La pre-doped high-cerium neodymium-iron-boron magnet and grain boundary diffusion preparation method

Abstract

The invention discloses a La pre-doped high-cerium neodymium-iron-boron magnet and a preparation method thereof. According to the method, the La element with the specific content is pre-doped in the magnet base material, and the two-step grain boundary diffusion process that the continuous grain boundary network is firstly constructed and then the heavy rare earth hardened shell layer is constructed is combined, so that the problem that the grain boundary diffusion efficiency of the high-Ce magnet is low is successfully solved, the high-performance La pre-doped high-Ce neodymium-iron-boron magnet is obtained with the lowest heavy rare earth cost, and the high-performance La pre-doped high-Ce neodymium-iron-boron magnet is obtained. The La pre-doped high-Ce neodymium-iron-boron magnet has high coercive force, high residual magnetism and good temperature stability. The manufacturing process disclosed by the invention is extremely low in heavy rare earth consumption, remarkable in comprehensive cost advantage and excellent in industrial application prospect.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet materials technology, specifically to a high-performance La (lanthanum) pre-doped high-cerium (Ce) neodymium iron boron (NdFeB) magnet and its grain boundary diffusion preparation method. Background Technology

[0002] Neodymium iron boron (NdFeB) permanent magnets are widely used in wind power generation, new energy vehicles, and consumer electronics due to their excellent magnetic properties. However, the high price and tight supply of their core raw materials, such as Nd, Pr, Tb, and Dy, severely restrict the sustainable development of the industry. Ce, as the most abundant and inexpensive rare earth element, is an effective way to reduce magnet costs by partially replacing Nd / Pr. However, magnets with high Ce content often suffer from low coercivity and poor temperature stability. This is because the introduction of Ce tends to reduce the anisotropy field of the main phase and disrupt the continuity of the grain boundary phase, which is detrimental to subsequent grain boundary diffusion treatment.

[0003] Grain boundary diffusion technology has become a key technology for improving the coercivity of magnets in recent years. By coating the magnet surface with a heavy rare earth source and then heat-treating it, heavy rare earth elements (such as Tb and Dy) selectively diffuse along grain boundaries into the magnet's interior, forming (Tb,Dy)₂Fe on the surface of the main phase grains. 14 A thin shell of boron can significantly improve coercivity with minimal consumption of heavy rare earth elements. However, for high-Ce magnets, the diffusion channels for heavy rare earth elements are blocked due to the discontinuous and incomplete grain boundary phases, resulting in low diffusion efficiency and making it difficult to form an effective hardened shell.

[0004] Therefore, how to design a high Ce magnet substrate and develop a matching efficient grain boundary diffusion process to achieve high coercivity while maintaining low cost has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a La-predoped high-cerium NdFeB magnet and its grain boundary diffusion preparation method.

[0006] This invention provides a La-predoped high-cerium NdFeB magnet, characterized in that the substrate composition of the magnet is as shown in the formula: (Nd 1−x Pr x ) a Ce b Fe bal M c La d B eWhere M is one or more of Al, Cu, and Ti, x is 20-30%, a is 16.5-18.0%, b is 15.0-35.0%, c is 0.35-1.5%, d is 0.3-0.6%, e is 0.88-0.95%, and the balance is Fe and unavoidable impurities. The 15.0-35.0% Ce in the substrate composition of this invention significantly reduces the cost of magnet manufacturing processes. However, when the Ce content is high, sufficient (Nd+Pr) must be ensured to "dilute" or "isolate" the negative impact of Ce on the main phase magnetism, ensuring that Br and Hcj do not decrease. The addition of 16.5-18.0% (Nd+Pr) in this invention is a key guarantee for maintaining the magnet's performance without collapse, determining the "lower limit" of the magnet's performance. In (Nd... 1−x Pr x In this combination, Nd is primarily responsible for maintaining high remanence and magnetic energy product, while adding 20-30% Pr effectively enhances the intrinsic coercivity Hcj of the magnet without significantly reducing its saturation magnetization. This is crucial for improving the magnet's resistance to demagnetization and its temperature stability.

