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Research Progress of SmFeN Rare-Earth Permanent Magnet

SmFeN permanent magnet material has high coercivity, high saturation magnetization and high Curie temperature. It has important application value in emerging fields such as aerospace, electric vehicle, wind power generation and artificial intelligence, and has been paid attention to by researchers again. This paper introduces the crystal structure and nitriding mechanism of SmFeN magnet, and summarizes the research progress of subsequent nitriding treatment of SmFeN magnetic powder by melt rapid quenching, mechanical alloying, reduction diffusion and hydrogenation disproportionation. In addition, the research progress in the preparation of bulk SmFeN magnets and the effect of alloy elements on the magnetic properties of SmFeN magnets are introduced. Based on the current research progress, the key scientific problems faced by SmFeN permanent magnet materials are clarified, and the development prospect of SmFeN magnets is prospected.

Current Research Status of Permanent Magnet Materials

Permanent magnet materials are widely used in electrical and electronic applications such as electric motors, generators, speakers, computers, and medical devices. With the rapid development of green energy-related applications such as electric vehicles and wind power generation, the demand for permanent magnet materials has been increasing year by year. This growing demand has promoted the development of permanent magnets in three directions: first, the magnetic energy product must be maximized to produce smaller, lighter, and more energy-efficient devices [1]; second, the coercivity needs to be increased to ensure good thermal stability of the magnets in high-temperature applications; and finally, a high cost performance.
Figure 1 illustrates the history of more than 100 years of development experienced by industrial production of tungsten to rare earth permanent magnets [2]-[8]. As can be seen from the maximum energy product (BH)max, the magnetic energy has increased from 1 MGOe for steel-based magnets to 60 MGOe for Nd-Fe-B sintered magnets.Since the discovery of Nd-Fe-B-based magnets in the early 1980s, no new permanent magnet material with a better magnetic energy product than Nd-Fe-B magnets has been discovered, despite extensive experimental and theoretical studies on ternary alloys. Although many new ternary alloys and compounds have been discovered in the past 30 years [9]-[15], efforts to improve the magnetic properties of existing permanent magnets have never stopped.
20220109003043 15809 - Research Progress of SmFeN Rare-Earth Permanent Magnet Figure 1. Development in the energy product (BH)max of permanent magnetic materials in the 20th century [8]

The Sm2Fe17N3 compound was discovered shortly after the discovery of Nd2Fe14B [9]. At that time, high hopes were placed on Sm2Fe17N3 because of its excellent endogenous magnetic properties and its potential as a permanent magnet equivalent to Nd2Fe14B magnets [10]-[16]. However, the number of studies on Sm2Fe17N3 magnets declined rapidly into the 2000s. The main reason is the difficulty to prepare high performance Sm2Fe17N3 sintered magnets. However, with the development of social intelligence and the renewed interest in rare-earth depleted permanent magnets triggered by the 2011 rare-earth supply crisis and the consideration of global climate change, especially because of the strong desire of those involved in electric vehicle motor technology to find magnets that can replace Nd2Fe14B magnets with high magnetic properties and high thermal resistance, while also reducing the risks associated with resource availability and cost, researchers The researchers started to refocus their attention on Sm2Fe17N3 magnets, a compound with not only a saturation magnetization strength (1.57 T) and anisotropic field (20.7 MA∙m-1) comparable to Nd2Fe14B, but also a high Curie temperature (743 K). Based on the experimental data of powders for estimation, Sm2Fe17N3 sintered magnets have a higher magnetic energy product than Dy-doped Nd2Fe14B magnets in the high temperature environment of automotive motors [17] [18]. Table 1 shows the theoretical magnetic properties of different rare earth permanent magnet materials [19][20][21], which shows that the excellent endogenous magnetic properties of Sm2Fe17N3 are very attractive. From the cost-performance point of view, Sm2Fe17N3 also offers considerable advantages because of its high coercivity and the absence of heavy rare earth elements, and the fact that Sm metal is much cheaper than Nd metal.

