JP2003166004A - Iron-based powder and dust core using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an iron-base powder having such a heat resistant insulating layer as not to be easily peeled even when the iron-base powder is pressure- formed, and further not to cause an electrical breakdown even when annealed in order to reduce a hysteresis loss, and provide a powder magnetic core using the same. <P>SOLUTION: The iron-base powder has a coated film including an epoxy resin and alumina-containing silica, on the surface of a raw powder containing iron as a main component. The film preferably includes 3-300 pts.mass of alumina-containing silica in (alumina + silica) terms, with respect to 100 pts. mass of the epoxy resin. The powder magnetic core is made by means of annealing after compacting the coated iron-base powder into a predetermined shape. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an iron-based powder and a dust core obtained by compacting the iron-based powder.

[0002]

2. Description of the Related Art In recent home appliances and electronic devices, there is a strong demand for downsizing of the devices and high power conversion efficiency. Switching power supplies are characterized by their small size and high efficiency compared to the power supplies used in the past, so in recent years they have been considered for use in large equipment where switching power supplies were not used in the past, and their large output. There is a strong demand for realization.

Further, in response to recent global warming problems, further energy saving of electric equipment is demanded, and further efficiency of switching power supplies is also demanded. As described above, increasing the output and increasing the efficiency have become important issues in the recent development of switching power supplies. by the way,
Increasing the switching frequency, that is, increasing the operating frequency is effective for increasing the output and efficiency of the switching power supply. Increasing the switching frequency can realize not only high efficiency but also device size reduction, and the switching frequency tends to gradually increase. At present, switching power supplies operating in a high frequency range of 10 kHz to 100 kHz are predominant.

It is well known that noise called switching noise is generated in a switching power supply due to its switching. This noise affects peripheral devices to cause malfunction, and may cause an accident due to momentary excessive current. Therefore, noise removal using a noise filter is essential in the switching power supply. With the recent increase in the frequency of switching power supplies, it has become necessary for this noise filter to operate reliably in the high frequency range.

As a noise filter for a switching power supply, a choke coil having a core wound is widely used. As the core, a laminated iron core using an electromagnetic steel plate, a ferrite core, or the like has been conventionally used. However, in the laminated core, as the frequency range used increases, the eddy current generated inside the core becomes excessive and the loss increases, resulting in a significant decrease in efficiency. There was a problem that the value of was also significantly reduced.

On the other hand, the ferrite core is characterized by a low iron loss in a high frequency range and a high AC relative initial permeability, but has a problem of a low saturation magnetic flux density. It is known that when a large amount of current flows through the noise filter and magnetic saturation occurs, the AC ratio initial permeability drops sharply and the filter performance also drops significantly.However, higher output power of switching power supplies is required. However, the low saturation magnetic flux density of the ferrite core was a big problem.

At present, a dust core has been attracting attention as a core material of a noise filter. Dust magnetic core, after appropriately adding a binder such as resin to iron-based powder,
It is produced by pressure molding and may be subjected to heat treatment for the purpose of curing the resin after molding. The powder magnetic core uses a fine iron-based powder as a raw material and also mixes the iron-based powder with a resin having a high insulating property. It has the characteristic of high magnetic susceptibility. Further, there is also a feature that the saturation magnetic flux density can be made higher than that of ferrite by using a metal powder having a high saturation magnetic flux density as a raw material.

Therefore, in recent years, the dust core has attracted a great deal of attention as an iron core material which can replace the laminated iron core and the soft ferrite core. However, iron loss in the current operating frequency range of switching power supplies, 10 kHz to 100 kHz, for example, cannot be said to be sufficiently low, and in order to respond to future high efficiency switching power supplies, iron Loss reduction was needed.

By the way, the iron loss of the dust core is roughly classified into a hysteresis loss and an eddy current loss. Conventionally, various studies have been conducted to reduce the eddy current loss. For example, JP-A-58-147106 discloses that
A method for controlling the particle size of the metal powder is disclosed in JP-A-62-71202, JP-A-62-29108, JP-A-2-153003, etc., in which an insulating substance such as a metal powder and a resin is used. A method of mixing is disclosed.

