IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18, No. 1; February 2011 49 1070-9878/11/$25.00 © 2011 IEEE porous layer metal electrode solid layer V P, d P F, d F F, d F Figure 1. Model of a 3-layer sandwich composed of a porous ePTFE film positioned between two solid dielectric films of FEP Importance of Geometry and Breakdown Field on the Piezoelectric d 33 Coefficient of Corona Charged Ferroelectret Sandwiches Heinz von Seggern, Sergey Zhukov Institute of Materials Science, Technische Universität Darmstadt, 64287 Petersenstrasse 23 Darmstadt, Germany and Sergey Fedosov Department of Physics and Materials Science, Odessa National Academy of Food Technologies, 65039 ul. Kanatnaya 112 Odessa, Ukraine ABSTRACT Electrical and mechanical properties of corona poled three layer FEP/ePTFE/FEP sandwiches were studied theoretically and experimentally. Modeling of the properties has been performed, as well as their experimental verification. The piezoelectric d 33 coefficient has been studied with the objective of its possible optimization. It has been shown that the maximum value of the d 33 piezocoefficient depends on interrelation between solid and porous layers thicknesses, the intrinsic Young modulus of the porous layer and dependence of the breakdown field on the thickness of the porous layer. The best geometry of the sample can be designed considering the obtained results. Index Terms — Ferroelectrets, piezoelectricity, Paschen breakdown, porous PTFE. 1 INTRODUCTION NEW materials called ferroelectrets or piezoelectrets having a voided or foam structure and possessing high piezoelectric d 33 coefficients have attracted considerable attention of researchers during the last years [1, 2]. Since originally very promising voided polypropylene (PP) films were not sufficiently stable at temperatures above 60 0 C [3], exploration of new materials and combinations still continues including layered structures consisting of blocking layers of solid fluorinated ethylene propylene (FEP Teflon) films and expanded (fibrous) polytetrafluoroethylene (ePTFE) in between them [4-9], as schematically shown in Figure 1 (index F relates to FEP and P stands for ePTFE). FEP layers and ePTFE layer have thicknesses d F and d P , respectively, with relative dielectric permittivities of F and P . The d 33 coefficient for the sandwich arrangements is high with values ranging between 100 pC/N and 1600 pC/N depending on the structure and measuring conditions [4, 6-9]. The porous structure of the ePTFE is composed of nano-sized (typical thickness of ~100 nm) fibers forming an open fibrillated morphology. The solid FEP layers are used to mechanically seal the open-fibrous ePTFE and to prevent undesirable discharging of the ePTFE film [4, 9]. The high piezocoefficient of the FEP/ePTFE/FEP sandwich results from charge layers of opposite polarity formed by breakdown generated charges trapped at the interfaces between FEP and ePTFE films. In a model proposed earlier for explanation the hysteresis phenomena in such sandwiches [9], a quasi-stationary approach has been used assuming that parameters depend only on geometry, applied voltage and dielectric properties of the utilized materials. Poling dynamics and thermal stability of the sandwiches have been also studied experimentally and theoretically [10]. Manuscript received on 20 August 2010, in final form 15 October 2010.