Non-isothermal crystallization in ZrF 4-BaF 2-GdF 3-AlF 3 fluoride glasses

Abstract

The crystallization kinetics of 50 mol.%ZrF 4-40 mol.%BaF 2,-4 mol.% GdF 3-6 mol.%AlF 3 were studied by DSC and optical microscopy. The crystallization kinetics were analysed on the basis of the non-isothermal kinetic theory. The viscosity of glass was measured by the penetration method. In the course of heating at a constant rate, metastable β-BaZrF 6 crystals were found to precipitate first and transform into stable α-BaZrF 6 crystals at higher temperatures. It was found that a constant number of crystal particles, irrespective of heating rate, grow three-dimensionally in bulk specimens. The activation energy determined for crystal growth was 469 kJ mol −1, much less than that for viscous flow. The apparent activation energy determined for viscous flow was 920 kJ mol −1 in the narrow temperature range of measurement but the activation energy was observed to decrease extraordinarily with temperature. The mechanism of crystal growth is discussed in relation to the discrepancy between the activation energies for crystal growth and viscous flow.