Sm-Fe-N-BASED SINTERED MAGNET AND MANUFACTURING METHOD THEREFOR

Abstract

Provided is an Sm-Fe-N-based sintered magnet containing a sintered body of a material containing Sm-Fe-N-based magnetic powder, wherein the Sm-Fe-N-based magnetic powder has an average particle diameter of 0.1-10 μm and an oxygen content of 0.8 mass% or less.

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 magnets are representative of rare earth-transition metal-nitrogen magnets, and have a high anisotropy field and saturation magnetization. Furthermore, their Curie temperature is relatively higher than other rare earth-transition metal-nitrogen magnets, giving them excellent heat resistance. For this reason, Sm—Fe—N magnets are used as one of the superior magnetic materials.

[0003] Sm—Fe—N magnetic powder is used as the raw material for Sm—Fe—N magnets. An alloy of Sm and Fe is produced, for example, by a reduction-diffusion method using calcium (Ca). This method produces by-products such as CaO and unreacted metallic Ca, which must be removed by rinsing with water. However, rinsing with water can increase the oxygen content of the alloy, potentially degrading the magnetic properties of the resulting magnetic powder.

[0004] For example, Patent Document 1 describes a method in which an alloy of Sm and Fe is subjected to a nitriding treatment and then washed with water, thereby converting unreacted metallic Ca into calcium nitride and quickly removing it.

[0005] For example, Patent Document 2 describes bubbling nitrogen gas into cleaning water to reduce the amount of dissolved oxygen in the cleaning water.

[0006] Japanese Patent Application Laid-Open No. 2000-034510 International Publication No. 2001 / 089747

[0007] K. Takagi, et al., "Possibility of high-performance Sm2Fe17N3 sintered magnets by low-oxygen powder metallurgy process", Journal of Magnetism and Magnetic Materials, 506, (2020), 16681.

[0008] In Patent Document 1, ion-exchanged water is used as the washing water, and therefore an increase in the oxygen content of the resulting powder cannot be sufficiently avoided.In Patent Document 2, washing is performed in an air atmosphere, and therefore oxygen from the air cannot be prevented from dissolving in the washing water, and therefore an increase in the oxygen content of the resulting powder cannot be sufficiently avoided.

[0009] It is known that the magnetic properties (typically, coercive force) of Sm—Fe—N magnetic powders tend to deteriorate during the sintering process. In recent years, it has been discovered that the deterioration of coercive force due to the sintering process is caused by the decomposition of the main phase by a film containing oxides and / or hydroxides on the surface of the magnetic powder, resulting in the generation of α-Fe (see Non-Patent Document 1).

[0010] An object of the present invention is to provide a Sm--Fe--N sintered magnet having high coercive force and a method for producing the same.

[0011] According to one aspect of the present invention, there is provided a sintered magnet including a sintered body of a material containing Sm-Fe-N magnetic powder, wherein the Sm-Fe-N magnetic powder has an average particle size of 0.1 μm or more and 10 μm or less, and an oxygen content of 0.8 mass % or less.

[0012] According to another aspect of the present invention, there is provided a method for producing a Sm-Fe-N sintered magnet, comprising: nitriding an alloy powder containing Sm and Fe to obtain a Sm-Fe-N precursor powder; washing the Sm-Fe-N precursor powder with low-dissolved-oxygen water having a dissolved oxygen content of 0.50 mg / L or less to obtain a Sm-Fe-N magnetic powder; and pressure-sintering a material containing the Sm-Fe-N magnetic powder, wherein all of the above steps are carried out in a low-oxygen atmosphere with a volume-based oxygen concentration of 2 ppm or less.

[0013] According to the present invention, it is possible to provide a Sm-Fe-N sintered magnet having high coercive force and a method for producing the same.

[0014] 1 is a flowchart showing a method for manufacturing a sintered magnet according to embodiment 1 of the present disclosure; FIG. 2 is a flowchart showing a method for manufacturing a sintered magnet according to embodiment 3 of the present disclosure; FIG. 3 is a flowchart showing a method for manufacturing a sintered magnet according to embodiment 4 of the present disclosure;

[0015] Sm—Fe—N sintered magnets according to embodiments of the present disclosure will be described in detail below along with their manufacturing methods, but the present invention is not limited to these embodiments.

[0016] [Embodiment 1] The Sm-Fe-N sintered magnet according to this embodiment includes a sintered body of a material containing Sm-Fe-N magnetic powder (hereinafter, sometimes referred to as "magnetic material").

