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

Abstract

To provide silicon oxide-coated soft magnetic powder that is excellent in increase rate of the dielectric breakdown voltage thereof.SOLUTION: There is provided 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. When 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 the following formula (1) is 55 or more. A=((Vs / Vc)-1)×100 / Ts...(1)SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Conventionally, a compacted magnetic core using soft magnetic powder such as iron powder, alloy powder containing iron, and intermetallic compound powder as a magnetic core for an inductor, choke coil, transformer, reactor, motor, etc. is known. 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 metal powder, it is common to first form an insulating film on the surface of the metal powder particles and then subject them to a process of compression molding and heat treatment.

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

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] One embodiment of the present invention aims to provide silicon oxide-coated soft magnetic powder having an excellent rate of increase in breakdown voltage.

Means for Solving the Problems

[0006] The first aspect of the present invention is silicon oxide-coated soft magnetic powder in which silicon oxide is coated on the surface of core particles made of a soft magnetic metal containing 20% by mass or more of iron, 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. The silicon oxide-coated soft magnetic powder is provided. A = ((Vs / Vc) - 1) × 100 / Ts ···(1)

[0007] The second aspect of the present invention is 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. The silicon oxide-coated soft magnetic powder according to the first aspect. B = Dx / Dy ···(2)

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

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

[0010] The fifth aspect of the present invention is A step of 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; A step of adding silicon alkoxide to the slurry and stirring and mixing; A step of 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; A step of subjecting the slurry to solid-liquid separation and drying the solid content to obtain silicon oxide-coated soft magnetic powder coated with the silicon oxide, 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), which is a method for producing silicon oxide-coated soft magnetic powder. (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.

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

[0012] A seventh aspect of the present invention is In the step of stirring and mixing, the method for producing silicon oxide-coated soft magnetic powder according to the fifth aspect, which is stirred and mixed 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.

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

[0014] According to an embodiment of the present invention, it is possible to provide silicon oxide-coated soft magnetic powder excellent in the rate of increase in breakdown voltage. [Brief Description of the Drawings]

[0015]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0016] <Findings Obtained by the Inventor> First, the findings obtained by the inventor will be described.

[0017] Silicon oxide-coated soft magnetic powder is generally used in a device state after being mixed with a resin. However, when there are cracks, pores, etc. in the silicon oxide, the specific surface area of the powder increases, and when mixed with the resin, the fluidity deteriorates, making it difficult to form a device.

[0018] In Patent Document 1, in order to reduce the amount of resin required during pressure molding, silicon oxide-coated soft magnetic powder having a silicon oxide coating layer that is thin, excellent in insulation, and has high film thickness uniformity and few defects on the particle surface is disclosed.

[0019] However, silicon oxide-coated soft magnetic powder has a problem that dielectric breakdown easily occurs in the insulating film covering the surface, and is inferior to a compacted magnetic core composed of ferrite powder in terms of dielectric breakdown voltage characteristics. However, in Patent Document 1 and the like, sufficient consideration has not been given to the technology for improving the dielectric breakdown voltage characteristics.

[0020] In response to this, the inventor conducted intensive research. As a result, in the process of forming a silicon oxide film, by devising the method of adding a hydrolysis catalyst (for example, ammonia) of silicon alkoxide, it was found that the variation in the film thickness of the silicon oxide film can be reduced, and the rate of increase in the dielectric breakdown voltage when forming the silicon oxide film can be improved. Here, in order to improve the rate of increase in the dielectric breakdown voltage, it is important not only to reduce micro defects such as cracks and pores in the silicon oxide film, but also to reduce macro unevenness and variation in the film thickness.

[0021] [Details of Embodiments of the Present Invention] Next, an embodiment of the present invention will be described below with reference to the drawings. It should be noted 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 equivalent to the claims.

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

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

[0024] As the core particles 20, for example, a powder made of a soft magnetic metal containing 20% by mass or more of iron can be used. Specifically, in addition to pure iron powder (for example, carbonyl iron powder), iron alloy powders such as Fe-Si alloy, Fe-Si-Cr alloy, Fe-Al-Si alloy (Sendust), and Fe-Ni alloy (30 to 80% by mass of Ni) having a permalloy composition can be mentioned. Further, Mo and Co may be added in a small amount (10% by mass or less) as necessary.

