A graphite box for reducing tensile force during NdFeB sintering shrinkage and its preparation method

Abstract

This invention belongs to the technical field of NdFeB sintering auxiliary materials, and discloses a graphite box for reducing the shrinkage and tensile force during NdFeB sintering and its preparation method. The box includes a high-density graphite body and an inner bottom siliconized graphite modified layer. This modified layer is formed by spraying and high-temperature aging diffusion of a silicon slurry containing silicon powder, binder, etc., with a thickness of 50-200 μm, and its main components are SiC phase and Si-C solid solution. The preparation involves graphite box pretreatment, silicon slurry preparation, spraying, and high-temperature diffusion steps to achieve a metallurgical bond between silicon and graphite. This invention reduces the friction coefficient of the inner bottom of the box to 0.15-0.25, the magnet crack rate to <1%, and the modified layer is firmly bonded. The thermal conductivity of the box is ≥135 W / (m·K). The process is simple, cost-controllable, suitable for industrialization, and solves the magnet defect problem caused by traditional boxes.

Description

Technical Field

[0001] This invention belongs to the technical field of NdFeB sintering auxiliary materials, and particularly relates to a graphite box for reducing tensile force during the shrinkage process of NdFeB sintering and its preparation method. Background Technology

[0002] Neodymium iron boron (NdFeB) permanent magnets, currently boasting the highest magnetic energy product, are widely used in high-end fields such as new energy vehicles, wind power generation, and electronic information. The fabrication of NdFeB magnets typically employs powder metallurgy, with the sintering process being a crucial step determining the final density, magnetic properties, and dimensional accuracy of the magnet. During sintering, the NdFeB blank needs to be held at a high temperature of 1040-1080℃ for 2-4 hours. During this process, the blank undergoes significant volume shrinkage, with a linear shrinkage rate of approximately 10-15%.

[0003] Currently, graphite boxes are commonly used in industrial production to support NdFeB (neodymium iron boron) blanks for sintering. Graphite, due to its excellent high-temperature resistance, good thermal conductivity, and chemical stability, is an ideal support material for NdFeB sintering. However, traditional graphite boxes present the following key problems during use: During high-temperature sintering shrinkage of the NdFeB blank, significant friction is generated between it and the bottom of the graphite box. This friction exerts a continuous tensile force on the blank during the shrinkage process. Since the NdFeB blank has relatively low strength at high temperatures, this continuous tensile force easily leads to stress concentration within the blank, resulting in defects such as cracks, deformation, and even fracture, severely affecting the yield and magnetic performance stability of the magnets. Statistical data shows that the crack rate of NdFeB magnets caused by sintering shrinkage tensile force is as high as 5-10%, resulting in significant production cost waste.

[0004] To address the aforementioned issues, existing technologies have proposed several solutions: First, a boron nitride (BN) coating is applied to the surface of the graphite cartridge to reduce the coefficient of friction due to the lubricating properties of boron nitride. However, the bonding force between the boron nitride coating and the graphite substrate is weak, making it prone to detachment during high-temperature sintering cycles, resulting in a short service life. Furthermore, the detached boron nitride particles can contaminate the magnet surface. Second, graphite paper is laid inside the cartridge as an insulating layer. However, graphite paper tends to adhere to the magnet surface at high temperatures, and its insulating effect is unstable, failing to fundamentally solve the tensile strength problem.

[0005] Therefore, developing a graphite cartridge modification technology with a low coefficient of friction, strong bonding with graphite substrate, no impact on thermal conductivity, and controllable preparation cost, to fundamentally reduce the tensile force during the sintering shrinkage process of NdFeB, has become an urgent technical challenge to be solved in this field. Attached Figure Description

[0006] Figure 1 is a schematic diagram of the graphite box structure, including the outer wall of the graphite box, the siliconized graphite modified layer, and the bottom of the graphite box. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention discloses a graphite box and its preparation method for reducing the tensile force during the sintering shrinkage process of NdFeB. By performing targeted siliconized graphite modification treatment on the bottom of the graphite box, the friction coefficient between the box and the NdFeB blank is significantly reduced, thereby reducing the tensile force during the sintering shrinkage process, while ensuring the thermal conductivity and service life of the box.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] On one hand, the present invention provides a graphite box for reducing the tensile force during the sintering shrinkage process of NdFeB, comprising a graphite box body and a siliconized graphite modified layer. The siliconized graphite modified layer is disposed in the key area of ​​the inner bottom of the graphite box body in contact with the NdFeB blank. The siliconized graphite modified layer is formed by spraying and high-temperature aging diffusion treatment of silicon slurry. The graphite box body is made of high-density graphite, and the thickness of the siliconized graphite modified layer is 50-200μm.