Chemical compound Saturation magnetization Js/T Maximum magnetic energy product (BH)max/(kJ∙m−3) Magnetocrystalline anisotropy HA/(MA∙m−1) Curie temperature Tc/K Rare earth atom content R/TM Base metal
SmCo5 1.15 248 32 1013 0.33 Co
Sm2Co17 1.56 480 ≈8 1163 0.105 Co
Nd2Fe14B 1.61 528 ≈6 583 0.117 Fe
Sm2Fe17 0.94 176 Easy base 392 0.105 Fe
Sm2Fe17Nx 1.54 472 11.2~20.8 743 0.0989 Fe

Table 1.  Theoretical magnetic properties of different rare earth permanent magnetic materials

Unit conversion: 1000 kg ∙ m − 3 = 1 g ∙ cm − 3; 1 emu∙g−1 = 1 A∙m2∙kg−1; 1 emu∙cm−1 = 1 kA∙m; 1 Gs = 10−4 T; 1 MGOe = 7.96 kJ∙m−3; 1 MA∙m−1 = 12.6 kOe。

Crystal structure and nitriding mechanism of SmFeN compounds

Sm2Fe17 structure has Th2Zn17 crystal structure, and Fig. 2 [22] shows the single cell structure of the crystal. One cell consists of three Sm2Fe17 molecules, 6 SM atoms occupy the C crystal position, 6 Fe atoms occupy the C crystal position, 9 Fe atoms occupy the D crystal position, 18 Fe atoms occupy the f crystal position, and 18 Fe atoms occupy the H crystal position. There are two large vacancies in this structure, one is the e crystal position located in the octahedral gap containing SM atoms, The other is the tetrahedral gap between two SM atoms along the c-axis. The Curie temperature of Sm2Fe17 is very low, only 392k, and it is easy to magnetize. The atomic spacing of Fe Fe is too small, resulting in negative exchange coupling. Therefore, Sm2Fe17 alloy has low Curie temperature. After the introduction of N atoms, N atoms enter the octahedral gap to form interstitial compounds. The lattice constant of the crystal increases, causing the unit cell volume expansion, but it will not change the crystal structure and slightly increase the Fe Fe axis, which will enhance the ferromagnetic coupling exchange and increase the Curie temperature of the magnet. Because a Th2Zn17 has only three octahedral voids, a unit cell can introduce up to three N atoms. Yang [23] found in his research that nitrogen atoms have a tendency to attract rare earth and iron electrons because the electronegativity of nitrogen is much greater than that of rare earth or iron ions. Therefore, this effect reduces the transfer of conduction electrons from rare earth ions to iron 3D band, and leads to the increase of Fe atomic moment, the increase of exchange coupling of Fe Fe atoms and the increase of Curie temperature. The magnetocrystalline anisotropy of rare earth sublattice is the anisotropy induced by single ion crystal field. In order to extend the anisotropy to the whole lattice and maintain it at high temperature, a strong magnetic coupling needs to be formed between the rare earth sublattice and the 3D lattice. In most rare earth permanent magnet materials, the exchange interaction exceeds the interaction of crystal field. In this case, the crystal field theory leads to the simple expression of the lowest order anisotropy constant K1 as follows:
20220109003719 49957 - Research Progress of SmFeN Rare-Earth Permanent Magnet(1)

Where NR is the rare earth content, AJ is the Stevens factor, ⟨ r2 ⟩ is the expected value of the square of 4f radius, A20 is the second-order crystal parameter, and ⟨ O20 ⟩ is 20220109010421 33314 - Research Progress of SmFeN Rare-Earth Permanent Magnet. ⟨ O20 ⟩ has a maximum value at 0k. When N atoms enter the Sm2Fe17 lattice, the second-order lattice parameter A20 increases, and finally the magnetocrystalline anisotropy field increases greatly [24] [25].

20220109003917 17166 - Research Progress of SmFeN Rare-Earth Permanent MagnetFigure 2.  Tb2Zn17 type Sm2Fe17N3 unit cell structure[23]

Preparation methods of SmFeN magnetic powders

The research on SmFeN rare-earth permanent magnetic materials has long been focused on the preparation of powders. Depending on the powder preparation process, the methods for preparing Sm2Fe17 magnets are mainly melt fast quenching method, mechanical alloying method, powder metallurgy method, hydrogenation disproportionation method and reduction diffusion method.

Rapidly Quenched (RQ)

