JP7670638B2 - Manufacturing method of rare earth magnet - Google Patents

Description

The present disclosure relates to a rare earth magnet and a manufacturing method thereof. In particular, the present disclosure relates to a rare earth magnet that contains Sm, Fe, and N and has a magnetic phase at least a portion of which has either a Th ₂ Zn ₁₇ type or a Th ₂ Ni ₁₇ type crystal structure.

High-performance rare earth magnets that have been put to practical use include Sm-Co rare earth magnets and Nd-Fe-B rare earth magnets, but in recent years, other rare earth magnets have been considered.

For example, rare earth magnets containing Sm, Fe, and N (hereinafter sometimes referred to as "Sm-Fe-N rare earth magnets") are being considered. Sm-Fe-N rare earth magnets are manufactured using, for example, magnetic powder containing Sm, Fe, and N (hereinafter sometimes referred to as "SmFeN powder").

SmFeN powder has a magnetic phase having either a Th ₂ Zn ₁₇ type or a Th ₂ Ni ₁₇ type crystal structure. This magnetic phase is thought to be a solid solution of N interstitially dissolved in Sm-Fe crystals. Therefore, SmFeN powder is easily decomposed by N being dissociated by heat. For this reason, Sm-Fe-N rare earth magnets are often manufactured by molding SmFeN powder with resin and/or rubber.

Other methods for manufacturing Sm-Fe-N rare earth magnets include, for example, the method disclosed in Patent Document 1. This method involves mixing SmFeN powder with a powder containing metallic zinc (hereinafter sometimes referred to as "metallic zinc powder"), compacting the mixed powder in a magnetic field, and sintering (including liquid phase sintering) the magnetically compacted body.

Methods for producing SmFeN powder are disclosed, for example, in Patent Documents 2 and 3.

International Publication No. 2015/199096 JP 2017-117937 A JP 2020-102606 A

Methods for sintering magnetically compacted bodies can be broadly divided into pressureless sintering and pressure sintering. In either sintering method, a high-density rare earth magnet (sintered body) is obtained by sintering the magnetically compacted body. In the pressureless sintering method, no pressure is applied to the magnetically compacted body during sintering, so to obtain a high-density sintered body, it is common to sinter the magnetically compacted body at a high temperature of 900°C or more for a long period of time of 6 hours or more. On the other hand, in the pressure sintering method, pressure is applied to the magnetically compacted body during sintering, so it is common to obtain a high-density sintered body even if the magnetically compacted body is sintered at a low temperature of 600-800°C for a short period of time of 0.1-5 hours.

When sintering a magnetic compact of a mixture of SmFeN powder and metal zinc powder, pressure sintering is used to avoid thermal decomposition of the SmFeN powder, but the sintering temperature is lower and shorter than the sintering temperature of normal pressure sintering. Sintering is possible at such a low temperature and in such a short time because the zinc component in the metal zinc powder diffuses to the surface of the magnetic powder during sintering and sinters (solidifies). In this way, the metal zinc powder in the magnetic compact functions as a binder. In addition, the metal zinc powder in the magnetic compact functions as a modifier, modifying the α-Fe phase in the SmFeN powder and absorbing oxygen in the SmFeN powder to improve the coercive force. Hereinafter, the powder used in the manufacture of Sm-Fe-N rare earth magnets and functioning as both a binder and a modifier may be referred to as a "modifier powder."

In both cases where the magnetic powder is molded using resin and/or rubber, or where a mixed powder of the magnetic powder and modifier powder is pressure sintered, the magnetization of the molded body (rare earth magnet) decreases by the amount of the resin and modifier content that do not contribute to magnetization. On the other hand, compared to molding the magnetic powder using resin and/or rubber, etc., when a mixed powder of the magnetic powder and modifier powder, or a coated magnetic powder, is pressure sintered, a high density molded body (rare earth magnet) is generally obtained, and as a result, it is easier to obtain high magnetization. However, when the magnetic powder is SmFeN powder, even if a mixed powder of the magnetic powder and modifier powder is pressure sintered, the magnetization is lower than predicted from the modifier content, and the desired magnetization cannot be obtained.

In addition, when permanent magnets such as Sm-Fe-N rare earth magnets are used in motors, the permanent magnets are placed in an external magnetic field environment that changes periodically. As a result, the permanent magnets are affected by the external magnetic field. In an ideal permanent magnet, the magnetization does not decrease due to the external magnetic field in the motor's operating range, but in an actual permanent magnet, the magnetization decreases due to the external magnetic field in the motor's operating range. If the decrease in magnetization (demagnetization) due to the external magnetic field is large in the motor's operating range, the current control on the stator side of the motor becomes complicated and the load on the inverter connected to the motor increases. To reduce the load on the inverter, a large-capacity inverter is required, which reduces economy. This is particularly noticeable when the motor operates at high output and the permanent magnets inside the motor become hot.

The inventors have therefore discovered that there is a need for a method for producing Sm-Fe-N rare earth magnets that can achieve even greater magnetization than conventional magnets and that are less susceptible to demagnetization than conventional magnets in the motor operating range, particularly at high temperatures.

This disclosure has been made to solve the above problems. In other words, the purpose of this disclosure is to provide a method for producing an Sm-Fe-N rare earth magnet that can further improve magnetization compared to conventional methods and is less susceptible to demagnetization than conventional methods, especially at high temperatures, in an environment where an external magnetic field is applied.

The present inventors have conducted intensive research to achieve the above object, and have completed the method for producing a rare earth magnet according to the present disclosure. The method for producing a rare earth magnet according to the present disclosure includes the following aspects.
<1> A method for producing a rare earth magnet containing Sm, Fe, and N and having a magnetic phase at least a portion of which has a crystal structure of either _{Th2Zn17 type} or _{Th2Ni17 type} , comprising the steps of:
Providing a magnetic powder comprising a first group of particles and a second group of particles;
forming a first coating on the surfaces of the particles of the first particle group of the magnetic powder to obtain a first coated magnetic powder;
forming a second coating on the surfaces of the particles of the second particle group of the magnetic powder to obtain a second coated magnetic powder;
mixing the first coated magnetic powder and the second coated magnetic powder to obtain a mixed powder;
The mixed powder is compression-molded in a magnetic field to obtain a magnetically molded body.
Pressure sintering the magnetic field compact to obtain a sintered body; and heat treating the sintered body.
Including,
The particle size distribution _D50 of the first particle group is expressed in terms of _d1 μm, and the particle size distribution _D50 of the second particle group is expressed in terms of _d2 μm;
The _d1 and _d2 satisfy the relationship of 0.350≦ _d2 / _d1 ≦0.500,
a ratio of a total volume of the first particle group to a total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) is in the range of 9:1 to 4:1,
the first coating contains zinc, and the content of the zinc component in the first coated magnetic powder is 3.0 mass% or more and 10.0 mass% or less with respect to the total mass of the first coated magnetic powder and the second coated magnetic powder; and
During the heat treatment, the first coating advances the modification of the surfaces of the particles of the first particle group, and the second coating inhibits the surfaces of the particles of the second particle group from being modified by the adjacent first coating.
A method for manufacturing rare earth magnets.
<2> The method for producing a rare earth magnet according to <1>, wherein the second coating contains phosphoric acid.
<3> The method for producing a rare earth magnet according to <1> or <2>, further comprising heat-treating the sintered body until an Fe-Zn alloy phase is formed on 75% or more of the surfaces of the particles of the first particle group in the sintered body.
<4> The method for producing a rare earth magnet according to any one of <1> to <3>, wherein the heat treatment is carried out at a temperature of 350° C. or higher and 410° C. or lower.
<5> The method for producing a rare earth magnet according to any one of <1> to <4>, wherein the heat treatment is carried out for 3 hours or more and 40 hours or less.
<6> The method for producing a rare earth magnet according to any one of <1> to <5>, wherein the magnetically compacted body is pressure-sintered at a pressure of 200 MPa to 1500 MPa and a temperature of 300°C to 400°C for 1 minute to 30 minutes.

According to the present disclosure, the density of a sintered body (rare earth magnet) is improved by using a magnetic powder containing a first particle group (large particle group) and a second particle group (small particle group), and during heat treatment of the sintered body, the modification of the surface of the particles of the first particle group is promoted and the modification of the surface of the particles of the second particle group is suppressed. As a result, it is possible to provide a method for producing a Sm-Fe-N rare earth magnet that is resistant to demagnetization, particularly at high temperatures, by improving the density of the sintered body, and by controlling the modification between the first particle group and the second particle group and reducing the difference in coercivity caused by each of the first particle group and the second particle group.

FIG. 1 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by the manufacturing method of the present disclosure. FIG. 2 is a schematic diagram showing an example of the structure in a sintered body before heat treatment. FIG. 3 is a graph showing the demagnetization curve of a compact of low coercivity powder and a demagnetization curve of a compact of a mixed powder of low coercivity powder and high coercivity powder at high temperatures. FIG. 4 is an explanatory diagram showing an example of a method for forming a zinc-containing coating (first coating) on the surfaces of particles of SmFeN powder (first particle group) by using a rotary kiln. FIG. 5 is an explanatory diagram showing an example of a method for forming a zinc-containing coating (first coating) on the surfaces of particles of SmFeN powder (first particle group) by a vapor deposition method. FIG. 6 is a schematic diagram showing the structures of the samples of Comparative Examples 1 and 2. FIG. 7 is a schematic diagram showing the structures of the samples of Comparative Examples 3 to 5. FIG. 8 is a schematic diagram showing the structure of the sample of Comparative Example 7. FIG. 9 is a schematic diagram showing the structures of the samples of Comparative Examples 8 and 9.

Below, an embodiment of the manufacturing method for rare earth magnets disclosed herein (hereinafter, sometimes referred to as the "manufacturing method of the present disclosure") will be described in detail. Note that the embodiment described below does not limit the manufacturing method of the present disclosure.

The reason why the rare earth magnets obtained by the manufacturing method disclosed herein have higher magnetization and are less susceptible to demagnetization, particularly at high temperatures, than rare earth magnets obtained by conventional manufacturing methods can be explained as follows, without being bound by theory, using drawings.

Figure 1 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by the manufacturing method of the present disclosure. As shown in Figure 1, a rare earth magnet 90 obtained by the manufacturing method of the present disclosure has SmFeN powder 10 including a first particle group 11 and a second particle group 12. A modified phase 20 is formed on the surface of the particles of the first particle group 11. This modified phase 20 bonds the particles of the first particle group 11 to each other, or bonds the particles of the first particle group 11 to the particles of the second particle group 12, forming the rare earth magnet 90.

In the rare earth magnet 90 obtained by the manufacturing method of the present disclosure, the particles of the second particle group 12, which have a small particle size, are present between the particles of the first particle group 11, which have a large particle size. This improves the density of the rare earth magnet 90, and as a result, improves the magnetization of the rare earth magnet 90. Here, in order to improve the density, it is necessary that the ratio of the particle size of the second particle group 12 to the particle size of the first particle group 11 is within a predetermined range, and that the ratio of the total volume of the first particle group 11 to the total volume of the second particle group 12 is within a predetermined range.

As described above, each particle of the SmFeN powder 10 is bonded by the modified phase 20 on the particle surface of the first particle group 11. The modified phase 20 on the particle surface of the first particle group 11 originates from a first coating formed on the particle surface of the first particle group 11 before pressure sintering. The formation of the modified phase 20 will be explained using the drawings.

Figure 2 is a schematic diagram showing an example of the structure in a sintered body before heat treatment. A first coated magnetic powder 31 and a second coated magnetic powder 32 are present in a sintered body 30 before heat treatment. In the first coated magnetic powder 31, a first coating 41 is present on the surface of the particles of the first particle group 11. In the second coated magnetic powder 32, a second coating 42 is present on the surface of the particles of the second particle group 12.

