Fabrication of High-Density Zn-Bonded Sm–Fe–N Bulk Magnets via High-Velocity Compaction

Abstract

High-velocity compaction (HVC) using a die was investigated as a compaction technique for the fabrication of high-density isotropic Zn-bonded Sm–Fe–N bulk magnets. The compaction characteristics of Zn-mixed Sm–Fe–N powders obtained by HVC were investigated while varying the Zn content. Moreover, the magnetic properties, flexural strengths (σ), and microstructures of the resulting magnets were studied. The relative density (d_r) steadily increased with the piston velocity during compaction and reached approximately 90% at a piston velocity of 11.2 m·s⁻¹ (equivalent to a compaction pressure of 3.45 GPa), regardless of the Zn content. Because the magnets were fabricated using a die, they also had a high dimensional precision. The resulting magnet, with 5 wt% Zn and d_r of 89.1%, exhibited a maximum energy product ((BH)_Max) of 54.4 kJ·m⁻³, remanence (B_r) of 0.56 T, and coercivity (H_cJ) of 965 kA·m⁻¹. σ exceeded 100 MPa when d_r was above 88%, which satisfies the required mechanical strength for applications such as permanent magnet motors.

Fig. 4 Dependences of d_r and magnetic properties (B_r, H_cJ, and (BH)_Max) on the piston velocity for the isotropic Zn-bonded Sm–Fe–N magnets with various Zn contents fabricated using HVC and subsequently annealed at 450°C. The inset shows a photograph of the green compact (5 wt% Zn) immediately after compaction.

1. Introduction

The demand for high-efficiency permanent magnet motors for various applications, including electric vehicles, increases because of the increasing awareness on global warming and carbon neutrality.¹⁾ To realize such high-efficiency permanent magnet motors, development of magnets with high heat resistances is required. Samarium–iron–nitrogen (Sm–Fe–N) is a promising candidate for the next generation of high-performance permanent magnets capable of replacing the Dy-added neodymium–iron–boron (Nd–Fe–B) magnets^2–5) widely used as high-heat-resistance magnets. Sm₂Fe₁₇N₃ has a high Curie temperature, high magnetic property values, and high temperature coefficient without addition of heavy rare-earth elements such as Dy and Tb.^6,7) Therefore, in the automotive industry, which is expected to further accelerate the mass consumption of magnets, the use of Sm–Fe–N magnets in the permanent magnet motors for electric vehicles has significant advantages in terms of supply risk and cost escalation of heavy rare-earth elements. However, the Sm–Fe–N decomposition into SmN, α-Fe, and N₂ at temperatures above approximately 600°C^6,8) prevents high-temperature sintering and hinders the fabrication of high-density Sm–Fe–N bulk magnets. Thus, Sm–Fe–N is practically used only as a bonded magnet mixed with resin, but its production volume is still small. In addition, because these bonded magnets contain resin (nonmagnetic phase), they cannot fully utilize their original advantages, such as the high heat resistance and high magnetic property values. Therefore, further research is ongoing to improve the performances of Sm–Fe–N magnets. For example, numerous studies demonstrated that the coercivity (H_cJ) of Sm–Fe–N magnets can be enhanced by using Zn instead of resins.^9–17) However, there are still serious problems with the high-densification technique of Sm–Fe–N, which is one of the reasons hindering the mass production of Sm–Fe–N bulk magnets.

High-velocity compaction (HVC)^18–21) is considered a high-densification technique for powder materials, which requires a shorter time, has a higher precision, and provides better mechanical properties and lower cost than those of conventional compaction techniques, such as high-pressure compaction and warm compaction. Therefore, in the field of powder metallurgy, few studies demonstrated densification of ferrous powder²¹⁾ and gear-parts¹⁸⁾ using HVC. In addition, HVC is capable of compacting large products (5–10 kg);¹⁸⁾ therefore, it is considered a suitable technique for mass production. The densification of Sm–Fe–N has been achieved through the explosive shock-compression method,²²⁾ which is a typical HVC method. Furthermore, swaging¹²⁾ and compression shearing²³⁾ have been reported as techniques based on a concept similar to the HVC. The resulting magnets had relative densities (d_r) above 90% or 95%. However, the reported explosive shock-compression method results in an excessively high velocity (above 0.9 × 10³ m·s⁻¹) and is not practical because of the use of explosives as a compression source. Moreover, swaging and compression shearing have disadvantages in terms of shape flexibility and material yield.

