Rare earth transition metal nitrogen-based magnetic powder

Abstract

This invention provides a rare-earth transition metal nitrogen-based magnetic powder that stably exhibits high maximum magnetic polarization and excellent heat resistance. The rare-earth transition metal nitrogen-based magnetic powder comprises at least rare-earth metal (R), transition metal (TM), and nitrogen (N) as main components, and has any one of the following crystal structures: Th2Zn17, Th2Ni17, and TbCu7. Furthermore, the average atomic ratio (R / TM ratio) of rare-earth metal (R) to transition metal (TM) in the rare-earth transition metal nitrogen-based magnetic powder is 0.107 to 0.121, the area ratio of rare-earth-rich phases with an R / TM ratio exceeding 0.130 is 1.0% or less, and when heated at 150°C for 1000 hours in atmosphere, the retention rate [HcJ(1000) / HcJ(0)] of the coercive force after heating (HcJ(1000)) to the coercive force before heating (HcJ(0)) is 83% or more.

Description

[Technical Field]

[0001] This invention relates to a rare earth transition metal nitrogen-based magnetic powder. [Previous Technology]

[0002] Rare earth transition metal nitrogen-based magnetic powders mainly consist of alloy powders of rare earth metals, transition metals, and nitrogen. Rare earth transition metal nitrogen-based magnetic powders are often used as permanent magnet materials. For example, Sm2Fe17N3 alloy powders have high saturation magnetization, anisotropic magnetic fields, and Curie temperatures. Therefore, they can be used as materials for permanent magnets with excellent properties, especially bonded magnets.

[0003] Rare earth transition metal nitrogen-based magnetic powders are typically manufactured by nitriding rare earth transition metal alloy powders. Previously known methods for manufacturing rare earth transition metal alloy powders include melt casting and reduction diffusion methods. The melt casting method involves using rare earth metals and transition metals as raw materials, conditioning these raw materials, melting them in an inert gas environment, heat-treating the resulting alloy ingot to homogenize it, and then pulverizing it.

[0004] On the other hand, the reduction-diffusion method is, for example, as follows: using rare earth oxides and transition metals as raw materials, these raw materials are mixed with a reducing agent such as metallic calcium and then heated in a non-oxidizing gas environment to obtain a rare earth transition metal alloy. During the heating treatment, the rare earth oxides are reduced to rare earth metals, which diffuse into the transition metals to form an alloy (intermetallic compound). The blocky reaction product obtained by heating treatment contains the target alloy and byproducts from the reducing agent. Therefore, the reaction product is added to water to remove the byproducts from the reducing agent, and the reaction product is broken down and pulverized. The alloy powder obtained by pulverization is subjected to acid washing and water washing to remove the remaining byproducts or unreacted substances, and then dried to obtain the target alloy powder.

[0005] The reduction-diffusion method has the following advantages: it can use inexpensive rare earth oxides and the like as raw materials, and the process is simple, enabling the production of alloy powders at a lower cost compared to the melting and casting method. Furthermore, by nitriding the alloy powder obtained using the reduction-diffusion method, rare earth transition metal alloy powders as nitrides can be obtained.

[0006] The master alloy supplied for nitriding is in powder form. In order to achieve uniform nitriding, it is desirable for the master alloy powder to have a steep particle size distribution. The reduction diffusion method can easily obtain a steep particle size distribution, and therefore it has become the mainstream method for manufacturing rare earth transition metal nitrogen-based magnetic powders.

[0007] Examples of documents disclosing the manufacture of rare earth transition metal nitrogen-based magnetic powders using a reduction diffusion method include Patent Document 1 and Patent Document 2. These documents disclose the process of mixing rare earth oxide powder, transition metal powder, and alkali metal reducing agents, heating the resulting mixture in an inactive environment, maintaining the resulting reaction product in a nitrogen environment, and performing wet treatment on the resulting heat-treated product to manufacture rare earth transition metal nitrogen-based alloy powders (claim 1 of Patent Document 1 and claim 1 of Patent Document 2).

[0008] Furthermore, it is also known that a phosphoric acid protective film is formed on the surface of magnetic powder particles to improve heat resistance and moisture resistance. For example, Patent Document 3 discloses the addition of 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 manufacturing rare earth iron-nitrogen-based magnetic powder, in which a raw material powder containing rare earth oxides is subjected to reduction diffusion and nitriding to obtain an alloy block, the alloy block is immersed in water to disintegrate, and then washed with water and acid treated. The manufacturing method is characterized in that the acid treatment consists of "treatment using a weak acid" and "subsequent treatment using an acid composed of group 5B elements and oxygen" (claim 1 of Patent Document 4). [Prior Art Documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. Hei 05-148517 [Patent Document 2] Japanese Patent Application Publication No. 2007-119909 [Patent Document 3] Japanese Patent Application Publication No. 2002-124406 [Patent Document 4] Japanese Patent Application Publication No. 2000-038608 [Summary of the Invention]

[0010] [Problem to be Solved by the Invention] Thus, it has been proposed to use the reduction diffusion method to manufacture rare earth transition metal nitrogen-based magnetic powders, or to form a phosphate protective film on the surface of magnetic powder particles to improve moisture resistance. However, there is room for improvement in the prior art.

[0011] For example, although Patent Document 1 or 2 discloses a method for manufacturing rare earth transition metal nitrogen-based magnetic powder, the maximum magnetic polarization in the magnetic powder obtained by this method is uneven. Therefore, there is a problem that the high maximum magnetic polarization originally possessed by the powder cannot be stably obtained. Furthermore, although Patent Document 3 or 4 discloses a method for forming a phosphoric acid protective coating, there is a problem that the uniformity of the obtained protective coating is poor, making it difficult to fully obtain the effect of improving heat resistance.

[0012] In view of this problem, the inventors have devoted themselves to research. As a result, the following insights have been obtained: In rare earth transition metal nitrogen-based magnetic powders that contain at least rare earth metals (R), transition metals (TM), and nitrogen (N) as main components and have any one of the crystal structures of Th2Zn17, Th2Ni17, and TbCu7, the ratio of rare earth-rich phases is very important. By controlling it, magnetic powders with stable high maximum magnetic polarization and excellent heat resistance can be obtained.

[0013] Furthermore, the following insights have been obtained: When manufacturing magnetic powder, the conditions of the reduction diffusion step, the nitriding step, and the wet processing step are particularly important. By controlling these conditions, the above-mentioned rare earth transition metal nitrogen-based magnetic powders can be easily manufactured.

[0014] This invention is based on this understanding, and its purpose is to provide a rare-earth transition metal nitrogen-based magnetic powder with stable high maximum magnetic polarization and excellent heat resistance, as well as a method for manufacturing the same. [Technical Means for Solving the Problem]

[0015] The present invention includes the states described in (1) to (7) below. Furthermore, in this specification, the expression "~" includes the values ​​at both ends. That is, "X~Y" is synonymous with "X or more and Y or less". Also, in this specification, any combination of preferred states may be adopted as long as technical integration is sought. For example, one of the preferred numerical ranges may be arbitrarily combined with the other.

[0016] (1) A method for manufacturing a rare earth transition metal nitrogen-based magnetic powder, wherein the rare earth transition metal nitrogen-based magnetic powder comprises at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as main components, and has any one of the crystal structures of Th2Zn17, Th2Ni17, and TbCu7, and the method comprises the following steps: heating a mixture of an alloy raw material containing at least a rare earth metal (R), a transition metal (TM), and oxygen (O) and a reducing agent in a reduced pressure gas or inactive gas environment to a temperature of 800°C to 1200°C to produce a reduction diffusion treated material (reduction diffusion step); exposing the reduction diffusion treated material to a hydrogen environment to break it down, thereby producing a broken material (hydrogen decomposition step). The process includes a step of nitriding the above-mentioned fragmented material to produce a nitride (nitriding step); and a step of wet processing the above-mentioned nitride to produce a wet-processed product (wet processing step); wherein the average atomic ratio (R / TM ratio) of the rare earth metal (R) to the transition metal (TM) contained in the above-mentioned raw material mixture is 0.117 or more and 0.160 or less; During the above-mentioned nitriding process, at least one procedure is performed of "reducing the pressure of the surrounding environment of the above-mentioned fragmented material to 50 kPa or less, and then restoring the pressure using a hydrogen-free nitriding gas", and then supplying a hydrogen-containing nitriding gas to the surrounding environment, and heating the above-mentioned fragmented material in the supplied nitriding gas environment to convert it into a nitride; The aforementioned wet processing steps include: a sub-step of immersing the aforementioned nitride in water and causing it to disintegrate to prepare a slurry containing nitride particles, followed by a water washing treatment of the aforementioned slurry containing nitride particles (first water washing step); a sub-step of acid washing treatment of the aforementioned slurry containing nitride particles after the first water washing step (acid washing step); a sub-step of water washing treatment of the aforementioned slurry containing nitride particles after the acid washing step (second water washing step); a sub-step of adding an aqueous phosphoric acid solution to the aforementioned slurry containing nitride particles after the second water washing step to form a phosphorus-containing protective film on the surface of the aforementioned nitride particles (surface treatment step); and a sub-step of water washing treatment of the aforementioned slurry containing nitride particles after the surface treatment step to obtain the aforementioned wet-processed product (third water washing step); The average value of the atomic ratio (R / TM ratio) of the rare earth metal (R) to the transition metal (TM) of the aforementioned rare earth transition metal nitrogen-based magnetic powder is 0.107 or more and 0.121 or less.

