KR20200106181A - Silicon oxide-coated iron powder, manufacturing method thereof, and molded article and inductor for inductor using same - Google Patents

Abstract

[Problem] To provide a silicon oxide-coated iron powder having a small particle diameter, high µ'in a high frequency band, and high insulating property, and a method for producing the same.
[Solution means] Disperse iron powder composed of iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less in a mixed solvent of water and organic material containing 1% by mass or more and 40% by mass or less of water. After adding the silicon alkoxide to the slurry, a silicon oxide-coated iron powder having a high μ'in a high frequency band and highly insulating is obtained by adding a catalyst for hydrolysis of the silicon alkoxide to coat the silicon oxide.

Description

Silicon oxide-coated iron powder, manufacturing method thereof, and molded article and inductor for inductor using same

The present invention relates to a silicon oxide-coated iron powder and a method for producing the same, and a molded article for an inductor and an inductor using the same, which are suitable for manufacturing a green powder core for an inductor.

Powders of iron-based metals, which are magnetic, are conventionally molded as green compacts and used as magnetic cores of inductors. Examples of iron-based metals include powders of iron-based alloys such as Fe-based amorphous alloys containing a large amount of Si or B (Patent Document 1), Fe-Si-Al-based Sendust, and Permalloy (Patent Document 2), or carbonyl iron powder ( Non-patent literature 1) and the like are known. Further, such iron-based metal powder is compounded with an organic resin to form a paint, and is also used in the manufacture of a surface-mounted coil component (Patent Document 2).

Power-based inductors, which are one of the inductors, have recently been increased in high frequency, and thus an inductor usable at a high frequency of 100 MHz or higher is required. As a method of manufacturing an inductor for a high frequency band, for example, in Patent Document 3, an inductor using a magnetic composition obtained by mixing a large particle diameter iron-based metal powder, a medium particle diameter iron-based metal powder, and a fine particle diameter nickel-based metal powder, and manufacturing the same The method is disclosed. Mixing the nickel-based metal powder having a fine particle diameter here is to improve the degree of filling of the magnetic body by mixing powders with different particle diameters, and as a result, to increase the magnetic permeability of the inductor (透磁率). However, in the technique disclosed in Patent Literature 3, there is a problem that the filling rate of the green compact is increased by mixing magnetic bodies of different particle diameters, but the increase in the magnetic permeability of the finally obtained inductor is small.

In general, soft magnetic powder for inductors is used by covering an insulating material. As a method for producing a soft magnetic powder coated with an insulating material, there is, for example, Patent Document 4, but the insulating material coated soft magnetic powder obtained in Patent Document 4 has a large average film thickness of the coating layer and the powder density of the magnetic powder decreases. There was a problem that magnetic properties deteriorated.

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-014162 Patent Document 2: Japanese Unexamined Patent Application Publication No. 2014-060284 Patent Document 3: Japanese Unexamined Patent Application Publication No. 2016-139788 Patent Document 4: Japanese Unexamined Patent Application Publication No. 2009-231481

Non-Patent Document 1: Yuichiro Sugawa et al., 12th MMM/INTERMAG CONFERENCE, CONTRIBUTED PAPER, HU-04, final manuscript.

It is considered that the reason why the permeability of the inductor obtained by the technique of Patent Document 3 is not so high is that the permeability of the nickel-based metal powder is lower than that of the iron-based metal powder. Therefore, it is expected that an inductor having a high magnetic permeability can be obtained by mixing iron powder having a fine particle diameter having a higher magnetic permeability than that of a nickel-based metal. However, conventionally, there is a limitation in improving the permeability of an inductor other than iron powder having an average particle diameter of 0.8 µm or less.

First, in Japanese Patent Application No. 2017-134617, the applicant of the present invention describes iron powder and silicon oxide-coated iron powder having a particle diameter of 0.25 to 0.80 µm and an axis ratio of 1.5 or less and having a high magnetic permeability μ'at 100 MHz, and a method for manufacturing the same. Started. In the manufacturing method disclosed in the above application, iron powder is prepared by a wet method in which phosphorus-containing ions are coexisted, but at this time, iron powder coated with silicon oxide containing a small amount of phosphorus is obtained. However, in the case of iron powder coated with silicon oxide containing a small amount of phosphorus, there is a problem in that the insulating property is low.

In view of the above problems, an object of the present invention is to provide a silicon oxide-coated iron powder having a small particle diameter, a high µ'in a high frequency band, and a high insulating property, and a method for producing the same.

In order to achieve the above object, in the present invention, the surface of iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less is a silicon oxide-coated iron powder coated with silicon oxide, and the Si content is 1.0 mass. % Or more and 10% by mass or less, and the volume resistivity of the green compact measured while applying an applied voltage of 10V to the green compact obtained by pressing the silicon oxide-coated iron powder vertically at 64 MPa is 1.0×10 ⁵ Ω·cm or more. , Silicon oxide coated iron is provided.

The silicon oxide-coated iron powder preferably has a P content of the iron particles of 0.1% by mass or more and 1.0% by mass or less with respect to the mass of the iron particles, and a green powder obtained by pressing the silicon oxide-coated iron powder at 64 MPa It is preferable that the green powder density of is 4.0 g/cm 3 or less.

In the present invention, the surface of the iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less has a Si content of 1.0 mass% or more and 10 mass% or less of the silicon oxide-coated iron powder coated with silicon oxide. , As a method for producing silicon oxide-coated iron powder, comprising an iron powder production step of preparing iron powder composed of iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less, and the iron powder obtained in the above step by 1 mass A slurry holding step of retaining a slurry obtained by dispersing in a mixed solvent of water and organic matter containing% or more and 40% by mass or less of water, and a silicon alkoxide in the retained slurry by dispersing the iron powder in the mixed solvent. An alkoxide addition step of adding, and a hydrolysis catalyst addition step of adding a hydrolysis catalyst of silicon alkoxide to the slurry to which the silicon alkoxide is added to obtain a slurry in which iron powder coated with silicon oxide is dispersed; and the silicon oxide There is provided a method for producing silicon oxide-coated iron powder comprising a recovery step of solid-liquid separating a slurry containing iron powder coated with a silicon oxide to obtain iron powder coated with silicon oxide.

By using the production method of the present invention, it has become possible to produce a silicon oxide-coated iron powder having a small particle diameter, a high µ'in a high frequency band, and having high insulating properties.

1 is a SEM photograph of iron powder obtained in Comparative Example 1.
[Fig. 2] It is a SEM photograph of iron powder obtained in Example 1. [Fig.

[Iron particle]

The iron particles used as the core of the silicon oxide-coated iron powder of the present invention are substantially pure iron particles, excluding P and other impurities that are unavoidably incorporated in the manufacturing process thereof. It is preferable that the iron particles have an average particle diameter of 0.25 µm or more and 0.80 µm or less, and an average axial ratio of 1.5 or less. By setting it as the range of this average particle diameter and the average axial ratio, it becomes possible to achieve both a large μ'and a sufficiently small tan δ. If the average particle diameter is less than 0.25 µm, µ'becomes small, which is not preferable. In addition, when the average particle diameter exceeds 0.80 µm, tan δ increases as the eddy current loss increases, which is not preferable. More preferably, the average particle diameter is 0.30 µm or more and 0.80 µm or less, still more preferably 0.31 µm or more and 0.80 µm or less, and even more preferably, the average particle diameter is 0.40 µm or more and 0.80 µm or less. About the average axial ratio, when it exceeds 1.5, since [micro]' falls due to an increase in magnetic anisotropy, it is not preferable. There is no lower limit in particular about the average axial ratio, but usually 1.10 or more is obtained. The coefficient of variation of the axial ratio is, for example, 0.10 or more and 0.25 or less. In addition, in the present specification, when an individual iron particle is targeted, it is expressed as an iron particle, but when an average characteristic of an aggregate of iron particles is targeted, it is sometimes expressed as an iron powder.

