Increase of energy products of Zn-bonded Sm-Fe-N magnets with low oxygen content

To increase the maximum energy product, (BH)_max, of Zn-bonded Sm-Fe-N magnets, a process for preparing a Sm-Fe-N powder with a low oxygen content was developed. The Sm-Fe-N powder containing 0.23 wt% oxygen was prepared by ball milling and nitriding, and the powder remanence and coercivity were 140 A·m²kg⁻¹ and 0.72 MA·m⁻¹, respectively. Arc plasma deposition (APD) was used for Zn addition. Fine Zn particles several tens of nanometers in size were deposited on the Sm-Fe-N powder under vacuum conditions, and APD prevented the oxygen content increasing in the Zn-deposited Sm-Fe-N powder. The low-oxygen Sm-Fe-N powder was pressed and sintered by spark plasma sintering (SPS), and 3.3 wt% Zn-bonded and Zn-free Sm-Fe-N magnets were prepared. For both Zn-bonded and Zn-free Sm-Fe-N magnets, the density increased with increasing sintering pressure, and the relative density of the Zn-bonded Sm-Fe-N magnet was 89% and that of the Zn-free Sm-Fe-N magnet was 85% after SPS. The coercivities of the Zn-bonded and Zn-free Sm-Fe-N magnets were both high at 1.1 and 0.8 MA·m⁻¹, respectively. (BH)_max of these magnets increased with increasing relative density, and the highest (BH)_max value of the Zn-bonded Sm-Fe-N magnets was 153 kJ·m⁻³ and that of the Zn-free Sm-Fe-N magnet was 179 kJ·m⁻³.