JP2000277314A - Dust core and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve saturation magnetic flux density, loss, and effective permeability in applying direct-current magnetic field by comprising a ferromagnetic metal powder and an insulating layer including isotropic carbon which exist between particles thereof. SOLUTION: A molding is produced through compression molding a mixture, including ferromagnetic metal powder and an insulating material. The molding is annealed so that insulating layers exist between ferromagnetic metal particles. The insulating layer is constituted to include isotropic carbon. In annealing, the isotropic carbon is produced by using a thermosetting resin including at least one of silicone resin, phenol resin and epoxy resin as the insulating layer. Alternatively, the insulating material including the isotropic carbon, and the ferromagnetic metal powder are mixed, molded and then annealed. Thereby, saturation magnetic flux density becomes high, a small loss is achieved, and moreover the direct-current superposition property is made satisfactory.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder magnetic core used for a magnetic core such as a transformer or an inductor, a magnetic core for a motor, and other electromagnetic components, and a method for manufacturing the powder magnetic core.

[0002]

2. Description of the Related Art In recent years, miniaturization of electric and electronic devices has progressed, and as a result, a compact and highly efficient powder magnetic core has been required. Ferrite powder or ferromagnetic metal powder is used for the dust core. The ferromagnetic metal powder has a higher saturation magnetic flux density than the ferrite powder, so that the magnetic core can be miniaturized. However, since the electric resistance is low, the eddy current loss of the magnetic core increases. Therefore, an insulating layer is usually provided on the surface of the ferromagnetic metal particles in the dust core.

In the process of manufacturing a dust core, coercive force increases due to distortion (stress) generated during molding, and high permeability cannot be obtained, and hysteresis loss increases. This is generally done. In order to sufficiently release the stress of the ferromagnetic metal particles, annealing at a high temperature is necessary. Further, in order to reduce the size of the magnetic core, it is important that not only the saturation magnetic flux density but also the DC superimposition characteristics (effective magnetic permeability when a DC magnetic field is applied) be excellent.

[0004]

Conventionally, insulating materials for forming an insulating layer provided on the surface of ferromagnetic metal particles include:
A silicone resin or a phenol resin having relatively high heat resistance is often used. However, according to a study by the present inventors, it has been found that even when a silicone resin is used, annealing at a high temperature increases the amount of resin loss, resulting in insufficient insulation between ferromagnetic metal particles. . If the insulation is insufficient, the eddy current loss in the high frequency region becomes extremely large, so that the core loss becomes large and the frequency characteristics of the magnetic permeability deteriorate. If the frequency characteristic of the magnetic permeability deteriorates, the DC superimposition characteristic deteriorates.

Further, according to the study of the present inventors, when the molding pressure is relatively low, even if the annealing temperature is relatively low, the resin is liable to be scattered and the resin weight loss is likely to be large. The insulation was found to be impaired. The molding pressure depends on the shape of the magnetic core, and the molding pressure cannot be increased with a magnetic core having a relatively complicated shape, for example, many magnetic cores other than a toroidal type such as an E type, an ER type, and a pot type. Therefore, means for reducing the molding pressure and ensuring sufficient insulation even when high-temperature annealing is performed is required.

The present invention has been made in view of such circumstances, and an object of the present invention is to realize a dust core having a high saturation magnetic flux density, a low loss, and a good effective magnetic permeability when a DC magnetic field is applied. It is to be.

[0007]

This and other objects are achieved by the present invention which is defined below as (1) to (7). (1) A dust core including a ferromagnetic metal powder and an insulating layer existing between particles constituting the ferromagnetic metal powder, wherein the insulating layer includes isotropic carbon. (2) The dust core according to (1), wherein the insulating layer contains a thermosetting resin. (3) The dust core according to (2), wherein the thermosetting resin contains at least one of a silicone resin, a phenol resin, and an epoxy resin. (4) The dust core according to any one of the above (1) to (3), which contains at least isotropic carbon powder as the isotropic carbon. (5) A method for producing a dust core by compression-molding a mixture containing a ferromagnetic metal powder and an insulating material, the method comprising the steps of: using a material containing a thermosetting resin as the insulating material; A method for producing a dust core in which isotropic carbon is produced from the thermosetting resin. (6) The method for manufacturing a dust core according to the above (5), wherein a material containing isotropic carbon powder in addition to the thermosetting resin is used as the insulating material. (7) A method for producing a dust core by compressing and molding a mixture containing a ferromagnetic metal powder and an insulating material, wherein the insulating material comprises a thermosetting resin and an isotropic carbon powder. A method for producing a dust core.