In this study, a hydraulic HVC using a die was investigated as a compaction technique for the fabrication of high-density isotropic Zn-bonded Sm–Fe–N bulk magnets. The compaction characteristics, i.e., the effects of the piston velocity on d_r, were experimentally investigated while varying the Zn content in a velocity range (significantly lower velocities than those in the previous explosive method²²⁾) that has not been sufficiently studied. In addition, the magnetic properties, flexural strengths (σ), and microstructures of the fabricated magnets were studied.

2. Experimental

Isotropic Zn-bonded Sm–Fe–N magnets were fabricated using HVC. The experimental procedures and specimen shape, size, and compaction direction are summarized in Fig. 1. Except for the main compaction and evaluation, the rest of the procedures were carried out in an Ar atmosphere to suppress the oxidation of Sm–Fe–N. A commercially available coarse Sm₂Fe₁₇N₃ powder with a particle size of approximately 35 µm was used as a raw material and pulverized to approximately 3 µm with a wet bead mill (MGF2, Ashizawa Finetech Ltd.). Isopropanol was used as a solvent for the slurry. 5 wt% methyl-caproate was added as a dispersant. To suppress the oxidation during pulverization, the inside of the slurry tank was sufficiently degassed with nitrogen gas and pulverized under positive pressure using nitrogen gas. A Zn raw powder (Sakai Chemical Co., Ltd.), with a particle size of approximately 4 µm, and Sm–Fe–N fine powder were mixed using a powder blender with varied Zn content from 5 to 20 wt%. The Zn and Sm–Fe–N mixed powders were filled into a die under vibration. A compact with a density of approximately 4 g·cm⁻³ was obtained by precompaction using a hand press. The precompaction was carried out to suppress the oxidation during the main compaction under atmospheric conditions. The main compaction was performed using HVC to fabricate a green compact with a high d_r. Cemented carbide dies and punches were used for this purpose. The configuration of the HVC equipment (125C, Morphic Technology AB) is shown in Fig. 2. By hydraulically lifting a piston with a mass of 35 kg while precisely adjusting the piston height, the piston velocity at compaction was varied from 8.0 to 11.2 m·s⁻¹. The piston velocity is not the compaction velocity, but the piston descent velocity. The pressure at compaction was measured using a strain gauge (EDX-11A, Kyowa Electronic Co., Ltd.) placed on the sides of the upper punch. The green compacts were annealed at 450°C for 30 min under a flowing Ar atmosphere, which yielded high-density isotropic Sm–Fe–N bulk magnets.

Fig. 1

Typical experimental procedure used in this study.

Fig. 2

Equipment configuration of the present HVC.

The particle sizes of the Zn raw powder and pulverized Sm–Fe–N fine powder were measured using a laser-diffraction-type particle size distribution meter. The shape of the powders was studied using scanning electron microscopy (SEM). The density of each annealed magnet was measured using the Archimedes method. d_r was calculated using the measured and theoretical densities. The magnetic properties of the magnets were evaluated using a pulse-excitation-type BH (PBH) tracer, while those of the Sm–Fe–N fine powder were measured using a vibrating-sample magnetometer (VSM) with paraffin. The flexural strength (σ), which is the stress at failure during bending, was measured by a three-point bending test using an autograph. Microstructural observations were performed using electron probe microanalysis (EPMA), X-ray diffraction (XRD), and scanning transmission electron microscopy (STEM). The oxygen contents of the raw powders and magnets were measured using an oxygen–nitrogen analyzer.

