Synthesis of aerogel foams through a pressurized sol-gel method

We report monolithic aerogel foams as solid materials with hierarchical porosity created by a foam-like structure embedded in the skeletal framework of a regular aerogel. The foam-like structure is prepared without chemical foaming agents or templates, resulting in a less expensive, more efficient, and more readily adaptable process. Specifically, pressurized air (7 bar) is injected into a suitable sol, which is allowed to gel under pressure, followed by slow depressurization. Voids are created from the air bubbles formed during depressurization. The model material used for validation of the technique is based on poly(isocyanurate-urethane) aerogels (PIR-PUR) and selected material properties of the resulted aerogel foams are compared with those of their pristine aerogel counterparts. With an eye on scalability, all wet-gels were dried under ambient conditions. Aerogel foams exhibit lower bulk densities by about 25%, and higher porosities by about 10% in comparison with their pristine PIR-PUR aerogel counterparts. Interestingly, the thermal conductivities of aerogel foams were found reduced significantly (by 25%) from 0.104 to 0.077 Wm−1K−1 compared to the corresponding pristine aerogels. In addition, aerogel foams absorb 36% w/w more oil and show better oil retention in comparison with regular PIR-PUR aerogel samples made from the same sols. As this technique does not alter the chemical composition of the aerogel, it is anticipated that it can be used for a variety of different types of aerogels and formulations in order to lower their bulk density and improve desired physical properties such as thermal conductivity.