Method for manufacturing grain boundary diffusion material using rare earth oxide and method for manufacturing r-fe-b-based permanent magnet using same

Abstract

The present invention relates to a method for manufacturing a grain boundary diffusion material using a rare earth oxide and a method for manufacturing a R-Fe-B-based permanent magnet using same, whereby a cheap grain boundary diffusion material can be manufactured using a rare earth oxide which is cheaper than expensive rare earth metals. In addition, the R-Fe-B-based permanent magnet in which the grain boundary diffusion material made of the rare earth oxide is grain boundary diffused can exhibit excellent coercive force.

Description

Method for producing grain boundary diffusion material using rare earth oxide and method for producing R-Fe-B permanent magnet using the same

[0001] This specification claims the benefit of Korean Patent Application No. 10-2023-0074839 filed with the Korean Intellectual Property Office on June 12, 2023, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a grain boundary diffusion material using rare earth oxide and a method for manufacturing an R-Fe-B permanent magnet using the same.

[0003] R-Fe-B permanent magnets, like Nd-Fe-B permanent magnets, are used in a variety of fields, including electronics, automotive, medical devices, energy, and transportation. In particular, in line with recent trends toward lighter weight and miniaturization, they are being used in a variety of products, including machine tools, electronic information devices, home appliances, mobile phones, robot motors, wind turbines, small automotive motors, and drive motors.

[0004] For these permanent magnets, a grain boundary diffusion process has been proposed as a post-processing method to improve their magnetic performance. The grain boundary diffusion process utilizes the highly chemically reactive nature of the interface between the permanent magnet and the magnet, coating the surface of the magnet with heavy rare earth elements and then heat-treating them. This grain boundary diffusion process concentrates the heavy rare earth elements around the grain boundaries, i.e., only on the surface of the ferromagnetic grains, forming a core-shell structure in which the grains are surrounded by a layer of high magnetic anisotropy, thereby achieving high coercivity.

[0005] However, there is a problem that the heavy rare earth elements such as Tb and Dy used in the existing interface diffusion method are very expensive.

[0006] The problem to be solved by the present invention is to provide a grain boundary diffusion material having a low melting point and being able to be manufactured inexpensively using rare earth oxides that are cheaper than expensive rare earth metals.

[0007] However, the problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0008] According to one aspect of the present invention, a method for producing a grain boundary diffusion material is provided, comprising: a step of mixing a rare earth oxide, Nd, Cu, and a reducing agent to produce a mixture; a step of heat-treating the mixture and then pulverizing the mixture to produce an alloy powder in which the rare earth oxide is reduced; a step of removing impurities from the alloy powder in which the rare earth oxide is reduced by mixing it with a solution containing ammonium nitrate and water; and a step of removing an oxide layer on the surface of the alloy powder by mixing the alloy powder in which the impurities are removed by mixing it with a solution containing acetic acid to obtain a grain boundary diffusion material.

[0009] According to another aspect of the present invention, a method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet is provided, comprising: a step of applying a grain boundary diffusion material manufactured by the above manufacturing method to an R-Fe-B permanent magnet; a step of drying the R-Fe-B permanent magnet to which the grain boundary diffusion material has been applied; and a step of heat-treating the dried R-Fe-B permanent magnet to perform grain boundary diffusion of the grain boundary diffusion material; wherein R is Nd, Pr, Dy, Ce, La, Gd, Tb or Y.

[0010] The method for manufacturing a grain boundary diffusion material according to one embodiment of the present invention can have a low manufacturing cost because it uses rare earth oxides that are cheaper than expensive rare earth metals.

[0011] An R-Fe-B permanent magnet in which a grain boundary diffusion material is manufactured from a rare earth oxide according to an embodiment of the present invention and is grain boundary diffused can exhibit a coercivity increasing effect similar to that of an R-Fe-B permanent magnet in which a grain boundary diffusion material is manufactured from a pure rare earth metal and is grain boundary diffused.

[0012] The effects of the present invention are not limited to the effects described above, and effects not mentioned will be clearly understood by those skilled in the art from the specification herein.

[0013] Figure 1 is Tb 30 Nd 40 Cu 30 This shows the results of measuring the melting point of an alloy.

[0014] Figure 2 shows the results of measuring the coercive force and magnetic flux density of the grain boundary-diffused permanent magnets of Example 2 and Comparative Example 2 and the base magnet without the grain boundary diffusion process.

