JP7430041B2 - Powder core and inductor - Google Patents

Description

The present invention relates to a powder magnetic core and an inductor.

A powder magnetic core is a magnetic core for electromagnetic components manufactured by compression molding soft magnetic powder whose surface is covered with an insulating coating. Since this magnetic core is manufactured using magnetic powder as a raw material, it has a superior degree of freedom in shape compared to conventionally widely used magnetic steel cores or ferrite cores. Furthermore, since the individual particles are insulated and have high electrical resistance, they have excellent electromagnetic conversion efficiency, and because the iron content in the magnetic powder can be increased, the saturation magnetic flux density can be increased. Taking advantage of these advantages, its applications are expanding year by year, and it is used in transformers such as DC-DC converters and inverters, transformer cores of power supplies such as switching power supplies equipped with these transformers, and noise-cutting choke coils. Applications are being expanded.

In recent years, from the viewpoint of resource and energy saving, there has been a demand for smaller powder cores and higher efficiency. In order to meet this demand, improvements in magnetic properties are required while increasing the frequency. In order to increase the frequency, for example, in the case of a chip inductor, it is necessary to support a frequency range of several 100 kHz to several MHz. Items for improving the magnetic properties include lower core loss, higher magnetic permeability, improved frequency characteristics, and improved DC superposition characteristics.

Among these, it is known that as a means to improve the frequency characteristics, the use of magnetic powder made of a high resistivity material with a high ratio of alloy components such as Fe-based amorphous and Fe-Si-Al; The proportion of Fe in the powder is low as the proportion of alloy components is high. Therefore, the saturation magnetic flux density of this type of high resistivity material is inevitably low, and there is a problem that the direct current superimposition characteristics of the powder magnetic core made of this high resistivity material are poor. Furthermore, as a means for improving DC superimposition characteristics, it is generally known to provide an air gap in a magnetic path formed by a magnetic core. Therefore, it seems possible to improve the DC superposition characteristics by adding an air gap to the powder magnetic core mentioned above, but since the original saturation magnetic flux density is too low, it is difficult to obtain the necessary level of DC superposition characteristics. .

For example, in Patent Document 1, a FeSiBPCu nanocrystalline material obtained by partially altering an Fe-based amorphous material is used as the Fe-based alloy soft magnetic powder, and an insulating powder is arranged between the Fe-based alloy soft magnetic powder. By using a powder magnetic core made of powder, it exhibits high saturation magnetic flux density and low loss, while ensuring insulation between soft magnetic powders and providing magnetic properties with an excellent balance between frequency characteristics and DC superposition characteristics. A powder magnetic core shown in the figure has been proposed.

JP 2017-34069 Publication

Incidentally, recently, as typified by the drive frequency of switching power supplies, the drive frequency of devices to which powder magnetic cores are applied has been shifting from the kHz range to the MHz range. In order to meet this demand, it is essential to provide a dust core with high effective magnetic permeability. Furthermore, when considering use in a high frequency range of the MHz level, the powder magnetic core is required not only to have high effective magnetic permeability but also to have good frequency characteristics and direct current superposition characteristics in the MHz band.

However, the powder magnetic core described in Patent Document 1 is not intended for use in the MHz band. Therefore, it could not be said that the frequency characteristics and DC superimposition characteristics at the level required under the usage environment in the high frequency range were satisfied.

In view of the above circumstances, an object of the present invention is to provide a powder magnetic core that can achieve both excellent frequency characteristics and DC superimposition characteristics.

In order to solve the above-mentioned problems, the present invention is a compacted powder formed by molding a soft magnetic powder made of Fe-based alloy particles coated with an insulating film, which is provided with an insulating layer on the surface and has an air gap. In the magnetic core, the Fe-based alloy particles contain Si of 2.0 mass% or more and 6.5 mass% or less, the volume average particle diameter of the soft magnetic powder is 10 μm or more and 50 μm or less, and the density of the powder magnetic core is It is characterized by exceeding 6.40 g/cm ³ .

As in this embodiment, Fe-based alloy particles containing 2.0 mass% to 6.5 mass% of Si (preferably those containing Si with the remainder being Fe and unavoidable impurities) are used as the soft magnetic powder. This results in a high specific resistance, which makes it possible to suppress eddy current loss. In addition, the volume average particle size of the soft magnetic powder is set to 10 μm or more and 50 μm or less, and the overall particle size is made small, so that eddy current loss can be suppressed. For these reasons, good frequency characteristics can be obtained. Furthermore, since the alloy components are small and the density of the powder magnetic core is high, the saturation magnetic flux density becomes large. Thereby, good DC superimposition characteristics can be obtained. Therefore, the dust core of this embodiment can achieve both good frequency characteristics and DC superposition characteristics even when used in a high frequency region such as the MHz band. By providing an air gap in the closed magnetic path, the DC superposition characteristics can be further improved.

The powder magnetic core preferably has a volume resistivity of 1×10 ⁴ Ωcm or more.

The insulating layer is preferably formed of an organic or inorganic compound containing Si and having a volume resistivity of 10 ¹² Ωcm or more, and the thickness of the insulating layer is preferably greater than 5 μm. This makes it possible to omit the bobbin disposed between the winding and the powder magnetic core, making it possible to reduce the size and cost of the inductor.

As the powder magnetic core, the soft magnetic powder has a small diameter Fe-based alloy powder having a relatively small average particle diameter and a large diameter Fe-based alloy powder having a relatively large average particle diameter, and the soft magnetic powder has a small diameter Fe-based alloy powder having a relatively large average particle diameter, and Both the small-diameter Fe-based alloy particles constituting the alloy powder and the large-diameter Fe-based alloy particles constituting the large-diameter Fe-based alloy powder contain 2.0 mass% or more and 6.5 mass% or less of Si, and the insulating coating contains Si. The soft magnetic powder may contain a larger amount of the small-diameter Fe-based alloy powder than the large-diameter Fe-based alloy powder.

The ratio of the small-diameter Fe-based alloy powder contained in the soft magnetic powder is 60% or more and 95% or less by volume, and the ratio of the large-diameter Fe-based alloy powder contained in the soft magnetic powder is 60% or more and 95% or less by volume. It is preferably 5% or more and 40% or less.

It is preferable that both the small-diameter Fe-based alloy particles and the large-diameter Fe-based alloy particles contain Si in an amount of 4.0 mass% or more and 5.0 mass% or less.

In the powder magnetic core described above, it is preferable that the insulating coating has a first coating layer formed by coating the surface of each of the Fe-based alloy particles with a methyl-based silicone resin.

The insulating coating further includes a second coating layer formed between each of the Fe-based alloy particles and the first coating layer, the surface of each of the Fe-based alloy particles being coated with a silane coupling agent. is preferred.

The total thickness of the first coating layer and the second coating layer is preferably 20 nm or more and 100 nm or less.

By attaching a winding to the powder magnetic core described above, an inductor having both frequency characteristics and DC superposition characteristics can be provided.

As described above, according to the present invention, it is possible to provide a powder magnetic core with excellent frequency characteristics and DC superposition characteristics. It is possible to obtain a powder magnetic core suitable for high frequency applications, particularly those used in the MHz band.

FIG. 2 is a plan view showing an example of an inductor. FIG. 3 is an enlarged cross-sectional view of the vicinity of the surface of the powder magnetic core. 2 is a table showing the results of testing the influence of the type of soft magnetic powder on frequency characteristics and DC superposition characteristics. 2 is a table showing the results of testing the influence of particle size on frequency characteristics and DC superposition characteristics. 2 is a table showing the results of testing the influence of the density of an insulating layer on frequency characteristics and DC superposition characteristics. 3 is a table showing the results of testing the influence of the type of insulating layer on frequency characteristics and DC superposition characteristics. 2 is a table showing the results of testing the influence of the thickness of an insulating layer on frequency characteristics and DC superposition characteristics. It is a flowchart which shows the whole flow of the manufacturing method of a powder magnetic core. 9 is a flowchart showing detailed steps of the preparation process shown in FIG. 8. It is a table showing test results with and without an air gap. It is a table showing test results when the material is changed. It is a table showing test results when the blending ratio of Fe-3Si powder and Fe-6.5Si powder was varied. It is a table showing test results when the average particle diameter was varied.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 conceptually shows an example of an inductor 2 using a powder magnetic core 1 according to the present invention.
The powder magnetic core 1 is manufactured by, for example, manufacturing two semicircular powder magnetic core parts 1a, forming an air gap 4 between both ends of each powder magnetic core part 1a, and forming the two powder magnetic core parts 1a into a ring shape. It has a form arranged in . An inductor 2 having an air gap 4 in a closed magnetic path is manufactured by arranging a winding 3 on each powder magnetic core 1a. The width dimension α of the air gap depends on the size of the powder compact, but is set to, for example, 0.5 mm or more and 2.0 mm or less. Note that the powder magnetic core portions 1a are in a mutually fixed state.

