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Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
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 [en]
Synthesis, Magnetism, Diffraction.
National Category
Inorganic Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-379177ISBN: 978-91-513-0594-3 (print)OAI: oai:DiVA.org:uu-379177DiVA, id: diva2:1295906
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
List of papers
1. Directly obtained tau-phase MnAl, a high performance magnetic material for permanent magnets
Open this publication in new window or tab >>Directly obtained tau-phase MnAl, a high performance magnetic material for permanent magnets
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2016 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 237, p. 300-306Article in journal (Refereed) Published
Abstract [en]

The metastable tetragonal iota-phase has been directly obtained from casting Mn0.54Al0.46 and (Mn0.55Al0.45)(100)C-2 using the drop synthesis method. The as-casted samples were ball milled to decrease the particle size and relaxed at 500 degrees C for 1 h. The phase composition, crystallographic parameters, magnetic properties and microstructure were systematically studied. The results reveal that the iota-phase could be directly obtained from drop synthesis. The highest M-s of 117 emu/g was achieved in the (Mn0.55Al0.45)(100)C-2 where the iota-phase was stabilized by doping with carbon. Carbon doping increased the c/a ratio of the tau-phase as it occupies specific interstitial positions (1/2, 1/2, 0) in the structure. Furthermore, ball milling increases the coercivity (H-c) at the expense of a decrease in magnetic saturation (M-s). The increase in coercivity is explained by a decrease of grain size in conjunction with domain wall pinning due to defects introduced during the ball milling process.

Keywords
Permanent magnets, Rare-earth free, High temperature synthesis, Diffraction, Magnetic measurements
National Category
Chemical Sciences Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-294651 (URN)10.1016/j.jssc.2016.02.031 (DOI)000373661100041 ()
Funder
Swedish Energy AgencySweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2016-06-02 Created: 2016-05-26 Last updated: 2019-03-13Bibliographically approved
2. Insights into formation and stability of tau-MnAlZ(x) (Z = C and B)
Open this publication in new window or tab >>Insights into formation and stability of tau-MnAlZ(x) (Z = C and B)
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2017 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 692, p. 198-203Article in journal (Refereed) Published
Abstract [en]

The tau-phase MnAl alloys are promising candidate for rare earth free permanent magnets. In this study, In order to better understand the MnAl epsilon ->tau phase transition mechanism in a continuous cooling process and metastable MnAl tau-phase high temperature stability, Mn0.54Al0.46, Mn0.55Al0.45C0.02 and Mn0.55Al0.45B0.02 alloys were systematically studied by in situ synchrotron X-ray powder diffraction (SR-XRD). The relationship between tau-phase formation tendency and different cooling rates of Mn0.55Al0.45C0.02 was investigated. Besides, the high temperature stabilities of undoped tau-MnAl and carbon/boron doped tau-MnAl were studied. Differential thermal analysis (DTA) was also employed to study the phase transformation as well. The research results show that a high cooling rate of 600 degrees C/min leads to a 50/50 wt% mixture of epsilon- and tau-phase; almost pure tau-phase was obtained when cooled at a moderate cooling rate of 10 degrees C/min; while for a slow cooling rate of 2 degrees C/min, the tau-phase partially decomposed into beta and gamma(2) phases. No intermediate epsilon'-phase was observed during the epsilon ->tau phase transition during the experiments. For the boron and carbon doped tau-MnAl, the 800 degrees C high temperature stability experiments reveal that C stabilizes the tau-MnAl while doped B destabilises the tetragonal structure and it decomposes into beta- and gamma(2)-phases.

Keywords
Phase transition, Thermal analysis, In situ, Powder diffraction, Phase stability, Permanent magnet
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-308613 (URN)10.1016/j.jallcom.2016.09.047 (DOI)000386231200025 ()
Funder
Swedish Energy AgencySweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2016-11-30 Created: 2016-11-29 Last updated: 2019-03-13Bibliographically approved
3. Structural, microstructural and magnetic evolution in cryo milled carbon doped MnAl
Open this publication in new window or tab >>Structural, microstructural and magnetic evolution in cryo milled carbon doped MnAl
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, no 1, article id 2525Article in journal (Refereed) Published
Abstract [en]

