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 disclosure relates to a method for preparing a grain boundary diffusion material using a rare earth oxide and a method for manufacturing an R-Fe-B based permanent magnet using the same, and an inexpensive grain boundary diffusion material can be prepared using a rare earth oxide, which is cheaper than an expensive rare earth metal. In addition, an R-Fe-B based permanent magnet in which the grain boundary diffusion material prepared using a rare earth oxide is grain boundary-diffused can exhibit excellent coercivity.

Description

Technical Field

[0001] The present 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 in the present disclosure.

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

[0003] R-Fe-B based permanent magnets, such as Nd-Fe-B based permanent magnets, are used in various fields including electronic information, automotive industry, medical devices, energy, transportation, etc. In particular, in line with the recent lightweight and miniaturization trends, they are being used in a variety of products including machine tools, electronic information devices, electronic appliances for home use, mobile phones, robot motors, wind power generators, small motors for automotives, and drive motors.

[0004] To improve the magnetic performance of these permanent magnets, the grain boundary diffusion process has been proposed as one of post-treatment methods. The grain boundary diffusion process is a method of performing heat treatment after coating the surfaces of the permanent magnets with heavy rare earth elements by using a point that the grain boundary diffusion process has a very high chemical reactivity on the interfaces of the permanent magnets. Such a grain boundary diffusion process enables the heavy rare earth elements to be intensively distributed around the grain boundaries, i.e., only on the surface area of the ferromagnetic crystal grains, thereby forming a core-shell structure in which the crystal grains are surrounded by a layer with high magnetic anisotropy to obtain high coercivity.

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

[0006] A technical object to be achieved by the present disclosure is to provide a grain boundary diffusion material which has a low melting point and can be prepared inexpensively using a rare earth oxide that is cheaper than an expensive rare earth metal.

[0007] However, objects to be achieved by the present disclosure are not limited to the object mentioned above, and other objects not mentioned will be clearly appreciated by those skilled in the art from the following description.Technical Solution

[0008] According to one aspect of the present disclosure, there is provided a method for preparing a grain boundary diffusion material, the method including steps of: mixing a rare earth oxide, Nd, Cu, and a reducing agent to prepare a mixture; heat-treating the mixture and then pulverizing the mixture to prepare an alloy powder in which the rare earth oxide has been reduced; mixing the alloy powder in which the rare earth oxide has been 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.

[0009] According to another aspect of the present disclosure, there is provided a method for manufacturing a grain boundary-diffused R-Fe-B based permanent magnet, the method including steps of: applying the grain boundary diffusion material prepared by the above-described preparation method to an R-Fe-B based permanent magnet; drying the R-Fe-B based permanent magnet to which the grain boundary diffusion material has been applied; and heat-treating the dried R-Fe-B based 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.Advantageous Effects

[0010] The method for preparing a grain boundary diffusion material according to one embodiment of the present disclosure can prepare the grain boundary diffusion material at a low preparation cost since it utilizes a rare earth oxide cheaper than an expensive rare earth metal.

[0011] An R-Fe-B based permanent magnet, in which a grain boundary diffusion material prepared using a rare earth oxide according to one embodiment of the present disclosure is grain boundary-diffused, can exhibit a coercivity increase effect similar to that of an R-Fe-B based permanent magnet, in which a grain boundary diffusion material prepared using a pure rare earth metal is grain boundary-diffused.

[0012] Effects of the present disclosure are not limited to the above-described effects, and effects not mentioned will be clearly appreciated by those skilled in the art from the present specification.Brief Description of Drawings

[0013] FIG. 1 shows results of measuring melting points of the Tb 30 Nd 40 Cu 30 alloy. FIG. 2 shows results of measuring coercivities and magnetic flux densities of grain boundary-diffused permanent magnets of Example 2 and Comparative Example 2 and a base magnet that does not undergo a grain boundary diffusion process. Best Mode for Carrying Out the Invention

[0014] When a part in the present specification is said to "include" a component, unless otherwise specifically stated, this means that other components are not excluded, but the other components may be further included.

