A method for preparing a high melting point metal gradient-doped layered neodymium-iron-boron magnet

Abstract

The application discloses a preparation method of a high-melting-point metal gradient-doped layered neodymium-iron-boron magnet, and comprises the following steps: S1), vacuum dehydrogenation of hydrogen broken thick and fast quenched thin strips to form powders; S2), fine grinding of the powders by using an airflow mill; S3), mixing of the airflow mill powders and high-melting-point metal powders according to a mass ratio of 99.6-99.9:0.1-0.4; S4), preparation of a green body; S5), cooling of the green body after sintering and then heat treatment to obtain a single-layer magnet; S6), preparation of a plurality of single-layer magnets with different high-melting-point metal contents according to steps S1-S5; and S7), layer-by-layer pressing of the plurality of single-layer magnets with sequentially increased high-melting-point metal contents from bottom to top to prepare a multi-layer magnet according to the sintering mode of step S5. The application introduces a multi-element high-melting-point alloy system into a magnet substrate in a gradient concentration distribution, constructs a magnet with a multi-stage layered composite structure, realizes a double effect of grain boundary migration inhibition and magnetic domain structure optimization, and improves the coercive force and reduces the loss of magnetic energy product.

Description

Technical Field

[0001] This invention relates to a method for preparing neodymium iron boron magnets, and more particularly to a method for preparing layered neodymium iron boron magnets with high melting point metal gradient doping. Background Technology

[0002] Neodymium iron boron (NdFeB) magnets, with their superior energy product (BH)max, have become core magnetic materials in fields such as new energy motors and wind power generation. However, the significant attenuation of their coercivity (Hcj) at high temperatures (>150℃) severely restricts their application expansion in high-end scenarios such as aerospace and new energy vehicles. Research has confirmed that performance breakthroughs can be achieved by controlling the microstructure of magnets, and current research mainly focuses on three technical paths: grain surface modification, grain boundary optimization, and grain refinement.

[0003] In terms of grain modification, grain boundary diffusion (GBD) technology infiltrates heavy rare earth elements Dy / Tb into the surface of the main phase grains, forming a (Nd,Dy / Tb)-Fe-B high anisotropic field (HA) shell. Grain boundary optimization, on the other hand, introduces low-melting-point nonmagnetic alloys / compounds to construct atomically flat nanograin boundary structures, effectively blocking the chain propagation of magnetic domain inversion. Current experiments show that the synergistic effect of these two technologies can achieve a coercivity leap while maintaining remanence stability.

[0004] Grain refinement technology is another effective way to improve the coercivity of magnets. Among them, air jet milling reduces the particle size by increasing the rotation speed, but the risk of oxidation needs to be balanced. High melting point metal doping achieves refinement by inhibiting grain growth, but the amount added needs to be precisely controlled in actual processing to avoid the deterioration of magnetic properties. Therefore, existing grain refinement technologies have many unstable factors in practical applications, poor process controllability and stability, and cannot provide a new and reliable technical path for industrial mass production, nor can they meet the production needs of the market. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art by providing a method for preparing layered NdFeB magnets with high melting point metal gradient doping, which can effectively improve the coercivity of the magnets and the process is stable, reliable and controllable.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a method for preparing layered NdFeB magnets with high melting point metal gradient doping, comprising the following steps:

[0007] S1) A 0.2–1.0 mm thick rapidly quenched strip with the following composition: Pr-Nd: 30%, Co: 0.35%, B: 1.0%, Ga: 0.05%, Al: 0.3%, Cu: 0.1%, and the balance being Fe, is used as raw material. It is subjected to hydrogen crushing in a 0.3–0.4 MPa hydrogen atmosphere, followed by vacuum dehydrogenation treatment at 550℃ for 4 hours after 1 hour to form powder.

[0008] S2) The dehydrogenated powder is refined to 3-6 μm by air jet milling under nitrogen protection;

[0009] S3) Mix the air jet mill powder and the high melting point metal powder at a mass ratio of 99.6-99.9:0.1-0.4, and disperse them evenly in a mixer for 2-4 hours;

[0010] S4) Antioxidant is added in the mixing stage, lubricant is added in the magnetic field orientation molding stage, orientation is performed under a 20kOe magnetic field, and green blank is prepared by isostatic pressing.

