Browsing by Author "Eisenmenger, Wolfgang"

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Open Access
Akustische Ortung kollabierender Kavitationsblasen in Wasser und in Gewebephantomen
(1993) Staudenraus, Joachim; Köhler, Michael; Lünstroth, Uwe; Eisenmenger, Wolfgang
Für moderne Ultraschall- und Stoßwellenquellen, wie sie z.B. medizintechnisch in der Sonographie und in der Lithotripsie eingesetzt werden, beobachtet man in Wasser intensive Kavitation. Unter Einwirkung des äußeren Drucks implodieren die als Folge hoher Unterdruckpulsanteile gebildeten Kavitationshohlräume, ein Vorgang der als Blasenkollaps bezeichnet wird. Mittels einer geeigneten Anordnung mehrerer Hydrophone gelingt es, Kavitationsblasen durch Rekonstruktion des Ausgangspunktes der bei Blasenkollaps abgestrahlten Stoßwellen zu orten.
Open Access
Akustische Schichtmikroskopie durch Pulsmessungen im ns-Bereich
(1982) Eisenmenger, Wolfgang
Im folgenden soll über Untersuchungen von Ladungs- und Polarisationsschichtungen in dielektrischen Folienmaterialien berichtet werden, sowie über deren zeitliche Entwicklung in der Polarisationsphase. Ein erster Vorschlag, akustische Sprungfunktionen oder Druckpulse für eine Analyse von Ladungsverteilungen einzusetzen, stammt von Laurengeau und Mitarbeitern. Zum Zeitpunkt des Vorschlages existierten noch nicht ausreichend gute elektronische und akustische Meßverfahren, um die erforderliche hohe Auflösung realisieren zu können. Inzwischen ist durch die Verfügbarkeit von Verstärkern im GHz-Bereich, aber auch durch neue Methoden der Anregung von akustischen Druckstufen bzw. von Druckpulsen eine Analyse von Schichtstrukturen im Auflösungsbereich von μm möglich.
Open Access
Analysis of the acoustlc transients in the pulse response of the linear electro-optic effect
(1976) Veeser, Hermann; Bogner, Udo; Eisenmenger, Wolfgang
The pulse response of the linear electro-optic effect is investigated with high temporal and spatial resolution in KDP, KD*P, and biaxial LiCOOH · H2O. Experimental results, which are explained theoretically, show that the piezo-optic transients induced by the piezoelectric effect, are generated by stress release waves. Starting from the surfaces, these excite by reflection, diffraction, and superposition, due to the finite crystal dimensions, normal vibration modes too. It is shown that the direct electro-optic effect can be measured for all crystal classes at the beginning of the pulse response. The pulse method also yields static electro-optic, piezo-optic, and elastic constants. The values of r63, r63, p66 and c66 measured in KDP, agree well with those of other authors. Measurement of the direct electro-optic effect of LiCOOH · H2O in some directions yields values within 1 to 4 × 10-12 m/V.
Open Access
Analysis of the phonon spectrum emitted by superconducting Al-tunneling junctions at high quasiparticle injection rates
(1981) Welte, Michael; Eisenmenger, Wolfgang
The phonon spectrum emitted by superconducting Al-tunneling junctions is analyzed by experiments, in which two junctions of different energy gaps are used as phonon generator and detector. The energy gap of the Al-films is varied by the evaporation conditions. The experiments show clearly that the phonon spectrum and the quasiparticle distributions in the generator are strongly nonthermal and depend markedly on the primary tunneling injection. At high injection rates also significant consequences of gap reductions and instabilities are observed.
Open Access
Anomalous recovery of the remanent polarization after heating polarized PVDF films up to 180°C
(1992) Eberle, Gernot; Eisenmenger, Wolfgang
Biaxially stretched 38-μm-thick PVDF (polyvinylidene fluoride) samples with about 80% β content were poled at room temperature at E=200 MV/m for 5 s. The remanent polarization was measured with the PPS method first at 20°C, then after heating to 180°C, and a third time after cooling the sample to room temperature again, to determine its temperature-dependent stability. To distinguish between irreversible and reversible polarization loss due to heating of polarized PVDF films, the polarization was measured at elevated temperatures and after cooling down to room temperature again. The reversible polarization loss causes the pyroelectricity of PVDF.
