Method for producing coated metal powder

Abstract

To provide a method of producing a coated metal powder that allows for forming a highly insulative and heat-resistive coating on a particle surface of a metal powder inexpensively and efficiently.SOLUTION: A method of producing a coated metal powder comprises the steps of: preparing a silanol solution having a silicon-containing substance dissolved in an alkaline water solution, obtaining a dispersion liquid by inputting a metal powder into the silanol solution, and forming a coating containing a silicon oxide on the particle surface of the metal powder by adding an acidic aqueous solution to the dispersion liquid.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing coated metal powder. [Background technology]

[0002] Patent Document 1 discloses a method for forming a silica coating on the surface of soft magnetic powder using a hydrolysis solution containing tetraethoxysilane, an organic solvent, an alkali, and water. By forming the silica coating, a soft magnetic powder for dust cores with high electrical resistance can be obtained.

[0003] Furthermore, soft magnetic powders for dust cores having such a silica coating can be heat-treated at high temperatures of 800°C or higher, which makes it possible to increase the crystal grain size and remove processing strain, thereby reducing the hysteresis loss of the dust core. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-231481 Summary of the Invention [Problem to be solved by the invention]

[0005] In the method described in Patent Document 1, an organic solvent is used together with tetraethoxysilane to prepare a hydrolysis solution, which increases the cost associated with the use of the organic solvent, thereby increasing the production cost of soft magnetic powder for dust cores. [Means for solving the problem]

[0006] A method for producing a coated metal powder according to an application example of the present invention includes: preparing a silanol solution in which a silicon-containing substance is dissolved in an alkaline aqueous solution; adding a metal powder to the silanol solution to obtain a dispersion; a step of forming a coating containing silicon oxide on the particle surfaces of the metal powder by adding an acidic aqueous solution to the dispersion; With death, the silicon-containing material is silicon, silicon oxide, silicon carbide, or silicon nitride; The alkaline aqueous solution is an amine-based aqueous solution exhibiting weak alkalinity, The pH of the dispersion before the addition of the acidic aqueous solution is higher by 0.5 to 4.0 than the pH of the dispersion after the addition of the acidic aqueous solution. . [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 2 is a cross-sectional view showing an example of a coated metal powder. [Figure 2] 1 is a flowchart illustrating a method for manufacturing a coated metal powder according to an embodiment. [Figure 3] 1 is a schematic diagram for explaining a method for producing a coated metal powder according to an embodiment. FIG. [Figure 4] 1 is a schematic diagram for explaining a method for producing a coated metal powder according to an embodiment. FIG. [Figure 5] 1 is a schematic diagram for explaining a method for producing a coated metal powder according to an embodiment. FIG. [Figure 6] 1 is a schematic diagram for explaining a method for producing a coated metal powder according to an embodiment. FIG. [Figure 7] 1 is a schematic diagram for explaining a method for producing a coated metal powder according to an embodiment. FIG. [Figure 8] 1 is a schematic diagram for explaining a method for producing a coated metal powder according to an embodiment. FIG. [Figure 9] 1 shows C1s spectra obtained by XPS for a coated metal powder before and after heat treatment. [Figure 10] 1 shows the N1s spectra obtained by XPS for the coated metal powder before and after heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0008] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the method for producing coated metal powder of the present invention will now be described in detail with reference to the accompanying drawings.

[0009] 1. Coated metal powder First, the coated metal powder will be described. Fig. 1 is a cross-sectional view showing an example of the coated metal powder.

[0010] The coated metal powder 1 shown in Fig. 1 is an aggregate of multiple particles. Hereinafter, the particles contained in the coated metal powder 1 will be referred to as coated metal particles 10. The coated metal particle 10 shown in Fig. 1 has a core particle 2 and a coating 3 provided on the surface of the core particle 2.

[0011] 1.1. Core particle The constituent material of the core particle 2 is not particularly limited as long as it is a metal material, and may be any metal material, but one example is a soft magnetic material. Coated metal particles 10 having core particles 2 made of a soft magnetic material have coatings 3 provided on the surfaces of the core particles 2, which easily improves the insulation between particles. For this reason, coated metal powder 1 is preferably used, for example, in dust cores with low core loss (iron loss).

