ZIRCON FORMATION FROM AMORPHOUS SPHERICAL ZRSIO4 PARTICLES OBTAINED BY HYDROLYSIS OF AEROSOLS

A procedure for the preparation of SiO2.ZrO2 spherical particles, based on the hydrolysis of liquid aerosols is described. A mixture of tetraethylortho-silicate (TEOS) and zirconium n-propoxide with the proper stoichiometry (Zr/Si atomic ratio = 1) was used as a liquid precursor. The alkoxide mixture had to be partially hydrolysed before aerosol generation in order to obtain solids with a zircon composition. The as-prepared powders consisted of particles in the micrometre range with an amorphous character. Energy dispersive X-ray spectroscopy (EDX), infrared (i.r.) and nuclear magnetic resonance (NMR) analyses indicated the existence of a good compositional homogeneity and a large number of Si-O-Zr bonds in the sample. Calcination of the powder up to 950-degrees-C gave rise to the segregation of silica and the crystallization of tetragonal zirconia, which transformed into the monoclinic phase after heating at 1300-degrees-C. Crystallization of zircon started on calcination at 1450-degrees-C; it was accompanied by the formation of some cristobalite. The complete transformation of the sample into zircon took place after prolonged heating (20 h) at 1500-degrees-C.