Grain boundary diffusion rare earth slurry, diffusion magnet and preparation method

Abstract

The application discloses a rare earth slurry for grain boundary diffusion, a diffusion magnet and a preparation method. The rare earth slurry comprises rare earth alloy particles and a composition; the rare earth alloy particles comprise rare earth elements and non-rare earth metal elements; the non-rare earth metal elements are selected from at least one of Cu, Al, Mg, Ga, Sn, Si, Ag, Co, Fe, Zr, Nb, V, Ti, Mo, Ta and Hf; the rare earth elements are selected from at least one of light rare earth elements and heavy rare earth elements, and must contain heavy rare earth elements; and the composition is formed by an ester coupling agent, inorganic fluoride, fluorine-containing borate, a polymer binder, fluorocarbon surfactant and an organic solvent with a weight ratio of 1.8-2.5:12.5-68:12-28:3:0.45-0.5:23-28. The rare earth slurry is suitable for being used as a slurry for grain boundary diffusion of sintered Nd-Fe-B magnets, and the coercive force improvement rate of the magnet can reach more than 100%.

Description

Technical Field

[0001] This invention relates to a rare earth slurry for grain boundary diffusion, a diffusion magnet, and a preparation method thereof. Background Technology

[0002] A suitable diffusion precursor can improve the depth and elemental distribution uniformity of the grain boundary diffusion layer in NdFeB magnets, thereby solving the problem of low coercivity. Before grain boundary diffusion, a diffusion source film needs to be formed on the surface of the magnet to be diffused.

[0003] Currently, the main film-forming technologies for diffusion source thin films are magnetron sputtering, screen printing, and spraying, while screen printing and spraying require the use of rare earth slurries.

[0004] The composition of rare earth slurry has a significant impact on the performance of magnets, especially in terms of how to substantially improve the coercivity of magnets while maintaining remanence or with only a slight decrease in remanence. Furthermore, the surface of the original magnet to be diffused generally needs to be cleaned before forming the diffusion source film; omitting this cleaning step can simplify the production process.

[0005] CN109695015A discloses a heavy rare earth thermal diffusion coating solution for NdFeB rare earth permanent magnets and its preparation method. The NdFeB rare earth thermal diffusion coating solution comprises: heavy rare earth powder, a penetration enhancer, a binder, a surfactant, a dispersant, and the balance being a diluent. The penetration enhancer is Al powder and / or Cu powder, the surfactant is a nonionic surfactant, and the diluent is one or more of N,N-dimethylformamide, N-methylpyrrolidone, toluene, cyclohexanone, methyl ethyl ketone, acetone, n-butanol, and diglycidyl ether. The method uses an original magnet with a coercivity of 14.16 kOe. The coercivity of the diffused magnet in Example 1 increased by 3.91 kOe, in Example 2 by 1.14 kOe, in Example 3 by 1.94 kOe, in Example 5 by 6.27 kOe, and in Example 7 by 9.54 kOe. The increase in magnet coercivity by this heavy rare earth thermal diffusion slurry is still relatively small. In addition, this method does not involve the relevant content of fluoroborate-assisted grain boundary diffusion.

[0006] CN116825526A discloses a surface treatment agent for improving the coercivity of magnets and its preparation method. The surface treatment agent comprises: an AB alloy, a large π-bonded substance, a composite surfactant, a dispersant, a coupling agent, a thickening agent, and sodium nitrite, with the remainder being a solvent. The composite surfactant is a compound product formed from triethanolamine, a surface wetting agent, and a fast-penetrating agent. This surface treatment agent is suitable for improving the coercivity of magnets containing La and Ce. However, this method does not address fluoroborate-assisted diffusion or the regulation of magnet performance by boron, particularly the surface modification of the diffusing substance by fluoroborate and the improvement of magnet remanence by boron.

[0007] CN118136395A discloses a paste for screen printing sintered NdFeB magnets. The paste comprises the following components by weight percentage: 65-90% rare earth alloy RM powder, 8-30% deionized water, 0.25-5% binder, and 0.25-2% dispersant. In the rare earth alloy RM, R is selected from one or more of La, Ce, Pr, Nd, Dy, and Tb, and M is selected from one or more of Cu, Al, Ga, Mg, Fe, Co, and Ni. The binder is selected from one or more of polyurethane, waterborne acrylic resin, and polyethylene ester. The dispersant is selected from one or more of polyvinylpyrrolidone, polyacrylic acid, polyethylene glycol, polyacrylate, ammonium citrate, castor oil, resin acetate, and nitrocellulose. This paste is suitable as a screen printing paste for cerium-containing magnets. The paste needs to be coated on a clean surface of the magnet to be diffused. The substances described in this document do not have any effect on improving surface wettability or enhancing the remanence of the diffused magnet. Summary of the Invention

[0008] In view of this, one object of the present invention is to provide a rare earth slurry for grain boundary diffusion, which is suitable for use as a grain boundary diffusion slurry for sintered NdFeB magnets and can significantly improve the coercivity of the magnet, especially for sintered NdFeB magnets that do not contain La and Ce, where the coercivity improvement rate can reach over 100%. Another object of the present invention is to provide a method for preparing the rare earth slurry as described above. A further object of the present invention is to provide a method for preparing a diffusion magnet. Yet another object of the present invention is to provide a diffusion magnet.

