Improvement in the Thermoelectric Figure-of-Merit of TAGS-85 by Rare Earth Additions
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Improvement in the Thermoelectric Figure-of-Merit of TAGS-85 by Rare Earth Additions B. A. Cook,1,2 J. L. Harringa,1 M. Besser, 1 and R. Venkatasubramanian2 1 2
Division of Materials Science & Engineering, Ames Laboratory, Ames, IA 50011, USA Center for Solid State Energetics, RTI, International, Research Triangle Park, NC 27709, USA
Abstract TAGS-85 is a well-known thermoelectric material based on germanium monotelluride that exhibits a second-order displacive transformation from a high-temperature cubic to a lowtemperature rhombohedral polymorph. Recent efforts to improve the thermoelectric figure-ofmerit through the addition of small amounts of the rare earth elements Ce and Yb have demonstrated a 25 percent increase in ZT at 700K in materials obtained by solidification from the melt. Preliminary analysis by x-ray diffraction of the chemically-modified alloy suggests a partial stabilization of the high-temperature cubic polymorph. 125Te NMR studies confirm the incorporation of rare earth cations into the GeTe-based lattice. Solid state synthesis has been successfully applied to the processing of rare-earth-doped TAGS-85 and has resulted in a further increase in ZT beyond the levels initially observed in melt-solidified materials. This is believed to be due to improved homogeneity in the distribution of the lanthanide. Introduction Energy harvesting is increasingly seen as a viable approach to reduced dependence on conventional fossil fuels. Among the various types of harvesting technologies, solid state thermoelectric direct conversion of waste heat to electrical energy is widely recognized as among the most reliable because of its absence of moving parts. A limiting factor to a more widespread use of thermoelectric technology in waste heat conversion applications has been the relatively low efficiency by which the process occurs, typically between 4 to 8%, depending on the temperature differentials and hot-side temperatures that are available. Recent breakthroughs in materials chemistry and processing science have led to significant improvements in the bulk thermoelectric materials used for direct energy conversion [1, 2, 3, 4]. The efficiency of a thermoelectric material is a function of three transport-related properties: Seebeck coefficient, S, electrical resistivity, ρ, and thermal conductivity, ț. as given by the expression (1) ZT = S2T/(ρ·ț) where ZT is the dimensionless thermoelectric figure-of-merit, and ț is comprised of an electronic contribution, țe, and a lattice, or phonon contribution, țl. Among materials exhibiting the highest ZT within the temperature range of 500 K to 800 K includes (AgSbTe2)x(GeTe)1-x, referred to as “TAGS.” This GeTe-based material exhibits a maximum ZT greater than unity, which is generally regarded as a consequence of an exceptionally low lattice thermal conductivity. Germanium-telluride based semiconductors of the form (AgSbTe2)1-x(GeTe)x have been of interest for thermoelectric energy conversion since the early 1960’s. Dismukes, Rosi, and Hockings reported a large densit
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