Experimental results on semiconductors with the skutterudite crystal structure show that these materials possess attractive transport properties and have a good potential for achieving ZT values substantially larger than for state-of-the-art thermoelectric materials. Associated with a low hole effective mass, very high carrier mobilities, low electrical resistivities and moderate Seebeck coefficients are obtained in p-type skutterudites. For a comparable doping level, the carrier mobilities of n-type samples are about an order of magnitude lower than the values achieved on p-type samples. But the much larger electron effective masses and Seebeck coefficients make n-type skutterudites even more promising candidates. However, the thermal conductivities of the binary skutterudite compounds are too large, particularly at low temperatures, to be useful for thermoelectric applications. Several approaches to the reduction of the lattice thermal conductivity in skutterudites were investigated: doping by ionized impurities, formation of solid solutions and alloys, study of novel ternary compounds and study of filled skutterudite compounds. While all those approaches did result in substantially lower thermal conductivity values in these materials, a combination of alloying and "filling" the skutterudite structure achieved superior thermoelectric properties in the moderate to high temperature range. By incorporating the novel skutterudite compositions into thermoelectric generators, the conversion efficiency of such devices could be increased very significantly.
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