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, ceramic-precursor powders. The basic approach for preparing such powders (where coprecipitation, cohydrolysis, or sol-gel preparation involving all components is difficult, or impossible) involves (i) preparation of an initial homogeneous hydrolysate containing as many components as possible and (ii) sorption of any additional components, as soluble species or as sols, onto the surface of the hydrolysate. This hydrolysate/sorption technique has been used to produce powders with such varied compositions as titanate-based dielectrics, lead zirconate titanate, lead lanthanum zirconate titanate, mullite, zirconia-based ceramics, etc. An example of this approach is provided by the preparation of the multicomponent titanate ceramic, Synroc, which has been produced on a 100 kg scale. The method involves the base-catalyzed hydrolysis of a concentrated solution of titanium, zirconium, and aluminum alkoxides in ethanol, and subsequent sorption of barium and calcium species under alkaline conditions. The initial formation and hydrolysis of Ti/Zr/Al alkoxide complexes was examined semiquantitatively by FT-IR spectroscopy and partial charge model analysis of the charge distribution in the metal complexes and hydrolysate particles. These calculations also identified the principal sites at which propagation and particle growth occurred during polymerization of hydrolyzed alkoxide monomers. In addition, variations in the conductivity of the bulk solution phase were used to characterize reactions occurring at the solid/solution interface during hydrolysis, particle growth, and sorption of alkaline-earth cations. The effects of processing parameters on particle nucleation and selected powder properties are discussed.