Large-size sintered magnet matrix, sintered neodymium-iron-boron permanent magnet and preparation method and application of sintered neodymium-iron-boron permanent magnet

Abstract

The invention discloses a large-size sintered magnet matrix, a sintered neodymium-iron-boron permanent magnet and a preparation method and application of the sintered neodymium-iron-boron permanent magnet. A base body of the sintered magnet is of a polyhedral structure; the sintered magnet matrix comprises four areas, and the oxygen content and the rare earth content of each area meet the following relational expression: O1 / O2 is greater than or equal to 2; w1 > W2 > W3 > W4, W2-W3 > = 0.15 wt%, and W3-W4 > = 0.25 wt%. The sintered magnet matrix has the characteristic that the content of rare earth on the surface is larger than that of rare earth in the center part, so that the enrichment time of diffusion elements at a surface layer grain boundary can be prolonged, and heavy rare earth elements cannot permeate into grains in a surface layer area, so that the overall high magnetic performance of a product can be ensured, and the service life of the product is prolonged. Therefore, the high-temperature-resistant requirement of the motor is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of rare earth sintered magnet preparation, and specifically relates to a large-size sintered magnet substrate, a sintered NdFeB permanent magnet, and a preparation method and application thereof. Background Art

[0002] Currently, it is known that sintered NdFeB has the optimal comprehensive magnetic properties among permanent magnets, and is widely used in various motors in automotive fields such as electric vehicles, renewable fields such as wind power generation, and household appliance fields such as air conditioners and consumer electronics. In addition, sintered NdFeB plays an irreplaceable role in the miniaturization, lightweight, and high efficiency of these motors. These motors require NdFeB to have high remanence and high coercivity to ensure high torque and high temperature resistance. The early method to improve the coercivity of sintered NdFeB was to add a certain amount of heavy rare earth elements Dy and / or Tb to NdFeB, but the reserves of these two heavy rare earth elements are limited and the price is expensive. With the development of technology, grain refinement technology and grain boundary diffusion technology have become the main methods developed recently. The grain refinement technology is that as the grain size decreases, the effective demagnetizing field factor of the grains becomes smaller, and the coercivity of the magnet increases, getting rid of the dependence on heavy rare earths. The grain boundary diffusion technology is to make the heavy rare earth diffuse along the grain boundary phase, significantly increasing the anisotropy constant at the grain boundary, achieving a significant increase in the coercivity of the magnet with a small amount of heavy rare earth used. Both technologies can achieve the effect of significantly reducing the use of heavy rare earths and increasing the coercivity of the magnet. However, currently, the improvement of coercivity by grain refinement technology is limited. Therefore, the grain boundary diffusion technology is the mainstream research direction. There are various types of motor designs, and the sizes of NdFeB used in motors are also diverse. For NdFeB magnets with a heavy rare earth diffusion direction of about 5 mm, the grain boundary diffusion technology can improve the performance of the surface and center of the NdFeB magnet to varying degrees, meeting the high temperature requirements of the motor. However, for large-size products with a diffusion direction of more than 10 mm, the performance improvement of the central region of the NdFeB magnet is limited and cannot meet the high temperature requirements of the motor.

[0003] Patent document WO2011004894A1 proposes that the main channel for heavy rare earth diffusion in the grain boundary diffusion technology is the rare earth-rich phase existing in the grain boundary. To achieve an ideal diffusion effect, the rare earth-rich phase in the grain boundary of the diffusion substrate needs to exist and be continuous.

[0004] Patent document JP2022115921A proposes a technical solution to improve the diffusion effect of heavy rare earth elements by controlling the content of elements such as Cu and Ga in the product, thereby improving the coercivity of sintered NdFeB. This technical solution limits the component content in NdFeB, which will limit some properties of the product and narrow the applicable range of the product. In addition, this technical solution still further improves the product performance in the diffusion of conventional sizes and fails to solve the problem of large-size diffusion.

[0005] Patent document CN116313468A obtains a composite diffusion source by sputtering a heavy rare earth layer on the surface of an ultra-thin aluminum foil, and processes the composite diffusion source at about 200 °C for 30 min so that the composite diffusion source covers the four surfaces of neodymium iron boron. By using Al as an auxiliary diffusing agent, the diffusion effect is improved to meet the large-size diffusion. However, when Al is used as an auxiliary agent, the performance improvement is restricted compared with pure heavy rare earth. At the same time, when Al enters neodymium iron boron, the remanence of the magnet will be reduced. In addition, this method requires heat treatment at 200 °C to achieve the coverage of the composite diffusion source on the surface of the neodymium iron boron magnet, with complex process and increased energy consumption.

