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

R P Harti,P Trtik,M Strobl,J Valsecchi,D Mannes,C Carminati,J Plomp,C P Duif,C Grünzweig

doi:10.1038/s41598-018-35845-y

R P Harti, P Trtik + Show 7 more

Open Access

https://doi.org/10.1038/s41598-018-35845-y

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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.

Highlights

Powders form the foundation of many technological and fundamental achievements in fields such as the production of nanocrystals[1] as well as the understanding of the complex interplay of forces[2,3,4]
In this article we will use the neutron dark-field image (DFI) contrast[13,14] of the neutron grating interferometry method[15] and especially the recently developed sub-pixel correlation length imaging technique[16,17,18] in combination with supporting methods such as scanning electron microscopy (SEM), X-ray computed tomography (X-CT) and spin-echo small-angle neutron scattering (SESANS) to study the compaction of a cohesive powder
All the knowledge gained from the supporting methods of SEM, X-CT and SESANS allowed us to use DFI and especially ξDFI to study the compaction of the powder sample with a pyramidal piston that introduces spatial variation in the microstructure on cm length scales

Summary

Conclusion

The experimental data presented in this work shows the application of a set of characterisation techniques to comprehensively study the heterogeneous transition from a fractal to a homogeneous microstructure. We combined SEM imaging with high resolution with X-CT with a large field-of-view to investigate the compaction of a cohesive powder at the micro and macro scale. In order to comprehensively study the microstructural changes we applied SESANS and confirmed a scattering model that describes two-phase random media. The combined knowledge gained from SEM, X-CT and SESANS was used to apply ξDFI to a compaction experiment of the same powder using a pyramidal shaped piston that introduces spatial variations in the sample. The combination of macroscopic imaging resolution with microstructural information by fitting the model confirmed with SESANS enabled us to visualise the propagation front of the breakdown of the fractal microstructure using ξDFI

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