Method for producing a thermoelectric object for a thermoelectric conversion device

Abstract

A method for producing a thermoelectric object for a thermoelectric conversion device is provided. A starting material which has elements in the ratio of a half-Heusler alloy is melted and then cooled to form at least one ingot. The ingot is homogenized at a temperature of 1000 C. to 1400 C. for a period of time t, wherein 0.5 ht<12 h or 24 h<t<100 h. The homogenized ingot is crushed and ground into a powder. The powder is cold-pressed and sintered at a maximum pressure of 1 MPa for 0.5 to 24 h at a temperature of 1000 C. to 1500 C.

Claims

1. A method for producing a thermoelectric object for a thermoelectric conversion device, comprising: providing a starting material which has elements in the ratio of a half-Heusler alloy described by the formula , wherein is one or more of the group consisting of Sc, Ti, V, Cr, Mn, Y, Zr, Nb, La, Hf, Ta and one or more of the rare earths, is one or more of the group consisting of Fe, Co, Ni, Cu and Zn, is one or more of the group consisting of Al, Ga, In, Si, Ge, Sn, Sb and Bi, and the sum of the valence electrons is between 17.5 and 18.5, melting of the starting material and subsequent cooling to form at least one ingot, heat treating of the ingot at a temperature of 1000 C. to 1400 C. for a period of time t, wherein 0.5 ht<12 h or 24 h<t<100 h, in order to produce a homogenized ingot, crushing the homogenized ingot to produce a crushed ingot, grinding the crushed ingot, whereby a powder is produced, the powder having an average value and median particle size distribution of less than 10 m, cold pressing the powder with a pressure of 1 t/cm.sup.2 to 10 t/cm.sup.2, whereby a green body is produced, the green body having a density ranging from 62% to 68% of a theoretical density of the green body, sintering of the green body for 0.5 to 24 h at a temperature of 1000 C. to 1500 C. under a maximum pressure of 1 MPa, whereby a thermoelectric object is produced.

2. The method according to claim 1, wherein the half-Heusler alloy has a composition of Ni.sub.1-y.sub.ySn.sub.1-z.sub.z, wherein is one or more of the group consisting of Zr, Hf and Ti, is one or more of the group consisting of Fe, Co, Cu and Zn and is one or more of the group consisting of Al, Ga, In, Si, Ge, Sb and Bi, wherein 0y0.9 and 0z0.3.

3. The method according to claim 1, wherein the half-Heusler alloy has a composition of Co.sub.1-y.sub.ySb.sub.1-z.sub.z, wherein is one or more of the group consisting of Zr, Hf and Ti, is one or more of the group consisting of Fe, Ni, Cu and Zn and is one or more of the group consisting of Al, Ga, In, Si, Ge, Sn and Bi, wherein 0y0.9 and 0z0.3.

4. The method according to claim 1, wherein the half-Heusler alloy has a composition based on XNiSn or XCoSb, wherein X is one or more of the elements of the group consisting of Zr, Hf and Ti.

5. The method according to claim 4, wherein the half-Heusler alloy is XNiSn and a proportion of Sn is replaced by Sb.

6. The method according to claim 4, wherein the half-Heusler alloy comprises Ti and Zr and Hf.

7. The method according to claim 1, wherein elements in the ratio of 0.25 Zr:0.25 Hf:0.5 Ti:1 Ni:0.998 Sn:0.002 Sb or 0.5 Zr:0.5 Hf:1 Co:0.8 Sb:0.2 Sn are provided as the starting material.

8. The method according to claim 1, further comprising casting of the melted starting material into an ingot.

9. The method according to claim 1, wherein the ingot is first cooled below 1000 C. and then homogenized.

10. The method according to claim 1, wherein the ingot is cooled from the temperature of the melted starting material to a temperature of 1000 C. to 1400 C., at which temperature the ingot is homogenized.

11. The method according to claim 1, wherein the starting material has a weight of at least 5 kg.

12. The method according to claim 1, wherein the ingot is crushed by means of a jaw breaker.

13. The method according to claim 1, wherein the crushing of the homogenized ingot to a powder is carried out by means of a mill, wherein a proportion of powder forms in a sieve and this proportion of powder is ground in a further grinding process.