[0025] The magnetic properties of the core particles 20 are not particularly defined, but it is preferable that they are soft magnetic powders having a low coercive force Hc and a high saturation magnetization σs. If Hc is high, the energy loss when reversing the magnetic field becomes large, which is unsuitable for the magnetic core. The Hc of the core particles 20 is preferably, for example, 3.98 kA / m (about 50 Oe) or less. Also, if the σs of the core particles 20 is low, the amount of magnetic powder required to form a magnetic core having predetermined magnetic properties increases, and the size of the magnetic core becomes large. σs is, for example, 100 Am 2It is preferably at 1 / kg (100 emu / g) or more.

[0026] The silicon oxide-coated soft magnetic powder 10 of the present embodiment is characterized in that when the coating of the silicon oxide 30 is formed from the state of only the core particles 20, the rate of increase in the breakdown voltage is high. 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 value 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] Note that the breakdown voltages Vs and Vc can be measured by, for example, a bridge-type measuring instrument. Also, the average film thickness Ts of the silicon oxide 30 is measured by the method described in the examples below.

[0028] The silicon oxide-coated soft magnetic powder 10 of the present embodiment preferably has a sphericity B of 0.13 or more when the BET diameter is Dx (μm) and the median diameter (D50) is Dy (μm). Thereby, the amount of resin required during pressure molding can be reduced. The upper limit value of the sphericity B is not particularly limited, but is, for example, 0.60 or less. B = Dx / Dy ···(2)

[0029] Note that the BET diameter Dx can be calculated from the BET specific surface area S (m 2 / g) and the true specific gravity ρ (g / cm 3 ) by the following formula. The BET specific surface area S can be measured by, for example, the BET one-point method using a mixed gas of nitrogen and helium (N2: 30% by volume, He: 70% by volume). The median diameter Dy can be measured by, for example, the laser diffraction particle size distribution measurement method. Dx = 6 / (S × ρ)

[0030] In the silicon oxide-coated soft magnetic powder 10 of the present 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. On the other hand, by setting the average film thickness Ts to 0.1 nm or more, it becomes easier to ensure stable insulation. On the other hand, if the average film thickness Ts exceeds 50 nm, the proportion of the silicon oxide 30 increases, and the magnetic properties may deteriorate. On the other hand, 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 the present 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, the particles may undergo secondary aggregation, and the green compact density may decrease. On the other hand, by setting the median diameter Dy to 0.1 μm or more, the green compact density can be increased. On the other hand, if the median diameter Dy exceeds 50 μm, the magnetic loss at high frequencies in the inductor tends to increase. On the other hand, by setting the median diameter Dy to 50 μm or less, the magnetic loss can be reduced.

[0032] (2) Method for manufacturing silicon oxide-coated soft magnetic powder Next, a method for manufacturing the silicon oxide-coated soft magnetic powder 10 of the present embodiment will be described. FIG. 2 is a flowchart showing an example of the method for manufacturing the silicon oxide-coated soft magnetic powder 10 of the present embodiment. As shown in FIG. 2, the method for manufacturing the silicon oxide-coated soft magnetic powder 10 of the present embodiment includes, for example, a step of 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 (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 of silicon alkoxide to the slurry to which silicon alkoxide has been added to coat the surface of the core particles with silicon oxide (hydrolysis catalyst addition step S3); and a step of subjecting the slurry to solid-liquid separation and drying the solid content to obtain a silicon oxide-coated soft magnetic powder coated with silicon oxide (solid-liquid separation / drying step S4).

[0033] (Dispersion step S1) The dispersion step S1 is, for example, a step of 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 20 made of a soft magnetic metal containing 20% by mass or more of iron to obtain a slurry. Specifically, first, a raw material powder (core particles 20) composed of soft magnetic metal particles containing 20% by mass or more of iron is prepared. An extremely thin oxide film of Fe exists on the surface of the core particles 20. As the solvent, a mixed solvent of water and an organic solvent is prepared. In the dispersion step S1, this oxide film of Fe is hydrated by the water contained in the mixed solvent. The surface of the hydrated Fe oxide is a kind of solid acid and exhibits behavior similar to that of 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 the 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 action of hydrating the Fe oxide on the surface of the core particles 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% by mass or more and 40% by mass or less of water is applied. 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 such as methanol, ethanol, 1-propanol, 2-propanol, butanol, pentanol, and hexanol that are affinity with water. However, when the solubility parameter of the organic solvent is close to that of water, the reactivity of water in the mixed solvent tends to decrease. Therefore, it is more preferable to use aliphatic alcohols having 3 to 6 carbon atoms, for example, 1-propanol, 2-propanol (isopropyl alcohol), butanol, pentanol, and hexanol.

