Aerogel The Lightest Material On Earth

Aerogel is making its way into all kinds of new applications

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component for 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, solid cloud, 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.

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.

 

This is an aerogel.

A classic silica aerogel monolith (image credit Prof. C. Jeffrey Brinker)

And so is this.

A flexible, mechanically strong silica aerogel made from methyltrimethoxysilane (image credit Prof. Venkateswara Rao)

And so are these.

A resorcinol-formaldehyde polymer aerogel (left) and an electrically-conductive carbon aerrogel (right)

And so are these.

Colorful lanthanide oxide aerogels made by epoxide-assisted gelation of metal salts (image credit Lawrence Livermore National Laboratory)

Transition metal oxide aerogels including an iron oxide (rust) aerogel (top) (image credit Lawrence Livermore National Laboratory)

Properties
A flower is on a piece of aerogel which is suspended over a flame from a Bunsen burner. Aerogel has excellent insulating properties, and the flower is protected from the flame.

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


Aerogels are a diverse class of porous, solid materials that exhibit an uncanny array of extreme materials properties. Most notably aerogels are known for their extreme low densities (which range from 0.0011 to ~0.5 g cm^-3). In fact, the lowest density solid materials that have ever been produced are all aerogels, including a silica aerogel that as produced was only three times heavier than air, and could be made lighter than air by evacuating the air out of its pores. That said, aerogels usually have densities of 0.020 g cm^-3 or higher (about 15 times heavier than air). But even at those densities, it would take 150 brick-sized pieces of aerogel to weigh as much as a single gallon of water! And if Michaelangelo’s David were made out of an aerogel with a density of 0.020 g cm^-3, it would only weigh about 4 pounds (2 kg)! Typically aerogels are 95-99% air (or other gas) in volume, with the lowest-density aerogel ever produced being 99.98% air in volume.
Essentially an aerogel is the dry, low-density, porous, solid framework of a gel (the part of a gel that gives the gel its solid-like cohesiveness) isolated in-tact from the gel’s liquid component (the part that makes up most of the volume of the gel). Aerogels are open-porous (that is, the gas in the aerogel is not trapped inside solid pockets) and have pores in the range of <1 to 100 nanometers (billionths of a meter) in diameter and usually <20 nm.
Aerogels are dry materials (unlike “regular” gels you might think of, which are usually wet like gelatin dessert). The word aerogel refers to the fact that aerogels are derived from gels–effectively the solid structure of a wet gel, only with a gas or vacuum in its pores instead of liquid. Learn more about gels, aerogels, and how aerogels are made.

Technical Definition

By definition,

An aerogel is an open-celled, mesoporous, solid foam that is composed of a network of interconnected nanostructures and that exhibits a porosity (non-solid volume) of no less than 50%.

The term “mesoporous” refers to a material that contains pores ranging from 2 to 50 nm in diameter.
Generally speaking, most of the pores in an aerogel fall within this size range. In practice, most aerogels exhibit somewhere between 90 to 99.8+% porosity and also contain a significant amount of microporosity (pores less than 2 nm in diameter).

What Are Aerogels Made Of?

The term aerogel does not refer to a particular substance, but rather to a geometry which a substance can take on–the same way a sculpture can be made out of clay, plastic, papier-mâché, etc., aerogels can be made of a wide variety of substances, including:

• Silica
• Most of the transition metal oxides (for example, iron oxide)
• Most of the lanthanide and actinide metal oxides (for example, praseodymium oxide)
• Several main group metal oxides (for example, tin oxide)
• Organic polymers (such as resorcinol-formaldehyde, phenol-formaldehyde, polyacrylates, polystyrenes, polyurethanes, and epoxies)
• Biological polymers (such as gelatin, pectin, and agar agar)
• Semiconductor nanostructures (such as cadmium selenide quantum dots)
• Carbon
• Carbon nanotubes

and

• Metals (such as copper and gold)

Aerogel composites, for example aerogels reinforced with polymer coatings or aerogels embedded with magnetic nanoparticles, are also routinely prepared.
Learn more about the different flavors of aerogels that exist.

Aerogel vs. Silica Aerogel

The term aerogel when used by itself is frequently used to refer specifically to silica aerogels like the blue one shown in the first picture above (although this is like saying “plastic” and specifically meaning, say, polyethylene, despite the fact that there are many other types of plastic such as polypropylene, acrylic, Teflon, Nylon, etc.). In general, the aerogel of a substance is chemically similar to the bulk form of that substance. This said, due to their low densities and length-scale effects arising from having nanostructured features, aerogels often exhibit many dramatically enhanced materials properties over the non-aerogel form of the same substance (for example, significant increases in surface area and catalytic activity), while frequently also exhibiting reductions in other materials properties (such as mechanical strength).

