JP2009272615A - Dust core, and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dust core having a constitution suitable for both high frequency characteristics and mass-productivity, and to provide a manufacturing method thereof. <P>SOLUTION: The manufacturing method of the dust core includes a process of obtaining a compact by pressure forming of a mixture of magnetic powder and a binder. The magnetic powder: is obtained by subjecting mixture powder of iron oxide powder and powder containing carbon to heat treatment in a non-oxidizing atmosphere; includes metal particulates having Fe as a main component and graphite coating the metal particulates; has an average particle size of 2.0 to 15.0 μm; and has a space factor of the magnetic powder in the dust core after pressure formation within a range of 70 to 98 vol.%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a dust core used for a noise removing element, an inductor, and other various inductance elements.

  As various electronic devices become more functional and multifunctional, power supplies that support large currents are required. A soft magnetic material used for a magnetic core of a winding component such as a choke coil used for it is also required to have a small characteristic change even at a large current, that is, excellent DC superposition characteristics and low loss. In response to these demands, conventionally, composite particles in which the surface of soft magnetic particles has been subjected to an insulating coating treatment, or composite particles in which insulator particles are dispersed on the surface of soft magnetic particles have been mixed with a binder to be compacted. Density bulk shapes and the like have been provided. For example, a dust core using magnetic metal particles having high saturation magnetization as soft magnetic particles does not saturate to a high magnetic flux density, and thus exhibits excellent DC superposition characteristics. In general, in the production of a powder magnetic core using metal particles, the magnetic metal particles to be used are difficult to be plastically deformed, so that a high pressure is required for consolidation. Therefore, a large pressure molding apparatus is required and the molding efficiency is low. Moreover, since the smaller the magnetic metal particles, the higher the pressure required for consolidation, magnetic metal particles having a large particle size of about 20 to 200 μm are generally used (for example, Patent Document 1). .

JP 2001-135514 A

  However, in applications that use high-frequency bands as the speed of electronic equipment increases, it is necessary for the magnetic permeability to be maintained up to high frequencies, that is, excellent high-frequency characteristics. A powder magnetic core is disadvantageous because it has poor frequency characteristics. Therefore, for example, it has been difficult to realize a dust core that is compatible with high-frequency applications of 1 MHz or more and has both high-frequency characteristics and mass productivity. Then, an object of this invention is to provide the powder magnetic core which has a suitable structure when aiming at coexistence of a high frequency characteristic and mass productivity, and its manufacturing method.

  The method for producing a powder magnetic core of the present invention is a method for producing a powder magnetic core comprising a step of pressing a mixture of magnetic powder and a binder to obtain a molded body, wherein the magnetic powder comprises iron oxide A powder obtained by heat-treating a mixed powder of powder and carbon-containing powder in a non-oxidizing atmosphere, comprising: metal fine particles mainly composed of Fe; and graphite covering the metal fine particles; The average particle diameter of the powder is 2.0 to 15.0 μm, and the space factor of the magnetic powder in the powder magnetic core after the pressure molding is in the range of 70 to 98 vol%. The magnetic powder obtained by such heat treatment can be easily consolidated even if it is a fine particle. According to the above configuration using such magnetic powder, a dust core excellent in high frequency characteristics can be manufactured by a simple method. In addition, that Fe is the main component means that the content of Fe is the largest in terms of weight among the constituent elements. The average particle diameter is d50 measured by a wet laser diffraction method. The space factor indicates the volume ratio of the magnetic powder in the dust core, and is calculated as follows. The saturation magnetization of the magnetic powder is measured, and the volume ratio Rp of the metal fine particles mainly composed of Fe in the magnetic powder is calculated. On the other hand, the saturation magnetization of the dust core is measured, and the volume ratio Rc of the metal fine particles mainly composed of Fe in the dust core is calculated. The space factor Ro is calculated from Ro = 100 × Rc / Rp.

  Further, in the method of manufacturing a powder magnetic core, the mixed powder is further mixed with a powder containing Cr, and a ratio of Cr to Fe and Cr as a whole is 7.5% by mass or less (excluding 0). It is preferable that a part of Fe is substituted with Cr so that By adding such Cr, the frequency dependence of the magnetic permeability of the dust core can be improved.

  Furthermore, in the manufacturing method of the said powder magnetic core, it is preferable that the molding pressure of the said pressure molding is 600 Mpa or less. Decreasing the molding pressure can reduce the attenuation rate and contribute to simplification of the molding apparatus.

