Densification and crystallization behaviour of colloidal cordierite-type gels

Three cordierite-type gels were prepared from an aqueous solution of Mg(NO3)2, a boehmite sol and silica sols of very small particle sizes. The effect of varying the silica particle size on the crystallization and densification behaviour was studied. Phase development was examined by thermal analysis and X-ray diffraction, while the densification behaviour was characterized by measuring the linear shrinkage of pellets. The activation energy of densification by viscous flow was determined using the Franckel model for non-isothermal conditions and a constant heating rate. The results show that spinel crystallizes from the colloidal gels prior to cristobalite, and their reaction gives -cordierite, which is specific for three-phase gels. Decreasing the silica particles size lowers the cristobalite crystallization temperature and the -cordierite formation temperature. The activation energy of densification by viscous flow is lower and the densification more efficient, the smaller the silica particles are. nema