JP7227737B2 - dust core - Google Patents

Description

The present invention relates to dust cores.

The development of powder magnetic core materials has been actively carried out due to their high degree of freedom in shape and applicability to high-frequency bands. A powder magnetic core is a core formed by coating the surface of magnetic metal particles of about 100 μm with an insulating substance such as phosphoric acid or silica and press-molding the magnetic metal particles coated with an insulating film.
Patent Literature 1 discloses a composite soft magnetic material in which the metal spaces between soft magnetic metal particles are composed of a high resistance soft magnetic substance as a material used for dust cores. In this document, by setting the cross-sectional area ratio of the soft magnetic metal particles in the composite soft magnetic material and the cross-sectional area ratio of the high resistance soft magnetic material to a specific range, the magnetic flux density Bm is sufficiently high in a practical magnetic field. , a composite soft magnetic material with good core loss Pcv can be obtained.
Further, Patent Document 2 discloses a technique of heat-treating (annealing) a molded body obtained by pressure-molding a magnetic core powder made of coated particles in which a ferrite raw material is attached to the surface of soft magnetic particles. there is This document discloses that heat treatment forms a ferrite film on the surface of the soft magnetic particles, thereby improving the specific resistance and the magnetic flux density.
Further, Patent Document 3 discloses a dust core powder in which the ferrite film covering the surfaces of soft magnetic metal particles is composed of ferrite crystal grains having a spinel structure. It is disclosed that this powder for a dust core improves the strength of the ferrite coating due to the coarsened ferrite crystal grains, and suppresses breakage of the ferrite coating due to high stress during compaction.

JP 2017-45892 A JP 2016-157753 A JP 2017-119908 A

By the way, in order to obtain a dust core with low iron loss, it is necessary to increase the magnetic permeability.
However, even if any of the above techniques are applied, the magnetic permeability is not necessarily sufficient, and a new technique for improving the magnetic permeability has been desired.
The present invention has been made in view of the above circumstances, and an object of the present invention is to improve magnetic permeability. The present invention can be implemented as the following modes.

[1] A powder magnetic core having a plurality of iron-based soft magnetic particles and ferrite formed on the surfaces of the iron-based soft magnetic particles,
When the average particle size of the iron-based soft magnetic particles is Da μm, when the cross-sectional structure of the dust core is observed in a square first field of view of 5 Da (μm) × 5 Da (μm), three or more There are N (N = an integer of 5 or more) grain boundary triple points formed by the iron-based soft magnetic particles of and substantially made of ferrite, and when observed in the first field of view, N A powder magnetic core, wherein the peak strength of ferrite at 60% or more of the grain boundary triple junctions is higher than the peak strength of ferrite at ferrite portions other than the grain boundary triple junctions.

[2] the iron-based soft magnetic particles have an aspect ratio of 1.25 or more and 2.5 or less;
When the average particle size of the iron-based soft magnetic particles is Da μm, pores existing in a state surrounded by ferrite when the cross-sectional structure of the dust core is observed in a square second field of view of 10 Da μm × 10 Da μm is 10% or less of the second field of view, the powder magnetic core according to [1].

[3] When the average particle size of the iron-based soft magnetic particles is Da μm, when the cross-sectional structure of the dust core is observed in a square third field of view of 10 Da μm × 10 Da μm, the third field of view is defined. Starting from a place where ferrite exists on one side of the square, ferrite is continuously formed to the side opposite to one side of the square, and has five or more continuous layers different from each other;
The dust core according to [1] or [2], wherein the continuous layer has an average path length of 11.5 Da μm or more from the one side to the opposite side.

[4] The dust core according to any one of [1] to [3], wherein the ferrite present in the dust core contains an alkali metal.

According to the above invention [1], the magnetic permeability of the dust core is increased.
According to the above invention [2], the density of the powder magnetic core can be increased, and as a result, the magnetic permeability of the powder magnetic core is further improved.
According to the above invention [3], the magnetic permeability of the powder magnetic core is further improved.
According to the above invention [4], the inclusion of the alkali metal densifies the ferrite and reduces the rate of change in the resistance value of the powder magnetic core.

It is a schematic diagram which shows a powder magnetic core. The figure on the right shows a schematic view of the cross-sectional structure of the powder magnetic core observed in the first visual field of a square of 5 Da μm×5 Da μm. It is a schematic diagram which shows the cross section of a composite particle. FIG. 4 shows a schematic view of the cross-sectional structure of a powder magnetic core observed in a 10 Da μm×10 Da μm square third field of view. FIG. 4 shows a schematic diagram of the cross-sectional structure of a powder magnetic core observed in a square second field of view of 10 Da μm×10 Da μm.

