NUCLEATION AND CRYSTALLIZATION KINETICS ON FLOAT GLASS SURFACES

The superficial crystallization kinetics of a commercial float glass is investigated in the temperature range from 665 to 925-degrees-C. The following phases were observed: cristobalite, devitrite, sporadically sodium metasilicate and additionally beta-wollastonite if a reducing atmosphere is applied. The devitrification process is divided into a primary crystallization of cristobalite and a secondary appearance of devitrite after an induction time (t > 7 min). The growth rates of devitrite are up to 4 times larger than those of cristobalite, thus, a fractured surface perpendicular to the float glass plate shows a two-stage devitrification image after longer times of heat treatment. The growth rates depend on the furnace atmosphere as well as on the compositional variations of the two float glass surfaces. The activation enthalpies of crystal growth rates are (in kJ/mol): almost-equal-to 110 for cristobatite, almost-equal-to 150 for beta-wollastonite and almost-equal-to 220 for devitrite. A superficial phase separation occurs at the tin bath side if oxidizing furnace atmosphere is applied. This can be related to a redox interaction of the tin ions and has consequences on nucleation, induction period, crystal growth mechanism and "bloom" effect. Heterogeneous surface nucleation arises from a certain number of sites which are specific for the various crystal phases. In the case of cristobalite the time and temperature dependence of the number of crystals are measured and nucleation rates were determined on the atmosphere side under oxidizing conditions.