Preparation method of layered neodymium-iron-boron magnet doped with high-melting-point metal in gradient mode

Abstract

The invention discloses a preparation method of a layered neodymium-iron-boron magnet doped with high-melting-point metal in a gradient mode. The preparation method comprises the following steps that S1, after hydrogen decrepitation is conducted on a thick rapid-quenching thin strip, vacuum dehydrogenation is conducted on the thick rapid-quenching thin strip to form powder; s2) refining the powder through a jet mill; s3) mixing the jet milling powder and the high-melting-point metal powder according to the mass ratio of (99.6-99.9): (0.1-0.4); s4) preparing a green body; s5) sintering the green body, cooling, and carrying out heat treatment to obtain a single-layer magnet; s6) preparing a plurality of high-melting-point single-layer magnets with different metal contents according to the steps S1 to S5; and S7) pressing a plurality of single-layer magnets of which the contents of the high-melting-point metals are sequentially increased layer by layer from bottom to top, and preparing the multi-layer magnet according to the sintering mode in the step S5. A multi-element high-melting-point alloy system is introduced into a magnet base body in a gradient concentration distribution mode, the magnet of a multi-stage layered composite structure is constructed, the dual effect of grain boundary migration inhibition and magnetic domain structure optimization is achieved, the coercive force is improved, and the magnetic energy product loss is reduced.

Description

Technical Field

[0001] The present invention relates to a method for preparing a neodymium iron boron magnet, in particular to a method for preparing a layered neodymium iron boron magnet doped with a high melting point metal gradient. Background Art

[0002] Neodymium iron boron magnets, with their exceptional magnetic energy product (BH)max, have become a core magnetic material in new energy motors, wind power generation, and other fields. However, the significant attenuation of their coercive force (Hcj) at high temperatures (>150°C) has severely restricted their application in high-end applications such as aerospace and new energy vehicles. Research has confirmed that performance breakthroughs can be achieved by manipulating the magnet's microstructure, with current focus on three key technical approaches: grain surface modification, grain boundary optimization, and grain refinement.

[0003] In terms of grain modification, grain boundary diffusion (GBD) infiltrates heavy rare earth elements (Dy / Tb) into the surface of the primary phase grains, forming a (Nd, Dy / Tb)-Fe-B high anisotropy field (HA) shell. Grain boundary optimization, on the other hand, introduces low-melting-point non-magnetic alloys / compounds to create atomically smooth nanograin boundaries, effectively blocking the chain propagation of magnetic domain reversal. Current experiments have shown that these two techniques work synergistically to achieve a dramatic increase in coercivity while maintaining stable remanence.

[0004] Grain refinement technology is another way to effectively improve the coercive force of magnets. The air flow milling process reduces the particle size by increasing the rotation speed, but the oxidation risk needs to be balanced. High-melting-point metal doping achieves refinement by inhibiting grain growth, but in actual processing, the addition amount needs to be precisely controlled to avoid deterioration of magnetic properties. Therefore, the existing grain refinement technology has many unstable factors in actual application, and the process has poor controllability and stability. It cannot provide a new and reliable technical path for industrial mass production and cannot adapt to market production needs. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for preparing a layered NdFeB magnet with a gradient doping of a high melting point metal, which can effectively improve the coercive force of the magnet and the process is stable, reliable and controllable.

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

[0007] S1) using a 0.2-1.0 mm thick rapidly quenched thin strip with the composition of Pr-Nd: 30%, Co: 0.35%, B: 1.0%, Ga: 0.05%, Al: 0.3%, Cu: 0.1% and the balance Fe as a raw material, hydrogen crushing in a 0.3-0.4 MPa hydrogen atmosphere, and then vacuum dehydrogenation at 550°C for 4 hours after 1 hour to form a powder;

[0008] S2) refining the dehydrogenated powder to 3-6 μm by jet milling under nitrogen protection;

[0009] S3) mixing the jet mill powder with the high melting point metal powder in a mass ratio of 99.6-99.9:0.1-0.4, and uniformly dispersing the mixture in a mixer for 2-4 hours;

[0010] S4) adding an antioxidant during the mixing stage, adding a lubricant during the magnetic field orientation molding stage, orienting under a 20 kOe magnetic field, and preparing a green body by isostatic pressing;

[0011] S5) sintering the green body at 1060° C. for 2 h in a vacuum of less than 3×10-3 Pa, followed by cooling and heat treatment to obtain a single-layer magnet;

[0012] S6) Repeating steps S1-S5 to prepare a plurality of single-layer magnets; wherein the content of the high-melting-point metal in each of the single-layer magnets is different;

[0013] S7) pressing the plurality of single-layer magnets layer by layer from bottom to top, wherein the content of the high-melting-point metal in the plurality of single-layer magnets is distributed in a gradient that decreases from top to bottom, and then processing them according to the sintering method of step S5 to obtain a multilayer magnet.

