Cerium-doped high-coercivity neodymium-iron-boron magnet and preparation process thereof

Abstract

The invention relates to the technical field of magnets, in particular to a cerium-doped high-coercivity neodymium-iron-boron magnet and a preparation process thereof. In order to solve the problems of insufficient coercive force and reduced residual magnetism caused by non-uniform cerium distribution of the existing cerium-doped magnet, the process comprises the following steps: main phase powder surface fluorination treatment, magnetic field orientation cold press molding, construction of a cerium fluoride skeleton by electrophoretic deposition, step-by-step vacuum impregnation of cerium salt and a fluorine source, and degreasing sintering and grain boundary heat treatment. According to the method, uniform enrichment of cerium at the grain boundary is controlled, a continuous and compact grain boundary phase is formed, the intrinsic coercive force, residual magnetism and high-temperature stability are effectively improved, meanwhile, corrosion resistance is improved, and dependence on heavy rare earth is reduced.

Description

Technical Field

[0001] This invention relates to the field of magnet technology, and in particular to a cerium-doped high coercivity neodymium iron boron magnet and its preparation process. Background Technology

[0002] Currently, the demand for rare earth permanent magnet materials continues to grow in high-tech fields such as energy conservation, environmental protection, and new energy vehicles, with neodymium iron boron magnets dominating due to their superior magnetic properties. However, the scarcity and price volatility of key heavy rare earth elements such as neodymium, dysprosium, and terbium severely restrict the cost controllability and supply chain security of their large-scale applications. To reduce dependence on heavy rare earths, the industry is attempting to partially replace neodymium with highly abundant light rare earth elements, especially cerium, hoping to significantly reduce raw material costs while maintaining basic magnetic properties.

[0003] This alternative approach faces significant challenges in practical applications. Due to its unique electronic structure, cerium exhibits significantly different solid-solution behavior in the NdFeB main phase lattice compared to NdFeB, easily leading to a reduction in the magnetocrystalline anisotropy of the main phase grains. More problematic is that cerium tends to undergo non-uniform segregation at grain boundaries during traditional smelting-sintering processes. This not only fails to effectively utilize its potential grain boundary modification function but may also form non-magnetic or weakly magnetic phases, disrupting the continuity and integrity of the grain boundary phases. This suboptimal distribution exacerbates the grain boundary demagnetization coupling effect, resulting in a limited increase in the intrinsic coercivity of the magnet, failing to meet the stringent requirements for demagnetization resistance in high-temperature applications. Furthermore, it is often accompanied by a significant reduction in remanence, making it difficult for the magnet's overall energy product to meet application standards.

[0004] To address the insufficient coercivity caused by cerium doping, existing technologies mainly focus on adjusting the composition of the base alloy or optimizing macroscopic process parameters such as sintering and heat treatment. For example, precisely controlling the total amount of cerium added or attempting to introduce multiple high-abundance rare earth elements for composite doping has yielded less than ideal results, with improvements in coercivity often coming at the cost of excessive remanence. Some studies have also attempted coating treatments or grain boundary diffusion on the surface of sintered magnets; however, these methods struggle to achieve effective and uniform modification of the internal grain boundaries of the magnet, especially deep grain boundaries. Cerium cannot be precisely enriched in the grain boundary regions most in need of strengthening, limiting the effectiveness of these methods.

[0005] Therefore, the core of the current technological dilemma lies in the lack of effective means to control the spatial distribution of cerium in the microstructure of NdFeB magnets. How to guide the selective and gradient distribution of cerium at key locations such as grain boundaries, so that it can both strengthen grain boundaries and isolate magnetic coupling between grains, and minimize its negative impact on the magnetic contribution of the main phase, is the key to breaking through the current technological bottleneck and achieving efficient utilization of high-abundance rare-earth cerium. This requires developing entirely new cerium source introduction and positioning strategies from the source of material preparation—the pretreatment stage of the main phase powder—and in the subsequent forming and sintering densification process. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a cerium-doped high coercivity NdFeB magnet and its preparation process, so as to solve the problem that existing cerium-doped NdFeB magnets suffer from insufficient coercivity and significant decrease in remanence due to uneven solid solution of cerium in the main phase and uncontrolled segregation at grain boundaries, making it difficult to achieve both high coercivity and high energy product at high temperature.

[0007] To achieve the above objectives, the present invention provides a process for preparing cerium-doped neodymium iron boron magnets, comprising the following steps: (1) Under inert gas protection, the cerium-containing neodymium iron boron main phase powder is mixed with anhydrous ethanol and ammonium fluoride aqueous solution, stirred and then subjected to solid-liquid separation, washing and drying to obtain surface fluorinated powder; (2) Orient the surface fluorinated powder under an external magnetic field and cold press it to obtain an oriented compact; (3) Prepare an electrophoretic deposition suspension containing cerium fluoride powder, magnesium chloride, anhydrous ethanol and deionized water, and perform electrophoretic deposition by applying a DC voltage under an inert atmosphere using the oriented compact as the cathode to obtain a compact with cerium fluoride framework deposition. (4) The compact deposited by the cerium fluoride framework is placed in a vacuum impregnation tank. After vacuuming and backfilling with inert gas, a cerium salt impregnation solution containing cerium nitrate hexahydrate and polyethyleneimine is first introduced for impregnation, and then a fluorine source impregnation solution containing ammonium fluoride aqueous solution and magnesium chloride is introduced for impregnation, and then dried. (5) The dried compact is degreased and vacuum sintered to densify it, and then subjected to grain boundary phase regulation heat treatment to obtain cerium-doped neodymium iron boron magnets.

