Rare earth transition metal nitrogen magnetic powder

Abstract

The present invention provides a rare earth transition metal nitrogen-based magnetic powder that stably exhibits high maximum magnetic polarization and has excellent heat resistance, and a method for producing the same. The present invention provides a ZnO-based ZnO catalyst containing at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as its main components. 17 type, Th2Ni 17 a rare earth transition metal nitrogen-based magnetic powder having either a TbCu7-type or a TbCu8-type crystal structure, wherein the rare earth transition metal nitrogen-based magnetic powder has an average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) of 0.107 or more and 0.121 or less, an area ratio of rare earth-rich phases having an R / TM ratio of more than 0.130 is 1.0% or less, and when heated in air at 150°C for 1000 hours, a retention factor [HcJ(1000) / HcJ(0)], which is the ratio of coercivity after heating (HcJ(1000)) to coercivity before heating (HcJ(0)), is 83% or more.

Description

[Technical Field]

[0001] The present invention relates to a rare earth transition metal nitrogen-based magnetic powder. [Background technology]

[0002] Rare earth transition metal nitrogen magnetic powder is an alloy powder mainly containing rare earth metals, transition metals, and nitrogen. Rare earth transition metal nitrogen magnetic powder is widely used as a permanent magnet material. For example, SmFe 17 N3-based alloy powder has high saturation magnetization, anisotropic magnetic field, and Curie temperature, making it useful as a material for permanent magnets, especially bonded magnets, with excellent properties.

[0003] Rare earth transition metal nitrogen-based magnetic powders are usually produced by nitriding rare earth transition metal alloy powders. Conventional methods for producing rare earth transition metal alloy powders include melting and casting and reduction and diffusion methods. The melting and casting method involves blending rare earth metals and transition metals as raw materials, melting the raw materials in an inert gas atmosphere, heat-treating the resulting alloy ingot to homogenize it, and then pulverizing it.

[0004] On the other hand, the reduction-diffusion method is a technique for obtaining a rare earth-transition metal alloy by mixing, for example, rare earth oxides and transition metals as raw materials with a reducing agent such as metallic calcium, followed by heat treatment in a non-oxidizing gas atmosphere. During the heat treatment, the rare earth oxides are reduced to rare earth metals, which then diffuse into the transition metals to form an alloy (intermetallic compound). The bulk reaction product obtained by the heat treatment contains by-products derived from the reducing agent as well as the desired alloy. Therefore, the reaction product is placed in water to remove the by-products derived from the reducing agent, and the reaction product is then disintegrated and pulverized. The pulverized alloy powder is then subjected to acid washing and water washing to remove excess by-products and unreacted materials, and then dried to obtain the desired alloy powder.

[0005] The reduction-diffusion method has the advantage that inexpensive rare earth oxides and the like can be used as raw materials, and the process is simple, making it possible to produce alloy powder at lower cost than the melt-casting method. Furthermore, by nitriding the alloy powder obtained by the reduction-diffusion method, it is possible to obtain rare earth transition metal alloy powder as a nitride.

[0006] The master alloy used for nitriding is in powder form, and for uniform nitriding, it is desirable for the master alloy powder to have a sharp particle size distribution. The reduction diffusion method is easy to obtain a sharp particle size distribution, and for this reason, it has become the mainstream method for producing rare earth transition metal nitrogen-based magnetic powders.

[0007] Examples of documents disclosing the production of rare earth transition metal nitrogen-based magnetic powder by a reduction diffusion method include Patent Document 1 and Patent Document 2. These documents disclose a method of producing a rare earth transition metal nitrogen-based alloy powder by mixing a rare earth oxide powder, a transition metal powder, and a reducing agent such as an alkali metal, heating the resulting mixture in an inert atmosphere, holding the resulting reaction product in a nitrogen atmosphere, and wet-treating the resulting heat-treated product (Claim 1 of Patent Document 1, Claim 1 of Patent Document 2).

[0008] It is also known to form a phosphoric acid protective coating on the particle surface of magnetic powder to improve heat resistance and moisture resistance. For example, Patent Document 3 discloses adding phosphoric acid when pulverizing iron-based magnetic alloy powder containing rare earth elements (Claim 1 of Patent Document 3). Patent Document 4 discloses a method for producing rare earth-iron-nitrogen-based magnetic powder, in which raw material powder containing rare earth oxides is subjected to reduction diffusion and nitriding to obtain an alloy block, which is then immersed in water to disintegrate, and then washed with water and treated with an acid, characterized in that the acid treatment comprises treatment with a weak acid and then treatment with an acid containing a Group 5B element and oxygen (Claim 1 of Patent Document 4). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 05-148517 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-119909 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-124406 [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-038608 Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, it has been proposed to produce rare earth transition metal nitrogen-based magnetic powders by reduction diffusion methods, and to form a protective phosphate coating on the particle surfaces of magnetic powders to improve moisture resistance. However, there is still room for improvement in these conventional techniques.

[0011] For example, Patent Documents 1 and 2 disclose methods for producing rare earth transition metal nitrogen-based magnetic powders, but the magnetic powders obtained using these methods have variations in maximum magnetic polarization. This poses a problem in that the high maximum magnetic polarization inherent to these powders cannot be consistently achieved. Furthermore, Patent Documents 3 and 4 disclose methods for forming a phosphate protective coating, but the resulting protective coating is poor in uniformity, making it difficult to fully achieve the effect of improving heat resistance.

[0012] The present inventors have conducted extensive research in light of these problems, and as a result have discovered a novel ZnO alloy containing at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as its main components. 17 type, Th2Ni 17 We have discovered that in rare earth transition metal nitrogen-based magnetic powders having either the TbCu7 type or TbCu8 type crystal structure, the proportion of the rare earth-rich phase is important, and that by controlling this, it is possible to obtain magnetic powders that have a stable high maximum magnetic polarization and excellent heat resistance.

[0013] Furthermore, when producing magnetic powder, the conditions of the reduction diffusion process, nitriding process, and wet treatment process are particularly important, and it has been discovered that by controlling these conditions, the above-mentioned rare earth transition metal nitrogen-based magnetic powder can be easily produced.

[0014] The present invention was completed based on such findings, and aims to provide a rare earth transition metal nitrogen-based magnetic powder that stably has a high maximum magnetic polarization and excellent heat resistance, and a method for producing the same. [Means for solving the problem]

[0015] The present invention encompasses the following embodiments (1) to (7). In this specification, the expression "to" includes both the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "X or more and Y or less." In addition, in this specification, any combination of suitable embodiments can be adopted as long as technical consistency can be achieved. For example, one of the suitable numerical ranges can be combined with the other.

[0016] (1) A material containing at least rare earth metals (R), transition metals (TM), and nitrogen (N) as its main components, and having a structure similar to that of Th2Zn 17 type, Th2Ni 17 A method for producing a rare earth transition metal nitrogen-based magnetic powder having either a TbCu7-type or a TbCu7-type crystal structure, comprising the steps of: a step of heating a raw material mixture containing an alloy raw material containing at least a rare earth metal (R), a transition metal (TM), and oxygen (O), and a reducing agent, to a temperature of 800°C or higher and 1200°C or lower in a reduced pressure gas or inert gas atmosphere to prepare a reduction-diffusion treated product (reduction-diffusion step); a step of exposing the reduction-diffusion treated product to a hydrogen atmosphere to crush it, thereby producing a crushed product (hydrogen crushing step); A step of subjecting the crushed material to nitriding treatment to produce a nitride (nitriding step); The method includes a step of subjecting the nitride to a wet treatment to prepare a wet-treated product (wet treatment step), the average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) contained in the raw material mixture is 0.117 or more and 0.160 or less; During the nitriding treatment, a process of reducing the pressure of the atmosphere surrounding the crushed material to a pressure of 50 kPa or less and then restoring the pressure using a nitriding gas that does not contain hydrogen is carried out at least once, and then nitriding gas that may contain hydrogen is supplied to the ambient atmosphere, and the crushed material is heated in the supplied nitriding gas atmosphere to convert it into a nitride, The wet treatment step includes a sub-step of adding the nitride particles to water and disintegrating the nitride particles to prepare a slurry containing nitride particles, followed by a sub-step of subjecting the slurry containing the nitride particles to a water washing treatment (first water washing step), a sub-step of subjecting the slurry containing the nitride particles after the first water washing step to an acid washing treatment (pickling step), a sub-step of subjecting the slurry containing the nitride particles after the acid washing step to a water washing treatment (second water washing step), a sub-step of adding a phosphoric acid aqueous solution to the slurry containing the nitride particles after the second water washing step to form a phosphorus-containing protective film on the surfaces of the nitride particles (surface treatment step), and a sub-step of subjecting the slurry containing the nitride particles after the surface treatment step to a water washing treatment to obtain the wet-treated product (third water washing step), The rare earth transition metal nitrogen-based magnetic powder has an average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) of 0.107 or more and 0.121 or less.

[0017] (2) The method according to (1) above, wherein the temperature of the slurry containing nitride particles is maintained at 13°C or higher and 30°C or lower in the pickling step.

[0018] (3) The method according to (1) or (2) above, wherein the transition metal (TM) concentration in the tap water of the slurry containing nitride particles just before the phosphoric acid aqueous solution is added in the surface treatment step is 1.0 g / L or less.

