JP7277194B2 - dust core - Google Patents

Description

The present invention relates to dust cores.

The development of powder magnetic cores is actively carried out due to their high degree of freedom in shape and applicability to high-frequency bands.
In Patent Document 1, a composite magnetic material powder obtained by uniformly mixing and dispersing a crystalline magnetic material and an amorphous magnetic material is added with an organic material such as silicone-based resin, phenol-based resin, or epoxy-based resin as an insulating material. A powder magnetic core for high frequency produced using polymer resin and water glass is disclosed.

JP 2005-294458 A

However, the iron loss of the dust core is not necessarily sufficiently suppressed, and further suppression of the iron loss has been desired.
The present invention has been made in view of the above circumstances, and aims to further suppress iron loss, and can be implemented as the following modes.

[1] A powder magnetic core comprising soft magnetic metal particles having an average particle size of 5 μm to 30 μm and a grain boundary phase in which a crystalline substance containing at least one of Al and Si is present,
When the cross-sectional structure of the powder magnetic core is observed in a square first field of view of 100 μm × 100 µm and the area ratio P (%) of the pores occupying the first field of view is obtained,
Assuming that the maximum value of the area ratio P is P1 and the minimum value of the area ratio P is P2,
A powder magnetic core, wherein the difference between P1 and P2 is 3% or less.

[2] When the cross-sectional structure of the interface between the soft magnetic metal particles and the grain boundary phase is observed in a square second field of view of 10 μm×10 μm, the surface roughness Ra of the soft magnetic metal particles is 0.05 μm or more. The dust core according to [1], having a thickness of 0.20 μm or less and an Rz of 0.20 μm or more and 0.70 μm or less.

[3] When the cross-sectional structure of the powder magnetic core is observed in a third field of view of a square of 50 μm × 50 µm for 10 consecutive fields in the vertical or horizontal direction so that the fields of view do not overlap each other,
For grain boundary multiple points surrounded by three or more of the soft magnetic metal grains,
[1] a first bonding line connecting the surfaces of the soft magnetic metal particles adjacent to each other at the closest portions, and [2] connecting the ends of the first bonding lines adjacent to each other to the soft magnetic metal Let S1 be the area of the region of the grain boundary phase obtained by excluding pores from the region surrounded by the second bonding line connecting the inner part of the outline of the grain on the side of the grain boundary multiple point, and
When the average area per one of the three or more soft magnetic metal particles is S2,
1/10≤S1/S2≤1/3
The dust core according to [1] or [2], wherein there are three or more grain boundary multiple points on average per field of view.

[4] When observing the cross-sectional structure of the powder magnetic core in a square fourth field of view of 50 μm × 50 μm,
[1] to [3] characterized in that the average area ratio obtained by measuring the area ratio of the grain boundary phase occupying the fourth field of view at three different points and obtaining the average value is 5 to 10% [1] to [3] The dust core according to any one of the items.

According to the above invention [1], iron loss is suppressed.
According to the above invention [2], the eddy current loss can be further reduced.
According to the above invention [3], the eddy current loss can be further reduced.
According to the above invention [4], the hysteresis loss can be further reduced.

It is a schematic diagram which shows a powder magnetic core. The figure on the right shows a schematic diagram of the cross-sectional structure of the powder magnetic core observed in a square field of view of 50 μm×50 μm. FIG. 3 is a perspective view of a powder magnetic core for explaining requirements regarding pores 35; FIG. 2 shows a perspective view of a dust core cut in half along the axis. A schematic diagram of the location D1 observed in a square field of view of 100 μm×100 μm is shown. A schematic view of the location D2 observed in a square field of view of 100 μm×100 μm is shown. 3 is a schematic diagram showing an enlarged view of the vicinity of the interface between the soft magnetic metal particles 3 and the grain boundary phase 6. FIG. FIG. 2 is an explanatory diagram showing an enlarged vicinity of a grain boundary multiple point for explaining requirements related to the grain boundary multiple point; FIG. 2 is an explanatory diagram showing soft magnetic metal grains 3A, 3B, and 3C surrounding grain boundary multiple points, in order to explain requirements related to grain boundary multiple points; It is process drawing which shows an example of the manufacturing method of a powder magnetic core.

The present invention will be described in detail below. In this specification, the description using "-" for the numerical range includes the lower limit and the upper limit unless otherwise specified. For example, the description “10 to 20” includes both the lower limit “10” and the upper limit “20”. That is, "10 to 20" has the same meaning as "10 or more and 20 or less".

