KR102376001B1 - Silicon oxide-coated iron powder, manufacturing method thereof, molded article for inductor using same, and inductor - 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 insulation, and a method for manufacturing the same.
[Solution] In a mixed solvent of water and an organic substance containing 1 mass % or more and 40 mass % or less of water, an 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 is dispersed. After adding a silicon alkoxide to a slurry, the silicon oxide-coated iron powder which has high micrometer in a high frequency band and is highly insulating is obtained by adding the hydrolysis catalyst of the said silicon alkoxide and performing silicon oxide coating.

Description

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

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

BACKGROUND ART A magnetic iron-based metal powder is conventionally molded as a green compact and used as a magnetic core of an inductor. Examples of iron-based metals include powders of iron-based alloys such as Fe-based amorphous alloys containing large amounts of Si or B (Patent Document 1), Fe-Si-Al-based sendust, and permalloy (Patent Document 2), and carbonyl iron powders ( Non-patent document 1) etc. are known. Moreover, this iron-type metal powder is used also for manufacture of a surface-mount type coil component by compounding with an organic resin to make a paint (patent document 2).

Power system inductors, which are one of inductors, have recently been increasing in frequency, and there is a demand for an inductor that can be used at a high frequency of 100 MHz or higher. As a method of manufacturing an inductor for a high-frequency band, for example, Patent Document 3 discloses an inductor using a magnetic composition obtained by mixing iron-based metal powder with a large particle diameter and an iron-based metal powder with a medium particle diameter and nickel-based metal powder with a small particle diameter in Patent Document 3, and manufacturing thereof A method is disclosed. Here, the mixing of the nickel-based metal powder having a small particle diameter is to improve the filling degree of the magnetic material by mixing the powders having different particle diameters, and, as a result, to increase the magnetic permeability of the inductor. However, in the technique disclosed in Patent Document 3, although the filling factor of the green body increases by mixing magnetic bodies of different particle diameters, there is a problem that the increase in the magnetic permeability of the finally obtained inductor is small.

The soft magnetic powder for inductors is generally used by coating an insulating material. For example, Patent Document 4 is available as a method for producing an insulating material-coated soft magnetic powder. However, the insulated material-coated soft magnetic powder obtained in Patent Document 4 has a large average film thickness of the coating layer, and the green density of the magnetic powder decreases. There was a problem that magnetic properties deteriorated.

Patent Document 1: Japanese Patent Laid-Open No. 2016-014162 Patent Document 2: Japanese Patent Laid-Open No. 2014-060284 Patent Document 3: Japanese Patent Laid-Open No. 2016-139788 Patent Document 4: Japanese Patent Laid-Open No. 2009-231481

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

The reason that the magnetic permeability of the inductor obtained by the technique of patent document 3 is not so high is considered because the magnetic permeability of a nickel-base metal powder is low compared with the magnetic permeability of an iron-type metal powder. Therefore, it is expected that an inductor with high magnetic permeability can be obtained by mixing iron powder having a fine particle diameter higher in magnetic permeability than nickel-based metals. However, conventionally, there has been a limit in improving the magnetic permeability of an inductor, which is not an iron powder having an average particle diameter of 0.8 µm or less.

First, in Japanese Patent Application No. 2017-134617, the applicant has a particle diameter of 0.25 to 0.80 μm, an axial ratio of 1.5 or less, and an iron powder coated with iron and silicon oxide having high magnetic permeability μ' at 100 MHz, and a method for manufacturing the same started. In the production method disclosed in the above application, iron powder is produced by a wet method in which phosphorus-containing ions coexist, but at this time, iron powder coated with silicon oxide containing a small amount of phosphorus is obtained. However, in the case of the iron powder coated with the silicon oxide containing a small amount of phosphorus, there is a problem that the insulation 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, high μ′ in a high frequency band and high insulation properties, and a method for manufacturing the same.

In order to achieve the above object, in the present invention, 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 is coated with silicon oxide on the surface of iron particles, wherein the Si content is 1.0 mass. % or more and 10% by mass or less, and the volume resistivity of the green compact measured in a state where an applied voltage of 10 V is applied to the green compact obtained by vertically press-molding the above silicon oxide-coated iron powder 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 mass% or more and 1.0 mass% or less with respect to the mass of the iron particles, and a green compact obtained by press-molding the silicon oxide-coated iron powder at 64 MPa. It is preferable that the density of the green powder is 4.0 g/cm 3 or less.

The present invention further relates to an iron particle 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 in which the surface of the iron particles is coated with silicon oxide is 1.0 mass% or more and 10 mass% or less , A method for producing silicon oxide-coated iron powder, comprising: an iron powder production step of preparing iron particles comprising 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 a slurry obtained by dispersing in a mixed solvent of water and an organic substance containing % or more and 40% by mass or less of water, and a silicon alkoxide in the holding slurry by dispersing the iron powder in the mixed solvent An alkoxide addition step of adding a silicon alkoxide, a hydrolysis catalyst addition step of adding a silicon alkoxide hydrolysis catalyst to the slurry to which the silicon alkoxide is added to obtain a slurry in which iron powder coated with silicon oxide is dispersed; There is provided a method for producing silicon oxide-coated iron powder, comprising a recovery step of solid-liquid separation of a slurry containing iron powder coated with silicon oxide to obtain iron powder coated with silicon oxide.

By using the manufacturing method of this invention, it became possible to manufacture the silicon oxide-coated iron powder which has a small particle diameter, can achieve high micrometer in a high frequency band, and has high insulation.

Fig. 1 is an SEM photograph of iron powder obtained in Comparative Example 1.
Fig. 2 is an SEM photograph of iron powder obtained in Example 1.