  Moreover, in the manufacturing method of the powder magnetic core of this invention, it has a process which heat-processes the said molded object, It is preferable that the temperature which heat-processes the said molded object is 250 degrees C or less. By setting the heat treatment temperature after forming to 250 ° C. or less, oxidation of metal fine particles containing Fe as a main component can be suppressed. In addition, since the heat treatment can be performed at a low temperature using a simple heat treatment apparatus, the manufacturing cost can be reduced.

The dust core of the present invention is a dust core formed by consolidating magnetic powder and a binder, and the magnetic powder comprises metal fine particles mainly composed of Fe and graphite covering the metal fine particles. The powder magnetic core has a composite structure composed of particles having a constriction and particles having no constriction, and a saturation magnetic flux density measured from an applied magnetic field of 1.36 MA / m at 25 ° C. is 1.40 to with a 1.70T, when the initial permeability at 1MHz and initial permeability _{mu 10} in _{μ 1, 10MHz, 100 × (} μ 1 -μ 10) / μ decay rate expressed by ₁ alpha is 20 % Or less. According to such a configuration, a dust core excellent in mass productivity and magnetic characteristics can be provided.

  ADVANTAGE OF THE INVENTION According to this invention, when aiming at coexistence with a magnetic characteristic and cost, the dust core which has a suitable structure, and its manufacturing method can be provided.

It is a figure which shows the structure | tissue of the powder magnetic core of one Embodiment which concerns on this invention. It is a figure which shows the structure | tissue of the powder magnetic core of other embodiment which concerns on this invention. It is a figure which shows the structure | tissue of the powder magnetic core of other embodiment which concerns on this invention. It is a figure which shows the structure | tissue of the powder magnetic core of other embodiment which concerns on this invention. It is a figure which shows the structure | tissue of the powder magnetic core of a comparative example. It is a figure which shows the structure | tissue of the powder magnetic core of a comparative example. It is a figure which shows the structure | tissue of the powder magnetic core of a comparative example. It is a figure which shows the structure | tissue of the powder magnetic core of a comparative example. It is a scanning electron micrograph of magnetic powder.

  Hereinafter, the present invention will be specifically described. The method for producing a dust core according to the present invention relates to a method for producing a dust core having a step of pressing a mixture of magnetic powder and a binder to obtain a compact. The magnetic powder is obtained by heat-treating a mixed powder of iron oxide powder and carbon-containing powder in a non-oxidizing atmosphere, and coats the metal fine particles mainly composed of Fe and the metal fine particles. The magnetic powder has an average particle diameter of 2.0 to 15.0 μm. Furthermore, the space factor Ro of the magnetic powder in the powder magnetic core after pressure molding is set to a range of 70 to 98 vol%. Each configuration will be described below.

  The dust core according to the present invention is obtained by compacting a magnetic powder comprising a metal fine particle mainly composed of Fe and graphite covering the metal fine particle and a binder. Here, “having Fe as a main component” means that the content of Fe among constituent elements is the largest in terms of weight. What is necessary is just to confirm content of this Fe by EDX. The metal fine particles according to the present invention may be those containing iron as a main component, and in addition to Fe alone, magnetic metal elements such as Co and Ni, Al capable of suppressing γ phase formation in an alloy structure with Fe, Elements containing elements such as Be, Cr, Ga, Mo, P, Sb, Si, Sn, Ti, V, W, and Zn, and other inevitable impurities may be used. The metal fine particles containing Fe as a main component need only be ferromagnetic, but Fe alone is more preferable from the viewpoint of obtaining a high saturation magnetic flux density.

For example, the high frequency characteristics of the powder magnetic core can be improved by adding Cr to the metal fine particles. Specifically, a configuration in which a part of Fe of the metal fine particle is replaced with Cr and the ratio of Cr in Fe and Cr as a whole is 7.5% by mass or less (however, not including 0) is preferable. By containing such Cr, the magnetic permeability is maintained up to a higher frequency, and the frequency dependence of the magnetic permeability of the dust core can be improved. Furthermore, it is more preferable to make the ratio of Cr into the range of 1.5-7.5 mass%. When the frequency at which the permeability .mu.i decreases by 10% against the values evaluated at 10MHz conditions as f _(μi-10%), if the range according to the ratio of Cr, the f _(μi-10%) It is also possible to increase it to 500 MHz or more and increase it by 1.7 times or more when Cr is not contained. Further, f _(μi−10%) of the dust core can be improved by replacing part of Fe of the metal fine particles with Si and adding Si to the metal fine particles. Furthermore, it is more preferable that the ratio of Si in the entire Fe and Si is in the range of 2.0 to 10.0 mass%. When the DC superposition characteristic is expressed by the magnitude H _(μ-1%) of the DC applied magnetic field when the permeability is decreased by 1% due to the increase of the DC applied magnetic field, the H _{(μ -1%)} is 2000 A / m or more, and it is also possible to increase it by 1.1 times or more when Si is not contained. More preferably, the ratio of Si is in the range of 6.0 to 10.0% by mass.

