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Progress in industrialization of samarium iron nitrogen rare earth permanent magnet materials

Sm2Fe17Nx rare-earth permanent magnetic material has excellent endowment magnetic properties, its saturation magnetization strength reaches 1.54T, which is comparable to 1.6T of Nd-Fe-B; Curie temperature is 470℃ (312℃ for Nd-Fe-B), anisotropic field is 14T (8T for Nd-Fe-B) are higher than Nd-Fe-B material, and its corrosion resistance, thermal stability and oxidation resistance are also better than Nd-Fe-B permanent magnetic material. The thermal stability and oxidation resistance are also better than Nd-Fe-B permanent magnetic materials have become a new generation of rare earth permanent magnetic materials. However, the industrialization research on coercivity mechanism, chemical composition optimization, nitriding process and preparation of high performance magnets is still not thorough, and a lot of work is needed to optimize the magnetic powder processing process and develop new binder and molding methods.

Review of the development history of rare-earth permanent magnetic materials

Rare earth permanent magnet materials are permanent magnet materials based on intermetallic compounds formed by rare earth metal elements and transition group metals, which are usually called rare earth intermetallic compound permanent magnets, referred to as rare earth permanent magnets. So far, rare earth permanent magnet materials have gone through three development stages, and the fourth generation of rare earth permanent magnets is in the pipeline.
In the 1950s, with the development and research of powder metallurgy and liquid-phase sintering process, the production of high-performance sintered SmCo5 permanent magnets stepped into the industrialization stage, and the first generation of rare-earth permanent magnet materials came into practical application. In 1977, Ojima et al. produced Sm2(Co, Cu, Fe, Zr)17 permanent magnets with (BH)max=30MGOe by powder metallurgy, making it a typical representative of the second generation of practical rare-earth permanent magnets. The emergence of high-performance Sm-CO permanent magnets has greatly promoted the development of instrumentation industry and other modern technologies, but the industrial production and market expansion of the first and second generation rare-earth permanent magnets were limited due to the fact that the main component of the first and second generation rare-earth permanent magnets is metallic Co, which is an expensive and strategic material.
In the 1970s, Clark et al. found that the coercivity of TbFe2 compound could be greatly improved after making it amorphous and annealing, which inspired people to make R-Fe compound amorphous and then precipitate a new non-equilibrium phase through heat treatment to achieve magnetic hardening. in the early 1980s, scientists have systematically studied the magnetic properties of Nd-Fe binary system alloy by using rapid solidification technique. Later, B element was added to the alloy to obtain amorphous Nd-Fe alloy, but it was unexpectedly found that this ternary alloy has high coercivity and high Curie temperature Tc, which triggered a boom in the research of rare earth permanent magnetic materials.
Unlike samarium-cobalt alloy, Nd-Fe-B permanent magnet material replaces the scarce and expensive cobalt with abundant and cheap iron, which greatly reduces the dependence on Co and drastically lowers the cost. More importantly, it has the advantages of high magnetic properties, good mechanical and mechanical properties, and high cost performance (140% of Sm-Co permanent magnet material), thus it becomes a unique material in the field of magnetic materials today and becomes the third generation permanent magnet material.
From the viewpoint of physical properties, Nd-Fe-B permanent magnets have their inherent weaknesses, especially the Curie temperature is not high, the temperature coefficient of coercive force is large and difficult to apply in high temperature fields, and there are also problems such as poor temperature stability and chemical stability. Therefore, since the 1990s, magnetic scientists have been improving the performance of Nd-Fe-B series rare-earth permanent magnet materials, while exploring other new iron-based rare-earth permanent magnet materials, mainly focusing on the following aspects.

  • ① The study of interstitial rare-earth compounds such as R2(Fe,M)17Ny and R2(Fe,M)17Cy.
  • ② The study of magnetic properties of 3:29 type R3(FeM)29 (Ti, V, Mo, etc.) compounds and their nitrides.
  • ③ Study of the structure and magnetic properties of R(Fe, M)12 (M=Ti, V, Mo, si) chemical carbon and nitrogen compounds with ThMn12 type structure and 1:7 type carbon and nitrogen compounds.
  • Research on high-performance nanocomposite permanent magnetic materials.

