Eckardt Bihler

Polarization dynamics of VDF-TrFE copolymers

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. >

Electric Field-Induced Gas Emission from PVDF Films

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 th prtmeable electrode negatively, gas emission was found mainly consisting of hydrogen, hydrogen fluoride, and fluorine. For comparison FEP-and PET-films were examined.

Influence of charge injection on the formation of remanent polarization in P(VDF-TrFE) copolymers

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.<<ETX>>

Polarization distributions in isotropic, stretched or annealed PVDF films

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 beta crystallite content, determines the spatial distribution of the permanent polarization in PVDF. >

Polarization profiles of polyvinylidene fluoride films polarized by a focused electron beam

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.

Dynamics of the polarization growth in P(VDF-TrFE) copolymers

The formation of the dielectric displacement under an applied electric field and the remanent polarization after switching-off of the electric field is studied in P(VDF-TrFE) copolymers as a function of the duration of the electric poling field. A time delay is observed between the orientation of the dipoles and their stabilization, leading to remanent polarization. This time delay is reduced with increasing crystallinity of the films. The poling time necessary to saturate the remanent polarization is determined and found to be dependent on the poling field strength.<<ETX>>

Dynamics of polarization growth and polarization reversal in PVDF films

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 mu s 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- mu m-thick films containing 37% beta -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.<<ETX>>

Polarization distribution in PVDF obtained by poling under constant current condition

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.<<ETX>>

Space charge and polarization in PETP at different temperatures

PETP (polyethyleneterephthalate) samples were poled at constant voltage and at different temperatures. Charge injection and orientation of dipoles as a function of the poling time at temperatures above 60 degrees C were measured with the PPS method under short circuit conditions. At room temperature the temporal injection and migration of space charges were observed. After long poling times a characteristic distribution of charges was observed. For temperatures above 60 degrees C, a strong additional orientation of dipoles in the field direction was observed. In cooling down to room temperature under field the dipoles remained oriented with strong piezoelectricity of the film. A few days later the injected space charges in excess of dipole compensation vanished. Most of the dipoles remained oriented for several months.<<ETX>>

Polarization profiles of electron-beam polarized VDF-TrFE copolymer films

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/m/sup 2/, which exceeds those produced in beta -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/sub a/ having its steepest rise around T/sub c/.<<ETX>>