Preparation method of high-performance sintered neodymium-iron-boron permanent magnet based on multi-element synergistic diffusion

Abstract

The application belongs to the field of permanent magnet materials, and discloses a high-performance sintered neodymium-iron-boron permanent magnet preparation method based on multi-element synergistic diffusion. The method comprises the following steps: S1, preparing a sintered neodymium-iron-boron permanent magnet blank with a grain size of 3-8 μm; S2, preparing a composite diffusion medium slurry, which comprises an inner diffusion medium slurry and an outer diffusion medium slurry; S3, coating a composite diffusion medium layer, the inner layer has a dry porosity of 15-20%, and the outer layer has a dry porosity of 30-35%; S4, performing multi-stage heat diffusion treatment, including pre-diffusion heat treatment, main diffusion heat treatment and post-diffusion annealing treatment; and S5, subsequent treatment. Through the synergistic effect of the composite diffusion medium and the multi-stage heat diffusion, the application realizes the uniform distribution of multi-elements, improves the intrinsic coercive force of the permanent magnet by 20-35%, reduces the batch fluctuation of the coercive force to below 3%, and reduces the irreversible magnetic flux loss at 120 DEG C high temperature by 15-25%, and is suitable for new energy and other fields with strict requirements on magnetic properties.

Description

Technical Field

[0001] This invention belongs to the field of permanent magnet material preparation technology, and relates to a method for preparing high-performance sintered NdFeB permanent magnets based on multi-element synergistic diffusion. Background Technology

[0002] The high performance of sintered NdFeB permanent magnets can no longer be achieved by diffusion of a single heavy rare earth element, but requires the synergistic effect of multiple key elements, including rare earth elements, transition metal elements, and dopant elements. These different elements differ in atomic radius, electronic structure, and chemical activity, resulting in significant differences in their diffusion activation energies. For example, the diffusion activation energy of the heavy rare earth element dysprosium (Dy) is about 20% higher than that of the transition metal element cobalt (Co). When a single diffusion medium is used, all elements to be diffused diffuse under the same external conditions, making it impossible to independently control the diffusion rate of different elements.

[0003] This single diffusion medium cannot effectively balance the diffusion kinetics of different elements, leading to an imbalance in element diffusion rates. For example, some elements with faster diffusion rates excessively penetrate into the magnet's interior, while heavy rare earth elements with slower diffusion rates prematurely accumulate near the magnet's surface. This uneven diffusion behavior easily forms a "rare earth-rich layer" on the magnet's surface, while a "rare earth-poor region" forms correspondingly inside the magnet. This macroscopic inhomogeneity and microscopic heterogeneity in element distribution fundamentally undermines the uniform and optimized structure desired for improving the coercivity of permanent magnets. Excessive rare earth on the surface forms a non-magnetic phase or an excessively thick grain boundary phase, which reduces the magnet's effective magnetic volume or causes magnetic performance loss; while the rare earth-poor region inside becomes a weak point for magnetization reversal, limiting the overall coercivity level of the magnet. As a result, the batch-to-batch fluctuation of the coercivity of sintered NdFeB permanent magnets increases, with measured results showing fluctuations exceeding 8%.

[0004] Furthermore, this non-ideal elemental distribution structure, especially the rare-earth-poor regions inside the magnet, will further exacerbate the decay of magnetic properties at high temperatures. This is because, under higher thermal disturbance energy, the magnetic domains in these regions are more likely to undergo irreversible flipping, thereby accelerating the demagnetization process of the magnet and affecting the reliability and service life of the permanent magnet under high-temperature service conditions. Summary of the Invention

[0005] This invention provides a method for preparing high-performance sintered NdFeB permanent magnets based on multi-element synergistic diffusion, aiming to overcome the inherent limitations of existing single diffusion medium technologies in controlling the diffusion kinetics of rare earth elements, transition metal elements, and other dopants, especially addressing the need to improve the coercivity, batch stability, and high-temperature magnetic properties of sintered NdFeB permanent magnets. This invention designs and applies a structured, functionally graded composite diffusion medium, combined with a precisely controlled multi-stage thermal diffusion process, to achieve precise synergistic control of the diffusion rates of multiple key elements. This results in a uniform and optimized elemental gradient distribution within the permanent magnet, ultimately producing sintered NdFeB permanent magnets with high coercivity, excellent batch stability, and superior high-temperature magnetic properties.

[0006] To achieve the above-mentioned objectives, this invention provides a method for preparing high-performance sintered NdFeB permanent magnets based on multi-element synergistic diffusion, characterized by comprising the following steps:

[0007] The first step is to prepare sintered NdFeB permanent magnet blanks: using conventional powder metallurgy processes, pre-alloyed powders (e.g., nominally composed of NdFeB) are formed. x Fe bal B y M z The process involves shaping, sintering, and preliminary heat treatment of NdFeB permanent magnet blanks (where M represents additional elements such as Co, Ga, Cu, Zr, and Al) to obtain sintered NdFeB permanent magnet blanks with predetermined dimensions and initial magnetic properties. The sintered NdFeB permanent magnet blanks have an average grain size between 3 μm and 8 μm and contain Nd₂Fe. 14 The primary phase consists of B, Nd-rich grain boundary phases, and secondary phases such as a small amount of borides.

[0008] The second step is to prepare the composite diffusion medium slurry:

[0009] The composite diffusion medium comprises an inner diffusion medium and an outer diffusion medium.

[0010] The preparation process of the inner diffusion medium slurry is as follows:

[0011] First, dysprosium-rich (Dy) oxide particles are prepared. These particles are prepared by mixing Dy₂O₃ powder and CuO powder in a specific ratio and then reacting them via solid-state reaction or co-precipitation. The Dy₂O₃ powder has an average particle size between 50 nm and 200 nm and a purity greater than 99.95%. The CuO powder has an average particle size between 20 nm and 100 nm and a purity greater than 99.9%.

[0012] Dy2O3 powder and CuO powder were mixed in a weight ratio of Dy2O3:CuO=(90-98):(2-10), and then thoroughly mixed by high-energy ball milling or mechanical grinding for 4 to 8 hours at a speed of 300 to 500 rpm and a ball-to-powder ratio of 10:1 to 20:1.

[0013] The mixed powder is calcined in an inert atmosphere at a temperature of 800°C to 950°C for 2 to 5 hours to promote the formation of composite oxides of Dy and Cu, or to form a Dy-Cu-O eutectic phase precursor with good sintering activity.

[0014] The calcined powder is crushed and sieved to obtain active particles of inner diffusion medium with an average particle size distribution between 100 nm and 300 nm.

[0015] Next, an inner diffusion medium slurry is prepared. The active particles of the inner diffusion medium are mixed with a dispersant, a binder, and a solvent. The dispersant is polyethylene glycol octylphenyl ether, added at an amount of 0.5 wt% to 2 wt% of the total weight of the active particles. The binder is polyvinyl butyral, added at an amount of 3 wt% to 7 wt% of the total weight of the active particles. The solvent is a mixed solution of isopropanol and deionized water, with a volume ratio of 1:1 to 3:1.

