Phonon backscatter, trapping, bottlenecking, and misalignment effects on thermal conductivity of Si Nanostructures

Nanostructured systems offer the ability to reduce thermal conductivity which, for instance, may be used to improve the thermoelectric efficiency. The internal surfaces of these nanostructures significantly curtail the ballistic free flight of phonons, and thus resist thermal transport. In this talk, we present a recent investigation of a simple one-parameter geometry that simultaneously modulates backscattering and trapping effects to enable directed study of controlling phonons. The geometry is a simple sequence of chambers offset from one another by a defined distance. We use the geometry to study the effects of phonon backscatter, trapping, bottlenecking, and corner-turning on the thermal conductance in Si nanowires (NWs). By creating a geometry that maximizes backscatter, a roughly 8-fold reduction in thermal conductance below the Casimir limit can be achieved at room temperature which is a factor of four smaller than the nearest reported value in the literature. The geometry is also useful for systematic investigation of other means of controlling phonons and affecting thermal transport. Specifically, we investigate the induced misalignment between the phonon flow and thermal flux due to the shape of the geometry as well as phonon filter-like behavior of the geometry.