[0017] (Sm—Fe—N based magnetic powder) The Sm—Fe—N based magnetic powder has an average particle size of 0.1 μm or more and 10 μm or less, and an oxygen content of 0.8 mass % or less.

[0018] By setting the average particle size of the Sm—Fe—N magnetic powder to 0.1 μm or more and 10 μm or less, the coercive force of the Sm—Fe—N sintered magnet (hereinafter sometimes simply referred to as "sintered magnet") using this powder can be increased.

[0019] The oxygen content of Sm—Fe—N magnetic powder (hereinafter sometimes simply referred to as "magnetic powder") is 0.8 mass % or less, which means that the formation of a film containing oxides and / or hydroxides (hereinafter sometimes simply referred to as "oxide film") on the surface of the magnetic powder is suppressed. As a result, decomposition of the main phase by the oxide film is suppressed during the sintering process, resulting in a sintered magnet with higher coercivity.

[0020] The magnetic powder used in this embodiment may have any composition consisting of Sm, Fe, and N. The magnetic powder is typically Sm ２ Fe １７ N ３ The composition may be, but is not limited to:

[0021] The average particle size of the magnetic powder is 0.1 μm or more and 10 μm or less. The average particle size of the magnetic powder may be 7.5 μm or less, or 5.0 μm or less, in order to further increase the coercive force. The average particle size of the magnetic powder may be 0.5 μm or more in order to suppress superparamagnetism.

[0022] The "average particle size" of a powder (such as a magnetic powder) refers to the particle size (D50) at the point where the cumulative value reaches 50% on a cumulative curve obtained by calculating the particle size distribution on a volume basis, with the total volume being 100%. The average particle size can be measured using a laser diffraction / scattering particle size / particle size distribution analyzer (e.g., HELOS & RODOS Lens R1, manufactured by Nippon Laser Co., Ltd.) or an electron scanning microscope (e.g., SU8230, manufactured by Hitachi High-Technologies Corporation).

[0023] The oxygen content of the magnetic powder is 0.80% by mass or less. The oxygen content of the magnetic powder may be 0.60% by mass or less, or may be 0.40% by mass or less. The oxygen content of the magnetic powder is measured within seven days after washing with low-dissolved-oxygen water, as described below. The oxygen content of the magnetic powder can be measured by inert gas fusion-non-dispersive infrared absorption (NDIR) or the like. The oxygen content of the magnetic powder can be measured, for example, using an EMGA-830 (oxygen, nitrogen, and hydrogen analyzer, manufactured by Horiba, Ltd.). The oxygen content is measured for the magnetic powder sealed in a metal capsule in a glove box where the oxygen concentration is controlled to 2 ppm or less, without being exposed to the atmosphere.

[0024] The moisture content of the magnetic powder may be 0.05% by mass or more. If the moisture content is 0.05% by mass or more, it can be said that a washing step using water (water washing) was carried out when producing the magnetic powder. According to this embodiment, even if the magnetic powder is washed with water, the oxygen content of the resulting sintered magnet is suppressed to 0.8% by mass or less. The moisture content of the magnetic powder may be 0.07% by mass or more, or 0.09% by mass or more. The moisture content of the magnetic powder may be 0.10% by mass or less.

[0025] The amount of moisture contained in the magnetic powder can be measured by the Karl Fischer method, for example, using an AQ-2250 trace moisture analyzer (manufactured by HIRANUMA Co., Ltd.).

[0026] At the surface of the Sm—Fe—N magnetic powder, the peak intensity ratio (P1 / P2) between the peak intensity P1 of the metal oxide containing Fe and Sm at a binding energy of 528.0 eV to 529.5 eV and the peak intensity P2 of the metal hydroxide containing Fe and Sm at a binding energy of 529.6 eV to 532.0 eV may be 0.5 or less.

[0027] A peak intensity ratio (P1 / P2) of 0.5 or less means that the oxide film on the surface of the magnetic powder contains a smaller proportion of metal oxides containing Fe and Sm than metal hydroxides containing Fe and Sm. Although the reason for this is not clear, this results in a higher coercive force for the sintered magnet.

[0028] The peak intensity ratio (P1 / P2) is reduced by, for example, performing washing using low-dissolved oxygen water with a dissolved oxygen content of 0.50 mg / L or less in the manufacturing process of Sm—Fe—N magnetic powder. On the other hand, as in Patent Documents 1 and 2, washing using ion-exchanged water or washing with water in the air increases the peak intensity ratio (P1 / P2) (i.e., the proportion of metal oxides containing Fe and Sm increases).