[0016] The above components were mixed and ground in a planetary ball mill for 8 to 12 hours at a speed of 200 to 400 rpm and a ball-to-material ratio of 5:1 to 10:1 to ensure uniform dispersion of the slurry. The solid content of the inner diffusion medium slurry was between 30 wt% and 50 wt%, and the viscosity was between 50 mPa·s and 200 mPa·s.

[0017] The preparation process of the outer diffusion medium slurry is as follows:

[0018] First, cobalt-rich (Co) fluoride and gallium-rich (Ga) fluoride particles were prepared. The cobalt-rich fluoride particles were prepared from CoF2 powder, with an average particle size between 100 nm and 500 nm and a purity greater than 99.9%. The gallium-rich fluoride particles were prepared from GaF3 powder, with an average particle size between 100 nm and 500 nm and a purity greater than 99.9%.

[0019] CoF2 powder and GaF3 powder were mixed at a weight ratio of CoF2:GaF3 = (90-99):(1-10) and thoroughly mixed by high-energy ball milling or mechanical grinding for 2 to 6 hours at a speed of 300 to 500 rpm and a ball-to-powder ratio of 10:1 to 20:1. This mixing process was carried out in an oxygen-free and anhydrous atmosphere to prevent hydrolysis of fluorides.

[0020] Next, an outer diffusion medium slurry is prepared. The mixed fluoride powder is mixed with a pore-forming agent, a dispersant, a binder, and a solvent. The pore-forming agent is polymethyl methacrylate microspheres with an average particle size of 500 nm to 2 μm, and the amount added is 10 wt% to 20 wt% of the total weight of the mixed fluoride powder.

[0021] The dispersant is a polycarboxylate dispersant, added at a rate of 0.5 wt% to 1.5 wt% of the total weight of the mixed fluoride powder. The binder is ethyl cellulose, added at a rate of 4 wt% to 8 wt% of the total weight of the mixed fluoride powder. The solvent is anhydrous ethanol.

[0022] The above components were mixed and ground in a planetary ball mill for 6 to 10 hours at a speed of 250 to 450 rpm and a ball-to-material ratio of 5:1 to 10:1. The solid content of the outer diffusion medium slurry was between 25 wt% and 45 wt%, and the viscosity was between 80 mPa·s and 250 mPa·s.

[0023] The third step is to coat the composite diffusion medium layer:

[0024] First, an inner diffusion medium is coated. The inner diffusion medium slurry is uniformly coated onto the surface of the sintered NdFeB permanent magnet blank by spraying, dipping, or brushing. After coating, it is dried at a temperature of 100°C to 150°C for 1 to 3 hours to remove the solvent and cure the binder, forming a uniform and dense inner diffusion medium.

[0025] The thickness of the inner diffusion medium is between 5 μm and 50 μm. The dry porosity of the inner diffusion medium is between 15% and 20%. This porosity is precisely controlled by adjusting the particle size distribution of the active particles in the inner diffusion medium, the solid content of the slurry, and the type and content of the binder.

[0026] Next, the outer diffusion medium is coated. After the inner diffusion medium is completely dry and cured, the outer diffusion medium slurry is uniformly coated onto the surface of the permanent magnet with the inner diffusion medium formed by spraying or dipping. After coating, it is dried at a temperature of 80°C to 120°C for 1 to 2 hours to remove the solvent and cure the adhesive, forming a uniform outer diffusion medium.

[0027] The thickness of the outer diffusion medium is between 10 μm and 100 μm. The dry porosity of the outer diffusion medium is between 30% and 35%. This porosity is precisely controlled by adjusting the amount of pore-forming agent added, the particle size distribution, and the solid content of the outer diffusion medium slurry.

[0028] The fourth step involves a multi-stage thermal diffusion process:

[0029] Sintered NdFeB permanent magnets coated with a composite diffusion medium layer are placed in a vacuum furnace or an inert atmosphere furnace for heat treatment. The heat treatment process is divided into the following three stages:

[0030] Phase 1: Pre-diffusion heat treatment.

[0031] The permanent magnet is heated to 300°C to 600°C at a heating rate of 5°C / min to 10°C / min, and held at this temperature for 1 hour to 3 hours. The objective of this stage is:

[0032] a. Thoroughly decompose and remove organic binders and pore-forming agents from the diffusion medium slurry. For the inner diffusion medium, the decomposition of the binder contributes to the initial curing of its ceramic matrix and the formation of a stable low-porosity structure; for the outer diffusion medium, the decomposition of the polymethyl methacrylate pore-forming agent will generate pre-defined high-porosity channels within it.

[0033] b. Promotes the initial interfacial reaction between the diffusion medium and the permanent magnet surface, forming a low-melting-point eutectic phase or intermediate reaction layer, providing a good diffusion channel for the subsequent main diffusion stage. For example, Dy2O3 and Nd2Fe 14 The Nd-rich phase at the surface or grain boundaries of the B main phase undergoes a preliminary reaction to form a Dy-Nd-O eutectic compound with a lower melting point. Simultaneously, CoF2 and GaF3 react with trace amounts of Nd, Fe, and other elements on and inside the permanent magnet to form fluorination compounds, generating volatile fluorides or promoting the formation of low-melting-point phases at grain boundaries.

[0034] c. Eliminate residual stress inside the diffusion medium layer to ensure the structural integrity and stability of the medium layer during subsequent high-temperature processing.

[0035] This stage of heat treatment is carried out in a vacuum environment with a vacuum degree greater than 10−2 Pa, or in a high-purity argon atmosphere with an argon flow rate of 100 sccm to 300 sccm.

[0036] Second stage: Main diffusion heat treatment.

[0037] After the pre-diffusion heat treatment is completed, the furnace temperature is further increased at a rate of 5°C / min to 15°C / min to a diffusion temperature of 850°C to 1000°C. This temperature is maintained for 4 to 12 hours to achieve synergistic diffusion of multiple elements. This stage is the step in achieving the core technical objective of this invention, and its mechanism includes:

[0038] a. Controlled diffusion of dysprosium (Dy): The Dy element in the inner diffusion medium mainly exists as Dy 3+In the form of ions, Dy ions diffuse into the interior of the permanent magnet blank via grain boundary diffusion at or near the surface. The low porosity design of the inner diffusion medium increases the tortuosity of the diffusion path of Dy ions and limits the propagation speed of its diffusion front. Simultaneously, copper (Cu) in the inner medium reacts with rare earth elements (Nd) inside the permanent magnet at high temperatures to form low-melting-point copper-rich rare earth compounds. These liquid phases wet the grain boundaries, providing additional fast channels for Dy diffusion and effectively reducing the effective diffusion activation energy of Dy, thus improving the overall diffusion efficiency of Dy. However, its diffusion rate is still effectively constrained by the overall low porosity of the inner medium. This ensures the formation of a uniform, controllable-thickness Dy-rich shell structure near the permanent magnet surface, avoiding premature enrichment of Dy and insufficient deep penetration.