The Sm2Fe17Nx powder is prepared by the Rapidly Quenched method, in which a certain ratio of Sm and Fe alloy or powder is melted and cast into alloy ingots by induction heating furnace or arc melting furnace and then rapidly cooled by high-speed rotating copper rollers to obtain amorphous alloy strips. -Katter [25] first prepared TbCu7 type Sm-Fe-N powder by melt fast quenching method, and then obtained Sm2Fe17Nx powder by crystallization and nitriding treatment, although it is isotropic, its remanent magnetization is as high as Js/2 (Js = 1.4 T) and coercivity is close to 21 kOe. Sm2Fe17Nx powders produced by the melt fast quenching method are related to their composition, fast quenching temperature, copper roll speed and nitriding time [26][27][28][29][30].
Liu [31] subjected fast-hardened thin strips to high-energy ball milling under Ar protection for 3-5 h to obtain the maximum number of short-range/medium-range ordered amorphous structures, which contributed to the formation of a homogeneous microstructure during subsequent crystallization annealing, and the nitride powder produced by crystallization of amorphous precursors showed improved magnetic properties compared to the nitride powder produced by direct melt fast quenching, with coercivity (Hcj) and remanence (Br ) by 1.28 kOe and 23.6 emu∙g-1, respectively.This microstructural modulation plays a key role in improving the magnetic properties after nitriding, laying the foundation for the future production of SmFeN bulk materials.
Coey [22] obtained magnetic powders with remanent magnetization ratio Mr/Ms > 0.6 and maximum magnetic energy product (BH) max > 62 kJ∙m-3 by rapid cooling of molten Sm-Fe alloy on a copper roll at 40 m∙s-1. The powders are well suited for the preparation of bonded magnets, and the maximum magnetic energy product (BH) max of 113.9 kJ∙m-3 was reported for compression molded isotropic SmFeN magnets by [25], which exceeds the level that can be achieved for isotropic Sm-Co and Nd-Fe-B magnets.
Lin [32] prepared Sm-Fe alloy ingots by vacuum melting and made them into Sm-Fe by melt fast quenching after coarse crushing.It was confirmed by XRD and SEM analysis that fine Sm2Fe17 columnar grains consisting of almost identical orientation could be obtained at a surface speed of 5-7 m∙s-1, suitable nozzle size and jet air pressure for copper rolls The thin strips, by adjusting the pressure, temperature and composition of the injected Sm-Fe melt to regulate the cooling rate and crystallization, laid the foundation for the preparation of anisotropic Sm2Fe17Nx magnetic powders.

Machanical Alloying (MA) Method

Starting from the early 1970s, Benjamin and colleagues [33][34] found that grinding Ni or Al alloy powders in an oxidizing atmosphere produced new powders in which each powder particle contained a dispersion of oxide particles. During the grinding process, the oxide layer formed on the surface of the powder particles breaks and is further incorporated into the solid powder particles by cold welding, a method known as the mechanical alloying method. This method can be applied to the preparation of magnetic materials. Sm and Fe powder are mixed and loaded into an argon protected ball mill tank for high energy ball milling (mechanical alloying), and the obtained Sm2Fe17 powder is tempered and held for a long time at high temperature (923K~1123K) and finally Sm2Fe17Nx powder is obtained by solid-gas phase reaction with N22Fe17Nx powder preparation.
Liu [35] used plasma-assisted ball milling technique in N22Fe17Nx phase on the particle surface by suppressing Sm volatilization during alloying, promoting atomic diffusion and increasing the initial free energy of Sm and Fe reaction, as shown in Figure 3. Normally, the surface of Sm2Fe17 powder produced by conventional mechanical alloying is prone to oxide layer formation, which can be prevented by introducing plasma field. The plasma-assisted ball mill mechanical alloying method can effectively suppress the oxidation of the powder and improve the nitriding effect.
20220109004157 83797 - Research Progress of SmFeN Rare-Earth Permanent Magnet
Figure 3. schematic diagram of conventional high energy ball milling and plasma assisted ball millingon pure Sm and Fe metals [35]
Popovich [36] studied the magnets made by adding Ti, Mo, and Nb mechanical alloying and found that the Ti, Mo, and Nb five-member alloy increased the Curie temperature from 412 K to 479 K. The powder particle size was about 7.5 μm with a more uniform particle size, as shown in Figure 4, which laid the foundation for the subsequent nitriding treatment to obtain high-performance Sm2Fe17Nx powders.

Reductionand Diffusion (R/D)

