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 a thermal 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 expanded polystyrene (Styrofoam) to the touch.
Aerogels are produced by extracting the liquid component of a gel through supercritical drying. This 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, chromia and tin oxide. Carbon aerogels were first developed in the late 1980s.
To the touch, aerogels feel like a light but rigid foam, something between Styrofoam and the green floral foam used for arranging 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 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. 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 smaller than 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 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.
Since it is 99.8% 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 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.
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 remarkable thermal insulative properties, having an extremely low thermal conductivity: from 0.03 W/m·K down to 0.004 W/m·K, which correspond to R-factors of 14 to 105 for 3.5 inch thickness. For comparison, typical wall insulation is 13 for 3.5 inch thickness. Its melting point is 1,473 K (1,200 °C or 2,192 °F).
Silica aerogel holds 15 entries in Guinness World Records for material properties, including best insulator and lowest-density solid.
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.
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.
There are a variety of tasks for which aerogels are used.
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.
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.
However, safety depends on the material from which the aerogel is made – it will be carcinogenic or toxic if made from a gel with such characteristics.
Research and Markets Adds Report: Aerogel Market by Form and Application (Building Insulation, Oil & Gas, Aerospace, Automotive, Industrial & Cryogenic) - Global Trends and Forecasts Up till 2017
Jul 10, 2012; Research and Markets has announced the addition of the "aerogel Market by Form and Application (Building Insulation, Oil &...
Evaluating and understanding the hydrothermal stability of alumina aerogel doped with yttrium oxide and used as a catalyst support for the thermo-catalytic cracking (TCC) process.(Report)
Feb 01, 2008; Abstract: The current work presents results concerning the hydrothermal stability of yttria-doped alumina aerogel, which...