Study of the anisotropic Sm₂Fe₁₇N₃ powders with high performance Open Access

Permanent magnets are widely used in electrical and electronic fields, and the demand for permanent magnet materials is also increasing yearly, especially in green energy-related applications such as electric vehicles and wind power generation.¹ Although Nd₂Fe₁₄B permanent magnet materials have excellent magnetic properties at room temperature, they are not suitable for high-temperature applications due to their low Curie temperature and stability. Furthermore, after years of development, the performance of the Nd₂Fe₁₄B magnet is quite close to the theoretical value, while the cost of Nd₂Fe₁₄B magnet keeps growing.² Therefore, there is a strong demand for developing a new generation of high-performance permanent magnet materials. Sm₂Fe₁₇N₃ is regarded as a prospective candidate since the it possesses intrinsic magnetic properties that are superior or comparable to those of Nd₂Fe₁₄B.³ Sm₂Fe₁₇N₃ was first reported in 1990 by Coey and Sun,⁴ and its crystal structure was determined by Yang.^5–7 The Sm₂Fe₁₇N₃ (Nd₂Fe₁₄B) compound has a saturation magnetization of 1.54 T (1.61 T), a magneto-crystalline anisotropic field of 14.0 T (7.6 T), a Curie temperature of 476 °C (315 °C) and a theoretical (BH)_max of 59.7 MGOe (64.7 MGOe).³ Unlike that used in Nd₂Fe₁₄B magnets, liquid phase sintering is not available for producing Sm₂Fe₁₇N₃ dense magnets. Therefore, the fabrication of high-performance anisotropic Sm₂Fe₁₇N₃ powders is the first step and the key to producing Sm-Fe-N sintered and bonded magnets.^8–12 

Due to Sm’s volatility and chemical activity, it is easy to generate a soft a-Fe phase or Sm-rich phase during the synthesis of Sm₂Fe₁₇N₃, which severely reduces the coercivity (H_c) of the powder.^13–15 The synthesis of a-Fe-free single-phase samples is a prerequisite for preparing Sm₂Fe₁₇N₃ with good magnetic properties. The diameter of single-domain particles of Sm₂Fe₁₇N₃ has been estimated to be less than 0.5 µm.⁸ The preparation of powder with high coercivity usually requires a long time of mechanical pulverization.¹⁶ The addition of stress and oxidation during pulverization reduces the powder’s saturation magnetization (M_s).¹⁷ Furthermore, the reduction of average particle size (D_m) increases the specific surface area of the powder and reduces the oxidation resistance. Therefore, it is helpful for practical applications to find a way of “soft grinding,” which can increase the H_c with less loss of the M_s and D_m.^18–20 

In this work, the strip casting technique was first used to prepare the Sm₂Fe₁₇ parent alloys. Next, the Sm₂Fe₁₇N₃ powders were synthesized using a gas–solid reaction of Sm₂Fe₁₇ powders with N₂. Then, the high-performance anisotropic Sm₂Fe₁₇N₃ powders were prepared by a surfactant-assisted grinding method in gasoline solvent. A systematic study was carried out on the relationship between the particle size of the powder and its magnetic properties.