[0036] The reaction temperature of the dispersion step S1 is not particularly defined, but for example, it is preferably 20 to 70°C. The retention time of the dispersion step S1 is also not particularly defined, 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) In the alkoxide addition step S2, for example, a hydrolysis catalyst for silicon alkoxide is added to a slurry to which silicon alkoxide has been added, and silicon oxide 30 is coated on the surface of the core particles 20. The coating method using silicon alkoxide is generally a method called the sol-gel method, which is excellent in mass productivity compared to the dry method. The slurry obtained in the dispersion step S1 is stirred by known mechanical means, and after adding silicon alkoxide, the slurry is held in that state for a certain period of time. In the alkoxide addition step S2, it is preferable to maintain the temperature of the slurry at 20 to 70 °C, and more preferably at 30 to 65 °C. When silicon alkoxide is added and stirring is maintained in this temperature range, a coating of silicon oxide 30 with significantly reduced formation of micropores (pores with a size of 2 nm or less) is likely to be formed. The reaction time in the alkoxide addition step S2 may be set, for example, in the range of 1 to 30 minutes.

[0038] When silicon alkoxide is hydrolyzed, part or all of the alkoxy groups are replaced with hydroxyl groups (OH groups) to form silanol derivatives. In this embodiment, the surface of the core particles 20 is coated with this silanol derivative. The silanol derivative coating the particle surface takes a polysiloxane structure by condensation or polymerization when heated, and becomes silica (SiO2) when the polysiloxane structure is further heated. In this specification, from the silanol derivative coating in which a part of the alkoxy group, which is an organic substance, remains to the silica coating is collectively referred to as silicon oxide 30.

[0039] The silicon alkoxide added in the alkoxide addition step S2 is hydrolyzed by the action of water contained in the mixed solvent to become a silanol derivative. The generated silanol derivative forms a reaction layer of silanol derivative on the surface of the core particles 20 by condensation, chemisorption, etc. In the alkoxide addition step S2, since no hydrolysis catalyst is added, the hydrolysis of silicon alkoxide occurs slowly, and it is considered that the above-mentioned reaction layer of silanol derivative is formed uniformly.

[0040] Examples of silicon alkoxides that can be used include trimethoxysilane, tetramethoxysilane, triethoxysilane, tetraethoxysilane, tripropoxysilane, tetrapropoxysilane, tributoxysilane, tributoxysilane, etc. The silicon alkoxide used may be a monomer or an oligomer. Tetraethoxysilane (TEOS) is particularly preferred 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 a step of adding a hydrolysis catalyst for silicon alkoxide to the slurry added with silicon alkoxide to coat the surface of the core particles 20 with silicon oxide 30. While stirring the slurry in which the particles having a reaction layer of silanol derivative are dispersed on the surface of the core particles 20 in the alkoxide addition step S2 by known mechanical means, a catalyst (hydrolysis catalyst) for promoting the hydrolysis of silicon alkoxide is added. In the hydrolysis catalyst addition step S3, the addition of the hydrolysis catalyst promotes the hydrolysis reaction of silicon alkoxide and increases the film formation rate of silicon oxide 30.

[0042] It is preferable to use a basic catalyst as the hydrolysis catalyst. When an acid catalyst is used, Fe, which is a component of the soft magnetic metal particles, may dissolve. As the basic catalyst, ammonia (aqueous ammonia) is preferably used because it is difficult for impurities 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, for example, the reaction temperature in the alkoxide addition step S2. The reaction time in the hydrolysis catalyst addition step S3 is not particularly defined, but since a long reaction time is economically disadvantageous, the conditions may be set, for example, to be 5 to 200 minutes.