[0006] Patent document CN113299476A forms a heavy rare earth layer on a metal foil, then places the metal foil between magnets and conducts diffusion treatment. Although patent document CN113299476A can achieve the diffusion and bonding of multiple magnets in a single heat treatment process, in order to ensure the consistency between magnets, a pressure of 0.1 - 10 MPa needs to be applied at both ends of the magnets, which increases the operation complexity and difficulty. In addition, in order to ensure the position of the metal foil between magnets, the placement of the metal foil needs to be strictly controlled, which also increases the difficulty and complexity of production.

[0007] The diffusion of sintered neodymium iron boron is to make the diffusing element penetrate from the magnet surface to the magnet interior along the grain boundaries, improve the magnetocrystalline anisotropy constant of the grain boundaries, and thus increase the coercivity. When diffusing along the direction with a size greater than 10 mm, the diffusing element first accumulates at the grain boundaries on the diffusion surface, and then penetrates into the grain boundaries inside the magnet. However, at this time, the resistance to the diffusion of the diffusing element from the magnet surface to the magnet interior is large. To improve the diffusion effect inside the magnet, the diffusion time is usually extended, which will cause the diffusing element on the magnet surface to penetrate from the grain boundaries on the magnet surface into the grain interior on the magnet surface, resulting in lower remanence and coercivity at the magnet surface, ultimately affecting the overall remanence and coercivity of the magnet and unable to meet the high-temperature requirements of the motor. Summary of the Invention

[0008] To improve the deficiencies of the prior art, the present invention provides the following solutions:

[0009] A sintered magnet substrate, the sintered magnet substrate is a polyhedral structure, preferably a cuboid.

[0010] According to an embodiment of the present invention, the sintered magnet substrate includes four regions:

[0011] Region 1 is the surface oxide layer, which refers to the region from the substrate surface to a distance of 100 μm from the substrate surface. The oxygen content of Region 1 is denoted as O1, and the rare earth content is denoted as W1;

[0012] Region 2 is the rare earth high-content layer, which refers to the region at a distance of 100 - 150 μm from the magnet surface. The oxygen content of Region 2 is denoted as O2, and the rare earth content is denoted as W2;

[0013] Region three is the middle rare-earth content layer, referring to the region 150 - 1000 μm away from the magnet surface. The oxygen content in region three is denoted as O3, and the rare-earth content is denoted as W3;

[0014] Region four is the low rare-earth content layer, referring to the region from 1000 μm away from the magnet surface to the geometric central axis of the magnet. The oxygen content in region four is denoted as O4, and the rare-earth content is denoted as W4;

[0015] Among them, the oxygen content and rare-earth content in each region satisfy the following relational expressions:

[0016] O1 / O2 ≥ 2; preferably, O1 ≥ 3000 ppm and O2 ≤ 1500 ppm;

[0017] W1 > W2 > W3 > W4, W2 - W3 ≥ 0.15 wt%, W3 - W4 ≥ 0.25 wt%.

[0018] According to the embodiments of the present invention, W2 - W3 is, for example, 0.2 wt%, 0.3 wt%; W3 - W4 is, for example, 0.3 wt%, 0.4 wt%, 0.6 wt%.

[0019] According to the embodiments of the present invention, the rare-earth content in the four regions of the sintered magnet substrate refers to the mass content of R.

[0020] According to the embodiments of the present invention, the thickness of the sintered magnet substrate is greater than 10 mm, for example, not less than 10.2 mm, 10 mm - 12 mm, 10 mm - 15 mm, 10 mm - 20 mm.

[0021] According to the embodiments of the present invention, the sintered magnet substrate comprises the following components:

[0022] R 27 - 33 wt%, R is selected from Nd and at least one of the following elements: Pr, Sm, La, Ce;

[0023] B 0.9 - 1.3 wt%;

[0024] M < 5 wt%, M is selected from one or more of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo;

[0025] RH 0 - 2 wt%; RH is a heavy rare-earth element, selected from at least one of Y, Ho, Gd, Dy, and Tb;

[0026] The balance is iron and unavoidable impurities.