[0019] (4) Any of the methods (1) to (3) above, wherein the amount of phosphoric acid aqueous solution added in the surface treatment step is 0.03% by mass or more and 1.0% by mass or less in terms of phosphorus (P) relative to the rare earth transition metal nitrogen-based magnetic powder.

[0020] (5) The method according to any one of (1) to (4) above, wherein the temperature of the slurry containing nitride particles is maintained at 13°C or higher and 30°C or lower in the surface treatment step.

[0021] (6) Any of the above-mentioned methods (1) to (5), wherein in the third water washing step, the water washing treatment is carried out until the phosphorus (P) concentration in the clean water of the slurry becomes 100 mg / L or less.

[0022] (7) A metal oxide containing at least rare earth metals (R), transition metals (TM), and nitrogen (N) as its main components, and having a structure similar to that of Th2Zn 17 type, Th2Ni 17 A rare earth transition metal nitrogen-based magnetic powder having either a TbCu7 type or a TbCu7 type crystal structure, The rare earth transition metal nitrogen-based magnetic powder has an average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) of 0.107 or more and 0.121 or less, and the area ratio of rare earth-rich phases having an R / TM ratio of more than 0.130 is 1.0% or less, A rare earth transition metal nitrogen-based magnetic powder that, when heated in air at 150°C for 1000 hours, has a coercive force retention rate [HcJ(1000) / HcJ(0)], which is the ratio of the coercive force after heating (HcJ(1000)) to the coercive force before heating (HcJ(0)), of 83% or more. [Effects of the Invention]

[0023] According to the present invention, there are provided rare earth transition metal nitrogen-based magnetic powders which have a high maximum magnetic polarization and excellent heat resistance, and a method for producing the same. [Brief explanation of the drawings]

[0024] [Figure 1] An SEM image (backscattered electron image) of magnetic powder is shown (Example 1). [Figure 2] An SEM image (backscattered electron image) of magnetic powder is shown (Example 7). [Figure 3] The results of thermogravimetry (TG) of magnetic powders are shown (Examples 1 and 8). DETAILED DESCRIPTION OF THE INVENTION

[0025] A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described below. However, the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

[0026] <<1. Manufacturing method of rare earth, transition metal, nitrogen-based magnetic powder>> This embodiment relates to a method for producing a rare earth transition metal nitrogen-based magnetic powder (hereinafter, sometimes simply referred to as "magnetic powder"). This magnetic powder contains at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as main components, and is a magnetic powder containing Th2Zn 17 type, Th2Ni 17 The magnetic powder has either a TbCu7 type or a TbCu7 type crystal structure. The average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) in this magnetic powder is 0.107 or more and 0.121 or less.

[0027] The manufacturing method of this embodiment includes the following steps: a step of heating a raw material mixture containing at least a rare earth metal (R), a transition metal (TM), and oxygen (O), and a reducing agent to a temperature of 800°C to 1200°C under a reduced pressure or inert gas atmosphere to produce a reduced-diffusion-treated product (reduction-diffusion step), a step of exposing the reduced-diffusion-treated product to a hydrogen atmosphere to crush it, thereby producing a crushed product (hydrogen crushing step), a step of nitriding the crushed product to produce a nitride (nitriding step), and a step of wet-treating the nitride to produce a wet-treat product (wet-treating step). Here, the average atomic ratio of rare earth metal (R) to transition metal (TM) (R / TM ratio) contained in the raw material mixture is 0.117 to 0.160. During the nitriding treatment, the ambient atmosphere of the crushed material is depressurized to a pressure of 50 kPa or less, and then the pressure is restored using a hydrogen-free nitriding gas, which is then supplied to the ambient atmosphere, and the crushed material is heated in the supplied nitriding gas atmosphere to convert it into nitrides.

[0028] The wet treatment process further includes the following sub-steps: adding nitride particles to water and disintegrating them to prepare a slurry containing nitride particles, followed by a water washing process (first water washing process), a sub-step of acid washing the slurry containing nitride particles after the first water washing process (pickling process), a sub-step of water washing the slurry containing nitride particles after the pickling process (second water washing process), a sub-step of adding a phosphoric acid aqueous solution to the slurry containing nitride particles after the second water washing process to form a phosphorus-containing protective film on the surfaces of the nitride particles (surface treatment process), and a sub-step of water washing the slurry containing nitride particles after the surface treatment process to obtain a wet-treated product (third water washing process). Each step will be described in detail below.

[0029] <Reduction and diffusion process> In the reduction-diffusion process, a raw material mixture containing an alloy raw material containing at least a rare earth metal (R), a transition metal (TM), and oxygen (O), and a reducing agent is prepared. The alloy raw material may contain at least a rare earth metal, a transition metal, and oxygen as constituent elements. During the reduction-diffusion process, oxygen in the alloy raw material is removed by the action of the reducing agent, and the rare earth metal diffuses into the transition metal and is alloyed. This results in a reduction-diffusion processed product (reaction product) containing a rare earth-transition metal alloy and by-products derived from the reducing agent.

[0030] [Alloy raw material - powder mixture] The alloy raw material may be a powder mixture, i.e., a mixture of rare earth oxide powder and transition metal powder. The rare earth oxide powder is the raw material of the rare earth metal that constitutes the desired magnetic powder. The type of rare earth metal may be selected depending on the composition of the desired magnetic powder. Examples of rare earth metals include, but are not limited to, one or more selected from the group consisting of yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), and ytterbium (Yb). As the rare earth oxide powder, one type of powder may be used alone, or two or more types of powders may be mixed and used.

[0031] The rare earth metal preferably contains samarium (Sm). By selecting Sm, samarium iron nitrogen (SmFe 17 It is now possible to manufacture Sm2Fe N3-based magnetic powder. 17 N3-based magnetic powder has excellent magnetic properties and is useful as a bonded magnet material. In this case, Sm may be used in combination with other rare earth metals, such as La and / or Ce. For example, 70 atomic % or more of the rare earth metals may be Sm, and 30 atomic % or less may be other elements (La, Ce, etc.).

[0032] The particle size of the rare earth oxide powder can be determined depending on the composition and application of the resulting alloy powder. However, it is desirable to determine the particle size of the rare earth oxide powder so that it is uniformly distributed near the transition metal particles in the resulting mixture. The average particle size D50 of the rare earth oxide powder is preferably 50 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. In particular, a powder in which particles with a particle size of 0.1 to 10 μm account for 80 mass% or more of the total is preferred. This improves the raw material mixability and improves the handleability of the raw material powder and reaction product. It also enables sufficient diffusion of the rare earth metal in the subsequent reduction-diffusion process. In this specification, the average particle size D50 refers to the cumulative 50% diameter in the particle size distribution on a volume basis. The particle size distribution on a volume basis can be determined using an airflow dispersion-type laser diffraction particle size distribution analyzer.

[0033] Rare earth oxide powders may contain moisture or organic matter as impurities. These impurities may increase the oxygen content of the final alloy powder. Therefore, it is preferable for the rare earth oxide powder to contain as few impurities as possible. For example, the weight loss after heating to 1000°C is preferably 2% by mass or less, and more preferably 1% by mass or less.

[0034] The transition metal powder is a raw material for the transition metal that constitutes the desired magnetic powder. The type of transition metal may be selected depending on the composition of the desired magnetic powder. Examples of transition metals include, but are not limited to, one or more selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), chromium (Cr), manganese (Mn), zinc (Zn), molybdenum (Mo), and tungsten (W). As the transition metal powder, one type of powder may be used alone, or two or more types of powder may be mixed and used.

[0035] The transition metal preferably includes iron (Fe). By selecting Fe, samarium iron nitrogen (SmFe 17 It is possible to produce N3)-based magnetic powders. In this case, Fe may be used in combination with other transition metals than Fe, such as one or more selected from the group consisting of Cr, Mn, Co, and Ni. For example, 90 atomic % or more of the transition metal may be Fe, with 10 atomic % or less being other elements (Cr, Mn, Co, Ni, etc.). When the transition metal powder is iron (Fe), cobalt (Co), nickel, or Ni powder, for example, reduced powder, gas atomized powder, water atomized powder, electrolytic powder, carbonyl powder, etc. can be used.

[0036] The transition metal powder may be a metal powder alone, or a part of the metal powder may be replaced with a metal oxide powder. Furthermore, not only transition metals but also other metals, such as rare earth metals, may be used. For example, the transition metal powder may be a metal powder (Fe, etc.), a metal oxide powder (FeO, FeO, etc.), a rare earth transition metal alloy powder (RFe, etc.), or a rare earth transition metal alloy powder (RFe). 17 It is possible to use rare earth transition metal composite oxide powders (RFeO3, etc.). However, when oxide powders are used, a thermite reaction occurs during the reduction-diffusion treatment. To prevent the rapid heat generation caused by this reaction, it is preferable to keep the proportion of oxide powders to 50 mass% or less of the total transition metal powder.

[0037] When the transition metal powder is a metal powder, the average particle size D50 of the transition metal powder is preferably 100 μm or less, more preferably 50 μm or less, taking into consideration the diffusion length of the rare earth metal at the reduction diffusion treatment temperature. On the other hand, when the transition metal powder is an oxide powder, the D50 of the transition metal powder is preferably 10 μm or less, more preferably 5 μm or less.

