An R-Fe-B sintered magnet, its preparation method and application

Abstract

This invention provides an R-Fe-B sintered magnet, its preparation method, and its applications. The R-Fe-B sintered magnet of this invention has an oxide coating layer on its surface. The R-Fe-B sintered magnet is obtained by heat treatment of an R-Fe-B magnet with a composite diffusion layer on its surface. The heat treatment includes alternating low-temperature and high-temperature heat treatments, wherein the low-temperature heat treatment temperature range is 750℃-830℃, and the high-temperature heat treatment temperature range is 830℃-970℃, resulting in a NdFeB blank with an oxide coating layer on its surface. The preparation method of the R-Fe-B sintered magnet of this invention optimizes grain boundary diffusion and improves the coercivity distribution of the magnet. This invention also provides applications of the above-mentioned R-Fe-B sintered magnet in the fields of automobiles, wind power generation, household motors, medical equipment, and mobile communication appliances.

Description

Technical Field

[0001] This invention relates to an R-Fe-B sintered magnet, its preparation method and application, belonging to the field of rare earth permanent magnet materials. Background Technology

[0002] R-Fe-B magnets are widely used in wind power generation, home motors, medical equipment, mobile communications and other fields due to their superior performance. In particular, due to the recent trend of low-carbon automotive technology worldwide, my country has the world's largest number of new energy vehicles. It is expected that high-end NdFeB permanent magnet materials will experience rapid growth in the next 10-15 years, and the market application of sintered NdFeB will be further expanded.

[0003] The coercivity of R-Fe-B magnets is a decisive factor in the magnitude and duration of magnetism in permanent magnet materials. Traditional processes primarily improve coercivity by directly adding heavy rare earth elements Dy and Tb to the smelting furnace. However, this method requires a large amount of heavy rare earth elements and sacrifices remanence, resulting in a significant decrease in remanence as coercivity increases. Furthermore, the rarity of heavy rare earth elements determines their high price, drastically increasing the cost of magnets. In the R-Fe-B field, grain boundary diffusion has been mass-produced. It improves the grain boundary microstructure by diffusing heavy rare earth elements such as Dy or Tb from the surface of the magnet along the grain boundary into the interior of the magnet, thereby increasing the coercivity of Nd-Fe-B sintered magnets. It also effectively reduces the grain boundary scattering field, weakens the magnetic exchange coupling effect, and magnetically hardens the grain boundary. Under the premise that the remanence of the magnet is not reduced by much, the coercivity is greatly improved. Currently, the main methods to achieve grain boundary diffusion include evaporation coating technology, arc ion plating technology, magnetron sputtering technology, roll coating technology, etc. (see Patent Documents 1, 2, 3, 4, and 5). The above methods and equipment have achieved the arrangement of heavy rare earth elements on the magnet surface. In patent document 5, the applicant achieved the effect of arranging heavy rare earth elements on the magnet surface by arranging an organic layer containing heavy rare earth elements on the magnet surface. This method has high controllability of the thickness and uniformity of the heavy rare earth layer, is not easy to fall off, and is easy to mass-produce. Moreover, the heavy rare earth powder is not easily oxidized when placed in the air because it is wrapped by organic matter. During the heat treatment process, the organic matter detaches from the magnet, and the heavy rare earth elements diffuse along the grain boundary into the interior of the magnet.

[0004] Grain boundary diffusion technology enhances the chromatic affinity (Hcj) of magnets by diffusing heavy rare earth elements (HREEs) along grain boundaries onto the magnet surface. It utilizes the concentration difference of HREEs between the magnet surface and interior at high temperatures as the diffusion driving force. For thinner magnets, HREEs easily diffuse to the center, resulting in good uniformity of Hcj distribution between the surface and interior. However, as magnet thickness increases, simply increasing the amount of HREEs on the surface significantly increases the Hcj difference between the surface and interior. Furthermore, the high HREE layer on the surface can damage the magnet's surface condition, requiring subsequent processing. Additionally, the abundant HREEs on the surface easily diffuse into the grains, leading to a significant reduction in remanence at the magnet surface and diminishing the effectiveness of grain boundary diffusion. Therefore, for thicker magnets, improving the diffusion depth and the consistency of Hcj between the surface and interior after diffusion is a pressing issue that needs to be addressed.

[0005] Cited patent documents:

[0006] Patent Document 1: CN101651038B;

[0007] Patent Document 2: CN101375352A;

[0008] Patent document 3: CN100565719C;

[0009] Patent document 4: CN101404195B;

[0010] Patent document 5: CN106158347A. Summary of the Invention

[0011] To improve the above-mentioned technical problems, the present invention provides an R-Fe-B magnet blank, the magnet blank comprising an R-Fe-B magnet and a composite diffusion layer, the composite diffusion layer being on the surface of the R-Fe-B magnet.

[0012] According to an embodiment of the present invention, the thickness of the R-Fe-B magnet in the magnet orientation direction is Z, and Z ≥ 3.95 mm. Preferably, 15.05 mm ≥ Z ≥ 3.95 mm. Preferably, the dimensional tolerance of Z is ±0.05 mm, for example ±0.03 mm.

[0013] According to an embodiment of the present invention, in the R-Fe-B magnet, R is selected from any one or more of the rare earth elements Nd, Pr, Tb, Dy, Gd, and Ho.

[0014] According to an embodiment of the present invention, the R content in the R-Fe-B magnet is preferably 27-34 wt%, for example, 27-30 wt%.

[0015] According to an embodiment of the present invention, the B content in the R-Fe-B magnet is preferably 0.8-1.3 wt%.