[0043] The method for producing the silicon oxide-coated soft magnetic powder 10 of the present embodiment is characterized in that, in the hydrolysis catalyst addition step S3, a hydrolysis catalyst is added so as to satisfy at least one of the following conditions (a) or (b). (a) Set the total addition amount of the hydrolysis catalyst to 0.8 mmol / g or less based on the weight of the slurry. (b) Set the addition rate of the hydrolysis catalyst to 9 (μmol / g) / min or less based on the weight of the slurry. When the addition amount of the hydrolysis catalyst is large or the addition rate is high, the reaction rate becomes too fast, uniform coating is inhibited, and the film thickness of the silicon oxide 30 tends to be non-uniform. On the other hand, by adding the hydrolysis catalyst so as to satisfy at least one of the conditions (a) or (b), the variation in the film thickness of the silicon oxide 30 can be reduced, and the increase rate of the breakdown voltage can be improved.

[0044] In the case of condition (a), when the total addition amount of the hydrolysis catalyst is too small, the hydrolysis reaction of the silicon alkoxide does not proceed sufficiently, and it is difficult to form a coating with sufficient thickness. Therefore, the total addition amount of the hydrolysis catalyst is preferably, for example, 0.1 mmol / g or more. In the case of condition (b), when the addition rate of the hydrolysis catalyst is too slow, the hydrolysis reaction of the silicon alkoxide also becomes slow, and the productivity decreases. Therefore, the addition rate of the hydrolysis catalyst is preferably, for example, 1 (μmol / g) / min or more.

[0045] In addition, in the alkoxide addition step S2, it is preferable to perform stirring and mixing so as to satisfy the following condition (c). (c) Set the stirring power to 3 W / kg or more based on the weight of the slurry. Thereby, the film thickness of the silicon oxide 30 can be made more uniform, and the increase rate of the breakdown voltage can be further increased. In addition, fine cracks, pores, etc. of the silicon oxide 30 film can be reduced, and the amount of resin required during pressure molding can be reduced. The stirring power Pz with respect to the weight of the slurry can be obtained by the following formula when the stirring torque of the slurry is T (N·cm), the weight of the slurry is W (kg), and the stirring rotation speed is R (rpm). Pz = 2π×(T / 100)×(R×60) / W

[0046] In condition (c), the upper limit of the stirring power with respect to the weight of the slurry is not particularly limited. However, considering the availability and miniaturization of power sources such as stirring motors, for example, it is preferably 15 W / kg or less.

[0047] In the hydrolysis catalyst addition step S3, it is preferable to add a hydrolysis catalyst so as to satisfy both conditions (a) and (b). Thereby, the film thickness of the silicon oxide 30 can be made more uniform, and the rate of increase in the breakdown voltage can be further increased. In addition, fine cracks, pores, etc. in the silicon oxide 30 coating can be reduced, and the amount of resin required during pressure molding can be reduced. For the same reason, it is more preferable to satisfy all of conditions (a), (b), and (c).

[0048] (Solid-liquid separation and drying step S4) The solid-liquid separation and drying step S4 is, for example, a step of obtaining the silicon oxide-coated soft magnetic powder 10 coated with silicon oxide 30 by subjecting the slurry to solid-liquid separation and drying the solid content. Powders composed of particles coated with silicon oxide 30 are recovered as the solid content from the slurry in which the powder after the hydrolysis catalyst addition step S3 is dispersed. As the solid-liquid separation means, known solid-liquid separation means such as filtration, centrifugation, and decantation can be used. An aggregating agent may be added for solid-liquid separation during solid-liquid separation. Thereafter, the recovered solid content is dried to obtain a dried product of the 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. For example, it can be dried in an air atmosphere. When it is desired to suppress the oxidation of the soft magnetic powder, it is advisable to dry it in an inert gas atmosphere or in a vacuum. The temperature during drying is preferably 80°C or higher. The temperature during drying is preferably 400°C or lower, and more preferably 150°C or lower so that the coating layer of the silicon oxide 30 does not peel off.