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

[0015] Furthermore, the high melting point metal powder in step S3 is a mixture of one or more of Ti, Zr, Mo, W, Nb, and Ta, and the particle size of the high melting point metal powder is in the range of 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 mass of the powder, and the amount of lubricant added is 0.2%-0.5% of the total mass of the powder.

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

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

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

[0021] Furthermore, in step S6, a total of four single-layer magnets are prepared, wherein the contents of the high-melting-point metal powder in the four single-layer magnets are 0.3%, 0.2%, 0.1%, and 0% from top to bottom, and the thickness of the single layer 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 method for preparing a layered NdFeB magnet with a gradient doping of high-melting-point metals of the present invention achieves the dual effects of inhibiting grain boundary migration and optimizing the magnetic domain structure by introducing a multi-component high-melting-point alloy system into a magnet matrix in a gradient concentration distribution, combined with the constructed multi-layer magnet with a multi-level layered composite structure, forming a directional pinning effect at the grain boundaries, thereby increasing the coercive force while reducing the magnetic energy product loss, which meets the actual market demand.

[0024] Among them, high-melting-point metal elements tend to exist in the form of solid particles at the grain boundaries due to their high melting points, hindering the growth of grains. This leads to a decrease in the grain size in the magnet's microstructure. Small grains increase the grain boundary area, and grain boundaries are the main obstacle to the movement of magnetic domain walls. The finer the grains, the more difficult it is for the magnetic domain walls to cross the grain boundaries under the action of the reverse magnetic field, thus requiring a higher reverse magnetic field to achieve demagnetization, that is, to increase the coercive force. The introduction of the multilayer structure avoids the situation in which the coercive force increases after the traditional addition of high-melting-point alloys, but the remanence is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0026] Figure 1 Schematic diagram of the sintering state of four single-layer magnets in one embodiment of the present invention;

[0027] Among them: 1. Single-layer magnet. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of this application.

[0029] The present invention provides a method for preparing layered NdFeB magnets with gradient doping of high-melting-point metals, so as to solve the problem in the prior art that when using grain refinement technology to improve the coercive force of the magnet, it is necessary to balance the oxidation risk and accurately control the addition amount to avoid deterioration of magnetic properties, thereby resulting in poor production process stability and reliability and inability to adapt to market demand.

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

[0031] S1) using a 0.2-1.0 mm thick rapidly quenched thin strip with the composition of Pr-Nd: 30%, Co: 0.35%, B: 1.0%, Ga: 0.05%, Al: 0.3%, Cu: 0.1% and the balance Fe as a raw material, hydrogen crushing in a 0.3-0.4 MPa hydrogen atmosphere, and then vacuum dehydrogenation at 550°C for 4 hours after 1 hour to form a powder;

[0032] S2) refining the dehydrogenated powder to 3-6 μm by jet milling under nitrogen protection;

[0033] S3) mixing the jet mill powder with the high melting point metal powder in a mass ratio of 99.6-99.9:0.1-0.4, and uniformly dispersing the mixture in a mixer for 2-4 hours;

[0034] S4) adding an antioxidant during the mixing stage, adding a lubricant during the magnetic field orientation molding stage, orienting under a 20 kOe magnetic field, and preparing a green body by isostatic pressing;

[0035] S5) sintering the green body at 1060° C. for 2 h in a vacuum of less than 3×10-3 Pa, followed by cooling and heat treatment to obtain a single-layer magnet;

[0036] S6) Repeating steps S1-S5 to prepare a plurality of single-layer magnets; wherein the content of the high-melting-point metal in each of the single-layer magnets is different;

[0037] S7) pressing the plurality of single-layer magnets layer by layer from bottom to top, wherein the content of the high-melting-point metal in the plurality of single-layer magnets is distributed in a gradient that decreases from top to bottom, and then processing them according to the sintering method of step S5 to obtain a multilayer magnet.

[0038] Furthermore, when hydrogen is crushed in a 0.3-0.4 MPa hydrogen atmosphere in step S1, the reaction temperature is controlled by cooling the furnace tube at a flow rate of 5-10 L / min, and the dehydrogenation vacuum is controlled at 1×10 -2 Pa, the reaction temperature can be effectively controlled by the above operation, thereby improving the stability of the entire process.