[0016] According to an embodiment of the present invention, the R-Fe-B magnet further comprises Fe and M, wherein M is selected from at least one of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo.

[0017] According to an embodiment of the present invention, the content of M in the R-Fe-B magnet can be 0-5wt%, preferably 0-3wt%, for example 2wt%.

[0018] According to an embodiment of the present invention, the total thickness of the composite diffusion layer is less than 200 μm, for example, 10 μm to 180 μm, such as 50 μm, 80 μm, 100 μm or 150 μm.

[0019] According to an embodiment of the present invention, the composite diffusion layer comprises heavy rare earth elements, preferably heavy rare earth elements, metal oxides, organic solids, and solvents that may or may not be present.

[0020] According to an embodiment of the present invention, the heavy rare earth element is selected from at least one of dysprosium metal, terbium metal, dysprosium hydride, terbium hydride, dysprosium fluoride, terbium fluoride, dysprosium oxide, and terbium oxide.

[0021] According to an embodiment of the present invention, the metal oxide is selected from at least one of zirconium oxide, calcium oxide, aluminum oxide, and holmium oxide.

[0022] According to an embodiment of the present invention, the metal oxide may be in powder form, for example, the particle size of the metal oxide may be selected from 0.5-10 μm. Preferably, the mass percentage of the metal oxide powder with a particle size between 0.5-3 μm is more than 70%.

[0023] According to an embodiment of the present invention, the organic solid may be in powder form, for example, the organic solid may be selected from at least one of rosin-modified alkyd resin, thermoplastic phenolic resin, urea-formaldehyde resin, and polyvinyl butyral.

[0024] According to an embodiment of the present invention, the solvent is selected from at least one of alcohol solvents (such as methanol, ethanol), ether solvents (such as diethyl ether), and aromatic solvents (such as benzene), preferably an alcohol solvent, for example ethanol.

[0025] According to an embodiment of the present invention, the composite diffusion layer comprises an RH layer and an RL layer, wherein:

[0026] The RH layer contains heavy rare earth elements, organic solids, and solvents that may or may not be present.

[0027] The RL layer contains metal oxides, organic solids, and solvents that may or may not be present.

[0028] According to an embodiment of the present invention, the RH layer and the RL layer are each at least one layer, for example selected from 1 layer, 2 layers, 3 layers, 4 layers, 5 layers or more.

[0029] According to an embodiment of the present invention, the RH layer and the RL layer are arranged alternately. Preferably, when the RH layer and the RL layer are arranged alternately, the outer layer furthest from the surface of the R-Fe-B magnet is preferably the RL layer.

[0030] According to an embodiment of the present invention, the single-layer thickness of the RH layer is selected from 0.5-40 μm, and exemplarily, the thickness of the RH layer is 20±5 μm or 25±5 μm.

[0031] According to an embodiment of the present invention, the single-layer thickness of the RL layer is selected from 0.5-15 μm, and exemplarily, the thickness of the RL layer is 3±2 μm.

[0032] According to an embodiment of the present invention, the weight of the composite diffusion layer is 0.1-3 wt% of the weight of the R-Fe-B magnet, for example, 0.9 wt% or 1.2 wt%.

[0033] The present invention also provides an R-Fe-B sintered magnet, wherein the R-Fe-B sintered magnet is obtained by heat treatment of the above-mentioned R-Fe-B magnet blank; after heat treatment, the metal oxide in the composite diffusion layer forms an oxide coating layer.

[0034] According to an embodiment of the present invention, the R-Fe-B sintered magnet, wherein:

[0035] The R-Fe-B sintered magnet has an oxide coating layer on its surface; and

[0036] The Hcj on the surface of the R-Fe-B sintered magnet along the magnet orientation direction is H1, and the Hcj from the magnet surface along the magnet orientation direction to a depth of 2.00±0.02 mm inside the magnet is H2. The relationship between H1 and H2 is as shown in equation (I):

[0037] H1-H2≤50kA / m (I);

[0038] According to an embodiment of the present invention, the thickness of the oxide adhesive layer is less than 20 μm, preferably less than or equal to 10 μm, for example, 5 μm.

[0039] Preferably, the oxide coating layer includes at least one of zirconium oxide, calcium oxide, aluminum oxide, and holmium oxide.

[0040] According to an embodiment of the present invention, the R-Fe-B sintered magnet contains at least one of R, B, Fe or M having the definitions and contents described above.

[0041] According to an embodiment of the present invention, the heat treatment includes alternating low-temperature heat treatment and high-temperature heat treatment.

[0042] Preferably, the temperature range of the low-temperature heat treatment is 750℃-830℃.

[0043] Preferably, the temperature range of the high-temperature heat treatment is 830℃-970℃.

[0044] According to an embodiment of the present invention, the oxide coating can be removed by non-mechanical grinding methods, such as brushing or ultrasonication.

[0045] The present invention also provides a method for preparing R-Fe-B sintered magnets, preferably the method for preparing R-Fe-B sintered magnets as described above, comprising the following steps:

[0046] (1) A composite diffusion layer is coated and arranged on the surface of the R-Fe-B magnet to form the above-mentioned R-Fe-B magnet blank;

[0047] (2) The R-Fe-B magnet blank is subjected to heat treatment in a vacuum or inert atmosphere to obtain a sintered magnet with an oxide coating on the surface;

[0048] Preferably, the heat treatment includes alternating low-temperature heat treatment and high-temperature heat treatment, wherein the temperature range of the low-temperature heat treatment is 750℃-830℃, and the temperature range of the high-temperature heat treatment is 830℃-970℃.

