Understanding the atomic local structure of thermoelectric materials.

摘要

There is renewed interested in novel thermoelectric materials driven by potential applications such as solid state refrigeration and waste heat recovery. I explore how the structure of several leading thermoelectric materials contributes to their performance, and how these materials could be made more efficient, and hence, economically viable. I approach this from a local atomic viewpoint using the extended x-ray absorption fine structure (EXAFS) technique, which is especially useful in determining the environment around specific atoms at dilute concentrations or in disordered states. I investigate the means by which Thallium doping in PbTe increases the thermoelectric performance and show how phase information unique to EXAFS gives information on whether Pb atoms are on center in the PbTe crystal lattice. I then present my work on skutterudites in which "rattling" atoms fill large voids and are consequently weakly bound to the rest of the lattice. Building on this project I present a theoretical model for predicting the interaction of the "cage" with the rattler atom modification of phonon dispersion curves which suggest new ways to decrease thermal conductivity and reduce the materials constraint between good electrical properties and low thermal conductivity. Finally, I present my findings on thermoelectric type I clathrates, examining cage buckling and the consequences this has on transport measurements. These studies on various materials all illustrate that small variations in the local structure from diffraction averages can greatly influence the electrical and thermal conductivities. Ultimately more efficient devices will be generated by utilizing these principals.