Piezoelectrets from sandwiched porous polytetrafluoroethylene (ePTFE) films: influence of porosity and geometry on charging properties

Sandwiched structures of open-porous and solid polymer dielectrics reveal a strong piezoelectric response after proper charging by high electric fields. Here, the electrical properties of corona-poled three-layer FEP/ePTFE/FEP sandwiches are studied theoretically and experimentally with the objective of its possible optimization for the highest piezoelectric activity. Modelling of the charging properties is performed for different sandwich geometries. It is shown that the maximum value of the remanent interface charge density accumulated during poling depends mainly on the ratio of the solid and porous layers thicknesses and on parameter E(B), which characterizes the electric breakdown strength of air in the porous layer. E(B) exhibits specific dependences on porosity and thickness of the porous layer. For a given porous layer E(B) is independent of the solid film thickness. The obtained results can be utilized to further optimize the sandwich structure as an electromechanically active device.