JP2001135515A - Dust core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize dust core excellent in DC superposing characteristic in a high magnetic field. SOLUTION: This dust core contains ferromagnetic metal powder and insulating material. Particles constituting the ferromagnetic metal powder contain spherical particles and different shape particles. Difference between the mean maximum width and the mean minimum width of gaps existing between the particles is at most 10 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dust core used for inductors and other electronic components.

[0002]

2. Description of the Related Art In recent years, miniaturization of electric and electronic devices has progressed, and as a result, a compact and highly efficient powder magnetic core has been required. Ferrite powder or ferromagnetic metal powder is used for the dust core. The ferromagnetic metal powder has a higher saturation magnetic flux density than the ferrite powder, so that the magnetic core can be miniaturized. However, since the electric resistance is low, the eddy current loss of the magnetic core increases. Therefore, an insulating layer is usually provided on the surface of the ferromagnetic metal particles in the dust core.

In order to reduce the size of the magnetic core, it is important to have not only a saturation magnetic flux density but also excellent DC superimposition characteristics (effective magnetic permeability when a DC magnetic field is applied), particularly excellent DC superposition characteristics under a high magnetic field. The reason why good DC bias characteristics under a high magnetic field is advantageous for miniaturization will be described below. When the magnetic core is miniaturized, the magnetic path length becomes shorter. Operating magnetic field (unit
A / m) is the current (A) divided by the magnetic path length (m).
When the magnetic path length becomes shorter, the operating magnetic field shifts to a higher magnetic field side.
Therefore, in order to obtain the same inductance as before downsizing when the magnetic core is downsized and the magnetic path length is shortened,
It is necessary that DC superposition characteristics under a high magnetic field be good.

[0004] In addition, it is considered that the necessity of an inductor corresponding to a large current is expected to increase in the future. However, even when a large current flows, the operating magnetic field strength also increases, so that the DC superposition characteristics under a high magnetic field are good. Then, it is possible to prevent a decrease in inductance when a large current flows.

In addition, if the DC superposition characteristics under a high magnetic field are good, that is, if the effective magnetic permeability does not suddenly decrease even if the magnetic field strength increases, the winding density of the inductor can be increased. Becomes Since the inductance of the inductor is proportional to the square of the number of windings, if the size of the inductor is reduced and the same number of windings is secured by increasing the winding density, a decrease in inductance due to the downsizing can be suppressed. .

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to realize a dust core excellent in DC superposition characteristics under a high magnetic field.

[0007]

This and other objects are achieved by the present invention which is defined below as (1) to (4). (1) A dust core containing a ferromagnetic metal powder and an insulating material, wherein the particles constituting the ferromagnetic metal powder include spherical particles and irregular particles, and the average voids existing between the particles. A dust core in which the difference between the large and the average minimum width is 10 μm or less. (2) The dust core according to (1), wherein the average value of the average maximum width and the minimum average width is 8 to 15 μm. (3) Formula I Circularity = 4πS / L ² (In the above Formula I, S is the area of the projected image of the particle,
L is the contour length of the projected image), the circularity of the spherical particles is 0.70 or more,
The dust core according to the above (1) or (2), wherein the irregular particles have a circularity of 0.60 or less. (4) The dust core according to (3), wherein in the ferromagnetic metal powder, the number of the spherical particles is 30% or more of the whole, and the number of the irregular particles is 20% or more of the whole.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION A dust core according to the present invention contains a ferromagnetic metal powder and also contains an insulating material for electrically insulating particles constituting the ferromagnetic metal powder. Further, when producing the dust core of the present invention, a lubricant is usually added to enhance lubricity between particles during molding or to improve releasability from a mold. You. Hereinafter, the dust core of the present invention will be described in detail.

Ferromagnetic Metal Powder In a dust core manufactured by compression-molding powder composed of ferromagnetic metal particles, voids exist between the particles. In the present invention, the difference between the average maximum width and the average minimum width of voids between particles is defined as 0.
10 to 10 μm, preferably 0 to 9 μm. As described above, by reducing the distribution of the width of the voids existing between the particles, the DC superimposition characteristics under a high magnetic field can be improved.