[0049] According to an embodiment of the present invention, the preparation method improves the distribution of coercivity in the magnet by optimizing grain boundary diffusion.

[0050] According to an embodiment of the present invention, the low-temperature heat treatment can be a low-temperature diffusion heat treatment, and the high-temperature heat treatment can be a high-temperature diffusion heat treatment. Exemplarily, the heat treatment includes a first low-temperature diffusion heat treatment, a first high-temperature diffusion heat treatment, a second low-temperature diffusion heat treatment, and a second high-temperature diffusion heat treatment.

[0051] According to an embodiment of the present invention, the total time of the heat treatment is ≥8 hours, wherein the time for low-temperature heat treatment and high-temperature heat treatment is the same or different. For example, the low-temperature heat treatment time is ≤5 hours, and the high-temperature heat treatment time is ≤5 hours.

[0052] Preferably, the process from low-temperature heat treatment to high-temperature heat treatment requires further heating at a rate of 4-10°C / min.

[0053] Preferably, the process from high-temperature heat treatment to low-temperature heat treatment requires further cooling. Cooling is achieved using a vacuum cooling method with no heating power output.

[0054] According to an embodiment of the present invention, the method further includes aging heat treatment followed by heat treatment. In this invention, the aging heat treatment refers to a heat treatment process in which alloy workpieces, after solution treatment, cold plastic deformation, or casting and forging, are placed at a higher temperature or at room temperature to maintain their properties, shape, and dimensions over time.

[0055] According to an embodiment of the present invention, the aging and heat preservation treatment includes heat preservation heat treatment, rapid cooling to room temperature, then heating to 430-650°C for aging treatment, holding for 1-72 hours, and then rapid cooling to room temperature again. For example, the aging and heat preservation treatment includes heat preservation heat treatment, rapid cooling to room temperature, then heating to 500°C for aging treatment, holding for 4 hours, and then rapid cooling to room temperature again.

[0056] According to an embodiment of the present invention, the coating of the composite diffusion layer includes coating the surface of the R-Fe-B magnet with a slurry and then drying it to form the composite diffusion layer.

[0057] According to an embodiment of the present invention, the drying can be achieved using drying equipment known in the art, such as a vacuum drying oven or a forced-air drying oven. The drying temperature and time are not specifically limited, as long as they are sufficient to dry the solvent in the slurry. For example, the drying temperature is 35-100℃, and the drying time is 5-600 seconds.

[0058] According to embodiments of the present invention, the coating method can be at least one of the following: brush coating, roller coating, dip coating, spray coating, etc. For example, for a regularly shaped square magnet, it is preferable to form a composite diffusion layer on the magnet surface by brush coating, roller coating, or other coating methods; for an irregularly shaped magnet, it is preferable to form a composite diffusion layer on the magnet surface by dip coating, spray coating, or other coating methods.

[0059] For example, when the dip coating method is selected, the R-Fe-B magnet is completely immersed in the slurry for dip, for example, for 1 to 10 seconds, or for example, 3 to 5 seconds.

[0060] According to an embodiment of the present invention, after drying, the weight of the R-Fe-B magnet blank increases by 0.1-3 wt% compared to the R-Fe-B magnet, for example, 0.9 wt% or 1.2 wt%.

[0061] According to an embodiment of the present invention, the solid content of the slurry is 30-90 wt%, preferably 40-60 wt%.

[0062] According to an embodiment of the present invention, the slurry is selected from RH layer slurry and / or RL layer slurry.

[0063] According to an embodiment of the present invention, the RH layer slurry comprises heavy rare earth elements, organic solids, and solvents. Preferably, the mass ratio of heavy rare earth elements, organic solids, and solvents is (40-70):(3-10):(20-50), for example, 60:5:35 or 55:5:40.

[0064] According to an embodiment of the present invention, the RL layer slurry comprises a metal oxide, an organic solid, and a solvent. Preferably, the mass ratio of the metal oxide, the organic solid, and the solvent is (30-70):(3-10):(20-50), for example, 55:5:40 or 50:6:44.

[0065] Preferably, the metal oxide, heavy rare earth element, organic solid and solvent have the meanings described above.

[0066] Preferably, the method for preparing the slurry includes: adding the metal oxide, heavy rare earth element, or organic solid to a solvent and stirring to form a homogeneous slurry. Preferably, the present invention does not specify the amount of the metal oxide, heavy rare earth element, organic solid, and solvent; the amount can be determined based on the performance of the R-Fe-B sintered magnet. Preferably, the present invention does not specifically limit the stirring conditions; any stirring method that can form a homogeneous slurry is applicable to the present invention. For example, using conventional stirring methods in the art, the stirring time can be 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, or any range between two of the above values.

[0067] For example, the slurry is selected from RH layer slurry and RL layer slurry.

[0068] In one specific embodiment, the RL layer slurry comprises 55 wt% zirconium oxide, 5 wt% rosin-modified alkyd resin, and 40 wt% ethanol; the RH layer slurry comprises 60 wt% terbium fluoride, 5 wt% rosin-modified alkyd resin, and 35 wt% ethanol.

[0069] In another specific embodiment, the RL layer slurry comprises 50 wt% alumina, 6 wt% rosin-modified alkyd resin, and 44 wt% ethanol; the RH layer slurry comprises 55 wt% terbium fluoride, 5 wt% rosin-modified alkyd resin, and 40 wt% ethanol.

[0070] According to an embodiment of the present invention, the coating process further includes applying the slurry multiple times to the surface of the R-Fe-B magnet.

