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Hengstler-Eger R.M.,AREVA | Baldo P.,Argonne National Laboratory | Beck L.,Maier Leibnitz Laboratorium MLL | Dorner J.,Max Planck Institute for Plasma Physics (Garching) | And 7 more authors.
Journal of Nuclear Materials | Year: 2012

Pressurized water reactor (PWR) Zr-based alloy structural materials show creep and growth under neutron irradiation as a consequence of the irradiation induced microstructural changes in the alloy. A better scientific understanding of these microstructural processes can improve simulation programs for structural component deformation and simplify the development of advanced deformation resistant alloys. As in-pile irradiation leads to high material activation and requires long irradiation times, the objective of this work was to study whether ion irradiation is an applicable method to simulate typical PWR neutron damage in Zr-based alloys, with AREVA's M5® alloy as reference material. The irradiated specimens were studied by electron backscatter diffraction (EBSD), positron Doppler broadening spectroscopy (DBS) and in situ transmission electron microscopy (TEM) at different dose levels and temperatures. The irradiation induced microstructure consisted of - and -type dislocation loops with their characteristics corresponding to typical neutron damage in Zr-based alloys; it can thus be concluded that heavy ion irradiation under the chosen conditions is an excellent method to simulate PWR neutron damage. © 2012 Published by Elsevier B.V. All rights reserved.

Redhammer G.J.,University of Salzburg | Senyshyn A.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II | Senyshyn A.,TU Darmstadt | Tippelt G.,University of Salzburg | And 4 more authors.
American Mineralogist | Year: 2012

The clinopyroxene-type compound FeGeO 3 was synthesized using ceramic sintering techniques at 1273 K in evacuated silica tubes and investigated by powder neutron diffraction between 4 and 300 K, X-ray diffraction, SQUID magnetometry, and 57Fe Mössbauer spectroscopy. The title compound shows space group C2/c symmetry (high pressure, HP-topology) between 4 and 900 K. No structural phase transition is present within this temperature interval, whereas lattice parameters show discontinuities around 50 and 15 K, which are due to magnetic phase transitions and the associated magneto-elastic coupling of the lattice. The magnetic susceptibility data show two maxima in their temperature dependence, one at ̃47 K, the second around 12 K (depending on the external field), indicative of two magnetic transitions in the title compound. Neutron data shows that for T < 45 K, FeGeO 3 orders magnetically, having a simple collinear structure, with space group C2/c, and with the spins aligned parallel to the crystallographic b-axis, both on M1 and M2. The coupling within the M1/M2 band is ferromagnetic, whereas between them it is antiferromagnetic. As the bulk magnetic measurements in the paramagnetic state revealed a dominating ferromagnetic coupling, the intra-chain interactions dominate the inter-chain interaction. At 12 K, additional magnetic reflections appear, revealing a second magnetic phase transition. Spins are rotated away from the b-axis toward the a-c plane. The coupling within the M1 chain is still ferromagnetic and antiferromagnetic between the M1 chains. However, spins on M1 and M2, are no longer collinear. The moment on the M2 site is rotated further away from the b-axis than on M1. Copyright © 1997-2012 Mineralogical Society of America.

Iturbe-Zabalo E.,Laue Langevin Institute | Iturbe-Zabalo E.,University of the Basque Country | Igartua J.M.,University of the Basque Country | Faik A.,CICEnergigune | And 3 more authors.
Journal of Solid State Chemistry | Year: 2013

Crystal structures of SrNdZnRuO6, SrNdCoRuO6, SrNdMgRuO6 and SrNdNiRuO6 double perovskites have been studied by X-ray, synchrotron radiation and neutron powder diffraction method, at different temperatures, and using the symmetry-mode analysis. All compounds adopt the monoclinic space group P21/n at room-temperature, and contain a completely ordered array of the tilted MO6 and RuO 6 octahedra, whereas Sr/Nd cations are completely disordered. The analysis of the structures in terms of symmetry-adapted modes of the parent phase allows the identification of the modes responsible for the phase-transition. The high-temperature study (300-1250 K) has shown that the compounds present a temperature induced structural phase-transition: P2 1/n→P42/n→Fm3̄m. © 2012 Elsevier Inc.

Li S.,CAS Institute of Physics | Zhang C.,University of Tennessee at Knoxville | Wang M.,CAS Institute of Physics | Wang M.,University of Tennessee at Knoxville | And 12 more authors.
Physical Review Letters | Year: 2010

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co,Ni)xAs2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe1-xTex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling. © 2010 The American Physical Society.

Schroder T.,Ludwig Maximilians University of Munich | Schwarzmuller S.,Ludwig Maximilians University of Munich | Stiewe C.,German Aerospace Center | De Boor J.,German Aerospace Center | And 2 more authors.
Inorganic Chemistry | Year: 2013

Exchanging one Ge2+ with two Li+ per formula unit in (GeTe)n(Sb2Te3) (n = 1, 2, 3,...) eliminates cation vacancies, because it leads to an equal number of cations and anions. This substitution results in the solid solution (GeTe)x(LiSbTe 2)2 (with x = n - 1, but n not necessarily an integer). For x < 6, these stable compounds crystallize in a rock-salt-type structure with random cation disorder. Neutron data show that a small fraction of Ge occupies tetrahedral voids for x = 2 and 3. For x > 6, (GeTe) x(LiSbTe2)2 forms a GeTe-type structure that shows a phase transition to a cubic high-temperature phase at ca. 280 C. The thermoelectric properties of (GeTe)11(LiSbTe2)2 have been investigated and show that this compound is a promising thermoelectric material with a ZT value of 1.0 at 450 C. The high ZT value of the thermodynamically stable compound is caused by a low phononic contribution to the thermal conductivity; probably, Li acts as a "pseudo-vacancy". © 2013 American Chemical Society.

