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Chemical synthesis and performance optimization of rare earth permanent magnets and their coupling magnets

Rare earth permanent magnet material is an indispensable part of today’s human society. It has been playing an important role in production and life since it came out in the 1960s. With the progress of science and technology and the improvement of living standards, people put forward higher expectations for industrial products and functional devices, so there are higher requirements for the performance of rare earth permanent magnet materials, including energy density In recent years, chemical method has become a common method for the synthesis of rare earth permanent magnet materials. Through the “top-down” process, the product morphology and size can be precisely controlled, which provides a new idea for the preparation of high-performance magnets. Chemical synthesis also has a unique advantage in the aspect of exchange coupling magnets, which can be precisely controlled The size of soft magnetic phase in two-phase coupling magnets can fully realize the exchange coupling effect, which has a broad prospect in improving the energy density of magnets. Starting from chemical synthesis, this paper introduces the preparation of different rare earth permanent magnet materials in recent years, and summarizes the progress and challenges in chemical preparation of rare earth permanent magnet based coupling magnets, which is of great significance to the future chemical synthesis of rare earth permanent magnets The development of magnetic materials is analyzed and prospected.

Preface

Magnetic materials have become an essential component in the production and life of modern human society, and are widely used in traditional manufacturing, national defense, power system, energy system and other aspects [1,2,3,4]. In recent years, with the progress of nano technology, magnetic nanomaterials have gradually entered people’s vision and played an indispensable role in biomedical, industrial catalysis, environmental purification and other fields [5,6,7,8].
Rare earth permanent magnet material is one of the most common hard magnetic materials. Since the advent of SmCo5 magnet in the last century, rare earth permanent magnet material has experienced the development of SmCo5, Sm2Co17 and Nd2Fe14B in three generations, compared with ferrite and other permanent magnetic materials, they have made great progress in comprehensive magnetic properties, especially magnetic energy product [9,10,11,12]. As shown in figure 1, the magnetic properties of permanent magnetic materials used by people in the 20th century are poor, and the maximum magnetic energy product is only less than 5mgoe, which limits the further application in aviation engines and other fields. Strnat discovered SmCo5 magnet in 1960s, and its magnetic crystal anisotropy length is as high as 400t. After further optimization, the coercive force at room temperature can reach more than 20mgoe [9]. In 1970s, people found a new type of rare earth permanent magnet material in SmCo alloy, namely Sm2Co17, which has higher Curie temperature and magnetic properties than SmCo5 and is considered as the second generation of rare earth permanent magnet material, the magnetic energy product of Sm2Co17 has been significantly improved after optimization processes such as domain wall nails binding by element doping [10,13,14]. In 1980s, japanese scientist Sagawa and others discovered a ternary permanent magnet material with Nd2Fe14B structure as matrix phase through a series of studies, Its room temperature magnetic energy product is as high as more than 35mgoe, marking the birth of the third generation of rare earth permanent magnet materials [11,12]. Nd2Fe14B has excellent magnetic properties at room temperature, occupying most of the market of permanent magnet materials and has the title of “Magneto. Since 90’s, in order to further improve the energy density of rare earth permanent magnet materials, researchers have begun to devote themselves to the study of exchange coupling magnets represented by Nd2Fe14B/α-fe [15,16,17,18,19,20], at present, the magnetic energy product of coupling magnet thin film prepared in laboratory can reach about 61MGOe, which lays a foundation for the research of coupling magnet [21]. In addition, the new rare earth-transition metal system and rare earth-iron-nitrogen permanent magnet materials have also attracted the attention of some researchers, and are expected to become a new generation of rare earth permanent magnet materials [22,23].
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Fig.1 The revolution of the magnetic energy product of permanent magnetic materials changes at room temperature[24]
