Rare earth-transition metal-nitrogen-based magnetic powder

Abstract

The present invention provides a rare earth-transition metal-nitrogen-based magnetic powder which has excellent heat resistance and stably maintains high maximum magnetic polarization. This rare earth-transition metal-nitrogen-based magnetic powder contains at least a rare earth metal (R), a transition metal (TM) and nitrogen (N) as main components, and has a crystal structure type which is a Th2Zn17 type, a Th2Ni17 type, or a TbCu7 type. The rare earth-transition metal-nitrogen-based magnetic powder exhibits: an atomic ratio (R / TM ratio) average value of the rare earth metal (R) to the transition metal (TM) of 0.107-0.121, inclusive; a rare earth-rich phase area ratio, in which the R / TM ratio is greater than 0.130, of 1.0% or less; and a retention ratio [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 heating in a 150°C atmosphere for 1000 hours, of at least 83%.

Description

Rare earth, transition metal, nitrogen-based magnetic powder

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

[0002] Rare earth transition metal nitrogen-based magnetic powder is an alloy powder mainly containing rare earth metals, transition metals, and nitrogen. Rare earth transition metal nitrogen-based magnetic powder is widely used as a permanent magnet material. For example, Sm ２ Fe １７ N ３ The alloy powders based on this system have high saturation magnetization, anisotropic magnetic field, and Curie temperature, making them useful as materials for permanent magnets, particularly 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 poured into 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 Documents 1 and 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, which involves reducing and nitriding raw material powder containing rare earth oxides to obtain an alloy block, immersing the alloy block in water to disintegrate it, and then washing with water and acid treatment, characterized in that the acid treatment consists of treatment with a weak acid and then treatment with an acid containing a Group 5B element and oxygen (claim 1 of Patent Document 4).

[0009] Japanese Patent Laid-Open No. 05-148517 Japanese Patent Laid-Open No. 2007-119909 Japanese Patent Laid-Open No. 2002-124406 Japanese Patent Laid-Open No. 2000-038608

[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 by these methods have variations in maximum magnetic polarization. As a result, 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] In view of these problems, the present inventors have conducted extensive research. As a result, they have discovered a novel ferroelectric material containing at least rare earth metals (R), transition metals (TM) and nitrogen (N) as main components, and containing Th ２ Zn １７ Type, Th ２ Ni １７ Type and TbCu ７ We have discovered that in rare earth transition metal nitrogen-based magnetic powders having either of these crystal structures, the proportion of the rare earth-rich phase is important, and that by controlling this proportion, 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.

[0015] The present invention encompasses the following aspects (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 aspects 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 containing Th２ Zn １７ Type, Th ２ Ni １７ Type and TbCu ７ a method for producing a rare earth transition metal nitrogen-based magnetic powder having any one of the crystal structures of the above types, the method comprising the following steps: 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 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 subjecting the crushed product to a nitriding treatment to produce a nitride (nitriding step); and a step of subjecting the nitride to a wet treatment to produce a wet treated product (wet treatment step), wherein an average atomic ratio (R / TM ratio) of the rare earth metal (R) to the transition metal (TM) contained in the raw material mixture is 0.117 or higher and 0.160 or lower, 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 hydrogen-free nitriding gas is carried out at least once, and then nitriding 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 a nitride, the wet treatment step includes a sub-step of adding the nitride particles to water and disintegrating them 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 an aqueous phosphoric acid solution to the slurry containing the nitride particles after the second water washing step to form a phosphorus-containing protective coating 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), wherein the rare earth transition metal nitrogen-based magnetic powder has an average atomic ratio of rare earth metal (R) to transition metal (TM) (R / TM ratio) 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 in the surface treatment step, the transition metal (TM) concentration in the tap water of the slurry containing nitride particles just before the phosphoric acid aqueous solution is added 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), 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) The method according to any one of (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 material containing at least rare earth metals (R), transition metals (TM) and nitrogen (N) as main components, and Th 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, 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, and the 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, the 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)), is 83% or more.

[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.

[0024] 1 shows an SEM image (backscattered electron image) of magnetic powder (Example 1). 2 shows an SEM image (backscattered electron image) of magnetic powder (Example 7). 3 shows the results of thermogravimetry (TG) of magnetic powder (Examples 1 and 8).

