Monolithic and shrinkage-free hydrophobic silica aerogels via new rapid supercritical extraction process

Aerogels have unusual mechanical and thermal properties and are useful in thermal insulation applications. However, aerogel production can be a tedious process if aerogels are made using supercritical extraction of a solvent from a sol-gel matrix. We employ a new rapid and simple supercritical extraction process that is better than conventional methods. This technique relies on rapid heating of the sol in a confined mold under a 50 bar initial pressure supplied by dry N₂ gas in an autoclave. The initial pre-pressure of N₂ gas and base catalyst concentration dictate whether aerogels or xerogels are formed. A model is presented based on the various experimental results that predicts that gelation occurred after the solvent reached a supercritical state. Aerogels produced using this new rapid supercritical extraction process are monolithic, shrinkage-free, have a prescribed shape and size, and possess high surface area (∼800 m²/g) and low thermal conductivity (∼0.035-0.041 W/(m K)). The hydrophobicity as measured by water contact angle was enhanced from 119° to 158° using a hydrophobic co-precursor in the sol.