ELASTIC PROPERTIES OF SILICA AEROGELS FROM A NEW RAPID SUPERCRITICAL EXTRACTION PROCESS

Silica aerogels were produced by a new process from tetramethoxysilane (TMOS) with ammonia as base catalyst. The process involves pouring th e liquid sol in a stainless steel mold and immediately heating to supe rcritical conditions. Gelation and aging occur during heating and reac tion rates are high due to high average temperatures. The gel fills th e container completely, which enables relatively fast venting of the s upercritical fluid by providing a constraint for swelling and failure of the gel monolith. The whole process can be completed within 1 h. Lo ngitudinal and shear moduli were measured in the dried aerogels by ult rasonic velocity measurements both as a function of chemical compositi on of the original sol. and of position in the aerogel. The sound velo city exhibits marked maxima close to the edges of the cylindrical, whe re the fluid left during venting. Specimens with high catalyst concent ration and high water: TMOS ratio exhibited higher average moduli. (C) 1998 Elsevier Science B.V. All rights reserved.