Method for producing coated rare earth transition metal alloy powder and coated rare earth transition metal alloy powder

Abstract

The present invention provides a rare-earth transition metal alloy powder having excellent magnetic properties and the like, and excellent heat resistance and weather resistance, and a method for producing the rare-earth transition metal alloy powder in a short time. The method for producing the coated rare-earth transition metal alloy powder of the present invention includes: a first step of pulverizing a rare-earth transition metal alloy coarse powder in a solvent, and adding a phosphoric acid compound to the solvent at the time of the pulverization, to obtain a fine powder coated with a phosphate film on the surface; and a second step of performing a heat treatment of maintaining the obtained fine powder at a predetermined temperature in a drying machine, to thereby obtain a rare-earth transition metal alloy powder coated with a phosphate film.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for producing a coated rare earth transition metal-based alloy powder and a coated rare earth transition metal-based alloy powder. BACKGROUND

[0002] Conventionally, permanent magnets such as ferrite magnets, alnico magnets, and rare earth magnets have been used for various applications such as motors. However, since these magnets are mainly produced by a sintering method, they are generally brittle and have a disadvantage that it is difficult to obtain a thin-walled or complex-shaped magnet. In addition, since the shrinkage at the time of sintering is as high as 15 to 20%, there is a disadvantage that a magnet with high dimensional accuracy cannot be obtained and post-processing such as polishing is required to improve the accuracy.

[0003] In order to solve the disadvantages of these sintered magnets and to open up new applications, bonded magnets have been developed in recent years. The bonded magnet is generally produced by using a thermoplastic resin such as a polyamide resin or a polyphenylene sulfide resin as a binder and filling a rare earth transition metal-based alloy powder such as a rare earth iron-nitrogen-based magnet powder therein.

[0004] Incidentally, a rare earth transition metal-based alloy powder is susceptible to rusting or a decrease in magnetic properties in a high-temperature and high-humidity environment. Therefore, a technique of performing surface treatment on the alloy powder using a phosphoric acid compound is known (Patent Literature 1). Specifically, when the alloy powder is pulverized in a pulverizing solvent, a phosphoric acid compound is added to the solvent, and a phosphate film is formed on the alloy powder. Then, the alloy powder is dried to produce a rare earth transition metal-based alloy powder coated with a film in which phosphoric acid is a main component. In addition, in order to sufficiently perform the drying treatment, a method of controlling the oxygen partial pressure at the time of drying within a prescribed range, thereby reducing the variation in coercive force or weather resistance, has been proposed (Patent Literature 2).

[0005] Patent Literature 1: Japanese Patent Application Publication No. 2002-124406;

[0006] Patent Literature 2: Japanese Patent Application Publication No. 2017-043832. SUMMARY

[0007] Thus, although a method of treating the surface of a rare earth transition metal-based alloy powder using a phosphoric acid compound has been proposed, there is still room for improvement in the existing method. For example, in the technology proposed in Patent Literature 1, when drying the pulverized alloy powder, the alloy powder is subjected to heat treatment in a non-reactive gas or in a vacuum at a temperature in the range of 100°C or higher and less than 400°C. However, with such a method, the properties of the alloy powder, such as coercive force or weather resistance, can vary between different manufacturing lots. In contrast, in the method proposed in Patent Literature 2, there is an advantage in that an alloy powder having high coercive force, excellent heat resistance, and weather resistance can be manufactured. However, there is a problem in that the total processing time is long, resulting in poor productivity. Therefore, in the existing technology, it is difficult to manufacture an alloy powder having high coercive force, excellent heat resistance, and weather resistance with high productivity.

[0008] The present inventors conducted intensive research in view of such existing problems. As a result, the present inventors obtained the following insight: in a method of manufacturing a rare earth transition metal-based alloy powder that has been subjected to surface treatment using a phosphoric acid compound, by focusing on the pressure variation during drying treatment, the internal pressure of the drying machine varies before and after the removal of the pulverizing solvent, and by controlling the drying environment using the internal pressure as an index, an alloy powder can be obtained in a shorter time than before, in addition, the properties of the obtained alloy powder, such as magnetic properties, are good, and the heat resistance and weather resistance are excellent.

[0009] The present application was completed based on such insight, and the object thereof is to provide a rare earth transition metal-based alloy powder having good properties, such as magnetic properties, and excellent heat resistance and weather resistance, and a method of manufacturing the rare earth transition metal-based alloy powder in a short time.

[0010] The present application includes the following (1) to (6). Note that the expression "to" in the present specification includes both numerical values. That is, "X to Y" is synonymous with "X or more and Y or less".

[0011] (1) A method of manufacturing a coated rare earth transition metal-based alloy powder, wherein

[0012] includes: a first step of pulverizing a rare earth transition metal-based alloy coarse powder in a solvent, and adding a phosphoric acid compound to the solvent at the time of the pulverization, to obtain a fine powder coated with a phosphate film on the surface; and

[0013] a second step of performing heat treatment of maintaining the obtained fine powder at a predetermined temperature in a drying machine, whereby a rare earth transition metal-based alloy powder coated with a phosphate film is obtained,

[0014] In the second step, the pressure P in the drying machine at the time of heat treatment is measured aat a drying initial stage where a pressure change P (unit: kPa) represented by the following (1) satisfies a relationship of x < P (where x is a pressure in a range of 0.8 kPa or more and 1.2 kPa or less), the fine powder is subjected to a heat treatment in a reduced pressure environment where an oxygen-containing gas is not introduced into a dryer, and thereafter, at a drying late stage where a relationship of P ≤ x is satisfied, the fine powder is subjected to a heat treatment in an environment where an oxygen partial pressure is 0.1 kPa or more and 5 kPa or less with the introduction of the oxygen-containing gas into the dryer.

[0015] P = |P a - P0|... (1)

[0016] (where P a is a pressure in the dryer at the time of the heat treatment, and P0 is a reduced pressure initial pressure in the dryer.)

[0017] (2) In the method of the above (1), the rare earth transition metal-based alloy powder is a samarium-iron-nitrogen (Sm-Fe-N) based magnet powder.

[0018] (3) In the method of the above (1) or (2), a holding temperature at the time of the heat treatment is 100°C or more and 200°C or less.

[0019] (4) In any one of the methods of the above (1) to (3), a heating time at the time of the heat treatment is 0.5 hours or more and 20 hours or less.

[0020] (5) An alloy powder which is a coated rare earth transition metal-based alloy powder, has a rare earth transition metal-based alloy powder and a phosphate film that coats a surface of the rare earth transition metal-based alloy powder, and an atomic concentration of phosphorus included in the phosphate film is 8 atomic % or more.

[0021] (6) In the alloy powder of the above (5), the rare earth transition metal-based alloy powder is a samarium-iron-nitrogen (Sm-Fe-N) based magnet powder.