Note that the shape of the powder magnetic core 1 is not limited to the shape shown in FIG. 1, and any shape other than a ring shape can be adopted. Although FIG. 1 illustrates a powder magnetic core 1 having two air gaps 4, the number of air gaps 4 is arbitrary, and one or three or more air gaps may be provided. Further, although FIG. 1 illustrates a powder magnetic core 1 formed of two powder magnetic core parts 1a, the number of powder magnetic core parts 1a is also arbitrary, and one or three or more powder magnetic core parts 1a The powder magnetic core 1 can be formed in this manner. The number of windings 3 is also arbitrary, and one or more windings 3 can be attached to the powder magnetic core 1. The mounting position of the winding 3 is not limited to the position shown in FIG.

Each powder magnetic core 1a consists of a preparation process for preparing soft magnetic powder as a raw material powder, a molding process for compression molding the soft magnetic powder, and an insulating layer 5 on the surface of the compact obtained by compression molding (Fig. (see) and magnetic annealing of the green compact. Note that the insulating layer 5 can also be formed after magnetic annealing.

In the preparation step, soft magnetic powder is prepared by coating Fe-based alloy particles with an insulating film.

As the Fe-based alloy particles, those containing Si are used. As Fe-based alloy particles, in addition to those containing Si with the balance being Fe and unavoidable impurities, those containing one or more types of components other than Fe and Si, such as Cr, can also be used. In this case, the proportion of Fe in the alloy particles should be in the first place, the proportion of Si should be in the second place, and components other than Fe and Si should be in the third place or lower. For example, a Fe-Si alloy or a Fe-Si-Cr alloy can be used as the Fe-based alloy particles. The content of Si in the Fe-based alloy particles is 2.0 mass% or more and 6.5 mass% or less (preferably 3.0 mass% or more and 6.5 mass% or less, more preferably 3.0 mass% or more and 4.0 mass% or less). . When the Si content is less than 2.0 mass%, the frequency characteristics deteriorate because it becomes close to pure iron. Furthermore, if the Si content exceeds 6.5 mass%, the powder becomes hard and difficult to plastically deform during compression molding, making it difficult to increase the density of the green compact. In particular, by using Fe-Si alloy powder such as Fe-3.5Si or Fe-6.5Si (mixed powder of Fe-3.5Si and Fe-6.5Si may also be used), DC superposition characteristics can be improved. High effects can be obtained. Soft magnetic powder may be prepared by blending an arbitrary material with high specific resistance based on Fe-based alloy particles.

As the Fe-based alloy particles, those manufactured by gas atomization method are preferable because they have high purity. However, Fe-based alloy particles produced by water atomization or other processes can also be used.

The smaller the particle size of the soft magnetic powder, the more eddy current loss can be suppressed in a high frequency range, thereby improving the frequency characteristics. The volume average particle size of the soft magnetic powder is preferably 10 μm or more and 50 μm or less. When the volume average particle size is smaller than 10 μm, cracks (lamination, etc.) tend to occur in the molded product, and when the volume average particle size exceeds 50 μm, eddy current loss increases and frequency characteristics deteriorate.

Assuming that the soft magnetic powder is a sphere with a uniform diameter, even if the powder is packed tightly, gaps will be created between the particles, making it impossible to achieve high density of the powder magnetic core. The soft magnetic powder is preferably prepared 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. If there is a large amount of fine powder, the fluidity of the powder will decrease, leading to problems such as segregation and powder intrusion into the clearance of the mold, so granulated powder in which fine powder is bound together with a binder can also be used.

Note that the volume average particle diameter MV is an average diameter weighted by volume, and in a group of particles, particle diameters d1, d2, ... di, ... dk are arranged in descending order of particle diameter. Assuming that there are n1, n2, ... ni, ... nk particles, and the volume per particle is Vi,
MV=(V1×d1+V2×d2+...Vi×di+...Vk×dk)/(V1+V2+...Vi+...Vk)
MV=Σ(Vi×di)/Σ(Vi)
It is expressed as The volume average particle diameter can be measured using a laser diffraction/scattering type particle size distribution measuring device.

The insulating film covering the Fe-based alloy particles can be formed of a known insulating material made of an organic or inorganic material that has heat resistance higher than the heating temperature during magnetic annealing (700° C. in this embodiment). When performing magnetic annealing, a material that exhibits small thermal contraction during annealing is suitable. This is because if the thermal contraction is too large, the insulation between the soft magnetic powders may be broken during magnetic annealing, and the soft magnetic powders may become energized. From this point of view, it is preferable to use silicone resin as the material for the insulating film.

When silicone resin is used as the material for the insulating coating, an intermediate layer made of a silane coupling agent, preferably an amino-based silane coupling agent, is provided between the silicone resin serving as the first coating layer and each Fe-based alloy particle. By providing this, the adhesion of the silicone resin to each Fe-based alloy particle is improved. This makes it possible to improve the strength of the green compact before magnetic annealing. In the powder magnetic core as a final product, the intermediate layer formed with the silane coupling agent becomes a second coating layer formed between each Fe-based alloy particle constituting the core and the first coating layer. . Note that the insulating coating material (the material for the first coating layer and the material for the second coating layer) may be one type or a combination of several types. Of course, if the adhesion between the first coating layer and the surface of the Fe-based alloy particles is sufficient, the Fe-based alloy particles can be coated with the insulating coating material forming the first coating layer without using a coupling agent forming the intermediate layer. may be directly coated on the surface. If silicone resin (for example, methyl silicone resin) is used as the material for the insulating coating in this way, the strength of the powder core after magnetic annealing can be improved. When a methyl silicone resin and a silane coupling agent are used, the first coating layer is composed of Si, O, C, and H as the main elements, and the second coating layer is composed of Si, O, C, N, It is composed of H as the main element.

In addition, the materials forming the insulating film include B, Ca, Mg, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Mo, Bi-based oxides, and their oxides. Complex oxides, Li, K, Ca, Na, Mg, Fe, Al, Zn, Mn-based carbonates and their complex carbonates, Ca, Al, Zr, Li, Na, Mg-based silicates and their complexes It can be selected from silicates, Si, Ti, Zr-based alkoxides, composite alkoxides thereof, and the like. The method of coating the Fe-based alloy particles with each of the above-mentioned materials is not particularly limited, and for example, a rolling fluid coating method, various chemical conversion treatments, etc. can be used.

In the molding step, the soft magnetic powder obtained in the preparation step is compression molded using a mold having a predetermined shape to form a green compact. A solid lubricant may be added to the powder magnetic core material from the viewpoint of extending the life of the mold used in compression molding or ensuring the fluidity of the soft magnetic powder. Solid lubricants include, for example, zinc stearate, calcium stearate, magnesium stearate, barium stearate, lithium stearate, iron stearate, aluminum stearate, stearamide, ethylene bisstearamide, oleic acid amide, and ethylene bisstearate. Oleic acid amide, erucic acid amide, ethylene bis-erucic acid amide, lauric acid amide, palmimic acid amide, behenic acid amide, ethylene biscapric acid amide, ethylene bishydroxystearic acid amide, montanic acid amide, polyethylene, polyethylene oxide, starch, It is possible to use molybdenum disulfide, tungsten disulfide, graphite, boron nitride, polytetrafluoroethylene, lauroyl lysine, melamine cyanurate, and the like. The above solid lubricants may be used alone or in combination of several types. Further, the above-mentioned fixed lubricant may be blended into the soft magnetic powder serving as the raw material powder before compression molding, or may be attached to the wall surface of the mold. The blending amount (or adhesion amount) at this time is preferably about 0.3 to 2.0 mass% based on the total soft magnetic powder, for example. If too much solid lubricant is added, the density of the green compact will decrease, leading to a decrease in magnetic properties and strength.

