A method for preparing high coercive force and high thickness NdFeB magnet

Abstract

A method for preparing a high-coercivity and high-thickness NdFeB magnet belongs to the technical field of rare earth permanent magnet material preparation and solves the technical problem of preparing high-thickness and high-coercivity NdFeB magnets. The method comprises the following steps: first, sequentially subjecting the NdFeB magnets to rapid solidification strip spinning, hydrogen crushing, airflow milling, magnetic field orientation molding, cold isostatic pressing, and vacuum low-temperature sintering to obtain a thin low-temperature sintered green body; second, subjecting the low-temperature sintered green body to a grain boundary thermal diffusion treatment after preparing a heavy rare earth element layer on the surface of the low-temperature sintered green body; third, subjecting the low-temperature sintered green body after the diffusion heat treatment to mechanical coarse crushing, hydrogen crushing, powder mixing, airflow milling, magnetic field orientation molding, and cold isostatic pressing to obtain a thick, high-thickness compact; finally, subjecting the high-thickness compact to vacuum low-temperature heat treatment, vacuum high-temperature sintering, and two heat treatments to obtain a high-coercivity and high-thickness NdFeB magnet. The method has a simple preparation process, significantly improves the coercivity of bulk NdFeB sintered magnets, and enables the magnets to have a high magnetic energy product.

Description

Technical Field

[0001] The invention belongs to the technical field of rare earth permanent magnet material preparation, and particularly relates to a method for preparing a high-coercivity and high-thickness neodymium iron boron magnet. Background Art

[0002] Neodymium iron boron permanent magnets, characterized by high magnetic energy product, high remanence, and high coercivity, are widely used in new energy vehicles, energy-saving appliances, intelligent machines, wind turbines, and other fields. Because permanent magnet motors typically operate at temperatures above 150°C, the permanent magnetic properties of neodymium iron boron magnets are susceptible to degradation or even irreversible loss under high-temperature operating conditions. To improve the thermal stability of the magnets, it is necessary to increase their coercivity.

[0003] The addition of heavy rare earth elements (HREs) increases the coercivity of NdFeB magnets, but high HRE content reduces the magnet's remanence. Furthermore, HREs are scarce in the Earth's crust and are expensive. In recent years, HRE grain boundary diffusion (GBD) has gained widespread application. This technique diffuses HREs along grain boundaries into the magnet's interior, distributing them at the edges of the main phase grains while replacing some of the praseodymium and neodymium elements on the grain surfaces, effectively increasing the magnet's coercivity. However, conventional GBD techniques are only effective for thinner magnets with a thickness of less than 6 mm; current GBD techniques cannot be applied to magnets thicker than 6 mm.

[0004] In summary, how to prepare high-thickness, high-coercivity NdFeB magnets, especially how to prepare high-thickness, high-coercivity NdFeB magnets with complex special-shaped structures, is still a hot issue that needs to be solved urgently. Summary of the Invention

[0005] The main purpose of the present invention is to overcome the shortcomings of the existing technology and solve the technical problem of preparing high-thickness, high-coercivity NdFeB magnets. The present invention provides a method for preparing high-coercivity and high-thickness NdFeB magnets. While significantly improving the coercivity of large-block NdFeB sintered magnets, the magnets can have a high magnetic energy product. Even complex and irregular structures can be achieved in the second magnetic field orientation molding process by selecting a mold.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for preparing a high-coercivity and high-thickness NdFeB magnet comprises the following steps:

[0008] S1. Using vacuum rapid solidification belt spinning technology to prepare rapid solidification flakes, and then using hydrogen cracking process to prepare NdFeB alloy powder;

[0009] S2, the NdFeB alloy powder prepared in step S1 is subjected to a jet milling process to obtain a powder, wherein the average particle size of the powder is 2-3 μm;

[0010] S3, orienting the powder prepared in step S2 in a magnetic field and compacting it into a compact, and then subjecting it to cold isostatic pressing to obtain a compact with a thickness of 2 to 6 mm;

