Silicon oxide-coated soft magnetic powder and method for producing silicon oxide-coated soft magnetic powder

Abstract

Disclosed is a silicon oxide-coated soft magnetic powder which is obtained by coating the surface of each core particle with silicon oxide, the core particle being formed of a soft magnetic metal that contains 20 mass% or more of iron. If Vs (V) is the dielectric breakdown voltage of the silicon oxide-coated soft magnetic powder, Vc (V) is the dielectric breakdown voltage of only the core particles, and Ts (nm) is the average film thickness of the silicon oxide, the increase rate A (% / nm) of the dielectric breakdown voltage per unit film thickness of the silicon oxide as defined by formula (1) is 55 or more.　(1): A = ((Vs / Vc) - 1) × 100 / Ts

Description

Silicon oxide coated soft magnetic powder and method for producing silicon oxide coated soft magnetic powder

[0001] The present invention relates to a silicon oxide-coated soft magnetic powder and a method for producing the silicon oxide-coated soft magnetic powder.

[0002] Conventionally, dust cores using soft magnetic powders such as iron powder, iron-containing alloy powders, and intermetallic compound powders have been known as magnetic cores for inductors, choke coils, transformers, reactors, motors, and the like. Metal powders such as iron powder and iron alloy powder have higher electrical conductivity than compound powders such as ferrite powder. Therefore, when manufacturing a magnetic core using a metal powder, it is common to first form an insulating coating on the surface of the metal powder particles, and then subject the resulting core to compression molding and heat treatment.

[0003] Various insulating coatings have been proposed in the past. For example, Patent Document 1 discloses a silicon oxide-coated soft magnetic powder having a silicon oxide coating layer with excellent insulating properties on the particle surface.

[0004] Japanese Patent Application Laid-Open No. 2021-34460

[0005] An object of one embodiment of the present invention is to provide a silicon oxide coated soft magnetic powder that has an excellent rate of increase in breakdown voltage.

[0006] A first aspect of the present invention is a silicon oxide-coated soft magnetic powder in which the surfaces of core particles made of a soft magnetic metal containing 20% ​​by mass or more of iron are coated with silicon oxide, wherein the silicon oxide-coated soft magnetic powder has a rate of increase A (% / nm) of breakdown voltage per unit film thickness of silicon oxide defined by the following formula (1) of 55 or more, where Vs (V) is the breakdown voltage of the silicon oxide-coated soft magnetic powder, Vc (V) is the breakdown voltage of the core particles alone, and Ts (nm) is the average film thickness of the silicon oxide. A=((Vs / Vc)-1)×100 / Ts (1)

[0007] A second aspect of the present invention is the silicon oxide-coated soft magnetic powder according to the first aspect, wherein the silicon oxide-coated soft magnetic powder has a sphericity B of 0.13 or more as defined by the following formula (2): B=Dx / Dy (2) where Dx (μm) is the BET diameter of the silicon oxide-coated soft magnetic powder and Dy (μm) is the median diameter (D50).

[0008] A third aspect of the present invention is the silicon oxide-coated soft magnetic powder according to the first aspect, wherein the silicon oxide has an average film thickness of 0.1 nm or more and 50 nm or less.

[0009] A fourth aspect of the present invention is the silicon oxide-coated soft magnetic powder according to any one of the first to third aspects, wherein the silicon oxide-coated soft magnetic powder has a median diameter (D50) of 0.1 μm or more and 50 μm or less.

[0010] A fifth aspect of the present invention is a method for producing a silicon oxide-coated soft magnetic powder, comprising the steps of: mixing a mixed solvent of water and an organic solvent containing 1% to 40% by mass of water with core particles made of a soft magnetic metal containing 20% ​​or more by mass of iron to obtain a slurry; adding silicon alkoxide to the slurry and stirring and mixing; adding a hydrolysis catalyst for the silicon alkoxide to the slurry containing the silicon alkoxide to coat the surfaces of the core particles with silicon oxide; and performing solid-liquid separation of the slurry and drying the solids to obtain a silicon oxide-coated soft magnetic powder coated with silicon oxide, wherein in the silicon oxide coating step, the hydrolysis catalyst is added so as to satisfy at least one of the following conditions (a) or (b): (a) the total amount of the hydrolysis catalyst added relative to the weight of the slurry is 0.8 mmol / g or less; and (b) the rate of addition of the hydrolysis catalyst relative to the weight of the slurry is 9 (μmol / g) / min or less.

[0011] A sixth aspect of the present invention is the method for producing a silicon oxide-coated soft magnetic powder according to the fifth aspect, wherein the step of coating with silicon oxide satisfies both of the conditions (a) and (b).

[0012] A seventh aspect of the present invention is the method for producing a silicon oxide-coated soft magnetic powder according to the fifth aspect, wherein the stirring and mixing step is performed so as to satisfy the following condition (c): (c) the stirring power relative to the weight of the slurry is 3 W / kg or more.

[0013] An eighth aspect of the present invention is the method for producing a silicon oxide-coated soft magnetic powder according to any one of the fifth to seventh aspects, wherein the hydrolysis catalyst is ammonia.

