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Subject: search.filters.subject.530Author: search.filters.author.Eisenmenger, WolfgangAuthor: search.filters.author.Bihler, EckardtStart date: 1987End date: 1989

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Now showing 1 - 6 of 6
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    Polarization distributions in isotropic, stretched or annealed PVDF films
    (1988) Bihler, Eckardt; Holdik, Karl; Eisenmenger, Wolfgang
    The spatial distribution of the polarization in polyvinylidene fluoride (PVDF) films was measured at room temperature with the PPS (piezoelectric pressure step) method. In order to investigate the time development under external fields, a thin insulation polyethylene terephthalate (PET) film covering an evaporated aluminium electrode was inserted between the sample and the measuring electrode. The observed development of inhomogeneous and internal polarization zones is attributed to charge injection and charge trapping at the polarization zone boundaries. The polarization zone develops at a position where the critical field strength for dipole orientation in the crystallites is exceeded by the approach of injected homocharges (or without injection by the depletion of internal homocharges and the excess of heterocharges). The critical field for α-crystallites corresponds to an electric field phase transition at 1.2 MV/cm. The development of central polarization zones in PVDF containing β-crystallites indicates injection of charges with both signs and almost equal mobility. The results indicate that the β-crystallites determine the charge injection rate or the mobility or both.
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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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    ItemOpen Access
    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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    ItemOpen Access
    Dynamics of polarization growth and polarization reversal in PVDF films
    (1988) Womes, Manfred; Bihler, Eckardt; Eisenmenger, Wolfgang
    The authors present measurements of the maximum displacement at the end of a rectangular high-voltage pulse together with the remanent polarization after the pulse under short-circuit condition as a function of the polarizing pulse end length. The pulse length varied from 10 μs to 100 s, and the field strength ranged from 0.8 MV/cm to 2 MV/cm. The original samples were reversely polarized or unpolarized 12-μm-thick films containing 37% β-material. It is concluded that the cooperative model combined with six-site potential alone cannot explain the observed switching back of the remanent polarization, its delayed development, and the long times necessary for polarization reversal. The results are qualitatively consistent with a model of charge injection and charge trapping at polarized crystallites.
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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.