Rare earth magnet powder, bonded magnet, compound for bonded magnet, sintered magnet, method for manufacturing rare earth magnet powder, and method for manufacturing rare earth permanent magnet

Abstract

To provide a rare earth magnet powder capable of sufficiently improving bulk density and suitable for manufacturing an anisotropic magnet.SOLUTION: A rare earth magnet powder has an average sphericity P1 defined by the formula (1) of P1=Ls / Ll of 0.65 or more. In the formula (1), Ll is the length of the long side of the circumscribing rectangle having the smallest area with respect to the rare earth magnet powder in a microscope image, and Ls in the formula (1) is the length of the short side of the circumscribing rectangle having the smallest area with respect to the rare earth magnet powder in the microscope image.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present invention relates to rare earth magnetic powder, bonded magnets, compounds for bonded magnets, sintered magnets, methods for producing rare earth magnetic powder, and methods for producing rare earth permanent magnets. [Background technology]

[0002] Rare earth permanent magnets are known to have excellent magnetic properties. RTB permanent magnets with even better magnetic properties are currently being developed. When manufacturing rare earth permanent magnets, the raw material powder used for sintering is refined to an average particle size of approximately 1 to 10 μm to ensure magnetic properties such as saturation magnetic flux density and coercive force. However, refinement of the raw material powder can hinder the dimensional accuracy and productivity of the compact.

[0003] Raw powder is compressed in a magnetic field to form a compact. In this compaction in a magnetic field, a static or pulsed magnetic field is applied to the raw powder to orient the particles of the raw powder. During this magnetic field compaction, the finer the raw powder, the poorer its fluidity, making it difficult to fill into a mold. If the powder's ability to fill into a mold is poor, the powder cannot be filled into the mold uniformly. This results in variations in the density of the compact and makes it difficult to achieve dimensional accuracy for the compact. Alternatively, the filling into the mold itself takes time, hindering productivity. It is particularly difficult to produce thin-walled or complex-shaped compacts accurately and efficiently.

[0004] For example, Patent Document 1 proposes a technique in which a lubricant is added to coarse powder, which is then finely pulverized in an airflow mill to produce a raw material powder that improves flowability into a mold during compaction. However, even with this technique, the flowability is still insufficient, and the bulk density of the compact cannot be sufficiently improved.

[0005] Patent Document 2 also discloses spherical Nd-Fe-B alloy magnetic powder produced using a thermal plasma method. However, the powder obtained by the method disclosed in this document has a large average particle size, and the internal structure of the particles is composed of a fine structure that is 1 / 10 to 1 / 100 the diameter of the spherical particles. As a result, each magnetic powder contains many crystallites internally, and their crystal orientation is random, making it unsuitable for producing anisotropic magnets in this state.

[0006] Furthermore, Patent Document 3 discloses a technique for rapidly solidifying a molten alloy using an inert gas atomization method to obtain spherical powder with an average grain size of 500 μm or less and a structure with an average crystal grain size of 1 to 30 μm. As disclosed in this document, the spherical powder obtained by the gas atomization method tends to have an internal structure that is a polycrystalline body consisting of an aggregate of crystallites smaller than the grain size of the spherical powder. When individual magnetic powder particles contain many crystallites, their crystal orientation becomes random, making them unsuitable for producing anisotropic magnets in this state. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 8-111308 [Patent Document 2] Japanese Patent Application Publication No. 9-143514 [Patent Document 3] Japanese Patent Application Publication No. 8-316016 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention was made in light of these circumstances, and its object is to provide rare earth magnetic powder that can sufficiently improve bulk density and is suitable for producing anisotropic magnets, and bonded magnets and compounds for bonded magnets molded using that powder. Another object of the present invention is to provide a method for producing rare earth magnetic powder that can easily produce rare earth magnetic powder that can sufficiently improve bulk density and is suitable for producing anisotropic magnets, and a method for producing rare earth permanent magnets using that method. [Means for solving the problem]

[0009] In order to achieve the above object, the rare earth magnet powder according to a first aspect of the present invention comprises: The rare earth magnetic powder has an average sphericity P1, defined by formula (1) P1=Ls / Ll, of 0.65 or more. In formula (1), Ll is the length of the long side of the rectangle circumscribing the rare earth magnetic powder in a microscope image, which has the smallest area, and Ls is the length of the short side of the rectangle circumscribing the rare earth magnetic powder in a microscope image, which has the smallest area. Furthermore, in the rare earth magnetic powder according to the first aspect of the present invention, the proportion of single-crystal rare earth magnetic powder particles is 65% or more.

[0010] The present inventors have discovered that the magnetic powder according to the first aspect can be sufficiently improved in bulk density (both loose bulk density and compacted bulk density), and that when compacted in a magnetic field, even if the magnetic powder is fine, its flowability is not easily reduced, improving its ability to be filled into a mold. As a result of the improved ability to be filled into a mold, the magnetic powder can be filled uniformly into the mold, reducing variations in the density of the compact and reducing deformation of the sintered body. Furthermore, since friction between powders is reduced, the degree of orientation of the magnetic powder can be increased.

[0011] Furthermore, the magnetic powder of the present invention can achieve a high degree of orientation of the magnetic powder in the compact. That is, the particles of the magnetic powder of the present invention are easily oriented in the magnetic field direction, resulting in a rare earth permanent magnet with excellent magnetic properties. Furthermore, in the first aspect of the present invention, the proportion of rare earth magnetic powder consisting of single crystals is 65% or more, which can improve the magnetic properties of the magnet, such as the coercive force (Hcj).

[0012] Preferably, the rare earth magnet powder has a sphericity P1 of 0.9 or more in a proportion of 30% or more. Even if not all of the magnet particles have a sphericity P1 of 0.9 or more, the effect is achieved as long as the raw material powder to be filled into the mold contains a certain amount of magnet particles with a specific sphericity P1.

[0013] Furthermore, in order to achieve the above object, the rare earth magnetic powder according to a second aspect of the present invention is a rare earth magnetic powder having an average value of 0.70 or more for sphericity P2, defined by formula (2) P2 = Lt / Lr. In formula (2), Lr is the perimeter of the rare earth magnetic powder in a microscope image, and Lt is the perimeter of a perfect circle having the same area as the area in the image of the rare earth magnetic powder from which the perimeter Lr was calculated. Furthermore, in the rare earth magnetic powder according to the second aspect of the present invention, the proportion of single-crystal rare earth magnetic powder particles is 65% or more.

[0014] The present inventors have discovered that the magnetic powder according to the second aspect of the present invention, like the first aspect of the present invention, can sufficiently improve bulk density, and that when pressure-molded in a magnetic field, even fine magnetic powders are less likely to lose fluidity, improving mold fillability. As a result of improving mold fillability, the magnetic powder can be filled uniformly into the mold, reducing variations in compact density and reducing deformation of the sintered body. Furthermore, the magnetic powder according to the second aspect of the present invention can increase the degree of orientation of the magnetic powder in the compact. Furthermore, the second aspect of the present invention can improve magnetic properties such as the coercive force (Hcj) of the magnet.

[0015] Preferably, the rare earth magnetic spherical powder contains 25% or more particles with a sphericity P2 of 0.8 or more. Even if not all magnetic particles have a sphericity P2 of 0.8 or more, the effect is achieved as long as the raw material powder before being filled into the mold contains a certain amount of magnetic particles with a specific sphericity P2.

[0016] Preferably, the average particle size of the rare earth magnet powder is 20 μm or less, more preferably 10 μm or less, and preferably 0.5 μm or more, and even more preferably 0.5 to 10 μm. If the average particle size of the rare earth magnet powder is too large, gaps are likely to occur between the particles, and the bulk density is likely to decrease. Furthermore, if the average particle size of the rare earth magnet powder is too large, crystal phases other than R2T14B crystals, etc., tend to be mixed within each magnet powder, which may result in a deterioration of the magnetic properties after compaction. Furthermore, if the average particle size of the rare earth magnet powder is too small, the particles are likely to aggregate frequently, which also tends to decrease the bulk density.

[0017] The rare earth magnet powder may include a plurality of coated particles, each of which is an individual particle made of a single main phase particle and at least a portion of the periphery of which is covered with a coating layer. Preferably, the average coverage, which indicates the proportion of the periphery of each individual particle covered with the coating layer, is 50% or more. Preferably, the coated particles include fully coated particles (individual particles entirely covered with the coating layer) with a coverage of 100%. Preferably, the coating layer is composed of a rare earth-rich component having a higher rare earth concentration than the rare earth concentration contained in the main phase particles.

[0018] Sintered magnets obtained by sintering magnet powder containing such coated particles have improved magnetic properties. For example, compared to sintered magnets obtained using magnet powder of the same composition obtained by conventional pulverization methods, sintered magnets obtained using rare earth magnet powder containing coated particles have a high coercive force (Hcj) because the subphase (coating layer) is uniformly and thinly distributed on the surface of the main phase (main phase particles). In addition, the segregation of the subphase is significantly reduced, and the main phase ratio is increased, resulting in a high remanence (Br).