[0038] [Alloy raw materials other than powder mixtures] The alloy raw material may be an oxide and / or a partially reduced oxide (partial oxide) containing a rare earth metal and a transition metal, an alloy containing a rare earth metal, a transition metal, and oxygen (e.g., an oxygen-containing SmFe alloy), or a mixture of an alloy containing a rare earth metal and a transition metal and a rare earth oxide (e.g., a mixture of an SmFe alloy and an Sm oxide). By using a partially reduced oxide as the alloy raw material, it is possible to reduce the amount of reducing agent added in the subsequent process. An example of a procedure for producing a partially reduced oxide containing a rare earth metal and a transition metal is described below.

[0039] First, a composite oxide containing a rare earth metal (Re) and a transition metal (TM) is prepared. The method for obtaining the composite oxide is not limited. For example, the composite oxide can be synthesized by a wet method. To synthesize the composite oxide by the wet method, a hydroxide is generated from an acid solution containing the rare earth metal and the transition metal by a neutralization reaction, and the obtained hydroxide is then heat-treated. Next, the prepared composite oxide is heated in a reducing atmosphere to obtain a partially reduced oxide containing the rare earth metal and the transition metal.

[0040] [Reducing agent] The reducing agent is added to reduce oxide components such as rare earth oxide powder during the subsequent reduction-diffusion treatment to promote alloy formation. The reducing agent is at least one selected from alkali metals, alkaline earth metals, and their hydrides. 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 their hydrides are preferred. From the viewpoints of safety during handling and cost, Li and / or Ca are more preferred, with Ca being particularly preferred.

[0041] The reducing agent is preferably granular. Increasing the coarseness of the reducing agent promotes the penetration and diffusion of the reducing agent, which is heated to form a melt during the reduction-diffusion treatment, into the raw material mixture. The particle size of the reducing agent is preferably 100 mesh or larger (coarser than a 100 mesh sieve), and more preferably 32 mesh or larger. On the other hand, if the reducing agent is finer, it is possible to further improve its dispersibility in the raw material mixture. The particle size of the reducing agent is preferably 4 mesh or smaller (finer than a 4 mesh sieve), and more preferably 9 mesh or smaller.

[0042] [Other ingredients] If necessary, other components may be added in addition to the rare earth oxide powder, transition metal powder, and reducing agent. For example, when producing a magnetic powder containing components other than rare earth metals, transition metals, and nitrogen, raw materials for other components may be added. Such components include, for example, zinc (Zn), boron (B), aluminum (Al), gallium (Ga), indium (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and / or bismuth (Bi).

[0043] [mixture] The raw materials may be mixed by a known method, either wet or dry. Dry mixing may be performed using a dry mixer such as a ribbon blender, tumbler, S-blender, V-blender, Nauta mixer, Henschel mixer, Miraro mixer, Nobilta mixer, Mechanofusion mixer, high-speed mixer, and / or vibration mill. Wet mixing may be performed using water or an organic solvent as a mixing medium, and the raw materials may be mixed in a wet mixer such as a ball mill and / or bead mill, followed by drying. However, in wet mixing, it is preferable to wet-mix the raw materials other than the reducing agent (metallic Ca, etc.), and then add the reducing agent.

[0044] In the manufacturing method of this embodiment, the average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) contained in the raw material mixture is 0.117 or more and 0.160 or less. This suppresses the formation of heterophases, making it possible to obtain magnetic powder with excellent magnetic properties. On the other hand, if the average R / TM ratio is less than 0.117, the diffusion of rare earth metals during the reduction-diffusion treatment may be insufficient. As a result, a transition metal-rich phase (such as an iron-rich phase) with an insufficient amount of rare earth metal remains in the reaction product (reduction-diffusion-treated product) after the reduction-diffusion treatment, which may degrade the magnetic properties of the magnetic powder, such as remanence and squareness. If the average R / TM ratio exceeds 0.160, the amount of the heterophase rare earth-rich phase increases, reducing the proportion of the main phase. As a result, the magnetic properties of the magnetic powder, such as remanence and squareness, may degrade. The average R / TM ratio is more preferably 0.123 or more and 0.135 or less. The average R / TM ratio of the raw material mixture is calculated from the blending amounts of the rare earth metal (R) raw material and the transition metal (TM) raw material added when preparing the raw material mixture.

[0045] The amount of reducing agent contained in the raw material mixture is preferably 1.05 equivalents or more and 3.0 equivalents or less, and more preferably 1.1 equivalents or more and 2.0 equivalents or less. Here, equivalent is an indicator of the amount required to reduce the oxides contained in the raw material mixture. The minimum amount required to reduce all oxides is 1 equivalent. By using an amount of reducing agent of 1.05 equivalents or more, the reduction of the oxides can be sufficiently promoted. On the other hand, if the amount of reducing agent is excessively large, excess reducing agent components will remain, which may inhibit nitridation in the subsequent nitridation step. By using an amount of reducing agent of 3.0 equivalents or less, nitridation inhibition in the nitridation step can be prevented.

[0046] In the reduction-diffusion process, the prepared raw material mixture is heated under a reduced pressure or inert gas atmosphere to produce a reduction-diffusion-treated product. During the reduction-diffusion process, a reducing agent (metallic Ca, etc.) reduces rare earth oxides to produce rare earth metals. The produced rare earth metals then diffuse into the transition metals to form a rare earth-transition metal alloy. Therefore, the reduction-diffusion-treated product contains the rare earth-transition metal alloy as the main phase, by-products derived from the reducing agent (CaO, etc.), and, in some cases, other heterogeneous phases.

[0047] During heating, the raw material mixture is filled into an iron reactor, heated to the heating temperature, held for a predetermined time, and then cooled. The heating temperature is between 800°C and 1200°C. A heating temperature below 800°C results in insufficient reduction of the oxides by the reducing agent. A heating temperature above 1200°C may lead to sintering of the alloy particles in the reaction product (reduction-diffusion-treated product) produced by the reduction-diffusion reaction, potentially hindering smooth nitrogen diffusion in the subsequent nitriding process. Alternatively, wet processing may be difficult to form a slurry, leaving large chunks behind. The temperature and holding time should be determined so that the rare earth metals diffuse to the core of the iron particles in the raw material mixture and no transition metal-rich phase remains. To reduce residual gases such as oxygen and moisture in the reactor, the reactor may be evacuated and replaced with an inert gas before or during heating. It is also preferable to maintain an inert gas atmosphere during the cooling process to prevent oxidation of the reaction product and maintain its activity.

[0048] <Hydro-crushing process> In the hydrogen crushing process, the reduction-diffusion-treated product obtained in the reduction-diffusion process is exposed to a hydrogen atmosphere to crush it, thereby producing crushed products. The reaction product obtained by the reduction-diffusion process (reduction-diffusion-treated product) contains a rare-earth-rich phase containing an excess of rare earth elements. For example, if the rare-earth metal is Sm, it contains Sm-rich phases such as SmFe5 phase, SmFe3 phase, and / or SmFe2 phase. When the reduction-diffusion-treated product is exposed to a hydrogen atmosphere, these rare-earth-rich phases absorb hydrogen and expand in volume. As a result, cracks occur in the reduction-diffusion-treated product, allowing it to be crushed. The size of the crushed product is not limited, but is, for example, 10 mm or less. If the crushed product is large, it may be crushed mechanically.

[0049] <Nitriding process> In the nitriding step, the crushed material obtained in the hydrogen crushing step is subjected to nitriding treatment to produce nitrides. The nitriding treatment nitrides the rare earth transition metal alloy contained in the crushed material to produce rare earth transition metal nitrogen-based compounds.

[0050] During nitriding, a pretreatment process is first performed at least once: the pressure of the atmosphere surrounding the crushed material is reduced to 50 kPa or less, and then the pressure is restored using a hydrogen-free pretreatment gas. The reaction product (crushed material) after the hydrocrushing process is usually a massive porous sintered body, and hydrogen remains in the pores and cracks. The inventors' investigations have revealed that residual hydrogen may hinder the diffusion of nitrogen through the pores and cracks. Therefore, by performing pretreatment using a hydrogen-free gas (pretreatment gas), the residual hydrogen can be removed, allowing the nitriding process to proceed uniformly and sufficiently.

[0051] If the ambient pressure after decompression exceeds 50 kPa, hydrogen removal may be insufficient, resulting in uneven and insufficient nitriding. It is preferable to use a nitrogen-containing gas (nitriding gas) such as hydrogen-free nitrogen and / or ammonia as the pretreatment gas introduced when the pressure is restored. If the pretreatment gas (nitriding gas) contains hydrogen, hydrogen that inhibits nitriding may remain in pores and cracks after pretreatment. The pretreatment process is carried out at least once, and preferably twice or more.

[0052] After the pretreatment, a nitriding gas (nitrogen-containing gas), which may contain hydrogen, is supplied to the ambient atmosphere, and the crushed material is heated in the supplied nitriding gas atmosphere to convert it into nitrides. The supplied nitriding gas may or may not contain hydrogen. Nitrogen-containing gases such as nitrogen and / or ammonia are preferably used as the nitriding gas. To control the nitriding reaction rate, other gases such as hydrogen, argon, and helium may also be added to the nitrogen-containing gas (nitriding gas). For example, a mixed gas containing ammonia and hydrogen can sufficiently nitride large alloy particles, even those with particle sizes exceeding 10 μm. In this case, the nitrogen (N) content of the final magnetic powder can be increased to 2.5% by mass or more and 5.0% by mass or less. When a mixed gas of ammonia and hydrogen is used as the nitriding gas, the ratio of the ammonia pressure to the total pressure (ammonia partial pressure) is preferably 0.20 or more and 1.00 or less, more preferably 0.30 or more and 0.95 or less. Nitriding the alloy particles is further promoted by setting the ammonia partial pressure to 0.20 or more. The amount of nitriding gas supplied cannot be determined in general, but it is preferable to adjust it so that the amount of N in the magnetic powder is sufficiently high, for example, between 2.5% and 5.0% by mass.

