JP6911401B2 - Powder magnetic core - Google Patents

Description

The present invention relates to a dust core, and more particularly to a powder core containing a metallic magnetic material having an insulating rust preventive coating.

In recent years, there has been a demand for miniaturization of coil parts such as inductors, choke coils, transformers, and motors. Therefore, a metallic magnetic material having a higher saturation magnetic flux density than ferrite and maintaining DC superimposition characteristics up to a high magnetic field. Has come to be widely used. Most of the metal magnetic materials are composed of Fe-based alloys, and are widely used as powder magnetic cores (cores) obtained by compression molding metal magnetic materials.

The dust core is used in various environments, and sufficient reliability and durability are required even in a humid environment such as a beach or the sea. In such an environment, particularly high corrosion resistance and rust prevention are required.

Most of the dust cores currently in practical use are composed of Fe-based alloys as described above, and are susceptible to erosion by moisture. For this reason, there is always a concern that rust will occur in equipment using a metallic magnetic material containing an Fe-based alloy.

Patent Document 1 discloses metal magnetic particles containing Cr that improve corrosion resistance, and further describes that a glass film containing _{SiO 2 and the like is formed on the surface of the metal magnetic particles to improve corrosion resistance.} ing. However, for the purpose of improving corrosion resistance, measures such as incorporating Cr into the metal composition are indispensable, and the range of material selection is narrowed. Further, the glass coating has a problem of low corrosion resistance. Therefore, it is necessary to increase the thickness of the glass coating in order to develop sufficient corrosion resistance. As a result, it is considered that the distance between the metal magnetic particles is widened and the magnetic permeability μ as the dust core is lowered.

Further, Patent Document 2 describes that the corrosion resistance is improved by coating a magnetic component having a coil formed therein with ceramics and a resin. However, in order to apply such a coating, it is necessary to heat-treat the dust core at a high temperature of 800 ° C. or higher. When the dust core contains an insulated copper winding or the like, there is a problem that the insulating property of the winding is destroyed by being exposed to a high temperature.

Patent Document 3 describes a method of forming a layer containing MgO on the surface of magnetic particles for the purpose of electrical insulation between magnetic particles. However, in the method described in Patent Document 3, the bondability between the magnetic particles and MgO is weak, and MgO may be peeled off during molding with a mold when molding the dust core. As a result, the magnetic metal is exposed, so that the corrosion resistance is significantly reduced.

JP-A-2010-62424 Japanese Unexamined Patent Publication No. 2010-118587 Special Table 2003-522298

Since rusting tends to proceed in an acidic or neutral environment, it is considered effective to make the inside of the dust core and the surface of the dust core alkaline in order to prevent rusting of the dust core. From this point of view, the teaching of Patent Document 3 in which the surface of magnetic particles is coated with MgO is considered to be promising. However, as described above, in the method of Patent Document 3, since the bondability between the magnetic particles and the MgO coating layer is low, there is a problem that the coating layer is peeled off during molding of the dust core and the corrosion resistance is lowered.

The present invention has been made in view of such an actual situation, and an object of the present invention is to provide a dust core having good corrosion resistance and durability.

As a result of examining the corrosion resistance, especially the rust prevention property, of the metal magnetic particles constituting the dust core, the present inventors have focused on the fact that rust easily progresses in an acidic or neutral environment, and the metals constituting the dust core. In order to make the surface of the magnetic particles alkaline, it was examined to coat them with a ceramic film containing an alkaline earth metal. Further, by providing an Fe-based oxide interlayer film between the ceramic film containing the alkaline earth metal and the metal magnetic particles, the bondability between the ceramic layer containing the alkaline earth metal and the metal magnetic particles was improved. .. As a result, it was found that the peeling of the ceramic layer containing an alkaline earth metal during compaction molding is suppressed, cracks in the dust core are also reduced, and the dust core exhibits good corrosion resistance and durability. The present invention has been completed.

That is, the present invention includes the following gist.
(1) A dust core containing a metallic magnetic material coated with a ceramic film containing an alkaline earth metal, wherein the film containing the alkaline earth metal has an average thickness of 10 to 500 nm.
A dust core characterized in that an Fe-based oxide interlayer film is present between the film containing an alkaline earth metal and the metal magnetic material.
(2) The dust core according to (1), wherein the Fe-based oxide interlayer film has an average thickness of 1 to 50 nm.
(3) The dust core according to (1) or (2), wherein a Si-based oxide film is present between the film containing the alkaline earth metal and the Fe-based oxide intermediate film.
(4) The dust core according to (3), wherein the Si-based oxide film has an average thickness of 5 to 30 nm.
(5) The dust core according to any one of (1) to (4), wherein the metal magnetic particles contain Fe.
(6) The dust core according to (5), wherein the metal magnetic particles further contain Si in addition to Fe.

