Non-isothermal crystallization kinetics and microstructure of a silver doped calcium aluminophosphate glass

The role of metallic silver in crystallization behavior of CaO–Al2O3–TiO2–P2O5 system glasses was investigated utilizing differential thermal analysis, X-ray diffractometry, atomic force microscope and scanning electron microscopy. Initial glasses were heat treated in hydrogen atmosphere to convert silver ions to the metallic state, prior to crystallization. The presence of metallic silver led to noticeable decreasing of crystallization temperatures as well as effective crystallization of calcium titanium phosphate (CaTi4(PO4)6), calcium metaphosphate (Ca(PO3)2) and calcium pyrophosphate (Ca2P2O7) phases. Kinetic of crystallization was also evaluated before and after hydrogen heat treatment under non-isothermal conditions. It was found that activation energy for crystallization was not significantly influenced by the metallic silver. However, the calculated Avrami exponents confirmed the dominant role of metallic silver as an effective nucleating agent in bulk crystallization of the reduced glasses. By leaching of the silver free and silver containing glass–ceramics, calcium pyrophosphate (Ca2P2O7) phase was selectively dissolved out and left porous microstructures with various pore morphologies and dimensions.