[iron particles]

The iron particles serving 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. The iron particles preferably 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 average axial ratio, it becomes possible only to make large mu' and sufficiently small tan δ compatible. If the average particle diameter is less than 0.25 mu m, mu' becomes small, which is not preferable. In addition, when the average particle diameter exceeds 0.80 µm, tan δ increases with an increase in eddy current loss, which is not preferable. More preferably, they are 0.30 micrometer or more and 0.80 micrometer or less, More preferably, they are 0.31 micrometer or more and 0.80 micrometer or less, More preferably, the average particle diameter is 0.40 micrometer or more and 0.80 micrometer or less. As for the average axial ratio, when it exceeds 1.5, it is undesirable because μ' decreases due to an increase in magnetic anisotropy. Although there is no minimum in particular about an average axial ratio, Usually 1.10 or more are obtained. The coefficient of variation of the axial ratio is, for example, 0.10 or more and 0.25 or less. Note that in the present specification, individual iron particles are expressed as iron particles, but when the average characteristic of an aggregate of iron particles is referred to as iron particles, it is sometimes expressed as iron particles.

[P content]

The iron particle used as the core of the silicon oxide-coated iron powder of this invention contains P substantially since it manufactures in the coexistence of a phosphorus containing ion by a wet method so that it may mention later. As content of average P in the iron powder comprised by the iron particle used for this invention, it is preferable to set it as 0.1 mass % or more and 1.0 mass % or less with respect to the mass of iron powder. When the P content is outside this range, it is not preferable because it becomes difficult to produce iron particles having the above average particle diameter and average axial ratio. As P content, it is more preferable that they are 0.1 mass % or more and 0.7 mass % or less, and it is still more preferable that they are 0.15 mass % or more and 0.4 mass % or less. Although the content of P does not contribute to the improvement of magnetic properties, it is permissible as long as it is contained within the above range.

[Silicone Oxide Coating]

In this invention, insulating silicon oxide is coat|covered on the surface of said iron particle by the wet coating method using silicon alkoxide. The coating method using silicon alkoxide is a method generally called a sol-gel method, and is excellent in mass productivity compared with a dry method.

When silicon alkoxide is hydrolyzed, some or all of the alkoxy groups are substituted with hydroxyl groups (OH groups) to form a silanol derivative. The silanol derivative is an organosilicon compound having a silanol group Si-OH in its molecular structure. In the present invention, the surface of the iron powder is coated with this silanol derivative. When heated, the coated silanol derivative takes a polysiloxane structure by condensation or polymerization, and when the polysiloxane structure is further heated, silica (SiO ₂ ). . In the present invention, from the coating of the silanol derivative in which a part of the alkoxy group, which is an organic substance, remains to the coating of silica, it is collectively referred to as the coating of silicon oxide.

The content of Si contained in the silicon oxide-coated iron powder is preferably 1.0 mass% or more and 10 mass% or less with respect to the mass of the silicon oxide-coated iron powder in order to ensure insulation and obtain high magnetic permeability μ' in a high-frequency region. In the case of 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 is equivalent to 0.5 to 8.0 nm in average film thickness do.

When content of Si contained in silicon-oxide-coated iron powder is less than 1.0 mass %, many defects exist in Si oxide coating layer, and it becomes difficult to ensure insulation. When 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 melt|dissolution method mentioned later.

[volume resistivity]

The silicon oxide-coated iron powder of the present invention preferably has a volume resistivity of 1.0×10 ⁵ Ω·cm or more, measured in a state in which an applied voltage of 10 V is applied to a green body obtained by vertical pressure molding at 64 MPa. When the volume resistivity is less than 1.0×10 ⁵ Ω·cm, the insulation between the particles is not sufficient, the loss increases due to the influence of the eddy current between the particles, and the characteristics of an inductor or the like are deteriorated, so it is not preferable. In the present invention, the upper limit of the volume resistivity of the green body is not particularly specified, but in the case of the Si content, a volume resistivity of about 1.0×10 ⁵ to 1.0×10 ⁹ Ω·cm can be obtained. In addition, if the thickness of the silicon oxide coating layer is increased, the volume resistivity is increased, but the silicon oxide coating is a non-magnetic component, and as described above, the magnetic properties are deteriorated.

[Pressure Density]

In the case of the present invention, the green compact obtained by press-molding the silicon oxide-coated iron powder at 64 MPa preferably has a green density of 4.0 g/cm 3 or less. This is because, when the above-described high magnetic permeability μ′ and high insulation properties are obtained in a state where the green density is small, weight reduction or miniaturization of the inductor is achieved.

[Iron manufacturing process]

The iron particle used as the core of the silicon oxide-coated iron powder of this invention can be manufactured by the manufacturing method disclosed in said Japanese Patent Application No. 2017-134617. The production method disclosed in the above application is characterized in that it is carried out by a wet method in the presence of phosphorus-containing ions, and there are roughly three types of embodiments. An iron powder composed of iron particles of 0.25 µm or more and 0.80 µm or less and having an average axial ratio of 1.5 or less can be obtained.

[starting material]

In the iron powder manufacturing process of the present invention, an acidic aqueous solution (hereinafter referred to as a raw material solution) containing trivalent Fe ions is used as a starting material of silicon oxide-coated iron oxide powder, which is a precursor of silicon oxide-coated iron powder. If a divalent Fe ion is used instead of a trivalent Fe ion as a starting material, a mixture containing, as a precipitate, a hydrated oxide of divalent iron, magnetite, etc. in addition to the hydrated oxide of trivalent iron is produced, and finally obtained iron particles Since the shape of becomes non-uniform, iron powder and silicon oxide-coated iron powder like the present invention cannot be obtained. Here, acidic means that the pH of a solution is less than 7. As such a Fe ion source, it is preferable to use a water-soluble inorganic acid salt such as nitrate, sulfate, and chloride from the viewpoint of availability and price. When this Fe salt is dissolved in water, Fe ions are hydrolyzed, and the aqueous solution is acidic. When this acidic aqueous solution containing Fe ions is neutralized by adding an alkali, 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).

Although the present invention does not specifically prescribe|regulate the Fe ion concentration in a raw material solution, 0.01 mol/L or more and 1 mol/L or less are preferable. When it is less than 0.01 mol/L, the amount of the precipitate obtained in one reaction is small, and it is economically unpreferable. When the Fe ion concentration exceeds 1 mol/L, the reaction solution tends to gel due to rapid precipitation of the hydrated oxide, which is not preferable.

