Sm-Fe-N-BASED SINTERED MAGNET AND METHOD FOR MANUFACTURING SAME

Abstract

Disclosed is an Sm-Fe-N-based sintered magnet that comprises crystal grains which contain samarium, iron, and nitrogen and which include main phase grains that contain not less than 9 atom% but less than 13 atom% of samarium and sub-phase grains that contain 13 atom% or more of samarium, iron, and nitrogen. The ratio (M2 / M1) of the average value M2 of the X-ray magnetic circular dichroism intensities of the iron of the sub-phase grains to the average value M1 of the X-ray magnetic circular dichroism intensities of the iron of the main phase grains, which is obtained in a plane that is perpendicular to the easy axis of magnetization in an environment in which a static magnetic field of 4T is applied in a direction that is parallel to the direction of the easy axis of magnetization is 10% to 83% inclusive.

Description

Sm-Fe-N sintered magnet and manufacturing method thereof

[0001] The present invention relates to an Sm—Fe—N sintered magnet and a method for producing the same.

[0002] Sm—Fe—N sintered magnets are representative of rare earth-transition metal-nitrogen magnets, and have a high anisotropy field and remanence. Furthermore, Sm—Fe—N sintered magnets have a relatively higher Curie temperature than other rare earth-transition metal-nitrogen magnets, giving them excellent heat resistance. However, it is said that the coercivity of Sm—Fe—N sintered magnets decreases during sintering.

[0003] In Patent Document 1, SmFe is used as a subphase. ３ N ｘ Patent Document 1 discloses an Sm—Fe—N rare earth magnet containing a subphase (1.0≦x≦2.5) in an area ratio of 5% or less (excluding 0) of the cross-sectional area. ３ N ｘ Since the phase is oxidized, the main phase Sm ２ Fe １７ N ３ Oxidation of the phase can be suppressed, and deterioration of magnetic properties can be suppressed.

[0004] Patent No. 6520337

[0005] However, the coercive force of the magnet in Patent Document 1 is not sufficient.

[0006] The present invention has been made in view of the above problems, and has as its object to provide a Sm—Fe—N based sintered magnet having high coercive force.

[0007] According to one aspect of the present invention, there is provided an Sm—Fe—N sintered magnet comprising: main-phase grains that are crystal grains containing samarium, iron, and nitrogen, the main-phase grains containing 9 atomic % or more but less than 13 atomic % samarium; and sub-phase grains that contain 13 atomic % or more samarium, iron, and nitrogen; wherein the ratio (M2 / M1) of the average X-ray magnetic circular dichroism intensity M2 of the sub-phase grains to the average X-ray magnetic circular dichroism intensity M1 of the iron of the main-phase grains, obtained in a plane perpendicular to the easy axis of magnetization under an environment in which a static magnetic field of 4 T is applied in a direction parallel to the easy axis of magnetization, is 10% or more and 83% or less.

[0008] According to another aspect of the present invention, there is provided a method for producing a Sm—Fe—N sintered magnet, comprising: pressure-sintering a magnetic material containing Sm—Fe—N magnetic powder in a sintering machine in a reduced-pressure atmosphere containing an inert gas at a pressure of 10 Pa or more and 200 Pa or less.

[0009] According to the present invention, a Sm-Fe-N sintered magnet having high coercive force is provided.

[0010] 2A is a flowchart showing an example of a method for producing a sintered magnet according to the present disclosure; FIG. 2B is an example of an SEM image (magnification 2000x) used for segmentation in the examples; FIG. 2C is an example of an SEM image obtained by binarizing FIG. 2A and showing main phase grains in black; and FIG. 2D is an example of an SEM image obtained by binarizing FIG. 2A and showing grain boundaries between main phase grains and subphase grains in black.

[0011] The Sm—Fe—N sintered magnet (samarium-iron-nitrogen sintered magnet) according to the present disclosure includes crystal grains containing samarium, iron, and nitrogen (Sm—Fe—N crystal grains). The Sm—Fe—N crystal grains are a sintered body of a magnetic material containing Sm—Fe—N magnetic powder.

[0012] The Sm—Fe—N crystal grains include main phase grains containing 9 atomic % or more and less than 13 atomic % samarium, and the Sm—Fe—N sintered magnet according to the present disclosure further includes subphase grains containing 13 atomic % or more samarium, iron, and nitrogen.

[0013] The main phase grains contain 9 atomic % or more and less than 13 atomic % of samarium, iron, and nitrogen. The main phase grains have an Sm—Fe—N crystalline structure. The main phase grains form the main phase of an Sm—Fe—N sintered magnet (hereinafter also simply referred to as a magnet). The main phase grains are the target substance when synthesizing the Sm—Fe—N crystalline grains.

[0014] The subphase grains contain 13 atomic % or more of samarium, iron, and nitrogen. The subphase grains may contain, on an atomic % basis, equal to or greater than the nitrogen content of the main phase grains. The subphase grains (hereinafter also referred to as Sm-rich phase) may be amorphous. The Sm-rich phase is a by-product that is inevitably produced when synthesizing Sm—Fe—N crystal grains in order to suppress the precipitation of the α-Fe phase, which significantly deteriorates magnetic properties. It has been discovered that if the average magnetic moment of iron in this unavoidably produced Sm-rich phase is large, the coercivity of the magnet decreases.

