Non-negligible Contributions to Thermal Conductivity From Localized Modes in Amorphous Silicon Dioxide

TL;DR

This study uses a new Green-Kubo modal analysis method to show that localized vibrational modes significantly contribute to thermal conductivity in amorphous silica, challenging previous assumptions.

Contribution

The paper introduces the first inclusion of anharmonic effects in thermal conductivity calculations for all modes in amorphous silica, revealing the substantial role of localized modes.

Findings

Localized modes contribute over 10% to thermal conductivity between 400K and 800K.

Localized modes are largely responsible for the increase in thermal conductivity above room temperature.

The Green-Kubo modal analysis accurately predicts experimental thermal conductivities.

Abstract

Thermal conductivity is an important property for almost all applications involving heat transfer, ranging from energy and microelectronics to food processing and textiles. The theory and modeling of crystalline materials is in some sense a solved problem, where one can now calculate the thermal conductivity of any crystalline line compound from first principles [1,2] using expressions based on the phonon gas model (PGM)[3,4]. However, modeling of amorphous materials still has many open questions, because the PGM itself becomes questionable when one cannot rigorously define the phonon velocities. New theories and methods are therefore needed to understand phonon transport in amorphous materials. In this letter, we used our recently developed Green-Kubo modal analysis (GKMA) method to study amorphous silica (a-SiO2). The predicted thermal conductivities exhibit excellent agreement with experiments at all temperatures and anharmonic effects are included in the thermal conductivity calculation for all types of modes in a-SiO2 for the first time. Previously, localized modes (locons) have been thought to have a negligible contribution to thermal conductivity, due to their highly localized nature, which conceptually should prevent them from moving energy to another location. However, in a-SiO2 our results indicate that locons contribute more than 10% to the total thermal conductivity from 400K to 800K and they are largely responsible for the increase in thermal conductivity of a-SiO2 above room temperature. This is an effect that cannot be explained by previous methods and therefore offers new insight into the nature of phonon transport in amorphous/glassy materials.