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Understanding the Compaction of Nanopowders Through Neutron and X-ray Diffraction

Research output: Book/anthology/dissertation/reportPh.D. thesis

This work explores the versatility of powder diffraction, both with X-rays and neutrons, with a particular focus on the information that the technique can provide at many length scales, spanning from atomic to bulk sizes. This is done through a series of studies, where the underlying theme is time-resolved powder diffraction of oxides, in particular the permanent magnet material SrFe12O19. Permanent magnet materials are ideal subjects for microstructure and texture studies using powder diffraction, as there exists a strong relationship between the magnetic properties and the micro- and texture structure of compacted bulk magnets. As a permanent magnet material, SrFe12O19 is relevant due to its large annual product on a global scale.
A brief introduction to magnetism from an applied point of view is provided to the reader, with a particular focus on the key parameters in the characterization of permanent magnets and the relation between magnetism and crystallite microstructure. This is followed by introductory theory on the subject of obtaining information on microstructure from powder diffraction. The theory segment moves on to texture analysis from diffraction data and lastly an introductory segment on time-resolved diffraction, including a discussion on some of the requirements and considerations regarding time-resolved experiments, data evaluation, and Rietveld refinement.
The time-resolved studies presented here, start out with a presentation of a laboratory diffractometer setup, utilizing a custom-built area detector Soller slit, allowing in-house in situ experiments with improved intensity and a limited spatial resolution. Results are presented from an in situ study of the annealing of SrFe12O19 nanopowders, using both the laboratory setup and a synchrotron setup. Results from the two setups are compared and a large discrepancy in the measured temperatures was found. The results provided limited information on microstructure, indicating an unusual anisotropic decrease in crystallite sizes. The inconclusive results prompted a further study, here conducted using neutron powder diffraction.
The neutron powder diffraction study takes a step toward the investigation of bulk behavior, as it uses in situ diffraction to investigate the sintering of cold-compacted powders of SrFe12O19. The diffraction data are analyzed using parametric refinement and the refined microstructure corroborates the unusual decrease in crystallite sizes, in particular along the width of platelet-shaped hydrothermally synthesized SrFe12O19. The magnetic properties of the sintered samples indicate a clear correlation between the refined preferred orientation and the magnetic alignment. Furthermore, a study investigating the parameterization degree of parametric refinements is presented, where one of the neutron diffraction datasets is used as a case study. It was seen that parametric refinements provide a tool for extracting further information from time-resolved diffraction data, and allows external parameters, such as temperature, to be included directly in the Rietveld refinement.
Continuing the bulk investigation, a technique for quantitative texture analysis is presented, using 2D diffraction images measured at a synchrotron using short exposure times. The technique uses binned azimuthal integration of a few images collected at various sample rotations to calculate the sample orientation distribution function. The calculations are performed in the refinement software MAUD. A case study implementing the technique is presented as a synopsis of a publication, investigating the alignment of strontium hexaferrite, by cold compaction of anisotropic non-magnetic crystallites. The ex situ 2D diffraction texture analysis serves as the foundation for the expansion towards in situ texture analysis.
The in situ texture analysis is made possible by a novel sample environment for time-resolved X-ray diffraction. The sample environment, dubbed ARΩS, implements the concept of ultrafast high-temperature sintering to heat and sinter compacted samples in seconds. A description of the sample environment design and considerations regarding the adaptation into an in situ sample environment is presented. A case study using compacted SrFe12O19 is presented in the form of a manuscript draft. The study was performed at the novel DanMAX beamline at MAX IV, Sweden, using a high heat rate of 150 K/s and a diffraction time-resolution of 250 Hz. The collected data proved refinable using both sequential Rietveld refinement and quantitative texture analysis.
The bulk in situ story ends with a discussion of a future sample environment for in situ hot-compaction, intended for the coming HEIMDAL neutron instrument at the European spallation source. The discussion includes considerations regarding requirements and design ideas for such a setup. The idea for the sample environment is a continuation of an induction furnace developed at Aarhus University for the neutron instrument POLARIS at the ISIS neutron source in the UK. The section ends with a presentation of the initial stages of a pressing frame, intended as a prototype and proof-of-concept for the in situ compaction setup.
Lastly, a series of publications to which I have contributed, but which do not take part in the main story, is presented in the form of synopses. The thesis is finished with a few final remarks and some of my perspectives and predictions for the future of the themes presented here, in particular concerning the future of time-resolved diffraction.
Translated title of the contributionForståelse af Sammenpressningen af Nanopulver Gennem Neutron- og Røntgendiffraktion
Original languageEnglish
PublisherAarhus Universitet
Number of pages271
Publication statusPublished - Aug 2022

Note re. dissertation

Termination date: 19-08-2022

    Research areas

  • PhD Thesis, Powder diffraction, Permanent magnet

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