A method for preparing permanent magnets by low-temperature orientation molding and hot pressing densification of omnidirectional NdFeB magnetic powder

Abstract

This invention discloses a method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder, relating to the field of permanent magnet materials technology. The invention includes the following steps: Fe, B-Fe alloy, and Co are melted, and Nd, Ga, Al, and Cu are added and melted. After casting, the mixture is vacuum annealed, crushed, and treated in a hydrogen atmosphere at 300-350℃, followed by vacuum treatment at 350℃, then treatment in a hydrogen atmosphere at 800-820℃ and 80kPa, and then vacuum treatment in a constant axial static magnetic field at 800℃, 0.08-0.1Pa, and 2.2-2.5T. After discharge, the material is pretreated with a dilute nitric acid aqueous solution, then modified with a modified suspension and modified nanosheet dispersion, and finally subjected to warm pressing, vacuum degreasing, and hot pressing to obtain an omnidirectional NdFeB permanent magnet with excellent remanence, intrinsic coercivity, maximum energy product, and squareness.

Description

Technical Field

[0001] This invention relates to the field of permanent magnet materials technology, specifically to a method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder. Background Technology

[0002] Omnidirectional neodymium iron boron permanent magnets, with their advantage of uniform magnetic properties in three-dimensional space, have broken through the application limitations of traditional anisotropic neodymium iron boron magnets, which can only achieve high magnetic properties along a single orientation. They have broad application prospects in fields such as new energy vehicles, high-end intelligent manufacturing, and consumer electronics, and are one of the core research and development directions in the field of high-performance rare earth permanent magnets.

[0003] However, in practical industrial applications, omnidirectional NdFeB permanent magnets still have the following core performance shortcomings: First, the control of grain orientation homogenization is difficult, easily leading to insufficient orientation degree and high grain orientation dispersion, resulting in magnet remanence far below the theoretical limit, failing to fully utilize the intrinsic magnetic properties of the NdFeB main phase; Second, the precision of grain boundary phase structure and distribution control is insufficient, easily leading to grain boundary phase segregation, depletion, or continuous grain boundary absence, making it difficult to effectively suppress the nucleation and expansion of antimagnetization domains, resulting in intrinsic coercivity. The remanence, intrinsic coercivity, maximum energy product, and squareness of existing omnidirectional NdFeB permanent magnets still need improvement. Firstly, the magnet's density and microstructure are not well controlled, leading to defects such as porosity, inclusions, and abnormal grain growth during fabrication. This weakens the effective magnetization volume, resulting in a significant gap between the maximum energy product and the theoretical value, making it difficult to achieve a compatible match between high remanence and high coercivity. Secondly, the poor consistency of the magnet's microstructure, with its wide grain size distribution and localized orientation disorder, results in poor squareness of the demagnetization curve. Under dynamic loads and high reverse magnetic fields, irreversible magnetic losses increase significantly, leading to insufficient device stability. Therefore, the remanence, intrinsic coercivity, maximum energy product, and squareness of existing omnidirectional NdFeB permanent magnets still need improvement. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder, thereby solving the following technical problems: Existing omnidirectional NdFeB permanent magnets still suffer from problems such as remanence, intrinsic coercivity, maximum energy product, and poor squareness.

[0005] The objective of this invention can be achieved through the following technical solutions: A method for preparing permanent magnets by low-temperature orientation molding and hot pressing densification of omnidirectional NdFeB magnetic powder includes the following steps: S1: Fe, B-Fe alloy, and Co are melted in a high-purity argon atmosphere at 1450-1480℃, and then Nd, Ga, Al, and Cu are added and stirred at the same temperature for 20-30 minutes. After removing the slag, the mixture is cast into a copper mold at 300-400℃. Then, it is vacuum annealed at 1050-1100℃ for 12-16 hours and cooled to obtain an alloy ingot. S2: After crushing the alloy ingot, it is treated in a high-purity hydrogen atmosphere at 300-350℃ for 2-3 hours, then vacuumed and treated at 350℃ for 2 hours. After cooling, it is sieved to obtain hydrogen-exploded coarse powder with a particle size of 100-300μm. S3: The hydrogen-exploded coarse powder is treated in a high-purity hydrogen atmosphere at 800-820℃ and 80kPa for 120-180min. Then, a constant axial static magnetic field of 2.2-2.5T is applied at 800℃, while the vacuum is evacuated to 0.08-0.1Pa and held for 30-60min. The magnetic field is then maintained and high-purity argon is introduced. Finally, the powder is cooled, the magnetic field is removed, and the powder is discharged to obtain uniaxial anisotropic NdFeB magnetic powder. S4: Add uniaxial anisotropic NdFeB magnetic powder to a dilute nitric acid aqueous solution and stir in a sealed environment for 30-60 seconds. Then filter, wash, and dry to obtain pretreated magnetic powder. S5: Add pretreated magnetic powder to anhydrous n-hexane, stir, then drop in modified suspension and stir at 50°C for 60-70 min. Add anhydrous ethanol and stir for 15 min. Then drop in modified nanosheet dispersion and stir at 50°C for 3-4 h. Finally, filter, wash and dry to obtain modified magnetic powder. S6: Add epoxy resin E-51 and silane coupling agent KH560 to ethyl acetate and stir well. Then spray it into modified magnetic powder and stir under vacuum at 45°C for 40-60 min. Then perform warm pressing, vacuum degreasing and hot pressing to obtain an omnidirectional neodymium iron boron permanent magnet.

