Preparation method of permanent magnet material with exchange bias effect

Abstract

The invention discloses a preparation method of a permanent magnet material with an exchange bias effect. The method comprises the following steps: weighing a Pr-Al-Cu alloy raw material, smelting, ball-milling, mixing with an organic solvent to prepare a diffusion source, uniformly coating the surface of a neodymium-iron-boron magnet with the diffusion source, and completing two-stage gradient diffusion heat treatment in a vacuum atmosphere. According to the neodymium-iron-boron magnet subjected to diffusion treatment, it is found through tests that a continuous and compact non-magnetic grain boundary phase is formed at the grain boundary of the diffused neodymium-iron-boron magnet, meanwhile, under the action of an external magnetic field, an antiferromagnetic grain boundary phase Pr6Fe13Cu generates a strong magnetic moment pinning effect on a main phase Nd2Fe14B, and then a reverse external magnetic field needed by ferromagnetic phase magnetic moment inversion is improved; therefore, an exchange bias phenomenon is generated, and the coercive force of the neodymium-iron-boron magnet is effectively improved. The method is well compatible with existing grain boundary diffusion process equipment, does not need additional transformation cost, is easy to implement industrially, is relatively low in cost, and can effectively improve the thermal stability of the magnet.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet materials technology, specifically to a method for improving coercivity by inducing an exchange bias effect on the surface layer of a neodymium iron boron magnet through grain boundary diffusion of a specific element. Background Technology

[0002] Neodymium iron boron (Nd-Fe-B) permanent magnet materials, with their high coercivity, high remanence, and high energy product, have rapidly captured the permanent magnet material market and are widely used in high-tech fields such as new energy vehicles, wind power generation, energy-saving home appliances, and the information industry. Coercivity, as a core parameter for measuring the demagnetization resistance of a magnet, is a physical quantity that is highly sensitive to the material structure. The microstructural characteristics, impurity content, and defect distribution within the permanent magnet material all have a significant impact on it. Existing literature 1 (YuY Q, Li KS, Peng HJ, Bai XY, Quan NT, Wu KW, Luo Y, Yu D B. Coercivity enhancement of hot‐deformed NdFeB permanent magnets with AlCuZn eutecticloy grain boundary diffusion. Rare Metals. 2021, 41(1): 226.) This study uniformly mixed Al-Cu-Zn powder with Nd-Fe-B permanent magnet powder according to the designed ratio, and then hot-pressed it at 700℃ and 100 MPa to prepare a fully dense cylindrical blank. The blank was then placed in an argon atmosphere and hot-deformed at 725℃ to compress its thickness by 70%. After annealing at 600℃ for 60 minutes in a tube furnace, the coercivity of the magnet was reduced from 828 kA·m -1 Increased to 987 kA·m -1 Furthermore, the remanence remains stable. Al-Cu-Zn alloying elements are enriched at grain boundaries, effectively isolating adjacent main phase grains and enhancing the pinning effect of domain wall displacement, thereby improving coercivity.

[0003] Existing literature 2 (Zhang T, Liu S, Wu Z, Zhang L, Yu R. Coercivity and remanence enhancement in hot-deformed Nd-Fe-B magnets by high-temperature short-term annealing process. Journal of Alloys and Compounds. 2022, 903.) describes a high-temperature, 5-minute short-term annealing treatment at 800℃ that allows the diffusion source Nd-Fe-Ga to penetrate into the grain boundaries of Nd-Fe-B magnets, forming a continuous non-magnetic grain boundary phase (Nd6Fe13Ga). This increases the magnet's coercivity from 1.05 T to 1.28 T.