14. The method according to claim 1, wherein material is crushed by means of a disc mill.

15. The method according to claim 1, wherein the ingot is crushed to a coarse powder, the coarse powder is ground to a fine powder in a further grinding process, and the fine powder is cold-pressed.

16. The method according to claim 15, wherein the further grinding process is carried out by means of a planetary ball mill or a jet mill.

17. The method according to claim 1, wherein the powder is mixed after each passage through a mill.

18. The method according to claim 17, wherein the coarse powder or the fine powder is mixed using rotation, translation and inversion.

19. The method according to claim 1, wherein the starting material is melted by means of vacuum induction melting.

20. The method according to claim 1, wherein the green body is sintered in an inert gas or in a vacuum.

21. The method according to claim 1, wherein the ingot is homogenized under argon or in a vacuum.

22. The method according to claim 1, wherein the ingot is heat-treated at a temperature of 1050 C. to 1180 C. for a period of time t, wherein 0.5 ht<12 h.

23. The method according to claim 1, wherein the thermoelectric object is processed into a plurality of working components by means of sawing and/or grinding processes.

24. The method according to claim 23, wherein the sawing process is performed by means of wire sawing, center hole sawing, wire spark erosion, water jet cutting and/or laser cutting.

25. The method according to claim 23, wherein the grinding process is performed by means of disc grinding, double disc grinding, belt grinding and/or with a flat grinding machine.

26. The method according to claim 1, wherein heat treatment conditions of the homogenization step are selected such that after the homogenization step, no reflexes of foreign phases can be seen in the homogenized ingot in a theta 2 theta diffractogram.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a light-micrograph and an X-ray diffractogram of a cast ingot from Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 according to a first embodiment.

(2) FIG. 2 shows a graph of the density of sintered samples from the cast ingot of the first embodiment as a function of sintering temperature.

(3) FIG. 3 shows an X-ray diffractogram of a homogenized cast ingot from Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 according to a second embodiment.

(4) FIG. 4 shows a graph of the density of sintered samples from the cast ingot of the second embodiment as a function of sintering temperature.

(5) FIG. 5 shows an X-ray diffractogram of a sample of the second embodiment sintered at 1190 C.

(6) FIG. 6 shows an X-ray diffractogram of a first comparative example.

(7) FIG. 7 shows a graph of the density of sintered samples of the first comparative example.

(8) FIG. 8 shows a graph of the density of sintered samples from Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 according to a third embodiment as a function of sintering temperature and sintering time.

(9) FIG. 9 shows a graph of the density of sintered samples according to a second comparative example as a function of sintering temperature and sintering time.

(10) FIG. 10 shows a graph of the density of sintered samples from Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 according to a third embodiment as a function of sintering temperature.

(11) FIG. 11 shows a graph of the density of sintered samples from Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 according to a fourth and fifth embodiment as a function of sintering temperature and sintering time.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(12) Various methods for producing a thermoelectric object suitable for a thermoelectric conversion device will now be described which are suitable for industrial scale production. In particular, a starting material is melted by means of vacuum induction melting (VIM) and then cast to produce a cast ingot. The cast ingot produced in this way is crushed and ground in several steps to produce a powder from the cast ingot. The powder is cold-pressed and sintered to produce sintered samples. These sintered samples can be used as thermoelectric components in thermoelectric conversion devices. In a further embodiment, the sintered samples are processed further in order to match their shape to the application. The samples are, for example, sawn to produce a plurality of working components from the sample.

(13) The thermoelectric object is formed from a half-Heusler alloy. Described below are four embodiments and two comparative examples with a half-Heusler alloy of a (Zr, Hf, Ti) NiSn-type and four embodiments with a half-Heusler alloy of a (Zr, Hf, Ti) CoSb-type.

Embodiment 1

(14) An ingot of the half-Heusler alloy with a composition of Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 is produced by means of vacuum induction melting. For charging the furnace, the elements are weighed in in accordance with their proportions in the alloy. The furnace is evacuated to a vacuum of 0.1 mbar or better and then heated up. After complete melting of the complete charge, an atmosphere of 800 mbar argon is set, and the melt is cast into a copper mold.