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 The dust core 1 has a plurality of iron-based soft magnetic particles 3, as shown in the right diagram (cross-sectional view) of FIG. The ferrite 6 forms a continuous layer 21 (21A, 21B, 21C, 21D, 21E) which will be described later. In addition, hatching (parallel lines) in FIG. 1 indicates the iron-based soft magnetic particles 3 . 1 and the like indicate the ferrite 6. FIG. The ferrite 6 in the powder magnetic core 1 indicated by stippling may contain 5.0% by weight or less of a binder-derived component.
The dust core 1 is composed of composite particles 7, which are iron-based soft magnetic particles 3 having ferrite 5 on their surfaces, as shown in FIG. By press-molding a plurality of composite particles 7, the dust core 1 shown in FIG. 1 is obtained.
Then, in the dust core 1 of the present embodiment, when the average particle diameter of the iron-based soft magnetic particles 3 is Da μm, the cross-sectional structure of the dust core 1 is a square of 5 Da (μm) × 5 Da (μm). When observed in the first field of view, there are N (N=an integer of 5 or more) grain boundary triple points T formed by three or more iron-based soft magnetic particles 3 and substantially made of ferrite. When observed in the first field of view, the peak intensity of the ferrite 6 at 60% or more of the grain boundary triple points T among the N grain boundary triple points T is the ferrite 6 at the ferrite portion F other than the grain boundary triple points. is higher than the peak intensity of The “grain boundary triple point” means a region surrounded by three or more iron-based soft magnetic grains, that is, a point where grain boundaries intersect at one point. In the present invention, as shown in FIG. 1, the grain boundary triple point T is defined as a point where the entire inside of the circle becomes ferrite when a perfect circle with a diameter of 500 nm is drawn.
In addition, "substantially composed of ferrite" means that the grain boundary triple point T may contain 5.0% by weight or less of a binder-derived component. The other ferrite part F may also contain ferrite as a main component (90% by weight or more), and may contain a binder-derived component.

An embodiment of the dust core 1 of the present invention will be described in detail below.
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) Iron-based soft magnetic particles 3
As the iron-based soft magnetic particles 3, a wide range of particles of soft magnetic pure iron and iron-based alloys can be used. Fe--Si alloys, Fe--Si--Cr alloys, Fe--Si--Al alloys, Ni--Fe alloys, Fe--Co alloys, Fe-based amorphous alloys and the like can be suitably used as iron-based alloys. In particular, an alloy containing Cr or Al in its composition is more preferable because it forms a metal oxide layer on the surface. When using a metal that does not contain Cr or Al, it is necessary to form a layer of Cr or Al on the surface of the metal in advance by plating or the like.
When using an Fe—Si—Cr alloy, for example, the alloy may have a composition of Si: 0.1 to 10% by mass, Cr: 0.1 to 10% by mass, and the balance: Fe and unavoidable impurities. .
The average particle size of the iron-based soft magnetic particles 3 is not particularly limited, but is preferably 2.0 μm or more and 100 μm or less, more preferably 10 μm or more and 80 μm or less, and even more preferably 20 μm or more and 60 μm or less. The average particle size of the iron-based soft magnetic 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 5 μm or more and 60 μm or less. The average particle diameter of the iron-based soft magnetic particles 3 was obtained by calculating the equivalent circle diameter from the particle area of the cross section of the dust core 1 observed with FE-SEM JSM-6330F.

The iron-based soft magnetic particles 3 may have a metal oxide layer (passive coating) on the surface. By providing the metal oxide layer on the surface, diffusion reaction of metal atoms between the iron-based soft magnetic particles 3 and the ferrite 5 can be suppressed when annealing (heat treatment) is performed.
The metal oxide forming the metal oxide layer 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, the above diffusion of metal atoms is effectively suppressed.
When Fe--Si--Cr alloy particles are used as the iron-based soft magnetic particles 3, a metal oxide layer having an effect of suppressing diffusion of metal atoms can be easily formed. That is, a metal oxide layer is formed on the outer edges of the iron-based soft magnetic particles 3 by oxidizing Cr in the Fe—Si—Cr alloy.
Moreover, the thickness of the metal oxide layer is not particularly limited. The thickness can preferably be between 1 nm and 20 nm. The thickness of the metal oxide layer can be measured using XPS (X-ray photoelectron spectroscopy).