How is Aerogel Made?

So how exactly do you make an aerogel?
As described in the What is Aerogel? section, an aerogel is the intact, dry, ultralow density, porous solid framework of a gel (that is, the part that gives a gel its solid-like cohesiveness) isolated from the gel’s liquid component (which takes up most of the volume in the gel). But how do you isolate such a material from a gel?

The Start of an Aerogel: A Gel

Aerogels start their life out as a gel, physically similar to Jell-O^®. A gel is a colloidal system in which a nanostructured network of interconnected particles spans the volume of a liquid medium. Gels have some properties like liquids, such as density, and some properties like solids, such as a fixed shape. In the case of Jell-O, this network of particles is composed of proteins and spans the volume of some sort of fruit juice. A gel is structurally similar to a wet kitchen sponge, only with pores a thousand to a million times smaller. Because a gel’s pores are so small, the capillary forces exerted by the liquid are strong enough to hold it inside the gel and prevent the liquid from simply flowing out. It’s important to remember that gelatin isn’t the only type of gel–in fact, chemists can prepare gels with backbones composed of many organic and inorganic substances and many liquid interiors.

Once a gel is prepared, it must be purified prior to further processing. This is because the chemical reactions that result in the formation of a gel leave behind impurities throughout the gel’s liquid interior that interfere with the drying processes used to prepare aerogel (as described below). Purification is done by simply soaking the gel under a pure solvent (depending on the gel this could be acetone, ethanol, acetonitrile, etc.), allowing impurities to diffuse out and pure solvent to diffuse in. The solvent in which the gel is soaked is typically exchanged with fresh solvent multiple times over the course several days. Depending on the volume and geometry of the gel, diffusive processes can take any where from hours to weeks. A ice-cube size sample can usually be purified in 1 or 2 days.

For more information about gels and gel preparation, see The Sol-Gel Process under The Science of Aerogel.

The Dire Consequences of Evaporatively Drying a Gel

Now, if you’ve ever left Jell-O uneaten and uncovered in the refrigerator for a long while (on the order of a week or so), you may have observed the gel shrinks gradually. This occurs when the liquid trapped in the gel evaporates from the gel’s surface. As molecules of liquid escape into the air, the surrounding liquid molecules are pulled together by capillary action and tug on the framework of the gel. Continued evaporation results in collapse of the framework of the gel, forming a dense, hard substance with less than 10% of the volume of the original gel. This is called xerogel (pronounced zeroGEL). In fact, 1980’s-style hard contact lenses used to be manufactured by drying silica gels into lens-shaped silica xerogels.

Aerogel is the solid framework of a gel isolated from its liquid component, prepared in such a way as to preserve the framework’s pore structure (or at least most of it). In other words, aerogel is what would be left over if you could remove the liquid from a gel without causing it to shrink. This is most effectively done through a special technique called supercritical drying (although as you will see below, there are other ways to make aerogel as well).

The Answer: Supercritical Drying

In general, supercritical drying is used when liquid needs to be removed from a sample that would be damaged by evaporative or other drying techniques. Biological specimens, for example, are often preserved through supercritical drying.

Supercritical drying is a clever technique by which we can pull the rug out from under capillary action (so to speak). As mentioned earlier, capillary action induced by liquid evaporating from a gel’s pores causes the gel to shrink. So what if there were some way to avoid capillary forces to begin with? This is where supercritical drying comes in.

All pure substances (that won’t decompose) have what’s called a critical point–a specific and characteristic pressure and temperature at which the distinction between liquid and gas disappears. For most substances, the critical point lies at a fairly high pressure (>70 atmospheres) and temperature (>400°F). At the critical point, the liquid and vapor phases of a substance merge into a single phase that exhibits the behavior of a gas (in that it expands to fill the volume of its container and can be compressed) but simultaneously possesses the density and thermal conductivity of a liquid. This phase is called a supercritical fluid.