[0015] The reason for this is not clear, but it is thought that if the average magnetic moment of iron in the Sm-rich phase is large, magnetization reversal of the main phase grains present around the Sm-rich phase is more easily induced.

[0016] The average magnetic moment m2 of iron in the Sm-rich phase changes depending on, for example, the degree of oxidation of the Sm-rich phase. When the Sm-rich phase is oxidized, oxygen (O) enters while nitrogen (N) is released. It is thought that the release of nitrogen reduces the distance between iron elements, thereby increasing the average magnetic moment m2 of iron in the Sm-rich phase.

[0017] Because the main phase grains are less susceptible to oxidation than the Sm-rich phase, it is believed that the average magnetic moment m1 of iron in the main phase grains does not change significantly depending on firing conditions, etc. In the present disclosure, the ratio of the average magnetic moment m2 of iron in the Sm-rich phase (corresponding to the average XMCD intensity M2 of iron in the Sm-rich phase described later) to the average magnetic moment m1 of iron in the main phase grains (corresponding to the average XMCD intensity M1 of iron in the main phase grains described later) is used as an index that influences the coercive force.

[0018] The average magnetic moment m1 of iron in the main phase grains can be obtained as the average value M1 of the X-ray magnetic circular dichroism (XMCD) intensity of iron in the main phase grains. The XMCD intensity is obtained in a plane perpendicular to the easy axis of magnetization in an environment where a static magnetic field of 4 T is applied in a direction parallel to the easy axis of magnetization. Similarly, the average magnetic moment m2 of iron in the Sm-rich phase can be obtained as the average value M2 of the XMCD intensity of iron in the subphase grains.

[0019] When the ratio (M2 / M1) of the average XMCD intensities is 10.0% or more and 83% or less, a high coercivity can be obtained. The ratio (M2 / M1) may be 82.5% or less, 82.0% or less, or 81.8% or less. The ratio (M2 / M1) may be 20.0% or more, or 30.0% or more. The ratio (M2 / M1) can be reduced, for example, by suppressing oxidation during pressure sintering of the magnetic material. Pressure sintering conditions will be described later.

[0020] The magnet may contain no oxygen or may contain 0.5% by mass or less of oxygen. A magnet with an oxygen content of 0.5% by mass or less indicates that oxidation of both the main phase grains and the subphase grains, particularly the Sm-rich phase, is suppressed during the sintering process. This tends to result in a smaller ratio (M2 / M1), which can lead to the expectation of a higher coercive force. The magnet's oxygen content may be 0.48% by mass or less, or 0.47% by mass or less.

[0021] The oxygen content can be measured by inert gas fusion non-dispersive infrared absorption (NDIR) for magnets stored immediately after pressure sintering or after pressure sintering in an atmosphere with a low oxygen concentration of 2 ppm or less on a volume basis.

[0022] The relative density (%) of the magnet may be, for example, 78% or more, or 79% or more. The relative density is determined by the ratio of the Sm content, which is a typical component of the main phase grains, to the total Sm content. ２ Fe １７ N ３ The known true density of the phase (7.67 g / cm ３ The relative density of a magnet is the ratio of its volume density to its mass. The volume density of a magnet can be calculated from the magnet's apparent volume and mass. Generally, the higher the relative density, the higher the magnetization. The relative density (%) of a magnet is 100% or less.

[0023] Sm—Fe—N sintered magnets are obtained by sintering a magnetic material containing Sm—Fe—N crystal grains at high temperature under pressure, and can consist essentially of a sintered body of main phase grains and subphase grains.

[0024] <Identification of Main Phase Grains and Subphase Grains> Identification of main phase grains and subphase grains can be performed as follows. First, the element distribution in the cross section of the magnet is obtained by energy dispersive X-ray (EDX) analysis. The element distribution is typically measured using a SEM-EDX analyzer.

[0025] The EDX analysis is carried out, for example, under the following conditions.

[0026]

[0027] An image using a SEM (scanning electron microscope) for EDX analysis can be obtained, for example, under the following conditions.

[0028]

[0029] Next, based on the element distribution obtained by SEM-EDX analysis, the region where the atomic percentage of samarium is 13 atomic % or more (corresponding to subphase grains (Sm-rich phase)) is separated (segmented) into a region where the atomic percentage of samarium is 9 atomic % or more but less than 13 atomic % (corresponding to main phase grains). By this segmentation, the grain boundaries between the main phase grains and the subphase grains are determined, and the main phase grains and the subphase grains are each identified.

[0030] Segmentation can be performed by processing the SEM image using deep learning image processing software (for example, "MIPAR" manufactured by MIPAR Software LLC). The specific procedure of this method is as follows.

