R-t-b based permanent magnet

Abstract

The permanent magnet includes a plurality of main phase particles. The plurality of main phase particles contain R, T, and B. A plurality of voids are formed in a cross section of the permanent magnet. The cross section is substantially parallel to a direction of an easy magnetization axis of the permanent magnet. An area ratio of the plurality of voids in the cross section is 1% or more and 5% or less. A direction perpendicular to the direction of the easy magnetization axis in the cross section is denoted as an AB direction. A direction in which each of the voids in the cross section extends is denoted as a VD. An angle between the AB direction and the VD is denoted as θ. A horizontal axis of a frequency distribution of the voids in the cross section indicates θ. A range of the horizontal axis of the frequency distribution is 0° or more and 180° or less. The frequency distribution is maximum in a range where θ is 60° or more and 120° or less.

Description

Technical Field

[0001] This invention relates to RTB-based permanent magnets. Background Technology

[0002] RTB-based permanent magnets contain rare earth elements R (such as Nd), transition metal elements T (such as Fe), and boron (B). The magnetic properties of RTB-based permanent magnets are superior to those of conventional permanent magnets (such as ferrite magnets), therefore, they are widely used in motors used in electric vehicles or hybrid vehicles. For example, RTB-based permanent magnets can form the rotor of an IPM (Interior Permanent Magnet Motor) or an SPM (Surface Permanent Magnet Motor). Multiple RTB-based permanent magnets are arranged circumferentially along the rotor, with the easy magnetization axis of each RTB-based permanent magnet perpendicular to the rotor's axis of rotation. A surface of the RTB-based permanent magnet that is approximately perpendicular to the easy magnetization axis (equivalent to the surface of the magnetic poles) faces the stator surrounding the rotor. An external magnetic field H, approximately parallel to the easy magnetization axis of each RTB-based permanent magnet, can be generated in the stator. The rotor rotates due to the attraction and repulsion between the stator and each RTB permanent magnet, which are associated with the change in the external magnetic field H applied to the surface of each RTB permanent magnet.

[0003] like Figure 1A As shown, with the change of the external magnetic field H applied to the RTB permanent magnet 2, eddy currents I induced by electromagnetic induction are generated within the RTB permanent magnet 2. According to Lenz's law, the eddy currents I induced by electromagnetic induction flow in a manner that generates a magnetic field that prevents the change of the external magnetic field H. The direction of the external magnetic field H is approximately parallel to the easy magnetization axis C of the RTB permanent magnet 2; therefore, the eddy currents I accompanying the change of the external magnetic field H circulate in a direction approximately perpendicular to the easy magnetization axis C.

[0004] RTB-based permanent magnets are metallic magnets; therefore, their resistivity is significantly lower than that of ferrite magnets, which are composed of metal oxides. Consequently, in motors using RTB-based permanent magnets, Joule heating caused by eddy currents is easily generated, leading to a decrease in motor efficiency. Therefore, attempts have been made to reduce eddy current losses in RTB-based permanent magnets used in motors.

[0005] For example, Japanese Patent Application Publication No. 2005-198365 discloses an RTB-based permanent magnet, which is composed of multiple plate-shaped permanent magnets stacked with layers of non-conductive resin. According to the RTB-based permanent magnet described in Japanese Patent Application Publication No. 2005-198365, the multiple plate-shaped permanent magnets are insulated by non-conductive resin, which can suppress eddy currents flowing between the multiple plate-shaped permanent magnets. For example, Japanese Patent Application Publication No. 2017-174962 discloses an RTB-based permanent magnet, which is composed of a magnet body and a resistive layer (oxide layer) formed on the surface of the magnet body. According to the RTB-based permanent magnet described in Japanese Patent Application Publication No. 2017-174962, the resistive layer (oxide layer) can suppress eddy currents. Summary of the Invention

[0006] In order to suppress eddy currents in a direction substantially perpendicular to the easy magnetization axis C, the inventors of this invention explored a method for increasing the resistivity of an RTB-based permanent magnet in this direction. The inventors discovered that by forming multiple voids extending in a predetermined direction within the RTB-based permanent magnet, the resistivity of the RTB-based permanent magnet in this direction substantially perpendicular to the easy magnetization axis C increases. However, with the formation of multiple voids in the RTB-based permanent magnet, the volume ratio of the multiple principal phase particles in the RTB-based permanent magnet relatively decreases, which can easily damage the orientation of the principal phase particles in the easy magnetization axis C. As a result, magnetic properties such as residual magnetic flux density are compromised. Therefore, it is necessary to balance high resistivity and excellent magnetic properties.

[0007] One aspect of the present invention is to provide an RTB-based permanent magnet having high resistivity and excellent magnetic properties in a direction substantially perpendicular to the easy magnetization axis.

[0008] For example, one aspect of the present invention relates to the following RTB-based permanent magnets.

[0009] [1] An RTB-based permanent magnet contains rare earth element R, transition metal element T and B, wherein the RTB-based permanent magnet contains at least Nd as R, the RTB-based permanent magnet contains at least Fe as T, the RTB-based permanent magnet contains multiple main phase particles, the multiple main phase particles contain at least R, T and B, multiple voids are formed in the cross section of the RTB-based permanent magnet, the cross section is approximately parallel to the easy magnetization axis of the RTB-based permanent magnet, the area ratio of the multiple voids in the cross section is more than 1% and less than 5%, the direction perpendicular to the easy magnetization axis in the cross section is denoted as the AB direction, the direction in which each of the multiple voids in the cross section extends is denoted as VD, the angle between the AB direction and VD is denoted as θ, the horizontal axis of the frequency distribution of the multiple voids in the cross section is denoted as θ, the horizontal axis of the frequency distribution is in the range of 0° to 180°, and the frequency distribution is maximum in the range of θ being more than 60° to 120°.

[0010] [2] According to the RTB system permanent magnet described in [1], multiple main phase particles are flat in the cross section and multiple main phase particles are stacked along the easy magnetization axis direction.

[0011] [3] According to [1] or [2], the average length of the short axis of the plurality of main phase particles in the cross section is more than 20 nm and less than 200 nm.

[0012] [4] The RTB permanent magnet according to any one of [1] to [3], wherein the R content in the RTB permanent magnet is 28% by mass or more and 33% by mass or less, and the B content in the RTB permanent magnet is 0.8% by mass or more and 1.1% by mass or less.

[0013] [5] The RTB system permanent magnet according to any one of [1] to [4] is a hot-worked magnet.

[0014] [6] According to any one of [1] to [5], the RTB permanent magnet is defined as follows: the width of the RTB permanent magnet in the easy magnetization axis direction is t; the portion in the easy magnetization axis direction with a depth of 0 to 0.25t from the surface of the RTB permanent magnet is defined as the surface portion of the RTB permanent magnet; the area ratio of the plurality of voids in the surface portion is ARs, where ARs is in %; ARs is measured on the surface portion exposed in the cross section; the portion in the easy magnetization axis direction with a depth greater than 0.25t and less than 0.5t from the surface of the RTB permanent magnet is defined as the central portion of the RTB permanent magnet; the area ratio of the plurality of voids in the central portion is ARc, where ARc is in %; ARc is measured on the central portion exposed in the cross section; and ARs-ARc is 1.0% to 4.0%.

[0015] According to the present invention, it is possible to provide an RTB-based permanent magnet having high resistivity and excellent magnetic properties in a direction substantially perpendicular to the easy magnetization axis. Attached Figure Description

[0016] Figure 1A This is a schematic perspective view of an RTB-based permanent magnet 2 according to one embodiment of the present invention. Figure 1B This is a schematic diagram of the cross section 2cs of the RTB permanent magnet 2 (a view of the RTB permanent magnet 2 along the bb line direction).

[0017] Figure 2 yes Figure 1B An enlarged view of a portion (region II) of section 2cs shown.

[0018] Figure 3A and Figure 3B This is a schematic diagram showing the direction VD of the extension of each of the multiple gaps 8 formed in the RTB system permanent magnet 2.

[0019] Figure 4A and Figure 4B This is a specific example of the frequency distribution of multiple gaps 8 in a cross section 2cs2 parallel to the direction of the easy magnetization axis.

[0020] Figure 5 This is a perspective view of the cavity 10 formed within a mold used in the manufacturing method of the RTB system permanent magnet 2.

[0021] Figure 6A , Figure 6B and Figure 6C This is a schematic diagram illustrating the mechanism by which multiple voids 8 are formed in the RTB system permanent magnet 2.

[0022] Figure 7A and Figure 7BThis is a cross-sectional image of Embodiment 1 of the present invention.

[0023] Figure 8A and Figure 8B This is a cross-sectional image of Embodiment 1 of the present invention.

[0024] Figure 9A and Figure 9B This is the frequency distribution of multiple gaps in the cross section of Embodiment 1 of the present invention.

[0025] Explanation of reference numerals in the attached figures

[0026] 2…RTB system permanent magnet, 2cs…cross section of permanent magnet, 4…main phase particle, 8…void, C…direction of easy magnetization axis, AB…direction approximately perpendicular to the direction of easy magnetization axis, F…frequency distribution of void. Detailed Implementation

[0027] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, the same reference numerals are used to denote the same constituent elements. The present invention is not limited to the embodiments described below. The term "permanent magnet" as used below refers to an RTB-based permanent magnet. The concentrations of each element in the permanent magnet described below are expressed in atomic percentages. Figure 5 The X, Y, and Z axes shown refer to three coordinate axes that are orthogonal to each other.

[0028] (permanent magnet)

[0029] The permanent magnet in this embodiment contains at least rare earth elements (R), transition metal elements (T), and boron (B). The permanent magnet in this embodiment is a hot-worked magnet. However, in another aspect of the invention, the permanent magnet can be a sintered magnet.

[0030] The permanent magnet contains at least neodymium (Nd) as a rare earth element R. The permanent magnet may contain other rare earth elements R besides Nd. These other rare earth elements R can be selected from at least one of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The permanent magnet 2 may not contain heavy rare earth elements (e.g., both Dy and Tb).

[0031] Permanent magnets must contain at least iron (Fe) as a transition metal element T. Permanent magnets may contain only Fe as a transition metal element T. Permanent magnets may also contain both Fe and cobalt (Co) as transition metal elements T.

