Grain boundary diffusion neodymium iron boron magnet and preparation method thereof

Abstract

The invention provides a grain boundary diffusion neodymium iron boron magnet and a preparation method thereof, and relates to the technical field of magnetic materials. The preparation method comprises the steps of surface pretreatment and purification, precise diffusion source construction through magnetron sputtering and diffusion promotion through vacuum heat treatment. The core function of pretreatment is to remove surface impurities and lay a foundation for subsequent deposition of alloy films; the proportion of Tb, Al and Cu in a diffusion source is accurately regulated and controlled in the magnetron sputtering stage, and the synergistic effect is achieved; and the vacuum heat treatment promotes Tb to diffuse into the neodymium iron boron. According to the method, a technical closed loop with pretreatment as a basis, a diffusion source as a core and heat treatment as power is formed, the microstructure of the neodymium-iron-boron magnet is optimized, a complete and continuous shell structure with the moderate thickness is formed, the diffusion depth of heavy rare earth is greatly increased, and the service life of the neodymium-iron-boron magnet is prolonged. Finally, the coercive force of the neodymium-iron-boron magnet is remarkably improved, and the utilization rate of heavy rare earth elements is effectively improved. The problem of low Tb utilization rate in the prior art is solved, and the bottleneck of coercive force improvement is broken through.

Description

Technical Field

[0001] This invention relates to the field of magnetic materials technology, specifically to a grain boundary diffused NdFeB magnet and its preparation method. Background Technology

[0002] Neodymium iron boron (NdFeB) magnets play a crucial role in green energy fields such as electric vehicles and wind power generation due to their excellent hard magnetic properties. However, their coercivity is significantly lower than theoretical values, and their low Curie temperature leads to insufficient thermal stability, which limits their applications. For example, the operating temperature of drive motors in hybrid / electric vehicles typically reaches 150°C. To meet the requirements of such high-temperature applications, the magnets must possess high coercivity to ensure resistance to demagnetization. Therefore, improving the coercivity of NdFeB magnets is key to expanding their application areas and promoting the development of advanced industrial technologies.

[0003] One key approach to improving coercivity is to utilize the properties of heavy rare earth elements: RE2Fe formed by Tb and Dy. 14 The anisotropic field of phase B is higher than that of Nd2Fe. 14 In the B phase, partially replacing Nd in NdFeB with heavy rare earth elements Tb and Dy can effectively improve its coercivity. However, traditional processes (such as fused alloying and dual alloying) require the addition of large amounts of expensive Tb and Dy, severely limiting the widespread application of magnets. In contrast, grain boundary diffusion technology, by covering the magnet surface with a heavy rare earth diffusion source and heat-treating it, allows heavy rare earth atoms to diffuse along the grain boundaries into the interior, replacing Nd₂Fe. 14 Nd on the surface of B grains forms a (Nd,HRE)2Fe coating on the grain surface. 14 The core-shell structure of B. This structure significantly improves coercivity while greatly reducing the amount of heavy rare earth elements by hardening the grain surface and suppressing the nucleation of reverse magnetic domains.

[0004] Among diffusion sources, heavy rare earth multi-element alloys perform best, significantly outperforming other diffusion sources in improving coercivity and reducing costs. Currently, the alloy ratio design of diffusion sources is mostly based on the eutectic point—low-melting-point alloys are more likely to penetrate into the magnet. According to the binary phase diagram of Tb / Dy and Cu / Al, when Tb / Dy:Cu / Al is 70at%:30at% it is close to the eutectic point, thus becoming the mainstream design standard.

[0005] However, this design logic is facing challenges: with the introduction of new elements or the adoption of multi-element alloy systems in the diffusion source, the original binary phase diagram can no longer provide effective guidance. Nevertheless, some studies still use the traditional heavy rare earth ratio to design the diffusion source, which not only limits further improvements in the magnet's magnetic properties but also restricts the effective utilization rate of heavy rare earth elements. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a grain boundary diffused NdFeB magnet and its preparation method. The method for preparing the grain boundary diffused NdFeB magnet, with a lower content of heavy rare earth elements in the diffusion source, optimizes the microstructure and increases the diffusion depth of heavy rare earth elements, thereby not only effectively improving the coercivity of the grain boundary diffused NdFeB magnet but also improving the effective utilization rate of heavy rare earth elements.

[0007] The specific technical solution of this invention is as follows: In a first aspect, the present invention provides a method for preparing a grain boundary diffused NdFeB magnet, comprising the following steps: S1. Pretreatment: After pretreatment, the neodymium iron boron magnet is placed in the sputtering chamber and evacuated until the vacuum level in the sputtering chamber is ≤5×10⁻⁶. -6 Pa; S2. Diffusion source preparation: Argon gas is introduced into the sputtering chamber to a pressure of 1.6-3.0 Pa, and Tb is deposited by magnetron sputtering. x Al y Cu 100-x-y An alloy thin film, wherein 45≤x≤60 and 15≤y≤35, is used to obtain a neodymium iron boron magnet with a surface-deposited diffusion source; S3. Vacuum heat treatment: The NdFeB magnet with the surface-deposited diffusion source is subjected to a first-stage diffusion treatment and a second-stage tempering treatment under vacuum conditions to obtain a grain boundary diffused NdFeB magnet.

[0008] In one possible implementation, the orientation direction of the neodymium iron boron magnet after pretreatment in step S1 is parallel to its thickness direction, and the alloy film in step S2 is perpendicular to the orientation direction.

[0009] In one possible implementation, the thickness of the alloy film in step S2 is 10-20 μm, and the Tb accounts for 0.3 wt.%-0.6 wt.% of the mass of the NdFeB magnet in the surface deposition diffusion source.

