Prosiding Pertemuan Ilmiah Sains M ateri IIISerpong, 20 -21 Oktober 1998 ISSN 1410-2897

PERMANENT MAGNETS IN EUROPE S:!Jb

B.A. Davies,Department of Engineering Materials, The University of Sheffield, Sheffield, S 1 3ill, UK

ABSTRACTRESEARCH, DEVELOPMENT AND APPUCATIONS OF PERMANENT MAGNETS IN EUROPE. The evolution

of pemlanent magnet materials during the twentieth century is briefly outlined and recent progress and developments, since theadvent ofNdFeB hard magnetic alloys, are reviewed, with particular emphasis on changes within Europe. Progress in understandingthe behaviour of rare earth-transition metal hard magnetic alloys, in developing new phases, microstructural variants andimproved and novel processing routes and procedures are discussed. The extensions to existing applications, the development ofnew applications and likely future trends for this family of pemlanent magnet materials are also outlined

1. INTRODUCnON AND HISTORICALBACKGROUND

service.The development ofhardferrites in the 1950's by

Philips in the Netherlands was a significant milestone ir.the evolution of magnets since they had much larger jHcthan Alnico's, arising from large magnetocrystallineanisotropy associated with the non-cubic crystal struc-tures of the barium- and strontium-iron oxides. This,together with their low cost, has led to their widespreadapplication, not only in more mundane contexts such asin rubber bonded magnetic seals for refrigerator doors,but also, for instance, in permanent magnet motors forwhich the ferrite could be magnetised perpendicular tothin sections, with good resistance to demagnetisationin service. On the other hand, they have low saturationmagnetisation because the compounds areferrimagneticand hence have only modest (BH)max. Nevertheless, theyare still by far the most widely used types of magnets inmany spheres because of their cost-effectiveness.

The success of these magnets, with propertiesbased on magnetocrystalline anisotropy led, in the 60 's,to a search for metallic materials having higt..magnetocrystalline anisotropies and attention focusedon rare earth-cobalt alloys since the lanthanide elementshave large magnetic moments as a result of unpaired 4felectrons and cobalt has a high Curie temperature T c.The SmCos phase was shown to have an excellent com-bination of properties, including an extremely large valueof anisotropy field HA associated with the Sm-Co inter-actions and a hexagonal crystal structure [2]. The firstcommercial SmCos alloy, processed by liquid phase sin-

tering of magnetically aligned single crystal powder par-ticles, was produced by General Electric Co in the USA.This represented a quantum leap in magnet evolutionsince (BH)max is typically up to-175 kJm-3 and,Hc reachesup to -750 kAm-1. This was followed soon afterwardsby magnets based on the Sm2Col7 compound [3] whichled, ultimately, to magnets having (BH)max -225 kJm-3.

However, Co is both an expensive element and issubject to marked insecurity of supply and a search be

Pemlanent, or hard, magnets are materials essen-tially for the storage of magnetic energy and are essen-tial features of modem life, with applications, for instance,in electric motors, actuators, television tubes, microwavedevices, C D units, microphones, speakers, magnetic res0-nance imaging and scientific instruments. A hard mag-netic material has good resistance to demagnetisationby a reverse field, i.e. it has a high coercivity jHc andpreferably also has a large remanent magnetisation (M)or induction (B ) in order to provide high flux density. Auseful figure of ~erit for permanent magnets is the maxi-

mum energy product (BH) mox

Developments in permanent magnet materialsover the past 100 years have been remarkable [1], with(BH)mox increasing from < 3 kJm-3 for the best steel mag-nets at the beginning of the century to values in excessof 400 kJm-3 for the best rare earth alloy magnets avail-

able today.An essential requirement for coercivity is that the

magnetic material should possess magnetic anisotropy.Three types are possible, namely strain, shape andmagnetocrystalline anisotropies. The early steel mag-nets were dependent on strain anisotropy induced byquench hardening to a martensitic structure but thisgives a limited effect. A significant advance was madein the 1930's with the development in Japan, and subse-quently in Britain, of new FeAlNi and then FeAlNiCoalloys (the latter being known as Alnico magnets), inwhich elongated single domain magnetic particles wereprecipitated by heat treatment and which induced sub-stantial shape anisotropy, leading initially to coercivi-ties about 2 Y2 times as large as that of the best cobaltsteel magnet and, eventually, to jHC up to -150 kAm-t

and (BH) Up to -90 kJm-3. However, H was stillmax I c

rather small for many applications such as permanentmagnet motors since they were easily demagnetised in

