Method for producing a rare-earth permanent magnet

Abstract

A method for producing a rare-earth permanent magnet, comprising: Manufacturing an RTB-based sintered magnet comprising at least one rare-earth element, R, at least one transition metal, T, boron, B and as a remainder iron and unavoidable impurities, Applying a first mixture containing a light rare-earth element to the surface of the RTB-based sintered magnet, and allowing the first mixture to diffuse into a grain boundary of the RTB-based sintered magnet to produce a light rare-earth permanent magnet containing the light rare-earth element, and Applying a second mixture containing a heavy rare-earth element to the surface of the light rare-earth permanent magnet, and allowing the heavy rare-earth element to diffuse into a grain boundary of the light rare-earth permanent magnet to produce the rare-earth permanent magnet. the application and diffusion of the first mixture exhibits: Preparing the first mixture containing the light rare earth element by mixing a light rare earth component with a solvent, Applying the first mixture to the surface of the RTB-based sintered magnet and Inserting the RTB-based sintering magnet, which has the first mixture applied to it, into a heating oven in a vacuum atmosphere, so that the first mixture diffuses into the grain boundary, and the light rare earth component NdH and the solvent alcohol.

Description

Technical field

[0001] The present invention relates to a method for producing a rare-earth permanent magnet (e.g., a permanent magnet which has at least one rare earth as an alloying element) in which a heavy rare-earth element may diffuse into a grain boundary of the permanent magnet. In particular, the method for producing a rare-earth permanent magnet can improve the magnetic properties of the rare-earth permanent magnet by diffusing a light rare-earth element into the grain boundary of the permanent magnet, so that a heavy rare-earth element can easily diffuse, and then diffusing the heavy rare-earth element into the grain boundary. Description of the related technology

[0002] Generally, a hybrid vehicle is a vehicle powered by an efficient combination of two or more types of drive or power sources. For example, a hybrid vehicle can be one that receives propulsion from both an internal combustion engine and an electric motor, and is referred to as a hybrid-electric vehicle (HEV). Recently, research into hybrid vehicles has been actively pursued in response to the demands of improving fuel efficiency and developing environmentally friendly products.

[0003] Such a hybrid vehicle has a combustion engine and an electric motor as its drive and power sources. The electric motor is powered by energy supplied by a battery installed in the vehicle and has a stator and a rotor as its main components, like a typical electric motor. The stator can be formed by winding a coil around a stator core, and the rotor can be located within the stator and formed by inserting a permanent magnet into a core of the rotor.

[0004] The electric motor for vehicles described above may require a high-performance permanent magnet to achieve high power and efficiency.

[0005] Therefore, a rare-earth permanent magnet, such as a sintered NdFeB magnet, which has a magnetic force three to five times greater than that of a conventional ferrite magnet, can be used to reduce the weight of the electric motor while improving the efficiency of the vehicle.

[0006] The magnetic characteristics of a rare-earth permanent magnet can include a residual magnetic flux density (Br), a coercive force (HcJ), and the like. The residual magnetic flux density can be determined by a major phase fraction, density, and a degree of magnetic orientation of the rare-earth permanent magnet, and the coercive force can be related to a microstructure of the rare-earth permanent magnet and can be determined by a reduction in the size of crystal grains or a uniform distribution of crystal grain boundary phases.

[0007] In the related technology, a technique for reducing the grain size in the production of rare-earth permanent magnets was developed to improve coercivity. However, reducing grain size can not only increase the degree of oxidation but also increase production costs. Therefore, the grain size cannot be reduced indefinitely.

[0008] Furthermore, because rare-earth permanent magnets have high conductivity and low resistivity, eddy currents can easily be generated within them. This can cause the permanent magnet's temperature to rise, potentially reducing the magnetic flux density or even leading to irreversible demagnetization. This reduction in magnetic flux density or irreversible demagnetization can significantly impair electric motor performance.

[0009] To solve the problem described above with the related technique, a grain boundary diffusion technique using a heavy rare-earth element, such as dysprosium (Dy) or terbium (Tb), was developed to improve the coercivity of the conventional rare-earth permanent magnet produced by sintering.

[0010] However, since the expensive and heavy rare-earth element cannot diffuse easily into the grain boundary during grain boundary diffusion, the magnetic characteristics of the rare-earth permanent magnet cannot be sufficiently improved. Furthermore, the consumption of the heavy rare-earth element used during grain boundary diffusion can significantly increase manufacturing costs.