[0008] Preferably, the cerium-containing NdFeB main phase powder is prepared by the following steps: melting NdFeB powder, cerium metal ingot, iron powder, boron powder and copper powder under an inert atmosphere and pouring them into a water-cooled copper roller to form a rapid quenching sheet; homogenizing and annealing the rapid quenching sheet and then hydrogenating and dehydrogenating it; and finally grinding it with an air jet mill to obtain the main phase powder.

[0009] Preferably, the mass ratio of neodymium powder, cerium metal ingot, iron powder, boron powder and copper powder is 210-260:40-90:642:10:2.

[0010] Preferably, the D50 of the main phase powder obtained after air jet milling is 3-5 μm.

[0011] Preferably, in step (1), 500 mL of anhydrous ethanol and 3-8 g of 40 wt% ammonium fluoride aqueous solution are added based on 100 g of main phase powder.

[0012] Preferably, in step (2), the external magnetic field is 2T and the cold pressing pressure is 160MPa.

[0013] Preferably, in step (3), the ratio of magnesium chloride, cerium fluoride powder, anhydrous ethanol and deionized water in the electrophoretic deposition suspension is 0.5-3g:8-14g:500mL:10-30mL.

[0014] Preferably, in step (3), the electrophoretic deposition is performed by applying a DC voltage of 40-60V for 5-10 minutes.

[0015] Preferably, in step (3), the cerium fluoride powder is prepared by the following method: dissolving cerium nitrate hexahydrate in deionized water to obtain a cerium salt solution, adding ammonium fluoride aqueous solution to deionized water to obtain a fluorine source solution, adding the fluorine source solution dropwise to the cerium salt solution to react and age to form a precipitate, and then washing and drying the precipitate and keeping it at 400°C for 2 hours under an argon atmosphere to obtain the cerium fluoride powder.

[0016] Preferably, in step (4), the cerium salt impregnation solution is prepared by dissolving cerium nitrate hexahydrate and polyethyleneimine in anhydrous ethanol and deionized water, and the ratio of cerium nitrate hexahydrate, polyethyleneimine, anhydrous ethanol and deionized water is 3-7g:0.5-1.5g:180mL:20mL.

[0017] Preferably, in step (4), the fluoride source impregnation solution in step (4) is prepared by adding magnesium chloride and a 40wt% ammonium fluoride aqueous solution to anhydrous ethanol and deionized water, and the ratio of magnesium chloride, 40wt% ammonium fluoride aqueous solution, anhydrous ethanol and deionized water is 1-3g:3-6g:180mL:20mL.

[0018] Preferably, in step (4), the immersion time of the cerium salt immersion solution is 8-15 min, and the immersion time of the fluorine source immersion solution is 8-15 min.

[0019] Preferably, step (5) includes: drying at 60°C under vacuum at 200 Pa for 2 hours, heating to 300°C and holding for 2 hours under an argon atmosphere, then switching to vacuum at 100 Pa and heating to 500°C and holding for 2 hours to complete degreasing and cooling to room temperature, then evacuating to 10 Pa and heating to 1080°C and holding for 2 hours to complete sintering densification, and finally cooling to 500°C under argon at 80 kPa and holding for 2 hours to regulate the grain boundary phase.

[0020] Furthermore, the present invention also provides a cerium-doped high coercivity NdFeB magnet, which is obtained by the above-mentioned preparation process of the cerium-doped high coercivity NdFeB magnet.

[0021] The beneficial effects of this invention are: This invention utilizes a specific surface treatment of the main phase powder under argon protection to effectively construct stable active reaction sites on the surface of the powder particles. This treatment provides preferential binding sites for subsequent anchoring of cerium species, promoting the formation of a denser and more uniform weakly magnetic interface layer at the grain boundaries of the cerium source. This interface layer effectively isolates the magnetic coupling between the main phase grains, significantly weakens the grain boundary demagnetization effect, and lays the microstructural foundation for a fundamental improvement in the intrinsic coercivity of the magnet.

[0022] This invention utilizes electrophoretic deposition technology to successfully construct a three-dimensional network-like micron-scale framework structure within the interconnected pores of an oriented compact. This framework serves as a slow-release carrier and support structure for the cerium source, significantly increasing the loading area and spatial distribution uniformity of the cerium source within the magnet. More importantly, it ensures that the subsequent solution-phase cerium source can penetrate deep into the compact via the pore channels for mass transfer, avoiding the drawback of cerium source enrichment only on the surface and creating conditions for achieving a gradient distribution of cerium from the surface inwards.

