Dependence of monolithicity and physical properties of TMOS silica aerogels on gel aging and drying conditions

In this paper the experimental results on the dependence of monolithicity and physical properties of TMOS silica aerogels as a function of alcogel aging and supercritical drying conditions, are reported. While very small and large alcogel aging periods have been found to result in cracked, high density and smaller surface area aerogels; monolithic, low density, and larger surface area aerogels have been obtained with aging periods varying from 6 to 40 days, It has been observed that the percentage of autoclave filling with solvent and the amount of initial pre-pressure, autoclave heating rates, and depressurization rates have an effect on monolithicity and other properties of the aerogels. The percentage of the autoclave filling and the amount of pre-pressure are to be adjusted in such a way that the liquid vapour phase boundary should not be crossed before reaching critical conditions in order to obtain crack-free aerogels. The autoclave heating rates and the depressurization rates in the range from 0.2 to 0.5 degrees C/min and from 0.8 to 2 bar/min, respectively, have been found to produce the best quality silica aerogels in terms of transparency, monolithicity and large surface area. Some properties of these aerogels have been measured by porosimetry and scanning electron microscopy techniques.