JP6911402B2 - 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. In order to develop sufficient corrosion resistance, it is necessary to increase the thickness of the glass coating. 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

In order to prevent rusting, it is considered promising to coat the metallic magnetic material with a coating layer. However, the glass coating has insufficient corrosion resistance, and it is necessary to thicken the glass coating in order to obtain sufficient corrosion resistance. The magnetic permeability decreases as the thickness of the coating layer increases.

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, the MgO coating layer is easily peeled off during the molding of the dust core, and there is a limit to the improvement of the corrosion resistance of the dust core.

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.

The present inventors aim to improve the corrosion resistance, particularly the rust prevention property, of the metal magnetic particles constituting the dust core, and bond the crystalline insulating film to the metal magnetic particles via the amorphous insulating film. It was investigated. Here, the crystallinity of the insulating coating ensures the uniformity and denseness of the coating due to its high crystallinity. This is effective in suppressing the invasion of oxygen molecules and contributes to the improvement of corrosion resistance. Further, the presence of an amorphous insulating film having high toughness between the metallic magnetic particles and the crystalline insulating film improves the bondability between the two. As a result, peeling of the coating film during mold molding is suppressed, and the rust preventive effect is maintained.

That is, the present invention includes the following gist.
(1) A powder magnetic core containing a metallic magnetic material coated with an insulating coating, wherein the insulating coating has an amorphous portion existing in the vicinity of the metallic magnetic material and a crystalline portion in contact with the amorphous portion. A dust core characterized by being composed of.
(2) The dust core according to (1), wherein the area ratio of the crystalline portion to the total area in the cross section of the insulating coating is 20 to 95%.
(3) The dust core according to (1) or (2), wherein the crystalline portion is composed of ceramics containing an alkaline earth metal.
(4) The dust core according to any one of (1) to (3), wherein the amorphous portion contains Si and O.
(5) The dust core according to any one of (1) to (4), wherein the total thickness of the insulating coating is 30 to 100 nm.

In the present invention, the metal magnetic material is covered with an insulating coating, and the insulating coating is composed of an amorphous portion existing in the vicinity of the metal magnetic material and a crystalline portion in contact with the amorphous portion. .. Due to its high crystallinity, the crystalline portion ensures the uniformity and denseness of the coating film, and is effective in suppressing the invasion of oxygen molecules. This contributes to the improvement of corrosion resistance. In addition, the amorphous portion existing in the coating film has high toughness, the resistance to rubbing and shearing force during molding of the dust core is improved, and the coating film is prevented from peeling off. Excellent corrosion resistance is achieved by the synergistic action of these two sites.

Further, by setting the area ratio of the crystalline portion in the insulating film to 20 to 95%, the synergistic action described above works well, and the corrosion resistance is further improved.

It is preferable that the crystalline part is composed of ceramics containing an alkaline earth metal. It is considered that the inside of the dust core and the surface of the dust core are maintained alkaline by introducing an alkaline earth metal into the insulating film. Since rusting tends to proceed in an acidic or neutral environment, rusting can be prevented more effectively by keeping the inside of the dust core and the surface of the dust core alkaline.

By forming the amorphous part with a material containing Si (silicon) and O (oxygen), for example, a Si-based oxide, the amorphous part can be easily formed, and the crystalline part can be made into a metallic magnetic material. Can be firmly joined.

By setting the total thickness of the insulating film to 30 to 100 nm, the corrosion resistance is further improved, and if the thickness is within this range, the decrease in magnetic permeability is 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 the particles having an insulating coating, which will be 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.

(Insulation film and its formation method)
In the present embodiment, the metallic magnetic material is covered with an insulating coating, and the insulating coating is composed of an amorphous portion existing in the vicinity of the metallic magnetic material and a crystalline portion in contact with the amorphous portion. ing.

In the present embodiment, the "crystalline part" means a part where the plaid is confirmed in the TEM observation of the insulating coating, and the "amorphous part" is a part other than that, that is, a part where the plaid is not confirmed. To say.

In the present embodiment, "coated" means that the insulating film is directly formed on the surface of the metal magnetic material, and that the insulating film is formed via another layer formed on the surface of the metal magnetic material. Includes both of these aspects. The insulating coating preferably covers the entire surface so that the surface of the metallic magnetic material is not exposed. When another layer is formed, at least a part of the particle surface may be covered with an insulating film, but it is preferable that the entire surface is covered. Further, the insulating coating may continuously or intermittently cover the surface of the particles.