[0011] S4, vacuum low-temperature sintering the green compact prepared in step S3 at a sintering temperature of 900-980°C for 3-5 hours to obtain a low-temperature sintered green body; lowering the sintering temperature at this stage can reduce the density of the NdFeB magnet and increase the pores and grain boundary channels inside the sintered green body, which is more conducive to the thermal diffusion of the heavy rare earth diffusion source (heavy rare earth element, alloy, compound, and mixture containing heavy rare earth, etc.) in the NdFeB green body in the subsequent step S5;

[0012] S5, preparing a heavy rare earth element layer on the upper and lower surfaces of the low-temperature sintered green body prepared in step S4, respectively, with the thickness of the heavy rare earth element layer on each side being 1-10 μm;

[0013] S6, placing the low-temperature sintered green body containing the heavy rare earth element layer into a vacuum diffusion furnace for vacuum diffusion heat treatment, wherein the diffusion heat treatment temperature is 850-980° C. and the holding time is 3-20 hours;

[0014] S7, first, mechanically coarsely crushing the sintered green body after the diffusion heat treatment;

[0015] Secondly, the coarsely crushed material is subjected to a hydrogenation process to produce alloy powder. This secondary hydrogenation process not only produces powder materials with a finer average particle size, but more importantly, it hydrogenates the material during the secondary hydrogenation process, that is, it reduces the NdFeB material that was oxidized during the preparation process in steps S5 and S6, thereby improving the NdFeB material's oxidation resistance. The secondary hydrogenation process is very effective in improving the coercive force and magnetic energy product of the final magnet, especially for the heavy rare earth layer coated and screen-printed. The effect of this secondary hydrogenation process is particularly significant.

[0016] Next, lubricants, antioxidants and dispersants are added to the alloy powder after hydrogen cracking and mixed evenly to obtain a mixed material powder. For the subsequent pressing lubrication and anti-oxidation, the addition of lubricants and antioxidants is necessary, and the dispersant is to reduce powder agglomeration.

[0017] Finally, the mixed material is processed into powder material through air jet milling process, and the average particle size of the powder material is 3.2~6μm;

[0018] In this step S7, mechanical coarse crushing is combined with the secondary hydrogen crushing process, which not only makes the grain boundary diffusion magnet pulverized again, but also produces a new superposition effect. The secondary hydrogen crushing process is more conducive to the secondary airflow milling of fine powder, and eliminates the oxidation that may be introduced by the grain boundary diffusion process in step S6, which is ultimately beneficial to reducing the oxygen content of the magnet. The average particle size of the powder material obtained by the secondary airflow milling process is 3.2~6μm, which can not only effectively retain the effect of the grain boundary diffusion of heavy rare earth elements, so that the diffusion effect of heavy rare earth elements can be better continued, but also makes the heavy rare earth element deposits that may exist on the surface of the sintered green body after the initial diffusion dispersed and wrapped on the particle surface again, so that the heavy rare earth source can be fully utilized. On the basis of the above, the secondary airflow milling process also realizes the further refinement of the magnet particles, providing preparation for the subsequent sintering to prepare high-density magnets;

[0019] The average particle size of the powder material after the secondary airflow milling of the present invention is 3.2-6 μm, which refines the particles and prepares for the subsequent step S8 powder secondary molding and step S10 high-density sintering. The control of the average particle size of the powder material after the secondary airflow milling also takes into account the balance between the inheritance of the fine structure after grain boundary diffusion and the high-density sintering.

[0020] S8, placing the powder material prepared in step S7 into a magnetic field for orientation and compaction, and then subjecting it to cold isostatic pressing to obtain a high-thickness compact;

[0021] S9, placing the thick compact prepared in step S8 into a vacuum sintering furnace for vacuum low-temperature heat treatment at a temperature of 400-450°C for a holding time of 0.5-1 hour; in this step S9, the lubricant, antioxidant, and dispersant added in step S7 can be discharged from the magnet;

