A fast supercritical extraction technique for aerogel fabrication

Aerogels are a class of ceramic materials fabricated from a sol–gel procedure followed by a carefully controlled solvent evacuation. This fabrication results in a porous nanostructure that is approximately 90–99% air by volume. The intricate pore structure of an aerogel results in remarkable properties. This paper describes a novel rapid supercritical extraction technique for fabricating aerogel monoliths. The technique uses a temperature controlled hydraulic hot press and a metal mold to contain and heat the precursors for a tetramethoxysilane-derived (TMOS) aerogel to supercritical state. During heating, the sol gels, strengthens, and ages. After a short soak at the supercritical state, the hot press restraining force is released and the supercritical gases are evacuated without causing damage to the silica nanostructure. Using a TMOS-based recipe (TMOS:MeOH:H2O:NH4OH with a molar ratio of 1.0:12.0:4.0:3.7 × 10−3), cylindrical silica aerogel monoliths have been fabricated as large as 22 mm in diameter and 17 mm high in a 5 h process. The thermo-physical properties are comparable to those of aerogels made using conventional techniques with bulk densities of 0.066 g cm−3, BET surface areas of 320 m2 g−1, thermal conductivities of 30–40 mW K−1 and optical transmittance rates of 80–90% in the near-infrared.