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

[0018] (3)As described in (1) or (2) above, wherein, in the above surface treatment step, the concentration of transition metal (TM) in the clear water on the slurry containing nitride particles before being immersed in the phosphoric acid aqueous solution is less than 1.0 g / L.

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

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

[0021] (6)The method of any one of (1) to (5) above, wherein in the third water washing step above, water washing treatment is performed until the phosphorus (P) concentration of the clear water above the slurry is below 100 mg / L.

[0022] (7) A rare earth transition metal nitrogen-based magnetic powder, comprising at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as main components, having any one of the crystal structures of Th2Zn17, Th2Ni17, and TbCu7, wherein the average atomic ratio (R / TM ratio) of the rare earth metal (R) to the transition metal (TM) in the above-mentioned rare earth transition metal nitrogen-based magnetic powder is 0.107 to 0.121, the area ratio of rare earth-rich phases with an R / TM ratio exceeding 0.130 is 1.0% or less, and when heated at 150°C for 1000 hours in the atmosphere, the ratio of the coercivity (HcJ(1000)) after heating to the coercivity (HcJ(0)) before heating, i.e., the retention rate of the coercivity [HcJ(1000) / HcJ(0)], is 83% or more. [Effects of the Invention]

[0023] According to the present invention, a rare earth transition metal nitrogen-based magnetic powder with stable high maximum magnetic polarization and excellent heat resistance is provided, and a method for manufacturing the same is provided.

Implementation Method

[0025] Hereinafter, specific embodiments of the present invention (hereinafter referred to as "this embodiment") will be described. However, the present invention is not limited to the following embodiments, and various modifications can be made without changing the spirit of the present invention.

[0026] <<1. Method for Manufacturing Rare Earth Transition Metal Nitrogen-Based Magnetic Powder>> This embodiment relates to a method for manufacturing a rare earth transition metal nitrogen-based magnetic powder (hereinafter sometimes simply referred to as "magnetic powder"). The magnetic powder contains at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as main components, and has any one of the following crystal structures: Th2Zn17 type, Th2Ni17 type, and TbCu7 type. Furthermore, the average value of the atomic ratio (R / TM ratio) of the rare earth metal (R) to the transition metal (TM) in the magnetic powder is 0.107 or more and 0.121 or less.

[0027] The manufacturing method of this embodiment includes the following steps: heating a mixture of alloy raw materials containing at least rare earth metal (R), transition metal (TM), and oxygen (O), and a reducing agent in a reduced pressure gas or inactive gas environment to a temperature of 800°C to 1200°C to produce a reduction-diffusion treated product (reduction-diffusion step); exposing the reduction-diffusion treated product to a hydrogen environment to break it down, thereby producing a broken product (hydrogen breaking down step); performing a nitriding treatment on the broken product to produce a nitride (nitriding step); and performing a wet treatment on the nitride to produce a wet treated product (wet treatment step). Here, the average value of the 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 to 0.160 or less. Furthermore, during the nitriding process, at least one procedure is performed to "depressurize the surrounding environment of the fragmented material to below 50 kPa and then restore the pressure using a hydrogen-free nitriding gas". Afterward, a nitriding gas containing hydrogen is supplied to the surrounding environment, and the fragmented material is heated in the supplied nitriding gas environment to turn it into a nitride.

[0028] Furthermore, the wet treatment step includes: a sub-step of adding nitride to water and causing it to disintegrate to prepare a slurry containing nitride particles, followed by a water washing treatment of the slurry containing nitride particles (first water washing step); a sub-step of acid washing treatment of the slurry containing nitride particles after the first water washing step (acid washing step); a sub-step of water washing treatment of the slurry containing nitride particles after the acid washing step (second water washing step); a sub-step of adding an aqueous phosphoric acid solution to the slurry containing nitride particles after the second water washing step to form a phosphorus-containing protective film on the surface of the nitride particles (surface treatment step); and a sub-step of water washing treatment of the slurry containing nitride particles after the surface treatment step to obtain a wet-treated product (third water washing step). Each step will be described in detail below.

[0029] <Reduction-Diffusion Step> In the reduction-diffusion step, a raw material mixture comprising an alloying material and a reducing agent is prepared. The alloying material comprises at least a rare earth metal (R), a transition metal (TM), and oxygen (O). The alloying material only needs to contain at least a rare earth metal, a transition metal, and oxygen as constituent elements. During the reduction-diffusion treatment, oxygen in the alloying material is removed by the action of the reducing agent, and the rare earth metal diffuses into the transition metal to alloy. In this way, a reduction-diffusion treated product (reaction product) comprising a rare earth transition metal alloy and byproducts from the reducing agent can be obtained.

[0030] [Alloy Raw Material - Powder Mixture Case] ​​The alloy raw material can also be a powder mixture, that is, 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 constituting the target magnetic powder. The type of rare earth metal can be selected according to the composition of the target magnetic powder. Although there is no limitation, examples of rare earth metals include one or more from the group consisting of yttrium (Y), lanthanum (La), cerium (Ce), plutonium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), thorium (Gd), tbium (Tb), dysprosium (Dy), holmium (Ho), and ytterbium (Yb). As for the rare earth oxide powder, one powder can be used alone, or two or more powders can be mixed and used.

[0031] Rare earth metals preferably include samarium (Sm). By selecting Sm, samarium iron nitrogen (Sm2Fe17N3) based magnetic powders can be manufactured. Sm2Fe17N3 based magnetic powders have excellent magnetic properties and can be used as bonding magnet materials. In this case, Sm can also be used in combination with other rare earth metals besides Sm, such as La and / or Ce. For example, more than 70 atomic percent of the rare earth metal can be Sm, and less than 30 atomic percent can be composed of other elements (La, Ce, etc.).

[0032] The particle size of the rare earth oxide powder can be determined based on the composition and intended use of the obtained alloy powder. However, it is more ideal to determine the particle size of the rare earth oxide powder in a manner that ensures it is uniformly distributed near the transition metal particles in the obtained 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. Particularly preferred is a powder in which particles with a particle size of 0.1 to 10 μm account for more than 80% by mass of the total. This improves the inmisivability of the raw materials and the workability of the raw material powder or reaction product. Furthermore, it allows for sufficient diffusion of the rare earth metal in the subsequent reduction diffusion step. Moreover, 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 measuring device.

[0033] Rare earth oxide powder may contain moisture or organic matter as impurities. These impurities can sometimes increase the oxygen content of the final alloy powder. Therefore, it is preferable that the rare earth oxide powder contains a small amount of impurities. For example, the loss on heating after heating to 1000°C is preferably less than 2% by mass, and more preferably less than 1% by mass.

[0034] The transition metal powder is a raw material for the transition metal constituting the target magnetic powder. The type of transition metal can be selected according to the composition of the target magnetic powder. Although there is no limitation, examples include one or more 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 can be used alone, or two or more types of powder can be mixed and used.

[0035] The transition metal is preferably iron (Fe). By selecting Fe, samarium iron nitrogen (Sm2Fe17N3) based magnetic powder can be manufactured. In this case, Fe can also be combined with one or more other transition metals besides Fe, such as those selected from the group consisting of Cr, Mn, Co, and Ni. For example, the transition metal can be composed of 90 atomic percent or more of Fe and less than 10 atomic percent of other elements (Cr, Mn, Co, Ni, etc.). When the transition metal powder is iron (Fe) powder, cobalt (Co) powder, or nickel (Ni) powder, reducing powder, gas atomized powder, water atomized powder, electrolytic powder, carbonyl powder, etc., can be used, for example.

[0036] As the transition metal powder, only metal powder may be used, or a portion thereof may be replaced with metal oxide powder. Furthermore, powders containing not only transition metals but also other metals, such as rare earth metals, may be used. For example, as the transition metal powder, metal powder (Fe, etc.), metal oxide powder (Fe2O3, Fe3O4, FeO, etc.), rare earth transition metal alloy powder (R2Fe17, etc.), rare earth transition metal composite oxide powder (RFeO3, etc.) may be used. However, if oxide powder is used, an aluminothermic reaction occurs during the reduction diffusion process. From the viewpoint of suppressing the rapid exothermic reaction, it is preferable to suppress the proportion of oxide powder in the overall transition metal powder to 50% by mass or less.

[0037] When the transition metal powder is a metal powder, considering the diffusion length of the rare earth metal at the reduction diffusion treatment temperature, the average particle size D50 of the transition metal powder is preferably 100 μm or less, and more preferably 50 μm or less. 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, and more preferably 5 μm or less.

[0038] [Other than Alloy Raw Materials - Powder Mixtures] The alloy raw material may also be: oxides and / or partially reduced oxides (partial oxides) containing rare earth metals and transition metals; alloys containing rare earth metals, transition metals and oxygen (e.g., oxygen-containing SmFe alloys); or mixtures of alloys containing rare earth metals and transition metals with rare earth oxides (e.g., mixtures of SmFe alloys and Sm oxides). By using partially reduced oxides as alloy raw materials, the amount of reducing agent added in subsequent steps can be suppressed. An example of the production sequence of partially reduced oxides containing rare earth metals and transition metals will be described below.

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

[0040] [Reducing Agent] The reducing agent is added to reduce oxide components such as rare earth oxide powder during subsequent reduction diffusion treatment, thereby promoting alloy formation. As a reducing agent, at least one is selected from alkali metals, alkaline earth metals, and their hydrides. Specifically, it is preferably 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. From the viewpoint of operational safety and cost, Li and / or Ca are more preferred, and Ca is particularly preferred.