[0010] In one possible implementation, during the magnetron sputtering deposition process described in step S2, the power of the Tb target is 100-150 W, the power of the Cu target is 20-80 W, and the power of the Al target is 30-70 W.

[0011] In one possible implementation, the flow rate of argon gas in step S2 is 30-50 sccm.

[0012] In one possible implementation, the vacuum degree of the vacuum condition described in step S3 is ≤1×10⁻⁶. -3 Pa.

[0013] In one possible implementation, the temperature of the primary diffusion treatment in step S3 is 850-950 ℃ and the holding time is 5-16 h, and the temperature of the secondary tempering treatment is 450-550 ℃ and the holding time is 2-5 h.

[0014] In one possible implementation, the heating rate of the primary diffusion treatment in step S3 is 5-15 °C / min, and the cooling rate of the secondary tempering treatment is 3-5 °C / min.

[0015] Secondly, the present invention provides a grain boundary diffused NdFeB magnet, wherein the microstructure of the grain boundary diffused NdFeB magnet includes NdFeB main phase grains and a hard magnetic shell continuously wrapped around the surface of the NdFeB grains, wherein the hard magnetic shell is composed of (Nd,Tb)₂Fe. 14 B, the grain boundary diffused NdFeB magnet is prepared by the above-described method for preparing grain boundary diffused NdFeB magnets.

[0016] Furthermore, the grain boundary diffusion depth of the grain boundary diffused NdFeB magnet is ≥ 800 μm.

[0017] The positive and progressive effects of this invention are as follows: Compared with existing technologies, this invention provides a grain boundary diffused NdFeB magnet and its preparation method. The preparation method includes surface pretreatment for purification, magnetron sputtering to construct a precise diffusion source, and vacuum heat treatment to promote diffusion. The core function of pretreatment is to remove surface impurities, laying the foundation for subsequent alloy film deposition; the magnetron sputtering stage precisely controls the ratio of Tb, Al, and Cu in the diffusion source to achieve their synergistic effect; and vacuum heat treatment promotes the diffusion of Tb into the NdFeB magnet. This method forms a closed-loop technology of "pretreatment as the foundation, diffusion source as the core, and heat treatment as the driving force," optimizing the microstructure of the NdFeB magnet, forming a complete, continuous, and moderately thick shell structure, and significantly improving the diffusion depth of heavy rare earth elements, ultimately achieving a significant improvement in the coercivity of the NdFeB magnet and an effective increase in the utilization rate of heavy rare earth elements. It solves the problem of low Tb utilization in existing technologies and breaks through the bottleneck of coercivity improvement. Attached Figure Description

[0018] Figure 1 This is a SEM image of the microstructure of the grain boundary diffused NdFeB magnet prepared in Example 1 at a depth of 50 μm from its surface.

[0019] Figure 2 This is a SEM image of the microstructure of the grain boundary diffused NdFeB magnet prepared in Example 1 at a depth of 800 μm from its surface.

[0020] Figure 3 The graph shows the demagnetization curves of the grain boundary diffused NdFeB magnets prepared in Example 1, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0021] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. It should be noted that the following embodiments are only used to illustrate the implementation methods and typical parameters of the present invention, and are not intended to limit the parameter range described in the present invention. Reasonable variations derived therefrom are still within the protection scope of the present invention.

[0022] It should be noted that the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0023] Unless otherwise defined, all terms, symbols, and other scientific terms used herein are intended to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In some instances, terms having a conventional meaning are defined herein for clarification or ease of reference, and such definitions should not be construed as indicating a significant difference from conventional understanding in the art. The technical methods described or referenced herein are generally well understood by those skilled in the art and employed by conventional methods. Unless otherwise stated, the use of commercially available kits, reagents, and instruments shall be performed according to the manufacturer's instructions and parameters.

[0024] Terminology Explanation Orientation direction: Magnetic materials are divided into two categories: isotropic magnets and anisotropic magnets. Isotropic magnets have the same magnetic properties in any direction, while anisotropic magnets have different magnetic properties in different directions. The direction in which anisotropic magnets can obtain the best magnetic properties is called the easy magnetization axis or orientation direction of the magnet.

[0025] The specific technical solution of this invention is as follows: In a first aspect, the present invention provides a method for preparing a grain boundary diffused NdFeB magnet, comprising the following steps: S1. Pretreatment: After pretreatment, the NdFeB magnets are placed in the sputtering chamber and evacuated until the vacuum level in the sputtering chamber is ≤5×10⁻⁶. -6 Pa; S2. Diffusion source preparation: Argon gas is introduced into the sputtering chamber to a pressure of 1.6-3.0 Pa, and Tb is deposited using magnetron sputtering. x Al y Cu 100-x-y An alloy thin film, wherein 45≤x≤60 and 15≤y≤35, is used to obtain a neodymium iron boron magnet with a surface-deposited diffusion source; S3. Vacuum heat treatment: The NdFeB magnet with surface-deposited diffusion source is subjected to a first-stage diffusion treatment and a second-stage tempering treatment under vacuum conditions to obtain a grain boundary diffused NdFeB magnet.

