Neodymium-iron-boron grain boundary reconstruction method and application
By using supercritical fluid infiltration and heat treatment, the problems of improving coercivity and maintaining remanence in NdFeB magnets were solved, achieving low-cost and efficient grain boundary reconstruction and producing high-performance 50UH-level magnets.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-01-27
- Publication Date
- 2026-03-13
AI Technical Summary
Existing technologies struggle to maintain high remanence while increasing the coercivity of NdFeB magnets. Traditional methods result in decreased remanence or high equipment costs, and are difficult to handle magnets with complex shapes. Furthermore, traditional processes suffer from impurity contamination and uneven diffusion.
Heavy rare earth organic complexes are infiltrated into the interior of neodymium iron boron magnets using supercritical fluid. Through infiltration and heat treatment with supercritical fluid, uniform deposition and diffusion of heavy rare earths at the grain boundaries are achieved, avoiding the use of organic binders, forming highly active oxides, and reconstructing the grain boundaries.
Significantly improves coercivity with low heavy rare earth consumption, maintains high remanence, achieves uniform penetration across the entire size, avoids impurity contamination, has low production cost, and produces magnets that meet the performance requirements of 50UH-class magnets.
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Figure CN121662536A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of rare earth permanent magnet material preparation technology, specifically to a method for reconstructing NdFeB grain boundaries and its application. Background Technology
[0002] In the current global materials science field, sintered neodymium iron boron (Nd-Fe-B) permanent magnets are hailed as the "King of Magnets" due to their excellent magnetic energy product (BH)max. With the explosive growth of high-end applications such as new energy vehicles (NEVs), wind power generation, industrial robots, and precision servo motors, the market's requirements for magnet performance have long surpassed the traditional single-indicator dimension, shifting to the simultaneous pursuit of the seemingly contradictory indicators of "high remanence (High Br)" and "high coercivity (High Hcj)".
[0003] According to internationally accepted NdFeB grade standards: The N50 grade represents extremely high magnetic energy product (approximately 50 MGOe) and remanence (Br≈14.0-14.5 kGs), but its intrinsic coercivity (Hcj) is typically only 11-12 kOe, and its maximum operating temperature is usually limited to 60-80℃, which cannot meet the demagnetization resistance requirements of drive motors at high temperatures of 150-180℃.
[0004] The 50UH grade requires maintaining the high remanence (Br≥13.9kGs) of the N50 level while significantly increasing the coercivity to above 25kOe (usually 25-28kOe is required to ensure a safety margin), thereby meeting the heat resistance standard of 180℃ (UH grade).
[0005] The physical challenge of this transformation lies in the fact that traditional methods of increasing coercivity (such as adding large amounts of heavy rare earth elements dysprosium (Dy) or terbium (Tb) during the alloy smelting stage) result in a significant decrease in saturation magnetization (Ms) due to the antiferromagnetic coupling between Dy / Tb atoms and Fe atoms. Experimental experience shows that for every 1 wt% of Dy added, the remanence Br decreases by approximately 0.1-0.12 T. If the coercivity is increased from 12 kOe to 25 kOe solely through alloying (requiring the addition of approximately 6-8 wt% Dy / Tb), the magnet grade will inevitably drop from N50 to UH35 or UH38, failing to meet the high energy product requirement of "50 UH".
[0006] Currently, the mainstream grain boundary diffusion technologies mainly include: 1. Physical Vapor Deposition (PVD / Sputtering): Tb / Dy is deposited on the surface of a magnet using magnetron sputtering. Disadvantages: Limited line-of-sight capability, making it difficult to handle complex shapes; expensive equipment and low deposition efficiency; for magnets thicker than 4mm, effective heavy rare earth element diffusion cannot be achieved in the central area, leading to a "soft core" phenomenon and a kink in the demagnetization curve.
[0007] 2. Dip Coating / Spraying: Tb / Dy oxide or fluoride powder is suspended in an organic solvent for coating. Defects: The organic binder in the slurry leaves carbon impurities upon high-temperature decomposition. Carbon reacts with rare earth elements to form carbides (RE2C3). These carbides are not only non-magnetic impurities at grain boundaries but also pin the flow of the liquid phase at grain boundaries, hindering diffusion depth. Furthermore, uneven surface coating thickness can lead to magnet dimensional deviations.