On the other hand, various studies have been conducted to reduce the hysteresis loss. It is well known that, in electromagnetic steel sheets, the hysteresis strain is reduced by removing the strain applied during processing by annealing, but it is also known that the processing strain applied during pressure forming is annealed even in dust cores. It has been pointed out that when released by, the hysteresis loss is reduced and the iron loss may be reduced. In particular, it is said that annealing at a temperature of 650 ° C or higher is effective.

However, since organic substances such as resin are generally decomposed when heated to 300 ° C. or higher, annealing the powder magnetic core causes the resin to disappear and the insulating property to be remarkably lowered. Therefore, in the dust core after annealing, an eddy current loss that greatly exceeds the reduction amount of hysteresis loss was generated, and it was very difficult to reduce the iron loss during annealing. Several methods have been proposed to solve this problem and reduce the iron loss due to strain relief annealing in the dust core, that is, reduce the hysteresis loss while suppressing the increase in the eddy current loss.

For example, Japanese Patent Laid-Open No. 61-222207 discloses that
Disclosed is a method of producing a powder magnetic core by contacting silica sol or alumina sol with a metal magnetic powder, and then drying to form an electrically insulating adhesion layer made of silica or alumina on the surface of the metal magnetic powder and press-molding. ing. In addition, JP-A-61-1
No. 47505 discloses that an electrically insulating inorganic fine powder having an electronegativity of 8.5 or more and 12.5 or less and a particle size of 0.05 μm or less is mixed with a metal magnetic powder to form an insulating adhesive layer on the surface of the metal magnetic powder. After that, a method for obtaining a powder magnetic core by pressure molding is disclosed.

It is said that the iron loss of each of these dust cores is reduced by heat treatment at 500 ° C. Further, Japanese Patent Application Laid-Open No. 4-219902 discloses a method of forming an electrically insulating adhesion layer made of silica by mixing ethyl silicate and metal magnetic powder and then drying.

However, in any of these methods, the force for binding the electrically insulating substance and the magnetic metal powder is very weak such as van der Waals force or Coulomb force, and therefore, the pressure insulation is used to obtain the electrically insulating property. There is a problem that the adhesion layer is easily peeled off, and the metals on the peeled surfaces come into contact with each other, resulting in a significant decrease in electrical insulation. Therefore, in order to suppress the eddy current loss and ensure sufficient insulation, it is necessary to significantly increase the amount of the electrically insulating substance added. However, if the amount of addition is increased, the molded body becomes extremely brittle, and there is a problem that it cannot be put to practical use.

In order to avoid the problem that the binding force between the electrically insulating substance (inorganic powder) and the metallic magnetic powder is weak in this way, in Japanese Examined Patent Publication No. 6-1727 or Japanese Examined Patent Publication No. 4-16004, etc. A method of using a mixture of an insulating inorganic powder and an electrically insulating binder resin is disclosed. In these methods, the binding force of the binding resin causes a strong binding between the inorganic powder and the magnetic metal powder, and the adhesion layer does not easily peel off even during pressure molding. Became significantly higher.

However, it is very difficult to knead the inorganic powder and the binder resin, and it is difficult to uniformly disperse them.
There has been a problem that the concentration of the inorganic powder in the adhered layer remarkably fluctuates, and where the concentration of the inorganic powder is small, the insulating property is deteriorated due to the direct contact between the metal magnetic powders after annealing.

[0017]

SUMMARY OF THE INVENTION The present invention has been made in view of the problems of the prior art as described above, and even if the powder is pressure-molded, the coating does not easily peel off, and the hysteresis loss It is an object of the present invention to provide an iron-based powder having a heat-resistant insulating coating that does not cause the insulation to be broken even when annealing is performed to reduce the above, and a dust core using the same.

[0018]

Means for Solving the Problems The inventors of the present invention have solved the above problems and have excellent film adhesion at the time of pressure molding at room temperature, which maintains insulation even after annealing and increases eddy current loss. In order to prevent the occurrence of the above phenomenon, the inventors conducted diligent studies on improving the adhesion and heat resistance of the insulating coating. As a result, in order to achieve both adhesion and heat resistance, coating with a solution obtained by mixing an epoxy resin and an inorganic fine powder is very effective, and silica powder containing alumina as the inorganic fine powder is further effective. It was found that when it is used, the dispersion in the solution becomes uniform and excellent insulation is obtained even after annealing.

