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PVDF-transducers and their use in NDE of engineering ceramics
Ultrasonic transducers made out of Polyvinylidene fluoride (PVDF) exhibit a broad bandwidth because the piezoelectric constant d sub 33 can have a gradient inside the material. Furthermore, the bandwidth of PVDF-transducers is also determined by the electrical matching network employed. A broadband characteristic can be obtained by matching the impedance with an inductance and by transforming the value of the effective impedance to 50 Omega 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 Trifluorethylene P(VDF-TrFE), a copolymer of PVDF, the insertion loss is about 10 dB rivaling the one obtainable with PZT-transducers. These transducers are well suited for defect-detection in ceramics. In ceramic components made out of SSiSiC and Si sub 3 N sub 4 with plane surfaces we have found defects with sizes down to 30 mym in a depth of 3 mm and with asignal/noise-r atio of 6 dB.