Rare earth paste for grain boundary diffusion, neodymium-iron-boron diffusion magnets and methods

Abstract

The application discloses a rare earth slurry for grain boundary diffusion, a Nd-Fe-B diffusion magnet and a method. The rare earth slurry for grain boundary diffusion is prepared from raw materials including a rare earth powder, a heat-conducting powder, a binder and an organic solvent; wherein the rare earth elements in the rare earth powder include light rare earth and / or heavy rare earth; the heat-conducting powder is selected from one of aluminum nitride, silicon carbide and silicon nitride; the weight of the heat-conducting powder is 3-10% of the weight of the rare earth powder; and the particle size of the heat-conducting powder is 15-40 times of the particle size of the rare earth powder. The rare earth slurry for grain boundary diffusion can improve the diffusion effect of a subsequent magnet and the utilization rate of rare earth.

Description

Technical Field

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

[0002] The development of emerging industries such as new energy vehicles, drones, high-speed trains, and industrial robots places higher demands on the coercivity, temperature stability, and material cost of sintered NdFeB magnets. Magnets without heavy rare earth elements such as terbium (Tb) or dysprosium (Dy) cannot be used in high-temperature environments due to their low magnetic properties and poor temperature resistance. Grain boundary diffusion of Tb or Dy can improve magnet performance to meet high-temperature service requirements. However, existing grain boundary diffusion technologies suffer from problems such as high Dy and Tb usage, high diffusion temperatures, shallow diffusion depths, and poor diffusion effects in high-cerium magnets. Limited heavy rare earth reserves cannot guarantee the huge demand for high-performance NdFeB magnets driven by my country's rapid technological development.

[0003] CN115440495A discloses a method for improving the coercivity of neodymium iron boron magnets. The specific method is as follows: (S1) A heavy rare earth slurry is prepared by mixing and stirring heavy rare earth diffusion source powder, organic binder, spherical high-temperature resistant ceramic powder, and organic solvent; (S2) The above heavy rare earth slurry is coated onto the surface of a neodymium iron boron magnet and dried to form a heavy rare earth coating; (S3) High-temperature diffusion and aging treatment. This patent uses heavy rare earth powder as the diffusion source and high-temperature ceramic powder (oxide) as the raw material. Firstly, oxide materials such as alumina and zirconium oxide react with the rare earth in the diffusion source when heated to high temperatures, consuming the diffusion source and reducing the utilization rate of rare earth. Secondly, these materials have low thermal conductivity, and the porous film layer formed by the removal of organic macromolecules from the cured slurry film layer, combined with the other two, generates a large thermal resistance. Therefore, a long period of heat preservation is required to eliminate the temperature gradient and obtain a uniform temperature field. This inevitably results in magnets closer to the heating element experiencing significantly longer high-temperature exposure times than magnets farther from the heating element. Given that the performance of grain boundary diffusion magnets is clearly related to the heating regime, the temperature gradient formed by large thermal resistance will also cause differences in magnet performance at different locations, increasing the difficulty of product quality control. Summary of the Invention

[0004] In view of this, one object of the present invention is to provide a rare earth slurry for grain boundary diffusion, which can improve the subsequent magnet diffusion effect and rare earth utilization rate. Another object of the present invention is to provide a method for preparing a rare earth slurry for grain boundary diffusion. A further object of the present invention is to provide a method for preparing a neodymium iron boron (NdFeB) diffusion magnet. A still other object of the present invention is to provide a NdFeB diffusion magnet.

[0005] The present invention achieves the above-mentioned technical objectives through the following technical solutions.

[0006] On one hand, the present invention provides a rare earth slurry for grain boundary diffusion, the rare earth slurry being prepared from raw materials including rare earth powder, thermally conductive powder, binder and organic solvent; wherein, the rare earth elements in the rare earth powder include light rare earth and / or heavy rare earth; the thermally conductive powder is selected from aluminum nitride, silicon carbide and silicon nitride; the weight of the thermally conductive powder is 3 to 10% of the weight of the rare earth powder; the particle size of the thermally conductive powder is 15 to 40 times the particle size of the rare earth powder.

