Abstract
Air-coupled ultrasonic transducers based on novel piezoelectrets with extremely small acoustic impedance have some distinctive advantages including no coupling agent, flexibility, and eco-friendliness, compared to conventional ultrasonic transducers. Such advantages make them unique in the applications of nondestructive testing. In this study, a focused air-coupled ultrasonic transducer featuring a sphere-shaped transduction surface is designed by introducing flexible irradiated cross-linked polypropylene (IXPP) films. The characteristic of focusing is evaluated by numerical and experimental analysis. The results show that, by bending the IXPP film into a sphere, the ultrasonic energy can be concentrated to a certain small area to achieve better sensitivity and higher resolution. Furthermore, the IXPP focusing transducer has an obvious focusing effect when the opening radius D of the transducer is increased. Meanwhile, with the reducing radius R of the spherical surface and increasing frequency f of excitation, the value of the deviation coefficient α can be made smaller so that the transducer designed in this way can obtain a better focusing effect.
Highlights
The air-coupled ultrasonic transducer, referring to the ultrasonic transducer with air as the medium, can avoid the pollution of the coupling agent to the target, which is an outstanding advantage compared to traditional transducers
For studying on focused air-coupled ultrasonic transducers, researchers mainly focus on discussing the characteristics of polyvinylidene fluoride (PVDF), polyester (PET) film, and scitation.org/journal/adv ceramics.[17]
PVDF was adopted to design the focused transducer, and a sort of photoacoustic microscopy combined with the PVDF transducer was proposed by Fang et al in 2020.18 In addition, Chimenti had a thorough study in the air-couple ultrasonic transducer made of a PET film, in which the design, fabrication, and characterization of a spherically focused transducer were discussed systematically.[19–21]
Summary
As an alternative to the piezoelectric ceramics and films, irradiated cross-linked polypropylene (IXPP) films show more excellent performance in thermal stability, mechanical properties, and electrical properties.[23]. It has the characteristics of flexibility and low acoustic impedance, and has a piezoelectric coefficient close to PZT, which is considered as a promising material for making acoustic–electric or air-coupled transducers.
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