The average maximum width and the average minimum width of the voids existing between the particles are determined by using a micrograph of the pressed surface of the dust core (the surface to which pressure was applied during compression molding). FIG.
Fig. 2 schematically shows a micrograph of the pressed surface. As shown in the figure, the maximum width and the minimum width are measured for the gap between adjacent particles. Next, a frequency distribution table is created for each of the maximum width and the minimum width, and the class value having the highest frequency is defined as an average maximum width and an average minimum width, respectively. The width of the class in the frequency distribution table is 3 μm or less.

The average value of the average maximum width and the minimum average width (average value of the gap width) is preferably 8 to 15 μm. If this average value is too small, the voids between the particles are not sufficiently secured, so that the electrical resistance becomes small,
As a result, eddy current loss increases. On the other hand, if the average value is too large, a gap larger than the required gap exists between the particles, so that the magnetic permeability is reduced.

It is not necessary to measure the gap width for all the gaps. The number of voids to be measured is preferably 50 or more, more preferably 100 or more.

In the present invention, in order to regulate the width of the gap existing between the particles as described above, spherical particles and irregular particles are contained in the dust core. By mixing these two types of particles in the dust core, the distribution of the gap width can be easily narrowed as described above. If spherical particles and irregular particles are mixed in a dust core, the density of the core increases, and the magnetic permeability under a low magnetic field increases. In general, when the magnetic permeability under a low magnetic field increases, the inclination of the minor loop under a high magnetic field generally decreases, so that the magnetic permeability under a high magnetic field significantly decreases. However, in the present invention, the distribution of the width of the voids existing between the particles is reduced, and as a result, the dependence of the magnetic permeability on the magnetic field strength is reduced. Superimposition characteristics are obtained.

The spherical particles preferably have a circularity of 0.70 or more, and the irregular particles preferably have a circularity of 0.60 or less. The circularity in the present invention is defined by the formula I: circularity = 4πS / L ² . In the above formula I, S is the area of the projected image of the particle, and L is the contour length (perimeter) of the projected image. The projection image is a two-dimensional image obtained by projecting a three-dimensional particle onto a plane. In the present invention, S and L are determined by using a particle image appearing in a micrograph of the pressed surface as the projected image.

The projected shape of a particle having a small circularity is an irregular shape having many projections on the contour, while the projected shape of the particle having a large circularity has a smooth contour such as a circle, an ellipse, or an array. Shape.

The number of spherical particles is 30% or more of the whole particles,
It is preferably at least 40%, and the number of irregular shaped particles is at least 20%, preferably at least 30%, of the whole particles.

The average particle size of the ferromagnetic metal powder before compression molding is preferably 20 to 150 μm, more preferably 25 to 80 μm. If the average particle size is too small, the coercive force increases, and if the average particle size is too large, the eddy current loss increases. The ferromagnetic metal powder having an average particle diameter in the above range may be obtained by classification using a sieve or the like.

If the degree of circularity and the abundance ratio of each of the spherical particles and the irregular particles in the dust core are selected from the above ranges, the distribution of the void width can be easily set within the above ranges. If the average particle size of the ferromagnetic metal powder before compression molding is selected from the above range, the average gap width can be easily set in the above range.

The ferromagnetic metal powder in the dust core is preferably composed of only the above-mentioned spherical particles and irregular-shaped particles, but may contain particles having the above-mentioned circularity intermediate between the two particles. . However, the number of such particles with respect to the whole particles is preferably 30% or less, more preferably 20% or less.

It is not necessary to measure the circularity and the abundance ratio of the particles for all the particles appearing on the pressed surface. The number of particles to be measured is preferably 50 or more,
More preferably, it is 100 or more.

The kind of the metal (simple or alloy) constituting the ferromagnetic metal powder is not particularly limited. For example, iron, sendust (Fe-Al-Si), iron silicide, permalloy (F
e-Ni), supermalloy (Fe-Ni-Mo), iron nitride, iron-aluminum alloy, iron-cobalt alloy, phosphorous iron and the like may be appropriately selected and used according to the purpose from known magnetic metal material powders. For example, it is preferable to use iron powder having high saturation magnetization in order to use a dust core that replaces a core for a relatively low frequency region currently manufactured using a laminated silicon steel sheet. The method for producing the iron powder may be any of an atomizing method, an electrolytic method, and a method of mechanically pulverizing electrolytic iron.

