JP2007146259A - Magnetite-iron composite powder for powder magnetic core, its production method and powder magnetic core using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a powder magnetic core of high performance combining excellent magnetic properties and high insulation properties using iron based metal powder having high saturated magnetic flux density Bs; to provide magnetite-iron composite powder as metal powder suitable for realizing the same; and to provide its production method. <P>SOLUTION: The magnetite-iron composite powder for a powder magnetic core comprises magnetite and is characterized by having an average primary particle diameter d of 0.7 to 5.0 μm, specific surface areas of 1.3×d<SP>-0.43</SP>to 4.0×d<SP>-0.58</SP>m<SP>2</SP>/g, and the content of chromium of 0.01 to 3.0 mass%. <P>COPYRIGHT: (C)2007,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 composite powder suitable for use as a base material, 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, Ni-based ferrites and dust cores are conventionally used for high-frequency signal magnetic cores. Here, the Ni-based ferrite has a high quality factor Q up to about 100 MHz and exhibits good magnetic characteristics. However, since it is affected by a resonance phenomenon caused by the crystal structure at a high frequency exceeding 100 MHz, the Ni-based ferrite has a stable initial permeability μi. And it is difficult to obtain the quality factor Q. On the other hand, the above-mentioned dust core is susceptible to eddy currents at high frequencies because the metal magnetic powder is a conductor, has a lower quality factor Q than ferrite, and is inferior in frequency characteristics of initial permeability μi. was there.

  Conventionally, MnZn ferrite and NiZn ferrite are used for power magnetic cores such as DCDC converter transformers used at high frequencies. However, since ferrite has a high electric 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 DCDC converter. There was a problem that it could not be used under excitation. For this reason, it is difficult to cope with the increase in 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 and Japanese Patent Application No. 2005-91559, an application was filed 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, since the electric resistance can be increased with an iron-based dust core having a high saturation magnetic flux density, eddy current loss can be suppressed even when used at a high frequency, and high initial permeability μi. In addition, a magnetite-iron composite powder for a dust core that can have both a quality factor Q and a low core loss, and a dust core using the same can be obtained. The electric resistance value of the dust core obtained here was about several kΩ, which was a sufficient level of electric resistance value from the viewpoint of improving the magnetic characteristics such as initial permeability μi, quality factor Q, and core loss.

  On the other hand, recently, in order to further reduce the size of electronic devices, the use of an inductor as a structure in which a coil is embedded in a magnetic powder and integrally formed and the coil and a magnetic core are integrated is increasing. Here, when the coil and the magnetic core are used in an integrated manner, there is a problem that the above-described electrical resistance of about kΩ is insufficient from the viewpoint of ensuring the safety of the parts. From the viewpoint of ensuring the safety of the parts, a high insulation property of 1 MΩ or more is essential as the electric resistance of the dust core.

  In addition, with conventional wire-wound inductor elements, with the recent development of high-density mounting technology, the mounting distance with other components tends to be narrowed, and it is higher from the viewpoint of ensuring safety between components. Insulation is required.

  As described above, in order to realize the safety suitable for the magnetic element for the high frequency signal and the high frequency power with the dust core, it is necessary to increase the electric resistance of the dust core to 1 MΩ or more.

The present invention has been made under such circumstances, and the present invention uses an iron-based metal powder having a high saturation magnetic flux density Bs to provide a high-performance pressure having both excellent magnetic properties and high insulating properties. It is an object of the present invention to provide a powder magnetic core, and to provide a magnetite-iron composite powder which is a metal powder suitable for realizing this and a method for producing the same.

  As means for increasing the electric resistance of the dust core, the present inventors paid attention to the presence or absence of protrusions on the particle surface, and various magnetic properties and mechanical properties including the particle shape of the iron powder and the electric resistance of the dust core. The relationship with characteristics was examined in detail.

  First, a magnetite-iron composite powder is prepared by the method of the above-mentioned Japanese Patent Application No. 2005-19338 or Japanese Patent Application No. 2005-91559 filed earlier by the present inventors, and mechanical treatment or chemical treatment is added thereto to Various magnetic powders having different protrusion states were prepared. The obtained magnetic powder and a binder resin were mixed, compression molded to produce a dust core, and electrical resistance and various magnetic properties were investigated. As a result, it was found that the electrical resistance of the dust core tends to increase when a powder whose surface is smoothed by applying post-treatment such as mechanical treatment or chemical treatment is used.