[0007] Preferably, the substrate composition of the magnet is as shown in the formula: (Nd 1−x Pr x ) a Ce b Fe bal M c La d B e M is one or more of Al, Cu and Ti, x is 20-30%, a is 17-18.0%, b is 20.0-30.0%, c is 0.40-1.0%, d is 0.4-0.5%, e is 0.88-0.95%, and the balance is Fe and unavoidable impurities.

[0008] This invention provides a method for preparing the La-predoped high-cerium NdFeB magnet by grain boundary diffusion, characterized by comprising the following steps: (1) Provide a La-predoped high-cerium NdFeB magnet substrate; (2) Perform a two-step grain boundary diffusion treatment on the substrate described in step (1): the two-step grain boundary diffusion is light rare earth transition layer diffusion and heavy rare earth strengthening diffusion; the light rare earth transition layer diffusion is to coat the surface of the La pre-doped high cerium NdFeB magnet substrate with Ce. 60 La 20 Al 10 Cu 10 An alloy diffusion source is used, followed by treatment at 860–900°C for 6–12 hours to form a continuous grain boundary network; the heavy rare earth strengthening diffusion is then applied to the substrate surface after diffusion in the light rare earth transition layer by coating with Tb. 30 Pr40 Al 20 Ga 10 A nanoribbon diffusion source is used, followed by gradient heating diffusion treatment at 830–890℃ for 4–10 hours to construct a (Tb,Pr) thin shell layer; the gradient heating diffusion rate is 5–10℃ / min. (3) The magnet after the two-step grain boundary diffusion treatment described in step (2) is subjected to magnetic field annealing and Al2O3 coating surface deposition to obtain a La predoped high cerium NdFeB magnet.

[0009] Preferably, step (1) of providing the La predoped high-cerium NdFeB magnet substrate includes sequentially processing raw materials according to the La predoped high-cerium NdFeB magnet substrate composition ratio, followed by melting, rapid solidification, hydrogen crushing, air jet milling, orientation forming, and then sintering at 1060-1080℃ for 2-4 hours to obtain the La predoped high-cerium NdFeB magnet substrate; wherein the rapid solidification rate is ≥10 4 K / s; the hydrogen crushing pressure is 0.15 MPa.

[0010] Preferably, in the La predoped high-cerium NdFeB magnet substrate, the La element is enriched at the magnet grain boundaries, with a grain boundary enrichment rate of 0.8–1.5 at%. The predoped La element of this invention preferentially segregates at the grain boundaries during sintering, optimizing the composition, wettability, and continuity of the grain boundary phase. This provides an inherent advantage for subsequent grain boundary diffusion and solves the fundamental problem of poor diffusion channels at grain boundaries in high-Ce magnets.

[0011] Preferably, in the La predoped high-cerium NdFeB magnet substrate, the La element is enriched at the magnet grain boundaries, with a grain boundary enrichment rate of 1.0 to 1.2 at.

[0012] Preferably, in step (1), the average particle size D of the substrate powder after air jet milling is... 50 The diameter is 2.5–3.5 μm; the orientation forming is carried out in a magnetic field of 1.8–2.0 T.

[0013] Preferably, in step (2), the Ce 60 La 20 Al 10 Cu 10 The particle size of the alloy powder is ≤5 μm; the Ce 60 La 20 Al 10 Cu 10 The alloy diffusion source coating amount is 0.3–0.7 wt% of the weight of the La-doped high-cerium NdFeB magnet; the formed continuous grain boundary network coverage is >95%. This invention uses a low-melting-point alloy rich in Ce and La (Ce... 60 La 20 Al10 Cu 10 As a diffusion source, it can effectively penetrate into the magnet at relatively high temperatures (860-900℃), repair and enhance the grain boundary network, form a continuous channel with a coverage of >95%, solve the problem of poor diffusion channels in high Ce magnets, and lay a solid foundation for the rapid and uniform diffusion of heavy rare earth elements in the second step.

