Thermoelectric and Thermomagnetic Transport in PbTe with Nanoscale Structures
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Thermoelectric and Thermomagnetic Transport in PbTe with Nanoscale Structures Joseph P. Heremans Department of Mechanical Engineering and Department of Physics The Ohio State University Columbus, OH 43220 ABSTRACT Quantum-dot superlattices (QDSLs) have demonstrated thermoelectric figure of merit (ZT) values double those of classical bulk materials, providing the proof that nanoscale materials can address this age-old limitation on thermoelectric technology. Since QDSL materials can only be obtained in thinfilm form, the development of bulk materials with nanoscale inclusions would be useful for many large-scale applications. Nanometer-scale inclusions in lead chalcogenides are known to improve the thermoelectric figure of merit through a combination of two factors: a strong decrease in lattice thermal conductivity, and an increase in the Seebeck coefficient over that of bulk PbTe for a given carrier concentration. This paper focuses on experimental results obtained on two types of PbTe nanocomposites, namely samples prepared by sintering powders with nanometer-sized grains, and samples prepared with nanoprecipitates of metallic Pb. The results are analyzed using the “method of four coefficients.” At each measurement temperature there are four unknowns. These are the carrier concentration, the mobility, the carrier effective mass, and the energy dependence of the relaxation time, which is modeled by a power law τ ∝ Eλ. Therefore, at each temperature, four measurements are taken: the electrical conductivity, and the Hall, Seebeck and transverse Nernst-Ettingshausen coefficients. This analysis concludes that the effect of the nanoscale inclusions on the power factor is due to an increase in the scattering parameter λ, and that the nanoscale inclusions affect the electron scattering in such a way as to increase the Seebeck coefficient. INTRODUCTION Nanometer-scale materials have been at the core of one of the most substantial advances in thermoelectric technology since the development of semiconductor technology, when T. Harman and co-workers1 developed quantum-dot superlattices (QDSLs) with room temperature thermoelectric figures of merit reaching 1.6 in the PbTe/PbTeSe system, and 2 in the quaternary PbSnTe/PbSnTeSe system. Since then the ZT values have reached 3 at 550 K2. The two mechanisms that led to this success were a strong reduction of the lattice thermal conductivity, and a slight increase in the power factor P=S2σ. In this equation, S is the Seebeck coefficient and σ the electrical conductivity. While one can imagine how the quantum-dots in the QDSLs scatter phonons, it is more difficult to analyze the increase in P. Indeed, there appear to be two factors in this. On the one hand, S is strongly increased in the QDSL system when compared to that of similarly doped bulk materials. On the other hand, the increase in S is partially offset by a loss in mobility, which is ascribed to scattering of the charge carriers on the nanometer-scale dost. This leads to the question of why the Seebeck coeffici
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