Grain boundary diffusion sintered neodymium-iron-boron magnet and preparation method and application thereof

Abstract

The invention discloses a grain boundary diffusion sintered neodymium-iron-boron magnet and a preparation method and application thereof. The sintered neodymium-iron-boron magnet comprises a surface layer area and a core layer area, the surface layer area refers to an area close to the surface of the magnet, the thickness of the surface layer area is H, and H is larger than 0 and smaller than 500 micrometers; the skin layer region includes a first skin layer, a second skin layer, and a third skin layer. According to the sintered neodymium-iron-boron magnet, the diffusion slurry serves as a diffusion source, the coercive force of the sintered neodymium-iron-boron magnet is improved more obviously, the use amount of heavy rare earth can be reduced, and the corrosion resistance of the sintered neodymium-iron-boron magnet can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of rare earth permanent magnet materials, and particularly relates to a sintered Nd-Fe-B magnet with grain boundary diffusion, a preparation method thereof and an application thereof. Background Art

[0002] The sintered Nd-Fe-B magnet R-T-B is a new generation of rare earth permanent magnet material introduced in the early 1980s. It has excellent comprehensive magnetic properties, high cost performance and easy processing, etc., and is now widely used in fields such as computers, motors, wind turbines, electric vehicles, instruments, high-fidelity speakers, nuclear magnetic resonance imagers and aerospace navigators. Due to its high magnetic energy product, it is particularly suitable for various replacement products that require miniaturization, lightweight and integration.

[0003] The surface of the sintered Nd-Fe-B magnet is attached with an alloy powder or compound of heavy rare earth elements such as Tb / Dy. After diffusion heat treatment, the heavy rare earth elements diffuse along the grain boundaries and form a (Nd,Tb / Dy)2Fe14B shell structure on the surface layer of the Nd2Fe14B grains, improving the surface anisotropy field of the grains and at the same time improving the grain boundary microstructure. This treatment will significantly increase the coercivity of the magnet while reducing the remanence very little. This technology is called the grain boundary diffusion technology.

[0004] Patent CN102568731A directly mixes a certain amount of Zn nanoparticles with Nd-Fe-B micron particles, so that the Zn nanoparticles are evenly distributed and coated on the surface of the Nd-Fe-B particles. The added Zn nanoparticles are enriched at the grain boundaries of the main phase Nd2Fe14B grains in the magnet through the sintering and heat treatment processes, thereby significantly improving the corrosion resistance of the magnet and achieving the effect that the magnet does not require plating. Summary of the Invention

[0005] In order to solve the above problems, the technical solution of the present invention is as follows:

[0006] A diffusion slurry, the diffusion slurry includes a solid component and an organic solvent, wherein the solid component includes:

[0007] RE1 60-90wt%, RE1 is selected from at least one of Dy and Tb;

[0008] RE2 0-10wt%, RE2 can be selected from one or two or more of Nd, La, Ce, Pr, Dy, Tb, Ho, Gd;

[0009] Zn-containing compound 10-40wt%, the Zn-containing compound is selected from one or two of zinc oxide and zinc carbonate.

[0010] According to an embodiment of the present invention, the organic solvent is selected from at least one or two or more of ethanol, dimethyl carbonate, diethyl carbonate, acetone, and butyl acetate.

[0011] According to an embodiment of the present invention, in the diffusion paste, the mass content of the organic solvent is not more than 30%, preferably 5%-30%, for example, 10%, 15%, 20%, 25%.

[0012] According to an embodiment of the present invention, in the solid component, RE1 is selected from the metal powder of RE1; RE2 is selected from the metal powder of RE2; the Zn-containing compound is a solid powder.

[0013] According to an embodiment of the present invention, in the diffusion paste, the mass content of the solid component is more than 70%, preferably 70%-95%, for example, 75%, 80%, 85%, 90%.

[0014] According to an embodiment of the present invention, in the diffusion paste, the mass ratio of the solid component to the organic solvent is (70-95):(5-30), for example, 74:25, 80:20, 85:15, 90:10.

