2 - Theory of energy conversion between heat and electricity

Abstract

Thermoelectric materials are a class of materials in which conversion between thermal energy and electrical energy can be realized. This conversion process involves charge and heat transport, with electrons and phonons mainly being the carriers. The efficiency of conversion is mainly dependent upon the figure of merit of the material, which is based on the electrical conductivity, Seebeck coefficient, electronic thermal conductivity, and lattice thermal conductivity. In this chapter, these transport coefficients are discussed to explain the conversion process and energy transport in thermoelectric materials. The electronic transport terms are discussed starting from the free electron theory then to semiclassical theory. The relation between electronic transport and electronic structure are discussed. Similarly, the relation between phonon transport and phonon spectrum are discussed. The phonon calculation methods discussed give an idea of how the phonon properties can be computed with the help of electronic structure methods. Furthermore, to show how materials’ electronic energy states are obtained, the key concepts behind the density functional theory (DFT) and DFT + U are summarized. Additionally, some of the theoretical aspects to be considered in the prediction of new thermoelectric materials are presented. Finally, a short review summarizing the theoretical and computational approaches in explaining the thermoelectric properties is given.