Grain boundary diffusion source for neodymium-iron-boron magnet, rare earth permanent magnet, preparation method and application

Abstract

The invention discloses a grain boundary diffusion source for a neodymium-iron-boron magnet, a rare earth permanent magnet, a preparation method and application. The grain boundary diffusion source for the neodymium-iron-boron magnet has any one of the following components: (a) a mixed solution formed by raw materials including ReF3, Ca metal particles and a solvent, and (b) a slurry containing ReF3 and a Ca metal sheet, and Re is selected from Dy or Tb. According to the grain boundary diffusion source for the neodymium-iron-boron magnet, ReF3 and Ca metal are combined for use, the coercive force of the obtained rare earth permanent magnet can be improved in a low-cost mode, and the coercive force of the obtained rare earth permanent magnet can reach or even exceed the coercive force of a rare earth permanent magnet obtained through heavy rare earth hydride grain boundary diffusion.

Description

Technical Field

[0001] The present invention relates to a grain boundary diffusion source for a neodymium iron boron magnet, a rare earth permanent magnet, a preparation method and applications. Background Art

[0002] At present, the demand for high coercive force NdFeB magnets in many fields such as rail transportation, wind power generation, and artificial intelligence is growing rapidly. Due to its excellent coercive force characteristics, this type of magnet exhibits stronger anti-demagnetization ability during operation, thereby maintaining a stable and long-lasting magnetic field. However, achieving a coercive force of more than 20kOe by using only light rare earth elements (La, Ce, Pr, Nd) faces major challenges. Traditionally, heavy rare earth elements such as dysprosium (Dy), holmium (Ho) or terbium (Tb) are added in large quantities during the alloying process of NdFeB materials to form (Nd,Dy / Tb)2Fe with high magnetocrystalline anisotropy. 14 The B phase is used to increase the coercivity. However, excessive heavy rare earth elements can cause antiferromagnetic coupling between Dy / Tb and Fe atoms, significantly weakening the remanence of the magnet. In addition, heavy rare earths such as Dy, Tb, and Ho are scarce and expensive rare earth resources, and their large-scale use significantly increases production costs. Therefore, improving the utilization efficiency of heavy rare earths and developing high-coercivity NdFeB magnets with low heavy rare earth content have become key research directions in this field.

[0003] Grain boundary diffusion (GBD) technology provides a new solution to break the inherent contradiction between remanence Br and coercivity Hcj in sintered NdFeB magnets. In the GBD process, diffusion sources such as Dy / Tb metal, oxide, fluoride, hydride or alloy are pre-coated on the magnet surface by vapor deposition, sputtering, electrophoretic deposition or impregnation. Then, during high temperature treatment, the diffusion source penetrates into the magnet along the liquefied grain boundary (GB) channel. 14 Dy / Tb-enriched (Nd,Dy / Tb)2Fe 14 B shell structure. This technology significantly improves the coercive force of the magnet by enhancing the local magnetic anisotropy field, improving the nucleation resistance of the antimagnetic domain and the intercrystalline expansion resistance. Its core value lies in optimizing the utilization rate of heavy rare earths, and it has rapidly shifted from laboratory research to industrial applications, becoming one of the fastest-growing technologies in recent years. At present, among the diffusion sources, heavy rare earth hydrides have excellent diffusion capabilities, but as a reprocessing product of heavy rare earth fluorides, they have storage requirements and cost disadvantages. Although heavy rare earth fluorides have cost advantages, their excessively high self-decomposition temperature (>1300°C) and the Nd-OF phase formed during the diffusion process block the grain boundary channels, resulting in self-limitation of the diffusion efficiency. Therefore, it is critical for the industry to fundamentally solve the grain boundary diffusion efficiency of heavy rare earth fluorides.

[0004] CN103219146A discloses a method for improving magnet performance, comprising: preparing a rare earth hydroxide sol solution from an aqueous rare earth chloride solution; immersing a NdFeB magnet block, which has been treated by pickling and cleaning processes, in the rare earth hydroxide sol solution and subjecting it to ultrasonic treatment to form a layer of rare earth hydroxide sol on the magnet surface; drying to remove the solvent; heating and insulating the NdFeB magnet and metallic calcium particles in a vacuum sintering furnace, cooling, and then subjecting them to aging treatment to obtain a rare earth permanent magnet with enhanced coercivity. However, this method still achieves a relatively small improvement in magnet coercivity, and the complex operation of generating the rare earth hydroxide sol makes it unsuitable for large-scale industrialization.

[0005] CN119296949A discloses a diffusion source and grain boundary diffusion process. The diffusion source is prepared using DyF3 and Sn as diffusing agents, mixed with a curing agent and a dispersant. Magnetic sheets are stacked and coated with the diffusion source, followed by primary and secondary heat treatments to produce rare earth permanent magnets. The coercivity of the rare earth permanent magnets produced by this process still needs to be improved, and the resulting coercivity increase is small, not comparable to the coercivity increase achieved when heavy rare earth hydrides are used as diffusing agents.