The present invention has solved the above problems by the following iron-based powder or powder magnetic core. An iron-based powder obtained by coating the surface of a powder containing iron as a main component with a film containing an epoxy resin and alumina-containing silica. The iron-based powder as described above, wherein the silica containing alumina is ₃ to 300 parts by mass in terms of Al ₂ O ₃ + SiO ₂ with respect to 100 parts by mass of the epoxy resin. The alumina-containing silica is silica 10 converted into SiO _2.
0 to 0.01 parts by weight of alumina in terms of Al ₂ O ₃ is 0.01 to
500 parts by mass of the iron-based powder as described in the above item 1 or 2. A powder magnetic core obtained by forming the iron-based powder according to any one of the above 1 to 3 into a predetermined shape and then annealing.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The iron-based powder of the present invention is characterized in that the surface of a raw material powder containing iron as a main component is coated with a film containing an epoxy resin and alumina-containing silica.
In particular, the iron-based powder of the present invention has an insulating coating having excellent heat resistance. Hereinafter, the iron-based powder of the present invention will be described in detail.

In the present invention, a raw material powder containing iron as a main component as a starting material (hereinafter, also simply referred to as raw material powder).
To use. The treatment for forming the heat resistant insulation coating is performed by mixing the raw material powder containing iron as a main component and the insulation treatment liquid. After mixing, dry the solvent by heating or air-drying, and further consider the properties, applications, required characteristics, etc. of the resin being used, and if necessary, cure and bake the resin to give the specified iron-based powder. obtain.

The insulating treatment liquid used in the present invention is a solution prepared by mixing a resin, alumina-containing silica and its stabilizer, and a solvent. In the present invention, an epoxy resin is used as the resin. This is because the strength and heat resistance of the epoxy resin can be adjusted by adjusting the molecular weight, and the epoxy resin becomes soluble in various solvents by substituting the functional group. Furthermore, in order to maintain the shape of the dust core during annealing, it is preferable to use a curable epoxy resin.

As the powder magnetic core resin, there are phenol resin, urethane resin, unsaturated polyester resin and the like. However, ammonia is generated during curing of the phenol resin, which is not preferable in terms of hygiene, and the urethane resin is not preferable in terms of safety because it uses a dangerous substance such as isocyanate as a curing agent. On the other hand, unsaturated polyester has problems that it is difficult to cure, the surface is sticky, and it is difficult to handle. From the above, a curable epoxy resin is suitable as the powder core resin.

The epoxy resin is dissolved to form a solution, or an emulsion or dispersion is formed into a colloidal solution and uniformly dispersed in the insulating treatment liquid.
When forming an emulsion or dispersion, the particle size of the epoxy resin is preferably 10 μm or less.
Further, when a resin that becomes liquid at room temperature is used as the epoxy resin, stickiness due to the epoxy resin occurs even after the solvent is dried after the insulation treatment, which makes the handling of the iron-based powder extremely difficult. It is preferable to use an epoxy resin that is solid under room temperature conditions.

Alumina-containing silica is further mixed with the insulating treatment liquid used in the present invention. The alumina-containing silica in the present invention is a mixture of a predetermined amount of alumina and silica, and in the present invention, it is particularly preferable that the silica surface is coated with the minimum necessary amount of alumina.
As described above, when alumina is contained in silica, the dispersibility in the insulating treatment liquid is remarkably improved.

In addition, since alumina and silica may aggregate due to impurities inevitably contained in the insulating treatment liquid, it is preferable to add an acid as a stabilizer in the present invention. As a stabilizer, an organic acid is suitable. However, nitric acid or hydrochloric acid is not preferable because it is highly corrosive, and when used as a stabilizer, it corrodes the surface of the iron-based powder and causes rusting, which significantly deteriorates the magnetic properties.

As the organic acid, carboxylic acids such as formic acid, acetic acid and propionic acid are preferable. It should be noted that the organic acid added in the present invention may be one containing at least one --COOH group having the characteristic of carboxylic acid, and for example, in order to improve compatibility with a solvent, other functional groups are added. You may use the one that you made. The amount of the stabilizer added is such that the surface charge of the alumina coating the silica is neutralized, and
The pH of the liquid is not particularly limited as long as the liquid is in a stable range.
Is preferably 6-8.