[0011] S5) The green blank is sintered at a vacuum degree of less than 3×10-3Pa and 1060℃ for 2h, then cooled and heat-treated to obtain a single-layer magnet;

[0012] S6) Repeat steps S1-S5 to prepare multiple monolayer magnets; wherein the content of high-melting-point metal in each monolayer magnet is different;

[0013] S7) Multiple single-layer magnets are pressed sequentially from bottom to top, and the content of high-melting-point metals in the multiple single-layer magnets is distributed in a gradient from top to bottom. Then, after processing according to the sintering method in step S5, a multilayer magnet is prepared.

[0014] Furthermore, during hydrogen crushing in step S1 at a hydrogen atmosphere of 0.3-0.4 MPa, the reaction temperature is controlled by a water cooling flow rate of 5-10 L / min in the furnace tubes, and the dehydrogenation vacuum is controlled at 1×10⁻⁶. -2 Below Pa.

[0015] Furthermore, the high melting point metal powder in step S3 is one or a mixture of Ti, Zr, Mo, W, Nb, and Ta, and the particle size range of the melting point metal powder is 30-50 nm.

[0016] Furthermore, the purity of the high-melting-point metal powder is 99.99%.

[0017] Furthermore, in step S4, the amount of antioxidant added is 0.05%-0.1% of the total powder mass, and the amount of lubricant added is 0.2%-0.5% of the total powder mass.

[0018] Furthermore, in step S5, the sintering heating rate is 5-10℃ / min.

[0019] Furthermore, in step S5, the cooling adopts a gradient cooling method: first, the temperature is reduced from 1060℃ to 800℃ within 1 hour, and then the temperature is reduced from 800℃ to 500℃ within 2 hours.

[0020] Furthermore, the heat treatment adopts a two-stage heat treatment method: first, heat treatment at 890℃ for 2 hours, and then heat treatment at 500℃ for 3 hours.

[0021] Furthermore, in step S6, a total of four monolayer magnets are prepared, wherein the content of high melting point metal powder in the four monolayer magnets is 0.3%, 0.2%, 0.1%, and 0% from top to bottom, and the thickness of the monolayer is 0.5-1.0 mm.

[0022] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0023] The present invention provides a method for preparing layered NdFeB magnets with high melting point metal gradient doping. By introducing a multi-element high melting point alloy system into the magnet matrix with a gradient concentration distribution, and combining it with a multi-layered magnet with a multi-level layered composite structure, the method achieves a dual effect of grain boundary migration suppression and magnetic domain structure optimization. A directional pinning effect is formed at the grain boundaries, which improves coercivity while reducing magnetic energy product loss, thus meeting the actual market demand.

[0024] High-melting-point metallic elements, due to their high melting points, tend to exist as solid particles at grain boundaries, hindering grain growth. This results in a reduction in grain size within the magnet's microstructure. Smaller grains increase the grain boundary area, and grain boundaries are the primary obstacle to the movement of magnetic domain walls. The smaller the grains, the more difficult it is for domain walls to cross them under a reverse magnetic field, thus requiring a higher reverse magnetic field to achieve demagnetization, i.e., increased coercivity. The introduction of a multilayer structure avoids the situation where adding high-melting-point alloys traditionally increases coercivity but significantly reduces remanence. Attached Figure Description

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings:

[0026] Figure 1 This is a schematic diagram of the state of four single-layer magnets during sintering in one embodiment of the present invention;

[0027] Among them: 1. Single-layer magnet. Detailed Implementation

[0028] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0029] This invention provides a method for preparing layered NdFeB magnets with high melting point metal gradient doping, which solves the problem that in the prior art, when using grain refinement technology to improve the coercivity of magnets, it is necessary to balance the risk of oxidation and precisely control the amount of additives to avoid the deterioration of magnetic properties, resulting in poor production process stability and reliability, and failing to meet market demands.

[0030] For ease of understanding, the specific process in the embodiments of this application is described below. A method for preparing a high-melting-point metal gradient-doped layered NdFeB magnet in the embodiments of this application includes the following steps:

[0031] S1) A 0.2–1.0 mm thick rapidly quenched strip with the following composition: Pr-Nd: 30%, Co: 0.35%, B: 1.0%, Ga: 0.05%, Al: 0.3%, Cu: 0.1%, and the balance being Fe, is used as raw material. It is subjected to hydrogen crushing in a 0.3–0.4 MPa hydrogen atmosphere, followed by vacuum dehydrogenation treatment at 550℃ for 4 hours after 1 hour to form powder.