(15) A portion of the ingot produced in this way is pre-crushed by means of a jaw crusher and then processed in a disc mill into a coarse powder with a maximum particle size of 315 m. For this purpose, the material to be ground passes through the disc mill in several steps. After each step, the fraction of the powder with a particle size of less than 315 m is separated out by means of a sieve with a mesh width of 315 m. The fraction which does not pass through the sieve passes through the disc mill with a reduced grinding gap in the next step, until the whole of the material is present as a powder with a particle size of less than 315 m. The powder is then mixed in a Turbula mixer for 30 minutes and thereby homogenized.

(16) To analyze the alloy in the cast state, light microscope micrographs of the cast structure and an X-ray diffractogram of the ground casted ingot after its passage through the disc mill were produced. These are shown in FIG. 1, wherein a light microscope micrograph of the structure and a corresponding X-ray diffractogram (Cu-Ku-radiation) of the cast ingot from the alloy Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 produced in embodiment 1 are shown. In the X-ray diffractogram, the Miller indices mark the reflexes of the half-Heusler phase. The arrows indicate reflexes associated with foreign phases.

(17) As can be seen in FIG. 1, the cast structure contains several foreign phases in addition to the matrix of the half-Heusler phases. By way of example, FIG. 1 shows foreign phases consisting of Hf-rich inclusions (dark grey) and islands of non-dissolved Sn (white).

(18) The existence of the foreign phases can also be seen in the X-ray diffractogram of FIG. 1. In the angle range between 30<2<40, the reflexes associated with foreign phases are marked by arrows. The remaining reflexes can be assigned to the half-Heusler phase in accordance with the indices specified in FIG. 1. Here, all indexed reflexes have a shoulder in the direction of higher diffraction angles. This shoulder is due to a reflection split, because the half-Heusler alloy of the composition Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 is segregated into at least two half-Heusler phases of different lattice constants, as disclosed in U.S. Pat. No. 7,745,720. The resulting splitting of the reflexes can also be seen in the following diffractograms.

(19) To produce the sintered samples, a portion of the coarse powder with the particle size 315 m is ground further in a planetary ball mill to a finer powder with a median particle size distribution of 2 m. This powder was in turn mixed in a Turbula mixer for 30 minutes and homogenized. Subsequently, it was pressed under a pressure of 2.5 t/cm.sup.2 into cylindrical green bodies with a diameter of approximately 9 mm.

(20) The density of the green bodies obtained in this way amounted to 62% of theoretical density. The green bodies were sintered in a furnace under argon as inert gas at temperatures between 1140 C. and 1220 C. The furnace was heated here to sintering temperature at a heating rate of 10 K/min, and the dwell time at sintering temperature was 1 hour.

(21) FIG. 2 shows a graph of the density of sintered samples from the cast ingot of the first embodiment as a function of sintering temperature. As shown in FIG. 2, a dense sintered body with a density of more than 95% of the theoretical density of 8.23 g/cm.sup.3 can be achieved by the method described at a sintering temperature of 1220 C.

Embodiment 2

(22) A cast ingot of the half-Heusler alloy with the composition Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 is produced by means of vacuum induction melting as described above in connection with the first embodiment.

(23) In this second embodiment, the portion of the cast ingot which is to be processed further is first aged for 24 hours at 1050 C. in an argon atmosphere for homogenization. The processing of the aged cast ingot into powder then takes place as described above in connection with the first embodiment.

(24) To assess the effectiveness of the aging treatment and heat treatment, an X-ray diffractogram as shown in FIG. 3 is produced from the powder with a particle size of 315 m as produced by the disc mill. The indices mark the reflexes of the half-Heusler phase.

(25) It is apparent from FIG. 3 that, after aging, no foreign phases can be detected by means of X-ray diffraction. Apart from the reflexes associated with the half-Heusler phase, no further reflexes are visible, in particular in the angle range between 30<2<40.

(26) The powder produced from the aged casted ingot is pressed under a pressure of 2.5 t/cm.sup.2 into rectangular green bodies with dimensions of approximately (17.210.45) mm.sup.3. The density of the green bodies is 62% of the theoretical density.