In the magnetic annealing process, the green compact obtained in the forming process is subjected to magnetic annealing treatment for the purpose of removing the magnetostriction of the green compact. This annealing treatment is preferably performed under an inert atmosphere such as nitrogen or argon, but depending on the conditions, it may be performed under an oxidizing atmosphere such as air, air, oxygen, or steam, or under a reducing atmosphere such as hydrogen. It is also possible to implement it by

The heating temperature during the magnetic annealing treatment (magnetic annealing temperature) is preferably set in consideration of the material of the target soft magnetic powder. For example, if each Fe-based alloy particle contains 2.0 to 6.5 mass% Si. In the case of forming a composition that includes the above, it is preferable to set the temperature to 700°C or higher and 850°C or lower. This is because at temperatures below 700° C., magnetostriction cannot be sufficiently removed and iron loss cannot be sufficiently suppressed. Moreover, if the temperature exceeds 850° C., eddy current loss increases due to deterioration of the insulating coating. In addition, when using a methyl-based silicone resin as a material for forming an insulating coating, from the viewpoint of increasing the strength of the dust core, it is preferable to set the magnetic annealing temperature to 800° C. or more and 850° C. or less.

It is important to set the magnetic annealing processing time (magnetic annealing temperature holding time) in consideration of the size, material, etc. 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, the magnetostriction within the compacted powder body is removed, and hysteresis loss can be reduced. When each Fe-based alloy particle is coated with a silicone resin and a silane coupling agent as in this embodiment, silica remains as an insulator, and the surface of each Fe-based alloy particle is coated with the silica. Two coating layers and a first coating layer covering the surface (outer surface) of the second coating layer are formed. Further, when a solid lubricant is blended into the raw material powder, the lubricant is also removed by the magnetic annealing process. As described above, the powder compact after magnetic annealing is in a state of being composed of Fe-based alloy particles and a first coating layer and a second coating layer that cover each Fe-based alloy particle.

As shown in FIG. 2, an insulating layer 5 is formed on the surface of each dust core portion 1a after magnetic annealing. The insulating layer 5 is made of an insulating organic compound or an inorganic compound, and covers the entire surface of the powder magnetic core 1a. The insulating layer 5 is preferably formed of a material containing Si as a main component and having a volume resistivity of 10 ¹² Ωcm or more. Examples of materials that meet the above conditions include silicone resins and layered silicates. As the layered silicate, one containing at least one element among Al, Mg, Na, and K can be used.

If the insulating layer 5 is too thin, the necessary volume resistivity cannot be obtained, so the thickness of the insulating layer 5 is set to be greater than 5 μm, preferably 10 μm or more. If the insulating layer 5 is too thick, the influence on dimensional tolerance cannot be ignored when it is applied to small components such as chip inductors. There is also a concern that if the insulating layer 5 is made extremely thick, the effective magnetic permeability will decrease. Therefore, it is appropriate that the thickness of the insulating layer 5 is 50 μm or less. The insulating layer 5 can be formed by dip coating, for example.

In addition, as already mentioned, the insulating layer 5 can be formed after magnetic annealing or can be formed before magnetic annealing. When forming the insulating layer 5 before magnetic annealing, it is necessary to form the insulating layer 5 with a material having heat resistance higher than the heating temperature during magnetic annealing.

It is preferable that the volume resistivity of the powder magnetic core 1a manufactured by the above procedure is 1×10 ⁴ Ωcm or more. If the volume resistivity is less than 1×10 ⁴ Ωcm, it will be difficult to achieve a bobbinless structure, which will be described later, and the specific resistance will decrease, causing problems such as increased eddy current loss. The term "volume resistivity" used herein refers to the electrical resistance between opposing surfaces of a 1 m ³ cube when a voltage is applied between the opposing surfaces (JIS C2560-1).

Frequency characteristics and DC superimposition characteristics are in a trade-off relationship, and if one is good, the other generally deteriorates. Here, the frequency characteristic is expressed by a change in inductance when the frequency is changed, and a magnetic core whose inductance does not easily change even when the frequency is changed is a magnetic core with excellent frequency characteristics. Further, the DC superposition characteristic is expressed by the change in inductance when a DC magnetic field is applied to the inductor, and a magnetic core whose inductance does not easily decrease even when a large DC magnetic field is applied is a magnetic core with excellent DC superposition characteristics.

As in this embodiment, by using Fe-based alloy particles containing 2.0 to 6.5 mass% of Si, a high specific resistance is achieved, so that eddy current loss can be suppressed. In addition, the volume average particle size of the soft magnetic powder is set to 10 μm or more and 50 μm or less, and the overall particle size is made small, so that eddy current loss can be suppressed. For these reasons, good frequency characteristics can be obtained. In addition, since the alloy component is small and the saturation magnetic flux density is large, good direct current superimposition characteristics can be obtained. Therefore, with the powder magnetic core of this embodiment, it is possible to achieve both frequency characteristics and DC superposition characteristics even when used in a high frequency range of MHz band or higher. Furthermore, since an air gap is provided in the closed magnetic path, the direct current superimposition characteristics can be improved.

In addition, the powder magnetic core 1a has a volume resistivity of 1×10 ⁴ Ωcm or more, and the insulating layer 5 is formed of an organic or inorganic compound containing Si and has a volume resistivity of 10 ¹² Ωcm or more, and Since the thickness of the insulating layer 5 is greater than 5 μm, the bobbin disposed between the winding 3 and the dust core 1 can be omitted.

Since the powder magnetic core 1 is manufactured by compacting soft magnetic powder, there is a risk that the insulation coating formed on the particle surface in submicron units may be deformed or peeled off, resulting in a decrease in the insulation between the particles. If left as is, there is a risk that dielectric breakdown will occur between the winding 3 and the powder magnetic core 1 when the winding 3 is energized, so in order to avoid this, insulation is provided between the winding 3 and the powder magnetic core. It is common to arrange a bobbin made of material. However, since the inductor becomes larger due to the arrangement of the bobbin, there is a problem that the degree of freedom in designing the electrical component module is reduced. This is a significant problem, especially in small-scale chip inductors. In some cases, insulating coating is applied instead of the bobbin, but insulating coating requires many steps, which increases the cost of the inductor. On the other hand, in the powder magnetic core of this embodiment, the bobbin and the insulating coating can be omitted, so the inductor can be made smaller or lower in cost.

Bobbin-less can also be achieved by adding a large amount of high-resistance material such as resin to the soft magnetic powder to improve insulation, but this increases the magnetic gap in the dust core, which reduces the effective magnetic permeability. There is a problem that the value becomes too low. In addition, it is also possible to form the core with a high resistivity material (volume resistivity of about 1 x 10 ⁷ Ωcm) such as Ni-Zn ferrite, but since ferrite has a low saturation magnetic flux density, it is useful for downsizing the inductor. There are concerns that this material is disadvantageous, and that at high temperatures of 150° C. or higher, iron loss increases and it no longer functions as a magnetic material.

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

<Test piece preparation conditions>
As a test piece (Example 1) serving as a standard for evaluating magnetic properties, an insulating coating was formed on the surface of Fe-3Si alloy particles using a silane coupling agent and a silicone resin. The thickness of the insulating layer 5 (the sum of the thicknesses of the first coating layer and the second coating layer) is 20 to 100 nm. This soft magnetic powder was mixed with a solid lubricant and then compression molded at room temperature under a predetermined pressure to obtain a ring-shaped test piece. This test piece was divided into two parts and each part was immersed in a silicone resin solution to form an insulating layer 5 with a thickness of 20 μm, and then magnetically annealed at a temperature of 700° C. or higher to produce two powder magnetic core portions 1a. As shown in FIG. 1, a winding 3 was attached to each powder magnetic core 1a, and both ends of each powder magnetic core 1a were opposed to each other with an air gap 4 interposed therebetween, to form an inductor 2. The winding 3 was formed to have a self-inductance of 10 μH.