[P content]

The iron particles used as the core of the silicon oxide-coated iron powder of the present invention are produced in the presence of phosphorus-containing ions by a wet method, as described later, and thus substantially contain P. The average P content in the iron powder composed of the iron particles used in the present invention is preferably 0.1% by mass or more and 1.0% by mass or less with respect to the mass of the iron powder. If the P content is out of this range, it becomes difficult to produce iron particles having both the average particle diameter and the average axis ratio, which is not preferable. As P content, it is more preferable that it is 0.1 mass% or more and 0.7 mass% or less, and it is still more preferable that it is 0.15 mass% or more and 0.4 mass% or less. The content of P does not contribute to the improvement of magnetic properties, but it is allowed if it is contained within the above range.

[Silicone oxide coating]

In the present invention, insulating silicon oxide is coated on the surface of the above iron particles by a wet coating method using silicon alkoxide. The coating method using a silicone alkoxide is a method commonly referred to as a sol-gel method, and is superior in mass productivity compared to the dry method.

When the silicone alkoxide is hydrolyzed, a part or all of the alkoxy group is substituted with a hydroxyl group (OH group) to obtain a silanol derivative. The silanol derivative is an organosilicon compound having a silanol group Si-OH in its molecular structure. In the present invention, this silanol derivative covers the surface of the above iron powder, but the coated silanol derivative condenses or polymerizes when heated to obtain a polysiloxane structure, and when the polysiloxane structure is further heated, it becomes silica (SiO ₂ ). . In the present invention, the covering of the silanol derivative in which a part of the alkoxy group, which is an organic substance, remains, to the coating of silica are collectively referred to as silicon oxide coating.

The content of Si contained in the silicon oxide-coated iron powder is preferably 1.0% by mass or more and 10% by mass or less with respect to the mass of the silicon oxide-coated iron powder in order to ensure insulation and to obtain a high magnetic permeability μ'in the high-frequency region. In the case of a silicon oxide-coated iron powder using iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less as a core, the Si content corresponds to 0.5 to 8.0 nm in terms of average film thickness. do.

When the content of Si contained in the silicon oxide-coated iron powder is less than 1.0% by mass, many defects exist in the Si oxide-coated layer, and it becomes difficult to ensure insulation. If the Si content exceeds 10% by mass, the insulating property is improved, but the green density is lowered and the magnetic properties are deteriorated, which is not preferable. In addition, Si content can be measured by the dissolution method mentioned later.

[Volume resistivity]

It is preferable that the silicon oxide-coated iron powder of the present invention has a volume resistivity of 1.0×10 ⁵ Ω·cm or more, measured while applying an applied voltage of 10 V to a green compact obtained by pressing vertically at 64 MPa. If the volume resistivity is less than 1.0×10 ⁵ Ω·cm, insulation between particles is not sufficient, loss increases due to the influence of eddy current between particles, and characteristics of an inductor or the like are deteriorated, which is not preferable. In the present invention, the upper limit of the volume resistivity of the green compact is not particularly defined, but in the case of the above Si content, the volume resistivity of the green compact is about 1.0×10 ⁵ to 1.0×10 ⁹ Ω·cm. Further, when the thickness of the silicon oxide coating layer is increased, the volume resistivity increases, but the silicon oxide coating is a non-magnetic component, and the magnetic properties deteriorate as described above.

[Dust density]

In the case of the present invention, the green powder density of the green compact obtained by pressing the silicon oxide-coated iron powder at 64 MPa is preferably 4.0 g/cm 3 or less. This is because, when the above-described high magnetic permeability μ'and high insulation property are obtained in a state where the green compaction density is small, the weight or shortening of the inductor can be achieved.

[Iron manufacturing process]

The iron particles used as the core of the silicon oxide-coated iron powder of the present invention can be produced by the production method disclosed in Japanese Patent Application No. 2017-134617 described above. The manufacturing method disclosed in the above application is characterized in that it is performed by a wet method in the presence of phosphorus-containing ions, and there are three types of embodiments broadly divided, but no matter which embodiment is used, the average particle diameter which becomes the above core is It is possible to obtain an iron powder composed of iron particles having an average axis ratio of 0.25 µm or more and 0.80 µm or less and having an average axis ratio of 1.5 or less.

[Starting material]

In the iron powder production process of the present invention, an acidic aqueous solution containing trivalent Fe ions (hereinafter referred to as a raw material solution) is used as a starting material for silicon oxide-coated iron oxide powder, which is a precursor of silicon oxide-coated iron powder. If divalent Fe ions are used instead of trivalent Fe ions as a starting material, a mixture containing divalent iron hydrated oxides or magnetite in addition to trivalent iron hydration oxides as precipitates is produced, and finally obtained iron particles Since the shape of is uneven, the iron powder and silicon oxide-coated iron powder similar to the present invention cannot be obtained. Here, acidic refers to that the pH of the solution is less than 7. As such an Fe ion source, it is preferable to use a water-soluble inorganic acid salt such as nitrate, sulfate, or chloride from the viewpoint of availability and price. When such an Fe salt is dissolved in water, Fe ions are hydrolyzed, and the aqueous solution exhibits acidity. When alkali is added and neutralized to the acidic aqueous solution containing Fe ions, a precipitate of a hydrated oxide of iron is obtained. Here, the hydrated oxide of iron is a substance represented by the general formula Fe ₂ O ₃ ·nH ₂ O, and when n=1, it is FeOOH (iron oxyhydroxide), and when n=3, it is Fe(OH) ₃ (iron hydroxide).

The concentration of Fe ions in the raw material solution is not particularly defined in the present invention, but is preferably 0.01 mol/L or more and 1 mol/L or less. If it is less than 0.01 mol/L, the amount of precipitate obtained in one reaction is small, which is not economically preferable. When the Fe ion concentration exceeds 1 mol/L, the reaction solution is liable to gel due to rapid precipitation of hydrated oxides, which is not preferable.

[Phosphorus-containing ions]

In the iron powder production process of the present invention, phosphorus-containing ions are added during the formation of a precipitate of the hydrated oxide of iron, or a silane compound is added to coat a hydrolysis product. In either case, when the silane compound is coated, phosphorus-containing ions coexist in the system. As a source of phosphorus-containing ions, soluble phosphoric acid (PO ₄ ^3- ) salts such as phosphoric acid, ammonium phosphate, Na phosphate, and monohydrogen salts and dihydrogen salts thereof can be used. Here, phosphoric acid is a tribasic acid, and since it dissociates in three stages in an aqueous solution, the presence of phosphate ions, phosphate dihydrogen ions, and monohydrogen phosphate ions can take the form of presence in the aqueous solution, but the present form is that of the drug used as a source of phosphate ions. Since it is determined not by the type but by the pH of the aqueous solution, the ions containing the above phosphate groups are collectively referred to as phosphate ions. Further, in the case of the present invention, it is also possible to use diphosphate (pyrophosphoric acid), which is condensed phosphoric acid, as a source of phosphate ions. In addition, in the present invention, instead of the phosphate ion (PO ₄ ^3- ), it is also possible to use a phosphorous acid ion (PO ₃ ^3- ) or a hypophosphorous acid ion (PO ₂ ^2- ) having different oxidation numbers of P. Such oxide ions containing phosphorus (P) are collectively referred to as phosphorus-containing ions.

