What is Aerogel?

Introduction

This is an aerogel.

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And so is this.

And so are these.

And so are these.

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. 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).

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. 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 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 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.

Aerogels of all sorts hold records for different properties. Here are some:

Records held by some specially-formulated silica aerogels:

• Lowest density solid (0. 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 ( 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.

Lots More to Learn About!

Aerogel.org has articles about many different types of aerogels, applications of aerogels, the history of aerogel, and how-to guides on how to make aerogels yourself. There is also an interactive properties database where you can compare the physical properties of different types of aerogels side-by-side. Explore and get involved!

Aerogel, a material created on a bet between two scientists in the late s, may be the most unique substance on Earth. It's the lightest solid in existence -- Guinness World Records even said so -- but it can support 500 to 4,000 times its own weight [source: NASA JPL, Guiness; Steiner, Zero-Gravity]. A cubic inch of aerogel could be spread out to cover an entire football field. It's breathable and fireproof, and it absorbs both oil and water.

Aerogel is also amazingly strong, considering its weight. Aerogel insulation can be a great electrical conductor, yet when made from different materials, it can also be an effective thermal insulator [source: Steiner, Zero-Gravity].

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In this article, we'll explore what makes aerogels unique, from their discovery in California in the late s, to their trip to collect space dust in . We'll also see what the future holds for aerogels and whether they can be made more cost-effective for the general public. Finally, we'll show you how to make your own aerogel -- surprisingly, it can be done!

Why Aren't Aerogel Particles Famous?

Having just read so many rare attributes, you must be wondering: Why doesn't aerogel insulation have the A-list name recognition it deserves? Unfortunately, producing such a unique product takes an extraordinary amount of time and money, in part because only a very small amount of aerogel is made in each batch.

Even though producing more aerogel at a time would bring its price down, the process and materials alone come with a high price tag of about $1.00 per cubic centimeter. At about $23,000 per pound, aerogel is currently more expensive than gold [source: NASA JPL, FAQs]!

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Such a valuable product would seem to belong next to diamonds and pearls in an heiress's jewelry box. But aerogel is more likely to be found insulating a rocket or thickening paint than adorning wealthy socialites. While aerogels may not be as glamorous as gold, they perform their tasks without peer.

Read on to learn more about how the world's lowest density solid first made an appearance and how this adaptable substance is made.

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Aerogel History

The legend of the aerogel is shrouded in mystery. What we do know is that in the late s, American chemistry professor Samuel Kistler had a bet with colleague Charles Learned. Kistler believed what made an object a gel was not its liquid properties but its structure: specifically, its network of tiny, microscopic pores known as nanopores. Trying to prove this by simply evaporating the liquid led to the gel deflating like a soufflé. So, the object of the game was to be the first to replace the liquid in "jellies" with gas, but without causing damage to the structure [source: Steiner, Zero Gravity].

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After much trial and error, Kistler was the first to successfully replace the gel's liquid with a gas, creating a substance that was structurally a gel, but without liquid. By he published his findings in an article called "Coherent Expanded Aerogels and Jellies" in the scientific journal Nature [source: Ayers, Pioneer].

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Aerogel begins as a gel, called alcogel. Alcogel is an amorphous silica gel with alcohol inside its pores. Simply evaporating the alcohol out of the silica gel structure would cause the structure to contract, much like a wet sponge will deform when left on a counter to dry. Instead of relying just on evaporation, the gel has to be supercritically dried. Here's what it takes:

1. Pressurize and heat the gel past its critical point -- at high temperatures, there's no difference between gas and liquids.
2. Depressurize the gel while it still remains above its critical temperature. As the pressure decreases, molecules are released as a gas and the fluid grows less dense.
3. Remove the gel from your heat source. After the structure cools, there's too little alcohol to recondense back into liquid, so it reverts to a gas.
4. Check out your final product. What's left behind is a solid made of silica, but now filled with gas (air) where there was once liquid.

Supercritical drying is how the liquid "alco" part of the alcogel turns into a gas within the silica's nanopores without the structure collapsing. The alcogel with its alcohol removed is now called aerogel, as the alcohol has been replaced by air. With only 50 to 99 percent of the original material's volume, aerogel is a porous structure that is light, flexible, and useful [source: Steiner, Zero Gravity].

