Photo induced surface near crystallization of a glass in the system Na2O/K2O/CaO/CaF2/Al2O3/SiO2

Glasses in the system Na2O/K2O/CaO/CaF2/Al2O3/SiO2 were doped with Ag2O, CeO2 and Sb2O3. They were irradiated with UV light and subsequently thermally annealed at 530 °C, a temperature just above Tg. This led to a slight yellow coloration. A second thermal annealing step was carried out at a temperature of 560 °C. This led to the crystallization of cubic CaF2 as proved by X-ray diffraction. The crystallization was only observed at that side, the sample was irradiated. Samples, annealed in a two step process at 530 °C for 1 h and 560 °C for 20 h without previous irradiation did not show crystallization. As shown by scanning electron microscopy, the size of the crystals was 30 nm. If this procedure was carried out in glass slides with a thickness of 0.5 mm, bended samples were obtained. This is due to a volume expansion which runs parallel to the crystallization.