R-T-B series permanent magnets

Abstract

To provide an R-T-B based permanent magnet having a high electric resistivity in a direction that is approximately orthogonal to an easy magnetization axis direction, and having excellent magnetic characteristics.SOLUTION: A permanent magnet 2 includes main phase grains 4. The main phase grains 4 contain R, T, and B. A plurality of voids 8 are formed in a cross section 2cs of the permanent magnet 2. The cross section 2cs is approximately parallel to an easy magnetization axis direction C of the permanent magnet 2. An area ratio of the voids 8 in the cross section 2cs is 1% or more and 5% or less. A direction orthogonal to the easy magnetization axis direction C in the cross section 2cs is expressed as AB direction. A direction in which each of the voids 8 extends in the cross section 2cs is expressed as VD. An angle between the AB direction and the VD is expressed as θ. A horizontal axis of a frequency distribution of the voids 8 in the cross section 2cs represents the θ. A range of the horizontal axis of the frequency distribution is from 0° or more and 180° or less. The frequency distribution is maximum in a range of the θ of from 60° or more to 120° or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This disclosure relates to R-T-B permanent magnets. [Background technology]

[0002] R-T-B permanent magnets contain rare earth elements R (such as Nd), transition metal elements T (such as Fe), and boron (B). Because R-T-B permanent magnets have superior magnetic properties compared to conventional permanent magnets (such as ferrite magnets), they are widely used in motors installed in electric vehicles or hybrid vehicles. For example, R-T-B permanent magnets constitute the rotor of an IPM motor (Interior permanent magnet) or SPM motor (Surface Permanent Magnet Motor). Multiple R-T-B permanent magnets are arranged along the circumferential direction of the rotor, and the easy magnetization axis direction of each R-T-B permanent magnet is positioned perpendicular to the rotation axis of the rotor. The surface of the R-T-B permanent magnet that is approximately perpendicular to the easy magnetization axis direction (the surface corresponding to the magnetic pole) faces the stator surrounding the rotor. An external magnetic field H is generated in the stator, approximately parallel to the easy magnetization axis of each R-T-B permanent magnet. The rotor rotates due to the attractive and repulsive forces between the stator and each R-T-B permanent magnet, which result from the change in the external magnetic field H applied to the surface of each R-T-B permanent magnet.

[0003] As shown in Figure 1(a), eddy currents I due to electromagnetic induction are generated within the R-T-B permanent magnet 2 as the external magnetic field H applied to the R-T-B permanent magnet 2 changes. According to Lenz's law, the eddy currents I due to electromagnetic induction flow in such a way that a magnetic field is created that opposes the change in the external magnetic field H. Since the direction of the external magnetic field H is approximately parallel to the easy magnetization axis C of the R-T-B permanent magnet 2, the eddy currents I associated with the change in the external magnetic field H circulate in a direction approximately perpendicular to the easy magnetization axis C.

[0004] Since the R-T-B system permanent magnet is a metal magnet, the electrical resistivity of the R-T-B system permanent magnet is significantly lower than that of the ferrite magnet composed of metal oxides. Therefore, in a motor using a conventional R-T-B system permanent magnet, Joule heat due to eddy currents is likely to occur, and the motor efficiency is likely to decrease. Therefore, attempts have been made to reduce eddy current losses in the R-T-B system permanent magnet for motors.

[0005] For example, Patent Document 1 below discloses an R-T-B system permanent magnet composed of a plurality of plate-shaped permanent magnets laminated via a non-conductive resin. According to the R-T-B system permanent magnet described in Patent Document 1 below, a plurality of plate-shaped permanent magnets are insulated by a non-conductive resin, and eddy currents flowing between the plurality of plate-shaped permanent magnets are suppressed. For example, Patent Document 2 below discloses an R-T-B system permanent magnet composed of a magnet body and a resistance layer (oxide layer) formed on the surface of the magnet body. According to the R-T-B system permanent magnet described in Patent Document 2 below, the resistance layer (oxide layer) suppresses eddy currents.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The inventors sought a method to increase the electrical resistivity of an R-T-B permanent magnet in a direction approximately perpendicular to the easy magnetization axis C, in order to suppress eddy currents in that direction. The inventors found that forming multiple voids extending in a predetermined direction within the R-T-B permanent magnet increases the electrical resistivity of the R-T-B permanent magnet in a direction approximately perpendicular to the easy magnetization axis C. However, the formation of multiple voids within the R-T-B permanent magnet relatively reduces the volume ratio of the multiple main phase particles within the R-T-B permanent magnet, making it easy to impair the orientation of each main phase particle in the easy magnetization axis C. As a result, magnetic properties such as residual magnetic flux density are impaired. Therefore, it is necessary to achieve both high electrical resistivity and excellent magnetic properties.

[0008] One objective of the present invention is to provide an R-T-B permanent magnet having high electrical resistivity and excellent magnetic properties in a direction substantially perpendicular to the easy magnetization axis. [Means for solving the problem]

[0009] For example, one aspect of the present invention relates to the following R-T-B permanent magnet. [1] An R-T-B permanent magnet containing a rare earth element R, a transition metal element T and B, wherein the R-T-B permanent magnet contains at least Nd as R, the R-T-B permanent magnet contains at least Fe as T, the R-T-B permanent magnet contains a plurality of main phase particles, the plurality of main phase particles contain at least R, T and B, a plurality of voids are formed in the cross-section of the R-T-B permanent magnet, the cross-section is substantially parallel to the easy magnetization axis of the R-T-B permanent magnet, the area ratio of the plurality of voids in the cross-section is 1% or more and 5% or less, the direction perpendicular to the easy magnetization axis in the cross-section is represented as the AB direction, the direction in which each of the plurality of voids extends in the cross-section is represented as VD, the angle between the AB direction and VD is represented as θ, and the frequency distribution of the plurality of voids in the cross-section The horizontal axis of the frequency distribution represents θ, the range of the horizontal axis is from 0° to 180°, and the frequency distribution is maximized within the range where θ is from 60° to 120°. R-T-B permanent magnet.

[0010] [2] The multiple main phase particles are flattened in the above cross-section, and the multiple main phase particles are stacked along the easy magnetization axis. [1] The R-T-B type permanent magnet described above.

[0011] [3] The average length of the short axis of multiple main phase particles in the above cross-section is between 20 nm and 200 nm. The R-T-B permanent magnet described in [1] or [2].

[0012] [4] The R content in the R-T-B permanent magnet is 28% by mass or more and 33% by mass or less, and the B content in the R-T-B permanent magnet is 0.8% by mass or more and 1.1% by mass or less. A permanent magnet of the R-T-B type as described in any one of the items [1] to [3].

[0013] [5] Hot-worked magnets, A permanent magnet of the R-T-B type as described in any one of the items [1] to [4].

[0014] [6] The width of the R-T-B permanent magnet in the easy magnetization axis direction is denoted as t, the surface portion of the R-T-B permanent magnet is defined as the portion at which the depth from the surface of the R-T-B permanent magnet in the easy magnetization axis direction is 0 or more and 0.25t or less, the area ratio of multiple voids in the surface portion is denoted as ARs%, where ARs is measured in the surface portion exposed in the above cross-section, the central portion of the R-T-B permanent magnet is defined as the portion at which the depth from the surface of the R-T-B permanent magnet in the easy magnetization axis direction is greater than 0.25t and 0.5t or less, the area ratio of multiple voids in the central portion is denoted as ARc%, where ARc is measured in the central portion exposed in the above cross-section, and ARs-ARc is 1.0% or more and 4.0% or less. A permanent magnet of the R-T-B type as described in any one of the items [1] to [5]. [Effects of the Invention]

[0015] According to the present invention, an R-T-B permanent magnet is provided that has high electrical resistivity and excellent magnetic properties in a direction substantially perpendicular to the easy magnetization axis. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1(a) is a schematic perspective view of an R-T-B permanent magnet 2 according to one embodiment of the present invention, and Figure 1(b) is a schematic view of the cross section 2cs of the R-T-B permanent magnet 2 (view in the direction of the b-b line in the R-T-B permanent magnet 2). [Figure 2] Figure 2 is an enlarged view of a portion (region II) of cross-section 2cs shown in Figure 1(b). [Figure 3] Figures 3(a) and 3(b) are schematic diagrams showing the directions VD to which each of the multiple air gaps 8 formed in the R-T-B permanent magnet 2 extends. [Figure 4] Figures 4(a) and 4(b) show specific examples of the frequency distribution of multiple air gaps 8 in a cross section 2cs2 parallel to the easy magnetization axis. [Figure 5] Figure 5 is a perspective view of the cavity 10 formed in the mold used in the manufacturing method of the R-T-B permanent magnet 2. [Figure 6] Figures 6(a), 6(b), and 6(c) are schematic diagrams illustrating the mechanism by which multiple air gaps 8 are formed in the R-T-B permanent magnet 2. [Figure 7] Figures 7(a) and 7(b) are images relating to a cross-section of Embodiment 1 of the present invention. [Figure 8] Figures 8(a) and 8(b) are images relating to a cross-section of Embodiment 1 of the present invention. [Figure 9] Figures 9(a) and 9(b) show the frequency distribution of multiple voids in a cross-section of Example 1 of the present invention. [Modes for carrying out the invention]

[0017] Preferred embodiments of the present invention will be described below with reference to the drawings. In the drawings, equivalent components are denoted by equivalent reference numerals. The present invention is not limited to the embodiments described below. The term "permanent magnet" as used below means an R-T-B system permanent magnet. The unit of concentration of each element in the permanent magnet described below is atomic percent. X, Y, and Z in Figure 5 represent three mutually orthogonal coordinate axes.

[0018] (Permanent magnet) The permanent magnet according to this embodiment contains at least a rare earth element (R), a transition metal element (T), and boron (B). The permanent magnet according to this embodiment is a hot-worked magnet. However, a permanent magnet according to another aspect of the present invention may be a sintered magnet.