[0049] By the above steps, the silicon oxide-coated soft magnetic powder 10 coated with silicon oxide 30 can be manufactured. The silicon oxide-coated soft magnetic powder 10 of the present embodiment, by the above-mentioned device, not only reduces micro defects such as cracks and pores in the silicon oxide 30 film, but also reduces the macro unevenness and variation in the film thickness of the silicon oxide 30. Therefore, it is characterized in that the rate of increase in the breakdown voltage when the film of the silicon oxide 30 is formed from the state of only the core particles 20 is high.

[0050] <Other Embodiments of the Present Invention> As described above, the embodiments of the present invention have been specifically described. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist thereof.

Examples

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

[0052] In this example, the following measurement methods were adopted. (True Specific Gravity) The true specific gravity was measured using "Micro UltraPyc 1200e" manufactured by Anton Paar Japan Co., Ltd. (BET Specific Surface Area) The BET specific surface area was measured by the BET one-point method while flowing a mixed gas of nitrogen and helium (N2: 30% by volume, He: 70% by volume) after degassing by flowing nitrogen gas at 105°C for 20 minutes in the measuring instrument using Macsorb manufactured by Mount Tech Co., Ltd. (BET Diameter) The BET diameter Dx was calculated by the following formula from the BET specific surface area S (m 2 / g) and the true specific gravity ρ (g / cm 3 ). Dx = 6 / (S × ρ)

[0053] (Si Content) Analysis was performed as follows by the dissolution method. First, hydrochloric acid and perchloric acid were added to a powder sample (raw material powder or silicon oxide-coated soft magnetic powder), and the mixture was heated for decomposition until white smoke of perchloric acid was generated. Subsequently, heating was continued until it dried up. After allowing it to cool, water and hydrochloric acid were added and heated to dissolve soluble salts. Subsequently, the insoluble residue was filtered using filter paper, and the residue was transferred to a crucible together with the filter paper, dried, and ashed. After allowing it to cool, the crucible was weighed. A small amount of sulfuric acid and hydrofluoric acid were added, heated until it dried up, and then strongly heated. After allowing it to cool, the crucible was weighed. Then, the second weighing value was subtracted from the first weighing value, the weight difference was calculated as SiO2, and the Si content in the powder sample was determined from that 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 (pneumatic dispersion module)), the 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) on a volume basis 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 ratio P (mass %) of the silicon oxide coating layer is calculated by the following formula from the Si atomic weight and the SiO2 molecular weight. P = D × SiO2 molecular weight / Si atomic weight = D × 60.08 / 28.09 The density of the silicon oxide coating layer is d (g / cm 3 ), and the BET specific surface area of the raw material powder (core particles) is S (m 2 / g), then the average film thickness Ts (nm) of the silicon oxide coating layer is expressed by the following formula. Ts = 10 × P / (d × S) Here, as the value of d, 2.65 (g / cm 3It is possible to adopt (). The 10 on the right side is a unit conversion factor. When the Si content of the raw material powder is unknown, the silicon oxide film of the silicon oxide-coated soft magnetic powder can be dissolved by alkali etching or the like, and the Si amount of the obtained raw material powder can be measured and obtained by a dissolution method or the like.

[0056] (Dielectric breakdown voltage of the powder) For the dielectric breakdown voltage, two brass plates with a thickness of 2 mm whose surfaces were electrolytically polished as electrodes were arranged so that the distance between the electrodes was 2 mm. After charging 200 mg of powder into the gap between the two electrode plates, magnets with a cross-sectional area of 240 mm2 were arranged 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 value flowing through the powder was measured by the four-terminal method. The DC voltage started from 10 V, with 10 V - 10 seconds as one step, and the applied voltage was increased. The voltage at which the current value flowing between the electrodes became 100 mA or more in the process of increasing the applied voltage was defined as the dielectric 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 2 / g, D50: 10.8 μm) was prepared. (Dispersion step S1) 306 g of pure water and 1650 g of isopropyl alcohol (IPA) were put into a 5000 mL reaction vessel at room temperature and mixed using a stirring blade to prepare a mixed solvent. 5550 g of the above 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: manufactured by Wako Pure Chemical Industries, Ltd., special grade reagent) aliquoted into a small-capacity beaker was added all at once to the slurry obtained in the dispersion step S1. The TEOS adhering to the small-capacity beaker was washed off with 200 g of IPA and added to the slurry. After the addition of TEOS, the temperature of the slurry was maintained at 40 °C and stirring was continued for 5 minutes to cause a reaction between the hydrolysis product of TEOS and the surface of the raw material powder particles. (Hydrolysis Catalyst Addition Step S3) Thereafter, 239 g of 25% by mass aqueous ammonia was continuously added to the slurry at an addition rate of 2.6 g / min. After the addition of the aqueous ammonia was completed, while stirring, a holding time of 60 min was provided to form a silicon oxide coating layer 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) Thereafter, the slurry was subjected to solid-liquid separation using a Nutsche suction filtration device to recover the solid content. The recovered solid content was dried at 100°C for 12 hr in a nitrogen atmosphere to obtain silicon oxide-coated soft magnetic powder. The above-described various measurements were performed on the obtained silicon oxide-coated soft magnetic powder.

