Karl Holdik

Investigation of piezoelectricity distributions in poly(vinylidene fluoride) by means of quartz‐ or laser‐generated pressure pulses

The direct determination of charge, polarization, or piezoelectricity profiles in thin dielectrics is now possible if piezoelectrically generated pressure steps or pulses and laser‐induced pressure pulses are employed. These recently developed high‐resolution methods were applied to the same piezoelectric poly(vinylidene fluoride) (PVDF) samples. Comparison of the respective results demonstrates the feasibility, the advantages, and the limitations of the new techniques for the study of piezoelectricity distributions. It is confirmed that, for relatively low poling fields, the piezoelectric activity of thermopoled PVDF foils is often confined to a layer near the positively biased surface. The same effect is found for poling with a positive corona discharge. For high‐field corona poling, the piezoelectric activity extends throughout the PVDF film.

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.

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.

Untersuchung von dielektrischer Polarisation und Ladungstransport in Polymerfilmen mit Hilfe der akustischen Drucksprungmethode PPS

Die mit der Drucksprungmethode PPS gewonnenen Resultate uber die zeitliche Entwicklung der Polarisation in Elektrofolien weisen als Ursachen inhomogener Piesoprofile vor allem elekrische Transportphanomene auf. Mechanisch-strukturelle Grunde sind dagegen von geringerer Bedeutung.

Quasiparticle recombination time of superconducting tin films in a parallel magnetic field

The influence of a parallel magnetic field on the quasiparticle recombination time of superconducting tin tunneling junctions has been determined experimentally. The results are compared with the predictions of a theoretical model that takes into account the modification of the quasiparticle density of states by the applied magnetic field.

Investigation of piezoelectricity distribution in PVDF by means of pressure-pulse techniques

Abstract Charge, polarization or piezoelectricity profiles in thin dielectrics can be directly determined by means of piezoelectrically-generated pressure steps1-3 (PPSs) or pulses (PPPs) and laser-induced pressure pulses4-6 (LIPPs). These recently developed high-resolution methods have been applied to the same piezoelectric PVDF samples, demonstrating the feasibility, advantages, and limitations of the new techniques for the study of piezoelectricity distributions. For relatively low poling fields, the piezoelectric activity of thermopoled PVDF films is often confined to a layer near the positively-biased surface. The same effect is observed for poling with a positive corona discharge. With high-field corona poling, the piezoelectric activity extends throughout the PVDF film.