JP7542714B2 - Silicon oxide coated soft magnetic powder - Google Patents

Description

The present invention relates to silicon oxide-coated soft magnetic powder with good insulating properties and a small specific surface area, suitable for the production of powder magnetic cores for electrical and electronic components such as inductors, choke coils, transformers, reactors, and motors.

Conventionally, dust cores using soft magnetic powders such as iron powder, iron-containing alloy powder, and intermetallic compound powder have been known as magnetic cores for inductors, choke coils, transformers, reactors, motors, and the like. However, metal powders such as iron powder and iron alloy powder have higher conductivity than compound powders such as ferrite powder. For this reason, when manufacturing magnetic cores using metal powder, it is common to first form an insulating coating on the surface of the metal powder particles, and then subject them to the process of compression molding and heat treatment.

Various insulating coatings have been proposed. Silicon oxide coating is known as a highly insulating coating. As a soft magnetic powder coated with silicon oxide by a dry method, for example, Patent Document 1 discloses an Fe-Si-Cr-Ni alloy powder on which a SiO ₂ coating with a thickness of 5 to 10 nm is formed by a vibration sputtering device. Patent Document 2 discloses an Fe-Si-Cr magnetic metal powder coated with borosilicate alkali glass containing 79% by weight of SiO ₂ by a mechanofusion method. As a soft magnetic powder coated with silicon oxide by a wet method, for example, Patent Document 3 discloses an Fe-6.5% Si powder coated with a hydrolysis product of tetraethoxysilane using an IPA (isopropanol) solution of tetraethoxysilane, and then dried at 120°C. Patent Document 4 discloses a technique for forming a _SiO2 film having a thickness of 1 to 13 nm by using tetraethyl orthosilicate (tetraethoxysilane) on a magnetic powder in which a hard magnetic Fe-Pd core is coated with a soft magnetic Fe.

International Publication No. 2007/013436 International Publication No. 2014/013896 JP 2009-231481 A JP 2017-152609 A

However, in the case of the sputtering method disclosed in Patent Document 1, although it is possible to form a very thin thin film on the surface of the powder, it is difficult to obtain a uniform thin film, and it was not possible to achieve both insulating and magnetic properties. In the case of the mechanofusion method disclosed in Patent Document 2, the surface coating obtained has many voids, and the surface of the soft magnetic powder is partially exposed, so there is a problem that good insulating properties cannot be ensured. The wet method is highly productive and is promising as an industrial manufacturing method for soft magnetic powder coated with an insulator. However, the insulator-coated soft magnetic powder obtained in Patent Document 3 has a problem that the average thickness of the coating layer is large, and the pressed powder density of the magnetic powder decreases, resulting in deterioration of the magnetic properties. In the technology disclosed in Patent Document 4, the insulator-coated hard magnetic powder is produced via reduction heat treatment, and the coated particles synthesized by this manufacturing method cause agglomeration, resulting in a decrease in the pressed powder density of the magnetic powder, resulting in a problem of deterioration of the magnetic properties. In this case, in order to obtain the specified magnetic properties, the powder core must be enlarged, which does not meet the demand for miniaturization of products. Furthermore, the method of Patent Document 4 requires a process of forming an insulating shell on the surface of the core via a reduction heat treatment, which makes the manufacturing process complicated.

In view of these problems, the present applicant developed a silicon oxide-coated soft magnetic powder composed of soft magnetic powder particles coated with silicon oxide having good film thickness uniformity and few defects, and disclosed the following invention in Japanese Patent Application No. 2019-025026.
A silicon oxide-coated soft magnetic powder comprising particles having a silicon oxide coating layer formed on the particle surface of a soft magnetic powder containing 20 mass% or more of iron, the silicon oxide coating layer having an average film thickness of 1 nm or more and 30 nm or less, a coverage rate R defined by the following formula (1) of 70% or more, and a pressed powder density of 4.0 g/ ^cm3 or more.
R=Si×100/(Si+M)…(1)
Here, Si is the molar fraction of Si obtained by X-ray photoelectron spectroscopy (XPS) measurement of the silicon oxide-coated soft magnetic powder, and M is the sum of the molar fractions of metal elements and non-metal elements, excluding oxygen, among the elements constituting the soft magnetic powder, obtained by XPS measurement.

The above invention makes it possible to provide a silicon oxide-coated soft magnetic powder that has excellent insulating properties and can achieve a high green density. However, technology for controlling the specific surface area of soft magnetic powders with excellent insulating properties has not been fully explored.

When making a dust core using soft magnetic powder, a process of pressure molding the powder is essential. In the pressure molding process, it is necessary to ensure sufficient fluidity between the powder particles, so the soft magnetic powder is usually mixed with resin before being subjected to pressure molding. The mixed resin also functions as a binder between the particles. However, if too much resin is mixed, the resin will remain in the dust core, causing a decrease in magnetic properties. Therefore, it is necessary to keep the amount of resin used to a minimum. The amount of resin required is thought to depend on the specific surface area of the soft magnetic powder. Reducing the specific surface area of the powder is extremely advantageous in reducing the amount of resin required when pressure molding the powder.

