Voided Space-Charge Electrets ? Piezoelectric Transducer Materials for Electro-Acoustic Applications
Voided space-charge electrets, such as cellular polypropylene, have recently been developed as piezoelectric materials that exhibit strong electromechanical thickness oscillations corresponding to high piezoelectric coefficients of around 500 pC/N and very good acoustical matching to air (low density of typically around 0.5 g/cm³ and low sound speed). Here, we discuss different aspects of the manufacture and the applicability of cellular polypropylene films as transducer materials at high frequencies and for ultrasound. The frequency response up to 90 kHz and the directivity patterns for several transducer geometries were investigated. Second- and third- order harmonic distortions and the power consumption of cellular polypropylene films in acoustic transducers are also described. Our results demonstrate that the relatively new ferroelectret films are very attractive for a range of device applications.
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