Aerogel

From Wikipedia, the free encyclopedia
  (Redirected from Silica aerogel)
Jump to: navigation, search
A block of aerogel in a person's hand

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density[1] and low thermal conductivity. Nicknames include frozen smoke,[2] solid smoke, solid air, or blue smoke owing to its translucent nature and the way light scatters in the material. It feels like fragile expanded polystyrene (Styrofoam) to the touch. Aerogels can be made from a variety of chemical compounds.[3]

Aerogel was first created by Samuel Stephens Kistler in 1931, as a result of a bet with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.[4][5]

Aerogels are produced by extracting the liquid component of a gel through supercritical drying. This allows the liquid to be slowly dried 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, chromia and tin dioxide. Carbon aerogels were first developed in the late 1980s.[6]

IUPAC definition

Gel comprised of a microporous solid in which the dispersed phase is a gas.

Note 1: Microporous silica, microporous glass, and zeolites are common examples of aerogels.

Note 2: Corrected from ref. [4], where the definition is a repetition of the incorrect definition
of a gel followed by an inexplicit reference to the porosity of the structure. [7]

Properties[edit]

A flower is on a piece of aerogel which is suspended over a flame from a Bunsen burner. Aerogel has excellent insulating properties, and the flower is protected from the flame.

Despite their name, aerogels are solid, rigid, and dry materials that do not resemble a gel in their physical properties: The name comes from the fact that they are made from gels. Pressing softly on an aerogel typically does not leave even a minor mark; pressing more firmly will leave a permanent depression. Pressing extremely firmly will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass – a property known as friability; although more modern variations do not suffer from this. Despite the fact that 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 just under 100 nm. The average size and density of the pores can be controlled during the manufacturing process.

Aerogels are good thermal insulators because they almost nullify two of the three methods of heat transfer (convection, conduction, and radiation). They are good conductive insulators because they are composed almost entirely from a gas, and gases are very poor heat conductors. Silica aerogel is especially good because silica is also a poor conductor of heat (a metallic aerogel, on the other hand, would be less effective). They are good convective inhibitors because air cannot circulate through the lattice. Aerogels are poor radiative insulators because infrared radiation (which transfers heat) passes right through silica aerogel.

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

The slight color it does have is due to Rayleigh scattering of the shorter wavelengths of visible light by the nano-sized dendritic structure. This causes it to appear smoky blue against dark backgrounds and yellowish 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.

Knudsen effect[edit]

Aerogels may have a thermal conductivity smaller than the gas they contain. This is caused by the Knudsen effect. Knudsen effect is the reduction of thermal conductivity in gases when the size of the cavity encompassing the gas becomes comparable to the mean free path. Effectively, the cavity restricts the movement of the gas particles, decreasing the thermal conductivity in addition to eliminating convection. For example, thermal conductivity of air is about 25 mW/m·K at STP and in a large container, but decreases to about 5 mW/m·K in a pore 30 nanometers in diameter.[8]

Materials[edit]

A 2.5 kg brick is supported by a piece of aerogel with a mass of only 2 g.

Silica[edit]

Silica aerogel is the most common type of aerogel, and the most extensively studied and used. It is silica-based, derived from silica gel. The lowest-density silica nanofoam weighs 1,000 g/m3,[9] which is the evacuated version of the record-aerogel of 1,900 g/m3.[10] The density of air is 1,200 g/m3 (at 20 °C and 1 atm).[11] As of 2013, aerographene had a lower density at 160 g/m3, or 0.13 times the density of air at room temperature.[12]

It has remarkable thermal insulative properties, having an extremely low thermal conductivity: from 0.03 W/m·K[13] in atmospheric pressure down to 0.004 W/m·K[9] in modest vacuum, which correspond to R-values of 14 to 105 (US customary) or 3.0 to 22.2 (metric) for 3.5 in (89 mm) thickness. For comparison, typical wall insulation is 13 (US customary) or 2.7 (metric) for the same thickness. Its melting point is 1,473 K (1,200 °C; 2,192 °F).

