THE FORMATION OF ZIRCON FROM AMORPHOUS ZRO2-CENTER-DOT-SIO2 POWDERS

The different factors affecting the mechanism of zircon formation from amorphous ZrO2 . SiO2 powders have been studied. It was shown that zi rcon was formed by solid state reaction between tetragonal zirconia an d silica (amorphous and cristobalite). The previously suggested Hedval l effect associated with the crystallization of amorphous silica into cristobalite did not play any role in this reaction. A high degree of Si-Zr mixing in the starting amorphous powders did not affect the mech anism of zircon formation, but speeded up the reaction rate due to the small particle size of the zirconia and silica particles segregated p reviously to zircon formation. It was also found that the formation re action was retarded by the presence of carbonaceous species coming fro m the alkoxide precursors, which were probably retained at grain bound aries after calcination, acting as a diffusion barrier. These factors can explain the observed differences in the temperatures of zircon for mation previously reported.