  Further, “coating the magnetic metal fine particles” means that the entire particle surface or a part thereof is coated. From the viewpoint of oxidation resistance, it is more preferable that the entire particle surface is coated. From the viewpoint of oxidation resistance, it is possible to improve oxidation resistance by coating a part, and excellent oxidation resistance can be expressed by coating the entire particle surface. Moreover, the space ratio of the magnetic powder in the powder magnetic core after the pressure molding can be controlled by the partially coated particles covering a part of the particle surface. Further, from the viewpoint of producing a green compact, if a portion is coated with graphite, the friction of particles can be reduced, and a high-density compact can be produced at a low pressure.

  By coating the core metal fine particles with graphite, consolidation becomes much easier. That is, it is possible to perform compaction at a low pressure while using fine magnetic powder. Therefore, stress due to pressurization can be suppressed and heat treatment at a high temperature can be omitted.

  In addition to the above effects, the graphite coating the metal fine particles also has a function of weakening the magnetic coupling between the magnetic metal fine particles. The thickness of the graphite coating is not particularly limited, but the average thickness of the graphite is preferably 5 nm or more in order to sufficiently weaken the magnetic coupling. On the other hand, if the graphite becomes too thick, the saturation magnetic flux density decreases, so the average thickness of graphite is preferably 300 nm or less. The average thickness of graphite is calculated from a transmission electron microscope (TEM) photograph or a TEM photograph of particles processed so that a cross section can be observed. If the thickness is not uniform, the average of the maximum thickness and the minimum thickness is calculated. May be the thickness of the coating. This is measured with five particles, and the average is taken to calculate the average thickness. The graphite coating the metal fine particles also contributes to the suppression of oxidation of the metal fine particles as nuclei during and after the production process of the dust core.

  A magnetic powder comprising metal fine particles mainly composed of Fe and graphite covering the metal fine particles was obtained by heat-treating a mixed powder of iron oxide powder and carbon-containing powder in a non-oxidizing atmosphere. Use magnetic powder. Such magnetic powder will be described in further detail below. When iron oxide powder and carbon-containing powder are mixed and the mixed powder is heat-treated in a non-oxidizing atmosphere, a powder in which metal fine particles mainly composed of Fe are coated with graphite is obtained. Since iron oxide is reduced to Fe and a graphite coating is formed on the surface thereof, it is possible to prevent oxidation of the Fe surface (the interface between Fe and graphite), which is difficult to avoid by other manufacturing methods. . In this case, the graphite coating has a form in which the hexagonal c-plane ((002) plane) is laminated so as to be parallel to the surface of the nucleus (magnetic metal fine particles) whose main component is Fe. It is considered that Fe as a constituent element of the starting material plays a role of a catalyst for forming a graphite layer. In addition, since the magnetic powder is obtained by gradual cooling after the heat treatment, it is possible to suppress the introduction of thermal stress and defects during powder production as in the case of atomized powder. As described above, the magnetic powder obtained by heat-treating a mixed powder of iron oxide powder and carbon-containing powder in a non-oxidizing atmosphere is particularly suitable for the method of manufacturing a dust core of the present invention. is there.

The powder containing carbon is suitably carbon powder such as graphite, carbon black, or natural graphite, but may be a compound containing carbon. That is, it may be a carbon containing carbon, activated carbon, coke, fatty acid, polymer such as polyvinyl alcohol, BC compound, or metal. However, carbon powder may be used to increase the carbon purity of the coating. As the iron oxide powder, Fe ₂ O ₃ , Fe ₃ O _4, or the like can be used. By changing the average particle size of the iron oxide powder, the average particle size of the obtained magnetic powder can be controlled. When the average particle size of the iron oxide powder is increased, the average particle size of the obtained magnetic powder is also increased. The average particle size of the iron oxide powder is preferably 0.01 to 3 μm. A powder having an average particle size of less than 0.01 μm is difficult to produce and is not practical. Production of iron oxide having an average particle size exceeding 3 μm is difficult and impractical. Moreover, it becomes difficult to reduce enough to the center part of a grain.

