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Publication Type: search.filters.UBSTyp.KonferenzbeitragSubject: search.filters.subject.530Author: search.filters.author.Eisenmenger, WolfgangAuthor: search.filters.author.Bihler, Eckardt

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Now showing 1 - 5 of 5
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    Polarization distribution in PVDF obtained by poling under constant current condition
    (1990) Neumann, Guido; Bihler, Eckardt; Eberle, Gernot; Eisenmenger, Wolfgang
    Since 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.
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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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    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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    Influence of charge injection on the formation of remanent polarization in P(VDF-TrFE) copolymers
    (1990) Bihler, Eckardt; Neumann, Guido; Eberle, Gernot; Eisenmenger, Wolfgang
    The 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.
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    Polarization profiles of electron-beam polarized VDF-TrFE copolymer films
    (1990) Schilling, Doris; Glatz-Reichenbach, Joachim; Dransfeld, Klaus; Bihler, Eckardt; Eisenmenger, Wolfgang
    In order to understand more clearly the poling mechanism in ferroelectric polymers, the PPS-technique was applied to copolymer films of vinylidenefluoride with trifluoroethylene P(VDF-TrFE), poled by a focused monoenergetic electron beam. Charges were injected in a well-defined way into the polymer films and thus provided information on the influence of externally introduced electrical charges on the poling process in ferroelectric polymers. The electron beam poling of P(VDF-TrFE) films is shown to produce very high values of polarization up to 180 mC/m2, which exceeds those produced in β-PVDF by electron irradiation by a factor of three. The distribution of the polarization across the film thickness is rather asymmetric, especially for samples irradiated with 20- and 30-KeV electrons. This may be caused by the rising electrical conductivity in samples with increasing electron energy. The annealing of the copolymer films before the poling procedure leads to a systematic increase of the polarization, with T a having its steepest rise around Tc.