WO2017047761A1

WO2017047761A1 - Dust core

Info

Publication number: WO2017047761A1
Application number: PCT/JP2016/077473
Authority: WO
Inventors: 加藤　哲朗; 西村　和則; 野口　伸
Original assignee: 日立金属株式会社
Priority date: 2015-09-16
Filing date: 2016-09-16
Publication date: 2017-03-23
Also published as: JPWO2017047761A1; JP6846016B2

Abstract

Provided is a dust core having exceptional corrosion resistance while ensuring high electric resistance and strong insulating properties. The present invention is a dust core that includes Fe-M (where M is Al or Cr) alloy particles, wherein the alloy particles are bonded by an oxide phase in which the M element is concentrated, and part of the surface of the dust core has a layered oxide which is a multilayered structure and in which Fe is the primary component.

Description

Dust core

The present invention relates to a dust core made of Fe-based soft magnetic material powder.

Conventionally, coil parts such as inductors, transformers and chokes have been used in a wide variety of applications such as home appliances, industrial equipment and vehicles. The coil component includes a magnetic core and a coil wound around the magnetic core. For such a magnetic core, ferrite having excellent magnetic properties, flexibility in shape and price is widely used.

In recent years, as power supply devices such as electronic devices have been miniaturized, the demand for coil parts that are small and low in profile and can be used even for large currents has become stronger. The adoption of powder magnetic cores using Fe-based soft magnetic material powder having a high saturation magnetic flux density is advancing. As the Fe-based soft magnetic material powder, for example, particles of an alloy such as Fe-Si, Fe-Si-Al, and Fe-Si-Cr are used. An insulating film is formed exclusively on the surface of the alloy particles.

The powder magnetic core obtained by compacting Fe-based soft magnetic material powder is filled with soft magnetic material powder in a die consisting of a punch and a die together with a binder, press-molded at high pressure, vacuum atmosphere, etc. It is formed by annealing at a temperature at which the binder does not decompose in a non-oxidizing atmosphere.

∙ The insulation coating on the surface of alloy particles may be destroyed by molding at high pressure. During molding, the soft magnetic material powder filled in the mold is in close contact with the die surface with a large surface pressure, and when the molded body is taken out of the mold, the alloy particles on the molded body surface side are greatly plastically deformed. In some cases, a number of streak marks are formed in the mold release direction on the close contact surface (hereinafter referred to as a sliding contact surface) with the die surface. In the part where the streak-like mark was formed on the surface of the molded body, the insulating film might be broken due to the particles extending in the release direction. The softer the alloy particles and the higher the malleability, the more likely direct contact of the alloy particles with no inclusions such as insulating coatings between the alloy particles occurs. The frequency increases as the molding is performed at a higher pressure. Finally, a thin metal layer (hereinafter referred to as a conductive portion) is formed on the sliding contact surface of the molded body, and the powder magnetic core obtained by annealing is The insulating film of the alloy particles is broken inside and on the surface, and the insulation tends to be insufficient. Similarly, when the formed body is machined, the alloy particles on the surface side are plastically deformed along with the destruction of the insulating coating, and the alloy particles may be in direct contact with each other.

If the insulation is insufficient and the electric resistance is small in the dust core, for example, as described in Patent Document 1, there is a problem that the core loss tends to increase due to an increase in eddy current loss. Therefore, Patent Document 1 and Patent Document 2 disclose performing surface treatment excluding the conductive portion on the surface of the molded body.

JP 2013-131676 A JP 2006-229203 A

The removal of the conductive part on the surface of the molded body has a certain effect on improving the electric resistance on the surface of the dust core, but it cannot be expected to improve the overall electric resistance including the inside of the dust core. In Patent Literature 1, mechanical removal means such as laser processing is used, and in Patent Literature 2, conductive portions are removed by chemical removal means that are immersed in concentrated hydrochloric acid to remove the conductive portions. However, in the conventional manufacturing process, an extra equipment for removing the conductive portion is necessary, and it is necessary to consider waste liquid treatment and the like, resulting in an increase in manufacturing cost. Further, there is a concern that such removal of the conductive portion may damage parts other than the conductive portion. Furthermore, in the portion where the conductive portion is removed, the alloy phase appears on the surface as it is and is in a state of being easily rusted. Therefore, it is necessary to separately perform a rust prevention treatment or the like.

Therefore, an object of the present invention is to provide a dust core having a large electric resistance and high insulation, and also excellent in rust prevention.

The present invention relates to a powder magnetic core including particles of an Fe-M (M is Al or Cr) based alloy, and the alloy particles pass through an oxide phase enriched in the M element. A dust core which is bonded and has a layered oxide mainly composed of Fe and having a multilayer structure on a part of the surface of the dust core.