[0010] The silicon slurry comprises the following components in parts by weight: 20-40 parts silicon powder, 3-8 parts binder, 0.5-2 parts dispersant, and 45-70 parts solvent;

[0011] The silicon powder has a particle size of 1-10μm and a purity of ≥99.5%. Selecting silicon powder within this particle size range can ensure the dispersion stability of the slurry and the uniformity of subsequent diffusion.

[0012] The binder is one of phenolic resin, epoxy resin or polyimide resin. The binder enables the silicon powder to form a stable pre-coating on the surface of the graphite substrate, which gradually decomposes during the high-temperature diffusion process without leaving any impurities.

[0013] The dispersant is one of polyethylene glycol, sodium dodecylbenzenesulfonate, or triethanolamine, which can effectively prevent silica powder agglomeration and ensure the uniform dispersion of the slurry.

[0014] The solvent is one or more of ethanol, propylene glycol methyl ether, or N-methylpyrrolidone, which can adjust the viscosity of the slurry to meet the requirements of the spraying process.

[0015] Furthermore, the mass fractions of each component in the silicone slurry are: 25-35 parts of silicon powder, 4-6 parts of binder, 0.8-1.5 parts of dispersant, and 55-65 parts of solvent. Silicone slurry within this ratio range has the best dispersibility and sprayability.

[0016] Furthermore, the high-density graphite box has a bulk density ≥1.8g / cm³, a Shore hardness ≥60HS, and a compressive strength ≥80MPa. Graphite box substrates with these performance parameters have good structural stability and thermal conductivity, ensuring uniform temperature transfer during sintering.

[0017] Furthermore, the silicon mass fraction in the siliconized graphite modified layer is 8-20%, mainly composed of SiC phase and Si-C solid solution. The SiC phase has excellent lubricity and wear resistance, while the Si-C solid solution can improve the bonding strength between the modified layer and the graphite substrate. The two work synergistically to achieve a balance between low friction coefficient and high bonding strength.

[0018] On the other hand, the present invention also provides a method for preparing the graphite box for reducing the tensile force during the sintering shrinkage process of NdFeB, comprising the following steps:

[0019] (1) Pretreatment of graphite cartridge substrate: Select high-density graphite cartridge as substrate, and polish the bottom of the substrate with 400-800 grit sandpaper to remove the surface oxide layer and impurities. After polishing, ultrasonically clean with anhydrous ethanol for 15-30 min, and then vacuum dry at 100-120℃ for 2-3 h to obtain pretreated graphite cartridge substrate. Pretreatment can improve the surface activity of the substrate and enhance the adhesion between the silicon slurry and the substrate.

[0020] (2) Preparation of silicon slurry: Weigh silicon powder, binder, dispersant and solvent according to the ratio, add them to the mixing tank in sequence, stir at 300-500 r / min for 1-2 h, and then grind them with a sand mill until the slurry particle size is ≤5μm to obtain a uniformly dispersed silicon slurry; the grinding media of the sand mill is zirconia beads with a particle size of 0.5-1 mm, and the grinding time is 2-3 h. The grinding treatment can further refine the slurry particles and ensure the uniformity of the coating after spraying.

[0021] (3) Spraying treatment: The silicon slurry is uniformly sprayed onto the inner bottom of the pretreated graphite substrate using air spraying method. The spraying pressure is 0.3-0.6MPa and the spraying distance is 15-25cm. After spraying, it is dried with hot air at 80-100℃ for 1-2h to form a pre-coating layer. The thickness of the pre-coating layer is 60-220μm, with a certain shrinkage allowance reserved to ensure that the thickness of the modified layer after high-temperature diffusion meets the design requirements.

