Optimizing composition in MnBi permanent magnet alloys

Journal Article · Mon Oct 07 00:00:00 EDT 2019 · Acta Materialia

DOI:https://doi.org/10.1016/j.actamat.2019.10.003· OSTI ID:1574017

Jensen, Brandt A. ^[1]; Tang, Wei ^[1]; Liu, Xubo ^[1]; Nolte, Alexandra I. ^[1]; Ouyang, Gaoyuan ^[2]; Dennis, Kevin W. ^[1]; Cui, Jun  ^[2]

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1. Ames Lab., Ames, IA (United States). Division of Materials Sciences and Engineering
2. Ames Lab., Ames, IA (United States). Division of Materials Sciences and Engineering; Iowa State Univ., Ames, IA (United States). Dept. of Materials Science and Engineering

MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 × 10⁶ J m^-3), and good magnetization (81 emu g^-1) at room temperature. Although the theoretical maximum energy product (BH)_max of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 °C, which makes it a potential candidate for high temperature applications such as permanent magnet motors. However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn₅₅Bi₄₅ resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. In this paper, we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (M_S) of 79 emu g^-1 at 300 K in the annealed Mn₅₂Bi₄₈ ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5–5 µm particles with M_S of 75.5 emu g^-1, coercivity H_ci of 10.8 kOe, and (BH)_max of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased H_ci by up to 21%, and also resulted in an increase in M_S up to 78.8 emu g^-1.

• Research Organization: Ames Lab., Ames, IA (United States); Iowa State Univ., Ames, IA (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE)
• Grant/Contract Number: EE0007794; AC02-07CH11358
• OSTI ID: 1574017
• Alternate ID(s): OSTI ID: 1573438; OSTI ID: 1864011
• Report Number(s): IS-J 10072
• Journal Information: Acta Materialia, Vol. 181, Issue C; ISSN 1359-6454
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
non-rare-earth
permanent magnet
MnBi