Mechanical and Civil Engineering Seminar

Thermal transport is a key issue limiting many important energy transfer and conversion applications such as thermoelectric energy conversion and thermal management of electronic devices. Due to the broadband nature of phonons, predicting and understanding spectral phonon properties (phonon spectroscopy), including phonon dispersion and spectral phonon mean free path (scattering rate), are essential for engineering nanomaterials and nanostructures to achieve desired thermal transport characteristics. This talk will cover several unusual findings on spectral phonon scattering in solids using predictive atomistic simulations. Despite the recent significant success of first principles predictions of lattice thermal conductivity from perturbation theory, it only considers three-phonon scattering. We have developed a formalism to explicitly determine four-phonon scattering rates, and found that they are comparable to three-phonon scattering rates at medium and high temperatures. As a consequence, the thermal conductivities of argon, diamond, silicon, and germanium are reduced considerably at high temperature when four-phonon scattering is considered. We have also investigated the validity of the spectral Matthiessen's (M's) rule. Despite its wide use, the M's rule assumes that the scattering mechanisms are independent, which is usually not justified. Here we take defective bulk Si as an example and calculate phonon-phonon scattering, phonon-impurity scattering, and the total scattering rates in three independent ways. It is found that the spectral M's rule is not accurate due to the neglect of the coupling between anharmonic phonon-phonon scattering and phonon-impurity scattering. Last, we show that asymmetric graphene nanoribbons show interesting nonlinear thermal transport phenomena such as thermal rectification. The mechanism is identified as phonon confinement in the lateral dimension, hence the lateral size needs to be smaller than the phonon mean free path. Such behaviors can potentially be used to make devices that can control heat flow such as thermal diodes.