Paper-Thin Speakers Made From Carbon Nanotubes

10 December 2008—Earlier this year, scientists at the University of Illinois at Urbana-Champaign demonstrated the first radio receiver made from carbon nanotubes (CNTs)—those impossibly thin tubes of carbon that are the darlings of nanotechnology research.Now Chinese researchers have built the speakers to go with it—out of the same material.

Researchers led by KaiLi Jiang, an associate professor at Tsinghua University, in Beijing, along with collaborators from the nearby Beijing Normal University, have developed CNT speakers that can reproduce music as well as the loudspeakers in your stereo. Jiang and his colleagues describe their unusual acoustic instruments in the current issue of the journal Nano Letters .

”The structure is extremely simple,” Jiang explains. He and his colleagues started with a very thin film of CNTs to make a device that’s probably less complex than a conventional speaker, which uses a vibrating diaphragm to produce sound. The diaphragm has a coil attached at its base, and when an audio-encoded electrical signal is passed through the coil in the presence of an electromagnet, the diaphragm vibrates, setting off sound waves. ”Our CNT loudspeaker is magnet free—just a piece of CNT thin film with two electrodes to feed in audio-frequency currents,” Jiang says. The films are transparent, and they can be made of various sizes and shapes.

The sound produced by the nanotube speakers has low distortion and high quality, rivaling that of conventional loudspeakers, says Jiang. But the mechanism by which they make sound is totally different. Instead of using electromagnetically induced vibrations, they operate by what’s called the thermoacoustic effect.

”The alternating current periodically heats the CNT thin films, resulting in a temperature oscillation,” explains Jiang. ”The temperature oscillation of thin film excites the pressure oscillation in the surrounding air, resulting in the sound generation.”

The CNT loudspeakers can’t just be plugged into, say, your stereo without some modification. The commercial audio amplifiers in consumer devices are tuned to drive conventional loudspeakers. When used to drive a CNT film, they make music and human voices sound much higher pitched. But Jiang says that a simple amplifier circuit tuned for CNT speakers fixes this problem.

Jiang has big dreams for the new speakers. ”Our CNT loudspeaker is transparent and can be directly mounted in front of flat-panel displays,” he says. Other potential applications include speakers on clothing, windows, and flags. Some of those applications might be possible because the CNT films continue to produce sound even if they’re torn, unlike a torn diaphragm in a conventional loudspeaker.

There will be some challenges in commercializing the technology, say experts. The biggest is probably the lack of an industrial process to create thin films of CNTs. Jiang thinks this will be overcome soon. ”The fabrication process is very simple,” he says. First you have to produce arrays of nanotubes on which they all face in one direction. Next, the arrays are drawn out into thin films. Then you attach two electrodes, and you’re done. As for mass production, Jiang and his team have made CNT arrays on 4-inch wafers in large numbers in a manner that lends itself to an industrial batch process, such as the one used in the semiconductor industry. One 4-inch wafer can produce 6 square meters of the thin film, he claims.

On the whole, CNT researchers seem to think that the prospects for the new loudspeaker technology are rather bright.

”I think it’s very cool,” says John Rogers, who led the development of the nanotube radio. ”It caps off a remarkable year in CNT research, in which people have been able to demonstrate realistic devices—speakers, transparent conductors, digital circuits, transistor radios—formed in manufacturable ways and with properties that can be benchmarked in a meaningful way against existing technologies.”