Open Access
Ausbreitung freifeldfokussierter Hochenergiedruckimpulse in Wasser
(1991) Staudenraus, Joachim; Eisenmenger, Wolfgang
Im Zuge der klinischen Einführung von Stoßwellengeneratoren zur Zertrümmerung von Harnsteinen sind in jüngerer Zeit physikalische Fragestellungen zur Fokussierung hochenergetischer Druck- und Stoßwellenimpulse in Flüssigkelten auf starkes Interesse gestoßen. Schalldruckimpulse hoher Amplitude erfahren dabei als Folge nichtlinearer Ausbreitung eine zunehmende Aufstellung der Wellenfront. Für die Fokussierungsexperimente steht eine elektromagnetische Impulsschallquelle in Kalottenform zur Verfügung.
Open Access
Binding energies of trapped charges in PVDF and P(VDF/TrFE)
(1994) Küssner, Bodo; Dehlen, Bernhard; Eberle, Gernot; Eisenmenger, Wolfgang
The remanent polarization in PVDF and P(VDF/TrFE) is stabilized by trapped charges. We measured the remanent polarization P as function of temperature during heating from room temperature to 180°C. The discharge current is then computed as the first derivative -dP/dT. Assuming a Debye relaxation and a continuous distribution of binding energies g(E) of the trapped charges we can show that the discharge current I(T) is proportional to g(E=mT), with m=const. The constant m can either be calculated numerically or by analytical approximation, With this method the distribution g(E) can be determined. These data are consistent with the extraordinary long life time of the remanent polarisation in PVDF and P(VDF/TrFE) at room temperature.
Open Access
Biological effects and physical characterization of shock waves by an XL-1 experimental lithotripter
(1989) Brümmer, Franz; Staudenraus, Joachim; Nesper, Martina; Suhr, Dierk; Eisenmenger, Wolfgang; Hülser, Dieter F.
Extracorporeal shock wave lithotripsy (ESWL) has become the clinical standard method for non-invasive disintegration not only of concrements in kidney and urinary tract but also of gallstones. Despite the widespread clinical use of ESWL, the mechanism of stone destruction is not yet really understood, but several possibilities (cavitation, shock wave reflection) are discussed. The final cause of various side effects is still under discussion. Nevertheless, during the last few years much effort was put into possible extensions of ESWL applications On the other hand , physical characterizations of shock waves are rarely reported and combined measurements of biological effects in vitro and physical characterization of the applied shock waves are not available. We, therefore, examined the influence of water temperature and gas content on the shock wave efficency in biological systems and determined several physical characteristics (pressure amplitudes, rise time etc.) of the shock waves under the same experimental conditions.
Open Access
Charakterisierung verschiedener Hydrophone unter Stosswellenbedingungen in Wasser
(1991) Staudenraus, Joachim; Köhler, Michael; Eisenmenger, Wolfgang
Als Drucksonde wird eine Glasfaser in das akustische Wellenfeld eingeführt und die durch zeitliche Druckänderung hervorgerufene zeitliche Variation der Lichtreflexion als Hydrophonsignal verwendet. Im gut reproduzierbaren Stoßwellensignal des optischen Sondenhydrophons kann Kavitation auf oder in nächster Nähe der Faserstirnfläche, aufgrund des wesentlich höheren Reflexionskoeffizienten gegen Wasserdampf, ausgeschlossen werden. Im Unterschied zu den anderen Hydrophonen ist somit hier von einer vollständigen Unterdruckübertragung auszugehen. Im übrigen stimmen die Stoßwellensignale des optischen Sondenhydrophons und des Membranhydrophons solange überein, bis im Signal des letzteren kavitationsbedingt der Unterdruckanteil verschwindet.
Open Access
Charge and polarization dynamics in polymer films
(1985) Holdik, Karl; Eisenmenger, Wolfgang
The 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.
Open Access
Diffusive scattering of high-frequency phonons at free silicon surfaces
(1983) Marx, Dieter; Eisenmenger, Wolfgang
Comparing measurements of high-frequency phonon reflection at the uncovered and optically polished (100)-silicon surface, with calculations considering phonon focusing, reveal complete diffusive scattering with at most 4% specular reflection contribution. Two possible mechanisms causing diffusive scattering are discussed.
Open Access
Down-conversion of high-frequency acoustic phonons
(1987) Galkina, Tatjana I.; Blinov, A. Y.; Bonch-Osmolovskii, M. M.; Koblinger, Otto; Lassmann, Kurt; Eisenmenger, Wolfgang
Measurements of phonon transport in amorphous media can give valuable information on the structural properties of these materials and may be of practical interest for its own concerning the question of thermalization in electronic devices. The existence of two-level systems in a-Si:H as one of these technically important materials has been concluded from measurements of dispersion and attenuation of acoustic surface waves.