[0031]

[0032] The segmentation may be performed by dividing the SEM image by contrast. The specific steps of this method are as follows:

[0033]

[0034] <Obtaining the average value of iron XMCD intensity> Based on the measurement results of XMCD imaging in an applied magnetic field of 4 T, the XMCD intensity and its distribution can be obtained as a signal proportional to the magnetic moment of iron. The number average of the XMCD intensity is taken as the average value of the XMCD intensity.

[0035] XMCD imaging measurements can be performed using a scanning X-ray microscope installed in BL25SU of the large synchrotron radiation facility SPring-8 (registered trademark), or, from April 2024 onwards, a scanning X-ray microscope installed in BL14U of the 3 GeV high-brilliance synchrotron radiation facility (NanoTerasu (registered trademark)) due to the relocation of equipment from SPring-8.

[0036] In the present disclosure, the ratio (M2 / M1) is based on the value measured using SPring-8 (registered trademark). When XMCD imaging is obtained based on the value measured using NanoTerasu (registered trademark), the ratio (M2 / M1) calculated from this is subtracted by 4.6% to obtain the ratio (M2 / M1) in the present disclosure.

[0037] Measurement and analysis of XMCD imaging are performed as follows: First, a cross section perpendicular to the easy axis of magnetization of a sample (sintered magnet) is exposed as the sample surface. The sample surface can be exposed by precision polishing in a water-free environment or by vertical Ar ion milling.

[0038] Right-handed circularly polarized and left-handed circularly polarized X-rays generated by a twin helical undulator light source are monochromatized to a predetermined absorption edge energy. Under an environment in which a 4T static magnetic field is applied parallel to the normal to the sample surface, these high-energy X-rays are focused onto the sample surface using a zone plate, while the sample is sequentially scanned in the in-plane direction. The X-ray absorption intensity obtained at each scan step using right-handed circularly polarized X-rays is designated R. The X-ray absorption intensity obtained at each scan step using left-handed circularly polarized X-rays is designated L. The ratio of the difference (R - L) between R and L to the sum (R + L) of R and L is calculated as the XMCD intensity at each scan step, and the in-plane distribution of these intensities is output as an XMCD image.

[0039] XMCD imaging is performed under the following conditions, for example: Measurement absorption edge: Fe L ３ Edge (energy value is determined so that the XMCD intensity is maximized) Detection method: Total electron yield method (measurement of sample current) X-ray beam size: 0.1 μm or less Scanning range: Rectangle with one side of 40 to 60 μm at any position on the sample surface Scanning step: 0.1 μm

[0040] <Obtaining average values ​​M1 and M2 of iron XMCD intensity in main phase grains and subphase grains> The above segmentation information is applied to the XMCD image to segment the XMCD image into main phase grains and subphase grains. The XMCD intensity of each particle can be determined from the XMCD image. The number average of the XMCD intensity of all main phase grains is taken as the average XMCD intensity of the main phase grains, M1. The number average of the XMCD intensity of all subphase grains is taken as the average XMCD intensity of the subphase grains, M2. This series of data processing can be performed using image processing software (for example, the above-mentioned MIPAR).

[0041] <Main Phase Grains> The main phase grains form the main phase of the magnet. The main phase grains are Sm—Fe—N crystal grains containing 9 atomic % or more and less than 13 atomic % samarium, iron, and nitrogen. The samarium content can be determined by EDX analysis.

[0042] The main phase grains are at least partially composed of Th ２ Zn １７ Type or Th ２ Ni １７ The main phase grains have a crystal structure of the type SmFe ９ N １．５ Structure, Sm ２ Fe １７ N ３ The crystal structure of the main phase grains is not limited to this, and may be any crystal structure composed of Sm, Fe, and N. A typical crystal structure of the main phase grains is Sm ２ Fe １７ N ３ It is a structure.

[0043] The average grain size of the main phase grains may be, for example, 0.5 μm or more and 3.0 μm or less. By having an average grain size of the main phase grains of 0.5 μm or more, oxidation and superparamagnetism of Sm—Fe—N crystal grains can be effectively suppressed. By having an average grain size of the main phase grains of 3.0 μm or less, many of the main phase grains can become single-domain grains (small regions in which the grain magnetic moment is aligned in one direction). By reducing the number of multi-domain grains with low coercivity, the coercivity of the magnet can be further improved. The average grain size of the main phase grains may be 2.0 μm or less, or 1.5 μm or less.

[0044] The method for calculating the average grain size of the main phase grains is as follows. First, a cross section of the magnet is photographed using a field emission scanning electron microscope (FE-SEM) so that a total of at least 50 grains are included. The main phase grains and subphase grains in the photographed image are identified in the same manner as above. Next, the total area A1 of the cross sections of the main phase grains in the photographed image and the number N1 of the main phase grains are determined. The average cross-sectional area per main phase grain (A1 / N1) is calculated. The diameter of a circle (equivalent circle) having the same area as the average cross-sectional area is the average grain size of the main phase grains.

[0045] <Subphase Grains> The subphase grains contain samarium at 13 atomic % or more. The subphase grains may be amorphous and contain samarium, iron, and nitrogen. The amount of nitrogen (atomic %) in the subphase grains may be equal to or greater than the amount of nitrogen (atomic %) in the main phase grains. The amount of samarium in the subphase grains may be 14 atomic % or less.