Sm-Fe-N rare earth permanent magnetic materials discovery

The R2Fe17 compounds have long been of interest to permanent magnet material workers because of their generally high Ms point, good stability, and iron-rich in the R-Fe binary system. But unfortunately in the binary R2Fe17 compounds, the Fe-Fe atomic spacing is too short and they have antiferromagnetic exchange, which leads to the R2Fe17 compounds with low Tc (110-119°C) and mostly easy basal plane with low anisotropic field. Therefore the binary R2Fe17 compounds have not been able to become permanent magnets of practical importance.
Over time, the situation has changed. First, it was found that the rare earth metal compounds could absorb large amounts of hydrogen, and the absorption of hydrogen caused significant changes in their magnetic properties; then it was found that partial replacement of iron in R2Fe17 with small amounts of magnetic or nonmagnetic elements such as Al could also improve the Tc of the 2:17 compound; later, it was found that introducing C atoms into the Sm2Fe17 compound would not change the crystal structure of the alloy, but made both its Tc and Ms increased, and the easy basal plane of the compound was transformed into an easy C axis, and its HA increased with the increase of C content, and the room temperature HA could reach 1.53T.
Inspired by these studies, people began to consider whether the Sm2Fe17 compound could be improved to become a material with permanent magnetic characteristics.
On the basis of the above, in 1990, Coey, as the chief scientist of the Rare Earth Permanent Magnet Development and Research Program of the European Community, successfully synthesized a series of R2Fe17Nx compounds by the gas-solid reaction method, and made a detailed study of their structures and magnetic properties, and found that Sm2Fe17Nx compounds have excellent endogenous magnetic properties and can be used as permanent magnetic materials, thus announcing the birth of SmFeN The Sm2Fe17Nx compound was found to have excellent endogenous magnetic properties and could be used as a permanent magnetic material, thus announcing the birth of SmFeN rare earth permanent magnetic material.
The Sm2Fe17Nx compound has excellent endogenous magnetic properties. Its Js can reach 1.54T and (BH)max can reach 472.0kJ/m3, which is completely comparable to Nd-Fe-B; meanwhile, its anisotropic field is about three times higher than that of Nd-Fe-B permanent magnets; the Curie temperature is 160℃ higher than that of Nd-Fe-B permanent magnets, and because of this, Sm2Fe17Nx rare-earth permanent magnet materials have become one of the hot spots in the field of permanent magnet materials research today. This is why Sm2Fe17Nx rare-earth permanent magnet materials have become one of the hot spots in the research of permanent magnet materials.
Since Coey et al. discovered the Sm2Fe17Nx series of rare-earth permanent magnets, there was a rapid boom in the research of Sm2Fe17Nx series of permanent magnets around the world, and hundreds of laboratories around the world were devoted to this research at that time. However, the subsequent series of tests proved that this kind of permanent magnet material was not successful on the road of industrialization, and there was a situation that the research was sometimes cold and sometimes hot.
In recent years, with the rapid development of miniaturization and light weight of automobile industry and electronic appliances, people put forward higher requirements for permanent magnets in terms of environmental use temperature and magnetic properties. The Sm2Fe17Nx permanent magnetic materials have also received a new wave of research and development.

Progress of industrialization of foreign Sm-Fe-N rare-earth permanent magnetic materials