[0006] Preferably, the alloy ingot described in S1 is composed of the following atomic percentages: Nd 11.8-12.5 at%, B 5.8-6.0 at%, Co 1.0-1.5 at%, Ga 0.3-0.5 at%, Cu 0.1-0.2 at%, Al 0.1-0.3 at%, with the balance being Fe.

[0007] Preferably, the ratio of the dilute nitric acid aqueous solution and the uniaxial anisotropic NdFeB magnetic powder in S4 is 5000mL:1000g; The dilute nitric acid aqueous solution described in S4 has a mass fraction of 0.8%-1% and a temperature of 2-5℃.

[0008] Preferably, the ratio of anhydrous n-hexane, pretreated magnetic powder, modified suspension, anhydrous ethanol, and modified nanosheet dispersion in S5 is 4000mL:1000g:70-90mL:4500mL:200mL.

[0009] Preferably, the method for preparing the modified suspension in S5 is as follows: Add dysprosium hydride and Nd to anhydrous n-hexane 70 Cu 30 Low-melting-point alloy powder and oleic acid are sealed and ultrasonically dispersed at 6-10℃ for 30-50 min to obtain a modified suspension. The anhydrous n-hexane, dysprosium hydride, and Nd 70 Cu 30 The ratio of low melting point alloy powder to oleic acid is 70-90mL: 4-6g: 4-6g: 0.2g.

[0010] Preferably, the preparation method of the modified nanosheet dispersion in S5 is as follows: Add deionized water and silane coupling agent KH550 to anhydrous ethanol and stir for 30-40 min. Then add hexagonal boron nitride nanosheets and seal and sonicate for 60-80 min to obtain a modified nanosheet dispersion. The ratio of anhydrous ethanol, deionized water, silane coupling agent KH550, and hexagonal boron nitride nanosheets is 200mL: 0.5-0.8mL: 1g: 10-15g.

[0011] Preferably, the mass ratio of ethyl acetate, epoxy resin E-51, silane coupling agent KH560, and modified magnetic powder in S6 is 100-110:2:0.5:1000.

[0012] Preferably, the temperature and pressure treatment in S6 is as follows: First, apply a constant axial static magnetic field of 2.5-3.0T, simultaneously heat to 100-110℃ and apply an axial pressure of 300MPa. After maintaining the pressure and magnetism for 30s, keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply an axial pressure of 300MPa. After maintaining the pressure and magnetism for 30s, keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply an axial pressure of 300MPa again. After maintaining the pressure and magnetism for 30s, keep the magnetic field cooled to room temperature. Finally, release the pressure, remove the magnetic field, and demold.

[0013] Preferably, the vacuum degreasing process in S6 is as follows: After loading the molybdenum material into the boat and placing it in the vacuum degreasing furnace, the furnace is evacuated and heated to 350°C. The temperature is maintained for 90-120 minutes and then cooled to 80-100°C with the furnace.

[0014] Preferably, the hot pressing process in S6 is as follows: Under an argon atmosphere, the material is transferred to a graphite mold and placed in a vacuum hot press furnace. The furnace is then evacuated and heated to 680-720℃. An axial pressure of 80-100MPa, consistent with the orientation direction, is applied and held at this temperature for 20-30 minutes. After depressurization, the material is cooled to 500-520℃ and held under vacuum for 120 minutes. Finally, the material is cooled with the furnace.

[0015] The beneficial effects of this invention are: This invention provides a method for preparing permanent magnets by low-temperature orientation molding and hot pressing densification of omnidirectional NdFeB magnetic powder. This invention simultaneously improves the remanence, intrinsic coercivity, maximum energy product, and squareness of omnidirectional NdFeB permanent magnets through the following methods.

[0016] (1) In this invention, an axial static magnetic field is applied throughout the dehydrogenation and recombination stage. The magnetic driving force introduces an additional static magnetic energy term into the system, making the c-axis of Nd2Fe parallel to the direction of the magnetic field. 14 The B-type nucleus exhibits significant thermodynamic stability, effectively reducing the critical nucleation work for this orientation and giving it an absolute advantage in the phase transformation competition nucleation process, significantly suppressing the nucleation of misaligned grains. Simultaneously, the magnetic field guides atoms to preferentially diffuse and attach to the surface of grains with matching orientations, preferentially promoting the growth of uniformly oriented grains and further suppressing the growth space of misaligned grains. These effects also result in a narrower grain size distribution and closer alignment with Nd₂Fe through the synchronous nucleation of orientation-matched nuclei. 14 The single-domain critical size of B indirectly suppresses abnormal local grain growth and weakens the risk of large grains acting as demagnetization nucleation centers. Improved texture is the core driving force for increased remanence; an increase in orientation factor directly leads to a significant increase in remanence. The dual improvement in grain size uniformity and orientation consistency significantly reduces the differences in reversal field and intrinsic coercivity between different grains, significantly suppressing the early drop in the demagnetization curve, optimizing the squareness of the demagnetization curve, and ultimately achieving a significant increase in the effective magnetic energy product. Furthermore, the constant magnetic field applied throughout the warm-pressing stage effectively avoids orientation disorder of the magnetic powder particles during mechanical pressing, fully preserving the high texture of HDDR magnetic powder (nanocrystalline NdFeB magnetic powder with uniaxial anisotropy prepared through a hydrogenation-disproportionation-dehydrogenation-recombination process), laying the foundation for the high remanence of the final magnet.

