PREPARATION OF MULTICOMPONENT POWDERS BY ALKOXIDE HYDROLYSIS .1. CHEMICAL-PROCESSING

A series of chemical processes, based on alkoxide hydrolysis, has been developed as a general technique for preparing multicomponent, cerami c-precursor powders. The basic approach for preparing such powders (wh ere coprecipitation, cohydrolysis, or sol-gel preparation involving al l components is difficult, or impossible) involves (i) preparation of an initial homogeneous hydrolysate containing as many components as po ssible and (ii) sorption of any additional components, as soluble spec ies or as sols, onto the surface of the hydrolysate. This hydrolysate/ sorption technique has been used to produce powders with such varied c ompositions as titanate-based dielectrics, lead zirconate titanate, le ad lanthanum zirconate titanate, mullite, zirconia-based ceramics, etc . An example of this approach is provided by the preparation of the mu lticomponent titanate ceramic, Synroc, which has been produced on a 10 0 kg scale. The method involves the base-catalyzed hydrolysis of a con centrated solution of titanium, zirconium, and aluminum alkoxides in e thanol, and subsequent sorption of barium and calcium species under al kaline conditions. The initial formation and hydrolysis of Ti/Zr/Al al koxide complexes was examined semiquantitatively by FT-IR spectroscopy and partial charge model analysis of the charge distribution in the m etal complexes and hydrolysate particles. These calculations also iden tified the principal sites at which propagation and particle growth oc curred during polymerization of hydrolyzed alkoxide monomers. In addit ion, variations in the conductivity of the bulk solution phase were us ed to characterize reactions occurring at the solid/solution interface during hydrolysis, particle growth, and sorption of alkaline-earth ca tions. The effects of processing parameters on particle nucleation and selected powder properties are discussed.