Heat conduction is central to numerous and diverse technologies, ranging from power generation, to electronics, to lasers. Increasingly in these applications, heat conduction occurs at scales much less than a micron, comparable to the fundamental length scales of the heat carriers themselves. For example, nanostructured thermoelectrics may contain engineered structures as small as a few nanometers; these nanoscale materials are 50% more efficient at converting heat to electricity than their macroscopic counterparts. However, further advances are difficult to achieve because of a poor understanding of the lattice vibration, or phonon, scattering processes that largely determine a material?s thermal conductivity. Despite over 50 years of investigation, the mean free paths (MFPs) of phonons, which describe how phonons scatter when they interact with other phonons, defects, and nanostructures, are unknown in most materials. This experimental and computational investigation will provide a comprehensive understanding of thermal phonon scattering. Novel experimental methods that have been developed only recently will be used to measure MFP distributions across length scales ranging from nanometers to millimeters. The effect of different scattering mechanisms on specific phonon modes will be obtained by observing the changes in the MFP distribution as mass defects, grain boundaries, nanoprecipitates, and other nanostructures are systematically introduced into the material. Analytical expressions for the MFPs, which are of great utility to researchers in the field, will be created and validated against the experimental results using numerical solutions of the Boltzmann transport equation.
This project will advance our scientific knowledge of heat conduction and enable many applications, particularly in the energy field. Scientifically, the investigation will provide a comprehensive understanding of the complex thermal phonon scattering process, which has eluded scientists for decades, as well as experimentally validate fundamental computational predictions of thermal phonon scattering rates. Practically, this research will enable engineers to design materials with precisely tailored thermal conductivities before fabrication rather than by trial-and-error, a major advance over present capabilities. This ability would lead to many applications, such as highly efficient cars that harvest useful electricity from wasted heat in the tailpipe, environmentally friendly refrigerators that do not use any harmful fluids, and electronic devices with reduced power consumption.