GROWTH AND STRUCTURE OF SINGLE-PHASE MULLITE GELS FROM CHELATED ALUMINUM ALKOXIDES AND ALKOXYSILANES

The growth and structure of single phase mullite gels on a molecular s cale and on a length scale up to a few nanometers is described. Tetraa lkoxysilane 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 e xistence 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 availabl e condensation sites. The alkoxysilane acts as a retarding group in th e early stages of the gelation, but plays an important role as an anch or group in network formation. A model of the molecular structure of t he gels including the covering of the particle surface by the chelatin g agent and the anchoring of alkoxysilane within this surface is prese nted.