(27) Samples are sintered at various temperatures and, afterwards, the density of the sintered samples is determined. The method described in connection with the first embodiment is used for sintering at various temperatures. The results of the sintering processes are represented in FIG. 4, which shows a graph of the density of the sintered samples as a function of sintering temperature.

(28) It can be seen from FIG. 4 that, through the aging of the cast ingot before crushing and grinding into powder, the sintering temperature required for obtaining a density of more than 95% of the theoretical density can be reduced by 40 C. to 1180 C. as compared to the first embodiment, which is not heat-treated for homogenization.

(29) An X-ray diffractogram as shown in FIG. 5 is taken for a sample produced in accordance with the second embodiment and which is sintered at 1190 C. Likewise, no foreign phases can be observed in the sintered sample by means of the X-ray diffractogram; the reflexes which can be seen can be attributed to the half-Heusler phase.

(30) The method having a heat treatment for homogenizing the cast ingot before processing into a powder is therefore capable of producing dense and phase-pure sintered bodies of the half-Heusler alloy made from the powder. Furthermore, the sintering temperature at which a density of at least 95% of the theoretical density is achieved is reduced. This can reduce manufacturing costs.

Comparative Example 1

(31) A cast ingot of the half-Heusler alloy with the composition Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 is produced as in embodiments 1 and 2 by means of vacuum induction melting.

(32) The portion of the cast ingot which is to be processed further is subjected to a homogenization treatment at a temperature below 1000 C., namely at 900 C., for 72 hours in an argon atmosphere.

(33) The cast ingot aged in this way is processed into a powder with a particle size of 315 m by means of a jaw crusher and a disc mill. The X-ray diffractogram shown in FIG. 6 is produced from this powder. Although the intensity of the foreign phase reflexes already found in the cast state, which can be seen in FIG. 1, is reduced, the reflexes are nevertheless still clearly detectable. In contrast to an aging treatment at temperatures above 1000 C., aging treatment at temperatures below 1000 C. does not result in a complete homogenization of the half-Heusler alloy. Even at the longer aging time of 72 hours, a complete homogenization of the half-Heusler alloy is not achieved.

(34) The coarse powder from the disc mill is, as in the first embodiment, processed further into a finer powder with a median particle size distribution of 2 m and then pressed and sintered.

(35) Samples are sintered at various temperatures and, afterwards, the density of the sintered samples is determined. The results of the sintering processes are represented in FIG. 7, which shows a graph of the density of the sintered samples as a function of sintering temperature.

(36) It is apparent from FIG. 7 that the aging treatment at 900 C. only allows the sintering temperature to be reduced by less than in the case of the second embodiment, regardless of the longer aging period. Constantly dense samples with a density above 95% of the theoretical density are only obtained at sintering temperatures from 1200 C. The density obtained at 1180 C., in contrast, varies strongly, with samples greater than 95% and less than 90% in outliers. Compared to this, the sintering temperature can reliably be reduced to 1180 C. in the case of the second embodiment, as a result of the 24 h aging process at 1050 C.

Embodiment 3

(37) An ingot of the half-Heusler alloy with the composition Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 was produced as in the context of the second embodiment, aged and processed into a powder. The powder is pressed with a pressure of 6.3 t/cm.sup.2 into rectangular green bodies with dimensions of approximately (17.210.44) mm.sup.3.

(38) The density of the green bodies obtained in this way amounted to 68% of theoretical density. The subsequent sintering process was carried out as in the context of the first and second embodiments. The samples were sintered for 1 hour or 4 hours. The results of the sintering are represented in FIG. 8, which shows a graph of the density of the sintered samples as a function of sintering temperature and sintering time.

(39) At a sintering time of one hour, the increased pressure did not lead to an improved result compared to the second embodiment, see FIG. 4, i.e. a sintering temperature of 1180 C. is still required for achieving samples with a density of above 95% of the theoretical density.

(40) In contrast, at a sintering time of 4 hours, sintered bodies with a density greater than 95% of the theoretical density were obtained at a temperature of 1140 C. The sintering temperature can therefore be reduced further by at least 40 C., compared to the first embodiment, which is not heat-treated for homogenization and which in which a lower pressure is used in the cold pressing process.