The low cost, rare earth free τ-phase of MnAl has high potential to partially replace bonded Nd2Fe14B rare earth permanent magnets. However, the τ-phase is metastable and it is experimentally difficult to obtain powders suitable for the permanent magnet alignment process, which requires the fine powders to have an appropriate microstructure and high τ-phase purity. In this work, a new method to make high purity τ-phase fne powders is presented. A high purity τ-phase Mn0.55Al0.45C0.02 alloy was synthesized by the drop synthesis method. The drop synthesized material was subjected to cryo milling and followed by a fash heating process. The crystal structure and microstructure of the drop synthesized, cryo milled and flash heated samples were studied by X-ray in situ powder diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy and electron backscatter diffraction. Magnetic properties and magnetic structure of the drop synthesized, cryo milled, flash heated samples were characterized by magnetometry and neutron powder diffraction, respectively. The results reveal that the 2 and 4hours cryo milled and flash heated samples both exhibit high τ-phase purity and micron-sized round particle shapes. Moreover, the fash heated samples display high saturation magnetization as well as increased coercivity.

National Category
Materials Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-341024 (URN)10.1038/s41598-018-20606-8 (DOI)000424189500012 ()29410462 (PubMedID)
Funder
Swedish Energy AgencySweGRIDS - Swedish Centre for Smart Grids and Energy StorageSwedish Research Council
Available from: 2018-02-06 Created: 2018-02-06 Last updated: 2019-03-13Bibliographically approved
4. Measured and calculated properties of B-doped τ-phase MnAl: A rare earth free permanent magnet
Open this publication in new window or tab >>Measured and calculated properties of B-doped τ-phase MnAl: A rare earth free permanent magnet
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2019 (English)In: Journal of Magnetism and Magnetic Materials, Vol. 474, p. 591-598Article in journal (Refereed) Published
Abstract [en]

The metastable tetragonal τ-phase MnAl has been doped interstitially with B through a drop synthesis method creating the (Mn0.55Al0.45)B0.02 compound. The as-casted samples were annealed, quenched and thereafter ball-milled and relaxed in order to decrease grain size and reduce the number of crystallographic defects. The Curie temperature of the quenched sample was estimated to 655 K. The magnetization, coercivity and anisotropy were analyzed with respect to flash-milling time, relaxation time and temperature. The results show that (Mn0.55Al0.45)B0.02 could be directly obtained from drop synthesis. The highest measured saturation magnetization of 393 kA/m (measured at ±1440kA/m) was achieved with a relaxation process after 1.5h milling, giving a theoretical maximum energy product of 48 kJ/m3. The highest value of the coercivity was 355 kA/m achieved by flash-milling for 10 h. However, the high coercivity was achieved at an expense of low saturation magnetization.

Keywords
Permanent magnets; Rare-Earth-free; Diffraction; Magnetometry; Computational modeling
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-368265 (URN)10.1016/j.jmmm.2018.11.006 (DOI)000459494600086 ()
Funder
Swedish Energy AgencySweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2019-03-21Bibliographically approved
5. Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux
Open this publication in new window or tab >>Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux
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2019 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 781, p. 308-314Article in journal (Refereed) Published
Abstract [en]

To effectively synthesize high purity ferromagnetic low temperature phase (LTP) MnBi with optimal microstructure is still a challenge that needs to be overcome for the system to reach its full potential. Here, the phase transitions and magnetic properties of MnBi crystals are reported. The phase transition between the low and high temperature structure of MnBi was systematically investigated at different heating/cooling rates using in situ synchrotron radiation X-ray diffraction. The material crystallizes in a layered hexagonal structure giving a platelike microstructure. The magnetic characterization of the crystals reveal that the saturation magnetization varies from 645 kA/m at 50 K to 546 kA/m at 300 K. Magnetization measurements also show that the sample upon heating becomes non-magnetic and transforms to the high temperature phase (HTP) at similar to 640 K, and that it regains ferromagnetic properties and transforms back to the LTP at similar to 610 K upon subsequent cooling.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
In situ synchrotron radiation X-ray diffraction, Phase transitions, Rare earth free permanent magnet, MnBi, Single crystals
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-378612 (URN)10.1016/j.jallcom.2018.12.146 (DOI)000457845900034 ()
Funder
Swedish Energy AgencySwedish Foundation for Strategic Research
Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-13Bibliographically approved
6. One step towards MnAl-based permanent magnets: Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis
Open this publication in new window or tab >>One step towards MnAl-based permanent magnets: Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis
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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.

Keywords
permanent magnet, rare earth free, microstructure
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-368283 (URN)10.1016/j.jssc.2019.03.035 (DOI)000467663700032 ()
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

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