[0032] Figure 1A This is a perspective view of the permanent magnet 2 in this embodiment. Figure 1BThis is a schematic diagram of the cross-section 2cs of permanent magnet 2. The cross-section 2cs of permanent magnet 2 is approximately parallel to the easy magnetization axis direction C of permanent magnet 2. The easy magnetization axis direction C is a direction parallel to the straight line connecting a pair of magnetic poles of permanent magnet 2. That is, the easy magnetization axis direction C is the direction from the S pole of permanent magnet 2 to the N pole of permanent magnet 2. The easy magnetization axis direction C can be determined based on the measurement of the magnetic flux distribution of permanent magnet 2. The easy magnetization axis direction C can also be determined based on the measurement of the magnetic flux distribution of an analytical sample separated from permanent magnet 2.

[0033] In this embodiment, the permanent magnet 2 is a cuboid (plate). However, the shape of the permanent magnet 2 is not limited to a cuboid. For example, the shape of the permanent magnet 2 can also be a cube, a polygonal prism, an arc segment, a sector, a sphere, a circular plate, a cylinder, a tube, or a ring. The shape of the cross-section 2cs of the permanent magnet 2 can also be, for example, a polygon, an arc (chord), a bow shape, an arch shape, a C shape, or a circle.

[0034] Figure 2 yes Figure 1B An enlarged view of a portion (region II) of section 2cs shown. (See diagram below.) Figure 2 As shown, the permanent magnet 2 contains multiple main phase particles 4.

[0035] In a cross section 2cs that is approximately parallel to the easy magnetization axis C, multiple voids 8 are formed. These voids 8 can also be referred to as multiple pores. At least a portion of the voids 8 can form at a two-grain boundary. At least a portion of the voids 8 can form at a multi-grain boundary. The direction in the cross section 2cs that is approximately perpendicular to the easy magnetization axis C is described as the "AB direction".

[0036] like Figure 3A and Figure 3BAs shown, the direction in which each of the multiple gaps 8 in section 2cs extends is denoted as VD. The direction in which each gap 8 extends can also be called its longitudinal direction. The shape (outline) of each gap 8 in section 2cs can be approximated by an ellipse 8E. The approximation of the shape of each gap 8 using ellipse 8E can be implemented through least squares fitting. The major axis direction of the ellipse 8E that approximates the shape of each gap 8 can be considered as VD. The angle between the AB direction and VD is denoted as θ (unit: °). Gaps with θ between 60° and 120° can also be called gaps extending along the easy magnetization axis direction C. Gaps with θ between 60° and 120° are described as "C-axis extending voids". Gaps with θ less than 60° and gaps with θ greater than 120° can also be called gaps extending along the AB direction. A gap with θ less than 60° and a gap with θ greater than 120° are described as "AB axis extending void".

[0037] The horizontal axis of the frequency distribution of multiple gaps 8 in a cross section 2cs, approximately parallel to the easy magnetization axis C, represents θ. The horizontal axis of the frequency distribution ranges from 0° to 180°. The vertical axis of the frequency distribution represents the frequency (number) of the gaps 8. For example, the frequency distribution can be obtained using... Figure 4A The frequency distribution curve FA is shown below. Frequency distribution can also be represented using... Figure 4B The histogram shown is represented by FB.

[0038] The frequency distribution of multiple gaps 8 in the cross section 2cs, which is approximately parallel to the easy magnetization axis C, is maximum in the range where θ is between 60° and 120°. In other words, the maximum value Fmax of the frequency distribution of multiple gaps 8 in the cross section 2cs, which is approximately parallel to the easy magnetization axis C, is within the range where θ is between 60° and 120°. In other words, in the cross section 2cs, which is approximately parallel to the easy magnetization axis C, the angle θ of the gap 8 with the highest frequency is... Fmax The angle is between 60° and 120°. In other words, the gap 8 with the highest frequency in the cross section 2cs, which is roughly parallel to the easy magnetization axis C, is the C-axis extension gap.

[0039] The frequency distribution of multiple voids 8 in the surface portion 2S, which belongs to section 2cs, can be maximized in the range of θ being between 60° and 120°.

[0040] The frequency distribution of multiple gaps 8 in the central part 2C of section 2cs (described later) can be maximized in the range of θ being between 60° and 120°.

[0041] As described above, the eddy current I generated in the permanent magnet 2 constituting the rotor of the motor flows in a direction approximately perpendicular to the easy magnetization axis direction C (AB direction). The path of the eddy current I flowing in the AB direction is composed of multiple principal phase particles 4 that are in contact with each other in the AB direction. However, due to the formation of the C-axis extension gap (8), the multiple principal phase particles 4 adjacent to each other in the AB direction are easily separated. That is, the path of the eddy current I formed in the AB direction is easily interrupted by the C-axis extension gap (8). Therefore, when the angle θ of the gap 8 with the highest frequency is between 60° and 120°, the resistivity of the permanent magnet 2 in the direction approximately perpendicular to the easy magnetization axis direction C (AB direction) is effectively increased due to the relatively large number of C-axis extension gaps (8).

[0042] Even with an AB-axis extending gap formed in section 2cs, adjacent principal phase particles 4 in the AB direction are not easily separated from each other. That is, the path of the eddy current I formed in the AB direction is not easily interrupted by the AB-axis extending gap. Therefore, when the gap 8 with the highest frequency in section 2cs, which is approximately parallel to the easy magnetization axis C, is an AB-axis extending gap, the resistivity of the permanent magnet 2 in the AB direction is difficult to increase. Specifically, when the angle θ of the gap 8 with the highest frequency is greater than 0° and less than 60°, or greater than 120° and less than 180°, the resistivity of the permanent magnet 2 in the AB direction is difficult to increase.

[0043] The area of ​​the cross section 2cs, approximately parallel to the easy magnetization axis C, can be represented as Acs. The total number of gaps 8 (C-axis extension gaps) with θ between 60° and 120° can be represented as N. N / Acs can be 50 gaps / (mm). 2 The above 3000 pieces / (mm) 2 Below, or 210.9 pieces / (mm) 2 The above is 742.2 pieces / (mm). 2 Below. When N / Acs is within the above range, the resistivity of the permanent magnet 2 in the direction approximately perpendicular to the easy magnetization axis C (direction AB) tends to increase. N / Acs can also be referred to as the number of C-axis extension gaps per unit area of ​​the cross section 2cs approximately parallel to the easy magnetization axis C.

[0044] The total area of ​​the gaps 8 (C-axis extension gaps) with θ between 60° and 120° can be expressed as A. 60-120 A 60-120 / Acs can be between 0.2% and 5.0%, or between 0.62% and 2.44%. In A 60-120When / Acs is within the above range, the resistivity of permanent magnet 2 in the direction approximately perpendicular to the easy magnetization axis direction C (AB direction) tends to increase.

[0045] For example, the area of ​​each gap 8 can be 5 (μm). 2 Above 5000 (μm) 2 the following.

[0046] The area ratio AR of the multiple voids 8 in a cross section 2cs that is approximately parallel to the easy magnetization axis C is between 1% and 5%. The area ratio AR can be expressed as Av / Acs (unit: %). Av can be the sum of the areas (opening areas) of all voids 8 measured in the cross section 2cs that is approximately parallel to the easy magnetization axis C. Acs can be the area of ​​the cross section 2cs (the cross section used to measure Av) that is approximately parallel to the easy magnetization axis C.

[0047] As described above, the gaps 8 can increase the resistivity of the permanent magnet 2. Therefore, as the area fraction AR of the multiple gaps 8 in the cross section 2cs increases, the resistivity of the permanent magnet 2 increases. When the area fraction AR of the multiple gaps 8 is 1% or more, the resistivity of the permanent magnet 2 in the direction approximately perpendicular to the easy magnetization axis direction C (AB direction) tends to increase. However, even when the area fraction AR is 1% or more, permanent magnets with an angle θ of the gaps 8 with the highest frequency of occurrence that is less than 60° or greater than 120° are unlikely to have high resistivity in the AB direction.

[0048] The main reason for the decrease in coercivity of permanent magnet 2 is the generation of reverse magnetic domains within it. With the application of a reverse magnetic field to permanent magnet 2, these reverse magnetic domains become the nuclei of magnetization reversal, and domain walls propagate from the reverse magnetic domains to the entire permanent magnet 2. This propagation of domain walls leads to magnetization reversal in each principal phase particle 4 within permanent magnet 2. This magnetization reversal in each principal phase particle 4 can be suppressed by the pinning of domain walls at pinning sites such as grain boundaries.

[0049] The greater the difference in anisotropic magnetic field strength between the pinning sites and the main phase particles 4, the easier it is to suppress domain wall movement through pinning. However, grain boundary phases in conventional hot-worked magnets cannot fully function as pinning sites. For example, conventional hot-worked magnets contain R-rich phases (secondary phases) with a higher concentration of rare earth elements R (such as Nd) than the main phase particles as grain boundary phases. The composition of the R-rich phase is close to that of Nd. 30 Fe 70 The R-rich phase is a ferromagnetic material, and also a soft magnetic material. Therefore, the difference in the intensity of the anisotropic magnetic field between the R-rich phase and the main phase particle 4 is small, and the R-rich phase cannot fully function as a pinning site.

[0050] The inventors of this invention discovered that by intentionally forming multiple gaps 8 as pinning sites in a hot-worked magnet, the coercivity of the hot-worked magnet increases. The strength of the anisotropic magnetic field in the gaps 8 is essentially zero, and the difference in strength of the anisotropic magnetic field between the gaps 8 and the main phase particles 4 is large, which can effectively suppress the movement of the domain walls through pinning at the gaps 8.

[0051] When the area fraction AR of the multiple voids 8 is 1% or more, the movement of the domain walls can be sufficiently suppressed by pinning the domain walls at the voids 8, and the permanent magnet 2 can have sufficiently high coercivity. However, as multiple voids 8 are formed in the permanent magnet 2, the volume ratio of the multiple principal phase particles 4 in the permanent magnet 2 is relatively reduced, which easily damages the orientation of each principal phase particle 4 in the easy magnetization axis direction C. As a result, the residual magnetic flux density of the permanent magnet 2 tends to decrease. When the area fraction AR of the multiple voids 8 is 5% or less, the decrease in residual magnetic flux density associated with the formation of voids 8 can be sufficiently suppressed. That is, when the area fraction AR of the multiple voids 8 is 1% or more and 5% or less, it is easy to balance high coercivity and high residual magnetic flux density. For the same reason, the area fraction AR of the multiple voids 8 can be 1.30% or more and 4.91% or less. Even when there are few grain boundary phases (R-rich phases, etc.) that function as pinning sites, this embodiment can achieve both high coercivity and high residual magnetic flux density. Even when the permanent magnet 2 does not contain heavy rare earth elements (both Dy and Tb), this embodiment can achieve both high coercivity and high residual magnetic flux density.