[0029] The peak intensity ratio (P1 / P2) may be 0.05 or more, 0.1 or more, or 0.2 or more. The peak intensity ratio (P1 / P2) may be 0.48 or less, 0.45 or less, 0.40 or less, or 0.30 or less.

[0030] The peak intensities P1 and P2 can be obtained by X-ray photoelectron spectroscopy (XPS). XPS is a surface analysis method for analyzing solid surfaces and is also known as ESCA (Electron Spectroscopy for Chemical Analysis). The peak intensity P1 is the intensity of the highest peak in the binding energy range of 528.0 eV to 529.5 eV in the XPS spectrum. The peak intensity P2 is the intensity of the highest peak in the binding energy range of 529.6 eV to 532.0 eV in the XPS spectrum. The peak intensities P1 and P2 can be measured, for example, using a Quantes (scanning dual X-ray photoelectron spectrometer, ULVAC-PHI, Inc.). The measurement conditions may be: X-ray source: Al, accelerating voltage: 15 kV, X-ray beam intensity: 25 W, and X-ray spot size: 100 μm.

[0031] Coercive force H of magnetic powder ｃｊｐ The coercive force H of the magnetic powder may be 700 kA / m or more, and may be 720 kA / m or more. ｃｊｐ can be obtained, for example, by using a VSM-5HSC10 (vibrating sample magnetometer, manufactured by Toei Kogyo Co., Ltd.).

[0032] (Sm—Fe—N sintered magnet) Sm—Fe—N sintered magnets are obtained by baking (sintering) a magnetic material containing the above-mentioned Sm—Fe—N magnetic powder at high temperatures. Because the Sm—Fe—N magnetic powder according to the present disclosure has a low oxygen content, oxide films are less likely to form, and the resulting magnets have high coercivity even after undergoing the baking process.

[0033] Coercive force H of sintered magnet ｃｊｓ The coercive force H of the sintered magnet is, for example, 780 kA / m or more. ｃｊｓ may be 785 kA / m or greater.

[0034] Coercive force H of magnetic powder ｃｊｐ and the coercive force H of the sintered magnet ｃｊｓ The ratio (H ｃｊｐ / H ｃｊｓ) may be 0.90 or more, 1.00 or more, or 1.05 or more. This means that the coercive force is unlikely to decrease even during the firing process.

[0035] In the present disclosure, a sintered magnet refers to a magnet obtained by sintering magnetic powder at high temperature.

[0036] The magnetic material used in this embodiment may consist essentially of Sm—Fe—N magnetic powder, but may also contain one or more other materials, such as a magnetic powder consisting of a rare earth element other than Sm, Fe, and N, or a magnetic powder consisting of a rare earth element including Sm, 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), and tungsten (W). The Sm—Fe—N sintered magnet of this embodiment may contain trace elements that are inevitably mixed in, such as carbon (C), silicon (Si), and aluminum (Al).

[0037] The average grain size of the crystal grains of the magnetic powder constituting the sintered magnet is 0.1 μm or more and 10 μm or less. The average grain size of the crystal grains can be considered to be the average grain size of the Sm—Fe—N magnetic powder according to the present disclosure contained in the magnetic material for producing the sintered magnet. The average grain size of the crystal grains may be 7.5 μm or less, or 5.0 μm or less. The average grain size of the crystal grains may be 1.0 μm or more.

[0038] The "average grain size" of the crystal grains is calculated as follows: First, a cross-section of the sintered magnet is photographed using an FE-SEM so that at least 50 crystal grains are included, and the total area A of the cross-sections of the crystal grains in the photographed image and the number N of crystal grains are determined. Next, the average cross-sectional area a1 of the crystal grains is calculated as A / N, and the square root of this average cross-sectional area a1 is calculated as the average grain size d of the crystal grains.

[0039] (Method for producing Sm-Fe-N sintered magnet) An Sm-Fe-N sintered magnet can be produced by a method comprising nitriding an alloy powder containing Sm and Fe to obtain an Sm-Fe-N precursor powder, washing the Sm-Fe-N precursor powder with low-dissolved oxygen water having a dissolved oxygen content of 0.50 mg / L or less to obtain an Sm-Fe-N magnetic powder, and pressure-sintering a material containing the Sm-Fe-N magnetic powder.