[0017] (2) This invention utilizes low-concentration, dilute nitric acid for low-temperature, short-time pickling to effectively remove the oxide layer and surface defects on the magnetic powder surface, while minimizing over-corrosion of the Nd-rich phase at the grain boundaries and the introduction of new defects, thus exposing a clean and fresh magnetic powder interface. In an anhydrous system, dysprosium hydride and Nd... 70 Cu 30 A composite diffusion source composed of low-melting-point alloy powder can be uniformly adhered to the surface of magnetic powder; during subsequent hot pressing, Nd...70 Cu 30 Low-melting-point alloy powder preferentially forms a low-melting-point eutectic liquid phase, significantly improving grain boundary wettability and providing a rapid intergranular diffusion channel for dysprosium. This promotes the selective diffusion of dysprosium to the surface of the main phase grains, forming a (Nd,Dy)₂Fe with a high magnetocrystalline anisotropy field. 14 The core-shell structure significantly enhances the antimagnetic nucleation field on the grain surface, effectively suppressing the nucleation and expansion of antimagnetic domains. Simultaneously, the low-melting-point Nd-rich liquid phase wets grain boundaries, repairs lattice defects on the magnetic powder surface, eliminates preferential nucleation sites for antimagnetic domains, and further enhances coercivity. This grain boundary diffusion mode avoids the solid solution of dysprosium within the main phase grains caused by melting doping, significantly reducing the amount of heavy rare earth elements and minimizing the loss of saturation magnetization in the main phase. The surface-modified hexagonal boron nitride nanosheets of this invention can uniformly coat the magnetic powder surface and, after hot pressing, stably distribute at grain / phase boundaries between magnetic powder particles, effectively severing the exchange coupling between adjacent main phase grains and suppressing the transcrystalline expansion of antimagnetic domains. Simultaneously, its high-temperature stability effectively pins grain boundaries during hot pressing, hindering grain boundary migration, suppressing abnormal growth of main phase grains, and maintaining fine and uniform grain size. Furthermore, the dense coating layer isolates water and oxygen throughout the process, preventing the oxidation of magnetic powder to Nd₂O₃ and FeO. x The presence of defective phases reduces the nucleation centers of demagnetizing domains, avoiding unnecessary attenuation of coercivity. This invention's synergistic process system achieves a significant increase in intrinsic coercivity with virtually no loss of remanence. Simultaneously, the uniform coating of the diffusion source ensures high consistency in the core-shell structure of each grain, while the hexagonal boron nitride grain boundary phase guarantees grain size uniformity. These two factors synergistically reduce the coercivity differences between grains, significantly improving the synchronicity of the demagnetization process and greatly optimizing the squareness of the demagnetization curve. The synergistic improvement in high remanence, high coercivity, and high squareness significantly enhances the effective energy product and energy product utilization rate of the magnet.

[0018] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. The embodiments described below are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Unless otherwise specified, the following information pertains to some of the raw materials used in the following embodiments and comparative examples of this invention: Fe (≥99.9wt%), Co (≥99.95%), B-Fe alloy (B≥20%, C≤0.05%, P≤0.02%), Nd (≥99.9%), Ga (≥99.99%), Al (≥99.9%), Cu (≥99.9%); Dysprosium hydride was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd., item number: PA92559; Nd 70 Cu 30 The low melting point alloy powder (D50=1-3μm) was purchased from Huainan Kedi Chemical Technology Co., Ltd.; the epoxy resin E-51 was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd., item number: WB98551.

[0021] Example 1: The method for preparing permanent magnets by low-temperature orientation molding and hot pressing densification of omnidirectional NdFeB magnetic powder is as follows: S1: Place Fe, B-Fe alloy, and Co in a magnesium oxide crucible of a medium-frequency induction furnace, seal it, and then evacuate to 4×10⁻⁶. - 3 Pa, then high-purity argon gas was introduced to 0.12 MPa and the temperature was raised to 1450℃. After the alloy was completely melted, Nd, Ga, Al, and Cu were added and the mixture was kept at this temperature and stirred for 20 min. The temperature was then lowered to 1400℃ and allowed to stand for 5 min to remove slag. The mixture was then poured into a copper mold preheated to 300℃ and subsequently placed under a vacuum of 0.8 × 10⁻⁶ MPa. -3 In a vacuum annealing furnace of Pa, the temperature is raised to 1050℃ and held for 12 hours. After cooling in the furnace, an alloy ingot is obtained. The alloy ingot is composed of the following atomic percentages: Nd 11.8at%, B 6.0at%, Co 1.0at%, Ga 0.5at%, Cu 0.1at%, Al 0.1at%, with the balance being Fe.