The reduction and diffusion method is to mix the powders of Sm2O3, Fe and Ca together and heat them under argon atmosphere for several hours.Sm2O3 is reduced to Sm metal by Ca and Sm diffuses into Fe to form Sm2Fe17 alloy The reaction product consists of Sm2Fe17 alloy and CaO. Since CaO is soluble in water, it is easy to obtain a solution that becomes alloy particles and Ca(OH)2 after water treatment. To separate Sm2Fe17 alloy powder, the slurry was washed and rinsed several times. Then the magnetic powder was prepared by nitriding treatment [37][38][39].
Japanese scholar Atsushi [40] produced high-performance Sm2Fe17N3 powder by diffusion reduction with saturation magnetization strength Js, remanent magnetization Br, and maximum magnetic energy product (BH)max of 1.40 T, 1.13 T, and 272 kJ-m-3, respectively.This method uses the cheaper Sm2O3 raw material instead of Sm metal, eliminating the need for The method utilizes the cheaper Sm2O3 raw material instead of Sm metal and eliminates the steps of alloy melting, homogenization and annealing and powder crushing. In the high temperature environment Sm is very volatile leading to the change of alloy composition, and when the CaO is not rinsed thoroughly with water, calcium oxide will be left in the alloy, which will corrode the subsequently generated Sm2Fe17Nx, while the SmFe5 soft magnetic phase will be generated during the rinsing process. The surface contamination of Sm2Fe17 alloy powder through the washing and rinsing process may prevent uniform nitriding and cause the particles to have an unnitrided state.
20220109004335 90509 - Research Progress of SmFeN Rare-Earth Permanent Magnet
Figure 4. Morphology of powder obtained by mechanical alloying [36]
Okada [37] dissolved the surface damaged impurity phase of Sm2Fe17N3 particles by acetic acid washing treatment. A new washing method was investigated to avoid the formation of coercivity-decreasing factors, and the coercivity reached 28.1 kOe. The heat resistance of Sm2Fe17N3 powders depends to a large extent on their oxygen content. Washing with ethylene glycol effectively suppressed the oxidation during the washing process. In addition, the washing atmosphere also influenced the increase of oxygen content in the powder. Sm2Fe17N3 powder washed with ethylene glycol in a glove box showed higher heat resistance and had the same microstructure before and after the heat resistance test. In contrast, the powder washed with water in air exhibited low heat resistance due to α-Fe precipitation during the heat resistance test.Ishikawa [39] prepared SmFeN powders with Br = 1.46 T, Hc = 874 kA∙m-1, by improving the reduction-diffusion method with nitriding heat treatment between the reduction-diffusion heat treatment and the wet process ( BH)max = 353 kJ∙m-3.
Matsuda [38] crushed Cr2O3 and Sm2O3 fine powders and mixed them by ball milling, after which the powder mixed with added Ca was heat treated. It was found that Cr could diffuse into Sm2Fe17 powders above 875°C during RD, and the diffusion distance of Cr increased with the increase of RD temperature to obtain Cr-diffusing Sm-Fe core-shell micropowders. After nitriding and washing, the Cr-diffused Sm-Fe-N core-shell powder was successfully obtained, and the coercivity and saturation magnetization strength of the core-shell powder were 855 kA∙m-1 and 122 Am2∙kg-1, respectively, and the saturation magnetization strength was higher than that of the non-core-shell Sm-Fe-Cr-N powder.
The surface of the Sm-Fe powder particles prepared by the reduction-diffusion method is easily damaged during the washing process.Chen [41] effectively prevented the formation of the deleterious α-Fe phase by CaH2 pretreatment, and additionally further increased the coercivity Hcj by reducing the nucleation center of magnetic inversion.The magnetic properties of Sm2Fe17Nx prepared from pretreated Sm-Fe powder far exceeded those of Sm2Fe17Nx prepared from normal Sm- Fe powder. The maximum magnetic energy product (BH)max of the prepared Sm2Fe17Nx powder was 192 kJ∙m-3, the remanent magnetic Br was 1 T, and the Hcj was 708 kA∙m-1.

Hydrogenation Disproportionation Desorption Recombination (HDDR)

The hydrogenation disproportionation process produces a fine crystalline SmFeN powder with homogeneous coercivity. The alloy is inductively melted from 99.9% Fe and 99.98% Sm under Ar atmosphere. The alloy was homogeneously annealed at 1000°C for 50 h. The annealed alloy ingots were almost single-phase with small amounts of free α-Fe and SmFe3. Sm2Fe17 alloy first absorbed hydrogen to undergo disproportionation reaction, and then pumped to vacuum, the hydrogen broke away from Sm2Fe17 alloy to undergo recombination reaction to refine the grains, thus improving the magnetic properties of Sm2Fe17Nx.
Shouzeng Zhou et al [42] first prepared isotropic Sm2Fe17Nx magnetic powders using the HDDR method, which has attracted extensive interest from many researchers. Some scholars in Germany [43] and Japan [44] investigated the phase composition and change laws of Sm-Fe alloys at various stages during the preparation of Sm2Fe17Nx magnetic powders by HDDR method; Zhao Xinguo [45] prepared anisotropic Sm2Fe17Nx magnetic powders by HDDR nitriding.
The HDDR process is simple, with good homogeneity, high powder yield and low powder oxygen content, and can prepare isotropic and anisotropic magnets, which is a new magnetic powder preparation process with good application prospects. However, the process of Sm2Fe17Nx preparation by HDDR method needs further study because of the numerous reactions involved in the process and the complexity of the process and mechanism, especially the microstructure evolution process and mechanism of Sm2Fe17 alloy in the HDDR process and the grain refinement mechanism are not fully understood.

