An R-T-B series sintered magnet with high magnetic property consistency, its preparation method and application

Abstract

The present invention provides an R-T-B series sintered magnet with high magnetic property consistency, a preparation method thereof, and an application thereof. The thickness of the R-T-B series sintered magnet of the present invention in the orientation direction and / or the non-orientation direction is greater than 3 mm and less than or equal to 10 mm; along the orientation direction and / or the non-orientation direction of the magnet to the geometric center of the magnet, the content of the heavy rare earth element RH shows a gradually decreasing trend; in the orientation direction of the sintered magnet, the maximum intrinsic coercivity iHc(1) at 100 μm from the magnet surface and the intrinsic coercivity iHc(2) at the geometric center of the magnet have the following relational expression: iHc(1) - iHc(2) ≤ 60 kA / m (Equation I). The R-T-B series sintered magnet of the present invention is prepared by the grain boundary diffusion method, has good consistency of the intrinsic coercivity inside, and is suitable for applications in environments with relatively high working temperatures such as automotive drive motors.

Description

Technical Field

[0001] The present invention relates to an R-T-B series sintered magnet with high magnetic property consistency, a preparation method thereof, and an application thereof, belonging to the field of rare earth permanent magnet materials. Background Art

[0002] Since the invention of the R-T-B series rare earth sintered magnet in the 1980s, due to its high strength, high magnetic flux density, temperature resistance characteristics, and low cost, it has been widely used in many industrial fields such as hybrid electric vehicles, electric vehicle drive motors, air-conditioning compressors, elevators, magnetic resonance equipment, and disk drives.

[0003] When the R-T-B series sintered magnet is used at high temperature, the residual magnetic flux density will be lower than that at room temperature, manifested as a decrease in the magnetic flux of the magnet; when the coercivity value of the magnet is too low, the reduced magnetic flux at high temperature cannot be fully restored when the magnet returns to room temperature, manifested as an irreversible decrease in the magnetic flux, thus affecting the normal operation of the motor.

[0004] As a method commonly used in the industry to improve the coercivity of the R-T-B series sintered magnet, in the initial stage of magnet production, adding Dy and / or Tb (hereinafter collectively referred to as RH) during the melting process is a commonly used method; or making a main phase alloy containing no or a small amount of RH, and a secondary alloy containing a large amount of RH, and mixing the alloy scales or powders of the two in a certain proportion for pressing and sintering to make a magnet is also commonly used in the industry. For a long time, the above methods have been widely used to improve the coercivity of the R-T-B series sintered magnet, and thus improve its use temperature. However, the above methods have significant disadvantages. On the one hand, since the magnetic moment of RH is opposite to that of Nd and Fe, the reverse magnetic coupling causes a decrease in the magnetic moment after RH enters the main phase; on the other hand, since the reserves of RH elements in nature are small and the price is expensive, both resource and cost allocation considerations limit its large-scale use.

[0005] In recent years, the grain boundary diffusion method has been popularized in the production industry of NdFeB magnets. It is an effective method to greatly improve the coercivity of sintered magnets of the R-T-B system, avoid a significant reduction in the remanent flux density of the magnets, and improve the utilization rate of RH. Its implementation method is to melt the grain boundaries of the R-T-B system sintered magnet at high temperature, and at the same time diffuse the RH attached to the magnet surface into the magnet interior through the grain boundaries. Since the main phase grains are wrapped by a highly anisotropic RH-rich phase near the grain boundaries, the coercivity of the R-T-B system sintered magnet is improved; at the same time, since the remanent flux density is mainly determined by the main phase of the magnet, and the speed of RH along the liquid grain boundary from the magnet surface to the center is much greater than the speed of RH diffusing into the main phase, and the volume of the grain boundary is much smaller than the volume of the main phase, so the grain boundary diffusion hardly affects the main phase of the magnet, and thus hardly causes a reduction in the remanent flux density; and making RH only distributed in the grain boundaries that account for a very small volume ratio of the magnet, enabling it to play a role in improving the coercivity of the magnet and avoiding its entry into the main phase causing waste, which also greatly improves the utilization rate of RH.

[0006] Currently, the commonly used grain boundary diffusion methods in the industry mainly include coating, dipping with RH compounds or metal powders, sputtering, evaporation plating of RH metals or their alloys, etc., covering the surface of the R-T-B system sintered magnet with RH, and then making the RH on the magnet surface enter the interior of the sintered magnet through the grain boundaries by heat treatment. In addition, through the adjustment of the base material formula, the process and parameter control during the normal production process, the R-T-B system sintered magnets manufactured by the grain boundary diffusion method exhibit different characteristics.

[0007] In Patent Document 1, a sintered magnet of the R-T-B-M system with R2T14B rich in Dy existing in the main phase shell of the grains is disclosed. In order to be able to achieve the existence of the above structure in the entire sintered magnet as a whole, the alloy flakes of the R-T-B-M system sintered magnet are pre-treated by diffusion, and then the processes of powder making, molding and heat treatment are carried out in sequence. Although using the above method can achieve the effect of increasing the coercivity of the sintered magnet and is not limited by the diffusion depth of the grain boundary diffusion, the increase in coercivity is very limited. The patent examples show that compared with not using the patent method, the coercivity of the magnet using the patent method increases by about 80 kA / m.

[0008] In Patent Document 2, a NdFeB sintered magnet is disclosed in which the grain boundaries reached by RH diffusion reach a depth of more than 2.5 mm from the surface, having a high coercive force HcJ and relatively high values of the maximum magnetic energy product (BH)max and the squareness ratio SQ. It is proposed in this patent that there is a large difference in the heavy rare earth between the magnet surface and the depth of 2.5 mm from the surface, and the heavy rare earth content at the center position is already very low and can hardly be measured at 3.0 mm. That is, for a magnet steel with a diffusion direction greater than 5 mm, the heavy rare earth diffusion at the center position of the magnet can hardly reach, or the difference in coercivity between the part of the magnet near the diffusion surface and the center part of the magnet is large.

[0009] Patent documents cited:

[0010] Patent document 1: CN102361998A;

[0011] Patent document 2: CN106098281A. Summary of the invention

[0012] To solve the above problems, the present invention provides an R-T-B series sintered magnet with high magnetic property consistency, a preparation method thereof and an application. The present invention is formed by the grain boundary diffusion method, in which the heavy rare earth metal RH diffuses along the grain boundaries of the R-T-B series sintered magnet substrate.