[0041] The reducing agent is preferably in granular form. By increasing the roughness of the reducing agent, the penetration and diffusion of the reducing agent, which becomes a molten liquid upon heating during the reduction diffusion process, into the raw material mixture is promoted. The particle size of the reducing agent is preferably 100 mesh or more (coarser than a 100-mesh sieve), and more preferably 32 mesh or more. On the other hand, the finer the reducing agent, the better its dispersibility in the raw material mixture. The particle size of the reducing agent is preferably 4 mesh or less (fineer than a 4-mesh sieve), and more preferably 9 mesh or less.

[0042] [Other Components] Other components besides rare earth oxide powder, transition metal powder, and reducing agent may also be added as needed. For example, when manufacturing magnetic powder containing components other than rare earth metals, transition metals, and nitrogen, other raw materials 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 mixing of raw materials can be carried out using known methods, either wet or dry. For dry mixing, a belt mixer, drum mixer, S-type mixer, V-type mixer, Nauta mixer, Henschel mixer, MIRALO, NOBILTA, mechanofusion, high-speed mixer, and / or vibratory mill can be used. For wet mixing, water or an organic solvent can be used as the mixing medium, and the raw materials can be mixed using a ball mill and / or bead mill, followed by drying. However, for wet mixing, it is preferable to wet mix all raw materials except for a reducing agent (such as metallic Ca), and then add the reducing agent.

[0044] In the manufacturing method of this embodiment, the average value of the 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 heterogeneous phases, enabling the acquisition of magnetic powder with excellent magnetic properties. Conversely, if the average value of the R / TM ratio is less than 0.117, there is a risk that the diffusion of rare earth metals during the reduction-diffusion treatment becomes insufficient. Therefore, a transition metal-rich phase (such as an iron-rich phase) with insufficient rare earth metal content remains in the reaction product (reduction-diffusion treated product) after the reduction-diffusion treatment, which risks decreasing the magnetic properties of the magnetic powder, such as remanent magnetic flux density or squareness. If the average value of the R / TM ratio exceeds 0.160, the rare earth-rich phase, as a heterogeneous phase, increases, and the proportion of the main phase decreases. Therefore, there is a risk that the magnetic properties of the magnetic powder, such as remanent magnetic flux density or squareness, decrease. The average R / TM ratio is preferably above 0.123 and below 0.135. Furthermore, the average R / TM ratio of the raw material mixture is calculated based on the amount of rare earth metal (R) raw materials and transition metal (TM) raw materials added during the preparation of 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, 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 setting the amount of reducing agent to 1.05 equivalents or more, the reduction of oxides can be sufficiently carried out. On the other hand, if there is too much reducing agent, there will be residual reducing agent components, which sometimes hinders nitriding in the subsequent nitriding step. By setting the amount of reducing agent to 3.0 equivalents or less, the nitriding in the nitriding step can be prevented from being hindered.

[0046] In the reduction-diffusion step, the prepared raw material mixture is heated in a reduced-pressure gas or inactive gas environment to produce a reduction-diffusion treated product. During the reduction-diffusion treatment, rare earth oxides are reduced to rare earth metals by the action of a reducing agent (such as metal Ca). The generated rare earth metals diffuse into transition metals to form rare earth transition metal alloys. Therefore, the reduction-diffusion treated product includes a rare earth transition metal alloy as the main phase and by-products (such as CaO) from the reducing agent, and may include other heterogeneous phases depending on the situation.

[0047] During heating, simply fill the raw material mixture into an iron reaction vessel and heat it to the specified temperature, hold it for a certain time, and then cool it. The heating temperature is set between 800°C and 1200°C. If the heating temperature is below 800°C, the reduction reaction of the oxides by the reducing agent becomes insufficient. If the heating temperature exceeds 1200°C, the alloy particles in the reaction product (reduction-diffusion treated product) generated by the reduction-diffusion reaction will be sintered, which may prevent nitrogen diffusion in the subsequent nitriding step. Alternatively, there is also the following problem: it is difficult to slurry in wet processing, and large lumps are easily left behind. Ideally, the temperature and holding time should be determined in a way that achieves the following state: rare earth metals diffuse to the center of the iron particles in the raw material mixture, leaving no transition metal-rich phase. Furthermore, to reduce residual gases such as oxygen or moisture in the reaction vessel, the vessel can be evacuated and replaced with inactive gases before or during heating. Furthermore, it is preferable to maintain an inactive gas environment during the cooling process to prevent the oxidation of the reaction products and maintain their activity.

[0048] <Hydrogen Decomposition Step> In the hydrogen decomposition step, the reduction-diffusion treated product obtained in the reduction-diffusion step is exposed to a hydrogen environment to decompose it, thereby producing a decomposed product. The reaction product (reduction-diffusion treated product) obtained by reduction-diffusion treatment contains a rare-earth-rich phase containing an excess of rare earth elements. For example, in the case of rare-earth metal Sm, it contains Sm-rich phases such as SmFe5, SmFe3, and / or SmFe2. When the reduction-diffusion treated product is exposed to a hydrogen environment, these rare-earth-rich phases absorb hydrogen and expand in volume. Therefore, cracks are formed in the reduction-diffusion treated product, which can be decomposed. The size of the decomposed product is not limited, but is, for example, 10 mm or less. In the case of larger decomposed products, mechanical decomposition can also be performed.

[0049] <Nitriding Step> In the nitriding step, the fragments obtained in the hydrogen decomposition step are subjected to nitriding treatment to produce nitrides. Through nitriding treatment, the rare earth transition metal alloys contained in the fragments are nitrided to generate rare earth transition metal nitrogen compounds.

[0050] During the nitriding process, as a pretreatment, at least once the following procedure is performed: the pressure around the fragmented material is reduced to below 50 kPa, and then the pressure is restored using a hydrogen-free pretreatment gas. The reaction product (fragmented material) after the hydrogen decomposition step is usually a blocky porous sintered body, in which hydrogen remains in the pores or cracks. The inventors conducted an investigation and found that if hydrogen remains, there is a risk that the diffusion of nitrogen through the pores or cracks will be hindered. Therefore, by performing pretreatment using a hydrogen-free gas (pretreatment gas), residual hydrogen can be removed, resulting in uniform and sufficient nitriding.

[0051] If the ambient pressure after depressurization exceeds 50 kPa, there is a risk that hydrogen removal will become insufficient, and nitriding will become uneven and incomplete. It is preferable to use a nitrogen-containing gas (nitriding gas) such as hydrogen-free nitrogen and / or ammonia as the pretreatment gas introduced during pressure recovery. If the pretreatment gas (nitriding gas) contains hydrogen, there is a risk that hydrogen that hinders nitriding may remain in the pores or cracks after pretreatment. Furthermore, the pretreatment process should be performed at least once, preferably more than twice.

[0052] After pretreatment, a nitriding gas (nitrogen-containing gas) containing hydrogen is supplied to the surrounding environment, and the fragmented material is heated in the supplied nitriding gas environment to convert it into nitride. The supplied nitriding gas may or may not contain hydrogen. As the nitriding gas, nitrogen and / or nitrogen-containing gases such as ammonia are preferred. In order to control the nitriding reaction rate, the nitrogen-containing gas (nitriding gas) may also contain other gases such as hydrogen, argon, and helium. For example, a mixture of ammonia and hydrogen can sufficiently nitride into the interior of larger alloy particles, such as those with a particle size 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 using a mixture of ammonia and hydrogen as the nitriding gas, the ratio of ammonia pressure to total pressure (ammonia partial pressure) is preferably 0.20 to 1.00, and more preferably 0.30 to 0.95 or less. By setting the ammonia partial pressure to 0.20 or higher, the nitriding of the alloy particles is further promoted. Furthermore, the supply amount of nitriding gas cannot be fixed in general. In order to ensure that the amount of nitrogen in the magnetic powder is sufficiently high, it is preferably adjusted to 2.5% by mass or more and 5.0% by mass or less.

[0053] The heating temperature (nitriding temperature) of the fragmented material is preferably 350°C to 500°C, and more preferably 400°C to 480°C. By setting the heating temperature to 350°C or higher, nitrogen can diffuse rapidly into the interior of the alloy particles. On the other hand, if the heating temperature is too high, there is a risk of alloy decomposition. For example, in the case of transition metal Fe, alloy decomposition can occur, generating heterogeneous phases such as α-Fe or Fe nitrides that degrade the magnetic properties of the magnetic powder. By setting the heating temperature to 500°C or lower, the formation of such heterogeneous phases can be suppressed. Furthermore, the optimal heating time varies depending on various conditions such as heating temperature or processing volume. Therefore, it is difficult to determine the heating time precisely. To ensure a sufficiently high N content in the magnetic powder, it is preferably adjusted to 2.5% by mass to 5.0% by mass.

[0054] Preferably, the nitride after nitriding is subjected to heat treatment under hydrogen, an inert gas, or a reduced pressure gas. Examples of inert gases include nitrogen, argon, and / or helium. More preferably, heat treatment is performed in hydrogen followed by heat treatment in nitrogen and / or argon. By performing heat treatment after nitriding, the uniformity of nitrogen distribution in the nitride can be achieved to a higher level. However, this heat treatment is not a necessary step. The magnetic powder of this embodiment can be obtained even without performing heat treatment after nitriding.