[0022] S10. The high-thickness compact after the vacuum low-temperature heat treatment in step S9 is vacuum-sintered at high temperature at a sintering temperature of 1020-1060°C for a holding time of 1.5-5 hours. After naturally cooling to 900°C, liquid argon is filled in for cooling to obtain a high-temperature sintered green body. The high-temperature sintering in step S10 further adjusts the distribution state of the heavy rare earth elements around the grain boundaries, and also causes the heavy rare earth elements diffused into the grain boundaries to enter the main phase lattice, which is conducive to further increase in coercivity. On this basis, on the one hand, since heavy rare earth elements further enter the lattice to form a new 2:14:1 phase replacing praseodymium and neodymium, the saturation magnetization intensity will be reduced, which will reduce the remanence of the magnet. On the other hand, high-temperature sintering is conducive to the formation of high-density sintered magnets, which can effectively reduce void defects and help increase the remanence of the magnet. Therefore, considering the above, the high-temperature sintering temperature should be neither too high nor too low, and needs to be specifically set to 1020~1060°C. More preferably, for cerium-containing magnets, sintering can be performed at 1020~1040°C, and for cerium-free magnets, sintering can be performed at 1030~1060°C.

[0023] S11, subjecting the high-temperature sintered green body obtained in step S10 to a vacuum primary heat treatment at a temperature of 800-920°C, keeping the temperature for 1.5-5 hours, and then filling with liquid argon for cooling;

[0024] S12, subjecting the high-temperature sintered green body after the vacuum primary heat treatment in step S11 to vacuum secondary heat treatment at a temperature of 450-650°C, keeping the temperature for 1.5-3 hours, then filling with liquid argon for cooling, and taking it out of the furnace to obtain a high coercive force and high thickness NdFeB magnet.

[0025] Furthermore, in step S1, the NdFeB alloy powder is a single alloy powder, or a mixed alloy powder consisting of a main alloy powder and a supplementary alloy powder.

[0026] Furthermore, in step S5, the heavy rare earth element layer is prepared by coating, immersion, sputtering, screen printing, evaporation or electroplating.

[0027] Furthermore, in step S5, the material of the heavy rare earth element layer is a heavy rare earth element, a heavy rare earth alloy, a heavy rare earth compound, or a mixture of heavy rare earth elements.

[0028] Furthermore, the heavy rare earth element is Dy, Tb or Ho.

[0029] Furthermore, in step S8, the thickness of the high-thickness green compact is determined according to the design size of the compacting mold and the pressing process.

[0030] The beneficial effects of the present invention are:

[0031] First, the green compact (thickness 2-6 mm) is obtained by rapid solidification belt spinning, hydrogen crushing, air flow milling (average particle size controlled at 2-3 microns), magnetic field orientation molding, and cold isostatic pressing. The green compact is then subjected to vacuum low-temperature sintering (900-980°C) to obtain a low-temperature sintered green body. Second, a heavy rare earth element layer is prepared on the upper and lower surfaces of the low-temperature sintered green body, followed by grain boundary thermal diffusion treatment (850-980°C). Third, the low-temperature sintered green body after grain boundary diffusion heat treatment is subjected to mechanical coarse crushing, hydrogen crushing, powder mixing (adding lubricants, antioxidants, etc.). The powder is then jet-milled to produce a powder material (average particle size of 3.2-6 μm). Finally, the jet-milled powder material is subjected to magnetic field orientation molding and cold isostatic pressing to produce a thick compact. The thick compact is then subjected to a vacuum low-temperature heat treatment (400-450°C), vacuum high-temperature sintering (1020-1060°C), vacuum primary heat treatment (800-920°C), and vacuum secondary heat treatment (450-650°C) to produce a high-coercivity and high-thickness NdFeB magnet. This method overcomes the size limitations of conventional grain boundary diffusion technology for NdFeB magnets, achieving the preparation of high-coercivity and high-thickness NdFeB magnets in the magnetization direction. The method is easy to operate, has significant advantages, is suitable for industrial production, and can achieve good economic benefits. DETAILED DESCRIPTION

[0032] The present invention is described in further detail below with reference to the embodiments. Example 1

[0033] A method for preparing a high-coercivity and high-thickness NdFeB magnet comprises the following steps:

[0034] S1. Weigh the raw materials according to the composition and proportion of the NdFeB magnet. The chemical formula of the NdFeB magnet in this embodiment 1 is (PrNd) 30 Cu 0.25 Co 0.25 Al 0.2 Ga 0.25 Zr 0.2 B 0.88 Fe 67.97 , using vacuum rapid solidification belt spinning technology to prepare rapid solidification flakes, and then using hydrogen cracking process to prepare NdFeB alloy powder;

[0035] S2, the NdFeB alloy powder prepared in step S1 is subjected to a jet milling process to obtain a powder having an average particle size of 2.8 μm;

[0036] S3, orienting the powder prepared in step S2 in a magnetic field and compacting it into a compact, and then subjecting it to cold isostatic pressing to obtain a compact with a thickness of 4 mm;

[0037] S4, vacuum sintering the green compact prepared in step S3 at a low temperature, with a sintering temperature of 900° C. and a holding time of 3 hours to obtain a low-temperature sintered green compact;

[0038] S5, using a sputtering process to prepare a heavy rare earth element Tb layer on the upper and lower surfaces of the low-temperature sintered green body prepared in step S4, wherein the thickness of the heavy rare earth element layer on each side is 3 μm;

[0039] S6, placing the low-temperature sintered green body containing the heavy rare earth element layer into a vacuum diffusion furnace for vacuum diffusion heat treatment at a temperature of 900° C. for 15 hours;

[0040] S7. First, the sintered green body after the diffusion heat treatment is mechanically coarsely crushed; second, the coarsely crushed material is subjected to a hydrogen crushing process to obtain alloy powder; third, 1 wt% lubricant (tributyl borate), 1.5 wt% antioxidant (a mixture of organic lipids and aviation gasoline), and 0.6 wt% dispersant (polyethylene glycol) are added to the hydrogen crushed alloy powder and mixed uniformly to obtain a mixed material powder; finally, the mixed material powder is subjected to a jet milling process to obtain a powder material, and the average particle size of the powder material is 3.3 μm;

[0041] S8, placing the powder material prepared in step S7 into the magnetic field again for orientation and compaction, determining the thickness of the high-thickness compact according to the design size of the compacting mold and the compacting process, and then subjecting it to cold isostatic pressing to obtain a high-thickness compact with a thickness of 35 mm;

[0042] S9, placing the thick compact prepared in step S8 into a vacuum sintering furnace for vacuum low-temperature heat treatment at a temperature of 400° C. for 1 hour;

[0043] S10, vacuum high-temperature sintering the thick green compact after the vacuum low-temperature heat treatment in step S9 at a sintering temperature of 1040°C for 3 hours, naturally cooling to 900°C, and then filling with liquid argon for cooling to obtain a high-temperature sintered green compact;

[0044] S11, subjecting the high-temperature sintered green body prepared in step S10 to a vacuum primary heat treatment at a temperature of 880°C, keeping the temperature for 2 hours, and then filling with liquid argon for cooling;

[0045] S12. The sintered green body after the primary vacuum heat treatment in step S11 is subjected to a secondary vacuum heat treatment at 560° C., kept at this temperature for 2 hours, and then filled with liquid argon for cooling. The green body is taken out of the furnace to obtain a high-coercivity and high-thickness NdFeB magnet. The room temperature properties of the high-coercivity and high-thickness NdFeB magnet prepared in Example 1 are: remanence Br=14.0 kGs, maximum magnetic energy product (BH)max=48 MGOe, and intrinsic coercivity Hcj=24.2 kOe. Example 2

[0046] A method for preparing a high coercive force and high thickness NdFeB magnet. The remaining steps and process parameters of Example 2 are the same as those of Example 1. The only difference is that the target material used in the sputtering process in step S5 is Dy 70 Cu 20 Al8Ga2, the room temperature performance of the high coercivity and high thickness NdFeB magnet prepared in Example 2 is: remanence Br = 13.98 kGs, maximum magnetic energy product (BH) max = 46.8 MGOe, and intrinsic coercivity Hcj = 19.8 kOe. Example 3

