Design Of Large-size Air-coupled Transducers Based On Phononic Crystal Theory

Air-coupled ultrasonic detection technology is widely used in industrial inspection due to the advantages of non-invasive,non-contact,no coupling agent,non-destructive to the detection object,etc.With the expansion of its application areas,the requirements for its core component piezoelectric transducer in terms of sensitivity,electromechanical coupling coefficient,frequency band width and other characteristics have also been improving.However,due to the high acoustic impedance of traditional piezoelectric materials,the reflection of ultrasonic waves at the interface,and the coupling of longitudinal vibration to transverse vibration,the performance of air-coupled transducers can not be effectively improved.This paper aims to improve the working performance of large-size aircoupled ultrasonic transducer and optimize the structure of the piezoelectric material and acoustic impedance matching layer in the conventional piezoelectric transducer by combining phonon crystal theory.The specific effects of the phononic crystal structure on the working performance of the piezoelectric transducer are investigated by numerical calculations,finite element simulations and experiments,and the structural optimization of the designed phononic crystal type air-coupled ultrasonic transducer is carried out using genetic algorithms to obtain the optimal structural parameters,the main research contents of the work are as follows:(1)A study of 1-3 piezoelectric composites in conjunction with phononic crystal theory to enhance their operating properties.The 1-3 piezoelectric composites are regarded as two-dimensional phononic crystals,and the suppression of transverse vibrations by their internal periodic structure is studied based on the band gap characteristics.The band structure,transport properties and vibrational modes of the phononic crystals are calculated by the finite element method,so that the piezoelectric materials can obtain a purer thickness vibration,and the effects of different filling rates and shapes of piezoelectric phases on the band gap width and position are analyzed;the effects of the lattice structure on the band gap and the overall working performance of the piezoelectric composites are also studied,and the working parameters such as the emitted voltage response level and the total radiated power of vibration of the square and hexagonal lattice piezoelectric composites are obtained by simulation.(2)An acoustic impedance gradient matching layer is designed based on the phononic crystal theory.The transverse structure of this matching layer can be regarded as a one-dimensional phononic crystal,which can effectively suppress the transverse vibration interference during the operation of the transducer and realize the gradient matching of the acoustic impedance of the ultrasonic transducer to the propagation medium,thus improving its working efficiency.The equivalent acoustic parameters of the matching layer are obtained through theoretical calculations,and the variation of the equivalent acoustic impedance in the thickness direction of the matching layer with respect to the sound velocity is studied.(3)To optimize the performance of a phononic crystal air-coupled transducer,the geometry of a1-3 piezoelectric composite with gradient matching layers is optimized using a numerical optimization method.Based on this,the NSGA-II genetic algorithm is used to optimise the structural parameters with the objective of improving the band gap width and operating bandwidth of the transducer,and the optimal solution is modelled and compared with the pre-optimized model.The results demonstrate the feasibility and effectiveness of the proposed optimization method.(4)Experimental study of a designed phononic crystal type air-coupled transducer.Based on the optimized design,a phononic crystal type air-coupled ultrasonic transducer prototype was prepared,an experimental platform was built,pulse echo tests were performed on the transducer prototype,and the performance parameters were actually compared with those of the conventional single/dual matching layer transducer and the pre-optimized transducer model in order to investigate the enhancement effect of the phononic crystal structure and the introduction of gradient matching layer on the operating performance of the air-coupled transducer,and to verify the accuracy of the previous simulation and optimization results.