[0007] According to the rare earth slurry for grain boundary diffusion of the present invention, preferably, the light rare earth is praseodymium or neodymium; and the heavy rare earth is dysprosium or terbium.

[0008] According to the rare earth slurry for grain boundary diffusion of the present invention, preferably, the thermally conductive powder is spherical, near-spherical, or elliptical.

[0009] According to the rare earth slurry for grain boundary diffusion of the present invention, preferably, the particle size of the rare earth powder is less than 4 μm.

[0010] According to the rare earth slurry for grain boundary diffusion of the present invention, preferably, the weight of the rare earth powder and the thermally conductive powder is 70-90% of the weight of the rare earth slurry.

[0011] On the other hand, the present invention provides a method for preparing a rare earth slurry for grain boundary diffusion, comprising the following steps:

[0012] Rare earth powder, thermally conductive powder, binder and organic solvent are mixed and stirred evenly to obtain rare earth slurry.

[0013] According to the preparation method of the present invention, preferably, the thermally conductive powder is a modified thermally conductive powder, and the following steps are further included before mixing:

[0014] The thermally conductive powder is modified by using a coupling agent to obtain a modified thermally conductive powder; the coupling agent is selected from one of silane coupling agents, titanate coupling agents, zirconate coupling agents, and aluminate coupling agents.

[0015] In another aspect, the present invention provides a method for preparing a neodymium iron boron diffused magnet, comprising the following steps:

[0016] (1) The rare earth slurry for grain boundary diffusion described above is uniformly applied to the surface of the neodymium iron boron magnet to be diffused and cured into a film to obtain the cured magnet.

[0017] (2) Heat-treat the cured magnet described in step (1) to obtain a neodymium iron boron diffused magnet.

[0018] According to the preparation method of the present invention, preferably, the vacuum heat treatment step includes:

[0019] After curing, the magnet is placed in a vacuum heat treatment furnace and a vacuum is drawn. It is subjected to primary heat treatment at 180-300℃ for 1-3 hours; secondary heat treatment at 320-480℃ for 0.5-4 hours; and tertiary heat treatment at 850-950℃ for 3-8 hours. After cooling, a neodymium iron boron diffused magnet is obtained.

[0020] In another aspect, the present invention also provides a neodymium iron boron diffusion magnet, which is prepared by the preparation method described above.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) The high thermal conductivity filler in the rare earth slurry of the present invention can inhibit the chemical reaction between the filler and the diffusing rare earth elements, thereby improving the diffusion effect and the utilization rate of rare earth.

[0023] (2) The high thermal conductivity filler in the rare earth slurry of the present invention has a high melting point and stable chemical properties. It is uniformly covered with the rare earth material of the diffusion source on the surface of the magnet, which inhibits the flow or surface diffusion of the liquefied rare earth alloy in the high temperature stage, and ensures the consistency and stability of the diffusion magnet performance.

[0024] (3) The neodymium iron boron diffused magnet of the present invention has high coercivity. Detailed Implementation

[0025] 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.

[0026] Rare earth slurry for grain boundary diffusion

[0027] The rare earth slurry for grain boundary diffusion of the present invention is prepared from raw materials including rare earth powder, thermally conductive powder, binder and organic solvent; wherein, the rare earth elements in the rare earth powder include light rare earth and / or heavy rare earth; the thermally conductive powder is selected from aluminum nitride, silicon carbide and silicon nitride; the weight of the thermally conductive powder is 3 to 10% of the weight of the rare earth powder; the particle size of the thermally conductive powder is 15 to 40 times the particle size of the rare earth powder.

[0028] The rare earth powder can be rare earth elemental metal powder, rare earth alloy powder, or rare earth compound; preferably, it is rare earth alloy powder or rare earth compound; more preferably, it is rare earth alloy powder or rare earth hydride.