[0023] This invention employs a stepwise sequential impregnation and in-situ precipitation strategy. First, a cerium salt solution is introduced to fully penetrate and adsorb onto the micron-scale framework and pretreated pore surface. Then, a fluorine source solution is introduced to trigger a uniform precipitation reaction. This sequential arrangement effectively prevents premature reaction between the cerium salt and the fluorine source in the solution, thus preventing the formation of coarse aggregates that could clog pore channels. The resulting nanoscale precipitates tend to nucleate and grow at pre-defined active sites and framework gaps, thereby forming a firmly attached and uniformly covered cerium-rich layer in the grain boundary region, further optimizing the composition and structure of the grain boundary phase.

[0024] This invention introduces specific organic components and inorganic electrolytes into the impregnation solution, significantly improving its wettability, permeability, and stability in complex porous structures. The steric hindrance effect of the organic components and the charge-regulating effect of the inorganic electrolyte synergistically prevent premature aggregation and sedimentation of the active components, ensuring that the cerium and fluorine sources remain in a dissolved state for a longer period to facilitate transport to the target site for reaction. This not only improves the uniformity and controllability of cerium distribution but also helps to form a denser grain boundary phase, indirectly enhancing the overall corrosion resistance of the magnet. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0026] Raw material sources and specifications: Neodymium powder is Merck Sigma-Aldrich brand neodymium powder, product number 460877, particle size 40 mesh, purity ≥99%; Cerium metal is Shanghai Aladdin Biochemical Technology Co., Ltd., in ingot form, item number C494046, purity 99.5%; Iron powder is Thermo Scientific Chemicals brand iron powder, catalog number 000737.30, particle size 200 mesh, purity ≥99%; Boron powder is Shanghai Aladdin Biochemical Technology Co., Ltd., item number B105883, purity ≥99.9%, D50 ≤20μm; Copper powder is Shanghai Aladdin Biochemical Technology Co., Ltd., item number C103842, purity ≥99.8%, particle size approximately 20μm; Polyethyleneimine is Shanghai Aladdin Biochemical Technology Co., Ltd., branched polyethyleneimine, item number P434400, average molecular weight approximately 25000 (branched).

[0027] Example 1: Step S1: In an argon glove box (argon protection, oxygen content ≤100ppm), weigh 240g neodymium powder, 60g cerium metal ingot, 642g iron powder, 10g boron powder and 2g copper powder in sequence. Mix the powder and metal block in a polytetrafluoroethylene-lined container for 10 minutes, then put them into an alumina crucible. Place the crucible in a vacuum induction melting furnace, evacuate to 50Pa, and then introduce argon to 80kPa. Repeat the evacuation and argon backfilling three times to remove residual oxygen and moisture. Then, under argon protection at 80kPa, heat to 1500℃ and hold for 20 minutes to completely melt the metal and stir for 5 minutes. Then, pour the melt into a water-cooled copper roller to form a rapid quenching sheet. Under argon protection, homogenize and anneal at 900℃ for 10 hours, and then cool to room temperature with the furnace. The mixture was then placed in a stainless steel pressure-resistant container, evacuated to 2000 Pa, and hydrogen was introduced at a flow rate of 300 mL / min for 10 min to replace the atmosphere. The hydrogen pressure was then increased to 150 kPa and the mixture was allowed to stand at 25 °C for 2 h to allow the alloy to undergo hydrogen fragmentation. Subsequently, the pressure was released under argon protection and the hydrogen fragmented powder was removed. Argon was then introduced at a flow rate of 500 mL / min for 30 min to remove hydrogen and replace it. The mixture was then held under vacuum (200 Pa) at 200 °C for 1 h to remove weakly bound hydrogen. The mixture was then fed into an air jet mill under nitrogen protection, using 99.999% nitrogen as the grinding gas at a flow rate of 8000 mL / min and a classifying wheel speed of 8000 rpm. The powder D50 at the outlet was controlled to be in the range of 3 μm-5 μm before collection to obtain the main phase powder. Step S2: Weigh 100g of main phase powder in an argon glove box and add it to a polytetrafluoroethylene reaction vessel with mechanical stirring. Then add 500mL of anhydrous ethanol and 5g of 40wt% ammonium fluoride aqueous solution. Stir at 20℃ for 30min and let stand for 10min. Then filter with a polytetrafluoroethylene filter membrane and wash twice with 200mL of anhydrous ethanol to remove soluble salt residue. Then dry under argon protection at 60℃ in vacuum (200Pa) for 2h to obtain surface fluorinated powder. Step S3: Weigh 44g of cerium nitrate hexahydrate and dissolve it in 400mL of deionized water to obtain a cerium salt solution. Then, add 30g of 40wt% ammonium fluoride aqueous solution to 200mL of deionized water to obtain a fluoride source solution. Heat the cerium salt solution to 80℃ and stir at 500rpm. Then, add the fluoride source solution dropwise to the cerium salt solution at a uniform rate over 60min and continue stirring at 80℃ for 120min to complete the aging process. After filtration, the precipitate is washed with 2000mL of deionized water until the conductivity of the filtrate is close to that of deionized water. Then, the surface water is replaced with 500mL of anhydrous ethanol. Finally, it is dried under vacuum (200Pa) at 80℃ for 12h and kept at 400℃ for 2h under an argon atmosphere to obtain cerium fluoride powder. Step S4: Weigh 100g of surface fluorination powder into an orientation pressing mold in an argon glove box. After orientation under an external magnetic field of 2T, cold press it at 160MPa to obtain an orientation compact with a diameter of 30mm and a height of 15mm. Then, prepare an electrophoretic deposition suspension: add 500mL of anhydrous ethanol, 20mL of deionized water, 2g of magnesium chloride and 10g of cerium fluoride powder to a beaker, ultrasonically disperse for 30min and magnetically stir for 20min. Fix the orientation compact to a stainless steel cathode plate with a stainless steel clamp, with a graphite plate as the anode and an electrode spacing of 30mm. Apply a 50V DC voltage in an argon hood for 8min of deposition, remove the compact and quickly rinse the surface with 200mL of anhydrous ethanol to remove free powder. Then dry it in a vacuum (200Pa) at 60℃ for 1h to obtain a compact with cerium fluoride framework deposition. Step S5: Weigh 100g of the cerium fluoride framework deposited compact and place it in a vacuum impregnation tank. First, prepare the cerium salt impregnation solution: add 5g of cerium nitrate hexahydrate and 1g of polyethyleneimine to 180mL of anhydrous ethanol and 20mL of deionized water, and stir for 20min to obtain the cerium salt impregnation solution; evacuate the impregnation tank to 2000Pa and maintain for 5min, then slowly backfill with argon gas to 80kPa and introduce the cerium salt impregnation solution to completely immerse the compact and maintain for 10min before removing and dripping dry; then prepare the fluorine source impregnation solution: add 2g of magnesium chloride and 4g of 40wt% ammonium fluoride aqueous solution to 180mL of anhydrous ethanol and 20mL of deionized water, and stir for 10min to obtain the fluorine source impregnation solution. The impregnation solution was introduced into the fluorine source impregnation solution and impregnated for 10 min using the same vacuum-argon backfilling process. After being removed and dripped dry, it was dried in a vacuum (200 Pa) at 60 °C for 2 h. Then it was placed in the degreasing section of the vacuum sintering furnace. First, it was heated to 300 °C and held for 2 h at an argon flow rate of 200 mL / min to remove residual solvents and low-boiling substances. Then, it was heated to 500 °C and held for 2 h in a vacuum of 100 Pa to remove organic matter and then cooled to room temperature. After being evacuated to 10 Pa, it was heated to 1080 °C and held for 2 h to complete sintering and densification. Then, it was cooled to 500 °C and held for 2 h in an argon flow rate of 80 kPa to regulate the grain boundary phase. Finally, it was cooled to room temperature to obtain cerium-doped high-coercivity NdFeB magnets.