[0014] Preferably, in step (2), the Tb 30 Pr 40 Al 20 Ga 10 The thickness of the nanoribbons is ≤200 nm; the Tb 30 Pr 40 Al 20 Ga 10 The nanoribbon diffusion source coating amount is 0.2–0.6 wt% of the mass of the La pre-doped high-cerium NdFeB magnet; the thickness of the (Tb,Pr) thin shell layer is ≤30 nm. This invention utilizes Tb nanoribbons in morphology. 30 Pr 40 Al 20 Ga 10 As a diffusion source, its large specific surface area and unique microstructure give it extremely high diffusion activity at relatively low temperatures (830–890 °C). Through the high-quality grain boundary network established in the first step, Tb / Pr can rapidly and selectively diffuse to the surface of the main phase grains, forming a uniformly thick (Tb,Pr)₂Fe. 14 B hardens the shell, greatly improving coercivity.

[0015] Preferably, the magnetic field annealing process in step (3) includes keeping the magnet at 500-600°C for 2-4 hours, while applying a steady magnetic field of 1.2-1.8 T in a direction perpendicular to the orientation of the magnet.

[0016] Preferably, the thickness of the Al2O3 protective coating in step (3) is 1 to 2 μm.

[0017] This invention optimizes the magnetic domain structure through magnetic field annealing and provides reliable surface protection through an Al2O3 coating. The resulting magnet not only has high remanence and high coercivity, but also good temperature stability and corrosion resistance.

[0018] In summary, the advantages of this invention are: 1. This invention solves the problem of low grain boundary diffusion efficiency in high Ce magnets by predoping a specific amount of La element in the magnet substrate and combining it with a two-step grain boundary diffusion process of "first building a continuous grain boundary network and then building a heavy rare earth hardened shell". This invention achieves high performance high Ce NdFeB magnets with the lowest heavy rare earth cost (low heavy rare earth content ≤1.2). 2. In the substrate composition of the present invention, 15.0–35.0 wt% Ce and 16.5–18.0 wt% (Nd+Pr) constitute the core rare earth combination, wherein Pr accounts for 20–30% of the total (Nd+Pr). This combination, through synergistic effect, significantly reduces raw material costs by utilizing the high abundance of Ce, while not only ensuring the high remanence (Br) and basic coercivity (Hcj) of the magnet, but more importantly, laying a key material foundation for significantly improving the intrinsic coercivity (Hcj) in subsequent processes; 3. This invention achieves a balanced and optimized improvement in real magnetic properties by reducing boron (0.88-0.95 wt%) and magnetic field annealing, resulting in increased HcJ and remanence Br < 4%. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to embodiments, but the scope of protection of the present invention is not limited thereto.