Currently, there is no established technology that combines excellent insulation properties with a low specific surface area.

The present invention aims to provide a silicon oxide-coated soft magnetic powder having a silicon oxide coating layer on the particle surface that is thin, has excellent insulation properties, and has a high degree of uniformity in thickness and few defects, and has a small specific surface area, which is advantageous in reducing the amount of resin required during pressure molding.

In order to achieve the above object, the present specification discloses the following invention.
[1] A soft magnetic powder composed of soft magnetic metal particles having an iron content of 20 mass% or more and a silicon oxide coating layer on the surface thereof, the silicon oxide coating layer having an average thickness of 0.5 to 30 nm and a BET specific surface area of 1.0 ^m2 /g or less.
[2] The silicon oxide-coated soft magnetic powder according to the above item [1], in which a load of 20 kN is applied to a powder sample having a mass of 4 g in an insulating cylinder having an inner diameter of 20 mm to prepare a disk-shaped compressed powder sample having a diameter of 20 mm, and when the compressed powder sample is subjected to a test for measuring its volume resistivity by the double ring electrode method while a load of 20 kN is applied to the compressed powder sample, the volume resistivity is 1.0 x 10 ⁸ Ω·cm or more.
[3] The silicon oxide-coated soft magnetic powder according to the above [1] or [2], wherein the volume-based cumulative 50% particle diameter _D50 measured by a laser diffraction/scattering method is 1.0 to 15.0 μm.
[4] The silicon oxide coated soft magnetic powder according to any one of the above [1] to [3], which has a BET specific surface area of 0.65 m ² /g or less.
[5] A step of mixing a mixed solvent of water and an organic solvent having a water content of 1 to 40 mass% with a raw material powder composed of soft magnetic metal particles having an iron content of 20 mass% or more to obtain a slurry;
A step of adding silicon alkoxide to the slurry and stirring and mixing the slurry while maintaining the temperature of the slurry at 50 to 65°C;
a step of adding a hydrolysis catalyst to the slurry after the stirring and mixing while maintaining the temperature of the slurry at 50 to 65°C, thereby obtaining a slurry of soft magnetic metal particles coated with silicon oxide, which is a hydrolysis product of silicon alkoxide;
A step of subjecting the slurry of the soft magnetic metal particles coated with the silicon oxide to solid-liquid separation to recover a solid content;
A step of drying the recovered solid content to obtain a soft magnetic powder composed of soft magnetic metal particles coated with silicon oxide;
The method for producing a silicon oxide-coated soft magnetic powder according to any one of the above [1] to [4],
[6] A step of mixing a mixed solvent of water and an organic solvent having a water content of 1 to 40 mass% with a raw material powder composed of soft magnetic metal particles having an iron content of 20 mass% or more to obtain a slurry;
adding silicon alkoxide to the slurry and stirring and mixing;
a step of adding a hydrolysis catalyst to the slurry after the stirring and mixing to obtain a slurry of soft magnetic metal particles coated with silicon oxide, which is a hydrolysis product of silicon alkoxide;
A step of subjecting the slurry of the soft magnetic metal particles coated with the silicon oxide to solid-liquid separation to recover a solid content;
A step of vacuum drying the recovered solid content under a pressure of 10 kPa or less to obtain a soft magnetic powder composed of metal particles coated with silicon oxide;
The method for producing a silicon oxide-coated soft magnetic powder according to any one of the above [1] to [3].

The present invention provides a silicon oxide-coated soft magnetic powder that is advantageous for producing powder magnetic cores with excellent insulation properties and high powder density. The silicon oxide-coated soft magnetic powder has a small specific surface area, making it possible to reduce the amount of resin required in pressure molding to obtain powder magnetic cores, etc., and reducing the amount of resin is expected to improve magnetic properties and reduce production costs.

[Raw material powder]
In the present invention, a soft magnetic powder composed of soft magnetic metal particles with an iron content of 20% by mass or more is used as the raw material powder. Specific examples of soft magnetic powders containing 20% by mass or more of iron include pure iron powder (e.g., carbonyl iron powder), as well as iron alloy powders such as Fe-Si alloy, Fe-Si-Cr alloy, Fe-Al-Si alloy (Sendust), and Fe-Ni alloy (Ni mass 30 to 80% by mass) which is a permalloy composition. In addition, small amounts (10% by mass or less) of Mo and Co may be added as necessary.