The amount of phosphorus-containing ions added to the raw material solution is preferably 0.003 or more and 0.1 or less in terms of a molar ratio (P/Fe ratio) to the total molar amount of Fe contained in the raw material solution. When the P/Fe ratio is less than 0.003, the effect of increasing the average particle diameter of the iron oxide powder contained in the silicon oxide-coated iron oxide powder is insufficient, and when the P/Fe ratio exceeds 0.1, the reason is unclear, but the effect of increasing the particle diameter. Is not obtained. A more preferable value of the P/Fe ratio is 0.005 or more and 0.05 or less.

The mechanism by which phosphorus-containing ions are coexisted to obtain iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less is unclear, but the present inventors and others believe that the silicon oxide coating layer described later contains phosphorus-containing ions. Since it contains, it is presumed that this is because the physical properties change.

In addition, as described above, the timing of adding phosphorus-containing ions to the raw material solution may be any time before the neutralization treatment described later, before the silicon oxide coating after the neutralization treatment, or during the addition of the silane compound.

[Neutralization treatment]

In the first embodiment of the iron powder production process of the present invention, an alkali is added to the raw material solution containing phosphorus-containing ions while stirring by a known mechanical means, and the pH is neutralized until the pH is 7 or more and 13 or less. A precipitate of hydrated oxide is formed. In addition, in Examples described later, explanation is mainly based on this first embodiment.

If the pH after neutralization is less than 7, it is not preferable because iron ions do not precipitate as hydrated oxides of iron. If the pH after neutralization exceeds 13, the hydrolysis of the silane compound added in the silicon oxide coating step described later becomes rapid, and the coating of the hydrolysis product of the silane compound becomes uneven, which is also not preferable.

In the production method of the present invention, in neutralizing the raw material solution containing phosphorus-containing ions with alkali, in addition to the method of adding alkali to the raw material solution containing phosphorus-containing ions, raw materials containing phosphorus-containing ions in alkali A method of adding a solution may be employed.

In addition, the value of pH described in this specification was measured using a glass electrode based on JIS Z8802. As a pH standard solution, it refers to a value measured by a pH meter calibrated using an appropriate buffer solution according to the pH range to be measured. In addition, the pH described in the present specification is a value obtained by directly reading a measurement value indicated by a pH meter compensated by a temperature compensation electrode under reaction temperature conditions.

The alkali used for neutralization may be any of an ammonium salt such as an alkali metal or alkaline earth metal hydroxide, ammonia water, or ammonium hydrogen carbonate, but ammonia water that is unlikely to remain impurities when the precipitate of iron hydrated oxide is finally heat treated. It is preferable to use ammonium hydrogen carbonate. Such an alkali may be added as a solid to the aqueous solution of the starting material, but from the viewpoint of securing the uniformity of the reaction, it is preferably added in the form of an aqueous solution.

After completion of the neutralization reaction, the slurry containing the precipitate is maintained at the pH for 5 min to 24 h while stirring, and the precipitate is aged.

In the production method of the present invention, the reaction temperature during the neutralization treatment is not particularly defined, but it is preferably 10°C or more and 90°C or less. When the reaction temperature is less than 10°C or more than 90°C, it is not preferable considering the energy cost required for temperature adjustment.

In the second embodiment of the production method of the present invention, an alkali is added to the raw material solution while stirring by a known mechanical means, and the pH is neutralized until the pH is 7 or more and 13 or less to form a precipitate of a hydrated oxide product of iron. After that, in the process of aging the precipitate, phosphorus-containing ions are added to the slurry containing the precipitate. The timing of addition of the phosphorus-containing ions may be immediately after formation of a precipitate or during aging. In addition, the aging time and reaction temperature of the precipitate in the second embodiment are the same as those in the first embodiment.

In the third embodiment of the production method of the present invention, an alkali is added to the raw material solution while stirring by a known mechanical means, and the pH is neutralized until the pH is 7 or more and 13 or less to produce a precipitate of hydrated oxide of iron. Then, the precipitate is aged. In this embodiment, phosphorus-containing ions are added when silicon oxide coating is performed.

[Coating by hydrolysis product of silane compound]

In the iron powder production process of the present invention, the hydrolysis product of the silane compound is coated on the precipitate of the hydrated oxide of iron produced in the above process. It is preferable to apply a so-called sol-gel method as a coating method for the hydrolysis product of the silane compound.

In the case of the sol-gel method, a silicon compound having a hydrolyzable group, such as tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), various silane coupling agents, etc. A silane compound is added to cause a hydrolysis reaction under stirring, and the surface of a precipitate of a hydrated oxide of iron is coated with a hydrolysis product of the resulting silane compound. In that case, although an acid catalyst and an alkali catalyst may be added, it is preferable to add such a catalyst in consideration of the treatment time. As a representative example, hydrochloric acid is used as an acid catalyst and ammonia is used as an alkali catalyst. In the case of using an acid catalyst, it is necessary to fix it in an amount in which the precipitate of the hydrated oxide of iron is not dissolved.

The specific method for coating the silane compound with the hydrolysis product can be the same as the sol-gel method in a known process, and the total number of moles of trivalent Fe ions injected into the raw material solution, and the silicon compound added dropwise to the slurry. The ratio of the total number of moles of Si contained (Si/Fe ratio) is set to be 0.05 or more and 0.5 or less. The reaction temperature for coating the hydrolysis product of the silane compound is 20°C or more and 60°C or less, and the reaction time is about 1h or more and 20h or less.

In the third embodiment of the iron powder production process of the present invention, between the start of addition of the silicon compound having a hydrolyzable group to the end of the addition to the slurry containing the precipitate of the hydrated oxide of iron obtained by aging after neutralization. To, phosphorus-containing ions are simultaneously added. The timing of addition of the phosphorus-containing ions may be at the same time as the start of the addition of the silicon oxide having a hydrolyzable group or the same time as the addition of the silicon oxide.

[Recovery of sediment]

A precipitate of a hydrated oxide of iron coated with a hydrolysis product of a silane compound is separated from the slurry obtained by the above process. As the solid-liquid separation means, known solid-liquid separation means such as filtration, centrifugation and decantation can be used. In the case of solid-liquid separation, a coagulant may be added to separate solid-liquid. Subsequently, after washing the precipitate of the hydrated oxide of iron coated with the hydrolysis product of the silane compound obtained by solid-liquid separation, it is preferable to perform solid-liquid separation again. As the cleaning method, known cleaning means such as repulp cleaning can be used. A drying treatment is performed on the precipitate of hydrated oxide of iron coated with the hydrolysis product of the finally recovered silane compound. In addition, this drying treatment is aimed at removing moisture adhering to the precipitate, and may be performed at a temperature of about 110°C above the boiling point of water.

[Heat treatment]

In the iron powder production process of the present invention, a precipitate of a hydrated oxide of iron coated with the hydrolysis product of the silane compound is heat treated to obtain a silicon oxide-coated iron oxide powder, which is a precursor of a silicon oxide-coated iron powder. Although it does not specifically define in the atmosphere of heat treatment, it does not matter with an atmospheric atmosphere. Heating can be performed in the range of approximately 500°C or more and 1500°C or less. If the heat treatment temperature is less than 500°C, the particles do not grow sufficiently, which is not preferable. If it exceeds 1500°C, it is not preferable because more than necessary particle growth or sintering of particles occurs. The heating time may be adjusted within the range of 10 min to 24 h. By this heat treatment, the hydrated oxide of iron changes to an iron oxide. The heat treatment temperature is preferably 800°C or more and 1250°C or less, and more preferably 900°C or more and 1150°C or less. In addition, during the heat treatment, the hydrolysis product of the silane compound that coats the precipitation of the hydrated oxide of iron also changes to silicon oxide. The silicon oxide coating layer also has an effect of preventing sintering during heat treatment between the hydration and oxidation precipitation of iron.

