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| − | [[Image:Aerogelbrick.jpg|250px|thumb|right|A 2.5 kg [[brick]] is supported by a piece of aerogel weighing only two grams.]]
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| − | [[Image:Aerogel nasa.jpg|thumb|300px|[[Peter Tsou]] of [[NASA]]'s [[Jet Propulsion Laboratory]] holds a sample of an aerogel.]]
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| − | '''Aerogel''' is a low-density [[solid|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''',<ref>Abul Taher, [http://www.timesonline.co.uk/tol/news/uk/science/article2284349.ece Scientists hail ‘frozen smoke’ as material that will change world]. Times Online. Retrieved October 12, 2007.</ref> '''solid smoke''', or '''blue smoke''', due to its semi-transparent nature and the way light scatters in the material. It feels like [[Polystyrene#Solid foam|expanded polystyrene]] ([[Styrofoam]]) to the touch.
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| − | 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 [[supercapacitor]]s. 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 [[dopant]]s, reinforcing structures, and hybridizing compounds to aerogels, the range of applications has been considerably broadened.
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| − | == Production ==
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| − | 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 [[Fruit preserves|jam]] (jelly) jar with gas without causing shrinkage.<ref>S. S. Kistler, "Coherent expanded aerogels and jellies." ''Nature'' (127) 3211(1931), 741.</ref><ref>S. S. Kistler. "Coherent Expanded-Aerogels." ''Journal of Physical Chemistry'' 36:(1)(1932), 52-64.</ref>
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| − | 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.
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| − | The first aerogels were produced from [[silica gel]]s. Kistler's later work involved aerogels based on [[alumina]], [[chromium(III) oxide]], and [[tin oxide]]. [[Carbon]] aerogels were first developed in the early 1990s.<ref>R. W. Pekala, "Organic aerogels from the polycondensation of resorcinol with formaldehyde." ''Journal of Material Science''. 24:(9)(1989), 3221-3227.</ref>
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| − | Silica aerogel is made by drying a [[hydrogel]] composed of [[colloid]]al [[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 (chemistry)|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.
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| − | 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 [[composite]]s with significantly improved mechanical properties.
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| − | [[Resorcinol]]-[[formaldehyde]] aerogel (RF aerogel) is made in a way similar to the production of silica aerogel.
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| − | 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.
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| − | == Properties ==
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| − | [[Image:Aerogel matches.jpg|right|thumb|200px|A demonstration of aerogel's insulation properties.]]
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| − | To the touch, aerogels feel like a light but rigid foam, something between [[Styrofoam]] and the [[Oasis (horticulture)|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]].''
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| − | Although it is prone to shattering, it is very strong structurally. Its impressive load bearing abilities are due to the [[dendrite (metal)|dendritic]] microstructure, in which spherical particles of average size 2-5 nm are fused together into clusters. These clusters form a three-dimensional highly [[porosity|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.
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| − | Aerogels are remarkable [[thermal insulation|thermal insulators]] because they almost nullify three methods of heat transfer: [[convection]], [[heat conduction|conduction]], and [[thermal radiation|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.
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| − | 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.
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| − | Given that it is 99 percent air, aerogel appears semi-transparent. Its color is due to [[Rayleigh scattering]] of the shorter [[wavelength]]s of [[visible light]] by the nanosized dendritic structure. This causes it to appear bluish against dark backgrounds and whitish against bright backgrounds.
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| − | 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]].
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| − | == Types of aerogel ==
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| − | === Silica aerogel ===
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| − | [[Image:AerogelElliot.jpg|left|thumb|200px|Aerogel produced at [[Florida State University]] by Elliot Schwartz and Robert Palmer.]]
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| − | 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/cm<sup>3</sup><ref name=terms>[http://www.llnl.gov/IPandC/technology/profile/aerogel/Terms/index.php Aerogels Terms]. LLNL. Retrieved October 12, 2007.</ref>, which is the evacuated version of the record-aerogel of 1.9 mg/cm<sup>3</sup><ref name="llnl03">[http://www.llnl.gov/str/October03/NewsOctober03.html Lab's aerogel sets world record]. ''LLNL Science & Technology Review.'' Retrieved October 12, 2007.</ref>. The density of [[air]] is 1.2 mg/cm<sup>3</sup>.
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| − | Silica aerogel strongly absorbs [[infrared]] radiation. It allows the construction of materials that let light into buildings but trap heat for solar heating.