[0012] The soft magnetic material is not particularly limited as long as it is a soft magnetic material whose main component is Fe, Ni, or Co, and examples thereof include pure iron, Fe-Si alloys such as silicon steel, Fe-Ni alloys such as Permalloy, Fe-Co alloys such as Permendur, Fe-Si-Al alloys such as Sendust, Fe-Cr-Si alloys, Fe-Cr-Al alloys, and various Fe alloys, as well as various Ni alloys and various Co alloys, etc. Among these, various Fe alloys are preferably used from the viewpoints of magnetic properties such as magnetic permeability and magnetic flux density, and cost, etc.

[0013] The crystal structure of the soft magnetic material is not particularly limited, and may be crystalline, amorphous, or microcrystalline (nanocrystalline).

[0014] Microcrystalline refers to a crystalline structure with a grain size of 1.0 nm to 30.0 nm. By including such microcrystalline in the crystal structure of a soft magnetic material, the soft magnetic properties of the material can be further improved. In other words, a soft magnetic material that combines low coercive force and high magnetic permeability can be obtained.

[0015] Examples of soft magnetic materials containing amorphous and soft magnetic materials containing microcrystalline include Fe-based alloys such as Fe-Si-B, Fe-Si-BC, Fe-Si-B-Cr-C, Fe-Si-Cr, Fe-B, Fe-PC, Fe-Co-Si-B, Fe-Si-B-Nb, Fe-Si-B-Nb-Cu, and Fe-Zr-B, Ni-based alloys such as Ni-Si-B and Ni-PB, and Co-based alloys such as Co-Si-B.

[0016] An oxide film may be present on the surface of the core particle 2. The oxide film refers to a coating made of an oxide of the element that constitutes the core particle 2.

[0017] 1.2.Coating The coating 3 is provided on the surface of the core particle 2. The coating 3 is preferably provided on the entire surface of the core particle 2, but may be provided on only a portion of the surface.

[0018] The coating 3 contains silicon oxide. Silicon oxide is an oxide of silicon and is a general term for, for example, SiO2, SiO3, SiO, and SiO. The coating 3 preferably contains silicon oxide as the main material. "Main material" refers to a volume ratio of 50% or more. The silicon oxide content in the coating 3 is preferably 70% or more by volume, and more preferably 90% or more by volume.

[0019] Such a volume ratio can be measured as the area ratio of silicon oxide when, for example, a surface analysis, that is, a two-dimensional composition analysis, is performed on a cross section of the coating 3 .

[0020] The coating 3 contains silicon oxide, and therefore imparts properties such as insulation, heat resistance, and corrosion resistance to the core particle 2. This can increase the added value of the core particle 2.

[0021] The O / Si ratio of the silicon oxide in the coating 3 affects the properties of the silicon oxide, and therefore it is desirable to optimize it. Specifically, the O / Si ratio of the silicon oxide in the coating 3 is preferably 2.0 or more and 4.0 or less, and more preferably 2.0 or more and 3.5 or less. If the O / Si ratio of the silicon oxide is within this range, the density of the coating 3 can be particularly improved. As a result, the properties of the coating 3, such as insulation, heat resistance, and corrosion resistance, can be further improved.

[0022] The O / Si ratio of silicon oxide refers to the ratio of the number of O atoms to the number of Si atoms, and can be calculated based on the analysis results of the coating 3 by, for example, X-ray photoelectron spectroscopy (XPS).

[0023] The thickness of the coating 3 is not particularly limited, but is preferably 1 nm or more and 100 nm or less, more preferably 2 nm or more and 50 nm or less, and even more preferably 3 nm or more and 20 nm or less. If the thickness of the coating 3 is within the above range, it is possible to avoid the coating 3 being excessively thick while ensuring the above-mentioned properties. In other words, if the thickness of the coating 3 is below the above-mentioned lower limit, the thickness of the coating 3 may be insufficient, and the above-mentioned properties may not be fully obtained. On the other hand, if the thickness of the coating 3 is above the above-mentioned upper limit, the thickness of the coating 3 may be excessive, and for example, the occupancy rate of the core particles 2 in the powder magnetic core may decrease, which may result in a deterioration in the magnetic properties of the powder magnetic core.

[0024] 2. Manufacturing method of coated metal powder First, a method for producing a coated metal powder according to an embodiment will be described.

[0025] Fig. 2 is a flowchart illustrating a method for producing a coated metal powder according to an embodiment. Figs. 3 to 8 are schematic diagrams illustrating a method for producing a coated metal powder according to an embodiment.

