Barrier discharges in cellular polypropylene ferroelectrets: How do they influence the electromechanical properties?

Abstract

Ferroelectrets (i.e., charged cellular polymers) are rendered piezoelectric by means of barrier discharges inside the air-filled voids. The light emission from barrier discharges in cellular polypropylene ferroelectrets was quantitatively studied. Light emission typically occurs above a threshold voltage of 3 kV and then significantly increases with the applied voltage. Time-resolved images reveal discharge processes in individual voids. In addition, a second "back discharge" emission is observed when the voltage is reduced to zero. The buildup of the "effective polarization" in cellular PP ferroelectrets was studied by an acoustic method and dielectric resonance spectroscopy. A polarization-voltage (P-V) hysteresis loop was obtained by analyzing the data with an existing model for the piezoelectric d33 coefficient of ferroelectrets, from which a threshold charging voltage of 3 kV and the back barrier discharges were confirmed and a zero-field "effective polarization" o f 0.5 mC/ m2 was determined. However, charge densities of up to 2 mC/ m2 were measured under an applied bias voltage, leading to the conclusion that the observed back discharges destroy a significant fraction of the effective charge density

Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications

“…Paschen breakdown in the gas has been suggested as the essential mechanism for the internal charging of the voids inside ferroelectrets. 4,7,16 According to Townsend's model, the critical breakdown field of common gases in a uniform electric field is a function of gas pressure p and electrode spacing d ͑which in our case is equal to the internal void height͒:…”

“…The characteristics of the light emission may also be used to investigate the DBDs in more detail. 7,8 Figure 3 shows the spatially resolved light emission from an FEP ferroelectret with tubular channels recorded by means of a PCcontrolled electron-multiplying charge-coupled-device camera ͑EMCCD͒ ͑iXon, Andor Technology͒. For such an investigation, samples are metallized on both sides with semitransparent Au electrodes having a diameter of 16 mm and a thickness of 20 nm each.…”

“…7,8 In addition, most of the piezoelectric sensitivity is already lost between 60 and 70°C in polypropylene ͑PP͒ ferroelectrets. 9 Consequently, cellular-foam ferroelectrets with higher thermal stability have been developed from polyethylene terephthalate ͑PET͒ and polyethylene naphthalate ͑PEN͒ by means of physical foaming with supercritical carbon dioxide.…”

Hidrofone ultrassônico com piezoeletreto como elemento transdutor

“…E de forma mais detalhada, o efeito piezoelétrico nesses polímeros porosos, pode ser explicado devido a dois fatores principais: as cavidades e ao processo de carregamento. Durante o carregamento elé-trico a estrutura celular é exposta à influência de um intenso campo elétrico, que provoca a ruptura do gás dentro das cavidades em forma de descargas elétricas (ALTAFIM et al, 2008), (QIU et al, 2007). Essas descargas promovem a ionização do gás dentro das cavidades e que pela ação do campo elétrico, são deslocadas em direções opostas (positivas e negativas).…”

Detecção acústica de descargas parciais com transdutor piezoelétrico

“…Durante o carregamento elétrico a estrutura celular é exposta à influência de um intenso campo elétrico, que provoca a ruptura do gás dentro das cavidades em forma de descargas elétricas (ALTAFIM et al, 2008;QIU et al, 2007). Essas descargas promovem a ionização do gás dentro das cavidades e que, pela ação do campo elétrico, são deslocadas em direções opostas (positivas e negativas) (MEDEIROS, 2014 …”