[0008] 3. Electrophoretic Deposition: This method uses an electric field to adsorb particles. Disadvantages: It also faces the problem of solvent residue and requires extremely high conductivity and cleanliness of the magnet surface. Summary of the Invention
[0009] To overcome the shortcomings of the above-mentioned related technologies, this application provides a method and application for NdFeB grain boundary reconstruction. By delivering heavy rare earth sources to the center of the magnet through supercritical fluid, the remanence retention rate and coercivity are effectively improved, while reducing production costs.
[0010] The technical solution adopted by this invention to solve the technical problem is: a method for reconstructing NdFeB grain boundaries, comprising the following steps: S1 involves immersing the magnet in a low-concentration nitric acid-ethanol solution for ultrasonic cleaning, creating tiny pores on the magnet surface. S2 supercritical fluid dissolves heavy rare earth organic complexes and then permeates the magnets treated in step S1 under pressure; S3 depressurization causes heavy rare earth organic complexes to be deposited in situ within the magnet; S4 involves heating the magnet treated in step S3 under a low vacuum or flowing inert gas protection to decompose the heavy rare earth organic complex into heavy rare earth oxides and organic matter, with the organic matter volatilizing upon heating. S5 involves heating the magnet treated in step S3 to 880-920℃ under low vacuum or flowing inert gas protection and holding it at that temperature for 6-10 hours. S6. The magnet treated in step S3 is cooled to 480-520℃ and held at that temperature for 3-5 hours, followed by gas quenching.
[0011] Preferably, in step S1, the concentration of nitric acid in the low-concentration nitric acid-ethanol solution is 0.5%-1.0%, and the ultrasonic time is 30-60 seconds.
[0012] Preferably, the supercritical fluid in step S2 is supercritical CO2, and the heavy rare earth organic complex is tris(2,2,6,6-tetramethyl-3,5-heptadecyl)terbium(III).
[0013] Preferably, step S2 specifically involves: placing the magnet treated in step S1 and tris(2,2,6,6-tetramethyl-3,5-heptadecyl)terbium(III) powder into a reaction vessel, ensuring that the magnet and the tris(2,2,6,6-tetramethyl-3,5-heptadecyl)terbium(III) powder do not come into contact, introducing CO2 gas, pressurizing to 18-25 MPa, heating to 40-60°C, and maintaining this state for 3-6 hours.
[0014] Preferably, the depressurization rate in step S3 is 5-10 MPa / min.
[0015] Preferably, step S4 specifically involves: placing the magnet processed in step S3 under a vacuum of 10... -1 Under the protection of Pa or flowing argon gas, the temperature is increased to 350-400℃ at a rate of 3-5℃ / min and held for 2 hours.
[0016] The aforementioned method for reconstructing NdFeB grain boundaries is applied to the fabrication of 50UH-level magnets from N50-level magnets.
[0017] A 50UH-class magnet is prepared from an N50-class magnet using the aforementioned neodymium iron boron grain boundary reconstruction method.
[0018] Compared with related technologies, the present invention has the following advantages: 1. Supercritical fluid carrier technology: Utilizing the characteristics of supercritical fluids, such as zero surface tension and high diffusion coefficient, heavy rare earth precursors are carried to overcome capillary resistance and penetrate deep into the micropores and grain boundary networks inside the magnet, achieving full-size uniform penetration.
[0019] 2. Adhesive-free cleaning process: No organic resin binders are used. The precursor ligands are completely volatilized during the pre-calcination stage, avoiding contamination of the grain boundaries by carbon and oxygen impurities and purifying the grain boundary phase.
[0020] 3. High-performance conversion: It can increase the coercivity by more than 12kOe while maintaining the high remanence of N50 with extremely low consumption of heavy rare earth elements (<1wt%), and stably prepare 50UH products. Attached Figure Description
[0021] Figure 1 This is a schematic diagram comparing the microscopic permeation mechanism of the present invention with that of Comparative Example 1. Detailed Implementation 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.
[0022] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.
[0023] Example 1 Step 1: Substrate Preparation and Surface Activation The magnets selected are N50 grade sintered NdFeB magnets (20mm×10mm×5mm in size).
[0024] Chamfering: Mechanical chamfering removes sharp edges and corners to prevent stress concentration and chipping during subsequent high-pressure processing.
[0025] Chemical activation: Immerse the magnet in a low-concentration nitric acid-ethanol solution (HNO3 concentration 0.5%-1.0%) and ultrasonically clean it for 30-60 seconds.