[0011] The foregoing is intended only to assist in understanding the background of the present invention and is not intended to make the present invention fall within the field of related technology which is already known to the person skilled in the art.

[0012] For example, a method for producing a rare-earth permanent magnet is known from e.g. JP 2012 - 043 968 A, comprising: producing an RTB-based sintered magnet, applying a first mixture comprising a light rare-earth element to the surface of the RTB-based sintered magnet, and allowing the first mixture to diffuse into a grain boundary of the RTB-based sintered magnet to produce a light rare-earth permanent magnet comprising the light rare-earth element, and applying a second mixture comprising a heavy rare-earth element to the surface of the light rare-earth permanent magnet, and allowing the heavy rare-earth element to diffuse into a grain boundary of the light rare-earth permanent magnet to produce the rare-earth permanent magnet.

[0013] Another method for producing a rare-earth permanent magnet is known, for example, from US 2016 / 0 284 452 A1. Brief explanation of the invention

[0014] The present invention provides a method for producing a rare-earth permanent magnet. A heavy rare-earth element can diffuse easily, thereby improving the magnetic characteristics of the permanent magnet, such as the coercive force or the residual magnetic flux (remanence).

[0015] Furthermore, the process for manufacturing a rare-earth permanent magnet reduces manufacturing costs by minimizing the consumption of a heavy rare-earth element.

[0016] According to the invention, the present invention provides a method for producing a rare-earth permanent magnet with the features of claim 1. Further embodiments of the method are described in the dependent claims. That is, the method comprises: producing an RTB-based sintered magnet, applying a first mixture comprising a light rare-earth element to the surface of the RTB-based sintered magnet to produce a light rare-earth permanent magnet, which has the light rare-earth element preferably diffused into a grain boundary, and applying a second mixture comprising a heavy rare-earth element to the surface of the light rare-earth permanent magnet to produce the rare-earth permanent magnet.

[0017] Preferably, the light rare-earth element can diffuse into a grain boundary of the RTB-based sintered magnet. This diffusion can occur adequately under reduced atmospheric conditions (vacuum). Likewise, the heavy rare-earth element can diffuse into (such as a grain boundary of) the light rare-earth permanent magnet. This diffusion can also occur adequately under reduced atmospheric conditions (vacuum).

[0018] The RTB-based sintered magnet can be produced, for example, by steps that include: producing an RTB-based alloy ingot (hereinafter referred to as ingot) by melting an RTB-based alloy, producing an RTB-based alloy powder with a mean grain size of 5.0 µm or less (excluding 0) by milling the RTB-based alloy ingot, producing an RTB-based green body by subjecting the RTB-based alloy powder to magnetic field shaping in an inert gas atmosphere, and producing the RTB-based sintered magnet by sintering the RTB-based green body.

[0019] The application and diffusion of the first mixture involves: producing the first mixture by mixing a light rare earth component or a light rare earth composition (hereinafter referred to as: component) with a solvent, applying the first mixture to a surface of the RTB-based sintering magnet, and placing the RTB-based sintering magnet, which has the first mixture applied to it, into a heating oven in a vacuum atmosphere, so that the first mixture diffuses into the grain boundary(s).

[0020] The term “RTB-based” as used herein refers to a material which contains (e.g. mainly): at least one rare earth element (R), at least one transition metal (T), boron (B) and, as a remainder, iron (Fe) and other unavoidable impurities.

[0021] The light rare earth component contains NdH and the solvent contains alcohol.

[0022] The light rare earth permanent magnet can be adequately produced by diffusing the light rare earth mixture (e.g., the first mixture) in a vacuum atmosphere at a temperature of approximately 800 to 1000°C for approximately 1 to 30 hours.

[0023] The process may, for example, further include, after the first mixture has diffused, cooling the light rare earth permanent magnet in an inert gas atmosphere and removing stresses (e.g. residual stresses) of the light rare earth permanent magnet by heat-treating the light rare earth permanent magnet at a temperature of about 400 to 600°C in an inert gas atmosphere for about 1 to 3 hours.

