Thermal conductivity of mixed alkali silicate glasses at low temperature

Thermal conductivity of glass is one of the fundamental properties of it. However, that has not been studied enough. That of only less than 20 compositions has been measured below the room temperature. In this study, we measured the thermal conductivity of xNa2O · (100 − x)SiO2 and (33 − y)Na2O · yCs2O · 67SiO2 glasses by a transient heating method in the temperature range from about 150 K to room temperature. The conductivity of xNa2O · (100 − x)SiO2 is found to decrease with the increase in alkali content. The dominant factor of this behavior is the decrease in phonon mean free path, which is due to the increase of non-bridge oxygen. Thermal conductivity of (33 − y)Cs2O · yNa2O · 67SiO2 is decreased with the increase in Cs2O/(Na2O + Cs2O) ratio. The dominant factor of this behavior is the decrease of sound velocity. However, composition dependence of the phonon mean free path also affects the thermal conductivity. Phonon mean free path of 33Cs2O · 67SiO2 glass is longer than that of 33Na2O · 67SiO2 glass, and should be related to the change in distribution of structural unit in glass. In addition, phonon mean free path of mixed alkali glasses are shorter than that of single alkali glasses.