Research progress of sintered NdFeB permanent magnets with high coercivity
Sintered NdFeB permanent magnets are widely used in the motor industry, medical equipment, wind power generation, electric vehicles, aerospace and many other fields due to their extremely high comprehensive performance. They are the permanent magnet materials with the best market application prospects. With the development of integration, miniaturization and intelligence of modern science and technology and the information industry, the emergence of sintered NdFeB permanent magnet materials with ultra-high comprehensive performance has strongly promoted the development of more emerging industries. Rare earth permanent magnet materials One of its main applications is to manufacture various permanent magnet motors. Compared with traditional motors, permanent magnet motors have the characteristics of light weight, small size, high efficiency and energy saving, and strong reliability. At present, most motors such as automobile launch motors and electric motors Bicycle motors, computer-driven motors, wind turbines, etc. all use permanent magnet motors. Especially in recent years, in order to protect the environment and save resources, the development of new energy vehicles such as electric vehicles has become a trend. In new energy vehicles, including drive motors, Generators, etc. require sintered NdFeB permanent magnet materials. Sintered NdFeB permanent magnets are small in size and high in performance, which can reduce the quality of the motor, improve the efficiency of the motor, and be more suitable for the miniaturization and light weight of the automobile. Sintered NdFeB permanent magnet materials play an indispensable role in the information industry, wind power generation industry, medical equipment industry, and magnetic levitation trains. Sintered NdFeB permanent magnet materials promote contemporary technology and social progress. One of the important material foundations, which provides the material foundation for the development of new industries.
Since it was discovered by Sagawa et al.  in 1983, after more than 30 years of development, the magnetic properties of NdFeB have been continuously improved. By 2006, the laboratory could obtain a magnetic energy product of 474.4kJ/m3. But because The temperature stability of sintered NdFeB is poor, so its application in the high temperature field is greatly restricted . In recent years, with the rapid development of electric vehicles and wind power generation industries, how to improve sintered NdFeB magnets The thermal stability has become a major issue in the field of industry research.
For a long time, a method to improve the high-temperature magnetic properties of magnets widely used in the sintered NdFeB manufacturing industry is to greatly increase its coercivity. Doping heavy rare earths such as Dy or Tb into the alloy is used to replace the main phase Nd2Fe14B. Nd, forming Dy2Fe14B or Tb2Fe14B to increase the anisotropy field, can well improve the coercivity and temperature stability of the magnet [4-5]. However, due to the antiferromagnetic coupling between Dy or Tb and Fe, this This method causes the magnet to lose part of the magnetic energy product due to the decrease in the remanence. More importantly, the abundance of Dy or Tb in the crust is much lower than that of Nd, and the price is more than 10 times that of Nd. It produces a large amount of high coercivity. NdFeB magnets containing Dy are facing cost and raw material supply difficulties. Therefore, under the premise of ensuring that NdFeB magnets obtain high coercivity, how to reduce the amount of heavy rare earths such as Dy or Tb becomes the current sintered NdFe. The focus of attention of researchers in the field of boron permanent magnets. In this regard, researchers have introduced a variety of new technologies into the preparation and post-processing of sintered NdFeB magnets, and have made good progress. Among them, by optimizing the crystal of the material Grain size, grain boundary composition and structure, grain refinement technology, grain boundary diffusion technology and grain boundary doping technology have become effective ways to prepare Dy-free or low-Dy high-coercivity magnets.
Grain refinement technology
Table of Contents
Studies have shown that as the grain size decreases, the coercive force of NdFeB permanent magnets will increase accordingly. Therefore, controlling the grain size during the manufacturing process becomes an effective way to obtain high coercivity. Uestuener et al.  The relationship between the particle size and distribution of different powders after jet milling on the grain size and performance of the sintered magnet was studied, and it was found that as the initial particle size and distribution width decrease, the average grain size of the magnet after sintering decreases linearly and the distribution is more uniform. Table 1 shows the relationship between grain size and magnetic properties. As the average grain size decreases, the coercive force of the magnet after sintering shows an increasing trend. When the grain size decreases from 7.6μm to 3.8μm, the coercivity of the magnet Increase from 883kA/m to 1178kA/m, and the remanence hardly decreases.
