Thermoelectric material

Abstract

Novel compounds with thermoelectric properties are presented. The novel compounds belong to the group of phosphides. They are characterized by excellent thermoelectric properties, in particularly in the temperature range of 400° C. to 700° C. Also a production method for the production of the compounds is presented, with which the thermoelectric substances can be prepared with high yield and quality.

Claims

1. A thermoelectric material with the formula R.sub.1-xMn.sub.xCuP, wherein x is at least 0.01 and at most 0.99 and R is selected from Mg, Ca, Ba, Sr and mixtures thereof.

2. The material according to claim 1, wherein x is not higher than 0.5.

3. The material according to claim 1, wherein x is at least 0.02.

4. The material according to claim 1, wherein R=Mg.

5. A method for the production of the material according to claim 1, comprising the following steps: providing the elements R, manganese, copper and phosphorus in a reaction vessel, evacuating the reaction vessel, heating the reaction vessel to a temperature of higher than 700° C., removal of the reaction product.

6. The method according to claim 5, with the further step of comminuting the reaction product, subsequent to the removal.

7. The method according to claim 5, wherein the reaction vessel is not heated to above 1000° C.

8. The method according to claim 5, wherein the reaction vessel is maintained at a temperature of higher than 700° C. for a period of time of at least 50 hours and at most 100 hours.

9. The method according to claim 5, with the further step of compacting the reaction product to a molded article, subsequently to the removal.

10. A use of a material according to claim 1 as thermoelectric substance.

11. The material according to claim 2, wherein x is not higher than 0.3.

12. The material according to claim 11, wherein x is not higher than 0.2.

13. The material according to claim 3, wherein x is at least 0.03.

14. The material according to claim 13, wherein x is at least 0.08.

15. The method according to claim 7, wherein the reaction vessel is not heated to above 850° C.

16. The method according to claim 6, wherein the comminuting is by grinding.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Shown are in

(2) FIG. 1 shows the result of a DSC/TG measurement on Mg.sub.0.90Mn.sub.0.10CuP;

(3) FIG. 2 shows the Seebeck coefficients of the synthesized compounds as a function of the temperature;

(4) FIG. 3 shows the electric conductivity of the synthesized compounds as a function of the temperature;

(5) FIG. 4 shows the thermal conductivity of the synthesized compounds as a function of the temperature;

(6) FIG. 5 shows the zT factors of the synthesized compounds as a function of the temperature.

DETAILED DESCRIPTION OF THE INVENTION

Examples

(7) Synthesis and Characterization by X-Ray Diffraction Analysis

(8) All compounds discussed here were synthesized in evacuated quartz glass ampoules with internal corundum crucibles. The elements magnesium, manganese, copper and phosphorus were weighed in in the ratio of (1.15−x):x:1:1 (x=0.05; 0.1; 0.15; 0.2) and transferred in the countercurrent argon flow into the internal crucible in a semi-ampoule which has been baked out in advance. An excess of magnesium was used, because at the reaction temperature magnesium volatilizes and forms a light precipitate on the quartz glass wall of the ampoule. The weighed portions of the elements for the ternary and the quaternary compounds can be found in Table 1. All batches were calculated such that ca. one gram of the desired product each was obtained.

(9) TABLE-US-00001 TABLE 1 Target compound m Mg/g m Mn/g m Cu/g m P/g MgCuP 0.2054 0 0.5438 0.2608 Mg.sub.0.95Mn.sub.0.05CuP 0.2285 0.024 0.5434 0.2668 Mg.sub.0.90Mn.sub.0.10CuP 0.193 0.043 0.5055 0.2463 Mg.sub.0.85Mn.sub.0.15CuP 0.2018 0.0646 0.5065 0.2439 Mg.sub.0.80Mn.sub.0.20CuP 0.1933 0.0988 0.5613 0.2742

(10) The evacuated ampoules were sealed and subjected to a temperature program in a tubular furnace. At first, with 50° C./h it was heated to 400° C., and this temperature was maintained for 36 hours, subsequently with 50° C./h it was heated to 850° C., and this temperature was maintained for 72 hours, and then it was cooled to room temperature with a cooling rate of 100° C./h. Temperatures of higher than 850° C. resulted in the formation of Cu.sub.3P and Mg.sub.3P.sub.2 as secondary phases. A larger excess of magnesium resulted in the formation of Mg.sub.2Cu. It has been shown that the obtained preparations of Mg.sub.1-xMn.sub.xCuP were not air- or humidity-sensitive.