[0040] All of the above steps are carried out in an atmosphere with a low oxygen concentration of 2 ppm or less by volume. This results in an Sm—Fe—N magnetic powder with an average particle size of 0.1 μm to 10 μm and an oxygen content of 0.8 mass% or less. The oxygen concentration may be 1 ppm or less, or 0.5 ppm or less.

[0041] All of 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.

[0042] Other steps performed after the nitriding treatment and before the pressure firing (for example, (5) orientation and magnetization, which will be described later) are also performed in the above-mentioned low-oxygen concentration atmosphere.

[0043] However, 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 capable of preventing oxidation between the time of washing and the time of pressure firing.

[0044] FIG. 1 is a flowchart showing a method for producing a sintered magnet according to the first embodiment.

[0045] (1) Preparation of Alloy Powder (S11) In this embodiment, an alloy powder containing Sm and Fe (Sm—Fe alloy) is prepared by a reduction diffusion method using its precursor powder.

[0046] In the Sm—Fe alloy powder, the amount of Sm relative to the total amount of Sm and Fe may be, for example, 9 atm % or more and 14 atm % or less, and the average particle size of the alloy powder may be, for example, 1 μm or more and 50 μm or less.

[0047] Examples of precursor powders for Sm—Fe alloys include a mixed powder of Sm compound powder, iron powder, and iron oxide powder, Sm—Fe-based oxide powder, and Sm—Fe-based hydroxide powder. The precursor powder is prepared, for example, by a coprecipitation method. The precursor powder may be pre-reduced in a reducing atmosphere. The pre-reducing step is performed, for example, by heating the precursor powder to 400° C. or higher in a hydrogen atmosphere.

[0048] The reduction diffusion method is, for example, a method of mixing a precursor powder of Sm—Fe alloy with Ca (calcium) or CaH ２ The Sm-Fe alloy powder is obtained by mixing the Sm compound with calcium hydroxide (calcium hydride) and heating the resulting mixture in an inert gas atmosphere at a temperature equal to or higher than the melting point of calcium (approximately 842°C). The heating time may be, for example, 1 hour to 10 hours. As a result, the Sm compound is reduced by calcium and reacts with iron, resulting in the production of a Sm-Fe alloy powder.

[0049] (2) Crushing, Pulverization, and Classification Prior to nitriding and washing, the Sm—Fe alloy may be crushed or pulverized, and then classified as necessary. Fine powder is removed from the crushed powder by classification. Crushing, pulverization, and classification are performed under conditions such that the average particle size of the resulting Sm—Fe—N magnetic powder is 0.1 μm or more and 10 μm or less.

[0050] 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.

[0051] (3) Nitriding (S12) The nitriding is typically performed by heat treatment in a nitrogen atmosphere, an ammonia atmosphere, a mixed atmosphere of ammonia and hydrogen, or a mixed atmosphere of nitrogen and hydrogen, whereby nitrogen is incorporated into the crystals of the alloy powder, thereby obtaining a Sm—Fe—N-based precursor powder.

[0052] When nitrogen gas is used, the partial pressure of nitrogen may be 10 kPa or more and 100 kPa or less, and the heating time may be 5 hours or more and 30 hours or less. When a mixed gas of ammonia and hydrogen is used, the partial pressure of ammonia may be 20 kPa or more and 40 kPa or less, and the heating time may be 10 minutes or more and 50 minutes or less, when the total pressure of the mixed gas is 100 kPa.

[0053] In the nitriding treatment, the heating temperature is preferably 350° C. or higher and 550° C. or lower, more preferably 400° C. or higher and 550° C. or lower. By using this heating temperature, it is possible to prevent decomposition into SmN and Fe, which may occur when the nitriding reaction is carried out at a higher temperature, and it is possible to allow the reaction to proceed more sufficiently compared to when the nitriding reaction is carried out at a lower temperature.

[0054] The nitriding treatment is typically carried out under atmospheric pressure, and can be carried out under a pressure of 90 kPa or more and 1.10 kPa or less, more preferably 95 kPa or more and 105 kPa or less.

[0055] (4) Washing with Low-Dissolved Oxygen Water (S13) The Sm—Fe—N precursor powder is washed with low-dissolved oxygen water with a dissolved oxygen content of 0.50 mg / L or less. When alloy powder is prepared by a reduction-diffusion method using Ca, the Sm—Fe—N precursor powder contains CaO and unreacted Ca as by-products. Therefore, washing with water is usually performed. By-products can be physically removed by washing. By-products can also be removed by chemical reaction with the low-dissolved oxygen water. For example, Ca reacts with the low-dissolved oxygen water to form calcium hydroxide, which dissolves in the low-dissolved oxygen water. The dissolved oxygen content of the liquid can be measured, for example, using a D210-PD (dissolved oxygen meter, manufactured by Horiba, Ltd.).

