A method for in-situ carbothermic reduction preparation of neodymium-iron-boron permanent magnet material

Abstract

This invention discloses a method for preparing neodymium iron boron permanent magnet materials by in-situ carbothermal reduction, comprising the following steps: S1 using sintered neodymium iron boron magnets as the magnetic substrate and removing the surface oxide layer; S2 coating with a diffusion source, preparing Tb-MOF nanocrystals into a suspension with a solid content of 18-22%, and uniformly coating it onto the surface of the magnetic substrate treated in step S1, wherein the average particle size of the Tb-MOF nanocrystals is 13-17 nm, and the coating amount of the Tb-MOF nanocrystals is 0.08-0.12 mg / mm. 2 S3 Vacuum heat treatment and in-situ reduction infiltration: In a vacuum environment, the temperature is slowly increased in the low-temperature range of 150℃-450℃ at a rate of 1-2℃ / min, and then increased to 900-1000℃ and held for 6-10 hours. S4 Segmented tempering: First, a high-temperature tempering is performed at 835-885℃ for 1.5-2.5 hours, followed by rapid quenching with high-pressure argon gas. Then, a low-temperature tempering is performed at 475-525℃ for 3-4 hours. Finally, rapid quenching with high-pressure argon gas is performed to improve coercivity while reducing remanence loss.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material manufacturing technology, specifically to a method for preparing neodymium iron boron permanent magnet materials by in-situ carbothermal reduction. Background Technology

[0002] Sintered neodymium iron boron (NdFeB) permanent magnets, known as the "King of Magnets" due to their extremely high magnetic energy product, are key materials for core components such as drive motors for new energy vehicles and high-power wind turbines. With technological upgrades, the industry has imposed stringent requirements on magnets at the "56SH" level, meaning that while maintaining high remanence (Br > 1.49T), the intrinsic coercivity (Hcj) must exceed 20 kOe, and the magnets must be able to operate stably at a high temperature of 150℃.

[0003] However, the improvement of magnet performance has long been constrained by the trade-off between remanence and coercivity. Early technical solutions mainly improved coercivity by adding heavy rare earth elements such as terbium (Tb) or dysprosium (Dy) to the alloy as a whole. However, this method causes antiferromagnetic coupling of the magnetic moments inside the main phase grains, resulting in a significant decrease in remanence. Moreover, it consumes a large amount of heavy rare earth elements, making it extremely costly.

[0004] To reduce costs and maintain remanence, grain boundary diffusion (GBD) technology has emerged. For example, the existing literature "The diffusion behavior and striking coercivity enhancement by Dip-coating TbH3 powders in sintered NdFeB magnets" explores a diffusion method using terbium hydride powder coating. While this technology can improve coercivity to some extent, the effective penetration depth of traditional powder diffusion sources is severely limited for thick magnets with a thickness greater than 10 mm. Furthermore, existing techniques such as "New grain boundary diffusion strategy of Nd-Fe-B sintered magnets to achieve breakthroughs in Tb-availability and thick limitation" point out that residual oxygen, nitrogen, and other impurities in the original grain boundaries of the magnet can form oxide films, hindering the wetting and migration of heavy rare earth atoms. This leads to uneven performance distribution within the thick magnet, and during high-temperature diffusion, the lack of effective grain boundary constraints easily causes coarsening of the main phase grains, resulting in a decrease in magnet rectangularity and performance degradation.

[0005] Therefore, how to achieve deep and uniform penetration of heavy rare earth elements into thick magnets while ensuring high remanence, and how to remove grain boundary impurities and suppress grain coarsening in situ, are the technical challenges that urgently need to be solved in the field of high-performance NdFeB manufacturing. Summary of the Invention

[0006] To overcome the shortcomings of the above-mentioned related technologies, this application provides a method for preparing neodymium iron boron permanent magnet materials by in-situ carbothermal reduction, which improves coercivity while reducing remanence loss.

[0007] The technical solution adopted by this invention to solve the technical problem is: a method for preparing neodymium iron boron permanent magnet materials by in-situ carbothermal reduction, characterized by comprising the following steps: S1 uses sintered NdFeB magnets as the magnetic substrate and removes the surface oxide layer; this step aims to expose fresh substrate grain boundary channels, laying the foundation for the subsequent close adhesion and efficient penetration of Tb-MOF nanocrystals.

