Recent developments in thermoelectric materials

Applied Energy 24 (1986) 139-162

Recent Developments in Thermoelectric Materials

D. M. Rowe University of Wales Institute of Science & Technology, Department of Physics, Electronics & Electrical Engineering, PO Box 25, Cardiff, CF1 3XE (Great Britain)

S UMMA R Y The increased activity in attempts to develop improved thermoelectric semiconductors for use in the direct conversion of heat into electrical energy results mostly from research sponsorship by the US Military and NASA. Thermoelectric generators have no moving parts and are difficult to detect by visual, aural or thermal infrared means. Fossil multifuelled thermoelectric generators are the leading candidates for replacing standard US Military engine generator sets up to 1.5 k W under the SLEEP programme (Signature Suppressed Lightweight Electric Energy Plants). When coupled to an isotopic heat source, thermoelectric generators are able to operate reliably and unattended for long periods of time and have a proven performance record in supplying electrical power to the Lunar Experimental Package (Apollo Program) and in providing onboard electrical power to the Voyager spacecrafts. In both military and space applications any improvement in the thermoelectric generators" conversion efficiency would result in a saving infuel--an important consideration. One way of improving the conversion efficiency is by increasing the so called 'Figure of merit'of the semiconductor material employed in the fabrication of the generators' thermocouples. In this paper an assessment is made of current thermoelectric materials; recent attempts to improve the figure of merit of existing materials are discussed and a number of new thermoelectric materials described. Significant headway has been made in reducing the lattice thermal conductivity of thermoelectric materials through the use of additives, small grain sizes or combinations of both. This development will result in substantial improvements in the thermoelectric figure of merit, provided the electrical properties can be maintained close to single crystal values. 139 Applied Energy 0306-2619/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

140

D. M. Rowe

It is concluded that, because in the past the development of new thermoelectric materials has occupied long periods of time, even during periods of intense research activity, it is likely that established or 'modified' established materials will remain the mainstay of military and space applications at least for the forseeable future.

1.

INTRODUCTION

The theory underlying the conversion of heat energy into electrical energy by the thermoelectric (Seebeck) effect has been exhaustively dealt with in the literature. ~-12 It is sufficient to state that for a given temperature difference across a thermocouple and a given temperature of operation, the conversion efficiency of the thermocouple is determined by the so-called 'figure of merit' Z of the material forming the limbs of the thermocouple, where Z = ot2a/2, ~ is the Seebeck coefficient and tr the electrical conductivity. The thermal conductivity (2) is the sum of a contribution 2 L arising from the lattice waves and a contribution 2e from the carriers present. All three parameters in the figure of merit vary with carrier concentration and Z is optimised at around 1024-1026 m-3, i.e. the materials are semiconductors. The resurgence of Western interest in the thermoelectric generation of electricity during the 1960s resulted from the United States Navy requirement for a silent source of electrical power for submarine propulsion. During this period, the thermoelectric properties of a large number of semiconductors were investigated and those based upon Bi2Te 3, PbTe and silicon-germanium (Si-Ge) alloy emerged as the best materials for operating up to temperatures of about 450 K, 750 K and 1250 K, respectively. Continued developments in thermoelectric semiconductor material technology are a consequence of their use in a number of military and space applications. Fossil-fuelled thermoelectric generators are used in a number of military applications ~3'~4 where their multifuel capability and silent operation in tactical situations prove very desirable features. Isotopic powered generators having a long-life heat source, coupled with a high energy density, provide a unique source of on-board electrical power for vehicles deployed in deep space. ~5 The conversion efficiency of a thermopile is low, typically < 7 - 8 % . The conversion efficiency can be increased by raising the operating

Recent developments in thermoelectric materials

141

temperature and by increasing the figure of merit. Consequently research effort has concentrated on developing materials which possess improved figures of merit and which are capable of operating with a hot junction temperature beyond the range of operation of Si-Ge alloy (namely 1250 K). In this paper the attempts to improve existing established thermoelectric materials are discussed, several new thermoelectric materials described and an assessment made of current material technology.

2.