Continue to the next page to learn about the most common types of aerogels in use today.

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Types of Aerogels

The three most common types of aerogels are silica, carbon and metal oxides, but it's silica that is most often used experimentally and in practical applications. When people talk about aerogels, chances are they're talking about the silica type [source: Aerogel.org, Silica]. Silica is not to be confused with silicon, which is a semiconductor used in microchips. Silica is a glassy material often used for insulation.

Carbon Aerogel Insulation Material

Unlike the smoky-blue silica aerogels, carbon-based ones are black and feel like charcoal to the touch. What they lack in looks, they make up for in high surface area and electrically conductive capabilities. These properties make carbon aerogels useful for supercapacitors, fuel cells, and desalination systems [source: Aerogel.org, Organic].

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Metal Aerogel Insulation Material

Metal oxide aerogels are made from metal oxides and are used as catalysts for chemical transformations. They are also used in the production of explosives and carbon nanotubes, and these aerogels can even be magnetic. What sets metal oxide aerogels such as iron oxide and chromia apart from their more common silica cousins is their range of startlingly bright colors. When made into an aerogel, iron oxide lends an aerogel in its trademark rust color. Chromia aerogels appear deep green or blue. Each type of metal oxide results in an aerogel of a slightly different color. [source: Aerogel.org, Metal].

Silica Aerogel Insulation Material

The most common type, silica aerogels are blue for the same reason the sky is blue. The blue color occurs when white light encounters the aerogel's silica molecules, which are larger than the wavelengths of light. The aerogel scatters, or reflects, the shorter wavelengths of light more easily than the longer ones. Because blue and violet light have the shortest wavelengths, they scatter more than other colors of the visible spectrum. We see scattered wavelengths as color, and since our eyes are more sensitive to blue wavelengths, we never see the violet ones [source: Steiner, Zero-Gravity].

Read on to learn more about aerogels' applications in space.

Water vs. Alcohol

Alcogels have their pores filled with alcohol, but what if you used water instead? In his first experiments, Kistler used hydrogels, which contained water. When drying, these gels behave much as Jell-O does. They break down into a gooey, messy blob because the liquid in the hydrogel evaporates too quickly for the substance to retain its shape. With each molecule that seeps out, others try to fill the gaps. This causes what's known as capillary stress within the pores of the gel, causing the entire structure to collapse [source: Hunt and Ayers, History].

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Aerogels in Space

Aerogel's versatility has made it very important both on Earth and in space. It has fulfilled a variety of roles on several NASA missions, from insulating the Mars rovers' electrical equipment (as aerogel blankets) to capturing space dust from a speeding comet.

On the latter mission, in , NASA launched a spacecraft that traveled 4.8 billion kilometers (the equivalent of 6,000 trips to the moon) to reach comet Wild 2. Once there, the tennis-racket-shaped dust collector opened up and used its 260 aerogel cubes to capture the speedy particles of interstellar dust and preserve them in their natural state [source: NASA JPL, Aerogel].

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What's more, as particles bombarded the dust collector, they left trails within the collector's aerogel cubes while slowing to a stop. These trails enabled scientists to more easily find the tiny particles from space. Aerogel's durability allowed the dust collector to return from space intact with not a single aerogel tile missing. Scientists have been able to study the dust and crystals contained in the aerogel and await the insights they may bring [source: Bridges].

Next, we'll learn about some of aerogel's commercial applications.

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Everyday Aerogel Uses

In their earliest days, aerogels were marketed as thickening agents and used in everything from makeup and paint to napalm. They were also used as cigarette filters and insulation for freezers. Monsanto was the first company to market aerogel's commercial applications. However, Kistler's supercritical drying method, though effective, was also dangerous, time-consuming and expensive. After 30 years of production, all these factors led Monsanto to discontinue its focus on aerogels in the s.