[0055] <Wet Processing Step> In the wet processing step, a wet-processed product is prepared by wet processing the nitride. The nitride obtained in the nitriding step is usually a bulk porous body. More specifically, it is a condensate formed by the bonding of nitride particles, which are the main phase. Furthermore, the nitride particles are particles of rare earth transition metal nitrogen compounds. In addition to nitride particles, the nitride also contains byproducts (such as CaO) or heterogeneous phases (such as rare earth-rich phases) from the reducing agent. By wet processing, the bonding of the nitride particles is broken, and a magnetic powder composed of rare earth transition metal nitrogen compounds can be obtained. Furthermore, by wet processing, byproducts or heterogeneous phases other than the main phase can be removed, and a heat-resistant protective film can be formed on the surface of the nitride particles. The wet processing step includes a first water washing step, an acid washing step, a second water washing step, a surface treatment step, and a third water washing step as sub-steps.

[0056] [First Washing Step] In the wet processing step, firstly, the nitride is added to water and allowed to disintegrate to prepare a slurry containing nitride particles. Then, the slurry containing nitride particles is subjected to a water washing process (first washing step). When preparing the slurry, the nitride is simply added to water. Upon addition to water, the byproducts (calcium oxide or calcium nitride, etc.) contained in the nitride from the reducing agent become calcium hydroxide. Subsequently, the nitride disintegrates, yielding a suspension (slurry) containing nitride particles and calcium hydroxide microparticles as insoluble components.

[0057] Furthermore, the method of water washing is not limited. For example, a series of operations including stirring, settling, and decantation can be performed on a slurry containing nitrides. At this time, water can also be added to the slurry as needed. The byproducts (such as CaO) from the reducing agent contained in the nitrides are converted into hydroxides (such as Ca(OH)2) in the slurry. The specific gravity of these hydroxides is different from that of the nitride particles. Therefore, by decantation, the supernatant water and the hydroxides from the reducing agent can be removed from the slurry. From the viewpoint of obtaining nitride particles with higher purity, it is preferable to repeat the series of operations. Furthermore, the supernatant water refers to the liquid on the top of the slurry obtained after the nitride particles with a higher specific gravity have settled after the slurry has been settling. The supernatant water can also be called supernatant liquid. However, the supernatant water (supernatant liquid) sometimes contains components with a lower specific gravity than the nitride particles, such as hydroxides. Therefore, the supernatant water is not necessarily transparent.

[0058] There are no limitations on the concentration, temperature, or number of decantations of the slurry in the first washing step. However, it is preferable that the concentration of the reducing agent component in the slurry at the end of the first washing step is low. For example, when using calcium (Ca) as a reducing agent, water washing is performed until the slurry is allowed to stand, and the concentration of Ca in the supernatant obtained is preferably below 1.0 g / L, more preferably below 0.1 g / L. In this case, the supernatant exhibits an alkaline pH of 8 to 13. The acid added in the subsequent acid washing step is first consumed to neutralize the slurry and lower the pH to below 7, and then consumed to dissolve byproducts or heterogeneous phases. If the concentration of the reducing agent component (Ca, etc.) in the slurry at the end of the first washing step is uneven, the amount of byproducts or heterogeneous phases dissolved will be uneven, which may lead to uneven magnetic properties of the final magnetic powder. By keeping the concentration of reducing agent in the slurry low, magnetic powder with high purity and excellent magnetic properties can be obtained stably.

[0059] [Acid Washing Step] Subsequently, the slurry containing nitride particles obtained in the first water washing step is subjected to acid washing treatment (acid washing step). Even if byproducts (such as Ca(OH)2) or heterogeneous phases (such as rare earth-rich phases) from the reducing agent remain in the nitride particles, they can be removed by acid washing.

[0060] In acid cleaning, the acid is simply added to the slurry containing nitride particles. It is preferable to ensure uniform contact between the nitride particles and the acid in the slurry; therefore, it is preferable to stir the slurry. The acid is not particularly limited as long as it can dissolve and remove byproducts (such as Ca(OH)2) or heterogeneous phases (such as rare earth-rich phases). However, it is preferable to select at least one from the group consisting of acetic acid, hydrochloric acid, and nitric acid.

[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) in the final magnetic powder is between 0.107 and 0.121." By adjusting the amount of acid added, magnetic powders with superior magnetic properties can be produced. Conversely, if the amount of acid added is so high that the average R / TM ratio is less than 0.107, the acid cleaning is excessively strong. There is a risk of strong etching marks on the surface of the particles constituting the powder, and a decrease in magnetic properties sensitive to the surface structure of the particles, such as coercivity. On the other hand, if the amount of acid added is so low that the average R / TM ratio exceeds 0.121, the acid cleaning effect cannot be sufficiently obtained. Due to residual byproducts or heterogeneous phases, there is a risk of a decrease in the magnetic properties of the magnetic powder, such as maximum magnetic polarization.

[0062] Preferably, during the pickling step, the temperature of the slurry containing nitride particles is maintained between 13°C and 30°C. By adjusting the slurry temperature, magnetic powders with superior magnetic properties can be produced. Conversely, if the slurry temperature is below 13°C, the pickling effect cannot be fully achieved. Due to residual byproducts or heterogeneous phases, the magnetic properties of the magnetic powder, such as maximum magnetic polarization, decrease. 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 rare earth ions or iron ions dissolve in the slurry, contaminating the surface of the nitride particles. If the nitride particle surface is contaminated, the formation of a protective film in subsequent surface treatment steps is hindered. Therefore, there is a risk of decreased homogeneity of the protective film, resulting in decreased heat resistance of the final magnetic powder. The slurry temperature is more preferably between 15°C and 25°C.

[0063] [Second Washing Step] Subsequently, the slurry containing nitride particles after the acid washing step is subjected to a water washing treatment (Second Washing Step). In the acid washing step, the nitride particles in the slurry are acid-washed. At this time, the following situations occur: by-products (Ca(OH)2, etc.) or heterogeneous phases (rare earth-rich phases, etc.) dissolve, and reducing agent ions (Ca ions, etc.), rare earth ions (R ions), and transition metal ions (TM ions) are generated in the slurry. These generated ions are diluted and removed by water washing.

[0064] There is no limitation to the method of water washing. For example, a series of operations including stirring, settling, and decantation can be performed on a slurry containing nitride particles. Water may also be added as needed. From the viewpoint of thoroughly removing generated ions, it is preferable to repeatedly perform the series of operations.

[0065] Perform water rinsing until the transition metal ions in the slurry are sufficiently removed. For example, perform water rinsing until the concentration of transition metals (TM) (such as Fe concentration) in the clear water on the slurry before it is added to the phosphoric acid aqueous solution in the subsequent surface treatment step drops to preferably 1.0 g / L or less, more preferably 0.3 g / L or less.

[0066] [Surface Treatment] Subsequently, an aqueous phosphoric acid solution is added to the slurry containing nitride particles after the second water washing step to form a phosphorus-containing protective film on the surface of the nitride particles (surface treatment step). By setting the phosphorus-containing protective film, the heat resistance of the magnetic powder is finally improved.

[0067] In the surface treatment step, the slurry obtained in the second water washing step is re-injected with water. While stirring the resulting slurry, a phosphoric acid aqueous solution is added. By adding the phosphoric acid aqueous solution, the pH of the slurry decreases from its initial value (usually 6-9). As the phosphoric acid reacts with the nitride particles, the pH of the slurry rises again. Furthermore, the phosphoric acid aqueous solution is an aqueous solution of phosphoric acid. Also, the concept of phosphoric acid includes both phosphoric acid and phosphates.

[0068] Because the second water washing step performed previously is sufficient, the amount of transition metals in the clear water on the slurry before it is added to the phosphoric acid aqueous solution is sufficiently reduced. Preferably, the concentration of transition metals (TM) (such as Fe concentration) in the clear water on the nitride-containing slurry before it is added to the phosphoric acid aqueous solution is 1.0 g / L or less. Therefore, a protective film with excellent uniformity is formed, and the heat resistance of the final magnetic powder is further improved. In contrast, if the TM concentration is high, most of the added phosphoric acid is consumed to generate transition metal phosphates such as ferrous phosphate or ferric phosphate. Therefore, the formation of the protective film becomes insufficient, and the uniformity of the obtained protective film deteriorates. From the viewpoint of improving the heat resistance of the magnetic powder, the TM concentration in the clear water on the slurry is preferably 0.3 g / L or less.

[0069] Examples of phosphoric acid aqueous solutions 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, converted to phosphorus (P), is 0.03% by mass to 1.0% by mass relative to the magnetic powder. In other words, the amount of phosphorus (P) contained in the added phosphoric acid aqueous solution is 0.03% by mass to 1.0% by mass relative to the magnetic powder. By controlling the amount of P within this range, magnetic powder with high magnetic properties and excellent heat resistance can be obtained. Conversely, if the amount of P is less than 0.03% by mass, the formation of the protective film and the resulting improvement in the heat resistance of the magnetic powder become insufficient. If the amount of P exceeds 1.0% by mass, the amount of residual phosphoric acid becomes excessive, and the magnetic properties of the final obtained magnetic powder, such as maximum magnetic polarization, decrease. From the viewpoint of improving magnetic properties and heat resistance, the amount of phosphoric acid aqueous solution, converted to P, is more preferably 0.07% by mass to 0.8% by mass relative to the magnetic powder.

[0071] Preferably, during the surface treatment step, the temperature of the slurry containing nitride particles is maintained between 13°C and 30°C. If the slurry temperature is below 13°C, the added phosphate and nitride particles cannot react sufficiently, making it difficult to form a protective film with excellent uniformity. If the slurry temperature exceeds 30°C, the phosphate and nitride particles react excessively. Therefore, the thickness of the protective film becomes uneven, resulting in a risk that the effect of improving heat resistance becomes insufficient. Furthermore, there is a risk of strong etching marks forming on the surface of the nitride particles, and a decrease in magnetic properties sensitive to the surface structure of the particles, such as coercivity. From the viewpoint of improving the magnetic properties and heat resistance of the magnetic powder, the slurry temperature is more preferably between 16°C and 25°C.

