THERMAL EVOLUTION OF FE-SUBSTITUTED AND CR-SUBSTITUTED SOL-GEL MULLITES

The study of Fe- and Cr- mullites reveals that Fe substitutes for Al d uring the whole annealing process yielding a decrease of the crystalli zation temperature whereas Cr only reacts at high temperature. In the case of iron, crystallization via spinel phase has been explained by t he formation of heterogeneities which reactivity Increase can be due t o the fact that Fe3+ has no stabilization energy for octahedral sites inside the spinel structure. The evolution of the mullite cell paramet ers allows to explain the thermal behavior of the samples. In the case of chromium, its reaction at high temperature has been related to the refractory character of Cr2O3.