JP2023069772A - Powder magnetic core - Google Patents

Abstract

To provide a powder magnetic core which becomes a high magnetic permeability, a low iron loss, and a high volume resistance.SOLUTION: A powder magnetic core includes: a soft magnetic particle; and an isolation layer formed in a front face of the soft magnetic particle. The soft magnetic particle contains Fe and an element which becomes easier oxidation than Fe. A 50% particle size (D50) in the case of being measured in a volume reference is 10 to 40 μm, a ratio D10/D50 of 10% particle sizes (D10) and (D50) is 0.3 to 0.7, a ratio D90/D50 of 90% particle sizes (D90) and D50 is 1.5 to 2.5, and a density is 5.8 g/cm3 or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to dust cores.

A powder magnetic core is manufactured by coating the surface of soft magnetic powder with an insulating film and compressing the soft magnetic powder with the insulating film. Typical applications of dust cores include DC-DC converters, inverters, transformers used in switching power supplies, and choke coils for noise reduction.

Among transformers, inductors (chip inductors) mounted on a substrate of a power supply circuit, in particular, are often used in a high frequency range of several 100 kHz to several MHz. Therefore, the powder magnetic core is also required to have a material composition suitable for use in a high frequency range. The higher the frequency, the greater the loss (iron loss) that is absorbed by the dust core and turned into heat. Since most of this loss is caused by eddy current loss, how to reduce eddy current loss is an important issue in studying the material and composition of the powder magnetic core. In addition, high volume resistivity and high magnetic permeability are required for powder magnetic cores for chip inductors. In addition, the required level of withstand voltage is also increasing.

In order to suppress eddy current loss, it is necessary to appropriately select the soft magnetic powder that constitutes the dust core. As described in Non-Patent Document 1, it is known that the eddy current loss is proportional to the square of the particle diameter of the soft magnetic powder and inversely proportional to the volume resistivity. Therefore, in order to suppress the eddy current loss, it is preferable to select a soft magnetic powder having a small particle size and a high volume resistivity. A soft magnetic powder having a high volume resistivity is synonymous with a soft magnetic powder containing a large amount of an alloy component.

On the other hand, if soft magnetic powder with a large amount of alloy components and a small particle size is used to produce a powder magnetic core, there is a problem that the magnetic permeability, which is another important index, is lowered. Although it depends on the conditions of use, soft magnetic powder for power supply system chip inductors generally has an alloy component of 10 mass% or less at most and an average particle size of 10 μm or more at least. With the intention of increasing the magnetic permeability of the powder magnetic core, a high magnetic permeability material such as amorphous or nanocrystal may be applied.

By the way, as in Patent Documents 1 and 2, a method is known in which a powder magnetic core made of soft magnetic powder is magnetically annealed in an oxidizing atmosphere to form an insulating coating around the soft magnetic powder, thereby improving frequency characteristics. . In this case, the composition of the soft magnetic powder must contain Fe and an element that is more easily oxidized than Fe, and FeSiCr and FeAlCr are commonly used. Further, as in Patent Document 3, a method of heat-treating FeNi soft magnetic powder to provide a high-resistance layer at the grain boundary is also known. As a result, a homogeneous insulating coating can be obtained as in Patent Documents 1 and 2, and a powder magnetic core excellent in iron loss and volume resistivity can be obtained.

In some cases, by limiting the particle size distribution, it is possible to cope with high frequencies. For example, as shown in Patent Document 4, D ₅₀ (particle size at cumulative 50% of cumulative particle size distribution) is 5 μm or less, and D ₉₀ (cumulative particle size distribution) for D ₁₀ (particle size at cumulative 10% of cumulative particle size distribution) The particle size distribution of the soft magnetic powder is limited so that the ratio D ₉₀ /D ₁₀ of the cumulative 90% particle size of the soft magnetic powder is 19 or less. By combining this with the hardness range of the soft magnetic powder, good magnetic properties can be obtained in the high frequency range. Furthermore, as in Patent Documents 5 and 6, by adjusting the cumulative particle size distribution of the soft magnetic powder, it is possible to obtain a powder magnetic core with an excellent balance between the filling rate and the magnetic properties.