[0047] A method for preparing a high-coercivity and high-thickness NdFeB magnet comprises the following steps:

[0048] S1. Weigh the raw materials according to the composition and proportion of the NdFeB magnet. The chemical formula of the NdFeB magnet in Example 3 is:

[0049] (PrNd) 30 Cu 0.25 Co 0.25 Al 0.2 Ga 0.25 Zr 0.2 B 0.88 Fe 67.97 , using vacuum rapid solidification belt spinning technology to prepare rapid solidification flakes, and then using hydrogen cracking process to prepare NdFeB alloy powder;

[0050] S2, the NdFeB alloy powder prepared in step S1 is subjected to a jet milling process to obtain a powder having an average particle size of 3 μm;

[0051] S3, orienting the powder prepared in step S2 in a magnetic field and compacting it into a compact, and then subjecting it to cold isostatic pressing to obtain a compact with a thickness of 4 mm;

[0052] S4, vacuum sintering the green compact prepared in step S3 at a low temperature, with a sintering temperature of 950° C. and a holding time of 3 hours to obtain a low-temperature sintered green compact;

[0053] S5, applying a TbH2 ethanol-based solution with a thickness of 2 μm on the upper and lower surfaces of the low-temperature sintered green body prepared in step S4, and drying the solution to form a heavy rare earth element TbH2 layer on the upper and lower surfaces of the low-temperature sintered green body;

[0054] S6, placing the low-temperature sintered green body containing the heavy rare earth element layer into a vacuum diffusion furnace for vacuum diffusion heat treatment at a temperature of 900° C. for 15 hours;

[0055] S7. First, the sintered green body after the diffusion heat treatment is mechanically coarsely crushed; second, the coarsely crushed material is subjected to a hydrogen crushing process to obtain alloy powder; third, 1.5 wt% of a lubricant (tributyl borate), 2 wt% of an antioxidant (a mixture of organic lipids and aviation gasoline), and 0.8 wt% of a dispersant (polyethylene glycol) are added to the hydrogen crushed alloy powder and mixed uniformly to obtain a mixed material powder; finally, the mixed material powder is subjected to a jet milling process to obtain a powder material, and the average particle size of the powder material is 3.4 μm;

[0056] S8, placing the powder material prepared in step S7 into the magnetic field again for orientation and compaction, determining the thickness of the high-thickness compact according to the design size of the compacting mold and the compacting process, and then subjecting it to cold isostatic pressing to obtain a high-thickness compact with a thickness of 35 mm;

[0057] S9, placing the thick compact prepared in step S8 into a vacuum sintering furnace for vacuum low-temperature heat treatment at a temperature of 420° C. for 1 hour;

[0058] S10, vacuum high-temperature sintering the thick green compact after the vacuum low-temperature heat treatment in step S9 at a sintering temperature of 1040°C for 2 hours, naturally cooling to 900°C, and then filling with liquid argon for cooling to obtain a high-temperature sintered green compact;

[0059] S11, subjecting the high-temperature sintered green body prepared in step S10 to a vacuum primary heat treatment at a temperature of 900°C, keeping the temperature for 2 hours, and then filling the green body with liquid argon for cooling;

[0060] S12. The sintered green body after the primary vacuum heat treatment in step S11 is subjected to a secondary vacuum heat treatment at 600° C., kept at this temperature for 3 hours, and then filled with liquid argon for cooling. The green body is taken out of the furnace to obtain a high-coercivity and high-thickness NdFeB magnet. The room temperature properties of the high-coercivity and high-thickness NdFeB magnet prepared in Example 3 are as follows: remanence Br = 13.8 kGs, maximum magnetic energy product (BH) max = 47 MGOe, and intrinsic coercivity Hcj = 23.4 kOe. Example 4