[0015] Throughout the present specification, the unit "parts by weight" or "wt%" refers to the weight ratio of each component.

[0016] Throughout the present specification, the unit "at%" refers to the elemental ratio of each component.

[0017] Hereinafter, the present disclosure will be described in more detail.

[0018] One embodiment of the present disclosure provides a method for preparing a grain boundary diffusion material, the method including steps of: preparing a mixture by mixing a rare earth oxide, Nd, Cu, and a reducing agent; heat-treating the mixture and then pulverizing it to prepare an alloy powder in which the rare earth oxide has been reduced; removing impurities by mixing the alloy powder in which the rare earth oxide has been reduced with a solution containing ammonium nitrate and water; 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.

[0019] According to one embodiment of the present disclosure, the rare earth oxide may be one or more selected from TbO 2 , Tb 2 O 3 , Tb 4 O 7 , DyO 2 , Dy 2 O 3 , Dy 4 O 7 , NdO 2 , Nd 2 O 3 , Nd 4 O 7 , PrO 2 , Pr 2 O 3 , and Pr 4 O 7 . The above-described method for preparing a grain boundary diffusion material using a rare earth oxide can prepare a grain boundary diffusion material at a lower cost than the existing method for preparing a grain boundary diffusion material using a pure rare earth metal.

[0020] According to one embodiment of the present disclosure, the reducing agent may be one or more selected from Ca, Mg, K, Na, NaK, CaH 2 , CaF 2 , CaS, CaCl 2 , MgH 2 , MgF 2 , MgS, MgCl 2 , KH, KF, K 2 S, KCl, NaH, NaF, and Na 2 S. Specifically, the molar ratio of the rare earth oxide to the reducing agent may be 1:1.2 to 1:2, 1:1.4 to 1:1.8, or 1:1.5. Effective reduction can be achieved so that the rare earth oxide does not remain by using the rare earth oxide and reducing agent at the molar ratio described above.

[0021] According to one embodiment of the present disclosure, the step of preparing an alloy powder in which the rare earth oxide has been reduced may be performing heat treatment at a temperature of 600°C to 1200°C. Effective reduction can be achieved so that the rare earth oxide does not remain by performing heat treatment in the temperature range described above.

[0022] The alloy powder in which the rare earth oxide has been reduced may be prepared by preparing an alloy in which the rare earth oxide has been reduced by performing heat treatment, and then pulverizing the alloy. The pulverization may allow the alloy powder to have a size suitable for grain boundary diffusion, and enable effective grain boundary diffusion of a grain boundary diffusion material in the method for manufacturing a grain boundary-diffused R-Fe-B based permanent magnet mentioned below.

[0023] The step of removing impurities by mixing the alloy powder in which the rare earth oxide has been reduced with a solution containing ammonium nitrate and water may be removing residual impurities such as Ca, CaO, Mg, or MgO through a reaction as described in Reaction Formula 1 or Reaction Formula 2 below, or a reaction similar thereto. Further, the solution containing ammonium nitrate and water may further contain a solvent of alcohols such as methanol or ethanol.         [Reaction Formula 1]      CaO + H 2 O -> Ca(OH) 2         [Reaction Formula 2]     Ca + 2H 2 O -> Ca(OH) 2 + H 2

[0024] However, the alloy powder may be exposed to oxygen in the solution in the step of removing impurities, 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 based permanent magnet mentioned below, and may degrade the magnetic properties of the grain boundary-diffused R-Fe-B based permanent magnet manufactured, such as coercivity and magnetic flux density.

[0025] According to one embodiment of the present disclosure, the step of removing impurities may be performing mixing for 15 to 120 minutes. Residual impurities such as Ca, CaO, Mg, or MgO can be effectively removed by mixing the alloy powder in which a rare earth oxide has been reduced with a solution containing ammonium nitrate and water for the aforementioned time, and the oxide layer formed on the surface of the alloy powder can be minimized by shortening the time for removing impurities using ammonium nitrate. However, the temperature is not limited to the aforementioned temperature, and the mixing time may be appropriately adjusted depending on the amount of an alloy powder being mixed.

