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
and low thermal conductivity. Nicknames include frozen smoke, 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 to the touch. Aerogels can be made from a variety
of chemical compounds. 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.
Properties
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
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. 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. They are good convective inhibitors
because air cannot circulate through the lattice. Aerogels are poor radiative insulators because
infrared radiation 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
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. Materials
Silica 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, which is the evacuated
version of the record-aerogel of 1,900 g/m3. The density of air is 1,200 g/m3. As of 2013,
aerographene had a lower density at 160 g/m3, or 0.13 times the density of air at room
temperature. 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 or 3.0
to 22.2 for 3.5 in thickness. For comparison, typical wall insulation is 13 or 2.7 for the
same thickness. Its melting point is 1,473 K. 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 graphene aerogel in 2013. Carbon
Carbon aerogels are composed of particles with sizes in the nanometer range, covalently
bonded together. They have very high porosity 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. Alumina
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 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 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. Applications
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.
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.
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. 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 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. 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. Aerogel heaters could be left on continuously at low
power, to prevent ice from forming. Chevrolet Corvettetransmission tunnel
Production
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 or tetraethyl orthosilicate. A hydrolysis reaction forms
particles of silicon dioxide forming a sol solution. The oxide suspension begins to undergo
condensation reactions which result in the creation of metal oxide bridges linking the
dispersed colloidal particles. 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.
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.
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 initial aqueous pore liquid for a CO2 miscible liquid
such as ethanol or acetone, 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 exchanges the initial liquid from the gel with carbon dioxide, 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. Resorcinol–formaldehyde 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 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. See also
Carbon nanofoam Nanogel
References
Further reading
External links Open source aerogel
NASA photos of aerogel LBL article covering the development of aerogels