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.
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.
Aerogel is not a single material with a set chemical formula, instead the term is used to group all materials with a certain geometric structure.
IUPAC definition
Aerogel: 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., where the definition is a repetition of the incorrect definition of a gel followed by an inexplicit reference to the porosity of the structure.
Properties
Despite the 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.
Aerogel is a material that is 99.8% air. Aerogels have a porous solid network that contains air pockets, with the air pockets taking up the majority of space within the material. The lack of solid material allows aerogel to be almost weightless.
Aerogels are good thermal insulators because they almost nullify two of the three methods of heat transfer – conduction (they are mostly composed of insulating gas) and convection (the microstructure prevents net gas movement). They are good conductive insulators because they are composed almost entirely of gases, which are very poor heat conductors. (Silica aerogel is an especially good insulator because silica is also a poor conductor of heat; a metallic or carbon 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 through them.
Owing to its hygroscopic nature, aerogel feels dry and acts as a strong desiccant. People 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.
Knudsen effect
Aerogels may have a thermal conductivity smaller than that of the gas they contain. This is caused by the Knudsen effect, a 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.
Structure
Aerogel structure results from a sol-gel polymerization, which is when monomers (simple molecules) react with other monomers to form a sol or a substance that consists of bonded, cross-linked macromolecules with deposits of liquid solution between them. When the material is critically heated the liquid is evaporated out and the bonded, cross-linked macromolecule frame is left behind. The result of the polymerization and critical heating is the creation of a material that has a porous strong structure classified as aerogel. Variations in synthesis can alter the surface area and pore size of the aerogel. The smaller the pore size the more susceptible the aerogel is to fracture.
Waterproofing
Aerogel contains particles that are 2–5 nm in diameter. After the process of creating aerogel, it will contain a large amount of hydroxyl groups on the surface. The hydroxyl groups can cause a strong reaction when the aerogel is placed in water, causing it to catastrophically dissolve in the water. One way to waterproof the hydrophilic aerogel is by soaking the aerogel with some chemical base that will replace the surface hydroxyl groups (–OH) with non-polar groups (–OR), a process which is most effective when R is an aliphatic group.
Porosity of aerogel
There are several ways to determine the porosity of aerogel: the three main methods are gas adsorption, mercury porosimetry, and scattering method. In gas adsorption, nitrogen at its boiling point is adsorbed into the aerogel sample. The gas being adsorbed is dependent on the size of the pores within the sample and on the partial pressure of the gas relative to its saturation pressure. The volume of the gas adsorbed is measured by using the Brunauer, Emmit and Teller formula (BET), which gives the specific surface area of the sample. At high partial pressure in the adsorption/desorption the Kelvin equation gives the pore size distribution of the sample. In mercury porosimetry, the mercury is forced into the aerogel porous system to determine the pores' size, but this method is highly inefficient since the solid frame of aerogel will collapse from the high compressive force. The scattering method involves the angle-dependent deflection of radiation within the aerogel sample. The sample can be solid particles or pores. The radiation goes into the material and determines the fractal geometry of the aerogel pore network. The best radiation wavelengths to use are X-rays and neutrons. Aerogel is also an open porous network: the difference between an open porous network and a closed porous network is that in the open network, gases can enter and leave the substance without any limitation, while a closed porous network traps the gases within the material forcing them to stay within the pores. The high porosity and surface area of silica aerogels allow them to be used in a variety of environmental filtration applications.
Materials
Silica
Silica aerogel is the most common type of aerogel, and the most extensively studied and used. It is silica-based and can be derived from silica gel or by a modified Stober process. The lowest-density silica nanofoam weighs 1,000 g/m3, which is the evacuated version of the record-aerogel of 1,900 g/m3. The density of air is 1,200 g/m3 (at 20 °C and 1 atm). As of 2013, aerographene had a lower density at 160 g/m3, or 13% the density of air at room temperature.
The silica solidifies into three-dimensional, intertwined clusters that make up only 3% of the volume. Conduction through the solid is therefore very low. The remaining 97% of the volume is composed of air in extremely small nanopores. The air has little room to move, inhibiting both convection and gas-phase conduction.
Silica aerogels also have a high optical transmission of ~99% and a low refractive index of ~1.05.
It has remarkable thermal insulative properties, having an extremely low thermal conductivity: from 0.03 W/(m•K) in atmospheric pressure down to 0.004 W/(m•K) 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 and then aerographene in 2013.
Carbon
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 resorcinol–formaldehyde 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.
Metal oxide
Metal oxide aerogels are used as catalysts in various chemical reactions/transformations or as precursors for other materials.
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.