3. Results and Discussion

Figure 3 shows the particle size distributions and SEM images of the Sm–Fe–N fine powder (D₁₀ = 1.5 µm, D₅₀ = 3.2 µm, and D₉₀ = 5.7 µm) and Zn raw powder (D₁₀ = 1.6 µm, D₅₀ = 4.1 µm, and D₉₀ = 10.8 µm). Because the Sm–Fe–N coarse powder passed between the beads and was pulverized by the shearing force, the shape of the obtained Sm–Fe–N fine powder was not spherical, but flat with rough corners. The resulting Sm–Fe–N fine powder exhibited a residual magnetization (M_r) of 67.5 A·m²·kg⁻¹ and coercivity (H_cJ) of 928 kA·m⁻¹ in the isotropic state. On the contrary, the Zn raw powder was prepared using a gas atomization technique, and thus had a clear spherical shape.

Fig. 3

Particle size distributions and SEM images of the Sm–Fe–N fine powder and Zn raw powder. The insets show photographs of the powders.

Figure 4 shows the dependences of d_r and magnetic properties (B_r, H_cJ, and (BH)_Max) of isotropic Zn-bonded Sm–Fe–N magnets on the piston velocity. The inset shows a photograph of the green compact (5 wt% Zn) immediately after compaction. Figure 5 shows typical demagnetization curves of the resulting magnets fabricated with varying Zn content and piston velocity of 11.2 m·s⁻¹; the corresponding d_r and magnetic properties are summarized in Table 1. Although the annealing temperature was constant (450°C), this parameter was studied beforehand. The magnetic properties obtained in the annealing temperature range of 400–475°C for the Zn-bonded Sm–Fe–N magnets (5 wt% Zn) fabricated with a piston velocity of 11.2 m·s⁻¹ are shown in Table 2. The annealing temperature is not the actual specimen temperature, but the set temperature of the outer-heating-type furnace (muffle furnace). In this study, to achieve a balance between the H_cJ and B_r, the annealing temperature was set to 450°C. The densification of the Zn-bonded Sm–Fe–N magnets was achieved by HVC with a piston velocity of 8.0 to 11.2 m·s⁻¹ (Fig. 4). The surface of the resulting green compacts was smooth without chips and other defects and exhibited a metallic luster. d_r steadily increased with the piston velocity. d_r corresponding to the piston velocity of 11.2 m·s⁻¹ (maximum limit of the equipment) was 89.1% (5 wt% Zn), 89.9% (10 wt% Zn), and 90.4% (20 wt% Zn). Notably, these d_r values are higher than those of resin-bonded magnets (d_r = 75–80%).²⁴⁾ Moreover, for given piston velocity, d_r increases with the Zn content. This indicates that Zn effectively acts as a binder to fill voids during compaction. However, B_r and (BH)_Max decreased with the increase in Zn content, because Zn is a nonmagnetic phase. Therefore, the Zn content in the bulk magnet affects B_r. In other words, the decrease in the magnetic moment per unit volume due to the increase in the Zn content could not be compensated by the slight increase in d_r. In addition, the rate of increase in d_r with the piston velocity decreased when the piston velocity was above 10 m·s⁻¹, because the deformation resistance of the raw powder increased with the piston velocity, or strain rate. This behavior is common in material processing.²⁵⁾ Mashimo et al.²²⁾ densified the Sm–Fe–N powder using an explosive method wherein the compaction velocity (described as impact velocity in the report) was 1.233 × 10³ m·s⁻¹, which is approximately hundred times that in this study, and the resulting d_r was approximately 95%. The previous²²⁾ and this studies suggest that, in an excessively HVC (such as the explosive method), most of the applied energy is spent on increasing the deformation resistance of the raw material. H_cJ was substantially enhanced by the addition of Zn, compared to the Sm–Fe–N starting fine powder (H_cJ = 928 kA·m⁻¹), which reached 1658 kA·m⁻¹ at a Zn content of 20 wt% and piston velocity of 11.2 m·s⁻¹ (Fig. 5). The increased values in H_cJ with the Zn content almost matches that of a previous report.¹¹⁾ According to previous reports,^{9,10,14–17)} the enhancement in H_cJ is due to the role of Zn in forming a paramagnetic or nonmagnetic Zn–Fe alloy phase via the annealing. The microstructures of the resulting magnets are discussed later. H_cJ decreased slightly with the increase in piston velocity. The reduction in H_cJ was approximately 2% when the piston velocity increased from 8.0 to 11.2 m·s⁻¹. This suggests generation of nucleation sites such as surface defects in Sm–Fe–N particles due to the HVC.

