Sintered magnet, method for producing powder for sintered magnet, and method for producing sintered magnet

Abstract

This sintered magnet comprises: a first phase, the main phase of which is a Sm-Fe-N crystal grain that has a Th2Zn17 structure; and a second phase which is composed of a metal binder that includes one or more element from among at least one of group 2 elements and rare earth elements, and that has a melting point of not higher than 620°C. The volume ratio of the main phase is not less than 90%.

Description

Sintered magnet, method for producing powder for sintered magnet, and method for producing sintered magnet

[0001] The present disclosure relates to an Sm—Fe—N based sintered magnet.

[0002] In recent years, Sm (samarium)-Fe (iron)-N (nitrogen) magnets have been developed as high-performance magnets. Sm—Fe—N compounds are known to exhibit both high spontaneous magnetization and high anisotropy fields, as well as high heat resistance. However, because Sm—Fe—N compounds tend to thermally decompose at temperatures around 620°C, powder of a magnetic material containing an Sm—Fe—N compound cannot be heated above the decomposition temperature when obtaining a compact. Therefore, when molding a sintered magnet using powder of an Sm—Fe—N compound, a method using a resin binder or a low-melting-point metal or alloy binder is known. However, this method poses the problem of a decrease in the volume fraction of the main phase by the amount of binder, which simultaneously reduces the saturation magnetization.

[0003] To address this issue, no technology has been developed to increase the main phase ratio of Sm—Fe—N sintered magnets. Meanwhile, recent reports have attempted to bulk Sm—Fe—N magnets using alloy binders. For example, Patent Document 1 discloses an La—Cu—(Al, Mg, Zn) binder with a low melting point, excellent wettability with the main phase, and excellent crushability. Patent Document 2 discloses a metal zinc powder or zinc alloy powder that functions as a binder and a modifier that improves coercivity when forming a sintered magnet using Sm—Fe—N powder. However, none of these documents aims to increase the volume fraction of the Sm—Fe—N main phase when the resulting sintered magnet is produced.

[0004] JP 2020-053435 A JP 2020-155740 A

[0005] First, the technique described in Patent Document 1 requires a binder in an amount greater than that of the main phase, and the volume fraction of the Sm—Fe—N main phase is not fully satisfactory.

[0006] Furthermore, the technology described in Patent Document 2 uses zinc or a zinc alloy as a binder. The zinc component corrodes the Sm—Fe—N main phase to form a low-magnetization phase, which significantly reduces the magnetic properties of the resulting sintered magnet. Therefore, this binder is not necessarily suitable for Sm—Fe—N sintered magnets.

[0007] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a technique for increasing the volume fraction of the Sm—Fe—N phase in an Sm—Fe—N based sintered magnet.

[0008] The present disclosure has been made to solve at least one of the above-mentioned problems, and can be realized in the following forms.

[0009] <1> According to one aspect of the present disclosure, a sintered magnet is provided. This sintered magnet comprises Th ２ Zn １７ The composite material comprises a first phase having Sm-Fe-N crystal grains having a Sm-Fe-N type structure as a main phase, and a second phase made of a metal binder containing at least one element selected from the group consisting of Group 2 elements and rare earth elements and having a melting point of 620°C or less, and the volume fraction of the main phase is 90% or more.

[0010] Here, the metal binder is a concept that includes a binder made of one kind of metal and a binder made of a so-called alloy containing two or more kinds of metals.

[0011] In this type of sintered magnet, a second phase made of a metal binder is formed as a grain boundary layer at the interfaces between the Sm—Fe—N crystal grains. Although Sm—Fe—N crystal grains are a material that is difficult to liquid-phase sinter, the use of a metal binder with the above composition allows for pseudo-liquid-phase sintering to occur, resulting in a dense sintered body.

[0012] Furthermore, by using a metal binder with the above composition, it is possible to achieve excellent wettability with respect to Sm—Fe—N crystal grains, which in turn allows the volume fraction of the main first phase to be 90% or more, thereby improving the volume magnetization (magnetic force per volume) of the sintered magnet.

[0013] <2> In the sintered magnet of the above embodiment, the second phase may satisfy at least one of the following conditions (1) and (2): (1) The content of the Group 2 element is 20 atomic % or more and 90 atomic % or less, and (2) The content of the rare earth element is 10 atomic % or more and 80 atomic % or less.

