Kinetics of isothermal crystallization of fluoride glasses

Fluoride glasses, for the production of IR optical fibers, have a tendency to crystallization. The crystallization of fluoride glasses is one of the main reasons for non-intrinsic optical losses due to scattering in fibers. The crystallization kinetics of 53ZrF4–20BaF2–4LaF3–3AlF3–20NaF (ZBLAN), 53.8HfF4–20.1BaF2–5CeF3–3.1AlF3–17NaF–1.2InF3 (HBCANI), 12.3BaF2–12.3SrF2–12.3CaF2–12.3MgF2–36AlF3–12.3YF3–2.5LaF3 (BSCMAYL), 30InF3–10YF3–25BaF2–5SrF2–10ZnF2–10GaF3–6NaF–4PbF2 (InYBSNZnGaPb) fluoride glasses was investigated at the early stages of glass-to-crystal transformation. The experiment consisted in measuring the contents and sizes of microcrystals in glass interiors as a function of treatment time of the sample at constant temperature. The contents and sizes of crystals were determined by the method of laser ultramicroscopy. Investigations were carried out in the range between glass-forming and softening temperatures. The nucleation rate, W, the linear growth rate of crystals, V, and the temperature dependence of these kinetic parameters were found at the level of conversion of glass-crystal between 10−6% and 10−3%. The nucleation rate was maximum of ∼200 nuclei/(s cm3) near 260°C in ZBLAN, ∼300 nuclei/(s cm3) near 280°C in HBCANI, ∼280 nuclei/(s cm3) near 400°C in BSCMAYL and ∼650 nuclei/(s cm3) near 310°C in InYBSNZnGaPb glass. The results of investigation are useful for optimization of the heat treatment mode of fluoride glasses and to understanding of the processes critical to the optical homogeneity of these glasses.