[0072] [Third Water Washing Step] Subsequently, the slurry containing nitride particles after the surface treatment step is subjected to water washing to obtain a wet-treated product (Third Water Washing Step). During the surface treatment step, transition metal phosphates or rare earth phosphates, or complex salts thereof, may be produced as byproducts in areas other than the protective coating. For example, phosphates may react with nitride particles to produce phosphate particles such as ferrous phosphate or ferric phosphate. Water washing removes these byproducts (phosphate particles).

[0073] There is no limitation to the method of water washing. For example, a series of operations including stirring, settling, and decantation can be performed on a slurry containing nitride particles. Water may also be added as needed. From the viewpoint of thoroughly removing byproducts, it is preferable to repeatedly perform the series of operations.

[0074] There are no limitations on the concentration, temperature, or number of decantations of the slurry in the third washing step. However, it is preferable that the phosphorus (P) concentration in the slurry is low at the end of the third washing step. Specifically, it is preferable to perform water washing until the phosphorus (P) concentration in the supernatant of the slurry is below 100 mg / L. If the P concentration is too high, the magnetic properties of the final magnetic powder, such as the maximum magnetic polarization, may decrease. From the viewpoint of improving magnetic properties, the P concentration in the supernatant is preferably below 50 mg / L.

[0075] <Drying Step> A drying step (drying step) may also be provided as needed for drying the wet-processed material (nitride particles) after washing. The drying method is not limited. For example, a desliming process can be performed on a slurry containing the wet-processed material to obtain a cake containing magnetic powder, and the obtained cake can be dried. Desliming can be performed using any known solid-liquid separation device such as a Nutsche funnel, a filter press, and / or a centrifuge. Drying can be performed under reduced pressure or by heating. To improve drying efficiency, the cake can be stirred during drying. Furthermore, to further improve drying efficiency, the solvent in the slurry before desliming can be replaced. For example, the water in the slurry can be replaced with alcohols such as methanol, ethanol, and / or propanol.

[0076] <Pulverization Step> A pulverization step (pulverization step) may also be provided as needed for pulverizing the wet-treated material (nitride particles) after washing or drying. The pulverization method is not limited, as long as it is performed wet or dry. In the case of wet pulverization, pulverization in an organic solvent is preferred. In the case of dry pulverization, pulverization in a non-reactive environment is preferred. Furthermore, a surface treatment for imparting weather resistance may be applied simultaneously with pulverization or to the pulverized material.

[0077] Thus, the magnetic powder of this embodiment is manufactured. That is, the wet-processed material after wet treatment, drying, and / or pulverization can be used as magnetic powder. The average particle size D50 of the manufactured magnetic powder is generally 1 μm or more and 10 μm or less, preferably 1 μm or more and 5 μm or less, and even 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 nitrogen-based magnetic powder of this embodiment contains at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as its main components. Details of the rare earth metal (R) are as described above. Although not limited, examples of rare earth metals include one or more selected from the group consisting of yttrium (Y), lanthanum (La), cerium (Ce), plutonium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), thorium (Gd), tbium (Tb), dysprosium (Dy), holmium (Ho), and ytterbium (Yb). Among these, Sm is particularly preferred. Furthermore, Sm may be used in combination with other rare earth metals besides Sm, such as La and / or Ce. For example, 70 atomic% or more of the rare earth metal may be Sm, and less than 30 atomic% may be composed of other elements (La, Ce, etc.).

[0079] Details of the transition metal (TM) are as described above. While not limited, examples of transition metals include 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, it is particularly preferred that it includes Fe. Furthermore, Fe may be used in combination with at least one other transition metal besides Fe, such as selected from the group consisting of Cr, Mn, Co, and Ni. For example, the transition metal may be composed of 90 atomic percent or more of Fe, and less than 10 atomic percent of other elements (Cr, Mn, Co, Ni, etc.).

[0080] The magnetic powder of this embodiment has any one of the crystal structures of Th2Zn17, Th2Ni17, and TbCu7. As an example of a magnetic powder having such a crystal structure, a powder with a basic composition represented by Sm2Fe17N3 (Sm2Fe17N3-based powder) can be cited. Sm2Fe17N3-based powder exhibits excellent magnetic properties and can be used as a bonding magnet material. Furthermore, the magnetic powder of this embodiment is not limited to those having a stoichiometric composition. As long as the crystal structure is maintained and the following R / TM ratio condition is met, deviations in composition are permissible.

[0081] The average value of the atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) in the magnetic powder of this embodiment is 0.107 or more and 0.121 or less. The average value of the R / TM ratio is evaluated by the following method.

[0082] The average R / TM ratio is a standard for the main phase composition of magnetic powder. By ensuring the average value falls within the above range, the main phase composition of the magnetic powder is optimized, resulting in improved magnetic properties. Conversely, if the average value is less than 0.107, the amount of transition metals in the magnetic powder is excessive. It is impossible to suppress transition metal-rich phases containing α-Fe or Fe nitrides as heterogeneous phases. Therefore, the magnetic properties of the magnetic powder, such as maximum magnetic polarization or coercivity, decrease. On the other hand, if the average value exceeds 0.121, the amount of rare earth metals is excessive. Since it is impossible to suppress rare earth-rich phases containing SmFe5, SmFe3, and / or SmFe2 as heterogeneous phases, the magnetic properties decrease. From the viewpoint of improving magnetic properties, the average R / TM ratio is preferably 0.108 or more and 0.118 or less.

[0083] Furthermore, the average R / TM ratio is obtained as follows. First, a thermosetting resin is mixed with magnetic powder, and the resulting mixture is pressurized and then thermosetting to produce a compression molded body. Second, the cross-section of the obtained compression molded body is exposed by grinding to produce a sample for SEM observation. The ground cross-section of the prepared SEM observation sample is observed using a scanning electron microscope (SEM) at a magnification of at least 1000 particles entering the field of view, and the R / TM atomic ratio is determined by EDS analysis of the entire field of view using the standard-free FP method.

[0084] In the cross-section of the compressed magnetic powder of this embodiment, the area ratio of rare earth-rich phases with an atomic ratio (R / TM ratio) of rare earth metal (R) to transition metal (TM) exceeding 0.130 is 1.0% or less. The area ratio of rare earth-rich phases is evaluated by the following method.

[0085] As described above, the average R / TM ratio of the magnetic powder in this embodiment, which serves as the main phase composition, is limited to a specific range. However, upon local observation, uneven composition can occur. In particular, magnetic powders containing rare earth metals and transition metals as main constituent elements often contain rare earth-rich heterogeneous phases (SmFe5, SmFe3, SmFe2, etc.) due to various reasons caused by manufacturing conditions, such as insufficient element diffusion or heterogeneous phase formation. Since such rare earth-rich phases lead to a decrease in magnetic properties, they are excluded as much as possible in this embodiment. The area ratio of the rare earth-rich phase is preferably 0.8% or less, more preferably 0.5% or less. There is no limitation on the lower limit of the area ratio. For example, it can be 0.1% or more, 0.3% or more, or 0.5% or more.

[0086] Furthermore, the area ratio of the rare earth-rich phase is determined as follows: A SEM image is obtained by observing the polished cross-section of the sample used for SEM observation. The area ratio of the rare earth-rich phase is determined by analyzing the obtained SEM image using different contrast ratios. That is, since the rare earth-rich phase region contains more mass rare earth metals than the main phase region of the magnetic powder, it appears brighter in the SEM image. Therefore, the area ratio of the brighter region in the SEM image can be determined. Specifically, a contrast threshold can be set in the SEM image to detect only the rare earth-rich phase region (regions with an R / TM atomic ratio exceeding 0.130), and the SEM image can be binarized. Then, the area of ​​the rare earth-rich phase is determined in the binarized SEM image, and the area ratio of the rare earth-rich phase is obtained by dividing it by the area of ​​the particle cross-section of the entire field of view.

[0087] The magnetic powder of this embodiment has the effect of stably having a high maximum magnetic polarization. The maximum magnetic polarization Jm 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 film with excellent uniformity. Therefore, it has excellent heat resistance. The heat resistance can be evaluated by the retention rate of coercivity [HcJ(1000) / HcJ(0)], which is the ratio of the coercivity (HcJ(1000)) after heating the magnetic powder at 150°C for 1000 hours in the atmosphere to the coercivity (HcJ(0)) before heating. The retention rate of coercivity [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 using the following embodiments and comparative examples. However, the present invention is not limited to the following embodiments.

[0090] (1) Evaluation of magnetic powder The magnetic powder prepared in the examples and comparative examples was evaluated for various properties as described below.

[0091] <Particle size> The particle size of magnetic powder is evaluated using an airflow dispersion type laser diffraction particle size distribution measuring device, and the cumulative 50% diameter in the particle size distribution under volume reference is defined as D50.

[0092] <R / Fe atomic ratio> The composition (R / Fe ratio) of the magnetic powder is determined by observation using a scanning electron microscope (SEM). Specifically, a thermosetting resin and magnetic powder are mixed, and the resulting mixture is pressurized and thermosetting to produce a compression molded body. At this time, the amount of thermosetting resin mixed is adjusted to be 4-5% by mass relative to the magnetic powder. Next, the cross-section of the obtained compression molded body is exposed by grinding to prepare a sample for SEM observation.

