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

Abstract

The invention belongs to the field of permanent magnet materials, and discloses a preparation method of a high-performance sintered neodymium-iron-boron permanent magnet based on multi-element synergistic diffusion. The method comprises the following steps: S1, preparing a sintered neodymium-iron-boron permanent magnet blank with the grain size of 3-8 microns; s2, preparing composite diffusion medium slurry, wherein the composite diffusion medium slurry comprises inner-layer diffusion medium slurry and outer-layer diffusion medium slurry; s3, a composite diffusion medium layer is coated, the drying porosity of the inner layer is 15-20%, and the drying porosity of the outer layer is 30-35%; s4, performing multi-stage thermal diffusion treatment, including pre-diffusion heat treatment, main diffusion heat treatment and post-diffusion annealing treatment; and S5, performing subsequent treatment. Through the synergistic effect of the composite diffusion medium and multi-stage thermal diffusion, uniform distribution of multiple elements is achieved, the intrinsic coercive force of the permanent magnet is improved by 20-35%, the coercive force batch fluctuation is reduced to 3% or below, the irreversible magnetic flux loss at the high temperature of 120 DEG C is reduced by 15-25%, and the permanent magnet is suitable for the fields of new energy and the like with strict requirements for magnetic performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of permanent magnet material preparation, and relates to a high-performance sintered neodymium-iron-boron permanent magnet preparation method based on multi-element synergistic diffusion. BACKGROUND

[0002] The high performance of sintered neodymium-iron-boron permanent magnets can no longer be satisfied by single heavy rare earth element diffusion, but requires the synergistic effect of rare earth elements, transition metal elements and doping elements and the like. These different elements differ in atomic radius, electronic structure and chemical activity, resulting in obvious differences in their diffusion activation energies. For example, the diffusion activation energy of heavy rare earth element dysprosium (Dy) is about 20% higher than that of transition metal element cobalt (Co). When a single diffusion medium is used for treatment, all the elements to be diffused are diffused under the same external conditions, and independent control of the diffusion rates of different elements cannot be achieved.

[0003] This single diffusion medium cannot effectively balance the diffusion kinetics of different elements, resulting in an imbalance in the diffusion rates of the elements. For example, some elements with a faster diffusion rate excessively penetrate into the interior of the magnet, while the heavy rare earth elements with a slower diffusion rate are excessively enriched near the surface layer of the magnet. This unbalanced diffusion behavior easily forms a "rich rare earth layer" in the surface layer region of the magnet, and a "poor rare earth zone" in the interior of the magnet. This macroscopic unevenness and microscopic heterogeneity in element distribution fundamentally destroys the uniform optimized structure expected for the improvement of the coercivity of the permanent magnet. The excessive rare earth in the surface layer forms a non-magnetic phase or a too thick grain boundary phase, which in turn reduces the effective magnet volume of the magnet or causes a loss of magnetic performance; and the poor rare earth zone in the interior becomes a weak link for magnetization reversal, which restricts the overall coercivity level of the magnet. As a result, the batch fluctuation of the coercivity of the sintered neodymium-iron-boron permanent magnet increases, and the measured results show that the fluctuation range even exceeds 8%.

[0004] Further, this non-ideal element distribution structure, especially the poor rare earth zone in the interior of the magnet, further exacerbates the decay of the magnetic performance at high temperatures, 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

[0005] The application provides a high-performance sintered Nd-Fe-B permanent magnet preparation method based on multi-element synergistic diffusion, aiming to overcome the existing limitations of single diffusion medium technology in regulating the diffusion kinetics of rare earth elements, transition metal elements and other doped elements, especially for the improvement of coercivity, batch stability and high-temperature magnetic properties of sintered Nd-Fe-B permanent magnets. The application designs and applies a structured and functionally graded composite diffusion medium, combined with an accurately controlled multi-stage thermal diffusion process, to achieve fine synergistic control of the diffusion rate of multiple key elements, thereby building a uniform and optimized element gradient distribution inside the permanent magnet, and finally preparing a sintered Nd-Fe-B permanent magnet with high coercivity, excellent batch stability and excellent high-temperature magnetic properties.

[0006] To achieve the above-mentioned application purposes, the application provides a high-performance sintered Nd-Fe-B permanent magnet preparation method based on multi-element synergistic diffusion, characterized by comprising the following steps: Step 1: Preparation of sintered Nd-Fe-B permanent magnet blank: through a conventional powder metallurgy process, pre-alloyed powder (for example, nominal composition Nd x Fe bal B y M z , wherein M represents additional elements such as Co, Ga, Cu, Zr, Al) is formed, sintered and preliminarily heat-treated to obtain a sintered Nd-Fe-B permanent magnet blank with a certain size and initial magnetic properties. The sintered Nd-Fe-B permanent magnet blank has an average grain size of 3-8 μm and contains Nd2Fe 14 B main phase, Nd-rich grain boundary phase and a small amount of secondary phases such as borides.

[0007] Step 2: Preparation of composite diffusion medium slurry: The composite diffusion medium comprises an inner diffusion medium and an outer diffusion medium.