(11) All products were ground and analyzed by powder diffractometry (CuKα,1 radiation). On the basis of the known structure model of MgCuP for all compounds Rietveld adjustments were conducted. Substituted products with x=0.05; 0.10 and 0.15 were obtained nearly phase-pure with only low proportions of secondary phases. In the case of a substitution of Mn for Mg with x=0.20 multiphase products with higher proportions of Cu.sub.3P as a secondary phase were obtained.

(12) Compaction

(13) For the mixed crystal series Mg.sub.1-xMn.sub.xCuP the compaction was conducted with the help of SPS. Ca. 0.6 to 0.8 g of the samples each were finely pulverized and placed in a female graphite mold with an inner diameter of 10 mm. The pressing parameters were determined by a plurality of experiments.

(14) Highly compacted test pieces were obtained as follows. At first, without pressure with ca. 50° C./min it was heated to ca. 400° C. Then a pressure of 30 MPa (corresponds to 2.3 kN) was applied and it was further heated. At 650° C. the pressure was increased to 50 MPa (corresponds to 3.7 kN), and at constant temperature it was maintained for a period of time of half an hour. After the completion of the compaction and the shutdown of the power of the SPS apparatus no pressure was applied onto the test piece. In the context of this work in many compaction experiments was found that it is possible that the test pieces of intermetallic phases break, when during the cooling phase pressure is applied onto them.

(15) After the samples had been cooled down, the female graphite molds were removed. With the help of a bench vise the samples were removed from the female molds, because it was difficult to loosen them from the female graphite mold. Thoroughly the compacted test pieces were mechanically treated with sand paper for removing optionally present graphite and for guaranteeing a good contact of the test pieces in the apparatuses for the determination of the thermoelectric properties. The densities of the samples were determined by pycnometry. Here, values of >95% of the theoretical, crystallographic density were measured.

(16) The obtained compacted test pieces of the solid solution Mg.sub.1-xMn.sub.xCuP were analyzed with X-ray diffraction in reflection for examining, whether the compaction had changed the sample. Diffractograms have shown that no changes in crystal structure had been caused by the compaction. After the compaction the samples have shown no change in identity.

(17) Thermal Analysis

(18) The compound Mg.sub.0.90Mn.sub.0.10CuP was analyzed thermoanalytically by means of DSC/TG (FIG. 1). The measurement was performed under argon in platinum crucibles with internal corundum crucibles at temperatures of 25° C. to 700° C. with a heating rate of 10° C./min. No changes of the mass or peaks which are indicative for a physical or chemical conversion of the compound can be seen. Thus, the compound is thermostable.

(19) Energy Dispersive Spectroscopy

(20) Compacted test pieces of the compounds MgCuP, Mg.sub.0.90Mn.sub.0.10CuP, Mg.sub.0.85Mn.sub.0.15CuP and Mg.sub.0.80Mn.sub.0.20CuP were analyzed by means of energy dispersive X-ray spectroscopy with respect to their composition and the distribution of the elements. The measurements have shown that the substitution of manganese for magnesium has resulted in a lower proportion of manganese as was envisaged by the weighed portion. In table 2 the proportions measured by means of energy dispersive spectroscopy (EDS) in comparison to the proportions envisaged by the weighed portion are listed. For all compounds a lower proportion of manganese as was envisaged by the weighed portion was measured each. This is consistent with the lattice parameters and the population factors which were determined by means of the Rietveld method. The reason for this is that the ratio of the weighed portions between magnesium and manganese was not an ideal one; an excess of magnesium was used, because magnesium partially volatilizes during the reaction. Therefore, the ratio between magnesium in the case of the intended compound Mg.sub.0.90Mn.sub.0.10CuP is not exactly 0.9 to 0.1.