Research progress of bulk SmFeN permanent magnetic materials

Block preparation is the key to limit the development and application of SmFeN permanent magnetic materials, Figure 5 [22] shows the development history of the maximum magnetic energy product of Sm2Fe17N3 powder and block, although the magnetic energy product of Sm2Fe17N3 powder is developing rapidly, close to 380 kJ∙m-3, but the magnetic energy product of block is only less than 200 kJ∙m -3, which greatly limits the application of Sm2Fe17N3 magnets. This is due to the inevitable decomposition of Sm2Fe17N3 into non-hard magnetic phases of SmN and α-Fe at temperatures above 893 K [46], which is well below the eutectic point of the Sm-Fe system (993 K), and therefore Sm2Fe17N3 cannot form a high coercivity weave using liquid-phase sintering as in the case of Nd2Fe14B magnets. Currently solid-phase sintering and organic binders are the only methods for solidifying Sm2Fe17N3 powders, and the conventional powder orientation-sintering method cannot be used, so commercial Sm2Fe17N3 magnets can still only be produced by bonding techniques.
Researchers have also not stopped studying hot-press sintering and cold-press forming. The main methods currently available are discharge plasma sintering and high-pressure hot compaction. These techniques usually have one or more of the following advantages in the solidification process, including low temperature (down to room temperature), short time (down to microseconds) and high pressure (up to tens of GPa), so that grain growth can be inhibited to achieve grain refinement. Moreover, the solidified magnets always maintain the microstructure and therefore the magnetic properties of the original strip or powder, which proves the possibility of these techniques in the fabrication of bulk Sm2Fe17Nx.
20220109004602 10476 - Research Progress of SmFeN Rare-Earth Permanent Magnet
Figure 5. progress in energy products of Sm-Fe-N powders and permanent magnets [22]

Preparation of bonded SmFeN bulk magnets

Bonded magnets are a type of magnet prepared by mixing Sm2Fe17N3 powder with a binder, which can modulate the magnetic properties and can be made into various shapes. The production process of bonded magnets is divided into four types, calendering, injection molding, extrusion molding and molding. The binder can be an organic binder and a low melting point metal. Therefore the main goal of preparing high performance bonded magnets is to find binders that are suitable for Sm2Fe17N3.
Otani [47] et al. studied the magnetic properties of Sm2Fe17N3 magnets bonded with low melting point Zn, Bi, Sn and Al metals and found that Zn improves the coercivity of Sm-Fe-N bonded magnets and the magnets have higher coercivity than Sm-Fe-N magnets bonded with other metals. They also found that the presence of Zn7Fe3 (which can be called G-FeZn phase) can improve the coercivity.
Matsuura [48] further used fine Zn particles with low oxygen content prepared by hydrogen plasma metal reaction technique as a binder, and during annealing Zn melted and diffused from the surface of Sm2Fe17N3 particles and reacted with Fe-Fe to form G-FeZn phase, which is nonmagnetic at room temperature [23]. Thus coercivity and maximum magnetic energy product increase after annealing, increasing to 2.66 MA∙m-1 and 53.1 kJ∙m-3.
In recent years, new alloy metals have also been tried as binders.Otogawaa [49] prepared Sm2Fe17N3 magnets with no decrease in coercivity over a very high temperature range using a new tetrameric Sm-based alloy as a binder. This is due to the fact that the Sm-Fe-Cu-Al binder suppresses the precipitation of α-Fe phase, and on the other hand, the coercivity decrease of the magnets is also suppressed due to the separation of α-Fe precipitates from the surface of Sm2Fe17N3 grains into the grain boundary layer consisting of Sm-based alloys.