[0022] According to the present application, a rare earth transition metal-based alloy powder having excellent characteristics such as magnetic characteristics, and excellent heat resistance and weather resistance, and a method of manufacturing the rare earth transition metal-based alloy powder in a short time are provided. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a graph showing a temperature at the time of the heat treatment and an environmental pressure change. DETAILED DESCRIPTION

[0024] A specific embodiment of the present application (hereinafter, referred to as "the present embodiment") will be described. Note that the present application is not limited to the following embodiment, and various modifications can be made within the scope of the gist of the present application.

[0025] <<1. Coated rare earth transition metal alloy powder>>

[0026] The coated rare earth transition metal alloy powder (hereinafter, sometimes referred to as "coated alloy powder") of the present embodiment has a rare earth transition metal alloy powder and a phosphate film covering the surface of the rare earth transition metal alloy powder. The coated rare earth transition metal alloy powder is useful as a permanent magnet material, a hydrogen storage material, an optical magnetic material, a magnetic refrigeration material, and the like in various applications.

[0027] The rare earth transition metal alloy powder coated with the phosphate film (hereinafter, sometimes referred to as "alloy powder") is composed of an alloy containing at least a rare earth metal (Re) and a transition metal (TM). Here, the concept of the alloy includes not only a solid solution but also a eutectic or an intermetallic compound. As the intermetallic compound, a compound having a crystal structure of CaCu5type, Th2Zn 17 type, Th2Ni 17 type, TbCu7type, ThMn 12 type, NaZn 13 type, Nd2Fe 14 B type, MgCu2type, and the like can be given. In addition, the powder is a collection of many particles. That is, many particles are collected to constitute the powder. The particle diameter of the particles constituting the alloy powder is not particularly limited. However, it is preferable that the particle size distribution of the particles be relatively concentrated. For example, the alloy powder is preferably composed of particles having a particle diameter in the range of 5 μm or more and 80 μm or less. Note that the particle diameter is measured by a publicly known measuring device such as a dry-type particle size distribution measuring device or a wet-type particle size distribution measuring device.

[0028] The rare earth metal (Re) is a general term for a metal (element) constituting a group consisting of scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium (Lu) having an atomic number of 71. The rare earth metal (Re) is not particularly limited. However, in the case where the alloy powder is applied to a use as a permanent magnet, it is preferable that one or more selected from a group consisting of samarium (Sm), gadolinium (Gd), and cerium (Ce), or one or more selected from a group consisting of praseodymium (Pr), neodymium (Nd), dysprosium (Dy), and ytterbium (Yb) be selected. In particular, from the viewpoint of obtaining a magnet material having excellent magnetic properties, it is more preferable that one or more selected from a group consisting of samarium (Sm), neodymium (Nd), and praseodymium (Pr) be selected, and from the viewpoint of obtaining a magnet material having not only very excellent magnetic properties but also very excellent heat resistance and weather resistance, it is most preferable that samarium (Sm) be selected. The rare earth metal contained in the alloy powder can be only one kind or can be a plurality of kinds.

[0029] The transition metal (TM) refers to a general term for metals (elements) present between the elements of Group 3 to Group 11 of the periodic table. The transition metal (TM) is not particularly limited. However, in the case where the alloy powder is applied to a use of a permanent magnet, from the viewpoint of magnetic properties, one or more selected from the group consisting of iron (Fe), manganese (Mn), and cobalt (Co) is preferable, and iron (Fe) is most preferable. The transition metal contained in the alloy powder can be only one kind, or can be a plurality of kinds.

[0030] The alloy powder can contain only the rare earth metal (Re) and the transition metal (TM). As such an alloy powder, for example, a samarium-cobalt (Sm-Co) powder can be mentioned. The samarium-cobalt powder is a permanent magnet material having a basic composition of SmCo5or Sm2Co 17 . Alternatively, the alloy powder can also contain other metal elements or non-metal elements other than the rare earth metal (Re) and the transition metal (TM). As such an alloy powder, for example, a samarium-iron-nitrogen (Sm-Fe-N) powder or a neodymium-iron-boron (Nd-Fe-B) powder can be mentioned. The samarium-iron-nitrogen powder is a permanent magnet material having a basic composition of Sm2Fe 17 N x , and the neodymium-iron-boron powder is a permanent magnet material having a basic composition of Nd2Fe 14 B.

[0031] Preferably, the rare earth transition metal-based alloy powder is a samarium-iron-nitrogen (Sm-Fe-N) based magnet powder. The samarium-iron-nitrogen based magnet powder is excellent in heat resistance and weather resistance, and is useful as a magnet powder of a bonded magnet. The composition of the samarium-iron-nitrogen powder is not particularly limited as long as the magnet properties can be obtained. However, from the viewpoint of magnetic properties, the content of samarium (Sm) in the powder is preferably 14 mass% or more and 27 mass% or less, and more preferably 15 mass% or more and 25 mass% or less. In addition, in the case of the samarium-iron-nitrogen powder, the saturation magnetization is the largest when x = 3 in the basic composition (Sm2Fe 17 N x ). Therefore, x is preferably 2.5 or more and 3.5 or less, and more preferably 2.8 or more and 3.2 or less.

[0032] The phosphate film coats each particle constituting the rare earth transition metal-based alloy powder. The composition of the phosphate film is not particularly limited. For example, a rare earth phosphate such as samarium phosphate or a transition metal phosphate such as iron phosphate. The thickness and the coating rate of the film are not particularly limited. However, the average thickness is preferably 2 nm or more and 50 nm or less. In addition, it is preferable that 80% or more of the surface of the powder (particle) is coated, and it is further preferable that 90% or more is coated.

[0033] It is preferable that the atomic concentration of phosphorus contained in the phosphate film be 8 atomic % or more. The concentration of phosphorus in the phosphate film is important in improving the heat resistance and weather resistance of the coated alloy powder. The higher the concentration of phosphorus, the higher the heat resistance and weather resistance of the coated alloy powder. The atomic concentration of phosphorus is more preferably 8.5 atomic % or more. Note that the atomic concentration of phosphorus can be measured by X-ray photoelectron spectroscopy (XPS). That is, by etching the surface of the coated alloy powder by ion sputtering while performing XPS analysis, elemental analysis in the depth direction from the surface is performed, and the atomic concentration of phosphorus can be calculated based on the analysis results.

[0034] The coated alloy powder of the present embodiment has excellent heat resistance. For example, the ratio of the coercivity after keeping at 250°C for 1 hour in a reduced pressure environment (iHC (heat resistance 1h)) to the coercivity before keeping (iHc (0h)) (iHC (heat resistance 1h) / iHc (0h)) is preferably 90% or more, and more preferably 95% or more.

[0035] In addition, the coated alloy powder of the present embodiment also has excellent weather resistance. For example, the ratio of the coercivity after keeping at 80°C for 300 hours in a 90% RH environment (iHC (weather resistance 300h)) to the coercivity before keeping (iHc (0h)) (iHC (weather resistance 300h) / iHc (0h)) is preferably 90% or more, and more preferably 95% or more.