[Reduction heat treatment]

In the iron powder production process of the present invention, silicon oxide-coated iron powder is obtained by heat-treating the silicon oxide-coated iron oxide powder, which is a precursor obtained in the above process, in a reducing atmosphere. Examples of the gas forming the reducing atmosphere include hydrogen gas or a mixed gas of hydrogen gas and inert gas. The temperature of the reduction heat treatment can be in the range of 300°C or more and 1000°C or less. If the reduction heat treatment temperature is less than 300°C, the reduction of iron oxide becomes insufficient, which is not preferable. If it exceeds 1000°C, the effect of reduction is saturated. The heating time may be adjusted in the range of 10 to 120 min.

[Stabilization treatment]

Usually, since the surface of iron powder obtained by reduction heat treatment is chemically very active, a stabilization treatment by slow oxidation is often performed. The iron powder obtained by the iron powder manufacturing process method of the present invention is coated with a chemically inert silicon oxide, but a part of the surface is not covered. Therefore, a stabilization treatment is preferably applied to the iron powder surface. An oxide protective layer is formed on the exposed portion of the. As a procedure of stabilization treatment, the following means can be mentioned as an example.

After the atmosphere in which the silicon oxide-coated iron powder is exposed after the reduction heat treatment is replaced with an inert gas atmosphere in the reducing atmosphere, oxidation of the exposed portion at 20 to 200°C, more preferably 60 to 100°C, gradually increasing the oxygen concentration in the atmosphere. The reaction proceeds. As the inert gas, at least one gas component selected from rare gas and nitrogen gas can be applied. As the oxygen-containing gas, pure oxygen gas or air can be used. Water vapor may be introduced together with the oxygen-containing gas. When the silicon oxide-coated iron powder is held at 20 to 200°C, preferably at 60 to 100°C, the oxygen concentration is finally set at 0.1 to 21% by volume. The oxygen-containing gas can be introduced continuously or intermittently. In the initial stage of the stabilization process, it is more preferable to hold the time when the oxygen concentration is 1.0 vol% or less for 5 min or more.

[Dissolution treatment of silicon oxide coating]

The silicon oxide-coated iron powder obtained by the series of treatments described above is, for example, a material for an inductor, and satisfactory pressure molding cannot be performed. In addition, the conventional silicon oxide is an aid for obtaining iron powder by reaction as described above, and is functionally different from the coating film described later. Once the silicon oxide coating layer is dissolved and removed in an aqueous alkali solution to obtain a non-coated iron powder, it is necessary to coat the iron powder with a highly insulating silicon oxide again.

The reason for the low volume resistivity of the green compact is not clear at this point, but the incorporation of a phosphorus-containing compound in the silicon oxide coating layer lowers the volume resistivity of the silicon oxide coating layer, or changes in the physical properties of the silicon oxide coating layer. It is considered that the defect density has increased.

As the aqueous alkali solution used for the dissolution treatment, a conventional aqueous alkali solution used industrially, such as sodium hydroxide solution, potassium hydroxide solution, and aqueous ammonia, can be used. In consideration of treatment time and the like, it is preferable that the pH of the treatment liquid is 10 or more and the temperature of the treatment liquid is 60°C or more and less than or equal to the boiling point.

[Disintegration treatment]

The iron powder obtained by the dissolution treatment of the silicon oxide coating described above is provided in a series of steps of the second silicon oxide coating treatment described later, but the iron powder may be pulverized before being applied to the next step. By performing disintegration, the cumulative 50% particle diameter on a volume basis by the microtrack measuring device of iron powder can be reduced. As the pulverization means, a known method such as a method using a pulverizing device using a medium such as a bead mill or a method using a medialess pulverizing device such as a jet mill can be adopted. In the case of the method using a pulverizing device using a media, the shape of the particles of the iron powder obtained is deformed, resulting in an increase in the axial ratio, and as a result, the degree of filling of the iron powder at the time of forming the molded body in the post process is poor, and the magnetic properties of the iron powder are deteriorated. Since there is a possibility that defects such as may occur, it is preferable to employ a medialess pulverizing device, and it is particularly preferable to pulverize using a jet mill pulverizing device. Here, the jet mill pulverizing device refers to a pulverizing device of a method in which an object to be pulverized or a slurry obtained by mixing the object to be pulverized and a liquid is sprayed with high-pressure gas to collide with a collision plate or the like. The type in which the object to be pulverized is sprayed with high-pressure gas without using a liquid is referred to as a dry jet mill pulverizing device, and a type using a slurry obtained by mixing the object and liquid is called a wet jet mill pulverizing device. As the object to collide with the object to be pulverized or the slurry in which the object to be pulverized and the liquid are mixed, it may not be a stationary object such as a collision plate, etc. You may adopt a method of colliding.

In addition, as a liquid in the case of pulverizing using a wet jet mill pulverizing device, a general dispersion medium such as pure water or ethanol can be used, but ethanol is preferably used.

When a wet jet mill grinding device is used for pulverization, a slurry after pulverization treatment, which is a mixture of pulverized iron powder and a dispersion medium, is obtained, and pulverized iron powder can be obtained by drying the dispersion medium in this slurry. As the drying method, a known method can be employed, and the atmosphere may be air. However, from the viewpoint of preventing oxidation of iron powder, drying in a non-oxidizing atmosphere such as nitrogen gas, argon gas, or hydrogen gas, or vacuum drying is preferably performed. In addition, in order to accelerate the drying rate, it is preferable to perform heating at, for example, 100°C or higher. Further, when the iron powder obtained after drying is mixed with ethanol again to measure the microtrack particle size distribution, the D50 of the iron powder in the slurry after the disintegration treatment can be almost reproduced. That is, the D50 of iron powder does not change before and after drying.

[Slurry holding process]

Hereinafter, a process of coating the iron powder obtained in the above-described series of iron powder production processes with highly insulating silicon oxide will be described.

In the production method of the present invention, the iron powder obtained in the above iron powder production process is dispersed in a mixed solvent of water and organic matter containing 1% by mass or more and 40% by mass or less water while stirring by a known mechanical means, After making it, hold it for a certain time. An oxide with very soft Fe exists on the surface of the iron powder, but in this slurry holding step, the Fe oxide is hydrated by water contained in the mixed solvent. The surface of the hydrated Fe oxide is a solid acid and exhibits a behavior similar to that of a weak acid as a Bronsted acid, so when silicon alkoxide is added to the slurry containing iron in the mixed solvent in the next step, the hydrolysis product of silicon alkoxide is silane. The reactivity between the all derivative and the iron powder surface is improved, and as a result, the uniformity of the finally formed silicon oxide coating layer is improved.

The content of water in the mixed solvent is preferably 1% by mass or more and 40% by mass or less. More preferably, it is 10 mass% or more and 35 mass% or less, More preferably, it is 15 mass% or more and 30 mass% or less. If the water content is less than 1% by mass, the above-described function of hydrating the Fe oxide is insufficient. If the water content exceeds 40% by mass, the rate of hydrolysis of the silicon alkoxide increases and a uniform silicon oxide coating layer cannot be obtained, which is not preferable respectively.

As the organic solvent used for the mixed solvent, it is preferable to use an aliphatic alcohol such as methanol, ethanol, 1-propanol, 2-propanol, butanol, pentanol, and hexanol that has an affinity for water. However, if the solubility parameter of the organic solvent is too close to that of water, the reactivity of water in the mixed solvent decreases, so it is more preferable to use 1-propanol, 2-propanol (isopropyl alcohol), butanol, pentanol, and hexanol. .