In the present invention, a thermosetting resin capable of producing isotropic carbon by heat treatment, for example, a silicone resin or a phenol resin is used as the insulating material, and isotropic carbon is produced from these at the time of annealing. Isotropic carbon has a high electric resistance and thus has excellent insulation properties. Therefore, a magnetic core with less loss is realized. A proposal to use isotropic carbon as an insulating material for a dust core is made for the first time in the present invention.

In a preferred embodiment of the present invention, an isotropic carbon powder is used as the insulating material in addition to the thermosetting resin. If the annealing is performed with the isotropic carbon powder added in advance, the scattering of the resin during high-temperature annealing can be suppressed, and the amount of resin loss can be reduced. Even when the resin is carbonized, the insulation between the ferromagnetic metal particles is improved. As the thermosetting resin in this case, a resin that does not generate isotropic carbon in addition to a resin that generates isotropic carbon by high-temperature annealing can be used. The present inventors have found for the first time that isotropic carbon suppresses resin loss during high-temperature annealing. In addition, when isotropic carbon powder is added, the gap between ferromagnetic metal particles can be made more uniform by the lubricating effect, so that the DC superposition characteristics are improved.

As described above, according to the present invention, the effect of the high-temperature annealing, that is, the effect of reducing the hysteresis loss and the effect of suppressing the deterioration of the magnetic permeability by releasing the stress of the ferromagnetic metal powder generated at the time of powdering and molding, and the effect of insulating The effect of maintaining the properties, that is, the effect of reducing the eddy current loss and the effect of improving the frequency characteristics of the magnetic permeability are both realized. Therefore, the dust core of the present invention has a small total loss (core loss), and has good magnetic permeability and its frequency characteristics.

A dust core using carbon as an insulating material is described in Japanese Patent Application Laid-Open No. 7-111209. In this powder magnetic core, an insulating binding layer composed of a carbon layer (carbon and / or carbide) containing at least a part of a cation exchangeable carbon body is formed on the surface of the metal magnetic powder particles. In the example of the publication, the cation exchangeable carbon body is generated by heat treatment of a polycarbodiimide resin. However, the cation-exchangeable carbon body described in the publication and the isotropic carbon used in the present invention are completely different from each other as described later, and when a cation-exchangeable carbon body is used instead of the isotropic carbon, Does not realize the effect of the present invention.

Dust cores using a phenol resin or a silicone resin as an insulating material include, for example, the following.

In Example 2 of JP-A-56-155510, water glass and a phenolic resin are added to pure iron powder, zinc stearate is added thereto, and a pressure of 7 t / cm ² is applied. After molding, a heat treatment is performed at 150 ° C. for 30 minutes to obtain a metal dust core. However, the publication does not describe carbon. In the embodiment of the publication, 1
Since the heat treatment is performed at a low temperature of 50 ° C., the insulation does not deteriorate and the frequency characteristics of the magnetic permeability are good. However, the stress release of the metal magnetic powder is insufficient at such a low-temperature heat treatment. Is not high enough.

JP-A-61-288403 discloses that pure iron powder of 60 mesh or less obtained by atomization is prepared by adding
A powder magnetic core obtained by adding 5% by volume of a phenol resin, performing compression molding and curing treatment is disclosed. In the example of the publication, a phenol resin was added to pure iron powder, zinc stearate was added as a lubricant, and the mixture was pressed at 5 t / cm ² and then molded at 80 ° C. for 2 hours and at 180 ° C. for 2 hours. The powder magnetic core is obtained by performing the hardening process for a long time. However, even in the publication, there is no description about carbon as in the above-mentioned JP-A-56-155510, and a sufficient magnetic permeability cannot be obtained because the curing treatment temperature is low.