Until 2011, silica aerogel held 15 entries in Guinness World Records for material properties, including best insulator and lowest-density solid, though it was ousted from the latter title by the even lighter materials aerographite in 2012[14] and then graphene aerogel in 2013.[15][16]

Carbon[edit]

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 400–1,000 m2/g. They are often manufactured as composite paper: non-woven paper made of carbon fibers, impregnated with resorcinolformaldehyde 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 density 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" to describe airy masses of carbon nanotubes produced through certain chemical vapor deposition techniques is incorrect. 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[edit]

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

Other[edit]

Organic polymers can be used to create aerogels. SEAgel is made of agar. Cellulose from plants can be used to create a flexible aerogel.[17]

Chalcogel is an aerogel made of chalcogens (the column of elements on the periodic table beginning with oxygen) such as sulfur, selenium and other elements.[18] Metals less expensive than platinum have been used in its creation.

Aerogels made of cadmium selenide quantum dots in a porous 3-D network have been developed for use in the semiconductor industry.[19]

Aerogel performance may be augmented for a specific application by the addition of dopants, reinforcing structures and hybridizing compounds. Aspen Aerogels makes products such as Spaceloft[20] which are composites of aerogel with some kind of fibrous batting.[21]

Applications[edit]

The Stardust dust collector with aerogel blocks. (NASA)

Aerogels are used for a variety of applications:

  • In the last decade aerogels, particularly silica aerogels, have entered various niche insulation market segments. This represents the economically most substantial impact of these materials today. The potential to replace conventional insulation by aerogel solutions in the building and construction sector as well as in industrial insulation is quite significant.[22]
  • In granular form to add insulation to skylights. Georgia Institute of Technology's 2007 Solar Decathlon House project used an aerogel as an insulator in the semi-transparent roof.[23]
  • A chemical adsorber for cleaning up spills.
  • A catalyst or a catalyst carrier.
  • Thickening agents in some paints and cosmetics.
  • Laser targets for the National Ignition Facility.
  • Commercial manufacture of aerogel 'blankets' began around the year 2000, combining 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 an 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.[24][25]
  • The US Navy is evaluating aerogel undergarments as passive thermal protection for divers.[26]
  • In particle physics as radiators in Cherenkov effect detectors, such as the ACC system of the Belle detector, used in the Belle Experiment at KEKB. 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.
  • Resorcinolformaldehyde aerogels (polymers chemically similar to phenol formaldehyde resins) are 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 prepared by impregnating the hydrogel with solution containing ions of a transition metal and irradiating the result with gamma rays, precipitates nanoparticles of the metal. Such composites can be used as catalysts, sensors, electromagnetic shielding, and in waste disposal. A prospective use of platinum-on-carbon catalysts is in fuel cells.
  • As a drug delivery system owing to its biocompatibility. Due to its high surface area and porous structure, drugs can be adsorbed from supercritical CO
    2
    . The release rate of the drugs can be tailored by varying the properties of the aerogel.[27]
  • 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 1/2000th to 1/5000th the size of similarly rated electrolytic capacitors.[28] Aerogel supercapacitors can have a very low impedance compared to normal supercapacitors and can absorb or produce very high peak currents. At present, such capacitors are polarity-sensitive and need to be wired in series to achieve a working voltage of greater than about 2.75 V.
  • Dunlop Sport uses aerogel in some of its racquets for tennis, squash and badminton.
  • In water purification, chalcogels have shown promise in absorbing the heavy metal pollutants mercury, lead, and cadmium from water.[29]
  • Aerogel can introduce disorder into superfluid helium-3.[30]
  • In aircraft de-icing, a new proposal uses a carbon nanotube aerogel. A thin filament is spun on a winder to create a 10 micron-thick film, equivalent to an A4 sheet of paper. The amount of material needed to cover the wings of a jumbo jet weighs 80 grams (2.8 oz). Aerogel heaters could be left on continuously at low power, to prevent ice from forming.[31]
  • Chevrolet Corvette (C7)transmission tunnel[32]

Production[edit]

Production of aerogels is done by the sol-gel process. First a gel is created in solution and then the liquid is carefully removed to leave the aerogel intact.

