A method for preparing a grain boundary diffusion thin film and a NdFeB magnet with a multi-layer structure

Abstract

The present invention discloses a grain boundary diffusion thin film with a multi-layer structure and a preparation method of a NdFeB magnet; the above-mentioned multi-layer grain boundary diffusion thin film includes a first auxiliary diffusion layer, a heavy rare earth diffusion source layer, a second auxiliary diffusion layer and a functional protection layer arranged successively from bottom to top. In the process of preparing the NdFeB magnet according to the present invention, the first auxiliary diffusion layer diffuses into the magnet interior along the grain boundaries under the action of capillary effect to construct a diffusion channel suitable for the diffusion of heavy rare earths. After the material in the second auxiliary diffusion layer melts, it continuously diffuses into the heavy rare earth diffusion layer to form a coating on the material in the heavy rare earth diffusion layer, reducing the direct contact between the material in the heavy rare earth diffusion layer and the magnet surface, avoiding excessive waste of heavy rare earths, and improving the coercivity of the magnet; the high temperature resistance and oxidation resistance characteristics of the functional protection layer avoid the oxidation of the diffused substances, reduce the consumption of the HRE diffusion source, prevent the adhesion phenomenon between magnets, and improve the loading capacity.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of neodymium-iron-boron magnets, and particularly relates to a grain boundary diffusion thin film with a multi-layer structure and a preparation method of a neodymium-iron-boron magnet. Background Art

[0002] Rare earth permanent magnets are widely used in electric motors, wind turbines and other electronic devices. Sintered neodymium-iron-boron magnets have excellent magnetic properties and are important rare earth permanent magnets. However, ternary alloy sintered neodymium-iron-boron magnets have problems of insufficient intrinsic coercivity and low-temperature stability. Heavy rare earth elements (HRE, Dy or Tb) can significantly improve the magnetic anisotropy field (Ha) and temperature stability of the RE2Fe14B phase by substituting Nd atoms in the Nd2Fe14B phase. Due to the increasing demand for rare earth elements and the limited rare earth resources, the grain boundary diffusion process (GBDP) is used to efficiently utilize rare earth resources. GBDP can weaken the magnetic coupling between adjacent Nd2Fe14B grains by increasing the proportion of the grain boundary phase and optimizing the distribution of the grain boundary phase. At the same time, the substitution of HRE for Nd atoms can form a (Nd, HRE)2Fe14B shell layer on the surface of Nd2Fe14B grains, providing a higher anisotropy field Ha. Both of these aspects contribute to improving the coercivity of sintered neodymium-iron-boron magnets.

[0003] Considering the role and cost of the diffusion source, various types of diffusion sources have been developed for GBDP, such as oxides, hydrides, fluorides, alloys and pure metals. However, due to the different diffusion rates of different elements and the lattice and grain boundary diffusion of HRE in GBDP, there must be a gradient distribution of HRE in the diffusion magnet. In addition, simulation results show that an overly thick (Nd, HRE)2Fe14B shell layer (exceeding 15 nm) cannot effectively further improve the coercivity of the magnet. The diffusion depth of HRE in thick magnets is limited, the gradient is large, and the formation of the anti-core / shell structure will also have a negative impact on the magnetic properties. Therefore, increasing the diffusion depth and reducing the formation of overly thick (Nd, HRE)2Fe14B shell layers are of great significance for further improving the utilization rate of HRE.

[0004] To improve the uniformity of HRE in diffusion magnets, a two-step diffusion process is currently proposed. By performing low-temperature diffusion before high-temperature diffusion of the Pr60Tb10Cu15Al15 alloy, it promotes the diffusion of Tb atoms along the grain boundaries into the magnet interior. Additionally, a combined diffusion method using the Pr65Cu15Al20 alloy to construct continuous grain boundaries has been proposed to improve the Tb distribution during the subsequent Pr50Tb30Al20 diffusion process. Another method is to use the Nd80Cu20 alloy for the first-step diffusion, followed by the diffusion of the Tb20Dy10Nd40Cu30 alloy containing HRE. In hot-deformed magnets, this method improves the directivity of the demagnetization curve of the diffusion magnet through Nd80Cu20 diffusion.