[0032] S2) The dehydrogenated powder is refined to 3-6 μm by air jet milling under nitrogen protection;

[0033] S3) Mix the air jet mill powder and the high melting point metal powder at a mass ratio of 99.6-99.9:0.1-0.4, and disperse them evenly in a mixer for 2-4 hours;

[0034] S4) Antioxidant is added in the mixing stage, lubricant is added in the magnetic field orientation molding stage, orientation is performed under a 20kOe magnetic field, and green blank is prepared by isostatic pressing.

[0035] S5) The green blank is sintered at a vacuum degree of less than 3×10-3Pa and 1060℃ for 2h, then cooled and heat-treated to obtain a single-layer magnet;

[0036] S6) Repeat steps S1-S5 to prepare multiple monolayer magnets; wherein the content of high-melting-point metal in each monolayer magnet is different;

[0037] S7) Multiple single-layer magnets are pressed sequentially from bottom to top, and the content of high-melting-point metals in the multiple single-layer magnets is distributed in a gradient from top to bottom. Then, after processing according to the sintering method in step S5, a multilayer magnet is prepared.

[0038] Furthermore, during hydrogen crushing in step S1 at a hydrogen atmosphere of 0.3-0.4 MPa, the reaction temperature is controlled by a water cooling flow rate of 5-10 L / min in the furnace tubes, and the dehydrogenation vacuum is controlled at 1×10⁻⁶. -2 Below Pa, the above operations can effectively control the reaction temperature, thereby improving the stability of the entire process.

[0039] Furthermore, the high-melting-point metal powder in step S3 is one or a mixture of Ti, Zr, Mo, W, Nb, and Ta, with a particle size range of 30-50 nm.

[0040] Furthermore, in step S4, the amount of antioxidant added is 0.05%-0.1% of the total powder mass, and the amount of lubricant added is 0.2%-0.5% of the total powder mass. The above contents can be adjusted according to actual use. In this embodiment, the antioxidant is zinc stearate and the lubricant is paraffin wax.

[0041] Furthermore, the sintering heating rate in step S5 is 5-10℃ / min, which can stably and effectively sinter the green blank.

[0042] Furthermore, a gradient cooling method is employed: the temperature is first reduced from 1060℃ to 800℃ within 1 hour, and then from 800℃ to 500℃ within 2 hours. Gradient cooling significantly enhances the magnetic properties of the monolayer magnet by directionally controlling phase transition, stress release, and microstructure.

[0043] Furthermore, the heat treatment adopts a two-stage heat treatment method: first, heat treatment at 890℃ for 2 hours, and then heat treatment at 500℃ for 3 hours, which can better improve the relevant performance of the single-layer magnet.

[0044] For further details, please refer to [link / reference]. Figure 1 In step S6, a total of four single-layer magnets are prepared, wherein the contents of high melting point metal powder from top to bottom are 0.3%, 0.2%, 0.1%, and 0%, and the thickness of the single layer is 0.5-1.0 mm.

[0045] Of course, it can also be made by pressing single-layer magnets with two, three, or more than four layers one by one, and ensuring that the content of high melting point metal powder in the single-layer magnets pressed one by one decreases from top to bottom in a trapezoidal distribution.

[0046] This invention forms a multilayer magnet by sintering multiple single-layer magnets pressed layer by layer, and the content of high-melting-point metal powder in the multiple single-layer magnets is distributed in a gradient from top to bottom. This creates a directional pinning effect at the grain boundaries, which increases coercivity while reducing magnetic energy product loss.

[0047] Several embodiments are listed below for illustration.

[0048] Example 1

[0049] 1) A 0.2 mm thick rapidly quenched strip with the composition of 30% Pr-Nd, 0.35% Co, 1.0% B, 0.05% Ga, 0.3% Al, 0.1% Cu, and the balance Fe was used. Hydrogen crushing was carried out in a 0.35 MPa hydrogen atmosphere. The reaction temperature was controlled by water cooling of the furnace tube at a flow rate of 8 L / min. After 1 hour, vacuum dehydrogenation treatment was carried out at 550℃ for 4 hours (vacuum degree 5×10-3 Pa).

[0050] 2) The dehydrogenation powder was fined to 3μm by air jet milling under nitrogen protection, with the classifier rotating at 5000rpm, and 0.08% zinc stearate was added simultaneously.