Prosiding Pertemuan Ilmiah Sains Materi IIISerpong, 20 -21 Oktober 1998 ISSN1410-2897

gan for a rare earth-Fe alloy phase with at least equiva-lent magnetic properties to those of the SmCo alloys.This culminated in the announcement in 1984, simulta-neously by workers at Sumitomo Special Metals in Ja-pan [4] and at the Delco-Remy Division of General Mo-tors [5] in the USA, of NdFeB alloys having excellentmagnetic properties and based on the tetragonalN~FeI4B compound (although the phase had, in fact,been reported earlier by workers in the Ukraine but with-out addressing its magnetic properties). These alloyshad the advantage over the SmCo alloys of being basedon more plentiful and cheaper component elements andin having higher saturation magnetisation M,.

The process used for producing the sinteredNdFeB magnets was virtually identical to that already inuse for SmC05 magnets and the technology was thuswell-established. In this magnetically aligned, anisotro-pic fom. B, and (BH)max (initially -280 kJm-3) were higherthan for the SmCo alloys, principally because of the higherM .On the other hand, the corrosion resistance and T

I C

are inferior to the SmCo magnets.The process route employed by General Motors

was rapid solidification by melt spinning to ribbon whichinduced a chemically homogeneous, i.e. practically singlephase, microcrystalline structure with a mean grain sizeof typically -60 om. This material, following crushing topowder, was sufficiently inert to be fabricated into mag-nets by compression or injection moulding in a polymermatrix. The disadvantage of this melt spun form ofNdFeBis that the crystallites are randomly oriented and so can-not be magnetically aligned, with the result that Br isrestricted to a maximum value of 0.5 M" thus placing alimit of about 80 kJm-3 on the (BH)max of the bonded mag-net (which generally contains -20% of polymer in thecompression moulded form). However, fully dense mag-nets were also produced by hot pressing the ribbon-derived powder at -700 °C and, on hot forging, a strongcrystallographic texture and magnetic anisotropy couldbe introduced to raise (BH)max up to the level achievable

by sintering.By today, the highest (BH)max for commercially

available NdFeB sintered magnets has been raised toabove 400 kJm-3 [6], an increase by a factor of more than100 in one century, the progression with time beingap-proximately exponential. In terms of the development ofnew hard magnetic materials over this periO<i, the honoursare approximately even between Europe, Japan and theUSA.

In 1984, very soon after the announcement ofNdFeB magnets, the European Community funded aStimulation Action Programme, known as C.E.A.M., tostudy, develop further and to explore the applicationsfor these new supermagnets. The collaborativeprogramme was based on a consortium of more than 60research groups, drawn from universities, industriallabo-ratories, research institutes and industrial alloy and mag-net producers across the countries of the EU and someassociate member countries. The consortium, headedinitially by the late Dr Rene Pauthenet of the Lab LouisNeel in Grenoble, France and subsequently by ProfMichael Coey of Trinity College in Dublin, Ireland, wasdivided into four sub-groups: (i) Materials I; (n) Materi-als II; (in) Processing and (iv) Applications, with each ofthese sub-groups headed by a co-ordinator. Theprogramme proved to be highly successful in terms ofstimulating new research projects, in many casesmultidisciplinary. TheSe were aimed at (i) gaining basicunderstanding of the structures and behaviour ofN~FeI4B and related phases, (ii) devising innovativeimprovements to the processing of the magnets and (iii)designing and optimising new or improved electricalmachine, drive and actuator systems, using magneticcircuit design techniques generally based on computermodelling, in order to exploit the new magnets in themost effective ways, on the basis of energy efficiencyand cost.

