Method for manufacturing permanent magnet

Abstract

The present invention relates to a method for manufacturing a permanent magnet, and provides a method for manufacturing a permanent magnet, the method comprising: a manufacturing step of manufacturing a Mn-Bi alloy; and a heat treatment step of heat-treating the Mn-Bi alloy to form a Mn-Bi low-temperature phase (LTP), wherein, in the heat treatment step, a load is applied to induce stress in the Mn-Bi alloy.

Description

Method for manufacturing permanent magnets

[0001] The present invention relates to a method for manufacturing a permanent magnet. More specifically, it relates to a method for manufacturing a Mn-Bi permanent magnet having excellent maximum energy and saturation magnetization values.

[0002] Permanent magnets are widely used in electrical products worldwide, including motors and generators. Because their properties directly impact energy consumption and power generation efficiency, there's a constant demand for the development of superior magnetic materials to reduce energy consumption and enhance product performance and efficiency.

[0003] The most commonly used permanent magnet materials are ferrite magnets and Nd-based magnets that use rare earth elements. Ferrite magnets are inexpensive as they are made of iron ore, but their magnetic properties, such as maximum magnetic energy (BHmax), are low at around 3-5 MGOe. On the other hand, Nd-based magnets, which are made of rare earth elements, have excellent characteristics such as a BHmax of 40-50 MGOe, and are therefore increasingly being used in motors and generators that require high performance. In particular, when high-performance magnets are used in motors for hybrid and electric vehicles, they can reduce power consumption, volume, and weight, which significantly improves vehicle performance. Therefore, the demand for motors using high-performance magnets is expected to continue to increase in the future.

[0004] However, Nd-based magnets exhibit declining coercivity at higher temperatures. Therefore, heavy rare earth elements, such as Dy and Tb, are added to improve their high-temperature properties for use in motors used in automobiles. However, rare earths are expensive and are only produced in certain regions, including China. Heavy rare earths are also very expensive due to their limited reserves, and their concentration in certain regions of China creates significant uncertainty in their production and supply. Furthermore, refining rare earths generates significant amounts of radioactive material and contaminated wastewater, increasing the cost of disposal. This is expected to further increase production costs. Recently, China, the largest producer, has designated rare earths as strategic materials, restricting production and exports, posing significant risks to pricing and the supply chain.

[0005] Due to these issues, there has been a continuous demand for the development of new magnetic materials with superior magnetic properties and low prices, and new magnet development is actively underway, mainly in the United States, Japan, and Europe. Among these new magnets, Mn-based magnets, especially Mn-Bi magnets, have recently been actively researched for practical use due to their high theoretical maximum energy product of approximately 18 MGOe and their composition of inexpensive Mn and Bi. Furthermore, Mn-Bi magnets show a characteristic in which coercivity increases with temperature, attracting attention as applications in automotive motors that require high high-temperature reliability.

[0006] However, the melting point temperatures of Mn and Bi, the two elements that make up the Mn-Bi magnet, are different, with Mn at 1246℃ and Bi at 271℃, making alloying difficult. In addition, Mn is easily oxidized, making it difficult to obtain a high-quality magnetic phase with a maximum energy of about 18 MGOe and a saturation magnetization of about 80 emu / g, which are the theoretical values ​​of the Mn-Bi magnet. In particular, the Mn-Bi magnet exhibits ferromagnetic properties in the Mn-Bi low-temperature phase (LTP) as shown in the phase change diagram of Fig. 1. However, the Mn-Bi low-temperature phase is a low-temperature phase generated below 355℃, and at temperatures higher than that, it easily separates into other phases and loses its ferromagnetic properties, making manufacturing difficult.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] Korean Patent Publication No. 10-2020-0086181

[0010] Korean Patent No. 10-1535487

[0011] The present invention was invented to solve the problems of the prior art, and an object of the present invention is to provide a method for manufacturing a Mn-Bi permanent magnet having excellent magnetic properties such as maximum energy and saturation magnetization value.

