Aerogel

Source: Wikipedia, the free encyclopedia.

A block of silica aerogel in a hand.
IUPAC definition

aerogel: gel comprised of a microporous solid in which the dispersed phase is a gas. (See Gold Book entry for note.) [1]

Aerogels are a class of

styrofoam
to the touch, while some polymer-based aerogels feel like rigid foams.

Aerogels are produced by extracting the liquid component of a gel through

tin dioxide. Carbon aerogels were first developed in the late 1980s.[5]

History

The first documented example of an aerogel was created by

Samuel Stephens Kistler in 1931,[6] as a result of a bet[7] with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.[8][9]

Properties

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.

An aerogel material can range from 50% to 99.98% air by volume, but in practice most aerogels exhibit somewhere between 90 and 99.8% porosity.[10] Aerogels have a porous solid network that contains air pockets, with the air pockets taking up the majority of space within the material.[11]

Aerogels 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 colour it does have is due to

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

hydrophobic
, via a chemical treatment. Aerogels with hydrophobic interiors are less susceptible to degradation than aerogels with only an outer hydrophobic layer, especially if a crack penetrates the surface.

Structure

Aerogel structure results from a

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.[12] 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.[13]

Porosity of aerogel

There are several ways to determine the porosity of aerogel: the three main methods are gas

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.[14]
The high porosity and surface area of silica aerogels allow them to be used in a variety of environmental filtration applications.

Knudsen effect

Aerogels may have a

thermal conductivity smaller than that of the gas they contain.[15][16] 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−1·K−1 at STP and in a large container, but decreases to about 5 mW·m−1·K−1 in a pore 30 nanometers in diameter.[17]

Waterproofing

Aerogel contains particles that are 2–5 nm in diameter. After the process of creating aerogel, it will contain a large amount of

aliphatic group.[18]

Production

Comparison of aerogel fabrication strategies showing typical transitions into an aerogel: (a) the supercritical drying process where precursor materials undergo gelation prior to supercritical drying. (b) A standard freeze-drying technique where an aqueous solution is frozen.
A typical phase diagram for pure compounds. Two methods are shown for the gel to aerogel transition: The solid-gas transition (during freeze-drying) and the transition from a liquid to gas during supercritical drying.

Overview

The preparation of silica aerogels typically involves three distinct steps:[19] the sol-gel transition (gelation),[20] the network perfection (aging), and[21] the gel-aerogel transition (drying).

Gelation

Silica aerogels are typically synthesized by using a sol-gel process. The first step of the sol-gel process is the creation of a

catalysts are used to improve the processing speed. Basic catalysts tend to produce more transparent aerogels and minimize the shrinkage during the drying process and also strengthen it to prevent pore collapse during drying.[23]

For some materials, the transition from a colloidal dispersion into a gel happens without the addition of crosslinking materials.[25] For others, crosslinking materials are added to the dispersion to promote the strong interaction of the solid particles in order to form the gel.[26][27] The gelation time depends heavily on a variety of factors such as the chemical composition of the precursor solution, the concentration of the precursor materials and additives, the processing temperature, and the pH.[28][29][30][31][32] Many materials may require additional curing after gelation (i.e., network perfection) in order to strengthen the aerogel network.[33][34][35][36][37][38]

Drying

Once the gelation is completed, the liquid surrounding the silica network is carefully removed and replaced with air, while keeping the aerogel intact. It is crucial that the gel is dried in such a way as to minimize the surface tension within the pores of the solid network. This is typically accomplished through supercritical fluid extraction using supercritical carbon dioxide (scCO2) or freeze-drying.This section briefly describes and compares the processing strategies of supercritical drying and freeze-drying.

Gels where the liquid is allowed to evaporate at a natural rate are known as

lyophilization (freeze-drying). Depending on the concentration of the fibers and the temperature to freeze the material, the properties such as porosity of the final aerogel will be affected.[40]

In 1931, to develop the first aerogels, Kistler used a process known as supercritical drying which avoids a direct phase change.[41] 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.[42] A variant on this process involves the direct injection of supercritical carbon dioxide into the pressure vessel containing the aerogel. The result of either process exchanges the initial liquid from the gel with carbon dioxide, without allowing the gel structure to collapse or lose volume.[23]

Supercritical Drying

To dry the gel, while preserving the highly porous network of an aerogel, supercritical drying employs the use of the liquid-gas transition that occurs beyond the critical point of a substance. By using this liquid-gas transition that avoids crossing the liquid-gas phase boundary, the surface tension that would arise within the pores due to the evaporation of a liquid is eliminated, thereby preventing the collapse of the pores.[43] Through heating and pressurization, the liquid solvent reaches its critical point, at which point the liquid and gas phases become indistinguishable. Past this point, the supercritical fluid is converted into the gaseous phase upon an isothermal de-pressurization. This process results in a phase change without crossing the liquid-gas phase boundary. This method is proven to be excellent at preserving the highly porous nature of the solid network without significant shrinkage or cracking. While other fluids have been reported for the creation of supercritically dried aerogels, scCO2 is the most common substance with a relatively mild supercritical point at 31 °C and 7.4 MPa. CO2 is also relatively non-toxic, non-flammable, inert, and cost-effective when compared to other fluids, such as methanol or ethanol.[44] While being a highly effective method for producing aerogels, supercritical drying takes several days, requires specialized equipment, and presents significant safety hazards due to its high-pressure operation.

