Zircon formation from amorphous powder and melt in the silica-rich region of the alumina-silica-zirconia system

Chemically homogeneous amorphous powders were prepared by sol-gel method from alkoxide mixtures in the silica-rich region of the alumina-silica-zirconia system. A glass with the same composition was obtained by quenching in water from the melt. The evolution with temperature in the range up to 1625 °C was studied by means of X-ray diffraction, infrared and UV-Raman spectroscopy, scanning electron microscopy and energy-dispersive X-ray analysis. Zircon crystallization and stability at temperatures higher than the liquidus temperature were confirmed in both melt-quenched and gel-glasses. Crystallization of cristobalite from the amorphous gel-glasses was observed above 1200 °C. The crystallization of zircon particles was observed after thermal treatment at 1550 °C. At 1625 °C zircon was the only stable crystalline phase due to the complete melting of the silica-rich matrix. The XRD pattern of the ternary gel treated at 1625 °C shows clearly the disappearance of the cristobalite phase, while at 1550 °C there is already partial melting. These results can help to determine the phase equilibrium in the ternary system.