Supercritical drying media modification for silica aerogel preparation

We modified a method of methanol supercritical drying by aiding water, nitr ic acid and ammonia when preparing silica aerogels from tetramethoxysilane; . We found that gel-component extraction Juring supercritical drying and th e microstructure of prepared aerogels were affected by these additives. We compared the effects of the additives to the effects created during the nor mal supercritical drying process. Ethanol, I-propanol, and carbon dioxide s upercritical drying were the methods studied for comparison. We found that gel-component extraction decreased in the modified-method cases. We also fo und that adding water was the most effective modification to decrease gel-c omponent extraction. The constituting particle and pore sizes were increase d when water or base was added. The specific surface area was increased whe n acid was added. These results are discussed in this paper in terms of the sol-gel reaction during supercritical drying. (C) 1999 Elsevier Science B. V. Ail rights reserved.