A nonisothermal study of the kinetics of the nanoporosity elimination in monolithic silica xerogels, prepared from acid and ultrasound catalyzed hydrolysis of tetraethylortosilicate (TEOS), has been carried out by means of in situ linear shrinkage measurements performed with different heating rates. The study could be applied up to almost alpha approximately 0.6 of the volume fraction alpha of eliminated pores. The activation energy was found increasing from about 3.2 x 10(2) kJ/mol for alpha approximately 0.06 up to about 4.4 x 10(2) kJ/mol for alpha approximately 0.44. The sintering process accompanying the nanopore elimination in this set of xerogels is in agreement with a viscous flux sintering process with the hydroxyl content diminishing with the volume fraction of eliminated pores. All the volume fraction of eliminated pores versus temperature (T) curves can be matched onto a unique curve with an appropriate rescaling of the T axis, independent of the heating rate. This scaling property suggests that the path of sintering seems the same, regardless of the heating rate; the difference is that the rate is faster at higher temperature.
The structural characteristics of saturated silica sonogels were studied by means of small-angle x-ray scattering ͑SAXS͒ and thermogravimetric analysis ͑TG͒, after a long time of aging in saturated conditions. The sonogels were obtained by a sol-gel routine from ultrasound stimulated tetramethoxysilane ͑TMOS͒ hydrolysis carried out with the initial water/TMOS molar ratio ͑r͒ ranging from 2 to 10. The saturated sonogel structure can be described as composed by mass fractal-like aggregates ͑clusters͒ of primary silica particles, all imbibed in a liquid phase. The values of the mass fractal dimension ͑D͒ of the clusters was found all around 2.5, while the characteristic size of the clusters () was found generally increasing with r, going from approximately 2.3 nm (rϭ2) to 4.5 nm (rϭ10). The volume fraction of the clusters was estimated from the SAXS data. The results were compared to the values of weight loss fraction at the inflection point that has been found in the derivative of the TG curve, which should accounts for the instant in which the meniscus of the liquid phase penetrates into the clusters under a rapid evaporation process as in a TG test.
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