[0024] The rare-earth permanent magnet can be produced, for example, by steps which include: producing the second mixture containing the heavy rare-earth element by mixing a (e.g., heavy) rare-earth component with a solvent, applying the second mixture to the surface of the light-rare-earth permanent magnet, and placing the rare-earth permanent magnet, which has the second mixture applied to it, into a heating oven in a vacuum atmosphere, so that the second mixture diffuses into the grain boundary.

[0025] Preferably, the heavy rare earth component may contain, for example, TbF or TbH, and may contain the solvent alcohol.

[0026] The rare-earth permanent magnet can be adequately produced by diffusing the second mixture in a vacuum atmosphere at a temperature of about 800 to 1000°C for about 1 to 30 hours.

[0027] The process may, for example, further include, after the second mixture has diffused, cooling the rare-earth permanent magnet in an inert gas atmosphere and removing stresses (e.g., residual stresses) of the rare-earth permanent magnet by heat-treating the rare-earth permanent magnet at a temperature of approximately 400 to 600°C in an inert gas atmosphere for approximately 1 to 3 hours.

[0028] Furthermore, a vehicle is provided which can have the rare-earth permanent magnet produced by the method described here.

[0029] Other aspects of the invention are discussed below. Brief description of the drawings

[0030] The above and other problems, features and other advantages of the present invention will become clearer from the following detailed description when viewed together with the accompanying drawings, in which: Fig. 1 is a flowchart which represents an exemplary method for producing a rare-earth permanent magnet according to an embodiment of the present invention, Fig. 2 a schematic view to illustrate a grain boundary diffusion step in an exemplary process according to an embodiment of the present invention, Fig. 3 is an illustration showing a grain boundary of an exemplary rare-earth permanent magnet according to an embodiment of the present invention, Fig.4 is a diagram representing a grain boundary composition of a rare-earth permanent magnet produced by a conventional grain boundary diffusion method, and Fig. 5 is a diagram which is a grain boundary composition of an exemplary rare-earth permanent magnet produced by the method according to an embodiment of the present invention. Detailed description

[0031] The terminology used herein serves solely to describe certain embodiments and is not intended to limit the invention. The singular forms "a," "an," and "the" are used herein to include the plural forms unless the context clearly indicates otherwise. Furthermore, it should be understood that the terms "exhibit," "exhibiting," "have," etc., when used in this description, specify the presence of the aforementioned features, areas, integers, steps, processes, elements, and / or components thereof, but do not exclude the presence or addition of one or more other features, areas, integers, steps, processes, elements, components, and / or groups thereof.

[0032] It should be understood that the term "vehicle" or "vehicle-..." or any other similar terms used herein include motor vehicles in general, such as passenger cars, including so-called sport utility vehicles (SUVs), buses, trucks, numerous commercial vehicles, as well as, for example, watercraft, including a variety of boats and ships, and also, for example, aircraft and the like, and furthermore, hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles, and other vehicles powered by alternative fuels (e.g., fuels produced from resources other than petroleum). A so-called hybrid vehicle, to which reference is made herein, is a vehicle that has two or more energy sources, e.g., vehicles that are powered by both gasoline and electricity.

[0033] Furthermore, unless otherwise stated or clear from the context, the term "approximately" used here is to be understood as "within the range of usual tolerances for this technique," for example, as within two standard deviations of the mean. "Approximately" can be understood as being within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values ​​provided here are modified by the term "approximately."

[0034] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as they are normally understood by a person skilled in the art to whom this invention relates.

[0035] Exemplary embodiments of the invention are described in detail below with reference to the accompanying drawings; however, the present invention is not limited by these embodiments. For the purposes of this description, identical reference numerals represent the same elements. Thus, elements shown in one drawing can be described with reference to content shown in other drawings, and content that is deemed obvious to a person skilled in the art, or duplicate content, can be omitted.

[0036] The present invention provides a method for producing a rare-earth permanent magnet. Preferably, the method comprises a primary diffusion of a light rare-earth element into the grain boundary of an RTB-based sintered magnet, followed by a secondary diffusion of a heavy rare-earth element to replace the light rare-earth element that has diffused into the grain boundary with the heavy rare-earth element. The method can maximize the content of the heavy rare-earth element in the grain boundary of the produced rare-earth permanent magnet, thereby improving the magnetic characteristics of the produced rare-earth permanent magnet, such as the coercive force and the density of the remaining magnetic flux (remanence).

