solid-state
sciences.) This need for chemists trained in the synthesis and elementary
characterization of solid compounds is critical to all studies of new
materials. The major synthetic efforts of this sort are in Europe,
particularly Germany, the United Kingdom and France. More support for
this kind of work in the United States would facilitate a more effective
national effort. In the past, immigration policies permitted the United
States community to advance by absorbing Europeans into the research
effort here with l i t t le difficulty. Such an approach is not possible
unde.'' current immigration practice. If these research efforts are to
continue, the U.S. would do well to foster the growth of such activities
in its own university system more than it has.
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Ti.CHNOLOGY NtLDS
Ionic or molecular /notion in the solid s' .te is critical in a
wide range of technologies, This impact is not only found in the
materials during actual use, but also in the formation and degradation
of key components. The technology needs are described here in several
rather broad categories and are not all inclusive but tend to cover
what were felt to be the presently conceived needs.
BATTERIES
In a society relying progressively more on electrical energy, there
is a need for storing this ennrgy, particularly in a mobile form. Batter
ies ire one way of doing this. A battery consists of three active
components: the anode (reducing electrode), the electrolyte (the medium
through which the charged ions, but no electrons, are transferred from
one Mectrode to the other), and the cathode (oxidizing electrode). In
addition, there are various inert components, for example current
collectors, separators and containment materials. Other related electro
chemical devices, such as electrochromic displays (HxW0 ) and oxygen
sensors (ZrOp), have similar components/properties and will not be
considered. In batteries of interest in the energy f ie ld, solid materials
play differing roles depending on the configuration of the particular
system as high discharge rates preclude solid/solid interfaces so that
these cells tend to have either both electrodes solids with a liquid
electrolyte, e.g. LiA£/molten salt/FeS, Li/organic/TiS2 or both
electrodes liquids with a solid electrolyte, e.g. Na/& alumina/S.
The various components will now be considered.
The need here is in the understanding and perfection of the
electrodeposition of metals, which is crit ical ly important in re
charge of cells using lithium and zinc anodes. In these cases, dendritic,
or even non-metal 1ic, deposition is l i fe determining. Electrodeposition
is an art not a science at the present time, and a fundamental under
standing of the crit ical parameters would be important not only in energy
storage but also in the energy intensive metal electrowinning industry
and in the electrosynthesis of new compounds (that might have impact in
the energy area as well as elsewhere). One way around this problem is
to use alloy nodes such as LiA& where dendrites cannot form under
proper voltage control; however, this approach incurs both weight and
voltage penalties but is worthy of further study particularly for high
lithium content alloys, e.g., those with silicon.
2. The Cathode
New materials can have a marked impact on the energy storage
capacity of batteries and particularly on the numbers of discharge/charge
cycles that can be obtained,. In the case of lithium cells , most work has
been performed on titanium disulfide’. 1 3 , and on transition metal oxides.
1 5)for example, VgO . There is s t i l l need for new inorganics that s t i l l
have higher energy densities without significant changes in the voltage
during discharge. Although several new compounds come close to TiS in
actual storage capacity, none show significant improvements from capacity/
cost/availability/electronic conductivity (over whole range of discharge)/re-
versibility/or rate of discharge point of view. A whole new class of
cathode materials that are just now being investigated are those that are
amorphous in structure, e„g, MoS * VgS5, MoS , 7> V i s * ’ 3’
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1 „ The Anode
It should, however, be emphasized that the cathode is no longer
the rate-control 1 ing step in the development of advanced lithium
batteries, and the fundamental understanding of their mode of operation
is in pretty good shape. It is therefore recommended that most support
be aimed at the understanding of the other parts of the system, e.g.
anode materials (alloys/plating, etc.), and on the properties of electro
lytic solutions which are the limiting steps (not materials science per sc),
3• The Solid Electrolyte
Much excellent work has been reported on the 3 aluminas since
their ion.conducting properties were reported in 1967 at the Ford Motor
Company, Subsequently, the Lincoln Laboratory Group^ have
discovered through a basic understanding of the structural needs of ionic
mobility a number of comparable materials which are isotropic in their
properties. Such isotropic materials help both the ionic conductivity
and the mechanical properties. Ideally, higher conductivities are needed
at lower temperatures to reduce corrosion problems associated with the
present high temperature sodium/sulfur cells. Preferably, one would
want to operate cells at temperatures sufficiently low, 200°C, where
plastic components can be used, However, i f such electrolytes are
discovered, then an alternate cathode will be needed because of the
■solidification temperature and high viscosity of sulfide melts which
restricts the use of the present sodium/sulfur cells to a lower temper-
ature limit of about 300°C.
Various fluorite-type materials containing non-bonding valence
pair species such as Bi exhibit high anionic conductivity and could
conceivably lead to electrochemical devices involving oxide or halide
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- 100 -
containing cathodes or batteries of the type alkali/alkali+ and fluoride"/
fluorine material. A fundamental understanding of the diffusion process
in such materials is highly desirable, and might be accomplished by high
resolution electron microscopy combined with synthetic chemistry.
