Parametrization of viscosity-temperature relations of aluminosilicate melts

A new approach to parameterize the viscosity-temperature relationships of silicate melts is applied to a data set for 28 aluminosilicate melts based on a haplogranitic chemistry. The approach is based on the introduction of a crossover temperature T_c. Such a crossover temperature has been predicted by the recently developed mode coupling theory (MCT). For simple liquids, this temperature distinguishes a fluid regime with power-law temperature dependence of viscosity from a viscous regime with Vogel-Fulcher-Tammann temperature dependence of viscosity. The viscosities of melts used in the present application, have been obtained in a temperature range from 1650° to 450°C at 1-atm pressure. The viscosity was determined with three different methods (concentric cylinder, micropenetration and centrifuge assisted falling sphere), which span a measurable viscosity range from 10° to 10¹² Pa s. By introducing the crossover temperature T_c we are able to collapse these data onto a master curve, which has been already obtained for simple glass formers like o-terphenyl. The resulting systematic variation of the fragility and T_c can be related to cationic properties such as electronegativity. The success of the master plot raises the possibility that an universal viscosity behavior exists for all liquids between T_C and T_g. The question remains as to whether T_c for non-fragile systems can be linked with the critical temperature of the mode coupling theory.