[0036] <<2. Method for producing coated rare earth transition metal-based alloy powder>>

[0037] The method for producing the coated rare earth transition metal-based alloy powder of the present embodiment includes a first step (pulverization step) and a second step (drying step). In the first step (pulverization step), a rare earth transition metal-based alloy coarse powder is pulverized in a solvent, and a phosphoric acid compound is added to the solvent at the time of the pulverization, to obtain a fine powder coated with a phosphate film on the surface. In the second step (drying step), the obtained fine powder is subjected to a heating treatment in which the fine powder is kept at a predetermined temperature in a drying machine, to thereby obtain a rare earth transition metal-based alloy powder coated with a phosphate film. In the second step (drying step), the pressure P in the drying machine at the time of the heating treatment is measured. a In the initial stage of drying in which the pressure change P (unit: kPa) represented by the following (1) satisfies the relationship of x < P (where x is a pressure in the range of 0.8 kPa or more and 1.2 kPa or less), the fine powder is subjected to a heating treatment in a reduced pressure environment in which an oxygen-containing gas is not introduced into the drying machine. In the later stage of drying in which the relationship of P ≤ x is satisfied, an oxygen-containing gas is introduced into the drying machine, and the fine powder is subjected to a heating treatment in an environment in which the oxygen partial pressure is 0.1 kPa or more and 5 kPa or less.

[0038] P = |P a - P0|... (1)

[0039] (where, P is the pressure in the dryer at the time of the heat treatment, and P0 is the initial pressure of the reduction of pressure in the dryer.) a

[0040] <First process>

[0041] In the first process (pulverization process), the rare earth transition metal alloy coarse powder (hereinafter, sometimes referred to as "alloy coarse powder") is pulverized in a solvent. In addition, a phosphoric acid compound is added to the solvent at the time of the pulverization, and a micro powder having a surface covered with a phosphate film is obtained. Here, the rare earth transition metal alloy coarse powder is a raw material powder which becomes the above-mentioned rare earth transition metal alloy powder by pulverization. Therefore, although the particle size is different from the alloy powder, the composition is almost the same. The rare earth transition metal alloy coarse powder can be manufactured by the method described later.

[0042] In the first process, the alloy coarse powder is pulverized in the presence of a phosphoric acid compound. Specifically, the phosphoric acid compound can be added to the solvent before or during the pulverization of the alloy coarse powder. The fresh surface of the micro powder generated by the pulverization is immediately treated with the phosphoric acid compound, and a phosphate film is formed. In this regard, although the treatment of forming a phosphate film on the surface of the alloy powder has been performed until now, since the alloy powder after the pulverization is agglomerated in the conventional method, a portion not covered with the phosphate film is easily generated on the contact surface of the alloy powder. Especially, in the case where the alloy powder is composed of a magnet material, since the agglomeration is caused by the magnetic force, the problem of the coating defect becomes significant. By adding the phosphoric acid compound to the solvent before or during the pulverization, such a problem can be avoided.

[0043] As the phosphoric acid compound, there is no particular limitation as long as a phosphate film can be formed on the surface of the alloy micro powder. For example, orthophosphoric acid, disodium hydrogen phosphate, pyrophosphoric acid, metaphosphoric acid, manganese phosphate, zinc phosphate, and / or aluminum phosphate, etc. can be mentioned. Orthophosphoric acid is preferred from the viewpoint of cost. The phosphoric acid compound can be used singly or in combination of two or more.

[0044] The optimum addition amount of the phosphoric acid compound depends on the particle diameter and the surface area of the alloy micro powder after the pulverization. Therefore, it is difficult to uniquely determine the optimum addition amount. It is preferred to adjust the addition amount so that the phosphorus analysis value of the finally obtained coated alloy powder is 0.2 mass% or more and 0.6 mass% or less. By adjusting the phosphorus analysis value to 0.2 mass% or more, the formation of the phosphate film becomes sufficient, and in addition, by adjusting to 0.6 mass% or less, the excessive formation of the phosphate film can be prevented, and the deterioration of the original characteristics of the alloy powder such as magnetic characteristics.

[0045] ​The method of adding the phosphoric acid compound is not particularly limited. For example, a method of adding the phosphoric acid compound to a solvent into which alloy coarse powder is put at the time of pulverizing the alloy coarse powder can be mentioned. At this time, the amount of the phosphoric acid compound to be added is only required to achieve the final desired concentration of phosphoric acid. The entire amount can be added at once before the start of pulverization. However, it is more preferable to gradually add so as to make the concentration of phosphoric acid in the solvent constant.

[0046] As the solvent (pulverization solvent), in addition to water, an organic solvent such as isopropyl alcohol, ethanol, toluene, methanol, hexane, etc. can be used, of which isopropyl alcohol is particularly preferable. Further, the pulverization device used for the pulverization is not particularly limited as long as it is a wet-type pulverization device capable of pulverizing to the desired particle size. For example, an attritor, a bead mill, or the like can be preferably used. The average particle size of the fine powder obtained by the pulverization is preferably 1 μm or more and 10 μm or less, and more preferably 1 μm or more and 3 μm or less.

[0047] <Second Step>

[0048] In the second step (drying step), the fine powder obtained in the first step (pulverization step) is subjected to a heating treatment in a drier maintained at a predetermined temperature. By heating the slurry containing the fine powder and the pulverization solvent in the drier, the pulverization solvent is volatilized and removed, leaving the rare earth transition metal-based alloy powder coated with a phosphate film.

[0049] In the second step, there are two purposes other than the removal of the solvent. The first purpose is to remove hydrogen from the alloy powder, i.e., so-called dehydrogenation. When the alloy powder is reacted with the phosphoric acid compound in the solvent, a part of the alloy is dissolved on the surface, and hydrogen is generated at the same time. At this time, the hydrogen is not discharged to the outside of the system, but is absorbed by the alloy powder or the phosphate film. The absorbed hydrogen can lower the properties of the coated alloy powder finally obtained. For example, in the case where the alloy powder is a samarium iron nitride powder having a Th2Zn 17 type crystal structure, hydrogen is occluded between the lattices of the crystal structure, and the magnetic anisotropy of the powder is lowered. When the magnetic anisotropy is lowered, a problem of lowering of the coercive force occurs. Therefore, it is preferable to dehydrogenate the alloy powder by the drying treatment.

[0050] The second object is to grow the phosphate film. When the alloy powder is reacted with the phosphoric compound, a phosphate film covering the surface of the particles of the alloy powder is formed. At this time, in order to impart excellent heat resistance and weather resistance to the alloy powder, the phosphorus concentration in the phosphate film is important. There is a relationship between the drying method and the phosphorus concentration, and when drying is insufficient, there is a tendency for the phosphorus concentration to decrease. The reason is not clear, but it is considered that when drying is insufficient, most of the water or hydrogen remains in the phosphate film, and the phosphorus concentration is relatively decreased. In addition, when the phosphorus concentration is low, the heat resistance and weather resistance of the alloy powder decrease. Therefore, in order to impart excellent heat resistance and weather resistance to the alloy powder, it is important to sufficiently grow the phosphate film and increase the phosphorus concentration.