[Phosphorus-containing ions]

In the iron production process of the present invention, phosphorus-containing ions coexist during the formation of a precipitate of the hydrated oxide of iron, or phosphorus-containing ions are added during addition of a silane compound to cover the hydrolysis product. In either case, phosphorus-containing ions coexist in the system at the time of coating with the silane compound. As a source of phosphorus-containing ions, phosphoric acid, ammonium phosphate, sodium phosphate, and soluble phosphoric acid (PO ₄ ^3- ) salts such as monohydrogen salts and dihydrogen salts thereof can be used. Here, phosphoric acid is a tribasic acid, and since it dissociates in three stages in aqueous solution, phosphate ions, dihydrogen phosphate ions, and monohydrogen phosphate ions can exist in aqueous solution, but their forms are those of drugs 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 phosphate group are collectively referred to as phosphate ions. In the present invention, it is also possible to use diphosphate (pyrophosphate), which is a condensed phosphoric acid, as a source of phosphate ions. In the present invention, it is also possible to use a phosphite ion (PO ₃ ^3- ) or a hypophosphite ion (PO ₂ ^2- ) having a different oxidation number of P instead of the phosphate ion (PO ₄ ^3- ). These 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 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. 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.

Although the mechanism by which 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 are obtained by coexistence of phosphorus-containing ions is unclear, the present inventors have found that the silicon oxide coating layer described later contains phosphorus-containing ions. Since it contains, it is estimated that it is because the physical property changes.

In addition, as described above, the timing of adding the phosphorus-containing ions to the raw material solution may be at 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 processing]

In the first embodiment of the iron powder manufacturing process of the present invention, an alkali is added to a raw material solution containing phosphorus-containing ions while stirring by a known mechanical means, and neutralized until the pH thereof becomes 7 or more and 13 or less to obtain iron Produces a precipitate of hydrated oxides. In addition, in the Example mentioned later, description is mainly based on this 1st Embodiment.

When the pH after neutralization is less than 7, it is not preferable because iron ions do not precipitate as a hydrated oxide of iron. When the pH after neutralization exceeds 13, the hydrolysis of the silane compound added in the silicon oxide coating step described later is rapid, and the coating of the hydrolyzed product of the silane compound becomes non-uniform.

Further, 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 an alkali to the raw material solution containing phosphorus-containing ions, the raw material 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 the 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 according to the pH range to be measured. In addition, the pH described herein is a value obtained by directly reading a measured 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 ammonium salts such as hydroxides of alkali metals or alkaline earth metals, aqueous ammonia, and ammonium bicarbonate, but ammonia water in which impurities are hardly left behind when the precipitate of hydrated oxides of iron is finally heat treated to iron oxides. It is preferable to use ammonium bicarbonate. Although you may add this alkali to the aqueous solution of a starting material as a solid, it is preferable to add in the state of aqueous solution from a viewpoint of ensuring the uniformity of reaction.

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

In the manufacturing method of this invention, although the reaction temperature at the time of a neutralization process is not specifically prescribed|regulated, It is preferable to set it as 10 degreeC or more and 90 degrees C or less. When the reaction temperature is less than 10°C or greater than 90°C, it is not preferable in consideration of 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 neutralized until the pH thereof becomes 7 or more and 13 or less to produce a precipitate of a hydrated oxide product of iron Then, in the process of aging the precipitate, phosphorus-containing ions are added to the slurry containing the precipitate. The timing of adding the phosphorus-containing ions does not matter immediately after formation of the precipitate or during aging. In addition, the aging time and reaction temperature of the deposit in 2nd Embodiment are the same as that of 1st 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 neutralized until the pH thereof becomes 7 or more and 13 or less to produce a precipitate of hydrated oxide of iron. After that, the precipitate is aged. In this embodiment, phosphorus-containing ions are added when coating the silicon oxide.

[Coating with hydrolysis product of silane compound]

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

In the case of the sol-gel method, a silicone compound having a hydrolyzable group, such as tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), or various silane coupling agents, is added to the slurry of the precipitate of hydrated oxide of iron. A silane compound is added to cause a hydrolysis reaction under stirring, and the resulting hydrolysis product of the silane compound covers the surface of the precipitate of the hydrated oxide of iron. In that case, although an acid catalyst or an alkali catalyst may be added, it is preferable to add such a catalyst in consideration of the processing 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 by addition in an amount in which the precipitate of the hydrated oxide of iron does not dissolve.

The specific method for coating with the hydrolysis product of the silane compound 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 silicone compound dropped into the slurry Ratio (Si/Fe ratio) of the total number of moles of Si contained shall 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 1 h or more and 20 h or less.

In the third embodiment of the iron powder production process of the present invention, from the start of the addition of the silicone compound having a hydrolyzed group to the end of the addition to the slurry containing the precipitate of the hydrated oxide of iron obtained by the aging after neutralization , phosphorus-containing ions are simultaneously added. The timing of addition of the phosphorus-containing ions may be simultaneous with the start of addition of the silicon oxide having a hydrolyzed group or simultaneously with the completion of the addition.

[Recovery of sediment]

From the slurry obtained by the above process, a precipitate of hydrated oxide of iron coated with a hydrolysis product of a silane compound is separated. As a solid-liquid separation means, well-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 and solid-liquid separation may be carried out. Then, it is preferable to perform solid-liquid separation again after washing the precipitate of the iron hydrated oxide coated with the hydrolysis product of the silane compound obtained by solid-liquid separation. As the cleaning method, known cleaning means such as repulping cleaning can be used. A drying treatment is performed on the precipitate of the hydrated oxide of iron coated with the finally recovered hydrolysis product of the silane compound. In addition, the said drying process is aimed at removing the water|moisture content adhering to a deposit, and you may perform it at the temperature of about 110 degreeC more than the boiling point of water.