[0046] Whether the subphase grains are amorphous or not can be confirmed by an electron diffraction image of a TEM (transmission electron microscope) or an electron backscatter diffraction (EBSD) device. If the TEM electron diffraction image shows a halo pattern or if a Kikuchi pattern cannot be obtained by EBSD, the subphase grains are considered to be amorphous. In the case of main phase grains, a spot diffraction pattern can be obtained in the TEM electron diffraction image, and a Kikuchi pattern can be obtained by EBSD.

[0047] The average particle size of the subphase grains is not particularly limited and is, for example, 0.2 μm or more and 5.0 μm or less.

[0048] The average grain size of the subphase grains can also be calculated from the total area A2 of the cross sections of the subphase grains in the photographed image and the number N2 of the subphase grains, in the same way as the average grain size of the main phase grains.

[0049] The subphase grain content in the magnet is, for example, 0.3 volume % or more and 5 volume % or less. A subphase grain content of 0.3 volume % or more is likely to improve coercivity. A subphase grain content of 5 volume % or less suppresses a decrease in remanence.

[0050] The content of the subphase grains may be 0.5% by volume or more, or 0.7% by volume or more, and 4.8% by volume or less, or 4.6% by volume or less.

[0051] The subphase grain content can be considered to be the ratio of the total area of ​​the subphase grains to the cross-sectional area of ​​the magnet. The above area percentage of the subphase grains (%) is obtained by the formula 100×A2 / A, where A2 is the total area of ​​the subphase grain cross sections, obtained in the same manner as when determining the average grain size of the main phase grains and subphase grains, and A is the cross-sectional area of ​​the entire magnet. The area percentage (%) obtained by 100×A2 / A can be directly considered to be the subphase grain content (volume %) in the magnet.

[0052] <Other Containing Components> The magnet contains at least the above-described main phase grains and subphase grains. The magnet may contain other substances, such as α-Fe, and unavoidably mixed trace elements. Examples of unavoidably mixed trace elements include carbon (C), silicon (Si), and aluminum (Al).

[0053] The magnet may contain a sintered body of a magnetic powder other than Sm—Fe—N magnetic powder. Examples of other magnetic powders include magnetic powders composed of a rare earth element other than Sm, Fe, and N, and magnetic powders composed of a rare earth element including Sm, Fe, a transition metal element other than Fe, and N. Examples of rare earth elements other than Sm include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), and ytterbium (Yb). Examples of transition metal elements other than Fe include cobalt (Co), nickel (Ni), manganese (Mn), chromium (Cr), titanium (Ti), Zr (zirconia), niobium (Nb), tungsten (W), and vanadium (V).

[0054] The presence or absence of sintered bodies of other magnetic powders and the region in which they are present can be determined using a method similar to the segmentation described above. The content ratio of the sintered bodies of other magnetic powders can be considered as the ratio of the total area of ​​the sintered bodies of other magnetic powders to the cross-sectional area of ​​the magnet, similar to the area ratio of subphase grains to the cross-sectional area of ​​the magnet. The area ratio (%) of the sintered bodies of other magnetic powders can be obtained from the total area A3 of the sintered bodies of other magnetic powders and the cross-sectional area A of the magnet using the formula 100 × A3 / A.

[0055] 100×A3 / A (the area ratio of the sintered body of the other magnetic powder to the cross-sectional area of ​​the magnet) may be 5% or less, or may be 3% or less.

[0056] The magnet may contain, other than magnetic powder, at least one of a metal or alloy having a melting point below the pressure sintering temperature and an alloy having a eutectic, peritectic, or monotectoid point below the pressure sintering temperature (hereinafter also referred to as a low-melting-point metal). The low-melting-point metal can coat at least a portion of the Sm—Fe—N crystal grains (especially the main phase grains), or can form a magnetically decoupling phase at the grain boundaries between the Sm—Fe—N crystal grains. The low-melting-point metal is expected to further improve coercivity.

[0057] When the pressure firing is carried out at a temperature of 300° C. or higher and 600° C. or lower, at least one of the melting point, eutectic point, peritectic point, and monotectic point of the low-melting-point metal needs to be 600° C. or lower. Examples of metals or alloys having at least one of the melting point, eutectic point, peritectic point, and monotectic point of 600° C. or lower include zinc (Zn), gallium (Ga), germanium (Ge), indium (In), tin (Sn), bismuth (Bi), Zn—Al alloy, Zn—Ga alloy, Zn—Ge alloy, Zn—In alloy, Zn—Sn alloy, and Zn—Bi alloy.

[0058] The presence or absence of low-melting-point metal and the region in which it is present can be determined using a method similar to the segmentation described above. The content ratio of low-melting-point metal can be considered as the ratio of the total area of ​​low-melting-point metal to the cross-sectional area of ​​the magnet, similar to the ratio of the area of ​​subphase grains to the cross-sectional area of ​​the magnet. The above-mentioned area ratio (%) of low-melting-point metal can be obtained from the total area Am of low-melting-point metal and the cross-sectional area A of the magnet using the formula 100 × Am / A.