Traditional rare earth permanent magnet is generally prepared by powder metallurgy, and magnetic powder is prepared through alloy smelting, crushing and other processes. After mixing the powder evenly, it is pressed, oriented, sintering, heat treatment and other processes to prepare bulk magnets, in addition, hot pressing/thermal deformation has also become a common method for preparing rare earth permanent magnet. These traditional physical methods play an irreplaceable role in the preparation of rare earth permanent magnet materials. However, due to the complexity of crushing and other processes, it is difficult to achieve accurate control, resulting in uneven performance of magnetic powder, it will also have certain influence on the performance of the final magnet. The “bottom-up” chemical method realizes more accurate control of product morphology and size, and also has the advantages of short cycle and low energy consumption. It is gradually applied to magnetic preparation of nanostructured materials [4,25,26]. Therefore, researchers have tried to prepare magnetic materials such as SmCo5 and Nd2Fe14B by chemical method, and have made preliminary progress [27,28,29,30,31,32].
After decades of research, people have proposed a variety of chemical methods to prepare rare earth permanent magnet nanomaterials, such as sol-gel method, surfactant-assisted high-energy ball milling method, microwave-assisted method, etc. Although there are many methods, the general preparation process is divided into two basic stages, the preparation of precursor and the reduction stage of precursor: firstly, different elements (such as Nd/Fe/B) are fully mixed on the molecular scale through ball milling and other processes, then, the dried precursor and strong reducing agent (such as metal Ca particles) are mixed and reduced at high temperature to realize the co-reduction of various high-priced ions so as to form the desired phase. In the preparation process, the mixing of the precursor is the most important stage, because the reduction potential of the high-priced rare earth elements is much higher than that of the 3d group elements, if the full mixing of the molecular scale is not realized, the elements of the 3d family will be reduced separately, and the non-magnetic substances formed by the rare earth elements will be removed in the magnetic separation, and the final product will be Fe(Co) single substance is the main substance. In the annealing process of the second step, special attention should be paid to the factor that is the energy of reduction annealing: the annealing energy is too low (the annealing temperature is low or the annealing time is short), and it is difficult to fully reduce; too high annealing energy (low annealing temperature or short annealing time) and serious volatilization of rare earth elements will also affect the performance of the final product.
In order to further improve the energy density of rare earth permanent magnet materials, researchers put forward the concept of exchange coupling magnet in 1990 s, by integrating the advantages of higher saturation magnetization of soft magnetic materials and higher magnetic crystal anisotropic field of hard magnetic materials, the maximum magnetic energy product can be further improved [15,16,33]. However, in order to fully realize the exchange coupling effect, the size of soft magnetic phase and hard magnetic phase has higher requirements, which brings challenges to the preparation of high-performance coupling magnets. In rare earth permanent magnet-based coupling nanomaterials, the nuclear field of the opposite magnetization domain of soft magnetic decreases with the increase of the size of nanocrystalline, and its growth is controlled by the dual-phase exchange coupling effect of soft and hard magnetic, the short-range interaction between hard magnetic phase nanocrystalline particles will determine the non-uniformity of magnetization, which is beneficial to the nucleation of anti-magnetization domains [34]. Therefore, the hysteresis loop of the two-phase exchange coupling magnet can be regarded as the superposition of the hysteresis loop of the soft magnetic phase and the hard magnetic phase, as shown in figure 2 [19,34]. Generally speaking, exchange coupling effect can be fully realized only when the size of soft magnetic phase is less than twice the size of domain wall of hard magnetic phase. The accuracy of size control in the “bottom-up” synthesis process of chemical method makes this requirement possible.
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Fig.2 Hysteresis loop of (a) hard magnetic materials, (B) soft magnetic materials and (c) exchange-coupled magnetic materials[19]
In this paper, we briefly introduce different rare earth permanent magnet materials, and summarize the common chemical methods of synthesizing rare earth permanent magnet nanomaterials in recent years. Further, we mainly focus on chemical synthesis, introduce the typical preparation methods of exchange coupling rare earth permanent magnet nanomaterials, and analyze the development and challenges of chemical synthesis in the preparation process of exchange coupling magnets. We hope this paper can provide guidance for the synthesis of rare earth permanent magnet nanomaterials and the design and preparation of exchange coupling magnets based on this, and make some contributions to the industry of rare earth permanent magnet materials in our country.