[0008] The preparation process of the inner diffusion medium slurry is as follows: First, prepare Dy-rich oxide particles. The Dy-rich oxide particles are prepared by mixing Dy2O3 powder and CuO powder in a specific ratio and through solid phase reaction or coprecipitation method. The average particle size of the Dy2O3 powder is 50-200 nm, and the purity is greater than 99.95%. The average particle size of the CuO powder is 20-100 nm, and the purity is greater than 99.9%.

[0009] Mix the Dy2O3 powder and CuO powder in a weight ratio of Dy2O3:CuO=(90-98):(2-10), and then mix them thoroughly by high-energy ball milling or mechanical grinding, with a grinding time of 4-8 hours, a rotation speed of 300-500 rpm and a ball-to-material ratio of 10:1-20:1.

[0010] The mixed powder is calcined at a temperature of 800-950°C for 2-5 hours in an inert atmosphere to promote the formation of Dy-Cu-O intermetallic compound or Dy-Cu-O eutectic phase precursor with good sintering activity.

[0011] The calcined powder is crushed and sieved to obtain inner diffusion medium active particles with an average particle size distribution of 100-300 nm.

[0012] Secondly, the inner diffusion medium slurry is prepared. The inner diffusion medium active particles are mixed with a dispersant, a binder and a solvent. The dispersant is polyethylene glycol octylphenyl ether, and the addition amount is 0.5-2 wt% of the total weight of the active particles. The binder is polyvinyl butyral, and the addition amount is 3-7 wt% of the total weight of the active particles. The solvent is a mixed solution of isopropyl alcohol and deionized water with a volume ratio of 1:1 to 3:1.

[0013] The above components are placed in a planetary ball mill for mixing and grinding, the grinding time is 8-12 hours, the rotation speed is 200-400 rpm, and the ball-to-material ratio is 5:1 to 10:1 to ensure uniform dispersion of the slurry. The solid content of the inner diffusion medium slurry is 30-50 wt%, and the viscosity is 50-200 mPa・s.

[0014] The preparation process of the outer diffusion medium slurry is as follows: First, cobalt (Co) fluoride-rich and gallium (Ga) fluoride-rich particles are prepared. The CoF2 powder is used to prepare the CoF2-rich particles, with an average particle size of 100-500 nm and a purity of more than 99.9%. The GaF3 powder is used to prepare the GaF3-rich particles, with an average particle size of 100-500 nm and a purity of more than 99.9%.

[0015] The CoF2 powder and GaF3 powder are mixed in a weight ratio of CoF2:GaF3=(90-99):(1-10), and are thoroughly mixed by high-energy ball milling or mechanical grinding, with a grinding time of 2-6 hours, a rotation speed of 300-500 rpm, and a ball-to-material ratio of 10:1 to 20:1. This mixing process is carried out in an oxygen-free and water-free atmosphere to prevent hydrolysis of the fluoride.

[0016] Secondly, the 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 addition amount is 10-20 wt% of the total weight of the mixed fluoride powder.

[0017] The dispersant is a polycarboxylate dispersant, and the addition amount is 0.5wt% to 1.5wt% of the total weight of the mixed fluoride powder. The binder is ethyl cellulose, and the addition amount is 4wt% to 8wt% of the total weight of the mixed fluoride powder. The solvent is anhydrous ethanol.

[0018] The above components are placed in a planetary ball mill for mixing and grinding, the grinding time is 6 to 10 hours, the rotation speed is 250 to 450 rpm, and the ball-to-material ratio is 5:1 to 10:1. The solid content of the outer layer diffusion medium slurry is between 25wt% and 45wt%, and the viscosity is between 80mPa・s and 250mPa・s.

[0019] Third step, coating the composite diffusion medium layer: First, coat the inner layer diffusion medium. The inner layer diffusion medium slurry is uniformly coated on the surface of the sintered Nd-Fe-B permanent magnet blank by spraying, dipping or brushing. After coating, drying treatment is carried out at a temperature of 100℃ to 150℃, and the drying time is 1 to 3 hours, to remove the solvent and solidify the binder, forming a uniform and dense inner layer diffusion medium.

[0020] The thickness of the inner layer diffusion medium is between 5μm and 50μm. The dry porosity of the inner layer diffusion medium is between 15% and 20%. The accurate control of this porosity is achieved by adjusting the particle size distribution of the active particles in the inner layer diffusion medium, the solid content of the slurry, and the type and content of the binder.

[0021] Second, coat the outer layer diffusion medium. After the inner layer diffusion medium is completely dried and solidified, the outer layer diffusion medium slurry is uniformly coated on the surface of the permanent magnet with the inner layer diffusion medium formed by spraying or dipping. After coating, drying treatment is carried out at a temperature of 80℃ to 120℃, and the drying time is 1 to 2 hours, to remove the solvent and solidify the binder, forming a uniform outer layer diffusion medium.

[0022] The thickness of the outer layer diffusion medium is between 10μm and 100μm. The dry porosity of the outer layer diffusion medium is between 30% and 35%. The accurate control of this porosity is achieved by adjusting the addition amount, particle size distribution of the pore-forming agent, and the solid content of the outer layer diffusion medium slurry.