[0013] The present invention provides an R-T-B series sintered magnet, the thickness of the sintered magnet along the orientation direction and / or the non-orientation direction is greater than 3 mm and less than or equal to 10 mm; along the orientation direction and / or the non-orientation direction of the magnet to the geometric center of the magnet, the content of the heavy rare earth element RH shows a gradually decreasing trend; in the orientation direction of the sintered magnet, the maximum intrinsic coercivity iHc(1) at 100 μm from the magnet surface and the intrinsic coercivity iHc(2) at the geometric center of the magnet have the following relationship:

[0014] iHc(1) - iHc(2) ≤ 60 kA / m (Equation I).

[0015] It should be noted that in the present invention, the orientation direction refers to the magnetic field direction of the magnet; the non-orientation direction refers to any direction orthogonal to the orientation direction. The diffusion direction in the present invention refers to the direction in which the heavy rare earth element RH on any surface of the magnet diffuses along the orientation direction or the non-orientation direction of the magnet to the geometric center of the magnet. In the present invention, the content of the heavy rare earth element RH in the sintered magnet specifically shows a gradually decreasing trend along any diffusion direction.

[0016] In the present invention, 100 μm from the magnet surface and the geometric center of the magnet mean that a corresponding part of the section is intercepted on the sintered magnet, and the intrinsic coercivity and the content of the heavy rare earth element RH are respectively measured. Preferably, both the intrinsic coercivity and the content of the heavy rare earth element RH refer to the average value on the section.

[0017] According to the present invention, the heavy rare earth element RH is selected from at least one of Dy and Tb.

[0018] According to the present invention, in the R-T-B series sintered magnet, after being treated by the grain boundary diffusion method, the content increment of the heavy rare earth element RH is ≥ 0.2 wt%, preferably 0.2 - 1.5 wt%.

[0019] According to the present invention, the R-T-B series sintered magnet includes:

[0020] R is selected from at least one of Nd, Pr, Dy, Tb, Ho, and Gd, and its content is preferably 27-34 wt%, for example, 27-30 wt%.

[0021] B, and its content is preferably 0.8-1.3 wt%.

[0022] T is selected from Fe and M, where M is selected from at least one of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, and Mo; the content of M is preferably 0-5 wt%, preferably 0-3 wt%, for example, 2 wt%.

[0023] According to the present invention, in the R-T-B series sintered magnet, along the orientation direction, the content C1 (mass%) of the heavy rare earth element RH at 100 μm from the magnet surface to the geometric center of the magnet and the content C2 (mass%) of the heavy rare earth element RH at the geometric center of the magnet satisfy at least one of the following formula II or formula ①:

[0024] C1 - C2 ≤ 0.4 wt% (formula II);

[0025] 0.64e -0.185t1 + 0.15 ≤ C1 - C2 ≤ 0.64e -0.185t1 + 0.25 formula ①;

[0026] Wherein, the units of C1 and C2 are wt%; t1 is the thickness value of the sintered magnet along the diffusion direction, and the unit of t1 is mm.

[0027] According to the present invention, in the non-orientation direction of the sintered magnet, the maximum value of the intrinsic coercivity iHc(1) at 100 μm from the magnet surface and the intrinsic coercivity iHc(2) at the geometric center of the magnet have the following relationship:

[0028] iHc(3) - iHc(4) ≤ 80 kA / m (formula III).

[0029] According to the present invention, in the R-T-B series sintered magnet, along the non-orientation direction, the content C3 (mass%) of the heavy rare earth element RH at 100 μm symmetric from the magnet surface to the geometric center of the magnet and the content C4 (mass%) of the heavy rare earth element RH at the geometric center of the magnet satisfy at least one of formula IV or formula ②:

[0030] C3 - C4 ≤ 0.6 wt% (formula IV);

[0031] -0.28ln(t2) + 0.83 ≤ C3 - C4 ≤ -0.28ln(t2) + 1.33 formula ②;

[0032] Among them, the units of C3 and C4 are wt%; t2 is the thickness value of the sintered magnet along the diffusion direction, and the unit of t1 is mm.

[0033] According to the present invention, the sintered magnet has a regular or irregular three-dimensional structure, and the three-dimensional structure is, for example, a cube, a cuboid, a C-shaped (tile-shaped) or a D-shaped (bread-shaped).

[0034] The present invention also provides a method for preparing the above R-T-B series sintered magnet, preferably the grain boundary diffusion method, and the preparation method includes the following steps:

[0035] 1) Prepare a sintered magnet substrate;

[0036] 2) Arrange an RH diffusion layer on at least two surfaces of the sintered magnet substrate;

[0037] 3) Diffuse the sintered magnet substrate with an RH diffusion layer arranged on the surface in step 2) to obtain the R-T-B series sintered magnet.

[0038] According to the present invention, in step 1), the raw materials of the substrate include:

[0039] R, selected from at least one of Nd, Pr, Dy, Tb, Ho, Gd;

[0040] T is selected from Fe and M, where M is selected from one or several of Ti, V, Cr, Mn, Co, Ni, Ga, Ca, Cu, Zn, Si, Al, Mg, Zr, Nb, Hf, Ta, W, Mo;

[0041] B.

[0042] According to the present invention, the raw materials of the substrate have the stoichiometric numbers as described above.

[0043] According to the present invention, in step 1), preparing the sintered magnet substrate specifically includes separately preparing a first powder and a second powder, and after mixing the first powder and the second powder, obtaining the sintered magnet substrate.

[0044] Preferably, the average powder particle size of the first powder is s1, the average powder particle size of the second powder is s2, and 0.3 μm ≤ s1 - s2 ≤ 1.0 μm. Further preferably, 2.5 μm ≤ s1 ≤ 3.2 μm. Further preferably, 2.0 μm ≤ s2 ≤ 2.7 μm.

[0045] Preferably, the mass ratio of the first powder to the second powder is 1:(1 - 5), preferably 1:(1 - 3), for example, 1:1, 1:2, 1:3, 1:4, 1:5.

[0046] Preferably, in the present invention, the method for preparing the sintered magnet substrate is not specifically limited and can be selected from the methods known in the technical field. Exemplarily, the method for preparing the sintered magnet substrate includes: melting, powder making, pressing, and sintering. Exemplarily, the melting includes melting the raw materials of the above-mentioned base material in an inert gas environment and obtaining a rapidly solidified sheet by rapid cooling, wherein the melting temperature is 1400 - 1500 °C, the rotation speed of the rapid cooling roll is 10 - 100 r / min, and the thickness of the rapidly solidified sheet is 0.3 mm - 0.4 mm. Exemplarily, the powder making includes subjecting the rapidly solidified sheet to hydrogen explosion powder making and jet milling to obtain the first powder and the second powder having the above-mentioned particle sizes. Exemplarily, the pressing includes mixing the first powder and the second powder with a mass ratio of 1:1 and subjecting them to orientation pressing and forming under the action of a magnetic field to obtain a green body, for example, in a magnetic field of 15 kOe, and for example, adding a lubricant known in the technical field such as calcium stearate during mixing. Exemplarily, the sintering includes sintering the pressed green body in an Ar atmosphere at 100 - 1500 °C for 1 - 10 h to obtain the sintered magnet substrate.