[0093] Next, the ground cross-section of the sample for SEM observation was observed using a scanning electron microscope (SEM, JST-IT200, Japan Electronics Corporation). Observations were performed at 1000x magnification, with three fields of view approximately 100 μm square each. Each field of view contained more than 5000 particles. Then, the R / Fe atomic ratio was determined by EDS analysis of the entire field of view using the standard-free FP method. The analysis was performed at an accelerating 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, SEM observation was performed on the ground cross-section of the aforementioned SEM observation sample prepared for determining the R / Fe atomic ratio. Observation was performed at 1000x magnification, and reflectance electron images were captured for a field of view approximately 100 μm square. The number of particles contained in one field of view was more than 5000. Then, image analysis was performed on the obtained reflectance electron images using different contrast ratios. Specifically, in the reflectance electron images, a contrast threshold was set to detect only the rare earth-rich phase region (the region with an R / Fe atomic ratio greater than 0.13), and the reflectance electron images were binarized. Then, the area s of the rare earth-rich phase was determined in the binarized reflectance electron images. Subsequently, the contrast threshold was changed to include both the rare earth-rich phase region and the main phase region, and the reflectance electron images were binarized again. Then, the area S of the entire particle is determined from the binarized reflected electron image. The area s is divided by the area S to obtain the area ratio of the rare earth-rich phase.

[0095] <Magnetic Properties> Regarding the magnetic properties of the magnetic powder, in accordance with the Guidelines for Test Methods of Bonded Magnetic Powders BM-2002 and BM-2005 of the Japan Association of Bonded Magnetic Industries, a 1600 kA / m alignment magnetic field was applied to orient the magnetic powder in stearic acid to prepare a sample. After magnetization under a magnetic field of 4.0 MA / m, the specific gravity of the magnetic powder was set to 7.67 g / cm3. The maximum magnetic polarization: Jm (T), coercivity: HcJ (MA / m), and squareness Hk (MA / m) were measured using a vibrating sample magnetometer with a maximum magnetic field of 1.6 MA / m. 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 drops to 90% of the residual magnetic polarization Jr.

[0096] <Heat Resistance> Magnetic powder was heated at 150°C for 1000 hours in the atmosphere, and the coercivity (HcJ(0)) before heating and the coercivity (HcJ(1000)) after heating were measured. The coercivity was measured using the same method as the above-mentioned magnetic property measurement. Furthermore, as an index for evaluating heat resistance, the ratio of the coercivity (HcJ(1000)) after heating to the coercivity (HcJ(0)) before heating [HcJ(1000) / HcJ(0)] was calculated as the coercivity retention rate.

[0097] <Thermogravimetric determination> Thermogravimetric determination (TG) of the magnetic powder was performed. The determination was carried out by circulating dry air (N2-21%O2) at a flow rate of 100 mL / min while heating the sample at a heating rate of 20°C / min.

[0098] (2)Preparation and evaluation results of magnetic powder [Example 1] <Reduction-diffusion step> 624 g of samarium oxide powder with an average particle size D50 of 2.3 μm, 1553 g of iron powder with a D50 of 40 μm, and 248 g of granular metallic calcium were mixed using a mixer to obtain a mixture with an R / Fe atomic ratio (Sm / Fe atomic ratio) of 0.129. The mixture was placed in an iron crucible and heated at 1150°C for 6 hours under argon atmosphere to obtain the reaction product (reduction-diffusion treated product).

[0099] <Hydrogen Decomposition Step> After cooling, the container is evacuated, and then hydrogen is supplied while adjusting the flow rate to maintain a pressure gauge reading of +20 kPa. The obtained reaction products are decomposed through hydrogen adsorption reaction.

[0100] <Nitriding Step> The hydrogen-treated reaction product (crushed material) is fed into a tubular furnace. Then, the following procedure is repeated twice: the pressure is reduced to 10 kPa absolute pressure, and then restored to atmospheric pressure using nitrogen. Next, a mixture of ammonia (flow rate: 280 mL / min) and hydrogen (flow rate: 90 mL / min) is circulated while the temperature is increased to 430°C for 9 hours. Then, a mixture of hydrogen (flow rate: 1000 mL / min) is circulated while heat treatment is performed for 1 hour, followed by a mixture of nitrogen (flow rate: 1000 mL / min) for 1 hour.

[0101] <Wet Processing Steps> 1360 g of the cooled and recovered nitriding reaction product (nitride) was added to 4 L of water for slurrying. This process of adding water, stirring, settling, and decanting was repeated 7 times (First Washing Step). The final calcium concentration in the supernatant was 0.02 g / L. Water was added again, and the slurry at 22°C was stirred while acetic acid was added for acid washing (Acid Washing Step). The pH of the slurry was lowered from 11 to 4-5 by adding acetic acid. Stirring continued after the acetic acid addition was completed, and the process ended when the pH of the slurry exceeded 7. The supernatant was then drained, and the process of adding water, stirring, settling, and decanting was repeated until the Fe concentration in the supernatant was below 1.0 g / L (Second Washing Step).

[0102] Next, 4 L of water was added again for surface treatment, and the mixture was stirred. The Fe concentration of the supernatant 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 0.31% by mass of P based on the mass of the magnetic powder was added (surface treatment step). By adding the orthophosphoric acid aqueous solution, the pH of the slurry was lowered from 8 to 4.1, and then rose again to 7. At this point, stirring was stopped, and decantation was performed.

[0103] Subsequently, water was injected again, and the process of water injection, stirring, settling-decanting was repeated twice (the third water washing step). At this point, the P concentration of the final slurry supernatant was 40 mg / L.

[0104] <Drying Step> After the final decantation in the third water washing step, 1 L of 2-propanol was added and stirred, followed by suction filtration to obtain a block. The obtained block was stirred using a mixer while being dried under reduced pressure at 130°C, and then cooled to obtain samarium iron nitrogen alloy powder. The obtained alloy powder has a D50 of 21 μm, an Sm composition of 23.1% by mass, and a P composition of 0.15% by mass.

[0105] <Pulverization Step> 900 g of the obtained samarium iron nitrogen alloy powder was micronized using a media stirring mill. Here, 2-propanol with 85% orthophosphoric acid aqueous solution was used as the pulverizing solvent. The pulverized slurry was heated and dried under reduced pressure to obtain magnetic powder. The manufacturing conditions of 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 has a Th2Zn17 type crystal structure with a D50 of 2.1 μm. Furthermore, the magnetic properties are: Jm (maximum magnetic polarization): 1.42 T, HcJ (coercivity): 0.90 MA / m, Hk (squareness): 0.52 MA / m. The P composition of the magnetic powder is 0.52% by mass.

[0107] A compressed molded body was fabricated from the obtained magnetic powder. SEM observations were performed on three arbitrary locations along its ground cross-section at a magnification of 1000x. The obtained SEM reflected electron images are shown in Figures 1-3. EDS analysis was performed on the overall Sm and Fe content of a 100 μm square region to calculate the R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio). The results showed that the R / Fe atomic ratios were 0.113, 0.113, and 0.114, with an average value (average R / Fe atomic ratio) of 0.113.

[0108] On the other hand, in each image, for any brighter contrast particles, logarithmic points were analyzed using EDS to determine the R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio), and the results were all values ​​exceeding 0.13. Then, the ratio of the cross-sectional area of ​​these brighter contrast particles to the cross-sectional area of ​​all particles was calculated by image analysis. The area ratios were 0.5%, 0.4%, and 0.5%, respectively, and their average value (area ratio of rare earth-rich phases) was 0.5%.

[0109] The magnetic powder was heated at 150°C for 1000 hours in the atmosphere. After heating, it was removed and the coercivity HcJ(1000) at room temperature was evaluated. The result was 0.77 MA / m. The retention rate of coercivity [HcJ(1000) / HcJ(0)], which is an indicator of heat resistance, was calculated and the result was 86%.

[0110] [Examples 2-4] The manufacturing conditions were changed as shown in Table 1 below. Otherwise, the magnetic powder was prepared in the same order as in Example 1. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure.

[0111] [Example 5] 200 g of samarium oxide powder and 2 kg of Sm2Fe17 alloy powder with a D50 of 30 μm obtained by reduction diffusion were premixed using a mixer. The mixed powder was pulverized to a D50 of 1.4 μm using a media stirring mill with a solvent of 4.4 kg of 2-propanol and 46 g of 85% phosphoric acid. The resulting slurry was subjected to reduced pressure while being heated to 210°C using the mixer. Once the vacuum level drop caused by gas release ceased and the vacuum level was confirmed to have risen sufficiently, the slurry was cooled.

[0112] 466 g of granular metallic calcium was added as a reducing agent to 2.00 kg of the pulverized mixture thus obtained (consisting of 1.82 kg of Sm2Fe17 alloy powder and 0.18 kg of samarium oxide), and the mixture was mixed using a mixer under argon atmosphere. This reducing agent dosage is 2.5 times the amount (equivalent) required for reduction calculated based on the oxygen analysis value of the pulverized mixture. The R / Fe atomic ratio of the raw material mixture thus obtained is 0.160.