[0051] 3) The air jet mill powder and 40nm high-purity Ta powder (purity 99.99%) were precisely mixed at mass ratios of 100:0, 99.9:0.1, 99.8:0.2, 99.7:0.3, and 99.6:0.4, and dispersed for 3 hours using a three-dimensional motion mixer.

[0052] 4) Add 0.3% paraffin during the magnetic field orientation stage, and shape it into a 10mm×10mm×1.5mm green body under a 20kOe magnetic field, and densify it by 200MPa isostatic pressing.

[0053] S5) Heat the green billet to 1060℃ at a rate of 8℃ / min, with a vacuum degree of 2×10⁻⁶. -3 Sintering at Pa for 2 h, followed by gradient cooling (1060℃→800℃ / 1h, 800℃→500℃ / 2h), and then heat treatment at 890℃ / 2h+500℃ / 3h to obtain single-layer magnets A0, A1, A2, A3, and A4, in which the mass of Ta powder accounts for 0%, 0.1%, 0.2%, 0.3%, and 0.4% of the total mass, respectively.

[0054] S6) Design a four-layer gradient magnet A5: the bottom layer A0 (0% Ta), the next lower layer A1 (0.1% Ta), the next upper layer A2 (0.2% Ta), and the top layer A3 (0.3% Ta), with a single layer thickness of 0.8 mm. After pressing each layer, the layered neodymium iron boron magnet is obtained by sintering in step 5.

[0055] S7) Finally, the magnetic properties of the diffused magnet were tested using the Mianyang Bipolar 264Y Permanent Magnet Characteristic Automatic Measuring Instrument at a temperature of 21℃.

[0056] The final properties of the prepared magnets are shown in the table below:

[0057]

[0058] Table 1

[0059] As shown in Table 1, the Ta content in Examples 1.1-1.4 gradually increases, and the remanence (Br) first increases and then decreases with the increase of Ta content, reaching a maximum of 14.22 kGs when the Ta content is 0.1%. After that, the remanence gradually decreases as the Ta content continues to increase.

[0060] The coercivity (Hcj) gradually increases with the increase of Ta content, reaching a maximum of 14.45 kOe when the Ta content is 0.4%.

[0061] The main reason for the increased coercivity is that high-melting-point metal elements, due to their high melting point, tend to exist as solid particles at grain boundaries, hindering grain growth. This leads to a reduction in grain size in the magnet's microstructure. The smaller the grains, the greater the grain boundary area. Grain boundaries are the main obstacle to the movement of magnetic domain walls. The smaller the grains, the more difficult it is for magnetic domain walls to cross the grain boundaries under the influence of a reverse magnetic field. Therefore, a higher reverse magnetic field is required to achieve demagnetization, i.e., increased coercivity.

[0062] The main reason for the decrease in remanence is that high-melting-point metallic elements partially replace the main phase (Nd2Fe). 14 The presence of Fe or rare earth element Nd in B) leads to a slight distortion or dilution of the main phase crystal structure. Since the main phase is the primary source of magnetization in a magnet, a reduction in its volume fraction directly decreases the overall magnetization, thereby reducing remanence.

[0063] This is similar to the traditional method of adding high-melting-point alloys, which increases coercivity but continuously reduces remanence. Especially when the Ta content is 0.4%, the remanence decreases significantly while the coercivity increases only slightly. Such magnetic properties cannot meet the optimal requirements.

[0064] To improve the situation in Examples 1.1-1.5 above and optimize the magnetic properties, in Example 1.6, a four-layer gradient magnet A5 is designed, which is supported by pressing and sintering magnets A0-A3 sequentially from bottom to top, i.e., Ta addition amounts of 0%, 0.1%, 0.2% and 0.3% are combined. Since the magnetic properties deteriorate significantly when the Ta addition amount is 0.4%, it is not considered because the remanence (Br) decreases the most at this point.

[0065] While significantly improving coercivity in A5 magnets, the reduction in remanence is also significantly reduced, resulting in good reliability and stability, making them well-suited for commercial production.

