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Publication Type: search.filters.UBSTyp.KonferenzbeitragStart date: 1977Institute: search.filters.UBSInstitut.1. Physikalisches Institut

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    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.
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    Phononenphysik : Akustik bei Terahertz-Frequenzen
    (1985) Eisenmenger, Wolfgang
    Mechanische oder akustische Schwingungen und Wellen in Flüssigkeiten und Festkörpern sind wie elektromagnetische Wellen quantisiert und man bezeichnet die entsprechenden Energiequanten als akustische Phononen. Für Existenz und Ausbreitung dieser mechanischen Wellen besteht wegen des atomaren Aufbaus der Materie eine obere Frequenzgrenze, die je nach Substanz im Bereich bis etwa 10 THz liegt. Die Akustik bei THz-Frequenzen ist daher eine Erweiterung des Ultraschallbereichs zu den höchstmöglichen Frequenzen und dient der Untersuchung von Wellenausbreitung, Absorption und Streuung im Volumen oder an Grenzflächen von Körpern. Streuung und Absorption werden u.a. durch Resonanzübergänge bei Fremdatomen oder Gitterdefekten, z.B. Versetzungen verursacht. Solche Untersuchungen gehören zur akustischen Absorptionsspektroskopie, wobei die Nachweisempfindlichkeit um einige Größenordnungen höher sein kann als die im vergleichbaren Frequenzbereich liegende Ferninfrarotspektroskopie.
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    Phonon detection by the fountain pressure in superfluid 4Helium films
    (1980) Eisenmenger, Wolfgang
    The importance of phonon focussing was first demonstrated by Taylor, Maris and Elbaum, who calculated the phonon intensities or different modes in single crystals from the detailed form of the angle or k-dependent sound velocity surfaces. The results of these calculations are well-confirmed by experiment. It is quite surprising that phonon focussing leads in several crystals to very narrow beams of phonon energy propagation especially for transverse modes, as recently calculated by Rosch and Weis, who presented their computer results in a very instructive form. From these theoretical results it appears worthwhile to devise methods by which the intensity distribution of incoherent phonons propagating in single crystals can be measured in detail or directly imaged.
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    Resonance and relaxation attenuation by neutral acceptors in a magnetic field
    (1977) Schad, Hanspeter; Lassmann, Kurt; Zeile, Heinrich
    Fine structure, level splitting, and relaxation times of the acceptor ground state in cubic semiconductors can be deduced from ultrasonic attenuation. By the application of a magnetic field it is possible to vary the coupling strength and the number of acceptors on speaking terms with the ultrasonic wave. Making use of this possibility, we have determined directly effective coupling constants for relaxation attenuation and the temperature dependence of the critical intensity for saturation of the resonant interaction. Results obtained for p-type GaAs and Si are discussed in comparison with analogous results for glasses.
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    Formung fokussierter akustischer Druckpulse
    (1987) Staudenraus, Joachim; Holdik, Karl; Eisenmenger, Wolfgang
    Das 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.
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    Phono-conduction spectroscopy of shallow states in semiconductors
    (1990) Lassmann, Kurt; Gienger, Martin; Groß, Peter
    Phonoconduction spectroscopy of shallow states in semiconductors by help of superconduction Al-junctions as phonon sources with high spectral resolution is a sensitive new technique with phonon frequencies well up in the Debye range. The large k-vector of the phonons involved opens new possibilities for the investigation of electron dynamics and states in semiconductors.
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    Field induced gas emission of polymer films
    (1992) Selle, Michael; Eberle, Gernot; Gompf, Bruno; Eisenmenger, Wolfgang
    Poling polyvinylidene fluoride under vacuum conditions causes strong gas emission, which is analyzed with a quadrupole mass spectrometer. The dominant peak in the mass spectrum is HF, which is produced by electrochemical reactions at the sample surface. After the electric field is switched off, the gas emission unexpectedly increases to a value higher than under the field. This can be explained by ionic charge detrappingand charge migration
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    Spectral dependence of the Kapitza resistance between O,5 K and 2,3 K
    (1984) Koblinger, Otto; Dittrich, Ehrenfried; Heim, Ulrich; Welte, Michael; Eisenmenger, Wolfgang
    Summarizing, we observed strongly enhanced phonon transport from a real solid into 4 He starting at about 85 GHz which is independent of the He gas pressure up to liquid helium. With the isotope 3 He the onset frequency for enhanced phonon transport shifts to 100 GHz. The threshold frequencies are almost independent of the He temperature in the range between 0.5 K and 2.3 K. Only slight shifts to lower frequencies with decreasing He temperature have been observed. No significant influence of the generator material (Sn, Al, SiO) or the generator preparation on the onset frequency has been observed. Moreover at low temperatures and thin He films we observed an additional structure which might be interpreted as film thickness resonances in the He film layer on the generator.
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    Shallow traps correlated with deep impurities in silicon as obtained by phonon induced conductance
    (1986) Burger, Wilfried; Lassmann, Kurt; Holm, Claus; Wagner, Peter
    At low temperatures shallow neutral donors and acceptors in silicon can bind an extra carrier to form the so-called D- and A+ centers. With the method of phonon-induced electrical conductivity (PIC) we find the same threshold energies for the detachment of these carriers associated with the shallow impurities P and B, as have been obtained previously by FIR measurements. This shows that the detachment is by a one-phonon process. We find that there is no central cell correction for the binding to the deeper acceptors Al and Ga, whereas for In+ the binding energy is as large as 5,8 meV. We interprete this dependence on acceptor species as another example of the shallow-deep instability of the binding energy with the variation of the central cell potential.