阅读说明：本技术 复合颗粒和压粉磁芯 (Composite particle and dust core ) 是由 高桥毅 于 2020-08-18 设计创作，主要内容包括：本发明提供一种复合颗粒(100),其具有：软磁性铁基颗粒(10)、设置于软磁性铁基颗粒(10)的表面的包覆层(20)和至少一部分配置于包覆层(20)内的球状的纳米粉体(30)。包覆层(20)是含有Fe、Si、O、B和N的化合物的层,纳米粉体(30)是含有选自Fe、Si、Zr、Co、Al、Mg、Mn和Ni中的至少一种元素、以及O和N的化合物的粉体。(The present invention provides a composite particle (100) having: the soft magnetic iron-based particle comprises a soft magnetic iron-based particle (10), a coating layer (20) provided on the surface of the soft magnetic iron-based particle (10), and spherical nanopowder (30) at least a part of which is disposed in the coating layer (20). The coating layer (20) is a layer containing a compound of Fe, Si, O, B and N, and the nano-powder (30) is a powder containing at least one element selected from the group consisting of Fe, Si, Zr, Co, Al, Mg, Mn and Ni, and a compound of O and N.)

1. A composite particle, comprising:

soft magnetic iron-based particles;

a coating layer provided on the surface of the soft magnetic iron-based particles; and

a spherical nano-powder, at least a part of which is disposed in the coating layer,

in the composite particle, the particle size distribution of the composite particle,

the clad layer is a layer containing a compound of Fe, Si, O, B and N,

the nano powder is a powder containing at least one element selected from Fe, Si, Zr, Co, Al, Mg, Mn and Ni, and a compound of O and N.

2. The composite particle of claim 1, wherein:

the nano powder is powder containing compounds of Fe, Si, O and N.

3. The composite particle of claim 1 or 2, wherein:

the average thickness of the coating layer is 5-100 nm.

4. The composite particle according to any one of claims 1 to 3, wherein:

the oxygen content of the composite particles is 0.3 mass% or less.

5. The composite particle according to any one of claims 1 to 4, wherein:

the nitrogen content of the composite particles is 0.2-0.6 mass%.

6. The composite particle according to any one of claims 1 to 5, wherein:

in an enlarged photograph of the surface of the composite particle, the proportion of the area of the surface of the composite particle to the entire area of the nano powder contained in the area is 1 to 5%.

7. The composite particle according to any one of claims 1 to 6, wherein:

the average particle size of the nano powder is 5-200 nm.

8. The composite particle according to any one of claims 1 to 7, wherein:

the soft magnetic iron-based particles include oxidized regions along the interface with the cladding layer.

9. A dust core comprising the composite particle according to any one of claims 1 to 8.

Technical Field

The invention relates to a composite particle and a dust core.

Background

Soft magnetic materials used for electronic components such as inductors, reactors, choke coils, and noise absorbers are required to have the surfaces of magnetic metal particles coated with a substance having high insulating properties such as phosphate and silica. As a coating film having high insulation properties, for example, patent document 1 discloses an insulating film characterized by containing boron nitride as a main component and containing an oxide.

Patent document 2 discloses that an electrically insulating ferrite layer is formed on the surface of soft magnetic metal particles, and an electrically insulating ferrite particle group is disposed on the ferrite layer.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 6477124

Patent document 2: japanese patent laid-open publication No. 2019-33107

Disclosure of Invention

Problems to be solved by the invention

High density and high electrical insulation between particles are required for the dust core. However, the conventional methods cannot be said to be sufficient in terms of both density and electrical insulation. In general, in the case of a powder magnetic core molded by press molding, if the coated portion is thin, the density of the molded body tends to be high. However, when the coated portion is too thin, insulation breakdown may be caused due to friction between the powders at the time of molding. In addition, when the coated portion is too thick, the density of the molded body is lowered. Therefore, the density of the compact of the dust core and the electric resistance are in a trade-off relationship.

The present invention has been made in view of the above problems, and an object thereof is to provide composite particles that can provide a product having both high density and high electrical insulation properties even when pressure-molded.

Means for solving the problems

The present invention provides a composite particle having: soft magnetic iron-based particles; a coating layer provided on the surface of the soft magnetic iron-based particles; and spherical nano-powder, at least a part of which is disposed in the coating layer.