[0052] For example, the resistivity (ρ) of the permanent magnet 2 in the direction perpendicular to the easy magnetization axis C can be 1.70 μΩ·cm or more and 2.50 μΩ·cm or less, or 1.78 μΩ·cm or more and 2.29 μΩ·cm or less.

[0053] For example, the coercivity (HcJ) of permanent magnet 2 at 23°C 23 It can be above 800kA / m and below 3000kA / m, or above 800kA / m and below 1113kA / m.

[0054] For example, the coercivity (HcJ) of permanent magnet 2 at 150℃ 150 It can be above 300kA / m and below 1500kA / m, or above 303kA / m and below 473kA / m.

[0055] For example, the temperature coefficient β of coercivity can be above -0.50% / ℃ and below -0.35% / ℃, or above -0.50% / ℃ and below -0.45% / ℃. The temperature coefficient β is defined by Equation 1 below. In Equation 1 below, HcJ... 150 The coercivity is at 150℃. HcJ in Formula 1 below...23 The coercivity is 23℃.

[0056] β=100×(HcJ 150 -HcJ 23 ) / HcJ 23 (150-23) (1)

[0057] For example, the residual magnetic flux density (Br) of the permanent magnet 2 at room temperature can be above 1245mT and below 1500mT, or above 1248mT and below 1278mT.

[0058] For example, the rectangularity ratio (Hk / HcJ) of permanent magnet 2 can be 94.0% to 100% or 94.0% to 99.5%. Hk is the strength of the demagnetizing field in the second quadrant of the magnetization curve, which is equivalent to 90% of the residual magnetic flux density.

[0059] Frequency distribution of multiple gaps 8, N / Acs, A 60-120 / Acs, the area of ​​each void 8, and the area ratio AR can be found in a portion of a section 2cs that is approximately parallel to the easy magnetization axis direction C (e.g., Figure 1B and Figure 2 The measurements are shown in region II. For example, a portion of the cross section 2cs used for these measurements may have dimensions of 300 μm in length and 426 μm in width. The cross section 2cs, which is approximately parallel to the easy magnetization axis C, can be observed using a scanning electron microscope (SEM), and a frequency distribution can be constructed based on all the voids 8 contained within an observation field of view.

[0060] like Figure 2 As shown, multiple flat principal phase particles 4 can be observed in a cross section 2cs that is approximately parallel to the easy magnetization axis C of the permanent magnet 2. In other words, each principal phase particle 4 observed in cross section 2cs can be plate-shaped. The multiple flat principal phase particles 4 can be stacked along the easy magnetization axis C. The permanent magnet 2 may also contain secondary particles composed of multiple principal phase particles 4 bonded together. The permanent magnet 2 can contain multiple secondary particles. At least a portion of the voids 8 can be located at grain boundaries between the multiple secondary particles.

[0061] Each main phase particle 4 contains at least R (Nd, etc.), T, and B. Each main phase particle 4 can also be referred to as a grain (i.e., a primary particle). Each main phase particle 4 contains R²T. 14 Crystallization of B (single crystal or polycrystalline). R2T 14 B is a ferromagnetic ternary intermetallic compound. The main phase particle 4 can be composed solely of R2T. 14 The crystalline structure of B. R2T 14 Bo can crystallize into tetragonal crystals. That is, R2T 14The crystallographic axes of B are the a-axis, b-axis, and c-axis, which are orthogonal to each other. R²T 14 The lattice constant of B along the a-axis can be related to R²T. 14 The lattice constants of B along the b-axis are equal, R²T 14 The lattice constant of B along the c-axis can be different from the lattice constants along the a-axis and b-axis. (R²T) 14 The a-axis direction of B can be approximately parallel to the AB direction of permanent magnet 2. R2T 14 The b-axis direction of B can be approximately parallel to the AB direction of permanent magnet 2. R2T 14 The c-axis direction of B can be approximately parallel to the easy magnetization axis direction C of permanent magnet 2.

[0062] The main phase particle 4 may contain elements other than R, T, and B. For example, R2T constituting the main phase particle 4 14 B can be represented as (Nd) 1-x Pr x )2(Fe 1-y Co y ) 14 B. x can be greater than 0 and less than 1. y can be greater than 0 and less than 1. The main phase particle 4 can contain not only light rare earth elements but also heavy rare earth elements such as Tb and Dy as R. R2T 14 A portion of the B in B can be replaced by other elements such as carbon (C). The composition within the main phase particle 4 can be homogeneous. Alternatively, the composition within the main phase particle 4 can be non-homogeneous. For example, the concentration distributions of R, T, and B within the main phase particle 4 can exhibit gradients.

[0063] The main phase particle 4 can be composed of a surface portion and a central portion covered by the surface portion. The surface portion can also be called a shell, and the central portion can also be called a core. The surface portion of the main phase particle 4 can contain at least one heavy rare earth element selected from Tb and Dy. It is possible that the surface portion of each of all the main phase particles 4 contains at least one heavy rare earth element selected from Tb and Dy. It is also possible that the surface portion of a portion of all the main phase particles 4 contains at least one heavy rare earth element selected from Tb and Dy. By containing heavy rare earth elements in the surface portion, the anisotropic magnetic field tends to increase locally near the grain boundaries, making it less likely for magnetization reversal nuclei to form near the grain boundaries. As a result, the coercivity of the permanent magnet 2 increases at high temperatures (e.g., 100–200 °C). From the viewpoint of easily balancing the residual magnetic flux density (Br) and coercivity of the permanent magnet 2, it is preferable that the total concentration of heavy rare earth elements in the surface portion is higher than the total concentration of heavy rare earth elements in the central portion.

[0064] The volume ratio of the main phase (the proportion of the volume of all main phase particles 4 in the permanent magnet 2) is not particularly limited. For example, the volume ratio of the main phase can be 80% to 99%, 90% to 99%, 95% to 99%, or 95.09% to 98.7%. As the volume ratio of the main phase increases, the residual magnetic flux density of the permanent magnet 2 increases.

[0065] The permanent magnet 2 may also contain multiple R-rich phases as secondary phases. The R-rich phases may be located between multiple main phase particles 4. That is, the R-rich phase can be one of the grain boundary phases contained in the grain boundaries between multiple main phase particles 4. Grain boundaries containing R-rich phases can be multi-grain boundaries surrounded by three or more main phase particles 4. Grain boundaries containing R-rich phases can also be two-grain boundaries between two main phase particles 4. The R-rich phase can be a ferromagnetic or soft magnetic material. The R-rich phase contains at least R. For example, the R-rich phase can contain Nd as R. The R-rich phase 6 may also contain one or more other rare earth elements as R in addition to Nd. The R-rich phase may contain one or more elements other than R in addition to R. The R-rich phase may contain at least one component selected from metals, alloys, intermetallic compounds, and oxides. For example, part or all of the R-rich phase may consist only of at least one component selected from R monomers, R-containing alloys, and R-containing metallic compounds. Part or all of the R-rich phase may contain R oxides (R-oxide). For example, the oxide of R can be an oxide of Nd. The oxidized surface of the main phase particle 4 can also be an oxide of R. A portion of the R-rich phase can consist solely of oxides of R.

[0066] It is preferable that the concentration of R in the R-rich phase is higher than the average concentration of R in the main phase particles 4. It is also preferable that the concentration of R in the R-rich phase is higher than the average concentration of R in the aforementioned cross section 2cs. When the permanent magnet 2 contains multiple types of R, the concentration of R can be the sum of the concentrations of all types of R.

[0067] The average length of the minor axis of the principal phase particles 4 (primary particles) observed in the aforementioned cross section 2cs can be between 20 nm and 200 nm. When the average length of the minor axis of the principal phase particles 4 is within the aforementioned range, each principal phase particle 4 (R2T) 14 The anisotropic growth of B crystals is sufficient, and the main phase particles 4 are easily oriented along the easy magnetization axis direction C, which easily increases coercivity, residual magnetic flux density, and rectangularity ratio. The average length of the major axis of the main phase particles 4 (primary particles) observed in the above cross section 2cs can be, for example, between 100 nm and 1000 nm.

[0068] The minor axis of each principal phase particle 4 observed in section 2cs can be approximately parallel to the easy magnetization axis direction C. The major axis of each principal phase particle 4 can be approximately perpendicular to the easy magnetization axis direction C. The shape of the principal phase particles 4 in section 2cs is not limited to rectangles. The shape of the principal phase particles 4 in section 2cs can be deformed. The shape of the principal phase particles 4 in section 2cs can be different. In the case of deformed shape of the principal phase particles 4 in section 2cs, the shape of the principal phase particles 4 can be approximated by the quadrilateral with the smallest area among the quadrilaterals circumscribed by the principal phase particles 4. The quadrilateral can be rectangular. The length of the short side of the quadrilateral can be regarded as the length of the minor axis of the principal phase particles 4, and the length of the long side of the quadrilateral can be regarded as the length of the major axis of the principal phase particles 4. The average value of the length of the minor axis of the principal phase particles 4 can be calculated based on the measured values ​​of the lengths of the minor axes of all the principal phase particles 4 present in the reflected electron image of section 2cs taken by scanning electron microscopy (SEM). The average length of the major axis of the principal phase particle 4 can also be calculated based on the measured lengths of the major axes of all principal phase particles 4 present in the aforementioned reflected electron images. However, the size of the principal phase particles 4 spilling out of the reflected electron images is excluded from the calculation of the average value. The maximum size of the reflected electron images used in measuring the lengths of the minor and major axes of the principal phase particles 4 can be, for example, 120 μm x 80 μm or 80 μm x 120 μm. Several representative locations within these reflected electron images taken at low magnification can be selected, and reflected electron images of each location can be taken at high magnification. Furthermore, the average lengths of the major and minor axes can be calculated based on the lengths of the major and minor axes of all principal phase particles 4 measured in the high-magnification reflected electron images. In determining the shape (outline) of the principal phase particles 4 and measuring the size of the principal phase particles 4 (the quadrilateral circumscribed with the principal phase particles 4), commercially available image analysis software can be used.