4. Separators
There is a need in many systems (batteries, electrolysis cells,
fuel cells, etc.) using liquid electrolytes for separators that are
stable at the same time to both highly reducing and oxidizing environments.
In many cases these are preferably ion-selective; an example is Nafion
(DuPont product), but this is too expensive for many large applications.
Another example of a system where development progress is being held up
for the lack of a suitable material is the high-temperature lithium
molten-salt battery (Argonne National Laboratory); in this case a
separator is required to replace the expensive boron nitride now in use.
CATALYSTS
Catalysis is at the heart of petroleum refining and will become more
critical as the feeds become heavier, as such crudes whether from tra
ditional sources or from shale, tar sands, coal, etc., will require much
greater conversion. Particularly important is the ability to both break
down large carbonaceous molecules and to remove undesirable species such
as sulfur, nitrogen, oxygen and various metals, 1n particular vanadium
and nickel. It is quite likely that petroleum crudes will become the
nation's largest single source of vanadium.
The diffusion of ions and molecules is important in several areas
of catalysis. These include: the sintering of metallic particles, of
compound catalysts and of the supports themselves; the migration of
oxygen ions in oxidation catalysts, which is critical in their operation;
and the diffusion, or non-diffusion as the desire rnay be, of molecules
into shape-selective catalysts prior to reaction. In this last case
there is a need for a strong long-term activity in these shape-selective
catalysts in terms of synthesis, structure and characterization. Such
catalysts are one hope in significantly improving the selectivity of
reactions, thus both reducing feed wastage and potentially environmentally
unacceptable byproducts. These catalysts may be of application both in
the petroleum and chemicals area, e.g., in the conversion of CO +
obtained from coal gasification to petroleum, and the conversion of
xylenes to the para isomer. There is very l i t t le synthetic materials
work (outside Mobil/Union Carbide) to create compounds with specific
structures (molecular engineering), be they zeolitic in nature, clay
based, or use transition metal layer or tunnel structures (including
clathration reactions),,
In the case of oxide oxidation catalysts, evidence is mounting
that ionic mobility coupled with a variable oxygen composition is critical
to their operation. The oxygen used to oxidize the organic comes from
the oxide lattice not from the gaseous oxygen; the latter merely
re-oxidizes the oxide. Apart from the early work of Winter, who showed
that the oxide oxygen could be readily exchanged throughout the whole
lattice with gaseous trace oxygen, there is no direct measurement of
the diffusion of oxide ions ''n these lattices under catalytic conditions.
One of the predominant ways that catalysts sinter and thereby
lose activity is by the surface diffusion of atomic/ionic species. Here
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- 102 -
again there is very l i t t le actual diffusion data to guide the engineer.
Various ways have been found to minimize diffusion, e<>g0 by increasing
the interaction between catalysts and support,^ or to undo its effects,
e.g. by redispersing the catalyst.
METALLURGY
Diffusion of ions/atoms are important in several areas of techno
logical importance to metals. These include corrosion (diffusion of
oxygen or metal ions), hydrogen embrittlement (hydrogen diffusion), and
hydrogen storage materials (again, hydrogen diffusion). In the f irst ,
means for minimizing the diffusion of the ions in protective films are
needed. In the other cases the requirements are opposites to one another.
Thus, to minimize hydrogen embrittlement both the solubility and diffusion
coefficient need to be minimized, whereas for storage these need maxi
mizing, What is learned in studies in the one will surely help in
advancing the other. The specific needs for hydrogen storage media,
be they metallic or inorganic in nature, are high and reversible incorpor
ation of hydrogen coupled with light weight and low cost, the temperature
and pressure at which this should occur depends on the application in
mind. Some intercalated graphites show metallic conductivity and are
highly resistant to oxidizing agents, and so may displace metals as
electrodes in electrochemical synthesis, e.g. CgAsFg in fluorine
reactions.
A better understanding of the diffusion of atomic species should
assist in enhancing the utilization of diffusion techniques for producing
and doping semiconductors; these could be crystalline, amorphous, bulk
or thin film in nature.' In addition, critical to the operation of so
many semiconductor devices, e.g. solar photovoltaic ce l ls , is the
minimization of diffusion of species across interfaces which can degrade
the device. One material where diffusion data would be very useful is
amorphous si licon, S1 Hx , where very l i t t le is known about the mobility
of the hydrogen.
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SEMICONDUCTORS
- 104 -
REFERENCES:
1. M.S. Whittingham, J. Solid State Chem., 29, 301 (1979).
2. A.J. Heeger, A.G. MacDiarmid in Molecular Metals, ed. W.E, Hatfield,(New York: Plenum Press, 1979) p. 424;
B.D. Cutforth, C.G. Davies, T.A.W, Dean, R.J. Gillespie, P.R. Irelandand P.K. Ummat, Inorg. Chem. 6 , 1343 (1973).