In the present invention, the layered oxide has a second oxide layer and a first oxide layer in order from the surface of the powder magnetic core, and the second oxide layer includes the first oxidation layer. It is preferable that the M element is contained more than the physical layer. Furthermore, it is preferable that the first oxide layer is mainly composed of Fe ₂ O ₃ , and the second oxide layer is mainly composed of Fe ₃ O ₄ , and the electric resistance of the first oxide layer is It is preferably greater than the electrical resistance of the second oxide layer.

In the present invention, the Fe-M alloy is an Fe-Al alloy, and it is preferable that Al is concentrated in the oxide phase. Further, the Fe-M alloy is an Fe-Al-Cr alloy, and contains more Al than Cr, and it is preferable that Al is concentrated in the oxide phase.

According to the present invention, it is possible to provide a dust core having a large electric resistance and high insulation, and also excellent in rust prevention.

It is a SEM photograph of the section of the dust core concerning one embodiment of the present invention. It is the SEM photograph which expanded the surface part of the cross section of the powder magnetic core shown in FIG. It is a process flow figure for explaining the manufacturing method of the dust core concerning one embodiment of the present invention. It is a figure for demonstrating the 2nd process of the manufacturing method of the powder magnetic core which concerns on one Embodiment of this invention. It is a perspective view of the molded object obtained by the 2nd process. It is a SEM photograph of the slidable contact surface of the molded object obtained by the 2nd process. It is the SEM photograph which expanded and observed the sliding contact surface of the molded object obtained by the 2nd process. It is the SEM photograph which expanded and observed the surface part in which the electroconductive part of the sliding contact surface of a molded object is not formed. It is the SEM photograph which expanded and observed the surface part in which the electroconductive part of the sliding contact surface of a molded object was formed. It is a SEM photograph of the section of the dust core of an example. It is a mapping figure which shows distribution of Fe corresponding to the observation visual field of the SEM photograph of FIG. 8A. It is a mapping figure which shows distribution of Al corresponding to the observation visual field of the SEM photograph of FIG. 8A. It is a mapping figure which shows distribution of Cr corresponding to the observation visual field of the SEM photograph of FIG. 8A. It is a mapping figure which shows distribution of O corresponding to the observation visual field of the SEM photograph of FIG. 8A. It is a SSRM photograph of a cross section of a dust core.

Hereinafter, embodiments of the powder magnetic core according to the present invention will be specifically described, but the present invention is not limited thereto. In the dust core of the present invention, particles of an Fe-M (M is Al or Cr) alloy, which is a soft magnetic material powder, are bonded via an oxide phase containing the M element, and Part of the surface has a layered oxide mainly composed of Fe and having a multilayer structure.

FIG. 1 is a cross-sectional SEM photograph of a dust core according to an embodiment of the present invention. Although the details will be described later, the dust core shown here is an Fe-M type alloy using particles containing both Al and Cr as M elements. It is observed that a layered structure covering the plurality of alloy particles 3 is formed on the surface side so as to cover them. FIG. 2 is a cross-sectional SEM photograph in which the vicinity of the surface of the powder magnetic core cross section shown in FIG. 1 is enlarged. A layered structure is formed so as to cover a plurality of alloy particles, and it is composed of two layers having different brightness. Although details will be described later, they are all layered oxides 150 mainly composed of Fe. Further, there is a grain boundary phase thinner than the oxide layer between the grains 3 of the alloy, and Al is concentrated.

The specific composition of the Fe-based soft magnetic material powder is not particularly limited as long as it can constitute a powder magnetic core having desired magnetic properties, but the preferred form has the largest content. An alloy powder in which the base element is Fe and the content of Al or Cr is the next highest. Here, Al or Cr means either Al or Cr. However, even when the content of Al is large, Cr may be included, and even when the content of Cr is large, Al may be included. Examples of such Fe-based alloys include Fe-Si-Cr-based, Fe-Si-Al-based, Fe-Al-Cr-based, and Fe-Al-Cr-Si-based alloys. Since these alloy powders contain Al and Cr in addition to the base element Fe, the alloy powder itself is superior in corrosion resistance compared to pure Fe.