[0012] Another object of the present invention is to provide a method for manufacturing a high-quality Mn-Bi permanent magnet having a high-purity Mn-Bi low-temperature phase by increasing the fraction of the Mn-Bi low-temperature phase during the manufacturing process.

[0013] The present invention provides a method for manufacturing a permanent magnet, comprising: a manufacturing step of manufacturing a Mn-Bi alloy; and a heat treatment step of heat-treating the Mn-Bi alloy so that a Mn-Bi low-temperature phase (LTP) is formed; and characterized in that a load is applied during the heat treatment process so that stress is generated in the Mn-Bi alloy.

[0014] At this time, a load may be applied so that the stress generated in the Mn-Bi alloy includes at least one of tensile stress, compressive stress, bending stress, shear stress, and torsional stress.

[0015] Additionally, the load per unit area applied to the Mn-Bi alloy may be 1 Mpa or more.

[0016] Additionally, the load applied to the above Mn-Bi alloy can be applied as a compressive load of 10 MPa per unit area.

[0017] In addition, the heat treatment process is performed by placing the Mn-Bi alloy inside a separate heat treatment chamber, and a load can be applied to the Mn-Bi alloy from a load application device placed inside the heat treatment chamber.

[0018] Additionally, the load applying device may include a press device capable of applying a compressive load to the Mn-Bi alloy.

[0019] Additionally, the process of applying a load to the Mn-Bi alloy can be continuously performed during the heat treatment process.

[0020] Additionally, in the above heat treatment process, the heat treatment temperature may be applied at 355°C or lower.

[0021] In addition, the Mn-Bi alloy can be manufactured by mixing 40 to 60 at% of Mn and the remainder of Bi in the above manufacturing step.

[0022] Additionally, in the above manufacturing step, the atomic ratio (at%) of Mn and Bi can be applied as 51:49.

[0023] In addition, the method for manufacturing the permanent magnet further includes an addition step of adding at least one of additives Sb, Mg, Al, Ga, and C, and the manufacturing step can manufacture the Mn-Bi alloy with the additives added.

[0024] Additionally, in the above adding step, the additive may be added in an amount of 10 at% or less out of the total 100 at% of the Mn-Bi alloy.

[0025] In addition, the method for manufacturing the permanent magnet may further include a step of manufacturing magnetic powder by crushing the Mn-Bi magnetic material in which the Mn-Bi low-temperature phase is formed through the heat treatment step; a step of molding the magnetic powder while applying a magnetic field; and a step of sintering the molded magnetic powder.

[0026] Meanwhile, the present invention provides a permanent magnet characterized by being manufactured through the above permanent magnet manufacturing method.

[0027] According to the present invention, by applying a load to generate stress in a Mn-Bi alloy during a heat treatment process of the Mn-Bi alloy, the formation of a Mn-Bi low-temperature phase is promoted, and at the same time, diffusion of the Mn-Bi low-temperature phase is induced, thereby increasing the fraction of the Mn-Bi low-temperature phase within the Mn-Bi magnetic body, thereby having the effect of improving the magnetic properties of the Mn-Bi magnet.

[0028] Figure 1 is a diagram illustrating a state change diagram of a Mn-Bi alloy.

[0029] Figure 2 is a process flow diagram illustrating a manufacturing process of a permanent magnet manufacturing method according to one embodiment of the present invention step by step.

[0030] FIG. 3 is a conceptual diagram conceptually illustrating the configuration of a heat treatment chamber and a load application device according to one embodiment of the present invention.

[0031] FIG. 4 is a graph showing the magnetic characteristics of a permanent magnet manufactured according to another permanent magnet manufacturing method according to one embodiment of the present invention.

[0032] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. First, when assigning reference numerals to components in each drawing, it should be noted that, where possible, identical components are assigned the same reference numerals even if they appear in different drawings. Furthermore, when describing the present invention, detailed descriptions of known related structures or functions will be omitted if they are deemed to obscure the gist of the present invention.

