Sm-Fe-N magnet

Abstract

To provide an Sm-Fe-N system magnet having high coercive force comparable to Sm-Fe-N system powder.SOLUTION: An Sm-Fe-N system magnet includes Sm-Fe-N-based particles 110 having a surface, and a coating layer 120 disposed on at least a part of the surface and / or at least a part of the interface between the Sm-Fe-N-based particles. The coating layer has a first layer 122 and a second layer 124 in order of proximity to the surface or interface. The first layer has α-Fe, the second layer has a Sm-Fe-Zn alloy, and the Zn content is 1 at% or more and 20 at% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an Sm—Fe—N based magnet. [Background technology]

[0002] Sm-Fe-N magnets are expected to be high-performance magnets because they have a high Curie temperature of 477°C, small temperature changes in their magnetic properties, and a high anisotropy field of 20.6 MA / m, which is the theoretical limit of coercivity.

[0003] Patent Document 1 describes a method for producing fine Sm—Fe—N powder by reducing and diffusing a precursor powder of an Sm—Fe alloy to form an alloy powder, and then nitriding the alloy powder.

[0004] To produce a high-performance magnet from magnetic powder with high coercivity, it is necessary to sinter the Sm-Fe-N powder.

[0005] However, Sm-Fe-N powder suffers from the problem of its magnetic properties deteriorating when sintered at high temperatures, particularly the coercive force of Sm-Fe-N magnets, which is significantly reduced by the sintering process.

[0006] Furthermore, Patent Document 2 proposes that the surface of Sm-Fe-N powder be coated with a subphase containing a metal such as zirconium to prevent a decrease in the coercive force of the magnet obtained after sintering. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 150557 [Patent Document 2] International Publication No. 2019 / 189440 Summary of the Invention [Problem to be solved by the invention]

[0008] According to experiments conducted by the present inventors, even when the methods described in Patent Documents 1 and 2 are used, the coercive force of the Sm-Fe-N magnets produced is still lower than that of powder, and it is difficult to say that conventional countermeasures are sufficient.

[0009] Therefore, there is a demand for more effective measures that can suppress the decrease in coercivity of Sm-Fe-N magnets.

[0010] The present invention has been made in view of the above background, and an object of the present invention is to provide an Sm—Fe—N based magnet having a high coercive force comparable to that of Sm—Fe—N based powder. [Means for solving the problem]

[0011] In the present invention, A Sm-Fe-N magnet, Sm-Fe-N based particles having a surface; a coating layer disposed on at least a portion of the surface and / or at least a portion of the interface between the Sm—Fe—N based particles; and the coating layer has a first layer and a second layer in order of proximity to the surface or interface, the first layer comprises α-Fe; the second layer comprises a Sm-Fe-Zn alloy; The Sm-Fe-N magnet is provided, wherein the Zn content in the second layer is 1 at % or more and 20 at % or less. [Effects of the Invention]

[0012] The present invention can provide an Sm-Fe-N magnet that has a high coercive force comparable to that of Sm-Fe-N powder. [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is a diagram showing a schematic example of the cross-sectional structure of an Sm—Fe—N based particle contained in an Sm—Fe—N based magnet according to one embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing an example of a flow chart of a method for producing an Sm—Fe—N based magnet according to an embodiment of the present invention. [Figure 3] FIG. 1 is a diagram showing a schematic diagram of an example of a flow of a method for producing magnetic powder in a method for producing an Sm—Fe—N based magnet according to an embodiment of the present invention. [Figure 4] FIG. 1 shows a high-angle annular dark-field (HAADF) image of a cross section of a Sm—Fe—N magnet according to one embodiment of the present invention, and the mapping results of each element obtained by EDS (energy dispersive X-ray spectroscopy). [Figure 5] FIG. 1 is a TEM image (bright-field image) of an enlarged portion of an interface between Sm—Fe—N based particles in an Sm—Fe—N based magnet according to one embodiment of the present invention. [Figure 6] FIG. 2 shows the results of mapping Fe and Zn obtained by EDS analysis of an enlarged portion of the interface between Sm—Fe—N based particles in an Sm—Fe—N based magnet according to one embodiment of the present invention. [Figure 7] FIG. 2 is an electron diffraction image of an Fe-rich layer in an enlarged portion of an interface between Sm—Fe—N based particles in an Sm—Fe—N based magnet according to one embodiment of the present invention. [Figure 8] FIG. 1 shows an example of the results of EDS line analysis of the interface between Sm—Fe—N based particles in an Sm—Fe—N based magnet according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the present invention will be described below.

[0015] In one embodiment of the present invention, A Sm-Fe-N magnet, Sm-Fe-N based particles having a surface; a coating layer disposed on at least a portion of the surface and / or at least a portion of the interface between the Sm—Fe—N based particles; and the coating layer has a first layer and a second layer in order of proximity to the surface or interface, the first layer comprises α-Fe; the second layer comprises a Sm-Fe-Zn alloy; The Sm-Fe-N magnet is provided, wherein the Zn content in the second layer is 1 at % or more and 20 at % or less.