  Furthermore, as a result of examining the relationship between the powder characteristics of the magnetic powder and the electrical resistance of the powder magnetic core in detail, the electrical resistance of the powder magnetic core increases when the specific surface area of the magnetic powder is within a predetermined range. I understood.

The present invention has been made based on the above findings and has the following characteristics.
[1] Contains magnetite, average primary particle diameter d is 0.7 to 5.0 μm, specific surface area is 1.3 × d ^{−0.43 to} 4.0 × d ^−0.58 m ² / g, chromium Magnetite-iron composite powder for dust core, characterized in that the content is 0.01 to 3.0 mass%.
[2] A method for producing a magnetite-iron composite powder for a dust core according to [1] above,
An iron oxide containing magnetite and containing chromium in an amount such that the chromium content of the magnetite-iron composite powder after production is 0.01 to 3.0 mass% is reduced in a reducing atmosphere, and then further oxidized. Of a magnetite-iron composite powder for a dust core, wherein particles of particles obtained by gradual oxidation treatment in a neutral atmosphere collide with each other to smooth the particle surface of the particles Method.
[3] A dust core obtained by mixing and molding the magnetite-iron composite powder according to [1] above and a resin and / or an inorganic insulating material.

  According to the present invention, an iron-based dust core having a high saturation magnetic flux density, a dust core having both a high electrical resistance of 1 MΩ or more, a high initial permeability μi and a quality factor Q, and a low core loss, and A magnetite-iron 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 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 3.0 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 and the initial permeability μi value of the dust core is lowered. In the range where the average primary particle diameter d exceeds 5.0 μm, the magnetic characteristics in the high frequency range are deteriorated 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.

The magnetite-iron composite powder of the present invention has a specific surface area SSA (Specific Surface Area) of 1.3 × d ^{−0.43 to} 4.0 × d ^−0.58 m ² / g. It is important to be. Here, d is an average primary particle size obtained by analyzing the above SEM photograph.

When the specific surface area SSA is less than 1.3 × d ^−0.43 , the particle shape is nearly spherical, so that the mechanical strength of the dust core becomes weak, which is not practically suitable as a coil integrated inductor. On the other hand, if the specific surface area SSA exceeds 4.0 × d ^−0.58 , the smoothness of the particle surface decreases and the electrical resistance of the dust core decreases to less than 1 MΩ, which is not preferable.

  Methods for controlling the specific surface area of the powder within the above range include (1) a method of selecting a powder with an appropriate specific surface area at the stage of iron oxide as a starting material, and (2) smoothing the particle shape during the reduction process. (3) A method of smoothing the particle surface by causing the powders after reduction to collide with each other at a high speed, (4) ) There is a method of chemically etching the reduced powder with hydrochloric acid or the like. Note that the method (1) is limited in raw materials, and the method (4) is inferior in mass productivity. Therefore, it is preferable to use the method (2) and / or (3).

  Moreover, it is important that the magnetite-iron composite powder of the present invention has a chromium content of 0.01 to 3.0 mass%. Preferably, it is 0.05-0.5 mass%. If the chromium content is less than 0.01 mass%, the particle smoothing effect in the reduction process is small, so that the electrical resistance does not increase, and the effect of improving the initial permeability μi, the quality factor Q, and the core loss is small. Absent. On the other hand, if the chromium content exceeds 3.0 mass%, the electrical resistance is lowered, and the initial permeability μi, the quality factor Q, and the core loss are lowered.

  Here, the magnetite-iron composite powder of the present invention contains magnetite and contains chromium (Cr) in an amount such that the chromium content of the magnetite-iron composite powder after production is 0.01 to 3.0 mass%. Iron oxide to be used as a starting material, this was reduced in a reducing atmosphere such as hydrogen or nitrogen, and the surface was stabilized in a oxidizing atmosphere having an oxygen concentration of 1 to 10 vol. Later, it can be manufactured by taking it out of the furnace.