[0014] According to one embodiment of the present invention, it is possible to provide a silicon oxide coated soft magnetic powder that has an excellent rate of increase in breakdown voltage.

[0015] Fig. 1 is a cross-sectional schematic view of a silicon oxide-coated soft magnetic powder 10 according to a first embodiment of the present invention. Fig. 2 is a flowchart showing an example of a method for producing the silicon oxide-coated soft magnetic powder 10 according to the first embodiment of the present invention.

[0016] <Insights Gained by the Inventors> First, insights gained by the inventors will be described.

[0017] Silicon oxide coated soft magnetic powder is generally mixed with resin to be used in devices, but if the silicon oxide has cracks, pores, etc., the specific surface area of ​​the powder increases, which causes a problem of poor fluidity when mixed with resin, making it difficult to create devices.

[0018] In response to the above-mentioned problems, Patent Document 1 discloses a silicon oxide-coated soft magnetic powder having a silicon oxide coating layer on the particle surface that is thin, has excellent insulating properties, and has a high degree of film thickness uniformity and few defects, and the powder has a small specific surface area, which is advantageous in reducing the amount of resin required during pressure molding.

[0019] However, silicon oxide-coated soft magnetic powders have a problem in that dielectric breakdown is likely to occur in the insulating film covering the surface, and the dielectric breakdown voltage characteristics are inferior to those of powder cores made of ferrite powder. However, in Patent Document 1 and other documents, sufficient consideration has not been given to techniques for improving the dielectric breakdown voltage characteristics.

[0020] In response to this, the inventors conducted extensive research and found that by devising a method for adding a silicon alkoxide hydrolysis catalyst (e.g., ammonia) in the process of forming a silicon oxide film, it is possible to reduce the variation in the film thickness of the silicon oxide film and improve the rate of increase in the breakdown voltage when the silicon oxide film is formed. Here, in order to improve the rate of increase in the breakdown voltage, it is important not only to reduce microscopic defects such as cracks and pores in the silicon oxide film, but also to reduce macroscopic unevenness and variation in film thickness.

[0021] [Details of the embodiment of the present invention] Next, an embodiment of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0022] In this specification, "A to B" means a numerical range of "A or more and B or less."

[0023] First Embodiment of the Present Invention (1) Silicon Oxide-Coated Soft Magnetic Powder First, a silicon oxide-coated soft magnetic powder 10 of this embodiment will be described. Fig. 1 is a cross-sectional schematic diagram of the silicon oxide-coated soft magnetic powder 10 of this embodiment. As shown in Fig. 1, the silicon oxide-coated soft magnetic powder 10 of this embodiment has a core particle 20 and silicon oxide 30, and the silicon oxide 30 coats the surface of the core particle 20.

[0024] For example, powder made of a soft magnetic metal containing 20% ​​by mass or more of iron can be used as the core particle 20. Specific examples include pure iron powder (e.g., carbonyl iron powder), as well as iron alloy powders such as Fe-Si alloy, Fe-Si-Cr alloy, Fe-Al-Si alloy (Sendust), and Fe-Ni alloy (Ni mass 30 to 80% by mass) having a permalloy composition. Furthermore, small amounts (10% by mass or less) of Mo and Co may be added as needed.

[0025] Although the magnetic properties of the core particles 20 are not particularly specified, it is preferable that the core particles 20 be soft magnetic powder with a low coercive force Hc and a high saturation magnetization σs. If Hc is high, the energy loss when reversing the magnetic field increases, making it unsuitable for a magnetic core. The Hc of the core particles 20 is preferably, for example, 3.98 kA / m (approximately 50 Oe) or less. Furthermore, if the σs of the core particles 20 is low, a large amount of magnetic powder is required to form a magnetic core with the specified magnetic properties, resulting in a large size of the magnetic core. σs is, for example, 100 Am ２ / kg (100 emu / g) or more is preferred.

[0026] The silicon oxide-coated soft magnetic powder 10 of this embodiment is characterized by a high rate of increase in breakdown voltage when a coating of silicon oxide 30 is formed from a state in which only the core particles 20 are present. Specifically, when the breakdown voltage of the silicon oxide-coated soft magnetic powder 10 is Vs (V), the breakdown voltage of only the core particles 20 is Vc (V), and the average film thickness of the silicon oxide 30 is Ts (nm), the rate of increase A (% / nm) of the breakdown voltage per unit film thickness of the silicon oxide 30, defined by the following formula (1), is 55 or more. The upper limit of the rate of increase A is not particularly limited, but is, for example, 200 or less. A=((Vs / Vc)-1)×100 / Ts (1)

[0027] The breakdown voltages Vs and Vc can be measured, for example, by a bridge-type measuring instrument. The average film thickness Ts of the silicon oxide 30 is measured by a method described in the examples below.

