Research progress of SmFeN rare earth permanent magnetic materials
The crystal structure, endogenous magnetic properties, magnetic properties of SmFeN rare-earth permanent magnetic materials in relation to the number of nitrogen atoms; the preparation process of several major SmFeN magnetic powders; the effect of added elements on magnetic properties; the preparation of bonded magnets; and the latest progress are reviewed.
Table of Contents
- Sm2Fe17Nx interstitial compound permanent magnetic materials
- Process of preparation of Sm2Fe17Nx compound magnetic powder
- Effect of added elements on the magnetic properties of Sm2Fe17Nx
- Preparation of bonded magnets
- Recent progress
Despite the excellent magnetic properties of NdFeB-based rare-earth permanent magnetic materials, there is a desire to develop a new generation of iron-based rare-earth permanent magnetic materials due to their low Curie temperature, poor corrosion resistance, and the limited potential of NdFeB-based rare-earth permanent magnetic materials after years of development as their actual properties are very close to the theoretical values. One of the approaches is to use the introduction of interstitial atoms to influence the magnetic properties of the compounds. The first interstitial atom introduced into intermetallic compounds was the H atom, which was first thought of because of its small size. In 1984, H was introduced into the R2Fe14B series, where the crystal structure remained unchanged, the Curie temperature increased, and the magnetic crystal anisotropy field decreased . Then, H was introduced into R2Fe17 [2-4], which increased the Curie temperature. Later, it was found that the introduction of C atoms into Sm2Fe17 compound formed Sm2Fe17Cx compound, which did not change the crystal structure of the alloy, but increased its Curie temperature and saturation magnetization intensity, and transformed the compound from easy basal anisotropy to easy c-axis anisotropy, with the increase of C content, the anisotropy field of magnetic crystal increased, and its room temperature magnetic crystal anisotropy field reached 5.3T . On this basis, in 1990, Coey  et al. reported the synthesis of R2Fe17Nx interstitial atomic intermetallic compounds using gas-solid phase reaction, which attracted great attention from the magnetic community and rapidly entered into a research climax.After R2Fe17 compounds absorb N The crystal structure of R2Fe17Nx compounds is unchanged, the volume of the single cell is expanded, the Curie temperature and the saturation magnetization intensity are significantly increased, and among all R2Fe17Nx compounds, only when R is Sm at room temperature, the easy c-axis anisotropy is shown. NdFeB, and the upper limit of theoretical magnetic energy product is 447 kJ/m3 (56.2 MGOe), which is comparable to NdFeB , and its thermal stability, oxidation resistance and corrosion resistance are better than Nd2Fe14B . In this paper, the relationship between the crystal structure and magnetic properties of SmFeN rare-earth materials is analyzed, and several main preparation processes of SmFeN magnetic powders, the preparation methods of its bonded magnets and the latest research progress are given.
Sm2Fe17Nx interstitial compound permanent magnetic materials
At room temperature, Sm2Fe17 compound is a Th2Zn17 type structure , and the Th2Zn17 type crystal structure is one of the most basic types of crystal structures of rare-earth permanent magnetic compounds.The spatial stereo diagram of the Th2Zn17 type crystal structure is shown in Fig. 1, which belongs to the rhombic square crystal system (or tripartite crystal system) with the space group R3m, and a single cell contains three Sm2Fe17 molecules. There are 57 atoms in a single cell, 6 Th (or R) atoms occupy the c-site, 9 out of 51 Zn (or Co, Fe, etc.) atoms occupy the d-site, 18 occupy the f-site, 18 occupy the h-site, and 6 occupy the c-site. There are two large interstitial positions in this structure: one is an octahedral interstitial position, located at the 9e crystal site, on the atomic plane containing rare earth atoms; the other is a 3b crystal site located between two rare earth atoms along the c-axis. The Sm2Fe17Nx-type compound has the same structure as Sm2Fe17, but the dotting constants have changed, with an increase in both a and c, and an increase in the volume of the single cell by about 6%. As shown in Figure 2.
Figure.1 Th2Zn17 type rhombic crystal structure
Figure.2 Single cell structure of 2:17:N compound
Endogenous magnetic properties of Sm2Fe17Nx
In the R_TM compound (R stands for rare earth element, TM stands for 3d transition group metal element) there are three kinds of exchange interactions : TM_TM interchange; R_R interchange; R_TM interchange.