Since the first coating 41 contains zinc, Fe diffuses from the particle surfaces of the first particle group 11 into the first coating 41 during the heat treatment of the sintered body 30. Then, this Fe and the Zn in the first coating 41 are alloyed to form an Fe-Zn alloy phase in the first coating 41. This Fe-Zn phase is the modified phase 20. The Fe that diffuses into the first coating 41 is mainly derived from the α-Fe phase present on the particle surfaces of the first particle group 11. The α-Fe phase is composed of Fe that did not contribute to the composition of the magnetic phase having either the Th ₂ Zn ₁₇ type or the Th ₂ Ni ₁₇ type crystal structure in the SmFeN powder. The α-Fe phase is a soft magnetic phase, which causes a decrease in coercive force. Therefore, the coercive force is improved by modifying the α-Fe phase to obtain a nonmagnetic modified phase 20 (Fe-Zn phase).

On the other hand, the second coating 42 suppresses the formation of a non-magnetic modified phase (Fe-Zn phase) on the particle surfaces of the second particle group 12 due to Zn diffusing from the adjacent first coating 41 during heat treatment of the sintered body 30. That is, the second coating 42 suppresses the modification of the particle surfaces of the second particle group 12 by the first coating 41. The second coating 42 contains a substance that suppresses the modification of the particle surfaces of the second particle group 12, and such a substance will be described later. In the manufacturing method disclosed herein, the particle surfaces of the first particle group 11 are modified, while the modification of the particle surfaces of the second particle group 12 is suppressed for the following reasons.

When a magnetic powder contains a magnetic powder having high coercivity (hereinafter sometimes referred to as "high coercivity powder") and a magnetic powder having low coercivity (hereinafter sometimes referred to as "low coercivity powder"), the squareness of a compact of such a magnetic powder, particularly at high temperatures, may decrease. This can be explained as follows using the drawings. In this specification, unless otherwise specified, "high temperature" in relation to magnetic properties means 100 to 200°C, and squareness is evaluated at 10% demagnetization _Hk .

Figure 3 is a graph showing the demagnetization curves of a compact made of low coercivity powder and a compact made of a mixed powder of low coercivity powder and high coercivity powder at high temperatures. It can be seen from Figure 3 that compared to the compact made of the mixed powder of low coercivity powder and high coercivity powder, the compact made of low coercivity powder has inferior coercivity but superior squareness.

In SmFeN magnetic powder, magnetic powders with large particle sizes have low coercivity, while magnetic powders with small particle sizes have high coercivity. In the magnetic powders used in the manufacturing method disclosed herein, the first particle group corresponds to low coercivity powder, and the second particle group corresponds to high coercivity powder.

As shown in FIG. 2, in the sintered body 30 before heat treatment, the first particle group 11 has a first coating 41 on the particle surface, and the second particle group 12 has a second coating 42 on the particle surface. By heat treating such a sintered body 30, as described above, the first particle group 11 is modified and the coercive force is improved, and the second particle group 12 is suppressed from being modified by the second coating 42 and the increase in the coercive force is suppressed. As a result, in the sintered body 30 after heat treatment, the difference between the coercive force of the first particle group 11 and the coercive force of the second particle group 12 can be suppressed from increasing, and the squareness can be improved. As a result, the density of the sintered body is improved between the first particle group and the second particle group, and even if the magnetization is improved, the squareness, especially the squareness at high temperatures, is improved. If the squareness is improved, it is less likely to be demagnetized in an environment where an external magnetic field is applied.

For these reasons, the rare earth magnet 90 (sintered body 30 after heat treatment) obtained by the manufacturing method of the present disclosure has a structure in which the modified phase 20 is formed on the particle surface of the first particle group 11, but the modified phase 20 is not substantially formed on the particle surface of the second particle group 12. Also, in the example of FIG. 1, in the rare earth magnet 90 (sintered body 30 after heat treatment) obtained by the manufacturing method of the present disclosure, the particle surface of the second particle group 12 is substantially free of a coating. This is thought to be because the second coating 42 present in the sintered body 30 before heat treatment typically decomposes into the elements that make up the second coating 42 when it performs the function of suppressing modification during heat treatment, and the elements are dispersed on the particle surface of the modified phase 20 or the second particle group 12, but this is not limited thereto, and part or all of the second coating 42 may remain on the rare earth magnet 90 (sintered body 30 after heat treatment).

The constituent elements of the manufacturing method disclosed herein, which was completed based on the findings discussed above, are described below.

<<Manufacturing method of rare earth magnets>>
The manufacturing method of the present disclosure includes a magnetic powder preparation step, a first coated magnetic powder preparation step, a second coated magnetic powder preparation step, a mixing step, a magnetic field compaction step, a pressure sintering step, and a heat treatment step. Each step will be described below.

<Magnetic powder preparation process>
A magnetic powder (SmFeN powder) is prepared. The magnetic powder (SmFeN powder) used in the manufacturing method of the present disclosure is not particularly limited as long as it contains Sm, Fe, and N and has a magnetic phase having at least a part of a crystal structure of either Th ₂ Zn ₁₇ type or Th ₂ Ni ₁₇ type. In addition to the above-mentioned structure, the crystal structure of the magnetic phase may be a phase having a crystal structure of TbCu ₇ type. Sm is samarium, Fe is iron, and N is nitrogen. Th is thorium, Zn is zinc, Ni is nickel, Tb is terbium, and Cu is copper.

The SmFeN powder may contain a magnetic phase represented by the composition formula (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17Nh} . The rare earth magnet obtained by the manufacturing method of the present disclosure (hereinafter sometimes referred to as the "result") exhibits magnetization due to the magnetic phase in the SmFeN powder. Note that i, j, and h are molar ratios.

The magnetic phase in the SmFeN powder may contain R to the extent that it does not impair the effects of the manufacturing method of the present disclosure and the magnetic properties of the product. The range of R content is represented by i in the above composition formula. i may be, for example, 0 or more, 0.10 or more, or 0.20 or more, and may be 0.50 or less, 0.40 or less, or 0.30 or less. R is one or more elements selected from rare earth elements other than Sm and Zr. In this specification, rare earth elements are Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In addition, Zr is zirconium, Sc is scandium, Y is yttrium, La is lanthanum, Ce is cerium, Pr is praseodymium, Nd is neodymium, Pm is promethium, Sm is samarium, Eu is europium, Gd is gadolinium, Tb is terbium, Dy is dysprosium, Ho is holmium, Er is erbium, Tm is thulium, Yb is ytterbium, and Lu is lutetium.

In the case of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17Nh} , typically, R is substituted at the position of Sm in _Sm2 (Fe _{(1-j) Coj} ) _17Nh , but _this is not limited thereto. For example, part of R may be interstitially disposed in _Sm2 (Fe _{(1-j) Coj ) 17Nh} .

The magnetic phase in the SmFeN powder may contain Co within a range that does not impair the effects of the manufacturing method of the present disclosure or the magnetic properties of the resulting product. Such a range is represented by j in the above composition formula. j may be 0 or more, 0.10 or more, or 0.20 or more, and may be 0.52 or less, 0.40 or less, or 0.30 or less.

In the case of (Sm _{(1-i) Ri} ) ₂ (Fe _{( 1-j) Coj} ) _17Nh , typically, Co is substituted at the Fe position _of (Sm _{(1-i) Ri ) 2Fe17Nh ,} but this is not limited thereto. For example, part of Co may be disposed in an interstitial manner in (Sm _{(1-i) Ri} ) _{2Fe17Nh .}

The magnetic phase in the SmFeN powder has crystal grains represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ , and N exists in an interstitial form, which contributes to the development and improvement of magnetic properties.

With respect to (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) C0j} ) _17Nh , h can be 1.5 to 4.5, but is typically (Sm _{( 1-i) Ri} ) ₂ (Fe _{(1-j) C0j} ) _17N3 . h may be 1.8 or more _, 2.0 or more, or 2.5 or more, and may be 4.2 or less, 4.0 or less, or 3.5 or less. _The content of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17N3 relative to the entire (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17Nh is preferably ₇₀ mass% or more, more preferably 80 mass% or more, and even more preferably 90 mass%. On the other hand, all of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17Nh does not have to be (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17N3 . The} content of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17N3 relative to the entire (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17Nh may be ₉₈ mass% or less, 95 mass% or less, or 92 mass% or less.

In addition to the magnetic phase represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ N _h , the SmFeN powder may contain oxygen, M ¹ , and unavoidable impurity elements to the extent that the effect of the manufacturing method of the present disclosure and the magnetic properties of the product are not substantially impaired. From the viewpoint of ensuring the magnetic properties of the product, the content of the magnetic phase represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ N _h relative to the entire SmFeN powder may be 80 mass% or more, 85 mass% or more, or 90 mass% or more. On the other hand, there is no practical problem even if the content of the magnetic phase represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ N _h relative to the entire SmFeN powder is not excessively high. Therefore, the content may be 97 mass % or less, 95 mass % or less, or 93 mass % or less. The remainder of the magnetic phase represented by (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17Nh} is the content of oxygen and ^M1 . In addition, a part of oxygen and ^M1 may be present in the magnetic phase in the form of interstitial and/or substitutional type.

The above-mentioned M ¹ includes one or more elements selected from Ga, Ti, Cr, Zn, Mn, V, Mo, W, Si, Re, Cu, Al, Ca, B, Ni, and C. An inevitable impurity element refers to an impurity element whose inclusion cannot be avoided when manufacturing raw materials and/or magnetic powders, or whose avoidance would lead to a significant increase in manufacturing costs. These elements may be present in the above-mentioned magnetic phases in a substitutional and/or interstitial form, or in phases other than the above-mentioned magnetic phases. Alternatively, they may be present in the grain boundaries of these phases. Note that Ga is gallium, Ti is titanium, Cr is chromium, Zn is zinc, Mn is manganese, V is vanadium, Mo is molybdenum, W is tungsten, Si is silicon, Re is rhenium, Cu is copper, Al is aluminum, Ca is calcium, B is boron, Ni is nickel, and C is carbon.

The SmFeN powder includes a first particle group and a second particle group. The particles of the first particle group have a large particle size, and the particles of the second particle group have a small particle size. The particle size of each of the first particle group and the second particle group can be represented by the particle size distribution D _50. The particle size distribution D ₅₀ of the first particle group is represented by d ₁ μm, and the particle size distribution D ₅₀ of the second particle group is represented by d ₂ μm. And, d ₁ and d ₂ satisfy the relationship of 0.350≦d ₂ /d ₁ ≦0.500. Since the above relationship is satisfied, it is clear that d ₂ <d ₁ , that is, the first particle group has a large particle size, and the second particle group has a small particle size.

When _d2 / _d1 is 0.350 or more, 0.360 or more, 0.370 or more, or 0.378 or more and 0.500 or less, 0.490 or less, 0.486 or less, 0.480 or less, 0.470 or less, or 0.467 or less, the particles of the second particle group are advantageously present between the particles of the first particle group, increasing the density of the molded body (rare earth magnet) and, as a result, improving magnetization.

As long as the above relationship is satisfied, the particle diameters of the first particle group and the second particle group are not particularly limited, but in order to easily satisfy the above relationship, it is preferable that _d1 and _d2 are each independently within the following ranges. _d1 is preferably 3.0 μm or more, 3.2 μm or more, or 3.4 μm or more, and is preferably 3.7 μm or less, 3.6 μm or less, or 3.5 μm or less. _d2 is preferably 1.4 μm or more or 1.5 μm or more, and is preferably 1.8 μm or less, 1.7 μm or less, or 1.6 μm or less.