[0069] The width (size) of the permanent magnet 2 in the easy magnetization axis direction C can be, for example, several mm to hundreds of mm, or tens of mm to hundreds of mm. The size of the permanent magnet 2 in the AB direction can be, for example, several mm to hundreds of mm, or tens of mm to hundreds of mm.

[0070] The grain boundaries may contain grain boundary phases other than the R-rich phase. For example, the grain boundaries may contain grain boundary phases containing elements introduced into the permanent magnet 2 through a grain boundary diffusion process described later. The elements introduced into the permanent magnet 2 through the grain boundary diffusion process may be at least one heavy rare earth element selected from Tb and Dy. The elements introduced into the permanent magnet 2 through the grain boundary diffusion process may also be heavy rare earth elements and light rare earth elements, and the light rare earth elements may be at least one selected from Nd and Pr. The elements introduced into the permanent magnet 2 through the grain boundary diffusion process may also be heavy rare earth elements, light rare earth elements, and copper.

[0071] The width of the permanent magnet 2 along the easy magnetization axis C is denoted as t. The surface portion 2S of the permanent magnet 2 is defined as the portion along the easy magnetization axis C with a depth of 0 to 0.25t from the surface of the permanent magnet 2. The area ratio of the plurality of voids 8 in the surface portion 2S is denoted as ARs%. ARs is measured on the surface portion 2S exposed in the cross section 2cs. The central portion 2C of the permanent magnet 2 is defined as the portion along the easy magnetization axis C with a depth greater than 0.25t and less than 0.5t from the surface of the permanent magnet 2. The central portion 2C can also be the portion along the easy magnetization axis C with a depth greater than 0.25t and less than 0.75t from the surface of the permanent magnet 2. The area ratio of the plurality of voids 8 in the central portion 2C is denoted as ARc%. ARc is measured on the central portion 2C exposed in the cross section 2cs. ARs higher than ARc is preferable. As will be described later, the eddy current in the surface portion 2S tends to be greater than the eddy current in the central portion 2C. Therefore, the larger ARs is than ARc, the higher the resistivity of the surface portion 2S is than the resistivity of the central portion 2C, and the easier it is to reduce eddy current losses in the permanent magnet 2. For the same reason, ARs-ARc can be 1.0% to 4.0% or 1.5% to 4.0%. ARs and ARc can each be 1% to 5%.

[0072] Permanent magnet synchronous motors (PMSMs), such as IPM or SPM motors, are a type of alternating-current synchronous motor (ACSM). AC synchronous motors are driven by a rotating magnetic field generated by alternating current. In electric vehicles or hybrid vehicles, a wide range of rotational speeds is required for driving AC synchronous motors. The speed of an AC synchronous motor is proportional to the frequency of the alternating current; therefore, a wide range of frequency alternating currents is required to drive an AC synchronous motor within a wide rotational speed range. This wide range of frequency alternating currents can be generated by an inverter.

[0073] For example, the fundamental frequency range of the inverter used in the AC synchronous motor of a car is about 100 to 1500 Hz. On the other hand, the inverter usually outputs a pseudo-sine wave through PWM (Pulse Width Modulation) control. In this case, the carrier frequency is about 20 kHz.

[0074] The aforementioned alternating current is a factor in the penetration depth of the eddy currents generated in the permanent magnet 2. Typically, the penetration depth is the depth from the metal surface that is (1 / e) times (approximately 36.8%) the eddy current at the metal surface. e is the Napier constant. For example, the penetration depth δ can be expressed by the following formula 2.

[0075] δ=[ρ / (πfμ)] 1 / 2 (2)

[0076] In Formula 2, ρ represents the resistivity of the metal (unit: ×10). -8 Ω·m). In Formula 2, f is the frequency of the alternating current. μ is the permeability of the metal (unit: H / m). Based on the resistivity and permeability of a typical RTB-type permanent magnet and the carrier frequency of the inverter (20kHz), the penetration depth of the permanent magnet 2 calculated by Formula 2 is approximately 1mm. That is, the eddy current decreases exponentially with increasing depth from the surface of the permanent magnet. In other words, the eddy current in the surface portion 2S of the permanent magnet 2 is significantly greater than the eddy current in the central portion 2C of the permanent magnet 2. Therefore, the more voids in the portion with a smaller depth from the surface of the permanent magnet 2 (i.e., the larger ARs), the higher the resistivity of the surface portion 2S, and the easier it is to reduce eddy current losses in the permanent magnet 2.

[0077] On the other hand, the eddy currents in the central portion 2C of the permanent magnet 2 are significantly smaller than those in the surface portion 2S. Therefore, the voids 8 formed in the central portion 2C are less likely to help reduce eddy current losses compared to the voids 8 formed in the surface portion 2S. That is, ARc is less likely to help reduce eddy current losses compared to ARs. On the contrary, the more voids 8 there are in the central portion 2C (i.e., the larger ARc is), the easier it is for the volume ratio of the main phase particles 4 in the central portion 2C to be relatively reduced. As a result, magnetic properties such as residual magnetic flux density are impaired. Therefore, for the improvement of magnetic properties, it is preferable that ARc is smaller than ARs.

[0078] Based on the above reasons, when ARs is greater than ARc, it is easy to balance the suppression of eddy currents in the surface part 2S and the excellent magnetic properties of the central part 2C.

[0079] The main phase particles 4, voids 8, and grain boundary phases can each be identified based on the contrast of images of the cross-section 2cs of the permanent magnet 2 taken by scanning electron microscopy (SEM) or scanning transmission electron microscopy (STEM). The composition of the main phase particles 4 and the grain boundary phases can be analyzed by an electron probe microanalyzer (EPMA) equipped with energy-dispersive X-ray spectroscopy (EDS).

[0080] The overall composition of permanent magnet 2 will be described below. However, the composition of permanent magnet 2 is not limited to the composition described below. The content of each element in permanent magnet 2 may also be outside the ranges described below.

[0081] The R content in RTB-based permanent magnets can be between 28.00% and 33.00% by mass. Within this range, the residual magnetic flux density and coercivity of the permanent magnet 2 tend to increase. When the R content is 28.00% by mass or higher, crack formation in the permanent magnet 2 is easily suppressed during the thermoplastic forming process. When the R content is 28.00% by mass or higher, R2T, constituting the main phase particles 4, tends to form more readily. 14 B does not readily form the soft magnetic α-Fe phase. As a result, coercivity tends to increase. On the other hand, when the R content is 33.00% by mass or less, the segregation of the liquid phase (R-rich phase) on the surface of the permanent magnet 2 can be suppressed during the thermoplastic processing, and the sintering of the mold and the permanent magnet 2 can be suppressed. When the R content is 33.00% by mass or less, the formation of the R-rich phase 6 can be moderately suppressed, and the residual magnetic flux density tends to increase. From the viewpoint that the residual magnetic flux density and coercivity tend to increase, the combined proportion of Nd and Pr to the total rare earth element R can be 80 atomic% to 100 atomic% or 95 atomic% to 100 atomic%.

[0082] The combined content of Tb and Dy in permanent magnet 2 can be between 0.00% and 5.00% by mass. By including at least one heavy rare earth element, Tb or Dy, the magnetic properties (especially coercivity at high temperatures) of permanent magnet 2 are easily increased. However, permanent magnet 2 may also be free of Tb and Dy.

[0083] The boron (B) content in RTB-based permanent magnets can be between 0.8% and 1.1% by mass. When the B content is 0.8% or higher, it can suppress R2Fe. 17 The formation of identical heterogeneous phases easily increases coercivity and residual magnetic flux density. When the B content is below 1.1% by mass, R can be suppressed. 1+ε The formation of heterogeneous phases such as Fe4B4 (Boride) easily increases coercivity and residual magnetic flux density. When the B content is within the above-mentioned range, the rectangularity of permanent magnet 2 easily approaches 1.0.

[0084] When the total content of rare earth element R in permanent magnet 2 is more than 28.00% by mass and less than 33.00% by mass, and the content of B in permanent magnet 2 is more than 0.8% by mass and less than 1.1% by mass, the content of rare earth element R in permanent magnet 2 is greater than R2T. 14 The stoichiometry of B. As a result, during the thermoplastic processing described later, a liquid phase is easily generated at the grain boundaries. The liquid phase at the grain boundaries can promote grain growth (R2T). 14B) Anisotropic growth, grain boundary slip, and grain rotation. As a result, the c-axis of the grains tends to align in the stress direction, the proportion of grains with the same easy magnetization axis direction in all grains of permanent magnet 2 tends to increase, and the residual magnetic flux density of permanent magnet 2 tends to increase.

[0085] The permanent magnet 2 may contain gallium (Ga). The Ga content may be 0.03% by mass to 1.00% by mass, or 0.20% by mass to 0.80% by mass. When the Ga content is within the above range, the formation of secondary phases (e.g., phases containing R, T, and Ga) can be moderately suppressed, and the residual magnetic flux density and coercivity of the permanent magnet 2 can be easily increased. However, the permanent magnet 2 may also not contain Ga.

[0086] The permanent magnet 2 may contain aluminum (Al). The Al content in the permanent magnet 2 may be 0.01% by mass to 0.2% by mass, or 0.04% by mass to 0.07% by mass. When the Al content is within the above range, the coercivity and corrosion resistance of the permanent magnet are easily improved. However, the permanent magnet 2 may also not contain Al.

[0087] The permanent magnet 2 may contain copper (Cu). The Cu content in the permanent magnet 2 may be between 0.01% and 1.50% by mass, or between 0.04% and 0.50% by mass. By keeping the Cu content within the above range, the coercivity, corrosion resistance, and temperature characteristics of the permanent magnet 2 are easily improved. However, the permanent magnet 2 may also not contain Cu.

[0088] The permanent magnet 2 may contain cobalt (Co). The Co content in the permanent magnet may be between 0.30% and 6.00% by mass, or between 0.30% and 4.00% by mass. By containing Co, the Curie temperature of the permanent magnet 2 is easily increased. In addition, by containing Co, the corrosion resistance of the permanent magnet 2 is easily improved. However, the permanent magnet 2 may also not contain Co.

[0089] The remaining portion of the permanent magnet 2, excluding the aforementioned elements, may consist solely of Fe, or of Fe and other elements. For the permanent magnet 2 to possess sufficient magnetic properties, the total content of elements other than Fe in the remaining portion may be less than 5% by mass relative to the total mass of the permanent magnet 2.

