MICROSTRUCTURE EVOLUTION OF NONHYDROLYTIC ALUMINA GELS

Nonhydrolitic sol-gel processes of aluminum chloride and aluminum brom ide with isopropyl ether and aluminum sec-butoxide were performed at v arious temperatures. Based on the Arrhenius type variation of the gela tion time with temperature, activation energies for the gelation were found to be in the range 19-25 Kcal/mol range. The energies were found to be sensitive to the nature of the aluminum ligands and the chemica l scheme. Due to the large activation energy, it is possible to stop t he reaction at any time before gelation by cooling the sol to room tem perature. Small angle X-ray scattering (SAXS) of sols from the AlCl3/( Pr2O)-O-i system shows unique development of a fractal like structure with nanometer scale order, demonstrated by discrete peaks in the SAXS data. A fractal dimension D = 1.64 was found. An aggregation scheme i s proposed to explain this phenomenon. A fractal dimension of 2.4 with out small scale ordering found for xerogels prepared from the AlCl3/AS B system reflects the effect of the different precursors on the micros tructure of nonhydrolytic gels.