Nd2Fe14B-based coupling magnet

Chemical synthesis of Nd2Fe14B nanomaterials

Nd2Fe14B crystal has a tetragonal structure with lattice constants a = B = 0.882nm,c = 1.224nm,α = β = γ = 90 °. Each Nd2Fe14B unit cell consists of four Nd2Fe14B molecules, including 8 Nd atoms, 64 Fe atoms and 4B atoms. There is a strong atom exchange between Fe-Fe and Fe-Nd in the crystal, which makes Nd2Fe14B crystal have strong intrinsic magnetism, and the c axis is its easy magnetization axis [24,35]. In fact, all rare earth elements can form 2:14:1 phase magnets under certain conditions, but due to the differences in the structure of atoms of different elements, different 2:14: there are great differences in the internal performance of phase 1 [36]. Nd2Fe14B structure has the highest magnetic polarization intensity 1.60T among all 2:14:1 phases, and the relatively high magnetic crystal anisotropy field 73kOe. The theoretical maximum magnetic energy product can reach about 60mgoe at room temperature, it has excellent comprehensive performance. In the preparation process of Nd2Fe14B, heavy rare earth elements such as Dy are usually added to improve the magnetic crystal anisotropic field, thus further improving the comprehensive performance of the magnet. In the chemical synthesis of Nd2 Fe 14B, the precursor is usually prepared by the oxide or salt of the three elements Nd, Fe and B, and the precursor and reductant Ca particles are mixed and then annealed at high temperature, under appropriate annealing conditions, single-phase 2 can be obtained: 14:1 phase.
Hou and other researchers recently invented a new surfactant assisted high-energy ball milling method for the preparation of single-phase Nd2Fe14B nanostructure [32]. In this method, the oxide of the three elements Nd, Fe and B is fully and evenly mixed by high-energy ball milling under the action of surfactant according to a certain proportion, thus realizing the preparation of precursor, the precursor is mixed with metal Ca particles under the conditions of KCl as molten salt and pressing respectively, reduction annealing in the standard gas of 900℃ ~ 1000℃ can obtain a relatively pure 2:14:1 structure. However, due to the different combination forms of precursor and metal Ca particles, the dynamic and thermodynamic behaviors during annealing are also different, in the pressed sintered sample, Nd element diffuses to the grain boundary under the action of compressive stress, thus forming a non-magnetic Nd-rich grain boundary phase (as shown in Figure 3(a)~ (B)), plays a role in magnetic isolation. The properties of the sample have been significantly improved under the action of grain boundary phase. As shown in figure 3(d), the coercive force of the sample with KCl as molten salt is only less than 1kOe without adding heavy rare earth elements, however, the coercive force of the pressed and sintered samples reached about 3.5Koe. On this basis, the author introduces two-dimensional graphene oxide to construct a new grain boundary phase, forming a core @ triple-shell structure as shown in figure 3(c), the coercive force of Nd2Fe14B nanostructure is further increased to above 8kOe. Research shows that, The construction of grain boundary phase significantly enhances the exchange coupling between Nd2Fe14B magnetic structures, thus improving the coercive force of the nanostructure.
Sol-gel method is one of the first methods that researchers try to use in the chemical synthesis of Nd2Fe14B [28,37,38]. In 2010, Ramanujan and his collaborators had already prepared sol as precursor through the reaction of NdCl3 · 6H2O,FeCl3 · 6H2O,H3BO3 in citric acid and ethylene glycol, nd2Fe14B nanoparticles [28] were obtained by the reaction of precursor and CaH2 at high temperature. In this process, citric acid and ethylene glycol are used as chelating agents and gelling agents respectively to ensure sufficient mixing in the precursor. Further analysis shows that the annealing process can be divided into three stages. Firstly, Fe 203 and b203 are reduced to Fe and B respectively at 300℃, and when the temperature rises to 620℃, nd203 and Nd Fe O3 are decomposed to form NdH2 and Fe. When the temperature rises to 692 ℃, NdH2 and Fe react with B to form Nd2 Fe 14B structure of hard magnetic phase. Similarly, Rahimi et al. also prepared Nd2Fe14B particles of hard magnetic phase based on metal nitrate and boronic acid by Pechini sol-gel method, on this basis, the influence of chelating agent and gelling agent on the morphology and properties of the product was investigated by changing the content of chelating agent and gelling agent [37]. Research shows that with the increase of gelling agent content in the preparation process, the particle size of the obtained product increases while the coercive force decreases, in addition, it will also lead to the increase of flake structure.
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Fig.3 Nd2Fe14B nanostructures prepared on the surfactant-assistant high-energy ball mill method, the HRTEM images of (a) S1, (B) S2 and (c) S3-1, (d) the demagnetization curves and (e) Henkel plots of the Nd2Fe14B nanostructures. S1 S2 and S3 represent the nanostructures with KCl as melt salt, as pellet and as pellet with the graphene oxide, respectively. S0 represents the sample prepared by powder metallurgy method[32]
In addition, researchers also proposed methods such as microwave-assisted method and co-precipitation method to prepare Nd2Fe14B nanostructures with different morphology and properties, it provides unit materials and new ideas for further preparation of high-performance sintered magnets [39].