[0015] When a part in this specification is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0016] Throughout this specification, the unit “parts by weight” or “wt%” means the weight ratio between each component.

[0017] Throughout this specification, the unit “at%” means the ratio of elements between each component.

[0018] Hereinafter, the present invention will be described in more detail.

[0019] One embodiment of the present invention provides a method for producing a grain boundary diffusion material, comprising the steps of: mixing a rare earth oxide, Nd, Cu, and a reducing agent to produce a mixture; heat-treating the mixture and then pulverizing the mixture to produce an alloy powder in which the rare earth oxide is reduced; mixing the alloy powder in which the rare earth oxide is reduced with a solution containing ammonium nitrate and water to remove impurities; and mixing the alloy powder from which the impurities have been removed with a solution containing acetic acid to remove an oxide layer on the surface of the alloy powder, thereby obtaining a grain boundary diffusion material.

[0020] According to one embodiment of the present invention, the rare earth oxide may be at least one selected from TbO2, Tb2O3, Tb4O7, DyO2, Dy2O3, Dy4O7, NdO2, Nd2O3, Nd4O7, PrO2, Pr2O3, and Pr4O7. The method for producing a grain boundary diffusion material using the above-described rare earth oxide can produce a grain boundary diffusion material at a lower cost than the existing method for producing a grain boundary diffusion material using pure rare earth metal.

[0021] According to one embodiment of the present invention, the reducing agent may be at least one selected from Ca, Mg, K, Na, NaK, CaH2, CaF2, CaS, CaCl2, MgH2, MgF2, MgS, MgCl2, KH, KF, K2S, KCl, NaH, NaF and Na2S. Specifically, the molar ratio of the rare earth oxide and the reducing agent may be 1:1.2 to 1:2, 1:1.4 to 1:1.8 or 1:1.5. By using the rare earth oxide and the reducing agent in the molar ratio described above, it is possible to effectively reduce the rare earth oxide so that no rare earth oxide remains.

[0022] According to one embodiment of the present invention, the step of manufacturing the alloy powder in which the rare earth oxide is reduced may be heat treating at a temperature of 600°C to 1200°C. By heat treating in the above-described temperature range, it is possible to effectively reduce the rare earth oxide so that no rare earth oxide remains.

[0023] The above rare earth oxide reduced alloy powder may be manufactured by heat treating the rare earth oxide reduced alloy and then pulverizing the alloy powder. Due to the pulverization, the alloy powder may have a size suitable for grain boundary diffusion, and the grain boundary diffusion material may be effectively grain boundary diffused in the method for manufacturing a grain boundary diffused R-Fe-B permanent magnet mentioned below.

[0024] The step of removing impurities by mixing the alloy powder in which the rare earth oxide is reduced with a solution containing ammonium nitrate and water may be a step of removing impurities such as Ca, CaO, Mg or MgO remaining by the following reaction formula 1 or reaction formula 2 or a similar reaction. In addition, the solution containing ammonium nitrate and water may further contain an alcohol solvent such as methanol or ethanol.

[0025] [Reaction Formula 1]

[0026] CaO + H2O -> Ca(OH)2

[0027] [Reaction Formula 2]

[0028] Ca + 2H2O -> Ca(OH)2+ H2

[0029] However, in the step of removing the impurities, the alloy powder may be exposed to oxygen in the solution, which may cause an oxide layer to form on the surface of the alloy powder. The oxide layer on the surface of the alloy powder may inhibit the grain boundary diffusion of the grain boundary diffusion material in the method for manufacturing a grain boundary diffused R-Fe-B permanent magnet mentioned below, and may deteriorate the magnetic properties of the grain boundary diffused R-Fe-B permanent magnet, such as coercivity and magnetic flux density.

[0030] According to one embodiment of the present invention, the step of removing the impurities may be mixing for 15 to 120 minutes. By mixing the alloy powder in which the rare earth oxide is reduced with a solution containing ammonium nitrate and water for the above-mentioned time, the remaining impurities such as Ca, CaO, Mg or MgO can be effectively removed, and the time for removing the impurities using ammonium nitrate can be shortened, thereby minimizing the oxide layer formed on the surface of the alloy powder. However, it is not limited to the above-mentioned temperature, and the mixing time can be appropriately adjusted depending on the amount of alloy powder to be mixed.