Preparation of sintered SmFeN bulk magnets

In rare-earth transition metal compounds, rare-earth atoms are oxidized more than transition metal atoms (oxides of rare-earth elements have lower Gibbs free energy than transition metals). Therefore, as reported in the literature, large amounts of α-Fe impurities are easily formed in the sintered pure Sm2Fe17N3 magnets. The addition of a suitable highly reactive oxygen absorber to the sintering process to suppress Sm2Fe17Nx is the main method to obtain high coercivity, high magnetic energy product magnets. So far, the coercivity of Sm2Fe17N3 sintered magnets can only reach the level of a few percent of their anisotropic field. Therefore, there is a high expectation for technologies that can realize the potential properties of this material.
Discharge plasma sintering (SPS) uses a combination of uniaxial pressure and pulsed direct current to heat and sinter the powder. The whole apparatus is shown in Figure 6 [50], where the sample is placed in a mold made of graphite, and when the powder is pressed in the mold is conductive, an electric current passes directly through the sample and heats the material. The non-conductive material is heated by heat conduction through the mold walls. Switching pulses generate a constant movement of heat at the contacts between the sample particles during the sintering cycle, and the whole process is carried out in a vacuum so that the material is not oxidized. Compared to conventional pressureless furnace sintering and hot pressing, discharge plasma sintering allows for shorter densification times, lower sintering temperatures, and the production of near-fully densified or fully densified materials with limited grain growth.
20220109004656 82174 - Research Progress of SmFeN Rare-Earth Permanent Magnet
Figure 6. A schematic of the SPS process [50]
Takagi et al [51] obtained magnets with 86.2% theoretical density at 1200 MPa using a strip of coarse powder with a thickness of 20 μm or more. The coercivity of the magnets reached 772.8 kA∙m-1 , remanence 0.862 T, and magnetic energy product 121 kJ∙m-3. The study suggested the effect of the pre-pressing method on the properties: repeated pre-pressing at 1500 MPa followed by hot pressing under the same conditions increased the density to 92.4%, coercivity to 774.4 kA∙m-1 , remanence 0.91 kJ∙m-1 , and remanence 0.91 kJ∙m-1 . -1, remanent magnetization 0.91 T, and maximum magnetic energy product to 129 kJ∙m-3.
Saito et al [52] prepared Sm2Fe17N3 magnets using Cu-plated Sm2Fe17N3 powder, which can solidify into a block at 473°C to 673°C. Sm-Fe-N magnets produced from Cu-plated Sm-Fe-N powder showed a high coercivity of 9.5 kOe at 473 K, close to that of Sm-Fe-N powder.
The magnetic properties and demagnetization curves of Sm2Fe17N3 magnets with 30 wt% SmCu added by Lu [53] et al. are shown in Fig. 7. Unlike the sintered pure Sm2Fe17N3 magnets, the decomposition of Sm2Fe17N3 phase is suppressed in the sintered magnets with SmCu added due to the oxidation of SmCu powder. smCu powder effectively prevents anisotropy Sm2Fe17N3 sintered magnets from forming α-Fe precipitation and improving the coercivity. The SmCu-added magnets have a coercivity and squareness comparable to that of the original Sm2Fe17N3 powder. The coercivity reached 10.3 kOe, which greatly exceeded that of the sintered pure Sm2Fe17N3 magnets.
Saito [54] prepared Sm-Fe-N bulk magnets from Sm2Fe17N3 fast-quenched thin strips using discharge plasma sintering, and the magnetic material obtained had a high density of 90% to 94%. The prepared Sm-Fe-N bulk material retains the Sm2Fe17N3 phase of the fast-quenched thin strip, and a high coercivity of 16.9 kOe is obtained.
In recent years, the speculation that the mechanism of coercivity reduction is not thermal decomposition has been proposed [55][56]. According to this mechanism, the precipitation of Fe, which causes a sharp decrease in coercivity due to the redox reaction between the surface oxide film and the Sm2Fe17N3 matrix, is not related to thermal decomposition.The surface oxide layer on Sm2Fe17N3 particles usually consists of Sm2O3 and Fe2O3, and during the sintering process, Sm2Fe17N3 reduces Fe2O3 by the following reaction.

  • Sm2Fe17N3 + Fe2O3 = Sm2O3 + 19Fe + (3N) (2)

This equation suggests that even a slight redox reaction is sufficient to produce a significant amount of α-Fe phase as a result of endogenous confinement reactions in the powder, rather than thermal decomposition or exogenous surface oxidation during sintering. Sm2Fe17N3 powders with minimal surface oxides have been shown to neither precipitate out Fe nor reduce the coercivity [55]. This provides an idea for the preparation of high-performance Sm2Fe17N3 sintered magnets.Matsuura [48] et al. investigated that the Sm2Fe17N3 sintered magnets could be produced while suppressing the decrease in coercivity by a low-oxygen process that prevents surface oxidation, and Takagi [55] obtained the same results as However, the maximum magnetic energy product (BH)max only reached 191 kJ∙m-3 due to the decrease in remanence caused by the decrease in saturation magnetization strength.
20220109004837 44454 - Research Progress of SmFeN Rare-Earth Permanent Magnet Figure 7. The normalized demagnetization curves (a) and a comparison of magnetic properties (b) of raw Sm2Fe17N3 powder A, sintered Sm2Fe17N3 magnets B and 30 wt% SmCu-added sintered Sm2Fe17N3 magnets C [53]

Other techniques for preparing SmFeN bulk magnets

In addition to bonding and sintering, other bulk preparation techniques have been cited for the preparation of bulk SmFeN rare earth permanent magnet materials, including high pressure torsional deformation process, forging process, etc.