[0022] (4) High-temperature aging diffusion treatment: The pre-coated graphite box substrate is placed in a sintering furnace and subjected to high-temperature diffusion under an argon protective atmosphere. The argon purity is ≥99.99%, the oxygen content in the furnace is ≤50ppm, the heating rate is 5-10℃ / min, the temperature is raised to 1150-1200℃, and held for 2-4 hours. Then, it is cooled to room temperature with the furnace, allowing silicon to diffuse to the graphite surface to form a silicified graphite modified layer. At high temperature, silicon powder reacts with graphite to generate the SiC phase, and at the same time, some silicon diffuses into the graphite lattice to form a Si-C solid solution, forming a gradient-distributed silicified graphite modified layer, which achieves metallurgical bonding with the substrate and has high bonding strength. Finally, the graphite box with reduced tensile force during the NdFeB sintering shrinkage process is obtained.

[0023] The beneficial effects of this invention are:

[0024] Significantly reduced tensile force: The SiC phase in the siliconized graphite modified layer has an extremely low coefficient of friction (0.15-0.25), compared to the coefficient of friction of 0.4-0.6 of traditional graphite cartridges. This can reduce the friction between the cartridge and the NdFeB blank by more than 50%, thereby effectively reducing the tensile force on the blank during sintering shrinkage and reducing the magnet crack rate to below 0.3%.

[0025] The modified layer is firmly bonded: using a high-temperature aging diffusion process, silicon diffuses to the surface of the graphite substrate to form a gradient silicide layer, achieving metallurgical bonding with the substrate. The bonding strength is ≥15MPa, and it is not easy to fall off during repeated sintering and cycling. Its service life is significantly better than that of existing boron nitride coated cartridges.

[0026] To ensure thermal conductivity: Only the bottom of the material box in contact with the blank is modified in a targeted manner. The graphite material box body retains its original excellent thermal conductivity, with a thermal conductivity coefficient of ≥135W / (m・K). This can ensure the uniformity of magnet temperature during sintering and avoid magnetic performance fluctuations caused by poor thermal conductivity.

[0027] The manufacturing process is simple: it adopts a process route of silicon slurry spraying + high temperature diffusion, which does not require complex equipment, has controllable costs, and is suitable for large-scale industrial production. Compared with the whole silicon graphite cartridge, the production cost is reduced by 30-40%.

[0028] Excellent stability: The siliconized graphite modified layer has good high-temperature stability and chemical inertness. It does not react chemically with the magnet at the NdFeB sintering temperature of 1000-1100℃, and will not contaminate the magnet surface, thus ensuring the quality of the magnet product. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0031] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods; unless otherwise specified, the experimental materials used in the following embodiments are all purchased from commercial channels.

[0032] Example 1: A graphite box for reducing tensile force during the sintering shrinkage process of NdFeB iron boron, comprising a graphite box body and a siliconized graphite modified layer. The graphite box body is made of high-density graphite with a bulk density of 1.8 g / cm³, a Shore hardness of 60 HS, and a compressive strength of 80 MPa. The siliconized graphite modified layer is disposed at the inner bottom and has a thickness of 50 μm.

[0033] The silicone slurry comprises the following components in parts by weight: 20 parts of silicone powder with a particle size of 1-5 μm and a purity of 99.5%, 3 parts of phenolic resin, 0.5 parts of polyethylene glycol, and 45 parts of ethanol.

[0034] The preparation method of this graphite cartridge includes the following steps:

[0035] (1) Pretreatment of graphite substrate: Select a high-density graphite box as the substrate, polish the bottom of the inner part with 400-grit sandpaper to remove the surface oxide layer and impurities, ultrasonically clean with anhydrous ethanol for 15 min after polishing, and then vacuum dry at 100℃ for 2 h to obtain the pretreated graphite substrate.

[0036] (2) Preparation of silicon slurry: Weigh silicon powder, phenolic resin, polyethylene glycol and ethanol according to the ratio, add them to the mixing tank in sequence, stir at 300 r / min for 1 h, and then grind them for 2 h with zirconia beads with a particle size of 0.5 mm until the slurry particle size is ≤5 μm, so as to obtain a uniformly dispersed silicon slurry.

[0037] (3) Spraying treatment: The silicon slurry is sprayed onto the bottom of the pretreated graphite substrate by air spraying. The spraying pressure is 0.3MPa and the spraying distance is 15cm. After spraying, it is dried with hot air at 80℃ for 1h to form a pre-coating layer with a thickness of 60μm.

