Rare earth magnets

Abstract

To provide a high-performance rare-earth magnet which is superior in workability in production.SOLUTION: A rare-earth magnet is produced by hot plastic working on an R-Fe-B based alloy containing R (R represents a rare earth element including Nd), Fe and B. The rare-earth magnet has a structure of main phases containing an R2Fe14B compound, in which an easy-magnetization axis is oriented toward a short axis direction of a crystal grain of the main phase. A thickness dimension in a direction of the easy-magnetization axis is 0.07 mm or more and 0.2 mm or less, and a decreasing rate of residual magnetization is 5% or less. (The decreasing rate (%) of residual magnetization is a rate of decrease of residual magnetization on the supposition that the residual magnetization is 100% when the thickness dimension of the rare-earth magnet is 0.5 mm.)SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to rare earth magnets.

Background Art

[0002] Since Nd-Fe-B-based permanent magnets have excellent magnetic properties, they are used in computer-related devices, various electronic devices, home appliances, and various actuators. In recent years, there has been a demand for miniaturization and thinning of these electronic devices, home appliances, various actuators, etc. Among Nd-Fe-B-based permanent magnets, miniaturization and thinning of sintered magnets, which are particularly excellent in magnetic properties, are required.

[0003] Conventionally, when miniaturizing and thinning an Nd-Fe-B-based sintered magnet, generally, after cutting a sintered block-shaped magnet into a predetermined shape, it is necessary to grind the surface to a desired thickness. However, it is known that the magnetic properties of the sintered magnet deteriorate as the Nd-Fe-B-based sintered magnet is ground and thinned (see, for example, Patent Document 1).

[0004] Patent Document 1 discloses forming an alloy thin film layer containing at least one of Ti, W, Pt, Au, Cr, Ni, Cu, Co, Al, Ta, and Ag in an amount of 1.0 atomic% to 50.0 atomic% on the surface to be machined of a sintered magnet body in order to prevent deterioration of magnetic properties accompanying grinding. The balance is composed of R' (R' is at least one of Ce, La, Nd, Pr, Dy, Ho, and Tb).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, Patent Document 1 states that, as a means of depositing an alloy thin film layer onto the ground surface of a sintered magnet body, a thin film formation method such as vacuum deposition, ion sputtering, ion plating, ion deposition thin film formation method, or plasma deposition thin film formation method can be appropriately selected and used, and that after forming the alloy thin film layer, it is necessary to perform a heat treatment at least once in a vacuum or inert atmosphere at 400°C to 900°C for 1 minute to 3 hours.

[0007] Thus, after grinding the surface of the sintered magnet to the desired thickness, additional processing steps are required, resulting in poor workability.

[0008] Therefore, the object of the present invention is to provide a high-performance rare-earth magnet that offers excellent workability during manufacturing. [Means for solving the problem]

[0009] To solve the above-mentioned problems and achieve the objective, a rare earth magnet according to one aspect of the present invention is a rare earth magnet obtained by hot plastic deformation of an R-Fe-B alloy containing R (where R represents a rare earth element including Nd), Fe, and B, wherein the rare earth magnet is R2Fe 14 The magnet has a main phase structure containing compound B, the easy magnetization axis is oriented in the direction of the short axis of the crystal grains of the main phase, the thickness dimension in the direction of the easy magnetization axis is 0.07 mm or more and 0.2 mm or less, and the rate of decrease in remanent magnetization is 5% or less (wherein the rate of decrease in remanent magnetization (%) is the percentage of decrease relative to the remanent magnetization when the thickness dimension of the rare earth magnet is 0.5 mm, which is set to 100%). [Effects of the Invention]