One of the most notable difference between silica aerogels and metal oxide aerogel is that metal oxide aerogels are often variedly colored.
| Aerogel | Color |
|---|---|
| Silica, alumina, titania, zirconia | Clear with Rayleigh scattering blue or white |
| Iron oxide | Rust red or yellow, opaque |
| Chromia | Deep green or deep blue, opaque |
| Vanadia | Olive green, opaque |
| Neodymium oxide | Purple, transparent |
| Samaria | Yellow, transparent |
| Holmia, erbia | Pink, transparent |
Other
Organic polymers can be used to create aerogels. SEAgel is made of agar. Cellulose from plants can be used to create a flexible aerogel.
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. 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.
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 which are composites of aerogel with some kind of fibrous batting.
Bio-based alternatives (bioaerogels)
The best-known airgel is silica-based, but researchers are looking to produce bio- sourced aerogels, possibly stronger than silica.
The seagel is a material similar to the organic airgel made of agar, with a taste and texture reminiscent of rice cakes.
The Maerogel consists in its basic rice (mostly threw in the rice industry) and reduces costs compared with other processes. This process makes it possible to divide by six the costs.
The aéropectine is produced from citrus peel (2015) but is too hygroscopic to make an insulator,
Starch airgel (actually a mixture of amylose and amylopectin) that can come for example corn or better pea. It is also very hygroscopic but could perhaps be covered with a coating making it more stable and hydrophobic. It is stronger than the silica airgel but with a coefficient of thermal conductivity slightly less good but nevertheless around 0.021 W m -1 K -1 (0.025 to about 0.035 and the air W m -1 K -1 for rockwool and polystyrene.
Their thermal performance could be improved during manufacture: the starch dissolved in water stirred under a certain pressure and temperature and mechanically stirred to break and disperse the grains is then cooled to 4° C ("retrogradation" phase) and gel formation before replacing it with a solvent during a supercritical drying phase (acetone could replace ethanol there) and then the solvent is desorbed and replaced by air. The Fitness Center of Materials (Cemef) Mines Paris-Tech studies this material.
Manufacturing
In principle, the manufacture of the airgel consists of replacing the liquid component of a silica gel (for silica airgel) with gas. Technically, the process is more complex. Indeed, the structure of the gel tends to collapse when it is simply dried. It becomes porous and crumbles.
In practice, hydrogel, a silica gel used in particular for soft contact lenses, is dried in extreme temperature and pressure conditions by replacing the water with a liquid such as ethanol in the presence of a " precursor ", the silica alkoxide. The alkoxide is a kind of catalyst for the reaction. It is composed of an alcohol and silicone. Its formula is Si (OR) 4. This reaction produces silica:
If (OCH 2 CH 3 ) 4 (Liq.) + 2H 2 O (Liq.) → SiO 2 (solid) + 4HOCH 2 CH 3 (Liq.) .
Silica is a stable mineral compound of formula SiO 2. Then comes a process called supercritical drying (in English: supercritical drying). In thermodynamics, the critical point is a transition phase between liquids and gases. Basically, the liquid and vapor states are microscopically identical: they are characterized by a disorder of atoms or molecules. Also, there is a pressure and a temperature (called critical) for which this liquid-vapor coexistence curve suddenly stops. Beyond, the body is neither liquid nor gaseous: it is a fluid phase. It is by this process that alcohol is removed from the gel. This operation is done in an autoclave at pressures ranging from 50 to 60 bar, temperatures of 5 to 10 ° C and for 12 hours to 6 days. The goal is then reached, the liquid has been replaced by a gas without the structure of the gel collapsing or reducing volume.
There are processes for making airgel at ambient temperature and pressure, but they are, for the moment, kept secret by industrialists.
Production
Aerogels are made by drying a gel of gelatinous material, mostly silica, under extreme conditions. The first synthesis of silicate aerogels was accomplished by Samuel Stephens Kistler in 1931/32. He developed a method first to dry gels without them it had a shrinkage.
Silicate airgel according to Kistler
Kistler used sodium silicate, which he mixed with water to make a solution (water glass). After the addition of a precipitating - reagent acting hydrochloric acid fell with time silica of (precipitation reaction), which caused by the Brownian movement distributed uncoordinated in the solution and thereby also collided.
Or:
Due to the gradual adhesion, these particles aggregated over time and within about one day, a gel with reticulated structure resulted. From this, the sodium chloride and the excess hydrochloric acid was rinsed with water (Aquagel) and followed by a displacement with alcohol (alkogel). This step is necessary as otherwise the water would destroy the gel structure as the process progresses. If the alcohol evaporates slowly, menisci are formed due to the surface forces acting on the gel, which "buries" itself in the gel and causes a ganglike structure in the gel. This would be associated with a shrinkage of the gel and as a result, a porous structure with only about 50% porosity, but this was just to avoid. Kistler used toDrying therefore an autoclave and elevated temperature and pressure over the critical point of alcohol, so that a supercritical fluid was formed. This procedure is called supercritical drying. The phase boundary between gas and liquid was thus canceled; Surface forces, which would have led to the formation of menisci in the other case, no longer existed. The supercritical fluid was then blown out of the autoclave, causing the product to dry and eventually become an airgel. The airgel had retained the size and shape of the original gel, with the silicate aerogels manufactured by Kistler having a density of around 30 to 300 kg / m 3and have a porosity in the range of 86 to 98%. However, the production method according to Kistler had the disadvantage of being long and expensive, which particularly concerned the solvent exchange before the evaporation of the alcohol.