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| − | It has extremely low [[thermal conductivity]] (0.03 [[watt|W]]·[[metre|m]]/m<sup>2</sup>·[[kelvin|K]] down to 0.004 W·m/m<sup>2</sup>·K),<ref name=terms/> which gives it remarkable insulative properties. Its [[melting point]] is 1,473 K (1,200 °C or 2,192 °F).
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| − | === Carbon aerogels ===
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| − | [[Carbon]] aerogels are composed of particles with sizes in the [[nanometer]] range, [[covalent bond|covalently bonded]] together. They have very high [[porosity]] (over 50 percent, 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 fiber]]s, impregnated with [[resorcinol]]-[[formaldehyde]] aerogel, and [[pyrolisis|pyrolyzed]]. Depending on the density, carbon aerogels may be electrically conductive, making composite aerogel paper useful for electrodes in [[capacitor]]s or deionization electrodes. Due to their extremely high surface area, carbon aerogels are used to create [[supercapacitor]]s, with values ranging up to thousands of [[farad]]s 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 percent of radiation between 250 nm and 14.3 µm, making them efficient for solar energy collectors.
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| − | The term "aerogel" has been incorrectly used to describe airy masses of [[carbon nanotube]]s 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.
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| − | === Alumina aerogels ===
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| − | Aerogels made with [[alumina|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.
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| − | === Other aerogels ===
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| − | [[SEAgel]] is a material similar to organic aerogel, made of [[agar]].
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| − | [[Chalcogel]]s are a type of aerogel made of [[chalcogen]]s (the column of elements on the periodic table beginning with oxygen) such as sulfur and selenium, platinum, and other elements.<ref>David Biello. [http://sciam.com/article.cfm?chanId=sa003&articleId=044B7489-E7F2-99DF-3433709C76B127DF Heavy Metal Filter Made Largely from Air.] ''Scientific American.'' 2007. Retrieved October 12, 2007.</ref> Research is ongoing, and metals less expensive than platinum have also been used in its creation.
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| − | == Uses ==
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| − | [[Image:Stardust Dust Collector with aerogel.jpg|thumb|right|The [[Stardust (spacecraft)|Stardust]] dust collector with aerogel blocks. (NASA)]]
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| − | Aerogels are used for a variety of tasks. Commercially, aerogels have been used in granular form to add [[Thermal insulation|insulation]] to [[Window#Skylight|skylights]]. After several trips on the [[Vomit Comet]], one research team<ref>[http://zerogaerogel.com Zero-Gravity Aerogel Formation.] Retrieved October 25, 2007.</ref> has shown that producing aerogel in a [[weightlessness|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.
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| − | 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 agent]]s in some [[paint]]s and [[cosmetics]].
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| − | Aerogels are being tested for use in targets for the [[National Ignition Facility]].
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| − | 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.
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| − | Commercial manufacture of aerogel 'blankets' began around the year 2000. An aerogel blanket is a [[composite material|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.
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| − | [[NASA]] used aerogel to trap [[space dust]] particles aboard the [[Stardust (spacecraft)|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 suit]]s.<ref>[http://marsrovers.jpl.nasa.gov/mission/sc_rover_temp_aerogel.html Preventing heat escape through insulation called "aerogel"]. NASA CPL. Retrieved October 12, 2007.</ref><ref>[http://www.aero.org/publications/crosslink/fall2006/backpage.html Down-to-Earth Uses for Space Materials]. The Aerospace Corporation. Retrieved October 12, 2007.</ref>
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| − | 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 (accelerator)|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.
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| − | [[Resorcinol]]-[[formaldehyde]] aerogels (polymers chemically similar to [[phenol formaldehyde resin]]s) 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.
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| − | Metal-aerogel nanocomposites can be prepared by impregnating the hydrogel with solution containing ions of the suitable [[noble metal|noble]] or [[transition metal|transition]] metals. The impregnated hydrogel is then irradiated with [[gamma ray]]s, leading to precipitation of nanoparticles of the metal. Such composites can be used as eg. [[catalyst]]s, sensors, [[electromagnetic shielding]], and in waste disposal. A prospective use of platinum-on-carbon catalysts is in [[fuel cell]]s.
