Visualizing the heterogeneous breakdown of a fractal microstructure during compaction by neutron dark-field imaging

Abstract

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.

Achromatic Non-Interferometric Single Grating Neutron Dark-Field Imaging

“…The model and parameters found through small angle scattering measurements in Ref. [21], where a = 1.56 µm and S = 0.45, fit the single grating data as depicted in Fig. 3a.…”

“…Systematic quantitative reference measurements have been performed on a second set of samples. The study focusses on quantitative dark-field imaging [15,[17][18][19][20][21] which has established as the dominant application of interferometeric modulated beam imaging in material research. The first reference samples were two commercially available fractal powders, namely Sipernat-350 and Sipernat-310 with characteristic particle sizes of 4 and 8.5 micrometers, respectively [36].…”

“…The large surface area together with the micrometer sized particle characteristics makes Sipernat-310 a well-suited material to investigate basic characteristics of cohesive powders. It has recently been used in a study observing with spatial resolution the heterogeneous breakdown of the fractal microstructure [21]. This study provided the required reference data and model fit from more conventional spin-echo small angle neutron scattering measurements with no direct spatial resolution.…”

A high visibility Talbot-Lau neutron grating interferometer to investigate stress-induced magnetic degradation in electrical steel

“…For Talbot-Lau types of interferometers higher Talbot orders have been used. This allows easier positioning of samples between G 1 and G 2 , as well as tuning of the correlation length without changing the neutron wavelength 8,15 . Typically a visibility of 0.25 has been achieved.…”

Characterization of oriented microstructures through anisotropic small-angle scattering by 2D neutron dark-field imaging

“…This contrast modality provides information on the microstructures of the sample on (sub)micrometer length scales, beyond the resolution capability of conventional neutron imaging [21][22][23][24][25] . A major breakthrough for the development of this approach could be established with the pursuit of quantitative evaluation of the neutron dark-field signal providing information about the macroscopic variations of the microstructure [26][27][28][29] . Furthermore, the DFI contrast proved to be a powerful tool for the investigation of magnetic domains in particular in bulk ferromagnetic grain-oriented electrical steels and of magnetic phenomena in superconductors as well [30][31][32][33][34][35][36][37][38][39][40] .…”