[0056] Washing with water typically promotes the formation of oxides and / or hydroxides (particularly hydroxide formation) of the contained metals on the surface of the Sm—Fe—N precursor powder. Washing with low-dissolved-oxygen water suppresses the formation of these oxides and / or hydroxides, making it difficult for an oxide film to form on the surface of the magnetic powder, resulting in an oxygen content of 0.8 mass% or less. Use of this magnetic powder suppresses decomposition of the main phase by the oxide film during the sintering process, resulting in a sintered magnet with high coercivity.

[0057] In this embodiment, washing is performed using low-dissolved-oxygen water at a temperature of more than 10° C. and not more than 30° C. and a pH of 4.0 or more and less than 9.0. This results in a magnetic powder with an oxygen content of 0.80 mass % or less.

[0058] Low-dissolved oxygen water can be obtained, for example, by a degassing device equipped with reduced pressure, ultrasonic vibration, helium purging, a gas-permeable membrane, or a combination thereof. The lower the dissolved oxygen content, the more desirable it is, and it may be 0.10 mg / L or less, or 0.05 mg / L or less.

[0059] The washing is carried out, for example, by adding the Sm—Fe—N precursor powder to low-dissolved-oxygen water and then stirring the mixture. The stirring is then stopped, and the resulting precipitate is taken out and dried to obtain the Sm—Fe—N magnetic powder.

[0060] Washing may be performed multiple times. For example, the Sm—Fe—N precursor powder is added to low-dissolved oxygen water, stirred, allowed to stand, the supernatant liquid is removed, and new low-dissolved oxygen water is added. The same operation as above may then be repeated a desired number of times. The stirring time may be 1 minute or more and 30 minutes or less. The number of repetitions may be 2 or more and 10 or less.

[0061] Prior to this washing, the magnetic powder may be washed with acetic acid or hydrochloric acid, etc. This will further remove any remaining Ca.

[0062] Drying can be performed by evacuation. The degree of vacuum may be, for example, -95 kPa or less. The treatment time may be, for example, 1 hour or more and 10 hours or less. The inside of the vacuum device is maintained in a low-oxygen atmosphere, and may be at a vacuum of, for example, 5 Pa or less, or may be filled with an inert gas atmosphere, or an inert gas may be flowed in under reduced pressure.

[0063] (5) Orientation and Magnetization Prior to pressure sintering, an orientation process, a magnetization process, and a molding process may be performed. This aligns the easy magnetization axis of the Sm—Fe—N magnetic powder, resulting in higher magnetic properties. The applied magnetic field may be, for example, a static magnetic field of 1 T or more, or a pulsed magnetic field.

[0064] (6) Filling: The resulting material containing the Sm—Fe—N magnetic powder is filled into a mold. (3) After the nitriding treatment (S12), at least up to this filling step, the process is carried out in an atmosphere with a low oxygen concentration, where the volumetric oxygen concentration is 2 ppm or less. The mold used may have any shape, and for example, a cylindrical mold can be used, but is not limited thereto.

[0065] (7) Pressure Sintering (S14) The material containing the Sm—Fe—N magnetic powder packed into the mold is pressure sintered. This produces a Sm—Fe—N sintered magnet. The treatment after the Sm—Fe—N magnetic powder is packed into the mold (the pressure sintering) may be carried out in the air.

[0066] Any pressure sintering method, including electric pressure sintering, can be used for the pressure sintering. Pressure sintering may be performed, for example, by hot pressing or electric sintering. Hot pressing is a common sintering method in which heating is performed while applying pressure in an inert atmosphere such as Ar. Electric sintering is a method in which a certain pressure is applied to a mold and an electric current is applied while maintaining this pressure. The interior of the pulse electric sintering machine is maintained in a vacuum of, for example, 5 Pa or less. The applied pressure may be higher than atmospheric pressure and may be a pressure capable of forming a sintered magnet, for example, in the range of 100 MPa to 2000 MPa. Electric sintering is performed, for example, at a temperature of 400°C to 600°C for a time of 30 seconds to 10 minutes.

[0067] [Embodiment 2] This embodiment differs from Embodiment 1 in the method for preparing the Sm—Fe alloy powder used to manufacture the Sm—Fe—N sintered magnet. This difference is explained below. In this embodiment, the other steps in the method for manufacturing the Sm—Fe—N sintered magnet are the same as in Embodiment 1, so their explanations are omitted. In this embodiment, the configuration of the sintered magnet is the same as in Embodiment 1, so their explanations are omitted.