[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. Method for producing rare earth transition metal nitrogen-based magnetic powder>> This embodiment relates to a method for producing 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 ２ Zn １７ Type, Th ２ Ni １７ Type and TbCu ７ The magnetic powder has a crystal structure of either one of the following types: The magnetic powder has an average atomic ratio of rare earth metal (R) to transition metal (TM) (R / TM ratio) of 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 a sub-process (first water washing process) of preparing a slurry containing nitride particles by adding nitride particles to water and disintegrating them, followed by a sub-process (pickling process) of subjecting the slurry containing nitride particles after the first water washing process to an acid washing process, a sub-process (second water washing process) of subjecting the slurry containing nitride particles after the acid washing process to an acid washing process, a sub-process (surface treatment process) of adding an aqueous phosphoric acid solution to the slurry containing nitride particles after the second water washing process to form a phosphorus-containing protective coating on the surfaces of the nitride particles, and a sub-process (third water washing process) of subjecting the slurry containing nitride particles after the surface treatment process to an acid washing process to obtain a wet-treated product. Each process will be described in detail below.

[0029] <Reduction-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 to form an alloy. 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] [In the case of 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 a raw material for 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 (Sm ２ Fe １７ N ３ ) based magnetic powder can be produced. ２ Fe １７ N ３ The Sm-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 metal may be Sm, and 30 atomic % or less of other elements (La, Ce, etc.).

[0032] The particle size of the rare earth oxide powder may 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 particle sizes of 0.1 to 10 μm account for 80 mass% or more of the total is preferred. This improves raw material mixability and improves the handleability of the raw material powder and reaction products. 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 that the amount of impurities contained in the rare earth oxide powder is small. 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 (Sm ２ Fe １７ N ３ This makes it possible to produce a ferrous-based magnetic powder. In this case, Fe may be used in combination with one or more transition metals other 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, and 10 atomic % or less may be 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 (such as Fe), a metal oxide powder (such as Fe), or a mixture thereof. ２ O ３ , Fe ３ O ４ , FeO, etc.), rare earth transition metal alloy powder (R ２ Fe １７ etc.), rare earth transition metal composite oxide powder (RFeO ３ However, if oxide powder is used, a thermite reaction occurs during the reduction-diffusion treatment. In order to prevent the rapid heat generation caused by this reaction, it is preferable to keep the proportion of oxide powder 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] [Cases other than alloy raw material-powder mixture] 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 is 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 hydrides thereof. Specifically, one or more selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and hydrides thereof are preferred. From the viewpoints of safety during handling and cost, Li and / or Ca are more preferred, and Ca is 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 more (coarser than a 100 mesh sieve), more preferably 32 mesh or more. On the other hand, if the reducing agent is finer, it is possible to further improve the dispersibility in the raw material mixture. The particle size of the reducing agent is preferably 4 mesh or less (finer than a 4 mesh sieve), more preferably 9 mesh or less.

[0042] [Other Components] 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. Examples of such components include zinc (Zn), boron (B), aluminum (Al), gallium (Ga), indium (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and / or bismuth (Bi).

[0043] [Mixing] 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. Therefore, 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 reduce 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. Therefore, the magnetic properties of the magnetic powder, such as remanence and squareness, may be reduced. The average R / TM ratio is more preferably 0.123 or more and 0.135 or less. The average value of the 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 necessary to reduce the oxides contained in the raw material mixture. The minimum amount necessary 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 components of the reducing agent 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 a 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 reaction vessel, 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 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 are preferably determined so that the rare earth metals diffuse to the center 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 reaction vessel, the vessel 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] <Hydrogen Crushing Step> In the hydrogen crushing step, the reduction-diffusion treated product obtained in the reduction-diffusion step is crushed by exposing it to a hydrogen atmosphere, thereby producing a crushed product. The reaction product obtained by the reduction-diffusion treatment (reduction-diffusion treated product) contains a rare earth-rich phase containing an excess of rare earth elements. For example, when the rare earth metal is Sm, SmFe ５ Phase, SmFe ３ phase and / or SmFe ２ The reduction-diffusion-treated product contains Sm-rich phases such as rare earth phases. When the reduction-diffusion-treated product is exposed to a hydrogen atmosphere, these rare earth-rich phases absorb hydrogen and expand in volume. This causes cracks to form 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 step> 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 atmosphere surrounding the crushed material is depressurized to a pressure of 50 kPa or less, and then the pressure is restored using a hydrogen-free pretreatment gas. The reaction product (crushed material) after the hydro-crushing step 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, thereby enabling 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. Furthermore, the pretreatment process is carried out at least once, preferably twice or more.