1. Configuration of dust core 1 As shown in the right diagram (cross-sectional view) of FIG. and a grain boundary phase 6 in which a crystalline material containing at least one of
When the cross-sectional structure of the dust core 1 is observed in a square first field of view of 100 μm × 100 μm and the area ratio P (%) of the pores 35 occupying the first field of view is obtained,
Assuming that the maximum value of the area ratio P is P1 and the minimum value of the area ratio P is P2,
The difference (porosity difference) between P1 and P2 is 3% or less.

In FIG. 1, a toroidal dust core 1 is taken as an example. The shape of the dust core 1 is not particularly limited. FIG. 1 shows a cross section of a dust core 1 taken along its axial direction.

(1) Soft magnetic metal particles 3
The soft magnetic metal particles 3 are not particularly limited as long as they are soft magnetic metal particles, and can be widely used. As the soft magnetic metal particles 3, a wide range of soft magnetic pure iron particles and iron-based alloy particles can be used. As iron-based alloys, Fe--Si--Cr alloys, Fe--Si--Al alloys (sendust), Ni--Fe alloys (permalloy), Ni--Fe--Mo alloys (supermalloy), Fe-based amorphous alloys, Fe--Si Alloys, Ni--Fe alloys, Fe--Co alloys, etc. can be suitably used. Among these, Fe--Si--Cr alloys, Ni--Fe alloys (permalloy), Ni--Fe--Mo alloys (supermalloy) and Fe-based amorphous alloys are preferable from the viewpoint of magnetic permeability, coercive force and frequency characteristics.
When using an Fe—Si—Cr alloy, for example, Si: 0.1% by mass to 10% by mass, Cr: 0.1% by mass to 10% by mass, balance: Fe and unavoidable impurities. can be used.
The average particle size of the soft magnetic metal particles 3 is 5 μm or more and 30 μm or less, preferably 10 μm or more and 25 μm or less, and more preferably 15 μm or more and 22 μm or less. The average particle size of the soft magnetic metal particles 3 can be appropriately changed depending on the frequency band used. In particular, when assuming use in a high frequency band exceeding 100 kHz, the thickness is more preferably 10 μm or more and 25 μm or less. The average particle diameter of the soft magnetic metal particles 3 is obtained by calculating the equivalent circle diameter from the particle area of the powder magnetic core 1 observed by FE-SEM JSM-6330F.

The soft magnetic metal particles 3 may have an oxide film (metal oxide layer, passive film) formed on the surface. By forming an oxide film on the surface, adhesion to the grain boundary phase can be improved.
The metal oxide forming the oxide film is not particularly limited. For example, one or more metal oxides selected from the group consisting of chromium oxide, aluminum oxide, molybdenum oxide, and tungsten oxide are preferred. In particular, the metal oxide preferably contains at least one of chromium oxide and aluminum oxide. By using these preferred metal oxides, eddy current loss is effectively suppressed.
When Fe--Si--Cr alloy particles are used as the soft magnetic metal particles 3, an oxide coating containing chromium oxide _{Cr.sub.2 O.sub.3} can be easily formed. That is, a metal oxide layer is formed on the outer edges of the soft magnetic metal particles 3 by oxidizing Cr in the Fe—Si—Cr alloy.
Moreover, the thickness of the oxide film is not particularly limited. The thickness is preferably 1 nm or more and 20 nm or less, more preferably 5 nm or more and 15 nm or less.
The thickness of the oxide film can be measured using XPS (X-ray photoelectron spectroscopy).

Moreover, a reaction phase may exist between the oxide film and the grain boundary phase 6 . If a reaction phase exists between the oxide film and the grain boundary phase 6, the interfacial resistance between the soft magnetic metal grains 3 and the grain boundary phase 6 increases, and the insulation resistance of the grain boundary phase 6 also increases. As a result, an effect of reducing eddy current loss can be expected. The reaction phase consists of a product produced by the reaction of the elements contained in the oxide film and the elements contained in the grain boundary phase 6 . The reaction phase has a composition gradient in which the content of elements derived from the oxide film increases as it approaches the oxide film, and the content of elements derived from the grain boundary phase 6 increases as it approaches the grain boundary phase 6 . In addition, in each figure, the layer composed of the oxide film and the reaction phase is drawn as a single layer 4 .
The reaction phase is preferably 10 nm or less, more preferably 8 nm or less.
The existence and composition of the reaction phase can be confirmed by XPS. Also, the thickness of the reaction phase can be measured using XPS.

Moreover, the aspect ratio of the soft magnetic metal particles 3 is not particularly limited. The aspect ratio of the soft magnetic metal particles 3 is preferably 1.15 or more and 1.40 or less, more preferably 1.2 or more and 1.35 or less.
By setting the aspect ratio of the soft magnetic metal particles 3 within this range, the hysteresis loss can be further reduced.