[0033] FIG. 2 is a process flow diagram illustrating a manufacturing process of a permanent magnet manufacturing method according to one embodiment of the present invention step by step, and FIG. 3 is a conceptual diagram conceptually illustrating the configuration of a heat treatment chamber and a load application device according to one embodiment of the present invention.

[0034] A method for manufacturing a permanent magnet according to one embodiment of the present invention is a method for manufacturing a Mn-Bi permanent magnet, comprising a manufacturing step (S20) of manufacturing a Mn-Bi alloy, and a heat treatment step (S30) of heat-treating the Mn-Bi alloy so that a Mn-Bi low-temperature phase (LTP) is formed.

[0035] In particular, in the method for manufacturing a permanent magnet of the present invention, a load is applied so as to generate stress in the Mn-Bi alloy during the heat treatment process of the heat treatment step (S30). That is, in the method for manufacturing a permanent magnet of the present invention, the heat treatment step (S30) is performed in a manner that the heat treatment process is performed while applying a load so as to generate stress in the Mn-Bi alloy.

[0036] In order to improve the magnetic properties of a Mn-Bi permanent magnet, forming the Mn-Bi alloy into a Mn-Bi low-temperature phase (LTP) is a very important process, and in one embodiment of the present invention, a process of applying a load to generate stress in the Mn-Bi alloy during a heat treatment process is applied to form the Mn-Bi alloy into the Mn-Bi low-temperature phase.

[0037] First, let's look at the manufacturing method of a general Mn-Bi permanent magnet according to the prior art. In the step of melting Mn and Bi raw materials for Mn-Bi alloying, the two materials are mixed at a certain ratio and melted at a high temperature through heating. The mixing ratio is 50 at % of Mn and Bi each, and the atomic ratio is mixed at a 1:1 ratio. At this time, in order to melt the Mn, the alloying is performed through a high temperature process of 1000℃ or higher, and the alloying is generally performed using an induction furnace. Through this alloying process, a heat treatment process is performed to form the molten liquid Mn-Bi alloy into a low-temperature ferromagnetic phase, Mn-Bi low-temperature phase (LTP).

[0038] The heat treatment process mainly uses the Rapid Solidification Process (RSP), which is a rapid cooling method. The RSP method sprays the Mn-Bi liquid phase through a nozzle onto a high-speed rotating metal wheel, allowing the Mn-Bi liquid phase to rapidly cool on the surface of the metal wheel, thereby obtaining the Mn-Bi low-temperature phase. Another heat treatment method is a method of obtaining the Mn-Bi low-temperature phase through a homogenization process in which the liquid Mn-Bi alloy is slowly cooled and heat-treated in a bulk state for a long time.

[0039] However, these heat treatment methods cause some oxidation of Mn during the heat treatment process and also create large crystal phases of Mn and Bi, making it difficult to form a low-temperature Mn-Bi phase even when homogenizing heat treatment is performed, resulting in a deterioration in magnetic properties.

[0040] Therefore, in order to increase the fraction of the Mn-Bi low-temperature phase, which is a ferromagnetic phase, during the heat treatment process of the Mn-Bi alloy, a method has been used in the past to suppress the formation of the crystal phase by increasing the wheel speed and form the Mn-Bi low-temperature phase through post-heat treatment, or a method has been used to form the Mn-Bi low-temperature phase by heat treating the Mn-Bi bulk alloy for a long time. However, even in this case, the non-magnetic Mn and Bi crystal phases cannot be completely removed, and thus the magnetic properties fall far short of the theoretical values ​​of the Mn-Bi system magnet, which are the maximum energy product of 18 MGOe or the saturation magnetization value of 80 emu / g. The maximum energy product of the Mn-Bi magnet manufactured by a general manufacturing method according to the conventional technology shows a value of 7 to 10 MGOe.

