Impact of grain sizes on phonon thermal conductivity of bulk thermoelectric materials

We report a phonon transport model for a bulk thermoelectric material to investigate the effect of the grain diameter and the grain-boundary thickness on its thermal conductivity. The analysis results display that the bulk thermal conductivity is significantly reduced by decreasing the grain sizes when the grain diameter is less than 500 nm, which provides us one feasible way to enhance the figure of merit ZT of the material, and the thermal conductivity is mainly attributed to the contribution of grains, while the grain-boundary thermal conductivity has to be considered when the grain diameter is less than 100 nm.