[0016] In this application, the "surface" of a particle contained in a magnet means the surface of the particle excluding the interface where the particle is bonded to other particles, that is, the "exposed surface."

[0017] In this application, the term "layer" refers to a coating that covers an object, where the ratio (t / L) of the maximum dimension t in the normal (thickness) direction to the coating length L in a cross-sectional view is less than 1. Therefore, the term "layer" includes not only a "full coating layer" that covers the entire surface of a particle, but also an "intermittent layer" that covers the surface of a particle intermittently, and a "local layer" that covers only a portion of the surface of a particle. However, even in a "local layer," the coating length L is 50 nm or more.

[0018] As mentioned above, Sm-Fe-N powder suffers from the problem of its magnetic properties deteriorating when sintered at high temperatures. In particular, the coercive force of Sm-Fe-N magnets is significantly reduced by the sintering process.

[0019] In contrast, in one embodiment of the present invention, a coating layer comprising a first layer and a second layer is formed on the surfaces of the Sm-Fe-N particles contained in the magnet and / or on at least a portion of the interfaces between the Sm-Fe-N particles, where the first layer comprises α-Fe and the second layer comprises an Sm-Fe-Zn alloy.

[0020] By adopting such a configuration, one embodiment of the present invention can provide an Sm-Fe-N based magnet having a high coercive force comparable to that of Sm-Fe-N based powder.

[0021] At present, the reason why the magnet configuration according to one embodiment of the present invention can suppress a decrease in coercive force has not been fully elucidated.

[0022] However, the following mechanism is suggested: Generally, when Sm-Fe-N particles are heat-treated, clumps of α-Fe often form on the surface of the particles. If such α-Fe is present on the particle surface, when a magnetic field is applied to the Sm-Fe-N magnet, the magnetization of the Sm-Fe-N particles will be reversed, resulting in a decrease in the coercive force of the magnet.

[0023] On the other hand, in one embodiment of the present invention, the α-Fe is formed in a "layered" state rather than a bulk state. A thin "layered" state is less likely to act as a starting point for magnetization reversal than bulk α-Fe. As a result, in one embodiment of the present invention, it is believed that a decrease in coercivity is suppressed in the magnet produced.

[0024] In one embodiment of the present invention, a second layer is formed to cover the first layer, and it is believed that the presence of this second layer suppresses magnetization reversal caused by the first layer, thereby suppressing a decrease in coercivity.

[0025] Furthermore, the second layer is expected to have the effect of repairing damage to the particle surface. That is, if scratches or defects occur on the particle surface during the preparation process of Sm-Fe-N based particles, such damage may adversely affect the magnetic properties of the manufactured magnet. However, in one embodiment of the present invention, the surface of the Sm-Fe-N based particles is coated with the second layer, so such damage is repaired and the effect of surface damage on the magnetic properties is thought to be reduced.

[0026] In any case, in one embodiment of the present invention, by forming a coating layer on the surface of the Sm-Fe-N based particles and / or on the interface between the particles, it is possible to provide an Sm-Fe-N based magnet with high coercivity.

[0027] (Sm-Fe-N based magnet according to one embodiment of the present invention) Hereinafter, an embodiment of the present invention will be described in more detail with reference to the drawings.

[0028] FIG. 1 shows a schematic diagram of an example of a cross section of an Sm—Fe—N based particle contained in an Sm—Fe—N based magnet according to one embodiment of the present invention.

[0029] Sm-Fe-N magnets are produced by sintering Sm-Fe-N particles together, so in reality, each Sm-Fe-N particle is at least partially bonded to another Sm-Fe-N particle within the Sm-Fe-N magnet.

[0030] Therefore, it should be noted that FIG. 1 is a hypothetical shape of one Sm—Fe—N based particle, drawn for the sake of clarity of explanation.

[0031] 1, an Sm—Fe—N based particle 110 contained in an Sm—Fe—N based magnet has a surface 112. A coating layer 120 is disposed on at least a portion of the surface 112.

[0032] The coating layer 120 has a first layer 122 and a second layer 124 in order of proximity to the surface 112 of the Sm—Fe—N based particle 110 .

[0033] The first layer 122 is composed of a phase mainly composed of α-Fe.

[0034] The second layer 124 is composed of a phase mainly made of an Sm-Fe-Zn alloy. The amount of Zn contained in the second layer 124 is 1 at% or more and 20 at% or less, and preferably 5 at% or more and 15 at% or less.

[0035] By forming such a coating layer 120 on the surface 112 of the Sm—Fe—N based particles 110, the coercive force of the magnet can be increased.

[0036] 1, the coating layer 120 is provided over the entire surface 112 of the Sm-Fe-N-based particle 110. However, as described above, the coating layer 120 may be provided only on a part of the surface 112 of the Sm-Fe-N-based particle 110.

[0037] For example, the coverage of the coating layer 120 with respect to the surface 112 of the Sm-Fe-N based particle 110 is 20% or more, preferably 40% or more, and more preferably 60% or more. On the other hand, the coverage of the coating layer 124 with respect to the surface 112 of the Sm-Fe-N based particle 110 or the surface of the first layer 122 is 50% or more, preferably 70% or more, and more preferably 90% or more.