In the present invention, since the ceramic film containing the metallic magnetic material and the alkaline earth metal is bonded via the Fe-based oxide intermediate layer, the ceramic film containing the alkaline earth metal is peeled off even if compression molding is performed. The obtained dust core shows good corrosion resistance and durability.

Further, by setting the thickness of the Fe-based oxide interlayer film to 50 nm or less, a decrease in magnetic permeability can be suppressed.

Further, by interposing a Si-based oxide film between the ceramic film containing an alkaline earth metal and the Fe-based oxide intermediate film, the ceramic film containing the alkaline earth metal and the Fe-based oxide intermediate film can be separated from each other. Corrosion resistance is further improved because the small space created between them is filled. By setting the thickness of the Si-based oxide film to 30 nm or less, a decrease in magnetic permeability can be suppressed.

Hereinafter, the present invention will be described based on specific embodiments, but various modifications are permitted without departing from the gist of the present invention.

(Powder magnetic core)
The dust core according to the present embodiment contains a metallic magnetic material having an insulating rust preventive coating, and usually contains a small amount of resin as a binder. A plurality of metallic magnetic particles constituting the metallic magnetic material are bonded to each other via a resin to be formed into a predetermined shape.

Such a dust core is preferably used as a magnetic core of a coil type electronic component. For example, it may be a coil-type electronic component in which an air-core coil around which a wire is wound is embedded inside a dust core having a predetermined shape, or a wire may be wound on the surface of a dust core having a predetermined shape. It may be a coil-type electronic component that is simply wound. Examples of the shape of the magnetic core around which the wire is wound include an FT type, an ET type, an EI type, a UU type, an EE type, an ER type, a UI type, a drum type, a toroidal type, a pot type, and a cup type. can.

(Metallic magnetic material)
The metal magnetic material constituting the dust core according to the present embodiment is not particularly limited in shape, but is usually in the form of particles. In the present embodiment, the particle size of the metal magnetic material is not particularly limited, but the median particle size (D50) of 1 μm to 100 μm is preferable in that the magnetic permeability is high. The above particle size means the particle size of particles having a film such as a ceramic film described later.

The metal magnetic material is not particularly limited, but is preferably soft magnetic particles, and the effect of the present invention works particularly effectively as long as it is Fe-based soft magnetic particles that are required to have improved corrosion resistance and rust prevention. Specifically, the Fe-based magnetic particles include pure iron, Fe-based alloys, Fe-Si-based alloys, Fe-Al-based alloys, Fe-Ni-based alloys, Fe-Si-Al-based alloys, and Fe-Co-based alloys. Examples thereof include Fe-based amorphous alloys and Fe-based nanocrystalline alloys, and pure iron or Fe—Si based alloys are more preferable.

In the Fe—Si based alloy, which is a preferable metallic magnetic material, the total content of Fe and Si is 80% by weight or more. Further, the ratio of Fe and Si is not particularly limited, but when the weight ratio is Si / Fe = 0/100 to 10/90, the saturated magnetic charge becomes high, which is preferable.

In the present embodiment, the metal magnetic material may be composed of metal magnetic particles of the same material, or may be composed of a mixture of a plurality of types of metal magnetic particles having different materials. For example, the metal magnetic material may be a mixture of Fe-based alloy particles and Fe—Si-based alloy particles.

The method for producing the metallic magnetic material is not particularly limited, and examples thereof include a gas atomizing method and a water atomizing method.

(Coating part and its forming method)
In the present embodiment, the metallic magnetic material is coated with a ceramic film containing an alkaline earth metal, and the ceramic film containing the alkaline earth metal and the metal magnetic particles are bonded via an Fe-based oxide intermediate layer. It is characterized by doing.

In the present embodiment, the ceramic film containing an alkaline earth metal is composed of an inorganic compound containing an alkaline earth metal, specifically, an oxide or composite oxide of the alkaline earth metal, or a hydroxide of the alkaline earth metal. Examples thereof include objects, carbonates of alkaline earth metals, halides of alkaline earth metals, and phosphates of alkaline earth metals. Of these, oxides of alkaline earth metals are preferable.

