Processing of transparent glass-ceramics by nanocrystallisation of LaF3

Transparent glass-ceramics have been prepared by heat-treating oxyfluoride glasses in the Na₂O–Al₂O₃–SiO₂–LaF₃ system. The nanocrystallisation of LaF₃ was achieved by controlling time and temperature parameters. Glasses and glass-ceramics were characterised by dilatometry, DTA, XRD and TEM. The mean crystal size (<20 nm) and the crystal fraction increase with the temperature of heat treatment, while they reach a maximum at about 20 h at a temperature close to T_g. The crystallisation of phases containing glass modifier elements as well as F anions leads to the increase in the viscosity of the remaining glass matrix. Phase separation occurs in glass-ceramics depending on the glass composition which affects nanocrystallisation.