[0031] The step of removing the oxide layer on the surface of the alloy powder by mixing the alloy powder from which the impurities have been removed with a solution containing acetic acid to obtain a grain boundary diffusion material may be to remove the oxide layer on the surface of the alloy powder produced in the step of removing the impurities. In addition, the solution containing acetic acid may further include an alcohol solvent such as methanol or ethanol.

[0032] The above-mentioned manufactured grain boundary diffusion material may include Tb in an amount of 20 at% to 40 at%, 20 at% to 35 at%, 25 at% to 40 at%, 25 at% to 35 at%, or 30 at% based on the total elements of the grain boundary diffusion material, Nd in an amount of 30 at% to 50 at%, 30 at% to 45 at%, 35 at% to 50 at%, 35 at% to 45 at%, or 40 at% based on the total elements of the grain boundary diffusion material, and Cu in an amount of 20 at% to 40 at%, 20 at% to 35 at%, 25 at% to 40 at%, 25 at% to 35 at%, or 30 at% based on the total elements of the grain boundary diffusion material. When the grain boundary diffusion material has a content of Tb, Nd or Cu in the aforementioned range, the melting point can be lowered and the grain boundary diffusion efficiency can be increased compared to pure rare earth. Specifically, FIG. 1 shows Tb30 Nd 40 Cu 30 This shows the results of measuring the melting point of the alloy. The melting point of pure Tb is 1356 ℃, but Tb 30 Nd 40 Cu 30 The melting point of the alloy was confirmed to be 704 ℃, which is lower than that of pure Tb.

[0033] One embodiment of the present invention provides a method for manufacturing a grain-boundary diffused R-Fe-B permanent magnet, comprising the steps of: applying a grain-boundary diffusion material manufactured by the above manufacturing method to an R-Fe-B permanent magnet; drying the R-Fe-B permanent magnet to which the grain-boundary diffusion material has been applied; and heat-treating the dried R-Fe-B permanent magnet to perform grain-boundary diffusion of the grain-boundary diffusion material; wherein R is Nd, Pr, Dy, Ce, La, Gd, Tb, or Y. The grain-boundary diffused R-Fe-B permanent magnet manufactured by the above manufacturing method may have superior magnetic properties, such as coercivity, to those of a non-grain-boundary diffused R-Fe-B permanent magnet.

[0034] According to one embodiment of the present invention, the step of drying the R-Fe-B permanent magnet may be drying at 30°C to 500°C for 1 to 60 minutes. By drying under the above-mentioned temperature and time conditions, the polyvinyl alcohol and ethanol mentioned below can be effectively dried, and the grain boundary diffusion material can be smoothly diffused at the grain boundary.

[0035] According to one embodiment of the present invention, the step of grain boundary diffusion of the grain boundary diffusion material may be a first heat treatment at 700°C to 1100°C for 1 to 30 hours, and a second heat treatment at 400°C to 700°C for 1 to 10 hours. By performing the first heat treatment under the above-described temperature and time conditions, the grain boundary diffusion material may be melted, and grain boundary diffusion may be smoothly performed inside the R-Fe-B permanent magnet. In addition, by performing the second heat treatment under the above-described temperature and time conditions, the R-rich phase may increase inside the R-Fe-B permanent magnet due to the grain boundary diffused rare earth, Nd, and Cu elements, and the coercivity of the R-Fe-B permanent magnet may be improved.

[0036] According to one embodiment of the present invention, before the step of applying the grain boundary diffusion material, a step of applying polyvinyl alcohol and ethanol to the R-Fe-B permanent magnet may be further included. The polyvinyl alcohol acts as an adhesive, making it easy to apply the grain boundary diffusion material and smoothly allow grain boundary diffusion.

[0037] According to one embodiment of the present invention, before the step of applying polyvinyl alcohol and ethanol, the method may further include the steps of applying a metal fluoride to an R-Fe-B permanent magnet; drying the R-Fe-B permanent magnet to which the metal fluoride has been applied; and heat-treating the dried R-Fe-B permanent magnet to cause grain boundary diffusion of the metal fluoride.

[0038] According to one embodiment of the present invention, the metal fluoride may be at least one selected from TaF5, TiF4, MoF5, MoF6, WF6, ZrF4, and NbF5. By first subjecting the R-Fe-B permanent magnet to grain boundary diffusion with the aforementioned metal fluoride, the grain growth within the R-Fe-B permanent magnet can be effectively restricted, and the coercivity of the R-Fe-B permanent magnet can be improved.

