Sm-fe-n-based sintered magnet and manufacturing method therefor

Abstract

The present invention provides a sintered magnet including a sintered body of a material containing Sm-Fe-N-based magnetic powder, which comprises a main phase containing Sm, Fe, and N, and a grain boundary phase containing a Zn-M alloy (where M is Al, Sn, Ge, or Mg) surrounding the main phase, the Zn-M alloy having a solid phase and a liquid phase that coexist at a temperature between 200-600°C.

Description

Sm-Fe-N sintered magnet and manufacturing method thereof

[0001] The present disclosure relates to an Sm—Fe—N based 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 remanent magnetization. Furthermore, their Curie temperature is relatively higher than that of other rare earth-transition metal-nitrogen magnets, giving them excellent heat resistance. For this reason, Sm—Fe—N magnetic powder is used as one of the superior magnet materials.

[0003] A surface modification method using Zn, a low-melting metal, is known as a method for obtaining a high-coercivity Sm-Fe-N sintered magnet (Patent Documents 1 and 2).

[0004] JP 2021-122061 A JP 2020-53437 A

[0005] In the methods of Patent Documents 1 and 2, Sm—Fe—N sintered magnets are sintered by pressure sintering. However, because Zn has a low melting point of 420°C, if pressure sintering is performed at a temperature higher than this, the spaces between the main phase particles are crushed and the liquid phase Zn is pushed out, making it impossible to obtain a sufficient reaction area with the main phase surface, and the effect of improving coercivity is reduced.

[0006] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a Sm—Fe—N sintered magnet that has high coercive force.

[0007] According to one aspect of the present invention, there is provided a sintered magnet comprising a sintered body of a material containing Sm-Fe-N magnetic powder, the Sm-Fe-N magnetic powder comprising a main phase containing Sm, Fe, and N, and a grain boundary phase surrounding the main phase, the Sm-Fe-N alloy (wherein M is Al, Sn, Ge, or Mg), wherein the Zn-M alloy exists in a solid phase and a liquid phase at any temperature between 200°C and 600°C.

[0008] According to another aspect of the present invention, there is provided a method for producing an Sm—Fe—N sintered magnet, comprising: mixing a magnetic powder containing Sm—Fe—N crystal grains with a powder of a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg) to obtain a raw material mixture; heat-treating the raw material mixture to obtain an Sm—Fe—N magnetic powder; and pressure-sintering the Sm—Fe—N magnetic powder, wherein a temperature T1 in the heat treatment is higher than a temperature T2 in the pressure-sintering, and the Zn-M alloy is in a state where a solid phase and a liquid phase coexist at T1, and a state where a solid phase and a liquid phase coexist at T2, or is in a completely solid phase state; or is in a completely liquid phase at T1, and a state where a solid phase and a liquid phase coexist at T2, or is in a completely solid phase state.

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

[0010] Fig. 1 is a diagram showing the equilibrium correlation between Zn and Al. Fig. 2 is a diagram showing the equilibrium correlation between Zn and Sn. Fig. 3 is a diagram showing the equilibrium correlation between Zn and Ge. Fig. 4 is a diagram showing the equilibrium correlation between Zn and Mg. Fig. 5 is a flowchart showing an example of a method for producing a sintered magnet according to the present disclosure.

[0011] (Sm—Fe—N based magnetic powder) The Sm—Fe—N based magnetic powder of the present disclosure comprises a main phase containing Sm, Fe, and N, and a grain boundary phase surrounding the main phase and containing a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg), in which a solid phase and a liquid phase coexist at any temperature between 200°C and 600°C.

[0012] (Main Phase) The main phase includes Sm—Fe—N crystal grains. The main phase is typically Sm—Fe—N crystal grains. The Sm—Fe—N crystal grains are crystal grains containing samarium, iron, and nitrogen. The Sm—Fe—N crystal grains are composed of Th ２ Zn １７ Type or Th ２ Ni １７ The crystal structure can be determined by X-ray diffraction.