Although the magnetic properties of the raw material powder are not particularly specified, it is preferable that the powder be a soft magnetic powder with low coercivity Hc and high saturation magnetization σs. If Hc is high, the energy loss when reversing the magnetic field becomes large and it is unsuitable for a magnetic core. It is preferable that the Hc of the raw material powder is, for example, 3.98 kA/m (about 50 Oe) or less. If the σs of the raw material powder used is low, a large amount of magnetic powder is required to form a magnetic core having the specified magnetic properties, and the size of the magnetic core will become large. It is preferable that σs is, for example, 100 Am ² /kg (100 emu/g) or more.

The average particle size of the primary particles of the raw material powder is not particularly specified, but traditionally the average particle size of the primary particles has been greater than 0.80 μm and less than 5.0 μm, and soft magnetic powders with an average particle size of any primary particle within this range can be used depending on the purpose.

[Silicon oxide coating layer]
The silicon oxide-coated soft magnetic powder according to the present invention is a soft magnetic powder composed of soft magnetic metal particles (particles of the raw material powder) having an iron content of 20% by mass or more and having a silicon oxide coating layer on the surface thereof. The silicon oxide coating layer has an average thickness of 0.5 to 30 nm. If the average thickness is less than 0.5 nm, it becomes difficult to stably ensure good insulation. The average thickness may be 3 nm or more. The thicker the average thickness of the silicon oxide coating layer, the better the insulation, but on the other hand, the proportion of silicon oxide, which is a non-magnetic phase, in the powder compact increases, and the magnetic properties tend to deteriorate. As a result of the investigation, it was found that the average thickness of the silicon oxide coating layer needs to be 30 nm or less, and is more preferably 25 nm or less.

The average thickness of the silicon oxide coating layer may be measured by the dissolution method described below. Alternatively, instead of the dissolution method, the average thickness may be determined by observing the cross section of the silicon oxide coating layer with a transmission electron microscope (TEM) or a scanning electron microscope (SEM). In this case, a TEM or SEM photograph of the cross section is taken, and the average thickness is determined by averaging the values of 50 randomly selected measurement points on the particle. The thickness determined by this method is also equivalent to that determined by the dissolution method.

[BET specific surface area]
In the present invention, the BET specific surface area of the silicon oxide-coated soft magnetic powder is specified to be 1.0 m ² /g or less. As described above, in the process of compacting the powder, resin is usually added to ensure the fluidity of the powder particles. If the amount of resin used is insufficient, smooth compaction cannot be performed. In the case of a powder with a large specific surface area, a large amount of resin is required. If the amount of resin used is large, the resin remains in the powder core, which is likely to cause a decrease in magnetic properties. As a result of various studies, it is considered that the effect of reducing the amount of resin used can be achieved when the BET specific surface area is reduced to 1.0 m ² /g or less in a silicon oxide-coated soft magnetic powder with excellent insulation. From the viewpoint of this effect, it is more preferable that the BET specific surface area is 0.8 m ² /g or less. In particular, when the BET specific surface area is reduced to 0.65 m ² /g or less in a soft magnetic powder having a highly insulating coating layer, insulation and a reduction in the amount of resin can be achieved at a high level. It is more preferable that the BET specific surface area is 0.60 ^m2 /g or less. Such silicon oxide-coated soft magnetic powders having high insulation and small BET specific surface area can be obtained by the manufacturing method described below. There is no particular lower limit for the BET specific surface area, but it may be controlled to, for example, 0.1 ^m2 /g or more, or 0.2 ^m2 /g or more.

The BET specific surface area in this specification can be measured by a BET single point method using a mixed gas of nitrogen and helium ( _N2 : 30 volume %, He: 70 volume %). The details of the method for measuring the BET specific surface area will be described in the examples below.

[Volume resistivity of powder compact sample]
The silicon oxide-coated soft magnetic powder according to the present invention is covered with a thin and highly homogeneous silicon oxide coating layer, similar to that disclosed in Japanese Patent Application No. 2019-025026. Even if the amount of coating is small (i.e., the average film thickness is small), a coating layer with a high film thickness uniformity prevents a decrease in insulation caused by the presence of exposed parts of the soft magnetic metal particles or parts where the coating layer is extremely thin, and a high volume resistivity is exhibited when a compact is formed. Here, the volume resistivity measured for a compact sample using the silicon oxide-coated soft magnetic powder is used as an index for evaluating the uniformity of the film thickness of the silicon oxide coating layer. Specifically, the following test method is applied.

(Test method for measuring volume resistivity of powder compact samples)
A load of 20 kN is applied to a powder sample having a mass of 4 g in an insulating cylinder having an inner diameter of 20 mm to prepare a disk-shaped compressed powder sample having a diameter of 20 mm. With a load of 20 kN applied to the compressed powder sample, the volume resistivity is measured by the double ring electrode method.