[0051] In the production method of the present embodiment, in the second step (drying step), the environment is changed at the initial stage and the late stage of the heating treatment of the slurry containing the fine powder. Specifically, at the drying initial stage, the heating treatment is performed in a reduced pressure environment without oxygen introduction, and at the drying late stage, the heating treatment is performed under a condition of a prescribed oxygen partial pressure. Thereby, it is possible to shorten the time required for the drying treatment while maintaining excellent characteristics of the obtained alloy powder.

[0052] The drying initial stage is a stage in which the pulverization solvent remains in the slurry. By heating under reduced pressure at this stage, the pulverization solvent is effectively volatilized and removed. The timing at which the drying initial stage ends can be determined based on the internal pressure of the drying machine. That is, when the slurry contains a large amount of solvent, the internal pressure of the drying machine is large, and the internal pressure decreases as the amount of solvent decreases. Therefore, when the internal pressure reaches a prescribed value, it can be determined that the pulverization solvent has been removed. Note that, in the drying initial stage, the internal pressure can temporarily change to the positive pressure side along with volatilization of the pulverization solvent. However, when the pulverization solvent is removed from the drying machine, the internal pressure improves to the negative pressure side.

[0053] In the present embodiment, as an index of the internal pressure of the drying machine, the pressure change P (unit: kPa) represented by the following (1) is used. Specifically, at a stage at which the pressure change P satisfies the relationship x < P (where x is a pressure in a range of 0.8 kPa or more and 1.2 kPa or less), the drying initial stage in which the pulverization solvent remains is determined. Here, x is in the vicinity of 1 kPa, and specifically, is a value of 0.8 kPa or more and 1.2 kPa or less. x can be 0.9 kPa or more and 1.1 kPa or less, and can be 1 kPa. Note that, in the following (1), P a is the pressure in the drying machine at the time of the heating treatment (unit: kPa), and P0 is the pressure in the drying machine at the time when the drying machine is empty and is reduced in pressure for 3 minutes, that is, the initial pressure of the reduction in pressure in the drying machine (unit: kPa).

[0054] P = |P a - P0|... (1)

[0055] In order to monitor the internal pressure (pressure change P), a vacuum gauge is preferably provided in the dryer. The vacuum gauge is not particularly limited as long as it can monitor the pressure. For example, a diaphragm type vacuum gauge or a Bourdon tube, or the like can be used. In addition, in order to correctly grasp the pressure change, a vacuum gauge capable of digital display of the pressure value or a vacuum gauge capable of output to an external device is preferable.

[0056] The late-stage of drying is a stage in which substantially all of the pulverizing solvent in the slurry is removed by volatilization. That is, it is a stage in which the pressure change P represented by the above formula (1) satisfies the relationship P < x. Oxygen is introduced into the dryer only after this stage is reached. In addition, in the late-stage of drying, the fine powder is subjected to a heat treatment in an environment in which the oxygen partial pressure is 0.1 kPa or more and 5 kPa or less.

[0057] Specifically, when P < x, it is determined that the pulverizing solvent has been removed, and in a state in which the vacuum pump is operating, an oxygen-containing gas such as air or oxygen is introduced into the dryer to control the oxygen partial pressure. The oxygen introduced into the dryer reacts with hydrogen contained in the alloy powder and / or the phosphate film to generate water, which is discharged to the outside of the system. Thus, the hydrogen content of the final obtained coated alloy powder is small, and as a result, a coated alloy powder having excellent characteristics such as coercive force and excellent heat resistance and weather resistance can be obtained. In addition, since drying is promoted, the drying time is shortened.

[0058] The oxygen partial pressure in the late-stage of drying is limited to 0.1 kPa or more and 5 kPa or less. When the oxygen partial pressure is less than 0.1 kPa, the drying time is prolonged and the productivity is reduced. In addition, there is a risk that the dehydrogenation effect is insufficient, and the characteristics, heat resistance, and weather resistance of the final obtained coated alloy powder are reduced. On the other hand, when the oxygen partial pressure exceeds 5 kPa, the oxidation of the alloy powder proceeds rapidly. Thus, there is a risk that the characteristics, heat resistance, and weather resistance of the final obtained coated alloy powder are reduced.

[0059] The holding temperature (heating temperature) at the time of the heat treatment in the initial-stage of drying and the late-stage of drying is not particularly limited as long as the above object can be achieved. However, the holding temperature is preferably 100°C or more and 200°C or less. When the holding temperature is less than 100°C, drying cannot be sufficiently performed. Thus, there is a risk that the time taken to remove the solvent is prolonged and the productivity is reduced, and the formation of a stable surface film is hindered. In addition, when the holding temperature exceeds 200°C, the surface of the particles constituting the powder is thermally damaged, and thus there is a risk that the characteristics such as magnetic characteristics are extremely reduced. In addition, the holding temperature in the late-stage of drying can be the same as or different from the holding temperature in the initial-stage of drying.

[0060] The drying machine used in the second step is not particularly limited. Examples include an agitating type drying machine that agitates the slurry while heating under reduced pressure, a stationary type electric furnace, and the like. However, in order to uniformly heat and dry the slurry, a drying machine having an agitation mechanism is preferred. In addition, in order to perform the heating treatment under a reduced pressure environment, a vacuum pump is preferably provided as a vacuum generating device in the drying machine. In the case where an organic solvent is used as the pulverizing solvent, a vacuum pump subjected to an explosion-proof treatment is preferably used.

[0061] The heating time (drying time) during the heating treatment depends on the size of the drying machine used, the exhaust capacity of the vacuum pump, and the amount of the treatment object, and thus it is difficult to uniquely specify it. However, the heating time (drying time) is preferably 0.5 hours or more and 20 hours or less, further preferably 0.5 hours or more and 10 hours or less, and more preferably 0.5 hours or more and 5 hours or less. When the heating time is too short, there is a risk that the drying of the solvent and the dehydrogenation are not sufficient. When the heating time is too long, the cycle time is prolonged. Note that the heating time refers to the time required from the start time of the drying treatment to the end time of the drying treatment. In addition, the start time of the drying treatment refers to the time when the slurry of the alloy fine powder is put into the drying machine and the reduced pressure and heating are started. The end time of the drying treatment refers to the time when the cooling is started after the prescribed holding time elapses.

[0062] As described above, in the production method of the present embodiment, by combining the heating treatment under a reduced pressure environment and the heating treatment under a prescribed oxygen partial pressure condition, the pulverizing solvent can be removed in a short time, and at the same time, the oxygen contained in the alloy powder or the phosphate film can be effectively removed. Thus, a rare earth transition metal-based alloy powder having excellent characteristics such as coercive force, and excellent heat resistance and weather resistance can be produced in a short time.

[0063] Note that in the above description, a magnet powder such as a samarium iron nitride magnet powder is mentioned, but the alloy powder of the present embodiment is not limited to a magnet powder, and includes an alloy powder for uses other than magnets. For example, it can be an alloy powder used as a hydrogen storage material, an optical magnetic material, a magnetic refrigeration material, or the like.

