Reducing thermal conductivity through lattice softening

Two fundamentally different avenues for controlling a materials thermal conductivity are phonon scattering and lattice softening. Lattice softening recognizes that lattice defects alter the phonon dispersion relation and thus reduce the lattice thermal conductivity (κ_L) by reducing phonon frequencies and group velocities. I will discuss experimental data on several systems (Si, PbTe, and SnTe) which demonstrate that microstructural defects such as grain boundaries, dislocations, and vacancies can significantly softening a materials lattice, reducing the materials speed of sound. By analyzing the data on elasticity and thermal conductivity through transport modeling, it is shown that lattice softening is a dominate mechanism for the reduction of κ_L in these systems. Additionally, it will be shown that lattice softening is theoretically expected to be more effective than phonon scattering effects in anharmonic materials and at high temperatures. This work demonstrates how lattice softening is emerging as an important mechanism for controlling a materials thermal conductivity, and provides new avenues to engineer a materials κ_L, beyond phonon scattering.