Further, examples of the alkaline earth metal include Mg (magnesium), Ca (calcium) and Ba (barium), and Mg is particularly preferable in this embodiment.

The ceramic film containing the alkaline earth metal may be composed of only the inorganic compound containing the alkaline earth metal, or may contain a compound other than the inorganic compound containing the alkaline earth metal. The compound other than the inorganic compound containing an alkaline earth metal may be an inorganic compound or an organic compound. In the present embodiment, as compounds other than the inorganic compound containing an alkaline earth metal, phosphates, oxides containing Al, oxides containing Si and the like are exemplified, and oxides containing Al (alumina) are used. Especially preferable. Compounds other than inorganic compounds containing alkaline earth metals may be contained in the film in a proportion of less than 30% by mass.

In the present embodiment, "coated" means an embodiment in which a ceramic film containing an alkaline earth metal is formed on the surface of a metallic magnetic material. The ceramic film containing an alkaline earth metal is formed on the metal magnetic material via an Fe-based oxide interlayer film described later. The Fe-based oxide interlayer film may be formed on the entire surface of the metal magnetic material, or may be partially formed. The ceramic film containing an alkaline earth metal preferably covers the entire surface so that the surface of the metallic magnetic material is not exposed. Further, the ceramic film containing an alkaline earth metal may continuously cover the surface of the particles or may intermittently cover the surface of the particles.

The average thickness of the ceramic film containing an alkaline earth metal is in the range of 10 to 500 nm, preferably 10 to 100 nm, and more preferably 30 to 90 nm. If the thickness of the ceramic film containing an alkaline earth metal is too thin, sufficient corrosion resistance cannot be obtained. If it is too thick, the strength of the dust core will decrease. This causes cracks in the dust core and may reduce durability. Further, if the ceramic film containing the alkaline earth metal is thick, the distance between the metal magnetic materials may be widened, and the magnetic permeability μ as the dust core may decrease.

The reason why the dust core according to the present embodiment has good corrosion resistance is not necessarily clear, but it is presumed as follows, for example. That is, it is considered that the environment near the surface of the metallic magnetic material is appropriately controlled to be alkaline by the ceramic film containing the alkaline earth metal that coats the metallic magnetic material. As a result, even when water, especially water containing salt, is present on or inside the dust core, oxidation of the metal magnetic particles (progression of rust) is suppressed, and good corrosion resistance, especially rust prevention. Show sex.

As a method for determining that the surface of the metal magnetic particles is covered with a ceramic film containing an alkaline earth metal, it is preferable to use a method capable of quantitatively measuring the amount of metal elements contained in the coated portion. In the present embodiment, the cross section of the dust core is excavated, flakes are cut out, and element mapping is performed by TEM-EDS observation. Among the obtained element mappings, the presence and thickness of the ceramic film containing the alkaline earth metal is confirmed from the composition mapping image of the alkaline earth metal. The specific method will be described later.

The ceramic film containing the alkaline earth metal is formed on the metal magnetic material via the Fe-based oxide interlayer film. That is, it is preferable that a ceramic film containing an alkaline earth metal is formed after the Fe-based oxide interlayer film is formed on the surface of the metal magnetic material. The Fe-based oxide interlayer film may be formed on the entire surface of the metal magnetic material, or may be partially formed.

The Fe-based oxide interlayer film is an oxide film containing Fe, and is formed of, for example, iron oxide such as _{Fe 2} O _{3 and Fe O.} The average thickness of the Fe-based oxide interlayer film is preferably in the range of 1 to 50 nm, more preferably 10 to 30 nm, and particularly preferably 15 to 25 nm.

By interposing an Fe-based oxide interlayer film between the ceramic film containing an alkaline earth metal and the metal magnetic material, the ceramic film containing an alkaline earth metal is made of a metal via the Fe-based oxide interlayer film. It is firmly bonded to the magnetic material, the ceramic film containing the alkaline earth metal is hard to peel off, and the corrosion resistance of the dust core is improved. Further, when the thickness of the Fe-based oxide interlayer film is 50 nm or less, the distance between the metal magnetic materials is not excessively separated, so that the decrease in the magnetic permeability μ as the dust core is suppressed.