Comparative Example 2

(41) Green bodies from the second embodiment, which are produced with a pressure of 2.5 t/cm.sup.2 and have a green density of 62% of the theoretical density, are, in addition to the sintering time of one hour used in the second embodiment, likewise sintered for four hours. The results of these tests are shown in FIG. 9. In contrast to the samples from the third embodiment, which are produced at an increased pressure, the extending of the sintering time did not result in a sufficiently dense sintered body at sintering temperatures below 1180 C. From the comparative example 2, it can thus be concluded that the increased pressure is responsible for this effect and that this effect can therefore not be attributed to the extending of the sintering time alone.

Embodiment 4

(42) In a fourth embodiment, the sintering process takes place in a vacuum.

(43) An ingot of a half-Heusler alloy with the composition Zr.sub.0.25Hf.sub.0.25Ti.sub.0.5NiSn.sub.0.998Sb.sub.0.002 is produced as in the context of the second embodiment, aged, processed into a powder and pressed into green bodies. The sintering of the green bodies is carried out for one hour in a vacuum at a maximum pressure of 510.sup.2 mbar. The results of the sintering tests are shown in FIG. 10. It can be seen therefrom that, through sintering in a vacuum, even higher densities of 99% of the theoretical density can be achieved in comparison with sintering under argon, see here FIG. 4. Furthermore, these high densities can be produced at a sintering temperature of 1140 C., which has again been reduced by up to 40 C.

(44) XCoSb-based half-Heusler alloys are also produced in processes which are suitable for industrial-scale production.

Embodiment 5

(45) A cast ingot of a half-Heusler alloy with the composition Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 is accordingly produced by means of vacuum induction melting as in the context of the first embodiment and processed into a powder with a median particle size distribution of approximately 2 m. This powder is pressed in a tool press with a pressure of 2.5 t/cm.sup.2 into rectangular green bodies with dimensions of approximately (17.210.45) mm.sup.3 and a green density of 62% of the theoretical density.

(46) The subsequent sintering is carried out with argon as inert gas at temperatures of 1350 C. and 1400 C. for 30 minutes and one hour, at a ramp rate of 10 K/min. As the results in Table 1 show, a dense sintered body with a density of more than 95% of the theoretical density of 9.16 g/cm.sup.3 is obtained at a temperature of 1400 C.

(47) TABLE-US-00001 TABLE 1 Sintering Sintering Sample Time Temperature Density 1 30 Min 1400 C. 95.4%, 2 30 Min 1400 C. 95.0%, 3 30 Min 1400 C. 95.2%, 4 1 h 1350 C. 93.9%, 5 1 h 1400 C. 95.8%,

(48) Table 1 shows the density of the alloy Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 according to embodiment 4 after sintering at various temperatures and dwell times. The density is specified as a percentage of the theoretical density of 9.16 g/cm 3.

Embodiment 6

(49) In the sixth embodiment, the cast ingot of Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 is subjected to a heat treatment for homogenization. The half-Heusler alloy with the composition Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 is produced as in the context of the fifth embodiment. In accordance with the second embodiment, however, the cast ingot is aged in an argon atmosphere for 24 hours at 1100 C. for homogenization before processing into powder.

(50) After the powder has been pressed into green bodies, these are sintered for 30 minutes at various temperatures with argon as inert gas. The results of the sintering trials compared to the data from embodiment 5 are shown in FIG. 11.

(51) For the half-Heusler alloy with the composition Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2, the aging treatment carried out on the cast ingot likewise results in a significantly increased sintering ability. Even at sintering temperatures of 1340 C., dense sintered bodies with a density greater than 95% of the theoretical density can be obtained as a result of the aging, while without aging, this result would require a temperature of 1400 C. Furthermore, significantly higher densities than in embodiment 5 are obtained in the temperature range between 1360 C. and 1420 C.

