Thermodynamic Barriers to the Formation of Critical-Sized Crystalline Embryos in Silicate Glasses

The author proposes and describes a way of estimating the magnitude of the thermodynamic barrier to the formation of a critical-sized crystalline nucleus. The frequency of the formation of such nuclei that are stable and capable of further growth determines the rate of nucleation. For nucleation to occur, the nucleus must overcome the thermodynamic barrier. The measure of the magnitude of this barrier is the work needed to create the surface of a nucleus W*, determined from the values of surface energy σ and size r* of the critical nucleus for different temperatures of the preliminary heat treatment of glasses and their subsequent manifestation (crystal growth). The values of σ and r* are obtained from experimentally determined rates Ist of steady-state crystal nucleation; periods τ of non-stationary crystal nucleation, the measure of which is time tind of the induction period; and difference Δφ between the specific free energies of the glass and crystal. Values of W* are obtained for a series of silicate glass compositions for promoting crystal nucleation in these glasses.