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What is Frozen Smoke?

Aerogels are known as frozen smoke because of their ghostly blue look.

Penelope Truce
Penelope Truce
May 11, 2011
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NASA/JPL-Caltech
Aerogels are known as "frozen smoke" because of their ghostly blue look.

 

Aerogel, a material created on a bet between two scientists in the late 1920s, 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 (depending upon whom you ask) [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. Aerogels can be great electrical conductors, yet when made from different materials, they are also one of the best insulators ever known [source: Steiner, Zero-Gravity]. So why don't aerogels have the A-list name recognition they deserve?

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]!

Such a valuable product would seem to belong next to the 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.

In this article, we'll explore what makes aerogels unique, from their discovery in California in the late 1920s, to their trip to collect space dust in 1999. We'll also see what the future holds for aerogels and if there is indeed a way to make them more cost-effective for the general public. Finally, we'll show you how you can make your own aerogel -- surprisingly, it can be done.

Aerogel History

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:

    ♦  Pressurize and heat the gel past its critical point -- the point at which there's no difference between gas and liquid.
    ♦ 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.
    ♦ 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.
    ♦ 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 light, flexible and useful material [source: Steiner, Zero Gravity].

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.

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].

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].

Aerogels in Space

 

NASA/JPL-Caltech
This dust collector for the spacecraft STARDUST was outfitted with 260 aerogel panels.


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].

 

Everyday Aerogel Uses

 

NASA/JPL-Caltech The crayons on top of the aerogel are protected from the flame underneath. Similar silica aerogels were used to insulate the Mars rover.

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:

    ♦ Wetsuits
   
Firefighter suits
   
Skylights
   
Windows
   
Rockets
   
Paints
   
Cosmetics
   
Nuclear weapons

[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.

The Future Of Aerogels

 

NASA/JPL-Caltech A 5.5 pound brick is supported by a piece of silica aerogel weighing only 2 grams (0.07 ounces).

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:

    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.

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.

More Great Links

  1. Aerogel.org
  2. Aerogel: Stardust's "Butterfly Net"
  3. Aspen Aerogels
  4. Making Silica Aerogels

Sources

  1. Aerogel.org. "Build A Supercritical Dryer." (July 13, 2010) http://www.aerogel.org/?cat=33
  2. Aerogel.org. "Metal Oxide Aerogels." (July 14, 2010) http://www.aerogel.org/?cat=44
  3. Aerogel.org. "Organic and Carbon Aerogels." (July 13, 2010) http://www.aerogel.org/?p=71
  4. Aerogel.org. "Silica Aerogel." (July 13, 2010) http://www.aerogel.org/?p=16
  5. Aerogel.org. "Strong and Flexible Aerogels." (July 13, 2010) http://www.aerogel.org/?p=1058
  6. Aerogel.org. "Supercritical Drying." (July 13, 2010) http://www.aerogel.org/?p=345
  7. Aerogel.org. "The Modern History of the Aerogel." (July 13, 2010) http://www.aerogel.org/?cat=46
  8. Aspen Aerogels. "New Spaceloft® Insul-Cap(TM) from Aspen Aerogels Improves Thermal Efficiency of Wall Framing." September 18, 2007. (July 13, 2010)
  9. http://news.thomasnet.com/fullstory/Aerogel-Insulation-Product-is-based-on-nanotechnology-804777
  10. Aspen Aerogels. "Thermal Properties." (July 13, 2010). http://www.aerogel.com/features/termal.html
  11. Ayers, Michael. "The Enigmatic Discovery of Our Favorite Material." The Early Days of Aerogel. May, 2000. (July 13, 2010) http://www.aerogel.org/?p=416
  12. Ayers, Michael. "The Pioneer: Samuel Kistler." May, 2000. (July 13, 2010)
  13. http://eetd.lbl.gov/ECS/Aerogels/kistler-elevated.html
  14. Bridges, Andrew. "Aerogel: Stardust's 'Butterfly Net.'" February 19, 2000. (July 14, 2010)
  15. http://www.space.com/businesstechnology/technology/stardust_aerogel_000219.html
  16. Cabot Corporation. "Nanogel Aerogel: Creating What Matters." (July 14, 2010)
  17. http://www.cabot-corp.com/Aerogel
  18. Hunt, Arlon and Michael Ayers. "History of Silica Aerogels." (July 13, 2010)
  19. http://eetd.lbl.gov/ecs/aerogels/aerogels.htm
  20. Hunt, Arlon and Michael Ayers. "Making Silica Aerogels." (July 13, 2010)
  21. http://eetd.lbl.gov/ECS/aerogels/sa-making.html
  22. Leventis, Nicholas. "Mechanically Strong, Lightweight Porous Materials Developed (X-Aerogels)." NASA Glenn Research Center. July 20, 2005. (July 13, 2010)
  23. http://www.grc.nasa.gov/WWW/RT/2004/RM/RM11P-leventis.html
  24. NASA Jet Propulsion Laboratory. "Aerogel." March 31, 2005. (July 13, 2010) http://stardust.jpl.nasa.gov/tech/aerogel.html
  25. NASA Jet Propulsion Laboratory. "FAQ's: Frequently Asked Questions and Gee Whiz Facts." September 29, 2005. (July 13, 2010) http://stardust.jpl.nasa.gov/overview/faq.html#aerogel
  26. NASA Jet Propulsion Laboratory. "Guinness Records Names JPL's Aerogel World's Lightest Solid." May 7, 2002. (July 13, 2010) http://www.spaceref.com/news/viewpr.html?pid=8270
  27. Steiner, Stephen. "How to Make Silica Aerogel: Part 1." October, 2009. (July 14, 2010)
  28. http://www.aerogel.org/wp-content/uploads/2009/10/how_to_make_silica_aerogel_part_1_64_kbps.mp3
  29. Steiner, Stephen. "Zero-Gravity Aerogel Formation: Research on the Formation of Aerogel in Weightlessness." (July 13, 2010). http://homepages.cae.wisc.edu/~aerogel/aboutaerogel.html
  30. Wray, Rachel. "Aerogel: Emerging Eco-Friendly Insulation." Re-nest: Abundant Design for Green Homes. March 17, 2010. (July 13, 2010) http://www.re-nest.com/re-nest/green-architect/aerogel-emerging-ecofriendly-insulation-green-architect-111377

 

Author's note: Written By Heather Quinlan
Keywords: Heather Quinlan, How Stufs Work, Aerogels, gel, charcoal , Water , Alcohol, What is Frozen Smoke, spacecraft , NASA, Silica ,metal oxides



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