[0027] According to an embodiment of the present invention, in the sintered magnet substrate, M is, for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%.

[0028] The present invention also provides a method for preparing the above-mentioned sintered magnet substrate, and the preparation method includes: performing multi-stage sintering treatment on the green compact to obtain the sintered magnet substrate; wherein, the multi-stage sintering treatment includes:

[0029] (1) The first stage of sintering: the sintering temperature T1 is 900 - 1100 °C, the sintering time H1 is 120 - 300 min, and the vacuum degree P1 of the sintering furnace satisfies: 10 -1 Pa < P1 < 10 Pa (for example, 0.5 Pa, 1 Pa, 5 Pa);

[0030] (2) The second stage of sintering, after the end of the first stage of sintering, evacuate to a vacuum degree P2 ≤ 10 -1 Pa (for example, 0.05 Pa), the sintering temperature T2 is the same as T1, and the sintering time H2 is 30 - 60 min (for example, 40 min);

[0031] (3) The third stage of sintering, after the end of the second stage of sintering, fill with inert gas until the pressure in the sintering furnace satisfies 50 kPa ≤ P3 ≤ 150 kPa (for example, 100 kPa), the sintering temperature T3 is the same as T1, and the sintering time H3 is 10 - 30 min (for example, 20 min);

[0032] (4) After the end of the third stage of sintering, cool to T40 ≤ 200 °C and then perform tempering treatment. The tempering treatment specifically includes: performing primary tempering treatment at a tempering temperature T41 of 700 - 900 °C (for example, 800 °C), and the treatment time H41 is 3 - 7 h; further performing secondary tempering treatment at a tempering temperature T42 of 450 - 600 °C (500 °C), and the treatment time H42 is 3 - 7 h.

[0033] According to an embodiment of the present invention, the green compact is prepared by the following method: each raw material is prepared by a method known in the art according to the composition of the above-mentioned sintered magnet substrate. Exemplarily, an R-Fe-B-M alloy sheet is prepared by the rapid solidification strip method for each raw material, and after the alloy sheet is subjected to hydrogen explosion treatment and airflow milling to obtain magnetic powder, it is pressed into a green compact. Preferably, the average particle size of the magnetic powder is 2 - 5 μm. Preferably, 0.1 - 0.6 wt% of a lubricant can also be added to the magnetic powder, and then mixed for 0.1 - 6 h, and then pressed into a green compact.

[0034] According to an embodiment of the present invention, the thickness of the green compact is greater than 10 mm, preferably greater than 10.2 mm, such as 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 20 mm. In the present invention, the thickness of the green compact can be adjusted as needed as long as the thickness required for the sintered magnet substrate can be obtained.

[0035] According to an embodiment of the present invention, the ratio of the thickness of the green compact to the thickness of the sintered magnet substrate is greater than 1 and not greater than 1.5, such as 1.01 to 1.5, and for another example, 1.1, 1.2, 1.3, 1.4.

[0036] According to an embodiment of the present invention, the sintered magnet substrate can be further subjected to a diffusion treatment. Before the diffusion treatment, the surface oxide layer of the sintered magnet substrate can be optionally removed, or it can be further cut and processed into the required size along the non-diffusion direction. In the present invention, the diffusion direction refers to the thickness direction of the magnet, and the non-diffusion direction refers to the length direction or width direction of the magnet.

[0037] The present invention also provides a sintered Nd-Fe-B permanent magnet. The thickness δ of the sintered Nd-Fe-B permanent magnet is ≥10 mm; along the thickness direction, the region from 0 to 1 mm away from the surface of the sintered Nd-Fe-B permanent magnet is denoted as the surface layer region, and the coercivity of the surface layer region is Hcj1; the region from (δ / 2 - 0.5 mm) to (δ / 2 + 0.5 mm) away from the surface of the sintered Nd-Fe-B permanent magnet is denoted as the geometric center region, and the coercivity of the geometric center region is Hcj2;

[0038] Both Hcj1 and Hcj2 simultaneously satisfy the following relational expressions:

[0039] ① Hcj1 > Hcj2,

[0040] ② 160 ≤ Hcj1 - Hcj2 ≤ 0.3×δ 2 +15×δ - 20.