[0015] The present invention also provides a diffusion method, which includes arranging a diffusion paste on the surface of a neodymium iron boron magnet substrate, and preparing a sintered neodymium iron boron magnet through diffusion treatment, and the diffusion paste has the meaning as described above.

[0016] According to an embodiment of the present invention, the sintered neodymium iron boron magnet obtained by the diffusion method has high coercivity.

[0017] According to an embodiment of the present invention, when arranging the diffusion paste on the surface of the neodymium iron boron magnet substrate, the dosage of the diffusion paste is 0.5-2 wt% of the total mass of the neodymium iron boron magnet substrate, for example, 1 wt%.

[0018] According to an embodiment of the present invention, the conditions of the diffusion treatment include: vacuum heat treatment and aging treatment.

[0019] According to an embodiment of the present invention, the temperature of the vacuum heat treatment is 800-950 °C, for example, 850 °C.

[0020] According to an embodiment of the present invention, the conditions of the vacuum heat treatment are specifically: under vacuum conditions, treat at a temperature of 800-950 °C for 5-40 h.

[0021] According to an embodiment of the present invention, the temperature of the aging treatment is 400-600 °C, for example, 500 °C.

[0022] According to an embodiment of the present invention, the conditions for the aging treatment are as follows: under a vacuum condition, treatment is carried out at a temperature of 400 - 600 °C for a period of time, for example, 5 - 20 h.

[0023] According to an embodiment of the present invention, the composition of the neodymium iron boron magnet substrate includes:

[0024] RE 29 - 35 wt%, RE includes Nd, and optionally includes at least one of the following elements in addition to Nd: La, Ce, Pr, Dy, Tb, Ho, Gd;

[0025] B 0.85 - 1.2 wt%;

[0026] Cu 0.1 - 0.8 wt%;

[0027] Ga 0.1 - 0.8 wt%;

[0028] M 0 - 5 wt%, M is selected from one or more of Al, Zr, Ti, Co;

[0029] The balance is Fe and unavoidable impurities.

[0030] The present invention also provides a sintered neodymium iron boron magnet, which includes a surface layer region and a core layer region.

[0031] According to an embodiment of the present invention, the surface layer region refers to the region close to the surface of the magnet, and the thickness of the surface layer region is H, 0 < H < 500 μm.

[0032] Preferably, the surface layer region includes a first surface layer, a second surface layer, and a third surface layer. The thickness H1 of the first surface layer is 0 < H1 ≤ 200 μm. Preferably, the thickness H2 of the second surface layer is 200 < H2 ≤ 400 μm. Preferably, the thickness H3 of the third surface layer is 400 < H2 < 500 μm.

[0033] According to an embodiment of the present invention, the surface layer region includes Zn, and Zn shows a gradient distribution in the surface layer region.

[0034] Preferably, the gradient distribution of Zn means that Zn is contained in the surface layer region of the sintered neodymium iron boron magnet. Preferably, Zn is contained in the first surface layer and the second surface layer, and optionally contains or does not contain Zn in the third surface layer. Moreover, the content of Zn in the second surface layer is less than the content of Zn in the first surface layer, and the content of Zn in the third surface layer is less than the content of Zn in the second surface layer; the core layer region (for example, at 500 μm and deeper from the surface of the magnet) does not contain Zn.

[0035] According to an embodiment of the present invention, in the first surface layer and the second surface layer, the content of Zn is 0.1 to 0.6 wt%, for example, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%.

[0036] According to an embodiment of the present invention, in the third surface layer, the content of Zn is 0 - 0.1 wt%.

[0037] According to an embodiment of the present invention, the sintered neodymium iron boron magnet is prepared by the above diffusion method, that is, by arranging the above diffusion paste on the surface of the neodymium iron boron magnet substrate and performing diffusion treatment.