JP-A-7-211531 and JP-A-7-215153
Japanese Patent No. 2111532 describes a dust core containing an alloy powder containing Fe, Si and Al as main components, a silicone resin, and stearic acid. In the examples of both these publications, after compression molding at a pressure of 10 t / cm ² ,
The heat treatment is performed at 700 ° C. for 2 hours in the air or Ar atmosphere. However, there is no description about carbon in the publication.

In Japanese Patent Application Laid-Open No. 9-260126, a silicone resin is used as an insulating material, and 550-65 is used.
It has been proposed to anneal at 0 ° C. But,
The publication also does not describe carbon.

[0017]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a mixture containing a ferromagnetic metal powder and an insulating material, preferably containing a lubricant, is compression-molded to obtain a compact, and the compact is annealed by subjecting the compact to annealing. A dust core having an insulating layer between ferromagnetic metal particles is manufactured. Hereinafter, details of the present invention will be described.

Insulating Material In the present invention, in the dust core after annealing, it is necessary that the insulating layer contains isotropic carbon.

Isotropic carbon, also called hard carbon or glassy carbon, is hardly graphitizable, has the lowest crystallinity among carbons, and has a structure close to amorphous. The basic unit of the structure is a hexagonal network of carbon atoms, and since they are extremely small and are randomly assembled, the properties as a block are isotropic. Carbon is generally produced by heat treatment of organic compounds, whereas isotropic carbon is produced when the organic compound contains a large amount of oxygen or has a low hydrogen content. When such an organic compound is heat-treated, dehydrogenation is rapidly performed in a relatively low temperature range, and there is no generation of a hydrocarbon that generates a tar-like substance by thermal decomposition, so that a three-dimensional crosslinked structure is formed. Highly developed and solidifies at low temperatures.
Therefore, since the movement of the planar molecules is hindered and a sufficient orientation cannot be obtained, the structure becomes isotropic.

The carbon described in JP-A-7-111209 is a cation-exchangeable carbon material. To have a cation exchange function, for example, "ion exchange" (published by Kodansha Scientific) No. 151
It is necessary to have a layered structure as described on page １５152, such carbon being a so-called soft carbon such as graphite. Since soft carbon has a layered structure, its electric resistance is several orders of magnitude lower than that of isotropic carbon, so that the effect of insulating between ferromagnetic metal particles is significantly reduced. Specifically, the specific resistance of isotropic carbon powder is generally 0.2 Ω · cm or more, and the specific resistance of soft carbon powder is generally 5 × 10 ⁻⁵ Ω · cm or less. Also, unlike isotropic carbon, soft carbon does not show the effect of suppressing resin loss caused during high-temperature annealing.

Whether the carbon present in the dust core is isotropic carbon or soft carbon can be determined by X-ray diffraction. FIG. 1 shows the X-ray diffraction patterns of both carbons. The upper part in the figure is soft carbon (graphite)
Which have two sharp peaks characteristic of graphite. On the other hand, the lower side in the figure is made of isotropic carbon (Nikabeads manufactured by Nippon Carbon Co., Ltd.).
Isotropic carbon is characterized in that there is no peak on the high angle side observed in soft carbon, and the peak on the low angle side shifts to a lower angle side and is broadened.

Although isotropic carbon powder is generally known as various fillers, no proposal has been made to use it as an insulating material for a dust core, and of course, the effect in that case is also known. Absent.

In the present invention, when it is necessary to generate isotropic carbon during annealing, a thermosetting resin capable of generating isotropic carbon is used as an insulating material. As such a thermosetting resin, a phenol resin or a silicone resin is preferable, and it is also possible to use a mixture of both.

The phenolic resin is synthesized by reacting a phenol with an aldehyde. The one that uses a base catalyst during synthesis is a Resol resin, and the one that uses an acid catalyst is Novolak.
Mold resin. The resol type resin is cured by heating or an acid catalyst, and becomes insoluble and infusible. The novolak-type resin is a fusible resin that does not thermoset by itself and is cured by heating with a crosslinking agent such as hexamethylenetetramine.

