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You searched for +publisher:"EPFL" +contributor:("Lehmann, Eberhard Helmar"). Showing records 1 – 2 of 2 total matches.

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EPFL

1. Peetermans, Steven Luc X. Energy-selective neutron imaging for materials science.

Degree: 2015, EPFL

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample. Advisors/Committee Members: Van Swygenhoven, Helena, Lehmann, Eberhard Helmar.

Subjects/Keywords: Neutron imaging; neutron tomography; energy-selective; diffraction contrast; imaging; topography; monochromator; crystal

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Peetermans, S. L. X. (2015). Energy-selective neutron imaging for materials science. (Thesis). EPFL. Retrieved from http://infoscience.epfl.ch/record/205011

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16th Edition):

Peetermans, Steven Luc X. “Energy-selective neutron imaging for materials science.” 2015. Thesis, EPFL. Accessed February 17, 2019. http://infoscience.epfl.ch/record/205011.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7th Edition):

Peetermans, Steven Luc X. “Energy-selective neutron imaging for materials science.” 2015. Web. 17 Feb 2019.

Vancouver:

Peetermans SLX. Energy-selective neutron imaging for materials science. [Internet] [Thesis]. EPFL; 2015. [cited 2019 Feb 17]. Available from: http://infoscience.epfl.ch/record/205011.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Peetermans SLX. Energy-selective neutron imaging for materials science. [Thesis]. EPFL; 2015. Available from: http://infoscience.epfl.ch/record/205011

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation


EPFL

2. Betz, Benedikt Karl-Josef Gerhard. Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry.

Degree: 2016, EPFL

Industrial transformers cores are built from stacked sheets of an iron silicon alloy, called laminations. The magnetic domain structures of these highly anisotropic electrical steels with a sharp (110)[001]-texture, the so-called Gosstexture, determines the magnetic properties of transformers. Commonly used investigation techniques for these laminations are inductive B-H-measurements. This technique reveals global magnetic properties such as hysteresis, remanence, saturation and losses. Locally resolved information about the underlying domain structure cannot be obtained. In this thesis, the neutron grating interferometer (nGI) is used to investigate the domain structures in Goss-oriented (GO) electrical steels at the Swiss Spallation Neutron Source using the cold neutron imaging facility ICON. In contrast to the attenuation based transmission image, the dark-field image (DFI) is related to multiple refraction of unpolarised neutrons at magnetic domain walls. Thereby the use of the DFI allows for the visualisation of bulk magnetic domain structures in two and three dimensions with a spatial resolution of down to 70 um in a field of view of 64mm x 64mm. The DFI is used for the visualisation of the locally resolved response of the bulk magnetic domain structures under the influence of externally applied magnetic fields. In the first part, the domain formation and growth along the initial magnetisation curve (0-6000 A/m) up to saturation in static DC magnetic fields was studied. For decreasing field values, the visualisation of the recurrence of the hysteretic domain structure down to the remanent (0 A/m) state is given. A correlation of the grain orientation and the corresponding basic and supplementary domain structure is given. In the second part, the DFI is used to investigate the response of magnetic domain walls to dynamic AC magnetic excitations. The visualisation of the domain wall motion under influence of alternating magnetic fields is performed. In detail, scans combining varying levels of an offset DC (0-30 A/m), oscillation amplitude A (0-1500 A/m), and oscillation frequency f (0-200 Hz) are conducted. By increasing the amplitude while maintaining constant values of DC and f, the transition from a frozen domain wall structure to a mobile one is recorded. Vice versa, increasing f while keeping A and DC constant led to the reverse transition from a mobile domain wall structure into a frozen one. It is shown that varying both, A and f shifts the position of transition region. Higher frequencies require higher oscillation amplitudes to overcome the freezing. The DFI allows for the analysis of the laminations coatings impact to the magnetic structures. To visualise the stress effect of the coating to the underlying domain formation an uncoated lamination is investigated with stepwise increasing applied external tensile stresses up to 20MPa. The domain configurations of the intermediate stress states are imaged and the original domain structure of the coated state is reproduced. To verify the results,… Advisors/Committee Members: Van Swygenhoven, Helena, Lehmann, Eberhard Helmar.

Subjects/Keywords: Neutron imaging; neutron grating interferometry; magnetism; dark-field imaging; magnetic domains; electrical steel; Goss-orientation

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Betz, B. K. G. (2016). Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry. (Thesis). EPFL. Retrieved from http://infoscience.epfl.ch/record/218434

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Chicago Manual of Style (16th Edition):

Betz, Benedikt Karl-Josef Gerhard. “Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry.” 2016. Thesis, EPFL. Accessed February 17, 2019. http://infoscience.epfl.ch/record/218434.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

MLA Handbook (7th Edition):

Betz, Benedikt Karl-Josef Gerhard. “Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry.” 2016. Web. 17 Feb 2019.

Vancouver:

Betz BKG. Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry. [Internet] [Thesis]. EPFL; 2016. [cited 2019 Feb 17]. Available from: http://infoscience.epfl.ch/record/218434.

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

Council of Science Editors:

Betz BKG. Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry. [Thesis]. EPFL; 2016. Available from: http://infoscience.epfl.ch/record/218434

Note: this citation may be lacking information needed for this citation format:
Not specified: Masters Thesis or Doctoral Dissertation

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