[0039] b. Accelerated diffusion of cobalt (Co) and gallium (Ga): Co and Ga elements in the outer diffusion medium, through the open channels provided by their high porosity (30% to 35%) and the catalytic effect of the fluoride medium, diffuse into the permanent magnet at a high rate at high temperatures. The high porosity reduces the resistance to Co and Ga diffusion, while CoF2 and GaF3 react with Fe or Nd elements inside the permanent magnet at high temperatures to form low-melting-point eutectic phases or gas-phase transport intermediates, further accelerating the transport of Co and Ga. Co elements diffuse mainly along grain boundaries and partly within the grain regions inside the permanent magnet, entering Nd2Fe. 14 The substitution of Fe atoms in the boron (B) main phase lattice enhances the magnetocrystalline anisotropy of the main phase, thereby increasing its intrinsic coercivity and improving its high-temperature magnetic properties. Ga is mainly enriched in the grain boundary region, refining the grains, optimizing the grain boundary structure, inhibiting abnormal grain growth, improving coercivity, and enhancing oxidation resistance.

[0040] c. Multi-element synergistic regulation: By effectively slowing down the diffusion of Dy in the inner medium and assisting in the penetration of Cu, and accelerating and guiding the diffusion of Co and Ga in the outer medium, this invention achieves an effective diffusion rate difference of less than 5% between key elements such as Dy, Co, and Ga within the permanent magnet. This means that these elements can advance into the permanent magnet at more similar rates, forming a more uniform and gradient-distributed element concentration, thus avoiding the problem of a surface "rare earth-rich layer" and an internal "rare earth-poor region" that occurs in traditional single-medium diffusion. This synergistic diffusion mechanism ensures the formation of a continuous and optimized element concentration gradient from the surface to the interior of the permanent magnet, especially in Nd2Fe. 14 A uniform, Dy-rich hard magnetic shell with ideal composition was formed on the surface of the B main phase grains. At the same time, Co and Ga elements were uniformly distributed inside the permanent magnet, optimizing the overall magnetic properties.

[0041] This stage of heat treatment is carried out in a high vacuum environment with a vacuum degree greater than 10−3Pa, or in a high-purity argon atmosphere with an argon flow rate of 50sccm to 200sccm.

[0042] Third stage: Post-diffusion annealing.

[0043] After completing the main diffusion heat treatment, the furnace temperature is slowly reduced to 500°C to 700°C at a cooling rate of 2°C / min to 8°C / min, and held at this temperature for 2 to 6 hours. The objective of this stage is:

[0044] a. Homogenization diffusion products: At relatively low temperatures, these products promote further homogenization and redistribution of Dy, Co, and Ga elements that have diffused into the permanent magnet within the grain boundary phase and main phase grains. For example, they promote the homogenization and redistribution of Dy... 3+ Ions form more stable (Nd,Dy)₂Fe in the Nd-rich grain boundary phase on the surface of the main phase grains. 14 The core-shell structure was optimized, and the thickness and composition of the Dy-rich grain boundary phase were further improved.

[0045] b. Eliminate internal stress: Reduce the residual stress inside the permanent magnet caused by the high-temperature diffusion treatment, and improve the mechanical strength of the permanent magnet and the smoothness of the magnetic domain wall movement.

[0046] c. Optimize the domain structure: promote smooth movement of domain walls, reduce pinning effect during magnetization reversal, and thus further enhance intrinsic coercivity and remanent magnetic induction.

[0047] This stage of heat treatment is carried out in a vacuum environment with a vacuum degree greater than 10−2 Pa, or in a high-purity argon atmosphere with an argon flow rate of 50 sccm to 150 sccm.

[0048] Step 5, follow-up processing:

[0049] After completing the multi-stage thermal diffusion process, the permanent magnet is removed from the furnace and subjected to necessary surface cleaning to remove the residual diffusion medium layer and surface oxides. Subsequently, surface passivation, magnetization, and testing are performed to obtain the final high-performance sintered NdFeB permanent magnet product.

[0050] The technical solution provided by this invention achieves precise control of multi-element diffusion behavior during the preparation of sintered NdFeB permanent magnets through the synergistic effect of the above steps.

[0051] In this invention, the low porosity of the inner diffusion medium is achieved through the following specific engineering methods:

[0052] Synergistic effect of Dy2O3 and CuO nanoparticles: The selected Dy2O3 powder has an average particle size between 50 nm and 200 nm, while the CuO powder has an average particle size between 20 nm and 100 nm. During slurry preparation, these nanoscale particles, due to their high specific surface area and surface energy, can achieve closer packing through van der Waals forces or electrostatic repulsion. During drying and pre-diffusion heat treatment, the sintering neck formation temperature between small-sized particles is lower, which is beneficial for forming a dense ceramic matrix. CuO, as a sintering aid, forms a liquid phase or solid solution between Dy2O3 particles, further promoting particle densification and neck growth, thereby reducing the porosity of the final layer.

[0053] Optimization of slurry solids content and binder: The solids content of the inner diffusion medium slurry is precisely between 30 wt% and 50 wt%. A higher solids content ensures a higher number of active particles per unit volume of slurry, which is beneficial for forming a dense coating. Polyvinyl butyral is used as a binder, with a content between 3 wt% and 7 wt%. During drying, polyvinyl butyral forms a high-strength, low-shrinkage organic network that stably binds the active particles together.

[0054] During the pre-diffusion heat treatment stage, polyvinyl butyral decomposes and volatilizes within a temperature range of 300℃ to 500℃. Its decomposition products have a negligible impact on pore formation, or form extremely fine, non-connected pores, further ensuring the compactness of the ceramic matrix. The binder-solvent ratio is precisely optimized to ensure the slurry has suitable rheological properties, enabling the formation of a uniform and defect-free wet film after coating, avoiding the introduction of unnecessary macropores due to excessive shrinkage or cracking during drying.

[0055] Precise control of the drying process: The drying temperature and time after the inner layer of the medium coating are precisely controlled using a programmed temperature rise profile. In the initial stage, a lower temperature is used to slowly evaporate the solvent, preventing rapid surface curing and the formation of a "hard shell effect" that hinders the escape of internal solvent, potentially leading to large pores or cracks. As the solvent is gradually removed, the temperature is increased to ensure full curing of the binder and the mechanical stability of the medium layer. This gradient drying method effectively suppresses the formation and interconnection of pores within the coating, ensuring that the final medium layer has a uniform low porosity.

[0056] In this invention, the high porosity of the outer diffusion medium is achieved through the following specific engineering methods:

[0057] Polymethyl methacrylate (PMMA) microspheres are used as pore-forming agents: PMMA microspheres are added as transient pore-forming agents during the preparation of the outer diffusion medium slurry. These microspheres have an average particle size of 500 nm to 2 μm and are added at 10 wt% to 20 wt% of the total weight of the mixed fluoride powder. The PMMA microspheres begin to thermally decompose at low temperatures and completely decompose and volatilize during the pre-diffusion heat treatment stage, leaving no solid residue. The decomposition of the PMMA microspheres leaves regularly distributed and interconnected μm-sized pores within the medium layer. These pore networks increase the overall porosity of the medium layer. By precisely controlling the particle size and addition amount of the PMMA microspheres, the size, distribution, and connectivity of the formed pores can be precisely controlled, thereby achieving the target high porosity.