3. R.J. Arganer and G.R. Landott, U.S. Patent 3,702,886.
4. K. Fredenhagen and G. Cadenbach, Z. Anorg. Allgem. Chem. 158,249 (1926).
5. W. Rudorff and H.H. Sick, Angew. Chem., 7J_, 127 (1959).
6 . A.H. Thompson, Phys. Rev. Lett., 40, 1511 (1978).
7. A.J. Jacobson, R.R. Chianelli, and M.S. Whittingham, U.S. Patents 4,144,384 and 4,166,160;
____ , J. Electrochem, Soc. and Mat. Res. Bull., in press.
8 . R. Schollhorn and H.D. Zogefka, Angew. Chem. Int. Ed., 16,199 (1977).
9. F.R, Gamble, et. a l . , Science, V74, 493 (1971).
10. M.S. Whittingham, Mat. Res. Bull., 9, 1681 (1974).
11. M.S. Whittingham, Science, 192., 11 26 (1976) and
U.S. Pat. 4,009,052.
12. R.R. Chianelli and M.B. Dines, Inorg. Chem., 2.7, 2758 (1978).
13. M.S. Whittingham, Progress in Solid State Chem., ]_2, 41 (1978).
14., G.C. Farrington and J.L. Briant, Science, 204, 1371 (1979).
15. D.W. Murphy, P.S. Christian, F.J. DiSalvo, and J.N. Carides,J. Electrochem. Soc., 1_26, 497 (1979).
16. Work at Bell Labs., reported by J.H. Wernick.
17. N. Weber and J.T. Kuminer, Proc. Ann. Power Sources Conf., y , 37(1967).
18. H.Y-P Hong, Solid State Chemistry of Energy Conversion and Storage,eds., J .B. GbodenbugTh and M.S7~Wfilttinghani, Amer. Chem. Soc.Adv. Chem., .162, 179 (1977).
19. S.J. Tauster, S.C. Fung, and R.L. Garten, J. Am. Chem. Soc., 100,170 (1978).
- 105 -
CHAPTER 7
CATALYSIS
Heterogeneous catalysts currently play a major role with energy
in many different ways, and it can be confidently predicted that this
role will become increasingly more important. Improvements in catalysts
which might appear minor have already had dramatic impacts on the energy
situation. For example, the replacement of part of the amorphous alumina
si l ica cracking catalyst with a crystalline alumina silicate (a zeolite)
saves this country more than two-billion dollars per year by converting
a larger fraction of crude petroleum into gasoline.
The number -of different chemical reactions which require
catalysts is too large to enumerate here. There are hundreds of such
reactions, and all can be related to energy in various ways. Furthermore,
catalysts can serve critical roles in terms of solving pollution problems
and conserving raw materials. In coal conversion,catalysts are important
for liquefaction, gasification, and the CO/H reaction. Photocatalysts
may play an essential role in utilization of solar energy, rioctro-
catalysts are important both for producing electricity in fuel cells and
for efficiently using electricity in electrolysis cells of various kinds.
There are many ways in which a catalyst advance could be success
ful from a technological point of view. Generally the more important
considerations are higher activity, better selectivity to a desired product,
and longer l i fe . Potentially, the most dramatic advance would be the
discovery of a catalyst which would catalyze a reaction which had never
before been effectively catalyzed, e .g . , the oxidation of methane to
methanol.
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Heterogeneous catalysts have become very advanced materials by
a largely empirical approach. A big problem in suggesting future di
rections for improved or new catalysts is that we do not know where we
are now. That is, we know very l i t t le about some of the most successful
and important catulysts that are presently in use. Therefore an area
which should receive special emphasis is the application of state-of-the-
art materials characterization techniques to important catalysts. A
critical characterization tool would be electron microscopy with microprobe
and microdiffraction. Examples of other useful techniques are EXAFS,
XPS, UPS, AUGER, NMR, ESR, IR and Raman.
Catalysts frequently have several different stages of l i fe .
The phases present in a material may change dramatically when it is used
as a catalyst. For enmple, cobalt molybdate is converted to a mixture
of cobalt and molybdenum sulfides when used as a hydrodesulfurization
catalyst. All catalysts eventually degrade with use, and sometimes they
can be regenerated. Thus, the characterization of catalysts should
include the fresh, activated, deactivated and regenerated stages. Once
current catalysts become better characterized, ideas will naturally
emerge for new and improved catalysts.
The importance to surface studies of catalyst characterization
cannot be overemphasized. Surface studies may not be relevant to catal
ysis unless they are performed on the phases and the crystallographic
faces actually present in real catalysis. Frequently, such information
is lacking even for very important catalysts.