[0005] The above research shows that the methods of using heavy rare earth low-melting point alloy diffusion and using auxiliary alloys to construct diffusion channels and then performing heavy rare earth diffusion can effectively increase the diffusion depth, reduce the formation of overly thick shells, and improve the utilization rate of HRE. However, there are still many deficiencies in the above methods:

[0006] 1. When using a low-melting point alloy containing heavy rare earth for coating diffusion, although it can increase the diffusion depth, the single-layer diffusion substance is directly coated on the magnet surface. During high-temperature diffusion, the diffusion substance is in direct contact with the magnet surface, and the Tb element will still inevitably exchange elements with the surface Nd-Fe-B grains, forming a relatively thick outer shell, and even forming an anti-core / shell structure.

[0007] 2. Using a low-melting point alloy containing heavy rare earth for coating diffusion will increase the coating amount of the diffusion substance on the magnet surface. During mass production, to ensure production efficiency, the magnets coated with the diffusion substance need to be stacked and loaded into the sintering furnace. However, during the high-temperature diffusion sintering process, as the surface diffusion substance increases, it is easy to cause adhesion on the contact surfaces between the stacked magnets, and seriously, the sintered products will be damaged and scrapped.

[0008] 3. The method of using auxiliary alloys to construct diffusion channels and then performing heavy rare earth diffusion requires two coatings and two heat treatment processes, resulting in low production efficiency and increased production costs and delivery cycles. Summary of the Invention

[0009] The purpose of the present invention is to provide a method for preparing a grain boundary diffusion thin film and a neodymium iron boron magnet with a multi-layer structure to overcome the deficiencies of the prior art. It has a relatively thin (Nd, HRE)2Fe14B outer shell, no adhesion will occur between the magnets, the production efficiency is high, and the performance of the magnets is excellent.

[0010] To achieve the above object, the technical solution adopted by the present invention is:

[0011] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0012] 1. During the thermal diffusion process of the grain boundary diffusion thin film with a multi-layer structure of the present invention, the first auxiliary diffusion layer diffuses into the magnet along the grain boundaries under the action of capillary effect to construct a diffusion channel suitable for heavy rare earth diffusion. The material in the second auxiliary diffusion layer melts and continuously diffuses into the heavy rare earth diffusion layer, coating the material in the heavy rare earth diffusion layer, reducing the direct contact between the material in the heavy rare earth diffusion layer and the magnet surface, thereby avoiding excessive waste of heavy rare earth and the formation of a relatively thick outer shell.

[0013] At the same time, with the further progress of diffusion, heavy rare earth enters the magnet interior along the grain boundaries from the magnet surface under the synergistic action of the first auxiliary diffusion layer, diffuses to the surface layer of the main phase grains to replace Nd in the Nd2Fe14B phase, and forms a (HRE,Nd)2Fe14B phase with a higher Ha. During the tempering process, due to the existence of the second auxiliary diffusion layer, the grain boundary microstructure is improved, making the grain boundary rare earth-rich phase more continuous, the grain boundaries clearer, and more effective in isolating the exchange coupling effect. Eventually, the coercivity of the magnet is significantly increased, and the remanence and maximum magnetic energy product of the magnet do not decrease significantly.

[0014] 2. During the entire diffusion process, the high temperature resistance and oxidation resistance characteristics of the functional protection layer avoid the oxidation of the diffused substances and reduce the unnecessary consumption of the HRE diffusion source.

[0015] 3. During the high temperature diffusion sintering process, due to the existence of the functional protection layer, the contact between magnets does not cause adhesion, thereby increasing the loading capacity and improving the production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The technical solution of the present invention will be further described below with reference to the accompanying drawings:

[0017] Figure 1 It is a schematic structural diagram of the grain boundary diffusion thin film with a multi-layer structure in an embodiment of the present invention;

[0018] Figure 2 It is a schematic flow diagram of the preparation method of the neodymium iron boron magnet in an embodiment of the present invention;

[0019] Figure 3 It is a schematic structural diagram of the grain boundary diffusion thin film with a multi-layer structure prepared by the casting method attached to a peelable carrier in an embodiment of the present invention;

[0020] Figure 4 It is a schematic structural diagram of the diffused magnets stacked and loaded into a sintering furnace for diffusion heat treatment in an embodiment of the present invention;

[0021] Among them: the first auxiliary diffusion layer 1, the heavy rare earth diffusion source layer 2, the second auxiliary diffusion layer 3, the functional protection layer 4, the peelable carrier 5, and the substrate 6. Detailed implementation mode

[0022] In order to enable those skilled in the art to better understand the solution of this application, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of this application.

