Effect of anharmonicity on the thermal conductivity of amorphous silica

TL;DR

This study investigates how anharmonicity affects the thermal conductivity of amorphous silica, revealing that anharmonic effects increase conductivity and influence mode interactions, providing new insights into heat transfer mechanisms in amorphous materials.

Contribution

It compares harmonic and anharmonic theories for thermal conductivity prediction in amorphous silica, highlighting the role of anharmonicity-induced frequency shifts and mode interactions.

Findings

Anharmonic theory predicts higher thermal conductivity than harmonic theory.

Anharmonic frequency shifts contribute to positive temperature dependence.

Diffusons' increased diffusivity mainly drives conductivity enhancement.

Abstract

Proper consideration of anharmonicity is important for the calculation of the thermal conductivity. However, how the anharmonicity influences the thermal conduction in amorphous materials is still an open question. In this work, we uncover the role of anharmonicity on the thermal conductivity of amorphous silica (a-SiO2) by comparing the thermal conductivity predicted from the harmonic theory and the anharmonic theory. Moreover, we explore the effect of anharmonicity-induced frequency shift on the prediction of the thermal conductivity. It is found that the thermal conductivity calculated by the recently developed anharmonic theory (quasi-harmonic Green-Kubo approximation, QHGK) is higher than that by the harmonic theory developed by Allen and Feldman. The use of anharmonic vibrational frequencies also leads to a higher thermal conductivity compared with that calculated using harmonic vibrational frequencies. The anharmonicity induced frequency shifts is a mechanism for the positive temperature dependence of the thermal conductivity of a-SiO2 at higher temperatures. Further investigation on mode diffusivity suggests that although anharmonicity has larger influence on locons than diffusons, the increase of the thermal conductivity due to the anharmonicity is mainly contributed by the anharmonicity induced increase of the diffusivity of diffusons. Finally, it is found that the cross-correlations between diffusons and diffusons contribute most to the thermal conductivity of a-SiO2, and the locons contribute to the thermal conductivity mainly through collaboration with diffusons. These results offer new insights into the nature of the thermal conduction in a-SiO2.