[0090] The permanent magnet 2 may contain at least one element selected from silicon (Si), titanium (Ti), manganese (Mn), zirconium (Zr), vanadium (V), chromium (Cr), nickel (Ni), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), tungsten (W), bismuth (Bi), tin (Sn), calcium (Ca), carbon (C), nitrogen (N), oxygen (O), chlorine (Cl), sulfur (S), and fluorine (F) as other elements (e.g., unavoidable impurities). The total content of other elements in the permanent magnet 2 may be more than 0.001% by mass and less than 0.50% by mass.

[0091] The overall composition of the permanent magnet 2 can be analyzed by methods such as X-ray fluorescence (XRF) analysis, high-frequency inductively coupled plasma (ICP) luminescence analysis, non-dispersive infrared absorption (NDIR) method of inert gas melting, combustion in oxygen flow-infrared absorption method, and non-dispersive gas melting-thermal conductivity method.

[0092] Permanent magnet 2 can be applied to motors, generators, or actuators. For example, permanent magnet 2 can be used in a wide variety of fields such as hybrid vehicles, electric vehicles, hard disk drives, magnetic resonance imaging (MRI) devices, smartphones, digital cameras, slim TVs, scanners, air conditioners, heat pumps, cold storage, vacuum cleaners, washer-dryers, elevators, and wind turbines.

[0093] (Manufacturing method of permanent magnets)

[0094] The method for manufacturing a permanent magnet according to this embodiment includes at least a strip fabrication process, a hot pressing process, and a hot plastic deforming process. The method may also include other processes such as a grain boundary diffusion process following the hot plastic deforming process. However, the grain boundary diffusion process is not mandatory.

[0095] To suppress oxidation of permanent magnets and their semi-finished products during the manufacturing process, the manufacturing method of permanent magnets can be carried out in a non-oxidizing atmosphere. For example, a non-oxidizing atmosphere can be an inert gas such as argon (Ar). In addition to inert gases, a non-oxidizing atmosphere can also contain reducing gases such as hydrogen (H2).

[0096] The ribbon fabrication process involves creating alloy ribbons from raw metal using a rapid-solidification method. In this method, molten metal (the liquid form of the molten metal) is ejected from a nozzle at the front of a crucible onto the surface of a cooling roll. Upon contact with the cooling roll's surface, the molten metal is instantly propelled away by the high-speed rotating roll, forming multiple elongated ribbons. Through this contact with the cooling roll's surface, the molten metal is rapidly cooled and solidified. The result is the formation of multiple elongated alloy ribbons. A container is positioned in the direction in which the alloy ribbons are propelled by the cooling roll, and the ribbons are collected back into the container.

[0097] The molten metal is a metal (raw material metal) containing the elements that constitute the permanent magnet. The raw material metal can be, for example, a monomer of a rare earth element (metal monomer), an alloy containing rare earth elements, pure iron, a ferroborone alloy, or an alloy containing both. These raw material metals are weighed in a manner consistent with the desired composition of the permanent magnet.

[0098] Molten metal can be obtained by heating the raw material metal in a container using high-frequency induction heating. The temperature of the molten metal injected from the nozzle (injection temperature) can be, for example, about 1400°C. The heating rate until the raw material metal reaches the injection temperature can be, for example, about 20 to 100°C / second.

[0099] The surface of the cooling roller can be made of a metal with high thermal conductivity, such as Cu. The surface temperature of the cooling roller can be controlled by a refrigerant flowing inside the roller. The higher the cooling rate of the molten metal on the surface of the cooling roller, the higher the crystal density (R2T) within the alloy ribbon. 14 The finer the particle size (B), the higher the coercivity of the permanent magnet. The less molten material sprayed onto the surface of the cooling roller per unit time, the thinner the molten material adhering to the surface of the cooling roller, the faster the cooling rate, and the thinner the alloy strip. The higher the circumferential speed of the cooling roller, the thinner the molten material adhering to the surface of the cooling roller, the faster the cooling rate, and the thinner the alloy strip. The thickness of the main phase particles in the direction of the easy magnetization axis (the length of the minor axis of the main phase particles) depends on the thickness of the alloy strip (as well as the crushing and grading of the alloy strip). There is a trend that the thinner the alloy strip, the smaller the thickness (particle size) of the main phase particles, and the higher the coercivity of the permanent magnet. The thickness of the alloy strip can, for example, be 20 μm to 60 μm or 30 μm to 50 μm. The width of the alloy strip can, for example, be 1.0 mm to 5.0 mm.

[0100] Following the ribbon fabrication process, a crushing / grading process can be performed. The crushing / grading process involves using a crushing device to crush the alloy ribbon into coarse powder, and then grading the coarse powder to recover alloy powder with a specified particle size and aspect ratio. The alloy powder is a precursor to the main phase particles contained in the permanent magnet. The shape of each alloy particle constituting the alloy powder can be plate-like or sheet-like. The crushing method for the alloy ribbon can be, for example, using at least one of a cutting mill and a propeller mill. The device for grading the coarse powder can be a sieve. The particle size and particle size distribution of the alloy powder obtained through grading can be measured, for example, by laser diffraction scattering. The particle size of the alloy powder obtained through grading can be, for example, 60 μm to 2800 μm or 150 μm to 2800 μm.

[0101] Hot forming is a process of forming a shaped body by heating and pressurizing an alloy strip (alloy powder). For example, the alloy powder can be heated in a mold while being compressed using the mold. Pressurizing the alloy powder reduces the voids between the powder particles, resulting in a dense shaped body. Furthermore, heating the alloy powder in conjunction with pressurization allows a liquid phase (such as an Nd-rich phase or R-rich phase) to form from the surface of the powder. This liquid phase fills the voids (grain boundaries) between the powder particles, and the powder becomes lubricated by the liquid phase, thus obtaining a dense shaped body. A cold forming process can be performed before the hot forming process. In the cold forming process, the alloy powder can be pressurized at room temperature to form a shaped body. The shaped body obtained from the cold forming process can be heated and pressurized simultaneously during the hot forming process to densify it. The temperature of the alloy powder in the hot forming process (hot forming temperature) can be, for example, between 650°C and 750°C. If the hot forming temperature is too high, crystallization (R2T) of the alloy powder can occur. 14 B) Excessive particle growth can easily reduce the coercivity of the permanent magnet. The pressure applied to the alloy powder during the hot forming process (hot forming pressure) can be between 50 MPa and 200 MPa. The time for which the hot forming temperature and hot forming pressure are maintained within the above range (hot forming time) can be, for example, between tens of seconds and hundreds of seconds.

[0102] Following the thermoforming process, a thermoplastic processing step is performed. The thermoplastic processing step involves hot extrusion of the molded body obtained from the thermoforming process to produce multiple principal phase particles (R2T) oriented in a specified direction, containing a c-axis (easy magnetization axis). 14The process of making a magnet substrate (with B-rich grains). For example, in a thermoplastic processing step, the molded body is heated while being extruded from a mold. Inside the mold, the grain boundary phase in the heated molded body liquefies to form a liquid phase (R-rich phase), and stress acts on the molded body in a predetermined direction, causing deformation of the alloy particles constituting the molded body. With the formation of the liquid phase and the deformation of the alloy particles, anisotropic growth of the grains occurs in the direction perpendicular to the c-axis of the grains. In addition, the liquid phase lubricates the grains, and forces act on the grains in response to the stress. As a result, the grains rotate by grain boundary slip, and the c-axis of each grain (main phase particle) is oriented approximately parallel to the stress direction. In other words, multiple flat main phase particles extending in a direction approximately perpendicular to the c-axis are stacked along the stress direction.

[0103] The temperature of the molded body in the thermoplastic processing process (thermoplastic processing temperature) can be, for example, above 700°C and below 900°C, or above 700°C and below 850°C.

[0104] When the thermoplastic processing temperature is too low, it is difficult for a liquid phase (such as an Nd-rich phase or an R-rich phase) to form at the grain boundaries within the molded body, making grain growth difficult and preventing grain rotation caused by grain boundary slip. As a result, the average length of the short axis of the main phase particles is easily less than 20 nm, and the c-axis of each main phase particle (grain) is difficult to be oriented approximately parallel to the stress direction.

[0105] At excessively high thermoplastic processing temperatures (e.g., above 900°C), the liquid phase (R-rich phase) excessively diffuses from the alloy particles, segregating at the particle surfaces and interparticle interfaces. Most of the liquid phase is consumed by grain growth. Because most of the liquid phase is consumed by grain growth, the growth of the main phase particles (grains) proceeds abnormally, easily forming coarse main phase particles with an average minor axis length exceeding 200 nm. These coarse main phase particles are difficult to orient along their easily magnetized axes.

[0106] The extrusion speed of hot extrusion molding can be 10. -2 The extrusion speed is between 9.9 mm / s and 10 mm / s. When the extrusion speed is too high (e.g., 10 mm / s or higher), the anisotropic growth of the main phase particles (grains) in the molded body cannot proceed sufficiently. Therefore, the average length of the minor axis of the main phase particles (primary particles) tends to be less than 20 nm. That is, when the extrusion speed is too high, the molded body is extruded from the die before the anisotropic growth of the grains in the molded body can proceed sufficiently. As a result, the c-axis of each main phase particle (grain) is difficult to align approximately parallel to the stress direction.

[0107] The pressure applied to the molded body during the thermoplastic processing step (thermoplastic processing pressure) can be between 50 MPa and 200 MPa. The time for which the thermoplastic processing temperature and thermoplastic processing pressure are maintained within the above range (thermoplastic processing time) can be, for example, tens of seconds.

[0108] The molds used in thermoplastic processing are cylindrical. That is, the cavity formed within the mold extends through the mold from the end face of the mold with the inlet for the molded part (starting face) to the end face of the mold with the extrusion port for the molded part (ending face). The starting face and the ending face are parallel planes. The direction from the starting face to the ending face is the extrusion direction of the molded part, and the extrusion direction is perpendicular to both the starting and ending faces. The opening area of ​​the extrusion port for the molded part is smaller than the opening area of ​​the inlet for the molded part.