[0026] This invention provides a method for preparing a grain boundary diffused NdFeB magnet. Through the synergistic effect of surface pretreatment for purification, magnetron sputtering to construct a precise diffusion source, and vacuum heat treatment to promote Tb diffusion, the microstructure of the NdFeB magnet is optimized, forming a complete, continuous, and moderately thick shell structure. This significantly increases the diffusion depth of heavy rare earth elements, ultimately achieving a significant improvement in the coercivity of the NdFeB magnet and an effective increase in the utilization rate of heavy rare earth elements. First, pretreatment removes contaminants and oxides from the surface of the NdFeB magnet, followed by vacuuming to achieve a vacuum degree ≤ 5 × 10⁻⁵. -6 A vacuum environment of Pa is maintained to eliminate impurity gases in the sputtering chamber, preventing the composition of the subsequently deposited diffusion source film from deviating from the designed Tb. x Al y Cu 100-x-y The proportions are crucial. Secondly, precise control of the diffusion source composition guides Tb diffusion along grain boundaries and enhances Tb diffusion depth, thereby improving coercivity and Tb utilization. Tb with an atomic percentage of 45-60 serves as the core element. On one hand, it ensures the film provides sufficient "raw materials" for deep diffusion. If the Tb percentage is < 45%, the total Tb amount is insufficient for the same film thickness, making deep diffusion difficult; if the Tb percentage is > 60%, the Al and Cu percentages are insufficient, reducing diffusion motive force and hindering deep diffusion. On the other hand, it ensures the formation of a complete and continuous shell structure, effectively suppressing reverse magnetic domain nucleation and enhancing the coercivity of NdFeB magnets. Al with an atomic percentage of 15-35 is a key element for enhancing diffusion depth. On one hand, it inhibits excessive Tb diffusion into the NdFeB main phase grains, forming a shell of moderate thickness; on the other hand, it effectively reduces the diffusion activation energy of Tb, making Tb easier to diffuse. Cu can improve the grain boundary wettability of NdFeB magnets, constructing continuous and effective diffusion channels for Tb diffusion and promoting deep Tb diffusion. During the sputtering deposition of the alloy thin film, the argon gas pressure was controlled at 1.6-3.0 Pa. This resulted in moderate bombardment energy of argon ions on the target material, leading to high density, uniform composition, and good adhesion of the deposited Tb-based alloy thin film, which is beneficial for improving Tb utilization. Finally, a primary diffusion process and a secondary tempering process under vacuum conditions were performed to allow Tb to diffuse along the grain boundaries and form a (Nd,Tb)₂Fe₂ composition on the surface of the NdFeB main phase grains. 14 The hard magnetic shell of B significantly optimizes the microstructure of the magnet. The synergistic effect of the above factors ultimately achieves "a higher coercivity improvement effect with a lower Tb content", which not only solves the problem of low Tb utilization in the prior art, but also breaks through the bottleneck of coercivity improvement.

[0027] In one possible implementation, in step S1, the orientation direction of the NdFeB magnet after pretreatment is parallel to its thickness direction, and in step S2, the alloy film is perpendicular to the orientation direction. The grain boundaries of the NdFeB magnet are the main channels for the diffusion of the heavy rare earth element Tb, and the distribution direction of the grain boundaries is closely related to the orientation of the main phase particles. When the orientation direction is parallel to the thickness direction, the main phase particles are arranged in an orderly manner along the thickness direction, and the grain boundaries exhibit a "directional extension" characteristic along the thickness direction. When the orientation direction of the NdFeB magnet after pretreatment is parallel to its thickness direction and the alloy film is perpendicular to the orientation direction, the diffusion resistance along the orientation direction is small when Tb atoms diffuse from the magnet surface to the interior, which is beneficial for increasing the diffusion depth of Tb.

[0028] In one possible implementation, the alloy film thickness in step S2 is 10-20 μm, and Tb accounts for 0.3 wt.%-0.6 wt.% of the mass of the NdFeB magnet in the surface-deposited diffusion source. Limiting the alloy film thickness to 10-20 μm and the Tb content to 0.3 wt.%-0.6 wt.% of the NdFeB magnet in the surface-deposited diffusion source provides a sufficient Tb source to meet the requirements for diffusion along grain boundaries. Simultaneously, the matching of thickness and proportion ensures that there is neither excess Tb residue nor a supply shortage, thus improving Tb utilization.

[0029] In one possible implementation, during the magnetron sputtering deposition process in step S2, the power of the Tb target is 100-150 W, the power of the Cu target is 30-60 W, and the power of the Al target is 50-70 W. At these power levels, the element deposition rate is fast and the target material is less prone to overheating, resulting in good deposition stability. This power combination of Tb, Cu, and Al targets ensures that the film composition is strictly matched to Tb. x Al y Cu 100-x-y The design range is (45≤x≤60, 15≤y≤35).

[0030] In one possible implementation, the argon gas flow rate in step S2 is 30-50 sccm. Excessive gas flow rate reduces the energy of sputtered atoms, resulting in poor film density; conversely, insufficient flow rate leads to instability in the sputtering process, affecting film uniformity. Films sputtered at this gas flow rate exhibit high density and good uniformity.

[0031] In one possible implementation, the vacuum level of the vacuum condition in step S3 is ≤1×10⁻⁶. -3 Pa. When the vacuum degree is ≤1×10⁻⁶ -3 At Pa, the oxygen content is extremely low, the oxidation effect of the diffusion source is limited, and the effect on the diffusion resistance of Tb is small, which is conducive to the diffusion of Tb into the interior of the magnet.

