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One step towards MnAl-based permanent magnets: Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
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2019 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 274, p. 229-236Article in journal (Refereed) Published
Abstract [en]

The influence of an additional annealing step during synthesis on the preparation of MnAl based permanent magnet alloys has been investigated. Bulk samples of Mn55Al45C2 alloys were synthesized using induction heating through drop synthesis from 1400 °C. Samples produced using cooling directly from 1400 °C (from the melt), and from 1400 °C to an intermediate annealing step at 1200 °C for ~ 30 min before cooling were compared with respect to differences in phase purity, microstructure and magnetic properties. We found that the phase purity was significantly enhanced using the route with an intermediate annealing step at 1200 °C. From XRD the phase purity of the tau-phase was improved from ~ 91 wt% for the sample cooled directly from 1400 °C to ~ 95.1 - 99.5 wt% for the sample exposed to an intermediate annealing step before cooling. Additionally, EBSD, and SEM with EDS indicates a clear difference in the phase composition and differences in the distribution of the magnetic tau phase and the non-magnetic epsilon-, beta-, and gamma-phases. Magnetic properties also indicate, an improvement in saturation magnetization for the sample exposed to the extra annealing step during synthesis. Our results suggest that an intermediate annealing step in the production of MnAl based alloys will provide a simple way of achieving better phase purity and magnetic properties in the bulk alloy.

Place, publisher, year, edition, pages
2019. Vol. 274, p. 229-236
Keywords [en]
permanent magnet, rare earth free, microstructure
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-368283DOI: 10.1016/j.jssc.2019.03.035ISI: 000467663700032OAI: oai:DiVA.org:uu-368283DiVA, id: diva2:1267720
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy StorageSwedish Foundation for Strategic Research Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2019-06-11Bibliographically approved
In thesis
1. Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals
Open this publication in new window or tab >>Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

How to generate and use electricity in a more efficient way is a major challenge for humankind. In this context, permanent magnets play an important role within a very broad range of electric power applications. The strongest magnets used today are mainly based on alloys that contain rare-earth metals, which are neither economical nor sustainable. The search for new alternative alloys with satisfactory magnetic properties is the major motivation for the investigations summarized in this thesis. Interesting candidates for alternative rare-earth free alloys were selected with τ-MnAl as the basis. Theoretical studies suggest that such alloys may show good magnetic properties after chemical modifications to optimize them. Another compound with promising magnetic properties is MnBi, included in this study.

MnAl-Z (Z= C, B, Ga as doping elements) and MnBi compounds were synthesized through carefully devised high-temperature methods, followed by various milling and annealing steps. The structural phase analysis of the samples was based on X-ray and neutron diffraction. A systematic microstructural investigation was also performed for selected samples. The phase transitions of MnAl and MnBi during heating and cooling at different rates were studied by in situ X-ray diffraction from a synchrotron source. The magnetic properties were characterized by various methods.

By strict control of experimental parameters, the metastable τ-MnAl was found to be directly obtainable using a "drop synthesis” process. A cooling rate of 10 K/min yielded an almost pure ferromagnetic τ-MnAl phase. A microstructural characterization of similarly synthesized MnAl-C samples revealed the presence of phase segregation, a Mn-rich region and an Al-rich grain boundary phase.

A cryomilling process was employed which decreased the particle size of the MnAl-C sample. Neutron diffraction data disclosed accompanying amorphous features, related to changes in Mn and Al atom occupancies during the milling process. A flash heating procedure regenerated the structural ordering between Mn and Al in the structure, where the initial magnetic properties were recovered.

The MnBi compound was synthesized by a self-flux method in order to isolate single crystals. As for τ-MnAl, in situ diffraction studies were applied for following phase transitions and the magnetic properties were studied.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 57
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1782
Keywords
Synthesis, Magnetism, Diffraction.
National Category
Inorganic Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-379177 (URN)978-91-513-0594-3 (ISBN)
Public defence
2019-05-03, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 14:15 (English)
Opponent
Supervisors
Available from: 2019-04-08 Created: 2019-03-13 Last updated: 2019-05-07

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Shafeie, SamrandFang, HailiangHedlund, DanielSvedlindh, PeterGunnarsson, KlasSahlberg, Martin

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