[0053] The heating temperature (nitriding temperature) of the crushed material is preferably 350°C to 500°C, more preferably 400°C to 480°C. A heating temperature of 350°C or higher allows nitrogen to diffuse rapidly into the alloy particles. However, excessively high temperatures can cause the alloy to decompose. For example, if the transition metal is Fe, the alloy may decompose, resulting in the formation of heterophases such as α-Fe and Fe nitrides, which degrade the magnetic properties of the magnetic powder. Setting the heating temperature to 500°C or lower can prevent the formation of such heterophases. The optimal heating time varies depending on various conditions, such as the heating temperature and processing volume. Therefore, it is difficult to determine a specific heating time. It is preferable to adjust the N content in the magnetic powder to a sufficiently high level, for example, to 2.5% by mass to 5.0% by mass.

[0054] Preferably, the nitride after nitriding is heat-treated under hydrogen gas, inert gas, or reduced pressure gas. Examples of inert gas include nitrogen gas, argon gas, and / or helium gas. More preferably, the heat treatment is performed in hydrogen gas followed by heat treatment in nitrogen and / or argon gas. The heat treatment after nitriding can achieve a more uniform nitrogen distribution in the nitride. However, this heat treatment is not an essential step. It is possible to obtain the magnetic powder of this embodiment without performing the heat treatment after nitriding.

[0055] <Wet processing process> In the wet treatment process, a nitride is subjected to wet treatment to produce a wet-treated product. The nitride obtained in the nitriding process is usually a porous mass. More specifically, it is an aggregate formed by the bonding of nitride particles, which are the main phase. The nitride particles are particles of a rare earth, transition metal, and nitrogen-based compound. In addition to the nitride particles, the nitride also contains by-products derived from the reducing agent (e.g., CaO) and heterogeneous phases (e.g., rare earth-rich phases). The wet treatment loosens the bonds between the nitride particles, resulting in a magnetic powder made of a rare earth, transition metal, and nitrogen-based compound. The wet treatment can also remove by-products and heterogeneous phases other than the main phase and provide a heat-resistant protective coating on the surface of the nitride particles. The wet treatment process includes sub-processes: a first water washing process, an acid washing process, a second water washing process, a surface treatment process, and a third water washing process.

[0056] [First washing process] In the wet treatment process, first, nitride is put into water and disintegrated to prepare a slurry containing nitride particles, and then the slurry containing nitride particles is subjected to a water washing process (first water washing process). The slurry is prepared by simply putting the nitride into water. By putting it into water, by-products derived from the reducing agent contained in the nitride (calcium oxide, calcium nitride, etc.) become calcium hydroxide. As a result, the nitride disintegrates, and a suspension (slurry) is obtained in which the water-insoluble components of nitride particles and calcium hydroxide fine particles are dispersed.

[0057] The water washing method is not limited. For example, a series of operations including stirring, standing, and decantation may be performed on a nitride-containing slurry. Water may be added to the slurry as needed. By-products (e.g., CaO) derived from the reducing agent contained in the nitride are converted into hydroxides (e.g., Ca(OH)2) in the slurry. These hydroxides have a significantly different specific gravity from the nitride particles. Therefore, decantation can remove the hydroxides derived from the reducing agent along with the clean water from the slurry. From the viewpoint of obtaining high-purity nitride particles, it is preferable to repeat the series of operations. Note that clean water refers to the liquid above the slurry obtained after the slurry is settling and the nitride particles, which have a high specific gravity, settle. Clean water can also be called the supernatant. However, clean water (supernatant) may contain components with a lower specific gravity than the nitride particles, such as hydroxides. Therefore, clean water is not necessarily transparent.

[0058] Conditions for the first water washing step, such as the slurry concentration, temperature, or number of decantation steps, are not limited. However, it is preferable that the concentration of the reducing agent component in the slurry at the end of the first water washing step is low. For example, when calcium (Ca) is used as the reducing agent, the slurry is washed with water until the Ca concentration in the clean water obtained by leaving the slurry to stand is preferably 1.0 g / L or less, more preferably 0.1 g / L or less. In this case, the clean water exhibits an alkaline pH of 8 to 13. The acid added in the subsequent pickling step is first consumed to neutralize the slurry and lower the pH to 7 or less, and then consumed to dissolve by-products and other phases. Variations in the concentration of the reducing agent component (Ca, etc.) in the slurry at the end of the first water washing step can cause variations in the amount of dissolved by-products and other phases, which may lead to variations in the magnetic properties of the final magnetic powder. By maintaining a low concentration of the reducing agent component in the slurry, magnetic powder with high purity and excellent magnetic properties can be consistently obtained.

[0059] [Pickling process] Next, the slurry containing the nitride particles obtained in the first water washing step is subjected to an acid washing treatment (acid washing step). Even if by-products derived from the reducing agent (e.g., Ca(OH)2) or heterogeneous phases (e.g., rare earth-rich phases) remain in the nitride particles, they can be removed by acid washing.

[0060] In acid washing, acid is added to a slurry containing nitride particles. It is preferable to uniformly contact the nitride particles in the slurry with the acid, and for this purpose, it is preferable to stir the slurry. There are no particular limitations on the acid, as long as it can dissolve and remove by-products (e.g., Ca(OH)2) and other phases (e.g., rare earth-rich phases). However, at least one acid selected from the group consisting of acetic acid, hydrochloric acid, and nitric acid is preferred.

[0061] The amount of acid added is adjusted so that the average atomic ratio (R / TM ratio) of rare earth metals (R) to transition metals (TM) contained in the final magnetic powder is between 0.107 and 0.121. Adjusting the amount of acid added enables the production of magnetic powder with even better magnetic properties. On the other hand, if the amount of acid added is so large that the average R / TM ratio is less than 0.107, the acid washing will be too strong. This may result in strong etching marks on the particle surfaces that make up the powder, resulting in a decrease in magnetic properties that are sensitive to the particle surface structure, such as coercivity. On the other hand, if the amount of acid added is so small that the average R / TM ratio exceeds 0.121, the acid washing effect will not be fully achieved. Residual by-products and other phases may result in a decrease in the magnetic properties of the magnetic powder, such as maximum magnetic polarization.

[0062] Preferably, in the pickling process, the temperature of the slurry containing nitride particles is maintained between 13°C and 30°C. Adjusting the slurry temperature enables the production of magnetic powder with even better magnetic properties. On the other hand, if the slurry temperature is below 13°C, the effect of the pickling process cannot be fully achieved. By-products and heterogeneous phases remain, resulting in a decrease in the magnetic properties of the magnetic powder, such as the maximum magnetic polarization. If the slurry temperature exceeds 30°C, even if the average R / TM ratio is within the range of 0.107 to 0.121, excessive amounts of rare earth ions and iron ions are eluted into the slurry, contaminating the nitride particle surfaces. Contamination of the nitride particle surfaces inhibits the formation of a protective coating in the subsequent surface treatment process. This reduces the uniformity of the protective coating, potentially resulting in a decrease in the heat resistance of the final magnetic powder. The slurry temperature is more preferably between 15°C and 25°C.

[0063] [Second washing process] Next, the slurry containing the nitride particles after the pickling process is subjected to a water washing treatment (second water washing process). In the pickling process, the nitride particles in the slurry are washed with acid. During this process, by-products (e.g., Ca(OH)2) and heterogeneous phases (e.g., rare earth-rich phases) dissolve, and ions of reducing agent components (e.g., Ca ions), rare earth ions (R ions), and transition metal ions (TM ions) may be generated in the slurry. These generated ions are diluted and removed by water washing.

[0064] The method of water washing is not limited. For example, a series of operations including stirring, leaving, and decantation may be performed on the slurry containing nitride particles. At this time, water may be added as needed. From the viewpoint of sufficiently removing the generated ions, it is preferable to repeat the series of operations.

[0065] The water washing is continued until the transition metal ions in the slurry are sufficiently removed, for example, until the transition metal (TM) concentration (e.g., Fe concentration) in the clean water of the slurry immediately before adding an aqueous phosphoric acid solution in the subsequent surface treatment step is reduced to preferably 1.0 g / L or less, more preferably 0.3 g / L or less.

[0066] [Surface treatment] Next, a phosphoric acid solution is added to the slurry containing the nitride particles after the second water washing step to form a phosphorus-containing protective coating on the surface of the nitride particles (surface treatment step). By providing the phosphorus-containing protective coating, the heat resistance of the final magnetic powder is improved.

[0067] In the surface treatment process, water is poured again into the slurry obtained in the second water washing process, and then an aqueous phosphoric acid solution is added to the resulting slurry while stirring it after pouring. By adding the aqueous phosphoric acid solution, the pH of the slurry drops from the value before addition (usually 6 to 9), and then rises again as the reaction between the phosphoric acids and the nitride particles progresses. Note that the aqueous phosphoric acid solution is an aqueous solution of phosphoric acids. Furthermore, the concept of phosphoric acids encompasses phosphoric acid and phosphates.