[0028] The silicon oxide coated soft magnetic powder 10 of this embodiment preferably has a sphericity B defined by the following formula (2) of 0.13 or more, where Dx (μm) is the BET diameter and Dy (μm) is the median diameter (D50). This makes it possible to reduce the amount of resin required during pressure molding. The upper limit of the sphericity B is not particularly limited, but is, for example, 0.60 or less. B=Dx / Dy (2)

[0029] The BET diameter Dx is calculated by multiplying the BET specific surface area S (m ２ / g) and true specific gravity ρ (g / cm ３) can be calculated by the following formula: The BET specific surface area S can be calculated by the following formula: ２ The median diameter Dy can be measured by a BET single-point method using 30% by volume of S and 70% by volume of He. The median diameter Dy can be measured, for example, by a laser diffraction particle size distribution measurement method. Dx = 6 / (S × ρ)

[0030] In the silicon oxide-coated soft magnetic powder 10 of this embodiment, the average film thickness Ts of the silicon oxide 30 is preferably, for example, 0.1 nm or more and 50 nm or less. If the average film thickness Ts is less than 0.1 nm, it may be difficult to ensure stable insulation. In contrast, by setting the average film thickness Ts to 0.1 nm or more, stable insulation can be more easily ensured. On the other hand, if the average film thickness Ts exceeds 50 nm, the proportion of silicon oxide 30 increases, and magnetic properties may deteriorate. In contrast, by setting the average film thickness Ts to 50 nm or less, good magnetic properties can be maintained.

[0031] The median diameter Dy of the silicon oxide-coated soft magnetic powder 10 of this embodiment is preferably, for example, 0.1 μm or more and 50 μm or less. If the median diameter Dy is less than 0.1 μm, secondary aggregation of particles may occur, which may result in a decrease in green density. In contrast, by setting the median diameter Dy to 0.1 μm or more, the green density can be increased. On the other hand, if the median diameter Dy exceeds 50 μm, magnetic loss at high frequencies in an inductor is likely to increase. In contrast, by setting the median diameter Dy to 50 μm or less, magnetic loss can be reduced.

[0032] (2) Manufacturing Method of Silicon Oxide-Coated Soft Magnetic Powder Next, a manufacturing method of the silicon oxide-coated soft magnetic powder 10 of this embodiment will be described. Fig. 2 is a flowchart showing an example of a manufacturing method of the silicon oxide-coated soft magnetic powder 10 of this embodiment. As shown in Fig. 2, the manufacturing method of the silicon oxide-coated soft magnetic powder 10 of this embodiment includes, for example, a step of mixing a mixed solvent of water and an organic solvent containing 1% to 40% by mass of water with core particles made of a soft magnetic metal containing 20% ​​by mass or more of iron to obtain a slurry (dispersion step S1), a step of adding silicon alkoxide to the slurry and stirring and mixing (alkoxide addition step S2), a step of adding a hydrolysis catalyst for the silicon alkoxide to the slurry containing the silicon alkoxide to coat the surfaces of the core particles with silicon oxide (hydrolysis catalyst addition step S3), and a step of performing solid-liquid separation of the slurry and drying the solid content to obtain a silicon oxide-coated soft magnetic powder (solid-liquid separation / drying step S4).

[0033] (Dispersion Step S1) The dispersion step S1 is a step of, for example, mixing a mixed solvent of water and an organic solvent containing 1% to 40% by mass of water with core particles 20 made of a soft magnetic metal containing 20% ​​or more by mass of iron to obtain a slurry. Specifically, first, a raw material powder (core particles 20) composed of soft magnetic metal particles containing 20% ​​or more by mass of iron is prepared. An extremely thin Fe oxide film is present on the surface of the core particles 20. A mixed solvent of water and an organic solvent is prepared as the solvent. In the dispersion step S1, the Fe oxide film is hydrated with water contained in the mixed solvent. The surface of the hydrated Fe oxide is a type of solid acid and behaves similarly to a weak acid as a Bronsted acid. Therefore, when silicon alkoxide is added in the next step, the reactivity between the silanol derivative, which is a hydrolysis product of silicon alkoxide, and the surface of the raw material powder particles is improved.

[0034] If the water content in the mixed solvent is low, the effect of hydrating the Fe oxide on the surface of the core particle 20 may be insufficient. On the other hand, if the water content is high, the hydrolysis rate of the silicon alkoxide increases, making it difficult to form a highly uniform silicon oxide coating layer. In this embodiment, a mixed solvent containing 1% to 40% by mass of water is used. The water content in the mixed solvent is more preferably 5 to 30% by mass, and even more preferably 10 to 20% by mass.

[0035] As the organic solvent used in the mixed solvent, it is preferable to use aliphatic alcohols that have an affinity for water, such as methanol, ethanol, 1-propanol, 2-propanol, butanol, pentanol, hexanol, etc. However, if the solubility parameter of the organic solvent is close to that of water, the reactivity of water in the mixed solvent tends to decrease, so it is more preferable to use aliphatic alcohols having 3 to 6 carbon atoms, such as 1-propanol, 2-propanol (isopropyl alcohol), butanol, pentanol, or hexanol.

[0036] The reaction temperature in the dispersion step S1 is not particularly limited, but is preferably set to, for example, 20 to 70° C. The retention time in the dispersion step S1 is also not particularly limited, but it is preferable to obtain a slurry by stirring for 1 to 30 minutes so that the hydration reaction of the Fe oxide occurs uniformly.