[0013] The Sm—Fe—N system crystal grains may have any composition of Sm, Fe, and N. The Sm—Fe—N system crystal grains are typically, but not limited to, SmFe ９ N １．５ or Sm ２ Fe １７ N ３ Composition of, typically Sm ２ Fe １７ N ３ The composition may be:

[0014] The average grain size of the Sm—Fe—N crystal grains in the main phase is preferably 3.0 μm or less, more preferably 2.5 μm or less. By setting the average grain size of the Sm—Fe—N crystal grains to 3.0 μm or less, the coercive force is increased. Furthermore, the average grain size of the Sm—Fe—N crystal grains is preferably 0.1 μm or more, more preferably 0.5 μm or more. By setting the average grain size of the Sm—Fe—N crystal grains to 0.1 μm or more, superparamagnetism can be suppressed. The average grain size of the Sm—Fe—N crystal grains is preferably 0.1 μm or more and 3.0 μm or less, more preferably 0.5 μm or more and 2.5 μm or less.

[0015] The method for calculating the average grain size of the Sm—Fe—N crystal grains in the main phase is as follows: First, a cross section of the magnet is photographed using a field emission scanning electron microscope (FE-SEM) so that at least 50 Sm—Fe—N crystal grains are included. Next, the total area A1 and number N1 of Sm—Fe—N crystal grains in the photographed image are determined. A1 / N1 is defined as the average cross-sectional area per Sm—Fe—N crystal grain, and the circle-equivalent diameter of the average cross-sectional area is defined as the average grain size.

[0016] (Grain boundary phase) A grain boundary phase containing a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg) exists around the main phase, and the Zn-M alloy exists in a solid phase and a liquid phase at any temperature between 200°C and 600°C.

[0017] The grain boundary phase refers to a subphase present between a plurality of Sm-Fe-N crystal grains.

[0018] In the Sm—Fe—N magnetic powder of the present disclosure, the Zn—M alloy contained in the grain boundary phase can coexist in a solid phase and a liquid phase at any temperature between 200°C and 600°C. That is, when heat-treated, particularly when pressure-sintered, the Zn—M alloy does not completely become liquid at that temperature, but remains solid, thereby preventing the Sm—Fe—N crystal grains of the main phase from coming too close to or contacting each other. This prevents the Zn—M alloy present between the Sm—Fe—N crystal grains of the main phase from being extruded and flowing out. This magnetically separates the Sm—Fe—N crystal grains, suppressing magnetization reversal and resulting in an Sm—Fe—N magnetic powder and an Sm—Fe—N sintered magnet that are resistant to demagnetization. In other words, the Sm—Fe—N magnetic powder and Sm—Fe—N sintered magnet of the present disclosure have suppressed magnetization reversal in the second quadrant and have high coercivity (Hcj), anisotropy field (Hk), and squareness (Hk / Hcj).

[0019] In the Zn-M alloy, M is Al, Sn, Ge, or Mg, and is preferably Al. By using an alloy containing Zn and the above M, a state in which a solid phase and a liquid phase coexist can be achieved at the heat treatment temperature and / or sintering temperature.

[0020] "The Zn-M alloy coexists in a solid phase and a liquid phase at any temperature between 200°C and 600°C" means that in the equilibrium diagram, there is a region in which the solid phase and the liquid phase coexist in at least a part of the range between 200°C and 600°C.

[0021] The Zn-M alloy preferably coexists in a solid phase and a liquid phase at a temperature between 350° C. and 550° C., more preferably between 400° C. and 500° C. The ability of the Zn-M alloy to coexist in a solid phase and a liquid phase within such a temperature range makes it possible to select the pressure sintering temperature from among temperatures more suitable for sintering.

[0022] The M content in the Zn-M alloy is an amount that allows the solid and liquid phases to coexist in the temperature range of 200°C to 600°C, preferably 350°C to 550°C, and more preferably 400°C to 500°C in the Zn-M equilibrium phase diagram. These equilibrium phase diagrams are equilibrium correlation diagrams at atmospheric pressure (1 atm). Figures 1 to 4 show equilibrium phase diagrams for Zn-Al, Zn-Sn, Zn-Ge, and Zn-Mg, respectively. In the equilibrium phase diagrams, solid and liquid phases coexist in the shaded regions. These equilibrium phase diagrams are described in Binary Alloy Phase Diagrams II Ed., Ed. T.B. Massalski, 1990, 3, 2571-2572, Clark, J.B., and Moser, Z.

