Grain boundary diffusion method of sintered neodymium-iron-boron magnet

Abstract

The invention discloses a grain boundary diffusion method of a sintered neodymium-iron-boron magnet. The grain boundary diffusion method comprises the following steps: providing a magnet to be diffused; coating the surface of the magnet to be diffused with coating slurry containing heavy rare earth metal powder, and performing grain boundary diffusion after vacuum heat preservation and dehydrogenation; the vacuum heat preservation dehydrogenation process comprises the steps that under the vacuum condition, the temperature is increased to 220-250 DEG C from the normal temperature after 1-1.5 h, heat preservation is conducted for 2-3 h, then the temperature is increased to 520-560 DEG C after 1-2 h, and heat preservation is conducted for 1-2 h; the grain boundary diffusion process comprises the following steps: heating to 650-750 DEG C after 1-1.5 hours, and preserving heat for 24-30 hours for permeation; and finally, the temperature is increased to 970-1000 DEG C after 1-1.5 h, and heat preservation is conducted for 2-3 h. According to the grain boundary diffusion method, the coercive force of the center part of the sintered neodymium-iron-boron magnet with the high permeation thickness can be remarkably improved, and the intrinsic coercive force gradient difference of the surface layer and the center part of the magnet is reduced.

Description

Technical Field

[0001] The present application belongs to the technical field of rare earth permanent magnet material preparation, and specifically relates to a grain boundary diffusion method for sintering NdFeB magnets. Background Art

[0002] As a high-performance permanent magnet material, sintered NdFeB magnets require enhanced intrinsic coercivity (Hcj) to optimize their performance. The grain boundary diffusion process introduces heavy rare earth elements (such as Dy and Tb) onto the magnet surface, allowing them to penetrate into the magnet through high-temperature diffusion along the grain boundaries. This creates a shell structure with a high magnetocrystalline anisotropy field, effectively increasing coercivity while reducing heavy rare earth content.

[0003] The current mainstream grain boundary diffusion process primarily involves the following steps: first, the magnet surface is coated with a heavy rare earth metal or alloy coating; then, a long-term, high-temperature diffusion heat treatment is performed in a high vacuum environment. During this process, the heavy rare earth elements first enter the surface grain boundary phase and gradually penetrate into the interior of the magnet over time. However, as the magnet thickness increases, the difficulty of heavy rare earth elements penetrating into the center of the magnet increases, resulting in a limited increase in the magnet's coercivity. Furthermore, if the magnet substrate has a high carbon content, the diffusion effect is affected when the diffusion substrate thickness is large, resulting in a significant difference in the coercivity between the surface and center of the magnet. Although gradient temperature control can be used to increase the diffusion depth of heavy rare earth elements and thus enhance the magnet's intrinsic coercivity, when the magnet thickness exceeds 8mm, the amount of heavy rare earth elements penetrating into the center decreases, resulting in a smaller increase in the intrinsic coercivity of the magnet in the center. This results in a significant gradient difference in intrinsic coercivity between the surface and center of the magnet. Taking the 10mm thick magnet commonly used in mass production as an example, existing data show that the gradient difference in the intrinsic coercive force between the surface and the center of the magnet is as high as more than 3kOe.

[0004] Excessively large coercive force gradients can lead to demagnetization rates on the surface and center of the magnet at high temperatures, causing non-uniform demagnetization of the magnet due to temperature changes during use, and causing the risk of local demagnetization under dynamic working conditions, seriously affecting the stability and life of the magnet in application scenarios such as motors and sensors. Summary of the Invention

[0005] In view of this, the primary purpose of this application is to provide a grain boundary diffusion method for sintered NdFeB magnets, which significantly increases the coercive force in the central part of sintered NdFeB magnets with high permeability thickness (above 8 mm) and reduces the intrinsic coercive force gradient difference between the surface and central parts of the magnet.

