Sm-fe-n-based magnet

Abstract

Disclosed is an Sm-Fe-N-based magnet which includes: crystal grains that contain samarium, iron, and nitrogen, containing first crystal grains that contain samarium in an amount of not less than 9% by atom but less than 13% by atom and second crystal grains that contain samarium in an amount of 13% by mass or more. In a distribution of average distances between adjacent grains among the second crystal grains, the average distance between adjacent grains having a cumulative frequency of 50% is 7 μm or less, and the average distance between adjacent grains having a cumulative frequency of 90% is 12 μm or less.

Description

Sm-Fe-N magnet

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

[0002] Sm—Fe—N 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 crystal grains have been used as one of the excellent materials for magnetic powders. However, it is said that Sm—Fe—N magnets tend to lose coercive force when heat-treated.

[0003] Therefore, Patent Document 1 discloses a magnet powder of an Sm-Fe-N magnet in which the surface of a main phase containing samarium and iron is covered with a subphase containing a larger amount of rare earth elements.

[0004] International Publication No. 2019-189440

[0005] According to Patent Document 1, covering the main phase with a specific subphase increases the coercive force of the magnet after heat treatment. However, because the magnet in Patent Document 1 contains the subphase, the proportion of the main phase in the magnet is reduced, resulting in a decrease in remanence.

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

[0007] In order to solve the above-mentioned problems, one aspect of the present invention provides an Sm—Fe—N based magnet comprising: first crystal grains containing samarium, iron, and nitrogen, the first crystal grains containing 9 atomic % or more but less than 13 atomic % samarium; and second crystal grains containing 13 atomic % or more samarium, wherein in the distribution of the average distance between adjacent particles of the second crystal grains, the average distance between adjacent particles at a cumulative frequency of 50% is 7 μm or less, and the average distance between adjacent particles at a cumulative frequency of 90% is 12 μm or less.

[0008] According to the present invention, there is provided an Sm--Fe--N magnet having high coercive force and high remanent magnetization.

[0009] This is an example of a SEM image (2000x magnification) used for segmentation. It is an example of a SEM image obtained by binarizing Fig. 1A and showing the first crystal grains (main phase) in black. It is an example of a SEM image obtained by binarizing Fig. 1A and showing the grain boundaries between the first crystal grains and the second crystal grains in black. It is a figure explaining the method for calculating the inter-particle distance. It is a SEM image (2000x magnification) of the cross-section of the sintered magnet obtained in Example 1. It is a SEM image (2000x magnification) of the cross-section of the sintered magnet obtained in Comparative Example 1. It is a SEM image (2000x magnification) of the cross-section of the sintered magnet obtained in Comparative Example 2.

[0010] (Configuration) The Sm—Fe—N-based magnet (samarium-iron-nitrogen-based magnet) according to the present disclosure includes crystal grains containing samarium, iron, and nitrogen, and includes first crystal grains containing 9 atomic % or more and less than 13 atomic % of samarium. The Sm—Fe—N-based magnet according to the present disclosure further includes second crystal grains (hereinafter, also referred to as Sm-rich crystal grains) containing 13 atomic % or more of samarium.

[0011] The first crystal grains contain samarium, iron, and nitrogen. The crystal grains containing samarium, iron, and nitrogen (hereinafter, also referred to as Sm—Fe—N-based crystal grains) refer to particles having a Sm—Fe—N-based crystal structure. The second crystal grains are crystal grains containing samarium oxide (not containing iron and nitrogen), or crystal grains containing samarium, iron, and nitrogen.

[0012] The first crystal grains form the main phase of the Sm—Fe—N-based magnet. The first crystal grains are the target substances when synthesizing Sm—Fe—N-based crystal grains (an alloy containing samarium, iron, and nitrogen). On the other hand, the Sm-rich crystal grains are by-products that are inevitably generated when synthesizing an alloy containing samarium, iron, and nitrogen. It has been found that the coercive force is improved when the inevitably generated Sm-rich crystal grains are in a specific distribution state.

[0013] That is, in the distribution of the average distances between adjacent second crystal grains in the magnet of the present disclosure, the average distance D50 between adjacent particles at a cumulative frequency of 50% is 7 μm or less, and the average distance D90 between adjacent particles at a cumulative frequency of 90% is 12 μm or less. In other words, 50% of the Sm-rich crystal grains are located within an average distance of 7 μm from other Sm-rich crystal grains, and 90% of the Sm-rich crystal grains are located within an average distance of 12 μm from other Sm-rich crystal grains. This indicates that the Sm-rich crystal grains are not unevenly distributed but are uniformly dispersed within the magnet.

