Length dependence of thermal conductivity by approach-to-equilibrium molecular dynamics

TL;DR

This study systematically investigates how thermal conductivity depends on sample length across various materials using approach-to-equilibrium molecular dynamics, revealing the link to phonon mean-free-path and evaluating extrapolation methods.

Contribution

It demonstrates that AEMD with finite supercell length probes the maximum phonon MFP and introduces a new extrapolation method for infinite-length conductivity.

Findings

Conductivity correlates strongly with maximum phonon MFP.

Poorer conductors like amorphous silica have shorter phonon MFPs.

The Matthiessen rule is not suitable for AEMD extrapolation.

Abstract

The length dependence of the thermal conductivity over more than two decades is systematically studied for a range of materials, interatomic potentials and temperatures, by the atomistic approach-to-equilibrium molecular dynamics method (AEMD). By comparing the values of conductivity obtained for a given supercell length and maximum phonon mean-free-path (MFP), we find that such values are strongly correlated, demonstrating that the AEMD calculation with a supercell of finite length, actually probes the thermal conductivity corresponding to a maximum phonon MFP. As a consequence, the less pronounced length dependence usually observed for poorer thermal conductors, such as amorphous silica, is physically justified by their shorter average phonon MFP. Finally, we compare different analytical extrapolations of the conductivity to infinite length, and demonstrate that the frequently used Matthiessen rule is not applicable in AEMD. An alternative extrapolation more suitable for transient-time, finite-supercell simulations is derived. This approximation scheme can also be used to classify the quality of different interatomic potential models with respect to their capability of predicting the experimental thermal conductivity.