Charging behavior and thermal stability of porous and non-porous polytetrafluoroethylene (PTFE) electrets

In view of the high piezoelectric activity found on some cellular polymer electret films, polymeric space-charge electrets were studied more frequently during the last ten years. An alternative concept -between cellular electret materials and devices with non-porous electret films -are layer systems of porous and non-porous electret films. Such layer systems exhibit the typical properties of ferroelectrets. In the late nineties, there were several investigations on the charging behavior of porous FIFE, but the main research focus shifted to new cellular ferroelectrets such as polyolefins and polyesters. Today, the additional expertise with polymer ferroelectrets may help to study and better understand the charging behavior of porous PTFE films. In this contribution, maximum charging levels and thermal charge stabilities of porous PTFE films are analyzed. As expected, the charging behavior is strongly influenced by the porosity of the polymer film. Mechanisms that limit the maximum charging levels -such as breakdown-induced conduction -are identified. The thermal stability of trapped charges is tested, and the light emission during the dielectric-barrier discharges of the charging process is briefly described.