[0061] A method for preparing a high-coercivity, high-thickness NdFeB magnet is disclosed. Example 4 shares the same remaining steps and process parameters as Example 3, differing only in step S5: immersing the sintered body prepared in step S4 in an ethanol-based suspension of DyH2+ nano-Cu, followed by drying to form a heavy rare earth element DyH2+ nano-Cu layer on the upper and lower surfaces of the sintered body. The room-temperature performance of the high-coercivity, high-thickness NdFeB magnet prepared in Example 4 is as follows: remanence Br = 13.5 kGs, maximum magnetic energy product (BH)max = 45 MGOe, and intrinsic coercivity Hcj = 17.9 kOe. Example 5

[0062] A method for preparing a high coercive force and high thickness NdFeB magnet. The remaining steps and process parameters of Example 5 are the same as those of Example 3, with the only difference being step S5: printing Tb by screen printing on the sintered green body prepared in step S4. 70 Cu 20 Ga 10 The room temperature performance of the high coercivity and high thickness NdFeB magnet prepared in Example 5 is as follows: remanence Br = 13.7 kGs, maximum magnetic energy product (BH) max = 46 MGOe, and intrinsic coercivity Hcj = 22 kOe. Comparative Example 1

[0063] Preparation of Neodymium Iron Boron Alloy (PrNd) by Vacuum Rapid Solidification Spinning Technology 30 Cu 0.25 Co 0.25 Al 0.2 Ga 0.25 Zr 0.2 B 0.88 Fe 67.97 The flakes were rapidly solidified and NdFeB alloy powders were prepared by hydrogenation. Powders were then prepared by jet milling to an average particle size of 2.8 μm. Appropriate amounts of 1 wt% lubricant (tributyl borate), 1.5 wt% antioxidant (a mixture of organic lipids and aviation gasoline), and 0.6 wt% dispersant (polyethylene glycol) were added to the powders and mixed for 2 hours. The powders were then compacted in a magnetic field and cold isostatically pressed to produce compacts 35 mm thick. The compacts were then vacuum sintered at 1070°C for 3 hours, heat treated at 880°C for 2 hours, and then cooled by argon gas at room temperature. The compacts were then vacuum heat treated at 560°C for 2 hours, and then cooled by argon gas at room temperature. The room temperature properties of the sintered NdFeB magnets obtained in Comparative Example 1 were: remanence Br = 14.2 kGs, maximum magnetic energy product (BH) max = 50 MGOe, and intrinsic coercivity Hcj = 14.6 kOe.

[0064] The initial NdFeB composition of Examples 1 to 5 and Comparative Example 1 is the same. Comparative Example 1 is a conventional sintered magnet. By comparing Examples 1 to 5 with Comparative Example 1, it can be found that the present invention can produce high-performance and large-thickness magnets. The remanence and magnetic energy product of the magnet of the present invention are slightly lower than those of the conventional sintered magnet, but the intrinsic coercive force of the magnet is significantly improved. Example 6

[0065] A method for preparing a high-coercivity and high-thickness NdFeB magnet comprises the following steps:

[0066] S1. Weigh the raw materials according to the composition and proportion of the NdFeB magnet. The chemical formula of the NdFeB magnet in Example 6 is (PrNd) 30.2Cu 0.22 Co 0.25 Al 0.2 Ga 0.1 B 0.9 Fe 68.13 , using vacuum rapid solidification belt spinning technology to prepare rapid solidification flakes, and then using hydrogen cracking process to prepare NdFeB alloy powder;

[0067] S2, the NdFeB alloy powder prepared in step S1 is subjected to a jet milling process to obtain a powder, wherein the average particle size of the powder is 3 μm;

[0068] S3, orienting the powder prepared in step S2 in a magnetic field and compacting it into a compact, and then subjecting it to cold isostatic pressing to obtain a compact with a thickness of 5 mm;

[0069] S4, vacuum sintering the green compact prepared in step S3 at a low temperature, with a sintering temperature of 980°C and a holding time of 5 hours to obtain a low-temperature sintered green compact;

[0070] S5, immersing the low-temperature sintered green body prepared in step S4 in an ethanol-based suspension solution of TbH2+ nano-Cu, and drying the green body to form a heavy rare earth element TbH2+ nano-Cu layer on the upper and lower surfaces of the green body, wherein the thickness of the single-sided heavy rare earth element TbH2+ nano-Cu layer is 3 μm;