Takemoto and Saito: Electric Steelmaking, Vol.81, No.2 (2010) pp117-122. Japanese Patent No. 4866971 Japanese Patent No. 5626672 Japanese Patent No. 6855936 JP 2021-68749 A Japanese Patent Application Laid-Open No. 2021-48176 JP 2016-139748 A

As in Patent Documents 1 and 2, when a dust core made of soft magnetic powder is magnetically annealed in an oxidizing atmosphere, alloys such as Si, Al, and Cr are used in order to obtain stable characteristics (frequency characteristics, volume resistivity). The amount of ingredients should be increased. As the alloying component increases, the soft magnetic powder becomes harder, resulting in lower compressibility. A decrease in compressibility causes a decrease in relative permeability, which is one of the important properties. In addition, oxygen that enters from the surface of the powder magnetic core reacts with the alloy components to form an insulating film, but the higher the density of the powder magnetic core, the more difficult it is for oxygen to diffuse into the interior. It becomes difficult to manufacture a relatively thick dust core.

As in Patent Document 3, the method of heat-treating FeNi soft magnetic powder to form a high-resistance layer at the grain boundary requires a high Ni content. There is a problem of a decrease in relative magnetic permeability due to a decrease in properties.

In the method of limiting the particle size distribution as in Patent Document 4, the average particle size is too small, so there is a concern that it is difficult to obtain a high magnetic permeability even with high-pressure molding. Further, in the method of adjusting the cumulative particle size distribution as in Patent Documents 5 and 6, unnecessary iron powder is generated by classification, which raises manufacturing costs and wastes resources.

In view of the above circumstances, an object of the present invention is to provide a powder magnetic core that achieves both high magnetic permeability, low iron loss, and high volume resistivity.

A dust core according to the present invention is a dust core having soft magnetic particles and an insulating layer formed on the surface of the soft magnetic particles, wherein the soft magnetic particles contain Fe and an element that is more easily oxidized than Fe, The 50% particle size (D ₅₀ ) measured on a volume basis is 10 to 40 μm, the ratio D ₁₀ /D ₅₀ of the 10% particle size (D ₁₀ ) to (D ₅₀ ) is 0.3 to 0.7, and 90 The ratio D ₉₀ /D ₅₀ between % particle size (D ₉₀ ) and D ₅₀ is 1.5 to 2.5, and the density is 5.8 g/cm ³ or more.

In the present invention, the soft magnetic particles contain Fe and an element that is more easily oxidized than Fe, and by adjusting the D ₁₀ , D ₅₀ , and D ₉₀ of the soft magnetic particles within the above ranges, the grain boundaries of the powder are prevented from becoming too large. Therefore, it is possible to obtain the required magnetic permeability and prevent an increase in iron loss. In addition, since there is no need to adjust the particle size by classification, the productivity is improved, the film thickness of the insulating layer can be maintained, and deterioration of iron loss can be prevented. Furthermore, since the dust core has a density of 5.8 g/cm ³ or more, the required relative permeability and strength can be obtained.

The element that is more easily oxidized than Fe is preferably Si or Si and an element M (M=Cr, Zn, Mn, Ti or Al). By containing Si, the magnetic permeability of the material can be effectively increased. In addition, as the amount of Si or Si and the element M mixed increases, the volume resistivity of the material increases, so the core loss of the powder magnetic core can be reduced.

It is preferable that the ratio of the elements more easily oxidized than Fe to the soft magnetic particles is 3.0 mass% to 7.0 mass%. This makes it possible to achieve both high relative permeability and low core loss.

The insulating layer is preferably made of a material mainly composed of a compound composed of an element having a Pauling electronegativity difference of 1.7 or less.