[0041] According to a method for manufacturing a permanent magnet according to one embodiment of the present invention, when manufacturing a Mn-Bi permanent magnet, the formation of a Mn-Bi low-temperature phase is promoted, thereby increasing the fraction of the Mn-Bi low-temperature phase, thereby significantly improving the magnetic properties. To this end, in one embodiment of the present invention, a load is applied so as to generate stress in a Mn-Bi alloy during a heat treatment process for forming a Mn-Bi low-temperature phase, thereby promoting the formation of the Mn-Bi low-temperature phase, reducing crystal phases such as Mn and Bi, and increasing the purity of the Mn-Bi low-temperature phase, thereby manufacturing a high-performance Mn-Bi permanent magnet.

[0042] Generally, stress in solid materials can accelerate the diffusion rate of solids. This is because stress causes particles or molecules within the solid to move with greater energy. Stress in solid materials increases the energy of thermal motion, and thermal motion is the primary mechanism driving diffusion. In other words, increased thermal motion of particles within a solid allows them to move further, thus increasing the diffusion rate. Furthermore, stress can influence the creation and movement of defects within solid materials, and stress in solid materials can accelerate the creation or movement of defects. An increase in defects within a solid material creates more diffusion paths for particles or molecules, and the movement of defects facilitates the movement of particles or molecules. Therefore, stress in a solid material can accelerate the diffusion rate of particles or molecules through defect-related diffusion mechanisms.

[0043] In one embodiment of the present invention, by applying a load to generate stress in the Mn-Bi alloy during the heat treatment process (S30), the formation of a Mn-Bi low-temperature phase from the Mn-Bi alloy is promoted and diffusion is induced, thereby increasing the fraction of the Mn-Bi low-temperature phase and forming a Mn-Bi magnetic body with improved magnetic properties.

[0044] At this time, the stress generated in the Mn-Bi alloy may be applied so as to include at least one of tensile stress, compressive stress, bending stress, shear stress, and torsional stress, and the load per unit area applied to the Mn-Bi alloy may be set to 1 Mpa or more.

[0045] For example, as illustrated in FIG. 3, the heat treatment process (S30) may be performed by placing the Mn-Bi alloy (10) inside a separate heat treatment chamber (100), and a separate load application device (200) may be placed inside the heat treatment chamber (100) to apply a load to the Mn-Bi alloy (10). The load application device (200) may be applied as a press device that can apply a compressive load to the Mn-Bi alloy (10) by compressing the Mn-Bi alloy (10) in the vertical direction. In this case, compressive stress is generated in the Mn-Bi alloy (10), and depending on its shape and structure, various stresses such as tensile stress and shear stress may be generated.

[0046] The load application device (200) illustrated in FIG. 3 applies a press device that compresses the Mn-Bi alloy (10) in the vertical direction, but in addition, a press device that compresses the Mn-Bi alloy (10) in the front-back direction and the left-right direction may be additionally applied. In this case, since a compressive load is applied to the Mn-Bi alloy (10) in three directions (up-down direction, front-back direction, and left-right direction), not only is the compressive load on the Mn-Bi alloy (10) strengthened, but the compressive load is applied uniformly throughout, and accordingly, stress can be generated more uniformly within the Mn-Bi alloy (10).

[0047] As the stress generated within the Mn-Bi alloy (10) is generated more strongly and uniformly, the diffusion speed of the particles increases further, and accordingly, the formation of the Mn-Bi low-temperature phase from the Mn-Bi alloy is further promoted, and at the same time, diffusion is induced, so that the fraction of the Mn-Bi low-temperature phase further increases, and the purity of the Mn-Bi low-temperature phase can be further improved.

[0048] Of course, the load application device (200) can be applied in various forms, such as a press device that applies a compressive load to the Mn-Bi alloy (10), a traction device that can apply a tensile load, a torsion device that can apply a torsional load, etc.