[0009] The present invention achieves the above objectives using the following technical solutions.

[0010] On one hand, the present invention provides a rare earth slurry for grain boundary diffusion, comprising rare earth alloy particles and a composition;

[0011] The rare earth alloy particles include rare earth elements and non-rare earth metal elements; the non-rare earth metal elements are selected from at least one of Cu, Al, Mg, Ga, Sn, Si, Ag, Co, Fe, Zr, Nb, V, Ti, Mo, Ta and Hf; the rare earth elements are selected from at least one of light rare earth elements and heavy rare earth elements, and must contain heavy rare earth elements.

[0012] The composition is formed from an ester coupling agent, an inorganic fluoride, a fluorinated borate, a polymeric binder, a fluorocarbon surfactant, and an organic solvent in a weight ratio of 1.8–2.5:12.5–68:12–28:3:0.45–0.5:23–28.

[0013] According to the rare earth slurry of the present invention, preferably, the content of the rare earth alloy particles is 35-88 wt% based on the total weight of the rare earth slurry; the rare earth alloy particles are formed from substances containing rare earth elements and substances containing non-rare earth elements.

[0014] Among them, the substances containing rare earth elements are selected from at least one of rare earth compounds, rare earth alloys and rare earth non-rare earth alloys; the substances containing non-rare earth elements are selected from one of non-rare earth oxides, non-rare earth alloys, rare earth non-rare earth alloys and non-rare earth elemental metals.

[0015] The rare earth compound is selected from at least one of rare earth oxides, rare earth hydroxides, rare earth fluorides, rare earth nitrides, rare earth chlorides, rare earth hydrides, rare earth carbonates, rare earth nitrates, rare earth oxalates, rare earth citrates, rare earth sulfates, rare earth carbides, rare earth sulfides, and rare earth basic carbonates.

[0016] In the rare earth slurry of the present invention, preferably, the ester coupling agent is selected from at least one of titanate coupling agents, aluminate coupling agents, and zirconate coupling agents.

[0017] According to the rare earth slurry of the present invention, preferably, the inorganic fluoride is selected from at least one of calcium fluoride, sodium fluoride, potassium fluoride, ammonium fluoride and ammonium hydrogen fluoride; and the fluoroborate is selected from one of sodium fluoroborate and potassium fluoroborate.

[0018] According to the rare earth slurry of the present invention, preferably, the polymer binder is selected from one or more of thermosetting resins, thermoplastic resins, synthetic rubbers, polyacrylic acid, polytetrafluoroethylene, polyimide and polyvinylpyrrolidone; the organic solvent is selected from at least one of p-methoxybenzyl alcohol, diethylene glycol butyl ether acetate, ethylene glycol diacetate, ethylene glycol ethyl ether acetate, tributyl citrate and α-terpineol.

[0019] On the other hand, the present invention also provides a method for preparing the rare earth slurry as described above, comprising the following steps:

[0020] (1) Provide rare earth alloy particles;

[0021] (2) Rare earth alloy particles, ester coupling agent, inorganic fluoride and fluorinated borate are mixed and ball-milled to obtain precursor A;

[0022] (3) Mix the polymer binder, fluorocarbon surfactant and organic solvent to obtain solution B;

[0023] (4) Mix precursor A and solution B to obtain the rare earth slurry.

[0024] In another aspect, the present invention also provides the use of the rare earth slurry as described above in improving the coercivity of a magnet used for grain boundary diffusion, said magnet being a neodymium iron boron magnet that does not contain Ce or La elements.

[0025] Furthermore, the present invention also provides a method for preparing a diffusion magnet, comprising the following steps:

[0026] 1) Provide the original magnet;

[0027] 2) Apply the rare earth slurry as described above to the surface of the original magnet and cure it to obtain a coated magnet;

[0028] 3) The coated magnet is subjected to vacuum heat treatment to obtain a diffused magnet.

[0029] According to the preparation method of the present invention, preferably:

[0030] In step 2), rare earth paste is applied to the surface of the original magnet by spraying or screen printing.

[0031] In step 3), the coated magnet is placed in a vacuum heat treatment furnace and heated to 180–280°C with a vacuum degree of less than 0.1 Pa, and held at this temperature for 0.5–3 hours; then the temperature is raised to 320–650°C and held at this temperature for 0.5–3 hours, followed by holding at 650–850°C for 1–5 hours, and then holding at 850–950°C for 3–8 hours, and then cooled to below 45°C; then the temperature is raised to 400–660°C and held at this temperature for 1–5 hours to obtain a diffused magnet.

[0032] In another aspect, the present invention also provides a diffusion magnet prepared according to the preparation method described above.

[0033] The rare earth slurry of this invention can be used as a grain boundary diffusion slurry for sintered NdFeB magnets, significantly improving the coercivity of the magnets. Especially for sintered NdFeB magnets without La and Ce, the coercivity improvement rate can reach greater than or equal to 95%, even exceeding 100%, with minimal decrease in remanence. Furthermore, the rare earth slurry of this invention can be prepared using rare earth compounds, reducing the difficulty and improving safety. The method for preparing the diffusion magnet of this invention, through multiple staged heating and holding phases, facilitates control over the diffusion sequence of the diffusion source, maximizing diffusion efficiency and thus further enhancing the coercivity of the magnet. In addition, this invention omits the cleaning step of the original magnet to be diffused, simplifying the process. Detailed Implementation

[0034] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0035] The "remanence" mentioned in this invention refers to the magnetic flux density at the point where the magnetic field strength on the saturation hysteresis loop is zero, expressed in Tesla (T) or Gauss (Gs). 1 Gs = 0.0001 T.