The coating layer is a layer containing a compound of Fe, Si, O, B and N,

the nano-powder is a powder containing at least one element selected from the group consisting of Fe, Si, Zr, Co, Al, Mg, Mn and Ni, and a compound of O and N.

Here, the nano powder may be a powder containing compounds of Fe, Si, O and N.

In addition, the average thickness of the coating layer can be 5 to 100 nm.

The oxygen content of the composite particles may be 0.3% by mass or less.

The nitrogen content of the composite particles may be 0.2 to 0.6 mass%.

In addition, in an enlarged photograph of the surface of the composite particle, the ratio of the area of the surface of the composite particle to the entire area of the nano-powder contained in the area may be 1 to 5%.

The average particle diameter of the nano powder can be 5 to 200 nm.

The soft magnetic iron-based particles may include an oxidized region along an interface with the clad layer.

The present invention provides a dust core comprising the composite particle described in any one of the above.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide composite particles that can achieve both high density and high electrical insulation properties in the case of press molding.

Drawings

Fig. 1 is a schematic cross-sectional view of a composite particle according to an embodiment of the present invention.

Fig. 2 is a cross-sectional TEM photograph of the vicinity of the surface of the composite particle of example 1.

Description of the symbols

10 … soft magnetic iron-based particles, 20 … coating layer, 30 … nano powder and 100 … composite particles.

Detailed Description

(composite particles)

A composite particle according to an embodiment of the present invention is described with reference to fig. 1.

The composite particle 100 of the present embodiment has: the soft magnetic iron-based particles 10, the coating layer 20 provided on the surface of the soft magnetic iron-based particles 10, and the nano-powder 30 disposed in the coating layer 20.

(Soft magnetic iron-based particles)

The soft magnetic iron-based particles 10 are particles that have soft magnetism and Fe is the element that exhibits the largest atomic fraction among the elements in the particles. The atomic fraction of Fe may be 50 at% or more.

Examples of soft magnetic iron-based materials are pure iron, carbonyl iron (carbonyl iron), Fe-Si alloys, Fe-Al alloys, Fe-N compounds, Fe-Ni alloys, Fe-C compounds, Fe-B compounds, Fe-Co alloys, Fe-Al-Si alloys, Fe-Al-Cr alloys, Fe-Al-Mn alloys, Fe-Al-Ni alloys, Fe-Si-Cr alloys, Fe-Si-Mn alloys, Fe-Si-Ni alloys. The soft magnetic iron-based material may be a crystalline material, an amorphous material, or a nanocrystalline material.

The particle diameter of the soft magnetic iron-based particles 10 is not particularly limited, and may be 1 to 100 μm. The lower limit of the particle size may be 3 μm or 5 μm. The upper limit may be 50 μm or 30 μm.

The particle size of the soft magnetic iron-based particles 10 is the particle size of D50 (average particle size) in the volume-based particle size distribution measured by a laser diffraction particle size distribution measuring apparatus (HELOS or the like).

The roundness of the soft magnetic iron-based particles 10 in the cross section is preferably 0.80 or more. By increasing the circularity, the points of approach of the soft magnetic iron-based particles 10 to each other are reduced, and electrical insulation is easily ensured.

The circularity in this specification is the circularity of Wadell. The roundness is defined as a ratio of the diameter of a circle equal to the area of the particle in the cross section of the particle to the diameter of a circle circumscribing the particle. In the case of perfect circles, Wadell has a roundness of 1, and as the roundness approaches 1, the roundness is higher, and if it is 0.80 or more, it can be regarded as a substantially spherical shape in appearance. Images obtained by an optical microscope, SEM, TEM, or the like can be used for observation of the cross section, and image analysis can be used for calculation of the circularity.

In addition, the soft magnetic iron-based particles 10 may have one or more oxidized regions 10a inside thereof along the interface of the clad layer 20. For example, the oxidized region 10a can have a spherical shape. The diameter of the oxidized region 10a can be 1 to 20 nm. The diameter can be measured in the same manner as the diameter of the nano-powder described later. By including the oxidized region 10a in the soft magnetic iron-based particles 10, the resistivity becomes high, and the eddy current loss in the particles is reduced. When the diameter of the oxidized region 10a is less than 1nm, the effect of reducing eddy current loss becomes low. When the diameter of the oxidized region exceeds 20nm, the magnetic permeability tends to decrease.