The ratio of the total volume of the first particle group to the total volume of the second particle group, i.e., (total volume of the first particle group):(total volume of the second particle group), must be in the range of 9:1 to 4:1. A ratio of (total volume of the first particle group):(total volume of the second particle group) of 9:1 means, for example, that the total volume of the first particle group is 90% and the total volume of the second particle group is 10% of the total volume of the SmFeN powder. A ratio of (total volume of the first particle group):(total volume of the second particle group) of 4:1 means, for example, that the total volume of the first particle group is 80% and the total volume of the second particle group is 20% of the total volume of the SmFeN powder.

If the ratio of (total volume of the first particle group):(total volume of the second particle group) is 9:1 or the total volume of the second particle group is greater than this, the particles of the second particle group will be advantageously present between the particles of the first particle group, increasing the density of the rare earth magnet and, as a result, improving magnetization. From this perspective, it is preferable that the ratio of (total volume of the first particle group):(total volume of the second particle group) is 8.8:1.2 or more, or 8.6:1.4 or more.

If the particles of the second particle group are present in excess, the gaps between the particles of the first particle group will be warped and expanded. In order to avoid this, the ratio of (total volume of the first particle group): (total volume of the second particle group) must be 4:1 or the total volume of the second particle group must be less than that. In addition, since the small particle size SmFeN powder particles such as the second particle group have a small particle remanence magnetization _σr , if the second particle group is present in excess, this leads to a decrease in the magnetization of the entire rare earth magnet. For this reason, by avoiding the expansion of the gaps between the particles of the first particle group, the decrease in the density of the rare earth magnet is suppressed, and the magnetization of the rare earth magnet is improved by reducing the number of the second particle group having a small _σr . From these viewpoints, it is preferable that the ratio of (total volume of the first particle group): (total volume of the second particle group) is 8.2:1.8 or less or 8.4:1.6 or less.

The magnetic powder containing the first particle group and the second particle group is typically obtained by classifying the SmFeN powder obtained by the manufacturing method described below into the first particle group and the second particle group. There are no particular limitations on the classification method, and any well-known method may be used. Examples of classification methods include sieve classification and air classification, and these may be combined.

The _D50 of the SmFeN powder is calculated from the particle size distribution of the SmFeN powder, which is measured (investigated) by the following method. In this specification, unless otherwise specified, descriptions regarding the particle size (particle size) of the SmFeN powder are based on the following measurement method (investigation method). Note that _D50 means the median diameter.

A sample was prepared by embedding SmFeN powder in resin, and the surface of the sample was polished and observed under an optical microscope. A straight line was then drawn on the optical microscope image, and the length of the line segment where the line was separated by SmFeN particles (bright field) was measured, and the particle size distribution of the SmFeN powder was obtained from the frequency distribution of the line segment lengths. The particle size distribution obtained by this method is approximately equal to the particle size distribution obtained by the intersection method or the dry laser diffraction/scattering method.

In the SmFeN powder, fine particles may be present due to manufacturing convenience, etc., but as long as d ₁ and d ₂ satisfy the above-mentioned relationship, there is no particular restriction on the ratio of magnetic particles (fine particles) having a particle size of 1.0 μm or less in the SmFeN powder. The ratio of magnetic particles (fine particles) having a particle size of 1.0 μm or less in the SmFeN powder is preferably as low as possible from the viewpoint of ensuring the mechanical strength of the compact (rare earth magnet). The ratio of fine particles to the total number of magnetic particles in the SmFeN powder is preferably 10.0% or less, 8.0% or less, 6.0% or less, or 4.0% or less. From the viewpoint of manufacturing convenience of the SmFeN powder, fine particles do not have to be completely absent, and there is no practical problem even if the lower limit of the ratio of fine particles is 1.0%, 2.0%, or 3.0%. Even if fine particles are present, by heat treating the sintered body, fine Fe-Zn alloy phase derived from the fine particles is generally not observed. The reason for this is believed to be as follows. In the fine particles of the SmFeN powder, during the heat treatment of the sintered body, the Fe-Zn alloy phase is formed not only on the particle surface but also in almost the entire particle. This is because the fine particles have a large proportion of portions in which the crystal structure of Th ₂ Zn ₁₇ type and/or Th ₂ Ni ₁₇ type is not complete. Furthermore, most of the Fe-Zn alloy phase derived from the fine particles is integrated with the modified phase (Fe-Zn alloy phase) formed on the particle surface of the first particle group.

Oxygen in the SmFeN powder causes a decrease in coercivity. The decrease in coercivity is often a problem with magnetic powders having large particle sizes. In the manufacturing method disclosed herein, the first particle group (magnetic powders having large particle sizes) is modified. During the modification, the zinc in the first coating absorbs oxygen, thereby suppressing the decrease in coercivity. The oxygen content in the SmFeN powder may be determined in consideration of the amount of oxygen absorbed by the zinc in the first coating in the SmFeN powder. The oxygen content of the SmFeN powder is preferably low relative to the entire SmFeN powder. The oxygen content of the SmFeN powder is preferably 2.0 mass% or less, more preferably 1.5 mass% or less, and even more preferably 1.0 mass% or less relative to the entire SmFeN powder. On the other hand, extremely reducing the oxygen content in the SmFeN powder leads to an increase in manufacturing costs. For this reason, the oxygen content of the SmFeN powder may be 0.1 mass% or more, 0.2 mass% or more, or 0.3 mass% or more, based on the entire SmFeN powder.

As long as the above-described conditions are satisfied, there is no particular restriction on the method of producing the SmFeN powder, and a commercially available product may be used. Examples of methods for producing SmFeN powder include a method in which Sm-Fe powder is produced from samarium oxide and iron powder by a reduction diffusion method, and then heat-treated at 600°C or less in an atmosphere of a mixed gas of nitrogen and hydrogen, nitrogen gas, ammonia gas, or the like to obtain Sm-Fe-N powder. Alternatively, examples include a method in which an Sm-Fe alloy is produced by a melting method, the alloy is coarsely crushed to obtain coarsely crushed particles, which are then nitrided and further crushed to the desired particle size. For example, a dry jet mill, a dry ball mill, a wet ball mill, or a wet bead mill can be used for the crushing. These may also be used in combination.

In addition to the above-mentioned manufacturing method, SmFeN powder can be obtained by a manufacturing method including a pretreatment step of heat-treating an oxide containing Sm and Fe in a reducing gas-containing atmosphere to obtain a partial oxide, a reduction step of heat-treating the partial oxide in the presence of a reducing agent to obtain alloy particles, and a nitriding step of heat-treating the alloy particles in a nitrogen or ammonia-containing atmosphere at a first temperature of 400°C to 470°C, and then at a second temperature of 480°C to 610°C to obtain nitrides. In particular, with alloy particles with a large particle size, such as alloy particles containing La, nitriding may not progress sufficiently to the inside of the oxide particles. However, when nitriding is performed at two temperatures, the inside of the oxide particles is also sufficiently nitrided, and anisotropic SmFeN powder with a narrow particle size distribution and high remanent magnetization can be obtained.

[Oxide preparation process]
The oxide containing Sm and Fe used in the pretreatment step described below may be prepared, for example, by mixing Sm oxide and Fe oxide. However, it is preferable to produce the oxide containing Sm and Fe by a step of mixing a solution containing Sm and Fe with a precipitant to obtain a precipitate containing Sm and Fe (precipitation step), and a step of calcining the precipitate to obtain an oxide containing Sm and Fe (oxidation step).

[Precipitation step]
In the precipitation step, a Sm raw material and an Fe raw material are dissolved in a strongly acidic solution to prepare a solution containing Sm and Fe. When Sm ₂ Fe ₁₇ N ₃ is obtained as the main phase, the molar ratio of Sm and Fe (Sm:Fe) is preferably 1.5:17 to 3.0:17, more preferably 2.0:17 to 2.5:17. Raw materials such as La, W, Co, Ti, Sc, Y, Pr, Nd, Pm, Gd, Tb, Dy, Ho, Er, Tm, and/or Lu may be added to the above-mentioned solution. In terms of residual magnetic flux density, it is preferable to contain La. In terms of coercive force and squareness ratio, it is preferable to contain W. In terms of temperature characteristics, it is preferable to contain Co and/or Ti.

The Sm raw material and the Fe raw material are not limited as long as they can be dissolved in a strong acidic solution. For example, in terms of ease of availability, the Sm raw material may be samarium oxide, and the Fe raw material may be FeSO _4. The concentration of the solution containing Sm and Fe may be appropriately adjusted within a range in which the Sm raw material and the Fe raw material are substantially dissolved in the acidic solution. In terms of solubility, the acidic solution may be sulfuric acid.

By reacting a solution containing Sm and Fe with a precipitant, an insoluble precipitate containing Sm and Fe is obtained. Here, the solution containing Sm and Fe may be a solution containing Sm and Fe when reacted with the precipitant. For example, raw materials containing Sm and Fe may be prepared as separate solutions, and each solution may be dropped to react with the precipitant. Even when preparing them as separate solutions, the raw materials are appropriately adjusted to a range in which each raw material is substantially soluble in the acidic solution. The precipitant is not limited as long as it is an alkaline solution that reacts with the solution containing Sm and Fe to obtain a precipitate, and examples of the precipitant include ammonia water and caustic soda, with caustic soda being preferred.

The precipitation reaction is preferably carried out by dropping the solution containing Sm and Fe and the precipitant into a solvent such as water, because the properties of the precipitate particles can be easily adjusted. By appropriately controlling the supply rate of the solution containing Sm and Fe and the precipitant, the reaction temperature, the concentration of the reaction solution, the pH during the reaction, etc., a precipitate with a uniform distribution of the constituent elements, a narrow particle size distribution, and a regular powder shape can be obtained. By using such a precipitate, the magnetic properties of the final product, SmFeN powder, are improved. The reaction temperature can be set to 0°C or higher and 50°C or lower, and preferably 35°C or higher and 45°C or lower. The reaction solution concentration is preferably 0.65 mol/L or higher and 0.85 mol/L or lower, and more preferably 0.7 mol/L or higher and 0.85 mol/L or lower, in terms of the total concentration of metal ions. The reaction pH is preferably 5 to 9, and more preferably 6.5 to 8.

The solution containing Sm and Fe preferably further contains one or more metals selected from the group consisting of La, W, Co, and Ti in terms of magnetic properties. For example, it is preferable to contain La in terms of residual magnetic flux density, it is preferable to contain W in terms of coercive force and squareness ratio, and it is preferable to contain Co and/or Ti in terms of temperature properties. The La raw material is not limited as long as it can be dissolved in a strongly acidic solution, and examples of the La raw material include La ₂ O ₃ and LaCl ₃ in terms of ease of availability. Together with the Sm raw material and the Fe raw material, the La raw material, the W raw material, the Co raw material, and the Ti raw material are appropriately adjusted within a range in which they are substantially dissolved in the acidic solution, and the acidic solution includes sulfuric acid in terms of solubility. The W raw material includes ammonium tungstate, the Co raw material includes cobalt sulfate, and the titanium raw material includes titania sulfate.

When the solution containing Sm and Fe further contains one or more metals selected from the group consisting of La, W, Co, and Ti, an insoluble precipitate containing Sm, Fe, and one or more selected from the group consisting of La, W, Co, and Ti is obtained. Here, the solution only needs to contain one or more selected from the group consisting of La, W, Co, and Ti when reacted with the precipitant. For example, each raw material may be prepared as a separate solution, and each solution may be dropped to react with the precipitant, or the solution may be prepared together with the solution containing Sm and Fe.