[0014] In this sintered magnet, the second phase contains at least one of a Group 2 element and a rare earth element in the above-mentioned proportions. When the second phase is an alloy with the above-mentioned element contents, the melting point of the metal binder that forms the second phase by eutectic reaction can be further lowered, and the reactivity between the Sm—Fe—N crystal grains and the metal binder can be improved while ensuring wettability with the Sm—Fe—N crystal grains. As a result, the volume fraction of the first phase in the sintered magnet can be further increased.

[0015] <3> In the sintered magnet of the above embodiment, the volume fraction of the second phase may be 0.1% or more and 2.4% or less. This also makes it possible to further increase the degree of densification during sintering and further increase the volume fraction of the first phase.

[0016] <4> In the sintered magnet of the above embodiment, the average grain size of the Sm—Fe—N crystal grains of the first phase may be 0.1 μm or more and 20 μm or less, thereby further improving the densification of the sintered magnet.

[0017] <5> According to another aspect of the present disclosure, there is provided a method for producing a powder for a sintered magnet used in molding the sintered magnet of the above aspect, the method for producing a powder for a sintered magnet including: a crushing step of crushing a coarse powder containing Sm—Fe—N single crystals to obtain the Sm—Fe—N crystal grains; a binder powder preparation step of obtaining a metal binder powder to serve as the metal binder; and a mixing step of dispersing the Sm—Fe—N crystal grains and the metal binder powder to obtain the powder for a sintered magnet, the crushing step, the binder powder preparation step, and the mixing step being carried out in a low-oxygen concentration atmosphere.

[0018] According to this method for producing sintered magnet powder, the above steps are carried out in a low-oxygen atmosphere, which suppresses oxidation of the Sm—Fe—N crystal grains and the metallic binder and ensures wettability between the Sm—Fe—N crystal grains and the metallic binder. As a result, a sintered magnet with a first phase volume fraction of 90% or more can be produced.

[0019] <6> According to another aspect of the present disclosure, there is provided a method for producing a sintered magnet according to the above aspect, which includes a sintering step of pressure-sintering the powder for a sintered magnet produced by the method for producing a powder for a sintered magnet according to the above aspect at a sintering temperature of 600°C or less in an atmosphere with a low oxygen concentration.

[0020] According to this method of producing a sintered magnet, the thermal decomposition of the Sm—Fe—N main phase can be suppressed by setting the sintering temperature at 600°C or less, and the oxidation of the Sm—Fe—N crystal grains and the metal binder powder can be suppressed by performing this process in a low-oxygen atmosphere. As a result, the density of the sintered magnet can be improved, and the volume fraction of the Sm—Fe—N main phase (first phase) can be increased.

[0021] The present disclosure can be realized in various forms, for example, in the form of a permanent magnet for a motor.

[0022] 1 is an explanatory diagram conceptually showing a cross-sectional configuration of a sintered magnet according to an embodiment; 2 is a process diagram showing an example of a method for producing a sintered magnet; 3 is a diagram showing evaluation results of a sample; 4 is an explanatory diagram of a sample for SEM observation; 5 is a diagram showing an example of an SEM image of a sample; and 6 is a diagram showing the results of wettability evaluation of a metal binder.

[0023] 1 is an explanatory diagram conceptually showing the cross-sectional structure of a sintered magnet 100 according to an embodiment. ２ Zn １７The composite material includes a first phase 10 having Sm-Fe-N (samarium-iron-nitrogen) crystal grains having a samarium-iron-nitrogen (Sm-Fe-N) type structure as a main phase, and a second phase 20 made of a metal binder containing at least one element selected from the group 2 elements and rare earth elements and having a melting point of 620°C or lower. The volume fraction of the first phase is 90% or higher. Here, Th is thorium, and Zn is zinc.

[0024] In Figure 1, the first phase 10 is hatched with diagonal lines that slope upward to the right, and the second phase 20 is hatched with diagonal lines that slope downward to the right. As shown in the figure, the first phase 10 has a plurality of Sm—Fe—N crystal grains 10G. The second phase 20 is located at the grain boundaries between the Sm—Fe—N crystal grains 10G and the Sm—Fe—N crystal grains 10G. The sintered magnet 100 also has voids V at the grain boundaries between the Sm—Fe—N crystal grains 10G and the Sm—Fe—N crystal grains 10G.

[0025] The Sm—Fe—N crystal grains 10G, which are the main phase, are composed of Th ２ Zn １７ Sm having a type structure ２ Fe １７ N ３ The sintered magnet 100 exhibits magnetism due to the Sm—Fe—N crystal grains 10G (main phase). The crystal structure of the main phase can be identified by, for example, subjecting the sintered magnet 100 to X-ray diffraction analysis. The main phase refers to the compound that determines the properties of the sintered magnet.

