Optical transmission properties of silica aerogels prepared from polyethoxidisiloxanes

Transparent, crack-fret low density silica monolithic aerogels synthesized from a series of polyethoxidisiloxanes have been characterized in the UV-visible-NIR optical range (0.3-2.5 mu m). Normal hemispherical transmittances in the visible range tau(v)(nh) were measured as well as extinction coefficients at 550 nm. These quantities are valuable parameters to quantify the image quality seen through a piece of aerogel by an empirical index called the transparency ratio (TR). The transparency ratios depend upon catalysis, volumic percentage of solvent and chemical nature of precursors, at the sol-gel step, as well as the supercritical temperature of drying and the sample thickness. Results show that the transparency ratio of the best aerogel is relatively close to that of a simple window pane of comparable thicknesses (0.92 versus 0.99, respectively) while their respective extinction coefficients at 550 nm are similar (16.3 m(-1) versus 16.4 m(-1), respectively).