[0006] In order to achieve the above objectives, the following technical solutions are adopted in this application:

[0007] One aspect of the present application provides a grain boundary diffusion method for sintering NdFeB magnets, comprising the following steps:

[0008] Providing a magnet to be diffused, wherein the magnet to be diffused is a sintered NdFeB magnet blank with a thickness of more than 8 mm and a carbon content of less than 500 ppm;

[0009] Coating a coating slurry containing heavy rare earth metal powder on the surface of the magnet to be diffused, performing vacuum heat preservation dehydrogenation, and then performing grain boundary diffusion;

[0010] The vacuum insulation dehydrogenation process is as follows: under vacuum conditions, heating from room temperature to 220°C to 250°C over 1 to 1.5 hours and then heating to 520°C to 560°C over 1 to 2 hours, and then heating to 1 to 2 hours;

[0011] The grain boundary diffusion process is as follows: heating to 650°C to 750°C over 1 to 1.5 hours, keeping the temperature for 24 to 30 hours for infiltration; and finally heating to 970°C to 1000°C over 1 to 1.5 hours, keeping the temperature for 2 to 3 hours.

[0012] Another aspect of the present application discloses a sintered NdFeB magnet obtained by the grain boundary diffusion method described above. The sintered NdFeB magnet has the characteristics of being thick (over 8 mm) and having a small difference in coercivity between the center and the surface (within 2 kOe).

[0013] Beneficial effects of this application:

[0014] The present application is to coat the surface of a sintered NdFeB magnet with a heavy rare earth element coating, and first heat it to a relatively low temperature of 650°C to 750°C under vacuum conditions and keep it warm for a long time, so that the heavy rare earth elements coated on the surface penetrate deeper into the magnet along the grain boundary phase; then heat it to a high temperature of 970°C to 1000°C and keep it warm, so as to promote the diffusion of heavy rare earth elements at the grain boundary to the surface of the grains, improve the penetration effect, and avoid the reduction of remanence caused by long-term high-temperature diffusion, effectively overcoming the current penetration diffusion process. The heavy rare earth is difficult to enter the center of the magnet, resulting in a large difference in the intrinsic coercive force gradient between the surface and the center of the magnet, and a significant reduction in remanence. At the same time, the low carbon content of the magnet facilitates the diffusion of heavy rare earth elements to the center, thereby increasing the coercive force of the center and reducing the intrinsic coercive force gradient difference between the surface and the center of the magnet. The grain boundary diffusion method in the present application increases the coercive force of the center of a thicker magnet and reduces the intrinsic coercive force gradient difference between the surface and the center of the magnet. DETAILED DESCRIPTION

[0015] The following will clearly and completely describe the embodiments of the present application. The technical solutions in the embodiments described below are exemplary and are only possible technical implementations of the present application, not all possible implementations. Those skilled in the art can fully combine the embodiments of the present application to obtain other embodiments without creative work, and these embodiments are also within the scope of protection of the present application.

[0016] This application discloses a grain boundary diffusion method for sintering NdFeB magnets, comprising the following steps:

[0017] Providing a magnet to be diffused, wherein the magnet to be diffused is a sintered NdFeB magnet blank with a thickness of more than 8 mm and a carbon content of less than 500 ppm;

[0018] The coating slurry containing heavy rare earth metal powder is coated on the surface of the magnet to be diffused, and after vacuum insulation dehydrogenation, grain boundary diffusion is carried out.

[0019] In the present application, a coating slurry containing heavy rare earth metal powder is coated on the surface of the magnet to be diffused, and then vacuum insulation dehydrogenation and grain boundary diffusion processes are performed to reduce the intrinsic coercive force gradient difference between the surface and the center of the magnet.

[0020] In this application, vacuum insulation dehydrogenation and grain boundary diffusion process are two key technical points, wherein the vacuum insulation dehydrogenation process is: under vacuum conditions, the temperature is raised from room temperature to 220℃~250℃ after 1~1.5 hours and kept warm for 2~3 hours, and then heated to 520℃~560℃ after 1~2 hours, and kept warm for 1~2 hours. Through vacuum insulation dehydrogenation, the hydrogen element in the medium and heavy rare earth powders is removed, so that the hydrogen content brought into the grain boundary phase during heavy rare earth infiltration is extremely low, thereby facilitating the diffusion of heavy rare earth metals in the grain boundary phase. In some examples, the vacuum condition refers to a high vacuum environment with a vacuum degree of less than 0.01Pa.