[0026] Sm ２ Fe １７ N ３ has excellent saturation magnetization and a huge anisotropic magnetic field, the sintered magnet 100 of the embodiment can withstand heat and reverse magnetic fields and generate a strong magnetic field.

[0027] The first phase 10 is composed of Sm—Fe—N-based crystal grains 10G, and is composed of Th ２ Ni １７ Type structure, TbCu ７ The phase may include a structure different from the main phase, such as a ternary structure, where Tb is terbium and Cu is copper.

[0028] The average grain size of the Sm—Fe—N crystal grains 10G of the first phase 10 is not particularly limited, but is preferably 0.1 μm or more and 20 μm or less, more preferably 0.4 μm or more and 10 μm or less, and even more preferably 1 μm or more and 5 μm or less. ２ Fe １７ N ３ The critical diameter of the single magnetic domain particles is 0.356 μm, and if the particle diameter is equal to or greater than this critical diameter, the magnet powder can exist in a stable state in terms of energy, and a sintered magnet with a higher density can be obtained. ２ Zn １７ Not limited to type structure, Th ２ Ni １７ Type structure, TbCu ７ Other structures such as mold structures may also be included.

[0029] The average particle size of the Sm—Fe—N crystal grains 10G of the first phase 10 in the sintered magnet 100 was evaluated based on the volumetric results obtained using a dry particle size distribution measuring device, HELOS & RODOS (manufactured by Sympatec).

[0030] The second phase 20 is made of a metal binder. The concept of a metal binder encompasses binders made of a single metal and binders made of alloys containing two or more metals. Group 2 elements are elements belonging to Group 2 of the periodic table and include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Rare earth elements include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), eurobium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0031] The metal binder may contain elements other than Group 2 elements and rare earth elements, such as silver (Ag), aluminum (Al), copper (Cu), and zinc (Zn).

[0032] The melting point of the metal binder that forms the second phase 20 is 620°C or lower, a temperature at which the main phase does not decompose. Therefore, when producing the sintered magnet 100, the main phase does not decompose even if the metal binder is heated to a temperature at which it melts. Therefore, by using a metal binder with the above composition for the second phase 20, liquid phase sintering can occur, and Sm ２ Fe １７ N ３ / Sm ２ Fe １７ N ３ The second phase 20 made of the metal binder can be formed as a grain boundary phase at the interface between the main phase (Sm ２ Fe １７ N ３ ) has excellent wettability to the sintered magnet 100. This allows for a dense sintered body to be obtained. As a result, the volume fraction of the main phase of the sintered magnet 100 can be increased to 90% or more, thereby improving the volume magnetization (magnetic force per unit volume).

[0033] The melting point of the second phase was measured using a differential scanning calorimeter (DSC). 10 to 20 mg of the liquid-quenched foil obtained by melt spinning, described below, was weighed out and used as the measurement sample. A BN (boron nitride) pan was used for the measurement, with the measurement temperature range set to room temperature to 700°C and the heating rate set to 10°C / min. The melting point was determined using the peak melting temperature that appeared within the measurement temperature range. Considering the usage environment of the sintered magnet, it is preferable that the lower limit of the binder's melting point be higher than the usage environment temperature. For example, when used in an EV motor, the binder's melting point should preferably be 180°C or higher.

[0034] The element contents of the metal binder are not particularly limited, but it is preferable that at least one of the following (1) and (2) is satisfied: (1) The content of Group 2 elements is 20 atomic % or more and 90 atomic % or less, and (2) The content of rare earth elements is 10 atomic % or more and 80 atomic % or less.

[0035] By using an alloy of these metal elements in the above ratio as the metal binder, the melting point of the metal binder can be further lowered through a eutectic reaction, and the reactivity between the main phase and the metal binder can be improved while ensuring wettability with the main phase, thereby enabling a higher volume fraction of the main phase in sintered magnet 100.

[0036] The content of the metal binder (volume fraction of the second phase) is not particularly limited, but is preferably 0.1% or more and less than 10%. Even in this case, the degree of densification during sintering can be ensured, and the volume fraction of the main phase can be made 90% or more.

[0037] The sintered magnet 100 may contain unavoidable impurity elements, etc., to the extent that the magnetic properties of the main phase are not impaired. An unavoidable impurity element is an impurity element whose inclusion cannot be avoided during the production, etc., of the sintered magnet 100 of the embodiment, or whose avoidance would result in a significant increase in production costs. Examples of such unavoidable impurity elements include impurity elements in raw materials and elements contained in lubricants, etc. used during molding.

