Permanent Magnets Beyond Nd-Dy-Fe-B

Abstract

Permanent magnets are one of the key parts in energy conversion devices to realize the utmost efficiency. DC motors up to the 100-kW class are used in some ‘‘strong’’ hybrid electric vehicles (HEV), in which the top-grade Nd-Dy-Fe-B-type permanent magnets are required. Such motors have inner permanent magnet (IPM) rotors with which the combination of magnet torque and reluctance torque is optimized. For such an optimized design, permanent magnets are shaped in thin plate-like blocks with a thickness of about 5 mm, inserted into slots in a laminated rotor core, and magnetized perpendicularly to the largest surfaces of the block. The weight of the magnets may amount to about 2 kg or less per vehicle. The reason why Nd-Dy-FeB permanent magnets are used is simply because the highest performance of this class of material in terms of residual flux density, Br, can be generated with them. These magnets have high intrinsic coercivity, HcJ, typically about 2.0–2.4 MA/m. Such high HcJ values are certainly not necessary at room temperature, but the magnet temperature can rise to about 200C at which HcJ is only about 0.6–0.8 MA/m, being marginally enough to withstand the armature magnetic fields generated by driving current. Dysprosium (Dy) or terbium (Tb) is used in the magnet in order just to obtain such high HcJ but have been indispensable in the permanent magnets. Since Dy and Tb can be mined industrially only in a southern province of China in a limited quantity, Dy and Tb are now typical critical elements, overreliance on these elements should be avoided. For this very reason, permanent magnets beyond Nd-Dy-Fe-B are now extensively sought. Development of permanent magnets beyond the Nd-Dy-Fe-B is a challenge because Nd2Fe14B, the main hard magnetic phase of the magnet, is the compound that has the largest magnetization at room temperature among all existing hard magnetic compounds, and that, in terms of elemental abundance, already consists mainly of Fe and Nd, both of which are the most abundant elements carrying magnetic moments in the 3d and 4f transition metal series, respectively. One of the approaches is to reduce the overall average Dy content over the entire commercial grades down to a sustainable level, which may be about 1–1.5 mass percent in the magnets, even if the crustal abundance ratio with Nd is considered. Another approach is to seek for an entirely new material with or without rare earth elements. In advanced economic blocks that have benefitted from functional critical elements, governments have taken strategic actions to promote developments in technologies that will lead to reduced society’s dependence upon those elements. In Japan, well