Thompson, Matthew2024-01-052024-01-052021-05https://hdl.handle.net/11299/259602University of Minnesota M.S. thesis. May 2021. Major: Mechanical Engineering. Advisor: Xiaojia Wang. 1 computer file (PDF); viii, 53 pages.Thermal transport is a critical aspect of device and material design for a wide range of applications including electronics, energy generation, construction materials and even nuclear fuel cladding. In materials with reduced dimensions and structural variations, the thermal transport behavior deviates significantly from bulk materials. Thus, a comprehensive understanding is necessary of the thermal transport mechanisms for micro/nanostructured materials, based on proper characterizations enabled by advanced metrologies. One such metrology, known as time-domain thermoreflectance (TDTR), utilizes the ultrafast laser pump-probe technique to achieve high temporal and spatial resolutions. In my master thesis project, I apply TDTR metrology to study the thermal conductivities of commercially available filler materials used in composites by conducting direct measurements of individual fillers. Such direct measurements of “true” filler thermal properties are absent in literature due to the challenges in thermal characterization resulting from the small dimensions of filler materials. Composites combine heterogeneous materials in a wide range of geometrical configurations in an attempt to take advantage of selected physical properties of individual components. Thermal transport in composites has emerged as a rich and promising area due to the unique capability of this class of materials to generate a suite of material functionalities (e.g., adhesion, strength, thermal conductivity, electrical conductivity). My samples are individual glass and ceramic microspheres (diameter ranges from 100 to 150 μm) embedded in an epoxy matrix. This sample configuration represents a typical composite-based thermal interface material (TIM) that is widely used in electronics packaging for better heat dissipation. The measured thermal conductivities of both the borosilicate glass and yttria stabilized zirconia (YSZ) microspheres agree well with literature values for bulk materials, whereas the thermal conductivity of alumina microspheres is nearly half the conductivity of α-phase alumina bulk crystals. The reduction in thermal conductivity of the alumina microspheres is attributed to enhanced phonon scattering due to structural heterogeneity, such as defects induced by phase mixing and microvoids. This thesis combines sample preparation, structural characterization, and direct thermal measurements using TDTR to elucidate the interplay between structural and thermal properties of microspherical composite filler materials. The structure-thermal property relationship of filler materials revealed from these direct thermal measurements can be readily integrated into the design and optimization of currently existing composite-based thermally conductive products. Ultimately, the results of my thesis work can provide further guidance in the design and optimization of composite-based thermal materials for applications in thermal management and energy storage.enMicrospheresThermal ConductivityThermal TransportTime-Domain ThermoreflectanceThesis or Dissertation