[0020] Example 1

[0021] A method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion, the specific steps of which are as follows: 1) Formulating according to atomic percentage (Nd 0.7 Pr 0.3 ) 17 Ce 25 Fe 50 Al 0.4 Cu 0.3 La 0.5 B 0.88 The raw materials are batched and then vacuum induction melted under argon protection, casting them into alloy ingots. These ingots are then rapidly solidified in a vacuum rapid quenching furnace to prepare rapidly solidified flakes, with a solidification rate of approximately 1.2 × 10⁻⁶. 4 K / s. The quick-setting sheet was hydrogen-crushed at room temperature and 0.15 MPa hydrogen pressure. The coarse powder after hydrogen crushing was subjected to air jet milling to obtain fine powder with an average particle size D50 of about 3.0 μm. The fine powder was oriented and pressed into shape in a magnetic field of 1.9 T, and then densified by applying a pressure of 200 MPa in an isostatic press. Finally, the compact was sintered in a vacuum sintering furnace at 1070℃ for 3 hours and cooled with the furnace to obtain La predoped high cerium NdFeB magnet substrate. According to the transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS), the enrichment rate of La at the grain boundaries of the La predoped high cerium NdFeB magnet substrate was about 1.1 at%. (2) The substrate in step (1) was subjected to two-step grain boundary diffusion treatment: the two-step grain boundary diffusion was light rare earth transition layer diffusion and heavy rare earth strengthening diffusion; the light rare earth transition layer diffusion was to coat the surface of the La predoped high cerium NdFeB magnet substrate with Ce. 60 La 20 Al 10 Cu 10Alloy diffusion source, the Ce 60 La 20 Al 10 Cu 10 The alloy diffusion source consists of Ce particles with a diameter of approximately 3 μm. 60 La 20 Al 10 Cu 10 The alloy powder was mixed with anhydrous ethanol to form a coating of 0.5 wt% of the La-predoped cerium-NdFeB magnet. It was then treated at 900°C for 12 hours to form a continuous grain boundary network; SEM observation showed that the coverage of the formed continuous grain boundary network was >96%. The heavy rare earth strengthening diffusion was achieved by coating the substrate surface with Tb after diffusion in the light rare earth transition layer. 30 Pr 40 Al 20 Ga 10 Nanoribbon diffusion source, the Tb 30 Pr 40 Al 20 Ga 10 The nanoribbon diffusion source is Tb 30 Pr 40 Al 20 Ga 10 A mixture of nanoribbons and anhydrous ethanol. The Tb 30 Pr 40 Al 20 Ga 10 The amount of nanoribbon diffusion source coating is 0.4 wt% of the weight of the La predoped high-cerium NdFeB magnet; then it is heated to 830℃ for 1 hour, and then heated to 870℃ for 5 hours to construct a (Tb,Pr) thin shell layer; the thickness of the (Tb,Pr) thin shell layer is ≤30 nm; (3) The magnet after the two-step grain boundary diffusion treatment in step (2) is subjected to magnetic field annealing and Al2O3 coating surface deposition to obtain a La predoped high-cerium NdFeB magnet, which is placed in a magnetic field annealing furnace, a steady magnetic field of 1.5 T is applied perpendicular to the orientation direction, and it is heated to 550℃ for 3 hours. Finally, an Al2O3 coating with a thickness of about 1.5 μm is deposited on the magnet surface using atomic layer deposition (ALD) technology to obtain a La predoped high-cerium NdFeB magnet. Performance testing: According to GB / T 3217-2013 "Magnetic Test Methods for Permanent Magnet (Hard Magnet) Materials", the magnetic properties of the La predoped high-cerium NdFeB magnet prepared according to the method of this embodiment were tested at room temperature (23±5°C) using the NIM-500C permanent magnet material measurement system. The remanence Br: 15.8 kGs; intrinsic coercivity Hcj: 25.6 kOe; magnetic energy product (BH)max: 45.8 MGOe.