[0019] Preferably, the rare earth magnet powder is at least partially composed of RTB permanent magnet powder, because RTB permanent magnet powder has excellent magnetic properties.

[0020] A bonded magnet according to one aspect of the present invention preferably contains any of the rare earth magnetic powders described above. The bonded magnet may also contain a resin. Furthermore, a bonded magnet compound (which may also contain a resin, etc.) of the present invention preferably contains any of the rare earth magnetic powders described above. When the above rare earth magnetic powder is used as a raw material powder for a bonded magnet (for example, as a raw material powder contained in a bonded magnet compound), a high filling factor and a high degree of orientation can be easily achieved, making it possible to realize a bonded magnet with a high Br.

[0021] A rare earth sintered magnet according to one aspect of the present invention has a cross section in which a main phase and a subphase are observed, with the subphase accounting for 2% or less of its area. Preferably, the main phase is an R2T14B crystal. Preferably, a sintered magnet according to another aspect of the present invention has a degree of orientation of 94% or more, calculated by dividing the remanence in the orientation direction by the saturation magnetic flux density.

[0022] A rare earth sintered magnet according to one aspect of the present invention has a high degree of orientation of the main phase, resulting in a high remanence. Furthermore, the subphase is distributed on the surface of the main phase, resulting in a high coercivity of the sintered magnet. Although the subphase tends to be unevenly distributed within the main phase, the area ratio of the subphase can be reduced, thereby suppressing a decrease in remanence. With such a sintered magnet, the uneven distribution of the subphase components within the sintered body is reduced, allowing for both a high remanence and a high coercivity to be achieved.

[0023] A method for producing rare earth magnetic powder according to one aspect of the present invention includes the steps of pulverizing an alloy of a desired composition to obtain a raw material powder, and then quenching the raw material powder in a thermal plasma to obtain a spherical powder. The use of a thermal plasma method, preferably a high-frequency induction thermal plasma method, makes it possible to spheroidize the rare earth magnetic powder while suppressing the incorporation of impurities. Furthermore, the method of the present invention facilitates the production of spherical powder with an average particle size of preferably 20 μm or less, more preferably 10 μm or less, and preferably 0.5 μm or more.

[0024] Preferably, the raw material powder is introduced into the thermal plasma tail flame. The temperature of the thermal plasma tail flame is, for example, 2000 to 5000 K, which is a relatively low temperature for a thermal plasma. By introducing the raw material powder (alloy powder) into this region, nano-sized particles due to evaporation and coarsening due to excessive melting can be suppressed, making it possible to obtain spherical powder with a particle size equivalent to the particle size of the introduced raw material powder. Furthermore, this manufacturing method makes it easy to obtain powder in which the number ratio of single-crystal rare earth magnet powder is 65% or more, thereby improving the magnetic properties of the magnet, such as the coercive force (Hcj).

[0025] Furthermore, for example, by introducing raw material powder into the tail flame of a thermal plasma, heating it, and then rapidly cooling it, the raw material powder particles are melted in whole or in part, making the powder more likely to become spherical. Furthermore, the subphase components are more likely to be uniformly distributed on the powder surface, making it easier to form spherical coated particles. The spherical shape of the powder makes it easier to improve bulk density and increase the degree of orientation.

[0026] Preferably, a method for producing a rare earth permanent magnet according to one aspect of the present invention comprises the steps of: pulverizing an alloy of a desired composition to obtain a raw material powder; charging the raw material powder into a thermal plasma tail flame, heating it, and then quenching it to obtain a spherical powder; and sintering the spherical powder to obtain a sintered body. Preferably, the magnetic powder containing the spherical powder is molded into a predetermined shape by, for example, press molding, and then sintered to form the sintered body.

[0027] In this manufacturing method, the spherical shape of the powder makes it easy to improve bulk density and increase the degree of orientation. This makes it easy to increase the remanence of the magnet after sintering. Furthermore, the subphase components are more likely to be uniformly distributed on the powder surface, making it easier to form spherical coated particles. When the subphase components are uniformly distributed on the powder surface, the subphase is uniformly distributed (thinly) at the grain boundaries between two particles during densification after sintering, making it difficult for the remanence to decrease. Furthermore, this manufacturing method makes it easy to obtain powder in which the number ratio of single-crystal rare earth magnet powder is 65% or more, thereby improving the magnetic properties of the magnet, such as the coercive force (Hcj). [Brief explanation of the drawings]

[0028] [Figure 1A] FIG. 1A is a micrograph of rare earth magnet powder according to an embodiment of the present invention. [Figure 1B] FIG. 1B is a micrograph of rare earth magnet powder according to a comparative example of the present invention. [Figure 2A] FIG. 2A is a schematic explanatory diagram for calculating the sphericity of a particle. [Figure 2B] FIG. 2B is a schematic diagram showing a perfect circle having the same cross-sectional area as the particle shown in FIG. 2A. [Figure 3] FIG. 3 is a flow chart showing a method for producing rare earth magnet powder according to one embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram of an apparatus used in the spheroidizing step shown in FIG. [Figure 5A] FIG. 5A is a schematic explanatory diagram for calculating the coverage of the particles shown in FIG. 1A. [Figure 5B] FIG. 5B is a schematic explanatory diagram for calculating the coverage of the particles shown in FIG. 1B. [Figure 5C] FIG. 5C is a cross-sectional micrograph of a particle showing a coated particle with a coverage rate of 100% among the particles shown in FIG. 1A. [Figure 6A] FIG. 6A is a photomicrograph of a cross section of a rare earth sintered magnet according to one embodiment of the present invention. [Figure 6B] FIG. 6B is a photomicrograph of a cross section of a rare earth sintered magnet according to a comparative example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, the present invention will be described based on an embodiment.

[0030] A rare earth magnet powder according to one embodiment of the present invention is, for example, an RTB magnet powder, in which R represents at least one rare earth element, T represents an iron group element, and B represents boron.

[0031] The term "rare earth elements" refers to Sc, Y, and lanthanoid elements, which belong to Group 3 of the long periodic table. Examples of lanthanoid elements include La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Rare earth elements are classified into light rare earth elements and heavy rare earth elements. In this application, "heavy rare earth elements" refers to rare earth elements with atomic numbers 64 to 71, i.e., Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and "light rare earth elements" refers to rare earth elements other than heavy rare earth elements. In this application, Y is classified as a light rare earth element. Hereinafter, heavy rare earth elements may be abbreviated as RH. The RTB magnet powder according to this embodiment may also contain a heavy rare earth element RH.

[0032] T, which represents an iron group element, may be Fe alone, or part of the Fe may be substituted with Co. When part of the Fe is substituted with Co, the temperature characteristics and corrosion resistance can be improved without deteriorating the magnetic properties.

[0033] B represents boron, but some of the boron can be substituted with carbon. Substituting some of the boron with carbon, i.e., including boron and carbon at the B site, facilitates the formation of thick two-particle grain boundaries during aging treatment after sintering the powder into a predetermined shape, which has the effect of improving coercivity. Note that when substituting some of the boron with carbon, the substitution amount may be approximately 20 at% or less of the total B contained in the R2 T14 B phase observed after sintering.

[0034] The RTB magnet powder may contain other elements. Examples of such elements include Ti, V, Cr, Mn, Ni, Cu, Zr, Nb, Mo, Hf, Ta, W, Al, Ga, Si, Bi, and Sn. The R content in the RTB magnet powder is arbitrary. The R content may be 26% by weight or more and 33% by weight or less. The B content in the RTB magnet powder is arbitrary. The boron content contained as B may be 0.8% by weight or more and 1.2% by weight or less.

[0035] The content of T in the RTB magnet powder is essentially the remainder of the constituent elements of the RTB magnet powder. Furthermore, when Co is contained as T, the content of Co may be 3.0 wt% or less relative to the sum of the contents of the iron-group elements. When Ni is contained as T, the content of Ni may be 1.0 wt% or less relative to the sum of the contents of the iron-group elements.

[0036] The oxygen (O) content of the RTB magnet powder is optional. For example, it may be 200 ppm or more and 3000 ppm or less. A higher O content is preferable from the viewpoint of improving corrosion resistance, while a lower O content is preferable from the viewpoint of improving magnetic properties.

[0037] The carbon (C) content of the RTB magnet powder is optional. For example, it may be between 200 ppm and 3000 ppm. If the C content is outside this range, the magnetic properties tend to deteriorate. As mentioned above, the RTB magnet powder may contain carbon by substituting some of the boron in the B site of the RTB magnet powder with carbon.

[0038] The nitrogen (N) content of the RTB magnet powder is optional. For example, it may be between 200 ppm and 1500 ppm. If the N content is outside this range, the magnetic properties tend to deteriorate.