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| − | 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.<ref>I. Smirnova, S. Suttiruengwong, and W. Arlt. "Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems." ''Journal of Non-Crystalline Solids'' 350 (2004), 54-60.</ref><ref>[http://www.tvt.cbi.uni-erlangen.de/eng/research/thermo_pharma/thermo_pharmazie_e.htm Research group Pharmaceutical Thermodynamics]. University Erlangen. Retrieved October 12, 2007.</ref>
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| − | 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 2,000 to 5,000 times smaller than similarly rated electrolytic capacitors.<ref>Marc Juzkow[http://powerelectronics.com/portable_power_management/batteries/power_aerogel_capacitors_support/ Aerogel Capacitors Support Pulse, Hold-Up, and Main Power Applications]. ''Power Electronics Technology.''(Feb. 1, 2002). Retrieved October 12, 2007.</ref> Aerogel supercapacitors can have a very low impedance compared to normal supercapacitors and can absorb/produce very high peak currents.
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| − | Dunlop tennis has recently incorporated Aerogel into the mold of its new series of racquets. Dunlop have also used it in squash racquets.
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| − | Chalcogels has shown promise in absorbing heavy metal pollutants mercury, lead, and cadmium from water.<ref>Mary Carmichael. [http://www.msnbc.msn.com/id/20123389/site/newsweek/ First Prize for Weird: A bizarre substance, like 'frozen smoke,' may clean up rivers, run cell phones and power spaceships.] ''Newsweek International.'' Retrieved October 12, 2007.</ref>
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| − | Aerogel is used to introduce disorder into superfluid 3-helium. <ref>W. P. Halperin and J. A. Sauls. [http://arxiv.org/PS_cache/cond-mat/pdf/0408/0408593v1.pdf Helium-Three in Aerogel]. Retrieved October 12, 2007.</ref>
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| − | == See also ==
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| − | * [[Capacitor]]
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| − | * [[Fiberglass]]
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| − | * [[Gel]]
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| − | * [[Silica gel]]
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| − | * [[Supercritical fluid]]
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| − | == Notes ==
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| − | <references/>
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| − | == References ==
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| − | * [http://stardust.jpl.nasa.gov/tech/aerogel.html Catching Comet Dust]. ''NASA JPL.'' Retrieved October 12, 2007.
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| − | * Fricke, J., A. Emmerling. "Aerogels—Preparation, properties, applications." ''Structure & Bonding'' 77 (1992), 37-87.
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| − | * Hüsing, N. and U. Schubert. "Aerogels - Airy Materials: Chemistry, Structure, and Properties." ''Angewandte Chemie International Edition'' 37(12) (1998), 22-196.
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| − | * Pierre A. C. and G. M. Pajonk. "Chemistry of aerogels and their applications." ''Chemical Reviews'' 102 (11)(2002), 4243-4266.
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| − | == External links ==
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| − | * [http://stardust.jpl.nasa.gov/photo/aerogel.html NASA photos of aerogel]. Retrieved October 12, 2007.
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| − | * [http://fourier.mech.virginia.edu/~microhx/thermalproperties.html Copy of Lawrence Berkeley National Laboratory page on the Thermal Properties of Silica Aerogels]. Retrieved October 12, 2007.
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| − | * [http://lbl.gov/Science-Articles/Archive/aerogel-insulation.html Another LBL article covering the development of aerogels]. Retrieved October 12, 2007.
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| − | * [http://stardust.jpl.nasa.gov/overview/faq.html#aerogel Aerogel FAQ at NASA JPL]. Retrieved October 12, 2007.
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| − | * [http://connectexpress.com/~ips/aerogel/faq.html Aerogel FAQ]. Retrieved October 12, 2007.
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| − | * David Tweney, [http://wired.com/news/technology/1,70268-1.html A Solid That's Light As Air].'' Wired'' 2006. Retrieved October 12, 2007.
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| − | * [http://science.nasa.gov/newhome/help/tutorials/housefuture.htm Aerogel insulates the The House of the Future?]. Retrieved October 12, 2007.
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| − | * [http://aerogel.com/ American company researching and producing flexible aerogel blankets for insulation]. Retrieved October 12, 2007.
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| − | * [http://www.airglass.se/ Swedish company researching aerogel glass for windows]. Retrieved October 12, 2007.
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| − | * [http://www.timesonline.co.uk/tol/news/uk/science/article2284349.ece Scientists hail ‘frozen smoke’ as material that will change world]. Retrieved October 12, 2007.
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| − | [[Category:Physical sciences]]
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| − | [[Category:Physics]]
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| − | [[Category:Chemistry]]
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| − | [[Category:Materials science]]
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