[0058] Synergistic effect of slurry solid content and particle size: The solid content of the outer diffusion medium slurry is between 25 wt% and 45 wt%. Compared to the inner layer, its solid content can be slightly lower to complement the effect of the pore-forming agent. The average particle size of the CoF2 and GaF3 powders is between 100 nm and 500 nm, slightly larger than the particles of the inner medium. The relatively larger particles naturally form larger gaps during stacking, which, combined with the pore-forming effect of the polymethyl methacrylate microspheres, jointly constructs an open and interconnected high-porosity network.

[0059] Characteristics of ethyl cellulose binder: Ethyl cellulose was chosen as the binder because its decomposition temperature is slightly higher than that of polyvinyl butyral, but it can still decompose completely during the pre-diffusion heat treatment stage. Its addition amount was optimized to ensure slurry stability and coating strength without over-filling interparticle gaps, thus not affecting the pore structure formed by the accumulation of polymethyl methacrylate microspheres and particles.

[0060] Furthermore, the mechanism of action of copper (Cu) in the inner diffusion medium is as follows:

[0061] Copper exists in the inner diffusion medium as CuO, with its content precisely measured between 1 wt% and 5 wt% of the total weight. During the main diffusion heat treatment stage, CuO reacts with Nd-rich elements in the grain boundary phase of the sintered NdFeB permanent magnet to form a eutectic phase with a lower melting point. This liquid phase can wet the grain boundaries of the main phase at high temperatures, providing a rapid pathway for the diffusion of dysprosium (Dy) ions across the grain boundaries. Specifically:

[0062] Lowering the activation energy for Dy diffusion: The presence of the liquid phase lowers the activation energy required for Dy diffusion at the solid-solid interface, promoting the movement of Dy ions. The disordered atomic arrangement in the liquid phase facilitates atomic migration.

[0063] Expanding the effective diffusion path: The liquid phase can penetrate into the original nanoscale grain boundary gaps, effectively increasing the diffusion cross-sectional area available to Dy ions.

[0064] Promoting grain boundary modification: The introduction of Cu can promote the enrichment of Dy at the grain boundary and form a (Nd,Dy)-rich phase with Nd, optimizing the composition and structure of the grain boundary phase, such as forming a thin-layered or continuous shell structure that covers the main phase grains, thereby enhancing the magnetocrystalline anisotropy of the grains.

[0065] This synergistic effect ensures that Dy can still permeate uniformly in a controlled and efficient manner under the constraint of the compactness of the inner medium, ultimately forming an optimized Dy-rich core-shell structure and improving coercivity.

[0066] Furthermore, the mechanism of action of fluorides in the outer diffusion medium is as follows:

[0067] CoF2 and GaF3 in the outer diffusion medium play a key role in the main diffusion heat treatment stage, and the mechanism is mainly reflected in:

[0068] Vapor transport enhancement: Fluorides exhibit volatility at high temperatures and can react with rare earth elements and iron on or inside the permanent magnet to generate volatile fluorides. These volatile species act as transport media, delivering Co and Ga elements to the open pores and grain boundaries inside the permanent magnet via a vapor transport mechanism. Vapor transport has a higher transport rate than solid-phase diffusion, thereby accelerating the overall penetration depth and uniformity of Co and Ga.

[0069] Lowering Melting Point and Wetting Properties: At high temperatures, CoF2 and GaF3 can form low-melting-point eutectic phases with trace amounts of rare earth elements or the main phase at the grain boundaries of permanent magnets. These liquid phases can better wet the surface and grain boundaries of permanent magnet grains, providing a rapid channel for the diffusion of Co and Ga across grain boundaries. This liquid-phase assisted diffusion mechanism is somewhat similar to the role of inner-layer Cu, but the effect of the fluoride matrix is ​​stronger, especially with the addition of high porosity, further enhancing the efficiency and rate of diffusion.

[0070] Purifying grain boundaries: Fluorides have a certain "purifying" effect, and can react with some non-magnetic oxides or other impurities at the grain boundaries to form volatile products or inert solid phases, thereby optimizing the grain boundary structure, reducing pinning centers of magnetization reversal, and further improving coercivity.

[0071] By introducing fluoride media, Co and Ga can permeate at a higher rate and over a wider range, effectively overcoming the diffusion barriers they encounter in single oxide media.

[0072] The sintered NdFeB permanent magnet blank used in this invention has an average grain size between 3 μm and 8 μm. This grain size range is chosen to ensure a high remanent magnetic induction while providing suitable grain boundary density and diffusion paths for subsequent grain boundary diffusion. If the grain size is too large, the grain boundary density will be insufficient, making it difficult for the diffusing elements to penetrate uniformly; if the grain size is too small, it will lead to excessive grain growth during subsequent diffusion, affecting the magnetic properties.

[0073] Through the above technical solution, the present invention achieves the following beneficial technical effects:

[0074] This invention, through the design of inner and outer layers of a gradient composite diffusion medium combined with a multi-stage heat treatment process, successfully minimizes the effective diffusion rate difference of key elements such as Dy, Co, Cu, and Ga in sintered NdFeB permanent magnets to within 5%. This precise control is superior to traditional single diffusion medium methods, which typically lead to element diffusion rate differences exceeding 20%, resulting in uneven element distribution.

[0075] This invention effectively avoids the macroscopic inhomogeneities and microscopic heterogeneities such as "rare earth-rich layers" and "rare earth-poor regions" that occur in traditional technologies. Permanent magnets treated by this method exhibit a continuous and uniform Dy, Co, and Ga concentration gradient from the surface to the interior, especially in Nd₂Fe₂. 14 A uniform, Dy-rich hard magnetic shell with ideal composition is formed on the surface of the B main phase grains, while Co and Ga are uniformly distributed at the grain boundaries or within the grains, thus comprehensively optimizing the overall magnetic properties.

[0076] Due to the formation of a uniform and optimized Dy-rich shell structure and uniformly distributed Co and Ga elements, the magnetocrystalline anisotropy field is effectively enhanced, and the grain boundary phase structure is optimized. Experimental data show that the intrinsic coercivity of the sintered NdFeB permanent magnets prepared using the method of this invention is increased by 20% to 35% compared with the untreated magnet blank, and with the same amount of rare earth elements, the coercivity performance surpasses that of existing single diffusion medium technologies.

[0077] This invention ensures a high degree of consistency in the microstructure and elemental distribution of permanent magnets in each batch through refined synergistic diffusion, thereby reducing the batch volatility of coercivity to below 3%, which is far lower than the volatility range of more than 8% in the prior art.

[0078] The Co element introduced during the main diffusion stage permeates into Nd2Fe. 14 The substitution of Fe atoms in the B-dominant lattice effectively improves the Nd2Fe... 14The Curie temperature and magnetocrystalline anisotropy of the B phase improve the problem of magnetic performance decay in permanent magnets at high temperatures. Simultaneously, the introduction of Ga element also optimizes the thermal stability of the grain boundary phase. At 120℃, the irreversible flux loss of the permanent magnet prepared by this invention is reduced by 15% to 25% compared to existing technologies, and its magnetic stability at 150℃ is also improved. Detailed Implementation

[0079] This invention provides a method for preparing high-performance sintered NdFeB permanent magnets based on multi-element synergistic diffusion. The aim is to improve the coercivity, batch stability, and high-temperature magnetic properties of sintered NdFeB permanent magnets by precisely controlling the diffusion kinetics of multiple elements in the permanent magnet. The technical solution of this invention will be described in detail below with reference to specific embodiments to ensure that those skilled in the art can clearly understand and reproduce all the technical contents of this invention.