(coating layer)

The coating layer 20 coats the surface of the soft magnetic iron-based particles 10. The coating layer 20 preferably covers the entire surface of the soft magnetic iron-based particles 10.

The average thickness of the clad layer 20 can be 1nm or more, preferably 5nm or more. The average thickness of the clad layer 20 can be 100nm or less, preferably 50nm or less. When the coating layer 20 is too thick, the density after press molding tends to decrease, while when it is too thin, the soft magnetic iron-based particles 10 may come into contact with each other during press molding. The average thickness of the clad layer 20 can be an arithmetic average of the thicknesses measured at 10 points arranged at equal intervals along the interface between the clad layer and the soft magnetic iron-based particles in the sectional photograph.

The coating layer is an electrically insulating layer and contains at least compounds of Fe, Si, O, B and N. This increases the adhesion to the soft magnetic iron-based particles 10 and also increases the insulation properties.

The oxygen content of the composite particle 100 may be 0.3 mass% or less. When the amount of oxygen is high, the composite particles tend to be hard, and it is difficult to increase the density.

The nitrogen content of the composite particles 100 can be 0.2 to 0.6 mass%. When the amount of nitrogen is small, the thickness of the coating layer tends to be insufficient, and the electric resistance may be low. When the amount of nitrogen is too large, the coating layer tends to become thick, and it is difficult to increase the density.

(nanopowder)

At least a part of each particle constituting the nano-powder 30 is disposed in the coating layer 20. In other words, the distance R between each particle of the nano-powder 30 and the soft magnetic iron-based particles 10 is smaller than the average thickness D of the coating layer 20. R may be D/2 or less and may be 0. Typically, a part of the particles constituting the nano-powder 30 is disposed in the coating layer 20, and the rest is exposed from the coating layer 20. Some of the particles constituting the nano-powder 30 may be completely embedded in the coating layer 20.

The shape of the nano-powder 30 is spherical. Spherical means that the Wadell roundness of the nanopowder in an electron micrograph (for example, TEM) of the cross section of the composite particle is 0.8 or more. The roundness of Wadell is defined as the ratio of the diameter of a circle equal to the area of a particle to the diameter of a circle circumscribing the particle. The roundness measurement can be performed by taking the arithmetic average of the roundness of about 30 nanoparticles.

The average particle diameter of the nano-powder 30 is 200nm or less. The upper limit of the average particle diameter may be 200nm, or 150nm, preferably 80 nm. The lower limit of the average particle diameter is not particularly limited, and may be 1nm, preferably 5 nm. The particle diameter of the nano powder 30 is an equivalent diameter of an equivalent circle of an equal area in an electron micrograph (for example, TEM) of a cross section of the composite particle, and the average particle diameter can be an arithmetic average of particle diameters of about 30 nano particles.

The average particle diameter d of the nano-powder 30 is preferably larger than the average thickness t of the coating layer 20. d/t may be 1.5 or more, 2 or more, 2.5 or more, or 3 or more.

The nano-powder 30 is a powder containing at least one element selected from Fe, Si, Zr, Co, Al, Mg, Mn, and Ni, and a compound of O and N. Such an oxynitride of a metal and/or semimetal element is preferable because it ensures electrical insulation and toughness. In particular, by containing N, the toughness is improved as compared with an oxide, and contact between soft magnetic particles is easily suppressed even when pressure is applied.

The nano-powder 30 is preferably a powder containing compounds of Fe, Si, O and N. The nano-powder 30 is a compound containing Fe and Si, and thus the magnetic properties such as magnetic permeability can be further improved.

In an enlarged photograph (for example, SEM) of the particle surface of one composite particle 100, the ratio of the total area of the particle areas of the nanopowder 30 observed in the area, which is occupied by the area of the surface of the composite particle 100, may be 1 to 5%.