[0064] <<3. Production of Rare Earth Transition Metal-Based Alloy Coarse Powder>>

[0065] The rare-earth transition metal-based alloy coarse powder (alloy coarse powder) is a raw material of the above-mentioned coated rare-earth transition metal-based alloy powder. The manufacturing method of the alloy coarse powder is not limited, and known methods such as a melting casting method, a reduction diffusion method, a liquid quenching method, a hydrogen absorption / desorption (HDDR) method, etc. can be used. Among them, the melting casting method is a method in which rare-earth metals and transition metals are used as raw materials, these raw materials are mixed, then melted in a non-active gas environment, and the obtained alloy ingot is homogenized by heat treatment and then pulverized. The liquid quenching method is a method in which an alloy melt is poured onto a cooled roller in a rotating state to quench to form a thin strip, and the thin strip is pulverized. The HDDR method is a method in which hydrogen is absorbed and desorbed in the alloy under high temperature conditions to obtain fine grains.

[0066] In relation to this, the reduction diffusion method is a method in which rare-earth oxides and transition metals are used as raw materials, these raw materials are mixed with a reducing agent, and then subjected to a heat treatment in a non-active gas environment to obtain a rare-earth transition metal alloy. The rare-earth oxides are reduced to rare-earth metals during the heat treatment, and the rare-earth metals diffuse and alloy in the transition metals. In the reaction product obtained by the heat treatment, a by-product from the reducing agent remains in addition to the rare-earth transition metal alloy. Therefore, the reaction product is put into water to remove the by-product from the reducing agent. In addition, by being put into water, the alloy components in the reaction product are disintegrated and pulverized. The reduction diffusion method has the advantages that inexpensive rare-earth oxides can be used as raw materials, and the process is simple, and the alloy powder can be manufactured at a lower cost than the melting casting method. Furthermore, by nitriding the alloy powder obtained by the reduction diffusion method, a rare-earth transition metal-based alloy powder as a nitride can be obtained. The manufacturing of the rare-earth transition metal-based coarse alloy powder based on the reduction diffusion method can be performed according to the following steps.

[0067] <Raw material mixing>

[0068] In the raw material mixing process, at least a rare-earth oxide powder, a transition metal powder, and a reducing agent are mixed to prepare a raw material mixture. In addition, other components can also be added as needed.

[0069] The rare earth oxide powder is a raw material of the rare earth metal that constitutes the object alloy powder. Although not limited, for example, it is preferable to contain an oxide of at least one metal selected from the group consisting of samarium (Sm), gadolinium (Gd), and cerium (Ce), or at least one metal selected from the group consisting of praseodymium (Pr), neodymium (Nd), and ytterbium (Yb). In particular, from the viewpoint of obtaining a magnet material having excellent magnetic properties, it is more preferable to contain an oxide of one or more metals selected from the group consisting of samarium (Sm), neodymium (Nd), and praseodymium (Pr), and from the viewpoint of obtaining a magnet material having not only excellent magnetic properties but also excellent heat resistance and weather resistance, it is most preferable to contain an oxide of samarium (Sm). The particle diameter of the rare earth oxide powder is not particularly limited. However, it is preferable that the particle size distribution be relatively concentrated. For example, the particle diameter of the rare earth oxide is preferably 5 μm or more and 80 μm or less.

[0070] The rare earth oxide powder is selected in accordance with the composition of the object alloy powder. For example, in the case of manufacturing a samarium iron nitride (Sm2Fe 17 N3) system alloy powder or a samarium cobalt (Sm2Co 17 ) system alloy powder, samarium oxide (Sm2O3) is selected. In addition, in the case of manufacturing a neodymium iron boron (Nd2Fe 14 B) system alloy powder, neodymium oxide (Nd2O3) is selected.

[0071] The transition metal powder is a raw material of the transition metal that constitutes the object alloy powder. The transition metal (TM) is not particularly limited. However, in the case where the alloy powder is applied to a permanent magnet use, from the viewpoint of magnetic properties, it is preferable to select one or more selected from the group consisting of iron (Fe), manganese (Mn), and cobalt (Co), and most preferable to select iron (Fe). The particle size of the transition metal powder can be determined in consideration of the ease of nitriding treatment or wet treatment performed after the reduction diffusion described later. For example, the particle diameter of the transition metal powder is preferably 10 μm or more and 100 μm or less, and more preferably 20 μm or more and 80 μm or less. The manufacturing method of the transition metal powder is not limited. A general method such as atomization or electrolysis can be used for the manufacturing.

[0072] The transition metal powder is selected in accordance with the composition of the object alloy powder. For example, in the case of manufacturing a samarium iron nitride (Sm2Fe 17 N3) system alloy powder or a neodymium iron boron (Nd2Fe 14 B) system alloy powder, iron (Fe) powder is selected. As the iron powder, reduced iron powder, gas atomized powder, water atomized powder, and / or electrolytic iron powder, etc. can be used. In addition, in the case of manufacturing a samarium cobalt (Sm2Co5, Sm2Co 17 ) system alloy powder, cobalt (Co) powder is selected.

[0073] The reducing agent is added in order to reduce the oxide components such as the rare earth oxide powder in the subsequent reduction diffusion process to promote the formation of the alloy. As the reducing agent, at least one selected from alkali metals, alkaline earth metals, and hydrides of these is used. Specifically, one or more selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and hydrides of these is preferred. From the viewpoint of operational safety and cost, lithium (Li) and / or calcium (Ca) is more preferred, and calcium (Ca) is particularly preferred.

[0074] If necessary, other components than the rare earth oxide powder, the transition metal powder, and the reducing agent can be added. For example, in the case of manufacturing an alloy powder containing components other than rare earth metals and transition metals, a raw material of the other components can be added. Specifically, in order to manufacture an Nd2Fe 14 B alloy powder, boron (B) or a boron source such as boron oxide (B2O3) can be added.

[0075] The rare earth oxide powder, the transition metal powder, the reducing agent, and the other components added as necessary, which are raw materials, are uniformly mixed. The mixing can use a publicly known mixing machine such as a ribbon blender, a tumbler, an S-type blender, a V-type blender, a NAUTA mixer, a Henschel mixer, a super mixer, a high-speed mixer, a ball mill, a vibration mill, an Attritor, a jet mill, and the like.

[0076] <Reduction Diffusion Process>

[0077] In the reduction diffusion process, the mixture is subjected to a heat treatment in a non-oxidizing environment such as argon (Ar) to obtain a reaction product. By the heat treatment, the rare earth oxide is reduced to a rare earth metal, and the reduced rare earth metal diffuses to the transition metal due to heat generated at the time of reduction, as a result of which a rare earth transition metal alloy is synthesized. The heating is performed under temperature conditions higher than the melting temperature of the reducing agent and lower than the melting temperature of the obtained rare earth transition metal alloy. For example, in the case where calcium (Ca) is used as the reducing agent, since the melting point of calcium is 838°C, it is sufficient to maintain at a temperature of 1100°C or higher and 1200°C or lower for 3 hours or more and 10 hours or less. The equipment used in the heat treatment is not particularly limited. A treatment furnace having a structure in which a gas can flow and a material that can withstand the heating temperature can be used.

