Porous PTFE space-charge electrets for piezoelectric ... - IEEE Xplore

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ABSTRACT. Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage ...
IEEE Transactions on Dielectrics and Electrical Insulation

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Vol. 7 No. 4, August 2000

Porous PTFE Space-Charge Electrets for Piezoelectric Applications R. Gerhard-Multhaupt, W. Kunstler, T. Gorne, A. Pucher, T. Weinhold, M. SeiO Department of Physics University of Potsdam Potsdam, Germany

Zhongfu Xia Pohl Institute of Solid-state Physics Tongji University Shanghai, China

A. Wedel, and R. Danz Fraunhofer Institute of Applied Polymer Research Potsdam/Golm, Germany

ABSTRACT Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d 3 3 coefficients of up to approximately 600 pClN or 600 pmlV. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed.

1 INTRODUCTION

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and electro-mechanical transducers for a wide range of applications often are based either on magnetic or on electrical effects. In comparison to magnetic devices, the electrical sensors or actuators do not require current loops for the generation or detection of the quintessential field changes. Instead, two electrodes suffice so that smaller or more flexible geometries can be employed and ohmic losses can be avoided almost completely However, the energy density and consequently also the transducer efficiency or sensitivity are usually much smaller with an electric field than with a magnetic one. Therefore, electrically insulating materials with ever higher piezoelectric coefficients are being investigated by researchers all over the world. Until recently, the main contenders for reasonably large piezoelectric effects were single-crystalline materials such as barium titanate ECHANO-ELECTRICAL

and polycrystalline ceramics such as lead zirconate titanate (PZT). The highest known piezoelectric coefficients of 2500 pC/N have been observed on single crystals of the inorganic relaxor perovskite PZN-PT (solid solution of lead zirconate niobate with 8% lead titanate) [l-31. During the 10th International Symposium on Electrets [4] in September 1999, however, as well as in subsequent publications (see below), several groups reported about surprisingly large quasi-piezoelectric effects in heterogeneous or porous polymer systems. These 'novel' piezoelectric spacecharge electrets should not be confused with the more classical piezoelectric polymer electrets based on polar polymers such as polyvinylidenefluoride (PVDF) and its copolymers or odd-numbered polyamides [5,6]. The principle of (quasi-)piezoelectricity in non-polar, mechanically non-uniform dielectrics with space charge already had been predicted theoretically in the early seventies [7-91. The first, and more or less accidental, findings of such an effect [lo] and also of pyroelectricity

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IEEE Transactions on Dielectrics and Electrical Insulation [ll]in polypropylene (PP) were, however, not very promising so that the search for and the optimization of heterogeneous spacecharge electrets with piezoelectric properties was not pursued at this time. It should be mentioned here that a much larger piezoelectric coefficient of 700 pC/N was estimated recently from quasi-static piezoelectric measurements on highly charged fluoroethylenepropylene copolymer (TeflonTM-FEP)electrets [12]. The situation changed in the mid-eighties when a process for manufacturing highly porous PP films was invented in Finland [13,14]. Since then, this so-called electro-thermo-mechanical (ETMF) or electromechanical (EMFI) film, was investigated, further optimized, and applied in various devices by several scientists and engineers [15-311. Piezoelectric coefficients >200 pC/N are now routinely achieved in this material [29-311, the theoretically predicted maximum coefficients being even higher [31]. It seems that the only major disadvantage of the flexible and versatile porous PP piezoelectrics is their rather low thermal stability [29]. An alternative principle was proposed and investigated in Poland [32,33]: A two or four layer sandwich of ‘soft’ (i.e. relatively low Young’s modulus) and ’hard’ (i.e. relatively high Young‘s modulus) polymers with space charge at their interface(s) also exhibits rather high piezoelectric coefficients. Much more stable fluoropolymer films could be employed for the ’hard’ layer in this case, but the soft layer consisted still of the electrically less well insulating and thermally much less stable PP [33]. At about the same time, commercial as well as experimental films of porous polytetrafluoroethylene (PTFE)became available. An experimental porous PTFE material from China proved to possess particularly good electret properties [34,36]. With this material, quasi-static piezoelectric coefficients of 5 35 and 150 pC/N were reported on sandwiches of ’hard’ and ’soft’ layers [37] and on single films of porous PTFE [38], respectively. With sandwiches of commercially available porous PTFE films and particularly ’hard’perfluorinated cyclobutene polymerelectret layers [39], piezoelectric coefficients as high as 600 pC/N were found recently [40]. Theoretical estimates yielded maximum possible values of 3500 pC/N [40],i.e. comparable to the above-mentioned single-crystal perovskites [l]. In the following, we report on our ongoing investigation of single and multiple-layer spacecharge electrets with porous PTFE films. First, we describe the materials employed in our study and the methods used for their preparation and characterization. Next, the results of charging and discharging experiments and of piezoelectric measurements on multiple and single layer samples are discussed. Finally, a few proposed device applications are listed.

