An assessment of crystal growth behavior in lithium disilicate glass

Growth rates of lithium disilicate crystals are measured as a function of water concentration in the glass and at temperatures in the deeply undercooled regime (T = 454-635 degreesC). The observed crystal growth rates do not appear to be significantly affected by the amounts of water present in glasses prepared under normal conditions. Combining our growth rate data with data reported in the literature, we assess the applicability of standard models of crystal growth for the description of experimental results over a very broad temperature range. We find the reduced growth rate versus undercooling graph to consist of three regimes. For undercoolings less than 140 degreesC, the reduced growth rate curve is suggestive of either 2D surface nucleation or screw dislocation growth. For undercoolings greater than 400 degreesC, the reduced growth rate plot suggests that the operative crystal growth mechanism is 2D surface nucleation, but detailed calculations cast doubt upon this conclusion. In the intermediate undercooling range, there appears to be some sort of transitional behavior for which none of the standard models appear to be applicable. Further, we find that small differences in the viscosity data employed can produce enormous differences in the predicted growth rates at larger undercoolings. The results of our kinetic analyses seem to indicate that the nature of the kinetic rate coefficient used in the standard growth models may be incorrect. (C) 2001 Elsevier Science B.V. All rights reserved.