[0022] S2: After crushing the alloy ingot, load it into the stainless steel trolley of the explosion-proof hydrogen explosion furnace, and evacuate to a vacuum level of 0.8 × 10⁻⁶. -3 After the pressure is reduced to 0.1 MPa, high-purity argon is introduced into the furnace, and this process is repeated three times. Then, high-purity hydrogen is introduced into the furnace to 0.12 MPa. The temperature is then increased to 300-350℃ at a rate of 5℃ / min and held for 2 hours. Finally, a vacuum is drawn to 0.8 × 10⁻⁶ MPa. -2 Pa was held at 350℃ for 2 hours, and after cooling to room temperature in the furnace, it was discharged and screened in an argon glove box with a water oxygen content of 0.6-1ppm to obtain hydrogen-exploded coarse powder with a particle size of 100-300μm. S3: Load the hydrogen-explosive coarse powder into a non-magnetic stainless steel trolley (16mm thick), place it in an explosion-proof tubular HDDR furnace equipped with an axial magnetic field module, seal it, and then evacuate to 0.8×10⁻⁶ mm. -3Pa, then high-purity argon gas is introduced to 0.1 MPa, and the gas exchange is repeated 3 times. Then high-purity hydrogen gas is introduced into the furnace to 80 kPa. Then the temperature is increased to 800℃ at 10℃ / min and held for 120 min. Then a 2.2T axial constant static magnetic field is applied at 800℃, while the vacuum is evacuated to 0.08 Pa and held for 30 min. Then the magnetic field is kept unchanged and high-purity argon gas is introduced. Finally, the temperature is cooled to room temperature at 80℃ / min, the magnetic field is removed, and the material is discharged to obtain uniaxial anisotropic NdFeB magnetic powder. S4: In an argon glove box with an oxygen content of 0.6-1ppm, 1000g of uniaxial anisotropic NdFeB magnetic powder was added to 5000mL of 0.8% dilute nitric acid aqueous solution at 2℃ and stirred in a sealed container for 30s. After that, the mixture was filtered and the precipitate was washed 6 times with anhydrous ethanol. Then, it was vacuum dried at 60℃ for 60min to obtain the pretreated magnetic powder. S5: In an argon glove box with a water and oxygen content of 0.6-1ppm, add 4g of dysprosium hydride with a particle size of 20-50nm and 4g of Nd2 to 70ml of anhydrous n-hexane. 70 Cu 30 Low-melting-point alloy powder and 0.2g of oleic acid were sealed and ultrasonically dispersed at 6℃ for 30min to obtain a modified suspension. S6: In an argon glove box with an oxygen content of 0.6-1ppm, add 0.5mL of deionized water and 1g of silane coupling agent KH550 to 200mL of anhydrous ethanol and stir for 30min. Then add 10g of hexagonal boron nitride nanosheets and seal and sonicate for 60min to obtain a modified nanosheet dispersion. S7: In an argon glove box with an oxygen content of 0.6-1ppm, 4000mL of anhydrous n-hexane and 1000g of pretreated magnetic powder were added to a high-pressure reactor and stirred at 500r / min for 15min. Then, 70mL of modified suspension was added dropwise at 1mL / min and stirred at 50℃ for 60min. Next, 4500mL of anhydrous ethanol was added and stirred for 15min. Then, 200mL of modified nanosheet dispersion was added dropwise at 5mL / h and stirred at 50℃ for 3h. Finally, the mixture was vacuum filtered and the precipitate was washed 4 times with anhydrous ethanol. After vacuum drying at 60℃ for 3h, the modified magnetic powder was obtained. S8: In an argon glove box with a water and oxygen content of 0.6-1ppm, add 2g of epoxy resin E-51 and 0.5g of silane coupling agent KH560 to 100g of ethyl acetate and stir evenly. Then spray it into 1000g of modified magnetic powder stirred at 30r / min at a uniform speed. After spraying, vacuum stir at 45℃ for 40min to obtain granulated powder. S9: After loading the granulated powder into a non-magnetic steel mold, place it into a temperature-pressure hydraulic press with a multi-axis magnetic field module. Then, apply a 2.5T constant axial static magnetic field, simultaneously heat up to 100℃ and apply 300MPa axial pressure. Hold the pressure and magnetism for 30s, then keep the magnetic field uninterrupted and release the pressure to 0MPa. Next, rotate the mold 90° along the axis perpendicular to the magnetic field and apply 300MPa axial pressure. Hold the pressure and magnetism for 30s, then keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply 300MPa axial pressure again. Hold the pressure and magnetism for 30s and keep the magnetic field cooled to room temperature. Finally, release the pressure, remove the magnetic field, and demold to obtain the green body. S10: Load the green billet into the molybdenum billet boat and place it in the vacuum degreasing furnace, then evacuate to 4×10. -3 Pa was heated to 350℃ at a rate of 1℃ / min, held at that temperature for 90 min, and then cooled to 80℃ in the furnace. The mixture was then transferred to a graphite mold under an argon atmosphere and placed in a vacuum hot press furnace, after which a vacuum of 0.8 × 10⁻⁶ was applied. -3 Pa was applied and the temperature was increased to 680℃ at 15℃ / min. Then, an axial pressure of 80MPa was applied in the same direction as the orientation and the temperature and pressure were maintained for 20min. After depressurization, the temperature was reduced to 500℃ at 50℃ / min and vacuum maintained for 120min. Finally, the furnace was cooled and the surface was ground to obtain an omnidirectional NdFeB permanent magnet.

[0023] Example 2: The method for preparing permanent magnets by low-temperature orientation molding and hot pressing densification of omnidirectional NdFeB magnetic powder is as follows: S1: Place Fe, B-Fe alloy, and Co in a magnesium oxide crucible in a medium-frequency induction furnace, seal it, and then evacuate to 4.5 × 10⁻⁶. -3 Pa, then high-purity argon gas was introduced to 0.13 MPa and the temperature was raised to 1470℃. After the alloy was completely melted, Nd, Ga, Al, and Cu were added and the mixture was kept at this temperature and stirred for 25 min. The temperature was then lowered to 1410℃ and allowed to stand for 6 min to remove slag. The mixture was then poured into a copper mold preheated to 350℃ and subsequently placed under a vacuum of 0.9 × 10⁻⁶ MPa. -3 In a vacuum annealing furnace of Pa, the temperature is raised to 1070℃ and held for 14 hours. After cooling in the furnace, an alloy ingot is obtained. The alloy ingot is composed of the following atomic percentages: Nd 12.2at%, B 5.9at%, Co 1.2at%, Ga 0.4at%, Cu 0.1at%, Al 0.2at%, with the balance being Fe.