[0038] Fig. 2 is a process diagram showing an example of a method for manufacturing a sintered magnet 100. The method for manufacturing the sintered magnet 100 of this embodiment is not particularly limited, but it can be manufactured by the following method, for example. As shown in Fig. 2, in the method for manufacturing the sintered magnet 100, steps P0 for manufacturing powder for a sintered magnet and P4 for sintering are carried out in this order. In the step P0 for manufacturing powder for a sintered magnet, steps P1 for crushing, P2 for preparing binder powder, and P3 for mixing are carried out in this order.

[0039] In the pulverization step P1, Th ２ Zn １７ Coarse powder containing Sm—Fe—N single crystals having a ZnO-type structure is pulverized to obtain Sm—Fe—N crystal grains. ２ Fe １７ N ３The average particle size of the Sm—Fe—N crystal grains after pulverization is not particularly limited, but is preferably 0.1 μm to 20 μm, more preferably 0.4 μm to 10 μm, and even more preferably 1 μm to 5 μm.

[0040] In the binder powder preparation step P2, powder of a predetermined size of a metal (pure metal or alloy) to be used as a metal binder is prepared to obtain metal binder powder.

[0041] In the mixing step P3, the Sm—Fe—N crystal grains and the metal binder powder are dispersed to obtain a mixed powder for a sintered magnet. The mixing step P3 may be a wet method in which the Sm—Fe—N crystal grains and the metal binder powder are dispersed in a solvent (e.g., ethanol), or a dry method in which the Sm—Fe—N crystal grains and the metal binder powder are dispersed in an inert gas (e.g., argon gas, helium gas, nitrogen gas, etc.).

[0042] The above-mentioned grinding step P1, binder powder preparation step P2, and mixing step P3 are all performed in a low-oxygen concentration atmosphere. The oxygen concentration is adjusted by controlling the atmosphere in each step. A low-oxygen concentration is a concentration lower than the oxygen concentration in the atmosphere (approximately 21 vol%). The oxygen concentration is, for example, preferably 100 ppm or less, more preferably 10 ppm or less, and even more preferably 0.5 ppm or less. For example, a low-oxygen concentration atmosphere can be achieved by injecting an inert gas into a vacuum chamber. Performing the above steps in a low-oxygen atmosphere can prevent oxidation of the Sm—Fe—N crystal grains and the metal binder powder, thereby ensuring wettability between the Sm—Fe—N crystal grains and the metal binder powder.

[0043] In the sintering step P4, the sintered magnet powder produced in the sintered magnet powder production step P0 is molded and pressure-sintered at a sintering temperature of 600°C or less in a low-oxygen atmosphere. In the sintering step P4, firing is performed in an atmosphere with an oxygen concentration similar to that of the crushing step P1 through the mixing step P3. By setting the sintering temperature to 600°C or less, thermal decomposition of the Sm—Fe—N crystal grains can be suppressed. Furthermore, by performing the sintering step P4 in a low-oxygen atmosphere, oxidation of the Sm—Fe—N crystal grains and the metal binder powder can be suppressed. As a result, the density can be improved, and the volume fraction of the first phase can be increased.

[0044] The sintered magnet of this embodiment can be used as a permanent magnet for various motors, such as motors for EVs (electric vehicles), motors built into robots, motors built into drones, and elevator motors.

[0045] The present disclosure will be explained in more detail with reference to examples. Fig. 3 shows the evaluation results for Samples 1 to 24. The volume fractions of the first phase 10 and the second phase 20 were evaluated for Samples 1 to 24 of sintered magnet 100, each having a different second phase 20 composition. In Fig. 3, of the elements contained in the second phase 20, Group 2 elements and rare earth elements are indicated by double-lined frames. In the examples, magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) were used as Group 2 elements, and lanthanum (La), samarium (Sm), and praseodymium (Pr) were used as rare earth elements.

[0046] 1. Sample Production Samples 1 to 24 were produced by the production method (FIG. 2) exemplified in the following embodiment.

