JP2007262490A - Magnetite-iron-cobalt composite powder for powder magnetic core, method for manufacturing the same, and powder magnetic core using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a powder magnetic core of high performance having a low core loss, high initial magnetic permeability μi, quality coefficient Q, flat DC superposition characteristics, and excellent insulation characteristics in combination by using iron-based metal powder of a high saturation magnetic flux density Bs and to provide magnetite-iron-cobalt composite powder which is the metal powder appropriate for realization of the same and a method for manufacturing the same. <P>SOLUTION: The magnetite-iron-cobalt composite powder for powder magnetic core contains magnetite, and has the average primary particle diameter of 0.7 to 5.0 μm, a bulk density of 0.6 to 2.5 g/cm<SP>3</SP>, and a cobalt content of 0.01 to 3mass%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-performance metallic composite magnetic material used as a magnetic core material for inductor elements, transformers, etc. used at high frequencies, and in particular, a soft magnetic material for a dust core obtained by molding metallic magnetic powder. The present invention relates to a magnetite-iron-cobalt composite powder suitable for use as a magnetic powder, a production method thereof, and a dust core using the same.

  With the downsizing of electronic equipment and higher driving frequencies, inductance components used as one of the circuit parts of these equipment are miniaturized and highly efficient magnetic elements even when used at high frequencies. Therefore, the use of a high-performance magnetic material capable of realizing the above has been demanded.

  Under such circumstances, MnZn ferrite and NiZn ferrite have been conventionally used for power magnetic cores such as transformers for DC-DC converters used at high frequencies. However, since ferrite has a high electrical resistance, eddy current loss is small even in a high frequency range, and low core loss is shown in the region of 100 to 3 MHz, which is the driving frequency of the DC-DC converter, but because the saturation magnetic flux density Bs is small. There was a problem that it could not be used under large current excitation. For this reason, it is difficult to cope with the increase in the current of the DC-DC converter for CPU drive accompanying the recent drop in CPU drive voltage, and the demand for a dust core having a high saturation magnetic flux density Bs is increasing. However, the powder magnetic core has a large eddy current loss because the metal magnetic powder is a conductor, and also has the disadvantage that the distortion during molding remains and the hysteresis loss is large, so it is used as a transformer or choke coil. It was necessary to reduce the core loss.

  In order to deal with the above problems, the present inventors have considered that the introduction of finer metal powder is advantageous in order to cope with the recent increase in driving frequency in electronic devices. In Japanese Patent Application No. 2005-19338, Japanese Patent Application No. 2005-91559, and Japanese Patent Application No. 2005-345370, an application was made for a technique of using a magnetite-iron composite powder having an average primary particle size of 0.7 to 3 μm for a dust core.

  According to the above invention, the iron oxide containing a predetermined trace component is reduced in a reducing atmosphere and then subjected to a gradual oxidation treatment in an oxidizing atmosphere, and then compacted using the fine magnetite-iron composite powder. By producing the magnetic core, it is possible to obtain a dust core having both a high initial permeability μi and a quality factor Q, a low core loss, a flat DC superposition characteristic, and an excellent insulating property.

  On the other hand, in order to meet the recent demands for smaller and lighter portable electronic devices such as notebook personal computers, further downsizing and higher performance of magnetic cores are indispensable. Magnetic cores are required. Moreover, it is desired to further reduce the core loss of the magnetic core from the viewpoint of extending the usage time of the battery of the portable electronic device.

  As described above, in the dust core used in portable electronic devices, in addition to high initial permeability μi and quality factor Q, flat DC superposition characteristics, excellent insulation, core loss should be kept as low as possible. Is required.

  The present invention has been made under such circumstances. The present invention uses an iron-based metal powder having a high saturation magnetic flux density Bs, and has a low core loss, a high initial permeability μi and a quality factor Q, Providing a high-performance dust core having flat direct current superimposition characteristics and excellent insulating properties, and a magnetite-iron-cobalt composite powder, which is a metal powder suitable for realizing this, and a method for producing the same The purpose is to provide.

  The present inventors analyzed in detail the frequency dependence of the core loss of each dust core obtained in the inventions of the Japanese Patent Application Nos. 2005-19338, 2005-91559 and 2005-345370 filed earlier. . As a result, it was found that 90% or more of the core loss is hysteresis loss, and the contribution of eddy current loss is small.

