Fatigue and recovery of cellular-polypropylene piezoelectrets during and after dielectric-barrier discharges

Ferroelectrets (also called piezoelectrets) are internally charged polymer foams that combine high direct and inverse piezoelectricity with high mechanical flexibility and good elastic compliance. The poling process of the cavities inside the material requires dielectric barrier discharges (DBDs). In DBDs, at least one of the inner surfaces of the discharge gap is provided by a dielectric layer. DBDs produce an internal plasma with - among others - energetic and often reactive charged particles, electrons, and neutral species. The existence of these particles facilitates the charging process. However, the inner surfaces of the cavities are thus also exposed to a highly reactive plasma. Repeated DBDs in air may result in significant fatigue of the effective polarization and hence also of the piezoelectricity and of the ferroic behavior in cellular-polypropylene (PP) ferroelectrets - mainly because of plasma-surface interactions in the cavities. In this paper, the recovery of the piezoelectricity in fatigued cellular-polypropylene ferroelectrets is studied, and the influence of storage times and different storage conditions on the stability of the piezoelectricity in fatigued samples is explored.