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Ames Lab team improves rare-earth-free MnBi magnets through microstructure engineering

Researchers from the Department of Energy’s Critical Materials Institute (CMI) and Ames National Laboratory have improved the properties of a rare-earth-free permanent magnet material and demonstrated the process can be upscaled for manufacturing. The researchers developed a new method of manufacturing manganese bismuth (MnBi) magnets based on microstructure engineering. This process is a step towards making compact, energy-efficient motors without the use of rare earths.

A paper on their work is published in the Journal of Magnetism and Magnetic Materials.

MnBi is a candidate material for high-temperature magnets because of its increasing coercivity with increasing temperatures up to 255 °C. However, most efforts in fabricating bulk MnBi magnets have run into the problem of preserving the coercivity (Hcj) of its feedstock powders. About 70% of powder’s Hcj would be lost during the densification process.

Our micromagnetic modeling shows that the coercivity mechanism of the MnBi bulk magnet is controlled by nucleation of the reversal magnetization domains, and the large Hcj loss that occurred during the powder consolidation process can be attributed to the inter-grain magnetic coupling. To attain a high Hcj, the grains in the MnBi bulk magnet must be separated with a non-magnetic grain boundary phase (GBP).

To validate this GBP hypothesis, we engineered MnBi bulk magnets with two different types of GBP. The first type of GBP was created in-situ by precipitating excessive Bi from the grains; the second type was created ex-situ by coating silicates on the feedstock powders before the consolidation. While both GBP work, the ex-situ approach resulted in a better Hcj due to a more uniform GBP distribution. The Hcj loss was reduced from 70% to 15%, and the (BH)max of a warm sintered bulk magnet reached 8.9 MGOe.

—Tang et al.

Permanent magnets used for motors require high energy density, or high levels of magnetism and coercivity. Coercivity is a magnet’s ability to maintain its current level of magnetism, despite exposure to high heat and outside influences that could demagnetize it.

The challenge with MnBi is that traditional manufacturing methods require high heat to transform the individual materials into a large magnet. The necessary heat reduces the energy density of the magnet. To address this problem, the team developed an alternative process.

The researchers started with a very fine powder for each of the materials, which increases the starting magnetic energy level. Next, they used a warm heating method rather than a high-temperature method for forming the magnet. Finally, the key to their new process was to add a non-magnetic component that would keep the grain particles from touching each other. This additional element, called a grain boundary phase, provides more structure to the magnet, and keeps the magnetism running through individual particles/grains from affecting one another.

Graph abstract

Grain boundaries and magnetism. (a) Shows the magnetic MnBi particles/grains separately in the powder. (b) Shows the “bulk” version, where the particles/grains are now touching because of the process of forming the magnet without any added grain boundary materials. (c) Shows the MnBi grains coated with the grain boundary material, illustrating how the grains are no longer touching. The graph illustrates the demagnetization of the magnet in the bulk and powder phases.


The effect of the warm temperature on the magnetic properties of MnBi is unique. The researchers expected the coercivity and magnetism to decrease with increasing temperature, which is true for most magnetic materials. However, for MnBi, the warm temperature increased the coercivity and decreased the magnetization. This increased coercivity helps to keep the magnet more stable at elevated temperatures than other known magnets.

The team also focused on making larger magnets, compared to the typically small magnets developed in labs. Upsizing the magnets helps to demonstrate to the manufacturing companies that they can build large magnets on a commercial scale.

The team is currently working with PowderMet Inc., using their patent-pending techniques to pursue mass production of the MnBi magnets for use in novel electric motors. That project is funded by the DOE Small Business Innovation Research program. The project has already entered phase II.

Resources

  • Wei Tang, Gaoyuan Ouyang, Xubo Liu, Jing Wang, Baozhi Cui, Jun Cui (2022) “Engineering microstructure to improve coercivity of bulk MnBi magnet,” Journal of Magnetism and Magnetic Materials, doi: 10.1016/j.jmmm.2022.169912

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