Therefore, as a result of intensive studies on means for reducing the hysteresis loss of the dust core, it was found that the core loss of the dust core can be greatly reduced by introducing the following four methods.
(1) Substitute part of iron, which is the basic composition of the dust core, with another element.
(2) A reduction treatment is performed after adding to the iron oxide a component that controls the reactivity and grain growth behavior during the reduction.
(3) The surface of the particles is smoothed by hitting the powder after reduction at high speed.
(4) The bulk density of the powder is adjusted to a predetermined range.

  Regarding the above (1), various components that can control physical properties such as magnetocrystalline anisotropy, magnetostriction constant, and saturation magnetic flux density of the material by substituting a part of iron were studied. As a result, it was found that the core loss can be reduced by substitution with a small amount of cobalt.

  In addition, regarding the viewpoint of (2) above, when cobalt oxide is added to iron oxide before reduction, neck growth between particles during reduction can be suppressed, and particles having excellent dispersibility can be obtained. I understood. When a dust core is produced using particles with good dispersibility, hysteresis loss associated with molding distortion can be suppressed, and thus core loss can be reduced. Moreover, since the contact frequency between particles decreases, the electrical resistance of the dust core can be increased.

  By performing the treatment (3), it becomes possible to further improve the dispersibility of the particles, and to obtain a lower core loss and a higher electric resistance.

  Further, the bulk density of (4) affects the uniformity of mixing in the process of mixing the magnetic powder, the rust inhibitor and the resin, and the residual degree of molding strain. It is desirable to adjust to the range.

The present invention has been made based on the above findings and has the following characteristics.
[1] It contains magnetite, has an average primary particle size of 0.7 to 5.0 μm, a bulk density of 0.6 to 2.5 g / cm ³ , and a cobalt content of 0.01 to ³ mass%. A magnetite-iron-cobalt composite powder for a dust core.
[2] A method for producing a magnetite-iron-cobalt composite powder for a dust core according to [1] above,
Iron oxide containing cobalt or a cobalt compound containing magnetite in an amount such that the cobalt content of the magnetite-iron-cobalt composite powder after production is 0.01 to 3 mass% is reduced to 450 to 900 ° C. in a reducing atmosphere. A method for producing a magnetite-iron-cobalt composite powder for a dust core, which is obtained by performing a slow oxidation treatment in an oxidizing atmosphere after reduction at a temperature of 1.
[3] A method for producing a magnetite-iron-cobalt composite powder for a dust core according to [1] above,
Iron oxide containing cobalt or a cobalt compound containing magnetite in an amount such that the cobalt content of the magnetite-iron-cobalt composite powder after production is 0.01 to 3 mass% is reduced to 450 to 900 ° C. in a reducing atmosphere. A magnetite for a dust core, which is obtained by colliding particles of a granular material obtained by slow oxidation treatment in an oxidizing atmosphere after reduction at a temperature of 50 m / sec or more. -Manufacturing method of iron-cobalt composite powder.
[4] A dust magnet obtained by mixing and molding the magnetite-iron-cobalt composite powder according to any one of [1] to [3] above and a resin and / or an inorganic insulating material. core.

  According to the present invention, an iron-based dust core having a high saturation magnetic flux density has a high electric resistance of 1 MΩ or more, a high initial permeability μi and a quality factor Q, a flat DC superposition characteristic, and a low core loss. A dust core, a magnetite-iron-cobalt composite powder for a dust core suitable for obtaining such a dust core, and a method for producing the same are provided.

  Hereinafter, an example of the best mode for carrying out the present invention will be described.

  First, the magnetite-iron-cobalt composite powder of the present invention exhibits good high-frequency magnetic properties when the average primary particle size d is in the range of 0.7 to 5.0 μm, more preferably 0.8 to 4.0 μm. When the average primary particle size d is less than 0.7 μm, the frequency of particles having a single domain structure increases, so that the retention force of the particles is remarkably increased, the hysteresis loss of the dust core is increased, and the initial permeability μi is increased. The value drops. In the range where the average primary particle size d exceeds 5.0 μm, the core loss and the quality factor Q in the high frequency range are lowered due to the influence of eddy current and domain wall resonance. The average primary particle size d is a value obtained by analyzing an SEM (scanning electron microscope) photograph. SEM photographs were taken at a magnification such that about 10 to 20 particles were placed on the diagonal of the visual field, and the average primary particle size d was calculated from the number and magnification of the particles on the diagonal.