[0023] Fourth step, multi-stage heat diffusion treatment: The sintered Nd-Fe-B permanent magnet coated with the composite diffusion medium layer is placed in a vacuum furnace or an inert atmosphere furnace for heat treatment, and the heat treatment process is divided into the following three stages: First stage: pre-diffusion heat treatment.

[0024] The permanent magnet is heated to 300-600℃ at a heating rate of 5-10℃ / min and kept at this temperature for 1-3 hours. The goal of this stage is to: a. Thoroughly decompose and remove the organic binder and pore-forming agent in the diffusion medium slurry. For the inner layer diffusion medium, the decomposition of the binder helps to preliminarily solidify its ceramic matrix and form a stable low-porosity structure; for the outer layer diffusion medium, the decomposition of the polymethyl methacrylate pore-forming agent will generate pre-set high-porosity channels inside it.

[0025] b. Promote the preliminary interfacial reaction between the diffusion medium and the surface of the permanent magnet, forming a low-melting eutectic phase or intermediate reaction layer, providing a good diffusion channel for the subsequent main diffusion stage. For example, Dy2O3 reacts with Nd2Fe 14 B The Nd-rich phase on the surface or grain boundary of the main phase grains undergoes preliminary reaction, forming a Dy-Nd-O eutectic compound with a lower melting point. At the same time, CoF2 and GaF3 react with a small amount of Nd, Fe, etc. on the surface and inside the permanent magnet, generating volatile fluorides or promoting the formation of low-melting phases at the grain boundaries.

[0026] c. Eliminate residual stress inside the diffusion medium layer, ensuring the structural integrity and stability of the medium layer during subsequent high-temperature processing.

[0027] The heat treatment in this stage 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 100-300 sccm.

[0028] Second stage: main diffusion heat treatment.

[0029] After completing the pre-diffusion heat treatment, the furnace temperature is continued to be heated to a diffusion temperature of 850-1000℃ at a heating rate of 5-15℃ / min. It is kept at this temperature for 4-12 hours to achieve the synergistic diffusion of multiple elements. This stage is the step to achieve the core technical goal of the invention, and its mechanism includes: a. Controlled diffusion of dysprosium (Dy): Dy elements in the inner layer diffusion medium mainly exist in the form of Dy2O3, which is decomposed into Dy and O2 at high temperatures. The diffusion of Dy is mainly driven by the concentration gradient between the inner layer diffusion medium and the permanent magnet, and the diffusion rate is controlled by the diffusion coefficient of Dy in the permanent magnet matrix. 3+Dy ions penetrate into the interior of the permanent magnet through a grain boundary diffusion mechanism at or near the surface of the permanent magnet blank. The low porosity design of the inner layer diffusion medium increases the tortuosity of the diffusion path of Dy ions and limits the advancement speed of the diffusion front. At the same time, the copper (Cu) element in the inner layer medium and the rare earth element (Nd) in the interior of the permanent magnet form a low-melting-point copper-rich rare earth compound at high temperature, which wets the grain boundary to provide an additional fast channel for the grain boundary diffusion of Dy element, effectively reducing the effective diffusion activation energy of Dy, thereby improving the diffusion efficiency of Dy as a whole, but the diffusion rate is still effectively restricted by the overall low porosity of the inner layer medium. This ensures that Dy forms a uniform and thickness-controllable Dy-rich shell structure near the surface of the permanent magnet, avoiding premature enrichment of Dy and insufficient deep penetration.

[0030] b. Accelerated diffusion of cobalt (Co) and gallium (Ga): Co elements and Ga elements in the outer diffusion medium penetrate into the interior of the permanent magnet at a higher diffusion rate through the open channels provided by their high porosity (30% to 35%) at high temperature, combined with the catalytic effect of the fluoride medium. High porosity reduces the resistance to diffusion of Co and Ga, while CoF2 and GaF3 react with Fe or Nd elements in the interior of the permanent magnet at high temperature to form low-melting-point eutectic phases or gas-phase transport intermediates, further accelerating the transport of Co and Ga. Co elements mainly diffuse along the grain boundaries and part of the intragranular regions in the interior of the permanent magnet, entering the Nd2Fe 14 B substitute Fe atoms in the main phase lattice, improve the magnetic crystal anisotropy field of the main phase, and thus improve its intrinsic coercivity and high-temperature magnetic performance. Ga elements are mainly enriched in the grain boundary region, refining the grain size, optimizing the grain boundary structure, and inhibiting abnormal grain growth, thereby improving the coercivity and enhancing the oxidation resistance.

[0031] c. Multi-element synergistic regulation: By effectively slowing down the diffusion of Dy in the inner layer medium and supplementing the diffusion of Cu, and by accelerating and guiding the diffusion of Co and Ga in the outer layer medium, the present application realizes that the effective diffusion rates of key elements such as Dy, Co and Ga in the interior of the permanent magnet differ by less than 5%. This means that these elements can advance into the interior of the permanent magnet at a more similar rate, forming a more uniform and gradient distribution of element concentration, thereby avoiding the problems of surface "rich rare earth layer" and internal "poor rare earth region" that occur 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 the Nd2Fe 14 B, forming a uniform and ideal composition of the Dy-rich hard magnetic shell on the surface of the main phase grains, while obtaining uniformly distributed Co and Ga elements in the interior of the permanent magnet, optimizing the overall magnetic performance.