The amount of alumina-containing silica based on 100 parts by mass of the epoxy resin is preferably 3 to 300 parts by mass, more preferably 10 to 300 parts by mass in terms of Al ₂ O ₃ + SiO ₂ . If the amount of alumina-containing silica is less than 3 parts by mass, the insulation performance after annealing will be significantly reduced. On the other hand, if the alumina-containing silica exceeds 300 parts by mass, the proportion of the epoxy resin becomes too small, the adhesive force of the insulating coating remarkably decreases, and the insulating performance deteriorates.

It is preferable that the amount of alumina is 0.01 to 500 parts by mass in terms of Al ₂ O ₃ with respect to 100 parts by mass of silica in terms of SiO ₂ . When the amount of alumina is less than 0.01 parts by mass, the amount of alumina added is too small and the dispersion of silica is significantly deteriorated, so that a uniform insulating treatment liquid cannot be obtained. Further, if the amount of alumina exceeds 500 parts by mass, there is a problem that the strength of the insulating coating is reduced and the insulating property after annealing is significantly deteriorated. Therefore, the amount of alumina is 0.01
To 500 parts by mass, preferably 1 to 300 parts by mass, more preferably 1 to 100 parts by mass.

The solvent used in the insulation treatment liquid in the present invention can be a ketone solvent such as acetone or methyl ethyl ketone, or an aromatic solvent such as benzene, xylene or toluene as long as it can dissolve and disperse the epoxy resin and the silica containing alumina. It is also possible to use an organic solvent typified by an alcohol solvent such as ethanol and methanol. Alternatively, water may be used.

Of course, other solvents can be used as long as they can dissolve and disperse the epoxy resin and alumina-containing silica. Among them, water is particularly suitable for preparing the insulation treatment liquid used in the present invention, because water is inexpensive and can be easily recovered.
Needless to say, two or more kinds of solvents arbitrarily selected from these may be used in combination with an appropriate formulation.

The insulating treatment liquid used in the present invention can be obtained by dispersing the above-mentioned epoxy resin, alumina-containing silica and a stabilizer organic acid in a solvent. The epoxy resin, the alumina-containing silica and the organic acid as the stabilizer may be prepared by dissolving and dispersing them in a solvent all at once.
Alternatively, the resin and the silica containing alumina may be made into a powder form and mixed before being mixed with the solvent, and the mixed powder may be mixed with the solvent.

It is also possible to dissolve or disperse either the epoxy resin or the alumina-containing silica first, and then add the remaining substances to the dispersion liquid all at once or sequentially. In the present invention, colloidal alumina or silica may be used as a raw material for alumina-containing silica. If the alumina and silica particles used have a large particle size, it becomes difficult to disperse them uniformly in the solvent. Therefore, when using alumina and silica powders in the present invention, the particle size of 10 μm or less is used to improve the dispersibility. It is preferable to use one.

The stabilizer may be mixed at any time during the preparation of the solution. Further, the predetermined addition amount may be added all at once or may be added in several times. The concentration of the insulating treatment liquid may be determined in consideration of ease of construction and drying time. In order to control the viscosity, thixotropy, leveling property, tack time, etc. of the insulation treatment liquid, some additives may be added to the insulation treatment liquid. Examples of such additives include metal soaps such as metal stearates that control the curing of the epoxy resin, and surfactants such as perfluoroalkyl. In addition, a small amount of rust preventive material may be added to prevent rusting of the raw material powder containing iron as a main component.

When the insulating treatment liquid used in the present invention has non-uniform concentration due to precipitation of components, the ratio of the epoxy resin to the silica containing alumina may deviate from the preferable range and the iron-based powder may be coated. . Therefore, when the insulating treatment liquid used in the present invention and the iron-based powder are mixed, it is preferable that the insulating treatment liquid is sufficiently stirred with a stirrer, a stirrer, a homogenizer or the like and then mixed.

