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

Abstract

Abstract 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. Tetraalkoxysilane and chelated aluminum alkoxide in alcoholic solution were employed as precursors. The precursor solutions were characterized by nuclear magnetic resonance (NMR) spectroscopy. The particle formation and the sol-gel transition were examined by NMR and small-angle X-ray scattering (SAXS). The dried xerogels and aerogels were characterized by solid state magic angle spinning NMR (MAS-NMR). The data show the existence of AlOSi bonds at different stages of processing, from the sol to the dried gel. The influence of chelating 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 covering of the particle surface by the chelating agent and the anchoring of alkoxysilane within this surface is presented.