[0029] The rare earth elements in rare earth powder can be only light rare earth elements, only heavy rare earth elements, or both light and heavy rare earth elements. Praseodymium or neodymium is preferred as the light rare earth element; dysprosium or terbium is preferred as the heavy rare earth element.

[0030] The thermally conductive powder can be selected from aluminum nitride, silicon carbide, and silicon nitride, preferably silicon carbide or silicon nitride. The thermally conductive powder can be spherical, near-spherical, or elliptical, preferably spherical or near-spherical.

[0031] The thermal conductivity of aluminum nitride can range from 50 to 320 W / m. -1 K -1 Preferably, it is 200–320 W / m -1 K -1 More preferably 300-320W m -1 K -1 The thermal conductivity of silicon carbide can range from 100 to 400 W / m. -1 K -1 Preferably, it is 200-350W m -1 K -1 More preferably 250–300 W / m -1 K -1 The thermal conductivity of silicon nitride can range from 100 to 200 W / m. -1 K -1 Preferably, it is 120–180 W / m -1 K -1 More preferably, 140–180 W / m -1 K -1 .

[0032] The particle size of the rare earth powder can be less than 4 μm, preferably less than 3 μm, and more preferably less than 2 μm. The particle size of the thermally conductive powder can be 15 to 40 times the particle size of the rare earth powder; preferably, the particle size of the thermally conductive powder is 20 to 35 times the particle size of the rare earth powder; more preferably, the particle size of the thermally conductive powder is 23 to 30 times the particle size of the rare earth powder.

[0033] The weight of the thermally conductive powder can be 3 to 10% of the weight of the rare earth powder; preferably, the weight of the thermally conductive powder is 5 to 9.8% of the weight of the rare earth powder; more preferably, the weight of the thermally conductive powder is 7.5 to 8.5% of the weight of the rare earth powder.

[0034] The weight of rare earth powder and thermally conductive powder can be 70-90% of the weight of rare earth slurry; preferably, the weight of rare earth powder and thermally conductive powder is 75-85% of the weight of rare earth slurry; more preferably, the weight of rare earth powder and thermally conductive powder is 80-85% of the weight of rare earth slurry.

[0035] The adhesive may be selected from at least one of thermoplastic resin adhesives, thermosetting resin adhesives, or synthetic rubber. The thermoplastic resin adhesive may be one or more of polyvinyl acetate, ethylene-vinyl acetate resin, polyvinyl alcohol, polyvinyl acetal, acrylic resin, and polymethyl methacrylate, preferably one or more of polyvinyl acetate, polyvinyl acetal, acrylic resin, and polymethyl methacrylate, more preferably one or more of polyvinyl acetate, polyvinyl acetal, and acrylic resin. Thermosetting resin adhesive may be one or more of phenolic resin, resorcinol-formaldehyde resin, urea-formaldehyde resin, melamine-formaldehyde resin, epoxy resin, and polyurethane, preferably one or more of phenolic resin, urea-formaldehyde resin, epoxy resin, and polyurethane, more preferably phenolic resin or polyurethane. The synthetic rubber may be one or more of styrene-butadiene rubber, chloroprene rubber, butyl rubber, and nitrile rubber, preferably nitrile rubber or chloroprene rubber, more preferably nitrile rubber.

[0036] The weight of the binder can be 2 to 5% of the weight of the rare earth powder, preferably 3 to 4.5%, and more preferably 3 to 4%.

[0037] The organic solvent may be selected from at least one of propylene oxide, propylene oxide butyl ether, diglycidyl ether, p-methoxybenzyl alcohol, tributyl citrate, α-terpineol, ethanol, ethyl acetate, diepoxybutadiene, n-butanol, acetone, petroleum ether, methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, and isopropanol; preferably one or more of ethanol, ethyl acetate, n-butanol, acetone, petroleum ether, methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, and isopropanol; more preferably one or more of ethanol, ethyl acetate, methyl isobutyl ketone, n-butanol, acetone, methyl ethyl ketone, and isopropanol.