[0023] The content of M in the Zn-M alloy is preferably greater than 5 at% and more preferably greater than 10 at%, for example, 15 at% or greater, 20 at% or greater, 25 at% or greater, or 30 at% or greater, relative to the total amount of Zn and M. By making the content of M greater than 5 at%, the temperature range in which the solid phase and the liquid phase coexist can be widened. Furthermore, the content of M in the Zn-M alloy is preferably less than 50 at%, and more preferably less than 40 at%, for example, 35 at% or less, or 30 at% or less, relative to the total amount of Zn and M. By making the content of M less than 50 at%, the effect of Zn, i.e., the effect of increasing the retention force, is further improved. The content of M in the Zn-M alloy is preferably greater than 5 at% and less than 50 at%, and more preferably greater than 10 at% and less than 40 at%, for example, 15 at% and less than 35 at%, or 20 at% and less than 30 at%.

[0024] In one embodiment, M is Al.

[0025] The Al content in the Zn—Al alloy is preferably 12 at% or more, more preferably 15 at% or more, for example, 20 at% or more, 25 at% or more, or 30 at% or more, relative to the total amount of Zn and Al. The M content in the Zn—Al alloy is preferably less than 80 at%, more preferably 70 at% or less, for example, 50 at% or less, or 30 at% or less, relative to the total amount of Zn and Al. The Al content in the Zn—Al alloy is preferably 12 at% or more and less than 80 at%, more preferably 15 at% or more and 70 at% or less, for example, 20 at% or more and 50 at% or less, or 25 at% or more and 30 at% or less, relative to the total amount of Zn and Al.

[0026] In one embodiment, M is Sn.

[0027] The Sn content in the Zn—Sn alloy is preferably 1 at% or more, more preferably 5 at% or more, for example, 10 at% or more, 15 at% or more, 20 at% or more, or 30 at% or more, relative to the total amount of Zn and Sn. Furthermore, the Sn content in the Zn—Sn alloy is preferably less than 85 at%, more preferably 50 at% or less, for example, 40 at% or less, or 30 at% or less, relative to the total amount of Zn and Sn. The Sn content in the Zn—Sn alloy is preferably 1 at% or more and less than 85 at%, more preferably 5 at% or more and 50 at% or less, for example, 10 at% or more and 40 at% or less, or 15 at% or more and 30 at% or less, relative to the total amount of Zn and Sn.

[0028] In one embodiment, M is Ge.

[0029] The Ge content in the Zn—Ge alloy is preferably 6 at% or more, more preferably 10 at% or more, for example, 15 at% or more, 20 at% or more, 25 at% or more, or 30 at% or more, relative to the total amount of Zn and Ge. The Ge content in the Zn—Ge alloy is preferably less than 50 at%, more preferably 40 at% or less, for example, 35 at% or less, or 30 at% or less, relative to the total amount of Zn and Ge. The Ge content in the Zn—Ge alloy is preferably 6 at% or more and less than 50 at%, more preferably 10 at% or more and 40 at% or less, for example, 15 at% or more and 35 at% or less, or 20 at% or more and 30 at% or less, relative to the total amount of Zn and Ge.

[0030] In one embodiment, M is Mg.

[0031] The Mg content in the Zn—Mg alloy is preferably 8 at% or more, more preferably 10 at% or more, for example, 15 at% or more, 20 at% or more, 25 at% or more, or 30 at% or more, relative to the total amount of Zn and Mg. Furthermore, the Mg content in the Zn—Mg alloy is preferably less than 95 at%, more preferably 50 at% or less, for example, 30 at% or less, or 20 at% or less, relative to the total amount of Zn and Mg. The Mg content in the Zn—Mg alloy is preferably 8 at% or more and less than 95 at%, more preferably 10 at% or more and 50 at% or less, for example, 15 at% or more and 30 at% or less, relative to the total amount of Zn and Mg.