[0059] 100×Am / A (the area ratio of the low-melting point metal to the cross-sectional area of ​​the magnet) may be 2.0% or more, or 3.0% or more. 100×Am / A may be 15.0% or less, or 10.0% or less.

[0060] (Manufacturing Method) The magnet of the present disclosure can be manufactured by a method comprising pressurizing and sintering a magnetic material containing Sm—Fe—N magnetic powder in a reduced pressure atmosphere containing an inert gas of 10 Pa or more and 200 Pa or less.

[0061] Firing is usually carried out in a vacuum atmosphere (see Patent Document 1) or an inert gas atmosphere. A vacuum atmosphere is created by evacuating the gas from the furnace using a vacuum pump or the like. However, as the gas is evacuated, it is unavoidable that air will flow into the furnace through gaps in the equipment, making it difficult to prevent oxidation of the magnetic material. An inert gas atmosphere is created by filling a sealed furnace with an inert gas. In this case, while the inflow of air from the outside can be suppressed, it is not possible to remove moisture contained in the magnetic powder or moisture generated during heating. As a result, oxidation of the magnetic material due to moisture may occur.

[0062] In the present disclosure, the pressure inside the sintering machine is reduced to 10 Pa or more and 200 Pa or less (within the range of low vacuum to medium vacuum as classified by JIS Z 8261-1) in the presence of an inert gas. Because the inside of the furnace is at a low to medium vacuum level, air is less likely to flow in from the outside. In addition, the presence of an inert gas prevents contact between the magnetic material and the air, even if air does flow in. Furthermore, moisture is also expelled from the furnace during evacuation to maintain the reduced pressure. Therefore, sintering in a reduced-pressure atmosphere containing an inert gas effectively prevents oxidation of the magnetic material.

[0063] When pressure sintering is carried out under the above conditions, the ratio (M2 / M1) of the average XMCD intensity M1 of iron in the main phase grains to the average XMCD intensity M2 of iron in the Sm-rich phase of the resulting magnet is 10% or more and 83% or less.

[0064] The reduced pressure atmosphere containing the inert gas is formed, for example, by discharging the gas inside the sintering machine while introducing the inert gas into the sintering machine. In this case, moisture is more easily removed, and the oxidation suppression effect is further improved.

[0065] From the viewpoint of suppressing oxidation, the pressure firing may be carried out in an atmosphere with a low oxygen concentration, in which the volumetric oxygen concentration is 2.0 ppm or less.

[0066] An example of a method for producing a sintered magnet will be described below: Fig. 1 is a flowchart showing an example of a method for producing a sintered magnet according to the present disclosure.

[0067] (1) Preparation of Sm—Fe—N based magnetic powder (S11) The Sm—Fe—N based magnetic powder may be a commercially available product. The Sm—Fe—N based magnetic powder may be prepared by nitriding an alloy powder containing Sm and Fe (Sm—Fe alloy).

[0068] (2) Pulverization or Crushing, and Classification (S12) The Sm—Fe—N magnetic powder may be pulverized (or crushed) and classified. The pulverization (crushing) and classification are carried out under conditions such that the average particle size of the Sm—Fe—N magnetic powder is 0.5 μm or more and 5 μm or less. Fine powder is removed by classification.

[0069] Crushing or grinding can be carried out using, but is not limited to, an agate mortar, a jet mill (airflow grinding type, etc.), a ball mill, etc. Examples of airflow grinding type jet mills include, but are not limited to, the MC44 manufactured by Micromachinazione. Classification can be carried out using, but is not limited to, an airflow classifier, etc.

[0070] (3) Addition of Low-Melting Point Metal (S13) Prior to pressure sintering, a low-melting point metal powder may be added to the Sm-Fe-N magnetic powder. The amount of low-melting point metal added may be, for example, 1% by mass or more and 20% by mass or less of the Sm-Fe-N magnetic powder. When the amount of low-melting point metal added is 1.0% by mass or more, the effect of the low-melting point metal is easily exerted. When the amount of low-melting point metal added is 20.0% by mass or less, the effect on the remanence magnetization of the resulting magnet is small. The amount of low-melting point metal added may be 2.0% by mass or more, or 3.0% by mass or more. The amount of low-melting point metal added may be 15.0% by mass or less, or 10.0% by mass or less.

[0071] (4) Magnetic Field Compaction (S14) Before pressure sintering, the magnetic material may be compacted in a magnetic field. Magnetic field compaction is a process of compacting a magnetic material while applying a magnetic field. For magnetic field compaction, for example, a powder press equipped with a magnetic field generator is used. By compacting in a magnetic field, the easy magnetization axis of the Sm—Fe—N magnetic powder is aligned, resulting in higher magnetic properties.

[0072] The conditions for compaction in a magnetic field are not particularly limited. The magnetic field to be applied may be, for example, a static magnetic field of 1 T or more, or a pulsed magnetic field. Compaction in a magnetic field may be performed on a magnetic material filled in a mold used for pressure sintering.

