STRUCTURE AND TOPOLOGY OF SILICA AEROGELS DURING DENSIFICATION

A combination of scattering techniques, neutron powder diffraction, sm all-angle X-ray scattering (SAXS), neutron spin echo (NSE) and neutron time-of-flight. (TOF), is used to follow the structure and topology o f silica aerogels during densification. NSE and TOF show that short-sc ale branching increases at the expense of large-scale connectivity lea ding to micron-size fluctuations, the existence of which is confirmed by SAXS. On shorter length scales, heat treatment leads to a transitio n from a spaghetti-like polymeric network to a colloidal microstructur e. With increasing exposure to temperatures of the order of 1000 degre es C, the chord length of the colloidal solid phase increases exponent ially with bulk density, even though the microscopic skeletal density remains at half that of amorphous silica.