(2) Grain boundary phase 6
The grain boundary phase 6 includes crystals containing at least one of Al and Si, as described above.
An example of the crystal containing at least _{one of Al and Si is a silicate compound, for example, selected from the group consisting of Al2SiO5, Y2Si2O7 , Zn2SiO4 , CaSiO3 , and MgSiO3 .} At least one is preferred.
Further, as a crystal containing Al (aluminum), a crystalline compound derived from alumina sol can be exemplified. Crystals containing Al (aluminum) are produced by heat-treating alumina sol. The crystal containing Al (aluminum) is a compound different from α-Al ₂ O ₃ , such as boehmite.
From the viewpoint of eddy current suppression, the particle size of the crystal containing at least one of Al and Si is preferably 25 nm or more and 200 nm or less.
The crystal can be detected by FE-SEM (for example, JSM-6330F) on the cross section of the powder magnetic core 1 . The grain diameter of the crystal is obtained by calculating the area circle equivalent diameter from the grain area observed by FE-SEM.
The grain boundary phase 6 has a property of high resistance.

(3) Requirements for Pores 35 (3.1) Requirements for Porosity Difference Observing the cross-sectional structure of the powder magnetic core 1 in a square first field of view of 100 μm×100 μm, the area ratio P of the pores 35 occupying the first field of view is (%). In the dust core 1 of the present invention, the difference (porosity difference) between P1 and P2 is 3% or less, where P1 is the maximum value of the area ratio P and P2 is the minimum value of the area ratio P. The difference (porosity difference) between P1 and P2 is preferably 2% or less, more preferably 0.5% or less. Although the lower limit of the average porosity is not particularly defined, it may be 0% or more.
In determining the area ratio P (porosity) (%), a square area of 100 μm×100 μm on the SEM image of the cross section of the dust core 1 is taken as the first field of view. Image analysis using EPMA is performed within the range of the first field of view, and the area ratio P (%) of the pores 35 in the entire first field of view is obtained.
This requirement will now be described with reference to FIGS.
First, when observing the cross-sectional structure of the powder magnetic core 1, a location D1 where the area ratio P of the pores 35 occupying the first field of view is the largest and a location D2 where the area ratio P of the pores 35 occupying the first field of view is the smallest. Explain how to decide. The dust core 1 is manufactured by press molding with a pair of dies. The surface to which the pressure is applied by the pair of dies is specified by the shape of the dust core 1 . For example, in the toroidal dust core 1 of FIG. 2, the surfaces to which pressure is applied are the press surface PS1 and the press surface PS2. The places where the highest pressure is applied are in the vicinity of the press surfaces PS1 and PS2, and can be uniquely identified by a person skilled in the art through simulation, experience, or the like. For example, in the case of the dust core 1 of FIG. 2, the location indicated by symbol D2 is the location where the highest pressure is applied. On the other hand, the place where the lowest pressure is applied can be uniquely identified by a person skilled in the art through simulation, experience, or the like. For example, in the case of the dust core 1 of FIG. 2, the location indicated by symbol D1 is the location where the lowest pressure is applied.
At the location D1 where the lowest pressure is applied, the cross-sectional structure of the powder magnetic core 1 is observed in the first visual field of a square of 100 μm × 100 μm, and the area ratio P (%) of the pores 35 occupying the first visual field is obtained ( See Figure 3). The area ratio P (%) at the location D1 where the lowest pressure is applied corresponds to the maximum value of the area ratio P, P1 (%). That is, location D1 has the lowest applied pressure and is the location where most of the pores 35 are likely to remain.
On the other hand, at the location D2 where the highest pressure is applied, the cross-sectional structure of the powder magnetic core 1 is observed in a square first field of view of 100 μm×100 μm, and the area ratio P (%) of the pores 35 in the first field of view is calculated. (See Figure 4). The area ratio P (%) at the location D2 where the lowest pressure is applied corresponds to the minimum value of the area ratio P, P2 (%). That is, location D2 has the highest applied pressure and the fewest pores 35 .
In this manner, the difference between P1 and P2 can be obtained by setting the maximum value of the area ratio P to P1 and the minimum value of the area ratio P to P2.