[0041] According to an embodiment of the present invention, the thickness δ of the sintered Nd-Fe-B permanent magnet is, for example, 10 - 12 mm, 10 - 15 mm, 10 - 20 mm.

[0042] According to an embodiment of the present invention, the ratio of the thickness δ of the sintered Nd-Fe-B permanent magnet to the thickness of the sintered magnet substrate is greater than 1 and not greater than 1.2, such as 1.01 to 1.2, and for another example, 1.01, 1.05, 1.1, 1.15.

[0043] According to an embodiment of the present invention, the sintered Nd-Fe-B permanent magnet comprises the following components:

[0044] R 27 - 33 wt%, R is selected from Nd, and optionally at least one of the following elements: Pr, Sm, La, Ce;

[0045] B 0.9 to 1.3 wt%;

[0046] M is less than 5 wt%, and M is selected from one or more of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo;

[0047] RH 0.2 to 3.5 wt%; RH is a heavy rare earth element and is selected from at least one of Y, Ho, Gd, Dy, and Tb;

[0048] The balance is iron and unavoidable impurities.

[0049] According to an embodiment of the present invention, the sintered neodymium-iron-boron permanent magnet is obtained by subjecting the sintered magnet substrate to a diffusion treatment.

[0050] Preferably, before the diffusion treatment, the surface oxide layer of the sintered magnet substrate is optionally removed.

[0051] Preferably, the diffusion treatment includes arranging a diffusion source on the surface of the sintered magnet substrate.

[0052] Further, the diffusion source includes a rare earth element; the rare earth element is selected from at least one of Nd, Y, Pr, Ho, Gd, Dy, and Tb.

[0053] The present invention also provides a method for preparing the above-mentioned sintered neodymium-iron-boron permanent magnet, and the preparation method includes: obtaining the sintered neodymium-iron-boron permanent magnet by subjecting the sintered magnet substrate to a diffusion treatment.

[0054] According to an embodiment of the present invention, before the diffusion treatment, the surface oxide layer of the sintered magnet substrate is optionally removed, or it is further cut and processed into a required size along the length direction or the width direction.

[0055] According to an embodiment of the present invention, before the diffusion treatment, the sintered magnet substrate can be subjected to treatments such as degreasing and pickling known in the art.

[0056] According to an embodiment of the present invention, the diffusion treatment includes arranging a diffusion source on the surface of the sintered magnet substrate.

[0057] According to an embodiment of the present invention, the diffusion source includes a diffused rare earth element. Preferably, the diffused rare earth element is selected from at least one of Nd, Y, Pr, Ho, Gd, Dy, and Tb.

[0058] According to an embodiment of the present invention, the method for arranging the diffusion source can be at least one of thermal spraying, coating, sputtering, impregnation, etc.

[0059] According to an embodiment of the present invention, the dosage of the diffusion source can be selected from the dosages known in the art. Exemplarily, the dosage of the diffused rare earth element in the diffusion source is 0.1 to 10 wt% of the total mass of the sintered magnet matrix, for example, 1.0 wt%.

[0060] According to an embodiment of the present invention, the conditions for the diffusion treatment include: the diffusion temperature T5 is 850 to 950 °C, and the diffusion time H5 is not less than 20 h, for example, 30 h.

[0061] According to an embodiment of the present invention, after the diffusion treatment is completed, optionally, a part of the surface of the sintered Nd-Fe-B permanent magnet is processed. For example, the surface coated with the diffusion layer is ground and / or polished, and the thickness of the grinding and / or polishing is 10 to 30 μm, for example, 20 μm.

[0062] Through research, it is found that by using the manufacturing method of the present invention, especially for the sintered magnet matrix with a thickness greater than 10 mm obtained after multi-stage sintering treatment, rare earth elements are enriched on the surface of the matrix, so that the rare earth content in the first region of the matrix is greater than that in the fourth region. After removing the surface oxide layer of the sintered Nd-Fe-B matrix with a thickness greater than 10 mm, diffusion is directly carried out without reducing the thickness, because the rare earth content in the surface layer is higher, and it takes a longer time for the rare earth elements in the diffusion source to be enriched at the grain boundaries in the surface layer, thus prolonging the diffusion treatment time, and the rare earth elements in the diffusion source will not penetrate into the grains in the surface layer region due to too long diffusion treatment time. Therefore, the magnetic properties of both the surface layer region and the geometric center region can be ensured, and finally a large-size sintered Nd-Fe-B permanent magnet with high remanence and high coercivity is obtained, which can meet the requirements of the motor for high temperature resistance.

