Nanostructure formation in bulk thermoelectric compounds in the pseudo binary PbTe-Sb 2 Te 3 system
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1267-DD06-07
Nanostructure formation in bulk thermoelectric compounds in the pseudo binary PbTe-Sb2Te3 system Teruyuki Ikeda1,2 and G. Jeffrey Snyder2 1 PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan 2 Materials Science, California Institute of Technology, Pasadena, CA 91125, USA ABSTRACT Studies on microstructures in thermoelectric compounds in the pseudobinary PbTe-Sb2Te3 system are overviewed and strategies to control the microstructure of thermoelectric compounds are discussed on the basis of the phase diagram and phase transformation theories. The morophology of solidification from the melt results in dendrite or lamellar structure depending on composition. The size-scales of the microstructures obtained by solidification can be controlled from the order of micrometers to tens of micrometers by controlling cooling rates (dendrites) or solidification velocity (lamellae). Lamellar and Widmanstätten structures are obtained by eutectoid (Pb2Sb6Te11 → PbTe + Sb2Te3) and precipitation (PbTe (Sb2Te3) → PbTe + Sb2Te3) reactions, respectively. These solid-state transformations show features with nanometer size-scales. For the eutectoid reaction the size-scale depends on annealing temperature and time. For precipitation, the size-scale depends on composition as well as cooling rate or annealing temperature. Such behavior can be understood in terms of phase transformation theories. INTRODUCTION In recent years, it has been reported that superlattice structures formed in thin film materials results in improved thermoelectric figure of merit zT [1-3], defined as S2σT/κ, where S is the Seebeck coefficient, σ the electrical conductivity, and κ the thermal conductivity, which is expressed as κ = κe + κp, where κe and κp stand for thermal conduction by electrons and phonons, respectively. This is thought to be due to enhanced phonon scattering [4], which reduces κp. Ideally, electrical conduction is not disturbed very much presumably because of the epitaxial interfaces. It has also been demonstrated that nanoparticles embedded in epitaxially grown materials are also effective in scattering phonons [5]. Such reduction of lattice thermal conductivity has become one of key routes to enhance zT. However, it is challenging to make efficient devices from thin-film materials because they are more degraded by electrical contact resistance [6] than devices made from bulk materials. Bulk materials with high thermoelectric performance should be readily fabricated into devices and could be inexpensive to produce. The capability of scattering phonons in epitaxial thin-film materials is not due to its periodic structure but due to the high density of interfaces [7]. Thus, for practical use, polycrystalline bulk materials would be preferable. While it was considered for many years that phonon scattering by boundaries is essentially low temperature phenomena, it is now thought that boundary scattering may occur at larger grain sizes and higher temperatures than was previously thought possible [8] since Goldsmi
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