(2) Ferrite 6
(2.1) Structure of Ferrite 6 It is preferable that the powder magnetic core 1 of the present embodiment is provided with a continuous layer 21 in which the ferrite 6 (soft magnetic ferrite) is continuously formed. Moreover, it is preferable that the ferrite 6 has a three-dimensional network structure.
A material for the ferrite 6 is not particularly limited. The material of the ferrite 6 is preferably one or more selected from the group consisting of magnetite, Ni ferrite, Zn ferrite, Mn ferrite, Ni--Zn ferrite, Mn--Zn ferrite, and Ni--Zn--Cu ferrite. Furthermore, it is preferable to use ferrite having an electric resistivity of 10 ⁴ Ω·cm or more. Therefore, at least one selected from the group consisting of Ni ferrite, Zn ferrite, Mn ferrite, Ni--Zn ferrite, Mn--Zn ferrite, and Ni--Zn--Cu ferrite is more preferable.
As the ferrite 6, for example, a soft magnetic ferrite of the following formula [1] or [2] can be preferably used.

[ ₁ ] _Fe3O4
[2] Mx _- Zny _- Fe _{(3-xy) O4}
(In the formula, M is Ni or Mn, and 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1.)

(2.2) Grain boundary triple point T
In the dust core 1 of the present embodiment, when the average particle size of the iron-based soft magnetic particles 3 is Da μm, the cross-sectional structure of the dust core 1 is a 5 Da (μm) × 5 Da (μm) square first A grain boundary triple point T exists as follows when observed in the field of view (see FIG. 1). That is, there are N grain boundary triple points T (N=an integer equal to or greater than 5) in the first field of view. Here, “5 Da μm” means five times the average particle diameter Da μm of the iron-based soft magnetic particles 3 . For example, when the average particle size of the iron-based soft magnetic particles 3 is 10 μm, it means 5 Da μm=5×10 μm=50 μm. Thus, the length of one side of the first field of view is proportional to the average particle size of the iron-based soft magnetic particles 3 . The right diagram of FIG. 1 schematically shows a first visual field of a square of 5 Da μm×5 Da μm when observing the cross-sectional structure of the dust core 1 .
N is not limited as long as it is an integer of 5 or more, but an integer of 5 to 25 is preferable, and an integer of 8 to 20 is more preferable.
The peak intensity of the ferrite 6 at 60% or more of the grain boundary triple junctions T among the N grain boundary triple junctions T is higher than the peak intensity of the ferrite 6 at the ferrite portions F other than the grain boundary triple junctions. The peak intensity of ferrite 6 can be determined by EPMA analysis. The ratio of the grain boundary triple point T having a higher peak intensity than the other ferrite portions F is not limited as long as it is 60% or more, but is preferably 70% or more, more preferably 80% or more. It is particularly preferable that the proportion of the grain boundary triple point T is 100%. If the peak intensity of the ferrite portion F other than the grain boundary triple point is not constant, the average value of the peak intensities of the other ferrite portions F is adopted as the peak intensity of the ferrite portion F.
The requirement for the grain boundary triple point T may be satisfied in at least one of a plurality of 5 Da μm×5 Da μm square fields of view observed when observing the cross-sectional structure of the dust core 1 .
Note that there are N grain boundary triple junctions T (N = an integer of 5 or more), and the peak intensity of the ferrite 6 at 60% or more of the grain boundary triple junctions T is the peak intensity of the ferrite 6 in the other ferrite portions F. The reason why the magnetic permeability of the dust core 1 increases when the strength is higher than the strength is presumed as follows. That is, the voids generated during press forming tend to be concentrated at the grain boundary triple point T, and the high peak intensity at the grain boundary triple point T indicates that the press forming is performed densely enough to fill the voids. . Since the air gap is a factor that significantly lowers the magnetic permeability, when the peak intensity of the grain boundary triple point T is high, the magnetic permeability of the powder magnetic core 1 increases.
In addition, the above-mentioned requirement for the number of grain boundary triple points T and the requirement for peak strength can be controlled by controlling the pressing pressure within the range of 1000 MPa to 2000 MPa.