In the present invention, it is preferable to use a resol type resin as the phenol resin. Among the resole resins, those containing N in the form of a tertiary amine are particularly preferred because of their good heat resistance. On the other hand, when novolak-type resin is used, the strength of the molded body is reduced.
Handling in the process after molding becomes difficult. When a novolak resin is used, it is preferable to perform molding (hot pressing or the like) while applying a temperature. In this case, the temperature during molding is usually about 150 to 400 ° C. The novolak type preferably contains a crosslinking agent.

A commercially available phenol resin can be used. For example, BRS-3 manufactured by Showa Polymer Co., Ltd.
801, ELS-572, 577, 579, 580, 5
82, 583 (above, resol type) and the like can be used.

The silicone resin used in the present invention preferably has a weight average molecular weight of about 700 to 3300.

In the present invention, the entire amount of isotropic carbon may be generated from the insulating resin. However, after the insulating material containing the isotropic carbon powder is mixed with the ferromagnetic metal powder, molding and annealing are performed. By doing so, the above-described effect is realized. When isotropic carbon powder is added in advance, it is preferable to use a thermosetting resin that can generate isotropic carbon, or use a thermosetting resin that does not generate isotropic carbon. it can.

The amount of the isotropic carbon powder to be added to the insulating material before molding is preferably 1.
0 wt% or less, more preferably 0.8 wt% or less. If the amount of the isotropic carbon powder is too large, it is necessary to reduce the amount of the resin to be added so as not to increase the total amount of nonmagnetic components, so the amount of the resin as a binder is insufficient,
The mechanical strength of the magnetic core may be reduced. In order to sufficiently exert the effect of the addition of the isotropic carbon powder, the amount of the isotropic carbon powder added to the ferromagnetic metal powder is preferably 0.05% by weight or more, more preferably 0.1% by weight. % Or more.

The addition amount of the thermosetting resin depends on the addition amount of the isotropic carbon powder, but is preferably 1 to 30% by volume, more preferably 2 to 2% by volume based on the ferromagnetic metal powder.
0% by volume. If the amount of the resin is too small, the mechanical strength of the magnetic core is reduced or insulation failure occurs. On the other hand, if the amount of the resin is too large, the ratio of the non-magnetic component in the dust core increases, and the magnetic permeability and the magnetic flux density of the core decrease.

The dust core after annealing may contain soft carbon in addition to isotropic carbon.
For example, when a resin that generates soft carbon and isotropic carbon powder are used as the insulating material, soft carbon is generated from the resin by annealing, and the soft carbon may be present in the magnetic core. Even in this case, the effect of the present invention is realized because isotropic carbon exists in the insulating layer.

When mixing the thermosetting resin and the ferromagnetic metal powder, a solid or liquid resin may be made into a solution and mixed, or a liquid resin may be directly mixed. The viscosity of the liquid resin is preferably 10 to 1000 at 25 ° C.
0 CPS, more preferably 50 to 9000 CPS. If the viscosity is too low or too high, it becomes difficult to form a uniform coating on the surface of the ferromagnetic metal powder. When the isotropic carbon powder is added, it may be added simultaneously with the mixing.

When the resin used as the insulating material remains after annealing, it also functions as a binder and improves the mechanical strength of the magnetic core.

In the present invention, an inorganic insulating material as described below may be used in addition to the thermosetting resin and the isotropic carbon powder. As the inorganic insulating material, titanium oxide sol and / or zirconium oxide sol are preferable. Titanium oxide sol and zirconium oxide sol are amorphous titanium oxide particles and zirconium oxide particles that are negatively charged and are dispersed in water or an organic dispersion medium to form a colloid. TiOH group, -ZrO
The H group is present. By adding a sol in which fine particles are uniformly dispersed in a solvent, such as titanium oxide sol and zirconium oxide sol, to a ferromagnetic metal powder, a uniform insulating film can be formed in a small amount, resulting in high magnetic flux density and high insulation properties. Can be realized.

The average particle size of the titanium oxide particles and zirconium oxide particles contained in the sol is preferably from 10 to 100.
nm, more preferably 10 to 80 nm, even more preferably 2 nm.
0 to 70 nm. The content of particles in the sol is 15
It is preferably about 40% by weight.