[0063] The present invention also provides the application of the above-mentioned sintered Nd-Fe-B permanent magnet in a motor.

[0064] The beneficial effects of the present invention:

[0065] The present invention provides a sintered magnet matrix, which can improve the coercivity of a permanent magnet with a diffusion direction size greater than 10 mm by using a small amount of heavy rare earth, reduce the cost, and meet the requirements of the motor for high temperature resistance at the same time.

[0066] Due to the characteristic that the surface rare earth content of the sintered magnet matrix of the present invention is greater than the rare earth content in the center, and after only removing the surface oxide layer in the diffusion direction with a size greater than 10 mm, diffusion can be directly carried out without segmentation. This can not only increase the time for the diffusion elements to be enriched at the surface grain boundaries, ensure the diffusion treatment time, but also prevent the heavy rare earth elements from penetrating into the grains in the surface layer region. Therefore, the magnetic properties of both the surface layer of the permanent magnet and the center are ensured, and the high magnetic properties of the whole product can be guaranteed, thus meeting the requirements of the motor for high temperature resistance. Brief Description of the Drawings

[0067] Figure 1 : Schematic diagram of the sintered Nd-Fe-B matrix before diffusion.

[0068] Figure 2 : Schematic diagram of the regional distribution from the surface to the inside of any surface after removing the surface oxide layer of the sintered Nd-Fe-B matrix before diffusion.

[0069] Figure 3 : Microstructural diagram of the rare earth distribution at the center of the magnet after diffusion in Example 1 and Comparative Example 1. Detailed Description of the Invention

[0070] The technical solution of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only for illustrative explanation 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.

[0071] 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.

[0072] Testing methods:

[0073] 1) Testing method for the surface coercivity Hcj1 of the magnet: Select any surface of the sintered Nd-Fe-B magnet along the diffusion direction, randomly take 5 sampling points in the surface layer area 0 - 1 mm away from this surface, measure with a TPM ultra-high coercivity permanent magnet measuring instrument, and calculate the average value of the coercivity of these 5 sampling points, denoted as Hcj1.

[0074] 2) Testing method for the coercivity Hcj2 at the geometric center of the magnet: Select any surface of the sintered Nd-Fe-B magnet along the diffusion direction, randomly take 5 sampling points in the magnet geometric center area (δ / 2 - 0.5 mm) - (δ / 2 + 0.5 mm) away from this surface, measure with a TPM ultra-high coercivity permanent magnet measuring instrument, and calculate the average value of the coercivity of these 5 sampling points, denoted as Hcj2.

[0075] 3) Testing method for the rare earth content W in Region 1, Region 2, Region 3, and Region 4 of the Nd-Fe-B magnet matrix: Use an X-ray fluorescence spectrometer (XRF) to randomly take 30 sampling points in Region 1, Region 2, Region 3, and Region 4 respectively, and calculate the average value of the rare earth content W of the 30 sampling points in different regions respectively, denoted as W1, W2, W3, and W4.

[0076] 4) Test method for oxygen content O in regions one, two, three, and four of the NdFeB magnet substrate: Random samples of the above regions are prepared. Five sampling points are taken and analyzed using a CSON oxygen and nitrogen analyzer respectively. The average value of the oxygen content of these five samples is calculated and denoted as O1, O2, O3, and O4.

[0077] Preparation Example 1

[0078] The green compact is prepared according to the following method:

[0079] Alloy sheets are made by high-frequency melting of at least 99% by weight pure NdPr, Co, Al, Fe, Cu, Ga, Ti, and ferroboron in an argon atmosphere and casting the melt onto a chill roll using the rapid solidification strip method. Among them, the contents of each raw material are 29.8 wt% NdPr, 1.5 wt% Co, 0.1 wt% Al, 0.2 wt% Cu, 0.15 wt% Ga, 0.18 wt% Ti, 1.0 wt% B, and the balance is iron and unavoidable impurities;

[0080] The above alloy sheets are subjected to hydrogen explosion treatment to obtain alloy micropowders;

[0081] The above alloy micropowders are ground by a jet mill to obtain jet mill powder with an average particle size D50 = 3.6 μm;

[0082] 0.3 wt% lubricant is added to the above jet mill powder, and the mixture is mixed for 3 h. Then, it is compacted under an orientation field with a magnetic field strength of 2 T to obtain a green compact for standby.

