Thermal aging of silica gels in water

The classical synthesis route for highly porous silica aerogels comprises supercritical drying (SCD) to prevent the collapse of the delicate gel backbone upon the extraction of the liquid phase. Although the capillary forces approach zero at supercritical conditions, shrinkage on the order of 20–30 vol.% is still observed by different authors. Heat treatment of silica gels in water has been shown previously to increase the mechanical stability of the aerogel backbone. The detailed investigation of the effects that thermal aging in water has on the structural and mechanical characteristics of the silica aerogel and its precursor demonstrates that the change in microporosity and surface groups is a key factor for drying shrinkage upon supercritical extraction.