[0038] The organic solvent can be 10-30% of the weight of the rare earth powder, preferably 15-25%, and more preferably 16-20%.

[0039] <Preparation method of rare earth slurry for grain boundary diffusion>

[0040] The method for preparing the rare earth slurry for grain boundary diffusion of the present invention includes the following steps:

[0041] Rare earth powder, thermally conductive powder, binder and organic solvent are mixed and stirred evenly to obtain rare earth slurry.

[0042] In this invention, the thermally conductive powder can prevent the agglomeration of fine-particle rare earth powder and improve powder dispersibility. By selecting appropriate rare earth elements and controlling the particle size of rare earth powder, the shape and particle size of thermally conductive powder, the ratio between rare earth powder and thermally conductive powder, and the total amount of both, the film-forming performance can be greatly improved, and a film with uniform thickness and good adhesion can be obtained.

[0043] In some embodiments, the thermally conductive powder can be modified with a coupling agent before mixing the raw materials to obtain modified thermally conductive powder. The coupling agent can be selected from one of silane coupling agents, titanate coupling agents, zirconate coupling agents, and aluminate coupling agents; preferably selected from one of silane coupling agents, titanate coupling agents, and aluminate coupling agents; more preferably selected from silane coupling agents or titanate coupling agents.

[0044] In some embodiments, an anti-settling agent may also be added. The anti-settling agent may be selected from 12-hydroxystearic acid, polyolefin wax (e.g., homopolymer polyethylene wax, ethylene-acrylic acid copolymer wax), polyamide wax, and liquid rheology modifier; preferably selected from 12-hydroxystearic acid or polyamide wax.

[0045] <Preparation Method of Neodymium Iron Boron Diffused Magnets>

[0046] The rare earth slurry for grain boundary diffusion described above is uniformly applied to the surface of the neodymium iron boron magnet to be diffused and cured into a film to obtain the sample to be diffused at the grain boundary.

[0047] In this invention, after the rare earth slurry is uniformly coated onto the magnet surface, pressure can be applied before it solidifies into a film to obtain a smooth, dense, and high-quality adhesive film with good adhesion to the magnet surface and uniform thickness. The pressure application process performs secondary densification and arrangement of solid particles in the slurry, avoiding problems such as material agglomeration and uneven thickness caused by surface tension during drying. Applying pressure can accelerate the rate of solvent removal from the slurry, thus speeding up the curing rate and reducing the organic solvent content in the cured film, resulting in a rare earth alloy adhesive film with a high loading capacity.

[0048] The thickness of the densified cured film can be 65-90% of the initial wet film thickness; preferably, the thickness of the densified cured film is 68-85% of the initial wet film thickness, and more preferably, the thickness of the densified cured film is 70-83% of the initial wet film thickness.

[0049] The sample to be diffused at the grain boundary as described above was subjected to vacuum heat treatment to obtain a NdFeB diffusion magnet.

[0050] In this invention, the sample undergoes segmented heat treatment, specifically as follows:

[0051] S1. Place the sample to be diffused at the grain boundary in a vacuum heat treatment furnace and evacuate the vacuum. Heat the sample to 180-300℃ and keep it at that temperature for 1-3 hours.

[0052] S2. Continue heating the sample obtained in step S1 to 320-480℃ and keep it at that temperature for 0.5-4 hours.

[0053] S3. The sample obtained in step S2 is heated to 850-950℃ and kept at that temperature for 3-8 hours. After cooling, a neodymium iron boron diffused magnet is obtained.

[0054] In step S1 of this invention, the heating temperature can be 180–300°C, preferably 200–300°C, and more preferably 250–300°C. The holding time can be 1–3 hours, preferably 1–1.6 hours, and more preferably 1.2–1.5 hours.

[0055] In step S2, the heating temperature can be 320–480°C, preferably 400–480°C, and more preferably 430–480°C. The holding time can be 0.5–4 hours, preferably 0.8–2.5 hours, and more preferably 1.2–1.8 hours.