[0068] Because sufficient water washing is performed in the preceding second water washing step, the amount of transition metals in the slurry after water injection, i.e., the slurry immediately before the phosphoric acid aqueous solution, is sufficiently reduced. Preferably, the transition metal (TM) concentration (e.g., Fe concentration) of the nitride-containing slurry immediately before the phosphoric acid aqueous solution is added is 1.0 g / L or less. This results in the formation of a highly uniform protective coating, further improving the heat resistance of the final magnetic powder. In contrast, if the TM concentration is high, much of the added phosphoric acid is consumed in the production of transition metal phosphates such as ferrous phosphate and ferric phosphate. This results in insufficient formation of the protective coating, and the uniformity of the resulting protective coating deteriorates. From the perspective of improving the heat resistance of the magnetic powder, the TM concentration of the slurry water is more preferably 0.3 g / L or less.

[0069] Examples of raw materials for the phosphoric acid aqueous solution include one or more phosphoric acids (phosphoric acid, phosphates) such as orthophosphoric acid, disodium hydrogen phosphate, and sodium dihydrogen phosphate. Only one type of phosphoric acid may be used, or multiple types of phosphoric acids may be used in combination.

[0070] Preferably, the amount of phosphoric acid solution added in the surface treatment step is 0.03% to 1.0% by mass, calculated as phosphorus (P), relative to the magnetic powder. In other words, the amount of phosphorus (P) contained in the phosphoric acid solution added is 0.03% to 1.0% by mass, calculated as phosphorus (P), relative to the magnetic powder. By controlling the P amount within this range, it is possible to obtain magnetic powder with excellent heat resistance while maintaining high levels of magnetic properties. On the other hand, if the P amount is less than 0.03% by mass, the formation of a protective coating and the resulting improvement in the heat resistance of the magnetic powder will be insufficient. If the P amount exceeds 1.0% by mass, the amount of excess phosphoric acid will be excessive, resulting in a decrease in the magnetic properties, such as the maximum magnetic polarization, of the final magnetic powder. From the perspective of improving the magnetic properties and heat resistance, the amount of phosphoric acid solution is more preferably 0.07% to 0.8% by mass, calculated as P, relative to the magnetic powder.

[0071] Preferably, in the surface treatment step, the temperature of the slurry containing the nitride particles is maintained at 13°C or higher and 30°C or lower. If the slurry temperature is lower than 13°C, the reaction between the added phosphoric acids and the nitride particles does not proceed sufficiently, making it difficult to form a highly uniform protective coating. If the slurry temperature exceeds 30°C, the reaction between the phosphoric acids and the nitride particles proceeds excessively. This results in greater variation in the thickness of the protective coating, which may result in insufficient improvement in heat resistance. Furthermore, strong etching marks may appear on the surface of the nitride particles, which may reduce magnetic properties sensitive to the particle surface structure, such as coercive force. From the viewpoint of improving the magnetic properties and heat resistance of the magnetic powder, the slurry temperature is more preferably 16°C or higher and 25°C or lower.

[0072] [Third water washing process] Next, the slurry containing the nitride particles after the surface treatment step is subjected to a water washing process to obtain a wet-treated product (third water washing process). In the surface treatment step, particles of transition metal phosphates, rare earth phosphates, or composite salts of these may be produced as by-products in areas other than the protective coating. For example, phosphoric acids may react with the nitride particles to produce phosphate particles such as ferrous phosphate or ferric phosphate as by-products. These by-products (phosphate particles) are removed by water washing.

[0073] The method of water washing is not limited. For example, a series of operations including stirring, standing, and decantation may be performed on the slurry containing the nitride particles. At this time, water may be added as needed. From the viewpoint of sufficiently removing by-products, it is preferable to repeat the series of operations.

[0074] Conditions such as the concentration and temperature of the slurry or the number of decantation steps in the third water washing step are not limited. However, it is preferable that the phosphorus (P) concentration in the slurry at the end of the third water washing step is low. Specifically, it is preferable to perform the water washing process until the phosphorus (P) concentration in the clean water of the slurry is 100 mg / L or less. If the P concentration is excessively high, the magnetic properties of the final magnetic powder, such as the maximum magnetic polarization, may be reduced. From the viewpoint of improving magnetic properties, it is more preferable that the P concentration in the clean water is 50 mg / L or less.

[0075] <Drying process> If necessary, a step (drying step) of drying the wet-processed product (nitride particles) after water washing may be provided. The drying method is not limited. For example, a method may be used in which a slurry containing the wet-processed product is subjected to a deliquification process to obtain a cake containing magnetic powder, and the obtained cake is dried. Deliquification may be performed using a known solid-liquid separation device such as a Nutsche separator, a filter press, and / or a centrifuge. Drying may be performed by vacuum drying or heat drying. To improve the efficiency of drying, the cake may be stirred during drying. Furthermore, to further improve the efficiency of drying, the solvent in the slurry before deliquification may be replaced. For example, the water contained in the slurry may be replaced with an alcohol such as methanol, ethanol, and / or propanol.

[0076] <Crushing process> If necessary, a step (pulverization step) of pulverizing the wet-treated product (nitride particles) after washing or drying may be provided. The pulverization method is not limited, and may be wet or dry. In the case of wet pulverization, pulverization is preferably carried out in an organic solvent. In the case of dry pulverization, pulverization is preferably carried out in an inert atmosphere. Furthermore, the treated product may be subjected to a surface treatment to impart weather resistance during or after pulverization.

[0077] In this way, the magnetic powder of this embodiment is produced. That is, the wet-treated product after wet treatment, drying, and / or pulverization can be used as the magnetic powder. The average particle size D50 of the produced magnetic powder is usually 1 μm or more and 10 μm or less, preferably 1 μm or more and 5 μm or less, and more preferably 1 μm or more and 3 μm or less.

[0078] <<2. Rare earth transition metal nitrogen-based magnetic powder>> The rare earth transition metal magnetic powder of this embodiment contains at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as main components. Details of the rare earth metal (R) are as described above. Examples of rare earth metals include, but are not limited to, one or more selected from the group consisting of yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), and ytterbium (Yb). Among these, it is particularly preferable to include Sm. Furthermore, Sm may be used in combination with other rare earth metals other than Sm, such as La and / or Ce. For example, the rare earth metal may be composed of 70 atomic % or more of Sm and 30 atomic % or less of other elements (La, Ce, etc.).

[0079] Details of the transition metal (TM) are as described above. Examples of transition metals include, but are not limited to, one or more selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), chromium (Cr), manganese (Mn), zinc (Zn), molybdenum (Mo), and tungsten (W). However, among these, it is particularly preferable to include Fe. Furthermore, Fe may be used in combination with another transition metal other than Fe, such as at least one selected from the group consisting of Cr, Mn, Co, and Ni. For example, 90 atomic % or more of the transition metal may be Fe, and 10 atomic % or less of other elements (such as Cr, Mn, Co, and Ni) may be used.

[0080] The magnetic powder of this embodiment is Th2Zn 17 type, Th2Ni 17 The magnetic powder having such a crystal structure is SmFe 17 Powder (SmFe) with the basic composition represented by N3 17 N3-based powders). 17 N3-based powders exhibit excellent magnetic properties and are useful as bonded magnet materials. The magnetic powders of this embodiment are not limited to those with a stoichiometric composition. Deviations in composition are permissible as long as the crystal structure is maintained and the R / TM ratio requirements described below are satisfied.

[0081] The magnetic powder of this embodiment has an average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) of 0.107 to 0.121. The average R / TM ratio is evaluated by the method described below.

[0082] The average R / TM ratio serves as a guide for the main phase composition of the magnetic powder. By keeping the average value within the aforementioned range, the main phase composition of the magnetic powder is optimized, resulting in improved magnetic properties. On the other hand, if the average value is less than 0.107, the amount of transition metal in the magnetic powder will be excessive. This makes it impossible to suppress the inclusion of heterogeneous transition metal-rich phases such as α-Fe and Fe nitrides. This results in a decrease in the magnetic properties of the magnetic powder, such as maximum magnetic polarization and coercivity. On the other hand, if the average value exceeds 0.121, the amount of rare earth metal will be excessive. This makes it impossible to suppress the inclusion of heterogeneous rare earth-rich phases such as SmFe5, SmFe3, and / or SmFe2, resulting in a decrease in magnetic properties. From the viewpoint of improving magnetic properties, the average R / TM ratio is preferably 0.108 or more and 0.118 or less.

[0083] The average R / TM ratio can be determined as follows: First, a thermosetting resin is mixed with magnetic powder, and the resulting mixture is press-molded and then thermally cured to produce a compression-molded body. Next, the cross section of the resulting compression-molded body is polished to expose it, preparing a sample for SEM observation. The polished cross section of the prepared SEM observation sample is observed using a scanning electron microscope (SEM) at a magnification that allows at least 1,000 particles to fit within the field of view, and the entire field of view is subjected to EDS analysis using the standardless FP method to determine the R / TM atomic ratio.

[0084] In the magnetic powder of this embodiment, the area ratio of rare earth-rich phases in which the atomic ratio of rare earth metal (R) to transition metal (TM) (R / TM ratio) exceeds 0.130 is 1.0% or less in the cross section of the compression-molded body. The area ratio of the rare earth-rich phases is evaluated by the method described below.