[0010] According to one aspect of the present invention, a high-performance rare-earth magnet with excellent workability during manufacturing can be obtained. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic cross-sectional view in the thickness direction of a rare earth magnet according to the embodiment. [Figure 2]Figure 2 schematically shows a cross-sectional view in the thickness direction of a rare earth magnet obtained when a surface parallel to the thickness direction of a flat plate is machined during the grinding process. [Figure 3] Figure 3 is a schematic cross-sectional view in the thickness direction of an Nd sintered magnet that has undergone a grinding process. [Figure 4] Figure 4 shows the demagnetization curve of the test specimen in the example. [Figure 5] Figure 5 shows the demagnetization curve of the comparative example test specimen (a test specimen prepared using sample A). [Figure 6] Figure 6 shows the demagnetization curve of the comparative example test specimen (a test specimen prepared using sample B). [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited by these embodiments. Furthermore, some of the components in the following embodiments may be easily substituted or substantially identical to those that are easily substituted by those skilled in the art.

[0013] The rare earth magnet (also called a rare earth permanent magnet) according to this embodiment is a rare earth magnet obtained by hot plastic deformation of an R-Fe-B alloy containing R (where R represents a rare earth element including Nd), Fe, and B. Specifically, it can be manufactured by the following method.

[0014] <Preparation of rare earth magnet powder> First, prepare R-Fe-B magnet powder as the rare earth magnet powder. The R-Fe-B (boron) magnet that makes up the R-Fe-B magnet powder is R2Fe, a ternary tetragonal compound. 14 Phase B (e.g., R2Fe) 14It contains a B-type compound phase as the main phase. Also, the R-Fe-B-based magnet usually further contains an R-rich phase or the like. R represents a rare earth element containing Nd. That is, R contains Nd as an essential component. As rare earth elements, in addition to neodymium (Nd) and praseodymium (Pr), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) can be mentioned. Along with Nd, one other rare earth element may be used, or two or more may be used in combination. As long as at least Nd is used as R. A part of Fe may be substituted with Co. When a part of Fe is substituted with Co, it is preferable that Fe is contained in an amount of 50 atomic% or more when the total amount of Fe and Co is 100 atomic%. The R-Fe-B-based magnet may contain other elements. Examples of other elements include titanium (Ti), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), and tungsten (W). One of the other elements may be used alone, or two or more may be used in combination. In the R-Fe-B-based magnet, R is preferably contained in an amount of 12 atomic% or more and 16 atomic% or less. B is preferably contained in an amount of 6 atomic% or more and 8 atomic% or less. Also, when containing the above-mentioned other elements, the other elements are preferably contained in a total amount exceeding 0 atomic% and 3 atomic% or less. Here, the remainder is the total amount of Fe and inevitably contained elements.

[0015] Here, as the R-Fe-B-based magnet, for example, Nd2Fe 14 The Nd-Fe-B-based magnet using a Nd-Fe-B-based alloy having B as the main phase will be described as an example.

[0016] The Nd-Fe-B-based magnet powder is produced, for example, by the melt spinning method (ultra-rapid cooling method). Specifically, the Nd-Fe-B-based alloy is melted by high-frequency induction heating under reduced pressure or in an argon atmosphere. Next, the molten alloy is sprayed onto a rotating copper roll and rapidly cooled (ultra-rapidly cooled) to produce ribbon-shaped thin strips. Next, these thin strips are pulverized. For example, after breaking the thin strips into pieces about several millimeters to several tens of millimeters, it is preferable to pulverize them with a pulverizer or the like. The thin strips are pulverized to obtain pulverized powder.

[0017] After pulverizing the ribbon-shaped thin strips to obtain pulverized powder, heat treatment is performed on this to obtain magnet powder. At this stage, since the directions of the easy magnetization axes of the crystal grains of the magnet powder are not aligned in one direction, it is magnetically isotropic. Instead of actually manufacturing the magnet powder, pre-manufactured magnet powder can be substituted. For example, Nd-Fe-B-based thin strips produced by the melt spinning method are pulverized to obtain magnetically isotropic magnet powder, which is provided by Magnequench as magnet powder with increased density by hot press molding.

[0018] <Hot Pressing Process: Fabrication of Sintered Magnet Body> Next, a first mold is prepared. The first mold is composed of a hollow cylindrical die, a hollow cylindrical upper punch and a lower punch inserted inside the die, and a cylindrical core disposed inside the upper punch and the lower punch. The die, the upper punch, the lower punch, and the core are formed of a conductive material (for example, graphite, cemented carbide, etc.).