[0038] The present invention also provides a method for preparing the above sintered neodymium iron boron magnet, and the preparation method includes the following steps:

[0039] (1) Prepare the above diffusion paste;

[0040] (2) Select a neodymium iron boron magnet substrate, and uniformly arrange the diffusion paste in step (1) on the surface of the magnet substrate to form a film, obtaining a pre-sintered body;

[0041] (3) After subjecting the pre-sintered body obtained in step (2) to diffusion treatment, obtain the sintered neodymium iron boron magnet.

[0042] According to an embodiment of the present invention, the neodymium iron boron magnet substrate has the meaning as described above.

[0043] According to an embodiment of the present invention, the thickness of the neodymium iron boron magnet substrate is 1 - 18 mm. Preferably, the neodymium iron boron magnet substrate can be ultrasonically cleaned and / or dried, and the ultrasonic cleaning and drying can use conditions known in the art, for example, drying in a drying tunnel at 60 - 90 °C.

[0044] According to an embodiment of the present invention, in step (2), the thickness of the film is 10 - 400 μm.

[0045] According to an embodiment of the present invention, in step (2), the formation conditions of the film are: drying at a temperature of 50 - 120 °C for a period of time, for example, drying for 5 - 10 min.

[0046] According to an embodiment of the present invention, in step (3), the conditions of the diffusion treatment include: vacuum heat treatment and aging treatment.

[0047] Preferably, the conditions of the vacuum heat treatment are: under vacuum conditions, treating at a temperature of 800 - 950 °C (for example, 850 °C) for 5 - 40 h.

[0048] According to an embodiment of the present invention, in step (3), the conditions for the aging treatment are as follows: under a vacuum condition, treat at a temperature of 400 - 600 °C (for example, 500 °C) for a period of time, for example, 5 - 20 h.

[0049] The inventors found that when Zn metal is added to the diffusion paste, although the coercivity of the sintered neodymium iron boron magnet can be improved, since the Zn element is easily volatilized during vacuum heat treatment, it is difficult to control the Zn content entering the magnet interior, and there will be large fluctuations, resulting in a reduction in the performance consistency during the production of sintered neodymium iron boron magnets. When using the Zn compound of the present invention as the Zn diffusion source, during vacuum heat treatment, since the Zn compound is more stable, the Zn content entering the interior of the sintered magnet is controllable and stable, greatly improving the performance consistency of the product.

[0050] The present invention also provides the application of the above-mentioned sintered neodymium iron boron magnet, preferably for use in an electric motor.

[0051] Advantages of the present invention:

[0052] The present invention discovers that for a neodymium iron boron magnet matrix containing 0.1 - 0.8 wt% of Cu and 0.1 - 0.8 wt% of Ga, when using the diffusion paste containing the Zn compound and rare earth (such as RE1 and / or RE2) of the present invention as the diffusion source, under the condition of the same diffusion amount of pure heavy rare earth, the coercivity of the sintered neodymium iron boron magnet is more significantly improved, and the usage amount of heavy rare earth can be reduced. In addition, after treating the neodymium iron boron magnet matrix by the diffusion method of the present invention, the corrosion resistance of the sintered neodymium iron boron magnet can also be improved. Description of the drawings

[0053] Figure 1 is the backscattered image at a distance of 200 μm from the magnet surface;

[0054] Figure 2 is the backscattered image at a distance of 500 μm from the magnet surface. Detailed implementation manners

[0055] The technical solutions of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only for illustrative and explanatory purposes of the present invention, and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0056] Unless otherwise specified, the raw materials and reagents used in the following embodiments are all commercially available products, or can be prepared by known methods.

[0057] Instruments and equipment

[0058] The grain boundary phase and main phase structure analysis method of the present invention is as follows: using backscattering and energy spectrum scanning measurement, randomly select any grain and grain boundary on any section of the permanent magnet, measure 5 point values ​​respectively, and take the average value.

[0059] Magnetic property test: Use NIM-62000 for testing, the test temperature is 20±3℃.

[0060] Weight loss test: using HAST equipment, the test conditions are 130℃, 95%RH, 2.6Bar.

