Dynamic Processes in Silica and Alumina Glasses

Reference	Presenter	Authors (Institution)	Abstract
10-024	Marcio Luis Ferreira Nascimento	Nascimento, M.L.(Federal University of Bahia);	A collection and critical analysis of extensive literature data on at least four important kinetic processes - viscous flow, conductivity, crystal growth and diffusivity data (measured or from molecular dynamics simulations) - in type I silica (${\mathrm{SiO}}_2$) as well as alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) are shown. Such data are in a wide temperature range, from above $T_m$ to below $T_g$, where $T_g=$ 727 K and 1063 K (estimated) are the glass transition temperatures and $T_m=$ 1307 K and 2323 K are the melting points of silica and alumina, respectively. We verified that the normal crystal growth is the most likely growth mechanism in silica (there are no crystal growth measurements in alumina case). The similarity of the temperature dependencies of $1/\mathrm{\eta }$, where $\mathrm{\eta }\left(T\right)$ is shear viscosity, and effective diffusion coefficients $D_{eff}$ corroborates the validity of the Stokes-Einstein / Eyring equation (SEE) at high temperatures around $T_m$. Using the equality of $D_{eff}$ and $D_{eff}^{\eta }$ (i.e. that uses viscosity), we estimated the jump distance $\mathrm{\lambda }$ of few angstroms from the SEE equation and showed that the $D_{eff}^U$ (that takes into account growth rate data) have the same temperature dependence but exceeds $D_{eff}^{\eta }$ by some orders of magnitude. The difference between $D_{eff}^{\eta }$ and $D_{eff}^U$ in silica indicates that the first determines the process of mass transport in the bulk whereas the second relates to the mobility of the structural units on the crystal/liquid interface. Summarizing, we have shown that at low undercoolings one can employ viscosity data for quantitative analyses of crystal growth rates, but in the deeply supercooled liquid state, mass transport for crystal growth is not controlled by viscosity.
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