[Heat treatment]

In the iron powder manufacturing process of this invention, the silicon oxide-coated iron oxide powder which is a precursor of silicon-oxide-coated iron powder is obtained by heat-processing the precipitate of the hydrated oxide of iron which coat|covered the hydrolysis product of the said silane compound. Although it does not prescribe|regulate in particular in the atmosphere of heat processing, it does not matter to atmospheric atmosphere. Heating can be performed in the range of about 500 degreeC or more and 1500 degrees C or less. If the heat treatment temperature is less than 500°C, it is not preferable because the particles do not sufficiently grow. When it exceeds 1500 degreeC, since excessive grain growth and particle|grain sintering will occur, it is unpreferable. The heating time may be adjusted within the range of 10 min to 24 h. By the said heat treatment, the hydrated oxide of iron changes to 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 covering the precipitation of the hydrated oxide of iron also changes to silicon oxide. The silicon oxide coating layer also has a function of preventing sintering during heat treatment of iron hydrated oxidation precipitation.

[Reduction heat treatment]

In the iron powder manufacturing process of this invention, silicon oxide-coated iron powder is obtained by heat-processing the silicon oxide-coated iron oxide powder which is a precursor obtained in said process in a reducing atmosphere. Examples of the gas for forming the reducing atmosphere include hydrogen gas and a mixed gas of hydrogen gas and inert gas. The temperature of reduction heat treatment can be made into the range of 300 degreeC or more and 1000 degrees 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. When it exceeds 1000 degreeC, the effect of reduction is saturated. The heating time may be adjusted within the range of 10 to 120 min.

[Stabilization processing]

Usually, since the surface of iron powder obtained by reduction heat treatment is chemically very active, stabilization treatment by slow oxidation is performed in many cases. Although the surface of the iron powder obtained by the method for manufacturing iron powder of the present invention is coated with a chemically inert silicon oxide, there are cases where a part of the surface is not covered. An oxidation protective layer is formed on the exposed portion of the As a procedure of stabilization process, the following means are mentioned as an example.

After the atmosphere to which the silicon oxide-coated iron powder after the reduction heat treatment is exposed is replaced by an inert gas atmosphere in the reducing atmosphere, the exposed portion is oxidized at 20 to 200°C, more preferably at 60 to 100°C, while gradually increasing the oxygen concentration in the atmosphere. Let the reaction proceed. As the inert gas, one or more gas components selected from rare gas and nitrogen gas can be applied. As the oxygen-containing gas, pure oxygen gas or air can be used. You may introduce water vapor|steam together with oxygen-containing gas. The oxygen concentration at the time of holding the silicon oxide-coated iron powder at 20 to 200°C, preferably at 60 to 100°C, is finally 0.1 to 21% by volume. The introduction of the oxygen-containing gas may be performed continuously or intermittently. In the initial stage of the stabilization process, it is more preferable to hold the time for which the oxygen concentration is 1.0 vol% or less for 5 minutes or more.

[Dissolution treatment of silicon oxide coating]

The silicon oxide-coated iron powder obtained by the above-mentioned series of processes cannot be press-molded satisfactorily as an inductor material, for example. In addition, conventional silicon oxide is an auxiliary agent for obtaining iron powder by reaction as mentioned above, and is functionally different from the coating film mentioned later. After dissolving and removing the silicon oxide coating layer in an aqueous alkali solution once to obtain uncoated iron powder, it is necessary to again coat the iron powder with highly insulating silicon oxide.

The reason for the low volume resistivity of the green compact is not clear at present, but the volume resistivity of the silicon oxide coating layer is lowered due to mixing of a phosphorus-containing compound in the silicon oxide coating layer, or the physical properties of the silicon oxide coating layer are changed. It is conceivable that the defect density increased.

As an aqueous alkali solution used for a dissolution treatment, a normal aqueous alkali solution industrially used, such as a sodium hydroxide solution, a potassium hydroxide solution, and aqueous ammonia, can be used. In consideration of processing time and the like, the pH of the processing liquid is preferably 10 or more, and the temperature of the processing liquid is preferably 60°C or more and below the boiling point.

[Disintegration processing]

Although the iron powder obtained by the dissolution treatment of the silicon oxide coating described above is subjected to a series of steps of the second silicon oxide coating treatment described later, the iron powder may be pulverized before being used in the next step. By pulverizing, the 50% cumulative particle diameter of the iron powder by volume by the microtrack measuring device can be made small. As the pulverizing means, a known method such as a method using a pulverizing device using a media 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 media, the particle shape of the obtained iron powder is deformed and the axial ratio becomes large. Since there is a possibility that such defects may occur, it is preferable to employ a medialess pulverizing device, and it is particularly preferable to perform pulverization using a jet mill pulverizing device. Here, the jet mill pulverizing device refers to a pulverizing device in which the pulverization object or a slurry in which the pulverization object and the liquid are mixed is sprayed with a high-pressure gas to collide with a collision plate or the like. A type that sprays the object to be pulverized with high-pressure gas without using a liquid is called a dry jet mill pulverizer, and a type that uses a slurry in which the object to be pulverized and a liquid is mixed is called a wet jet mill pulverizer. The object to be pulverized or the object to which the pulverization object and the liquid mixed slurry are collided may not be a static object such as a collision plate. You may employ the method of collide.

In addition, as a liquid in the case of performing pulverization using a wet jet mill grinding device, although a general dispersion medium, such as pure water and ethanol, can be employ|adopted, it is preferable to use ethanol.

When a wet jet mill pulverizer 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 the slurry. A well-known method can be employ|adopted as a drying method, and air|atmosphere may be sufficient as an atmosphere. However, from the viewpoint of preventing oxidation of iron powder, drying in a non-oxidizing atmosphere such as nitrogen gas, argon gas, hydrogen gas or the like, or vacuum drying is preferably performed. Moreover, in order to speed up the drying rate, it is preferable to carry out by heating to 100 degreeC or more, for example. Moreover, when the iron powder obtained after drying is mixed with ethanol again and microtrac particle size distribution measurement is performed, D50 of the iron powder in the slurry after the said pulverization process can be reproduced substantially. That is, the D50 of iron content does not change before and after drying.

