Research On Double Matching-Lavers Air-Coupled Ultrasonic Transducer

Air-coupled ultrasonic transducer is an important component of ultrasonic detection technology.When the piezoelectric material of ultrasonic transducer excites the ultrasonic radiation to air medium,the acoustic energy to be reflected at the gas-solid interface due to the great difference of acoustic impedance between piezoelectric material and air medium,which results in low efficiency of sound energy radiation and low sensitivity.The purpose of this paper is to design,fabricate and optimize the high sensitivity air coupled ultrasonic transducer with center frequency about 450kHz.In this paper,the basic theory of air-coupled ultrasonic transducer is systematically analyzed,and the air-coupled ultrasonic probe with double-layer matching structure was designed.In order to solve the problem of impedance mismatch,the impedance characteristics of 1-3 type piezoelectric composites were studied in this paper.The attenuation characteristics of high frequency ultrasonic wave in air,the transmission property of acoustic wave at the solid-gas interface and the impedance matching theory were analyzed.Low density and low impedance composites were fabricated experimentally by using hollow glass beads as filler and epoxy resin as matrix.By controlling the ratio of filler to matrix,the required density and acoustic impedance of the materials can be obtained flexibly.Based on the KLM model,the equivalent circuit model of air-coupled probe with two-layer matched layer structure is established,and the cascade structure of double-matched layer transducer is obtained by using two-port network,and the total transfer matrix from the end to the load end of the transducer is obtained.Then the transfer function of the transducer is calculated.In addition,the sound field of a thick circular piston piezoelectric wafer was simulated by Comsol Multiphysics and the required material parameters were obtained by theoretical analysis.The matched layers with different acoustic impedance were prepared,and the air-coupled transducer samples were made with having many relevant tests.Through a lot of experiments and parameter fitting method,the best material parameters are obtained.The optimum volume ratio of piezoelectric composite wafer,the optimum proportion of matched layer composite material and the optimum thickness of two layers matching layer were obtained.