In addition, the magnetite-iron-cobalt composite powder of the present invention has a powder bulk density of 0 in order to uniformly mix the magnetic powder with the rust inhibitor and resin during the rust prevention treatment and granulation process of the magnetic powder. It is important to adjust to the range of 6 to 2.5 g / cm ³ . When the bulk density exceeds 2.5 g / cm ³ , the magnetic powder easily falls and settles in the process of mixing the magnetic powder with the rust inhibitor and the resin, and uniform mixing becomes difficult. Therefore, as a result, the electrical resistance of the dust core decreases and eddy current loss increases. On the other hand, if the bulk density is less than 0.6 g / cm ³ , the compression ratio before and after molding increases, so that the residual strain after molding increases and the hysteresis loss increases. Further, since the volume of the powder becomes large, it is not preferable from the viewpoint of reducing the efficiency of powder transportation. The bulk density is measured according to JIS Z 2504.

  Moreover, it is important that the magnetite-iron-cobalt composite powder of the present invention has a cobalt content of 0.01 to 3 mass%. Preferably, it is 0.1-2 mass%. If the cobalt content is less than 0.01 mass%, the effect of reducing hysteresis loss is small, such being undesirable. Moreover, when cobalt content exceeds 3 mass%, since a hysteresis loss increases on the contrary and a core loss increases, it is unpreferable.

  Here, the magnetite-iron-cobalt composite powder of the present invention contains magnetite, and the cobalt content of the magnetite-iron-cobalt composite powder after production is 0.01 to 3 mass% of cobalt (Co). As a starting material, this is reduced at a temperature of 450 to 900 ° C. in a reducing atmosphere such as hydrogen or nitrogen, and further the surface is exposed in an oxidizing atmosphere having an oxygen concentration of 1 to 10 vol. It can manufacture by taking out from a furnace, after stabilizing by a slow oxidation process.

  Although the mechanism by which the core loss of the dust core is reduced by including Co in the iron oxide as the raw material is not yet clear, the crystal magnetic anisotropy and magnetostriction constant of the material are formed by iron and Co alloying. It is presumed that hysteresis loss is reduced by approaching zero. In addition, when SEM photographs of the powder before and after the reduction were compared, reducing the iron oxide containing Co tends to suppress neck growth between the particles during the reduction, thereby improving the dispersibility of the particles. Thus, it is conceivable that the influence of molding distortion is reduced and hysteresis loss is reduced.

  Moreover, the direct current superimposition characteristic of a powder magnetic core is improved by containing Co in the iron oxide which is a raw material. When it is necessary to make the DC superimposition characteristic compatible with a high DC current, a method of providing a gap in a part of the magnetic path is generally used. As described above, reducing the iron oxide containing Co tends to improve the dispersibility of the particles, so that the particles forming the green compact after molding are uniformly separated by the resin, effectively It is presumed that the formation of the gap makes it possible to cope with higher DC currents.

  Furthermore, the magnetite-iron-cobalt composite powder of the present invention causes particles to collide with each other by swirling particles that are powder particles at a high speed of a peripheral speed of 50 m / sec or more after the above reduction treatment step. It is preferable to smooth the particle surface. This is because the electrical resistance of the dust core is further increased and the core loss is further reduced. The reason why the electrical resistance of the dust core is increased by this smoothing treatment is that the protrusions on the particle surface disappear due to the collision, so that the contact frequency between the particles in the compact is reduced and the insulation between the particles is improved. It is thought to do. In addition, the reason why the core loss is reduced is thought to be that the projection on the particle surface disappears due to the collision, the molding distortion is reduced, and the hysteresis loss is reduced.

  If the peripheral speed of collision between particles is less than 50 m / sec, the impact force due to the collision is small, so that the smoothing effect is insufficient, and a sufficient electrical resistance increasing effect and a core loss reducing effect cannot be obtained. Here, as means for causing particles to collide at a high speed of 50 m / sec or more, for example, a mechanical surface reformer such as a hybridizer system manufactured by Nara Machinery Co., Ltd. can be used. However, it is not limited to this as long as the same effect can be obtained.

  By the method as described above, the magnetite-iron-cobalt composite powder for dust core of the present invention can be obtained.