Next, the method for producing the iron-based powder of the present invention will be described in detail. In the present invention, a raw material powder containing iron as a main component is used as a starting raw material. The powder may be any powder containing iron as a main component, which exhibits ferromagnetism and high saturation magnetic flux density, and its kind is not particularly limited. However, among them, iron powder, Fe-Si alloy powder represented by Fe-3% -Si alloy powder, Fe-Al alloy powder, Fe-Ni alloy powder, Sendust powder, iron-based amorphous alloy, etc. are used. It is preferable.

In the present invention, it is preferable to use, as a raw material powder, one or more powders selected from the powders containing iron as a main component. Further, as the raw material powder containing iron as a main component, a flat iron-based powder which has been flattened by a manufacturing method or some machining method may be used. Further, among iron-based powders, atomized iron powder, pure iron powder represented by electrolytic iron powder, and the like, not only have excellent magnetic characteristics such as saturation magnetic flux density and magnetic permeability, but also excellent compressibility, It is inexpensive. Therefore, it is suitable as the raw material powder in the present invention. Examples of the pure iron powder include KIP-MG270H and KIP-304AS manufactured by Kawasaki Steel Co., Ltd.

The particle size of the raw material powder used in the present invention is not particularly limited, but it is desirable to appropriately determine it according to the intended use of the dust core and the required characteristics. For example, when particles having a large particle size are taken out by classification and used, the compressibility is improved and the magnetic gap generated between the particles is significantly reduced. As a result, it is possible to obtain a dust core having a high magnetic permeability and a high magnetic flux density, and further, the hysteresis loss due to the reduction of the magnetic gap is significantly reduced.

Such a dust core is suitable for applications where the operating frequency is, for example, about 1 kHz or less and a high magnetic flux density is required. In this case, the preferable particle size is 75 μm or more, and the more preferable particle size is 106 μm or more. Conversely, when particles with a small particle size are taken out and used, the size of the particles becomes smaller, and the loss due to the eddy current generated in the particles is greatly reduced. as a result,
Iron loss in the high frequency range is significantly reduced. Such a dust core is suitable for applications where low loss is required in the operating frequency range of about 100 kHz to 500 kHz, for example. In this case, the preferable particle size is 75 μm or less.

The present invention is applicable to iron-based powders of any particle size. The particle size of the iron-based powder can be appropriately determined depending on the application of the dust core and required characteristics. Further, the iron-based powder may be prepared by adjusting the elements contained within a range that does not adversely affect the compressibility and the magnetic characteristics of the dust core. In the present invention, the iron-based mixed powder is obtained by adding the raw material powder selected from the above to the insulating treatment liquid adjusted under the above conditions, stirring and mixing, further drying and evaporating the solvent.

Regarding the addition to the insulating treatment liquid used in the present invention, the whole amount may be added first, or the components may be divided and added during stirring. Moreover, you may spray through a spray nozzle at the time of stirring. When the insulating treatment liquid is sprayed and added by the spray nozzle, the insulating treatment liquid is uniformly added to the raw material powder and the coating becomes uniform, which is preferable. For stirring and mixing,
Attritors, henschel mixers, ball mills, fluidized granulators, tumbling granulators and the like are generally used. It is preferable to perform stirring in a fluidized tank as in a fluidized granulator or a tumbling granulator because aggregation of powder particles is suppressed.

Further, it is particularly preferable to spray the insulating treatment liquid to the fluidized tank by using a spray, because the effect of spray spraying and the effect of utilizing the fluidized tank are combined and a more uniform film can be obtained. Note that heat treatment may be performed during or after the mixing for the purpose of accelerating the drying of the solvent and curing the epoxy resin. The amount of the insulating coating adhered to the raw material powder is preferably 0.01 to 10% by mass, more preferably 0.1 to 5% by mass, and most preferably the calculated amount of the solid residue when the solvent is volatilized. 0.1 to 2% by mass.

If the adhered amount is 0.01% by mass or less, the entire raw material powder cannot be coated, and the insulating property is significantly lowered. On the other hand, if the adhered amount is more than 10% by mass, the strength of the powder magnetic core obtained by molding the iron-based powder becomes extremely low, which is not suitable. The iron-based powder of the present invention is added with a lubricant or the like if necessary, and then pressure-molded using a mold or the like to obtain a powder-molded product. Examples of the lubricant include metal soaps such as lithium stearate, zinc stearate and calcium stearate, and waxes such as fatty acid amides.