[0021] In this application, the grain boundary diffusion process is as follows: heating to 650℃~750℃ after 1~1.5 hours, keeping warm for 24~30 hours for infiltration; finally heating to 970℃~1000℃ after 1~1.5 hours, keeping warm for 2~3 hours. First, the temperature is kept at 650℃~750℃ for a long time. At this temperature, the grain boundary phase of the magnet partially melts into liquid, the diffusion channel becomes better, and the heavy rare earth elements coated on the surface enter deeper into the magnet surface along the grain boundary phase; then heating to a high temperature of 970℃~1000℃ for a short time to promote the diffusion of heavy rare earth elements at the grain boundary to the surface of the grain, thereby improving the infiltration effect. At high temperature, the heavy rare earth elements in the surface grain boundary phase can enter the deeper central part with the flowing liquid grain boundary phase, thereby improving the coercivity of the central part, effectively overcoming the problems of the current infiltration diffusion process in which heavy rare earths are difficult to enter the central part of the magnet, resulting in a large difference in the intrinsic coercivity gradient between the surface and the central part of the magnet, and a significant reduction in remanence. At the same time, the low carbon content of the magnet facilitates the diffusion of heavy rare earth elements to the center, which also increases the coercivity of the center. The grain boundary diffusion method in this application increases the coercivity of the center and reduces the intrinsic coercivity gradient difference between the surface and the center of the magnet.

[0022] In this application, the combination of the vacuum insulation dehydrogenation process and the grain boundary diffusion process is used to reduce the intrinsic coercive force gradient difference between the surface and the center of the magnet, thereby ensuring the stability and life of the magnet.

[0023] In this application, the magnet to be diffused refers to a sintered NdFeB magnet blank with a thickness of more than 8 mm, and the carbon content of the blank is less than 500 ppm; the sintered NdFeB magnet blank can be a rare earth permanent magnet material with neodymium (Nd), iron (Fe), and boron (B) as the main components, which is well known in the art, and its main phase is Nd2Fe 14 B tetragonal crystal structure. In the present application, the magnet to be diffused can be self-made or commercially available, such as magnet blanks with grades of 54M, 48H or N50, but is not limited thereto.

[0024] It is understandable that the sintered NdFeB magnet blank needs to be sliced ​​and cleaned before diffusion. Among them, slicing is to process the blank magnet into a suitable size and thickness through conventional mechanical processing methods in this field. Preferably, it is processed into a magnet blank with a thickness of more than 8mm; more preferably, it is processed into a magnet blank with a thickness of 8mm to 11mm; more preferably, it is processed into a magnet blank with a thickness of 10mm to 11mm. This is because when the thickness of the magnet is relatively thick (more than 8mm), the intrinsic coercive force gradient difference between the surface and the center of the magnet caused by grain boundary diffusion will be more significant, and only by adopting the preparation method of the present application can a more significant effect be obtained. The thickness of the magnet generally does not exceed 11mm. This is because if the thickness is too thick, the grain boundary diffusion process has no cost advantage compared to the conventional process, and the industry generally does not use the grain boundary diffusion process at this time.

[0025] Furthermore, the carbon content of the sintered NdFeB magnet blank is less than 500 ppm; a lower carbon content can promote the diffusion of heavy rare earth elements along the grain boundaries into deeper parts of the magnet, thereby increasing the diffusion depth of thick magnets, and further increasing the coercive force of the central part of the magnet, and reducing the coercive force gradient difference between the surface and the central part of the magnet.

[0026] Furthermore, the cleaning process described in this application refers to cleaning the surface of the magnet blank through a process of degreasing, pickling, and drying, thereby improving the penetration and diffusion of heavy rare earth metal elements. The degreasing process is generally performed using a conventional degreasing agent in the art, and the pickling process is performed using an acid solution well known in the art, such as, but not limited to, 2% to 3% by weight dilute nitric acid.

[0027] In the present application, the coating slurry includes heavy rare earth metal powder, a solvent, and a binder. In some examples, the coating slurry may further contain other functional additives as needed, such as a dispersant to improve the dispersion or uniformity of the heavy rare earth metal powder in the coating slurry. Those skilled in the art can add these additives as needed and have the ability to do so.

[0028] In some examples, the heavy rare earth metal powder is dysprosium hydride (DyH) or terbium hydride (TbH), preferably terbium hydride (TbH). Using hydrogenated heavy rare earth metal powder can protect the heavy rare earth metal from oxidation, thereby improving the diffusion effect of the heavy rare earth metal.

[0029] The heavy rare earth metal powder is obtained by crushing heavy rare earth metal (Dy or Tb) in a vacuum environment (vacuum degree less than 0.1 Pa) by absorbing hydrogen (hydrogen purity >99.999%), and then jet milling to produce a powder with a particle size of 2μm-5μm. Powders within this particle size range not only avoid oxidation but also cooperate with subsequent coating processes to enhance grain boundary diffusion.