[Slurry holding process]

Hereinafter, the process of giving highly insulating silicon oxide coating to the iron powder obtained in the above-mentioned series of iron powder manufacturing processes is described.

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

It is preferable that content of the water in a mixed solvent is 1 mass % or more and 40 mass % or less. More preferably, they are 10 mass % or more and 35 mass % or less, More preferably, they are 15 mass % or more and 30 mass % or less. When the content of water is less than 1% by mass, the effect of hydrating the above-mentioned Fe oxide is insufficient. Since the hydrolysis rate of a silicon alkoxide becomes quick when content of water exceeds 40 mass %, and a uniform silicon oxide coating layer will not be obtained, respectively, it is unpreferable, respectively.

As an organic solvent used for a mixed solvent, it is preferable to use aliphatic alcohols, such as methanol, ethanol, 1-propanol, 2-propanol, a butanol, a pentanol, and hexanol, which have 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, or hexanol. .

In this invention, although the temperature in particular of a slurry holding process is not prescribed|regulated, it is preferable to set it as 20 degreeC or more and 60 degrees C or less. If the holding temperature is less than 20°C, the rate of hydration reaction of Fe oxide becomes slow, so it is not preferable. In addition, 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 not particularly stipulated, but the 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 compound occurs uniformly.

[Alkoxide addition process]

After adding a silicon alkoxide, stirring the slurry which disperse|distributed the iron powder in the mixed solvent obtained by said slurry holding process by a well-known mechanical means, the slurry is hold|maintained in that state for a fixed period of time. As the silicon alkoxide, as described above, trimethoxysilane, tetramethoxysilane, triethoxysilane, tetraethoxysilane, tripropoxysilane, tetrapropoxysilane, tributoxysilane, tetrabutoxysilane, etc. can be used

The amount of silicon alkoxide to be added can be set according to a desired value of the volume resistivity of the green compact. Specifically, it is 10 mass % or more. For this reason, by setting the axial ratio of the iron particles to 1.5 or less, since they are close to a circular shape, the possibility that the coating will be unevenly distributed at different locations within the particles is low, and it is not unevenly distributed even between particles, and silicon alkoxide is mostly deposited on the surface of the iron particles. Investigate Moreover, when it is added in excess, since it exists free from the surface of an iron particle, it is unpreferable, and specifically, it will be 100 mass % or less.

The silicon alkoxide added in this step is hydrolyzed by the action of water contained in the mixed solvent to obtain a silanol derivative. The produced 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 slowly, and it is considered that the reaction layer of the silanol derivative is uniformly formed.

In the present invention, the reaction temperature of the alkoxide addition step is not particularly defined, but is preferably set to 20°C or higher and 60°C or lower. When the reaction temperature is less than 20°C, the reaction rate between the iron surface and the silanol derivative becomes slow, so it is not preferable. Moreover, since the hydrolysis reaction rate of the added silicon alkoxide increases when reaction temperature exceeds 60 degreeC, and the uniformity of a silicon oxide coating layer deteriorates, it is unpreferable. In the present invention, the reaction time of the alkoxide addition step is not particularly stipulated, 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 surface and the silanol derivative occurs uniformly.

[hydrolysis catalyst addition process]

In the production method of the present invention, after the reaction layer of the silanol derivative is formed on the surface of the iron powder in the alkoxide addition step described above, while stirring the slurry in which the iron powder is dispersed in a mixed solvent by a known mechanical means, the valence of the silicon alkoxide is A decomposition catalyst is added. In this step, the addition of the hydrolysis catalyst accelerates the hydrolysis reaction of the silicon alkoxide and increases the film-forming rate of the silicon oxide coating layer. In addition, after this process, it becomes the method similar to the film-forming method by the normal sol-gel method.

The hydrolysis catalyst uses an alkali catalyst. The use of an acid catalyst is undesirable because iron dissolves. As the alkali catalyst, it is preferable to use aqueous ammonia from the viewpoints of impurity hardly remaining in the silicon oxide coating layer and easiness of availability.

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

[Solid-liquid separation and drying]

Silicon oxide-coated iron powder is collect|recovered using a well-known solid-liquid-separation means from the slurry containing the silicon oxide-coated iron powder obtained by a series of processes mentioned above. 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 and solid-liquid separation may be carried out.

After wash|cleaning the collect|recovered silicon oxide-coated iron powder using the pure water of about 50 times, it is made to dry 50 degreeC or more and 200 degrees C or less, 2 hours or more, for example, 100 degreeC, 10 hours in nitrogen atmosphere. 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 diameter of the iron particles constituting the silicon oxide-coated iron powder and the particle diameter of the iron oxide particles constituting the silicon oxide-coated iron oxide powder were measured after dissolving and removing the silicon oxide coating using a 10% by mass aqueous sodium hydroxide solution, respectively. It was calculated|required by scanning electron microscope (SEM) observation. S-4700 by Hitachi Seisakusho was used for SEM observation.

The dissolution removal of silicon oxide was performed by filtration, washing with water and drying after putting silicon oxide-coated iron powder or silicon oxide-coated iron oxide powder in a 60 degreeC 10 mass % sodium hydroxide aqueous solution, stirring for 24 hours. In addition, the quantity of the said sodium hydroxide aqueous solution was made into the ratio of 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 long side of the circumscribed rectangle having the smallest area for a certain particle is determined as the particle diameter (major diameter) of the particle. Specifically, from among the SEM photographs taken at a magnification of about 3,000 to 30,000 times, 300 randomly selected particles in which the entire outer edge is observed, their particle diameters are 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.