[0036] The "coercivity" described in this invention, also known as intrinsic coercivity, 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, causing its magnetization intensity to decrease to zero along the saturation hysteresis loop. The unit is Oersted (Oe) or Ampere per meter (A / m). 1 Oe = 79.6 A / m.

[0037] Rare Earth Slurry for Grain Boundary Diffusion

[0038] The rare earth slurry for grain boundary diffusion of the present invention can form a thin film on the surface of a neodymium iron boron magnet that does not contain La or Ce, and significantly improve the coercivity of the magnet after grain boundary diffusion.

[0039] The rare earth slurry for grain boundary diffusion of the present invention comprises rare earth alloy particles and a composition. In some specific embodiments, the rare earth slurry for grain boundary diffusion of the present invention is composed of rare earth alloy particles and a composition. The present invention has found that using the specific composition of the present invention in combination with rare earth alloy particles is more conducive to grain boundary diffusion, improves coercivity, has a higher rate of coercivity improvement, and does not significantly reduce remanence.

[0040] Based on the total weight of the rare earth slurry, the content of rare earth alloy particles is 35-88 wt%, preferably 45-88 wt%, more preferably 60-85 wt%, and even more preferably 70-85 wt%.

[0041] In this invention, the rare earth alloy particles comprise rare earth elements and non-rare earth metal elements. In some embodiments, the weight content of rare earth elements is greater than the weight content of non-rare earth metal elements. The non-rare earth metal elements are selected from at least one of Cu, Al, Mg, Ga, Sn, Si, Ag, Co, Fe, Zr, Nb, V, Ti, Mo, Ta, and Hf, preferably one of Cu, Al, Mg, Ga, Sn, Si, Ag, Zr, Nb, V, Ti, Mo, Ta, and Hf, and more preferably one or more of Cu, Al, and Ga. The rare earth elements are selected from at least one of light rare earth elements and heavy rare earth elements, and must contain heavy rare earth elements.

[0042] Light rare earth elements include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), and europium (Eu). Heavy rare earth elements include gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and yttrium (Y).

[0043] According to one embodiment of the present invention, the light rare earth element is selected from at least one of Pr and Nd, and the heavy rare earth element is selected from Dy or Tb.

[0044] According to a specific embodiment of the present invention, the rare earth elements are composed of light rare earth elements and heavy rare earth elements, and the weight content of the light rare earth elements is greater than or equal to the weight content of the heavy rare earth elements. The light rare earth elements are selected from at least one of Pr and Nd, and the heavy rare earth elements are selected from Dy or Tb.

[0045] In this invention, the rare earth alloy particles are formed from a substance containing rare earth elements and a substance containing non-rare earth elements. In some embodiments, the weight ratio of the substance containing rare earth elements to the substance containing non-rare earth elements is 65-70:30-35. According to one specific embodiment of the invention, the weight ratio of the substance containing rare earth elements to the substance containing non-rare earth elements is 65:35. According to another specific embodiment of the invention, the weight ratio of the substance containing rare earth elements to the substance containing non-rare earth elements is 70:30.

[0046] Among them, substances containing rare earth elements are selected from at least one of rare earth compounds, rare earth alloys, and rare earth-non-rare earth alloys. Substances containing non-rare earth elements are selected from one of non-rare earth compounds, non-rare earth alloys, rare earth-non-rare earth alloys, and non-rare earth elemental metals.

[0047] Specifically, rare earth alloy particles can be composed of any of the following:

[0048] (a) Composed of rare earth compounds and non-rare earth compounds;

[0049] (b) Composed of rare earth compounds, rare earth-non-rare earth alloys and non-rare earth compounds;

[0050] (c) Composed of rare earth compounds, rare earth non-rare earth alloys and non-rare earth alloys;

[0051] (d) Composed of rare earth compounds and non-rare earth alloys;

[0052] (e) Composed of rare earth alloys and non-rare earth alloys;

[0053] (f) Composed of rare earth alloys and non-rare earth elemental metals.

[0054] In this invention, the rare earth compound is selected from at least one of rare earth oxides, rare earth hydroxides, rare earth fluorides, rare earth nitrides, rare earth chlorides, rare earth hydrides, rare earth carbonates, rare earth nitrates, rare earth oxalates, rare earth citrates, rare earth sulfates, rare earth carbides, rare earth sulfides, and rare earth basic carbonates, preferably at least one of rare earth oxides, rare earth oxalates, rare earth carbonates, and rare earth basic carbonates.

[0055] This invention discovers that by using rare earth oxides or compounds that decompose to form rare earth oxides as diffusion sources, and by using added inorganic fluorides and / or fluorinated borates to convert them into rare earth fluorides through a fluorination process, not only is the melting point of the diffusion material lowered, but the in-situ formed material has high activity, low pollution, and excellent diffusion effect. Furthermore, using rare earth oxides or related rare earth compounds that decompose to form rare earth oxides as diffusion precursors reduces the difficulty of processing the diffusion source, improves safety, increases production efficiency, and eliminates the need for harsh operating environments such as vacuum and atmosphere protection.

