Tuning into Carbon Nanotubes

Researchers apply thermoacoustic effect to create audio speakers

Image courtesy of Nano Letters

In the not so distant future, you could be jamming to your favourite tunes with carbon nanotube speakers thanks to a team from Tsinghua University in Beijing, who recently published their work on the potential of aligned carbon nanotubes as thermoacoustic speakers.

Unlike conventional loudspeakers, which use the interaction of magnetic fields to produce mechanical oscillations, thermoacoustic speakers don’t use moving parts. Instead, they pass an alternating current through a conductive material, thereby creating sound waves from the thermal expansion and contraction of air around the conductor. While the thermoacoustic effect is a well-known phenomenon, it has been difficult to apply to speakers due to factors such as the lack of heat dissipation.

The team from Tsinghua University, however, believes that their new speaker design offers a possible solution. The manufacturing process begins by imparting grooves on a silicon chip through the use of photolithography and etching. Electrodes are then added to each end of the chip and it’s coated in a thin film of aligned carbon nanotubes. Next, the film is laser-cut into strips and treated with ethanol, causing the film to shrink into an array of thin “yarns”. When an alternating current is passed through the electrodes, the highly conductive thin yarn array causes thermal oscillations thereby producing sound.

“We found that processing carbon nanotubes into thin yarn arrays doesn’t weaken the thermoacoustic effect but can greatly improve the device robustness and durability,” said mechanical engineer Yang Wei. “And the new design mounts the nanotube structure 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,” he added.

The Tsinghua team was also able to integrate the speakers into a pair of ear buds, which have been working successfully for about one year without any significant degradation.

To read more, see Wei, Yang, Xiaoyang Lin, Kaili Jiang, Peng Liu, Qunqing Li, and Shoushan Fan. “Thermoacoustic Chips with Carbon Nanotube Thin Yarn Arrays.”  Nano Letters . American Chemical Society. 16 Oct. 2013.