In the present invention, the temperature of the slurry holding step is not particularly defined, but it is preferably 20°C or more and 60°C or less. If the holding temperature is less than 20°C, the rate of the hydration reaction of the Fe oxide is slowed, which is not preferable. Further, when the holding temperature exceeds 60°C, in the alkoxide addition step of the next step, the hydrolysis reaction rate of the added silicon alkoxide increases, and the uniformity of the silicon oxide coating layer is deteriorated, which is not preferable. In the present invention, the holding time is also not particularly defined, but conditions are appropriately selected so that the holding time is 10 min or more and 180 min or less so that the hydration reaction of the Fe product occurs uniformly.

[Alkoxide addition process]

The slurry obtained by the above-described slurry holding step in which iron powder is dispersed in a mixed solvent is stirred by a known mechanical means, while silicon alkoxide is added, and the slurry is held in that state for a certain period of time. As the silicone alkoxide, as described above, trimethoxysilane, tetramethoxysilane, triethoxysilane, tetraethoxysilane, tripropoxysilane, tetrapropoxysilane, tributoxysilane, tetrabutoxysilane, etc. Can be used.

The amount of silicon alkoxide added can be set by a desired value of the volume resistivity of the green compact. Specifically, it is 10 mass% or more. For this reason, when the axial ratio of the iron particles is set to 1.5 or less, it is close to the circular shape, so the possibility of the coating being unevenly distributed at the release sites in the particles is low, and the silicon alkoxide is mostly deposited on the surface of the iron particles. I guess. In addition, when added in excess, it is not preferable because it is released from the surface of the iron particles, and is specifically 100% by mass or less.

The silicone alkoxide added in this step is hydrolyzed by the action of water contained in the mixed solvent to become a silanol derivative. The resulting silanol derivative forms a reaction layer of the silanol derivative on the surface of the iron powder by condensation, chemical adsorption, or the like. In this step, since the hydrolysis catalyst is not added, the hydrolysis of the silicon alkoxide occurs gently, and it is considered that the reaction layer of the silanol derivative described above is formed uniformly.

In the present invention, the reaction temperature in the alkoxide addition step is not particularly defined, but it is preferably 20°C or more and 60°C or less. If the reaction temperature is less than 20°C, the rate of reaction between the surface of the iron powder and the silanol derivative is slowed, which is not preferable. Further, when the reaction temperature exceeds 60°C, the rate of hydrolysis reaction of the added silicon alkoxide increases, and the uniformity of the silicon oxide coating layer is deteriorated, which is not preferable. In the present invention, the reaction time of the alkoxide addition step is not particularly defined, but conditions are appropriately selected so that the reaction time is 5 min or more and 180 min or less so that the reaction between the iron powder surface and the silanol derivative occurs uniformly.

[Hydrolysis catalyst addition process]

In the production method of the present invention, after forming a reaction layer of a silanol derivative on the surface of the iron powder in the alkoxide addition step, a slurry obtained by dispersing iron powder in a mixed solvent is stirred by a known mechanical means, A decomposition catalyst is added. In this step, the hydrolysis reaction of the silicon alkoxide is accelerated by the addition of the hydrolysis catalyst, and the film formation rate of the silicon oxide coating layer is increased. In addition, after this step, the same method as the film formation method by a conventional sol-gel method is performed.

As the hydrolysis catalyst, an alkali catalyst is used. The use of an acid catalyst is not preferable because iron is dissolved. As the alkali catalyst, it is preferable to use aqueous ammonia in terms of difficulty in remaining impurities in the silicon oxide coating layer and ease of availability.

In the present invention, the reaction temperature in the hydrolysis catalyst addition step is not particularly defined, and may be the same as the reaction temperature in the previous step, the alkoxide addition step. Further, in the present invention, the reaction time of the hydrolysis catalyst addition step is not particularly defined, but since the long reaction time is economically disadvantageous, conditions are appropriately selected so that the reaction time is 10 min or more and 180 min or less.

[Solid-liquid separation and drying]

From the slurry containing the silicon oxide-coated iron powder obtained in the series of steps up to the above, the silicon oxide-coated iron powder is recovered using a known solid-liquid separation means. As the solid-liquid separation means, known solid-liquid separation means such as filtration, centrifugation and decantation can be used. In the case of solid-liquid separation, a coagulant may be added to separate solid-liquid.

The recovered silicon oxide-coated iron powder is washed with 50 times the amount of pure water, and then dried under a nitrogen atmosphere at 50°C or higher and 200°C or lower, for 2 hours or longer, such as 100°C and 10 hours. After drying, in order to improve the magnetic properties of the magnetic body, a firing treatment at a high temperature may be further added.

[Particle diameter]

The particle diameters of the iron particles constituting the silicon oxide-coated iron powder, and the particle diameters of the iron oxide particles constituting the silicon oxide-coated iron oxide powder are respectively determined after dissolving and removing the silicon oxide coating using 10% by mass aqueous sodium hydroxide solution. It was determined by observation with a serpentine electron microscope (SEM). For SEM observation, Hitachi Seisakusho S-4700 was used.

The dissolution and removal of the silicon oxide was performed by putting silicon oxide-coated iron powder or silicon oxide-coated iron oxide powder in a 10% by mass sodium hydroxide aqueous solution at 60°C, stirring for 24 hours, filtration, washing with water, and drying. In addition, the amount of the aqueous sodium hydroxide solution was 0.8 L with respect to 5 g of silicon oxide coated iron powder or silicon oxide coated iron oxide powder.

After dissolution and removal of the silicon oxide, SEM observation is performed, and the length of the circumscribed rectangular long side at which the area is minimum is determined as the particle diameter (long diameter) of the particle. Specifically, among the SEM photographs taken at a magnification of about 3,000 to 30,000 times, 300 particles in which the entire outer edge is observed is randomly selected, the particle diameter is measured, and the average value is calculated as the silicon oxide coated iron powder. It was set as the average particle diameter of the iron particle which comprises. In addition, the particle diameter obtained by this measurement is a primary particle diameter.

[Celebration]

For a certain particle on the SEM image, the length of the short side of the circumscribed rectangle with the minimum area is referred to as "shorter diameter", and the ratio of the longest diameter/shorter diameter is referred to as the "axis ratio" of the particle. The "average axis ratio", which is an average axis ratio as a powder, can be determined as follows. By SEM observation, ``long diameter'' and ``short diameter'' were measured for 300 randomly selected particles, and the average value of the long diameter and the average value of the short diameter for all the particles to be measured were set as the ``average long diameter'' and the ``average short diameter'', respectively. And, the ratio of the average long diameter/average short diameter is set as the "average axis ratio". For each of the long axis, the short axis, and the axis ratio, a coefficient of variation can be calculated as an index indicating the magnitude of the unevenness.

[Measurement of Si Content]

The Si content of the iron powder (uncoated treated product) as a starting material and the iron powder coated with silicon oxide was determined by the following method. After the sample was weighed and dissolved with hydrochloric acid, perchloric acid was added and heated until the liquid disappeared, and then hydrochloric acid was added again to dissolve all components soluble in the acid. Thereafter, the residue mainly composed of silicon dioxide was filtered, placed in a platinum crucible, ignited in an electric furnace, and the mass was measured after standing to cool. Hydrofluoric acid and sulfuric acid were added to the platinum crucible after the mass measurement, silicon dioxide was dissolved, and further heated to evaporate and remove the silicon component as silicon tetrafluoride. Then, the platinum crucible was ignited again, and the mass was measured after standing to cool, and the difference from the mass measured earlier was taken as the amount of silicon dioxide. From the obtained amount of silicon dioxide, the amount of silicon in the sample was calculated.