Here, the surface area of the particles is preferably set to 5 μm²～15μm²。

When the ratio of the above area of the nano powder 30 is too small compared to 1%, there is a tendency that the soft magnetic iron-based particles 10 easily contact each other. When the ratio of the area of the nano powder 30 is too large as compared with 5%, it is difficult to increase the density.

(Effect)

According to the composite particles of the present embodiment, by providing the electrically insulating coating layer 20 with a specific composition, the adhesion between the coating layer 20 and the soft magnetic iron-based particles 10 is improved, and the coating layer 20 easily maintains the insulation between the soft magnetic iron-based particles 10 even if the composite particle group is press-molded to increase the density. Further, since the nano powder 30 is disposed in the coating layer 20, the coating layer 20 is less likely to be peeled off by the nano powder 30 at the time of pressure molding, and since the nano powder contains not only O (oxygen) but also N (nitrogen), not only electrical insulation but also toughness is improved, and contact between the soft magnetic iron-based particles 10 at the time of pressure molding is further suppressed.

(method for producing composite particles)

Next, an example of the method for producing the composite particles will be described.

First, a case where Fe — Si alloy particles are used as soft magnetic iron-based particles will be described. Fe — Si alloy particles and BN (boron nitride) powder were prepared as the soft magnetic iron-based particles 10.

As the Fe — Si alloy particles, for example, particles having high circularity obtained by a gas atomization method are preferably used. The particle size is as described above.

The average particle diameter of the BN powder is preferably 20nm to 4 μm. BN is preferably hexagonal (h-BN). The average particle diameter of the BN powder is D50 of the volume-based particle size distribution measured by a wet laser diffraction scattering method.

Next, the soft magnetic iron-based particles 10 and the BN powder were mixed to obtain a mixed powder. In the mixing, various mixers such as a V-type mixer, a W-type mixer, a drum-type mixer, a high-speed stirrer, a ribbon mixer, a conical screw-type mixer, an FM stirrer, a high-speed flow-type mixer, an air stirrer, a shaking stirrer, an SPEX stirrer, and a mixing shaker can be used as examples.

Next, the mixed powder is subjected to a heat treatment at 800 to 1100 ℃, preferably 900 to 1000 ℃ in a nitrogen atmosphere, thereby forming a coating layer 20 containing Fe, Si, O, B, and N and a nano-powder 30 containing a plurality of Fe, Si, O, and N disposed in the coating layer 20. In addition, the oxidized region 10a may be generated at this time.

The heat treatment time is not limited, and may be 30 minutes to 6 hours.

When the heat treatment temperature is about 400 to 700 ℃, the nano powder is not formed, and a coating layer containing Fe, Si, O, B and N is formed. Therefore, in this case, the composite particles including the coating layer and the spherical nanopowder of the present embodiment can also be obtained by adding the nanopowder containing at least one element selected from Fe, Si, Zr, Co, Al, Mg, Mn and Ni, and O and N to the mixed powder in advance.

When the soft magnetic iron-based particles 10 do not contain Si, the coating layer 20 can be formed by further adding a Si source compound such as tetraethylorthosilicate or a silane coupling agent to the mixed powder.

Then, if necessary, the composite particles are washed with an organic solvent such as alcohol, unreacted BN powder or the like adhering to the composite particles is removed from the composite particles, and the solvent is dried, whereby composite particles for pressure molding can be obtained.

(dust core)

The powder magnetic core (magnetic core) according to the embodiment of the present invention includes the above-described composite particle group. The powder magnetic core may contain a binder for binding the composite particle groups to each other, in addition to the composite particle groups. Examples of the binder are epoxy resin, phenol resin, polyamide resin, silicone resin. In particular, silicone resins are preferred from the viewpoint of heat resistance. These resins can be thermosetting resins. The binder fills the composite particles. The binder may be the resin itself, or at least a part or all of the binder may be a thermal decomposition product of the resin. The mixing ratio of the binder is not particularly limited, and the amount of the binder may be 0.05 to 2.00 mass% with respect to the mass of the composite particle group.

The shape of the powder magnetic core is not particularly limited. For example, the core may be a cylindrical core, a toroidal (annular) core, a cut core (cut core) such as an E-shape or U-shape, or a motor core.

(method of manufacturing dust core)

The method of manufacturing the powder magnetic core will be described by taking a cylindrical magnetic core as an example.