  The mechanism of improving the electromagnetic properties of the dust core by adding Cr to the raw material iron oxide is not clear, but iron oxide (hematite) is transformed into iron (α-Fe) during the reduction process. When Cr is present, the particle shape tends to be rounded and the particle size distribution tends to be uniform, so that a particle surface smoothing effect can be obtained, and Cr is reduced during the reduction process. The possibility of improving the insulating properties of the particles by forming a surface insulating layer by concentrating on the surface is considered. Therefore, in the present invention, it is important that Cr is present in the raw material during the reduction process of iron oxide, and the method of adding Cr later to the iron powder provides the effect of increasing electrical resistance as obtained in the present invention. It is not possible.

  Furthermore, the magnetite-iron composite powder according to the present invention has a particle surface which is made to collide with each other by swirling particles, which 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 perform the smoothing process. 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 the collision between particles is less than 50 m / sec, the impact force due to the collision is small, the smoothing effect is insufficient, and a sufficient electrical resistance increasing 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 composite powder for dust core of the present invention can be obtained.

  Next, after mixing the magnetite-iron composite powder according to the present invention and a resin and / or an inorganic insulating material, compression molding is performed, and if necessary, the resin is subjected to a thermosetting treatment to have a resistance of 1 MΩ or more. It is possible to obtain a dust core exhibiting high insulating properties and excellent magnetic properties under high frequency excitation.

  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 Chromium oxide (Cr ₂ O ₃ ) is added so that the Cr content in Table 1 is the mass (mass%) shown in Table 1 below, wet-mixed with a ball mill using pure water and steel balls, and then dried and sized. Thus, a Cr-containing iron oxide was produced. This was heat-treated at a temperature of 550 to 800 ° C. in a hydrogen atmosphere to obtain various iron powders having different average primary particle sizes. Then, before opening the furnace, it is held in a 5 vol.% O ₂ —N ₂ atmosphere to generate magnetite on the surface layer of the iron powder, and then taken out of the furnace. Magnetite-iron composite powders with various Cr contents Got. Furthermore, the particles are swirled at a peripheral speed of 100 m / sec using a mechanical surface reformer (hybridizer system manufactured by Nara Machinery Co., Ltd.) with respect to the magnetite-iron composite powder. 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 average primary particle size and specific surface area SSA 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, 5 mass% phenol resin was mixed with the magnetite-iron composite powder and compression molded 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, and 150 ° C. × 30 The phenolic resin was cured by heat treatment for a minute. 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 are 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, and the core loss is measured using an AC BH analyzer. The measurement was performed under the conditions of 85 volumes, N2 = 10 volumes, frequency f = 50 kHz, and magnetic flux density Bm = 100 mT.

  Table 1 also shows the evaluation results of the electrical resistance, initial magnetic permeability μi, quality factor Q, and core loss of the inventive examples and the comparative examples. As shown in Table 1, by using the magnetite-iron composite powder in the range according to the present invention, a high resistance of 1 MΩ or more, a high initial permeability μi, a high quality factor Q, and a low core loss can be satisfied at the same time.

  As shown in the above examples, by using the magnetite-iron composite powder according to the present invention, a metal-based dust core having a high saturation magnetic flux density Bs can be used to simultaneously achieve high insulation and high magnetic properties in a high frequency range. It was possible to obtain the effect of the present invention.

Claims (3)

It contains magnetite, the average primary particle diameter d is 0.7 to 5.0 μm, the specific surface area is 1.3 × d ^{−0.43 to} 4.0 × d ^−0.58 m ² / g, and the chromium content is Magnetite-iron composite powder for dust core, characterized by being 0.01 to 3.0 mass%. A method for producing a magnetite-iron composite powder for a dust core according to claim 1,
An iron oxide containing magnetite and containing chromium in an amount such that the chromium content of the magnetite-iron composite powder after production is 0.01 to 3.0 mass% is reduced in a reducing atmosphere, and then further oxidized. Of a magnetite-iron composite powder for a dust core, wherein particles of particles obtained by gradual oxidation treatment in a neutral atmosphere collide with each other to smooth the particle surface of the particles Method.   A powder magnetic core comprising the magnetite-iron composite powder according to claim 1 mixed with a resin and / or an inorganic insulating material and molded.