Browsing by Subject "Thermal transport"

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    Thermal Transport In Nanostructured Materials By Ultrafast Pump-Probe Techniques
    (2019-08) Wu, Xuewang
    Thermal transport in nanostructured materials is crucial to nanotechnology-initiated applications such as electronics, solid-state energy conversion, and biomedical applications. At reduced size, the thermal properties of nanostructured materials can differ greatly from their bulk counterparts due to events such as the scattering of heat carriers. New experimental techniques, which can detect the nanoscale thermal properties, are needed to promote further study in this field. Pump-probe optical techniques, which utilize ultrafast laser pulses with a very short duration time and high-power objective lenses to achieve high temporal and spatial resolutions, make it feasible. Our studies were motivated to advance the understandings of thermal transport in nanostructured materials as functions of various structural parameters, utilizing pump-probe optical techniques and numerical/theoretical methods. In this dissertation, I have presented three research projects of thermal transport in different novel nanostructured materials including ultrathin films, nanoparticles, and nanocomposite. First, we extract the glass-like thermal conductivities of single-crystalline La0.5Sr0.5CoO2.9 (LSCO) epitaxial films with “built-in ordered oxygen vacancies”, through Time-domain thermoreflectance (TDTR) and linear extrapolation. Molecular dynamics simulation (MD) and Boltzmann Transport Equation (BTE) are applied to reveal the suppression mechanisms on thermal conductivity of LSCO due to structural parameters including the oxygen vacancies and orderings, film thickness, and substitution. Second, we study thermal transport across cetyltrimethylammonium bromide (CTAB) and polyethylene glycol (PEG) surfactants on gold nanorods (GNRs) in water solution, utilizing transient absorption (TA) technique. We notice a better thermal performance in PEG compared to that in CTAB on GNRs. Through a multiscale thermal modeling with the incorporation of MD simulation, we reveal such better thermal performance in PEG is due to water penetration and strong covalent bonding between GNR and PEG, which are not present in CTAB. Finally, we report thermal conductivities of direct-contact ZnO nanocrystal (NC) networks with infill materials of ZnO and/or Al2O3 by TDTR, as functions of contact radius between adjacent NCs, doping concentration, and the infill composition. A modified effective medium approximation model is applied to validate the experiment results and reveal the influences of various parameters on the thermal conductivity of this nanocomposite sample system.
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    Understanding and Tailoring Thermal Transport in Materials and Across Interfaces
    (2022-12) Zhang, Yingying
    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.