[0023] For the convenience of understanding, the specific process in the embodiments of this application will be described below. Please refer to Figure 1 , a grain boundary diffusion thin film with a multi-layer structure in the embodiments of this application includes a first auxiliary diffusion layer 1, a heavy rare earth diffusion source layer 2, a second auxiliary diffusion layer 3, and a functional protection layer 4 arranged from bottom to top in sequence.

[0024] In this embodiment, the materials of the first auxiliary diffusion layer 1 and the second auxiliary diffusion layer 3 are selected from low-melting-point alloys, the low-melting-point alloy is Nd-M alloy or Pr-M alloy, and M is one or more selected from Al, Cu or Ni. The low-melting-point alloy is more preferably Nd70Cu30, Nd70Al15Cu15, Pr70Cu30 or Pr70Al15Cu15.

[0025] In this embodiment, the heavy rare earth diffusion source layer 2 is selected from HRE compounds or HRE-M alloy powders, the HRE is one or more selected from Dy, Tb, Gd or Ho, the HRE compound is one or more selected from oxides, fluorides, hydrides, chlorides or nitrates of HRE, and M is one or more selected from Fe, Co, Bi, Al, Ca, Mg, O, C, N, Cu, Zn, In, Si, S, P, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta or W; among them, the thickness of the heavy rare earth diffusion source layer 2 is 5-200um.

[0026] In this embodiment, the functional protection layer 4 is a high-temperature resistant and oxidation-resistant coating, and the coating material is selected from a metal powder of one of the IVB, VB, VIB or VIIB groups of the periodic table of Mo, W, Nb, Ta, Ti, Hf, Zr, Ti, V, Re or an alloy powder of the above materials, or one or more of oxides, fluorides, hydrides, chlorides or nitrates of the above materials. Among them, the thickness of the functional protection layer 4 is 1-10um.

[0027] In addition, in this embodiment, the ratio of the sum of the thicknesses of the first auxiliary diffusion layer 1 and the second auxiliary diffusion layer 3 to the thickness of the heavy rare earth diffusion source layer 2 is 2:3, and the ratio of the thickness of the auxiliary diffusion layer 1 to the thickness of the auxiliary diffusion layer 2 is 1:1.

[0028] In the grain boundary diffusion thin film of the multi-layer structure of this embodiment, the ratio of the particle size of the alloy powder to the particle size of the alloy crystal alloy powder in the auxiliary diffusion layer is 2:1.

[0029] Refer to Figure 2 , the present invention also discloses a preparation method of a NdFeB magnet based on a grain boundary diffusion thin film with a multi-layer structure, including the following steps:

[0030] S1) Prepare a sintered NdFeB blank for grain boundary diffusion treatment;

[0031] Among them, the components of the prepared sintered NdFeB blank are composed by mass percentage, including: Pr-Nd: 29% - 32%, Co: 0.8 - 1.2%, Nb: 0.5 - 1.0%, B: 1.0% - 1.2%, Cu: 0 - 0.2%, Ti: 0 - 0.2%, Zr: 0.1 - 0.3%, and the balance is Fe.

[0032] S2) Process the sintered NdFeB blank into a substrate for grain boundary diffusion treatment and perform surface treatment;

[0033] S3) Deposit a multi-layer structure grain boundary diffusion thin film on the substrate to obtain a magnet to be diffused;

[0034] In step S3, methods such as casting, spin coating, spraying, screen printing, etc. can be used to deposit a multi-layer structure grain boundary diffusion thin film on the substrate to obtain a magnet to be diffused.

