Breakdown-induced polarization buildup in porous fluoropolymer sandwiches: a thermally stable piezoelectret

The buildup of air-breakdown-induced polarization in a one-side-metallized three-layer sandwich structure consisting of fluorinated ethylene propylene copolymer (FEP) / expanded polytetrafluoroethylene (ePTFE) / FEP has been studied utilizing a corona triode for voltage application. The FEP layers form structurally and electrically dense layers, whereas the ePTFE layer consists of 91% air and 9% fibrous PTFE. Upon negative corona charging, breakdown sets in within the pores of the ePTFE, as soon as the electric field strength exceeds the Paschen breakdown value of air. The resulting ion-plasma then separates in the strong electric field of the corona-deposited surface charges whereby ions of the two polarities drift towards opposite FEP layers, where they are trapped, and macroscopic dipoles are formed. These dipoles are responsible for a strong piezoelectricity. It will be demonstrated by thermally stimulated discharge currents that when poling is performed at elevated temperatures, for example, 150 degrees C, the polarization is temperature stable, and consequently so is the piezoelectric d(33) coefficient. The d(33) coefficients, however, decay from 800 to 400 pC/N under atmospheric pressures within six days, and repeated loading also shows a similar loss of piezoelectricity, related to mechanical relaxation of the highly porous ePTFE. (c) 2006 American Institute of Physics.