<Measurement conditions for effective magnetic permeability, frequency characteristics, and DC superposition characteristics>
With respect to Example 1, changes in magnetic properties when the type, average particle size, and density of the soft magnetic powder were changed were evaluated. The magnetic properties to be evaluated are frequency characteristics, direct current superimposition characteristics, and volume resistivity of the powder magnetic core. First, using an impedance analyzer (IM3570) manufactured by Hioki Electric Co., Ltd., at a frequency of 1 kHz, the effective Magnetic permeability was calculated. Regarding the frequency characteristics, when the effective magnetic permeability at 1 kHz is 100%, the case where the rate of decrease in the effective magnetic permeability at 1 MHz is 10% or less is considered as passing. The DC superimposition characteristics are such that the effective permeability when AC current is 0.01A, frequency is 1kHz, and DC magnetic field is 0 is 100%, and the rate of decrease in effective permeability when a DC magnetic field of 10kA/m is superimposed is 10% or less. The test was considered to have passed if A case where the volume resistivity was 10 ⁴ Ωcm or more was considered to be a pass. Evaluation of frequency characteristics and DC superposition characteristics was performed with an air gap provided in each test piece.

The results of testing the influence of various elements on frequency characteristics and DC superposition characteristics are shown below.

(1) Effects of the type of soft magnetic powder We measured changes in frequency characteristics and DC superimposition characteristics when changing the type of Fe-based alloy particles constituting the soft magnetic powder. There are four types of Fe-based alloy particles: Fe-3Si (Example 1), Fe-6.5Si (Example 2), pure iron (Comparative Example 1), and Fe-based amorphous alloy (Comparative Example 2). The test results are shown in Figure 3.

As shown in Figure 3, in Example 1 using Fe-3Si as the Fe-based alloy particles and Example 2 using Fe-6.5Si, good results were obtained in both frequency characteristics and DC superimposition characteristics. . On the other hand, in Comparative Example 1 in which pure iron was used as the Fe-based alloy particles, the frequency characteristics did not reach the required level. Furthermore, in Comparative Example 2 in which an Fe-based amorphous alloy was used as the Fe-based alloy particles, the DC superposition characteristics did not reach the required level. In this way, there is a trade-off relationship between frequency characteristics and DC superposition characteristics, and if one is good, the other usually deteriorates, but in Examples 1 and 2, frequency characteristics and DC superposition characteristics It has become clear that both can be achieved.

(2) Effect of particle size Next, regarding the soft magnetic powder using Fe-3Si and the soft magnetic powder using Fe-6.5Si, the volume average particle size of the soft magnetic powder will affect the frequency characteristics and DC superimposition characteristics. The impact was evaluated. The volume average particle size of the soft magnetic powder using Fe-3Si was 20 μm (Example 1), 10 μm (Example 3), 50 μm (Example 4), and 75 μm (Comparative Example 3). The volume average particle diameter of the soft magnetic powder using .5Si was 50 μm (Example 5). The test results are shown in Figure 4.

As shown in Figure 4, regardless of whether it is soft magnetic powder using Fe-3Si or soft magnetic powder using Fe-6.5Si, the volume average particle diameter of soft magnetic powder is 20 μm, 10 μm, In Examples 1 to 5 of 50 μm, results were obtained that satisfied the required levels in each of the frequency characteristics and DC superimposition characteristics. On the other hand, in Comparative Example 3 in which the volume average particle size of the soft magnetic powder was 75 μm, the frequency characteristics did not reach the required level. It was also revealed that, although the smaller the particle size, the lower the DC superimposition properties, the DC superimposition properties at the predetermined level could be ensured even at the minimum volume average particle diameter (10 μm) examined. On the other hand, it was also revealed that the larger the particle size, the more eddy current loss in the high frequency range, resulting in a decrease in frequency characteristics. In particular, when the volume average particle diameter exceeds 50 μm, the result is that the frequency characteristics do not meet the required level. Therefore, it has become clear that if the volume average particle diameter of the soft magnetic powder is set to 10 μm to 50 μm, both frequency characteristics and direct current superimposition characteristics can be achieved.

(3) Effect of the density of the powder magnetic core Next, for soft magnetic powder using Fe-3Si, the influence of the density of the powder magnetic core on the frequency characteristics and DC superposition characteristics was evaluated. Three types of density were used: 6.40 g/cm ³ (Comparative Example 4), 6.60 g/cm ³ (Example 1), and 6.80 g/cm ³ (Example 6). The test results are shown in Figure 5.

As shown in FIG. 5, in Examples 1 and 6 with densities of 6.60 g/cm ³ and 6.80 g/cm ³ , results were obtained that satisfied the required levels in each of the frequency characteristics and DC superimposition characteristics. On the other hand, in Comparative Example 4 with a density of 6.40 g/cm ³ , the DC superimposition characteristics did not reach the required level. Therefore, it has become clear that frequency characteristics and direct current superimposition characteristics can be achieved at the same time by setting the density to a level exceeding 6.40 g/cm ³ .

(4) Effect of type of insulating layer Next, the effect of different materials of the insulating layer 5 (see FIG. 2) on frequency characteristics and DC superposition characteristics was measured. In Example 1, the insulating layer 5 was formed using silicone resin, which is an organic compound, and in Example 7, the insulating layer 5 was formed using layered silicate, which is an inorganic compound. In either case, the insulating layer 5 contains Si. In addition, as a comparative example, a powder magnetic core (comparative example 5) in which the insulating layer 5 was formed using an epoxy resin was also prepared. The test results are shown in Figure 6.

As shown in FIG. 6, it was revealed that Examples 1 and 7 satisfied the required levels in each of the frequency characteristics and DC superimposition characteristics. Although the epoxy resin used in Comparative Example 5 had a volume resistivity of 1×10 ¹² Ωcm or more, deformation of the powder core was observed after magnetic annealing, so it was rejected. As described above, when a material having a resistance of 1×10 ¹² Ωcm or more is used as the insulating layer 5, the condition for passing is that there is no problem in strength and shape retention after magnetic annealing. From the results shown in FIG. 6, silicone resin and layered silicate are preferred materials for forming the insulating layer 5. Of course, other inorganic compounds and organic compounds can also be used as the material for the insulating layer 5, as long as they have a volume resistivity of 1×10 ¹² Ωcm or more and do not cause problems in strength and shape retention after magnetic annealing. It is possible to use.

(5) Effect of the thickness of the insulating layer Next, the effect of the thickness of the insulating layer on the frequency characteristics and DC superposition characteristics was measured. In Example 1, the thickness of the insulating layer 5 was 20 μm, in Example 8, the thickness was 10 μm, and in Comparative Example 6, the thickness was 5 μm. The test results are shown in Figure 7.

As shown in FIG. 7, it was revealed that Example 1 with a film thickness of 20 μm and Example 8 with a film thickness of 10 μm satisfied the required levels in each of the frequency characteristics and DC superimposition characteristics. On the other hand, in Comparative Example 6 where the film thickness was 5 μm, the volume resistivity of the dust core did not meet the required level. Therefore, the thickness of the insulating layer 5 is preferably greater than 5 μm.

[Description of second embodiment]
Next, a second embodiment of the powder magnetic core will be described.

In recent years, there has been a demand for smaller and more efficient powder magnetic cores from the viewpoint of resource and energy conservation. There is a need for a powder magnetic core that can exhibit good frequency characteristics and direct current superposition characteristics while exhibiting a required level of magnetic permeability.

Recently, the drive frequency of devices to which powder magnetic cores are applied has been shifting from the kHz range to the MHz range, as typified by the drive frequency of switching power supplies. For example, in the case of a transformer, the higher the frequency range used, the more compact the transformer can be. Therefore, when applying a powder magnetic core to a transformer core, it is necessary to improve the structure of the powder magnetic core so that it can be made smaller. For this purpose, it is important to use a powder magnetic core with excellent magnetic permeability. On the other hand, when considering use in a high frequency range at the MHz level, the powder magnetic core is required not only to have high magnetic permeability but also to have good frequency characteristics and DC superposition characteristics in the MHz range as described above.

In view of the above circumstances, it is desirable to provide a powder magnetic core that can exhibit a level of magnetic permeability required for a magnetic core while also exhibiting frequency characteristics and DC superposition characteristics at a level required in a usage environment in a high frequency range. is desired.

