Does viscosity describe the kinetic barrier for crystal growth from the liquidus to the glass transition?

Abstract

An analysis of the kinetic coefficient of crystal growth, Ukin , recently proposed by Ediger and colleagues [J. Chem. Phys. 128 (2008) 034709] indicates that the Stokes-Einstein / Eyring (SE/E) equation does not describe the transport controlling crystal growth rates in fragile glass forming liquids. Ukin was defined using the normal growth model and tested for crystal data for inorganic and organic liquids covering a viscosity range of about 104 to 1012 Pa.s. Here we revisit this interesting finding considering two other models: the screw dislocation (SD) and the two-dimensional surface nucleated (2D) growth models for nine undercooled oxide liquids, in a wider temperature range, from slightly below the melting point down to the glass transition region, Tg, thus covering a wider viscosity range: 101 - 1013 Pa.s. We then propose and use normalized kinetic coefficients (Mkin) for the SD and 2D growth models. These new kinetic coefficients restore the ability of viscosity to describe the transport part of crystal growth rates (Mkin ~ 1/η and ξ ~ 1) from low to moderate viscosities (eta < 106 Pa.s), and thus the SE/E equation works well in this viscosity range for all systems tested. For strong glasses, the SE/E equation works well from low to high viscosities, from the melting point down to Tg! However, for at least three fragile liquids [diopside (kink at 1.08Tg, eta = 1.6x108 Pa.s), lead metasilicate (kink at 1.14Tg, eta = 4.3x106 Pa.s) and lithium disilicate (kink at 1.11Tg, eta = 1.6x108 Pa.s) there are clear signs of a breakdown of the SE/E equation at these higher viscosities. Our results corroborate the findings of Ediger et al., and demonstrate that viscosity data cannot be used to describe the transport part of crystal growth (via the SE/E equation) in fragile glasses in the neighborhood of Tg.