Fig. 4

Dependences of d_r and magnetic properties (B_r, H_cJ, and (BH)_Max) on the piston velocity for the isotropic Zn-bonded Sm–Fe–N magnets with various Zn contents fabricated using HVC and subsequently annealed at 450°C. The inset shows a photograph of the green compact (5 wt% Zn) immediately after compaction.

Fig. 5

Demagnetization curves of the Zn-bonded Sm–Fe–N magnets with various Zn contents fabricated by HVC at a piston velocity of 11.2 m·s⁻¹.

Table 1 Summary of d_r and magnetic properties of the Zn-bonded Sm–Fe–N magnets fabricated by HVC at a piston velocity of 11.2 m·s⁻¹.

Table 2 Magnetic properties of the Zn-bonded Sm–Fe–N magnets (5 wt% Zn) fabricated at the piston velocity of 11.2 m·s⁻¹ at various annealing temperatures.

Figure 6(a) shows the relationship between the piston velocity and pressure in the HVC, while Fig. 6(b) shows the pressure waveform at a piston velocity of 11.2 m·s⁻¹. The strong positive correlation (r = 0.980) between the piston velocity and pressure was confirmed (see Fig. 6(a)). The pressure reached a maximum of 3.45 GPa when the piston velocity was 11.2 m·s⁻¹. According to the pressure waveform (Fig. 6(b)), the maximum pressure of 3.45 GPa was applied only for a very short time of a few milliseconds. For comparison with previous reports on densification of Sm–Fe–N powder using a conventional high-pressure technique, Machida et al.²⁶⁾ achieved d_r above 95% by warm compaction with a pressure of 3.0 GPa and temperature of 550°C. Takagi et al.²⁷⁾ achieved d_r of 90.8% by 100 cycles of compaction at a pressure of 1.2 GPa. Notably, in both studies,^26,27) only Sm–Fe–N powder was compacted without Zn addition. Moreover, Takagi et al.²⁷⁾ used a coarse Sm–Fe–N powder as a raw material and applied current sintering of the green compacts at a pressure of 1.2 GPa after the compaction. This suggests that the reported d_r of the Sm–Fe–N bulk magnets fabricated using conventional high-pressure techniques is high. However, considering the processing cost and time, warm compaction above 500°C or 100 cycles of compaction are difficult to apply to mass production. In contrast, in this study, d_r of approximately 90% could be achieved by HVC with single and cold compaction in a very short time period. Furthermore, in this study, because die compaction was combined with HVC, the dimensional precision of the fabricated magnets was high (within ±0.02 mm for both thickness and width under constant conditions). Such high dimensional precision is a characteristic of die compaction and is difficult to achieve using an explosive method,²²⁾ compression shearing,²³⁾ or swaging,¹²⁾ which have been reported for the compaction of Sm–Fe–N.

Fig. 6

(a) Relationship between the piston velocity and pressure in the HVC and (b) pressure waveform at the piston velocity of 11.2 m·s⁻¹.

Figure 7 shows the relationship between d_r and flexural strength (σ) of the Zn-bonded Sm–Fe–N magnets with varying Zn content fabricated using HVC and subsequently annealed at 450°C. The inset shows a photograph of the magnet (10 wt% Zn and d_r of 89.9%) after the flexural test. Because magnets in a permanent magnet motor are subjected to centrifugal force,^28,29) a high σ is vital for the magnets. The required σ of the magnets in permanent magnet motors is approximately 50–100 MPa,²⁹⁾ which has been easily achieved by Nd–Fe–B sintered magnets.³⁰⁾ In this study, σ of the fabricated magnets increased steadily with the increase in d_r. Moreover, when d_r exceeded 88%, σ of approximately 100 MPa could be achieved regardless of the Zn content. In addition, after the flexural test, the magnet was cleanly split into two equal parts. This suggests that the density of the magnets fabricated using the HVC was uniform throughout. Furthermore, σ decreased with the increase in Zn content. The increase in the content of Zn (soft metal) was considered to progress the fracture behavior caused by Zn.