[0056] In this invention, the non-rare earth compound is a non-rare earth oxide.

[0057] In this invention, the composition content is 12-65 wt%, preferably 22-55 wt%, based on the total weight of the rare earth slurry. The composition is formed from an ester coupling agent, inorganic fluoride, fluoroborate, polymeric binder, fluorocarbon surfactant, and organic solvent in a weight ratio of 1.8-2.5:12.5-68:12-28:3:0.45-0.5:23-28. Preferably, the weight ratio of the ester coupling agent, inorganic fluoride, fluoroborate, polymeric binder, fluorocarbon surfactant, and organic solvent is 1.8-2.2:12.5-58:12-20:3:0.45-0.5:24-28, more preferably 1.8-2:12.5-48:12-15:3:0.45-0.5:25-28. This facilitates obtaining a high-quality diffusion source precursor film layer with uniform thickness, good adhesion, and high density on the magnet surface, enabling efficient and stable diffusion during the heat treatment stage and significantly improving the coercivity of the magnet. Because it contains inorganic fluorides, fluorinated borates, and other substances, it can change the activity of the diffusion source and the magnet surface layer when heated. Therefore, before coating with rare earth slurry, it is not necessary to perform deep cleaning treatment on the magnet to be diffused (i.e., the original magnet), which not only simplifies the coating process but also avoids the generation of waste acid, waste liquid, waste gas, and other substances.

[0058] In this invention, the organic solvent is selected from at least one of p-methoxybenzyl alcohol, diethylene glycol butyl ether acetate, ethylene glycol diacetate, ethylene glycol ethyl ether acetate, tributyl citrate, and α-terpineol, preferably from at least one of diethylene glycol butyl ether acetate, ethylene glycol diacetate, ethylene glycol ethyl ether acetate, and tributyl citrate, and more preferably from ethylene glycol diacetate or ethylene glycol ethyl ether acetate.

[0059] In this invention, the ester coupling agent is selected from at least one of titanate coupling agents, aluminate coupling agents, and zirconate coupling agents, preferably one of titanate coupling agents, aluminate coupling agents, and zirconate coupling agents, and more preferably titanate coupling agents or aluminate coupling agents.

[0060] In this invention, the inorganic fluoride is selected from at least one of calcium fluoride, sodium fluoride, potassium fluoride, ammonium fluoride, and ammonium bifluoride, preferably from one of ammonium fluoride and ammonium bifluoride, and more preferably from ammonium bifluoride. This invention finds that the preferred use of ammonium bifluoride facilitates the escape of decomposition or reaction gaseous products during heating, forming channels that allow for the removal of harmful impurity elements near the magnet surface. This prevents these impurities from failing to escape in time and carbonizing in subsequent high-temperature stages, thus avoiding the formation of impurity carbon elements that could negatively impact the performance of the diffusion magnet.

[0061] In this invention, the fluorinated borate is selected from sodium fluoroborate and potassium fluoroborate, preferably potassium fluoroborate. By adding the fluorinated borate, this invention can remove the surface oxide layer of the magnet to be diffused during high-temperature heating, improve the wettability between the diffusion source and the magnet, promote heat transfer, accelerate the melting of the diffusion source, and prevent the volatilization and oxidation of the diffusion source during diffusion. This eliminates the need for pretreatment steps (i.e., cleaning surface contaminants) of the magnet to be diffused, and also helps to minimize the reduction in remanence. Furthermore, when used in combination with fluorinated inorganic substances and fluorocarbon surfactants, it further enhances the coercivity of the magnet. In addition, when simultaneously vacuum heat-treating multiple magnets, the fluorinated borate helps to prevent adhesion between magnets, which is beneficial for maintaining relatively intact geometric dimensions and a clean surface quality after diffusion.

[0062] In this invention, the fluorocarbon surfactants may be those known to be used.

[0063] In this invention, the polymeric binder is selected from one or more of thermosetting resins, thermoplastic resins, synthetic rubbers, polyacrylic acid (PAA), polytetrafluoroethylene (PTFE), polyimide (PI), and polyvinylpyrrolidone (PVP). Examples of thermosetting resins include, but are not limited to, epoxy resins, phenolic resins, urea-formaldehyde resins, and polyurethanes. Examples of thermoplastic resins include, but are not limited to, polyvinyl alcohol acetal and chlorinated polyethylene. Examples of synthetic rubbers include, but are not limited to, styrene-butadiene rubber, chloroprene rubber, and nitrile rubber. This invention discovers that by using the polymeric binder and organic solvents of this invention to undergo redox reactions with non-rare earth metal surface oxide layers and non-rare earth metal oxide particles, the removal of organic substances such as hydrocarbons as gaseous substances such as CO, CO2, and H2O is accelerated. This not only avoids the carbonization of the polymeric binder, but also the highly active metal formed in situ is beneficial for accelerating the diffusion rate and improving the diffusion effect.