Freeze-Drying

Freeze-drying, also known as freeze-casting or ice-templating, offers an alternative to the high temperature and high-pressure requirements of supercritical drying. Additionally, freeze-drying offers more control of the solid structure development by controlling the ice crystal growth during freezing.[45][46][47][48] In this method, a colloidal dispersion of the aerogel precursors is frozen, with the liquid component freezing into different morphologies depending on a variety of factors such as the precursor concentration, type of liquid, temperature of freezing, and freezing container.[49][50][51] As this liquid freezes, the solid precursor molecules are forced into the spaces between the growing crystals. Once completely frozen, the frozen liquid is sublimed into a gas through lyophilization, which removes much of the capillary forces, as was observed in supercritical drying.[52][53] Though typically classified as a “cryogel”, aerogels produced through freeze-drying often experience some shrinkage and cracking while also producing a non-homogenous aerogel framework.[54] This often leads to freeze-drying being used for the creation of aerogel powders or as a framework for composite aerogels.[55][56][57][58][59]

Preparation of non-silica aerogels

Resorcinol–formaldehyde aerogel (RF aerogel) is made in a way similar to production of silica aerogel. A carbon aerogel can then be made from this resorcinol–formaldehyde aerogel by pyrolysis in an inert gas atmosphere, leaving a matrix of carbon.[60] The resulting carbon aerogel may be used to produce solid shapes, powders, or composite paper.[61] Additives have been successful in enhancing certain properties of the aerogel for the use of specific applications. 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.

Materials

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

Silica aerogel

Silica aerogels are the most common type of aerogel, and the primary type in use or study.

air is 1,200 g/m3 (at 20 °C and 1 atm).[66]

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.[67]

Silica aerogel also has a high optical transmission of ~99% and a low refractive index of ~1.05.[68] It is very robust with respect to high power input beam in continuous wave regime and does not show any boiling or melting phenomena.[69] This property permits to study high intensity nonlinear waves in the presence of disorder in regimes typically unaccessible by liquid materials, making it promising material for nonlinear optics.

This aerogel has remarkable thermal insulative properties, having an extremely low

thermal conductivity: from 0.03 W·m−1·K−1[70] in atmospheric pressure down to 0.004 W·m−1·K−1[64] 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). It is also worth noting that even lower conductivities have been reported for experimentally produced monolithic samples in the literature, reaching 0.009 W·m−1·K−1 at 1atm.[71]

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[72] and then aerographene in 2013.[73][74]

Carbon

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

fluoresce
at the particle impact site, with the amount of fluorescence dependent on impact energy.

One of the most notable differences between silica aerogels and metal oxide aerogel is that metal oxide aerogels are often variedly colored.[75]

Aerogel Color
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. AeroZero film is made of polyimide. Cellulose from plants can be used to create a flexible aerogel.[76]

GraPhage13 is the first graphene-based aerogel assembled using

graphene oxide and the M13 bacteriophage.[77]

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.[78] Metals less expensive than platinum have been used in its creation.

Aerogels made of

quantum dots in a porous 3-D network have been developed for use in the semiconductor industry.[79]

Aerogel performance may be augmented for a specific application by the addition of dopants, reinforcing structures, and hybridizing compounds. For example, Spaceloft is a composite of aerogel with some kind of fibrous batting.[80]

Applications

  • The "Stardust" dust collector with aerogel blocks. (NASA)
    The "Stardust" dust collector with aerogel blocks. (NASA)
  • Cosmic dust caught in aerogel blocks from "Stardust". (NASA)
    Cosmic dust caught in aerogel blocks from "Stardust". (NASA)
  • Oil absorption by an aerogel.[81] (Scientific Reports)
    Oil absorption by an aerogel.[81] (Scientific Reports)
  • An aerogel held up by hair.[81] (Scientific Reports)
    An aerogel held up by hair.[81] (Scientific Reports)
  • An aerogel holding crayons, with a flame lit underneath, demonstrating its excellent insulation from heat. (NASA)
    An aerogel holding crayons, with a flame lit underneath, demonstrating its excellent insulation from heat. (NASA)

Aerogels are used for a variety of applications:

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. They can also induce dryness of the skin, eyes, and mucous membranes.[122] Therefore, it is recommended that protective gear including respiratory protection, gloves and eye goggles be worn whenever handling or processing bare aerogels, particularly when a dust or fine fragments may occur.[123]

See also

References

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Further reading

External links

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