[0049] The heat treatment process (S30) can be performed by heat treating the Mn-Bi alloy (10) for a long time at a low temperature (e.g., 300°C) of 355°C or lower inside a heat treatment chamber (100), and at this time, the process of applying a load to the Mn-Bi alloy (10) can be continuously performed during the heat treatment process.

[0050] Meanwhile, in the step (S20) of manufacturing a Mn-Bi alloy, Mn and Bi are mixed and melted at a high temperature (e.g., 1200°C). At this time, Mn and Bi can be mixed with 40 to 60 at% of Mn and the remainder as Bi. More specifically, Mn and Bi can be mixed at an atomic ratio (at%) of 1:1, but considering that some of the Mn is lost during the manufacturing process due to oxidation, etc., it can be mixed at a ratio of 51 at% of Mn and 49 at% of Bi. After mixing Mn and Bi at this mixing ratio and melting them at a high temperature, a bulk Mn-Bi alloy can be manufactured by slow cooling. The bulk Mn-Bi alloy manufactured in this way undergoes a heat treatment process and a load application process.

[0051] In addition, in the process of mixing Mn and Bi prior to the step (S20) of manufacturing the Mn-Bi alloy, a separate additive may be added (S10). The additive is a material capable of improving magnetic properties, and may be at least one of Sb, Mg, Al, Ga, and C, and such additive may be added in an amount of 10 at% or less based on 100 at% of the entire Mn-Bi alloy.

[0052] After the Mn-Bi alloy manufacturing step (S20), a heat treatment step (S30) is performed to apply a load to the Mn-Bi alloy and perform heat treatment as described above, and during this process, a very high fraction of the Mn-Bi low-temperature phase is formed in the Mn-Bi alloy.

[0053] After this heat treatment step (S30), the Mn-Bi magnetic body in which the Mn-Bi low-temperature phase is formed is pulverized to manufacture magnetic powder (S40). Thereafter, the magnetic powder is molded while a magnetic field is applied (S50), and the molded magnetic powder is sintered (S60) to complete the manufacture of the final Mn-Bi permanent magnet. The process of pulverizing the magnetic body to manufacture magnetic powder, molding the magnetic powder, and sintering the magnetic powder can be applied in the same manner as a general magnet manufacturing method, and therefore, a detailed description thereof is omitted here.

[0054]

[0055] Example

[0056] First, Mn and Bi pellets with a purity of 99.9% or higher were mixed at 51 at% and 49 at%, respectively, and placed in a crucible, and heated to 1200°C or higher using a vacuum induction heater to perform alloying. No additives were added, and the molten liquid Mn-Bi alloy was slowly cooled to produce a bulk Mn-Bi alloy.

[0057] The Mn-Bi alloy thus produced was placed in a heat treatment chamber and a load was applied to transform it into the ferromagnetic Mn-Bi low-temperature phase (LTP). The heat treatment chamber was set to an Ar atmosphere at 300°C, and a compressive load of 10 MPa was applied through a press device in the heat treatment chamber, and a homogenization heat treatment was performed for 48 hours.

[0058] The Mn-Bi magnetic material produced through this heat treatment process was placed in a mortar and pulverized, and the pulverized powder was then finely pulverized using a ball mill. The ball mill was performed under low-energy conditions (150 rpm, 6 hours) by mixing the powder in an organic solvent.

[0059] Afterwards, the organic solvent was removed, and the powder with an average particle size of 5 ㎛ was molded under a magnetic field of 1 Tesla and sintered in a 300°C chamber in an Ar atmosphere to manufacture the final Mn-Bi permanent magnet.

[0060] The magnetic properties of the Mn-Bi permanent magnets manufactured in this manner are shown in Table 1 below. Table 1 also shows the properties of a conventional magnet sample manufactured without applying a load during homogenization heat treatment for comparison with the magnetic properties of the conventional magnet.

