Synthesis of High Coercivity Sm₂Fe₁₇N₃ Powder

Abstract

In this paper, we report on our group’s efforts to improve the coercivity of Sm₂Fe₁₇N₃ powder, especially on reducing the particle size to submicron scale, smoothing the particle surface, and suppression of the formation of coarse particles by developing a new reduction-diffusion process. During the course of a series of these works, it was revealed that the washing step, which is performed to remove excess Ca, supplied hydrogen into the Sm₂Fe₁₇N₃ crystal structure, and induced unfavorable elongation of the crystal structure along the c-axis. To avoid this problem, the powders were subjected to dehydrogenation treatment, demonstrating reasonably high coercivity values that we expect from the known relationship between particle size and coercivity. It was also found that the conventional dissolution and the removal of impurities by acetic acid were roughening the particle surfaces. Thus, development of an alternative process to acetic acid cleaning prevented the surface roughening and showed the further improvement of the coercivity. Finally, the development of a new uniform reduction-diffusion reaction using a rotary furnace brought about a breakthrough for further improvement of coercivity by suppressing the formation of coarse particles. As a result, we succeeded in synthesizing Sm₂Fe₁₇N₃ anisotropic powder with an ultra-high coercivity (i.e. the current world record) of 31.7 kOe, and also showed that the powder can maintain a coercivity higher than 10 kOe at 200°C.