[0071] Preferably, the coating is applied at least twice, for example, three, four or five times.

[0072] According to an embodiment of the present invention, when the slurry is applied multiple times, the slurry may be the same or different, preferably different slurries.

[0073] Preferably, when applying the slurry multiple times, the composite diffusion layer is applied alternately using RH layer slurry and RL layer slurry respectively.

[0074] Preferably, the slurry used for the final coating is an RL layer slurry.

[0075] According to an embodiment of the present invention, the dried composite diffusion layer is composed of alternating RH layers and RL layers.

[0076] Preferably, in the dried composite diffusion layer, the thickness of the RH layer coated in a single application is 0.5-40 μm, for example, 0.5 μm, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 30 μm or any two of the above values.

[0077] Preferably, in the dried composite diffusion layer, the thickness of the RL layer coated in a single application is 0.5-15 μm, for example, 0.5 μm, 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, 15 μm or any two of the above values.

[0078] According to an embodiment of the present invention, before applying the slurry, the R-Fe-B magnet may be washed sequentially with an acid solution and deionized water, and then dried. The acid solution may be any acid solution known in the art, such as an aqueous solution of hydrogen chloride or nitric acid.

[0079] According to an embodiment of the present invention, the heat treatment or aging treatment is carried out in a vacuum or inert atmosphere. For example, the inert atmosphere is selected from nitrogen, argon, etc.

[0080] According to an embodiment of the present invention, during heat treatment, the R-Fe-B magnet blanks are arranged in contact, thereby saving space.

[0081] The present invention also provides an application of the above-mentioned R-Fe-B sintered magnet in the fields of automobiles, wind power generation, household motors, medical equipment or mobile communication appliances, preferably in the field of new energy vehicles.

[0082] The present invention also provides an electric motor comprising the above-described R-Fe-B sintered magnet.

[0083] Preferably, the motor includes a power output motor, a steering EPS motor, or a micro motor.

[0084] Preferably, the micro motor includes an electric water pump motor, a fog light motor for steering linkage, a sunroof motor, an air conditioning motor, a windshield wiper motor, etc.

[0085] Beneficial effects

[0086] This invention provides an R-Fe-B sintered magnet and its preparation method. By alternately arranging a composite diffusion layer containing heavy rare earth RH and metal oxide powder RL on the surface of the R-Fe-B magnet, the structure of the heavy rare earth layer on the surface is improved, the grain boundary diffusion of the R-Fe-B magnet is optimized, and the uniformity of the magnet's coercivity distribution is improved. Compared with patent CN106158347A, this patent improves the distribution of heavy rare earth RH on the magnet surface by alternately adding RL layers. During the diffusion process, the specific RL layer effectively reduces the concentration difference between the heavy rare earth in the RH layer and the diffusion process inside the magnet, thereby reducing the driving force of the heavy rare earth diffusion process from the magnet surface to the magnet interior during the grain boundary diffusion process. Meanwhile, during the grain boundary diffusion stage, alternating high and low temperature insulation is used to control the diffusion rate and concentration of heavy rare earth elements. This optimizes the problem of poor uniformity of Hcj distribution inside thick diffusion magnets, where heavy rare earth elements are concentrated on the magnet surface and do not easily diffuse into the magnet center. It reduces the difference in Hcj between the magnet surface and the center. For magnets with a thickness ≥4mm, the difference between the coercivity H1 at the magnet surface and the coercivity H2 at a position 2mm inside the magnet is ≤50kA / m, improving the overall squareness of the magnet and thus enhancing its temperature resistance. In addition, the selected metal oxide powder forms an oxide coating layer on the magnet surface after heat treatment, solving the problem of magnets directly contacting and sticking together during high-temperature treatment. This increases the amount of material loaded during heat treatment, which is beneficial for mass production. Furthermore, the organic solid powder and metal oxide powder do not enter the magnet interior after diffusion, and the C and O element content in the magnet is not significantly increased.

[0087] Compared to patent CN106158347A, this patent improves the distribution of heavy rare earth elements (RH) on the magnet surface by alternately adding RL layers. During the diffusion process, the RL layer effectively reduces the concentration difference of heavy rare earth elements in the RH layer, preventing excessive accumulation of heavy rare earth elements on the magnet surface, which could lead to their entry into the main phase of the grains and a significant reduction in the magnet's remanence. In actual mass production, the total number of RL and RH layers is ≤5. Too many alternating layers increase production costs, and too many RL layers reduce the diffusion rate, requiring longer diffusion holding times and reducing overall diffusion efficiency. Furthermore, during the alternating arrangement of RL and RH, the outermost RL layer protects the RH layer from damage, and the RL layer can form an oxide coating on the magnet surface after heat treatment. This oxide coating isolates the magnet and prevents it from sticking together during high-temperature holding.

[0088] The sintered magnet surface after heat treatment has an oxide coating layer with a thickness of less than 20 μm, preferably 10 μm, which is easy to clean, for example, by brushing or ultrasonic cleaning, and can be completely removed without relying on mechanical grinding; the oxide coating layer can isolate the magnet during the diffusion treatment of the magnet and increase the diffusion loading. In terms of thickness, it is preferably less than 10 μm, more preferably less than 6 μm.

