Abstract
Zircon powders were prepared by means of a sol-gel method from an equimolar mixed solution of Si(OC2H5)4 and ZrOCl2·8H2O dissolved by hot ethanol, and the crystallite growth and phase change in the powders caused by heat treatment were investigated. The formation mechanism and the thermal-decomposition temperature of zircon powders were discussed on the basis of X-ray diffractometry. The results obtained are as follows:
(1) The starting temperature of zircon formation in the as-dried powders without grinding treatment was ca. 1500°C and the rate of formation was very slow. On the other hand, the formation of zircon was accelerated by the grinding treatment of the as-dried powders (the starting temperature was ca. 1200°C), and especially the formation reaction was remarkably promoted in the temperature range of 1300-1450°C with the aid of “Hedvall effect” induced by the crystallization of amorphous silica.
(2) It was considered that the promotion effect of zircon formation by grinding was attained by the breakdown of siloxane polymers. The formation of zircon took place via the solid-state reaction of tetragonal ZrO2 and SiO2, and silica changed its phase from amorphous (lower temperature) to transition and then to crystal-like (higher temperature) with heat-treatment temperature.
(3) It was pointed out that the thermal decomposition temperature of high purity zircon was higher than that reported in literature (1676°C).