(2.3) Continuous layer 21
In the dust core 1 of the present embodiment, the continuous layer 21 in which the ferrite 6 is continuously formed preferably satisfies the following first and second requirements.
Note that the first requirement and the second requirement are that when observing the cross-sectional structure of the dust core 1, a plurality of square fields of view of 10 Da μm × 10 Da µm, which will be described later, are observed, and if at least one of them satisfies good.

(2.3.1) First requirement First, the first requirement will be explained. FIG. 3 schematically shows a third field of view of a square of 10 Da μm×10 Da μm when observing the cross-sectional structure of the dust core 1 . Here, “10 Da μm” means 10 times the average particle diameter Da μm of the iron-based soft magnetic particles 3 (the same shall apply hereinafter). For example, when the average particle size of the iron-based soft magnetic particles 3 is 10 μm, it means 10 Da μm=10×10 μm=100 μm. Thus, the length of one side of the first field of view is proportional to the average particle size of the iron-based soft magnetic particles 3 .
Let the starting point S be the place where the ferrite 6 exists on one side 11 of the square defining the third visual field. First, it is found that five or more different routes (paths) are present when tracing the place where the ferrite 6 continues from the starting point S on one side 11 to the side 13 opposite to one side 11 of the square. It is a requirement. That is, the first requirement is that there are five or more continuous layers 21 different from each other. It should be noted that when a branch point is reached on the way, the shortest route to reach the opposite side 13 is selected. As long as the number of different routes is 5 or more, there is no upper limit for the number of routes, but the normal upper limit is 30.
FIG. 3 shows five different continuous layers 21A, 21B, 21C, starting from five different starting points S1, S2, S3, S4, S5 on one side 11 and ending at different ending points E1, E2, E3, E4, E5, respectively. 21D and 21E are shown.
When this first requirement is satisfied, many continuous layers 21 are present in the powder magnetic core 1, and the adjacent iron-based soft magnetic particles 3 are effectively insulated by the ferrite 6, resulting in high withstand voltage characteristics. get higher In addition, the continuous layer 21 of the ferrite 6 binds the iron-based soft magnetic particles 3 together, improving the mechanical strength of the dust core 1 .
The average length of the continuous layer 21 is controlled by the press pressure or the like during press molding, which will be described later. By appropriately changing the pressing pressure, the particles become intricate and meandering.

(2.3.2) Second Requirement Next, the second requirement will be described. The second requirement is that the average length of the path from one side 11 to the opposing side 13 of the continuous layer 21 is 11.5 Da μm or more. For example, when the average particle size of the iron-based soft magnetic particles 3 is 10 μm, 11.5 Da μm=11.5×10 μm=115 μm, so this requirement means 115 μm or more.
The average length of the paths of the continuous layer 21 is more preferably 12.0 Da μm or more, and even more preferably 12.5 Da μm or more. The upper limit of the average length of the paths of the continuous layer 21 is 15.0 Da μm.
In the example of FIG. 1, the second requirement is that the average path length of the continuous layers 21A, 21B, 21C, 21D, and 21E is 11.5 Da μm or more.
When this second requirement is satisfied, the average length of the continuous layer 21 is longer than the length of one side of the first field of view, 10 Da μm. That is, the continuous layer 21 meanders from one side 11 to the opposite side 13 . When the continuous layer 21 is meandering, the area of the continuous layer 21 in contact with the iron-based soft magnetic particles 3 increases compared to when the continuous layer 21 is linear. Therefore, the binding effect between the iron-based soft magnetic particles 3 is enhanced by the continuous layer 21 of the ferrite 6 . In addition, when this requirement is satisfied, the continuous layer 21 meanders between the iron-based soft magnetic particles 3 and leads to more penetration, so the iron-based soft magnetic particles 3 are effectively insulated and the withstand voltage characteristics are high. Become.

