where TC was 300 K. Although BiSb4Tehas the highest figure of merit of the materials in the table, its low maximum operating temperature makes it relatively inefficient. On the other hand, lead telluride, with a much smaller figure of merit, can operate at 900 K and can, therefore, yield 12.6% efficiency when its cold side is at 300 K.
The product, ZT, is the dimensionless figure of merit. Each semiconductor has a range of temperature over which its ZT is best. For low temperatures around 100 C, Bi2Te3 is normally used. At much higher temperatures around 500 C, PbTe, popular in the 1960s, was the choice. For this range, the so-called TAGS (tellurium-antimony-germanium-silver) is now preferred, as is the Michigan State University synthesized family of lead-antimony-silver-tellurium alloy (LAST) with ZT=1.4 at some 450 C. The Jet Propulsion Laboratory, JPL, (2003) developed Zn4Sb3, an excellent solution for the intermediary range of around 350 C. For the very high operating temperature of the radioisotope thermal generators used in space missions to the outer planets (where there is insufficient sunlight to operate photovoltaic panels), the thermoelectric material used is the SiGe alloy.
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