(3.2) Guessing Reasons for Suppression of Iron Loss of Dust Core 1 The present inventors have made earnest studies to suppress the iron loss of the dust core 1 . In this case, the density of the powder magnetic core is not sufficient in the conventional grain boundary phase composed of inorganic substances and the grain boundary phase in which ceramic particles are dispersed in inorganic substances, and the interface between the soft magnetic metal particles and the grain boundary phase Focusing on the fact that the insulation resistance is not sufficient, the relationship between the porosity difference of the powder magnetic core 1 and the iron loss of the powder magnetic core 1 was studied. Then, in the powder magnetic core 1 using the soft magnetic metal particles 3 having an average particle size within a specific range, the grain boundary phase 6 contains a crystalline substance containing at least one of Al and Si, and the above porosity A new knowledge was obtained that iron loss of the powder magnetic core 1 can be suppressed when the requirements for the difference are satisfied. The present invention has been made based on this finding.
Although the reason why the desired effect is obtained in this embodiment is not clear, it is presumed as follows.
In the powder magnetic core 1 in which the difference between P1-P2 is 3% or less, the press pressure is sufficiently applied even at the place where the area ratio P of the pores 35 occupying the first field of view is the maximum, and the density of the entire powder magnetic core 1 is increased. can be made high enough. As a result, it is presumed that the hysteresis loss of the powder magnetic core 1 can be suppressed and the iron loss of the powder magnetic core 1 can be suppressed.
In addition, in the powder magnetic core 1 in which the difference between P1-P2 is 3% or less, the soft magnetic metal particles 3 and the grain boundary phase 6 are sufficient even in the place where the area ratio P where the pores 35 occupy the first field of view is the maximum. , the insulation resistance at the interface between the soft magnetic metal particles 3 and the grain boundary phase 6 can be increased. As a result, it is presumed that the eddy current loss of the powder magnetic core 1 can be suppressed, and the iron loss of the powder magnetic core 1 can be suppressed.

(4) Requirements for Surface Roughness of Soft Magnetic Metal Particles 3 From the viewpoint of further reducing eddy current loss, the powder magnetic core 1 satisfies the following requirements for surface roughness of the soft magnetic metal particles 3. More preferred.
In the dust core 1, the surface roughness Ra of the soft magnetic metal particles 3 is 0 when the cross-sectional structure of the interface between the soft magnetic metal particles 3 and the grain boundary phase 6 is observed in a square second field of view of 10 μm×10 μm. It is preferably 0.05 μm or more and 0.20 μm or less, and Rz is preferably 0.20 μm or more and 0.70 μm or less.
Ra is the arithmetic mean roughness defined in JIS B0601 (2001). Rz is the maximum height roughness defined in JIS B0601 (2001). As a method for measuring Ra and Rz of the soft magnetic metal particles 3, a method based on JIS B0601 (2001) may be used. For example, from a TEM (transmission electron microscope) image of the cross section of the interface between the soft magnetic metal particles 3 and the grain boundary phase 6 as shown in FIG. A method for calculating Rz can be used.
The requirements for Ra and Rz may be satisfied in at least one of a plurality of 10 μm×10 μm square fields of view when observing the cross-sectional structure of the dust core 1 .
This requirement can be controlled by appropriately setting the surface roughness of the soft magnetic metal powder (soft magnetic metal particles 3) that is the raw material of the soft magnetic metal particles. When the powder magnetic core 1 is given an uneven shape due to friction between the soft magnetic metal particles 3 during press molding, etc., the surface roughness of the raw material should be set in consideration of the uneven shape that may be imparted during press molding. Just do it.

