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We report on electric, thermal, and optical properties of Si subjected to severe plastic deformation. Single-crystalline Si wafers were processed by high-pressure torsion (HPT) under a nominal pressure of 6 GPa. The HPT-processed samples consisted of metastable body-centered-cubic Si-III and rhombohedral Si-XII as well as diamond-cubic Si-I and amorphous phases. The metastable phases increased with increasing the number of anvil rotations (N). The resistivity of the single-crystalline Si (20 Ω cm) increased to 50 Ω cm after HPT processing for N = 10 and then it decreased to ∼0.7 Ω cm when increasing N to 100. Such an increase and a subsequent decrease in resistivity were attributed to the grain refinement and the increase in the volume fraction of semimetallic Si-III, respectively. The thermal conductivity was reduced by two orders of magnitude (∼3 W m−1 K−1) after HPT processing for N ≥ 50. A weak broad photoluminescence peak originating from Si-I nanograins appeared in the visible light region after annealing at 600 °C. These results indicate that the resistivity, thermal conductivity, and photoluminescence of the HPT-processed Si strongly depend on the formation of metastable phases and grain refinement, which are induced by shear strain under high pressure. [ABSTRACT FROM AUTHOR]

Reducing the lattice thermal conductivity (κL) of dense solid materials is critical for thermal insulation and thermoelectrics. Although nanocrystalline materials formed on a large-scale by hot pressing or sintering nanoparticles can achieve low κL, there is still considerable room for further reduction. In this study, a moderate high-pressure torsion (HPT) process is applied on nanocrystalline silicon to further reduce the κL by directly introducing finer nanostructures and internal strain without causing phase transition. Unlike conventional approaches that manipulate the phonon mean free path through the classical "size effect", the inhomogeneous internal strain induced by HPT leads to overall lattice softening and a significant boundary softening effect, which can reduce the phonon group velocity, and enhance the phonon scattering at grain boundaries, respectively. This can thereby bring extra suppression on κL, achieving an record low κL of 1.49 W m−1 K−1 for being a fully dense bulk silicon without any amorphous or metastable phases, which is comparable to its amorphous counterpart. This study demonstrates a practical and feasible strain engineering strategy for realizing low κL that is applicable to various nanocrystalline materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]