Zhang, Yingying2023-02-162023-02-162022-12https://hdl.handle.net/11299/252520University of Minnesota Ph.D. dissertation. December 2022. Major: Mechanical Engineering. Advisor: Xiaojia Wang. 1 computer file (PDF); xvi, 122 pages.As a rapid development of synthesis and processing of materials at the nanometer scale, the continuous trend of electronic miniaturization leads to unprecedented power densities within today’s devices. This results in excessive operating temperatures, which becomes the bottleneck of device performance. The challenges in the thermal management of devices (i.e., the ability to remove and/or redistribute heat generation during the device operation) lead to a desire to engineer new materials to lower temperatures and improve reliability. Especially, at reduced length and time scales, size effects and interfaces become dominant, and physical properties such as thermal conductivity of materials may deviate from their bulk properties significantly. Therefore, a better understanding and further manipulation of thermal transport at the micro/nanoscale is necessary to improve overall device performance. In this dissertation, I present four research projects on thermal transport in different material systems, including ultrawide bandgap semiconductors, amorphous materials, perovskite oxides, and metal/semiconductor interfaces. First, I experimentally study the thickness-dependent thermal conductivity of beta-phase gallium oxide (beta-Ga2O3) by conducting time-domain thermoreflectance (TDTR) measurements. By comparing with model calculations, I attribute the dependence of thermal conductivity on thickness to the pronounced phonon-boundary scattering as the film thickness gets smaller. Second, I reveal the impact of hydrogen atoms on thermal transport in hydrogenated amorphous silicon (a-Si:H) films. It is found that the thermal conductivity of a-Si:H films monotonically decreases as the hydrogen concentration increases. By combining the experimental results with the theoretical calculations based on the effective medium approximation, I successfully decompose the impacts of different heat carriers (propagons and diffusons) on thermal transport. I conclude that such a significant reduction in the thermal conductivity of a-Si:H originates from the hydrogenation-induced material softening, the decrease in density, and phonon-defect scattering. Third, after understanding the impacts of different factors on thermal transport, I further investigate the thermal properties of La0.5Sr0.5CoO3-delta (LSCO) films, as a model system offering great flexibility in structural engineering. I demonstrate the ability to continuously tune the thermal conductivity of LSCO films by a factor of over 5. Such a large modulation factor is achieved via a room-temperature electrolyte-gate-induced non-volatile topotactic phase transformation from perovskite (with oxygen non-stoichiometry delta ≈ 0.1) to an oxygen-vacancy-ordered brownmillerite phase (with delta = 0.5), accompanied by a metal-insulator transition. Combining TDTR and electronic transport measurements, model analyses based on molecular dynamics and Boltzmann transport, and structural characterization by X-ray diffraction, I uncover and deconvolve the effects of these transitions on heat carriers, including electrons and lattice vibrations. In parallel with experimental studies, I also make advances in theories to better evaluate interfacial thermal conductance. I come up with a modified model, named the mixed mismatch model, to consider the roughness/bonding at the interface. The model uses interfacial roughness as an input to determine the proportions of specular and diffuse transmission and thus can predict the interfacial thermal conductance for real materials to a certain degree of accuracy.enThermal conductivityThermal transportTime-domain thermoreflectanceThesis or Dissertation