There is good reason to believe that the highly empirical
approach to new catalysts is no longer successful enough to be encouraged.
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Further advances are most likely to occur If a systematic approach is
stressed. Such studies would combine and correlate careful synthesis,
characterization, and reactor studies. For the synthesis part the
composition, structure, and synthesis technique would be systematically
varied. Materials prepared would then be characterized by techniques
such as x-ray diffraction, adsorption measurements and electron microscopy.
Reactor studies should be carried out over a wide variety of conditions,
and an attempt should be made to derive kinetic expressions. There are
many possibilities for both materials and reactions. An example would
be the study of Chevrel phases (AxMOgSg) where A can be Pb, Sn, rare
earths, etc. for hydrodesulfurization.
Selective oxidation catalysts are continually being reduced at
one type of surface site and being reoxidized at a different site. There
must be an effective method of passing electrons from the site of catalyst
reduction to the site of its reoxidation. Valence degeneracy is involved,
and detailed knowledge of the electronic structure of the catalyst may
be very important in certain types of catalysis. This should be further
studied.
Bulk, as well as surface diffusion, is of great importance in
catalysis. It is believed that high mobility of oxygen anions is a
characteristic of some of the best catalysts for selective oxidation.
However, there are no reliable data on oxygen mobility in such materials
as molybdenum trioxide and bismuth molybdates. The selectivity obtained
with zeolite catalysts is generally assumed to be related to diffusion
of molecules in the zeolite pores. More and better diffusion data for
molecules in zeolites would improve our understanding and point the way
to better catalysts and processes. Many catalysts deactivate due to
sintering. A study of diffusion in such catalysts could suggest methods
of improving catalyst l i fe .
Many important catalysts consist of several phases. The
catalytic properties of this mixture are clearly superior to those of
any of the individual phases. One example is the cobalt sulfide/molyb
denum sulfide mixture used as a hydrodesulfurization catalyst. The
nature of the synergistic mechanism is not known in such catalysts, and
there should be increased effort in this area, particularly to look for
replacements for cobalt.
Many oxides have acid sites which are crit ical to their cat
alytic properties. There have been many attempts to characterize such
sites by chemical adsorption with molecules such as ammonia and by
spectroscopic techniques such as IR. However, none of these techniques
have been completely satisfactory, and there is a need for the develop
ment of a better method to characterize surface acidity.
Many of the conclusions reached 1n the Panel Report on High
Temperature Ceramics (Dept, of Energy, January, 1979) are highly per
tinent to catalysis. For example, a rational synthesis of a catalyst
generally depends on an understanding of the physics and chemistry of
particulates; also, defects in materials can strongly influence
catalytic properties and lifetime of catalysts.
Recent advances in catalysis have come about primarily from
research activities in industrial laboratories. Frequently, information
is not made available on the actual composition or the optimum synthesis
method for advanced catalysts. This presents a problem for those outside
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- 109 -
a particular company who desire to carry out research on a relevant
catalyst . However, this problem cannot be allowed to inhibit progress
in this critical field. Industrial laboratories will generally welcome
more research on catalytic materials by scientists in academic and
government laboratories, and they would certainly be willing to identify
many of the important materials relevant to catalysis. Examples of
simple, but very important, catalysts are:
• Silver metal for oxidation of methanol to formaldehyde and
for the oxidation of ethylene to ethylene oxide
• Co/Mo/S for hydrodesulfurization reactions
• BigMOgO-j for the oxidation of propylene to acrolein, for the
ammoxidation of propylene to acrylonitrile, and for the
oxidation of 1-butene to butadiene.
no -
CHAPTER 3
SEMICONDUCTORS
INTRODUCTION
Semiconductors are the key materials of the second industrial revo
lution. They are able to act as switches, and as detectors and emitters of
light at high functional densities. Thus semiconductors afford inexpensive
intelligent machines that have begun to pervade nearly all aspects of
industrial economies.
The importance of a s«rmH research base in semiconductors was recog
nized early. Vigorous and continuous research, in fact, has contributed to
making the United States the world leader in electronic information processing
and in communication. Improved information technology contributes to energy
conservation; thus can be considered energy technology in a broad sense.
Studies of future research needs in these areas include pertinent materials
aspectsJ ^
This chapter covers aspects of bulk semiconductors for all applications,
all materials and some device aspects for solar cells, and some aspects of2)
thermoelectric converters. '
STATUS OF SEMICONDUCTOR TECHNOLOGY, WITH EMPHASIS ON SOLAR CELLS
Because of the existing and at present more economic alternatives
for producing energy, the potential of semiconductors in terrestrial photo
voltaic conversion did not receive adequate attention until the tnid-1970's.