At present, Europe and Japan are world leaders in the production and research of Sm2Fe17Nx system rare earth permanent magnetic materials.
Under the support of “Rare Earth Permanent Magnet Development and Research Program”, European scientists in the field of magnetic materials, represented by Professor Coey, have done a lot of research work in the basic fields of Sm2Fe17Nx microstructure, magnetic formation mechanism, nitriding mechanism, temperature characteristics, etc. They have made indelible contributions to the birth and development of Sm2Fe17Nx series of rare earth permanent magnetic materials, but unfortunately, they have made a lot of efforts in the industrialization of Sm2Fe17Nx series of rare earth permanent magnetic materials. But unfortunately, they failed to make a breakthrough in the industrialization of Sm2Fe17Nx series rare-earth permanent magnetic materials.
Although Japan started later than Europe in the research of Sm2Fe17Nx series rare earth permanent magnetic materials, there are many companies and units with strong technical force in the field of magnetic materials, such as Hiroshima University, Nichia Chemical Industries, Hitachi Metals, Toshiba, Sumitomo Metal Mining, Daido Electronics, TDK, etc. in Japan have been conducting research on Sm2Fe17Nx series permanent magnetic materials. In 2000, Niyachem Industries, Inc. of Japan considered its recycling from the beginning and used the existing ferrite production equipment to produce Sm2Fe17Nx rare earth permanent magnet powder and its bonded magnets by reduction diffusion method. The maximum magnetic energy product (BH)m has reached more than 103kJ/m3, and the thermal stability and corrosion resistance is good and has been industrialized, and now it is further improving its magnetic properties and striving for its maximum magnetic energy product (BH)max to reach 119kJ/m3. Toshiba uses the method of melt fast quenching to prepare amorphous strip of (Sm0.7Zr0.3)(Fe0.8Co0.2)9B0.1, and after crystallization and heat treatment, microfine and homogeneous grains can be obtained, and after crushing and nitriding, (Sm0.7Zr0.3)(Fe0.8Co0.2)9B0.1Nx magnetic powder can be obtained, and the company has already mass-produced (BH)max of 123kJ/m3 Sm In autumn 2002, Daido Electronics purchased Toshiba’s fast-quenched magnetic powder in bulk and prepared it into isotropic bonded magnets, and in 2001, the production scale of 70 tons of isotropic Sm2Fe17Nx magnets was established. Hitachi has studied the effect of various additive elements on improving the performance of Sm-Fe-N system compounds, and studied in detail the effect of refining the grains by hydrogen treatment, and found that by adding trace amounts of Ti and B in combination and controlling the hydrogen reaction conditions, the formation of α-Fe can be effectively suppressed and a uniform and fine structure can be obtained. The reliable Sm-Fe-Ti-B-N magnetic powder with excellent and stable magnetic properties can be obtained after crushing into particles below 75 μm and then nitriding in nitrogen atmosphere for several hours.
Ltd. has also made some progress in the development of duplex nanocrystalline composite permanent magnet materials. The Sm2Fe17Nx/α-Fe isotropic composite magnet recently introduced by the company is a nanobimetallic magnet, whose magnetic properties (remanence, coercivity, maximum magnetic energy product) can reach: Br=0.99T, HcJ=656 kA/m, (BH)max=140 kJ/m3. the magnetic powder can reproduce (BH)max up to 96 kJ/m3, and the (BH)max of the bonded magnet made with this magnet powder (BH) max of bonded magnets made with this magnet powder can reach 70 kJ/m3.
In the research of alloy nitriding, foreign scholars have taken various advanced methods to explore actively. Korean scholars used Sm2Fe17 alloy as a target from the cathodic vacuum sputtering method to study the effect of nitrogen pressure, flow rate, heat treatment temperature, etc. on the microstructure and magnetic properties of the alloy after nitriding, and found that the coercivity of Sm2Fe17Nx reached l000Oe and the saturation magnetization strength reached 6500G when the heat treatment temperature was 530℃ and the nitrogen rate was 20%; Brazilian scientists studied the effect of nitrogen pressure, flow rate and heat treatment temperature on the microstructure and magnetic properties of the alloy after nitriding. The Brazilian scientists plasma nitriding in the flowing N2/H2 compound gas atmosphere, increasing the playing concentration in the compound gas can reduce the amount of nitrogen-rich phase Sm2Fe17N8 and Sm2Fe17N11, and increase the content of Sm2Fe17N3 and α-Fe, and the content of Sm2Fe17N3 phase reaches the maximum when the H2 content is 70%.

