EPITACTIC NUCLEATION OF SPINEL IN ALUMINOSILICATE GELS AND ITS EFFECT ON MULLITE CRYSTALLIZATION

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the approximately 1000-degrees-C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at approximately 1000-degrees-C in gels that otherwise crystallize directly to mullite at approximately 1000-degrees-C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at approximately 1000-degrees-C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at approximately 1000-degrees-C, the release of alumina from the spinel-type crystal structure becomes the rate-controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.