It is particularly preferable that the insulating layer contains silica as a main component. As a result, a dust core having high volume resistivity, strength, and withstand voltage can be obtained.

As described above, according to the present invention, it is possible to obtain a powder magnetic core that achieves both high magnetic permeability, low iron loss, and high volume resistivity.

1 is an enlarged cross-sectional view schematically showing the microstructure of a dust core; FIG. 4 is a table showing basic characteristics of a powder magnetic core according to the present embodiment; 4 is a table showing the effect of particle size distribution of soft magnetic powder on properties. 4 is a table showing the effects of the composition of soft magnetic powder on properties. 4 is a table showing the effect of powder magnetic core density on properties.

An embodiment of the present invention will be described below.

The powder magnetic core according to the present embodiment can be used as a core for winding a winding in a power system inductor used especially in the range of several 100 kHz to several MHz.

The powder magnetic core is prepared by an adjustment process for adjusting the powder for the powder magnetic core, a molding process for obtaining a compact by compression molding the adjusted material for the powder magnetic core, and a magnetic annealing process for magnetically annealing the compact. It is manufactured by going through

The powder magnetic core material includes soft magnetic powder and an insulating coating covering the surface of the soft magnetic powder. The composition and particle size distribution of the soft magnetic powder greatly affect the magnetic properties of the powder magnetic core, so it must be optimized according to the conditions of use.

As the soft magnetic powder, a soft magnetic alloy powder containing Fe as a main component (approximately 80 mass% or more), containing alloying elements that are more easily oxidized than Fe, and the balance being inevitable impurities is used. As an element that is more easily oxidized than Fe, it is preferable to include Si from the viewpoint of increasing the magnetic permeability. Thus, iron loss of the powder magnetic core can be suppressed. The proportion of the elements that are more easily oxidized than Fe in the soft magnetic powder is 3.0 mass% to 7.0 mass%. If it exceeds 7.0 mass%, the hardness of the material becomes too high, and it is not possible to obtain a powder magnetic core with a high relative density even with high-pressure molding. It is not suitable because it cannot obtain magnetic permeability and low iron loss).

In addition to the essential alloying elements described above, the soft magnetic powder may contain other alloying elements (for example, one or more of Ni, Co, Cu, B, Nb, Zr, etc.). may be included. As the soft magnetic powder, Fe-based amorphous alloys and Fe-based nanocrystalline alloys (materials in which nanocrystals of αFe of nm order are dispersed in an amorphous phase) can also be used.

Assuming that the soft magnetic powder is spheres with a uniform diameter, even if the powder is densely packed, gaps are generated between the particles, making it impossible to achieve high density of the powder magnetic core. It is preferable to prepare the soft magnetic powder so as to have a particle size distribution in the range of, for example, 1 μm to 100 μm so that the gaps can be filled with the fine powder. At this time, the particle size distribution may have a single peak, or may have a plurality of peaks. Also, two or more different soft magnetic powders can be mixed and used.

The soft magnetic powder has a 50% particle diameter (median diameter) ( _D50 ) measured on a volume basis of 10 to 40 μm, preferably 20 to 30 μm, and the ratio of the 10% particle diameter ( _D10 ) to _D50 D ₁₀ /D ₅₀ is 0.3 to 0.7, and the ratio of 90% particle diameter (D ₉₀ ) to D ₅₀ is adjusted so that D ₉₀ /D ₅₀ is 1.5 to 2.5. Here, D ₁₀ , D ₅₀ and D ₉₀ are particle diameters at cumulative 10% by volume, cumulative 50% and cumulative 90% from the small particle side in the cumulative particle size distribution of the powder. If the _D50 is lower than the above range, the grain boundaries of the powder become too large and the required magnetic permeability cannot be obtained. If the _D50 is higher than the above range, iron loss increases. , is not suitable. Further, when _D10 and _D90 are outside the above ranges, the particle size adjustment by classification is unavoidable, and the productivity is poor, which is unsuitable. Furthermore, if D ₁₀ is too small, the film thickness of the insulating coating becomes thin, and the core loss becomes worse, which is not suitable.

