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 o rder to obtain appropriate gel-derived glasses for further thermal processi ng. After annealing for several time periods at temperatures between 750 an d 1500 degrees C, differences in crystallization pathways were observed. Po lymeric 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 te mperatures between 1100 and 1200 degrees C, except for the higher substitut ed 3:2 mullite in which it was produced at 1000 degrees C. In constrast, ga mma-Al2O3 and NiAl2O4 spinel were the first crystalline phases identified a t 750 degrees C in specimens from colloidal gels, whereas mullite was forme d at temperatures higher than 1200 degrees C. In specimens with high substi tution, 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 lon g annealing at 1400 degrees C was quite similar. Differences in the microst ructure of specimens from either type of gel precursor after annealing at 1 400 degrees C concerned the size of mullite particles and the presence of s econdary phases in specimens derived from colloidal precursors. (C) 1998 Kl uwer Academic Publishers.