[0030] In some examples, in the coating slurry, the weight ratio of the solvent to the heavy rare earth metal powder is 1:(5.5-6), and the weight ratio of the binder to the heavy rare earth metal powder is 1:(99-102).

[0031] In some examples, the solvent is terpineol, but is not limited thereto. The adhesive can be any commercial adhesive known in the art without particular limitation.

[0032] Furthermore, after the coating slurry is applied, the weight gain ratio of the magnet to be diffused can be further selected based on the required magnet performance, preferably 0.7% to 1.0%. If the weight gain is too low, the performance improvement of the magnet will be limited; while if the weight gain is too high, although more heavy rare earth elements enter the center of the magnet, which can improve the coercivity of the center of the magnet, the magnetic remanence of the magnet surface will be greatly reduced, and the cost will also increase. Therefore, in some examples, the preferred weight gain ratio in this application is 0.7% to 1.0%.

[0033] In this application, the coating method used is not particularly limited and can be conventional spraying, dipping, or screen printing in the art. In some examples, in this application, the coating method used is screen printing. The use of screen printing coating method has better consistency in the weight gain of the magnet, and thus better consistency in the magnetic properties of the magnet. The specific mesh size of the screen printing can be selected as needed. In some examples, the mesh size of the screen printing is preferably 120 to 180 mesh.

[0034] It is understandable that aging treatment is generally included after grain boundary diffusion. This is a conventional processing step in this field. It is generally carried out after grain boundary diffusion in order to ensure the performance of the magnet. There is no special limitation on aging treatment. Those skilled in the art can carry out it according to conventional methods in this field, or determine the optimal parameter conditions through experiments. In some examples, the aging treatment includes primary aging and secondary aging; specifically, the primary aging is: under the condition of vacuum degree less than 0.1Pa, the temperature is raised to 880℃~900℃ and kept warm for 2~3 hours, and then cooled; the secondary aging is: under the condition of vacuum degree less than 0.1Pa, the temperature is raised to 480℃~500℃ and kept warm for 4~6 hours, and then cooled.

[0035] Through the grain boundary diffusion method of the present application, the intrinsic coercive force gradient difference between the end surface and the center of the final magnet can be controlled within 2kOe, and the minimum is only 1.83kOe, which can significantly ensure the stability and life of the magnet and has important practical significance.

[0036] The following are specific embodiments of the present application. It should be noted that the following specific embodiments are only for illustrative purposes and do not limit the scope of the present application in any way.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0038] In addition, unless otherwise specified, methods without specific conditions or steps are conventional methods, and the reagents and materials used are all commercially available.

[0039] Example 1

[0040] This embodiment discloses a grain boundary diffusion method for sintering NdFeB magnets, and the specific steps are as follows:

[0041] (1) Select 54M brand blanks with a carbon content of 380ppm, cut into specifications of 40mm×30mm×10mm, degrease and clean, and then pickle with 2% dilute nitric acid.

[0042] (2) The heavy rare earth metal terbium Tb is crushed in a hydrogen crushing furnace at a vacuum of less than 0.08 Pa, filled with high-purity hydrogen to absorb hydrogen, crush, and cool to prepare terbium hydride powder (TbH); and the terbium hydride powder is ground into fine powder with a particle size of 3.8μm to 4.0μm by air flow milling.

[0043] (3) mixing the fine powder with the solvent and the adhesive in a ratio of solvent to terbium hydride = 1:5.8 (weight ratio) and adhesive to terbium hydride = 1:100 (weight ratio) to obtain a uniform mixed slurry; and printing and coating the mixed slurry on the surface of the magnet in step (1) using a 120-mesh screen, with a weight gain ratio of 0.8% to 0.9%.

[0044] (4) Grain boundary diffusion: The coated magnet in step (3) is placed in a high vacuum sintering furnace, and the temperature is raised from room temperature to 230°C after 1.2 hours and kept at this temperature for 2.5 hours; then the temperature is raised to 540°C after 1.5 hours and kept at this temperature for 1.5 hours; then the temperature is raised to 650°C after 1 hour and kept at this temperature for 30 hours; finally, the temperature is raised to 1000°C after 1.5 hours and kept at this temperature for 3 hours; and then the temperature is cooled to below 70°C by filling with argon gas.

