Optically transparent silica aerogels based on sodium silicate by a two step sol-gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol-gel process. To produce optically transparent silica aerogel granules, NH4F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol gel process-based aerogels is higher than the conventional single step aerogels. The N-2 adsorption-desorption analysis depicts that the two step sal gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of similar to 0.125 g/cc, low thermal conductivity of similar to 0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of similar to 425 m(2)/g were obtained by using NH4F (1 M), HCl (4 M),and a molar ratio of Na2SiO3::H2O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 degrees C. (C) 2012 Elsevier Masson SAS. All rights reserved.