In this test, when the volume resistivity of a soft magnetic powder having a silicon oxide coating layer with an average thickness of 0.5 to 30 nm is 1.0 x 10 ⁸ Ω·cm or more, the powder can be evaluated as having a highly uniform silicon oxide coating layer. The volume resistivity is more preferably 5.0 x 10 ⁸ Ω·cm or more. The upper limit of the volume resistivity is not particularly limited, but is usually 1.0 x 10 ¹² Ω·cm or less.

The coverage rate R defined in the above formula (1) disclosed in Japanese Patent Application No. 2019-025026 can also be used as an index for identifying a highly uniform silicon oxide coating layer. In this case, it is preferable that the coverage rate R is 70% or more. According to the present invention, it is possible to obtain a silicon oxide-coated soft magnetic powder having a coverage rate R of 70% or more. The XPS measurement method for applying formula (1) is as described in Japanese Patent Application No. 2019-025026.

[Volume-based cumulative 50% particle size]
Regarding the particle size of the silicon oxide-coated soft magnetic powder, it is desirable that the cumulative 50% particle size _D50 based on volume as measured by the laser diffraction/scattering method is 1.0 to 15.0 μm. If _D50 is too small, the particles are likely to undergo secondary aggregation, which is a factor in causing a decrease in the green density. In that case, the magnetic permeability is reduced. If _D50 is large, the magnetic loss at high frequencies in the inductor is likely to increase. It is more preferable that _D50 is 1.5 to 10.0 μm, and even more preferable that it is 1.8 to 8.0 μm.

[Powder density]
The green density of the silicon oxide-coated soft magnetic powder is preferably 4.0 g/ ^cm3 or more in the green compact obtained by applying a pressure of 63.66 MPa. More preferably, it is 5.0 g/ ^cm3 or more. The green density affects the magnetic permeability of the green compact. If the green compact density is low, the magnetic permeability of the green compact will be low, and as a result, the size of the green compact will be large in order to obtain a predetermined magnetic permeability. The green compact density is preferably high, but due to the composition of the soft magnetic powder, it is practically 8.0 g/ ^cm3 or less. The green compact for measuring the green compact density can be prepared in the same manner as in the above-mentioned volume resistivity measurement test, by placing a powder sample with a mass of 4 g in a cylinder with an inner diameter of 20 mm and applying a load of 20 kN. In this case, a pressure equivalent to 63.66 MPa is applied with a load of 20 kN.

[Manufacturing process]
The silicon oxide coated soft magnetic powder according to the present invention can be manufactured using the following process.
(1) A step of mixing a mixed solvent of water and an organic solvent, the mixed solvent having a water content of 1 to 40% by mass, with a raw material powder composed of soft magnetic metal particles having an iron content of 20% by mass or more to obtain a slurry;
(2) adding silicon alkoxide to the slurry and stirring and mixing;
(3) A step of adding a hydrolysis catalyst to the slurry after the stirring and mixing to obtain a slurry of soft magnetic metal particles coated with silicon oxide, which is a hydrolysis product of silicon alkoxide;
(4) subjecting the slurry of the silicon oxide-coated soft magnetic metal particles to solid-liquid separation to recover a solid content;
(5) A step of drying the recovered solid content to obtain a soft magnetic powder composed of soft magnetic metal particles coated with silicon oxide.

The inventors have found that the following two patterns can be adopted as a method for controlling the BET specific surface area to a small value in a silicon oxide-coated soft magnetic powder having excellent insulating properties.
(Pattern 1)
The slurry temperature when silicon alkoxide is added and stirred and mixed in the above step (2), and the slurry temperature when the hydrolysis catalyst is added to coat the hydrolysis product of silicon alkoxide in the above step (3) are maintained at 50 to 65°C.
(Pattern 2)
In the above step (5), the solid content is dried by vacuum drying.
It is sufficient to apply at least one of these patterns 1 and 2. However, in order to control the BET specific surface area to 0.65 m ² /g or less, the above pattern 1 is adopted.

[Dispersion process]
The above step (1) is called the "dispersion step". A raw powder consisting of soft magnetic metal particles with an iron content of 20% by mass or more is prepared. An extremely thin oxide film of Fe is present on the surface of this raw powder. A mixed solvent of water and an organic solvent is prepared as the solvent. In this dispersion step, this oxide film of Fe is hydrated with the water contained in the mixed solvent. The surface of the hydrated Fe oxide is a kind of solid acid, and behaves similarly to a weak acid as a Bronsted acid. Therefore, when silicon alkoxide is added in the next step, the reactivity between the silanol derivative, which is a hydrolysis product of silicon alkoxide, and the surface of the raw powder particles is improved.

If the water content in the mixed solvent is low, the effect of hydrating the Fe oxide on the surface of the raw powder particles is insufficient. If the water content is high, the hydrolysis rate of the silicon alkoxide increases, making it difficult to form a highly uniform silicon oxide coating layer. In the present invention, a mixed solvent with a water content in the range of 1 to 40 mass% is used. The water content in the mixed solvent is more preferably 5 to 30 mass%, and even more preferably 10 to 20 mass%.