Table 1 The relationship between grain size and magnetic properties 
However, Li et al.  found that the coercivity of the magnet first increases and then decreases with the decrease of the grain size. When the grain size is smaller than a certain size, the rare earth Nd element is prone to oxidation, and the dhcp-rich The Nd phase is oxidized to the NdOx phase, which leads to the destruction of the continuity of the neodymium-rich phase and the decrease of the coercivity, as shown in Figure 1. Wang Qingkai et al.  also have similar research results, that is, when the particle size of the magnetic powder decreases to a certain value, the magnet The rare earth-rich phase at the grain boundary is significantly affected by oxidation, which reduces the coercivity of the magnet. Therefore, inhibiting the oxidation of the neodymium-rich phase during the preparation of ultra-fine-grained sintered NdFeB magnets is the key to obtaining high coercivity.
In recent years, with the development of new technologies such as helium jet milling and pressureless sintering, the research of high coercivity Dy-free sintered NdFeB magnets has made certain progress. Une et al.  used in the jet milling process Helium replaces nitrogen as the grinding medium to obtain magnetic powder with an average particle size of only 1.1μm, as shown in Figure 2. In the subsequent process, they use an oxygen-controlled pressureless sintering process (as shown in Figure 3) to strictly control Due to the oxidation problem caused by the magnetic powder is too fine, the coercive force and the maximum magnetic energy product have been improved. Without adding Dy, a magnet with a coercivity of 1592kA/m and a maximum magnetic energy product of 382kJ/m3 is obtained.
Figure.1 SEM images of Nd2Fe14B phase, dhcp-Nd-rich phase and NdOx phase when the grain size is 4.5 and 3.0μm, respectively 
Figure.2 SEM image of D50=1.1μm powder
Fig.3 Plan view of experimental device for pressureless process
Chen et al.  obtained particles with a powder size of 2.6μm through jet milling process, and obtained Dy-free sintered NdFeB magnet with a coercivity of 1513.2kA/m after low-temperature sintering and heat treatment. After high-frequency induction heat treatment, the magnet The coercive force is increased to 1636.6kA/m, as shown in Figure 4. This is mainly because the stirring effect of the induced eddy current promotes the continuous distribution of the neodymium-rich phase grain boundary, improves the magnetic isolation effect, and thus increases the magnet coercive force.
Figure.4 The demagnetization curve of the sample after sintering at room temperature, annealing at 500℃, and high frequency induction heat treatment
Zhang et al.  refined the particle size from 400nm to 250nm (below the single domain size 275nm) by optimizing the HDDR process parameters, and the coercivity of the magnetic powder increased from 920kA/m to 1440kA/m, and the remanence was still up to 1.36T. The morphology of the HDDR powder before and after the improvement process is shown in Figure 5.
Figure.5 Microstructure and particle size distribution of HDDR powder 
In order to ascertain the nature of the influence of grain size on the magnetic properties of magnets, Hono et al. [4,12] compared the initial magnetization of ultrafine sintered NdFeB with a grain size of 1μm and a traditional sintered magnet with a grain size of 3μm. Curve (see Figure 6). Studies have shown that: under a lower magnetic field, the initial magnetization curve of the traditional sintered NdFeB magnet has reached saturation; while the initial magnetization curve of the ultrafine crystal sintered magnet has two steps Since the grain size of the ultra-fine-grained sintered magnet is in the same order of magnitude as the critical size of the single domain, some grains do not contain magnetic domain walls. The interaction between the magnetic domain walls and the grain boundary becomes an important factor in determining the magnetic hardening mechanism of the material Factor. When the magnetic domain wall is pinned to the grain boundary, only when the applied magnetic field is higher than the pinning field can the single domain particles be reversed. This pinning field is similar to the coercivity of the ultrafine crystal magnet. The above results It shows that when the grain size is equivalent to the critical size of a single domain, the pinning of the magnetic domain wall becomes very important.