There are now substantial applied research and engineering programs
under wa in the United States,^ and to a lesser degree in other countries.
m -
These programs are justified through economic analyses that indicate a
realistic chance for photovoltaics to replace/substitute conventional
electric generating capacity,^
Three types of technology are pursued currently to obtain photo
voltaic converters for economic production of electricity: concentrators>
flat plate silicon, and thin film arrays.
In concentrators, the semiconductor cell area may be ten to ten-thou
sand times smaller than the system aperture. Cells can be comparably
expensive {approximately $1000/n ) but must be highly efficient (>30%)^.
The best research results to date are 20.5% in silicon ce l ls ,^ 25% in
cells based on gallium arsenide,^ 28% in a converter that employed
spectral splitting for rrore efficient conversion by a pair of silicon and
gal 1ium-arsenide based cells ,^ and 26% for a silicon thermophotovoltaic
cell designed to convert radiation from a black body at 2000°C (ultimately
to be heated by sunlight).^ Device theory, materials, and cell fabri
cation are closely related to those employed in conventional semiconductor
technology.
Flat plate silicon arrays represent the only current commercial tech
nology, drawing on twenty years of development for small-scale applications
primarily in spacecraft. Conventional single crystal silicon is used as
the starting material but alternatives, such as coarsely polycrystalline
(multigrain) cast materials, are in sight. One principal barrier to the
rapid introduction of cast and other non-single-crystal silicon is the
lack of understanding of growth, electronic properties, and processing of
polycrystalline semiconductors and junction devices. For silicon cells ,
a specific fabrication technology, distinct from those employed in microcircuit
production, has been developed over the years. Due to the small size of
the market, processing steps have evolved incrementally. The challenge now
is for rapid introduction of efficient and highly automated production of
pure silicon, its crystal 1 ization, device processing, and module fabrication.
Cost (less than 70rf per peak watt generating capacity), performance
(10% efficiency) and operating l i fe (20 years) of thin film modules have
been well defined, but no such cell has been demonstrated in the laboratory.
This lack of an obvious candidate which combines minimal materials consump
tion with acceptable performance in a device and module structure that is
highly amenable to automation explains the very broad "screening" programs
carried out at present. These programs are severely hampered by the absence
of elementary information about optical (e.g., energy gap) and electronic
(e.g. conductivity type) properties of the majority of materials of
interest. Moreover, the constitution of these materials ranges from amor
phous to polycrystalline, but adequate information on device theory,
processing characteristics, or aging behavior does not exist.
Thin film modules more expensive than 70jd/Wp or less efficient than
10% can address limited but s t i l l substantial markets. Commercial
production is imminent for cuprous sulfide/cadmium sulfide modules for
solar applications. The production of small cells based on amorphous silicon
hydrogen alloys for incorporation in watches has been announced for early
1 980.
Emphasis on inexpensive thin-film devices likewise will provide the
motivation for the screening of candidates for solar thermoelectric
converters. Here new materials such as the amorphous silicon-hydrogen
alloys or polyacetylene will be characterized for the three properties that
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contribute to the figure of merit: the Seebeck coefficient, the electrical
and the thermal conductivity.
OPPORTUNITIES FOR BASIC RESEARCH
1, Preparation and understanding of perfect crystals
The group IV semi conductors silicon and germanium have been pre
pared as more perfect crystals and with higher purity than any other
material. This quest for materials with ideal properties that can be com
pared with theoretical predictions has generated advanced purification,
crystal!ization and measurement techniques. These techniques, as well as
the better theoretical unaerstanding of crystalline solids, has been
valuable in controlling the properties of a wide range of materials of use
in the energy field. Moderately funded but long-range work in the growth
of silicon or germanium crystals with zero dislocation density and with
minimum content of low Z impurities (carbon, nitrogen, oxygen) is recommended.
2. Interaction between defects
Numerous properties of solids (plastic behavior, electronic
conductivity in semiconductors, impurity diffusion) can be dominated by
defects rather than by the matrix materials Since more methods for analysis
can be brought to bear on semiconductors than on most other classes of
materials, defect studies can attain a scope and a degree of complementarity
dif f icu lt to reach elsewhere. Efforts should be undertaken to promote
comprehensive understanding of interactions between point defects, line
defects, and surfaces/interfaces. These studies should cover structural»
electronic thermodynamic (equilibrium) and kinetic (transport) aspects.
Examples are the.theory and experiments on the energetics of point defect
interaction, or the correlation of electronic with metallurgical (e.g.,
chemical activity) properties at high concentrations of impurities.
• Relation of bulk to surface properties.
Much of current surface research focusses on aspects of surfaces
that differ from the bulk. It would be desirable, however, to come up
with guidelines that permit prediction of surface properties from known
bulk data. Clearly, this must be a theoretical effort carried out with a
large base of reliable experimental data. Examples of questions are:
• How do surface layer space charges and the associated field
superpose on bulk properties?
• Are surface and bulk electron affinities identical?
• Are bulk thermodynamic data applicable to surface reactions?