[0037] (Alkoxide Addition Step S2) The alkoxide addition step S2 is, for example, a step of adding a silicon alkoxide hydrolysis catalyst to a slurry containing silicon alkoxide to coat the surfaces of the core particles 20 with silicon oxide 30. Coating methods using silicon alkoxide are generally referred to as sol-gel methods, and are superior in terms of mass productivity compared to dry methods. The slurry obtained in the dispersion step S1 is stirred by a known mechanical means while adding silicon alkoxide, and the slurry is then maintained in this state for a certain period of time. In the alkoxide addition step S2, the temperature of the slurry is preferably maintained at 20 to 70°C, more preferably at 30 to 65°C. Adding silicon alkoxide in this temperature range and maintaining stirring facilitates the formation of a coating of silicon oxide 30 with significantly reduced formation of micropores (pores with a size of 2 nm or less). The reaction time in the alkoxide addition step S2 may be set, for example, within the range of 1 to 30 minutes.

[0038] When silicon alkoxide is hydrolyzed, some or all of the alkoxy groups are replaced with hydroxyl groups (OH groups), resulting in a silanol derivative. In this embodiment, the surface of the core particle 20 is coated with this silanol derivative. When heated, the silanol derivative that has coated the particle surface undergoes condensation or polymerization to form a polysiloxane structure, and when the polysiloxane structure is further heated, silica (SiO ２ In this specification, the silicon oxide 30 is a general term for the silanol derivative coating with some of the alkoxy groups remaining as organic matter, through to the silica coating.

[0039] The silicon alkoxide added in the alkoxide addition step S2 is hydrolyzed by the action of water contained in the mixed solvent to form a silanol derivative. The generated silanol derivative forms a reaction layer of the silanol derivative on the surface of the core particle 20 through condensation, chemical adsorption, etc. Since no hydrolysis catalyst is added in the alkoxide addition step S2, the hydrolysis of the silicon alkoxide occurs slowly, which is thought to result in the uniform formation of the reaction layer of the silanol derivative described above.

[0040] Examples of silicon alkoxides that can be used include trimethoxysilane, tetramethoxysilane, triethoxysilane, tetraethoxysilane, tripropoxysilane, tetrapropoxysilane, tributoxysilane, and tributoxysilane. The silicon alkoxide used may be a monomer or an oligomer. Tetraethoxysilane (TEOS) is particularly suitable because it has good wettability with the core particles 20 and easily forms a uniform coating layer.

[0041] (Hydrolysis Catalyst Addition Step S3) The hydrolysis catalyst addition step S3 is, for example, a step of adding a hydrolysis catalyst for silicon alkoxide to a slurry to which silicon alkoxide has been added, thereby coating the surfaces of the core particles 20 with silicon oxide 30. A catalyst (hydrolysis catalyst) for promoting the hydrolysis of silicon alkoxide is added while stirring, by known mechanical means, the slurry in which particles having formed a reaction layer of a silanol derivative on the surface of the core particles 20 in the alkoxide addition step S2 are dispersed. In the hydrolysis catalyst addition step S3, the addition of the hydrolysis catalyst promotes the hydrolysis reaction of the silicon alkoxide, thereby increasing the film formation rate of the silicon oxide 30.

[0042] A basic catalyst is preferably used as the hydrolysis catalyst. If an acid catalyst is used, Fe, a component of the soft magnetic metal particles, may dissolve. As the basic catalyst, ammonia (aqueous ammonia) is preferably used because impurities are less likely to remain in the silicon oxide coating layer and it is easily available. The reaction temperature in the hydrolysis catalyst addition step S3 may be the same as the reaction temperature in the alkoxide addition step S2, for example. The reaction time in the hydrolysis catalyst addition step S3 is not particularly specified, but a long reaction time is economically disadvantageous, so it is advisable to set the conditions so that it is, for example, 5 to 200 minutes.

[0043] The method for producing the silicon oxide-coated soft magnetic powder 10 of this embodiment is characterized in that in the hydrolysis catalyst addition step S3, the hydrolysis catalyst is added so as to satisfy at least one of the following conditions (a) or (b): (a) the total amount of hydrolysis catalyst added relative to the weight of the slurry is 0.8 mmol / g or less; and (b) the addition rate of the hydrolysis catalyst relative to the weight of the slurry is 9 μmol / g / min or less. If the amount of hydrolysis catalyst added is large or the addition rate is fast, the reaction rate becomes fast, which hinders uniform coating and makes the film thickness of the silicon oxide 30 prone to become non-uniform. In contrast, by adding the hydrolysis catalyst so as to satisfy at least one of conditions (a) or (b), it is possible to reduce the variation in the film thickness of the silicon oxide 30 and improve the rate of increase in the breakdown voltage.

[0044] In condition (a), if the total amount of hydrolysis catalyst added is too small, the hydrolysis reaction of silicon alkoxide does not proceed sufficiently, making it difficult to form a coating of sufficient thickness, so the total amount of hydrolysis catalyst added is preferably, for example, 0.1 mmol / g or more. In condition (b), if the addition rate of hydrolysis catalyst is too slow, the hydrolysis reaction of silicon alkoxide also slows down, resulting in reduced productivity, so the addition rate of hydrolysis catalyst is preferably, for example, 1 (μmol / g) / min or more.

