Aerogels are not new. Dr. Samuel Kistler is acknowledged as the creator of the first aerogel around 1930 and he published a paper in Nature, Vol. 127, No. 3211, pg. 741, copyright May 16, 1931.
An aerogel is created when the liquid part of the gel is replaced by a gas. In a gel such as Jell-O ®, proteins form chains and build a matrix that incorporates water, which is the biggest component of the gel. However, if the water is left to evaporate naturally, the gel shrinks and hardens. Dr. Kistler found a way to replace the fluid in the gel without causing any breakdown of the matrix. Using the technique is known as supercritical drying, the liquid is slowly drawn off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation.
The result is an extremely low-density solid with several remarkable properties, most notably its effectiveness as a thermal insulator and its extremely low density. It is usually described as feeling like expanded polystyrene (styrofoam) or the green flower foam called Oasis. It is dry and rigid, unlike a "wet" gel.
Monsanto Corporation produced aerogels commercially in the 1940’s, producing a powdered silica aerogel product called “Santocel.” This was used as a thickening agent for paints, makeup, and napalm; as cigarette filters; and as insulation for a national line of freezers. Monsanto abandoned production of Santocel in the 1970s, due to high manufacturing costs and competition from other types of product.
Types of Aerogel
- Silica - Silica aerogel is the most common type of aerogel.It is the type of aerogel most frequently seen in photographs. It has a typical blue color cast and is nicknamed frozen smoke or solid smoke.
- Carbon - Carbon aerogels are totally black and not transparent. To the touch they are very similar in consistency to activated charcoal. They can be electrically conductive.
- Alumina - Alumina aerogels are being considered by NASA to capture of hypervelocity particles. See below.
- Others - There are numerous examples of both inorganic and organic aerogels. Many of these are biological in nature. Supercritical drying has even been used with collagen-based gels to prepare grafts for artificial skin.
Aerogel Insulation and Other Uses
There are a variety of uses for aerogels:
- Insulation - As shown in the accompanying photographs, aerogels are an excellent insulator. Transparent silica aerogel is suitable as a thermal insulation material for windows, significantly reducing the thermal losses of buildings. The Georgia Institute of Technology uses aerogel as an insulator in the semi-transparent roof in its Solar Decathlon House.The US Navy is evaluating aerogel undergarments as passive thermal protection for divers. NASA has used aerogel for thermal insulation of the Mars Rover and space suits.
- Liquid absorption - The high surface area leads to its application as a chemical absorber for cleaning up spills. They have shown promise in absorbing heavy metal pollutants such as mercury, lead, and cadmium from water.
- Thickening agents - Aerogel particles are used in some paints and cosmetics.
- NASA uses aerogel to trap space dust particles aboard the Stardust spacecraft. The particles vaporize on impact with solids and pass through gases, but can be trapped in aerogels.See below.
- Drug delivery - Aerogel is used as a drug delivery system due to its biocompatibility. The release rate of the drugs can be tailored based on the properties of aerogel.
- Supercapacitance - Carbon aerogels are used in the construction of small electrochemical double layer supercapacitors. Aerogel supercapacitors can have a very low impedance compared to normal supercapacitors and can absorb or produce very high peak currents.
Particle Collection and the Stardust Space Mission
Of particular interest to space scientists is the ability of aerogels to trap dust. Not because space scientists have particularly dirty offices, or that they are particularly scrupulous about cleanliness, but because space is full of dust particles cast off from comets, asteroid collisions, and planetary explosions. The dust, traveling at up to six times the speed of a rifle bullet, is slowed to a stop in the structure of the aerogel, leaving a tapered track showing where the particle has been captured (see photo.) The dust can tell physicists and geologists much about the make-up of other astronomical objects. In particular, the possibility of capturing some indication of extra-terrestrial life from objects that exist, or have existed, in deep space.
In 1999, the first Stardust space mission took off from Earth. Its task was to loop around the sun and capture interstellar dust samples from comets. It carried a dust collection grid filled with an aerogel. The ability of aerogels to capture dust particles traveling at high speed and slow them down with little or no degradation was critical to this mission.
On August 17th, 2009, NASA issued a press release indicating that they had identified glycine in the particles captured by Stardust.
"Glycine is an amino acid used by living organisms to make proteins, and this is the first time an amino acid has been found in a comet," said Dr. Jamie Elsila of NASA's Goddard Space Flight Center in Greenbelt, Md. "Our discovery supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts."
A second Stardust mission is currently in progress.
Sources
NASA Jet Propulsion Laboratory
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Roger Tunsley - Roger has been a professional writer for over 25 years and is now enjoying the life of a freelance writer. Until August of 2009, Roger was ...