The powder obtained in the precipitation process largely determines the powder particle size, powder shape, and particle size distribution of the final SmFeN powder. When the particle size of the obtained powder is measured using a laser diffraction type wet particle size distribution analyzer, it is preferable that the size and distribution of all the powder is approximately in the range of 0.05 μm to 20 μm, preferably 0.1 μm to 10 μm.

After separating the precipitate, it is preferable to desolvate the separated material in order to prevent the precipitate from redissolving in the remaining solvent during the heat treatment in the subsequent oxidation step, causing the precipitate to aggregate and change in particle size distribution, powder particle size, etc. when the solvent evaporates. Specific examples of methods for desolvation include drying in an oven at 70°C to 200°C for 5 hours to 12 hours when water is used as the solvent.

The precipitation step may be followed by a step of separating and washing the resulting precipitate. The washing step is appropriately performed until the conductivity of the supernatant solution becomes 5 mS/ ^m2 or less. As a step of separating the precipitate, for example, a solvent (preferably water) may be added to the resulting precipitate and mixed, followed by filtration, decantation, or the like.

[Oxidation process]
The oxidation step is a step of obtaining an oxide containing Sm and Fe by firing the precipitate formed in the precipitation step. For example, the precipitate can be converted into an oxide by heat treatment. When the precipitate is heat treated, it needs to be performed in the presence of oxygen, for example, in an air atmosphere. In addition, since it needs to be performed in the presence of oxygen, it is preferable that the non-metallic part in the precipitate contains oxygen atoms.

The heat treatment temperature in the oxidation step (hereinafter sometimes referred to as "oxidation temperature") is not particularly limited, but is preferably 700°C or higher and 1300°C or lower, and more preferably 900°C or higher and 1200°C or lower. If the temperature is lower than 700°C, the oxidation will be insufficient, and if the temperature exceeds 1300°C, the desired shape, average particle size, and particle size distribution of the SmFeN powder will tend not to be obtained. The heat treatment time is also not particularly limited, but is preferably 1 hour or higher and 3 hours or lower.

The resulting oxide particles have sufficient microscopic mixing of Sm and Fe within the oxide particles, and reflect the shape and particle size distribution of the precipitates.

[Pretreatment process]
The pretreatment step is a step in which the oxide containing Sm and Fe is heat-treated in an atmosphere containing a reducing gas to obtain a partial oxide in which a part of the oxide is reduced.

Here, partial oxide refers to an oxide in which a part of an oxide has been reduced. There are no particular limitations on the oxygen concentration of the partial oxide, but it is preferably 10% by mass or less, and more preferably 8% by mass or less. If it exceeds 10% by mass, the reduction heat generated by Ca in the reduction process increases, and the firing temperature increases, tending to produce particles that have undergone abnormal particle growth. Here, the oxygen concentration of the partial oxide can be measured by non-dispersive infrared absorption (ND-IR).

The reducing gas is appropriately selected from hydrocarbon gases such as hydrogen (H ₂ ), carbon monoxide (CO), and methane (CH ₄ ), but hydrogen gas is preferred from the viewpoint of cost, and the flow rate of the gas is appropriately adjusted within a range in which the oxide does not scatter. The heat treatment temperature in the pretreatment step (hereinafter, pretreatment temperature) is preferably 300° C. or more and 950° C. or less, and the lower limit is more preferably 400° C. or more, and even more preferably 750° C. or more. The upper limit is more preferably less than 900° C. When the pretreatment temperature is 300° C. or more, the reduction of the oxide containing Sm and Fe proceeds efficiently. Furthermore, when the pretreatment temperature is 950° C. or less, the particle growth and segregation of the oxide particles are suppressed, and the desired particle size can be maintained. The heat treatment time is not particularly limited, but can be 1 hour or more and 50 hours or less. Furthermore, when hydrogen is used as the reducing gas, it is preferable to adjust the thickness of the oxide layer used to 20 mm or less, and further adjust the dew point in the reaction furnace to −10° C. or less.

[Reduction process]
The reduction step is a step of obtaining alloy particles by heat treating the partial oxide in the presence of a reducing agent, for example, by contacting the partial oxide with calcium melt or calcium vapor. From the viewpoint of magnetic properties, the heat treatment temperature is preferably 920° C. or more and 1200° C. or less, more preferably 950° C. or more and 1150° C. or less, and even more preferably 980° C. or more and 1100° C. or less.

The reducing agent, metallic calcium, is used in granular or powder form, with the particle size preferably being 10 mm or less. This makes it possible to more effectively suppress agglomeration during the reduction reaction. In addition, metallic calcium is preferably added in an amount 1.1 to 3.0 times the reaction equivalent (the stoichiometric amount required to reduce the rare earth oxide, and if the Fe component is in the form of an oxide, includes the amount required to reduce it), and more preferably 1.5 to 2.5 times.

In the reduction step, a disintegration promoter can be used as necessary together with the reducing agent, calcium metal. This disintegration promoter is used as appropriate to promote disintegration and granulation of the product in the post-treatment step described below, and examples of such disintegration promoters include alkaline earth metal salts such as calcium chloride and alkaline earth oxides such as calcium oxide. These disintegration promoters are used in a ratio of 1% by mass to 30% by mass, preferably 5% by mass to 30% by mass, per samarium oxide.

[Nitriding process]
The nitriding step is a step of obtaining anisotropic magnetic particles by heat-treating the alloy particles obtained in the reduction step at a first temperature of 400°C to 470°C in a nitrogen or ammonia-containing atmosphere, and then heat-treating the alloy particles at a second temperature of 480°C to 610°C in a nitriding process. Since the particulate precipitate obtained in the precipitation step is used, the alloy particles in the form of porous aggregates are obtained in the reduction step. This allows the alloy particles to be heat-treated and nitrided immediately in a nitrogen atmosphere without pulverization, so that the nitriding can be performed uniformly. If the alloy particles are heat-treated at a high second temperature without nitriding at the first temperature, the nitriding process will proceed rapidly, causing abnormal heat generation, which may cause SmFeN to decompose and greatly reduce the magnetic properties. In addition, the atmosphere in the nitriding step is preferably a substantially nitrogen-containing atmosphere, since the nitriding process can slow down the progress of nitriding. The term "substantially" used here is used in consideration of the fact that elements other than nitrogen are inevitably included due to the inclusion of impurities, etc., and for example, the ratio of nitrogen in the atmosphere is 95% or more, preferably 97% or more, and more preferably 99% or more.

The first temperature in the nitriding step is 400°C or higher and 470°C or lower, but preferably 410°C or higher and 450°C or lower. Below 400°C, nitriding proceeds very slowly, while above 470°C, over-nitriding or decomposition is likely to occur due to heat generation. The heat treatment time at the first temperature is not particularly limited, but is preferably 1 hour or higher and 40 hours or lower, and more preferably 20 hours or lower. Below 1 hour, nitriding may not proceed sufficiently, and above 40 hours, productivity decreases.

The second temperature is 480°C or more and 610°C or less, but preferably 500°C or more and 550°C or less. If the temperature is less than 480°C, nitriding may not proceed sufficiently if the particles are large, and if the temperature exceeds 610°C, over-nitriding or decomposition is likely to occur. The heat treatment time at the second temperature is preferably 15 minutes to 5 hours, and more preferably 30 minutes to 2 hours. If the temperature is less than 15 minutes, nitriding may not proceed sufficiently, and if the temperature exceeds 5 hours, productivity decreases.

The heat treatment at the first temperature and the heat treatment at the second temperature may be performed consecutively, and a heat treatment at a temperature lower than the second temperature may be included between these heat treatments, but from the viewpoint of productivity, it is preferable to perform them consecutively.

[Post-processing process]
The product obtained after the nitriding step contains, in addition to the magnetic particles, by-produced CaO, unreacted metallic calcium, etc., and these may be in the form of a composite sintered mass. The product obtained after the nitriding step can be put into cooling water to separate the CaO and metallic calcium as a calcium hydroxide (Ca(OH) ₂ ) suspension. Furthermore, the remaining calcium hydroxide may be sufficiently removed by washing the magnetic powder with acetic acid or the like. When the product is put into water, the composite sintered mass reaction product is disintegrated, i.e., pulverized, by the oxidation of metallic calcium by water and the hydration reaction of the by-produced CaO.

[Alkaline treatment step]
The product obtained after the nitriding step may be put into an alkaline solution. Examples of the alkaline solution used in the alkaline treatment step include an aqueous calcium hydroxide solution, an aqueous sodium hydroxide solution, and an aqueous ammonia solution. Among these, an aqueous calcium hydroxide solution and an aqueous sodium hydroxide solution are preferred in terms of wastewater treatment and high pH. The alkaline treatment of the product leaves an Sm-rich layer containing a certain amount of oxygen, which functions as a protective layer, thereby suppressing an increase in oxygen concentration due to the alkaline treatment.

The pH of the alkaline solution used in the alkaline treatment step is not particularly limited, but is preferably 9 or higher, and more preferably 10 or higher. If the pH is less than 9, the reaction rate when calcium hydroxide is produced is fast and heat is generated, so the oxygen concentration of the final SmFeN powder tends to be high.

In the alkaline treatment step, the moisture content of the SmFeN powder obtained after treatment with the alkaline solution can be reduced by a method such as decantation if necessary.

[Acid treatment step]
The alkaline treatment step may be followed by an acid treatment step in which the SmFeN powder is treated with an acid. In the acid treatment step, at least a part of the Sm-rich layer is removed to reduce the oxygen concentration in the entire SmFeN powder. In addition, in the manufacturing method according to the embodiment of the present invention, since no pulverization or the like is performed, the average particle size of the SmFeN powder is small, the particle size distribution is narrow, and no fine powder generated by pulverization or the like is included, so that it is possible to suppress an increase in the oxygen concentration.

The acid used in the acid treatment step is not particularly limited, and examples include hydrogen chloride, nitric acid, sulfuric acid, and acetic acid. Among these, hydrogen chloride and nitric acid are preferred because they do not leave behind any impurities.

The amount of acid used in the acid treatment step is preferably 3.5 parts by mass or more and 13.5 parts by mass or less, more preferably 4 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of SmFeN powder. If the amount is less than 3.5 parts by mass, oxides remain on the surface of the SmFeN powder, and the oxygen concentration becomes high. If the amount of acid exceeds 13.5 parts by mass, reoxidation is likely to occur when exposed to the atmosphere, and the SmFeN powder is dissolved, so the cost tends to be high. By setting the amount of acid to 3.5 parts by mass or more and 13.5 parts by mass or less per 100 parts by mass of SmFeN powder, it is possible to make the Sm-rich layer oxidized to a degree that reoxidation is unlikely to occur when exposed to the atmosphere after acid treatment cover the surface of the SmFeN powder, so that SmFeN powder with a low oxygen concentration, small average particle size, and narrow particle size distribution can be obtained.

In the acid treatment step, the moisture content of the SmFeN powder obtained after the acid treatment can be reduced by a method such as decantation if necessary.

[Dehydration process]
It is preferable to include a dehydration step after the acid treatment step. The dehydration step reduces the moisture content in the solid content before vacuum drying, and suppresses the progress of oxidation during drying that occurs when the solid content before vacuum drying contains more moisture. Here, the dehydration step means a process in which pressure or centrifugal force is applied to reduce the moisture content of the solid content after treatment compared to the solid content before treatment, and does not include simple decantation, filtration, or drying. The dehydration method is not particularly limited, but examples include squeezing, centrifugation, and the like.

The amount of moisture contained in the SmFeN powder after the dehydration process is not particularly limited, but in order to suppress the progression of oxidation, it is preferably 13% by mass or less, and more preferably 10% by mass or less.