The first step is the creation of a colloidal suspension of solid particles known as a “sol”. Silica aerogel is made by the creation of colloidal silica. The process starts with a liquid alcohol such as ethanol which is mixed with a silicon alkoxide precursor, for example tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). A hydrolysis reaction forms particles of silicon dioxide forming a sol solution.[33] The oxide suspension begins to undergo condensation reactions which result in the creation of metal oxide bridges (either M–O–M, “oxo” bridges or M–OH–M, “ol” bridges) linking the dispersed colloidal particles.[34]

When this interlinking has stopped the flow of liquid within the material, this is known as a gel. This process is known as gelation. These reactions generally have moderately slow reaction rates, and as a result either acidic or basic catalysts are used to improve the processing speed. Basic catalysts tend to produce more transparent aerogels with less shrinkage.[33]

The removal of the liquid from a true aerogel involves special processing. Gels where the liquid is allowed to evaporate normally are known as xerogels. As the liquid evaporates, forces caused by surface tensions of the liquid-solid interfaces are enough to destroy the fragile gel network. As a result xerogels cannot achieve the high porosities and instead peak at lower porosities and exhibit large amounts of shrinkage after drying.[35]

In 1931, to develop the first aerogels, Kistler used a process known as supercritical drying. By increasing the temperature and pressure he forced the liquid into a supercritical fluid state where by dropping the pressure he could instantly gasify and remove the liquid inside the aerogel, avoiding damage to the delicate three-dimensional network. While this can be done with ethanol, the high temperatures and pressures lead to dangerous processing conditions. A safer, lower temperature and pressure method involves a solvent exchange. 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 of either process removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.[33]

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.

Resorcinolformaldehyde 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.

Safety[edit]

Silica-based aerogels are not known to be carcinogenic or toxic. However, they are a mechanical irritant to the eyes, skin, respiratory tract, and digestive system. Small silica particles can potentially cause silicosis when inhaled. They also can induce dryness of the skin, eyes, and mucous membranes. Therefore, it is recommended that protective gear including respiratory protection, gloves and eye goggles be worn whenever handling aerogels.[36]

See also[edit]

References[edit]