[0039] The amounts of O, C, and N in RTB magnet powder can be measured by commonly known methods. The O amount is measured, for example, by inert gas fusion-nondispersive infrared absorption spectroscopy. The C amount is measured, for example, by oxygen flow combustion-infrared absorption spectroscopy. The N amount is measured, for example, by inert gas fusion-thermal conductivity spectroscopy.

[0040] As shown in Figure 1A, most of the particles in the RTB magnet powder of this embodiment are spherical. That is, the RTB magnet powder of this embodiment has an average sphericity P1, defined by formula (1) P1 = Ls / Ll, of 0.65 or more, preferably 0.7 or more, and more preferably 0.75 or more. Furthermore, the percentage of magnet powder particles with a sphericity P1 of 0.9 or more is preferably 30% or more, more preferably 35% or more, and particularly preferably 40% or more.

[0041] In the above formula (1), Ll is the length of the long side of the rectangle that circumscribes each particle 2 of the RTB-based magnet powder in a microscope image, such as that shown in Figure 2A, and Ls in the above formula (1) is the length of the short side of the rectangle that circumscribes each particle 2 of the RTB-based magnet powder in a microscope image, such as that shown in Figure 2A, and

[0042] The microscope image is a cross-sectional SEM image obtained by embedding RTB magnet powder shown in Figure 1A in resin, hardening it, polishing the cut surface, and then observing the polished surface with an SEM, in which at least 100 magnet particles are observed, and the average of 100 randomly selected particles is obtained. The number proportion (number density) is also found by measuring P1 of 100 randomly selected particles and calculating the number proportion of magnet powder particles with a sphericity P1 of 0.9 or more out of the 100.

[0043] From another perspective, the RTB magnet powder of this embodiment has an average sphericity P2, defined by formula (2) P2 = Lt / Lr, of 0.70 or more, preferably 0.73 or more, and even more preferably 0.75 or more. The percentage of magnet powder particles having a sphericity P2 of 0.8 or more is preferably 25% or more, more preferably 30% or more, and particularly preferably 35% or more.

[0044] In the formula (2), Lr is the perimeter of each particle 2 of the RTB-based magnet powder in a microscope image such as that shown in Figure 2A, and Lt in the formula (2) is the perimeter of a perfect circle 20 shown in Figure 2B that has the same area as the area in the image of each particle 2 from which the perimeter Lr was calculated. The methods for obtaining the microscope image, calculating the average, and calculating the number ratio are the same as those for the formula (1) described above. In the case of sphericity P2, the number ratio can be found by calculating the number ratio of magnet powder particles with a sphericity P2 of 0.8 or more.

[0045] The average particle size of the RTB magnet powder of this embodiment is preferably 20 μm or less, more preferably 10 μm or less, and preferably 0.5 μm or more, and even more preferably 0.5 to 10 μm. The average particle size of the RTB magnet powder is calculated by calculating the equivalent circle diameter of each particle from the microscope image as described above and averaging the values ​​of at least 100 particles. The average particle size can also be measured by a light scattering method using a laser diffraction particle size distribution analyzer, for example.

[0046] If the average particle size of the RTB magnet powder is too large, gaps are likely to form between the particles, resulting in a decrease in bulk density. Furthermore, if the average particle size of the rare earth magnet powder is too large, crystal phases other than R2T14B crystals, etc., tend to be mixed within each magnet powder, which may result in a decrease in magnetic properties. For example, if the average particle size is large, different crystal phases are more likely to be mixed, and the larger the particle size, the more difficult it is to obtain coercivity. Furthermore, if the average particle size of the rare earth magnet powder is too small, the particles are more likely to aggregate, which also tends to decrease bulk density.

[0047] As shown in Fig. 5A, most of the particles 2 in the RTB magnet powder of this embodiment are composed of coated particles 30 with a substantially circular cross section. Each coated particle 30 is composed of individual particles 31, each consisting primarily of a single main phase particle 31a, and at least a portion of the peripheral surface of each individual particle 31 is covered with a coating layer 32. The main phase particles 31a are composed of a single crystal, such as an R2 T14B crystal.

[0048] The proportion of single-crystal main phase particles 31a contained in the RTB magnet powder of this embodiment is 65% or more, preferably 70% or more, and more preferably 75% or more. Single-crystal particles have a higher degree of orientation than polycrystalline particles, making it easier to improve the Br of the magnet when using these particles to produce bonded magnets or sintered magnets.

[0049] To determine whether particle 2 is a single crystal, the cross section of the particle is exposed and evaluated using an electron backscatter diffraction (EBSD) analyzer attached to the FE-SEM device. The number ratio is the number ratio obtained by evaluating, for example, 100 randomly selected particles. Specifically, the number of particles in which a single crystal plane is recognized in each particle is counted in the image obtained with the EBSD analyzer, and the ratio is calculated.

[0050] As shown in Fig. 5A, the peripheral surfaces of the single-crystal main phase particles 31a contained in the RTB magnet powder of this embodiment are preferably at least partially covered with coating layers 32. Coating layers 32 are composed of rare earth-rich components with a higher rare earth concentration than the rare earth concentration contained in the main phase particles. The thickness of coating layer 32 is thin, approximately 10 to 200 nm, and the average particle size of coated particles 30 is within the range of the average particle size of the RTB magnet powder described above.

[0051] As shown in FIG. 5B, the individual particles 31 may be composed of multiple main phase particles 31a connected via a grain boundary phase 33. However, in the present embodiment, it is preferable that at least a portion of the peripheral surface of a single main phase particle 31a is covered with a coating layer 32.

[0052] Furthermore, in this embodiment, the average coverage, which indicates the proportion of the periphery of a single main phase particle 31a that is covered with the coating layer 32, is preferably 50% or more, more preferably 60% or more, and particularly preferably 80% or more. The higher the average coverage of the main phase particles 31a that are covered with the coating layer 32 composed of a rare-earth-rich component with high magnetic anisotropy, the less likely magnetization reversal occurs in the magnet, and the higher the Hcj.

[0053] The average coverage can be determined, for example, as the average value of the coverage of 100 randomly selected particles 2. The coverage of a particle 2 can be calculated, for example, as follows. As shown in FIG. 5A, when an individual particle 31 is composed of only a single main phase particle 31a, the circumferential interface lengths Lb1 and Lb2 of the interface between the outer surface of the main phase particle 31a and the coating layer 32 in the cross section of the particle 2 are determined by image analysis.

[0054] Similarly, the circumferential exposed lengths La1 and La2 of the outer surface of the main phase particle 31a that is not covered with the coating layer 32 are determined for each particle 2 by image analysis. The sum (La1 + La2 + Lb1 + Lb2) of the circumferential exposed lengths La1 and La2 and the circumferential interface lengths Lb1 and Lb2 for each particle 2 is the total circumferential length Lt of the main phase particle 31a (individual particle 31). The coverage for each particle 2 can be expressed as 100 × circumferential interface length (Lb1 + Lb2) / total circumferential length Lt.

[0055] In addition, when there is one circumferential interface length Lb1 and Lb2 (circumferential exposed length La1 and La2) or three or more circumferential interface lengths, the number n (n is an integer greater than or equal to 1) in the total circumferential interface length ΣLbn (total circumferential exposed length ΣLan) increases or decreases.

[0056] In this embodiment, for example, 100 particles 2 randomly selected from magnet powder contain at least one, preferably 10 or more, and more preferably 30 or more fully coated particles with a coverage of 100%, as shown in FIG. 5C. The greater the number of fully coated particles, the higher the average coverage and the improved Hcj. Note that fully coated particles can also be defined as coated particles in which each individual particle is completely covered with a coating layer.

[0057] In the above example, the calculation of the coverage ratio is shown for the case where the individual particle 31 is a single main phase particle 31a as shown in FIG. 5A. However, the calculation of the coverage ratio can also be performed in the same manner for the case where the individual particle 31 is an aggregate of multiple main phase particles 31a as shown in FIG. 5B.

[0058] 5B , for example, when multiple main phase particles 31a are connected by a grain boundary phase 33, an aggregate of these multiple main phase particles 31a is regarded as an individual particle 31. Then, the length of the interface (dotted line portion) between the grain boundary phase 33 and the main phase particle 31a is not counted, and the circumferential interface lengths (two-dot chain line portions) Lb1 and Lb2 of the individual particle 31 and the circumferential exposed lengths (solid line portions) La1 and La2 are calculated by image analysis.

[0059] The magnetic powder of this embodiment may also contain individual particles 31 each consisting of an aggregate of a plurality of such main phase particles 31a, but it is preferable that the average coverage be within the above range. Furthermore, with regard to the sphericity P1 or P2, it is preferable that the sphericity of each main phase particle 31a be within the above range.

[0060] In each individual particle 31, which is an aggregate of a plurality of main phase particles 31a, the grain boundary phase 33 is composed of components containing a higher amount of rare earth than the main phase particles 31a, and is composed of a similar composition, but not necessarily the same, as the coating layer 32. The grain boundary phase 33 exists between the main phase particles 31a, and the coating layer 32 is defined as a portion that covers the outer surface of the main phase particle 31a and is not in contact with other main phase particles 31a, with the outer surface being exposed.

