Influence of the ambient gas on the charging efficiency of tubular-channel fluoroethylenepropylene (FEP) copolymer ferroelectrets

Ferroelectrets (also called piezoelectrets) are non-polar polymer foams or polymer systems with internally charged cavities. The charging process represents a series of dielectric barrier discharges (DBDs), and is critical for the piezoelectric properties of ferroelectrets. DBD charging relies on the breakdown of the gas molecules, and therefore strongly depends on the gas components inside the cavities. In this paper, we study the influence of gases with lower (helium) and with higher (nitrogen or air) breakdown strength on the charging efficiency of tubular-channel fluoroethylenepropylene (FEP) ferroelectrets. It is found that filling the cavities with helium results in low threshold voltage, facilitating the DBD charging process. However, a lower threshold voltage leads to a lower value for the remanent polarization because back discharges occur at lower charge level when the charging voltage is turned off. Via gas exchange involving helium and nitrogen during charging, the charging efficiency of tubular-channel FEP ferroelectrets can be significantly enhanced.