[0064] <Preparation Method of Rare Earth Slurry>

[0065] The present invention also provides a method for preparing the rare earth slurry as described above, comprising the following steps: (1) providing rare earth alloy particles; (2) mixing and ball milling the rare earth alloy particles, ester coupling agent, inorganic fluoride and fluorinated borate to obtain precursor A; (3) mixing a polymer binder, fluorocarbon surfactant and organic solvent to obtain solution B; (4) mixing precursor A and solution B to obtain the rare earth slurry. This is beneficial for obtaining a rare earth slurry with stable performance.

[0066] In step (1), substances containing rare earth elements and substances containing non-rare earth elements are mixed and crushed to form rare earth alloy particles with a particle size of less than 5 μm. Crushing can be done by ball milling. According to one specific embodiment of the present invention, light rare earth oxides, heavy rare earth oxides, and non-rare earth oxides are mixed and ball-milled to form rare earth alloy particles with a particle size distribution of 1.5–3.5 μm. According to another specific embodiment of the present invention, light rare earth heavy rare earth alloys and non-rare earth alloys are mixed and ball-milled to form rare earth alloy particles with a particle size distribution of 1.5–3.5 μm.

[0067] In steps (2) and (3), the ingredients are prepared according to the weight ratios described above. In step (3), it is preferable to first add the fluorocarbon surfactant to the organic solvent, and then add the polymer binder for mixing to obtain solution B.

[0068] <Preparation Method of Diffusing Magnet>

[0069] This invention provides a method for preparing a diffused magnet, comprising the following steps: 1) providing a raw magnet; 2) a coating step; and 3) a vacuum heat treatment step. These are described in detail below.

[0070] Provide original magnet

[0071] In this invention, the original magnet is a sintered NdFeB magnet, which is made of RE2Fe 14 Sintered permanent magnets with type B compounds as the main phase. The original magnets do not contain La or Ce elements. This makes it easier to significantly improve the coercivity of the magnets using the rare earth slurry of this invention.

[0072] RE stands for rare earth elements, including light rare earth elements and heavy rare earth elements.

[0073] In some embodiments, RE is a light rare earth element, preferably praseodymium (Pr) and / or neodymium (Nd), and RE does not include heavy rare earth elements.

[0074] According to a specific embodiment of the present invention, RE is selected from at least one element selected from Pr and Nd, and must contain Nd.

[0075] In this invention, the original magnet may contain M, which is selected from one or more of Cu, Co, Al, Ga and Zr.

[0076] In certain specific embodiments, the original magnet has the following specific composition: RE, Fe, B, Cu, Al, Ga, Zr, and other unavoidable impurities. Based on all elements of the original magnet, RE is selected from at least one of Pr and Nd, and must contain Nd; the total RE content is 29.0–33.0 wt%, preferably 29.5–32.5 wt%. Based on all elements of the original magnet, the Cu content is 0.10–0.60 wt%, preferably 0.1–0.3 wt%. The Al content is 0.1–1.2 wt%, preferably 0.3–1.0 wt%, more preferably 0.5–0.8 wt%. Based on all elements, the B content is 0.88–0.98 wt%, preferably 0.89–0.95 wt%. Based on all elements of the original magnet, the Ga content is 0.05–0.6 wt%, preferably 0.1–0.5 wt%, more preferably 0.2–0.4 wt%. Based on all elements of the original magnet, the Zr content is 0.05–0.55 wt%, preferably 0.1–0.50 wt%, and more preferably 0.18–0.45 wt%. Fe is the balance.

[0077] In practical applications, the grade of the original magnet includes, but is not limited to, N50.

[0078] Coating steps

[0079] In this invention, the rare earth slurry described above is applied to the surface of the original magnet to obtain a coated magnet.

[0080] In this invention, it is not necessary to clean and pre-treat the surface of the original magnet; the rare earth slurry can be directly coated or covered onto the surface of the original magnet.

[0081] In this invention, rare earth paste is applied to the surface of the original magnet using spraying or screen printing. All surfaces of the original magnet are covered with the rare earth paste.

[0082] According to one specific embodiment of the present invention, the rare earth slurry for grain boundary diffusion as described above is screen-printed onto the surface of the original magnet. The magnet covered with the rare earth slurry is dried at 110–150°C for 5–15 minutes to complete curing. After curing, the uncoated surface is screen-printed again and dried at 110–150°C for 5–15 minutes to complete curing, resulting in a coated magnet. The thickness of the film in the obtained coated magnet is 10–100 micrometers, preferably 20–90 micrometers, more preferably 40–85 micrometers, and even more preferably 50–80 micrometers. In some embodiments, the thickness of the film parallel to the C-axis orientation direction is 70–90% of the thickness of the film perpendicular to the C-axis orientation direction.

[0083] Vacuum heat treatment steps

[0084] Vacuum heat treatment of the coated magnet yields a diffused magnet. This facilitates pre-diffusion and grain boundary diffusion, thereby improving the magnet's coercivity.

[0085] According to one embodiment of the present invention, a coated magnet is placed in a vacuum heat treatment furnace and heated to 180–280°C with a vacuum degree of less than 0.1 Pa, and held at this temperature for 0.5–3 hours; then the temperature is raised to 320–650°C and held at this temperature for 0.5–3 hours, followed by holding at 650–850°C for 1–5 hours, and then holding at 850–950°C for 3–8 hours, and then cooled to below 45°C; then the temperature is raised to 400–660°C and held at this temperature for 1–5 hours to obtain a diffused magnet.

