Hot nanotube sheets produce music on demand

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.

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 temperatures of up to 80 °C - although for consumer use the oscillation will be to only slightly above room temperature. 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 Kaili Jiang, a member of Fan's team. "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 more efficiently than a platinum sheet - the researchers estimate their sheet is 20 to 30 decibels louder.

'Singing jackets'

The nanotube loudspeakers have several key advantages over standard speaker systems, says Jiang. "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: Nano Letters (DOI: 10.1021/nl802750z)

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