Figure.6 Initial magnetization curves of sintered NdFeB magnets with grain sizes of 1 and 3μm 
In order to ascertain the influence of grain size on the coercivity of NdFeB magnets, the researchers used various techniques to prepare NdFeB sintered magnets, HDDR magnetic powders and fast-quenched tapes with different grain sizes. The relationship of the size is shown in Figure 7 [4, 12]. As the grain size decreases, the coercivity tends to increase. However, when the grain size is less than 3μm, it is equivalent to 10 times the critical size of the single domain of the Nd2Fe14B phase. , The coercivity deviated from expectations and gradually decreased. In particular, the coercivity of HDDR magnetic powders with a grain size close to the critical size of a single domain and the coercivity of the rapid quenching belt with a smaller grain size is particularly prominent. HDDR magnetic powder and Nd2Fe14B in the rapid quenching belt The crystal grains are close to direct contact, not only the neodymium-rich phase is thinner and the rare earth is less, but also the recent three-dimensional atom probe (3DAP) and Lorentz-TEM (Lorentz-TEM) results show that there is a large amount of iron at the grain boundary Magnetic elements, the grain boundary phase is not a non-magnetic phase. There is ferromagnetic coupling between Nd2Fe14B grains, so the coercivity of HDDR magnetic powder and rapid quenching tape is reduced . In order to improve the coercivity of HDDR magnetic powder and rapid quenching tape Lin et al.  diffused Pr68Cu32 alloy powder into HDDR powder, and the coercivity increased from 1034.8kA/m to 1432.8kA/m. At the same time, Sepehri-Amin et al.  performed Nd80Cu20 diffusion heat treatment on NdFeB magnet powder , The coercive force is increased to 2069.6kA/m. The results show that the effective magnetic isolation of the neodymium-rich phase through grain boundary diffusion is also one of the important factors that increase the coercive force of the magnet.
Fig.7 The relationship between coercivity and grain size of sintered NdFeB magnet, HDDR magnetic powder and rapid quenching tape 
Grain boundary diffusion technology
Grain boundary diffusion technology is a technical means developed in recent years that can effectively improve the magnetic properties of sintered NdFeB magnets. Compared with the traditional method of adding heavy rare earths in the alloy melting process, the new technology makes the Dy or Tb is preferentially distributed to the epitaxial layer of the main phase Nd2Fe14B crystal grains, and this unique microstructure has a good effect of greatly improving the coercivity of the magnet [16-18]. At the same time, the addition of heavy rare earths is significantly reduced, which has achieved The win-win goal of saving resources and reducing costs.
There have been related reports on grain boundary diffusion technology for a long time. As early as 2000, Park et al.  attached Dy and Tb to the surface of NdFeB magnets by sputtering. After heat treatment, they found that the coercivity was significantly improved. The remanence is almost not reduced. In 2006, Shin-Etsu Corporation of Japan first proposed the concept of grain boundary diffusion technology . They attached heavy rare earth Dy and Tb compounds to the surface of sintered NdFeB magnets by dip coating. Heat treatment increased the coercivity by 820kA/m. They also compared the amount of Dy when the grain boundary diffusion process and the traditional dual alloy process reached the same coercivity, as shown in Figure 8. In thin-film magnets, grain boundary diffusion is used The amount of Dy added in the process is less than 10% of the traditional double alloy process. This is mainly due to the formation of a grain boundary layer with a shell structure inside the magnet after the grain boundary diffusion, which improves the microstructure of the grain boundary.
Figure.8 The relationship between the coercivity of different magnets and the mass fraction of Dy or Tb 
Because the grain boundary diffusion technology is the diffusion and infiltration of heavy rare earth elements in the sintered magnet, the diffusion depth is limited, and the diffusion concentration gradually decreases from the surface to the inside. Then, how deep can the grain boundary diffusion technology penetrate into the magnet sample? In order to explore this issue, Nakamura et al.  coated N52 magnets with a thickness of 14.5mm with different amounts of TbF3 mixed solution, and tested samples at different depths. As the distance from the surface increases, the correction The smaller the increase in coercive force; when the depth approaches 4mm, the coercive force of the magnet no longer increases, which is similar to the initial magnet performance, and the diffusion depth has nothing to do with the coating amount, as shown in Figure 9.
