Exploring local measurements to investigate the effects of defects and crystallinity on heat flow in polycrystalline materials

Crystalline defects have been utilized to control the performance of thermoelectric materials, which convert heat to electricity and vice versa. For example, impurity dopants have been used to optimize the energy conversion efficiency of thermoelectric materials by enhancing the electronic carrier concentration and electrical conductivity and suppressing the bulk thermal conductivity due to the scattering of microscopic heat carriers. However, the effects of single-crystal imperfections on the local thermal conductivity are poorly understood. In this paper, I will present recent spatially resolved frequency domain thermoreflectance (FDTR) measurements, in which the effect of grain boundary (GB) roughness, nano-twinning, and porosity on the local thermal conductivity and GB thermal boundary resistance in multi- and polycrystalline silicon is investigated. These measurements provide valuable insights into how the microstructure of polycrystalline materials leads to spatially heterogeneous thermal properties. These studies can potentially improve the current understanding of the local effects of crystal imperfections on microscale heat flow and facilitate microstructure-engineered bulk thermal properties of polycrystalline materials.