[0066] Example 2

[0067] The difference between Example 2 and Example 1 is that Ta was replaced with the high-melting-point metal Ti. The final magnet performance is shown in the table below:

[0068]

[0069]

[0070] Table 2

[0071] As shown in Table 2, when the high-melting-point metal Ti replaces Ta, in Example 2.6, a four-layer gradient magnet A5 is designed. Magnets A0-A3 are sequentially pressed from bottom to top and then sintered with Ti additions of 0%, 0.1%, 0.2%, and 0.3%, respectively, and then combined. In the A5 magnet, the coercivity is significantly improved while the remanence reduction is significantly reduced, resulting in good reliability and stability.

[0072] Example 3

[0073] The difference between Example 3 and Example 1 is that Example 3 adds an example with a two-layer magnet and an example with a three-layer magnet. Specifically, the two-layer magnet A4 comprises A0 and A1 pressed layer by layer, the three-layer magnet A5 comprises A0, A1, and A2 pressed layer by layer, and the four-layer magnet A6 comprises A0, A1, A2, and A3 pressed layer by layer.

[0074]

[0075] Table 3

[0076] As can be seen from Table 3, in the three embodiments 3.5-3.6, the number of multilayer magnets increases from two layers to four layers. It can be seen that the coercivity is improved the most in A6, while the reduction in remanence is also reduced the least. Therefore, the magnetic performance is better when designed as a four-layer magnet.

[0077] In summary, the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit it. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing a layered NdFeB magnet with high melting point metal gradient doping, characterized in that, Includes the following steps: S1) A 0.2–1.0 mm thick rapidly quenched strip with the following composition: Pr-Nd: 30%, Co: 0.35%, B: 1.0%, Ga: 0.05%, Al: 0.3%, Cu: 0.1%, and the balance being Fe, is used as raw material. It is hydrogen-crushed in a 0.3–0.4 MPa hydrogen atmosphere, and after 1 hour, it is vacuum dehydrogenated at 550℃ for 4 hours to form powder. S2) The dehydrogenated powder is refined to 3~6μm by air jet milling under nitrogen protection; S3) The air jet mill powder and high melting point metal powder are mixed at a mass ratio of 99.6~99.9:0.1~0.4, and uniformly dispersed using a mixer for 2-4 hours; the high melting point metal powder is one or a mixture of Ti, Zr, Mo, W, Nb, and Ta. S4) Add antioxidants during the mixing stage and lubricants during the magnetic field orientation molding stage. Orient the material under a 20kOe magnetic field and prepare the green body by isostatic pressing. S5) The green body is placed in a vacuum degree lower than 3×10 -3 Sintering at 1060℃ for 2 hours, followed by cooling and heat treatment to obtain a single-layer magnet; S6) Repeat steps S1-S5 to prepare multiple monolayer magnets; wherein the content of high-melting-point metal in each monolayer magnet is different; S7) Multiple single-layer magnets are pressed sequentially from bottom to top, and the content of high-melting-point metals in the multiple single-layer magnets is distributed in a gradient from top to bottom. Then, after processing according to the sintering method in step S5, a multilayer magnet is prepared.

2. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that: In step S1, during hydrogen crushing in a 0.3-0.4 MPa hydrogen atmosphere, the reaction temperature is controlled by a water cooling flow rate of 5-10 L / min in the furnace tubes, and the dehydrogenation vacuum is controlled at 1×10⁻⁶. -2 Below Pa.

3. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that: The particle size range of the high melting point metal powder in step S3 is 30-50 nm.

4. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that: The purity of the high melting point metal powder is 99.99%.

5. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that: In step S4, the amount of antioxidant added is 0.05%-0.1% of the total powder mass, and the amount of lubricant added is 0.2%-0.5% of the total powder mass.

6. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that: In step S5, the sintering heating rate is 5-10℃ / min.

7. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that, In step S5, the cooling adopts a gradient cooling method: first, the temperature is reduced from 1060℃ to 800℃ within 1 hour, and then the temperature is reduced from 800℃ to 500℃ within 2 hours.

8. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that, The heat treatment adopts a two-stage heat treatment method: first, heat treatment at 890℃ for 2 hours, and then heat treatment at 500℃ for 3 hours.

9. The method for preparing a layered NdFeB magnet with high melting point metal gradient doping as described in claim 1, characterized in that: In step S6, a total of four monolayer magnets are prepared. The content of high melting point metal powder in the four monolayer magnets is 0.3%, 0.2%, 0.1%, and 0% from top to bottom, and the thickness of the monolayer is 0.5-1.0 mm.

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