[0068] In this embodiment, the Sm—Fe alloy powder is prepared by melt spinning. Melt spinning is a method in which a molten Sm—Fe alloy is injected into a metal roll rotating at high speed and quenched. The Sm—Fe alloy solidifies into a thin ribbon shape, which is then crushed and heat-treated in an inert atmosphere to obtain the Sm—Fe alloy powder.

[0069] The alloy powder prepared by the melt spinning method is subjected to (2) crushing, grinding, and classification, (3) nitriding, and then (4) washing with low-dissolved-oxygen water, (5) orientation and magnetization, (6) packing, and (7) pressure sintering in a low-oxygen atmosphere, as in embodiment 1. This results in a Sm—Fe—N magnetic powder with an average particle size of 0.1 μm to 10 μm and an oxygen content of 0.8 mass % or less, and a sintered magnet with a coercive force of 780.0 kA / m or more.

[0070] [Embodiment 3] This embodiment differs from Embodiment 1 in the temperature of the low-dissolved-oxygen water used for cleaning. This difference is explained below. In this embodiment, the other configurations of the method for producing a Sm—Fe—N based sintered magnet and the configuration of the sintered magnet are the same as those in Embodiment 1, so explanations thereof will be omitted.

[0071] FIG. 2 is a flowchart showing a method for producing a sintered magnet according to the third embodiment.

[0072] In this embodiment, the temperature of the low-dissolved oxygen water used for washing is 5°C or higher and 10°C or lower. As described above, Ca reacts with low-dissolved oxygen water to form calcium hydroxide, which dissolves in the low-dissolved oxygen water. The reaction between Ca and low-dissolved oxygen water is an exothermic reaction. As described above, water washing can produce oxides and / or hydroxides of the contained metals on the surface of the Sm—Fe—N precursor powder. The production of these oxides and / or hydroxides is accelerated by heat. Washing with cold low-dissolved oxygen water at a temperature of 5°C or higher and 10°C or lower suppresses the production of these oxides and / or hydroxides. As a result, the oxygen content of the resulting magnetic powder can be further reduced.

[0073] In this embodiment, similar to the first embodiment, (2) crushing, pulverization, and classification, and (3) nitriding (S12) are performed, followed by (4) washing with low-dissolved-oxygen water at 5°C to 10°C (S13), (5) alignment and magnetization, (6) filling, and (7) pressure sintering (S14) in a low-oxygen atmosphere. This results in a Sm—Fe—N magnetic powder with an average particle size of 0.1 μm to 10 μm and an oxygen content of 0.8 mass% or less, and a sintered magnet with a coercive force of 780.0 kA / m or more.

[0074] [Embodiment 4] This embodiment differs from Embodiment 1 in the pH of the low-dissolved-oxygen water used for cleaning. This difference is explained below. In this embodiment, the other configurations of the method for producing a Sm—Fe—N based sintered magnet and the configuration of the sintered magnet are the same as those in Embodiment 1, so explanations thereof will be omitted.

[0075] FIG. 3 is a flowchart showing a method for producing a sintered magnet according to the fourth embodiment.

[0076] In this embodiment, the pH of the low-dissolved oxygen water used for cleaning is 9 or more and 14 or less. As described above, Ca reacts with the low-dissolved oxygen water to form calcium hydroxide, which dissolves in the low-dissolved oxygen water. Because calcium hydroxide is basic, the production of calcium hydroxide is suppressed by adjusting the pH of the low-dissolved oxygen water to 9 or more and 14 or less (i.e., alkaline). In other words, the exothermic reaction between Ca and the low-dissolved oxygen water is suppressed, thereby suppressing heat generation. As a result, the production of oxides and / or hydroxides of the contained metals on the surface of the Sm—Fe—N-based precursor powder is suppressed, and the oxygen content of the resulting magnetic powder can be further reduced.

[0077] In this embodiment, similar to the first embodiment, (2) crushing, pulverization, and classification, and (3) nitriding treatment (S12) are performed, followed by (4) washing with low-dissolved-oxygen water having a pH of 9 to 14 (S13), (5) alignment and magnetization, (6) filling, and (7) pressure sintering (S14) in a low-oxygen atmosphere. This results in a Sm—Fe—N magnetic powder having an average particle size of 0.1 μm to 10 μm and an oxygen content of 0.8 mass % or less, and a sintered magnet with a coercive force of 780.0 kA / m or more.