A titanium oxide sol for a ferromagnetic metal powder,
The added amount of zirconium oxide sol in terms of solid content, that is, the total added amount of titanium oxide particles and zirconium oxide particles is preferably 15% by volume or less, more preferably 5.0% by volume or less. If this total addition amount is too large, the nonmagnetic content in the dust core increases, so that the magnetic permeability and the magnetic flux density decrease. In order to sufficiently exhibit the effect of adding these sols, the total amount is preferably 0.1% by volume or more, more preferably 0.2% by volume or more, and still more preferably 0.1% by volume or more. 5% by volume or more.

The titanium oxide sol and the zirconium oxide sol may be used alone or in combination. When used in combination, the ratio is arbitrary.

These sols are commercially available products [Nissan Chemical Industries, Ltd., NZS-20A, NZS-30A, NZS-30].
B etc.] can be used. When the available sol has a low pH value, it is preferable to adjust the pH value to about 7. If the pH value is low, the ferromagnetic metal powder is oxidized and non-magnetic oxides increase, and the magnetic permeability and the magnetic flux density may decrease, or the coercive force may deteriorate.

These sols include those using an aqueous solvent and those using a non-aqueous solvent. In the present invention, those using a solvent compatible with the above resin are preferable. Particularly, ethanol and butanol are preferable. It is preferable to use a non-aqueous solvent such as toluene, xylene or the like. When the available sol uses an aqueous solvent, the solvent may be replaced as needed.

The sol may contain chlorine ions, ammonia and the like as a stabilizer.

These sols are usually in the form of a milky white colloid.

Lubricant A lubricant is added at the time of molding to enhance the lubricity between particles and to improve the releasability from a mold. As the lubricant, it is preferable to use at least one selected from magnesium stearate, calcium stearate, strontium stearate and barium stearate. Of these, strontium stearate is most preferred.

The content of these divalent metal stearates is preferably 0.2 to 1.5 with respect to the ferromagnetic metal powder.
%, More preferably 0.2 to 1.0% by weight.
If the content is too small, the insulation between the ferromagnetic metal particles in the dust core becomes insufficient, and problems such as the magnetic core becoming difficult to remove from the mold during molding are likely to occur. On the other hand, if the content is too large, the nonmagnetic content in the dust core increases, so that the magnetic permeability and the magnetic flux density decrease and the strength of the core tends to be insufficient.

In addition to the above-mentioned divalent metal stearate, other divalent metal salts of higher fatty acids, in particular, divalent metal laurate may be used as the lubricant. However, the amount used is preferably not more than 30% by weight of the amount of the divalent metal stearate used.

Ferromagnetic metal powder The ferromagnetic metal powder used in the present invention is not particularly limited. For example, iron, sendust (Fe-Al-Si), iron silicide, permalloy (Fe-Ni), supermalloy (Fe
-Ni-Mo), iron nitride, iron-aluminum alloy, iron-cobalt alloy, phosphorous iron, and other known magnetic metal powders. For example, it is preferable to use iron powder having high saturation magnetization in order to use a dust core that replaces a core for a relatively low frequency region currently manufactured using a laminated silicon steel sheet. The method for producing the iron powder may be any of an atomizing method, an electrolytic method, and a method of mechanically pulverizing electrolytic iron.

When an alloy is used for the ferromagnetic metal powder, the particles are harder than when iron is used, so that the stress at the time of molding increases. Therefore, annealing at a higher temperature is required. Therefore, the effect of the present invention in which the insulating property is maintained up to a high annealing temperature appears more remarkably.

The average particle size of the ferromagnetic metal powder is preferably 20 to 150 μm, more preferably 25 to 80 μm. If the average particle size is too small, the coercive force increases, and if the average particle size is too large, the eddy current loss increases. The ferromagnetic metal powder having an average particle diameter in the above range may be obtained by classification using a sieve or the like.