[0038] The coverage is the coverage of the surface of the Sm-Fe-N particle 110, excluding the bonding interface with other particles, in cross section, that is, the coverage of the exposed surface. In the present application, the "coverage" refers to the average value of the coverage measured for 20 particles.

[0039] Alternatively, as described above, the coating layer 120 may be formed on at least a part of the interface between two Sm—Fe—N based particles 110 that are bonded to each other.

[0040] In this case, the coating layer 120 may be formed on, for example, about 20% or more of the interface, preferably 40% or more, and more preferably 60% or more. On the other hand, the coverage of the coating layer 124 with respect to the surface 112 of the Sm-Fe-N particle 110 or the surface of the first layer 122 is 50% or more, preferably 70% or more, and more preferably 90% or more.

[0041] As mentioned above, an Sm-Fe-N magnet having such a structure has a significantly high coercive force.

[0042] For example, the coercive force of an Sm—Fe—N based magnet according to one embodiment of the present invention may be 25 kOe or more.

[0043] The Sm-Fe-N magnet according to one embodiment of the present invention may contain Zn in the range of 1 wt% to 20 wt%. The oxygen content of the Sm-Fe-N magnet according to one embodiment of the present invention is preferably less than 1.0 wt%, and more preferably less than 0.8 wt%.

[0044] (Details of each part) Next, each part included in the Sm-Fe-N based magnet according to one embodiment of the present invention will be described in more detail.

[0045] (Sm-Fe-N particles 110) The Sm—Fe—N based particles 110 are particles containing Sm (samarium), Fe (iron), and N (nitrogen).

[0046] The Sm—Fe—N-based particles 110 may contain other additional elements, such as at least one selected from the group consisting of rare earth elements (excluding samarium), such as neodymium and praseodymium, and cobalt.

[0047] The total content of the additional elements is preferably less than 30 at % in terms of anisotropic magnetic field and magnetization.

[0048] The average particle size of the Sm-Fe-N particles 110 is preferably less than 2.0 μm. If the average particle size of the Sm-Fe-N particles 110 is less than 2.0 μm, the coercive force of the Sm-Fe-N magnet will be even higher. Furthermore, the average particle size of the Sm-Fe-N particles 110 is preferably greater than 0.1 μm. If the average particle size of the Sm-Fe-N particles 110 is 0.1 μm or less, it becomes difficult to suppress oxidation of the Sm-Fe-N particles, and heterogeneous phases are more likely to occur in the magnet according to one embodiment of the present invention.

[0049] In a magnet according to one embodiment of the present invention, the proportion of Sm-Fe-N based particles 110 with an aspect ratio of 2.0 or more is preferably 10% or less, and more preferably 8% or less. When the proportion of Sm-Fe-N based particles 110 with an aspect ratio of 2.0 or more is 10% or less, the coercive force of the magnet according to one embodiment of the present invention is further increased.

[0050] The amount of oxygen contained in the Sm—Fe—N particles 110 is, for example, less than 1 wt %, and preferably less than 0.8 wt %. If the oxygen content is high, heterogeneous phases are more likely to occur in the magnet according to one embodiment of the present invention.

[0051] (Coating layer 120) The coating layer 120 is disposed on at least a portion of the surface 112 of the Sm—Fe—N based particle 110. Alternatively, the coating layer 120 may be disposed on at least a portion of the interface between two Sm—Fe—N based particles 110 bonded to each other.

[0052] In particular, the coating layer 120 is preferably disposed on both the surface 112 of each Sm—Fe—N based particle 110 and the interface between two bonded Sm—Fe—N based particles 110 .

[0053] The thickness of the coating layer 120 is, for example, in the range of 2 nm or more and 200 nm or less, and preferably in the range of 21 nm or more and 71 nm or less.

[0054] In the present application, the layer thickness means the average thickness measured at 20 locations.

[0055] (First layer 122) The first layer 122 is composed of a phase mainly containing α-Fe. The first layer 122 may further contain a trace amount of Zn.

[0056] The thickness of the first layer is, for example, in the range of 1 nm or more and 100 nm or less, and preferably in the range of 1 nm or more and 21 nm or less.

[0057] It should be noted that the first layer 122 does not necessarily have to be disposed over the entire area below the area where the second layer 124 is disposed. In other words, there may be an area below the second layer 124 where the first layer 122 is not present. However, such an area is not referred to as the "coating layer 120" but simply as an area where the Sm-Fe-Zn alloy layer is disposed.

[0058] (Second layer 124) The second layer 124 is composed of a phase mainly made of an Sm-Fe-Zn alloy. The amount of Zn contained in the second layer 124 is in the range of 1 at % to 20 at %, and preferably in the range of 5 at % to 15 at %.

[0059] The second layer 124 may further include elements such as oxygen, nitrogen, and carbon.

[0060] The crystal structure of the second layer 124 may be, but is not limited to, SmFe2, SmFe3, SmFe5, Sm2Fe 17 , SmFe7, or SmFe 12 Alternatively, the second layer 124 may be amorphous.

