THE EFFECT OF PRECURSOR CHARACTERISTICS ON THE CRYSTALLIZATION AND DENSIFICATION OF DIPHASIC MULLITE GELS

The crystallization and densification behavior of diphasic mullite gels was studied with the gels prepared by using various sources of boehmite powder and silica sol to find the effect of precursor characteristics such as the pH of mixed sols, particle size, and the type of the stabilizing alkali ion in silica sol. Both low pH and fine particle size were found to promote mullite crystallization and thus result in low densification. However, even with a high pH and a large particle size, densification was low without the presence of sodium because of the crystallization of cristobalite and mullite before densification. A small amount of sodium (0.5 wt% as Na2O), present as a stabilizing ion in coilloidal silica or intentionally added, suppressed the crystallization of cristobalite and mullite before densification. As a result, bulk densities of more than 95% of the theoretical density were obtained by sintering at 1320 degrees C for 1 h without a controlled heating.