[0028] Example 2: Compared with Example 1, in step S1, the amount of neodymium powder used was 260g and the amount of cerium metal ingot used was 40g; in step S2, the amount of 40wt% ammonium fluoride aqueous solution used was 3g, the stirring time was 20min, and the drying conditions were 55℃ vacuum (200Pa) for 2h; in step S4, the amount of deionized water used in the electrophoretic deposition suspension was 15mL, the amount of magnesium chloride used was 1g, the amount of cerium fluoride powder used was 8g, the ultrasonic dispersion time was 20min, and the magnetic stirring time was [not specified]. For step S5, the cerium salt impregnation solution contained 4g of cerium nitrate hexahydrate and 0.8g of polyethyleneimine. The impregnation tank was evacuated to 2500Pa and held for 4 minutes, with an impregnation time of 8 minutes. The fluorine source impregnation solution contained 1g of magnesium chloride and 3g of a 40wt% ammonium fluoride aqueous solution. The impregnation tank was evacuated to 2500Pa and held for 4 minutes, with an impregnation time of 8 minutes. All other conditions were the same as in Example 1.

[0029] Example 3: Compared with Example 1, in step S1, the amount of neodymium powder used was 235g and the amount of cerium metal ingot used was 65g; in step S2, the amount of 40wt% ammonium fluoride aqueous solution used was 6g, the stirring time was 35min, and the drying conditions were 65℃ under vacuum (200Pa) for 2h; in step S4, the amount of deionized water in the electrophoretic deposition suspension was 25mL, the amount of cerium fluoride powder used was 12g, the applied DC voltage was 45V, and the deposition time was 10min; in step S5, the amount of cerium nitrate hexahydrate in the cerium salt impregnation solution was 6g, the amount of polyethyleneimine used was 1.2g, the impregnation tank was evacuated to 1800Pa and held for 6min, and the impregnation time was 12min; the amount of 40wt% ammonium fluoride aqueous solution in the fluorine source impregnation solution was 5g, the impregnation tank was evacuated to 1800Pa and held for 6min, and the impregnation time was 12min. All other conditions were the same as in Example 1.

