Aerogel

Fig. XVII-29. Nitrogen isotherms the volume adsorbed is plotted on an arbitrary scale. The upper scale shows pore radii corresponding to various relative pressures. Samples A, Oulton catalyst B, bone char number 452 C, activated charcoal F, Alumina catalyst F12 G, porous glass S, silica aerogel. (From Ref. 196).

This article also aims at estabHshing the stmcture—property—appHcation relationships of aerogels. Selected examples are given to show what some desirable properties are and how they can be deHvered by design based on an understanding of the preparation and preservation of a gel s microstmcture. 

Producing aerogel-like materials without supercritical drying at all preparation of inorganic—organic hybrid materials. ... 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

Fig. 4. Comparison of physical properties of silica xerogels and aerogels. Note the similar properties of the aerogels prepared with and without supercritical...

Fig. 5. Schematic diagram of an extractive drying process that produces aerogels at ambient pressure. Reproduced from Ref. 49.

A detailed discussion of the properties of aerogels can be found ia several recent review articles (51—55) and the references thereia. This section provides a physical basis for these properties by focusiag on the microstmcture of an aerogel. The latent is to provide a bridge between the two previous sections, which discuss the preparative and dryiag parameters that affect microstmcture, and the next one, which oudines the potential appHcations made possible by unique stmctural features. The emphasis is on siUca aerogels because they have been the most extensively characteri2ed. 

Thus, the porosity of an aerogel is ia excess of 90% and can be as high as 99.9%. As a consequence of such a high porosity, aerogels have large internal surface area and pore volume. 

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