[0035] The following lists two methods of casting and spin coating to specifically illustrate the process of step S3:

[0036] Refer to Figure 3 , in the casting method, the finally obtained multi-layer structure grain boundary diffusion thin film is attached to the peelable carrier 5.

[0037] The steps of preparing the magnet to be diffused by the casting method are as follows:

[0038] S31) Take the binder and put it into the liquid dispersant, stir to dissolve the binder, and note that the weight ratio of the binder to the liquid dispersant is 1:9;

[0039] S32) Put the dissolved colloid into the glove box, open the nitrogen outlet valve of the glove box, open the nitrogen inlet valve, let nitrogen evacuate the air in the glove box, and make the oxygen content lower than 0.01%;

[0040] S33) Add the low-melting-point metal powder of the first auxiliary diffusion layer material to the colloid in multiple batches. Stir evenly each time until it is completely added, and then extend the stirring time by 5 minutes to ensure that the low-melting-point metal powder is completely covered by the colloid, obtaining the first auxiliary diffusion slurry. Among them, the material of the first auxiliary diffusion layer 1 has a mass percentage of 1%-90% in the slurry, a purity of more than 99.5%, and a powder particle size of 0.1μm-100μm;

[0041] S34) Repeat the steps of S31 to S34 to respectively prepare the heavy rare earth diffusion source slurry, the second auxiliary diffusion slurry, and the functional protective layer slurry;

[0042] S35) Place the first auxiliary diffusion slurry, the heavy rare earth diffusion source slurry, the second auxiliary diffusion slurry, and the functional protective layer slurry in different feed tanks, and then form the auxiliary diffusion layer 1, the heavy rare earth diffusion source layer, the auxiliary diffusion layer 2, and the functional protective layer on the flexible peelable carrier 5 in sequence by the casting method, finally obtaining the grain boundary diffusion thin film with a multi-layer structure; the thickness deviation of the formed diffusion source thin film can be minimized to 0.1μm at least, and the diffusion source thin film is flexible and has excellent adhesion to the diffusion substrate;

[0043] S36) Peel the obtained grain boundary diffusion thin film with a multi-layer structure from the peelable carrier and stick it to the substrate, finally obtaining the magnet to be diffused with the grain boundary diffusion thin film with a multi-layer structure deposited on it.

[0044] Among them, the adhesive is PVB resin, the liquid dispersant is terpineol, and PVB resin and terpineol are green environmental protection materials and will not cause a burden on the environment.

[0045] In this embodiment, the grain boundary diffusion thin film with a multi-layer structure provided by the casting method can obtain the HRE diffusion source with any shape corresponding to the tile-shaped magnet, the ring-shaped magnet, or the irregular non-planar special-shaped magnet by arbitrary cutting. By laminating the flexible HRE diffusion source on the surface of the non-planar special-shaped magnet, the related problems of grain boundary diffusion of the special-shaped magnet can be solved, and a magnet with an increased Hcj (coercivity), and without a sharp decrease in Br (residual magnetism) and SQ (squareness) can be obtained.

[0046] In addition, the preparation method using the spin coating method is as follows:

[0047] S31) Add 10-80wt.% of organic solvent and 5wt.% of adhesion promoter to the container, heat it evenly to 60°C, and continuously stir to form a mixture. Among them, the rotation speed of the mixer used for stirring the mixture is 60-120 revolutions per minute, and the stirring time is 15 minutes;

[0048] S32) Cool down the dissolved mixture to 30 °C, place it in a glove box, open the nitrogen outlet valve of the glove box, and open the nitrogen inlet valve to evacuate the air in the glove box with nitrogen, so that the oxygen content is lower than 0.01%. Then add 20 - 80 wt.% of the material powder corresponding to the first auxiliary diffusion layer and 0 - 5 wt.% of the particle dispersant and continue stirring to form the first auxiliary diffusion layer slurry for spin coating. Among them, the rotation speed of the stirrer used for stirring is 50 - 60 revolutions per minute, and the stirring time is 40 minutes;

[0049] S33) Repeat the steps of S21 - S22 for the material powders corresponding to the heavy rare earth diffusion source layer, the second auxiliary diffusion layer, and the functional protection layer respectively to obtain the heavy rare earth diffusion source slurry, the second auxiliary diffusion slurry, and the functional protection layer slurry for spin coating respectively;

