Piezoelectrets from sandwiched porous polytetrafluoroethylene films with different porosity

The present study deals with the investigation of the poling behavior of individual and sandwiched porous films of polyfluoroethylene (ePTFE) having different porosities ranging from 70% to 98%. In the stand alone porous films (90-98%) the maximum breakdown electric field strength E_P^B follows Paschen´s law for air. Therefore those ePTFE films can be considered as a single large air pore with respect to breakdown phenomenon. In the sandwich arrangement the same film exhibits a 2 times higher E_P^B value. On lowering porosity up to 70% the E_P^B value considerably increases. The model proposed in earlier work had no parameter for introducing porosity, but nevertheless well describes the changes of sandwich poling and switching behaviour if the experimental value for E_P^B on porosity is taken into account. The model and the gained insight into the poling process can also be utilized to further optimization of the sandwich structure as an electromechanically active device.