Understanding Electrical Transport and the Large Power Factor Enhancements in Co-Nanostructured PbTe
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1166-N04-01
Understanding Electrical Transport and the Large Power Factor Enhancements in CoNanostructured PbTe Joseph R. Sootsman1, Vladimir Jovovic2, Christopher M. Jaworski2, Joseph P. Heremans2, Jiaqing He3, Vinayak P. Dravid3, Mercouri G. Kanatzidis1
1. Department of Chemistry, Northwestern University, Evanston, IL 60208 2. Department of Mechanical Engineering, Ohio State University, Columbus, OH 43210 3. Department of Materials Science, Northwestern University, Evanston, IL 60208
ABSTRACT We previously reported the synthesis of nanostructured composite PbTe with excess Pb and Sb metal inclusions. The electrical conductivity shows an unusual temperature dependence that depends on the inclusion Pb/Sb ratio, resulting in marked enhancements in power factor and ZT at 700 K. Additional investigation of the transport and structure of these materials is reported here. Measurements of the scattering parameter reveals there is little change in electron scattering with respect to pure PbTe. High resolution electron microscopy was used to determine additional information about the nature of the precipitate phases present in the samples. High temperature transmission electron microscopy reveals that the precipitates begin to dissolve at high temperatures and completely disappear at T > 619K. A qualitative explanation of the unusual transport behavior of these materials is presented. INTRODUCTION Emerging trends from thermoelectric research indicate that nanostructured materials can achieve a high figure of merit.1-3 The figure of merit, which determines thermoelectric device conversion efficiency, is related to the transport properties of a material through ZT=S2σΤ/κ where S is the Seebeck coefficient, σ is the electrical conductivity, κ the thermal conductivity, and T the temperature.4 Nanostructured materials can have increased ZT because of low thermal conductivity resulting from phonon scattering at the interfaces of nanoscale features.5 Recently, we reported the synthesis and thermoelectric properties of PbTe nanostructured with nanoscale precipitates of Pb and Sb.6 These nanoprecipitates profoundly impact the temperature dependence of the mobility. Typically in degenerately doped PbTe the mobility decreases by a power law relationship (µ=aTα) where the exponent (α) is -2.2 to -2.5. This is a result of acoustic phonon scattering of the electrons at high temperature. In samples with both Pb and Sb nanoinclusions this power law relationship is no longer valid and in several cases the mobility actually increases before it falls at high temperature. This behavior is tuned by the Pb/Sb ratio.6 In these materials the lattice thermal conductivity is also tuned by the Pb/Sb ratio where samples rich in Sb tend to have a reduced thermal conductivity. Here we present additional transport measurements and transmission electron microscopy (TEM) results that add
to our understanding of the novel behavior of these materials. First, measurements were performed on several samples using the method of 4 coefficients in order to de
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