Is Bismuth the gift that keeps on giving U.S. Department of Energy Pacific Northwest National Laboratories
Creating Ultra-Pure Manganese Bismuth for Permanent Magnets
Battelle Number: 30264 | N/A
TECHNOLOGY OVERVIEW
From wind turbines to electric vehicle motors, permanent magnets play an essential role in many of today’s energy-producing devices. These magnets maintain their properties even when an inducing field is absent, but there’s a limited supply of the rare earth minerals that traditionally have been used in their construction. The manganese bismuth materials have shown lower energy production near room temperature compared with state-of-the-art neodymium-iron-boron magnets. However, magnets using iron coupled with manganese bismuth showed higher energy production than other magnets under development (Alnico, Co-Sm, BaO-Fe2O3, SrO-Fe2O3, and some low-grade iron-neodymium-boron). Moreover, in situations where the temperature will be greater than 100 degrees centigrade, a magnet of iron coupled with manganese bismuth shows energy production even greater than the state-of-the-art magnet. This manganese bismuth material holds great promise as a replacement for rare earth minerals. However, ultra-pure manganese bismuth material is needed to achieve high energy production and coupling with iron or soft magnets. Until now, no process has been able to fabricate the high-purity, low-temperature phase of the manganese bismuth material needed for viable mass production.
Pacific Northwest National Laboratory has developed such a process, yielding mass or bulk quantities of high purity (greater than 90%) low-temperature-phase manganese bismuth alloys suitable for the scalable production of permanent magnets. The method combines selected atomic ratios of manganese and bismuth metals in either an arc-melter or induction melter to form a stable composite alloy in the form of either an ingot or pellet. Using two temperature regimes with an oxygen-free or reducing gas environment, heat treatment, and particle size control results in a ferromagnetic material with coercivity values (the ability to maintain magnetic properties) that increase significantly with increasing temperatures.
Once thought impossible, the method has higher productivity and lower cost than traditional melt-spinning processes, and it is compatible with current industrial practices.
APPLICABILITY
This high-purity alloy is an ideal candidate for use in electric devices, as well as other applications with elevated temperatures (above 100 degrees centigrade). Specific areas include the following:
- Energy production with high efficiency (for example, wind turbines)
- Electrical motors for vehicles
- Magnetic materials for military applications
- Magnetic materials for toys
- Special high-temperature applications.
ADVANTAGES
- Contains no rare earth elements, which are currently scarce in the United States
- Has high productivity, higher purity, and lower cost than any other process used commercially or experimentally
- Is compatible with current industrial practices
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Ames Lab team improves rare-earth-free MnBi magnets through microstructure engineering
09 November 2022
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.