Synthesis of Nd2Fe14B-based coupling magnet

Chemical method has unique advantages in precise control of product size and morphology, so it can be an effective means to prepare soft and hard magnetic exchange coupling magnets. In the preparation process of precursor, introducing soft magnetic phase nanoparticles (such as Fe/Fe3B) is a common method, the soft magnetic phase particles dispersed in the precursor avoid the change of the aggregation phase during the high temperature reduction process, and finally evenly distribute in the formed hard magnetic phase, realizing the full coupling of the soft magnetic phase and the hard magnetic phase.
Hou et al. proposed a high-temperature organic liquid phase method for the preparation of Nd2Fe14B-based nanostructure. Through the reaction of Nd and Fe at organic slat high temperature in oil amine, after degassing and purification, borane triethylamine is added as B source, borane triethylamine is decomposed at high temperature to provide element B, mixed with Nd and Fe, NdFeBO precursor [40] can be obtained. In this process, a proper amount of Fe(CO)5 can be injected as the source of soft magnetic phase. Fe(CO)5 can be decomposed at high temperature to obtain α-Fe nanoparticles, after washing and drying by ethanol, the black precursor in the dispersion distribution of α-Fe nanoparticles can be obtained (Fig. 4(d)). When the content of α-Fe is appropriate, Nd2Fe14B of hard magnetic phase and α-Fe of soft magnetic phase are fully coupled, and the hysteresis loop shows relatively smooth characteristics, it can also be seen from Henkel curve that there is strong exchange coupling inside the magnet, as shown in figure 4 (e)~(f).
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Fig.4 (a) Schematic of the Nd2Fe14B/α-Fe exchange-coupled nanocomposites, (B )&(c) TEM images of the NdFeBO precursor, (d) HRTEM image, (e) hysteresis loops and (f) Henkel plot of the as-synthesized Nd2Fe14B/α-Fe exchange-coupled nanocomposites[40]
Based on this, researchers have prepared Nd2Fe14B/Fe3B coupling magnet by high temperature organic liquid phase method, and its basic process is similar to that of Nd2Fe14B/α-Fe, when introducing soft magnetic dissociator, Fe3B synthesized in advance is directly injected into precursor solution, and Nd2Fe14B/Fe3B composite magnet can be obtained after high temperature reduction of the dried precursor, its interior shows a strong exchange coupling effect [41].
Preparation of Nd2Fe14B-based composite films by low-voltage deposition, magnetron sputtering and other methods is also one of the effective ways to fully realize coupling effect. Hono et al. prepared Ti/Nd Fe B- Nd/Ta/ Fe/Ti thin films under low pressure (, by adjusting the thickness of Ta layer and Fe layer, the regulation of exchange coupling and static magnetic effect is realized [21]. When the thickness of Ta layer is 2nm, it can be seen that there is a strong exchange coupling between soft magnetic phase and hard magnetic phase, and the hysteresis loop is relatively smooth. When the thickness of Ta layer is further increased, the exchange coupling effect of the composite film is weakened while the static magnetic effect is obviously increased. Further research shows that in order to regulate the relative strength of exchange coupling and static magnetic interaction, it is necessary to insert a suitable separation layer between hard magnetic phase and soft magnetic phase, therefore, high coercive force can be maintained in the anisotropic Nd2 Fe 14B/Fe composite structure, thus ensuring excellent maximum magnetic energy product.