[0019] The permanent magnet contains at least neodymium (Nd) as a rare earth element R. In addition to Nd, the permanent magnet may also contain other rare earth elements R. The other rare earth elements R contained in the permanent magnet may be at least one selected from the group consisting 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 does not have to contain heavy rare earth elements (e.g., both Dy and Tb).

[0020] A permanent magnet contains at least iron (Fe) as the transition metal element T. A permanent magnet may contain only Fe as the transition metal element T. A permanent magnet may contain both Fe and cobalt (Co) as the transition metal element T.

[0021] Figure 1(a) is a perspective view of the permanent magnet 2 according to this embodiment, and Figure 1(b) is a schematic diagram of the cross-section 2cs of the permanent magnet 2. The cross-section 2cs of the permanent magnet 2 is substantially parallel to the easy magnetization axis direction C of the permanent magnet 2. The easy magnetization axis direction C is parallel to the straight line connecting the pair of magnetic poles of the permanent magnet 2. In other words, the easy magnetization axis direction C is the direction from the south pole of the permanent magnet 2 to the north pole of the permanent magnet 2. The easy magnetization axis direction C may be determined based on the measurement of the magnetic flux distribution of the permanent magnet 2. The easy magnetization axis direction C may also be determined based on the measurement of the magnetic flux distribution of an analytical sample separated from the permanent magnet 2.

[0022] The permanent magnet 2 according to this embodiment is a rectangular parallelepiped (plate). However, the shape of the permanent magnet 2 is not limited to a rectangular parallelepiped. For example, the shape of the permanent magnet 2 may be a cube, polygonal prism, arc segment, annular sector, sphere, disc, cylinder, tube, or ring. The shape of the cross-section 2cs of the permanent magnet 2 may be a polygon, arc (chole chord), bow-shaped, arch-shaped, C-shaped, or circle.

[0023] Figure 2 is an enlarged view of a portion (region II) of the cross-section 2cs shown in Figure 1(b). As shown in Figure 2, the permanent magnet 2 contains multiple main phase particles 4.

[0024] In a cross-section 2cs that is approximately parallel to the easy magnetization axis C, multiple voids 8 are formed. These multiple voids 8 may be rephrased as multiple holes. At least some of these multiple voids 8 may be formed at two-particle grain boundaries. At least some of these multiple voids 8 may be formed at grain boundary multipoints. In a cross-section 2cs that is approximately parallel to the easy magnetization axis C, the direction approximately perpendicular to the easy magnetization axis C is denoted as the "AB direction".

[0025] As shown in Figures 3(a) and 3(b), the direction in which each of the multiple voids 8 extends in cross-section 2cs is denoted as VD. The direction in which each void 8 extends can also be described as the longitudinal direction of each void 8. The shape (profile) of each void 8 in cross-section 2cs can be approximated by an ellipse 8E. The approximation of the shape of each void 8 by the ellipse 8E can be performed by least squares fitting. The direction of the major axis of the ellipse 8E that approximates the shape of each void 8 can be considered as VD. The angle between the AB direction and VD is denoted as θ (unit: °). A void where θ is between 60° and 120° can be described as a void extending along the easy magnetization axis C. A void where θ is between 60° and 120° is described as a "C-axis extending void". Voids where θ is less than 60° and voids where θ is greater than 120° can be described as voids extending along the AB direction. Voids where θ is less than 60° and voids where θ is greater than 120° are referred to as "AB axis extending voids".

[0026] The horizontal axis of the frequency distribution of multiple air gaps 8 in a cross-section 2cs approximately parallel to the easy magnetization axis C represents θ. The range of the horizontal axis of the frequency distribution is from 0° to 180°. The vertical axis of the frequency distribution represents the frequency (number) of air gaps 8. For example, the frequency distribution may be represented by the frequency distribution curve FA shown in Figure 4(a). The frequency distribution may also be represented by the histogram FB shown in Figure 4(b). The frequency distribution of multiple air gaps 8 in a cross section 2cs that is approximately parallel to the easy magnetization axis C is maximum within the range where θ is between 60° and 120°. In other words, the maximum value Fmax of the frequency distribution of multiple air gaps 8 in a cross section 2cs that is approximately parallel to the easy magnetization axis C is within the range where θ is between 60° and 120°. In other words, in a cross section 2cs that is approximately parallel to the easy magnetization axis C, the angle θ of the air gap 8 with the maximum frequency is... Fmax The angle is between 60° and 120°. In other words, the gap 8 with the maximum angle in the cross section 2cs, which is approximately parallel to the easy magnetization axis C, is the C-axis extended gap. The frequency distribution of multiple voids 8 in the surface portion 2S, which belongs to the cross-section 2cs, as described later, may be maximum within the range where θ is between 60° and 120°. The frequency distribution of multiple voids 8 in the central portion 2C, which belongs to section 2cs and is described later, may be maximum within the range where θ is between 60° and 120°.

[0027] As described above, the eddy currents I generated in the permanent magnets 2 that make up the rotor of the motor flow in a direction approximately perpendicular to the easy magnetization axis C (AB direction). The path of the eddy currents I flowing in the AB direction consists of multiple main phase particles 4 that are in contact with each other in the AB direction. However, due to the formation of C-axis extending gaps (8), multiple adjacent main phase particles 4 tend to separate from each other in the AB direction. In other words, the path of the eddy currents I formed in the AB direction is easily interrupted by the C-axis extending gaps (8). Therefore, when the angle θ of the gap 8 with the maximum degree is between 60° and 120°, the electrical resistivity of the permanent magnets 2 in the direction approximately perpendicular to the easy magnetization axis C (AB direction) is effectively increased due to a relatively large number of C-axis extending gaps (8). Even if an AB-axis extending gap is formed in the cross section 2cs, a plurality of main phase particles 4 adjacent in the AB direction are difficult to be separated from each other. That is, the path of the eddy current I formed in the AB direction is difficult to be interrupted by the AB-axis extending gap. Therefore, when the gap 8 with the maximum frequency in the cross section 2cs substantially parallel to the magnetization easy axis direction C is the AB-axis extending gap, the electrical resistivity of the permanent magnet 2 in the AB direction is difficult to increase. That is, when the angle θ of the gap 8 with the maximum frequency is 0° or more and less than 60°, or more than 120° and 180° or less, the electrical resistivity of the permanent magnet 2 in the AB direction is difficult to increase.

[0028] The area of the cross section 2cs substantially parallel to the magnetization easy axis direction C may be represented as Acs. The total number of gaps 8 (C-axis extending gaps) with θ being 60° or more and 120° or less may be represented as N. N / Acs is 50 pieces / (mm) 2 or more and 3000 pieces / (mm) 2 or less, or 210.9 pieces / (mm) 2 or more and 742.2 pieces / (mm) 2 or less. When N / Acs is within the above range, the electrical resistivity of the permanent magnet 2 in the direction substantially perpendicular to the magnetization easy axis direction C (AB direction) is likely to increase. N / Acs may be paraphrased as the number of C-axis extending gaps per unit area of the cross section 2cs substantially parallel to the magnetization easy axis direction C. The total area of the gaps 8 (C-axis extending gaps) with θ being 60° or more and 120° or less is A 60-120 and may be represented as such, and A 60-120 / Acs may be 0.2% or more and 5.0% or less, or 0.62% or more and 2.44% or less. When A 60-120 / Acs is within the above range, the electrical resistivity of the permanent magnet 2 in the direction substantially perpendicular to the magnetization easy axis direction C (AB direction) is likely to increase. For example, the area of each gap 8 may be 5 (μm) 2 or more and 5000 (μm) 2 or less.

[0029] The area ratio AR of multiple air gaps 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 may be expressed as Av / Acs (unit: %). Av may be the sum of the areas (aperture areas) of all air gaps 8 measured in a cross section 2cs that is approximately parallel to the easy magnetization axis C. Acs may be the area of ​​the cross section 2cs that is approximately parallel to the easy magnetization axis C (the cross section in which Av is measured).

[0030] As described above, the air gaps 8 increase the electrical resistivity of the permanent magnet 2. Therefore, the electrical resistivity of the permanent magnet 2 increases with increasing area ratio AR of the multiple air gaps 8 in the cross-section 2cs. When the area ratio AR of the multiple air gaps 8 is 1% or more, the electrical resistivity of the permanent magnet 2 in the direction approximately perpendicular to the easy magnetization axis C (AB direction) tends to increase. However, even if the area ratio AR is 1% or more, permanent magnets where the angle θ of the air gap 8 with the largest degree is less than 60° or greater than 120° tend not to have high electrical resistivity in the AB direction.