[Measurement of Fe and P content]

The iron powder (uncoated treated product) as a starting material and the Fe and P contents of the iron powder coated with silicon oxide were determined by the following method. The sample was weighed and dissolved by heating in an aqueous solution at 100°C in which a 36% by mass aqueous hydrogen chloride solution and an aqueous 60% by mass nitric acid solution were mixed in a volume ratio of 1:1, and then the residue was filtered, and the filtrate was placed in a volumetric flask for determination. ). After diluting this solution, the concentrations of Fe and P were measured by ICP emission spectrometry (ICP-AES).

Further, the residue obtained above was put into a platinum crucible as a filter paper, heated in an electric furnace to incinerate the filter paper, and after standing to cool, sodium carbonate and potassium carbonate were added and dissolved in an electric furnace. After standing to cool, the melt was leached into hot water, and hydrochloric acid was added to dissolve by heating. After the solution was put into a volumetric flask and measured, the concentrations of Fe and P were measured by ICP emission spectroscopy (ICP-AES). The content of each element was determined from the ICP measurement value of the filtrate and the ICP measurement value of the solution after dissolving the residue.

[Calculation of average thickness of silicon oxide coating]

In addition, the average film thickness t of the silicon oxide coating in the silicon oxide coating iron powder was calculated by the following equation.

Average film thickness t=Si content (mass%)/100×(SiO ₂ molecular weight/Si atomic weight)/(SiO ₂ density×BET specific surface area of iron powder (uncoated product))

In addition, the SiO ₂ density was calculated as 2.65 (g/cm 3 ). In the present invention, it is preferable that the average film thickness t of the silicon oxide is 1.0 nm or more and 6.0 nm or less. By setting the average film thickness t in the above range, it is possible to achieve both a high μ'and a high volume resistivity of the green compact. When the average film thickness t is less than 1.0 nm, the volume resistivity of the green compact decreases, which is not preferable. In addition, when the average film thickness t exceeds 6.0 nm, μ'decreases, which is not preferable.

[Magnetic properties]

Using VSM (VSM-P7 manufactured by Toei Kogyo Corporation), the B-H curve was measured at an applied magnetic field of 795.8 kA/m (10 kOe), and the coercive force Hc, saturation magnetization sigma s, and angle ratio SQ were evaluated.

[Complex Permeability]

Iron powder or silicon oxide-coated iron powder and bisphenol F type epoxy resin (manufactured by Tesco; one-component epoxy resin B-1106) were weighed in a mass ratio of 90:10, and a rotating orbiting mixer (manufactured by THINKY: ARE-250) These were kneaded using, and the test powder was obtained as a paste dispersed in an epoxy resin. This paste was dried on a hot plate at 60° C. for 2 hours to form a composite of a metal powder and a resin, and then disintegrated into powder to obtain a composite powder. 0.2 g of this composite powder was placed in a donut-shaped container, and a load of 9800 N (1 TON) was applied by a hand press to obtain a toroidal shaped body having an outer diameter of 7 mm and an inner diameter of 3 mm. With respect to this molded body, an RF impedance analyzer (manufactured by Keysight Technologies; E4990A), a terminal adapter (manufactured by Keysight Technologies; 42942A), and a test fixture (manufactured by Keysight Technologies; 16454A) were used to achieve a complex relative magnetic permeability at 100 MHz. The real part μ'and the imaginary part μ" were measured, and the loss factor tanδ=μ"/μ' of the complex relative magnetic permeability was obtained. Here, the real part of the complex relative magnetic permeability is simply "permeability" and "μ" in this specification. It may be referred to as "." By using the silicon oxide-coated iron powder of the present invention, a molded article having a magnetic permeability µ'at 100 MHz of 3.0 or more can be obtained.

The molded article manufactured using the silicon oxide-coated iron powder of the present invention exhibits excellent complex permeability characteristics, and can be suitably used for applications such as magnetic cores of inductors.

[BET specific surface area]

The BET specific surface area was determined by the BET single point method using MACSORB MODEL-1210 manufactured by MUNTECH Co., Ltd.

[Microtrack particle size distribution measurement]

For the measurement of the cumulative 50% particle diameter on a volume basis and the cumulative 90% particle diameter by the microtrack measuring device of iron powder, a microtrack particle size distribution measuring device MT3300EXII manufactured by Microtrac Bell was used. In addition, ethanol was used as a liquid to be put into the sample circulator of the measuring device. In addition, in the form of a slurry in which iron powder and ethanol or pure water were mixed, the slurry was stirred to the extent that uneven spots were not visible with the naked eye immediately before supply, and then supplied to the measuring device.

[Measurement of volume resistivity and green density]

The measurement of the volume resistivity of silicon oxide-coated iron powder is a powder resistance measurement unit (MCP-PD51) manufactured by Mitsubishi Kagaku Analytech Co., Ltd., and a high resistance resistivity meter Hirester UP (MCP) manufactured by Mitsubishi Kagaku Analytech Co., Ltd. -HT450), using a high-resistance powder measurement system software manufactured by Mitsubishi Kagaku Analytech Co., Ltd., by double ring electrode method, to a green compact obtained by vertically pressing and molding 4.0 g of powder at 64 MPa (20 kN), voltage Was determined by measuring in a state where 10V was applied.

Specifically, the volume resistivity ρv was calculated by the following equation.

ρv=R×πd ² /4t

Here, R is the measured volume resistance, d is the diameter of the inner ring of the surface electrode, and t is the thickness of the powder sample. In the following examples, the diameter d of the inner ring of the surface electrode was all set to 2.0 cm.

The green compact density was calculated from the sample volume and the sample weight of the green compact obtained by pressure molding at the above 64 MPa (20 kN).

Example

[Comparative Example 1]

In a 5L reactor, to 4113.24 g of pure water, 566.47 g of iron (III) nitrate 9 hydrate having a purity of 99.7% by mass, and 1.39 g of 85% by mass H ₃ PO ₄ as a source of phosphorus-containing ions were mechanically stirred by a stirring blade in an air atmosphere. While dissolving (Procedure 1). The pH of this solution was about 1. In addition, the P/Fe ratio is 0.0086 under this condition.

In an atmospheric atmosphere, under the condition of 30°C, while mechanically stirring with a stirring blade, 409.66 g of a 23.47% by mass ammonia solution was added over 10 minutes (about 40 g/L), followed by stirring for 30 minutes after the dropping was completed. Subsequently, the resulting precipitate was aged. At that time, the pH of the slurry containing the precipitate was about 9 (procedure 2).

While stirring the slurry obtained in step 2, 55.18 g of tetraethoxysilane (TEOS) having a purity of 95.0 mass% was added dropwise at 30° C. in the air over 10 minutes. After that, stirring was continued as it is for 20 hours, and the precipitate was coated with a hydrolysis product of the silane compound produced by hydrolysis (Procedure 3). In addition, the Si/Fe ratio is 0.18 under this condition. Table 1 shows the Si/Fe ratio and the P/Fe ratio of this comparative example.

The slurry obtained in step 3 was filtered, the moisture of the precipitate coated with the hydrolysis product of the obtained silane compound was removed as much as possible, and then again dispersed in pure water, followed by repulping washing. The washed slurry was filtered again, and the resulting cake was dried at 110°C in air (Procedure 4).

The dried product obtained in step 4 was subjected to heat treatment at 1050° C. in air using a box-shaped sintering furnace to obtain silicon oxide-coated iron oxide powder (Step 5).

Reduction heat treatment was performed by placing the silicon oxide-coated iron oxide powder obtained in Step 5 into a ventilable bucket, charging the bucket into a through-type reduction furnace, and holding the bucket at 630°C for 40 minutes while flowing hydrogen gas in the furnace (D). 6).