Multi-Main phase Ce rare earth doped Nd2Fe14B magnet

Nd2Fe14B magnet is the most widely used and consumed permanent magnet material in the world. The excessive use of indispensable Nd element and Pr/Dy and other heavy rare earth elements is one of the serious problems faced by modern rare earth industry. China’s current rare earth permanent magnet industry relies too much on the above elements, while the application of high abundance rare earth elements such as La/Ce is relatively small, resulting in the unbalanced use of rare earth resources, the separation process also causes environmental pollution. Although Ce and other elements can form 2:14:1 phase, due to the influence of its own atomic structure, Ce2Fe14B has poor performance, and its magnetic crystal anisotropy field is only 26kOe, limits direct applications. Some researchers have reduced the amount of Nd element by directly doping Ce element into Nd2Fe14B magnet, but the dilution effect of Ce element is obvious, and the general doping amount is below 10% [42].
In recent years, researchers have invented a preparation technology of Multi-main-phase (mmps) with high abundance of rare earth, which greatly weakens the magnetic dilution effect of Ce element on Nd2Fe14B magnet, it provides a new idea for the application of Ce-doped magnet [43,44]. At present, the most common method for preparing multi-phase magnets in industry is double alloy method, specifically, two alloy powders with different nominal contents (Nd-based powder and Ce-based powder respectively) are prepared first, and then the two different magnetic powders are mechanically mixed, followed by orientation, the pressing, sintering and heat treatment processes are similar to the preparation of single-phase Nd2Fe14B magnets [45,46,47,48]. Due to the limited diffusion effect of rare earth elements during sintering and heat treatment, Nd enrichment region and Ce enrichment region are generated in the finally obtained Ce-doped Nd2Fe14B magnet, which forms more than one main phase, therefore, it is called a multi-phase magnet. Jin et al. prepared the multi-main phase Nd2Fe14B magnet produced by Ce-La by double alloy method. Compared with the single main phase Nd2Fe14B magnet formed by direct doping of Ce-La, the uneven distribution of rare earth elements is its main feature (as shown in figure 5(a)~ (B)), The performance of multi-phase magnet is obviously improved under the condition of the same doping amount [45]. As shown in figure 5 (c)~(d), the performance of Ce-La multi-phase magnet with 18 wt.% dopant is even higher than that of Ce-La single-phase magnet with 9 wt.% dopant.
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Fig.5 Morphology and element distribution of the (a) MMP and (B) SMP magnets, (c)&(d) magnetic properties of the MMP and SMP magnets[45]
Some researchers have realized the doping of Ce element in Nd2Fe14B nanostructure through chemical synthesis, and further analyzed the performance enhancement mechanism of multi-main phase magnets on the nanometer scale. Research shows that the non-uniformity of components in the multi-main phase magnet further enhances the exchange coupling between different main phases, thus significantly weakening the dilution effect of Ce element on the performance of the magnet.

