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The spatial distribution of the piezoelectric coefficient in polarized PVDF has been investigated by several authors using different methods / 1-6/. In addition also the time deve lopment of the polarization distribution in PVDF under external electfic fields was studied for a variety of commercially available PVDF films from different suppliers using the piezoelectric pressure step (PPS) technique / 7 J. For a field strength ranging from 0.5 to 1 MV fem, samples from different suppliers developped either i) a centra! polarization zone with no piezoeffect in the boundary regions close to the fi lm surface or Ii) a polarization maximum close to the positive charg ing electrode (anode) without polarization from about the centre of the film to the negative electrode. Since sample inhomogeneities can be excluded , the development of polarization zones are to be explained by charge injection and trapping / 3,9 f.The different behaviour i) and ii) in principle must be attributed either to possible differences in the general chem ical composition of the films of different suppliers or to differences in the crystall ite structu re. So far it was found / 11 / that the central polarization zone i) appeared in films with a finite content o f /3-form crystallites, whereas the polarization maximum at the positive electrode ii) appeared for pure a-material. In order to check the possible influence of the crystall ite structure independently from the' chemical sample composition, we investigated the po larizatron distributions for the same material as received first with dominant a-crystallite content. then after biaxially stretching in order to increase the ,8-crvstallite content and flnaiiy after anneal ing again in order to reduce the /3-crystallite content. Experimental TechniqueThe spatial distribution of the polarization in PVDF ~i lms was measured at room temperature with the PPS-method / 3.7 f. In order to investigate the time development under external fields a thin insulating PET -film covering an evaporated aluminium electrode was inserted between sample and the measuring electrode. This additional polymer ads as a broadband coupling capacitor to the measured displacement current. It does not influence the shape of the signal. The crystallite phase composition of the PVOF films was determined by IR absorption. The ratio between the relative absorption at 530 em-1 (a-crystallites) and 510 cm-1 t,B -crystalli1es) is taken as a standard for the a/,B-phase content (according to /8/ ).

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.

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.

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.