  In addition, when elements other than Fe are contained, these elements may be mixed with iron oxide powder or the like in the form of a simple substance, oxide powder or other compound powder, and then heat-treated. For example, when Cr is contained in the metal fine particles as described above, the mixed powder is further mixed with a powder containing Cr in addition to the iron oxide powder and the powder containing carbon. It is only necessary that a part of Fe is replaced with Cr so that the ratio of is 7.5 mass% or less (excluding 0). As the powder containing Cr, powder of Cr alone, CrN, or CrC may be used. Further, for example, when Si is contained in the metal fine particles as described above, the mixed powder is further mixed with the powder containing Si in addition to the iron oxide powder and the powder containing carbon, and the Fe and Si as a whole are mixed. Of these, a part of Fe may be substituted with Si so that the ratio of Si becomes a predetermined ratio. As the powder containing Si, powder of Si alone, SiC, SiN, or the like may be used.

  Moreover, 0.01-100 micrometers is preferable and, as for the average particle diameter of the powder containing carbon, 0.1-50 micrometers is more preferable. Carbon powder having an average particle size of less than 0.1 μm is expensive and impractical. On the other hand, when the average particle size exceeds 100 μm, the dispersion in the mixed powder is biased, and finally it becomes difficult to uniformly coat the metal fine particles. The mixing ratio of the iron oxide powder and the carbon-containing powder is preferably in the range of 20 to 95% of the total weight of the carbon-containing powder. If the blending ratio of the powder containing carbon is less than 20%, the reduction reaction tends to be insufficient due to the lack of carbon. On the other hand, if the blending ratio of the powder containing carbon exceeds 95%, the volume ratio of the metal to be reduced becomes extremely small, which is not practical. In addition, the average particle size of the magnetic powder having a graphite coating decreases as the amount of carbon increases.

  For mixing the iron oxide powder and the carbon-containing powder, a wet or dry ball mill or bead mill, a V-type mixer, a pulverizer (for example, a device that combines pulverization and mixing like a raikai machine), a mortar, or the like is used. . In order to realize a uniform mixed state, it is preferable to employ a wet mixing method using water or an organic solvent. The mixed powder is filled with a heat-resistant crucible such as alumina, boron nitride, graphite or the like in a predetermined amount and heat-treated under a predetermined condition to obtain a magnetic powder in which fine metal particles mainly composed of Fe are coated with graphite. can get. The atmosphere during the heat treatment is a non-oxidizing atmosphere. As the non-oxidizing atmosphere, for example, an inert gas such as an argon gas, a nitrogen gas, a mixed gas of an inert gas mainly containing nitrogen and nitrogen, or the like can be used. The heat treatment temperature is preferably 600 ° C to 1600 ° C, more preferably 900 ° C to 1400 ° C. Especially preferably, it is the range of 900 to 1150 degreeC. When the heat treatment temperature is lowered, the particle size of the magnetic powder obtained is increased. Below 600 ° C, the reaction itself does not proceed. If it is less than 900 degreeC, the time required until reaction is completed will become long. In addition, when the temperature exceeds 1400 ° C. in a non-oxidizing atmosphere, there is a concern that oxygen may be released due to decomposition of oxide ceramics used as a furnace member. At the same time, for example, when an alumina crucible breaks in a short period There is. If the temperature exceeds 1600 ° C., it is indispensable to use a heat-resistant member not only for the crucible but also for the equipment itself, resulting in high cost and not suitable for industrialization. For heat treatment, heat is applied in a state where the powder itself is scattered, such as a stationary static electric furnace with a tubular core, an electric furnace in which the furnace core tube moves dynamically during heat treatment, such as a rotary kiln, and a fluidized bed. A device having a mechanism to be used can be used.

