UC Irvine

Nanocrystalline silicon can have unique thermal transport and mechanical properties governed by the constituent grain microstructure. In this thesis work, we use phonon ray-tracing and molecular dynamics simulations to demonstrate the largely tunable thermomechanical behaviors with varying grain sizes (a0) and aspect ratios (ξ). We show that by selectively increasing the grain size along the heat transfer direction while keeping the grain area constant, the in-plane thermal conductivity (kx) increases more significantly than the cross-plane thermal conductivity (ky), originating from anisotropic phonon-grain boundary scattering. The kx increases with increasing aspect ratio ξ until a critical value, at which kx reaches a maximum. Further increase in ξ leads to a decrease in kx thermal conductivity decrease, steaming from substantial scattering of low-frequency phonon with anisotropic grain boundaries. In addition, we find the elastic modulus shows strong size-dependence, and the softening effect leads to significant reductions in the phonon group velocity and the thermal conductivity. By accounting for both thermal and mechanical size effects, we identify two distinct regimes of thermal transport, in which anisotropic phonon-grain boundary scattering becomes more appreciable at low temperatures and phonon softening becomes more appreciable at high temperatures. Through spectral analysis, we attribute the significant thermal conductivity anisotropy in the nanocrystalline silicon to grain boundary scattering of low-frequency phonons and the softening-driven thermal conductivity reductions to Umklapp scattering of high-frequency phonons. These findings suggest new pathways to manipulate thermomechanical properties of nanocrystalline silicon via microstructure engineering, having profound implication for future anisotropic nanomaterials.