ESTABLISHED MATERIALS

(i) Bismuth telluride The figures of merit of a number of thermoelectric semiconductor materials are displayed in Fig. 1. Bismuth telluride has a multivalley structured conduction and valence band, with an indirect bandgap of 0.15 eV; 16 it is essentially a low-temperature thermoelectric material

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142

D. M. Rowe

with a maximum operating temperature of around 450 K. It is the only one of the established materials that exhibits anisotropic properties with maximum Z values for both n- and p-type materials obtained with current flows parallel to the cleavage planes.17 Bismuth telluride can be doped n-type with halogens, silver or copper (although in practice silver and copper are not used because of their fast diffusion) and p-type with cadmium, tin or lead. Its lattice thermal conductivity can be reduced by forming isomorphic alloys with bismuth selenide or antimony telluride, which are n- and p-type respectively. Minimum 2 L values for (Bi-Sb)2Te 3 occur near 70% Sb, and both BiSbTe 3 and Bio.sSbl.sTe 3 are used as p-type materials.

(ii) Lead telluride Lead telluride and its alloys possess a cubic structure and exhibit isotropic electric properties. Lead telluride's energy gap is 0.3 eV and its maximum temperature of operation is considerably higher than that of the bismuth telluride family. It can be made to be n- or p-type as a result of departure from stoichiometry, but to optimize the Seebeck coefficient, doping is necessary. As with bismuth telluride, doping is achieved by the addition of halogens to produce n-type material or of alkali metals to produce p-type materials. Materials developed by the 3M company are identified by designations such as TEGS-2N. This indicates a specific semiconductor composed of essentially a compound of lead and tellurium with small additions of an electrically active impurity (0-03 mol% PbI 2 in order to provide the desired concentration of electrical carriers (n-type), while TEGS-3N is doped n-type with 0.055mo1% PbI 2. On the other hand, - 2 P signifies that the lead telluride is doped with sodium to give p-type material while 3 P signifies a lead-tin telluride combination doped with sodium and manganese. The lattice thermal conductivity can be reduced by the addition of SnTe and this reduction more than offsets any increase in electrical resistivity that has resulted from a fall in carrier mobility. Unfortunately, the improvement is offset by a reduction in energy gap, and the material is optimized at a composition of Pbo.vsSno.25Te. la More complex materials such as TAG-85 (silver-tin-tellurium) and TAGS (tellurium-antimony-germanium-silver) have also been developed. 19

Recent developments in thermoelectric materials -300

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(iii) Silicon-germanium alloys Although individually silicon and germanium are of no use in thermoelectric applications because of their high values of 2 e, an alloy of the two displays a marked reduction in 2 e and exhibits a broad maximum in 1/2 near the middle of the alloy system. The melting point and band

144

D. M . Rowe

gap of the alloy increase with silicon content and consequently alloys of composition Sia 5Ge 15 and Si 70Ge 30 have been extensively studied. 20 The maximum of the Seebeck coefficient occurs at the composition where the conduction bands of silicon and germanium become equienergetic. 8 As can be seen from Fig. 1 these materials are amongst the best for high-temperature operation. In order to approach optimum material performance, the silicon-germanium alloys are doped to the solid solubility limits of the dopants within the alloys. The dopants possess retrograde characteristics, which are strongly temperature dependent. Consequently, the amount of dopant in solution at a particular temperature can exceed the solid solubility limit at that temperature and the dopant precipitates out as a function of time. 21 This can significantly affect the thermoelectric properties. An overview of the effect of phosphorus precipitation on the room temperature Seebeck coefficient during isothermal heat treatment of 5i63.5Ge36. 5 alloy is shown in Fig. 2. 22 Considerable success has been enjoyed in applying the LifshitzSlyozov model to the precipitation process and the long-term behaviour of the thermoelectric transport properties of silicon-germanium alloys can be predicted from the results of relatively short-term heat treatment. 23 3.