[0006] CN115101281B discloses a method for preparing NdFeB magnetic materials. The method involves depositing a coating on the surface of a magnetic material. The coating comprises a heavy rare earth layer on the surface of the magnetic material and a light rare earth layer on the opposite surface of the magnetic material. The heavy rare earth slurry of the heavy rare earth layer includes dysprosium trifluoride, dysprosium nitride, a dysprosium-nickel alloy, and iron, while the light rare earth slurry of the light rare earth layer is primarily made of a copper-niobium alloy and a praseodymium-copper alloy. The coercivity of the rare earth permanent magnets obtained using this method still needs to be improved, and the grain boundary diffusion efficiency of dysprosium trifluoride cannot be effectively improved. Summary of the Invention

[0007] In view of this, one object of the present invention is to provide a grain boundary diffusion source for NdFeB magnets, which can improve the coercive force of the resulting rare earth permanent magnet in a low-cost manner by combining ReF3 (such as DyF3) with Ca metal, and the coercive force of the resulting rare earth permanent magnet reaches or even exceeds the coercive force of the rare earth permanent magnet obtained by grain boundary diffusion of heavy rare earth hydrides. Another object of the present invention is to provide an application of the grain boundary diffusion source for NdFeB magnets as described above. Another object of the present invention is to provide a method for preparing rare earth permanent magnets. Yet another object of the present invention is to provide a rare earth permanent magnet prepared according to the preparation method as described above. The present invention adopts the following technical solutions to achieve the above-mentioned objects.

[0008] In one aspect, the present invention provides a grain boundary diffusion source for NdFeB magnets, comprising any one of the following components:

[0009] (a) a mixed solution formed from raw materials including ReF3, Ca metal particles and a solvent,

[0010] (b) consists of a paste containing ReF3 and Ca metal flakes,

[0011] Here, Re is selected from Dy or Tb.

[0012] The grain boundary diffusion source for NdFeB magnets according to the present invention preferably has any one of the following compositions:

[0013] (a) a mixed solution formed by a slurry containing ReF3 and Ca metal particles, wherein the mass ratio of the Ca metal particles to the slurry containing ReF3 is 1:5 to 3:1;

[0014] (b) consisting of a slurry containing ReF3 and a Ca metal flake, wherein the thickness of the Ca metal flake is 0.8 to 1.5 mm;

[0015] wherein Re is selected from Dy or Tb;

[0016] The ReF3-containing slurry is a slurry formed by ReF3, a solvent and a dispersant; the concentration of ReF3 in the ReF3-containing slurry is 55-65 wt%.

[0017] According to the grain boundary diffusion source for NdFeB magnets of the present invention, preferably, Re is Dy.

[0018] According to the grain boundary diffusion source for NdFeB magnets of the present invention, preferably, the dispersant is selected from one or more of polypyrrolidone, polyvinyl butyral and polyvinyl alcohol, and the solvent is selected from one or more of C1-C4 alkyl alcohol, acetone and petroleum ether.

[0019] On the other hand, the present invention also provides a use of the grain boundary diffusion source for NdFeB magnets as described above in improving the coercive force of rare earth permanent magnets.

[0020] In another aspect, the present invention further provides a method for preparing a rare earth permanent magnet, comprising the following steps:

[0021] 1) coating two opposite surfaces of an initial NdFeB magnet with a mixture of a slurry containing ReF3 and Ca metal particles as a grain boundary diffusion source for the NdFeB magnet to obtain a coated magnet;

[0022] wherein the two opposing surfaces are both perpendicular to the C-axis orientation direction;

[0023] wherein Re is selected from Dy or Tb; the ReF3-containing slurry is a slurry formed by ReF3, a solvent, and a dispersant; the concentration of ReF3 in the ReF3-containing slurry is 55 to 65 wt%;

[0024] The mass ratio of Ca metal particles to ReF3-containing slurry is 1:5 to 3:1;

[0025] wherein the weight increase of the coated magnet is 0.6 to 3 wt % relative to the initial NdFeB magnet;

[0026] 2) The coated magnet is subjected to heat treatment, and then subjected to tempering treatment after cooling to obtain a rare earth permanent magnet.

[0027] In another aspect, the present invention further provides a method for preparing a rare earth permanent magnet, comprising the following steps:

[0028] 1-1) applying a slurry containing ReF3 on two opposite surfaces of an initial NdFeB magnet to form a coating to obtain a coated magnet;

[0029] wherein the two opposing surfaces are both perpendicular to the C-axis orientation direction;

[0030] wherein Re is selected from Dy or Tb; the ReF3-containing slurry is a slurry formed by ReF3, a solvent, and a dispersant; the concentration of ReF3 in the ReF3-containing slurry is 55 to 65 wt%;

[0031] wherein the weight increase of the coated magnet is 0.5 to 3.2 wt % relative to the initial NdFeB magnet;

[0032] 1-2) Covering the coating with two Ca metal sheets respectively to obtain a coated magnet; wherein the thickness of each Ca metal sheet is 0.8 to 1.5 mm;

[0033] 1-3) The coated magnet is subjected to heat treatment, and then tempered after cooling to obtain a rare earth permanent magnet.

[0034] According to the preparation method of the present invention, preferably, the initial NdFeB magnet is composed of R, B, Fe and M elements; wherein R is a rare earth element and must contain Nd; M is selected from one or more of Cu, Al, Ti, Co, Ga, Zr and Zn.

[0035] According to the preparation method of the present invention, preferably:

[0036] The heat treatment is a vacuum heat treatment with a vacuum degree of less than or equal to 0.1 Pa. The heat treatment comprises the following specific steps: firstly heating the temperature to 820-870°C and keeping the temperature at 820-870°C for 20-50 minutes, then heating the temperature to 880-950°C and keeping the temperature at 880-950°C for 150-480 minutes; after the heat treatment, cooling the temperature to below 50°C;

[0037] The tempering treatment includes the following specific steps: heating to 450-550° C. and keeping the temperature at this temperature for 120-240 minutes.

[0038] In another aspect, the present invention further provides a rare earth permanent magnet, which is prepared according to the preparation method described above.