(2.4) Method for Forming Ferrite The ferrite 6 forming the continuous layer 21 is derived from the ferrite 5 of the composite particles 7 as described above. A method for forming the ferrite 5 of the composite particles 7 is not particularly limited. Here, an example of the forming method will be described.
For example, the iron-based soft magnetic particles 3 having a metal oxide layer on the surface are subjected to a plating reaction using an ultrasonically excited ferrite plating apparatus. This reaction utilizes the oxidation reaction of Fe ²⁺ →Fe ³⁺ in an aqueous solution, and is a method of depositing spinel-type ferrite on the surface of a substrate, particles, etc. by reacting metal ions and water molecules in the plating solution. In this method, the surface of the metal oxide layer can be coated with ferrite 5 by adjusting the plating conditions and plating time. The thickness of the ferrite 5 can be adjusted by adjusting the plating time.
Normally, when a metal oxide layer such as chromium oxide or aluminum oxide exists on an object to be plated, the reaction rate of plating is significantly lowered, so the ferrite 5 cannot be formed. In order to densely cover the surface of the metal oxide layer (for example, 80% or more of the surface of the metal oxide layer), it is necessary to finely adjust the pH of the plating solution. In the pH-oxidation-reduction potential diagram of the ferrite 5 to be coated, it is necessary to adjust the pH of the plating solution to the high pH side of the ferrite formation conditions. Although this condition changes depending on the composition of the ferrite 5 to be plated, pH=10 to 11 is preferable for Mn--Zn ferrite, and pH=11 to 12 is preferable for Ni--Zn ferrite.

Iron-based soft magnetic particles 3 to be plated are added to a buffer solution adjusted to a desired pH, and a reaction solution in which metal ions as raw materials are dissolved and an oxidizing solution are gradually added to obtain ferrite. 5 is deposited to form a deposited portion. The ultrasonic horn causes the iron-based soft magnetic particles 3 to be violently dispersed while generating heat, and the ferrite formation reaction is accelerated together with the heating from the constant temperature bath. Moreover, as can be seen from the reaction formula below, protons are generated as the reaction progresses, so that the pH in the plating bath gradually changes to acidic. Since variations in pH greatly affect the formation of ferrite, it is necessary to keep the pH in the plating tank constant at all times. By optimizing the plating conditions, suppression of the plating reaction by the metal oxide layer can be minimized.

3Fe ²⁺ +4H ₂ O→Fe ₃ O ₄ +8H ⁺ +2e ⁻

An example of a method for producing iron-based soft magnetic particles 3 coated with ferrite 5 is shown below. A reaction solution containing metal ions in water is prepared. Prepare an oxidizing solution in which an oxidizing agent is dissolved in water. Iron-based soft magnetic particles 3 having a metal oxide layer on their surface are dispersed in a buffer solution adjusted to a predetermined pH. When the reaction liquid and the oxidizing liquid are dropped into the buffer solution in which the iron-based soft magnetic particles 3 are dispersed while applying ultrasonic waves, the ferrite 5 is formed on the metal oxide layer. The pH of the buffer is preferably 11-12, as described above, in the case of Ni--Zn ferrite. The type of buffer solution is not particularly limited.
Although the mechanism by which the ferrite 5 is formed on the surface of the iron-based soft magnetic particles 3 has not been elucidated, it is presumed as follows. Specifically, it is presumed that the hydroxyl groups on the surface of the iron-based soft magnetic particles 3 initiate the reaction and the formation of the ferrite 5 begins.
Thus, iron-based soft magnetic particles 3 covered with ferrite 5 can be produced. By press-molding and annealing the iron-based soft magnetic particles 3, the dust core 1 having the ferrite 6 is manufactured.
The amount of ferrite 5 covering the iron-based soft magnetic particles 3 can be controlled by adjusting the reaction time or the like. By controlling the amount of the ferrite 5 to cover, the thickness of the continuous layer 21 made of the ferrite 6 in the dust core 1 can be adjusted. The reaction time for forming the ferrite 5 on the surface of the iron-based soft magnetic particles 3 is preferably 1 minute or more and 60 minutes or less, more preferably 1 minute or more and 25 minutes or less. Since the reaction rate differs depending on the composition of the iron-based soft magnetic particles 3 used, the reaction time may be appropriately changed according to the type of the iron-based soft magnetic particles 3 .

2. Method for Manufacturing Dust Core 1 (1) Press Molding Press molding is usually used to form the shape of the dust core 1 . The molding pressure during press molding is preferably 500 MPa to 2000 MPa, 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 50° C. to 200° C. during press molding. Heating the mold facilitates plastic deformation of the iron-based soft magnetic particles 3, 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 iron-based soft magnetic particles 3 and is not so preferable.

(2) Annealing It is preferable to anneal the molded body obtained above in order to release the strain applied during press molding. Annealing temperature is preferably 500° C. or higher. The annealing atmosphere is preferably an inert atmosphere such as argon or nitrogen, or a reducing atmosphere such as hydrogen, and may be annealed in a vacuum. Annealing conditions are appropriately changed depending on the types of iron-based soft magnetic particles 3 and ferrite 5 to be used.

