Direct Measurements of Homogeneous Nucleation Rates in a Glass-Forming System

Abstract

A metastable liquid miscibility gap was shown to exist in the Na2O·CaO·SiO2 system. Nucleation rates were measured at a number of undercoolings for a composition at the edge of this gap. Accurate measurements were possible due to the extremely slow approach to equilibrium because of the high viscosities (>1010 P) of the liquid at the nucleation temperatures. The Lumsden‐solution model and experimental miscibility‐gap data were used to calculate volume free energies driving the separation at the various undercoolings. Measuring the variation of solubility temperature with particle size of the separating phase a value of 4.6 ergs cm−2 was obtained for the interfacial energy. An activation energy for diffusion of 94.5 kcal mole−1 was obtained from measured growth and miscibility‐gap data. At most of the seven undercoolings the agreement between measured and calculated homogeneous nucleation rates when using the Lumsden model was excellent.