[0047] According to the present invention, in step 2), the RH diffusion layer includes the heavy rare earth metal RH. Preferably, the heavy rare earth metal RH has the meaning as described above. Exemplarily, the RH diffusion layer includes at least one of a metal of RH, a compound of RH, or an alloy of RH, preferably a pure metal of RH or a hydride of RH.

[0048] According to the present invention, the thickness of the sintered magnet substrate in the orientation direction and / or non-orientation direction is greater than 3 mm and less than or equal to 10 mm.

[0049] According to the present invention, the sintered magnet has a regular or irregular three-dimensional structure, and the three-dimensional structure is, for example, a cube, a cuboid, a C-type (tile type), or a D-type (bread type).

[0050] According to the present invention, in step 2), the at least two surfaces include two opposite surfaces parallel and / or perpendicular to the orientation direction, preferably including two opposite surfaces parallel and perpendicular to the orientation direction.

[0051] According to the present invention, in step 2), arranging the diffusion layer can be achieved by methods known in the industry, for example, by at least one method such as sputtering, evaporation coating, or organic coating.

[0052] According to the present invention, in step 3), the diffusion treatment includes performing a first heat treatment and a second heat treatment successively under temperature conditions of T1 and T2, wherein T1 < T2.

[0053] Preferably, 850 °C ≤ T1 ≤ 930 °C.

[0054] Preferably, the time of the first heat treatment is 5 - 20 h.

[0055] Preferably, 930 °C ≤ T2 ≤ 980 °C.

[0056] Preferably, the time of the second heat treatment is 10 - 24 h.

[0057] The present invention also provides an R-T-B sintered magnet prepared by the above preparation method of the R-T-B sintered magnet, and the R-T-B sintered magnet has the meaning as described above.

[0058] The present invention also provides the application of the above R-T-B sintered magnet in the fields of hybrid electric vehicles, electric vehicle drive motors, air-conditioning compressors, elevators, magnetic resonance equipment, disk drives, etc.

[0059] The beneficial effects of the present invention are:

[0060] The R-T-B sintered magnet of the present invention is prepared by the grain boundary diffusion method, which can save the usage amount of heavy rare earths; the thickness of the sintered magnet is greater than 3 mm and less than or equal to 10 mm, so that the application range of the magnet prepared by the grain boundary diffusion method is wider.

[0061] For the R-T-B sintered magnet of the present invention, the internal intrinsic coercivity has good consistency and has a low coercivity deviation. Since magnetization reversal is likely to occur at positions with relatively low coercivity, the risk of demagnetization at positions with low coercivity in the magnet at high temperature can be greatly reduced. Therefore, the magnet with a small coercivity deviation also has better thermal demagnetization characteristics, and is particularly suitable for applications in working environments with relatively high temperatures such as automotive drive motors.

[0062] The preparation method of the R-T-B neodymium iron boron sintered magnet of the present invention has the characteristics of simple operation, high efficiency and easy implementation, and has extremely high practical significance. Detailed embodiments

[0063] The present invention provides an R-T-B sintered magnet, and the R-T-B sintered magnet at least includes: a heavy rare earth element RH, and the heavy rare earth element RH is selected from at least one of Dy and Tb.

[0064] According to the present invention, after the R-T-B sintered magnet is treated by the grain boundary diffusion method, the content increment of the heavy rare earth element RH is ≥ 0.2 wt%, preferably 0.2 - 1.5 wt%.

[0065] According to the present invention, the R-T-B sintered magnet further includes:

[0066] R is selected from at least one of Nd, Pr, Dy, Tb, Ho, and Gd, and its content is preferably 27 - 34 wt%, for example, 27 - 30 wt%;

[0067] B, and its content is preferably 0.8 - 1.3 wt%.

[0068] T is selected from Fe and M, where M is selected from at least one of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, Mo; the content of M is preferably 0 - 5 wt%, more preferably 0 - 3 wt%, for example, 2 wt%.

[0069] According to the present invention, the thickness of the sintered magnet along the orientation direction is greater than 3 mm and less than or equal to 10 mm; along the orientation direction of the magnet to the geometric center of the magnet, the content of the heavy rare earth element RH shows a gradually decreasing trend; in the orientation direction of the sintered magnet, the relationship between the maximum intrinsic coercivity iHc(1) at 100 μm from the magnet surface and the intrinsic coercivity iHc(2) at the geometric center of the magnet is as follows:

[0070] iHc(1) - iHc(2) ≤ 60 kA / m (Equation I).

[0071] The inventors found that when the thickness along the orientation direction of the magnet is greater than 10 mm, for the sintered magnet prepared by the preparation method of the present invention, iHc(1) - iHc(2) > 60 kA / m. The inventors also found that although extending the heat treatment time of the diffusion process may satisfy the range of iHc(1) - iHc(2) ≤ 60 kA / m, considering cost factors, practicality, and mass producibility, the thickness of the sintered magnet is limited within the above range.

[0072] In the technical field, the grain boundary diffusion method mostly adopts processes such as evaporation diffusion, magnetron sputtering, surface coating, electrophoresis, etc., to deposit the metal of the heavy rare earth element RH, the compound of RH, or the alloy of RH on the surface of the substrate or the magnet, forming a thin film containing the heavy rare earth element. The above methods make the surface of the magnet contact with a large amount of heavy rare earth elements, and the surface of the magnet will contain an excessive amount of heavy rare earth elements. At the same time, through high-temperature heat treatment and low-temperature aging, the surface of the magnet treated by the above methods shows good magnetic properties: the coercivity increases significantly and the remanence hardly decreases or decreases very little. However, the performance of the magnet surface does not represent the overall performance of the magnet. To more accurately grasp the performance of the magnet, a cross-section at 100 μm from the surface is selected. At this cross-section, the heavy rare earth element RH has been fully diffused but will not accumulate excessively. The performance of the magnet is relatively more stable, and the continuous grain boundary phase is obvious. The maximum intrinsic coercivity iHc, the content of the heavy rare earth element RH, etc. at this cross-section can all be used as parameter indicators for the stable performance of the magnet.

