Filled skutterudites have shown promise as semiconducting materials with superior thermoelectric properties at temperatures up to at least 650 °C. This finding is a breakthrough in a continuing investigation of the potential utility of skutterudites as thermoelectric materials. Previous results of this investigation were reported in several articles in NASA Tech Briefs; namely, "Skutterudite Compounds for Power Semiconductor Devices" (NPO-19378), NASA Tech Briefs, Vol. 20, No. 3 (March 1996), page 60; "Two Potentially Useful Ternary Skutterudite Compounds" (NPO-19409), NASA Tech Briefs, Vol. 20, No. 11 (November 1996), page 66; and "Preparation and Some Properties of n-Type IrxCo1 — xSb3" (NPO-19852), NASA Tech Briefs, Vol. 20, No. 11 (November 1996), page 94.
Filled skutterudites are derived from the skutterudite crystal structure and can be represented by the formula LnT4Pn12; where "Ln" denotes one of the rare-earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Th, or U; "T" denotes Fe, Ru, Os, Co, Rh, or Ir; and "Pn" denotes one of the pnicogen elements P, As, or Sb. A skutterudite is said to be filled when empty octants in the skutterudite structure of T4Pn12 are filled with rare-earth atoms.
Some of the filled skutterudites of various compositions prepared by a combination of melting and powder-metallurgy techniques have shown exceptional thermoelectric properties in the temperature range of 350 to 700 °C. Both p-type (electron-acceptor) and n-type (electron-donor) conductivities have been obtained; this is fortunate in that to be functional, a thermoelectric device must contain layers of both types.
The thermoelectric figure of merit, ZT, is given by ZT = S2T/rl, where S is the Seebeck coefficient, T is the absolute temperature, r is the electrical resistivity, and l is the thermal conductivity. The figure shows ZT values obtained from measurements on several filled skutterudites and on other, state-of-the-art thermoelectric materials. One specimen exhibited ZT of almost 1.8 at a temperature of 650 °C; this is the highest ZT ever obtained since the beginning of thermoelectric technology in the 1950s. By manipulating the nominal compositions and doping concentrations of filled skutterudites, it may be possible to obtain similarly high ZT at lower and/or higher temperatures. These high-performance thermoelectric materials could be used to make thermoelectric power generators, coolers, and detectors that would operate with efficiencies greater than those of the corresponding devices now in use and could thus be useful in a greater variety of applications.
This work was done by Jean-Pierre Fleurial, Alexander Borshchevsky, Thierry Caillat, Donald Morelli, and Gregory Meisner of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
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