The oxide of Fe constituting the alloy and the oxide of non-ferrous metal such as Al and Cr have a larger electric resistance than a single metal or an alloy thereof. Even if the insulating coating of the alloy particles breaks in the manufacturing process of the dust core, the present inventors intervene and bond an oxide phase containing M element of Al or Cr as a grain boundary phase between the alloy particles. At the same time, an oxide mainly composed of Fe liberated from the alloy is formed on the surface of the powder magnetic core, and the oxide mainly composed of Fe is formed in a multilayered manner so as to cover a plurality of alloy particles. Thus, it has been found that the electrical resistance can be increased to improve the insulation. In other words, even if there is a conductive part connected with particles of an alloy of soft magnetic material powder, it is actively oxidized without removing it to form an oxide of Fe or M element, so that it functions as an insulating layer This is the idea. As an oxidation method, heat treatment in an atmosphere containing oxygen is employed. In particular, in order to reduce the manufacturing cost, it is preferable to carry out in the atmosphere that does not require special equipment.

Al is an element that improves the corrosion resistance of the alloy particles themselves and is effective in improving the strength of the dust core. Further, as the Al content increases, the magnetic anisotropy constant decreases and the magnetic permeability increases. Moreover, since the coercive force of the alloy is proportional to the magnetic anisotropy constant, the hysteresis loss can be reduced and the magnetic core loss can be improved. On the other hand, the saturation magnetic flux density decreases. In the case of an Fe—Al-based alloy, from these viewpoints, for example, Al is preferably 4.0% by mass or more and 14.0% by mass or less. More preferably, it is 5.0 mass% or more and 13.0 mass% or less.

Cr has the effect of increasing the corrosion resistance of the alloy particles themselves. If the amount is too large, the saturation magnetic flux density is lowered. Therefore, in the case of an Fe—Cr alloy, for example, Cr is preferably 1.0% by mass or more. More preferably, it is 2.5 mass% or more. On the other hand, Cr is preferably 9.0% by mass or less. More preferably, it is 7.0 mass% or less, More preferably, it is 4.5 mass% or less.

In the case of an Fe-Al-Cr alloy, Al is in the above-mentioned range, and Cr is 16.5% by mass or less in total with Al, and the Al content is larger than the Cr content. preferable.

Furthermore, adding Si has the effect of improving magnetic properties. On the other hand, if the amount of Si becomes excessive, the strength of the powder magnetic core decreases, so Si is preferably 5.0% by mass or less. From the viewpoint of strength, Si is preferably at an inevitable impurity level. For example, Si is preferably regulated to less than 0.5% by mass.

In addition, as long as the soft magnetic material powder exhibits advantages such as formability and magnetic properties, it can contain other elements. However, since a nonmagnetic element causes a decrease in saturation magnetic flux density or the like, it is more preferably 1.0% by mass or less except for inevitable impurities. The soft magnetic material powder is preferably composed of Fe, Al or Cr except for inevitable impurities, and further composed of Si.

FIG. 3 shows a flowchart of the manufacturing process of the dust core according to the embodiment of the present invention. A first step of mixing the soft magnetic material powder and the binder, a second step of pressure-molding the mixture obtained through the first step, and a heat treatment of the molded body through the second step. 3 steps. Hereinafter, the dust core of the present embodiment will be described along the steps.

[First step]
First, the soft magnetic material powder used in the first step will be described. The Fe-based soft magnetic material powder is not particularly limited as long as it has a magnetic property capable of forming a dust core and can form an oxide layer containing the contained element. Magnetic alloys can be used.

The average particle size of the alloy particles of the soft magnetic material powder (here, the median diameter d50 in the cumulative particle size distribution is used) is not limited to this, but has an average particle size of, for example, 1 μm or more and 100 μm or less Can be used. By reducing the average particle size, the strength, magnetic core loss, and high frequency characteristics of the powder magnetic core are improved, so the median diameter d50 is more preferably 30 μm or less, and even more preferably 15 μm or less. On the other hand, when the average particle size is small, the magnetic permeability is low, so the median diameter d50 is more preferably 5 μm or more.

Also, the form of alloy particles is not particularly limited. For example, it is preferable to use granular powder represented by atomized powder from the viewpoint of fluidity and the like. Atomization methods such as gas atomization and water atomization are suitable for producing powders of alloys that have high malleability and ductility and are difficult to grind. The atomization method is also suitable for obtaining a substantially spherical soft magnetic material powder.

Note that an oxide film of Fe, M element, or Si may be formed in a film shape with a thickness of about 5 to 20 nm on the surface of the alloy particles obtained by the water atomization method. Here, the island shape means a state where oxides containing Al and Cr are scattered on the surface of the alloy particles constituting the soft magnetic material powder. In this embodiment, alloy particles are bonded to each other through an oxide derived from an alloy of soft magnetic material powder by heat treatment described later, and an oxide is also formed on the surface of the powder magnetic core. However, it is not always necessary to form an insulating film actively. However, the natural oxide film functions as an insulating film and provides an antirust effect to the alloy particles, so that the soft magnetic material powder can be stored in the atmosphere. It is preferable because unnecessary oxidation can be prevented until the molded body is heat-treated.