The radius of the 4f electron cloud in rare-earth metals is about 0.6-0.8 A゜, while in rare-earth compounds, the interatomic distance is more than 10 times larger than it. At the same time, there are 5s and 6p electron layers outside the 4f electron layer to play a shielding role, whether it is between the 4f electron cloud or 3d and 4f electron cloud can not overlap, 4f electrons or 3d and 4f can not have a direct exchange between the role of the 3d and 4f electron magnetic moment coupling. In rare-earth metal compounds, the spin magnetic moments of 3d metal and 4f metal are always in antiparallel arrangement. In light rare-earth compounds, the 3d and 4f electron magnetic moments are ferromagnetic coupling, i.e., the spin magnetic moments of the 3d metal and the light rare-earth metal atoms are aligned in the same direction and in parallel. At the Curie temperature, without the action of external magnetic field, the small area inside the ferromagnet is magnetized to saturation spontaneously. These regions, which have been spontaneously magnetized to saturation, are called magnetic domains. The magnetic properties of permanent magnet alloys are sensitive to the microstructure, and the addition of rare earth elements mainly increases the magnetic crystal anisotropy of the material. The orbital magnetic moments of 3d electrons are quenched under the action of crystal fields, thus the magnetic materials consisting of only 3d transition group elements have less magnetic crystal anisotropy, while 4f is the inner layer of electrons with strong spin-orbit coupling, which can produce strong magnetic crystal anisotropy . This is an intrinsic factor for the strong coercivity of rare earth-transition group alloys.
The Curie temperature of R2Fe17 is generally very low, about 240-480 K. The reason is that the Fe_Fe atomic spacing in R2Fe17 is too small, which causes them to become partially antiferromagnetic coupled and the exchange effect is very weak, so the Curie temperature is low. According to the empirical rule of Bethe_Slates, the exchange effect is enhanced when the Fe_Fe atomic spacing is enlarged. The most significant effect of the introduction of N atoms into the R2Fe17 compound is to increase the Fe_Fe atomic spacing in the R2Fe17Nx compound, which leads to a significant enhancement of the Fe_Fe atomic exchange effect and an increase in the Curie temperature, generally raising the Curie temperature by 400 K on average. At the same time, the entry of N atoms into the 9e position of the Sm2Fe17 cell generates a strong electric field gradient in the 4f shell layer of Sm, which changes the crystal field coefficient A20 and increases the anisotropy constant K1, leading to a substantial increase in the coercivity [5,11,12].
The introduction of N atoms into the R2Fe17 compound resulted in a substantial increase in the room temperature saturation magnetization intensity of the R2Fe17Nx compound.
Relationship between magnetic properties and the number of nitrogen atoms
The N content has a crucial influence on the magnetic properties of Sm2Fe17Nx compounds. Earlier it was thought that the maximum number of N atoms x of Sm2Fe17Nx compounds would not exceed 3. Later it was found that when NH3 or NH3_H2 mixture was used instead of N2 as the nitrogen source gas for nitriding, the number of N atoms could exceed 3 and reach up to 6 [13,14].When the N content x=6, three N atoms occupy 9e crystal sites, which play an enhanced role in magnetic properties, and the other three N atoms The other three N atoms occupy half of the 18g crystalline sites, which weaken the magnetic energy. Figure 3 shows the variation of magnetic properties of Sm2Fe17Nx with the number of N atoms.
Fig.3 Magnetic properties of Sm2Fe17Nx as a function of N content [13,14]
Process of preparation of Sm2Fe17Nx compound magnetic powder
The preparation of Sm2Fe17Nx compound magnetic powder is mainly divided into two steps: the first step is to prepare the single-phase Sm2Fe17 compound, and the second step is to nitride the Sm2Fe17 compound to produce Sm2Fe17Nx. The methods of preparing Sm2Fe17 compound include rapid quenching (RQ), reductive diffusion (R/D), powder metallurgy (PM), hydrogenation disproportionation (HDDR), and hydrogenation disproportionation (HDDR). (HDDR), and mechanical alloying (MA).
