METHOD FOR PRODUCING A THERMOELECTRIC OBJECT FOR A THERMOELECTRIC CONVERSION DEVICE
A method is provided for producing a thermoelectric object for a thermoelectric conversion device in which a powder having a bulk density d s and elements in the ratio of a Half-Heusler alloy with a theoretical density d i is provided. The powder is mechanically compressed and a green body with a tap density d K is formed, the tap density d K being up to 30% of the theoretical density d i greater than the bulk density d s of the powder. The green body with the tap density d K is sintered at a temperature of 1000° C. to 1500° C. for 0.5 h to 100 h, a thermoelectric object with a density d s of greater that 95% and preferably 99% of the theoretical density d i being produced.
1 . A method for producing a thermoelectric object for a thermoelectric conversion device comprising:
providing a powder with a bulk density d s comprising elements in the ratio of a Half-Heusler alloy that is described by the formula αβχ and has a theoretical density d i , α being one or more of the elements in the group consisting of Ti, Zr and Hf, β being Co or Ni, χ being Sn and/or Sb, the composition being described by Zr a Hf b Ti c NiSn 1-d Sb d or Zr a Hf b Ti c CoSb 1-e Sn e , where 0≦a≦0.8, 0≦b≦0.8, 0≦c≦0.8, 0≦d≦0.1 and 0≦e≦0.3 and the sum (a+b+c)=1,
mechanically compressing the powder, wherein a green body with a tap density d K is formed, the tap density d K being up to 30% of the theoretical density d i higher than the bulk density d s of the powder,
sintering the green body with tap density d K at a temperature of 1000° C. to 1500° C. for 0.5 h to 100 h, wherein a thermoelectric object with a density d G greater than 95%, preferably greater than 99%, of the theoretical density d i is produced.
2 . A method for producing a thermoelectric object for a thermoelectric conversion device comprising:
providing a powder with a bulk density d s comprising elements in the ratio of a Half-Heusler alloy that is described by the formula αβχ and has a theoretical density d i , α being one or more of the elements in the group consisting of Sc, Ti, V, Cr, Mn, Y, Zr, Nb, La, Hf, Ta and one or more of the rare earth elements, β being one or more of the group consisting of Fe, Co, Ni, Cu and Zn, χ being one or more of the group consisting of Al, Ga, In, Si, Ge, Sn, Sb and Bi, and the sum of the valence electrons lying between 17.5 and 18.5,
mechanically compressing the powder, wherein a green body with a tap density d K is formed, the tap density d K being up to 30% of the theoretical density d i higher than the bulk density d s of the powder,
sintering the green body with tap density d K at a temperature of 1000° C. to 1500° C. for 0.5 h to 100 h, wherein a thermoelectric object with a density d G greater than 95%, of the theoretical density d i is produced.
3 . A method in accordance with claim 1 , wherein a plurality of green bodies is formed simultaneously by mechanical compression and simultaneously sintered.
4 . A method in accordance with claim 1 , wherein the powder is introduced into a mould and the powder in the mould has a bulk density d s where d s ≦40% of the theoretical density d i .
5 . A method in accordance claim 4 , wherein the powder is introduced continuously into the mould and compressed mechanically in order to fill the mould with a green body with a tap density d K .
6 . A method in accordance with claim 4 , wherein the mould is coated with a release agent before the powder is introduced into it.
7 . A method in accordance with claim 4 , wherein the mould comprises a ceramic or a refractory metal.
8 . A method in accordance with claim 4 , wherein the green body is sintered in the mould.
9 . A method in accordance with claim 4 , wherein the mould comprises at least one cavity in which the green body is formed.
10 . A method in accordance with claim 9 , wherein the cross section of the cavity has a clearance w≦6 mm.
11 . A method in accordance with claim 9 , wherein the cavity has a height h and a cross-sectional area A, where h≦0.2 √A.
12 . A method in accordance with claim 4 , wherein the mould comprises a plurality of cavities that are filled with the powder, and a plurality of green bodies are formed simultaneously by mechanical compression.