SmCo-based coupling magnet

Chemical synthesis of SmCo-based nanomaterials

SmCo5 crystal is a CuCa5 cubic structure with lattice constants a = 0.4495 nm and c = 0.3978 nm. SmCo5 crystal molecules can be regarded as Sm atoms and Co atoms arranged in two layers in a certain way. Odd layers consist of Sm atoms and Co atoms, while even layers consist of Co atoms alone, form hexagonal structure [35]. This structure makes SmCo5 Crystal have high magnetic crystal anisotropic field, up to 400kOe, magnetic crystal anisotropic constant K1 = 1.5~1.9kJ/m3. Although Nd2Fe14B magnet occupies most of the Rare earth permanent magnet Market after its advent, SmCo5 magnet still has its irreplaceable role. SmCo5 magnet has a higher curie temperature, which can reach ℃. Therefore, compared with Nd2Fe14B magnet, it has better high temperature performance and is more suitable for high temperature occasions such as engines.
Hou and other researchers have prepared SmCoO precursor with core-shell structure through the reaction of Co nanoparticles and sm203 in 2007, smCo5 nanostructure was further obtained by reduction reaction of precursor and metal Ca at high temperature (as shown in Fig. 6)[27]. The coercive force of this nanostructure is as high as 26kOe at 100k, the coercive force at room temperature can also reach more than 10kOe, and the remanence can reach 40 ~ 50emu/g. It is worth noting that by changing the shell thickness of Co @ sm203 to adjust the ratio of Sm content and Co content in the precursor, Sm2Co17 structure can be obtained after annealing.
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Fig.6 TEM images of the (a) Co nanoparticles, (B) Co@Sm2O3 and (c) SmCo5, the hysteresis loops of SmCo5 at (d) 100 K and (e) room temperature[27]
Dong and other researchers conducted in-depth research on the coercive force of SmCo5 structure synthesized by chemistry in 2019, and raised this value to above 70 kOe [29]. In this process, the researchers used CaCl2 to replace CaO in the traditional method in the preparation process of the precursor, through amorphous phase Sm(OH)3 and crystalline phase Co(OH)3/Ca(OH) the co-precipitation reaction of 3 obtains the “reaction vessel” shown in Fig. 7(a). CaO acts as a blocking agent in the reduction reaction with metal Ca, on the one hand, it affects the element diffusion and alloying process in the formation process of SmCo5, and on the other hand, it also inhibits the growth of SmCo5 grains. It can be seen from figure 7 (B) ~(c) that the SmCo5 grains prepared by the above method are smaller and more uniform than the samples prepared by adding CaO directly, therefore, the performance will be enhanced. As shown in figure 7(d)~(e), after being oriented in a magnetic field of 0.8t, the coercive force of SmCo5 prepared by the above method can reach 72 kOe, and the remanence ratio reaches 0.96, compared with the samples directly added with CaO (coercive force 17 kOe, remanence ratio 1.8), it has obvious improvement.
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Fig.7 (a) Schematic of the synthesis of the precursor and subsequent reduction process, SEM images and hysteresis loops of the (B )&(d) experimental and (c)&(e) control samples[29]
Considering the lattice structure of SmCo5 crystal, it is also of interest for researchers to directly use chemical synthesis to prepare anisotropic SmCo5 nanostructure “from bottom to top. Sun et al. prepared the precursor through the self-assembly characteristics of Co nanoparticles and Sm(OH)3 nanorods, and obtained SmCo5 nanosheets by reducing the precursor, after magnetization orientation in epoxy resin, the coercive force of the sample is about 30 kOe, and the saturation magnetization reaches 66.1 emu/g[30].
Sm2Co17 is the second generation of rare earth permanent magnet material, which can be divided into two types according to different structures: one is Th2Ni17 type high temperature stable structure, which belongs to hexagonal crystal system, the other is Th2Zn17 type low temperature stable structure, it belongs to Diamond Square crystal system. Compared with the first generation of SmCo5 magnets, Sm2Co17 has higher curie temperature, up to ℃, and can maintain higher performance at 500℃, so it is especially suitable for high temperature environment [24,35]. However, the magnetic crystal anisotropic field is only about 100 kOe, which limits the further improvement of performance. Based on Sm2Co17, researchers prepared Sm(Co,Cu,Fe Cr)z alloy through alloying process. After subsequent optimization treatment, the magnetic nail binding effect was enhanced, the comprehensive performance of Sm2Co17 magnet is greatly improved [49,50,51].
In the chemical synthesis process of SmCo5 magnet, the element ratio of the final product can be effectively adjusted by changing the molar ratio of Sm element and Co element, so smco, Sm2Co7, sm2Co17 and other products [52]. Choa and other researchers use hydrated nitrate of Sm and Co elements as raw materials, stirring and heating under the action of glycine [31]. Nano-sized metal oxide powder is generated by self-combustion reaction of glycine-nitrate complex, which can be used as the precursor of Sm2Co17. Sm2Co17 nanoparticles can be obtained through subsequent reduction annealing reaction with CaH2 at high temperature.