[0073] According to the present invention, in the R-T-B series sintered magnet, along the orientation direction, the RH content C1 (mass %) at a position 100 μm from the magnet surface to the geometric center of the magnet and the heavy rare earth element RH content C2 (mass %) at the geometric center of the magnet satisfy at least the following formula II:

[0074] C1 - C2 ≤ 0.4 wt% (Formula II);

[0075] Wherein, the units of C1 and C2 are wt%.

[0076] The inventors found that when RH diffuses along the orientation direction of the sintered magnet to the geometric center, along the orientation direction of the magnet to the geometric center of the magnet, the content of the heavy rare earth element RH shows a gradually decreasing trend. When the thickness of the sintered magnet along the diffusion direction is less than or equal to 10 mm, using the preparation method of the present invention, the sintered magnet can satisfy both Formula I and Formula II; when the thickness of the sintered magnet along the diffusion direction exceeds 10 mm, the preparation method of the present invention cannot satisfy the ranges of the above Formula I and Formula II at the same time.

[0077] The inventors further studied and experimented and found that when the thickness of the sintered magnet along the diffusion direction does not exceed 10 mm, for the R-T-B series sintered magnet of the present invention, the RH content C1 at a position 100 μm from the surface to the center of the magnet along the orientation direction of the magnet and the heavy rare earth content C2 (mass %) at the center of the magnet satisfy 0.64e -0.185t1 + 0.15 ≤ C1 - C2 ≤ 0.64e -0.185t1 + 0.25 (Relationship ①), where t1 is the thickness value of the magnet along the diffusion direction.

[0078] According to the present invention, the heavy rare earth content of each part of the R-T-B series sintered magnet is measured by an X-ray fluorescence spectrometer (XRF).

[0079] According to the present invention, in the non-orientation direction of the sintered magnet, the maximum value of the intrinsic coercivity iHc(3) at a position 100 μm from the magnet surface and the intrinsic coercivity iHc(4) at the geometric center of the magnet have the following relationship:

[0080] iHc(3) - iHc(4) ≤ 80 kA / m (Formula III).

[0081] The inventor found that when the magnet diffuses from the surface of the magnet along one of the non-oriented directions towards the center, the difference in the intrinsic coercivity iHc(3) at 100 μm from the surface towards the center along the non-oriented direction and the intrinsic coercivity iHc(4) at the center position of the magnet, iHc(3) - iHc(4), is higher than the difference in iHc(1) - iHc(2). The inventor found that when the thickness of the magnet along the non-oriented direction is greater than 10 mm, for the sintered magnet prepared by the preparation method of the present invention, iHc(1) - iHc(2) > 60 kA / m. The inventor also found that although using the method in the patent, although extending the heat treatment time of the diffusion process may meet the range of iHc(3) - iHc(4) ≤ 80 kA / m, considering cost factors, practicality, and mass producibility, the thickness of the sintered magnet is limited within the above range.

[0082] According to the present invention, along the non-oriented direction, the content C3 (mass %) of the heavy rare earth element RH at 100 μm from the surface of the magnet to the geometric center of the magnet and the content C4 (mass %) of the heavy rare earth element RH at the geometric center of the magnet at least satisfy Equation IV:

[0083] C3 - C4 ≤ 0.6 wt% (Equation IV);

[0084] wherein the units of C3 and C4 are wt%.

[0085] The inventor found that when RH diffuses along the non-oriented direction of the sintered magnet to the geometric center, along the non-oriented direction of the magnet to the geometric center of the magnet, the content of the heavy rare earth element RH shows a gradually decreasing trend. When the thickness of the sintered magnet along the diffusion direction is less than or equal to 10 mm, using the preparation method of the present invention, the sintered magnet can also satisfy Equation III and Equation IV simultaneously; while when the thickness of the sintered magnet along the diffusion direction exceeds 10 mm, it is impossible to satisfy the ranges of the above Equation III and Equation IV simultaneously using the preparation method of the present invention.

[0086] The patent inventor further studied and experimented and found that when the thickness of the sintered magnet along the diffusion direction does not exceed 10 mm, the content C3 of RH at 100 μm from the surface to the center of the magnet along the diffusion direction of the magnet and the content C4 (mass %) of the heavy rare earth element at the center of the magnet can also satisfy -0.28ln(t2) + 0.83 ≤ C3 - C4 ≤ -0.28ln(t2) + 1.33 (Relationship Equation ②), where t2 is the thickness value of the magnet along the diffusion direction.

[0087] According to the present invention, the sintered magnet is a regular or irregular three-dimensional structure, and the three-dimensional structure is, for example, a cube, a cuboid, a C-shaped (tile-shaped), or a D-shaped (bread-shaped).

[0088] [Preparation Method of R-T-B Series Sintered Magnet]

[0089] The present invention also provides a method for preparing the above-mentioned R-T-B series sintered magnet, and the preparation method specifically includes:

[0090] 1) Prepare an R-T-B sintered magnet substrate by a method known to those skilled in the art.

[0091] 2) Arrange an RH diffusion layer on at least two surfaces of the sintered magnet substrate;

[0092] 3) Perform a diffusion treatment on the sintered magnet substrate with an RH diffusion layer arranged on its surface in step 2) to obtain the R-T-B series sintered magnet.

[0093] According to the present invention, in step 1), the raw materials of the sintered magnet substrate include:

[0094] R is selected from at least one rare earth element among Nd, Pr, Dy, Tb, Ho, and Gd;

[0095] T is selected from Fe and M, where M is selected from one or several of Ti, V, Cr, Mn, Co, Ni, Ga, Ca, Cu, Zn, Si, Al, Mg, Zr, Nb, Hf, Ta, W, and Mo;

[0096] B.

[0097] According to the present invention, the raw materials of the substrate have the stoichiometric numbers as described above.

[0098] According to the present invention, in the present invention, the method for preparing the sintered magnet substrate is not specifically limited, and a method known in the technical field can be selected. Exemplarily, the method for preparing the sintered magnet substrate includes: melting, powder making, pressing, and sintering.

[0099] Exemplarily, the melting includes melting the raw materials of the above substrate in an inert gas environment and obtaining a rapidly solidified sheet through rapid cooling, where the melting temperature is 1400 - 1500 °C, the rotation speed of the rapid cooling roll is 10 - 100 r / min, and the thickness of the rapidly solidified sheet is 0.3 mm - 0.4 mm. Exemplarily, the powder making includes subjecting the rapidly solidified sheet to hydrogen explosion powder making and jet milling to obtain a first powder and a second powder with the above particle sizes, and the mass ratio of the first powder to the second powder is 1:(1 - 5), preferably 1:(1 - 3), such as 1:1, 1:2, 1:3, 1:4, 1:5. Exemplarily, the pressing includes mixing the first powder and the second powder with a mass ratio of 1:1 and orienting and pressing them into a green body under the action of a magnetic field, for example, in a magnetic field of 15 kOe, and for example, adding a lubricant known in the technical field such as calcium stearate during mixing. Exemplarily, the sintering includes sintering the pressed green body in an Ar atmosphere at 100 - 1500 °C for 1 - 10 h to obtain the sintered magnet substrate.