4. Electronic structure and optical properties of polycrystalline
semiconductors.
With a few notable exceptions, fundamental research on polycrys
talline semiconductors for junction devices has been carried out only91during the past two or three years. ' Electronic equilibrium and transport
properties, the segregation of impurities and the formation of phases
in the grain boundary, grain boundary diffusion and passivation (the re
duction of grain boundary effects on carrier transport), and the effects of
intersecting grain boundaries arid p-n junctions require advances in basic
understanding. Very fruitful studies could be carried out in the following
areas:
• Explanation of the known differences between group IV, I I I - V ,
and I I - V I semiconductors, in majority carrier transport across
grain boundaries
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• Measurement of the electronic band Structure at grain boundaries
and comparison to the at least four existing models
• Using present high lateral and depth resolution surface analysis
methods to study the mechanism of formation and segregation of
impurity phases within grain boundaries, and
• Applying controlled preparation techniques in conjunction with
methods for structural study to identify the relative uptake
of strain by grain boundary and grain.
5. Nucleation and growth
Structures with preferred grain orientation, and coatings are
needed in a number of energy technologies to perform a variety of tasks.
Polycrystalline and thin film cells depend on controlled nucleation and
growth with specified grain dimensions. Progress in cell research depends
on practical solutions addressing particular combinations of materials.
Guidance from a better theoretical understanding of nucleation and growth
could shorten the recurring and wasteful but entirely empirical search for
proper conditions for grain growth. It has been shown recently that
nucleation and growth phenomena also affect the structure, and apparently
the electrical characteristics, of hydrogenated amorphous silicon.
In this context the question of directional solidification in a
low technology environment does merit attention. A number of techniques
for the preparation of polycrystalline bulk or sheet silicon would benefit
from quantitative guidelines.
Because of the wide applicability of a useful theory of nucleation
and growth this area is ranked among the opportunities for basic research.
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6 • Amorphous Semiconductors
The discovery that hydrogenated amorphous silicon can be doped
n- and p-type1( and the subsequent demonstration of a photovoltaic device
based on a-SiH ^ has stimulated substantial research activity. ThisA
material, a challenge to theorists and experimenters alike, is under in
vestigation in university, government and industrial laboratories because
it is regarded as a candidate for "breakthroughs" both in basic physics
and in device applications.
Research on amorphous semiconductors does not appear in need
of extra support. However, a brief listing of recognized problems is
presented to provide a better balance to this chapter on research opportun
ities. It should be mentioned that a-SiH is considered primarily as a
candidate for an inexpensive thin-film solar cell , but th?t applications
as a selective solar absorber or1 as a thermo-electric material also have
been discussed.
a-SiH can be mad by d.c. or a.c. glow discharge in silane,
reactive sputtering of silicon in the presence of hydrogen, chemical
vapor deposition from silane, and a number of less used techniques such
as electrodeposition, ion plating, ion implantation and evaporation. The
properties of the product film are affected by a large number of growth
parameters including the deposition geometry. The effect of the respective
technique, e.g., plasma deposition, on the film properties is not under
stood. In response to this situation, several laboratories are augmenting
their diagnostic capabilities.
The role of dopant impurities (boron, phosphorus) and of so-
called modifiers (fluorine) is not understood, neither is the creation of
new defects upon the introduction of the p-type dopant, boron. There is no
theoretical framework for understanding and interpreting charge transport.
Moreover measurement techniques for carrier lifetime and diffusion length
are d if f icu lt and controversial. The determination of the residual density
of states within the band gap also is d iff icu lt . There is no routine
technique for the precise determination of hydrogen content.
Needless to say, fabrication of junction devices with a-SiH
identifies additional opportunities for thin films oriented materials
research.
7. Interaction of energy beams with semiconductors
Vet another area which is receiving much current attention from
physicists, materials scientists, and electrical engineers is the processing
of semiconductors with laser or electron beams. Fo> instance, heavily
damaged ion-implanted semiconductor layers have been recrystal 1ized with
a single laser pulse. Ion-implanted solar cells have been rendered elec
tr ica lly active with one electron pulse covering a 3-inch diameter wafer.
The mechanisms of recrystallization, the distribution of impurities
in the temporarily liquid phase, and the nature of residual defects under
conditions of extremely rapid cooling should be studied. Basic under
standing of these issues is likely to lead to new techniques of treating
and processing surface layers.
NEEDS FOR BASIC RESEARCH
1. Exploratory preparation of new semiconductors
Particularly in the area of thin film photovoltaicss a continuous
search for new semiconductors would be welcome to broaden the choice of
available materials. This search need not be carried out in the context
of a separate program. Instead, support could be provided to all pertinent
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synthesis groups in the country for the measurement of the optical absorp
tion edge (between one and two electron volts), the conductivity type
(preferably both n and p) and the conductivity of a new compound.