The SmFeN powder obtained by acid treatment, or the SmFeN powder obtained by dehydration after acid treatment, is preferably vacuum dried. The drying temperature is not particularly limited, but is preferably 70°C or higher, and more preferably 75°C or higher. The drying time is also not particularly limited, but is preferably 1 hour or more, and more preferably 3 hours or more.

<First coated magnetic powder preparation process>
A first coating is formed on the particle surface of a first particle group of SmFeN powder to obtain a first coated magnetic powder. The first coating typically contains zinc. The coating containing zinc means at least one of a coating containing metallic zinc and a coating containing a zinc alloy. The metallic zinc means unalloyed zinc.

As long as the first coating can be formed on the particle surfaces of the first particle group, the method of formation is not particularly limited. In the heat treatment process described below, the first coating on the particle surfaces of the first coated magnetic powder modifies the vicinity of the interface between the particle surfaces of the first particle group and the first coating, and the above-mentioned modified phase is formed (see FIG. 1). At the stage of obtaining the first coated magnetic powder, the vicinity of the interface between the particle surfaces of the first particle group and the first coating may or may not be modified. Typically, at the stage of obtaining the first coated magnetic powder, the vicinity of the interface between the particle surfaces of the first particle group and the first coating is not modified.

In the manufacturing method disclosed herein, the particle powder of the first particle group and the powder containing zinc are not simply mixed, but a first coated magnetic powder is obtained in which a first coating containing zinc is formed on the particle surfaces of the first particle group, and the first coated magnetic powder is subjected to magnetic field compaction, pressure sintering, and heat treatment. Therefore, even if the content of the zinc component in the first coated magnetic powder is relatively low, a modified phase such as that shown in FIG. 1 is obtained. This is because, compared to the case of simple mixing, in the manufacturing method disclosed herein, the zinc component is advantageously brought into contact with the particle surfaces of the first particle group to form the first coating.

Specifically, if the content of the zinc component in the first coated magnetic powder is 3.0% by mass or more, 3.5% by mass or more, 4.0% by mass or more, or 5.0% by mass or more relative to the total mass of the first coated magnetic powder and the second coated magnetic powder, most of the particle surfaces of the first particle group are covered with the modifier phase as shown in FIG. 1, and demagnetization can be suppressed. In other words, the Fe-Zn alloy phase as the modifier phase is advantageously formed in a coating on the particle surfaces of the first particle group.

On the other hand, if the content of the zinc component in the first coated magnetic powder is 10.0% by mass or less relative to the total mass of the first coated magnetic powder and the second coated magnetic powder, the decrease in magnetization due to the use of the zinc component can be suppressed. From this perspective, the content of the zinc component in the first coated magnetic powder may be less than 10.0% by mass, 9.0% by mass or less, 8.0% by mass or less, 7.0% by mass or less, or 6.0% by mass or less relative to the total mass of the first coated magnetic powder and the second coated magnetic powder.

Methods for forming the first coating include, for example, a method using a rotary kiln furnace and a vapor deposition method. Each of these methods will be briefly explained below using the example in which the first coating is a coating containing zinc.

[Method using a rotary kiln]
4 is an explanatory diagram showing an example of a method for forming a zinc-containing coating (first coating) on the particle surface of SmFeN powder (first particle group) using a rotary kiln furnace. In the following description of the "method using a rotary kiln furnace", "SmFeN powder" means the particle powder of the first particle group unless otherwise specified.

The rotary kiln furnace 100 includes an agitating drum 110. The agitating drum 110 has a material storage section 120, a rotating shaft 130, and an agitating plate 140. A rotating means (not shown) such as an electric motor is connected to the rotating shaft 130.

SmFeN powder 150 and zinc-containing powder 160 are loaded into the material storage section 120. Then, while rotating the stirring drum 110, the material storage section 120 is heated by a heater (not shown).

When the material storage section 120 is heated to a temperature lower than the melting point of the zinc-containing powder 160, the zinc component of the zinc-containing powder 160 undergoes solid-phase diffusion onto the surface of the particles of the SmFeN powder 150. As a result, a coating containing zinc is formed on the surface of the particles of the SmFeN powder 150. When the material storage section 120 is heated to a temperature higher than the melting point of the zinc-containing powder 160, a molten solution of the zinc-containing powder 160 is obtained, and the molten solution comes into contact with the SmFeN powder 150. When the material storage section 120 is cooled in this state, a coating containing zinc is formed on the surface of the particles of the SmFeN powder 150.

The operating conditions of the rotary kiln furnace can be determined appropriately to obtain the desired coating.

The heating temperature of the material storage section may be, for example, (T-50)°C or more, (T-40)°C or more, (T-30)°C or more, (T-20)°C or more, (T-10)°C or more, or T°C or more, and may be (T+50)°C or less, (T+40)°C or less, (T+30)°C or less, (T+20)°C or less, or (T+10)°C or less, where T is the melting point of the zinc-containing powder. Note that if the zinc-containing powder is a powder containing metallic zinc, T is the melting point of zinc. Also, if the zinc-containing powder is a powder containing a zinc alloy, T is the melting point of the zinc alloy.

The rotation speed (number of rotations) of the stirring drum may be, for example, 5 rpm or more, 6 rpm or more, 10 rpm or more, or 20 rpm or more, and may be 200 rpm or less, 100 rpm or less, or 50 rpm or less. The atmosphere during rotation is preferably an inert gas atmosphere to prevent oxidation of the powder and the formed coating. The inert gas atmosphere also includes a nitrogen gas atmosphere. The rotation time of the stirring drum (coating treatment time) can be appropriately determined so as to form a coating containing the desired zinc. The rotation time of the stirring drum (coating treatment time) may be, for example, 1 minute or more, 5 minutes or more, 10 minutes or more, 15 minutes or more, 30 minutes or more, 45 minutes or more, or 60 minutes or more, and may be 240 minutes or less, 210 minutes or less, 180 minutes or less, 150 minutes or less, 120 minutes or less, or 90 minutes or less.

After forming a zinc-containing coating on the surface of the SmFeN powder particles, if the particles of the coated magnetic powder are bonded together, the bonded bodies may be pulverized. The pulverization method is not particularly limited, and examples include pulverization methods using a ball mill, a jaw crusher, a jet mill, a cutter mill, and combinations of these.

[Vapor deposition method]
5 is an explanatory diagram showing an example of a method for forming a coating (first coating) containing zinc on the particle surface of SmFeN powder (first particle group) by a vapor deposition method. In the following description of the "vapor deposition method", "SmFeN powder" means the particle powder of the first particle group unless otherwise specified.

The SmFeN powder 150 is stored in a first container 181, and the zinc-containing powder 160 is stored in a second container 182. The first container 181 is stored in a first heat treatment furnace 171, and the second container 182 is stored in a second heat treatment furnace 172. The first heat treatment furnace 171 and the second heat treatment furnace 172 are connected by a connecting path 173. The first heat treatment furnace 171 and the second heat treatment furnace 172, and the connecting path 173 are airtight, and a vacuum pump 180 is connected to the second heat treatment furnace 172.

The vacuum pump 180 reduces the pressure inside the first heat treatment furnace 171, the second heat treatment furnace 172, and the connecting passage 173, and then heats the inside of these furnaces. This causes zinc-containing vapor to be generated from the zinc-containing powder 160 stored in the second container 182. The zinc-containing vapor moves from the inside of the second container 182 to the inside of the first container 181, as shown by the solid arrow in Figure 5.

The zinc-containing vapor that moves into the inside of the first container 181 is cooled and forms (vapor-deposits) a coating on the particle surfaces of the SmFeN powder 150.

By making the first container 181 a rotating container, it can be made to resemble a rotary kiln, and the coverage percentage of the coating formed on the particle surfaces of the SmFeN powder 150 can be further increased. The coverage percentage will be described later.

The conditions for forming a coating using the method shown in Figure 5 can be determined appropriately to obtain the desired coating.

The temperature of the first heat treatment furnace (heating temperature of the SmFeN powder) may be, for example, 120°C or more, 140°C or more, 160°C or more, 180°C or more, 200°C or more, or 220°C or more, and may be 300°C or less, 280°C or less, or 260°C or less.

The temperature of the second heat treatment furnace (heating temperature of the zinc-containing powder), where T is the melting point of the zinc-containing powder, may be, for example, T°C or higher, (T+20)°C or higher, (T+40)°C or higher, (T+60)°C or higher, (T+80)°C or higher, (T+100)°C or higher, or (T+120)°C or higher, or may be (T+200)°C or lower, (T+180)°C or lower, (T+160)°C or lower, or (T+140)°C or lower. Note that, when the zinc-containing powder is a powder containing metallic zinc, T is the melting point of zinc. Also, when the zinc-containing powder is a powder containing a zinc alloy, T is the melting point of the zinc alloy. Although the second container may contain a bulk material containing zinc, it is preferable to store a zinc-containing powder in the second container from the viewpoint of quickly melting the charge in the second container and generating a zinc-containing vapor from the molten liquid.

The first heat treatment furnace and the second heat treatment furnace are in a reduced pressure atmosphere to promote the generation of vapor containing zinc and to prevent oxidation of the powder and the formed coating, etc. The atmospheric pressure is, for example, preferably 1×10 ⁻⁵ MPa or less, more preferably 1×10 ⁻⁶ MPa or less, and even more preferably 1×10 ⁻⁷ MPa or less. On the other hand, there is no practical problem even if the pressure is not excessively reduced, and the atmospheric pressure may be 1×10 ⁻⁸ MPa or more as long as the above-mentioned atmospheric pressure is satisfied.

When the first container is a rotating container, its rotation speed (number of rotations) may be, for example, 5 rpm or more, 10 rpm or more, or 20 rpm or more, and may be 200 rpm or less, 100 rpm or less, or 50 rpm or less.

In the vapor deposition method, after forming a zinc-containing coating on the surface of the SmFeN powder particles, if the particles of the coated magnetic powder are bonded together, the bonded bodies may be pulverized. The pulverization method is not particularly limited, and examples include pulverization methods using a ball mill, a jaw crusher, a jet mill, a cutter mill, and combinations of these.

[Coverage rate of zinc component]
Even when the first coated magnetic powder is obtained by forming the first coating on the particle surface of the SmFeN powder particles (first particle group) by any method, it is preferable that the coating rate of the zinc component in the first coated magnetic powder is high. This is because if the coating rate of the zinc component is high, the coating rate of the modified phase formed on the particle surface of the first particle group in the rare earth magnet will be high after the heat treatment process described below, and the coercive force will be advantageously improved. Next, a method for determining the coating rate of the zinc component will be described.

In the first coated magnetic powder, the coverage of the zinc component is the ratio (percentage) of the zinc component covering the entire particle surface of the first particle group. The coverage (%) of the zinc component is determined as follows. In the following explanation of the "coverage of the zinc component," "SmFeN powder" means the particle powder of the first particle group unless otherwise specified.

X-ray photoelectron spectroscopy (XPS) is used to obtain composition information about the SmFeN powder (first particle group) and the constituent elements of the first coating for the first coated magnetic powder. The coverage rate (%) of the zinc component is then calculated using the following formula.

