Modifications of the sol-gel method for the preparation of ultrafine/ultrapure ceramic oxide powders-properties of the powders and microstructure of the derived ceramic bodies

Synthesis of ultrafine/ultrapure powders was carried out by hydrolysis of metal alkoxides in anhydrous solvents, at various temperatures, using superheated steam for the process. The precursor powders of alumina, titania, tialite, zirconia, and magnesia obtained were characterized by chemical analysis, physical properties (e.g., BET surface measurements and XRD phase analysis). Thermal treatment of the powders (cold pressing) in the range up to 1400°C showed dehydration in the lower range, phase transitions that not always corresponded to the usual thermodynamic order, and a gradual increase of crystallinity at higher temperatures. Tialite was formed only at temperatures above 1200°C, despite the intimate contact between alumina and magnesia extant in the precursor powders, similarly to the behavior of stoichiometric mixtures of pulverized commercial corundum and rutile. However, reaction with ultrafine precursors was complete, as opposed to the latter mixtures. Densification of powders obtained with superheated steam showed that the particles are probably of less homogeneous shape, possibly more agglomerated, than in the case of ultrafine powders obtained by the usual sol-gel methods, and ways have to be found to improve this characteristic.