[0100] Exemplarily, the powder making includes separately preparing a first powder with an average powder particle size of s1 and a second powder with an average powder particle size of s2, and then mixing the two powders, where 0.3 μm ≤ s1 - s2 ≤ 1.0 μm. Preferably, 2.5 μm ≤ s1 ≤ 3.2 μm. Preferably, 2.0 μm ≤ s2 ≤ 2.7 μm. Preferably, in step 2) of the present invention, the RH diffusion layer includes the heavy rare earth metal RH. Preferably, the heavy rare earth metal RH has the meaning as described above. Exemplarily, the RH diffusion layer includes at least one of a metal of RH, a compound of RH, or an alloy of RH, preferably a pure metal of RH or a hydride of RH.

[0101] According to the present invention, the thickness of the sintered magnet substrate in the orientation direction and / or the non-orientation direction is greater than 3 mm and less than or equal to 10 mm.

[0102] According to the present invention, the sintered magnet has a regular or irregular three-dimensional structure, and the three-dimensional structure is, for example, a cube, a cuboid, a C-type (tile type), or a D-type (bread type).

[0103] According to the present invention, in step 2), the at least two surfaces include two opposite surfaces parallel and / or perpendicular to the orientation direction, preferably including two opposite surfaces parallel and perpendicular to the orientation direction.

[0104] According to the present invention, in step 2), the arrangement of the diffusion layer can be achieved by methods known in the industry, for example, by at least one of sputtering, evaporation coating, organic coating, etc.

[0105] According to the present invention, in step 3), the diffusion treatment includes performing a first heat treatment and a second heat treatment successively under temperature conditions of T1 and T2, where T1 < T2.

[0106] Preferably, 850 °C ≤ T1 ≤ 930 °C.

[0107] Preferably, the time of the first heat treatment is 5 - 20 h.

[0108] Preferably, 930 °C ≤ T2 ≤ 980 °C.

[0109] Preferably, the time of the second heat treatment is 10 - 24 h.

[0110] The inventors found that for the sintered magnet matrix prepared by directly mixing magnet powders of different particle sizes with the same raw materials in the present invention, although the particle sizes of the magnet powders are different, they have the same main phase. Therefore, the sintered magnet matrix obtained by pressing has the characteristics of uniform heating, fast heating rate, low sintering temperature, short sintering time, high production efficiency, and fine and uniform grain size. By mixing magnet powders of the same raw materials but different particle sizes and then pressing and sintering to obtain the matrix, the internal bonding force between the magnetic powders during the sintering process can be improved, the possibility of the NdFeB matrix collapsing and the deformation of the NdFeB matrix can be reduced, the forming rate of the obtained sintered magnet matrix can be ensured, and the workload of surface treatment can be reduced. Furthermore, after arranging the diffusion layer, the better the solid solution and penetration effects occur between the heavy rare earth source melt and the NdFeB matrix, the greater the driving force generated, which can better promote the diffusion of the diffusion source into the magnet interior. At the same time, in the present invention, by adopting a sintering process of staged heat treatment with a short holding time, a magnetization layer is formed at the grain boundaries, enabling the heavy rare earth RH to better diffuse along the grain boundary interior rather than excessively entering the main phase, avoiding the formation of an inverse core-shell structure. Thus, less heavy rare earth RH can be used to more effectively increase the coercivity while maintaining the remanence and magnetic energy product.

[0111] [Application]

[0112] The present invention also provides the applications of the above R-T-B series sintered magnets in the fields of hybrid electric vehicles, electric vehicle drive motors, air-conditioning compressors, elevators, magnetic resonance equipment, disk drives, etc.

[0113] The technical solutions 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 only for illustrative explanation of the present invention and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0114] Unless otherwise specified, the raw materials and reagents used in the following embodiments are all commercially available products or can be prepared by known methods.

[0115] In the following embodiments, the prepared sintered magnet is tested for its magnetic properties on a NIM-62000 device; and an X-ray fluorescence spectrometer (XRF) is used to measure the heavy rare earth content at the corresponding position thereof, including: along the magnetic field orientation direction of the sintered magnet, from the magnet surface to 100 μm inside the magnet along a section perpendicular to the orientation direction, and the heavy rare earth content at the section is measured and recorded as C1 (specifically: the heavy rare earth content of a total of 5 measurement points at the four corners and the center of the section is measured, and the average is calculated and recorded as C1); the permanent magnet is oriented along the magnetic field from the magnet surface After cutting the permanent magnet in a direction perpendicular to the orientation direction at half of its length in the orientation direction, the heavy rare earth content at the geometric center of the permanent magnet is measured and recorded as C2 (the calculation method is the same as C1); along the non-magnetic field orientation direction of the sintered magnet, it is cut perpendicularly to the non-orientation direction from the magnet surface to 100 μm inside the magnet, and the heavy rare earth content at the cross section is measured and recorded as C3 (the calculation method is the same as C1); after cutting the permanent magnet in a direction perpendicular to the non-orientation direction from the magnet surface along the non-magnetic field orientation direction at half of its length in the non-orientation direction, the heavy rare earth content at the center of the permanent magnet is measured and recorded as C4 (the calculation method is the same as C1).

[0116] Example 1

[0117] Neodymium, praseodymium, dysprosium, terbium, electrolytic iron, cobalt, copper, gallium, aluminum, zirconium, boron and other raw materials are prepared in the following weight ratios: Nd-23.8wt%, Pr-5wt%, Dy-0.6wt%, Tb-0.4wt%, Co-0.5wt%, Cu-0.13wt%, Ga-0.1wt%, Al-0.1wt%, Zr-0.12wt%, B-1wt%, and the balance is Fe and unavoidable impurities. In an inert gas environment, a quick-setting belt throwing method is used to complete the casting. The casting temperature is 1450°C, and the speed of the quenching roller is 60r / min. Flakes are obtained, and the average thickness thereof is 0.36mm. The flakes are subjected to HD pulverization and air flow milling to be made into powder particles with an average particle size of S1=3.0μm.

