Preparation of Sm−Fe−N Bulk Magnets with High Maximum Energy Products

Abstract

In an effort to increase the maximum energy product ((BH)max) and coercivity (HcJ) of Zn-bonded Sm−Fe−N magnets, we developed a process for preparing Sm−Fe−N and Zn powders with low oxygen contents and subjected them to spark plasma sintering. The oxygen content, remanence, and coercivity of the Sm−Fe−N powder were 0.22 wt%, 151 A•m 2 •kg −1 , and 0.72 MA•m −1 , respectively. The oxygen content and secondary average particle size of the Zn powder were 0.083 wt% and 0.93 μm, respectively. The magnetic properties of the Zn-free Sm−Fe−N magnets included an HcJ of 0.86 MA•m −1 and a (BH)max of 188 kJ•m −3 , while the Zn-bonded (10 wt%) Sm−Fe−N magnets exhibited excellent magnetic properties with a (BH)max of 200 kJ•m −3 and an HcJ of 1.28 MA•m −1. Compared with previous studies, this is the high (BH)max observed for a Sm−Fe−N bulk magnet simultaneously displaying a high HcJ. The (BH)max of the Zn-bonded magnets was greater than that of the Zn-free magnets owing to the higher relative density of the former. Therefore, Zn is an effective binder for increasing not only the coercivity but also the density of Sm−Fe−N magnets. Consequently, the procedure reported herein permits the successful preparation of high-performance Sm−Fe−N bulk magnets.

Recent progress in the development of high-performance bonded magnets using rare earth–Fe compounds

“…The HPMR-Zn powder and Sm-Fe-N powder with low oxygen content were used to fabricate high-(BH) max Zn-bonded magnets via SPS, which is shown as "(2)" in Figure 12. As reported in our previous papers [81,84], it is found that the HPMR method can prepare Zn powder with sub-micrometer particle size and low oxygen content. The primary average particle and secondary average particle size of Zn were…”

“…this kind of magnet with high (BH) max , it is necessary to develop a densification process and decrease the Zn content while maintaining high coercivity. Several processes have been used to increase the relative density of Zn-bonded Sm-Fe-N magnets, including hot isostatic pressing [74], hot-rolling [78], swaging [80], and spark plasma sintering (SPS) [77,79,83,84].…”

“…Figure 15. Magnetic properties of Zn-bonded Sm-Fe-N magnet prepared using lowoxygen-content Sm-Fe-N and Zn mixed powders[84].…”

“…They also pointed out that Zn 7 Fe 3 phase, which is indexed as -FeZn phase in the Fe-Zn binary alloy phase diagram, was observed in Zn-bonded Sm-Fe-N magnets after annealing. After this paper, many researchers attempted to improve the magnetic properties by investigating the phase changes of Zn-bonded Sm-Fe-N magnets[74][75][76][77][78][79][80][81][82][83][84]. The influence of the -FeZn phase can be explained as follows:An oxidation layer is present on the surface of Sm 2 coercivity Zn-bonded magnets, and consisted of fine grains of -FeZn, -FeZn, and Sm-O.…”

Novel powder processing technologies for production of rare-earth permanent magnets

“…They applied this process to prepare Zn-bonded magnets, and succeeded in improving the coercivity while suppressing the decrease in remanence [39]. As the result, the obtained magnets exhibited a very excellent (BH) max of 200 kJ/m -3 .…”