Joined: 16 Mar 2004
|Posted: Mon Aug 31, 2009 9:31 am Post subject: CNTs Will Rock You
|Hot Nanotube Sheet Sounds Like a Plan for Flat Speakers
Sheets made of carbon nanotubes behave like a loudspeaker when zapped with a varying electric current, say Chinese researchers. The discovery could lead to new generation of cheap, flat speakers.
Since the early 1990s, nanotubes have been intensively studied by researchers across the globe. The tiny structures are widely touted as potential drug delivery devices but might also be useful in more exotic gadgets including artificial photosynthesis devices and space elevators. But no one has thought to test their acoustic properties until now.
Shoushan Fan and his research team at Tsinghua University in Beijing , China , working with colleagues at Beijing Normal University, created a thin sheet by roughly aligning many 10-nanometer-diameter carbon nanotubes. When they sent an audio frequency current through the sheet, they discovered it acted as a loudspeaker.
The CNT thin film was pulled out from a super-aligned CNT array grown on a 4 inch silicon wafer and put on two electrodes of a frame to make a loudspeaker
A standard loudspeaker consists of three basic elements – a speaker cone, a voice coil and a magnet. The cone and coil are attached and sit in a permanent magnetic field created by the magnet. When an audio frequency current passes through the voice coil, it creates a temporary magnetic field, and the coil and cone shift relative to the permanent magnetic field. Those shifts induce vibrations in the air molecules near the speaker cone, generating sound.
How it works
Fan's team wondered whether the nanotube speaker behaved in a similar way. They used a laser vibrometer to look for vibrations in their nanotube speaker as it produced sound, but the sheet remained resolutely static throughout. Instead, they think that the nanotube speaker functions as a thermoacoustic device.
When an alternating current passes through the nanotube sheet, the sheet alternates between room temperature and 80 °C. Those rapid temperature oscillations lead to pressure oscillations in the air next to the film. It is those thermally induced pressure oscillations that are responsible for the sound, rather than any physical movement of the nanotube sheet itself.
In fact, the researchers realised this phenomenon was first observed over a century ago independently by William Henry Preece and Karl Ferdinand Braun. Those nineteenth century researchers realised they could get sound from a thin metal foil by passing an alternating current through it, a discovery that led to the invention of a device called the "thermophone".
But the thermophone produced a very weak sound, whereas the nanotube sheets can be very loud (see video). That's because of the unusual properties of carbon nanotubes, says Fan. "A key parameter that determines the sound generation efficiency is the heat capacity per unit area," he says. Put simply, that's a measure of how much heat energy must be applied to a material to raise its temperature. The heat capacity per unit area of a carbon nanotube sheet is 260 times smaller than that of a platinum foil sheet. That means a nanotube sheet can generate sound waves 260 times more efficiently than a platinum sheet, and so produce a much louder sound.
Carbon nanotube thin film loudspeakers. (a) The CNT thin film was pulled out from a super-aligned CNT array grown on a 4 inch silicon wafer and put on two electrodes of a frame to make a loudspeaker. (b) SEM image of the CNT thin film showing that the CNTs are aligned in the drawing direction. (c) A4 paper size CNT thin film loudspeaker. (d) The cylindrical cage shape CNT thin film loudspeaker can emit sounds to all directions, diameter 9 cm, height 8.5 cm. Image Credit: American Chemical Society
The nanotube loudspeakers have several key advantages over standard speaker systems, says Fan. "Conventional loudspeakers which [produce sound] due to the vibration of the cone will fail to emit sound if the cone is broken," he says. "The carbon nanotube loudspeaker does not vibrate, which means it will still emit sound if part of the film is broken."
The flexible nanotube sheets can be stretched or flexed into complicated shapes and they still produce sound, Fan says. When fully stretched, the sheets are transparent and so they could be attached to the front of an LCD screen to replace standard speakers.
But more exotic uses might see nanotube sheets stitched into clothing to create "singing and speaking jackets", Fan's team thinks.
Cees Dekker, a nanoscience expert at Delft University of Technology in the Netherlands , finds the new study very interesting. "It's just amazing how widespread the diversity of applications of these nanotubes are," he says.
Journal reference: http://pubs.acs.org/doi/abs/10.1021/nl802750z
Source: Tsinghua University via http://www.newscientist.com/article/dn15098?DCMP=ILC-hmts&nsref=news9_head_dn15098