Kinetics of crystal nucleation in silicate glasses

The kinetics of volume nucleation are analysed in a number of “simple” systems in which the crystallising phase has the same composition as the parent glass. The compositions considered are Li 2O·2SiO 2, Na 2O·2CaO·3SiO 2, BaO·2SiO 2, 2Na 2O·CaO·3SiO 2, Na 2O·SiO 2, 3BaO·5SiO 2 and CaO·SiO 2. In the first three cases classical homogeneous nucleation theory provides a satisfactory description of both the temperature dependence and magnitude of the nucleation rate if the crystal-liquid interfacial energy σ is temperature dependent. All seven compositions showed striking similarities. The values of T MAX/ T m, where T MAX is the temperature of maximum nucleation and T m the melting point (or effective melting point) were all in the range 0.54 to 0.59. T MAX was always at, or somewhat above, T g. Values of T d/ T m where T d is the “just detectable” nucleation temperature ( I = 10 6 m −3 s −1) were 0.62, 0.64 and 0.66 in the first three systems. These represent somewhat higher undercoolings for the onset of homogeneous nucleation than observed in “droplet” nucleation studies of non-silicate systems. However, values of the Turnbull α parameter calculated from the σ results were in general accord with droplet studies for non-metals. Values of W ∗/kT (W ∗ is thermodynamic barrier to nucleation) in the various systems also showed close similarities being ∼ 30 armat T MAX remarkably consistent pattern of the results suggests that the nucleation observed is predominantly homogeneous. Reasons for the failure to observe homogeneous nucleation in other “simple” compositions are discussed.