[0061] The thickness of the second layer is, for example, in the range of 1 nm or more and 100 nm or less, and preferably in the range of 20 nm or more and 50 nm or less.

[0062] (Method of manufacturing an Sm-Fe-N magnet according to one embodiment of the present invention) Next, with reference to FIG. 2, a method for producing an Sm—Fe—N magnet according to one embodiment of the present invention will be described in more detail.

[0063] FIG. 2 shows a schematic diagram of an example of the flow of a method for producing an Sm—Fe—N based magnet according to one embodiment of the present invention (hereinafter referred to as the “first method”).

[0064] As shown in Figure 2, the first method is A step of preparing Sm-Fe-N based magnet powder (step S110); A step (step S120) of mixing the magnet powder with Zn powder to prepare a mixed powder; A step (step S130) of molding the mixed powder to obtain a molded body; A step of sintering the compact (step S140); It has.

[0065] Each step will be described below.

[0066] (Step S110) First, Sm-Fe-N based magnet powder is prepared.

[0067] The method for producing the magnet powder is not particularly limited.

[0068] An example of a method for producing magnet powder will be described below with reference to FIG.

[0069] FIG. 3 shows a schematic flow of the method for producing Sm—Fe—N based magnetic powder.

[0070] As shown in Figure 3, the manufacturing method of this magnetic powder is as follows: A step (S10) of preparing a precursor powder of an Sm-Fe alloy; a step (S20) of reducing and diffusing the precursor powder in an inert gas atmosphere to produce an Sm-Fe alloy powder; a step (S30) of nitriding the Sm-Fe-based alloy powder to produce Sm-Fe-N-based alloy powder; a step (S40) of washing the Sm-Fe-N alloy powder; It has.

[0071] Each step will be briefly described below.

[0072] (Process S10) First, a precursor powder of an Sm-Fe alloy is prepared.

[0073] The precursor powder may be, for example, an Sm—Fe-based oxide powder or an Sm—Fe-based hydroxide powder, which will hereinafter be collectively referred to as Sm—Fe-based (hydr)oxide powder.

[0074] The precursor powder may be prepared, for example, by a coprecipitation method. In this method, a precipitant such as an alkali is first added to a solution containing a samarium salt and an iron salt to cause precipitation, and the precipitate is then collected by filtration, centrifugation, or the like. Next, the precipitate is washed, dried, and then pulverized to obtain a Sm-Fe (hydr)oxide powder.

[0075] However, since the magnetic properties of Sm-Fe-N magnet powder deteriorate when it contains metallic iron, it is preferable to add samarium in excess of the stoichiometric ratio when producing the precursor powder.

[0076] The counter ions in the samarium salt and iron salt may be inorganic ions such as chloride ions, sulfate ions, and nitrate ions, or organic ions such as alkoxides.

[0077] The solvent contained in the solution containing the samarium salt and the iron salt can be water, but an organic solvent such as ethanol may also be used.

[0078] As the alkali, hydroxides of alkali metals and alkaline earth metals, and ammonia can be used. Compounds such as urea that decompose under external influence such as heat and act as a precipitant can also be used.

[0079] The obtained precursor powder may then be handled in an atmosphere-shielded environment, such as a glove box, until the Sm-Fe-N magnet powder is produced. When an inert gas atmosphere is used, the oxygen concentration is preferably 1 ppm or less.

[0080] The obtained precursor powder is preferably pre-reduced in a reducing atmosphere, which can reduce the amount of calcium used in the subsequent reduction-diffusion step (step S20) and suppress the generation of coarse Sm-Fe alloy particles.

[0081] The pre-reduction of the precursor powder may be carried out, for example, by heating the precursor powder to 400° C. or higher in a hydrogen atmosphere. The treatment temperature is preferably in the range of 500° C. to 800° C. When the pre-reduction is carried out in this temperature range, a Sm—Fe alloy powder with a uniform particle size can be obtained.

[0082] (Process S20) Next, the precursor powder is subjected to a reduction diffusion treatment under an inert gas atmosphere.

[0083] Examples of methods for reducing and diffusing the precursor powder include a method in which the precursor powder is mixed with calcium (Ca) or calcium hydride (CaH2) and then heated to a temperature equal to or higher than the melting point of Ca (approximately 842°C).

[0084] During this treatment, Sm reduced by Ca diffuses in the Ca melt and reacts with Fe to form Sm-Fe alloy powder.

[0085] There is a correlation between the temperature of the reduction diffusion treatment and the particle size of the Sm-Fe alloy powder, and the higher the temperature of the reduction diffusion, the larger the particle size of the Sm-Fe alloy powder.

[0086] The average particle size of the Sm-Fe alloy powder is preferably less than 2.0 μm. When the average particle size of the Sm-Fe alloy powder is less than 2.0 μm, the coercive force of the magnet is further increased. Furthermore, the average particle size of the Sm-Fe alloy powder is preferably greater than 0.1 μm and less than 2.0 μm.

[0087] In order to obtain Sm—Fe alloy powder with a uniform particle size, it is preferable to subject the precursor powder to reduction diffusion treatment in an inert gas atmosphere at 850° C. to 1050° C. for about 1 minute to 2 hours.