In addition, it was hoped that the deeper under-standing of the physics of rare earth-transition metal(RE- TM) magnetic structures would lead to the success-ful prediction and realisation of new phases and of newphenomena. This aim was achieved, either within theprogramme or in related projects within member labora-tories, through the development of (i)S~FeI7N) mag-nets (although contemporaneous developments in Ja-pan also focused on this compound) and (ii) ofnanocomposite melt spun alloys based on Nd2Fel4BIFe)B and Nd2Fe14Bla-Fe systems with exchange-en-hanced Band (BH) .

In'the initial phase, funds were provided for equip-ment, staff salaries, student stipends and for studentand staff exchanges, in addition to numerous thematicmeetings and annual meetings, but this was progres-sively tapered down through three successive stages ofthe Programme with funding renewals. The Programmeextended over a record ten-year period, such was itssuccess.

The aim of this paper now is to outline brieflysome of the recent developments in permanent magnetsand their realised and potential applications in Europesince the advent of NdFeB alloys some fifteen years

ago.

3. PROCESS DEVELOPMENTS

3.1. Hydrogen Decrepitation

A major advance in the production of sinteredNdFeB resulted from the use of hydrogen decrepitationfor the first stage of fragmentation of cast alloy ingots(7]. The rare earth alloy readily absorbs hydrogen and

2.

CONCERTED EUROPEAN ACTIONON MAGNETS (C.E.A.M)

H.A. Davies2

Prosiding Pertemuan Ilmiah Sains Materi IIISerpong, 20 -21 Oktober 1998 ISSN 1410-2897

ing parameters, notably milling energy and time and theoxygen content of the milling chamber, microcrystallineor nanoscale exchange-enhanced structures can be pro-duced. It offers considerable flexibility in terms of acces-sibility of various phases and microstructures, probablygreater than is availability by melt spinning. Although aEC-sponsoredBrite-Euram programme involving Inoo inthe UK and Siemens in Germany investigated the largerscale processing of REFeB magnets by this route, theoutcome of the study in terms of establishing the com-mercial viability of the process in comparison with otherroutes is not clear.

especially so the paramagnetic rare earth-rich phase ly-ing around the boundaries of the coarse Nd2Fel4B grains.The expansion resulting from this absorption leads tointergranular fracture or decrepitation. This process con-siderably reduces the amount of mechanical fragmenta-tion r~uired and, following desorption of the hydrogenat reduced pressure, the powder is then coarse-ground,jet-milled down to the requirOO 5 J.Utl particle size, pressed,aligned and sintered. This process has now been widelyadopted by commercial producers of sintered NdFeBmagnets worldwide.

An evolution of this process is hydrogen-dis-proporti 0 nation -desorpti on -reco mbinati on (HD DR)which was subsequently developed and patented in Ja-pan [8]. Here, hydrogen is absorbed into the NdFeB al-loy at elevated temperature (~800°C) which leads to theformation of a finely divided mixture of N~, Fe andFe2B. Applying a vacuum in the same temperature rangereverses the reaction, ie the Nd2Fe14B is reconstitutedbut with a much finer grain size than the the originalmaterial, typically 0.2J.Utl. vs > l00~m. The resulting pow-der in this fine grained state is fully coercive, in contrastto the initial material, and is thus suitable for consolida-tion to magnets by polymer bonding or by hot pressing.For suitable processing conditions and alloy composi-tions, magnetically anisotropic powders can also beachieved and polymer bonded magnets with (BH)max upto 180 kJrn-3 [9] have been achieved which could pose agreater threat to melt spin-derived magnets than the iso-tropic variant. However, the reason for the developmentof texture is not yet understood and some difficultiesremain to be overcome in order that the process can besuccessfully operated on a commercial scale.

4. IMPROVED AND NEW MAGNETMATERIALS

4.1. NdFeB Magnets with Enhanced Energy Products

The original magnetically aligned, sintered NdFeBmagnets introduced by Sumitomo in 1983 had (BH) of-280 kJ m-3 [4]. These had Nd concentrations conside;ablyin excess of the value of 11. 76 a~/o for the stoichiometricNdfe,4B compound in order: (i) to promote the formationof a low melting point Nd-rich phase, which is essentialfor liquid phase sintering of the Nd2Fe,4B grainsfollowing pressing and aligning and (ii) to promote highcoercivity by damping the nucleation of reverse magneticdomains. However, this Nd-rich phase is paramagneticand thus acts to magnetically dilute the Nd2Fe14B grainsand lower the remanence. Other diluting phases that arisefrom the non-stoichiometric composition of the magnetsare paramagnetic Nd4FeB 4 and Nd oxide and also a-Feacts to reduce H