[0026] The method for producing a coated metal powder according to the embodiment includes an alkaline aqueous solution preparation step S102, a silanol solution preparation step S104, a dispersion preparation step S106, and an acidic aqueous solution addition step S108, as shown in Figure 2. Each step will be described below in order.

[0027] 2.1. Alkaline aqueous solution preparation process In the alkaline aqueous solution preparation step S102, an alkaline aqueous solution 4 shown in Fig. 3 is prepared. The alkaline aqueous solution 4 is an aqueous solution obtained by dissolving an alkali in water.

[0028] Examples of alkalis include strong alkalis such as sodium hydroxide, potassium hydroxide, and calcium hydroxide, and weak alkalis such as ammonia, tetramethylammonium, and sodium hydrogen carbonate.

[0029] For example, when ammonia is used as the alkali, the ammonia is ionized according to the reaction shown in formula (1) below, thereby producing ammonium ions and hydroxide ions. NH3+H2O→NH4 + +OH - … (1)

[0030] The alkaline aqueous solution 4 may be strongly alkaline, but is preferably weakly alkaline. The weakly alkaline aqueous solution 4 can prevent the pH of the dispersion liquid, which will be described later, from becoming too high. This can prevent the pH of the dispersion liquid from changing suddenly when the acidic aqueous solution is added to the dispersion liquid in the acidic aqueous solution addition step S108, which will be described later. As a result, adverse effects caused by a sudden change in pH, such as a decrease in the density of the coating film 3, can be suppressed.

[0031] The pH of the alkaline aqueous solution 4 is preferably 8.0 or higher, more preferably 8.0 or higher and 12.0 or lower, and even more preferably 8.0 or higher and 11.0 or lower.

[0032] Here, "weakly alkaline" means that the pH of the alkaline aqueous solution 4 is 8.0 or more and 11.0 or less, and "strongly alkaline" means that the pH of the alkaline aqueous solution 4 is higher than 11.0.

[0033] The pH of the alkaline aqueous solution 4 is measured by the pH measurement method using a pH meter with a glass electrode, as specified in JIS Z 8802:2011.

[0034] The alkaline aqueous solution 4 is the aforementioned alkaline aqueous solution, but is preferably an amine-based aqueous solution. An amine-based aqueous solution is an aqueous solution that uses an amine compound such as ammonia or tetramethylammonium as the alkali. By using an amine-based aqueous solution as the alkaline aqueous solution 4, even if cations generated by ionization of the alkali are captured in the coating 3, deterioration of the properties of the coating 3 can be suppressed.

[0035] Instead of preparing the alkaline aqueous solution 4, a commercially available product may be purchased. In that case, this step is omitted. In addition, any additive may be added to the alkaline aqueous solution 4.

[0036] 2.2. Silanol solution preparation process In the silanol solution preparation step S104, a silanol solution 5 shown in FIG. 4 is prepared. The silanol solution 5 refers to an alkaline aqueous solution 4 containing silanol. Silanol is a hydroxyl group (OH - ) is a general term for silicon compounds having the formula Si(OH)4, and examples thereof include orthosilicic acid, its oligomers, and precursors of orthosilicic acid (silicate ions). In the drawings of this application, orthosilicic acid is shown as an example of silanol.

[0037] In order to dissolve silanol in the alkaline aqueous solution 4, in FIG. 4, Si chips 6 are added to the alkaline aqueous solution 4. The Si chips 6 are individual pieces of a Si wafer and are made of Si. The Si dissolves in the alkaline aqueous solution 4 and becomes silanol. This reaction is represented by the following formula (2). Si + 4H2O → Si(OH)4 + 2H2… (2)

[0038] In the above formula (2), orthosilicic acid is used as an example of silanol. The reaction represented by the above formula (2) produces silanol and hydrogen.

[0039] The method for dissolving silanol in alkaline aqueous solution 4 is not limited to the method using Si chip 6, and may be a method using, for example, silicon oxide such as quartz or quartz glass, silicon carbide, silicon nitride, etc. In other words, the substance to be dissolved in alkaline aqueous solution 4 may be any silicon-containing substance, including elemental silicon and silicon compounds.

[0040] The amount of Si chips 6 added to the alkaline aqueous solution 4 is not particularly limited, but is preferably 0.01 g or more and 0.50 g or less, and more preferably 0.03 g or more and 0.20 g or less, per 40 mL of the alkaline aqueous solution 4. This allows for a smaller amount of Si chips 6 to be added to the alkaline aqueous solution 4, thereby reducing the cost of the method for producing coated metal powder.

