Astrid Pundt
University of Göttingen
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Featured researches published by Astrid Pundt.
Journal of Alloys and Compounds | 2003
Mohammed Suleiman; N.M. Jisrawi; O. Dankert; M.T. Reetz; C. Bähtz; R. Kirchheim; Astrid Pundt
Abstract In situ X-ray diffraction (XRD) measurements for Pd-clusters (3.8 and 6.0 nm) are performed during hydrogen loading and unloading. The lattice parameter increases as a function of the hydrogen partial pressure. The expansion is smaller than that of bulk palladium and is shown to be cluster-size dependent. An (α–α′) phase transition was observed for the large clusters but small clusters do not show this transition. XRD analysis of the as-prepared clusters show that the 3.8-nm sized clusters predominantly have an icosahedral structure, while the 6.0-nm sized clusters have a cubic structure. The effect of size and structure of the cluster on the lattice expansion and on the phase transition will be discussed.
Journal of Alloys and Compounds | 1999
Astrid Pundt; C. Sachs; M. Winter; M.T. Reetz; D. Fritsch; R. Kirchheim
Abstract Thermodynamic properties of the Pd–H system are studied on 3.1- and 3.6-nm clusters by performing gasvolumetric measurements. To avoid interfacial stress that occurs between an adhering particle and the substrate, the clusters are embedded in an elastic soft polymer matrix and in a surfactant shell, respectively. A largely enhanced H-solubility was found in the α-phase region. Additionally the solubility limit of the α-phase is shifted to a higher H-concentration whereas the minimum α′-phase solubility is lowered compared to that of bulk Pd. This can be attributed to an occupation of subsurface sites. Sorption–desorption isotherms show the existence of a small hysteresis. This hysteresis can not be related to the formation of misfit dislocations because of the small cluster size, but can be explained with regard to the thermodynamics of an open two-phase system with coherent interfaces.
Applied Physics Letters | 2002
Olof Dankert; Astrid Pundt
Hydrogen absorption in a metal leads to significant lattice expansion. This effect can be applied for discontinuous films to make them exceed the percolation limit, turning the film from nonconducting to conducting. Furthermore, in a percolating system, the conductivity should depend strongly on the external hydrogen pressure. This letter discusses the idea of a hydrogen-induced percolation mechanism. The process is illustrated by measurements on agglomerated palladium films, showing a maximum decrease of the resistivity by 50% at an exposure of the films to 5×104 to 9×104 Pa. The agglomerated films are prepared by Joule heating, using heat dissipation in the film itself.
Journal of Alloys and Compounds | 1999
U. Laudahn; Astrid Pundt; M. Bicker; U. v. Hülsen; U. Geyer; T Wagner; R. Kirchheim
Abstract Niobium films prepared by molecular beam epitaxy (MBE) and electron beam evaporation (EB) were loaded electrolytically with hydrogen. Out-of-plane strain and in-plane stresses during hydrogen loading were determined using X-ray diffraction and substrate bending measurements. Stress and strain development during loading can be explained using a one-dimensional elastic expansion model up to concentrations of X H =0.05 H/Nb. Deviations from elastic behavior were observed above X H =0.05 H/Nb and at X H =0.20 H/Nb for MBE and EB samples, respectively. These concentrations are where phase separation occurs. Additionally, the stress increase in the EB films deviates from a linear elastic dependence above X H =0.07 H/Nb within the α-phase. The maximum measured stress is about −2.6 GPa.
Acta Materialia | 2001
M. Maxelon; Astrid Pundt; W. Pyckhout-Hintzen; J. Barker; R. Kirchheim
Abstract Small angle neutron scattering (SANS) measurements on Pd samples containing dislocations with a density of a few 1011 cm−2 reveal an additional intensity for a scattering vector of 0.02 A−1 to 0.2 A−1 after loading with hydrogen (H) or deuterium (D). The corresponding net cross section is inversely proportional to the scattering vector as expected for line type scattering objects with a superimposed exponential decrease stemming from scattering within the Guinier-regime. This experimental finding is in accordance with a model where extended segregation of H or D within the dilated regions of edge dislocations occurs. In a first order approximation this corresponds to a precipitation of cylindrically shaped hydrides along the dislocation line and can be treated quantitatively yielding radii in agreement with SANS data. Whereas gas volumetric measurements at the same total concentration reveal no difference for the amount of H- and D-segregation, there is a pronounced effective difference in SANS intensities which cannot be explained by the different scattering lengths alone. However, the different sign of the latter quantity in combinations with an expected volume expansion within the hydride/deuteride region provides a reasonable explanation of the intensity difference observed. Knowing the amount of segregated H or D from gas volumetry and the dislocation density from electron microscopy the SANS results can be explained in a self consistent way.
