GEL TO GLASS TRANSFORMATION IN BASE-CATALYZED SILICA

The effects of heat-treatments on silica xerogels prepared from base catalyzed hydrolysis and condensation of TMOS are studied by low-frequency Raman scattering, scanning electron microscopy, small angle X-ray scattering and BET measurements. Raman scattering in the range 3-700 cm-1 reveals a low-frequency band that arises from the discrete particulate character of base-catalysed gels at very small scale. A continuous evolution of the structure inside particles is observed during the thermal treatment and the Raman spectra is similar to the fused silica one after treatment at 1050-degrees-C. Electron microscopy observations of the nanostructure of the xerogels correlate with the particle Raman band maximum. Complementary SAXS and BET measurements allows us to propose a model for the gel-to-glass transformation.