Mechanisms of temperature-dependent thermal transport in amorphous silica from machine-learning molecular dynamics

TL;DR

This study develops a machine-learned potential for amorphous silica, enabling accurate molecular dynamics simulations that reveal how phonons and liquid-like diffusion contribute to temperature-dependent thermal transport.

Contribution

The paper introduces a highly efficient machine-learning potential that closely replicates experimental amorphous silica, allowing detailed analysis of thermal transport mechanisms across temperatures.

Findings

Phonons remain well-defined below the Ioffe-Regel crossover frequency.

Thermal conductivity increases at low temperatures due to phonon activity.

High-temperature thermal conductivity is governed by phonon scattering and liquid-like diffusion.

Abstract

Amorphous silica (a-SiO$_2$) is a foundational disordered material for which the thermal transport properties are important for various applications. To accurately model the interatomic interactions in classical molecular dynamics (MD) simulations of thermal transport in a-SiO$_2$, we herein develop an accurate yet highly efficient machine-learned potential model that allowed us to generate a-SiO$_2$ samples closely resembling experimentally produced ones. Using the homogeneous nonequilibrium MD method and a proper quantum-statistical correction to the classical MD results, quantitative agreement with experiments is achieved for the thermal conductivities of bulk and 190 nm-thick a-SiO$_2$ films over a wide range of temperatures. To interrogate the thermal vibrations at different temperatures, we calculated the current correlation functions corresponding to the transverse acoustic (TA) and longitudinal acoustic (LA) collective vibrations. The results reveal that below the Ioffe-Regel crossover frequency, phonons as well-defined excitations, remain applicable in a-SiO$_2$ and play a predominant role at low temperatures, resulting in a temperature-dependent increase in thermal conductivity. In the high-temperature region, more phonons are excited, accompanied by a more intense liquid-like diffusion event. We attribute the temperature-independent thermal conductivity in the high-temperature range of a-SiO$_2$ to the collaborative involvement of excited phonon scattering and liquid-like diffusion in heat conduction. These findings provide physical insights into the thermal transport of a-SiO$_2$ and are expected to be applied to a vast range of amorphous materials.