Elastic and dielectric losses in piezoelectric PVDF, P(VDF-TrFE) copolymer, and selected passive polymers and their effect on acoustic performance of broadband sonar transducers

Design tools for simulating the response characteristics of acoustic transducers fabricated from inherently lossy materials are presented. Classic electromechanical models are extended to several commercially available polymer piezoelectrics by (1) incorporating dielectric and mechanical losses in a general manner, and (2) considering the frequency, temperature, and pressure dependence of both the storage and loss components of the elastic and dielectric moduli of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymers. Extensive data regarding the structure, morphology, and known loss processes of these materials are compiled. Phenomenological models and molecular estimation techniques are then used to reconstruct loss tangent surfaces from the sparse material data typically supplied by the manufacturer. It is shown that for PVDF and P(VDF-TrFE), the standard measurement point often used for reporting loss characteristics is situated in a trough between two main relaxation processes; over the extended range of interest in sonar the dynamic behavior of the glass transition process causes loss tangents to increase by 1-2 orders of magnitude. Structure-property correlations applicable to passive polymers commonly found in underwater transducers are also discussed.; The improved material description is used to assess the acoustic performance of a typical thickness-mode PVDF hydrophone over a range of environmental parameters of interest in sonar. Variations in elastic parameters shift the resonance frequency, cause a departure from a truly 'pc-matched' condition, and introduce a dynamic variation in the open-circuit receive sensitivity. Variations in the dielectric parameters of similar magnitude occurring at the same frequency, temperature, and pressure coordinates ensure that the wideband response remains flat to within 1 dB, depending upon the geometry of the device and operating parameters chosen. The effects of intrinsic material losses are most pronounced in the internal noise spectrum of the PVDF hydrophone. In warm waters noise levels average 10 dB lower than predicted by a constant loss model, whereas in cool waters noise levels are 6-10 dB below the predictions based upon these models at low frequencies ({dollar}{lcub}cal O{rcub}{dollar}(10{dollar}sp2{dollar}) Hz), rising to 10-18 dB above the constant loss predictions at frequencies on the order of 10{dollar}sp4{dollar}-10{dollar}sp5{dollar} Hz.