3. Preferred aspect ratio of iron-based soft magnetic particles 3 and preferred area ratio of pores 35 of dust core 1 The aspect ratio of the iron-based soft magnetic particles 3 is 1.25 or more and 2.5 or less, and the dust core 1 is observed in a square second field of view of 10 Da μm×10 Da μm, the area ratio of pores 35 existing in a state surrounded by ferrite 6 is preferably 10% or less of the second field of view. In this range, the filling property is improved, and the density of the powder magnetic core 1 can be increased. As a result, the magnetic permeability of the powder magnetic core 1 is improved.
The aspect ratio of the iron-based soft magnetic particles 3 is more preferably 1.30 or more and 2.30 or less, and even more preferably 1.35 or more and 2.10 or less. When observed in the second field of view, the area ratio of the pores 35 surrounded by the ferrite 6 is more preferably 4% or less, more preferably 3% or less of the second field of view.
FIG. 4 shows the second field of view.
The aspect ratio of the iron-based soft magnetic particles 3 can be measured by FE-SEM. Also, the area ratio of the pores 35 can be calculated by image processing of the SEM image or the like.
In addition, the requirements for the area ratio of the pores 35 described in this column are obtained by observing a plurality of 10 Da μm × 10 Da μm square fields of view when observing the cross-sectional structure of the dust core 1, and as long as it is satisfied.

4. Requirements Concerning Alkali Metal The ferrite 6 present in the dust core 1 preferably contains an alkali metal (Li (lithium), Na (sodium), K (potassium), etc.).
If an alkali metal exists in the ferrite 6, the ferrite 6 becomes denser, and the magnetic permeability and resistance change rate of the dust core 1 become smaller.
The presence of alkali metals can be confirmed by TOF-SIMS measurement.

5. Effects of the Dust Core 1 of the Present Embodiment The dust core 1 of the present embodiment has a high magnetic permeability.
The aspect ratio of the iron-based soft magnetic particles 3 is 1.25 or more and 2.5 or less, and when the cross-sectional structure of the powder magnetic core 1 is observed in the second field of view, the area ratio of the predetermined pores 35 is When it is 10% or less of the second field of view, the filling property is improved and the density of the powder magnetic core 1 can be increased. As a result, the magnetic permeability of the powder magnetic core 1 is improved.
When the continuous layer 21 satisfies the first and second requirements, it has good withstand voltage characteristics and high strength. Furthermore, according to this configuration, the magnetic permeability of the dust core 1 is improved, and the electrical resistance of the dust core 1 is increased.
When an alkali metal is present in the ferrite 6, the ferrite 6 becomes denser, and the magnetic permeability and resistance change rate of the powder magnetic core 1 become smaller.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples.
Experimental Examples 1 to 20 are examples, and Experimental Examples 21 to 27 are comparative examples.
In the table, experimental examples are indicated using "no." Also, in the table, when "*" is attached, such as "21*", it indicates that it is a comparative example.

1. Preparation of dust core (1) Experimental example 1 (no. 1)
Fe-5.5% by mass Si-4.0% by mass Cr particles (average particle size: 10 μm) produced by water atomization were used as the iron-based soft magnetic particles (raw material powder).
10 g of iron-based soft magnetic particles were dispersed in 100 ml of an aqueous potassium acetate solution, and the pH was adjusted to 11 with potassium hydroxide. In addition, predetermined amounts of nickel chloride and iron (II) chloride were added to 100 ml of pure water and sufficiently dissolved to obtain a reaction solution. Similarly, potassium nitrite as an oxidizing agent was added to 100 ml of pure water to prepare an oxidizing solution.
The resulting plating solution (solution in which the iron-based soft magnetic particles 3 are dispersed) was heated to 70° C. while flowing nitrogen. While stirring the plating solution, the reaction solution and the oxidizing solution were dropped into the plating solution to form a ferrite film. The reaction was carried out for 25 minutes, and the iron-based soft magnetic particles were washed with pure water and collected with a magnet. After that, the iron-based soft magnetic particles were dried, pulverized and passed through a sieve. Thus, ferrite-coated iron-based soft magnetic particles (composite particles) were obtained.
For molding, the obtained composite particles were filled in a mold and press-molded into a toroidal shape (outer diameter: 8 mm, inner diameter: 4.5 mm, height: 1 mm) at a pressure of 1000 MPa. The obtained pressed body was subjected to heat treatment at 700° C. for 15 minutes in an Ar atmosphere to obtain a powder magnetic core according to Experimental Example 1.

