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Self propagating high temperature synthesis of ferrites in magnetic fields.

Affleck, Louise; (2002) Self propagating high temperature synthesis of ferrites in magnetic fields. Doctoral thesis (Ph.D.), University College London. Green open access

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Abstract

Self propagating high temperature synthesis (SHS) reactions have been performed on mixtures of BaO2, Fe and Fe2O3 to form barium ferrite, BaFe12O19. Reactions were conducted in zero field and in an applied magnetic field of 1.1 T with the aim of exploring the influence of the field. The temperature and velocity of the reactions were measured and the products, both post-SHS and post-annealing, were characterised by techniques including X-ray diffraction, Mossbauer spectroscopy, vibrating sample magnetometry and electron microprobe analysis. The applied magnetic field was found to lead to hotter and faster reactions, a greater degree of conversion of the reactants, a needle-like micro structure in the post-SHS product, and a reduced coercive field (?20-30 %) in the annealed product, compared to zero field. Sodium perchlorate was used as an internal oxidising agent, and found to produce similar effects. Correlations were observed between the temperature reached in the SHS reaction, the microstructure and reaction completeness of the post-SHS product and the magnetic parameters of the annealed product. One consequence is that it appears to be possible to control magnetic properties of the BaFe12O19 product by controlling the SHS conditions. Reactions were also performed in magnetic fields of up to 20 T. Strongly and weakly magnetic systems m powder and pellet form were studied. Evidence was found to show that the magnetic field acts by reorganising strongly magnetic material along the field lines. Some evidence was found to suggest that the field affects weakly magnetic systems, so the field may also act by influencing ions at the SHS wavefront and/or by acting on paramagnetic oxygen. The first time-resolved X-ray diffraction studies of SHS reactions to form ferrites, and the first such studies in a magnetic field, were performed. The experimental procedure was established and the important variables were determined. Reaction pathways were observed to be different in a magnetic field compared to zero field.

Type: Thesis (Doctoral)
Qualification: Ph.D.
Title: Self propagating high temperature synthesis of ferrites in magnetic fields.
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
URI: https://discovery.ucl.ac.uk/id/eprint/10104474
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