[0073] All of the above steps may be carried out in an atmosphere with a low oxygen concentration of 10 ppm or less (particularly 2 ppm or less) on a volume basis. The above steps are carried out, for example, in a glove box purged with an inert gas (one or a mixture of two or more gases such as nitrogen, argon, and helium), preferably in a glove box connected to a gas circulation type oxygen and moisture purifier.

[0074] After the preparation of the Sm-Fe-N magnetic powder and before pressure sintering (or compaction in a magnetic field), for material handling purposes, it is permissible to place the Sm-Fe-N magnetic powder in an air atmosphere while immersed in an organic solvent that can prevent oxidation.

[0075] (5) Pressure Sintering (S15) The material containing the Sm—Fe—N magnetic powder filled into the mold is pressure sintered. This produces a Sm—Fe—N sintered magnet. The mold used may have any shape, including, but not limited to, a cylindrical mold.

[0076] The pressure firing is carried out in the sintering machine by reducing the pressure in the sintering machine to 10 Pa or more and 200 Pa or less in the presence of an inert gas. This suppresses oxidation of the magnetic material during firing, reduces the ratio (M2 / M1) to 83% or less, and produces a magnet with high coercivity. The gas inside the sintering machine may be discharged while introducing an inert gas into the sintering machine, thereby forming a reduced-pressure atmosphere containing the inert gas.

[0077] The pressure inside the sintering machine may be 15 Pa or more, or may be 20 Pa or more. The pressure inside the sintering machine may be 150 Pa or less, or may be 100 Pa or less.

[0078] Examples of inert gases include helium, neon, argon, and nitrogen. These may be used alone or in combination of two or more. The inert gas may be argon.

[0079] Any pressure firing method, including electric pressure firing, can be used for the pressure firing. Pressure firing may be performed, for example, by hot pressing or by electric sintering. The pressure applied to the magnetic material in the mold may be any pressure higher than atmospheric pressure and capable of forming a sintered magnet. The pressure applied to the magnetic material in the mold may be, for example, in the range of 100 MPa to 2000 MPa. The pressure firing time is, for example, from 30 seconds to 10 minutes.

[0080] The pressure firing is performed at a temperature of, for example, 300° C. or higher and 600° C. or lower. The pressure firing temperature may be 350° C. or higher, or 400° C. or higher. The pressure firing temperature may be 580° C. or lower, or 550° C. or lower.

[0081] The present disclosure is not limited to the above-described embodiments, and design modifications are possible within the scope of the present disclosure.

[0082] Hereinafter, the present disclosure will be described in more detail with reference to examples. However, the present disclosure is not limited to the following examples, and it is of course possible to make appropriate modifications within the scope applicable to the above and below-described aims, and all such modifications are included in the technical scope of the present disclosure.

[0083] Example 1 A Sm-Fe-N magnet was produced by the following procedure: (i) Preparation of Sm-Fe-N magnetic powder. ２ Fe １７ N ３ An Sm--Fe--N magnetic powder having an average particle size of about 25 μm was prepared.

[0084] (ii) Pulverization and Classification Using an airflow pulverization type jet mill, the Sm-Fe-N magnetic powder was pulverized at a pulverization pressure of 0.7 MPa.

[0085] The pulverization was carried out in a glove box under a low-oxygen atmosphere of 2 ppm or less. After pulverization, an air classifier was used to remove fine powder (particles with a particle size of less than 0.04 μm). This adjusted the average particle size of the Sm—Fe—N magnetic powder to 1.6 μm.

[0086] (iii) Magnetic Field Molding 0.2 g of the obtained magnetic powder was immersed in heptane in a glove box under a low-oxygen atmosphere of 2 ppm or less to prepare a slurry. The obtained slurry was filled into a cemented carbide mold. The mold was removed from the glove box and placed in a powder press equipped with a magnetic field generator. While applying a static magnetic field of 1.5 T to the powder press, press molding was performed at a pressure of 1250 MPa in the direction perpendicular to the magnetic field.

[0087] (iv) Pressure Sintering Next, the above mold was placed in a pulse current sintering machine equipped with a pressure mechanism using a servo-controlled press. While evacuating the interior of the pulse current sintering machine with a vacuum pump, Ar gas was introduced to create a reduced pressure atmosphere of 100 Pa. While maintaining this reduced pressure atmosphere, a pressure of 1250 MPa was applied to the mold, and current sintering was performed at 350°C for 2 minutes to obtain a sintered magnet.

[0088] [Example 2] A sintered magnet was obtained in the same manner as in Example 1, except that a magnetic material was used in which 100% by mass of Sm-Fe-N magnetic powder was mixed with 9.2% by mass of a Zn-Al alloy (Al content: 10% by mass, melting point: 381°C), and the pressure sintering conditions were changed as shown in Table 1.

[0089] Comparative Examples 1 and 2 Sintered magnets were obtained in the same manner as in Example 1, except that the pressure sintering conditions were changed as shown in Table 5.

[0090] [Evaluation] (Oxygen Content) The oxygen content of the Sm-Fe-N magnetic powder was measured by inert gas fusion-non-dispersive infrared absorption method (NDIR method).