The above problems are solved by the powder magnetic core described below. That is, this powder magnetic core is a powder magnetic core that includes a powder compact obtained by compression molding soft magnetic powder serving as a raw material powder, and an air gap provided in the powder compact. The magnetic powder has a small-diameter Fe-based alloy powder with a relatively small average particle diameter and a large-diameter Fe-based alloy powder with a relatively large average particle diameter, and is a small-diameter Fe-based alloy powder that constitutes the small-diameter Fe-based alloy powder. The particles and the large-diameter Fe-based alloy particles constituting the large-diameter Fe-based alloy powder both contain Si at 2.0 mass% or more and 6.5 mass% or less and are covered with an insulating coating, and the soft magnetic powder contains: It is characterized by the fact that it contains more small-diameter Fe-based alloy powder than large-diameter Fe-based alloy powder.

The present inventors focused on the composition and particle size of Fe-based alloy particles constituting the soft magnetic powder, and as a result of intensive study on the relationship between the composition and particle size distribution of these particles and the air gap, the following composition was found. We have found that when an air gap is applied to a powder magnetic core made of iron-based alloy powder that exhibits a particle size distribution of Specifically, firstly, the decrease in magnetic permeability that may occur when the average particle size of Fe-based alloy particles constituting Fe-based alloy powder is reduced in order to reduce iron loss is It has been found that by blending an appropriate amount of Fe-based alloy particles, it is possible to suppress a reduction in magnetic permeability while exhibiting good frequency characteristics. Second, as mentioned above, we need to create an air gap in the powder core so that the permeability of the powder core is above a predetermined level and the Fe-based alloy powder itself that forms the powder core has good direct current superposition characteristics. It has been found that when applied, a significant improvement effect on DC superimposition characteristics is observed.

The second embodiment below has been made based on this knowledge, and the soft magnetic powder constituting the powder magnetic core is a small-diameter Fe-based alloy powder with a relatively small average particle diameter, and a small-diameter Fe-based alloy powder with a relatively small average particle diameter. The small-diameter Fe-based alloy particles constituting the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy particles constituting the large-diameter Fe-based alloy powder both have Si The soft magnetic powder contains 2.0 mass% or more and 6.5 mass% or less and is covered with an insulating film, and the soft magnetic powder contains a larger amount of small-diameter Fe-based alloy powder than large-diameter Fe-based alloy powder. In this way, the soft magnetic powder constituting the dust core contains more small-diameter Fe-based alloy powder with a relatively small average particle size than large-diameter Fe-based alloy powder with a relatively large average particle size. By doing so, it is possible to suppress a decrease in magnetic permeability while exhibiting good frequency characteristics. In addition, at that time, by controlling the content ratio of Si contained in the Fe-based alloy powder to 2.0 mass% or more and 6.5 mass% or less, it becomes possible to suppress iron loss and improve frequency characteristics. In addition, it is possible to ensure the DC superposition characteristics at the minimum required level. In addition, an air gap is provided in the powder compact formed from the soft magnetic powder having the above composition and particle size distribution. Since the gap is applied, it is possible to effectively enjoy the effect of improving the DC superposition characteristics due to the air gap. From the above, this powder magnetic core is capable of exhibiting the level of permeability required for a magnetic core, as well as the frequency characteristics and DC superimposition characteristics required in a high frequency usage environment. becomes. Therefore, the powder magnetic core can be miniaturized and can be suitably applied to electronic devices that are becoming increasingly energy-saving and miniaturized.

Further, in this powder magnetic core, the average particle diameter of the mixed powder of the small diameter Fe-based alloy powder and the large diameter Fe-based alloy powder may be 20 μm or more and 60 μm or less on a volume average.

As shown in the experimental results described below, when the average particle diameter of each Fe-based alloy powder is less than 20 μm, there is a possibility that the formability of the dust core is significantly reduced. Further, by making the average particle diameter too small, there is a possibility that the magnetic permeability will decrease significantly. On the other hand, if the average particle diameter of each Fe-based alloy powder exceeds 60 μm, there is a possibility that the frequency characteristics will be significantly deteriorated. From the above, by setting the average particle diameter of the mixed powder of small-diameter Fe-based alloy powder and large-diameter Fe-based alloy powder to a volume average of 20 μm or more and 60 μm or less, excellent magnetic permeability, frequency characteristics, and formability can be achieved. It becomes possible to obtain a dust core.

In addition, in this powder magnetic core, the ratio of small-diameter Fe-based alloy powder contained in the soft magnetic powder is 60% or more and 95% or less in terms of volume ratio, and the ratio of large-diameter Fe-based alloy powder contained in the soft magnetic powder is 60% or more and 95% or less. , the volume ratio may be 5% or more and 40% or less.

In this way, if the ratio of the small-diameter Fe-based alloy powder contained in the soft magnetic powder is 60% or more by volume, the effect of incorporating the small-diameter Fe-based alloy powder mentioned above, specifically, the iron loss It becomes possible to enjoy the effect of improving frequency characteristics due to the reduction. Further, if the proportion of the large-diameter Fe-based alloy powder contained in the soft magnetic powder is 5% or more by volume, it is possible to enjoy the above-mentioned magnetic permeability improvement effect.

Moreover, in this powder magnetic core, both the small diameter Fe-based alloy particles and the large-diameter Fe-based alloy particles may contain 2.0 mass% or more and 6.5 mass% or less of Si.

As described above, if the proportion of Si contained in each Fe-based alloy particle is too high, it is difficult to expect a significant improvement in the DC superimposition characteristics by applying an air gap. On the other hand, by setting the proportion of Si contained in each Fe-based alloy particle to 6.5 mass% or less, a significant improvement effect on the DC superimposition characteristics can be expected by applying an air gap. Furthermore, by setting the proportion of Si contained in each Fe-based alloy particle to 2.0 mass% or more, it is possible to increase specific resistance and obtain better frequency characteristics.

Moreover, in the powder magnetic core described above, the insulating coating may have a first coating layer formed by coating the surface of each Fe-based alloy particle with a methyl-based silicone resin.

In this way, by forming the first coating layer as an insulating coating by coating the surface of each Fe-based alloy particle with silicone resin, preferably methyl-based silicone resin, magnetic The strength of the powder magnetic core after annealing can be significantly improved.

In addition, when the powder magnetic core described above has a first coating layer, an insulating coating is formed between each Fe-based alloy particle and the first coating layer, and a silane coupling agent is applied to each Fe-based alloy particle. It may further include a second coating layer that covers the surface of the substrate.

As mentioned above, when using a silicone resin for the insulating coating, an intermediate layer consisting of a silane coupling agent, preferably an amino-based silane coupling agent, is provided between the silicone resin serving as the first coating layer and each Fe-based alloy particle. By providing the intermediate layer consisting of, the adhesion of the silicone resin to each Fe-based alloy particle is improved. This makes it possible to improve the strength of the green compact before magnetic annealing.

Further, when the powder magnetic core described above has first and second coating layers, the sum of the thickness of the first coating layer and the thickness of the second coating layer is 20 nm or more and 100 nm or less. There may be.

By setting the sum of the thickness of the first coating layer and the second coating layer constituting the insulating coating within the above-mentioned range, the insulation required between each Fe-based alloy particle is ensured, while the transparency necessary for the magnetic core is maintained. This is preferable because it is possible to obtain magnetic properties.

As described above, according to the second embodiment, while exhibiting magnetic permeability at a level required for a magnetic core, it also exhibits frequency characteristics and DC superposition characteristics at a level required in a usage environment in a high frequency range. It becomes possible to provide a powder magnetic core that can be obtained.

The second embodiment will be described below based on the drawings.

FIG. 8 is a flowchart showing the overall flow of the method for manufacturing a powder magnetic core according to the second embodiment. As shown in FIG. 8, the method for manufacturing a powder magnetic core according to the present embodiment includes a preparation step S1 in which soft magnetic powder serving as a raw material powder is obtained by preparation, and a compression molding of the soft magnetic powder obtained in the preparation step S1. A powder compacting step S2 for obtaining a powder compact, a magnetic annealing step S3 for subjecting the powder compact to magnetic annealing, and an air gap forming step for forming an air gap in the magnetically annealed powder compact. S4.

Further, as shown in FIG. 9, the adjustment step S1 includes a mixing step S11 in which a small-diameter Fe-based alloy powder having a relatively small average particle size and a large-diameter Fe-based alloy powder having a relatively large average particle size are mixed. and a coating step S12 of covering small-diameter Fe-based alloy particles constituting the small-diameter Fe-based alloy powder and large-diameter Fe-based alloy particles constituting the large-diameter Fe-based alloy powder with an insulating coating material.