[0022] Example 2

[0023] A method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion, the specific steps of which are as follows: 1) Formulating according to atomic percentage (Nd 0.8 Pr 0.2 ) 18 Ce 20 Fe 45 Al 0.4 La 0.6 B 0.95 The raw materials are batched and then vacuum induction melted under argon protection, casting them into alloy ingots. These ingots are then rapidly solidified in a vacuum rapid quenching furnace to prepare rapidly solidified flakes, with a solidification rate of approximately 1.2 × 10⁻⁶. 4 K / s. The quick-setting sheet was hydrogen-crushed at room temperature and 0.15 MPa hydrogen pressure. The coarse powder after hydrogen crushing was subjected to air jet milling to obtain fine powder with an average particle size D50 of about 3.5 μm. The fine powder was oriented and pressed into shape in a magnetic field of 1.8T, and then densified by applying a pressure of 200 MPa in an isostatic press. Finally, the compact was sintered in a vacuum sintering furnace at 1080℃ for 3 hours and cooled with the furnace to obtain La predoped high cerium NdFeB magnet substrate. According to the transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS), the enrichment rate of La at the grain boundaries of the La predoped high cerium NdFeB magnet substrate was about 1.5 at%. (2) The substrate in step (1) was subjected to two-step grain boundary diffusion treatment: the two-step grain boundary diffusion was light rare earth transition layer diffusion and heavy rare earth strengthening diffusion; the light rare earth transition layer diffusion was to coat the surface of the La predoped high cerium NdFeB magnet substrate with Ce. 60 La 20 Al 10 Cu 10 Alloy diffusion source, the Ce 60 La 20 Al 10 Cu 10 The alloy diffusion source consists of Ce particles with a diameter of approximately 5 μm. 60 La 20 Al 10 Cu 10 The alloy powder was mixed with anhydrous ethanol to form a coating amount of 0.7% of the La-predoped cerium-NdFeB magnet. It was then treated at 860°C for 12 hours to form a continuous grain boundary network; SEM observation showed that the coverage of the formed continuous grain boundary network was >96%. The heavy rare earth strengthening diffusion was achieved by coating the substrate surface with Tb after diffusion in the light rare earth transition layer. 30 Pr 40 Al 20 Ga 10 Nanoribbon diffusion source, the Tb 30 Pr 40 Al 20 Ga 10 The nanoribbon diffusion source is Tb30 Pr 40 Al 20 Ga 10 A mixture of nanoribbons and anhydrous ethanol. The Tb 30 Pr 40 Al 20 Ga 10 The amount of nanoribbon diffusion source coating is 0.2wt% of the weight of the La predoped high-cerium NdFeB magnet; then it is heated to 830℃ for 1 hour, and then heated to 890℃ for 5 hours to construct a (Tb,Pr) thin shell layer; the thickness of the (Tb,Pr) thin shell layer is ≤30 nm; (3) The magnet after the two-step grain boundary diffusion treatment in step (2) is subjected to magnetic field annealing and Al2O3 coating surface deposition to obtain a La predoped high-cerium NdFeB magnet, which is placed in a magnetic field annealing furnace, and a steady magnetic field of 2.0T is applied perpendicular to the orientation direction and held at 500℃ for 3 hours. Finally, an Al2O3 coating with a thickness of about 1μm is deposited on the magnet surface using atomic layer deposition (ALD) technology to obtain a La predoped high-cerium NdFeB magnet. Performance testing: According to GB / T 3217-2013 "Magnetic Test Methods for Permanent Magnet (Hard Magnet) Materials", the magnetic properties of the La predoped high-cerium NdFeB magnet prepared according to the method of this embodiment were tested at room temperature (23±5°C) using the NIM-500C permanent magnet material measurement system. The remanence Br: 14.2kGs; intrinsic coercivity Hcj: 22.3kOe; magnetic energy product (BH)max: 45.8MGOe.