13 . A method in accordance with claim 1 , wherein the powder is compressed mechanically by means of tapping, shaking, vibration and/or ultrasound.
14 . A method in accordance with claim 1 , wherein the powder is further pressed at a pressure of less than 10 MPa during the mechanical compression.
15 . A method in accordance with claim 1 , wherein one or more additives are mixed into the powder.
16 . A method in accordance with claim 15 , wherein a stearate, a fatty acid and/or a liquid organic solvent is mixed in as the additive.
17 . A method in accordance with claim 1 , wherein the powder has a particle size D 50 of 6 μm to 150 μm.
18 . A method in accordance with claim 1 , wherein the green body is sintered in two steps and the green body is removed from the mould before the second step.
19 . A method in accordance with claim 18 , wherein the green body is heat treated at a temperature of 1000° C. to 1200° C. in a first step and sintered at a temperature of 1200° C. to 1500° C. in a second step.
20 . A method in accordance with claim 1 , wherein the green body is sintered under protective gas or a vacuum.
21 . A method in accordance with claim 1 , wherein the powder is compressed under protective gas or a vacuum.
22 . A method in accordance with claim 1 , wherein after sintering, the object has a cross section corresponding to that of the end cross section and the height of the object is adjusted by further processing.
23 . A method in accordance with claim 1 , further comprising:
melting a starting material comprising elements in the ratio of a Half-Heusler alloy and subsequently casting to form an ingot,
heat treating the ingot at a temperature of 1000° C. to 1200° C. for 0.5 h to 100 h to produce a homogenised ingot,
crushing the homogenised ingot,
milling the crushed ingot to form the powder.
24 . A method in accordance with claim 23 , wherein the ingot is processed to a powder in multiple stages.
25 . A method in accordance with claim 24 , wherein the ingot is crushed using a jaw crusher and then ground using a mill.
26 . A method in accordance with claim 23 , wherein the starting material is melted by means of vacuum induction melting.
27 . A method in accordance with claim 1 , wherein the powder is produced directly from a molten mass by a powder gas atomisation system.
28 . A method in accordance with claim 1 , wherein a starting material comprising elements in the ratio of a Half-Heusler alloy is melted, solidified using a rapid solidification technology and then ground to produce the powder.
29 . A method in accordance with claim 2 , wherein the Half-Heusler alloy comprises a composition of αNi 1-y β y S 1-z χ z , where α is one of more of the group comprising Zr, Hf and Ti, β is one or more of the group comprising Fe, Co, Cu and Zn and χ is one or more of the group consisting of Al, Ga, In, Si, Ge, Sn and Bi, where 0≦y≦0.9 and 0≦z≦0.3.
30 . A method in accordance with claim 2 , wherein the Half-Heusler alloy comprises a composition of αCo 1-y β y Sb 1-z χ z , where α is one of more of the group comprising Zr, Hf and Ti, β is one or more of the group comprising Fe, Ni, Cu and Zn and χ is one or more of the group consisting of Al, Ga, In, Si, Ge, Sn and Bi, where 0≦y≦0.9 and 0≦z≦0.3.
31 . A method in accordance with claim 2 , wherein the Half-Heusler alloy comprises a composition based on XNiSn or XCoSb, where X is one or more elements from the group comprising Zr, Hf and Ti.
32 . A method in accordance with claim 31 , wherein the Half-Heusler alloy comprises XNiSn and a proportion of the Sn being replaced by Sb.
33 . A method in accordance with claim 31 , wherein the Half-Heusler alloy comprises Ti and Zr and Hf.
34 . A method in accordance with claim 2 , wherein elements in the ratio 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 SB are provided as the starting material.
35 . A method in accordance with claim 2 , wherein the thermoelectric object produced has a density d G greater than 99%.
36 . A method in accordance with claim 1 , wherein the powder has a particle size D 50 of 6 μm to 10 μm.