FATTY ACID-COATED Sm-Fe-N-BASED MAGNETIC POWDER, BONDED MAGNET, AND PRODUCTION METHODS THEREFOR

Abstract

[Problem] To provide a Sm-Fe-N-based magnetic powder that has good heat resistance in an atmospheric environment (that is, good oxidation resistance when the temperature has been raised in the atmosphere) and that, when held in an atmospheric environment, is capable of maintaining good magnetic properties as compared to conventional, typical powders that are surface-treated by phosphate coating. [Solution] Provided is a fatty acid-coated Sm-Fe-N-based magnetic powder that is composed of particles having a structure in which one or more fatty acid molecules having 12-20 carbon atoms per molecule are adhered to the surface of Sm-Fe-N-based magnetic particles, wherein, when C content (mass%) in the powder is denoted as [% C], and a cumulative 50% particle diameter (μm) of the powder in a volume-based particle size distribution by a dry laser diffraction scattering method is denoted as [D50], an adhesion amount index Ci defined by formula (1) is 0.5-50.0 mass%･μm.　(1): Ci = [% C] × [D50]

Description

Fatty acid coated Sm-Fe-N magnetic powder, bonded magnet, and manufacturing method thereof

[0001] The present invention relates to an Sm-Fe-N magnetic powder having excellent storage stability in the air atmosphere, a method for producing the same, and a bonded magnet using the Sm-Fe-N magnetic powder and a method for producing the same.

[0002] Sm ２ Fe １７ A material in which nitrogen is introduced into an intermetallic compound (the typical composition formula is Sm ２ Fe １７ N ３ ) is known to be a ferromagnetic material that exhibits excellent hard magnetic properties. ２ Fe １７ Powder of a substance in which nitrogen is introduced into an Sm—Fe alloy having a stoichiometric composition or a composition close to this, and which is a ferromagnetic material, is called "Sm—Fe—N magnetic powder." Sm—Fe—N magnetic powder is useful as a material for bonded magnets.

[0003] Sm—Fe—N magnetic powders typically oxidize easily when exposed to the air, resulting in a significant decrease in coercive force. When manufacturing bonded magnets using Sm—Fe—N magnetic powders, measures must be taken to prevent oxidation by the air until the powder is embedded in resin. Forming a coating of a phosphate compound on the particle surface of the magnetic powder is known as a method for imparting oxidation resistance to Sm—Fe—N magnetic powders in the air (see, for example, Patent Documents 1 to 3).

[0004] On the other hand, a technique of adding a fatty acid to Sm—Fe—N magnetic powder is known. For example, Patent Document 4 describes adding a fatty acid such as oleic acid, linoleic acid, or linolenic acid to the powder and then grinding the powder in a ball mill in order to shorten the grinding time (paragraphs 0010 and 0011). In Example 1, Sm ２ Fe １７ N ３5 g of the base raw material powder was ground in a glass ball mill using 300 cc of normal hexane as a solvent and 2.4 cc of linolenic acid (paragraph 0036). In this case, the concentration of linolenic acid added was low, so the surface of the magnetic particles could not be covered with linolenic acid (fatty acid) molecules.

[0005] Patent Document 5 describes the addition of an organic compound to Sm-Fe-N powder for sintered magnets, which contains Cr, Si, or other additives to increase the decomposition temperature. This reacts with the oxygen atoms of the organic compound during sintering to form a fine oxide phase, reducing the specific surface area of ​​the α-Fe phase that forms between the crystal grains. As a result, the decrease in coercivity of the permanent magnet due to the formation of the α-Fe phase can be suppressed (paragraph 0044). In the working example, Cr- and Si-containing Sm-Fe-N powder was sieved through a 25 μm mesh sieve and then nitrided. The powder was then mixed with 5% oleic acid by weight in a ball mill, and packed into a mold while being pressed to orient the powder in a magnetic field. The powder was then subjected to spark plasma sintering at a pressure of 1.0 GPa and a sintering temperature of 600°C (paragraph 0054). No heating process was performed in air after the addition of oleic acid. Patent Document 5 does not teach a method for stably improving the heat resistance in air of fine Sm—Fe—N magnetic powder with an average particle size of approximately 1 to 5 μm that is suitable for bonded magnets and does not contain added elements such as Cr or Si.

[0006] Patent Document 6 describes the use of long-chain fatty acids such as stearic acid and oleic acid as lubricants to improve injection moldability (particle dispersibility) when manufacturing bonded magnets using polyamide resins (paragraphs 0056 and 0057). It also describes the use of magnetic powder that is pre-coated with a heat-resistant coating layer made of an inorganic phosphate compound (paragraphs 0048, 0082, and 0083). There is no mention of using fatty acids to improve the heat resistance of magnetic powder in the atmosphere.

[0007] JP 2003-7521 A JP 2003-297618 A JP 2004-111515 A JP 8-316015 A JP 2020-80336 A JP 2018-41882 A

[0008] During the manufacture of bonded magnets, magnetic powder particles are generally heated to around 200°C. Depending on the type of resin, the temperature may rise to around 300°C. Furthermore, when the manufactured bonded magnet is in use, it is possible that it will be exposed to temperatures of several tens of degrees Celsius or more for long periods of time, depending on the application.

[0009] In conventional Sm-Fe-N magnetic particles coated with a phosphate compound, the phosphate compound coating acts as a barrier layer against external oxidizing gases, such as the atmosphere. However, after the bonded magnet is molded, the phosphate compound or a substance derived from it remains near the magnetic particles after its function is completed. The phosphate compound is thought to be primarily composed of iron (III) phosphate, which is produced by the reaction of phosphoric acid with Fe, a constituent element of the magnetic particles. This iron (III) phosphate can be reduced by Sm during the production of the bonded magnet or during use of the bonded magnet product, to form a nano-sized α-Fe phase. The formation of the α-Fe phase is a factor that reduces the magnetic properties of the bonded magnet.

