Exploring Heat Transfer At The Atomistic Level For Thermal Energy Conversion And Management

Heat transfer at the scales of atoms plays an important role in many applications such as thermoelectric energy conversion and thermal management of microelectronic devices. While nanoengineering offers unique opportunities to manipulate heat to our advantages, it also imposes challenges on the fundamental understanding of nanoscale heat transfer. As the characteristic lengths of the system size become comparable to the mean free paths of heat carriers, macroscopic theories based on heat diffusion are no longer valid due to size effects. Atomistic level simulation can provide powerful insights into the microscopic processes governing heat conduction, and is the focus of this thesis. In this thesis, we first introduce atomistic techniques to investigate phonon transport in bulk crystals. We start with normal mode analysis within the classical molecular dynamics framework to estimate the spectral phonon transport properties. Although it can provide the detailed phonon properties adequately, classical molecular dynamics with empirical potentials do not always yield accurate predictions. Then, we move to first-principles density functional theory (DFT) to compute mode-dependent phonon properties. Such simulations can well reproduce experimental values of phonon dispersion and thermal conductivity with no adjustable parameters, establishing confidence that such an approach can provide reliable information about the microscopic processes. These detailed calculations not only unveil which phonon modes are responsible for heat conduction in bulk crystals, but also expand our fundamental understanding of phonon transport, such as the importance of optical phonons. Next, we study thermal transport across single and multiple interfaces via the atomistic Green's function method, especially the impact of interface roughness on phonon transmission across a single interface and coherent phonon transport in superlattices. Both the DFT and Green's function techniques provide fundamental parameters that then can be used to understand mesoscale transport. This paves the way for multiscale modeling from first-principles. Through these multiscale modeling efforts, we are able to obtain a comprehensive understanding of heat transfer from the atomistic to the macroscale, with important implications for energy applications. Complementary to the theoretical work, we measure the interface thermal conductance using ultrafast time-domain thermoreflectance experiments, examining thermal transport across solid-liquid interfaces modified by self-assembled monolayers. We find that an extra molecular layer can enhance the thermal transport across solid-liquid interfaces. In summary, theoretical, computational and theoretical approaches have been applied to study heat transfer at the atomistic level. The findings from this thesis have improved our fundamental understanding of phonon transport properties with important implications for energy applications and beyond, and build a foundation for multiscale simulation of phonon heat conduction at the mesoscale.