[0061] The RTB magnet powder according to this embodiment can sufficiently improve bulk density (both loose bulk density and compacted bulk density), and when pressure-molded in a magnetic field, even fine particles of the magnet powder are less likely to lose fluidity, improving mold fillability. As a result of the improved mold fillability, the magnet powder can be filled uniformly into the mold, reducing density variations in the compact and reducing deformation of the sintered body.

[0062] Furthermore, the RTB magnet powder of this embodiment can increase the degree of orientation of the magnet powder in the compact. That is, each particle of the magnet powder of the present invention is easily oriented in the magnetic field direction, resulting in an RTB magnet with excellent magnetic properties. Furthermore, since friction between powder particles is reduced, the degree of orientation of the magnetic powder can be increased. Furthermore, in this embodiment, the proportion of single-crystal rare earth magnet powder is 65% or more, which can improve the magnetic properties of the magnet, such as the coercive force (Hcj).

[0063] As an RTB permanent magnet, a sintered magnet is preferred from the viewpoint of high magnetic properties and heat resistance. The sintered magnet preferably has a sintered body obtained by sintering a compact obtained by compacting the rare earth magnet powder described above in a magnetic field. As shown in FIG. 6A, a main phase and a subphase are preferably observed in the cross section of the sintered body, and the area ratio of the subphase is 2% or less. Preferably, the main phase is an R2 T14B crystal. Furthermore, the sintered magnet preferably has an orientation degree of 94% or more, more preferably 95% or more, obtained by dividing the remanence magnetic flux density in the orientation direction by the saturation magnetic flux density.

[0064] The sintered magnet according to this embodiment has a high degree of orientation of the main phase, resulting in a high remanence. Furthermore, the subphase is uniformly distributed on the surface of the main phase, resulting in a high coercivity of the sintered magnet. Although the subphase tends to be unevenly distributed between the main phases, the area ratio of the subphase can be reduced, thereby suppressing a decrease in remanence. With such a sintered magnet, the uneven distribution of the subphase components within the sintered body is reduced, allowing for both a high remanence and a high coercivity to be achieved.

[0065] The area ratio of the subphase can be calculated from a backscattered electron image obtained using, for example, an FE-SEM (field emission scanning electron microscope). When using an FE-SEM, a sample for the FE-SEM is first prepared. Specifically, the RTB permanent magnet is embedded in an epoxy or phenolic resin and polished so that a cross section parallel to the orientation direction of the RTB permanent magnet can be observed. Specifically, the polishing is performed by rough polishing using a conventional method, followed by finish polishing. The finish polishing is performed so that the cross section becomes glossy. There are no particular limitations on the method of finish polishing.

[0066] It is preferable to perform the final polishing using dry polishing, which does not use a polishing liquid such as water. If a polishing liquid such as water is used, corrosion of the grain boundary phase may prevent proper analysis. Next, the cross section of the polished RTB permanent magnet is subjected to ion milling to remove oxide films, nitride films, etc.

[0067] Next, a cross section of the obtained RTB permanent magnet is observed using an FE-SEM, and a backscattered electron image is obtained at a magnification of 1000x to 3000x, with a size of 50µm square to 100µm square. The contrast of the backscattered electron image confirms that the RTB permanent magnet is composed of multiple types of phases. By comparing the results of point analysis using an EDS (energy dispersive X-ray spectrometer) attached to the FE-SEM with the contrast of the backscattered electron image, it is possible to classify the magnet into main phase crystal grains (main phase) consisting of R2 T14B and phases (subphases) such as R-rich phases. The EDS measurement results can be used to determine whether a grain is a main phase crystal grain (main phase) or not.

[0068] To calculate the area ratio of the subphase, the backscattered electron image is subjected to image processing and binarized. In this embodiment, in the backscattered electron image of the RTB permanent magnet, areas having a contrast brighter than the main phase crystal grains at a predetermined level or more are extracted and designated as subphase areas. For example, the backscattered electron image of the RTB permanent magnet shown in FIG. 6A or 6B is subjected to image processing and binarized. The area ratio of the subphase (area ratio of the subphase) can be calculated by dividing the area of ​​the subphase detected by binarization by the area of ​​the RTB permanent magnet.

[0069] Secondary phases such as the R-rich phase generally have a higher content of rare earth element R than the main-phase crystal particles. Here, the rare earth element R is an element with a particularly large atomic number among the elements usually contained in the R-T-B system permanent magnet. It is known that the signal intensity of the backscattered electron image becomes stronger and appears brighter as the content of the element with a large atomic number increases. In the present embodiment, the volume ratio can also be calculated assuming that the area ratio and the volume ratio are equal.

[0070] The permanent magnet of the present embodiment may be a bonded magnet in which the above-described R-T-B system magnet powder is kneaded into a resin, or a compound for bonded magnets (for example, in the form of pellets). When the rare earth-based magnet powder of the present embodiment is used as a raw material powder of a bonded magnet (for example, a raw material powder contained in a compound for bonded magnets), a high filling rate can be achieved, a high degree of orientation can be easily obtained, and a bonded magnet with a high Br can be realized.

[0071] Hereinafter, a method for manufacturing an R-T-B system sintered magnet (hereinafter, also referred to as an R-T-B system permanent magnet) will be described in detail.

[0072] <Method for Manufacturing R-T-B System Permanent Magnet> The method for manufacturing an R-T-B system permanent magnet according to the present embodiment includes the following steps. (a) Alloy preparation step of preparing a raw material alloy (b) Crushing step of crushing the raw material alloy (c) Step of spheroidizing the crushed raw material alloy powder (d) Forming step of forming the spheroidized raw material powder (e) Sintering step of sintering the formed body to obtain an R-T-B system permanent magnet base material (f) Processing step of processing the R-T-B system permanent magnet base material

[0073] [Alloy Preparation Step] A raw material alloy for the RTB permanent magnet according to this embodiment is prepared. Raw material metals corresponding to the composition of the RTB permanent magnet according to this embodiment are melted in a vacuum or in an inert gas atmosphere such as Ar gas, and then the melted raw material metals are cast to produce a raw material alloy having the desired composition. Note that while this embodiment describes the case of a single-alloy method, a two-alloy method in which a main phase alloy and a grain boundary alloy are separately produced may also be used.

[0074] Examples of raw material metals that can be used include rare earth metals or rare earth alloys, pure iron, ferroboron, and alloys and compounds thereof. Casting methods for casting the raw material metals include ingot casting, strip casting, book molding, and centrifugal casting. If the obtained raw material alloy has solidification segregation, it is subjected to a homogenization treatment as necessary.

[0075] [Crushing process] After the raw alloy is produced, the raw alloy is pulverized.

[0076] The pulverization process can be carried out in two stages: a coarse pulverization process (step S1 shown in FIG. 3) in which the material is pulverized until the particle size is about several hundred μm to several mm, and a fine pulverization process (step S3 shown in FIG. 3) in which the material is pulverized until the particle size is about several μm.

[0077] (coarse grinding process) The raw alloys are coarsely pulverized until the particle size is on the order of several hundred μm to several mm (step S1 shown in FIG. 3). This produces a coarsely pulverized powder of the raw alloy. The coarse pulverization is performed by allowing the raw alloy to absorb hydrogen, then releasing the hydrogen based on the difference in the amount of hydrogen absorbed between different phases, and dehydrogenating the alloy to cause self-destructive pulverization (hydrogen absorption pulverization). The coarse pulverization step does not have to use the hydrogen absorption pulverization described above, and may instead be performed in an inert gas atmosphere using a coarse pulverizer such as a stamp mill, jaw crusher, or Braun mill.

[0078] Furthermore, to obtain high magnetic properties, it is preferable to maintain a low oxygen concentration in the atmosphere during each process from the crushing process to the sintering process described below. The oxygen concentration is adjusted by controlling the atmosphere during each manufacturing process. If the oxygen concentration is high during each manufacturing process, the rare earth elements in the raw alloy powder will oxidize to form R oxides, which will not be reduced during sintering and will precipitate as R oxides at the grain boundaries, resulting in a lower remanence Br of the resulting RTB permanent magnet. Therefore, it is preferable to maintain the oxygen concentration in each process at 100 ppm or less.

[0079] (Fine grinding process) After the raw alloy is coarsely pulverized, the obtained coarsely pulverized powder of the raw alloy is finely pulverized until the average particle size is about several μm (Step S3 shown in FIG. 3). This results in a finely pulverized powder of the raw alloy. By further finely pulverizing the coarsely pulverized powder, a finely pulverized powder having particles preferably of 1 μm to 10 μm, more preferably of 3 μm to 5 μm, can be obtained.