The crystalline portion is not particularly limited as long as it is composed of a crystallized material. Since the insulating film contains crystalline parts, the uniformity and denseness of the film are ensured. This is effective in suppressing the invasion of oxygen molecules and contributes to the improvement of corrosion resistance. From the viewpoint of further improving the corrosion resistance, it is preferable that the crystalline portion is made of a material that prevents rusting. As a material for preventing rust, a substance capable of making the atmosphere basic is more preferable, and specific examples thereof include ceramics containing an alkaline earth metal. In the present embodiment, as the rust preventive material, ceramics containing an alkaline earth metal, which is easy to form crystals and has excellent rust preventive properties, is particularly preferable.

In the present embodiment, the ceramics containing an alkaline earth metal is composed of an inorganic compound containing an alkaline earth metal, specifically, an oxide or a composite oxide of an alkaline earth metal, or a hydroxide of an alkaline earth metal. , Carbonates of alkaline earth metals, halides of alkaline earth metals, phosphates of alkaline earth metals and the like are exemplified. 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 ceramics may be composed of only an inorganic compound containing an alkaline earth metal, or may contain a compound other than the inorganic compound containing an 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.

The reason why the dust core has good corrosion resistance by forming the crystalline part with ceramics containing an alkaline earth metal is not always clear, but it is presumed as follows, for example. That is, it is considered that the environment near the surface of the metal magnetic material is appropriately controlled to be alkaline by including ceramics containing an alkaline earth metal as a crystalline part in the insulating film of the metal 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.

In another aspect, the crystalline moiety may be composed of an oxide containing Al, an oxide containing Si, or an oxide containing Cr. Examples of such oxides include Al ₂ O ₃ , SiO ₂ , Cr ₂ O _{3 and the} like.

The crystalline part can be confirmed by TEM observation. By TEM observation of the checkered fringes of the insulating film, it is confirmed that the part where the checkered pattern is confirmed and the part where the checkered pattern is not confirmed. Here, the place where the plaid is confirmed is determined to be a crystalline part, and the part where no plaid is confirmed is determined to be an amorphous part. The composition of each part is confirmed by elemental mapping of each part.

As a method for confirming the composition of the insulating coating of the metal magnetic particles, it is preferable to use a method capable of quantitatively measuring the amount of metal elements contained in the insulating coating. 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. The composition of the insulating coating is confirmed by the obtained element mapping. These specific methods will be described later.

The amorphous portion is not particularly limited as long as it is composed of a non-crystalline material, but it is more likely that the amorphous portion is composed of a material having high toughness from the viewpoint of improving the bondability between the metal magnetic material and the insulating coating. preferable. Preferred amorphous materials include materials containing Si (silicon) and O (oxygen), B ₂ O ₃ , Al ₂ O ₃ , B ₂ O _3- Al ₂ O _{3, and the} like. In the present embodiment, it is particularly preferable that the amorphous portion is made of a material containing Si and O, that is, a Si-based oxide. The Si-based oxide is an oxide containing Si, and is formed of, for example, silicon oxide such as _{SiO 2.}

Since the insulating film contains an amorphous portion, the resistance to rubbing and shearing force during molding of the dust core is improved, and the insulating film is prevented from peeling off.

The ratio of the crystalline portion in the insulating coating is the area ratio of the crystalline portion to the total area of the insulating coating, preferably in the range of 20 to 95%, more preferably 40 to 80%, and particularly preferably 60 to 80%. The area ratio of the crystalline part can be confirmed by TEM observation of the cross section of the insulating film. If the area ratio of the crystalline part becomes too high, the function (prevention of peeling) by the amorphous part is impaired, and the corrosion resistance may decrease.

By controlling the area ratio of the crystalline part within the above range, excellent corrosion resistance is achieved by the synergistic effect of the function of the crystalline part (improvement of corrosion resistance) and the function of the amorphous part (prevention of peeling). ..

The thickness of the insulating coating is preferably in the range of 5 to 160 nm, more preferably 30 to 100 nm, and particularly preferably 50 to 95 nm. If the thickness of the insulating film is too thin, sufficient corrosion resistance cannot be obtained, and if it is too thick, the distance between the metal magnetic materials may be widened, and the magnetic permeability μ as the dust core may decrease. In another preferred embodiment, the thickness of the insulating coating may be 5 to 50 nm. The thinner the insulating film, the better the magnetic permeability. In yet another preferred embodiment, the thickness of the insulating coating may be 95-160 nm. The thicker the insulating coating, the better the corrosion resistance.

