WET CHEMICAL SYNTHESES OF ULTRAFINE MULTICOMPONENT CERAMIC POWDERS THROUGH GEL TO CRYSTALLITE CONVERSION

Coarse (BOn/2)-O-n+/xH(2)O (10<x<120) gels, free of anionic contaminan ts react with A(OH), solutions under refluxing conditions at 70-100 de grees C giving rise to nanoparticles of multicomponent oxides (A=Ba, S r, Ca, Mg or Pb; B=Zr, Ti, Sn, Fe, Al or Cr). These include ABO, perov skites and their solid solutions, polytitanates, hexaferrites and rela ted phases, aluminates with spinel or tridymite structure and chromate s. The nanosized crystallites are often in metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Th rough the same route, luminescent phosphors of aluminates doped with r are-earth metals could be prepared. The present results indicate the g eneral features of the gel-crystallite (G-C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A(2+) ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B-(OH )-B or B-O-B, leading to the breakdown of the gel into crystallites. G -C conversion has advantages as a method of synthesis of ceramics in t erms of operational cost and procedural simplicity.