NEW MATERIALS

(i) Improved silicon-germanium alloys The relatively low figures of merit of silicon-germanium alloys are attributed to their relatively high thermal conductivities. In semiconductors, even when heavily doped, the lattice component of the thermal conductivity predominates and attempts have, and are, being made to reduce 2 L through the introduction of further disorder in the lattice. A variety of methods have been tried such as the addition of impurity atoms, creation of vacancies at lattice sites, introduction of defects and increasing the weight of the lattice components. Usually these methods result in an increase in the electrical resistivity. Thus any reduction in thermal conductivity must overcompensate for the increase in electrical resistivity if the figure of merit of a material is to be improved. Two methods have been developed which reduce preferentially the lattice thermal conductivity: (a) The addition of small amounts of Group III-V elements. (b) Use of very small grain-size material.

Recent developments in thermoelectric materials

145

Significant reductions in the thermal conductivity of silicon-germanium alloys accompany the additions of small amounts of Groups III and V elements. 24 The symbolic notation for modified p-type material, for example, is (p-SixGe 1 _ x) (Ill-V). Gallium phosphide is isostructural and isoelectronic with silicon-germanium alloys and it has a lower vapour pressure than other Group III-V compounds at higher temperattures. Consequently, attention has concentrated on this material. The reduction in lattice thermal conductivity is greater when the GaP content is varied between 4 and 8 mol%. The thermal conductivity of n- and ptype 80 atomic% Si-20 atomic% Ge alloy to which an unspecified amount of gallium phosphide has been added is displayed in Figs 3 and 4 as a function of temperature. Evidently reductions in thermal conductivity of up to 50% compared with that of the unmodified alloy have been achieved. Generally the electrical resistivity of SiGe-GaP is higher than that of the unmodified alloy when in the unprepared state. High-temperature annealing, however, significantly reduces the electrical resistivity and after long annealing times (ca. 5000 hours) attains values comparable with those for unmodified alloys. Peculiar effects have, however, been noted during the annealing of boron-doped SiGe-GaP alloy. On preparation at 1400K, the modified alloy exhibits n-type Seebeck polarity, which vanishes, typically after annealing for 400 hours. Further annealing slowly restores its expected p-type character. The anomalous behaviour is attributed to cross-doping effects. A comprehensive investigation of the properties of SiGe-GaP material, as well as theoretical studies of phenomena such as cross doping, have been carried out at the Syncal laboratories. 25"26 The lattice thermal conductivity of silicon-germanium alloys can also be reduced by employing small grain-size material, 2~ an effect attributed to phonon-grain boundary scattering. In semiconductor alloys, whose constituent elements have large differences in atomic masses, the shortwavelength phonons are scattered by alloy disorder, resulting in the heat being carried by phonons of long wavelength. These longer wavelength phonons are effectively scattered by grain boundaries. In a Si-Ge alloy compact with a grain size of 5 pm, a reduction in the lattice thermal conductivity of 28% compared with a single-crystal alloy is observed. At 1000K this reduction is about 35%. Preliminary measurements of electrical resistivity and Seebeck coefficient indicate that their properties do not change with grain size over the range indicated. The thermoelectric figure of merit and conversion efficiency are substantially higher for small grain-size material as shown in Figs

D. M. Rowe

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Recent developments in thermoelectric materials

147

5 and 6, respectively. 2s Information on the long-term behaviour of small grain-size material is scant. Preliminary results at UWIST indicate that some grain growth accompanies an extended period of heat treatment at high temperatures, although in very heavily doped samples, grain growth appears to be inhibited by precipitated dopant. It has also been reported that the addition of GaP to Si-Ge alloy reduces grain growth. 2a

(ii) Improved lead telluride materials Attempts have been made to reduce the lattice thermal conductivity of lead telluride type material through the introduction of additives and through the use of small grain-size material. 3° Short-wavelength components of the phonon spectrum are reduced by the addition of GeTe to the n-type material and PbSe to the p-type. Recent measurements of material parameters at The General Electric Company indicate a potential increase of 40% in thermopile converter efficiency compared with standard PbTe material. In addition, this improved material has been life-tested for over 4000 hours at 823 K without any significant degradation in material parameters. 1

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D. M. Rowe

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The results of an analysis of the lattice thermal conductivity of lead telluride as a function of grain size, doping level and alloying 31 are shown in Fig. 7(a) and 7(b). It is evident that in moderately doped material, having a grain size of the order of 1/~m, the reduction in lattice thermal conductivity would be in the range 4-6% for unalloyed lead telluride and 11-13% for highly disordered alloys such as PbTe-SnTe or PbTe-GeTe.