Further, in the present embodiment, the crystalline portion and the amorphous portion are unevenly distributed in the insulating film. That is, in the thickness direction of the insulating coating, many amorphous portions are unevenly distributed in the region close to the metallic magnetic material, and many crystalline portions are unevenly distributed near the surface of the insulating coating. Specifically, with respect to the total thickness of the insulating coating, 80% or more of the amorphous parts are unevenly distributed in the region of 30% or less from the surface of the metal magnetic material, and 70% or less of the surface of the insulating coating. It is preferable that 80% or more of the crystalline parts are unevenly distributed.

Since the amorphous part is unevenly distributed near the surface of the metal magnetic material, the amorphous part easily adheres to the metal magnetic material, the bonding strength between the insulating film and the metal magnetic material is increased, and the powder magnetic core is formed. The resistance to rubbing and shearing force at times is improved, and the insulating film is prevented from peeling off. The uneven distribution of crystalline portions near the surface of the insulating coating ensures the uniformity and denseness of the surface of the insulating coating. As a result, the invasion of oxygen molecules into the metal magnetic material is suppressed, and the corrosion resistance is improved. Even when water is present in the surroundings, oxidation of the metal magnetic particles (progress of rust) is suppressed, and good corrosion resistance, especially rust prevention, is exhibited.

The insulating film may be formed directly on the surface of the metal magnetic material, or may be formed on the metal magnetic material via an Fe-based oxide film. The Fe-based oxide film is an oxide film containing Fe, and is formed of, for example, iron oxide such as _{Fe 2} O _{3 and Fe O.} When the Fe-based oxide film is provided, its average thickness 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 film between the insulating film and the metal magnetic material, the insulating film is firmly bonded to the metal magnetic material via the Fe-based oxide film, and the insulating film is hard to peel off, resulting in pressure. The corrosion resistance of the powder magnetic core is further improved. Further, when the thickness of the Fe-based oxide 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.

In the present embodiment, the metallic magnetic material is covered with an insulating coating, and the metallic material is substantially not exposed. That is, it is preferable that substantially the entire surface of the metallic magnetic material is covered with an insulating 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 the insulating coating, and all (100%) particles are covered with the insulating coating. It is more preferable that it is coated. That is, the metal magnetic material constituting the dust core may contain particles that are not coated with the insulating coating 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 method for producing the metallic magnetic material coated with the insulating film including the crystalline portion and the amorphous portion is not particularly limited. For example, the metal magnetic material is coated with the precursor material of the amorphous portion, and the metal magnetic material coated with the precursor material is heat-treated to form the amorphous portion. Further, the metal magnetic material is coated with the precursor material of the crystalline portion, and the metallic magnetic material coated with the precursor material is heat-treated to form the crystalline portion.

The formation of the crystalline portion and the formation of the amorphous portion may be performed simultaneously or sequentially. Preferably, after forming the amorphous part, the crystalline part is formed. By forming each part in this order, an insulating film in which the amorphous part is unevenly distributed on the metal magnetic material side can be obtained. Hereinafter, a method for forming an insulating film will be described by taking as an example a case where the amorphous portion is a Si-based oxide containing Si and O and the crystalline portion is made of ceramics containing an alkaline earth metal.

Amorphous moieties made of Si-based oxides can be formed by spraying a Si source such as alkoxysilane onto a metallic magnetic material, drying it, and heat-treating it. The heat treatment conditions may be, for example, about 400 to 550 ° C. for about 10 hours in an argon atmosphere. If the heat treatment temperature is too high, it may crystallize. The area ratio of the amorphous part can be controlled by the wet coating amount of alkoxysilane, and the area ratio of the amorphous part (Si-based oxide) can be controlled by performing the alkoxysilane coating and heat treatment multiple times. You can also.

After forming a Si-based oxide on the metallic magnetic material, a crystalline portion can be formed by spraying, drying, and heat-treating a solution of a precursor substance of the crystalline portion (alkaline earth metal-containing ceramics). The solution of the precursor substance is obtained, for example, _{by dissolving a predetermined ionic crystal (Mg (NO 3} ) _2, etc.) in a solvent such as acetone. The heat treatment conditions may be, for example, about 600 to 1200 ° C. for about 10 hours in the atmosphere. If the heat treatment temperature is too high during the formation of the crystalline part, the amorphous part may crystallize. Therefore, the crystallization temperature of the constituent material of the crystalline part and the crystal of the constituent material of the amorphous part. It is preferable to determine the constituent material of each part and the heat treatment temperature in consideration of the crystallization temperature. The area ratio of the crystalline portion can be controlled by the amount of coating of the coating raw material, and the area ratio of the crystalline portion can also be controlled by performing the coating and heat treatment of the coating raw material solution a plurality of times.