[0052] After the pretreatment is completed, 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 nitriding gas supplied here may or may not contain hydrogen. Nitrogen-containing gases such as nitrogen and / or ammonia are preferably used as the nitriding gas. Furthermore, to control the nitriding reaction rate, other gases such as hydrogen, argon, and helium may be added to the nitrogen-containing gas (nitriding gas). For example, a mixed gas containing ammonia and hydrogen can sufficiently nitride the interior of 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. By setting the ammonia partial pressure to 0.20 or more, the nitriding of the alloy particles is further promoted. 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 or higher and 500°C or lower, and more preferably 400°C or higher and 480°C or lower. By setting the heating temperature at 350°C or higher, nitrogen can be rapidly diffused into the interior of the alloy particles. On the other hand, excessively high heating temperatures may cause the alloy to decompose. For example, if the transition metal is Fe, the alloy may decompose and produce 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 suppress the formation of such heterophases. Furthermore, the optimal heating time varies depending on various conditions, such as the heating temperature and processing volume. Therefore, it is difficult to uniquely determine the 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 or higher and 5.0% by mass or lower.

[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 Treatment Step> In the wet treatment step, a nitride is subjected to wet treatment to produce a wet-treated product. The nitride obtained in the nitriding step is usually a porous mass. More specifically, it is an aggregate formed by the bonding of nitride particles, which constitute the main phase. The nitride particles refer to particles of a rare earth, transition metal, and nitrogen-based compound. In addition to the nitride particles, the nitride also contains by-products (e.g., CaO) derived from the reducing agent and heterophases (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 heterophases other than the main phase and provide a heat-resistant protective coating on the surface of the nitride particles. The wet treatment step includes sub-steps: a first water washing step, an acid washing step, a second water washing step, a surface treatment step, and a third water washing step.

[0056] [First Water Washing Step] In the wet treatment step, first, the nitride is introduced into water and disintegrated to prepare a slurry containing nitride particles, and then the slurry containing the nitride particles is subjected to a water washing treatment (first water washing step). The slurry is prepared by introducing the nitride into water. By introducing the nitride 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, nitride particles and calcium hydroxide fine particles, are dispersed.

[0057] 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 nitrides. At this time, water may be poured into the slurry as needed. By-products (such as CaO) derived from the reducing agent contained in the nitrides are converted into hydroxides (Ca(OH) ２ The hydroxides have a significantly different specific gravity from the nitride particles. Therefore, by performing decantation, the hydroxides derived from the reducing agent can be removed along with the clean water from the slurry. From the viewpoint of obtaining nitride particles with high purity, it is preferable to repeat the series of operations. Note that clean water refers to the liquid above the slurry that is obtained after the slurry is left to stand and the nitride particles with a high specific gravity settle. Clean water can also be called the supernatant liquid. However, clean water (supernatant liquid) may contain components with a lower specific gravity than the nitride particles, such as hydroxides. Therefore, clean water is not necessarily transparent.

[0058] Conditions such as the slurry concentration, temperature, or number of decantations in the first water washing step 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 (e.g., Ca) 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. Maintaining a low concentration of the reducing agent component in the slurry allows for the consistent production of magnetic powder with high purity and excellent magnetic properties.

[0059] [Pickling Process] Next, the slurry containing the nitride particles obtained in the first water washing process is subjected to an acid washing treatment (pickling process). ２Even if rare earth-rich phases or other phases (rare earth-rich phases, etc.) remain in the nitride particles, they can be removed by acid washing.

[0060] In the acid washing, acid is added to a slurry containing nitride particles. At this time, 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. As the acid, a by-product (Ca(OH) ２ There are no particular limitations on the acid, as long as it can dissolve and remove the rare earth-rich phase and other phases (rare earth-rich phase, etc.). However, at least one 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 0.107 or more and 0.121 or less. Adjusting the amount of acid added makes it possible to produce 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 performed too strongly. This may result in strong etching marks on the particle surfaces constituting the powder, which may reduce magnetic properties 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 effect of the acid washing cannot be fully achieved. Residual by-products and heterogeneous phases may reduce 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 homogeneity 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 Water Washing Step] Next, the slurry containing the nitride particles after the pickling step is subjected to a water washing treatment (second water washing step). In the pickling step, the nitride particles in the slurry are washed with an acid. At this time, the by-product (Ca(OH) ２ In some cases, ions of reducing agent components (e.g., Ca ions), rare earth ions (R ions), and transition metal ions (TM ions) are generated in the slurry. These generated ions are diluted and removed by washing with water.

