Sandwiched porous polytetrafluoroethylene ferroelectrets: The piezoelectric d33 coefficient

The present study deals with a 3-layer sandwich of metal/FEP/ePTFE/FEP/metal, where the fluoroethylene propyl-ene (FEP) acts as electrical blocking layer and ePTFE is a 91% porous polyfluoroethylene layer which can be compressed mechanically. After proper charging such hybrid structures exhibit a large piezoelectric d₃₃ coefficient comparable with conventional ferroelectric PZT ceramics. This piezoelectric d₃₃ coefficient is investigated theoretically and the limits are evaluated with respect to charging and mechanical properties. In order to optimize the d₃₃ coefficient the quasi-remanent polarization and the mechanical properties are taken into account which are both dependent on the sample geometry such as on the relative thicknesses of the FEP and ePTFE layers and on the breakdown strength. It will be demonstrated that each sandwich exhibits a certain poling limit above which the induced quasi-remanent polarization becomes unstable due to insufficient charge compensation. In addition, the dependence of Young´s modulus on the geometry is estimated by the so called Reuss model for layer structures and by a model from Gibson and Ashby for the porous PTFE layer. Finally, the model predictions are compared to experimental d₃₃ coefficients for selected sandwich geometries.