[0019] Next, the first mold is filled with rare earth magnet powder (Nd-Fe-B-based magnet powder), set in a sintering apparatus (SPS apparatus: Spark Plasma Sintering apparatus) and sintered, and the sintered magnet body (hot press magnet body) is taken out from the first mold.

[0020] The magnetic powder filled in the cavity of the first mold is pressurized by the upper and lower punches due to the pressure applied between the upper and lower electrodes. Furthermore, an electric current flows from the upper electrode to the upper punch, which then flows through the die, core, and magnetic powder, and through the lower punch to the lower electrode. This generates Joule heat and a discharge plasma within the magnetic powder, thus heating it. For example, it is heated to 600-700°C while being pressurized at 30-50 MPa (hot press). Sintering is preferably carried out under reduced pressure in an inert atmosphere, specifically in an argon or nitrogen atmosphere.

[0021] After heating, the current is cut off and the device is cooled. After cooling to a predetermined temperature, the first mold is removed from the sintering apparatus. Specifically, a ring-shaped sintered magnet body formed by sintering magnet powder is removed from the first mold. In this state, the ring-shaped sintered magnet body has, for example, an outer diameter of Φ30 mm, an inner diameter of Φ15 mm, and a thickness of 10 mm, with a relative density of approximately 90%, and the orientation of the easy magnetization axes of its crystal grains is random and magnetically isotropic.

[0022] <Hot plastic deformation process: Manufacturing of hot-formed magnets> Next, prepare the second mold. Note that the preparation of the second mold may be carried out in parallel with the preparation of the first mold, or it may be carried out before the preparation of the first mold.

[0023] The sintered magnet body produced in the aforementioned hot pressing process is placed in the second mold, and the second mold is then placed in the sintering apparatus to perform hot plastic deformation.

[0024] In the sintering apparatus, an upper electrode is positioned at the upper end of the punch, and a lower electrode is positioned at the lower end of the die. The upper and lower electrodes are made of a conductive material (e.g., graphite, cemented carbide, etc.). The sintering apparatus includes a power supply device and a control device that apply a predetermined voltage between the upper and lower electrodes and supply a predetermined current. The sintering apparatus may be the same as the sintering apparatus used in the hot pressing process described above, or it may be a separate apparatus.

[0025] The sintered magnet body, positioned between the die and punch of the second mold, is pressurized by the die and punch. It is also heated by discharge plasma and Joule heat generated by the flow of current through the path upper electrode → punch → sintered magnet body → die → lower electrode. Hot plastic deformation begins with the application of pressure at 30-100 MPa, followed by heating, and is pressurized while heating to, for example, 600°C to 700°C. During heating, ON-OFF DC pulse current is applied to the sintered magnet body. During hot plastic deformation, it is desirable to adjust the pressure so that the processing speed does not increase, preferably so that the processing speed remains constant. Hot plastic deformation is preferably performed under reduced pressure or in an inert atmosphere, specifically in an argon or nitrogen atmosphere. Hot plastic deformation is preferably performed while monitoring the displacement, from the start of displacement until completion. Here, the displacement is typically monitored by the displacement amount of a pressure-controlled servo motor.

[0026] The crystal grains of hot-worked magnets produced by hot plastic deformation have a flattened shape, and the easy magnetization axis of the crystal grains is oriented perpendicular to the flattened plane of the crystal grains. While the crystal grains produced during the ultra-rapid cooling method are isotropic, hot plastic deformation causes the crystal grains to grow into a flattened shape, and the flattened plane of the particles aligns mechanically in the direction of pressure. In other words, the easy magnetization axis aligns with the direction of pressure (the direction of the short axis of the crystal grain). Therefore, the easy magnetization axis of the crystal grains within the magnet aligns in the thickness direction of the hot-worked magnet (a sintered magnet body transformed into a hot-worked magnet by hot plastic deformation). Hot-worked magnets are a manufacturing method that applies the process of producing anisotropic magnets by so-called hot extrusion.

