[meteorite-list] A Solid That's Light as Air (Aerogel)
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Feb 24 13:46:54 2006
A Solid That's Light as Air
February 23, 2006
If you wanted to catch a few particles of comet dust speeding through
the vacuum of space at 6 kilometers per second -- without damaging or
destroying those particles -- how would you do it?
Faced with exactly this problem, scientists at the Jet Propulsion
Laboratory focused on aerogel -- an extremely lightweight, porous
material that is chemically identical to glass, but weighs only a little
more than air.
Aerogel is the lightest solid known to science. It's also one of the
most insulating materials on Earth, the most porous, and it's nearly
transparent. Those last two properties made it an ideal choice for
catching flecks of comet and interstellar dust on the recently-returned
Stardust mission launched by NASA and JPL.
Since the satellite returned to Earth on January 15, NASA scientists
have been busy slicing open Stardust's aerogel cells and carefully
extracting the bits of dust it collected from Comet Wild 2.
"Aerogel is unique in having so many superlative properties, and a huge
range of properties too," said Donald Brownlee, a University of
Washington astronomer and the principal investigator for Stardust.
Aerogel isn't exactly space-age technology. It was invented in 1931 by
Steven Kistler, in response to a bet made by a fellow scientist. Kistler
found a way to remove the liquid from a silica gel without destroying
the long silica molecule chains that gave the gel its structure.
Holding a piece of aerogel is an uncanny experience. It's so light it
feels nearly weightless, like a chunk of solidified fog or smoke. It
feels a bit like Styrofoam, and it squeaks when you rub your finger on
it. It's strong enough to support many times its own weight if the load
is distributed evenly. But bend it or squeeze it too hard, as one Wired
News editor discovered, and a chunk of aerogel will shatter into tiny
Ordinary gels, like Jell-O, are comprised of tangled chains of molecules
-- polymers -- surrounding empty pockets of a liquid, such as water. If
you try to dry out a cube of Jell-O at room temperature, the surface
tension of the liquid will cause the polymer structures to collapse as
the liquid evaporates. The result is that the gel cracks, shrinks and
eventually crumbles to dust.
Modern scientists make aerogel by pressurizing and heating an ordinary
gel to its "supercritical" point, where the liquid's fluid and gaseous
phases are indistinguishable, and then draining off the supercritical
liquid. Because there's no gas-liquid interface, there is no surface
tension and so the liquid can be removed without destroying the gel's
polymer structure. With the liquid gone, air fills up the spaces
between the polymers, and the result is a meringue-like aerogel.
Scientists aren't sure why aerogel works so well as a cosmic duster. One
theory, says Brownlee, is that the porousness of the material gives
particles a chance to slow down as they smash through the
nanometer-scale silica structures. As they go, the particles pick up a
"paint" of melted glass on their front edge, which protects them from
further collisions with the structure until they come to rest.
The transparency of aerogel was also critical to the Stardust mission
because it allowed scientists to find the particles by following their
tracks through the material.
Back on Earth, that porousness (aerogel can be up to 99-percent air)
makes aerogel an ideal thermal insulator. So it's no surprise that
companies are investigating commercial uses for this material, ranging
from windows to home insulation to clothing.
Such a wealth of useful properties makes aerogel interesting not only to
rocket scientists, but to entrepreneurs and venture capitalists, who
sunk $50 million last year into Aspen Aerogels, a company devoted to
"As an insulator, aerogel is two to four times more efficient than
anything else out there," said George Gould, the director of research
for Aspen Aerogels.
Aspen Aerogels makes economical aerogel textiles by impregnating
"blankets" of fabric with silica gel, then pressurizing the impregnated
fabric and extracting the now-supercritical liquid. The result is a
flexible fabric with aerogel integrated into its matrix.
Prices for the material vary, but a typical price is a few dollars per
square foot for quarter-inch thick material. When Aspen Aerogel's second
factory is completed later this year, Gould said, the company will be
able to produce 100 million square feet per year of its aerogel
textiles, bringing costs even lower.
Aspen's products have been used to to insulate the pipelines used in
deep-sea oil drilling operations, in winter jackets by Burton Snowboards
and even to make shoe inserts.
Other commercial producers of Aerogel include Aerogel Composite
and a Swedish company, Airglass, which sells aerogel-based insulated
The problem these companies face is that, while aerogel is a vastly
superior insulator, the alternatives (like fiberglass or plain glass
windows) are dirt-cheap.
The high pressure needed to create aerogel (around 800 pounds per square
inch) means that producing even a tiny amount requires costly lab
equipment. You can buy aerogel samples on eBay but they cost around $30
to $50 for small, nickel-sized chunks.
That means aerogel is unlikely to play a major role in construction or
clothing unless its makers can bring the price down much further -- or
capitalize on its space-age reputation enough to make customers willing
to pay extra for cachet.
"The costs are not necessarily prohibitive," said Gould. "Relative to
something like fiberglass, the costs are certainly greater but a lot of
it has to do with capacity."
Received on Fri 24 Feb 2006 01:45:11 PM PST