[0038] (4) High-temperature aging diffusion treatment: The pre-coated graphite substrate is placed in a sintering furnace, and argon gas with a purity of 99.99% is introduced. The oxygen content in the furnace is ≤50ppm. The temperature is raised to 1200℃ at a heating rate of 5℃ / min, held for 2h, and cooled to room temperature with the furnace to form a siliconized graphite modified layer, thus obtaining the graphite box.

[0039] Example 2: A graphite box for reducing tensile force during the sintering shrinkage process of NdFeB, comprising a graphite box body and a siliconized graphite modified layer. The graphite box body is high-density isostatic graphite with a bulk density of 1.9 g / cm³, a Shore hardness of 65HS, and a compressive strength of 90 MPa. The siliconized graphite modified layer is disposed at the inner bottom and has a thickness of 120 μm.

[0040] The silicone slurry comprises the following components in parts by weight: 30 parts of silicone powder with a particle size of 3-8 μm and a purity of 99.8%; 5 parts of epoxy resin; 1.2 parts of sodium dodecylbenzenesulfonate; and 60 parts of a mixed solvent of propylene glycol methyl ether and N-methylpyrrolidone in a volume ratio of 1:1.

[0041] The preparation method of this graphite cartridge includes the following steps:

[0042] (1) Pretreatment of graphite substrate: High-density isostatic graphite was selected as the substrate of the material box. The bottom of the inner part was polished with 600-grit sandpaper to remove the surface oxide layer and impurities. After polishing, it was ultrasonically cleaned with anhydrous ethanol for 20 minutes and then vacuum dried at 110℃ for 2.5 hours to obtain the pretreated graphite substrate.

[0043] (2) Preparation of silicone slurry: Weigh out the silicone powder, epoxy resin, sodium dodecylbenzene sulfonate and mixed solvent according to the ratio, add them to the mixing tank in sequence, stir at 400 r / min for 1.5 h, and then grind in a sand mill for 2.5 h until the slurry particle size is ≤5 μm to obtain a uniformly dispersed silicone slurry;

[0044] (3) Spraying treatment: The silicon slurry is sprayed onto the bottom of the pretreated graphite substrate by air spraying. The spraying pressure is 0.45MPa and the spraying distance is 20cm. After spraying, it is dried with hot air at 90℃ for 1.5h to form a pre-coating layer with a thickness of 150μm.

[0045] (4) High-temperature aging diffusion treatment: The graphite substrate with pre-coated coating is placed in a sintering furnace, and argon gas with a purity of 99.99% is introduced. The oxygen content in the furnace is ≤50ppm. The temperature is raised to 1180℃ at a heating rate of 8℃ / min and held for 3h. The substrate is then cooled to room temperature with the furnace to form a siliconized graphite modified layer with a silicon mass fraction of 15%, thus obtaining the graphite box.

[0046] Example 3: A graphite box for reducing tensile force during the sintering shrinkage process of NdFeB iron boron, comprising a graphite box body and a siliconized graphite modified layer. The graphite box body is high-density isostatic graphite with a bulk density of 2.0 g / cm³, a Shore hardness of 70 HS, and a compressive strength of 100 MPa. The siliconized graphite modified layer is disposed at the inner bottom and has a thickness of 200 μm.

[0047] The silicone slurry comprises the following components in parts by weight: 40 parts of silicone powder with a particle size of 5-10 μm and a purity of 99.5%, 8 parts of polyimide resin, 2 parts of triethanolamine, and 70 parts of N-methylpyrrolidone.

[0048] The preparation method of this graphite cartridge includes the following steps:

[0049] (1) Pretreatment of graphite substrate: High-density isostatic graphite was selected as the substrate of the material box. The bottom of the inner part was polished with 800-grit sandpaper to remove the surface oxide layer and impurities. After polishing, it was ultrasonically cleaned with anhydrous ethanol for 30 minutes and then vacuum dried at 120℃ for 3 hours to obtain the pretreated graphite substrate.

[0050] (2) Preparation of silicon slurry: Weigh silicon powder, polyimide resin, triethanolamine and N-methylpyrrolidone according to the ratio, add them to the mixing tank in sequence, stir at 500 r / min for 2 h, and then grind them for 3 h with zirconia beads with a particle size of 1 mm until the slurry particle size is ≤5 μm, so as to obtain a uniformly dispersed silicon slurry.