(21) TABLE-US-00002 TABLE 2 Ideal x (EDS) Mg.sub.0.90Mn.sub.0.10CuP 0.089 Mg.sub.0.85Mn.sub.0.15CuP 0.101 Mg.sub.0.80Mn.sub.0.20CuP 0.182

(22) Thermoelectric Characterization

(23) All synthesized and compacted compounds in the system Mg.sub.1-xMn.sub.xCuP were characterized thermoelectrically. The measurements of the Seebeck coefficients and the electric conductivities were conducted simultaneously from 50° C. to 650° C. or 800° C. In a further measurement the temperature and thermal conductivities were determined with the laser flash method.

(24) The measurements of the Seebeck coefficients (FIG. 2) show that MgCuP and the quaternary compounds are p-thermoelectric substances. The course of the Seebeck coefficients is similar for all compounds. At first, the Seebeck coefficients strongly increase up to ca. 450° C. At higher temperatures differences for the different compounds can be seen. Mg.sub.0.90Mn.sub.0.10CuP and Mg.sub.0.85Mn.sub.0.15CuP show further increasing Seebeck coefficients without a maximum and with a value of 165 μV/K at 680° C. For MgCuP and Mg.sub.0.90Mn.sub.0.20CuP decreasing Seebeck coefficients can be seen. But in the case of MgCuP the Seebeck coefficient again increases at temperatures of higher than 600° C.

(25) The changes in the course of the Seebeck coefficients at 550° C. correspond with the course of the electric conductivities. At low temperatures of up to ca. 300° C. all compounds show metallic behavior. Then, in a range of between 300° C. and 400° C. all compounds show a transition from metallic behavior to semiconductor behavior. This is shown in FIG. 3. The electric conductivities of different samples show a similar course. The proportion of manganese in Mg.sub.1-xMn.sub.xCuP does not correspond with the values of the electric conductivities.

(26) With an Arrhenius plot of the natural logarithms of the electric conductivities against the reciprocal temperature the band gaps of the compounds in the temperature range starting at 450° C. were determined (table 3). Mg.sub.0.90Mn.sub.0.10CuP and Mg.sub.0.85Mn.sub.0.15CuP are characterized by band gaps of ca. 0.1 eV which are lower than the band gaps of MgCuP and Mg.sub.0.80Mn.sub.0.20CuP.

(27) TABLE-US-00003 TABLE 3 Ideal Band gap/eV MgCuP 0.2 Mg.sub.0.90Mn.sub.0.10CuP 0.1 Mg.sub.0.85Mn.sub.0.15CuP 0.09 Mg.sub.0.80Mn.sub.0.20CuP 0.28

(28) The thermal conductivities of Mg.sub.1-xMn.sub.xCuP were calculated on the basis of the thermal capacities of Mg.sub.0.90Mn.sub.0.10CuP which were measured by means of DSC (FIG. 4). All compounds show considerable changes at a temperature of ca. 400° C. Up to 400° C. the thermal conductivities decrease, above 400° C. the thermal conductivities increase. MgCuP with values of 2.4 to 3.2 W/mK shows the highest thermal conductivities. Mg.sub.0.90Mn.sub.0.10CuP and Mg.sub.0.85Mn.sub.0.15CuP have the lowest values which above 600° C. are approximately constant. The substitution of Mn for Mg has resulted in lower thermal conductivities.

(29) The high Seebeck coefficients, the relatively high electric conductivities and the low thermal conductivities result in high thermoelectric figures of merit (zT factors) which increase with the temperature (FIG. 5). The zT factors of all compounds do not show a maximum. MgCuP and Mg.sub.0.80Mn.sub.0.20CuP have zT values of ca. 0.3 at 650° C. With the substitution of Mn for Mg a significant improvement of the thermoelectric properties was achieved. Mg.sub.0.90Mn.sub.0.10CuP and Mg.sub.0.85Mn.sub.0.15CuP with zT values of higher than 0.6 have factors of merit which are comparable with the zT values of the best phosphides described in literature till today.