[0030] Example 4: Compared with Example 1, in step S1, the amount of neodymium powder used was 220g and the amount of cerium metal ingot used was 80g; in step S2, the amount of 40wt% ammonium fluoride aqueous solution used was 8g, the stirring time was 40min, and the drying conditions were 70℃ vacuum (200Pa) for 2h; in step S4, the amount of deionized water used in the electrophoretic deposition suspension was 30mL, the amount of magnesium chloride used was 3g, and the amount of cerium fluoride powder used was 14g, the ultrasonic dispersion time was 40min, and the magnetic stirring time was [not specified]. For step S5, the cerium salt impregnation solution contained 7g of cerium nitrate hexahydrate and 1.5g of polyethyleneimine. The impregnation tank was evacuated to 1500Pa and held for 8 minutes, with an impregnation time of 15 minutes. The fluorine source impregnation solution contained 3g of magnesium chloride and 6g of a 40wt% ammonium fluoride aqueous solution. The impregnation tank was evacuated to 1500Pa and held for 8 minutes, with an impregnation time of 15 minutes. All other conditions were the same as in Example 1.

[0031] Example 5: Compared with Example 1, in step S1, the amount of neodymium powder used was 250g and the amount of cerium metal ingot used was 50g; in step S2, the amount of 40wt% ammonium fluoride aqueous solution used was 4g, and the stirring time was 25min; in step S4, the amount of magnesium chloride used in the electrophoretic deposition suspension was 2.5g and the amount of cerium fluoride powder used was 10g, the applied DC voltage was 50V, and the deposition time was 7min; in step S5, the impregnation tank was pumped to 2200Pa and held for 5min, the cerium salt impregnation time was 10min, the fluorine source impregnation time was 10min, and the amount of magnesium chloride used in the fluorine source impregnation solution was 2.5g. All other conditions were the same as in Example 1.

[0032] Example 6: Compared with Example 1, in step S1, the amount of neodymium powder used was 210g and the amount of cerium metal ingot used was 90g; in step S2, the amount of 40wt% ammonium fluoride aqueous solution used was 3g, the stirring time was 20min, and the drying conditions were 50℃ vacuum (200Pa) for 2h; in step S4, the amount of deionized water used in the electrophoretic deposition suspension was 10mL, the amount of magnesium chloride used was 0.5g, the amount of cerium fluoride powder used was 9g, the ultrasonic dispersion time was 20min, and the magnetic stirring time was... The deposition time was 5 minutes, with a DC voltage of 40V applied. In step S5, the amount of cerium nitrate hexahydrate in the cerium salt impregnation solution was 3g and the amount of polyethyleneimine was 0.5g. The impregnation tank was pumped to 3000Pa and held for 3 minutes, with an impregnation time of 8 minutes. In the fluorine source impregnation solution, the amount of magnesium chloride was 1g and the amount of 40wt% ammonium fluoride aqueous solution was 3g. The impregnation tank was pumped to 3000Pa and held for 3 minutes, with an impregnation time of 8 minutes. The remaining conditions were the same as in Example 1.

[0033] Comparative Example 1: The difference from Example 1 is that: in step S2, instead of adding a 40wt% ammonium fluoride aqueous solution, only 500mL of anhydrous ethanol is added, stirred at 20°C for 30min, and then allowed to stand for 10min. The mixture is then filtered through a polytetrafluoroethylene filter membrane, washed twice with 200mL of anhydrous ethanol, and dried under vacuum (200Pa) at 60°C for 2h to obtain an unfluorinated powder. The remaining conditions are the same as in Example 1.

[0034] Comparative Example 2: The difference from Example 1 is that in step S4, after orientation under an external magnetic field of 2T and cold pressing at 160MPa to obtain the orientation compact, the operation of preparing the electrophoretic deposition suspension and applying a 50V DC voltage for 8 minutes is not performed, and the orientation compact without cerium fluoride framework deposition is directly obtained; the other conditions are the same as in Example 1.

[0035] Comparative Example 3: The difference from Example 1 is that in step S5, the fluorine source impregnation solution is prepared according to the method described in Example 1, and the fluorine source impregnation solution is introduced into the solution for 10 minutes according to the vacuum-backfilling argon process, and then taken out and dripped dry. Then, the cerium salt impregnation solution is prepared according to the method described in Example 1, and the cerium salt impregnation solution is introduced into the solution for 10 minutes according to the same vacuum-backfilling argon process, and then taken out and dripped dry. That is, the order of cerium salt impregnation and fluorine source impregnation is reversed; the other conditions are the same as in Example 1.

[0036] Comparative Example 4: The difference from Example 1 is that: in step S5, polyethyleneimine is not added to the preparation of the cerium salt impregnation solution. The amount of cerium nitrate hexahydrate is kept at 5g, the solvent is 180mL of anhydrous ethanol and 20mL of deionized water and stirred for 20min to obtain the cerium salt impregnation solution; the other conditions are the same as in Example 1.