[0088] As the reduction and diffusion in the precursor progresses, crystallization progresses, and an Sm-Fe alloy powder is formed. In the resulting Sm-Fe alloy powder, an Sm-rich phase is formed on at least a portion of the surface of each particle.

[0089] In the Sm-Fe alloy powder, the proportion of particles with an aspect ratio of 2.0 or more is preferably 10% or less, and more preferably 8% or less. When the proportion of particles with an aspect ratio of 2.0 or more is 10% or less, the coercive force of the magnetic powder is further increased.

[0090] The residual oxygen content in the Sm-Fe alloy powder obtained after step S20 is preferably less than 1.0 wt%. If the residual oxygen content in the Sm-Fe alloy powder is less than 1.0 wt%, the coercive force of the magnet will be even higher.

[0091] (Process S30) Next, the obtained Sm-Fe alloy powder is subjected to a nitriding treatment.

[0092] Examples of a method for nitriding Sm-Fe alloy powder include a method in which the Sm-Fe alloy powder is heat-treated at 300°C to 500°C in an atmosphere of ammonia, a mixed gas of ammonia and hydrogen, nitrogen, or a mixed gas of nitrogen and hydrogen.

[0093] Using ammonia allows for the nitriding of Sm-Fe alloy powder in a short time. However, the nitrogen content in the Sm-Fe-N magnet powder may be higher than optimal. In this case, it is preferable to anneal the Sm-Fe alloy powder in hydrogen after nitriding. This allows excess nitrogen to be expelled from the crystal lattice.

[0094] The nitriding treatment produces Sm-Fe-N alloy powder.

[0095] The composition of the particles contained in the Sm-Fe-N alloy powder is Sm2Fe 17 N3 is preferred.

[0096] For example, Sm-Fe alloy powder is heat-treated in an ammonia-hydrogen mixed atmosphere at 350-450°C for 10 minutes to 2 hours, and then annealed in a hydrogen atmosphere at 350-450°C for 30 minutes to 2 hours. This allows the nitrogen content in the Sm-Fe-N magnet powder to be optimized.

[0097] (Process S40) Next, the Sm-Fe-N alloy powder formed in step S30 is washed.

[0098] The Sm-Fe-N alloy powder formed in step S30 contains calcium compounds, and the washing process is carried out to remove such calcium compounds.

[0099] The cleaning treatment is carried out using a cleaning liquid such as water and / or alcohol. Alternatively, the cleaning liquid may be an acid such as amidosulfuric acid. Alternatively, the Sm-Fe-N alloy powder may be cleaned using water and / or alcohol, and then further cleaned using amidosulfuric acid. The temperature of the cleaning liquid is not particularly limited, but it is preferable to select a temperature at which CaO and Ca(OH)2 have high solubility. For example, when the cleaning liquid is water, it is preferable that the temperature be between 0°C and room temperature.

[0100] The cleaning step may be carried out before the nitriding treatment.

[0101] The washed Sm-Fe-N alloy powder is then preferably dried.

[0102] The drying temperature is not particularly limited, but is preferably from room temperature to 100° C. By setting the drying temperature to 100° C. or less, oxidation of the Sm—Fe—N alloy powder can be suppressed.

[0103] The Sm-Fe-N alloy powder may be subjected to a dehydrogenation treatment, which can remove hydrogen that has entered between the crystal lattices during the cleaning treatment.

[0104] The dehydrogenation method is not particularly limited. For example, the dehydrogenation may be performed by heating the Sm-Fe-N alloy powder under vacuum or in an inert gas atmosphere. For example, the dehydrogenation may be performed by heat treating the Sm-Fe-N alloy powder at 150°C to 250°C for 1 hour under a vacuum atmosphere.

[0105] Through the above steps, Sm-Fe-N based magnet powder can be produced. The amount of residual oxygen in the magnet powder is less than 1.0 wt%.

[0106] The average particle size of the resulting Sm-Fe-N based magnet powder is preferably greater than 0.1 μm and less than 2.0 μm.

[0107] The residual oxygen content is preferably less than 1.0 wt%, more preferably less than 0.8 wt%.

[0108] (Process S120) Next, Zn powder is mixed with the Sm-Fe-N magnet powder produced by the above method to prepare a mixed powder.

[0109] The average particle size of the Zn powder is, for example, in the range of 5 μm to 100 μm. In particular, it is preferable that the average particle size of the Zn powder is larger than that of the Sm—Fe—N based magnet powder.

[0110] The amount of Zn powder mixed is not particularly limited, but may be, for example, 1 wt % or more and 20 wt % or less with respect to the entire mixed powder.

[0111] Although there are no particular limitations on the method for mixing the Sm-Fe-N magnetic powder and Zn powder, it is preferable to mix them in a way that does not physically damage the surfaces of the particles of the Sm-Fe-N magnetic powder. For example, it is preferable to avoid methods such as mixing using a ball mill and crushing.

[0112] (Step S130) The mixed powder is then compacted to form a compact.

