Final Technical Report for the project titled "Manganese Based Permanent Magnet with 40 MGOe at 200°C"
The objective of project was to develop MnBi based permanent magnet for high temperature application (~150°C). This objective is derived based on MnBi’s unique positive temperature dependence of coercivity, which is doubled from ~1 T at RT to ~2.5 T at 200°C. Because of its limited magnetization (<0.9 T at RT), the MnBi magnet is best suited to fill in the gap between rare earth based NdFeB-Dy or SmCo magnet (20 MGOe) and the AlNiCo magnet (10 MGOe) at 150°C. It is expected that if successfully developed, MnBi will effectively mitigate the world’s demand on Dy. Before this project, the highest LTP content in MnBi powder is about 90% if the quantity of the powder is less than 5 gram (using melt-spin method); or 80% if the quantity is greater than 100 gram (using conventional powder metallurgical method such as arc melting and annealing). After this project, large quantities (5kg/batch) with high LPT phase content (>92 wt%) can be routinely synthesized. This achievement is made possible by the newly developed synthesis method based on conventional metallurgical processing technique involving arc melting, two-stage ingot annealing, grinding, sieving, and vacuum annealing. Before this project, the finest powder particle size is about 35 μm with overall powder composition maintaining at about 85% LTP phase. The reason why LTP phase content is listed along with particle size is because LTP MnBi is easy to decompose when exposed to temperature higher than 350 °C. As result, only low energy ball milling can be used to refine the particle size; moreover, the ball milling time cannot exceed 4 hrs, or else the decomposed LTP MnBi phase will exceed 10%. After this project, the finest powder size is reduced to 1~5 μm while maintain the 90% LTP MnBi phase content. This achievement is made possible by a newly developed cryogenic ball milling system, which provides -70 °C ambient for the rolling container. Before this project, it is not clear if MnBi will ferromagnetically exchange-couple with soft magnetic phase such as Fe or Co. After this project, it is established that MnBi will ferromagnetically exchange couple with Co, but not with Fe. It is also possible for MnBi to ferromagnetically exchange couple with Fe-Co alloy, but the amount of Fe cannot be more than 50 at.%. This conclusion is made possible by a series of electronic structure calculation followed by a series of thin film experimentation. As the result, 25 MGOe energy product was demonstrated using a MnBi-Co film. Before this project, the highest energy product for a bulk MnBi magnet is about 5 MGOe with 70% green density, and near-fully dense magnet is not available. After this project, the highest energy density is about 8.6 MGOe with 95% green density. This achievement is made possible by a modified warm-compaction system developed at University of Texas at Arlington. This system has 2.1 T alignment field vs the previous 1.8 T, and the compaction ambient maintains <1 ppm oxygen partial pressure. The estimated cost of MnBi magnet is about $110/kg when conventional magnet fabrication method is used, and about $84/kg when warm extrusion method is used. In comparison the cost of NdFeB, SmCo, AlNiCo, and Sr-Ferrite magnets is $150/kg, $180/kg, $119/kg, and $20/kg, respectively. The near term future work should focus on further improve the purity of the LTP MnBi, pushing it from the current 91 wt.% to 99 wt.%. If successful, the increased 8% LTP phase will increase the remanent magnetization, which in turn, increase the energy product. In addition, high reduction ratio warm extrusion method should be investigated to further push the texture to >90%.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE Advanced Research Projects Agency - Energy (ARPA-E)
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1234967
- Report Number(s):
- Final Report
- Country of Publication:
- United States
- Language:
- English
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