Fast quenching method
The hard magnetic properties of Sm2Fe17Nx were firstly reported by Katter  using RapidlyQuenched (RQ) method, where the endogenous coercivity HcJ=21kOe at room temperature, and the Sm2Fe17Nx magnetic powder prepared by the rapid quenching method remains isotropic. The core technology of this process is to manufacture thin strips by fast quenching of the melt, and to make the strips into magnetic powder, which can directly obtain fast quenched commercial magnetic powder. This method uses inert gas as a protective atmosphere, the alloy ingot in the quartz container melting, the alloy melt under pressure by the bottom of the container hole sprayed into the high-speed rotation of water-cooled copper roller outer surface, the formation of amorphous or microcrystalline narrow thin strip. Due to the demanding conditions of this process, there are difficulties in mass production.
Powder metallurgy method
Powder metallurgy (PowderMetallurgy, referred to as PM) is a common process for the preparation of interstitial nitride, its product performance is excellent, high magnetic energy product, is incomparable with other methods. The basic process flow is: master alloy melting * homogenization annealing * coarse crushing * ball milling powder.
This process and the gas-solid phase reaction method can produce high-performance anisotropic Sm2Fe17Nx magnetic powders .
Hydrogenated disproportionation method
The process flow is as follows: master alloy melting * homogenization annealing * coarse crushing * HDDR treatment.
The basic principle of HDDR (HydrogenationDisproportionationDesorptionRecombination) method is to use the phase transformation of rare earth metal interstitial compounds under the action of hydrogen in order to refine the grains. The HDDR process is characterized by fine powder grain size and low oxygen content, but the disadvantage is the high α_Fe content, which affects the magnetic properties of the material.
Reduction diffusion method
The basic principle of reduction and diffusion (RD) method is to reduce the rare-earth oxides with reducing agent to become rare-earth metals, and then obtain rare-earth permanent magnetic powder directly through the interdiffusion of rare-earth metals and transition group metals. The advantage of this method is the low cost of raw materials, the disadvantage is that it is difficult to implement. At present, Japanese scientists have achieved great success and industrialization with this method, but the research progress in China is slow.
Mechanical alloying method (MA)
MechanicalAlloying (MA) or HighEnergyBallMilling was successfully developed as a new material preparation process by Benjamin [17,18] of IN-CO, USA in 1970. The mechanical alloying technology is used in the ball mill to make the powder particles repeatedly squeezed, deformed, fractured, welded and finally alloyed by mechanical forces, i.e., frequent collisions between grinding balls, grinding jars and powders. By mechanically alloying Sm and Fe powders, a mixture of nanoscale amorphous Sm_Fe alloy and α_Fe will be obtained, which will be crystallized and heat treated at 700~750℃ and then nitrided to obtain single-domain Sm2Fe17Nx magnetic powder. Compared with the conventional melting alloying, the MA method has the following features: simple process conditions; continuously adjustable composition; the ability to cover the range of alloys formed by melting alloying, and the ability to alloy systems that cannot or are difficult to be alloyed by melting.
In addition, Sm2Fe17Nx can be synthesized directly by the MA method, using Sm powder ball-milled with Fe powder in NH3 atmosphere [19,20]. Some work has been done in this area by Jia Chengzhuang et al  in China.
Effect of added elements on the magnetic properties of Sm2Fe17Nx
Wendhausen  investigated the effect of partial substitution of Fe by different metallic elements (Nb, V, Ti and Co) on the magnetic properties of the alloy. It was found that the Curie temperatures of the formed Sm2(Fe,M)17 compounds were all higher than those of the Sm2Fe17 compounds. The addition of alloying elements had a great influence on the single-phase nature of the parent alloy Sm2(Fe,M)17, with Ga and Ti as unfavorable elements and Al and Cr as favorable elements. The Curie temperature and spontaneous magnetization intensity of the formed Sm2(Fe,M)17Nx compound were lower than those of Sm2Fe17Nx after nitriding treatment. In contrast, for Co, the Curie temperature was increased by nearly 100 K compared to Sm2Fe17Nx to 845 K and the spontaneous magnetization intensity was 1.41 T. Moreover, the Curie temperature and the magneto-crystal anisotropy field increased with the increase of Co content in a certain range . With the addition of Al and Ga, all the endogenous magnetic parameters of nitrides gradually decreased; with the partial replacement of Sm by Pr, the magnetocrystalline anisotropy field of nitrides decreased, but the saturation magnetization intensity increased, and Br also increased. The addition of Cr and Ga significantly increased the HcJ of Sm2(Fe1-xMx)Ny compounds; the addition of Zr promoted the amorphization of Sm_Fe alloy and inhibited the grain growth; the addition of Co increased the Curie temperature but decreased the HcJ because Sm(Fe,Co)2Nx and Sm(Fe,Co)3Nx were easily formed after the addition of Co. interstitial compounds, which decompose below 500°C, resulting in the formation of the soft magnetic phase α_(Fe,Co), leading to a decrease in coercivity.