[0039] The present invention's grain boundary diffusion source for neodymium iron boron magnets primarily contains ReF3 and Ca metal, and offers the advantage of lower cost compared to heavy rare earth hydrides. The present invention can improve the coercivity of the resulting rare earth permanent magnet, minimizing the reduction in remanence. The resulting coercivity can reach or even exceed that of a rare earth permanent magnet obtained by grain boundary diffusion using heavy rare earth hydrides. The present invention's method for preparing rare earth permanent magnets facilitates the formation of Re from the grain boundary diffusion source, thereby enabling better diffusion of Re and improving the coercivity of the magnet. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 The XRD results of the powder on the surface of the rare earth permanent magnet obtained in Example 1 are shown. DETAILED DESCRIPTION

[0041] The present invention will be further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

[0042] The "remanence" mentioned in the present invention refers to the value of the magnetic flux density corresponding to the zero magnetic field intensity on the saturation hysteresis loop, usually expressed as Br or Mr, and the unit is Tesla (T) or Gauss (Gs). 1Gs = 0.0001T.

[0043] The "coercive force" mentioned in this invention, also known as intrinsic coercive force, refers to the magnetic field strength when the magnetic field is monotonically reduced to zero and then increased in the opposite direction from the saturated magnetization state of the magnet, so that the magnetization intensity decreases to zero along the saturation hysteresis loop. It is usually denoted as Hcj or MHc, and the unit is Oersted (Oe) or Ampere / meter (A / m). 1Oe = 79.6A / m.

[0044] The "vacuum degree" mentioned in the present invention refers to absolute vacuum degree; the smaller the value, the higher the vacuum degree.

[0045] <Grain Boundary Diffusion Source for NdFeB Magnets>

[0046] The present invention provides a grain boundary diffusion source for NdFeB magnets, which has any one of the following compositions:

[0047] (a) a mixed solution formed from raw materials including ReF3, Ca metal particles and a solvent,

[0048] (b) Composed of ReF3-containing slurry and Ca metal flakes.

[0049] In the present invention, Re is selected from Dy or Tb. According to a specific embodiment of the present invention, Re is Dy.

[0050] The raw materials of the present invention may also include a dispersant, etc. Such a grain boundary diffusion source for NdFeB magnets can improve the coercivity of the resulting rare earth permanent magnet after grain boundary diffusion, and can reach or even exceed the coercivity that can be achieved after grain boundary diffusion of heavy rare earth hydrides. In the prior art, it is impossible to improve the grain boundary diffusion effect of DyF3 to the same level as DyH2, and it is impossible to improve the grain boundary diffusion effect of TbF3 to the same level as TbH2. This is particularly true for magnets with a thickness of 5mm or more. The present invention uses Ca metal with a very high cost-effectiveness to carry out in-situ chemical reaction, achieving highly efficient grain boundary diffusion.

[0051] When pure heavy rare earth elements (such as Dy) are used as grain boundary diffusion sources, heavy rare earth elements are soft metals with poor crushing effects and are expensive. In industrial applications, the dosage cannot be accurately controlled and is not suitable for large-scale production.

[0052] The diffusion source (a) can be a mixture of a ReF3-containing slurry and Ca metal particles. The mass ratio of the Ca metal particles to the ReF3-containing slurry is 1:5 to 3:1, preferably 1:3 to 3:1, and can be, for example, 1:3, 1:2, 1:1, 3:1, or 2:3, more preferably 1:1 to 1:2. The Ca metal particles have a particle size of less than 2 mm. Ca represents elemental calcium.

[0053] The diffusion source (b) may be composed of a slurry containing ReF3 and a Ca metal flake. The thickness of the Ca metal flake is 0.8 to 1.5 mm, preferably 0.8 to 1.2 mm, and more preferably 0.8 to 1.0 mm. When in use, the slurry containing ReF3 is first applied to two opposing surfaces of the initial NdFeB magnet to form a coating, and then the Ca metal flakes are respectively applied to the coating. The shape and area of ​​the Ca metal flakes are substantially the same as the shape and area of ​​the surface of the initial NdFeB magnet perpendicular to the C-axis orientation direction.

[0054] The ReF3-containing slurry is composed of ReF3, a solvent, and a dispersant. The ReF3 concentration in the ReF3-containing slurry is 55-65wt%, preferably 58-65wt%, and more preferably 60-63wt%. The mass of the dispersant is 2-8wt% of the mass of the slurry, preferably 3-7wt%, and more preferably 4-6wt%. This helps increase the coercive force of the magnet.

[0055] In the present invention, the dispersant may be selected from one or more of polypyrrolidone, polyvinyl butyral, and polyvinyl alcohol, preferably polyvinyl butyral (PVB).

[0056] In the present invention, the solvent can be selected from one or more of C1-C4 alkyl alcohol, acetone, and petroleum ether, preferably C1-C4 alkyl alcohol. Examples of C1-C4 alkyl alcohol include, but are not limited to, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, and the like. The solvent is preferably ethanol, commonly known as alcohol. This helps to prevent a decrease in remanence.

[0057] According to a specific embodiment of the present invention, the solvent used in the slurry containing ReF3 is ethanol.

[0058] <Application>

[0059] The present invention also provides the use of the aforementioned NdFeB grain boundary diffusion source for increasing the coercivity of rare earth permanent magnets. This can improve the coercivity of the resulting rare earth permanent magnets at a low cost. The specific application steps can be found in the steps of the rare earth permanent magnet preparation method described below.