[0031] The reason the coercivity of the permanent magnet 2 decreases is the presence of reversed magnetic domains within the permanent magnet 2. When a reverse magnetic field is applied to the permanent magnet 2, the reversed magnetic domains become nuclei for magnetization reversal, and domain walls propagate from the reversed magnetic domains throughout the permanent magnet 2. Due to the propagation of domain walls, the magnetization of each main phase particle 4 in the permanent magnet 2 is reversed. The magnetization reversal of each main phase particle 4 is suppressed by pinning of domain walls at pinning sites such as grain boundaries. The greater the difference in anisotropic magnetic field strength between the pinning site and the main phase particles 4, the easier it is for domain wall movement to be suppressed by pinning. However, the grain boundary phase in conventional hot-worked magnets does not function adequately as a pinning site. For example, conventional hot-worked magnets contain an R-rich phase as a grain boundary phase (subphase) in which the concentration of rare earth elements R (such as Nd) is higher than that of the main phase particles. The composition of the R-rich phase is Nd 30 Fe 70 The R-rich phase is both ferromagnetic and soft magnetic. Therefore, the difference in anisotropic magnetic field strength between the R-rich phase and the main phase particle 4 is small, and the R-rich phase does not function adequately as a pinning site. The inventors found that intentionally forming multiple voids 8 as pinning sites within a hot-worked magnet increases the coercivity of the magnet. The anisotropic magnetic field strength in the voids 8 is virtually zero, and the difference in anisotropic magnetic field strength between the voids 8 and the main phase particles 4 is large, so pinning of the magnetic domain walls in the voids 8 effectively suppresses the movement of the magnetic domain walls. When the area ratio AR of the multiple air gaps 8 is 1% or more, the movement of the domain walls is sufficiently suppressed by pinning of the domain walls in the air gaps 8, and the permanent magnet 2 can have a sufficiently high coercivity. However, with the formation of multiple air gaps 8 in the permanent magnet 2, the proportion of the volume of the multiple main phase particles 4 in the permanent magnet 2 decreases relatively, and the orientation of each main phase particle 4 in the easy magnetization axis axis C is easily impaired. As a result, the residual magnetic flux density of the permanent magnet 2 tends to decrease. When the area ratio AR of the multiple air gaps 8 is 5% or less, the decrease in residual magnetic flux density due to the formation of air gaps 8 is sufficiently suppressed. In other words, when the area ratio AR of the multiple air gaps 8 is 1% or more and 5% or less, high coercivity and high residual magnetic flux density are easily achieved simultaneously. For similar reasons, the area ratio AR of the multiple air gaps 8 may 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, according to this embodiment, high coercivity and high residual magnetic flux density can be achieved simultaneously. Even if the permanent magnet 2 does not contain heavy rare earth elements (both Dy and Tb), this embodiment achieves both high coercivity and high residual magnetic flux density.

[0032] For example, the electrical resistivity (ρ) of the permanent magnet 2 in a direction perpendicular to the easy magnetization axis C may 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. For example, the coercivity of permanent magnet 2 at 23°C (HcJ) 23 The current may be between 800 kA / m and 3000 kA / m, or between 800 kA / m and 1113 kA / m. For example, the coercivity of permanent magnet 2 at 150°C (HcJ) 150 The current may be between 300 kA / m and 1500 kA / m, or between 303 kA / m and 473 kA / m. For example, the temperature coefficient β of coercivity may be between -0.50% / °C and -0.35% / °C, or between -0.50% / °C and -0.45% / °C. The temperature coefficient β is defined by the following formula 1. In formula 1 below, HcJ 150 This is the coercivity at 150°C. (HcJ in Equation 1 below) 23 This is the coercivity at 23°C. β = 100 × (HcJ) 150 -HcJ 23 ) / HcJ 23 (150-23) (1) For example, the residual magnetic flux density (Br) of the permanent magnet 2 at room temperature may be 1245 mT or more and 1500 mT or less, or 1248 mT or more and 1278 mT or less. For example, the aspect ratio (Hk / HcJ) of permanent magnet 2 may be between 94.0% and 100%, or between 94.0% and 99.5%. Hk is the demagnetizing field strength corresponding to 90% of the remanent magnetic flux density in the second quadrant of the magnetization curve.

[0033] Frequency distribution of multiple voids 8, N / Acs, A 60-120 / Acs, the area of ​​each void 8, and the area ratio AR may be measured in a portion of the cross section 2cs that is substantially parallel to the easy magnetization axis C (for example, region II shown in Figures 1(b) and 2). For example, the dimensions of the portion of cross section 2cs in which these measurements are taken may be 300 μm in length × 426 μm in width. The cross section 2cs substantially parallel to the easy magnetization axis C is observed by a scanning electron microscope (SEM), and a frequency distribution is created based on all the voids 8 included in a single observation field.

[0034] As shown in Figure 2, in a cross-section 2cs of the permanent magnet 2 that is substantially parallel to the easy magnetization axis C, a plurality of flattened main phase particles 4 may be observed. In other words, each main phase particle 4 observed in the cross-section 2cs may be plate-shaped. The plurality of flattened main phase particles 4 may be stacked along the easy magnetization axis C. The permanent magnet 2 may further include secondary particles composed of a plurality of bonded main phase particles 4. The permanent magnet 2 may contain a plurality of secondary particles. At least some of the voids 8 may be located at grain boundaries between the plurality of secondary particles.

[0035] Each main phase particle 4 contains at least R (Nd, etc.), T, and B. Each main phase particle 4 can be rephrased as a single crystal grain (i.e., a primary particle). Each main phase particle 4 contains R2T 14 Contains crystals of B (single crystal or polycrystalline). R2T 14 B is a ferromagnetic ternary intermetallic compound. The main phase particle 4 is R2T 14 It may consist only of crystals B. R2T 14 The crystal of B may be tetragonal. That is, R2T 14 The crystal axes of B are the a-axis, b-axis, and c-axis, and the a-axis, b-axis, and c-axis are orthogonal to each other, R2T 14 The lattice constant of B in the a-axis direction is R²T 14 The lattice constant in the b-axis direction of B may be equal to R²T. 14 The lattice constant in the c-axis direction of B may be different from the lattice constants in the a-axis and b-axis directions, respectively. R2T 14 The a-axis direction of B may be approximately parallel to the AB direction of permanent magnet 2. 14 The b-axis direction of B may be approximately parallel to the AB direction of permanent magnet 2. R2T 14 The c-axis direction of B may be approximately parallel to the easy magnetization axis C of the permanent magnet 2.

[0036] The main phase particles 4 may contain other elements in addition to R, T, and B. For example, R2T may be the element that constitutes the main phase particles 4. 14 B is (Nd 1-x Pr x )2(Fe 1-y Co y ) 14 It may be represented as B. x may be between 0 and 1 (inclusive). y may be between 0 and 1 (inclusive). The main phase particle 4 may contain heavy rare earth elements such as Tb and Dy in addition to light rare earth elements as R. R2T 14 A portion of B in B may be substituted with other elements such as carbon (C). The composition within the main phase particles 4 may be uniform. The composition within the main phase particles 4 may be non-uniform. For example, the concentration distributions of R, T, and B in the main phase particles 4 may have gradients.

[0037] The main phase particle 4 may consist of a surface layer and a central part covered by the surface layer. The surface layer may be referred to as a shell, and the central part as a core. The surface layer of the main phase particle 4 may contain at least one heavy rare earth element from among Tb and Dy. The surface layer of each of the main phase particles 4 may contain at least one heavy rare earth element from among Tb and Dy. The surface layer of some of the main phase particles 4 may contain at least one heavy rare earth element from among Tb and Dy. The inclusion of heavy rare earth elements in the surface layer makes it easier for the anisotropic magnetic field to increase locally near the grain boundaries, and makes it difficult for magnetization reversal nuclei to be generated near the grain boundaries. As a result, the coercivity of the permanent magnet 2 increases at high temperatures (e.g., 100-200°C). Since the residual magnetic flux density (Br) and coercivity of permanent magnet 2 are easily compatible, the total concentration of heavy rare earth elements in the surface layer may be higher than the total concentration of heavy rare earth elements in the center.

[0038] The volume ratio of the main phase (the ratio 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 may be 80% to 99% by volume, 90% to 99% by volume, 95% to 99% by volume, or 95.09% to 98.7% by volume. As the volume ratio of the main phase increases, the residual magnetic flux density of the permanent magnet 2 increases.

[0039] The permanent magnet 2 may further contain multiple R-rich phases as subphases. The R-rich phases may be located between multiple main phase particles 4. That is, the R-rich phases may be a type of grain boundary phase contained in the grain boundaries between multiple main phase particles 4. The grain boundary containing the R-rich phases may be a grain boundary multipoint surrounded by three or more main phase particles 4. The grain boundary containing the R-rich phases may also be a two-particle grain boundary between two main phase particles 4. The R-rich phases may be ferromagnetic or soft magnetic materials. The R-rich phases contain at least R. For example, the R-rich phases may contain Nd as R. The R-rich phases may further contain one or more other rare earth elements in addition to Nd as R. The R-rich phases may further contain one or more elements other than R in addition to R. The R-rich phases may contain at least one component selected from the group consisting of metals, alloys, intermetallic compounds, and oxides. For example, part or all of the R-rich phase may consist of at least one component from elemental R, alloys containing R, and metallic compounds containing R. Part or all of the R-rich phase may contain oxides of R. For example, the oxide of R may be an oxide of Nd. The oxidized surface of the main phase particles 4 may be an oxide of R. Part of the R-rich phase may consist of only oxides of R.

[0040] The concentration of R in the R-rich phase may be higher than the average value of the R concentration in the main phase particles 4. The concentration of R in the R-rich phase may be higher than the average value of the R concentration in the cross-section 2cs described above. If the permanent magnet 2 contains multiple types of R, the concentration of R may be the sum of the concentrations of the multiple types of R.