[0045] (5) Primary aging: After the diffusion is completed, cool to below 50°C, stop cooling, evacuate, and start heating when the vacuum degree is below 0.095 Pa. After 2.5 hours, heat to 900°C and keep warm for 2 hours, then fill with argon and cool.

[0046] (6) Secondary aging: After air cooling, heat up, evacuate, and start heating when the vacuum degree is below 0.1 Pa. After 1.5 hours, heat up to 480℃ and keep warm for 5 hours, then fill with argon and cool again until it cools to below 70℃. Take it out of the furnace to obtain a magnet with good penetration and diffusion.

[0047] Comparative Example 1-1

[0048] This comparative example discloses a grain boundary diffusion method for sintering NdFeB magnets, which is carried out with reference to the implementation method in Example 1, except that the dehydrogenation treatment at 540° C. for 1.5 hours is not performed.

[0049] The specific steps are as follows:

[0050] (1) to (3) are the same as in Example 1.

[0051] (4) Grain boundary diffusion: Place the coated magnet in step (3) into a high vacuum sintering furnace, evacuate the vacuum to below 0.09 Pa, and then heat it from room temperature to 230°C over 1.2 hours and keep it at that temperature for 2.5 hours; then heat it to 650°C over 2.5 hours and keep it at that temperature for 30 hours; finally heat it to 1000°C over 1.5 hours and keep it at that temperature for 3 hours; then fill it with argon and cool it to below 70°C.

[0052] (5) to (6) are the same as in Example 1.

[0053] Comparative Example 1-2

[0054] This comparative example discloses a grain boundary diffusion method for sintering NdFeB magnets, which is carried out with reference to the implementation method in Example 1, except that the heat preservation at 650° C. for 30 hours is not performed.

[0055] The specific steps are as follows:

[0056] (1) to (3) are the same as in Example 1.

[0057] (4) Grain boundary diffusion: Place the coated magnet prepared in step (3) into a high vacuum sintering furnace, evacuate the vacuum to below 0.09 Pa, and then heat it from room temperature to 230°C over 1.2 hours and keep it at that temperature for 2.5 hours; then heat it to 540°C over 1.5 hours and keep it at that temperature for 2 hours; then heat it to 1000°C over 2.5 hours and keep it at that temperature for 33 hours; then fill it with argon and cool it to below 70°C.

[0058] (5) to (6) are the same as in Example 1.

[0059] Performance Test 1

[0060] After infiltration, the 40mm×30mm×10mm magnet was peeled 4mm on both sides along the 10mm direction to obtain a 40mm×30mm×4mm surface end magnet and a 40mm×30mm×2mm center magnet; the surface end magnet was processed into a 10mm×10mm×4mm sample, and the center magnet was processed into a 10mm×10mm×2mm sample. The magnetic properties at room temperature (23°C) were tested according to the test standard "Guide to Pulse Measurement Methods of Permanent Magnets (Hard Magnets)" GB / T 29628-2013. Each sample was tested three times and the average value was taken. The test results are shown in Table 1.

[0061] Table 1 Magnetic property test results of Example 1 and Comparative Example 1-1 and Comparative Example 1-2

[0062]

[0063] The test results in Table 1 show that the intrinsic coercivity Hcj gradient difference between the magnet ends and the center in Example 1 is 1.87 kOe; while it is 2.18 kOe in Comparative Example 1-1 and 2.66 kOe in Comparative Example 1-2. This indicates that the grain boundary diffusion process described in this application can significantly increase the diffusion of heavy rare earth elements in the center of the magnet, reduce the intrinsic coercivity gradient difference between the magnet ends and the center, and improve magnet performance.

[0064] Example 2

[0065] This embodiment discloses a grain boundary diffusion method for sintering NdFeB magnets, and the specific steps are as follows:

[0066] (1) Select 48H brand blanks with a carbon content of 450ppm, cut into specifications of 40mm×30mm×11mm, degrease and clean, and then pickle with 3% dilute nitric acid.

[0067] (2) The heavy rare earth metal terbium Tb is crushed in a hydrogen crushing furnace at a vacuum of less than 0.09 Pa, filled with high-purity hydrogen to absorb hydrogen, crush, and cool to prepare terbium hydride powder (TbH); and the terbium hydride powder is ground into fine powder with a particle size of 4.5μm to 5.0μm by air flow milling.