Synthesis of SmCo-based coupling magnet

SmCo5 magnet has high coercive force, but its saturation magnetization is not high, thus limiting the further improvement of its saturation magnetization, the coupling magnet provides a new idea for improving the energy density of SmCo magnet. In 2007, Hou and other researchers mixed hydrophobic Fe3O4 nanoparticles with water-soluble SmCl3 and CoCl2, and then added tetrabutylammonium hydroxide to the mixed solution for precipitation process. After heating at 120℃, SmCo matrix with Fe 3O4 nanoparticles uniformly embedded in it can be obtained, which can be used as the precursor of SmCo5/Fe coupling magnet [53]. After mixing the precursor and metal Ca particles, high-temperature reduction annealing is carried out. SmCo5/Fe coupling magnets with different Fe contents can be obtained according to the ratio of Fe 3O4 nanoparticles and Sm/Co elements, 8(a). When x = 0.23, the coercive force of the magnet is the largest, which can reach 11.6kOe. Further analysis shows that there is a strong exchange coupling effect inside the sample (figure 8 (B)).
Similarly, Sun et al. also introduced Fe nanoparticles in the process of preparing the precursor to realize the preparation of SmCo5/Fe coupling magnet. The process is shown in figure 8(c) [54]. Researchers first prepared Fe nanoparticles coated with sio_2 to maintain the size of Fe morphology during the reduction process. The above core @ shell structure, flake Co(OH)2 and rod Sm(OH)3 the precursor is prepared after mixing, and SmCo5-Fe/sio_2 can be obtained through high-temperature reduction process. SmCo5-Fe coupling magnet is obtained after etching sio_2 (figure 8(d)). The properties of samples with different Fe contents are shown in figure 8(e). It can be seen that there is a strong exchange coupling between soft magnetic and hard magnetic phases.
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Fig.8 (a) HRTEM image of the as-synthesized SmCo5/Fe1, (B) magnetic properties of the as-synthesized SmCo5/Fex[53]. (c) Schematic of the synthesis process of the SmCo5/Fe, (d) HADDF-STEM image and (e) magnetic properties of the SmCo5-Fe exchange-coupled magnets[54]
Co can also exist as a soft magnetic phase in the coupling magnet. Yang and his cooperators use two-dimensional graphene oxide as the aid. Firstly, K3[Co(CN)6] and Sm (no3.) 3.6h2o was mixed to prepare Sm[Co(CN)6] 4H2O @ GO coated with single-layer graphene oxide as the precursor, as shown in fig. 9(a) [55]. After mixing with metal Ca, the single-domain SmCo5 @ Co coupling magnet can be obtained (figure 9(a)& (B)), whose size is about 200nm, the coercive force can reach 20.7kOe and the saturation magnetization is 82emu/g. When the ratio of K3[Co(CN)6] and Sm(NO3) 3.6h2o is changed, SmCo5 @ Sm2Co17 coupling magnets with different composition ratios can also be obtained, its properties show different characteristics with the change of components, as shown in Figure 9(d).
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Fig.9 (a) TEM image of the Sm[Co(CN)6]·4H2O@GO, TEM images of the (B) exterior part and (c) arbitrary interior part of the as-synthesized SmCo5@Co, (d) hysteresis loops of the as-synthesized exchange coupled magnets[55]
Some researchers first obtained single-phase Sm2Co17 magnet through high-temperature reduction, and successfully prepared one-dimensional Sm2Co17 by electrospinning. On this basis, a layer of FeCo alloy was deposited on the outside of Sm2Co17 nanoparticles and one-dimensional Sm2Co17 through chemical plating, thus preparing Sm2Co17 @ FeCo composite structure [31]. Further research shows that with the increase of chemical plating time, the saturation magnetization of the product increases while the coercive force gradually decreases. Under appropriate chemical plating conditions, the one-dimensional Sm2Co17 @ FeCo composite structure shows strong exchange coupling effect, and the maximum magnetic energy product increases from less than 8 MGOe to nearly 10 MGOe.