[0080] Example 1: Preparation of sintered NdFeB permanent magnet blanks:

[0081] The nominal composition of the pre-alloyed powder is Nd. 30.5 Fe bal B 1.0 Co 1.0 Ga 0.2 Cu 0.1 Zr 0.05 (Atomic percentage), prepared by hydrogen explosion pulverization and jet milling, with an average particle size of 4.2 μm and an oxygen content of 550 ppm; pressed under a 180 MPa pressure in a 1.8 T directional magnetic field; and under a vacuum degree better than 4 × 10⁻⁶. -4 In a vacuum furnace, the temperature was increased to 1065°C at a rate of 10°C per minute and held for 3 hours; subsequently, it was aged at 600°C for 2 hours to obtain a blank with an average grain size of 5.1 μm. The initial magnetic properties were remanence of 1.42 T, coercivity of 920 kA / m, and maximum energy product of 405 kJ / m. 3 .

[0082] Preparation of inner diffusion medium slurry: Dysprosium-rich oxide particles were prepared by mixing dysprosium oxide with an average particle size of 120 nm and a purity of 99.95% with copper oxide with an average particle size of 50 nm and a purity of 99.9% at a weight ratio of 95:5. After being processed by a high-energy planetary ball mill, the mixture was calcined at 900°C for 3 hours under a high-purity argon atmosphere of 200 standard cubic centimeters per minute. After pulverization and sieving, the average particle size was 200 nm. The slurry was prepared by mixing 42% by weight of active particles, 1.0% by weight of polyethylene glycol octylphenyl ether, and 5% by weight of polyvinyl butyral. The solvent was a mixture of isopropanol and deionized water in a 2:1 ratio. After being processed by a planetary ball mill, the solid content was 43% by weight and the viscosity was 120 mPa·s.

[0083] Preparation of outer diffusion medium slurry: Cobalt-rich fluoride and gallium-rich fluoride particles were mixed at a weight ratio of 97:3, consisting of cobalt fluoride with an average particle size of 300 nm and a purity of 99.9% and gallium fluoride with an average particle size of 300 nm and a purity of 99.9%. The mixture was then processed in a high-purity argon glove box using a dry high-energy planetary ball mill. The slurry was prepared by mixing fluoride powder (38% by weight), polymethyl methacrylate microspheres with an average particle size of 1.2 μm (15% by weight), polycarboxylate dispersant (1.0% by weight), and ethyl cellulose (6% by weight). The solvent was anhydrous ethanol. The mixture was processed by a planetary ball mill, with a solid content of 38% by weight and a viscosity of 170 mPa·s.

[0084] The composite diffusion medium layer is coated: the inner layer is sprayed and then dried at 100℃ for 1 hour and 130℃ for 1.5 hours to form an inner diffusion medium with a thickness of 25μm and a dry porosity of 18.5%; the outer layer is dip-coated and then dried at 80℃ for 1 hour and 100℃ for 1 hour to form an outer diffusion medium with a thickness of 50μm and a dry porosity of 33.0%.

[0085] Multi-stage thermal diffusion treatment: Pre-diffusion heat treatment at 5×10 -3 Under vacuum conditions, the temperature was increased to 450°C at a rate of 8°C per minute and held for 2 hours; the main diffusion heat treatment was carried out at 4×10⁻⁶ ℃. -4 Under vacuum, the temperature was increased to 950°C at a rate of 10°C per minute and held for 8 hours; followed by diffusion annealing at 8×10⁻⁶. -4 Under vacuum conditions, the temperature is reduced to 600°C at a rate of 5°C per minute and held for 4 hours.

[0086] Subsequent treatment: Ultrasonic cleaning with a 1% (w / w) dilute oxalic acid solution for 5 minutes, followed by rinsing with deionized water and hot air drying; surface passivation by immersion in a phosphate solution at 50℃ and pH 4.0 for 15 minutes; and final magnetic properties after 5T pulse magnetization: remanence 1.42T, intrinsic coercivity 1242 kA / m, and maximum energy product 410 kJ / m. 3 Irreversible flux loss at 120℃ is 8%, and coercivity fluctuates by 2.5% batch.

[0087] Example 2: Preparation of sintered NdFeB permanent magnet blanks:

[0088] The pre-alloyed powder composition was the same as in Example 1, with an average particle size of 4.0 μm and an oxygen content of 530 ppm; the pressing conditions were the same as in Example 1; sintering was carried out under a vacuum degree better than 4 × 10⁻⁶. -4 In a vacuum furnace, the temperature was increased to 1060°C at a rate of 10°C per minute and held for 3 hours; then aged at 600°C for 2 hours to obtain a blank with an average grain size of 3.0 μm. The initial magnetic properties were remanence of 1.40 T, coercivity of 910 kA / m, and maximum energy product of 400 kJ / m. 3 .

[0089] Preparation of inner diffusion medium slurry: Dysprosium-rich oxide particles were prepared by mixing dysprosium oxide with an average particle size of 100 nm and a purity of 99.95% with copper oxide with an average particle size of 40 nm and a purity of 98:2 by weight. After being processed by a high-energy planetary ball mill, the mixture was calcined at 880°C for 4 hours under a high-purity argon atmosphere of 180 standard cubic centimeters per minute. After pulverization and sieving, the average particle size was 180 nm. The slurry was prepared by mixing 45% by weight of active particles, 0.8% by weight of polyethylene glycol octylphenyl ether, and 4% by weight of polyvinyl butyral. The solvent was a mixture of isopropanol and deionized water in a 3:1 ratio. After being processed by a planetary ball mill, the solid content was 45% by weight and the viscosity was 100 mPa·s.

[0090] Preparation of outer diffusion medium slurry: Cobalt-rich fluoride and gallium-rich fluoride particles were mixed at a weight ratio of 99:1, consisting of cobalt fluoride with an average particle size of 200 nm and a purity of 99.9% and gallium fluoride with an average particle size of 200 nm and a purity of 99.9%. The mixture was then processed in a high-purity argon glove box using a dry high-energy planetary ball mill (grinding for 3 hours at 380 rpm, ball-to-particle ratio of 12:1). The slurry was prepared by mixing 40% fluoride powder, 12% polymethyl methacrylate microspheres with an average particle size of 0.8 μm, 0.8% polycarboxylate dispersant, and 5% ethyl cellulose in anhydrous ethanol. The slurry was processed in a planetary ball mill (grinding for 7 hours at 320 rpm, ball-to-particle ratio of 7:1), with a solid content of 40% wt% and a viscosity of 150 mPa·s.

[0091] Coating of composite diffusion medium layer: The inner layer is sprayed (parameters same as in Example 1), dried at 110℃ for 1.5 hours and 140℃ for 1 hour to form an inner diffusion medium with a thickness of 20μm and a dry porosity of 15.0%; the outer layer is dip-coated (parameters same as in Example 1), dried at 90℃ for 1.2 hours and 110℃ for 0.8 hours to form an outer diffusion medium with a thickness of 40μm and a dry porosity of 30.0%.

