Thermal Conductivity Reduction Paths in Thermoelectric Materials
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Thermal Conductivity Reduction Paths in Thermoelectric Materials C. Godart 1, A.P. Gonçalves 2, E.B. Lopes 2, B. Villeroy 1 1 CNRS, ICMPE, CMTR, 2/8 rue Henri Dunant, 94320 Thiais, France 2 Dep. Química, Instituto Tecnológico e Nuclear/CFMC-UL, P-2686-953 Sacavém, Portugal ABSTRACT The figure of merit ZT = σS2T/κ (S the Seebeck coefficient, σ and κ the electrical and thermal conductivity respectively) is an essential element of the efficiency of a thermoelectric material for applications which convert heat to electricity or, conversely, electric current to cooling. From the expression of the power factor σS2 it was deduced that a highly degenerated semiconductor is necessary. In order to reduce the lattice part of the thermal conductivity, various mechanisms were tested in new thermoelectric materials and those had been the topics of several reviews. These include cage-like materials, effects of vacancies, solid solutions, complex structures (cluster, tunnel, …,), micro- and nano-structured systems, and more recently semiconducting glasses. We plan to review such aspects in the modern thermoelectric materials and include results of the very last years. Moreover, as micro- and nano-composites seem to be promising to increase ZT in large size samples, we will also briefly discuss the interest of spark plasma sintering technique to preserve the micro- or nano- structure in highly densified samples. INTRODUCTION Thermoelectric (TE) effects include the transformation of caloric energy to electric energy or its reverse, and their applications consequently include the two aspects: (micro)cooling or electricity generation from heat sources. However, the efficiency was not sufficient to compete with the cooling by compression/expansion cycles or for economically profitable electricity production. For both cooling or electricity generation, a TE module is made of couples, each couple includes a p-type material (S>0) and a n-type material (S
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