High Pressure Torsion (HPT)

High pressure torsional deformation is a severe torsional deformation under several Gpa high pressure to solidify the powder and refine the microstructure at the same time, and it has been studied that the density can even reach close to 100% in samples treated with high pressure of several GPa. High-pressure torsional deformation consolidation of Ni nanopowders prepared by ball milling [57] showed that the density of the prepared powder was close to 95% of the theoretical density of bulk coarse-grained Ni. The absence of porosity was found by TEM and the average grain size reached 17 nm. high-pressure torsional deformation is a method to obtain bulk nanostructured materials by forming a high defect density in the crystal structure. When using this method, the formation of crystalline weaving is usually observed [58][59][60], which is extremely important to obtain high magnetic properties.
Schchetinin [61] found that the Sm2Fe17N3 hard magnetic phase can be well coupled with the α-Fe soft magnetic phase of the main phase decomposition during high pressure torsional deformation, and the saturation magnetization intensity after deformation increases with the number of torsions, as shown in Table 2 and Figure 8. When n = 3, the coercivity of the obtained Sm2Fe17N3 magnet is Hc = 624.4 kA∙m-1, σr = 57.7 A∙m2∙kg-1, σs = 114.5 A∙m2∙kg-1. During the high-pressure torsion, Sm2Fe17N3 undergoes The phase change corresponds to a temperature rise to 1000 K. The Sm2Fe17Nx phase decomposes into α-Fe and SmN phases, which leads to the formation of exchange-coupled states. This process provides a new idea for the preparation of bulk SmFeN.

Turns, n

Phase composition

Magnetic properties

Sm2Fe17Nx

α-Fe

SmN

Hc/(kA∙m−1)

σr/(A∙m2∙kg−1)

σs, A∙m2∙kg−1

0

100

98.5 ± 0.5

81.5 ± 0.2

151.4 ± 0.2

3

95 ± 2

5 ± 1

621.4 ± 0.5

57.7 ± 0.2

114.5 ± 0.2

5

60 ± 3

35 ± 3

5 ± 2

451.1 ± 0.5

62.5 ± 0.2

130.2 ± 0.2

 Table 2. XRD phase analysis results and magnetic properties of Sm2Fe17Nxalloys after HPT [61]
20220109005137 17094 - Research Progress of SmFeN Rare-Earth Permanent Magnet Figure 8. magnetic hysteresis loops of Sm2Fe17Nx alloys after HPT [61]

Forging

Forging is the process of producing plastic deformation by using dies and tools to obtain forgings with certain mechanical properties, shapes and sizes. During the processing, defects such as cast looseness produced by the metal during the smelting process can be eliminated, the microstructure can be optimized, and high-performance materials can be obtained. Kataoka [62] prepared forged Sm2Fe17Nx high-density magnets bonded with Zn, with a maximum magnetic energy product (BH) max of 191 kJ∙m-3 and a coercivity Hc of 1.89 The Sm2Fe17Nx grains become smaller and better oriented when not annealed. Although the maximum magnetic energy product (BH)max is low, it lays the foundation for the preparation of high-density, high-coercivity magnets in the future.