[0092] Multi-stage thermal diffusion treatment: Pre-diffusion heat treatment at 6×10 -3 Under vacuum conditions, the temperature was increased to 400°C at a rate of 5°C per minute and held for 3 hours; the main diffusion heat treatment was carried out at 5×10⁻⁶ ℃. -4 Under vacuum conditions, the temperature was increased to 850°C at a rate of 8°C per minute and held for 4 hours; followed by diffusion annealing at 9×10⁻⁶. -4 Under vacuum conditions, the temperature is reduced to 550°C at a rate of 3°C per minute and held for 5 hours.

[0093] Subsequent processing: Same as in Example 1, the final magnetic properties are: remanence 1.40T, intrinsic coercivity 1184kA / m, and maximum energy product 402kJ / m. 3Irreversible flux loss at 120℃ is 10%, and coercivity fluctuates by 2.8% batch-to-batch.

[0094] Example 3: Preparation of sintered NdFeB permanent magnet blanks:

[0095] The pre-alloyed powder composition was the same as in Example 1, with an average particle size of 4.5 μm and an oxygen content of 580 ppm; the pressing conditions were the same as in Example 1; sintering was carried out under a vacuum degree better than 4 × 10⁻⁶. -4 In a vacuum furnace, the temperature was increased to 1070°C at a rate of 10°C per minute and held for 2.5 hours; then aged at 620°C for 1.5 hours to obtain a blank with an average grain size of 8.0 μm. The initial magnetic properties were remanence of 1.43 T, coercivity of 925 kA / m, and maximum energy product of 408 kJ / m. 3 .

[0096] Preparation of inner diffusion medium slurry: Dysprosium-rich oxide particles were prepared by mixing dysprosium oxide with an average particle size of 150 nm and a purity of 99.95% with copper oxide with an average particle size of 80 nm and a purity of 99.9% at a weight ratio of 90:10. After being processed by a high-energy planetary ball mill, the mixture was calcined at 920°C for 2.5 hours under a high-purity argon atmosphere of 220 standard cubic centimeters per minute. After pulverization and sieving, the average particle size was 220 nm. The slurry was prepared by mixing 40% by weight of active particles, 1.5% by weight of polyethylene glycol octylphenyl ether, and 6% by weight of polyvinyl butyral. The solvent was a 1:1 mixture of isopropanol and deionized water. After being processed by a planetary ball mill, the solid content was 40% by weight and the viscosity was 150 mPa·s.

[0097] Preparation of outer diffusion medium slurry: Cobalt-rich fluoride and gallium-rich fluoride particles were mixed at a weight ratio of 90:10, consisting of cobalt fluoride with an average particle size of 400 nm and a purity of 99.9% and gallium fluoride with an average particle size of 400 nm and a purity of 99.9%. The mixture was then processed in a high-purity argon glove box using a dry high-energy planetary ball mill. The slurry was prepared by mixing fluoride powder (35% by weight), polymethyl methacrylate microspheres with an average particle size of 1.5 μm (18% by weight), polycarboxylate dispersant (1.2% by weight), and ethyl cellulose (7% by weight). The solvent was anhydrous ethanol. The mixture was processed by a planetary ball mill, with a solid content of 35% by weight and a viscosity of 200 mPa·s.

[0098] The composite diffusion medium layer is coated: the inner layer is brush-coated and dried at 120°C for 2 hours and 150°C for 1 hour to form an inner diffusion medium with a thickness of 30 μm and a dry porosity of 20.0%; the outer layer is spray-coated (with parameters the same as in Example 1) and dried at 85°C for 1.5 hours and 115°C for 0.5 hours to form an outer diffusion medium with a thickness of 60 μm and a dry porosity of 35.0%.

[0099] Multi-stage thermal diffusion treatment: Pre-diffusion heat treatment at 4×10 -3Under vacuum conditions, the temperature was increased to 500°C at a rate of 10°C per minute and held for 1.5 hours; the main diffusion heat treatment was carried out at 3×10⁻⁶ ℃. -4 Under vacuum, the temperature was increased to 950°C at a rate of 12°C per minute and held for 12 hours; followed by diffusion annealing at 7×10⁻⁶. -4 Under vacuum conditions, the temperature is reduced to 650°C at a rate of 7°C per minute and held for 3 hours.

[0100] Subsequent processing: Same as in Example 1, the final magnetic properties are: remanence 1.43T, intrinsic coercivity 1240kA / m, and maximum energy product 415kJ / m. 3 Irreversible flux loss at 120℃ is 7%, and coercivity fluctuates by 2.2% batch.

[0101] Example 4: Preparation of sintered NdFeB permanent magnet blanks:

[0102] The pre-alloyed powder composition was the same as in Example 1, with an average particle size of 4.3 μm and an oxygen content of 560 ppm; the pressing conditions were the same as in Example 1; sintering was carried out under a vacuum degree better than 4 × 10⁻⁶. -4 In a vacuum furnace, the temperature was increased to 1065°C at a rate of 10°C per minute and held for 3 hours; then aged at 610°C for 2 hours to obtain a blank with an average grain size of 6.0 μm. The initial magnetic properties were remanence of 1.41 T, coercivity of 915 kA / m, and maximum energy product of 403 kJ / m. 3 .

[0103] Preparation of inner diffusion medium slurry: Dysprosium-rich oxide particles were prepared by mixing dysprosium oxide with an average particle size of 130 nm and a purity of 99.95% with copper oxide with an average particle size of 60 nm and a purity of 92:8 by weight. After being processed by a high-energy planetary ball mill, the mixture was calcined at 910°C for 3.5 hours under a high-purity argon atmosphere of 190 standard cubic centimeters per minute. After pulverization and sieving, the average particle size was 210 nm. The slurry was prepared by mixing 43% by weight of active particles, 1.2% by weight of polyethylene glycol octylphenyl ether, and 5.5% by weight of polyvinyl butyral. The solvent was a mixture of isopropanol and deionized water in a 2.5:1 ratio. After being processed by a planetary ball mill, the solid content was 43% by weight and the viscosity was 130 mPa·s.

[0104] Preparation of outer diffusion medium slurry: Cobalt-rich fluoride and gallium-rich fluoride particles were mixed at a weight ratio of 95:5, consisting of cobalt fluoride with an average particle size of 350 nm and a purity of 99.9% and gallium fluoride with an average particle size of 350 nm and a purity of 99.9%. The mixture was then processed in a high-purity argon glove box using a dry high-energy planetary ball mill. The slurry was prepared by mixing fluoride powder (37% by weight), polymethyl methacrylate microspheres with an average particle size of 1.3 μm (16% by weight), polycarboxylate dispersant (1.1% by weight), and ethyl cellulose (6.5% by weight). The solvent was anhydrous ethanol. The mixture was processed by a planetary ball mill, resulting in a solid content of 37% by weight and a viscosity of 180 mPa·s.