[0041] The average length of the short axis of the main phase particles 4 (primary particles) observed in the above cross-section 2cs may be between 20 nm and 200 nm. If the average length of the short axis of the main phase particles 4 is within the above range, each main phase particle 4 (R2T 14 The anisotropic growth of the crystals of B is sufficient, and each main phase particle 4 is easily oriented in the easy magnetization axis C, which easily increases coercivity, remanent magnetic flux density, and angular ratio. The average length of the major axis of the main phase particles 4 (primary particles) observed in the above cross-section 2cs may be, for example, between 100 nm and 1000 nm. The minor axis of each principal phase particle 4 observed in cross-section 2cs may be approximately parallel to the easy magnetization axis direction C. The major axis of each principal phase particle 4 may be approximately perpendicular to the easy magnetization axis direction C. The shape of the principal phase particle 4 in cross-section 2cs is not limited to a rectangle. The shape of the principal phase particle 4 in cross-section 2cs may be distorted. The shape of the principal phase particle 4 in cross-section 2cs does not have to be uniform. If the shape of the principal phase particle 4 in cross-section 2cs is distorted, the shape of the principal phase particle 4 may be approximated by the quadrilateral with the smallest area that circumscribing the principal phase particle 4. The quadrilateral may be a rectangle. The length of the short side of this quadrilateral may be considered as the length of the minor axis of the principal phase particle 4, and the length of the long side of the above quadrilateral may be considered as the length of the major axis of the principal phase particle 4. The average value of the minor axis length of the principal phase particle 4 may be calculated from the measured values ​​of the minor axis lengths of all principal phase particles 4 present in the backscattered electron image of cross-section 2cs taken with a scanning electron microscope (SEM). The average length of the major axis of the main phase particle 4 may also be calculated from the measured lengths of the major axes of all main phase particles 4 present in the backscattered electron image. However, the dimensions of main phase particles 4 that extend beyond the backscattered electron image are excluded from the calculation of the average value. The maximum dimensions of the backscattered electron image used to measure the lengths of the minor axis and major axis of the main phase particle 4 may be, for example, 120 μm vertically × 80 μm horizontally, or 80 μm vertically × 120 μm horizontally. Several representative locations within these low-magnification backscattered electron images may be selected, and backscattered electron images of each location may be taken at high magnification. Then, the average values ​​of the major axis and minor axis may be calculated from the lengths of the major axis and minor axis of all main phase particles 4 measured in the high-magnification backscattered electron image. Commercially available image analysis software may be used to identify the shape (contour) of the main phase particle 4 and to measure the dimensions of the main phase particle 4 (the rectangle circumscribing the main phase particle 4).

[0042] The width (dimensions) of the permanent magnet 2 in the easy magnetization axis direction C may be, for example, several millimeters to several hundred millimeters, or tens of millimeters to several hundred millimeters. The dimensions of the permanent magnet 2 in the AB direction may be, for example, several millimeters to several hundred millimeters, or tens of millimeters to several hundred millimeters.

[0043] Grain boundary phases other than the R-rich phase may be included in the grain boundary. For example, the grain boundary may include a grain boundary phase containing elements introduced into the permanent magnet 2 by the grain boundary diffusion process described later. The elements introduced into the permanent magnet 2 by the grain boundary diffusion process may be at least one heavy rare earth element from among Tb and Dy. The elements introduced into the permanent magnet 2 by the grain boundary diffusion process may be heavy rare earth elements and light rare earth elements, and the light rare earth element may be at least one from among Nd and Pr. The elements introduced into the permanent magnet 2 by the grain boundary diffusion process may be heavy rare earth elements, light rare earth elements and copper.

[0044] The width of the permanent magnet 2 in the easy magnetization axis direction C is expressed as t. The surface portion 2S of the permanent magnet 2 is defined as the portion where the depth from the surface of the permanent magnet 2 in the easy magnetization axis direction C is between 0 and 0.25t. The area ratio of the multiple voids 8 in the surface portion 2S is expressed as ARs%. ARs is measured in the surface portion 2S exposed in the cross-section 2cs. The central portion 2C of the permanent magnet 2 is defined as the portion where the depth from the surface of the permanent magnet 2 in the easy magnetization axis direction C is greater than 0.25t and less than or equal to 0.5t. The central portion 2C may also be the portion where the depth from the surface of the permanent magnet 2 in the easy magnetization axis direction C is greater than 0.25t and less than 0.75t. The area ratio of the multiple voids 8 in the central portion 2C is expressed as ARc%. ARc is measured in the central portion 2C exposed in the cross-section 2cs. ARs may be higher than ARc. As described later, the eddy currents in the surface portion 2S tend to be larger than those in the central portion 2C. Therefore, the greater ARs is than ARc, the higher the electrical resistivity of the surface portion 2S is compared to that of the central portion 2C, and the easier it is to reduce eddy current losses in the permanent magnet 2. For similar reasons, ARs-ARc may be between 1.0% and 4.0%, or between 1.5% and 4.0%. ARs and ARc themselves may be between 1% and 5%. Permanent magnet synchronous motors (PMSMs), such as IPM motors 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. Electric vehicles and hybrid vehicles require AC synchronous motors to operate across a wide range of rotational speeds. Since the rotational speed of an AC synchronous motor is proportional to the frequency of the alternating current, a wide range of alternating current frequencies is required to drive an AC synchronous motor across a wide range of rotational speeds. A wide range of alternating current frequencies can be generated by an inverter. For example, the fundamental frequency range of an inverter for an AC synchronous motor installed in an automobile is approximately 100 to 1500 Hz. On the other hand, inverters generally output a modified sine wave using PWM (Pulse Width Modulation) control. In this case, the carrier frequency is approximately 20 kHz. The alternating current described above is one factor in the penetration depth of the eddy currents generated in the permanent magnet 2. Generally, penetration depth is the depth from the metal surface to which the eddy currents are (1 / e) times (approximately 36.8%) the eddy currents at the metal surface. e is Napier's number. For example, the penetration depth δ is expressed by the following equation 2. δ = [ρ / (πfμ)] 1 / 2 (2) In equation 2, ρ represents the electrical resistivity of the metal (unit: ×10⁻¹⁰). -8The value is Ω·m. In equation 2, f is the frequency of the alternating current. μ is the permeability of the metal (unit: H / m). Based on the electrical resistivity, permeability, and inverter carrier frequency (20kHz) of a typical RTB permanent magnet, the penetration depth of the permanent magnet 2 calculated from equation 2 is approximately 1mm. In other words, the eddy current decreases exponentially with increasing depth from the surface of the permanent magnet. To put it another way, the eddy current at the surface portion 2S of the permanent magnet 2 is significantly larger than the eddy current at the central portion 2C of the permanent magnet 2. Therefore, the larger the air gap in the relatively shallow portion from the surface of the permanent magnet 2 (i.e., the larger the ARs), the higher the electrical resistivity of the surface portion 2S, and the easier it is to reduce eddy current losses in the permanent magnet 2. On the other hand, the eddy currents in the central part 2C of the permanent magnet 2 are significantly smaller than the eddy currents in the surface part 2S of the permanent magnet 2. Therefore, the air gap 8 formed in the central part 2C contributes less to reducing eddy current loss than the air gap 8 formed in the surface part 2S. In other words, ARc contributes less to reducing eddy current loss than ARs. Rather, the larger the air gap 8 in the central part 2C (i.e., the larger ARc), the more likely it is that the proportion of the volume of main phase particles 4 in the central part 2C will decrease relatively. As a result, magnetic properties such as residual magnetic flux density are impaired. Therefore, for improving magnetic properties, it is preferable that ARc be smaller than ARs. For the reasons stated above, when ARs is greater than ARc, it is easier to achieve both suppression of eddy currents in the surface region 2S and excellent magnetic properties in the central region 2C.

[0045] 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 with a scanning electron microscope (SEM) or scanning transmission electron microscope (STEM). The composition of the main phase particles 4 and grain boundary phases can be analyzed by an electron probe microanalyzer (EPMA) equipped with an energy-dispersive X-ray spectroscopy (EDS) instrument.

[0046] The overall composition of permanent magnet 2 is 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 be outside the ranges described below.

[0047] The R content in the R-T-B permanent magnet may be between 28.00% by mass and 33.00% by mass. When the R content is within the above 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 more, it is easier to suppress the formation of cracks in the permanent magnet 2 during the hot deforming process. When the R content is 28.00% by mass or more, the R2T constituting the main phase particles 4 14 B is easily formed, and the α-Fe phase, which has soft magnetism, is difficult to form. As a result, the coercivity tends to increase. On the other hand, when the R content is 33.00 mass% or less, segregation of the liquid phase (R-rich phase) on the surface of the permanent magnet 2 is suppressed during the hot plastic working process, and seizure of the mold and the permanent magnet 2 is suppressed. When the R content is 33.00 mass% or less, the formation of the R-rich phase 6 is moderately suppressed, and the residual magnetic flux density tends to increase. Since the residual magnetic flux density and coercivity tend to increase, the total proportion of Nd and Pr in the total rare earth element R may be 80 atomic% or more and 100 atomic% or less, or 95 atomic% or more and 100 atomic% or less.

[0048] The total content of Tb and Dy in the permanent magnet 2 may be between 0.00 mass% and 5.00 mass%. The presence of at least one heavy rare earth element among Tb and Dy in the permanent magnet 2 tends to increase its magnetic properties (especially its coercivity at high temperatures). However, the permanent magnet 2 does not necessarily have to contain Tb and Dy.

[0049] The B content in the R-T-B permanent magnet may be 0.8% by mass or more and 1.1% by mass or less. If the B content is 0.8% by mass or more, R2Fe 17 The formation of equivalent different phases is suppressed, and coercivity and residual magnetic flux density tend to increase. When the content of B is 1.1 mass% or less, R 1+ε The formation of different phases such as Fe4B4(Boride) is suppressed, and coercivity and residual magnetic flux density tend to increase. When the B content is within the above range, the square ratio of permanent magnet 2 tends to approach 1.0. If the total content of rare earth element R in permanent magnet 2 is 28.00% by mass or more and 33.00% by mass or less, and the content of B in permanent magnet 2 is 0.8% by mass or more and 1.1% by mass or less, then the content of rare earth element R in permanent magnet 2 is R2T 14 The stoichiometric ratio of B is large. As a result, a liquid phase is easily formed at the grain boundaries during the hot plastic deformation process described later. The liquid phase within the grain boundaries is formed at the crystal grain (R2T 14 B) promotes anisotropic growth, grain boundary sliding, and grain rotation. As a result, the c-axis of the grains is more likely to be oriented in the stress direction, the proportion of grains in the permanent magnet 2 whose easy magnetization axis direction is aligned is more likely to increase, and the residual magnetic flux density of the permanent magnet 2 is more likely to increase.