[0078] <Hydrogen Treatment>

[0079] The hydrogen treatment step can be set as needed. In the hydrogen treatment step, the reaction product obtained in the reduction diffusion step is exposed to a hydrogen atmosphere and is crushed, whereby a crushed product is obtained. When the reaction product is exposed to a hydrogen atmosphere, the rare earth metal in the reaction product occludes hydrogen, thereby expanding in volume. The reaction product is embrittled by this volume expansion, thereby increasing the disintegrating property. By increasing the disintegrating property, removal of components from the reducing agent and pulverization of the alloy in the wet treatment described later can be efficiently performed.

[0080] The hydrogen treatment is performed in a hydrogen atmosphere using a hydrogen-containing gas. As the hydrogen-containing gas, hydrogen (H2) gas alone can be used, or a mixed gas of an inactive gas such as argon (Ar) or helium (He) and hydrogen gas can be used. However, it is preferable to use hydrogen gas alone.

[0081] < Nitriding treatment >

[0082] As needed, a step of nitriding the product (nitriding treatment step) can be further provided. By providing this step, a nitride-based alloy powder such as samarium iron nitride (Sm2Fe 17 N3) can be obtained. The nitriding treatment can be performed at any time as long as it is after the hydrogen treatment step. The nitriding can be performed on the crushed product obtained in the hydrogen treatment step, or the nitriding treatment product obtained in the wet treatment step described later can be nitrided. However, when the alloy powder is surface-oxidized upon wet treatment, there is a case where the nitriding is performed unevenly. Therefore, it is preferable to provide the nitriding treatment before the wet treatment.

[0083] In the nitriding treatment, the product (crushed product, alloy powder) is heated while being exposed to an atmosphere containing nitrogen or ammonia. Thereby, a nitriding treatment product is obtained. In the case of treatment in an ammonia atmosphere, it is preferable to use a mixed gas of ammonia and hydrogen. The mixing ratio of ammonia and hydrogen is not particularly limited, but it is preferable to be 10 to 70: 30 to 90, and more preferably 30 to 60: 40 to 70. When the ammonia is too little, the nitriding efficiency decreases.

[0084] The heating temperature at the time of nitriding treatment is preferably 300°C or higher and 500°C or lower, and more preferably 400°C or higher and 450°C or lower. When the heating temperature is less than 300°C, the nitriding reaction cannot be sufficiently performed, and the time required for the nitriding treatment is prolonged. When the heating temperature exceeds 500°C, there is a case where the composition of the alloy powder changes, and the characteristics such as magnetic properties significantly decrease. The holding temperature at the time of heating also depends on the treatment amount, but is, for example, 5 hours or more and 10 hours or less.

[0085] < Wet treatment step >

[0086] In the wet treatment step, the crushed product after the hydrogen treatment step or the nitrided product after the nitriding treatment is washed with a cleaning solution to obtain an alloy powder. Specifically, the crushed product or the nitrided product is put into the cleaning solution and stirred. The product put into the cleaning solution is disintegrated into a slurry. At this time, the by-product from the reducing agent reacts with water and changes into a component from the solid by-product composed of a hydroxide. Therefore, the component from the solid by-product contains a hydroxide of an alkali metal and / or an alkaline earth metal. For example, in the case where metallic calcium (Ca) is used as the reducing agent, the product after the reduction diffusion step contains a rare earth transition metal alloy component and a by-product (CaO, Ca). At the time of water washing, the by-product (CaO, Ca) reacts with water and changes into calcium hydroxide (Ca(OH)2). Since the solubility of calcium hydroxide in water is low, most of it becomes a suspended matter and floats in water. By separating the component from the solid by-product from the rare earth transition metal alloy component in the slurry, a high-purity high-performance alloy powder can be obtained.

[0087] As the cleaning solution, water, a glycol, or a mixed solution of water and a glycol can be used. As the water, ion-exchanged water is preferred. As the glycol, one or more selected from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, and tripropylene glycol is preferred.

[0088] The separation of the component from the solid by-product can be performed by decantation. The decantation can be performed once or a plurality of times. For example, the crushed product can be repeatedly put into a cleaning solution, stirred, and left to stand, and then the supernatant is removed, further cleaning solution is added to the obtained residue, stirred, and left to stand, and then the supernatant is removed. The specific gravity of the rare earth transition metal alloy component is relatively large, whereas the specific gravity of the component from the solid by-product is small. Therefore, by decantation, the component from the solid by-product having a small specific gravity can be separated and removed together with the supernatant. Alternatively, instead of performing decantation, the separation and removal of the component from the solid by-product can be performed using a specific gravity separator such as a liquid cyclone or a centrifugal separator.

[0089] At the time of the wet treatment, the crushed product can be subjected to pickling treatment. By this, the component from the solid by-product or the heterogeneous phase can be more effectively removed. For example, the heterogeneous phase (SmFe3, etc.) in the product and the component from the solid by-product (Ca(OH)2, etc.) which is not completely removed in the treatment of the cleaning solution can be removed. The pickling treatment is performed, for example, by putting the product into water, adding an acid while stirring. As the kind of acid, inorganic acids such as hydrochloric acid, acetic acid, nitric acid, and sulfuric acid or organic acids can be used. In addition, the pickling treatment time also depends on the treatment amount, but is generally 1 hour or more and 2 hours or less. Excessive pickling treatment etches the surface of the alloy powder and can cause a decrease in performance.

[0090] < Drying Treatment >

[0091] The alloy powder obtained in the wet treatment is dried. The drying can be performed at a temperature of 50°C or higher and 200°C or lower. In order to improve the drying efficiency, it is preferable to dry under a reduced pressure. After the drying, the alloy powder is cooled to room temperature and taken out from the drying device. At this time, the average particle diameter of the alloy powder also depends on the transition metal raw material or the manufacturing conditions, but is approximately 10 μm or more and 30 μm or less. In this way, the rare earth transition metal-based coarse alloy powder used in the production of the coated rare earth transition metal-based alloy powder of the present embodiment can be obtained.

[0092] Example

[0093] The present application is further explained in detail using the following examples and comparative examples. However, the present application is not limited to the following examples.

[0094] (1) Production of Rare Earth Transition Metal-Based Alloy Powder

[0095] In Examples 1 to 7, the environment, the holding temperature, and the holding time at the time of the heating treatment in the drying step were changed to produce samarium iron nitride magnet powders (coated rare earth transition metal-based alloy powders) having a phosphate film. Here, Example 1 is an example satisfying the present embodiment, and Examples 2 to 7 are reference examples not satisfying the present embodiment. In addition, the changes in the holding temperature and the environment at the time of the heating treatment are shown in Table 1. Figure 1 .