Subsequently, the atmospheric gas in the furnace was converted from hydrogen to nitrogen, and the temperature in the furnace was lowered to 80°C at a temperature-fall rate of 20°C/min while nitrogen gas was flowed. Thereafter, as the initial gas for stabilization treatment, a gas (oxygen concentration about 0.17% by volume) of nitrogen gas and air is introduced into the furnace so that the volume ratio of nitrogen gas/air becomes 125/1, and the surface layer of metal powder particles The oxidation reaction of is started, and thereafter, the mixing ratio of air is gradually increased, and finally, a mixed gas (oxygen concentration of about 0.80% by volume) having a volume ratio of nitrogen gas/air being 25/1 is continuously introduced into the furnace. , An oxidation protective layer was formed on the surface layer of the particles. During the stabilization treatment, the temperature was maintained at 80°C, and the flow rate of gas introduction was also maintained substantially constant (Step 7).

The silicon oxide-coated iron powder obtained in step 7 was immersed in a 10% by mass, 60°C sodium hydroxide aqueous solution for 24 hours to dissolve the silicon oxide coating. The slurry containing the obtained iron powder was filtered by suction filtration using a membrane filter, washed with water, and then dried for 2 h at 110°C in nitrogen to obtain iron powder. In addition, the amount of the aqueous sodium hydroxide solution was made into a ratio of 3.2 L to 56 g of iron powder coated with silicon oxide.

1 shows the results of SEM observation of iron powder obtained by this comparative example. In addition, the length indicated by the 11 vertical white lines shown in the lower right of Fig. 1 is 5 µm (the same is applied to Fig. 2). About the obtained iron powder, the average particle diameter, average axial ratio, composition, BET specific surface area, and magnetic properties of the iron particles were measured. Table 2 shows the measurement results. The average particle diameter of the iron particles constituting the obtained iron powder was 0.51 µm, and the average axis ratio was 1.27. In addition, using the obtained iron powder, the volume resistivity of the green compact obtained by molding by the above method was measured, the resistance measurement value R is the result below the measurement limit, and the measurement limit also as the volume resistivity (volume resistivity 9.9×10 ⁴ Ω·cm) or less was the result. In addition, using the obtained iron powder, the volume resistivity and density of the green compact obtained by molding by the above method, and the high frequency characteristics of the toroidal compact obtained by molding by the above method are shown together in Table 2. The reason why the volume resistivity of the green compact obtained in this comparative example was lower than the measurement limit was because the iron powder was not covered with insulating silicon oxide.

[Example 1]

54.09 g of pure water and 271 g of isopropyl alcohol (IPA) were added to a 1 L reactor to prepare a mixed solvent, and 15.00 g of iron powder obtained in the same conditions as in Comparative Example 1 were added to the mixed solvent, and mechanically stirred with a stirring blade, It was purged with nitrogen for 30 min at room temperature. After 30 minutes elapsed, stirring and nitrogen purge were continued, and the reaction solution was heated up to 40 degreeC.

Then, 9.06 g of tetraethyl orthosilicate (TEOS) was added to the reaction solution at once, and the mixture was held for 10 minutes. After 10 minutes, 10.8 g of ammonia water having a concentration of 10% by mass was continuously added to the reaction solution over 45 minutes. After completion of the addition of the aqueous ammonia, the reaction solution was held for 60 minutes and aged, and the surface of the iron powder was coated with a hydrolysis product of the silane compound produced by hydrolysis. Table 1 shows the conditions of the iron powder production process and a series of processes for coating silicon oxide.

The resulting slurry was filtered by suction filtration using a membrane filter, washed with pure water, and the resulting iron cake was dried at 100°C in a nitrogen atmosphere. Fig. 2 shows the results of SEM observation of iron powder obtained by the above series of procedures and coated again after dissolution and removal of silicon oxide. With respect to the silicon oxide-coated iron powder, the BET specific surface area, composition, magnetic properties, complex permeability, density of the green compact, and volume resistivity were measured. The measurement results are also shown in Table 2. In addition, as the measurement result of the volume resistivity, the measured value R of the volume resistivity was 1.4×10 ⁶ (Ω), and the powder sample thickness t was 0.429 (cm).

[Examples 2 to 10]

In the same manner as in Example 1, 15.00 g of iron powder obtained under the same conditions as in Comparative Example 1 was used to variously change the conditions for coating silicon oxide to obtain silicon oxide-coated iron powder. Table 1 shows the conditions of the silicon oxide coating used in this example. In addition, in Example 10, iron powder is crushed before the silicon oxide coating treatment. Iron powder disintegration treatment conditions are shown below. The iron powder obtained in Comparative Example 1 was mixed with pure water to prepare an iron powder pure mixed slurry having an iron content of 10% by mass. This slurry was pulverized using a jet mill pulverizing device (manufactured by Rix Corporation; nano-atomizing device G-smasher LM-1000) to obtain a slurry after pulverization treatment. In addition, in the pulverization, the pulverization treatment was repeated 5 times with the supply rate of the pure iron powder mixed slurry being 100 ml/min and the air pressure being 0.6 MPa. The slurry after the disintegration treatment was dried in nitrogen gas at 100° C. for 2 h to obtain iron powder according to Example 10.

The BET specific surface area, composition, magnetic properties, complex permeability, density of green compact, and volume resistivity were measured for the silicon oxide-coated iron powder obtained in these examples. The measurement results are also shown in Table 2.

[Example 11]

Iron powder was obtained by the same procedure as the procedure 1 to procedure 8 of Comparative Example 1, except that the heat treatment temperature in the air was changed to 1020°C. About the obtained iron powder, the average particle diameter, average axial ratio, composition, BET specific surface area, and magnetic properties of the iron particles were measured. Table 2 shows the measurement results. The average particle diameter of the iron particles constituting the obtained iron powder was 0.31 µm, and the average axis ratio was 1.20.

The obtained iron powder was mixed with pure water, and an iron powder pure mixed slurry having an iron content of 10% by mass was prepared. This slurry was pulverized using a jet mill pulverizing device (Sugino Machine Co., Ltd. Starburst Mini, model number: HJP-25001) to obtain a slurry after pulverization treatment. In addition, in the pulverization, the pressure to pressurize the iron powder pure water mixed slurry was 245 MPa, and the pulverization treatment was repeated 10 times. The slurry after the disintegration treatment was dried in nitrogen gas at 100°C for 2 hours to obtain iron powder after the disintegration treatment (Procedure 19).

54.09 g of pure water and 196 g of isopropyl alcohol (IPA) were added to a 1 L reactor to prepare a mixed solvent, and 15.00 g of the iron powder obtained in step 19 was added to the mixed solvent, and mechanically stirred with a stirring blade, at room temperature for 30 minutes. Purged with nitrogen. After 30 minutes elapsed, stirring and nitrogen purge were continued, and the reaction solution was heated up to 40 degreeC.

Then, 2.55 g of tetraethyl orthosilicate (TEOS) was added to the reaction solution at once, and the mixture was held for 10 minutes. After 10 minutes, 9.4 g of ammonia water having a concentration of 10% by mass was continuously added to the reaction solution over 45 minutes. After completion of the addition of the aqueous ammonia, the reaction solution was held for 60 minutes and aged, and the surface of the iron powder was coated with a hydrolysis product of the silane compound produced by hydrolysis. Table 1 shows the conditions of the iron powder production process and a series of processes for coating silicon oxide.

