PVF 2 Transducers for NDE

We report on recent calculations and experiments on the broadband properties and impulse characteristics of PVF? transducers and arrays. Experimental wedge transducers show bandwidths of approximately 100% in the excitationofsurface acoustic waves at 7 MHz on nonpiezoelectric silicon nitride ceramic substrates. Computer calculations predict similar bandwidths for interdigital transducer arrays on PVF films for surface acoustic wave excitation on similar substrates. Insertion loss versus frequency m~asure­ ments on bulk longitudinal wave transducers in water at frequencies in the 1 to 30 MHz range show good agreement with theory. 0 A computer program for multilayer piezoelectric films predicts included angles of acceptance exceeding 60 , and control of acceptance angle profiles, in face plates using multilayer PVF2 films. PVF2 bulk wave transducers are made by bonding 25 11 PVF? film (~ l/2" x 1/2") onto brass backing rods witn V-6 epoxy. The PVF film is obtained from Kureha Corporation in st?etched, poled and electroded form. We etch off the aluminum electrodes, which erode quickly when placed in water, and put down a thin layer of chrome followed by a layer of gold (""' 1000 A). These bulk wave transducers have a very flat frequency spectrum up to 20 MHz when radiating into water. 1 This is expected, since the acoustic impedance and velocity of longitudinal waves in PVF2 (3.83 x 105 gm/cmZsec and 2.15 x 105 em/sec, respectively) match relatively wel~ to the impedance and veloc~ty of water (1.48 x 10 gm/cm2sec and 1.48 x 10 em/sec). As the impedance mismatch at the brass/PVF2 interface is large, little energy is radiated into the brass. These characteristics of PVF? result in a clean, nearly bipolar impulse response (Fig. 1). PVF transducers have shown a 60 dB two-way insertion foss at their resonance frequency (A= 4 x film thickness).