[0109] A specific example of the cavity formed within the mold is shown in Figure 5 The chamber 10 is divided along the extrusion direction Z into an inlet-side region 10A, an intermediate region 10B, and an extrusion outlet-side region 10C. The inlet-side region 10A is open on the starting end face. The extrusion outlet-side region 10C is open on the ending end face. The intermediate region 10B is located between the inlet-side region 10A and the extrusion outlet-side region 10C in the extrusion direction Z.

[0110] The outlet of the intermediate region 10B (i.e., the boundary between the intermediate region 10B and the extrusion port side region 10C) is referred to as the "first outlet". The outlet of the extrusion port side region 10C (i.e., the extrusion port of the shaped body on the end face) is referred to as the "second outlet".

[0111] The cavity 10 in the cross-section of the die perpendicular to the extrusion direction Z (the cross-section of the die parallel to the starting and ending faces) is a quadrilateral with all four corners being right angles. One opposite pair of sides of this quadrilateral is referred to as the first side, and the other opposite pair of sides of the quadrilateral is referred to as the second side.

[0112] The length xa of the first side in the inlet-side region 10A is constant. The length ya of the second side in the inlet-side region 10A is also constant. That is, the opening area of ​​the inlet-side region 10A in the cross-section perpendicular to the extrusion direction Z is constant. In the intermediate region 10B, the length xa of the first side gradually decreases along the extrusion direction Z, eventually becoming approximately the same as the length xc1 of the first side of the first outlet (the outlet of the intermediate region 10B). Therefore, the first side in the extrusion outlet-side region 10C is shorter than the first side in the inlet-side region 10A. Furthermore, in the intermediate region 10B, the length ya of the second side gradually increases along the extrusion direction Z, eventually becoming approximately the same as the length yc1 of the second side of the first outlet (the outlet of the intermediate region 10B). Therefore, the second side in the extrusion outlet-side region 10C is longer than the second side in the inlet-side region 10A. Additionally, the opening area of ​​the intermediate region 10B in the cross-section perpendicular to the extrusion direction Z gradually decreases along the extrusion direction Z, eventually becoming approximately the same as the opening area of ​​the extrusion outlet-side region 10C in the cross-section perpendicular to the extrusion direction Z. Therefore, the opening area of ​​the extrusion port side region 10C in the cross section perpendicular to the extrusion direction Z is smaller than the opening area of ​​the inlet side region 10A in the cross section perpendicular to the extrusion direction Z.

[0113] The length of the first side in the extrusion port side region 10C is constant. That is, the length xc2 of the first side of the second outlet (the outlet of the extrusion port side region 10C) is equal to the length xc1 of the first side of the first outlet (the outlet of the intermediate region 10B).

[0114] The length of the second side in the extrusion port side region 10C gradually increases along the extrusion direction Z. That is, the length yc2 of the second side of the second outlet (the outlet of the extrusion port side region 10C) is slightly larger than the length yc1 of the second side of the first outlet (the outlet of the middle region 10B).

[0115] The length of the second side in the extrusion port side region 10C gradually increases along the extrusion direction Z. Therefore, the opening area of ​​the extrusion port side region 10C in the cross-section perpendicular to the extrusion direction Z also gradually increases along the extrusion direction Z. Consequently, the opening area of ​​the second outlet (the outlet of the extrusion port side region 10C) is slightly larger than the opening area of ​​the first outlet (the outlet of the intermediate region 10B). However, the opening area of ​​the second outlet (the outlet of the extrusion port side region 10C) is smaller than the opening area of ​​the inlet side region 10A in the cross-section perpendicular to the extrusion direction Z.

[0116] Figure 6A This refers to the cross-section CS1 of the mold for the first outlet (the outlet of the intermediate region 10B) mentioned above. Additionally, Figure 6B The section CS2 represents the die section of the second outlet (the outlet of the extrusion port side region 10C) described above. Both the die sections CS1 and CS2 are perpendicular to the extrusion direction Z.

[0117] As described above, the opening area of ​​the extrusion port side region 10C in the cross-section perpendicular to the extrusion direction Z is smaller than the opening area of ​​the inlet side region 10A in the cross-section perpendicular to the extrusion direction Z, and the first side in the extrusion port side region 10C (terminal face) is shorter than the second side in the extrusion port side region 10C (terminal face). In the intermediate region 10B, the length xa of the first side gradually decreases along the extrusion direction Z, and in the intermediate region 10B, the length ya of the second side gradually increases along the extrusion direction Z. Therefore, in the intermediate region 10B and the extrusion port side region 10C, stress approximately parallel to the first side acts on the molded body, causing grain boundary slip and rotation of the main phase particles. As a result, the c-axis of the main phase particles is oriented along the stress direction (the direction of the first side). That is, the easy magnetization axis direction C of the molded body (the magnet substrate obtained by hot extrusion molding) is approximately aligned with the direction X of the first side in the extrusion port side region 10C (terminal face). In other words, the AB direction of the molded body is approximately aligned with the direction Y of the second side in the extrusion port side region 10C (terminal face).

[0118] The temperature of the extrusion port side region 10C gradually decreases along the extrusion direction Z. That is, the temperature T1 of the first outlet (the outlet of the intermediate region 10B) is higher than the temperature T2 of the second outlet (the outlet of the extrusion port side region 10C). Therefore, the temperature of the molded body gradually decreases during movement within the extrusion port side region 10C. For example, the temperature T1 of the first outlet (the outlet of the intermediate region 10B) can be adjusted to be above 780°C and below 790°C, and the temperature T2 of the second outlet (the outlet of the extrusion port side region 10C) can be adjusted to (T1-30)°C (i.e., above 750°C and below 760°C).

[0119] The main phase particles (Nd2Fe) along the easy magnetization axis direction C (c-axis direction) 14 The thermal expansion coefficient of tetragonal B is 6.5 × 10⁻⁶. -6 (1 / K). On the other hand, the thermal expansion coefficient of the principal phase particles in the AB direction (a-axis and b-axis directions) is -1.5 × 10⁻⁶. -6 (1 / K). Therefore, as the temperature of the molded body in the extrusion side region 10C decreases, the molded body tends to shrink in the direction of the easy magnetization axis C (direction X of the first side) and tends to expand in the AB direction (direction Y of the second side).

[0120] The shrinkage of the molded body in the easy magnetization axis direction C (direction X of the first side) and the expansion of the molded body in the AB direction (direction Y of the second side) can be considered. By adjusting the size ratio of the first side and the second side in the extrusion port side region 10C, the disorder of the orientation of the main phase particles in the molded body that accompanies the decrease in the temperature of the molded body can be suppressed.

[0121] As the pressure applied to the molded body by the mold decreases or is eliminated, the molded body expands. That is, the molded body expands due to springback.

[0122] As described above, the length of the first side in the extrusion port side region 10C is constant. That is, the length xc2 of the first side in the second outlet (the outlet of the extrusion port side region 10C) is equal to the length xc1 of the first side in the first outlet (the outlet of the intermediate region 10B). Therefore, the pressure acting on the molded body in the easy magnetization axis direction C (direction X of the first side) in the extrusion port side region 10C is approximately constant. As a result, the molded body in the extrusion port side region 10C is difficult to expand in the easy magnetization axis direction C (direction X of the first side). In other words, it is difficult for multiple adjacent main phase particles 4 in the molded body to be spaced apart in the easy magnetization axis direction C (direction X of the first side).

[0123] On the other hand, the length of the second side in the extrusion port side region 10C gradually increases along the extrusion direction Z. That is, the length yc2 of the second side in the second outlet (the outlet of the extrusion port side region 10C) is slightly larger than the length yc1 of the second side in the first outlet (the outlet of the intermediate region 10B). Therefore, the pressure acting on the molded body in the AB direction (the direction Y of the second side) in the extrusion port side region 10C gradually decreases. As a result, the molded body in the extrusion port side region 10C easily expands in the AB direction (the direction Y of the second side) due to springback. Due to the springback of the molded body in the AB direction (the direction Y of the second side), multiple adjacent main phase particles 4 in the molded body are easily separated in the AB direction (the direction Y of the second side). That is, due to the springback of the molded body in the AB direction (the direction Y of the second side), a gap 8 extending along the easy magnetization axis direction C (C-axis extended gap) can be formed between multiple adjacent main phase particles 4 (see reference). Figure 6B The cross section CS2 of the mold in the middle.

[0124] yc2 / yc1 can be 30.005 / 30.00 or higher, 30.04 / 30.00 or lower, or 30.01 / 30.00 or higher, 30.03 / 30.00 or lower.

[0125] As yc2 / yc1 increases, the number and volume of gaps 8 tend to increase. By adjusting yc2 / yc1 within the aforementioned range, it is easy to find the angle θ with the highest frequency of gap 8. Fmax N / Acs, A 60-120 / Acs, the area of ​​each void 8, and the area ratio AR are each controlled within the aforementioned desired range.

[0126] Assuming that the length xc2 of the first side of the second outlet is greater than the length xc1 of the first side of the first outlet, and the length yc2 of the second side of the second outlet is equal to the length yc1 of the second side of the first outlet, it is difficult to form a gap 8 extending along the easy magnetization axis direction C in the molded body. In other words, if the size of the extrusion port side region 10C in the easy magnetization axis direction C (direction X of the first side) gradually increases along the extrusion direction Z, and the size of the extrusion port side region 10C in the AB direction (direction Y of the first side) is constant, the molded body in the extrusion port side region 10C is prone to expand in the easy magnetization axis direction C (direction X of the first side), but difficult to expand in the AB direction (direction Y of the second side). As a result, multiple adjacent main phase particles 4 in the molded body are easily separated in the easy magnetization axis direction C, and a gap 8 extending along the AB direction (AB axis extension gap) is easily formed between multiple adjacent main phase particles 4 (refer to...). Figure 6C The cross section CS3 of the mold in the middle.

[0127] The purpose of thermoplastic processing is to obtain a dense magnet substrate (molded body). To obtain a fully dense molded body, thermoplastic processing under high temperature and high pressure is required, and time is also needed.

[0128] However, high temperatures coarsen the grain size of the raw material (alloy strip). Coarsening the grain size reduces coercivity, impairs forgeability, damages the orientation of the grains (main phase particles), and reduces residual magnetic flux density. High pressure accelerates die wear, leading to reduced productivity. Processing time also contributes to reduced productivity. Reduced forgeability further contributes to reduced productivity.