First, a composite particle group and a binder raw material in an amount according to need are mixed. When a solvent is added to the binder raw material, the solvent is preferably dried after mixing.

Next, a lubricant is mixed with the composite particle group including the binder raw material. An example of a lubricant is zinc stearate. The amount of the lubricant is not particularly limited, and may be set to 0.01 to 0.5 mass% with respect to the composite particle group.

Next, a mixed powder containing a lubricant, a binder material, and a composite particle group is filled in a metal mold having a void corresponding to the cylindrical magnetic core, and pressure molding is performed to obtain a powder magnetic core having a desired shape. The inner surface of the metal mold is also preferably coated with a lubricant in advance.

The pressure during the press molding is not particularly limited, and may be 981 to 1570 MPa.

After or during the press molding, heating, curing of the binder material, and/or annealing of the soft magnetic iron-based particles may be performed as necessary.

Examples

(production of composite particles)

(example 1)

A gas atomized Fe-Si (4.5 mass%) alloy powder having a particle size of 5 μm and a BN powder having a particle size of 4 μm were prepared. Mixing Fe-Si alloy powder and BN powder in a weight ratio of 5: 1 to obtain a powder mixture.

Next, the powder mixture was charged into a crucible, and heat treatment was performed at 900 ℃ for 30 minutes in a nitrogen atmosphere to form a coating layer containing compounds of B, O, N, Fe, and Si and a nano-powder containing compounds of O, N, Si, and Fe disposed in the coating layer on the surface of the Fe — Si alloy powder, thereby obtaining composite particles.

The composite particles were supplied with alcohol, and unreacted BN powder was washed off from the composite particles, followed by drying, to obtain the composite particles of example 1.

(example 2)

Composite particles of example 2 were obtained in the same manner as in example 1, except that the weight ratio of the Fe — Si alloy powder to the BN powder was set to 100: 1.

(example 3)

Composite particles of example 3 were obtained in the same manner as in example 1, except that the weight ratio of the Fe — Si alloy powder to the BN powder was set to 1: 1.

Comparative example 1

The same procedure as in example 1 was repeated, except that MgO powder having an amorphous average particle size of 700nm was added in addition to the Fe-Si alloy powder and BN powder of example 1, and the mixture was mixed at a weight ratio of 50: 10: 1, and the heat treatment temperature was 500 ℃. Since the heat treatment temperature is low, a coating layer containing compounds of B, O, N, Fe, and Si is formed on the surface of the Fe — Si alloy powder, but no nanopowder containing compounds of O, N, Si, and Fe is formed, and MgO nanopowder is disposed in the coating layer.

Comparative example 2

The Fe-Si alloy powder of example 1 was simply charged into a crucible without adding BN powder, and heat-treated at 900 ℃ for 30 minutes in a nitrogen atmosphere, whereby a coating layer was not formed on the surface of the Fe-Si alloy powder, and a nano-powder containing compounds of O, N, Si, and Fe was formed on the surface of the Fe-Si alloy powder. Next, an alcohol solution containing 1 wt% phosphoric acid based on the weight of the Fe — Si powder was supplied to the Fe — Si powder having the nano powder on the surface thereof, and then the alcohol was dried to form an iron phosphate coating layer having the nano powder inside on the surface of the Fe — Si alloy powder, thereby obtaining composite particles of comparative example 2.

Comparative example 3

Composite particles of comparative example 3 were obtained in the same manner as in example 1, except that the heat treatment temperature was set to 500 ℃, a coating layer containing a compound of B, O, N, Fe, and Si was formed on the surface of the Fe-Si alloy powder, but no nanopowder containing a compound of O, N, Si, and Fe was formed on the surface of the Fe-Si alloy powder, and after the BN powder was washed off from the Fe-Si alloy powder having the coating layer by alcohol washing, the spherical nanosilica powder was mixed on the surface of the Fe-Si alloy powder, and the spherical nanosilica powder was attached to the coating layer (outer side) of the Fe-Si alloy powder.

(example 4)

A gas atomized Fe-Ni (47.0 mass%) -Si (1.0 mass%) alloy powder having a particle size of 5 μm and a BN powder having a particle size of 4 μm were prepared. Mixing Fe-Ni-Si alloy powder and BN powder in a weight ratio of 5: 1 to obtain a powder mixture.