[0010] Meanwhile, as seen in Patent Documents 4 to 6, fatty acids such as stearic acid and oleic acid are sometimes added to magnetic powder during the magnet manufacturing process. However, no technology has been established that uses the addition of fatty acids to stably improve the heat resistance in air of fine Sm—Fe—N magnetic powder with an average particle size of approximately 1 to 5 μm.

[0011] The present invention aims to provide a fine Sm—Fe—N magnetic powder suitable for bonded magnets, which has good heat resistance in an atmospheric environment (i.e., oxidation resistance when heated in the atmosphere) and maintains better magnetic properties when stored in an atmospheric environment than powders surface-treated with conventional phosphate compounds. It also aims to provide a bonded magnet that uses this surface-treated powder as a filler.

[0012] To achieve the above object, the present specification discloses the following invention: [1] A powder composed of particles having a structure in which one or more fatty acid molecules, each having 12 to 20 carbon atoms per molecule, are coated on the surfaces of Sm—Fe—N magnetic particles, wherein the fatty acid-coated Sm—Fe—N magnetic powder has a coating amount index Ci, defined by the following formula (1): Ci = [%C] × [D50] (1), where [%C] is the carbon content (mass %) of the powder and [D50] is the cumulative 50% particle diameter (μm) in a volume-based particle size distribution of the powder measured by a dry laser diffraction / scattering method. [2] The fatty acid-coated Sm—Fe—N magnetic powder according to claim 1, which is for use in bonded magnets. [3] The fatty acid-coated Sm—Fe—N magnetic powder according to [1] or [2] above, wherein the [D50] is 1.0 μm or more and 5.0 μm or less. [4] The fatty acid-coated Sm—Fe—N magnetic powder according to any of [1] to [3] above, wherein the Sm / Fe molar ratio is 0.09 or more and 0.25 or less, and the N / Fe molar ratio is 0.06 or more and 0.30 or less. [5] The fatty acid-coated Sm—Fe—N magnetic powder according to any of [1] to [4] above, wherein the one or more fatty acid molecules are molecules of one or more fatty acids selected from oleic acid, linoleic acid, linolenic acid, and stearic acid. [6] When a sample of the powder is subjected to thermogravimetry in which the temperature is raised from room temperature (25° C.) to 300° C. in an air flow at a heating rate of 5° C. / s, the temperature T ２．０％The fatty acid-coated Sm—Fe—N magnetic powder according to any one of [1] to [5] above, having a temperature of 220° C. or higher. [7] A method for producing a fatty acid-coated Sm—Fe—N magnetic powder, comprising a coating step of mixing a powder of Sm—Fe—N magnetic particles with one or more fatty acids having 12 to 20 carbon atoms per molecule to cause the fatty acid molecules to adhere to the surfaces of the Sm—Fe—N magnetic particles, thereby obtaining a powder having an adhesion amount index Ci, defined by the following formula (1), of 0.5 to 50.0 mass% μm: Ci = [%C] × [D50] (1), where [%C] is the C content (mass%) in the fatty acid-coated Sm—Fe—N magnetic powder, and [D50] is the cumulative 50% particle size (μm) of the fatty acid-coated Sm—Fe—N magnetic powder in a volume-based particle size distribution determined by a dry laser diffraction / scattering method. [8] A method for producing a fatty acid-coated Sm—Fe—N magnetic powder according to the above [7], wherein in the coating step, a powder made of Sm—Fe—N magnetic particles is mixed with one or more fatty acids having 12 to 20 carbon atoms per molecule in a mixing ratio such that the A value, as defined by the following formula (2), is 0.5 μm to 50.0 μm: A value = 100 × [W Ｆ / (W Ｍ +W Ｆ ) × D50 Ｍ ...(2) where W Ｆ is the mass of the fatty acid to be mixed (g), W Ｍ is the mass (g) of the powder consisting of Sm—Fe—N magnetic particles to be mixed, D50 Ｍ is the cumulative 50% particle size (μm) in the volume-based particle size distribution of the powder consisting of Sm—Fe—N magnetic particles to be mixed, as determined by dry laser diffraction / scattering. [9] A bonded magnet using the fatty acid-coated Sm—Fe—N magnetic powder described in any one of [1] to [6] above.

[10] A method for producing a bonded magnet, comprising the step of mixing the fatty acid-coated Sm—Fe—N magnetic powder obtained by the production method described in [7] or [8] above with a resin binder component.

[0013] According to the present invention, a Sm-Fe-N magnetic powder composed of magnetic particles coated with fatty acid molecules has been realized, which has good heat resistance in the atmosphere. This fatty acid-coated Sm-Fe-N magnetic powder is advantageous over conventional phosphate-coated Sm-Fe-N magnetic powder in terms of the ability to maintain its magnetic properties when stored in an atmospheric environment.

[0014] Graph showing TG curves in air for test powders obtained in Examples and Comparative Examples.

[0015] [Sm—Fe—N-based magnetic powder] The Sm (samarium)—Fe (iron)—N (nitrogen)-based magnetic powder of the present invention is composed of particles having a structure in which one or more fatty acid molecules, each having 12 to 20 carbon atoms per molecule, are coated on the surface of Sm—Fe—N-based magnetic particles.

[0016] [Particle diameter] In this specification, the cumulative 50% particle diameter D in the volume-based particle size distribution of the fatty acid-coated Sm—Fe—N magnetic powder measured by a dry laser diffraction / scattering method is ５０ Similarly, the cumulative 10% particle diameter D (μm) in the volume-based particle size distribution of the fatty acid-coated Sm—Fe—N magnetic powder measured by the dry laser diffraction / scattering method is expressed as [D50]. １０ (μm) and cumulative 90% particle diameter D ９０ The particle sizes (μm) are expressed as [D10] and [D90], respectively. Considering bonded magnet applications, [D50] is preferably 0.5 μm or more and 5.0 μm or less, and more preferably 1.0 μm or more and 5.0 μm or less. It is also preferable that the proportion of excessively large particles and excessively small particles be as low as possible. For example, the [D10] / [D50] ratio is preferably 0.20 or more, and more preferably 0.30 or more. The [D90] / [D50] ratio is preferably 5.00 or less, more preferably 3.00 or less, and can also be adjusted to 2.50 or less.