[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 the addition of 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 aqueous 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). The provision of the phosphorus-containing protective coating improves the heat resistance of the final magnetic powder.

[0067] In the surface treatment step, water is again poured into the slurry obtained in the second water washing step, and then an aqueous phosphoric acid solution is added to the resulting slurry while stirring it after the water pouring. By adding the aqueous phosphoric acid solution, the pH of the slurry drops from the value before the 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 term "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. Therefore, a highly uniform protective coating is formed, further improving the heat resistance of the final magnetic powder. On the other hand, 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. Therefore, the formation of the protective coating is insufficient, and the uniformity of the resulting protective coating deteriorates. From the viewpoint 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 aqueous solution added in the surface treatment step is 0.03% by mass or more and 1.0% by mass or less of the magnetic powder in terms of phosphorus (P). In other words, the amount of phosphorus (P) contained in the phosphoric acid aqueous solution added is 0.03% by mass or more and 1.0% by mass or less of the magnetic powder. By controlling the P content 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 content 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 become insufficient. If the P content exceeds 1.0% by mass, the amount of excess phosphoric acid becomes excessive, resulting in a decrease in the magnetic properties of the final magnetic powder, such as the maximum magnetic polarization. From the viewpoint of improving magnetic properties and heat resistance, the amount of phosphoric acid aqueous solution is more preferably 0.07% by mass or more and 0.8% by mass or less of the magnetic powder in terms of P.

[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 protective coating with excellent uniformity. If the slurry temperature exceeds 30°C, the reaction between the phosphoric acids and the nitride particles proceeds excessively. As a result, the thickness of the protective coating varies greatly, which may result in an insufficient improvement in heat resistance. In addition, 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 Step] Next, the slurry containing the nitride particles after the surface treatment step is subjected to a water washing step to obtain a wet-treated product (third water washing step). In the surface treatment step, particles of transition metal phosphates, rare earth phosphates, or composite salts thereof may be by-produced 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 magnetic powder obtained in the end, such as the maximum magnetic polarization, may be reduced. From the viewpoint of improving the magnetic properties, it is more preferable that the P concentration in the clean water is 50 mg / L or less.

[0075] <Drying Step> If necessary, a step (drying step) of drying the wet-treated 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-treated product is subjected to a deliquoring process to obtain a cake containing magnetic powder, and the obtained cake is dried. Deliquoring 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 deliquoring 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] <Pulverization step> If necessary, a step (pulverization step) of pulverizing the wet-treated product (nitride particles) after washing with water or drying may be provided. The pulverization method is not limited, and may be performed wet or dry. In the case of wet pulverization, pulverization is preferably performed in an organic solvent. In the case of dry pulverization, pulverization is preferably performed 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 manner, 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-based 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. Sm may also 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 Sm and 30 atomic % or less 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 may be other elements (Cr, Mn, Co, Ni, etc.).

[0080] The magnetic powder of this embodiment is Th ２ Zn １７ Type, Th ２ Ni １７Type and TbCu ７ The magnetic powder having such a crystal structure includes Sm ２ Fe １７ N ３ Powder having a basic composition represented by Sm ２ Fe １７ N ３ Sm-based powders are included. ２ Fe １７ N ３ This system powder exhibits excellent magnetic properties and is useful as a bonded magnet material. The magnetic powder of this embodiment is not limited to one having a stoichiometric composition. Deviations in composition are permissible as long as the crystal structure is maintained and the R / TM ratio requirement, described below, is 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 value of the 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. It will not be possible to suppress the inclusion of transition metal-rich phases, such as α-Fe and Fe nitrides, which are different phases. As a result, the magnetic properties of the magnetic powder, such as maximum magnetic polarization and coercive force, will be reduced. On the other hand, if the average value exceeds 0.121, the amount of rare earth metal will be excessive. SmFe ５ , SmFe ３ and / or SmFe ２ From the viewpoint of improving the magnetic properties, the average value of the R / TM ratio is preferably 0.108 or more and 0.118 or less.

[0083] The average R / TM ratio is 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. The cross section of the resulting compression-molded body is then 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 such that at least 1,000 particles 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 the rare earth-rich phase 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 phase is evaluated by the method described below.

