Extremely suppressed thermal conductivity of large-scale nanocrystalline silicon through inhomogeneous internal strain engineering

Reducing the lattice thermal conductivity (κ_L) 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 κ_L, 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 κ_L 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 κ_L, achieving an record low κ_L 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 κ_L that is applicable to various nanocrystalline materials.