Magnetization Response of the Bulk and Supplementary Magnetic Domain Structure in High-Permeability Steel Laminations VisualizedIn Situby Neutron Dark-Field Imaging

Abstract

Industrial transformer cores are composed of stacked high-permeability steel laminations (HPSLs). The performance and degree of efficiency of transformers are directly determined by the magnetic properties of each HPSL. In this article, we show how the neutron dark-field image (DFI) allows for the in situ visualization of the locally resolved response of the bulk and supplementary magnetic domain structures in HPSLs under the influence of externally applied magnetic fields. In particular, we investigate the domain formation and growth along the initial magnetization curve up to the saturated state. For decreasing field, we visualize the recurrence of the hysteretic domain structure down to the remanent state. Additionally, the DFI allows us to derive a correlation between the grain orientation and the corresponding volume and supplementary domain structure. Furthermore, we visualize the influence of the insulation coating, introducing desired tensile stresses on the domain structures. To compare our DFI findings to traditional methods, we perform complementary surface-sensitive magneto-optical Kerr-microscopy investigations.

The influence of laser scribing on magnetic domain formation in grain oriented electrical steel visualized by directional neutron dark-field imaging

“…The general interpretation of the DFI signal in relation to the magnetic domain structure was discussed in refs 7, 8, 9, 10, 11, 12, 13, 14, 15. The spatially resolved magnetic structures illustrated as black vertical lines parallel to the RD can be attributed to domain walls of 180° basic domains.…”

“…The smaller volume domain wall spacing of the laser treated samples A, B, C in comparison to the untreated sample 1 can be explained by the refinement of the basic domains due to laser scribing. The dark areas in the DFIs for the ω = 0° orientation are grains with a certain misorientation to the RD and hence forming supplementary surface domains14. These domains are below the detection limit and are therefore detected as darker areas.…”

“…This method has been used to investigate GO steel1112131415 as well as non-grain oriented steel grades16, which demonstrates the general possibility to investigate bulk magnetic behavior of silicon steel sheet without any preparation (removing of the isolating coating layer) that has be shown recently influencing the domain structure13. In the following we present the observation of bulk and supplementary domain structures and the refinement of them of an untreated and differently laser treated high-permeability GO electrical steel laminations.…”

Small Angle Scattering in Neutron Imaging—A Review

“…Grating interferometry with neutrons has proven particularly useful to detect magnetic domain walls and visualize magnetic domains even in the bulk of samples, which are not accessed by any other technique [18][19][20][21][22][23][24][25][26][27][28][29]. The Talbot-Lau grating interferometer, being the current standard in neutron grating interferometry consists of a set of three gratings ( Figure 3).…”

“…Microstructural deviations hence provided additional image contrast from structures beyond direct real space resolution, and contrast for material compositions, which would otherwise not provide contrast in terms of attenuation. In neutron imaging the detection of magnetic structures providing a signal due to splitting of the spin states of an unpolarized beam became most prominent [18][19][20][21][22][23][24][25][26][27][28][29]. However, also the detection of regions with altered porosity or precipitations in particular in metallic samples has been demonstrated and applied [40,41] (Figure 4).…”