Fig. 7

Relationship between d_r and σ of the Zn-bonded Sm–Fe–N magnets with various Zn contents fabricated by HVC and subsequently annealed at 450°C.

The effect of the HVC on the microstructures of the magnets was also investigated. Figure 8 shows back-scattered electron (BSE) and EPMA images of Sm, Fe, N, Zn, and O of the Zn-bonded Sm–Fe–N magnet (5 wt% Zn and d_r of 88.9%) fabricated by HVC and subsequently annealed at 450°C. The observations were performed near the center of the cross section parallel to the compaction direction. The cross-sectional BSE images demonstrated a well-compacted structure with only a few voids, which explains the high d_r reported earlier. The EPMA images confirmed that Zn was scattered around the Sm–Fe–N particles; i.e., Zn exhibited a grain-boundary-like structure. This finding suggests that Zn dissolved and permeated into the void by annealing when the temperature was above the melting point of Zn. However, the detailed Zn distribution between the surface of the Sm–Fe–N particles and grain boundary phase could not be clearly determined at the resolution of EPMA. Because the Zn and Sm–Fe–N fine powders contained a certain degree of oxygen, oxygen was observed from Zn near the surface of the Sm–Fe–N particles. The oxygen contents in the Zn raw powder and Sm–Fe–N fine powder were 0.83 and 0.69 wt%, respectively, while the oxygen content in the resulting magnets was approximately 0.95–1.00 wt%. One reason for the increase in the oxygen content is that the HVC was performed under atmospheric conditions.

Fig. 8

BSE and EPMA images of Sm, Fe, N, Zn, and O of the Zn-bonded Sm–Fe–N magnet (5 wt% Zn and d_r of 88.9%) fabricated by HVC and subsequently annealed at 450°C.

Figure 9 shows the XRD patterns of the starting powders and magnets before and after annealing. Each specimen contained 5 wt% Zn. The HVC was conducted at a piston velocity of 11.2 m·s⁻¹. The XRD patterns show that the magnets fabricated using HVC did not exhibit significant phase decomposition and almost maintained the original Sm₂Fe₁₇N₃ phase structure. Peaks attributed to Sm₂Fe₁₇N₃ and Zn were observed for both starting powders and specimens obtained after the HVC. However, after annealing, the peaks of Zn disappeared and a few peaks related to ZnO and α-Fe were observed. Most Zn-related peaks were not observed after annealing, which implies that Zn was present as an amorphous or fine nanocrystalline phase near the surface of the Sm–Fe–N particles. The peaks of the Zn–Fe alloy phase, which contributed to the enhancement in H_cJ,^{9,10,14–17)} could not be clearly observed, probably because of the overlap with the Sm₂Fe₁₇N₃ peaks. A small amount of the α-Fe phase was formed owing to the partial decomposition of the Sm₂Fe₁₇N₃ phase during annealing at 450°C. Although this temperature is lower than the temperature at which Sm₂Fe₁₇N₃ undergoes thermal decomposition,^6,8) Yamaguchi et al.³¹⁾ reported that the α-Fe phase is generated because of the thin oxide layer covering the surface of Sm₂Fe₁₇N₃ particles. As mentioned earlier, the present fine Sm–Fe–N powder contained a certain degree of oxygen. Therefore, it could be assumed that the α-Fe phase formation observed in this study was also due to the presence of an initial thin oxide layer at the surface of the fine Sm–Fe–N powder.

Fig. 9

XRD patterns of the specimens with 5 wt% Zn: (a) starting powders, (b) after HVC, and (c) after annealing. HVC was conducted at a piston velocity of 11.2 m·s⁻¹.

