OVERVIEW OF CRYSTALLIZATION IN FLUORIDE GLASSES

Most fluoride glasses are likely to devitrify when heated for some time between the glass transition and melting temperatures. Their crystallization has been widely studied in order to measure kinetic parameters and to ease their development. Several experimental TTT curves have been reported, providing a global picture of the thermal instability range. Critical cooling rates have been deduced from systematic measurements. The Johnson-Mehl-Avrami approach provides theoretical relations which provide the basis for isothermal and non-isothermal methods using differential scanning calorimetry. The value of the Avrami exponent, n, and activation energy, E, for various fluoride glasses have been deduced from experimental measurements. They suggest that crystals grow tridimensionally from a constant number of nuclei. However, this mechanism appears different and probably more complex in fluorochloride and less stable fluoride glasses. Nucleation around T(g) appears very limited in the general case. Activation energy as measured by DSC is correlated with the difference between the crystallization temperature and T(g). The physical meaning of kinetic parameters is discussed. Among other questions, the role of anionic oxygen in nucleation and crystallization processes needs to be known more precisely.