Theoretical considerations towards an optimal d33-coefficient of sandwiched piezoelectrets

During the last years piezoelectrets having a voided or foamed structure possess high piezoelectric d33 coefficients after proper poling and have attracted considerable attention. Since very promising cellular polypropylene (PP) films turned out to be not sufficiently stable at temperatures above 60°C, new and hybrid materials still are explored including sandwich structures which consist of solid polyfluoroethylene propylene (Teflon FEP) blocking layers enveloping a layer of porous polytetrafluoroethylene (ePTFE). Figure 1 displays a schematic drawing of such a 3-layer sandwich. The respective notations for the thicknesses (dF, dρ) and dielectric constants (εF, ερ) are included in Fig. 1. The open-porous morphology of the ePTFE films consists of solid nodes and connecting fibres.