When the metal magnetic material is an Fe-based magnetic material, an Fe-based oxide 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 film can be controlled. By forming the Fe-based oxide film on the surface of the metal magnetic material and then forming the insulating film by the above method, the insulating film can be formed on the metal magnetic material via the Fe-based oxide film.

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

Although an example of the method for forming the insulating film is shown above, the present invention is not limited to these forming methods, and other wet methods and dry methods may be used. Further, a solid solution layer in which mutual components are partially dissolved may be formed between the crystalline part and the amorphous part.

(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 an insulating coating 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.

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.

(Formation of amorphous part)
(Samples 1-15, 17-24, 28-41)
An alkoxysilane solution was wet-sprayed onto the metallic magnetic material. A 50% by mass solution of trimethoxysilane was used as the alkoxysilane solution. Here, the wet coating was performed at 5 mL / min, and the coating amount was adjusted by adjusting the wet coating time to control the formation amount of the amorphous portion. The powder after wet spraying was heat-treated at 400 ° C. for 10 hours in an argon atmosphere to form an amorphous portion composed of a Si-based oxide.

(Sample 25)
The metal magnetic material was wet-sprayed with an acetone solution of _{Al (NO 3} ) _3. The concentration of Al (NO ₃ ) _{3 in} the solution was 5 wt%. Here, the wet coating was performed at 5 mL / min, and the coating amount was adjusted by adjusting the wet coating time to control the formation amount of the amorphous portion. The powder after wet spraying was heat-treated at 550 ° C. for 10 hours in an argon atmosphere to form an amorphous portion composed of _{Al 2} O _3.

(Sample 26)
Acetone solution of triethoxyborane was wet-sprayed on the metallic magnetic material. The concentration of triethoxyborane in the solution was 50% by mass. Here, the wet coating was performed at 5 mL / min, and the coating amount was adjusted by adjusting the wet coating time to control the formation amount of the amorphous portion. The powder after wet spraying was heat-treated at 500 ° C. for 10 hours in an argon atmosphere to form an amorphous portion composed of _{B 2} O _3.

(Sample 27)
Triethoxyborane and trimethoxysilane were mixed with the metallic magnetic material, and a solution dissolved in acetone was wet-sprayed. The total amount of triethoxyborane and trimethoxysilane in the solution was 50% by mass, and the ratio of both was 10% by mass: 40% by mass. Here, the wet coating was performed at 5 mL / min, and the coating amount was adjusted by adjusting the wet coating time to control the formation amount of the amorphous portion. The powder after wet spraying was heat-treated at 450 ° C. for 10 hours in an argon atmosphere to form an amorphous portion composed of _{B 2} O _3- SiO _2.

(Sample 16)
In Sample 16, the crystalline part described later was generated without forming the amorphous part.

(Formation of crystalline part)
(Samples 2-16, 25-41)
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-coated on the metallic magnetic material on which the amorphous portion was formed (however, only the metallic magnetic material in Sample 16). The powder after wet coating was heat-treated at 700 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion. Here, the wet coating amount was set to 5 mL / min, and the wet coating time was adjusted to adjust the coating amount and control the formation amount of the crystalline portion.

(Sample 17)
Mg (NO ₃ ) ₂ and Al (NO ₃ ) ₃ were dissolved or dispersed in acetone to prepare a solution having a concentration of 5% by weight. Here, the ratio of Mg (NO ₃ ) ₂ and Al (NO ₃ ) ₃ in the solution was set to 4.5% by weight: 0.5% by weight. The above solution was wet-coated on the metallic magnetic material on which the amorphous portion was formed. Here, the wet coating was performed at 5 mL / min, and the coating amount was adjusted by adjusting the wet coating time to control the formation amount of the crystalline portion. The powder after wet coating was heat-treated at 900 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 18)
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-coated on the metallic magnetic material on which the amorphous portion was formed. The powder after wet coating was heat-treated at 600 ° C. for 10 hours. Then, by storing in a humidity environment of 85 ° C. and 85% RH for 24 hours, MgO was changed to Mg (OH) _2, and a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion was obtained.

