Manganese-bismuth alloys are arguably the most advanced rare-earth-free alternative to the lower-grade NdFeB permanent magnets. Rare-earth-free MnBi-based magnets can fill the existing “gap” between the ferrite and rare-earth permanent magnets. The easily attainable magnetic hardness with the remarkable positive temperature coefficient of intrinsic coercivity Hc, together with the low cost of the raw materials, has attracted worldwide interest. A room-temperature maximum energy product (BH)max greater than 10 MGOe is sometimes considered the threshold for commercialization, whereas the intrinsic magnetic properties of the key α-MnBi compound allow for a (BH)max up to 20 MGOe. MnBi is a candidate for high-temperature magnets because of their increasing coercivity with increasing temperatures up to 255°C.
The alpha phase of MnBi is the only known material with a large positive temperature coefficient of coercivity (~1.5 %/ºC), which results in an Hcj as high as 28 kOe at 255ºC. In comparison, the most powerful high-temperature magnet SmCo5 has an Hcj of 16 kOe at 250ºC, while the 52 MGOe NdFeB magnet has an Hcj of only 2 kOe. It would take at least 8 wt.% of Dy, which is the most critical material according to the U.S. Department of Energy, to make the NdFeB magnet usable when the operating temperature exceeds 150ºC.
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. The challenge is how to achieve a high-volume fraction of the a-MnBi phase in a fully dense and highly textured bulk magnet. The melt solidification process must be fast enough to avoid Mn precipitation, while the temperature of the consolidation process must be low enough to avoid the alpha phase decomposition or transformation into the beta phase. Whereas the earlier approaches relied on sintering of fine and easily degrading single-crystal MnBi particles, the new method being commercialized by Magnet Energy achieves refinement of the key α-MnBi phase through melt-spinning combined with proprietary alloying.
