Takahiko Iriyama

Abstract:

The demand for high-performance rare-earth-iron permanent magnets in various applications such as motors/sensors for automotive, electronic devices, or home appliances has increased in recent years. Sintered or hot-deformed Nd-Fe-B magnets [1, 2] are the most attractive choice because they have the highest (BH)max value among all the commercial magnetic materials. The addition of heavy rare earth elements (HREEs), such as Dy or Tb, is a common way to increase the coercivity of Nd-Fe-B magnets thus allowing the use of such magnets at high temperatures. The problem with the HREE addition, however, is the reduction of (BH)max. Another problem, which is even more serious, is the supply risk of HREEs due to geo-political and environmental issues. Accordingly, efforts to reduce HREE use have been undertaken all over the world. Fabrication processes utilizing grain boundary diffusion (GBD) have recently been developed to effectively reduce HREE usage by more than 50%. The ultimate goal, however, is the creation of HREE-free Nd-Fe-B magnets with high coercivities.
It is well known that decreasing the grain size increases coercivity. The hot-deformed magnet has a fine microstructure that is one order of magnitude finer than that of the conventional sintered magnet. Thus, hot-deformed Nd-Fe-B is a promising material to create high performance HREE-free magnets. The PLP (Press-less Process) proposed by one of the authors also realized a fine-grained sintered Nd-Fe-B magnet. This is because the atmosphere in the PLP process is inert throughout the processes and the resulting magnets contain a low level of oxygen inclusions [3]. By optimizing the fabrication processes and microstructures, high coercivities of about 1600 kA/m (20 kOe) have been obtained for HREE-free magnets made with the above two types of fine-grained magnets [3, 4].
Sm-Fe-N materials also have demonstrated excellent magnetic properties and corrosion resistance when used as bonded magnets [5]. It is important to expand the market for the Sm-Fe-N from the resource management point of view because Sm is one of the surplus rare-earth elements. Various studies to improve the magnetic properties are now being conducted by many research groups.
In this symposium, recent developments for the above R-Fe-X permanent magnets will be presented.

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[2] J. F. Herbst, J. J. Croat, F. E. Pinkerton and W. B. Yelon, Phys. Rev., 29 (1984) 4176.
[3] M. Sagawa, Proceedings of the 21st International Workshop on Rare-Earth Permanent Magnets and their Applications, (2010) 183.
[4] K. Hioki, A. Hattori, and, T. Iriyama, J. Magn. Soc. Jpn., 38 (2014) 79.
[5] T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, H. Kato, Y. Nakagawa, IEEE Trans. Magn. 28 (1992) 2326-2331.