Figure.9 The relationship between the diffusion depth of the grain boundary diffusion magnet and the increase in coercive force 
With the continuous research on grain boundary diffusion technology, people have developed different diffusion media and diffusion processes. At present, the materials used in grain boundary diffusion mainly include rare earth metals [22-23], rare earth oxides [20, 24], rare earth Low melting point alloys such as fluoride [20, 25], Nd-Cu binary alloy  and Dy-Ni-Al ternary alloy [27-28]. The main technical methods include coating, deposition, sputtering, electroplating, etc. .
Li et al.  sputtered metal Dy and Tb on the surface of sintered NdFeB. After heat treatment and diffusion, the coercivity of the sputtered Dy sample increased by 470kA/m, and the coercivity of the sputtered Tb sample increased by about 830kA/m. m. Sepehri-Amin et al.  studied the effect of Dy diffusion on the properties of neodymium iron boron magnets by evaporation method, as shown in Figure 10. After 900 ℃ evaporation and subsequent 500 ℃ aging treatment, the coercivity of the magnet changes from 1042.5kA/m increased to 1623.4kA/m, while the remanence only decreased by 0.02T. Microstructure observation revealed that Dy diffused into the main phase grain epitaxial layer of Nd2Fe14B, while the substituted Nd precipitated to form a new Nd-rich Boundary phase. Researchers speculate that the formation of the Dy2Fe14B phase with higher anisotropy field in the main phase of the magnet Nd2Fe14B grain boundary region is the main reason for the increase of its coercivity. Nakamura et al.  immersed the sintered NdFeB magnet in DyF3 or In the solution of TbF3 and other compounds, a layer of compound coating was formed on the surface of the magnet, and then annealed. It was found that the remanence of the magnet did not change significantly, but the coercive force was greatly increased. The coercive force of the magnet after coating with TbF3 was 900kA /m is increased to 1700kA/m, and the magnetic energy product is slightly increased. Hirota et al.  also believe that the coercivity of the magnet can be significantly improved after grain boundary diffusion treatment, and the remanence change is small; even if a certain thickness is removed The surface layer of the magnet still has a certain improvement. They also compared the distribution of Dy element in the magnet and found that the Dy distribution in the magnet prepared by the traditional process is relatively uniform, and the Dy in the magnet after the grain boundary diffusion treatment is in the grain boundary zone Significant aggregation, and the thickness of the Dy-rich region is about 0.1μm. Compared with the traditional process, the Dy mass fraction at the grain boundary of the magnet prepared by the grain boundary diffusion process is higher, while the total mass fraction of Dy is lower. Sun Xuxin, etc.  A layer of Dy2O3 was plated on the surface of the sintered NdFeB magnet by surface permeation, and then diffused into the magnet by diffusion annealing. The coercivity of the magnet was increased from 1570kA/m to 1750kA/m The irreversible loss of magnetic flux at 100°C is reduced from -1.67% before Dy infiltration to -1.03%. Researchers believe that Dy2O3 infiltration on the surface of the magnet enhances the magnetocrystalline anisotropy field of the main phase grain surface layer, thereby effectively increasing Magnet coercivity. Liu et al. , Ji et al.  developed a method for coating nano-heavy rare earth hydride particles. They first used evaporation and condensation technology to obtain heavy rare earth hydride nanoparticles with a size of 50~200nm. Then it is coated on the surface of the sintered magnet and subjected to diffusion heat treatment. This method makes good use of the activity of the nano-hydride powder, has better adsorption, and does not introduce F, O and other harmful magnetic non-metal elements.