[0086] In this invention, the vacuum degree during vacuum heat treatment can be less than 0.1 Pa, preferably less than or equal to 0.01 Pa, and more preferably less than or equal to 0.001 Pa.

[0087] In this invention, multiple stages of heating and holding can be employed, which significantly improves the coercivity of the magnet. The holding temperature in the first holding stage can be 180–280°C, preferably 200–270°C, and more preferably 230–265°C. The holding time can be 0.5–3 hours, preferably 1–3 hours, and more preferably 2–3 hours. This invention believes that controlling the temperature within this range in this stage is beneficial for removing organic solvents from the film and for the reaction of fluorides and rare earth elements to form rare earth fluorides, thereby accelerating grain boundary diffusion.

[0088] The insulation temperature in the second insulation stage can be 320–650℃, preferably 380–600℃, and more preferably 450–580℃. The insulation time can be 0.5–3 hours, preferably 1–3 hours, and more preferably 2–3 hours. This invention believes that controlling the temperature within this range during this stage is beneficial for removing the polymer binder and for the continued reaction of fluorides and rare earth elements to form rare earth fluorides.

[0089] The insulation temperature in the third insulation stage can be 650–850℃, preferably 700–800℃, and more preferably 720–780℃. The insulation time can be 1–5 hours, preferably 2–4.5 hours, and more preferably 2–4 hours.

[0090] The holding temperature for the fourth holding stage can be 850–950℃, preferably 880–950℃, and more preferably 900–950℃. The holding time can be 3–8 hours, preferably 4–7 hours, and more preferably 5–6 hours. After the fourth holding stage, the temperature is cooled to below 45℃. This stage allows grain boundary diffusion to complete.

[0091] The holding temperature for the fifth holding stage can be 400–660℃, preferably 450–600℃, and more preferably 500–580℃. The holding time can be 1–5 hours, preferably 2–5 hours, and more preferably 3–4 hours. After the fifth holding stage, it is cooled to room temperature to obtain the diffusion magnet. This improves the diffusion effect. By controlling the temperature and time of different holding stages, it is beneficial to control the order in which the diffusion source diffuses, thereby maximizing the diffusion efficiency.

[0092] The heating rate during vacuum heat treatment can be 3 to 10 °C / min, preferably 3.5 to 7.5 °C / min, and more preferably 5 to 7 °C / min.

[0093] The coercivity of the diffused magnet obtained by this invention is significantly improved compared to the original magnet, with an increase rate of greater than or equal to 95%, and even exceeding 100%. Furthermore, the decrease in remanence is not significant.

[0094] The formula for calculating the coercivity improvement rate is:

[0095] Coercivity improvement rate = [(coercivity of the diffused magnet - coercivity of the original magnet) / coercivity of the original magnet] × 100%.

[0096] <Testing Methods>

[0097] Magnetic properties were measured using a BH magnetometer at room temperature, including room temperature remanence (Br) and room temperature coercivity (Hcj).

[0098] In the following examples, the sources of some of the raw materials used are explained:

[0099] Titanate coupling agent: Isopropyltrioleoyloxytitanate, model HY-105, CAS number: 136144-62-2, produced by Hangzhou Jessica Chemical Co., Ltd.

[0100] The aluminate coupling agent is a high-purity aluminate, model HY-985, produced by Hangzhou Jessica Chemical Co., Ltd.

[0101] The fluorocarbon surfactant is Novec™ fluorosurfactant produced by 3M, model number FC-4430.

[0102] Example 1 - Preparation of Rare Earth Slurry

[0103] Praseodymium oxide, terbium oxide, and copper oxide were mixed in a weight ratio of 50:20:30, and then ball-milled until the particle size distribution was 2-3.5 μm, thus obtaining rare earth alloy particles.

[0104] Two parts by weight of titanate coupling agent, 48 parts by weight of ammonium bifluoride, and 12 parts by weight of potassium fluoroborate were added to the obtained rare earth alloy particles, and then ball milling was continued for 30 minutes. After ball milling, precursor A was obtained.

[0105] 0.5 parts by weight of fluorocarbon surfactant and 3 parts by weight of polyvinylpyrrolidone were added sequentially to 28 parts by weight of ethylene glycol diacetate and stirred until homogeneous to obtain solution B.

[0106] Precursor A is added to solution B and stirred until homogeneous to obtain rare earth slurry for grain boundary diffusion.

[0107] Example 2 - Preparation of Diffusion Magnet

[0108] Sintered NdFeB magnets (i.e., original magnets) of grade N50 are provided, with a length, width, and height of 40 mm, 25 mm, and 7.5 mm, respectively, and the height direction is oriented along the C-axis. The magnets have the following composition: PrNd 30.2 wt%, B 0.95 wt%, Al 0.5 wt%, Cu 0.12 wt%, Zr 0.18 wt%, Ga 0.2 wt%, and Fe balance.

[0109] The rare earth slurry for grain boundary diffusion prepared in Example 1 was screen-printed onto the surface of a sintered NdFeB magnet. The magnet coated with the rare earth slurry was dried at 135°C for 10 minutes to complete curing. After curing, the uncoated surface was screen-printed again and dried at 135°C for 10 minutes to complete curing, resulting in a coated magnet. All surfaces of the magnet were coated, but the film thickness parallel to the C-axis orientation direction was 75% of the film thickness perpendicular to the C-axis orientation direction. The film thickness perpendicular to the C-axis orientation direction in the coated magnet was 75 micrometers.