In the present invention, the ferromagnetic metal particles may be flattened if necessary. Flattening is performed by applying pressure in the direction perpendicular to the direction of the magnetic path at the time of use, so-called laterally-pressable magnetic core, for example, a toroidal magnetic core or an E-shaped magnetic core, and all legs are rectangular parallelepiped. It is particularly effective in certain cases. That is, in the horizontal extrusion molding, it is easy to make the main surface of the flat particles substantially parallel to the magnetic path in the dust core, so that the use of the flat particles can easily improve the magnetic permeability. Means for flattening is not particularly limited, but it is preferable to use means utilizing rolling and shearing action such as a ball mill, a rod mill, a vibration mill, and an attrition mill. Although the flattening rate is not particularly limited, it is usually preferable to set the average aspect ratio to about 5 to 25. The aspect ratio in this case is a value obtained by dividing the average of the minor axis and major axis of the main surface by the thickness.

Manufacturing Method of Dust Core At the time of manufacturing the dust core of the present invention, first, a ferromagnetic metal powder and an insulating material are mixed.

When iron powder is used as the ferromagnetic metal powder,
Before mixing, it is preferable to subject the iron powder to a heat treatment (annealing) for strain relief. Before mixing, the iron powder may be subjected to an oxidation treatment. If a thin oxide film having a thickness of about several tens of nanometers is formed in the vicinity of the surface of the iron particles by this oxidation treatment, improvement in insulating properties can be expected. This oxidation treatment may be performed by heating at 150 to 300 ° C. for about 0.1 to 2 hours in an oxidizing atmosphere such as air. When the oxidation treatment is performed, a dispersant such as ethyl cellulose may be mixed in order to improve the wettability of the iron particle surface.

The mixing conditions are not particularly limited. For example, using a pressure kneader, a raikai machine or the like, at room temperature for about 20 to 60
Mix for a minute. After mixing, preferably 100 to 30
Dry at about 0 ° C. for 20-60 minutes.

After drying, a lubricant is added.

In the forming step, a desired magnetic core shape is formed. The shape of the magnetic core is not particularly limited, and so-called toroidal type, E type, I type, F type, C type, EE type, EI type, ER
The present invention can be applied to the production of magnetic cores of various shapes such as a mold, an EPC mold, a pot, a drum mold, a pot mold, and a cup mold. INDUSTRIAL APPLICABILITY The present invention is particularly effective when manufacturing a magnetic core having a complicated shape, because scattering of a resin which is likely to occur when molding pressure is small can be suppressed.

The molding conditions are not particularly limited, and may be appropriately determined according to the type, shape and size of the ferromagnetic metal particles, the desired magnetic core shape and size, the magnetic core density, and the like. ２０20 t / cm ² , and the time for maintaining the maximum pressure is about 0.1 second to 1 minute.

After the compacting, annealing is performed to improve the magnetic properties of the magnetic core. This annealing is for releasing the stress generated in the ferromagnetic metal particles during powdering or molding. When the particles are mechanically flattened, the stress due to the flattening can be released. In addition, the annealing hardens the insulating resin and improves the mechanical strength of the green compact.

The annealing conditions may be appropriately determined according to the type of the ferromagnetic metal powder, molding conditions, flattening conditions, and the like. If it is necessary to generate isotropic carbon from the insulating resin, The conditions at which the isotropic carbon can be generated may be selected, but the processing temperature is preferably 500
To 900 ° C, more preferably 600 to 850 ° C.
If the processing temperature is too low, the release of the stress becomes insufficient and the return to the original coercive force becomes insufficient, so that the direct current superposition characteristic is poor and the hysteresis loss is increased.
On the other hand, if the processing temperature is too high, the insulation coating is thermally destroyed and insulation becomes insufficient, so that the eddy current loss increases and the frequency characteristics of the magnetic permeability deteriorate. become worse. In order to generate isotropic carbon, the processing temperature is preferably set to 600 ° C. or higher. The processing time, that is, the time for passing through the above temperature range or the time for maintaining the temperature at a constant temperature within the above temperature range is preferably 10 minutes to 2 hours.
If the treatment time is too short, the annealing effect tends to be insufficient,
If it is too long, dielectric breakdown tends to occur.

Annealing is preferably performed in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen in order to prevent a decrease in magnetic permeability and magnetic flux density due to oxidation of the ferromagnetic metal powder.