[0058] <Example 2> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 1, except that the aqueous ammonia added to the slurry had an addition rate of 1.3 g / min.

[0059] <Example 3> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 1, except that the stirring of the slurry was set at 550 rpm.

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

[0061] <Example 5> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 3, except that the aqueous ammonia added to the slurry had an addition rate of 1.3 g / min.

[0062] <Example 6> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 3, except that the aqueous ammonia added to the slurry had 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 procedure as in Example 3, except that the ammonia water added to the slurry had an addition rate of 10.6 g / min and a holding time of 128 min.

[0064] <Comparative Example 1> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 1, except that the ammonia water added to the slurry was 954 g and the addition rate was 10.6 / min.

[0065] <Comparative Example 2> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 3, except that the ammonia water added to the slurry was 954 g and the addition rate was 10.6 / min.

[0066] <Comparative Example 3> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 4, except that the ammonia water added to the slurry had an addition rate of 5.3 g / min.

[0067] <Example 8> As the raw material powder, FeSiCr soft magnetic powder (BET specific surface area: 0.29 m 2 / g, D50: 10.6 μm) was prepared. (Dispersion step S1) Into a 5000 mL reaction vessel, 311 g of pure water and 1675 g of isopropyl alcohol (IPA) were charged at room temperature and mixed using a stirring blade to prepare a mixed solvent. 5625 g of the above 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) To the slurry obtained in the dispersion step S1, 223.2 g of tetraethoxysilane (TEOS: manufactured by Wako Pure Chemical Industries, Ltd., special grade reagent) separated into a small-volume beaker was added all at once. The TEOS adhering to the small-volume beaker was washed off with 200 g of IPA and added to the slurry. After the addition of TEOS, the temperature of the slurry was maintained at 40 °C and stirring was continued for 5 minutes to cause a reaction between the hydrolysis product of TEOS and the surface of the raw material powder particles. (Hydrolysis catalyst addition step S3) Thereafter, 242 g of 25% by mass aqueous ammonia was continuously added to the slurry at an addition rate of 2.7 g / min. After the addition of the aqueous ammonia was completed, a holding time of 60 min was provided while stirring to form a silicon oxide coating layer 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) Soft magnetic powder coated with silicon oxide was obtained in the same procedure as in Example 1.

[0068] <Comparative Example 4> Soft magnetic powder coated with silicon oxide was obtained in the same procedure as in Example 8, except that 968 g of aqueous ammonia was added to the slurry, the addition rate was 10.8 / min, and the stirring was 380 rpm.

[0069] <Example 9> Soft magnetic powder coated with silicon oxide was obtained in the same procedure as in Example 8, except that 74.4 g of tetraethoxysilane was added to the slurry.

[0070] <Comparative Example 5> Soft magnetic powder coated with silicon oxide was obtained in the same procedure as in Example 9, except that 968 g of aqueous ammonia was added to the slurry, the addition rate was 10.8 / min, and the stirring was 380 rpm.

[0071] <Example 10> The raw material powder added to the slurry was FeSiCr soft magnetic powder (BET specific surface area: 0.15 m 2 / g, D50: 24.1 μm), and soft magnetic powder coated with silicon oxide was obtained in the same procedure as in Example 8, except that 77.5 g of tetraethoxysilane was added.

