MCE Ph.D. Thesis Seminar

Heat is one of the most fundamental forms of energy, and the ability to precisely control heat plays a critical role in most current and future energy applications. Recently, interface engineering between heterogeneous solids has provided new approaches to manipulate heat transport at the scales of the energy carriers in solids, i.e. phonons, which are quantized lattice vibrations. For example, engineered interfaces in solids have led to dramatic increases in the efficiency of thermoelectric (TE) materials. Despite the importance of interfaces in phonon-mediated heat conduction, the basic phenomena of interfacial heat transport, i.e. phonon transmission coefficients at interfaces, remain among the most poorly understood transport processes. To elucidate this process, in this work we investigate interfacial thermal phonon transport using both modeling and experiment. In the first part of this work, we examined the impact of frequency-dependent transmission coefficient profiles in nanocrystalline silicon and silicon-germanium alloys with nanoscale grain sizes using a novel computational method. We find that the interface may not be as effective as commonly considered in scattering certain phonon, with a substantial amount of heat being carried by low frequency phonons with mean free paths longer than the grain size. While this computational study provides important insights, despite decades of work, an unambiguous determination of the transmission coefficients at an actual interface has not yet been reported.  In the second part of this work, we reported the first measurements of the thermal phonon transmission coefficients at a solid interface using ab-initio phonon transport modeling based on the Boltzmann transport equation and a thermal characterization technique, time-domain thermoreflectance. With our approach, we are able to directly link the atomic structure of an interface to the spectral content of the heat crossing it for the first time. Our work realizes the long-standing goal of directly measuring thermal phonon transmission coefficients and demonstrates a general route to study microscopic processes governing interfacial phonon transport