[0037] Comparative Example 5: The difference from Example 1 is that: when preparing the electrophoretic deposition suspension in step S4, magnesium chloride is not added, and the amount of anhydrous ethanol is kept constant at 500 mL, the amount of deionized water is 20 mL, and the amount of cerium fluoride powder is 10 g. Electrophoretic deposition is carried out after ultrasonic dispersion for 30 min and magnetic stirring for 20 min. The other conditions are the same as in Example 1.

[0038] Comparative Example 6: The difference from Example 1 is that: when preparing the fluoride source impregnation solution in step S5, magnesium chloride is not added, and the amount of ammonium fluoride aqueous solution with a concentration of 40wt% is 4g, the solvent is 180mL of anhydrous ethanol and 20mL of deionized water, and the solution is stirred for 10min to obtain the fluoride source impregnation solution; the other conditions are the same as in Example 1.

[0039] Performance testing: Sample preparation and pretreatment: Cerium-doped high-coercivity NdFeB magnets prepared in Examples 1-6 and Comparative Examples 1-6 were cut into 10mm×10mm×5mm samples along the orientation direction (orientation direction is 5mm direction). The six surfaces of the samples were ground stepwise with 800# silicon carbide sandpaper under ethanol lubrication. Then, they were ultrasonically cleaned in anhydrous ethanol for 10 min, dried with argon gas, and placed in a desiccator for later use. The samples used for magnetic property testing and irreversible demagnetization testing were all magnetized once in a 5T magnetic field using pulse magnetization and left to stand for 24 h. The magnetization direction was consistent with the orientation direction.

[0040] Density determination: The density of the sintered magnet was determined using the Archimedes method according to GB / T 3850-2015. The dry mass m0 (accurate to 0.1 mg) of the dried sample was weighed at 25℃. The sample was then immersed in deionized water at 25℃ for 30 seconds, and the immersed mass m1 was measured. After removing the sample and wiping off the surface water film, the surface dry mass m2 was immediately measured. The density was then calculated as ρ = m0 / (m2-m1) × ρ. w Calculate the density, where ρ w Take 0.9970 g / cm 3 Each sample was tested in parallel with 3 pieces, and the arithmetic mean was taken. After the test, the sample was immediately rinsed with anhydrous ethanol and dried in a vacuum (200Pa) at 60℃ for 30 minutes to avoid residual moisture affecting subsequent tests. The results are shown in Table 1.

[0041] Room temperature magnetic properties: Magnetic properties were tested according to GB / T 3217-2013, using a closed magnetic circuit permanent magnet material magnetic property testing system. The test temperature was 20℃±2℃, and the maximum applied magnetic field was 4.0T. The sample prepared and magnetized in test item one was placed in the testing system to obtain the demagnetization curve, and the remanence Br (T), intrinsic coercivity Hcj (kA / m), and maximum energy product (BH)max (kJ / m) were calculated. 3 Each sample was tested in parallel with 3 pieces, and the arithmetic mean was taken. The results are shown in Table 1.

[0042] High-temperature coercivity and irreversible demagnetization: The high-temperature magnetic properties and temperature stability tests were conducted according to GB / T 24270-2009. A closed magnetic circuit permanent magnet material testing system with a temperature-controlled cavity was used. The magnetized sample was placed in a constant temperature cavity at 150℃ and kept for 10 min. The intrinsic coercivity Hcj,150℃ (kA / m) was measured at 150℃. The irreversible demagnetization rate was tested using the fluxmeter-Helmholtz coil method. The Helmholtz coil had a diameter of 100 mm, 200 turns, and a coil spacing of 50 mm. The initial open-circuit magnetic flux Φ0 of the magnetized sample was measured at 20℃. The sample was then placed in a constant temperature chamber at 150℃ and kept for 2 h. After naturally cooling to 20℃, the magnetic flux Φ1 was measured. The irreversible demagnetization rate η150℃,2h (%) was calculated according to η=(Φ0-Φ1) / Φ0×100%. Three samples were tested in parallel for each sample and the average value was taken. The results are shown in Table 1.

[0043] Neutral Salt Spray Corrosion: Corrosion resistance was tested using a neutral salt spray (NSS) test according to GB / T 10125-2021. The unmagnetized sample prepared for test item one was degreased with anhydrous ethanol and dried. Non-test surfaces were sealed with PTFE tape, exposing only one 10mm × 10mm end face. The salt spray solution was a 5% sodium chloride solution, with the pH adjusted to 7.0. The test chamber temperature was 35℃, and the deposition rate was 1.5 mL / (80cm). 2 Spraying continuously for 96 hours; after the test, remove the corrosion products and weigh the mass loss Δmc (mg / cm³) using an analytical balance. 2 Meanwhile, the corrosion level was rated according to GB / T 6461-2002, and the results are shown in Table 1.

