Design of acoustic impedance gradient matching layers

•A phononic crystal type gradient matching layer is proposed to improve the efficiency of piezoelectric transducers.•The proposed structure effectively suppresses the lateral vibration of the transducer.•The gradient matching layer implements the gradient matching between the transducer and the transmission medium.•The -6dB bandwidth of the transducer sample equipped with the gradient matching layer is 90.8%.•This study provides a new idea for the design and application of new high-efficiency gradient matching layers. This article proposes a new acoustic impedance gradient matching layer based on phononic crystal theory to further expand the bandwidth of piezoelectric ultrasonic transducers and improve their working efficiency. The matching layer is made of a composite of two different materials and its transverse structure can be regarded as a one-dimensional phononic crystal. This structure can effectively reduce the transverse vibration of the transducer and enhance the longitudinal vibration. In addition, the gradient alteration of acoustic impedance in the longitudinal direction of the matching layer improves its acoustic impedance matching performance with the medium. Through theoretical calculations and finite element simulations, we obtained the equivalent acoustic parameters of the gradient matching layer and investigated the suppression of transverse vibration by its structural properties based on the band gap theory. Finally, a sample of the transducer equipped with the gradient matching layer was prepared and its working performance was measured by pulse echo experiments, which is a significant improvement compared to the conventional λ/4 matching layer transducer.