Eliminating Heavy Rare-Earth Elements in Permanent Magnets Through Grain Boundary Engineering

High performance electric motors for electric vehicles, efficient wind generators and a variety of commercial and industrial motors all require high-performance permanent magnets (PM) which can maintain their energy product at temperatures up to 200°C. For the Nd2Fe14B based PMs, this has been achieved primarily by replacing the Nd with heavy rare-earth elements (Tb, Dy and Ho). Of the rare-earth elements (REE), these are truly rare, with the cost of Dy typically about 10x that of Pr and Nd. The heavy REEs increase the temperature range by enhancing anisotropy field and lowering its temperature dependence. They also lower the remanence of the alloy relative to the light REE. While applying the Dy only to the surfaces of the magnetic reduces the overall amount needed, it does increase the processing steps needed for the final product. An alternative approach to enhancing the temperature range is to reduce the grain size (dia.< 2 µm), so that grain sizes are closer to those of the magnetic domain size, hence improving the resistance to demagnetization. However, reducing the grain size in sintered magnets is fraught with several challenges. Not only does the grain size need to be reduced, but the distribution of grain sizes also needs to be top-size limited since a small number of large grains can have a disproportionately large effect on decreasing coercivity. Finer grained samples are more susceptible to oxidation, harder to fully magnetically align and, most importantly, need to be sintered to full density without excessive grain growth. We will present results from various methods for reducing grain size, passivating the grains, and using sintering aids to minimize grain growth.