6533b7d7fe1ef96bd1267910

RESEARCH PRODUCT

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

subject

description

Abstract CoAl 2 O 4 spinel-mullite composites were prepared by double substitution of Al by Ti and Ni in stoichiometric 3:2 mullite. Gels with compositions 3(Al 2 − 2 x Co x Ti x O 3 )·2SiO 2 (x = 0.025, 0.05 and 0.2) and 3(Al 2 − 2 x M x O 3 )·2SiO 2 (x = 0.05; M = Co 2+ or Ti 4+ ) were synthesized by sol-gel techniques. The structure of the gels was investigated by infra-red spectroscopy. Heating gels at 750 °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 °C from the amorphous state. Al-Si spinel and/or CoAl 2 O 4 were the first crystalline phases detected, which appeared almost simultaneously. This finding indicated the important role played by Co 2+ and Ti 4+ in phase development and crystallization kinetics, even for compositions very close to the stoichiometric 3:2. The reaction sequence can understood by assuming that CoAl 2 O 4 spinel was developed by epitaxial growth on Al-Si spinel. This fact permitted direct control of the final microstructure of the CoAl 2 O 4 spinel-mullite composite as revealed by scanning and transmission electron microscopies.