Minimum thermal conductivity in the context of diffuson-mediated thermal transport

Abstract

A model for the thermal conductivity of bulk solids is proposed in the limit of diffusive transport mediated by diffusons as opposed to phonons. This diffusive thermal conductivity, κ_diff, is determined by the average energy of the vibrational density of states, ℏω_avg, and the number density of atoms, n. Furthermore, κ_diff is suggested as an appropriate estimate of the minimum thermal conductivity for complex materials, such that (at high temperatures): . A heuristic finding of this study is that the experimental ω_avg is highly correlated with the Debye temperature, allowing κ_diff to be estimated from the longitudinal and transverse speeds of sound: . Using this equation to estimate κ_min gives values 37% lower than the widely-used Cahill result and 18% lower than the Clarke model for κ_min, on average. This model of diffuson-mediated thermal conductivity may thus help explain experimental results of ultralow thermal conductivity.