Garching bei München, Germany
Garching bei München, Germany

Time filter

Source Type

Laurati M.,Rhodia | Laurati M.,Heinrich Heine University Düsseldorf | Sotta P.,Rhodia | Long D.R.,Rhodia | And 6 more authors.
Macromolecules | Year: 2012

We investigate the dynamics of water absorbed in amorphous and semicrystalline aromatic polyamide copolymers. The combination of dielectric spectroscopy and quasi-elastic neutron scattering experiments allows us to characterize the water dynamics over a wide range of temperatures (dielectric spectroscopy) and at microscopic length scales (neutron scattering). The dielectric investigation evidences two relaxations associated with water motions: a fast process corresponding to motions of loosely bonded water molecules and a slower process corresponding to motions of amide-water complexes. While the slower process presents the characteristic Arrhenius temperature dependence of a secondary local relaxation over the whole temperature range, the fast process shows a crossover from Arrhenius to Vogel-Fulcher-Tamman (VFT) behavior at T ≈ 225 K, characteristic of confined water dynamics. The microscopic investigation by neutron scattering shows than in the VFT regime of the fast process the dynamics present a diffusive nature similar to bulk water. A large distribution of diffusion coefficients indicates possible differences in the connectivity of the hydrogen bond network. Diffusive heterogeneous dynamics can arise from a nonuniform distribution of water. The confinement effect of the polymer matrix is detected as a considerable reduction of the diffusion coefficient of water with respect to bulk. The presence of a crystalline phase results in a slowing down of both the fast and slow processes involving water motions. This could give a hint to the presence of a rigid amorphous phase in the semicrystalline material. © 2012 American Chemical Society.

Schobel M.,Vienna University of Technology | Altendorfer W.,Vienna University of Technology | Degischer H.P.,Vienna University of Technology | Vaucher S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | And 3 more authors.
Composites Science and Technology | Year: 2011

Metal-matrix composites (MMC) are being developed for power electronic IGBT modules, where the heat generated by the high power densities has to be dissipated from the chips into a heat sink. As a means of increasing long term stability a base plate material is needed with a good thermal conductivity (TC) combined with a low coefficient of thermal expansion (CTE) matching the ceramic insulator. SiC particle reinforced aluminum (AlSiC) offers the high TC of a metal with the low CTE of a ceramic. Internal stresses are generated at the matrix-particle interfaces due to the CTE mismatch between the constituents of the MMC during changing temperatures. Neutron and synchrotron diffraction was performed to evaluate the micro stresses during thermal cycling. The changes in void volume fraction, caused by plastic matrix deformation, are visualized by synchrotron tomography. The silicon content in the matrix connecting the particles to a network of hybrid reinforcement contributes essentially to the long term stability by an interpenetrating composite architecture. © 2011 Elsevier Ltd.

Schobel M.,Vienna University of Technology | Degischer H.P.,Vienna University of Technology | Vaucher S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Hofmann M.,Forschungsneutronenquelle Heinz Maier Leibnitz | Cloetens P.,European Synchrotron Radiation Facility
Acta Materialia | Year: 2010

Aluminum reinforced by 60 vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC ≈ 1000 W mK-1) and low coefficient thermal expansion (CTE ≈ 1 ppm K -1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was determined in situ by neutron diffraction during thermal cycling between room temperature and 350 °C (soldering temperature). Tensile stresses build up in the Al/CD composites: during cooling <100 MPa in a pure Al matrix, but around 200 MPa in the Al in an AlSi7 matrix. Compressive stresses build up in Al during heating of the composite. The stress evolution causes changes in the void volume fraction and interface debonding by visco-plastic deformation of the Al matrix. Thermal fatigue damage has been revealed by high resolution synchrotron tomography. An interconnected diamond-Si 3D network formed with an AlSi7 matrix promises higher stability with respect to cycling temperature exposure. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Li Y.,Stanford University | Li Y.,Max Planck Institute for Solid State Research | Yu G.,University of Minnesota | Chan M.K.,University of Minnesota | And 14 more authors.
Nature Physics | Year: 2012

There exists increasing evidence that the phase diagram of the high-transition temperature (T c) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO 2 square. Below the 'pseudogap' temperature T* (T* >T c), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa 2 CuO 4+δ the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description. © 2012 Macmillan Publishers Limited. All rights reserved.

Yu G.,Stanford University | Li Y.,Stanford University | Motoyama E.M.,Stanford University | Zhao X.,Stanford University | And 10 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Inelastic neutron-scattering measurements of single- CuO2-layer HgBa2 CuO4+δ reveal an antiferromagnetic resonance with energy ωr =56meV (≈6.8 kBTc) below the superconducting transition temperature Tc ≈96K. The resonance is energy-resolution limited and exhibits an intrinsic momentum width of about 0.2 Å-1, consistent with prior work on several other cuprates. The rather large value of ωr is identical to the characteristic energy of the electron-boson spectral density obtained from recent optical conductivity work, consistent with the notion that the charge carriers are strongly coupled to magnetic fluctuations. © 2010 The American Physical Society.

Stock C.,University of Edinburgh | Dunsiger S.R.,TU Munich | Mole R.A.,Forschungsneutronenquelle Heinz Maier Leibnitz | Mole R.A.,Australian Nuclear Science and Technology Organisation | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

A neutron scattering investigation of the magnetoelectric coupling in PbFe1/2Nb1/2O3 has been undertaken. Ferroelectric order occurs below 400 K, as evidenced by the softening with temperature and subsequent recovery of the zone center transverse optic phonon mode energy (âΩ0). Over the same temperature range, magnetic correlations become resolution limited on a terahertz energy scale. In contrast to the behavior of nonmagnetic disordered ferroelectrics [namely Pb(Mg,Zn)1/3Nb2/3O3], we report the observation of a strong deviation from linearity in the temperature dependence of (âΩ0 )2. This deviation is compensated by a corresponding change in the energy scale of the magnetic excitations, as probed through the first moment of the inelastic response. The coupling between the short-range ferroelectric and antiferromagnetic correlations is consistent with calculations showing that the ferroelectricity is driven by the displacement of the body-centered iron site, illustrating the multiferroic nature of magnetic-lead-based relaxors in the dynamical regime. © 2013 American Physical Society.

Yu G.,Stanford University | Yu G.,University of Minnesota | Li Y.,Stanford University | Li Y.,Max Planck Institute for Solid State Research | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Neutron scattering for Nd2-xCexCuO 4+δ (x≈0.155, Tc =25 K) reveals two distinct magnetic energy scales in the superconducting state: ω1 ≈6.4 meV and ω2 ≈4.5 meV. These magnetic energies agree quantitatively with the B1g/B2g and A1g features observed in electronic Raman scattering, where the former is believed to indicate the maximum superconducting gap and the origin of the latter has remained unexplained. The data are inconsistent with previous claims of the existence of a magnetic resonance mode near 10 meV, but consistent with a resonance at ω2 and with the recently established universal ratio of resonance energy to superconducting gap in unconventional superconductors. © 2010 The American Physical Society.

Loading Forschungsneutronenquelle Heinz Maier Leibnitz collaborators
Loading Forschungsneutronenquelle Heinz Maier Leibnitz collaborators