Mechanisms and dynamics of crystal growth, viscous flow, and self-diffusion in silica glass

Abstract

We analyzed extensive literature data on crystal growth rate and viscosity in the temperature range between $1.1\phantom{\rule{0.3em}{0ex}}{T}_{g}$ (glass transition temperature) and the melting point of silica, ${T}_{m}$. We selected data for one silica glass type, having similar impurity contents, and confirmed that the normal growth model describes quite well the experimental growth rate data in this wide undercooling range. We then calculated effective diffusion coefficients from crystal growth rates, ${D}_{u}$, and from viscosity, ${D}_{\ensuremath{\eta}}$ (through the Eyring equation), and compared these two independent diffusivities with directly measured self-diffusion coefficients of silicon and oxygen in the same silica glass for which viscosity was measured. Our results show that silicon (not oxygen) controls the diffusion dynamics involved in both crystal growth and viscous flow in undercooled silica. This study not only unveils the transport mechanism in this important glass-forming material, but also validates the use of (easily measured) viscosity to account for the unknown transport term of the crystal growth expression in a wide range of undercoolings.