Structural evolution of aerogels prepared from TEOS sono-hydrolysis upon heat treatment up to 1100 °C

The structural evolution of aerogels prepared from TEOS sono-hydrolysis was studied as a function of the temperature of heat treatment up to 1100 °C by means of small angle X-ray scattering (SAXS) and density measurements. The mass fractal structure of the original wet sonogel (with scattering exponent α∼2.2) apparently transforms to a surface fractal structure in a length scale lesser than ∼1.5 nm, upon the process resulting in aerogel. Such a structural transformation is interpreted by the formation of new particles with characteristic dimension of ∼1.5 nm, with rough boundaries or electronic density fluctuations (or ultra-micropores) in their interior. The structural arrangement of these particles seem to preserve part of mass fractal characteristics of the original wet sonogel, now in a length scale greater than ∼1.5 nm. The electronic density heterogeneities in the particles start to be eliminated at around 800 °C and, at 900 °C, the particles become perfectly homogeneous, so the structure can be described as a porous structure with a porosity of ∼68% with ∼9.0 nm mean size pores and ∼4.3 nm mean size solid particles. Above 900 °C, a vigorous viscous flux sintering process sets in, eliminating most of the porosity and increasing rapidly the bulk density in an aerogel–glass transformation.