Aerogel

A 2.5 kg brick is supported by a piece of aerogel weighing only 2 grams.

Peter Tsou of NASA's Jet Propulsion Laboratory holds a sample of an aerogel.

Aerogel is a low-density solid-state material derived from gel in which the liquid component of the gel has been replaced with gas. The result is an extremely low density solid with several remarkable properties, most notably its effectiveness as an insulator. It is nicknamed frozen smoke, solid smoke, or blue smoke, due to its semi-transparent nature and the way light scatters in the material. It feels like expanded polystyrene (Styrofoam) to the touch.

Aerogels are useful for a variety of applications. In particular, they are good for thermal insulation and for cleaning up chemical spills. In medicine, they offer a useful drug delivery system. Carbon aerogels are used in the manufacture of small electrochemical double-layer supercapacitors. Aerogels are also being incorporated into tennis and squash racquets. In space exploration, aerogels have been used to trap space dust. By the addition of dopants, reinforcing structures, and hybridizing compounds to aerogels, the range of applications has been considerably broadened.

Production

Aerogel was first created by Steven Kistler in 1931, as a result of a bet with Charles Learned over who could replace the liquid inside a jam (jelly) jar with gas without causing shrinkage.

To produce an aerogel, the liquid component of a gel is extracted by a technique known as supercritical drying. This method allows the liquid to be slowly drawn off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation.

The first aerogels were produced from silica gels. Kistler's later work involved aerogels based on alumina, chromium(III) oxide, and tin oxide. Carbon aerogels were first developed in the early 1990s.

Silica aerogel is made by drying a hydrogel composed of colloidal silica in an extreme environment. Specifically, the process starts with a liquid alcohol, like ethanol, which is mixed with a silicon alkoxide precursor to form a silicon dioxide sol gel (silica gel). Then, through the process of supercritical drying, the alcohol is removed from the gel. This is typically done by exchanging the ethanol for liquid acetone, allowing a better miscibility gradient, and then onto liquid carbon dioxide and then bringing the carbon dioxide above its critical point. A variant of this process involves the direct injection of supercritical carbon dioxide into the pressure vessel containing the aerogel. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.

Aerogel composites have been made using a variety of continuous and discontinuous reinforcements. The high aspect ratio of fibers such as fiberglass have been used to reinforce aerogel composites with significantly improved mechanical properties.

Resorcinol-formaldehyde aerogel (RF aerogel) is made in a way similar to the production of silica aerogel.

Carbon aerogel is made from a resorcinol-formaldehyde aerogel by its pyrolysis in an inert gas atmosphere, leaving a matrix of carbon. It is commercially available as solid shapes, powders, or composite paper.

Properties

A demonstration of aerogel's insulation properties.

To the touch, aerogels feel like a light but rigid foam, something between Styrofoam and the green floral foam used for arranging flowers. Despite their name, aerogels are dry materials and do not resemble a gel in their physical properties. (The name comes from the fact that they are derived from gels.) Pressing softly on an aerogel typically does not leave a mark; pressing more firmly will leave a permanent dimple. Pressing firmly enough will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass—a property known as friability.

Although it is prone to shattering, it is very strong structurally. Its impressive load bearing abilities are due to the dendritic microstructure, in which spherical particles of average size 2-5 nm are fused together into clusters. These clusters form a three-dimensional highly porous structure of almost fractal chains, with pores smaller than 100 nm. The average size and density of the pores can be controlled during the manufacturing process.

Aerogels are remarkable thermal insulators because they almost nullify three methods of heat transfer: convection, conduction, and radiation. They are good convective inhibitors because air cannot circulate throughout the lattice. Silica aerogel is an especially good conductive insulator because silica is a poor conductor of heat—a metallic aerogel, on the other hand, would be a less effective insulator. Carbon aerogel is a good radiative insulator because carbon absorbs the infrared radiation that transfers heat. The most insulative aerogel is silica aerogel with carbon added to it.

Due to its hygroscopic nature, aerogel feels dry and acts as a strong desiccant. Persons handling aerogel for extended periods of time should wear gloves to prevent the appearance of dry brittle spots on their hands.