[Axial ratio]

With respect to any particle on the SEM image, the length of the short side of the circumscribed rectangle having the minimum area is called the "minor axis", and the ratio of the major axis/minor axis is called the "axial ratio" of the particle. The "average axial ratio" which is an average axial ratio as a powder can be determined as follows. By SEM observation, the "major axis" and "minor axis" are measured for 300 randomly selected particles, and the average value of the major axis and the average value of the minor axis for all the particles to be measured are defined as the "average major axis" and "average minor axis", respectively. and the ratio of the average major axis/average minor axis is defined as the “average axis ratio”. For each of the major axis, minor axis, and axis ratio, a coefficient of variation can be calculated as an index indicating the size of the non-uniformity.

[Measurement of Si content]

The Si content of the iron powder (uncoated product) as a starting material and the iron powder coated with silicon oxide was calculated|required by the following method. After the sample was weighed and dissolved with hydrochloric acid, perchloric acid was added and heated until the solution disappeared, and then hydrochloric acid was added to dissolve all components soluble in the acid. Then, the residue mainly containing silicon dioxide was filtered, put in a platinum crucible, it was ignited in an electric furnace, and the mass was measured after leaving to cool. Hydrofluoric acid and sulfuric acid were added to the platinum crucible after the mass measurement to dissolve silicon dioxide and further heated to evaporate and remove the silicon powder as silicon tetrafluoride. Then, the platinum crucible was ignited again, and the mass was measured after leaving to cool, and the difference with the mass previously measured was made into the amount of silicon dioxide. The amount of silicon in the sample was computed from the calculated|required amount of silicon dioxide.

[Measurement of Fe and P content]

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

In addition, the residue obtained above was put in a platinum crucible as a filter paper as a whole, 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 molten product was leached into warm water, and hydrochloric acid was added to dissolve it by heating. After the solution was put into a volumetric flask and adjusted, Fe and P concentrations were measured by ICP emission spectroscopy (ICP-AES). Content of each element was calculated|required from the ICP measurement value of the filtrate, and the ICP measurement value of the solution after dissolving the residue.

[Calculation of Average Film Thickness of Silicon Oxide Coating]

In addition, the average film thickness t of the silicon oxide coating in a silicon oxide coating iron powder was computed with the following formula.

Average film thickness _t =Si content (mass %)/100×(SiO2 molecular weight/Si atomic weight)/(SiO2 density×BET specific surface area _of iron powder (uncoated product))

_In addition, SiO2 density was calculated as 2.65 (g/cm<3>). In this invention, it is preferable that the average film thickness t of a silicon oxide shall be 1.0 nm or more and 6.0 nm or less. By setting the average film thickness t within the above range, it is possible to achieve both high μ′ and high volume resistivity of the green compact. When the average film thickness t is less than 1.0 nm, it is not preferable because the volume resistivity of the green body decreases. In addition, when the average film thickness t exceeds 6.0 nm, it is not preferable because μ′ decreases.

[magnetic properties]

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

[Complex Permeability]

Iron iron or silicon oxide-coated iron powder and bisphenol F-type epoxy resin (manufactured by Tekken Co., Ltd.; one-component epoxy resin B-1106) were weighed in a mass ratio of 90:10, and a rotating and revolving mixer (manufactured by THINKY: ARE-250) was used to knead them, and a paste was obtained in which the test powder (provided in the experiment, or a sample thereof) was dispersed in an epoxy resin. This paste was dried on a hot plate at 60 DEG C for 2 h to obtain a composite of metal powder and resin, and then granulated into powder to obtain a composite powder. 0.2 g of this composite powder was put in a donut-shaped container, and a load of 9800 N (1 TON) was applied with a hand press to obtain a toroidal shaped body having an outer diameter of 7 mm and an inner diameter of 3 mm. For this molded article, using an RF impedance analyzer (manufactured by Keysight Technologies, Inc.; E4990A), a terminal adapter (manufactured by Keysight Technologies; 42942A), and a test fixture (manufactured by Keysight Technologies; 16454A), The real part μ' and the imaginary part μ" were measured, and the loss coefficient tanδ = μ"/μ' of the complex relative magnetic permeability was obtained. 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 is obtained.

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

[BET specific surface area]

BET specific surface area was calculated|required by the BET one-point method using MACSORB MODEL-1210 by Muntech Corporation.

[Microtrack particle size distribution measurement]

Microtrac particle size distribution analyzer MT3300EXII manufactured by Microtrac Bell was used for the measurement of the cumulative 50% particle diameter and the cumulative 90% particle diameter on a volume basis by the Microtrac measuring device for iron powder. In addition, ethanol was used as a liquid put into the sample circulator of a measuring apparatus. In addition, in the form of a slurry in which iron powder and ethanol or pure water were mixed, the slurry was stirred to such an extent that no non-uniformity was observed with the naked eye immediately before supply, and then supplied to the measuring device.

[Measurement of volume resistivity and green density]

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

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

ρv=R×πd ² /4t

where R is the measured value of the 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 density was calculated from the sample volume and sample weight of the green compact obtained by press-molding at 64 MPa (20 kN).

Example

[Comparative Example 1]

In a 5L reaction tank, 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 atmospheric atmosphere. while dissolving (Step 1). The pH of this solution was about 1. In addition, under this condition, the P/Fe ratio is 0.0086.

409.66 g of 23.47 mass % ammonia solution was added over 10 min while mechanically stirring this pouring solution with a stirring blade under conditions of 30 ° C. in an atmospheric atmosphere (about 40 g/L), and after completion of the dropping, stirring was carried out for 30 min. Then, aging of the produced precipitate was performed. At that time, the pH of the slurry containing the precipitate was about 9 (Step 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 the air. After that, stirring was continued for 20 hours, and the precipitate was coated with a hydrolysis product of a silane compound produced by hydrolysis (Step 3). In addition, under this condition, the Si/Fe ratio is 0.18. Table 1 shows the Si/Fe ratio and the P/Fe ratio of this comparative example.

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

The dried product obtained in Step 4 was heat-treated in the air at 1050°C using a box-type kiln 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, the bucket was charged into a flow-through reduction furnace, and a reduction heat treatment was performed by holding at 630° C. for 40 minutes while flowing hydrogen gas in the furnace (procedure). 6).

