Influence of structure disorder on the lattice thermal conductivity of polycrystals: A frequency-dependent phonon-transport study

It is widely accepted that the lattice thermal conductivity of a polycrystal mainly depends on its grain sizes, phonon mean free paths, and grain-boundary thermal resistance. However, uncertainties always exist on how much grain misalignment and a wide grain size distribution in a real polycrystal could affect the thermal analysis. Considering frequency-dependent phonon mean free paths, the influence of these factors is carefully examined by phonon Monte Carlo simulations for a series of disordered silicon polycrystals with grain sizes ranging from 1 to 400 nm. More generally, simulations are also performed on thermally anisotropic polycrystals. Despite all structure variation, this work suggests that the direction-averaged lattice thermal conductivity of a polycrystal is always close to that of an aligned polycrystal, with an effective grain size matching the interface density of the studied polycrystal.