thosilicate (TMOS) and tetraethylorthosilicate (TEOS) have Silica alcogels were synthesized by the sol-gel polymerization of often been used as silicon alkoxides in the synthesis of silica tetraethylorthosilicate (TEOS). In the synthesis, HCl and NH 3 aerogels. TEOS is polymerized according to the following were used as hydrolysis and condensation catalysts. The gelation two steps: hydrolysis of TEOS and condensation. For examtime became short and the visible light transmittance increased ple, hydrogen chloride (HCl) is used as a hydrolysis catalyst, with increasing the amount of HCl or lengthening the hydrolysis and ammonia (NH 3 ) is used as a condensation catalyst. time. The alcogels were dried under supercritical conditions with Many studies have been carried out on the preparation and carbon dioxide, and silica aerogels were obtained. As a result of characterization of silica aerogels. It is well known that the characterization by visible light transmission and N 2 adsorption, catalysts used in the sol-gel polymerization greatly influence the aerogels are mesoporous materials with high surface areas.
The experimental results suggest that the aerogel properties are physical properties of silica aerogels (7-11). However, not influenced by the drying conditions such as extraction temperthere are few studies on the influence of supercritical drying ature, extraction time, depressurizing temperature, and depressuron the mesoporous structures of silica aerogels, as far as we izing rate. On the other hand, the properties are changed under know. The objectives of the present work are to elucidate the conditions of sol-gel polymerization. In the preparation of the influence of drying conditions on porous structures of highly transparent aerogels with high surface areas and large pore silica aerogels and to propose an index for preparing a transvolumes, it is necessary to synthesize highly transparent alcogels. parent silica aerogel with a high surface area and a large It is found that the visible light transmittance of alcogels is an pore volume.