[0026] The 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 may be removing the oxide layer formed on the surface of the alloy powder formed in the step of removing impurities. Further, the solution containing acetic acid may further contain a solvent of alcohols such as methanol or ethanol.

[0027] The prepared grain boundary diffusion material may include 20 at% to 40 at%, 20 at% to 35 at%, 25 at% to 40 at%, 25 at% to 35 at%, or 30 at% of Tb based on the total elements of the grain boundary diffusion material, may include 30 at% to 50 at%, 30 at% to 45 at%, 35 at% to 50 at%, 35 at% to 45 at%, or 40 at% of Nd based on the total elements of the grain boundary diffusion material, and may include 20 at% to 40 at%, 20 at% to 35 at%, 25 at% to 40 at%, 25 at% to 35 at%, or 30 at% of Cu based on the total elements of the grain boundary diffusion material. When the grain boundary diffusion material has the content of Tb, Nd, or Cu within the aforementioned range, the melting point can be lowered and the grain boundary diffusion efficiency can be increased compared to pure rare earths. Specifically, FIG. 1 shows results of measuring melting points of the Tb 30 Nd 40 Cu 30 alloy, and it was confirmed that the melting point of pure Tb is 1356°C, but the melting point of the Tb 30 Nd 40 Cu 30 alloy is 704°C, which is lower than that of pure Tb.

[0028] One embodiment of the present disclosure provides a method for manufacturing a grain boundary-diffused R-Fe-B based permanent magnet, the method including steps of: applying a grain boundary diffusion material prepared by the above-described preparation method to an R-Fe-B based permanent magnet; drying the R-Fe-B based permanent magnet to which the grain boundary diffusion material has been applied; and grain boundary-diffusing the grain boundary diffusion material by heat-treating the dried R-Fe-B based permanent magnet, wherein R is Nd, Pr, Dy, Ce, La, Gd, Tb or Y. The grain boundary-diffused R-Fe-B based permanent magnet manufactured by the manufacturing method may have superior magnetic properties, such as coercivity, to those of a non-grain boundary-diffused R-Fe-B based permanent magnet.

[0029] According to one embodiment of the present disclosure, the step of drying the R-Fe-B based permanent magnet may be performing drying at 30°C to 500°C for 1 to 60 minutes. Drying is performed under the aforementioned temperature and time conditions, thereby enabling polyvinyl alcohol and ethanol mentioned below to be effectively dried and enabling the grain boundary diffusion material to be smoothly grain boundary-diffused.

[0030] According to one embodiment of the present disclosure, the step of grain boundary-diffusing the grain boundary diffusion material may be performing a primary heat treatment at 700°C to 1100°C for 1 to 30 hours, and performing a secondary heat treatment at 400°C to 700°C for 1 to 10 hours. The primary heat treatment may be performed under the aforementioned temperature and time conditions, thereby melting the grain boundary diffusion material to enable smooth grain boundary diffusion into the inside of the R-Fe-B based permanent magnet. In addition, a secondary heat treatment may be performed under the aforementioned temperature and time conditions, thereby, due to the grain boundary-diffused rare earths, Nd, and Cu elements, increasing the R-rich phase inside the R-Fe-B based permanent magnet and improving the coercivity of the R-Fe-B based permanent magnet.

[0031] According to one embodiment of the present disclosure, the method may further include a step of applying polyvinyl alcohol and ethanol to the R-Fe-B based permanent magnet before the step of applying the grain boundary diffusion material. Polyvinyl alcohol acts as an adhesive, facilitating the application of the grain boundary diffusion material and enabling smooth grain boundary diffusion.

[0032] According to one embodiment of the present disclosure, before the step of applying polyvinyl alcohol and ethanol, the method may further include steps of: applying a metal fluoride to the R-Fe-B based permanent magnet; drying the R-Fe-B based permanent magnet to which the metal fluoride has been applied; and grain boundary-diffusing the metal fluoride by heat-treating the dried R-Fe-B based permanent magnet.