[0017] [Magnetic Material] The magnetic material (Sm—Fe—N magnetic material) in the Sm—Fe—N magnetic powder of the present invention is Th ２ Zn １７ Sm type crystal structure ２ Fe １７It is mainly composed of a magnetic phase in which N (nitrogen) atoms are introduced into the crystal lattice. ２ Fe １７ It is thought that the N atoms are interstitial in the crystal lattice, and the Th ２ Zn １７ The crystal structure is maintained. ２ Fe １７ When N atoms are introduced into the Sm—Fe—N system, the magnetic crystal anisotropy changes from in-plane to uniaxial, and the Curie point rises, making it a practical magnetic material. ２ Fe １７ N ３ The Sm / Fe molar ratio, which means the molar ratio of Sm to Fe, and the N / Fe molar ratio, which means the molar ratio of N to Fe, are ２ Fe １７ N ３ The closer to the stoichiometric composition, the more advantageous it is in terms of magnetic properties, but hard magnetism is also exhibited in the composition range around this. ２ Fe １７ N ３ The stoichiometric Sm / Fe molar ratio is 0.118, and the N / Fe molar ratio is 0.176. In order to stably obtain effective coercivity as a material for bonded magnets in a temperature range including room temperature, the Sm-Fe-N magnetic material of the present invention is preferably adjusted to a composition in which the Sm / Fe molar ratio is in the range of 0.09 to 0.25, and the N / Fe molar ratio is in the range of 0.06 to 0.30. The Sm-Fe-N magnetic material of the present invention contains Sm containing N. ２ Fe １７ In addition to the phase, for example, TbCu ７ SmFe with a crystalline structure ７ Although there may be cases where a different phase such as a hydroxy phase is mixed in, the presence of the different phase is permissible as long as it does not impede the object of the present invention.

[0018] In this specification, particles made of the above magnetic material are specifically referred to as "Sm-Fe-N magnetic particles." The particles that make up the fatty acid-coated Sm-Fe-N magnetic powder of the present invention have a structure in which fatty acid molecules are coated on the surface of Sm-Fe-N magnetic particles. Hereinafter, when simply referring to "magnetic particles," this refers to the above "Sm-Fe-N magnetic particles" unless otherwise specified.

[0019] [Fatty Acid Coating] According to the research of the inventors, by coating the surface of Sm—Fe—N based magnetic particles with one or more fatty acid molecules having 12 to 20 carbon atoms per molecule, heat resistance in the atmosphere can be imparted.

[0020] Powder particles generally have a positive charge on their surfaces, making them prone to adsorbing negatively charged substances. It is believed that the carboxyl group at the end of the carbon chain of a fatty acid molecule has a negative charge at the O (oxygen) portion of the double bond. Furthermore, Sm (samarium), a constituent element of Sm-Fe-N magnetic particles, has a strong affinity for O (oxygen). These factors allow fatty acid molecules to be strongly adsorbed to the surface of Sm-Fe-N magnetic particles.

[0021] By adsorbing fatty acid molecules at a high density to the surface of Sm—Fe—N magnetic particles, oxygen molecules in the air are less likely to come into contact with the metal elements (especially Sm, which is easily oxidized) on the surface of the magnetic particles. To fully utilize this effect, fatty acids with 12 or more carbon atoms per molecule are used. Fatty acid molecules with such a high carbon number provide steric hindrance to atmospheric oxygen reaching the magnetic particle surface, significantly improving the oxidation resistance of the magnetic particles in the atmosphere. From the perspective of exerting high steric hindrance, fatty acids with 14 or more carbon atoms per molecule are more preferable, and fatty acids with 16 or more carbon atoms per molecule are even more preferable. However, if the number of carbon atoms per molecule is excessively large, excessive steric hindrance will reduce the particle packing ability when producing a bonded magnet, which is undesirable from the perspective of ensuring sufficient fluidity and moldability. Therefore, in the present invention, fatty acids with 20 or less carbon atoms per molecule are used. A single type of fatty acid with 12 to 20 carbon atoms per molecule may be used, or two or more types may be used in combination.

[0022] When a fatty acid with 12 or more carbon atoms per molecule is heated in the atmosphere, it usually begins to volatilize around 200°C. However, it has been found that fatty acid molecules adsorbed on the surface of Sm-Fe-N magnetic particles contribute to maintaining the oxidation resistance of the magnetic particles even when heated to 200°C or higher in the atmosphere. This is presumably because, even if some of the fatty acid molecules adsorbed on the surface of the magnetic particles are desorbed and volatilized during heating, there are many fatty acid molecules that remain adsorbed on the surface of the magnetic particles. The reason for this is thought to be that, as mentioned above, fatty acid molecules have the property of being strongly adsorbed on the surface of Sm-Fe-N magnetic particles.

[0023] Fatty acids applicable to the present invention include lauric acid (12 carbon atoms), myristic acid (14 carbon atoms), palmitic acid (16 carbon atoms), stearic acid (18 carbon atoms), oleic acid, linoleic acid, linolenic acid, and dihomo-γ-linolenic acid (20 carbon atoms), arachidonic acid, and eicosapentaenoic acid. Fatty acids that are liquid at or near room temperature under atmospheric pressure have a wide range of coating treatment conditions, and are therefore advantageous in that a highly uniform fatty acid coating layer can be formed relatively easily on the surface of the magnetic particles. From this perspective, it is preferable to use unsaturated fatty acids. Suitable unsaturated fatty acids include, for example, oleic acid, linoleic acid, and linolenic acid.