However, it wasn't the end of this thermal insulation. Not long after it was abandoned by Monsanto, scientists developed a process that made the production of aerogels less toxic by using a safer alkoxide compound. They also made it less dangerous by replacing supercritical alcohol with supercritical carbon dioxide in the drying process. These developments reduced the time spent drying the aerogels and reduced the hazardous and flammable nature of their production. Such advances made aerogel a bit more commercially viable again, and scientists grew intrigued by the product's possibilities. [source: Hunt and Ayers, History]

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As production was made less complicated and dangerous, its unique properties (including thermal conductivity) have made aerogel popular within a range of industries. Silicon manufacturers, homebuilding materials manufacturers and space agencies have all put aerogel to use. Its popularity has only been hindered by cost, though there is an increasingly successful push to create aerogels that are cost-efficient. In the meantime, aerogels can be found in a range of products:

• Wetsuits
• Firefighter suits
• Skylights
• Windows
• Rockets
• Paints
• Cosmetics
• Nuclear weapons

[source: Aerogel.org, Modern History]

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A Wonder of Thermal Management

Thanks to aerogel's unique structure, its use as an insulator is a no-brainer. The super-insulating air pockets with the aerogel's structure almost entirely counteract the three methods of heat transfer: convection, conduction and radiation [source: Cabot Corporation]. Even though aerogel is still quite expensive, the good news is that studies have shown that aerogel insulation used in wall framing and hard-to-insulate areas such as window flashing can save a homeowner up to $750 per year.

In addition to helping homeowners save money, aerogel insulation can significantly reduce your carbon footprint. [source: Aspen Aerogels, New Spaceloft]. Companies are racing to find a way to bring costs down, but for now, aerogels are more affordable for NASA than the general public. Still, aerogels are put to use by construction companies, power plants and refineries. Perhaps when it's more affordable, aerogel will achieve its overdue A-list status.

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From Earth to space, aerogels undoubtedly have a place in our future. Read on to learn about recent aerogel advancements and how you, too, can experiment with aerogel.

The Future of Aerogels

Although aerogel is expensive, researchers are still experimenting with ways to make it stronger, cheaper and less hazardous. For example, Professor Nicholas Leventis from the Missouri University of Science and Technology amazed the science world in with the announcement that he had developed a method for making non-brittle aerogels.

Leventis's aerogels, known as x-aerogels, are not only stronger; they're also more flexible, waterproof and impact resistant. The downside is that x-aerogel production requires more hazardous chemicals and takes more time; these chemicals also decrease its insulation ability [source: Aerogel.org, Strong]. Despite some negatives, x-aerogels have the following possible applications:

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• Insulating skylights
• Armor
• Non-deflatable (or "run-flat") tires
• Membranes for electrochemical cells
• Aircraft structural components
• Heat shields for spacecraft reentry

[source: Leventis]

Additionally, aerogels could help with the push for more "green" technology. Carbon aerogel holds great potential for supercapacitors and fuel cells for energy-efficient automobiles. In fact, the energy storage capacity of carbon aerogel could bring about a slew of new technologies, but only if aerogel's production price becomes more affordable for large scale operations.

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Want to Make Your Own?

The good news is that you don't have to be a well-funded research scientist to experiment with making new aerogels. Want to make your own aerogel? Though it's possible to do this at home, it's best done in a laboratory that contains all the necessary materials, including an autoclave to supercritically dry your aerogel. (If you're feeling super productive, here are instructions on how to make your own supercritical dryer.) Ask around your local university or community college; chances are, if you tell them you have a recipe you want to work with, they may let you use their equipment [source: Hunt and Ayers, Making; Aerogel.org, Build].

Several web sites provide instruction on how to make aerogels, including aerogel.org. Regardless of where you make your aerogel, safety precautions are a must. Wear goggles, gloves (the best kind are dishwashing gloves), long pants, closed-toe shoes, and a painter's mask to protect yourself from hazardous fumes and flammable materials. [source: Steiner, How to Make; Hunt and Ayers, Making]

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Aerogels -- is there anything they can't do? Hopefully the public will be on a first-name basis with them in the near future. For more information on aerogels and related topics, check out the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

Sources

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• Aerogel.org. "Metal Oxide Aerogels." (July 14, ) http://www.aerogel.org/?cat=44
• Aerogel.org. "Organic and Carbon Aerogels." (July 13, ) http://www.aerogel.org/?p=71
• Aerogel.org. "Silica Aerogel." (July 13, ) http://www.aerogel.org/?p=16
• Aerogel.org. "Strong and Flexible Aerogels." (July 13, ) http://www.aerogel.org/?p=
• Aerogel.org. "Supercritical Drying." (July 13, ) http://www.aerogel.org/?p=345
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