[0044] Table 1 Performance test results of the examples and comparative examples

[0045] Data Analysis: As can be seen from the data in Table 1, the cerium-doped high-coercivity NdFeB magnets prepared in this invention exhibit a consistent trend of comprehensive optimization in terms of density, room-temperature magnetic properties, high-temperature coercivity, temperature stability, and neutral salt spray corrosion resistance. The high density in each embodiment indicates sufficient vacuum sintering densification, providing a structural basis for stable magnetic properties. The high room-temperature remanence and maximum energy product indicate that the effective magnetic contribution of the main phase was not excessively weakened under the conditions of cerium introduction and subsequent treatment. Simultaneously, the intrinsic coercivity and the intrinsic coercivity at 150℃ increased synchronously, while the overall irreversible demagnetization rate decreased, reflecting more sufficient grain boundary phase regulation and effective suppression of the sensitive region for magnetic reversal at high temperatures. The possible reasons are as follows: the ammonium fluoride surface treatment of the ethanol-deionized water system provides a stable reaction interface for the subsequent impregnation reaction; the electrophoretic deposition of the cerium fluoride framework constructs a continuous carrier in the interconnected pores of the oriented compact; the sequence of first impregnating with cerium salt and then with fluorine source makes the precipitation more likely to be generated in a controlled manner near the pore channels and framework and avoids premature aggregation in the solution; further introduction of polyethyleneimine and magnesium chloride helps the impregnation solution to be transported and retained in the pores, thereby improving the effective utilization of the cerium source in the potential channels of the grain boundaries, and finally achieving a synergistic improvement in heat resistance, temperature stability and corrosion resistance.

[0046] As can be seen from the data in Example 1 and Comparative Example 1 in Table 1, when the ammonium fluoride surface treatment lacks step S2, the intrinsic coercivity at room temperature, intrinsic coercivity at high temperature, and temperature stability all decrease. Simultaneously, the neutral salt spray mass loss increases, and the corrosion rating decreases. The main reason for this may be the lack of interfacial conditions on the main phase powder surface that can promote subsequent reactions. This makes it difficult for the cerium fluoride framework formed by subsequent electrophoretic deposition and the cerium salt introduced by impregnation to be effectively fixed on the pore walls. The precipitation sites are more random and more likely to accumulate at the pore inlets, thus weakening the continuity of grain boundary phase regulation. Therefore, surface treatment is not a dispensable pretreatment step, but rather a key factor that, together with framework deposition and impregnation timing, determines the cerium source distribution and pore channel stability. The combined effect of these three factors produces a comprehensive gain that is difficult to replace with a single measure.

[0047] As can be seen from the data in Table 1 for Example 1 and Comparative Example 2, omitting the electrophoretic deposition framework in step S4 significantly worsens the coercivity-related indicators and temperature stability, and also reduces corrosion resistance. This may be because the lack of a cerium fluoride framework acting as a carrier within the interconnected pores restricts the transport and residence of the cerium salt impregnation solution inside the compact. When a fluorine source is subsequently introduced, the precipitation is more likely to concentrate on the surface or in localized pores, making it difficult to form a continuous grain boundary phase control network. Simultaneously, localized precipitation enrichment may induce early blockage of pore channels, further reducing the effective depth of the vacuum impregnation process. This demonstrates that framework deposition and impregnation-precipitation are not simply additive processes, but rather a synergistic mechanism formed by the mutual cooperation of carrier construction and sequential reactions, exhibiting an effect greater than the sum of its parts.

[0048] As can be seen from the data in Example 1 and Comparative Example 3 in Table 1, when the order of cerium salt impregnation and fluorine source impregnation is reversed, the intrinsic coercivity and irreversible demagnetization at high temperature deteriorate most significantly, accompanied by a decrease in corrosion resistance. The main reason may be that after the fluorine source is introduced first, the pores and framework surface are preferentially occupied by the fluorine source. When cerium salt is introduced subsequently, it is easier for premature precipitation to occur at the pore inlet or solution phase, forming coarser precipitate aggregates, thereby blocking the transport channels connecting the pores. This blocking effect weakens the uniformity of grain boundary phase regulation, resulting in a lack of continuous regulation layer in the magnetic reversal sensitive region.

[0049] As can be seen from the data in Table 1 for Example 1 and Comparative Example 4, both coercivity and corrosion resistance decreased when polyethyleneimine was not added to the cerium salt impregnation solution. This may be because polyethyleneimine in the ethanol-deionized water system helps improve the wetting and carrying capacity of the cerium salt impregnation solution on the pore walls, making it easier for the cerium source to enter the interconnected pores during vacuum impregnation and form a more uniform precipitation layer when a fluorine source is subsequently introduced. When this component is absent, the cerium source is more likely to be lost or locally enriched during the dripping process, resulting in insufficient continuity of the grain boundary phase control layer. This indicates that there is a synergistic effect between the organic component and the inorganic impregnation system, which can achieve both improved coercivity and corrosion resistance stability without changing the sintering process.

[0050] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 5 and 6, when magnesium chloride is not added to the electrophoretic deposition suspension or fluorine source impregnation solution, the coercivity, high-temperature coercivity, and corrosion-related indicators all deteriorate to varying degrees. The main reason for this is likely that magnesium chloride helps improve the charge and deposition efficiency of particles in the suspension system during the electrophoretic deposition stage, resulting in a more uniform and firmly adhered carrier for the cerium fluoride framework on the surface of the oriented compact and at the pore inlets. During the fluorine source impregnation stage, the absence of magnesium chloride weakens the effective transport of the fluorine source within the pore channels and the reaction interface conditions, leading to more uneven precipitation. Therefore, magnesium chloride plays a role in both the framework construction and sequential precipitation stages, forming a synergistic system with surface fluorination treatment and polyethyleneimine, involving multiple coupled factors.