[0008] The S2 diffusion source coating involves preparing a suspension of Tb-MOF nanocrystals with a solid content of 18-22%, which is then uniformly coated onto the surface of the magnetic substrate treated in step S1. The average particle size of the Tb-MOF nanocrystals is 13-17 nm, and the coating amount of the Tb-MOF nanocrystals is 0.08-0.12 mg / mm². 2 ; S3 vacuum heat treatment and in-situ reduction infiltration: In a vacuum environment, the temperature is slowly increased in the low-temperature range of 150℃-450℃ at a rate of 1-2℃ / min, and then increased to 900-1000℃ and held for 6-10 h. In the low-temperature range of 150℃-450℃, the reducing gas released by the decomposition of the MOF framework is used to remove oxygen impurities at the grain boundaries in situ. Then, the temperature is increased to 900-1000℃, allowing Tb atoms to penetrate deep into the magnet along the liquid phase grain boundaries, forming a highly magnetically anisotropic "defense shell" structure by reconstructing the surface of the grains.

[0009] The S4 staged tempering process involves a first-stage high-temperature tempering at 835-885℃ for 1.5-2.5 hours, followed by rapid quenching with high-pressure argon. A second-stage low-temperature tempering at 475-525℃ for 3-4 hours is then performed, again followed by rapid quenching with high-pressure argon. The first-stage high-temperature tempering eliminates diffusion stress and locks in the desired microstructure distribution. The second-stage low-temperature tempering induces the reaction of excess carbon atoms with the matrix elements, generating pinned carbide phases with a diameter less than 15 nm. These "nanopinions" effectively fix grain boundaries, significantly improving the coercivity and thermal stability of the magnet at high temperatures.

[0010] Preferably, the suspension in step S2 is prepared by adding Tb-MOF nanocrystals to cyclohexane and adding an appropriate amount of dispersant.

[0011] Preferably, the dispersant is oleylamine.

[0012] Preferably, the Tb-MOF nanocrystals in step S2 are prepared by the following method: Tb salt and pyromellitic acid were dissolved in an organic solvent, and a morphology modifier was added. The mixture was heated at 130-150℃ for 48 h. After the reaction was completed, Tb-MOF nanocrystals were obtained by high-speed centrifugation and separation, wherein the molar ratio of Tb to pyromellitic acid was 1:1.

[0013] Preferably, the morphology modifier is 0.1 mol / L PVP.

[0014] Preferably, the organic solvent is DMF or ethanol.

[0015] Preferably, the removal of the surface oxide layer in step S1 specifically involves: acid washing the surface of the magnetic substrate with a 0.5% dilute nitric acid solution, followed by degreasing the magnetic substrate in an ultrasonic cleaner containing alcohol or organic solvent, and finally drying it.

[0016] Preferably, in step S4, the first stage is high-temperature tempering at 860℃ for 2 hours, and the second stage is low-temperature tempering at 500℃ for 3.5 hours.

[0017] Compared with related technologies, the present invention has the following advantages: Carbothermic reduction is performed using reducing gases (CO, H2) generated by the pyrolysis of Tb-MOF to remove oxygen impurities at grain boundaries in situ, making the grain boundaries "transparent" to heavy rare earth atoms. Simultaneously, quasi-atomic-level Tb clusters are released, allowing Tb atoms to penetrate deep into the magnet along the liquid-phase grain boundaries, forming a highly magnetically anisotropic "defense shell" structure on the grain surface. A two-stage tempering process is used, and at approximately 500℃, the derived nanoscale carbides pinnate to the grain boundary triangular regions, ensuring a fine-grained structure. These methods enhance coercivity while reducing remanence loss. Attached Figure Description

[0018] Figure 1 This is a scanning electron microscope image of the Tb-MOF nanocrystals of the present invention; Figure 2 This is a schematic diagram of the structure of the neodymium iron boron permanent magnet material prepared by the present invention. Detailed Implementation

[0019] First, those skilled in the art should understand that these embodiments are merely used to explain the technical principles of the embodiments of this application and are not intended to limit the scope of protection of the embodiments of this application. Those skilled in the art can make adjustments as needed to adapt to specific application scenarios.

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0021] Example 1: Step A: Deep cleaning and preparation of the substrate magnet Prepare a 10mm cubic N56M magnet. Next, acid-wash the magnet surface with a 0.5% dilute nitric acid solution for 30 seconds to remove the oxide scale. Then, place the magnet in an ultrasonic cleaner containing alcohol or organic solvent for degreasing to thoroughly remove any residual oil and grease, and dry it for later use.

