Structure Design And Sound Field Directivity Of The PVDF-based Ultrasonic Transducer

Nowadays with the rapid advancement of non-destruction detection technology,Piezoelectric ultrasonic transducer is drawing more and more attention with a prosperousapplication. As a new piezoelectric material for the ultrasonic transducer,polyvinylidenefluoride （PVDF） plays an increasingly important role in the field ofpiezoelectric sensing. Comparing with the traditional piezoelectric material, PVDF wasregarded as the hot research point for the piezoelectric composites and a significant researchdirection in the application of detecting and imaging techniques due to its unique advantages,such as low quality factor, wide bandwidth, high piezoelectric-voltage-constant and high-acoustic-impedance. However，PVDF polymer material has some inherent shortcomings，suchas low piezoelectric-strain-coefficients, low electero-mechanical coupling coefficients andhigh transition losses, which seriously limit its practical applications especially for the use oftransmitting transducer. Therefore a new structure of ultrasonic transducer based on the PVDFpiezoelectric film was proposed to increase the input/output gain, detection sensitivity andSNR of the transducers, and improve its sound field directivity as well, which are verysignificant for shortening the design cycle，reducing the product cost of ultrasonic transducer，and widening its range of practical utilization.In this work, an ultrasonic transducer with laminated structure based on PVDFpiezoelectric film is presented to solve one of the key technical issues concerning the lowinput/output gain in the flexible polymer film ultrasonic transducer. The amplitude-frequencycharacteristics of ultrasonic transducer is given by theory deduction and software simulation.Moreover the dependence of its gain, resonance frequency and bandwidth on the thicknessand the layer number of the film could be reasonably obtained by the simulation result. Inaddition, the theoretical analysis and the simulation results are agreed with the measured datasfrom self-made film ultrasonic transducer.In addition, the near acoustic field forming by the ultrasonic transducer with flexiblecurved-surface wafer and its equivalent dynamic phased array structure was emphaticallystudied. The directive property and the space distribution characteristics of near field pressurewere deduced by the physical acoustics principle and calculated, and plotted numerically by the means of the mathematics software. For a given operating frequency and materialproperties, comparing to the ultrasonic transducer with a plane structure, the ultrasonic wavesexcited by two-dimensional half sphere shell surface piezoelectric wafer or equivalent phasedarray can strongly focus in the near-acoustic-field with highest lateral/vertical resolution andstrongest sidelobe/grating-lobe suppression according to the simulation results of acousticfield directivity.