[0050] The permanent magnet 2 may contain gallium (Ga). The Ga content may be between 0.03% by mass and 1.00% by mass, or between 0.20% by mass and 0.80% by mass. When the Ga content is within the above range, the formation of subphases (e.g., phases containing R, T, and Ga) is moderately suppressed, and the residual magnetic flux density and coercivity of the permanent magnet 2 tend to increase. However, the permanent magnet 2 does not have to contain Ga.

[0051] The permanent magnet 2 may contain aluminum (Al). The Al content in the permanent magnet 2 may be between 0.01% by mass and 0.2% by mass, or between 0.04% by mass and 0.07% by mass. Having an Al content within the above range tends to improve the coercivity and corrosion resistance of the permanent magnet. However, the permanent magnet 2 does not need to contain Al.

[0052] The permanent magnet 2 may contain copper (Cu). The Cu content in the permanent magnet 2 may be 0.01% by mass or more and 1.50% by mass or less, or 0.04% by mass or more and 0.50% by mass or less. Having a Cu content within the above range tends to improve the coercivity, corrosion resistance, and temperature characteristics of the permanent magnet 2. However, the permanent magnet 2 does not need to contain Cu.

[0053] The permanent magnet 2 may contain cobalt (Co). The Co content in the permanent magnet may be 0.30% by mass or more and 6.00% by mass or less, or 0.30% by mass or more and 4.00% by mass or less. The presence of Co in the permanent magnet 2 tends to increase the Curie temperature of the permanent magnet 2. Furthermore, the presence of Co in the permanent magnet 2 tends to improve the corrosion resistance of the permanent magnet 2. However, the permanent magnet 2 does not have to contain Co.

[0054] The remainder of the permanent magnet 2 after removing the above elements may consist only of Fe, or Fe and other elements. For the permanent magnet 2 to have sufficient magnetic properties, the total content of elements other than Fe in the remainder may be 5% by mass or less of the total mass of the permanent magnet 2.

[0055] The permanent magnet 2 may contain, as other elements (e.g., unavoidable impurities), at least one selected from the group consisting of 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). The total content of other elements in the permanent magnet 2 may be 0.001% by mass or more and 0.50% by mass.

[0056] The overall composition of the permanent magnet 2 may be analyzed by methods such as X-ray fluorescence (XRF) analysis, inductively coupled plasma (ICP) emission spectrometry, inert gas fusion-nondispersive infrared absorption (NDIR) spectrometry, combustion in an oxygen stream-infrared absorption spectrometry, and inert gas fusion-thermal conductivity spectrometry.

[0057] The permanent magnet 2 may be applied to motors, generators, actuators, etc. For example, the permanent magnet 2 is used in various fields such as hybrid vehicles, electric vehicles, hard disk drives, magnetic resonance imaging (MRI) machines, smartphones, digital cameras, flat-screen TVs, scanners, air conditioners, heat pumps, refrigerators, vacuum cleaners, washer-dryers, elevators, and wind turbines.

[0058] (Method of manufacturing permanent magnets) The method for manufacturing a permanent magnet according to this embodiment includes at least a strip fabrication step, a hot pressing step, and a hot deforming step. The method for manufacturing a permanent magnet may further include other steps, such as a grain boundary diffusion step following the hot deforming step. However, the grain boundary diffusion step is not essential.

[0059] To suppress oxidation of permanent magnets and their work-in-progress during the manufacturing process, the manufacturing method of permanent magnets may be carried out under a non-oxidizing atmosphere. For example, the non-oxidizing atmosphere may be an inert gas such as argon (Ar) gas. In addition to the inert gas, the non-oxidizing atmosphere may further contain a reducing gas such as hydrogen gas (H2).

[0060] The thin-strip manufacturing process is a process for producing alloy strips from raw metals using the ultra-rapid solidification method. In the ultra-rapid solidification method, molten metal in a container is sprayed from a nozzle located at the front of the container onto the surface of a cooling roll. The molten metal comes into contact with the surface of the cooling roll and is instantly ejected by the rapidly rotating cooling roll, forming numerous elongated ribbon-like shapes. The molten metal is rapidly cooled and solidified upon contact with the surface of the cooling roll. As a result, numerous elongated alloy strips are formed. A container is placed in the direction from which the alloy strips are ejected by the cooling roll, and the alloy strips are collected into the container.

[0061] The molten metal is a metal (raw material) containing each element that makes up the permanent magnet. The raw material may be, for example, a rare earth element (elementary metal), an alloy containing a rare earth element, pure iron, ferroboron, or an alloy containing these. These raw material metals are weighed to match the composition of the desired permanent magnet.

[0062] The molten metal may be obtained by heating the raw metal in a container using high-frequency induction heating. The temperature of the molten metal sprayed from the nozzle (injection temperature) may be, for example, about 1400°C. The heating rate at which the temperature of the raw metal reaches the injection temperature may be, for example, about 20 to 100°C / second.

[0063] The surface of the cooling roll may be made of a metal with high thermal conductivity, such as Cu. The surface temperature of the cooling roll may be controlled by a coolant flowing inside the cooling roll. The higher the cooling rate of the molten metal on the surface of the cooling roll, the more the crystals (R2T) contained in the alloy strip will dissipate. 14 In B), the particle size tends to be finer, and the coercivity of the permanent magnet tends to increase. The less molten metal is sprayed onto the surface of the cooling roll per unit time, the thinner the molten metal adhering to the surface of the cooling roll, the higher the cooling rate, and the thinner the alloy strip. The higher the peripheral speed of the cooling roll, the thinner the molten metal adhering to the surface of the cooling roll, the higher the cooling rate, and the thinner the alloy strip. The thickness of the main phase particles in the easy magnetization axis direction (length of the short axis of the main phase particles) depends on the thickness of the alloy strip (and the crushing and classification of the alloy strip). 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 tends to be. The thickness of the alloy strip may be, for example, 20 μm to 60 μm, or 30 μm to 50 μm. The width of the alloy strip may be, for example, 1.0 mm to 5.0 mm.

[0064] After the thin strip manufacturing process, a crushing / classification process may be carried out. The crushing / classification process involves crushing the alloy thin strip using a crushing device to produce coarse powder, and then classifying the coarse powder to recover alloy powder having a predetermined particle size and aspect ratio. The alloy powder is a precursor of the main phase particles contained in the permanent magnet. The shape of each alloy particle constituting the alloy powder may be plate-shaped or flake-shaped. The crushing method for the alloy thin strip may be at least one of, for example, a cutter mill and a propeller mill. The means for classifying the coarse powder may be a sieve. The particle size and particle size distribution of the alloy powder obtained by classification may be measured, for example, by laser diffraction scattering. The particle size of the alloy powder obtained by classification may be, for example, 60 μm to 2800 μm, or 150 μm to 2800 μm.

[0065] The hot forming process is a process of forming a molded body by heating and pressurizing an alloy strip (alloy powder). For example, the alloy powder may be heated in a mold while being compressed in the mold. Pressurizing the alloy powder reduces the voids between the alloy powder particles, resulting in a dense molded body. Furthermore, the heating of the alloy powder accompanying the pressurization causes a liquid phase (R-rich phase such as an Nd-rich phase) to form from the surface of the alloy powder. This liquid phase fills the voids (grain boundaries) between the alloy powder particles, and the liquid phase also lubricates the alloy powder, resulting in a dense molded body. A cold forming process may be performed before the hot forming process. In the cold forming process, the molded body may be formed by pressurizing the alloy powder at room temperature. The molded body obtained in the cold forming process may be densified by heating and pressurizing it in the hot forming process. The temperature of the alloy powder in the hot forming process (hot forming temperature) may be, for example, 650°C or higher and 750°C or lower. If the hot forming temperature is too high, the crystals (R2T) constituting the alloy powder may be affected. 14 In B), excessive grain 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) may be between 50 MPa and 200 MPa. The time during which the hot forming temperature and hot forming pressure are maintained within the above range (hot forming time) may be, for example, between several tens of seconds and several hundred seconds.

[0066] After the hot forming process, a hot plastic deformation process is carried out. In the hot plastic deformation process, multiple main phase particles (R2T) whose c-axis (easy magnetization axis) are oriented in a predetermined direction are formed by hot extrusion molding of the molded body obtained in the hot forming process. 14 This is a process to obtain a magnetic substrate containing crystal grains of B. For example, in the hot plastic deformation process, the molded body is heated and extruded from the mold. Inside the mold, the grain boundary phase in the heated molded body liquefies to generate a liquid phase (R-rich phase), and stress acts on the molded body in a predetermined direction, causing each alloy particle constituting the molded body to deform. Along with the generation of the liquid phase and the deformation of the alloy particles, anisotropic growth of the crystal grains in the direction perpendicular to the c-axis of the crystal grains progresses. The liquid phase also lubricates each crystal grain, and a force acts on each crystal grain according to the stress. As a result, the crystal grains rotate due to grain boundary sliding, and the c-axis of each crystal grain (main phase particle) is oriented approximately parallel to the stress direction. In other words, multiple flattened main phase particles extending in a direction approximately perpendicular to the c-axis are stacked along the stress direction. The temperature of the molded body during the hot plastic deformation process (hot plastic deformation temperature) may be, for example, 700°C or more and less than 900°C, or 700°C or more and 850°C or less. If the hot plastic deformation temperature is too low, the liquid phase (R-rich phase such as Nd-rich phase) is less likely to form at the grain boundaries within the molded body, making it difficult for crystal grains to grow and reducing the likelihood of grain rotation due to grain boundary sliding. As a result, the average length of the short axis of the main phase particles tends to be less than 20 nm, and the c-axis of each main phase particle (crystal grain) is less likely to be oriented approximately parallel to the stress direction. If the hot plastic deformation temperature is too high (for example, if the hot plastic deformation temperature is 900°C or higher), the liquid phase (R-rich phase) seeps excessively from each alloy particle and segregates onto the surface of each alloy particle and the interfaces between alloy particles, and most of the liquid phase is consumed for grain growth of the crystal grains. Because most of the liquid phase is consumed for grain growth of the crystal grains, the grain growth of the main phase particles (crystal grains) proceeds abnormally, making it easy for coarse main phase particles to form, and the average length of the short axis of the main phase particles tends to exceed 200 nm. Coarse main phase particles are difficult to orient in the easy magnetization axis direction. The extrusion speed for hot extrusion molding is 10 -2The extrusion speed may be between 9.9 mm / second and 10 mm / second. If the extrusion speed is too high (for example, if the extrusion speed is 10 mm / second or higher), the anisotropic growth of the main phase particles (crystal grains) in the molded body does not proceed sufficiently, and the average length of the short axis of the main phase particles (primary particles) tends to be less than 20 nm. In other words, if the extrusion speed is too high, the molded body is pushed out of the mold before the anisotropic growth of the crystal grains in the molded body has progressed sufficiently. As a result, the c-axis of each main phase particle (crystal grain) is unlikely to be oriented approximately parallel to the stress direction. The pressure applied to the molded body during the hot plastic deformation process (hot plastic deformation pressure) may be between 50 MPa and 200 MPa. The time during which the hot plastic deformation temperature and hot plastic deformation pressure are maintained within the above range (hot plastic deformation time) may be, for example, several tens of seconds.