[0080] Fine pulverization is performed by further pulverizing the coarsely pulverized powder using a fine pulverizer such as a jet mill, ball mill, vibration mill, wet attritor, etc., while appropriately adjusting conditions such as pulverization time. Jet milling is a method in which high-pressure inert gas (e.g., N2 gas) is released from a narrow nozzle to generate a high-velocity gas flow, which accelerates the coarsely pulverized powder of the raw alloy, causing collisions between the coarsely pulverized powder of the raw alloy and with the target or the wall of the container, thereby pulverizing the powder.

[0081] When the coarsely pulverized powder of the raw alloy is pulverized, adding a grinding aid such as zinc stearate or oleic acid amide (lubricant addition step S2 shown in Figure 3) allows for the production of a pulverized powder with high orientation during compaction. An example of the RTB magnet powder obtained in the pulverization step S3 is shown in Figure 1(B). The RTB magnet powder obtained in the pulverization step S3 has an average sphericity P1 defined by the aforementioned formula (1) of less than 0.65, preferably 0.60 or less. Furthermore, the average sphericity P2 defined by the aforementioned formula (2) is less than 0.70, preferably 0.68 or less.

[0082] [Spheroidization process] Next, in this embodiment, a spheroidizing step is performed as shown in step S4 in Fig. 3. In the spheroidizing step, the finely pulverized powder obtained in the fine pulverizing step is spheroidized using, for example, an apparatus shown in Fig. 4.

[0083] The device 10 shown in Figure 4 can generate high-frequency induction thermal plasma 12 inside a plasma generation chamber 13 located in the upper center of a chamber 11. High-frequency induction thermal plasma 12 is generated by locally concentrating high-frequency power in a reduced-pressure atmosphere at atmospheric pressure or close to atmospheric pressure, and by electromagnetic induction, instantly transforming various gases into an ultra-high-temperature plasma state of approximately 10,000 degrees. Raw materials (powder, gas, liquid) are introduced into this plasma 12, and through evaporation, melting, decomposition, chemical reaction, etc., various processes can be carried out, such as the synthesis and reaction of nanoparticles, the modification and spheroidization of fine powders, film formation, and the decomposition of harmful gases.

[0084] The plasma generation chamber 13 is internally connected to the chamber 11 located below it, and a raw material powder supply section 14 is connected near the joint between the plasma generation chamber 13 and the chamber 11, from which fine raw material powder is introduced (sprayed) toward the tail flame section 12a of the thermal plasma 12.

[0085] A high-frequency coil is arranged around the plasma generation chamber 13, and high-frequency induction heating is performed inside the plasma generation chamber 13 to generate a flame of thermal plasma 12. The high-frequency (frequency) and voltage (or power) of the high-frequency voltage applied to the high-frequency coil are not particularly limited and may be selected appropriately depending on the properties of the thermal plasma, such as its temperature.

[0086] In this embodiment, the fine raw material powder obtained in the pulverization step is heated and spheroidized by thermal plasma. The plasma gas is converted to a thermal plasma of approximately 10,000 degrees by high-frequency induction, and the fine raw material powder is introduced into the plasma. The spheroidized raw material powder after the thermal plasma treatment may be classified using a cyclone or the like. This high-frequency induction thermal plasma method can produce spheroidized raw material powder that is close to a perfect sphere. Note that the thermal plasma method may be used to process only part of the fine raw material powder into spherical shapes, rather than all of it.

[0087] In this embodiment, it is more preferable to spray the fine raw material powder obtained in step S3 shown in Fig. 3 toward the tail flame 12a of the thermal plasma 12 of the device 10 shown in Fig. 4. This prevents the fine raw material powder from being excessively evaporated or melted, making it easier to obtain spherical fine raw material powder (the RTB magnet powder of this embodiment) with an average particle size of about 0.5 to 20 µm.

[0088] The particle size and particle size distribution of the spheroidized raw material powder can be controlled by controlling the particle size of the fine raw material powder, the amount of fine raw material powder sprayed per unit time into the thermal plasma 12, the flow rate of the carrier gas, etc. The tail flame 12a of the thermal plasma 12 is near the tip of the flame of the thermal plasma 12 (the lower end in the figure), and the temperature of the tail flame 12a is about 2000 to 5000K.

[0089] The temperature of the tail flame 12a is somewhat low for a thermal plasma, and by adding fine raw material powder (alloy powder) to this region, nanoparticles due to evaporation and coarsening due to excessive melting can be suppressed. If the raw material powder is added from above the thermal plasma (upstream of the tail flame), a large amount of nanopowder is likely to be generated, and the nanopowder tends to aggregate, resulting in a decrease in bulk density.

[0090] The raw material powder introduced into the tail flame portion 12a is exposed to high temperatures in the tail flame portion 12a of the thermal plasma 12, and then quenched and spheroidized by quenching gas 15 in the upper interior of the chamber 11. The atmosphere of the quenching gas 15 in the upper interior of the chamber 11 is, for example, an inert atmosphere made of argon or a reducing atmosphere made of argon containing hydrogen.

[0091] The plasma gas ejected from gas ejection port 16 at the top of plasma generation chamber 13 is a mixture of hydrogen gas and argon gas. The internal pressure of plasma generation chamber 13 is preferably a reduced pressure atmosphere of 700 Torr or less, similar to that of chamber 11, and more preferably 75 to 675 Torr. Inert gases such as helium gas and nitrogen gas may be used instead of or in addition to argon gas. Hydrogen gas does not necessarily need to be included, but is preferably included. Instead of hydrogen gas, oxygen or hydrocarbon gases such as methane, ethane, propane, butane, acetylene, ethylene, propylene, and butene may be used depending on the purpose.

[0092] The spherical powder that has been spheroidized inside chamber 11 is collected and recovered in a box-shaped powder recovery unit 17 provided below chamber 11. The spherical powder (RTB magnet powder of this embodiment) recovered in powder recovery unit 17 is classified as necessary and transferred to an apparatus for carrying out the orientation molding step (step S5) shown in FIG.

[0093] [Orientation molding process] Next, the RTB magnet powder of this embodiment, as shown in FIG. 1A, is molded into the desired shape. This results in a green compact. In the molding process, the magnet powder is filled into a mold placed between electromagnets and pressurized to form the desired shape (step S5 shown in FIG. 3). At this time, pressure is applied while a magnetic field is applied, causing a predetermined orientation in the spheroidized powder, and molding is performed in a magnetic field with the crystal axes aligned. Because the resulting green compact is oriented in a specific direction, an RTB permanent magnet base material with stronger magnetic anisotropy is obtained.

[0094] [Sintering process] Next, the green compact obtained by molding into the desired shape in step S5 shown in Figure 3 is sintered in a vacuum or in an inert gas atmosphere (step S6) to obtain an RTB permanent magnet. The sintering temperature needs to be adjusted depending on various conditions, such as the composition, pulverization method, particle size and particle size distribution, but the green compact is sintered, for example, by heating it in a vacuum or in the presence of an inert gas at 1000°C to 1200°C for 1 hour to 10 hours. This causes the spheroidized powder to undergo liquid phase sintering, resulting in an RTB permanent magnet base material with an increased volume fraction of the main phase. Furthermore, it is preferable to rapidly cool the sintered RTB permanent magnet base material to improve production efficiency.

[0095] If the magnetic properties are to be measured at this stage, an aging treatment is performed. Specifically, after sintering, the RTB permanent magnet substrate is aged by, for example, maintaining the resulting RTB permanent magnet substrate at a temperature lower than that used during sintering. The aging treatment may be performed, for example, in two stages: heating at a temperature between 700°C and 900°C for 1 to 3 hours, followed by heating at a temperature between 500°C and 700°C for 1 to 3 hours, or in one stage heating at a temperature around 600°C for 1 to 3 hours. The treatment conditions are adjusted appropriately depending on the number of aging treatments. This type of aging treatment can improve the magnetic properties of the RTB permanent magnet substrate. The aging treatment may also be performed after the processing step.

[0096] After the RTB permanent magnet substrate is subjected to aging treatment, it is rapidly cooled in an Ar gas atmosphere. This results in the RTB permanent magnet substrate according to this embodiment. The cooling rate is not particularly limited, but is preferably 30°C / min or faster.

[0097] The obtained RTB permanent magnet base material may be machined into a desired shape as needed (step S7 shown in FIG. 3). Examples of processing methods include shaping such as cutting and grinding, and chamfering such as barrel polishing. However, in this embodiment, molding is performed using powder (the RTB magnet powder according to this embodiment) that has been spheroidized in the spheroidizing step. Because of its excellent fluidity, molding can also be performed using a mold with a shape similar to that of the final product in the orientation molding step of step S5 shown in FIG. 3. In other words, this embodiment makes it possible to mold thin-walled products, which was previously difficult to achieve. Therefore, in the processing step, it is possible to produce a product without cutting the sintered magnet base material. After the processing step, a diffusion step, as described below, may be performed as needed.