[0050] S34) Use a pipette to suck up the prepared first auxiliary diffusion layer slurry, drop the first auxiliary diffusion layer slurry onto the surface of the substrate, and then spin coat the first auxiliary diffusion layer on the surface of the substrate using a spin coater. The rotation speed used for spin coating is 1500 - 3500 r / min, and the spin coating time is 10 - 60 s;

[0051] S35) Dry the substrate with the first auxiliary diffusion layer spin - coated. Among them, the drying temperature is 100 - 120 °C, and the time is 8 - 15 min;

[0052] S36) Repeat the steps of S34 - S35 for the heavy rare earth diffusion source slurry, the second auxiliary diffusion slurry, and the functional protection layer slurry in sequence, so as to form the heavy rare earth diffusion source layer, the second auxiliary diffusion layer, and the functional protection layer in sequence above the first auxiliary diffusion layer, and finally obtain the magnet to be diffused with a multi - layer structure grain boundary diffusion film deposited.

[0053] In this embodiment, the organic solvent is terpineol, the adhesion promoter is PVB resin promoter, and the particle dispersant is cellulose derivative.

[0054] S4) Stack and load the magnet to be diffused into a sintering furnace for diffusion heat treatment.

[0055] In this step S4, first perform diffusion treatment at 650 - 750 °C for 10 - 20 h, which is the first - stage diffusion heat treatment; then perform diffusion treatment at 900 - 1000 °C for 3 - 6 h, which is the second - stage diffusion heat treatment, and finally perform low - temperature tempering treatment at 400 °C - 600 °C for 4 - 8 hours to finally obtain the diffused magnet.

[0056] To sum up, refer to Figure 4 , the structure of stacking and loading the magnet to be diffused into the sintering furnace is as shown in the figure, that is, one layer of grain boundary diffusion film, one layer of substrate, and then continue with one layer of grain boundary diffusion film and one layer of substrate for continuous thermal diffusion treatment.

[0057] When the magnet to be diffused is subjected to diffusion heat treatment, during the first-stage diffusion heat treatment process, the materials in the first auxiliary diffusion layer 1 and the second auxiliary diffusion layer 3 first melt. After melting, the first auxiliary diffusion layer diffuses along the grain boundaries into the magnet under the action of capillary effect, thus constructing a diffusion channel suitable for heavy rare earth diffusion.

[0058] And after the materials in the second auxiliary diffusion layer 3 melt, they continuously diffuse into the heavy rare earth diffusion layer 2, covering the materials in the heavy rare earth diffusion layer 2, reducing the direct contact between the materials in the heavy rare earth diffusion layer and the magnet surface, thereby avoiding excessive waste of heavy rare earth.

[0059] With the further progress of diffusion, heavy rare earth enters the magnet interior along the grain boundaries from the magnet surface under the synergistic action of the first auxiliary diffusion layer 1, diffuses to the surface layer of the main phase grains. The first-stage diffusion heat treatment can effectively increase the diffusion depth, reduce the formation of an overly thick shell layer, and improve the utilization rate of HRE.

[0060] During the second-stage diffusion heat treatment process, the heavy rare earth diffused to the surface layer of the main phase grains replaces Nd in the Nd2Fe14B phase to form a (HRE,Nd)2Fe14B phase with a higher Ha; during the tempering process, due to the existence of the first auxiliary diffusion layer 1 and the second auxiliary diffusion layer 3, the grain boundary microstructure is improved, making the grain boundary rare earth-rich phase more continuous and the grain boundaries clearer, which is more effective in isolating the exchange coupling effect. Eventually, the coercivity of the magnet is significantly improved, and the remanence and maximum magnetic energy product of the magnet do not decrease significantly.

[0061] During the entire diffusion process, the high-temperature resistance and oxidation resistance characteristics of the functional protective layer 4 avoid the oxidation of the diffused substances and reduce the unnecessary consumption of HRE.

[0062] At the same time, during the high-temperature diffusion sintering process, due to the existence of the functional protective layer 4, the contact between magnets does not cause adhesion phenomena, thereby increasing the loading capacity and improving the production efficiency.