(5) Requirements for Grain Boundary Multiple Points From the viewpoint of further reducing eddy current loss, the dust core 1 more preferably satisfies the following requirements for grain boundary multiple points. Here, the grain boundary multiple point is a portion surrounded by three or more soft magnetic metal particles 3 in an image obtained by observing the cross section of the dust core 1, and is a grain surrounded by three soft magnetic metal particles 3 A boundary triple point, a grain boundary quadruple point surrounded by four soft magnetic metal particles 3, and the like are included (usually, a portion surrounded by five or less soft magnetic metal particles 3).
When the cross-sectional structure of the powder magnetic core 1 is observed in a 50 μm × 50 μm square third field of view for 10 consecutive fields in the vertical or horizontal direction so that the fields do not overlap each other, three or more soft magnetic metals For the grain boundary multiple points surrounded by the grains 3, the area of the grain boundary phase 6 region excluding the pores 35 from the region surrounded by the first bonding line and the second bonding line defined in [1] [2] below is S1.
[1] The first bonding line is a line connecting the surfaces of the soft magnetic metal particles 3 adjacent to each other at the closest portions.
[2] The second bond line is a line that connects the ends of the first bond lines that are adjacent to each other by following the inner part of the outline of the soft magnetic metal grain 3 on the grain boundary multiple point side.
Also, let S2 be the average area per one of the three or more soft magnetic metal particles 3 .
In calculating the area S1 of the region of the grain boundary phase 6, if part of the region surrounded by the first bonding line and the second bonding line is missing from the third field of view, S1 is calculated for that region. do not. Further, when calculating the average area S2 of the soft magnetic metal particles 3, if a part of the soft magnetic metal particles 3 is missing from the third visual field, the observation field is expanded to obtain the area of the entire soft magnetic metal particles 3. . The area S1 and the area S2 are calculated by image analysis using EPMA on the SEM image of the cross section of the powder magnetic core 1 in the same manner as (3.1) Requirement for porosity difference.
In the powder magnetic core 1, it is preferable that grain boundary multiple points satisfying 1/10≦S1/S2≦1/3 exist on average at 5 or more locations per field of view, and 8 or more on average exist. more preferred. The average number of grain boundary multiple points per field of view (average number of agglomerates) that satisfies 1/10≦S1/S2≦1/3 is rounded off to the nearest whole number. The average number of aggregated portions is usually 15 or less.
In the range of 1/10≦S1/S2≦1/3, when the value is equal to or more than the lower limit, the grain boundary phase 6 aggregates at the grain boundary multiple points where pores 35 are likely to occur, forming aggregated portions 7. The insulation resistance of the interface phase 6 can be ensured.
In the range of 1/10 ≤ S1/S2 ≤ 1/3, when the value is equal to or less than the upper limit, the adjacent soft magnetic metal particles 3 are arranged sufficiently close to each other, so that the softness in the dust core 1 The filling rate of the magnetic metal particles 3 can be made sufficient.
In the powder magnetic core 1 in which the average number of aggregated portions 7 is 5 or more, the grain boundary multiple points where the pores 35 are likely to occur are sufficiently filled with the grain boundary phase 6, thereby ensuring the insulation resistance of the grain boundary phase 6. , eddy current loss can be reduced.
This requirement will now be described with reference to FIGS.
A method for obtaining the area S1 of the grain boundary phase 6 will be described. As shown in FIG. 6, at the grain boundary triple point surrounded by the soft magnetic metal particles 3A, the soft magnetic metal particles 3B, and the soft magnetic metal particles 3C, the distance between the surfaces of the soft magnetic metal particles 3A and the soft magnetic metal particles 3B is the maximum. A line connecting the adjacent portions A2 and B1, a line connecting the surfaces of the soft magnetic metal particles 3B and the soft magnetic metal particles 3C between the surfaces of the soft magnetic metal particles 3B and the soft magnetic metal particles 3C and connecting the closest portions B2 and C1, and the soft magnetic metal particles 3C and the soft magnetic A line connecting the closest portions C2 and A1 between the surfaces of the metal particles 3A is defined as a first bonding line L1. A line that connects A1 and A2 along the inner part of the outline of the soft magnetic metal particle 3A on the side of the grain boundary multiple point, and B1 and B2 along the outline of the soft magnetic metal particle 3B. A line connecting the inner part on the side of the field multiple point and a line connecting C1 and C2 by tracing the inner part on the side of the grain boundary multiple point of the outline of the soft magnetic metal particle 3C are the second coupling line L2. and Then, the area S1 of the grain boundary phase 6 is obtained by calculating the area excluding the pores 35 from the area surrounded by the first bonding line L1 and the second bonding line L2. If the surface of the soft magnetic metal particles 3 is formed with a layer 4 made of an oxide film or a reaction phase, the area of this layer 4 is also included in S1.
A method for obtaining the average area S2 of each of the soft magnetic metal particles 3A, 3B, and 3C will be described. As shown in FIG. 4, the area S2A of the soft magnetic metal particles 3A, the area S2B of the soft magnetic metal particles 3B, and the area S2C of the soft magnetic metal particles 3C are calculated. Then, these areas are summed up and divided by the number of particles to obtain an average area S2 (S2=(S2A+S2B+S2C)/3).
Then, the number of grain boundary multiple points that satisfy the requirement of 1/10≦S1/S2≦1/3 is counted for each of the 10 consecutive visual fields observed. By adding up the number of grain boundary multiple points that satisfy the above requirements in each field of view and dividing by 10, which is the number of observed fields of view, the average number of points per field of view (average number of aggregated portions 7) can be obtained. can.
The number of grain boundary multiple points satisfying the requirement of 1/10≦S1/S2≦1/3 depends on the fluidity of the material forming the grain boundary phase 6, the blending amount of the material forming the grain boundary phase 6, It can be controlled by the press pressure or the like when the powder magnetic core 1 is manufactured.