[0041] The Si chips 6 that remain undissolved are removed, thereby obtaining the silanol solution 5 shown in FIG.

[0042] If necessary, the alkaline aqueous solution 4 and the silanol solution 5 may be heated. This promotes the reaction represented by the above formula (2). The temperatures of the alkaline aqueous solution 4 and the silanol solution 5 are preferably 30°C or higher and 100°C or lower, and more preferably 40°C or higher and 90°C or lower. By setting the temperatures of the alkaline aqueous solution 4 and the silanol solution 5 within the above ranges, the Si chips 6 can be efficiently dissolved and the silanol solution 5 can be prepared in a short time.

[0043] Furthermore, the time for which the above temperature is maintained is not particularly limited, but is preferably 3 hours or more, more preferably 10 hours to 120 hours, and even more preferably 20 hours to 72 hours. This allows the silanol concentration in the silanol solution 5 to be increased until it reaches an over-dissolved state. As a result, in the process described below, a dense coating 3 with minimal thickness variation can be efficiently formed.

[0044] In this step, instead of preparing the silanol solution 5, a commercially available product of the silanol solution 5 may be obtained. Furthermore, the silanol solution 5 may contain any additive.

[0045] Furthermore, prior to the dispersion liquid preparation step S106 described below, an operation for removing cations generated by ionization of the alkali may be carried out as necessary. Such an operation may be, for example, an ion exchange treatment. An ion exchange resin is used for the ion exchange treatment.

[0046] 2.3. Dispersion liquid preparation process In the dispersion liquid preparation step S106, as shown in Fig. 6, core particles 2 (metal powder) are added to a silanol solution 5. This results in a dispersion liquid 7 in which the core particles 2 are dispersed in the silanol solution 5, as shown in Fig. 7.

[0047] The amount of core particles 2 added to the dispersion 7 is not particularly limited, but is preferably 0.5 g to 20 g, and more preferably 1.0 g to 10 g, per 40 mL of the dispersion 7. This allows a coating 3 of sufficient thickness to be formed efficiently.

[0048] The method for producing the core particles 2 is not particularly limited, but examples thereof include various atomization methods such as water atomization, gas atomization, and rotary water flow atomization, as well as pulverization.

[0049] 2.4. Acidic aqueous solution addition process In the acidic aqueous solution addition step S108, as shown in FIG. 7, an acidic aqueous solution 8 is added to the dispersion liquid 7. As a result, the silanols dissolved in the dispersion liquid 7 tend to aggregate on the surfaces of the core particles 2 as the acidic aqueous solution 8 is added, increasing the concentration. Finally, the silanols undergo condensation polymerization and gel to form silicon oxide. As a result, a coating 3 containing silicon oxide is formed on the surfaces of the core particles 2, and the coated metal particles 10 shown in FIG. 8 are obtained. The obtained coated metal particles 10 may then be subjected to a washing treatment and a drying treatment.

[0050] The acidic aqueous solution 8 is an aqueous solution that exhibits acidity. Examples of such an acidic aqueous solution 8 include carboxylic acid aqueous solutions such as acetic acid aqueous solution, citric acid aqueous solution, and oxalic acid aqueous solution, weak acid aqueous solutions such as hypochlorous acid, and strong acid aqueous solutions such as hydrochloric acid and sulfuric acid. In FIG. 7, an acetic acid aqueous solution is illustrated as an example of the acidic aqueous solution 8.

[0051] When an aqueous acetic acid solution is used as the acidic aqueous solution 8, silicon oxide is produced by the reaction shown in formula (3) below. Si(OH)4 → SiO2 + 2H2O … (3) In the above formula (3), SiO2 is given as an example of silicon oxide.

[0052] The acidic aqueous solution 8 may be strongly acidic, but is preferably weakly acidic. When added to the dispersion 7, the weakly acidic acidic aqueous solution 8 suppresses a sudden change in pH in the dispersion 7. This makes it possible to suppress adverse effects caused by a sudden change in pH, such as a decrease in the density of the coating 3.

[0053] The pH of the acidic aqueous solution 8 is preferably 6.0 or less, and more preferably 3.0 or more and 5.0 or less.