Zinc component coverage (%) = [(total composition information for each of the constituent elements of the first coating) / {(total composition information for each of the constituent elements of the SmFeN powder (first particle group)) + (total composition information for each of the constituent elements of the first coating)}] x 100

For example, when the SmFeN powder is composed of Sm, Fe, and N, the sum of the composition information for each of the constituent elements of the SmFeN powder means the sum of the composition information for each of Sm, Fe, and N. Even when the SmFeN powder contains elements other than Sm, Fe, and N, the content ratio of elements other than Sm, Fe, and N is small. Therefore, even when the SmFeN powder contains elements other than Sm, Fe, and N, the sum of the composition information for each of the constituent elements of the SmFeN powder may be approximated by the sum of the composition information for each of Sm, Fe, and N. Also, when the first coating is, for example, metallic zinc, the sum of the composition information for each of the constituent elements of the first coating means the composition information for Zn. When the first coating is, for example, a zinc alloy, the sum of the composition information for each of the constituent elements of the first coating means the sum of the composition information for Zn and each of the alloy elements. If the zinc alloy is, for example, a Zn-Al alloy, the sum of the composition information for each of the constituent elements of the first coating means the sum of the composition information for Zn and Al.

For example, the composition information about Zn means the mass of Zn present, which is obtained by measuring the XPS spectrum of the first coated magnetic powder and calculating the peak intensity of the obtained XPS spectrum. In the case where the SmFeN powder is composed of, for example, Sm, Fe, and N, and the first coating is, for example, zinc, the coverage rate (%) of the zinc component is calculated as follows.
Coverage rate (%) of zinc component = (abundant mass of Zn) / (total of abundant mass of Sm, Fe, N, and Zn) x 100

The coverage rate of the zinc component determined in this manner is preferably 80% or more, 83% or more, 90% or more, or 94% or more, with 100% being ideal.

The particles of SmFeN powder are very hard. In comparison, the particles of zinc-containing powder are generally soft. For this reason, simply mixing SmFeN powder with a powder containing zinc may result in deformed particles of the zinc-containing powder adhering to the surface of the SmFeN powder particles, forming a first coating. However, it is difficult to stably achieve a coverage rate of the zinc component of 80% or more by simply mixing. Therefore, when preparing the first coated magnetic powder, it is preferable to adopt the method using a rotary kiln furnace or the vapor deposition method, as described above.

As described above, the first coating is typically a coating containing zinc, and the coating containing zinc means at least one of a coating containing metallic zinc and a coating containing a zinc alloy. When the zinc alloy is represented by Zn-M ² , M ² may be selected from an element that is alloyed with Zn (zinc) to lower the melting start temperature of the zinc alloy below the melting point of Zn and an inevitable impurity element. This improves the sinterability in the pressure sintering process described below, and improves the modifiability in the heat treatment process described below. Examples of M ² that lowers the melting start temperature of the zinc alloy below the melting point of Zn include elements that form a eutectic alloy with Zn and M ^2. Examples of such M ² include typically Sn, Mg, and Al, as well as combinations thereof. Sn is tin, Mg is magnesium, and Al is aluminum. Elements that do not impair the melting point lowering effect of these elements and the properties of the product can also be selected as M ² . In addition, unavoidable impurity elements refer to impurity elements, such as impurities contained in the raw materials of zinc-containing powder, whose inclusion cannot be avoided, or whose avoidance would result in a significant increase in production costs.

In the zinc alloy represented by Zn- ^M2 , the ratio (molar ratio) of Zn and ^M2 may be appropriately determined so that the sintering temperature and/or the heat treatment temperature of the sintered body is appropriate. The ratio (molar ratio) of ^M2 to the entire zinc alloy may be, for example, 0.05 or more, 0.10 or more, or 0.20 or more, and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less.

The zinc-containing powder used in the above-mentioned "Method using a rotary kiln furnace" and "Vapor deposition method" may contain, in addition to metallic zinc and/or zinc alloy, any substance having a binder function and/or a modifying function and other functions, so long as the effect of the present invention is not impaired. Examples of other functions include a function to improve corrosion resistance.

The particle size of the powder containing zinc is not particularly limited, but is preferably smaller than the particle size of the SmFeN powder. This makes it easier for the particles of the powder containing zinc to spread between the particles of the SmFeN powder, especially when a rotary kiln is used. The particle size of the powder containing zinc may be, for example, 0.1 μm or more, 0.5 μm or more, or 1.0 μm or more in terms of D ₅₀ (median diameter), and may be 12.0 μm or less, 11.0 μm or less, 10.0 μm or less, 9.0 μm or less, 8.0 μm or less, 7.0 μm or less, 6.0 μm or less, 5.0 μm or less, 4.0 μm or less, or 2.0 μm or less. The particle size D ₅₀ (median diameter) of the powder containing zinc is measured, for example, by a dry laser diffraction/scattering method.

In particular, when using a rotary kiln furnace, it is preferable that the oxygen content of the zinc-containing powder is low, since it can absorb a large amount of oxygen in the SmFeN powder. From this viewpoint, the oxygen content of the zinc-containing powder is preferably 5.0 mass% or less, more preferably 3.0 mass% or less, and even more preferably 1.0 mass% or less, relative to the entire zinc-containing powder. On the other hand, extremely reducing the oxygen content of the modifier powder leads to an increase in manufacturing costs. For this reason, the oxygen content of the zinc-containing powder may be 0.1 mass% or more, 0.2 mass% or more, or 0.3 mass% or more, relative to the entire zinc-containing powder.

<Second coated magnetic powder preparation process>
A second coating is formed on the particle surface of the second particle group of SmFeN powder to obtain a second coated magnetic powder. There is no particular restriction on the second coating, so long as it can prevent the particle surface of the second particle group from being modified by the adjacent first coating and does not adversely affect the magnetic properties of the rare earth magnet obtained by the manufacturing method of the present disclosure. Such a second coating contains, for example, phosphoric acid, but is not limited thereto.

When the second coating is a coating containing phosphoric acid, the phosphoric acid content in the second coating may be 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more, or may be 100% by mass, based on the entire second coating. Also, when the second coating is a coating containing phosphoric acid, its thickness may be 5 to 100 nm.

As long as the second coating can be formed on the particle surfaces of the second particle group, the method of formation is not particularly limited.

When a coating containing phosphoric acid is formed as the second coating on the particle surfaces of the second particle group, the following method can be used as the method for forming the second coating.

By subjecting the particles of the second particle group to a phosphate treatment, a passivation film having a P-O bond is formed on the particle surface of the second particle group. In the phosphate treatment process, a phosphate treatment agent is reacted with the particles of the second particle group. Examples of the phosphate treatment agent include inorganic phosphoric acids and organic phosphoric acids such as phosphate salts such as orthophosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, zinc phosphate, and calcium phosphate, hypophosphorous acid, hypophosphite salts, pyrophosphoric acid, and polyphosphoric acid. These phosphate sources are basically dissolved in water or an organic solvent such as an IPN, and the particles of the second particle group are placed in a phosphate bath to which a reaction accelerator such as nitrate ions and a crystal refiner such as V ions, Cr ions, and Mo ions have been added as necessary, to form a passivation film having a P-O bond on the particle surface of the second particle group.

<Mixing process>
The first coated magnetic powder and the second coated magnetic powder are mixed to obtain a mixed powder. The mixing method is not particularly limited, and examples thereof include a method of mixing using an agitator mixer and a V-type mixer, and these may be combined.

<Magnetic field molding process>
The mixed powder is compressed in a magnetic field to obtain a magnetically compacted body, which gives orientation to the magnetically compacted body and anisotropy to the resulting product (rare earth magnet), improving remanence.

The magnetic field molding method may be a well-known method such as a method of compressing and molding the mixed powder using a mold around which a magnetic field generating device is installed. The molding pressure may be, for example, 10 MPa or more, 20 MPa or more, 30 MPa or more, 50 MPa or more, 100 MPa or more, or 150 MPa or more, and may be 1500 MPa or less, 1000 MPa or less, or 500 MPa or less. The time for applying the above-mentioned molding pressure may be, for example, 0.5 minutes or more, 1 minute or more, or 3 minutes or more, and may be 10 minutes or less, 7 minutes or less, or 5 minutes or less. The magnitude of the applied magnetic field may be, for example, 500 kA/m or more, 1000 kA/m or more, 1500 kA/m or more, or 1600 kA/m or more, and may be 20000 kA/m or less, 15000 kA/m or less, 10000 kA/m or less, 5000 kA/m or less, 3000 kA/m or less, or 2000 kA/m or less. Examples of methods for applying a magnetic field include a method of applying a static magnetic field using an electromagnet and a method of applying a pulse magnetic field using an alternating current. In addition, in order to suppress oxidation of the mixed powder, it is preferable that the magnetic field molding is performed in an inert gas atmosphere. The inert gas atmosphere includes a nitrogen gas atmosphere.

<Pressure sintering process>
The magnetic field compact is pressure-sintered to obtain a sintered body. The pressure-sintering method is not particularly limited, and any known method can be applied. As the pressure-sintering method, for example, a method is given in which a die having a cavity and a punch capable of sliding inside the cavity are prepared, the magnetic field compact is inserted inside the cavity, and the magnetic field compact is sintered while applying pressure to the magnetic field compact with the punch. In this method, typically, the die is heated using a high-frequency induction coil. Alternatively, a discharge plasma sintering (SPS) method may be used.

The pressure sintering conditions can be appropriately selected so that the magnetic field compact can be sintered (hereinafter sometimes referred to as "pressure sintered") while pressure is applied to the magnetic field compact.

If the sintering temperature is 300°C or higher, the sintering of the mixed powder of the first coated magnetic powder and the second coated magnetic powder proceeds favorably in the magnetic field compact. From this viewpoint, the sintering temperature may be, for example, 310°C or higher, 320°C or higher, 340°C or higher, or 350°C or higher. On the other hand, if the sintering temperature is 430°C or lower, the Fe on the particle surface of the first particle group of the first coated magnetic powder and the zinc component in the first coating of the first coated magnetic powder do not diffuse excessively between each other. As a result, there is no problem with the heat treatment process described later, or there is no adverse effect on the magnetic properties of the obtained sintered body. From this viewpoint, the sintering temperature may be 420°C or lower, 410°C or lower, 400°C or lower, 390°C or lower, 380°C or lower, 370°C or lower, or 360°C or lower. From the viewpoint of sintering the mixed powder in a relatively short time during pressure sintering and minimizing mutual diffusion between the Fe on the particle surfaces of the first particle group of the first coated magnetic powder and the zinc component in the first coating of the first coated magnetic powder, the sintering temperature may be 350°C or higher, 360°C or higher, or 370°C or higher, and may be 400°C or lower, 390°C or lower, or 380°C or lower.

As for the sintering pressure, a sintering pressure capable of increasing the density of the sintered body may be appropriately selected. The sintering pressure may typically be 100 MPa or more, 200 MPa or more, 400 MPa or more, 500 MPa or more, 600 MPa or more, 800 MPa or more, or 1000 MPa or more, and may be 2000 MPa or less, 1800 MPa or less, 1600 MPa or less, 1500 MPa or less, 1300 MPa or less, or 1200 MPa or less. In the above-mentioned range, the higher the pressure, the more advantageous it is for improving the density of the sintered body, but the frequency of contact between particles in the magnetic field compact increases during sintering, and cracks and/or deformations are likely to occur in the sintered body itself and/or the particles in the sintered body, and as a result, distortions are likely to remain in the sintered body itself and/or the particles in the sintered body. Improving the density of the sintered body contributes to improving the residual magnetization, but if distortions remain in the sintered body itself and/or the particles in the sintered body, the saturation magnetization decreases, and therefore the residual magnetization decreases. For these reasons, the sintering pressure is preferably 200 MPa or more, 400 MPa or more, 500 MPa or more, 600 MPa or more, 800 MPa or more, or 1000 MPa or more, and 1500 MPa or less, 1300 MPa or less, or 1200 MPa or less.