In the present invention, if spherical particles and irregular particles are present in the dust core and the distribution of the gap width is within the above range, the effect of improving the DC superimposition characteristic is realized. Also,
Ferromagnetic metal particles are generally deformed to varying degrees by compression molding. Therefore, in the present invention, the spherical particles and the irregular particles need not be present in the ferromagnetic metal powder before compression molding. For example, a mixture of particles having a high degree of circularity made of a material that is easily deformed and particles having a high degree of circularity made of a material that is hardly deformed may be used, and the particles that are easily deformed may be deformed by compression molding to form irregular-shaped particles. . However, for example, as shown in FIG. 5, it is preferable to use only one kind of particle material because the flatness of the DC superimposition characteristic can be improved and the magnetic permeability at a high frequency is easily increased.

Insulating Material The insulating material used in the present invention is not particularly limited, and at least one kind may be appropriately selected from various inorganic materials and organic materials. Specifically, water glass, a phenol resin, a silicone resin, an epoxy resin, metal oxide particles, and the like may be selected. Preferably, a resin, particularly, a phenol resin and / or a silicone resin is used. In addition, metal oxide particles are used.

The phenolic resin is synthesized by reacting a phenol with an aldehyde. The one that uses a base catalyst during synthesis is a Resol resin, and the one that uses an acid catalyst is Novolak.
Mold resin. The resol type resin is cured by heating or an acid catalyst, and becomes insoluble and infusible. The novolak-type resin is a fusible resin that does not thermoset by itself and is cured by heating with a crosslinking agent such as hexamethylenetetramine. It is preferable to use a resole type resin as the phenol resin. Among the resole resins, those containing N in the form of a tertiary amine are particularly preferred because of their good heat resistance. On the other hand, when a novolak-type resin is used, the strength of the molded body is weakened, so that handling in the steps after molding becomes difficult. When a novolak resin is used, it is preferable to perform molding (hot pressing or the like) while applying a temperature. In this case, the temperature during molding is usually about 150 to 400 ° C.
The novolak type preferably contains a crosslinking agent.

The raw materials for synthesizing the phenol resin include:
As phenols, for example, at least one of phenol, cresols, xylenols, bisphenol A, resorcin, etc. may be used, and as aldehydes, for example, at least one of formaldehyde, paraformaldehyde, acetaldehyde, benzaldehyde and the like are used. I just need.

The weight average molecular weight of the phenol resin is preferably from 300 to 7000, more preferably from 500 to 70.
00, more preferably 500-6000. The smaller the weight-average molecular weight, the greater the strength of the molded body, and the less the powder drops off at the edge of the molded body. However, if the weight-average molecular weight is less than 300, the amount of reduction in the resin during annealing at a high temperature increases, so that the insulation between the ferromagnetic metal particles in the dust core cannot be maintained.

A commercially available phenol resin can be used. For example, BRS-3 manufactured by Showa Polymer Co., Ltd.
801, ELS-572, 577, 579, 580, 5
82, 583 (above, resol type), BRP-5417
(Novolak type) or the like can be used.

As the silicone resin, those having a weight average molecular weight of about 700 to 3300 are preferred.

The amount of the resin used as the insulating material is preferably 1 to 30% by volume, more preferably 2 to 20% by volume, based on the ferromagnetic metal powder. If the amount of the resin is too small, the mechanical strength of the magnetic core is reduced or insulation failure occurs. On the other hand, if the amount of the resin is too large, the ratio of the non-magnetic component in the dust core increases, and the magnetic permeability and the magnetic flux density of the core decrease.

The dust core of the present invention is preferably annealed after molding. Some or all of the resin added as an insulating material may be carbonized after annealing.

When mixing the insulating resin and the ferromagnetic metal powder, a solid or liquid resin may be made into a solution and mixed, or a liquid resin may be directly mixed. The viscosity of the liquid resin is preferably 10 to 10,000 at 25 ° C.
CPS, more preferably 50-9000 CPS. If the viscosity is too low or too high, it becomes difficult to form a uniform coating on the surface of the ferromagnetic metal particles.

The insulating resin also functions as a binder and improves the mechanical strength of the magnetic core.

When metal oxide particles are used as the insulating material, it is preferable to use titanium oxide sol and / or zirconium oxide sol. Titanium oxide sol and zirconium oxide sol are amorphous titanium oxide particles and zirconium oxide particles that are negatively charged and are dispersed in water or an organic dispersion medium to form a colloid. TiOH groups and -ZrOH groups are present. Like titanium oxide sol and zirconium oxide sol,
By adding a sol in which fine particles are uniformly dispersed in a solvent to a ferromagnetic metal powder, a uniform insulating film can be formed with a small amount, so that a high magnetic flux density and a high insulating property can be realized.