Note that the alloy particles may be heat-treated in the atmosphere and oxidized at a high temperature. As another method, an insulating film may be formed on the alloy particles of the soft magnetic material powder by a sol-gel method or the like.

Next, the binder used in the first step will be described. The binder binds the powder alloy particles to each other during press molding, and imparts strength to the molded body to withstand handling after molding. Although the kind of binder does not limit this, For example, various organic organic binders, such as polyethylene, polyvinyl alcohol (PVA), an acrylic resin, can be used. The organic binder is thermally decomposed by heat treatment after molding, but if carbon derived from the organic binder remains, the formation of oxides of M elements is suppressed in the oxide phase between the alloy particles formed by high-temperature oxidation, and the M elements In some cases, the ratio of Fe oxide or the like is higher than that of the oxide, and the electric resistance of the dust core is lowered. For this reason, it is preferable to remove the binder under conditions such that residual carbon is not generated as much as possible, for example, by slowing the rate of temperature rise in a temperature range including the decomposition temperature of the organic binder.

Furthermore, a silicone resin may be used together with an organic binder as an inorganic binder. When the silicone resin is used in combination, the oxide phase contains Si.

The amount of the binder added may be an amount that can be sufficiently distributed between the soft magnetic material powders and can secure a sufficient compact strength. On the other hand, if the amount is too large, the density and strength are lowered. For example, the amount is preferably 0.25 to 3.0 parts by weight with respect to 100 parts by weight of the soft magnetic material powder.

The mixing method of the soft magnetic material powder and the binder in the first step is not particularly limited, but it is preferable to use a mixing / dispersing device such as an attritor.

The mixture obtained by mixing is preferably subjected to a granulation process from the viewpoint of moldability and the like. Various methods can be applied to the granulation process, but it is particularly preferable to have a spray drying step as the granulation method. In the spray drying step, a slurry-like mixture containing soft magnetic material powder and binder and a solvent such as water is spray dried using a spray dryer. By spray drying, a granulated powder having a sharp particle size distribution and a small average particle size can be obtained. According to spray drying, a substantially spherical granulated powder can be obtained, so that the powder feeding property (powder fluidity) during molding is also improved. The average particle diameter (median diameter d50) of the granulated powder is preferably 40 to 150 μm, more preferably 60 to 100 μm, although it depends on the average particle diameter of the alloy particles of the soft magnetic material powder.

A method such as rolling granulation may be applied as the granulation method. The granulated powder obtained by rolling granulation is an agglomerated powder having a wide particle size distribution, but it is suitable for pressure molding by passing the granulated powder through a sieve using, for example, a vibrating sieve. Desired granulated powder can be obtained.

In order to reduce friction between the powder and the mold during pressure molding, it is preferable to add a lubricant such as stearic acid, stearate, zinc stearate to the granulated powder. The addition amount of the lubricant is preferably 0.1 to 2.0 parts by weight with respect to 100 parts by weight of the soft magnetic material powder. On the other hand, the lubricant can be applied to or sprayed on the mold. When the lubricant is used, Zn derived from the lubricant is included in the oxide phase.

[Second step]
Next, the 2nd process of press-molding the granulated powder obtained through the 1st process is explained. The granulated powder obtained in the first step is preferably granulated as described above and provided for the second step. The granulated powder is pressure-molded into a predetermined shape such as a cylindrical shape, a rectangular parallelepiped shape, a toroidal shape, an E shape, a U shape, a pin shape, or a drum shape using a molding die. The molding in the second step may be room temperature molding or warm molding performed by heating to such an extent that the organic binder does not disappear.

FIG. 4 is a view for explaining pressure molding, and FIG. 5 is a perspective view showing an appearance of a molded body obtained by pressure molding. The molding die can take various forms depending on the shape of the molded body and the like. However, in the illustrated example, the configuration of the molding die for press-molding a rectangular flat plate-shaped molded body is shown. As shown in FIG. 4, the molding die 200 includes an upper punch 201, a lower punch 202, and a die 205. An opening into which the upper punch 201 and the lower punch 202 can be inserted is provided at the center of the die 205. The granulated powder 300 is filled in a cavity that appears when the lower punch 202 is combined with the opening of the die 205. An upper punch 201 is inserted into the opening of the die 205 so as to close the cavity. The granulated powder is pressed in the Z direction in the figure so that the pair of upper and

lower punches

201 and 202 come closer to each other, and is formed into a predetermined shape. The upper and

lower punches

201 and 202 are pulled away from each other in the Z direction, and the lower punch 202 is moved in the Z direction so that the molded body 100 appears on the upper side of the die 205, and the molded body 100 is released and molded. Remove from mold.