The sintering time may be appropriately determined in consideration of the relationship with the sintering temperature, etc., so that the mixed powder in the magnetic compact can be sintered while avoiding excessive mutual diffusion between the Fe on the particle surface of the first particle group of the first coated magnetic powder and the zinc component in the first coating of the first coated magnetic powder. The sintering time does not include the time required for the temperature to rise to the heat treatment temperature. The sintering time may be, for example, 1 minute or more, 2 minutes or more, or 3 minutes or more, and 30 minutes or less, 20 minutes or less, 10 minutes or less, or 5 minutes or less. When the sintering time is within the above-mentioned range, the sintering temperature is set to be equal to or less than the melting point of zinc, i.e., 300°C or more, 310°C or more, 320°C or more, 340°C or more, or 350°C or more, and 400°C or less, 390°C or less, 380°C or less, 370°C or less, or 360°C or less, thereby preventing the zinc component from excessively diffusing to the surface of the SmFeN particles, and as a result, the decrease in remanent magnetization can be suppressed.

After the sintering time has elapsed, the sintered body is cooled to terminate the sintering. A faster cooling rate can suppress oxidation of the sintered body and can also suppress coarsening of the magnetic phase in the sintered body. The cooling rate may be, for example, 0.5°C/sec or more, 1°C/sec or more, or 10°C/sec or more, and may be 200°C/sec or less, 150°C/sec or less, 100°C/sec or less, or 50°C/sec or less.

As for the sintering atmosphere, an inert gas atmosphere is preferable to suppress oxidation of the magnetic compact and the sintered body. Inert gas atmospheres include argon gas atmosphere and nitrogen gas atmosphere. Alternatively, sintering may be performed in a vacuum.

<Heat treatment process>
The sintered body is subjected to a heat treatment, whereby, during the heat treatment, in the first coated magnetic powder in the sintered body, the first coating advances the modification of the particle surfaces of the first particle group, and, in the second coated magnetic powder in the sintered body, the second coating inhibits the particle surfaces of the second particle group from being modified by the adjacent first coating.

As the modification progresses, a coating-like modified phase (Fe-Zn alloy phase) is formed on the particle surfaces of the first particle group, bonding the particles of the first particle group together even more strongly (hereinafter, this may be referred to as "solidifying" or "solidification"). At the same time, the modification is promoted. At this time, due to the solidification of the particles of the first particle group, the particles of the second particle group present between the first particle group are also bonded firmly to the particles of the first particle group.

When the first particle group and the second particle group contain fine powder particles, an Fe-Zn alloy phase is formed in almost the entire fine powder particle, and most of the Fe-Zn alloy phase is integrated with the coating-like modified phase (Fe-Zn alloy phase) formed on the particle surface of the first particle group.

As described above, in the manufacturing method of the present disclosure, even if the modification progresses in the first coated magnetic powder, in the second coated magnetic powder, the particle surface of the second particle group is suppressed from being modified by the adjacent first coating by the second coating. This promotes the improvement of the coercive force of the first particle group and suppresses the improvement of the coercive force of the second particle group. Since the first particle group has a large particle size, it has a low coercive force without modification, whereas the second particle group has a small particle size and has a high coercive force even without modification. As the improvement of the coercive force of the first particle group progresses and the improvement of the coercive force of the second particle group is suppressed, the difference in coercive force between the first particle group and the second particle group becomes smaller, and the squareness, especially at high temperatures, is improved (see FIG. 3). As a result, the sintered body after heat treatment (rare earth magnet obtained by the manufacturing method of the present disclosure) is less likely to demagnetize in an environment where an external magnetic field is applied, especially at high temperatures.

Ideally, the heat treatment should be performed until all of the particle surfaces of the first particle group are covered with the modified phase, that is, until 100% of the particle surfaces of the first particle group are covered with the modified phase (modified phase coverage of 100%). However, it is sufficient to perform the heat treatment with the aim of covering 75% or more, 76% or more, 80% or more, or 90% or more of the particle surfaces of the first particle group with the modified phase. This provides high-temperature demagnetization characteristics that are practically advantageous. The method for measuring the modified phase coverage is as follows. In this specification, unless otherwise specified, descriptions regarding the modified phase coverage are based on the following measurement method (investigation method).

The cross section of the sintered body after the heat treatment is polished, and the polished surface is subjected to component analysis (surface analysis) for Fe and Zn, respectively, to obtain an Fe mapping image and a Zn mapping image. The Fe mapping image and the Zn mapping image are superimposed to obtain an integrated mapping image. In the integrated mapping image, the region of the SmFeN powder (first particle group) is identified, and the periphery length L of the SmFeN powder (first particle group) is measured. In the integrated mapping image, the length _Lc of the portion sandwiched between the Fe detection region and the Zn detection region, and the length Lg of the portion sandwiched between the Fe detection region and the non-detection region of the periphery of the SmFeN _powder (first particle group) are measured. The non-detection region means a region where neither Fe nor Zn is detected. Then, the modified phase coverage rate (%) is calculated from the following formula (1).
Modified phase coverage (%) = L _c / (L _c + L _g ) x 100...Formula (1)

In the above formula (1), ( _Lc + _Lg ) means the total circumferential length of the particle surface of the first particle group in cross section, and _Lc means the covering length (modified phase length) of the particle surface of the first particle group.

As shown in FIG. 1, the sintered body after heat treatment (rare earth magnet 90 obtained by the manufacturing method of the present disclosure) has a structure in which the modified phase 20 is formed on the particle surfaces of the first particle group 11, but the modified phase 20 is not substantially formed on the particle surfaces of the second particle group 12. For convenience, FIG. 1 shows an example in which the modified phase coverage is 100%, but this is not limited to this and may be the modified phase coverage described above.

In the example of FIG. 1, in the sintered body after heat treatment (rare earth magnet 90 obtained by the manufacturing method of the present disclosure), substantially no coating is observed on the particle surfaces of the second particle group 12, but this is not limited to this. That is, the second coating 42 present in the sintered body before heat treatment is typically decomposed into the elements that make up the second coating 42, and the elements are thought to be dispersed on the modified phase 20 or the particle surfaces of the second particle group 12, but this is not limited to this. For this reason, in the sintered body after heat treatment (rare earth magnet 90 obtained by the manufacturing method of the present disclosure), the second coating may remain on some or all of the particle surfaces of the second particle group.

For example, if the heat treatment temperature is 350°C or higher, the above-mentioned modified phase can be obtained. From this perspective, the heat treatment temperature may be 360°C or higher, 370°C or higher, or 380°C or higher. On the other hand, if the heat treatment temperature is 410°C or lower, Fe and Zn do not diffuse excessively between each other. However, although solidification, modification, and rendering the fine particles harmless can be achieved at a heat treatment temperature of 410°C, knicks occur, so the heat treatment temperature is preferably 400°C or lower or 390°C or lower. Note that knicks refer to a sudden decrease in magnetization in response to a slight decrease in the magnetic field in a region other than the region showing the coercive force of the magnetization-magnetic field curve (M-H curve).

The heat treatment time is not particularly limited, but may be determined by the following formulas (2) and (3) where the heat treatment temperature is x° C. and the heat treatment time is y hours.
y≧-0.32x+136...Formula (2)
350≦x≦410...Formula (3)

The above formulas (2) and (3) have been confirmed by experiments, and specifically show that the higher the heat treatment temperature, the shorter the heat treatment time for solidification and the formation of the above-mentioned modified phase.

From the viewpoint of increasing the coverage rate of the modified phase as much as possible, the above formula (2) is preferably y ≧ -0.32x + 137, more preferably y ≧ -0.32x + 140, and even more preferably y ≧ -0.32x + 145.

The above-mentioned modified phase is formed by alloying the α-Fe phase present on the particle surface of the first particle group with the zinc component in the first coating in the first coated magnetic powder. To form the modified phase, the heat treatment time may typically be 3 hours or more, 4 hours or more, 5 hours or more, 8 hours or more, 10 hours or more, 12 hours or more, 15 hours or more, 17 hours or more, or 20 hours or more. On the other hand, the amount of α-Fe phase present on the particle surface of the first particle group is limited, and the depth to which the zinc component in the first coating diffuses into the particles of the first particle group is also limited. Therefore, even if the heat treatment is performed for an excessively long time, the formation of the modified phase is saturated. From this viewpoint, the heat treatment time y (hours) is preferably 40 hours or less, 35 hours or less, 30 hours or less, 25 hours or less, or 24 hours or less.

In order to suppress oxidation of the sintered body, it is preferable to heat treat the sintered body in a vacuum or in an inert gas atmosphere. The inert gas atmosphere includes a nitrogen gas atmosphere. The heat treatment of the sintered body may be performed in the mold used for pressure sintering, but in that case, no pressure is applied to the sintered body during the heat treatment. If the above-mentioned heat treatment conditions are satisfied, the normal magnetic phase is decomposed to generate an α-Fe phase, and as a result of the generation, it is possible to suppress excessive interdiffusion of Fe and Zn. When the heat treatment is performed in a vacuum, the absolute pressure of the atmosphere may be 1×10 ⁻⁷ Pa or more, 1×10 ⁻⁶ Pa or more, or 1×10 ⁻⁵ Pa or more, and may be 1×10 ⁻² Pa or less, 1×10 ⁻³ Pa or less, or 1×10 ⁻⁴ Pa or less.

《Transform》
In addition to what has been described above, the manufacturing method of the present disclosure can be modified in various ways within the scope of the claims.

For example, when the first particle group and/or the second particle group contain fine particles, a part or all of the fine particles may be removed in advance before the magnetic field compaction, as long as the _D50 of the first particle group and the second particle group satisfies the above-mentioned range. There is no particular limitation on the fine particle removal operation (fine particle removal method). Examples of the fine particle removal operation (fine particle removal method) include a method using a centrifugal field classification device, a method using a sieve, a method using a magnetic field, and a method using static electricity. A combination of these methods may also be used. By removing the fine particles, the density of the molded body (rare earth magnet) can be further increased, and the magnetization and mechanical strength can be further increased.

The manufacturing method of the present disclosure will be explained in more detail below with reference to examples and comparative examples. Note that the manufacturing method of the present disclosure is not limited to the conditions used in the following examples.

<Sample preparation>
Samples of Examples 1 to 8 and Comparative Examples 1 to 9 were prepared as follows.

Examples 1 to 8 and Comparative Examples 1 to 6
5.0 kg of FeSO ₄ · 7H ₂ O was mixed and dissolved in 2.0 kg of pure water. 0.49 kg of Sm ₂ O ₃ , 0.74 kg of 70% sulfuric acid, and 0.035 kg of La ₂ O ₃ were further added and thoroughly stirred to completely dissolve. Next, pure water was added to the obtained solution, and the final Fe concentration was adjusted to 0.726 mol / L and the Sm concentration to 0.112 mol / L to obtain a SmFeLa sulfuric acid solution.

[Precipitation step]
The entire amount of the prepared SmFeLa sulfuric acid solution was dropped into 20 kg of pure water kept at a temperature of 40°C over a period of 70 minutes from the start of the reaction while stirring, and at the same time, 15% ammonia solution was dropped to adjust the pH to 7 to 8. This resulted in a slurry containing SmFeLa hydroxide. The obtained slurry was washed with pure water by decantation, and the hydroxide was subjected to solid-liquid separation. The separated hydroxide was dried in an oven at 100°C for 10 hours.

[Oxidation process]
The hydroxide obtained in the precipitation step was calcined in air at 1000° C. for 1 hour. After cooling, red SmFeLa oxide was obtained as a raw material powder.