[0027] After heating, the sintering apparatus is cooled by cutting off the current. After cooling to a predetermined temperature, the second mold is removed from the sintering apparatus, and the ring-shaped hot-worked magnet obtained from the sintered magnet body by hot plastic deformation is removed from the second mold.

[0028] Hot-worked magnets obtained by hot plastic deformation exhibit magnetic anisotropy, and their shape is, for example, an outer diameter of Φ40 mm × inner diameter of Φ11 mm × thickness of 3 mm. Their relative density is approximately the true density, and they possess high magnetic properties. The fabricated hot-worked magnets have an average crystal grain size of 0.02 μm to 0.5 μm, and a Curie point of 250°C to 400°C.

[0029] <Grinding process> The hot-worked magnets obtained by hot plastic deformation are ground into a predetermined shape. Specifically, the hot-worked magnets are removed from the apparatus, set in a wire cutting device, and cut into the predetermined shape. For example, they are cut into flat plates ranging from 2mm x 2mm to 10mm x 10mm, with a thickness of 1mm to 5mm. Here, the thickness direction of the flat plate is cut so that it coincides with the thickness direction of the hot-worked magnet (the direction of pressure applied during the production of the hot-worked magnet, i.e., the direction of easy magnetization axis).

[0030] Next, both end faces of the cut flat plate (faces perpendicular to the thickness direction of the plate) are ground using a surface grinder to achieve the desired thickness. Specifically, the thickness after grinding is 0.07 mm or more and 0.2 mm or less, preferably 0.1 mm or more and 0.2 mm or less. In detail, the grinding is performed only in the direction of the short axis of the flattened crystal grains in the hot-worked magnet. Grinding is performed using a rotary grinding wheel, for example, made of a diamond grinding wheel.

[0031] For example, as can be obtained as described above, the rare earth magnet according to this embodiment is R2Fe 14 The magnet has a microstructure of a main phase containing compound B. Preferably, the average grain size of the main phase is 0.02 μm or more and 0.5 μm or less. Furthermore, in the rare earth magnet according to this embodiment, the easy magnetization axis is oriented in the direction of the short axis of the main phase grains. Additionally, the thickness dimension in the direction of the easy magnetization axis is 0.07 mm or more and 0.2 mm or less. It is preferable that the magnet be a flat plate measuring 2 mm × 2 mm square to 10 mm × 10 mm square.

[0032] Furthermore, the rare earth magnet according to the embodiment has a residual magnetization reduction rate of 5% or less. Thus, the rare earth magnet according to the embodiment has a low residual magnetization reduction rate and is high performance. Here, the residual magnetization reduction rate (%) is determined as follows. First, the residual magnetization of the rare earth magnet when the thickness dimension is 0.5 mm is measured. Specifically, a rare earth magnet with a thickness dimension of 0.5 mm can be prepared as a sample whose thickness dimension is adjusted to 0.5 mm in the grinding process of the rare earth magnet according to the embodiment. Next, the residual magnetization of the rare earth magnet according to the embodiment is measured. Then, with the residual magnetization of the rare earth magnet with a thickness dimension of 0.5 mm set to 100%, the percentage of the residual magnetization reduction of the rare earth magnet according to the embodiment is determined. For example, if the residual magnetization of the rare earth magnet according to the embodiment is 95% when the residual magnetization of the rare earth magnet with a thickness dimension of 0.5 mm is set to 100%, then the residual magnetization reduction rate is 5% (100% - 95%).

[0033] Furthermore, the rare earth magnet according to the embodiment preferably has a thickness dimension of 0.1 mm or more and 0.2 mm or less in the easy magnetization axis direction, and a remanent magnetization reduction rate of 1% or less. When the thickness dimension in the easy magnetization axis direction is within the above range, the remanent magnetization reduction rate is further suppressed.

