Diffraction de neutrons

Résumé

La diffractométrie de neutrons est une technique d'analyse basée sur la diffraction des neutrons sur la matière. Elle est complémentaire à la diffractométrie de rayons X. L'appareil de mesure utilisé s'appelle un diffractomètre. Les données collectées forment le diagramme de diffraction ou diffractogramme. La diffraction n'ayant lieu que sur la matière cristalline, on parle aussi de radiocristallographie. Pour les matériaux non-cristallins, on parle de diffusion. La diffraction fait partie des méthodes de diffusion élastique. Histoire La première expérience de diffraction de neutrons fut effectuée en 1945 par Ernest O. Wollan au Laboratoire national d'Oak Ridge. Il fut rejoint en 1946 par Clifford Shull. Ensemble, ils établirent les principes de base de la technique et l'appliquèrent avec succès à plusieurs matériaux différents, s'occupant de problèmes comme la structure de la glace et les arrangements microscopiques de moments magnétiques dans les matériaux. Pour ces rés

Source officielle

https://fr.wikipedia.org/wiki/Diffraction_de_neutrons

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Mg and Ti alloys are attractive materials for structural applications in the transportation and biomedical industries due to their high strength-to-weight ratios. However, due to their hexagonally close-packed (HCP) lattice structure, they exhibit poor formability at room temperature and strong plastic anisotropy. The anisotropy often results in non-intuitive deformation behaviors. In situ test methods such as x-ray diffraction, neutron diffraction, and high-resolution digital image correlation are powerful tools that make it possible to examine the evolution of the microstructure during deformation. In this thesis, the plastic deformation behavior of three HCP metals is examined using various in situ mechanical test techniques. Each study focuses on a particular aspect of the plastic deformation: the effect of strain path changes on Mg AZ31B, the high work hardening of a Ti-6Al-4V alloy, and the three-stage work hardening and springback of a Grade 2 Ti.

EPFL2019

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Magnetic x-ray and neutron scattering from UPd2Si2

EPFL1998

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Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

2020

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