Supercritical Drying of the Initial Solvent

Near atmospheric pressure, the autoclave is flushed with an inert gas, for example, nitrogen, to prevent condensation in small pores (Sakka and Kozuka, 2005). Finally, the autoclave is cooled down - again slowly to avoid mechanical damage. 

It may be concluded that the supercritical route is most successful in preserving the gel during the drying step. Due to the absence of interfadal forces, both shrinkage and cracks can be successfiilly avoided. The standard technique uses carbon dioxide for its moderate critical values, and the necessary washing step is mostly done under supercritical conditions. In the frame of two recent European research projects (HILIT and HILIT +, 1998-2005), large monolithic silica aerogels for windows (55 X 55 X 1.5 cm, see Fig. 5.24) could be successfully produced by this route (Masmoudi, 2006). 

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