Abstract
Carbon nanotubes (CNTs) have attracted considerable attention from researchers since their discovery only two decades ago. Among their many extremely valuable properties are their mechanical strength, electrical conductivity, thermal conductivity and solar-thermal conversion efficiency. Due to its low density and low heat capacity per unit area (HCPUA), CNT thin film, which can be drawn directly from a CNT array, is an excellent candidate for a new type of loudspeaker or acoustic transducer: magnet free and thermo-acoustics. The mechanism of thermo-acoustic generation differs from that of a conventional electro-acoustic device, which produces sound by mechanical vibration. Thermo-acoustic effects are produced by variation in the temperature of the medium surrounding the source, whether air or liquid. The device induces temperature variation in its surrounding medium by generating Joule heating, typically via an alternating current.To design, manufacture and develop an effective thermo-acoustic device, the first and most important task is to construct an accurate theoretical model of the thermo-acoustic structure and sound response of the device. In this thesis, accurate measurements of the thermo-acoustic radiation emitted by a suspended CNT thin film are obtained for both the near field and the far field by constructing a coupled thermo- mechanical model and solving for the thermally induced acoustic response. The acoustic pressure is approximated within a suitable frequency range and expressed briefly and concisely. The results of an experiment using CNT thin film are compared with the theoretical results and the analytical prediction. The experimental results are found to be consistent with the analytical approximation. As HCPUA is a key determinant of acoustic pressure, the use of a CNT thin film with a small HCPUA significantly improves acoustic pressure. The acoustic wave radiating from a CNT thin film is shown to be a plane wave in the near field, which transforms into a spherical wave in the far field. Higher-frequency acoustic waves remain plane waves across greater distances. In addition, acoustic pressure is found to show a wideband constant (flat) amplitude frequency response in the near field.
Due to its different sound-response characteristics, a model of a gas-filled encapsulated thermo-acoustic transducer is also developed, and exact and approximate solutions are derived. The theoretical prediction is shown to be highly consistent with the experimental data. The frequency response of the acoustic transducer is investigated, and its acoustic response is discussed as a function of the distance between the window and the thin film within the encapsulated transducer. The optimal distance between the window and the thin film is successfully derived and used in some practical examples. As resonance occurs at a suitable input frequency, this type of transducer can be used either to generate acoustic waves of a specific frequency or to filter specific resonance frequencies from a wide spectrum of signals. It can also operate while immersed in various liquid media. When immersed in a gaseous medium, the transducer performs better at lower frequencies; in a liquid medium, better performance is achieved at higher frequencies. The findings of this research provide useful guidelines for enhancing the efficiency of thermo-acoustic conversion and optimizing the design of thermo-acoustic transducers.
Completely suspended CNT thin film, which can be drawn from a CNT array in an open space, has been identified as an excellent source of powerful thermo-acoustic waves. CNT thin film is widely known to be structurally weak, and its lack of structural stability considerably limits its practical applications. However, suspending CNT thin film on a substrate significantly increases its structural stability. It is thus crucial to investigate the operation of a structurally reinforced thermo-acoustic transducer. In this study, an analytical model of the thermo-acoustic pressure response of CNT thin film suspended on a substrate is established. The model is validated by comparison with experimental results. The analytical model can be used to examine the influence on acoustic-pressure response of the distance separating the thin film from the substrate. Compared with completely suspended CNT thin film in an open space, the performance of a CNT thin film suspended on substrate is far more efficient if the gap separation is within a few micrometers. In addition, the new analytical model developed in this study can be used to measure acoustic-pressure response in a range of surrounding gaseous media. The effects of the thermal properties of different substrates on the performance of such suspended CNT thin film devices are also analyzed theoretically.
A mirage is a naturally occurring optical phenomenon created by the bending of light as it passes through a medium with a temperature gradient determined by variation in the refractive index. The mirage effects generated in both gaseous and liquid media by heated CNT thin film are analyzed theoretically. The theoretical prediction is found to be highly consistent with the experimental findings reported in previous literature. Both the theoretical and the experimental results indicate that the mirage effect is more likely to occur in liquid. The phase of a deflected optical beam and the method used to theoretically measure the thermal diffusivity of a medium are discussed. In addition, an experimental method for measuring the refractive index of a gas by detecting optical-beam deflection is presented.
Next, a model of high intensity focused ultrasound generation by the irradiation of a composite CNT thin film and elastomeric polymer is presented. The composite CNT thin film is deposited on the surface of a concave lens, and focused ultrasound is generated by applying an incident pulsed laser to the lens. The performance of the focused ultrasound is analyzed. It should be emphasized that although the energy comes from an irradiated laser rather than an electrical current, thermo-acoustic sound generation is similar to opto-acoustic sound generation. The results of the analysis are compared with previously reported experimental data, and a very good agreement is recorded. The opto-acoustic pressure on the symmetric axis and in the lateral focal plane is investigated analytically, and excellent acoustic performance is identified in the focal region. The temporal performance of the focused lens is also investigated both at the focal point and in the pre-focal zone, and the results are found to agree with the experimental data. Consequently, it is demonstrated theoretically that the performance of the focused lens can be dramatically enhanced by setting the pulsed laser to an optimal input frequency. In summary, this novel analytical model may provide new guidelines for the design of high intensity ultrasound lens devices with promising applications in medical ultrasonographic treatment.
In this research project, CNT-based thermo- and opto-acoustic transducers are theoretically and numerically analyzed. The performance of the transducers in a variety of scenarios is discussed in detail. The theoretical models and results presented in the study are expected to offer useful guidelines for the design, manufacture and optimization of thermo- and opto-acoustic transducers.
| Date of Award | 27 May 2015 |
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| Original language | English |
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| Supervisor | C W LIM (Supervisor) |