Next, after mixing the magnetite-iron-cobalt composite powder according to the present invention and a resin and / or an inorganic insulating material, compression molding is performed, and if necessary, a thermosetting treatment of the resin is performed. It is possible to obtain a dust core having both initial permeability μi and quality factor Q, excellent DC superposition characteristics, high insulation property of 1 MΩ or more, and low core loss of less than 1600 kW / m ³ at 50 kHz and 100 mT.

  Here, although the resin is used for bonding, for example, a phenol resin, an acrylic resin, a silicone resin, an epoxy resin, or the like can be used.

Further, as the inorganic insulating material, insulating powder, for example, fine powder such as SiO ₂ and Al ₂ O ₃ can be used.

  In addition, the compression molding method is not particularly limited, and compression molding methods such as warm compression molding and injection molding can be used in addition to compression molding that is usually used.

Specific examples of the present invention will be described below.

[Example 1]
After iron oxide for ferrite (JC-DC made by JFE Chemical Co., average particle size 0.8μm measured by air permeation method) is reduced and further stabilized by gradual oxidation treatment of the surface in an oxidizing atmosphere Cobalt oxide (CoO) was added so that the Co content in the following Table 1 (mass%) was added, and wet-mixed with a ball mill using pure water and steel balls, and then dried and sized to obtain Co. Containing iron oxide was prepared. This was heat-treated at a temperature of 600 ° C. in a hydrogen atmosphere to obtain various Co-containing iron powders having different average primary particle sizes. Then, by holding in a 5 vol.% O ₂ —N ₂ atmosphere before opening the furnace, magnetite is generated on the surface of the iron powder and then taken out of the furnace, and magnetite-iron-cobalt with various Co contents. A composite powder was obtained. Further, this magnetite-iron-cobalt composite powder is swung between particles at a peripheral speed of 100 m / sec using a mechanical surface reformer (hybridizer system manufactured by Nara Machinery Co., Ltd.). The particle surface was smoothed.

  As a result of examining the constituent phases of the obtained powder by X-ray diffraction, the α-Fe phase was 99.7 to 100 mass% and the remaining 0 to 0.3 mass% was the magnetite phase in all samples. The results of measuring the average primary particle size and bulk density calculated from the SEM photograph are shown in Table 1 below.

  Here, when the average primary particle size is calculated from the SEM photograph, the SEM photograph is taken at a magnification such that about 10 to 20 particles enter the diagonal line of the field of view, and the number and magnification of the particles on the diagonal line are taken. From this, the average primary particle size was calculated.

Subsequently, 3 mass% phenol resin was mixed with the magnetite-iron-cobalt composite powder, and compression molding was performed at a molding pressure of 7 t / cm ² (about 700 MPa) to produce a ring-type sample having an outer diameter of 12 mmφ. A phenol resin was cured by heat treatment for × 30 minutes. Both ends of the obtained ring-shaped sample were sandwiched between alligator clips, and the electrical resistance was measured at an applied voltage of 10V. The frequency characteristics of the initial permeability μi and the quality factor Q were measured using an LCR meter under the conditions of N = 10 windings, an applied current of 0.2 mA, and a frequency of 100 k to 30 MHz. The core loss was measured using an AC BH analyzer under the conditions of N1 = 85, N2 = 10, frequency f = 50 kHz, and magnetic flux density Bm = 100 mT. The direct current superimposition characteristics were measured by measuring permeability μ under the conditions of N = 85, DC applied magnetic field Hdc = 0 to 110 (Oe), AC current Iac = 0.2 mA (100 kHz), and Δμ = μ (100 Oe) / It calculated from μ (0Oe) × 100 (%).

Table 1 also shows the evaluation results of the electrical resistance, core loss, direct current superimposition characteristics, initial permeability μi, and quality factor Q of the inventive examples and comparative examples. As shown in Table 1, by using a magnetite-iron-cobalt composite powder containing Co in the range according to the present invention, a high core resistance of 1 MΩ or higher, a low core loss of less than 1600 kW / m ³ at 50 kHz and 100 mT is simultaneously satisfied. can do.

