Phonon transport in multiphase nanostructured silicon fabricated by high-pressure torsion

We present a combined experimental and numerical investigation of phonon transport in multiphase nanostructured silicon. The sample was synthesized by high-pressure torsion with a nominal pressure of 24 GPa. Based on the x-ray diffraction measurement, we have identified the existence of three phases of silicon in the sample: Si-I, Si-III, and Si-XII, with volume fractions of 66%, 25%, and 9% and average grain sizes of 25, 14, and 11 nm, respectively. The measured thermal conductivities of the sample in the temperature range of 150-330 K are on the order of 5 W/(m K) and exhibit weak temperature dependence. A multiscale modeling that incorporates first-principles lattice dynamics, the Monte Carlo ray-tracing method, and effective medium theory was used to understand the mechanism of phonon transport in multiphase nanostructured silicon as well as the weak temperature dependence. We found that the thermal conductivity of single-phase nanostructured silicon decreases with decreasing average grain size and is about an order of magnitude lower than the corresponding bulk counterpart when the average grain size is O (10 nm). The weak temperature-dependent thermal conductivity in the nanostructured silicon is attributed to the strong elastic phonon-boundary scattering at the grain boundary. The thermal conductivity predicted from the multiscale modeling matches reasonably well with the measurement. This work provides insights into phonon transport in multiphase nanostructured materials and suggests that the effective thermal conductivity of nanostructured silicon from high-pressure torsion can be further reduced by increasing the volume fractions of the Si-III and Si-XII phases.