Optimization of the porous polytetrafluoroethylene sandwiches for piezoelectric applications

The three- and multi-layered sandwiched structures of polyfluoroethylene propylene (FEP) and porous polytetrafluoroethylene (ePTFE) have gained considerable interest as ferroelectrets [1-6]. Being initially completely non-polar, after proper poling in a high electric field they reveal a large piezoelectric d₃₃ coefficient comparable to conventional ferroelectric Pb(Zr,Ti)O₃ (PZT) ceramics. It has been recognized that a unique combination of charge storage ability together with a low elastic modulus typical for foams is responsible for high piezoelectric response of such hybrid structures. The ePTFE films are commercially produced by many companies such as Gore Ltd., DuPont Ltd., DeWal Industries Ltd. and Tetratex Ltd. The open-porous morphology of the ePTFE films strongly depends on preparation, stretching and expansion processes and usually consists of separated phases with solid nodes and fibres. Fig. 1 displays the scanning electron micrographs of the 91% porous ePTFE film from GoodFellow. Moreover, the possible high thermal piezoelectric stability of sandwiched arrangements has also been demonstrated [1-3,5]. All this makes the polymer porous structures a very promising object for applications in electromechanically active devices. Several ways to realize a full potential of such hybrid structures will be considered in the current contribution. To this end the charging and mechanical properties of different sandwich arrangements will be analysed.