[0022] Step B: Synthesis of Tb-MOF nanocrystals 1 mmol Tb(NO3)3·6H2O and 1 mmol trimesic acid (BTC) were dissolved in 30 mL DMF / ethanol, and 0.1 mol / L PVP was added as a morphology modifier. The mixture was poured into a reaction vessel and heated at 140 °C for 48 hours. After the reaction, the mixture was washed and separated using a high-speed centrifuge at 4000 rpm, finally obtaining ultrafine Tb-MOF crystals with an average diameter of 15 nm. Figure 1 As shown.

[0023] Step C: Uniform spraying of Tb-MOF nanocrystals Using cyclohexane as a solvent and adding a small amount of oleylamine dispersant, Tb-MOF nanocrystals were prepared into a cyclohexane suspension with a solid content of 20 wt%. Subsequently, a multi-station ultrasonic atomization spraying device was used to break the liquid into extremely fine droplets using ultrasound, which were then evenly sprayed onto the six surfaces of the magnet. The spraying amount was precisely controlled to ensure that the weight increase per square millimeter of the magnet surface was between 0.08 and 0.12 milligrams, with an error not exceeding 3%.

[0024] Step D: Conduct "high-temperature permeation and conversion" in a vacuum furnace. The coated magnets are placed in a high-vacuum diffusion furnace for heat treatment. Heating is performed at a very slow rate (only 1.5°C per minute) between 150°C and 450°C, allowing the organic framework to slowly decompose and release reducing gases (such as carbon monoxide and hydrogen). The temperature is then raised to 900-1000°C, causing the filling phase inside the magnet to liquefy. At this point, active terbium atoms produced by the MOF decomposition rapidly penetrate into the center of the magnet through these liquid channels. After holding at this temperature for 8 hours, the terbium atoms can penetrate more than 1 mm into the thick magnet.

[0025] Step E: Segmented tempering After infiltration, segmented tempering is performed. The first step is tempering at 870℃ for 2 hours to eliminate internal stress generated during diffusion and to make the heavy rare earth elements more smoothly distributed. Then, quenching is performed for 10-15 minutes with argon gas at 0.5-1.0 MPa to lock in the ideal microstructure and prevent the formation of impurity phases. The second step is a secondary tempering at 500℃ for 3.5 hours, followed by rapid quenching with high-pressure argon gas after approximately 3.5 hours.

[0026] The core performance indicators of the magnetic substrate (N56M magnet) before treatment in this invention are as follows: Remanence Br: 15.20 kGs Intrinsic coercivity Hcj: 14.1 kOe Rectangularity Hk / Hcj: 0.98 Test results of key indicators of neodymium iron boron permanent magnet materials after treatment according to this invention: Remanence Br: 14.91 kGs Intrinsic coercivity Hcj: 23.34 kOe Rectangularity Hk / Hcj: 0.97 Comparative Example 1: Substrate Blank Control Experimental procedure: Take N56M substrates from the same batch and do not apply any spray coating.

[0027] Step A: Deep cleaning and preparation of the substrate magnet Prepare a 10mm cubic N56M magnet, identical to that used in Example 1. Next, acid-wash the magnet surface with a 0.5% dilute nitric acid solution for 30 seconds to remove the oxide scale. Then, place the magnet in an ultrasonic cleaner containing alcohol or organic solvent for degreasing to thoroughly remove any residual oil and grease, and dry it for later use.

[0028] Step B: Conduct "high-temperature infiltration and conversion" in a vacuum furnace. The magnet was placed in a high-vacuum diffusion furnace for heat treatment. It was first heated at a very slow rate (only 1.5°C per minute) between 150°C and 450°C. The temperature was then raised to 900°C and held for 8 hours.

[0029] Step C: Segmented tempering as in Example 1 The first step is to temper at 870℃ for 2 hours, followed by rapid quenching with high-pressure argon. The second step is to temper again at 500℃ for 3.5 hours, followed by rapid quenching with high-pressure argon again after approximately 3.5 hours.

[0030] Test results of core indicators of NdFeB permanent magnet material after treatment in Comparative Example 1: Remanence Br: 15.12 kGs Intrinsic coercivity Hcj: 12.8 kOe Rectangularity Hk / Hcj: 0.95 Comparative Example 2: Traditional TbH3 Powder Diffusion Process Step A: Deep cleaning and preparation of the substrate magnet Prepare a 10mm cubic N56M magnet. Next, acid-wash the magnet surface with a 0.5% dilute nitric acid solution for 30 seconds to remove the oxide scale. Then, place the magnet in an ultrasonic cleaner containing alcohol or organic solvent for degreasing to thoroughly remove any residual oil and grease, and dry it for later use.