(2) Experimental Examples 2 to 27 (no.2 to 27)
In Experimental Examples 2 to 27, instead of "Fe-5.5% by mass Si-4.0% by mass Cr particles", "nickel chloride, iron (II) chloride", and "25 minutes" in Experimental Example 1, the table 1, "iron-based soft magnetic particles", "salt dissolved in the reaction solution", and "reaction time". A dust core was obtained in the same manner as in Experimental Example 1 except for these.
Table 1 also describes the "iron-based soft magnetic particles", the "salt dissolved in the reaction liquid", and the "reaction time" in Experimental Example 1. Also, in the table, the symbol "↑" indicates that it is the same as the one directly above.

2. Evaluation method (1) FE-SEM observation A cross-sectional observation of the powder magnetic core after annealing was performed. A powder magnetic core was cut in half by a dicing device (cutting device), cured in an epoxy resin, and a diced cut surface was mirror-finished to obtain an evaluation sample. Evaluation samples were observed by FE-SEM.
When obtaining the average particle size of the iron-based soft magnetic particles 3, a square field of view of 100 μm×100 μm was used. The area circle equivalent diameter was calculated from the observed particle area of the iron-based soft magnetic particles 3 and taken as the average particle diameter. In Experimental Examples 1 to 27, the average particle size was 10 μm.

(1.1) Aspect Ratio of Iron-Based Soft Magnetic Particles The aspect ratio of the iron-based soft magnetic particles 3 was obtained from the above FE-SEM observation. The observation was performed in a square field of view of 10 Da μm×10 Da μm, where the average particle size of the iron-based soft magnetic particles 3 was Da μm. For example, when the average particle size of the iron-based soft magnetic particles was 10 μm, a square field of view of 100 μm×100 μm was adopted. The aspect ratio was calculated from cross-sectional observation of the dust core. It was calculated from the ratio of the longest side to the shortest side of the iron-based soft magnetic particles 3 that can be confirmed in the cross-sectional image. This was done for 30 particles, and the average value was taken as the aspect ratio.

(1.2) Ferrite Film The composition of the ferrite film was measured using TOF-SIMS for the ferrite film observed in the above field of view. The elements contained in the ferrite film and their concentrations were measured using TOF-SIMS.
The locations (number) of grain boundary triple points T were observed using a 5 Da (μm)×5 Da (μm) square visual field. Then, for all the observed grain boundary triple points T and ferrite portions F other than the triple points, peak intensities of ferrite were determined by EPMA analysis. In Table 2, when the ratio of the grain boundary triple point T having a peak intensity higher than that of the other ferrite portions F is higher than 60%, it is marked with "o" in the column of high concentration. For example, when 10 grain boundary triple junctions T are observed, if 6 or more of them have a higher peak intensity than the other ferrite portions F, the high concentration column is marked with "O". In this column, when the ratio of the grain boundary triple point T with high peak intensity is less than 60%, "x" is given.
The number of continuous layers was counted by the method described in "(2.3.1) First Requirement" using a square field of view of 10 Da μm×10 Da μm. The path length of each observed continuous layer was determined and their average was calculated as the average length. In Experimental Examples 1 to 27, the average particle size of the iron-based soft magnetic particles 3 is 10 μm, so 11.5 Da μm is 115 μm.
Alkali metals were obtained in the above FE-SEM observation using a square field of view of 3 Da μm×3 Da μm as described in “4. Requirements for Alkali Metals”.

(1.3) Area ratio of pores in dust core Preferable Area Ratio of Pores 35 of Powder Magnetic Core 1”. The area ratio of pores thus determined is indicated as "porosity (%)".

(1.4) Field of View In the observation using the field of view in this test, the same field of view was used when the field of view had the same size. That is, in this case, the same location of the cross-sectional structure of the powder magnetic core was commonly adopted.

(2) Performance evaluation of powder magnetic core (2.1) Complex magnetic permeability The complex magnetic permeability of the powder magnetic core (also simply referred to as “magnetic permeability”) was measured using an impedance analyzer E-4991A manufactured by Agilent Technologies, and the frequency Measurements were made in the range of 1 MHz to 1 GHz. Permeability values were compared at 10 MHz.