Process according to Teichner - the sol-gel process
Stanislas Teichner attempted to reproduce Kistler's procedure at the University of Lyon in the 1960s, although it took him weeks to produce smaller airgel samples. As an alternative, he developed in 1968 the sol-gel process used today as a standard method, which was further improved in 1986. Starting material here is the toxic tetramethyl orthosilicate (TMOS), which hydrolyzes slowly to ortho silicic acid and methanol according to the reaction equation below with a defined amount of water after the addition of a catalyst.
As a result, water is split off from the silica and SiO 2 tetrahedra is formed. These then network to form a gel. The drying of the resulting alkogel is again equal to the method Kistler, wherein the methanol has critical values of 239.4 ° C and 80.9 bar. The properties of the airgel thus formed, in particular structure and density, can be controlled by the choice of the catalyst, the pH or the proportion of the substances used, in particular of the methanol. The procedure is used today at DESY and in Lund.
Other Procedures
In another process, a research group under Arlon Hunt at the University of California at Berkeley produces airgel pieces instead of the toxic TMOS from tetraethyl orthosilicate (TEOS). In addition, the combustible ethanol replaced by carbon dioxide, which is very time consuming. One advantage is the relatively low critical temperature of the carbon dioxide at 31 ° C, which considerably facilitates the drying process.
Another process is used at BASF in Ludwigshafen am Rhein, where in particular airgel pellets (granules) with a diameter of about one to six millimeters and a density of about 200 kg / m 3 are produced. Sulfuric acid and sodium silicate are reacted by spraying them on a piston with a mixing nozzle. This leads to the formation of alkali salts, which must be washed out by a post-processing. The advantage of this process lies in the comparatively lower costs, the disadvantage is to be seen in the worse, in particular optical properties of the granules.
Carbon aerogels (CRF) are predominantly produced by the pyrolysis of resorcinol - formaldehyde aerogels (RF). In the preparation of the resorcinol-formaldehyde aerogels cheaper air drying can be used instead of supercritical drying.
Applications
Since the refractive index of the aerogels is within a range which is not achievable by gases, liquids or conventional solids, they play an important role as so-called radiator material for Cherenkov detectors; Carbon aerogels also because of their high electrical conductivity and stability in materials research for electrode material in primary and fuel cells, vehicle catalysts and supercapacitors.
Storage medium
Due to their high porosity, aerogels were initially developed with the intention of conserving storage possibilities for gases and solids. In the 1960s, aerogels were tested for their suitability as storage media for liquid rocket fuel.
Filtering
Due to their fine structure, aerogels can be used as collecting matrix for the smallest dust particles. They were therefore used aboard the "comet dust spacecraft " Stardust. The trapped dust particles and molecules are slowly slowed down in the airgel, so that they are not thermally destroyed. So u succeeded. a. also for the first time, without bringing material from a comet (Wild 2) to Earth.
Thermal insulation
Especially silicate aerogels have a very low thermal conductivity and are therefore often used as insulating material for special applications (eg as transparent thermal insulation); Since the beginning of 2013, a corresponding special plaster with added airgel granules has been sold in Switzerland.
Cosmetics and Hair Care
Fine, hydrophobic airgel particles made from silica silylates are used, among other things, as fixing powders in cosmetics and as volume and styling powders in hair care.
Pharmacy
Pharmaceutically silica airgel is used as a drying and solvent, as well as a carrier.
Aerogels are used for a variety of applications:
In 2004 about US$25 million of aerogel insulation product were sold, which had risen to about US$500 million by 2013. This represents the most substantial economic 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.
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.
A chemical adsorber for cleaning up spills.
A catalyst or a catalyst carrier.
Silica aerogels can be used in imaging devices, optics, and light guides.
A material for filtration due to its high surface area and porosity, to be used for the removal of heavy metals.
Thickening agents in some paints and cosmetics.
As components in energy absorbers.
Laser targets for the United States National Ignition Facility.
A material used in impedance matchers for transducers, speakers and range finders.
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.
The US Navy is evaluating aerogel undergarments as passive thermal protection for divers.
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.
Resorcinol–formaldehyde 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 CO2. The release rate of the drugs can be tailored by varying the properties of the aerogel.
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. 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.
Aerogel can introduce disorder into superfluid helium-3.
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.
Thermal insulation transmission tunnel of the Chevrolet Corvette (C7).
CamelBak uses aerogel as insulation in a thermal sport bottle.
45 North uses aerogel as palm insulation in its Sturmfist 5 cycling gloves.
Safety
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 can also 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 or processing bare aerogels.
Source from Wikipedia
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