[0004] Existing literature 3 (Mohapatra J, Xing M, Wu R, Yang J, Liu JP. Giant exchange bias by tuning Co / CoO core / shell structure. Scripta Materialia. 2023, 230.) established Co / CoO core-shell nanoparticles as a model system to prepare pure Co nanoparticles and room-temperature oxidized Co / CoO core-shell particles. Magnetic tests revealed that the Co / CoO core-shell particles exhibited an exchange bias field of 3.6 kOe at 5 K, significantly enhancing magnetic moment pinning ability. This study elucidates that the exchange bias originates from the interfacial magnetic coupling between the Co core (ferromagnetic phase) and the CoO shell (antiferromagnetic phase), effectively enhancing the magnet's resistance to demagnetization. Related research indicates that the exchange bias effect generated between the antiferromagnetic and ferromagnetic phases is also an effective method to improve the coercivity of magnets. The exchange bias effect is a unidirectional magnetic anisotropy effect generated by the interfacial coupling between ferromagnetic (FM) and antiferromagnetic (AFM) materials. Specifically, it involves the pinning effect of the antiferromagnetic phase on the ferromagnetic phase at the interface, which enhances the coercivity of the magnet. Its core manifestation is the shift of the hysteresis loop of the ferromagnetic layer material along the magnetic field axis from the origin. Therefore, developing a method to enhance the coercivity of materials by generating exchange bias through the exchange coupling of ferromagnetic / antiferromagnetic phases is of great significance for developing permanent magnet materials with excellent performance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for producing permanent magnet materials with exchange bias effect, by introducing an antiferromagnetic phase (Pr6Al) through a diffusion process. 13 Cu), so that it combines with the Nd2Fe main phase (Nd2Fe). 14 B) It generates an exchange coupling effect, thereby enhancing the coercivity of the magnet.

[0006] A method for preparing a permanent magnet material with exchange bias effect includes the following steps:

[0007] (1) Light rare earth low melting point alloy ingots are obtained by multiple meltings in a vacuum arc melting furnace;

[0008] (2) After the ingot is coarsely crushed, it is ball-milled under the protection of acetone and then sieved through a screen to obtain diffused powder.

[0009] (3) Mix the diffusion powder with the organic binder evenly to obtain a mixed alloy grain boundary diffuser;

[0010] (4) After uniformly coating the surface of the NdFeB magnet with the mixed grain boundary diffuser, dry it to obtain the magnet to be diffused;

[0011] (5) The coated magnet is subjected to diffusion heat treatment under vacuum or inert atmosphere protection to obtain a diffusion magnet;

[0012] (6) Under the condition of maintaining a vacuum or inert atmosphere, an external magnetic field is applied to the diffuse magnet along the easily magnetized C-axis, thereby inducing an exchange bias effect;

[0013] The alloy composition of the light rare earth low melting point alloy ingot selected in step (1) is Pr. x Al y Cu z x, y, z are atomic percentages, and 25 ≤ x ≤ 60, 10 ≤ y ≤ 25, 15 ≤ z ≤ 40, x + y + z = 100. More preferably, x = 50, y = 15, z = 35.

[0014] In step (1), the light rare earth low melting point alloy powder is Pr 50 Al 15 Cu 35 Alloys and alloying elements are expressed as a percentage by mass.

[0015] In step (2), the ball milling speed is 200 r / min and the ball milling time is 18 h.

[0016] In step (2), the material is sieved through a 300-mesh sieve.

[0017] In step (3), the organic binder and its mass ratio are terpineol:resin:alloy powder = 4.5:0.25:10.

[0018] In step (4), the neodymium iron boron magnet is a sintered, hot-pressed or hot-deformed neodymium iron boron magnet, and the magnet is in block shape.

[0019] The thickness of the neodymium iron boron magnet in step (4) is between 4 and 6 mm.

[0020] The drying temperature in step (4) is 80 °C and the time is 30 min.

[0021] The coating amount in step (4) is based on 3wt% of the mixed powder in the mixed grain boundary diffuser as a percentage of the NdFeB magnet mass.

[0022] The vacuum condition in step (5) is 5 × 10⁻⁶. -2 Pa.

[0023] In step (5), the diffusion heat treatment is divided into a primary heat treatment and a secondary heat treatment. The temperature of the primary heat treatment is 800℃ and the holding time is 3h; the temperature of the secondary heat treatment is 580℃ and the holding time is 3h.