[0026] Principle: Dilute acid preferentially corrodes the neodymium-rich phase at the grain boundaries, which not only removes the oxide scale but also artificially creates tiny pores on the surface, facilitating the entry of supercritical fluid.
[0027] It was then cleaned with anhydrous ethanol and isopropanol, and then vacuum dried.
[0028] Step Two: Supercritical Impregnation Equipment: High-pressure autoclave equipped with precision temperature control and back pressure valve.
[0029] Selection of heavy rare earth organic complex: Tris(2,2,6,6-tetramethyl-3,5-heptadecane)terbium(III), abbreviated as Tb(TMHD)3, was selected. It has high solubility in supercritical CO2, does not contain water of crystallization, and the methylated ligand not only increases the solubility in nonpolar CO2, but also has a moderate decomposition temperature (250-300℃) and clean decomposition products.
[0030] Operating conditions: The magnet and Tb(TMHD)3 powder are mixed at a ratio of 1% of the magnet weight and placed in the reaction vessel. The magnet is suspended or placed on a stainless steel mesh frame to avoid direct contact with the powder (relying on supercritical fluid transport).
[0031] High-purity CO2 gas was introduced and pressurized to 20 MPa.
[0032] When the temperature is raised to 50℃, CO2 is in a supercritical state (critical point: 31.1℃, 7.38MPa).
[0033] Pressure-holding permeation: Maintain this state for 4 hours. During this time, Tb(TMHD)3 molecules dissolve in supercritical CO2 fluid, freely travel through the fluid, and permeate into the open pore network of the magnet.
[0034] Step 3: Rapid Expansion Deposition Operation: Open the pressure relief valve to quickly release the pressure at a rate of 5-10 MPa / min.
[0035] As the pressure drops sharply, the density of CO2 decreases instantaneously, resulting in a loss of its solubility for heavy rare earth organic complexes (Solubility Drop). Tb(TMHD)3 molecules dissolved in the fluid instantly precipitate out in situ (i.e., at the pore walls and grain boundaries inside the magnet), forming a uniform nano-coating. This step achieves "volume-based" pre-positioning of the heavy rare earth source across the entire size range of the magnet.
[0036] Step 4: Ligand Removal The magnets processed in step three are then transferred to a vacuum sintering furnace.
[0037] In low vacuum (10 -1 Under the protection of Pa or flowing argon gas, the temperature is increased to 350-400℃ at a rate of 3-5℃ / min and held for 2 hours. This causes Tb(TMHD)3 to decompose, the organic ligands to volatilize and be removed by the vacuum system, leaving highly active terbium oxide (Tb4O7) or terbium oxide nanoparticles attached to the grain boundaries, preventing organic matter from being introduced into the high-temperature section and forming carbide impurities.
[0038] Step 5: Grain Boundary Diffusion Heat Treatment (GBD Heat Treatment) Continue heating to 900℃, high vacuum (10 -3 Pa), and held at this temperature for 8 hours. At this temperature, the Nd-rich grain boundary phase inside the magnet melts into a liquid state. Tb atoms deposited at the grain boundaries rapidly dissolve into the liquid phase and, driven by the concentration gradient, diffuse towards the surface of the main phase grains, undergoing an Nd-Tb substitution reaction to form (Nd,Tb)₂Fe. 14 B-type highly anisotropic shell.
[0039] Step two has already delivered the Tb source to the center of the magnet, so the diffusion in this step only requires the transport of "microscopic distances" (from the grain boundary to the grain surface, only a few micrometers), instead of the "macroscopic distances" (from the magnet surface to the center, several millimeters) required in traditional processes. This significantly reduces the time and ensures uniformity.
[0040] Step Six: Aging Treatment The temperature was lowered to 500℃ and held for 4 hours, followed by gas quenching. The distribution of the grain boundary phase was optimized to make it straight, smooth, and continuous, thereby severing the magnetic exchange coupling between grains and further "squeezing out" the coercivity potential.
[0041] Test results: Br: 14.15 kGs (a decrease of only 1.0% compared to the original N50 magnet's 14.30 kGs).
[0042] Hcj: 26.8 kOe (an increase of 14.3 kOe compared to the original N50 magnet's 12.5 kOe).