As shown in FIG. 5, the surface of the obtained rectangular flat plate-shaped molded body 100 is a surface that is pressed by upper and

lower punches

201 and 202 and a surface that is in contact with a die 205, When the molded body 100 is released, a sliding contact surface 101 that slides on the surface of the die 205 appears.

FIG. 6 is an SEM photograph in which the sliding surface of the molded body is observed with a scanning electron microscope (SEM: Scanning Electron Microscope). A plurality of linear streak marks 50 are formed on the sliding contact surface 101 of the molded body 100 across the two surfaces of the pressing surface 102 of the molded body 100 in the Z direction of FIG. 5 (the vertical direction of the photograph in FIG. 6). It is formed. As the molding pressure increases, the number of streak marks 50 increases, and a plurality of streak marks 50 are connected and appear in a planar shape as a conductive portion. FIG. 7A is an SEM photograph in which the sliding contact surface of the molded body is enlarged and observed, and FIG. 7B is an enlarged view of a surface portion (a region surrounded by a solid line in FIG. 7A) where no clear streak is confirmed. FIG. 7C is an SEM photograph obtained by magnifying and observing a surface portion (a region surrounded by a broken line in FIG. 7A) on which clear streak marks are formed. In the figure, particles of the soft magnetic material powder alloy are observed in a light color, and a binder or a void portion is observed in a relatively dark color between the alloy particles. When the surface portion of the molded body 100 where the streak-like marks 50 are formed is observed in an enlarged manner, as shown in FIG. 7C, a region (conductive portion) where a plurality of alloy particles are in direct contact in the Z direction is observed. Further, as shown in FIG. 7B, it was confirmed that there is a portion where the alloy particles are in direct contact with each other in a relatively small region, although it is not clearly observed as the streak 50 on the sliding contact surface 101. . In addition, although the surface state of the upper and

lower punches

201 and 202 is transferred to the pressing surface 102 of the molded body 100, the streak marks 50 like the sliding contact surface 101 are not observed.

[Third step]
Next, the 3rd process of heat-processing the molded object which passed through the said 2nd process is demonstrated. In the third step, the molded body is heat-treated in an oxidizing atmosphere to perform annealing to relieve stress strain applied to the alloy particles during molding, and also perform oxide formation by oxidation (high-temperature oxidation). Oxides are formed inside and on the surface of the dust core. Inside the dust core, alloy particles are bonded via an oxide phase containing M element. The oxide phase and the surface oxide intervening between the alloy particles are formed by the surface oxidation of the alloy particles by the heat treatment, but the structure differs depending on the alloy composition and heat treatment conditions.

The oxide phase intervening between the alloy particles is, for example, an Al-concentrated alloy in the case of an Fe—Al alloy, and the oxide is a corundum type in which Fe and Al are dissolved in addition to Al ₂ O ₃ . Oxides ((Fe, Al) ₂ O ₃ ), FeO, Fe ₂ O ₃ , Fe ₃ O _{4 and the} like may be present. Further, in the case of an Fe-Cr alloy, the oxide phase intervening between the alloy particles is enriched with Cr, and the oxide is a corundum type in which Fe and Cr are dissolved in addition to Cr ₂ O _3. Oxide ((Fe, Cr) ₂ O ₃ ), FeO, Fe ₂ O ₃ , Fe ₃ O _4, etc. may be present. If the alloy is an Fe-Al-Cr alloy and contains more Al than Cr, the oxide phase intervening between the alloy particles will be Al-concentrated, and the oxide will be Al ₂ O ₃ . In addition, there are corundum type oxides ((Fe, Al, Cr) ₂ O ₃ ), Cr ₂ O ₃ , FeO, Fe ₂ O ₃ , Fe ₃ O _4, etc. in which Fe, Al, and Cr are dissolved. Also good. Furthermore, even when Si is contained in the alloy, an oxide of Si may be contained in the oxide phase. Here, the concentration of the M element means that the ratio of the M element to the sum of the Fe and M elements is higher than the ratio in the alloy composition.

When the molded body is oxidized at a predetermined temperature in an atmosphere containing oxygen at a predetermined temperature, an oxide of M element and Fe having a high affinity for O is formed on the surface of the powder magnetic core, and on the inner side of the powder magnetic core. In the oxide phase, M element having a large affinity with O is concentrated. The formation process of oxides derived from alloys by high-temperature oxidation is complicated and the mechanism is unclear and the reason is not clear. However, the affinity of each element with oxygen (O), the ion radius, the oxygen partial pressure in the oxidation process, etc. Inferred to have an effect. The M element which is Al or Cr constituting the soft magnetic material powder has a greater affinity with O than Fe, and Al contains Al and Cr as M elements having a greater affinity with O than Cr. If the composition contains a larger amount of Al than that, Al is concentrated in the oxide phase. Such an oxide covers the particle surface of the alloy of the soft magnetic material powder, and further fills the space between the particles of the alloy to firmly connect the particles and functions as an insulating layer between the particles.

