Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films

Abstract

Cellular and porous polymers with internal bipolar space charge can exhibit large piezoelectric thickness coefficients and have therefore led to significant advances in the understanding and the application of piezoelectricity in polymer electrets. As possible alternatives to these cellular ferroelectrets, other potentially useful configurations of space-charge electrets, such as bi-layer or multi-layer stacks of at least one "soft" porous and one "hard" non-porous electret film have been suggested and investigated. Extending the concept of cellular or porous polymer electrets with microscopic voids, we propose and describe novel piezoelectret structures with regular arrays of millimeter-sized bubbles that are formed between fluoro-ethylene-propylene (Teflon®-FEP) films via a vacuum-assisted thermal process. After internal charging by means of high impulse voltages, the bubble structures exhibit rather large piezoelectric activities in their thickness direction, with phenomenological quasi-static d 33 coefficients of up to 500 pC/N (or pm/V) which quite strongly decrease with the amplitude of the applied force. The electromechanical behavior of the new piezoelectrets has been modeled in analogy to the operation of electret microphones. , he is a visiting professor at the Institute of Physics of the University of São Paulo in São Carlos, Brazil. The main research areas of Prof. Gerhard-Multhaupt are polymer electrets and ferroelectrets, in particular the mechanisms of space-charge storage and dipole polarization in dielectric polymers and polymer composites, their ferro-, pyro-and piezoelectrical properties, and their applications in sensors and actuators, as well as the nonlinear optical properties of polymers, and more recently also the physics of musical instruments. From 1974From until 1979 he was a fellow of the Studienstiftung des Deutschen Volkes. In 1988, he was awarded an ITG-Preis by the Informationstechnische Gesellschaft im VDE. In 1989, he received a Silver medal from the Stiftung Werner-von-Siemens-Ring. In 2001, he was awarded the first Technologietransfer-Preis by the Technologie-Stiftung Brandenburg and the Prof.-Adalbert-Seifriz-Preis by the Verein Technologie-Transfer Handwerk for his technological collaborations with small industrial companies. Reimund Gerhard-Multhaupt is a member of the American, European and German physical societies. Since 2002, he serves as Digest Editor of the IEEE Dielectrics and Electrical Insulation Society.

DC-Biased Piezoelectret Film Transducers for Airborne Ultrasound

“…It was also found that the d 33 -coefficient of piezoelectrets decreases slowly throughout the infrasonic, audio, and ultrasonic frequency ranges due to the increase of Young's modulus in this region [14,15]. The d 33 -coefficients of ferroelectrets have been further increased by pressure expansion methods [16,17], while the thermal stability of the piezoelectric effect was improved by using layered arrangements based on fluoropolymer films, where the air enclosures consist either of voids between the layers or of air volumes in a porous layer [18][19][20][21].…”

Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets

“…So far, ferroelectrets have been successfully developed with poly(ethylene terephthalate), 41,42 poly(ethylene naphthalene-2,6-dicarboxylate), 43,44 cyclo-olefine copolymers, 45 and fluoropolymers. 35,[46][47][48][49] The consequences of plasma-polymer interactions might be different for different polymers. For example, an oxygen-plasma treatment of polytetrafluoroethylene can improve the stability of a negative charge.…”

Fluoropolymer piezoelectrets with tubular channels: resonance behavior controlled by channel geometry

“…[10,11] and references cited therein), since they usually show ferroelectric-like behavior such as a polarization-voltage hysteresis as well as significant piezoelectricity. The piezoelectricity is directly related to the charge-storage properties of the polymer films and to the charging conditions.…”

“…In addition, voided structures were prepared by means of thermal fusion of various polymers, which resulted in multilayer piezoelectrets from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) films with piezoelectric activities comparable to the ferroelectret foams [11,18]. The two approaches essentially rely on molding several dome-shaped semi-bubbles into a fluoropolymer film and then fusing it to another flat polymer film, thus creating closed and more or less hemispherical voids.…”