Diffusion, crystallization, viscous flow and relaxation kinetics in diopside and lithium diborate glasses

Abstract

The Stokes-Einstein/Eyring (SEE) equations are frequently used to estimate the effective diffusion coefficients that control crystallization by using viscosity data. Such equation is accepted for low to medium values of viscosity (10^-1 - 10^5 Pa.s) but has been questioned for higher viscosities (near 10^12 Pa.s). Nucleation and crystal growth mechanisms have received increasing attention, not only for the development or improvement of glass-ceramics, but also for providing indirect means to infer the effective diffusion coefficients in a wider temperature range (from Tg to near the melting point). In this research we plan to calculate the diffusion coefficients DI(T) and DU(T) using experimental determination of viscosity, nucleation and crystal growth rates, for diopside and lithium diborate glasses from the same batch. We will then compare DI(T) and DU(T) with the coefficients provided by the SEE equations.We also plan to measure the average relaxation times below Tg for both glasses. These values can be compared with viscosity models using Maxwell's equation, and be used to test the main viscosity models: VFT, MYEGA, Adam-Gibbs and Isostructural. This method is necessary due to the impossibility to obtain experimental viscosity data above 10^14 Pa.s. Finally, with extrapolations from the crystallization and structural relaxation data we will be able to check Kauzmann's proposal to solve his own paradox: that any glass must crystallize before it reaches the equilibrium liquid structure at the Kauzmann temperature, Tk. Such hypothesis has not been rigorously tested in the way we propose here. (AU)