Research On Subminiature Capacitive Ultrasonic Transducer Of Non-flat Membrane

Ultrasonic waves have characteristics such as good direction, penetrating ablility, sound energy concentrated and cavitation, which is commonly used in many applications including non-destructive testing, medical ultrasonic imaging and SONAR system in military. As a critical part of ultrasonic system, ultrasonic transducers determinate the system's performance. Now, piezoelectric materials have dominated the ultrasonic transducer technology. However, the piezoelectric transducers exhibit disadvantages such as the impedance mismatch between the piezoceramic and the air or the fluid of interest, difficult of fabricating large 2D arrays, and unsuitable being used in high temperature environments. The capacitive subminiature ultrasonic transducers receive increasing acceptance as an alternative to conventional piezoelectric transducers, they offer performance advantages of wide bandwidth,low impedance,high sensitivity and small size,and they are ideally suitable for medical diagnostics,air-coupled inspection and micro-flow measurement.For the application in audio directional system, the capacitive subminiature ultrasonic transducers of non-flated membrane are studied deeply in this dissertation, which covers the aspects including theory model, structural designing, FEM simulations and characteristic testing.The major research works in this dissertation can be summarized as follows:1. Aimming at increasing the electromechanical coupling efficiency of transducers, two improved structures with non-flat membrane are proposed, one is a boss on the membrane and another is trench on the membrane. Finite element analysis results demonstrated that both improved structures achieved more uniform membrane deformation, and larger average membrane displacement, and higher effective electromechanical coupling efficiency.2. The acoustic finite element models of transducers are developed to demonstrate the distributing characteristic of sound field. The FEA simulations have illustrated that the output pressure comparision between flat membrane and non-flated membrane by same size and same displacement, the non-flated membranes can radiate larger sound pressure.3. The behaviors of the transducers with non-flated membrane in terms of collapse voltage, frequency, bandwidth and transformer ratio are investigated by varying the critical structural parameters such as membrane radius, membrane thickness, gap thickness, boss size and trench size. The detailed analysis results have been presented in this paper, which are beneficial for the design and optimization of capacitive ultrasonic transducers.4. The capacitive transducers of non-flated membrane for audio directional system have been fabricated, and the experiment setups for transducers are established. The electrical, mechanical and acoustical characteristic performances of transducers have been tested. The results of finite element analysis in this paper are successfully verified by those experiments.