The average particle diameter of the titanium oxide particles and zirconium oxide particles contained in the sol is preferably from 10 to 100.
nm, more preferably 10 to 80 nm, even more preferably 2 nm.
0 to 70 nm. The content of particles in the sol is 15
It is preferably about 40% by mass.

A titanium oxide sol for a ferromagnetic metal powder,
The added amount of zirconium oxide sol in terms of solid content, that is, the total added amount of titanium oxide particles and zirconium oxide particles is preferably 15% by volume or less, more preferably 5.0% by volume or less. If this total addition amount is too large, the nonmagnetic content in the dust core increases, so that the magnetic permeability and the magnetic flux density decrease. In order to sufficiently exhibit the effect of adding these sols, the total amount is preferably 0.1% by volume or more, more preferably 0.2% by volume or more, and still more preferably 0.1% by volume or more. 5% by volume or more.

The titanium oxide sol and the zirconium oxide sol may be used alone or in combination. When used in combination, the ratio is arbitrary.

These sols are commercially available products (Nissan Chemical Industries, Ltd., NZS-20A, NZS-30A, NZS-30).
B etc.] can be used. When the available sol has a low pH value, it is preferable to adjust the pH value to about 7. If the pH value is low, the ferromagnetic metal powder is oxidized and non-magnetic oxides increase, and the magnetic permeability and the magnetic flux density may decrease, or the coercive force may deteriorate.

These sols include those using an aqueous solvent and those using a non-aqueous solvent, and those using a solvent compatible with the resin used in combination are preferable. In particular, ethanol, butanol, toluene, Those using a non-aqueous solvent such as xylene are preferred. When the available sol uses an aqueous solvent, the solvent may be replaced as needed.

The sol may contain chlorine ions, ammonia and the like as a stabilizer.

These sols are usually in the form of a milky white colloid.

Lubricant A lubricant is added at the time of molding to enhance the lubricity between particles and to improve the releasability from a mold. As the lubricant, it is preferable to use at least one selected from magnesium stearate, calcium stearate, strontium stearate and barium stearate. Of these, strontium stearate is most preferred.

The content of these divalent metal stearates is preferably 0.2 to 1.5 with respect to the ferromagnetic metal powder.
%, More preferably 0.2 to 1.0% by mass.
If the content is too small, the insulation between the ferromagnetic metal particles in the dust core becomes insufficient, and problems such as the magnetic core becoming difficult to remove from the mold during molding are likely to occur. On the other hand, if the content is too large, the nonmagnetic content in the dust core increases, so that the magnetic permeability and the magnetic flux density decrease and the strength of the core tends to be insufficient.

As the lubricant, other divalent metal salts of higher fatty acids, especially divalent metal salts of lauric acid, may be used in addition to the divalent metal stearate. However, it is preferable that the amount used does not exceed 30% by mass of the amount of the divalent metal stearate used.

Manufacturing Method of Dust Core At the time of manufacturing a dust core, first, a ferromagnetic metal powder and an insulating material are mixed.

When iron powder is used as the ferromagnetic metal powder,
Before mixing, it is preferable to subject the iron powder to a heat treatment (annealing) for strain relief. Before mixing, the iron powder may be subjected to an oxidation treatment. If a thin oxide film having a thickness of about several tens of nanometers is formed in the vicinity of the surface of the iron particles by this oxidation treatment, improvement in insulating properties can be expected. This oxidation treatment may be performed by heating at 150 to 300 ° C. for about 0.1 to 2 hours in an oxidizing atmosphere such as air. When the oxidation treatment is performed, a dispersant such as ethyl cellulose may be mixed in order to improve the wettability of the iron particle surface.

The mixing conditions are not particularly limited. For example, a pressure kneader, a raikai machine or the like is used at room temperature for about 20 to 60 hours.
Mix for a minute. After mixing, preferably 100 to 30
Dry at about 0 ° C. for 20-60 minutes.

After drying, a lubricant is added.

In the molding step, compression molding is performed to obtain a desired magnetic core shape. The shape of the magnetic core is not particularly limited, and so-called toroidal type, E type, I type, F type, C type, EE type, EI
Type, ER type, EPC type, pot type, drum type, pot type,
Any of a cup type or the like may be used.