[0078] Although four embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to these. For example, any two or more of the features of the above-described embodiments may be combined.

[0079] In the above-described embodiment, the Sm—Fe alloy powder is prepared by a reduction-diffusion method or a melt spinning method, but is not limited thereto. The Sm—Fe alloy powder may be prepared by, for example, an atomization method, a mold casting method, a mechanical alloying method, or an electrolytic deposition method.

[0080] The present invention will be explained in more detail with reference to the following examples, but the present invention is not limited to these examples.

[0081] Example 1 (i) Preparation of alloy powder by reduction diffusion method 2.87 g of a mixed powder of samarium oxide powder and iron powder and 0.36 g of metallic calcium with an average particle size of 2 mm were mixed and placed in a furnace. After evacuating the furnace, argon gas was introduced. The temperature was raised to 950°C and maintained for 5 hours to prepare a Sm—Fe alloy.

[0082] (ii) Crushing The obtained Sm-Fe alloy was crushed using an agate mortar to obtain Sm-Fe alloy powder having an average particle size of 4.83 μm.

[0083] (iii) Nitriding Treatment The obtained Sm—Fe alloy powder was heat treated at 475° C. for 23 hours in a nitrogen atmosphere to obtain a Sm—Fe—N-based precursor powder.

[0084] (iv) Washing The obtained Sm—Fe—N precursor powder was poured into washing water (liquid temperature 26° C., pH 7) with the amount of dissolved oxygen shown in Table 1, and stirred for 10 minutes. After leaving to stand, the supernatant was drained by decantation. The addition of washing water, stirring, and decantation were repeated three times. Next, acetic acid of pH 4.5 was added, and the mixture was stirred for 10 minutes. After leaving to stand, the supernatant was drained by decantation. Thereafter, the addition of washing water, stirring, and decantation were repeated once, and after solid-liquid separation, the mixture was vacuum dried for 1 hour to obtain magnetic powder.

[0085] (v) Filling 0.2 g of the magnetic powder obtained was immersed in heptane to prepare a slurry, which was then filled into a cemented carbide mold.

[0086] (vi) Pressure Sintering: The mold was placed in a pulse current sintering machine equipped with a servo-controlled press mechanism. Next, a pressure of 1500 MPa was applied to the sintered magnet in an Ar atmosphere maintained at 20 Pa. While maintaining this pressure, the magnet was subjected to current sintering at 400°C for 2 minutes, yielding a sintered magnet.

[0087] In Example 1, the processes from (ii) crushing to (v) filling were carried out in a glove box (substituted with nitrogen) connected to a gas circulation type oxygen and moisture purifier. The oxygen concentration in the glove box was set to 1 ppm or less. The sample was moved between each device so as not to be exposed to the atmosphere. (vi) Pressurized firing was carried out in an Ar atmosphere.

[0088] Comparative Example 1 (iv) A sintered magnet was obtained in the same manner as in Example 1, except that the washing was carried out in the atmosphere (oxygen concentration 20.9% by volume) using washing water with a dissolved oxygen content of 4.8 mg / L.

[0089] [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) under the conditions described above.

[0090] (Coercive Force) The coercive force of the Sm-Fe-N magnetic powder and the sintered magnet was measured using a vibrating sample magnetometer (VSM).

[0091] (Water Content) The water content of the Sm—Fe—N based magnetic powder was measured by the Karl Fischer method. The water content of the Sm—Fe—N based precursor powder before washing was 0.040 mass % in both the Examples and Comparative Examples.

[0092] (Peak Intensity by XPS Analysis) The peak intensity by XPS analysis of the Sm—Fe—N based magnetic powder was measured by an X-ray photoelectron spectrometer.

[0093]

[0094] [Example 2 and Comparative Example 2] Except for preparing the Sm-Fe alloy powder by melt spinning, Sm-Fe-N sintered magnets were obtained and evaluated in the same manner as in Example 1 and Comparative Example 1. The results are shown in Table 2.