After the heat treatment, the magnetic core may be impregnated with a resin or the like, if necessary. By impregnating the resin, the strength is further improved. Examples of the resin used for the impregnation include a phenol resin, an epoxy resin, a silicone resin, and an acrylic resin. Among them, a phenol resin is preferable. These resins may be used by dissolving them in a solvent such as ethanol, acetone, toluene and pyrrolidone.

As a method of impregnating the magnetic core with the resin, the magnetic core is placed on a container such as a vat and a mixed solution of a resin and a solvent (for example, a 10% ethanol solution of phenol resin) is poured into the container. Be completely hidden.
After holding for about 1 to 30 minutes as it is, the magnetic core is taken out, the resin solution attached to the surroundings is removed to some extent, and then heat treatment is performed. In this heat treatment, first,
Using an oven or the like, the temperature is raised to about 80 to 120 ° C. in the air atmosphere and maintained for about 1 to 2 hours. In addition, 13
The temperature is raised to about 0 to 170 ° C. and maintained for about 1.5 to 3 hours.
Hold for about an hour.

After the heat treatment, if necessary, an insulating film is formed on the surface of the magnetic core in order to secure insulation between the winding and the core. Do.

The powder magnetic core of the present invention is suitable for magnetic cores such as transformers and inductors, magnetic cores for motors, and other electromagnetic components. It can also be used for choke coils and airbag sensors in electric vehicles. The frequency used is preferably 10 Hz to 500 kHz, more preferably 500 Hz to 20 kHz.
It is 0 kHz.

[0062]

EXAMPLE 1 A dust core sample was prepared in the following procedure.

Ferromagnetic metal powder: Permalloy powder [manufactured by Daido Steel Co., Ltd., average particle diameter 30 μm] Zirconia sol: Nissan Chemical Co., Ltd. ZrO ₂ sol (NZ
S-30A, an average particle diameter of 62 nm) was adjusted to pH 7, and then a dispersion was obtained by replacing the aqueous solvent with an ethanol solvent. Phenol resin: resole type resin [ELS-582 manufactured by Showa Polymer Co., Ltd., weight average molecular weight 1500] , Isotropic carbon powder: Nicabeads (manufactured by Nippon Carbon Co., Ltd.), and lubricant: strontium stearate (manufactured by Sakai Chemical Co., Ltd.).

First, ferromagnetic metal powder, zirconia sol of 2.0% by volume in terms of solid content, 7.1% by volume
Was mixed with a isotropic carbon powder having a weight ratio shown in Table 1 by a pressure kneader at room temperature for 30 minutes. Next, it was dried in the air at 250 ° C. for 30 minutes. The dried mixture was added to the ferromagnetic metal powder in an amount of 0.6%.
After adding 15% by weight of a lubricant and mixing with a V mixer for 15 minutes, the outer diameter was 17.5 mm and the inner diameter was 1 at a pressure of 9 t / cm ^2.
It was formed into a toroidal shape having a height of 0.2 mm and a height of about 6 mm. After molding, annealing was performed in an N ₂ atmosphere at a temperature shown in Table 1 for 30 minutes to obtain a dust core sample.

A sample to which no isotropic carbon powder was added was also prepared. For comparison, a sample using a polycarbodiimide resin described in JP-A-7-111209 as a raw material of a cation-exchangeable carbon material, instead of the phenol resin, was also prepared.

For each sample, 100 kHz, 400
The effective magnetic permeability (μ _DC ) at 0 A / m and the core loss at 100 kHz and 100 mT [total loss (Pc), hysteresis loss (Ph) and eddy current loss (Pe)] were determined. In addition,
The magnetic permeability was calculated from the inductance measured with an LCR meter [HP4284A, manufactured by Yokogawa Hewlett-Packard Co., Ltd.]. The magnetic core loss was measured with a BH analyzer [SY-8232 manufactured by Iwasaki Communication Equipment Co., Ltd.]. Table 1 shows the results.

