08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Dynamics of poling PVDF between 25°C and 120°C(1991) Eberle, Gernot; Eisenmenger, WolfgangPVDF films with high β-content are poled in an electric field of 60 MV/m at temperatures between 25°C and 120°C. At 25°C the alignment of dipoles takes place in a central poling zone during several hours of poling. When the temperature is increased to 120°C the poling time necessary to align the dipoles in this narrow zone is reduced to several seconds. In addition, at temperatures higher than 90°C and increased poling times the central poling zone first increases but later decreases in its dielectric displacement. Simultaneously in a 10 μm regime adjoining the positive electrode a secondary displacement zone starts to grow. The strong reduction of this peak under short circuit conditions indicates hetero-charge accumulation in front of an anode.Item Open Access Field induced gas emission of polymer films(1992) Selle, Michael; Eberle, Gernot; Gompf, Bruno; Eisenmenger, WolfgangPoling polyvinylidene fluoride under vacuum conditions causes strong gas emission, which is analyzed with a quadrupole mass spectrometer. The dominant peak in the mass spectrum is HF, which is produced by electrochemical reactions at the sample surface. After the electric field is switched off, the gas emission unexpectedly increases to a value higher than under the field. This can be explained by ionic charge detrappingand charge migrationItem Open Access Polarization distribution in PVDF obtained by poling under constant current condition(1990) Neumann, Guido; Bihler, Eckardt; Eberle, Gernot; Eisenmenger, WolfgangSince the resistivity of PVDE films increases when the films are poled, the voltage across the samples must be increased during the poling process in order to keep the current constant. Under these conditions, the formation of a polarization zone in the center of the film depth is observed. The thickness of the polarization-free zone close to the film surfaces is found to decrease with larger current densities. The results are consistent with a model assuming charge trapping in deep traps associated with the orientation of polar crystallites.Item Open Access Binding energies of trapped charges in PVDF and P(VDF/TrFE)(1994) Küssner, Bodo; Dehlen, Bernhard; Eberle, Gernot; Eisenmenger, WolfgangThe remanent polarization in PVDF and P(VDF/TrFE) is stabilized by trapped charges. We measured the remanent polarization P as function of temperature during heating from room temperature to 180°C. The discharge current is then computed as the first derivative -dP/dT. Assuming a Debye relaxation and a continuous distribution of binding energies g(E) of the trapped charges we can show that the discharge current I(T) is proportional to g(E=mT), with m=const. The constant m can either be calculated numerically or by analytical approximation, With this method the distribution g(E) can be determined. These data are consistent with the extraordinary long life time of the remanent polarisation in PVDF and P(VDF/TrFE) at room temperature.Item Open Access Electric-field profiles in corona- or electron-beam-charged and thermally treated Teflon PTFE, FEP, and PFA films(1992) Gerhard-Multhaupt, Reimund; Eberle, Gernot; Xia, Zhongfou; Yang, Guomao; Eisenmenger, WolfgangCharge spreading in three different types of Teflon electrets was studied by means of piezoelectrically generated pressure steps, FEP and PFA samples corona-charged at room temperature usually exhibited only a surface charge layer. Uniform charge spreading throughout the bulk was found in FEP charged at or heated to high temperatures. Charge spreading was much less prominent in PFA because of a smaller retrapping efficiency. In PTFE (polytetrafluorethylene), charges from the surface and the rear electrode were injected into the bulk during charging at any temperature. Electron-beam-deposited charge layers broadened significantly upon heating.Item Open Access Influence of poling conditions on the gas emission of PVDF(1993) Eberle, Gernot; Schmidt, Helmut; Eisenmenger, WolfgangThe influence of the poling time and the polarity of the applied voltage on the time dependence of the gas emission of PVDF is investigated. If PVDF films are poled in electric fields of about 100MV/m, charges are injected and ions are formed by electrochemical reactions. By recombination and by other reactions of these ions, HF gas is evolved, diffusing out of the sample through the porous electrode. This gas emission increases with increasing field strength. At constant field it decreases to a steady state value comparable to the space-charge-limited poling current of PVDF. Free ions are trapped in the polarization zone. Under short-circuit conditions shallow trapped ions are freed and the gas emission increases strongly. The longer the poling time, the broader the polarization distribution. If the polarization zone is located near the nonporous electrode, the recombined gas molecules need longer time to reach the porous electrode. Therefore, the gas emission under short-circuit conditions is delayed by poling with negative polarity compared to positive polarity.Item Open Access Time development of multiple polarization zones in PVDF(1993) Eberle, Gernot; Dehlen, Bernhard; Eisenmenger, WolfgangThe polarization distribution in PVDF which causes the piezo- and pyroelectricity of this polymer depends strongly on the polarization distribution. Applying an electric field of E = 60M/m causes a ca. 10μm thick polarization zone in the middle of the sample. Applying the same field in opposite direction a bimorph structure results. If the same field is applied to a homogeneously prepolarized PVDF sample a trimorph structure is formed with three polarization zones under field. But under short circuit conditions the polarization zone in the new field direction vanishes. The polarization distribution is measured with the acoustic PPS method with a spatial resolution of about 2μm.Item Open Access Influence of charge injection on the formation of remanent polarization in P(VDF-TrFE) copolymers(1990) Bihler, Eckardt; Neumann, Guido; Eberle, Gernot; Eisenmenger, WolfgangThe influence of charge injection on the formation of remanent polarization of P(VDF-TrFE) copolymers was studied by inserting an insulating barrier between the metal electrodes and the P(VDF-TrFE) copolymer during the poling process. The resulting remanent polarization is significantly smaller than that of identically poled samples for the same field strength and time duration, but with directly contacting metal electrodes. This shows that the orientation of the dipoles alone is not a sufficient condition for the formation of remanent polarization in PVDF and its copolymers. The experiments indicate that the stabilization of the polarization is mediated by charge injection and charge trapping at polarized crystallite surfaces.Item Open Access Influence of solvents on the polarization distribution in PVDF(1992) Dehlen, Bernhard; Eberle, Gernot; Eisenmenger, WolfgangThe authors report on polarization reductions when poled PVDF films are immersed in polar solvents like acetone, propanol, ethanol, and methanol for a few minutes. Also, immersion in nonpolar solvents, like hexane or carbon tetrachloride for some hours results in a small reduction of polarization. The amount of the reduction depends on the dipole moment of the solvent molecules, the length of the influence of the solvent, and the poling parameters. The authors also observe an influence on the remanent polarization if the films have been immersed in a solvent for some minutes before poling. They find inhomogeneous polarization distributions in these films that are different from distributions in untreated films before poling. In the case of a polar solvent pretreatment, the development of polarization profiles with several maxima is observed.Item Open Access Thermal depolarization of PVDF : anomaly at 180°C(1991) Eberle, Gernot; Eisenmenger, WolfgangPVDF (polyvinylidene fluoride) samples with high β-content (80%), prepolarized at room temperature, were heated to higher temperatures. The polarization was measured first under open circuit conditions at the high temperature and then cooling down again to room temperature. For elevated temperatures up to 175°C the polarization is reduced to 2% of the original value, yet covers roughly to 8% of the original value after cooling down again to room temperature. In contrast, when heating up the films to 180°C a significantly different behavior was observed: in cooling down to room temperature the polarization returns from about 1% at 180°C to about 40% of the original prepolarized value.