Progress of industrialization of domestic Sm-Fe-N rare-earth permanent magnetic materials

While Sm2Fe17Nx rare-earth permanent magnetic materials were industrialized in Japan, many domestic universities and research institutes with foresight and strong R&D strength, such as Peking University, University of Science and Technology Beijing, Hebei University of Technology, Sichuan University, General Research Institute of Iron and Steel, Institute of Physics, Chinese Academy of Sciences, Institute of Metals, Chinese Academy of Sciences, Zhejiang University, etc., have also started to conduct research in this area.
Zhou Shouzeng et al. of University of Science and Technology Beijing obtained isotropic Sm2Fe17Nx permanent magnetic powder with high coercivity by HDDR method, and studied the relationship between the process of this method and the permanent magnetic properties of Sm2Fe17Nx. It was found that the original powder particle size, hydrogen treatment temperature and time, and nitriding temperature and time had a large effect on the properties of Sm2Fe17Nx magnetic powder, and when the original particle size was larger than 10 μm, Br and Hcj decreased sharply, the hydrogen extraction time was less than 1 hour, the alloy recombination was incomplete, and the coercivity of magnetic powder was low; they also studied the effect of single addition of trace alloying elements such as Cr, Ga and Co on the alloy properties, and found that Cr and Ga could significantly improve the coercivity of magnetic powder, while Co reduced HCj.
In addition, Hu Guohui of Beihang University used Sm2O3 and Ca, Fe as raw materials and redoxed at 1100°C for 4-7 h, then water washed to obtain single-phase Sm2Fe17 compounds, and prepared Sm-Fe-N alloy powder by nitriding treatment with NH3+H2 gas mixture, and studied the effect of crushing and ball milling process on magnetic properties, and pointed out that the damage to grains and α Fe generation are the main factors to reduce the coercivity, but this aspect of the process needs further study.
In addition, Qiu et al. from Beihang University prepared Sm-Fe alloy powders using low-temperature liquid nitrogen ball milling technology, and the tissue evolution of Sm2Fe17 alloy powders during liquid nitrogen ball milling was investigated by XRD, HRTEM and inert gas pulse-IR-thermal conduction. The results show that liquid nitrogen ball milling accelerates the refinement of Sm-F alloy powders. With the increase of ball milling time, the Sm2Fe17 phase transforms to amorphous state, and the reduction of ball milling speed can delay the formation of amorphous process. The liquid nitrogen ball milling process can realize simultaneous nitriding and shorten the preparation process of Sm-Fe-N magnetic materials, which is expected to provide a new way to prepare Sm2Fe17Nx nanocrystalline composite permanent magnetic materials with high magnetic properties.
Professor ChuNxiang Cui et al. of Hebei University of Technology also used the method of HDDR and systematically studied the effects of trace added elements Zr, Nb and Ti on the process and the magnetic properties of the alloy, pointing out that Nb, Zr and Ti can effectively reduce the content of α-Fe in the alloy, in which the cast state organization is basically free of α-Fe when the Zr content is 1%at, and a well-organized master alloy is obtained; subsequently, using the conventional nitriding It was found that for particles with a diameter of 20 μm, nitriding at 500°C for 6 h was the best, and the decomposition of Sm2Fe17N3 alloy would occur at too high a temperature or too long a time.
The Shenyang Institute of Metals Research, Chinese Academy of Sciences has prepared Sm2Fe17Nx rare earth permanent magnet powder with magnetic properties up to Br=0.81T, HcJ=1670kA/m, (BH)max=103.5kJ/m3 when a comparative and systematic study was conducted on the change of composition and structure of HDDR process and the influence of alloying element Ti on the structure and magnet performance. In general, the research on Sm2Fe17Nx rare-earth permanent magnetic materials in China is mainly focused on the preparation of Sm2Fe17Nx magnetic powder by HDDR method, traditional powder metallurgy method and reduction-diffusion method combined with conventional nitriding process and the addition of trace amount of mono-alloy elements, etc. The research and development of Sm2Fe17Nx magnetic powder is only at the laboratory level, and there is a big gap compared with foreign countries, especially in the industrial development. There is still a long way to go in terms of industrialization and development.

Sm-Fe-N rare-earth permanent magnet materials industrialization main sticking points

Although the research of Sm2Fe17Nx permanent magnet material has made great progress and there is still great potential to be explored compared with the third generation permanent magnet Nd-Fe-B, so far, Sm2Fe17Nx permanent magnet material has not really become the fourth generation permanent magnet material with practical significance. In today’s permanent magnet field, Nd-Fe-B still dominates the world, which indicates that there are still some problems in the research and development of Sm2Fe17Nx permanent magnet materials, mainly in the following aspects.