[0024] S2: After crushing the alloy ingot, load it into the stainless steel trolley of the explosion-proof hydrogen explosion furnace, and evacuate to 0.9×10⁻⁶. -3After the pressure is reduced to 0.1 MPa by purging with high-purity argon, and this process is repeated three times, followed by purging with high-purity hydrogen to 0.14 MPa. The temperature is then increased to 330℃ at a rate of 5℃ / min and held for 2.5 hours. Finally, a vacuum is applied to 0.9 × 10⁻⁶ MPa. -2 Pa was held at 350℃ for 2 hours, and after cooling to room temperature in the furnace, it was discharged and screened in an argon glove box with a water oxygen content of 0.6-1ppm to obtain hydrogen-exploded coarse powder with a particle size of 100-300μm. S3: Load the hydrogen-explosive coarse powder into a non-magnetic stainless steel trolley (18mm thick), place it in an explosion-proof tubular HDDR furnace equipped with an axial magnetic field module, seal it, and then evacuate to 0.9×10⁻⁶. -3 Pa, then high-purity argon gas is introduced to 0.1 MPa, and the gas exchange is repeated 3 times. Then high-purity hydrogen gas is introduced into the furnace to 80 kPa. Then the temperature is increased to 810℃ at 10℃ / min and held for 150 min. Then a 2.4T axial constant static magnetic field is applied at 800℃, while the vacuum is evacuated to 0.09 Pa and held for 45 min. Then the magnetic field is kept unchanged and high-purity argon gas is introduced. Finally, the temperature is cooled to room temperature at 90℃ / min, the magnetic field is removed, and the material is discharged to obtain uniaxial anisotropic NdFeB magnetic powder. S4: In an argon glove box with an oxygen content of 0.6-1ppm, 1000g of uniaxial anisotropic NdFeB magnetic powder was added to 5000mL of 0.9% dilute nitric acid aqueous solution at 4℃ and stirred in a sealed container for 45s. After that, the mixture was filtered and the precipitate was washed 7 times with anhydrous ethanol. Then, it was vacuum dried at 60℃ for 75min to obtain the pretreated magnetic powder. S5: In an argon glove box with a water and oxygen content of 0.6-1ppm, add 5g of dysprosium hydride with a particle size of 20-50nm and 5g of Nd2 to 80ml of anhydrous n-hexane. 70 Cu 30 Low-melting-point alloy powder and 0.2g of oleic acid were sealed and ultrasonically dispersed at 8℃ for 40min to obtain a modified suspension. S6: In an argon glove box with an oxygen content of 0.6-1ppm, add 0.7mL of deionized water and 1g of silane coupling agent KH550 to 200mL of anhydrous ethanol and stir for 35min. Then add 13g of hexagonal boron nitride nanosheets and seal and sonicate for 70min to obtain a modified nanosheet dispersion. S7: In an argon glove box with an oxygen content of 0.6-1ppm, 4000mL of anhydrous n-hexane and 1000g of pretreated magnetic powder were added to a high-pressure reactor and stirred at 550r / min for 13min. Then, 80mL of modified suspension was added dropwise at 1mL / min and stirred at 50℃ for 65min. Next, 4500mL of anhydrous ethanol was added and stirred for 15min. Then, 200mL of modified nanosheet dispersion was added dropwise at 5mL / h and stirred at 50℃ for 3.5h. Finally, the mixture was vacuum filtered and the precipitate was washed 5 times with anhydrous ethanol. After vacuum drying at 60℃ for 4h, the modified magnetic powder was obtained. S8: In an argon glove box with a water and oxygen content of 0.6-1ppm, add 2g of epoxy resin E-51 and 0.5g of silane coupling agent KH560 to 105g of ethyl acetate and stir evenly. Then spray it into 1000g of modified magnetic powder stirred at 30r / min at a uniform speed. After spraying, vacuum stir at 45℃ for 50min to obtain granulated powder. S9: After loading the granulated powder into a non-magnetic steel mold, place it into a temperature-pressure hydraulic press with a multi-axis magnetic field module. Then, apply a 2.8T axial constant static magnetic field, simultaneously heat up to 105℃ and apply 300MPa axial pressure. Hold the pressure and magnetism for 30s, then keep the magnetic field uninterrupted and release the pressure to 0MPa. Next, rotate the mold 90° along the axis perpendicular to the magnetic field and apply 300MPa axial pressure. Hold the pressure and magnetism for 30s, then keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply 300MPa axial pressure again. Hold the pressure and magnetism for 30s and keep the magnetic field cooled to room temperature. Finally, release the pressure, remove the magnetic field, and demold to obtain the green body. S10: Load the green billet into a molybdenum billet boat and place it in a vacuum degreasing furnace, then evacuate to 4.5 × 10⁻⁶. -3 Pa was heated to 350℃ at a rate of 1℃ / min, held at that temperature for 105 min, and then cooled to 90℃ in the furnace. The mixture was then transferred to a graphite mold under an argon atmosphere and placed in a vacuum hot press furnace, after which a vacuum of 0.9 × 10⁻⁶ was applied. -3 Pa is applied and heated to 700℃ at 15℃ / min. Then, an axial pressure of 90MPa is applied in the same direction as the orientation and held at that temperature for 25min. After depressurization, the temperature is lowered to 510℃ at 50℃ / min and held under vacuum for 120min. Finally, the furnace is cooled and the surface is ground to obtain an omnidirectional NdFeB permanent magnet.