[0071] S6, placing the low-temperature sintered green body containing the heavy rare earth element layer into a vacuum diffusion furnace for vacuum diffusion heat treatment at a temperature of 910° C. for 5.5 hours;

[0072] S7. First, the sintered green body after diffusion heat treatment is mechanically coarsely crushed; second, the coarsely crushed material is subjected to a hydrogen crushing process to obtain alloy powder; third, 1.5 wt% of a lubricant (tributyl borate), 1.5 wt% of an antioxidant (a mixture of organic lipids and aviation gasoline), and 0.5 wt% of a dispersant (polyethylene glycol) are added to the hydrogen crushed alloy powder and mixed uniformly to obtain a mixed material powder; finally, the mixed material powder is subjected to a jet milling process to obtain a powder material, and the average particle size of the powder material is 3.8 μm;

[0073] S8, placing the powder material prepared in step S7 into the magnetic field again for orientation and compaction, determining the thickness of the high-thickness compact according to the design size of the compacting mold and the compacting process, and then subjecting it to cold isostatic pressing to obtain a high-thickness compact with a thickness of 30 mm;

[0074] S9, placing the high-thickness compact prepared in step S8 into a vacuum sintering furnace for vacuum low-temperature heat treatment at a temperature of 460° C. for 1.5 hours;

[0075] S10, vacuum high-temperature sintering the thick green compact after the vacuum low-temperature heat treatment in step S9 at a sintering temperature of 1030° C. for 1 hour, naturally cooling to 900° C., and then filling with liquid argon for cooling to obtain a high-temperature sintered green compact;

[0076] S11, subjecting the high-temperature sintered green body prepared in step S10 to a vacuum primary heat treatment at a temperature of 850°C, keeping the temperature for 2 hours, and then filling the green body with liquid argon for cooling;

[0077] S12. The sintered green body after the primary vacuum heat treatment in step S11 is subjected to a secondary vacuum heat treatment at 620° C., kept at this temperature for 2 hours, and then filled with liquid argon for cooling. The green body is taken out of the furnace to obtain a high-coercivity and high-thickness NdFeB magnet. The room temperature properties of the high-coercivity and high-thickness NdFeB magnet prepared in Example 6 are: remanence Br = 13.5 kGs, maximum magnetic energy product (BH) max = 45.3 MGOe, and intrinsic coercivity Hcj = 19.1 kOe. Comparative Example 2

[0078] Preparation of Neodymium Iron Boron Alloy (PrNd) by Vacuum Rapid Solidification Spinning Technology 30.2 Cu 0.22 Co 0.25 Al 0.2 Ga 0.1 B 0.9 Fe 68.13 The flakes were rapidly solidified and prepared using a hydrogenation process to produce NdFeB alloy powder. Powder was then produced using a jet milling process to an average particle size of 3.2 μm. The powder was then mixed with 1 wt% lubricant (tributyl borate), 1.5 wt% antioxidant (a mixture of organic lipids and aviation gasoline), and 0.6 wt% dispersant (polyethylene glycol) for 2 hours. The powder was then compacted in a magnetic field and cold isostatically pressed to produce a 35 mm thick compact. The compact was then vacuum sintered at 1070°C for 1 hour, heat treated at 850°C for 2 hours, and then cooled with room temperature argon gas. The resulting sintered NdFeB magnets from Comparative Example 2 exhibited room temperature properties: remanence Br = 13.9 kGs, maximum magnetic energy product (BH) max = 47 MGOe, and intrinsic coercivity Hcj = 12.5 kOe.

[0079] Comparative Example 2 is a conventional sintered magnet. The NdFeB alloy composition of Example 6 and Comparative Example 2 is the same. By comparing the two, it can be found that the present invention can produce high-performance and large-thickness magnets. The remanence and magnetic energy product of the magnet of the present invention are slightly lower than those of the conventional sintered magnet, but the intrinsic coercive force of the magnet is significantly improved.