[0024] Example 3

[0025] A method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion, the specific steps of which are as follows: 1) Formulating according to atomic percentage (Nd 0.8 Pr 0.2 ) 18 Ce 30 Fe 48 Al 0.4 La 0.6 B 0.95 The raw materials are batched and then vacuum induction melted under argon protection, casting them into alloy ingots. These ingots are then rapidly solidified in a vacuum rapid quenching furnace to prepare rapidly solidified flakes, with a solidification rate of approximately 1.2 × 10⁻⁶. 4K / s. The quick-setting sheet was hydrogen-crushed at room temperature and 0.15 MPa hydrogen pressure. The coarse powder after hydrogen crushing was subjected to air jet milling to obtain fine powder with an average particle size D50 of about 2.5 μm. The fine powder was oriented and pressed into shape in a magnetic field of 2.0T, and then densified by applying a pressure of 200 MPa in an isostatic press. Finally, the compact was sintered in a vacuum sintering furnace at 1060℃ for 3 hours and cooled with the furnace to obtain La predoped high cerium NdFeB magnet substrate. According to the transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS), the enrichment rate of La at the grain boundaries of the La predoped high cerium NdFeB magnet substrate was about 0.8 at%. (2) The substrate in step (1) was subjected to two-step grain boundary diffusion treatment: the two-step grain boundary diffusion was light rare earth transition layer diffusion and heavy rare earth strengthening diffusion; the light rare earth transition layer diffusion was to coat the surface of the La predoped high cerium NdFeB magnet substrate with Ce. 60 La 20 Al 10 Cu 10 Alloy diffusion source, the Ce 60 La 20 Al 10 Cu 10 The alloy diffusion source consists of Ce particles with a diameter of approximately 5 μm. 60 La 20 Al 10 Cu 10 The alloy powder was mixed with anhydrous ethanol to form a coating amount of 0.4% of the La-predoped cerium-NdFeB magnet. It was then treated at 860°C for 12 hours to form a continuous grain boundary network; SEM observation showed that the coverage of the formed continuous grain boundary network was >96%. The heavy rare earth strengthening diffusion was achieved by coating the substrate surface with Tb after diffusion in the light rare earth transition layer. 30 Pr 40 Al 20 Ga 10 Nanoribbon diffusion source, the Tb 30 Pr 40 Al 20 Ga 10 The nanoribbon diffusion source is Tb 30 Pr 40 Al 20 Ga 10 A mixture of nanoribbons and anhydrous ethanol. The Tb 30 Pr 40 Al 20 Ga 10The amount of nanoribbon diffusion source coating is 0.6 wt% of the weight of the La predoped high-cerium NdFeB magnet; then it is heated to 830℃ for 1 hour, and then heated to 890℃ for 5 hours to construct a (Tb,Pr) thin shell layer; the thickness of the (Tb,Pr) thin shell layer is ≤30 nm; (3) The magnet after the two-step grain boundary diffusion treatment in step (2) is subjected to magnetic field annealing and Al2O3 coating surface deposition to obtain a La predoped high-cerium NdFeB magnet, which is placed in a magnetic field annealing furnace, and a steady magnetic field of 2.0T is applied perpendicular to the orientation direction and held at 600℃ for 3 hours. Finally, an Al2O3 coating with a thickness of about 2 μm is deposited on the magnet surface using atomic layer deposition (ALD) technology to obtain a La predoped high-cerium NdFeB magnet. Performance testing: According to GB / T 3217-2013 "Magnetic Test Methods for Permanent Magnet (Hard Magnet) Materials", the magnetic properties of the La predoped high-cerium NdFeB magnet prepared according to the method of this embodiment were tested at room temperature (23±5°C) using the NIM-500C permanent magnet material measurement system. The remanence Br: 13.5kGs; intrinsic coercivity Hcj: 18.7kOe; magnetic energy product (BH)max: 40.3MGOe.

[0026] Comparative Example 1

[0027] Using the substrate from Example 1 as raw material, without the two-step grain boundary diffusion treatment described above, only the same magnetic field annealing and ALD coating treatment as in Example 1 were performed to obtain a magnet. Performance testing: Remanence Br of the magnet in Comparative Example 1: 15.7 kGs; Intrinsic coercivity Hcj: 15.5 kOe; Magnetic energy product (BH)max: 45.1 MGOe. Comparing Example 1 and Comparative Example 1: Both have the same substrate, but after the two-step diffusion process in Example 1, the coercivity increased significantly from 15.5 kOe to 25.6 kOe, while the remanence and maximum magnetic energy product decreased only slightly. This fully demonstrates the significant advantage of the two-step grain boundary diffusion process in improving coercivity and verifies its minimization of the magnetic dilution effect on the main phase.