Effect of alloying elements on the magnetic properties of SmFeN magnets

The magnetic properties of Sm2Fe17N3 compounds can be modified by the introduction of various interstitial or substitution impurities [63][64][65][66][67]. These substitutions can improve the magnetic properties of Sm2Fe17 compounds, and there has been extensive work on the magnetic properties of compounds with light interstitial atoms.
Popovich [68] investigated the effect of Ti and Mo on the magnetic properties of Sm2Fe17, and alloying Sm2Fe17 with Ti and Mo resulted in an increase in lattice parameters and lattice volume, but did not change the lattice symmetry. When the hysteresis properties of Sm2Fe17-based alloys were measured, it was found that the introduction of titanium and molybdenum resulted in a broadening of the hysteresis loop, as shown in Figure 9, and an increase in the Curie temperature of the magnets, from 412 K to 449 K when Ti and Mo were doped at 0.5 wt%. The increase in Ti content resulted in a slight increase in coercivity and a slight decrease in remanence.
20220109005340 31866 - Research Progress of SmFeN Rare-Earth Permanent Magnet
Figure 9. the hysteresis loops for Sm-Fe-Ti, Sm-Fe-Mo, and Sm-Fe-Ti-Mo systems alloys obtainedby mechanical alloying[28]
The ternary compound Sm2Fe(17-x)Alx has been studied in sufficient detail [69][70][71]. It has been determined that these intermetallic compounds Sm2Fe17 have a rhombic structure of the Th2Zn17 type. In addition, regularities concerning the relationship between magnetization intensity and Curie temperature and Al substitution level have been found in [70][71]. For example, it has been determined that the Curie temperature increases first (when x ≤ 3) and then decreases rapidly with increasing Al concentration.Al substitution also affects the magnetic anisotropy of Sm2Fe17-xAlx. Thus low Al content in Sm2Fe17 (x ≤ 5) causes a reorientation of the easy magnetization axis from the basal plane to the c-axis, but this effect will be suppressed as Al continues to increase (5 ≤ x ≤ 7).
Saito [72] studied the effect of Ti and Zr doping on Sm2Fe17 and found that Sm2Fe17 magnets doped with higher levels of Zr and Ti had higher coercivity than those with lower levels of Zr and Ti, as shown in Figure 10. And the highest coercivity of 4.0 kOe was obtained in annealed (Sm0.7Zr0.3)2(Fe0.7Co0.3)15.5Ti1.5 fast quenched strips.
Xu [73] studied the magnetic properties of Sm2Fe17Nx after Y substitution for Sm and found that the saturation magnetization intensity σs increased with increasing Y concentration. With increasing Y content, the remanent magnetization Br first increases slightly and reaches a maximum at y = 0.4. As y continues to increase, Br decreases. The endogenous coercivity Hcj decreases with increasing Y content. The (BH)max of (Sm1-yYy)2Fe17Nx powder increases when the doped Y is below 40 at%. When replacing Sm with 20 at% Y, the (BH)max increased from 131.7 kJ∙m-3 to 151.6 kJ∙m-3, an increase of 15.1%. This is related to the improved σs due to lower grain size, increased hysteresis line rectangularity and improved exchange pegging field.
20220109005716 79463 - Research Progress of SmFeN Rare-Earth Permanent Magnet
Figure 10. Dependence of the coercivity of the (Sm1-xZrx)2(Fe0.7Co0.3)17-yTiy (x = 0~0.3, y = 0~2.0) melt-spunribbonsannealed at 1173 K on the Ti content[72].

Outlook

SmFeN rare-earth permanent magnetic materials are expected to develop into the fourth generation of rare-earth permanent magnetic materials by virtue of their excellent endogenous magnetic properties.The preparation of SmFeN powder materials has relatively mature process technology, however, for the preparation of bulk SmFeN rare-earth permanent magnetic materials, there is still an urgent need to study the work including magnetization mechanism, microstructure optimization, coercivity mechanism, and control of thermal decomposition. Especially for the preparation of bulk high-performance SmFeN rare-earth permanent magnetic materials, there is not a proven pathway, and new ideas and technologies are needed. Recently, in order to break through the bottleneck of achieving simultaneous control of hard magnetic phase weaving and soft magnetic phase morphology in bulk nanocomposite permanent magnetic materials, Prof. Xiangyi Zhang’s research group has successfully induced the orientation growth of nanocrystals with high soft magnetic content (28 wt%) and fine grain size (~10 nm) by applying high stress and large strain during amorphous crystallization using crystal strain energy anisotropy using high pressure thermal compression technique. The weaving of SmCo hard magnetic nanocrystals along their easy magnetization axis was induced to obtain magnetic energy products up to 28 MGOe [74]. Subsequently, this technique was successively extended and applied to the preparation of other systems of bulk nanocrystalline permanent magnetic materials with promising results, achieving the formation of hard magnetic nanocrystal weaving and the enhancement of magnetic properties in bulk nanocrystalline magnets [75][76][77]. We envision that if the preparation of bulk SmFeN rare-earth permanent magnetic materials is studied using high-pressure thermal compression technique, it will be promising to realize the preparation of high-performance bulk SmFeN rare-earth permanent magnetic materials. As magnetic researchers continue to conduct in-depth research on SmFeN magnets, it is expected that the development of cost-effective permanent magnetic materials will be realized for application in various high-tech industries.
Source: China Permanent Magnet Manufacturer – www.rizinia.com

Authors: Li Wei, Guo Jiarui, Zuo Siyuan, Wang Yana, Huang Guangwei *, Zheng Liyun

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