The molding conditions are not particularly limited, and may be appropriately determined according to the type, shape and size of the ferromagnetic metal particles, the desired magnetic core shape and size, the magnetic core density, and the like. The pressure is maintained at about 2000 MPa and the maximum pressure for about 0.1 second to 1 minute.

After molding, annealing is performed to improve the magnetic properties of the magnetic core. This annealing is for releasing the stress generated in the ferromagnetic metal particles during powdering or molding. When the particles are mechanically flattened, the stress due to the flattening can be released. In addition, the annealing hardens the insulating resin and improves the mechanical strength of the green compact.

The annealing conditions may be appropriately determined according to the type of ferromagnetic metal powder, molding conditions, flattening conditions, etc., and the treatment temperature is preferably 500 to 900 ° C., more preferably 600 to 850 ° C. It is. If the processing temperature is too low, the release of the stress becomes insufficient and the return to the original coercive force becomes insufficient, so that the direct current superposition characteristic is poor and the hysteresis loss is increased. On the other hand, if the processing temperature is too high, the insulation film is thermally destroyed and insulation becomes insufficient, so that eddy current loss increases,
Since the frequency characteristic of the magnetic permeability also deteriorates, the direct current superposition characteristic also deteriorates. The processing time, that is, the time for passing through the above temperature range or the time for maintaining the temperature at a constant temperature within the above temperature range is preferably 10 minutes to 2 hours.
If the treatment time is too short, the annealing effect tends to be insufficient,
If it is too long, dielectric breakdown tends to occur.

Annealing is preferably performed in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen in order to prevent a decrease in magnetic permeability and magnetic flux density due to oxidation of the ferromagnetic metal powder.

After the heat treatment, the magnetic core may be impregnated with a resin or the like, if necessary. By impregnating the resin, the strength is further improved. Examples of the resin used for the impregnation include a phenol resin, an epoxy resin, a silicone resin, and an acrylic resin. Among them, a phenol resin is preferable. These resins may be used by dissolving them in a solvent such as ethanol, acetone, toluene and pyrrolidone.

As a method of impregnating the magnetic core with the resin, the magnetic core is placed on a container such as a vat, and a mixed solution of a resin and a solvent (for example, a 10% ethanol solution of phenol resin) is poured into the container. Be completely hidden.
After holding for about 1 to 30 minutes as it is, the magnetic core is taken out, the resin solution attached to the surroundings is removed to some extent, and then heat treatment is performed. In this heat treatment, first,
Using an oven or the like, the temperature is raised to about 80 to 120 ° C. in the air atmosphere and maintained for about 1 to 2 hours. In addition, 13
The temperature is raised to about 0 to 170 ° C. and maintained for about 1.5 to 3 hours.
Hold for about an hour.

After the heat treatment, if necessary, an insulating film is formed on the surface of the magnetic core in order to secure insulation between the winding and the core. Do.

The powder magnetic core of the present invention is suitable for magnetic cores such as transformers and inductors, magnetic cores for motors, and other electromagnetic components. It can also be used for choke coils and airbag sensors in electric vehicles. The frequency used is preferably 10 Hz to 500 kHz, more preferably 500 Hz to 20 kHz.
It is 0 kHz.

[0057]

EXAMPLE A dust core sample was prepared by the following procedure.

Ferromagnetic metal powder: a first permalloy powder composed of particles having a relatively high circularity (average particle size: 30 μm)
And a second permalloy powder (average particle size: 28 μm) composed of particles having a relatively low circularity mixed at a weight ratio of 1: 1. Zirconia sol: ZrO ₂ sol (Nissan Chemical Industries, Ltd.) (NZ
S-30A, an average particle diameter of 62 nm) was adjusted to pH 7, and then a dispersion was obtained by replacing the aqueous solvent with an ethanol solvent. Phenol resin: resole type resin [ELS-582 manufactured by Showa Polymer Co., Ltd., weight average molecular weight 1500] , Lubricant: Strontium stearate (Sakai Chemical Co., Ltd.) was prepared.