[0061] The Zn gradient content test on the magnet surface adopts backscattering and energy spectrum test. Along the diffusion direction, at 0-200μm (first surface layer), 200-400μm (second surface layer), 400-500μm (third surface layer), the grain boundary phase position is selected for component point analysis, and the average value is taken for each measurement of 5 points, that is, the Zn element content in the first surface layer to the third surface layer is obtained.

[0062] Example 1

[0063] The preparation method of sintered NdFeB permanent magnet is as follows:

[0064] (1) Prepare the raw materials for sintering NdFeB permanent magnets according to the following weight percentages: PrNd is 31wt%, Co is 2wt%, Cu is 0.3wt%, Ga is 0.1wt%, Al is 0.4wt%, Zr is 0.1wt%, B is 1wt%, and Fe is the remainder. The above raw materials are used to make alloy flakes using the rapid solidification and strip throwing method, and are formed under normal temperature and an orientation field environment with a magnetic field strength of 2T. Then the blank is placed in a vacuum sintering furnace, sintered at 1070°C for 6h, and aged at 520°C for 5h to obtain the NdFeB magnet matrix. The above NdFeB matrix is ​​cut into 10cm magnet pieces along the orientation direction. The magnetic properties of the magnet pieces are tested with reference to the above test method, recorded as matrix Br and matrix Hcj, and recorded in Table 1.

[0065] (2) After ultrasonic cleaning, the magnet pieces are dried in a drying oven at 90° C.

[0066] (3) Preparation of Zn-containing heavy rare earth slurry: zinc oxide powder: RE1 (Tb powder) powder: RE2 (Pr powder) powder = 1:8:1 after thorough mixing to obtain mixed powder. The mixed powder and dimethyl carbonate are prepared into a slurry in a ratio of 9:1 as a diffusion source.

[0067] (4) The slurry in step (3) is evenly arranged on the surface of the magnet piece in step (1) to form a thin film, the thickness of which accounts for 0.7 wt% of the weight of the magnet piece, and then dried at 60° C. for 6 min to obtain a pre-sintered body.

[0068] (5) Diffusion treatment: The pre-sintered body obtained in step (4) is diffused in a vacuum environment at 850 °C for 20 h, and then vacuum aging treatment is carried out at 500 °C for 15 h to obtain a sintered NdFeB permanent magnet after diffusion treatment.

[0069] Example 2

[0070] The preparation method of the sintered NdFeB permanent magnet in this example is basically the same as that in Example 1, except that:

[0071] In the raw materials of step (1), Cu is 0.4 wt%, and Ga is 0.2 wt%;

[0072] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder: RE2 (Pr powder) powder = 2:7:1, mixed powder: dimethyl carbonate = 86:14;

[0073] In step (4): the weight of the film thickness accounts for 0.9 wt% of the weight of the magnet sheet, and then it is dried at 60 °C for 8 min to obtain a pre-sintered body.

[0074] Example 3

[0075] The preparation method of the sintered NdFeB permanent magnet in this example is basically the same as that in Example 1, except that:

[0076] In the raw materials of step (1): Cu is 0.6 wt%, and Ga is 0.8 wt%;

[0077] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder: RE2 (Pr powder) powder = 30:65:5, mixed powder: dimethyl carbonate = 82:18;

[0078] In step (4): the weight of the film thickness accounts for 1.5 wt% of the weight of the magnet sheet, and then it is dried at 60 °C for 9 min.

[0079] Example 4

[0080] The preparation method of the sintered NdFeB permanent magnet in this example is basically the same as that in Example 1, except that:

[0081] In the raw materials of step (1): Cu is 0.8 wt%, and Ga is 0.6 wt%;

[0082] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder = 2:3, mixed powder: dimethyl carbonate = 4:1;

[0083] In step (4): the weight of the film thickness accounts for 1.8 wt% of the weight of the magnet sheet, and then it is dried at 60 °C for 10 min.