[0039] According to one embodiment of the present invention, the step of diffusion of the metal fluoride into the grain boundary may be heat treatment at 700°C to 1100°C for 0.5 to 10 hours. By heat treating under the above-described temperature and time conditions, the metal fluoride can be effectively diffused into the grain boundary, and the grain growth within the R-Fe-B permanent magnet can be effectively restricted.

[0040] Hereinafter, the present invention will be described in detail with examples and experimental examples to specifically explain the present invention. However, the examples and experimental examples according to the present invention may be modified in various different forms, and the scope of the present invention is not construed as being limited to the examples and experimental examples described below. The examples and experimental examples in this specification are provided to more fully explain the present invention to those of average skill in the art.

[0041] Example 1. Preparation of grain boundary diffusion material using rare earth oxides

[0042] A mixture was prepared by uniformly mixing 4.056 g of Tb4O7, 4.697 g of Nd, 1.532 g of Cu, 3.029 g of Ca, and 0.3366 g of Mg. To reduce Tb4O7, the mixture was tapped into a mold of any shape, and then heated at 900°C for 30 minutes to prepare an alloy. The prepared alloy was ground to a size suitable for grain boundary diffusion to obtain alloy powder. To remove impurities such as Ca, CaO, Mg, and MgO, the alloy powder was mixed with 7 g of ammonium nitrate (NH4NO3), 36 ml of water (H2O), and 36 ml of methanol at room temperature for 20 minutes using a homogenizer or ultrasonicator. Next, the alloy powder was washed three times with water and twice with acetone, and the oxide layer on the surface of the alloy powder was removed by mixing the alloy powder with acetic acid and methanol at room temperature for 2 minutes. Finally, the alloy powder was washed three times with methanol and twice with acetone, and a grain boundary diffusion material containing Tb, Nd, and Cu was obtained. The composition ratio of the grain boundary diffusion material is Tb 30 at%, Nd 40 at%, and Cu 30 at%.

[0043] Comparative Example 1. Preparation of grain boundary diffusion material using Tb

[0044] Tb (15.324 g), Nd (18.548 g), and Cu (6.128 g) were melted by applying 3.5 kW of electricity, and then poured onto a rapidly rotating wheel to rapidly cool, thereby obtaining a ribbon-shaped grain boundary diffusion material. The composition ratio of the grain boundary diffusion material is Tb (30 at%), Nd (40 at%), and Cu (30 at%).

[0045] Example 2. Manufacturing of a permanent magnet in which the grain boundary diffusion material of Example 1 is grain boundary diffused.

[0046] An adhesive material mixed with polyvinyl alcohol and ethanol was applied to an R-Fe-B permanent magnet (Nd-Fe-B permanent magnet) having a size of 12.6 mm (width) × 13.5 mm (length) × 3.7 mm (thickness). 3 wt% of the grain boundary diffusion material of Example 1 was applied to the permanent magnet based on 100 wt% of the permanent magnet, and dried at 80°C for 10 minutes. The dried permanent magnet was first heat-treated at 970°C for 15 hours, and then second heat-treated at 550°C for 2 hours, thereby obtaining a permanent magnet in which the grain boundary diffusion material of Example 1 was grain boundary diffused.

[0047] Comparative Example 2. Manufacturing of a permanent magnet in which the grain boundary diffusion material of Comparative Example 1 is grain boundary diffused.

[0048] A permanent magnet in which the grain boundary diffusion material of Comparative Example 1 was grain boundary diffused was obtained in the same manner as in Example 2, except that the grain boundary diffusion material of Comparative Example 1 was used instead of the grain boundary diffusion material of Example 1.

[0049] Experimental Example 1. Magnetic Characteristics Evaluation

[0050] The grain boundary-diffusion permanent magnets of Example 2 and Comparative Example 2 and the base magnet (Nd-Fe-B permanent magnet) without the grain boundary diffusion process were magnetized by applying a pulsed magnetic field at a magnetic field of 5 T. Then, the demagnetization curves (coercivity and magnetic flux density) of the magnetized magnets were measured at room temperature by changing the applied magnetic field from 0 kOe to -30 kOe using a BH hysteresis loop tracer. At this time, the magnetic field was applied in the opposite direction to the magnetization direction, and the results are shown in Fig. 2.