[0037] The Fig.Figure 1 is a flowchart showing an exemplary process for producing an exemplary rare-earth permanent magnet according to an exemplary embodiment of the present invention, and the Fig. Figure 2 is a schematic representation showing a grain boundary diffusion step in an exemplary process according to an exemplary embodiment of the present invention.

[0038] As it is in the Fig. 1 and Fig.As shown in Figure 2, the method for producing a rare-earth permanent magnet according to an exemplary embodiment of the present invention can comprise a manufacturing step of producing an RTB-based sintered magnet, a first grain boundary diffusion step of forming a phase 100 rich in light rare earth in a grain boundary of the RTB-based sintered magnet by grain boundary diffusion of a light rare-earth element into the grain boundary, and a second grain boundary diffusion step of substituting the diffused light rare-earth element with a heavy rare-earth element, thereby producing the rare-earth permanent magnet which has a phase 200 rich in heavy rare earth in the grain boundary.

[0039] The manufacturing step according to an embodiment of the present invention may comprise: an alloy manufacturing step of producing an RTB-based alloy ingot by, for example, strip casting of an RTB-based alloy; a grinding process of producing an RTB-based alloy powder by grinding the RTB-based alloy ingot; a forming process of producing an RTB-based green body by subjecting the RTB-based alloy powder to magnetic field forming (e.g., shaping the green body under the influence of pressure with a simultaneously applied external magnetic field); and a sintering process of producing an RTB-based sintered magnet by sintering the RTB-based green body.

[0040] The alloy manufacturing process according to one embodiment of the present invention can involve producing the RTB-based alloy ingot by melting, for example, "ferroboron" (e.g., CAS number 11108-67-1), a rare-earth metal such as neodymium (Nd) or dysprosium (Dy) with 99 wt.% purity, copper (Cu), and iron (Fe) (e.g., steel). Preferably, the RTB-based alloy ingot can have a content of approximately 20 to 30 wt.% R (rare-earth element), a content of approximately 0 to 5 wt.% T (transition metal), a content of approximately 0 to 2 wt.% B (boron), and, as the remainder, iron (Fe) and other unavoidable impurities. All wt. percent are based on the total weight of the RTB-based alloy ingot.

[0041] The RTB-based alloy ingot can be manufactured in a vacuum atmosphere. Since the vacuum atmosphere can minimize the oxygen content of the rare-earth alloy ingot and subsequently allow a light rare-earth element and a heavy rare-earth element to diffuse easily (e.g., into it), the magnetic characteristics of the manufactured rare-earth permanent magnet can be improved.

[0042] When the RTB-based alloy ingot is manufactured, it can be exposed to hydrogen gas to react with the hydrogen during the milling process. The ingot can then be subjected to a vacuum and heated to a temperature of approximately 500°C, causing the hydrogen gas to be partially released (e.g., at least partially removed from the ingot). A jet mill using cooling or high-pressure nitrogen can then be used to produce the RTB-based alloy powder.

[0043] The RTB-based alloy ingot can be milled in such a way that the RTB-based alloy powder has a mean particle size of approximately 5.0 µm or less. Accordingly, reducing the grain size in the fabricated rare-earth permanent magnet can improve the magnetic characteristics, such as the coercivity.

[0044] Once the RTB-based alloy powder is produced, the RTB-based green body can be manufactured by mixing the RTB-based alloy powder with a lubricant during the forming process. The RTB-based green body can then be produced by a magnetic field forming process using an external magnetic field of 3 T and a pressure of 1 ton / cm² in an inert gas atmosphere.

[0045] Once the RTB-based green body is produced, it can be sintered in a sintering furnace at a temperature of approximately 1080 °C in a vacuum or inert gas atmosphere for about 4 hours. The sintered body can then be heat-treated for approximately 2 hours each at temperatures of 850 °C, 550 °C, and 500 °C to produce the RTB-based sintered magnet.

[0046] Once the RTB-based sintered magnet is fabricated, a light rare earth permanent magnet can be produced by diffusing a light rare earth element into the grain boundary(s) of the RTB-based sintered magnet in a first grain boundary diffusion step, and a rare earth permanent magnet can be produced by substituting the light rare earth element present in the grain boundary of the light rare earth permanent magnet with a heavy rare earth element in a second grain boundary diffusion step.