[0060] <Preparation Method of Rare Earth Permanent Magnet>

[0061] The method for preparing the rare earth permanent magnet comprises the following steps: 1) providing an initial NdFeB magnet; 2) forming a coated magnet; and 3) forming a rare earth permanent magnet, which will be described in detail below.

[0062] In step 1), the initial NdFeB magnet is composed of R, B, Fe and M elements; it may contain unavoidable impurities. Among them, R is a rare earth element and must contain Nd; M is selected from one or more of Cu, Al, Ti, Co, Ga, Zr and Zn. Rare earth elements include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), as well as scandium (Sc) and yttrium (Y). Preferably, R is selected from one of Pr, Nd, Dy, Tb and Ho, and must contain Nd.

[0063] According to one embodiment of the present invention, in the initial NdFeB magnet, the R content is 28-34 wt%, the B (boron) content is 0.89-0.98 wt%, the M content is 0.5-2.5 wt%, M is selected from one or more of Cu, Al, Ti, Co, Ga, Zr and Zn, and Fe is the balance. Based on the weight of the initial NdFeB magnet, the Cu content can be 0.1-0.3wt%, preferably 0.15-0.25wt%, more preferably 0.18-0.22wt%, for example, 0.18wt%, 0.19wt%, 0.20wt%, 0.21wt%, 0.22wt%; the Al content can be 0.1-0.3wt%, preferably 0.15-0.25wt%, more preferably 0.18-0.22wt%, for example, 0.18wt%, 0.19wt%, 0.20wt%, 0.21wt%, 0.22wt%; the Co content can be 0.2-0.6wt%, preferably 0.3-0.5wt%, more preferably 0.35-0.45wt%, for example, 0.35wt%, 0.36wt%, 0.37wt%, 0.38wt%, 0.39wt%. .39wt%, 0.40wt%, 0.41wt%, 0.42wt%, 0.43wt%, 0.44wt%, 0.45wt%; the Zr content can be 0.1-0.3wt%, preferably 0.15-0.25wt%, more preferably 0.18-0.22wt%, for example, it can be 0.18wt%, 0.19wt%, 0.20wt%, 0.21wt%, 0.22wt%; the Ga content is 0-1.5wt%, preferably 0.05-1wt%, more preferably 0.08-0.3wt%, further preferably 0.1-0.2wt%; the Ti content can be 0.05-0.15wt%, preferably 0.08-0.12wt%, more preferably 0.09-0.11wt%, for example, it can be 0.09wt%, 0.10wt%, 0.11wt%. The Zn content may be 0 to 1.5 wt %, preferably 0 to 1.2 wt %, and more preferably 0 to 0.08 wt %.

[0064] According to a specific embodiment of the present invention, M is Cu, Al, Ti, Co, Ga, and Zr, wherein, based on the weight of the initial NdFeB magnet, the Cu content is 0.15-0.25wt%, the Al content is 0.15-0.22wt%, the Co content is 0.35-0.45wt%, the Zr content is 0.15-0.25wt%, the Ga content is 0.08-0.15wt%, and the Ti content is 0.08-0.12wt%.

[0065] Initial NdFeB magnets can be purchased commercially or prepared in-house. In the present invention, the initial NdFeB magnets can be obtained by smelting, hydrogen crushing, jet milling, pressing, and sintering and tempering. The smelting, hydrogen crushing, jet milling, and pressing processes can be those known in the art and are not described in detail here.

[0066] In step 2), according to one embodiment of the present invention, a mixed solution comprising ReF3, Ca metal particles, and a solvent (e.g., a mixed solution comprising ReF3-containing slurry and Ca metal particles) is applied to two opposing surfaces of an initial NdFeB magnet as a grain boundary diffusion source for the NdFeB magnet, thereby producing a coated magnet. The two opposing surfaces are both perpendicular to the C-axis orientation direction; Re is selected from Dy or Tb; the ReF3-containing slurry is a slurry formed from ReF3, a solvent, and a dispersant; the ReF3 concentration in the ReF3-containing slurry is 55-65 wt%; the mass ratio of Ca metal particles to the ReF3-containing slurry is 1:5-3:1; and the weight gain of the coated magnet relative to the initial NdFeB magnet is 0.6-3 wt%. The detailed description of the specific formation and concentration of the ReF3-containing slurry, as well as the mass ratio of Ca metal particles to the ReF3-containing slurry can be found in the above description and is not further elaborated here. The weight gain of the coated magnet can be 0.6-3wt%, preferably 0.7-2.5wt%, more preferably 0.8-2wt%, for example 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.6wt%, 1.8wt%, 2wt%. This can reduce costs while ensuring a significant increase in coercivity.

[0067] In step 2), according to another embodiment of the present invention, a slurry containing ReF3 is applied to two opposing surfaces of an initial NdFeB magnet to form two coatings, thereby obtaining a coated magnet. The two opposing surfaces are both perpendicular to the C-axis orientation direction. Re is selected from Dy or Tb. The ReF3-containing slurry is formed from ReF3, a solvent, and a dispersant. The ReF3 concentration in the ReF3-containing slurry is 55-65 wt%. The weight gain of the coated magnet relative to the initial NdFeB magnet is 0.5-3.2 wt%. Two Ca metal flakes are then applied to the coatings of the coated magnet to obtain a film-coated magnet. The thickness of each Ca metal flake is 0.8-1.5 mm. The detailed description of the specific formation and concentration of the ReF3-containing slurry can be found above and is not repeated here. The weight gain of the coated magnet can be 0.5-3.2 wt%, preferably 0.6-3 wt%, more preferably 0.7-2.2 wt%, and even more preferably 0.8-1.3 wt%. The thickness of each Ca metal sheet may be 0.8 to 1.5 mm, for example, 0.8 mm, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, or 1.5 mm.

