Rocking Out With Carbon Nanotubes

A new type of headphone heats up carbon nanotubes to crank out tunes. The tiny speaker doesn’t rely on moving parts and instead produces sound through the thermoacoustic effect (Nano Lett. 2013, DOI: 10.1021/nl402408j). The nanotube speaker could be manufactured at low cost in the same facilities used to make computer chips, the researchers say.

In conventional speakers, mechanical drivers create sound waves via vibrations that compress the air in front of them. But that’s not the only way to rock out. About a hundred years ago, researchers discovered the thermoacoustic effect. When an alternating current passes through a conductor, the material heats and cools the air around it; as the air warms, it expands, and as it cools, it contracts. This expansion and contraction creates sound waves.

Engineers think that the lack of moving parts would make speakers that use the thermoacoustic effect more durable than conventional ones. The problem has been that most conducting materials don’t have a thermoacoustic effect strong enough to produce sound efficiently. However, the effect is enhanced in carbon nanotubes, which are superb conductors of electricity and heat.

In 2008, a group at Tsinghua University, in China, demonstrated thermoacoustic loudspeakers that use stretchable, transparent films of nanotubes. They showed they could use the films to turn flexible surfaces into speakers, making a waving flag that played music (Nano Lett. 2008, DOI: 10.1021/nl802750z). Unfortunately, the devices struggled to dissipate the heat created while generating sounds, says Yang Wei, a mechanical engineer at the university.

The Tsinghua team, including Yang and Kaili Jiang, now report a new nanotube speaker design that overcomes the issues in the earlier devices. The researchers start by patterning a silicon chip with grooves using photolithography and etching processes commonly used to make computer chips. The chip supports the nanotube material, and the grooves help dissipate heat. After adding electrodes to each end of the chip, the team coats the chips with a film of carbon nanotubes that are all aligned in the same direction. The engineers then cut the carbon film into strips with a laser and treat the strips with ethanol, causing them to shrink into about 1-µm-thick, yarn-like strands.

These carbon nanotube strands run from one electrode to the other, so that when the researchers connect the silicon chip to a circuit, current flows through the yarn and heats it. The team tested how the depth of the chip’s grooves affected the generated sound pressure and found that the deeper the grooves, the greater the sound pressure. The best device had grooves about 200 µm deep.

The Tsinghua researchers integrated these thermoacoustic chips into a pair of earbud headphones and connected them to a computer to play music from videos and sound files. They’ve used the headphones to play music for about a year without significant signs of wear, Yang says. According to him, this is the first thermoacoustic device to be integrated with commercial electronics and used to play music.

“We found that processing the carbon nanotube film into thin yarn arrays doesn’t weaken the thermoacoustic effect but can greatly improve the device robustness and durability,” Yang says. And the new design mounts the nanotube structures on silicon chips that are compatible with existing manufacturing methods. The thermoacoustic chips could be easily integrated into circuit boards for speakers with other electronic elements, such as control circuits, Yang says.

Ray H. Baughman, director of the NanoTech Institute at the University of Texas, Dallas, who also works on thermoacoustic speakers, is impressed by the device’s design, but says there are some remaining challenges for these devices. For example, he says, the speakers consume relatively high levels of power, because of their low efficiency of converting electrical energy into sound.