[0024] To improve the heat resistance of Sm-Fe-N magnetic powders in the atmosphere (i.e., their oxidation resistance when heated in an oxygen-containing environment), it is necessary to fully utilize the "steric hindrance" provided by fatty acid molecules. To achieve this, it is important that the surfaces of the magnetic particles are densely coated with fatty acid molecules. The amount of fatty acid molecules attached can be determined by the C (carbon) content (mass%) of the Sm-Fe-N magnetic powder. However, since the specific surface area varies depending on the particle size, it is necessary to consider the particle size in addition to the C content (mass%) to evaluate whether the surfaces of the magnetic particles are densely coated with fatty acid molecules. After various studies, it was found that the range of fatty acid attachment amounts that can stably and sufficiently exert steric hindrance can be determined by the attachment amount index Ci, defined by the following formula (1): Ci = [%C] × [D50] (1) where [%C] is the C content (mass%) in a powder composed of Sm-Fe-N magnetic particles to which fatty acid molecules are attached.

[0025] [Fatty Acid Coated Sm—Fe—N Magnetic Powder] Among fatty acid coated Sm—Fe—N magnetic powders using one or more fatty acids with 12 to 20 carbon atoms per molecule, powders that exhibit excellent heat resistance in air are specified by having an adhesion amount index Ci of 0.5 to 50.0 mass% μm inclusive, as defined by the above formula (1). The adhesion amount index Ci is preferably 1.0 to 10.0 mass% μm inclusive, and more effectively 3.0 to 8.0 mass% μm inclusive. In particular, fatty acid-coated Sm—Fe—N-based magnetic powders for bonded magnets are extremely effective when they have a particle size distribution in which [D50] is 0.5 μm or more and 5.0 μm or less, a [D10] / [D50] ratio of 0.20 or more, and a [D90] / [D50] ratio of 5.00 or less, and a coating amount index Ci is 1.0 mass% μm or more and 10.0 mass% μm or less.

[0026] The heat resistance of the fatty acid-coated Sm—Fe—N magnetic powder according to the present invention in the atmosphere can be measured, for example, by measuring the temperature T at which a sample of the powder increases in weight by 2.0% compared to the weight of the powder sample at the start of heating when the powder sample is subjected to thermogravimetry in which the sample is heated from room temperature (25° C.) to 300° C. at a rate of 5° C. / s in an air flow. ２．０％ is 220°C or higher.

[0027] The fatty acid-coated Sm—Fe—N magnetic powder according to the present invention preferably has a coercive force of 490 kA / m or more and 1600 kA / m or less, and more preferably 500 kA / m or more and 1200 kA / m or less, when left in air at room temperature for 24 hours and then subjected to magnetic measurement by the method described below.

[0028] [Manufacturing Method] The fatty acid-coated Sm—Fe—N magnetic powder of the present invention can be obtained by using a powder consisting of Sm—Fe—N magnetic particles as a raw material and, for example, by utilizing the coating process described below to adhere fatty acid molecules to the surfaces of the Sm—Fe—N magnetic particles.

[0029] The powder (hereinafter sometimes referred to as "raw material powder") made of Sm-Fe-N magnetic particles can be manufactured by a reduction-diffusion method, a gas atomization method, or the like. Of these, the gas atomization method has the advantage of not causing an environmental burden due to alkaline waste liquid. In order to obtain fatty acid-coated Sm-Fe-N magnetic powder having the above-mentioned preferred particle size distribution, it is desirable to prepare a raw material powder having a particle size distribution close to the desired one in advance.

[0030] [Coating Process] By mixing a powder of Sm—Fe—N magnetic particles as the starting material with one or more fatty acids having 12 to 20 carbon atoms per molecule in a non-oxidizing atmosphere, fatty acid molecules can be deposited on the surfaces of the Sm—Fe—N magnetic particles. If necessary, a solvent in which the fatty acid is soluble may be added during the mixing. Examples of the non-oxidizing atmosphere include an inert atmosphere such as nitrogen, argon, or helium, and a reducing atmosphere such as hydrogen or carbon monoxide. From the standpoint of cost and safety, a nitrogen gas atmosphere is preferred. Mixing devices such as a vibrating mill, sample mill, Henschel mixer, and fluidized bed mixer can be used as a grinding or mixing device. However, it is desirable to apply a relatively light stirring force or shorten the operating time of the device to minimize the introduction of lattice strain into the magnetic particles during this mixing. Therefore, the purpose of this mixing is not to cause additional grinding of the magnetic particles, but to deposit fatty acid molecules on the surfaces of the magnetic particles.

[0031] The amount of fatty acid used is set according to the particle size of the raw material powder so that the coating amount index Ci according to the above-mentioned formula (1) falls within the above-mentioned predetermined range in this coating process. Specifically, for example, a method of mixing a powder made of Sm-Fe-N magnetic particles with one or more fatty acids having 12 to 20 carbon atoms per molecule in a mixing ratio such that the A value defined by the following formula (2) is 0.5 μm to 50.0 μm. It is more preferable that the A value is 1.0 μm to 10.0 μm, and even more effective that it is 3.0 μm to 8.0 μm. A value = 100 × [W Ｆ / (W Ｍ +W Ｆ ) × D50 Ｍ ...(2) where W Ｆ is the mass of the fatty acid to be mixed (g), W Ｍ is the mass (g) of the powder consisting of Sm—Fe—N magnetic particles to be mixed, D50 Ｍ is the cumulative 50% particle size (μm) in the volume-based particle size distribution determined by the dry laser diffraction / scattering method for the powder made of Sm—Fe—N magnetic particles to be mixed.

[0032] If a solvent is used, the solvent components will remain in the Sm—Fe—N magnetic powder after the coating process has been completed. Therefore, it is desirable to volatilize and remove the solvent components before subjecting the powder to the bonded magnet manufacturing process.

