Modeling Of Thermo-Acoustic Emission From Porous Silicon

The phenomenon of thermally induced ultrasonic emission from porous silicon (PS) was first reported by H.Shinoda et al in1999. Since then, as a new way that generates ultrasonic, it has attracted many researchers’ attention. They revealed that the thermo-acoustic (TA) ultrasound shows a lot of advantages compared with the conventional electro-acoustic ultrasound due to its unique constant (flat) amplitude-frequency response over a wide frequency range:larger frequency bandwidth and acoustic pressure, less reverberation and distortion, higher accuracy and spatial resolution for sensing a distance, reliable response under impulse operation and the availability and controllability for finely structured phase array operation, etc. But so far, compared with lots of experimental studies, theoretical investigations of TA ultrasound characteristics are very few. After H.Shinoda et al presented a formula calculating acoustic pressure of flat frequency response for PS thermal-ultrasonic emission by utilizing the fundamental equations of McDonald and Wetsel’s photoacoustic (PA) model, Boullosa and Santillan also derived an expression from the gas thermal piston model for ultrasound radiation from TA transducer. Although those formulas of acoustic pressure contain the mainly characters of TA emission, there is still some defects. Therefore, more work should be done to fully understand the features and regularities of TA ultrasound. In this thesis, the phenomenon of TA emission from PS is systematically analyzed by the way of thermal-mechanical coupling. And the theories of TA emission are developed which allow us to understand the principles and regularities of TA ultrasound better. We build three models from simple to complex in order to adapt to the different situations, so the thesis is divided into three parts.Firstly, in order to further understand the principle of TA emission and rapidly master its characters and regularities, the basic model of thermal-mechanical coupling of TA emission from solids is established. We only consider the thermal-mechanical coupling of fluid medium and neglect the thermal expansion of solids and the heat capacity of surface heating film, so as to avoid the intricate matrix manipulation. The distinct expression of sound pressure for TA emission form solids is proposed, and the mainly features-the formula of sound flat amplitude-frequency response and the existing conditions are obtained. In this part, we research the dependence of TA frequency response on the distance from the emitter surface, the thickness of the sample, and the sound pressure changing with the location under different frequencies, et al.Secondly, we build the model of multi-layer thermal-mechanical coupling of TA emission from solids, so that the effect of the physical properties of every layer on TA ultrasound can be fully gasped. This model considers the thermal, physical and geometry properties and the influence of thermal contact resistance of every layer. The advantage of this model is that it can be used for TA emission from arbitrary multi-layer construct solid materials. Based on this model, we systematically research the effect of the heat capacity of surface heating film, the stiffness and the thermal expansion of every solid layer on TA emission. In addition, the contribution of solid vibration for TA signal is also studied.Finally, in order to resolve the problem of thermally induced ultrasound emission in the complex fluid with the worse heat conductivity and the bigger viscosity, a model of double multi-layer thermal-mechanical coupling for solid TA emission in the viscous fluid is established. With this model, we explore the TA emission in ocean which is divided into lots of layers on account of adapting to the changes of temperature, salinity, pressure and other thermal physical properties. In this way, the effects of the temperature, salinity and depth of the ocean on TA signal are studied, and we also research TA positive and reverse propagation and the problem of ocean bottom reflectivity. Due to the fact that the flat frequency response can reach a longer distance in the ocean than in the air, so this research of TA exploration in the ocean play a significant role in the development of the new-style TA sonar.