[0054] Here, "weakly acidic" means that the pH of the acidic aqueous solution 8 is 3.0 or more and 6.0 or less, and "strongly acidic" means that the pH of the acidic aqueous solution 8 is less than 3.0.

[0055] The pH of the acidic aqueous solution 8 is measured by the pH measurement method using a pH meter with a glass electrode, as specified in JIS Z 8802:2011.

[0056] The acidic aqueous solution 8 is preferably a carboxylic acid aqueous solution. A carboxylic acid aqueous solution is an aqueous solution of an organic compound having a carboxyl group. By using a carboxylic acid aqueous solution as the acidic aqueous solution 8, even if anions obtained by ionization of the carboxylic acid are captured in the coating 3, deterioration of the properties of the coating 3 can be suppressed.

[0057] The pH of the dispersion 7 after the addition of the acidic aqueous solution 8 is preferably 7.0 or more and 11.0 or less, and more preferably 8.5 or more and 9.5 or less. By setting the pH of the dispersion 7 after the addition of the acidic aqueous solution 8 within the above range, silicon oxide is produced at an appropriate rate on the surfaces of the core particles 2. As a result, a coating 3 that is particularly dense and has excellent properties is formed.

[0058] If the pH of the dispersion 7 after the addition of the acidic aqueous solution 8 is below the lower limit, the reaction represented by the above formula (3) will proceed in a short time, which may result in a decrease in the density of the coating 3. On the other hand, if the pH of the dispersion 7 after the addition of the acidic aqueous solution 8 is above the upper limit, the rate of the reaction represented by the above formula (3) will decrease, which may result in a long time being required to form the coating 3 or the thickness of the coating 3 being too thin.

[0059] The pH of the dispersion 7 after the addition of the acidic aqueous solution 8 is measured by the pH measurement method using a pH meter with a glass electrode, as specified in JIS Z 8802:2011.

[0060] Furthermore, the thickness of the coating 3 can be adjusted by controlling the pH of the dispersion 7 after the addition of the acidic aqueous solution 8. For example, the thickness of the coating 3 can be increased by lowering the pH of the dispersion 7 after the addition of the acidic aqueous solution 8 within the above range.

[0061] Furthermore, the pH of the dispersion 7 before the addition of the acidic aqueous solution 8 is preferably 0.5 to 4.0 higher, and more preferably 1.0 to 3.0 higher, than the pH of the dispersion 7 after the addition of the acidic aqueous solution 8. This makes it possible to suppress the change in pH before and after the addition of the acidic aqueous solution 8 relatively gradually. As a result, the rate of the reaction represented by the above formula (3) can be prevented from becoming too fast or too slow, and a dense coating 3 can be formed at a sufficient rate. Therefore, coated metal particles 10 having coatings 3 with excellent properties can be efficiently produced.

[0062] As described above, the method for producing coated metal powder 1 according to this embodiment includes a silanol solution preparation step S104, a dispersion preparation step S106, and an acidic aqueous solution addition step S108. In the silanol solution preparation step S104, a silanol solution 5 in which a silicon-containing substance is dissolved in an alkaline aqueous solution 4 is prepared. In the dispersion preparation step S106, core particles 2, which are metal powder, are added to the silanol solution 5 to obtain a dispersion 7. In the acidic aqueous solution addition step S108, an acidic aqueous solution 8 is added to the dispersion 7 to form a coating 3 containing silicon oxide on the surface of the core particles 2.

[0063] According to this configuration, the coating 3 having high insulating properties and heat resistance can be formed inexpensively and efficiently on the surface of the core particle 2. That is, in this embodiment, no organic solvent is used, and the alkaline aqueous solution 4 and the acidic aqueous solution 8 are mainly used, so that the cost required for forming the coating 3 can be reduced by eliminating the cost of procuring the organic solvent and the cost of treating the waste liquid. In addition, the coating 3 is dense and has a low content of organic components derived from the raw materials, and therefore has excellent insulating properties and heat resistance.

[0064] Furthermore, the process of producing silicon oxide using the acidic aqueous solution 8 is characterized by a high rate of silicon oxide production due to the extremely high reaction promotion effect that accompanies the addition of the acidic aqueous solution 8. Therefore, in this embodiment, the coating 3 can be efficiently formed at low cost.