[0025] Example 3: The method for preparing permanent magnets by low-temperature orientation molding and hot pressing densification of omnidirectional NdFeB magnetic powder is as follows: S1: Place Fe, B-Fe alloy, and Co in a magnesium oxide crucible of a medium-frequency induction furnace, seal it, and then evacuate to 5×10⁻⁶. - 3Pa, then high-purity argon gas was introduced to 0.15 MPa and the temperature was raised to 1480℃. After the alloy was completely melted, Nd, Ga, Al, and Cu were added and the mixture was kept at this temperature and stirred for 30 min. The temperature was then lowered to 1420℃ and allowed to stand for 7 min to remove slag. The mixture was then poured into a copper mold preheated to 400℃ and subsequently placed under a vacuum of 1×10⁻⁶. 3 In a vacuum annealing furnace of Pa, the temperature is raised to 1100℃ and held for 16 hours. After cooling in the furnace, an alloy ingot is obtained. The alloy ingot is composed of the following atomic percentages: Nd 12.5at%, B 5.8at%, Co 1.5at%, Ga 0.3at%, Cu 0.2at%, Al 0.3at%, with the balance being Fe.

[0026] S2: After crushing the alloy ingot, load it into the stainless steel trolley of the explosion-proof hydrogen explosion furnace, and evacuate to 1×10⁻⁶. -3 After Pa, high-purity argon gas was introduced to 0.1 MPa, and this process was repeated three times. Then, high-purity hydrogen gas was introduced into the furnace to 0.15 MPa. The temperature was then increased to 350℃ at a rate of 5℃ / min and held for 3 hours. Finally, a vacuum was drawn to 1×10⁻⁶ MPa. -2 Pa was held at 350℃ for 2 hours, and after cooling to room temperature in the furnace, it was discharged and screened in an argon glove box with a water oxygen content of 0.6-1ppm to obtain hydrogen-exploded coarse powder with a particle size of 100-300μm. S3: Load the hydrogen-explosive coarse powder into a non-magnetic stainless steel trolley (20mm thick), place it in an explosion-proof tubular HDDR furnace equipped with an axial magnetic field module, seal it, and then evacuate to 1×10⁻⁶. -3 Pa, then high-purity argon gas is added to 0.1 MPa, and the gas exchange is repeated 3 times. Then high-purity hydrogen gas is added to 80 kPa. Then the temperature is increased to 820℃ at 10℃ / min and held for 180 min. Then a 2.5T axial constant static magnetic field is applied at 800℃, while the vacuum is evacuated to 0.1 Pa and held for 60 min. Then the magnetic field is kept unchanged and high-purity argon gas is added. Finally, the temperature is cooled to room temperature at 100℃ / min, the magnetic field is removed, and the material is discharged to obtain uniaxial anisotropic NdFeB magnetic powder. S4: In an argon glove box with an oxygen content of 0.6-1ppm, 1000g of uniaxial anisotropic NdFeB magnetic powder was added to 5000mL of 1% (w / w) dilute nitric acid aqueous solution at 5℃ and stirred in a sealed container for 60s. After that, the mixture was filtered and the precipitate was washed 8 times with anhydrous ethanol. Then, it was vacuum dried at 60℃ for 90min to obtain the pretreated magnetic powder. S5: In an argon glove box with a water and oxygen content of 0.6-1ppm, add 6g of dysprosium hydride with a particle size of 20-50nm, 6g of Nd70Cu30 low melting point alloy powder, and 0.2g of oleic acid to 90m of anhydrous n-hexane. After sealing, ultrasonically disperse at 10℃ for 50min to obtain a modified suspension. S6: In an argon glove box with an oxygen content of 0.6-1ppm, add 0.8mL of deionized water and 1g of silane coupling agent KH550 to 200mL of anhydrous ethanol and stir for 40min. Then add 15g of hexagonal boron nitride nanosheets and seal and sonicate for 80min to obtain a modified nanosheet dispersion. S7: In an argon glove box with an oxygen content of 0.6-1ppm, 4000mL of anhydrous n-hexane and 1000g of pretreated magnetic powder were added to a high-pressure reactor and stirred at 600r / min for 15min. Then, 90mL of modified suspension was added dropwise at 1mL / min and stirred at 50℃ for 70min. Next, 4500mL of anhydrous ethanol was added and stirred for 15min. Then, 200mL of modified nanosheet dispersion was added dropwise at 5mL / h and stirred at 50℃ for 4h. Finally, the mixture was vacuum filtered and the precipitate was washed 5 times with anhydrous ethanol. After vacuum drying at 60℃ for 5h, the modified magnetic powder was obtained. S8: In an argon glove box with a water and oxygen content of 0.6-1ppm, add 2g of epoxy resin E-51 and 0.5g of silane coupling agent KH560 to 110g of ethyl acetate and stir evenly. Then spray it into 1000g of modified magnetic powder stirred at 30r / min at a uniform speed. After spraying, vacuum stir at 45℃ for 60min to obtain granulated powder. S9: After loading the granulated powder into a non-magnetic steel mold, place it into a temperature-pressure hydraulic press with a multi-axis magnetic field module. Then, apply a 3.0T constant axial static magnetic field, simultaneously heat up to 110℃ and apply 300MPa axial pressure. Hold the pressure and magnetism for 30s, then keep the magnetic field uninterrupted and release the pressure to 0MPa. Next, rotate the mold 90° along the axis perpendicular to the magnetic field and apply 300MPa axial pressure. Hold the pressure and magnetism for 30s, then keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply 300MPa axial pressure again. Hold the pressure and magnetism for 30s and keep the magnetic field cooled to room temperature. Finally, release the pressure, remove the magnetic field, and demold to obtain the green body. S10: Load the green billet into the molybdenum billet boat and place it in the vacuum degreasing furnace, then evacuate to 5×10. -3 Pa was heated to 350℃ at a rate of 1℃ / min, held at that temperature for 120 min, and then cooled to 100℃ in the furnace. The mixture was then transferred to a graphite mold under an argon atmosphere and placed in a vacuum hot press furnace, after which a vacuum of 1×10⁻⁶ was applied. -3 Pa is applied and heated to 720℃ at 15℃ / min. Then, 100MPa axial pressure is applied in the same direction as the orientation and the temperature and pressure are maintained for 30min. After depressurization, the temperature is lowered to 520℃ at 50℃ / min and vacuum maintained for 120min. Finally, the furnace is cooled and the surface is ground to obtain an omnidirectional NdFeB permanent magnet.