I C

Thus, considerable attention has been focusedby magnet manufacturers on reductions in Nd content.in the starting alloys and on process modifications andoptimisation to avoid the deleterious effects of suchreductions and to reduce the median and standarddeviation of the Nd2Fel4B particle size, both prior topressing and sintering and after sintering. These effortshave met with considerable success. Both Pechiney/UGIMAG [12] and Vacuumschmelze [13] now marketgrades of sintered NdFeB magnets with (BH)mox of 400kJm-3. The use ofstripcastNdFeB has proved beneficialin avoiding the presence of unwanted a-Fe phase. (Byusing robber isostatic pressing, Sumitomo in Japan havesucceeded also in improving significantly theeffectiveness of magnetic alignment of the particles priorto sintering [6]). It is estimated that the best mass-produced NdFeB magnets have achieved (BH)mox ofabout 95% of the maximum practicable limit forthisfarnilyof magnets. The principal demand for these very high(BH)max magnets is for magnetic resonance imaging (MRI)units and for head positioning actuators in hard disc

drives.

3.2. Mechanical Alloying

Mechanical alloying is a process developed origi-nally by Inco in the USA and first exploited commer-cially in the UK for the production of oxide dispersionstrengthened superalloys, notably for high temperatureapplications in gas turbine engines. It involves the highenergy ball milling of constituent elemental powders orof partly prealloyed powders for extended periods ofseveral hours. Thus, the repeated mechanical deforma-tion, pressure welding and fracture of particles, throughthe action of colliding hardened steel balls, combinedwith the heating resulting from the mechanical work, leadsto intimate di:ffilsive mixing of atomic species to formphases with elevated free enthalpies or ultra-fine grainedstructures. These include nanocrystalline or amorphousphases, particularly for rare earth-transition metal mix-tures, and the latter are amenable to devitrification to

nanophase stuctures by annealing.This technique has been quite widely employed

for scientific studies of the processing of Nd/Pr-Fe-B[10] and SmFe alloys, the latter prior to nitriding to pro-duce S~FeI7Nx alloys [1 I]. Depending on the process-

4.2. Nanostroctured Magnets

3H. A. Davies

Prosiding Pertemuan Ilmiah Sains Materi /IISerpong, 20 -2l Oktoher 1998 ISSN1410-2897

has a significantly larger M. than does Fe3B (2.2 T vs. 1.6

T). A corollary of enhanced B is a reduction in.H and,, I C

for x < ~ 8 afO/o, (BH)max rOOuces with decreasing x because

iHc is reduced to a level where the B-H second quadrant

characteristic becomes non-linear.

Similar B,/(BH)max enhancements have been

demonstrated for nanocomposite alloys in theisomorphous PrFeB system ,[19J but these have H ~

25% larger than for the corresponding NdFeB ~l~ys

because of the correspondingly larger anisotropy field

for the Pr2Fel4B phase. Thus, better (BH)max/iHc

combinations are obtained for a given RE2Fe14B mean

grain size.The successful commercial exploitation of these

promising alloys is dependent on the ability to obtain

similar levels of grain refinement, by overquenching to

the amorphous state followed by a devitrification anneal,

as can be achieved by direct quenching. The latter route

is not suitable for large scale operation because the

processing window is too narrow.

4.3. Samarium-Iron-Nitrogen Alloys

The report in 1972 [20] of a large saturationmagnetisation, well in excess of pure Fe, for the Fel6N2compound later stimulated, in part, investigations of theeffects of introducing nitrogen atoms into RE-Fecompounds with potential for improved hard magneticproperties. Success was achieved when Coer and Sunat Trinity College, Dublin [21] reported very goodmagnetic properties for S~FeI7N) processed by nitridingS~FeI7 alloy at elevated temperature (-450 °C). Thisphase has HA of >2 I MAm-1 (comparedwith-5.6MAm-I for N~FeI4B). Another principal feature of this phase

