Transport Behavior and Thermal Conductivity Reduction in the Composite System PbTe-Pb-Sb
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1044-U08-01
Transport Behavior and Thermal Conductivity Reduction in the Composite System PbTePb-Sb Joseph Sootsman1, Huijun Kong2, Ctirad Uher2, Adam Downey3, Jonathan James D'Angelo3, Chun-I Wu3, Timothy Hogan3, Thierry Caillat4, and Mercouri Kanatzidis1 1 Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL, 60208 2 Department of Physics, University of Michigan, Ann Arbor, MI, 48109 3 Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI, 48824 4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109 ABSTRACT We report the synthesis of nanostructured composite PbTe with excess Pb and Sb metal inclusions. Scanning and transmission electron microscopy reveal these inclusions in both the nano- and macroscales. The electrical conductivity and Seebeck coefficient dependence on temperature show unusual trends which depend on the inclusion Pb/Sb ratio. Several ratios showed marked enhancements in power factor at 700 K. The thermal conductivity of these composites is reported. INTRODUCTION Significant expansion of thermoelectric power generation and cooling to new applications may be realized if improvements in the efficiency of thermoelectric materials continues. Research in thermoelectrics aims to increase the dimensionless figure of merit, ZT, because it is directly related to the efficiency of a thermoelectric device. ZT is defined (S2σT/κ) where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity and T the temperature.[1] Primarily, advances have come from research on nanostructured semiconductors both in the thin-film [2] and bulk form which showed dramatically reduced thermal conductivities.[3-7] In the thin films, ZT values above 3 have been claimed for the PbTe/PbSe superlattice structures [8] while a ZT of 1.7 at 700K was reported in nanostructured AgPb18SbTe20 [4]. It has been demonstrated that nanoscale inclusions of appropriate nature and size can significantly reduce the thermal conductivity thereby raising ZT.[9] In the PbTe-Sb system we observed that an optimum, not a maximum concentration of Sb nanoparticles is most efficient in reducing the lattice thermal conductivity although the electronic transport was not optimized.[10] In order to obtain the large gains predicted in ZT both enhanced power factors and reduced thermal conductivity must simultaneously occur. Recently, it was reported that the Seebeck coefficient can be increased in bulk PbTe by the inclusion of nanoscale Pb precipitates. This was attributed to an increase in the scattering parameter λ.[11] Given that Pb particles embedded in PbTe have been shown to enhance the Seebeck coefficient and Sb particles can reduce the lattice thermal conductivity we decided to explore the combination of the two types inclusions (i.e. Pb and Sb) in PbTe and investigate the effects on the thermoelectric figure of merit. We report here that we have achieved unique electronic transport in nanostructured PbTe which leads
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