(iii) Selenides The selenides employed in thermoelectric application have been given the designation TPM-217. Typical members of this class include p-type material composed of copper, silver and selenium in proportions similar

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150

D. M. Rowe

t o C u l . 9 7 A g o . o a S e l . o o 8 1 and n-type materials composed of gadolinium

and selenium with compositions similar to GdSel.49. Good thermoelectric properties are derived from a disordered lattice structure and a non-stoichiometric doping mechanism. The disordered lattice structure is advantageous in that it results in a short p h o n o n mean free path and a consequent low lattice thermal conductivity. It also results in these compounds existing over a wide range of compositions. The following relationships hold between extrinsic carrier concentration and composition: n h = 4 x 1028y holes/m 3 for Cu 1.97Ago.03Se1 +r (1) n~ = 3"33 x 1028yelectrons/m 3 for GdSel. 5_y

(2)

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Recent developments in thermoelectric materials

151

temperature. 32 Although it is the low thermal conductivities of the selenides which are largely responsible for their superior thermoelectric performances, information on this parameter is scant. 3a An interesting and exciting property of p-type TPM-217 results from the high mobility of a major constituent, copper. The relatively short response time of copper to electric and thermal fields results in a steady state being quickly reached in which the concentration of dopant increases significantly from the cold to the hot end. This effectively serves to optimize the average leg efficiency.34 The preparation of the gadolinium metal material with consistent properties presents a major problem in the use of this material. 35 In addition, although the selenides possess the highest figure of merit of any other known material in the 800 to 1200 K temperature range, considerable difficulties are encountered if operated below 550 K 3 6 for a variety of reasons. Consequently, attempts have been made to segment p-type material with BiSbTe and the n-type with 2N-PbTe. 37 The potential thermoelectric efficiency of paired thermoelectric materials is shown in Fig. 10.34 The characteristics of the thermoelectric properties of TPM-217 are incomplete. Nevertheless this material was chosen for use in the low-cost high-performance generator (LCHPG) technology programme initiated by the US Air Force. These materials were in

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Recent developments in thermoelectric materials

153

competition with silicon-germanium alloys for use in the Galileo and International Solar Polar missions. 38 Serious difficulties were encountered in producing stable selenide thermoelectrics and it has been confirmed that generators based upon silicon-germanium alloys will be used on these missions. 29

(iv) Boron-based materials A number of boron-based compounds possess physico-chemical properties which make them attractive candidates for thermoelectric applications. Attention has concentrated on r-boron; this is a semiconductor with an intrinsically low lattice thermal conductivity, 4° a band gap of 1.3 eV, a melting point of 2315 K, is mechanically strong and has a low vapour pressure ( < 1 0 - 1 1 a t m at 1473K). It has an open crystal structure which facilitates the introduction of interstitial dopant atoms. A systematic investigation into the doping of r-boron indicates that the most effective dopants are the transition metals used in the concentration range between 0.5 and 2.5 atomic°/o. 41 Both n- and p-type materials have been made with the addition of suitable dopants reducing the electrical resistivity and thermal conductivity. Copper is found to be the most effective p-type dopant. The addition of V, Cr or Cu reduces the room-temperature resistivity from around 104 Dm to around 10-2 f~m, while the addition of 1 atomic% Zr lowers the room-temperature thermal conductivity from 26 to 4.5 W m - l K - 1 The room-temperature electrical resistivity versus dopant for a number of dopants is shown in Fig. 11, 42 while in Fig. 12 is displayed the Seebeck coefficient versus composition. The temperature variation of the Seebeck coefficient for n- and p-type material is displayed in Fig. 13. Evidently the change from n- to p-type Seebeck coefficient in material doped with V and Cr makes them unsuitable for thermoelectric applications. Confining attention to p-type material, the thermal conductivity of r-boron doped with 1 atomic% copper is about 3W m-1 K over the temperature range 600 to 1300 K giving a corresponding figure of merit of about 7.1 × 10-SK -~. The figure of merit of silicongermanium alloy over the same range is about 9 x 1 0 - 4 K -1. The property responsible for the relatively low figure of merit is the low electrical conductivity and specifically the low carrier mobility, i.e. less than 10-4m 2 V -1 s-1. It is thought that conduction in r-boron takes place by polaron hopping, and it has been suggested that a possible way

154

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of increasing the mobility is by doping with more than one element, the intention being to decrease the distance between the dopant-occupied lattice sites.