[0091] (Average Grain Size and Area Ratio) The cross section of the magnet was segmented as described above, and the average grain size and area ratio of the main phase grains and subphase grains were calculated.

[0092] FIG. 2A shows an example of an SEM image of a cross section of the sintered magnet used in the calculation. In the SEM image, the light gray areas represent the Sm-rich phase, and the black or dark gray areas represent main phase grains. FIG. 2B is an image obtained by binarizing FIG. 2A, with the main phase grains shown in black. FIG. 1C is an image obtained by binarizing FIG. 2A, with the Sm-rich phase shown in black.

[0093] (Average Value of Iron XMCD Intensity) Segmentation information and XMCD imaging at an applied magnetic field of 4 T were obtained as described above to obtain average values ​​M1 and M2 of the iron XMCD intensity of the main phase grains and the Sm-rich phase grains. XMCD imaging measurements were performed using the large synchrotron radiation facility, Spring-8 (registered trademark). For Comparative Example 2, XMCD imaging measurements were also performed using a 3 GeV high-brilliance synchrotron radiation facility (NanoTerasu (registered trademark)).

[0094] (Relative density) Typical Sm constituting the main phase grains ２ Fe １７ N ３ The known true density of the phase (7.67 g / cm ３ The ratio of the volume density of the sintered magnet to the total mass was calculated as the relative density. The volume density of the sintered magnet was calculated from the volume and mass of the sintered magnet.

[0095] (Coercive force, residual magnetization) Measured using a vibrating sample magnetometer (VSM).

[0096]

[0097] Example 3 A Sm—Fe—N based magnet was produced by the following procedure: (i) Preparation of Sm—Fe—N based magnetic powder Sm—Fe—N based magnetic powder with a composition of Sm2Fe17N3 and an average particle size of approximately 25 μm was prepared.

[0098] 100% by mass of Sm-Fe-N magnetic powder was mixed with 5.0% by mass of Zn-Al alloy (Al content: 20% by mass, melting point: 395° C.).

[0099] (ii) Pulverization and Classification Using an airflow pulverization type jet mill, the Sm-Fe-N magnetic powder was pulverized at a pulverization pressure of 0.7 MPa.

[0100] The pulverization was carried out in a glove box under a low-oxygen atmosphere of 2 ppm or less. After pulverization, an air classifier was used to remove fine powder (particles with a particle size of less than 0.04 μm). This adjusted the average particle size of the Sm—Fe—N magnetic powder to 2.0 μm.

[0101] (iii) Magnetic Field Molding 0.2 g of the obtained magnetic powder was immersed in heptane in a glove box under a low-oxygen atmosphere of 2 ppm or less to prepare a slurry. The obtained slurry was filled into a cemented carbide mold. The mold was removed from the glove box and placed in a powder press equipped with a magnetic field generator. While applying a static magnetic field of 1.5 T to the powder press, press molding was performed at a pressure of 1250 MPa in the direction perpendicular to the magnetic field.

[0102] (iv) Pressure Sintering Next, the above mold was placed in a pulse current sintering machine equipped with a pressure mechanism using a servo-controlled press. While evacuating the interior of the pulse current sintering machine with a vacuum pump, Ar gas was introduced to create a reduced pressure atmosphere of 200 Pa. While maintaining this reduced pressure atmosphere, a pressure of 1470 MPa was applied to the mold, and current sintering was performed at 400°C for 2 minutes to obtain a sintered magnet.

[0103] Example 4 A sintered magnet was obtained in the same manner as in Example 3, except that the pressure sintering conditions were changed as shown in Table 6.

[0104] [Example 5] A sintered magnet was obtained in the same manner as in Example 3, except that 10.0 mass% of a Zn-Al alloy (Al content 20 mass%, melting point 395°C) was added and the pressure sintering conditions were changed as shown in Table 6.

[0105] Comparative Example 3 A sintered magnet was obtained in the same manner as in Example 3, except that the pressure sintering conditions were changed as shown in Table 6.

[0106] [Evaluation] Evaluation was carried out in the same manner as in Example 1. The results are shown in Table 6. However, for Examples 3 to 5 and Comparative Example 3, the average values ​​M1 and M2 of the XMCD intensities of iron in the main phase grains and Sm-rich phase grains were obtained based on XMCD imaging measurements using a 3 GeV high-brilliance synchrotron radiation facility (NanoTerasu (registered trademark)), and the ratio (M2 / M1) was calculated, after which 4.6% was subtracted from the average values.