[0118] Using the same raw materials and weight ratio as above, in an inert gas environment, the casting is completed by the rapid solidification belt method, the casting temperature is 1550°C, the speed of the quenching roller is 70r / min, and the flakes are obtained, the thickness of which is 0.30mm on average; the flakes are made into powder particles with an average particle size of S2=2.4μm by HD powder making and air flow grinding;

[0119] Take 50 kg of the powder with particle size S1 and 50 kg of the powder with particle size S2, add 5 kg of lubricant calcium stearate, and mix in a mixer for 1 hour to obtain mixed powder;

[0120] The above-mentioned mixed powder particles are oriented and pressed into a green compact in a magnetic field of 15 KOe; the green compact is placed in a sintering furnace under an Ar atmosphere and sintered at 1020 °C for 9 h to obtain a green body, and then aged at 500 °C for 5 h to obtain a sintered magnet substrate.

[0121] The sintered magnet substrate is machined into magnets M1 with dimensions of 40 mm * 20 mm * 8 mm, magnets M2 with dimensions of 40 mm * 20 mm * 10 mm, and magnets M3 with dimensions of 40 mm * 20 mm * 10 mm. Among them, the dimensions along the magnetic field orientation direction are all the minimum dimension directions, which are 8 mm, 10 mm, and 12 mm respectively.

[0122] After degreasing, pickling, activation and washing with deionized water, M1, M2, and M3 are dried. The heavy rare earth hydrogenated terbium (TbH) with an average particle size of 3.4 μm is deposited on the entire surface of magnets M1, M2, and M3 by magnetron sputtering to form an RH diffusion layer with a thickness of 7 μm; then heat treatment is carried out. The heat treatment process includes a first-stage heat treatment at a diffusion temperature of 900 °C for 16 h; a re-heat treatment at a diffusion temperature of 940 °C for 20 h; and a subsequent second-stage heat treatment at 500 °C for 10 h to obtain sintered magnets N1, N2, and N3.

[0123] After degreasing, pickling, activation and washing with deionized water, M1, M2, and M3 are dried. The heavy rare earth hydrogenated terbium (TbH) with an average particle size of 3.4 μm is deposited on the two opposite surfaces perpendicular to the orientation direction of magnets M1, M2, and M3 to form a diffusion layer with a thickness of 7 μm; then heat treatment is carried out. The heat treatment process includes a first-stage heat treatment at a diffusion temperature of 900 °C (within the pressure range of 10 -3 ~10 -4 Pa) for 16 h; a re-heat treatment at a diffusion temperature of 940 °C (within the pressure range of 10 -3 ~10 -4 Pa) for 20 h; and a subsequent second-stage heat treatment at 500 °C for 10 h to obtain sintered magnets P1, P2, and P3.

[0124] After degreasing, pickling, activation and washing with deionized water, M1, M2, and M3 are dried. The heavy rare earth hydrogenated terbium (TbH) with an average particle size of 3.4 μm is deposited on any two opposite surfaces parallel to the orientation direction of magnets M1, M2, and M3 to form a diffusion layer with a thickness of 7 μm; then heat treatment is carried out. The heat treatment process includes a first-stage heat treatment at a diffusion temperature of 900 °C for 16 h; a re-heat treatment at a diffusion temperature of 940 °C for 20 h; and a subsequent second-stage heat treatment at 500 °C for 10 h to obtain sintered magnets Q1, Q2, and Q3.

[0125] Comparative Example 1

[0126] Using M1 in Example 1, after degreasing, pickling, activation and washing with deionized water followed by drying, terbium heavy rare earth hydride (TbH) with an average particle size of 3.4 μm was deposited on the entire surface of the magnet M1 by magnetron sputtering to form a diffusion layer with a thickness of 7 μm; at a temperature of 970 °C, under vacuum conditions (pressure in the range of 10 -3 ~10 -4 Pa), it was treated for 24 h, and then aged at 500 °C for 5 h, and cooled to room temperature by passing Ar to obtain the sintered magnet Q4.

[0127] The performances of magnets N1, N2, N3, P1, P2, P3, Q1, Q2, Q3, and Q4 were detected. For specific data, please refer to Table 1.

[0128] At the same time, for the products of Example N1 and Q4, using the above method, the surface heavy rare earth content C0 of the permanent magnet in the orientation direction was measured by an X-ray fluorescence spectrometer (XRF) to be 5.7 wt% and 5.8 wt% respectively (that is, at four corners + center on the magnet surface, a total of 5 measurement points, and the average of the heavy rare earth contents at these 5 positions was taken). For specific data, please refer to Table 2.

[0129] Table 1 Magnetic properties of magnets in Example 1 and Comparative Example 1 (1)

[0130]

[0131] As can be seen from Table 1, after preparing powder by mixing powder particles with an average particle size of S1 = 3.0 μm and powder particles with an average particle size of S2 = 2.4 μm in a ratio of 1:1 and then sintering the prepared NdFeB sintered magnet, after grain boundary diffusion, the remanence Br of N1, N2, N3, P1, P2, P3, Q1, Q2, and Q3 shows little difference. However, in the sintered magnets with a thickness of 8 mm and 10 mm in the orientation direction, the effect of improving the intrinsic coercivity is obvious. Moreover, for the sintered magnets prepared by the method of the present invention, for the diffusion method of fully coating diffusion and only diffusing two surfaces, although there is a slight difference in the intrinsic coercivity of the diffused products (wherein, the intrinsic coercivity refers to the difference between the maximum intrinsic coercivity iHc(1) at a cross-section 100 μm from the surface to the center along the orientation direction of the magnet and the intrinsic coercivity iHc(2) at the center position of the magnet, or the difference between the maximum intrinsic coercivity iHc(3) at a cross-section 100 μm from the surface to the center along the orientation direction of the magnet and the intrinsic coercivity iHc(4) at the center position of the magnet), for the sintered magnets with a thickness of 8 mm and 10 mm, both can satisfy iHc(1) - iHc(2) ≤ 60 kA / m and iHc(3) - iHc(4) ≤ 80 kA / m, and satisfy the relational expressions ① and ② of the intrinsic coercivity iHc. The sintered magnets with a thickness of 8 mm and 10 mm in the orientation direction satisfy C1 - C2 ≤ 0.4 and C3 - C4 ≤ 0.6. When the thickness of the magnet in the orientation direction is 12 mm and the same diffusion layer is arranged, the product does not satisfy C1 - C2 ≤ 0.4 and C3 - C4 ≤ 0.6, nor does it satisfy the relational expressions ① and ② of the intrinsic coercivity iHc.

