Thermal conductivity in modified oxide glasses is governed by modal phase changes

TL;DR

This study investigates how modal phase changes influence thermal conductivity in modified oxide glasses, revealing that increased sodium oxide content induces vibrational localization and reduces thermal conductivity due to disordered vibrational phases.

Contribution

The paper provides a detailed analysis of vibrational modal contributions in sodium silicate glasses, highlighting the role of phase changes in governing thermal conductivity, which advances understanding beyond simple glasses.

Findings

Increased Na2O content leads to vibrational localization.

Vibrational phases of sodium are more disordered than silicon and oxygen.

Higher Na2O content decreases thermal conductivity.

Abstract

The thermal conductivity of glasses is well-known to be significantly harder to theoretically describe compared to crystalline materials. Because of this fact, the fundamental understanding of thermal conductivity in glasses remain extremely poor when moving beyond the case of simple glasses, e.g., glassy SiO$_2$, and into so-called 'modified' oxide glasses, that is, glasses where other oxides (e.g. alkali oxides) have been added to break up the network and alter e.g. elastic and thermal properties. This lack of knowledge is apparent despite how modified glasses comprise the far majority of known glasses. In the present work we study an archetypical series of sodium silicate ($x\text{Na}_2\text{O}\text{-}(100\text{-}x)\text{SiO}_2$) glasses. Analyses of modal contributions reveal how increasing Na$_2$O content induces increasing vibrational localization with a change of vibrations to be less ordered, and a related general decrease in modal contributions to thermal conductivity. We find the vibrational phases (acoustic vs. optical) of sodium vibrations to be relatively disordered compared to the network-forming silicon and oxygen species, explaining how increasing Na$_2$O content decreases thermal conductivity. Our work sheds new light on the fundamentals of glassy heat transfer as well as the interplay between thermal conduction and modal characteristics in glasses.