[0028] Comparative Example 2

[0029] The preparation method is the same as in Example 2, except that La is removed from the substrate composition formulation and adjusted to (Nd) 0.8 Pr 0.2 ) 18 Ce 20 Fe 45 Al 0.4 La 0.6 B 0.95(i.e., La content is 0), to obtain a high-cerium NdFeB magnet substrate. The subsequent two diffusion steps and post-processing are exactly the same as in Example 2, to obtain a high-cerium NdFeB magnet. Performance testing: Remanence Br of Comparative Example 1 magnet: 13.6 kGs; Intrinsic coercivity Hcj: 18.5 kOe; Magnetic energy product (BH)max: 42.1 MGOe. Comparing Example 2 and Comparative Example 2: Both have the same Ce content and both undergo two diffusion steps, but Example 2, due to the presence of pre-doped La, has a significantly higher coercivity (22.3 kOe) than DM1 (18.5 kOe). This indicates that the pre-doping of La effectively optimizes the grain boundaries, significantly improving the diffusion efficiency and hardening effect of subsequent heavy rare earth (Tb / Pr) elements. By enriching La to pre-construct a continuous grain boundary network, the REFe2 phase blockage problem in high-cerium magnets is solved.

[0030] Comparative Example 3

[0031] The preparation method was exactly the same as in Example 3, except that the heavy rare earth strengthening step was not performed. The magnet performance of Comparative Example 3 was tested: remanence Br: 13.8 kGs; intrinsic coercivity Hcj: 14.5 kOe; magnetic energy product (BH)max: 41.5 MGOe. Comparing Comparative Example 3 and Example 3, it can be seen that Hcj in Comparative Example 3 decreased significantly, while the remanence Br decreased only slightly. This proves that without the aforementioned (Tb,Pr) thin shell layer, optimization solely through a continuous grain boundary network is insufficient to obtain high coercivity.

[0032] Comparative Example 4

[0033] The preparation method was exactly the same as in Example 3, except that the light rare earth diffusion step was omitted. The magnet performance of Comparative Example 4 was tested: remanence Br: 13.0 kGs; intrinsic coercivity Hcj: 14.1 kOe; magnetic energy product (BH)max: 37.5 MGOe. Comparing Comparative Example 4 and Example 3, it can be seen that the expensive Tb / Pr cannot effectively penetrate into the magnet interior; only the magnet surface hardens, the overall Hcj is very low, and the diffusion efficiency is low. Tb / Pr is excessively enriched on the surface, causing local magnetic dilution, and Br is even lower than in Example 3. Comparative Example 4 demonstrates that without a pre-constructed grain boundary network, heavy rare earth diffusion cannot function.

Claims

1. A La-predoped high-cerium NdFeB magnet, characterized in that, The substrate composition of the magnet is shown in the formula: (Nd) 1− x Pr x ) a Ce b Fe bal M c La d B e M is one or more of Al, Cu and Ti; by mass percentage, x is 20-30%, a is 16.5-18.0%, b is 15.0-35.0%; c is 0.35-1.5%, d is 0.3-0.6%, e is 0.88-0.95%; the balance is Fe and unavoidable impurities.

2. The La-predoped high-cerium NdFeB magnet according to claim 1, characterized in that, The substrate composition of the magnet is shown in the formula: (Nd) 1−x Pr x ) a Ce b Fe bal M c La d B e M is one or more of Al, Cu and Ti, x is 20-30%, a is 17-18.0%, b is 20.0-30.0%, c is 0.40-1.0%, d is 0.4-0.5%, e is 0.88-0.95%, and the balance is Fe and unavoidable impurities.

3. The La-predoped high-cerium NdFeB magnet according to claim 1, characterized in that, In the La-predoped high-cerium NdFeB magnet substrate, La element is enriched at the magnet grain boundaries, with a grain boundary enrichment rate of 0.8 to 1.5 at.