[0089] The applicant discovered that, during high-temperature heat treatment, the metal oxide powder does not react with heavy rare earth RH or reduce its activity. Furthermore, the metal oxide powder does not react with NdFeB magnets, and its contact with the magnet surface at high temperatures does not damage the magnet's surface condition. However, when the selected metal oxide powder has a particle size greater than 10μm, excessively large particles can easily damage the magnet's surface condition during high-temperature diffusion treatment due to contact between magnets, causing pits to form on the magnet surface. To maintain the product's appearance, these pits need to be ground off in subsequent processing, increasing processing costs and requiring additional dimensions in the substrate blank design, resulting in significant waste. When the particle size is <0.5μm, the diffusion rate cannot be effectively controlled due to the small particle size of the metal oxide powder, and the concentration difference of heavy rare earth content between the magnet surface and the center position after diffusion cannot be effectively reduced. In addition, the effect of reducing adhesion between magnets will be reduced when the metal oxide powder is too small, and the magnets are easy to stick together and difficult to separate. When the proportion of powder with a particle size in the range of 0.5-3μm is above 70%, the particle size of the metal oxide powder is slightly smaller than the size of the main phase grain of the magnet. Batch verification shows that, on the one hand, the particle size of the powder metal oxide can effectively control the diffusion rate of heavy rare earth. On the other hand, since the particle size of the metal oxide powder is slightly smaller than the size of the grain, it will not hinder the diffusion of heavy rare earth along the grain boundary on the grain surface during the diffusion process, and will not damage the surface state of the magnet under the high temperature of diffusion.

[0090] When ethanol is selected as the organic solvent in this invention, it helps to further reduce the additional burden on equipment sealing, venting capacity, safety, etc., and increase equipment costs.

[0091] The total thickness of the composite diffusion layer described in this invention is less than 200 μm, preferably 10 μm to 100 μm. Controlling the thickness of the composite diffusion layer within a certain range is based on the fact that when the thickness is too small, the distribution of heavy rare earth elements is uneven, leading to uneven distribution of heavy rare earth elements diffused into the magnet across the entire magnet. The effect of the RL layer arranged on the magnet surface in adjusting the diffusion concentration is not significant, ultimately resulting in poor magnet uniformity. Conversely, when the thickness is too large, on the one hand, it contains excessive heavy rare earth elements. Excessive heavy rare earth elements cannot completely diffuse into the magnet's interior during heat treatment, forming agglomerates on the magnet surface, eroding the magnet's surface, affecting its surface condition, and wasting raw materials. On the other hand, it contains excessive organic matter, which leads to a large amount of organic matter being released during heat treatment, affecting the atmosphere of the heat treatment apparatus, causing an increase in carbon and oxygen elements in the magnet, ultimately affecting the magnet's performance.

[0092] The heat treatment stage described in this invention employs an alternating low-temperature and high-temperature heat treatment process. In general diffusion processes, the diffusion temperature is relatively uniform. The aim is to ensure that the heavy rare earth elements (RH) arranged on the surface diffuse into the magnet at a consistently high concentration gradient, thereby improving diffusion efficiency. However, in actual diffusion processes, if a single high temperature is consistently used, the grain boundary channels are fixed, leading to a large amount of heavy rare earth elements entering the main phase during diffusion. This fails to achieve the optimal grain boundary diffusion effect. Furthermore, the entry of heavy rare earth elements into the main phase significantly reduces the remanence of the magnet, especially for magnets with a thickness ≥ 4 mm. Due to the larger magnet thickness, simply using a larger layer of heavy rare earth RH on the surface and a higher diffusion temperature will reduce the diffusion effect. Most of the heavy rare earth RH is distributed near the grain boundaries close to the magnet surface, and a large amount of heavy rare earth penetrates into the main phase, resulting in a large difference in Hcj between the magnet surface and the interior, and a significant reduction in the remanence Br of the magnet relative to the matrix. This patent utilizes an alternating arrangement of RL and RH layers on the magnet surface and a low-temperature and high-temperature alternating heat treatment process. During the high-temperature heat treatment holding stage, the grain boundaries are in a fully molten state, providing the driving force for heavy rare earth RH to diffuse into the grain boundaries. The RL layer adjusts the structure of the RH layer to reduce the RH layer concentration difference, preventing the RH layer from accumulating on the magnet surface at high temperatures, which would otherwise cause RH to diffuse into the main phase and reduce the effectiveness of grain boundary diffusion. During the low-temperature heat treatment holding stage, the diffusion rate of the surface-arranged RH layer into the magnet decreases. At this time, the RH diffused into the grain boundaries replaces the Nd elements at the grain boundaries, and the replaced Nd elements are precipitated outside the magnet. By reducing the diffusion of RH into the magnet during the low-temperature heat treatment holding stage, Tb and Nd elements at the grain boundaries are replaced, and some Nd elements are precipitated from the magnet, improving diffusion efficiency, enhancing the uniformity of RH distribution on the magnet, and facilitating RH diffusion into the magnet interior.

[0093] Conventional diffusion processes tend to use higher diffusion temperatures of 870-970℃, based on the assumption that higher temperatures provide greater diffusion motive force, allowing heavy rare earth elements (HREEs) placed on the magnet surface to diffuse deeper along the magnet. However, in actual experiments with magnets thicker than 4mm, since the grain boundaries between the grains in the diffusion channel are fixed, higher diffusion temperatures cause a large amount of heavy rare earth elements to diffuse into the main phase. On the other hand, the accumulation of heavy rare earth elements on the grain boundaries hinders diffusion along the grain boundaries into the interior of the magnet. This results in a significant increase in Hcj on the magnet surface, while the diffusion of heavy rare earth elements along the grain boundaries is hindered in the center, leading to a large difference in Hcj between the magnet surface and the interior. The applicant discovered that during alternating low-temperature and high-temperature heat treatment, with the low-temperature heat treatment temperature range being 750℃-830℃ and the high-temperature heat treatment temperature range being 830℃-970℃, the low-temperature and high-temperature heat treatment times being ≤5h and the holding heat treatment time being ≥8h, the following issues were observed: During the low-temperature heat treatment diffusion process, when the holding temperature was below 750℃, the heavy rare earth RH arranged on the magnet surface could not diffuse effectively due to the low temperature; when the temperature was above 830℃, the diffusion efficiency was too high, and Tb and Nd at the grain boundaries could not be fully replaced, reducing the RH diffusion depth. During the high-temperature heat treatment diffusion process, when the holding temperature was above 970℃, the heavy rare earth RH arranged on the surface directly entered the main phase due to the excessively high temperature, failing to achieve the effect of grain boundary diffusion.