The molding pressure may be appropriately determined according to the application. After molding, in order to release the strain applied to the iron-based powder during pressurization and reduce the hysteresis loss, it is preferable to hold the molded body at a temperature of 600 ° C. or higher and anneal it. Particularly, the annealing temperature is preferably 800 ° C or higher. The annealing time is preferably 30 minutes or longer. The annealing atmosphere may be an inert atmosphere such as Ar or N ₂ , a reducing atmosphere such as hydrogen, or a vacuum. The dew point of water vapor may be appropriately determined according to the application. The temperature rising rate and the temperature lowering rate during annealing may be appropriately determined depending on the application and the equipment. A step of maintaining a constant temperature may be provided when the temperature is raised.

Since the iron-based powder of the present invention can maintain a highly insulating film even after annealing, it is possible to maintain the insulating property even after strain relief annealing. Therefore, it is possible to reduce the hysteresis loss without increasing the eddy current loss. In the present invention, by containing alumina in silica, it is important not only that the silica is uniformly dispersed in the insulating treatment liquid but also that the insulating property after annealing can be remarkably improved. is there. The details of the reason why the heat-resistant insulation is improved by adding alumina is not clear, but by using alumina and silica at the same time, a glass-like bond is formed between them, and the alumina-silicate glass structure is formed. It is speculated that this may have been achieved.

[0046]

EXAMPLES Examples of the present invention will be described below. As raw material powder, iron powder KIP-270H, KI manufactured by Kawasaki Steel Co., Ltd.
P-304AS and Sendust powder were used. The present invention is
It is effective for all raw material powders containing iron as a main component, and the raw material powders are not limited to the examples.

First, the production of iron-based powder will be described.
As the resin, two kinds of epoxy resins were used: an emulsion of epoxy resin (average particle diameter 1 μm) and a bisphenol A type epoxy resin having a molecular weight of 3000. As alumina, 2 of an alumina sol having an average particle diameter of 1 μm and an alumina powder are used.
The type used.

As silica, silica sol having an average particle diameter of 1 μm or amorphous silica was used. The alumina stabilizer was added in such an amount that the charge of the added alumina was 100% neutralized and became electrically medieval. This addition amount was calculated from the amount of alumina to be added.

A Henschel mixer or a tumbling granulator was used for mixing the insulating treatment liquid and the raw material powder. When using a henschel mixer, first, the whole amount of the insulation treatment liquid was added to the raw material powder, and then the mixture was stirred and mixed. The mixing time was 400 seconds. Then, after stirring, it was air-dried at room temperature for 10 hours to obtain an iron-based powder. Next, manufacturing of the dust core will be described.

Unless otherwise specified, a lubricant was added to and mixed with the above iron-based powder. Zinc stearate was used as the lubricant. The amount of the lubricant added was 0.25 parts by mass with respect to 100 parts by mass of the iron-based powder. Lubricant addition and mixing were performed according to the following procedure. First, the iron-based powder is put in a bag, and then a predetermined amount of lubricant is added to the bag. Then, the mouth of the bag is strictly closed, and then the entire bag is vibrated to mix the lubricant so that the lubricant becomes uniform.

The iron-based powder thus obtained was pressure-molded to give a ring sample for magnetic measurement (outer diameter φ38 mm, inner diameter φ25).
mm, height 6.2 mm) and a rectangular parallelepiped sample (width 10
mm, length 35 mm, height 6.2 mm). The molding pressure is 68
6 MPa. The strain relief annealing temperature was 800 ° C., and heating was performed for 1 hour in a nitrogen atmosphere. Using the sample thus obtained, the powder compact density, the specific resistance, the inductance at 1 kHz to 1 MHz, the iron loss at 10 kHz and 0.1 T, and the hand bending test were performed.