[0068] (3) mixing the fine powder with the solvent and the adhesive in a ratio of solvent: terbium hydride = 1:5.5 (weight ratio) and adhesive: terbium hydride = 1:99 (weight ratio) to obtain a uniform mixed slurry; and printing and coating the mixed slurry on the surface of the magnet in step (1) using a 180-mesh screen, with a weight gain ratio of 0.9% to 1.0%.

[0069] (4) Grain boundary diffusion: The coated magnet in step (3) is placed in a high vacuum sintering furnace, and the vacuum is evacuated to below 0.08 Pa and the temperature is raised from room temperature to 250°C over 1.5 hours and kept at this temperature for 3 hours; then the temperature is raised to 560°C over 2 hours and kept at this temperature for 2 hours; then the temperature is raised to 750°C over 1 hour and kept at this temperature for 30 hours; finally, the temperature is raised to 1000°C over 1.5 hours and kept at this temperature for 3 hours; and then the temperature is cooled to below 70°C by filling with argon gas.

[0070] (5) Primary aging: After the infiltration and diffusion is completed, cool to below 50°C, stop cooling, evacuate, and start heating when the vacuum degree is below 0.095 Pa. After 2.5 hours, heat to 890°C and keep warm for 2 hours, then fill with argon and cool.

[0071] (6) Secondary aging: After air cooling, heat up, evacuate, and start heating when the vacuum degree is below 0.1 Pa. After 1.5 hours, heat up to 500℃ and keep it at this temperature for 5 hours, then fill it with argon and cool it down to below 70℃. Then take it out of the furnace to obtain a magnet with good penetration and diffusion.

[0072] Comparative Example 2-1

[0073] This comparative example discloses a grain boundary diffusion method for sintering NdFeB magnets, which is carried out with reference to the implementation method in Example 2, except that the dehydrogenation treatment at 560° C. for 2 hours is not performed.

[0074] The specific steps are as follows:

[0075] (1) to (3) are the same as in Example 2.

[0076] (4) Grain boundary diffusion: Place the coated magnet in step (3) into a high vacuum sintering furnace, evacuate the vacuum to below 0.08 Pa, and then heat it from room temperature to 250°C over 1.5 hours and keep it at that temperature for 3 hours; then heat it to 750°C over 3 hours and keep it at that temperature for 30 hours; finally heat it to 1000°C over 1.5 hours and keep it at that temperature for 3 hours; then fill it with argon and cool it to below 70°C.

[0077] (5) to (6) are the same as in Example 2.

[0078] Comparative Example 2-2

[0079] This comparative example discloses a grain boundary diffusion method for sintering NdFeB magnets, which is carried out with reference to the implementation method in Example 2, except that the heat preservation at 750° C. for 30 hours is not performed.

[0080] The specific steps are as follows:

[0081] (1) to (3) are the same as in Example 2.

[0082] (4) Grain boundary diffusion: Place the coated magnet in step (3) into a high vacuum sintering furnace, evacuate the vacuum to below 0.08 Pa, and then heat it from room temperature to 250°C over 1.5 hours and keep it at that temperature for 3 hours; then heat it to 560°C over 2 hours and keep it at that temperature for 5 hours; then heat it to 1000°C over 2.5 hours and keep it at that temperature for 33 hours; then fill it with argon and cool it to below 70°C.

[0083] (5) to (6) are the same as in Example 2.

[0084] Performance Test 2

[0085] After infiltration, the 40mm×30mm×11mm magnet was peeled 4.5mm on both sides along the 10mm direction to obtain a 40mm×30mm×4.5mm surface end magnet and a 40mm×30mm×2mm center magnet. The surface end magnet was processed into a 10mm×10mm×4.5mm sample, and the center magnet was processed into a 10mm×10mm×2mm sample. The magnetic properties at room temperature (23°C) were tested according to the test standard "Guide to Pulse Measurement Methods of Permanent Magnets (Hard Magnets)" GB / T 29628-2013. Each sample was tested three times and the average value was taken. The test results are shown in Table 2.