[0094] Furthermore, when the thickness of the magnet in the orientation direction is less than 4 mm, the performance improvement of the magnet is not significant after diffusion by arranging an RL layer on the magnet surface and using an alternating heat preservation process. However, when the magnet thickness is greater than 15 mm, during the heat treatment process, heavy rare earth elements diffuse into the magnet through the liquid-phase grain boundaries. The diffusion process is mainly driven by the concentration difference, and the low concentration difference results in a weak driving force, so the diffusion is a slow process. When the magnet thickness is greater than 15 mm, the RH and RL layers arranged on the magnet surface are thicker, resulting in a large difference in magnetic properties between the magnet surface and the center after diffusion. This leads to a deterioration in magnetic properties such as the squareness of the magnet, ultimately affecting the temperature resistance of the magnet. Attached Figure Description

[0095] Figure 1 This is an EPMA analysis of magnets at different positions on the sintered magnet of Example 1. Detailed Implementation

[0096] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention.

[0097] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0098] The method for analyzing the grain boundary phase and main phase structure in this invention is as follows: the fracture surface of the blank is scanned using EPMA, and the CP image of the EPMA is processed using ImagePRO software to analyze the width and length of the grain boundaries.

[0099] Example 1

[0100] First, prepare neodymium iron boron magnets (R-Fe-B magnets). After processing, the magnets are formed into magnetic sheets. The magnetic sheets are washed with acid solution and deionized water, and then dried to obtain neodymium iron boron magnet M1. The magnetic sheet size is 40mm*20mm*6mm, and the dimensional tolerance is ±0.03mm. The thickness Z in the orientation direction of magnet M1 is 6mm. The composition of M1 is shown in the table below.

[0101] A RH slurry was prepared using terbium hydride powder (a heavy rare earth element), organic solid pine-modified alkyd resin powder, and ethanol, with weight percentages of 60 wt%, 5 wt%, and 35 wt%, respectively. An RL slurry was prepared using zirconium oxide as the metal oxide, organic solid pine-modified alkyd resin powder, and ethanol, with weight percentages of 55 wt%, 5 wt%, and 40 wt%, respectively. A composite diffusion layer was formed by sequentially depositing RH layers, RL layers, and so on, on the magnet surface using a dip-dip heating and air-drying method, resulting in an R-Fe-B magnet blank. The thickness of the RH layer was 20 ± 5 μm, the thickness of the RL layer was 3 ± 2 μm, and the composite diffusion layer accounted for 1.2 ± 0.2% of the weight of magnet M1.

[0102] The magnet blank containing the aforementioned composite diffusion layer is placed in a material box and subjected to diffusion heat treatment in a heat treatment apparatus. The heat treatment process is carried out under vacuum, with heating starting at a vacuum level ≤10Pa. The diffusion heat treatment process is set as follows:

[0103] (1) Heating: (50~780)℃*100min;

[0104] (2) Low-temperature diffusion heat treatment: 780℃*240min;

[0105] (3) Heating: (780~920)℃*30min;

[0106] (4) High-temperature diffusion heat treatment: 920℃*240min;

[0107] (5) Low-temperature diffusion heat treatment: 780*240min (no heating power output during the cooling stage to 780℃);

[0108] (6) Heating: (780~920)℃*30min;

[0109] (7) High-temperature diffusion heat treatment: 920℃*240min.

[0110] After diffusion heat treatment, the magnet was rapidly cooled, followed by aging treatment at 500℃ (aging treatment refers to a heat treatment process in which the properties, shape, and dimensions of an alloy workpiece change over time after solution treatment, cold plastic deformation, casting, or forging, and is placed at a higher temperature or kept at room temperature). After holding at this temperature for 4 hours, it was rapidly cooled again to room temperature to obtain sintered magnet M2. During the heat treatment process, the magnet blanks were arranged in contact, and after diffusion, the sintered magnets showed no adhesion. Energy dispersive spectroscopy (EDS) analysis showed that the surface of M2 was coated with a zirconia powder layer with an average thickness of 5 μm.

[0111] The above products were subjected to the following tests:

[0112] Table 1 Comparison of overall performance between sintered magnet M2 and magnet M1

[0113] Test method: Take a 7mm*7mm*6mm sample from magnets M1 and M2.

[0114] Test equipment: NIM-62000

[0115]

[0116] Table 2 Comparison of Hcj at the surface of sintered magnet M2 and Hcj at a distance of 2mm from the magnet surface

[0117] Testing method: such as Figure 1 As shown, 1mm*1mm*1mm samples were taken from the surface of the sintered magnet M2 and at a distance of 2mm from the surface, respectively, designated as H1 and H2. The difference in Hcj between positions H1 and H2 was 39kA / m. EPMA analysis of Tb showed that in field of view 1 selected at position H1, Tb was mainly distributed on the grain boundaries, and no large amount of Tb was found to enter the main phase. In field of view 2 selected at position H2, there was obvious Tb element on the grain boundaries, and the distribution of Tb element at the grain boundaries was relatively uniform after diffusion.