[0096] [Example 1]

[0097] <Production of Raw Material Mixture>

[0098] Samarium oxide (Sm2O3), iron (Fe) powder, and granular calcium (Ca) were combined and mixed as raw materials to produce a raw material mixture. The combination and mixing of the raw materials were performed in a manner that 1 kg of samarium iron nitride magnet coarse powder could be obtained after the nitriding treatment described later.

[0099] <Reduction Diffusion Treatment>

[0100] The obtained raw material mixture was heated under an argon gas environment at 1150°C for 10 hours to obtain a reduction diffusion treatment product.

[0101] <Nitriding Treatment>

[0102] The obtained reduction diffusion treatment product was subjected to a nitriding treatment to obtain a nitriding product. Specifically, the reduction diffusion treatment product was held under an environment in which ammonia gas and hydrogen gas were flowing at 450°C for 7 hours. At this time, the ammonia gas flow rate was made to be 1.5 L / minute, and the hydrogen gas flow rate was made to be 1 L / minute.

[0103] <Wet Treatment>

[0104] The obtained nitrided product was subjected to a wet treatment to obtain a wet-treated product. Specifically, the nitrided product was first put into ion-exchange water. Thereby, the nitrided product was disintegrated into a slurry. Next, a series of operations consisting of stirring of the slurry, standing, and removal of the supernatant was repeated 5 times. Further, ion-exchange water and acetic acid were added to adjust the pH of the slurry to 5 to 6, and the slurry was left in this state for 1 hour before the supernatant was removed. Next, a series of operations consisting of putting into ion-exchange water, stirring, standing, and removal of the supernatant was repeated 5 times. Thereafter, the obtained wet-treated product was heat-dried under reduced pressure at 200°C for 0.5 hours to obtain a crude samarium-iron-nitrogen magnet powder. The average particle diameter of the obtained crude powder was 21 μm.

[0105] <Crushing treatment>

[0106] The obtained samarium-iron-nitrogen crude powder was crushed using a media agitator mill to produce a slurry containing samarium-iron-nitrogen fine powder. Specifically, the crushing chamber of the mill was charged with the crude powder 1000 g and isopropyl alcohol 1500 g as a crushing solvent, and further, orthophosphoric acid was charged at the initial stage of crushing. The amount of orthophosphoric acid charged was adjusted so that the phosphorus amount would be 0.5 mass% with respect to the obtained fine powder. In addition, crushing was performed until a samarium-iron-nitrogen fine powder having an average particle diameter of 2.0 μm was obtained.

[0107] <Drying treatment>

[0108] The slurry containing the samarium-iron-nitrogen fine powder obtained in the crushing treatment was put into a dryer. Next, the inside of the dryer was depressurized using a vacuum pump, and the inside of the dryer was heated using a heater in this state. At this time, the internal pressure of the dryer was monitored using a vacuum gauge attached to the dryer. At the initial stage of drying where the pressure change P satisfied 1 kPa < P, the fine powder was subjected to a heat treatment in a reduced pressure environment. Thereafter, at the time when P = 1 kPa, it was judged that isopropyl alcohol had been removed from the slurry, and at the later stage of drying where P ≤ 1 kPa was satisfied, air was introduced so that the oxygen partial pressure in the inside of the dryer would be 2 kPa, in a state where the vacuum pump was kept operating. In this state, heating was continued, and after the temperature in the inside of the dryer reached 135°C, the temperature was maintained at this temperature for 2 hours. After the maintenance, heating was stopped, and the samarium-iron-nitrogen fine powder was cooled to room temperature. At this time, condensation of water, which was considered to be produced due to dehydrogenation, was found in the exhaust piping of the dryer. After the temperature in the inside of the dryer reached room temperature, the samarium-iron-nitrogen fine powder was taken out of the dryer. In this way, a coated samarium-iron-nitrogen fine powder was produced. Note that, Figure 1 Point A shown in FIG. 1 indicates the time when the introduction of air into the dryer was started.

[0109] [Example 2]

[0110] In Example 2, during the drying treatment, oxygen was not introduced, and the temperature in the dryer was maintained at 135°C for 2 hours under a state where a reduced pressure environment was maintained, after which the temperature was cooled to room temperature. Other than this, the coated samarium-iron-nitrogen micropowder was produced in the same manner as in Example 1.

[0111] [Example 3]

[0112] In Example 3, during the drying treatment, oxygen was not introduced, and the temperature in the dryer was maintained at 210°C for 2 hours under a state where a reduced pressure environment was maintained, after which the temperature was cooled to room temperature. Other than this, the coated samarium-iron-nitrogen micropowder was produced in the same manner as in Example 1.

[0113] [Example 4]

[0114] In Example 4, during the drying treatment, oxygen was not introduced, and the temperature in the dryer was maintained at 135°C for 10 hours under a state where a reduced pressure environment was maintained, after which the temperature was cooled to room temperature. Other than this, the coated samarium-iron-nitrogen micropowder was produced in the same manner as in Example 1.

[0115] [Example 5]

[0116] In Example 5, during the drying treatment, the temperature in the dryer was maintained at 135°C for 1 hour under a state where a reduced pressure environment was maintained. Subsequently, air was introduced so that the oxygen partial pressure was 2 kPa, and the temperature was maintained for 2 hours under this state, after which the temperature was cooled to room temperature. Other than this, the coated samarium-iron-nitrogen micropowder was produced in the same manner as in Example 1.

[0117] [Example 6]

[0118] In Example 6, during the drying treatment, air was introduced so that the oxygen partial pressure in the dryer was 0.05 kPa under a state where the vacuum pump was kept operating. Other than this, the coated samarium-iron-nitrogen micropowder was produced in the same manner as in Example 1.

[0119] [Example 7]

[0120] In Example 7, during the drying treatment, air was introduced so that the oxygen partial pressure in the dryer was 6 kPa under a state where the vacuum pump was kept operating. Other than this, the coated samarium-iron-nitrogen micropowder was produced in the same manner as in Example 1.

[0121] (2) Evaluation

[0122] The samarium-iron-nitrogen micropowder of Examples 1 to 7 and the method for producing the same were evaluated as described below.

[0123] < Drying Time >

[0124] The time required for the drying treatment (drying time) was evaluated. Specifically, the slurry containing the samarium-iron-nitrogen micropowder was put into a drier, the time when the vacuum pump and the heater were operated was regarded as the start of the drying treatment, the time when the desired holding time had elapsed and the cooling was started was regarded as the end of the drying treatment, the time elapsed from the start of the drying treatment to the end of the drying treatment was found, and this was taken as the drying time.

[0125] < Magnetic properties (VSM) >

[0126] The magnetic properties (remanence, coercive force) of the produced coated samarium-iron-nitrogen micropowder were evaluated using a vibrating sample magnetometer. Specifically, after the coated samarium-iron-nitrogen micropowder was filled into a sample holder together with paraffin, the entire sample holder was heated to orient the micropowder and further cooled and solidified to produce a sample. Next, using a vibrating sample magnetometer (Toei Industry Co., Ltd., VSM), the hysteresis curve of the sample was plotted under the condition of a maximum applied magnetic field of 1670 kA / m (21 kOe). The remanence (Br) and the coercive force (iHc) were read from this hysteresis curve.