[0072] <Example 11> Soft magnetic powder coated with silicon oxide was obtained in the same procedure as in Example 10, except that the stirring of the slurry was 550 rpm.

[0073] <Comparative Example 6> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 10, except that 968 g of aqueous ammonia added to the slurry was used and the addition rate was 10.8 / min.

[0074] <Example 12> As the raw material powder, FeSiCr soft magnetic powder (BET specific surface area: 0.29 m 2 / g, D50: 10.6 μm) was prepared. (Dispersion step S1) Into a 1000 mL reaction vessel, 70 g of pure water and 400 g of isopropyl alcohol (IPA) were charged at room temperature and mixed using a stirring blade to prepare a mixed solvent. 250 g of the above 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) To the slurry obtained in the dispersion step S1, 8.0 g of tetraethoxysilane (TEOS: manufactured by Wako Pure Chemical Industries, Ltd., special grade reagent) separated into a small beaker was added all at once. The TEOS adhering to the small beaker was washed off with 5 g of IPA and added to the slurry. After adding TEOS, the temperature of the slurry was maintained at 40°C and stirring was continued for 5 minutes to cause a reaction between the hydrolysis product of TEOS and the surface of the raw material powder particles. (Hydrolysis catalyst addition step S3) Thereafter, 45 g of 28% by mass aqueous ammonia was continuously added to the slurry at an addition rate of 0.3 g / min. After the addition of aqueous ammonia was completed, a holding time of 60 min was provided while stirring to form a silicon oxide coating layer 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) Thereafter, solid-liquid separation was performed using a Nutsche suction filtration device to recover the solid content. The recovered solid content was vacuum dried at 120°C for 3 hours to obtain a silicon oxide-coated soft magnetic powder.

[0075] <Comparative Example 7> A silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 12, except that the addition rate of aqueous ammonia added to the slurry was 1.0 / min and the stirring was 600 rpm.

[0076] <Example 13> The raw material powder added to the slurry was FeSiCr soft magnetic powder (BET specific surface area: 0.90 m 2 / g, D50: 3.2 μm). Silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 8, except that the added tetraethoxysilane was 93.3 g, the added aqueous ammonia was at an addition rate of 1.3 g / min, and the stirring was 604 rpm.

[0077] <Comparative Example 8> Silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 13, except that the added aqueous ammonia was 968 g, the addition rate was 10.8 / min, and the stirring was 380 rpm.

[0078] <Example 14> The raw material powder added to the slurry was FeSiCr soft magnetic powder (BET specific surface area: 0.035 m 2 / g, D50: 40.2 μm). Silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 8, except that the added tetraethoxysilane was 36.2 g.

[0079] <Comparative Example 9> Silicon oxide-coated soft magnetic powder was obtained in the same procedure as in Example 14, except that the added aqueous ammonia was 968 g, the addition rate was 10.8 / min, and the stirring was 380 rpm.

[0080] Table 1 shows the results of various measurements 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.

[0081]

Table 1

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

[0083] From the above, it was confirmed that by adding a hydrolysis catalyst so as to satisfy at least one of the above conditions (a) or (b) in the hydrolysis catalyst addition step S3, the increase rate of the breakdown voltage can be improved. Also, it was confirmed that the sphericity can be increased and the amount of resin required during pressure molding can be reduced.

Explanation of Reference Signs

[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 and 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 step of 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; a step of adding silicon alkoxide to the slurry and stirring and mixing; a step of adding a hydrolysis catalyst of the silicon alkoxide to the slurry to which the silicon alkoxide has been added, and coating the surface of the core particles with silicon oxide; a step of subjecting the slurry to solid-liquid separation and drying the solid content to obtain the silicon oxide-coated soft magnetic powder coated with the silicon oxide, and in the step of coating with the silicon oxide, the hydrolysis catalyst is added so as to satisfy at least one of the following conditions (a) or (b). A method for producing a silicon oxide-coated soft magnetic powder. (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 a silicon oxide-coated soft magnetic powder according to claim 5, which satisfies both the conditions (a) and (b).

7. In the step of stirring and mixing, the method for producing a 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 hydrolysis catalyst is ammonia, and the method for producing a silicon oxide-coated soft magnetic powder according to any one of claims 5 to 7.

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