[0067] The mold used in the hot plastic deformation process is cylindrical. That is, the cavity formed inside the mold penetrates the mold from the end face of the mold where the inlet for the molded body opens (start face) to the end face of the mold where the extrusion port for the molded body opens (end face). The start face and the end face are parallel planes to each other. The direction from the start face to the end face is the extrusion direction of the molded body, and the extrusion direction is perpendicular to the start face and the end face. The opening area of ​​the extrusion port of the molded body is smaller than the opening area of ​​the inlet for the molded body. A specific example of a cavity formed in a mold is shown in Figure 5. The cavity 10 is divided along the extrusion direction Z into an inlet-side region 10A, an intermediate region 10B, and an extrusion-side region 10C. The inlet-side region 10A opens at the start end face. The extrusion-side region 10C opens at the end face. The intermediate region 10B is located between the inlet-side region 10A and the extrusion-side region 10C in the extrusion direction Z. The exit 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 exit." The exit of the extrusion port side region 10C (i.e., the extrusion port of the molded body at the end face) is referred to as the "second exit." The shape of the cavity 10 in the cross-section of the mold perpendicular to the extrusion direction Z (the cross-section of the mold parallel to the start and end faces) is a quadrilateral with all four corners being right angles. In this quadrilateral, opposite pairs of sides are denoted as the first side, and the other pair of opposite sides in the above quadrilateral are denoted as the second side. The length xa of the first side in the inlet region 10A is constant. The length ya of the second side in the inlet region 10A is also constant. In other words, the opening area of ​​the inlet region 10A in a 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, and eventually approximately coincides with the length xc1 of the first side at the first exit (exit of the intermediate region 10B). Therefore, the first side in the extrusion port side region 10C is shorter than the first side in the inlet region 10A. Also in the intermediate region 10B, the length ya of the second side gradually increases along the extrusion direction Z, and eventually approximately coincides with the length yc1 of the second side at the first exit (exit of the intermediate region 10B). Therefore, the second side in the extrusion port side region 10C is longer than the second side in the inlet region 10A. Furthermore, the opening area of ​​the intermediate region 10B in a cross section perpendicular to the extrusion direction Z gradually decreases along the extrusion direction Z, and eventually approximately coincides with the opening area of ​​the extrusion port side region 10C in a cross section perpendicular to the extrusion direction Z. Therefore, the opening area of ​​the extrusion port side region 10C in a cross section perpendicular to the extrusion direction Z is smaller than the opening area of ​​the inlet side region 10A in a cross section perpendicular to the extrusion direction Z. The length of the first side in the extrusion port side region 10C is constant. That is, the length of the first side xc2 at the second outlet (outlet of the extrusion port side region 10C) is equal to the length of the first side xc1 at the first outlet (outlet of the intermediate region 10B). The length of the second side in the extrusion port side region 10C gradually increases along the extrusion direction Z. That is, the length of the second side yc2 at the second exit (exit of the extrusion port side region 10C) is slightly greater than the length of the second side yc1 at the first exit (exit of the intermediate region 10B). Since the length of the second side in the extrusion port side region 10C gradually increases along the extrusion direction Z, the opening area of ​​the extrusion port side region 10C in a cross-section perpendicular to the extrusion direction Z also gradually increases along the extrusion direction Z. Therefore, the opening area of ​​the second outlet (outlet of the extrusion port side region 10C) is slightly larger than the opening area of ​​the first outlet (outlet of the intermediate region 10B). However, the opening area of ​​the second outlet (outlet of the extrusion port side region 10C) is smaller than the opening area of ​​the inlet side region 10A in a cross-section perpendicular to the extrusion direction Z. Figure 6(a) shows the cross-section CS1 of the mold at the first exit (exit of the intermediate region 10B) described above. Furthermore, Figure 6(b) shows the cross-section CS2 of the mold at the second exit (exit of the extrusion port side region 10C) described above. Both the cross-section CS1 and the cross-section CS2 of the mold are perpendicular to the extrusion direction Z. As described above, the opening area of ​​the extrusion port side region 10C in a cross section perpendicular to the extrusion direction Z is smaller than the opening area of ​​the inlet side region 10A in a cross section perpendicular to the extrusion direction Z, and the first side in the extrusion port side region 10C (end face) is shorter than the second side in the extrusion port side region 10C (end 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, a stress approximately parallel to the first side acts on the molded body, causing grain boundary sliding and rotation of the main phase particles. As a result, the c-axis of the main phase particles is oriented along the stress direction (direction of the first side). In other words, the easy magnetization axis direction C of the molded body (magnetic substrate obtained by hot extrusion molding) approximately coincides with the direction X of the first side in the extrusion port side region 10C (end face). In other words, the AB direction of the molded body approximately coincides with the direction Y of the second edge in the extrusion port side region 10C (end face).

[0068] The temperature of the extrusion port side region 10C gradually decreases along the extrusion direction Z. That is, the temperature T1 at the first outlet (outlet of the intermediate region 10B) is higher than the temperature T2 at the second outlet (outlet of the extrusion port side region 10C). Therefore, the temperature of the molded body gradually decreases as it moves through the extrusion port side region 10C. For example, the temperature T1 at the first outlet (outlet of the intermediate region 10B) may be adjusted to 780°C or higher and 790°C or lower, and the temperature T2 at the second outlet (outlet of the extrusion port side region 10C) may be adjusted to (T1-30)°C (i.e., 750°C or higher and 760°C or lower). Main phase particles (Nd2Fe) in the easy magnetization axis direction C (c axis direction) 14 The thermal expansion coefficient of B (tetragonal crystal) is 6.5 × 10⁻⁶. -6 (1 / K). On the other hand, the thermal expansion coefficient of the main phase particles in the AB direction (a-axis and b-axis directions) is -1.5 × 10 -6 The ratio is (1 / K). Therefore, as the temperature of the molded body decreases in the extrusion port region 10C, the molded body tends to shrink in the easy magnetization axis direction C (direction X of the first side) and expand in the AB direction (direction Y of the second side). By adjusting the dimensional ratio of the first and second sides in the extrusion port region 10C, taking into account 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), the disorder of the orientation of the main phase particles in the molded body that occurs as the temperature of the molded body decreases is suppressed.

[0069] As the pressure applied to the molded body by the mold decreases or disappears, the molded body expands. In other words, the molded body expands due to springback. 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 at the second outlet (outlet of the extrusion port side region 10C) is equal to the length xc1 of the first side at the first outlet (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 less likely to expand in the easy magnetization axis direction C (direction X of the first side). In other words, multiple adjacent main phase particles 4 within the molded body are less likely to separate in the easy magnetization axis direction C (direction X of the first side). 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 at the second outlet (outlet of the extrusion port side region 10C) is slightly greater than the length yc1 of the second side at the first outlet (outlet of the intermediate region 10B). Therefore, the pressure acting on the molded body in the AB direction (direction Y of the second side) in the extrusion port side region 10C gradually decreases. As a result, due to springback, the molded body in the extrusion port side region 10C is prone to expansion in the AB direction (direction Y of the second side). Due to the springback of the molded body in the AB direction (direction Y of the second side), multiple adjacent main phase particles 4 within the molded body are prone to separating in the AB direction (direction Y of the second side). That is, due to the springback of the molded body in the AB direction (direction Y of the second side), gaps 8 (C-axis extended gaps) extending along the easy magnetization axis C are formed between multiple adjacent main phase particles 4. (See cross-section CS2 of the mold in Figure 6(b).) yc2 / yc1 may be between 30.005 / 30.00 and 30.04 / 30.00, or between 30.01 / 30.00 and 30.03 / 30.00. As yc2 / yc1 increases, the number and volume of voids 8 tend to increase. By adjusting yc2 / yc1 within the above range, the angle θ at which the degree of voids 8 is maximized can be determined. Fmax , N / Acs, A 60-120 The area of ​​each void 8 and the area ratio AR can be easily controlled to fall within the desired range described above. If the length xc2 of the first side at the second exit is greater than the length xc1 of the first side at the first exit, and the length yc2 of the second side at the second exit is equal to the length yc1 of the second side at the first exit, then a void 8 extending along the easy magnetization axis C is unlikely to form in the molded body. In other words, if the dimensions of the extrusion port side region 10C in the easy magnetization axis C (direction X of the first side) gradually increase along the extrusion direction Z, and the dimensions of the extrusion port side region 10C in the AB direction (direction Y of the first side) remain constant, then the molded body within the extrusion port side region 10C tends to expand in the easy magnetization axis C (direction X of the first side) and does not tend to expand in the AB direction (direction Y of the second side). As a result, multiple adjacent main phase particles 4 within the molded body tend to separate in the easy magnetization axis C, and voids 8 extending along the AB direction (AB axis extended voids) tend to form between multiple adjacent main phase particles 4. (See cross-section CS3 of the mold in Figure 6(c).)

