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

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).