Open Access
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.
Open Access
Dynamics of polarization growth and reversal in PVDF films
(1989) Womes, Manfred; Bihler, Eckardt; Eisenmenger, Wolfgang
Measurements are presented of the time development of the dielectric displacement and the remanent polarization in polyvinylidene fluoride (PVDF) for poling times ranging from 1 μs to 1000 s and poling fields between 0.8 and 2.0 MV/cm. For longer times (0.1 to 1000 s), the time dependence of the polarization distribution across the film thickness is also determined. After application of a steep rectangular high-voltage pulse, the sample is shorted to zero voltage. The remanent polarization under the short-circuit conditions is compared to the maximum dielectric displacement under the external poling field. A significant time delay of the buildup of the remanent polarization was observed as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal, a flipping back' of the polarization was observed for shorter poling times of up to 200 μs. Under these conditions, a large part of the polarization is reversed under the field, but after the removal of the field, most of the polarization returns to the original direction. The results can be explained by the ferroelectric cooperative coupling of oriented crystallite dipoles to charges trapped at the surface of polarized crystallites.
Open Access
Dynamics of poling PVDF between 25°C and 120°C
(1991) Eberle, Gernot; Eisenmenger, Wolfgang
PVDF films with high β-content are poled in an electric field of 60 MV/m at temperatures between 25°C and 120°C. At 25°C the alignment of dipoles takes place in a central poling zone during several hours of poling. When the temperature is increased to 120°C the poling time necessary to align the dipoles in this narrow zone is reduced to several seconds. In addition, at temperatures higher than 90°C and increased poling times the central poling zone first increases but later decreases in its dielectric displacement. Simultaneously in a 10 μm regime adjoining the positive electrode a secondary displacement zone starts to grow. The strong reduction of this peak under short circuit conditions indicates hetero-charge accumulation in front of an anode.
Open Access
Effect of laser annealing on specular and diffuse scattering of 285 GHz phonons at polished silicon surfaces
(1986) Mok, Erich; Burger, Susanne; Döttinger, Siegfried; Lassmann, Kurt; Eisenmenger, Wolfgang
We have investigated the time resolved backscattering of high frequency phonons (greater-or-equal, slanted 285 GHz) at laser annealed silicon surfaces at low temperatures. It is found that the scattering off the free surface becomes predominantly specular up to frequencies well above 285 GHz and that the anomalous transmission into liquid helium (Kapitza effect) is strongly reduced.
Open Access
Effective quasiparticle recombination times and electronic density of states at the Fermi level in superconducting films
(1978) Epperlein, Peter W.; Lassmann, Kurt; Eisenmenger, Wolfgang
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Open Access
Electric field profiles in electron-beam-charged polymer electrets
(1983) Gerhard-Multhaupt, Reimund; Haardt, Martin; Eisenmenger, Wolfgang; Sessler, Gerhard M.
A recently developed method, which uses piezoelectrically generated pressure steps for the determination of electric-field profiles in dielectrics, has been applied to electron-beam-charged polyfluoroethylenepropylene (FEP) and polyethyleneterephthalate (PETP) electrets. The results indicate that the technique can be employed to study volume charge effects in thin dielectrics. If properly calibrated, the method provides a quantitative measure of charge-integral functions or electric-field distributions in polymer foils.
Open Access
Electric field-induced gas emission from PVDF films
(1987) Bihler, Eckardt; Holdik, Karl; Eisenmenger, Wolfgang
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.
Open Access
Electric-field profiles in corona- or electron-beam-charged and thermally treated Teflon PTFE, FEP, and PFA films
(1992) Gerhard-Multhaupt, Reimund; Eberle, Gernot; Xia, Zhongfou; Yang, Guomao; Eisenmenger, Wolfgang
Charge spreading in three different types of Teflon electrets was studied by means of piezoelectrically generated pressure steps, FEP and PFA samples corona-charged at room temperature usually exhibited only a surface charge layer. Uniform charge spreading throughout the bulk was found in FEP charged at or heated to high temperatures. Charge spreading was much less prominent in PFA because of a smaller retrapping efficiency. In PTFE (polytetrafluorethylene), charges from the surface and the rear electrode were injected into the bulk during charging at any temperature. Electron-beam-deposited charge layers broadened significantly upon heating.