[0083] Examples 1 - 3 and Comparative Examples 1 - 5

[0084] The steps for preparing the sintered NdFeB magnet are as follows:

[0085] 1) Sintering treatment: The green compacts obtained in Preparation Example 1 are respectively subjected to sintering treatment in a vacuum sintering furnace. The sintering temperature T, holding time H, and heat treatment atmosphere vacuum degree P of different examples and comparative examples are carried out respectively according to the method in Table 1.

[0086] 2) Tempering treatment: After the sintering treatment is completed, the sintered green compact is cooled to 100 °C, then heated to 900 °C, and a primary tempering treatment is carried out at 900 °C for 3 h; then a secondary tempering treatment is carried out at 500 °C for 5 h; after cooling and taking out of the furnace, a sintered NdFeB substrate is obtained.

[0087] The size of the sintered NdFeB substrate is 46 mm in length, 30 mm in width, and 12 mm in thickness.

[0088] Table 1: Heat treatment processes of Examples 1 - 3 and Comparative Examples 1 - 5

[0089]

[0090] Take a sintered NdFeB substrate and sample in Region 1 (also denoted as Region One), Region 2 (also denoted as Region Two), Region 3 (also denoted as Region Three), and Region 4 (also denoted as Region Four) along the thickness direction. Use an oxygen and nitrogen analyzer to test the oxygen content in different regions. The test results are shown in Table 2.

[0091] Table 2: Oxygen Content in Different Regions of Examples 1-3 and Comparative Examples 1-5

[0092] <![CDATA[O1(ppm)]]> <![CDATA[O2(ppm)]]> <![CDATA[O3(ppm)]]> <![CDATA[O4(ppm)]]> Example 1 3500 650 638 630 Example 2 3320 666 629 616 Example 3 3400 655 633 639 Comparative Example 1 4400 1200 757 681 Comparative Example 2 3700 730 669 630 Comparative Example 3 3500 680 647 622 Comparative Example 4 3400 660 636 620 Comparative Example 5 3600 700 670 634

[0093] Take a sintered NdFeB substrate and take spot samples in Region 1, Region 2, Region 3, and Region 4 along the thickness direction. Use an X-ray fluorescence spectrometer to test the rare earth content in different regions. The test results are shown in Table 3.

[0094] Table 3: Rare Earth Content in Different Regions of Examples 1-3 and Comparative Examples 1-5

[0095]

[0096]

[0097] 3) After removing the surface oxide layer of the sintered magnet substrate obtained in the above step 2), process it into the dimensions required for motor assembly: length 22.9 mm, width 14.9 mm, and thickness 11.9 mm.

[0098] 4) Diffusion treatment: After degreasing and pickling the substrates with the surface oxide layer removed in the above different examples and comparative examples, perform diffusion treatment along the thickness direction. The content of Tb in the diffusion source is 1.0 wt% of the total mass of the substrate (the diffusion source is metal Tb powder with an average particle size of 3.5 μm). The diffusion temperature is 900 °C, and the diffusion time is 30 h. Among them, after the diffusion source is coated on the substrate surface, diffusion treatment is carried out, and finally, the diffused sintered NdFeB magnet is obtained.

[0099] Perform coercivity tests on the diffused sintered NdFeB magnets obtained in step 4). Refer to the above test methods 1) and 2) respectively, and test Hcj1 and Hcj2 of the diffused sintered NdFeB magnets in different examples and comparative examples, denoted as H1 and H2 respectively. The test results are shown in Table 3.

[0100] Table 3: Rare Earth Content in Different Regions of Examples 1-3 and Comparative Examples 1-5

[0101] <![CDATA[Hcj1(kA / m)]]> <![CDATA[Hcj2(kA / m)]]> <![CDATA[Hcj1-Hcj2 (kA / m)]]> <![CDATA[0.3*δ 2 +15*δ - 20]]> Example 1 2250 2071 177 201 Example 2 2230 2058 172 201 Example 3 2265 2082 183 201 Comparative Example 1 2247 1988 259 201 Comparative Example 2 2245 2002 243 201 Comparative Example 3 2243 2014 229 201 Comparative Example 4 2238 2020 218 201 Comparative Example 5 2245 2023 222 201