[0014] The reason why the coercive force of a magnet improves when Sm-rich crystal grains are uniformly dispersed within the magnet is thought to be as follows: When Sm-rich crystal grains are uniformly dispersed within the magnet, the Sm-rich crystal grains can also be considered to be uniformly dispersed in the magnet precursor (for example, the magnet after molding and before firing or hardening treatment).

[0015] When manufacturing a magnet from Sm—Fe—N crystal grains (here, at least the first and second crystal grains), heat treatment may be performed. For example, heat treatment may be performed during the sintering or hardening process. During this process, a redox reaction occurs in which Sm oxides are formed, and α-iron (α-Fe) may precipitate. α-Fe is soft magnetic, and magnets containing it tend to have a low coercive force. However, if Sm-rich crystal grains are uniformly dispersed in the magnet precursor, Fe can easily react with the Sm abundant in the Sm-rich crystal grains before precipitation. As a result, α-Fe precipitation is suppressed. Therefore, magnets manufactured in the presence of Sm-rich crystal grains can have a high coercive force.

[0016] As described above, the present disclosure controls the distribution of the unavoidably generated Sm-rich crystal grains, thereby preventing a decrease in the proportion of the main phase in the magnet. As a result, the high remanence inherent in Sm—Fe—N magnets can be maintained, and the resulting magnet also has high remanence.

[0017] The distribution of Sm-rich crystal grains can be controlled by, for example, the pulverization conditions when synthesizing Sm—Fe—N crystal grains, which will be described later.

[0018] The average distance D50 between adjacent particles at a cumulative frequency of 50% of the Sm-rich crystal grains (hereinafter simply referred to as "adjacent particle distance D50") may be 6.5 μm or less, 6.3 μm or less, or 5.4 μm or less. The adjacent particle distance D50 may be 3.0 μm or more, 3.5 μm or more, or 4.0 μm or more.

[0019] The average distance D90 between adjacent grains at a cumulative frequency of 90% of the Sm-rich crystal grains (hereinafter simply referred to as "adjacent grain distance D90") may be 11.5 μm or less, or 11.3 μm or less. The adjacent grain distance D90 may be 5.0 μm or more, or 8.0 μm or more.

[0020] The magnet according to the present disclosure includes at least the first and second crystal grains described above. The Sm—Fe—N magnet may also contain other materials, such as α-Fe, and trace elements that are inevitably mixed in.

[0021] The magnet may be a bonded magnet or a sintered magnet. The magnet may be a sintered magnet. A sintered magnet is obtained by sintering Sm—Fe—N crystal grains and the like at high temperatures. The Sm—Fe—N magnet, which is a sintered magnet according to the present disclosure, may essentially consist of a sintered body of first crystal grains and second crystal grains. Since Sm-rich crystal grains are uniformly dispersed even in the magnet precursor (the magnet after molding and before sintering), precipitation of α-Fe during the sintering process can be effectively suppressed.

[0022] <Method for Identifying Crystal Grains> The first and second crystal grains can be identified as follows. First, the element distribution in the cross section of the magnet is obtained by energy dispersive X-ray (EDX) analysis. The element distribution is typically measured using a SEM-EDX analyzer.

[0023] The EDX analysis is carried out, for example, under the following conditions.

[0024]

[0025] An image using a SEM (scanning electron microscope) for EDX analysis can be obtained, for example, under the following conditions.

[0026]

[0027] Next, based on the element distribution obtained by SEM-EDX analysis, a region where the atomic percentage of samarium is 13 atomic % or more (corresponding to second crystal grains (Sm-rich crystal grains)) is separated (segmented) into a region where the atomic percentage of samarium is 9 atomic % or more but less than 13 atomic % (corresponding to first crystal grains). By segmentation, the grain boundaries between the first and second crystal grains are determined, and the first and second crystal grains are identified.

[0028] Segmentation can be performed by processing the SEM image using deep learning image processing software (for example, "MIPAR3.4" manufactured by Lightstone). The specific procedure of this method is as follows.

[0029]

[0030] The segmentation may be performed by binarizing the SEM image. The specific procedure for this method is as follows.

