Production of Sm₂Fe₁₇N₃ Bulk Magnets

Abstract

Sm₂Fe₁₇N₃ powder exhibits excellent magnetic properties but is unstable and decomposes into α-Fe and SmN phases at high temperatures. Therefore, the key to producing Sm₂Fe₁₇N₃ bulk magnets is to reduce the deterioration of Sm₂Fe₁₇N₃ powder during sintering. Herein, Sm₂Fe₁₇N₃ bulk magnets were made using the spark plasma sintering (SPS) method with the addition of zinc stearate powder and zinc powder. Adding small amounts of zinc stearate powder and zinc powder improved the magnetic anisotropy and the coercivity of the magnets, respectively. The magnets produced by the SPS method using zinc stearate powder and zinc powder exhibited enhanced magnetic properties almost comparable to those of Sm₂Fe₁₇N₃ powder.

1. Introduction

High-performance neodymium-iron-boron (Nd-Fe-B) magnets have been applied to various advanced electromagnetic devices, including hard disk drives, electric vehicles, and medical equipment [1,2,3]. In particular, the production of electric cars equipped with Nd-Fe-B magnet motors has considerably increased owing to the global trend of enacting regulations to reduce the production of internal combustion engines, which emit large volumes of greenhouse gases [4]. The growing demand for high-performance Nd-Fe-B magnets has raised concerns over the prices of rare-earth materials and their availability [5,6,7,8,9,10]. Therefore, the development of permanent magnets using the relatively abundant rare-earth element Sm has recently attracted interest.

Samarium-iron-nitride (Sm₂Fe₁₇N₃) powder consists of the Sm₂Fe₁₇N₃ compound. The Sm₂Fe₁₇N₃ compound has been produced by the nitrogenation of Sm₂Fe₁₇ alloy powder, and thus, the Sm₂Fe₁₇N₃ compound has been made in powder form. The Sm₂Fe₁₇N₃ compound possesses not only high saturation magnetization comparable to the Nd₂Fe₁₄B compound but also a higher Curie temperature and a higher uniaxial anisotropy [11,12,13]. However, the phase decomposition of the Sm₂Fe₁₇N₃ compound at high temperatures above 873 K makes it difficult to produce Sm₂Fe₁₇N₃ bulk magnets in practice [14]. Several attempts have been made to integrate Sm₂Fe₁₇N₃ powder into bulk magnets [15,16,17,18]. Sm₂Fe₁₇N₃ bulk magnets have been produced by nonconventional consolidation techniques such as dynamic compaction using explosion or gun methods [15,16]. The dynamic compaction has realized high-density Sm₂Fe₁₇N₃ bulk magnets. Another nonconventional consolidation technique, the so-called compression shearing method, has been successfully applied to produce the Sm₂Fe₁₇N₃ bulk magnets [17]. The compression shearing method has also realized high-density Sm₂Fe₁₇N₃ bulk magnets. However, there has yet to be a practical application of these techniques due to the difficulty in production.

The SPS method has recently been effectively applied to produce Sm₂Fe₁₇N₃ bulk magnets [18,19]. The addition of zinc powder has been suggested to improve the coercivity of Sm₂Fe₁₇N₃ bulk magnets [20,21]. However, these magnets were magnetically isotropic, and their remanence magnetization was not comparable to that of Sm₂Fe₁₇N₃ powder. The key to producing high-performance Sm₂Fe₁₇N₃ bulk magnets using the SPS method is to increase their remanence. As Sm₂Fe₁₇N₃ powder possesses high magnetization, one solution is to align Sm₂Fe₁₇N₃ powder before sintering. Herein, anisotropic Sm₂Fe₁₇N₃ bulk magnets were successfully produced using the SPS method with the addition of zinc stearate powder and zinc powder, and the effects of their addition on the magnetic properties of the magnets were examined.

2. Results and Discussion

The images of the Sm₂Fe₁₇ alloy powder and Sm₂Fe₁₇N₃ powder are shown in Figure 1. The Sm₂Fe₁₇ alloy powder was made using the diffusion reduction process [22]. The Sm₂Fe₁₇ alloy powder was a coarse powder with an average particle size of about 30 μm. The Sm₂Fe₁₇N₃ powder was first made by nitrogenation of the Sm₂Fe₁₇ alloy powder and then pulverized into fine powder. The average particle size of the Sm₂Fe₁₇N₃ powder was about 3 μm. The fine Sm₂Fe₁₇N₃ powder was used in this study.

Before the consolidation of the Sm₂Fe₁₇N₃ powder using the SPS method, the powder was characterized by X-ray diffraction and thermomagnetic studies. Figure 2 shows the X-ray diffraction pattern of the Sm₂Fe₁₇N₃ powder, together with that of the Sm₂Fe₁₇ alloy powder. The X-ray diffraction pattern of the Sm₂Fe₁₇ alloy powder shows a typical Th₂Zn₁₇-type phase. The diffraction peaks of the Sm₂Fe₁₇ alloy powder were indexed to be the Th₂Zn₁₇-type phase, and no clear diffraction peaks of Sm-oxides and α-Fe phase were detected in the X-ray diffraction pattern. This indicates that the Sm₂Fe₁₇ alloy powder made using the diffusion reduction process consisted of the Th₂Zn₁₇-type Sm₂Fe₁₇ phase. The X-ray diffraction pattern of the Sm₂Fe₁₇N₃ powder also exhibits a typical Th₂Zn₁₇-type phase. Although it is known that the Sm₂Fe₁₇N₃ phase decomposes into the α-Fe and SmN phases at high temperatures [14], no diffraction peaks of these phases were noticed in the X-ray diffraction pattern. It is confirmed that the Sm₂Fe₁₇N₃ powder consisted of the Sm₂Fe₁₇N₃ phase.

Figure 3 shows the thermomagnetic curve of the Sm₂Fe₁₇N₃ powder, together with that of the Sm₂Fe₁₇ alloy powder. It was found that the thermomagnetic curves of the Sm₂Fe₁₇ alloy powder and the Sm₂Fe₁₇N₃ powder are pretty different. The thermomagnetic curve of the Sm₂Fe₁₇ alloy powder shows a magnetic transition at around 490 K. According to the results of the X-ray diffraction studies, the Sm₂Fe₁₇ alloy powder consisted of the Th₂Zn₁₇-type Sm₂Fe₁₇ phase. Thus, the magnetic transition is believed to be the Curie temperature of the Sm₂Fe₁₇ phase. On the other hand, a large magnetic transition at around 740 K is observed in the thermomagnetic curve of the Sm₂Fe₁₇N₃ powder. Since the Sm₂Fe₁₇N₃ powder consisted of the Sm₂Fe₁₇N₃ phase, the magnetic transition is believed to be the Curie temperature of the Sm₂Fe₁₇N₃ phase. The Sm₂Fe₁₇N₃ phase has a significantly high Curie temperature of 740 K, compared with the Nd₂Fe₁₄B phase in the Nd-Fe-B magnets (585 K) [23]. The observed Curie temperature of the Sm₂Fe₁₇N₃ phase is consistent with the reported value [11]. No magnetic transition of the Sm₂Fe₁₇ phase was noted in the thermomagnetic curve, indicating that the Sm₂Fe₁₇N₃ powder solely consisted of the Sm₂Fe₁₇N₃ phase.

However, a slight increase in magnetization at temperatures over the Curie temperature is observed in the thermomagnetic curve of the Sm₂Fe₁₇N₃ powder. This suggests that a new magnetic phase formed during the thermomagnetic measurement. The new magnetic phase is believed to be the α-Fe phase since the new magnetic phase shows the Curie temperature near 1050 K. The thermomagnetic study indicates that the Sm₂Fe₁₇N₃ phase decomposes into the α-Fe and SmN phases at high temperatures over 800 K. The observed decomposition temperature is slightly lower than the reported value (873 K) [14]. In any case, it is essential to sinter the Sm₂Fe₁₇N₃ powder at the lowest possible temperature because the Sm₂Fe₁₇N₃ phase in Sm₂Fe₁₇N₃ powder decomposes into the α-Fe and SmN phases at high temperatures.

In order to confirm the decomposition of the Sm₂Fe₁₇N₃ powder, the specimen after the thermomagnetic measurement was examined using X-ray diffraction. The result is shown in Figure 4. No diffraction peaks of the Sm₂Fe₁₇N₃ phase were seen in the X-ray diffraction pattern. Instead, diffraction peaks of the α-Fe and SmN phases were noticed in the X-ray diffraction pattern. This confirms that the Sm₂Fe₁₇N₃ phase decomposes into the α-Fe and SmN phases during thermomagnetic measurement.

Figure 5 shows the hysteresis loops of the Sm₂Fe₁₇ alloy powder and the Sm₂Fe₁₇N₃ powder. The Sm₂Fe₁₇ alloy powder showed a narrow hysteresis loop, but the Sm₂Fe₁₇N₃ powder exhibited a wide one. This indicates that the Sm₂Fe₁₇ alloy powder without nitrogenation did not display large coercivity. On the other hand, Sm₂Fe₁₇N₃ powder, prepared by nitriding the Sm₂Fe₁₇ alloy powder, exhibited a large coercivity. The Sm₂Fe₁₇N₃ powder exhibits a large coercivity of 12.4 kOe with a remanence of 75.3 emu/g.

In order to confirm the magnetic anisotropy of the Sm₂Fe₁₇N₃ powder, the magnetic properties of the Sm₂Fe₁₇N₃ powder were measured parallel to the magnetic alignment after the Sm₂Fe₁₇N₃ powder had been magnetically aligned. Figure 6 shows the hysteresis loops of the Sm₂Fe₁₇N₃ powder measured parallel and perpendicular to the direction of the magnetic alignment. The respective hysteresis loops are quite different. This confirms that the Sm₂Fe₁₇N₃ powder is magnetically anisotropic. The powder exhibited a high remanence of 107 emu/g and a large coercivity of 10.3 kOe when measured parallel to the direction of the magnetic alignment.

In this study, the Sm₂Fe₁₇N₃ powder was sintered into bulk magnets carried out at 673 K, far below its decomposition temperature of 800 K. Since the Sm₂Fe₁₇N₃ powder is magnetically anisotropic, the Sm₂Fe₁₇N₃ powder was magnetically aligned before sintering (see Figure 7). Despite the low sintering temperature, Sm₂Fe₁₇N₃ bulk magnets were produced from the magnetically aligned Sm₂Fe₁₇N₃ powder using the SPS method.

Carbon contamination from carbon dies might occur in the SPS method [24]. To avoid this, the SPS experiment was done with a BN-coated carbon die, and the surface of the Sm₂Fe₁₇N₃ bulk magnets was ground to remove carbon contamination before the property measurements.

A photograph of the Sm₂Fe₁₇N₃ bulk magnet is shown in Figure 8. No significant cracks were found in the Sm₂Fe₁₇N₃ bulk magnet. The relative density of the Sm₂Fe₁₇N₃ bulk magnet was as high as 91.7% (compared with the density of the Sm₂Fe₁₇N₃ powder). A higher consolidation temperature is known to promote the densification of bulk materials produced using the SPS method [25]. A further increase in the consolidation temperature may result in an increase in density, but the consolidation temperature was kept below 673 K in this study because the Sm₂Fe₁₇N₃ phase in Sm₂Fe₁₇N₃ powder easily decomposes into the α-Fe and SmN phases at high temperatures.

Figure 9 shows the magnetic hysteresis loops of the Sm₂Fe₁₇N₃ powder and the magnet produced. The powder exhibited a high remanence of 107 emu/g and a large coercivity of 10.3 kOe. However, the magnets showed a remanence of 68.6 emu/g and a coercivity of 3.05 kOe. This indicates that the magnetic properties of the powder deteriorated during sintering. Thus, it is essential to reduce the deterioration of the magnetic properties of Sm₂Fe₁₇N₃ powder during sintering.

As remanence is related to the magnetic alignment of the powder in the magnet, small amounts of sintering aid powder were added to the Sm₂Fe₁₇N₃ powder before magnetic alignment, and its effects on the magnetic properties of the magnets were examined. In this study, various sintering aid powders, such as graphite and silica powders, were applied for this purpose. It was found that the zinc stearate powder could improve the density and magnetic alignment of the Sm₂Fe₁₇N₃ powder. The effects of zinc stearate powder addition on the relative density of the Sm₂Fe₁₇N₃ bulk magnets are shown in Figure 10. The relative density of the Sm₂Fe₁₇N₃ bulk magnet increased from 91.7% to 95.8% as the zinc stearate content increased. Although the density of the zinc stearate powder was 1.095 g/cm³, much smaller than that of the Sm₂Fe₁₇N₃ powder (7.66 g/cm³), the zinc stearate powder drastically improved the density of the Sm₂Fe₁₇N₃ bulk magnet. The resultant Sm₂Fe₁₇N₃ bulk magnet with small amounts of zinc stearate powder had a high density of over 95%.

