In the spirit of my last waffle on structural materials (Duralumin and Alclad) we shall have a look at an incredible material that has been around a long time - and very few know much about it. In 1932, only three decades after Duralumin was first produced, the lightest known solid material was first prepared. Called aerogel, this stuff is incredible. It looks like smoke, but you can grab it - not too tightly, mind you! Under moderate pressure the material gives and springs back. Press too hard and it shatters and crumbles. If you wet it, it turns to snot.
Quite bizarre, the 'smoke' appears moulded into a shape, with nothing wrapped around it to hold it together. Looking closely at it, your eyes have difficulty figuring out where to focus, like a hologram that refuses to disclose the image that it is storing. Incredibly light, it is around four times the density of air. Looking at it against a dark background, it looks like a blue fog, against a light background it looks like a yellow fog.
Besides having the lowest density of any known solid, aerogel has numerous other properties that are making it a sought-after material in a growing number of areas. It happens to be the best thermal insulator (100 times better than glass) and it has the lowest dielectric constant (1,1). It is incredibly strong for its weight - a cubic centimetre can support a brick - not bad for smoke!
Want to make some?
A metal alkoxide reacts with water, forming a metal hydroxide. In a condensation stage, two metal hydroxide molecules combine to produce a metal oxide molecule and the reaction liberates one water molecule.
Over time more and more condensation occurs and the metal oxide begins to link together in a three-dimensional network. When linked oxide particles span the container holding the solution, it has reached the gel point and has become what is called an alcogel (wet gel).
This alcogel consists of a solid part, formed by the three-dimensional network of linked oxide particles, and liquid water that fills the spaces within the network. Simply evaporating the water out of the matrix would cause it to shrink dramatically (keeping its shape) but often crack, and so a more crafty method is used to remove the water.
This is where it becomes awkward and very dangerous. It was discovered that alcohol could be made a supercritical fluid at attainable temperatures and pressures in a lab. (Supercritical fluids exhibit properties of both liquid and gaseous phases while having extremely low viscosity.)
Pressurised alcohol vapour and air is potentially explosive
The watery lump of alcogel is placed in this dangerous pressure chamber, and by carefully controlling the pressure and temperature of the chamber, the water is gently replaced with alcohol. The alcohol is then allowed to gently escape the gel, its supercritical properties enabling it to leave the gel without causing areas of vacuum within it, which would collapse the structure. With the liquid removed, the material is called an aerogel - as it is a gel filled with air instead of water.
Aerogels were making big news recently when they were being used as a 'net' to catch fast-moving comet material. The idea was that they needed to be able to capture these particles without altering them, so that they could be studied. As aerogels are mostly nothing, damage done to the collected particles would be kept to a minimum.
If you would like to know more about aerogel or the man who first made it, visit the Berkeley Lab's Microstructured Materials Group website: http://eetd.lbl.gov/ECS/aerogels
John Gibbs, Editor, SA Instrumentation & Control,
031 764 0593, [email protected], www.technews.co.za
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