Given that it is 99% air, aerogel appears semi-transparent. Its color is due to Rayleigh scattering of the shorter wavelengths of visible light by the nanosized dendritic structure. This causes it to appear bluish against dark backgrounds and whitish against bright backgrounds.

Aerogels by themselves are hydrophilic, but chemical treatment can make them hydrophobic. If they absorb moisture they usually suffer a structural change, such as contraction, and deteriorate, but degradation can be prevented by making them hydrophobic. Aerogels with hydrophobic interiors are less susceptible to degradation than aerogels with only an outer hydrophobic layer, even if a crack penetrates the surface. Hydrophobic treatment facilitates processing because it allows the use of a water jet cutter.

Types of aerogel

Silica aerogel

Aerogel produced at Florida State University by Elliot Schwartz and Robert Palmer.

Silica aerogel is the most common type of aerogel and the most extensively studied and used. It is a silica-based substance, derived from silica gel. The world's lowest-density solid is a silica nanofoam at 1 mg/cm³, which is the evacuated version of the record-aerogel of 1.9 mg/cm³. The density of air is 1.2 mg/cm³.

Silica aerogel strongly absorbs infrared radiation. It allows the construction of materials that let light into buildings but trap heat for solar heating.

It has extremely low thermal conductivity (0.03 W·m/m²·K down to 0.004 W·m/m²·K), which gives it remarkable insulative properties. Its melting point is 1,473 K (1,200 °C or 2,192 °F).

Carbon aerogels

Carbon aerogels are composed of particles with sizes in the nanometer range, covalently bonded together. They have very high porosity (over 50%, with pore diameter under 100 nm) and surface areas ranging between 400–1000 m²/g. They are often manufactured as composite paper: non-woven paper made of carbon fibers, impregnated with resorcinol-formaldehyde aerogel, and pyrolyzed. Depending on the density, carbon aerogels may be electrically conductive, making composite aerogel paper useful for electrodes in capacitors or deionization electrodes. Due to their extremely high surface area, carbon aerogels are used to create supercapacitors, with values ranging up to thousands of farads based on a capacitance of 104 F/g and 77 F/cm³. Carbon aerogels are also extremely "black" in the infrared spectrum, reflecting only 0.3% of radiation between 250 nm and 14.3 µm, making them efficient for solar energy collectors.

The term "aerogel" has been incorrectly used to describe airy masses of carbon nanotubes produced through certain chemical vapor deposition techniques—such materials can be spun into fibers with strength greater than kevlar and unique electrical properties. These materials are not aerogels, however, since they do not have a monolithic internal structure and do not have the regular pore structure characteristic of aerogels.

Alumina aerogels

Aerogels made with aluminium oxide are known as alumina aerogels. These aerogels are used as catalysts, especially when "metal-doped" with another metal. Nickel-alumina aerogel is the most common combination. Alumina aerogels are also examined by NASA for capturing of hypervelocity particles; a formulation doped with gadolinium and terbium could fluoresce at the particle impact site, with amount of fluorescence dependent on impact velocity.

Other aerogels

SEAgel is a material similar to organic aerogel, made of agar.

Chalcogels are a type of aerogel made of chalcogens (the column of elements on the periodic table beginning with oxygen) such as sulfur and selenium, platinum, and other elements. Research is ongoing, and metals less expensive than platinum have also been used in its creation.

Uses

The Stardust dust collector with aerogel blocks. (NASA)

Aerogels are used for a variety of tasks. Commercially, aerogels have been used in granular form to add insulation to skylights. After several trips on the Vomit Comet, one research team has shown that producing aerogel in a weightless environment can produce particles with a more uniform size and reduce the Rayleigh scattering effect in silica aerogel, thus making the aerogel less blue and more transparent. Transparent silica aerogel would be very suitable as a thermal insulation material for windows, significantly limiting thermal losses of buildings.

Its high surface area leads to many applications, such as a chemical absorber for cleaning up spills. This feature also gives it great potential as a catalyst or a catalyst carrier. Aerogel particles are also used as thickening agents in some paints and cosmetics.

Aerogels are being tested for use in targets for the National Ignition Facility.

