Journal of Materials Science, Vol.33, No.17, 4435-4446, 1998
Effect of NiO and/or TiO2 mullite formation and microstructure from gels
Polymeric and colloidal gels with a constant molar ratio of (Al + Ni and/or Ti)/Si = 3/1 and various (Al/Ni and/or Ti) ratios (up to 21.42 mol% NiO + TiO2) were prepared and used to study the effect of the precursor chemical homogeneity on mullite formation processes and the resulting microstructure. Both kinds of gel precursors were preheated at 750 degrees C for 3 h in order to obtain appropriate gel-derived glasses for further thermal processing. After annealing for several time periods at temperatures between 750 and 1500 degrees C, differences in crystallization pathways were observed. Polymeric gels crystallized Al-Si and NiAl2O4 spinels from the amorphous form at temperatures in the range between 900 and 1000 degrees C, depending on the amount of aluminium substitution. Mullite formation was initiated at temperatures between 1100 and 1200 degrees C, except for the higher substituted 3:2 mullite in which it was produced at 1000 degrees C. In constrast, gamma-Al2O3 and NiAl2O4 spinel were the first crystalline phases identified at 750 degrees C in specimens from colloidal gels, whereas mullite was formed at temperatures higher than 1200 degrees C. In specimens with high substitution, mullite was observed at lower temperatures. Although the sequences of reaction from either kind of ge I we re rather different, main ly at low temperatures (as could be inferred from the chemical homogeneity attained in both gel-derived glasses), the final set of crystalline phases after long annealing at 1400 degrees C was quite similar. Differences in the microstructure of specimens from either type of gel precursor after annealing at 1400 degrees C concerned the size of mullite particles and the presence of secondary phases in specimens derived from colloidal precursors.