Figure 11 shows the effects of zinc stearate powder addition on the remanence and coercivity of the magnets. The remanence significantly increased but then decreased as the amount of zinc stearate powder was increased. In contrast, the coercivity gradually decreased as the amount of zinc stearate increased. This confirms that the addition of zinc stearate powder before magnetic alignment improved the magnetic alignment of the Sm₂Fe₁₇N₃ powder.

Figure 12 shows the hysteresis loops of the Sm₂Fe₁₇N₃ bulk magnet with 1 wt% zinc stearate powder measured parallel and perpendicular to the magnetic alignment in order to examine the magnetic anisotropy. The Sm₂Fe₁₇N₃ bulk magnet is anisotropic and shows a higher remanence, 90.6 emu/g, in the parallel direction than the perpendicular direction. The magnet exhibits magnetic anisotropy due to the magnetic alignment of the Sm₂Fe₁₇N₃ powder.

As the magnetic alignment of the Sm₂Fe₁₇N₃ powder is determined via the crystallographic alignment in the Sm₂Fe₁₇N₃ phase, the crystallographic alignment of the Sm₂Fe₁₇N₃ phase in the magnets was examined using X-ray diffraction patterns. Figure 13 shows the X-ray diffraction patterns of the magnet with 1% zinc stearate powder. As shown in the figure, X-ray diffraction patterns were measured perpendicularly and parallel to the direction of magnetic alignment. The X-ray diffraction pattern of the magnet shows a typical powder pattern of the Sm₂Fe₁₇N₃ phase when measured perpendicular (⊥) to the direction of magnetic alignment. However, when measured parallel (//) to the direction of magnetic alignment, the X-ray diffraction pattern exhibited a prominent (006) peak of the Sm₂Fe₁₇N₃ phase owing to the crystallographic alignment of the c-axis of the Sm₂Fe₁₇N₃ phase in the magnet. This confirms that the crystallographic alignment of the Sm₂Fe₁₇N₃ phase was retained in the Sm₂Fe₁₇N₃ bulk magnet even after sintering using the SPS method.

In the Nd-Fe-B magnets, the crystallographic alignment of the tetragonal Nd₂Fe₁₄B phase in the Nd-Fe-B magnets has been evaluated by the ratio, I(006)/I(0410), of the intensity of the c-axis-related peak (006) to that of the strongest peak (410) [26,27]. As the same token, the crystallographic alignment of the Sm₂Fe₁₇N₃ phase can be evaluated by the ratio I(006)/I(303). The ratio of I(006)/I(303) was 0.27 for the Sm₂Fe₁₇N₃ bulk magnet measured perpendicular to the direction of magnetic alignment, while the ratio of I(006)/I(303) was 10.8 for the Sm₂Fe₁₇N₃ bulk magnet measured parallel to the direction of magnetic alignment. This indicates the crystallographic alignment of the Sm₂Fe₁₇N₃ phase in the magnet.

As the coercivity of the magnets was improved by adding a small amount of Zn [20,21], zinc powder and 1% zinc stearate powder were added to the Sm₂Fe₁₇N₃ powder before magnetic alignment. Consequently, the effects of this addition on the density and magnetic properties of the magnets were examined. Figure 14 shows the effects of zinc powder addition on the relative density of the Sm₂Fe₁₇N₃ bulk magnets with 1% zinc stearate powder. Unlike in the case of the zinc stearate powder addition, the relative density of the Sm₂Fe₁₇N₃ bulk magnets decreased from 95.4% to 91.5% as the zinc content increased. This indicates that the addition of zinc powder deteriorated the density of the Sm₂Fe₁₇N₃ bulk magnets with 1% zinc stearate powder.