Note that the volume average particle diameter MV is the average diameter weighted by volume, and the particle diameters of d1, d2, ... di, ... dk in the population of particles in descending order of particle diameter. Suppose that there are n1, n2, . . . ni, .
MV=(V1*d1+V2*d2+...Vi*di+...Vk*dk)/(V1+V2+...Vi+...Vk)
MV=Σ(Vi×di)/Σ(Vi)
is represented by The volume average particle diameter can be measured by using a laser diffraction/scattering particle size distribution analyzer.

When a large amount of fine powder is contained in the soft magnetic powder, the fluidity of the powder is lowered, causing problems such as segregation and penetration of the powder into the clearance of the mold. In order to prevent this, it is also possible to increase the apparent particle size of the soft magnetic powder by binding the fine powder together with a binder. Various organic binders and inorganic binders can be used as the binder for granulation. In particular, it is preferable to use a silicone resin because improvement in insulation and strength can be expected after magnetic annealing. As the granulation method, general methods such as tumbling granulation, fluidized bed granulation, stirring granulation, compression granulation, extrusion granulation, crushing granulation, melt granulation and spray granulation can be used. The granulation method may be wet or dry.

Any insulation film covering the soft magnetic powder may be used as long as the powder magnetic core has a volume resistivity of 10 ⁶ Ωcm or more after magnetic annealing as described later. From this point of view, it is preferable to use a material with high covalent bond as the coating. “Highly covalent” means that the difference in Pauling electronegativity of the elements is small. If this difference is less than 1.7, the covalent bonding is 50% or more, and the material is generally regarded as having high covalent bonding. Examples of _such materials include _SiO2 , SiC, _Al2O3 , _{P2O5 ,} and the like.

As the insulating coating, the surface of the soft magnetic powder may be preliminarily provided with the above-described highly covalent-bonding material, or a material that changes to the above-described highly covalent-bonding material by heat treatment may be used. Examples of materials that can be used include various silanes, various silane coupling agents, various silicone oils, and various silicone resins (eg, methyl silicone resins). These materials change to SiO ₂ during magnetic annealing regardless of the type of atmosphere gas. These materials may be used alone or in combination of multiple types.

The method of forming the insulating coating is not particularly limited, and for example, mixing using a mixer, kneading using a pressure kneader, coating using a fluidized bed, various chemical conversion treatments, and the like can be used.

In the molding step, the soft magnetic powder obtained in the preparation step is compression-molded with a mold having a predetermined shape to form a green compact. Friction between the mold and the powder magnetic core material, and between the powder magnetic cores during compaction, may shorten the life of the mold and deteriorate the insulating coating. Therefore, from the viewpoint of extending the life of the mold used in compression molding or ensuring the fluidity of the soft magnetic powder, it is necessary to add an internal lubricant (mixed with the soft magnetic powder that becomes the raw material powder before compression molding), Either or both lubrication (sticking to the walls of the mold) may be employed. Various kinds of internal lubricants can be used, for example, various metal soaps, various amide waxes, graphite, MoS and the like can be used. These may be used alone, or may be used in combination.

Compression molding is adjusted so as to obtain the required magnetic properties, but it is preferable to use a molding pressure as low as possible. Generally, the molding pressure is in the range of 588MPa to 1960MPa. Also, warm compaction may be applied to lower the yield point of the soft magnetic powder.

The soft magnetic powder in the green compact immediately after compression molding is highly distorted, resulting in poor soft magnetic properties. Therefore, in the magnetic annealing step, the green compact is magnetically annealed for the purpose of removing the strain and improving the soft magnetic properties. Although the type of atmosphere gas for this annealing treatment is not particularly limited, it is desirable to use an inert or reducing atmosphere gas so as not to oxidize the soft magnetic powder and degrade its magnetic properties. Examples of these atmospheric gases include inert gases such as nitrogen and argon, and reducing gases such as hydrogen and AX gas (ammonia decomposition gas).