Say we have a sealed container containing a liquid below its critical point inside and equipped with a pressure gauge on top. In fact, a certain amount of liquid will evaporate in the container until the vapor pressure of the liquid is reached in the container, after which no more liquid will evaporate and the gauge will read a corresponding stable pressure. Now if we heat this container, we will notice the pressure in the container increases, since the vapor pressure of a liquid increases with increasing temperature. As the critical point draws near, the pressure in the container squeezes molecules in the vapor close enough together that the vapor becomes almost as dense as a liquid. At the same time, the temperature in the container gets high enough that the kinetic energy of the molecules in the liquid overwhelms the attractive forces that hold them together as a liquid. In short, as the pressure and temperature in the container get closer to the critical point, the liquid phase becomes more gas-like and the vapor phase more liquid-like. Finally, the critical point is reached and the meniscus dividing the two phases blurs away, resulting in a single supercritical phase. As this occurs, the surface tension in the fluid  gradually drops to zero, and thus the ability of the fluid to exert capillary stress does too.

Aerogelification

In the case of making aerogels, a gel is placed in a pressure vessel under a volume of the same liquid held within its pores (lets say ethanol for example). The pressure vessel is then slowly heated to the liquid’s critical temperature. As this happens, the vapor pressure of the liquid increases, causing the pressure in the vessel to increase and approach the critical pressure of the liquid. The critical point is then surpassed, gently transforming the liquid in the gel (as well as the liquid and vapor surrounding the gel) into a supercritical fluid. Once this happens, the ability of the fluid in the gel to exert capillary stress on the gel’s solid framework structure of the gel has decreased to zero.

With supercritical fluid now present throughout the entire vessel and permeating the pores of the gel, the fluid in the gel can be removed. This is done by partially depressurizing the vessel, but not so much as to cause the pressure in the vessel to drop below the critical pressure. The temperature of the vessel must also remain above the critical temperature during this step. The goal is to remove enough fluid from the vessel while the fluid is still supercritical so that when the vessel is fully depressurized/cooled down and drops below the fluid’s critical point, there will simply not be enough substance left in the vessel left for liquid to recondense. This might require several cycles of heating (and thus pressurizing) followed by depressurization (again all done above the critical point). Once enough fluid has been removed from the vessel, the vessel is slowly depressurized and cooled back to ambient conditions. As this happens, the fluid in the vessel passes back through the critical point, but since much of the fluid has been removed and the temperature is still elevated as the vessel depressurizes, the fluid reverts to a gas phase instead of a liquid phase. What was liquid in the gel has been converted into a gas without capillary stress every arising, and an aerogel is left behind.

It is important to note, however, that most of the liquids used in the preparation of gels are organic solvents such as methanol, ethanol, acetone, and acetonitrile, and such liquids are potentially dangerous at the temperatures and pressures required to make them supercritical. To make the aerogelification process less dangerous, the liquid component of a gel can be exchanged with a non-flammable solvent that mixes well with organic solvents–liquid carbon dioxide (see below).

For more information about supercritical drying, see The Science of Aerogel section.

The Hunt Process: Making Supercritical Drying Safer With Liquid Carbon Dioxide

In the early 1980’s, Dr. Arlon Hunt at Lawrence Berkeley National Laboratory developed a technique for preparing aerogels without needing to supercritically extract potentially explosive solvents. In this technique, a gel containing an organic solvent (such as methanol, ethanol, acetone, or acetonitrile) is soaked under liquid carbon dioxide to replace the liquid in the gel with liquid CO₂. CO₂, which is the product of combustion reactions, is inherently non-flammable (since it’s already oxidized), and has a low critical point of only 31.13°C (88.03°F) and 7.375 MPa (1069.7 psi, or 72.786 times atmospheric pressure). This is compared with, say, methanol, which is very flammable and has a critical point of 239.5°C (463.1°F) and 8.084 MPa (1172.5 psi, 79.783 times atmospheric pressure).

One drawback, however, is that unlike methanol or other organic solvents, CO₂ does not exist as a liquid at ambient conditions. In fact, dry ice, the solid form of CO₂ (which you can buy at some gas stations and grocery stores), sublimes directly to gaseous CO₂ at atmospheric pressure instead of melting. As a result, in order to work with liquid CO₂ so that we can soak a gel in it, we have to use CO₂ at a pressure where it can exist as a liquid (around 58 times atmospheric pressure at room temperature). This doesn’t really pose much of a problem, though, since we need to do the supercritical extraction in a pressure vessel eventually anyway.

