How Aerogels Work

The legend of the aerogel is shrouded in mystery. What we do know is that in the late 1920s, 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].

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 1931 he published his findings in an article called "Coherent Expanded Aerogels and Jellies" in the scientific journal Nature [source: Ayers, Pioneer].

Aerogel begins as a gel, called alcogel. Alcogel is a silica gel with alcohol inside its pores. Simply evaporating the alcohol out of the silica 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:

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 The alcogel with its alcohol removed is now called aerogel, as the alcohol has been replaced by air.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 light, flexible and useful material [source: Steiner, Zero Gravity].

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

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.

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].

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 aerogels -- the most common 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.

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 to capturing space dust from a speeding comet.

Comets are primitive objects that date back to the birth of the solar system. As they fly through space, they cast off particles called space dust. This space dust is much sought-after by scientists who hope it will teach us how our world began.

On a mission to capture comet samples and space dust in 1999, 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]. 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.

When the spacecraft arrived home in 2006, it brought back the first samples returned to Earth from space in more than 30 years. 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.

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 1970s.

However, this wasn't the end of aerogel. 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]

As aerogel's production was made less complicated and dangerous, its unique properties have made aerogel popular with 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:

[source: Aerogel.org, Modern History]

Because of aerogel's unique structure, its use as an insulator 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 that A-list status.

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.

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 2002 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:

[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.

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 and this one from the University of California. 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]

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.