[0118] Test equipment: PFM06

[0119]

[0120] Table 3 Comparison of main components of sintered magnet M2 and magnet M1

[0121] Test equipment: Spectrometer

[0122]

[0123] Note: The rest are Fe.

[0124] The results show that, using this method, the remanence Br of M2 is reduced by only about 0.011T compared to M1, while Hcj is increased by about 820kA / m. The composition test shows that M2 has about 0.41wt% more Tb than M1.

[0125] Table 4 Comparison of C and O element content analysis between sintered magnet M2 and magnet M1

[0126] Test equipment: CS analyzer, ONH analyzer

[0127]

[0128] Table 4 shows that the CSON element content before and after magnet diffusion did not increase significantly, indicating that the impurity elements formed by the slurry during the diffusion process did not enter the magnet.

[0129] Example 2

[0130] The NdFeB magnet M1 of the same embodiment 1 has a magnetic sheet size of 40mm*30mm*8mm and a dimensional tolerance of ±0.03mm. A composite diffusion layer is arranged on its surface: an RH slurry is prepared using terbium fluoride powder (a heavy rare earth element), organic solid pine modified alkyd resin powder, and ethanol, with weight percentages of 55wt%, 5wt%, and 40wt%, respectively. An RL slurry is prepared using alumina as the metal oxide, organic solid pine modified alkyd resin powder, and ethanol, with weight percentages of 50wt%, 6wt%, and 44wt%, respectively. The RH layer, RL layer, and RH layer are sequentially arranged on the magnet surface using a roller coating heating and air drying method to form a composite diffusion layer, resulting in an R-Fe-B magnet blank with an oxide surface. The thickness of the RH layer is 25±5μm, the thickness of the RL layer is 3±2μm, and the composite diffusion layer accounts for 0.9±0.2% of the weight of magnet M1.

[0131] The aforementioned magnet blanks were placed in a material box and subjected to diffusion heat treatment in a heat treatment apparatus. The heating process was set as follows: 50~780℃*100min + 780℃*180min + 780~920℃*30min + 920℃*240min + 780℃*360min (no heating power output during heat treatment) + 780~920℃*30min + 920℃*360min rapid cooling. After rapid cooling, the temperature was raised to 520℃ for aging treatment, held for 4 hours, and then rapidly cooled to room temperature to obtain magnet M3. Energy dispersive spectroscopy (EDS) analysis showed that the surface of M3 was coated with alumina powder. During the heat treatment process, the magnet blanks were arranged in contact, and after diffusion sintering, the magnets did not adhere together.

[0132] Table 5 Performance Comparison of Sintered Magnet M3 and Magnet M1

[0133]

[0134] Table 5 shows that, using this method, the remanence Br of M3 decreases by only about 0.015T compared to M1, while Hcj increases by about 868kA / m.

[0135] Table 6 Comparison of Hcj at the surface of sintered magnet M3 and Hcj at a distance of 2mm from the magnet surface

[0136] Test method: Take 1mm*1mm*1mm samples from the surface of sintered magnet M3 and at a distance of 2mm from the surface. The difference in Hcj between positions H1 and H2 is 38kA / m.

[0137] Test equipment: PFM06

[0138]

[0139] The exemplary embodiments of the present invention have been described above. However, the scope of protection of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An R-Fe-B sintered magnet, characterized in that, The R-Fe-B sintered magnet has an oxide coating layer on its surface; and The Hcj on the surface of the R-Fe-B sintered magnet along the magnet orientation direction is H1, and the Hcj from the magnet surface along the magnet orientation direction to a depth of 2.00±0.02 mm inside the magnet is H2. The relationship between H1 and H2 is as shown in equation (I): H1-H2≤50kA / m (I); The R-Fe-B sintered magnet is obtained by heat treatment of an R-Fe-B magnet blank; the R-Fe-B magnet blank includes an R-Fe-B magnet and a composite diffusion layer, the composite diffusion layer being on the surface of the R-Fe-B magnet; the thickness of the R-Fe-B magnet in the magnet orientation direction is Z, and Z≥3.95mm; The composite diffusion layer comprises an RH layer and an RL layer, wherein the RH layer and the RL layer are each at least one independent layer; the RH layer and the RL layer are arranged alternately in sequence, with the outermost layer furthest from the surface of the R-Fe-B magnet being the RL layer, wherein: The RH layer contains heavy rare earth elements, organic solids, and solvents that may or may not be present; the heavy rare earth elements are selected from at least one of dysprosium metal, terbium metal, dysprosium hydride, terbium hydride, dysprosium fluoride, terbium fluoride, dysprosium oxide, and terbium oxide. The RL layer comprises a metal oxide, an organic solid, and a solvent, which may or may not be present; the metal oxide is selected from at least one of zirconium oxide, calcium oxide, aluminum oxide, and holmium oxide. After the composite diffusion layer undergoes heat treatment, the metal oxides therein form an oxide coating layer; the heat treatment includes alternating low-temperature heat treatment and high-temperature heat treatment; the temperature range of the low-temperature heat treatment is 750℃-830℃; the temperature range of the high-temperature heat treatment is 830℃-970℃.