[0051] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A process for preparing a cerium-doped NdFeB magnet with high coercivity, characterized in that, Includes the following steps: (1) Under inert gas protection, the cerium-containing neodymium iron boron main phase powder is mixed with anhydrous ethanol and ammonium fluoride aqueous solution, stirred and then subjected to solid-liquid separation, washing and drying to obtain surface fluorinated powder; (2) Orient the surface fluorinated powder under an external magnetic field and cold press it to obtain an oriented compact; (3) Prepare an electrophoretic deposition suspension containing cerium fluoride powder, magnesium chloride, anhydrous ethanol and deionized water, and perform electrophoretic deposition by applying a DC voltage under an inert atmosphere using the oriented compact as the cathode to obtain a compact with cerium fluoride framework deposition. (4) The compact deposited by the cerium fluoride framework is placed in a vacuum impregnation tank. After vacuuming and backfilling with inert gas, a cerium salt impregnation solution containing cerium nitrate hexahydrate and polyethyleneimine is first introduced for impregnation, and then a fluorine source impregnation solution containing ammonium fluoride aqueous solution and magnesium chloride is introduced for impregnation, and then dried. (5) The dried compact is degreased and vacuum sintered to densify it, and then subjected to grain boundary phase regulation heat treatment to obtain cerium-doped neodymium iron boron magnets; In step (4), the cerium salt impregnation solution is prepared by dissolving cerium nitrate hexahydrate and polyethyleneimine in anhydrous ethanol and deionized water, and the ratio of cerium nitrate hexahydrate, polyethyleneimine, anhydrous ethanol and deionized water is 3-7g:0.5-1.5g:180mL:20mL; the impregnation time of the cerium salt impregnation solution is 8-15min. In step (4), the fluorine source impregnation solution is prepared by adding magnesium chloride and a 40wt% ammonium fluoride aqueous solution to anhydrous ethanol and deionized water, and the ratio of magnesium chloride, 40wt% ammonium fluoride aqueous solution, anhydrous ethanol and deionized water is 1-3g:3-6g:180mL:20mL; the impregnation time of the fluorine source impregnation solution is 8-15min.

2. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 1, characterized in that, The cerium-containing NdFeB main phase powder is prepared by the following steps: NdFeB powder, cerium metal ingot, iron powder, boron powder and copper powder are melted in an inert atmosphere and poured into a water-cooled copper roller to form a rapid quenching sheet. The rapid quenching sheet is homogenized and annealed, then hydrogenated and dehydrogenated, and finally ground by an air jet mill to obtain the main phase powder.

3. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 2, characterized in that, The mass ratio of neodymium powder, cerium metal ingot, iron powder, boron powder and copper powder is 210-260:40-90:642:10:

2.

4. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 1, characterized in that, In step (1), 500 mL of anhydrous ethanol and 3-8 g of 40 wt% ammonium fluoride aqueous solution are added based on 100 g of main phase powder.

5. The preparation process of the cerium-doped high coercivity NdFeB magnet according to claim 1, characterized in that, In step (2), the external magnetic field is 2T and the cold pressing pressure is 160MPa.

6. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 1, characterized in that, In step (3), the ratio of magnesium chloride, cerium fluoride powder, anhydrous ethanol and deionized water in the electrophoretic deposition suspension is 0.5-3g:8-14g:500mL:10-30mL.

7. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 1, characterized in that, In step (3), the electrophoretic deposition is performed by applying a DC voltage of 40-60V for 5-10 minutes.

8. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 1, characterized in that, In step (3), cerium fluoride powder is prepared by the following method: cerium nitrate hexahydrate is dissolved in deionized water to obtain a cerium salt solution, ammonium fluoride aqueous solution is added to deionized water to obtain a fluorine source solution, the fluorine source solution is added dropwise to the cerium salt solution to react and age to form a precipitate, and the precipitate is washed, dried and then kept at 400°C for 2 hours under an argon atmosphere to obtain the cerium fluoride powder.

9. The preparation process of the cerium-doped high-coercivity NdFeB magnet according to claim 1, characterized in that, Step (5) includes: drying at 60°C under vacuum at 200 Pa for 2 hours, heating to 300°C and holding for 2 hours under an argon atmosphere, then switching to vacuum at 100 Pa and heating to 500°C and holding for 2 hours to complete degreasing and cooling to room temperature, then pumping to 10 Pa and heating to 1080°C and holding for 2 hours to complete sintering densification, and finally cooling to 500°C under argon at 80 kPa and holding for 2 hours to regulate the grain boundary phase.

10. A cerium-doped neodymium iron boron magnet with high coercivity, characterized in that, It is obtained by the preparation process of cerium-doped high coercivity NdFeB magnets as described in any one of claims 1-9.

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