[0113] The compaction is preferably carried out in an environment where a magnetic field is applied, such as a static magnetic field. When compacted in a static magnetic field, a compact is obtained in which the easy axes of magnetization of the particles are oriented along the static magnetic field, and an anisotropic magnet can be obtained after sintering.

[0114] For example, a compact can be obtained by pressing the mixed powder in a mold while applying a static magnetic field to the mixed powder in the mold.

[0115] The pressure that the mold exerts on the mixed powder may be, for example, 10 MPa or more and 3000 MPa or less, and the pressure is preferably 500 MPa or less for uniform diffusion of Zn.

[0116] The strength of the magnetic field applied to the mixed powder may be 5 kOe or more and 40 kOe or less.

[0117] (Process S140) Next, the compact is subjected to a sintering process.

[0118] The sintering process melts the Zn powder contained in the compact. The molten Zn spreads throughout the Sm-Fe-N magnet powder during the sintering process, ultimately forming the coating layer described above.

[0119] The sintering process may be carried out by, for example, a discharge plasma method, a hot press method, or an electric pressure sintering method, among which an electric pressure sintering method is preferred because it can achieve low thermal load sintering by rapid heating and short sintering time.

[0120] The sintering conditions may be set appropriately depending on the composition of the magnet to be manufactured, the average particle size of the powder contained therein, and the like.

[0121] The sintering step may include a temperature-raising step and a temperature-holding step following the temperature-raising step, or may include only the temperature-raising step.

[0122] The temperature reached during the temperature increase process may be, for example, 420°C or higher and 600°C or lower.

[0123] The temperature increase rate during the temperature increase process may be, for example, 5° C. / min or more and 100° C. / min or less.

[0124] The sintering time during the temperature holding step is, for example, 5 hours or less, and may be 0 hour.

[0125] The method for heating the compact is not particularly limited. The compact may be sintered by resistance heating, electrical heating, or high-frequency heating.

[0126] The atmosphere for the sintering treatment is, for example, a nitrogen atmosphere, an argon atmosphere, or a vacuum (reduced pressure atmosphere). The oxygen concentration and moisture concentration in the atmosphere are preferably 1 ppm or less, and 0.5 ppm or less, respectively. These concentrations are expressed as mole fractions.

[0127] After the sintering treatment, the sintered body may be cooled at a cooling rate of, for example, 5°C / min or more and 100°C / min or less.

[0128] By the above steps, an Sm-Fe-N magnet having the above-mentioned characteristics can be manufactured. [Example]

[0129] Examples of the present invention will be described below, in which Examples 1 to 3 are examples, and Examples 11 to 13 are comparative examples.

[0130] (Example 1) An Sm-Fe-N magnet was produced by the following method.

[0131] (Preparation of mixed powder) First, a mixed powder was prepared by the following method.

[0132] (Preparation of Sm-Fe-(hydr)oxide powder) 64.64 g of iron nitrate nonahydrate and 12.93 g of samarium nitrate hexahydrate were dissolved in 800 ml of water, and 120 ml of 2 mol / L potassium hydroxide solution was added dropwise while stirring. The mixture was then stirred overnight at room temperature to produce a suspension. The suspension was then filtered, and the residue was washed and then dried overnight at 120 °C in an air atmosphere using a hot air oven. The residue was then coarsely pulverized using a blade mill and then finely pulverized in ethanol using a rotary mill with stainless steel balls. The pulverized residue in ethanol was then centrifuged and vacuum dried to produce Sm-Fe-(hydr)oxide powder.

[0133] (Pre-reduction) The Sm-Fe-(hydr)oxide powder was pre-reduced by heat treatment at 600°C for 6 hours in a hydrogen atmosphere to produce a powder (referred to as Powder A).

[0134] (Reduced diffusion) 5.0 g of Powder A and 2.5 g of calcium powder were placed in an iron crucible and then heated at 900° C. for 1 hour for reduction and diffusion to produce a powder (referred to as Powder B).

[0135] (nitriding) Powder B was cooled to room temperature and then heated to 380°C in a hydrogen atmosphere. Next, Powder B was nitrided by heating to 420°C in an atmosphere of ammonia-hydrogen mixture with a volume ratio of 1:2 and maintaining the temperature for 1 hour.

[0136] Next, the powder was annealed in a hydrogen atmosphere at 420°C for 1 hour, and then in an argon atmosphere at 420°C for 0.5 hours to optimize the nitrogen content in the powder. Powder C was thus obtained.

[0137] (Washing) Powder C was washed five times with pure water. After washing, the washed powder C was added to an aqueous solution of amidosulfuric acid, the pH was adjusted to 5, and the mixture was maintained for 15 minutes to remove calcium compounds. Next, powder C was washed with pure water to remove the amidosulfuric acid. This yielded powder D.

[0138] (vacuum drying) The water remaining in Powder D was replaced with 2-propanol, and then the powder was dried in vacuum at room temperature.

[0139] The vacuum-dried powder D was dehydrogenated under vacuum at 200°C for 3 hours.

[0140] The steps after the pre-reduction were carried out in a glove box under an argon atmosphere without exposure to the air.