[0032] The heat treatment in this stage is carried out in a high vacuum environment with a vacuum degree greater than 10-3 Pa, or in a high-purity argon atmosphere with an argon flow rate of 50-200 sccm.

[0033] Third stage: post-diffusion annealing treatment.

[0034] After the main diffusion heat treatment is completed, the furnace temperature is slowly reduced to 500-700 DEG C at a cooling rate of 2-8 DEG C / min, and is kept at this temperature for 2-6 hours. The goal of this stage is to: a. Homogenize the diffusion products: at a relatively low temperature, promote the further homogenization and redistribution of the Dy, Co and Ga elements that have diffused into the interior of the permanent magnet within the grain boundary phase and the main phase grains. For example, promote the Dy 3+ ions form a more stable (Nd, Dy)2Fe 14 B core-shell structure, and optimize the thickness and composition of the Dy-rich grain boundary phase.

[0035] b. Eliminate internal stress: reduce the residual stress in the permanent magnet caused by high-temperature diffusion treatment, improve the mechanical strength of the permanent magnet and the smoothness of the magnetic domain wall movement.

[0036] c. Optimize the magnetic domain structure: promote the smooth movement of the magnetic domain wall, reduce the pinning effect in the magnetization reversal process, thereby further improving the intrinsic coercivity and residual magnetic induction.

[0037] The heat treatment in this stage 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 50-150 sccm.

[0038] Fifth step, subsequent treatment: After the multi-stage heat diffusion treatment is completed, the permanent magnet is taken out of the furnace, and necessary surface cleaning is carried out to remove the residual diffusion medium layer and surface oxides, followed by surface passivation treatment, magnetization and detection, to obtain the final high-performance sintered neodymium-iron-boron permanent magnet product.

[0039] The technical solution provided by the present application realizes precise control of the multi-element diffusion behavior in the preparation process of sintered neodymium-iron-boron permanent magnets through the synergistic effect of the above steps.

[0040] In the present application, the low porosity of the inner layer diffusion medium is realized through the following specific engineering means: Synergistic effect of Dy2O3 and CuO nanoparticles: The Dy2O3 powder used has an average particle size of 50-200 nm, and the CuO powder has an average particle size of 20-100 nm. These nanoscale particles can be more closely packed during slurry preparation due to their high specific surface area and surface energy, through van der Waals forces or electrostatic repulsion. During the drying and pre-diffusion heat treatment, the sintering necks between small-sized particles form at a lower temperature, which is conducive to the formation of a dense ceramic matrix. CuO as a sintering aid forms a liquid phase or solid solution between Dy2O3 particles, further promoting the densification of the particles and the growth of the necks, thereby reducing the porosity of the final layer.

[0041] Optimization of slurry solid content and binder: The solid content of the inner layer diffusion medium slurry is precisely between 30wt% and 50wt%. Higher solid content ensures the number of active particles in a unit volume of slurry, which is conducive to the formation of a dense coating. Polyvinyl butyral as a binder, its content is 3wt% to 7wt%. Polyvinyl butyral forms a high-strength, low-shrinkage organic network during drying, which stably fixes the active particles together.

[0042] During the pre-diffusion heat treatment stage, polyvinyl butyral decomposes and volatilizes at a temperature range of 300-500°C, and its decomposition products have little effect on pore formation, or form very fine, non-connected pores, further ensuring the density of the ceramic matrix. The ratio of binder to solvent is precisely optimized to ensure that the slurry has suitable rheological properties, and after coating, a uniform and defect-free wet film can be formed, avoiding the introduction of unnecessary macroscopic pores due to excessive shrinkage or cracking during the drying process.

[0043] Precise control of the drying process: The drying temperature and time of the inner layer medium after coating are precisely controlled by a programmed temperature curve. A lower temperature is used in the initial stage to slowly evaporate the solvent, avoiding the formation of a "hard shell effect" that hinders the escape of internal solvents, thereby producing large pores or cracks. As the solvent is gradually removed, the temperature is increased to ensure the complete solidification of the binder and the mechanical stability of the medium layer. This gradient drying method effectively inhibits the generation and connection of internal pores in the coating, ensuring that the final medium layer has uniform low porosity.

[0044] In the present application, the high porosity of the outer layer diffusion medium is achieved through the following specific engineering means: Poly(methyl methacrylate) microspheres as pore-forming agent: In the preparation of the outer layer diffusion medium slurry, poly(methyl methacrylate) microspheres are added as a transient pore-forming agent, with an average particle size of 500 nm to 2 μm and an addition amount of 10 wt% to 20 wt% of the total weight of the mixed fluoride powder. Poly(methyl methacrylate) 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 poly(methyl methacrylate) microspheres leaves a regularly distributed and interconnected μm-level pore network inside the medium layer, which increases the overall porosity of the medium layer. By precisely controlling the particle size and addition amount of poly(methyl methacrylate) microspheres, the size, distribution, and connectivity of the formed pores can be precisely controlled, thereby achieving the target high porosity.

