NanoLetters – Underwater Sound Generation Using Carbon Nanotube Projectors
Water’s high heat capacity and low thermal expansion would normally absorb any temperature fluctuations. But the nanotubes’ hydrophobic properties result in an air pocket around the sheet which expands and contracts as the sheet heats and cools, pushing the surrounding water in and out as a piston would. The sheets can generate a wide range of frequencies, including those below 4 kilohertz, which are used for long-range sonar and are traditionally generated using heavy sound projectors. There is a high demand for lighter sonar devices
The application of solid-state fabricated carbon nanotube sheets as thermoacoustic projectors is extended from air to underwater applications, thereby providing surprising results. While the acoustic generation efficiency of a liquid immersed nanotube sheet is profoundly degraded by nanotube wetting, the hydrophobicity of the nanotube sheets in water results in an air envelope about the nanotubes that increases pressure generation efficiency a hundred-fold over that obtained by immersion in wetting alcohols. Due to nonresonant sound generation, the emission spectrum of a liquid-immersed nanotube sheet varies smoothly over a wide frequency range, 1−105 Hz. The sound projection efficiency of nanotube sheets substantially exceeds that of much heavier and thicker ferroelectric acoustic projectors in the important region below about 4 kHz, and this performance advantage increases with decreasing frequency. While increasing thickness by stacking sheets eventually degrades performance due to decreased ability to rapidly transform thermal energy to acoustic pulses, use of tandem stacking of separated nanotube sheets (that are addressed with phase delay) eliminates this problem. Encapsulating the nanotube sheet projectors in argon provided attractive performance at needed low frequencies, as well as a realized energy conversion efficiency in air of 0.2%, which can be enhanced by increasing the modulation of temperature.