Recent Developments in Bulk Thermoelectric Materials
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Recent Developments in Bulk Thermoelectric Materials
George S. Nolas, Joe Poon, and Mercouri Kanatzidis Abstract Good thermoelectric materials possess low thermal conductivity while maximizing electric carrier transport. This article looks at various classes of materials to understand their behavior and determine methods to modify or “tune” them to optimize their thermoelectric properties. Whether it is the use of “rattlers” in cage structures such as skutterudites, or mixed-lattice atoms such as the complex half-Heusler alloys, the ability to manipulate the thermal conductivity of a material is essential in optimizing its properties for thermoelectric applications. Keywords: alloy, compound, thermal conductivity, thermoelectricity.
scattering centers, thus greatly reducing the lattice thermal conductivity of these compounds, has resulted in improvements in the TE properties of skutterudites.1,2 The smaller and heavier the ion in the voids, the larger the disorder that is produced and thus the larger the reduction in the lattice thermal conductivity. Skutterudite antimonides possess the largest voids and are therefore of particular interest for TE applications. Results from Sales et al.4 and Fleurial et al.5 show high ZT values (the common figure of merit for comparing different TE materials) at elevated temperatures in LaFe3CoSb12 and CeFe3CoSb12 for both p-type and n-type specimens. ZT values approaching 1.4 above 900°C for these materials have been reported,5 indicating their successful optimization for TE powerconversion applications. It should be noted, however, that a small concentration of void-fillers results in a large reduction in thermal conductivity. Five percent of La6 or Ce,7 for example, in the voids of CoSb3 results in a thermalconductivity reduction of 50%, as compared with CoSb3. In certain cases, higher power factors have also been obtained with partial filling, as compared with more fully filled, charge-compensated compositions. The aim in investigating partially filled skutterudites is realizing an optimum electron concentration while re-
Introduction As the search for promising bulk thermoelectric materials intensifies, certain material systems stand out as possessing the highest potential for achieving thermoelectric figures of merit well above unity. These materials possess relatively good electrical properties while maintaining very low thermal conductivities. In some cases, enhancements in the electrical properties have been realized. These results have attracted great attention, and many research laboratories worldwide are now working on one or more of these material systems in order to achieve further improvements in their thermoelectric properties. We discuss some of the bulk thermoelectric materials of primary interest in this article.
Skutterudites The physical properties of skutterudites depend sensitively on their compositions. This compositional dependence not only provides a means to investigate the structure–property relationships in this material system but also allows the
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