Room Temperature Phonon Focusing As a Tool for Tuning Thermal Conductivity in Nanostructured Materials

Phonon focusing arises from the anisotropy in the phonon dispersions in crystalline materials. This effect has been extensively studied at low temperatures in bulk materials, where relatively low phonon scattering rates lead long mean-free-paths (MFP) with respect to the material’s specimen [1]. Here we investigate this phenomenon at room temperature in Si nanostructured materials. These systems can have a feature size as small as a few nanometers thus are comparable with a large fraction of phonon MFPs. By using the MFP-Boltzmann transport equation [2], we compute heat transport of nanoporous materials with different crystal orientations with respect to the pore lattice, finding a significant variation in the thermal conductivity. These results illustrate a route for tuning thermal transport in nanostructured materials beyond shape optimization.
[1] G. A., Northrop and J. P. Wolfe. Physical Review Letters 43.19 (1979): 1424.
[2] G. Romano and J. C. Grossman. Journal of Heat Transfer 137.7 (2015): 071302.