aerogel

 

Aerogel

The First Encounter
aerogel
Facts About Aerogel
The history of Aerogel
How is Aerogel Made?
What can you do with it?
The Future
Where do I get some?
Links to related sites
 

 

The First Encounter
I first heard about the amazing properties of aerogel in a series called 'High Resolution', broadcast on BBC Radio4 on 18th of April 2001.  It was the last in the series and I was lucky enough to catch it. That evening's edition, sandwiched between "It's a Funny Old World" and "Midweek" started at 9pm and was entitled "Lord Kelvin's Bedspring".  Adam Hart-Davis looked at the history and future of foam.  Aerogel, commonly described as 'frozen smoke' and included in the series as 'Foamed Glass' to be used to collect stardust.  The description of aerogel offered by a NASA scientist was so intriguing that I wanted to own or at least see aerogel for myself.

Facts About Aerogel
A nearly transparent, very lightweight material that is a dry gel principally made from silica (silicon dioxide) and 96% air.  Dubbed a "Super Material", aerogel is the world's lightest solid, weighing as little as three times that of air and exhibiting superb insulating properties.  Although aerogel looks like it could float away, it has very high compression strength.  Theoretically, a block weighing less than a pound could support a weight of half a ton.  Aerogels real strength is its incredible insulating effects on any kind of energy transfer; thermal, electrical or acoustic.  Aerogel can damp out almost any kind of energy.  A one-inch thick Aerogel window has the same insulation value as 15 panes of glass and trapped air - which means a conventional window would have to be ten-inches thick to equal a one-inch thick aerogel window.

That's the primary reason aerogel was used as insulation on the Sojourner Mars rover in 1997.  As night fell on Mars, the temperature dropped down to -67 C (-88 F). Although the temperature outside was colder than Antarctica in winter, it remained a balmy 21 C (70 F) inside the Rover, where sensitive electronics were protected from the hard freeze.

The history of Aerogel
Steven. S. Kistler of the College of the Pacific in Stockton, California set out to prove that a "gel" contained a continuous solid network of the same size and shape as the wet gel.  It is believed that Kistler's interest was stimulated by a friendly wager with fellow worker Charles Learned.  They competed to see if one of them could replace the liquid inside a jelly jar with gas without causing any shrinkage. Kistler won the bet, and published his findings in a 1931 edition of the journal Nature.

As is often the case, the obvious route included many obstacles.  If a wet gel were simply allowed to dry on its own, the gel would shrink, often to a fraction of its original size. This shrinkage was frequently accompanied by severe cracking of the gel.  Kistler surmised, correctly, that the solid component of the gel was micro porous, and that the liquid-vapor interface of the evaporating liquid exerted strong surface tension forces that collapsed the pore structure.  Kistler then discovered the key aspect of aerogel production:

"Obviously, if one wishes to produce an aerogel [Kistler is credited with coining the term "aerogel"], he must replace the liquid with air by some means in which the surface of the liquid is never permitted to recede within the gel.  If a liquid is held under pressure always greater than the vapor pressure, and the temperature is raised, it will be transformed at the critical temperature into a gas without two phases having been present at any time."
(S. S. Kistler, J. Phys. Chem. 34, 52, 1932).

Samuel S. Kistler's Patents
Aerogel-Related (US)

· Aerogels of Silica, etc. U. S. Patent 2,093,454. September 21, 1937.
· Inorgranic Aerogel Compositions as Catalysts. U. S. Patent 2,188,007. January 23, 1940.
· Aerogels, U. S. Patent 2,249,767. July 22, 1941.
· Dentifrice Comprising an Aerogel and Dicalcium Phosphate. U. S. Patent 2,222,969. November 26, 1941.
· Water Repellent Aerogels. U. S. Patent 2,589,705. March 18, 1952.

Aerogels had been largely forgotten when, in the late 1970s, the French government approached Stanislaus Teichner at University Claud Bernard, Lyon seeking a method for storing oxygen and rocket fuels in porous materials.  There is a legend passed on between researchers in the aerogel community concerning what happened next.  Teichner assigned one of his graduate students the task of preparing and studying aerogels for this application.  However, using Kistler's method, which included two time-consuming and laborious solvent exchange steps, their first aerogel took weeks to prepare.  Teichner then informed his student that a large number of aerogel samples would be needed for him to complete his dissertation.  Realising that this would take many, many years to accomplish, the student left Teichner's lab with a nervous breakdown.  Upon returning after a brief rest, he was strongly motivated to find a better synthetic process.  This directly lead to one of the major advances in aerogel science, namely the application of sol-gel chemistry to silica aerogel preparation.  This process replaced the sodium silicate used by Kistler with an alkoxysilane, (tetramethyorthosilicate, TMOS).  Hydrolyzing TMOS in a solution of methanol produced a gel in one step (called an "alcogel").  This eliminated two of the drawbacks in Kistler's procedure, namely, the water-to-alcohol exchange step and the presence of inorganic salts in the gel.  Drying these alcogels under supercritical alcohol conditions produced high-quality silica aerogels.

How is Aerogel Made?
Aerogel starts as a silica dioxide gel, similar to the gelatine dessert you might make at home.  Then the liquid in the gel is removed without collapsing the gel (normal evaporation causes the gel to collapse).  Through a process called supercritical drying, the material does not collapse but retains its original size and shape

Typical Recipes

Single-Step Base Catalyzed Silica Aerogel

This will produce an aerogel with a density of approx. 0.08 g/cm3. The gel time should be 60-120 minutes, depending on temperature.