[0047] (1) Pulverization step P1: As a coarse powder containing Sm—Fe—N single crystals, Sm having an average particle size of 30 μm was used. ２ Fe １７ N ３Powder was used. First, in a glove box in which the oxygen concentration was controlled to 0.5 ppm or less, stainless steel balls and the above-mentioned coarse powder were placed in a stainless steel pot, and ethanol was added as a solvent. The pot was rotated, and the coarse powder was ball milled. Here, the rotation time was 48 hours and the rotation speed was 120 rpm. The slurry obtained by the above process was sieved through a sieve with 25 μm openings in the same glove box and dried in a vibration dryer. The obtained powder was then sieved through a sieve with 250 μm openings to obtain Sm—Fe—N-based crystal grains as a dry powder. The average particle size of the Sm—Fe—N-based crystal grains obtained by this process was 0.1 μm or more and 20 μm or less. Here, the particle size distribution was measured by dry measurement.

[0048] (2) Binder Powder Preparation Process P2 The following materials were used as raw materials (granular metal raw materials) for the metal binder: Mg metal (granular), Ca metal (granular), Sr metal (granular), Ba metal (granular), Cu metal (granular), Ag metal (granular), Zn metal (granular), Al metal (granular), Fe metal (granular), Sm metal (granular), La metal (granular), Pr metal (granular). These granular metal raw materials were weighed out to the composition ratio shown in Figure 3 and melted in an arc melting furnace under reduced pressure of argon (Ar) to produce alloy ingots. The produced ingots were high-frequency melted under reduced pressure of Ar using a liquid quenching foil manufacturing apparatus, and then melt-spun with a single copper roll rotated at 5000 rpm to produce quenched alloy foils. In a glove box with an oxygen concentration controlled to 0.5 ppm or less, a quenched alloy foil and stainless steel balls were placed in a stainless steel pot, along with ethanol or heptane as a solvent. The pot was rotated, and the quenched alloy foil was ground in a planetary ball mill. The rotation time was 12 hours and the rotation speed was 200 rpm. The slurry obtained by the above process was sieved through a 25 μm mesh sieve in the glove box and dried in a vibration dryer. The resulting powder was then sieved through a 63 μm mesh sieve to obtain a dry metal binder powder. The metal binder powder had an average particle size of 100 μm or less.

[0049] (3) Mixing step P3: The Sm—Fe—N crystal grains (Sm ２ Fe １７ N ３ ) and a metal binder powder were ball milled using ethanol as a solvent to obtain a mixed powder (slurry) for a sintered magnet. In the mixing step P3, the metal binder was added in an amount (vol%) shown in Table 3. Here, the amount (vol%) is determined based on the amount of Sm—Fe—N crystal grains (Sm ２ Fe １７ N ３ The amount (volume) of the metal binder powder relative to the amount of the base material (amount of the base material) plus the amount of the metal binder powder.

[0050] (4) Sintering Step P4 The sintered magnet powder obtained in the mixing step P3 was placed in a non-magnetic cemented carbide die (mold) in a glove box with an oxygen concentration controlled to 0.5 ppm or less, and a pressure of 600 MPa was applied using a uniaxial press. This was subjected to electric sintering in a vacuum atmosphere at a sintering temperature of 400°C to 600°C for 10 minutes. This resulted in the formation of Sm—Fe—N crystal grains (Sm ２ Fe １７ N ３ A sintered body (sintered magnet 100) was obtained, which included a first phase 10 having a main phase of ZnO (ZnO), and a second phase 20 made of a metal binder.

[0051] 2. Evaluation Method The volume fractions of the first phase 10 and the second phase 20 were calculated using SEM (Scanning Electron Microscope) photographs. FIG. 4 is an explanatory diagram of a sample 100S for SEM observation. First, the sintered body (sintered magnet) produced by the above method was cut into a cylindrical shape with a diameter of 10 mm and a thickness of 3 mm (FIG. 4(A)). Then, a cross section was exposed using waterproof abrasive paper (FIG. 4(B)). The cross section was subjected to Ar ion milling to produce a sample 100S for SEM observation (FIG. 4(C)).

[0052] (1) Volume Fraction of Second Phase 20 Using SEM observation sample 100S, an SEM image of a cross section formed by ion milling was taken, and the volume fraction of the second phase was calculated from image analysis by binarization using image processing software Image-J. The SEM image used to calculate the volume fraction of the second phase was taken in an area (wider than 70 μm × 45 μm) with a display magnification of 3000 times or less on the SEM device, and the area fraction occupied by the second phase 20 (metal binder phase) was calculated. During the calculation, a backscattered electron image was used for binarization so that the first phase 10 and voids V were not reflected in the second phase 20, and EDS mapping analysis was performed in some cases to clearly show the compositional difference between the first phase 10 and the second phase 20. Based on stereology, the calculated area fraction was assumed to be equal to the volume fraction, and the volume fraction of the second phase was calculated. The same process was carried out for five different fields of view for each sample to calculate the volume fraction of the second phase 20, and the average value was taken as the second phase volume fraction for each sample.