is its relatively large T c of ~ 470°C, considerably higher

than that for Ndfel4B (312°C).However, significant difficulties exist in its

characteristics and processing, quite apart from the factthat Sm is more costly than Nd. The nitrogenation isperformed on the alloy in finely powdered form to avoidexcessive diffusion times. However, the Sm2FeI7N) phasecannot be sintered in the conventional way to a bulkform since it decomposes below 600 K. The particle sizeneeds to be of the order of 2 ~m to develop usefulcoercivity but, on the other hand, such a small particlesize introduces difficulties in achieving high packingdensities in compression moulding of polymer bondedmagnets. Nevertheless, mechanical alloying [II], HDDR[22] or rapid quenching by melt spinning [23] can beemployed for obtaining higher coercivities through grainrefinement in coarse powders. Many of the developmentsthat may lead to commercialisation of this class ofmagnetic alloy have, however, been undertaken in Japan.For isotropic, bonded SmFeN magnets, containing smallfractions of Co and Zr, processed by HDDR and havinga nanocomposite Sm2FeI7N)/a.-Fe structure, (BH)max upto80kJm-)incombinationwith.H of710kAm-1 [23] has

I c

It was reported by Keem et al in the USA in 1987[14] that Br significantly in excess of the Stoner-WohJfarthlimit of 0.5 M, for non-interacting uniaxial particlesoccurred for melt spun NdFeB based alloy ribboncontaining a small concentration (1.0-2.5 at%) ofSi orAI. It was claimed that the Si and AI acted as grain refinersand that the Br enhancement resulted from exchangeinteraction between the resulting crystallites, which hadmean diameters d of -25 om compared, typically, with 60

gom for the commercial melt spun Magnequench ribbon.(BH) up to -185 kJm-) were claimed for an isotropicstruct:re. Studies at Sheffield [15], in fact, showed thatthe critical d at which B enhancement becameg r

significant was -40 om and that the effect increasedprogressively with decreasing d as the cooling rate

gduring melt spinning was increased.

Meanwhile, workers at Philips Laboratories inEindhoven [16J had developed an exchange-enhancedNdFeB hard magnetic alloy containing a much smallerconcentration of Nd (4 at%) than the commercialMagnequench alloy (- 13 at%). This was processed bymelt spinning the Nd.Fe77sBl8s composition to theamorphous state, followed by a devitrification anneal toyield a nanocomposite structure with the major phasebeing soft magnetic Fe)B interspersed with a smallervolume fraction ofNd2Fel4B and a minor fraction of a.-Fe. This was shown to have Br-l.2T, some 50% largerthan that for microcrystalline N~FeI4B, typified byMagnequench, and consistent with d -20 om. This was

8

combined with goodcoercivity of280 kAm-I, consideringthe large volume fraction of soft magnetic phase. Thisnanocomposite so-called 'lean' (i.e. low Nd) alloy thushad the characteristics of a single phase magnet becauseof inter-crystalline exchange coupling between the hardand soft phases. For this reason, the alloy also manifestsa so-called 'exchange-spring' effect [17], i.e. a muchlarger second quadrant recoil permeability thanconventional magnets. This is a useful practical featurein that there is smaller loss of magnetic flux in cases ofaccidental reverse magnetisation, such as in cases ofstalling in an electric motor. Because of the low Ndcontent, the alloys are cheaper than the Magnequenchalloys and, in spite of the rather low jHC' it has potentialmass-market applications in, for instance, spindle motors.The technology has now been licenced to other magnetmanufacturers.

Subsequently, a range of nanocompositeNd2FeI4B/a.-Fe alloys was developed and studied atSheffield [18J. This work demonstrated that Si and AIwere not prerequisites for the formation of sub-50 omdiameter Ndfel4B grains in melt spun NdFeB alloys andalso that Br and (BH)mu were progressively increasedwith decreasing Nd content in the NdxFe94_xB6 alloy series, i.e. with increasing volume fraction of the soft magnetica.-Fe phase. For these alloys, the exchange couplingbetween hard and soft magnetic grains leads to greaterenhancement of Br and (BH)mu' for a given Nd2Fe14Bgrain size, than for the 'lean' Nd4Fe77sB18.s alloy since Fe

Prosiding Pertemuan Ilmiah Sains Materi /IISerpong, 20 -21 Oktober 1998 ISSN 1410-2897

been achieved. Also, by stabilising the powder particlesurfaces with a layer of Zn, anisotropic bonded magnetshaving (BH)max as high as 176 kJrn-) have been reported

[24].