[0056] In step S3, the heating temperature can be 850–950°C, preferably 880–920°C, and more preferably 880–900°C. The holding time can be 3–8 hours, preferably 5–7.5 hours, and more preferably 6–7 hours.

[0057] In the vacuum heat treatment stage of the magnet, this invention discovers that the thermally conductive material of this invention can prevent the filler from reacting with the diffused rare earth elements, reducing the diffusion effect and rare earth utilization rate; secondly, it can suppress the flow or surface diffusion of liquefied rare earth alloys in the high-temperature stage, ensuring consistent and stable magnet performance; furthermore, it can reduce thermal resistance, accelerate the uniform distribution of the temperature field, shorten the time required for temperature uniformity, save energy consumption, and maintain consistent performance; at the same time, the high thermal conductivity material can suppress direct contact between magnets and interdiffusion of elements, avoid adhesion, and ensure the quantity of filler and surface quality; finally, in the cooling stage, it reduces thermal resistance, achieves rapid and uniform cooling, shortens the time, and avoids performance deviation.

[0058] Neodymium iron boron diffused magnets

[0059] This invention also provides a neodymium iron boron (NdFeB) diffused magnet. This NdFeB diffused magnet is prepared using the method described above. The NdFeB diffused magnet obtained by this invention possesses excellent magnetic properties.

[0060] The performance testing method is described in detail below:

[0061] Magnetic properties: tested using a NIM-10000HC permanent magnet tester; test temperature: 20℃; test standard: GB / T 3217-2013.

[0062] The raw materials are described below:

[0063] Spherical aluminum nitride powder: prepared according to the method of Example 1 in patent CN108862216A.

[0064] Spherical aluminum nitride powder: prepared according to the method of Example 1 in patent CN109790027A.

[0065] Spherical silicon carbide powder: prepared according to the method of Example 1 in patent CN108483447A.

[0066] Spherical silicon nitride powder: prepared according to the method of Example 1 in patent CN118125389A.

[0067] Preparation Example 1

[0068] 10 mL of N,N-diethyl-3-aminopropyltrimethoxysilane was dissolved in 90 mL of anhydrous ethanol and stirred to form a mixed solution. 50 mL of the mixed solution was added to 200 g of spherical aluminum nitride powder, and the mixture was heated in a water bath at 68 °C to evaporate the anhydrous ethanol, thus obtaining the modified spherical aluminum nitride filler.

[0069] 100 parts by weight of dysprosium hydride powder (D 50 9.8 parts by weight of modified spherical aluminum nitride filler (D) with a diameter of 1.89 μm and a thickness of 1.89 μm. 50 The thickness is 46.5 μm, and the thermal conductivity is 310 W / m. -1 K -1 3.4 parts by weight of acrylic resin and 20 parts by weight of anhydrous ethanol are mixed and stirred evenly to obtain a mixed slurry. Then, 0.5 wt% (based on the mass of the mixed slurry) of 12-hydroxystearic acid is added to the mixed slurry to obtain a rare earth slurry.

[0070] Preparation Example 2

[0071] 10 mL of isopropyltrioleoyl oxytitanate was dissolved in 90 mL of anhydrous ethanol and stirred to form a mixed solution. 50 mL of the mixed solution was added to 200 g of spherical aluminum nitride powder, and the mixture was heated in a water bath at 68 °C to evaporate the anhydrous ethanol, thus obtaining modified spherical aluminum nitride filler.

[0072] 100 parts by weight of terbium hydride powder (D 50 5.8 parts by weight of modified spherical aluminum nitride filler (D) with a diameter of 1.89 μm and a thickness of 1.89 μm. 50 Its diameter is 54 μm and its thermal conductivity is 310 W / m. -1 K -1 3.2 parts by weight of polyvinyl acetate and 18 parts by weight of methyl isobutyl ketone are mixed and stirred evenly. Then, 0.5 wt% (based on the mass of the mixed slurry) of polyamide wax is added to the mixed slurry to obtain a rare earth slurry.