[0034] In the rare earth magnet according to this embodiment, the reason why the rate of decrease in remanent magnetization is suppressed even when the thickness dimension is small is thought to be as follows. Figure 1 is a schematic cross-sectional view in the thickness direction of the rare earth magnet according to this embodiment. In the rare earth magnet according to this embodiment, the flattened crystal grains 10 are stacked so that their short axis direction (easy magnetization axis) coincides with the thickness direction of the flat plate rare earth magnet. In Figure 1, the upward arrow indicates the direction of the easy magnetization axis. In the grinding process, the surface perpendicular to the thickness direction of the flat plate is processed, so the crystal grains exposed on the processed surface after processing deteriorate. These crystal grains are shown in Figure 1 as crystal grains 12 that deteriorate due to grinding. In the rare earth magnet according to this embodiment, the portion occupied by crystal grains 12 that deteriorate due to grinding is small, so it is thought that the rate of decrease in remanent magnetization is suppressed.

[0035] On the other hand, Figure 2 schematically shows a cross-sectional view in the thickness direction of a rare earth magnet obtained when a surface parallel to the thickness direction of a flat plate is processed during the grinding process. In Figure 2, the upward arrow indicates the direction of the easy magnetization axis. In this case, the portion occupied by the degraded crystal grains 12 due to grinding becomes larger, making it difficult to suppress the rate of decrease in residual magnetization. Therefore, since the crystal grains 10 have a flattened shape, it is considered that if the easy magnetization axis and the processed surface are aligned, as in the rare earth magnet according to the embodiment, the degradation due to processing will be smaller and the rate of decrease in residual magnetization will be suppressed.

[0036] Furthermore, for comparison, we will describe the case in which an Nd sintered magnet is ground to obtain a magnet of the same size as the rare earth magnet according to the embodiment. Figure 3 is a schematic cross-sectional view in the thickness direction of an Nd sintered magnet that has undergone a grinding process. In Figure 3, the upward arrow indicates the direction of the easy magnetization axis. The crystal grains 20 of the Nd sintered magnet are almost isotropic in shape. In the grinding process, processing is performed in the direction of the easy magnetization axis. The crystal grains exposed on the processed surface after processing are degraded. These crystal grains are shown in Figure 3 as crystal grains 22 that are degraded by grinding. In the Nd sintered magnet, the portion occupied by crystal grains 22 that are degraded by grinding is large, so it is thought that the rate of decrease in residual magnetization is also large. However, since the crystal grains 20 of the Nd sintered magnet are almost isotropic in shape, it is thought that the degradation due to grinding does not depend on the easy magnetization axis and the processed surface.

[0037] Thus, the rare-earth magnet according to this embodiment offers excellent workability during manufacturing, as it does not require the formation of an alloy thin film layer on its surface or heat treatment. Furthermore, even with a small thickness, the rate of decrease in residual magnetization is suppressed, resulting in high performance. Therefore, the rare-earth magnet according to this embodiment is suitable as a magnet for ultra-small, thin motors, ultra-small, thin magnetic actuators, and the like.

[0038] [Examples] <Preparation of test specimens for the examples> As described above, the hot press process is used to produce Nd-Fe-B magnetic powder (R2Fe 14 A sintered magnet body was prepared using a material having a main phase structure containing compound B, and a hot-worked magnet was fabricated by hot plastic deformation. Next, the hot-worked magnet obtained by hot plastic deformation was cut into a sample piece of a predetermined flat plate shape (3 mm × 3 mm). Here, the thickness direction of the plate was cut to coincide with the thickness direction of the hot-worked magnet (the direction of pressure applied during the production of the hot-worked magnet, i.e., the axis of easy magnetization). Next, a sample piece (3 mm × 3 mm × thickness t) was prepared by grinding. Specifically, test pieces were prepared with sample piece thickness t of 0.5 mm, 0.2 mm, 0.1 mm and 0.07 mm. The test pieces were made of R2Fe 14 The rare-earth magnet had a main phase structure containing compound B, and its easy magnetization axis was oriented in the direction of the short axis of the main phase's crystal grains. The average crystal grain size of the main phase was between 0.02 μm and 0.5 μm.