[0024] In step (6), the applied magnetic field has an intensity of 0 to 2T.

[0025] The present invention has the following advantages and beneficial effects:

[0026] 1. This method is compatible with existing grain boundary diffusion process equipment, requiring no additional modification costs. It not only has controllable preparation costs and is easy to achieve industrial mass production, but also effectively improves the thermal stability of magnets. The process is highly practical and has broad industrialization prospects.

[0027] 2. This invention does not add heavy rare earth elements. It only relies on light rare earth Pr-Al-Cu diffusion source and two-stage gradient heat treatment to improve magnet performance through exchange bias effect, thus getting rid of dependence on heavy rare earth resources and being green and environmentally friendly.

[0028] 3. A two-step diffusion process combining low and high temperatures was developed. A primary heat treatment at 800℃ optimizes the atomic diffusion effect of the Pr-Al-Cu alloy diffusion source, and this temperature is lower than that of the Nd2Fe main phase in NdFeB. 14 The phase transition temperature of B does not destroy the ferromagnetic properties of the main phase. After a two-stage heat treatment at 580℃, Pr6Fe... 13 Cu compounds exhibit an antiferromagnetic state, which can affect the main phase Nd2Fe. 14 B grains form strong magnetic moment pinning, generating an exchange bias effect, which in turn enhances the coercivity and thermal stability of NdFeB magnets.

[0029] 4. Compared with existing methods for preparing materials with exchange bias effects, the preparation process is simple and the cost is low. Attached Figure Description

[0030] Figure 1 The hysteresis loops of the magnet after mixed diffusion and the initial neodymium iron boron magnet in Example 1 are shown.

[0031] Figure 2The hysteresis loops of the mixed-diffusion magnet with a first-stage heat treatment time of 7 hours and the initial NdFeB magnet in Comparative Example 1 are shown.

[0032] Figure 3 The hysteresis loops of the mixed-diffusion magnet with a secondary heat treatment temperature of 500℃ and the initial NdFeB magnet in Comparative Example 2 are shown.

[0033] Figure 4 The hysteresis loop of the mixed-diffusion magnet with a coating amount of 1 wt% of the NdFeB magnet mass in Comparative Example 3 is compared with that of the initial NdFeB magnet.

[0034] Figure 5 For Comparative Example 4, Pr 50 Fe 15 Cu 35 For the diffusion source, the diffusion magnet and Pr 50 Al 15 Cu 35 A comparison diagram of the hysteresis loops of the diffusion magnets serving as diffusion sources.

[0035] Figure 6 The image shows the surface morphology of the initial magnet in Example 1, captured by a scanning electron microscope (SEM).

[0036] Figure 7 The image shows the surface morphology of the magnet after mixed diffusion in Example 1, taken using a scanning electron microscope (SEM). Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0038] This invention provides an innovative method for achieving an exchange bias effect by performing grain boundary diffusion on the surface of sintered NdFeB magnets. The core of this method lies in utilizing a low-melting-point alloy (Pr-Al-Cu) of a specific composition as a diffusion source. Through optimized coating and heat treatment processes, a microstructure with ferromagnetic / antiferromagnetic exchange coupling characteristics is formed on the magnet surface, thereby achieving a macroscopic exchange bias effect. This effect can effectively pin magnetic domains, improve the coercivity and thermal stability of the magnet, and is of great significance for the development of high-performance, high-stability permanent magnet materials. More specifically, it includes the following steps:

[0039] Step 1, in the vacuum arc melting furnace, press Pr 50 Al 15 Cu 35 High-purity Pr, Al, and Cu metals are weighed by atomic percentage and smelted four times under an inert atmosphere to ensure uniform composition, thus obtaining a Pr-Al-Cu alloy ingot.

[0040] Step 2: After mechanically crushing the ingot into coarse particles, high-energy ball milling (milling speed 200 r / min, time 18h) is carried out under acetone protection to prevent oxidation and excessive heat generation. The ball-milled slurry is dried and sieved through a 300-mesh sieve to obtain Pr-Al-Cu alloy powder with uniform particle size and high activity.