[0051] (3) Spraying treatment: The silicon slurry is sprayed onto the bottom of the pretreated graphite substrate by air spraying. The spraying pressure is 0.6MPa and the spraying distance is 25cm. After spraying, it is dried with hot air at 100℃ for 2h to form a pre-coating layer with a thickness of 220μm.

[0052] (4) High-temperature aging diffusion treatment: The pre-coated graphite substrate is placed in a sintering furnace, and argon gas with a purity of 99.99% is introduced. The oxygen content in the furnace is ≤50ppm. The temperature is raised to 1160℃ at a heating rate of 10℃ / min, held for 4h, and cooled to room temperature with the furnace to form a siliconized graphite modified layer, thus obtaining the graphite box.

[0053] Comparative Example 1: A conventional high-density graphite cartridge without any modification was used, with performance parameters consistent with the graphite substrate of Example 2.

[0054] Comparative Example 2: A graphite cartridge with a boron nitride coating on the inner bottom, the boron nitride coating thickness being 100 μm, and the other substrate properties being consistent with those of Example 2.

[0055] Results Analysis

[0056] Friction coefficient test: The friction coefficient between the bottom of each material box and the NdFeB blank was measured at 1050℃ using a high-temperature friction and wear tester. The results are shown in Table 1.

[0057] Tensile force test: Neodymium iron boron blanks of the same specifications were placed into the material boxes of each embodiment and comparative example, and sintered according to the standard sintering process of 1060℃×3h. The maximum tensile force on the blank during the sintering shrinkage process was monitored in real time using a built-in stress sensor. The results are shown in Table 1.

[0058] Magnet crack rate statistics: Each set of material boxes was used 20 times continuously. The number of cracks in the NdFeB magnets after each sintering was counted, and the crack rate was calculated. The results are shown in Table 1.

[0059] Thermal conductivity test: The thermal conductivity of each material box was determined by laser flash method, and the results are shown in Table 1.

[0060] Service life test: The number of times the modified layer or coating of each material box began to peel off during use was recorded. The results are shown in Table 1.

[0061] Table 1 Performance Test Results Comparison Table

[0062] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Coefficient of friction (1050℃) 0.23 0.18 0.2 0.48 0.21 Maximum tensile force (N) 185 142 156 328 168 Magnet crack rate (%) 0.8 0.3 0.5 8.6 1.5 Thermal conductivity (W / (m・K)) 135 142 138 145 143 Service life (times) ≥100 ≥100 ≥100 (Uncoated) 45

[0063] The data in Table 1 shows that:

[0064] The friction coefficient of the hull in Examples 1-3 is all below 0.25, the maximum tensile force is all below 190N, and the crack rate of the magnet is all below 1%, showing excellent tensile force reduction effect. This is due to the low friction characteristics of the siliconized graphite modified layer.

[0065] Example 2 exhibits the best overall performance, with a friction coefficient as low as 0.18, a maximum tensile force of only 142N, and a magnet crack rate of 0.3%, which is significantly better than the conventional graphite cartridge of Comparative Example 1 and the boron nitride coated cartridge of Comparative Example 2.

[0066] The thermal conductivity of the example group was maintained above 135 W / (m·K), which is close to that of the conventional graphite box comparative example 1, ensuring the uniformity of magnet temperature during sintering;

[0067] The service life of the example group was ≥200 cycles, which was significantly better than that of the boron nitride coated cartridge of Comparative Example 2, which only had 45 cycles.

[0068] Comparative Example 1 shows that the traditional graphite cartridge has the highest coefficient of friction and tensile force, and the magnet crack rate is as high as 8.6%, which cannot meet the production requirements of high-performance magnets.

[0069] Although the boron nitride coated cartridge of Comparative Example 2 had a low initial coefficient of friction, the coating was easy to peel off, had a short service life, and poor long-term stability.

[0070] In summary, this invention, through siliconization modification treatment of the bottom of the graphite box by spraying silicon slurry and high-temperature aging diffusion, significantly reduces the friction coefficient and tensile force during the sintering shrinkage of NdFeB while ensuring the thermal conductivity and structural stability of the box, effectively reducing the crack rate of the magnet. It also has the advantages of simple preparation process, controllable cost, and long service life, making it suitable for large-scale industrial application.