To perform CO₂ exchange, a gel is placed in a pressure vessel which is then sealed and slowly pressurized with a tank of liquid CO₂ equipped with a siphon tube (like a liquid soap dispenser). Liquid CO₂ siphon tanks are common, and can be found in almost any restaurant or bar as the source of carbonation in a soda fountain system. Liquid CO₂ entering the vessel will boil instantly, pressurizing the vessel until the vapor pressure of liquid CO₂ at room temperature (~58 atm) is reached. At that point, liquid CO₂ will siphon into the vessel and cover the gel. Depending on the size of the vessel and the gel, it is common to pre-fill the vessel with organic solvent (whatever is in the gel) to prevent the gel from drying out while waiting for CO₂ to siphon in. This organic solvent is then drained off as soon as CO₂ starts to siphon in. After liquid CO₂ has siphoned in, the gel is simply allowed to soak for a number of hours. The liquid in the vessel is drained out and replaced with new liquid every few hours for a period of time of 1-3 days for small samples and up to a week or two for large samples. As mentioned, liquid CO₂ doesn’t exist at ambient conditions, so when liquid CO₂ is drained from a pressurized vessel, although the liquid level goes down in the vessel, only gaseous CO₂ and churnings of dry ice evolve from the drain valve.

As the gel soaks in the liquid CO₂, the organic solvent held within its pores diffuses out, and liquid CO₂ diffuses in its place. Once the gel has been thoroughly diffused through with CO₂ (and this is up to the researcher’s discretion), supercritical extraction can be performed just as described above.

Learn how to build a supercritical dryer of your own and find a fully-illustrated step-by-step process of performing supercritical drying with CO₂ under the Make section.

How Big Can an Aerogel Be Made?

Just as you can only bake a pie as big as your oven, you can only supercritically dry an aerogel as large as your pressure vessel. This means one of three things–either you need a big supercritical dryer, you limit yourself to making small aerogels, or you use a non-supercritical drying technique (see below). Additionally, large continuous volumes (such as cubes or spheres) are generally difficult to make since it takes exponentially longer for solvent from the interior of the gel to diffuse out of the gel as the gel thickness is increased. However, hollow cubes and spheres, flat plates and discs, and rods with thicknesses less than two inches (5 cm), regardless of how big the gel’s other dimensions are, can be easily made.

This said, there are many techniques for preparing aerogel materials called ambigels (often just referred to as aerogels) with subcritical drying techniques. These materials typically have porosities of 50-95% and so they are usually (but not always) a little less dense than supercritically-dried aerogels. Subcritical drying techniques typically require specially-modified gels, in which the solid framework of the gel is chemically changed so that liquid is less able to stick to it and thus exerts only minimal stress on the gel upon evaporation.  Additionally, the liquid in the pores of the gel is frequently replaced with a liquid that has a low surface tension, such as pentane or hexane, so that when the liquid is evaporated little capillary stress can result.  Cabot Corp.’s Nanogel® aerogel granules are made through a subcritical drying technique.

Special Properties of Aerogels

Many aerogels boast a combination of impressive materials properties that no other materials possess simultaneously. Specific formulations of aerogels hold records for the lowest bulk density of any known material (as low as 0.0011 g cm^-3), the lowest mean free path of diffusion of any solid material, the highest specific surface area of any monolithic (non-powder) material (up to 3200 m² g^-1), the lowest dielectric constant of any solid material, and the slowest speed of sound through any solid material. It is important to note that not all aerogels have record properties (in fact most don’t, although they may have very good values for many properties)!
By tailoring the production process, many of the properties of an aerogel can be adjusted. Bulk density is a good example of this, adjusted simply by making a more or less concentrated precursor gel. The thermal conductivity of an aerogel can be also be adjusted this way, since thermal conductivity is related to density. Typically, aerogels exhibit bulk densities ranging from 0.5 to 0.01 g cm^-3 and surface areas ranging from 100 to 1000 m² g^-1, depending of course on the composition of the aerogel and the density of the precursor gel used to make the aerogel. Other properties such as transparency, color, mechanical strength, and susceptibility to water depend primarily on the composition of the aerogel.
For example, silica aerogels, which are the most widely researched type of aerogel (and the type people typically see in photographs), are usually transparent with a characteristic blue cast due to Rayleigh scattering of the short wavelengths of light off of nanoparticles that make up the aerogel’s framework. Carbon aerogels, on the other hand, are totally opaque and black. Furthermore, iron oxide aerogels are just barely translucent and can be either rust-colored or yellow. As another example, low-density (<0.1 g cm^-3) inorganic aerogels are both excellent thermal insulators and excellent dielectric materials (electrical insulators), whereas most carbon aerogels are both good thermal insulators and electrical conductors. Thus it can be seen that by adjusting processing parameters and exploring new compositions, we can make materials with a versatile range of properties and abilities.