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

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

[Alkoxide Addition Step]
The above step (2) is called the "alkoxide addition step". The coating method using silicon alkoxide is generally called the sol-gel method, and is superior in mass productivity compared to the dry method. The slurry obtained by the above dispersion step is stirred by a known mechanical means, and then silicon alkoxide is added, and the slurry is held in that state for a certain period of time. According to the research of the inventors, it was found that in order to obtain a silicon oxide-coated soft magnetic powder having a small BET specific surface area, it is extremely effective to keep the temperature of the slurry in this step at 50 to 65°C (the method of the above pattern 1). As a result of investigating the pore distribution, when silicon alkoxide is added in this temperature range and stirring is maintained, a silicon oxide coating is formed in which the formation of micropores (pores with a size of 2 nm or less) is significantly reduced. It is considered that the formation of micropores is reduced, resulting in the formation of a silicon oxide-coated soft magnetic powder having a small BET specific surface area. When the method of the above pattern 1 is applied, the reaction time in this step may be set in the range of, for example, 1 to 30 minutes.

Even if it is not necessary to maintain the temperature of the slurry at 50 to 65°C in this step, it is possible to obtain a silicon oxide-coated soft magnetic powder with a small BET specific surface area by applying the method of vacuum drying in the step described below (the method of pattern 2 above). However, if the reaction temperature is too low, the reaction rate between the surface of the raw powder and the silanol derivative will be slow. If the reaction temperature is too high, the hydrolysis reaction rate of the added silicon alkoxide will increase, and the uniformity of the silicon oxide coating layer will tend to deteriorate. When the method of pattern 2 above is applied, it is preferable that the slurry temperature in this step be in the range of 20 to 70°C. In that case, the reaction time in this step may be set in the range of 1 to 30 minutes, for example.

When silicon alkoxide is hydrolyzed, some or all of the alkoxy groups are replaced with hydroxyl groups (OH groups) to form a silanol derivative. In the present invention, the surface of the raw material powder particles is coated with this silanol derivative. When the silanol derivative coating the particle surface is heated, it condenses or polymerizes to form a polysiloxane structure, and when the polysiloxane structure is further heated, it becomes silica (SiO ₂ ). In this specification, the silanol derivative coating in which some of the alkoxy groups, which are organic substances, remain, and the silica coating are collectively referred to as silicon oxide coating.

The silicon alkoxide added in this process 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 raw powder particles through condensation, chemical adsorption, etc. In this process, since no hydrolysis catalyst is added, the hydrolysis of the silicon alkoxide occurs slowly, and it is believed that the reaction layer of the silanol derivative is formed uniformly.

Examples of silicon alkoxides that can be used include trimethoxysilane, tetramethoxysilane, triethoxysilane, tetraethoxysilane, tripropoxysilane, tetrapropoxysilane, tributoxysilane, and tributoxysilane. Among these, tetraethoxysilane (TEOS) is particularly suitable because it has good wettability with the raw material powder particles described above and is easy to form a uniform coating layer.

The silicon alkoxide added in this step is almost entirely used for forming the silicon oxide coating layer, so the amount added is an amount that results in an average film thickness of the silicon oxide coating layer of 0.5 to 30 nm. The amount of silicon alkoxide added is specifically determined by the following method.
If the mass of the raw material powder contained in the slurry is Gp (g), the BET specific surface area of the raw material powder (the BET specific surface area before being subjected to the coating treatment in this step) is S ( ^m2 /g), and the target thickness of the silicon oxide coating layer is t (nm), then the total volume of the silicon oxide coating layer is V = Gp x S x t ( ^{10-5 m3} ). If the density of the silicon oxide coating layer is d = 2.65 (g/ ^cm3 = ¹⁰⁶ g/ ^m3 ), then the mass of the silicon oxide coating layer is Gc = 0.1V x d (g). Therefore, the number of moles of Si contained in the silicon oxide coating layer is calculated by dividing Gc by the molecular weight of _SiO2 , 60.08. In the manufacturing method of the present invention, the number of moles of silicon alkoxide corresponding to the above-mentioned target thickness t (nm) is added to the slurry.
In addition, the average film thickness of the silicon oxide coating layer measured by cutting the silicon oxide-coated soft magnetic powder using a focused ion beam (FIB) processing device and observing it with a transmission electron microscope (TEM) has been confirmed to accurately match the film thickness determined by the dissolution method described below, assuming the density of the silicon oxide coating layer to be d = 2.65 (g/ ^cm3 ).