[Example 2]
After iron oxide for ferrite (JC-DC made by JFE Chemical Co., average particle size 0.8μm measured by air permeation method) is reduced and further stabilized by gradual oxidation treatment of the surface in an oxidizing atmosphere Cobalt oxide (CoO) was added so that the Co content of 0.5 mass% was obtained, and after wet-mixing with a ball mill using pure water and steel balls, drying and sizing were performed to produce Co-containing iron oxide. . This was heat-treated at a temperature of 520 to 800 ° C. in a hydrogen atmosphere to prepare various Co-containing iron powders having different average primary particle sizes. Thereafter, by holding in a 5 vol.% O ₂ —N ₂ atmosphere before opening the furnace, magnetite is generated on the surface layer of the iron powder and then taken out of the furnace, and various magnetite-irons having different average primary particle sizes are obtained. -Cobalt composite powder was obtained. Further, this magnetite-iron-cobalt composite powder is swung between particles at a peripheral speed of 100 m / sec using a mechanical surface reformer (hybridizer system manufactured by Nara Machinery Co., Ltd.). The particle surface was smoothed.

  As a result of examining the constituent phases of the obtained powder by X-ray diffraction, the α-Fe phase was 99.7 to 100 mass% and the remaining 0 to 0.3 mass% was the magnetite phase in all samples. The results of measuring the average primary particle size and bulk density calculated from the SEM photograph are shown in Table 2 below.

  Here, when the average primary particle size is calculated from the SEM photograph, the SEM photograph is taken at a magnification such that about 10 to 20 particles enter the diagonal line of the field of view, and the number and magnification of the particles on the diagonal line are taken. From this, the average primary particle size was calculated.

Subsequently, 2.5 mass% phenol resin was mixed with the magnetite-iron-cobalt composite powder, and compression molding was performed at a molding pressure of 7 t / cm ² (about 700 MPa) to produce a ring-type sample having an outer diameter of 12 mmφ. A phenol resin was cured by heat treatment at 150 ° C. for 30 minutes. The electrical resistance, initial permeability μi, quality factor Q, core loss, and direct current superposition characteristics of the obtained dust core were measured in the same manner as in Example 1.
Table 2 also shows the evaluation results of the electrical resistance, core loss, direct current superimposition characteristics, initial permeability μi, and quality factor Q of the inventive example and the comparative example. As shown in Table 2, by using the magnetite-iron-cobalt composite powder produced under the conditions of the present invention, a high core resistance of 1 MΩ or higher, a low core loss of less than 1600 kW / m ³ at 50 kHz and 100 mT can be satisfied at the same time. it can.

  As shown in Examples 1 and 2 above, by using the magnetite-iron-cobalt composite powder according to the present invention, a metal-based dust core with a high saturation magnetic flux density Bs, high insulation and low core loss, Excellent direct current superimposition characteristics, high initial permeability μi and quality factor Q up to a high frequency range can be obtained simultaneously, and the effects of the present invention were confirmed.

Claims (4)

A pressure containing magnetite, having an average primary particle size of 0.7 to 5.0 μm, a bulk density of 0.6 to 2.5 g / cm ³ , and a cobalt content of 0.01 to ³ mass%. Magnetite-iron-cobalt composite powder for powder magnetic core. A method for producing a magnetite-iron-cobalt composite powder for a dust core according to claim 1,
Iron oxide containing cobalt or a cobalt compound containing magnetite in an amount such that the cobalt content of the magnetite-iron-cobalt composite powder after production is 0.01 to 3 mass% is reduced to 450 to 900 ° C. in a reducing atmosphere. A method for producing a magnetite-iron-cobalt composite powder for a dust core, which is obtained by performing a slow oxidation treatment in an oxidizing atmosphere after reduction at a temperature of 1. A method for producing a magnetite-iron-cobalt composite powder for a dust core according to claim 1,
Iron oxide containing cobalt or a cobalt compound containing magnetite in an amount such that the cobalt content of the magnetite-iron-cobalt composite powder after production is 0.01 to 3 mass% is reduced to 450 to 900 ° C. in a reducing atmosphere. A magnetite for a dust core, which is obtained by colliding particles of a granular material obtained by slow oxidation treatment in an oxidizing atmosphere after reduction at a temperature of 50 m / sec or more. -Manufacturing method of iron-cobalt composite powder.   A powder magnetic core comprising the magnetite-iron-cobalt composite powder according to any one of claims 1 to 3 mixed with a resin and / or an inorganic insulating material and molded.