Seminar"Interplay of Electronic and Lattice Degrees of Freedom in Thermoelectric Materials"
Title: "Interplay of Electronic and Lattice Degrees of Freedom in Thermoelectric Materials"
Speaker: Prof. David J. Singh, Department of Physics and Astronomy, University of Missouri
Thermoelectric materials are important for spacecraft power, waste heat recovery, thermal management and cooling applications. The efficiency of thermoelectric systems is limited by materials performance. This is measured by the figure of merit ZT=sS2T/k, where S is the thermopower and the other symbols have their usual meaning. ZT is therefore a composite property involving electronic and thermal transport, with high ZT favored by high conductivity, high thermopower and low thermal conductivity. However, these properties are inter-related, often to the detriment of high ZT. This talk discusses these correlations, and points out one positive correlation and ways to exploit it. This is the correlation between lattice thermal conductivity and mobility that occurs in materials near polar instabilities. PbTe and GeTe are examples of materials in this category. This proximity to polar distortions is associated with strong anharmonicity of the phonons and also lattice contributions to the dielectric constant. High dielectric constant in turn favors screening of ionized impurity scattering and high carrier mobility. The chemical basis for these effects in terms of electronic structure is discussed along with directions for using this to discover new thermoelectric systems. We also discuss ways of overcoming the contradictions inherent in high ZT and approaches that can be used to identify favorable compositions in relation to overcoming these contradictions.