Viscosity of Silica and Doped Silica Melts: Evidence for a Crossover Temperature

TL;DR

This study investigates how trace dopants affect silica melt viscosity, revealing a universal crossover temperature where the dominant transport mechanism shifts from intrinsic to dopant-induced defects.

Contribution

It provides experimental evidence of a crossover temperature in silica melts, highlighting the impact of impurities on viscosity and transport mechanisms.

Findings

Viscosity curves show a crossover around 2200-2500 K.

Dopant type significantly alters the glass transition temperature.

Transport mechanism shifts from intrinsic to dopant-induced defects at crossover.

Abstract

Silica is known as the archetypal strong liquid, exhibiting an Arrhenius viscosity curve with a high glass transition temperature and constant activation energy. However, given the ideally isostatic nature of the silica network, the presence of even a small concentration of defects can lead to a significant decrease in both the glass transition temperature and activation energy for viscous flow. To understand the impact of trace level dopants on the viscosity of silica, we measure the viscosity-temperature curves for seven silica glass samples having different impurities, including four natural and three synthetic samples. Depending on the type of dopant, the glass transition temperature can vary by nearly 300 K. A common crossover is found for all viscosity curves around ~2200-2500 K, which we attribute to a change of the transport mechanism in the melt from being dominated by intrinsic defects at high temperature to dopant-induced defects at low temperatures.