[0132] The difference between N1 in Example 1 and Q4 in Comparative Example 1 is only the difference in the sintering system of the diffusion process. For Q4 using single-temperature sintering diffusion, both its Br and Hcj are slightly lower than those of N1. Thus, it can be seen that the present invention adopts a segmented sintering process to perform diffusion treatment on the sintered magnet matrix formed by mixing and compacting powder particles with two different particle sizes. The heavy rare earth RH can just diffuse along the grain boundary phase at different temperature segments and does not diffuse too much into the main phase Nd 2 Fe 14 B, and thus does not damage the crystal structure. Therefore, although it will not cause the Hcj of the sintered magnet to decrease during conventional diffusion, the situation where Br significantly decreases will not occur.

[0133] Table 2 Magnetic Properties of Magnets in Example 1 and Comparative Example 1 (2)

[0134] Product Thickness (mm) Hcj after diffusion (KA / m) C1 (wt%) C2 (wt%) C0 (wt%) N1 8 2177.85 1.67 1.35 5.7 wt% Q4 8 2074.32 1.73 1.21 5.8 wt%

[0135] As can be seen from Table 2, the difference in the RH content on the surfaces of the diffused N1 and Q4 magnets is not significant, and the RH content on the surface of the diffused magnet of Q4 is greater than that of N1. However, it is obvious that the Hcj value of N1 is higher, indicating that the N1 magnet has a stronger anti-demagnetization ability, that is, the effect of high temperature on the demagnetization of N1 is smaller. Thus, it can be seen that the RH content on the magnet surface cannot accurately reflect the magnetic properties and stability of the magnet itself. In addition, through the data comparison in Table 2, the RH content on the surface of the N1 magnet is 5.7 wt%, while the RH content C1 at the cross-section 100 μm from the surface to the center along the magnet orientation direction, the RH content C2 of the heavy rare earth elements at the geometric center of the magnet, and the RH content in the raw materials are 1.67 wt% and 1.35 wt% respectively. Therefore, in the present invention, C1, C2, C3, and C4, which can more accurately reflect the heavy rare earth content inside the magnet, are used to judge the magnetic property stability of the magnet.

[0136] Example 2

[0137] Using the same raw materials and mass ratios as in Example 1, the melting process is adopted, and the casting is completed by the method of rapid solidification and strip casting in an inert gas environment. The casting temperature is 1450 °C, and the rotational speed of the chill roll is 60 r / min, and the average scale thickness obtained is about 0.36 mm; the scales are subjected to HD powder making and air jet milling to make powder particles with an average particle size of S1 = 3.0 μm;

[0138] Using the same raw materials and mass ratios as above, the casting is completed by the method of rapid solidification and strip casting in an inert gas environment. The casting temperature is 1550 °C, and the rotational speed of the chill roll is 70 r / min, and the average scale thickness obtained is about 0.30 mm; the scales are subjected to HD powder making and air jet milling to make powder particles with an average particle size of S2 = 2.4 μm;

[0139] Using the same raw materials and mass ratios as above, the casting is completed by the method of rapid solidification and strip casting in an inert gas environment. The casting temperature is 1500 °C, and the rotational speed of the chill roll is 45 r / min, and the average scale thickness obtained is about 0.41 mm; the scales are subjected to HD powder making and air jet milling to make powder particles with an average particle size of S3 = 3.6 μm;

[0140] Take 100 Kg of the air jet milled powder with a particle size of S1, add 5 Kg of the lubricant calcium stearate, and mix in a mixer for 1 h. Orient and press into a compact in a magnetic field of 15 KOe; put the compact into a sintering furnace under an Ar atmosphere and sinter at 1020 °C for 9 h to obtain a green compact, and then age at 500 °C for 5 h to obtain a sintered magnet matrix. The sintered magnet matrix is machined into a magnet M4 with dimensions of 40 mm * 20 mm * 8 mm.

[0141] Take 100 Kg of airflow milled powder with S2 particle size, and use the same mixing, molding, and sintering processes as the airflow milled powder with S1 particle size. Through machining, the sintered magnet substrate is processed into a magnet M5 with dimensions of 40 mm * 20 mm * 8 mm.

[0142] Take 100 Kg of airflow milled powder with S3 particle size, and use the same mixing, molding, and sintering processes as the airflow milled powder with S1 particle size. Through machining, the sintered magnet substrate is processed into a magnet M6 with dimensions of 40 mm * 20 mm * 8 mm.

[0143] Take 40 Kg of airflow milled powder with S2 particle size and 60 Kg of airflow milled powder with S3 particle size respectively, and use the same mixing, molding, and sintering processes as the airflow milled powder with S1 particle size. Through machining, the sintered magnet substrate is processed into a magnet M7 with dimensions of 40 mm * 20 mm * 8 mm.

[0144] After degreasing, pickling, activation, and washing with deionized water, M4, M5, M6, and M7 are dried. The average particle size of heavy rare earth terbium fluoride (TbF) is 2.8 μm, and it is arranged on the entire surface of magnets M4, M5, M6, and M7 by spraying to form a diffusion layer with a thickness of 7 μm. Then, heat treatment is carried out. The heat treatment process includes a first-stage heat treatment with a diffusion temperature of 900 °C and a holding time of 16 h; a re-heat treatment with a diffusion temperature of 940 °C and a holding time of 20 h; and a subsequent second-stage heat treatment at 500 °C for 10 h to obtain rare earth permanent magnets N4, N5, N6, and N7.

[0145] Detect the performance of magnets N4, N5, N6, and N7.

[0146] Magnetic properties of the magnets in Example 2 in Table 3

[0147]

[0148] From the results in Table 3, it can be seen that NdFeB sintered magnet substrates with the same size are prepared using magnet powder particles with different particle sizes. After being treated by the same diffusion method, the remanence Br of the sintered magnets after diffusion is equivalent. Along the orientation direction and non-orientation direction, the differences between the maximum intrinsic coercivity values iHc(1) and iHc(3) at 100 μm from the magnet surface to the center and the intrinsic coercivity values iHc(2) and iHc(4) at the magnet center position do not all satisfy iHc(1) - iHc(2) ≤ 60 kA / m and iHc(3) - iHc(4) ≤ 80 kA / m. Moreover, when the content of heavy rare earth RH in the sintered magnet does not simultaneously satisfy C1 - C2 ≤ 0.4 and C3 - C4 ≤ 0.6, it does not necessarily simultaneously satisfy the relational expressions ① and ② of the intrinsic coercivity iHc.