[0107]

[0108] <1> An Sm—Fe—N sintered magnet comprising: main-phase crystal grains containing samarium, iron, and nitrogen, the main-phase grains containing 9 atomic % or more but less than 13 atomic % samarium; and sub-phase grains containing 13 atomic % or more samarium, iron, and nitrogen, wherein the ratio (M2 / M1) of the average X-ray magnetic circular dichroism intensity M2 of the sub-phase grains to the average X-ray magnetic circular dichroism intensity M1 of the main-phase grains, obtained in a plane perpendicular to the easy axis of magnetization under an environment in which a static magnetic field of 4 T is applied in a direction parallel to the easy axis of magnetization, is 10% or more and 83% or less. <2> The Sm—Fe—N sintered magnet according to <1>, wherein the content of the sub-phase grains is 0.3 volume % or more and 5 volume % or less. <3> The Sm—Fe—N sintered magnet according to <1> or <2>, wherein the average grain size of the main-phase grains is 0.5 μm or more and 3.0 μm or less. <4> The Sm—Fe—N sintered magnet according to any one of <1> to <3>, which contains no oxygen or 0.5 mass% or less of oxygen. <5> A method for producing a Sm—Fe—N sintered magnet, comprising pressure-sintering a magnetic material containing Sm—Fe—N magnetic powder in a sintering machine in a reduced-pressure atmosphere containing an inert gas at a pressure of 10 Pa to 200 Pa. <6> A method for producing a Sm—Fe—N sintered magnet according to <5>, which forms the reduced-pressure atmosphere by discharging gas from the sintering machine while introducing the inert gas into the sintering machine. <7> A method for producing a Sm—Fe—N sintered magnet according to <5> or <6>, which performs the pressure-sintering in an atmosphere with a low oxygen concentration, with a volume-based oxygen concentration of 2 ppm or less. <8> A method for producing a Sm—Fe—N sintered magnet according to any one of <5> to <7>, which performs the pressure-sintering at a temperature of 300°C to 600°C. <9> The method for producing a Sm—Fe—N sintered magnet according to any one of <5> to <8>, wherein the magnetic material comprises at least one of a metal or alloy having a melting point at or below the pressure-sintering temperature, and an alloy having a eutectic point, peritectic point, or monotectoid point at or below the pressure-sintering temperature. <10> The method for producing a Sm—Fe—N sintered magnet according to any one of <5> to <9>, further comprising compacting the magnetic material in a magnetic field before the pressure-sintering.<11> The method for producing a Sm—Fe—N sintered magnet according to any one of <5> to <10>, further comprising pulverizing and classifying the Sm—Fe—N magnetic powder before the pressure sintering.

[0109] The sintered magnet and magnet powder of the present invention can be used in a wide range of applications in the field of various motors, such as in-vehicle accessory motors and main motors for EVs and HEVs, and more specifically, in oil pump motors, electric power steering motors, and EV / HEV drive motors.

[0110] This application claims priority based on Japanese Patent Application No. 2024-111634, filed on July 11, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. A Sm-Fe-N sintered magnet comprising crystal grains containing samarium, iron, and nitrogen, wherein the main phase grains contain 9 atomic % or more but less than 13 atomic % samarium, and subphase grains contain 13 atomic % or more samarium, iron, and nitrogen, and wherein the ratio (M2 / M1) of the average X-ray magnetic circular dichroism intensity M2 of the subphase grains to the average X-ray magnetic circular dichroism intensity M1 of the iron of the main phase grains, obtained in a plane perpendicular to the easy axis of magnetization under an environment in which a static magnetic field of 4 T is applied in a direction parallel to the easy axis of magnetization, is 10% or more and 83% or less.

2. The Sm-Fe-N sintered magnet according to claim 1, wherein the content of the subphase grains is 0.3% by volume or more and 5% by volume or less.

3. The Sm-Fe-N sintered magnet according to claim 1 or 2, wherein the average grain size of the main phase grains is 0.5 μm or more and 3.0 μm or less.

4. A Sm-Fe-N sintered magnet according to any one of claims 1 to 3, which contains no oxygen or not more than 0.5 mass % of oxygen.

5. A method for producing a Sm-Fe-N sintered magnet, comprising pressurizing and sintering a magnetic material containing Sm-Fe-N magnetic powder in a sintering machine in a reduced pressure atmosphere of 10 Pa or more and 200 Pa or less, including an inert gas.

6. The method for producing a Sm-Fe-N sintered magnet according to claim 5, wherein the reduced pressure atmosphere is formed by discharging gas from the sintering machine while introducing the inert gas into the sintering machine.

7. The method for producing a Sm-Fe-N sintered magnet according to claim 5 or 6, wherein the pressure sintering is carried out in an atmosphere with a low oxygen concentration, where the volumetric oxygen concentration is 2 ppm or less.

8. The method for producing a Sm-Fe-N sintered magnet according to any one of claims 5 to 7, wherein the pressure sintering is carried out at a temperature of 300°C or higher and 600°C or lower.

9. A method for producing a Sm-Fe-N sintered magnet as recited in any one of claims 5 to 8, wherein the magnetic material comprises at least one of a metal or alloy having a melting point below the pressure-sintering temperature, and an alloy having a eutectic point, peritectic point, or monotectonic point below the pressure-sintering temperature.

10. The method for producing a Sm-Fe-N sintered magnet according to any one of claims 5 to 9, further comprising compacting the magnetic material in a magnetic field before the pressure sintering.

11. The method for producing a Sm—Fe—N sintered magnet according to any one of claims 5 to 10, further comprising pulverizing and classifying the Sm—Fe—N magnetic powder before the pressure sintering.

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