[0073] Preparation Example 3

[0074] 10 mL of isopropyltrioleoyl oxytitanate was dissolved in 90 mL of anhydrous ethanol and stirred to form a mixed solution. 50 mL of the mixed solution was added to 200 g of spherical silicon carbide powder, and the mixture was heated in a water bath at 68 °C to evaporate the anhydrous ethanol, thus obtaining modified spherical silicon carbide filler.

[0075] 100 parts by weight of terbium copper alloy powder (D 50 The particle size is 2.4 μm, the terbium content is 76 wt%, and the remainder is Cu), and 7.5 parts by weight of modified spherical silicon carbide filler (D 50 Its thickness is 54 μm and its thermal conductivity is 270 W / m. -1 K -1 3.5 parts by weight of polymethyl methacrylate and 16 parts by weight of methyl ethyl ketone are mixed and stirred evenly. Then, 0.5 wt% (based on the mass of the mixed slurry) of polyamide wax is added to the mixed slurry to obtain a rare earth slurry.

[0076] Preparation Example 4

[0077] 10 mL of isopropyltrioleoyl oxytitanate was dissolved in 90 mL of anhydrous ethanol and stirred to form a mixed solution. 50 mL of the mixed solution was added to 200 g of spherical silicon nitride powder, and the mixture was heated in a water bath at 68 °C to evaporate the anhydrous ethanol, thus obtaining modified spherical silicon nitride filler.

[0078] 100 parts by weight of terbium hydride powder (D 50 7.85 parts by weight of modified spherical silicon nitride filler (D, with a thickness of 1.89 μm) 50 Its diameter is 48 μm and its thermal conductivity is 146 W / m. -1 K -1 1.3 parts by weight of phenolic resin, 2.4 parts by weight of polyvinyl acetal, and 18 parts by weight of anhydrous ethanol are mixed and stirred evenly. Then, 0.5 wt% (based on the mass of the mixed slurry) of polyamide wax is added to the mixed slurry to obtain a rare earth slurry.

[0079] Preparation Example 5

[0080] 10 mL of isopropyltrioleoyl oxytitanate was dissolved in 90 mL of anhydrous ethanol and stirred to form a mixed solution. 50 mL of the mixed solution was added to 200 g of spherical silicon nitride powder, and the mixture was heated in a water bath at 68 °C to evaporate the anhydrous ethanol, thus obtaining modified spherical silicon nitride filler.

[0081] 100 parts by weight of praseodymium-terbium-gallium alloy powder (D 50 The particle size is 2.1 μm, the praseodymium content is 45 wt%, the praseodymium content is 35 wt%, the gallium content is 20 wt%), and 10 parts by weight of modified spherical silicon nitride filler (D 50The diameter is 38 μm and the thermal conductivity is the same as in Preparation Example 4). 3 parts by weight of nitrile rubber and 18 parts by weight of ethyl acetate are mixed and stirred evenly. Then, 0.5 wt% (based on the mass of the mixed slurry) of polyamide wax is added to the mixed slurry to obtain a rare earth slurry.

[0082] Comparative Preparation Example 1

[0083] Without adding spherical aluminum nitride powder, the other conditions were the same as in Preparation Example 1.

[0084] Example 1

[0085] The magnet to be diffused is a sintered NdFeB magnet with the following composition: PrNd 26.4wt%, Dy 3.4wt%, B 0.94wt%, Al 0.7wt%, Cu 0.32wt%, Co 1.1wt%, Zr 0.2wt%, Ga 0.35wt%, and Fe balance.

[0086] The magnet to be diffused was placed in anhydrous ethanol and hydrocarbon cleaning agent in sequence for ultrasonic treatment to obtain the treated magnet.

[0087] The rare earth slurry obtained in Preparation Example 1 was uniformly coated onto the surface of the treated magnet to form a wet film. The magnet coated with the wet film was then subjected to infrared heating for curing while a pressure of 5 MPa was applied for densification. The thickness of the cured film was 82.4% of the initial wet film thickness, resulting in a cured magnet.