(v) Boron-carbon Boron--carbon compositions hold out prospects for applications at temperatures above 1473K. 43 At present, developments are directed towards improving the performance of these materials over the temperature range 773-1373 K for possible replacement of the silicon-germanium alloys. Typically thermoelectric parameter values at 1273 K are: Seebeck coefficient 250pV K-1; electrical resistivity 3.5 x 10-5 ~m; and thermal conductivity 5 W m -1 K - l ; so giving a figure of merit of around 0.30"4 × 10 - 3 K - I. Examination of structural models of boron-carbon

Recent developments in thermoelectric materials

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156

D.M.

Rowe

composition suggest that the maximum electrical conductivity occurs in B~3C2and that high carbon compositions such as B4C should be avoided; emphasis has concentrated on B9C composition. Attempts are being made to increase the thermoelectric figure of merit by reducing the thermal conductivity and the electrical resistivity. It was anticipated that, in common with the majority of thermoelectric semiconductors, the thermal conductivity could be reduced by alloying; dissolving a few per cent silicon in a B9C composition had the opposite effect (Fig. 14).44 Some success in reducing the thermal conductivity has been achieved by producing material less-chemically homogeneous on a microscopic scale. Other possibilities being explored are preparation by chemical vapour deposition and the pyrolysis of organoboron precursors. Preliminary results indicate that impurity doping can occur in boron-carbon compositions and the addition of calcium is observed to reduce both the Seebeck coefficient and the electrical resistivity.

(vi) The lanthanum sulphides Stoichiometric compounds of sulphur and lanthanum are formed with the composition LaS, La3S 4, L2S 3 and LaS 2. Their electrical properties range from metallic LaS and LaaS 4 to the insulating La2S 3 and LaS 2. Compounds of composition LaS R where 1.33 < R < 1.50 are semiconductors and can be prepared by pressure-assisted reaction q ~

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Recent developments in thermoelectric materials

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sintering *s and form a continuous series of solid solutions. The electron concentration is determined by the stoichiometry of the compound and varies from a maximum of 6.03 x 1027m -3 for LaS1.33 to zero for LaSt.s0 as shown in Fig. 15. *6 This self-doping feature of the lanthanum sulphides, together with their low thermal conductivities (1.2 to 2 - 0 W m - I K -t) and high melting point (intermediate between the 2393 K of La3S , and 2223 K of La2S3) makes them attractive for hightemperature thermoelectric applications. Measurements on the non-stoichiometric LaS R with 1 " 3 3 < R < 1.5046'47 indicate that the electrical resistance and Seebeck coefficient increase with temperatures from 473 to 1373 K. The power factor, defined as the ratio Seebeck coefficient/electrical resistivity, generally increases with temperature and as the compound composition is varied from LaSI.,s to LaS~.35. Combining the power factor with estimates of the thermal conductivity values for LaS1.38 and LaSt.,1 gives figures of merit greater than 0.5 × 10 -3 at 1273K.

(vii) Ytterbium sulphides These are one of the few rare-earth sulphides which possess a p-type electrical conductivity. The low thermal conductivities of some of I La

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1.36

1.38

1.40

1.42

1.44

1.46

1.48

1.50

Electron concentration versus S/La ratio for the lanthanum sulfides (after Danielson et al., ref. 46).