[0085] As described above, the magnetic powder of this embodiment has an average R / TM ratio, which serves as a guide for the main phase composition, limited to a predetermined range. However, local variations in the composition may occur. In particular, magnetic powders containing rare earths and transition metals as their main constituent elements often contain rare-earth-rich heterophases (SmFe5, SmFe3, SmFe2, etc.) due to various factors resulting from manufacturing conditions, such as insufficient element diffusion and heterophase formation. Because such rare-earth-rich phases cause deterioration of magnetic properties, this embodiment strives to eliminate them as much as possible. The area ratio of the rare-earth-rich phase is preferably 0.8% or less, more preferably 0.5% or less. There is no lower limit to the area ratio. For example, it may be 0.1% or more, 0.3% or more, or 0.5% or more.

[0086] The area ratio of the rare earth-rich phase is determined as follows. The polished cross section of the SEM observation sample described above is observed using an SEM to obtain a backscattered electron image. Image analysis utilizing contrast differences is then performed on the obtained backscattered electron image to determine the area ratio of the rare earth-rich phase. That is, the rare earth-rich phase region appears brighter in the backscattered electron image than the main phase region of the magnetic powder because it contains a large amount of rare earth metals, which have a large mass. Therefore, the area ratio of the rare earth-rich phase can be determined by determining the area ratio of the region that appears brighter in the backscattered electron image. Specifically, a contrast threshold is set so that only the rare earth-rich phase region (region where the R / TM atomic ratio exceeds 0.130) is detected in the backscattered electron image, and the backscattered electron image is binarized. The area of ​​the rare earth-rich phase is then determined in the binarized backscattered electron image, and divided by the area of ​​the particle cross section of the entire field of view to determine the area ratio of the rare earth-rich phase.

[0087] The magnetic powder of this embodiment has the effect of stably having a high maximum magnetic polarization, Jm of which is preferably 1.33 T or more, and more preferably 1.40 T or more.

[0088] The magnetic powder of this embodiment has a protective coating with excellent uniformity. This effectively results in excellent heat resistance. Heat resistance can be evaluated by the coercive force retention rate [HcJ(1000) / HcJ(0)], which is the ratio of the coercive force (HcJ(1000)) after heating to the coercive force (HcJ(0)) before heating when the magnetic powder is heated in air at 150°C for 1000 hours. The coercive force retention rate [HcJ(1000) / HcJ(0)] is preferably 83% or more, and more preferably 87% or more. [Example]

[0089] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

[0090] (1) Evaluation of magnetic powder The magnetic powders produced in the examples and comparative examples were evaluated for various properties as follows.

[0091] <Particle size> The particle size of the magnetic powder was evaluated using an air-flow dispersion type laser diffraction particle size distribution measuring device, and the cumulative 50% diameter in the particle size distribution based on volume was measured as D50.

[0092] <R / Fe atomic ratio> The composition (R / Fe ratio) of the magnetic powder was determined by observation with a scanning electron microscope (SEM). Specifically, a thermosetting resin was mixed with the magnetic powder, and the resulting mixture was press-molded and then thermally cured to produce a compression molded body. At this time, the mixing amount of the thermosetting resin was adjusted to be 4 to 5% by mass with respect to the magnetic powder. Next, the cross-section of the obtained compression molded body was exposed by polishing to prepare a sample for SEM observation.

[0093] Next, the polished cross-section of the sample for SEM observation was observed using a scanning electron microscope (SEM; JEOL Ltd., JST-IT200). The observation was carried out under the condition of a magnification of 1000 times, and was performed for three fields of view having a size of about 100 μm square. The number of particles contained in one field of view was 5000 or more. Then, the entire field of view was subjected to EDS analysis by the standardless FP method to determine the R / Fe atomic ratio. The analysis was carried out under the condition of an acceleration voltage of 25 kV.

[0094] <Area ratio of rare earth-rich phase> The area ratio of the rare-earth-rich phase was determined by SEM observation. Specifically, the polished cross section of the SEM observation sample prepared for the R / Fe atomic ratio measurement was observed using SEM. Observation was performed at 1000x magnification, and backscattered electron images were taken of a field of view approximately 100 μm square. The number of particles contained in each field of view was 5000 or more. The obtained backscattered electron images were then subjected to image analysis utilizing differences in contrast. Specifically, a contrast threshold was set to detect only the rare-earth-rich phase region (region where the R / Fe atomic ratio exceeds 0.13) in the backscattered electron image, and the backscattered electron image was then binarized. The area s of the rare-earth-rich phase was then calculated from the binarized backscattered electron image. Next, the contrast threshold was changed to include both the rare-earth-rich phase region and the main phase region, and the backscattered electron image was then binarized. The area S of the entire particle was then calculated from the binarized backscattered electron image. The area ratio of the rare earth-rich phase was determined by dividing the area s by the area S.

[0095] <Magnetic properties> The magnetic properties of the magnetic powder were measured by applying an orienting magnetic field of 1600 kA / m to the magnetic powder in stearic acid in accordance with the Bond Magnetic Testing Method Guidebook, BM-2002 and BM-2005, published by the Japan Bond Magnetic Industry Association. After magnetizing the magnetic powder in a magnetic field of 4.0 MA / m, the specific gravity of the magnetic powder was measured at 7.67 g / cm. 3 Using a vibrating sample magnetometer with a maximum magnetic field of 1.6 MA / m, the maximum magnetic polarization (Jm (T)), coercivity (HcJ (MA / m)), and squareness (Hk) (MA / m) were measured. Here, squareness (Hk) (MA / m) is the magnetic field H at which the magnetic polarization J has decreased to 90% of the remanent magnetic polarization Jr in the second quadrant of the magnetization curve.

[0096] <Heat resistance> The magnetic powder was heated in air at 150°C for 1000 hours, and the coercive force before heating (HcJ(0)) and after heating (HcJ(1000)) were measured. The coercive force was measured using the same method as in the measurement of magnetic properties described above. Then, as an index for evaluating heat resistance, the ratio of the coercive force after heating (HcJ(1000)) to the coercive force before heating (HcJ(0)) [HcJ(1000) / HcJ(0)] was calculated as the coercive force retention rate.

[0097] <Thermogravimetry> Thermogravimetry (TG) of the magnetic powder was performed by heating the sample at a temperature increase rate of 20°C / min while flowing dry air (N2-21% O2) at a flow rate of 100 mL / min.

[0098] (2) Preparation and evaluation of magnetic powder [Example 1] <Reduction and diffusion process> Average particle size D 50 624 g of samarium oxide powder with a diameter of 2.3 μm and D 50 1553 g of iron powder with a particle size of 40 μm and 248 g of granular metallic calcium were mixed in a mixer to obtain a mixture with an R / Fe atomic ratio (Sm / Fe atomic ratio) of 0.129. This mixture was placed in an iron crucible and heat-treated at 1150°C for 6 hours in an argon gas atmosphere to obtain a reaction product (reduced and diffused product).

[0099] <Hydro-crushing process> After cooling, the inside of the vessel was evacuated to a vacuum, and then hydrogen gas was supplied while adjusting the flow rate so as to maintain a gauge pressure of +20 kPa, and the obtained reaction product was decomposed by a hydrogen absorption reaction.

[0100] <Nitriding process> The hydrogen-treated reaction product (crushed material) was placed in a tubular furnace. The pressure was then reduced to 10 kPa absolute, followed by a pressure recovery to atmospheric pressure with nitrogen gas, a process repeated twice. The temperature was then raised to 430°C for 9 hours while a mixture of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min) was passed through the furnace. The mixture was then heat-treated for another hour while hydrogen gas (flow rate: 1000 mL / min) was passed through the furnace, and for another hour while nitrogen gas (flow rate: 1000 mL / min) was passed through the furnace.

[0101] <Wet processing process> After cooling, 1,360 g of the recovered nitriding reaction product (nitride) was added to 4 L of water to form a slurry, and the process of pouring water, stirring, leaving to stand, and decanting was repeated seven times (first water washing process). The final calcium concentration of the clean water was 0.02 g / L. Water was poured again, and the slurry was stirred at 22°C while acetic acid was added for acid washing (pickling process). The addition of acetic acid reduced the slurry pH from 11 to 4-5, and stirring continued even after the addition of acetic acid, until the slurry pH exceeded 7. The clean water was then drained, and water pouring, stirring, leaving to stand, and decantation were repeated seven times (second water washing process) until the Fe concentration in the clean water was 1.0 g / L or less.

[0102] Next, 4 L of water was again poured in for surface treatment and stirred. The Fe concentration in the clean water was 0.1 g / L, and the slurry temperature was 20°C. While stirring the slurry, a 25 mass% aqueous solution of orthophosphoric acid containing P, which accounted for 0.31 mass% of the magnetic powder mass, was added (surface treatment step). The addition of the orthophosphoric acid aqueous solution caused the slurry pH to drop from 8 to 4.1, then rise again to 7, at which point stirring was stopped and the slurry was decanted.

[0103] Next, water was poured again, and the process of pouring water, stirring, leaving to stand, and decantation was repeated twice (third water washing step). At this time, the P concentration of the final slurry water was 40 mg / L.

[0104] <Drying process> After the final decantation after the third water washing step, 1 L of 2-propanol was added, stirred, and suction filtered to obtain a cake. The obtained cake was dried at 130°C under reduced pressure while stirring in a mixer, and then cooled to obtain a samarium-iron-nitrogen alloy powder. The obtained alloy powder was D 50 The grain size was 21 μm, the Sm composition was 23.1 mass %, and the P composition was 0.15 mass %.