[0098] [Diffusion process] The heavy rare earth element RH may be diffused into the grain boundaries of the RTB permanent magnet substrate. It is preferable to subject the magnet substrate to an etching treatment as a pretreatment for grain boundary diffusion. Specifically, a mixed solution of 100% ethanol and 3% nitric acid by mass is prepared, and the magnet substrate is immersed in the mixed solution for 3 minutes to etch, and then immersed in ethanol for 1 minute to wash.

[0099] The diffusion can be carried out by a method in which a compound containing a heavy rare earth element is attached to the surface of an RTB permanent magnet substrate and then a heat treatment is performed, or by a method in which the RTB permanent magnet substrate is subjected to a heat treatment in an atmosphere containing vapor of the heavy rare earth element.

[0100] There are no particular limitations on the method for attaching the heavy rare earth element RH, and examples thereof include vapor deposition, sputtering, electrodeposition, spray coating, brush coating, jet dispenser, nozzle, screen printing, squeegee printing, and sheet coating.

[0101] Any type of heavy rare-earth element RH may be used, but it is preferable to use Dy or Tb, and it is particularly preferable to use Tb. Furthermore, for example, when Tb is diffused as the heavy rare-earth element RH, the effect of diffusion can be made more favorable by appropriately controlling the amount of Tb attached, the diffusion temperature, and the diffusion time.

[0102] When the heavy rare earth element RH is applied by coating, it is common to apply a coating material comprising a heavy rare earth compound containing the heavy rare earth element RH and a solvent. There are no particular limitations on the form of the coating material. Furthermore, any type of heavy rare earth compound may be used. Examples include alloys, oxides, halides, hydroxides, and hydrides. It is particularly preferable to use hydrides.

[0103] When a Tb compound is to be deposited, it is possible to deposit, for example, Tb hydride (TbH2), Tb oxide (Tb2 O3, Tb4 O7) or Tb fluoride (TbF3).

[0104] The heavy rare earth compound is preferably in the form of particles, and the average particle size is preferably 100 nm to 50 μm, and more preferably 1 μm to 20 μm.

[0105] The solvent used in the coating material is preferably one that can uniformly disperse the heavy rare earth compound without dissolving it, such as alcohol, aldehyde, or ketone, with ethanol being particularly preferred.

[0106] There are no particular restrictions on the content of the heavy rare earth compound in the paint. For example, it may be 50% by weight to 90% by weight. The paint may further contain components other than the heavy rare earth compound as needed. For example, the paint may contain a dispersant to prevent aggregation of the heavy rare earth compound particles, a powder of a transition metal or base metal, or a powder mainly composed of a light rare earth element.

[0107] In the diffusion process of this embodiment, there is no particular limit to the number of surfaces of the RTB permanent magnet substrate to which the paint containing a heavy rare earth compound is applied. For example, the paint may be applied to all surfaces, or to only two surfaces: the largest surface and the surface opposite the largest surface. Furthermore, as necessary, masking may be performed on surfaces other than the surface to which the paint containing a heavy rare earth element is applied. Furthermore, it is preferable that the surface to which the paint containing a heavy rare earth element is applied is the magnetic pole surface.

[0108] The amount of Tb deposited can be, for example, 0.2 to 3.0% by weight, with the entire RTB permanent magnet being 100% by weight. The heat treatment temperature during diffusion can be 800 to 950°C. The heat treatment time during diffusion is preferably 1 to 30 hours. The atmosphere during the diffusion step can be any, but an Ar atmosphere is preferred.

[0109] [Aging treatment process] After the diffusion step, the RTB permanent magnet may be subjected to an aging treatment. After the diffusion step, the resulting RTB permanent magnet is subjected to aging treatment by, for example, holding it at a temperature lower than that used during diffusion. The aging treatment is carried out, for example, at a temperature of 450°C to 700°C for 0.5 to 4 hours, but this time is adjusted appropriately depending on the number of aging treatments performed. The aging treatment can improve the magnetic properties of the RTB permanent magnet. The aging treatment can be carried out in any atmosphere, but an Ar atmosphere is preferred.

[0110] [Cooling process] After the RTB permanent magnet is subjected to aging treatment, it is cooled in an Ar gas atmosphere. This results in the RTB permanent magnet according to this embodiment. The cooling rate is arbitrary, but is, for example, 30°C / min to 300°C / min.

[0111] [Surface treatment process] The RTB permanent magnet obtained by the above steps may be subjected to surface treatment such as plating, resin coating, oxidation treatment, chemical conversion treatment, etc. depending on the application and desired properties. The surface treatment step may also be omitted.

[0112] The RTB permanent magnet according to this embodiment is magnetized in accordance with a conventional method to obtain a magnet product.

[0113] According to the method of this embodiment, it is possible to obtain spherical powder with a particle size equivalent to that of the raw material powder that was added in the spheroidizing step. Furthermore, this manufacturing method makes it easy to obtain powder in which the proportion of single-crystal rare earth magnet powder is 65% or more, thereby improving the magnetic properties of the magnet, such as the coercive force (Hcj).

[0114] Furthermore, for example, by introducing raw material powder into the tail flame of a thermal plasma, heating it, and then rapidly cooling it, the raw material powder particles are melted in whole or in part, making the powder more likely to become spherical. Furthermore, the subphase components are more likely to be uniformly distributed on the powder surface, making it easier to form spherical coated particles. The spherical shape of the powder makes it easier to improve bulk density and increase the degree of orientation.

[0115] In this manufacturing method, the spherical shape of the powder makes it easy to improve bulk density and increase the degree of orientation. This makes it easy to increase the remanence of the magnet after sintering. Furthermore, the subphase components are more likely to be uniformly distributed on the powder surface, making it easier to form spherical coated particles. When the subphase components are uniformly distributed on the powder surface, the subphase is uniformly distributed (thinly) at the grain boundaries between two particles during densification after sintering, making it difficult for the remanence to decrease. Furthermore, this manufacturing method makes it easy to obtain powder in which the number ratio of single-crystal rare earth magnet powder is 65% or more, thereby improving the magnetic properties of the magnet, such as the coercivity.

[0116] The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the present invention.

[0117] For example, the rare earth magnet powder of the present invention may be further subjected to a microstructural refinement process using the HDDR method. By further subjecting the rare earth magnet powder of the present invention to the HDDR method, the crystallite size can be reduced while largely maintaining the particle shape, particle size, and magnetic anisotropy, resulting in an RTB permanent magnet with high coercivity.

[0118] Furthermore, the RTB permanent magnets of the above-described embodiments can be modified and combined in various ways, and the same can be applied to other rare earth magnets. For example, the RTB permanent magnets are not limited to RTB sintered magnets manufactured by sintering as described above. They may also be RTB permanent magnets manufactured by hot compacting and hot working instead of sintering.

[0119] If a cold-molded body obtained by molding raw material powder at room temperature is subjected to hot molding, in which the body is heated and pressurized, the pores remaining in the cold-molded body disappear, and the body can be densified without sintering. Furthermore, if the molded body obtained by hot molding is subjected to hot extrusion as hot processing, an RTB-based permanent magnet having the desired shape and magnetic anisotropy can be obtained.

[0120] The RTB permanent magnet according to this embodiment can be used in any application, including motors for electric vehicles and wind power generation, for example.

[0121] Furthermore, the rare earth magnet powder of the present invention can be used for purposes other than molding magnets, such as magnetic refrigeration, magnetic fluids, magnetic sheets, and magnetic recording. [Example]

[0122] The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples.

[0123] Example 1 First, the raw alloy was cast by strip casting (SC) to obtain RTB-based magnet powder with a composition of Nd: 30.5, Al: 0.23, Co: 0.5, Cu: 0.06, Zr: 0.15, B: 1.01, and Fe: balance (unit: wt%).

[0124] Next, the raw alloy was allowed to absorb hydrogen at room temperature, and then dehydrogenated at 600°C for 1 hour, followed by hydrogen pulverization (coarse pulverization) to obtain coarsely pulverized powder. Note that each step from the hydrogen pulverization to sintering (fine pulverization and molding) was carried out in an atmosphere with an oxygen concentration of less than 50 ppm.

[0125] Next, 0.2 wt% of oleic acid amide was added as a grinding aid to the coarsely ground powder of the raw alloy, and the mixture was mixed using a Nauta mixer. After that, the mixture was finely ground using a jet mill with high-pressure N2 gas to obtain a finely ground powder with an average particle size of 4.0 μm (fine grinding step).

[0126] Next, the finely pulverized powder obtained in this manner was spheroidized as follows (spheroidization step). Specifically, the finely pulverized powder was passed through the raw material powder supply section 14 of the apparatus 10 shown in FIG. 4 and sprayed toward the tail flame 12a of the thermal plasma 12. A modified version of the TP-40020NPS apparatus manufactured by JEOL Ltd. was used as the apparatus 10. The powder was supplied to the apparatus 10 using a TP-99010FDR apparatus manufactured by JEOL Ltd. as the powder feeder, and the powder was fed at various powder introduction rates into the generated thermal plasma.