[0095]

[0096] <1> A sintered magnet including a sintered body of a material containing Sm—Fe—N magnetic powder, wherein the Sm—Fe—N magnetic powder has an average particle size of 0.1 μm or more and 10 μm or less and an oxygen content of 0.8 mass% or less. <2> The Sm—Fe—N sintered magnet according to <1>, having a coercive force of 780 kA / m or more. <3> The Sm—Fe—N sintered magnet according to <1> or <2>, wherein the Sm—Fe—N magnetic powder contains a moisture content of 0.05 mass% or more. <4> The Sm—Fe—N sintered magnet of any one of <1> to <3>, in which, at the surface of the Sm—Fe—N magnetic powder, the peak intensity ratio (P1 / P2) between the peak intensity P1 of a metal oxide containing Fe and Sm at a binding energy of 528.0 eV to 529.5 eV and the peak intensity P2 of a metal hydroxide containing Fe and Sm at a binding energy of 529.6 eV to 532.0 eV is 0.5 or less. <5> The Sm—Fe—N sintered magnet of any one of <1> to <4>, in which the Sm—Fe—N magnetic powder has an average particle size of 0.5 μm or more and 5 μm or less. <6> The Sm—Fe—N sintered magnet of any one of <1> to <5>, in which the Sm—Fe—N magnetic powder has an oxygen content of 0.4 mass% or less. <7> A method for producing an Sm-Fe-N sintered magnet, comprising: nitriding an alloy powder containing Sm and Fe to obtain an Sm-Fe-N precursor powder; washing the Sm-Fe-N precursor powder with low-dissolved oxygen water having a dissolved oxygen content of 0.50 mg / L or less to obtain an Sm-Fe-N magnetic powder; and pressure-sintering a material containing the Sm-Fe-N magnetic powder, wherein all of the above steps are carried out in a low-oxygen atmosphere having a volumetric oxygen concentration of 2 ppm or less. <8> The method for producing an Sm-Fe-N sintered magnet according to <7>, wherein the low-dissolved oxygen water used for the washing has a temperature of 5°C or higher and 10°C or lower. <9> The method for producing an Sm-Fe-N sintered magnet according to <7> or <8>, wherein the low-dissolved oxygen water used for the washing has a pH of 9.0 or higher and 14.0 or lower.

[0097] The Sm—Fe—N sintered magnet 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 (electric vehicles) and HEVs (hybrid electric vehicles), and more specifically, in oil pump motors, electric power steering motors, and EV / HEV drive motors.

[0098] This application claims priority based on Japanese Patent Application No. 2024-049836, filed on March 26, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. A sintered magnet comprising a sintered body of a material containing Sm-Fe-N magnetic powder, wherein the Sm-Fe-N magnetic powder has an average particle size of 0.1 μm or more and 10 μm or less, and an oxygen content of 0.8 mass % or less.

2. The Sm-Fe-N sintered magnet according to claim 1, having a coercive force of 780 kA / m or more.

3. The Sm-Fe-N sintered magnet according to claim 1 or 2, wherein the moisture content of the Sm-Fe-N magnetic powder is 0.05 mass % or more.

4. The Sm—Fe—N sintered magnet according to any one of claims 1 to 3, wherein the peak intensity ratio (P1 / P2) at the surface of the Sm—Fe—N magnetic powder is 0.5 or less, between the peak intensity P1 of the metal oxide containing Fe and Sm at a binding energy of 528.0 eV to 529.5 eV and the peak intensity P2 of the metal hydroxide containing Fe and Sm at a binding energy of 529.6 eV to 532.0 eV.

5. The Sm-Fe-N sintered magnet according to any one of claims 1 to 4, wherein the average particle size of the Sm-Fe-N magnetic powder is 0.5 μm or more and 5 μm or less.

6. The Sm-Fe-N sintered magnet according to any one of claims 1 to 5, wherein the oxygen content of the Sm-Fe-N magnetic powder is 0.4 mass % or less.

7. A method for producing a Sm-Fe-N sintered magnet, comprising: nitriding an alloy powder containing Sm and Fe to obtain a Sm-Fe-N precursor powder; washing the Sm-Fe-N precursor powder with low-dissolved-oxygen water having a dissolved oxygen content of 0.50 mg / L or less to obtain a Sm-Fe-N magnetic powder; and pressure-sintering a material containing the Sm-Fe-N magnetic powder, wherein all of the above steps are carried out in a low-oxygen atmosphere with a volume-based oxygen concentration of 2 ppm or less.

8. The method for producing a Sm-Fe-N sintered magnet according to claim 7, wherein the temperature of the low-dissolved-oxygen water used for the washing is 5°C or higher and 10°C or lower.

9. The method for producing a Sm-Fe-N sintered magnet according to claim 7 or 8, wherein the low-dissolved-oxygen water used for the washing has a pH of 9.0 or more and 14.0 or less.

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