[0041] Step 3: The alloy powder and organic binder are stirred evenly until a viscous slurry is formed. The organic binder is a compound of terpineol and resin.

[0042] Step 4. Apply the prepared slurry evenly to the polished NdFeB magnet surface. The coating amount should be 3 wt% of the magnet mass by weight of the alloy powder. Then dry at 80 ℃ for 30 min to allow the solvent to evaporate and form a well-adhesive dispersant coating on the magnet surface, thus obtaining the "magnet to be diffused".

[0043] Step 5, place the magnet to be diffused in a vacuum (5×10⁻⁶). -2 In a heat treatment furnace protected by a high-purity inert atmosphere (Pa) or a high-purity inert atmosphere, the first heat treatment is carried out: holding at 800 ℃ for 3 hours. During this stage, Pr, Al, and Cu elements diffuse into the depth of the magnet grain boundary, and some Pr atoms replace Nd in the main phase of the grain, forming a specific Pr-rich phase structure in the grain boundary and near-surface region. Then, the second heat treatment is carried out: holding at 580 ℃ for 3 hours.

[0044] Step 6: After the secondary heat treatment and holding period, maintain vacuum or inert atmosphere conditions and apply an external strong magnetic field with an intensity of 0 to 2 T along the easy magnetization axis (c-axis) of the magnet. The applied external field magnetizes and orients the ferromagnetic phase. At the same time, during the cooling process, unidirectional anisotropy is established at the heterogeneous interface formed by diffusion (such as between the ferromagnetic phase and the antiferromagnetic phase), exhibiting the exchange bias phenomenon of the hysteresis loop.

[0045] The magnetic properties of the diffused magnet were measured using a vibrating sample magnetometer (VSM), and the test results are as follows: Figure 1 As shown, the coercivity is 9.69 kOe and the remanence is 143.63 emu / g.

[0046] The microstructure test results observed using scanning electron microscopy (SEM) are as follows: Figure 5 As shown, the main phase with black contrast is Nd2Fe 14 B) and gray-contrast intercrystalline phases (Pr6Al) 13 Composed of Cu.

[0047] By employing the above preparation method and adjusting the diffusion source concentration and heat treatment time to improve the interfacial synergistic effect and pinning behavior, thereby influencing the exchange bias effect, an excellent exchange bias effect can be obtained, thus enhancing the performance of high-performance rare-earth permanent magnet materials.

[0048] It has opened up new avenues for functional design.

[0049] The following explanation uses proportions to further illustrate this point.

[0050] Example 1

[0051] A method for preparing permanent magnet materials with exchange bias effect, wherein Pr is melted in a vacuum arc furnace at an atomic percentage ratio. 50 Al 15 Cu 35 High-purity Pr, Al, and Cu metals were weighed and repeatedly smelted four times under an inert atmosphere to obtain an alloy ingot. After mechanical crushing, the ingot was ball-milled at 200 r / min for 18 h under acetone protection, dried, and passed through a 300-mesh sieve to obtain Pr-Al-Cu alloy powder. The powder was mixed with an organic binder composed of terpineol and resin to form a viscous slurry, which was then uniformly coated onto the polished NdFeB magnet surface at a ratio of 3 wt% of the alloy powder by magnet mass. The slurry was dried at 80 ℃ for 30 min to form a dispersant coating. The magnet was then placed in a 5×10⁻⁶ m² / h ... -2 In a vacuum or high-purity inert atmosphere furnace, a first-stage diffusion heat treatment is performed by holding at 800 ℃ for 3 h, followed by a second-stage heat treatment by holding at 580 ℃ for 3 h. After the second-stage heat treatment is completed, an external reinforcing magnetic field of 0–2 T is applied along the easy magnetization axis of the magnet under vacuum or inert atmosphere to induce an exchange bias.