[0032] The content of M in the Zn-M alloy can be measured by SEM-EDX analysis.

[0033] The content of the Zn-M alloy is preferably 20 wt % or less, more preferably 10 wt % or less, for example, 8 wt % or less, or 5 wt % or less, relative to the total amount of Sm, Fe, and N in the main phase. The content of the Zn-M alloy is preferably 1 wt % or more, more preferably 3 wt % or more, relative to the total amount of Sm, Fe, and N in the main phase. The content of the Zn-M alloy is preferably 1 wt % or more and 20 wt % or less, more preferably 3 wt % or more and 10 wt % or less, for example, 3 wt % or more and 8 wt % or less, or 3 wt % or more and 5 wt % or less, relative to the total amount of Sm, Fe, and N in the main phase. By keeping the content of the Zn-M alloy within the above range, it is possible to obtain Sm-Fe-N magnetic powder and Sm-Fe-N sintered magnets that have high coercivity while minimizing the decrease in magnetization that occurs with an increase in nonmagnetic components.

[0034] The content of the Zn-M alloy can be measured by SEM-EDX analysis.

[0035] The EDX analysis is carried out, for example, under the following conditions.

[0036]

[0037] An image using a SEM (scanning electron microscope) for EDX analysis can be obtained, for example, under the following conditions.

[0038]

[0039] The grain boundary phase may include a reaction phase resulting from a reaction between Zn and the Sm—Fe—N crystal grains of the main phase. The reaction phase may be present so as to cover the entire Sm—Fe—N crystal grains, or may be present on only part of the surface of the Sm—Fe—N crystal grains. The presence of this reaction phase in the Sm—Fe—N magnetic powder of the present disclosure makes it possible to obtain an Sm—Fe—N magnetic powder and an Sm—Fe—N sintered magnet with high coercivity.

[0040] (Sm—Fe—N Sintered Magnet) The Sm—Fe—N sintered magnet of the present disclosure includes a sintered body of a material containing the above-described Sm—Fe—N magnetic powder of the present disclosure.

[0041] The Sm—Fe—N sintered magnet of the present disclosure is obtained by sintering a material containing Sm—Fe—N magnetic powder at high temperature.

[0042] Sm—Fe—N sintered magnets contain at least Sm—Fe—N crystal grains and a Zn-M alloy. Sintered magnets may also contain other materials, such as α-Fe, and unavoidably mixed trace elements, such as carbon (C), silicon (Si), and aluminum (Al).

[0043] The sintered magnet may include a sintered body of another magnetic powder. Examples of the other magnetic powder 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, 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).

[0044] (Manufacturing Method) The Sm—Fe—N sintered magnet of the present disclosure includes: mixing a magnetic powder containing Sm—Fe—N crystal grains with a powder of a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg) to obtain a raw material mixture; heat-treating the raw material mixture to obtain an Sm—Fe—N magnetic powder; and pressure-sintering the Sm—Fe—N magnetic powder. The temperature T1 in the heat treatment is higher than the temperature T2 in the pressure-sintering, and the Zn-M alloy is in a state where a solid phase and a liquid phase coexist at T1 and a state where a solid phase and a liquid phase coexist at T2 or is in a completely solid phase state; or is in a completely liquid phase at T1 and a state where a solid phase and a liquid phase coexist at T2 or is in a completely solid phase state.

[0045] An example of a method for producing a sintered Sm—Fe—N magnet according to the present disclosure is shown below: Fig. 5 is a flowchart showing an example of a method for producing a sintered Sm—Fe—N magnet according to the present disclosure.

[0046] (1) Preparation of Alloy Powder (S11) Alloy powder containing Sm and Fe (Sm—Fe alloy) is prepared, for example, by a reduction diffusion method using its precursor powder.

[0047] In the Sm—Fe alloy powder, the amount of Sm relative to the total amount of Sm and Fe may be, for example, 9 atomic % or more and 14 atomic % or less, and the average particle size of the alloy powder may be, for example, 1 μm or more and 50 μm or less.

[0048] 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.

[0049] 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.