[0105] <Crushing process> 900 g of the resulting samarium-iron-nitrogen alloy powder was finely pulverized in a media agitation mill. 2-Propanol containing 85% orthophosphoric acid was used as the pulverization solvent. The pulverized slurry was heated and dried under reduced pressure to obtain a magnetic powder. The manufacturing conditions for Example 1 are summarized in Table 1 below.

[0106] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It has a D type crystal structure. 50 The particle size was 2.1 μm. The magnetic properties were Jm (maximum magnetic polarization): 1.42 T, HcJ (coercive force): 0.90 MA / m, and Hk (squareness): 0.52 MA / m. The P composition of the magnetic powder was 0.52 mass %.

[0107] A compression molded body was made from the obtained magnetic powder, and three randomly selected locations on the polished cross section were observed using an SEM at 1000x magnification. The resulting SEM backscattered electron images are shown in Figures 1 to 3. EDS analysis was performed on Sm and Fe over the entire 100 μm square area, and the R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio) was calculated. The resulting R / Fe atomic ratios were 0.113, 0.113, and 0.114, respectively, with an average value (average R / Fe atomic ratio) of 0.113.

[0108] In each image, EDS analysis was performed on several random bright contrast particles to determine the R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio). All values ​​exceeded 0.13. Image analysis was then used to calculate the ratio of the cross-sectional area of ​​these bright contrast particles to the total particle cross-sectional area. The respective area fractions were 0.5%, 0.4%, and 0.5%, with the average (area fraction of the rare-earth-rich phase) being 0.5%.

[0109] This magnetic powder was heated in air at 150°C for 1000 hours, and after heating it was removed and the coercive force HcJ(1000) at room temperature was evaluated to be 0.77 MA / m. From this, the coercive force retention rate [HcJ(1000) / HcJ(0)], an index of heat resistance, was calculated to be 86%.

[0110] [Examples 2-4] The manufacturing conditions were changed as shown in Table 1 below. Other than that, magnetic powder was manufactured using the same procedure as in Example 1. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It had the type crystal structure.

[0111] [Example 5] D obtained by reduction diffusion method 50 Sm2Fe with a thickness of 30 μm 17 2 kg of alloy powder was premixed with 200 g of samarium oxide powder in a mixer. This mixed powder was mixed with a mixed solution of 4.4 kg of 2-propanol and 46 g of 85% phosphoric acid as a solvent in a media agitation mill. 50 The resulting slurry was heated to 210°C while reducing the pressure in a mixer, and was cooled after confirming that the decrease in the degree of vacuum due to gas release had ceased and that the degree of vacuum had increased sufficiently.

[0112] The resulting ground mixture was 2.00 kg (SmFe 17To a mixture of 1.82 kg of alloy powder and 0.18 kg of samarium oxide, 466 g of granular calcium metal was added as a reducing agent and mixed in a mixer under an argon atmosphere. The amount of reducing agent was 2.5 times the amount (equivalent) required for reduction calculated from the oxygen analysis value of the pulverized mixture. The R / Fe atomic ratio of the raw material mixture obtained in this way was 0.160.

[0113] This raw material mixture was placed in an iron crucible and heated under an argon gas atmosphere for the reduction-diffusion process. It was then held at 830°C for two hours and then cooled (reduction-diffusion process). The reaction product (reduced-diffusion-treated product) recovered from the crucible was then crushed by hydrogen absorption (hydrogen treatment), followed by nitriding. For the hydrogen treatment, the reaction product (reduced-diffusion-treated product) was placed in a tubular furnace, the pressure inside the furnace was reduced to -100 kPa, hydrogen gas was introduced to atmospheric pressure, and the temperature was raised to 300°C while hydrogen was flowing. Because the reaction product absorbed hydrogen while generating heat during the heating process, the exhaust valve of the tubular furnace was closed while the furnace was under negative pressure, and the hydrogen supply was continued. After the heat generation and hydrogen absorption ceased, the product was cooled (hydrogen crushing process). After the hydrogen treatment, the reaction product (crushed product) was in a crushed state. Next, for the nitriding treatment, the pressure inside the tubular furnace was reduced to 5 kPa absolute pressure, and then nitrogen gas was introduced to atmospheric pressure. After that, the flow rate of nitrogen gas was increased to 1 L / min, and the temperature was raised to 450°C, and the material was maintained at this temperature for 24 hours before being cooled. This yielded a nitriding reaction product (nitride) (nitriding step).

[0114] Next, in the wet treatment process, 2.0 kg of the nitriding reaction product (nitride) was added to 4 L of water to form a slurry, and the process of water injection, stirring, settling, and decantation was repeated 10 times (first water washing process). The final calcium concentration of the clean water was 0.1 g / L. Water was again poured in, and acetic acid was added while stirring the slurry at 13°C (pickling process). The addition of acetic acid reduced the slurry pH from 12 to approximately 5, and stirring continued even after the addition of acetic acid, until the slurry pH exceeded 7. The clean water was then drained, and water injection, stirring, settling, and decantation were repeated 10 times (second water washing process) until the Fe concentration in the clean water reached 1.0 g / L or less.

[0115] Next, 4 L of water was added again and stirred. The Fe concentration of this clean water was 0.9 g / L, and the slurry temperature was 13°C. While stirring the slurry, a 25 mass% aqueous solution of orthophosphoric acid containing P, equivalent to 1.0 mass% of the magnetic alloy powder mass, was added (surface treatment process). The addition of the orthophosphoric acid aqueous solution caused the slurry pH to drop from 8 to 3.3, and then rose again to 7, at which point the stirring was stopped and the slurry was decanted. Water was then added again, and the cycle of water addition, stirring, settling, and decantation was repeated eight times (third water washing process). At this point, the P concentration of the final clean water slurry was 20 mg / L.

[0116] After the final decantation in the third water washing step, 2 L of 2-propanol was added and stirred, and the cake obtained by suction filtration was dried at 170°C under reduced pressure in a mixer, and then cooled to obtain a samarium-iron-nitrogen alloy powder (drying step). 50 The particle size was 2.9 μm. The Sm composition was 23.8 mass % and the P composition was 0.83 mass %. This alloy powder was finely pulverized with alumina balls having a diameter of 0.2 mm in a 2-propanol solvent containing an aqueous orthophosphoric acid solution, and the slurry was dried to obtain a magnetic powder (pulverization step).

[0117] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It has a D type crystal structure. 50 The particle size was 1.3 μm. The magnetic properties were Jm (maximum magnetic polarization): 1.13 T, HcJ (coercive force): 1.44 MA / m, and Hk (squareness): 0.40 MA / m. The P composition was 0.83 mass %.

[0118] The R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio) was calculated for the polished cross section of a compression molded product made from the magnetic powder, and the average value was 0.121. Furthermore, EDS analysis revealed that the average percentage of regions where the R / Fe atomic ratio exceeded 0.13 (area percentage of rare earth-rich phases) was 0.8%. Furthermore, the coercivity retention rate [HcJ(1000) / HcJ(0)] of this magnetic powder was 91%.

[0119] [Example 6] The manufacturing conditions were changed as shown in Table 1 below. Otherwise, magnetic powder was manufactured using the same procedure as in Example 5. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It had the type crystal structure.

[0120] [Example 7] The production conditions were changed as shown in the following Table 1. The raw materials were weighed and subjected to a reduction diffusion step and a hydrogen crushing step in the same manner as in Example 1. Next, prior to the nitriding step, a hydrotreatment step and a drying step were performed to obtain a samarium-iron alloy powder.

[0121] Next, in the nitriding step, the samarium-iron alloy powder was placed in a tubular furnace and heat-treated for 9 hours at 430°C while flowing a mixed gas of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min). Further heat treatment was performed for 1 hour while flowing hydrogen gas (flow rate: 1000 mL / min) and for another 1 hour while flowing nitrogen gas (flow rate: 1000 mL / min), to obtain a samarium-iron-nitrogen alloy powder.

[0122] The obtained alloy powder was finely pulverized in a 2-propanol solvent containing an aqueous solution of orthophosphoric acid, as in Example 1, and then the slurry was dried to obtain a magnetic powder.

[0123] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It has a D type crystal structure. 50 The particle diameter was 2.1 μm. The magnetic properties were Jm: 1.41 T, HcJ: 0.86 MA / m, and Hk: 0.39 MA / m. The P composition was 0.49 mass %.

[0124] A compression molded body was made from the obtained magnetic powder, and three randomly selected locations on the polished cross section were observed using an SEM at 1000x magnification. The obtained SEM backscattered electron images are shown in Figures 4 to 6. The R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio) of the polished cross section was calculated in the same manner as in Example 1, and the average value was 0.117. Furthermore, the area ratio of regions where the R / Fe atomic ratio exceeded 0.13 by EDS analysis (area ratio of rare earth-rich phase) was 2.2%. Furthermore, the retention rate of the coercive force of this magnetic powder was 85%.

[0125] [Example 8] The reduction-diffusion step through the second water-washing step of the wet treatment step were carried out under the conditions shown in Table 1 below. When the Fe concentration in the clean water reached 1 g / L or less in the final wash of the second water-washing step, the samarium-iron-nitrogen alloy powder was recovered by a drying step without going through the surface treatment step and the third water-washing step. The alloy powder recovered at this stage had not been surface-treated.