[0032] In one possible implementation, the temperature of the primary diffusion treatment in step S3 is 850-950 ℃, and the holding time is 5-16 h; the temperature of the secondary tempering treatment is 450-550 ℃, and the holding time is 2-5 h. The core function of the primary diffusion treatment is to activate the migration ability of Tb atoms using high temperature, enabling them to diffuse efficiently from the surface alloy film along the grain boundaries into the interior of the magnet. The diffusion rate of Tb in the 850-950 ℃ temperature range is relatively high, which can overcome the diffusion resistance of the grain boundary phase, achieving deep diffusion of Tb from the surface to the interior without causing deterioration of the microstructure of the NdFeB magnet. The 5-16 h holding time, combined with the temperature, allows Tb to be continuously released from the surface alloy film, ensuring the formation of a highly anisotropic hard magnetic shell on the surface of the NdFeB magnet's main phase grains, and promoting deep diffusion of Tb through gradient diffusion. The secondary tempering treatment, after the primary diffusion treatment, achieves a triple effect of "stress relief, structure optimization, and improved magnetic properties" through a medium-temperature treatment. A tempering temperature of 450-550℃ combined with a holding time of 2-5 hours can activate local atomic rearrangement, release stress, reduce internal defects, improve the phase composition and structure at grain boundaries, optimize the arrangement of magnetic domains, and thus improve multiple magnetic properties such as coercivity, remanence, and rectangularity.

[0033] In one possible implementation, the heating rate of the primary diffusion treatment in step S3 is 5-15 ℃ / min, and the cooling rate of the secondary tempering treatment is 3-5 ℃ / min. For small magnets, where the heat conduction path is short, a higher rate of 10-15 ℃ / min can be used, which can shorten the heating time and ensure a consistent starting temperature for Tb diffusion. For large magnets, where heat conduction lag is significant, a lower rate of 5-8 ℃ / min can extend the heating time, thereby reducing the temperature difference between the magnet core and the surface, preventing Tb diffusion "stratification" caused by temperature gradients, and ensuring an overall improvement in the coercivity of large magnets.

[0034] Secondly, the present invention provides a grain boundary diffused NdFeB magnet, the microstructure of which includes NdFeB main phase grains and a hard magnetic shell continuously wrapped around the surface of the NdFeB main phase grains, the hard magnetic shell being composed of (Nd,Tb)₂Fe. 14 B. Grain boundary diffused NdFeB magnets are prepared by the above-described method for preparing grain boundary diffused NdFeB magnets. The grain boundary diffused NdFeB magnets prepared by the above method form a complete and continuous hard magnetic shell on the surface of the NdFeB main phase grains, and have a deep Tb diffusion depth. They possess advantages such as high coercivity, low heavy rare earth element content, and high utilization rate of heavy rare earth elements.

[0035] Furthermore, the grain boundary diffusion depth of the grain boundary-diffused NdFeB magnet is ≥ 800 μm. The coercivity of the NdFeB magnet mainly depends on the substitution of Nd in the NdFeB main phase by heavy rare earth elements, forming a highly anisotropic (Nd,Tb)₂Fe₂ phase. 14 Phase B suppresses reverse magnetization. Because heavy rare earth elements can only penetrate to the surface, the diffusion depth is limited, resulting in insufficient core-shell structure; therefore, the majority of the magnet's structure remains Nd₂Fe. 14 The predominance of B leads to a performance stratification of "strong surface and weak core," while increasing the diffusion depth of heavy rare earth elements can effectively increase the proportion of core-shell structure, thereby further improving coercivity.

[0036] The technical solution of the present invention will be further described below with reference to specific embodiments and comparative examples. All reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing. The instruments used in the embodiments are also commercially available.

[0037] Example 1 This embodiment provides a grain boundary diffused NdFeB magnet, which is prepared by the following steps: D1. Pretreatment: Commercially available NdFeB magnets with the following properties (Hcj=16.73 kOe, Br=14.20 kG, (BH)max=47.67 MGOe) were wire-cut into rectangular magnets measuring 8.2 mm × 8.2 mm × 6.2 mm, with the orientation direction parallel to the thickness direction (6.2 mm). The surface of the rectangular magnets was successively polished with 180-grit, 400-grit, and 800-grit SiC sandpaper, followed by ultrasonic treatment with acetone and drying with a nitrogen gun. The magnets were then wiped clean with lint-free paper to ensure a bright surface free of contaminants or oxides, ultimately yielding rectangular NdFeB magnets measuring 8 mm × 8 mm × 6 mm. These magnets were then vacuum-sealed for storage to prevent oxidation.

[0038] D2. Determination of Diffusion Source Components: The silicon substrate was cut into suitable sizes and ultrasonicated in anhydrous ethanol for 15 minutes. Surface particles were then wiped away with a clean cotton swab, followed by ultrasonication in acetone for 15 minutes to further clean the silicon substrate surface. The treated silicon substrate was placed in the sputtering chamber of a magnetron sputtering system, and a vacuum was evacuated until the vacuum level in the sputtering chamber was less than 5 × 10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was deposited on the silicon substrate surface by magnetron sputtering. The diffusion source composition corresponding to this power was determined by analyzing the alloy composition. The target sputtering power required to achieve the desired diffusion source ratio was determined by adjusting the working power of the Tb, Cu, and Al targets. Simultaneously, the time required for the diffusion source to deposit the target thickness was determined by measuring the film deposition thickness at a fixed sputtering time.

[0039] D3. Preparation of diffusion source: The rectangular NdFeB magnet obtained in step D1 is placed in the sputtering chamber of a magnetron sputtering device, and the vacuum level of the sputtering chamber is ≤5×10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was co-sputtered onto the surface of a rectangular NdFeB magnet. During co-sputtering, the Tb target power was 120 W, the Cu target power was 50 W, and the Al target power was 50 W. The sputtering time was 310 min, and the sputtering temperature was maintained below 50 °C. After sputtering, the film was cooled to room temperature, removed, and sealed for storage, resulting in a 10 μm thick Tb film deposited on the surface. 50 Al 25 Cu 25 Neodymium iron boron magnets for diffusion sources.

