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

• Supercritical CO2 drying of aerogels was investigated using a view cell.
• Aerogel size remains constant while the drying cell is pressurized.
• Shrinkage of aerogels occurs during the depressurization step.
• After solvent is flushed out of the gel diffusion is the controlling mechanism.

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 carbon dioxide supercritical drying (T = 45 °C; P = 10.5 MPa). 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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