[0068] In the present invention, the molar mass of the generated Dy or Tb is relatively large and is deposited on the surface of the magnet for diffusion, while the generated CaF2 is relatively light and does not enter the magnet. In this way, the diffusion of heavy metals Dy or Tb can be achieved and the diffusion of heavy rare earths can be promoted, thereby improving the coercive force of the resulting rare earth permanent magnet.

[0069] In step 3), in some embodiments, the coated magnet is subjected to heat treatment, and after cooling, it is tempered to obtain a rare earth permanent magnet. In other embodiments, the coated magnet is subjected to heat treatment, and after cooling, it is tempered to obtain a rare earth permanent magnet. The above-mentioned heat treatment is a vacuum heat treatment, and the vacuum degree can be less than or equal to 0.1 Pa, preferably less than or equal to 0.05 Pa, and more preferably less than or equal to 0.001 Pa. The heat treatment includes a first heating stage, a first heat preservation stage, a second heating stage, and a second heat preservation stage. The heating rate of the first heating and heat preservation stage is 5 to 10°C / min, preferably 7 to 10°C / min, and more preferably 8 to 9°C / min. The final temperature reached in the first heating stage is the heat preservation temperature of the first heat preservation stage. The heat preservation temperature of the first heat preservation stage can be 820 to 870°C, preferably 830 to 860°C, and more preferably 840 to 850°C. The holding time of the first holding stage can be 20 to 50 minutes, preferably 30 to 40 minutes, and more preferably 30 to 35 minutes. This allows the Ca metal to fully melt and begin to react with ReF3, and because of the strong reducing properties of the Ca metal, the oxide impurities in the shallow surface layer of the magnet are removed, forming more diffusion channels. The heating rate of the second heating stage can be 3 to 7°C / min, preferably 4 to 6°C / min, and more preferably 5 to 5.5°C / min. The final temperature reached in the second heating stage is the holding temperature of the second holding stage. The holding temperature of the second holding stage can be 880 to 950°C, preferably 890 to 940°C, and more preferably 900 to 930°C. The holding time of the second holding stage can be 150 to 480 minutes, preferably 180 to 360 minutes, and more preferably 240 to 300 minutes. This allows the reaction product heavy rare earth (such as Dy or Tb) to be fully diffused. After the heat treatment is completed, the temperature is lowered to below 50°C.

[0070] The tempering temperature may be 450 to 550° C., preferably 470 to 550° C., more preferably 500 to 520° C. The tempering time may be 120 to 240 min, preferably 150 to 200 min, more preferably 180 to 200 min.

[0071] According to one embodiment of the present invention, the heat treatment includes the following specific steps: first heating the temperature to 820-870°C and keeping it at 820-870°C for 20-50 minutes, then heating the temperature to 880-950°C and keeping it at 880-950°C for 150-480 minutes; after the heat treatment is completed, cooling the temperature to below 50°C; the tempering treatment includes the following specific steps: heating the temperature to 450-550°C and keeping it at this temperature for 120-240 minutes.

[0072] In the present invention, powder or floccules will exist on the surface of the prepared rare earth permanent magnet. The powder or floccules are mainly CaF2 and can be collected.

[0073] <Rare earth permanent magnets>

[0074] The present invention also provides a rare earth permanent magnet prepared according to the above-described preparation method. The rare earth permanent magnet of the present invention can have a coercive force of 28.5 to 35 kOe, preferably 29 to 33 kOe, and more preferably 29.5 to 32.5 kOe. The remanence can be greater than 12.0 kGs.

[0075] The coercive force of the rare earth permanent magnet obtained by the present invention is relatively high, which is comparable to the coercive force of the rare earth permanent magnet obtained by heavy rare earth hydride, and the remanence is slightly reduced.

[0076] <Test Method>

[0077] Performance measurement: The NIM-6500C permanent magnet material magnetic measurement device developed and produced by the China National Institute of Metrology was used to measure the magnetic properties of rare earth permanent magnets at room temperature (20°C) to obtain room temperature remanence and room temperature coercive force.

[0078] In the following examples and comparative examples, the initial NdFeB magnets used are composed of Nd 21.8 Pr 7.2 Dy 2.8 B 0.95 M 1.2 Fe bal Each subscript represents the weight percentage. The composition of M is Cu 0.2 Al 0.2 Co 0.4 Zr 0.2 Ga 0.1 Ti 0.1 The initial NdFeB magnet has a remanence of 12.45 kGs and a coercive force of 24.35 kOe. The initial NdFeB magnet is obtained by processing the NdFeB magnet, as detailed below.

[0079] Preparation Example 1 - Preparation of Grain Boundary Diffusion Source for NdFeB Magnets

[0080] DyF3, ethanol, and PVB (polyvinyl butyral) were mixed in a mass ratio of 60:36:4 to obtain a slurry containing DyF3.

[0081] In a hypoxic glove box, use a shear to cut Ca metal particles to a maximum size of no more than 1 mm. Use the particles as soon as possible after cutting to prevent oxidation of Ca.

[0082] The Ca metal particles and the slurry containing DyF3 are mixed in a mass ratio of 1:2 to form a mixed solution, thereby obtaining a grain boundary diffusion source for NdFeB magnets.

[0083] Example 1

[0084] The NdFeB magnets were processed and cut into 10 mm diameter and 5 mm thick magnetic sheets. The thickness direction was oriented along the C-axis. The surfaces of the magnetic sheets were sandblasted and cleaned to remove surface oil and oxides, yielding the initial NdFeB magnets.