[0040] D4. Grain boundary diffusion treatment: Place the NdFeB magnet with the surface-deposited diffusion source from step D3 into a quartz tube, and evacuate to a vacuum level less than 1.0 × 10⁻⁶. -3 After Pa, the quartz tube wall and internal quartz columns were melted using a flame torch, and then cooled and solidified to a seal. The sealed quartz tube was then subjected to diffusion treatment in a muffle furnace. First, a primary diffusion treatment was performed at 900 °C and held for 15 h. Then, the temperature inside the muffle furnace was lowered to 500 °C for a secondary tempering treatment of 2 h. The heating rate of the muffle furnace was 10 °C / min, and the cooling rate was 5 °C / min, resulting in a grain boundary diffused NdFeB magnet.

[0041] Example 2 This embodiment provides a grain boundary diffused NdFeB magnet, which is prepared by the following steps: D1. Pretreatment: Commercial neodymium iron boron magnets with the following properties (Hcj=16.73 kOe, Br=14.20 kG, (BH)max=47.67 MGOe) were wire-cut into rectangular magnets with dimensions of 8.2 mm × 8.2 mm × 6.2 mm, with the orientation direction parallel to the thickness direction (6.2 mm). The surface of the rectangular magnets was successively polished with 180-grit, 400-grit, and 800-grit SiC sandpaper, followed by ultrasonic treatment with acetone and drying with a nitrogen gun. The magnets were then wiped clean with lint-free paper to ensure a bright surface free of contaminants or oxides, ultimately yielding rectangular neodymium iron boron magnets with dimensions of 8 mm × 8 mm × 6 mm. These magnets were then vacuum-sealed for storage to prevent oxidation.

[0042] D2. Determination of Diffusion Source Components: The silicon substrate was cut into suitable sizes and ultrasonicated in anhydrous ethanol for 15 minutes. Surface particles were then wiped away with a clean cotton swab, followed by ultrasonication in acetone for 15 minutes to further clean the silicon substrate surface. The treated silicon substrate was placed in the sputtering chamber of a magnetron sputtering system, and a vacuum was evacuated until the vacuum level in the sputtering chamber was less than 5 × 10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was deposited on the silicon substrate surface by magnetron sputtering. The diffusion source composition corresponding to this power was determined by analyzing the alloy composition. The target sputtering power required to achieve the desired diffusion source ratio was determined by adjusting the working power of the Tb, Cu, and Al targets. Simultaneously, the time required for the diffusion source to deposit the target thickness was determined by measuring the film deposition thickness at a fixed sputtering time.

[0043] D3. Preparation of diffusion source: The rectangular NdFeB magnet obtained in step D1 is placed in the sputtering chamber of a magnetron sputtering device, and the vacuum level of the sputtering chamber is ≤5×10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was co-sputtered onto the surface of a rectangular NdFeB magnet. During co-sputtering, the Tb target power was 130 W, the Cu target power was 20 W, and the Al target power was 60 W. The sputtering time was 310 min, and the sputtering temperature was maintained below 50 °C. After sputtering, the film was cooled to room temperature, removed, and sealed for storage, resulting in a 10 μm thick Tb film deposited on the surface. 60 Al 35 Neodymium iron boron magnets with Cu5 diffusion source.

[0044] D4. Grain boundary diffusion treatment: Place the NdFeB magnet with the surface-deposited diffusion source from step D3 into a quartz tube, and evacuate to a vacuum level less than 1.0 × 10⁻⁶. -3 After Pa, the quartz tube wall and internal quartz columns were melted using a flame torch, and then cooled and solidified to a seal. The sealed quartz tube was then subjected to diffusion treatment in a muffle furnace. First, a primary diffusion treatment was performed at 900 °C for 15 h. Then, the temperature inside the muffle furnace was lowered to 500 °C for a secondary tempering treatment of 2 h. The heating rate of the muffle furnace was 10 °C / min, and the cooling rate was 5 °C / min, resulting in a grain boundary diffused NdFeB magnet.

[0045] Example 3 This embodiment provides a grain boundary diffused NdFeB magnet, which is prepared by the following steps: D1. Pretreatment: Commercial neodymium iron boron magnets with the following properties (Hcj=16.73 kOe, Br=14.20 kG, (BH)max=47.67 MGOe) were wire-cut into rectangular magnets with dimensions of 8.2 mm × 8.2 mm × 6.2 mm, with the orientation direction parallel to the thickness direction (6.2 mm). The surface of the rectangular magnets was successively polished with 180-grit, 400-grit, and 800-grit SiC sandpaper, followed by ultrasonic treatment with acetone and drying with a nitrogen gun. The magnets were then wiped clean with lint-free paper to ensure a bright surface free of contaminants or oxides, ultimately yielding rectangular neodymium iron boron magnets with dimensions of 8 mm × 8 mm × 6 mm. These magnets were then vacuum-sealed for storage to prevent oxidation.

[0046] D2. Determination of Diffusion Source Components: The silicon substrate was cut into appropriate sizes and ultrasonicated in anhydrous ethanol for 15 minutes. Surface particles were then wiped away with a clean cotton swab, followed by ultrasonication in acetone for 15 minutes to further clean the silicon substrate surface. The treated silicon substrate was placed in the sputtering chamber of a magnetron sputtering system, and a vacuum was evacuated until the vacuum level in the sputtering chamber was less than 5 × 10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was deposited on the silicon substrate surface by magnetron sputtering. The diffusion source composition corresponding to this power was determined by analyzing the alloy composition. The target sputtering power required for the sputtering target diffusion source ratio was determined by adjusting the working power of the Tb, Cu, and Al targets. Simultaneously, the time required for the diffusion source to deposit the target thickness was determined by measuring the film deposition thickness at a fixed sputtering time.

