Effect of Precursor Particle Size on the Densification and Crystallization Behavior of Mullite

Stoichiometric mullite, 3Al₂O₃·2SiO₂, has been prepared from a variety of colloidal and solution precursors. In order to change the level of mixing, the size of the silica particles has been varied while keeping the alumina (boehmite) particle size constant. The effect of varying particle size on the crystallization and densification behavior has been studied. Densification behavior was characterized by measuring the bulk density of pellets as a function of heat treatment temperature and by dilatometry. Phase development was examined by thermal analysis and X-ray diffraction. Results showed that the formation of crystalline phases including mullite inhibited densification. The formation of mullite was controlled by the initial segregation of alumina and silica in the gel. A heat treatment time-temperature profile designed to optimize the densification of the colloidal powders was investigated in order to compare the ability of the different gels to densify.