While neutron imaging is a wellestablished technique for investigations of inner structures and processes in materials, complex systems, and devices, the utilization of polarized neutron beams to visualize magnetic phenomena has been introduced only about a decade ago. In contrast to neutron scattering studies, where the interaction of the neutron's magnetic moment with magnetic states of matter has been exploited for a long time, the direct visualization of magnetic fields in neutron imaging is a relatively new field and is still developing. Here, we give an overview of the status and provide approaches to visualizing magnetic fields with polarized neutrons, together with a report on the latest developments in attempting to record neutron tomographies for 3D reconstructions of magnetic vector fields.

Structural properties of cohesive powders are dominated by their microstructural composition. Powders with a fractal microstructure show particularly interesting properties during compaction where a microstructural transition and a fractal breakdown happen before compaction and force transport. The study of this phenomenon has been challenging due to its long-range effect and the subsequent necessity to characterize these microstructural changes on a macroscopic scale. For the detailed investigation of the complex nature of powder compaction for various densification states along with the heterogeneous breakdown of the fractal microstructure we applied neutron dark-field imaging in combination with a variety of supporting techniques with various spatial resolutions, field-of-views and information depths. We used scanning electron microscopy to image the surface microstructure in a small field-of-view and X-ray tomography to image density variations in 3D with lower spatial resolution. Non-local spin-echo small-angle neutron scattering results are used to evaluate fitting models later used as input parameters for the neutron dark-field imaging data analysis. Finally, neutron dark-field imaging results in combination with supporting measurements using scanning electron microscopy, X-ray tomography and spin-echo small angle scattering allowed us to comprehensively study the heterogeneous transition from a fractal to a homogeneous microstructure of a cohesive powder in a quantitative manner.

Visualizing the water distribution in porous gas diffusion layers (GDLs) of operating polymer electrolyte fuel cells (PEFCs) is indispensable to understand the impact of water management on performance. For this purpose, neutron and X-ray transmission imaging have been used for nearly two decades. Certain limitations inherent to attenuation based imaging methods can be overcome by applying neutron dark-field imaging, which has the ability to selectively visualize structures in the micrometer size range. In this study, we compare dark-field images and transmission images of GDLs filled with water through an injection channel. The high contrast of the dark-field value between a heavy water filled and a dry GDL is suitable to reveal water distribution patterns in the GDL. The water present in the 1 mm wide water injection channel of the test device does not alter the dark-field signal, as this technique is selectively sensitive to microstructures. Therefore, neutron dark-field imaging can be applied for the selective analysis of the water distribution in the GDL overlapping with channel water. In addition to the selective visualization of water distributed in a GDL, we show that neutron dark-field imaging can also be used to visualize GDL damages.

4D wavelength-resolved neutron tomography of a reference sample made of several polycrystalline materials, namely nickel, iron, titanium, lead, copper and aluminium, is presented. Data were acquired using the time-of-flight transmission imaging method at the IMAT beamline at the ISIS pulsed neutron source. Wavelength-dispersive tomography reconstruction was computed using filtered back projection, allowing wavelength-resolved total-cross-section retrieval for each voxel in the reconstructed volume of the sample. The need for background correction to enable quantitative results and analysis is discussed, and the achieved 3D spatial resolution with respect to the obtained Bragg-edge pattern quality is investigated.