Two years ago, Chinese scientists coated one side of a flag with a thin sheet of nanotubes, then played a song using the flapping sheet-coated flag as a speaker. It was a demonstration of flexible speaker technology, in which nanotubes can be made to generate sound waves via a thermoacoustic effect – every time an electrical pulse is sent through the microscopic layer of nanotubes, it causes the air around them to heat up, which in turn creates a sound wave. Now, an American scientist has taken that technology underwater, where he claims it could allow submariners to detect other submarines, and to remain hidden themselves.

Research scientist Ali Aliev, of the University of Texas at Dallas, has determined that the low-frequency sound waves created by carbon nanotube sheets can be used by sonar systems to determine the location, depth, and speed of underwater objects. Aliev and his team also determined that the sheets could be tuned to transmit specific frequencies that would cancel out certain noises... noises such as those that a submarine makes while passing through the water, for instance.

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One obstacle that Aliev had to overcome was the fact that the sheets do not do well in direct contact with water. The sheets can oxidize when in contact with water at high temperatures, the high surface tension and vibrational frequency of water causes the nanotubes to bundle into acoustically-poor ropes, and ocean water can cause the sheets to short circuit. On the plus side, however, the hydrophobic (water-repellent) nature of the sheets causes an air envelope to form around the nanotubes, which in turn acts as a kind of resonating chamber for the sound waves, boosting their strength.

Be that as it may, the sheets still needed to be protected from the water. In order to do so, Aliev encapsulated them in thin, flat gas-filled containers with acoustically-transparent windows. As with the air envelopes, the resonance that resulted from the sound waves being generated in such an enclosed space proved to be a benefit – the encapsulated sheets were actually ten times more effective at transmitting low-frequency sound underwater than non-encapsulated sheets.

The researchers also experimented with stacking the sheets several deep, but found that this negatively affected the desired thermoacoustics. The optimum arrangement turned out to be a layer of just two separated sheets, which received their electrical pulses alternately instead of simultaneously.

The research has just been published in the journal Nano Letters.

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