Thermal energy transport in micro/nanostructures

As the ongoing technological revolution progresses towards nanoscale engineering components, the ability to predict and control thermal energy transport in and across these complex structures becomes increasingly important. Nonetheless, there is limited understanding of the mechanisms (e.g. phonon confinement, thermal boundary resistance, interface roughness and phonon wave interaction) that affect heat transport in small-scale systems. In this dissertation, thermal transport in micro/nanostructures using both experimental and computational methods is explored. Better insight into these phenomena will offer the opportunity to create customized nanostructures for enhancing the performance of energy conversion devices and/or for manipulating the heat flow in complex micro/nanoscale systems.; Molecular dynamics computational simulations are employed to examine how thermal transport is affected by the presence of one or more interfaces in bi-material films and simplified superlattices. Parameters such as film thickness, the ratio of respective material composition, the number of interfaces per unit length, and lattice strain are considered. For solids suitably represented as Lennard-Jones materials, results indicate that for simple nanoscale strained heterostructures containing a single interface, the effective thermal conductivity may be less than half the value of an average of the thermal conductivities of the respective unstrained thin films. Increasing the number of interfaces per unit length, however, does not necessarily result in a corresponding decrease in the effective thermal conductivity of the superlattice. Furthermore, strain appears to play an important role in the thermal transport through certain materials.; The thermal conductivities of silicon-germanium superlattices are experimentally investigated using the 3-ω measurement technique over a temperature range from 50 to 340 K. Results indicate that the cross-plane thermal conductivity decreases with decreasing period thickness over the thickness range studied if the acoustic impedance mismatch is large. In-plane thermal conductivity results further demonstrate interesting behavior that provides insight into phonon transport mechanisms.; A novel thermoelectric device comprised of arrays of semiconductor nanowires embedded in a polymer matrix is also presented. Scanning electron microscope pictures are provided to demonstrate the fabrication processes. By exploiting the low thermal conductivity of the polymer and presumably high power factor of the nanowires, a high figure of merit results. Using the 3-ω technique, the effective thermal conductivity of the matrix embedded array of nanowires is measured.