Role of Structures on Thermal Conductivity in Thermoelectric Materials

The figure of merit ZT = sigma alpha T-2/lambda (alpha the Seebeck coefficient, sigma and lambda 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, sigma alpha(2), it was deduced that a highly degenerated semiconductor is necessary. In order to reduce the lattice part of the thermal conductivity, various mechanisms, mainly related to the structure of the materials, were tested in new thermoelectric materials and had been the topics of different reviews. These include cage-like materials, effects of vacancies, solid solutions, complex structures (cluster, tunnel,...), nano-structured systems. We plan to review structural aspects in the modern thermoelectric materials and to include results of the very recent 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.