[0033] [Bonded Magnet] The fatty acid coated Sm—Fe—N magnetic powder according to the present invention can be molded into a bonded magnet using commonly known techniques, similar to conventional phosphate compound coated Sm—Fe—N magnetic powder.

[0034] The particle size distribution measurement and elemental analysis of the powder were carried out by the following methods.

[0035] (Measurement of Particle Size Distribution) The volumetric particle size distribution was measured using a dry laser diffraction particle size distribution analyzer (Helos / Rodos, manufactured by Sympatec).

[0036] (Elemental Analysis) Metal element analysis was performed by heating and dissolving the analytical sample in hydrochloric acid in a glove box filled with argon (Ar) gas, followed by dilution to prepare a sample solution for analysis, and analyzing this solution with an ICP optical emission spectrometer (Agilent Technologies, Agilent 720). Nitrogen analysis was performed by the inert gas fusion-thermal conductivity method using an oxygen / nitrogen analyzer (Horiba, EMGA-920). Carbon analysis was performed by the infrared absorption method using a carbon / sulfur analyzer (LECO, CS-744).

[0037] Example 1 (Synthesis of Sm—Fe-based powder by gas atomization) A pre-melted Sm—Fe alloy was used as the raw material alloy. Elemental analysis revealed that the Sm / Fe molar ratio of this raw material alloy was 0.14. 1000 g of this raw material was placed in a boron nitride (BN) crucible and melted by high-frequency induction heating in an argon (Ar) atmosphere. After the raw material alloy was completely molten, 30 minutes after the start of heating, the entire amount of 1550°C molten metal was ejected from a boron nitride (BN) nozzle with an inner diameter of 3.0 mm into the lower gas phase space. The maximum supply pressure of the molten metal ejection gas was 65 kPa, calculated as the differential pressure from the atmospheric gas pressure. Argon was used as the ejection gas. The lower gas phase space was also filled with argon. The resulting powder (hereinafter referred to as "gas-atomized powder") was then completely recovered.

[0038] As a result of elemental analysis, the Sm / Fe molar ratio of the gas atomized powder was 0.13, which was equivalent to that of the raw material alloy. The obtained gas atomized powder was classified using a sieve with a mesh size of 16 μm in a glove box under a nitrogen atmosphere to obtain a powder from which fine particles had been removed (hereinafter referred to as "classified gas atomized powder"). As a result of particle size distribution measurement, the cumulative 50% particle diameter D of the classified gas atomized powder was ５０ was 33.2 μm.

[0039] (Heat Treatment) The classified gas atomized powder was placed in a rotary tubular furnace and heat-treated by holding at 850°C for 5 minutes while flowing argon (Ar) gas into the furnace, followed by cooling to a temperature near room temperature (50°C or less) while flowing argon gas.

[0040] (Nitriding Treatment) After the temperature reached near room temperature, the flow gas flowing into the rotary tubular furnace was changed from argon gas to ammonia (NH ３ ) 35% by volume, hydrogen (H ２The temperature was then increased and the mixture was held at 370°C for 60 minutes in the mixed gas. Subsequently, the supply of ammonia was stopped while heating was maintained, and the mixture was exposed to a flow gas of 100% hydrogen for 60 minutes. The flow gas was then switched to argon gas, and the mixture was held at 370°C for 60 minutes. Thereafter, the heating was stopped, and the mixture was cooled to a temperature near room temperature while flowing argon gas. Next, the flow gas flowing into the rotary tubular furnace was changed from argon gas to ammonia (NH ３ ) 35% by volume, hydrogen (H ２ ) 65% by volume composition. The temperature was then increased and the mixture was held at 470°C for 60 minutes. While maintaining the heating, the ammonia supply was stopped and the mixture was exposed to a 100% hydrogen flow gas for 60 minutes. The flow gas was then switched to argon gas and the mixture was held at 470°C for 60 minutes. The heating was then stopped and the mixture was cooled to near room temperature while argon gas was flowing, yielding nitrided powder. Elemental analysis revealed that the Sm / Fe molar ratio of the nitrided powder was 0.13 and the N / Fe molar ratio was 0.16.

[0041] (Pulverization) 210 g of the above nitrided powder, 2.1 g of ethanol, and 4,500 g of 1.6 mm diameter stainless steel balls were placed in a 1.2 L stainless steel pot filled with nitrogen gas and sealed. Pulverization was performed using a vibration mill (YAMP-2SND, manufactured by Uras Techno Co., Ltd.) at an amplitude of ±2.5 mm and a frequency of 29.1 Hz for 168 minutes. This pulverization process crushes the particles along the grain boundaries embrittled by the nitriding process, thereby preventing excessive lattice strain from being introduced into the magnetic material. After operation, the sample was separated from the balls in a glove box filled with nitrogen gas, and a powder consisting of Sm—Fe—N magnetic particles was obtained. Particle size distribution measurement revealed that the cumulative 50% particle diameter D of the powder consisting of Sm—Fe—N magnetic particles was 0.015 μm. ５０ The powder made of Sm—Fe—N magnetic particles was pulverized to such an extent that almost no breakable embrittled crystal grain boundaries remained during the 168-minute pulverization process, and therefore further pulverization (pulverization) of the particles hardly occurred during the mixing operation in the coating step described below.

[0042] (Coating Step) In this example, oleic acid (manufactured by Kanto Chemical Co., Inc.) with 18 carbon atoms per molecule was used as the fatty acid. In a nitrogen gas atmosphere glove box, 20 g of powder (raw material powder) consisting of Sm—Fe—N magnetic particles obtained as described above, 0.6 g of oleic acid, and 2 mL of industrial ethanol as a solvent were placed in a 70 mL mixer with a rotating stainless steel propeller, and mixing was performed at 25,000 rpm for 60 seconds to coat the surfaces of the magnetic particles with the fatty acid. The amount of fatty acid added per 100 parts by mass of the raw material powder was 3.0 parts by mass. In addition, in the above formula (2), the mass W of the fatty acid Ｆ = 0.6 g, the mass W of the powder made of Sm-Fe-N magnetic particles to be mixed Ｍ = 20 g, cumulative 50% particle diameter D50 in the volume-based particle size distribution by dry laser diffraction / scattering method for the powder consisting of Sm-Fe-N based magnetic particles to be mixed Ｍ = 1.6 μm, the A value defined in the above formula (2) is 4.66 μm.