Preparation of bonded magnets
The production process of bonded permanent magnets can be divided into four types: calendering, injection molding, extrusion molding and molding. Since Sm2Fe17Nx decomposes at higher temperatures, only bonded magnets can be made, and organic substances such as epoxy resins are usually used as binders. In addition to organic binders, low melting point metals such as Zn and Sn can also be used as binders, and Otanti et al  obtained bonded magnets with a magnetic energy product of (BH)max=10.5 MGOe by bonding Zn powder using a quasi-powder metallurgy method.
Suzuki et al  studied the magnetic properties and magnetic stability of resin-bonded Sm2Fe17Nx permanent magnets.Sm2Fe17Nx powder was prepared by the fast quenching method with an average particle size of about 3 μm.The magnetic properties of the resin-bonded magnets were: Br=0.97 T, HcJ=676.4 kA/m, (BH)max=154 kJ/m3, and The environmental stability is better than that of NdFeB bonded permanent magnets. The (BH)max=85.9kJ/m3 of Sm2Fe17N3 bonded permanent magnets prepared with Zn as binder is better than that of Sm2Fe17N3 bonded permanent magnets made with Sn as binder. Since low melting point metals such as Zn are used as binders, they reduce the saturation magnetization strength, which in turn leads to a lower (BH)max.
In recent years, a new process has been developed, i.e., explosive sintering, which works by instantaneously (<10-6s) forming the powder under the high pressure (~103 MPa) generated by the explosion to achieve the purpose that cannot be achieved by conventional sintering. Zhang Dengxia  first prepared SmFeN magnets with a density of 6.5 g/cm3 and a magnetic energy product of 11 MGOe by using the explosion sintering process, which can improve the density and remanence of magnets and the squareness of the demagnetization curve, but it is still difficult to enter the practical applications because of the dangerous working environment, the difficulty to master the process parameters, and the poor repeatability.
In Japan, TsutomuMashimo, XinshengHuang et al  successfully prepared fully dense bulk Sm2Fe17Nx magnets with porosity of 2%-8% by ShockPressure method, and they used the raw powder with (BH)max=27.5MGOe at instantaneous shock pressure up to 16GPa. They obtained centimeter-sized anisotropic magnets with a (BH)max of 22.5 MGOe by molding under high pressure at instantaneous impact pressure of 16 GPa or more.
Sumitomo Metal Mining Company of Japan has made a breakthrough in the preparation of Sm2Fe17Nx magnetic powder by reduction-diffusion process [28,29], and they have produced anisotropic Sm2Fe17Nx magnets with good performance by injection molding technology, which have been released to the market. 100 t of Sm2Fe17Nx magnets were produced annually in 2002. Sm2Fe17Nx magnets are mainly used as an alternative to ferrite sintered magnets for applications such as amplifiers and fan motors for air conditioners, and it is expected that the development of these magnets will increase.
The best performance was Br=1.12T, HcJ=597kA/m, (BH)max=178kJ/m3. The coercive force of Sm2(Fe,Mn)17Nx magnet with the addition of alloying element Mn was only decreased by 7% at 433K after 1000h exposure to air. The temperature coefficient of coercivity was -0.26%/K in the range from room temperature to 423 K, which was comparable to that of SmCo5.
Ryo Osatsuzawa et al  used the super-rapid cooling method to produce SmFeN powder. The (BH)max of the bonded specimens pressed from Sm9.2Fe90.8 and Sm9.2(Fe0.85Co0.15)90.8 (Vs=40m/s) nitride powders with high magnetic energy product were 106kJ/m3 and 116kJ/m3, respectively, which have surpassed the existing NdFeB bonded magnets.
SmFeN series permanent magnet materials are expected to be a new generation of rare earth permanent magnet materials due to their excellent endogenous magnetic properties. There is a clear understanding of their structure and magnetic properties, but there is still a lot of work to be done in the optimization of the chemical composition of the elements, the preparation of alloy powder, the coercivity mechanism and the preparation of high performance magnets using their high endowment magnetic properties.
Source: China Permanent Magnet Manufacturer – www.rizinia.com
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