The ceramic film containing an alkaline earth metal may be formed directly on the surface of the Fe-based oxide interlayer film, or may be formed on the Fe-based oxide interlayer film via the Si-based oxide film. .. That is, a ceramic film containing an alkaline earth metal may be formed after the Si-based oxide film is further formed on the Fe-based oxide intermediate film.

The Si-based oxide film is an oxide film containing Si, and is formed of, for example, silicon oxide such as _{SiO 2.} The average thickness of the Si-based oxide film is preferably in the range of 5 to 30 nm, more preferably 8 to 20 nm.

Although it depends on the forming method of the Fe-based oxide interlayer film, minute irregularities are usually formed on the surface of the Fe-based oxide interlayer film. When a ceramic film containing an alkaline earth metal is formed on an Fe-based oxide interlayer film having a high surface roughness, a void is generated between the Fe-based oxide intermediate film and the ceramic film containing an alkaline earth metal. There is. If water enters this void, it causes rust. Therefore, by forming the Si-based oxide film on the Fe-based oxide interlayer film, the voids are filled and the ingress of moisture and rusting are prevented. Further, when the thickness of the Si-based oxide film is 30 nm or less, the distance between the metal magnetic materials is not excessively separated, so that the decrease in the magnetic permeability μ as the dust core is suppressed.

In the present embodiment, the metallic magnetic material is coated with a ceramic film containing an alkaline earth metal and an Fe-based oxide interlayer film, more preferably a Si-based oxide film, and the metal material is substantially not exposed. That is, it is preferable that substantially the entire surface of the metallic magnetic material is covered with a coating film. However, it is unavoidable that a part of the metal surface is exposed due to restrictions on the manufacturing method or the like. Therefore, in the present embodiment, it is permissible that the metal surface is exposed in an area of 10% or less of the entire surface of the metal magnetic material.

Further, in the present embodiment, it is preferable that 90% or more of the metal magnetic materials constituting the dust core are covered with a ceramic film containing an alkaline earth metal, and all (100%). ) Is more preferably covered with a ceramic film containing an alkaline earth metal. That is, the metal magnetic material constituting the dust core may contain particles that are not coated with a ceramic film containing an alkaline earth metal or are insufficiently coated at a ratio of less than 10%. The particles with insufficient coating mean particles in which the metal surface is exposed in an area of 50% or more of the entire surface of the particles.

The metallic magnetic material is covered with a ceramic film containing an alkaline earth metal, an Fe-based oxide interlayer film, and a Si-based oxide film formed as needed, and the total thickness of the film (alkali earth metal). The ceramic film containing the above (Fe-based oxide intermediate film + Si-based oxide film) is preferably in the range of 5 to 90 nm, more preferably 10 to 70 nm on average. If the thickness of the coating film is too thick, the distance between the metal magnetic materials may be increased, and the magnetic permeability μ as the dust core may decrease. If the thickness of the coating film is too thin, sufficient corrosion resistance may not be obtained. be.

When the metal magnetic material is an Fe-based magnetic material, an Fe-based oxide interlayer film can be formed by surface oxidation of the powder. The oxide film is formed by heat treatment in a weak oxidizing atmosphere. The heat treatment conditions are not particularly limited, but can be performed, for example, in the air at about 800 ° C. By adjusting the heat treatment time, the thickness of the Fe-based oxide interlayer film can be controlled. After forming an Fe-based oxide interlayer film on the surface of the metal magnetic material, a ceramic film containing an alkaline earth metal is formed by the method described later, so that the alkaline earth metal is contained through the Fe-based oxide film. A ceramic film can be formed on a metallic magnetic material.

When the metal magnetic material is a non-Fe-based magnetic material, an Fe-based oxide interlayer film can be formed by spraying a solution containing Fe on the metal magnetic material and heat-treating the metal magnetic material.

The method for producing a metallic magnetic material coated with a ceramic film containing an alkaline earth metal is not particularly limited. The ceramic film containing an alkaline earth metal can be formed by using an ionic crystal containing an alkaline earth metal element constituting the ceramic film containing an alkaline earth metal. For example, a predetermined ionic crystal (Mg (NO ₃ ) _2, etc.) is dissolved in a solvent such as acetone, sprayed onto a metallic magnetic material on which an Fe-based oxide film is formed, and the surface is coated with a ceramic precursor film. By heat-treating the powder at about 600 ° C. for about 10 hours, a metallic magnetic material having a ceramic film is obtained. The thickness of the ceramic film can be controlled by the thickness of the ceramic precursor film, and the thickness of the ceramic film can also be controlled by performing the coating and heat treatment of the ceramic precursor film a plurality of times.