[0053] (2) Volume Ratio of First Phase 10 FIG. 5 is a diagram showing an example of an SEM image of a sample. As in the calculation of the volume ratio of the second phase, the volume ratio of the first phase was calculated from image analysis of the SEM image. The SEM image used to calculate the volume ratio of the first phase was taken in an area (wider than 70 μm × 45 μm) at an SEM device display magnification of 3000 times or less. FIG. 5(A) shows an image binarized so that the white portions represent voids V. FIG. 5(B) shows an image binarized so that the white portions represent the second phase 20. The threshold values ​​for the binarization process were determined with 0 representing black and 255 representing white, with reference to section 28.2.4 Threshold in the Image 1.46 ver. manual, with the Auto Threshold method set to default. The sum of the area ratios of the voids V and the second phase 20 obtained by this binarization process was subtracted from 100% to obtain the area ratio of the first phase 10. Based on stereology, the calculated area ratio was assumed to be equal to the volume ratio, and the volume ratio of the first phase 10 was calculated. The same process was performed on five different fields of view for each sample to calculate the volume ratio of the first phase 10, and the average value was used as the first phase volume ratio of each sample. In Figure 5, only some of the voids V and the second phase 20 are indicated by symbols.

[0054] 3. Measurement Results As shown in FIG. 3, in Samples 1 to 16, the volume fraction of the first phase 10 is 90% or more.

[0055] Samples 1 to 16 satisfy the following requirements [1] to [3] (Figure 3). [1] Th ２ Zn １７ [1] The composite material includes a first phase mainly composed of Sm-Fe-N crystal grains having a Zn-Fe-N structure. [2] The composite material includes a second phase composed of a metal binder containing at least one element selected from the group consisting of Group 2 elements and rare earth elements and having a melting point of 620°C or less. [3] The volume fraction of the first phase is 90% or more.

[0056] Samples 1 to 16 satisfied the above requirement [2], allowing the Sm—Fe—N crystal grains to undergo liquid phase sintering, resulting in dense sintered bodies (sintered magnets). Furthermore, Samples 1 to 16 satisfied the above requirement [2], allowing the Sm—Fe—N crystal grains to exhibit excellent wettability. As a result, the volume fraction of the first phase was able to be increased to 90% or more (satisfying the above requirement [3]). This allowed the volume magnetization of the sintered magnet to be improved.

[0057] In contrast, Samples 17 to 24 do not satisfy the above requirement [2], and therefore do not satisfy the above requirement [3]. Therefore, sufficient volume magnetization cannot be obtained. In Samples 17 to 22, the metal binder forming the second phase contains neither a Group 2 element nor a rare earth element. In Sample 23, the metal binder forming the second phase contains calcium (Ca), a Group 2 element, but has a melting point of 650°C, and therefore does not satisfy the above requirement [2]. In Sample 24, the metal binder forming the second phase contains barium (Ba), a Group 2 element, and lanthanum (La), a rare earth element, but has a melting point of 650°C, and therefore does not satisfy the above requirement [2].

[0058] Samples 3 to 16 further satisfy the following requirement [4]. [4] The second phase satisfies at least one of the following requirements (I) and (II): (I) The content of the Group 2 element is 20 atomic % or more and 90 atomic % or less. (II) The content of the rare earth element is 10 atomic % or more and 80 atomic % or less. Samples 3 to 16 contain at least one of the Group 2 element and the rare earth element in the above proportions in the second phase. When the second phase is an alloy with the above element contents, the melting point of the metal binder that forms the second phase by eutectic reaction can be further lowered, and the reactivity between the Sm—Fe—N crystal grains and the metal binder can be improved while ensuring wettability with the Sm—Fe—N crystal grains. As a result, the volume fraction of the first phase in the sintered magnet was able to be higher than that of Samples 1 and 2.

[0059] Samples 14 to 16 further satisfy the following requirement [5]: [5] The volume fraction of the second phase is 0.1% or more and 2.4% or less. Samples 14 to 16 satisfy the above requirement [5], and therefore the degree of densification during sintering can be further increased, and the volume fraction of the first phase can be increased more than that of Samples 1 to 13.

[0060] As explained above, Samples 1 to 16 satisfy all of the requirements [1] to [3] above and are examples of the sintered magnet 100 of the above embodiment. These samples have a high first phase volume fraction of 90% or more, and therefore can be sintered magnets with higher volume magnetization than those of the prior art.

