We have developed a new simulation technique to probe phonon-nanoparticle scattering in anisotropic materials. This technique enables, for the first time, direct observation of the effects of mode conversion, lattice mismatch strain, elastic anisotropy, and atomistic granularity on the spectral-directional scattering of phonons from nanoparticles. The technique will be useful for the design of novel nanoparticle-based thermal insulating materials for thermoelectric energy conversion.

We have created the first molecular dynamics approach capable of studying phonon focusing. This approach, which generates multidimensional acoustic phonon wavepackets and visualizes their arrival at an image plane "detector," provides new capabilities for identifying regions where thermal energy is preferentially channeled and offers an alternative to existing experimental techniques. Additionally, it enables direct observation of phonon frequency redistribution as the wave packet propagates.

Compare simulation movie above to experimental movie from Wolfe group

We have used molecular dynamics simulations to probe how thermal expansion and impurities affect the thermal conductivity of materials with point defects. We found that strain induced by the defect (and not mass difference between the defect and its surroundings) is the dominant mechanism contributing to the thermal conductivity reduction occurring in such materials. Also we have found that inclusion of temperature dependent lattice constants is essential to recover the correct thermal conductivity temperature dependence. Finally, we have found that molecular dynamics is a viable alternative to phonon scattering theory for obtaining phonon scattering parameters for use in thermal conductivity models such as the Callaway model. This discovery provides a step toward expedient calculation of thermal conductivity in materials with defects.