Strengthening of silica gels and aerogels by washing and aging processes

Gels were prepared from a polyethoxydisiloxane precursor by using HF as a c atalyst. During washing in water solution a significant increase in the per meability of the gels was observed, showing that dissolution-reprecipitatio n occurs. After washing, the gels were further soaked in a solution of poly ethoxydisiloxane precursor to strengthen and stiffen the gel. As expected, a significant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability do es not decrease below the value for the as-prepared gels. Hence, a promisin g process has been developed where both the stiffness and the strength have been increased as well as the permeability. The increase in permeability i s of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laborato ry scale has been achieved, and monolithic and transparent gels are obtaine d. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small- angle X-ray scattering(SAXS). (C) 2001 Elsevier Science B.V. All rights reserved.