[0063] Multiple embodiments are listed below for illustration.

[0064] Example 1

[0065] The grain boundary diffusion is carried out by depositing a grain boundary diffusion thin film with a multi-layer structure on the magnet surface by the tape casting method.

[0066] 1) Take the binder and put it into the liquid dispersant, stir to dissolve the binder, and note that the weight ratio of the binder to the liquid dispersant is 1:9;

[0067] 2) The dissolved colloid is placed in the glove box. Open the nitrogen outlet valve of the glove box and the nitrogen inlet valve to evacuate the air in the glove box with nitrogen, so that the oxygen content is lower than 0.01%.

[0068] 3) Add Pr70Al15Cu15 alloy powder with a particle size of 3 μm to the colloid in multiple batches. Stir evenly each time until it is completely added, and then extend the stirring time by 5 minutes to ensure that the alloy powder is completely covered by the colloid, obtaining the first auxiliary diffusion slurry; the mass percentage of Pr70Al15Cu15 alloy powder in the slurry is 75 wt.%, and the purity is above 99.5%;

[0069] 4) Repeat the above steps 1 - 3 to separately prepare a heavy rare earth diffusion source slurry (Tb70Al15Cu15 powder with a particle size of 6 μm, the mass percentage of Tb70Al15Cu15 alloy powder in the slurry is 75 wt.%), a second auxiliary diffusion slurry (Pr70Al15Cu15 alloy powder with a particle size of 3 μm, the mass percentage of Pr70Al15Cu15 alloy powder in the slurry is 75 wt.%), and a functional protective layer slurry (molybdenum sulfide powder with a particle size of 3 μm, the mass percentage of molybdenum sulfide powder in the slurry is 75 wt.%);

[0070] 5) Place the first auxiliary diffusion slurry, heavy rare earth diffusion source slurry, second auxiliary diffusion slurry, and functional protective layer slurry in different tanks, and then sequentially form a first auxiliary diffusion layer, a heavy rare earth diffusion source layer, a second auxiliary diffusion layer, and a functional protective layer on a flexible and peelable substrate by the doctor - blade method, finally obtaining a grain - boundary diffusion thin film with a multi - layer structure; in this thin film, the film thickness of the heavy rare earth diffusion source layer is 10 - 50 μm (Examples 1.1 - 1.5), and the film thickness of the functional protective layer is 5 μm;

[0071] 6) Prepare a rare earth sintered magnet, which has the following mass percentage composition: Pr - Nd: 30%, Co: 0.8%, Nb: 0.5%, B: 0.97%, Cu: 0.1%, Ti: 0.2%, Zr: 0.1%, and the balance is Fe; it is prepared according to the existing processes of melting, strip casting, hydrogen decrepitation, jet milling, orientation pressing, sintering, and heat treatment of rare earth magnets;

[0072] 7) Process the processed sintered magnet into a square magnet with dimensions of 10 mm * 10 mm * 4 mm, and the 4 - mm direction is the magnetic field orientation direction;

[0073] 8) Then peel the obtained multi - layer structure grain - boundary diffusion thin film from the peelable carrier and stick it onto a substrate to obtain the magnet to be diffused;

[0074] 9) Stack and load the magnet to be diffused into a sintering furnace for diffusion heat treatment;

[0075] 10) Detect the magnetic properties of the diffused magnet using a 264Y permanent magnet characteristic automatic measuring instrument from Mianyang Bipolar, and the measurement temperature is 20 °C;

[0076] 11) A single-layer Tb70Al15Cu15 diffusion thin film was deposited on the diffusion substrate by traditional screen printing as a comparative example, where the film thickness of the Pr70Al15Cu15 diffusion layer was controlled at 10 - 50 μm (Comparative Examples 1.1 - 1.5).

[0077] The magnetic property evaluation of the examples is shown in Table 1.

[0078] Table 1 Magnetic property evaluation of the examples

[0079]

[0080]

[0081] In Examples 1.1 - 1.5, no adhesive sheets were found, indicating that the functional protective layer can effectively avoid the problem of diffusion magnet adhesion; no oxidation and rusting were found on the magnet surface and no blank areas of the diffusion source powder were observed, indicating that the multi-layered grain boundary diffusion thin film prepared by the casting method has excellent adhesion to the diffusion substrate.

