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Effect of substitution of SiO2 by CaO/CaF 2 on structure and synthesis of transparent glass-ceramics containing CaF2 nanocrystals

Wang, Zhenlin, Cheng, Laifei
Journal of materials science 2015 v.50 no.11 pp. 4066-4074
aluminum oxide, calcium oxide, ceramics, crystallization, glass, glass transition temperature, heat treatment, microstructure, nanocrystals, optical properties, silica, thermodynamics
Oxyfluoride glass ceramics with varied composition in system SiO₂–Al₂O₃–CaO–CaF₂with 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 SiO₂by CaO and/or CaF₂leads to rupture of Si–O network and hence escalating disorder of glass structure. Glass transition temperature increases with substituting SiO₂by CaO but decreases by CaF₂. Depending on glass composition and treating temperature, the initial crystalline phase CaF₂and the secondary phase anorthite can successively crystallized. Substitution of SiO₂by CaO and/or CaF₂may facilitate the precipitation of CaF₂crystal but suppress the crystallization of anorthite. Transparent glass ceramic containing nanosized CaF₂can be synthesized using glass with a molar ratio of SiO₂to (CaO + CaF₂) approaching 1.3 via heat-treatment close to the initial crystallization temperature. Elevating treating temperature improves the crystallization of CaF₂but in return decreases transparency of the glass ceramics (GC).