[0050] (2) 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 precursor powder.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] Prior to the nitriding treatment, 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, crushing, and classification are performed under conditions such that the average particle size of the resulting Sm—Fe—N-based precursor powder is preferably 0.1 μm or more and 10 μm or less.

[0055] The crushing or pulverizing can be carried out using a mortar, a jet mill, a ball mill, a vibration mill, a planetary mill, or the like, but is not limited to these.

[0056] After the nitriding treatment, the Sm—Fe—N-based precursor powder may be washed with water. By-products (CaO, unreacted Ca, etc.) contained in the Sm—Fe—N-based precursor powder can be physically removed by washing. By-products can also be removed by chemical reaction with water. For example, Ca reacts with water to become calcium hydroxide, which dissolves in water.

[0057] The washing is carried out, for example, by putting the Sm—Fe—N precursor powder into water and stirring the mixture, then stopping the stirring and taking out the resulting precipitate, which is then dried.

[0058] Washing may be performed multiple times. For example, the Sm—Fe—N precursor powder is placed in water, stirred, and then allowed to stand, the supernatant liquid is removed, and new water is added. The above-described procedure 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 to 10 times.

[0059] Prior to this washing, the magnetic powder may be washed with acetic acid or hydrochloric acid, etc. This will further remove any remaining Ca.

[0060] 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.

[0061] (3) Addition of low melting point metal (Zn-M alloy) (S13) Zn-M alloy powder is added to the Sm-Fe-N precursor powder.

[0062] The amount of the Zn-M alloy added is, for example, preferably 20 wt % or less, more preferably 10 wt % or less, for example, 8 wt % or less, or 5 wt % or less, of the Sm-Fe-N precursor powder. The amount of the Zn-M alloy added is, for example, preferably 1 wt % or more, more preferably 3 wt % or more, of the Sm-Fe-N precursor powder.

[0063] Methods for producing Zn-M alloys include, but are not limited to, atomization, rotating electrode, thermal plasma, PVD or CVD, liquid phase methods, and the like.

[0064] (4) Mixing (S14) The Sm-Fe-N precursor powder and the Zn-M alloy powder are mixed together. The mixing method can be, but is not limited to, a mortar mixer, a ball mill, a rocking mixer, or the like.

[0065] (5) Heat Treatment (S15) The mixture of the Sm-Fe-N precursor powder and the Zn-M alloy powder is heat treated to obtain Sm-Fe-N magnetic powder.

[0066] The temperature T1 in the heat treatment may be, for example, 200 to 600°C, preferably 300 to 600°C, more preferably 400 to 550°C, and particularly preferably 450 to 500°C.

[0067] (6) Crushing (or Pulverization) (S16) After the heat treatment, the Sm—Fe—N based magnetic powder may be crushed (or pulverized). Furthermore, the Sm—Fe—N based magnetic powder may be classified. Crushing (or pulverization) and classification may be performed in the same manner as the treatment for the Sm—Fe based precursor powder.

[0068] (7) Magnetic Field Orientation (S17) Next, an orientation process, a magnetization process, and a molding process may be performed. This process involves molding the magnetic material while applying a magnetic field (hereinafter also referred to as "magnetic field molding"). For magnetic field molding, for example, a powder press equipped with a magnetic field generator is used. Magnetic field molding aligns the easy magnetization axis of the Sm-Fe-N magnetic powder, resulting in higher magnetic properties.

[0069] 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.

[0070] (8) Pressure Sintering (S18) The material containing the Sm—Fe—N magnetic powder is filled into a mold and 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.

[0071] Pressure sintering is carried out in a sintering machine in a vacuum or in the presence of an inert gas. This suppresses oxidation of the magnetic material during sintering, resulting in a magnet with high coercivity. While introducing an inert gas into the sintering machine, the gas inside the sintering machine may be discharged to form a reduced pressure atmosphere containing the inert gas.

[0072] 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.

[0073] Any pressure sintering method, including electric pressure sintering, can be used for pressure sintering. Pressure sintering may be performed, for example, by hot pressing or electric current sintering. The pressure applied 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, preferably 1000 MPa to 1500 MPa, and more preferably 1200 MPa to 1400 MPa. The pressure sintering time is, for example, 30 seconds to 10 minutes.