Guidelines for exploring new semiconductors, or analogue com
pounds of known semiconductors could be of value. Such guidelines,
possibly derived from semi-empirical theory, should indicate the nature
(direct or indirect) and the magnitude of the energy gap.
2. Refractory materials
Since the performance of semiconductor devices is highly sus
ceptible to impurities nonreactive materials for containers, reactors,
and substrates are important. However for fabrication temperatures above
500°C, the number of available refractories is so limited as to be re
strictive. In essence, fused silica and graphite are used for crucibles
and shaping dies, fused silica for reactors, and alumina, mullite, carbon,
tungsten, and molybdenum as substrates. A program on new refractory
materials as well as on the mechanisms of wetting and reaction will be
useful not only to semiconductor and solar cell research but also to other
energy conversion technologies,
3• Classification of preparation techniques
A large number of preparation techniques has been used to produce
device-quality semiconductors. These include crystallization from the melt
or from liquid solutions, evaporation, chemical vapor deposition, sputter
ing and related techniques, 1on or neutral beams* electrodeposition,
spraying, screen printing, etc. Patterns have begun to emerge on the
■applicability of'these techniques. For instance, 11- VI semiconductors can
be screen printed while group IV or 111-V1s do not yield useful material.
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Sputtering has shown l i t t le promise with conventional semiconductors (as
opposed to a-SiH ), apparently on account of concomitant radiation damageA
and of diff icult control of grain growth. A survey of published literature
on this question could provide very useful guidelines to the experimenter
and could avoid unnecessary duplication.
4• Theories for mass and heat transfer during so l id i f i cation and
condensation
Two areas in semiconductor preparation require improved mass and
heat transfer theories: crystal 1ization of sheet si licon, and deposition
of films from vapors. A drawback, common to current silicon sheet growth
techniques, is the need for extensive instrumentation and highly trained
operators to prevent or rectify instabilities that develop easily at the
solidification front. Improved theoretical support in the design of film
shaping parts and of crucible, heater, and heat-shield geometries could
contribute substantially to cost reduction. In chemical vapor deposition,
the problems of non-uniform deposition over the length of a reactor, and
of efficient use of reactant gases need to be addressed, the former in the
interest of improved device yield, the latter as a means of materials
conservation.
5• Methods for simultaneous structural, electrical, or chemical
analysi s
The most promising techniques for characterizing defects in
semiconductors- appear to be those combining structural or chemical
analysis with electrical measurements. For instance, electron beam
induced currents produced in a scanning transmission electron microscope
have provided the f irs t direct observation of the electrical activity of
semiconductor interfaces.12 Other techniques that could produce useful
fundamental information are:
• Characterization of electrical and of chemical properties
on a lateral scale of 10 nm
• Direct measurement of space charge or of the associated
field with an energy resolution of better than 0.1 eV.
6 . Theories of deep level, interfaces, and of carrier_/ecombin-
_atjion
Although the performance of most junction devices is limited
by carrier recombination or generation at deep levels (within the energy
gap) or at surfaces, no adequate predictive theories exist. At present
i t is not possible to calculate the energy level of a given impurity, the
density of states within the gap at a surface or interface, or tne rate
of carrier generation and recombination which proceeds by these defects.
Therefore the effect of introducing impurities or of an interface cannot
be predicted quantitatively, nor can one put limits on the ultimate per
formance of devices.
7. Photoexcitation and charge separation
Photovoltaic processes different from charge excitation across
a band gap and charge separation by a space charge layer do exist. Ex
amples can be found in the photosynthetic apparatus which employs molecular
levels and tunnelling, and in metal-ins,ulator-metal structures that can
exhibit internal photoemission albeit with low efficiency. 0' Tunnelling
barriers (1-5nm) are typically more than one order of magnitude narrower
than semiconductor space charges (100 nm). Hence, the materials saving
for tunnelling converters would be significant. Provided that very highly
absorbing materials ( a > 10“5 cm"1) can be employed, fundamental research
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on alternative excitation and separation processes could lead to less
expensive photoconversion technologies.
TECHNOLOGY NEEDS
1 • Materials needs
a. Semiconductors
Semiconductor technology depends on continued advances in
the preparation of crystals and crystalline films of known and well char
acterized materials. Silicon, gallium arsenide, indium phosphide, and
alloys of 111-V and of 11-VI compounds with high purity and crystalline
perfection are needed for high-performance devices such as concentrator
cells and other optoelectronic devices, and for microcircuits.
Preparation and purification techniques need to be explored
for "solar grade" silicon. This is a yet undefined material containing
more impurity than silicon used in microelectronics. Solar-grade silicon
should be inexpensive but exhibit adequate photovoltaic performance.
Demands on purity may vary with the method that is used for silicon
crystal 1ization. For instance, polycrystalline material may offer better
opportunities for impurity rejection (into grain boundaries and grain
boundary phases) than single crystals.