(6) Requirements for Area Ratio of Grain Boundary Phase 6 From the viewpoint of further reducing hysteresis loss, the powder magnetic core 1 more preferably satisfies the following area ratio (occupancy) (%) requirements.
When the cross-sectional structure of the powder magnetic core 1 was observed in a 50 μm×50 μm square fourth field of view, the area ratio of the grain boundary phase 6 occupying the fourth field of view was measured at three different points, and an average value was obtained. The average area ratio is preferably 5-10%, more preferably 6-9%. Note that the area of the grain boundary phase 6 does not include the area of the pores 35 .
The area ratio (%) of the grain boundary phase 6 is calculated using the same analysis means as in (3.1) Requirement for porosity difference. On the SEM image of the cross section of the dust core 1, a square area of 50 μm×50 μm is defined as a fourth visual field. Image analysis using EPMA is performed within the range of this fourth field of view, and the area ratio (%) of the grain boundary phase 6 in the entire fourth field of view is determined. The area ratio (%) of the grain boundary phase 6 is determined at three locations, and the average value is calculated. The calculated average value is the average area ratio (%) of the grain boundary phase 6 .
The requirement for this area ratio (%) is that when observing the cross-sectional structure of the powder magnetic core 1, three or more square fields of view of 50 μm × 50 µm are observed, and three fields of view arbitrarily selected from among them are observed. as long as it is satisfied.
This requirement can be controlled by adjusting the amount of the material that forms the grain boundary phase 6 .

(7) Porosity Requirements From the viewpoint of further reducing hysteresis loss, the powder magnetic core 1 more preferably satisfies the following porosity (%) requirements.
When the cross-sectional structure of the powder magnetic core 1 is observed in a 50 μm×50 μm square fifth field of view, the ratio of the pores 35 occupying the fifth field of view is measured at 10 different points, and the average porosity is calculated. is preferably 10% or less, more preferably 8% or less, and even more preferably 5% or less. Although the lower limit of the average porosity is not particularly defined, it may be 1% or more.
The requirement for this porosity (%) is that when observing the cross-sectional structure of the powder magnetic core 1, 10 or more 50 μm×50 μm square fields of view are observed, and 10 fields of view arbitrarily selected from among them are observed. as long as it is satisfied.
This requirement can be controlled by adjusting the molding pressure during press molding.

2. Method for Manufacturing Dust Core 1 The method for manufacturing the dust core 1 is not particularly limited. FIG. 8 shows an example of a method for manufacturing the dust core 1, and this manufacturing method will be described below.
(1) Preparation of Soft Magnetic Metal Powder First, a soft magnetic metal powder (soft magnetic metal particles 3) is prepared as a raw material (step S1).
(2) Heat Treatment Next, the soft magnetic metal powder is heat treated (step S2). Conditions for this heat treatment are not particularly limited. Heat treatment conditions are, for example, heat treatment temperature: 700° C. to 900° C., heating rate: 1° C. to 10° C./min, holding time: 1 minute to 120 minutes, inert atmosphere (N ₂ atmosphere, Ar atmosphere). It is preferably adopted.
(3) Binder coating Next, the soft magnetic metal powder is coated with a binder (step S3). A coating method is not particularly limited, and for example, a spray coating method, a dipping method, and a wet mixing method are preferably used. The binder contains aluminum compound particles. That is, alumina sol, which is a colloidal solution of alumina hydrate, can be preferably used as the binder. The coated soft magnetic metal powder is dried, for example, under the conditions of drying temperature: 60° C. to 150° C. and drying time: 30 minutes to 120 minutes.
(4) Molding (press molding)
In order to make the shape of the powder magnetic core 1, press molding (for example, uniaxial mold molding) is usually used (step S4). The molding pressure during press molding is preferably 1.2 GPa to 2.4 GPa, and it is better to press at a high pressure in order to obtain a high-density molded body. Also, the mold may be heated in the range of room temperature to 200° C. during press molding. Heating the mold facilitates plastic deformation of the soft magnetic metal powder, making it possible to obtain a high-density compact. On the other hand, press molding at a temperature exceeding 200° C. poses a problem of oxidation of the soft magnetic metal powder and is not so preferable.

(5) Heat treatment The obtained compact is heat treated (annealed) to release the strain applied during press molding (step S5). Heat treatment conditions are, for example, heat treatment temperature: 700° C. to 900° C., heating rate: 1° C. to 10° C./min, holding time: 1 minute to 120 minutes, inert atmosphere (N ₂ atmosphere, Ar atmosphere). It is preferably adopted.
The heat treatment conditions are changed as appropriate depending on the type of soft magnetic metal powder used.

3. Effects of the Dust Core 1 of the Present Embodiment According to the dust core 1 of the present embodiment, iron loss is suppressed.
The dust core 1 can further reduce eddy current loss by satisfying the surface roughness requirements of the soft magnetic metal particles 3 .
The dust core 1 can further reduce the eddy current loss by satisfying the requirements for grain boundary multiple points.
The powder magnetic core 1 can further reduce the hysteresis loss by satisfying the requirements for the average area ratio of the grain boundary phase 6 .