Synthesis of other rare earth permanent magnet materials

As a strategic element, Co greatly increases the cost of SmCo permanent magnet materials and limits their use to some extent. Fe is rich in resources and low in cost, so the alloy compounds of Sm and Fe have always been of interest to researchers. Sm-Fe alloy has many forms, but its magnetism is generally weak, and its performance needs to be further improved by other means. For example, Sm2Fe17 is a typical Sm-Fe alloy, it only shows weak ferromagnetism, but if N atom can enter the lattice to change its crystal structure and form a magnetic structure, the magnetic energy will be significantly improved. This rare earth-iron-nitrogen permanent magnet material represented by magnetic magnet has been paid more and more attention by researchers and is expected to become a new generation of permanent magnet material.
However, it is a metastable phase, which brings challenges to its synthesis process. The most common chemical method for synthesizing nano-structures is to prepare Sm2Fe17 by conventional reduction process, and then use substances containing n elements such as nh3. under special conditions (such as high temperature and high pressure) carry out nitridation treatment [56,57,58]. Okada and its cooperators firstly mixed the hydrated nitrate of Sm and Fe according to the ratio of 1:5.5, then included in the KOH solution to prepare suspension, and obtained the precursor powder through centrifugal, drying, grinding and other processes. The precursor is firstly reduced in H2 for 4 hours, and the temperature is controlled at 973k. The precursor for initial reduction is mixed with metal Ca here, and fully reduced between 1173k and 1223k. The sample after full reduction is nitrided after 1 hour in NH3-H2 atmosphere. The nitrided temperature is 693k, and then it is kept in H2 atmosphere for 693k for one hour to adjust the balance of n element, finally, after the annealing and heat preservation process in Ar atmosphere for 0.5 hours to remove the adsorbed H2, the matte particles with an average particle size of about 800 nm can be obtained [57].
Sun et al. prepared SmCo-O precursor through the reaction of Sm and Co acetylacetone salt, and further coated a layer of CaO on the outer layer of the precursor to reduce the grain growth during surface annealing. SmCo-O precursor with different Sm: Co content ratio was reduced and annealed after mixed metal Ca particles, and SmCo5 and Sm2Co17 nanoparticles with controllable grain size were obtained [58]. On this basis, the researchers changed the Co(acac)3 in the process of preparing Sm2Co17 nanoparticles into Fe(acac)3. After the same CaO coating, cleaning, the process of removing impurities and so on successfully reached the Sm Fe-O precursor with uniform size, and its morphology is shown in figure 10(a). Compared with the pure Sm2Fe17 nanostructure, the SmFe-O precursor obtained after reduction annealing shows weak hard magnetic properties at room temperature, and its coercive behavior is not obvious (fig. 10 (B)). The obtained Sm2Fe17 nanoparticles are nitrided at 600℃ for 6 hours under the action of melamine (C3H6N6), and the obtained Sm2Fe17 nanoparticles can be obtained through subsequent cleaning and magnetic separation processes, as shown in figure 10(c) as shown, the particle size is about 100nm. It can be seen that the coercive force of nano-particles after nitridation is above 15kOe (figure 10(d)), compared with Sm2Fe17, the magnetic properties of the sample will be further improved after the orientation process.
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Fig.10 (a) TEM image of the SmFe-O precursor, (B) hysteresis loop of the as-synthesized Sm2Fe17, (c) TEM image and (d) hysteresis loops of the Sm2Fe17N3 nanoparticle[58]

Conclusion and prospect

Starting from SmCo, Nd2Fe14B and other rare earth permanent magnet materials that have been widely used, this paper focuses on chemical synthesis and introduces the progress in recent years. The preparation of rare earth permanent magnet nanoparticles by chemical reduction is generally divided into two processes, the preparation stage of precursor and the reduction stage of precursor. Although particle agglomeration is also easy to occur in the chemical reduction process, the problem of equality is formed separately, but by controlling the morphology of the precursor such as oxide in the first stage, the precise control of the morphology and size of the final formed product can be realized. SmCo-based magnetic powder with high performance can be directly synthesized through chemical synthesis, which simplifies the preparation process and reduces energy consumption compared with powder metallurgy and other methods. For Nd2Fe14B, a co-reduction reaction of three elements is required, therefore, higher requirements are put forward for the uniformity of the precursor. On the other hand, the formation mechanism of the coercive force of Nd2Fe14B includes two aspects of nucleating nailing, therefore, grain boundary phase can be formed by specific means (such as pressing and sintering) in the reduction process to strengthen the magnetic pin binding effect so as to prepare high-performance magnetic powder, it provides unit materials and new ideas for the preparation of higher performance magnets; It has superior theoretical maximum magnetic energy product (above 60MGOe) and higher curie temperature (749k) than Nd2Fe14B than SmCo5, therefore, excellent magnetic properties can be maintained at temperatures close to 240℃. Chemical synthesis provides new ideas and methods for the high-performance preparation of stable magnet, and is expected to promote the rare earth permanent magnet industry to a new height.
Rare earth permanent magnet materials, especially nano-rare earth permanent magnet, have broad prospects in further improving the performance of functional magnetic materials. Although powder metallurgy and other methods have become mature preparation methods, however, due to the uneven size of magnetic powder particles, large size and other factors, its performance will also be adversely affected. Chemical method is more accurate in controlling the morphology and size of products, so it has unique advantages in nano-preparation of magnets, etc. In addition, chemical synthesis has unique advantages in the preparation of two-phase coupling magnets, which can realize accurate control of soft magnetic phase size and significantly improve exchange coupling effect, combining the characteristics of soft magnetic compared with high magnetic polarization intensity and the characteristics of large magnetic anisotropic field compared with hard magnetic, the energy density of the magnet is further improved to realize the application in the field of higher requirements.
Author:Zhu Kai, Xu Junjie, Hou Yanglong, Gao Song
Source: China Permanent Magnet Manufacturer – www.rizinia.com
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