[0061] 4. Wettability of Metal Binder Prior to the production of the above samples, Sm—Fe—N crystal grains (Sm ２ Fe １７ N ３ A preliminary experiment was conducted to narrow down the element group and metal binder composition that exhibit good wettability with Sm—Fe—N system crystal grains (Sm ２ Fe １７ N ３The Sm—Fe—N crystal grains (Sm fine powder) were prepared by the (1) pulverization step P1 in the manufacturing method of the above sample. ２ Fe １７ N ３ The above metal binder powder was added to the Sm-Fe-N based crystal grains (Sm) at a ratio of 20 vol %, and the resulting mixture was subjected to electric pressure sintering at a sintering temperature of 450°C to 600°C, a sintering time of 10 min, and a pressure of 600 MPa in a vacuum atmosphere. The resulting sintered bodies (samples S1 to S17) were subjected to cross-sectional ion milling, and SEM observation was carried out, and secondary electron images were taken at 1000x magnification at five different points in the field of view, in the same manner as in the evaluation method described above. The secondary electron images were binarized using the image processing software Image-J to calculate the porosity, and these values ​​were compared for each metal binder to determine the porosity of the Sm-Fe-N based crystal grains (Sm ２ Fe １７ N ３ The alloy composition was selected to have good wettability with the molten metal (fine powder).

[0062] As shown in Figure 6, the metal binders of samples S1 to S4 contain calcium (Ca), a Group 2 element; sample S5 contains magnesium (Mg), a Group 2 element; sample S6 contains strontium (Sr), a Group 2 element; and samples S7 to S9 contain barium (Ba), a Group 2 element. The metal binders of samples S10, S11, and S14 contain lanthanum (La), a rare earth element; and samples S12 and S13 contain praseodymium (Pr), a rare earth element. The metal binders of samples S15 to S21 contain neither a Group 2 element nor a rare earth element. The sintered compact of sample S22 does not contain a metal binder.

[0063] Preliminary experiments confirmed that the porosity was significantly reduced in the sintered bodies (samples S1 to S14) to which a metal binder containing a Group 2 element and a rare earth element was added. That is, the metal binder of samples S1 to S14 was used to form Sm-Fe-N crystal grains (Sm ２ Fe １７ N ３ It is believed that it exhibits good wettability with respect to the surface of the material (fine powder).

[0064] In the preliminary experiments, magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) were used as Group 2 elements, but for the following reasons, it is believed that similar effects can be obtained by using other Group 2 elements such as beryllium (Be) and radium (Ra).

[0065] In the Ellingham diagram, calcium (Ca) is located below Sm, so Ca has the effect of reducing Sm in all temperature ranges. ２ Fe １７ N ３ It is also used as a reducing material for Sm oxide when chemically synthesizing particles. ２ Fe １７ N ３ Since Ca is an element that undergoes a redox reaction with the inevitable oxides present on the particle surface of a main phase of Sm—Fe—O, alloys containing Ca also have a similar effect. "Occurrence of a redox reaction" can be rephrased as "having a certain level of reactivity with the main phase particle surface," and having wettability can be considered one aspect of that reactivity. Ca belongs to Group 2 of the periodic table, and its homologous elements, Be, Mg, Sr, Ba, and Ra, also have a reducing effect on Sm—Fe—O. Therefore, similar effects can be expected with other elements in the same group as Ca. In addition, in binary phase diagrams, element X (Be, Mg, Sr, Ba, and Ra) have a eutectic point when alloyed with Ca, and can achieve the minimum required low melting point as a binder. In fact, even when element X is Mg, Sr, or Ba, preliminary experiments showed that alloying with Sm achieved a low melting point, and the alloy exhibited a low melting point. ２ Fe １７ N ３ It has been revealed that it exhibits good wettability.

[0066] In addition, in the preliminary experiments, lanthanum (La) and praseodymium (Pr) were used as rare earth elements, but for the following reasons, it is believed that similar effects can be obtained by using other rare earth elements such as scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), promethium (Pm), samarium (Sm), eurobium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0067] The main phase particles are Sm ２ Fe １７ N ３ As shown in the composition, it contains the rare earth element Sm. Sm and other lanthanoid elements (such as La and Pr) are expected to exhibit high similarity / substitution properties, as in a complete solid solution system, and to have high reactivity. In fact, it has been experimentally demonstrated that one of the reactivity properties is good wettability.