  • (1) Because Sm is easy to volatilize at high temperature, the solid melting zone of Sm2Fe17 is too narrow, and it is difficult to obtain Sm2Fe17 alloy close to the positive component during the melting of Sm-Fe master alloy; if the Sm content is too small, the obtained alloy contains a lot of α-Fe, and even Sm2Fe17 phase cannot be formed; if the Sm content is too much, the alloy will form a lot of shirt-rich phase, and the formation of these phases will reduce the magnet performance. performance.
  • (2) It is difficult to obtain uniform composition and phase composition of single Sm2Fe17 phase alloy. Sm2Fe17 phase alloy is generated by the inclusion of crystal reaction, and there are inevitably more or less α-Fe, rich shirt and other heterogeneous phases in the solidification process. This process is both time-consuming and energy-consuming, leading to an increase in the cost of the entire production process, resulting in the industrial mass production of Sm2Fe17Nx magnetic materials is hampered.
  • (3) The problem of high temperature decomposition of Sm2Fe17Nx has not been solved. Sm2Fe17Nx is a sub-stable phase, which will decompose when the temperature is higher than 600℃, and it is irreversible, i.e., it cannot be restored to Sm2Fe17Nx even if the temperature is lowered. The magnetic properties of bonded magnets are generally lower than those of sintered magnets, so how to make a breakthrough in the process and prepare Sm2Fe17Nx bulk magnets that can work at high temperatures is the key to give full play to the internal magnetic properties of Sm2Fe17Nx permanent magnets and whether they can compete with Nd-Fe-B magnets.
  • (4) The nitriding reaction kinetics of Sm2Fe17 alloy is very poor. Sm2Fe17 nitriding is different from ordinary steel nitriding, which requires all Sm2Fe17 grains to be fully nitrided, otherwise the unnitrided Sm2Fe17 will act as a soft magnetic phase and seriously affect the permanent magnetic properties of Sm2Fe17Nx. In addition, according to the theory of micromagnetism, high endowment coercivity must be obtained when the grain size is close to the theoretical single-domain particle size (about 0.3 μm). Although such fine powder can be obtained by techniques such as airflow milling, the powder is too fine and the increase of oxygen content will deteriorate the magnetic powder performance.
  • (5) The role of alternative elements has not yet formed a unified understanding. Since the invention of Sm-Fe-N to the end, people in the study of ⅣB, ⅤB, ⅥB, ⅦB, ⅧB elements such as Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Mn, Co, Ni and ⅠB group of Cu and Pr, Ce, Gd and other alloying elements added to the SmFe alloy structure and the impact of performance, in some aspects have been widely recognized, but there are still There are still differences in many aspects, especially the influence of SmFeN compounds obtained by different preparation methods with the addition of alloying elements, and there is no unified conclusion on the structure and properties of SmFeN compounds, which needs further study.
  • (6) The magnet properties are unstable, poorly reproducible, and there is a large gap between the magnetic properties and the theoretical values. The preparation of Sm2Fe17Nx bonded magnets requires a series of strict and complicated procedures, and the repeatability of Sm2Fe17Nx is poor because the magnetic properties of Sm2Fe17Nx are extremely sensitive to the process parameters; in addition, the magnetic properties of Sm2Fe17Nx magnetic powders produced are much lower than the theoretical values due to the limitations of the preparation means and theoretical studies.

The development trend of industrialization of high-performance Sm-Fe-N rare-earth permanent magnetic materials

