Influence of water content on the rates of crystal nucleation and growth in lithia-silica and soda-lime-silica glasses

Two sets of glasses were studied with compositions close to Li₂O·2SiO₂ and Na₂O·2CaO·3SiO₂, and with water contents ranging from 0.019 to 0.136 wt% and 0.007 to 0.040 wt%, respectively. The crystal nucleation and growth rates increased markedly with increase in water content, whereas the viscosities of the glasses decreased. For the lithia glasses, increases in nucleation rates at various temperatures closely corresponded to reductions in viscosity, indicating that the main effect of water was to lower the kinetic barrier to nucleation (G_D), rather than to alter the thermodynamic barrier to nucleation (W*). For the soda-lime glasses, G_D was also lowered by water content but additional effects due to differences in base compositions were observed. The kinetic barriers to growth were lowered by water content for both sets of glasses, increases in growth rates corresponding closely to reductions in the viscosities. It is suggested that the large effects of water on nucleation and growth may be due to an increase in the oxygen ion diffusion coefficient. In the soda-lime glasses addition of sodium fluoride produced similar effects to the addition of water. Liquidus temperature measurements, and the results of DTA, optical microscopy and electron microscopy are also reported.