[0045] Synergistic effect of slurry solid content and particle size: The solid content of the outer layer diffusion medium slurry is between 25 wt% and 45 wt%. Compared to the inner layer, its solid content can be slightly lower to cooperate with the action of the pore-forming agent. The average particle size of the CoF2 and GaF3 powder is between 100 nm and 500 nm, slightly larger than the particles of the inner layer medium. The relatively large particles naturally form larger gaps when packed, which, combined with the pore-forming effect of poly(methyl methacrylate) microspheres, together build an open and interconnected high-porosity network.

[0046] Properties of ethyl cellulose binder: Ethyl cellulose is chosen as the binder, with a decomposition temperature slightly higher than that of polyvinyl butyl, but it can still completely decompose during the pre-diffusion heat treatment stage. Its addition amount is optimized to ensure the stability of the slurry and the strength of the coating without excessively filling the interstitial gaps, thereby not affecting the pore structure formed by the packing of poly(methyl methacrylate) microspheres and particles.

[0047] Further, the mechanism of action of copper (Cu) elements in the inner layer diffusion medium is as follows: Copper elements exist in the form of CuO in the inner layer diffusion medium, with a content precisely between 1 wt% and 5 wt% of the total weight. During the main diffusion heat treatment stage, CuO reacts with the Nd-rich elements in the grain boundary phase of the sintered neodymium-iron-boron 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 fast channel for the grain boundary diffusion of dysprosium (Dy) ions. Specifically: Lowering the activation energy for Dy diffusion: The presence of the liquid phase lowers the activation energy required for Dy to diffuse across the solid-solid interface, promoting the movement of Dy ions. The disordered atomic arrangement in the liquid phase makes atomic migration easier.

[0048] Expanding the effective diffusion path: The liquid phase can penetrate into the original nanoscale intergranular gaps, effectively increasing the available diffusion cross-sectional area for Dy ions.

[0049] Promote grain boundary modification: The introduction of Cu can promote the enrichment of Dy at the grain boundary and form (Nd, Dy)-rich phase with Nd, optimize the composition and structure of the grain boundary phase, such as forming a thin layer or a continuous shell structure that wraps the main phase grain, thereby enhancing the magnetic crystal anisotropy of the grain.

[0050] This synergistic effect ensures that Dy can still penetrate uniformly in a controlled and efficient manner under the density constraints of the inner layer medium, ultimately forming an optimized Dy-rich core-shell structure, thereby improving the coercivity.

[0051] Further, the mechanism of fluorides in the outer diffusion medium is as follows: CoF2 and GaF3 in the outer diffusion medium play a key role in the main diffusion heat treatment stage, and the mechanism mainly reflects in: Gas phase transmission promotion: Fluorides have a certain volatility at high temperatures and can react with rare earth elements and iron elements on the surface or inside of the permanent magnet to form volatile fluorides. These volatile species act as transmission media and transport Co and Ga elements to the open pores and grain boundaries inside the permanent magnet through the gas phase transmission mechanism. Gas phase transmission has a higher transmission rate than solid phase diffusion, thereby accelerating the overall penetration depth and uniformity of Co and Ga.

[0052] Lower melting point and wettability: CoF2 and GaF3 can form low-melting eutectic phases with a small amount of rare earth elements or main phases at the grain boundaries of the permanent magnet at high temperatures. These liquid phases can better wet the surface of the permanent magnet grain and the grain boundary, providing a fast channel for the grain boundary diffusion of Co and Ga. This liquid phase assisted diffusion mechanism is similar to the role of the inner layer Cu to some extent, but the fluorine matrix has a stronger effect, especially in combination with high porosity, which further enhances the efficiency and rate of diffusion.

[0053] Purification of grain boundaries: Fluorides have a certain "purification" 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 and reducing the pinning centers of magnetization reversal, further improving the coercivity.

[0054] By introducing fluorine medium, Co and Ga can penetrate at a higher rate and in a wider range, effectively overcoming the diffusion barriers encountered in single oxide medium.

[0055] The sintered neodymium-iron-boron permanent magnet blank used in the present application has an average grain size of 3-8 μm. This range of grain size is selected to ensure a high residual magnetic induction while providing suitable grain boundary density and diffusion path for subsequent grain boundary diffusion. If the grain size is too large, the grain boundary density is insufficient, and the diffusion elements are difficult to penetrate uniformly; if the grain size is too small, the subsequent diffusion process will cause excessive grain growth, affecting the magnetic properties.

[0056] By the above technical solution, the application achieves the following beneficial technical effects: The application successfully controls the effective diffusion rate of key elements such as Dy, Co, Cu and Ga in sintered neodymium-iron-boron permanent magnets within 5% by designing the inner layer and outer layer of the gradient composite diffusion medium and combining a multi-stage heat treatment process. This precise control is superior to the traditional single diffusion medium method, which usually results in a difference in element diffusion rate of more than 20%, thereby causing uneven element distribution.

[0057] The application effectively avoids the macroscopic unevenness and microscopic heterogeneity problems such as "rare earth-rich layer" and "rare earth-poor zone" in traditional technologies. The permanent magnet treated by the method has a continuous and uniform Dy, Co and Ga concentration gradient from the surface to the inside, especially in the Nd2Fe 14 A thick, uniform and ideal composition Dy-rich hard magnetic shell layer is formed on the surface of the B main phase grains, and Co and Ga are uniformly distributed in the grain boundaries or inside the grains, which comprehensively optimizes the overall magnetic performance.

[0058] Due to the formation of a uniform and optimized Dy-rich shell structure and uniformly distributed Co and Ga elements, the magnetic crystal anisotropy field is effectively enhanced, and the grain boundary phase structure is optimized. Experimental data show that the intrinsic coercive force of the sintered neodymium-iron-boron permanent magnet prepared by the application is increased by 20% to 35% compared to the undiffused magnet blank, and the coercive force performance exceeds that of the existing single diffusion medium technology under the same rare earth usage.

[0059] The application ensures the high consistency of the microstructure and element distribution of each batch of permanent magnets through fine and coordinated diffusion, thereby reducing the batch fluctuation of the coercive force to less than 3%, which is much lower than the fluctuation range of more than 8% in the existing technology.

[0060] The Co element introduced in the main diffusion stage penetrates into the Nd2Fe 14 B main phase lattice to replace Fe atoms, effectively improving the Curie temperature and magnetic crystal anisotropy field of the Nd2Fe 14 B phase, and improving the magnetic performance decay problem of the permanent magnet at high temperatures. At the same time, the introduction of Ga elements also optimizes the thermal stability of the grain boundary phase. The irreversible magnetic flux loss of the permanent magnet prepared by the application is reduced by 15% to 25% compared to the existing technology at a high temperature of 120℃, and the magnetic stability at a high temperature of 150℃ is also improved. DETAILED DESCRIPTION

[0061] The application provides a high-performance sintered neodymium-iron-boron permanent magnet preparation method based on multi-element synergistic diffusion, aiming to improve the coercivity, batch stability and high-temperature magnetic properties of the sintered neodymium-iron-boron permanent magnet by fine regulation of the diffusion kinetics of multi-elements in the permanent magnet. The technical solutions of the application will be described in detail below in combination with specific embodiments to ensure that those skilled in the art can clearly understand and reproduce the entire technical content of the application.

[0062] Example 1: Preparation of sintered neodymium-iron-boron permanent magnet blank The nominal composition of the pre-alloyed powder is neodymium 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 crushing and jet stream grinding, with an average particle size of 4.2 μm and an oxygen content of 550 ppm; pressed at a pressure of 180 megapascals under a 1.8T directional magnetic field; heated to 1065°C at a rate of 10°C per minute in a vacuum furnace with a vacuum degree better than 4×10 -4 Pa, and kept for 3 hours; then aged at 600°C for 2 hours to obtain a blank with an average grain size of 5.1 μm, and initial magnetic properties of remanence 1.42T, coercivity 920kA / m, and maximum magnetic energy product 405kJ / m 3 .

[0063] Preparation of inner layer diffusion medium slurry: the dysprosium oxide-rich particles are mixed with dysprosium oxide with an average particle size of 120 nm and a purity of 99.95% and copper oxide with an average particle size of 50 nm and a purity of 99.9% at a weight ratio of 95:5, treated by a high-energy planetary ball mill, calcined at 900°C for 3 hours under a high-purity argon atmosphere of 200 standard cubic centimeters per minute, crushed and sieved to an average particle size of 200 nm; the slurry is prepared with active particles at 42 weight percent, polyethylene glycol octylphenyl ether at 1.0 weight percent, and polyvinyl butyral at 5 weight percent, the solvent is a mixture of isopropyl alcohol and deionized water at a ratio of 2:1, treated by a planetary ball mill, with a solid content of 43 weight percent and a viscosity of 120 mPa・s.

[0064] Outer layer diffusion medium slurry preparation: cobalt fluoride-rich and gallium fluoride-rich particles were mixed by 97:3 weight ratio 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%, and treated by a dry high-energy planetary ball mill in a high-purity argon glove box; the slurry was prepared by mixing 38 weight percent of fluoride powder, 15 weight percent of polymethyl methacrylate microspheres with an average particle size of 1.2 μm, 1.0 weight percent of polycarboxylate dispersant, and 6 weight percent of ethyl cellulose, and treated by a planetary ball mill, with a solid content of 38 weight percent and a viscosity of 170 mPa・s.

[0065] Coated composite diffusion medium layer: the inner layer was sprayed, then dried at 100℃ for 1 hour and at 130℃ for 1.5 hours, forming an inner layer diffusion medium with a thickness of 25 μm and a dry porosity of 18.5%; the outer layer was dip-coated, then dried at 80℃ for 1 hour and at 100℃ for 1 hour, forming an outer layer diffusion medium with a thickness of 50 μm and a dry porosity of 33.0%.

[0066] Multi-stage thermal diffusion treatment: the pre-diffusion heat treatment was at 5×10 -3 at a temperature rise of 8℃ per minute to 450℃ and held for 2 hours in a vacuum environment; the main diffusion heat treatment was at 4×10 -4 at a temperature rise of 10℃ per minute to 950℃ and held for 8 hours in a vacuum environment; the post-diffusion annealing treatment was at 8×10 -4 at a temperature drop of 5℃ per minute to 600℃ and held for 4 hours in a vacuum environment.