From the cross-sectional SEM photograph of the powder magnetic core shown in FIG. 1 described above, the surface of the powder magnetic core is not covered with the region covered with the layered oxide 150 of Fe, which is a multilayer structure composed of two layers. A region is observed. As a result of examining such a difference in configuration, the region covered with the layered oxide of Fe and the region where the streak traces of the sliding surface of the dust core were in good agreement, It has been found that a layered oxide having a multilayer structure is selectively formed. Further, according to composition mapping in SEM / EDX (EDX: Scanning Electron Microscope / energy dispersive X-ray spectroscopy), the region 250 located on the surface of the dust core and not covered with the layered oxide of Fe (Fig. The particle surface of the alloy (see 1) was covered with an oxide enriched with M element such as the oxide phase.

Although details will be described later, the layered oxide 150 of Fe having a multilayer structure is an oxide mainly composed of Fe according to SEM / EDX mapping, and includes the first oxide layer 150a and the first oxidation layer. A second oxide layer 150b containing more M element (Al, Cr) than the product 150a, and the second oxide layer 150b and the first oxide layer 150a from the surface of the dust core. It is formed so as to be in close contact with each other in order. The layered oxide 150 mainly composed of Fe and having a multilayer structure is formed on the surface of the dust core. This is because the alloy particles 3 are destroyed by plastic deformation of the oxide film on the surface. In addition to this, it is presumed that O is sufficiently supplied during high-temperature oxidation compared to the inside of the dust core.

Furthermore, as a result of performing point analysis on each layer and quantifying the composition, the first oxide layer 150a is an oxide mainly containing Fe and containing a small amount of Al and Cr and having a corundum structure (Fe , Al, Cr) ₂ O ₃ or Fe ₂ O ₃ , and the second oxide layer 150b is an oxide that contains a relatively large amount of Al and Cr and has a reverse spinel structure (Fe, Al, Cr). ₃ has been a O _4. According to the observation of the cross-sectional SEM photograph of the powder magnetic core in FIG. 2, the second oxide layer 150b is filled with irregularities so as to straddle between the alloy particles 3 located on the surface of the powder magnetic core, and the upper layer thereof. A first oxide layer 150a having a thickness of about 3 μm is formed. The layered oxide 150 mainly composed of Fe is firmly adhered to the dust core, and the second oxide layer 150b contributes to the improvement of the adhesion.

The heat treatment can be performed in an atmosphere in which oxygen exists, such as in the air or in a mixed gas of oxygen and an inert gas. Of these, heat treatment in the air is simple and preferable. Further, the pressure of the heat treatment atmosphere is not particularly limited, but is preferably an atmospheric pressure that does not require pressure control. The heat treatment in the third step may be performed at a temperature at which the oxide layer is formed, but is preferably performed at a temperature at which the soft magnetic material powder is not significantly sintered. As the sintering of the soft magnetic material powder proceeds, necking that connects the alloy particles occurs, and the electrical resistance decreases. Specifically, in order to prevent an increase in magnetic core loss and to form an oxide phase between alloy particles and a layered oxide 150 mainly composed of Fe, a range of 700 to 900 ° C. is preferable, and 700 to 800 is preferable. A range of ° C is more preferred. The holding time is appropriately set depending on the size of the dust core, the processing amount, the allowable range of characteristic variation, and the like, and preferably 0.5 to 3 hours, for example. Further, after isothermal oxidation is performed at the peak temperature and time, cooling is performed to room temperature. In order to obtain the layered oxide 150 mainly composed of Fe having the above-described configuration, a temperature decreasing rate from the peak temperature to 200 ° C. is set to 300 ° C. It is preferable to set it to at least ° C / hr. When the temperature decreasing rate is slow, the layered oxide 150 mainly composed of Fe does not have a two-layer structure, and the effect of improving the electric resistance cannot be obtained, or the layered oxide 150 mainly composed of Fe adheres to the dust core. Sexuality may be weakened.

More preferably, the space factor, which is the ratio of the soft magnetic material powder in the dust core subjected to heat treatment, is in the range of 80 to 95%. The reason why such a range is preferable is that increasing the space factor improves the magnetic properties, but if the space factor is excessively increased, cracks tend to occur inside the molded body. A more preferable range of the space factor is 84 to 92%. The dust core obtained as described above exhibits an excellent effect of the dust core itself. That is, high insulation and excellent corrosion resistance are realized.