[0084] Example 5

[0085] The preparation method of the sintered NdFeB permanent magnet in this example is basically the same as that in Example 1, except that:

[0086] In the raw materials of step (1): Cu is 0.7wt%, and Ga is 0.6wt%;

[0087] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder = 1:9, and the mixed powder: dimethyl carbonate = 4:1;

[0088] In step (4): the weight of the thin film thickness accounts for 0.5wt% of the weight of the magnet sheet, and then it is dried at 60°C for 5 minutes.

[0089] After repeating the experiment 5 times, 5 groups of different diffused sintered NdFeB permanent magnets are obtained respectively, and are denoted as Example 5-1, Example 5-2, Example 5-3, Example 5-4 and Example 5-5 respectively.

[0090] Example 6

[0091] The preparation method of the sintered NdFeB permanent magnet in this example is basically the same as that in Example 1, except that:

[0092] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Dy powder) powder = 1:9, and the mixed powder: dimethyl carbonate = 4:1;

[0093] In step (4): the weight of the thin film thickness accounts for 1.0wt% of the weight of the magnet sheet, and then it is dried at 60°C for 8 minutes.

[0094] Example 7

[0095] The preparation method of the sintered NdFeB permanent magnet in this example is basically the same as that in Example 1, except that:

[0096] In the Zn-containing heavy rare earth slurry of step (3): zinc carbonate powder: RE1 (Tb powder) powder = 1:9, and the mixed powder: ethanol = 4:1;

[0097] Comparative Example 1

[0098] The preparation method of the sintered NdFeB permanent magnet in this comparative example is basically the same as that in Example 1, except that:

[0099] In the raw materials of step (1): Cu is 0.05wt%, and Ga is 0.1wt%;

[0100] Comparative Example 2

[0101] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 1, except that:

[0102] In the raw materials of step (1): Cu is 0.85 wt%, and Ga is 0.1 wt%;

[0103] Comparative Example 3

[0104] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 1, except that:

[0105] In the raw materials of step (1): Cu is 0.3 wt%, and Ga is 0.05 wt%;

[0106] Comparative Example 4

[0107] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 1, except that:

[0108] In the raw materials of step (1): Cu is 0.3 wt%, and Ga is 0.85 wt%;

[0109] Comparative Example 5

[0110] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 3, except that:

[0111] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder: RE2 (Pr powder) powder = 3:5:2.

[0112] Comparative Example 6

[0113] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 5, except that:

[0114] In step (4): the weight of the film thickness accounts for 0.2 wt% of the weight of the magnet sheet, and then it is dried at 60 °C for 5 min.

[0115] Comparative Example 7

[0116] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 5, except that:

[0117] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder = 8:92.

[0118] Comparative Example 8

[0119] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that in Example 5, except that:

[0120] In the Zn-containing heavy rare earth slurry of step (3): zinc oxide powder: RE1 (Tb powder) powder = 1:1.

[0121] Comparative Example 9

[0122] The preparation method of the sintered Nd-Fe-B permanent magnet in this comparative example is basically the same as that of Example 5, except that:

[0123] In the Zn-containing heavy rare earth slurry in step (3): zinc metal powder: RE1 (Tb powder) powder = 1:9.

[0124] After repeating the experiment 5 times, 5 groups of different diffused sintered Nd-Fe-B permanent magnets were obtained respectively, and were denoted as Comparative Example 9-1, Comparative Example 9-2, Comparative Example 9-3, Comparative Example 9-4 and Comparative Example 9-5 respectively.

[0125] Test Example

[0126] Take the diffused sintered Nd-Fe-B permanent magnets prepared in the above examples and comparative examples, and test them according to the above test method, and record the test results in Table 1 and Table 2, where the magnetic properties are denoted as diffused Br and diffused Hcj respectively.

[0127] Table 1 Summary Table of Detection Performance of Examples and Comparative Examples

[0128]

[0129] Table 2 Summary Table of Detection Performance of Example 5 and Comparative Example 9

[0130]

[0131] It can be seen from the above test results that:

[0132] Compared with Comparative Examples 1-2, the only difference in Example 1 is the content of matrix Cu. Judging from the increase in Hcj before and after diffusion, when the Cu content is too high or too low, the matrix performance has little effect basically. However, with the same type and amount of diffusion source used, the increase in Hcj brought by grain boundary diffusion in Comparative Examples 1-2 is relatively low.