[Pretreatment process]
100 g of SmFeLa oxide was placed in a steel container so that the bulk thickness was 10 mm. The container was placed in a furnace, and after reducing the pressure to 100 Pa, the temperature was raised to the pretreatment temperature of 850° C. while introducing hydrogen gas, and the temperature was maintained for 15 hours. The oxygen concentration was measured by non-dispersive infrared absorption (ND-IR) (EMGA-820 manufactured by Horiba, Ltd.) and found to be 5 mass %. This revealed that the oxygen bonded to Sm was not reduced, and 95% of the oxygen bonded to Fe was reduced to obtain a black partial oxide.

[Reduction process]
60 g of the partial oxide obtained in the pretreatment process and 19.2 g of metallic calcium having an average particle size of about 6 mm were mixed and placed in a furnace. After evacuating the furnace, argon gas (Ar gas) was introduced. The temperature was raised to 1090° C. and held for 45 minutes, and then cooled to obtain SmFe powder particles.

[Nitriding process]
Subsequently, the temperature inside the furnace was cooled to 100° C., and then the furnace was evacuated to a vacuum, and while introducing nitrogen gas, the temperature was raised to a first temperature of 430° C. and held for 3 hours. Then, the temperature was raised to a second temperature of 500° C. and held for 1 hour, and then cooled to obtain an aggregated product containing magnetic powder particles.

[Post-processing process]
The aggregated product obtained in the nitriding step was poured into 3 kg of pure water and stirred for 30 minutes. After standing, the supernatant was drained by decantation. The process of pouring into pure water, stirring and decantation was repeated 10 times. Then, 2.5 g of 99.9% acetic acid was poured into the product and stirred for 15 minutes. After standing, the supernatant was drained by decantation. The process of pouring into pure water, stirring and decantation was repeated twice.

[Acid treatment step]
A 6% aqueous hydrochloric acid solution was added to 100 parts by mass of the powder obtained in the post-treatment process so that the hydrogen chloride content was 4.3 parts by mass, and the mixture was stirred for 1 minute. After standing, the supernatant was drained by decantation. The mixture was poured into pure water, stirred, and decanted twice. After solid-liquid separation, the mixture was vacuum dried at 80°C for 3 hours to obtain SmFeN powder having a composition of Sm _9.2 Fe _77.1 N _13.59 La _0.11 .

The SmFeN powder was packed into a sample container together with paraffin wax, and the paraffin wax was melted in a dryer, after which the easy axis of magnetization was aligned in an orientation magnetic field of 16 kA/m. This magnetically oriented sample was pulse magnetized in a magnetizing magnetic field of 32 kA/m, and the magnetic properties were measured at room temperature using a VSM (vibrating sample magnetometer) with a maximum magnetic field of 16 kA/m, revealing a residual magnetization of 1.44 T and a coercive force of 750 kA/m.

The SmFeN powder obtained as described above was classified to obtain a powder of the first particle group and a powder of the second particle group. The particle size distribution (D ₅₀ ) of each of the first particle group and the second particle group was as shown in Table 1-1. The ratio of the total volume of the first particle group to the total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) was as shown in Table 1-1. Table 1-1 also lists the particle remanence σ _r and particle coercivity H _c of each of the first particle group and the second particle group.

A first coating was formed on the particle surface of the first particle group by the method shown in Fig. 4 to obtain a first coated magnetic powder. Metallic zinc powder was prepared as a powder containing zinc so that the first coating was a coating containing zinc. The _D50 of the metallic zinc powder was 0.5 μm. The purity of the metallic zinc powder was 99.5 mass%.

The zinc component content ratio relative to the total mass of the first coated magnetic powder and the second coated magnetic powder, i.e., the amount of metal zinc powder charged into the rotary kiln furnace together with the particle powder of the first particle group, relative to the total mass of the first coated magnetic powder and the second coated magnetic powder, was as shown in Table 1-1. The treatment conditions in the rotary kiln furnace were as follows: the atmosphere was a nitrogen gas atmosphere, the treatment temperature was 380°C, the treatment time was 1 minute, and the rotation speed of the stirring drum was 6 rpm. The zinc component coverage rate of the coated magnetic powder particles obtained in this manner is also shown in Table 1-1.

A coating containing phosphoric acid was formed as a second coating on the particle surfaces of the second particle group to prepare a second coated magnetic powder. The coating containing phosphoric acid was formed by carrying out a dispersion process and a surface treatment process as a preparation process before the phosphoric acid treatment process. Details of the dispersion process, surface treatment process, and phosphoric acid treatment process are as follows.

[Dispersion process]
The powder of the second particle group and the media were placed in a container so that the powder of the second particle group was 5 volume % and the media (iron-core nylon media, diameter 10 mm, Vickers constant of nylon coating 7, specific gravity 7.48 g/cm3) was 60 volume % relative to the volume of the container used for the vibration mill. The mixture was dispersed in a nitrogen atmosphere for 60 minutes using the vibration mill to obtain an intermediate powder.

[Surface treatment process]
The obtained intermediate powder was put into pure water and stirred for 1 minute. An acid solution was added to this slurry to perform etching. The acid solution used was a hydrochloric acid solution. 50 g or more of 5% hydrochloric acid was added to 100 g of the intermediate powder while stirring. Then, after confirming that the pH was 3 or more, decantation was performed until the electrical conductivity of the slurry was 100 μS/cm or less.

[Phosphating process]
A phosphoric acid solution was added to the obtained slurry. The phosphoric acid solution was added in an amount of 1 mass% as _PO4 relative to the solid content of the particles of the second particle group. After stirring for 5 minutes and performing solid-liquid separation, the mixture was vacuum-dried at 120°C for 3 hours to obtain a second coated magnetic powder having a coating containing phosphoric acid.

The first coated magnetic powder and the second coated magnetic powder were mixed to obtain a mixed powder. The mixed powder was compression molded in a magnetic field to obtain a magnetically molded body. The compression molding pressure was 50 MPa. This pressure was applied for 1 minute. The applied magnetic field was 1600 kA/m. The compression molding was performed in a nitrogen atmosphere.

The magnetic field compact was pressure sintered. Pressure sintering was performed in an argon gas atmosphere (97,000 Pa) using a high-frequency induction coil. The sintering temperature was 380°C, the sintering pressure was 500 MPa, and the sintering pressure was applied for 5 minutes.

The sintered body was heat-treated in a vacuum (10 ⁻² Pa) at a temperature of 380° C. for a period of 24 hours.

Comparative Example 7
A sample of Comparative Example 7 was prepared in the same manner as in Example 3, except that the zinc component coverage rate of the particle surfaces of the first particle group was 72%, the second coating film was not formed on the particle surfaces of the second particle group, and the σ _r of the second particle group was 141 emu/g.

Comparative Example 8
A sample of Comparative Example 8 was prepared in the same manner as in Example 1, except that the magnetic powder contained only the first particle group and did not contain the second particle group.

Comparative Example 9
A sample of Comparative Example 9 was prepared in the same manner as in Example 3, except that the magnetic powder contained only the first particle group and did not contain the second particle group.

"evaluation"
The modified phase coverage and magnetic properties of each sample were measured. The magnetic properties were measured at room temperature and 120°C using a vibrating sample magnetometer (VSM). Demagnetization was evaluated by the magnetic field _Hk at 120°C when the magnetization decreased by 10% from the residual magnetization _Br . The density of the sintered body after the heat treatment was also measured by the Archimedes method.

The evaluation results are shown in Tables 1-1 and 1-2. In Table 1-2, the residual magnetization and coercivity are the results of measurements taken at room temperature.

From Tables 1-1 and 1-2, it can be seen that in all of the example samples, the residual magnetization was 0.9 T or more, and the _Hk at 120° C. was 700 kA/m or more. From this, it can be seen that the rare earth magnets obtained by the manufacturing method of the present disclosure have excellent magnetization and are resistant to demagnetization in an environment where an external magnetic field is applied, particularly at high temperatures.

In contrast, in the samples of Comparative Examples 1 and 2, d ₂ /d ₁ is large, and as shown in FIG. 6, the particle diameter of the second particle group 12 is large, so the density is reduced and the magnetization is low. In the samples of Comparative Examples 3 to 5, the total volume ratio of the first particle group 11 and the second particle group 12 is not appropriate, and as shown in FIG. 7, the volume of the second particle group 12 is excessive, so the density is reduced and the magnetization is low. In the sample of Comparative Example 6, the zinc content in the first coated magnetic powder is low, so the modification of the particle surface of the first particle group is insufficient, and H _k is low. In the sample of Comparative Example 7, the second coating was not formed on the particle surface of the second particle group, so as shown in FIG. 8, a thick modified phase 20 was formed on the particle surface of the second particle group, and the modification was excessive, so H _k is low. In the samples of Comparative Examples 8 to 9, as shown in FIG. 9, only the first particle group 11 is included, and the second particle group 12 is not included, so the density is low and the magnetization is low.

These results confirm the effectiveness of the disclosed method for manufacturing rare earth magnets.

REFERENCE SIGNS LIST 10 SmFeN powder 11 First particle group 12 Second particle group 20 Modified phase 30 Sintered body 31 First coated magnetic powder 32 Second coated magnetic powder 41 First coating 42 Second coating 90 Rare earth magnet 100 Rotary kiln furnace 110 Stirring drum 120 Material storage section 130 Rotating shaft 140 Stirring plate 150 SmFeN powder 160 Powder containing zinc 171 First heat treatment furnace 172 Second heat treatment furnace 173 Connecting passage 180 Vacuum pump 181 First container 182 Second container

Claims (6)

A method for producing _a rare earth magnet containing Sm, Fe, and N and having a magnetic phase at least a part of which has a crystal structure of either _Th2Zn17 type or _{Th2Ni17 type} , comprising the steps of:
Providing a magnetic powder comprising a first group of particles and a second group of particles;
forming a first coating on the surfaces of the particles of the first particle group of the magnetic powder to obtain a first coated magnetic powder;
forming a second coating on the surfaces of the particles of the second particle group of the magnetic powder to obtain a second coated magnetic powder;
mixing the first coated magnetic powder and the second coated magnetic powder to obtain a mixed powder;
The mixed powder is compression-molded in a magnetic field to obtain a magnetically molded body.
Pressure sintering the magnetic field compact to obtain a sintered body; and heat treating the sintered body.
Including,
The particle size distribution _D50 of the first particle group is expressed in terms of _d1 μm, and the particle size distribution _D50 of the second particle group is expressed in terms of _d2 μm;
The _d1 and _d2 satisfy the relationship of 0.350≦ _d2 / _d1 ≦0.500,
a ratio of a total volume of the first particle group to a total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) is in the range of 9:1 to 4:1,
the first coating contains zinc, and the content of the zinc component in the first coated magnetic powder is 3.0 mass% or more and 10.0 mass% or less with respect to the total mass of the first coated magnetic powder and the second coated magnetic powder; and
During the heat treatment, the first coating film causes the particle surfaces of the first particle group to be modified, and the second coating film suppresses the particle surfaces of the second particle group from being modified by the adjacent first coating film.
A method for manufacturing rare earth magnets. The method for producing a rare earth magnet according to claim 1, wherein the second coating contains phosphoric acid. The method for producing a rare earth magnet according to claim 1 or 2, wherein heat treatment is performed until an Fe-Zn alloy phase is formed on 75% or more of the surface of the particles of the first particle group in the sintered body. The method for manufacturing a rare earth magnet according to any one of claims 1 to 3, wherein the heat treatment is carried out at a temperature of 350°C or higher and 410°C or lower. The method for manufacturing a rare earth magnet according to any one of claims 1 to 4, wherein the heat treatment is carried out for 3 hours or more and 40 hours or less. The method for producing a rare earth magnet according to any one of claims 1 to 5, wherein the magnetic compact is pressure sintered at a pressure of 200 MPa to 1500 MPa and a temperature of 300°C to 400°C for 1 minute to 30 minutes.

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