The Flower, the Mona Lisa of aerogel pictures, dramatically demonstrates the superinsulating properties of silica aerogel by insulating a delicate, moist flower from the raging heat of a Bunsen burner (image credit Lawrence Berkeley National Laboratory)

Aerogels of all sorts hold records for different properties. Here are some:
Records held by some specially-formulated silica aerogels:

• Lowest density solid (0.0011 g cm^-3)
• Lowest optical index of refraction (1.002)
• Lowest thermal conductivity (0.016 W m^-1 K^-1)
• Lowest speed of sound through a material (70 m s^-1)
• Lowest dielectric constant from 3-40 GHz (1.008)

Record held by a specially-formulated carbon aerogel:

• Highest specific surface area for a monolithic material (3200 m g^-1)

A more in-depth discussion of the properties of silica aerogel and other historically underrepresented types of aerogel can be found in the Flavors of Aerogel section.

What Does an Aerogel Feel Like? How Strong Are They?

To the touch, an inorganic aerogel (such as a silica or metal oxide aerogel) feels something like a cross between a Styrofoam peanut, that green floral potting foam used for potting fake flowers, and a Rice Krispie. Unlike wet gels such as Jell-O, inorganic aerogels are dry, rigid materials and are very lightweight.
In general aerogels are pretty fragile. Inorganic aerogels are friable and and will snap when bent or, in the case of very low density aerogels, when poked, cleaving with an irregular fracture. This said, depending on their density, aerogels can usually hold a gently applied load of up to 2,000 times their weight and sometimes more. But since aerogels are so low in density, it doesn’t take much force to achieve a pressure concentration equivalent to 2,000 times the material’s weight at a given point. The amount of pressure required to crush most aerogels with your fingers is about what it would take to crush a piece of Cap’n Crunch cereal.
Organic polymer aerogels are less fragile than inorganic aerogels and are more like green potting foam in consistency in that they are squish irreversibly. Carbon aerogels, which are derived from organic aerogels, have the consistency of activated charcoal and are very much not squishy.
There are several examples, however, of remarkably strong aerogels that can withstand tens of thousands of times their weight in applied force. A class of polymer-crosslinked inorganic aerogels called x-aerogels are such materials and can even be made flexible like rubber in addition to being mechanically robust (see Flavors of Aerogels). One type of x-aerogel made from vanadia (vanadium oxide) is extraordinarily strong in compression with the highest compressive strength to weight ratio of any known type of aerogel and rivals that of materials such as aerospace-grade carbon fiber composites! Regardless of composition, most types of aerogel can be made stronger simply by making them denser (between 0.1 and 0.5 g cm^-3), however only at the expense of their light weight and ultralow thermal conductivity.

A Note About the Spelling and Use of the Word Aerogel

Aerogel is correctly spelled just like that–aerogel–and is pronounced like “air-o-jel”. It is not a proper noun nor is it a trade name and thus should not be specially capitalized, noted with trademark, or placed in quotes in normal use. It should only be capitalized at the beginning of sentence and in titles, like other nouns. It is also not a compound word and should not be spelled with a hyphen or a space. Frequent misspellings include “AeroGel”, “aerojell”, “areogel”, “aerojel”, “aerojell”, “airojell”, “aero-gel”, “aero gel”, and “airgel”. Aerogel is occasionally referred to as “air glass” or “frozen smoke” but these are just nicknames. Brand names that refer to some commercial aerogel materials include Santocel (obsolete), Nanogel, Pyrogel, Cryogel, and Spaceloft - each of which consists of aerogel with a different formulation and composition.
Furthemore, aerogels are most definitely not aerosols, which are colloidal sprays such as those used for hairspray.
When confronted with the question of how to properly use the word aerogel in a sentence, try replacing “aerogel” in your mind with the word “plastic” and think of how you would use that word in a similar context. For example: “plastics are useful materials” = “aerogels are useful materials”, or “plastic has greatly impacted society” = “aerogel can greatly impact society”. Additionally, a sample of aerogel can be referred to as “an aerogel”. As mentioned earlier, “aerogel” by itself is frequently used to refer specifically to silica aerogel, even though there many types of aerogels other than silica aerogel.