2. The R-Fe-B sintered magnet according to claim 1, characterized in that, The thickness of the oxide adhesive layer is less than 20 μm; The oxide coating layer includes at least one of zirconium oxide, calcium oxide, aluminum oxide, and holmium oxide.

3. The R-Fe-B sintered magnet according to claim 1, characterized in that, The thickness of the oxide adhesive layer is less than or equal to 10 μm.

4. The R-Fe-B sintered magnet according to claim 1, characterized in that, The oxide coating layer is removed by non-mechanical grinding. The R-Fe-B magnet blank includes an R-Fe-B magnet and a composite diffusion layer, wherein the composite diffusion layer is on the surface of the R-Fe-B magnet.

5. The R-Fe-B sintered magnet according to claim 1, characterized in that, 15.05mm ≥ Z ≥ 3.95mm; the dimensional tolerance of Z is ±0.05mm; In the R-Fe-B magnet, R is selected from any one or more of the rare earth elements Nd, Pr, Tb, Dy, Gd, and Ho; In the R-Fe-B magnet, the R content is 27-34 wt%; The B content in the R-Fe-B magnet is 0.8-1.3 wt%. The R-Fe-B magnet further comprises Fe and M, wherein M is selected from at least one of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo; In the R-Fe-B magnet, the content of M is 0-5wt%.

6. The R-Fe-B sintered magnet according to claim 1, characterized in that, The total thickness of the composite diffusion layer is less than 200 μm; The organic solid is selected from at least one of rosin-modified alkyd resin, thermoplastic phenolic resin, urea-formaldehyde resin, and polyvinyl butyral; The solvent is selected from at least one of alcohol solvents, ether solvents, and aromatic solvents; The metal oxide is in powder form, and the particle size of the metal oxide is selected from 0.

5. 10μm, of which the particle size is between 0.5 The mass percentage of metal oxide powder with a particle size between 3 μm is over 70%; The thickness of the RH layer is selected from 0.5-40 μm; The thickness of the RL layer is selected from 0.5-15 μm; The weight of the composite diffusion layer is 0.1-3 wt% of the weight of the R-Fe-B magnet.

7. The R-Fe-B sintered magnet according to claim 1, characterized in that, The solvent is selected from alcohol solvents.

8. The method for preparing the R-Fe-B sintered magnet according to any one of claims 1-7, characterized in that, The preparation method includes the following steps: (1) A composite diffusion layer is coated and arranged on the surface of an R-Fe-B magnet to form the R-Fe-B magnet blank; the coating and arrangement of the composite diffusion layer includes coating the R-Fe-B magnet surface with RH layer slurry and RL layer slurry, and then drying to form the composite diffusion layer; the dried composite diffusion layer is composed of alternating RH layer and RL layer. (2) The R-Fe-B magnet blank is subjected to heat treatment in a vacuum or inert atmosphere to obtain a sintered magnet with an oxide coating on the surface; The heat treatment includes alternating low-temperature heat treatment and high-temperature heat treatment, wherein the temperature range of the low-temperature heat treatment is 750℃-830℃, and the temperature range of the high-temperature heat treatment is 830℃-970℃.

9. The method for preparing R-Fe-B sintered magnets according to claim 8, characterized in that, The coating method shall be at least one of the following: brush coating, roller coating, dip coating, or spray coating. After drying, the weight of the R-Fe-B magnet blank increased by 0.1-3 wt% compared to the R-Fe-B magnet; the total time of the heat treatment was ≥8 hours. The preparation method further includes heat treatment followed by aging heat treatment; The aging and heat preservation treatment includes heat preservation heat treatment, rapid cooling to room temperature, then heating to 430-650℃ for aging treatment, heat preservation for 1-72 hours, and then rapid cooling to room temperature.

10. The method for preparing R-Fe-B sintered magnets according to claim 8, characterized in that, The RH layer slurry comprises heavy rare earth elements, organic solids, and solvents; the mass ratio of heavy rare earth elements, organic solids, and solvents is 40-70:3-10:20-50. The RL layer slurry comprises metal oxides, organic solids, and solvents; the mass ratio of metal oxides, organic solids, and solvents is 30-70:3-10:20-50. The coating process also includes applying RH layer slurry and RL layer slurry multiple times to the surface of the R-Fe-B magnet; The heat treatment or aging heat treatment is carried out in a vacuum or inert atmosphere.

11. The method for preparing R-Fe-B sintered magnets according to claim 8, characterized in that, The RL layer slurry comprises 55 wt% zirconium oxide, 5 wt% rosin-modified alkyd resin, and 40 wt% ethanol; the RH layer slurry comprises 60 wt% terbium fluoride, 5 wt% rosin-modified alkyd resin, and 35 wt% ethanol. Alternatively, the RL layer slurry comprises 50 wt% alumina, 6 wt% rosin-modified alkyd resin, and 44 wt% ethanol; the RH layer slurry comprises 55 wt% terbium fluoride, 5 wt% rosin-modified alkyd resin, and 40 wt% ethanol.

12. The application of the R-Fe-B sintered magnet according to any one of claims 1-7 in the fields of automobiles, wind power generation, household motors, medical devices or mobile communication appliances.

13. The application according to claim 12, characterized in that, The aforementioned R-Fe-B sintered magnet is used in the field of new energy vehicles.

14. An electric motor, characterized in that, The motor includes the R-Fe-B sintered magnet as described in any one of claims 1-7.

15. The motor according to claim 14, characterized in that, The motors include power output motors, steering EPS motors, and micro motors; the micro motors include electric water pump motors, steering linkage fog light motors, sunroof motors, air conditioning motors, and windshield wiper motors.

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