[0129] For the reasons stated above, complete densification of the molded body may lead to a decrease in the magnetic properties of the final permanent magnet and a reduction in productivity. Therefore, obtaining a fully densified molded body is not necessary. The degree of densification should be determined from the perspective of balancing the magnetic properties of the permanent magnet with productivity.

[0130] Generally, thermoforming processes differ from cold isostatic pressing (CIP) or hot isostatic pressing (HIP) by using uniaxial or biaxial pressure forming. Uniaxial or biaxial pressure forming creates a stress distribution on the magnet substrate (molded body) within the mold. Due to pressure transmission within the mold, the stress is lower near the inner wall of the mold (i.e., the surface of the molded body). As a result, the density of the molded body tends to be lower on the surface portion in contact with the inner wall of the mold. That is, voids tend to remain on the surface portion of the molded body in contact with the inner wall of the mold. These voids remain in the molded body and do not disappear after the thermoplastic processing steps; they are not removed from the molded body. In other words, the voids remaining on the surface portion of the molded body will remain on the surface portion of the final permanent magnet.

[0131] For the reasons mentioned above, the area ratio ARs of multiple voids in the surface portion of a permanent magnet is more likely to be higher than the area ratio ARc of multiple voids in the central portion of the permanent magnet.

[0132] The magnet substrate obtained through the above process can be a finished permanent magnet. The magnet substrate after the following grain boundary diffusion process can also be a finished permanent magnet.

[0133] Following the thermoplastic processing, the following grain boundary diffusion process can be performed. The grain boundary diffusion process involves attaching a diffusion material containing heavy rare earth elements to the surface of a magnet substrate and heating both the diffusion material and the magnet substrate. Through heating the magnet substrate with the attached diffusion material, the heavy rare earth elements in the diffusion material diffuse from the surface of the magnet substrate into its interior. Inside the magnet substrate, the heavy rare earth elements diffuse through grain boundaries to the vicinity of the surface of the main phase particles. Near the surface of the main phase particles, some light rare earth elements (such as Nd) are replaced by heavy rare earth elements. Because the heavy rare earth elements are locally present near the surface of the main phase particles and at the grain boundaries, the anisotropic magnetic field locally increases near the grain boundaries, making it difficult to generate nuclei with magnetization reversal near the grain boundaries. As a result, a permanent magnet with high coercivity can be obtained.

[0134] The temperature (diffusion temperature) of the diffusion material and the magnet substrate in the grain boundary diffusion process can be, for example, 550°C or higher and 900°C or lower. The time (diffusion time) for maintaining the diffusion temperature within the above range can be, for example, 1 minute or higher and 1440 minutes or lower.

[0135] The diffusion material may contain at least one heavy rare earth element selected from Tb and Dy. In addition to heavy rare earth elements, the diffusion material may also contain at least one light rare earth element selected from Nd and Pr. In addition to heavy and light rare earth elements, the diffusion material may also contain Cu. The diffusion material may, for example, be a metal composed of one of the above elements, a hydride of one of the above elements, an alloy containing multiple of the above elements, or a hydride of the alloy. The diffusion material may be a powder. In the grain boundary diffusion process, a slurry containing the diffusion material and an organic solvent may be applied to the surface of the magnet substrate. In the grain boundary diffusion process, the surface of the magnet substrate may also be covered with a sheet containing the diffusion material and an adhesive. In the grain boundary diffusion process, the surface of the magnet substrate may also be covered with an alloy foil (ribbon) composed of the diffusion material.

[0136] To promote the diffusion of the diffusion material, the surface of the magnet substrate can be ground before the grain boundary diffusion process. To remove the diffusion material remaining on the surface of the magnet substrate after the grain boundary diffusion process, the surface of the magnet substrate can be ground after the grain boundary diffusion process.

[0137] The size and shape of the magnet substrate can be adjusted through cutting and grinding. A passivation layer can be formed on the surface of the magnet substrate through oxidation or chemical treatment. The surface of the magnet substrate can be covered with a protective film such as a resin film. The corrosion resistance of permanent magnets can be improved by utilizing passivation layers or protective films.

[0138] This invention is not necessarily limited to the embodiments described above. Various modifications can be made to this invention without departing from its spirit, and these modifications are also included in this invention.

[0139] [Example]

[0140] The present invention will be described in detail through the following embodiments and comparative examples. The present invention is not limited to the following embodiments.

[0141] <Making of Permanent Magnets>

[0142] (Example 1)

[0143] The following steps of Example 1 were carried out in a non-oxidizing atmosphere (Ar).

[0144] In the strip fabrication process, alloy powder (alloy strip) is produced from raw metal using an ultra-rapid solidification method. The raw metal (molten metal) used in the strip fabrication process contains Nd, Fe, Co, Ga, Al, and B.

[0145] The Nd content in the raw metal was 30.17% by mass.

[0146] The Co content in the raw metal is 3.96% by mass.

[0147] The Ga content in the raw material metal is 0.59% by mass.

[0148] The Al content in the raw metal is 0.04% by mass.

[0149] The boron content in the raw metal is 0.97% by mass.

[0150] The remaining portion of the raw material metals other than Nd, Co, Ga, Al and B is Fe.

[0151] In the thermoforming process, an alloy powder is heated within a mold while simultaneously being compressed using the mold to create a shaped body. The shaped body is a cuboid. Its dimensions are 22mm × 11mm × 80mm. The thermoforming temperature T is... HP The temperature is 750℃. The thermoforming pressure P HP The pressure is 100 MPa. The thermoforming time is 300 seconds.

[0152] Thermoplastic processing step following the thermoforming process. In the thermoplastic processing step, the aforementioned mold (forming a...) is used... Figure 5 The permanent magnet is made by hot extrusion molding of the molded body of the cavity 10 shown.

[0153] The length xa of the first side of the mold entrance (entrance side area 10A) is 22mm.

[0154] The length ya of the second side of the mold entrance (entrance side area 10A) is 11mm.

[0155] The temperature Ti at the mold inlet (temperature of the inlet-side region 10A) is maintained at the values ​​shown in Table 1 below.

[0156] The length xc1 of the first side of the first exit (the exit of the middle area 10B) is 7mm.

[0157] The length yc1 of the second side of the first exit (the exit of the middle area 10B) is 30mm.

[0158] The temperature T1 of the first outlet (the temperature of the intermediate region 10B) is maintained at the value shown in Table 1 below.

[0159] The length xc2 of the first side of the second outlet (the outlet in the extrusion side region 10C) is 7mm.

[0160] The length yc2 of the second side of the second outlet (the outlet of the extrusion side region 10C) is the value shown in Table 1 below.

[0161] The temperature T2 of the second outlet (the temperature of the outlet in the extrusion side region 10°C) is maintained as shown in Table 1 below.

[0162] The thermoplastic processing pressure (maximum pressure) is 60 MPa.

[0163] The extrusion speed for hot extrusion molding is 1 mm / s.

[0164] The permanent magnet of Example 1 is fabricated using the method described above. The direction of the first side is the same as the easy magnetization axis direction C of the permanent magnet. The direction of the second side is the same as the AB direction of the permanent magnet. The width t of the permanent magnet in the easy magnetization axis direction C is 7 mm.

[0165] (Examples 2-5 and Comparative Examples 1-7)

[0166] In the strip manufacturing processes of Examples 4, 6, and 7, the following raw material metals (molten metal) were used, which were different from those in Example 1.

[0167] In addition to Nd, Fe, Co, Ga, Al, and B, the raw material metals also contain Pr and Dy.

[0168] The Nd content in the raw metal is 10.65% by mass.

[0169] The Pr content in the raw material metal is 17.40% by mass.

[0170] The Dy content in the raw metal is 2.07% by mass.

[0171] The Co content in the raw metal is 3.40% by mass.

[0172] The Ga content in the raw material metal is 0.50% by mass.

[0173] The Al content in the raw metal is 0.07% by mass.

[0174] The boron content in the raw metal is 0.97% by mass.

[0175] The remaining portion of the raw material metals other than Nd, Pr, Dy, Co, Ga, Al, and B is Fe.

[0176] Thermoforming pressure P in Example 5 HP The pressure was 150 MPa. That is, in Example 5, a further densification of the permanent magnet was attempted through a thermoforming process under higher pressure. The thermoforming temperature T in Comparative Example 5 was... HP It is 740℃.

[0177] The inlet temperature Ti of the molds in Examples 2-5 and Comparative Examples 1-7 was maintained at the values ​​shown in Table 1 below.

[0178] The temperature T1 of the first outlet of each of Examples 2-5 and Comparative Examples 1-7 was maintained at the values ​​shown in Table 1 below.

[0179] The length yc2 of the second side of the second outlet of each of Examples 2-5 and Comparative Examples 1-7 was adjusted to the values ​​shown in Table 1 below.

[0180] The temperature T2 of the second outlet of each of Examples 2-5 and Comparative Examples 1-7 was maintained as shown in Table 1 below.

[0181] In addition to the above, permanent magnets for Examples 2-5 and Comparative Examples 1-7 were manufactured using the same method as in Example 1.

[0182] Analysis of Permanent Magnets

[0183] (Composition and microstructure of permanent magnets)

[0184] The cross-sections of the permanent magnets in Examples 1-5 and Comparative Examples 1-7 were observed using a scanning electron microscope (SEM). The observed cross-sections of each permanent magnet were parallel to the easy magnetization axis of each permanent magnet. The composition of the cross-sections of each permanent magnet was analyzed using an electron probe microanalyzer (EPMA) and energy-dispersive X-ray spectroscopy (EDS).

[0185] In any of Examples 1-5 and Comparative Examples 1-7, the permanent magnet has the following characteristics.

[0186] The permanent magnet contains multiple main phase particles (Nd2Fe). 14 B grains).

[0187] Multiple voids are formed in the permanent magnet.

[0188] The principal phase particles observed in the cross-section are flat.

[0189] Multiple main phase particles are stacked along the easy magnetization axis direction C.

[0190] (Regarding the measurement of voids)

[0191] like Figure 7A As shown, a reflected electron image i1a of a portion of the cross-section of the permanent magnet of Example 1 was captured using SEM. The cross-section of the reflected electron image i1a of Example 1 is parallel to the easy magnetization axis direction C. The longitudinal direction of the reflected electron image i1a is the easy magnetization axis direction C, and the transverse direction of the reflected electron image i1a is the AB direction. Figure 7B Image i1b in Figure 8A The images in ilc and Figure 8BImage ild is obtained from the reflected electron image i1a of Example 1. The vertical direction of each of images i1b, i1c, and i1d is the easy magnetization axis direction C. The horizontal direction of each of images i1b, i1c, and i1d is the AB direction. The size of each pixel (1 pixel) of images i1b, i1c, and i1d is (1 / 3) μm × (1 / 3) μm.

