08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Neuartige PVDF-Dünnschichtwandler für Ultraschallmessungen im GHz-Bereich(1985) Ambrosy, Anton; Holdik, Karl; Eisenmenger, WolfgangWegen seines hohen Piezoeffekts wird auch das Pol:ymer Polyvinylidenfluorid (PVDP) als Ultraschallwandler benutzt. Bisher war es jedoch nicht möglich PVDP-Wandler im GHz-Bereich zu verwenden, da für hochfrequente Anwendungen dünnere und ebenere Folien. als sie kommerziell angeboten werden, notwendig sind. Mit der nun folgenden Methode wurden nun erstmals PVDF-Dünnschichtwandler, die noch bei 24 GHz funktionieren, aus der Lösung hergestellt.Item Open Access Polarization distributions in isotropic, stretched or annealed PVDF films(1988) Bihler, Eckardt; Holdik, Karl; Eisenmenger, WolfgangThe 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.Item Open Access Electric field-induced gas emission from PVDF films(1987) Bihler, Eckardt; Holdik, Karl; Eisenmenger, WolfgangCharge 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.Item Open Access Polarization distributions in isotropic, stretched or annealed PVDF films(1989) Bihler, Eckardt; Holdik, Karl; Eisenmenger, WolfgangThe 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.Item Open Access Observation of charge compensated polarization zones in Polyvinylidenefluoride (PVDF) by piezoelectrically generated pressure step wave (PPS) response(1982) Eisenmenger, Wolfgang; Haardt, Martin; Holdik, Karl-Item Open Access Formung fokussierter akustischer Druckpulse(1987) Staudenraus, Joachim; Holdik, Karl; Eisenmenger, WolfgangDas Schallfeld eines räumlich begrenzten Strahlers besteht bei Freifeldausbreitung aus einer "direkten geometrischen Welle" und der "Randbeugungswelle" mit entgegengesetztem Vorzeichen. Daher ist die Abstrahlung eines "reinen Überdruckimpulses" stets von einem nachfolgenden Unterdrucksignal begleitet. Aus den Hochpaßeigenschaften eines Strahlers endlicher Dimension folgt ebenso, daß das Zeitintegral von Überdruckpuls und Unterdrucksignal insgesamt verschwindet. Bei medizinisch eingesetzten fokussierenden Druckpulsgeneratoren kann daher z.B. die Amplitude des immer vorhandenen Unterdrucksignals zur Vermeidung von Kavitation und Gewebeschäden durch eine möglichst große zeitliche Dehnung herabgesetzt werden. Die geringste Unterdruckamplitude wird in diesem Sinne durch eine radial quadratisch abfallende Druckanregung in der Fläche des Strahlers sowie durch einen möglichst sägezahnförmigen zeitlichen Druckverlauf mit steiler Front erzielt. Hierzu wurden theoretische und experimentelle Untersuchungen an einem Modellstrahler aus PVDF-Piezofilm sowie einem selbstfokussierenden elektromagnetischen Stoßwellengenerator durchgeführt, wobei über die Resultate an letzterem System im Folgenden berichtet wird.Item Open Access Untersuchung von dielektrischer Polarisation und Ladungstransport in Polymerfilmen mit Hilfe der akustischen Drucksprungmethode PPS(1985) Holdik, Karl; Eisenmenger, WolfgangDie mit der Drucksprungmethode PPS gewonnenen Resultate über die zeitliche Entwicklung der Polarisation in Elektrofolien weisen als Ursachen inhomogener Piesoprofile vor allem elekrische Transportphänomene auf. Mechanisch-strukturelle Gründe sind dagegen von geringerer Bedeutung.Item Open Access Quasiparticle recombination time of superconducting tin films in a parallel magnetic field(1985) Holdik, Karl; Welte, Michael; Eisenmenger, WolfgangThe 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.Item Open Access Investigation of piezoelectricity distributions in poly(vinylidene fluoride) by means of quartz- or laser-generated pressure pulses(1984) Gerhard-Multhaupt, Reimund; Sessler, Gerhard M.; West, James E.; Holdik, Karl; Haardt, Martin; Eisenmenger, WolfgangThe 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.Item Open Access Charge and polarization dynamics in polymer films(1985) Holdik, Karl; Eisenmenger, WolfgangThe PPS-method has been shown to be a powerful instrument to reveal the polarization evolution within a dielectric under a forming field. Because the final polarization found in a material is not generally a monotonic function of the formation time this can be used to control the polarization pattern by variation of external parameters during the process. The reason for inhomogenious polarization patterns and thus the cause of inhomogenious piezo- (and pyroelectric) reply of the examined dielectric materials was found to be predominantly charge carrier transport.