. Thin small grain polycrystalline films (of direct gap
materials) for photovoltaic conversion have been plagued notoriously
with non-uniform crystalline size and porosity with the effect that thuse
these films have to be grown much thicker (20 to 30 pm) than what 1s
thought to be economically acceptable (2 to 5 nm). Here, technology needs
to encompass the selection of proper substrates, control of nucleation and
122 -
growth, as well as growth techniques that make efficient use of the
starting material. In this context, thought should be given to the
recycling of unused reactants.
The need for controllable and scaleable techniques for the
deposition of hydrogenated amorphous silicon films has been emphasized
earl ier.
A separate category of semiconductors demanding material
development is electrodes for photoelectrochemical conversion. Here, the
need is for stable electrodes inert against dark and photocorrosion with
band gaps low enough for substantial solar absorption,
b. Ancillary materials
Inexpensive or reusable substrates and containers should be
explored with emphasis on the principles governing introduction of im
purit ies materials losses, and reactivity in the case of re-use.
New transparent, electrically and thermally conducting,
materials such as tin oxide, indium tin oxide, and cadmium stagnate should
be developed for application in contacts or in connecting layers, with
emphasis on low contact resistance.
A clear need does exist for inexpensive contact metalli
zation to reduce the demand for silver, and possibly gold. Stable
intermetallic compounds that do not release impurities into the semi"
conductor are required, or metals that form a diffusion barrier upon
initia l reaction with the underlying semiconductor.
2. Growth and processing
Non-conventional, potentially cheaper methods are needed for
the fabrication of device grade materials. The search for such methods
that have not been adequately explored are spraying, screen printing*
casting of sheets, melt-spinning, horizontal growth, and electrodeposition.
Processing techniques that are amenable to automation need to
be explored. Examples of such techniques are sheet growth, sputtering
techniques, and energy beam techniques for annealing or recrystal li.zation.
The problem of grain boundary passivation should be examined in
a systematic fashion. In polycrystalline silicon, promising results have
been obtained by introducing hydrogen and by in-diffusion of phosphorus
(the dopant for the top layer in the solar cell). The mechanisms of
passivation are under dispute. Preferential oxidation at grain boundaries
has been demonstrated in polycrystal 1ine gallium arsenide films.
Two device-oriented materials problems that also fall in the
domain of thin film research are the preparation of reproducible and
stable tunnelling barriers, and the very general quest for cell structures
that are compatible with polycrystalline materials. With polycrystal 1ine
semiconductors, junction formation at low temperature (by producing a
Schottky barrier) may be preferable to high temperature techniques (in
diffusion of a dopant) to avoid shorting or shunting of cells along
grain boundaries.
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1 • Mic rostructure Science, Engineering, and Technology, National Academy of Sciences", Washing to~n7D7 C. (19791.
2. D.M, Rowe, Proc. IEEE 125, 1113-1136 (1978).
3. Solar Photovol taic Energy Conversion, ed,, H. Ehrenreich, AmericanThysTca\ ’Soci e'ty7 Nev7 York~TT9T9).
H. Ehrenreich and J.H. Martin, Physics Today 32, 25-32 (Sep. 1979).
4. E.A. DeMeo and P,B. £ios, Perspectives on Ut i l i ty Central Stat ionPhotovoltaic ApplicationsT Electric Power Research iristitute,PaTo A T to, Cali fof7r?a~(T978).
5. Proceedings, Fourth Project Integration Meeting, PhotovoltaicConcentrator Technology Development Project, Albuquerque, New Mexico, October 16-17, 1979. Sandia Report SAND 79-1791.
6 . R. Sahai, O.D. Edwall, and J.S. Harris, Jr. , Appl. Phys. Lett. 34,147-9 (1979).
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8 . R.N. Bracewel 1 and R.M. Swanson, S i l icon Photovoltaic Cells in Thermo-Photovoltaic Conversion, Electric Power Research Institute, Palo Alto, ra TTfo rni a , TrojecFW~790-2, Report (September 1979).
9• Propert ies of Polycrystal 1ine and Amorphous Thin Films and Devices,ed., L.L.. Kazmerski ,7New York: Aead~e7nlc Press, 198077.............
10. W.E. Spear and P.G. LeComber, Solid-State Comm. 17, 1193-6 (1975).
11. D.E. Carlson and C. Wronski, Appl. Phys. Lett. 28, 671-3 (1976).
12. P.M. Petroff, Scanning Electron Microscopy, vol. 1,(SEM Inc., AMSO'Ha re, Illinois 1978); pp. 155r 332*.
13. T.K. Gustafson, R.V. Schmidt, and J.R. Perruca, Appl, Phys. Lett.24, 620-2 (1974),
G.M. Elchinger, A. Sanchez, C.F. Davis Jr. , and A. Javan,J. Appl. Phys. 47, 591-4 (1976).