[0070] The hot plastic deformation process aims to obtain a dense magnetic substrate (molded body). Obtaining a completely dense molded body requires hot plastic deformation at high temperature and high pressure, and also takes time. However, high temperatures coarseen the crystal grain size of the raw material (alloy strip). Coarser crystal grain size reduces coercivity, impairs forgeability, impairs the orientation of crystal grains (main phase particles), and reduces residual magnetic flux density. High pressure accelerates mold wear, leading to decreased productivity. Increased processing time leads to decreased productivity. Decreased forgeability leads to decreased productivity. 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 completely dense molded body is not essential. The degree to which the molded body is densified should be determined in terms of balancing the magnetic properties of the permanent magnet and productivity. Generally, hot forming processes differ from cold isostatic pressing (CIP) or hot isostatic pressing (HIP) in that they are performed by uniaxial or biaxial forming. Uniaxial or biaxial forming results in a stress distribution acting on the magnet substrate (molded body) within the mold. This stress distribution is due to pressure transmission within the mold, with stress being 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. In other words, voids tend to remain on the surface portion of the molded body in contact with the inner wall of the mold. These voids remaining in the molded body do not disappear even after the hot plastic working process and are not discharged outside the molded body. That is, the voids remaining in the surface portion of the molded body remain on the surface of the permanent magnet that is ultimately obtained. For the reasons stated above, the area ratio ARs of multiple voids on the surface of a permanent magnet tends to be higher than the area ratio ARc of multiple voids in the central part of the permanent magnet.

[0071] The magnetic substrate obtained through the above process may be a finished permanent magnet. Alternatively, the magnetic substrate that has undergone the grain boundary diffusion process described below may also be a finished permanent magnet.

[0072] After the hot plastic deformation process, the following grain boundary diffusion process may 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 the diffusion material and the magnet substrate. Heating the magnet substrate to which the diffusion material is attached causes the heavy rare earth elements in the diffusion material to diffuse from the surface of the magnet substrate into the interior of the magnet substrate. Inside the magnet substrate, the heavy rare earth elements diffuse through the 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. Due to the localization of heavy rare earth elements near the surface of the main phase particles and at the grain boundaries, the anisotropic magnetic field becomes locally larger near the grain boundaries, making it difficult for magnetization reversal nuclei to be generated near the grain boundaries. As a result, a permanent magnet with high coercivity can be obtained.

[0073] The temperature of the diffusion material and the magnetic substrate in the grain boundary diffusion process (diffusion temperature) may be, for example, 550°C to 900°C. The time during which the diffusion temperature is maintained within the above range (diffusion time) may be, for example, 1 minute to 1440 minutes.

[0074] The diffusion material may contain at least one heavy rare earth element from among Tb and Dy. In addition to the heavy rare earth element, the diffusion material may further contain at least one light rare earth element from among Nd and Pr. In addition to the heavy rare earth element and the light rare earth element, the diffusion material may further contain Cu. The diffusion material may be, for example, a metal made 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 be covered with a sheet containing the diffusion material and a binder. In the grain boundary diffusion process, the surface of the magnet substrate may be covered with an alloy foil (ribbon) made of the diffusion material.

[0075] To promote the diffusion of the diffusion agent, the surface of the magnetic substrate may be polished before the grain boundary diffusion process. After the grain boundary diffusion process, the surface of the magnetic substrate may be polished to remove any remaining diffusion agent.

[0076] The dimensions and shape of the magnetic substrate may be adjusted by cutting and polishing. A passive layer may be formed on the surface of the magnetic substrate by oxidation or chemical treatment. The surface of the magnetic substrate may be covered with a protective film such as a resin film. The passive layer or protective film improves the corrosion resistance of the permanent magnet.

[0077] The present invention is not necessarily limited to the embodiments described above. Various modifications to the present invention are possible without departing from the spirit of the invention, and such modifications are also included in the present invention. [Examples]

[0078] The present invention will be described in detail by the following examples and comparative examples. The present invention is not limited to the following examples.

[0079] <Manufacturing of permanent magnets> (Example 1) Each step in Example 1 below was carried out in a non-oxidizing atmosphere (Ar gas).

[0080] In the thin-strand fabrication process, alloy powder (alloy strips) was produced from the raw metals by an ultra-rapid solidification method. The raw metals (molten metal) used in the thin-strand fabrication process contained Nd, Fe, Co, Ga, Al, and B. The Nd content in the raw metal was 30.17% by mass. The Co content in the raw metal was 3.96% by mass. The Ga content in the raw metal was 0.59% by mass. The Al content in the raw metal was 0.04% by mass. The content of B in the raw metal was 0.97% by mass. The remaining raw material metal after Nd, Co, Ga, Al, and B was Fe.

[0081] In the hot forming process, a molded body was produced by heating the alloy powder in the mold and compressing it within the mold. The molded body was a rectangular parallelepiped. The dimensions of the molded body were 22 mm × 11 mm × 80 mm. Hot forming temperature T HP The temperature was 750°C. Hot forming pressure P HP The pressure was 100 MPa. The hot forming time was 300 seconds.

[0082] A hot plastic deformation process was carried out following the hot forming process. In the hot plastic deformation process, a permanent magnet was manufactured by hot extrusion molding of the molded body using the aforementioned mold (a mold in which the cavity 10 shown in Figure 5 was formed). The length xa of the first side at the mold entrance (entrance side region 10A) was 22 mm. The length ya of the second side at the mold entrance (entrance side region 10A) was 11 mm. The mold inlet temperature Ti (temperature in the inlet region 10A) was maintained at the values ​​shown in Table 1 below. The length of the first side xc1 at the first exit (the exit of intermediate region 10B) was 7 mm. The length of the second side yc1 at the first exit (the exit of intermediate region 10B) was 30 mm. The temperature T1 at the first outlet (temperature in the intermediate region 10B) was maintained at the values ​​shown in Table 1 below. The length of the first side xc2 at the second exit (extrusion port side region 10C) was 7 mm. The length of the second side yc2 at the second outlet (outlet of the extrusion port side region 10C) was the value shown in Table 1 below. The temperature T2 at the second outlet (the outlet temperature in the extrusion port side region 10C) was maintained at the temperatures shown in Table 1 below. The hot plastic deformation pressure (maximum pressure) was 60 MPa. The extrusion speed for hot extrusion molding was 1 mm / second.

[0083] The permanent magnet of Example 1 was fabricated using the method described above. The direction of the first side was equal to the easy magnetization axis C of the permanent magnet. The direction of the second side was equal to the AB direction of the permanent magnet. The width t of the permanent magnet in the easy magnetization axis C was 7 mm.

[0084] (Examples 2-5 and Comparative Examples 1-7) In the thin strip fabrication process of Example 4, Comparative Examples 6 and 7, the following raw metals (molten metals) different from those used in Example 1 were employed. The raw metals contained Nd, Fe, Co, Ga, Al, and B, in addition to Pr and Dy. The Nd content in the raw metal was 10.65% by mass. The Pr content in the raw metal was 17.40% by mass. The Dy content in the raw metal was 2.07% by mass. The Co content in the raw metal was 3.40% by mass. The Ga content in the raw metal was 0.50% by mass. The Al content in the raw metal was 0.07% by mass. The content of B in the raw metal was 0.97% by mass. The remaining raw material metal, excluding Nd, Pr, Dy, Co, Ga, Al, and B, was Fe.

[0085] Hot forming pressure P in Example 5 HP The pressure was 150 MPa. In other words, in Example 5, a further densification of the permanent magnet was attempted by a hot forming process at a higher pressure. Hot forming temperature T of Comparative Example 5 HP The temperature was 740℃.

[0086] The mold inlet temperature Ti for each of Examples 2-5 and Comparative Examples 1-7 was maintained at the values ​​shown in Table 1 below. The temperature T1 at the first outlet of each of Examples 2-5 and Comparative Examples 1-7 was maintained at the values ​​shown in Table 1 below. The length of the second side yc2 at the second outlet of each of Examples 2-5 and Comparative Examples 1-7 was adjusted to the values ​​shown in Table 1 below. The temperature T2 of the second outlet in each of Examples 2-5 and Comparative Examples 1-7 was maintained at the temperatures shown in Table 1 below.

[0087] Except for the points mentioned above, permanent magnets for Examples 2-5 and Comparative Examples 1-7 were fabricated in the same manner as in Example 1.

[0088] <Analysis of permanent magnets> (Composition and microstructure of permanent magnets) 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 the permanent magnet. The composition of the cross-sections of each permanent magnet was analyzed using an electron probe microanalyzer (EPMA) and an energy-dispersive X-ray spectroscopy (EDS) system. In all of the real-world examples 1-5 and comparative examples 1-7, the permanent magnets had the following characteristics. A permanent magnet consists of numerous main phase particles (Nd2Fe 14 It contained crystal grains of B. Multiple voids were formed within the permanent magnet. Each principal phase particle observed in the cross-section was flattened. Numerous main phase particles were stacked along the easy magnetization axis C.

