Aerogel
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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; however it feels like extruded polystyrene to the touch.
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. The first results were silica gels. Aerogel can be made of many different materials; Kistler's work involved aerogels based on silica, alumina, chromia, and tin oxide. Carbon aerogels were first developed in the early 1990s.
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[edit] Technical definition
Aerogels are a class of open-celled mesoporous solid materials possessing no less than 50% porosity by volume. Typically, aerogels are composed of 90-99.8% air, with densities ranging from 1.9 to around 150 mg/cm³. At the nanoscale, an aerogel structurally resembles a sponge and is composed of a network of interconnected nanoparticles. The term aerogel does not refer to a particular substance itself but rather to a geometry a substance can take on--in fact, aerogels can be composed of a variety of materials including silica (SiO2), alumina (Al2O3), transition and lanthanide metal oxides, metal chalcogenides (such as CdS and CdSe), organic and inorganic polymers, and carbon. However the term "aerogel" is frequently used to refer specifically to silica aerogel.
[edit] Properties
To the touch, aerogels feel like a light but rigid foam, something between a Styrofoam peanut and the green floral foam used for potting fake flowers. Despite what their name may suggest, 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 harder will leave a permanent dimple. Pressing hard enough will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass--a property known as friability. Despite the fact that it is prone to shattering, it is very strong structurally, able to hold over 2000 times its own weight. 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. (Metallic aerogel, on the other hand, would be a better heat conductor.) 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. SEAgel is a material similar to organic aerogel, made of agar.
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 the hands.
Since it is mostly air, it appears semi-transparent. The color it does have 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 moisture is absorbed, they will usually cause a structural change of contraction etc. and deteriorate; however, degradation can be prevented by turning them hydrophobic. The aerogel which has hydrophobicity to the interior can prevent degradation, even if a crack reaches deeper than its surface, compared with an aerogel that is only hydrophobic on its surface. Hydrophobic treatment makes processing easy because it allows the use of a water jet cutter.
[edit] Silica aerogels
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 aerogel (the latest and lightest versions of this substance have a density 1.9 mg/cm³, 1/530 as dense as water), produced by the Lawrence Livermore National Laboratory.
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 (0.003 W/(m·K)) [1], which gives it remarkable insulative properties. Its melting point is 1,200 °C (2,192 °F).
Silica aerogel holds 15 entries in the Guinness Book of Records for material properties, including best insulator and lowest-density solid.
Silica aerogel can protect the human hand from the heat of a blowtorch at point blank range.
[edit] Carbon aerogels
Carbon aerogels can be electrically conductive, depending heavily on the density of the aerogel. They 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. The composite aerogel paper is frequently used for electrodes in capacitors, or deionization electrodes. Due to their extremely high surface area (about 800 m²/g), carbon aerogels are used to create supercapacitors, with values ranging up to thousands of farads. The capacitances achieved were 104 F/g and 77 F/cm³. Carbon aerogels are also extremely black, 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 have no internal structure giving them monolithicity (structural continuity) and do not have the regular pore structure characteristic of aerogels.
[edit] Alumina aerogels
Aerogels made with aluminum 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.
[edit] Uses
There are a variety of tasks for which aerogels are used. 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 (see adsorption). 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.
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. One manufacturer of this aerogel composite may be found in the link below.
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 biocompatability. Due to its high surface area and porous structure, drugs can be adsorbed from supercritical CO2. The release rate of the drugs can be taliored based on the properties of aerogel.www.tvt.cbi.uni-erlangen.de/eng/research/index_research.html
Dunlop tennis has recently incorporated Aerogel into the mold of its new series of racquets.
[edit] Production
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 a process called 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 on 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 production of silica aerogel.
Carbon aerogel is made from a resorcinol-formaldehyde aerogel by its pyrolysis in inert gas atmosphere, leaving a matrix of carbon. It is commercially available as solid shapes, powders, or composite paper.
[edit] See also
[edit] References
- ^ Thermal conductivity from the CRC Handbook of Chemistry and Physics, 85th Ed. section 12, p. 227
2. From NASA's Stardust comet return mission on AEROGEL.
[edit] External links
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Wikimedia Commons has media related to: |
- NASA photos of aerogel.
- Lawrence Berkeley National Laboratory page on the Thermal Properties of Silica Aerogels
- Another LBL article covering the development of aerogels
- Aerogel FAQ at NASA JPL
- Aerogel FAQ
- Aerogel Jewelry by Aerogem - Includes Aerogel Photo Gallery
- "A Solid That's Light As Air", by Dylan Tweney, Wired, 23 February 2006
- Small samples of aerogel available for purchase
- Aerogel insulates the The House of the Future?
- American company researching and producing flexible aerogel blankets for insulation
- Swedish company researching aerogel glass for windows