First, as a soft magnetic material powder used for the production of a dust core, Fe-Al-Cr having an alloy composition of 91.0% Fe-5.0% Al-4.0% Cr in terms of mass percentage is as follows. A soft magnetic material powder, which is a base alloy, was prepared. The soft magnetic material powder is a spherical water atomized powder, and a natural oxide film made of Al ₂ O ₃ having a thickness of about 10 nm is formed on the alloy surface. The average particle diameter (median diameter d50) of the soft magnetic material powder measured with a laser diffraction / scattering particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.) was 18.5 μm.

To 100 parts by weight of the soft magnetic material powder, PVA (Poval PVA-205 manufactured by Kuraray Co., Ltd .; solid content 10%) as a binder was mixed at a ratio of 2.5 parts by weight (first step). The obtained mixture was dried at 120 ° C. for 1 hour and then passed through a sieve to obtain granulated powder. To 100 parts by weight of the granulated powder, 0.4 part by weight of zinc stearate was added and mixed. A mixture for pressure molding was obtained. The obtained mixture was subjected to pressure molding at room temperature with a molding pressure of 0.8 GPa using a press machine to obtain a disk-shaped molded body (second step). The dimension of the obtained molded body is φ6.5 × 5 mm. The space factor and density evaluated by the molded body were 84.9% and 6.22 × 10 ³ kg / m ³ . The opposing flat surface of the molded body is a pressing surface that comes into contact with the punch of the molding die, and the peripheral surface connecting the flat surfaces is a sliding contact surface that comes into contact with the die. In the visual confirmation with the metal microscope, no streak marks were observed on the pressing surface during mold release, but on the sliding contact surface, many streak marks were formed in the thickness direction of the molded body and connected to each other, and the alloy particles A region (conductive portion) in direct contact was observed in a planar shape. Ten samples of the molded body were prepared. In any case, the area of the conductive portion was approximately 70% with respect to the total area of the sliding contact surface.

The compact was heat treated in the atmosphere at a heat treatment temperature of 800 ° C. for 1.0 hour, and the temperature was lowered to room temperature at 375 ° C./hr to obtain a disk-shaped dust core (third step). The space factor and density evaluated by the dust core after the heat treatment were 88.9% and 6.40 × 10 ³ kg / m ³ .

As an evaluation of electrical resistance, the specific resistance of a disk-shaped dust core was measured. First, a conductive adhesive was applied to two flat surfaces (pressurized surfaces) at both ends of the powder magnetic core, dried and solidified, and a measurement object was prepared. An object to be measured was set between the electrodes, and a resistance value R (Ω) was measured by applying a DC voltage of 50 V using an electric resistance measuring device (8340A manufactured by ADC Corporation). From the planar area A (m ² ), thickness t (m), and resistance value R (Ω) of the object to be measured, the specific resistance ρ (Ωm) was calculated by the following equation.
Specific resistance ρ (Ωm) = R × (A / t)

Although the specific resistance of each of the molded bodies not subjected to heat treatment was in a conductive state, the specific resistance was 1 × 10 ² Ωm to 1 × 10 ³ Ωm in the dust core of the example, and the insulation was improved.

While observing a cross section in the thickness direction of the dust core of the example, the distribution of each constituent element was examined using an SEM. 8A to 8E show SEM photographs of the cross section of the dust core shown in FIG. 2 and mapping diagrams showing the element distribution in the corresponding visual field. 8A is a SEM photograph of a cross section of the dust core according to the invention, FIG. 8B is a mapping diagram showing the distribution of Fe, FIG. 8C is a mapping diagram showing the distribution of Al, and FIG. 8D shows the distribution of Cr. FIG. 8E is a mapping diagram showing the O distribution. In the SEM photograph, the portion with high brightness is an alloy particle of soft magnetic material powder, and the portion with low brightness is a grain boundary portion or a void portion. In the mapping diagram, the brighter color tone indicates that there are more target elements. As shown in FIG. 8C, the Al concentration is high on the surface of the alloy of the soft magnetic material powder, and there is a lot of O and oxides are formed, and the alloy grains are bonded to each other with the oxides as grain boundaries. You can see how they are doing. Al has a remarkably high concentration between the alloy particles (grain boundaries) of the soft magnetic material powder. 8B and 8D, the concentration of Fe is lower at the grain boundaries than the inside of the alloy particles, and Cr does not show a large concentration distribution. As a result, an oxide phase containing the element contained in the soft magnetic material powder is formed at the grain boundary, and the oxide phase is an oxide having a higher ratio of Al to the sum of Fe, Al, and Cr than the alloy. It was confirmed. Prior to the heat treatment, such a concentration distribution of each constituent element was not observed, and it was also found that the oxide phase was formed by the heat treatment. In addition, the oxides at the grain boundaries having a high Al ratio were connected to each other, and the oxide phase was also formed on the alloy particles on the surface of the magnetic material.