[0033] According to one embodiment of the present disclosure, the metal fluoride may be one or more selected from TaF 5 , TiF 4 , MoF 5 , MoF 6 , WF 6 , ZrF 4 , and NbF 5 . The R-Fe-B based permanent magnet may be first grain boundary-diffused with the aforementioned metal fluoride, thereby effectively limiting crystal grain growth within the R-Fe-B based permanent magnet, and improving the coercivity of the R-Fe-B based permanent magnet.

[0034] According to one embodiment of the present disclosure, the step of grain boundary-diffusing the metal fluoride may be performing heat treatment at 700°C to 1100°C for 0.5 to 10 hours. Heat treatment may be performed under the aforementioned temperature and time conditions, thereby effectively grain boundary-diffusing the metal fluoride, and effectively limiting crystal grain growth within the R-Fe-B based permanent magnet.Mode for Carrying Out the Invention

[0035] Hereinafter, in order to specifically describe the present disclosure, Examples and Experimental Examples will be described in detail. However, Examples and Experimental Examples according to the present disclosure may be modified in various different forms, and the scope of the present disclosure is not construed as being limited to Examples and Experimental Examples described below. Examples and Experimental Examples in the present specification are provided to more fully explain the present disclosure to those skilled in the art.Example 1. Preparation of Grain Boundary Diffusion Material Using Rare Earth Oxide

[0036] 4.056 g of Tb 4 O 7 , 4.697 g of Nd, 1.532 g of Cu, 3.029 g of Ca, and 0.3366 g of Mg were uniformly mixed to prepare a mixture. To reduce the Tb 4 O 7 , the mixture was contained in a mold of arbitrary shape, tapped, and then heated at 900°C for 30 minutes to prepare an alloy. The prepared alloy was pulverized to a size suitable for grain boundary diffusion to obtain an alloy powder. To remove impurities such as Ca, CaO, Mg, and MgO, the alloy powder was mixed with 7 g of ammonium nitrate (NH 4 NO 3 ), 36 ml of water (H2O), and 36 ml of methanol at room temperature for 20 minutes using a homogenizer or ultrasonication. After that, the mixture 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 mixture 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 30 at% of Tb, 40 at% of Nd, and 30 at% of Cu.Comparative Example 1. Preparation of Grain Boundary Diffusion Material Using Tb

[0037] 15.324 g of Tb, 18.548 g of Nd, and 6.128 g of Cu were melted by applying 3.5 kW of electricity, then poured onto a wheel rotated at a rapid speed and rapidly cooled, yielding a ribbon-shaped grain boundary diffusion material. The composition ratio of the grain boundary diffusion material is 30 at% of Tb, 40 at% of Nd, and 30 at% of Cu.Example 2. Fabrication of a permanent magnet in which the grain boundary diffusion material of Example 1 is grain boundary diffused

[0038] An R-Fe-B based permanent magnet (Nd-Fe-B based permanent magnet) having a size of 12.6 mm (width) × 13.5 mm (length) × 3.7 mm (thickness) was applied with an adhesive material mixed with polyvinyl alcohol and ethanol. With respect to 100 wt% of the permanent magnet, 3 wt% of the grain boundary diffusion material of Example 1 was applied to the permanent magnet and dried at 80°C for 10 minutes. The dried permanent magnet was primarily heat-treated at 970°C for 15 hours and secondarily heat-treated at 550°C for 2 hours to obtain a permanent magnet in which the grain boundary diffusion material of Example 1 was grain boundary-diffused.Comparative Example 2. Manufacturing of Permanent Magnet Having Grain Boundary Diffusion Material of Comparative Example 1 Grain Boundary-Diffused thereto

[0039] A permanent magnet having the grain boundary diffusion material of Comparative Example 1 grain boundary-diffused thereto was obtained in the same method 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.Experimental Example 1. Evaluation of Magnetic Properties

[0040] The grain boundary-diffused permanent magnets of Example 2 and Comparative Example 2 and the base magnet (Nd-Fe-B based permanent magnet) that did not undergo the grain boundary diffusion process were magnetized by applying a pulsed magnetic field at a magnetic field of 5 T. After that, the demagnetization curves (coercivity and magnetic flux density) of the magnetized magnets were measured by using a BH hysteresis loop tracer at room temperature and varying the applied magnetic field from 0 kOe to -30 kOe. At this time, the magnetic field was applied in the direction opposite to the direction in which the magnets were magnetized, and the results are shown in FIG. 2.

