March 1, 2018
11:00 AM - 12:00 PM
Speaker: Jeff Snyder
Professor, Department of Materials Science & Engineering
Title: "Dislocation strain as the mechanism of phonon scattering at grain boundaries"
For 50 years, we have commonly been using Casimir’s theory that describes the scattering of heat-carrying lattice vibrations (phonons) on the sample boundaries to also describe the reduction of thermal conductivity due to grain boundaries. In the frequency-independent Casimir model, phonons simply cannot travel across the boundaries, which is not the case in grain boundaries. This and a growing body of experimental and computational evidence shows that the modification of the Casimir model is necessary for grain boundaries. In this talk I will discuss our analysis of phonon scattering that controls the thermal conductivity of many common thermoelectric materials. We find that the grain boundary dislocation strain model can substitute for the Casimir model. More importantly, the two models can be distinguished at low temperature in fine-grained materials such that experimental evidence supports the grain boundary dislocation strain model. In this way, we suggest that grain boundaries themselves are best conceptualized as a collection of dislocations. Since strain and grain boundary structures can vary, we should be able to engineer grain boundaries or grain complexions (including extrinsic atoms) to disrupt phonon transport without harming electron transport.