[0039] <Preparation of comparative test specimens> Comparative test specimens were prepared using Sample A and Sample B. Both Sample A and Sample B are Nd-Fe-B sintered magnets manufactured by Shin-Etsu Chemical Co., Ltd., and are magnetically anisotropic. Sample A is model number N50, and Sample B is model number N39UH. From this block with a predetermined shape, sample pieces of a predetermined shape (3 mm × 3 mm) were cut. Here, the thickness direction of the flat plate was cut so that it coincided with the easy magnetization axis direction. Next, sample pieces (3 mm × 3 mm × thickness t) were prepared by grinding. Specifically, test specimens were prepared with thickness t of 0.5 mm, 0.2 mm, 0.1 mm, and 0.07 mm.

[0040] <Measurement of magnetic properties> The magnetic properties of each sample were measured using a vibrating magnetometer (VSM) after applying a 5T magnetic field to each sample and magnetizing it in the thickness direction (easy magnetization axis direction). The results are shown in Figures 4 to 6. Specifically, Figure 4 shows the demagnetization curve of the example test piece, Figure 5 shows the demagnetization curve of the comparative example test piece (a test piece made using sample A), and Figure 6 shows the demagnetization curve of the comparative example test piece (a test piece made using sample B).

[0041] As shown in Figures 4 to 6, in both the example and comparative example specimens, a deterioration in both remanent magnetization and intrinsic coercivity is observed when the specimen thickness decreases compared to 0.5 mm. Table 1 shows the ratio of the magnetic properties (remanent magnetization) values ​​for specimens with thicknesses of 0.2 mm, 0.1 mm, and 0.07 mm to the magnetic properties (remanent magnetization) for specimens with a thickness of 0.5 mm. Table 2 shows the rate of decrease in remanent magnetization.

[0042] Table 1 shows that in the example specimens, the percentage of degradation was 99% for both the 0.2mm and 0.1mm thicknesses, indicating almost no degradation. However, when the sample thickness was 0.07mm, the percentage was 95%, showing slight degradation. In contrast, the comparative specimens (specimens prepared using sample A and sample B) showed significant degradation, with a percentage of 97% for the 0.2mm thickness and 83% for the 0.07mm thickness.

[0043] Thus, the test specimens of the examples showed that when the thickness of the processed magnet was up to 0.07 mm, the rate of decrease in remanent magnetization was kept to approximately 5%, and when the thickness of the processed magnet was up to 0.1 mm, there was almost no decrease in remanent magnetization, indicating that they are high-performance rare-earth magnets. Therefore, as stated above, the test specimens of the examples are suitable as magnets for ultra-small and thin motors, ultra-small and thin magnetic actuators, and the like.

[0044] [Table 1]

[0045] [Table 2] [Explanation of symbols]

[0046] 10: Crystal grain, 12: Crystal grain degraded by grinding, 20: Crystal grain, 22: Crystal grain degraded by grinding

Claims

1. A rare earth magnet made by hot-plastically working an R-Fe-B alloy containing R (where R represents a rare earth element including Nd), Fe, and B, The aforementioned rare earth magnet is R 2 Fe 14 Having a structure with a main phase containing compound B, The easy magnetization axis is oriented in the direction of the short axis of the crystal grains of the main phase. The thickness dimension in the direction of the easy magnetization axis is 0.07 mm or more and 0.2 mm or less. A rare earth magnet in which the rate of decrease in remanent magnetization is 5% or less (where the rate of decrease in remanent magnetization (%) is the percentage decrease relative to the remanent magnetization when the thickness dimension of the rare earth magnet is 0.5 mm, which is set to 100%).

2. The rare earth magnet according to claim 1, wherein the average crystal grain size of the main phase is 0.02 μm or more and 0.5 μm or less.

3. The rare earth magnet according to claim 1 or 2, wherein the thickness dimension of the rare earth magnet is 0.1 mm or more and 0.2 mm or less.

4. The rare earth magnet according to claim 1 or 2, wherein the rare earth magnet is in the shape of a flat plate.

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