Next, the powder mixture was charged into a crucible, and heat treatment was performed at 900 ℃ for 30 minutes in a nitrogen atmosphere to form a coating layer containing compounds of B, O, N, Fe, and Si and a nano-powder containing compounds of O, N, Si, and Fe disposed in the coating layer on the surface of the Fe — Ni — Si alloy powder, thereby obtaining composite particles.

The composite particles were supplied with alcohol, and unreacted BN powder was washed off from the composite particles, followed by drying, to obtain composite particles of example 4.

Comparative example 4

The same operation as in example 4 was carried out except that MgO powder having an amorphous average particle size of 700nm was added in addition to the Fe-Ni-Si alloy powder and the BN powder of example 4, and the mixture was mixed at a weight ratio of 50: 10: 1, and the heat treatment temperature was 500 ℃. Since the heat treatment temperature is low, a coating layer containing compounds of B, O, N, Fe, and Si is formed on the surface of the Fe — Ni — Si alloy powder, but no nanopowder containing compounds of O, N, Si, and Fe is formed, and MgO nanopowder is disposed in the coating layer.

Comparative example 5

The Fe-Ni-Si alloy powder of example 4 was added to a crucible without adding BN powder, and heat-treated at 900 ℃ for 30 minutes in a nitrogen atmosphere, whereby a coating layer was not formed on the surface of the Fe-Ni-Si alloy powder, and a nano-powder containing compounds of O, N, Si, and Fe was formed on the surface of the Fe-Ni-Si alloy powder. Next, an alcohol solution containing 1 wt% phosphoric acid based on the weight of the Fe — Ni-Si powder was supplied to the Fe — Ni-Si powder having the nano powder on the surface thereof, and then the alcohol was dried to form an iron phosphate coating layer having the nano powder inside on the surface of the Fe — Ni-Si alloy powder, thereby obtaining composite particles of comparative example 5.

Comparative example 6

Composite particles of comparative example 6 were obtained in the same manner as in example 4, except that the heat treatment temperature was set to 500 ℃, a coating layer containing compounds of B, O, N, Fe, and Si was formed on the surface of the Fe-Ni-Si alloy powder, a nanopowder containing compounds of O, N, Si, and Fe was not formed on the surface of the Fe-Ni-Si alloy powder, BN powder was washed from the Fe-Ni-Si alloy powder having the coating layer by alcohol washing, and then spherical nanosilica powder was mixed on the surface of the Fe-Ni-Si alloy powder, and the spherical nanosilica powder was attached to (outside) the coating layer of the Fe-Ni-Si alloy powder.

(sample preparation)

To the composite particles of examples 1 to 3 and comparative examples 1 to 3, 1 wt% of a silicone resin was added, mixed and dried. To the dry matter was added 0.05 wt% of a lubricating material (zinc stearate) and further mixed. A molding was prepared by filling 1g of the final mixture in a metal mold having an inner diameter of 8mm and pressing the mixture at a molding pressure of 1570 MPa. Then, the molded body was heat-treated at 900 ℃ for 30 minutes in a nitrogen atmosphere to obtain a cylindrical magnetic core having a diameter of 8 mm. The density and resistivity of the cylindrical magnetic core were measured as follows.

To the composite particles of example 4 and comparative examples 4 to 6, 1 wt% of a silicone resin was added, mixed and dried. To the dry matter was added 0.05 wt% of a lubricating material (zinc stearate) and further mixed. A molding was prepared by filling 1g of the final mixture in a metal mold having an inner diameter of 8mm and pressing the mixture at 1180 MPa. Then, the molded body was heat-treated at 600 ℃ for 30 minutes in a nitrogen atmosphere to obtain a cylindrical magnetic core having a diameter of 8 mm. The apparent density and resistivity of the cylindrical magnetic core were measured as follows.

Further, the apparent density of the dust core varies not only depending on the filling ratio but also depending on the compositions of the metal and semimetal elements in the composite particles, and therefore, it is difficult to compare the performance by the apparent density of the dust core in the case where the soft magnetic iron-based material is different or the like. Therefore, the following filling ratio is adopted as an index relating to the density without changing the composition of the metal and semimetal elements.