Fig.10 Demagnetization curve of magnet before and after grain boundary diffusion 
Soderžnik et al.  used the method of electrodeposition sample treatment, immersing a thin magnet as an anode in a sub-micron TbF3 alcohol suspension for diffusion treatment. The device is shown in Figure 11. After a certain period of diffusion and heat treatment, the magnet The coercivity is increased to 1536kA/m. Microstructure analysis shows that inside the magnet, the main phase crystal grains form a “core-shell structure”. Tb is only on the surface, and there are only Nd and Pr rare earth elements inside, which is the corrective effect. The main reason for the increase in coercivity is shown in Figure 12.
Figure.11 Electrophoresis equipment 
Figure.12 SEM images of the core-shell structure at different magnifications 
Lee et al.  improved the electrodeposition device and developed a rotating chamber as a counter electrode, as shown in Figure 13. The device uses the reaction chamber as the working electrode, Pt mesh as the counter electrode, and saturated Ag/AgCl as the reference electrode. During the reaction, the spherical small magnet is in contact with the cavity, and the Cu(NO3)2 aqueous solution is used as the Cu precursor, and Nd(NO3)3 is used as the Nd precursor as the electrolyte. After heat treatment, the Nd and Cu are diffused into the magnet , The coercivity increased from 942.46kA/m to 1162.75kA/m. Compared with the traditional horizontal or vertical arrangement of electrodes, the improved post-treatment magnet can obtain a more uniform coating.
Figure.13 Principle of rotating studio device for sintered Nd-Fe-B magnet electrodeposition Cu-Nd alloy 
In recent years, in order to further reduce the mass fraction of heavy rare earth elements in magnetic parts, low melting point alloys have been used as diffusers, combined with HDDR powders, hot-formed magnets, etc., to develop new diffusion processes [26, 35-41].
The relatively low coercivity of nanocrystals is mainly due to the lack of non-magnetic neodymium-rich phase in the grain boundary phase, which leads to exchange coupling between the grains. Too much Nd is distributed at the triangular grain boundary instead of the boundary phase. Therefore, through the crystal The introduction of non-magnetic neodymium-rich phase into the boundary phase can increase the coercivity of the magnet. In order to increase the coercivity of HDDR magnetic powder, Sepehri-Amin et al.  adjusted the grain boundary phase by surface diffusion of Nd-transition metal alloy. HDDR powder After being mixed with Nd70Cu30 alloy powder and annealed at 700℃, the Nd70Cu30 alloy melts and diffuses into the HDDR powder through the grain boundary, forming a neodymium-rich layer along the grain boundary layer, which has a good magnetic isolation effect on the Nd2Fe14B grains, and the magnetic powder is corrected. The coercivity increased from 1321.3kA/m to 1552.2kA/m. After Nd-Cu diffusion, the thickness of the grain boundary phase increased significantly. However, the TEM results showed that not all the Nd2Fe14B grains in the Nd-Cu diffused HDDR magnetic powder were The neodymium-rich grain boundary phase is completely isolated. Therefore, after better organization optimization treatment, it is expected to obtain a coercivity up to 1990kA/m. Using the same Nd-Cu diffusion, they also reported a low melting point in a thermally deformed magnet The diffusion of the alloy increases the coercivity of the 1mm thick thermally deformed magnet from 1194.0kA/m to 1830.8kA/m. And Wan et al.  will approach the calculated value of Nd2Fe14B ternary alloy HDDR powder and Pr- After the Cu alloy rapid quenching powder is mixed, it is diffused, and the coercivity is increased from 208.6kA/m to 980.1kA/m. Sawatzki  combined the rapid quenching powder with Dy-Cu, Dy-Ni-Al, Nd-Cu, Nd-Al and other types of low melting point eutectic alloy quick quenched powders are mixed and then hot pressed. The diffusion effect of different types of low melting point alloys is also studied, and it is found that Dy-Cu alloy contributes best to the coercivity, as shown in the figure. 14 shows. And Liu et al.  systematically studied the influence of various low melting point Nd-M (M=Cu, Al, Ga, Zn) alloy diffusion on hot-pressed magnets. They found that at room temperature, Nd90Al10 alloy After diffusion, the coercive force of the magnet reaches the highest 1990.0kA/m, but as the temperature increases, the performance of the magnet deteriorates significantly after diffusion. At 200℃, the coercive force of the Nd70Cu30 alloy after diffusion into the hot deformed magnet still has 559.3kA/m m, this is the highest performance among all diffusion magnets.