[0043] The obtained powder was removed from the mixer and placed in a glove box for 30 minutes to volatilize the solvent component (ethanol), thereby obtaining a test powder. The obtained test powder was examined for the following:

[0044] (Carbon content) The C (carbon) content of the test powder was measured using the above-mentioned elemental analysis method. As a result, the C content was 2.8 mass%. Since this C (carbon) was due to the mixed fatty acid, the test powder obtained in this example can be said to be a fatty acid-coated Sm—Fe—N based magnetic material composed of particles with fatty acid molecules coated on the surface of the magnetic particles (the same applies to each of the following examples).

[0045] (Particle size distribution) When the particle size distribution of the test powder in this example was measured using the dry laser diffraction particle size distribution analyzer, the [D10] was 0.62 μm, the [D50] was 1.86 μm, and the [D90] was 4.11 μm. The [D10] / [D50] ratio was 0.33, and the [D90] / [D50] ratio was 2.21. The coating amount index Ci according to the formula (1) was 5.21 mass% μm.

[0046] (Thermogravimetry) The weight change of the test powder during the heating process in a dry air stream of 100 mL / min was measured using a differential thermal-thermogravimetry simultaneous analyzer (TG-DTA8122, manufactured by Rigaku Corporation). A 20 mg sample of the test powder was placed in an aluminum container and heated from room temperature (25°C) to 300°C at a heating rate of 5°C / s. Alumina was used as the standard material.

[0047] Figure 1 shows an example of a TG (thermogravimetric) curve (the same applies to each example described later). The weight increase with increasing temperature is due to the oxidation of the magnetic particles. Here, the TG curve shows the temperature T at which a weight increase of 1.5% was observed compared to the original sample weight. １．５％ (°C) and the temperature T at which a 2.0% weight increase was observed ２．０％ (°C) was calculated. １．５％ (°C) and T ２．０％ The higher the T (°C), the better the heat resistance in the atmosphere. １．５％ is 219℃, T ２．０％ The temperature was 291°C.

[0048] (Magnetic Measurement) After the above-described coating process, the test powder sample was left in an air atmosphere at room temperature (25°C) for 24 hours and subjected to magnetic measurements. A sample cell containing 20 mg of the sample powder and paraffin was placed in the center of an electromagnet, and the sample cell was heated to 80°C for 2 minutes using a hot air generator. The amount of paraffin filled was determined so that the sample cell's container was filled with the sample powder and paraffin. Next, while still heated at 80°C, an external magnetic field of 1.0 T (tesla) was applied to the sample cell using the electromagnet for 2 minutes. Next, the sample cell was cooled to room temperature with the 1.0 T magnetic field still applied. In this way, a magnetically oriented measurement sample was obtained. The measurement sample was placed in a VSM (VSM-P7 model, manufactured by Toei Kogyo Co., Ltd.) with the direction of the applied magnetic field parallel to the magnetic orientation direction of the measurement sample, and the coercive force Hc was measured. The measurement conditions were a maximum applied magnetic field of 1.27 MA / m (1.6 T).

[0049] The Sm-Fe-N magnetic powder of this example left in the air for 24 hours had a coercive force Hc of 569.3 kA / m and a saturation magnetization σs of 104.8 A·m ２ / kg, residual magnetization σr is 84.3 A m ２ The results are shown in Tables 1 and 2 (the same applies to the following examples).

[0050] [Example 2] A test powder was prepared and tested under the same conditions as in Example 1, except that in the coating process, the amount of oleic acid mixed was 0.2 g and the amount of industrial ethanol mixed was 1 mL. The amount of fatty acid added per 100 parts by mass of raw material powder was 1.0 part by mass. The raw material powder used was the same as in Example 1. In addition, in the above formula (2), the mass W of the fatty acid Ｆ = 0.2 g, the mass W of the powder made of Sm—Fe—N based magnetic particles to be mixed Ｍ = 20 g, cumulative 50% particle diameter D50 in the volume-based particle size distribution by dry laser diffraction / scattering method for the powder consisting of Sm-Fe-N based magnetic particles to be mixed Ｍ = 1.6 μm, the A value defined in the above formula (2) is 1.58 μm.

[0051] The test powder in this example had a C content of 0.91 mass%, a [D10] of 0.56 μm, a [D50] of 1.51 μm, and a [D90] of 3.44 μm. The [D10] / [D50] ratio was 0.37, and the [D90] / [D50] ratio was 2.28. The coating amount index Ci calculated by the formula (1) was 1.37 mass% μm.

[0052] As a result of thermogravimetry, the T １．５％ is 205℃, T ２．０％ The magnetic measurement results showed that the Sm-Fe-N magnetic powder of this example, left in the air for 24 hours, had a coercive force Hc of 538.0 kA / m and a saturation magnetization σs of 105.1 A·m ２ / kg, residual magnetization σr is 79.1 A m ２ / kg.

[0053] [Example 3] A test powder was prepared and tested under the same conditions as in Example 1, except that in the coating process, linoleic acid (manufactured by Nacalai Tesque, Inc.), which has 18 carbon atoms per molecule, was used as the fatty acid and the amount of linoleic acid mixed was 0.6 g. The amount of fatty acid added per 100 parts by mass of raw material powder was 3.0 parts by mass. The raw material powder used was the same as in Example 1. The A value defined in the above formula (2) was 4.66 μm, the same as in Example 1.

