Microstructural evolution and crystallization mechanism of zircon from frit glaze

Zircon (ZrSiO4) is widely used as a glaze opacifier in the ceramic industry. The crystallization mechanism of ZrSiO4 from a CaO-riched zirconium-based frit glaze was studied by XRD, SEM, and DSC in this work. Isothermal and non-isothermal heat treatments were used to study the phase development and microstructural evolution during the firing process. It was found that t-ZrO2 and Ca2ZrSi4O12 were first crystallized at a relative low temperature and then converted to ZrSiO4 with the increasing of temperature. ZrSiO4 originating from t-ZrO2 has an inward growth and an outward growth, while ZrSiO4 transformed from Ca2ZrSi4O12 only has an outward growth. SEM showed that ZrSiO4 appeared to be a rod shape in fired samples. The calculated activation energy for ZrSiO4 is 311.2 kJ/mol, and the Avrami index (n) is 0.35-0.39. This indicates the one-dimensional diffusion-controlled growth of ZrSiO4, which explains the formation of rod-shape ZrSiO4 crystal.