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the sulphides, for example YbSI.as has a thermal conductivity of 1 . 1 3 W m - l K -1 at room temperature to 0.58Win - I K -~ at 1000K, makes them possible candidates for high-temperature thermoelectric applications. The preparation and measurement of the electrical conductivity and Seebeck coefficient of binary ytterbium sulphides and ternary ytterbium-calcium-barium and vanadium sulphides have been reported. 4s The electrical conductivity of members such as Yb2S a can be increased by four or five orders of magnitude, i.e. from 10- 2 f~- 1 m - 1 to 10311-1 m-~ and the Seebeck coefficient increased by the addition of 5 atomic% of Ti, Zr, Hf or Nb. However, a more systematic investigation of the effect of alloying transition metals with the ytterbium sulphides is required before an assessment can be made of its thermoelectric potential.

4.

CONCLUSIONS

Lead telluride and silicon-germanium alloys are the established thermoelectric materials currently being investigated with the view to improving their thermoelectric figures of merit. The reported reduction in thermal conductivity of lead telluride due to the introduction of additives into the crystal structure and consequent very substantial (up to 40%) increase in efficiency represents a significant breakthrough in lead telluride technology. Reducing the thermal conductivity through the use of small grain-size lead telluride is less dramatic with the 15 % reduction predicted for highly disordered alloys with a grain size of around 0.25/~m, about the largest attainable before being accompanied by an unwanted reduction in carrier mobility. The reduction in the thermal conductivity of Si-Ge of between 4050%, depending upon the particular starting alloy, which is achieved by the addition of gallium phosphide, is not accompanied by a change in the Seebeck coefficient. However, there does appear to be a problem in obtaining electrical conductivity values similar to those of normal silicon germanium. The annealing process employed in achieving this objective can be complicated and considerably more investigations are needed to obtain consistent results. The use of small grain size, however, can reduce the lattice thermal conductivity by 28% (5/~m grain size) without a change in the other electrical parameters being observed to date. Considerably higher figures of merit are feasible using small

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grain material but information is required on the long-term stability of these materials. A new class of thermoelectric materials, the selenides, was developed during the 1970's. Their relatively high-conversion efficiency of 10% was the prime motivation for their initial consideration for use in unmanned spacecraft. Unfortunately, this family of materials is sensitive to oxygen and water vapour and needs to operate in an inert atmosphere. Following their rejection in 1979 in favour of the known technology of the silicon-germanium alloys for use in the NASA Galileo Mission, interest in these materials has waned, although the US Department of Energy has investigated whether advances can be made using selenide material segmented with established material. 49 fl-Boron is capable of operating at very high temperatures, but both n- and p-type material doped with single elements possess figures of merit less than silicon-germanium alloys. However, its competitiveness may improve through the use of mixed dopants to increase the carrier mobility. Recent results report that a sample of composition BlooVNi has a room-temperature Seebeck coefficient of - 7 0 # V K - 1 and an electrical conductivity of 2-3 x 103 ~ - 1 m - 1 compared with the values for a sample of composition Bl0oV 2 which are - 7 6 # V K - t and 3"8 x 1 0 3 ~ - 1 m - 1 Boron-carbon composites are also capable of operating at temperatures above 1500 K; the present effort is directed at developing these materials as replacements for silicon-germanium alloys. It seems unlikely that the necessary two-fold increase in figure of merit required for them to effectively substitute for silicon-germanium will be achieved. The rare-earth sulphides have the potential for operating at high temperatures (1300 K or more) with a high figure of merit: both n- and p-type doping is possible and their stabilities at the intended temperatures of operation appear satisfactory. However, they are at an early stage of development. In conclusion, it is likely that the established thermoelectric materials will remain the main stay of military and space applications for some time to come. Approximately ten years of concentrated effort was needed to develop silicon-germanium alloys to the stage when devices could be fabricated and their long-term performance guaranteed. Investigation into the use of fl-boron and boron-carbon composites commenced in the early 1980's, and the associated technology of operating devices at higher temperatures will present additional problems. Modification of

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the established material either through the use of additives, small grain sizes or combination of both holds out most promise. An evaluation programme for the performance of bicouples based on S i G e / G a P and fine-grained technology is underway, and preliminary results are encouraging 5° although there do appear to be problems encountered with bonding the fine-grained materials.

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