[0086] Table 2 Magnetic property test results of Example 2 and Comparative Example 2-1 and Comparative Example 2-2

[0087]

[0088]

[0089] The test results in Table 2 show that the intrinsic coercivity Hcj gradient difference between the magnet ends and the center is 1.94 kOe in Example 2, 2.35 kOe in Comparative Example 2-1, and 2.68 kOe in Comparative Example 2-2. This indicates that the grain boundary diffusion process described in this application can significantly increase the diffusion of heavy rare earth elements in the center of the magnet, reduce the intrinsic coercivity gradient difference between the magnet ends and the center, and improve magnet performance.

[0090] Example 3

[0091] This embodiment discloses a grain boundary diffusion method for sintering NdFeB magnets, and the specific steps are as follows:

[0092] (1) Select N50 brand blank with a carbon content of 270ppm, cut into specifications of 40mm×30mm×8mm, degrease and clean, and then pickle with 3% dilute nitric acid.

[0093] (2) The heavy rare earth metal terbium Tb is crushed in a hydrogen crushing furnace at a vacuum degree of less than 0.09 Pa, filled with high-purity hydrogen to absorb hydrogen, crush, and cool to prepare terbium hydride powder (TbH); and the terbium hydride powder is ground into fine powder with a particle size of 2.0μm to 2.4μm by air flow milling.

[0094] (3) mixing the fine powder with the solvent and the adhesive in a ratio of solvent: terbium hydride = 1:6 (weight ratio) and adhesive: terbium hydride = 1:101 (weight ratio) to obtain a uniform mixed slurry; and printing and coating the mixed slurry on the surface of the magnet in step (1) using a 180-mesh screen, with a weight gain ratio of 0.7% to 0.8%.

[0095] (4) Grain boundary diffusion: The coated magnet in step (3) is placed in a high vacuum sintering furnace, and the temperature is raised from room temperature to 220°C after 1 hour and kept at this temperature for 2 hours; then the temperature is raised to 520°C after 2 hours and kept at this temperature for 3 hours; then the temperature is raised to 650°C after 1.5 hours and kept at this temperature for 24 hours; finally the temperature is raised to 970°C after 1.5 hours and kept at this temperature for 3 hours; and then the temperature is cooled to below 70°C by filling with argon gas.

[0096] (5) Primary aging: After the infiltration and diffusion is completed, cool to below 50°C, stop cooling, evacuate, and start heating when the vacuum degree is below 0.095 Pa. After 2.5 hours, heat to 890°C and keep warm for 3 hours, then fill with argon and cool.

[0097] (6) Secondary aging: After air cooling, heat up, evacuate, and start heating when the vacuum degree is below 0.1 Pa. After 1.5 hours, heat up to 500℃ and keep it at this temperature for 5 hours, then fill it with argon and cool it down to below 70℃. Then take it out of the furnace to obtain a magnet with good penetration and diffusion.

[0098] Comparative Example 3-1

[0099] This comparative example discloses a grain boundary diffusion method for sintering NdFeB magnets, which is carried out with reference to the implementation method in Example 3, except that the dehydrogenation treatment at 520° C. for 3 hours is not performed.

[0100] The specific steps are as follows:

[0101] (1) to (3) are the same as in Example 3.

[0102] (4) Grain boundary diffusion: Place the coated magnet prepared in step (3) into a high vacuum sintering furnace, evacuate the vacuum to below 0.09 Pa, and then heat it from room temperature to 220°C over 1 hour and keep it at that temperature for 2 hours; then heat it to 650°C over 3.5 hours and keep it at that temperature for 24 hours; finally heat it to 970°C over 1.5 hours and keep it at that temperature for 4 hours; then fill it with argon and cool it to below 70°C.

[0103] (5) to (6) are the same as in Example 3.

[0104] Comparative Example 3-2

[0105] This comparative example discloses a grain boundary diffusion method for sintering NdFeB magnets, which is carried out with reference to the implementation method in Example 3, except that the 24-hour heat preservation at 650° C. is not performed.

[0106] The specific steps are as follows:

[0107] (1) to (3) are the same as in Example 3.

[0108] (4) Grain boundary diffusion: Place the coated magnet prepared in step (3) into a high vacuum sintering furnace, evacuate the vacuum to below 0.09 Pa, and then heat it from room temperature to 220°C over 1 hour and keep it at that temperature for 2 hours; then heat it to 520°C over 2 hours and keep it at that temperature for 4 hours; then heat it to 970°C over 3 hours and keep it at that temperature for 27 hours; then fill it with argon and cool it to below 70°C.

