Characteristic of synthesis and transformations of hardystonite in willemite glass-crystalline glaze based on thermal analysis

Abstract

A new crystalline phase hardystonite (Ca2ZnSi2O7) has been obtained in willemite glass–ceramic glaze containing more than 20 % by mass ZnO. To our knowledge, there was no report about the presence of this calcium zinc silicate in glazes. The aim of this study was to investigate the synthesis and thermal behavior of Ca2ZnSi2O7 in ceramic glaze. A hardystonite formed in glaze at 750 °C and exist to 850 °C, as confirmed by X-ray phase powder diffraction findings and scanning electron microscope image (SEM) with chemical elemental analysis. The second formed of zinc calcium silicate is corresponding with liquid phase presence in glaze observed at 1150 °C and also decompose below 1200 °C. Because glaze is complicated multi-component system with different crystalline phases, it is developed for research transformations in stoichiometric hardystonite. On this basis, an oxide system was prepared from SiO2, ZnO and CaCO3 to designated precise chemical synthesis temperature of pure hardystonite powders. Thermal analysis method such as differential scanning calorimetry has confirmed that in 1175 °C formed hardystonite according to X-ray phase powder diffraction of a reaction mixture. Hot-stage microscope is useful to detailed description of the processes and transformations which occur when mixture and glaze were heated. SEMs of the new phase in mixture and in glaze are presented. Phenomena related with the influence of heating cycle and liquid phase on the transformation of hardystonite are examined and analytically discussed.