[0082] In addition, as can be seen from Table 1, the multi-layered grain boundary diffusion thin films with different film thicknesses all have a promoting effect on the coercivity of the diffusion substrate. As the film thickness increases, the increment of coercivity continues to increase, but Br (residual magnetism) and SQ (squareness) do not decrease sharply.

[0083] At the same time, the magnetic properties of the magnets in Examples 1.1 - 1.5 are significantly better than those of the comparative examples, indicating that using the multi-layered grain boundary diffusion thin film for grain boundary diffusion can achieve more excellent magnetic properties.

[0084] Example 2

[0085] Grain boundary diffusion was carried out by directly depositing a multi-layered grain boundary diffusion thin film on the magnet surface using the spin coating method.

[0086] 1) Prepare a rare earth sintered magnet with the following mass percentage composition, including: Pr-Nd: 30%, Co: 0.8%, Nb: 0.5%, B: 0.97%, Cu: 0.1%, Ti: 0.2%, Zr: 0.1% and the balance Fe;

[0087] 2) Process the processed sintered magnet into a square magnet of 10 mm * 10 mm * 4 mm, with the 4 mm direction being the magnetic field orientation direction;

[0088] 3) Add 20 wt.% of terpineol and 5 wt.% of PVB resin promoter into a container, heat it evenly to 60 °C, and continuously stir to form a mixture;

[0089] 4) Cool the dissolved first mixture to 30 °C, place it in a glove box, open the nitrogen outlet valve of the glove box, and open the nitrogen inlet valve to evacuate the air in the glove box with nitrogen, so that the oxygen content is lower than 0.01%. Then add 70 wt.% of Pr70Al15Cu15 alloy powder with a particle size of 3 um and 5 wt.% of cellulose derivative and continue stirring to form the first auxiliary diffusion layer slurry for spin coating;

[0090] 5) Repeat the steps of 3-4 to respectively prepare a heavy rare earth diffusion source slurry for spin coating (Tb70Al15Cu15 powder with a particle size of 6 um, and the mass percentage of Tb70Al15Cu15 alloy powder in the slurry is 75 wt.%), a second auxiliary diffusion slurry (Pr70Al15Cu15 alloy powder with a particle size of 3 um, and the mass percentage of Pr70Al15Cu15 alloy powder in the slurry is 75 wt.%), and a functional protective layer slurry (molybdenum sulfide powder with a particle size of 3 um, and the mass percentage of molybdenum sulfide powder in the slurry is 75 wt.%).

[0091] 6) Use a pipette to suck up the prepared first auxiliary diffusion layer slurry, drop the first auxiliary diffusion layer slurry onto the surface of the substrate, and then spin coat the first auxiliary diffusion layer on the surface of the substrate using a spin coater. The rotation speed used for spin coating is 2500 r / min, and the spin coating time is 30 s;

[0092] 7) Bake the substrate with the first auxiliary diffusion layer spin-coated. Among them, the baking temperature is 100-120 °C, and the time is 8-15 min;

[0093] 8) Repeat the steps of 6-7 to sequentially form a heavy rare earth diffusion source layer, a second auxiliary diffusion layer, and a functional protective layer on the substrate with the first auxiliary diffusion layer 1, and finally obtain a magnet to be diffused with a multi-layer structure grain boundary diffusion thin film deposited;

[0094] 9) Stack and load the magnet to be diffused into a sintering furnace for diffusion heat treatment;

[0095] 10) Detect the magnetic properties of the diffused magnet using a 264Y permanent magnet characteristic automatic measuring instrument of Mianyang Bipolar, and the measurement temperature is 20 °C;

[0096] 11) Deposit a single-layer Pr70Al15Cu15 diffusion thin film on the diffusion substrate by traditional screen printing as a comparative example, where the film thickness of the Pr70Al15Cu15 diffusion layer is controlled at 20 um (Comparative Example 2.1), and at the same time, Example 1.2 is also used as a comparative example (Comparative Example 2.2).