Further, on the surface side of the powder magnetic core, which is a part corresponding to the streak marks on the sliding contact surface, the concentration of Fe is high, and there is much O, and a layered oxide mainly composed of Fe is formed, It can be seen that the oxide layer mainly composed of Fe layered oxide is layered and has two layers with different brightness. Moreover, as shown in Table 1, among the two oxide layers, the lower second oxide layer contains a large amount of Al and Cr, and Al is concentrated and observed. There is an oxide phase in which Al is concentrated between the second oxide layer and the alloy particles.

Regarding the alloy particles of the oxide layer (first oxide layer, second layer oxide layer) of the multilayer structure mainly composed of Fe and soft magnetic material powder, O and Fe, Al which are constituent elements of the soft magnetic material powder , Cr and the composition were quantified by SEM / EDX point analysis. Spectrum 4 in FIG. 8A is a measurement point in the first oxide layer, spectrum 5 is a measurement point in the second oxide layer, and spectrum 7 is a measurement point of alloy particles. The results are shown in Table 1. The outer first oxide layer is substantially composed of a layered oxide mainly composed of Fe. It can also be seen that the inner second oxide layer contains a large amount of Al and Cr.

In the compact before the heat treatment, such a concentration distribution of each constituent element was not observed, and it was also found that both the first oxide layer and the second oxide layer were formed by the heat treatment.

The spreading resistance of the cross-section of the dust core was measured by scanning spreading resistance microscopy (SSRM) (Scanning Spreading Resistance Microscopy). The spreading resistance is measured by visualizing the spreading resistance directly under the probe by scanning the surface of the sample to which a bias is applied with a conductive probe and measuring the distribution of resistance values two-dimensionally. is there. Such local resistance is spread and called resistance, and the level of the resistance value is displayed by the brightness of the color. The spreading resistance can be measured for each of the first oxide layer and the second oxide layer, and the relative magnitude relationship of the electrical resistance can be clarified. It was measured using an atomic force microscope (AFM: NanoScope IVa AFM Dimension 3100 stage AFM system + SSRM option, manufactured by Bruker AXS, Digital Instruments. FIG. 9 shows an SSRM photograph. From the difference in color tone, it was confirmed that the spreading resistance of the second oxide layer was smaller than that of the first oxide layer, and that the first oxide layer had higher resistance than the alloy particles. Even if the second oxide layer 150b that contributes to the adhesion of the layered oxide mainly composed of Fe has a relatively low resistance, the second oxide layer 150b is covered with the first oxide layer having a high resistance. When the electrode is directly formed on the surface of the powder magnetic core by plating, it is advantageous for suppressing the elongation of the plating.

In addition, corrosion resistance was evaluated separately by a salt spray test. The salt spray test was performed based on JIS 22371 (2000) by using a 5% NaCl aqueous solution and exposing the powder magnetic core at 35 ° C. for 24 hours. As a result of visual confirmation, generation of red rust was not confirmed on the surface of the dust core after the test, and good corrosion resistance was exhibited by the formation of oxides by high-temperature oxidation.

DESCRIPTION OF SYMBOLS 1 Powder magnetic core 3 Alloy particle 50 Streaks 60 Terminal electrode 100 Molded body 101 Sliding contact surface 102 Pressurization surface 150 Layered oxide 150a mainly composed of Fe First oxide layer 150b Second oxide layer 200 Molding Die 201 Upper punch 202 Lower punch 205 Die

Claims

A dust core containing particles of an alloy of Fe-M (M is Al or Cr) series,
The alloy particles are bonded via an oxide phase enriched in the M element,
A dust core having a layered oxide mainly composed of Fe and having a multilayer structure on a part of a surface of the dust core.
The dust core according to claim 1,
The layered oxide has a second oxide layer and a first oxide layer in order from the surface of the dust core.
The second oxide layer is a dust core containing more M element than the first oxide layer.
The dust core according to claim 2,
A powder magnetic core in which the electric resistance of the first oxide layer is larger than the electric resistance of the second oxide layer.
The dust core according to any one of claims 1 to 3,
The Fe-M alloy is an Fe-Al alloy,
A dust core in which Al is concentrated in the oxide phase.
The dust core according to any one of claims 1 to 3,
The Fe-M alloy is an Fe-Al-Cr alloy and contains more Al than Cr.
A dust core in which Al is concentrated in the oxide phase.