The resulting slurry was filtered by suction filtration using a membrane filter and then washed with pure water, and the resulting iron cake was dried at 100°C in a nitrogen atmosphere. With respect to the silicon oxide-coated iron powder, the BET specific surface area, composition, magnetic properties, complex permeability, density of the green compact, and volume resistivity were measured. The measurement results are also shown in Table 2. In addition, as a measurement result of the volume resistivity, the measured value R of the volume resistivity was 3.9×10 ⁴ (Ω), and the powder sample thickness t was 0.381 (cm).

[Example 12]

Iron powder was obtained by the same procedure as in Comparative Example 1, except that the heat treatment in the air using a box-shaped kiln was performed at 1090°C. Using 15.00 g of the obtained iron powder, a silicon oxide coating treatment was performed under the same conditions as in Example 11 except that the TEOS addition amount was changed to 1.27 g to obtain a silicon oxide coated iron powder. Table 1 shows the conditions of the iron powder production process and the series of processes for coating silicon oxide.

The resulting slurry was filtered by suction filtration using a membrane filter and then washed with pure water, and the resulting iron cake was dried at 100°C in a nitrogen atmosphere. With respect to the silicon oxide-coated iron powder, the BET specific surface area, composition, magnetic properties, complex permeability, density of the green compact, and volume resistivity were measured. The measurement results are also shown in Table 2. In addition, as the measurement result of the volume resistivity, the measured value R of the volume resistivity was 3.8×10 ⁴ (Ω), and the powder sample thickness t was 0.412 (cm).

[Comparative Example 2]

A silicon oxide-coated iron powder was obtained using the same conditions as in Example 2, except that the amount of TEOS added was 0.91 g. Table 1 shows the conditions of the silicon oxide coating used in this comparative example. In addition, the measurement results of the BET specific surface area, composition, magnetic properties, complex permeability, density of the green compact, and volume resistivity of the silicon oxide-coated iron powder obtained in this comparative example are shown together in Table 2.

Since the silicon oxide-coated iron powder Si content obtained in this comparative example was 0.9%, and the thickness of the silicon oxide coating layer was not sufficient, the volume resistivity of the green compact was 9.9×10 ⁴ Ω·cm or less. This volume resistivity was markedly inferior to the volume resistivity for Examples 1 to 10.

[Comparative Example 3]

In a 5L reactor, in 4113.24 g of pure water, 566.47 g of iron (III) nitrate 9 hydrate with a purity of 99.7% by mass, and 1.39 g of 85% by mass H ₃ PO ₄ as a source of phosphorus-containing ions were mechanically stirred with a stirring blade in an air atmosphere. While dissolving (Procedure 1). The pH of this solution was about 1. In addition, the P/Fe ratio is 0.0086 under this condition.

In an air atmosphere, under the condition of 30°C, while mechanically stirring with a stirring blade, 409.66 g of a 23.47 mass% ammonia solution was added over 10 minutes (about 40 g/L), and stirring was performed for 30 minutes after completion of the dropwise addition. Subsequently, the resulting precipitate was aged. At that time, the pH of the slurry containing the precipitate was about 9 (procedure 2).

While stirring the slurry obtained in step 2, 55.18 g of tetraethoxysilane (TEOS) having a purity of 95.0 mass% was added dropwise over 10 minutes at 30° C. in air. After that, stirring was continued as it is for 20 hours, and the precipitate was coated with a hydrolysis product of the silane compound produced by hydrolysis (Procedure 3). Also. In this condition, the Si/Fe ratio is 0.18.

The slurry obtained in step 3 was filtered, the moisture of the precipitate coated with the hydrolysis product of the obtained silane compound was removed as much as possible, and then again dispersed in pure water, followed by repulping washing. The washed slurry was filtered again, and the resulting cake was dried at 110°C in air (Procedure 4). The dried product obtained in step 4 was subjected to heat treatment at 1050° C. in air using a box-shaped sintering furnace to obtain silicon oxide-coated iron oxide powder (Step 5). The silicon oxide-coated iron oxide powder obtained in step 5 was placed in a ventilable bucket, and the bucket was charged in a through-type reduction furnace, and the reduction heat treatment was performed by holding the bucket at 630°C for 40 minutes while flowing hydrogen gas in the furnace (Step 6).

Subsequently, the atmospheric gas in the furnace was converted from hydrogen to nitrogen, and the temperature in the furnace was lowered to 80°C at a temperature-fall rate of 20°C/min while nitrogen gas was flowed. After that, as the initial gas for stabilization treatment, a gas (oxygen concentration of about 0.17% by volume) of nitrogen gas and air is introduced into the furnace so that the volume ratio of nitrogen gas/air is 125/1, and the surface layer of metal powder particles By starting the oxidation reaction of, and then gradually increasing the mixing ratio of air, and finally introducing a mixed gas (oxygen concentration of about 0.80% by volume) into the furnace with a volume ratio of nitrogen gas/air being 25/1. , An oxidation protective layer was formed on the surface layer of the particles. During the stabilization process, the temperature was maintained at 80° C., and the flow rate of the gas was kept almost constant (Step 7).

For the silicon oxide-coated iron powder obtained by the above series of procedures, magnetic properties, BET specific surface area, particle diameter of iron particles, and complex permeability were measured. The measurement results are also shown in Table 2.

The silicon oxide coating of the silicon oxide-coated iron powder obtained in this comparative example contained a phosphorus-containing compound, and the volume resistivity of the green compact was 9.9×10 ⁴ Ω·cm or less.

From the above Examples and Comparative Examples, by applying a predetermined silicon oxide coating to the iron powder specified in the present invention, a silicon oxide-coated iron powder having a small particle diameter, a high μ'in a high frequency band can be achieved, and a high insulating property is obtained. It can be seen that it is obtained.

Claims (6)

A silicon oxide-coated iron powder having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less, the surface of which is coated with silicon oxide, wherein the Si content is 1.0 mass% or more and 10 mass% or less, and the above silicon oxide Silicon oxide-coated iron powder having a volume resistivity of 1.0×10 ⁵ Ω·cm or more, measured while applying an applied voltage of 10 V to a green body obtained by pressing the coated iron powder vertically at 64 MPa. The silicon oxide-coated iron powder according to claim 1, wherein the P content of the iron particles is 0.1% by mass or more and 1.0% by mass or less with respect to the mass of the iron particles. The silicon oxide-coated iron powder according to claim 1 or 2, wherein the green powder obtained by pressing the silicon-oxide-coated iron powder at 64 MPa is 4.0 g/cm 3 or less. Preparation of a silicon oxide-coated iron powder having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less, wherein the Si content of the silicon oxide-coated iron powder coated with silicon oxide is 1.0 mass% or more and 10 mass% or less As a method,
An iron powder manufacturing process for preparing iron powder composed of iron particles having an average particle diameter of 0.25 µm or more and 0.80 µm or less and an average axial ratio of 1.5 or less,
A slurry holding step of holding the slurry obtained by dispersing the iron powder obtained in the above step in a mixed solvent of water and organic matter containing 1% by mass or more and 40% by mass or less of water; and
An alkoxide addition step of dispersing the iron powder in the mixed solvent and adding silicon alkoxide to the retained slurry,
A hydrolysis catalyst addition step of adding a hydrolysis catalyst of silicon alkoxide to the slurry to which the silicon alkoxide is added to obtain a slurry in which iron powder coated with silicon oxide is dispersed;
Recovery process of solid-liquid separation of the slurry containing iron powder coated with silicon oxide to obtain iron powder coated with silicon oxide
Containing, a method for producing a silicon oxide coated iron powder. A molded article for an inductor comprising the silicon oxide-coated iron powder according to any one of claims 1 to 3. An inductor using the silicon oxide-coated iron powder according to any one of claims 1 to 3.

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