[0113] As a reduction-diffusion step, the raw material mixture is placed in an iron crucible and heated under an argon atmosphere. After maintaining the temperature at 830°C for 2 hours, it is cooled (reduction-diffusion treatment step). Subsequently, the reaction product (reduction-diffusion treated product) recovered from the crucible is broken down by hydrogen absorption (hydrogen treatment), followed by nitriding treatment in the nitriding step. In the hydrogen treatment, the reaction product (reduction-diffusion treated product) is placed in a tubular furnace. The pressure inside the furnace is reduced to -100 kPa, and hydrogen is introduced until it reaches atmospheric pressure. While allowing hydrogen to flow, the temperature is raised to 300°C. Since the reaction product releases heat and absorbs hydrogen during the heating process, the exhaust valve of the tubular furnace is closed while the furnace is under negative pressure, and hydrogen is continuously supplied. After the heat release and hydrogen absorption are completed, cooling is performed (hydrogen breaking down step). The reaction product (broken product) after hydrogen treatment is in a broken-down state. Subsequently, in order to perform nitriding, the pressure inside the tubular furnace was reduced to 5 kPa using an absolute pressure gauge, and nitrogen gas was introduced until the pressure reached atmospheric pressure. Then, the nitrogen flow rate was directly set to 1 L / min and the temperature was increased, maintained at 450°C for 24 hours, and then cooled. This yielded the 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 for slurrying, and the process of adding water, stirring, settling, and decanting was repeated 10 times (first water washing step). Finally, the calcium concentration of the supernatant was 0.1 g / L. Water was added again, and the slurry at 13°C was stirred while acetic acid was added for acid washing (acid washing step). The pH of the slurry was lowered from 12 to about 5 by adding acetic acid. Stirring continued after the acetic acid was added until the pH of the slurry exceeded 7. Then the supernatant was drained, and the process of adding water, stirring, settling, and decanting was repeated until the Fe concentration of the supernatant was below 1.0 g / L (second water washing step).

[0115] Next, 4 L of water was added again and stirred. The Fe concentration of the supernatant was 0.9 g / L, and the slurry temperature was 13°C. While stirring the slurry, a 25% by mass aqueous solution of orthophosphoric acid containing 1.0% by mass of P based on the mass of the magnetic alloy powder was added (surface treatment step). By adding the orthophosphoric acid aqueous solution, the pH of the slurry decreased from 8 to 3.3, and then rose again to 7. Stirring was stopped, and decantation was performed. Then, water was added again, and the process of adding water, stirring, settling, and decantation was repeated 8 times (third water washing step). At this point, the P concentration of the supernatant of the final 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. The mixture was then filtered by suction to obtain a block. This block was dried under reduced pressure at 170°C using a mixer, followed by cooling to obtain samarium iron nitrogen alloy powder (drying step). The obtained alloy powder has a D50 of 2.9 μm. Furthermore, the Sm composition is 23.8% by mass, and the P composition is 0.83% by mass. This alloy powder was then micronized in a 2-propanol solvent containing an aqueous solution of orthophosphoric acid using alumina balls with a diameter of 0.2 mm. The slurry was then dried to obtain magnetic powder (pulverization step).

[0117] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure with a D50 of 1.3 μm. Furthermore, it exhibits magnetic properties of Jm (maximum magnetic polarization): 1.13 T, HcJ (coercivity): 1.44 MA / m, and Hk (squareness): 0.40 MA / m. Moreover, the P composition is 0.83% by mass.

[0118] The R / Fe atomic ratio (Sm / Fe atomic ratio; R / TM atomic ratio) was calculated for the ground cross section of the compressed molded body made from the obtained magnetic powder, and the average value was 0.121. Furthermore, the average ratio of regions with an R / Fe atomic ratio exceeding 0.13 (area ratio of rare earth-rich phases) obtained by EDS analysis was 0.8%. Moreover, the coercivity retention rate [HcJ(1000) / HcJ(0)] of the magnetic powder was 91%.

[0119] [Example 6] The manufacturing conditions were changed as shown in Table 1 below. Otherwise, the magnetic powder was prepared in the same order as in Example 5. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure.

[0120] [Example 7] The manufacturing conditions were modified as shown in Table 1 below. Except as otherwise provided, the raw materials were weighed in the same order as in Example 1, and a reduction-diffusion step and a hydrogen decomposition step were performed. Subsequently, a wet treatment step and a drying step were performed before the nitriding step to obtain samarium iron alloy powder.

[0121] Next, as a nitriding step, samarium-iron alloy powder is placed in a tubular furnace, and a mixture of ammonia (flow rate: 280 mL / min) and hydrogen (flow rate: 90 mL / min) is circulated while the temperature is increased, and heat treatment is performed at 430°C for 9 hours. Then, hydrogen (flow rate: 1000 mL / min) is circulated while heat treatment is performed for 1 hour, and then nitrogen (flow rate: 1000 mL / min) is circulated while heat treatment is performed for 1 hour to obtain samarium-iron nitrogen alloy powder.

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

[0123] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure with a D50 of 2.1 μm. Furthermore, the magnetic properties are Jm: 1.41 T, HcJ: 0.86 MA / m, Hk: 0.39 MA / m. Moreover, the P composition is 0.49% by mass.

[0124] A compressed molded body was fabricated from the obtained magnetic powder, and SEM observations were performed on three arbitrary locations at 1000x magnification of its polished cross-section. The obtained SEM reflected electron images are shown in Figures 4-6. 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 (area ratio of rare earth-rich phase) of regions with an R / Fe atomic ratio exceeding 0.13, as obtained by EDS analysis, was 2.2%. Moreover, the coercivity retention rate of the magnetic powder was 85%.

[0125] [Example 8] The reduction-diffusion step to the second water washing step of the wet treatment step was carried out under the conditions shown in Table 1 below. When the Fe concentration in the supernatant water in the final rinsing of the second water washing step became less than 1 g / L, a drying step was performed without a surface treatment step and a third water washing step to recover the samarium-iron-nitrogen alloy powder. The alloy powder recovered in this stage was not surface treated.

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

[0127] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure with a D50 of 2.1 μm. Furthermore, the magnetic properties are Jm: 1.44 T, HcJ: 0.93 MA / m, Hk: 0.55 MA / m. Moreover, the P composition is 0.43% by mass.

[0128] [Examples 9-15] The manufacturing conditions were changed as shown in Table 1 below. Otherwise, the magnetic powder was prepared in the same order as in Example 1. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure.

[0129] [Example 16] In the nitriding step, the reaction product treated with hydrogen was fed into a tubular furnace, and then the following procedure was repeated twice: after depressurization to 10 kPa (absolute pressure gauge), the pressure was restored to atmospheric pressure using a gas mixture of ammonia and hydrogen in a 4:1 ratio. Next, while circulating a mixture of ammonia (flow rate: 280 mL / min) and hydrogen (flow rate: 90 mL / min), the temperature was increased, and heat treatment was performed at 430°C for 9 hours. Otherwise, magnetic powder was prepared in the same order as in Example 1.

[0130] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure.

[0131] [Example 17] In the nitriding step, the hydrogen-treated reaction product was fed into a tubular furnace, and then the following procedure was performed once: the pressure was reduced to 80 kPa using an absolute pressure gauge, and then the pressure was restored to atmospheric pressure using nitrogen. Next, while circulating a mixture of ammonia (flow rate: 280 mL / min) and hydrogen (flow rate: 90 mL / min), the temperature was increased, and heat treatment was performed at 430°C for 9 hours. Otherwise, magnetic powder was prepared in the same order as in Example 1.

[0132] The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure.

[0133] [Example 18] The manufacturing conditions were changed as shown in Table 1 below. Otherwise, the magnetic powder was prepared in the same order as in Example 5. The evaluation results of the obtained magnetic powder are shown in Table 2 below. The magnetic powder has a Th2Zn17 type crystal structure.

[0134] [Table 1] Table 1 Manufacturing conditions for magnetic powder raw material Reduction diffusion step Hydrogen decomposition steps Nitriding step Wet treatment steps pickling Surface treatment Third wash Sm2Fe 17 Alloy powder (g) Sm2O3 powder (g) Fe powder (g) Metallic Ca(g) R / Fe atomic ratio Temperature (°C) Pressure (kPa) Pressure (kPa) Nitrogen gas Number of processing times Slurry temperature (°C) Fe concentration in the upper clarified water (g / L) Slurry temperature (°C) P concentration (mass%) Phosphorus concentration (mg / L) in the upper clarified water Example 1 - 624 1553 248 0.129 1150 20 10 nitrogen 2 twenty two 0.1 20 0.31 40 Example 2 - 564 1524 228 0.119 1050 20 10 nitrogen 2 30 0.1 30 0.13 70 Example 3 - 581 1512 235 0.123 1140 20 10 nitrogen 2 17 0.2 twenty four 0.04 30 Example 4 - 629 1440 264 0.140 1180 20 10 nitrogen 2 20 0.1 18 0.30 40 Example 5 1820 180 - 466 0.160 830 20 5 nitrogen 1 13 0.9 13 1.00 20 Example 6 1820 180 - 466 0.160 930 20 5 nitrogen 1 15 0.6 16 0.50 90 Example 7* - 624 1553 248 0.129 1150 20 - nitrogen - 20 - - - - Example 8* - 624 1553 248 0.129 1150 20 10 nitrogen 2 twenty two - - - - Example 9* - 460 1270 182 0.116 1130 20 10 nitrogen 2 twenty two 0.1 20 0.31 40 Example 10* - 610 1200 250 0.163 1120 20 10 nitrogen 2 twenty three 0.1 20 0.32 50 Example 11* - 624 1553 248 0.129 1210 20 10 nitrogen 2 25 0.2 18 0.30 40 Example 12* - 624 1553 248 0.129 1150 20 10 nitrogen 2 33 0.3 twenty four 0.15 40 Example 13* - 624 1553 248 0.129 1150 20 10 nitrogen 2 10 0.3 twenty four 0.15 40 Example 14* - 624 1553 248 0.129 1150 20 10 nitrogen 2 17 0.3 twenty four 0.02 20 Example 15* - 624 1553 248 0.129 1150 20 5 nitrogen 1 17 0.3 twenty four 1.20 60 Example 16* - 624 1553 248 0.129 1150 20 10 ammonia + hydrogen 2 twenty two 0.1 20 0.31 40 Example 17* - 624 1553 248 0.129 1150 20 80 nitrogen 1 twenty two 0.1 20 0.31 40 Example 18* 1820 180 - 466 0.160 790 20 5 nitrogen 1 13 0.9 13 1.00 50 Note 1) "*" indicates a comparative example.