[Hydrolysis catalyst addition step]
The above step (3) is called the "hydrolysis catalyst addition step". The slurry in which the raw material powder particles having a silanol derivative reaction layer formed on the surface thereof in the alkoxide addition step are dispersed is subjected to a known mechanical treatment. While stirring the mixture by a stirring means, a catalyst (hydrolysis catalyst) for promoting the hydrolysis of silicon alkoxide is added. In this step, the addition of the hydrolysis catalyst promotes the hydrolysis reaction of silicon alkoxide, and silicon oxide is formed. The deposition rate of the material coating layer is increased.

It is preferable to use a basic catalyst as the hydrolysis catalyst. If an acid catalyst is used, Fe, a component of the soft magnetic metal particles, may dissolve. It is preferable to use ammonia water as the basic catalyst because impurities are unlikely to remain in the silicon oxide coating layer and it is easy to obtain. The reaction temperature of the hydrolysis catalyst addition step is kept at 50 to 65°C when the above-mentioned pattern 1 method is applied, but there is no need to be particular about this when the above-mentioned pattern 2 method is applied, and it can be the same as the reaction temperature of the previous alkoxide addition step. There is no particular restriction on the reaction time of the hydrolysis catalyst addition step, but a long reaction time is economically disadvantageous, so it is advisable to set the conditions so that the reaction time is, for example, 5 to 200 minutes.

[Solid-liquid separation process]
The above step (4) is called the "solid-liquid separation step". From the slurry in which the powder after the hydrolysis catalyst addition step is dispersed, a powder composed of particles coated with silicon oxide is recovered as a solid content. As a solid-liquid separation means, known solid-liquid separation means such as filtration, centrifugation, decantation, etc. may be used. During solid-liquid separation, a flocculant may be added to perform solid-liquid separation.

[Drying process]
The above step (5) is called the "drying step". The collected solid content is dried to obtain a dried soft magnetic powder composed of soft magnetic metal particles coated with silicon oxide. When the method of pattern 1 is adopted in the above steps (2) and (3), there is no need to be particularly particular about the drying method in this step. For example, drying can be performed in an air atmosphere. If it is desired to suppress oxidation of the soft magnetic powder, it is recommended to dry in an inert gas atmosphere or in a vacuum. The temperature during drying is preferably 80°C or higher. In order to prevent the silicon oxide coating layer from peeling off, it is preferable to set the temperature to 400°C or lower, and more preferably 150°C or lower. In this way, a silicon oxide-coated soft magnetic powder having a very small BET specific surface area of, for example, 0.65 ^m2 /g or less can be obtained.

On the other hand, when the method of Pattern 2 is adopted in which the holding temperature is not particularly important in steps (2) and (3), vacuum drying is adopted in this step. Specifically, the material is dried by heating to a predetermined temperature in a container evacuated under a pressure of 10 kPa or less. The heating temperature is preferably in the range of 80 to 400°C, more preferably in the range of 80 to 150°C, as described above. The processing time of the vacuum drying may be set in the range of 12 to 24 hours, for example. This method makes it possible to obtain silicon oxide-coated soft magnetic powder having a BET specific surface area of 1.0 ^m2 /g or less. The mechanism by which vacuum drying makes it possible to reduce the BET specific surface area is not yet understood.

In each example, the following measurement methods were used.
(BET specific surface area)
The BET specific surface area was measured by a single-point BET method using a Macsorb manufactured by Mountec Co., Ltd., with nitrogen gas flowing through the measuring device for 20 minutes at 105° C. to degas the device, and then a mixed gas of nitrogen and helium ( _N2 : 30 vol.%, He: 70 vol.%) was flowed through the measuring device.

(TAP density)
The powder sample was filled into a bottomed cylindrical die having an inner diameter of 6 mm and a height of 11.9 mm to 80% of the volume to form a powder layer, and a pressure of 0.160 N/ ^m2 was uniformly applied to the upper surface of this powder layer. The powder layer was compressed at this pressure until the powder could no longer be packed densely. The height of the powder layer was then measured, and the density of the powder was calculated from the measured height of the powder layer and the weight of the filled powder, which was used as the tap density of the powder sample.

(Si content)
The analysis was carried out by the dissolution method as follows. First, hydrochloric acid and perchloric acid were added to the powder sample (raw material powder or silicon oxide-coated soft magnetic powder) to decompose it by heating, and the powder sample was heated until white smoke of perchloric acid was generated. The powder sample was then heated to dryness. After cooling, water and hydrochloric acid were added and heated to dissolve soluble salts. The insoluble residue was then filtered using filter paper, and the residue was transferred to a crucible together with the filter paper, dried, and incinerated. After cooling, the crucible was weighed together. A small amount of sulfuric acid and hydrofluoric acid were added, heated to dryness, and then ignited. After cooling, the crucible was weighed together. The second weighing value was subtracted from the first weighing value, and the weight difference was calculated as _SiO2 , and the Si content in the powder sample was calculated from that value.

