Jf& __ __ !iB ELSEVIER Solid State Ionics 101-103 (1997) l-7

SOLID STATE IONICS

Highly stable ceramics through single source precursors

Martin Jansen

Institut fiir Anorganisrhe Chemir Gerhurd-Domcrgk-Strife 1. D-53/2/ Bonn. Germany

Abstract

A brief review of polymer routes for the production of ceramics is given. The main emphasis is laid on systems derived from the single source precursor Cl,Si(NH)BCl,. This precursor has been developed in order to meet the objective of synthesizing amorphous multinary nitridic or carbidic ceramics. The synthesis of the single source precursor, its

polymerization and finally pyrolysis to extended solids are reported. The amorphous materials obtained show outstanding properties with respect to thermal stability and resistivity against oxidation and crystallization.

Keywords: Ceramics; Single source precursors; Siliconboron carbonitride; Ceramic fibers

Materials: Trichlorosilyi-amino-dichloroborane

1. Introduction

The starting materials for the production of

ceramics originate from three main sources, ( 1) natural alumosilicates, (2) synthetic materials like

binary oxides, nitrides and carbides that have been discovered already in the past century [l-3] and, most recently, (3) molecular precursors that are processed via polymeric intermediates to a solid

ceramic 14-71. This paper focuses on the latter approach. The two main objectives of the work

presented are finding a preparative access to multin- ary nitridic or carbidic ceramics which would allow

for a tuning of properties by changing the com- positions, and producing these phases in an amor- phous state thus suppressing intra-grain crack propa- gation along crystallographic planes. As the diffusion coefficients of silicon or boron in e.g. nitrides are extremely low, and binary components that are expected to form powerful multinary carbonitridic ceramics do not melt without decomposition, only

one way to prepare the desired materials seems to be left, the synthetic route via polymeric intermediates.

An (amorphous) material designated for an appli-

cation as a high performance ceramic must be

extremely durable and must not change its micro- structure when exposed to peak loads. An amorphous solid that is expected to resist to crystallization should be based on strong covalent bonds, and

consequently show negligible small contributions from long range bonding interactions to the enthalpy

of formation. These requirements seem to be fulfilled best by combinations of Si, B or P and N or C. In order to avoid spatially varying concentrations,

which might induce phase separation and even crystallization during pyrolysis, the polymers must show a homogeneous distribution of the components on an atomic level. Thus, in a first step, molecular single source precursors have to be synthesized which already contain linkages that in the most favorable case would survive the processing into the final ceramic material.

0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl67-2738(97)00234-S

2 M. Jensen I Solid Stute Ionics 101-103 (1997) l-7

2. The synthesis of single source precursors

A broad variety of synthetic routes for the forma-

tion of element-nitrogen bonds has been developed (cf. Table 1). As an example, the synthesis of

Cl,Si(NH)BCl, is given explicitly in Fig. 1. This molecule contains an Si-N-B bridge and is fully

functionalized by chlorine which allows for rapid polymerization via ammonolysis (or aminolysis) and

subsequent polycondensation [&lo]. With respect to

Table I Synthetic routes for the formation of element-nitrogen-bonds

Dehydrohalogenation

E-X +2HNRR’ - E-NRR’ +HX NH2RR’

Salt Elimination

E-X +LiNRR’ B E-NRR’ +LiX

Cleavage of Silazanes or Stannazanes

‘: R

E-X + Me$i(Sn)-N-M F E-N-M

Dehydrogenation

‘: R

E-H + H-N-M - E&-M +H2

Transamination

E-NRa +HNR’, B E--NR’* +HNR,

c MqSi(Sn)X

WhKdWCW3 SiCb _

A CSSi,N,WW3

-cWCHd, k hemmethyl disiie l,l,l trichtoro-3,3,3 trimettlytdisiie

Fig. 1. Synthesis of (trichlorosilyl)-amino-dichloroborane.

F’ Cl&, B.

/ Y 1 C~3S+%,,SiC!,

Fig. 2. Selection of single source precursors derived from hexa-

methyl disilazane.

a possible technical application this reaction se- quence has several favorable aspects. ( 1) All starting

materials are well available and relatively cheap, (2)

all by-products can be reused or recycled, in princi- ple, (3) the yield in each step is close to quantitative

and (4) production on an industrial scale is tech- nologically feasible. By applying these tools offered by inorganic molecular chemistry, an almost innum- erable variety of single source precursors is access- ible. A selection of those derived from hexa-

methyldisilazane is presented in Fig. 2 [ 11,121.