[0102] Combined with the above heat treatment process, the rare earth content at different positions of the pre-diffusion sintered NdFeB matrix, and the coercivity difference between the surface and the center of the matrix after diffusion, it can be seen that: for the sintered NdFeB matrix manufactured by the method of Example 1 according to the present invention, the rare earth difference between Region 1 and Region 2 is 0.3 wt%, and the rare earth difference between Region 2 and Region 3 is 0.4 wt%. The rare earth difference in different regions provides a better diffusion path for diffusion in the large-size direction, which not only inhibits the penetration of heavy rare earths into the interior of the main phase grains in the surface region of the magnet, but also ensures the diffusion of heavy rare earths to the geometric center region of the magnet, achieving the required coercivity difference in different regions, and ultimately ensuring that the product has a high coercivity.

[0103] In Comparative Example 1, the vacuum degree in the first sintering stage was poor, resulting in an increase in the oxygen content of the magnet matrix before diffusion, and some rare earth elements in the magnet matrix became rare earth oxides. Although there are differences in the rare earth content between different regions of the magnet matrix, the rare earth oxides in the magnet will seriously affect the diffusion of the heavy rare earth Tb in the diffusion source. Therefore, generally speaking, the coercivity of the geometric center region of the permanent magnet in Comparative Example 1 is low. Electron probe microanalysis was performed on the content of the rare earth element Tb in the geometric center region (5.45 - 6.45 mm) of the permanent magnets of Example 1 and Comparative Example 1 respectively, as Figure 3 shown. It can be clearly seen from the figure that after diffusion, the content of the heavy rare earth Tb in the geometric center region of the permanent magnet of Example 1 is greater than that in the geometric center region of the permanent magnet of Comparative Example 1. Therefore, the permanent magnet of Example 1 exhibits a higher coercivity.

[0104] Since Comparative Examples 2 to 5 did not meet the requirements of the present invention in the second and third sintering stages, the rare earth difference between different regions did not meet the requirements of the present invention, which is not conducive to the diffusion of the rare earth element Tb in the diffusion source, resulting in a large difference in coercivity between the surface and the center, and affecting the high-temperature resistance of the product.

[0105] The above describes the exemplary embodiments of the present invention. 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 in the protection scope of the present invention.

Claims

1. A sintered magnet substrate, characterized in that, The sintered magnet substrate has a polyhedral structure; The sintered magnet substrate includes four regions: Region 1 is the surface oxide layer, which refers to the region from the surface of the substrate to a distance of 100 μm from the surface of the substrate. The oxygen content in Region 1 is denoted as O1, and the rare earth content is denoted as W1; Region 2 is the high rare earth content layer, which refers to the region at a distance of 100 - 150 μm from the surface of the magnet. The oxygen content in Region 2 is denoted as O2, and the rare earth content is denoted as W2; Region 3 is the medium rare earth content layer, which refers to the region at a distance of 150 - 1000 μm from the surface of the magnet. The oxygen content in Region 3 is denoted as O3, and the rare earth content is denoted as W3; Region 4 is the low rare earth content layer, which refers to the region from a distance of 1000 μm from the surface of the magnet to the geometric center axis of the magnet. The oxygen content in Region 4 is denoted as O4, and the rare earth content is denoted as W4; Among them, the oxygen content and rare earth content in each region satisfy the following relational expressions: O1 / O2 ≥ 2; W1 > W2 > W3 > W4, W2 - W3 ≥ 0.15 wt%, W3 - W4 ≥ 0.25 wt%.

2. The sintered magnet substrate according to claim 1, wherein O1 ≥ 3000 ppm, O2 ≤ 1500 ppm; The thickness of the sintered magnet substrate is greater than 10 mm.

3. The sintered magnet substrate according to claim 1, characterized in that, The sintered magnet substrate includes the following components: R 27 - 33 wt%, R is selected from Nd, and at least one of the following elements: Pr, Sm, La, Ce; B 0.9 - 1.3 wt%; M < 5 wt%, M is selected from one or several of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo; RH 0 - 2 wt%; RH is a heavy rare earth element, selected from at least one of Y, Ho, Gd, Dy, Tb; The balance is iron and unavoidable impurities.

