Silica aerogels supercritical drying – in view –

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Silica aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and low thermal conductivity. Therefore these materials have been used in a variety of applications such as lightweight thermal and acoustic insulation, catalytic supports, microfilters, supercapacitors for electric cars, controlled reléase of drugs, inertial confinement fusion (ICF) targets, Cerenkov radiation detectors and oil-spill clean-up applications.

View cell investigation of silica aerogels during supercritical drying: Analysis of size variation and mass transfer mechanisms.

Luis Miguel Sanz-Moral, Miriam Rueda, Rafael Mato, Ángel Martín

Keywords: silica aerogel; supercritical CO2; advection; shrinkage; drying front

On the other hand, silica aerogels are very brittle structures and use to crack during the drying step of their production. Several techniques have been used in order to dry the aerogels like the atmospheric or freeze drying but the supercritical drying in which the alcohol is solved in a supercritical fluid seems to be the way to overcome the collapse of the structure. But even employing supercritical methods, certain shrinkage of aerogels due to morphological variations is frequently observed after drying.

We report the synthesis and supercritical drying of silica aerogels made via a sol–gel process. Tetramethylortosilicate has been used as precursor. Hydrolysis and poly-condensation steps were followed by CO2 supercritical drying. The complete supercritical drying step was video recorded in order to study the evolution of the size of the gels, concluding that a noticeable shrinkage only takes place during the decompression of CO2 at the end of the drying process, being the total shrinkage of 3-4%. The mass transfer mechanisms during drying have also been studied through analysis of the evolution transparency of the aerogels along the supercritical drying process. The mass transfer processing during drying was observed to be dominated by convection in the earliest stages, where a direct relationship between drying rate and CO2 flow were found. In the later stages, diffusion of the remaining organic solvent through the alcogel determined the mass transfer process.

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A full report of our results is is available at the following link:
http://www.sciencedirect.com/science/article/pii/S0896844614001193

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