The heating temperature (magnetic annealing temperature) during magnetic annealing treatment should be set in consideration of the material of the target soft magnetic powder. However, when using the various silanes, various silane coupling agents, various silicone oils, and various silicone resins described above, the temperature is preferably in the range of 700° C. to 900° C. in order to chemically transform them into SiO ₂ .

It is important to set the magnetic annealing treatment time (holding time of the magnetic annealing temperature) in consideration of the size and material of the powder magnetic core so that the inside of the powder magnetic core can be sufficiently heated.

By performing the magnetic annealing treatment described above, strain in the powder compact is removed. As shown in FIG. 1, the powder magnetic core after magnetic annealing has soft magnetic particles 10 derived from soft magnetic powder, an insulating layer 11 derived from an insulating coating and covering the soft magnetic particles 10, and the insulating layer 11. It is formed in a porous shape having a large number of pores 12 formed therebetween.

The type, content, and grain size of the alloying element contained in the soft magnetic particles 10 are substantially the same as the type, content, and grain size of the alloying element contained in the soft magnetic powder before magnetic annealing. Therefore, in the dust core, the soft magnetic particles 10 have a 50% particle diameter (D ₅₀ ) of 10 to 40 μm when measured on a volume basis, and the ratio D ₁₀ between the 10% particle diameter (D ₁₀ ) and D ₅₀ /D ₅₀ is 0.3 to 0.7, and the ratio D ₉₀ /D ₅₀ between the 90% particle size (D ₉₀ ) and D ₅₀ is 1.5 to 2.5. Thus, by setting D ₁₀ , D ₅₀ , and D ₉₀ within the above ranges, it is possible to obtain a powder magnetic core excellent in iron loss and withstand voltage. In addition, the proportion of elements that are more easily oxidized than Fe is 3.0 mass% to 7.0 mass%, which makes it possible to achieve both high relative permeability and low core loss. In particular, if the element that is more easily oxidized than Fe is Si, or Si and element M (M = one of Cr, Zn, Mn, Ti, and Al), the inclusion of Si effectively increases the magnetic permeability of the material. In addition, the iron loss of the powder magnetic core can be suppressed.

Since the insulating layer is composed of a material mainly composed of a compound having a Pauling electronegativity difference of less than 1.7, it has a high withstand voltage. In addition, since the insulating layer contains silica (SiO ₂ ) as a main component, it has a high withstand voltage. Insulating layers mainly composed of _SiO2 are obtained by heating substances containing Si and O (silane coupling agents, silicone oligomers, silicone resins, etc.) with magnetic annealing, so they rely on atmospheric gases. It is possible to form an insulating layer without Therefore, it is possible to form an insulating layer even inside a dust core having a large thickness without the need for oxygen to enter when forming the insulating layer.

The density of the powder magnetic core after magnetic annealing shall be 5.8 g/cm ³ or more. If the density is less than 5.8 g/cm ³ , it is difficult to ensure the required relative permeability and strength. By setting the density to 5.8 g/cm ³ or more, the required relative permeability and strength can be obtained.

In order to further secure the withstand voltage, the surface of the dust core may be coated with an insulating coating. Although the coating method is not limited, various techniques such as dip coating, spray coating, CVD, PVD, and various etchings can be used. It is preferable to use a coating material having a high covalent bond, and a material containing a siloxane bond (Si--O--Si bond) is particularly preferable. Examples of such materials include various silanes, various silane coupling agents, various silicone oils, various silicone resins, and the like. The film thickness of the coating is preferably 100 μm or less, although it depends on the size of the product.

The volume resistivity of the powder magnetic core manufactured by the above procedure is preferably 1×10 ⁶ Ωcm or more. If the volume resistivity is less than 1×10 ⁶ Ωcm, problems such as increased eddy current loss may occur depending on the frequency used. The "volume resistivity" referred to here means the electric resistance between the opposite sides of a 1 m ³ cube in the magnetic core when a voltage is applied between the opposite sides (JIS C2560-1).