[0141] Through the above steps, Sm-Fe-N based magnet powder (hereinafter referred to as "powder E") was obtained.

[0142] (Evaluation of Powder E) At this stage, various evaluations of the obtained powder E were carried out.

[0143] (Evaluation of coercive force) The coercive force of Powder E was measured using the following method.

[0144] First, powder E and a thermoplastic resin were mixed and then oriented in a magnetic field of 20 kOe to prepare a sample for measuring the powder coercivity. Next, the coercivity of the sample for measuring the powder coercivity was measured using a vibrating sample magnetometer (VSM). The measurement temperature was 27°C, and the maximum applied magnetic field was 90 kOe.

[0145] As a result of the measurement, the coercive force of the powder coercive force measurement sample was 32.2 kOe.

[0146] (Measurement of average particle size) Powder E was mixed with a thermosetting epoxy resin and thermally cured, and then the mixture was irradiated with a focused ion beam (FIB) and etched to expose the cross section, thereby preparing a sample.

[0147] Using a field emission scanning electron microscope (FE-SEM), the cross section of the sample was observed and outlines were drawn on more than 200 randomly selected particles.

[0148] The contour lines correspond to the surfaces of particles and / or surfaces of touching particles, although touching particles can be distinguished by FE-SEM backscattered electron imaging or energy dispersive X-ray spectroscopy (EDS) mapping.

[0149] Next, the diameter of a circle with the same area as the region surrounded by the outline was defined as the particle diameter of the particle. The average particle diameter of Powder E was calculated by taking the volume-weighted average of these particle diameters.

[0150] The average particle size of Powder E was 1.4 μm.

[0151] (Preparation of mixed powder) Next, powder E (that is, Sm—Fe—N based magnet powder) and Zn powder were slowly mixed in a V-type mixer to prepare a mixed powder.

[0152] The amount of Zn powder added was 5 wt % based on the total mixed powder. The average particle size of the Zn powder was 6 μm to 9 μm.

[0153] The prepared mixed powder is referred to as "Mixed Powder 1."

[0154] (Magnetic manufacturing) Next, mixed powder 1 was molded by the following method, and the resulting molded body was sintered to produce a magnet.

[0155] The molding pressure was set to 200 MPa.

[0156] The sintering temperature of the compact was 470°C, and the sintering time was 1 minute.

[0157] This resulted in a sintered magnet, which will be referred to as "Magnet 1."

[0158] (Example 2) A sintered magnet was produced in the same manner as in Example 1. However, in this Example 2, the amount of Zn powder added to the mixed powder was set to 10 wt %. The other production conditions were the same as in Example 1.

[0159] The resulting sintered magnet is referred to as "Magnet 2."

[0160] (Example 3) A sintered magnet was produced in the same manner as in Example 1. However, in this Example 3, the amount of Zn powder added to the mixed powder was set to 20 wt %. The other production conditions were the same as in Example 1.

[0161] The resulting sintered magnet is referred to as "Magnet 3."

[0162] (Example 11) A sintered magnet was produced in the same manner as in Example 1. However, in Example 11, no Zn powder was added to Powder E, and Powder E was directly molded and sintered to produce a magnet.

[0163] The resulting sintered magnet is referred to as "Magnet 11."

[0164] (Example 12) A sintered magnet was produced in the same manner as in Example 2. However, in this Example 12, when preparing the mixed powder, a ball mill was used to disperse and mix Powder E and Zn powder. The other production conditions were the same as in Example 2.

[0165] The resulting sintered magnet is referred to as "Magnet 12."

[0166] (Example 13) A sintered magnet was produced using the same method as in Example 2. However, in this Example 13, when preparing the mixed powder, Powder E and Zn powder were dispersed and mixed using a ball mill. Zn powder with an average particle size of 1 μm was used. The other production conditions were the same as in Example 2.

[0167] The resulting sintered magnet is referred to as "Magnet 13."

[0168] Table 1 below shows the manufacturing conditions for each magnet.

[0169] [Table 1] (evaluation) The following evaluations were carried out using each of the manufactured magnets.

[0170] (Evaluation of the shape of Sm-Fe-N particles) The average particle size of the Sm-Fe-N particles contained in each magnet was measured, and the aspect ratio of the Sm-Fe-N particles was also evaluated.

[0171] The average particle size of the Sm-Fe-N particles was measured in the same manner as in the measurement of the average particle size of Powder E described above.

[0172] The aspect ratio was evaluated as follows.

[0173] For each particle, a rectangle was determined that circumscribed the outline and had the smallest area. The aspect ratio of each particle was calculated by dividing the length of the long side of the rectangle by the length of the short side. Furthermore, the proportion of particles with an aspect ratio of 2 or more was evaluated.

[0174] (Evaluation of the coating layer) In each magnet, the surfaces and interfaces of the Sm-Fe-N particles were observed.

[0175] FIG. 4 shows a high-angle annular dark field (HAADF) image of the cross section of magnet 2 and the mapping results of each element obtained by EDS (energy dispersive X-ray spectroscopy).