[0074] The temperature T2 in pressure sintering may be, for example, 200 to 600°C, preferably 300 to 550°C, more preferably 350 to 500°C, and particularly preferably 400 to 500°C.

[0075] The temperature T1 in the heat treatment is higher than the temperature T2 in the pressure sintering. The Zn-M alloy is in a state where a solid phase and a liquid phase coexist at T1, and in a state where a solid phase and a liquid phase coexist or a completely solid phase at T2, or in a completely liquid phase at T1, and in a state where a solid phase and a liquid phase coexist or a completely solid phase at T2. That is, the Zn-M alloy is (i) in a state where a solid phase and a liquid phase coexist at heat treatment and a state where a solid phase and a liquid phase coexist at pressure sintering, (ii) in a state where a solid phase and a liquid phase coexist at heat treatment and a completely solid phase at pressure sintering, (iii) in a state where a solid phase and a liquid phase coexist at heat treatment and a state where a solid phase and a liquid phase coexist at pressure sintering, or (iv) in a state where a liquid phase is completely at heat treatment and a completely solid phase at pressure sintering. The Zn-M alloy preferably satisfies any one of the above (i) to (iii), and more preferably satisfies the above (i).

[0076] All of the above steps are preferably 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.

[0077] After cleaning and before pressure sintering, for the sake of material handling, 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.

[0078] The present disclosure is not limited to the above-described embodiments, and design modifications are possible within the scope of the present disclosure.

[0079] 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.

[0080] (Sm—Fe—N-based precursor powder) 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.

[0081] The obtained Sm--Fe alloy was crushed in an agate mortar to obtain Sm--Fe alloy powder having an average particle size of 4.83 μm.

[0082] The obtained Sm--Fe alloy powder was heat-treated at 475° C. for 23 hours in a nitrogen atmosphere and then washed to obtain a Sm--Fe--N based precursor powder.

[0083] (Zn—Al Alloy) The following two types of Zn—Al alloy were prepared by disk atomization: Zn—Al alloy 1: Zn / Al=62.27 at% / 37.73 at% Zn—Al alloy 2: Zn / Al=78.78 at% / 21.22 at%

[0084] (Heat Treatment) The Sm—Fe—N precursor powder and Zn—Al alloy prepared above were mixed in a ball mill and heat-treated at the temperatures shown in the table below to obtain Sm—Fe—N magnetic powder. Note that if the heat treatment temperature is not listed, no heat treatment was performed.

[0085] (Crushing) Next, the Sm—Fe—N magnetic powder was crushed using an airflow crushing jet mill. Note that the magnetic material that was not subjected to the heat treatment was crushed as it was. In addition, some of the Sm—Fe—N magnetic powder was not crushed.

[0086] (Magnetic Field Orientation) Next, the obtained magnetic powder was immersed in heptane to prepare a slurry. The prepared slurry was filled into a cemented carbide mold. The cemented carbide mold filled with the slurry was set in a press, and magnetic field orientation was performed by pressure molding while applying a magnetic field of 1.5 T or more.

[0087] (Pressure Sintering) This mold was placed in a pulse current sintering machine equipped with a pressure mechanism using a servo-controlled press device.

[0088] The pulse current sintering machine was evacuated while introducing Ar gas into it, and the pressure inside the machine was maintained at the pressure shown in the table below. A pressure of 1250 MPa was applied to the mold, and while maintaining this pressure, current sintering was carried out for 2 minutes at the temperature shown in the table below to obtain a sintered magnet.

[0089] All processes from crushing to filling during pressure sintering were carried out in a glove box (nitrogen substituted) connected to a gas circulation type oxygen and moisture purifier. The oxygen concentration in the glove box was set to 2 ppm or less. The samples were moved between each device without being exposed to the atmosphere.

[0090] [Evaluation] (Measurement of magnetic properties (coercive force, residual magnetization, Hk)) The obtained sintered body was magnetized by applying an external magnetic field of 9 T in the direction of easy magnetization at room temperature. The residual magnetization, coercive force, and Hk of this magnetized sintered body were measured using a vibrating magnetometer (VSM).