[Example]
Tests conducted to confirm the usefulness of the present invention are described below.

<Conditions for preparing test piece>
FIG. 2 shows the measurement results of the above evaluation items for Example 1. As shown in FIG. In Example 1, Fe-4.5Si-2.0Cr (D ₁₀ =10 μm, D ₅₀ =20 μm, D ₉₀ =40 μm) was used as the soft magnetic powder. The shape of the particle size distribution is a single peak with only one peak. _D10 / _D50 : 0.5, _D90 / _D50 : 2.0. An insulating coating was formed on the surface of the soft magnetic powder using a silane coupling agent, and the soft magnetic powder with the insulating coating was granulated using a silicone resin. An appropriate amount of lubricant (solid lubricant) is added to the powder after granulation, compression molding is performed at room temperature under a predetermined pressure, magnetic annealing is performed in nitrogen at 750° C., and a ring-shaped test piece (powder magnetic core) is obtained. made. The density of the specimen is 6.2g/ ^cm3 .

Evaluation items were relative magnetic permeability, iron loss, withstand voltage, volume resistivity and radial crushing strength. The relative magnetic permeability was measured according to the initial magnetic permeability measurement method described in JIS C 2560-2:2006 with an LCR meter (1 kHz 1 mA constant current mode). Iron loss was measured with a BH analyzer (0.05T 100kHz). At this time, the powder magnetic core was wound with two windings and measured. The withstand voltage was measured according to the method described in JIS C 2110-1:2016, and the volume resistivity was measured according to the method described in JIS K 7194:1994. Radial crushing strength was measured according to the method described in JIS Z 2507:2000.

The measurement results will be described with reference to FIGS. 2 to 5. FIG. A relative magnetic permeability of 50 or more, a core loss of less than 700 kW/m ³ , a withstand voltage of 100 V/mm or more, a volume resistivity of 10 ⁶ Ω cm or more, and a radial crushing strength of 40 MPa or more were evaluated as passing (○). As shown in FIG. 2, in Example 1, good results were obtained in all cases.

Next, each evaluation item was measured for a test piece in which the particle size distribution of the soft magnetic powder was changed from that of Example 1. The results are shown in FIG. 3 together with Example 1.

Example 2 has _D50 = 20 µm, _D10 / _D50 : 0.7, _D90 / _D50 : 1.8, and the density of the test piece is 6.3 g/ ^cm3 . Comparative Example 1 has _D50 = 20 µm, D ₁₀ / _D50 : 0.3, _D90 / _D50 : 2.5, and the density of the test piece is 6.5 g/ ^cm3 . In Example 3 and Comparative Example 2, _D50 was changed to 10 μm.
Example 3 has _D10 / _D50 : 0.5, _D90 / _D50 : 2.5, and the density of the test piece is 5.9 g/ ^cm3 . Comparative Example 2 has _D10 / _D50 : 0.3, _D90 / _D50 : 3.5, density of specimen is 6.1 g/ ^cm3 . Example 4
, in Comparative Example 3, the D ₅₀ was changed to 40 μm. Example 4 has _D10 / _D50 : 0.6, _D90 / _D50 : 1.5, and the density of the test piece is 6.6 g/ ^cm3 . Comparative Example 3 has _D10 / _D50 : 0.3, _D90 / _D50
: 3.0, the density of the specimen is 6.9 g/ ^cm3 .

