INFLUENCE OF TRANSITION-METAL OXIDES ON SOL-GEL MULLITE CRYSTALLIZATION

The influence of transition metal oxides on the crystallization of sol -gel mullite samples with nominal compositions (MyAl4.5-ySi1.5O9.75)-A l-III with y=0.25 and 0.50 for trivalent elements (M=Fe and Cr) and M- y(II) Al4.5-y Si1.5+y/4O9.75 with y=0.05 and 0.50 for divalent element s (M=Mn, Co, Ni, Cu and Zn) reveals two different behaviours. Whereas only one exotherm is observed for M=Co, Cu and Mn with y=0.50, all oth er samples exhibit two exotherms. From X-ray diffraction data, it has been shown that a spinel phase is always observed after the first exot herm and that direct orthorhombic mullite crystallization is evidenced only for y=0.50 with M=Co, Cu and Mn samples at the same temperature. For all other samples mullitization occurs during the second exotherm . Fe(III) is the only element completely soluble during the whole anne aling process. The phase evolution has been explained by considering t he role of the spinel phase that controls the composition of the remai ning aluminosilicate amorphous phase and its further-evolution. (C) 19 98 Elsevier Science S.A.