(52) TABLE-US-00002 TABLE 2 Theoretical Sintering Sintering Density Time Temperature Density Embodiment Compound (g/cm.sup.3) (Min) ( C.) Obtained 7 Ti.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 8.9 30 1300 97.1%, 30 1320 98.2%, 30 1340 98.3%, 8 Ti.sub.0.5Zr.sub.0.5CoSb.sub.0.8Sn.sub.0.2 7.7 30 1300 96.3%, 30 1320 97.8%, 30 1340 98.1%,

Embodiment 7

(53) In the seventh embodiment, a cast ingot of a half-Heusler alloy with the composition Ti.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 is produced by means of vacuum induction melting, aged for 24 h at 1100 C. for homogenization, processed into a powder and pressed into green bodies. The green bodies were then sintered for 0.5 hours at 1300 C., 1320 C. and 1340 C. The composition and the measured density of these samples are shown in Table 2. For each temperature, the method described produces sintered bodies with a density of more than 95% of the theoretical density. This density increases with sintering temperature, so that at a temperature of 1340 C., the sample has 98.2% of the theoretical density.

Embodiment 8

(54) In the eighth embodiment, a cast ingot of a half-Heusler alloy with the composition Ti.sub.0.5Zr.sub.0.5CoSb.sub.0.8Sn.sub.0.2 is produced by means of vacuum induction melting, aged for 24 h at 1100 C. for homogenization, processed into a powder and pressed into green bodies. The green bodies were then sintered for 0.5 hours at 1300 C., 1320 C. and 1340 C. The composition and the measured density of these samples are shown in Table 2. For each temperature, the method described produces sintered bodies with a density of more than 95% of the theoretical density. This density increases with sintering temperature, so that at a temperature of 1340 C., the sample has 98.4% of the theoretical density.

(55) In embodiments 9 and 10, a shorter time of 1 hour is used for homogenization. A density of above 95% of the theoretical density is obtained in the sintered object.

(56) TABLE-US-00003 TABLE 3 Theoretical Sintering Density Sintering Time Temperature Density Embodiment Compound (g/cm.sup.3) (Min) ( C.) Obtained 9 Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 9.2 4 1320 97.0%, 4 1340 98.9%, 4 1350 99.7%, 10 Zr.sub.0.4Hf.sub.0.6NiSn.sub.0.98Sb.sub.0.02 9.6 4 1320 94.8%, 4 1340 96.4%, 4 1350 97.2%,

Embodiment 9

(57) A cast ingot of the half-Heusler alloy with the composition Zr.sub.0.5Hf.sub.0.5CoSb.sub.0.8Sn.sub.0.2 is produced by means of vacuum induction melting, aged for 1 h at 1100 C. for homogenization, processed into a powder with a median particle distribution of approximately 5 m and pressed to form green bodies. The green bodies are subsequently sintered for 4 hours in a vacuum at a maximum pressure of 510.sup.2 mbar at different temperatures, namely 1320 C., 1340 C. and 1350 C. The densities of the sintered samples are shown in Table 3. Densities greater than 95% of the theoretical density can be obtained from a sintering temperature of 1320 C.

Embodiment 10

(58) A cast ingot of the half-Heusler alloy with the composition Zr.sub.0.4Hf.sub.0.6NiSn.sub.0.98Sb.sub.0.02 is produced by means of vacuum induction melting, aged for 1 h at 1100 C. for homogenization, processed into a powder with a median particle distribution of approximately 5 m and pressed to form green bodies. The green bodies are each subsequently sintered for 4 hours in a vacuum at a maximum of 510.sup.2 mbar at 1320 C., 1340 C. and 1350 C. The densities of the sintered samples are shown in Table 3. Densities greater than 95% of the theoretical density can be obtained from a sintering temperature of 1340 C.

(59) The manufacturing processes described herein are based on the use of industrial scale processing methods, such as vacuum induction melting, cold pressing and sintering. With these methods, half-Heusler alloys can be produced in industrial quantities. If the cast ingot is subjected to a heat treatment at a temperature between 1000 C. and 1400 C., the sintering temperature for the cold pressed samples, in which a high density is obtained, can be reduced. The sintering method may be carried out at a pressure of less than 1 MPa, for example without external pressure, in order to obtain a density in the sintered object of more than 95% of the theoretical density. Objects produced thereby are suitable for use as thermoelectric objects in thermoelectric conversion devices, as they can be produced cost-effectively in large quantities and in varying shapes by means of the production method described herein.