Gatta G.D.,University of Milan | Gatta G.D.,CNR Institute for the Dynamics of Environmental Processes | Meven M.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II | Bromiley G.,University of Edinburgh
Physics and Chemistry of Minerals | Year: 2010

The effects of temperature on the crystal structure of a natural epidote [Ca1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH), a = 8.890(6), b = 5.630(4), c = 10. 50(6) Å and β = 115.36(5)°, Sp.Gr. P21/m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe3+ at the M3 site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)-H(1)···O(2) and O(10)-H(1)···O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)-H(1)···O(4) is energetically more favourable, as H(1)···O(4) = 1.9731(28) Å, O(10)···O(4) = 2.9318(22) Å and O(10)-H(1)···O4 = 166.7(2)°, whereas H(1)···O(2) = 2.5921(23) Å, O(10)···O(2) = 2.8221(17)Å and O(10)-H(1)···O2 = 93.3(1)°. The O(10)-H(1) bond distance corrected for "riding motion" is 0.9943 Å. The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe3+ share the M(3) site along with Al3+ [%Fe(M3)1,070K = 74(2)%]. The refined amount of Fe3+ at the M(1) is not significant [%Fe(M1)1,070K = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for "rigid body motions". The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)-O(9)-Si(1) of the Si2O7 group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)···O(4) and H(1)···O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe3+ between different octahedral sites and/or convergent ordering on M(3) are discussed. © 2009 Springer-Verlag.

Churakov S.V.,Paul Scherrer Institute | Gimmi T.,Paul Scherrer Institute | Gimmi T.,University of Bern | Unruh T.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II | And 2 more authors.
Applied Clay Science | Year: 2014

Since no single experimental or modeling technique provides data that allow a description of transport processes in clays and clay minerals at all relevant scales, several complementary approaches have to be combined to understand and explain the interplay between transport relevant phenomena. In this paper molecular dynamics simulations (MD) were used to investigate the mobility of water in the interlayer of montmorillonite (Mt), and to estimate the influence of mineral surfaces and interlayer ions on the water diffusion. Random Walk (RW) simulations based on a simplified representation of pore space in Mt were used to estimate and understand the effect of the arrangement of Mt particles on the meso- to macroscopic diffusivity of water. These theoretical calculations were complemented with quasielastic neutron scattering (QENS) measurements of aqueous diffusion in Mt with two pseudo-layers of water performed at four significantly different energy resolutions (i.e. observation times). The size of the interlayer and the size of Mt particles are two characteristic dimensions which determine the time dependent behavior of water diffusion in Mt. MD simulations show that at very short time scales water dynamics has the characteristic features of an oscillatory motion in the cage formed by neighbors in the first coordination shell. At longer time scales, the interaction of water with the surface determines the water dynamics, and the effect of confinement on the overall water mobility within the interlayer becomes evident. At time scales corresponding to an average water displacement equivalent to the average size of Mt particles, the effects of tortuosity are observed in the meso- to macroscopic pore scale simulations. Consistent with the picture obtained in the simulations, the QENS data can be described using a (local) 3D diffusion at short observation times, whereas at sufficiently long observation times a 2D diffusive motion is clearly observed. The effects of tortuosity measured in macroscopic tracer diffusion experiments are in qualitative agreement with RW simulations. By using experimental data to calibrate molecular and mesoscopic theoretical models, a consistent description of water mobility in clay minerals from the molecular to the macroscopic scale can be achieved. In turn, simulations help in choosing optimal conditions for the experimental measurements and the data interpretation. © 2014 Elsevier B.V.

Adams T.,TU Munich | Chacon A.,TU Munich | Wagner M.,TU Munich | Bauer A.,TU Munich | And 5 more authors.
Physical Review Letters | Year: 2012

We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu 2OSeO 3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe, and Fe 1-xCo xSi, i.e., binary isostructural siblings of Cu 2OSeO 3 that order helimagnetically. The temperature dependence of the specific heat of Cu 2OSeO 3 is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far. © 2012 American Physical Society.

Breitkreutz H.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II | Rohrmoser A.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II | Petry W.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II
IEEE Transactions on Nuclear Science | Year: 2010

The neutronic Monte Carlo code MCNPX and the thermal-hydraulic CFD code CFX have been coupled to satisfy the increasing demand for 3-dimensional results with high spatial resolution in nuclear reactor physics. In a first step, results for an evolute shaped fuel plate of a symmetric compact core and an attached cooling channel in the fuel element of FRM II were regarded. © 2010 IEEE.

Pikart P.,TU Munich | Hugenschmidt C.,Forschungsneutronenquelle Heinz Maier Leibnitz FRM II
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2014

In Coincident Doppler Broadening Spectroscopy (CDBS) the sum energy of the annihilation photons is checked to be 1022 keV, to validate the measurement of an undisturbed two-gamma electron-positron decay event. The events are stored in a two-dimensional acquisition matrix. A new algorithm is presented, which optimizes the extraction of the one-dimensional CDBS spectrum from this matrix by enhanced background suppression by the use of variable size bins. © 2014 Elsevier B.V.

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