[0089] (Measurements related to voids) As shown in Figure 7(a), a backscattered electron image i1a of a portion of the cross-section of the permanent magnet of Example 1 was captured by SEM. The cross-section from which the backscattered electron image i1a of Example 1 was captured was parallel to the easy magnetization axis C. The vertical direction of the backscattered electron image i1a was the easy magnetization axis C, and the horizontal direction of the backscattered electron image i1a was the AB direction. Image i1b in Figure 7(b), image i1c in Figure 8(a), and image i1d in Figure 8(b) are images obtained from the backscattered electron image i1a of Example 1. The vertical direction of images i1b, i1c, and i1d is the easy magnetization axis C. The horizontal direction of images i1b, i1c, and i1d is the AB direction. The dimensions of the pixels (1 pixel) in images i1b, i1c, and i1d were (1 / 3) μm × (1 / 3) μm. The brightness of backscattered electron beams in any part of the backscattered electron image i1a (in arbitrary units) increases with increasing atomic weight of the element present in that part. Therefore, relatively bright areas in the backscattered electron image i1a correspond to areas with relatively high concentrations of elements with relatively large atomic weights (e.g., Nd). Conversely, the darkest areas in the backscattered electron image i1a correspond to voids where no elements are present. A monochrome image i1b was obtained by thresholding (binarization) of the backscattered electron image i1a based on the RGB color model (Red-Green-Blue color model). The black areas in the monochrome image i1b represent voids. The area of ​​each void in the monochrome image i1b was measured. Based on the measured area of ​​each void, the area ratio AR of the voids in the cross-section of the permanent magnet (backscattered electron image i1a) was calculated. The area ratio AR for Example 1 is shown in Table 1. Image processing of the monochrome image i1b identified the contours of each void in image i1b. Each of the multiple closed curves contained in image i1c corresponds to the contour of each void in image i1b. In the image processing of the monochrome image i1b, the area of ​​the black parts in the monochrome image i1b was 16.67 (μm). 2 The following parts were removed from the image as noise. In the image processing of the monochrome image i1b, gaps that were interrupted at the edges of the image were removed. Image i1d was obtained by approximating the contours of each void in image i1c with ellipses. The approximation of the contours of each void with ellipses was performed by fitting based on the least squares method. The major axis direction of the ellipse approximating the contour of each void in image i1d (i.e., the direction VD in which each void 8 extends) was identified. Furthermore, the angle θ between the AB direction and VD of each void was measured. Since it is not possible to define the direction VD in which a void approximated by a perfect circle extends, voids approximated by a perfect circle were excluded from the measurement of the angle θ. The image processing described in Example 1 above was performed using ImageJ, a public domain image processing software. Based on the image processing of Example 1 described above, the frequency distribution F of the voids in the backscattered electron image i1a (cross-section approximately parallel to the easy magnetization axis C) was obtained. Furthermore, the weighted frequency distribution WF of the voids, which is weighted according to the area of ​​each void, was 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 9(a). The weighted frequency distribution WF of Example 1 is shown in Figure 9(b).

[0090] Using the same method as described above, backscattered electron images were captured and image processing was performed on the surface portion exposed to the cross-section of the permanent magnet of Example 1. The surface portion was the part where the depth from the surface of the permanent magnet in the easy magnetization axis axis C was 0 to 1.75 mm. Based on the capture and image processing of backscattered electron images on the surface portion, the area ratio ARs of the voids on the surface portion was calculated. Using the same method as described above, backscattered electron images were captured and image processing was performed on the central portion of the cross-section of the permanent magnet in Example 1 that was exposed. The central portion was the part where the depth from the surface of the permanent magnet in the easy magnetization axis axis C was greater than 1.75 mm and less than or equal to 3.5 mm. Based on the capture and image processing of backscattered electron images in the central portion, the area ratio ARc of the void in the central portion was calculated. The ARs, ARc, average values ​​of ARs and ARc, and ARs-ARc for Example 1 are shown in Table 3 below.

[0091] Using the same method as in Example 1, the area ratio AR, frequency distribution F, and load frequency distribution WF for Examples 2-5 and Comparative Examples 1-7 were obtained, respectively. The respective area ratios AR for Examples 2-5 and Comparative Examples 1-7 are shown in Table 1 below. In Table 1 below, "θ-F" represents the angle θ at which the frequency reaches its maximum value, Fmax, in the frequency distribution F. Fmax This means the range of θ that includes this range. In Table 1 below, "θ-WF" represents the angle θ at which the load frequency reaches its maximum value, WFmax, in the load frequency distribution WF. WFmax This means the range of θ that includes this range. In Table 1 below, "0-30" means "0° or more and less than 30°". In Table 1 below, "30-60" means "30° or more and less than 60°". In Table 1 below, "60-90" means "60° or more and 90° or less." In Table 1 below, "90-120" means "90° or more and 120° or less." In Table 1 below, "120-150" means "greater than 120° and less than or equal to 150°". In Table 1 below, "150~180" means "greater than 150° and less than or equal to 180°". In Table 1 below, N / Acs represents the number of C-axis-extended air gaps per unit area in the backscattered electron image (cross-section of a permanent magnet). The detailed definition of N / Acs is as described above. A in Table 1 below 60-120 / Acs is the ratio of the total area of ​​the C-axis extended void to the area of ​​the backscattered electron image (cross-section of the permanent magnet).60-120 The details of the definition of / Acs are as described above. In Comparative Examples 1 and 2, the number of detected voids was very small, making it difficult to accurately identify θ-WF and θ-WF. In Comparative Example 6, no C-axis extension void was detected. The ARs and ARc for Example 5 were calculated using the same method as in Example 1. The ARs, ARc, the average values ​​of ARs and ARc, and ARs-ARc for Example 5 are shown in Table 3 below.

[0092] (Electrical resistivity of permanent magnets) A sample for measuring electrical resistivity was prepared from the permanent magnet of Example 1. The sample was a rectangular parallelepiped. The dimensions of the sample were 10.0 mm (length) x 1.0 mm (width) x 0.5 mm (thickness). The thickness direction of the sample (the direction of the side with a length of 0.5 mm) was the easy magnetization axis C of the sample. The electrical resistivity ρ of the sample in the direction perpendicular to the easy magnetization axis C was measured by 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 plane perpendicular to the easy magnetization axis C (the plane with dimensions of 10.0 mm (length) x 1.0 mm (width)). The spacing between the probes was 1.5 mm. The measurement current was adjusted to 100 mA. A resistivity meter (Loresta GP) manufactured by Nitto Seikou Analytech Co., Ltd. (formerly Mitsubishi Chemical Analytech Co., Ltd.) was used for measuring electrical resistivity by the four-probe method. The electrical resistivity ρ of Examples 2-5 and Comparative Examples 1-7 was measured using the same method as in Example 1. The electrical resistivity ρ of Examples 1-5 and Comparative Examples 1-7 are shown in Table 2 below.

[0093] (Magnetic properties of permanent magnets) The coercivity, remanent magnetic flux density, and aspect ratio of the permanent magnets in Examples 1-5 and Comparative Examples 1-7 were measured. The remanent magnetic flux density, coercivity, and aspect ratio were measured using a BH tracer. The coercivity was defined as the coercivity at 23°C (HcJ). 23 ), and coercivity at 150° (HcJ) 150The following measurements were taken: Residual magnetic flux density (Br) was measured at room temperature. The aspect ratio (Hk / HcJ) was measured at 23°C. The measured values ​​of the magnetic properties and the temperature coefficient β of coercivity for each permanent magnet are shown in Table 2 below. The definition of the temperature coefficient β is as described above.

[0094] [Table 1]

[0095] [Table 2]

[0096] [Table 3] [Industrial applicability]

[0097] For example, the R-T-B permanent magnet according to the present invention is suitable for use as a magnet in the rotor of an IPM motor or SPM motor. [Explanation of Symbols]

[0098] 2…R-T-B permanent magnet, 2cs…Cross-section of the permanent magnet, 4…Main phase particles, 8…Air gap, C…Easy magnetization axis direction, AB…Direction approximately perpendicular to the easy magnetization axis direction, F…Frequency distribution of the air gap.

Claims

1. A permanent magnet containing rare earth elements R, transition metal elements T and B, The R-T-B permanent magnet contains at least Nd as R, The R-T-B permanent magnet contains at least Fe as T, The R-T-B system permanent magnet comprises a plurality of main phase particles, The plurality of main phase particles contain at least R, T, and B, In the cross-section of the R-T-B permanent magnet, multiple voids are formed. The cross-section is substantially parallel to the easy magnetization axis direction of the R-T-B permanent magnet. The area ratio of the plurality of voids in the cross-section is 1% or more and 5% or less. In the aforementioned cross-section, the direction perpendicular to the easy magnetization axis is represented as the AB direction. In the aforementioned cross-section, the direction in which each of the plurality of voids extends is represented as VD. The angle between the direction AB and the direction VD is represented as θ. The horizontal axis of the frequency distribution of the plurality of voids in the cross-section represents θ. The range of the horizontal axis of the frequency distribution is between 0° and 180°. The frequency distribution is maximized within the range where θ is 60° or more and 120° or less. R-T-B series permanent magnet.

2. The plurality of main phase particles are flattened in cross-section, The plurality of main phase particles are stacked along the easy magnetization axis. The R-T-B permanent magnet according to claim 1.

3. 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. The R-T-B permanent magnet according to claim 1.

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

5. Hot-worked magnets, The R-T-B permanent magnet according to claim 1.

6. The width of the R-T-B permanent magnet in the easy magnetization axis direction is represented by t, The surface portion of the R-T-B permanent magnet is defined as the portion where the depth from the surface of the R-T-B permanent magnet in the easy magnetization axis direction is 0 or more and 0.25t or less. The area ratio of the plurality of voids in the surface portion is expressed as ARs%. The ARs are measured on the surface portion exposed in the cross-section. The central portion of the R-T-B permanent magnet is defined as the portion where the depth from the surface of the R-T-B permanent magnet in the easy magnetization axis direction is greater than 0.25t and less than or equal to 0.5t. The area ratio of the multiple voids in the central portion is expressed as ARc%, The ARc is measured at the central portion exposed in the cross-section, ARs-ARc is between 1.0% and 4.0%. The R-T-B permanent magnet according to claim 1.

Images