[0027] Comparative Example 1: Compared with Example 1, this comparative example only did not apply a "2.2T axial constant static magnetic field" during the preparation process of S3. All other steps and parameters were the same, and will not be repeated here. The final result was an omnidirectional neodymium iron boron permanent magnet.

[0028] Comparative Example 2: Compared with Example 1, this comparative example only did not add "modified suspension" during the preparation process of S7. All other steps and parameters were the same, and will not be repeated here. The final result was an omnidirectional neodymium iron boron permanent magnet.

[0029] Comparative Example 3: Compared with Example 1, this comparative example differs only in the preparation process of S7: "Adding 4000 mL of anhydrous n-hexane and 1000 g of pretreated magnetic powder to a high-pressure reactor and stirring at 500 r / min for 10 min; then adding 70 mL of modified suspension dropwise at 1 mL / min and stirring at 50°C for 60 min; then adding 4500 mL of anhydrous ethanol and stirring for 15 min; then adding 200 mL of modified nanosheet dispersion dropwise at 5 mL / h and stirring at 50°C for 3 h; finally, vacuum filtration and washing the precipitate four times with anhydrous ethanol; and finally vacuum drying at 60°C for 3 h to obtain modified magnetic powder." This is replaced with "Adding 4000 mL of anhydrous n-hexane to a high-pressure reactor." 1000g of pretreated magnetic powder was stirred at 500r / min for 10min, then 4500mL of anhydrous ethanol was added and stirred for 15min. After that, the temperature was raised to 50℃ and 10g of silane coupling agent KH550 was added and stirred for 30min. Then, 200mL of modified nanosheet dispersion was added dropwise at 5mL / h and stirred at 50℃ for 3h. Vacuum filtration was performed and the precipitate was washed 4 times with anhydrous ethanol. After vacuum drying at 60℃ for 3h, 70mL of modified suspension was added dropwise at 1mL / min and stirred at 50℃ for 60min to obtain modified magnetic powder. The remaining steps and parameters were the same, and will not be repeated in this comparative example. Finally, an omnidirectional neodymium iron boron permanent magnet was obtained.

[0030] Performance testing: Measurement of remanence: Referring to GB / T 3217-2013 standard, a closed-loop DC magnetic property tester for permanent magnet materials was used to measure the magnetic induction intensity B value at the intersection of the normal hysteresis loop of the omnidirectional NdFeB permanent magnets BH prepared in Examples 1-3 and Comparative Examples 1-3 with the vertical axis (H=0), which is the remanence (T). The test results are shown in Table 1. Measurement of intrinsic compensatory power: Referring to the GB / T 3217-2013 standard, a closed-loop DC magnetic property tester for permanent magnet materials was used to measure the magnetic field strength H value at the intersection of the intrinsic hysteresis loop JH of the omnidirectional NdFeB permanent magnets prepared in Examples 1-3 and Comparative Examples 1-3 with the horizontal axis (J=0). This value is the intrinsic coercivity (kOe). The test results are shown in Table 1. Determination of the maximum magnetic energy product: Referring to GB / T 3217-2013 standard, a closed-loop DC magnetic property tester was used to measure the maximum magnetic energy product (MGOe) of the omnidirectional NdFeB permanent magnets prepared in Examples 1-3 and Comparative Examples 1-3. The test results are shown in Table 1. Determination of squareness: Referring to the GB / T 3217-2013 standard, a closed-loop DC magnetic property tester for permanent magnet materials was used to measure the corresponding point in the intrinsic hysteresis loop of the omnidirectional NdFeB permanent magnets JH prepared in Examples 1-3 and Comparative Examples 1-3 where J equals 0.9 × Jr. The ratio of the absolute value of the reverse magnetic field strength to the intrinsic coercivity at the corresponding point, multiplied by 100%, is the squareness (%). The test results are shown in Table 1.