Aerogel performance may be augmented for a specific application by the addition of dopants, reinforcing structures, and hybridizing compounds. Using this approach, the breadth of applications for the material class may be greatly increased.

Commercial manufacture of aerogel 'blankets' began around the year 2000. An aerogel blanket is a composite of silica aerogel and fibrous reinforcement that turns the brittle aerogel into a durable, flexible material. The mechanical and thermal properties of the product may be varied based upon the choice of reinforcing fibers, the aerogel matrix, and opacification additives included in the composite.

NASA used aerogel to trap space dust particles aboard the Stardust spacecraft. The particles vaporize on impact with solids and pass through gases, but can be trapped in aerogels. NASA also used aerogel for thermal insulation of the Mars Rover and space suits.

Aerogels are also used in particle physics as radiators in Cherenkov effect detectors. ACC system of the Belle detector, used in the Belle Experiment at KEKB, is a recent example of such use. The suitability of aerogels is determined by their low index of refraction, filling the gap between gases and liquids, and their transparency and solid state, making them easier to use than cryogenic liquids or compressed gases. Their low mass is also advantageous for space missions.

Resorcinol-formaldehyde aerogels (polymers chemically similar to phenol formaldehyde resins) are mostly used as precursors for manufacture of carbon aerogels, or when an organic insulator with large surface is desired. They come as high-density material, with surface area about 600 m²/g.

Metal-aerogel nanocomposites can be prepared by impregnating the hydrogel with solution containing ions of the suitable noble or transition metals. The impregnated hydrogel is then irradiated with gamma rays, leading to precipitation of nanoparticles of the metal. Such composites can be used as eg. catalysts, sensors, electromagnetic shielding, and in waste disposal. A prospective use of platinum-on-carbon catalysts is in fuel cells.

Aerogel can be used as drug delivery system due to its biocompatibility. Due to its high surface area and porous structure, drugs can be adsorbed from supercritical CO2. The release rate of the drugs can be tailored based on the properties of aerogel.

Carbon aerogels are used in the construction of small electrochemical double layer supercapacitors. Due to the high surface area of the aerogel, these capacitors can be 2000 to 5000 times smaller than similarly rated electrolytic capacitors. Aerogel supercapacitors can have a very low impedance compared to normal supercapacitors and can absorb/produce very high peak currents.

Dunlop tennis has recently incorporated Aerogel into the mold of its new series of racquets. Dunlop have also used it in squash racquets.

Chalcogels has shown promise in absorbing heavy metal pollutants mercury, lead, and cadmium from water.

Aerogel is used to introduce disorder into superfluid 3-helium.

See also

• Capacitor
• Fiberglass
• Gel
• Silica gel
• Supercritical fluid

References

ISBN links support NWE through referral fees

• Catching Comet Dust. NASA JPL. Retrieved October 12, 2007.
• Fricke, J., A. Emmerling. 1992. Aerogels—Preparation, properties, applications. Structure & Bonding. 77:37-87.
• Hüsing,N., U. Schubert. 1998. Aerogels - Airy Materials: Chemistry, Structure, and Properties. Angewandte Chemie International Edition. 37:1/2:22-196.
• Pierre A.C., G.M. Pajonk. 2002. Chemistry of aerogels and their applications. Chemical Reviews. 102:11:4243-4266.

External links

• NASA photos of aerogel. Retrieved October 12, 2007.
• Copy of Lawrence Berkeley National Laboratory page on the Thermal Properties of Silica Aerogels. Retrieved October 12, 2007.
• Another LBL article covering the development of aerogels. Retrieved October 12, 2007.
• Aerogel FAQ at NASA JPL. Retrieved October 12, 2007.
• Aerogel FAQ. Retrieved October 12, 2007.
• Tweney, David. 2006. A Solid That's Light As Air. Wired. Retrieved October 12, 2007.
• Aerogel insulates the The House of the Future?. Retrieved October 12, 2007.
• American company researching and producing flexible aerogel blankets for insulation. Retrieved October 12, 2007.
• Swedish company researching aerogel glass for windows. Retrieved October 12, 2007.
• Scientists hail ‘frozen smoke’ as material that will change world. Retrieved October 12, 2007.

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