[0085] The NdFeB magnets prepared in Preparation Example 1 were coated on two opposing surfaces of the initial NdFeB magnets perpendicular to the C-axis orientation using a grain boundary diffusion source to obtain coated magnets. The average weight gain per coated magnet relative to the initial NdFeB magnets was 1.4 wt%.

[0086] The coated magnets were stacked without drying and placed in a vacuum sintering furnace. - 3 Pa, firstly, the temperature was increased to 850°C at a heating rate of 9°C / min and kept at 850°C for 30 minutes. Then, the temperature was increased to 900°C at a heating rate of 5°C / min and kept at 900°C for 240 minutes. After the holding period, the material was naturally cooled to room temperature. Then, the temperature was increased to 500°C and kept at 500°C for 180 minutes. Then, the material was cooled to room temperature to obtain a rare earth permanent magnet.

[0087] The surface of the obtained rare earth permanent magnet has powder. These powders are collected and the powder product is mainly CaF2 with a small amount of CaO. The XRD results of the powder and DyF3 are compared in Figure 1 .Depend on Figure 1 As can be seen, the Ca metal particles transform into CaF2 and a small amount of CaO, while the DyF3 transforms into Dy metal. The resulting Dy metal is absorbed by the magnet, achieving grain boundary diffusion. The test results of the remanence and coercivity of the resulting rare earth permanent magnet are shown in Table 1.

[0088] Comparative Example 1

[0089] The NdFeB magnets were processed and cut into 10 mm diameter and 5 mm thick magnetic sheets. The thickness direction was oriented along the C-axis. The surfaces of the magnetic sheets were sandblasted and cleaned to remove surface oil and oxides, yielding the initial NdFeB magnets.

[0090] The DyF3 slurry prepared in Preparation Example 1 was applied only to two opposing surfaces of the initial NdFeB magnet perpendicular to the C-axis orientation to obtain coated magnets. The average weight gain per coated magnet relative to the initial NdFeB magnet was 1.4 wt%.

[0091] The coated magnets were stacked without drying and placed in a vacuum sintering furnace. -3 Pa below, first raise the temperature to 850℃ and keep it at 850℃ for 30 minutes. Then raise the temperature to 900℃ at a heating rate of 5℃ / min, keep it at 900℃ for 240 minutes, and then naturally cool it to room temperature. Then raise the temperature to 500℃ and keep it at 500℃ for 180 minutes, and then cool it to room temperature to obtain a rare earth permanent magnet.

[0092] Comparative Example 2

[0093] The NdFeB magnets were processed and cut into 10 mm diameter and 5 mm thick magnetic sheets. The thickness direction was oriented along the C-axis. The surfaces of the magnetic sheets were sandblasted and cleaned to remove surface oil and oxides, yielding the initial NdFeB magnets.

[0094] DyH2, ethanol, and PVB (polyvinyl butyral) were mixed in a mass ratio of 60:36:4 to produce a DyH2-containing slurry. The DyH2-containing slurry was then applied to two opposing surfaces of a starting NdFeB magnet, perpendicular to the C-axis orientation, to produce coated magnets. The average weight gain per coated magnet relative to the starting NdFeB magnet was 1.4 wt%.

[0095] The coated magnets were stacked without drying and placed in a vacuum sintering furnace. - 3 Pa below, first raise the temperature to 850℃ and keep it at 850℃ for 30 minutes. Then raise the temperature to 900℃ at a heating rate of 5℃ / min, keep it at 900℃ for 240 minutes, and then naturally cool it to room temperature. Then raise the temperature to 500℃ and keep it at 500℃ for 180 minutes, and then cool it to room temperature to obtain a rare earth permanent magnet.

[0096] Example 2

[0097] The only difference from Example 1 is that the average weight increase of each coated magnet is 0.6 wt % relative to the initial NdFeB magnet.

[0098] Comparative Example 3

[0099] The difference from Example 2 is that only the slurry containing DyF3 is coated without Ca metal particles. The average weight increase of each coated magnet relative to the initial NdFeB magnet is 0.6 wt%.

[0100] Comparative Example 4

[0101] The difference from Example 2 is that only the DyH2-containing slurry is coated (refer to Comparative Example 2 for the specific coating steps), and no Ca metal particles are used. The average weight gain of each coated magnet relative to the initial NdFeB magnet is 0.6 wt%.

[0102] Example 3

[0103] The only difference from Example 1 is that the average weight increase of each coated magnet is 3 wt % relative to the initial NdFeB magnet.

[0104] Table 1

[0105] serial number Remanence (kGs) Coercive force (kOe) Example 1 12.31 29.54 Comparative Example 1 12.28 26.52 Comparative Example 2 12.25 29.30 Example 2 12.34 27.52 Comparative Example 3 12.32 26.04 Comparative Example 4 12.36 27.44 Example 3 12.18 29.32

[0106] As shown in Table 1, the coercivity of the rare earth permanent magnet obtained in Example 1 by combining DyF3 and Ca metal for grain boundary diffusion is significantly higher than that of the rare earth permanent magnet obtained in Comparative Example 1 using only DyF3 for grain boundary diffusion, and is also slightly higher than that of the rare earth permanent magnet obtained in Comparative Example 2 using DyH2 for grain boundary diffusion. In addition, the remanence of the rare earth permanent magnet obtained in Example 1 is slightly higher.