Then, the atmospheric gas in the furnace was converted from hydrogen to nitrogen, and the furnace temperature was lowered to 80°C at a temperature decrease rate of 20°C/min while nitrogen gas was flowing. After that, as the initial gas to be subjected to stabilization treatment, a gas (oxygen concentration of about 0.17% by volume) mixed with nitrogen gas (oxygen concentration of about 0.17% by volume) is introduced into the furnace so that the volume ratio of nitrogen gas/air is 125/1, and the metal powder particle surface layer portion by continuously introducing a mixed gas (oxygen concentration of about 0.80 vol%) into the furnace by starting the oxidation reaction of , 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 introduction flow rate of the gas was also maintained almost constant (procedure 7).

The silicon oxide-coated iron powder obtained in step 7 was immersed in 10 mass %, 60 degreeC sodium hydroxide aqueous solution for 24 hours, and the silicon oxide coating was melt|dissolved. After filtering the obtained slurry containing iron powder by suction filtration using a membrane filter and washing with water, it dried at 110 degreeC in nitrogen for 2h and obtained iron powder. In addition, the quantity of the said sodium hydroxide aqueous solution was made into the ratio of 3.2L with respect to 56g of silicon oxide-coated iron powders.

In FIG. 1, the SEM observation result of the iron powder obtained by this comparative example is shown. In addition, the length indicated by 11 white vertical lines shown in the lower right of FIG. 1 is 5 μm ( FIG. 2 is also the same). About the obtained iron powder, the average particle diameter of an iron particle, an average axial ratio, a composition, BET specific surface area, and magnetic properties 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 axial ratio was 1.27. In addition, when the volume resistivity of the green compact obtained by molding by the above method was measured using the obtained iron powder, the resistance measurement value R was a result below the measurement limit, and also as the volume resistivity, the measurement limit (volume resistivity 9.9 × 10 ⁴ ) Ω·cm) or less. Table 2 also shows the volume resistivity and density of the green compact obtained by molding by the above method using the obtained iron powder, and the high frequency characteristics of the toroidal formed product obtained by molding by the above method. The reason that the volume resistivity of the green compact obtained in this comparative example was low below 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 put into a 1L reaction tank to prepare a mixed solvent, and 15.00 g of iron powder obtained under the same conditions as in Comparative Example 1 was added to the mixed solvent, and mechanically stirred with a stirring blade, Nitrogen purged for 30 min at room temperature. After 30 min, the reaction solution was heated to 40°C while stirring and nitrogen purge were continued.

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

After filtering the obtained slurry by suction filtration using a membrane filter, it wash|cleaned with pure water, and the obtained iron powder cake was dried at 100 degreeC in nitrogen atmosphere. In FIG. 2, the SEM observation result of the iron powder obtained by the above-mentioned series of procedures again coat|covered after dissolving and removing a silicon oxide is shown. For the silicon oxide-coated iron powder, the BET specific surface area, composition, magnetic properties, complex magnetic permeability, density of the green compact, and volume resistivity were measured. A measurement result is shown together in Table 2. Moreover, as a measurement result of volume resistivity, the measured value R of volume resistance was 1.4x10 ⁶ (Ω), and the powder sample thickness t was 0.429 (cm).

[Examples 2 to 10]

In the same manner as in Example 1, using 15.00 g of iron powder obtained on the same conditions as in Comparative Example 1, various conditions for coating silicon oxide were changed to obtain silicon oxide coated iron powder. Table 1 shows the conditions of the silicon oxide coating used in these examples. Moreover, in Example 10, the crushing process of iron powder is performed before the silicon oxide coating process. The conditions for the pulverization treatment of iron powder are shown below. The iron powder obtained in Comparative Example 1 was mixed with pure water to prepare a pure iron powder mixed slurry having an iron content of 10% by mass. This slurry was pulverized using a jet mill pulverizer (manufactured by Ricks, Inc.; nano atomization apparatus G-smasher LM-1000) to obtain a slurry after pulverization treatment. In the pulverization, the pulverization treatment was repeated 5 times at a supply rate of 100 ml/min and an air pressure of 0.6 MPa of the iron powder pure water mixed slurry. The slurry after the pulverization treatment was dried in nitrogen gas at 100°C for 2 h to obtain an iron powder according to Example 10.

For the silicon oxide-coated iron powder obtained in these examples, the BET specific surface area, composition, magnetic properties, complex magnetic permeability, density of the green compact, and volume resistivity were measured. A measurement result is shown together in Table 2.

[Example 11]

Iron powder was obtained in the same manner as in Steps 1 to 8 of Comparative Example 1 described above, except that the heat treatment temperature in the air was changed to 1020°C. About the obtained iron powder, the average particle diameter of an iron particle, an average axial ratio, a composition, BET specific surface area, and magnetic properties 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 axial ratio was 1.20.

The obtained iron powder was mixed with pure water, and the iron content pure water mixing slurry of 10 mass % of iron content was produced. This slurry was pulverized using a jet mill pulverizer (Starburst Mini, model number: HJP-25001 manufactured by Sugino Machinery Co., Ltd.), and the slurry after pulverization treatment was obtained. In addition, in the pulverization, the pressure to pressurize the iron powder pure water mixed slurry was 245 Mpa, and the pulverization process was repeated 10 times. The slurry after the pulverization process was dried at 100 degreeC in nitrogen gas for 2 hours, and the iron powder after a pulverization process was obtained (procedure 19).

54.09 g of pure water and 196 g of isopropyl alcohol (IPA) were put into a 1 L reactor to prepare a mixed solvent, and 15.00 g of iron powder obtained in step 19 was added to the mixed solvent, and mechanically stirred with a stirring blade for 30 min at room temperature Nitrogen purged. After 30 min, the reaction solution was heated to 40°C while stirring and nitrogen purge were continued.