(S1) Preparation step (S11) Mixing step In this step, two types of Fe-based alloy powders as soft magnetic powders are mixed. Specifically, a small-diameter Fe-based alloy powder with a relatively small average particle size and a large-diameter Fe-based alloy powder with a relatively large average particle size are mixed. Here, the small-diameter Fe-based alloy particles and the large-diameter Fe-based alloy particles constituting the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy powder, respectively, have a composition containing 2.0 mass% or more and 6.5 mass% or less of Si. to do. In addition, as mentioned above, from the viewpoint of expecting further improvement in DC superimposition characteristics, each Fe-based alloy particle preferably has a composition containing 5.0 mass% or less of Si, and contains 4.0 mass% or less of Si. It is more preferable to have a composition containing the above. In addition, as described above, each Fe-based alloy particle preferably has a composition containing 2.0 mass% or more of Si, from the viewpoint of further improving the frequency characteristics by improving the specific resistance. More preferably, the composition contains 0 mass% or more.

Each Fe-based alloy particle may have an alloy structure containing one or more components other than Fe and Si, such as Cr, but the proportion of Fe in the alloy structure is the highest, and the proportion of Si is It is preferable that the ratio be in the second place and components other than Fe and Si be in the third place or lower. Of course, Fe-based alloy particles containing only Fe and Si may be used. Further, the particles may further contain other components that do not constitute the alloy structure, but it is preferable that the ratio of the other components to the particles is the third or lower. Moreover, the compositions of the small-diameter Fe-based alloy particles and the large-diameter Fe-based alloy particles may be different or may be the same. As an example of a case where the compositions are different, for example, the small-diameter Fe-based alloy particles and the large-diameter Fe-based alloy particles are mixed so that the ratio of Si contained in the small-diameter Fe-based alloy particles is larger than the ratio of Si contained in the large-diameter Fe-based alloy particles. An example of this is setting the composition of alloy particles.

The average particle diameter of the mixed powder of the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy powder is set to, for example, a volume average of 20 μm or more and 60 μm or less. Here, the average particle diameter of each Fe-based alloy powder can be measured by known means, for example, by using a laser diffraction method. Further, when each Fe-based alloy powder is composed of Fe-based alloy particles containing 2.0 to 6.5 mass% of Si as described above, the average particle diameter of the small-diameter Fe-based alloy powder is preferably a volume average. The thickness is set to 10 μm or more and 30 μm or less, more preferably 10 μm or more and 20 μm or less. Further, the average particle diameter of the large-diameter Fe-based alloy powder is preferably set to be 60 μm or more and 80 μm or less on a volume average, and preferably set to 70 μm or more and 80 μm or less.

Furthermore, the mixing ratio of the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy powder having the above configuration is adjusted so that the small-diameter Fe-based alloy powder is included more than the large-diameter Fe-based alloy powder in the mixed powder. Ru. Specifically, the ratio of small-diameter Fe-based alloy powder is 60% or more and 95% or less in volume ratio, and the ratio of large-diameter Fe-based alloy powder is 5% or more and 40% or less in volume ratio. Preferably, the proportion of small-diameter Fe-based alloy powder is 85% or more and 95% or less by volume, and the proportion of large-diameter Fe-based alloy powder is 5% or more by volume. It is preferable to set the mixing ratio so that it is 15% or less.

Note that the means for obtaining each Fe-based alloy powder to be mixed is arbitrary. For example, two types of Fe-based alloy powder each having a desired average particle size are manufactured or purchased, and the above-mentioned two types of Fe-based alloy powders that are manufactured or purchased are A system alloy powder may be mixed. Alternatively, one type of Fe-based alloy powder having a predetermined average particle diameter and particle size distribution is manufactured or purchased, and the manufactured or purchased one type of Fe-based alloy powder is subjected to classification treatment once or multiple times, Two types of Fe-based alloy powders having different average particle diameters may be obtained and these two types of Fe-based alloy powders may be mixed. Further, the means for producing each of the above Fe-based alloy powders is also arbitrary; for example, a powder produced by a water atomization method is used as a small-diameter Fe-based alloy powder, and a powder produced by a gas atomization method is used as a large-diameter Fe-based alloy powder. Is possible. Moreover, the above-mentioned powders can be mixed using any means, for example, using a V-type or double-cone mixer.

(S12) Coating step In this step, the small-diameter Fe-based alloy particles constituting the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy particles forming the large-diameter Fe-based alloy powder are coated with an insulating coating material. Known materials can be used as the insulation coating material used here, such as B, Ca, Mg, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Mo. , Bi-based oxides and their composite oxides, Li, K, Ca, Na, Mg, Fe, Al, Zn, Mn-based carbonates and their composite carbonates, Ca, Al, Zr, Li, Na, Mg The material can be selected from silicates based on these, composite silicates thereof, Si, Ti, Zr based alkoxides, composite alkoxides thereof, and the like. In addition, as in this embodiment, when magnetically annealing the powder compact (step S3 in FIG. 1), a material having a heat resistance higher than the heating temperature during annealing (hereinafter simply referred to as magnetic annealing temperature) is used. suitable. When each Fe-based alloy particle has a composition containing 2.0 to 6.5 mass% of Si as described above, a material having heat resistance of 700° C. or higher is suitable. Furthermore, when performing magnetic annealing, a material that exhibits small thermal contraction during annealing is suitable. In this embodiment, for example, a silicone resin, particularly a methyl-based silicone resin, which can significantly improve the strength of the powder compact after magnetic annealing is preferably used. In a powder magnetic core as a final product, the coating layer formed of this silicone resin becomes a first coating layer that covers the surface of each Fe-based alloy particle constituting the magnetic core.

In addition, when a silicone resin is used as the insulation coating material, an intermediate layer consisting of a silane coupling agent, preferably an amino-based silane coupling agent, is formed between the silicone resin serving as the first coating layer and each Fe-based alloy particle. Providing the intermediate layer improves the adhesion of the silicone resin to each Fe-based alloy particle. This makes it possible to improve the strength of the green compact before magnetic annealing. In the powder magnetic core as a final product, the intermediate layer formed with this silane coupling agent is the second coating layer formed between each Fe-based alloy particle constituting the magnetic core and the first coating layer. Become. Note that the insulating coating material (the material for the first coating layer and the material for the second coating layer) may be one type or may be used in combination of several types. Of course, if the adhesion between the first coating layer and the surface of the Fe-based alloy particles is sufficient, the Fe-based alloy particles can be coated with the insulating coating material forming the first coating layer without using a coupling agent forming the intermediate layer. may be directly coated on the surface. Further, the method of coating each Fe-based alloy particle with the above-mentioned material is not particularly limited, and for example, it is possible to use a rolling fluid coating method, various chemical conversion treatments, etc. In this embodiment, the silane coupling agent that forms the second coating layer is supplied to the mixed powder obtained in the mixing step S11, and then the silicone resin that forms the first coating layer is supplied. These insulating coating materials are supplied to each Fe-based alloy powder while stirring (mixing) each Fe-based alloy powder.

(S2) Powder compacting step In this step, the soft magnetic powder as the raw material powder obtained in the preparation step S1 (mixing step S11, coating step S12) is compression molded in a mold of a predetermined shape. Obtain a powder compact.

Note that a solid lubricant may be used from the viewpoint of extending the life of the mold or ensuring fluidity of the soft magnetic powder. Solid lubricants include, for example, zinc stearate, calcium stearate, magnesium stearate, barium stearate, lithium stearate, iron stearate, aluminum stearate, stearamide, ethylene bisstearamide, oleic acid amide, and ethylene bisstearate. Oleic acid amide, erucic acid amide, ethylene bis-erucic acid amide, lauric acid amide, palmimic acid amide, behenic acid amide, ethylene biscapric acid amide, ethylene bishydroxystearic acid amide, montanic acid amide, polyethylene, polyethylene oxide, starch, It is possible to use molybdenum disulfide, tungsten disulfide, graphite, boron nitride, polytetrafluoroethylene, lauroyl lysine, melamine cyanurate, and the like. The above solid lubricants may be used alone or in combination of several types. Further, the above-mentioned fixed lubricant may be blended into the soft magnetic powder serving as the raw material powder before compression molding, or may be attached to the wall surface of the mold. The blending amount (or adhesion amount) at this time is, for example, 0.3 to 2.0 mass% based on the total soft magnetic powder.