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MATERIALS AND METHODS

2.1 ‘HARD’ AND ‘SOFT’ PTFE FILMS As the ’hard’ layer of the two, three and four layer sandwiches, we employed commercial PTFE films with thickness of either 15 or 25 p m as obtained from Plastpolymer, St. Petersburg, Russia and from Goodfellow, Cambridge, UK, respectively. 20 p m thick PTFE films from Plastpolymer were used in order to protect the single porous PTFE films after

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bipolar charging (see below). For some of the charge-stability measurements, 25 p m thick PTFE films from Dilectrix were utilized as well. In comparison to the spin-coated perfluorinated cyclobutene layers studied by Neugschwandtner et d. [40],nonporous PTFE with an elastic modulus of only 300 MPa is softer, and thus less suitable. However, the nonporous PTFE films are still hard enough to form useful multilayer sandwiches with the even softer porous PTFE films. These PTFE films with a porosity of 50% were obtained from the Shanghai Plastics Institute in thicknesses of 40,80, and 160 p m [34-361. Soft and hard films were left either without electrodes or evaporated with 50 nm of aluminum on one side in vacuum. Since the porous PTFE films have open pores ( c j the micrographs shown in [35], it is usually not possible to coat them with aluminum on both sides. For the sandwiches (layer sequences given below), the individually charged or uncharged films were piled on top of each other.

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2.2

POSITIVE AND NEGATIVE CHARGING AT ROOM AND ELEVATED T EMPE RATU R ES

Unmetalized or one-side metalized films were placed on a heated and electrically grounded metal plate and held flat by means of an electrically insulated ring. Positive or negative charging was performed with a point-to-plane corona discharge in air. The needle electrode was biased to HV between f 6 and f 20 kV. Charging times between 15 and 90 min, and charging temperatures between room temperature and 300°C were employed. An air gap of 5 cm was used between needle electrode and sample so that the plasma around the needle tip could not affect the sample surface directly and so that the charged sample area was 5 cm in diameter. Because of the different stabilities of positive and negative space charge in porous PTFE (see below), the temperatures for positive charging usually could be chosen N 100°C lower than for negative charging so that the negative charge was not destroyed during the subsequent positive charging procedure in the case of bipolar charging. For the charge-stability measurements, the samples were charged at room temperature with a control grid inserted between corona needle and sample plate. Grid voltages of 625 and 1000 V were used on the 25 p m thick nonporous and the 40 p m thick porous samples, respectively, so that the effective field across the film thickness always was 25 MV/m corresponding to equivalent surface-charge densities of 0.44 and 0.33 mC/m2, respectively. N

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2.3 SURFACE POTENTIAL AND

PIEZOELECTRIC MEASUREMENTS After charging, effective surface potentials were determined by means of noncontacting field-compensating electrostatic voltmeters (Monroe Isoprobe Model 244 or Trek Model 341). For these measurements, the samples usually were left on the same heated and grounded metal plate as during charging. For the quasi-static measurement of their piezoelectric response, the single or multiple film samples were mounted in a grounded metal

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box with polished electrode plates and connected to a vibrating capacitor electrometer (Statron Model 6305) via a large parallel capacitor of 101.5 nF. In relation to the parallel capacitor, the sample capacitances are very small (at most 0.19/0).Therefore, the charges generated on the sample electrodes upon loading or unloading are transferred almost entirely to the parallel capacitor whose voltage can then be measured easily. Loading of the samples was achieved with a calibrated mass of 1.0 kg. In order to homogenize the pressure across the sample area, a conductive rubber pad was inserted between plate electrode and sample. From the compressive stress produced by the loading mass and the voltage generated across the parallel capacitor, the quasi-static direct piezoelectric d 3 3 coefficient was calculated. Further details are found in previous reports [37,38]. The inverse piezoelectric effect of porous PTFE films was measured with an application-relevant acoustical technique: The frequency response of the absolute sound pressure level (i.e. the sound pressure relative to 20 pPa) generated by a porous PTFE sample was recorded with a 6 mm electret microphone from Audio Teknology Inc. in the near field at a distance of