Air-Coupled Low Frequency Ultrasonic Transducers and Arrays with PMN-32%PT Piezoelectric Crystals

Abstract

Air-coupled ultrasonic techniques are being increasingly used for material characterization, non-destructive evaluation of composite materials using guided waves as well as for distance measurements. Application of those techniques is mainly limited by the big losses of ultrasonic signals due to attenuation and mismatch of the acoustic impedances of ultrasonic transducers and air. One of the ways to solve this problem is by application of novel more efficient piezoelectric materials like lead magnesium niobate-lead titanate (PMN-PT) type crystals. The objective of this research was the development and investigation of low frequency (<50 kHz) wide band air-coupled ultrasonic transducers and arrays with an improved performance using PMN-32%PT crystals. Results of finite element modelling and experimental investigations of the developed transducers and arrays are presented. For improvement of the performance strip-like matching elements made of low acoustic impedance, materials such as polystyrene foams were applied. It allowed to achieve transduction losses for one single element transducer −11.4 dB, what is better than of commercially available air-coupled ultrasonic transducers. Theoretical and experimental investigations of the acoustic fields radiated by the eight element ultrasonic array demonstrated not only a good performance of the array in a pulse mode, but also very good possibilities to electronically focus and steer the ultrasonic beam in space.

Photoacoustic Tomography System for Roughly Painted Micro Objects

“…To yield optimal efficiency, the air-couple transducer has been developed by combining several impedance-matching materials so that the increase of impedance can be adjusted gradually. However, the minimum focal diameter of such technology is still greater than 1 mm [7,8]. Therefore, the air-couple technique for acoustic imaging still leads to problems when dealing with micro objects.…”

Air-Coupled Ultrasonic Receivers with High Electromechanical Coupling PMN-32%PT Strip-Like Piezoelectric Elements

“…It means that in this case there are no losses due to a wave leaking to air, that allows covering longer distances for a non-destructive testing [ 11 , 16 ]. For this purpose, we have proposed air-coupled multi-element array in which as individual elements piezoelectric strips made of PMN-32%PT crystals vibrating in a transverse-extension mode are used [ 42 ]. PMN-32%PT crystals possess a very high piezoelectric properties.…”

“…PMN-32%PT crystals possess a very high piezoelectric properties. The electromechanical coupling factor for the transverse extension mode is k 32 > (0.84–0.91) [ 42 , 43 , 44 ]. The Curie temperature for the PMN-32%PT crystals is 90 °C [ 44 ].…”

“…It allows using them in majority of NDT applications. Performance of such elements and the array as a whole in a transmitting mode was described in detail in [ 42 ]. It was shown that exploitation of a transverse extension mode allows developing efficient air-coupled ultrasonic transmitters in a low frequency range up to 100 kHz.…”

“…The aperture in this case is rather small 1 × 5 mm, e.g., a few times smaller than the wavelength of ultrasonic wave in air at 40 kHz, but such dimensions enable to use such strips in electrically steered ultrasonic arrays. The developed transducer possesses low transduction losses of −11.4 dB at the frequency f r = 41.5 kHz [ 42 ]. However, the performed investigation limited to only a transmission mode.…”