[0110] The coated magnet is placed in a vacuum heat treatment furnace at a vacuum level of less than 10. -3 The temperature is raised starting at Pa, first to 260℃ and held for 3 hours. During this process, the organic solvent in the film is removed, and rare earth fluorides are formed. Then, the temperature is raised to 520℃ and held for 3 hours. During this process, the polymer binder is removed, and the rare earth fluorides are formed again. Next, the temperature is raised to 750℃ and held for 2 hours to complete pre-diffusion. After this holding period, the temperature is raised to 920℃ and held for 5 hours to complete grain boundary diffusion. Then, the temperature is rapidly cooled to room temperature. Finally, the temperature is raised to 520℃ and held for 3 hours, and then cooled to room temperature to obtain the diffused magnet.

[0111] The magnet performance test results are shown in Table 1. The coercivity increased by 12.44 kOe, reaching 24.72 kOe, and the magnet grade was upgraded from the original N50 to 50SH.

[0112] Table 1

[0113] project Remanence / kGs Coercivity / kOe Coercivity improvement rate % Initial sintering of NdFeB magnets N50 14.26 12.28 -- Diffusion magnet of Example 2 14.18 24.72 101.30 Diffusion magnet of Comparative Example 2 14.04 22.20 80.78％

[0114] Comparative Example 1 - Preparation of Rare Earth Slurry

[0115] The only difference between this comparative example and Example 1 is that 12 parts by weight of potassium fluoroborate are replaced with 12 parts by weight of potassium borate. The remaining steps and parameters are the same as in Example 1. A rare earth slurry for grain boundary diffusion is obtained.

[0116] Comparative Example 2 - Preparation of Diffused Magnets

[0117] The only difference between this comparative example and Example 2 is that the rare earth slurry used for grain boundary diffusion was prepared in Comparative Example 1. The remaining steps and parameters are the same as in Example 2. A diffusion magnet was obtained. The magnet performance results are shown in Table 1.

[0118] Example 3 - Preparation of Rare Earth Slurry

[0119] Pr 60 Tb 40 (at%) alloy powder, Cu 80 Al20 (at%) alloy powder is prepared in a weight ratio of 65:35, 100 parts by weight, mixed, and then ball-milled until the particle size distribution is 1.8 to 2.4 μm to obtain rare earth alloy particles.

[0120] Add 1.8 parts by weight of aluminate coupling agent, 12.5 parts by weight of ammonium bifluoride, and 14.8 parts by weight of potassium fluoroborate to the obtained rare earth alloy particles, and then continue ball milling for 30 minutes. After ball milling, precursor A is obtained.

[0121] 0.45 parts by weight of fluorocarbon surfactant and 3 parts by weight of polyacrylic acid were added sequentially to 25 parts by weight of ethylene glycol diacetate and stirred until homogeneous to obtain solution B.

[0122] Precursor A is added to solution B and stirred until homogeneous to obtain rare earth slurry for grain boundary diffusion.

[0123] Example 4 - Preparation of Diffusion Magnet

[0124] Sintered NdFeB magnets (i.e., original magnets) of grade N50 are provided, with a length, width, and height of 40 mm, 25 mm, and 7.5 mm, respectively, and the height direction is oriented along the C-axis. The magnets have the following composition: PrNd 30.2 wt%, B 0.95 wt%, Al 0.5 wt%, Cu 0.12 wt%, Zr 0.18 wt%, Ga 0.2 wt%, and Fe balance.

[0125] The rare earth slurry prepared in Example 3 was sprayed onto the surface of a sintered NdFeB magnet using compressed gas, then dried at 135°C for 10 minutes. After curing, the uncoated surface was sprayed again and dried to obtain a coated magnet. All surfaces of the magnet were coated, but the film thickness in the direction parallel to the C-axis orientation was 80% of the film thickness in the direction perpendicular to the C-axis orientation. The film thickness in the direction perpendicular to the C-axis orientation of the coated magnet was 65 micrometers.

[0126] The coated magnet is placed in a vacuum heat treatment furnace at a vacuum level of less than 10. -3 The temperature was initially raised to 255°C and held for 3 hours to remove organic solvents from the film and to form rare earth fluorides. The temperature was then raised to 550°C and held for 3 hours to remove polymer binders and continue the reaction to form rare earth fluorides. The temperature was then raised to 780°C and held for 4 hours to complete pre-diffusion. After this holding period, the temperature was raised to 900°C and held for 5 hours to complete grain boundary diffusion. The film was then rapidly cooled to room temperature. Finally, the temperature was raised to 510°C and held for 3 hours, followed by cooling to room temperature to obtain the diffused magnet.

[0127] The magnet performance test results are shown in Table 2. The coercivity increased by 13.08 kOe to 25.36 kOe, improving from the original N50 grade to the 50 UH grade.