[0061] Table 1

[0062]

[0063] As shown in Table 1, the Mn-Bi permanent magnet manufactured according to one embodiment of the present invention has a saturation magnetization value and a remanent magnetization value improved by more than 20% compared to a magnet manufactured by a conventional process.

[0064] In addition, FIG. 4 shows the magnetic properties of a Mn-Bi permanent magnet manufactured according to an embodiment of the present invention and a conventional magnet in the form of a graph, through which it can be seen that the saturation magnetization value and the residual magnetization value of the Mn-Bi permanent magnet manufactured according to an embodiment of the present invention are improved compared to the conventional magnet, and it can be seen that the maximum energy is also improved by more than 20%.

[0065] The above description is merely an illustrative illustration of the technical idea of ​​the present invention, and those skilled in the art will appreciate that various modifications and variations can be made without departing from the essential characteristics of the present invention. Therefore, the embodiments disclosed in the present invention are intended to illustrate rather than limit the technical idea of ​​the present invention, and the scope of the technical idea of ​​the present invention is not limited by these embodiments. The scope of protection of the present invention should be interpreted by the following claims, and all technical ideas within a scope equivalent thereto should be interpreted as being included in the scope of the rights of the present invention.

[0066] [Explanation of symbols]

[0067] 10: Mn-Bi alloy

[0068] 100: Heat treatment chamber

[0069] 200: Load application device

Claims

1. Manufacturing step for manufacturing Mn-Bi alloy; and A heat treatment step of heat treating the Mn-Bi alloy so that a Mn-Bi low-temperature phase (LTP) is formed. A method for manufacturing a permanent magnet, characterized in that a load is applied so as to generate stress in the Mn-Bi alloy during the heat treatment process.

2. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that a load is applied so that the stress generated in the Mn-Bi alloy includes at least one of tensile stress, compressive stress, bending stress, shear stress, and torsional stress.

3. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that the load per unit area applied to the above Mn-Bi alloy is 1 Mpa or more.

4. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that the load applied to the above Mn-Bi alloy is a compressive load of 10 Mpa per unit area.

5. In paragraph 1, The above heat treatment process is performed by placing the Mn-Bi alloy inside a separate heat treatment chamber, A method for manufacturing a permanent magnet, characterized in that a load is applied to the Mn-Bi alloy from a load applying device placed inside the heat treatment chamber.

6. In paragraph 5, A method for manufacturing a permanent magnet, characterized in that the load applying device includes a press device capable of applying a compressive load to the Mn-Bi alloy.

7. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that the process of applying a load to the Mn-Bi alloy is continuously performed during the heat treatment process.

8. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that the heat treatment temperature in the above heat treatment process is 355°C or lower.

9. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that the Mn-Bi alloy is manufactured by mixing 40 to 60 at% of Mn and the remainder of Bi in the above manufacturing step.

10. In paragraph 1, A method for manufacturing a permanent magnet, characterized in that the atomic ratio (at%) of Mn and Bi in the above manufacturing step is 51:

49.

11. In paragraph 1, Further comprising an addition step of adding at least one of additives Sb, Mg, Al, Ga, and C, A method for manufacturing a permanent magnet, characterized in that the above manufacturing step manufactures the Mn-Bi alloy with the above additive added.

12. In paragraph 11, A method for manufacturing a permanent magnet, characterized in that in the above adding step, the additive is added in an amount of 10 at% or less out of the total 100 at% of the Mn-Bi alloy.

13. In paragraph 1, A step of producing magnetic powder by crushing the Mn-Bi magnetic material in which the Mn-Bi low-temperature phase is formed through the above heat treatment step; A step of molding the magnetic powder while applying a magnetic field; and A step of sintering the above-mentioned molded magnetic powder A method for manufacturing a permanent magnet, characterized in that it further includes:

14. A permanent magnet characterized by being manufactured through the permanent magnet manufacturing method described in Article 13.

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