(Carbon Content)
The carbon content in the powder samples (raw material powder or silicon oxide-coated soft magnetic powder) was measured using a carbon/sulfur analyzer (EMIA-22V manufactured by Horiba, Ltd.).
(Oxygen Content)
The oxygen content in the powder samples (raw material powder or silicon oxide-coated soft magnetic powder) was measured using an oxygen/nitrogen/hydrogen analyzer (EMGA-920 manufactured by Horiba, Ltd.).

(Particle size distribution)
Using a laser diffraction particle size distribution measuring device (SYMPATEC's HELOS particle size distribution measuring device; HELOS & RODOS (airflow type dispersion module)), the volume-based cumulative 10% particle size (D ₁₀ ), cumulative 25% particle size (D ₂₅ ), cumulative 50% particle size (D ₅₀ ), cumulative 75% particle size (D ₇₅ ), cumulative 90% particle size (D ₉₀ ), and cumulative 99% particle size (D ₉₉ ) were determined at a dispersion pressure of 5 bar (0.5 MPa).

(Volume resistivity of compact sample)
The volume resistivity of the compacted sample using the powder sample (silicon oxide-coated soft magnetic powder) was measured by the double ring electrode method using a Mitsubishi Chemical Analytech Co., Ltd. powder resistance measurement unit (MCP-PD51), a Mitsubishi Chemical Analytech Co., Ltd. high resistance resistivity measurement unit (MCP-HT450), and a Mitsubishi Chemical Analytech Co., Ltd. high resistance powder measurement system software. The compacted sample was prepared by placing 4.0 g of the powder sample in an insulating cylinder with an inner diameter of 20 mm equipped on a Mitsubishi Chemical Analytech Co., Ltd. powder resistance measurement unit (MCP-PD51) and vertically pressing with a load of 20 kN. The compacted sample was in the shape of a disk with a diameter of 20 mm, and the pressure applied by a load of 20 kN was 63.66 MPa. With a load of 20 kN applied to this compacted sample, the electrical resistance was measured by the upper and lower electrodes, and the volume resistivity was obtained by the double ring electrode method.
In the case of the raw material powder, the volume resistivity was determined by the four-terminal method with a load of 20 kN applied to the same compressed powder sample as above, using MCP-PD51, a low resistivity measurement unit (MCP-T610) manufactured by Mitsubishi Chemical Analytech Co., Ltd., and low resistance powder measurement system software manufactured by Mitsubishi Chemical Analytech Co., Ltd.

(Powder Density)
The thickness of the green compact sample used in the above-mentioned volume resistivity measurement was measured, and the green density of the green compact obtained by compacting at 63.66 MPa was calculated from the mass and volume of the green compact sample.

(Average thickness of silicon oxide coating layer)
If the Si content of the silicon oxide-coated soft magnetic powder measured by the above method is A (mass%), the mass proportion B (mass%) of the silicon oxide coating layer is calculated from the Si atomic weight and the _SiO2 molecular weight by the following formula.
B=A× _SiO2 molecular weight/Si atomic weight=A×60.08/28.09
When the density of the silicon oxide coating layer is d (g/ ^cm3 ) and the BET specific surface area of the raw material powder (powder before being coated with silicon oxide) is S ( ^m2 /g), the average film thickness t (nm) of the silicon oxide coating layer is expressed by the following formula:
t=10×B/(d×S)
Here, the value of d can be set to 2.65 (g/cm ³ ). The number 10 on the right side is a unit conversion coefficient.

(Magnetic properties of powder)
Using a high-temperature superconducting VSM (VSM-5HSC-6T manufactured by Toei Kogyo Co., Ltd.), the magnetic properties were measured with an applied magnetic field of 798 kA/m (10 kOe), an M measurement range of 0.002 A·m ² (2 emu), a step bit of 300 bits, a time constant of 0.03 seconds, and a wait time of 0.2 seconds. The coercive force Hc, saturation magnetization σs, squareness ratio SQ, and switching field distribution SFD were obtained from the BH curve.