[0067] Subsequent treatment: ultrasonic cleaning with 1 weight percent dilute oxalic acid solution for 5 minutes, rinsing with deionized water and then blowing dry with hot air; surface passivation in a phosphate solution at 50℃ and pH=4.0 for 15 minutes; final magnetic properties after 5T pulse magnetization: remanence 1.42T, intrinsic coercive force 1242kA / m, maximum magnetic energy product 410kJ / m 3 , irreversible magnetic flux loss at 120℃ 8%, coercive force batch fluctuation 2.5%.

[0068] Example 2: Preparation of sintered neodymium-iron-boron permanent magnet blanks: The pre-alloyed powder had the same composition 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 in a vacuum furnace with a vacuum better than 4×10 -4 at a temperature rise of 10℃ per minute to 1060℃ and held for 3 hours; aging at 600℃ for 2 hours, resulting in a blank with an average grain size of 3.0 μm, with initial magnetic properties of remanence 1.40T, coercive force 910kA / m, and maximum magnetic energy product 400kJ / m 3 .

[0069] Inner layer diffusion medium slurry preparation: Dysprosium oxide-rich particles with an average particle size of 100 nm and a purity of 99.95% were mixed with copper oxide with an average particle size of 40 nm and a purity of 99.9% at a weight ratio of 98:2, treated by a high-energy planetary ball mill, calcined at 880°C for 4 hours in a high-purity argon atmosphere of 180 standard cubic centimeters per minute, crushed and sieved to an average particle size of 180 nm; the slurry was prepared with active particles at 45% by weight, polyethylene glycol octylphenyl ether at 0.8% by weight, polyvinyl butyral at 4% by weight, a solvent of isopropyl alcohol and deionized water mixed at a ratio of 3:1, treated by a planetary ball mill, a solid content of 45% by weight, and a viscosity of 100 mPa・s.

[0070] Outer layer diffusion medium slurry preparation: Cobalt fluoride-rich and gallium fluoride-rich particles with an average particle size of 200 nm and a purity of 99.9% were mixed with cobalt fluoride with an average particle size of 200 nm and a purity of 99.9% at a weight ratio of 99:1, treated by a dry high-energy planetary ball mill (grinding for 3 hours at a speed of 380 revolutions per minute, ball-to-material ratio of 12:1) in a high-purity argon glove box; the slurry was prepared with mixed fluoride powder at 40% by weight, polymethyl methacrylate microspheres with an average particle size of 0.8 μm at 12% by weight, polycarboxylate dispersant at 0.8% by weight, and ethyl cellulose at 5% by weight, a solvent of anhydrous ethanol, treated by a planetary ball mill (grinding for 7 hours at a speed of 320 revolutions per minute, ball-to-material ratio of 7:1), a solid content of 40% by weight, and a viscosity of 150 mPa・s.

[0071] Coating of the composite diffusion medium layer: The inner layer was sprayed (parameters same as in Example 1), dried at 110°C for 1.5 hours and at 140°C for 1 hour, forming an inner layer diffusion medium with a thickness of 20 μm and a dry porosity of 15.0%; the outer layer was dip-coated (parameters same as in Example 1), dried at 90°C for 1.2 hours and at 110°C for 0.8 hours, forming an outer layer diffusion medium with a thickness of 40 μm and a dry porosity of 30.0%.

[0072] Multi-stage thermal diffusion treatment: The pre-diffusion heat treatment was at 6×10 -3 at a temperature of 400°C for 3 hours at a rate of 5°C per minute in a vacuum environment; the main diffusion heat treatment was at 5×10 -4 at a temperature of 850°C for 4 hours at a rate of 8°C per minute in a vacuum environment; the post-diffusion annealing treatment was at 9×10 -4 at a temperature of 550°C for 5 hours at a rate of 3°C per minute in a vacuum environment.

[0073] Subsequent treatment: same as in Example 1, the final magnetic properties were a remanence of 1.40 T, an intrinsic coercive force of 1184 kA / m, and a maximum magnetic energy product of 402 kJ / m 310%, irreversible magnetic flux loss at 120℃ 10%, coercivity batch variation 2.8%.

[0074] Example 3: Preparation of sintered NdFeB permanent magnet green body: The pre-alloyed powder had the same composition as in Example 1, with an average particle size of 4.5 pm and an oxygen content of 580 ppm. The compaction conditions were the same as in Example 1. Sintering was performed in a vacuum furnace at a vacuum level better than 4x10 -4 at 10℃ per minute to 1070℃ and held for 2.5 hours. Aging at 620℃ for 1.5 hours resulted in a green body with an average grain size of 8.0 pm, initial magnetic properties of remanence 1.43T, coercivity 925kA / m, maximum magnetic energy product 408kJ / m 3 .

[0075] Inner layer diffusion medium slurry preparation: Dysprosium-rich oxide particles were mixed with copper oxide of average particle size 80 nm and purity 99.9% at a weight ratio of 90:10, treated by high-energy planetary ball mill, calcined at 920℃ for 2.5 hours in a high-purity argon atmosphere of 220 standard cubic centimeters per minute, crushed and sieved to an average particle size of 220 nm. The slurry was prepared with active particles at 40 weight percent, polyethylene glycol octylphenyl ether at 1.5 weight percent, polyvinyl butyral at 6 weight percent, solvent being a 1:1 mixture of isopropyl alcohol and deionized water, treated by planetary ball mill, solid content 40 weight percent, viscosity 150 mPa・s.

