Aerogels
Aerogel ('frozen smoke') is a truly remarkable material. It is the lightest (lowest-density) solid known to exist and holds the top spot in 14 categories in the Guinness Book of World Records, including the best insulator and the lowest-density solid. Aerogel is composed of 99.8% air. Still it is not a conventional foam, but is a special porous material with extreme porosity on the micron scale. It is composed of individual particles of only a few nanometers in size, which are linked in a three-dimensional network.
Aerogels can be synthesized from silicon oxide (silica aerogels) as well as from different organic and inorganic substances, for example titanium oxide, aluminium oxide, carbon etc.
These novel materials have many unusual properties, such as a low thermal conductivity, refractive index, sound speed, along with a high surface area and thermal stability. An aerogel can be made with a density only three times larger than that of air. Aerogels have many applications in different fields of science and industry. One of the most fascinating applications is the insulation of space shuttles with aerogels, practiced by NASA (USA). Many scientific groups are currently working with aerogels, which is evident by the International Symposia on Aerogels, taking place every three years [ISA 1-6]. Both synthesis and innovative applications of aerogels are of great interest at the present time. Being environmentally friendly and non-toxic, silica aerogels can be used in the pharmaceutical industry. Their large surface area and open pore structure make them an ideal potential carrier material.
Silica Aerogel Properties
Aerogels produced at our chair
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Production methods
The process in which aerogels are made can be explained in simplest terms by its creator, Steven S. Kistler, who proclaimed: “Obviously, if one wishes to produce an aerogel, he must replace the liquid with air by some means in which the surface of the liquid is never permitted to recede within the gel. If a liquid is held under pressure always greater than the vapour pressure, and the temperature is raised, it will be transformed at the critical temperature into a gas without two phases having been present at any time” (S. S. Kistler, J. Phys. Chem. 34, 52, 1932).
An aerogel is made by the so called “sol-gel process”. During this process, all necessary compounds are mixed with a solvent and undergo a chemical reaction producing highly crosslinked particles. The mixture is a liquid at the beginning of the reaction, and becomes more and more viscous as the reaction proceeds. When the reaction is finished, the solution loses its fluidity and the whole reacting mixture turns into a gel. This gel consists of a three-dimensional network filled with the solvent. During the special drying procedure (supercritical drying), the solvent is extracted from the gel body leaving the solid network filled with air. The network retains its original shape and size.
Preparation of aerogel can be summarized in simple four steps (see the sketch below):
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Dissolution of the ingredients (an alkoxide) in a solvent (an alcohol) |
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Hydrolysis by addition of water |
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Condensation / Gelation - formation of the three-dimensional network of colloidal particles |
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Supercritical drying - CO2 extraction, (CO2 critical point, |