[0047] D3. Preparation of diffusion source: The rectangular NdFeB magnet obtained in step D1 is placed in the sputtering chamber of a magnetron sputtering device, and the vacuum level of the sputtering chamber is ≤5×10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was co-sputtered onto the surface of a rectangular NdFeB magnet. During co-sputtering, the Tb target power was 120 W, the Cu target power was 50 W, and the Al target power was 70 W. The sputtering time was 310 min, and the sputtering temperature was maintained below 50 °C. After sputtering, the film was cooled to room temperature, removed, and sealed for storage, resulting in a 10 μm thick Tb film deposited on the surface. 45 Al 35 Cu 20 Neodymium iron boron magnets for diffusion sources.

[0048] D4. Grain boundary diffusion treatment: Place the NdFeB magnet with the surface-deposited diffusion source from step D3 into a quartz tube, and evacuate to a vacuum level less than 1.0 × 10⁻⁶. -3After Pa, the quartz tube wall and internal quartz columns were melted using a flame torch, and then cooled and solidified to a seal. The sealed quartz tube was then subjected to diffusion treatment in a muffle furnace. First, a primary diffusion treatment was performed at 900 °C for 15 h. Then, the temperature inside the muffle furnace was lowered to 500 °C for a secondary tempering treatment of 2 h. The heating rate of the muffle furnace was 10 °C / min, and the cooling rate was 5 °C / min, resulting in a grain boundary diffused NdFeB magnet.

[0049] Example 4 This embodiment provides a grain boundary diffused NdFeB magnet, which is prepared by the following steps: D1. Pretreatment: Commercial neodymium iron boron magnets with the following properties (Hcj=16.73 kOe, Br=14.20 kG, (BH)max=47.67 MGOe) were wire-cut into rectangular magnets with dimensions of 8.2 mm × 8.2 mm × 6.2 mm, with the orientation direction parallel to the thickness direction (6.2 mm). The surface of the rectangular magnets was successively polished with 180-grit, 400-grit, and 800-grit SiC sandpaper, followed by ultrasonic treatment with acetone and drying with a nitrogen gun. The magnets were then wiped clean with lint-free paper to ensure a bright surface free of contaminants or oxides, ultimately yielding rectangular neodymium iron boron magnets with dimensions of 8 mm × 8 mm × 6 mm. These magnets were then vacuum-sealed for storage to prevent oxidation.

[0050] D2. Determination of Diffusion Source Components: The silicon substrate was cut into appropriate sizes, sonicated in anhydrous ethanol for 15 minutes, and then the surface particles were wiped off with a clean cotton swab. Next, it was sonicated in acetone for 15 minutes to further clean the silicon substrate surface. The treated silicon substrate was placed in the sputtering chamber of a magnetron sputtering equipment, and the vacuum level in the sputtering chamber was evacuated to less than 5 × 10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was deposited on the silicon substrate surface by magnetron sputtering. The diffusion source composition corresponding to this power was determined by analyzing the alloy composition. The target sputtering power required to achieve the desired diffusion source ratio was determined by adjusting the working power of the Tb, Cu, and Al targets. Simultaneously, the time required for the diffusion source to deposit the target thickness was determined by measuring the film deposition thickness at a fixed sputtering time.

[0051] D3. Preparation of diffusion source: The rectangular NdFeB magnet obtained in step D1 is placed in the sputtering chamber of a magnetron sputtering device, and the vacuum level of the sputtering chamber is ≤5×10⁻⁶. -6Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was co-sputtered onto the surface of a rectangular NdFeB magnet. During co-sputtering, the Tb target power was 120 W, the Cu target power was 50 W, and the Al target power was 30 W. The sputtering time was 310 min, and the sputtering temperature was maintained below 50 °C. After sputtering, the film was cooled to room temperature, removed, and sealed for storage, resulting in a 10 μm thick Tb film deposited on the surface. 60 Al 15 Cu 25 Neodymium iron boron magnets for diffusion sources.

[0052] D4. Grain boundary diffusion treatment: Place the NdFeB magnet with the surface-deposited diffusion source from step D3 into a quartz tube, and evacuate to a vacuum level less than 1.0 × 10⁻⁶. -3 After Pa, the quartz tube wall and internal quartz columns were melted using a flame torch, and then cooled and solidified to a seal. The sealed quartz tube was then subjected to diffusion treatment in a muffle furnace. First, a primary diffusion treatment was performed at 900 °C for 15 h. Then, the temperature inside the muffle furnace was lowered to 500 °C for a secondary tempering treatment of 2 h. The heating rate of the muffle furnace was 10 °C / min, and the cooling rate was 5 °C / min, resulting in a grain boundary diffused NdFeB magnet.

[0053] Example 5 This embodiment provides a grain boundary diffused NdFeB magnet, which is prepared by the following steps: D1. Pretreatment: Commercial neodymium iron boron magnets with the following properties (Hcj=16.73 kOe, Br=14.20 kG, (BH)max=47.67 MGOe) were wire-cut into rectangular magnets with dimensions of 8.2 mm × 8.2 mm × 6.2 mm, with the orientation direction parallel to the thickness direction (6.2 mm). The surface of the rectangular magnets was successively polished with 180-grit, 400-grit, and 800-grit SiC sandpaper, followed by ultrasonic treatment with acetone and drying with a nitrogen gun. The magnets were then wiped clean with lint-free paper to ensure a bright surface free of contaminants or oxides, ultimately yielding rectangular neodymium iron boron magnets with dimensions of 8 mm × 8 mm × 6 mm. These magnets were then vacuum-sealed for storage to prevent oxidation.