[0076] Outer layer diffusion medium slurry preparation: Cobalt-rich fluoride and gallium-rich fluoride particles were mixed with cobalt fluoride of average particle size 400 nm and purity 99.9% and gallium fluoride of average particle size 400 nm and purity 99.9% at a weight ratio of 90:10, treated by dry high-energy planetary ball mill in a high-purity argon glove box. The slurry was prepared with mixed fluoride powder at 35 weight percent, polymethyl methacrylate microspheres of average particle size 1.5 pm at 18 weight percent, polycarboxylate dispersant at 1.2 weight percent, ethyl cellulose at 7 weight percent, solvent being anhydrous ethanol, treated by planetary ball mill, solid content 35 weight percent, viscosity 200 mPa・s.

[0077] Coating composite diffusion medium layer: The inner layer was coated by brushing, dried at 120℃ for 2 hours and at 150℃ for 1 hour, forming an inner layer diffusion medium with a thickness of 30 pm and a dry porosity of 20.0%; the outer layer was coated by spraying (parameters same as in Example 1), dried at 85℃ for 1.5 hours and at 115℃ for 0.5 hours, forming an outer layer diffusion medium with a thickness of 60 pm and a dry porosity of 35.0%.

[0078] Multi-stage thermal diffusion treatment: The pre-diffusion heat treatment was performed at 4x10 -3The pre-alloyed powder was sintered in a vacuum furnace with a vacuum degree better than 4x10 -4 The pre-alloyed powder was sintered in a vacuum furnace with a vacuum degree better than 4x10 -4 The pre-alloyed powder was sintered in a vacuum furnace with a vacuum degree better than 4x10

[0079] Subsequent treatment: same as example 1, the final magnetic properties are remanence 1.43T, intrinsic coercive force 1240kA / m, maximum magnetic energy product 415kJ / m 3 , irreversible flux loss at 120℃ 7%, coercive force batch fluctuation 2.2%.

[0080] Example 4: Preparation of sintered Nd-Fe-B permanent magnet blank: The pre-alloyed powder composition is same as example 1, the average particle size is 4.3μm, the oxygen content is 560ppm; the pressing condition is same as example 1; the sintering is in a vacuum furnace with a vacuum degree better than 4x10 -4 The pre-alloyed powder was sintered in a vacuum furnace with a vacuum degree better than 4x10 3 .

[0081] The inner layer diffusion medium slurry was prepared: the dysprosium oxide-rich particles were mixed with copper oxide with an average particle size of 60nm and a purity of 99.9% at a weight ratio of 92:8, treated by a high-energy planetary ball mill, calcined at 910℃ for 3.5 hours in a high-purity argon atmosphere of 190 standard cubic centimeters per minute, crushed and sieved to an average particle size of 210nm; the slurry was prepared with active particles at 43wt%, polyethylene glycol octylphenyl ether at 1.2wt%, polyvinyl butyral at 5.5wt%, isopropyl alcohol and deionized water mixed at 2.5:1 as solvent, treated by a planetary ball mill, with a solid content of 43wt% and a viscosity of 130mPa・s.

[0082] The outer layer diffusion medium slurry was prepared: the cobalt fluoride-rich and gallium fluoride-rich particles were mixed with cobalt fluoride with an average particle size of 350nm and a purity of 99.9% and gallium fluoride with an average particle size of 350nm and a purity of 99.9% at a weight ratio of 95:5, treated by a dry high-energy planetary ball mill in a high-purity argon glove box; the slurry was prepared with mixed fluoride powder at 37wt%, polymethyl methacrylate microspheres with an average particle size of 1.3μm at 16wt%, polycarboxylate dispersant at 1.1wt%, ethyl cellulose at 6.5wt%, anhydrous ethanol as solvent, treated by a planetary ball mill, with a solid content of 37wt% and a viscosity of 180mPa・s.

[0083] 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%.

[0084] 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.

[0085] 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.

[0086] Comparative Example 1: Traditional single dysprosium oxide diffusion: 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 .

[0087] 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.

[0088] 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%.

[0089] Thermal diffusion treatment: exactly the same as in Example 1 (pre-diffusion, main diffusion, and post-diffusion parameters are the same).

[0090] 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.

[0091] Comparative Example 2, composite diffusion but without copper oxide in the inner layer: 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 .

[0092] 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.

[0093] Preparation of outer diffusion medium slurry: exactly the same as in Example 1.

[0094] 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%).

[0095] Multi-stage thermal diffusion treatment: exactly the same as in Example 1.

[0096] 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.

[0097] 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: 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.

[0098] 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.

[0099] 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). 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; 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%).

[0100] 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.

[0101] 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 crushed and sieved to obtain inner 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.

10. The preparation method according to claim 9, characterized in that, The post-diffusion annealing process in S4 includes: After the main diffusion heat treatment is completed, 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 hours to 6 hours. The post-diffusion annealing process 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 50sccm to 150sccm.

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