[0065] Furthermore, while a method of using a silicon oxide precursor as a raw material has been known in the past, the silicon oxide precursor also incurs procurement costs. In this embodiment, a readily available Si chip 6 or the like can be used, which reduces the manufacturing cost in terms of raw material procurement costs as well.

[0066] The temperature of the dispersion 7 when the acidic aqueous solution 8 is added is preferably 30°C or higher and 100°C or lower, and more preferably 40°C or higher and 90°C or lower. By setting the temperature of the dispersion 7 within this range, the reaction represented by the above formula (3) is promoted. This allows the coating 3 to be formed in a shorter time. Furthermore, for the same formation time, the thickness of the coating 3 can be made thicker.

[0067] Furthermore, the time for which the temperature is maintained is not particularly limited, but is preferably 3 hours or more, more preferably 10 hours to 240 hours, and even more preferably 20 hours to 120 hours, which allows a dense coating 3 of necessary and sufficient thickness to be formed evenly.

[0068] The coated metal powder 1 obtained as described above can be used for any purpose, including the production of the above-mentioned dust cores. Examples of applications of the coated metal powder 1 include powder for 3D printers and powder for powder metallurgy.

[0069] The average particle size of the coated metal powder 1 is not particularly limited, but is preferably 1.0 μm or more and 30.0 μm or less, and more preferably 2.0 μm or more and 10.0 μm or less. This allows for a reduction in eddy current loss within particles in a powder core, for example, when a powder core is manufactured using the coated metal powder 1. Furthermore, the powder density can be increased, which facilitates the improvement of the magnetic properties of the powder core.

[0070] The average particle size of the coated metal powder 1 is determined as the particle size D50 at which the cumulative 50% from the smallest diameter side is reached in the volume-based particle size distribution obtained by laser diffraction.

[0071] The method for producing coated metal powder of the present invention has been described above based on the illustrated embodiment, but the present invention is not limited to this. For example, the method for producing coated metal powder of the present invention may be one in which any desired process is added to the above embodiment. [Example]

[0072] Next, specific examples of the present invention will be described. 3. Manufacturing of coated metal powder Example 1 First, an alkaline aqueous solution was prepared. A sodium hydroxide aqueous solution was used as the alkaline aqueous solution. The pH of the alkaline aqueous solution was 12.0.

[0073] Next, Si chips were added to the obtained alkaline aqueous solution. The temperature of the alkaline aqueous solution was maintained at 50°C for 1 day (24 hours). After that, the remaining Si chips were removed from the alkaline aqueous solution to obtain a silanol solution.

[0074] Next, a metal powder was added to the resulting silanol solution to prepare a dispersion. The metal powder used was an Fe-Si-B amorphous alloy powder with an average particle size of 5.0 μm, produced by water atomization.

[0075] Next, an acidic aqueous solution was added to the obtained dispersion. An acetic acid aqueous solution was used as the acidic aqueous solution. The pH of the acidic aqueous solution was 4.0. After the addition of the acidic aqueous solution, the temperature of the dispersion was maintained at 50°C and kept for one day (24 hours). The pH of the dispersion before the addition of the acidic aqueous solution was 12.0, and the pH of the dispersion after the addition of the acidic aqueous solution was 7.5. As a result, a coating containing silicon oxide was obtained on the particle surfaces of the metal powder.

[0076] The obtained coated metal powder was then removed from the dispersion and dried by vacuum drying. The above manufacturing conditions are shown in Table 1. In Table 1, the pH of the dispersion after the addition of the acidic aqueous solution is referred to as the "pH of the dispersion." In Table 1, the difference between the pH of the dispersion before the addition of the acidic aqueous solution and the pH of the dispersion after the addition of the acidic aqueous solution is referred to as the "pH difference."

[0077] Example 2 First, an alkaline aqueous solution was prepared. Ammonia water was used as the alkaline aqueous solution. The pH of the alkaline aqueous solution was 10.5.

[0078] Next, Si chips were added to the obtained alkaline aqueous solution. The temperature of the alkaline aqueous solution was maintained at 50°C for 1 day (24 hours). After that, the remaining Si chips were removed from the alkaline aqueous solution to obtain a silanol solution.

[0079] Next, a metal powder was added to the resulting silanol solution to prepare a dispersion. The metal powder used was an Fe-Si-B amorphous alloy powder with an average particle size of 5.0 μm, produced by water atomization.