3. Polymerization and pyrolysis

Polymerization is best achieved by adding am-

monia or methylamine. In both cases the am- monolysis is followed immediately by polycondensa- tion:

{Cl,Si(NH)BCl, + (10 - xI2)NH,},, 2

[si(NH)B(NH),,2(NH2)~s~,~l,, + 5nNWL (1)

{Cl,Si(NH)BCl, + (10 - xI2)CH,NH,}, *

[si(NH)B(NCH,),,,(NHcH,),,_,,l,,

+ SnNH,CH,Cl. (2)

Using methylamine has the advantage that the oligo- merit condensation products are liquid and can be separated from the salt by filtration (Eq. (2)). The

hf.

. 5

. 0

. -5 exe

-50

=,; 2d0 .~o;G40;

27 (indcating HCN) 30 (indicating HpNCHd

Fig. 3. DTA, TG and DTG of the N-methylpolyborosilaaane

obtained via Eq. (2) and the development of the intensities of the

most frequent gaseous decomposition products during the

pyrolysis.

Further routes to pre-ceramic polymers are the

sol-gel process in the ammonia system where elementamides are co-condensated with ammonia

[ 141 or the so called hydride-route to inorganic

polymers with excellent yields via intermolecular hydrogen elimination [ 15,161.

4. Properties of the ceramics

solid-solid separation, necessary in route (1) can be

rather tedious (e.g. the ammonium chloride has to be washed out using liquid ammonia).

Both, Si,B,N, and SiBN,C do not show any Bragg reflection of X-rays. Their compositions have

been determined by chemical analysis (cf. Table 2). According to the composition, $ of the carbon atoms

in SiBN,C have an anionic function, and $ a cationic one, and C-N-linkages must be present. The

first coordination sphere of silicon is clearly tetra- hedral and that for boron, trigonal planar, as shown

by MAS-NMR spectroscopy (Fig. 4).

The polymer obtained via Eq. (1) is principally unmeltable, while the viscoelastic properties of the

N-methylpolyborosilazane can be tailored by an appropriate heat treatment to be liquid, meltable, soluble or unmeltable.

The thermal stability is impressingly high. The

onset of nitrogen evolution of Si,B3N, is shifted by

160°C to higher temperatures, as compared to crys- talline (thermodynamic stable) silicon nitride. SiBN,C is stable up to 1900°C at which temperature

it starts decomposing into Si, N,, BN, and Sic. Surprisingly, during the pyrolysis only three well The most critical drawback of nitride ceramics as

resolvable stages are passed. Fig. 3 shows the results compared to oxides is their susceptibility to oxida- of a simultaneous thermal and mass spectroscopic tion. With this respect an unforeseen feature makes analysis [ 131. From 200 to 6OO”C, simply the poly- SiBN,C superior to all known carbides and nitrides. condensation is completed while methylamine is By exposing this material to air at 14OO”C, a evolved, at about 600°C the biggest weight loss protecting double layer is formed. The outer part accompanied by fragmentations occurs, eventually, being enriched in silicon and oxygen (Fig. 5) acts as above 1000°C small amounts of nitrogen and mainly a diffusion barrier for oxygen, the inner part which is hydrogen separate. If ammonia as a reactive gas is enriched with respect to carbon and boron suppresses applied during pyrolysis, in both cases a ternary cation diffusion. Thus the maximum temperature for

Table 2

Chemical Analysis of Si,B,N, and SiBN,C

Si,B,N,

Element

Si

B

N

0

Sum

Anal. c/c Calc. 7~

38.7 39.2

15.1 15.1

44.2 45.7

1.5 0.0

99.5 100.0

SiBN,C

Element

Si

B

N

C

0

Sum

Anal. B Calc. %

32.1 30.2

11.7 11.7

40.5 45.2

12.7 12.9

0.5 0.0

97.5 100.0

nitride Si,B,N, is formed, while in nitrogen, the

N-methylpolyborosilazane yields SiBN,C.

M. Jansen I Solid State tonics 101-103 (1997) 1-7

b)

11.6ppm

22.95 pp

i’ 1

250 200 150 loo 50 0 -50 -100 -150 -200 -250 wm 23.0wm,jl.3wm

d)

250 200 150 loo 50 0 -50 -100 -150 -200 -250 wm

Fig. 4. ‘“Si-MAS-NMR spectra of SiBN,C (a) and wSi,N, (b), “B-MAS-NMR spectra of SiBN,C (c) and h-BN (d)

Fig. 5. SN-MS (secondary neutral particle mass spectroscopy) of the surface of SiBN,C powder oxidized at 1500°C (up to 3000 s a layer of

about 2 p,m is removed).

an application in air of = 1500°C is significantly

higher than for all competing systems known, so far. Finally, there are no changes in microstructure detectable up to 1900°C (Fig. 6), another outstanding and unprecedented feature. Its hardness is compar- able to sapphire, and the density is 1.8 g cme3.