[0133] Compared with Comparative Examples 3-4, the only difference in Example 1 is the content of matrix Ga. Although the matrix performance increases when the Ga content is relatively low, the increase in Hcj after diffusion decreases.

[0134] It has been found through research that the contents of Cu and Ga in the base material affect the Hcj of the grain boundary diffusion magnet. The preferred content of Cu is 0.1-0.8 wt%, and the preferred content of Ga is 0.1-0.8 wt%. During the diffusion process, the presence of Cu and Ga elements can improve the diffusion efficiency and increase the Hcj increase. When Cu and Ga exceed the preferred range, the beneficial effects are very small, and too high contents of Cu and Ga will partially reduce the remanence of the base material.

[0135] Comparative Example 5 is compared with Example 3. Only the RE2 content of the diffusion source is different. When the proportion of RE2 is relatively high and the RE1 in the diffusion source is lower than a certain level, the increase in Hcj shows a significant decrease. A similar situation is also found in the performance results of Comparative Example 6. Comparative Example 6 is the same as Example 5 in other conditions except for the diffusion weight gain. The difference is that due to too little RE1 on the surface of the matrix, the rapid decrease in the increase of Hcj is caused, and the weight loss performance of the magnet is also significantly deteriorated, and the corrosion resistance becomes worse.

[0136] Comparative Examples 7-8 are compared with Example 5. Only the proportion of zinc oxide in the diffusion source is different. Comparative Example 7 contains a lower zinc oxide content, and the weight loss level is higher than that of Example 5. When the zinc oxide content in Comparative Example 8 is too high, the weight loss level is the same as that of Example 5, without further improvement effect and without causing further performance growth.

[0137] Comparative Example 9 is compared with Example 5. Zn metal is used as the diffusion source. Since Zn elements are relatively volatile during vacuum treatment, the product performance stability is low and the consistency is poor. It can be found from the test data of Comparative Examples 9-1 to 9-5 that with the same process, the performance differences of different samples are relatively large. While the stability and consistency are poor, the Hcj of Comparative Example 9-2 is the same as that of Example 5-2, and the same increase in coercivity is obtained. However, the remanence Br of Comparative Example 9-2 decreases more, and the comprehensive performance is also slightly worse.

[0138] The exemplary embodiments of the present invention have been described above. However, the protection scope of this application is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made by those skilled in the art within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A diffusion paste, characterized in that, the diffusion paste comprises a solid component and an organic solvent, wherein the solid component comprises: RE1 60 - 90 wt%, RE1 is selected from at least one of Dy and Tb; RE2 0 - 10 wt%, RE2 can be selected from one or two or more of Nd, La, Ce, Pr, Dy, Tb, Ho, Gd; Zn-containing compound 10 - 40 wt%, the Zn-containing compound is selected from one or two of zinc oxide and zinc carbonate.

2. The diffusion paste according to claim 1, characterized in that, the organic solvent is selected from at least one or two or more of ethanol, dimethyl carbonate, diethyl carbonate, acetone, and butyl acetate. Preferably, in the diffusion paste, the mass content of the organic solvent is not more than 30%. Preferably, in the solid component, RE1 is selected from the metal powder of RE1; RE2 is selected from the metal powder of RE2; the Zn-containing compound is a solid powder. Preferably, in the diffusion paste, the mass content of the solid component is more than 70%. Preferably, in the diffusion paste, the mass ratio of the solid component to the organic solvent is (70 - 95):(5 - 30).

3. A diffusion method, characterized in that, the diffusion method includes arranging the diffusion paste according to claim 1 or 2 on the surface of a neodymium iron boron magnet substrate, and preparing a sintered neodymium iron boron magnet through diffusion treatment.

