Effect of substitution of SiO2 by CaO/CaF2 on structure and synthesis of transparent glass-ceramics containing CaF2 nanocrystals

Abstract

Oxyfluoride glass ceramics with varied composition in system SiO2–Al2O3–CaO–CaF2 with fixed content of alumina were synthesized by isothermal heat-treatment of melt-quenched glasses. The as-prepared glasses and glass-ceramics were characterized in thermodynamics, microstructure, crystalline phase, and morphology and optical properties. The results show that substitution of SiO2 by CaO and/or CaF2 leads to rupture of Si–O network and hence escalating disorder of glass structure. Glass transition temperature increases with substituting SiO2 by CaO but decreases by CaF2. Depending on glass composition and treating temperature, the initial crystalline phase CaF2 and the secondary phase anorthite can successively crystallized. Substitution of SiO2 by CaO and/or CaF2 may facilitate the precipitation of CaF2 crystal but suppress the crystallization of anorthite. Transparent glass ceramic containing nanosized CaF2 can be synthesized using glass with a molar ratio of SiO2 to (CaO + CaF2) approaching 1.3 via heat-treatment close to the initial crystallization temperature. Elevating treating temperature improves the crystallization of CaF2 but in return decreases transparency of the glass ceramics (GC).