Pore structures and microstructures of silica gel monoliths at different stages of sintering

The pore structures and microstructures of silica gel monoliths derived from the polycondensation of tetramethoxysilane (TMOS) in basic pH were characterized after various stages of sintering, using N{sub 2} gas adsorption-desorption, mercury porosimetry, and TEM. Gels dried by supercritical drying, i.e., aerogels and gels dried slowly in air have different pore structures and sinter with contrasting results when heat-treated under identical conditions. The air-dried gels do not sinter to full density and exhibit bloating on heating to high temperatures (1,100{degree}C), whereas under the same condition aerogels sinter to dense, transparent glass without bloating. Transmission electron microscopy shows that the microstructure of gels is composed of 5- to 10-nm primary spherical particles which clustered to form large (100 nm) spherical aggregates loosely bound to each other. In aerogels, two types of porosity exit: macroporosity and ultraporosity.