[0091] (Measurement of Density) The density was calculated from the volume calculated from the outer dimensions of the sample and the weight of the sintered body actually obtained.

[0092]

[0093] The present disclosure includes the following aspects. [Item 1] A sintered magnet comprising a sintered body of a material containing Sm—Fe—N magnetic powder, the Sm—Fe—N magnetic powder comprising a main phase containing Sm, Fe, and N, and a grain boundary phase surrounding the main phase and containing a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg), the Zn-M alloy existing in a solid phase and a liquid phase at temperatures between 200°C and 600°C. [Item 2] The sintered magnet according to Item 1, wherein the content of the Zn-M alloy is 10 wt % or less relative to the total amount of Sm, Fe, and N in the main phase. [Item 3] The sintered magnet according to Item 1 or 2, wherein M is Al. [Item 4] The sintered magnet according to Item 3, wherein the content of Al in the Zn—Al alloy is 12 at % or more relative to the total amount of Zn and Al. [Item 5] The sintered magnet according to any one of Items 1 to 4, wherein the average grain size of the Sm—Fe—N crystal grains in the main phase is 3.0 μm or less. [Item 6] A method for producing an Sm—Fe—N sintered magnet, comprising: mixing a magnetic powder containing Sm—Fe—N crystal grains with a powder of a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg) to obtain a raw material mixture; heat-treating the raw material mixture to obtain an Sm—Fe—N magnetic powder; and pressure-sintering the Sm—Fe—N magnetic powder, wherein a temperature T1 in the heat treatment is higher than a temperature T2 in the pressure-sintering, and the Zn-M alloy is in a state where a solid phase and a liquid phase coexist at T1, and a state where a solid phase and a liquid phase coexist at T2, or is in a completely solid phase state; or is in a completely liquid phase at T1, and a state where a solid phase and a liquid phase coexist at T2, or is in a completely solid phase state. [Item 7] The method for producing a Sm—Fe—N sintered magnet according to Item 6, wherein the temperature T2 during the pressure sintering is 400° C. or higher and 500° C. or lower.

[0094] The Sm—Fe—N magnetic powder and Sm—Fe—N sintered magnet of the present disclosure 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.

Claims

1. A sintered magnet comprising a sintered body of a material containing Sm-Fe-N magnetic powder, the Sm-Fe-N magnetic powder comprising a main phase containing Sm, Fe, and N, and a grain boundary phase surrounding the main phase, the grain boundary phase containing a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg), the Zn-M alloy existing in both solid and liquid phases at any temperature between 200°C and 600°C.

2. The sintered magnet according to claim 1, wherein the content of said Zn-M alloy is 10 wt % or less relative to the total amount of Sm, Fe, and N in said main phase.

3. The sintered magnet according to claim 1 or 2, wherein M is Al.

4. The sintered magnet according to claim 3, wherein the Al content in the Zn-Al alloy is 12 at % or more relative to the total amount of Zn and Al.

5. The sintered magnet according to any one of claims 1 to 4, wherein the average grain size of the Sm-Fe-N crystal grains in the main phase is 3.0 µm or less.

6. A method for producing an Sm-Fe-N sintered magnet, comprising: mixing a magnetic powder containing Sm-Fe-N crystal grains with a powder of a Zn-M alloy (wherein M is Al, Sn, Ge, or Mg) to obtain a raw material mixture; heat-treating the raw material mixture to obtain an Sm-Fe-N magnetic powder; and pressure-sintering the Sm-Fe-N magnetic powder, wherein temperature T1 in the heat treatment is higher than temperature T2 in the pressure-sintering, and the Zn-M alloy is in a state where a solid phase and a liquid phase coexist at T1 and a state where a solid phase and a liquid phase coexist at T2 or is in a completely solid phase state, or is in a completely liquid phase at T1 and a state where a solid phase and a liquid phase coexist at T2 or is in a completely solid phase state.

7. The method for producing a Sm-Fe-N sintered magnet according to claim 6, wherein the temperature T2 during the pressure sintering is 400°C or higher and 500°C or lower.

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