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Subject: search.filters.subject.530Author: search.filters.author.Eisenmenger, WolfgangAuthor: search.filters.author.Bihler, EckardtPublication Type: search.filters.UBSTyp.Zeitschriftenartikel

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Now showing 1 - 5 of 5
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    Electric field-induced gas emission from PVDF films
    (1987) Bihler, Eckardt; Holdik, Karl; Eisenmenger, Wolfgang
    Charge injection, conduction and trapping are important processes for stabilizing the electric polarization in the piezoelectric polymer PVDF. In order to study the nature of the pertinent charges in PVDF we measured the gas emission from PVDF using a permeable electrode under an applied electric field up to 0.7 MV/ cm. The films were covered on one side with evaporated copper, on the other side a copper wire gauze was used as a permeable electrode. The polymer films were mounted in an UHV-system with a built-in quadrupole mass spectrometer for residual gas analysis. Charging the permeable electrode negatively, gas emission was found mainly consisting of hydrogen, hydrogen fluoride, and fluorine. For comparison FEP-and PET-films were examined.
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    Polarization distributions in isotropic, stretched or annealed PVDF films
    (1989) Bihler, Eckardt; Holdik, Karl; Eisenmenger, Wolfgang
    The time development of the polarization distribution across the film thickness in polyvinylidene fluoride (PVDF) was observed using the pressure step response technique. The crystallite phase composition of the samples was changed by annealing and stretching at elevated temperatures. It is shown that the crystallite phase composition, e.g., the β crystallite content, determines the spatial distribution of the permanent polarization in PVDF.
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    Dynamics of polarization growth and reversal in PVDF films
    (1989) Womes, Manfred; Bihler, Eckardt; Eisenmenger, Wolfgang
    Measurements are presented of the time development of the dielectric displacement and the remanent polarization in polyvinylidene fluoride (PVDF) for poling times ranging from 1 μs to 1000 s and poling fields between 0.8 and 2.0 MV/cm. For longer times (0.1 to 1000 s), the time dependence of the polarization distribution across the film thickness is also determined. After application of a steep rectangular high-voltage pulse, the sample is shorted to zero voltage. The remanent polarization under the short-circuit conditions is compared to the maximum dielectric displacement under the external poling field. A significant time delay of the buildup of the remanent polarization was observed as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal, a flipping back' of the polarization was observed for shorter poling times of up to 200 μs. Under these conditions, a large part of the polarization is reversed under the field, but after the removal of the field, most of the polarization returns to the original direction. The results can be explained by the ferroelectric cooperative coupling of oriented crystallite dipoles to charges trapped at the surface of polarized crystallites.
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    Polarization dynamics of VDF-TrFE copolymers
    (1991) Eberle, Gernot; Bihler, Eckardt; Eisenmenger, Wolfgang
    The polymer polyvinylidenefluoride (PVDF) and its copolymers with trifluoroethylene (TrFE) exhibit a strong piezoelectric effect after poling in high electric fields (100 MV/m). HV impulses of definite duration have been applied to the polymer in order to study the dynamics of the poling process. The dielectric displacement during the HV impulse was recorded. After the impulse, the remanent polarization under short-circuit conditions was also measured. It was thus possible to obtain the minimum poling time dependent on the applied field strength necessary to stabilize the remanent polarization. Comparing the time development of the dielectric displacement with the corresponding remanent polarization revealed a time delay between the orientation of the dipoles and their stabilization. It is concluded that the process of orientation of the dipoles itself is not sufficient to lead to a remanent polarization and that an additional interaction between the trapped charges and the oriented dipoles can explain the stability of the remanent polarization and the observed time delay.
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    Polarization profiles of polyvinylidene fluoride films polarized by a focused electron beam
    (1989) Schilling, Doris; Dransfeld, Klaus; Bihler, Eckardt; Holdik, Karl; Eisenmenger, Wolfgang
    The depth profiles of the polarization in films of polyvinylidene fluoride (PVDF) as well as in vinylidene‐fluoride–trifluoroethylene (VDF‐TrFE) copolymer films polarized by a focused electron beam were investigated using the piezoelectrically generated pressure step method. The dominant polarization exhibits a broad maximum inside the film. The position of this maximum depends not only on the energy of the incident electrons but also on the material parameters of the sample. Close to the surface exposed to the electron beam we have in addition observed a small secondary maximum of opposite polarization (amounting to about 1 mC/m2). A qualitative model is presented for the poling of films of PVDF and its copolymers with TrFE by focused electron beam accounting for most of the observed features. The application of electron beams for the poling of ferroelectric films allows the production of piezoelectric bimorphs. By using a well‐focused electron beam also ferroelectric domains of very small lateral dimensions can be created which could become important for ferroelectric data storage.