The parent Sm₂Fe₁₇ alloy was synthesized by the strip casting method. High purity Sm (99.9%) with no oxide layer and Fe (99.9%) were used, and excess Sm was added to compensate the evaporation. The obtained strips of 0.7 mm in thickness were annealed at 1273 K under an atmosphere of high-purity argon to form the Th₂Zn₁₇ phase. The annealed strips were crushed into 150 mesh powders under inert gas protection. The Sm₂Fe₁₇N₃ powder was synthesized using a gas–solid reaction of Sm₂Fe₁₇ powder with N₂. Nitrogenation was conducted at 700–800 K using N₂ gas for 10–24 h to ensure that the powder was fully nitrided. The composition of Sm₂Fe₁₇N₃ powder was strictly controlled. To reduce the particle size of Sm₂Fe₁₇N₃ powders, the surfactant-assisted grinding method was carried out in a vial with steel balls on a Roll Ball Mill. The ball-to-powers weight ratio was about 145:1, and gasoline had been used a solvent to prevent Sm₂Fe₁₇N₃ powders from severe oxidation. A small amount of hexadecenoic acid by powder mass was added as a surfactant to reduce the particle size more effectively.⁸ XRD measurement was carried out To characterize the crystal structure of the sample using X’pert Pro MPD diffractometer in theta–2theta geometry with Cu Ka radiation (k = 1.5418 Å). The morphology of powders with different ball milling times was investigated by scanning electron microscopy (SEM). The particle size of the powder was obtained from the SEM image measured by Image J software. The hysteresis loops at different temperatures and room-temperature demagnetization curves were measured by the physical properties measurement system (PPMS) produced by Quantum Design. Samples for magnetic measurement were prepared by mixing the powders with epoxy resin and orienting them under a 1.5 T magnetic field.

During the gas–solid reaction process, it is easy to generate a soft a-Fe phase reducing the coercivity of the permanent magnet material. Therefore, synthesizing single-phase samples is a prerequisite for preparing Sm₂Fe₁₇N₃ with good magnetic properties. Since the Sm is easy to volatilize at high temperatures and the gas–solid reaction rate is limited, the preparation of Sm₂Fe₁₇ and Sm₂Fe₁₇N₃ requires accurate control of the Sm:Fe ratio and the heat treatment process to avoid Sm deficiency and insufficient nitrogen absorption. As shown in Figs. 1(a) and (b), single-phase Sm₂Fe₁₇ and Sm₂Fe₁₇N₃ were successfully synthesized. The annealed strips of Sm₂Fe₁₇ formed the Th₂Zn₁₇-type Structure with no detectable Sm-rich phase or a-Fe. After nitriding [Fig. 1(b)], Sm₂Fe₁₇N₃ remained Th₂Zn₁₇-type Structure, while the diffraction peak shifted to a low angle, indicating that the lattice parameters were enlarged due to the entry of N atoms into the interstitial position. There was no appearance of the a-Fe in the nitrides, and the composition of Sm₂Fe₁₇N₃ powder was strictly controlled. In order to characterize the magnetic anisotropy of the Sm₂Fe₁₇N₃ powder sample, XRD measurement for a magnetic-field oriented sample was carried out. The powders mixed with epoxy resin were solidified under a magnetic field. In Fig. 1(d), the direction of the magnetic field was along the found direction of the XRD test plane, and only the (00l) peak remained, indicating that the powder had a strong c-axis uniaxial magnetic anisotropy. In addition, in order to verify the stability of the Sm₂Fe₁₇N₃ phase during the pulverization process, XRD test was carried out on the sample ground for 4 h. As shown in Fig. 1(c), diffraction peaks were broadened due to the reduction of particle size and the increase of crystal stress, while the peak positions were not changed, and no impurity peak appeared, proving that no decomposition or significant oxidation occurred during the ball milling process.

The coercivity mechanism of Sm₂Fe₁₇N₃ is mainly nucleation; therefore, reducing the particle size is an effective method to improve the coercivity.⁸ The particle size of Sm₂Fe₁₇N₃ was reduced by the grinding method to study the relationship between the particle size and magnetic properties. The powders ground at different times were observed by SEM shown in Fig. 2, and the software counted the average particle size (D_m). The relationship between the particle size and the ball milling time is shown in Fig. 3(a). The unground powder took on an irregular, lumpy shape with D_m about 25 µm. The D_m decreased rapidly to 4.4 µm after the first hour of grinding, and the shape of the powder ground for 3 h became more uniform. After 6 h of ball milling, the powder presented a homogeneous flaky shape, and D_m was reduced to 1.5 µm. Due to the limited energy of ball milling, the particle size did not reduce significantly for a longer grinding time.