After forming the Fe-based oxide interlayer film, a Si-based oxide film can be formed by spraying, drying, and heat-treating a Si source such as alkoxysilane on the obtained powder. The heat treatment conditions may be, for example, about 800 ° C. for about 10 hours in the atmosphere. The thickness of the Si-based oxide film can be controlled by the coating thickness of the alkoxysilane, and the thickness of the Si-based oxide film can also be controlled by performing the alkoxysilane coating and heat treatment a plurality of times. After forming the Si-based oxide film, by forming the ceramic film containing the alkaline earth metal by the above method, the ceramic film containing the alkaline earth metal is formed into the Fe-based oxide via the Si-based oxide film. It can be formed on the interlayer film.
The Fe-based oxide interlayer film and the Si-based oxide film can be confirmed by element mapping based on the above TEM-EDS observation.

The above shows an example of a method for forming a ceramic film containing an alkaline earth metal, an Fe-based oxide intermediate film, and a Si-based oxide film, but the present invention is not limited to these forming methods. , Other wet method or dry method may be used. Further, between the ceramic film containing an alkaline earth metal and the Fe-based oxide intermediate film, between the ceramic film containing an alkaline earth metal and the Si-based oxide film, and between the Fe-based oxide intermediate film and the Si-based oxide film. A solid-dissolved layer in which mutual components are partially dissolved may be formed between the oxide film and the oxide film.

(resin)
A known resin can be used as the resin constituting the dust core. Specifically, various organic polymer resins, silicone resins, phenol resins, epoxy resins, water glasses and the like are exemplified. The contents of the metallic magnetic material and the resin are not particularly limited. The content of the metal magnetic material in the entire dust core is preferably 90% by weight to 98% by weight, and the content of the resin is preferably 2% by weight to 10% by weight.

(Manufacturing method of dust core)
The method for producing the dust core is not particularly limited, and a known method can be adopted. First, a metallic magnetic material coated with a ceramic film containing an alkaline earth metal and a known resin as a binder are mixed to obtain a mixture. Further, if necessary, the obtained mixture may be used as a granulated powder. Then, the mixture or granulated powder is filled in a mold and compression molded to obtain a molded product having the shape of a magnetic material (compact magnetic core) to be produced. By heat-treating the obtained molded product, a dust core having a predetermined shape in which metal magnetic particles are fixed can be obtained. The conditions of the thermosetting treatment are not particularly limited, and heat treatment is performed at 150 to 220 ° C. for 1 to 10 hours, for example. Further, the atmosphere at the time of heat treatment is not particularly limited, and the heat treatment may be performed in the atmosphere. A coil-type electronic component such as an inductor can be obtained by winding a wire around the obtained dust core a predetermined number of times.

Further, even if the above mixture or granulated powder and an air-core coil formed by winding a wire a predetermined number of times are filled in a mold and compression-molded to obtain a molded body in which the coil is embedded inside. good. By heat-treating the obtained molded product, a dust core having a predetermined shape in which a coil is embedded can be obtained. Since a coil is embedded in such a dust core, it functions as a coil-type electronic component such as an inductor.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and may be modified in various ways within the scope of the present invention.

Hereinafter, the invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

(Experimental Example 1) Samples 1-37
As a metallic magnetic material, Fe—Si alloy particles having a weight ratio of Si / Fe = 4.5 / 95.5 were produced by a gas atomization method. The median particle size (D50) of the Fe—Si alloy particles was 5 μm.

In Sample 1 (Comparative Example), an MgO film was directly formed on the metal magnetic material by a method described later without oxidizing the metal magnetic material to obtain a metal magnetic material having the MgO film. Using this metallic magnetic material, a dust core was produced by the method described later.

In Samples 2-37, the metallic magnetic material was heat-treated in the air at 800 ° C. for the time shown in Table 1 to form an Fe-based oxide film on the surface. The thickness of the Fe-based oxide film was controlled by adjusting the heat treatment time.