[0107] Similarly, in Example 2, by using DyF3 and Ca metal together for grain boundary diffusion, the coercive force of the rare earth permanent magnet obtained is significantly higher than the coercive force of the rare earth permanent magnet obtained in Comparative Example 1 using only DyF3 for grain boundary diffusion, and is also slightly higher than the coercive force of the rare earth permanent magnet obtained in Comparative Example 2 using DyH2 for grain boundary diffusion.

[0108] The coercivity of the rare earth permanent magnet obtained in Example 3 is also high, but the remanence is slightly low. Therefore, the coating weight gain of the coated magnet should be controlled within a specific range.

[0109] Example 4

[0110] The NdFeB magnets were processed and cut into 10 mm diameter and 5 mm thick magnet sheets. The thickness direction was oriented along the C-axis. The surfaces of the magnet sheets were sandblasted and cleaned to remove surface oil and oxides, yielding the initial NdFeB magnets.

[0111] The DyF3-containing slurry prepared in Preparation Example 1 was applied to two opposing surfaces of a primary NdFeB magnet perpendicular to the C-axis orientation to form two coating layers, yielding coated magnets. The average weight gain per coated magnet relative to the primary NdFeB magnet was 0.8 wt%.

[0112] The Ca metal block is rolled into a thin sheet using a rolling press to obtain a Ca metal thin sheet having a thickness of 0.8 mm and a diameter of 10 mm.

[0113] Two Ca metal sheets are respectively attached to the two coating layers of the coated magnet to obtain a coated magnet.

[0114] The multiple coated magnets are stacked without drying and placed in a vacuum sintering furnace. - 3 Pa, first increase the temperature to 850°C at a heating rate of 8°C / min and hold at 850°C for 30 minutes. Then increase the temperature to 900°C at a heating rate of 5°C / min and hold at 900°C for 240 minutes. After the holding period, naturally cool to room temperature. Then increase the temperature to 500°C and hold at 500°C for 180 minutes. Then cool to room temperature to obtain a rare earth permanent magnet.

[0115] Comparative Example 5

[0116] The difference from Example 4 is that only the slurry containing DyF3 is applied without using Ca metal flakes. The average weight increase of each coated magnet relative to the initial NdFeB magnet is 0.8 wt%.

[0117] Comparative Example 6

[0118] The difference from Example 4 is that only the DyH2-containing slurry is applied (the coating steps can be referred to Comparative Example 2), without using Ca metal flakes. The average weight gain of each coated magnet relative to the initial NdFeB magnet is 0.8 wt%.

[0119] Table 2

[0120] serial number Remanence (kGs) Coercive force (kOe) Example 4 12.31 28.95 Comparative Example 5 12.37 26.46 Comparative Example 6 12.32 29.06

[0121] As can be seen from Table 2, Example 4 of the present invention uses a combination of DyF3 and Ca metal flakes for grain boundary diffusion, and the coercive force of the rare earth permanent magnet obtained is significantly higher than the coercive force of the rare earth permanent magnet obtained in Comparative Example 5 using only DyF3 for grain boundary diffusion; and is basically equivalent to the coercive force of the rare earth permanent magnet obtained in Comparative Example 6 using DyH2 for grain boundary diffusion.

[0122] Example 5

[0123] The only difference from Example 1 is that DyF3 is replaced by TbF3.

[0124] Table 3

[0125] serial number Remanence (kGs) Coercive force (kOe) Example 5 12.09 32.22

[0126] Preparation Examples 2 to 6 - Preparation of Grain Boundary Diffusion Sources for NdFeB Magnets

[0127] The only difference from Preparation Example 1 is that the mass ratio of the DyF3-containing slurry to the Ca metal particles is different, as detailed below:

[0128] In Preparation Example 2, the mass ratio of Ca metal particles to the slurry containing DyF3 is 1:1.

[0129] In Preparation Example 3, the mass ratio of Ca metal particles to the slurry containing DyF3 is 3:1.

[0130] In Preparation Example 4, the mass ratio of Ca metal particles to the slurry containing DyF3 is 1:3.

[0131] In Preparation Example 5, the mass ratio of Ca metal particles to the slurry containing DyF3 is 2:3.

[0132] In Preparation Example 6, the mass ratio of Ca metal particles to the slurry containing DyF3 is 1:5.

[0133] Examples 6 to 10 - Preparation of rare earth permanent magnets

[0134] The only difference from Example 1 is that the grain boundary diffusion source for the NdFeB magnets used is different, the details are as follows: Example 6 adopts the grain boundary diffusion source for the NdFeB magnets of Preparation Example 2.

[0135] Example 7 uses the grain boundary diffusion source for the NdFeB magnet prepared in Example 3.

[0136] Example 8 uses the grain boundary diffusion source for the NdFeB magnet prepared in Example 4.

[0137] Example 9 uses the grain boundary diffusion source for the NdFeB magnet prepared in Example 5.

[0138] Example 10 uses the grain boundary diffusion source for the NdFeB magnet prepared in Example 6.

[0139] Comparative Preparation Examples 1-2 - Preparation of Grain Boundary Diffusion Sources for NdFeB Magnets

[0140] The only difference from Preparation Example 1 is that the mass ratio of the DyF3-containing slurry to the Ca metal particles is different, as detailed below:

[0141] In Comparative Preparation Example 1, the mass ratio of Ca metal particles to the slurry containing DyF3 is 4:1.

[0142] In Comparative Preparation Example 2, the mass ratio of Ca metal particles to the DyF 3 -containing slurry is 1:7.