[0127] A high-frequency voltage of approximately 4 MHz and approximately 6 kV was applied to the high-frequency generator coil for generating the thermal plasma 12, and a mixed gas of 100 liters / minute of argon and 10 liters / minute of hydrogen was used as the plasma gas ejected from the gas ejection port 16. At this time, the atmosphere of the thermal plasma 12 formed in the plasma generation chamber 13 was a reduced-pressure atmosphere of approximately 375 Torr.

[0128] The fine raw material powder was carried by argon as a carrier gas at 5 liters / minute and introduced into the tail flame portion 12a of the thermal plasma 12. The introduction rate was 15.0 g / minute. The temperature of the tail flame portion 12a was approximately 2000 to 5000 K. The quenching gas 15 in the upper interior of the chamber 11 was a reducing gas consisting of argon containing hydrogen.

[0129] The average particle size and sphericity P1 and P2 of the spheroidized powder (magnet powder) obtained in the powder recovery section 17 were determined using the methods described above. The results are shown in Table 1A. Furthermore, the percentage by number of main phase particles composed of single crystals contained in the magnet powder (single crystal magnet powder content) was determined using the methods described above. The results are shown in Table 1A. Furthermore, the percentage by number of magnet powder particles having a sphericity P1 of 0.9 or more and the percentage by number of magnet powder particles having a sphericity P2 of 0.8 or more were determined using the methods described above. The results are shown in Table 1B.

[0130] Whether or not a particle is single crystal was determined using an EBSD analyzer. EBSD analyzers can image a specified crystal plane in each particle. That is, if a single crystal plane is observed in each particle in the image obtained with the EBSD analyzer, the particle is presumed to be single crystal, and if multiple crystal planes are observed in each particle, the particle can be determined to be polycrystalline.

[0131] Furthermore, the bulk density of the magnet powder was determined by the following method. Specifically, the bulk density was measured using a powder property measuring instrument (manufactured by Hosokawa Micron) to measure the packed bulk density (g / cm 3 ) and loose bulk density (g / cm 3 The sieve used had a mesh size of 710 μm, and the metal funnel had an inner diameter of 0.8 cm. Vibration was set to 2.0 (power supply: AC 100 V, 50 Hz). The results are shown in Table 1B.

[0132] A micrograph of the magnet powder obtained in Example 1 is shown in Figure 1A. Furthermore, the average coverage of the coated particles contained in the magnet powder was determined using the method described above. The results are shown in Table 1B. The presence or absence of magnet powder with a coverage of 100% was also determined using the method described above. The results are shown in Table 1B.

[0133] Comparative Example 1 Magnet powder was obtained in the same manner as in Example 1, except that the spheroidizing step was not performed. That is, the finely pulverized powder immediately after the fine pulverization step using a jet mill with high-pressure N2 gas was used as the magnet powder. The average particle size, sphericity P1 and P2, and single crystal magnet powder content were determined in the same manner as in Example 1. The results are shown in Table 1A.

[0134] The percentage by number of magnetic powder particles having a sphericity P1 of 0.9 or more and the percentage by number of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 1. The results are shown in Table 1B. A micrograph of the magnetic powder obtained in Comparative Example 1 is shown in Figure 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0135] Example 2 Magnet powder was obtained in the same manner as in Example 1, except that the conditions for the pulverization step performed before the spheroidization step were changed to obtain pulverized powder with an average particle size of 10.0 μm. That is, the pulverized powder after pulverization with high-pressure N2 gas using a jet mill (pulverization step) was introduced into the tail flame portion 12a of the thermal plasma 12 in the same manner as in Example 1, and the obtained spheroidized powder was used as magnet powder. The average particle size, sphericity P1 and P2, and single-crystal magnet powder content were determined in the same manner as in Example 1. The results are shown in Table 1A.

[0136] The percentage by number of magnetic powder particles having a sphericity P1 of 0.9 or more and the percentage by number of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0137] Comparative Example 2 Magnet powder was obtained in the same manner as in Example 2, except that the spheroidizing step was not performed. That is, the finely pulverized powder immediately after the fine pulverization step using a jet mill with high-pressure N2 gas was used as the magnet powder. The average particle size, sphericity P1 and P2, and single crystal magnet powder content were determined in the same manner as in Example 2. The results are shown in Table 1A.

[0138] The number percentage of magnetic powder particles having a sphericity P1 of 0.9 or more and the number percentage of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 2. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0139] Comparative Example 3 Magnet powder was obtained in the same manner as in Example 1, except that in the spheroidizing step, the pulverized powder after the pulverizing step was fed into the upper part 12b of the thermal plasma 12 (see Figure 4). The temperature at the center of the thermal plasma 12 was 10,000 K or higher. The average particle size, sphericity P1 and P2, and single crystal magnet powder content were determined in the same manner as in Example 1. The results are shown in Table 1A.

[0140] The percentage by number of magnetic powder particles having a sphericity P1 of 0.9 or more and the percentage by number of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0141] Comparative Example 4 Magnet powder was obtained in the same manner as in Example 2, except that in the spheroidizing step, the pulverized powder after the pulverizing step was fed into the upper part 12b of the thermal plasma 12 (see Figure 4). The temperature at the center of the thermal plasma 12 was 10,000 K or higher. The average particle size, sphericity P1 and P2, and single crystal magnet powder content were determined in the same manner as in Example 2. The results are shown in Table 1A.

[0142] The number percentage of magnetic powder particles having a sphericity P1 of 0.9 or more and the number percentage of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 2. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 2. The results are shown in Table 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0143] Comparative Example 5 Magnet powder having the same composition as in Example 1 was formed by gas atomization without using the strip casting (SC) method, and magnet powder was obtained in the same manner as in Example 1, except that spheroidization treatment using thermal plasma was not performed. The average particle size, sphericity P1 and P2, and single crystal magnet powder content were determined in the same manner as in Example 1. The results are shown in Table 1A.

[0144] The percentage by number of magnetic powder particles having a sphericity P1 of 0.9 or more and the percentage by number of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0145] Examples 3 to 5 Magnet powders were obtained in the same manner as in Example 1, except that in the spheroidizing step, the atmospheric pressure of the thermal plasma formed in plasma generation chamber 13 was changed to 675 Torr, 300 Torr, and 75 Torr, respectively. The average particle size, sphericity P1 and P2, and single crystal magnet powder content were determined in the same manner as in Example 1. The results are shown in Table 1A.

[0146] The percentage by number of magnetic powder particles having a sphericity P1 of 0.9 or more and the percentage by number of magnetic powder particles having a sphericity P2 of 0.8 or more were determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the bulk density of the magnetic powder was determined in the same manner as in Example 1. The results are shown in Table 1B. Furthermore, the average coverage of the coated particles contained in the magnetic powder and the presence or absence of magnetic powder with a coverage of 100% are shown in Table 1B.

[0147] Rating 1 As shown in Tables 1A and 1B, compared to Comparative Examples 1 to 4, in Examples 1 to 5, both sphericity P1 and P2 were improved, bulk density was improved, and the number proportion of magnet powder particles with a sphericity P1 of 0.9 or more was also improved. It was also confirmed that the number proportion of magnet powder particles with a sphericity P2 of 0.8 or more was also improved. It was also found that the average particle size of the magnet powder could be controlled within a preferred range. Furthermore, it was confirmed that the particle size and particle size distribution of the spheroidized raw material powder (magnet powder), as well as the single crystal magnet powder content and average coverage, could be controlled by controlling the particle size of the fine raw material powder, the amount of fine raw material powder sprayed (input amount) per unit time into the thermal plasma, the flow rate of the carrier gas, the internal pressure of the plasma generation chamber, etc.

[0148] Furthermore, as shown in Tables 1A and 1B, the average coverage of each particle constituting the magnetic powder (average coverage) was improved in Examples 1 to 5 compared to Comparative Examples 1 to 5, and magnetic powders with a coverage of 100% were confirmed. It has been confirmed that the improved coverage improves the magnetic properties of the magnet, as will be described later.

[0149] The reason why the average particle size was small in Comparative Example 3 is thought to be as follows: When small particles are introduced from the upper part 12b of the thermal plasma 12, they pass through the plasma at tens of thousands of degrees, and as a result, excessive heat is transferred to the particles, causing them to completely evaporate. When these particles resolidify, nanopowder is generated. Furthermore, since small particles tend to aggregate, the generated nanopowder aggregates and the particle shape becomes distorted. In addition, it is thought that many irregularly shaped particles are generated during resolidification, resulting in poor sphericity.

[0150] Furthermore, in Comparative Example 4, when large particles are introduced from the upper portion 12b of the thermal plasma 12, they are heated by passing through the plasma at tens of thousands of degrees and become liquid. The liquid particles then coalesce to form enlarged particles. As a result, the average particle size of the powder after the spheroidization process is thought to be as large as 25.6 μm. Furthermore, it takes longer for the enlarged liquid particles to resolidify than for particles with smaller diameters, and during this time, the particles aggregate, presumably resulting in particles with poor sphericity.