[0192] The brightness (in arbitrary units) of the reflected electron beam in any part of the reflected electron image i1a increases with the atomic weight of the element present in that part. The brightness of the reflected electron beam in any part of the reflected electron image i1a also increases with the concentration of the element in that part. Therefore, the brighter parts of the reflected electron image i1a are those with higher concentrations of elements with larger atomic weights (e.g., Nd). On the other hand, the darkest parts of the reflected electron image i1a are voids where no element is present.

[0193] Thresholding (binarization) of the reflected electron image i1a based on the RGB color model (Red-Green-Blue color model) yields a monochromatic image i1b. The black portions in the monochromatic image i1b represent voids. The area of ​​each void in the monochromatic image i1b is measured. Based on the measured area of ​​each void, the area ratio AR of the voids in the cross-section of the permanent magnet (reflected electron image ila) is calculated. The area ratio AR of Example 1 is shown in Table 1.

[0194] Image processing of the monochrome image i1b determines the contours of each gap in image i1b. The multiple closed curves contained in image i1c each correspond to the contours of each gap in image i1b. In the image processing of the monochrome image i1b, the area of ​​the black portion in the monochrome image i1b is 16.67 (μm). 2 The following portion is removed from the image as noise. In the image processing of monochrome image i1b, gaps that interrupt the image at the ends are removed from the image.

[0195] Image i1d is obtained by approximating the contours of each gap in image i1c with an ellipse. The approximation of the gap contours using ellipses is performed through a least-squares fitting method. The major axis direction of the ellipse approximating the contours of each gap in image i1d (i.e., the direction VD in which each gap 8 extends) is determined. Additionally, the angle θ between the AB direction and VD of each gap is measured. Since the direction VD of the gap extension approximated by a perfect circle cannot be specified, gaps approximated by a perfect circle are excluded from the measurement of angle θ.

[0196] The image processing in Embodiment 1 described above is performed by ImageJ, an image processing software used in the public domain.

[0197] Based on the image processing described in Example 1, the frequency distribution F of the gaps in the reflected electron image i1a (a cross-section approximately parallel to the easy magnetization axis direction C) is obtained. Furthermore, a weighted frequency distribution WF, weighted according to the area of ​​each gap, is obtained. The horizontal axis of both the frequency distribution F and the weighted frequency distribution WF is θ. The frequency distribution F of Example 1 is shown in... Figure 9A The weighted frequency distribution WF of Example 1 is shown in... Figure 9B .

[0198] Using the same method as described above, reflected electron images were captured and image processing was performed on the exposed surface portion of the permanent magnet in Example 1 at the aforementioned cross-section. The surface portion is the part located at a depth of 0 to 1.75 mm from the surface of the permanent magnet along the easy magnetization axis C. Based on the captured and image processed reflected electron images of the surface portion, the area ratio ARs of the voids in the surface portion was calculated.

[0199] Using the same method as described above, a reflected electron image was captured and processed at the central portion of the exposed cross-section of the permanent magnet in Example 1. The central portion is the part located at a depth greater than 1.75 mm and less than 3.5 mm from the surface of the permanent magnet along the easy magnetization axis C. Based on the captured and processed reflected electron image of the central portion, the area ratio ARc of the voids in the central portion was calculated.

[0200] The ARs, ARc, average values ​​of ARs and ARc, and ARs-Arc of Example 1 are shown in Table 3 below.

[0201] Using the same method as in Example 1, the area ratio AR, frequency distribution F, and weighted frequency distribution WF of Examples 2-5 and Comparative Examples 1-7 were obtained respectively.

[0202] The area ratios AR of Examples 2-5 and Comparative Examples 1-7 are shown in Table 1 below.

[0203] In Table 1 below, "θ-F" refers to the angle θ where the frequency distribution F contains the maximum frequency Fmax. Fmax The range of θ.

[0204] In Table 1 below, "θ-WF" refers to the angle θ where the weighted frequency WF is the maximum value of the weighted frequency distribution WF. WFmax The range of θ.

[0205] In Table 1 below, "0-30" refers to "above 0° and less than 30°".

[0206] In Table 1 below, "30-60" refers to "30° or higher and less than 60°".

[0207] In Table 1 below, "60-90" refers to "60° or higher and 90° or lower".

[0208] In Table 1 below, "90-120" refers to "90° or higher and 120° or lower".

[0209] In Table 1 below, "120-150" refers to "greater than 120° and less than 150°".

[0210] In Table 1 below, "150~180" means "greater than 150° and less than 180°".

[0211] In Table 1 below, N / Acs represents the number of C-axis extension gaps per unit area of ​​the reflected electron image (cross-section of the permanent magnet). The detailed definition of N / Acs is as described above.

[0212] A in Table 1 below 60-120 / Acs is the ratio of the total area of ​​the C-axis extended gap to the area of ​​the reflected electron image (the cross-section of the permanent magnet). A 60-120 The detailed definition of / Acs is as described above.

[0213] In Comparative Examples 1 and 2, the number of detected voids was very small, making it difficult to accurately determine θ-F and θ-WF.

[0214] In Comparative Example 6, no C-axis extension gap was detected.

[0215] Using the same method as in Example 1, ARs and ARc for Example 5 were calculated. The ARs, ARc, average values ​​of ARs and ARc, and ARs-ARc for Example 5 are shown in Table 3 below.

[0216] (Resistivity of permanent magnets)

[0217] A sample for resistivity measurement was prepared using the permanent magnet described in Example 1. The sample was a cuboid with dimensions of 10.0 mm (length) × 1.0 mm (width) × 0.5 mm (thickness). The thickness direction of the sample (the direction of the 0.5 mm side) was the easy magnetization axis direction C. The resistivity ρ of the sample in the direction perpendicular to the easy magnetization axis direction C was measured using the four-probe method. In the four-probe method, the tips of four probes were pressed against the surface of the sample. The surface of the sample against which the four probes were pressed was the surface perpendicular to the easy magnetization axis direction C (a surface with dimensions of 10.0 mm (length) × 1.0 mm (width)). The probe spacing was 1.5 mm. The measuring current was adjusted to 100 mA. In the resistivity measurement using the four-probe method, a resistivity meter (LORESTA GP) manufactured by Nitto Seiko Analytech Co., Ltd. (formerly Mitsubishi Chemical Analytech Co., Ltd.) was used.

[0218] Using the same method as in Example 1, the resistivity ρ of Examples 2-5 and Comparative Examples 1-7 was measured. The resistivity ρ of Examples 1-5 and Comparative Examples 1-7 is shown in Table 2 below.

[0219] (Magnetic properties of permanent magnets)

[0220] The coercivity, residual magnetic flux density, and rectangularity ratio of the permanent magnets in Examples 1-5 and Comparative Examples 1-7 were measured. The residual magnetic flux density, coercivity, and rectangularity ratio were measured using a BH tracer. As the coercivity, the coercivity (HcJ) at 23°C was measured. 23 ) and coercivity of 150° (HcJ) 150 The residual magnetic flux density (Br) was measured at room temperature. The rectangularity ratio (Hk / HcJ) was measured at 23°C. The measured values ​​of the magnetic properties of each permanent magnet and the temperature coefficient β of the coercivity are shown in Table 2 below. The temperature coefficient β is defined as described above.

[0221]

[0222]

[0223] [Table 3]

[0224] Table 3 ARs ARc average value ARs-ARc unit ％ ％ ％ ％ Example 1 2.58 1.08 1.83 1.50 Example 5 1.72 1.01 1.37 0.71

[0225] Industrial availability

[0226] For example, the RTB-type permanent magnet of the present invention is suitable for use as a magnet constituting the rotor of an IPM motor or an SPM motor.

Claims

1. An RTB-based permanent magnet, comprising rare earth element R, transition metal elements T and B, wherein, The RTB-based permanent magnet contains at least Nd as R. The RTB-based permanent magnet contains at least Fe as T. The RTB-based permanent magnet has multiple main phase particles. The plurality of main phase particles contain at least R, T, and B. Multiple voids are formed in the cross-section of the RTB-based permanent magnet. The cross section is approximately parallel to the easy magnetization axis of the RTB system permanent magnet. The area ratio of the plurality of voids in the cross-section is more than 1% and less than 5%. The direction perpendicular to the easy magnetization axis in the cross section is represented by the AB direction. The direction in which each of the plurality of voids in the cross section extends is denoted as VD. The angle between the AB direction and the VD direction is denoted as θ. The horizontal axis of the frequency distribution of the plurality of voids in the cross section represents θ. The horizontal axis of the frequency distribution ranges from 0° to 180°. The frequency distribution is highest in the range where θ is between 60° and 120°.

2. The RTB-based permanent magnet according to claim 1, wherein, The plurality of main phase particles are flat in the cross-section. The plurality of main phase particles are stacked along the direction of the easy magnetization axis.

3. The RTB-based permanent magnet according to claim 1, wherein, The average length of the short axis of the plurality of main phase particles in the cross section is between 20 nm and 200 nm.

4. The RTB-based permanent magnet according to claim 1, wherein, The content of R is 28% by mass or more and 33% by mass or less. The content of B is more than 0.8% by mass and less than 1.1% by mass.

5. The RTB-type permanent magnet according to claim 1 is a hot-worked magnet.

6. The RTB-based permanent magnet according to claim 1, wherein, Let the width of the RTB system permanent magnet along the easy magnetization axis be t. The portion of the RTB-based permanent magnet located at a depth of 0 to 0.25t from the surface of the magnetized axis is defined as the surface portion of the RTB-based permanent magnet. Let ARs be the area ratio of the plurality of voids in the surface portion, where ARs is expressed as a percentage (%). The ARs are measured on the surface portion exposed in the cross-section. The portion of the RTB-based permanent magnet located at a depth greater than 0.25t and less than 0.5t from the surface of the magnetized axis is defined as the central portion of the RTB-based permanent magnet. Let ARc be the area ratio of the plurality of voids in the central portion, where ARc is expressed as a percentage (%). The ARc is measured at the central portion exposed in the cross-section. ARs-ARc is above 1.0% and below 4.0%.

Images