The filling ratio (a (%)) is calculated based on the ratio (a ═ C/B x 100) of the apparent density (C) of the dust core to the true density (B) calculated from the composition of the metal and semimetal elements contained in the dust core.

The true density (B) is calculated from the sum of the product of the mass ratio of the metal and semimetal elements contained in the dust core and the density of each element. For example, in the above examples and comparative examples, the calculation is as follows.

In the case of the Fe — Si (4.5 mass%) alloy particles of examples 1 to 3 and comparative examples 1 to 3, (7.87 × (100.0 to 4.5) +2.33 × 4.5)/100 ═ 7.62g/cm³。

In the case of the Fe — Ni (47.0 mass%) -Si (1.0 mass%) alloy particles of example 4 and comparative examples 4 to 6, the ratio was (7.87 × (100.0-47.0-1.0) +8.90 × 47.0+2.33 × 1.0)/100 ═ 8.30g/cm³。

The density of the cylindrical magnetic core (dust core) was calculated by measuring the weight and the size. For the resistivity, both surfaces of a cylindrical magnetic core having a diameter of 8mm were mirror-polished, and a resistance value of 0.05V between both surfaces was measured by an IR meter (R8340) made by ADVANYEST. Then, the resistivity was calculated from the thickness and the resistance value of the sample. When the soft magnetic iron-based particles are Fe-Si alloy particles, the resistivity is preferably 2.5 k.OMEGA.m or more, and the density is preferably 6.7g/cm³(filling ratio of 88% or more). When the soft magnetic iron-based particles are Fe-Ni-Si alloy particles, the particles preferably have a resistivity of 2.5 k.OMEGA.m or more and a density of 7.3g/cm³Above (filling rate of 88% or more). That is, the resistivity is preferably 2.5k Ω · m or more, and the filling ratio is preferably 88% or more.

The conditions and results are shown in table 1. The amount of oxygen and the amount of nitrogen contained in the composite particles were confirmed by an oxygen and nitrogen analyzer (TC600) manufactured by LECO corporation using a pulse heating melting extraction method. Using a graphite crucible containing Sn and graphite powder, a sample was accurately weighed in a Ni capsule at a bit number of about 100mg to 0.1mg, and the obtained sample was used as a measurement sample.

The composition and film thickness of the coating layer, and the composition, position, shape, and particle diameter of the nanoparticles were confirmed by performing STEM analysis of the cross section. After filling the composite particle resin, a sheet sample was prepared using FIB (Nova200i) manufactured by FEI corporation of Japan, and observed using STEM-EDS (JEM2100FCS) manufactured by JEOL corporation. The composition of the coating and nanoparticles was confirmed by both line and point analysis of STEM-EDS. The thickness of the coating layer was measured at 10 points, and the average value was obtained. The shape and particle size of the nanoparticles were calculated by measuring the circularity and average particle size of TEM images using QMP (ver.2.0.1) as free software.

The ratio of the area of the nano powder to the area of the composite particle was calculated as follows: the surface of the composite particle was observed by FE-SEM (SU5000) made by Hitachi High-Tech, and the SEM image (50k times) was analyzed by QMP (ver.2.0.1) as free software.

The conditions and results are shown in tables 1 and 2. The term "inside the coating layer" means that at least a part of the particles of the nano-powder are arranged inside the coating layer, and the term "outside the coating layer" means that the particles of the nano-powder are completely arranged outside the coating layer.

[ TABLE 1 ]

[ TABLE 2 ]

Fig. 2 shows a TEM cross-sectional view of the vicinity of the surface of the composite particle of example 1.

In examples 1 to 4, a ring core having both high density and high electric resistance was obtained. On the other hand, in comparative examples 1 and 4, the nano powder is amorphous MgO, and therefore, the electric resistance is lowered. In comparative examples 2 and 5, the iron phosphate-based coating layer was easily peeled off from the Fe — Si alloy powder or the Fe — Ni — Si alloy powder, and both the resistance and the density were reduced. In comparative examples 3 and 6, the density was not sufficiently increased because the nano-powder was outside the coating layer.