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Synthesis of Sm 2 Fe 17 N 3 powder by a reduction-diffusion technique requires a washing process to remove CaO and Ca residues, but it is typically associated with a decrease of coercivity. Synchrotron radiation X-ray diffraction study revealed forming of a SmFe 5 poor hard magnetic phase in a water washing treatment. Cross-sectional electron microscopic analysis of particles revealed an acetic acid washing treatment to dissolve the impurity phases accompanied surface damages on Sm 2 Fe 17 N 3 particles. A new washing process to avoid forming of the factors which decrease coercivity was examined, and Sm 2 Fe 17 N 3 powders having about 20% higher coercivity than that in the previous reports, achieving up to 28.1 kOe, were successfully demonstrated.

Submicron-sized Sm 2 Fe 17 N 3 powders having high coercivity were prepared by a newly-developed synthesis procedure for submicron-sized Sm 2 Fe 17 powders and nitridation process with a proper post-process for the powders. It was revealed that the washing step, which is performed to remove excess Ca, supplied hydrogen into the Sm 2 Fe 17 N 3 crystal structure, and elongation of the crystal structure along the c-axis by the supplied hydrogen reduced coercivity. This phenomenon was observed clearly in powder with a smaller size and good dispersity. When the powders were subjected to dehydrogenation treatment, they showed high coercivity, as expected from the particle size, and coercivity of 24.7 kOe was achieved with the 0.5 μm powder. The fact that the intrinsic good thermal stability of Sm 2 Fe 17 N 3 is maintained at the submicron-scale was also confirmed. In addition, the obtained powder exhibited a high maximum energy product after disintegration treatment under appropriate conditions. This study demonstrated the high potential of Sm 2 Fe 17 N 3 for surpassing the performance of Nd Fe B magnets under hot environments.

We investigated the cause of forming of aggregates which decreases magnetization of a submicron-sized Sm2Fe17N3 powder. The aggregation was considered to be caused through sintering and growth of α-Fe particles in a hydrogen reduction treatment. We newly synthesized cubic hematite powder which has tolerance for the sintering of particles in a hydrogen reduction treatment. The aggregation of submicron-sized Sm2Fe17N3 particles was significantly reduced by using the cubic hematite as a precursor, and the magnetization was improved without decreasing the high coercivity.