(S3) Magnetic annealing step In this step, the powder compact obtained in the powder compacting step S2 is subjected to magnetic annealing treatment for the purpose of removing the magnetostriction of the compact. This annealing treatment is preferably performed under an inert atmosphere such as nitrogen or argon, but depending on the conditions, it may be performed under an oxidizing atmosphere such as air, air, oxygen, or steam, or under a reducing atmosphere such as hydrogen. It is also possible to implement it by

The heating temperature during magnetic annealing (magnetic annealing temperature) is preferably set in consideration of the material of the soft magnetic powder. For example, if each Fe-based alloy powder contains 2.0 to 6.5 mass% Si, In the case of forming a composition that includes the above, it is preferable to set the temperature to 700°C or higher and 850°C or lower. This is because at temperatures below 700° C., magnetostriction cannot be sufficiently removed and iron loss cannot be sufficiently suppressed. Moreover, if the temperature exceeds 850° C., eddy current loss increases due to deterioration of the insulating coating. In addition, when using a methyl-based silicone resin as a material for forming an insulating coating, from the viewpoint of increasing the strength of the dust core, it is preferable to set the magnetic annealing temperature to 800° C. or more and 850° C. or less.

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

By performing the above-described treatment, the magnetostriction within the compacted powder body is removed. In addition, when each Fe-based alloy particle is coated with a silicone resin and a silane coupling agent as in this embodiment, silica remains as an insulator, and the surface of each Fe-based alloy particle is coated with this silica. A second coating layer is formed, and a first coating layer is formed to cover the surface (outer surface) of the second coating layer. Furthermore, when a solid lubricant is blended into the raw material powder, the lubricant is also removed by the magnetic annealing process. From the above, the powder compact after magnetic annealing is in a state composed of small-diameter Fe-based alloy particles, large-diameter Fe-based alloy particles, and a first coating layer and a second coating layer that cover each Fe-based alloy particle. becomes.

(S4) Air gap forming step In this step, an air gap is formed in the powder compact that has been subjected to magnetic annealing treatment. Specifically, a slit-shaped air gap is formed in the powder compact so as to block a magnetic path formed in the powder compact. For example, when the powder compact has an annular shape, a slit-shaped air gap extending in the radial and axial directions of the powder compact and penetrating the powder compact can be created at one location or Form in two or more places. In this case, the width direction dimension (dimension in the gap direction) of the air gap is set to, for example, 0.5 mm or more and 2.0 mm or less, although it depends on the size of the compacted powder body. As a result, a powder magnetic core with an air gap formed in the powder compact is completed.

The completed powder magnetic core is in a state in which adjacent small-diameter Fe-based alloy particles and small-diameter Fe-based alloy particles and large-diameter Fe-based alloy particles are bonded to each other via the first coating layer and the second coating layer. Since the ratio of small-diameter Fe-based alloy powder contained in the powder magnetic core is larger than the ratio of large-diameter Fe-based alloy powder, large-diameter Fe-based alloy particles are bonded to each other through the first coating layer and the second coating layer. The area covered is very small or non-existent.

As described above, according to the method for manufacturing a powder magnetic core according to the second embodiment, a powder compact obtained by compression molding a soft magnetic powder serving as a raw material powder, and The soft magnetic powder includes a small-diameter Fe-based alloy powder with a relatively small average particle diameter, and a large-diameter Fe-based alloy powder with a relatively large average particle diameter. Both the small-diameter Fe-based alloy particles constituting the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy particles constituting the large-diameter Fe-based alloy powder contain 2.0 mass% or more and 6.5 mass% or less of Si. A powder magnetic core is obtained in which the soft magnetic powder contains a larger amount of small-diameter Fe-based alloy powder than the large-diameter Fe-based alloy powder.

In this way, the soft magnetic powder constituting the dust core contains more small-diameter Fe-based alloy powder with a relatively small average particle size than large-diameter Fe-based alloy powder with a relatively large average particle size. By doing so, it is possible to suppress reduction in magnetic permeability while exhibiting good frequency characteristics. In addition, by controlling the content ratio of Si contained in the Fe-based alloy powder to 6.5 mass% or less, it is possible to suppress iron loss and improve frequency characteristics, and to maintain the minimum level of DC Superimposition characteristics can be ensured. In addition, in the powder magnetic core according to the second embodiment, an air gap is provided in the powder compact formed of soft magnetic powder having the above composition and particle size distribution. Furthermore, since the air gap is applied to the powder magnetic core that can exhibit DC superposition characteristics, it is possible to effectively enjoy the effect of improving the DC superposition characteristics due to the air gap. As described above, according to the powder magnetic core according to the second embodiment, while exhibiting magnetic permeability at a level required as a magnetic core, it also has frequency characteristics and DC superposition characteristics at a level required in a usage environment in a high frequency range. It becomes possible to demonstrate. Therefore, this powder magnetic core can be miniaturized, and can be suitably applied to electronic devices that are becoming increasingly energy-saving and miniaturized.

Although the second embodiment has been described above, the powder magnetic core and the method for manufacturing the powder magnetic core according to the second embodiment may have configurations other than those described above without departing from the spirit thereof. be.

For example, in the above embodiment, the air gap is formed after the compact is magnetically annealed, but it is of course possible to use other procedures. For example, the powder magnetic core according to the present invention can also be obtained by performing the air gap forming step S4 after the powder compacting step S2, and then performing the magnetic annealing step S3. Alternatively, the powder magnetic core according to the present invention may be obtained by forming a powder compact having an air gap in the powder compacting step S2, and then performing the magnetic annealing step S3.

Furthermore, in the above embodiment, a powder obtained by mixing two types of Fe-based alloy powder, a small-diameter Fe-based alloy powder and a large-diameter Fe-based alloy powder, is used as the soft magnetic powder serving as the raw material powder. Other compositions are also possible. For example, it is possible to use a soft magnetic powder obtained by further mixing Fe-based alloy powders having different average particle sizes or compositions with both the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy powder; It is also possible to further mix powders made of other metals to obtain the raw material powder. However, in any case, other powders should be adjusted so that the ratio of small-diameter Fe-based alloy powder to the total soft magnetic powder (raw material powder) is the first, and the ratio of large-diameter Fe-based alloy powder is second. It is better to adjust the blending ratio.

An experiment to prove the usefulness of the above configuration will be described below.

<Test piece preparation conditions>
Fe-6.5Si powder manufactured by Sanyo Special Steel Co., Ltd. was used as a soft magnetic powder serving as a raw material powder for the compacted compact. This soft magnetic powder was subjected to a classification process to produce a small-diameter Fe-based alloy powder with an average particle diameter of 10 μm and a large-diameter Fe-based alloy powder with an average particle diameter of 75 μm. The prepared small-diameter Fe-based alloy powder and large-diameter Fe-based alloy powder were mixed at a predetermined blending ratio, and the average particle size of the mixed powder was adjusted to 30 μm.

In addition, as an insulating coating material, an amino-based silane coupling agent (KBE-903) manufactured by Shin-Etsu Chemical Co., Ltd. is supplied to the mixed powder, and each Fe-based alloy particle is coated with the coupling agent. Methyl-based silicone resin (KR-242A) manufactured by Shin-Etsu Chemical Co., Ltd. was supplied to the mixed powder, and each Fe-based alloy particle was coated with the silicone resin. The amount supplied was adjusted so that the total thickness of the first coating layer and the second coating layer coated with these insulating coating materials was 20 to 100 nm. Further, as a solid lubricant, N,N'-ethylenebisoctadecaneamide (ACRAWAX C) manufactured by Lonza Co., Ltd. was blended at a ratio of 0.3 mass% to the soft magnetic powder to prepare a raw material powder.

A powder compact having a predetermined shape was obtained by filling the raw material powder having the above structure into a predetermined mold and compression molding it. Here, a powder compact having an annular shape of φ20 mm×φ12 mm×7 mm was obtained. Further, the pressing force was adjusted so that the green compact density of this green compact was 6.55 g/cm ³ . Next, the compact was subjected to magnetic annealing treatment. Then, the powder compact after magnetic annealing was subjected to a cutting process to form an air gap having a gap width of 1 mm passing through the powder compact in the radial and axial directions at one location in the circumferential direction.