[0045] Furthermore, in the alkoxide addition step S2, it is preferable to stir and mix the slurry so as to satisfy the following condition (c): (c) The stirring power relative to the weight of the slurry is set to 3 W / kg or more. This makes it possible to make the film thickness of the silicon oxide 30 more uniform and further increase the rate of increase in the breakdown voltage. It also reduces fine cracks, pores, etc. in the silicon oxide 30 coating, thereby reducing the amount of resin required for pressure molding. The stirring power Pz relative to the weight of the slurry can be calculated using the following formula, where T (N cm) is the stirring torque of the slurry, W (kg) is the weight of the slurry, and R (rpm) is the stirring rotation speed: Pz = 2π × (T / 100) × (R × 60) / W

[0046] In condition (c), the upper limit of the stirring power relative to the weight of the slurry is not particularly limited, but in consideration of the availability and miniaturization of power sources such as stirring motors, it is preferable to set it to, for example, 15 W / kg or less.

[0047] In the hydrolysis catalyst addition step S3, it is preferable to add the hydrolysis catalyst so as to satisfy both conditions (a) and (b). This makes it possible to make the film thickness of the silicon oxide 30 more uniform and further increase the rate of increase in the breakdown voltage. It also reduces fine cracks and pores in the silicon oxide 30 film, thereby reducing the amount of resin required for pressure molding. For the same reasons, it is more preferable to satisfy all of conditions (a), (b), and (c).

[0048] (Solid-Liquid Separation and Drying Step S4) In the solid-liquid separation and drying step S4, for example, the slurry is subjected to solid-liquid separation and the solid content is dried to obtain the silicon oxide-coated soft magnetic powder 10 coated with silicon oxide 30. Powder composed of particles coated with silicon oxide 30 is recovered as a solid content from the slurry in which the powder obtained after the hydrolysis catalyst addition step S3 is dispersed. Known solid-liquid separation methods such as filtration, centrifugation, and decantation can be used as the solid-liquid separation method. A flocculant may be added during solid-liquid separation. The recovered solid content is then dried to obtain a dried silicon oxide-coated soft magnetic powder 10 coated with silicon oxide 30. The drying method in the solid-liquid separation and drying step S4 is not particularly limited, and can be, for example, drying in an air atmosphere. To prevent oxidation of the soft magnetic powder, drying in an inert gas atmosphere or vacuum is recommended. The drying temperature is preferably 80°C or higher. The temperature during drying is preferably 400° C. or less, more preferably 150° C. or less, so that the coating layer of silicon oxide 30 does not peel off.

[0049] The above steps make it possible to produce silicon oxide-coated soft magnetic powder 10 coated with silicon oxide 30. The silicon oxide-coated soft magnetic powder 10 of this embodiment is characterized by a high rate of increase in breakdown voltage when a silicon oxide 30 coating is formed on the core particles 20 alone, since the above-mentioned measures not only reduce microscopic defects such as cracks and pores in the silicon oxide 30 coating, but also reduce macroscopic unevenness and variation in the film thickness of the silicon oxide 30.

[0050] <Other embodiments of the present invention> Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the present invention.

[0051] Next, examples of the present invention will be described. These examples are merely examples of the present invention, and the present invention is not limited to these examples.

[0052] In this example, the following measurement methods were employed. (True Specific Gravity) The true specific gravity was measured using a "Micro UltraPyc 1200e" manufactured by Anton Paar Japan K.K. (BET Specific Surface Area) The BET specific surface area was measured using a Macsorb manufactured by Mountec Co., Ltd. The measurement was carried out by flowing nitrogen gas into the measuring device at 105°C for 20 minutes to degas the sample, and then by flowing a mixed gas of nitrogen and helium (N ２ The BET diameter Dx was measured by the BET single-point method while flowing a mixture of 30% by volume of NaCl and 70% by volume of He. (BET diameter) The BET diameter Dx was calculated by the BET specific surface area S (m ２ / g) and true specific gravity ρ (g / cm ３ ) was calculated using the following formula: Dx = 6 / (S × ρ)

[0053] (Si content) Analysis was performed by the dissolution method as follows. First, hydrochloric acid and perchloric acid were added to a powder sample (raw material powder or silicon oxide-coated soft magnetic powder) to thermally decompose it, and the mixture was heated until white smoke of perchloric acid was generated. Heating was continued to dry it. After cooling, water and hydrochloric acid were added and the mixture was heated to dissolve soluble salts. Next, the insoluble residue was filtered using filter paper, and the residue together with the filter paper was transferred to a crucible, dried, and incinerated. After cooling, the crucible was weighed. A small amount of sulfuric acid and hydrofluoric acid was added, and the mixture was heated to dry it, and then ignited. After cooling, the crucible was weighed. The second weighing value was subtracted from the first weighing value, and the weight difference was used as the SiO ２ The Si content in the powder sample was calculated from the calculated value.