[0080] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for preparing a high coercive force and high thickness NdFeB magnet, characterized in that: The following steps are involved: S1. Using vacuum rapid solidification belt spinning technology to prepare rapid solidification flakes, and then using hydrogen cracking process to prepare NdFeB alloy powder; S2, the NdFeB alloy powder prepared in step S1 is subjected to a jet milling process to obtain a powder, wherein the average particle size of the powder is 2-3 μm; S3, orienting the powder prepared in step S2 in a magnetic field and compacting it into a compact, and then subjecting it to cold isostatic pressing to obtain a compact with a thickness of 2 to 6 mm; S4, vacuum sintering the green compact prepared in step S3 at a low temperature, with a sintering temperature of 900-980°C and a holding time of 3-5 hours to obtain a low-temperature sintered green compact; S5, preparing a heavy rare earth element layer on the upper and lower surfaces of the low-temperature sintered green body prepared in step S4, respectively, with the thickness of the heavy rare earth element layer on each side being 1-10 μm; S6, placing the low-temperature sintered green body containing the heavy rare earth element layer into a vacuum diffusion furnace for vacuum diffusion heat treatment, wherein the diffusion heat treatment temperature is 850-980° C. and the holding time is 3-20 hours; S7. First, the sintered green body after the diffusion heat treatment is mechanically coarsely crushed; second, the coarsely crushed material is subjected to a hydrogen crushing process to obtain alloy powder; third, a lubricant, an antioxidant, and a dispersant are added to the hydrogen-crushed alloy powder and mixed uniformly to obtain a mixed material powder; finally, the mixed material powder is subjected to a jet milling process to obtain a powder material, and the average particle size of the powder material is 3.2-6 μm; S8, placing the powder material prepared in step S7 into a magnetic field again for orientation and compaction, and then subjecting it to cold isostatic pressing to obtain a high-thickness compact; S9, placing the high-thickness compact prepared in step S8 into a vacuum sintering furnace for vacuum low-temperature heat treatment at a temperature of 400-450° C. for a holding time of 0.5-1 hour; S10, vacuum high-temperature sintering the thick green compact after the vacuum low-temperature heat treatment in step S9 at a sintering temperature of 1020-1060°C for 1.5-5 hours, naturally cooling to 900°C, and then filling with liquid argon for cooling to obtain a high-temperature sintered green compact; S11, subjecting the high-temperature sintered green body prepared in step S10 to a vacuum primary heat treatment at a temperature of 800-920°C, keeping the temperature for 1.5-5 hours, and then filling with liquid argon for cooling; S12, subjecting the high-temperature sintered green body after the vacuum primary heat treatment in step S11 to vacuum secondary heat treatment at a temperature of 450-650°C, keeping the temperature for 1.5-3 hours, then filling with liquid argon for cooling, and taking it out of the furnace to obtain a high coercive force and high thickness NdFeB magnet.

2. The method for preparing a high coercive force and high thickness NdFeB magnet according to claim 1, characterized in that: In the step S1, the NdFeB alloy powder is a single alloy powder, or a mixed alloy powder consisting of a main alloy powder and a supplementary alloy powder.

3. The method for preparing a high coercive force and high thickness NdFeB magnet according to claim 1, characterized in that: In step S5, the heavy rare earth element layer is prepared by coating, immersing, sputtering, screen printing, evaporation or electroplating.

4. The method for preparing a high coercive force and high thickness NdFeB magnet according to claim 1 or 3, characterized in that: In step S5 , the material of the heavy rare earth element layer is a heavy rare earth element, a heavy rare earth alloy, a heavy rare earth compound, or a mixture of heavy rare earth elements.

5. The method for preparing a high coercive force and high thickness NdFeB magnet according to claim 4, characterized in that: The heavy rare earth element is Dy, Tb or Ho.

6. The method for preparing a high coercive force and high thickness NdFeB magnet according to claim 1, characterized in that: In step S8, the thickness of the high-thickness green compact is determined according to the design size of the compacting mold and the pressing process.