After dissolving each sample with nitric acid to remove metal components, the samples were analyzed by X-ray diffraction to check for the presence of isotropic carbon. FIG. 1 shows X-ray diffraction patterns of soft carbon (graphite) and isotropic carbon (Nikabeads manufactured by Nippon Carbon Co., Ltd.). FIG. 2 shows the X of carbon generated in sample No. 101 of Table 1.
3 shows a line diffraction pattern. From the comparison of both figures, the sample No.
It is clear that isotropic carbon is generated at 101. Table 1 shows the presence / absence of isotropic carbon in each sample examined in this manner.

Further, the carbon content in the sample was examined by gas analysis. Table 1 shows the results.

[0069]

[Table 1]

Table 1 shows that a sample containing isotropic carbon, particularly a sample annealed with the addition of isotropic carbon powder, has good DC superimposition characteristics and low loss.

Example 2 A sendust powder [manufactured by Daido Steel Co., Ltd., average particle size 50 μm] was used as a ferromagnetic metal powder, and a silicone resin [KR15 manufactured by Shin-Etsu Chemical Co., Ltd.] was used instead of the phenol resin.
3, a weight average molecular weight of 2600], and a dust core sample was prepared in the same manner as in Example 1 except that the annealing temperature was set to the value shown in Table 2. In addition, a sample that was annealed at a temperature shown in Table 2 using a polycarbodiimide resin was also prepared. Table 2 shows the amount of isotropic carbon powder added, the annealing temperature, the presence or absence of isotropic carbon in the sample, and the carbon content in the sample for each sample.

The characteristics of these samples were measured in the same manner as in Example 1. Table 2 shows the results.

[0073]

[Table 2]

Table 2 shows that isotropic carbon can be produced even when a silicone resin is used. In addition, even when a silicone resin is used, in a sample containing isotropic carbon, in particular, in a sample annealed with the addition of isotropic carbon powder, the DC superimposition characteristics are improved and the loss is reduced. Understand.

Example 3 An epoxy resin [Araidite manufactured by Nippon Ciba Geigy Co., Ltd.] was used in place of the phenol resin, and the annealing temperature was set as shown in Table 3.
A powder magnetic core sample was prepared in the same manner as in Example 1 except that the values shown in Table 1 were used. Table 3 shows the amount of isotropic carbon powder added, the annealing temperature, the presence or absence of isotropic carbon in the sample, and the carbon content in the sample for each sample.
Shown in

The characteristics of these samples were measured in the same manner as in Example 1. Table 3 shows the results.

[0077]

[Table 3]

In sample No. 301 of Table 3, no isotropic carbon was generated from the resin. However, from Table 3,
It can be seen that by adding the isotropic carbon powder in advance, the improvement of the DC superimposition characteristics and the reduction of the loss are realized.

[0079]

According to the present invention, the saturation magnetic flux density is high,
In addition, a dust core having low loss and good DC superimposition characteristics is realized.

[Brief description of the drawings]

FIG. 1 is a graph showing X-ray diffraction patterns of soft carbon and isotropic carbon.

FIG. 2 is a graph showing an X-ray diffraction pattern of isotropic carbon generated by annealing.

Claims (7)

[Claims] 1. A dust core comprising: a ferromagnetic metal powder; and an insulating layer existing between particles constituting the ferromagnetic metal powder, wherein the insulating layer contains isotropic carbon. 2. The dust core according to claim 1, wherein the insulating layer contains a thermosetting resin. 3. The dust core according to claim 2, wherein the thermosetting resin contains at least one of a silicone resin, a phenol resin, and an epoxy resin. 4. The dust core according to claim 1, wherein the isotropic carbon contains at least isotropic carbon powder. 5. A method for producing a dust core by compressing and molding a mixture containing a ferromagnetic metal powder and an insulating material, wherein the insulating material comprises a thermosetting resin, A method for producing a dust core, wherein isotropic carbon is generated from the thermosetting resin during annealing. 6. The method for manufacturing a dust core according to claim 5, wherein the insulating material contains an isotropic carbon powder in addition to a thermosetting resin. 7. A method for producing a dust core by compressing and molding a mixture containing a ferromagnetic metal powder and an insulating material, wherein a thermosetting resin and an isotropic carbon powder are used as the insulating material. A method for producing a dust core using one containing: