Drying of Aerogels

The substitution/displacement of solvent molecules located within the solid matrix by dense CO2 is the basis for drying of highly porous materials. Supercritical carbon dioxide is not miscible with water, but completely miscible with most organic solvents and, therefore, very suitable for such a process. The specific volume of the C02-solvent mixture changes when the phase is changed from liquid to gas within the porous structure. In case part of the liquid remains in the structure, the surface tension at the solid-liquid interface creates capillary forces that destroy the structure, that is, delicate structures tend to break up and porous structures collapse. 

Currently, the supercritical drying process is the best solution to avoid such capillary forces. With the supercritical fluid drying, it is possible to move from the liquid phase via the supercritical fluid directly to the gas phase without touching the two-phase area. With optimized drying parameters, capillary forces can be avoided and nanoporous structures are maintained [55-57]. 

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Garcfa-Gonz ez, C.A., Camino-Rey, M.C., Alnaief, M., Zetzl, C., and Smirnova, L, Supercritical drying of aerogels using COj Effect of extraction time on the end material textural properties, Journal of Supercritical Fluids, 66, 297-306, 2012. 

Kirkbir F, Murata H, Meyers D, Ray Chaudhuri S (1998) Drying of aerogels in different solvents between atmospheric and supercritical pressures. J Non-Cryst Solids 225 14-18... 

ReuB M, Ratke L (2009) Characterization of Caibon-AeroSands. hit J Foundry Res 61 2-11 Milow B, Ratke L (2009) German patent apphcation 102009024013 Milow B, Ratke L, Nolte E (2006) German patent application 1020060560936 ReuB M, Ratke L (2010) Drying of aerogel bonded sands. J Mat Sci 45 3974—3980... 

Fig. 2. XRD patterns of titania-silica aerogels, (a) aerogel prepared by high-temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Solvents from pharmaceutical compounds and drugs from natural sources Extraction of volatile substances from substrates Drying and aerogel formation... 

Summary Hydrophobic aerogels were prepared by base-catalyzed hydrolysis and condensation of RSi(OMe)3 (R = Me, Ph, PrI1)/Si(OMe)4 (1 4) mixtures in methanol, followed by supercritical drying of the obtained alcogels with methanol. The organic substituents also increase the elasticity of the aerogels. 

A detailed discussion of the properties of aerogels can be found in several recent review articles (51—55) and the references therein. This section provides a physical basis for these properties by focusing on the microstructure of an aerogel. The intent is to provide a bridge between the two previous sections, which discuss the preparative and drying parameters that affect microstructure, and the next one, which outlines the potential applications made possible by unique structural features. The emphasis is on silica aerogels because they have been the most extensively characterized. 

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