Understanding Phonon Interactions with Defects in Functional Oxide Materials

Understanding of nano-scale thermal transport has enabled significant advances in a wide variety of fields including microelectronics, alternative energy, optics, and many more. Utilized in many of these applications are a class of materials known as functional oxides. These materials are oxygen compounds with specific functional properties of interest (e.g., dielectrics, piezoelectrics, photovoltaics, thermoelectrics). Often it is the defects in these materials that enable functionality. In this dissertation, I aim to identify the role that defects play in functional oxide materials with respect to thermal transport.

The major thermal carriers in most functional oxide crystals are collective lattice vibrations, or phonons. Phonon-dominated thermal conductivity is dictated by the scattering of these thermal carriers with a variety of other components or the material system, including defects in the crystal. I concentrate on three types of defects that are common in functional oxides: boundaries, extrinsic point defects, and intrinsic point defects. Specifically, I investigate spectral phonon scattering in thin films of nano-grained barium titanate, thermal conductivity modulation in dysprosium doped cadmium oxide, and the reduction of the thermal conductivity of rutile single crystals with high concentrations of vacancies and interstitials.

I use time domain thermoreflectance (TDTR) to measure the thermal conductivity in each these experiments. TDTR is an ultra-fast optical pump-probe measurement technique that is ideal for the measurement of nano-scale thermal transport.

The results in each of the studies discussed will provide valuable insight into the role of thermal transport in their respective applications. Furthermore, these results demonstrate a framework for understanding the impact defects on thermal transport in functional oxides and will lead to an advanced control of the thermal conductivity of oxides in general.