[0085] As described above, the magnetic powder of this embodiment has an average R / TM ratio, which is a measure of the main phase composition, limited to a predetermined range. However, when viewed locally, variations in the composition may occur. In particular, magnetic powders containing rare earths and transition metals as main constituent elements may contain rare earth-rich heterophases (SmFe, ５ , SmFe ３ , SmFe ２ Such rare earth-rich phases often contain rare earth elements, etc. Since these rare earth-rich phases cause deterioration of magnetic properties, they are eliminated as much as possible in this embodiment. The area ratio of the rare earth-rich phases is preferably 0.8% or less, and 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 above-mentioned SEM observation sample is observed using an SEM to obtain a backscattered electron image. Image analysis utilizing the contrast difference is 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 with 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 as to detect only the rare earth-rich phase region (region where the R / TM atomic ratio exceeds 0.130) 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 backscattered electron image after the binarization process, 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, 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. Therefore, it has the effect of having 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 after heating (HcJ(1000)) to the coercive force before heating (HcJ(0)) 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.

[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 Powders The magnetic powders prepared 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 airflow dispersion type laser diffraction particle size distribution measuring device, and the cumulative 50% diameter in the particle size distribution on a volume basis was measured as D50.

[0092] <R / Fe atomic ratio> The composition (R / Fe ratio) of the magnetic powder was determined by scanning electron microscope (SEM) observation. 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. In this case, the amount of thermosetting resin mixed was adjusted to 4 to 5 mass% relative to the magnetic powder. Next, the cross section of the resulting compression-molded body was polished to expose it, and a sample for SEM observation was prepared.

[0093] Next, the polished cross section of the SEM observation sample was observed using a scanning electron microscope (SEM; JEOL Ltd., JST-IT200). The observation was performed at a magnification of 1000x, and three fields of view, each approximately 100 μm square, were observed. The number of particles contained in each field of view was 5000 or more. The entire field of view was then subjected to EDS analysis using the standardless FP method to determine the R / Fe atomic ratio. The analysis was performed at an acceleration voltage of 25 kV.

[0094] <Area Percentage of Rare Earth-Rich Phase> The area percentage of the rare earth-rich phase was determined by SEM observation. Specifically, the polished cross section of the SEM observation sample prepared for measuring the R / Fe atomic ratio was observed using SEM. The observation was performed at a magnification of 1000x, and backscattered electron images were taken of a field of view approximately 100 μm square. The number of particles contained in one 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 binarized. The area s of the rare earth-rich phase was then determined from the backscattered electron image after binarization. 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 binarized. The area S of all particles was then determined from the backscattered electron image after binarization. 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 in accordance with the Bonded Magnetic Industry Association, Bonded Magnetic Testing Method Guidebook, BM-2002 and BM-2005, by preparing a sample in which the magnetic powder was oriented in stearic acid by applying an orienting magnetic field of 1600 kA / m, and then magnetizing it in a magnetic field of 4.0 MA / m, after which the specific gravity of the magnetic powder was measured to be 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 the point in the second quadrant of the magnetization curve where the magnetic polarization J has decreased to 90% of the remanent magnetic polarization Jr.

[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 the coercive force after heating (HcJ(1000)) were measured. The coercive force was measured using the same method as in the measurement of the 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] <Thermogravimetric measurement> Thermogravimetric measurement (TG) of the magnetic powder was carried out. The measurement was carried out in dry air (N ２ -21% O ２ The sample was heated at a temperature increase rate of 20° C. / min while flowing HCl at a flow rate of 100 mL / min.

[0098] (2) Preparation of magnetic powder and evaluation results [Example 1] <Reduction diffusion process> Average particle size D ５０ 624 g of samarium oxide powder having a diameter of 2.3 μm, ５０ 1553 g of iron powder having 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] <Hydrogen Decomposition Step> After cooling, the vessel was evacuated, and then hydrogen gas was supplied while adjusting the flow rate to maintain a gauge pressure of +20 kPa, to decompose the obtained reaction product by a hydrogen absorption reaction.

[0100] <Nitriding step> The hydrogen-treated reaction product (crushed material) was placed in a tubular furnace. The process of reducing the pressure to 10 kPa absolute and then returning it to atmospheric pressure with nitrogen gas was repeated twice. Next, the temperature was increased while a mixed gas of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min) was flowed, and the material was heat-treated at 430°C for 9 hours. The material was further heat-treated while flowing hydrogen gas (flow rate: 1000 mL / min) for 1 hour, and then while flowing nitrogen gas (flow rate: 1000 mL / min) for another 1 hour.