Applied Physics Letters | 1999
U. Laudahn; S. Fähler; Hans-Ulrich Krebs; Astrid Pundt; M. Bicker; U. v. Hülsen; U. Geyer; R. Kirchheim
By measuring stress and strain that build up in thin films during hydrogen absorption, the elastic constants of the films can be determined, if a one-dimensional elastic behavior occurs only. This will be demonstrated for hydrogen absorption in Nb films. The in-plane stress is determined from the substrate curvature that is measured by using a two-beam laser setup. The out-of-plane strain is measured via x-ray diffraction. Furthermore, this method allows us to distinguish whether the film is plastically or elastically deformed by checking the reversibility of the stress–strain curve. In the case of a 250-nm-thick Nb film, the elastic constants obtained are similar to that of bulk Nb.
Journal of Applied Physics | 2008
Jakub Čížek; N. Žaludová; Martin Vlach; S. Daniš; J. Kuriplach; I. Procházka; G. Brauer; W. Anwand; D. Grambole; W. Skorupa; Ryota Gemma; R. Kirchheim; Astrid Pundt
Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...
Journal of Applied Physics | 2003
M. Dornheim; Astrid Pundt; R. Kirchheim; S. J. van der Molen; Ernst S. Kooij; J.W.J. Kerssemakers; R. Griessen; H. Harms; U. Geyer
The present drive to make munitions as safe as is feasible and to develop predictive models describing their constitutive response, has led to the development and production of plastic bonded explosives and propellants. There is a range of elastomers used as binder materials with the energetic components. One of these is known as Kel-F-800™ (poly-chloro-trifluroethylene) whose structure is in some ways analogous to that of poly-tetrafluoroethylene (PTFE or Teflon). Thus, it is of interest to assess the mechanical behavior of Teflon and to compare the response of five different production Teflon materials, two of which were produced in pedigree form, one as-received product, and two from previous in-depth literature studies. The equations of state of these variants were quantified by conducting a series of shock impact experiments in which both pressure-particle velocity and shock velocity-particle velocity dependencies were measured. The compressive behavior of Teflon, based upon the results of this study, appears to be independent of the production route and additives introduced.
European Physical Journal B | 1992
Astrid Pundt; Gabriele Hampel; J. Hesse
Amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9 were studied by conversion electron and transmission Mössbauer effect measurements. The Curie temperatureTc of the ferromagnetic amorphous phase was determined to (602±2) K. The formation of nanocrystals from the amorphous alloy can be initiated by annealing the sample at 520 °C. The kinetics of crystallization was observed by annealing the samples for 2, 4, 8, 16, 32 and 60 min and studying the state of the alloy by Mössbauer spectroscopy. The sample annealed for 60 min consisted of about 50% of the amorphous and about 50% crystalline phase. The structure of the crystalline phase is dominated by Fe-Si (DO3) with 21% Si. Other crystalline phases such as Fe-B were found. For the remaining amorphous phase a description is given. It consists of a phase with a hyperfine field distribution like the as-quenched alloy and of a new “emaciated” amorphous phase that develops during the crystallization.
Applied Physics Letters | 2008
Stefan Wagner; Astrid Pundt
Thermodynamic properties of thin films deviate strongly from those of bulk. The deviations are reported to originate from microstructure and from mechanical stress, whereas the contribution of both is unknown in particular. Focussing on the mechanical stress contribution and by using Pd1−xFex–H as a model system, it is shown that mechanical stress strongly changes phase transition pressures. The measured loading pressures shift up to 400mbars in contrast to 18mbars for bulk. These shifts relate to the film bonding to the substrate and can be affected by film detachment.