新型电容-压电空耦超声换能器的特性研究

The various great advantages of the air-coupled ultrasonic non-destructive testing method, e.g. no contamination to the sample surface, good reproductivity of the measured signal, easy operation, low cost and fast on-line testing, have reveal its important role in the testing and monitoring of the air/aero craft composite component, metal plate, wood laminated structure, etc. The various testing demands of the advanced composite material have raised their requirement of the efficiency and reliability of this non-contact testing method. Since the inherent feature of the air-coupled ultrasonic testing, it has been the key technology to design and fabricate the air-coupled ultrasonic transducer with high efficiency and sensitivity. The existing air-coupled transducers, those based on piezoelectric material and those based on capacitive structure, reveals their limits in either efficiency or frequency bandwidth. The strength of the capacitive air-coupled transducer lies in its wide frequency bandwidth and good efficiency, which could be used in various applications. However, the limit of its sensitivity when used as a receiver and limit of exciting voltage when used as an emitter impair its applications. Therefore, this project aims to the designing of a novel capacitive-piezoelectric air-coupled ultrasonic transducer, which based on the structure of traditional capacitive one, while take advantage of the wide frequency response characterization of the PVDF piezo-film. This novel air-coupled transducer should perform improved feature in frequency bandwidth, exciting efficiency and received sensitivity. The characterization study of this capacitive-piezoelectric air-coupled transducer would be carried out through theoretical research and numerical simulation for the complication of the multi-physics coupling field.

空气耦合超声无损检测具有无需耦合剂、对材料无污染、信号测量重复性好、操作简便可以实现快速检测并且成本较低等无可替代的优势，在航空航天复合材料构件检测、金属薄板、木质层结构检测等领域显示出越来越重要的地位。各种先进复合材料的检测需求对于空耦超声检测技术的效率和可靠性提出越来越高的要求。由于空气耦合超声检测方法本身应用的限制，对于高灵敏、高效率的空耦超声换能器的研究是整个技术系统的核心关键技术。现在的两种基于压电的空耦换能器和电容的空耦换能器具有各自的优缺点，为了克服电容空耦换能器用作接收换能器时灵敏度不高，较易受到电路串扰，激发效率不高等缺陷，本项目在电容空耦换能器的结构基础之上，通过加入PVDF压电薄膜，结合其宽频响应压电性能等优点，设计了一种新型的电容-压电空耦超声换能器，目的是得到其宽频带、高发射功率、高接收灵敏度的优势。并通过理论分析与数值模拟对其进行多物理场耦合下的特性分析。

本项目是以空耦超声换能器的设计制造及其换能机理为主线提出的超声检测课题。当今各种先进复合材料的检测需求对于空耦超声检测技术的效率和可靠性提出越来越高的要求。由于空气耦合超声检测方法本身应用的限制，对于高灵敏、高效率的空耦超声换能器的研究是整个技术系统的核心关键技术。本项目在电容空耦换能器的结构基础之上设计了一种新型的电容-压电空耦超声换能器。首先进行了完成了对于新型空耦超声换能器的结构理论分析，并进行了有限元仿真建模，数值仿真结果显示上表面镀金属的薄膜材料经静电场作用在几十到几百微米尺度的金属基底下的空穴产生电场，并在时变静电场作用下由于电容作用产生振动的位移场。在该种空耦换能器在微小空腔产生的电容作用下，相当于为整个金属基底下的空腔为一个个微小的微声源，在超声的频段内，具有一定数量分布的微声源共同汇聚形成总的换能器发射声场。作为接收换能器时以同样的互易原理由薄膜的位移场变化产生电场变化。首先使用非压电薄膜材料构建成功了电容超声换能器，并改进了设计结构，以获得较好的稳定性。搭建完成了空耦检测测试平台，并取得了换能器测试结果。然后使用压电PVDF单层覆铝箔薄膜构建了电容式空耦超声换能器。使用光纤光栅对空耦换能器的声场时域响应进行了测试，显示电容空耦超声换能器具有非常良好的频率响应特性。并在空耦超声换能器基础上设计了多阵元环阵聚焦换能器，并申请了发明专利，已获授权。研究了空气中高声压高声强声场的测量方法，发明了一种测量装置，已获得实用新型专利授权。该种空耦超声换能器有望在非接触无损检测中获得转化应用，有望提高宽频无损检测的检测效率和速度。