Lithium borate glasses: a quantitative study of strength and fragility

Methods for quantifying the strength/fragility of glass-forming systems are described, with particular reference to the alkali borates. One parameter used is the limiting slope of the log viscosity versus T∗/T plot as T approaches T∗, the temperature at which the viscosity is 1012 Pa s. A comparison is made with ΔCp/Cp1, the normalised change in specific heat capacity at Tg, as an alternative measure of the degree of fragility in these glass-forming systems. Both approaches indicate that increasing alkali oxide content results in increasing fragility within the borate system, although anomalous correlations can result when different glassy system are compared. The extent to which the limiting viscosity slope and ΔCp/Cp1 probe different aspects of structural change and fragility is briefly explored.