[0031]

[0032] Fig. 1A is an example of an SEM image (magnification 2000x) used for segmentation. Fig. 1B is an example of an SEM image obtained by binarizing Fig. 1A and showing first crystal grains (main phase) in black. Fig. 1C is an example of an SEM image obtained by binarizing Fig. 1A and showing grain boundaries between first crystal grains and second crystal grains in black.

[0033] <Distance between adjacent particles> The distance between adjacent particles can be obtained by performing Delaunay triangulation on the cross section of the magnet. First, the centers of gravity of the multiple Sm-rich crystal grains identified by the segmentation above are regarded as points, and a set of points distributed discretely on a plane is created. Delaunay triangulation is then performed on this set of points.

[0034] Next, as shown in FIG. 2 , an arbitrary vertex T0 is selected from the multiple Delaunay triangles obtained. The vertex T0 is also the center of gravity of a certain Sm-rich crystal grain (particle of interest P1, not shown). The other two vertices T1 and T2 of the Delaunay triangle including the vertex T0 are also the centers of gravity of two other Sm-rich crystal grains that are close (adjacent) to the particle of interest P1. The distance D1 of the line segment connecting the vertex T0 and the vertex T1 represents the distance between the particle of interest P1 and another Sm-rich crystal grain that is close (adjacent) to it (the adjacent particle distance). Similarly, the distance D2 of the line segment connecting the vertex T0 and the vertex T2 represents the distance between the particle of interest P1 and yet another Sm-rich crystal grain that is close (adjacent) to it (the adjacent particle distance). There are multiple Delaunay triangles that include the vertex T0. For all Delaunay triangles including vertex T0, the distance D of the line segments connecting vertex T0 and other vertices TX (not shown, points surrounded by dashed lines) is calculated and the distances D are averaged. The distance D is set as the average distance Dave between the target particle P1 and the multiple Sm-rich crystal grains adjacent to the target particle P1.

[0035] Next, the same operation as for the particle P1 is performed on all other Sm-rich crystal grains as the target particle to determine the average distance Dave between all the Sm-rich crystal grains and other Sm-rich crystal grains adjacent to it. The obtained average distances Dave between the multiple neighboring particles are plotted as a histogram to obtain a frequency distribution of the average distances Dave. In the frequency distribution of the average distances Dave, the average distance D50 between neighboring particles with a cumulative frequency of 50% and the average distance D90 between neighboring particles with a cumulative frequency of 90% are determined.

[0036] <First Crystal Grains> The first crystal grains form the main phase of the magnet. The first crystal grains are Sm—Fe—N crystal grains containing 9 atomic % or more and less than 13 atomic % samarium. The crystal structure of the first crystal grains is, for example, SmFe ９ N １．５ Structure, Sm ２ Fe １７ N ３ The first grains may have any crystalline structure consisting of Sm, Fe, and N, but are not limited to this. ２ Fe １７ N ３It is a structure.

[0037] The average grain size of the first crystal grains is not particularly limited. The average grain size of the first crystal grains may be, for example, 0.04 μm or more and 5 μm or less. When the average grain size of the first crystal grains is 0.04 μm or more, it is possible to effectively suppress the Sm—Fe—N crystal grains from becoming superparamagnetic. When the average grain size of the first crystal grains is 5 μm or less, it is possible to effectively improve the coercive force.

[0038] The method for calculating the average grain size of the first crystal grains is as follows. First, a cross-section of the magnet is photographed using a field emission scanning electron microscope (FE-SEM) so that a total of at least 50 crystal grains are included. The first and second crystal grains in the photographed image are identified in the same manner as above. Next, the total area A1 of the cross-sections of the first crystal grains in the photographed image and the number N1 of crystal grains are determined. With A1 / N1 being the average cross-sectional area per first crystal grain, the average grain size of the first crystal grains is calculated from the square root of the average cross-sectional area a1.

[0039] <Second crystal grains> The second crystal grains contain 13 atomic % or more of samarium. ２ O ３ The amount of samarium in the second crystal grains may be 14 atomic % or less. The crystal structure of the second crystal grains, which are Sm—Fe—N crystal grains, may be, for example, Sm ５ Fe １７ N structure, SmFe ３ The second crystal grains may be any crystal structure consisting of Sm, Fe and N, but are not limited to this. The second crystal grains are uniformly dispersed in the magnet.

[0040] The average grain size of the second crystal grains may be 0.2 μm or more and 1 μm or less. By uniformly dispersing small second crystal grains with an average grain size of 0.2 μm or more and 1 μm or less throughout the magnet, the coercive force can be further improved. The average grain size of the second crystal grains may be 0.95 μm or less, 0.90 μm or less, or 0.86 μm or less. The average grain size of the second crystal grains may be 0.70 μm or more, 0.75 μm or more, or 0.76 μm or more.

