OT : Nanotubes

["Todd Matthews" <gtmatthews@gmail.com>]

I pulled this from 'The Economist" and thought it was interesting.


Nanotunes

Nov 20th 2008
>From The Economist print edition
Nanotubes made of carbon find an unexpected use

CARBON nanotubes have been all the rage in chemistry over the past
decade, but actual applications are thin on the ground. The tubes
(cylinders a few billionths of a metre across, whose walls are made of
carbon atoms) have found their way into tennis racquets and bicycles'
handlebars, where they provide strength and stiffness. But Lilliputian
nanoradios, nanomotors and the like have, so far, been confined to the
laboratory.

With luck, that fate will be avoided by the latest addition to the
list. Carbon nanotubes, it turns out, can be used to make paper-thin
loudspeakers. As they report in a forthcoming issue of Nano Letters, a
group of researchers led by Jiang Kaili of Tsinghua University in
Beijing have developed a transparent film made of nanotubes. When
electrodes are attached to the ends of this film, and a
signal-carrying current is passed through it, the result is a sound
that matches the signal. Carbon-nanotube speakers play music with a
fidelity similar to that of conventional loudspeakers. What is more,
they continue to play even while they are being bent and stretched.

A conventional "moving coil" loudspeaker—like the one in your
stereo—is made of a bulky permanent magnet, a coil of copper wire and
a flexible diaphragm, housed in an enclosure. When a varying
electrical current is passed through the coil it creates a varying
magnetic field that interacts with the field of the permanent magnet.
The diaphragm moves back and forth in response to this interaction,
causing pressure waves to form in the air in front of it. The listener
perceives these waves as sound.

Nanotubes produce sound by a different mechanism, known as the
"thermoacoustic effect", which is also responsible for the thunderclap
that follows a burst of lightning. Lightning is created when an
electrical arc jumps between clouds, or between clouds and the ground.
The arc heats the air surrounding it and causes that air to expand
rapidly, producing a shock wave that is heard as thunder. With the
nanotubes, variations in the electrical current cause the air
surrounding the tubes to heat up (and thus expand) or cool (and thus
contract), which produces pressure waves that register as sound. The
beauty of generating sound this way is that no bulky magnets or moving
diaphragms are needed.

In fact, the thermoacoustic effect has been employed in a similar way
before. In the 19th century, a device called a thermophone, which used
metal sheets to generate sound, was developed. But the effect was so
weak that the thermophone never took off. Dr Jiang says nanotubes
produce a bigger thermoacoustic effect than metal because carbon has a
lower heat capacity. More of the converted electrical signal thus ends
up in the air.

Potential applications of flexible and stretchable carbon-nanotube
loudspeakers include speakers on clothing, windows, flags, and video
and laptop screens. It helps that nanotube films continue to produce
sound if torn, unlike a torn diaphragm in a conventional loudspeaker.
Earphones and hearing aids might also benefit from the new approach.

There is some way to go before the technology can be commercialised.
The biggest task is devising a way to create the necessary films in
industrial quantities. But if that can be done, the next generation of
loudspeakers may be almost invisible. The speaker-banks at rock
concerts will never be the same again.