Wolfgang Künstler

High surface-charge stability of porous polytetrafluoroethylene electret films at room and elevated temperatures

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

Porous PTFE space-charge electrets for piezoelectric applications

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/sub 33/ coefficients of up to approximately 600 pC/N or 600 pm/V. 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.

Ferroelectric polarization in stretched piezo- and pyroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer films

After suitable preparation, films of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] show strong piezo- and pyroelectric effects. We investigated the polarization buildup in stretched P(VDF-HFP) copolymer films, cast from a dimethylsulfoxide/acetone solution. The films were poled under varying or constant electric fields. We observed a hysteresis of the polarization as a function of the electric field during poling of the stretched P(VDF-HFP) copolymer films with varying electric fields. From the hysteresis curve, a polarization of 24 mC/m2, and a coercive field of 100 MV/m were determined. The polarization values are lower than those of PVDF films cast from solution and stretched under the same conditions. It seems that HFP comonomer leads to a decrease in the amount of switchable dipoles. Nevertheless, the P(VDF-HFP) copolymer is very attractive because of its high piezo- and pyroelectric coefficients as well as its better chemical and thermal stability and its predicted longer lifetime. A ra...

Piezoelectric activity in a copolymer of acrylonitrile and methylacrylate

Results of investigations of piezoelectric properties in a copolymer of acrylonitrile and methylacrylate are summarized. A preliminary explanation is given of the observed phenomena on the basis of surface potential measurements. The maximum values of piezoelectric strain coefficient d/sub 31/ obtained after optimizing the poling conditions for stretched and unstretched films were around 3 and 0.85 pC/N, respectively. The time stability of piezoelectricity was found to be relatively low. A hysteresis loop of electric displacement versus electric field was observed and assumed to be a manifestation of a ferroelectriclike behavior. All the observations point to a dipolar nature of piezoelectricity where ferroelectric and frozen-in dipole orientations obviously superimpose. >

Infrared spectrometric study of poly(vinylidene fluoride) films in cyclic electric fields

Abstract Infrared spectra of uniaxially drawn β-poly(vinylidene fluoride) films were recorded under the influence of an electric field. The direction and strength of the applied field were changed in up to thirteen cycles with a maximum field strength of 240 MV m−1. The field-induced changes of the absorbances of the bands at 445 cm−1 (CF2 rocking) and at 510 cm−1 (CF2 bending) were evaluated. Certain amounts of 180° switching and a structure memory effect are discussed to explain the experimental results obtained for repeated repoling processes.

Anomalies of dielectric properties of vinylidene fluoride-trifluoroethylene copolymer films

The temperature dependences of the real and imaginary parts of the complex permittivity of vinylidene fluoride-trifluoroethylene copolymer films at temperatures ranging from −40 to 140°C are determined in the frequency range 10−1–108 Hz. An analysis of the experimental data has demonstrated that the behavior of the dielectric characteristics of the copolymer in the course of heating to temperatures above the ferroelectric phase transition point and during subsequent cooling differs substantially. In the latter case, the high-frequency dielectric response exhibits properties characteristic of relaxor ferroelectrics. The observed features are discussed in terms of the competition of two mechanisms associated with the short-range and long-range forces.

Piezo-, pyro- and ferroelectricity in poly (vinylidene fluoride-hexafluoropropylene) copolymer films

Thin films of poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) show significant electroactive properties, such as piezoelectricity, pyroelectricity and electrostriction. Suitable polar P(VDF-HFP) copolymer films can be prepared by melt-pressing or solution-casting. Dipolar orientation causes the macroscopic polarization and thus also the symmetry breaking necessary for electroactive properties. We discuss the polarization build-up in thin, stretched and non-stretched, films of P(VDF-HFP) copolymer with a HFP content of 15%. Poling currents measured in-situ during electric poling are analyzed and the polarization is calculated. Suitable electric poling leads to hysteresis phenomena of the polarization as a function of the electric field as well as to significant polarization during switching experiments. Our results indicate dipolar orientation also in non-stretched P(VDF-HFP) films. *Originally presented at 10th European Meeting on Ferroelectricity, Cambridge, UK, August 3–8, 2003.

Inverse piezoelectricity of porous PTFE films with bipolar space charge

After suitable charging at room or elevated temperatures, porous films of polytetrafluoroethylene (PTFE) exhibit direct as well as inverse piezoelectricity. For the present study, the inverse piezoelectric responses of charged porous PTFE films are measured by means of an interferometric technique as well as an acoustic method. In addition to materials issues such as sample and electrode preparation as well as electret charging, the two methods are briefly described and the respective results obtained with them are discussed in comparison to each other.

Coating of porous polytetrafluoroethylene films with other polymers for electret applications

Porous polytetrafluoroethylene (PTFE) films are attractive for electret applications because of their unusual mechanical properties and their excellent charge-storage capabilities. The direct application of electrodes is facilitated and the charge stability of the open-porous films is improved, if the porous PTFE films are coated with layers of other suitable polymers. Here, we report on the preparation of coated porous PTFE films and their investigation by means of scanning electron microscopy, corona charging and surface-potential measurements. For the coating, non-porous polymers like polystyrene (PS), polyethylene terephthalate (PETP) and Teflon/sup (R)/ AF were employed. After optimization, such layered films may be suitable for a range of electret-transducer applications under various environmental conditions.

Surface-charging behavior of plasma-treated polymer films

Thin films of the Teflon/sup TM/ homo- and copolymers TFE, FEP, PFA, MFA, AF, and PVDF-TFE (VF/sub 2/-VF/sub 4/) were treated with a microwave plasma in hydrogen or argon, and charged with a point-to-grid corona. It was found that a hydrogen-plasma treatment has a greater influence on charge stability than an argon-plasma process and that the stability of plasma-treated and negatively charged fluoropolymers is lower than that of untreated samples, while the stability of plasma-treated and positively charged fluoropolymers is higher. X-ray photoelectron-spectroscopy showed ablation of fluorine and incorporation of oxygen- and nitrogen-containing groups into the surface. Furthermore, Teflon-PFA and polyethylene terephthalate (PETP) samples were charged in a radio-frequency argon plasma by way of self-biasing. In this case, the charge level increases with increasing bias and charging time. However, enhanced sample heating must be taken into account at higher plasma power. With a plasma, PFA can be charged to higher initial values, but PETP shows better charge stability.