[0052] The magnetic performance test results of Example 1 are shown by the black line segment in Figure 1. The coercivity of the diffused magnet increased from the initial 4.817 kOe to 9.558 kOe. The remanence of the diffused magnet increased from 86.395 emu / g to 143.63 emu / g. The exchange bias field reached 3.96 kOe.

[0053] Comparative Example 1

[0054] Same as in Example 1, the steps not specifically described are the same as the preparation method described in Example 1, the difference being that the first-stage heat treatment time in step 5 is 7 hours.

[0055] The magnetic property test results of Comparative Example 1 are as follows: Figure 2As shown, in this comparative example, the holding time for the first-stage heat treatment was extended to 7 hours. After treatment with the same process, no exchange bias effect was generated in the magnet, and the improvement in magnetic properties was significantly reduced. The coercivity of the magnet increased from 4.817 kOe to 8.499 kOe, and the remanence of the magnet increased from 86.395 emu / g to 128.127 emu / g. It is speculated that the excessively long first-stage holding time led to excessive diffusion of diffusing elements at the grain boundaries of the magnet, which altered the microstructure of the grain boundary phase, reduced the magnetic isolation effect between the main phase grains, and limited the improvement in the coercivity and remanence of the magnet.

[0056] Comparative Example 2

[0057] Same as in Example 1, the steps not specifically described are the same as the preparation method described in Example 1, the difference being that: in step 4, the coating amount is 1 wt% of the mass of the alloy powder as the mass of the magnet.

[0058] The magnetic property test results of Comparative Example 2 are as follows: Figure 3 As shown, the alloy powder coating amount in this comparative example is only 1 wt% of the magnet mass. After the same process, no exchange bias effect is generated in the magnet, and the improvement in magnetic properties is significantly reduced. The coercivity only increases from the initial 4.817 kOe to 7.549 kOe, and the increase in coercivity is lower than that in Example 1. The remanence increases from the initial 86.395 emu / g to 101.051 emu / g, and the increase in remanence continues to decrease compared to Example 1. The reason is that the insufficient coating amount results in a lack of supply of diffusion elements, which makes it impossible to form a sufficient and continuously distributed antiferromagnetic functional phase at the magnet grain boundaries and near-surface region. It is difficult to establish an effective exchange coupling with the main phase grains, and therefore no observable exchange bias effect is generated.

[0059] Comparative Example 3

[0060] Same as in Example 1, the steps not specifically described are the same as the preparation method described in Example 1, the difference being that the secondary heat treatment temperature in step 5 is 500℃.

[0061] The magnetic property test results of Comparative Example 3 are as follows: Figure 4 As shown, in this comparative example, when the secondary heat treatment temperature was reduced to 500℃, the magnet exhibited no exchange bias effect after being treated with the same process, and the improvement in magnetic properties was significantly reduced. Specifically, the coercivity increased from the initial 4.817 kOe to 8.568 kOe, with the increase rate decreasing compared to Example 1. The remanence increased from the initial 86.395 emu / g to 115.765 emu / g, with the increase rate continuing to decrease compared to Example 1. When the secondary heat treatment temperature was reduced to 500℃, grain boundary diffusion and phase transformation were insufficient, and the antiferromagnetic interface phase could not be effectively formed. It was difficult to construct a sufficiently strong exchange coupling interface with the main phase grains, thus preventing the generation of an observable exchange bias effect.

[0062] Comparative Example 4

[0063] Similar to Example 1, the steps not specifically described are the same as the preparation method described in Example 1, except that the alloy ingot in step 1 is composed of Pr, Fe, and Cu. According to Pr... 50 Fe 15 Cu 35 Atom percentages were mixed with organic binders to create a diffusion source.

[0064] The magnetic property test results of Comparative Example 4 are as follows: Figure 5 As shown, this comparative example uses Pr 50 Fe 15 Cu 35 Using the alloy as a diffusion source, after treatment with the same grain boundary diffusion and magnetic field induction process, the NdFeB magnet did not produce an observable exchange bias effect. The Pr-Fe-Cu system could not form a functional phase with antiferromagnetic properties in the magnet grain boundary region, lacking the core phase support conditions for constructing the ferromagnetic / antiferromagnetic exchange coupling interface, and thus could not produce magnetic moment pinning.