Sm2Fe17Nx endogenous magnetic properties are almost comparable to the third generation permanent magnet Nd-Fe-B, and has the significant advantages of high Curie temperature (Tc=470℃) and good temperature stability, which is fully equipped to become a new type of permanent magnetic material. However, the biggest drawback of Sm2Fe17Nx is that the compound belongs to the sub-stable phase, which completely decomposes into α-Fe and SmN when the temperature is higher than 650°C. Therefore, it cannot prepare dense high-performance sintered magnets and can only be used to make bonded magnets. If the (BH)max of sintered magnets made of the same magnetic powder is taken as 100%, then the bonded magnets made by compression molding process are 60%, while the injection molded magnets are only 35%. Therefore, the ability to prepare large sintered Sm2Fe17Nx magnets is the key to their industrialization.
Although Sm2Fe17Nx is far from commercialization, it has developed extremely rapidly in just a few years and is likely to become another new generation of permanent magnets after Nd-Fe-B. Especially due to the rapid development of Nd-Fe-B magnets and the surge of demand in the market, neodymium resources are facing depletion, so the development of new permanent magnets that can replace Nd-Fe-B is imperative, and in order to meet the growing need for high temperature working environment, the practical development of Sm2Fe17Nx is on the agenda.
From the viewpoint of commercial production, the research on the preparation method and process of Sm2Fe17Nx permanent magnets is seriously hindered by the fact that Sm2Fe17Nx is a thermodynamically sub-stable phase. Once a preparation process with suitable conditions can be developed, then there is a possibility to fully develop its endogenous magnetic properties, thus greatly accelerating its commercialization process. To accelerate the commercialization of Sm2Fe17Nx, the future research priorities and requirements are.
(1) Optimize the existing magnetic powder processing process
Permanent magnets are various shapes of permanent magnetic components in practical applications. They are characterized by large batches, high dimensional accuracy and require fully automatic molding. Therefore, solving the magnetic powder process problem is the basic condition for its practical use.
At present, the preparation methods for preparing Sm2Fe17Nx magnetic powder and its nanocomposite magnetic powder mainly include: mechanical alloying method, powder crushing method, melt fast quenching method, hydrogenation disproportionation (HDDR) method and so on.
Among them, the mechanical alloying method can produce isotropic Sm2Fe17Nx magnetic powders with high coercivity, which does not require large equipment and is a simple method for manufacturing magnetic powders. However, due to the long time of ball milling, the mechanical alloying method is very easy to cause powder oxidation, which reduces the magnetic properties of magnetic powders, and coupled with the disadvantages of long cycle time and high energy consumption, it limits its popular application in production.
The Sm2Fe17Nx compound powder prepared by the melt fast quenching method has uniform organization and composition, fine grain size and simple process, which is conducive to industrial mass production, but due to the harsh requirements of this process conditions, there are certain difficulties in mass production, and how to effectively control its process parameters need to be further explored.
The reduction-diffusion method uses reducing agents to reduce rare-earth oxides to rare-earth metals, and then obtains rare-earth permanent magnetic powders directly through the interdiffusion of rare-earth metals and transition group metals. The advantage of this method to prepare Sm2Fe17Nx compound powder is the low cost of raw materials, and the disadvantage is that it is difficult to implement. At present, Japan has achieved great success and industrialization with this method, but the progress is slow in China. The HDDR process is a new type of magnetic powder preparation process with good application and development prospects due to its simple equipment, good homogeneity, low oxygen content, high yield, and the ability to prepare not only isotropic but also anisotropic magnets. However, the process of Sm2Fe17Nx preparation by HDDR method needs to be further investigated because of the many 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, as well as the grain refinement mechanism are not fully understood.
Although the permanent magnetic properties of Sm2Fe17Nx prepared in the laboratory have been greatly improved, poor repeatability and unstable performance in mass production are the key cruxes for the commercialization of Sm2Fe17Nx at present. The industrialization of Sm2Fe17Nx should be based on the existing process and develop the best process route that can produce excellent performance, moderate price and good repeatability.
(2) Efforts to develop new binders and molding methods
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. Initially, people used organic substances such as nylon and epoxy resin as binder. Since these binders can only be used below 200°C, they cannot give full play to the advantages of good high temperature performance of Sm2Fe17Nx, so how to make a breakthrough in the process and whether to develop a new type of binder is the key for Sm2Fe17Nx magnets to compete with Nd-Fe-B magnets.
In recent years, some low melting point metals have started to receive wide attention, and people use low melting point metals such as Zn and Sn as binders, but since low melting point metals such as Zn as binders will reduce the saturation magnetization strength, which in turn leads to a lower (BH)max. It can be seen that it is crucial to find a good binder in order to give full play to the performance of Sm2Fe17Nx.
Author: Li Dayong, Duan huanqiang

Source: China Permanent Magnet Manufacturer – www.rizinia.com
References

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