[0105] Coating of composite diffusion medium layer: The inner layer is coated by dip coating (parameters same as in Example 1), dried at 105℃ for 1.2 hours and 135℃ for 1.3 hours to form an inner diffusion medium with a thickness of 28μm and a dry porosity of 17.0%; the outer layer is coated by dip coating (parameters same as in Example 1), dried at 88℃ for 1.3 hours and 105℃ for 0.7 hours to form an outer diffusion medium with a thickness of 55μm and a dry porosity of 32.0%.

[0106] Multi-stage thermal diffusion treatment: Pre-diffusion heat treatment at 5.5×10 -3 Under vacuum conditions, the temperature was increased to 480°C at a rate of 9°C per minute and held for 2.2 hours; the main diffusion heat treatment was carried out at 3.5 × 10⁻⁶. -4 Under vacuum, the temperature was increased to 1000℃ at a rate of 15℃ per minute and held for 6 hours; followed by diffusion annealing at 7.5×10⁻⁶. -4 Under vacuum conditions, the temperature was reduced to 620°C at a rate of 6°C per minute and held for 4.5 hours.

[0107] Subsequent processing: Same as in Example 1, the final magnetic properties are: remanence 1.41T, intrinsic coercivity 1222kA / m, and maximum energy product 408kJ / m. 3 Irreversible flux loss at 120℃ is 9%, and coercivity fluctuates by 2.6% batch.

[0108] Comparative Example 1: Traditional single dysprosium oxide diffusion:

[0109] Preparation of sintered NdFeB permanent magnet blanks: The process was identical to that in Example 1, with an average grain size of 5.1 μm and initial magnetic properties of 1.42 T remanence, 920 kA / m coercivity, and a maximum energy product of 405 kJ / m. 3 .

[0110] Preparation of diffusion medium slurry: Dysprosium oxide slurry was prepared only: 45% by weight of dysprosium oxide with an average particle size of 120 nm and a purity of 99.95%, 1.0% by weight of polyethylene glycol octylphenyl ether, and 5% by weight of polyvinyl butyral. The solvent was a 2:1 mixture of isopropanol and deionized water. The mixture was processed by a planetary ball mill, with a solid content of 46% by weight and a viscosity of 130 mPa·s.

[0111] Coating diffusion medium layer: Using a spraying method (parameters same as in Example 1), dry at 100°C for 1 hour and at 130°C for 1.5 hours to form a single diffusion layer with a thickness of 75 μm and a dry porosity of 19.0%.

[0112] Thermal diffusion treatment: exactly the same as in Example 1 (pre-diffusion, main diffusion, and post-diffusion parameters are the same).

[0113] Subsequent processing: exactly the same as in Example 1, the final magnetic properties are: remanence 1.41T, intrinsic coercivity 1012kA / m, and maximum energy product 398kJ / m. 3 Irreversible flux loss at 120℃ is 22%, and coercivity fluctuates by 8.5% batch-to-batch.

[0114] Comparative Example 2, composite diffusion but without copper oxide in the inner layer:

[0115] Preparation of sintered NdFeB permanent magnet blanks: The process was identical to that in Example 1, with an average grain size of 5.1 μm and initial magnetic properties of 1.42 T remanence, 920 kA / m coercivity, and a maximum energy product of 405 kJ / m. 3 .

[0116] Preparation of inner diffusion medium slurry: Dysprosium-rich oxide particles were made using only dysprosium oxide with an average particle size of 120 nm and a purity of 99.95% (without copper oxide). After being processed by a high-energy planetary ball mill (parameters same as in Example 1), the particles were calcined at 900°C for 3 hours under a high-purity argon atmosphere of 200 standard cubic centimeters per minute. After pulverization and sieving, the average particle size was 200 nm. The slurry was prepared with 42% by weight of active particles, 1.0% by weight of polyethylene glycol octylphenyl ether, and 5% by weight of polyvinyl butyral. The solvent was a 2:1 mixture of isopropanol and deionized water. After being processed by a planetary ball mill (parameters same as in Example 1), the solid content was 43% by weight and the viscosity was 125 mPa·s.

[0117] Preparation of outer diffusion medium slurry: exactly the same as in Example 1.

[0118] Coated composite diffusion medium layer: exactly the same as in Example 1 (inner layer thickness 25 μm, porosity 18.5%, outer layer thickness 50 μm, porosity 33.0%).

[0119] Multi-stage thermal diffusion treatment: exactly the same as in Example 1.

[0120] Subsequent processing: exactly the same as in Example 1, the final magnetic properties are: remanence 1.42T, intrinsic coercivity 1086kA / m, and maximum energy product 400kJ / m. 3 Irreversible flux loss at 120℃ is 15%, and coercivity fluctuates by 4.8% batch-to-batch.

[0121] The magnetic properties and high-temperature stability data of the permanent magnets prepared through the above embodiments and comparative examples are shown in the table below:

[0122]

[0123] Comparing Examples 1-4 with Comparative Example 1, it can be seen that the composite diffusion medium using an inner layer of dysprosium-rich oxide and an outer layer of cobalt-rich gallium fluoride improves intrinsic coercivity by 29%-35%, while traditional single dysprosium oxide diffusion only improves it by 10%. Simultaneously, composite diffusion reduces irreversible flux loss at 120℃ to 7%-10%, far lower than the 22% of single diffusion, and batch-to-batch coercivity fluctuation is controlled at 2.2%-2.8%, significantly better than the 8.5% of single diffusion. This indicates that the composite diffusion medium can achieve multi-element synergistic diffusion, avoiding the "rare earth-rich layer" and "rare earth-poor region" problems caused by single diffusion, and optimizing the uniformity of element distribution.

[0124] Compared with Comparative Example 2 (without copper oxide in the inner layer), Example 1, containing copper oxide, showed a 14.4% increase in intrinsic coercivity (1242 kA / m) compared to Comparative Example 2 (1086 kA / m), a 46.7% decrease in irreversible flux loss at 120°C (8%) compared to Comparative Example 2 (15%), and a 47.9% decrease in batch coercivity fluctuation (2.5%) compared to Comparative Example 2 (4.8%). This is because copper oxide forms a low-melting-point eutectic phase with the neodymium-rich phase at the grain boundaries of the permanent magnet, reducing the dysprosium diffusion activation energy, expanding the diffusion path, promoting uniform dysprosium penetration, and forming an optimized dysprosium-rich shell structure.

[0125] Raw grain size: Both Example 2 (3.0 micrometers) and Example 3 (8.0 micrometers) maintained excellent performance, indicating that a grain size range of 3-8 micrometers can balance remanence and diffusion uniformity. Too large or too small a grain size will affect the performance (for example, if the grain size is too small, it will easily lead to excessive grain growth during diffusion, and if it is too large, the grain boundary density will be insufficient).

[0126] Main diffusion temperature: The intrinsic coercivity of Example 4 (1000℃) and Example 2 (850℃) is similar (1222 vs 1184 kA / m), indicating that 850-1000℃ is an effective temperature range and can be adjusted according to actual needs;

[0127] Porosity: The design of 15%-20% porosity in the inner layer and 30%-35% porosity in the outer layer can realize controlled diffusion of dysprosium and accelerated diffusion of cobalt and gallium respectively, which is the core guarantee for multi-element rate synergy (difference <5%).