[0097] The magnetic property evaluation of Example 2 is shown in Table 2.

[0098] Table 2 Magnetic property evaluation of the examples

[0099]

[0100] As can be seen from Table 2, when the spin coating method is used to directly prepare a multi-layered grain boundary diffusion film on the surface of the magnet for grain boundary diffusion, the magnetic properties obtained are not much different from those of the grain boundary diffusion of the multi-layered grain boundary diffusion film prepared by casting, and both are better than those of the single-layer Pr70Al15Cu15 diffusion film deposited on the diffusion substrate by traditional screen printing.

[0101] Therefore, it can be known that better magnetic properties can be obtained by using the multi-layered grain boundary diffusion film for grain boundary diffusion.

[0102] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A grain boundary diffusion thin film with a multi-layer structure, characterized in that, Including: A first auxiliary diffusion layer, a heavy rare earth diffusion source layer, a second auxiliary diffusion layer, and a functional protective layer arranged successively from bottom to top; Both the first auxiliary diffusion layer and the second auxiliary diffusion layer are low melting point alloys; the low melting point alloy is Nd-M alloy or Pr-M alloy, where M is one or more selected from Al, Cu, or Ni; The heavy rare earth diffusion source layer is a HRE compound or a HRE-M alloy powder; The HRE is one or more of Dy, Tb, Gd, or Ho, the HRE compound is one or more of oxides, fluorides, hydrides, chlorides, or nitrates of HRE, and M is one or more of Fe, Co, Bi, Al, Ca, Mg, O, C, N, Cu, Zn, In, Si, S, P, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, or W; The functional protective layer is molybdenum sulfide powder.

2. The grain boundary diffusion thin film of the multi-layer structure according to claim 1, wherein: The ratio of the sum of the thicknesses of the first auxiliary diffusion layer and the second auxiliary diffusion layer to the thickness of the heavy rare earth diffusion source layer is 2:

3.

3. A preparation method of a neodymium-iron-boron magnet with a grain boundary diffusion film based on a multi-layer structure, characterized in that, Including the following steps: S1) Prepare a sintered NdFeB blank for grain boundary diffusion treatment; S2) Process the sintered NdFeB blank into a substrate for grain boundary diffusion treatment and perform surface treatment; S3) Deposit the multi-layer structure grain boundary diffusion thin film as described in claim 1 on the substrate to obtain a magnet to be diffused; S4) Stack and load the magnet to be diffused into a sintering furnace for diffusion heat treatment; Among them, the steps of S3 are as follows: S31) Add 10-80 wt.% of an organic solvent and 5 wt.% of an adhesion promoter to a container, uniformly heat to 60°C, and continuously stir to form a mixture; S32) Cool the dissolved mixture to 30°C, place it in a glove box, open the nitrogen outlet valve of the glove box, open the nitrogen inlet valve, let nitrogen evacuate the air in the glove box, make the oxygen content less than 0.01%, and then add 20-80 wt.% of the powder of the material corresponding to the first auxiliary diffusion layer and 0-5 wt.% of a particulate dispersant and continue to stir to form a first auxiliary diffusion slurry for spin coating; S33) Repeat the steps of S31-S32 for the powder of the materials corresponding to the heavy rare earth diffusion source layer, the second auxiliary diffusion layer, and the functional protective layer to respectively obtain a heavy rare earth diffusion source slurry, a second auxiliary diffusion slurry, and a functional protective layer slurry for spin coating; S34) Use a pipette to suck the prepared first auxiliary diffusion layer slurry, drop the first auxiliary diffusion layer slurry onto the surface of the substrate, and then spin coat the first auxiliary diffusion layer on the surface of the substrate using a spin coater; S35) Dry the substrate with the first auxiliary diffusion layer spin-coated; S36) Repeat the steps of S34-S35 for the heavy rare earth diffusion source slurry, the second auxiliary diffusion slurry, and the functional protective layer slurry in sequence, so as to successively form a heavy rare earth diffusion source layer, a second auxiliary diffusion layer, and a functional protective layer above the first auxiliary diffusion layer, and finally obtain a magnet to be diffused deposited with a multi-layer structure grain boundary diffusion thin film.

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