[0054] The test powder in this example had a C content of 1.9 mass%, a [D10] of 0.82 μm, a [D50] of 2.35 μm, and a [D90] of 4.91 μm. The [D10] / [D50] ratio was 0.35, and the [D90] / [D50] ratio was 2.09. The coating amount index Ci calculated by the formula (1) was 4.47 mass% μm.

[0055] As a result of thermogravimetry, the T １．５％ is 250°C, T ２．０％ The magnetic measurement results showed that the Sm-Fe-N magnetic powder of this example, left in the air for 24 hours, had a coercive force Hc of 523.4 kA / m and a saturation magnetization σs of 102.5 A·m ２ / kg, residual magnetization σr is 75.1 A m ２ / kg.

[0056] [Example 4] Sample powders were prepared and tested under the same conditions as in Example 1, except that in the coating process, (9,12,15)-linolenic acid (manufactured by Nacalai Tesque, Inc.), which has 18 carbon atoms per molecule, was used as the fatty acid, and the amount of linolenic acid mixed was 0.6 g. The amount of fatty acid added per 100 parts by mass of raw material powder was 3.0 parts by mass. The raw material powder used was the same as in Example 1. The A value defined in the above formula (2) was 4.66 μm, the same as in Example 1.

[0057] The test powder in this example had a C content of 2.5 mass%, a [D10] of 0.91 μm, a [D50] of 2.40 μm, and a [D90] of 4.96 μm. The [D10] / [D50] ratio was 0.38, and the [D90] / [D50] ratio was 2.07. The coating amount index Ci calculated by the formula (1) was 6.00 mass% μm.

[0058] As a result of thermogravimetry, the T １．５％ is 274℃, T ２．０％ The magnetic measurement results showed that the Sm-Fe-N magnetic powder of this example, left in the air for 24 hours, had a coercive force Hc of 545.6 kA / m and a saturation magnetization σs of 101.2 A·m ２ / kg, residual magnetization σr is 72.5 A m ２ / kg.

[0059] [Example 5] In the coating process, stearic acid (manufactured by Katayama Chemical Industry Co., Ltd.), which has 18 carbon atoms per molecule, was used as the fatty acid. The test powder was prepared and tested under the same conditions as in Example 1, except that the amount of stearic acid mixed was 0.6 g and no solvent component was used. The amount of fatty acid added per 100 parts by mass of raw material powder was 3.0 parts by mass. The raw material powder used was the same as in Example 1. Furthermore, the A value defined in the above formula (2) was 4.66 μm, the same as in Example 1.

[0060] The test powder in this example had a C content of 2.4 mass%, a [D10] of 0.50 μm, a [D50] of 1.38 μm, and a [D90] of 3.07 μm. The [D10] / [D50] ratio was 0.36, and the [D90] / [D50] ratio was 2.22. The coating amount index Ci calculated by the formula (1) was 3.31 mass% μm.

[0061] As a result of thermogravimetry, the T １．５％ is 283℃, T ２．０％ The magnetic properties of the Sm-Fe-N magnetic powder of this example left in the air for 24 hours were as follows: coercivity Hc was 503.2 kA / m, saturation magnetization σs was 105.8 A·m ２ / kg, residual magnetization σr is 74.2 A m ２ / kg.

[0062] [Comparative Example 1] In this example, the coating process was omitted. However, as a preliminary experiment, when the above-mentioned raw material powder was removed from the nitrogen gas atmosphere into an air atmosphere, the powder temperature reached 245°C after 10 seconds, causing a sudden heat generation. This heat generation was caused by an oxidation reaction occurring when the Sm—Fe—N-based magnetic particles came into contact with oxygen in the air. Therefore, in this example, in order to prevent a sudden heat generation due to oxidation in an air environment, the raw material powder used in Example 1 was exposed to an atmosphere with an oxygen concentration of 1% by volume in a glove box for 24 hours, thereby gradually oxidizing it (stabilized powder), and this was used as the test powder. Except for using this stabilized powder as the test powder, a test was conducted under the same conditions as in Example 1.

[0063] The test powder in this example had a C content of 0.0 mass%, a [D10] of 0.64 μm, a [D50] of 1.56 μm, and a [D90] of 3.18 μm. The [D10] / [D50] ratio was 0.41, and the [D90] / [D50] ratio was 2.04. The deposition amount index Ci according to equation (1) was 0.00 mass% μm.

[0064] As a result of thermogravimetry, the T １．５％ is 163℃, T ２．０％ The magnetic measurement results showed that the Sm-Fe-N magnetic powder of this example, left in the air for 24 hours, had a coercive force Hc of 533.9 kA / m and a saturation magnetization σs of 103.0 A·m ２ / kg, residual magnetization σr is 77.4 A m ２ / kg.

[0065] Comparative Example 2: A test powder was prepared and tested under the same conditions as in Example 1, except that benzotriazole (special grade, manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of fatty acid in the coating process, and the amount of benzotriazole added was 0.6 g. The amount of benzotriazole added per 100 parts by mass of raw material powder was 3.0 parts by mass. The raw material powder used was the same as in Example 1.

[0066] The test powder in this example had a C content of 1.8 mass%, a [D10] of 0.56 μm, a [D50] of 1.51 μm, and a [D90] of 3.75 μm. The [D10] / [D50] ratio was 0.37, and the [D90] / [D50] ratio was 2.48. The coating amount index Ci calculated by the formula (1) was 2.72 mass% μm.

[0067] As a result of thermogravimetry, the T １．５％ is 199℃, T ２．０％ The magnetic measurement results showed that the Sm-Fe-N magnetic powder of this example, left in the air for 24 hours, had a coercive force Hc of 525.8 kA / m and a saturation magnetization σs of 97.6 A·m. ２ / kg, residual magnetization σr is 73.9 A m ２ / kg.