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

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

[0041] Furthermore, the sintering heating rate in step S5 is 5-10° C. / min. Such a heating rate can stably and effectively sinter the green compact.

[0042] Furthermore, the cooling method uses gradient cooling: first, the temperature is reduced from 1060°C to 800°C within 1 hour, and then the temperature is reduced from 800°C to 500°C within 2 hours. Gradient cooling significantly improves the magnetic properties of single-layer magnets by directional control of phase transition, stress release, and microstructure.

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

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

[0045] Of course, it can also be a two-layer, three-layer or more than four-layer single-layer magnet pressed layer by layer, and ensure that the high melting point metal powder content in the single-layer magnet pressed layer by layer decreases from top to bottom and is distributed in a trapezoidal shape.

[0046] The present invention forms a multi-layer magnet by sintering multiple single-layer magnets that are pressed layer by layer, and the content of high-melting-point metal powder in the multiple single-layer magnets is distributed in a gradient that decreases from top to bottom. In this way, a directional pinning effect is formed at the grain boundary, which increases the coercive force while reducing the magnetic energy product loss.

[0047] Several embodiments are listed below for illustration.

[0048] Example 1

[0049] 1) A 0.2 mm thick rapidly quenched thin strip with a 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 subjected to hydrogen crushing in a 0.35 MPa hydrogen atmosphere. The reaction temperature was controlled by a water cooling flow rate of 8 L / min in the furnace tube. After 1 hour, a vacuum dehydrogenation treatment was performed at 550°C for 4 hours (vacuum degree 5×10-3 Pa).

[0050] 2) The dehydrogenated powder was finely ground to 3 μm by air flow grinding under nitrogen protection, with a classifier wheel speed of 5000 rpm, and 0.08% zinc stearate was added simultaneously.

[0051] 3) The jet milled powder was precisely mixed with 40 nm high-purity Ta powder (purity 99.99%) 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 in a three-dimensional motion mixer for 3 h.

[0052] 4) 0.3% paraffin wax was added during the magnetic field orientation stage, and the green body was oriented and formed into a 10 mm × 10 mm × 1.5 mm green body under a 20 kOe magnetic field, and then densified by isostatic pressing at 200 MPa.

[0053] S5) The green body was heated to 1060°C at 8°C / min and the vacuum degree was 2×10 -3 Pa for 2h, 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.

[0054] S6) Design a four-layer gradient magnet A5: the bottom layer A0 (0% Ta), the next bottom layer A1 (0.1% Ta), the next top layer A2 (0.2% Ta), and the top layer A3 (0.3% Ta), with a single layer thickness of 0.8 mm. Press each layer one by one and sinter according to step 5 to obtain a layered RuFeB magnet.

[0055] S7) Finally, the magnetic properties of the diffusion magnet were tested using a Mianyang bipolar 264Y permanent magnet property automatic measuring instrument at a temperature of 21°C.

[0056] The performance of the magnets finally prepared is shown in the following table:

[0057]

[0058] Table 1

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

[0060] The coercive force (Hcj) gradually increases with the increase of Ta content and reaches a maximum of 14.45 kOe when the Ta content is 0.4%.

[0061] The main reason for the increase in coercivity is that 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 smaller grain size in the magnet's microstructure. Smaller grains increase the area of ​​grain boundaries, which are the main obstacle to the movement of magnetic domain walls. The finer the grains, the more difficult it is for magnetic domain walls to cross the grain boundaries under the action of a reverse magnetic field. Consequently, a higher reverse magnetic field is required to achieve demagnetization, which in turn increases the coercivity.

[0062] The main reason for the reduction of remanence is that high melting point metal elements partially replace the main phase (Nd2Fe 14 B) Fe or rare earth element Nd, resulting in slight distortion or dilution of the main phase crystal structure. Since the main phase is the main source of magnetization, the reduction of its volume fraction will directly reduce the overall magnetization intensity, thereby reducing the remanence.

[0063] This is similar to the traditional addition of high-melting-point alloys, where the coercivity increases but the remanence continues to decrease. Especially when the Ta content is 0.4%, the remanence is greatly reduced while the coercivity increases only slightly. Such magnetic properties cannot meet the optimization requirements.

[0064] To improve the conditions described in Examples 1.1-1.5 and optimize the magnetic properties, Example 1.6 employs a four-layer gradient magnet A5, which is formed by pressing and sintering magnets A0-A3 layer by layer from bottom to top. Ta additions of 0%, 0.1%, 0.2%, and 0.3% are used in these combinations. However, a Ta addition of 0.4% severely deteriorates the magnetic properties and is therefore disregarded because the remanence (Br) is most reduced at this level.