[0047] Preferably, the first grain boundary diffusion step according to an exemplary embodiment of the present invention may comprise the preparation of a first mixture comprising the light rare earth element, the application of the first mixture and the diffusion of the first mixture.

[0048] In the present embodiment, the first mixture can be prepared by mixing a light rare-earth component with a solvent. The light rare-earth component contains NdH. Ethanol can be used as the solvent, and the first mixture can be prepared to a slurry state by mixing the light rare-earth component with the solvent in a weight ratio of approximately 1:1. Other suitable light rare-earth components include, for example, lanthanum (La), cerium (Ce), praseodymium (Pr), promethium (Pm), or combinations thereof with other non-metallic elements such as F, H, N, or O. In general, a light rare-earth component referred to herein contains a rare-earth element having an atomic number of 57 to 61.

[0049] During the application of the first mixture, the mixture, in a slurry state, can be applied to the surface of the RTB-based sintering magnet. Then, while the first mixture diffuses, the RTB-based sintering magnet, which has the first mixture applied to it, can be placed in a heating oven so that the first mixture can diffuse into the grain boundary in a vacuum atmosphere.

[0050] Preferably, the diffusion process of the first mixture can be carried out at a temperature of about 800 to -1000°C for about 1 to 30 hours.

[0051] Since the light rare-earth element does not diffuse easily at temperatures below approximately 800°C, and the grains of the RTB-based sintered magnet can grow at temperatures above approximately 1000°C, the coercive force may be reduced.

[0052] The first diffusion step according to an embodiment of the present invention can further comprise a first cooling process of cooling the light rare earth permanent magnet after the diffusion process of the first mixture and a heat treatment process of removing stresses from the light rare earth permanent magnet by heat treatment of the cooled light rare earth permanent magnet.

[0053] Preferably, the first cooling process may include rapid cooling of the light rare earth permanent magnet, which is produced by subjecting the light rare earth element to grain boundary diffusion in an inert gas atmosphere, and the first heat treatment process may include the removal of residual stresses in the light rare earth permanent magnet by heat treatment of the cooled light rare earth permanent magnet at a temperature of approximately 400 to 600°C in an inert gas atmosphere for approximately 1 to 3 hours.

[0054] At this stage, if the heat treatment is performed at a temperature below approximately 400°C, stress relief can be time-consuming, thus reducing productivity. Furthermore, if the heat treatment is performed at a temperature above approximately 600°C, the distribution of the light rare-earth element diffused into the grain boundary can be altered, degrading magnetic characteristics such as coercivity. Therefore, the temperature may need to be limited to the range described above.

[0055] As described above, if the light rare earth permanent magnet, which has a high concentration of the light rare earth element in the grain boundary, is produced by diffusing the light rare earth element through the first diffusion step, the rare earth permanent magnet can be produced by diffusing the heavy rare earth element into the light rare earth permanent magnet through the second diffusion step.

[0056] The second diffusion step according to an exemplary embodiment of the present invention can comprise: producing a second mixture containing the heavy rare-earth element, applying the second mixture, and allowing the second mixture to diffuse. In particular, the light rare-earth element present in the grain boundary of the light rare-earth permanent magnet can be replaced by the heavy rare-earth element when the second mixture is applied to the surface of the light rare-earth permanent magnet.

[0057] In the present embodiment, the heavy rare earth mixture (e.g., the second mixture) can be prepared by mixing a heavy rare earth component with a solvent. The heavy rare earth component may, but is not limited to, containing TbF or TbH. Ethanol may be used as the solvent, and the second mixture is prepared to a slurry state by mixing the heavy rare earth component with the solvent in a weight ratio of approximately 1:1. Other suitable heavy rare earth components include, for example, europium (Eu), gadolinium (Gd), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), or combinations thereof with other non-metallic elements such as F, H, N, or O.In general, a heavy rare earth component referred to herein contains a rare earth element with an atomic number greater than 62.

[0058] During the application process of the heavy rare earth mixture, the heavy rare earth mixture, in a slurry state, can be applied to the surface of the light rare earth permanent magnet. During the diffusion of the second mixture, the light rare earth permanent magnet, which now has the second mixture applied to it, can be placed in the heating furnace and subjected to grain boundary diffusion in a vacuum atmosphere.

[0059] For the same reasons as in the process of applying the light rare earth mixture and the diffusion process of the light rare earth mixture, the process of applying the second mixture and the diffusion process of the second mixture can preferably be carried out under the same conditions as the process of applying the first mixture and the diffusion process of the first mixture.