[0065] Comparative Example 5

[0066] Figure 6 The image shows the surface morphology of an initial NdFeB magnet without Pr-Al-Cu grain boundary diffusion treatment. The distribution of the main phase grains and grain boundary phases on the magnet surface is observed. The grain boundary phases are unevenly distributed and there are local discontinuities. Figure 7 This is a scanning electron microscope (SEM) image of a NdFeB magnet after Pr-Al-Cu grain boundary diffusion treatment. Compared with the initial magnet, the surface microstructure of the magnet has been significantly optimized. The gray-contrast Pr6Al... 13 The Cu grain boundary phase achieves a uniform and continuous thin-layer distribution between the main phase grains, completely encapsulating the main phase grains and effectively isolating them from each other.

Claims

1. A method for preparing a permanent magnet material with exchange bias effect, characterized in that, Includes the following steps: (1) Preparation of light rare earth low melting point alloy ingots: In a vacuum arc melting furnace, Pr was added according to atomic percentage. x Al y Cu z High-purity Pr, Al, and Cu metals were weighed and smelted multiple times under an inert atmosphere to obtain Pr with uniform composition. x Al y Cu z Alloy ingots, wherein 25≤x≤60, 10≤y≤25, 15≤z≤40, and x+y+z=100; (2) After the alloy ingot is mechanically coarsely crushed, it is subjected to high-energy ball milling under acetone protection. After ball milling, it is sieved through a 300-mesh sieve to obtain PrxAlyCuz alloy diffusion powder. (3) Preparation of grain boundary diffuser: The alloy diffusion powder and the organic binder are uniformly stirred and mixed to form a viscous slurry, i.e. grain boundary diffuser. The organic binder is composed of terpineol and resin, and the mass ratio of terpineol:resin:alloy diffusion powder is 4.5:0.25:

10. (4) The grain boundary diffusing agent is uniformly coated on the polished NdFeB magnet surface, with the coating amount being 3 wt% of the alloy diffusion powder in the NdFeB magnet mass. Then, it is dried at 80°C for 30 min to obtain the magnet to be diffused. (5) Two-stage diffusion heat treatment: The magnet to be diffused is placed in a vacuum of 5×10⁻⁶. -2 In a heat treatment furnace protected by a high-purity inert atmosphere of Pa, a first-stage heat treatment and a second-stage heat treatment are carried out sequentially. The first-stage heat treatment temperature is 800℃ and the holding time is 3 h. The second-stage heat treatment temperature is 580℃ and the holding time is 3 h. (6) Maintain a vacuum or high-purity inert atmosphere and apply an external magnetic field of 0 to 2 T along the easily magnetized C-axis of the diffuse magnet.

2. The preparation method according to claim 1, characterized in that, The Pr mentioned in step (1) x Al y Cu z The atomic percentages of the alloy are x=50, y=15, z=35, i.e., Pr 50 Al 15 Cu 35 .

3. The preparation method according to claim 1, characterized in that, The number of melting operations mentioned in step (1) is 4 times to ensure that the alloy ingot composition is uniform.

4. The preparation method according to claim 1, characterized in that, The parameters for the high-energy ball milling in step (2) are: ball milling speed 200 r / min, ball milling time 18 h.

5. The preparation method according to claim 1, characterized in that, The neodymium iron boron magnet mentioned in step (4) is a sintered neodymium iron boron magnet, which is block-shaped and has a thickness of 4 to 6 mm.

6. The preparation method according to claim 1, characterized in that, The polished NdFeB magnets described in step (4) need to be ultrasonically cleaned first, then polished and leveled with sandpaper of different grits, and finally polished to a mirror finish with a polishing machine.

7. Rare earth permanent magnet material prepared according to any one of claims 1-6.

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