[0080] Next, an acidic aqueous solution was added to the obtained dispersion. An acetic acid aqueous solution was used as the acidic aqueous solution. The pH of the acidic aqueous solution was 4.0. After the addition of the acidic aqueous solution, the temperature of the dispersion was maintained at 50°C and kept for one day (24 hours). The pH of the dispersion before the addition of the acidic aqueous solution was 10.5, and the pH of the dispersion after the addition of the acidic aqueous solution was 8.0. As a result, a coating containing silicon oxide was obtained on the particle surfaces of the metal powder.

[0081] The obtained coated metal powder was then removed from the dispersion and dried by vacuum drying. The above manufacturing conditions are shown in Table 1. In Table 1, the pH of the dispersion after the addition of the acidic aqueous solution is referred to as the "pH of the dispersion." In Table 1, the difference between the pH of the dispersion before the addition of the acidic aqueous solution and the pH of the dispersion after the addition of the acidic aqueous solution is referred to as the "pH difference."

[0082] 3.3. Examples 3 to 5 Coated metal powders were obtained in the same manner as in Example 1, except that the production conditions were changed as shown in Table 1.

[0083] 3.4.Comparative Example First, a hydrolysis solution was prepared by mixing metal powder, tetraethoxysilane, isopropyl alcohol (organic solvent), aqueous ammonia, and water, and the resulting hydrolysis solution was stirred for 3 hours using a propeller stirrer.

[0084] Thereafter, the metal powder was separated from the hydrolysis solution, and the metal powder was then heat-treated at 120° C. for 1 hour, thereby obtaining a coated metal powder.

[0085] 4. Evaluation of coated metal powder 4.1.Breakdown voltage The dielectric breakdown voltage of the coated metal powders of each of the Examples and Comparative Examples was measured at room temperature (25° C.) by the following method.

[0086] Next, the coated metal powder was subjected to a heat treatment for 700 hours at 150° C. Thereafter, the dielectric breakdown voltage of the heat-treated coated metal powder was measured again by the following method.

[0087] The method for measuring the breakdown voltage is as follows. First, 2 g of the coated metal powder was filled into an alumina cylinder with an inner diameter of 8 mm, and brass electrodes were placed on both ends of the cylinder. Then, a digital force gauge was used to apply a force of 40 kg / cm between the electrodes on both ends of the cylinder. 2 While applying a pressure of 100 kJ / cm, a voltage of 50 V was applied between the electrodes for 2 seconds.

[0088] Next, the voltage applied between the electrodes was increased to 100 V and maintained for 2 seconds, and the electrical resistance between the electrodes at this time was measured using a digital multimeter to confirm whether or not dielectric breakdown had occurred.

[0089] The voltage applied between the electrodes was then increased in increments of 50 V from 150 V, while the electrical resistance between the electrodes was measured each time, and the occurrence of dielectric breakdown was confirmed with a digital multimeter. The voltage was then increased in increments of 50 V and the electrical resistance was measured until dielectric breakdown occurred, and the lowest voltage value at which dielectric breakdown occurred was taken as the dielectric breakdown voltage.

[0090] The breakdown voltage measured in this manner was evaluated for change before and after the heat treatment in accordance with the following evaluation criteria.

[0091] A: The breakdown voltage did not change before and after heat treatment. B: The breakdown voltage after heat treatment was slightly lower than before heat treatment (the change was less than 100 V). C: The breakdown voltage after heat treatment was significantly lower than before heat treatment (the change was 100V or more). The evaluation results are shown in Table 1.

[0092] [Table 1]

[0093] As shown in Table 1, no significant change in breakdown voltage was observed before and after heat treatment for the coated metal powders of each example. In particular, it was found that by optimizing the range of change in pH before and after adding the acidic aqueous solution, the range of change in breakdown voltage could be kept small even after heat treatment. Therefore, it is found that the coated metal powders of each example have coatings with high insulating properties and heat resistance.

[0094] 4.2. Evaluation of secondary ingredients derived from raw materials The coated metal powder of Example 2 was subjected to surface analysis by X-ray photoelectron spectroscopy (XPS). XPS not only analyzes the composition of the coating, but also identifies the atomic bonding state based on chemical shifts. The XPS measurement conditions are as follows:

[0095] ·X-ray source: AlKα radiation Irradiation area: 100μm diameter Beam angle: 45° Line voltage: 15kV ·Line source power: 25W

[0096] Using XPS set under these measurement conditions, C1s spectra were obtained for the coated metal powder before and after the heat treatment described in 4.1. The obtained C1s spectra are shown in Figure 9. The horizontal axis of Figure 9 represents binding energy in eV, and the vertical axis represents photoelectron intensity in arbitrary units.