[0060] The second diffusion step according to an exemplary embodiment of the present invention can also further comprise a second cooling process of cooling the rare-earth permanent magnet after the diffusion process of the second mixture and a heat treatment process of removing stresses from the rare-earth permanent magnet by heat treatment of the cooled rare-earth permanent magnet.

[0061] For the same reasons as for the first cooling process and the first heat treatment process, the second cooling process and the second heat treatment process can preferably be carried out under the same conditions as the first cooling process and the first heat treatment process.

[0062] The Fig. Figure 3 is an illustration to explain the grain boundary of the rare-earth permanent magnet, which Fig. Figure 4 is a representation showing a grain boundary composition of a rare-earth permanent magnet produced by a conventional grain boundary diffusion method, and the Fig. Figure 5 is a representation showing a grain boundary composition of the rare-earth permanent magnet according to an exemplary embodiment of the present invention.

[0063] As it is in the Fig.As shown in Figures 3 to 5, the content of the heavy rare earth element in the grain boundary of the rare earth permanent magnet produced by the conventional grain boundary diffusion process can be approximately 30 atomic percent, but the content of the heavy rare earth element in the grain boundary of the rare earth permanent magnet produced by the process according to an exemplary embodiment of the present invention can be approximately 60 atomic percent.

[0064] Numerous embodiments and comparative examples of the present invention are described below. (1 kG = 0.1 T; 1 Oe = 79.577 A / m) Table 1 Classification Light rare earth element Heavy rare earth element Magnetic characteristics Residual magnetic flux (kG) Coercivity (kOe) First comparison example - - 13.28 17.05 Second comparative example NdF - 13.29 18.24 Third comparative example NdH - 13.30 18.68 Fourth comparative example - TbF 13.25 23.56 Fifth comparative example - TbH 13.28 24.06 Sixth comparative example Y TbF 13.22 25.03 Seventh comparative example Y TbH 13.25 25.54 Eighth comparison example NdOF TbF 12.29 24.46 Ninth comparative example NdOF TbH 13.01 25.02 tenth comparative example NdF TbF 13.31 26.68 eleventh comparative example NdF TbH 13.29 27.36 first version NdH TbF 13.33 27.09 second version NdH TbH 13.26 27.96

[0065] Table 1 shows magnetic characteristics of numerous comparative examples and embodiments produced by applying different types of light and heavy rare earth components under the same diffusion conditions.

[0066] As shown in Table 1, the first comparison example, which was subjected to grain boundary diffusion, has improved magnetic characteristics than the other comparison examples and embodiments.

[0067] The fourth and fifth comparative examples, in which the heavy rare-earth elements were subjected to grain boundary diffusion, retained the same level of remaining magnetic flux density as the second and third comparative examples, in which the light rare-earth elements were subjected to grain boundary diffusion, but had a significantly improved coercive force than the second and third comparative examples.

[0068] In each of the sixth to the eleventh comparative example and in each of the first and second embodiments, a light rare earth element diffused to form a phase 100 rich in light rare earth at the grain boundary, and a heavy rare earth element diffused to form a phase 200 rich in heavy rare earth to produce a rare earth permanent magnet.

[0069] The tenth and eleventh comparative examples and the first and second embodiments show that when NdH or NdF is used as the light rare-earth component, the remaining magnetic flux density is maintained at the same level as when NdOF or Y is used as the light rare-earth component, but the coercive force (in the embodiments) was more improved than when NdOF or Y was used as the light rare-earth component, which means that the magnetic characteristics have been improved.

[0070] As described above, the method for producing a rare-earth permanent magnet according to numerous exemplary embodiments of the present invention can increase the content of the light rare-earth component, such as Nd, in the grain boundary by carrying out primary grain boundary diffusion using the light rare-earth component (NdH) in the first grain boundary diffusion step and by substituting the light rare-earth element in the grain boundary with the heavy rare-earth element, such as Tb, by carrying out secondary grain boundary diffusion using the heavy rare-earth component, such as TbF or TbH, in the second grain boundary diffusion step, thereby improving the magnetic characteristics of the rare-earth permanent magnet in the second grain boundary diffusion step.