(Sample 19)
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-coated on the metallic magnetic material on which the amorphous portion was formed. The powder after wet coating was heat-treated at 750 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 20)
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-coated on the metallic magnetic material on which the amorphous portion was formed. The powder after wet coating was heat-treated at 800 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 21)
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-coated on the metallic magnetic material on which the amorphous portion was formed. The powder after wet coating was heat-treated at 900 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 22)
Trimethoxysilane was dissolved in acetone to prepare a solution having a concentration of 50% by mass. This solution was wet-coated on the metallic magnetic material on which the amorphous part was formed. The powder after wet coating was heat-treated at 1000 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 23)
Al (NO ₃ ) ₃ was dissolved or dispersed in acetone to prepare a solution having a concentration of 5% by weight. This solution was wet-coated on the metallic magnetic material on which the amorphous part was formed. The powder after wet coating was heat-treated at 1200 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 24)
Cr (NO ₃ ) ₃ was dissolved or dispersed in acetone to prepare a solution having a concentration of 5% by weight. This solution was wet-coated on the metallic magnetic material on which the amorphous part was formed. The powder after wet coating was heat-treated at 700 ° C. for 10 hours to obtain a metallic magnetic material having an insulating film containing a crystalline portion and an amorphous portion.

(Sample 1)
In Sample 1, only the amorphous part was formed, and the crystalline part was not formed.

The area ratio between the crystalline part and the amorphous part is the ratio of the coating amount of the coating material (the ratio of "the coating time when forming the amorphous part" and "the coating time when forming the crystalline part"). Was controlled by adjusting.

(Manufacturing of dust core)
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 metal magnetic material having the insulating film formed above, 4% by mass of the 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 a dust core of Samples 1-41.

<Confirmation of insulating 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.

The TEM lattice fringes were observed in the portion (insulating coating) excluding the metallic magnetic material, and binarization was performed in the portion where the lattice fringes were confirmed and the portion where the lattice fringes were not confirmed. The part where the plaid was confirmed was judged to be a crystalline part, and the part where no plaid was confirmed was judged to be an amorphous part. The area ratio of the crystalline part was calculated from the area of the crystalline part with respect to the area of the entire visual field.

The composition of the crystalline part and the amorphous part was confirmed by TEM-EDS observation. It was confirmed that the crystalline part and the amorphous part were made of the materials shown in Table 1.

<Film thickness measurement>
The film thickness of the insulating film was measured by TEM observation. A perpendicular line was drawn from the surface of the metallic magnetic material to the insulating film, and the length of the line segment in contact with the insulating film was measured. The measurement was performed at 10 points, and the average value of the lengths of the line segments was taken as the thickness of the insulating coating.

<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.

<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 the table below. In the table, sample numbers marked with * indicate comparative experimental examples.

From Sample 1 and Sample 16, it can be seen that sufficient corrosion resistance cannot be obtained with an insulating coating that does not contain either a crystalline portion or an amorphous portion. From Samples 2 to 15, it can be seen that the corrosion resistance is improved by controlling the area ratio between the crystalline part and the amorphous part within an appropriate range. From Samples 17 to 24, it can be seen that good corrosion resistance is achieved even if the crystalline portion is an alkaline earth metal compound other than MgO, silica, alumina, or chromium oxide. From Samples 25 to 27, _{it can be seen that even if the amorphous portion is Al 2} O ₃ , B ₂ O _3, or a composite thereof, good corrosion resistance is achieved as in _{SiO 2.} It can be seen from Samples 28 to 41 that when the thickness of the insulating film is thicker, the corrosion resistance is good, but the initial magnetic permeability is lowered.

Claims (4)

It is a dust core containing a metallic magnetic material coated with an insulating coating, and the insulating coating is composed of an amorphous portion existing in the vicinity of the metallic magnetic material and a crystalline portion in contact with the amorphous portion. ,
A dust core characterized in that the crystalline portion is composed of ceramics containing an alkaline earth metal. The dust core according to claim 1, wherein the area ratio of the crystalline portion to the total area in the cross section of the insulating coating is 20 to 95%. The dust core according to claim 1 or 2 , wherein the amorphous portion contains Si and O. The dust core according to any one of claims 1 to 3 , wherein the total thickness of the insulating coating is 30 to 100 nm.