[0054] (Particle size distribution) Using a laser diffraction particle size distribution measuring device (HELOS particle size distribution measuring device manufactured by SYMPATEC; HELOS & RODOS (airflow type dispersion module)), the volume-based cumulative 10% particle diameter (D10), cumulative 25% particle diameter (D25), cumulative 50% particle diameter (D50), cumulative 75% particle diameter (D75), cumulative 90% particle diameter (D90), and cumulative 99% particle diameter (D99) were determined at a dispersion pressure of 5 bar (0.5 MPa).

[0055] (Average Film Thickness of Silicon Oxide Coating Layer) When the difference between the Si content of the silicon oxide coated soft magnetic powder measured by the above method and the Si content of the raw material powder is defined as D (mass %), the mass proportion P (mass %) of the silicon oxide coating layer is determined by the ratio of the Si atomic weight to the SiO ２ It is calculated from the molecular weight using the following formula: P = D x SiO ２ Molecular weight / Si atomic weight = D x 60.08 / 28.09 The density of the silicon oxide coating layer is d (g / cm ３ ), the BET specific surface area of ​​the raw material powder (core particle) is S (m ２ / g), the average film thickness Ts (nm) of the silicon oxide coating layer is expressed by the following formula: Ts = 10 × P / (d × S) where d is 2.65 (g / cm ３) can be used. The number 10 on the right side is a unit conversion coefficient. If the Si content of the raw material powder is unknown, the silicon oxide coating of the silicon oxide-coated soft magnetic powder can be dissolved by alkaline etching or the like, and the Si content of the obtained raw material powder can be measured by a dissolution method or the like.

[0056] (Breakdown voltage of powder) The breakdown voltage was measured by placing two 2 mm thick brass plates with electrolytically polished surfaces as electrodes at a distance of 2 mm between the electrodes, placing 200 mg of powder in the gap between the two electrode plates, and then placing a magnet with a cross-sectional area of ​​240 mm behind each electrode plate to form a bridge of the powder to be measured between the electrodes. A DC voltage was applied between the electrodes, and the current flowing through the powder was measured using a four-terminal method. The DC voltage was started at 10 V and increased in steps of 10 V for 10 seconds. The voltage at which the current flowing between the electrodes became 100 mA or more during the process of increasing the applied voltage was taken as the breakdown voltage in the bridge-type measuring instrument.

[0057] <Example 1> As the raw material powder (core particles), FeSiCr soft magnetic powder (BET specific surface area: 0.28 m ２A 5000 mL (1 / g, D50: 10.8 μm) dispersion was prepared. (Dispersion Step S1) 306 g of pure water and 1650 g of isopropyl alcohol (IPA) were added to a 5000 mL reaction vessel at room temperature and mixed using a stirring blade to prepare a mixed solvent. 5550 g of the raw material powder was added to this mixed solvent, the liquid temperature was adjusted to 40°C, and the mixture was stirred at 380 rpm for 5 minutes to obtain a slurry. (Alkoxide Addition Step S2) 177.4 g of tetraethoxysilane (TEOS: Wako Pure Chemical Industries, Ltd., special grade reagent) dispensed into a small beaker was added all at once to the slurry obtained in the dispersion step S1. The TEOS adhering to the small beaker was washed off with 200 g of IPA and added to the slurry. After the TEOS addition, the slurry temperature was maintained at 40°C and stirring was continued for 5 minutes to allow the TEOS hydrolysis product to react with the surface of the raw material powder particles. (Hydrolysis Catalyst Addition Step S3) Then, 239 g of 25% by mass ammonia water was continuously added to the slurry at an addition rate of 2.6 g / min. After the addition of the ammonia water was completed, a holding time of 60 minutes was provided while stirring, allowing a silicon oxide coating layer to form on the surface of the soft magnetic powder. Up to this point, the slurry temperature was maintained at 40°C. (Solid-Liquid Separation and Drying Step S4) Then, the slurry was subjected to solid-liquid separation using a Nutsche suction filtration device, and the solid content was recovered. The recovered solid content was dried at 100°C for 12 hours in a nitrogen atmosphere to obtain a silicon oxide-coated soft magnetic powder. The obtained silicon oxide-coated soft magnetic powder was subjected to the above-mentioned measurements.

[0058] Example 2 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 1, except that the ammonia water was added to the slurry at an addition rate of 1.3 g / min.

[0059] Example 3 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 1, except that the stirring speed of the slurry was set to 550 rpm.

[0060] Example 4 A silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 3, except that 477 g of aqueous ammonia was added to the slurry.

[0061] Example 5 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 3, except that the ammonia water was added to the slurry at an addition rate of 1.3 g / min.

[0062] Example 6 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 3, except that the ammonia water was added to the slurry at an addition rate of 5.3 g / min and the holding time was 105 min.

[0063] Example 7 A silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 3, except that the ammonia water was added to the slurry at a rate of 10.6 g / min and the holding time was 128 min.

[0064] Comparative Example 1 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 1, except that the amount of ammonia water added to the slurry was 954 g and the addition rate was 10.6 g / min.

[0065] Comparative Example 2 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 3, except that the amount of ammonia water added to the slurry was 954 g and the addition rate was 10.6 g / min.