Figure.14 The influence of the mass fraction of Dy or Nd added in the hot-pressed magnet on (BH)m, μ0iHc, Jr 
For sintered magnets, Chen et al.  put Nd70Cu30 alloy flakes on the top and bottom of the original sintered magnets, and then applied a pressure of 40MPa along the c-axis, and the treated magnets were subjected to diffusion treatment under a certain heating process. The results found that After diffusion under a certain pressure, the coercivity of the magnet increases from 1170kA/m to 1660kA/m, which is 178kA/m higher than the performance of the magnet after diffusion without pressure. The subsequent microstructure shows that after the diffusion heat treatment, the Under a certain pressure, a certain thickness of microcracks appear at the grain boundary phase (as shown in Figure 15), which provides a diffusion channel for the diffusion of Nd70Cu30 alloy, which is beneficial to the Nd70Cu30 alloy to diffuse into the grain boundary more fully.
Figure.15 The microstructure of the diffusion magnet under certain pressure 
Based on the study of the grain boundary diffusion mechanism, Oono et al.  mixed Dy-Ni-Al ternary alloy powder and paraffin powder and coated on the surface of the sintered NdFeB magnet. After a certain high-temperature diffusion process, high-resolution Transmission electron microscopy analyzed the microstructure of the infiltrated magnet, and studied the changes in the mass fraction of Dy in the magnet with different diffusion depths. They found that Dy is concentrated in the crystal grains and grain boundaries at a depth of 100~500μm. And the distribution is uniform; and at a depth of 1~2mm, Dy is concentrated in the grain boundary phase and grain edge in a thickness of 50nm. Subsequent research shows that Dy does not enter the main phase grain edge by diffusion, but replaces it. Nd atoms form the (Dy, Nd)2Fe14B phase. The existence of this structure significantly increases the coercivity of the magnet, while the remanence and magnetic energy product hardly decrease. They proposed a grain boundary diffusion mechanism model, as shown in Figure 16. Show. Through the diffusion process, Dy diffuses into the grain boundary and replaces the Nd atoms at the grain boundary of the main phase to form a thin shell layer of (Dy, Nd)2Fe14B, which is the main reason for the improvement of the performance of the sintered NdFeB magnet.
Figure.16 Schematic diagram of the mechanism of Dy replacing Nd 
Grain boundary doping technology
Compared with the grain boundary diffusion technology, the grain boundary doping technology is not limited by the size of the magnet, and can prepare large-size high coercivity sintered NdFeB magnets. Grain boundary doping refers to the raw powder of NdFeB in the powder making process After mixing with the dopant in a certain form, after pressing and forming, sintering and heat treatment, a higher performance sintered magnet is obtained. At present, the main dopants include heavy rare earth nanoparticles [42-44], heavy rare earth compounds Micron particles [45-55] and low melting point Cu-Al alloys [56-59].
Li et al.  found that the initial NdFeB powder was doped with different mass fractions of Dy2O3 and found that with the increase of the doping amount, the coercive force continued to increase. When doped with 2% Dy2O3, the coercive force increased from 656kA /m is increased to 824kA/m. Xu et al.  doped DyF3 powder with a particle size of about 1μm with different mass fractions into the initial magnetic powder without Dy, and found that the coercivity was increased from 1223kA/m to 1595kA/m, The remanence is slightly reduced. Kim et al.  doped different rare-earth fluorides RF3 (R=La, Ce, Pr, Nd, Dy) into the quenched powder, and then obtained the magnet through the hot pressing process. The study found that, The doping of RF3 (R=Dy, Nd and Pr) can increase the coercivity, because the introduction of Dy and Pr increases the anisotropic field of Nd2Fe14B, and the addition of Nd optimizes the shell structure of the grain boundary. Liu  After doping 1% DyHx micron particles into the magnetic powder, they studied and compared the influence of the subsequent sintering process on the performance of the magnet. They found that with the increase of the sintering temperature, the remanence continued to increase, and the non-doped DyHx magnet was corrected. The coercive force gradually decreases, and the coercivity of the doped DyHx magnet shows a trend of first increasing and then decreasing, and at 1010℃, the coercive force of the doped DyHx magnet reaches the maximum. Bae et al.  compared Dy2O3 with After the doping effect of DyHx powder, it is found that the effect of doping DyHx is significantly better than doping Dy2O3. They believe that this is mainly because DyHx is easier to decompose at the sintering temperature, resulting in the formation of core-shell crystals in the doped DyHx particle sample The boundary structure is better than the sample doped with Dy2O3. In addition, the entry of H into Nd2Fe14B increases the lattice constant and the density of point defects, which contributes to a better distribution of Dy.
Literature [42-44] used inert gas protection evaporation-condensation method to prepare Dy, Tb and Pr nanoparticles with a particle size of 10-50nm, and then uniformly coat them on the surface of NdFeB powder with an average diameter of 5μm, and Sintered magnets are prepared by traditional sintering and heat treatment processes. The coercivity of the new sintered NdFeB magnets prepared by this method is significantly improved, while the remanence and magnetic energy product change little. Among them, the average particle size of 20nm Tb nanoparticle package The coercivity of the coated sintered magnet increased significantly from 960kA/m to 1560kA/m, while the remanence and maximum energy product did not change significantly. Through careful observation with electron probes and transmission electron microscopy, they found that the Nd2Fe14B crystal was enriched in the main phase. The exact location of the Tb element in the grain boundary area is not the neodymium-rich phase of the grain boundary, but the epitaxial layer of the main phase grain. In addition, combined with the analysis of the micro-domain spectral composition and the test of the magnet anisotropy field, they believe that the Tb element It is in the form of Tb2Fe14B or (Tb, Nd)2Fe14B, as shown in Figure 17.
Figure.17 Electron probe image of Tb nanoparticles doped sintered NdFeB magnet and the distribution of Tb and Nd elements 
However, the doping of heavy rare earth Dy and Tb has greatly increased the production cost. Therefore, the industry has used Dy and Tb compounds to replace the metal element to improve the performance of the magnet while reducing the effective Dy doping amount.
Liang et al.  doped the ternary Dy2.5Fe62Cu5.5 alloy as an intergranular phase into the initial magnet powder. After sintering heat treatment, the coercivity of the magnet increased from 1010.9kA/m to 1209.9kA/m, and the magnetic energy The product has increased by 9.5kJ/m3. Later they changed different dopants and added (Pr, Dy, Cu)-Hx powder to the initial magnet powder. Follow-up studies found that the mass fraction of the dopant was 2% When the coercive force was increased from 1194kA/m to 1449kA/m, and only the effective Dy with an atomic fraction of 0.32% was introduced, the amount of Dy used was greatly saved . Subsequently, they compared the intergranular doping The effect of different types of Dy compounds on the coercivity is found to be better than other forms of (Pr, Dy, Cu)-Hx, as shown in Figure 18. The data in the figure comes from literature [44, 49, 51-54] .
