Carbon Nanotube Assemblies for Sound Generation and Heat Dissipation

Abstract

Strong thermo- and photoacoustic responses have been detected for aligned arrays of multi-walled carbon nanotube (MWNT) forests and solid drawn MWNT sheets. When heated using alternating current or a near-IR laser modulated in 100-20 000 Hz range, the nanotube assemblies generated loud, audible sound, with higher sound pressure being detected from the MWNT sheets. An evaluation of nonlinear distortions of the thermoacoustic signal revealed a highly peculiar behavior of the third and fourth harmonics produced from forests grown on silicon wafers. The peculiarities were especially pronounced for short forests and can be associated with the heat transfer from the MWNT layer to the substrate. The dependencies can be used for evaluating properties of thermal interfaces, in particular, those based on carbon nanotubes. The investigated nanotube sheets can be attached to any surface (e.g., wall of an apartment) making the surface acoustically active. They can also be attached to windows of buildings and cars and can be used for active noise cancellation in noisy environments. Because of good coupling with the surrounding air, MWNT sheets and forests can be used for dissipation of heat produced by various electronic devices. They can be easily patterned in such way, that upon scanning with a laser beam, the pattern generates a characteristic sound and can therefore be employed used for acoustically readable barcodes.

Introduction

Remarkable properties of carbon nanotube assemblies (forests, sheets, yarns) are expected to lead to a variety of applications. It has recently been reported¹ that freestanding multi-walled carbon nanotube (MWNT) sheets generate sound when heated with alternating current (ac).

Thermoacoustic (TA or current driven) properties are related to the widely studied photoacoustic (PA or laser radiation driven) behavior. In order to check if the outstanding performance of this unusual material can be achieved with the laser excitation, we carried out the comparative TA-PA measurements of single (S1) and multilayer MWNT sheets (S2-S10) [2]. We also discovered that quite strong TA and PA signals can be generated by vertically aligned arrays of MWNTs (MWNT forest) grown on silicon wafers. In spite of their topological differences from the sheets, heating the forests with alternating current or a laser beam modulated in the audio frequency range produced an acoustic signal that can be heard by ear or detected with a microphone. We also performed measurements of nonlinear distortions of the TA signal and discovered highly peculiar behavior of the third and fourth harmonics produced by forests from ac-dc voltage scans [2]. The reported data can find applications in devices for sound generation and manipulation, heating, and cooling.

Results

Aligned arrays of MWNTs (MWNT forest) were grown by the conventional chemical vapor deposition technique on silicon wafers and had heights of about 32, 138, and 233 µm (labeled F32, F138, and F233). MWNT sheets were fabricated by pulling a sidewall of the forest in the sample plane direction, Figure 1A.

The sound pressure generated by MWNT sheets and MWNT forests subjected to TA excitation is shown in Figure 1B. TA intensities were normalized by the supplied electric power. The TA efficiency (sound pressure per watt of input power) was the highest for the single layer MWNT sheet: about 820 mPa/W (92 dB/W). Although the sound pressure generated by the multilayer sample in our experiments was roughly proportional to the number of layers, the conversion efficiency for multilayer samples dropped quite significantly, to about 63% for S10 as compared with S1. This could be caused by some increase in the sheet density in the multilayer assembly.

TA sound pressure for the investigated MWNT forests was comparable to that for the S1 sample. However, the efficiency of sound generation by forests was found to be lower than the efficiency of the MWNT sheets. The highest value was recorded from the F138 forest: 153 mPa/W (78 dB/W), which is 19% the efficiency of S1 sample. This difference can be caused by the substantially higher forest density compared with the density of MWNT sheets.

References

1. L. Xiao, Z. Chen, C. Feng, L. Liu, Z.-Q. Bai, Y. Wang, L. Qian, Y. Zhang, Q. Li, K. Jiang, and S. Fan, Nano Lett. 8, 4539 (2008).
2. M. E. Kozlov, C. S. Haines, J. Oh, M. D. Lima, S. Fang, J. Appl. Phys., 106, 124311 (2009).

Copyright AZoNano.com, MANCEF.org

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.