Yb14MnSb11 has been found to be well-suited for use as a p-type thermoelectric material in applications that involve hot-side temperatures in the approximate range of 1,200 to 1,300 K. The figure of merit that characterizes the thermal-to-electric power-conversion efficiency is greater for this material than for SiGe, which, until now, has been regarded as the state-of-the art high-temperature p-type thermoelectric material. Moreover, relative to SiGe, Yb14MnSb11 is better suited to incorporation into a segmented thermoelectric leg that includes the moderate-temperature p-type thermoelectric material CeFe4Sb12 and possibly other, lower-temperature p-type thermoelectric materials.

Thermoelectric Figures of Merit and Compatibility Factors of three compounds of interest as functions of temperature are plotted to illustrate the superiority of Yb14MnSb11 over SiGe.

Interest in Yb14MnSb11 as a candidate high-temperature thermoelectric material was prompted in part by its unique electronic properties and complex crystalline structure, which place it in a class somewhere between (1) a class of semiconducting valence compounds known in the art as Zintl compounds and (2) the class of intermetallic compounds. From the perspective of chemistry, this classification of Yb14MnSb11 provides a first indication of a potentially rich library of compounds, the thermoelectric properties of which can be easily optimized.

The concepts of the thermoelectric figure of merit and the thermoelectric compatibility factor are discussed in “Compatibility of Segments of Thermo electric Generators” (NPO-30798), which appears on page 55. The traditional thermoelectric figure of merit, Z, is defined by the equation

Z = α2/ρκ

where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. Sometimes, in current usage, the term “thermoelectric figure of merit” signifies the product ZT, where T is the absolute temperature. The thermoelectric compatibility factor, s, is defined by the equation

s = [(1 + ZT)1/2 – 1]/αT.

For maximum efficiency, s should not change with temperature, both within a single material, and throughout a segmented thermoelectric-generator leg as a whole. It is in this sense that s serves as a basis for assessing both compatibility among segments and compatibility within a segment (self-compatibility). The degree to which s varies with temperature along a given segment or differs among adjacent segments in a thermoelectric leg thus serves as a measure of incompatibility that one strives to minimize.

As shown in the upper part of the figure, in the temperature range of 975 to 1,275 K, the ZT value of Yb14MnSb11 is approximately double that of SiGe. Moreover, as shown in the lower part of the figure, the s value of Yb14MnSb11 is much closer to that of CeFe4Sb12 than is the s value of SiGe. The net effect of the greater ZT and closer match of s of Yb14MnSb11, compared to those of SiGe, is that the thermal-to-electric power-conversion efficiency of a segmented Yb14MnSb11/CeFe4Sb12 leg operating between the given hot-side and cold-side temperatures is significantly greater than that of a SiGe/CeFe4Sb12 leg operating between the same hot- and cold-side temperatures. For example, for a hot-side temperature of 1,275 K and a cold-side temperature of 775 K, the thermal-to-electric power-conversion efficiency of a segmented Yb14MnSb11/CeFe4Sb12 leg is about 7.3 percent, while that of a segmented SiGe/CeFe4Sb12 leg is about 4.5 percent.

This work was done by G. Jeffrey Snyder and Franck Gascoin of Caltech and Shawna Brown and Susan Kauzlarich of U.C. Davis for NASA’s Jet Propulsion Laboratory. 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:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-42627, volume and number of this NASA Tech Briefs issue, and the page number.


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This article first appeared in the June, 2009 issue of NASA Tech Briefs Magazine.

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