[0109] (5) to (6) are the same as in Example 3.

[0110] Performance Test 3

[0111] After infiltration, the 40mm×30mm×8mm magnet was peeled 3mm on both sides along the 8mm direction to obtain a 40mm×30mm×3mm surface end magnet and a 40mm×30mm×2mm center magnet; the surface end magnet was processed into a 10mm×10mm×3mm sample, and the center magnet was processed into a 10mm×10mm×2mm sample. The magnetic properties at room temperature (23°C) were tested according to the test standard "Guide to Pulse Measurement Methods of Permanent Magnets (Hard Magnets)" GB / T 29628-2013. Each sample was tested three times and the average value was taken. The test results are shown in Table 3.

[0112] Table 3 Magnetic property test results of Example 3 and Comparative Example 3-1 and Comparative Example 3-2

[0113]

[0114]

[0115] The test results in Table 3 show that the intrinsic coercivity Hcj gradient difference between the magnet ends and the center is 1.83 kOe in Example 3, 2.37 kOe in Comparative Example 3-1, and 2.62 kOe in Comparative Example 3-2. This indicates that the grain boundary diffusion process described in this application can significantly increase the diffusion of heavy rare earth elements in the center of the magnet, reduce the intrinsic coercivity gradient difference between the magnet ends and the center, and improve magnet performance.

[0116] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A grain boundary diffusion method for sintering NdFeB magnets, characterized in that: The following steps are involved: Providing a magnet to be diffused, wherein the magnet to be diffused is a sintered NdFeB magnet blank with a thickness of more than 8 mm and a carbon content of less than 500 ppm; Coating a coating slurry containing heavy rare earth metal powder on the surface of the magnet to be diffused, performing vacuum heat preservation dehydrogenation, and then performing grain boundary diffusion; The vacuum insulation dehydrogenation process is as follows: under vacuum conditions, heating from room temperature to 220°C to 250°C over 1 to 1.5 hours and then heating to 520°C to 560°C over 1 to 2 hours, and then heating to 1 to 2 hours; The grain boundary diffusion process is as follows: heating to 650°C to 750°C over 1 to 1.5 hours, keeping the temperature for 24 to 30 hours for infiltration; and finally heating to 970°C to 1000°C over 1 to 1.5 hours, keeping the temperature for 2 to 3 hours.

2. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1, wherein: The thickness of the magnet to be diffused is 8 mm to 11 mm; Preferably, the thickness of the magnet to be diffused is 10 mm to 11 mm.

3. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1, wherein: The coating slurry includes heavy rare earth metal powder, solvent and adhesive; Preferably, the heavy rare earth metal powder is dysprosium hydride or terbium hydride; Preferably, the heavy rare earth metal powder is terbium hydride; Preferably, in the coating slurry, the weight ratio of the solvent to the heavy rare earth metal powder is 1:(5.5-6), and the weight ratio of the adhesive to the heavy rare earth metal powder is 1:(99-102).

4. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1 or 3, characterized in that: The particle size of the heavy rare earth metal powder is 2 μm to 5 μm.

5. The grain boundary diffusion method for sintering NdFeB magnets according to claim 3, wherein: The solvent is terpineol.

6. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1, wherein: After the coating slurry is applied, the weight increase ratio of the magnet to be diffused is 0.7% to 1.0%.

7. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1, wherein: The coating method is screen printing; Preferably, the mesh size of the screen printing is 120 to 180 meshes.

8. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1, wherein: The vacuum degree of the vacuum condition is lower than 0.01 Pa.

9. The grain boundary diffusion method for sintering NdFeB magnets according to claim 1, wherein: After the grain boundary diffusion, an aging treatment is also included; Preferably, the aging treatment includes primary aging and secondary aging; Preferably, the primary aging is as follows: under a vacuum degree of less than 0.1 Pa, the temperature is raised to 880°C to 900°C and kept warm for 2 to 3 hours, and then cooled; the secondary aging is as follows: under a vacuum degree of less than 0.1 Pa, the temperature is raised to 480°C to 500°C and kept warm for 4 to 6 hours, and then cooled.

10. A sintered NdFeB magnet, characterized in that: It is prepared by the grain boundary diffusion method according to any one of claims 1 to 9. The thickness of the sintered NdFeB magnet is more than 8 mm, and the intrinsic coercive force gradient difference between the end surface and the center is within 2 kOe.

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