4. The La-predoped high-cerium NdFeB magnet according to claim 1, characterized in that, In the La-predoped high-cerium NdFeB magnet substrate, La element is enriched at the magnet grain boundaries, with a grain boundary enrichment rate of 1.0–1.2 at.

5. A method for preparing a La-predoped high-cerium NdFeB magnet by grain boundary diffusion as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Provide a La-predoped high-cerium NdFeB magnet substrate; (2) Perform a two-step grain boundary diffusion treatment on the substrate described in step (1): the two-step grain boundary diffusion is light rare earth transition layer diffusion and heavy rare earth strengthening diffusion; the light rare earth transition layer diffusion is to coat the surface of the La predoped high cerium NdFeB magnet substrate with Ce. 60 La 20 Al 10 Cu 10 An alloy diffusion source is used, followed by treatment at 860–900°C for 6–12 hours to form a continuous grain boundary network; the heavy rare earth strengthening diffusion involves coating the substrate surface with Tb after diffusion in the light rare earth transition layer. 30 Pr 40 Al 20 Ga 10 A nanoribbon diffusion source was used, and then a gradient temperature diffusion treatment was carried out at 830–890℃ for 4–10 hours to construct a (Tb,Pr) thin shell layer. (3) The magnet after the two-step grain boundary diffusion treatment described in step (2) is subjected to magnetic field annealing and Al2O3 coating surface deposition to obtain a La predoped high cerium NdFeB magnet.

6. The method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion according to claim 5, characterized in that, Step (1) involves providing La-predoped high-cerium NdFeB magnet substrate, which includes sequentially processing raw materials according to the La-predoped high-cerium NdFeB magnet substrate composition ratio, followed by melting, rapid solidification, hydrogen crushing, air jet milling, orientation shaping, and then sintering at 1060–1080°C for 2–4 hours to obtain the La-predoped high-cerium NdFeB magnet substrate; wherein the rapid solidification rate is ≥10 4 K / s; the hydrogen crushing pressure is 0.15 MPa.

7. The method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion according to claim 6, characterized in that, In step (1), the average particle size D of the substrate powder after air jet milling 50 The diameter is 2.5–3.5 μm; the orientation forming is carried out in a magnetic field of 1.8–2.0 T.

8. The method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion according to claim 5, characterized in that, In step (2), the Ce 60 La 20 Al 10 Cu 10 The particle size of the alloy powder is ≤5 μm; the Ce 60 La 20 Al 10 Cu 10 The alloy diffusion source coating amount is 0.3–0.7 wt% of the weight of the La pre-doped high-cerium NdFeB magnet; the formed continuous grain boundary network coverage is >95%; the Ce... 60 La 20 Al 10 Cu 10 The diffusion source of the alloy is Ce 60 La 20 Al 10 Cu 10 A mixture of alloy and anhydrous ethanol.

9. The method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion according to claim 5, characterized in that, In step (2), the Tb 30 Pr 40 Al 20 Ga 10 The thickness of the nanoribbons is ≤200 nm; the Tb 30 Pr 40 Al 20 Ga 10 The nanoribbon diffusion source coating amount is 0.2–0.6 wt% of the weight of the La pre-doped high-cerium NdFeB magnet; the thickness of the (Tb,Pr) thin shell layer is ≤30 nm; the Tb 30 Pr 40 Al 20 Ga 10 The nanoribbon diffusion source is Tb 30 Pr 40 Al 20 Ga 10 A mixture of nanoribbons and anhydrous ethanol.

10. The method for preparing La-predoped high-cerium NdFeB magnets by grain boundary diffusion according to claim 5, characterized in that, The magnetic field annealing process in step (3) includes keeping the magnet at 500-600°C for 2-4 hours, while applying a steady magnetic field of 1.2-1.8 T in a direction perpendicular to the orientation of the magnet; the thickness of the Al2O3 protective coating in step (3) is 1-2 μm.