Cobalt aluminate spinel-mullite composites synthesized by sol-gel method

CoAl2O4 spinel-mullite composites were prepared by double substitution of Al by Ti and Ni in stoichiometric 3:2 mullite. Gels with compositions 3(Al2-2xCoxTixO3).2SiO(2) (x = 0.025, 0.05 and 0.2) and 3(Al(2-2x)M(x)O(3)).2SiO(2) (x = 0.05; M = Co2+ or Ti4+) were synthesized by sol-gel techniques. The structure of the gels was investigated by infra-red spectroscopy. Heating gels at 750 degrees C produced an amorphous silicoaluminate network. The reaction sequence was investigated by differential thermal analysis. X-ray diffraction and ultraviolet-visible spectroscopy. All samples crystallized at temperatures lower than 1000 degrees C from the amorphous state. Al-Si spinel and/or CoAl2O4 were the first crystalline phases detected, which appeared almost simultaneously. This finding indicated the important role played by Co2+ and Ti4+ in phase development and crystallization kinetics, even for compositions very close to the stoichiometric 3:2. The reaction sequence can understood by assuming that CoAl2O4 spinel was developed by epitaxial growth on Al-Si spinel. This fact permitted direct control of the final microstructure of the CoAl2O4 spinel-mullite composite as revealed by scanning and transmission electron microscopies. (C) 1996 Elsevier Science Limited.