(2.2) Resistivity The electrical resistivity (also referred to simply as “resistivity”) of the powder magnetic core was measured by the four-core method using Lorestar GX manufactured by Mitsubishi Chemical Analytech. The change in resistivity was calculated from the resistivity at applied voltages of 1 V and 90 V under the condition of an applied current of 1 μA. Specifically, the resistivity at an applied voltage of 1 V was used as a reference (100%), and the rate of change in resistivity at an applied voltage of 90 V was determined in comparison with this reference. A smaller resistance change rate is desirable.

3. Evaluation results Evaluation results are shown in Tables 2 and 3.
In addition, in Table 2, in the column of alkali content, "O" indicates that an alkali metal is contained, and "X" indicates that an alkali metal is not contained.

In Experimental Examples 1 to 20, which are examples, the number of grain boundary triple points T is 5 or more, and the ratio of grain boundary triple points T having higher peak strength than other ferrite portions F is 60% or more. .
In contrast, Experimental Examples 21 to 27, which are comparative examples, do not satisfy the following requirements.
Experimental Examples 21 to 26 have less than five grain boundary triple points T.
In Experimental Example 27, the ratio of the grain boundary triple point T, which has a higher peak intensity than the other ferrite portions F, is less than 60%.

Experimental Examples 1 to 20, which are working examples, were superior in magnetic permeability compared to Experimental Examples 21 to 27, which are comparative examples.

Further, among Experimental Examples 1 to 20, Experimental Examples 7 to 20, in which the iron-based soft magnetic particles have an aspect ratio of 1.25 or more and 2.5 or less and a porosity of 10% or less, Compared to Experimental Examples 1 to 6, the magnetic permeability and resistivity were improved, and the rate of change in resistance value was smaller.

Further, among Experimental Examples 1 to 20 which are examples, Experimental Examples 12 to 20 in which the number of continuous layers is 5 or more and the average length of the continuous layers is 11.5 Da μm (115 μm) or more, Experimental Examples 1 to 20 Compared to No. 11, the magnetic permeability and resistivity were improved, and the resistance value change rate was smaller.

Further, among Experimental Examples 1 to 20, which are Examples, Experimental Examples 17 to 20 in which the ferrite contains an alkali metal have improved magnetic permeability and resistivity compared to Experimental Examples 1 to 16, The rate of change in resistance value became smaller.

4. Effect of Example The dust core of this example has a high magnetic permeability.

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.
In addition, in the present invention, the same location of the cross-sectional structure of the powder magnetic core can be commonly used for the first and second fields of view. Also, different locations may be adopted for each field of view.

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

1: dust core 3: iron-based soft magnetic particles 5: ferrite 6: ferrite 7: composite particles 11: one side 13: opposite side 21: continuous layer 35: pores S (S1 to S5): starting point E (E1 to E5 ): end point T: grain boundary triple junction F: other ferrite parts than the grain boundary triple junction

Claims (4)

A dust core having a plurality of iron-based soft magnetic particles and ferrite formed on the surface of the iron-based soft magnetic particles,
When the average particle size of the iron-based soft magnetic particles is Da μm, when the cross-sectional structure of the dust core is observed in a square first field of view of 5 Da (μm) × 5 Da (μm), three or more There are N (N = an integer of 5 or more) grain boundary triple points formed by the iron-based soft magnetic particles of and substantially made of ferrite, and when observed in the first field of view, N The peak intensity of ferrite at 60% or more of the grain boundary triple junctions is higher than the peak intensity of ferrite at ferrite portions other than the grain boundary triple junctions,
A powder magnetic core characterized by having an electrical resistivity of 36 Ω·cm or more . The aspect ratio of the iron-based soft magnetic particles is 1.25 or more and 2.5 or less,
When the average particle size of the iron-based soft magnetic particles is Da μm, pores existing in a state surrounded by ferrite when the cross-sectional structure of the dust core is observed in a square second field of view of 10 Da μm × 10 Da μm 2. The dust core according to claim 1, wherein the area ratio of is 10% or less of the second field of view. When the average particle size of the iron-based soft magnetic particles is Da μm, when the cross-sectional structure of the dust core is observed in a square third field of view of 10 Da μm × 10 Da μm, one side of the square that defines the third field of view Above, starting from the place where the ferrite exists, the ferrite is continuously formed to one side of the square and the opposite side, and has 5 or more continuous layers different from each other,
3. The dust core according to claim 1, wherein an average length of a path from said one side to said opposite side of said continuous layer is 11.5 Da [mu]m or more. The dust core according to any one of claims 1 to 3, wherein the ferrite present in the dust core contains an alkali metal.

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