Ž . Journal of Non-Crystalline Solids 210 1997 224–231 Optical transmission properties of silica aerogels prepared from polyethoxidisiloxanes G.M. Pajonk a, ) , E. Elaloui a , B. Chevalier b , R. Begag a a UniÕersite Claude Bernard, Laboratoire d’Application de la Chimie a l’EnÕironnement, UMR CNRS 5634–43, Bld du 11 NoÕembre 1918, ´ ` 69622 Villeurbanne, France b CSTB, 24 rue Fourier, 38000 St. Martin D’Heres, France Received 24 October 1995; revised 22 July 1996 Abstract Transparent, crack-free low density silica monolithic aerogels synthesized from a series of polyethoxidisiloxanes have Ž . been characterized in the UV-visible-NIR optical range 0.3–2.5 mm . Normal hemispherical transmittances in the visible range t nh were measured as well as extinction coefficients at 550 nm. These quantities are valuable parameters to quantify v Ž . 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 Ž . is relatively close to that of a simple window pane of comparable thicknesses 0.92 versus 0.99, respectively while their Ž y1 y1 . respective extinction coefficients at 550 nm are similar 16.3 m versus 16.4 m , respectively . 1. Introduction It is widely accepted that silica aerogels, which are made by the sol–gel method, and supercritically dried with respect to the liquid phase, are very interesting transparent and thermal superinsulator w x materials for double windows 1–9 . As a solid very porous and light material, they also exhibit thermal conductivities lower than 20 mWrKrm at atmo- spheric pressure, but in general their transparencies, in the visible range, depend on the degree of Rayleigh ) Corresponding author. Tel.: q33-4 72 44 82 52; fax: q33-4 78 94 19 95; e-mail: pajonk@cpmol.univ-lyon1.fr. wx and surface-scattering 9 . Only rare gas can match Ž these values at ambient pressures Kr, Xe and A, exhibit thermal conductivities of 8.6, 5.8 and 16.2 . mWrmK, respectively , while still air has a thermal conductivity of 24.1 mWrmK under the same condi- tions. It is important to improve these aerogels to get closer to the transparency exhibited by a single window pane similar to that used currently in double window glazings. A recent series of papers demon- strated the advantage of making silica aerogels with a family of new prepolymerized precursors based upon the partial hydrolysis of tetraethoxisilane Ž . TEOS in the presence of sulfuric acid as catalyst w x 10–12 . The use of commercial prepolymers versus the corresponding monomers, presents the advan- 0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0022-3093 96 00600-X