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples.
Experimental Examples 1 to 15 are examples, and Experimental Examples 16 to 20 are comparative examples.
In the table, experimental examples are indicated using "no." In addition, when "*" is added, such as "16*" in the table, it indicates that it is a comparative example.

1. Preparation of dust core (1) Experimental examples 1 to 17, 19 (no.1 to 17, 19)
Various particles having an average particle size shown in Table 1 were used as the soft magnetic metal particles (raw material powder). In Table 1, "Fe--Si--Cr" means Fe-5.5 mass % Si-4.0 mass % Cr particles produced by the water atomization method.
First, the soft magnetic metal powder was heat-treated. The heat treatment conditions were heat treatment temperature: 200 to 400° C., temperature increase rate: 1.0 to 10° C./min, holding time: 10 to 45 minutes, inert atmosphere (Ar, N ₂ ) or vacuum atmosphere.
The soft magnetic metal particles were then coated with a coating liquid. Alumina sol was used as the coating liquid. In addition, in Table 1, when the column of "crystalline substance of Al and Si" is set to "○", it indicates that aluminum compound particles were used in the coating liquid, and "Crystals of Al and Si In the case where the column of "Positive Substance" is marked with "X", it means that the coating liquid does not contain aluminum compound particles or silicon compound particles.

Then, the coated soft magnetic metal particles were dried under conditions of 60 to 150° C. for a drying time of 60 to 180 minutes.
The coated soft magnetic metal powder was then heat treated. The heat treatment conditions were heat treatment temperature: 200 to 400° C., heating rate: 60 to 180° C./min, holding time: 10 to 45 minutes, inert atmosphere (Ar, N ₂ ) or vacuum atmosphere.
Then, it was press-molded at a molding pressure of 1.0 to 2.5 GPa to obtain a compact (toroidal shape (outer diameter: 8 mm, inner diameter: 4.5 mm, height: 1.5 mm)). This compact was heat-treated at a heat treatment temperature of 400 to 800° C., a heating rate of 1.0 to 10° C./min, a holding time of 10 to 45 minutes, and an inert atmosphere (Ar, N ₂ ) or vacuum atmosphere. . As described above, powder magnetic cores according to Experimental Examples 1 to 17 and 19 were obtained.
The porosity difference and the average number of aggregated portions were controlled by changing the molding pressure of the press molding.

(2) Experimental Examples 18 and 20 (no.18 and 20)
Particles having an average particle size shown in Table 1 were used as the soft magnetic metal particles (raw material powder). Then, the soft magnetic metal powder was heat-treated under the same conditions as in Experimental Examples 1-17 and 19.
The soft magnetic metal particles were then coated with a coating liquid. Silica sol was used as the coating liquid. Then, the coated soft magnetic metal particles were dried under the same conditions as in Experimental Examples 1 to 17 and 19, and the coated soft magnetic metal particles were heat-treated. Dust cores according to Experimental Examples 18 and 20 were obtained as described above.
The difference in porosity and the average number of aggregated portions were controlled by changing the molding pressure of the press molding.

Table 1 summarizes the characteristics of the soft magnetic metal particles and the grain boundary phase in each experimental example.
The columns of "Ra" and "Rz" are the arithmetic mean roughness Ra and the maximum height roughness Rz measured by the method described in the column "(4) Requirements for surface roughness of soft magnetic metal particles 3". is shown.
The column of "area ratio" shows the average area ratio (%) measured by the method described in the column of "(6) Requirements for the area ratio of the grain boundary phase 6".
The column "Average number of aggregates" indicates the average number of aggregates measured by the method described in the column "(5) Requirements for grain boundary multiple points".
The column "Porosity difference" shows the difference (%) of P1-P2 measured by the method described in the column "(3.1) Requirements for porosity difference".

2. Evaluation Method of Iron Loss Iron loss was evaluated under the following conditions using the following modified Steinmetz equation for iron loss using a measuring device (BH analyzer, manufactured by Iwasaki Tsushinki Co., Ltd., model number SY-8218).

Core conditions: outer diameter φ8 mm - inner diameter φ4.5 mm thickness 1.5 mm
Enameled wire φ0.3 15 rolls Bifilar roll

Evaluation was performed as follows.