[0149] The above describes the exemplary embodiments of the present invention. However, the scope of protection of this application is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made by those skilled in the art within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. An R-T-B series sintered magnet, characterized in that, after arranging an RH diffusion layer on at least two surfaces of the sintered magnet substrate, diffusion treatment is carried out to obtain the R-T-B series sintered magnet; wherein, the sintered magnet substrate is obtained by mixing a first powder with an average powder particle size of s1 and a second powder with an average powder particle size of s2, 0.3μm ≤ s1 - s2 ≤ 1.0μm; 2.5μm ≤ s1 ≤ 3.2μm, 2.0μm ≤ s2 ≤ 2.7μm; the mass ratio of the first powder to the second powder is 1:(1 - 5); the diffusion treatment includes performing a first heat treatment and a second heat treatment successively under temperature conditions of T1 and T2, wherein, T1 < T2, 850°C ≤ T1 ≤ 930°C, 930°C ≤ T2 ≤ 980°C; the thickness of the R-T-B series sintered magnet along the orientation direction and / or non-orientation direction is greater than 3mm and less than or equal to 10mm; along the orientation direction and / or non-orientation direction of the magnet to the geometric center of the magnet, the content of the heavy rare earth element RH shows a gradually decreasing trend; in the orientation direction of the R-T-B series sintered magnet, the maximum intrinsic coercivity iHc(1) at 100μm from the magnet surface and the intrinsic coercivity iHc(2) at the geometric center of the magnet have the following relationship: iHc(1) - iHc(2) ≤ 60kA / m (Equation I); along the orientation direction, the RH content C1 at 100μm from the magnet surface to the geometric center of the magnet and the heavy rare earth element RH content C2 at the geometric center of the magnet at least satisfy the following Equation II: C1 - C2 ≤ 0.4 wt% (Equation II); wherein, the units of C1 and C2 are wt%; in the non-orientation direction of the R-T-B series sintered magnet, the maximum intrinsic coercivity iHc(3) at 100μm from the magnet surface and the intrinsic coercivity iHc(4) at the geometric center of the magnet have the following relationship: iHc(3) - iHc(4) ≤ 80kA / m (Equation III); along the non-orientation direction, the heavy rare earth element RH content C3 at 100μm symmetric from the magnet surface to the geometric center of the magnet and the heavy rare earth element RH content C4 at the geometric center of the magnet at least satisfy one of the following Equation IV or Equation ②: C3 - C4 ≤ 0.6wt% (Equation IV); wherein, the units of C3 and C4 are wt%.

2. The R-T-B series sintered magnet according to claim 1, characterized in that, the heavy rare earth element RH is selected from at least one of Dy and Tb; in the R-T-B series sintered magnet, after being treated by the grain boundary diffusion method, the content increment of the heavy rare earth element RH is ≥ 0.2wt%; the R-T-B series sintered magnet includes: R is selected from at least one of Nd, Pr, Dy, Tb, Ho, Gd, and its content is 27 - 34wt%; B, and its content is 0.8 - 1.3wt%; T is selected from Fe and M, where M is selected from at least one of Ti, V, Cr, Mn, Co, Ga, Cu, Si, Al, Zr, Nb, W, Mo; the content of M is 0 - 5wt%.

3. The R-T-B system sintered magnet according to claim 1, characterized in that in the R-T-B system sintered magnet, along the orientation direction, the content C1 of RH at a distance of 100 μm from the magnet surface to the geometric center of the magnet and the content C2 of the heavy rare earth element RH at the geometric center of the magnet satisfy the following formula ①: 0.64e -0.185t1 +0.15 ≤ C1 - C2 ≤ 0.64e -0.185t1 +0.25 Equation ①; wherein, t1 is the thickness value of the R-T-B system sintered magnet along the diffusion direction, and the unit of t1 is mm.

4. The R-T-B system sintered magnet according to claim 1, characterized in that in the R-T-B system sintered magnet, along the non-orientation direction, the content C3 of the heavy rare earth element RH at a symmetric position 100 μm from the magnet surface to the geometric center of the magnet and the content C4 of the heavy rare earth element RH at the geometric center of the magnet satisfy formula ②: -0.28ln(t2) + 0.83 ≤ C3 - C4 ≤ -0.28ln(t2) + 1.33 Formula ②; wherein, t2 is the thickness value of the R-T-B system sintered magnet along the diffusion direction, and the unit of t1 is mm.

5. The method for preparing the R-T-B system sintered magnet according to any one of claims 1-4, characterized in that the preparation method is the grain boundary diffusion method, and the preparation method includes the following steps: 1) Prepare a sintered magnet substrate; specifically preparing a sintered magnet substrate includes respectively preparing a first powder and a second powder, and after mixing the first powder and the second powder, a sintered magnet substrate is obtained; the average powder particle size of the first powder is s1, the average powder particle size of the second powder is s2, and 0.3 μm ≤ s1 - s2 ≤ 1.0 μm; the mass ratio of the first powder to the second powder is 1:(1 - 5); 2) Arrange an RH diffusion layer on at least two surfaces of the sintered magnet substrate; 3) Perform diffusion treatment on the sintered magnet substrate with the RH diffusion layer arranged on the surface in step 2) to obtain the R-T-B system sintered magnet; the diffusion treatment includes performing a first heat treatment and a second heat treatment under temperature conditions of T1 and T2 in sequence, wherein, T1 < T2.

6. The preparation method according to claim 5, characterized in that in step 1), the raw materials of the substrate include: R, selected from at least one of Nd, Pr, Ho, Gd, and its content is 27 - 34 wt%; T is selected from Fe and M, where M is selected from one or more of Ti, V, Cr, Mn, Co, Ni, Ga, Ca, Cu, Zn, Si, Al, Mg, Zr, Nb, Hf, Ta, W, Mo; the content of M is 0 - 5 wt%; B, its content is 0.8 - 1.3 wt%; and optionally a heavy rare earth element RH, the heavy rare earth element RH is selected from at least one of Dy and Tb.

7. The preparation method according to claim 5, characterized in that in step 2), the RH diffusion layer includes the heavy rare earth element RH; in step 2), the at least two surfaces include two opposite surfaces parallel and / or perpendicular to the orientation direction; in step 2), arranging the diffusion layer adopts at least one of sputtering, evaporation coating, and organic coating.

8. The preparation method according to claim 5, characterized in that The RH diffusion layer comprises at least one of a metal of RH, a compound of RH, or an alloy of RH; In step 2), the at least two surfaces include two opposite surfaces parallel and perpendicular to the orientation direction.

9. The preparation method according to claim 5, characterized in that in step 3), 850 °C ≤ T1 ≤ 930 °C; the time of the first heat treatment is 5 - 20 h; 930℃≤T2≤980℃； the time of the second heat treatment is 10 - 24 h.

10. Application of the R-T-B series sintered magnet according to any one of claims 1 - 4 in a hybrid electric vehicle, an electric vehicle drive motor, an air conditioner compressor, an elevator, a magnetic resonance device, a disk drive.

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