Growth and structure of single phase mullite gels from chelated aluminium alkoxides and alkoxysilanes

The growth and structure of single phase mullite gels on a molecular scale and on a length scale up to a few nanometers is described. Traalkoxysilane and chelated aluminium alkoxide in alcoholic solution were employed as precursors. The prcursor solutions were characterized by nuclear magnetic resonance (NMR) spectroscopy. The particle formation and the sol-gel tranition were examined by NMR ans small-angle X-ray scattering (SAXS). The dried xerogels and arerogels were characterized by solid state magic angle spinning NMR (MAS-NMR). The data show the existance of Al-O-Si bonds at differnt stages of processing, from the sol to the dried gel. The influence of chelting agent on the particle formation manifests itself as the reduction of the number of available condensation sites. The alkoxysilane acts as a retarding group in the early stages of the gelation, but plays an important role as an anchor group in network formation. A model of the molecular structure of the gels including the co vering of the particle surface by the chelating agent and anchoring of alkoxysilane within this surface is presented.