The specific resistance was measured by a four-terminal method using a rectangular parallelepiped sample. In addition, the inductance measurement was carried out by an LCR meter HP4284A manufactured by Agilent Technologies, using a coil prepared by winding 11 round wire formal-coated wires of φ0.6 mm on a ring sample. Then, the AC ratio initial permeability μ _iAC was calculated from the obtained inductance value. On the other hand, the iron loss was measured by using a coil made by winding a φ0.6mm formal-coated conductor wire on the ring sample 40 times on both the primary side and the secondary side.
It was measured at 5060A. The green compact density was calculated from the measured values of the mass and volume of the sample. The hand-folding test is a test in which a sample prepared for measuring the specific resistance is tried to be folded by hand, and a hand-folded sample is judged to be unsuitable for use in dust cores.

Table 1 shows the raw material powder, the blending amount of the insulating treatment liquid, the addition amount, etc. for the examples of the present invention and the comparative examples.

[0054]

[Table 1]

Further, the powder density of the powder magnetic core sample prepared from the iron-based powders exemplified in Table 1, the specific resistance, the AC relative initial permeability (μ _iAC ) at 10 KHz, the iron at 10 kHz and 0.1 T Table 2 shows the results of the loss and the hand folding test. The name of the equipment used for mixing was described in the remarks.

[0056]

[Table 2]

Example 5 shows the results when Sendust was used as the raw material powder. Similar to the other examples, it retains its insulating property even after being strain-removed and purified, and has a high AC relative initial permeability,
It has low iron loss. From this result, it can be seen that the present invention is also effective for the alloy powder. In addition, Example 12
4 shows the results when 0.5 part of a rust preventive material (triazinethiol) was further added to the insulating treatment liquid of Example 6. By adding a rust preventive agent, the specific resistance increases and the iron loss decreases. From this, it can be seen that the addition of the rust preventive agent is effective in the present invention.

Examples 13 and 14 show the results when nitric acid and hydrochloric acid were used as the alumina stabilizer. Compared to Example 6 which was similar except that an organic acid was used,
The specific resistance is low and the iron loss is high. This is because when nitric acid or hydrochloric acid is used, the surface of the raw material powder rusts, causing a slight breakage of the insulating coating, and after annealing,
It is presumed that this is because the minute portion made electrical contact.

Next, Comparative Example 1 is the result when silica was not added to the insulating treatment liquid. Alumina was not uniformly dispersed in the solution, and the specific resistance after annealing of the iron-based powder obtained by adding it to the raw material powder was also low. The iron loss was too large to be measured. Comparative Example 2 is the result when alumina was not added to the insulation treatment liquid. Since silica could not be uniformly dispersed, the surface of the raw material powder could not be uniformly coated, and the specific resistance after annealing was extremely low. Further, the initial permeability of the AC ratio was also greatly reduced, and the iron loss value was significantly increased, making it impossible to measure.

Comparative Example 3 is the result when no epoxy resin was added to the insulating treatment liquid. Alumina and silica were only mixed with the raw material powder, and the surface of the raw material powder could not be completely covered. Therefore, the specific resistance after annealing is extremely low. Moreover, the initial magnetic permeability of the AC ratio was also significantly reduced, and the iron loss was significantly increased, and the measurement was impossible.

As explained above, according to the present invention,
Hysteresis loss can be reduced during annealing, insulation is not destroyed, and iron-based powder for a dust core having a heat-resistant insulating coating can be obtained. Further, since the value of the specific resistance before the strain relief annealing is high, it can be seen that the adhesion of the coating film is good even after the powder compaction.

[0062]

According to the present invention, the coating is not easily peeled off even when the iron-based powder is pressure-molded, and the insulation is not destroyed even during annealing. The present invention makes it possible to realize an iron-based powder having a heat-resistant insulating coating and a dust core using the same.

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Claims (4)

[Claims] 1. An iron-based powder obtained by coating the surface of a powder containing iron as a main component with a film containing an epoxy resin and silica containing alumina. 2. The alumina-containing silica is contained in an amount of 3 to 30 in terms of Al ₂ O ₃ + SiO ₂ with respect to 100 parts by mass of the epoxy resin.
The iron-based powder according to claim 1, wherein the iron-based powder is 0 part by mass. 3. The alumina-containing silica has an alumina content of 0.01 to 500 parts by mass in terms of Al ₂ O ₃ with respect to 100 parts by mass of silica in terms of SiO _2.
Or the iron-based powder according to 2. 4. A dust core obtained by forming the iron-based powder according to claim 1 into a predetermined shape and then annealing it.