[0041] Referring to FIG. 2, it was confirmed that the permanent magnet (Example 2) in which the grain boundary diffusion material prepared from the rare earth oxide of the present disclosure was grain boundary-diffused exhibited superior coercivity to the base magnet that did not undergo the grain boundary diffusion process.

[0042] Furthermore, it was confirmed that the permanent magnet (Example 2) in which the grain boundary diffusion material prepared from the rare earth oxide of the present disclosure was grain boundary-diffused exhibited a coercivity improvement similar to that of the permanent magnet (Comparative Example 2) in which the grain boundary diffusion material prepared from pure rare earth metal was grain boundary-diffused.

[0043] Although the present disclosure has been described above with limited Examples, the present disclosure is not limited thereto, and it is obvious that various modifications and variations may be made possible within technical ideas of the present disclosure and the equivalent scope of the claims to be described below by those skilled in the art to which the present disclosure pertains.

Claims

1. A method for preparing a grain boundary diffusion material, the method comprising steps of: mixing a rare earth oxide, Nd, Cu, and a reducing agent to prepare a mixture; heat-treating the mixture and then pulverizing the mixture to prepare an alloy powder in which the rare earth oxide has been reduced; mixing the alloy powder in which the rare earth oxide has been 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.

2. The method of claim 1, wherein the rare earth oxide is one or more selected from TbO2, Tb2O3, Tb4O7, DyO2, Dy2O3, Dy4O7, NdO2, Nd2O3, Nd4O7, PrO2, Pr2O3, and Pr4O7.

3. The method of claim 1, wherein the reducing agent is one or more selected from Ca, Mg, K, Na, NaK, CaH2, CaF2, CaS, CaCl2, MgH2, MgF2, MgS, MgCl2, KH, KF, K2S, KCl, NaH, NaF, and Na2S.

4. The method of claim 1, wherein the step of preparing an alloy powder in which the rare earth oxide has been reduced is performing heat treatment at a temperature of 600°C to 1200°C.

5. The method of claim 1, wherein the step of removing impurities is performing mixing for 15 to 120 minutes.

6. A method for manufacturing a grain boundary-diffused R-Fe-B based permanent magnet, the method comprising steps of: applying the grain boundary diffusion material prepared by the preparation method of claim 1 to an R-Fe-B based permanent magnet; drying the R-Fe-B based permanent magnet to which the grain boundary diffusion material has been applied; and heat-treating the dried R-Fe-B based 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.

7. The method of claim 6, wherein the step of drying the R-Fe-B based permanent magnet is performing drying at 30°C to 500°C for 1 to 60 minutes.

8. The method of claim 6, wherein the step of grain boundary-diffusing the grain boundary diffusion material is performing a primary heat treatment at 700°C to 1100°C for 1 to 30 hours, and performing a secondary heat treatment at 400°C to 700°C for 1 to 10 hours.

9. The method of claim 6, further comprising a step of applying polyvinyl alcohol and ethanol to the R-Fe-B based permanent magnet before the step of applying the grain boundary diffusion material.

10. The method of claim 9, before the step of applying polyvinyl alcohol and ethanol, further comprising steps of: applying a metal fluoride to the R-Fe-B based permanent magnet; drying the R-Fe-B based permanent magnet to which the metal fluoride has been applied; and grain boundary-diffusing the metal fluoride by heat-treating the dried R-Fe-B based permanent magnet.

11. The method of claim 10, wherein the metal fluoride is one or more selected from TaF5, TiF4, MoF5, MoF6, WF6, ZrF4, and NbF5.

12. The method of claim 10, wherein the step of grain boundary-diffusing the metal fluoride is performing heat treatment at 700°C to 1100°C for 0.5 to 10 hours.

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