Ferroelectret Ultrasonic Transducers for Pulse-Echo Water Immersion

Abstract

Ferroelectrets are thin and porous polymeric films with a cellular microstructure, high porosity, permanent polarization and piezoelectric response. They have been used for different applications, where one of the most interesting ones is for the fabrication of air-coupled ultrasonic transducers. More recently they have been tested as water immersion transducers, showing a promising wide bandwidth but limited sensitivity along with other technical problems. This paper investigates ultrasonic transducers for water immersion and pulse-echo operation based on ferroelectret films. Two different ferroelectret foams with different resonant frequencies, acoustic impedances and cellular structures were tried. Flat and spherically focused prototypes (radius of curvature of 22 and 35 mm) were produced and tested. Finally, different materials and methods were tried to provide a protective surface coating. Acoustic field measurements for the focused transducers confirm the possibility to efficiently focus the ultrasonic beam by the proposed fabrication method, with focal spot size of 1.86 mm at −6 dB. Results show that in spite of the reduced sensitivity (about −115 dB), some of the tried ferroelectret films provide a very wide band response (−6 dB band from 0.29 to 2.7 MHz) and short pulse duration (2–3 us) that can be of interest for different applications.