The results of Example 2 were as good as those of Example 1 except that the iron loss was lower than that of Example 1. It is considered that this is because the particle size distribution of Example 2 was sharper than that of Example 1, and the insulating coating was uniformly applied. Comparative Example 1, which has a wider particle size distribution than Example 1, has an improved relative magnetic permeability, but deteriorates in any one of iron loss, volume resistivity, and withstand voltage. It is presumed that the main reason for this is that Comparative Example 1 contained a large amount of fine powder and that the film thickness of the insulating coating between the particles varied greatly. Comparative Example 2 has a lower withstand voltage than Example 3. It is considered that this is due to the fact that Comparative Example 2 has a wide particle size distribution. Comparative Example 3 has a lower iron loss than Example 4. This is considered to be due to the wide particle size distribution of Comparative Example 3. From the above results, it was found that it is preferable to adjust D ₁₀ /D ₅₀ to more than 0.3 and 0.7 or less and D ₉₀ /D ₁₀ to more than 1.5 and 2.5 or less.

Next, with respect to Example 1, the composition of the soft magnetic powder was changed, and the change in the evaluation items was measured when the particle size distribution of the soft magnetic powder was at the same level as in Example 1. The results are shown in FIG.

Example 5 in FIG. 4 uses Fe-7.0Si as the soft magnetic powder, and the density of the test piece is 6.3 g/cm
³ , Example 6 uses Fe3.0Si as the soft magnetic powder, and the density of the test piece is 6.5 g/cm ³ . Comparative Example 4 uses Fe-3.5Si5.0Cr as the soft magnetic powder, and the density of the test piece is 5.8 g/cm ^{3 .}
and Comparative Example 5 uses Fe2.0Si as the soft magnetic powder, and the density of the test piece is 6.7 g/cm ³ .

As shown in FIG. 4, when the proportion of the element more easily oxidized than Fe constituting the soft magnetic powder was 3.0 mass% (Example 6) or 7.0 mass% (Example 5), the results were good regardless of the composition. is. On the other hand, when the ratio of elements more easily oxidized than Fe was 2.0 mass% (Comparative Example 5), the iron loss was worsened. In addition, when the ratio of elements more easily oxidized than Fe was 8.5 mass% (Comparative Example 4), the result was that the relative magnetic permeability was low. From the above, it was found that favorable results can be obtained when the ratio of easily oxidizable elements is 3.0 to 7.0 mass%.

Furthermore, the molding pressure of the test piece was changed to adjust the density. The conditions are the same as in Example 1 except for the density. The results are shown in FIG. As shown in Comparative Example 6, when the density was too low, the magnetic permeability was low and was below the acceptance criteria. Moreover, the radial crushing strength did not satisfy the required level. On the other hand, as shown in Examples 7 and 8, the higher the density, the higher the filling rate of the magnetic powder, so the relative magnetic permeability and radial crushing strength increased. In addition, there were no problems with iron loss, volume resistivity, and withstand voltage. From the above, it was found that favorable results were obtained when the density was 5.8 g/cm ³ or more.

10 soft magnetic particles 11 insulating layer 12 holes

Claims (5)

In a powder magnetic core having soft magnetic particles and an insulating layer formed on the surface of the soft magnetic particles,
The soft magnetic particles contain Fe and an element that is more easily oxidized than Fe, and have a 50% particle size (D ₅₀ ) measured on a volume basis of 10 to 40 μm, and a 10% particle size (D ₁₀ ) and (D ₅₀ ) has a ratio _D10 / _D50 of 0.3 to 0.7, a ratio _D90 / _D50 of 90% particle size ( _D90 ) to _D50 is 1.5 to 2.5,
A dust core having a density of 5.8 g/cm ³ or more. 2. The dust core according to claim 1, wherein the element that is more easily oxidized than Fe is Si or Si and an element M (M=Cr, Zn, Mn, Ti, or Al). 3. The powder magnetic core according to claim 1, wherein the proportion of the element more easily oxidized than Fe in the soft magnetic particles is 3.0 mass % to 7.0 mass %. 4. The pressure according to any one of claims 1 to 3, wherein the insulating layer is made of a material mainly composed of a compound composed of an element having a Pauling electronegativity difference of 1.7 or less. powder magnetic core. 5. The dust core according to claim 1, wherein the insulating layer contains silica as a main component.

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