Mix two solutions:
Silica solution containing 50 mL of TEOS, 40 mL of ethanol
Catalyst solution containing 35 mL of ethanol, 70 mL of water, 0.275 mL of 30% aqueous ammonia, and 1.21 mL of 0.5 M ammonium fluoride.
Slowly add the catalyst solution to the silica solution while stirring.
Pour the mixture into an appropriate mould until gelation.
Allow to age
Supercritical Dry

Supercritical Drying
The final, and most important, process in making silica aerogels is supercritical drying.  This is where the liquid within the gel is removed, leaving only the linked silica network.  The process can be performed by venting the ethanol above its critical point (high temperature -very dangerous) or by prior solvent exchange with CO2 followed by supercritical venting (lower temperatures -less dangerous).  It is imperative that this process only be performed in an autoclave specially designed for this purpose.

The process is as follows.  
The mixed alcogels are placed in the autoclave (which has been filled with ethanol).  The system is pressurised to at least 750-850 psi with CO2 and cooled to 5-10 degrees C.  Liquid CO2 is then flushed through the vessel until all the ethanol has been removed. When the gels are ethanol-free the vessel is heated to a temperature above the critical temperature of CO2 (31 degrees C). As the vessel is heated the pressure of the system rises. CO2 is carefully released to maintain a pressure slightly above the critical pressure of CO2 (1050 psi).  The system is held at these conditions for a short time, followed by the slow, controlled release of CO2 to ambient pressure.  The length of time required for this process is dependent on the thickness of the gels. The process may last anywhere from 12 hours to 6 days. At this point the vessel can be opened and the aerogels admired for their intrinsic beauty.

What can you do with it?
Well, to be honest I've not found any use for the aerogel I have, it's an interesting material that's for sure, and the experience of holding it has been worth owning some.  Although NASA have found several uses, the installation properties coupled with being the lightest material on earth has been exploited.  One other thing that NASA are using it for (and I wish I'd thought of it) is to collect stardust.

"Catching comet dust is no easy feat!  When the spacecraft flies past the comet, the impact velocity of the particles they are captured will be up to 9 times the speed of a bullet fired from a rifle. Although the captured particles will each be smaller than a grain of sand, high-speed capture could alter their shape and chemical composition - or vaporize them entirely" - read more at the NASA site

The Future
Dr. David Noeyer a member of Marshall's three-man aerogel experiment team, believes results from recent space research indicate that they are on the right track to making the hazy material transparent enough to see through clearly when made on the ground, it has a hazy or smoky appearance.  NASA scientists are experimenting with aerogel in space and believe that they may be able to learn how to make the foam-like material transparent.

On April 3, 1996, the first space-produced samples of aerogels were produced by NASA on a flight of a starfire rocket. The production of such materials in space is interesting because of the strong influence of gravity on how a gel is formed.  Comparison of gels manufactured in space and on the ground have shown large differences, and the production of gels in space can provide a higher-quality product with a more uniform structure.

Aerogel will probably be a common household name.  Although it is not yet ready for commercial use, Americas Fortune magazine's "Technology to Watch" column mentioned 800 potential products that could be manufactured out of aerogel, citing everything from surfboards to satellites.  With the use of aerogel in the Sojourner Mars rover, in the StarDust spacecraft, and in the hundreds of other possible products and applications, the "unobtainium" once only dreamed about may finally be just around the corner.

Where do I get some
Out of the blue in May 2012 I was contacted by Aerogel Technologies with a kind offer of Aerogel samples. They have started to sell Aerogel on their site Buy Aerogel and would appreciate a mention here, true to their word, within a couple of weeks from the USA came a package of Aerogel- I will be updating my aerogel pages over the next few weeks but I am very impressed with the professionalism of this new source of purchase.

My original source was from Airglass - the procedure is a little trickier than on-line ordering nevertheless it works fine. This is how I did it, locate the Sale link on there website, complete the form indicating how much aerogel you want, I opted for the 55x55cm at a cost of 500SEK. Within a day or two I received a e-mail from Team Airglass / Rasmus Gullberg thanking me for my order and requesting a transfer 500SEK (500 Swedish Krona is about 32 UKPounds) to their Swedish bank account and they would dully dispatch my aerogel. The money transfer is easily arranged with any high street bank. Barclays Bank charged 10 UKPounds to do this within 5 days (don't you just find it amazing how banks bend over backwards to please you), and they managed the transfer successfully. Within three weeks (the aerogel is produced on a batch basis thus the delay) the package had arrived. The box contained four various size pieces of aerogel, individually wrapped in tissue and soft polystyrene sheet, none of the samples sustained damaged. I have no affiliation with Airglass, it's just useful information - I suppose I could sell some of mine! you can mail me if you want.

Links to related sites

Buy Aerogel http://www.buyaerogel.com
Aerogel - thermal-insulation http://www.proctorgroup.com/products/thermal-insulation/spacetherm
Aerogel Super-Insulation http://www.aspensystems.com/aerogel.html
Aerogel Research at NASA/Marshall http://science.nasa.gov/aerogel/
Aspen Aerogel Inc http://www.aerogel.com/
Hubert's aerogel page http://p25ext.lanl.gov/~hubert/aerogel/
Microgravity Science - Aerogel in your House http://science.nasa.gov/newhome/help/tutorials/housefuture.htm
Aerogel Research on the Web http://science.nasa.gov/aerogel/webinfo.htm
Aerogel Rides Again http://science.nasa.gov/newhome/headlines/msad04feb99%5F1a.htm
Silica Aerogels
http://eande.lbl.gov/ECS/Aerogels/satoc.htm
Airglass http://www.airglass.se/
Zero-Gravity Aerogel Information http://homepages.cae.wisc.edu/~aerogel/index.html

 

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