[0143] Comparative Examples 7-8 - Preparation of Rare Earth Permanent Magnets

[0144] The only difference from Example 1 is that the NdFeB magnets used have different grain boundary diffusion sources, as detailed below:

[0145] In Comparative Example 7, the grain boundary diffusion source for the NdFeB magnet of Comparative Preparation Example 1 was used.

[0146] In Comparative Example 8, the grain boundary diffusion source for the NdFeB magnet of Comparative Preparation Example 2 was used.

[0147] Table 4

[0148]

[0149] In summary, the present invention utilizes grain boundary diffusion sources comprising DyF3 and Ca metal, achieving a DyF3 diffusion effect comparable to that of DyH2, resulting in a rare earth permanent magnet with a higher coercivity. Given the excellent technical effects of the present invention, the use of extremely inexpensive Ca metal to enhance the coercivity of rare earth permanent magnets is an unconventional approach.

[0150] As shown in Table 4, the mass ratio of Ca metal particles to DyF3-containing slurry has a significant impact on coercivity. When this mass ratio exceeds the range of the present invention, the improvement in coercivity is minimal. Therefore, the mass ratio of Ca metal particles to DyF3-containing slurry is not a conventional choice.

[0151] The present invention is not limited to the above-mentioned embodiments. Any modification, improvement, or substitution that can be conceived by those skilled in the art without departing from the essential content of the present invention shall fall within the scope of the present invention.

Claims

1. A grain boundary diffusion source for NdFeB magnets, characterized in that: It has any of the following components: (a) a mixed solution formed from raw materials including ReF3, Ca metal particles and a solvent, (b) consists of a paste containing ReF3 and Ca metal flakes, Here, Re is selected from Dy or Tb.

2. The grain boundary diffusion source for NdFeB magnets according to claim 1, wherein It has any of the following composition: (a) a mixed solution formed by a slurry containing ReF3 and Ca metal particles, wherein the mass ratio of the Ca metal particles to the slurry containing ReF3 is 1:5 to 3:1; (b) consisting of a slurry containing ReF3 and a Ca metal flake, wherein the thickness of the Ca metal flake is 0.8 to 1.5 mm; wherein Re is selected from Dy or Tb; The ReF3-containing slurry is a slurry formed by ReF3, a solvent and a dispersant; the concentration of ReF3 in the ReF3-containing slurry is 55-65 wt%.

3. The grain boundary diffusion source for NdFeB magnets according to claim 1 or 2, characterized in that Re is Dy.

4. The grain boundary diffusion source for NdFeB magnets according to claim 2, wherein The dispersant is selected from one or more of polypyrrolidone, polyvinyl butyral and polyvinyl alcohol, and the solvent is selected from one or more of C1-C4 alkyl alcohol, acetone and petroleum ether.

5. Use of the grain boundary diffusion source for NdFeB magnets according to any one of claims 1 to 4 in improving the coercive force of rare earth permanent magnets.

6. A method for preparing a rare earth permanent magnet, characterized in that: The steps include: 1) coating two opposite surfaces of an initial NdFeB magnet with a mixture of a slurry containing ReF3 and Ca metal particles as a grain boundary diffusion source for the NdFeB magnet to obtain a coated magnet; wherein the two opposing surfaces are both perpendicular to the C-axis orientation direction; wherein Re is selected from Dy or Tb; the ReF3-containing slurry is a slurry formed by ReF3, a solvent, and a dispersant; the concentration of ReF3 in the ReF3-containing slurry is 55 to 65 wt%; The mass ratio of Ca metal particles to ReF3-containing slurry is 1:5 to 3:1; wherein the weight increase of the coated magnet is 0.6 to 3 wt % relative to the initial NdFeB magnet; 2) The coated magnet is subjected to heat treatment, and then subjected to tempering treatment after cooling to obtain a rare earth permanent magnet.

7. A method for preparing a rare earth permanent magnet, characterized in that: The following steps are involved: 1-1) applying a slurry containing ReF3 on two opposite surfaces of an initial NdFeB magnet to form a coating to obtain a coated magnet; wherein the two opposing surfaces are both perpendicular to the C-axis orientation direction; wherein Re is selected from Dy or Tb; the ReF3-containing slurry is a slurry formed by ReF3, a solvent, and a dispersant; the concentration of ReF3 in the ReF3-containing slurry is 55 to 65 wt%; wherein the weight increase of the coated magnet is 0.5 to 3.2 wt % relative to the initial NdFeB magnet; 1-2) Covering the coating with two Ca metal sheets respectively to obtain a coated magnet; wherein the thickness of each Ca metal sheet is 0.8 to 1.5 mm; 1-3) The coated magnet is subjected to heat treatment, and then tempered after cooling to obtain a rare earth permanent magnet.

8. The preparation method according to claim 6 or 7, characterized in that The initial NdFeB magnet is composed of R, B, Fe and M elements; wherein R is a rare earth element and must contain Nd; M is selected from one or more of Cu, Al, Ti, Co, Ga, Zr and Zn.

9. The preparation method according to claim 6 or 7, characterized in that: The heat treatment is a vacuum heat treatment with a vacuum degree of less than or equal to 0.1 Pa. The heat treatment comprises the following specific steps: firstly heating the temperature to 820-870°C and keeping the temperature at 820-870°C for 20-50 minutes, then heating the temperature to 880-950°C and keeping the temperature at 880-950°C for 150-480 minutes; after the heat treatment, cooling the temperature to below 50°C; The tempering treatment includes the following specific steps: heating to 450-550° C. and keeping the temperature at this temperature for 120-240 minutes.

10. A rare earth permanent magnet, characterized in that: It is prepared according to the preparation method according to claim 6 or 7.

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