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

[0102] Next, 4 L of water was added again for surface treatment and stirred. The Fe concentration of the clean water was 0.1 g / L, and the slurry temperature was 20°C. While stirring the slurry, a 25% by mass aqueous solution of orthophosphoric acid containing P amounting to 0.31% by 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, and then rose again to 7, at which point stirring was stopped and the slurry was decanted.

[0103] Subsequently, 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 step> After the final decantation following 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 Sm composition was 23.1 mass %, and the P composition was 0.15 mass %.

[0105] <Pulverization step> 900 g of the obtained samarium-iron-nitrogen alloy powder was pulverized in a media agitation mill. 2-propanol containing an 85% aqueous solution of 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. ２ Zn 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, Hk (squareness): 0.52 MA / m. The P composition of the magnetic powder was 0.52 mass%.

[0107] A compression molded body was produced 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 1 to 3. Sm and Fe were analyzed over the entire 100 μm square area using EDS to calculate the R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio). 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] On the other hand, in each figure, 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), and 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 proportions were 0.5%, 0.4%, and 0.5%, and the average (area proportion of the rare earth-rich phase) was 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 to 4] The manufacturing conditions were changed as shown in Table 1 below. Other than that, magnetic powders were manufactured in the same manner as in Example 1. The evaluation results of the obtained magnetic powders are shown in Table 2 below. ２ Zn １７ It had the type crystal structure.

[0111] [Example 5] D obtained by reduction diffusion method ５０ Sm of 30 μm ２ Fe 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 obtained slurry was heated to 210° C. while reducing the pressure in a mixer, and 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, the slurry was cooled.

[0112] The resulting ground mixture was 2.00 kg (Sm ２ Fe １７ To a mixture (consisting 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 thus obtained was 0.160.

[0113] This raw material mixture was placed in an iron crucible and heated under an argon gas atmosphere in a reduction-diffusion process. The mixture was then held at 830°C for 2 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. In 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 temperature rise, the exhaust valve of the tubular furnace was closed while the furnace was under negative pressure, and the hydrogen supply was continued. Cooling was allowed to continue after the heat generation and hydrogen absorption had ceased (hydrogen crushing process). After the hydrogen treatment, the reaction product (crushed product) was in a crushed state. Next, for nitriding, the pressure inside the tubular furnace was reduced to 5 kPa absolute, and then nitrogen gas was introduced to atmospheric pressure, and the temperature was increased with the nitrogen gas flow rate at 1 L / min, and the mixture was maintained at 450°C for 24 hours and then cooled. This yielded a nitriding reaction product (nitride) (nitriding step).

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

[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, which accounted for 1.0 mass % of the magnetic alloy powder mass, was added (surface treatment step). 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 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 step). At this time, 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. ２ Zn １７ It has a D type crystal structure. ５０ 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 content 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 obtained magnetic powder, and the average value was 0.121. Furthermore, the average percentage of regions where the R / Fe atomic ratio exceeded 0.13 (area percentage of rare earth-rich phases) determined by EDS analysis 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 in the same manner as in Example 5. The evaluation results of the obtained magnetic powder are shown in Table 2 below. ２ Zn 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 the reduction diffusion step and 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 the temperature was raised to 430° C. for 9 hours while a mixed gas of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min) was passed through it. The heat treatment was further continued for 1 hour while a hydrogen gas (flow rate: 1000 mL / min) was passed through it, and for another 1 hour while a nitrogen gas (flow rate: 1000 mL / min) was passed through it, 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. ２ Zn １７ It has a D type crystal structure. ５０ The magnetic properties were Jm: 1.41 T, HcJ: 0.86 MA / m, and Hk: 0.39 MA / m. The P content was 0.49 mass %.

[0124] A compression molded body was produced 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 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 the 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. ２ Zn １７ It has a D type crystal structure. ５０ The magnetic properties were Jm: 1.44 T, HcJ: 0.93 MA / m, and Hk: 0.55 MA / m. The P content was 0.43 mass %.

[0128] [Examples 9 to 15] The manufacturing conditions were changed as shown in Table 1 below. Other than that, magnetic powders were manufactured in the same manner as in Example 1. The evaluation results of the obtained magnetic powders are shown in Table 2 below. ２ Zn １７ It had the type crystal structure.