[0126] The obtained alloy powder was finely pulverized in a 2-propanol solvent containing an aqueous solution of orthophosphoric acid, as in Example 1, and then the slurry was dried to obtain a magnetic powder.

[0127] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It has a D type crystal structure. 50 The particle size was 2.1 μm. The magnetic properties were Jm: 1.44 T, HcJ: 0.93 MA / m, and Hk: 0.55 MA / m. The P composition was 0.43 mass %.

[0128] [Examples 9-15] The manufacturing conditions were changed as shown in Table 1 below. Other than that, magnetic powder was manufactured using the same procedure as in Example 1. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It had the type crystal structure.

[0129] [Example 16] In the nitriding process, the hydrogen-treated reaction product was placed in a tubular furnace, and then the pressure was reduced to 10 kPa absolute, and then the pressure was returned to atmospheric pressure with a 4:1 mixture of ammonia gas and hydrogen gas. This process was repeated twice. Next, the temperature was raised while a mixture of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min) was passed through the furnace, and the furnace was heat-treated at 430°C for 9 hours. Other than that, the magnetic powder was produced using the same procedure as in Example 1.

[0130] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It had the type crystal structure.

[0131] [Example 17] In the nitriding process, the hydrogen-treated reaction product was placed in a tubular furnace, and the pressure was reduced to 80 kPa absolute, followed by a cycle of increasing the pressure to atmospheric pressure with nitrogen gas. The temperature was then increased while a mixed gas of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min) was passed through, and the mixture was heat-treated at 430°C for 9 hours. The magnetic powder was otherwise prepared using the same procedure as in Example 1.

[0132] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It had the type crystal structure.

[0133] [Example 18] The manufacturing conditions were changed as shown in Table 1 below. Otherwise, magnetic powder was manufactured using the same procedure as in Example 5. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder was Th2Zn 17 It had the type crystal structure.

[0134] [Table 1]

[0135] [Table 2]

[0136] (3) Summary of evaluation results Examples 1 to 4 are magnetic powders produced using a mixture containing samarium oxide, iron powder (metallic iron powder), and metallic calcium as raw materials. These magnetic powders had an R / Fe atomic ratio (R / TM atomic ratio) of 0.108 to 0.114, and the proportion of impurity phases (rare earth-rich phases) (0.5 to 0.7%) was 1.0% or less. As a result, the maximum magnetic polarization Jm was high, at 1.33 to 1.42 T. Furthermore, these magnetic powders had a coercive force retention ratio [HcJ(1000) / HcJ(0)], an index of heat resistance, of 83 to 88%.

[0137] In contrast, in Example 7, in which the order of the nitriding process and the wet treatment process was reversed, the R / Fe atomic ratio (0.123) exceeded 0.121. The proportion of impurity phases (rare earth-rich phases) (2.2%) exceeded 1.0%. Therefore, the maximum magnetic polarization Jm (1.40 T) was lower than that of Example 1 (1.42 T) with the same D50 (2.1 μm). The squareness Hk (0.39 MA / m) was also lower than that of Example 1 (0.52 MA / m).

[0138] In Example 8, in which the surface treatment step and the third water washing step in the wet treatment step were omitted, Jm was high at 1.44 T, but the coercive force retention rate was low at 68%.

[0139] Alloy powders of Examples 1 and 8 (D after drying process) 50 Figure 7 shows the results of thermogravimetry (TG) performed on a magnetic alloy powder (samarium-iron-nitrogen alloy with a particle size of approximately 20 μm). These results indicate that the alloy powder oxidizes and gains in mass when heated in air. The temperature at which the oxidized mass gain due to heating reaches 1% is 410°C in Example 8, while it is 570°C in Example 1, which has a protective coating. It is presumed that this difference in heat resistance (oxidation resistance) of the alloy powder before pulverization leads to a difference in the uniformity of the protective coating on the magnetic alloy powder pulverized while undergoing the surface treatment of Patent Document 3, resulting in a difference in the heat resistance of the magnetic powder.

[0140] Example 9 shows the results when the R / Fe atomic ratio (0.116) of the raw material mixture was below 0.117, and Example 10 shows the results when the R / Fe atomic ratio (0.163) of the raw material mixture was above 0.160. In the former case, the R / Fe atomic ratio (0.106) of the magnetic powder obtained after the milling process was below 0.107, and the coercive force HcJ was low at 0.73 MA / m. In the latter case, the R / Fe atomic ratio (0.122) of the magnetic powder milled in the same manner was above 0.121, and the proportion of impurity phases (rare earth-rich phases) (4.9%) exceeded 1.0%. As a result, the Jm was low at 1.17 T and the HcJ was low at 0.81 MA / m. The coercive force retention rates were low at 82% and 78%, respectively.

[0141] Example 11 shows the results when the treatment temperature (1210°C) in the reduction-diffusion process exceeded 1200°C. The high-temperature treatment sintered the particles, and sintered particles were observed in the magnetic powder obtained after the drying process. As a result, the magnetic powder obtained after the crushing process had a low Jm of 1.25T, a low Hk of 0.33MA / m, and a low coercive force retention rate of 79%.

[0142] Example 12 shows the results when the slurry temperature in the fourth acid washing step was high, resulting in an R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step being below 0.107. In contrast, Example 13 shows the results when the slurry temperature in the acid washing step was low, resulting in less dissolution of subphases during the acid washing, resulting in an R / Fe atomic ratio (0.122) exceeding 0.121. In the former case, excessive acid washing resulted in an R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step being below 0.107. Furthermore, the HcJ was low at 0.76 MA / m. On the other hand, in the latter case, the R / Fe atomic ratio (0.122) of the magnetic powder exceeded 0.121, resulting in a proportion of impurity phases (rare earth-rich phases) exceeding 1.0% (1.8%). Therefore, the Jm of the magnetic powder was low at 1.21 T. The coercivity retention rates were also low at 74% and 77%, respectively.

[0143] Example 14 shows the results when the amount of P in the phosphoric acid aqueous solution added in the surface treatment step (0.02% by mass relative to the alloy powder) was less than 0.03% by mass. In contrast, Example 15 shows the results when the amount of P (1.2% by mass relative to the alloy powder) was greater than 1.0% by mass. In the former, Jm was 1.28 T, which is not particularly low. However, the coercive force retention rate was low at 72%. On the other hand, in the latter, Jm of the magnetic powder was low at 1.19 T due to the addition of an excess amount of phosphoric acid aqueous solution.

[0144] In Example 16, the reaction product, which had been hydrogen-treated during the nitriding process, was placed in a tubular furnace, depressurized to 10 kPa absolute, and then repressurized to atmospheric pressure with a 4:1 mixture of ammonia and hydrogen gas. This process was repeated twice, and the product was then nitrided. Because the gas used to repressurize contained hydrogen, hydrogen remained in pores and cracks in the reaction product, inhibiting the diffusion of the nitriding gas during the nitriding process and resulting in a large amount of phase that was essentially free of nitrogen (N). The proportion of impurity phases (rare-earth-rich phases) (2.1%) exceeded 1.0%, and the Jm, HcJ, and Hk values ​​of the magnetic powder were all lower than those of Example 1. The coercivity retention rate was also low at 81%.

[0145] Example 17 shows the results of a nitriding treatment performed once after reducing the pressure to 80 kPa absolute and then returning it to atmospheric pressure with nitrogen gas. Because the atmosphere (80 kPa) of the crushed material was not sufficiently reduced in pressure, hydrogen remained in the pores and cracks in the reaction product, as in Example 16, inhibiting the diffusion of the nitriding gas during the nitriding treatment and resulting in a large amount of phase that was essentially free of nitrogen (N). As a result, the proportion of impurity phases (rare earth-rich phases) (2.6%) exceeded 1.0%, and the Jm, HcJ, and Hk of the magnetic powder were all lower than in Example 1. The coercivity retention rate was also low at 78%.

[0146] Examples 5 and 6 show the results of production using a mixture containing samarium oxide, rare earth iron alloy powder, and metallic calcium as the raw material, with reduction diffusion treatment temperatures of 830°C and 930°C, respectively. Jm was 1.13 to 1.20 T, and the coercive force retention rates were 91% and 93%, respectively.

[0147] In contrast, Example 18 used a mixture of the same composition and was subjected to reduction and diffusion treatment at a temperature (790°C) below 800°C. Jm was low at 0.98T, and the coercive force retention rate (79%) was lower than in Examples 5 and 6.

Claims

[Claim 1] It contains at least rare earth metals (R), transition metals (TM) and nitrogen (N) as main components, and ２ Zn １７ Type, Th ２ Ni １７ Type and TbCu ７ A rare earth transition metal nitrogen-based magnetic powder having any one of the following crystal structures: The rare earth transition metal nitrogen-based magnetic powder is a samarium iron nitrogen-based magnetic powder in which 70 atomic % or more of the rare earth metal is samarium (Sm) and 90 atomic % or more of the transition metal is iron (Fe), The rare earth transition metal nitrogen-based magnetic powder has an average atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) of 0.107 or more and 0.121 or less, and the area ratio of rare earth-rich phases having an R / TM ratio of more than 0.130 is 1.0% or less, A rare earth transition metal nitrogen-based magnetic powder having a retention rate [HcJ(1000) / HcJ(0)], which is the ratio of the coercive force after heating (HcJ(1000)) to the coercive force before heating (HcJ(0)), of 83% or more when heated at 150°C for 1000 hours in air.

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