5. Applications

The extremely high durability of amorphous silicon-boron carbonitride is one side of the coin, its low sinterability is the other side. Appropriate sinter- ing additives that would allow the production of

M. Jutwn I Solid State Ionics 101-10.3 (1997) 1-7 5

31.0 38.8 Xl---

Fig. 6. X-ray powder diffraction patterns of SiBN,C from 1400 to

1900°C (CuKa-radiation).

components via powder processing have not been

developed, yet. Likewise it is not possible to manu- facture monolithic ceramic pieces by shaping a

polymer green body because of severe shrinking and evolution of gaseous by-products during pyrolysis.

1 * 2”Nli

Fig. 7. REM photograph of SiBN,C fibers 171,

To start with, all types of applications that are

based on small dimensions seem to be the most

promising. Among these there are protective coatings

e.g. carbon fiber ceramic (CFC) components against oxidation, and ceramic fibers made by melt spinning

of the polymer and subsequent pyrolysis [ 17,181. To the present state of knowledge, theses fibers (Fig. 7)

are the only ones available that fulfill the very

ambitious requirements as formulated by the Euro- pean manufactures of jet propulsion engines for

ceramic fibers of the third generation (Table 3).

6. Competing approaches and conclusion

Meanwhile many polymer routes for the product- ion of ceramic materials based on only one metal are

known. Most of them just represent variations of the basic ones as published by Verbeek et al., Yajima et

al. and Seyferth et al. [22-271 (cf. Table 4). In the field of multinary ceramics, the variety is

not as broad yet. Seyferth and Plenio [25] have cross connected oligomeric silazanes using a borane di-

methylsulfide adduct. The pyrolysis of the resulting polyborosilazane yields composites of crystalline

Si,N, and BN (Table 4).

However by using a single source precursor, a homogeneous distribution of the constituting ele-

ments on an atomic level can be achieved. In the case of SiBN,C this elemental distribution and the

amorphous microstructure does not change to unpre-

cedented high temperatures [ 171.

The access to polyborocarbosilane via hydrobora- tion of vinylsilanes [26,27] as introduced by Ric-

citiello and modified by Riedel seems to be some- what awkward to use on a technical scale. Further- more the siliconboroncarbides seem to undergo microsructural changes at about 1600°C.

The role of the constituents at tailoring the prop-

erties is becoming clear. A high content of carbon is improving the thermal but lowering the oxidation stability, boron is slowing down cation diffusion and thus prevents (among others) grain coarsening.

The rather stringent economical and technical requirements that will have to be met at transferring these processes to an industrial scale will sift the chaff from the wheat. Our route presented here might be able to compete [7,8].

6 M. Jansen I Solid State Ionics 101-103 (1997) l-7

Table 3

Comparision of the properties of different fibers [19-211

Property Requirements of the

European

manufacturers

SiBN(C)

Bayer

Uni Bonn

Sic

Dow Coming

SIC

(Hi-Nicalon)

Nippon Carbon

SiCTiO

(Tyranno Lox E)

Ube Industries

Max. temperature usable

in air

Tensile strength (rt) Tensile strength

(1500”c)

e-modulus (rt) e-modulus

(1400°C)

Breaking elongation

Coefficient of expansion

Density

Diameter

1500-2000°C 1500°C 1300°C 1200°C 1000°C

3.0 GPa

2.5 GPa

3-4 GPa

2.3 GPa

3-4 GPa _

3 GPa ca. 3.2 GPa

300 GPa

250 GPa

1%

3-5X10 h/K

<5 g/cm’

lo-150 p,m

200-350 GPa

80-90% of

rt-value

0.7-1.5%

3.5X 10-“/K

2 g/cm’

8-14 km

420 GPa _

0.6%

4X 10mh/K

3.1 g/cm’

10 p,m

300 GPa

1.0%

3.3X IOmh/K

2.74 glcmZ

14 pm

200 GPa

1.5%

4.5X lO-h/K

2.5 g/cm

12 p,m

Flexibility good good medium good good

Table 4

Selection of polymeric routes

Synthetic route Product Reference

Sic (crystalline) P21

2H,SiCl + NH,_<zc,NH(SiH,)2 -+N(SiH,), -+ I”IOCI~YC

Si,N, (crystalline) ~231

Cl,SiMe + CllCH,_~~<,polymer~“~ 1

I OOOY BHTSMe, + (CH,SiHNH),RLpolymer--+

I O”,,T R,Si(CH=CH2) + H,B*D + polymer-+

Si,N,/SiC (crystalline) 1241

SilBINIC (nanocomposite) 1251

Si/B/C (nanocomposite) PWI

Acknowledgements

The results presented have been obtained in a close cooperation with the Bayer AG, Leverkusen. The contributions of individual partners have been

cited in each case. Especially, I would like to acknowledge the input and efficient cooperation with Dr. Baldus and Dr. Eiling from the Bayer company, and the contributions of my co-workers 0. Wagner, J. Loffelholz and M. Kroschel. This work has obtained financial support by the German Ministry for Research and Technology.

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