STEM observations were carried out to thoroughly investigate the microstructure of the interface between the Sm–Fe–N particles and Zn. STEM was performed in the same region as that for the EPMA. Figure 10(a) shows STEM high-angle annular dark-field (HAADF) images and STEM–energy-dispersive X-ray (EDX) spectroscopy elemental maps of Sm, Fe, N, Zn, and O of the Zn-bonded Sm–Fe–N magnet (5 wt% Zn) fabricated by HVC and subsequently annealed at 450°C. Two distinct phases with different color tones (gray and dark gray) were observed near the surface of the Sm–Fe–N particles. The bright gray contrast in the STEM-HAADF image corresponds to the Sm₂Fe₁₇N₃ phase. The STEM-EDX elemental maps confirmed that the gray phase was the surface region of the Sm–Fe–N particle with diffused Zn (hereinafter referred to as Zn-diffused phase), while the dark gray phase was composed mainly of Zn and Fe (hereafter referred to as Zn-rich phase), which is different from the Sm–Fe–N particles. The thicknesses of these phases were estimated to be approximately 30–50 nm (Zn-diffused phase) and 50–300 nm (Zn-rich phase) by the TEM images. Figure 10(b) shows concentration line profiles of Sm, Fe, N, Zn, and O at the interface of the Sm–Fe–N particles extracted by an STEM-EDX line analysis. The line analysis was performed along the direction of the yellow dashed line shown in Fig. 10(a) (STEM-HAADF image). The concentration line profiles revealed that the amount of Fe decreased stepwise from the Sm–Fe–N particle to the Zn-diffused phase, and then to the Zn-rich phase, while the amount of Zn increased. The concentrations of Fe and Zn were approximately 51.3 and 23.1 at% (Zn-diffused phase) and 27.3 and 63.6 at% (Zn-rich phase), respectively. In the Zn-diffused phase, the concentrations of Sm and N were approximately comparable to those of the Sm–Fe–N particles, which suggests presence of Sm–(Fe, Zn)–N phase on the surface of the Sm–Fe–N particle, as previously reported.^16,17) A typical composition of the Sm–(Fe, Zn)–N phase extracted by the EDX spectroscopy mapping is Sm_9.5Fe_52.3Zn_24.8N_13.4, which results in a Sm:(Fe + Zn) atomic ratio of 2:16.2, close to Sm:Fe = 2:17. This finding suggests that, during annealing at 450°C, the dissolved Zn diffused into the surface of the Sm–Fe–N particles and replaced Fe with Zn. The concentration line profiles also show some amount of O in the Zn-diffused phase, which indicates that the diffusion to the surface of the Sm–Fe–N particles proceeded with O in addition to Zn. On the contrary, because almost no Sm and N were detected in the Zn-rich phase, it is reasonable to assume that this phase is composed of Zn and Fe (Fe precipitated during the diffusion of Zn into Sm–Fe–N particles), which is a different phase from the Sm–Fe–N particles, as mentioned earlier. Figures 10(c) and (d) show the diffraction patterns obtained from the Zn-diffused and Zn-rich phases, respectively. The electron diffraction pattern revealed the presence of α-Fe(Zn) $[\bar{1}\ \bar{1}\ 3]$ (Zn-diffused phase) and Γ-FeZn $[\bar{1}\ \bar{1}\ 5]$ (Zn-rich phase). Because the α-Fe(Zn) phase has a body-centered cubic structure and contains more than 50 at% of Fe, it is assumed to be soft magnetic, while the Γ-FeZn phase is nonmagnetic at room temperature.³²⁾ Moreover, in Fig. 10(c), a ring-like pattern is observed (indicated by the red arrow), which suggests presence of an amorphous or fine nanocrystalline phase in addition to α-Fe(Zn) in the Zn-diffused phase.