[0031] Table 1: Performance test results of Examples 1-3 and Comparative Examples 1-3 Data Analysis: As can be seen from Table 1, the omnidirectional NdFeB permanent magnets prepared in the embodiments of the present invention simultaneously possess excellent remanence, intrinsic coercivity, maximum energy product, and squareness.

[0032] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder, characterized in that, Includes the following steps: S1: Fe, B-Fe alloy, and Co are melted, and Nd, Ga, Al, and Cu are added and melted. After casting, the mixture is vacuum annealed at 1050-1100℃ to obtain an alloy ingot. S2: After crushing the alloy ingot, it is treated in a hydrogen atmosphere at 300-350℃ for 2-3 hours, then vacuum treated at 350℃ for 2 hours. After cooling and sieving, it is treated in a hydrogen atmosphere at 800-820℃ and 80kPa for 120-180 minutes. Then, it is vacuum treated in an axial constant static magnetic field at 800℃, 0.08-0.1Pa, and 2.2-2.5T for 30-60 minutes. After purging with argon and cooling, the magnetic field is removed and the material is discharged. Finally, it is stirred in a dilute nitric acid aqueous solution in a closed container for 30-60 seconds, filtered, washed, and dried to obtain pretreated magnetic powder. S3: Add pretreated magnetic powder to anhydrous n-hexane, stir, then drop in modified suspension and stir at 50°C for 60-70 min. Add anhydrous ethanol and stir well, then drop in modified nanosheet dispersion and stir at 50°C for 3-4 h. After filtration, washing and drying, the modified magnetic powder is obtained. S4: Add epoxy resin E-51 and silane coupling agent KH560 to ethyl acetate and stir well. Then spray it into modified magnetic powder and stir under vacuum at 45°C for 40-60 min. Then perform warm pressing, vacuum degreasing and hot pressing to obtain an omnidirectional neodymium iron boron permanent magnet.

2. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The alloy ingot described in S1 is composed of the following atomic percentages: Nd 11.8-12.5 at%, B 5.8-6.0 at%, Co 1.0-1.5 at%, Ga 0.3-0.5 at%, Cu 0.1-0.2 at%, Al 0.1-0.3 at%, with the balance being Fe.

3. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The dilute nitric acid aqueous solution in S2 has a mass fraction of 0.8%-1% and a temperature of 2-5℃.

4. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The ratio of anhydrous n-hexane, pretreated magnetic powder, modified suspension, anhydrous ethanol, and modified nanosheet dispersion in S3 is 4000mL:1000g:70-90mL:4500mL:200mL.

5. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The preparation method of the modified suspension described in S3 is as follows: Add dysprosium hydride and Nd to anhydrous n-hexane 70 Cu 30 Low-melting-point alloy powder and oleic acid are sealed and ultrasonically dispersed at 6-10℃ for 30-50 min to obtain a modified suspension. The anhydrous n-hexane, dysprosium hydride, and Nd 70 Cu 30 The ratio of low melting point alloy powder to oleic acid is 70-90mL: 4-6g: 4-6g: 0.2g.

6. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The preparation method of the modified nanosheet dispersion described in S3 is as follows: Add deionized water and silane coupling agent KH550 to anhydrous ethanol and stir for 30-40 min. Then add hexagonal boron nitride nanosheets and seal and sonicate for 60-80 min to obtain a modified nanosheet dispersion. The ratio of anhydrous ethanol, deionized water, silane coupling agent KH550, and hexagonal boron nitride nanosheets is 200mL: 0.5-0.8mL: 1g: 10-15g.

7. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The mass ratio of ethyl acetate, epoxy resin E-51, silane coupling agent KH560, and modified magnetic powder in S4 is 100-110:2:0.5:1000.

8. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The temperature and pressure treatment described in S4 is as follows: First, apply a constant axial magnetic field of 2.5-3.0T, simultaneously heat to 100-110℃ and apply an axial pressure of 300MPa. After maintaining the pressure and magnetism for 30s, keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply an axial pressure of 300MPa. After maintaining the pressure and magnetism for 30s, keep the magnetic field uninterrupted and release the pressure to 0MPa. Then, rotate the mold 90° along the axis perpendicular to the magnetic field and apply an axial pressure of 300MPa again. After maintaining the pressure and magnetism for 30s, keep the magnetic field cooled to room temperature. Finally, release the pressure, remove the magnetic field, and demold.

9. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The vacuum degreasing process described in S4 is as follows: after loading the molybdenum material into a boat and placing it in a vacuum degreasing furnace, a vacuum is drawn and the temperature is raised to 350°C. After holding at this temperature for 90-120 minutes, the temperature is cooled to 80-100°C with the furnace.

10. The method for preparing permanent magnets by low-temperature orientation molding and hot-pressing densification of omnidirectional NdFeB magnetic powder according to claim 1, characterized in that, The hot pressing process described in S4 is as follows: under an argon atmosphere, the material is transferred to a graphite mold and placed into a vacuum hot press furnace. Then, a vacuum is drawn and the temperature is raised to 680-720℃. Then, an axial pressure of 80-100MPa consistent with the orientation direction is applied and the temperature and pressure are maintained for 20-30 minutes. After depressurization, the temperature is lowered to 500-520℃ and vacuum-held for 120 minutes. Finally, the material is cooled with the furnace.

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