Low-Density Silica Aerogels Observed by High-Resolution Electron Microscopy

High-resolution electron microscope observations of two types of silica aerogels (base catalyzed and neutrally prepared) have shown two types of structures. The base catalyzed has an open structure with a 3 D network of branched chains containing for very low density (0.03 g cm -3 ) two separate hierarchy levels. In neutrally or acid catalyzed aerogels self-similarity over two orders of magnitude is evidenced with a tendency to form several hierarchy levels in the lowest-density case (0.12 g cm -3 ). These results are in accordance with the range of fractal domains determined by small-angle neutron scattering.