Scientists Develop Airier Aerogels
Uses for the nearly invisible substance range from space experiments to efficient insulation
SCIENTISTS at the Lawrence Livermore Laboratory have spent much of the last five years trying to make something that almost doesn't exist. Called an aerogel, the bluish ghostlike material looks like a special effect from a science fiction movie. It is only four times heavier than air and is nearly invisible under many lighting conditions, yet a block of the substance is strong enough to support coins haphazardly placed on top.Skip to next paragraph
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``It looks like it shouldn't be there,'' says John F. Poco, one of the material scientists who makes the substance, which is more than 99 percent air.
Aerogels were discovered in the 1930s by S.S. Kistler at Stanford University. Since then, their main use has been for detecting particles in high-energy physics experiments.
The group at Livermore, headed by Lawrence Hrubesh, has developed techniques for making aerogels 10 times lighter than similar materials made anywhere in the world. The University of California, which oversees the lab, has applied for a patent on the process.
Livermore's interest in aerogels goes back five years, when the laboratory's laser-fusion program was looking for a new way to hold together the hydrogen gas used to make energy.
``We wanted to use a material that was a foam, and something that was transparent,'' recalls Mr. Poco.
Aerogels, which are riddled with tiny channels and pockets, seemed to be worth investigating. Then at the end of 1988, the National Aeronautics and Space Administration (NASA) awarded the group a contract to make aerogels for the space shuttle. NASA's plan: Use large blocks of aerogel to catch micro-meteorites and cosmic dust.
A particle in space that hit a gel would be gently brought to a stop. Later, the gels could be brought back to Earth for analysis, where scientists could study the gel's damage to determine the speed and weight of the meteorite. Since aerogels are chemically pure - their only ingredient is silica - they would not contaminate the material they caught.
But the real payoff from aerogels will probably be on the ground.
Aerogels conduct heat 12 times more slowly than glass does, making them phenomenal insulators. Since they are also transparent, aerogels might be used in the future as insulation between the panes of a double-pane window. They could also replace chlorofluorocarbon-blown foam as insulation inside refrigerators.
Another possibility, advocated by Jochen Fricke at the University of Wurzburg in West Germany, would be to use aerogels in passive solar collectors. By painting the sides of a house black and covering the paint with a layer of aerogel, a house in Northern Europe might be able to trap enough sunlight to cut its heating bill in half.
Scientists speculate that the silica in an aerogel is spun out in long fibers, like many strings of pearls tangled together, with the spaces between them filled by air. Aerogels have a bluish tint because they scatter light the same way the sky does.
To make an aerogel, scientists start with a chemical called tetramethylorthosilica, whose atomic structure looks like a silicon atom in the center of a pyramid made out of carbon and hydrogen. To this they add water and stir. The oxygen from the water removes the groups of carbon and hydrogen atoms, leaving long chains of silicon dioxide, or glass, in a solution of methanol.
``The tricky part of the process is how you get the liquid out without collapsing what you have made,'' says Poco. Simply evaporating or boiling off the methanol collapses the gel: As each molecule of methanol leaves, the surface tension of those remaining pulls the gel into a tight blob.