A SAXS study of the nanostructural characteristics of TEOS-derived sonogels upon heat treatment up to 1100 °C

Abstract Xerogels obtained from the acid-catalyzed and ultrasound stimulated hydrolysis of TEOS were submitted to heat treatment at temperatures ranging from 60 to 1100 °C and studied by small-angle X-ray scattering (SAXS). The SAXS intensity as a function of the modulus of the scattering vector q was obtained in the range from q 0 =0.19 to q m =4.4 nm −1 . At 60 °C the xerogels exhibit an apparent surface fractal structure with a fractal dimension D S ∼2.5 in a length scale ranging from 1/ q 1 ∼1 to 1/ q m ∼0.22 nm. This structure becomes extremely rough at 120 °C ( D S ∼3) and at 150 °C, it apparently converts to a mass fractal with a fractal dimension D ∼2.4. This may mean an emptying of the pores with preservation of a share of the original mass fractal structure of the wet aged gel, for it had presented a mass fractal dimension D ∼2.2. A well characterized porous structure formed by 2.0 nm mean size pores with smooth surface of about 380 m 2 /g is formed at 300 °C and remains stable until approximately 800 °C. At 900 °C the SAXS intensity vanishes indicating the disappearance of the pores in the probed length scale. The elimination of the nanopores occurs by a mechanism in which the number of pores diminishes keeping constant their mean size. The xerogels exhibit a foaming phenomenon above 900 °C and scatter following Porod's law as does a surface formed by a coarse structure.