[0041] The average grain size of the second crystal grains can also be calculated from the total area A2 of the cross sections of the second crystal grains in the captured image and the number N2 of the crystal grains, similar to the average grain size of the first crystal grains.

[0042] In the cross section of the magnet, the ratio of the total area of ​​the second crystal grains to the total area of ​​the first crystal grains may be, for example, 2% or more and 10% or less. When the area ratio of the second crystal grains is 2% or more, the effect of improving the coercive force is easily obtained. When the area ratio of the second crystal grains is 10% or less, the decrease in remanence is suppressed.

[0043] The area ratio of the second crystal grains is calculated from the total area A2 of the cross sections of the second crystal grains and the total area A1 of the cross sections of the first crystal grains, which are obtained in the same manner as when calculating the average grain size of the first and second crystal grains, by the formula 100×A2 / A1 (%).

[0044] 100×A2 / A1 (%) may be 3% or more, 3.5% or more, 4.0% or more, or 5.0% or more. 100×A2 / A1 (%) may be 9.0% or less, or 8.0% or less.

[0045] (Manufacturing Method) An example of a method for manufacturing a sintered magnet is shown below. First, the raw material, Sm—Fe—N magnet coarse powder, is pulverized. By carrying out the pulverization under appropriate conditions, it is possible to reduce the adjacent particle distance D50 to 7 μm or less and the adjacent particle distance D90 to 12 μm or less.

[0046] The Sm—Fe—N coarse magnet powder is pulverized, for example, by repeating the pulverization and classification cycle multiple times, which results in the Sm-rich crystal grains contained in the Sm—Fe—N coarse magnet powder being pulverized more finely and dispersed more uniformly, making it easier for the adjacent particle distances D50 and D90 to satisfy the above-mentioned ranges.

[0047] The pulverization process may be carried out, for example, at a pulverization pressure of 0.3 MPa or more and 1.2 MPa or less. The pulverization pressure may be 0.5 MPa or more, 0.6 MPa or more, or 0.7 MPa or more. The pulverization pressure may be 1.0 MPa or less, or 0.9 MPa or less. The number of cycles may be, for example, 2 or more, 3 or more, or 4 or more.

[0048] The pulverization can be carried out using, for example, a jet mill (airflow pulverization type, etc.), a ball mill, etc. An example of an airflow pulverization type jet mill is MC44 manufactured by Micromachinazione.

[0049] The pulverization is preferably carried out in a glove box purged with an inert gas (one or a mixture of two or more gases such as nitrogen, argon, and helium). The pulverization may also be carried out in a glove box connected to a gas circulation type oxygen and moisture purifier.

[0050] In the powder after pulverization, fine powder (corresponding to the particle portion with an extremely small particle size in the particle distribution of the powder) is more likely to be damaged by pulverization and has lower crystallinity than larger particles. To obtain a pulverized powder with good crystallinity, such fine powder with low crystallinity may be removed by classification. Fine powder removal can be performed using an air classifier or the like, but is not limited to this. The removed fine powder may be, for example, particles with a particle size of less than 0.04 μm.

[0051] Next, the mixture containing the first crystal grains and the second crystal grains is pressure sintered, thereby obtaining a sintered magnet.

[0052] Prior to pressure sintering, the mixture may be subjected to an orientation process and a magnetization process. This aligns the easy magnetization axes of the crystal grains, resulting in higher magnetic properties. The applied magnetic field may be, for example, a static magnetic field of 2 T or more.

[0053] Pressure sintering is performed, for example, in an atmosphere with a low oxygen concentration. Any pressure sintering method, including electric pressure sintering, can be used for pressure sintering. Pressure sintering may involve, for example, filling a die with magnet powder and placing it in a pulse current sintering machine equipped with a pressure mechanism using a servo-controlled press without exposing it to the atmosphere. Subsequently, while maintaining a vacuum inside the pulse current sintering machine, a certain pressure is applied to the die, and electric current sintering is performed while maintaining this pressure. The die used may have any shape, including, but not limited to, a cylindrical die. The pulse current sintering machine is maintained in a vacuum of, for example, 5 Pa or less. The applied pressure may be higher than atmospheric pressure and sufficient to form a sintered magnet, for example, in the range of 100 MPa to 2000 MPa. Electric current sintering is performed, for example, at a temperature of 400°C to 600°C for a time of 30 seconds to 10 minutes.