Hysteresis loss (kW/m ³ )
"☆" ... less than 600 "◎" ... 600 or more and less than 700 "○" ... 700 or more and less than 800 "△" ... 800 or more and less than 900 "×" ... 900 or more

Eddy current loss (kW/m ³ )
"☆" ... less than 15 "◎" ... 15 or more and less than 30 "○" ... 30 or more and less than 50 "△" ... 50 or more and less than 80 "×" ... 80 or more

3. Evaluation Results Table 1 shows the evaluation results.
Experimental Examples 1 to 15, which are examples, satisfy the following requirements (a), (b), and (c).
Requirement (a): The average particle size of the soft magnetic metal particles is 5 μm or more and 30 μm or less.
Requirement (b): A crystalline substance containing at least one of Al and Si is present in the grain boundary phase.
- Requirement (c): The difference between P1 and P2 is 3% or less (corresponding to the requirement for (3.1) porosity difference).

In contrast, Experimental Examples 16 to 20, which are comparative examples, do not satisfy the following requirements.
Experimental Example 16 does not satisfy the requirement (a).
Experimental Example 17 does not satisfy the requirement (a).
Experimental Example 18 does not satisfy the requirement (b).
Experimental Example 19 does not satisfy the requirement (c).
In Experimental Example 20, the requirements (b) and (c) are not satisfied.

In Experimental Examples 1 to 15, which are examples, hysteresis loss and eddy current loss were suppressed in a well-balanced manner, as compared with Experimental Examples 16 to 20, which are comparative examples.
Further, among Experimental Examples 1 to 15, Experimental Examples 6 to 15, which further satisfy the following requirement (d), had less eddy current loss.
Further, among Experimental Examples 6 to 15, Experimental Examples 10 to 15, which further satisfy the following requirement (e), had less eddy current loss.
Further, among Experimental Examples 10 to 15, Experimental Examples 13 to 15, which further satisfy the following requirement (f), had a smaller hysteresis loss.

Requirement (d): The surface roughness Ra of the soft magnetic metal particles is 0.05 μm or more and 0.20 μm or less, and Rz is 0.20 μm or more and 0.70 μm or less.
- Requirement (e): The average number of aggregated parts is 5 or more (corresponding to the requirement for (4) grain boundary multiple points).
Requirement (f): The area ratio of the grain boundary phase is 5 to 10% ((2.1) Requirements for the area ratio of the grain boundary phase 6).

4. Effect of Example The powder magnetic core of this example had low hysteresis loss and low eddy current loss.

The present invention is not limited to the embodiments detailed above, and various modifications and changes are possible within the scope of the claims of the present invention.

The powder magnetic core of the present invention is particularly suitable for applications such as motor cores, transformers, choke coils, and noise absorbers.

REFERENCE SIGNS LIST 1... Dust cores 3, 3A, 3B, 3C... Soft magnetic metal particles 4... Layers composed of oxide films and reaction phases 6... Grain boundary phases 35... Pores L1... First coupling lines L2... Second coupling lines D1... Pores D2 where the area ratio P occupying the first visual field is the largest D2 .

Claims (3)

A powder magnetic core comprising soft magnetic metal particles having an average particle size of 5 μm to 30 μm and a grain boundary phase in which a crystalline substance containing at least one of Al and Si is present,
When the cross-sectional structure of the powder magnetic core is observed in a square first field of view of 100 μm × 100 µm and the area ratio P (%) of the pores occupying the first field of view is obtained,
Assuming that the maximum value of the area ratio P is P1 and the minimum value of the area ratio P is P2,
The difference between P1 and P2 is 3% or less,
An oxide film is formed on the surface of the soft magnetic metal particles,
A reaction phase exists between the oxide film and the grain boundary phase,
A powder magnetic core, wherein the metal oxide forming the oxide film is one or more selected from the group consisting of chromium oxide, aluminum oxide, and molybdenum oxide. When the cross-sectional structure of the interface between the soft magnetic metal particles and the grain boundary phase is observed in a square second field of view of 10 μm×10 μm, the surface roughness Ra of the soft magnetic metal particles is 0.05 μm or more and 0.20 μm. 20 μm or more and Rz is 0.20 μm or more and 0.70 μm or less. When the cross-sectional structure of the powder magnetic core is observed in a third field of view of a square of 50 μm × 50 µm for 10 consecutive fields in the vertical or horizontal direction so that the fields of view do not overlap each other,
For grain boundary multiple points surrounded by three or more of the soft magnetic metal grains,
[1] a first bonding line connecting the surfaces of the soft magnetic metal particles adjacent to each other at the closest portions, and [2] connecting the ends of the first bonding lines adjacent to each other to the soft magnetic metal Let S1 be the area of the region of the grain boundary phase obtained by excluding pores from the region surrounded by the second bonding line connecting the inner part of the outline of the grain on the side of the grain boundary multiple point, and
When the average area per one of the three or more soft magnetic metal particles is S2,
1/10≤S1/S2≤1/3
3. The dust core according to claim 1, wherein there are three or more grain boundary multiple points on average per field of view.

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