[0097] In the C1s spectrum shown in Figure 9, a peak-like shape is observed at a position corresponding to the C1s bond energy. This is presumed to be due to contamination adhering to the analyte for the following reasons. Contaminants are generally thought to be organic substances containing C-H bonds. Because the bond energy of a C-H bond is approximately 284.8 eV, the peak-like shape shown in Figure 9 is presumed to be due to C-H bonds derived from contamination. This peak-like shape was observed in the C1s spectra obtained for both the coated metal powder before and after heat treatment.

[0098] On the other hand, examples of minor components derived from the raw materials include substances containing C-O bonds derived from an acetic acid aqueous solution, which is an acidic aqueous solution. Since the bond energy of a C-O bond is approximately 286.5 eV, it can be said that the C-O bond derived from the raw materials is not observed in the C1s spectrum shown in Figure 9.

[0099] Next, the N1s spectra were obtained by XPS for the coated metal powder before and after heat treatment. The obtained N1s spectra are shown in Figure 10. The horizontal axis of Figure 10 represents binding energy in eV, and the vertical axis represents photoelectron intensity in arbitrary units.

[0100] In the N1s spectrum shown in FIG. 10, no peak or the like was observed at a position corresponding to the N1s binding energy.

[0101] An example of a minor component derived from the raw material is a substance containing an N-H bond derived from aqueous ammonia, which is an alkaline solution. Since the binding energy of an N-H bond is approximately 402 eV, it can be said that the N1s spectrum shown in Figure 10 does not show any N-H bonds derived from the raw material.

[0102] Therefore, it is believed that the coated metal powders of each example contain almost no secondary components derived from the raw materials, and therefore the coated metal powders exhibit good insulating properties even after undergoing heat treatment. [Explanation of symbols]

[0103] 1... Coated metal powder, 2... Core particle, 3... Coating, 4... Alkaline aqueous solution, 5... Silanol solution, 6... Si chip, 7... Dispersion, 8... Acidic aqueous solution, 10... Coated metal particle, S102... Alkaline aqueous solution preparation step, S104... Silanol solution preparation step, S106... Dispersion preparation step, S108... Acidic aqueous solution addition step

Claims

1. preparing a silanol solution in which a silicon-containing substance is dissolved in an alkaline aqueous solution; adding a metal powder to the silanol solution to obtain a dispersion; a step of forming a coating containing silicon oxide on the particle surfaces of the metal powder by adding an acidic aqueous solution to the dispersion; and the silicon-containing material is silicon, silicon oxide, silicon carbide, or silicon nitride; The alkaline aqueous solution is an amine-based aqueous solution exhibiting weak alkalinity, A method for producing a coated metal powder, characterized in that the pH of the dispersion before the addition of the acidic aqueous solution is 0.5 to 4.0 higher than the pH of the dispersion after the addition of the acidic aqueous solution.

2. 2. The method for producing a coated metal powder according to claim 1, wherein the pH of the dispersion after the addition of the acidic aqueous solution is 7.0 or more and 11.0 or less.

3. A method for producing a coated metal powder as described in Claim 1, wherein the silicon-containing substance is silicon alone.

4. 4. The method for producing a coated metal powder according to claim 3, wherein the amine-based aqueous solution is ammonia water.

5. The method for producing a coated metal powder according to claim 1 , wherein the acidic aqueous solution exhibits weak acidity.

6. The method for producing a coated metal powder according to claim 5, wherein the acidic aqueous solution is an aqueous carboxylic acid solution.

7. A method for producing a coated metal powder as described in Claim 6, wherein the carboxylic acid aqueous solution is an acetic acid aqueous solution.

8. 8. The method for producing a coated metal powder according to claim 1, wherein the temperature of the dispersion when the acidic aqueous solution is added is 30°C or higher and 100°C or lower.

9. 9. The method for producing a coated metal powder according to claim 1, wherein the O / Si ratio of the silicon oxide in the coating is 2.0 or more and 4.0 or less.

10. The method for producing a coated metal powder according to any one of claims 1 to 9, wherein the coating has a thickness of 1 nm or more and 100 nm or less.

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