Ultralow density silica aerogels by alcohol supercritical drying

Synthesis of ultralow density aerogels requires full transformation of diluted monomers into solid. Shrinkage during ageing and supercritical drying must be controlled as low as possible. Reported investigations deal with achievement in the optimisation of the two-step process parameters performed using classical acid and basic steps. Acid step involves the formation of partially hydrolysed and partially condensed silica mixture in ethanol. Different compositions of tetraethoxysilane, ethanol and hydrochloric acid (TEOS:ETOH:HCl) have been prepared to provide suitable gelling time and to enhance mechanical properties of final gels. Supercritical drying was performed using modified solutions mixtures which prevent silica dissolution. Under these conditions gel texture is preserved as demonstrated by the low shrinkage. This route permits positioning of uncracked transparent aerogels with densities lower than 3 kg/m(3). Specific surface area values are in the range of usual aerogels. Therefore, because of their unusual large pores, specific surface area decreases with density. (C) 1998 Elsevier Science B.V. All rights reserved.