[0127] < Phosphorus concentration (XPS) >

[0128] The phosphorus atom concentration of the phosphate film of the coated samarium-iron-nitrogen micropowder was measured using X-ray photoelectron spectroscopy (XPS). Specifically, under the conditions shown in Table 1 below, sputter etching treatment using argon was performed on the particle surface of the coated samarium-iron-nitrogen micropowder having a phosphate film, and XPS analysis was performed to investigate the phosphorus atom concentration in the surface depth direction. At the time of analysis, the phosphorus atom concentration of each 5 nm portion from the particle surface in the depth direction was calculated from the peak area. In addition, if a peak of iron (Fe) in a metallic state was detected after sputter etching, it was judged that the particle of the samarium-iron-nitrogen micropowder under the phosphate film was exposed, and the thickness of the phosphate film was found from the sputter depth.

[0129] Table 1

[0130] Table 1 XPS analysis conditions

[0131]

[0132] < Heat resistance >

[0133] For the samarium-iron-nitrogen micropowder, the heat resistance was evaluated by comparing the coercive force before and after the heat resistance test. Specifically, while reducing the pressure with a vacuum pump, the samarium-iron-nitrogen micropowder was kept in an environment of 125°C for 1 hour. The coercive force before the keeping was denoted as iHc(0h), the coercive force after the keeping was denoted as iHC(heat resistance 1h), and the ratio (iHC(heat resistance 1h) / iHc(0h)) was calculated. The larger this ratio, the higher the heat resistance was judged to be.

[0134] < Weather resistance >

[0135] For the samarium iron nitride fine powder, the coercivity before and after the weather resistance test was measured to evaluate the weather resistance of the fine powder. Specifically, the samarium iron nitride fine powder was kept in an atmosphere at 80°C and 90% RH for 300 hours. The coercivity before the keeping was denoted as iHc(Oh), the coercivity after the keeping was denoted as iHC(weather resistance 300h), and the ratio (iHC(weather resistance 300h) / iHc(Oh)) was calculated. The larger the ratio, the higher the weather resistance was judged.

[0136] (3) Evaluation results

[0137] The evaluation results of the samarium iron nitride fine powder obtained in Examples 1 to 7 are shown in Table 2. The highest coercivity (iHc) was obtained in Example 1, and the value was 12.06 kOe. In addition, in Example 1, the phosphorus atom concentration in the phosphate film was the highest, and the value exceeded 8 atom%. In addition, the heat resistance and the weather resistance of the fine powder were excellent.

[0138] Comparing Example 1 with Example 5 of the conventional drying conditions, Example 1 obtained a higher coercivity (iHc) and a phosphorus atom concentration in a shorter time, and the heat resistance and the weather resistance were excellent. Comparing Example 1 with Examples 6 and 7, when the oxygen partial pressure at the time of the drying treatment was outside the prescribed range, the coercivity (iHc) was low, and the heat resistance and the weather resistance were deteriorated.

[0139] On the other hand, in Example 2, the drying time was the shortest, but the coercivity (iHc) of the obtained fine powder was low, and the heat resistance and the weather resistance were poor.

[0140] Note that the results of the XPS analysis showed that Fe in a metallic state was detected at a depth of 10 nm by Ar sputtering for any of the samples. Therefore, it was found that the thickness of the phosphate film was 10 nm. The phosphorus atom concentration shown in Table 2 is an average value of the phosphorus concentration calculated from the peak areas at the three positions of the surface, a depth of 5 nm, and a depth of 10 nm of the phosphate film.

[0141] From the above results, it was found that, in the drying process, by combining the heating treatment under a reduced pressure environment and the heating treatment under a prescribed oxygen partial pressure condition, a rare earth transition metal-based alloy powder having good characteristics and excellent heat resistance and weather resistance can be manufactured in a short time.

[0142] Table 2

[0143] Table 2 Evaluation results of samarium iron nitride fine powder

[0144]

Claims

1. A method for producing a coated rare earth transition metal-based alloy powder, wherein includes: a first step of pulverizing a rare earth transition metal alloy coarse powder in a solvent, and adding a phosphoric acid compound to the solvent at the time of the pulverization, to obtain a fine powder coated with a phosphate film on the surface; and a second step of performing a heat treatment of maintaining the obtained fine powder at a predetermined temperature in a drying machine, whereby a rare earth transition metal alloy powder coated with a phosphate film is obtained, an atomic concentration of phosphorus included in the phosphate film of the rare earth transition metal alloy powder is 8 atomic% or more, In the second process, the pressure P in the dryer at the time of heat treatment is measured a In the initial stage of drying where the pressure change P shown in the following (1) satisfies the relationship x < P, heat treatment is performed on the fine powder in a reduced pressure environment where an oxygen-containing gas is not introduced into the dryer, and thereafter, in the later stage of drying where the relationship P ≤ x is satisfied, heat treatment is performed on the fine powder in an environment where the oxygen partial pressure is 0.1 kPa or higher and 5 kPa or lower, the unit of P being kPa and x being a pressure in the range of 0.8 kPa or higher and 1.2 kPa or lower, P = |P a - P0| · · · (1), P a is the pressure in the drying machine at the time of heat treatment, and P0 is the initial pressure of the reduced pressure in the drying machine.

2. The method according to claim 1, wherein the rare earth transition metal alloy powder is a samarium iron nitride Sm-Fe-N based magnet powder.

3. The method according to claim 1, wherein the maintaining temperature at the time of the heat treatment is 100°C or more and 200°C or less.

4. The method according to claim 2, wherein the maintaining temperature at the time of the heat treatment is 100°C or more and 200°C or less.

5. The method according to claim 1, wherein the heating time at the time of the heat treatment is 0.5 hours or more and 20 hours or less.

6. The method according to claim 2, wherein the heating time at the time of the heat treatment is 0.5 hours or more and 20 hours or less.

7. The method according to claim 3, wherein the heating time at the time of the heat treatment is 0.5 hours or more and 20 hours or less.

8. The method according to claim 4, wherein the heating time at the time of the heat treatment is 0.5 hours or more and 20 hours or less.

9. An alloy powder which is a coated rare earth transition metal-based alloy powder, wherein, a rare earth transition metal alloy powder and a phosphate film coating a surface of the rare earth transition metal alloy powder, an atomic concentration of phosphorus included in the phosphate film being 8 atomic% or more, a ratio of a coercive force iHC after the heat resistance 1h at 250°C for 1 hour in a reduced pressure environment to a coercive force iHc before the heat resistance 1h, iHC(heat resistance 1h) / iHc(0h), is 95% or more, a ratio of a coercive force iHC after the weather resistance 300h at 80°C for 300 hours in a 90% RH environment to a coercive force iHc before the weather resistance 300h, iHC(weather resistance 300h) / iHc(0h), is 95% or more.

10. The alloy powder according to claim 9, wherein the rare earth transition metal alloy powder is a samarium iron nitride Sm-Fe-N based magnet powder.

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