[0151] Furthermore, in Comparative Example 4, the particles aggregate during resolidification, and the internal structure of the particles constituting the powder tends to be particles consisting of two or more crystallites (for example, individual particles 31 shown in FIG. 5B), unlike in Examples 1 to 5. In Examples 1 to 5, the particles constituting the powder are considered to be single crystals (for example, individual particles 31 shown in FIG. 5A), and are preferably single-crystal main phase particles 31a with an average particle size of 1 to 10 μm (more preferably 3 to 10 μm).

[0152] Furthermore, in Comparative Example 5, the sphericity is slightly inferior to that of Examples 1 to 5, but the single crystal magnet powder content is extremely lower than that of Examples 1 to 5. The average particle size of the magnet powder in Comparative Example 5 is also larger than that of Examples 1 to 5. Furthermore, the average coverage of the magnet powder in Comparative Example 5 is lower than that of Examples 1 to 5, and no magnet powder with a coverage of 100% was observed. Therefore, as will be described later, when a permanent magnet is made using the magnet powder in Comparative Example 5, the magnetic properties are inferior to those of Examples 1 to 5.

[0153] Examples 11 to 14, Comparative Examples 11 and 12 The magnet powders of Examples 1 and 3 to 5 and Comparative Examples 1 and 5 were used to carry out the orientation molding process and sintering process of the above-described embodiment to produce permanent magnet samples consisting of sintered bodies. The area ratio of the subphases was determined for each of the resulting magnet samples. The results are shown in Table 2. The remanence Br, coercivity Hcj, and degree of orientation Br / Js were also determined. The results are shown in Table 2. These magnetic properties were measured using a BH tracer and an X-ray diffractometer (XRD). An FE-SEM micrograph of a cross section of the sintered magnet sample obtained in Example 11 is shown in FIG. 6A, and an FE-SEM micrograph of a cross section of the sintered magnet sample obtained in Comparative Example 11 is shown in FIG. 6B.

[0154] Rating 2 Compared to Comparative Examples 11 and 12, it was confirmed that in Examples 11 to 14, the proportion (area ratio) of subphases decreased, and the remanence Br, coercivity Hcj, and orientation Br / Js improved. It is believed that the use of highly spherical magnet powder improved the packing density and orientation, resulting in the improved remanence Br. Furthermore, the use of single-crystal main-phase particles coated with a coating layer made of an R-rich phase with high magnetic anisotropy makes magnetization reversal less likely to occur, which is believed to have improved the coercivity Hcj. Note that the coating layer on the main-phase particles in the examples is very thin and uniform, so it does not contribute to an increase in the area ratio of the subphase in the cross section of the sintered body after sintering. In the examples, the area ratio of the subphase is preferably 2% or less, and more preferably 1% or less.

[0155] Furthermore, in alloys produced by strip casting, such as those in Comparative Example 1 (Comparative Example 11), dendrites consisting of R-rich phases can occur, causing compositional imbalances within the particles. When these particles are sintered and densified, the R-rich phases that are unevenly distributed within the particles affect the formation of many subphases (three-grain dots) in the sintered magnet structure, as shown in Figure 6B.

[0156] In contrast, for magnetic powder that has been plasma-treated, such as in Example 1 (Example 11), the magnetic powder is introduced into high-temperature plasma, which causes the magnetic powder to melt, eliminating structural irregularities and making it possible to obtain magnetic powder (spherical powder) with a uniform structure. As a result, after sintering, the occurrence of a subphase (three grain dots) made of an R-rich phase is suppressed, as shown in Figure 6A, and it is thought that it is possible to obtain a sintered magnet with a high main phase ratio.

[0157] Examples 21 and 22 and Comparative Examples 21 and 22 The magnetic powders obtained in Examples 4 and 5 and Comparative Examples 1 and 5 shown in Table 3 were first kneaded into polyphenylene sulfide resin to prepare compounds for bonded magnets. Using the compounds kneaded with these magnetic powders, samples of each bonded magnet were prepared. The residual magnetic flux density Br and coercive force Hcj of each of the obtained bonded magnet samples were determined in the same manner as in Examples 11 to 14. The results are shown in Table 3.

[0158] Rating 3 It was confirmed that the residual magnetic flux density Br and coercive force Hcj were improved in Examples 21 and 22 compared to Comparative Examples 21 and 22. It is believed that the use of highly spherical magnet powder improved the filling rate, resulting in the improved residual magnetic flux density Br. It is also believed that the use of main phase particles made of single crystals coated with a coating layer made of an R-rich phase with high magnetic anisotropy made it difficult for magnetization reversal to occur, thereby improving the coercive force Hcj.

[0159] [Table 1A]

[0160] [Table 1B]

[0161] [Table 2]

[0162] [Table 3] [Explanation of symbols]

[0163] 2. RTB magnet powder (rare earth magnet powder) particles 10... device 11...Chamba 12... Thermal plasma 12a… Tail flame part 12b... Upper 13... Plasma generation chamber 14... Raw material powder supply section 15...Quenching gas 16... Gas outlet 17... Powder recovery section 20...Perfect circle 30, 30a... Coated particles 31… Individual particles 31a… Main phase particles 32…Covering layer 33… Grain boundary phase

Claims

1. A rare earth magnet powder having an average sphericity P1, defined by the formula (1) of P1=Ls / Ll, of 0.65 or more, In the formula (1), Ll is the length of the long side of the rectangle that circumscribes the rare earth magnet powder in the microscope image and has the smallest area, In the formula (1), Ls is the length of the short side of the rectangle that circumscribes the rare earth magnet powder in the microscope image and has the smallest area, The proportion of the rare earth magnet powder consisting of single crystals is 65% or more and 89% or less, the rare earth magnet powder includes a plurality of coated particles, each of which is an individual particle made of a single main phase particle and at least a portion of the periphery of which is covered with a coating layer; The rare earth magnet powder has an average coverage rate, which indicates the percentage of the periphery of each individual particle that is covered with the coating layer, of 50% or more.

2. 2. The rare earth magnetic powder according to claim 1, wherein the number ratio of rare earth magnetic powder particles having a sphericity P1 of 0.9 or more is 30% or more.

3. A rare earth magnet powder having an average sphericity P2 of 0.70 or more, as defined by the formula (2) P2 = Lt / Lr, In the formula (2), Lr is the perimeter of the rare earth magnet powder in a microscope image, In the formula (2), Lt is the circumferential length of a perfect circle having the same area as the area in the image of the rare earth magnet powder from which the circumferential length Lr was calculated, The proportion of the rare earth magnet powder consisting of single crystals is 65% or more and 89% or less, the rare earth magnet powder includes a plurality of coated particles, each of which is an individual particle made of a single main phase particle and at least a portion of the periphery of which is covered with a coating layer; The rare earth magnet powder has an average coverage rate, which indicates the percentage of the periphery of each individual particle that is covered with the coating layer, of 50% or more.

4. 4. The rare earth magnetic powder according to claim 3, wherein the number ratio of rare earth magnetic powder particles having a sphericity P2 of 0.8 or more is 25% or more.

5. The rare earth magnet powder according to any one of claims 1 to 4, A rare earth magnet powder having an average particle size of 20 μm or less.

6. A rare earth magnet powder according to claim 1 or 3, The coated particles are rare earth magnet powders including fully coated particles in which the individual particles are entirely covered with the coating layer.

7. A rare earth magnet powder according to any one of claims 1 to 6, The rare earth magnetic powder includes RTB magnetic powder.

8. A bonded magnet containing the rare earth magnetic powder described in any one of claims 1 to 7.

9. A compound for bonded magnets containing the rare earth magnetic powder described in any one of claims 1 to 7.

10. A rare earth sintered magnet obtained by sintering the rare earth magnet powder according to any one of claims 1 to 7, a main phase and a subphase are observed in a cross section of the rare earth sintered magnet, and the area ratio of the subphase is 2% or less; The main phase is R 2 T 14 A rare earth sintered magnet that is a B-type crystal and has a degree of orientation of 94% or more, calculated by dividing the residual magnetic flux density in the orientation direction by the saturation magnetic flux density.

11. A step of pulverizing an alloy having a desired composition to obtain a raw material powder; 8. The method for producing rare earth magnet powder according to claim 1, further comprising the steps of: introducing the raw material powder into a thermal plasma tail flame, heating the raw material powder, and then quenching the raw material powder to obtain spherical powder.

12. A step of pulverizing an alloy having a desired composition to obtain a raw material powder; a step of introducing the raw material powder into a thermal plasma tail flame, heating it, and then quenching it to obtain spherical powder, which is the rare earth magnet powder according to any one of claims 1 to 7; and sintering the spherical powder to obtain a sintered body.

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