[0129] [Example 16] In the nitriding step, the hydrogen-treated reaction product was placed in a tubular furnace, and then the pressure was reduced to 10 kPa absolute, followed by a 4:1 mixture of ammonia gas and hydrogen gas. This process was repeated twice. Next, the temperature was increased while a mixture of ammonia gas (flow rate: 280 mL / min) and hydrogen gas (flow rate: 90 mL / min) was flowed, and the mixture was heat-treated at 430°C for 9 hours. A 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. ２ Zn １７ It had the type crystal structure.

[0131] [Example 17] In the nitriding step, 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 returning 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 flowed, and the mixture was heat-treated at 430°C for 9 hours. A magnetic powder was produced using the same procedure as in Example 1, except for the above.

[0132] The evaluation results of the obtained magnetic powder are shown in Table 2 below. ２ Zn 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 in the same manner as in Example 5. The evaluation results of the obtained magnetic powder are shown in Table 2 below. ２ Zn １７ It had the type crystal structure.

[0134]

[0135]

[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, ranging from 1.33 to 1.42 T. Furthermore, these magnetic powders had a coercive force retention ratio [HcJ(1000) / HcJ(0)], an indicator of heat resistance, of 83 to 88%.

[0137] In contrast, in Example 7, in which the order of the nitriding step and the wet treatment step 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 (a samarium-iron-nitrogen alloy with a particle size of approximately 20 μm) oxidized and increased in mass when heated in air. The temperature at which the oxidized mass increase due to heating was 1% was 410°C for Example 8, while it was 570°C for Example 1, which had a protective coating. It is presumed that this difference in heat resistance (oxidation resistance) of the alloy powder before pulverization led to a difference in the uniformity of the protective coating on the magnetic alloy powder pulverized while undergoing the surface treatment described in 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, 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, 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 phase (rare earth-rich phase) (4.9%) exceeded 1.0%. As a result, Jm was low at 1.17 T and 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 step exceeded 1200°C. The high-temperature treatment sintered the particles, and sintered particles were observed in the magnetic powder obtained after the drying step. As a result, the magnetic powder obtained after the crushing step had a low Jm of 1.25 T, a low Hk of 0.33 MA / m, and a low coercivity retention rate of 79%.

[0142] Example 12 shows the results when the slurry temperature in the fourth acid washing step was high and the R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step was 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 the subphase during the acid washing and an R / Fe atomic ratio (0.122) greater than 0.121. In the former, the R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step was below 0.107 due to excessive acid washing. In addition, the HcJ was low at 0.76 MA / m. On the other hand, in the latter, the R / Fe atomic ratio (0.122) of the magnetic powder exceeded 0.121, and the proportion of impurity phases (rare earth-rich phases) (1.8%) exceeded 1.0%. Therefore, the Jm of the magnetic powder was low at 1.21 T. Furthermore, the retention rates of the coercive force were 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 process (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, the Jm was 1.28 T, which was not particularly low. However, the coercive force retention rate was low at 72%. On the other hand, in the latter, the Jm of the magnetic powder was low at 1.19 T due to the addition of an excessive amount of phosphoric acid aqueous solution.

[0144] Example 16 shows the results of a nitriding process in which the reaction product, which had been hydrogen-treated in the nitriding step, was placed in a tubular furnace, depressurized to 10 kPa absolute, and then repressurized to atmospheric pressure with a 4:1 mixture of ammonia gas and hydrogen gas. This process was repeated twice. 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 phase (rare earth-rich phase) (2.6%) exceeded 1.0%, and the Jm, HcJ, and Hk of the magnetic powder were all lower than those of Example 1. The coercivity retention rate was also low at 78%.

[0146] Examples 5 and 6 show the results of fabricating magnets using a mixture containing samarium oxide, rare earth iron alloy powder, and metallic calcium as the raw material at reduction and diffusion treatment temperatures of 830° C. and 930° C., respectively. The 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 a reduction diffusion treatment at a temperature (790° C.) below 800° C. Jm was low at 0.98 T, and the coercive force retention rate (79%) was lower than those of Examples 5 and 6.

Claims

1. 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, 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, and the 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, the magnetic powder has a retention factor [HcJ(1000) / HcJ(0)] of 83% or more, which is the ratio of the coercive force after heating (HcJ(1000)) to the coercive force before heating (HcJ(0)).

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