[0068] Therefore, the inventors aimed to improve the volume fraction of the first phase (magnetic phase) by optimizing the composition and amount of the metal binder containing at least one of a Group 2 element and a rare earth element.

[0069] The present disclosure has been described above based on embodiments, examples, and modifications. However, the above-described embodiments are intended to facilitate understanding of the present disclosure and are not intended to limit the present disclosure. The present disclosure may be modified or improved without departing from the spirit and scope of the claims, and equivalents thereof are included in the present disclosure. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate.

[0070] The present disclosure can also be realized as the following application examples. [Application Example 1] A sintered magnet comprising: Th ２ Zn １７A sintered magnet comprising: a first phase primarily composed of Sm—Fe—N crystal grains having a Zn—Fe—N structure; and a second phase composed of a metal binder containing at least one Group 2 element and / or rare earth element and having a melting point of 620°C or lower, wherein the volume fraction of the first phase is 90% or higher. [Application Example 2] The sintered magnet according to Application Example 1, wherein the second phase satisfies at least one of the following (1) and (2): (1) The content of the Group 2 element is 20 atomic % or higher but 90 atomic % or lower. (2) The content of the rare earth element is 10 atomic % or higher but 80 atomic % or lower. [Application Example 3] The sintered magnet according to Application Example 1 or Application Example 2, wherein the volume fraction of the second phase is 0.1% or higher but 2.4% or lower. [Application Example 4] The sintered magnet according to any one of Application Examples 1 to 3, wherein the Sm—Fe—N crystal grains of the first phase have an average grain size of 0.1 μm or more and 20 μm or less. [Application Example 5] A method for producing a powder for a sintered magnet used in molding the sintered magnet according to any one of Application Examples 1 to 4, comprising: a crushing step of crushing a coarse powder containing Sm—Fe—N single crystals to obtain Sm—Fe—N crystal grains; a binder powder preparation step of preparing a metal binder powder to serve as the metal binder; and a mixing step of dispersing the Sm—Fe—N crystal grains and the metal binder powder to obtain the powder for the sintered magnet, which is a mixed powder, wherein the crushing step, the binder powder preparation step, and the mixing step are carried out in an atmosphere with a low oxygen concentration. [Application Example 6] The method for producing a sintered magnet according to any one of Application Examples 1 to 4, further comprising a sintering step of pressure-sintering the powder for a sintered magnet produced by the method for producing a powder for a sintered magnet according to Application Example 5 at a sintering temperature of 600°C or less in an atmosphere with a low oxygen concentration.

[0071] 10...First phase 10G...Sm-Fe-N crystal grains 20...Second phase 100...Sintered magnet 100S...Sample for SEM observation V...Void

Claims

1. A sintered magnet, ２ Zn １７ A sintered magnet comprising: a first phase whose main phase is Sm—Fe—N crystal grains having a Zn—Fe—N structure; and a second phase made of a metal binder containing at least one element selected from the group 2 elements and rare earth elements and having a melting point of 620°C or lower, wherein the volume fraction of the first phase is 90% or higher.

2. The sintered magnet according to claim 1, wherein the second phase satisfies at least one of the following conditions (1) and (2): (1) The content of Group 2 elements is 20 atomic percent or more and 90 atomic percent or less, and (2) The content of rare earth elements is 10 atomic percent or more and 80 atomic percent or less.

3. A sintered magnet according to claim 1, characterized in that the volume fraction of the second phase is 0.1% or more and 2.4% or less.

4. A sintered magnet according to claim 1, characterized in that the average grain size of the Sm-Fe-N crystal grains of the first phase is 0.1 μm or more and 20 μm or less.

5. A method for producing powder for sintered magnets used in molding the sintered magnets according to any one of claims 1 to 4, comprising: a crushing step of crushing coarse powder containing Sm-Fe-N single crystals to obtain Sm-Fe-N crystal grains; a binder powder preparation step of preparing metal binder powder to serve as the metal binder; and a mixing step of dispersing the Sm-Fe-N crystal grains and the metal binder powder to obtain the powder for sintered magnets, which is a mixed powder; wherein the crushing step, the binder powder preparation step, and the mixing step are carried out in an atmosphere with a low oxygen concentration.

6. A method for producing a sintered magnet according to any one of claims 1 to 4, characterized in that it comprises a sintering step in which the powder for a sintered magnet produced by the method for producing a powder for a sintered magnet according to claim 5 is pressure-sintered at a sintering temperature of 600°C or less in an atmosphere with a low oxygen concentration.

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