[0054] D2. Determination of Diffusion Source Components: The silicon substrate was cut into suitable sizes and ultrasonicated in anhydrous ethanol for 15 minutes. Surface particles were then wiped away with a clean cotton swab, followed by ultrasonication in acetone for 15 minutes to further clean the silicon substrate surface. The treated silicon substrate was placed in the sputtering chamber of a magnetron sputtering system, and a vacuum was evacuated until the vacuum level in the sputtering chamber was less than 5 × 10⁻⁶. -6Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was deposited on the silicon substrate surface by magnetron sputtering. The diffusion source composition corresponding to this power was determined by analyzing the alloy composition. The target sputtering power required to achieve the desired diffusion source ratio was determined by adjusting the working power of the Tb, Cu, and Al targets. Simultaneously, the time required for the diffusion source to deposit the target thickness was determined by measuring the film deposition thickness at a fixed sputtering time.

[0055] D3. Preparation of diffusion source: The rectangular NdFeB magnet obtained in step D1 is placed in the sputtering chamber of a magnetron sputtering device, and the vacuum level of the sputtering chamber is ≤5×10⁻⁶. -6 Argon gas was introduced before sputtering at a flow rate of 50 sccm, maintaining a working pressure of 2.8 Pa. An alloy thin film was co-sputtered onto the surface of a rectangular NdFeB magnet. During co-sputtering, the Tb target power was 120 W, the Cu target power was 80 W, and the Al target power was 40 W. The sputtering time was 310 min, and the sputtering temperature was maintained below 50 °C. After sputtering, the film was cooled to room temperature, removed, and sealed for storage, resulting in a 10 μm thick Tb layer. 45 Al 15 Cu 40 Neodymium iron boron magnets for diffusion sources.

[0056] D4. Grain boundary diffusion treatment: Place the NdFeB magnet with the surface-deposited diffusion source from step D3 into a quartz tube, and evacuate to a vacuum level less than 1.0 × 10⁻⁶. -3 After Pa, the quartz tube wall and internal quartz columns were melted using a flame torch, and then cooled and solidified to a seal. The sealed quartz tube was then subjected to diffusion treatment in a muffle furnace. First, a primary diffusion treatment was performed at 900 °C for 15 h. Then, the temperature inside the muffle furnace was lowered to 500 °C for a secondary tempering treatment of 2 h. The heating rate of the muffle furnace was 10 °C / min, and the cooling rate was 5 °C / min, resulting in a grain boundary diffused NdFeB magnet.

[0057] Comparative Example 1 This comparative example provides a grain boundary diffused NdFeB magnet, which differs from Example 1 in that: D3. Preparation of diffusion source: A 10 μm thick Tb layer was deposited on the surface. 65 Al 17.5 Cu 17.5 Neodymium iron boron magnets for diffusion sources.

[0058] Comparative Example 2 This comparative example provides a grain boundary diffused NdFeB magnet, which differs from Example 1 in that: D3. Preparation of diffusion source: A 10 μm thick Tb layer was deposited on the surface.40 Al 30 Cu 30 Neodymium iron boron magnets for diffusion sources.

[0059] The composition of the diffusion sources prepared in Examples 1-5 and Comparative Examples 1-2 is shown in Table 1.

[0060] Table 1. Composition of the diffusion sources prepared in Examples 1-5 and Comparative Examples 1-2 Example Components of the diffusion source Example 1 <![CDATA[Tb 50 the 25 With 25 ]]> Example 2 <![CDATA[Tb 60 Al 35 Cu5 <!-- 8 -->]]> Example 3 <![CDATA[Tb 45 the 35 With 20 ]]> Example 4 <![CDATA[Tb 60 the 15 With 25 ]]> Example 5 <![CDATA[Tb 45 the 15 With 40 ]]> Comparative Example 1 <![CDATA[Tb 65 the 17.5 With 17.5 ]]> Comparative Example 2 <![CDATA[Tb 40 the 30 With 30 ]]> The performance of the grain boundary diffused NdFeB magnets in Examples 1-5 and Comparative Examples 1-2 was tested, and the results are as follows.

[0061] Figure 1 This is a SEM image of the microstructure of the grain boundary diffused NdFeB magnet prepared in Example 1 at a depth of 50 μm from its surface. As shown in the image, the microstructure of the grain boundary diffused NdFeB magnet prepared in Example 1 exhibits a thin and continuous hard magnetic shell layer formed on the surface of the NdFeB grains, i.e., a core-shell structure. The composition of the hard magnetic shell layer is (Nd,Tb)₂Fe. 14 B. The hard magnetic shell effectively improves the anisotropic field on the grain surface, thereby enhancing the anti-demagnetization ability.

[0062] Figure 2 This is a SEM image of the microstructure of the grain boundary diffused NdFeB magnet prepared in Example 1 at a depth of 800 μm from its surface. As shown in the image, the grain boundary diffused NdFeB magnet prepared in Example 1 still exhibits clear thin grain boundaries and grain morphology at a depth of 800 μm, further confirming that the diffusion depth of the diffusion source in the grain boundary diffused NdFeB magnet prepared by the method provided in this invention is relatively deep. This continuous thin grain boundary phase at the grain boundaries can prevent direct contact between the main phase grains and effectively block magnetic coupling between adjacent grains, playing an important role in improving magnetism.