In Samples 1 to 18 and 24 to 37, Mg (NO ₃ ) ₂ was dissolved or dispersed in acetone to prepare an alkaline earth metal compound solution having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. Here, the wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 1. The powder after wet spraying was heat-treated at 600 ° C. for 10 hours to obtain a metallic magnetic material having a ceramic film made of MgO on an Fe-based oxide interlayer film. The thickness of the ceramic film was controlled by adjusting the wet spraying time as shown in Table 1.

In Sample 19, Mg (NO ₃ ) ₂ and Al (NO ₃ ) ₃ were dissolved or dispersed in acetone to prepare a solution of an alkaline earth metal compound and an aluminum metal compound having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. Here, the wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 1. 5 hours the powder after the wet spray at 700 ° C., subjected to a heat treatment to obtain a metallic magnetic material having a ceramic film made of Mg-Al ₂ O ₃ on the Fe-based oxide intermediate layer.

In Sample 20, Mg (NO ₃ ) ₂ was dissolved or dispersed in acetone to prepare an alkaline earth metal compound solution having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. Here, the wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 1. The powder after wet spraying was heat-treated at 600 ° C. for 10 hours, and further stored in a humidity environment of 85 ° C. and 85% RH for 24 hours. By this operation, a metallic magnetic material having a ceramic film made _{of Mg (OH) 2} on an Fe-based oxide interlayer film was obtained.

In Sample 21, Ba (NO ₃ ) ₂ was dissolved or dispersed in acetone to prepare an alkaline earth metal compound solution having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. Here, the wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 1. The powder after wet spraying was heat-treated at 700 ° C. for 10 hours to obtain a metallic magnetic material having a ceramic film made of BaO on an Fe-based oxide interlayer film.

In Sample 22, Ca (NO ₃ ) ₂ was dissolved or dispersed in acetone to prepare an alkaline earth metal compound solution having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. Here, the wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 1. The powder after wet spraying was heat-treated at 750 ° C. for 10 hours to obtain a metallic magnetic material having a ceramic film made of CaO on an Fe-based oxide interlayer film.

In Sample 23, Sr (NO ₃ ) ₂ was dissolved or dispersed in acetone to prepare an alkaline earth metal compound solution having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. Here, the wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 1. The powder after wet spraying was heat-treated at 800 ° C. for 10 hours to obtain a metallic magnetic material having a ceramic film made of SrO on an Fe-based oxide interlayer film.

As a binder, an epoxy resin as a thermosetting resin and an imide resin as a curing agent were prepared. To 100% by mass of the ceramic film-containing metal magnetic material obtained above, 4% by mass of a binder and acetone were added, and the solution was spray-dried to obtain granules. The obtained granules were sized with a 355 μm mesh. The powder after sizing was filled in a toroidal mold having an outer diameter of 17.5 mm and an inner diameter of 11.0 mm, and pressed at a molding pressure of 980 MPa to obtain a compact magnetic core molded body. The weight of the molded product was 5 g. The prepared compact magnetic core was heat-cured in the air at 200 ° C. for 5 hours to obtain dust cores of Samples 1-37.

(Experimental Example 2) Samples 38 to 49
The metallic magnetic material was heat-treated in the air at 800 ° C. for the time shown in Table 2 to obtain a metallic magnetic material having an Fe-based oxide film. An alkoxysilane solution was wet-sprayed onto a metallic magnetic material having an Fe-based oxide film. A 50% by mass aqueous solution of trimethoxysilane was used as the alkoxysilane solution. The wet spray amount was 5 mL / min, and spraying was performed for the time shown in Table 2. The powder after wet spraying was heat-treated at 800 ° C. for 10 hours to form a Si-based oxide film on the Fe-based oxide film. The thickness of the Si-based oxide film was controlled by adjusting the wet spraying time.

Mg (NO ₃ ) ₂ was dissolved or dispersed in acetone to prepare an alkaline earth metal compound solution having a concentration of 5% by weight. An alkaline earth metal compound solution was wet-sprayed on the metallic magnetic material on which the Si-based oxide film was formed. Here, the wet spray amount was set to 5 mL / min, and spraying was performed for the time shown in Table 2. The powder after wet spraying was heat-treated at 600 ° C. for 10 hours to obtain a metallic magnetic material having a ceramic film made of MgO on a Si-based oxide film. The thickness of the ceramic film was controlled by adjusting the wet spraying time as shown in Table 1. Next, the dust cores of Samples 38 to 49 were prepared in the same manner as in Experimental Example 1.

