Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Advisor

He, Jian

Committee Member

Daw, Murray

Committee Member

Rao, Apparao

Committee Member

Marinescu, Catalina

Abstract

Arguably the main focus of thermoelectric materials research over the last decade has been the reduction of lattice thermal conductivity through nanostructuring. This approach has proved quite effective in many instances, but has several inherent drawbacks including not only the metastability of many of the nanostructures used, but also difficulty decoupling the effects on the thermal properties of materials from the effects on their electrical properties. Some more recent research has focused on reduced thermal conductivity in materials with strong anharmonicity. In these systems anharmonicty in the crystal structure, whose strength can be gauged by the so-called Gruneissen parameter leads to amorophous-like thermal behavior in crystaline materials. Ag8GeTe6 is one such material which displays an unusually low thermal conductivity (~0.25 W/m*K at room temperature) that can be at least partially attributed to its large mode-averaged Gruneisen parameter, which we have estimated to be ~3.8 at room temperature. Beyond the small magnitude of the thermal conductivity of Ag8GeTe6 its temperature dependence is also surprising, and displays a positive temperature coefficient rather than the usual 1/T dependence expected for crystalline materials. Furthermore, Ag8GeTe6 is an unusual example of a material in which superionic conduction and promising thermoelectric performance coexist at 500 K and above. Such coexistence is a surprise as the crystal chemistry requirements for these two phenomena are distinct. Therefore understanding the interplay between the ionic, electronic, and phonon conduction in Ag8GeTe6 not only attracts interest in fundamental research but also bridges two realms of energy-related materials research, namely, thermoelectric materials (electronic conductors for direct heat-to-electricity energy conversion) and electrolyte materials (ionic conductors for energy storage in batteries). To better understand how Ag8GeTe6 evolves and sets the stage for such rare coexistence the coexistence of superionic conduction and promising thermoelectric performance at elevated temperatures, we have conducted temperature variable powder X-ray diffraction, photo-acoustic spectroscopy, heat capacity, thermal conductivity, electrical conductivity (DC and AC electrical conductivity, and ionic conductivity), Seebeck coefficient, and Hall coefficient measurements over a wide temperature range below room temperature. As previous studies on Ag8GeTe6 are scarce, these results have brought our understanding of Ag8GeTe6 to an unprecedented level.

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