NANOSTRUCTURE AND OPTICAL TRANSPARENCY OF SILICA AEROGELS

Silica aerogels are promising materials for transparent thermal insulation systems in solar architecture. The optical transparency of these highly porous substances is influenced by the reaction parameters upon preparation. The specific extinction due to bulk scattering was found to decrease both with increasing macroscopic density and increasing pH-value of the sol-gel starting solution. Since the inhomogeneities of the aerogel network are much smaller than the light wavelengths, nearly isotropic light (Rayleigh) scattering is expected and observed. hi order to check correlations between optical transparency and nanostructural build-up of the aerogels, small angle X-ray scattering measurements have been performed. Both, X-ray and light scattered intensity, have been reduced to an absolute scale. Within the measurement accuracy the light scattering intensity of the aerogel bulk is equal to the extrapolated X-rav scattering intensity towards scattering angle zero. As a consequence, the amount of isotropically scattered light and thus the optical extinction can directly be related to the correlation volume. It will be shown how the latter depends on the nanostructural features of the gel network, such as average particle size, interparticle arrangement, pore diameter and an ordering parameter, which accounts for concentration effects.