[0051] Referring to Figure 2, it was confirmed that the permanent magnet (Example 2) in which the grain boundary diffusion material manufactured from the rare earth oxide of the present invention was subjected to grain boundary diffusion had a superior coercivity than the base magnet that was not subjected to the grain boundary diffusion process.

[0052] In addition, it was confirmed that the permanent magnet (Example 2) in which the grain boundary diffusion material manufactured from the rare earth oxide of the present invention is grain boundary diffused exhibits a similar improvement in coercivity as the permanent magnet (Comparative Example 2) in which the grain boundary diffusion material manufactured from pure rare earth metal is grain boundary diffused.

[0053] Although the present invention has been described above through limited embodiments, the present invention is not limited thereto, and it is obvious that various modifications and variations are possible within the scope of the technical idea of ​​the present invention and the equivalent scope of the patent claims to be described below by a person having ordinary skill in the art to which the present invention pertains.

Claims

1. A step of preparing a mixture by mixing rare earth oxide, Nd, Cu and a reducing agent; A step of heat-treating the above mixture and then pulverizing it to produce an alloy powder in which rare earth oxide is reduced; A step of removing impurities by mixing the alloy powder in which the rare earth oxide is reduced with a solution containing ammonium nitrate and water; and A method for producing a grain boundary diffusion material, comprising: a step of mixing the alloy powder from which the impurities have been removed with a solution containing acetic acid to remove an oxide layer on the surface of the alloy powder, thereby obtaining a grain boundary diffusion material.

2. In paragraph 1, A method for producing a grain boundary diffusion material, wherein the rare earth oxide is at least one selected from TbO2, Tb2O3, Tb4O7, DyO2, Dy2O3, Dy4O7, NdO2, Nd2O3, Nd4O7, PrO2, Pr2O3, and Pr4O7.

3. In paragraph 1, A method for producing a grain boundary diffusion material, wherein the reducing agent is at least one selected from Ca, Mg, K, Na, NaK, CaH2, CaF2, CaS, CaCl2, MgH2, MgF2, MgS, MgCl2, KH, KF, K2S, KCl, NaH, NaF and Na2S.

4. In paragraph 1, A method for producing a grain boundary diffusion material, wherein the step of producing an alloy powder in which the rare earth oxide is reduced is heat treatment at a temperature of 600°C to 1200°C.

5. In paragraph 1, A method for producing a grain boundary diffusion material, wherein the step of removing the above impurities is mixing for 15 to 120 minutes.

6. A step of applying a grain boundary diffusion material manufactured by the manufacturing method of paragraph 1 to an R-Fe-B permanent magnet; A step of drying the R-Fe-B permanent magnet coated with the above grain boundary diffusion material; and A step of heat-treating the dried R-Fe-B permanent magnet to perform grain boundary diffusion of the grain boundary diffusion material; A method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet, wherein the above R is Nd, Pr, Dy, Ce, La, Gd, Tb or Y.

7. In paragraph 6, A method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet, wherein the step of drying the above R-Fe-B permanent magnet is to dry the R-Fe-B permanent magnet at 30°C to 500°C for 1 to 60 minutes.

8. In paragraph 6, A method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet, wherein the step of grain boundary-diffusion of the above grain boundary-diffusion material is a first heat treatment at 700°C to 1100°C for 1 to 30 hours, and a second heat treatment at 400°C to 700°C for 1 to 10 hours.

9. In paragraph 6, A method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet, further comprising a step of applying polyvinyl alcohol and ethanol to the R-Fe-B permanent magnet before the step of applying the grain boundary-diffusion material.

10. In paragraph 9, A step of applying metal fluoride to an R-Fe-B permanent magnet before the step of applying the polyvinyl alcohol and ethanol; A step of drying the R-Fe-B permanent magnet coated with the above metal fluoride; and A method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet, further comprising the step of heat-treating the dried R-Fe-B permanent magnet to diffuse metal fluoride into the grain boundary.

11. In paragraph 10, A method for manufacturing a grain boundary diffused R-Fe-B permanent magnet, wherein the metal fluoride is at least one selected from TaF5, TiF4, MoF5, MoF6, WF6, ZrF4, and NbF5.

12. In paragraph 10, A method for manufacturing a grain boundary-diffused R-Fe-B permanent magnet, wherein the step of diffusion of the above metal fluoride is heat treatment at 700°C to 1100°C for 0.5 to 10 hours.

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