High-Performance Thermoelectric Materials Based on -Zn4Sb3
Even better performances are obtained with solid solutions of -Zn4Sb3 and Cd4Sb3.
NASA's Jet Propulsion Laboratory, Pasadena, California
Materials based on -Zn4Sb3 have been found to exhibit unusually high values of the dimensionless thermoelectric figure of merit at temperatures between 200 and 400 °C. The discovery that p-type -Zn4Sb3 is a high-performance thermoelectric material was reported in "Zn4Sb3: A High-Performance Thermoelectric Material" (NPO-19677), NASA Tech Briefs, Vol. 23, No. 2 (February, 1999), page 54. The development reported here extends beyond that discovery to include solid solutions of -Zn4Sb3 and Cd4Sb3 (with general compositions given by Zn4-xCdxSb3) in the class of high-performance thermoelectric materials based on -Zn4Sb3.
The development has included studies of doping with impurities and of deviation from stoichiometry as means to affect the electrical properties of -Zn4Sb3. These studies have included the preparation of samples with electrical conductivities of both the p- type and the n-type. Theoretical modeling of the thermoelectric properties of p-type -Zn4Sb3 was also performed to predict the maximum achievable figure of merit for this compound as a function of temperature, and experimental values were found to approach the predicted values.
The thermoelectric figure of merit, ZT is given by ZT = 2T/, where is the Seebeck coefficient, T is the absolute temperature, is the electrical resistivity, and is the thermal conductivity. The figure illustrates ZT as a function of temperature, both from the theoretical prediction described above and as calculated from measurements on p-doped -Zn4Sb3, on other state-of-the-art p-doped thermoelectric materials, and on a p-type Zn4Sb3/Cd4Sb3 solid solution of nominal composition Zn3.2Cd0.8Sb3. In the cited prior article, the high ZT of p-type -Zn4Sb3 in the temperature range of interest was attributed partly to its low thermal conductivity, which was then the lowest known thermal conductivity of any thermoelectric material in that temperature range. Since then, the thermal conductivity of the Zn3.2Cd0.8Sb3 solid solution has been found to be even lower. The net result is that the ZT values of Zn3.2Cd0.8Sb3 exceed those of -Zn4Sb3 at temperatures > 50 °C, reaching a high value of 1.4 at a temperature of 250 °C.
Temperature-stability tests have shown that thermoelectric materials based on -Zn4Sb3 are stable in dynamic vacuum at temperatures up to about 250 °C and in static vacuum up to about 400 °C. A Zn/Cd eutectic brazing material has been developed for use in bonding these materials to copper electrodes. Contact electrical resistivities between samples of these materials and copper electrodes have been found to be very low. Thus, it should be relatively easy to incorporate these materials into thermoelectric power-generating and cooling devices.
The Dimensionless Figure of Merit (ZT) of Zn3.2Cd0.8Sb3 exceeds that of -Zn4Sb3, and exceeds the ZTs of other thermoelectric materials even more.
This work was done by Thierry Caillat, Alexander Borshchevsky, and Jean-Pierre Fleurial of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the category, or circle no. 40 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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