Wet chemical syntheses of ultrafine multicomponent ceramic powders through gel to crystallite conversion
Coarse Bn+On/2·xH2O (10 < x < 120) gels, free of anionic contaminants react with A(OH)2 solutions under refluxing conditions at 70–100 °C giving rise to nanoparticles of multicomponent oxides (A = Ba, Sr, Ca, Mg or Pb; B = Zr, Ti, Sn, Fe, Al or Cr). These include ABO3 perovskites and their solid solutions, polytitanates, hexaferrites and related phases, aluminates with spinel or tridymite structure and chromates. The nanosized crystallites are often in metastable phases, such as cubic BaTiO3 at room temperature or superparamagnetic hexaferrites. Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general 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 A2+ 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 terms of operational cost and procedural simplicity.