[0068] [Comparative Example 3] In the coating process, phosphoric acid (H ３ P.O. ４ The preparation of the test powder, measurement of the C content, thermogravimetry, and magnetic measurement were carried out under the same conditions as in Example 1, except that 0.6 g of phosphoric acid was added and 3.0 parts by mass of benzotriazole was added per 100 parts by mass of the raw material powder. The raw material powder used was the same as in Example 1.

[0069] The test powder of this example had a C content of 0.0 mass%, a [D10] of 0.70 μm, a [D50] of 1.68 μm, and a [D90] of 3.23 μm. The [D10] / [D50] ratio was 0.42, and the [D90] / [D50] ratio was 1.92. As a result of the thermogravimetric measurement, the T of the test powder of this example １．５％ is 216℃, T ２．０％ The magnetic measurement results showed that the Sm-Fe-N magnetic powder of this example, left in the air for 24 hours, had a coercive force Hc of 474.8 kA / m and a saturation magnetization σs of 97.4 A·m. ２ / kg, residual magnetization σr is 66.8 A m ２ / kg.

[0070]

[0071]

[0072] In the TG curves of the fatty acid-coated Sm—Fe—N magnetic powders of each example according to the present invention, a decrease in the weight gain rate (slope of the curve) is observed during the heating process, which is thought to be due to the desorption of fatty acids. However, no rapid weight gain due to oxidation occurs up to at least 300°C. Therefore, it is presumed that the fatty acid molecules remain adsorbed on the surface of the Sm—Fe—N magnetic particles up to high temperatures exceeding the volatilization temperature of the fatty acids, thereby exhibiting excellent heat resistance in air (oxidation resistance during heating). On the other hand, as seen in the example of the benzotriazole-coated Sm—Fe—N magnetic powder of Comparative Example 2, when coated with an organic compound that is thought to have a lower adsorptive power than fatty acids, the organic compound volatilizes and is removed during heating, resulting in reduced heat resistance in air.

[0073] When comparing the magnetic properties after 24 hours in air at room temperature, a significant decrease in magnetic properties (especially coercive force Hc) was observed in the conventional Sm-Fe-N magnetic powder with a coating layer made of phosphoric acid. It was confirmed that the magnetic properties could be maintained better when coated with a fatty acid than when coated with a phosphoric acid compound.

Claims

1. A powder composed of particles having a structure in which one or more fatty acid molecules, each having 12 to 20 carbon atoms per molecule, are coated on the surface of Sm-Fe-N magnetic particles, wherein the fatty acid-coated Sm-Fe-N magnetic powder has a coating amount index Ci, defined by the following formula (1), of 0.5 to 50.0 mass% μm, where [%C] is the carbon content (mass%) of the powder and [D50] is the cumulative 50% particle diameter (μm) in the volume-based particle size distribution of the powder measured by dry laser diffraction / scattering method: Ci = [%C] x [D50] (1) 2. The fatty acid coated Sm-Fe-N magnetic powder according to claim 1, which is for use in bonded magnets.

3. The fatty acid coated Sm-Fe-N magnetic powder according to claim 1, wherein the [D50] is 1.0 μm or more and 5.0 μm or less.

4. The fatty acid coated Sm-Fe-N magnetic powder according to claim 1, wherein the Sm / Fe molar ratio is 0.09 or more and 0.25 or less, and the N / Fe molar ratio is 0.06 or more and 0.30 or less.

5. The fatty acid coated Sm-Fe-N magnetic powder according to claim 1, wherein the one or more fatty acid molecules are molecules of one or more fatty acids selected from the group consisting of oleic acid, linoleic acid, linolenic acid, and stearic acid.

6. When a sample of the powder is subjected to thermogravimetric measurement in which the temperature is raised from room temperature (25°C) to 300°C at a rate of 5°C / s in an air stream, the temperature T at which the weight of the powder sample increases by 2.0% compared to the weight at the start of the heating ２．０％ 2. The fatty acid-coated Sm—Fe—N magnetic powder according to claim 1, wherein the temperature is 220° C. or higher.

7. A method for producing a fatty acid-coated Sm—Fe—N magnetic powder, comprising a coating step of mixing a powder of Sm—Fe—N magnetic particles with one or more fatty acids having 12 to 20 carbon atoms per molecule to deposit the fatty acid molecules onto the surfaces of the Sm—Fe—N magnetic particles, thereby obtaining a powder having an adhesion index Ci, defined by the following formula (1), of 0.5 to 50.0 mass % μm: Ci = [% C] × [D50] (1) where [% C] is the C content (mass %) in the fatty acid-coated Sm—Fe—N magnetic powder, and [D50] is the cumulative 50% particle size (μm) of the fatty acid-coated Sm—Fe—N magnetic powder in a volume-based particle size distribution determined by a dry laser diffraction / scattering method.

8. A method for producing fatty acid-coated Sm—Fe—N magnetic powder according to claim 7, wherein in the coating step, a powder made of Sm—Fe—N magnetic particles is mixed with one or more fatty acids having 12 to 20 carbon atoms per molecule in a mixing ratio such that the A value, as defined by the following formula (2), is 0.5 μm to 50.0 μm: A value = 100 × [W Ｆ / (W Ｍ +W Ｆ ) × D50 Ｍ ...(2) where W Ｆ is the mass of the fatty acid to be mixed (g), W Ｍ is the mass (g) of the powder consisting of Sm—Fe—N magnetic particles to be mixed, D50 Ｍ is the cumulative 50% particle size (μm) in the volume-based particle size distribution determined by the dry laser diffraction / scattering method for the powder made of Sm—Fe—N magnetic particles to be mixed.

9. A bonded magnet using the fatty acid-coated Sm-Fe-N magnetic powder according to any one of claims 1 to 6.

10. A method for producing a bonded magnet, comprising the step of mixing the fatty acid-coated Sm-Fe-N magnetic powder obtained by the method of claim 7 with a resin binder component.

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