Bandwidth of inhomogeneously polarized PVDF-films and their use in the design of efficient ultrasonic transducers

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Abstract
Ultrasonic transducers made out of Polyvinylidene fluoride (PVDF) exhibit a broad bandwidth because the piezoelectric constant d 33 can have a gradient inside the material. To understand the behavior of the bandwidth in more detail, we have solved the inhomogeneous wave-equation for ultrasound generation by PVDF-transducers with the aid of the Green's function formalism. Furthermore, the bandwidth of PVDF-transducers is also determined by the electrical matching network employed and a broadband characteristic can be obtained by matching the impedance with an inductance, and by transforming the value of the effective impedance to 50 Ω with a toroidal transformer. Experimentally, we achieved an insertion loss of about 20 dB for a 35 MHz-PVDF-transducer, its 3 dB bandwidth being about 17 MHz. For Polyvinylidene fluoride Trifluorethylene P(VDF-TrFE), a copolymer of PVDF, the insertion loss is about 10 dB rivaling the one obtainable with PZT-transducers.