[0065] While the coercive force of A5 magnets is greatly improved, the reduction of residual magnetism is significantly reduced, and the reliability and stability are good, so it can be well applied in market-oriented production.

[0066] Example 2

[0067] The difference between Example 2 and Example 1 is that Ta is replaced by high melting point metal Ti. The performance of the magnet finally prepared is shown in the following table:

[0068]

[0069]

[0070] Table 2

[0071] As shown in Table 2, when replacing Ta with the high-melting-point metal Ti, in Example 2.6, a four-layer gradient magnet A5 was designed. Magnets A0-A3 were pressed and sintered sequentially from bottom to top, with Ti additions of 0%, 0.1%, 0.2%, and 0.3% added. This significantly increased the coercivity of the A5 magnet while significantly reducing the loss of remanence, resulting in excellent reliability and stability.

[0072] Example 3

[0073] The difference between Example 3 and Example 1 is that a two-layer magnet and a three-layer magnet are added. The two-layer magnet A4 includes A0 and A1 pressed layer by layer, the three-layer magnet A5 includes A0, A1, and A2 pressed layer by layer, and the four-layer magnet A6 includes 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 multi-layer magnets increases from two to four layers, and it can be seen that the coercive force is most significantly improved in A6, and the reduction in residual magnetism is also significantly reduced the least. Therefore, the magnetic performance is better when the design is a four-layer magnet.

[0077] In summary, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a layered NdFeB magnet doped with a high melting point metal gradient, characterized in that: The steps include: S1) using a 0.2-1.0 mm thick rapidly quenched thin strip with the composition of Pr-Nd: 30%, Co: 0.35%, B: 1.0%, Ga: 0.05%, Al: 0.3%, Cu: 0.1% and the balance Fe as a raw material, hydrogen crushing in a 0.3-0.4 MPa hydrogen atmosphere, and then vacuum dehydrogenation at 550°C for 4 hours after 1 hour to form a powder; S2) refining the dehydrogenated powder to 3-6 μm by jet milling under nitrogen protection; S3) mixing the jet mill powder with the high melting point metal powder in a mass ratio of 99.6-99.9:0.1-0.4, and uniformly dispersing the mixture in a mixer for 2-4 hours; S4) adding an antioxidant during the mixing stage, adding a lubricant during the magnetic field orientation molding stage, orienting under a 20 kOe magnetic field, and preparing a green body by isostatic pressing; S5) sintering the green body at 1060° C. for 2 h in a vacuum of less than 3×10-3 Pa, followed by cooling and heat treatment to obtain a single-layer magnet; S6) Repeating steps S1-S5 to prepare a plurality of single-layer magnets; wherein the content of the high-melting-point metal in each of the single-layer magnets is different; S7) pressing the plurality of single-layer magnets layer by layer from bottom to top, wherein the content of the high-melting-point metal in the plurality of single-layer magnets is distributed in a gradient that decreases from top to bottom, and then processing them according to the sintering method of step S5 to obtain a multilayer magnet.

2. The method for preparing a layered NdFeB magnet with a gradient doping of a high melting point metal according to claim 1, wherein: In step S1, when hydrogen crushing is carried out in a 0.3-0.4 MPa hydrogen atmosphere, the reaction temperature is controlled by cooling the furnace tube at a flow rate of 5-10 L / min, and the dehydrogenation vacuum is controlled at 1×10 -2 Below Pa.

3. The method for preparing a layered NdFeB magnet with a gradient doping of a high melting point metal according to claim 1, wherein: The high melting point metal powder in step S3 is a mixture of one or more of Ti, Zr, Mo, W, Nb, and Ta, and the particle size of the high melting point metal powder is in the range of 30-50 nm.

4. The method for preparing a layered NdFeB magnet with gradient doping of a high melting point metal according to claim 1, wherein: The purity of the high melting point metal powder is 99.99%.

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

6. The method for preparing a layered NdFeB magnet with gradient doping of a high melting point metal according to claim 1, wherein: The sintering heating rate in step S5 is 5-10°C / min.

7. The method for preparing a layered NdFeB magnet doped with a high melting point metal gradient according to claim 1, wherein: In step S5, cooling is performed in a gradient cooling manner: first, the temperature is reduced from 1060° C. to 800° C. within 1 hour, and then the temperature is reduced from 800° C. to 500° C. within 2 hours.

8. The method for preparing a layered NdFeB magnet doped with a high melting point metal gradient according to claim 1, wherein: The heat treatment adopts a two-stage heat treatment method: first, heat treatment at 890°C for 2 hours, and then heat treatment at 500°C for 3 hours.

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

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