[0071] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A graphite sintering box for reducing tensile force during the sintering shrinkage process of NdFeB magnets, characterized in that: The device comprises a graphite cartridge body and a siliconized graphite modified layer. The siliconized graphite modified layer is disposed in the key area of ​​the inner bottom of the graphite cartridge body in contact with the NdFeB blank. The siliconized graphite modified layer is formed by spraying and high-temperature aging diffusion treatment of silicon slurry. The graphite cartridge body is made of high-density graphite. The thickness of the siliconized graphite modified layer after high-temperature aging diffusion treatment is 50-200 μm. The silicon slurry comprises the following components in parts by weight: 20-40 parts silicon powder, 3-8 parts binder, 0.5-2 parts dispersant, and 45-70 parts solvent. The silicon powder has a particle size of 1-10 μm and a purity ≥99.5%. The binder is one of phenolic resin, epoxy resin, or polyimide resin. The dispersant is one of polyethylene glycol, sodium dodecylbenzene sulfonate, or triethanolamine. The solvent is one or a mixture of ethanol, propylene glycol methyl ether, or N-methylpyrrolidone.

2. The graphite box for reducing tensile force during the sintering shrinkage process of NdFeB as described in claim 1, characterized in that: The mass fractions of each component in the silicon slurry are: 25-35 parts silicon powder, 4-6 parts binder, 0.8-1.5 parts dispersant, and 55-65 parts solvent.

3. The graphite box for reducing tensile force during the sintering shrinkage process of NdFeB according to claim 1, characterized in that: The high-density graphite has a bulk density ≥1.8 g / cm³, tested according to GB / T 14898-2018; a Shore hardness ≥60HS, tested according to GB / T 231.1-2018; and a compressive strength ≥80 MPa, tested according to GB / T 14390-2015.

4. The graphite box for reducing tensile force during the sintering shrinkage process of NdFeB as described in claim 1, characterized in that: In the siliconized graphite modified layer, based on the total mass of the modified layer, the mass fraction of silicon is 8-20%, and it is mainly composed of SiC phase and Si-C solid solution.

5. The method for preparing a graphite box for reducing tensile force during the sintering shrinkage process of NdFeB as described in any one of claims 1-4, characterized in that: Includes the following steps: (1) Pretreatment of graphite substrate: Select high-density graphite boxes as substrates, and polish the bottom of the substrate with 400-800 grit sandpaper to remove the surface oxide layer and impurities. After polishing, ultrasonically clean with anhydrous ethanol for 15-30 min, and then vacuum dry at 100-120℃ for 2-3 h to obtain pretreated graphite substrates; (2) Preparation of silicon slurry: Weigh silicon powder, binder, dispersant and solvent according to the ratio, and add them to the stirring tank in sequence. Stir at a speed of 300-500 r / min. (2) Stir for 1-2 hours, then grind the slurry to a particle size ≤5μm using a sand mill to obtain a uniformly dispersed silicon slurry; (3) Spraying treatment: The silicon slurry is uniformly sprayed onto the inner bottom of the pretreated graphite substrate using an air spraying method. The spraying pressure is 0.3-0.6MPa, the spraying distance is 15-25cm, and after spraying, it is dried with hot air at 80-100℃ for 1-2 hours to form a pre-coating layer; (4) High temperature aging diffusion treatment: The graphite substrate with the pre-coating layer is placed in a sintering furnace and subjected to high temperature diffusion under an argon protective atmosphere. The heating rate is 5-10℃ / min, the temperature is raised to 1150-1200℃, and the temperature is held for 2-4 hours. Then, it is cooled to room temperature with the furnace to allow silicon elements to diffuse to the graphite surface to form a siliconized graphite modified layer, thus obtaining the graphite box that reduces the tensile force during the NdFeB sintering shrinkage process.

6. The preparation method according to claim 5, characterized in that: In step (4), the purity of argon gas is ≥99.99%, and the oxygen content in the furnace is ≤50ppm.

7. The preparation method according to claim 5, characterized in that: In step (2), the grinding media of the sand mill is zirconia beads with a particle size of 0.5-1mm, and the grinding time is 2-3h.

8. The preparation method according to claim 5, characterized in that: The thickness of the pre-coating layer in step (3) is 60-220 μm, and the thickness of the siliconized graphite modified layer after high-temperature diffusion is 50-200 μm.

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