[0128] This invention utilizes the synergy of composite diffusion medium and multi-stage thermal diffusion to stably prepare sintered NdFeB permanent magnets with high coercivity, low high-temperature loss, and high batch stability, which has significant application value in fields with stringent requirements for magnetic properties, such as new energy vehicles and wind power.

[0129] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-performance sintered NdFeB permanent magnets based on multi-element synergistic diffusion, characterized in that, Includes the following steps: S1. Prepare a sintered NdFeB permanent magnet blank, wherein the sintered NdFeB permanent magnet blank has an average grain size between 3μm and 8μm; S2. Prepare a composite diffusion medium slurry, wherein the composite diffusion medium slurry comprises an inner diffusion medium slurry and an outer diffusion medium slurry; S3. Coating a composite diffusion medium layer, wherein coating the composite diffusion medium layer includes: firstly, uniformly coating the inner diffusion medium slurry onto the surface of the sintered NdFeB permanent magnet blank, and drying it to form the inner diffusion medium, wherein the dry porosity of the inner diffusion medium is between 15% and 20%; subsequently, uniformly coating the outer diffusion medium slurry onto the permanent magnet surface on which the inner diffusion medium has been formed, and drying it to form the outer diffusion medium, wherein the dry porosity of the outer diffusion medium is between 30% and 35%. S4. Perform multi-stage thermal diffusion treatment, which includes: pre-diffusion heat treatment, main diffusion heat treatment and post-diffusion annealing treatment; wherein, the main diffusion heat treatment is carried out at a diffusion temperature of 850°C to 1000°C and held for 4 to 12 hours to achieve synergistic diffusion of multiple elements, so that the effective diffusion rate difference of key elements such as dysprosium (Dy), cobalt (Co) and gallium (Ga) inside the permanent magnet is within 5%; S5. Subsequent processing.

2. The preparation method according to claim 1, characterized in that, In step S2, the preparation process of the inner diffusion medium slurry is as follows: First, dysprosium-rich oxide particles are prepared by mixing Dy2O3 powder and CuO powder in a specific ratio and then reacting them through a solid-state reaction or co-precipitation method. The average particle size of the Dy2O3 powder is between 50 nm and 200 nm, and the purity is greater than 99.95%. The average particle size of the CuO powder is between 20 nm and 100 nm, and the purity is greater than 99.9%. The Dy2O3 powder and CuO powder are mixed in a weight ratio of Dy2O3:CuO = (90-98):(2-10) and thoroughly mixed by ball milling. Subsequently, the mixed powder was calcined in an inert atmosphere at a temperature of 800°C to 950°C for 2 to 5 hours. Finally, the calcined powder is pulverized and sieved to obtain inner-layer diffusion medium active particles with an average particle size distribution between 100 nm and 300 nm.

3. The preparation method according to claim 2, characterized in that, The preparation process of the inner diffusion medium slurry in S2 also includes: The inner diffusion medium active particles are mixed with a dispersant, a binder, and a solvent, wherein the dispersant is polyethylene glycol octylphenyl ether, and the amount added is 0.5 wt% to 2 wt% of the total weight of the active particles; The binder is polyvinyl butyral, and the amount added is 3wt% to 7wt% of the total weight of the active particles; the solvent is a mixed solution of isopropanol and deionized water, with a volume ratio of 1:1 to 3:

1. The above components are placed in a planetary ball mill for mixing and grinding. The grinding time is 8 to 12 hours, the rotation speed is 200 to 400 rpm, and the ball-to-material ratio is 5:1 to 10:

1. The solid content of the inner diffusion medium slurry is between 30 wt% and 50 wt%, and the viscosity is between 50 mPa·s and 200 mPa·s.

4. The preparation method according to claim 1, characterized in that, The method of coating the inner diffusion medium slurry in S3 includes spraying, dipping or brushing. After the inner diffusion medium is coated, it is dried at a temperature of 100°C to 150°C for 1 to 3 hours to form a uniform and dense inner diffusion medium. The thickness of the inner diffusion medium is between 5 μm and 50 μm.

5. The preparation method according to claim 1, characterized in that, The preparation process of the outer diffusion medium slurry in S2 is as follows: First, cobalt-rich fluoride and gallium-rich fluoride particles are prepared. The cobalt-rich fluoride particles are prepared from CoF2 powder, with an average particle size between 100 nm and 500 nm and a purity greater than 99.9%. The gallium-rich fluoride particles were prepared from GaF3 powder, with an average particle size between 100 nm and 500 nm and a purity greater than 99.9%. CoF2 powder and GaF3 powder are mixed in a weight ratio of CoF2:GaF3 = (90-99):(1-10) and thoroughly mixed by high-energy ball milling or mechanical grinding for 2 to 6 hours at a speed of 300 to 500 rpm and a ball-to-powder ratio of 10:1 to 20:

1. The mixing process is carried out in an oxygen-free and water-free atmosphere.

6. The preparation method according to claim 5, characterized in that, The preparation process of the outer diffusion medium slurry in S2 further includes: The mixed fluoride powder is mixed with a pore-forming agent, a dispersant, a binder, and a solvent, wherein the pore-forming agent is polymethyl methacrylate microspheres with an average particle size of 500 nm to 2 μm, and the amount added is 10 wt% to 20 wt% of the total weight of the mixed fluoride powder; The dispersant is a polycarboxylate dispersant, and the amount added is 0.5 wt% to 1.5 wt% of the total weight of the mixed fluoride powder. The binder is ethyl cellulose, and the amount added is 4 wt% to 8 wt% of the total weight of the mixed fluoride powder; the solvent is anhydrous ethanol; The above components are placed in a planetary ball mill for mixing and grinding. The grinding time is 6 to 10 hours, the speed is 250 to 450 rpm, and the ball-to-material ratio is 5:1 to 10:

1. The outer diffusion medium slurry has a solid content between 25 wt% and 45 wt% and a viscosity between 80 mPa·s and 250 mPa·s.

7. The preparation method according to claim 1, characterized in that, The method of coating the outer diffusion medium slurry in S3 includes spraying or dipping. After the outer diffusion medium is coated, it is dried at a temperature of 80°C to 120°C for 1 to 2 hours to form a uniform outer diffusion medium. The thickness of the outer diffusion medium is between 10 μm and 100 μm.

8. The preparation method according to claim 1, characterized in that, The pre-diffusion heat treatment in S4 includes: The sintered NdFeB permanent magnet coated with the composite diffusion medium layer is heated to 300°C to 600°C at a heating rate of 5°C / min to 10°C / min, and held at this temperature for 1 hour to 3 hours. The pre-diffusion heat treatment is carried out in a vacuum environment with a vacuum degree greater than 10−2Pa, or in a high-purity argon atmosphere with an argon flow rate of 100sccm to 300sccm.

9. The preparation method according to claim 8, characterized in that, The main diffusion heat treatment in S4 includes: After the pre-diffusion heat treatment is completed, the permanent magnet is heated to a diffusion temperature of 850°C to 1000°C at a heating rate of 5°C to 15°C per minute, and held at this temperature for 4 to 12 hours.

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