Microstructure Size Control through Cooling Rate in Thermoelectric PbTe-Sb 2 Te 3 Composites
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TRODUCTION
THERMOELECTRIC devices can convert thermal energy to electric energy. The development of thermoelectric conversion technology has been of great interest.[1] However, thermoelectric conversion is not yet widely used because of its poor efficiency.[2] In order to enhance the efficiency in thermoelectric conversion, it is necessary to improve the figure of merit of thermoelectric materials zT, defined as S2rT/j, where S is the Seebeck coefficient, r the electrical conductivity, and j the thermal conductivity. The term zT > 2 has been reported in thin-film superlattice materials.[3,4] This is presumably due to enhanced phonon scattering,[5] which reduces lattice thermal conductivity. At present, thinfilm materials are challenging to fabricate into efficient devices because of relatively large contact resistance, among other issues. Therefore, bulk materials with high thermoelectric performance are desired for the widespread use of thermoelectric devices. Recently, Kanatzidis and co-workers have shown that the nanostructure existing in bulk thermoelectric materials such as PbTe-AgSbTe2 alloys[6] appears to be effective in reducing the lattice thermal conductivities, resulting in a large thermoelectric figure of merit. Because the size of the particles, the density of the microstructure, and the nature of the interfaces[7] are all expected to influence the thermoelectric transport TERUYUKI IKEDA, PRESTO Researcher, PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan, is with the Materials Science Department, California Institute of Technology, Pasadena, CA 91125. Contact e-mail: [email protected] VILUPANUR A. RAVI, Professor, is with the Chemical and Materials Engineering Department, California State Polytechnic University, Pomona, CA 91768. G. JEFFREY SNYDER, Faculty Associate, is with the Materials Science Department, California Institute of Technology. Manuscript submitted July 6, 2009. Article published online January 5, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
properties, the control of microstructures in bulk thermoelectric materials is rapidly becoming of great interest for the enhancement of thermoelectric performance. Our research group has focused on the control of a selfassembled microstructure in thermoelectric materials using solidification[8] or solid-state transformations[9–11] for the purpose of reduction of the lattice thermal conductivity. In the PbTe-Sb2Te3 composites obtained by solid-state transformations, PbTe and Sb2Te3 crystals form semicoherent interfaces with the orientation relationship (0001)Sb2Te3//{111}PbTe, h1120iSb2 Te3 == h110iPbTe in both a nanolamellar structure[9] and a Widmansta¨tten precipitation structure.[11] It has been found that nanolamellae obtained by the eutectoid reaction effectively reduces the lattice thermal conductivity.[12] While the microstructure obtained by solidification processing[8] was in the scale of tens of micrometers, it was suggested that a finer microstructure would be obtained by enhancing cooling rates. In the present solidification study,
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