As shown in Fig. 3(b), the room-temperature hysteresis loops were measured to characterize the magnetic properties of powders ground at different times. In Fig. 3(c), H_c increased with decreasing D_m, from 1.1 kOe (25 µm) to 17.5 kOe (1.3 µm); meanwhile, M_s decreased slowly with the decrease of D_m, from 155 emu/g (25 µm) to 140 emu/g (2.1 µm). In the 2.1–1.3 µm segment, M_s decreased rapidly. This was mainly due to the fact that with the increase of grinding time, the particle size did not decrease further, while the stress and defects of the grain kept increasing. The maximum value of H_c of 17.5 kOe was obtained at the D_m = 1.3 µm. Ground powder with D_m of 3.1 µm showed a remanence ratio M_r/M_s over 95% and a maximum energy product (BH)_max of 35 MGOe. As shown in Fig. 3(d), powders prepared in this work had a larger particle size at the same coercivity compared to previous work using the mechanical pulverization method. The increase in particle size reduced the specific surface area and improved the anti-oxidation performance of the Sm₂Fe₁₇N₃ powder, which had advantages in industrial production.

The powders ground for 4 h was selected for the M–H loop measurements at 10, 300 and 373 K. As shown in Fig. 4, the H_c of the powder increased rapidly with the decrease in temperature, reaching 36.1 kOe at 10 K, while the M_s increased slightly. When the temperature decreased, the effect of thermal disturbance on the magnetic moment was weakened, and the magneto-crystalline anisotropy field of Sm₂Fe₁₇N₃ increased; therefore, the M_s and the H_c increased. Above 300 K, the M_s and H_c decreased slowly with the temperature increase. At 373 K, the M_s and H_c were 128 emu/g and 8.5 kOe. Sm₂Fe₁₇N₃ powder mixed with epoxy resin showed a low remanence temperature coefficient of α_(RT-100°C) = −0.077% °C⁻¹, which was superior to the Nd₂Fe₁₄B magnets.

In this work, anisotropic single-phase Sm₂Fe₁₇N₃ powder was synthesized by the strip casting method. A systematic study was carried out on the relationship between the particle size of the powder and its magnetic properties. It was found that the coercivity of the powders has a strong negative correlation with their particle size. High magnetic performance of M_s ∼ 155 emu/g (under 5 T), H_c ∼ 17.5 kOe, M_r/M_s ∼ 95%, and (BH)_max ∼ 35 MGOe have been obtained for the Sm₂Fe₁₇N₃ powders prepared by the surfactant-assisted grinding method. The ground powder mixed with epoxy resin showed a low remanence temperature coefficient of α_(RT-100°C) = −0.077% °C⁻¹. The synthesis of high-performance anisotropic Sm₂Fe₁₇N₃ powder and the study of its magnetic properties are valuable to the development and application of Sm₂Fe₁₇N₃ as bonded and high-temperature magnets.

The work is supported by the National Key Research and Development Program of China under Grant Nos. 2021YFB3500301 and 2021YFB3500304, and the National Natural Science Foundation of China under Grant Nos. 51731001 and 52171167.

The authors have no conflicts to disclose.

Dong Liang: Conceptualization (equal); Data curation (equal); Funding acquisition (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Wenyun Yang: Data curation (equal); Formal analysis (equal); Visualization (equal); Writing – original draft (equal). Xinan Wang: Investigation (equal); Methodology (equal); Resources (equal). Qing Xu: Investigation (equal); Resources (equal). Jingzhi Han: Data curation (equal); Project administration (equal). Shunquan Liu: Funding acquisition (equal). Changsheng Wang: Data curation (equal); Formal analysis (equal); Visualization (equal). Honglin Du: Methodology (equal); Resources (equal). Xiaoyu Zhu: Funding acquisition (equal); Resources (equal); Supervision (equal). Tao Yuan: Funding acquisition (equal); Supervision (equal). Zhaochu Luo: Data curation (equal); Project administration (equal); Visualization (equal). Jinbo Yang: Conceptualization (lead); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.