<Confirmation of coating film>
The obtained dust core was cut and polished to expose the cross section of the dust core. The exposed cross section was excavated by a focused ion beam (FIB), and a thin piece having an area of 1 μm × 1 μm and a thickness of 100 nm was cut out. The obtained flakes were observed by TEM, and image analysis was performed in a field of view of 500 nm × 500 nm.

By TEM-EDS observation, a film containing Fe and O on the metallic magnetic material was confirmed, and it was judged to be an Fe-based oxide intermediate film. Next, in the outermost layer, a film containing an alkaline earth metal and oxygen was confirmed, and it was determined that the film was a ceramic film containing an alkaline earth metal. Finally, a film containing Si and O existing between the Fe-based oxide interlayer film and the ceramic film containing an alkaline earth metal was confirmed, and it was determined to be a Si-based oxide film.

<Film thickness measurement>
The thickness of the "ceramic film containing alkaline earth metal", "Fe-based oxide intermediate film", and "Si-based oxide film" was measured by TEM observation. Draw a perpendicular line from the surface of the metallic magnetic material to the "ceramic film containing alkaline earth metal", "Fe-based oxide interlayer film", and "Si-based oxide film", and "ceramic film containing alkaline earth metal", The lengths of the lines in contact with the "Fe-based oxide interlayer film" and the "Si-based oxide film" were measured, respectively. The measurement was performed at 10 locations, and the average value of the length of the line segment was taken as the average thickness of the "ceramic film containing alkaline earth metal", "Fe-based oxide intermediate film", and "Si-based oxide film", respectively. ..

<Corrosion resistance>
A test was conducted in which a 5% aqueous salt solution was sprayed onto the prepared compact magnetic core molded product and kept at 35 ° C. for 24 hours. After the powder magnetic core after the test was washed with ion-exchanged water and dried, the rusting condition was observed with an optical microscope (50 times), and the area ratio occupied by rust in a field of view of 3 mm × 3 mm was calculated. The number of measurement points was 10 per sample, and the average rust area ratio was calculated. The results are shown in Table 1.

<Crack of dust core>
The surface of the dust core prepared with an optical microscope (50 times) was observed. A streak of 100 μm or more was judged to be a crack.

<Permeability>
The initial magnetic permeability of the obtained dust core was measured. The initial magnetic permeability was measured by an LCR meter (HP LCR428A) with a wire wound around a dust core and the number of turns set to 50 turns.

The above results are summarized in Tables 1 and 2 below. In the table, sample numbers marked with * indicate comparative experimental examples.

From Sample 1 in Table 1, when a metal magnetic material in which an MgO film is directly formed on a metal magnetic material without forming an Fe-based oxide film is used, the corrosion resistance of the obtained dust core is inferior. I understand. From Samples 2 and 3, it can be seen that even when the Fe-based oxide interlayer film is formed, the corrosion resistance is inferior when the thickness of the ceramic film containing the alkaline earth metal is less than 10 nm. From the samples 17 and 18, it can be seen that when the thickness of the ceramic film containing the alkaline earth metal exceeds 500 nm, cracks occur in the dust core. Further, as shown in Table 2, the corrosion resistance was further improved in the samples 38 to 49 in which the Si-based oxide film was formed between the Fe-based oxide interlayer film and the metal magnetic material. Here, it can be seen that the initial magnetic permeability decreases when the thickness of the Fe-based oxide interlayer film is too thick (Sample 36) and when the thickness of the Si-based oxide film is too thick (Samples 48 and 49).

Claims (5)

A dust core containing a metallic magnetic material coated with a ceramic film containing an alkaline earth metal, wherein the film containing the alkaline earth metal has a thickness of 10 to 500 nm.
An Fe-based oxide interlayer film exists between the film containing the alkaline earth metal and the metal magnetic material .
A dust core characterized in that a Si-based oxide film is present between the film containing an alkaline earth metal and the Fe-based oxide intermediate film. The dust core according to claim 1, wherein the Fe-based oxide interlayer film has a thickness of 1 to 50 nm. The dust core according to claim 1 or 2 , wherein the Si-based oxide film has a thickness of 5 to 30 nm. The dust core according to any one of claims 1 to 3 , wherein the metal magnetic material contains Fe. The dust core according to claim 4 , wherein the metal magnetic material further contains Si in addition to Fe.