Figure 15 shows the effects of zinc powder addition on the remanence and coercivity of the Sm₂Fe₁₇N₃ bulk magnets. Since the 1 wt% zinc stearate powder addition effectively increased the magnetic alignment of the Sm₂Fe₁₇N₃ powder, the Sm₂Fe₁₇N₃ bulk magnets with 1 wt% zinc stearate powder were used as the base magnet. While the remanence slightly decreased as the amount of zinc powder increased, the coercivity considerably increased with increasing amounts. This verifies that the addition of zinc powder before the magnetic alignment improved the coercivity of the magnets.

Figure 16 shows the hysteresis loop of the magnet with 1 wt% zinc stearate and 5 wt% zinc powder. The hysteresis loop of the magnet without additives is also shown. The magnet with 1 wt% zinc stearate and 5 wt% zinc powder exhibited a wider hysteresis loop than the magnet without additives, indicating that the use of small amounts of zinc stearate powder and zinc powder increased the magnetic properties of the Sm₂Fe₁₇N₃ bulk magnet. The magnet with 1 wt% zinc stearate and 5 wt% zinc powder showed a high remanence of 86.8 emu/g and a large coercivity of 9.82 kOe.

Compared with the hysteresis loop of the Sm₂Fe₁₇N₃ powder (see Figure 6), the remanence and coercivity of the Sm₂Fe₁₇N₃ bulk magnets with 1 wt% zinc stearate powder were slightly smaller than those of the Sm₂Fe₁₇N₃ powder. Thus, further work is still necessary to improve the magnetic properties of the Sm₂Fe₁₇N₃ bulk magnets.

3. Materials and Methods

Sm₂Fe₁₇ alloy powder and Sm₂Fe₁₇N₃ powder, prepared by the nitrogenation of Sm₂Fe₁₇ alloy powder, were supplied by Sumitomo Metal Mining Co., Ltd. (Tokyo, Japan). The powder was poured into a BN-coated carbon die and magnetically aligned using an electromagnet before being subjected to spark plasma sintering. Sm₂Fe₁₇N₃ bulk magnets were produced from the Sm₂Fe₁₇N₃ powder using the SPS method under the applied pressure of 100 MPa using a spark plasma sintering apparatus (Plasman, S. S. Alloy, Hiroshima, Japan). The SPS method made the sintering in a vacuum at temperatures between 473 K and 673 K for 300 s. The specimens were cooled in the SPS chamber in a vacuum to room temperature. In order to improve the magnetic properties of the magnets, zinc stearate powder (0–3 wt%) and zinc powder (0–5 wt%) were added to the Sm₂Fe₁₇N₃ powder, and the mixed powders were magnetically aligned before sintering using the SPS method.

The properties of the original powders and the magnets produced using the SPS method were examined. The morphology of the original powders was investigated using a scanning electron microscope (JSM-IT300LA, JEOL, Tokyo, Japan). The densities of the magnets were measured using the Archimedes method using an electronic balance (GR-120, AND, Tokyo, Japan). The specimens were characterized by X-ray diffraction using Cu Kα radiation using an X-ray diffractometer (MiniFlex600, Rigaku, Tokyo, Japan) and magnetic measurements by a vibrating sample magnetometer (BHV-525RSCM, Riken Denshi, Tokyo, Japan). Before magnetic hysteresis measurements of the Sm₂Fe₁₇N₃ powder were performed, the powder was magnetically aligned and mixed with paraffin. To measure the magnetic properties of the Sm₂Fe₁₇N₃ bulk magnets, rod-like specimens (1 × 1 × 10 mm) were measured parallel to the length direction to avoid demagnetization correction.

4. Conclusions

Although Sm₂Fe₁₇N₃ powder was magnetically anisotropic and possessed high remanence and coercivity, Sm₂Fe₁₇N₃ bulk magnets produced from Sm₂Fe₁₇N₃ powder using the SPS method exhibited deteriorated remanence and coercivity. Herein, Sm₂Fe₁₇N₃ bulk magnets were made using the SPS method with the addition of zinc stearate powder and zinc powder. It was found that the magnetic anisotropy of the magnets was enhanced upon the addition of zinc stearate powder, and the coercivity of the magnets was enhanced upon the addition of zinc powder. The Sm₂Fe₁₇N₃ bulk magnets with 1 wt% zinc stearate powder and 5 wt% zinc powder exhibited a remanence of 86.8 emu/g and coercivity of 9.82 kOe.