[0088] The cured magnet was placed in a vacuum heat treatment furnace and a vacuum was drawn. When the vacuum degree of the heat treatment furnace reached 3 Pa, heating was started. The temperature inside the furnace was raised to 280℃ and held for 1.3 hours. After the holding period, the temperature was raised to 465℃ and held for another 1.5 hours. After the holding period, the temperature was raised to 920℃ and held for another 4.5 hours. Then the temperature was rapidly cooled to room temperature to obtain a neodymium iron boron diffused magnet.

[0089] Example 2

[0090] The magnet to be diffused (same as in Example 1) was placed in anhydrous ethanol and hydrocarbon cleaning agent in sequence for ultrasonic treatment to obtain the treated magnet.

[0091] The rare earth slurry obtained in Preparation Example 2 was uniformly coated onto the surface of the treated magnet, forming a wet film on the magnet surface. The magnet coated with the wet film was then subjected to infrared heating for curing while a pressure of 5 MPa was applied for densification. After densification, the thickness of the cured film layer was 75.8% of the initial wet film thickness, resulting in a cured magnet.

[0092] The cured magnet was placed in a vacuum heat treatment furnace and a vacuum was drawn. When the vacuum degree of the heat treatment furnace reached 3 Pa, heating was started. The sample was heated to 265℃ and held for 1.5 h. After the holding period, the temperature was raised to 430℃ and held for 1.2 h. After the holding period, the temperature was raised to 900℃ and held for 5.5 h. Then it was rapidly cooled to room temperature to obtain a neodymium iron boron diffused magnet.

[0093] Example 3

[0094] The magnet to be diffused (same as in Example 1) was placed in anhydrous ethanol and hydrocarbon cleaning agent in sequence for ultrasonic treatment to obtain the treated magnet.

[0095] The rare earth slurry obtained in Preparation Example 3 was uniformly coated onto the surface of the magnet to be diffused, forming a wet film on the magnet surface. The magnet coated with the wet film was heated and cured with infrared heat while being densified by applying a pressure of 5 MPa. After densification, the thickness of the cured film layer was 68.6% of the initial wet film thickness, resulting in a cured magnet.

[0096] The cured magnet was placed in a vacuum heat treatment furnace and a vacuum was drawn. When the vacuum degree of the heat treatment furnace reached 3 Pa, heating was started. The sample was heated to 300℃ and held for 1.2 h. After the holding period, the temperature was raised to 450℃ and held for 1.6 h. After the holding period, the temperature was raised to 895℃ and held for 7 h. Then it was rapidly cooled to room temperature to obtain a neodymium iron boron diffused magnet.

[0097] Example 4

[0098] The magnet to be diffused (same as in Example 1) was placed in anhydrous ethanol and hydrocarbon cleaning agent in sequence for ultrasonic treatment to obtain the treated magnet.

[0099] The rare earth slurry obtained in Preparation Example 4 was uniformly coated onto the surface of the magnet to be diffused, forming a wet film on the magnet surface. The magnet coated with the wet film was heated and cured with infrared heat while being densified by applying a pressure of 5 MPa. After densification, the thickness of the cured film layer was 70.6% of the initial wet film thickness, resulting in a cured magnet.

[0100] The cured magnet was placed in a vacuum heat treatment furnace and a vacuum was drawn. When the vacuum degree of the heat treatment furnace reached 3 Pa, heating was started. The sample was heated to 260℃ and held for 1.2 h. After the holding period, the temperature was raised to 460℃ and held for 1.5 h. After the holding period, the temperature was raised to 880℃ and held for 6 h. Then the sample was rapidly cooled to room temperature to obtain a neodymium iron boron diffused magnet.

[0101] Example 5

[0102] The magnet to be diffused (same as in Example 1) was placed in anhydrous ethanol and hydrocarbon cleaning agent in sequence for ultrasonic treatment to obtain the treated magnet.