4. The method for preparing a sintered magnet substrate according to any one of claims 1 to 3, characterized in that, The preparation method includes: subjecting the green compact to multi-stage sintering treatment to obtain the sintered magnet substrate; among them, the multi-stage sintering treatment includes: (1) The first stage of sintering: the sintering temperature T1 is 900 to 1100 °C, the sintering time H1 is 120 to 300 min, and the vacuum degree P1 of the sintering furnace satisfies: 10 -1 Pa < P1 < 10 Pa; (2) In the second stage of sintering, after the first stage of sintering ends, evacuate to a vacuum degree of P2 ≤ 10 -1 Pa, the sintering temperature T2 is the same as T1, and the sintering time H2 is 30 - 60 min; (3) The third stage of sintering. After the second stage of sintering, inert gas is filled until the pressure in the sintering furnace satisfies 50 kPa ≤ P3 ≤ 150 kPa. The sintering temperature T3 is the same as T1, and the sintering time H3 is 10 - 30 min; (4) After the third stage of sintering, it is cooled to T40 ≤ 200 °C and then tempering treatment is carried out. The tempering treatment specifically includes: performing primary tempering treatment at a tempering temperature T41 of 700 - 900 °C, and the treatment time H41 is 3 - 7 h; further performing secondary tempering treatment at a tempering temperature T42 of 450 - 600 °C, and the treatment time H42 is 3 - 7 h.

5. The preparation method according to claim 4, characterized in that, The thickness of the green compact is greater than 10 mm; The ratio of the thickness of the green compact to the thickness of the sintered magnet substrate is greater than 1 and not greater than 1.

5.

6. A sintered Nd-Fe-B permanent magnet, characterized in that, The thickness δ of the sintered NdFeB permanent magnet ≥ 10 mm; along the thickness direction, the region from a distance of 0 - 1 mm from the surface of the sintered NdFeB permanent magnet is denoted as the surface layer region, and the coercivity in the surface layer region is Hcj1; the region from a distance of (δ / 2 - 0.5 mm) - (δ / 2 + 0.5 mm) from the surface of the sintered NdFeB permanent magnet is denoted as the geometric center region, and the coercivity in the geometric center region is Hcj2; Hcj1 and Hcj2 simultaneously satisfy the following relational expressions: ① Hcj1 > Hcj2, ② 160 ≤ Hcj1 - Hcj2 ≤ 0.3×δ 2 + 15×δ - 20。 7. The sintered Nd-Fe-B permanent magnet according to claim 6, characterized in that, The ratio of the thickness δ of the sintered NdFeB permanent magnet to the thickness of the sintered magnet substrate is greater than 1 and not greater than 1.2; and / or, the sintered NdFeB permanent magnet comprises the following components: R 27 - 33 wt%, R is selected from Nd, and optionally at least one of the following elements: Pr, Sm, La, Ce; B 0.9 - 1.3 wt%; M is less than 5 wt%, M is selected from one or more of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo; RH 0.2 - 3.5 wt%; RH is a heavy rare earth element, selected from at least one of Y, Ho, Gd, Dy, Tb; The balance is iron and unavoidable impurities; and / or, the sintered NdFeB permanent magnet is obtained by subjecting the sintered magnet substrate to a diffusion treatment; Optionally, the surface oxide layer of the sintered magnet substrate is removed before the diffusion treatment.

8. The method for preparing the sintered Nd-Fe-B permanent magnet according to claim 6 or 7, characterized in that, The preparation method includes: obtaining the sintered NdFeB permanent magnet by subjecting the sintered magnet substrate to a diffusion treatment; The diffusion treatment includes arranging a diffusion source on the surface of the sintered magnet substrate.

9. The preparation method of the sintered neodymium iron boron permanent magnet according to claim 8, wherein Optionally, the surface oxide layer of the sintered magnet substrate is removed, or it is further cut and processed into the required size along the length direction or the width direction; and / or, the diffusion source includes a diffused rare earth element; the diffused rare earth element is selected from at least one of Nd, Y, Pr, Ho, Gd, Dy, Tb; and / or, the method of arranging the diffusion source is at least one of thermal spraying, coating, sputtering, and dipping; and / or, the conditions of the diffusion treatment include: the diffusion temperature T5 is 850 - 950 °C, and the diffusion time H5 is not less than 20 h.

10. Application of the sintered magnet substrate according to any one of claims 1 - 3 or the sintered NdFeB permanent magnet according to claim 6 or 7 in an electric motor.

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