Crystal Nucleation and Growth in Baria-Silica Glasses.

The kinetics of crystal nucleation and growth of barium disilicate were studied in baria-silica glasses containing 25 to 35 mole% baria. In this composition range, liquid-liquid immiscibility exerted a pronounced influence on crystal nucleation kinetics. The progressive shift in composition of the baria-rich matrix phase with time caused changes in both the thermodynamic driving force and in the kinetic barrier to nucleation which in turn caused a marked increase of nucleation rate. Study of the nucleation kinetics in relation to quantitative data of the morphology of the two liquid phases showed no evidence of heterogeneous nucleation at the liquid-liquid interfaces. Crystal growth measurements at lower temperatures showed appreciable induction times which were caused by a change in growth morphology from spheres to rapidly growing needles nucleated at the sphere-glass interfaces. The induction time decreased with rise in temperature. Comparison of crystal growth rates in phase separating and non-phase separating glasses showed that phase separation increased the growth rates due to the accompanying shift in composition of the baria-rich phase during heat treatment. This composition shift also caused an apparent reduction in the measured activation enthalpies for growth in the phase separated glasses. The morphology of the two liquid phases had no influence on crystal growth rates. All the glasses gave constant growth rates, except at high temperatures where growth rates increased with time. Reasons for this behaviour are discussed. No effects on the kinetics of nucleation and growth in a baria-silica based glass were observed on application of electric fields of 4 kV cm 1. A theoretical calculation showed that the field necessary to observe an effect would be much larger than is possible to achieve in practice, due to joule heating and electrical breakdown.