NANOSTRUCTURE AND OPTICAL TRANSPARENCY OF SILICA AEROGELS

Silica aerogels are promising materials for transparent thermal insula tion 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 p H-value of the sol-gel starting solution. Since the inhomogeneities of the aerogel network are much smaller than the light wavelengths, near ly isotropic light (Rayleigh) scattering is expected and observed. hi order to check correlations between optical transparency and nanostruc tural build-up of the aerogels, small angle X-ray scattering measureme nts have been performed. Both, X-ray and light scattered intensity, ha ve been reduced to an absolute scale. Within the measurement accuracy the light scattering intensity of the aerogel bulk is equal to the ext rapolated X-rav scattering intensity towards scattering angle zero. As a consequence, the amount of isotropically scattered light and thus t he optical extinction can directly be related to the correlation volum e. It will be shown how the latter depends on the nanostructural featu res of the gel network, such as average particle size, interparticle a rrangement, pore diameter and an ordering parameter, which accounts fo r concentration effects.