Figure.18 Correspondence between the increase in effective Dy atomic fraction and the increase in coercive force in different forms of Dy doping 
Because Dy is a scarce type of heavy rare earth metal with limited resources, the addition of Dy will inevitably lead to an increase in production costs. Therefore, the use of doped Dy-free grain boundary materials to replace Dy has increasingly become a research hotspot and trend. Ni et al. [57 ] The Cu-Al alloy was doped into neodymium iron boron powder during the powder making process, and the influence of Cu and Al doping on the alloy microstructure was studied. They found that Cu is mainly distributed at the triangular grain boundaries, which can be very The grain boundary is well repaired, the microstructure and chemical characteristics of the neodymium-rich phase are improved, and the magnetic isolation is strengthened, thereby increasing the coercive force. Al is mainly distributed more evenly inside the magnet, and a small amount enters the main phase to make the remaining The magnetism has decreased. Wan et al.  doped and diffused Pr-Cu alloy into the Dy-free NdFeB sintered magnetic powder, and the coercivity of the magnet increased from 1114.4kA/m to 1671.6kA/m, which they believe is currently reported. The highest coercivity achieved by a sintered NdFeB magnet without Dy. Subsequent microstructure analysis revealed that a layer of grain boundary layer was formed at the grain boundary of the magnet after doping with Pr-Cu, and the ferromagnetic Fe and Co atoms The fraction is reduced from 65% to 9%, as shown in Figure 19. Liang et al.  studied the effect of different doping amounts of Ho-Fe alloy on the magnetic properties of sintered NdFeB magnets and found that the mass fraction of dopants At 2.5%, the overall performance of the magnet is the highest. When the doping is continued, the remanence decreases more severely, as shown in Figure 20.
Figure.19 SEM images of the magnet before and after doping and the corresponding energy spectra of Fe, Pr and Cu elements 
Fig.20 Demagnetization curves of Ho63.4Fe36.6 alloy magnets with different doping content after sintering at 1050℃
Figure 20 Demagnetization curves of the magnets with different amounts of Ho63.4Fe36.6sintered at 1065℃
In summary, the use of grain refinement technology, grain boundary diffusion technology and grain boundary doping technology can all improve the magnetic performance to varying degrees, but each technology has a different degree of improvement on the magnetic performance. The use of grain refinement technology can Increase the coercivity by 120~520kA/m; while the grain boundary diffusion of Dy, Tb and their compounds can increase the coercivity by 180~800kA/m, and diffusion of Pr-Cu and other low melting point alloys can increase the coercivity by 200~770kA/ m; Grain boundary doping of Dy, Tb and their compounds can increase the coercivity by 160~600kA/m, and doping with low melting point alloys such as Pr-Cu can increase the coercivity by 190~550kA/m. At present, a large number of researches are devoted to To develop technology to obtain high-coercivity magnets without loss of remanence, especially under the background of the continuous shortage of rare earth resources, through the regulation of composition and microstructure, high-performance magnets with less and no Dy can be prepared. The development of Dy high coercivity magnets will always be the focus of research in the industry.
China’s rare earth resources are widely distributed and very rich, which provides a solid backing for the NdFeB industry. In recent years, the continuous application of NdFeB materials in the electronic industry, aerospace, medical equipment and other high-tech fields has given China NdFeB industry provides better development space. However, compared with developed countries, China’s sintered NdFeB industry still has a certain gap, especially in the field of high coercivity magnets. Improve the coercivity of NdFeB magnets to meet The requirement of higher operating temperature is the key to improving the quality, performance and grade of NdFeB products in our country.
In addition, since the light rare earth metals such as La and Ce separated from the rare earth metals of Pr and Nd have the highest mass fractions in the rare earths and are mostly idle, research and use cheap La and Ce rare earth metals to replace Pr and Nd to obtain better Good performance, broadening the application of NdFeB industry, while reducing production costs, is a major direction for the development of my country’s sintered NdFeB industry.
In addition, in the manufacturing and use of NdFeB permanent magnet materials, a large amount of waste materials are often generated, such as cutting, processing waste and scrap magnets. In order to avoid environmental damage and efficient use of rare earth resources, speed up development and effective The NdFeB recycling technology of NdFeB is the key to integrating rare earth resources and protecting the environment.
Author: LIU Weiqiang, ZHA Shanshun, YUE Ming, ZHANG Dongtao. Research Progress of Sintered Nd-Fe-B Permanent Magnets With High Coercivity[J]. Journal of Beijing University of Technology, 2017, 43(10): 1569-1581.
Source: China Permanent Magnet Manufacturer – www.ymagnet.com
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