[0054] The method for producing the mixture containing the first crystal grains and the second crystal grains is not limited to the above-described method, and any appropriate method may be used. The method for producing the magnet is not limited to the above-described method, and any appropriate method may be used.

[0055] Examples 1 to 4 and Comparative Examples 1 and 2 Sm-Fe-N sintered magnets were produced by the following procedure. ２ Fe １７ N ３ A coarse powder having an average particle size of about 25 μm was prepared. The prepared coarse powder was pulverized using an airflow pulverization jet mill under various conditions. The pulverization conditions are shown in the table below.

[0056]

[0057] The pulverization was carried out in a glove box under a low-oxygen atmosphere. After pulverization, fine powder (particles with a particle size of less than 0.04 μm) was removed using an air classifier. As a result, Sm—Fe—N crystal grains were obtained.

[0058] 0.5 g of the resulting Sm—Fe—N crystal grains was weighed and packed into a cylindrical cemented carbide die with an inner diameter of 6 mm. This was placed in a pulse current sintering machine equipped with a servo-controlled press mechanism without being exposed to the atmosphere. Next, while maintaining a vacuum of 2 Pa or less and an oxygen concentration of 0.4 ppm or less inside the pulse current sintering machine, a pressure of 1200 MPa was applied. While maintaining this pressure, the magnet was subjected to current sintering for 2 minutes at a sintering temperature of 500°C, yielding a sintered magnet. SEM images of the cross section of the sintered magnet are shown in Figures 3 to 5. In the SEM images, the light gray areas represent the second crystal grains, and the black or dark gray areas represent the first crystal grains.

[0059] (Evaluation) The obtained sintered magnets were evaluated as follows, and the evaluation results are shown in the table below.

[0060] (1) Average grain size, A2 / A1 of each crystal grain An SEM image of the cross section of the sintered magnet was taken, and the first and second crystal grains in the image were identified using the deep learning image processing software ("MIPAR3.4" manufactured by Lightstone) described above. The total area A1 and number N1 of the cross section of the first crystal grains in the image were determined, and the total area A2 and number N2 of the cross section of the second crystal grains were calculated. The average grain size and A2 / A1 of each crystal grain were calculated in the same manner as described above.

[0061] (2) D50 and D90 of Average Distance Between Adjacent Particles A cross-sectional SEM image of the sintered magnet was subjected to Delaunay triangulation, and the average value of the distance between adjacent particles was calculated in the same manner as above, and the resultant was plotted as a histogram.

[0062] (3) Residual Magnetization and Coercive Force These were measured using a vibrating sample magnetometer (VSM).

[0063]

[0064] The sintered magnet and magnet powder of the present invention can be used in a wide range of applications in the field of various motors, such as in-vehicle accessory motors and main motors for EVs and HEVs, and more specifically, in oil pump motors, electric power steering motors, and EV / HEV drive motors.

[0065] This application claims priority based on Japanese Patent Application No. 2023-170759, filed on September 29, 2023, the entire contents of which are incorporated herein by reference.

[0066] T0 is one of the vertices of the Delaunay triangle and is the center of gravity of the particle P1 of interest. T1 is another of the vertices of the Delaunay triangle and is the center of gravity of another Sm-rich crystal grain that is close (adjacent) to the particle P1 of interest. T2 is yet another of the vertices of the Delaunay triangle and is the center of gravity of yet another Sm-rich crystal grain that is close (adjacent) to the particle P1 of interest. D1 is the distance of the line segment connecting the vertex T0 and the vertex T1. D2 is the distance of the line segment connecting the vertex T0 and the vertex T2.

Claims

1. A Sm-Fe-N magnet comprising: first crystal grains containing samarium, iron, and nitrogen, the first crystal grains containing 9 atomic % or more but less than 13 atomic % of samarium; and second crystal grains containing 13 atomic % or more of samarium, wherein in a distribution of the average distance between adjacent particles of the second crystal grains, the average distance between adjacent particles with a cumulative frequency of 50% is 7 μm or less, and the average distance between adjacent particles with a cumulative frequency of 90% is 12 μm or less.

2. The Sm-Fe-N magnet according to claim 1, wherein the average grain size of the second crystal grains is from 0.2 μm to 1 μm.

3. The Sm-Fe-N magnet according to claim 1 or 2, which is a sintered body containing the first crystal grains and the second crystal grains.

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