[0066] Comparative Example 3 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 4, except that the ammonia water was added to the slurry at an addition rate of 5.3 g / min.

[0067] Example 8: As a raw material powder, FeSiCr soft magnetic powder (BET specific surface area: 0.29 m) was used. ２A 5000 mL (1 / g, D50: 10.6 μm) dispersion was prepared. (Dispersion Step S1) 311 g of pure water and 1675 g of isopropyl alcohol (IPA) were added to a 5000 mL reaction vessel at room temperature and mixed using a stirring blade to prepare a mixed solvent. 5625 g of the raw material powder was added to this mixed solvent, the liquid temperature was adjusted to 40°C, and the mixture was stirred at 550 rpm for 5 minutes to obtain a slurry. (Alkoxide Addition Step S2) 223.2 g of tetraethoxysilane (TEOS: special grade reagent, manufactured by Wako Pure Chemical Industries, Ltd.) dispensed into a small beaker was added all at once to the slurry obtained in the dispersion step S1. The TEOS adhering to the small beaker was washed off with 200 g of IPA and added to the slurry. After the TEOS addition, the slurry temperature was maintained at 40°C and stirring was continued for 5 minutes to allow a reaction between the TEOS hydrolysis product and the surface of the raw material powder particles. (Hydrolysis catalyst addition step S3) Then, 242 g of 25 mass% ammonia water was continuously added to the slurry at an addition rate of 2.7 g / min. After the addition of the ammonia water was completed, a holding time of 60 minutes was provided while stirring, thereby forming a silicon oxide coating layer on the surface of the soft magnetic powder. Up to this point, the slurry temperature was maintained at 40°C. (Solid-liquid separation and drying step S4) A silicon oxide-coated soft magnetic powder was obtained using the same procedure as in Example 1.

[0068] Comparative Example 4 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 8, except that the amount of ammonia water added to the slurry was 968 g, the addition rate was 10.8 / min, and the stirring speed was 380 rpm.

[0069] Example 9 Silicon oxide-coated soft magnetic powder was obtained in the same manner as in Example 8, except that 74.4 g of tetraethoxysilane was added to the slurry.

[0070] Comparative Example 5 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 9, except that the amount of ammonia water added to the slurry was 968 g, the addition rate was 10.8 / min, and the stirring speed was 380 rpm.

[0071] Example 10: The raw material powder to be added to the slurry was FeSiCr soft magnetic powder (BET specific surface area: 0.15 m ２A silicon oxide-coated soft magnetic powder was obtained in the same manner as in Example 8, except that the powder density was changed to 77.5 g (D50: 24.1 μm) and the amount of tetraethoxysilane added was changed to 77.5 g.

[0072] Example 11 A silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 10, except that the stirring speed of the slurry was set to 550 rpm.

[0073] Comparative Example 6 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 10, except that the amount of ammonia water added to the slurry was 968 g and the addition rate was 10.8 g / min.

[0074] <Example 12> As a raw material powder, FeSiCr soft magnetic powder (BET specific surface area: 0.29 m ２ A 1000 mL (1 / g, D50: 10.6 μm) dispersion was prepared. (Dispersion Step S1) 70 g of pure water and 400 g of isopropyl alcohol (IPA) were added to a 1000 mL reaction vessel at room temperature and mixed using a stirring blade to prepare a mixed solvent. 250 g of the raw material powder was added to this mixed solvent, the liquid temperature was adjusted to 40°C, and the mixture was stirred at 800 rpm for 5 minutes to obtain a slurry. (Alkoxide Addition Step S2) 8.0 g of tetraethoxysilane (TEOS: special grade reagent, manufactured by Wako Pure Chemical Industries, Ltd.) dispensed into a small beaker was added all at once to the slurry obtained in the dispersion step S1. The TEOS adhering to the small beaker was washed off with 5 g of IPA and added to the slurry. After the TEOS addition, the slurry temperature was maintained at 40°C and stirring was continued for 5 minutes to allow the TEOS hydrolysis product to react with the surface of the raw material powder particles. (Hydrolysis catalyst addition step S3) Then, 45 g of 28 mass % ammonia water was continuously added to the slurry at an addition rate of 0.3 g / min. After the addition of the ammonia water was completed, a holding time of 60 minutes was set while stirring, and a silicon oxide coating layer was formed on the surface of the soft magnetic powder. Up to this point, the temperature of the slurry was maintained at 40°C. (Solid-liquid separation and drying step S4) Then, solids were separated using a Nutsche suction filtration device and the solids were recovered. The recovered solids were vacuum dried at 120°C for 3 hours to obtain silicon oxide-coated soft magnetic powder.

[0075] Comparative Example 7 A silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 12, except that the ammonia water was added to the slurry at a rate of 1.0 / min and the stirring speed was 600 rpm.

[0076] Example 13 The raw material powder to be added to the slurry was FeSiCr soft magnetic powder (BET specific surface area: 0.90 m ２ A silicon oxide-coated soft magnetic powder was obtained in the same manner as in Example 8, except that the powder was changed to a powder having a viscosity of 1000 saturations (0.015 saturations / g, D50: 3.2 μm), the amount of tetraethoxysilane added was 93.3 g, the addition rate of aqueous ammonia added was 1.3 g / min, and the stirring speed was 604 rpm.