Next, zirconia sol of 2.0% by volume in solid content and phenol resin of 7.1% by volume were added to the ferromagnetic metal powder, and these were mixed by a pressure kneader at room temperature for 30 minutes. did. Then, in the atmosphere 2
Dry at 50 ° C. for 30 minutes. To the mixture after drying, a lubricant of 0.6% by mass based on the ferromagnetic metal powder was added and mixed for 15 minutes by a V mixer. Then, at a pressure of 1176 MPa, an outer diameter of 17.5 mm, an inner diameter of 10.2 mm, and a height of It was formed into a toroidal shape of about 6 mm. After the molding, annealing was performed at 775 ° C. for 30 minutes in an N ₂ atmosphere to obtain a dust core sample of the example.

Also, a comparative sample was prepared in the same manner as the example sample except that only the first permalloy powder was used as the ferromagnetic metal powder.

A plurality of scanning electron microscope photographs of the pressed surface of each sample were taken. From these photos,
The above-described distribution of the gap width and the average gap width were obtained. The number of voids measured was 100. FIG. 1 shows an example of a photograph of an example sample, and FIG. 2 shows an example of a photograph of a comparative sample.

FIG. 3A shows a histogram of the maximum width of the voids in the sample of the embodiment, and FIG. 3B shows a histogram of the minimum width of the voids. Further, FIG. 4A shows a histogram of the maximum width of the voids in the comparative example sample, and FIG. 4B shows a histogram of the minimum width of the voids. When the difference between the average maximum width and the average minimum width is obtained from these histograms, the difference is 15-6 = 9 (μm) in the example sample and 18-6 = 12 (μm) in the comparative example sample. Also,
The average value of the gap width is 10.5 μm for the example sample and 12 μm for the comparative example sample.

From the above photographs, the circularity of the particles that appeared on the pressed surface of the sample was measured. The number of particles measured is 1
The number was set to 20. As a result of this measurement, in the example sample,
The number of spherical particles having a circularity of 0.70 or more is 4
The number of irregular particles having a circularity of 8% and a circularity of 0.60 or less was 35% of the measured number. In contrast, in the comparative sample, the number of spherical particles having a circularity of 0.70 or more was 63% of the measured number, and the number of irregular particles having a circularity of 0.60 or less was 9% of the measured number.

Next, for each sample, the effective magnetic permeability (μ _DC ) at a frequency of 100 kHz when a DC superimposed magnetic field having the strength shown in FIG. 5 was applied was measured. The effective magnetic permeability was calculated from the inductance measured with an LCR meter [HP4284A, manufactured by Yokogawa Hewlett-Packard Co., Ltd.]. FIG. 5 shows the results.

FIG. 5 shows that the sample of the example has a higher effective magnetic permeability under a high magnetic field than the sample of the comparative example.

[0066]

According to the present invention, the DC superposition characteristics of the dust core under a high magnetic field can be improved, so that the dust core can be downsized.

[Brief description of the drawings]

FIG. 1 is a drawing substitute photograph showing a particle structure, and is a scanning electron micrograph of a dust core of the present invention.

FIG. 2 is a drawing substitute photograph showing a particle structure, and is a scanning electron microscope photograph of a conventional dust core.

FIG. 3A is a histogram showing a distribution of a maximum width of a void existing between particles in an example sample;
(B) is a histogram showing the distribution of the minimum width of the gap.

FIG. 4A is a histogram showing the distribution of the maximum width of voids existing between particles in a comparative sample.
(B) is a histogram showing the distribution of the minimum width of the gap.

FIG. 5 is a graph showing a relationship between a DC magnetic field intensity and an effective magnetic permeability.

FIG. 6 is a diagram schematically showing a maximum width and a minimum width of a void existing between particles.

Claims (4)

[Claims] 1. A powder magnetic core containing a ferromagnetic metal powder and an insulating material, wherein the particles constituting the ferromagnetic metal powder include spherical particles and irregular particles, and are used for forming voids existing between the particles. A dust core in which the difference between the average maximum width and the average minimum width is 10 μm or less. 2. The dust core according to claim 1, wherein an average value of the average maximum width and the minimum average width is 8 to 15 μm. 3. The formula I: circularity = 4πS / L ² (wherein S is the area of a projected image of a particle,
L is the contour length of the projected image), the circularity of the spherical particles is 0.70 or more,
The circularity of the irregular shaped particles is 0.60 or less.
Or 2 dust cores. 4. The dust core according to claim 3, wherein in the ferromagnetic metal powder, the number of the spherical particles is 30% or more of the whole, and the number of the irregular particles is 20% or more of the whole.