Example 1
Carbonyl iron powder (BET specific surface area: 0.67 m ² /g, cumulative 50% particle size D ₅₀ on a volume basis measured by a laser diffraction/scattering method: 5.4 μm, green density: 6.3 g/cm ³ ) was prepared as a raw material powder.
(1) Dispersion process 455 g of pure water and 2640 g of isopropyl alcohol (IPA) were charged into a 5000 mL reaction vessel at room temperature and mixed with a stirring blade to prepare a mixed solvent. 1650 g of the raw material powder was added to this mixed solvent, the liquid temperature was adjusted to 40° C., and the mixture was stirred at 380 rpm for 5 minutes to obtain a slurry.
(2) Alkoxide Addition Step 54.0 g of tetraethoxysilane (TEOS: Wako Pure Chemical Industries, Ltd., special grade reagent) dispensed in a small beaker was added all at once to the slurry. The TEOS adhering to the small beaker was washed off with 220 g of IPA and added to the slurry. After the TEOS addition, the temperature of the slurry was maintained at 40° C. and stirring was continued for 5 minutes to cause a reaction between the hydrolysis product of TEOS and the surface of the raw material powder particles.
(3) Hydrolysis catalyst addition step Then, 1135 g of 28 mass % ammonia water was continuously added to the slurry at an addition rate of 12.6 g/min. After the addition of the ammonia water was completed, the mixture was held for 2 hours while stirring to form a silicon oxide coating layer on the surface of the soft magnetic powder. Up to this point, the temperature of the slurry was maintained at 40°C.
(4) Solid-liquid separation step The slurry was then subjected to solid-liquid separation using a pressure filter to recover the solid content.
(5) Drying Step The recovered solid content was vacuum-dried by heating to 100° C. in a chamber evacuated to 1 kPa or less.
The silicon oxide-coated soft magnetic powder was obtained by the above steps. The above-mentioned measurements were carried out on the obtained silicon oxide-coated soft magnetic powder. The results are shown in Table 1.

Examples 2 to 4
The above steps (1) to (5) were carried out using the same raw material powder as in Example 1 to obtain a silicon oxide-coated soft magnetic powder. However, in Examples 2 to 4, the liquid temperature when obtaining a slurry in the dispersion step (1) was changed from 40° C. to 60° C., and the temperature of the slurry was maintained at 60° C. until the completion of the hydrolysis catalyst addition step (3).
In addition, the following conditions were adopted in the drying step (5).
Example 2: Air atmosphere at 100°C Example 3: Nitrogen gas atmosphere at 100°C Example 4: Vacuum drying at 100°C (same conditions as in Example 1)
Except for the changes from Example 1 described above, the experiment was carried out under the same conditions as Example 1. The above-mentioned measurements were carried out on the obtained silicon oxide-coated soft magnetic powder. The results are shown in Table 1.

Comparative Examples 1 and 2
The above steps (1) to (5) were carried out using the same raw material powder as in Example 1 to obtain a silicon oxide-coated soft magnetic powder. However, in Comparative Examples 1 and 2, the following conditions were adopted in the drying step (5).
Comparative Example 1: Air atmosphere at 100° C. Comparative Example 2: Nitrogen gas atmosphere at 100° C. The experiment was carried out under the same conditions as in Example 1, except that the drying conditions were as described above. The above-mentioned measurements were carried out on the obtained silicon oxide-coated soft magnetic powder. The results are shown in Table 1.

Control Example
The raw material powder (carbonyl iron powder described in Example 1) was also subjected to the above-mentioned measurements. The results are shown in Table 1.

The silicon oxide-coated soft magnetic powders of Comparative Examples 1 and 2 had a high BET specific surface area because the slurry temperature during silicon oxide coating was low and the gas atmosphere during the drying process was air (Comparative Example 1) or nitrogen gas (Comparative Example 2).
The silicon oxide-coated soft magnetic powder of Example 1 was dried in a vacuum gas atmosphere, and therefore had a reduced BET specific surface area compared to the comparative example.
In the silicon oxide-coated soft magnetic powders of Examples 2 to 4, the slurry temperature during silicon oxide coating was increased to the range specified by the present invention, and therefore an even lower BET specific surface area than that of Example 1 was achieved regardless of the gas atmosphere in the drying process.
In addition, all of the silicon oxide-coated soft magnetic powders obtained in Comparative Examples 1 and 2 and Examples 1 to 4 had excellent insulation properties, with the volume resistivity of the 63.66 MPa compacted powder sample being 1.0 x 10 ⁸ Ω·cm or more, and the compact density of the 63.66 MPa compacted powder sample was also sufficiently high.

Claims (3)

A soft magnetic powder (excluding those using flat soft magnetic powder ) consisting of particles (excluding those containing a fluorine-containing composition or silicone resin) having a silicon oxide coating layer on the surface of soft magnetic metal particles having an iron content of 20 mass% or more, wherein the silicon oxide coating layer has an average thickness of 0.5 to 30 nm, a BET specific surface area of 1.0 ^m2 /g or less, and a volume-based cumulative 50% particle diameter _D50 measured by a laser diffraction/scattering method of 1.0 to 15.0 μm. 2. The silicon oxide-coated soft magnetic powder according to claim 1, wherein a load of 20 kN is applied to a powder sample having a mass of 4 g in an insulating cylinder having an inner diameter of 20 mm to prepare a disk-shaped compressed powder sample having a diameter of 20 mm, and when the compressed powder sample is subjected to a test for measuring its volume resistivity by a double ring electrode method while a load of 20 kN is applied to the compressed powder sample, the volume resistivity is 1.0 x 10 ⁸ Ω·cm or more. 3. The silicon oxide coated soft magnetic powder according to claim 1, which has a BET specific surface area of 0.65 m ² /g or less.