[0077] Comparative Example 8 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 13, except that the amount of ammonia water added to the slurry was 968 g, the addition rate was 10.8 / min, and the stirring speed was 380 rpm.

[0078] Example 14: The raw material powder to be added to the slurry was FeSiCr soft magnetic powder (BET specific surface area: 0.035 m ２ A silicon oxide-coated soft magnetic powder was obtained in the same manner as in Example 8, except that the powder density was changed to 1 / g, D50: 40.2 μm, and the amount of tetraethoxysilane added was changed to 36.2 g.

[0079] Comparative Example 9 Silicon oxide coated soft magnetic powder was obtained in the same manner as in Example 14, except that the amount of ammonia water added to the slurry was 968 g, the addition rate was 10.8 / min, and the stirring speed was 380 rpm.

[0080] The results of various measurements carried out on the silicon oxide coated soft magnetic powders and raw material powders (core particles) of Examples 1 to 14 and Comparative Examples 1 to 9 are shown in Table 1.

[0081]

[0082] As shown in Table 1, in Examples 1 to 14, in which ammonia water was added in the hydrolysis catalyst addition step S3 so as to satisfy at least one of the above-mentioned conditions (a) and (b), the increase rate A was 55 or more. In addition, the sphericity B was 0.13 or more. On the other hand, in Comparative Examples 1 to 9, in which the hydrolysis catalyst addition step S3 did not satisfy the conditions (a) and (b), the increase rate A was less than 55. In addition, the sphericity B was less than 0.13.

[0083] From the above, it was confirmed that adding a hydrolysis catalyst in the hydrolysis catalyst addition step S3 so as to satisfy at least one of the above-mentioned conditions (a) and (b) can improve the rate of increase in breakdown voltage and also increase the sphericity, thereby reducing the amount of resin required for pressure molding.

[0084] 10 Silicon oxide coated soft magnetic powder 20 Core particles 30 Silicon oxide S1 Dispersion step S2 Alkoxide addition step S3 Hydrolysis catalyst addition step S4 Solid-liquid separation / drying step

Claims

1. A silicon oxide-coated soft magnetic powder in which the surface of core particles made of a soft magnetic metal containing 20% by mass or more of iron is coated with silicon oxide, wherein when the breakdown voltage of the silicon oxide-coated soft magnetic powder is Vs (V), the breakdown voltage of only the core particles is Vc (V), and the average film thickness of the silicon oxide is Ts (nm), the rate of increase A (% / nm) of the breakdown voltage per unit film thickness of the silicon oxide defined by the following formula (1) is 55 or more. A = ((Vs / Vc) - 1) × 100 / Ts... (1) 2. The silicon oxide-coated soft magnetic powder according to claim 1, wherein when the BET diameter of the silicon oxide-coated soft magnetic powder is Dx (μm) and the median diameter (D50) is Dy (μm), the sphericity B defined by the following formula (2) is 0.13 or more. B = Dx / Dy... (2) 3. The silicon oxide-coated soft magnetic powder according to claim 1, wherein the average film thickness of the silicon oxide is 0.1 nm or more and 50 nm or less.

4. The silicon oxide-coated soft magnetic powder according to any one of claims 1 to 3, wherein the median diameter (D50) of the silicon oxide-coated soft magnetic powder is 0.1 μm or more and 50 μm or less.

5. A method for producing a silicon oxide-coated soft magnetic powder, comprising: mixing a mixed solvent of water and an organic solvent containing 1% by mass or more and 40% by mass or less of water with core particles made of a soft magnetic metal containing 20% by mass or more of iron to obtain a slurry; adding a silicon alkoxide to the slurry and stirring and mixing; adding a hydrolysis catalyst of the silicon alkoxide to the slurry to which the silicon alkoxide has been added to coat the surface of the core particles with silicon oxide; solid-liquid separating the slurry and drying the solid content to obtain the silicon oxide-coated soft magnetic powder coated with the silicon oxide, wherein in the step of coating the silicon oxide, the hydrolysis catalyst is added so as to satisfy at least one of the following conditions (a) or (b). (a) The total addition amount of the hydrolysis catalyst with respect to the weight of the slurry is 0.8 mmol / g or less. (b) The addition rate of the hydrolysis catalyst with respect to the weight of the slurry is 9 (μmol / g) / min or less.

6. In the step of coating the silicon oxide, the method for producing the silicon oxide-coated soft magnetic powder according to claim 5, which satisfies both of the conditions (a) and (b).

7. In the step of stirring and mixing, the method for producing the silicon oxide-coated soft magnetic powder according to claim 5, wherein stirring and mixing are performed so as to satisfy the following condition (c). (c) The stirring power with respect to the weight of the slurry is 3 W / kg or more.

8. The method for producing the silicon oxide-coated soft magnetic powder according to any one of claims 5 to 7, wherein the hydrolysis catalyst is ammonia.

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