[0176] In addition, Fig. 5 shows a TEM image (bright field image) of an enlarged portion of the interface between Sm-Fe-N particles. In Fig. 5, for example, Sm-Fe-N particles are present at the bottom, left, and top.

[0177] Figure 6 shows the Fe and Zn mapping results obtained by EDS analysis of an enlarged portion of the interface between Sm-Fe-N particles. In Figure 6, the presence of an Fe-rich layer can be seen, for example, in the circled area. Furthermore, Figure 7 shows an electron diffraction image of the Fe-rich layer in an enlarged portion of the interface between Sm-Fe-N particles. The area marked with "*1" in Figure 6 corresponds to the diffraction image "(1)" in Figure 7. Furthermore, a comparison of Figure 7 and Figure 6 reveals that the area marked with "*1" in Figure 6 is the α-Fe layer.

[0178] Figure 4 shows that Zn is present on the surface and interface of the Sm-Fe-N particles, and Figure 6 shows that an Fe-rich layer (first layer) is present on the surface of the Sm-Fe-N particles, and that a Zn-containing layer (second layer) is present outside of that.

[0179] An example of the results of EDS line analysis of the interface between Sm-Fe-N particles is shown in Figure 8. In Figure 8, the position at a distance of approximately 230 nm on the horizontal axis corresponds to the surface of one particle, and the region at a distance of approximately 230 nm to approximately 290 nm corresponds to the coating layer.

[0180] In Figure 8, although it is difficult to see the α-Fe layer within the coating layer due to the magnification, the presence of a layer (second layer) containing Sm, Fe, and Zn is recognized. From this result, the thickness of the second layer is estimated to be approximately 50 nm.

[0181] (First layer evaluation) The thickness of the first layer (α-Fe phase) was measured at 20 selected interfaces between Sm-Fe-N particles, and the average thickness of the first layer was calculated by averaging the measurement results.

[0182] (Second layer evaluation) The thickness of the second layer (Sm-Fe-Zn phase) was measured at 20 selected interfaces between Sm-Fe-N particles, and the average thickness of the second layer was calculated by averaging the results.

[0183] Additionally, 20 interfaces between Sm-Fe-N particles were selected and the amount of Zn contained in the second layer was measured using EDS.These measurement results were averaged to determine the average amount of Zn contained in the second layer.

[0184] (Measurement of Zn content in magnets) The amount of Zn contained in the whole magnet was evaluated by high-frequency inductively coupled plasma (ICP) emission spectrometry.

[0185] (Measurement of residual oxygen in magnets) The amount of residual oxygen in each magnet was evaluated by inert gas fusion-non-dispersive infrared absorption spectroscopy.

[0186] (Evaluation of coercive force) The coercive force of each magnet was measured using a vibrating sample magnetometer (VSM) at a temperature of 27°C and a maximum applied magnetic field of 90 kOe.

[0187] Table 2 below shows the evaluation results for each magnet.

[0188] [Table 2] As shown in Table 2, it was found that a first layer and a second layer were formed in all of Magnets 1 to 3. The Zn content in the second layer was 20 at % or less.

[0189] On the other hand, in magnet 11, no Zn powder was added to the raw material, so no second layer was formed and only the first layer was observed.In addition, in magnets 12 and 13, the first and second layers were observed, but the Zn content in the second layer was found to exceed 20 at%.

[0190] Furthermore, it was found that magnets 11 to 13 had a lower coercive force than that of powder E at the raw material stage.

[0191] In contrast, it was found that all of Magnets 1 to 3 had improved coercive force compared to that of Powder E at the raw material stage.

[0192] Thus, it was confirmed that Magnets 1 to 3 have significantly high coercive forces. [Explanation of symbols]

[0193] 110 Sm-Fe-N particles 112 Surface 120 Covering layer 122 First Layer 124 Second Layer

Claims

1. A Sm—Fe—N based magnet, Sm—Fe—N-based particles having a surface; a coating layer disposed on at least a portion of the surface and / or at least a portion of the interface between the Sm—Fe—N based particles; and the coating layer has a first layer and a second layer in order from closest to the surface or interface, the first layer comprises α-Fe; the second layer comprises a Sm—Fe—Zn alloy; the Zn content in the second layer is 1 at% or more and 20 at% or less; The Sm-Fe-N magnet is characterized in that the Sm-Fe-N particles have an average particle size of less than 2.0 μm and the proportion of Sm-Fe-N particles having an aspect ratio of 2.0 or more is 10% or less.

2. 2. The Sm--Fe--N magnet according to claim 1, wherein the Zn content in said second layer is 5 at % or more and 15 at % or less.

3. An Sm-Fe-N based magnet as described in claim 1 or 2, wherein the first layer is composed of a phase mainly composed of α-Fe.

4. 4. The Sm--Fe--N magnet according to claim 1, wherein the average thickness of the second layer is 1 nm or more and 100 nm or less.

5. 5. The Sm--Fe--N magnet according to claim 1, wherein the Zn content in the Sm--Fe--N magnet is 1 wt % or more and 20 wt % or less.

6. 6. The Sm--Fe--N magnet according to claim 1, wherein the oxygen content is less than 1.0 wt %.

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