27Al and 29Si MAS–NMR studies of structural changes in hybrid aluminosilicate gels

Aluminosilicate gels with stoichiometric and nonstoichiometric compositions were synthesized by means of colloidal sol-gel method and their mullitization behavior was studied by X-ray diffraction (XRD), 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS–NMR) experiments. Particular attention was given to the structural changes of matrix accompanying the formation of mullite. The various coordinated Al occupancies were clarified by simulating the 27Al MAS–NMR spectra with Gaussian lines. The results demonstrate that the so-synthesized aluminosilicate gel is a hybrid gel containing a mixture of a single-phase gel and a diphasic gel. The mullitization of so-formed hybrid gel exhibits a consecutive one-step conversion process, but not a two-step process, much similar to that of a true diphasic mullite gel. The mullite formation from hybrid aluminosilicate gel mainly depends on the nature of dominant matrix part, but not on the nature of minor matrix part in gel. During the formation process of mullite, amorphous Si-rich phase appears as a transitional phase. The effects of gel composition and heating rate on the phase transformation behavior of hybrid aluminosilicate gel were also discussed here.