Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer–Tower circuit

Abstract

Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance—which is, however, still considerably larger than the sample capacitance—is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.

Polarization and Hysteresis in Tubular-Channel Fluoroethylenepropylene-Copolymer Ferroelectrets

“…The charging current includes three components that originate from charging of the sample capacitance, from the DBDs inside the tubular channels and from the conductivity in the sample, respectively. For highly insulating non-polar polymers such as Teflon TM FEP, the current from the sample conductivity is negligibly small [14]. Therefore, the current is essentially given by i (t) = i cap (t) + i p (t).…”

“…Hysteresis curves were determined by means of a modified Sawyer-Tower circuit as described in [14]. In this circuit, a high-voltage (HV) capacitor C 1 (3 nF) and a large standard capacitor C m (1 μF) are connected in series with the sample (Fig.…”

A matter of attraction: Electric charges localised on dielectric polymers enable electromechanical transduction

“…Recent progress in the experimental investigation of cellular-foam and tubular-channel [12] ferroelectrets led to clear indications of quasi-ferroelectric behavior in these materials [14]. It must be pointed out, however, that the observed hysteresis curves are caused by the Paschen breakdown Fig.…”

“…Current-voltage curves on (a) a cellular-PP ferroelectret film and (b) a Teflon TM −FEP tubular-channel ferroelectret system, as recorded during the second complete voltage loop (two positive-voltage semicycles (1 and 2) followed by two negative-voltage semicycles (3 and 4). The progress along the curves during the semicycles is indicated by arrows [14]. threshold of the internal dielectric-barrier discharges (DBDs) in the cavities of the heterogeneous polymer systems [15] and not by potential barriers for the re-orientation of dipoles.…”

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

“…Positive and negative charges are produced in DBDs and are trapped at the top and bottom internal surfaces of the gas-filled cavities, respectively. Thus, the charged cavities form macroscopic dipoles in ferroelectrets, and these dipoles can be reversed by applying sufficiently high electric fields [7]. Ferroelectrets contain no intrinsic molecular dipoles, and their effective polarization is related to the density of the internally charged cavities (i.e.…”