[0043] Grade determination: Meets 50UH (Standard: Br ≥ 13.9 kGs, Hcj ≥ 25 kOe).
[0044] Microstructure: SEM / EDS energy dispersive spectroscopy shows that the Tb concentration distribution at the grain boundaries is extremely uniform from the surface to a depth of 6 mm from the center of the magnet, with no obvious concentration gradient.
[0045] Comparative Example 1 (Conventional Dip Coating Diffusion) Object: N50 magnets from the same batch as in Example 1.
[0046] Processing: Disperse TbH2 powder in ethanol, dip it onto the surface of the magnet, dry it, and then perform the same heat treatment as steps five and six in Example 1 to obtain the magnet.
[0047] Test results: Br: 14.05 kGs. Hcj: 21.5 kOe. Compared to Example 1, the coercivity improvement was limited (only +9 kOe), failing to meet the UH level requirement (25 kOe). Dissection revealed that Hcj reached 26 kOe at 1 mm of the magnet's surface, but only 16 kOe at the center. This is a typical example of insufficient diffusion depth leading to a lower average performance.
[0048] Comparative Example 2 (Traditional Alloying Process) Target: Add 4.5 wt% Tb metal directly to the formula and remelt and sinter.
[0049] Test results: Br: 12.80 kGs. Hcj: 26.5 kOe. Compared to Example 1, although the coercivity reached the 50 UH standard, the remanence decreased significantly, and the actual magnet grade was 40 UH or 42 UH, failing to meet the high energy product standard of 50 UH. This proves that simple alloying cannot solve the conversion problem from N50 to 50 UH.
[0050] Table 1 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 reconstructing NdFeB grain boundaries, characterized in that, Includes the following steps: S1 involves immersing the magnet in a low-concentration nitric acid-ethanol solution for ultrasonic cleaning, creating tiny pores on the magnet surface. S2 supercritical fluid dissolves heavy rare earth organic complexes and then permeates the magnets treated in step S1 under pressure; S3 depressurization caused heavy rare earth organic complexes to be deposited in situ within the magnet; S4 involves heating the magnet treated in step S3 under a low vacuum or flowing inert gas protection to decompose the heavy rare earth organic complex into heavy rare earth oxides and organic matter, with the organic matter volatilizing upon heating. S5 The magnet treated in step S3 is heated to 880-920℃ under low vacuum or flowing inert gas protection and held at that temperature for 6-10 hours. S6. The magnet treated in step S3 is cooled to 480-520℃ and held at that temperature for 3-5 hours, followed by gas quenching.
2. The method for reconstructing NdFeB grain boundaries according to claim 1, characterized in that, In step S1, the concentration of nitric acid in the low-concentration nitric acid-ethanol solution is 0.5%-1.0%, and the ultrasonic time is 30-60 seconds.
3. The method for reconstructing NdFeB grain boundaries according to claim 1, characterized in that, The supercritical fluid in step S2 is supercritical CO2, and the heavy rare earth organic complex is tris(2,2,6,6-tetramethyl-3,5-heptadecyl)terbium(III).
4. The method for reconstructing NdFeB grain boundaries according to claim 3, characterized in that, Step S2 specifically involves placing the magnet treated in step S1 and tris(2,2,6,6-tetramethyl-3,5-heptanedione)terbium(III) powder into a reaction vessel, ensuring that the magnet and the tris(2,2,6,6-tetramethyl-3,5-heptanedione)terbium(III) powder do not come into contact. CO2 gas is introduced, the pressure is increased to 18-25 MPa, and the temperature is increased to 40-60°C. This state is maintained for 3-6 hours.
5. The method for reconstructing NdFeB grain boundaries according to claim 3, characterized in that, The pressure relief rate in step S3 is 5-10 MPa / min.
6. The method for reconstructing NdFeB grain boundaries according to claim 3, characterized in that, Specifically, step S4 involves: placing the magnet processed in step S3 under a vacuum of 10... -1 Under the protection of Pa or flowing argon gas, the temperature is increased to 350-400℃ at a rate of 3-5℃ / min and held for 2 hours.
7. A method for reconstructing NdFeB grain boundaries according to any one of claims 1-6, characterized in that, It is used in the fabrication of 50UH-level magnets from N50-level magnets.
8. A 50UH-class magnet, characterized in that, It is prepared from N50 grade magnets by a neodymium iron boron grain boundary reconstruction method as described in any one of claims 1-6.