Euram Programme following from the earlier CEAMProgramme in Euro~ [28J.

6. CONCLUSIONS

5. APPLICADONS

With the increased awareness for the need toreduce the consumption of fossil fuels, both from theviewpoint of conserving a finite resource and of reducingpollution and global warming, there has been an growingemphasis on increasing the efficiencies of electricalmachines, especially motors. These are, and are projectedto remain, the dominant application for permanentmagnets in Europe and the rest of the world. Thus, therehas been a rapid increase in the total market for permanentmagnets. However, the development of NdFeB magnetshas greatly increased the share of the rare earth-basedpermanent magnets, so that today they represent about40% of the total value. In the early 80's SmCo magnetsalready represented an important segment of the magnetindustry but the advent of NdFeB has led not only toreplacement for SmCo, on grounds of lower cost andbetter performance, but also, for these reasons, to abroadening of the range of cost-effective applicationsfor high performance magnets. SmCo, Alnico's andferrites still hold their own in applications where high T cor, for ferrites, low cost are paramount. However, theproduction ofNdFeB magnets has been growing at > 12%per annum and the demand for bonded NdFeB magnetsexpanding by -20% per annum [251. These growths havebeen stimulated in particular by the computer industrywith sintered NdFeB being used in voice coil motors andbonded NdFeB for spindle motors, both in disc drives.

The principal applications for sintered NdFeBmagnets, other than for voice coil motors in disc drives,which currently represent ~O % of the market, are formotors, loudspeakers andMRI [261. For bonded magnets,apart from the ~O % represented by the computerindustry, they are penetrating new areas, includingautomobiles, energy efficient motors, compressors,pumps and micro motors [26J. The commercialisation ofnanocomposite 'spring' magnets, with lower rare earthcontents than standard Magnequench magnets, shouldlead to cheaper magnets, on the basis of unit energyproduct, and to an expanding market for permanentmagnet motors, both rotary and linear. In particular,brushless d.c. motor drive systems for hybrid electricvehicles are seen as a major potential application forNdFeB magnets. Wind powered generators and highspeed turbo-alternators for combined heat and powerunits are other significant potential applications for rareearth magnets in Europe [271. Successful appraisals ofapplications in hybrid stepper motors, shaft torquetransducers, rotary actuators in the process engineeringindustry and electronically commutated actuators forauto motives have also been undertaken, under the Brite-

With the drive for increasing perfonnance andefficiency of devices in all spheres of activity, the futurefor pennanent magnets appears bright, not only in Eu-rope but throughout the world. However, since a largeproportion of the world's rare earth ore resources lie inChina, it is certain that Chinese companies will assumeincreasing importance, not only in the supply of rareearth metals but also in magnet production. The rate atwhich the production of rare earth pennanent magnetswill continue to rise will also, of course, depend on rawmaterial prices and measures have been taken recentlyto reduce the marked fluctuations that have occurred inthe past [29).

It is very unlikely that the rate of increase of(BH)ma with time will continue into the next millenium atanywhere near that which has occurred during thiscentury since, even if pure bulk Fe could be produced ina magnetically hard fonn with iHe of 900 kAm.\ and aperfectly square hysteresis loop shape, its (BH) wouldstill be only 1 MJm.) [30J. It seems improbable thatferromagnetic alloys with M. at room temperature greaterthan that of pure Fe or oxides with M. greater than Ba/Srferrites will be found although such a discovery shouldnot be completely ruled out. Thus, on this basis, futureprogress in magnetic materials is likely to be incremental,based on compositional and process optimisation. Newapplications are, however, likely to proliferate.

ACKNOWLEDGEMENTS

The author is grateful to the British Council officein Jakarta and to the organisers of this conference forfinancial support to attend and to the U.K. Engineeringand Physical Sciences Research Council and theEuropean Commission for financial support of hisresearch on rare earth pennanent magnets.

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