Effect of V2O5 addition on the crystallisation of glasses belonging to the CaO–ZrO2–SiO2 system

The crystallisation of CaO–ZrO2–SiO2 glasses doped with V2O5 (0.1–5 mol%) has been investigated in terms of microstructure and thermal parameters. Results indicate that crystallisation is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 2 mol% of doping oxide. As detected from differential thermal analysis curves, glass transition temperature and crystallisation temperature, are strongly dependent upon V2O5 content varying from 0.0 to 2.0 mol%, while the crystallisation activation energy values decrease with a parabolic trend from B-glass (0.0 mol% V2O5 content, 495±7) to V-0.7 (0.7 mol% V2O5 content, 420±6) composition, increasing again to 442±5 kJ/mol K with higher amount of V2O5. The microstructure of the glass-ceramic materials clearly showed a marked dependence upon the amount of V2O5, also due to the presence of phase separation for content higher than 0.7 mol%. Wollastonite, CaO·SiO2, and a calcia–zirconia–silicate, 2CaO·4SiO2·ZrO2, are the main crystalline phases whose ratio slightly varies with vanadium oxide content. The glass ceramics obtained from the studied materials are greenish and bluish coloured, so it is possible to use the studied glasses as coloured frits for tile glazes.