[0031] Step B: Synthesize a traditional terbium hydride (TbH3) powder dispersant and uniformly "spray" the dispersant onto the magnet surface. Industrial-grade terbium hydride (TbH3) powder with an average particle size of 5 μm was selected as the diffusion source, and cyclohexane was used as the solvent with the addition of a small amount of oleylamine dispersant to prepare a cyclohexane suspension with a solid content of 20 wt%. Subsequently, a multi-station ultrasonic atomization spraying device was used to break the liquid into extremely fine droplets using ultrasound, which were then evenly sprayed onto the six surfaces of the magnet. The spraying amount was precisely controlled to ensure that the weight increase per square millimeter of the magnet surface was between 0.08 and 0.12 milligrams, with an error not exceeding 3%.

[0032] Step C: Conduct "high-temperature permeation and conversion" in a vacuum furnace. The magnet was placed in a high-vacuum diffusion furnace for heat treatment. It was first heated at a very slow rate (only 1.5°C per minute) between 150°C and 450°C. The temperature was then raised to 900°C and held for 8 hours.

[0033] Step D: Segmented tempering as in Example 1 The first step is to temper at 870℃ for 2 hours, followed by rapid quenching with high-pressure argon. The second step is to temper again at 500℃ for 3.5 hours, followed by rapid quenching with high-pressure argon again after approximately 3.5 hours.

[0034] Test results of core indicators of NdFeB permanent magnet materials after treatment in Comparative Example 2: Remanence Br: 14.88 kGs Intrinsic coercivity Hcj: 19.1 kOe Rectangularity Hk / Hcj: 0.90 The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing neodymium iron boron permanent magnet materials by in-situ carbothermic reduction, characterized in that, Includes the following steps: S1 uses sintered NdFeB magnets as the magnetic substrate and removes the surface oxide layer; The S2 diffusion source coating involves preparing a suspension of Tb-MOF nanocrystals with a solid content of 18-22%, which is then uniformly coated onto the surface of the magnetic substrate treated in step S1. The average particle size of the Tb-MOF nanocrystals is 13-17 nm, and the coating amount of the Tb-MOF nanocrystals is 0.08-0.12 mg / mm². 2 ; S3 vacuum heat treatment and in-situ reduction infiltration: In a vacuum environment, the temperature is slowly increased in the low temperature range of 150℃-450℃ at a rate of 1-2℃ / min, and then increased to 900-1000℃ and held for 6-10 hours. The S4 staged tempering process involves first performing a high-temperature tempering at 835-885℃ for 1.5-2.5 hours, followed by rapid quenching with high-pressure argon gas, then a low-temperature tempering at 475-525℃ for 3-4 hours, and finally rapid quenching with high-pressure argon gas.

2. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 1, characterized in that, The suspension in step S2 is prepared by adding Tb-MOF nanocrystals to cyclohexane and adding an appropriate amount of dispersant.

3. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 2, characterized in that, The dispersant is oleylamine.

4. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 2, characterized in that, The Tb-MOF nanocrystals in step S2 were prepared by the following method: Tb salt and pyromellitic acid were dissolved in an organic solvent, and a morphology modifier was added. The mixture was heated at 130-150℃ for 48 h. After the reaction was completed, Tb-MOF nanocrystals were obtained by high-speed centrifugation and separation, wherein the molar ratio of Tb to pyromellitic acid was 1:

1.

5. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 4, characterized in that, The morphology modifier is 0.1 mol / L PVP.

6. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 4, characterized in that, The organic solvent is DMF or ethanol.

7. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 1, characterized in that, The removal of the surface oxide layer in step S1 specifically involves: acid washing the surface of the magnetic substrate with a 0.5% dilute nitric acid solution, followed by degreasing the magnetic substrate in an ultrasonic cleaner containing alcohol or organic solvent, and finally drying it.

8. The method for preparing NdFeB permanent magnet materials by in-situ carbothermal reduction according to claim 1, characterized in that, In step S4, the first stage is high-temperature tempering at 860℃ for 2 hours, and the second stage is low-temperature tempering at 500℃ for 3.5 hours.

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