[0071] The light rare-earth element that moves away from the grain boundary during the substitution process can be discharged to the outside of the rare-earth permanent magnet, and a post-treatment process, such as surface polishing after the second grain boundary diffusion step, can be performed to remove the light rare-earth element that remains on the surface of the rare-earth permanent magnet, while the light rare-earth element (e.g., the light rare-earth element at the grain boundary) can be substituted by the heavy rare-earth element and discharged to the outside of the rare-earth permanent magnet during the second grain boundary diffusion step.

[0072] According to the present invention, the method can easily diffuse the heavy rare-earth element of the rare-earth permanent magnet into the grain boundary(s) and can increase the content of the heavy rare-earth element that has diffused into the rare-earth permanent magnet, thereby improving the magnetic characteristics, such as the coercive force and the remaining or residual flux density.

[0073] Furthermore, the process can minimize the consumption of the heavy rare earth element compared to a rare earth permanent magnet with the same magnetic characteristics, thereby reducing manufacturing costs.

Claims

[1] A method for producing a rare-earth permanent magnet, comprising: Manufacturing an RTB-based sintered magnet comprising at least one rare-earth element, R, at least one transition metal, T, boron, B and as a remainder iron and unavoidable impurities, Applying a first mixture containing a light rare-earth element to the surface of the RTB-based sintered magnet, and allowing the first mixture to diffuse into a grain boundary of the RTB-based sintered magnet to produce a light rare-earth permanent magnet containing the light rare-earth element, and Applying a second mixture containing a heavy rare-earth element to the surface of the light rare-earth permanent magnet, and allowing the heavy rare-earth element to diffuse into a grain boundary of the light rare-earth permanent magnet to produce the rare-earth permanent magnet. the application and diffusion of the first mixture exhibits: Preparing the first mixture containing the light rare earth element by mixing a light rare earth component with a solvent, Applying the first mixture to the surface of the RTB-based sintered magnet and Inserting the RTB-based sintering magnet, which has the first mixture applied to it, into a heating oven in a vacuum atmosphere, so that the first mixture diffuses into the grain boundary, and the light rare earth component NdH and the solvent alcohol. [2] The method according to claim 1, wherein the RTB-based sintered magnet is produced by the steps comprising: Producing an RTB-based alloy ingot by melting an RTB-based alloy, Producing an RTB-based alloy powder having a mean particle size of 5.0 µm or less (excluding 0) by milling the RTB-based alloy ingot, Production of an RTB-based green body by subjecting the RTB-based alloy powder to magnetic field shaping in an inert gas atmosphere, and Production of the RTB-based sintered magnet by sintering the RTB-based green body. [3] The method according to any of the preceding claims, wherein the light rare earth magnet is produced by diffusing the first mixture in a vacuum atmosphere at a temperature of about 800 to 1000°C for about 1 to 30 hours. [4] The method according to any one of the preceding claims, further comprising, after diffusing the first mixture: Cooling the light rare-earth permanent magnet in an inert gas atmosphere and Removing stresses from the light rare earth permanent magnet by heat-treating the light rare earth permanent magnet at a temperature of approximately 400 to 600°C in an inert gas atmosphere for approximately 1 to 3 hours. [5] The method according to any one of the preceding claims, wherein the rare-earth permanent magnet is produced by the steps comprising: Producing the second mixture, which contains the heavy rare earth element, by mixing a heavy rare earth component with a solvent, Applying the second mixture to the surface of the light rare earth permanent magnet and The light rare earth permanent magnet, which has the second mixture applied to it, is placed in a heating oven in a vacuum atmosphere, so that the second mixture diffuses into the grain boundary. [6] The method according to claim 5, wherein the heavy rare earth component comprises TbF or TbH and the solvent comprises alcohol. [7] The method according to any one of the preceding claims, wherein the rare-earth permanent magnet is produced by diffusing the second mixture in a vacuum atmosphere at a temperature of about 800 to 1000°C for about 1 to 30 hours. [8] The method according to any one of the preceding claims, further comprising, after diffusing the second mixture: Cooling the rare-earth permanent magnet in an inert gas atmosphere and Removing stresses from the rare-earth permanent magnet by heat-treating the rare-earth permanent magnet at a temperature of approximately 400 to 600°C in an inert gas atmosphere for approximately 1 to 3 hours. [9] A vehicle comprising a rare-earth permanent magnet produced by a method according to any of the preceding claims.

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