[0103] The rare earth slurry obtained in Preparation Example 5 was uniformly coated onto the surface of the magnet to be diffused, forming a wet film on the magnet surface. The magnet coated with the wet film was heated with infrared heat and cured while a pressure of 5 MPa was applied to densify it. After densification, the thickness of the cured film layer was 75% of the initial wet film thickness, thus obtaining the cured magnet.

[0104] The cured magnet was placed in a vacuum heat treatment furnace and a vacuum was drawn. When the vacuum degree of the heat treatment furnace reached 3 Pa, heating was started. The sample was heated to 285℃ and held for 1 hour. After the holding period, the temperature was raised to 430℃ and held for another 0.8 hours. After the holding period, the temperature was raised to 895℃ and held for 8 hours. Then the sample was rapidly cooled to room temperature to obtain a neodymium iron boron diffused magnet.

[0105] Comparative Example 1

[0106] Except for uniformly applying the slurry obtained in Comparative Preparation Example 1 to the surface of the magnet to be diffused, the other operations are the same as in Example 1.

[0107] The performance of the neodymium iron boron diffused magnets obtained in Examples 1-5 and Comparative Example 1 was tested, and the results are shown in Table 1.

[0108] Table 1

[0109] Serial Number Br / kGs Hcj / kOe To be diffused magnet 13.04 21.43 Example 1 12.85 31.65 Example 2 12.92 32.46 Example 3 12.87 33.98 Example 4 12.76 33.24 Example 5 12.84 32.87 Comparative Example 1 12.65 29.80

[0110] 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, The rare earth slurry is prepared from raw materials including rare earth powder, thermally conductive powder, binder and organic solvent; wherein, the rare earth elements in the rare earth powder include light rare earth and / or heavy rare earth; the thermally conductive powder is selected from aluminum nitride, silicon carbide and silicon nitride; the weight of the thermally conductive powder is 3 to 10% of the weight of the rare earth powder; the particle size of the thermally conductive powder is 15 to 40 times the particle size of the rare earth powder.

2. The rare earth slurry for grain boundary diffusion according to claim 1, characterized in that, The light rare earth element is praseodymium or neodymium; the heavy rare earth element is dysprosium or terbium.

3. The rare earth slurry for grain boundary diffusion according to claim 1 or 2, characterized in that, The thermally conductive powder is spherical, near-spherical, or elliptical.

4. The rare earth slurry for grain boundary diffusion according to claim 1, characterized in that, The particle size of the rare earth powder is less than 4 μm.

5. The rare earth slurry for grain boundary diffusion according to claim 1, characterized in that, The weight of the rare earth powder and the thermally conductive powder is 70-90% of the weight of the rare earth slurry.

6. A method for preparing a rare earth slurry for grain boundary diffusion as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Rare earth powder, thermally conductive powder, binder and organic solvent are mixed and stirred evenly to obtain rare earth slurry.

7. The preparation method according to claim 6, characterized in that, The thermally conductive powder is a modified thermally conductive powder, and the following steps are included before mixing: The thermally conductive powder is modified by using a coupling agent to obtain a modified thermally conductive powder; the coupling agent is selected from one of silane coupling agents, titanate coupling agents, zirconate coupling agents, and aluminate coupling agents.

8. A method for preparing a neodymium iron boron diffused magnet, characterized in that, Includes the following steps: (1) The rare earth slurry for grain boundary diffusion according to any one of claims 1 to 5 is uniformly coated onto the surface of the NdFeB magnet to be diffused and cured into a film to obtain a cured magnet. (2) Heat-treat the cured magnet described in step (1) to obtain a neodymium iron boron diffused magnet.

9. The preparation method according to claim 8, characterized in that, The steps of vacuum heat treatment include: After curing, the magnet is placed in a vacuum heat treatment furnace and a vacuum is drawn. It is subjected to primary heat treatment at 180-300℃ for 1-3 hours; secondary heat treatment at 320-480℃ for 0.5-4 hours; and tertiary heat treatment at 850-950℃ for 3-8 hours. After cooling, a neodymium iron boron diffused magnet is obtained.

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