[0128] Table 2

[0129] project Remanence / kGs Coercivity / kOe Coercivity improvement rate % Initial sintering of NdFeB magnets N50 14.26 12.28 -- Diffusion magnet of Example 4 14.21 25.36 106.51 Diffusion magnet of Comparative Example 4 14.10 21.65 76.30

[0130] Comparative Example 3 - Preparation of Rare Earth Slurry

[0131] The only difference between this comparative example and Example 3 is that potassium fluoroborate is replaced with potassium borate. The remaining steps and parameters are the same as in Example 3. A rare earth slurry for grain boundary diffusion is obtained.

[0132] Comparative Example 4 - Preparation of Diffused Magnets

[0133] The only difference between this comparative example and Example 4 is that the rare earth slurry used for grain boundary diffusion was prepared in Comparative Example 3. The remaining steps and parameters are the same as in Example 4. A diffusion magnet was obtained. The magnet performance results are shown in Table 2.

[0134] This invention is not limited to the above-described embodiments. Any modifications, improvements, or substitutions that can be conceived by those skilled in the art without departing from the essential content of this invention fall within the scope of this invention.

Claims

1. A rare earth slurry for grain boundary diffusion, characterized in that, Including rare earth alloy particles and compositions; The rare earth alloy particles include rare earth elements and non-rare earth metal elements; the non-rare earth metal elements are selected from at least one of Cu, Al, Mg, Ga, Sn, Si, Ag, Co, Fe, Zr, Nb, V, Ti, Mo, Ta and Hf; the rare earth elements are selected from at least one of light rare earth elements and heavy rare earth elements, and must contain heavy rare earth elements. The composition is formed from an ester coupling agent, an inorganic fluoride, a fluorinated borate, a polymeric binder, a fluorocarbon surfactant, and an organic solvent in a weight ratio of 1.8–2.5:12.5–68:12–28:3:0.45–0.5:23–28.

2. The rare earth slurry according to claim 1, characterized in that, Based on the total weight of the rare earth slurry, the content of the rare earth alloy particles is 35-88 wt%; the rare earth alloy particles are formed from substances containing rare earth elements and substances containing non-rare earth elements. Among them, the substances containing rare earth elements are selected from at least one of rare earth compounds, rare earth alloys and rare earth non-rare earth alloys; the substances containing non-rare earth elements are selected from one of non-rare earth oxides, non-rare earth alloys, rare earth non-rare earth alloys and non-rare earth elemental metals. The rare earth compound is selected from at least one of rare earth oxides, rare earth hydroxides, rare earth fluorides, rare earth nitrides, rare earth chlorides, rare earth hydrides, rare earth carbonates, rare earth nitrates, rare earth oxalates, rare earth citrates, rare earth sulfates, rare earth carbides, rare earth sulfides, and rare earth basic carbonates.

3. The rare earth slurry according to claim 1, characterized in that, The ester coupling agent is selected from at least one of titanate coupling agents, aluminate coupling agents, and zirconate coupling agents.

4. The rare earth slurry according to claim 1, characterized in that, The inorganic fluoride is selected from at least one of calcium fluoride, sodium fluoride, potassium fluoride, ammonium fluoride, and ammonium hydrogen fluoride; the fluorinated borate is selected from one of sodium fluoroborate and potassium fluoroborate.

5. The rare earth slurry according to claim 1, characterized in that, The polymeric binder is selected from one or more of thermosetting resins, thermoplastic resins, synthetic rubbers, polyacrylic acid, polytetrafluoroethylene, polyimide, and polyvinylpyrrolidone; the organic solvent is selected from at least one of p-methoxybenzyl alcohol, diethylene glycol butyl ether acetate, ethylene glycol diacetate, ethylene glycol ethyl ether acetate, tributyl citrate, and α-terpineol.

6. A method for preparing a rare earth slurry according to any one of claims 1 to 5, characterized in that, Includes the following steps: (1) Provide rare earth alloy particles; (2) Rare earth alloy particles, ester coupling agent, inorganic fluoride and fluorinated borate are mixed and ball-milled to obtain precursor A; (3) Mix the polymer binder, fluorocarbon surfactant and organic solvent to obtain solution B; (4) Mix precursor A and solution B to obtain the rare earth slurry.

7. The use of a rare earth slurry according to any one of claims 1 to 5 in improving the coercivity of a magnet during grain boundary diffusion, characterized in that, The magnet is a neodymium iron boron magnet, which does not contain Ce or La elements.

8. A method for preparing a diffused magnet, characterized in that, Includes the following steps: 1) Provide the original magnet; 2) The rare earth slurry described in any one of claims 1 to 5 is applied to the surface of the original magnet and cured to obtain a coated magnet; 3) The coated magnet is subjected to vacuum heat treatment to obtain a diffused magnet.

9. The preparation method according to claim 8, characterized in that: In step 2), rare earth paste is applied to the surface of the original magnet by spraying or screen printing. In step 3), the coated magnet is placed in a vacuum heat treatment furnace and heated to 180–280°C with a vacuum degree of less than 0.1 Pa, and held at this temperature for 0.5–3 hours; then the temperature is raised to 320–650°C and held at this temperature for 0.5–3 hours, followed by holding at 650–850°C for 1–5 hours, and then holding at 850–950°C for 3–8 hours, and then cooled to below 45°C; then the temperature is raised to 400–660°C and held at this temperature for 1–5 hours to obtain a diffused magnet.

10. A diffused magnet, characterized in that, It is prepared by the preparation method according to any one of claims 8 to 9.