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

After filtering the obtained slurry by suction filtration using a membrane filter, it wash|cleaned with pure water, and the obtained iron powder cake was dried at 100 degreeC in nitrogen atmosphere. For the silicon oxide-coated iron powder, the BET specific surface area, composition, magnetic properties, complex magnetic permeability, density of the green compact, and volume resistivity were measured. A measurement result is shown together in Table 2. In addition, as a measurement result of the volume resistivity, the measured value R of the volume resistance was 3.9x10 ⁴ (Ω), and the powder sample thickness t was 0.381 (cm).

[Example 12]

Iron powder was obtained in the same manner as in Comparative Example 1 except that heat treatment in the atmosphere using a box-type kiln was performed at 1090°C. Using 15.00 g of the obtained iron powder, the silicon oxide coating process was performed on the conditions similar to Example 11 except having changed the TEOS addition amount into 1.27 g, and the silicon oxide coating iron powder was obtained. Table 1 shows the conditions for a series of steps for producing iron powder and silicon oxide coating.

After filtering the obtained slurry by suction filtration using a membrane filter, it wash|cleaned with pure water, and the obtained iron powder cake was dried at 100 degreeC in nitrogen atmosphere. For the silicon oxide-coated iron powder, the BET specific surface area, composition, magnetic properties, complex magnetic permeability, density of the green compact, and volume resistivity were measured. A measurement result is shown together in Table 2. In addition, as a measurement result of the volume resistivity, the measured value R of the volume resistance was 3.8x10 ⁴ (Ω), and the powder sample thickness t was 0.412 (cm).

[Comparative Example 2]

Silicon oxide-coated iron powder was obtained using the same conditions as in Example 2 except that the addition amount of TEOS was 0.91 g. The conditions of the silicon oxide coating used in this comparative example are shown together in Table 1. Table 2 also shows 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.

The silicon oxide-coated iron powder obtained in this comparative example had a Si content of 0.9%, and since 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 remarkably inferior compared with the volume resistivity with respect to Examples 1-10.

[Comparative Example 3]

In a 5L reaction tank, 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 atmospheric atmosphere. while dissolving (Step 1). The pH of this solution was about 1. In addition, under this condition, the P/Fe ratio is 0.0086.

409.66 g of 23.47 mass% ammonia solution was added over 10 min (about 40 g/L) while mechanically stirring with a stirring blade under the conditions of 30 ° C., and stirring was performed for 30 min after completion of the dropwise addition. Then, aging of the produced precipitate was performed. At that time, the pH of the slurry containing the precipitate was about 9 (Step 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 for 20 hours, and the precipitate was coated with a hydrolysis product of a silane compound produced by hydrolysis (Step 3). also. Under this condition, the Si/Fe ratio is 0.18.

The slurry obtained in step 3 was filtered, and after removing as much moisture as possible in the precipitate coated with the obtained hydrolysis product of the silane compound, it was re-dispersed in pure water, followed by repulping washing. The washed slurry was filtered again, and the resulting cake was dried in the air at 110°C (Step 4). The dried product obtained in Step 4 was heat-treated in the air at 1050°C using a box-type kiln 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, the bucket was charged into a flow-through reduction furnace, and a reduction heat treatment was carried out by holding at 630° C. for 40 minutes while flowing hydrogen gas in the furnace (Step 6).

Then, the atmospheric gas in the furnace was converted from hydrogen to nitrogen, and the furnace temperature was lowered to 80°C at a temperature decrease rate of 20°C/min while nitrogen gas was flowing. After that, as the initial gas to be subjected to stabilization treatment, a gas (oxygen concentration of about 0.17% by volume) mixed with nitrogen gas (oxygen concentration of about 0.17% by volume) is introduced into the furnace so that the volume ratio of nitrogen gas/air is 125/1, and the metal powder particle surface layer portion by continuously introducing a mixed gas (oxygen concentration of about 0.80% by volume) into the furnace by starting the oxidation reaction of , 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 gas introduction flow rate was also maintained 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 size of iron particles, and complex magnetic permeability were measured. A measurement result is shown together 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 prescribed in the present invention, the silicon oxide coated iron powder having a small particle size and high μ' in a high frequency band and having high insulating properties can be obtained. can be seen to be 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, wherein the surface of iron particles is coated with silicon oxide, wherein the Si content is 1.0 mass% or more and 10 mass% or less, the silicon oxide The green compact obtained by vertically press-molding the coated iron powder at 64 MPa had a volume resistivity of 1.0 × 10 ⁵ Ω·cm or more, measured in a state where an applied voltage of 10 V was applied, and the P content of the iron particles was The silicon oxide-coated iron powder which is 0.1 mass % or more and 1.0 mass % or less with respect to the mass of an iron particle. The silicon oxide-coated iron powder according to claim 1, wherein the green compact obtained by press-molding the silicon oxide-coated iron powder at 64 MPa has a green density of 4.0 g/cm 3 or less. Production of silicon oxide-coated iron powder, wherein the silicon oxide-coated iron powder having an average particle diameter of 0.25 µm or more and 0.80 µm or less and the surface of iron particles having an average axial ratio of 1.5 or less is coated with silicon oxide has a Si content of 1.0 mass% or more and 10 mass% or less As a method,
An iron powder manufacturing step of preparing iron particles comprising 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 an organic substance containing 1% by mass or more and 40% by mass or less of water;
an alkoxide addition step of dispersing the iron powder in the mixed solvent and adding a silicon alkoxide to the retained slurry;
A hydrolysis catalyst addition step of adding a silicon alkoxide hydrolysis catalyst to the slurry to which the silicon alkoxide is added to obtain a slurry in which iron powder coated with silicon oxide is dispersed;
A recovery process for obtaining iron powder coated with silicon oxide by solid-liquid separation of a slurry containing iron powder coated with silicon oxide
A method for producing a silicon oxide-coated iron containing iron. The molded object for inductors containing the silicon oxide-coated iron powder of Claim 1 or 2. An inductor using the silicon oxide-coated iron powder according to claim 1 or 2. delete

Images