[0135] [Table 2] Table 2 Manufacturing conditions for magnetic powders R / Fe atomic ratio Ratio of rare earth phases (%) D50 (μm) Jm(T) HcJ (MA / m) Hk (MA / m) P (mass%) HcJ(1000) / HcJ(0)(%) Example 1 0.113 0.5 2.1 1.42 0.90 0.52 0.52 86 Example 2 0.108 0.6 1.6 1.34 1.10 0.62 0.79 87 Example 3 0.110 0.7 1.5 1.33 1.13 0.62 0.83 83 Example 4 0.114 0.7 1.5 1.35 1.13 0.63 0.80 88 Example 5 0.121 1.0 1.3 1.13 1.44 0.40 0.83 91 Example 6 0.120 0.9 1.4 1.20 1.21 0.42 0.75 93 Example 7* 0.123 2.2 2.1 1.40 0.86 0.39 0.49 85 Example 8* 0.111 0.5 2.1 1.44 0.93 0.55 0.43 68 Example 9* 0.106 0.4 1.6 1.34 0.73 0.34 0.74 82 Example 10* 0.122 4.9 2.1 1.17 0.81 0.36 0.73 78 Example 11* 0.113 1.2 2.0 1.25 1.01 0.33 0.74 79 Example 12* 0.106 0.3 2.1 1.42 0.76 0.44 0.59 74 Example 13* 0.122 1.8 1.8 1.21 0.78 0.30 0.60 77 Example 14* 0.111 0.7 1.6 1.28 1.08 0.61 0.72 72 Example 15* 0.112 1.1 1.6 1.19 0.71 0.25 1.10 84 Example 16* 0.111 2.1 2.1 1.39 0.86 0.44 0.51 81 Example 17* 0.111 2.6 2.1 1.36 0.82 0.41 0.49 78 Example 18* 0.109 1.3 1.5 0.98 0.42 0.16 0.73 79 Note 1) "*" indicates a comparative example.

[0136] (3) Summary of Evaluation Results Examples 1 to 4 are magnetic powders made from a mixture containing samarium oxide, iron powder (metallic iron powder), and metallic calcium. The R / Fe atomic ratio (R / TM atomic ratio) of these magnetic powders is 0.108 to 0.114, and the ratio of impurity phase (rare earth rich phase) (0.5 to 0.7%) is less than 1.0%. Therefore, the maximum magnetic polarization Jm is as high as 1.33 T to 1.42 T. In addition, the coercivity retention rate [HcJ(1000) / HcJ(0)], which is an indicator of heat resistance, of these magnetic powders is 83% to 88%.

[0137] In contrast, in Example 7, where the order of the nitriding and wet treatment steps was changed, the R / Fe atomic ratio (0.123) exceeded 0.121. Furthermore, the ratio 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, which had the same D50 (2.1 μm) (1.42 T). Also, the squareness Hk (0.39 MA / m) was lower than that of Example 1 (0.52 MA / m).

[0138] In Example 8, where the surface treatment step and the third water washing step in the wet treatment process are omitted, although Jm is as high as 1.44 T, the retention rate of coercive force is as low as 68%.

[0139] The thermogravimetric analysis (TG) results obtained for the alloy powders (samarium iron nitrogen alloys with a D50 of approximately 20 μm after the drying step) of Examples 1 and 8 are shown in Figure 7. The results show that the alloy powders undergo an increase in oxidation due to heating in air. The temperature at which the increase in oxidation due to heating is 1% is 410°C in Example 8, compared to 570°C in Example 1, where a protective coating is provided. It is speculated that the difference in heat resistance (oxidation resistance) of the alloy powders before pulverization is related to the difference in the uniformity of the protective coating of the magnetic alloy powders obtained by surface treatment on one side and pulverization on the other side in Patent Document 3, leading to the difference in the heat resistance of the magnetic powders.

[0140] Example 9 shows the results when the R / Fe atomic ratio (0.116) of the raw material mixture is less than 0.117, and Example 10 shows the results when the R / Fe atomic ratio (0.163) of the raw material mixture is greater than 0.160. In the former case, the R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step is less than 0.107, and the coercivity HcJ is as low as 0.73 MA / m. In the latter case, similarly, the R / Fe atomic ratio (0.122) of the pulverized magnetic powder is greater than 0.121, and the ratio of impurity phase (rare earth rich phase) (4.9%) exceeds 1.0%. Therefore, Jm is as low as 1.17 T, and HcJ is as low as 0.81 MA / m. The retention rates of coercivity are as low as 82% and 78%, respectively.

[0141] Example 11 shows the results when the processing temperature (1210°C) in the reduction-diffusion step is higher than 1200°C. During the high-temperature treatment, the particles are sintered, and sintered particles are visible in the magnetic powder obtained after the drying step. Therefore, the magnetic powder obtained after the pulverization step has a Jm of 1.25 T, a Hk as low as 0.33 MA / m, and a coercivity retention rate as low as 79%.

[0142] Example 12 shows the result when the slurry temperature of the acid cleaning in step 4 is relatively high, and the R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step is lower than 0.107. In contrast, Example 13 shows the result when the slurry temperature of the acid cleaning is relatively low, the dissolution of the secondary phase during acid cleaning is less, and the R / Fe atomic ratio (0.122) is higher than 0.121. In the former case, due to excessive acid cleaning, the R / Fe atomic ratio (0.106) of the magnetic powder obtained after the pulverization step is lower than 0.107. Also, HcJ is as low as 0.76 MA / m. On the other hand, in the latter case, the R / Fe atomic ratio (0.122) of the magnetic powder is higher than 0.121, and the ratio of impurity phase (rare earth rich phase) (1.8%) exceeds 1.0%. Therefore, Jm of the magnetic powder is as low as 1.21 T. Furthermore, the maintenance rates of coercive force were as low as 74% and 77%, respectively.

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

[0144] Example 16 shows the result of nitriding after the hydrogen-treated reaction product was fed into a tubular furnace and the following procedure was repeated twice: the pressure was reduced to 10 kPa (absolute pressure), and the pressure was restored to atmospheric pressure using a gas mixture of ammonia and hydrogen in a 4:1 ratio. Because the gas used for pressure restoration contained hydrogen, hydrogen remained in the pores or cracks of the reaction product, hindering the diffusion of the nitriding gas during nitriding, resulting in a substantial increase in nitrogen-free phases. Furthermore, the proportion of impurity phases (rare earth-rich phases) (2.1%) exceeded 1.0%, and the Jm, HcJ, and Hk of the magnetic powder were all lower than in Example 1. The retention rate of coercivity was also as low as 81%.

[0145] Example 17 shows the results of nitriding after performing the following procedure once: the pressure was reduced to 80 kPa (absolute pressure gauge), and then restored to atmospheric pressure using nitrogen. Because the environment of the fragmented material (80 kPa) was not sufficiently depressurized, similar to Example 16, hydrogen remained in the pores or cracks of the reaction products, hindering the diffusion of nitriding gas during the nitriding process, resulting in a substantial increase in nitrogen-free phase transitions. 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 retention rate of coercivity was also as low as 78%.

[0146] Examples 5 and 6 show the results of preparations using a mixture containing samarium oxide, rare earth iron alloy powder, and metallic calcium as raw materials, with reduction diffusion treatment temperatures set at 830°C and 930°C, respectively. Jm ranges from 1.13 T to 1.20 T, and the maintenance rates of coercivity are 91% and 93%, respectively.

[0147] In contrast, Example 18 used a mixture of the same composition and performed a reduction diffusion treatment at a temperature below 800°C (790°C). Jm was as low as 0.98 T, and the retention rate of coercivity (79%) was lower than that of Examples 5 and 6. [Simplified Explanation of the Diagram]

[0024] [Figure 1] shows the SEM image (reflected electron image) of the magnetic powder (Example 1). [Figure 2] shows the SEM image (reflected electron image) of the magnetic powder (Example 7). [Figure 3] shows the thermogravimetric (TG) measurement results of the magnetic powder (Examples 1 and 8).

Claims

1. A rare earth transition metal nitrogen-based magnetic powder, comprising at least a rare earth metal (R), a transition metal (TM), and nitrogen (N) as main components, having any one of the crystal structures of Th2Zn17, Th2Ni17, and TbCu7, wherein the average atomic ratio (R / TM ratio) of the rare earth metal (R) to the transition metal (TM) in the above rare earth transition metal nitrogen-based magnetic powder is 0.107 to 0.121, the area ratio of rare earth-rich phases with an R / TM ratio exceeding 0.130 is 1.0% or less, and when heated in the atmosphere at 150°C for 1000 hours, the ratio of the coercivity (HcJ(1000)) after heating to the coercivity (HcJ(0)) before heating, i.e., the retention rate [HcJ(1000) / HcJ(0)], is 83% or more.

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