<Evaluation conditions for frequency characteristics and DC superposition characteristics>
Winding the powder magnetic core produced as described above so that the self-inductance is 10 μH, measuring the self-inductance when changing the frequency or alternating current, and evaluating the rate of decrease. The frequency characteristics and DC superposition characteristics were evaluated. Here, the self-inductance was measured using an impedance analyzer (IM3570) manufactured by Hioki Electric Co., Ltd. The frequency characteristics were evaluated as the rate of decrease in self-inductance when the frequency was increased from 1 kHz to 5000 kHz, assuming that the self-inductance at 1 kHz was 100%. In addition, the DC superposition characteristic is the self-inductance when the DC superimposition current is increased from 0A to 20kA, assuming that the self-inductance under the conditions of AC current 0.01A, frequency 1kHz, and DC superposition current 0A is 100%. It was evaluated as a rate of decrease. The DC magnetic field was calculated based on the DC superimposed current, the magnetic path length of the sample, and the number of windings. The frequency characteristics measured and evaluated under the above conditions were determined to be passed (○) when the inductance reduction rate at 2 MHz was 20% or less, and failed (×) when it exceeded 20%. Further, the DC superimposition characteristics measured and evaluated under the above conditions were evaluated as passed (○) when the inductance reduction rate at 10 kA was 10% or less, and failed (x) when it exceeded 10%. The results of investigating the effects of various elements on frequency characteristics and DC superposition characteristics are shown below.

(1) Effect of air gap The powder compacts produced under the above conditions were prepared with an air gap (Example 9) and without an air gap (Comparative Example 7), and the frequency characteristics and The DC superposition characteristics were evaluated. The points other than the presence or absence of an air gap are the same.

FIG. 10 shows test results with an air gap (Example 8) and without an air gap (Comparative Example 6). As shown in FIG. 10, although no major difference was observed in the frequency characteristics, it was found that the DC superimposition characteristics were significantly improved by providing an air gap.

(2) Effect of particle composition Next, as a powder magnetic core, the composition of the small-diameter Fe-based alloy particles and large-diameter Fe-based alloy particles that become the soft magnetic powder was changed from Fe-6.5Si (Example 9) to Fe-3Si (Example 9). A modified version of Example 10) was prepared. In addition, Fe-based alloy particles having a composition of Fe-9.7Si-5.5Al (Comparative Example 8), pure Fe (Comparative Example 9), and Fe-based amorphous (Comparative Example 10) were prepared. The magnetic annealing temperature in the case of pure Fe was 600°C, the magnetic annealing temperature in the case of Fe-based amorphous was 480°C, and the magnetic annealing temperature in other cases was 800°C. The points other than the particle composition are the same.

FIG. 11 shows the test results when the material was changed. When Fe-3Si (Example 10) and Fe-6.5Si (Example 9) were used, good results were obtained in both frequency characteristics and DC superposition characteristics. On the other hand, when Fe-9.7Si-5.5Al (Comparative Example 8) and Fe-based amorphous (Comparative Example 10) were used, although the frequency characteristics were excellent, the DC superimposition characteristics were poor. Further, when pure Fe (Comparative Example 9) was used, although the DC superimposition characteristics were excellent, the frequency characteristics were poor.

(3) Influence of Si Next, two types of Fe-based alloy powders with different Si contents were mixed at various blending ratios and prepared. These powders are the same as in Examples 9 and 10. Specifically, the blending ratio of the Fe-3Si composition was set to 0 mass% (Example 9), 25 mass% (Example 11), 50 mass% (Example 12), 75 mass% (Example 13), and 100 mass% (Example 13). 10) were prepared in five stages. In this case, the blending ratios of the Fe-6.5Si composition are 100 mass% (Example 9), 75 mass% (Example 11), 50 mass% (Example 12), 25 mass% (Example 13), and 0 mass% (Example 13), respectively. Example 10). The points other than the particle blending ratio are the same.

FIG. 12 shows test results when the blending ratios of Fe-3Si powder and Fe-6.5Si powder were varied. As shown in FIG. 12, no matter what the blending ratio (Examples 9 to 13), in other words, if an Fe-based alloy with a Si content within a certain range is used for the soft magnetic powder, It was found that good results were obtained in both frequency characteristics and DC superposition characteristics.

(4) Effect of average particle size Next, small-diameter Fe-based alloy powder and large-diameter Fe-based alloy powder were appropriately blended, and the average particle size was adjusted to 10 μm (Comparative Example 11), 20 μm (Example 14), and 30 μm (Example 14). Example 9), 60 μm (Example 15), and 70 μm (Comparative Example 12) were prepared. The points other than the average particle diameter are the same.

FIG. 13 shows test results when the average particle diameter was varied. As shown in FIG. 13, it was found that when the average particle diameter was too small (Comparative Example 11), cracks occurred in the molded product, resulting in insufficient moldability. Furthermore, it was found that when the average particle diameter was too large (Comparative Example 12), the frequency characteristics did not reach the target value. On the other hand, when the average particle diameter was 20 to 60 μm (Examples 9, 14, and 15), good results were shown in both frequency characteristics and DC superimposition characteristics.

1 Powder magnetic core 1a Powder magnetic core part 2 Inductor 3 Winding 4 Air gap 5 Insulating layer S1 Preparation process S11 Mixing process S12 Coating process S2 Powder forming process S3 Magnetic annealing process S4 Air gap forming process.

Claims (9)

In a powder magnetic core that is formed by molding soft magnetic powder made by covering Fe-based alloy particles with an insulating film, has an insulating layer on the surface, and has an air gap,
The Fe-based alloy particles contain 2.0 mass% or more and 6.5 mass% or less of Si,
The volume average particle size of the soft magnetic powder is 20 μm or more and 60 μm or less,
Density exceeds 6.40g/ ^cm3 ,
The soft magnetic powder has a small-diameter Fe-based alloy powder with a relatively small average particle size and a large-diameter Fe-based alloy powder with a relatively large average particle size,
The small diameter Fe-based alloy particles constituting the small-diameter Fe-based alloy powder and the large-diameter Fe-based alloy particles forming the large-diameter Fe-based alloy powder both contain Si at 2.0 mass% or more and 6.5 mass% or less, and covered with the insulating coating, water atomized powder is used as the small diameter Fe-based alloy particles, and gas atomized powder is used as the large-diameter Fe-based alloy particles,
The average particle size of the small-diameter Fe-based alloy powder is 10 μm or more and 30 μm or less on a volume average, and the average particle size of the large-diameter Fe-based alloy powder is 60 μm or more and 80 μm or less on a volume average,
The small-diameter Fe-based alloy powder is contained in the soft magnetic powder in a larger amount than the large-diameter Fe-based alloy powder, and the ratio of the small-diameter Fe-based alloy powder contained in the soft magnetic powder is 60% by volume. % or more and 95% or less, and the ratio of the large-diameter Fe-based alloy powder contained in the soft magnetic powder is 5% or more and 40% or less in terms of volume ratio. The powder magnetic core according to claim 1, wherein the Fe-based alloy particles contain Si with the remainder being Fe and unavoidable impurities. The powder magnetic core according to claim 1 or 2, having a volume resistivity of 1×10 ⁴ Ωcm or more. The insulating layer is formed of an organic compound or inorganic compound containing Si and having a volume resistivity of 10 ¹² Ωcm or more, and the thickness of the insulating layer is greater than 5 μm. powder magnetic core. The powder magnetic core according to claim 1, wherein both the small-diameter Fe-based alloy particles and the large-diameter Fe-based alloy particles contain Si in an amount of 4.0 mass% or more and 5.0 mass% or less. The powder magnetic core according to any one of claims 1 to 5, wherein the insulating coating has a first coating layer formed by coating the surface of each of the Fe-based alloy particles with a methyl-based silicone resin. The insulating coating further includes a second coating layer formed between each of the Fe-based alloy particles and the first coating layer, the surface of each of the Fe-based alloy particles being coated with a silane coupling agent. The powder magnetic core according to item 6. The powder magnetic core according to claim 7, wherein the sum of the thickness of the first coating layer and the thickness of the second coating layer is 20 nm or more and 100 nm or less. An inductor formed by attaching a winding to a powder magnetic core according to any one of claims 1 to 8.

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