  1. ^ "Guinness Records Names JPL's Aerogel World's Lightest Solid". NASA. Jet Propulsion Laboratory. 7 May 2002. Archived from the original on 25 May 2009. Retrieved 2009-05-25. 
  2. ^ Taher, Abul (19 August 2007). "Scientists hail ‘frozen smoke’ as material that will change world". News Article (London: Times Online). Retrieved 2007-08-22. 
  3. ^ Aegerter, M.A.; N. Leventis; M. M. Koebel (2011). Aerogels Handbook. Springer publishing. ISBN 978-1-4419-7477-8. 
  4. ^ Kistler S. S. (1931). "Coherent expanded aerogels and jellies". Nature 127 (3211): 741. Bibcode:1931Natur.127..741K. doi:10.1038/127741a0. 
  5. ^ Kistler S. S. (1932). "Coherent Expanded-Aerogels". Journal of Physical Chemistry 36 (1): 52–64. doi:10.1021/j150331a003. 
  6. ^ Pekala R. W. (1989). "Organic aerogels from the polycondensation of resorcinol with formaldehyde". Journal of Materials Science 24 (9): 3221–3227. Bibcode:1989JMatS..24.3221P. doi:10.1007/BF01139044. 
  7. ^ "Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC Recommendations 2007)". Pure and Applied Chemistry 79 (10): 1801–1829. 2007. doi:10.1351/goldbook.A00173. 
  8. ^ http://publications.lib.chalmers.se/records/fulltext/local_159807.pdf
  9. ^ a b Aerogels Terms. LLNL.gov
  10. ^ "Lab's aerogel sets world record". LLNL Science & Technology Review. October 2003. 
  11. ^ Groom, D.E. Abridged from Atomic Nuclear Properties. Particle Data Group: 2007.
  12. ^ "Ultra-light Aerogel Produced at a Zhejiang University Lab-Press Releases-Zhejiang University". Zju.edu.cn. 19 March 2013. Retrieved 2013-06-12. 
  13. ^ "Thermal conductivity" in Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5. . section 12, p. 227
  14. ^ Mecklenburg, Matthias; Arnim Schuchardt; Yogendra Kumar Mishra; Sören Kaps; Rainer Adelung; Andriy Lotnyk; Lorenz Kienle; Karl Schulte (July 2012). "Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance". Advanced Materials 24 (26). doi:10.1002/adma.201200491. PMID 22688858. 
  15. ^ Whitwam, Ryan (26 March 2013). Graphene aerogel is world’s lightest material. gizmag.com
  16. ^ Quick, Darren (24 March 2013). Graphene aerogel takes world’s lightest material crown. gizmag.com
  17. ^ "Plant material aligns to make tough aerogels". Royal Society of Chemistry. Retrieved July 17, 2014. 
  18. ^ Biello, David Heavy Metal Filter Made Largely from Air. Scientific American, 2007-07-26. Retrieved on 2007-08-05.
  19. ^ H. Yu, R. Bellair, R.M. Kannan, S. Brock (2008). "Engineering Strength, Porosity, and Emission Intensity of Nanostructured CdSe Networks By Altering The Building Block Shape". Journal of the American Chemical Society 130 (15): 5054–5055. doi:10.1021/ja801212e. PMID 18335987. 
  20. ^ "Spaceloft 6250". Aspen Aerogels. Retrieved April 25, 2014. 
  21. ^ "Strong and Flexible Aerogels". Aerogel.org. Retrieved July 17, 2014. 
  22. ^ Matthias Koebel, Arnaud Rigacci, Patrick Achard (2012). "Aerogel-based thermal superinsulation: an overview". Journal of Sol-Gel Science and Technology 63 (3): 315–339. doi:10.1007/s10971-012-2792-9. 
  23. ^ GATech Decathon
  24. ^ Preventing heat escape through insulation called "aerogel", NASA CPL
  25. ^ Down-to-Earth Uses for Space Materials, The Aerospace Corporation
  26. ^ Nuckols, M. L.; Chao J. C.; Swiergosz M. J. (2005). "Manned Evaluation of a Prototype Composite Cold Water Diving Garment Using Liquids and Superinsulation Aerogel Materials". United States Navy Experimental Diving Unit Technical Report. NEDU-05-02. Retrieved 2008-04-21. 
  27. ^ Smirnova I., Suttiruengwong S., Arlt W. (2004). "Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems". Journal of Non-Crystalline Solids 350: 54–60. Bibcode:2004JNCS..350...54S. doi:10.1016/j.jnoncrysol.2004.06.031. 
  28. ^ Marc Juzkow (1 February 2002). "Aerogel Capacitors Support Pulse, Hold-Up, and Main Power Applications". Power Electronic Technology. 
  29. ^ Carmichael, Mary. First Prize for Weird: A bizarre substance, like 'frozen smoke,' may clean up rivers, run cell phones and power spaceships. Newsweek International, 2007-08-13. Retrieved on 2007-08-05.
  30. ^ Halperin, W. P. and Sauls, J. A. Helium-Three in Aerogel. Arxiv.org (26 August 2004). Retrieved on 2011-11-07.
  31. ^ Stay informed today and every day (26 July 2013). "De-icing aeroplanes: Sooty skies". The Economist. Retrieved 2013-12-11. 
  32. ^ http://gmauthority.com/blog/2013/07/what-does-nasa-have-to-do-with-the-2014-corvette-stingray/
  33. ^ a b c "Making silica aerogels". Lawrence Berkeley National Laboratory. 
  34. ^ A.C. Pierre, G.M. Pajonk (2002). "Chemistry of Aerogels and their Applications". Chemical Reviews 102 (11): 4243–4265. doi:10.1021/cr0101306. PMID 12428989. 
  35. ^ J. Fricke, A. Emmerling (1992). "Aerogels". Journal of the American Ceramic Society 75 (8): 2027–2036. doi:10.1111/j.1151-2916.1992.tb04461.x. 
  36. ^ Cryogel® 5201, 10201 Safety Data Sheet. Aspen Aerogels. 13 November 2007
Further reading

External links[edit]