Fig. 10

(a) STEM-HAADF image and STEM-EDX elemental maps of Sm, Fe, N, Zn, and O of the Zn-bonded Sm–Fe–N magnet (5 wt% Zn) fabricated by HVC and subsequently annealed at 450°C. (b) Concentration line profiles of Sm, Fe, N, Zn, and O at the interface of the Sm–Fe–N particle extracted by the STEM-EDX line analysis (the direction of the analysis is shown by the yellow dashed line in (a)). Diffraction patterns obtained from the (c) Zn-diffused phase and (d) Zn-rich phase.

Thus, the microstructural observations showed that the surface region of the Sm–Fe–N particles with a size of approximately 50 nm has a microstructure in which the Sm–(Fe, Zn)–N and soft magnetic α-Fe(Zn) phases (Zn-diffused phase) coexist and the nonmagnetic Γ-FeZn phase (Zn-rich phase) is present around these particles. H_cJ of the Zn-bonded Sm–Fe–N magnet was enhanced even though the soft magnetic α-Fe(Zn) phase was present at the surface of the Sm–Fe–N particles (the soft magnetic phase such as α-Fe is considered to deteriorate H_cJ of permanent magnets). This occurred as the surface defects of the Sm–Fe–N particles generated by the fine pulverization using a bead mill (where the magnetic anisotropy is locally low and reversed domains are easily generated) are recovered by the formation of the Sm–(Fe, Zn)–N phase during the annealing.^16,17) In particular, Hiraga et al.¹⁷⁾ proposed that the presence of the Sm–(Fe, Zn)–N phase increased H_cJ of Zn-bonded Sm–Fe–N magnets by blocking the soft magnetic α-Fe phase penetration into the Sm–Fe–N phase. Notably, a previous study¹⁷⁾ suggested that the α-Fe phase is present outside the Sm–(Fe, Zn)–N phase; this assumption is well supported by the microstructure observed in this study. The Sm–(Fe, Zn)–N phase coexisting with the α-Fe(Zn) phase prevented the propagation of the reverse domains to the Sm–Fe–N particles. Furthermore, the presence of the nonmagnetic Γ-FeZn phase around the Sm–Fe–N particles, which decreased the exchange coupling between the particles, was also considered as a factor for the increase in H_cJ. However, because the thickness of the Γ-FeZn phase is typically 50–300 nm, considerably larger than the typical exchange length of Sm₂Fe₁₇N₃,³³⁾ it may not be the main reason for the increase in H_cJ, as reported by Prabhu et al.¹⁶⁾ However, Matsuura et al.¹⁴⁾ proposed that the Γ-FeZn phase surrounds the soft magnetic α-Fe(Zn) phase around the Sm–Fe–N particles and magnetically separates them from the Sm–Fe–N particles resulting in a high H_cJ; the role of the Γ-FeZn phase needs further studies.

4. Summary

HVC using a die was investigated to fabricate high-density isotropic Zn-bonded Sm–Fe–N bulk magnets. The compaction characteristics of the Zn-mixed Sm–Fe–N powders were experimentally investigated while varying the Zn content in the velocity range that has not been sufficiently studied. The densification of the Zn-bonded Sm–Fe–N magnets was achieved by HVC with a piston velocity of 8.0 to 11.2 m·s⁻¹ without significant phase decomposition of Sm₂Fe₁₇N₃. d_r of the fabricated magnets steadily increased with the piston velocity. Moreover, at a piston velocity of 11.2 m·s⁻¹, d_r was almost 90% regardless of the Zn content. The fabricated magnets also had high dimensional precision and mechanical strength. The resulting magnet (5 wt% Zn and d_r of 89.1%) exhibited (BH)_Max of 54.4 kJ·m⁻³, B_r of 0.56 T, and H_cJ of 965 kA·m⁻¹. Microstructural observations demonstrated that the resulting Zn-bonded Sm–Fe–N magnets were well packed with few voids. Furthermore, Zn dissolved during the annealing and diffused to the surface region of the Sm–Fe–N particle, forming the Sm–(Fe, Zn)–N phase and soft magnetic α-Fe(Zn) on the surface of the Sm–Fe–N particle and nonmagnetic Γ-FeZn phase around the particle. This microstructure enhanced H_cJ of the Zn-bonded Sm–Fe–N magnet.

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