4. The diffusion method according to claim 3, characterized in that, the sintered neodymium iron boron magnet obtained after being treated by the diffusion method has a high coercivity. Preferably, when arranging the diffusion paste on the surface of the neodymium iron boron magnet substrate, the dosage of the diffusion paste is 0.5 - 2 wt% of the total mass of the neodymium iron boron magnet substrate. Preferably, the conditions of the diffusion treatment include: vacuum heat treatment and aging treatment. Preferably, the temperature of the vacuum heat treatment is 800 - 950 °C. Preferably, the conditions of the vacuum heat treatment are specifically: under vacuum conditions, treating at a temperature of 800 - 950 °C for 5 - 40 h. Preferably, the temperature of the aging treatment is 400 - 600 °C. Preferably, the conditions of the aging treatment are: under vacuum conditions, treating at a temperature of 400 - 600 °C for a period of time.

5. The diffusion method according to claim 3 or 4, characterized in that, the composition of the neodymium iron boron magnet substrate includes: RE 29 - 35 wt%, RE includes Nd, and can optionally include at least one of the following elements in addition to Nd: La, Ce, Pr, Dy, Tb, Ho, Gd; B 0.85 - 1.2 wt%; Cu 0.1 - 0.8 wt%; Ga 0.1 - 0.8 wt%; M 0 - 5 wt%, M is selected from one or more of Al, Zr, Ti, Co; the balance is Fe and inevitable impurities.

6. A sintered neodymium iron boron magnet, characterized in that, the sintered neodymium iron boron magnet includes a surface layer region and a core layer region. Preferably, the surface layer region refers to the region close to the surface of the magnet, and the thickness of the surface layer region is H, 0 < H < 500 μm. Preferably, the surface layer region includes a first surface layer, a second surface layer, and a third surface layer. The thickness H1 of the first surface layer satisfies 0 < H1 ≤ 200 μm. Preferably, the thickness H2 of the second surface layer satisfies 200 < H2 ≤ 400 μm. Preferably, the thickness H3 of the third surface layer satisfies 400 < H2 < 500 μm.

7. The sintered neodymium iron boron magnet according to claim 6, wherein, the surface layer region includes Zn, and Zn exhibits a gradient distribution in the surface layer region. Preferably, the gradient distribution of Zn means that Zn is contained in the surface layer region of the sintered neodymium iron boron magnet, preferably contained in the first surface layer and the second surface layer, and optionally contained or not contained in the third surface layer. And the content of Zn in the second surface layer is less than the content of Zn in the first surface layer, and the content of Zn in the third surface layer is less than the content of Zn in the second surface layer; the core layer region does not contain Zn. Preferably, in the first surface layer and the second surface layer, the content of Zn is 0.1 - 0.6 wt%. Preferably, in the third surface layer, the content of Zn is 0 - 0.1 wt%.

8. A method for preparing the sintered neodymium iron boron magnet according to claim 6 or 7, wherein, the preparation method includes the following steps: (1) Prepare the diffusion slurry according to claim 1 or 2; (2) Select a neodymium iron boron magnet substrate, and uniformly arrange the diffusion slurry in step (1) on the surface of the magnet substrate to form a thin film, obtaining a pre-sintered body; (3) After subjecting the pre-sintered body obtained in step (2) to diffusion treatment, obtain the sintered neodymium iron boron magnet.

9. According to the preparation method of claim 8, wherein, the thickness of the neodymium iron boron magnet substrate is 1 - 18 mm. Preferably, in step (2), the thickness of the thin film is 10 - 400 μm. Preferably, in step (2), the formation conditions of the thin film are: drying for a period of time at a temperature of 50 - 120 °C. Preferably, in step (3), the conditions of the diffusion treatment include: vacuum heat treatment and aging treatment. Preferably, the conditions of the vacuum heat treatment are: under vacuum conditions, treating at a temperature of 800 - 950 °C for 5 - 40 h. Preferably, in step (3), the conditions of the aging treatment are: under vacuum conditions, treating at a temperature of 400 - 600 °C for a period of time.

10. Application of the sintered neodymium iron boron magnet according to claim 6 or 7 in an electric motor.

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