Aerogel
Aerogel is a solid-state substance similar to gel where the liquid component is 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; however it feels like foam 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 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 . Carbon aerogels were first developed in the early 1990s.
Properties
Aerogel is composed of 99.8% air with a typical density of 3 mg/cm3. It feels like hard foam. Pressing softly won't leave any 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 (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, with spherical particles of average size 2-5 nm fused together to clusters, forming a three-dimensional highly porous structure of fractal-like chains with pores smaller than 100 nanometers. The average size and density of pores can be controlled during manufacture.
Aerogel is a remarkable insulator because it almost nullifies three methods of heat transfer (convection, conduction or radiation). It is a good convective inhibitor because air cannot circulate throughout the lattice. Silica aerogel is a good conductive insulator because silica is a poor conductor of heat. (Metallic aerogel, on the other hand, is 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, with a taste and consistency similar to rice cakes.
Due to its hygroscopic nature, aerogel feels dry and acts as a strong desiccant. 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 of their surface can make them hydrophobic.
Silica aerogel
Silica aerogel is the most common type of aerogel and the most extensively studied and used. It is a silica-based substance and the world's lowest-density solid. It is an advanced version of silica gel. The latest and lightest versions of this substance have a density 1.9 mg/cm3 (i.e., 1/530 as dense as water), and are produced by the Lawrence Livermore National Laboratory.
It has extremely low thermal conductivity (approx. 0.017 W/(m·K)), 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.
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.
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 weight is also advantageous for space missions.
Silica aerogel strongly absorbs infrared radiation. It allows construction of materials that let the light into the buildings, but traps heat for solar heating.
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.
Carbon aerogels are electrically conductive. They are composed of particles with sizes ranging in nanometers, 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. Carbon aerogels made of carbon nanotubes instead of graphite particles are highly elastic. They can be spun into fibers with strength greater than kevlar and unique electrical properties.
Alumina aerogels, especially metal-doped, are used as catalysts. 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.
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. This 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.
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 carbon dioxide and then bringing the carbon dioxide above its critical point. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.
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.
See also
External links
