Diffusion processes in vitreous silica revisited

Abstract

We analyzed extensive literature data on crystal growth rate, u, viscosity, eta, and diffusivities of silicon and oxygen at deep and low undercoolings, between the glass transition, Tg, and the melting point, Tm, for four types of commercial silica glasses and thin films. The self-diffusion coefficients, Du, and the viscosity, Deta, in this network glass are extremely dependent on the impurity level, much more than in multi-component, depolymerized, silicate glasses. Despite this drawback, we combined such kinetics data in a systematic way and confirmed that normal growth is the operative mechanism of crystal growth. Then the effective diffusivity for viscous flow, Deta, and the controlling activation energy were compared with the activation energies and diffusivities calculated from crystal growth rates, Du, and with those of silicon and oxygen diffusion rates (DSi and DO, respectively). In the whole temperature range Du = Deta = DSi, but measured oxygen diffusivities were much higher than Du = Deta = DSi. We speculate that this fact can be explained because non-bridging oxygen diffuse much faster than bridging oxygen (more easily measured experimentally); or perhaps Si and bridging oxygen do not diffuse together. In addition, there is no sign of decoupling between silicon diffusivity and viscous flow from near the melting point to somewhat below Tg. We thus conclude that silicon controls the transport mechanism involved in crystal growth and viscous flow in this glass. The congruence of Du and Deta indicates that whatever the bond breaking and molecular reorientation mechanisms required for crystallization are, they are the same as those required for the atomic transport mechanism that controls viscous flow