[0063] Figure 3 This is a demagnetization curve diagram of Example 1 and Comparative Examples 1 and 2 of the present invention. The solid line (Tb) represents the demagnetization curve. 50 Al 25 Cu 25 The dashed line (Tb) represents the magnetization curve of the grain boundary diffused NdFeB magnet prepared in Example 1. 65 Al 17.5 Cu 17.5 The dotted line (Tb) represents the magnetization curve of the grain boundary diffused NdFeB magnet prepared in Comparative Example 1. 40 Al 30 Cu 30The figure shows the magnetization curve of the grain boundary diffused NdFeB magnet prepared in Comparative Example 2. As can be seen from the figure, the content of heavy rare earth elements in the optimized diffusion source in Example 1 is lower than that in the diffusion source of Comparative Example 1. However, the coercivity of the grain boundary diffused NdFeB magnet in Example 1 is increased by 1.53 kOe, while the remanence and maximum energy product show no significant change. This proves that the preparation method of the grain boundary diffused NdFeB magnet provided by this invention effectively improves the utilization rate of grain boundary diffused heavy rare earth elements and the coercivity of the magnet. However, when the content of heavy rare earth elements in the diffusion source is too low, such as the grain boundary diffused NdFeB magnet prepared in Comparative Example 2, the coercivity is also lower than that in Example 1. This is because the low content of heavy rare earth elements makes it difficult to form an effective hard magnetic shell inside the magnet.

[0064] Table 2 shows the magnetic performance data of the grain boundary diffused NdFeB magnets prepared in Examples 1-5 and Comparative Examples 1-2. The data in the table show that, compared to the diffusion source with high heavy rare earth content in Comparative Example 1 and the diffusion source with excessively low heavy rare earth content in Comparative Example 2, the NdFeB grain boundary diffusion source designed in this invention achieves a higher increase in coercivity for the magnet.

[0065] Table 2 Magnetic property data of the grain boundary diffused NdFeB magnets prepared in Examples 1-5 and Comparative Examples 1-2 Example Hcj (kOe) Br (kG) (BH)max (MGOe) Example 1 28.22 14.01 45.97 Example 2 28.16 13.96 47.70 Example 3 27.84 14.02 45.76 Example 4 28.12 13.97 46.21 Example 5 27.83 13.98 46.01 Comparative Example 1 26.69 14.06 47.48 Comparative Example 2 27.30 14.04 46.18 Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a grain boundary diffused NdFeB magnet, characterized in that, Includes the following steps: S1. Pretreatment: After pretreatment, the neodymium iron boron magnet is placed in the sputtering chamber and evacuated until the vacuum level in the sputtering chamber is ≤5×10⁻⁶. -6 Pa; S2. Diffusion source preparation: Argon gas is introduced into the sputtering chamber to a pressure of 1.6-3.0 Pa, and Tb is deposited by magnetron sputtering. x Al y Cu 100-x-y An alloy thin film, wherein 45≤x≤60 and 15≤y≤35, is used to obtain a neodymium iron boron magnet with a surface-deposited diffusion source; S3. Vacuum heat treatment: The NdFeB magnet with the surface-deposited diffusion source is subjected to a first-stage diffusion treatment and a second-stage tempering treatment under vacuum conditions to obtain a grain boundary diffused NdFeB magnet.

2. The method for preparing a grain boundary diffused NdFeB magnet according to claim 1, characterized in that, In step S1, the orientation direction of the neodymium iron boron magnet after pretreatment is parallel to its thickness direction, and in step S2, the alloy film is perpendicular to the orientation direction.

3. The method for preparing a grain boundary diffused NdFeB magnet according to claim 1, characterized in that, The thickness of the alloy film in step S2 is 10-20 μm, and the Tb accounts for 0.3wt.%-0.6wt.% of the mass of the NdFeB magnet in the surface deposition diffusion source.

4. The method for preparing a grain boundary diffused NdFeB magnet according to claim 1, characterized in that, During the magnetron sputtering deposition process described in step S2, the power of the Tb target is 100-150 W, the power of the Cu target is 20-80 W, and the power of the Al target is 30-70 W.

5. The method for preparing a grain boundary diffused NdFeB magnet according to claim 1, characterized in that, The flow rate of argon gas in step S2 is 30-50 sccm.

6. The method for preparing a grain boundary diffused NdFeB magnet according to claim 1, characterized in that, The vacuum degree of the vacuum condition described in step S3 is ≤1×10 -3 Pa.

7. The method for preparing a grain boundary diffused NdFeB magnet according to claim 3, characterized in that, The temperature of the first-stage diffusion treatment in step S3 is 850-950 ℃ and the holding time is 5-16 h. The temperature of the second-stage tempering treatment is 450-550 ℃ and the holding time is 2-5 h.

8. The method for preparing a grain boundary diffused NdFeB magnet according to claim 3, characterized in that, The heating rate of the first-stage diffusion treatment in step S3 is 5-15 ℃ / min, and the cooling rate of the second-stage tempering treatment is 3-5 ℃ / min.

9. A grain boundary diffused NdFeB magnet, characterized in that, The microstructure of the grain boundary diffused NdFeB magnet includes NdFeB main phase grains and a hard magnetic shell continuously wrapped around the surface of the NdFeB grains. The hard magnetic shell is composed of (Nd,Tb)₂Fe. 14 B, the grain boundary diffused NdFeB magnet is prepared by the preparation method of the grain boundary diffused NdFeB magnet according to any one of claims 1-8.

10. The grain boundary diffused NdFeB magnet according to claim 9, characterized in that, The grain boundary diffusion depth of the grain boundary diffused NdFeB magnet is ≥ 800 μm.

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