A magnetic powder having a form in which metal fine particles are coated with graphite is easy to be compacted, so that a powder magnetic core can be formed using a fine magnetic powder compared to a conventional powder magnetic core. If the magnetic powder is too large, the attenuation factor and loss of the magnetic permeability with respect to the frequency will be large, and if it is too small, handling will be difficult, and consolidation will be difficult and the magnetic permeability will be reduced. In the present application, as the attenuation factor, when the initial permeability at 1MHz was initial permeability _{mu 10} in _mu 1, 10 MHz, is expressed by _{100 × (μ 1 -μ 10)} / μ 1 attenuation The rate α is used. In order to obtain an attenuation factor α of 20% or less, the average particle size of the magnetic powder is preferably 2.0 μm to 15.0 μm in the state of the magnetic powder before consolidation. In order to obtain an attenuation factor α of 2% or less, the average particle size of the magnetic powder is more preferably 2.0 μm to 8.0 μm. In addition, the average particle diameter of magnetic powder is the value of d50 measured with the wet laser diffraction type particle size distribution measuring device. Since graphite covering the surface of the magnetic powder is hydrophobic, particles are aggregated when an aqueous solution is used as a dispersion medium. Therefore, it is preferable to use alcohol such as isopropyl alcohol, ethanol or methanol as the dispersion medium. Moreover, it is preferable that the particle size of the core (metal fine particle) portion containing iron as a main component and coated with graphite is distributed in the range of 0.01 μm to 50 μm. If there are nuclei of less than 0.01 μm, the magnetic properties such as saturation magnetization will be lowered due to the development of superparamagnetism. If there are nuclei exceeding 50 μm, eddy current loss increases and core loss tends to increase. Further, if the particle size of the core portion becomes too large, the formation of the graphite coating may be insufficient. Note that the particle size of the core (metal fine particles) containing Fe as a main component can be directly measured from a reflected electron image obtained by SEM. The maximum diameter in the SEM image may be the particle diameter of the core portion.

  The method for manufacturing a powder magnetic core according to the present invention includes a step of pressure-molding a mixture of magnetic powder and a binder, but conventional manufacturing methods can be applied except for the portions specified in the present invention. . As the binder, for example, a thermosetting resin that cures at 250 ° C. or lower can be used. Hereinafter, a case where a resin is used as the binder will be described as an example. Magnetic powder and resin are mixed before being subjected to pressure molding. The resin is mixed, for example, by dissolving the resin in an organic solvent, mixing the resin and magnetic powder, then drying the mixture, and volatilizing the solvent. As the resin, a thermosetting resin can be used. In this case, it is preferable to use an epoxy resin that contributes to the improvement of the strength of the molded body and is rich in insulation. The magnetic powder mixed with the resin is pressure-molded using a press to obtain a molded body. Since the magnetic powder according to the present invention can be easily consolidated, a high density can be obtained with a low molding pressure. By controlling the molding pressure and the amount of the resin as the binder, the space ratio Ro of the magnetic powder in the powder magnetic core after the pressure molding is set in the range of 70 to 98 vol%. When the space factor Ro is less than 70 vol%, the saturation magnetic flux density of the dust core decreases and the crushing strength decreases to less than 60. On the other hand, when the space factor Ro exceeds 98 vol%, the attenuation rate α exceeds 20%. The space factor Ro is calculated as follows. The saturation magnetization of the magnetic powder is measured, and the volume ratio Rp of the metal fine particles mainly composed of Fe in the magnetic powder is calculated. On the other hand, the saturation magnetization of the dust core is measured, and the volume ratio Rc of the metal fine particles mainly composed of Fe in the dust core is calculated. The space factor Ro is calculated from Ro = 100 × Rc / Rp. Saturation magnetization uses a value measured with an applied magnetic field of 1.36 MA / m using a VSM (sample vibration type magnetometer).

When the amount of the resin mixed is too large, the space factor Ro is lowered and the magnetic permeability is also lowered. On the other hand, if the amount is too small, the strength of the molded body is lowered and the insulating property is also lowered. Therefore, the amount of the resin is preferably in the range of 0.1 to 4.0 parts by weight with respect to 100 parts by weight of the magnetic powder. What is necessary is just to control a shaping | molding pressure so that it may become the said space factor. The molding pressure is particularly preferably 600 MPa or less. This is because it can be molded with a simple molding machine, and for example, the loss at 1 MHz and 10 mT can be suppressed to 400 kW / cm ³ or less. The lower limit of the molding pressure is not particularly limited as long as a desired maximum magnetic flux density is obtained, but is preferably 300 MPa or more from the viewpoint of obtaining a high-density powder magnetic core. In addition to the resin, a lubricant may be mixed in order to increase the fluidity of the particles. By doing so, a dust core having sufficient characteristics can be obtained with a lower molding pressure. In pressure molding, only the magnetic powder may be molded, or may be molded integrally with the coil. As described above, since the magnetic powder according to the present invention can be consolidated at a low pressure, a coil-incorporated (metal composite) type inductor can be manufactured by lowering the molding pressure.

  The obtained compact is subjected to a heat treatment for curing the binder and removing the distortion to obtain a dust core. Such heat treatment can be omitted. Since the magnetic powder is a powder obtained by heat-treating a mixed powder of iron oxide powder and carbon-containing powder in a non-oxidizing atmosphere, the temperature for heat-treating the compacted body after pressure molding is The mixed powder is made lower than the temperature for heat treatment. This is because the graphite coating is damaged when the heat treatment temperature after the pressure molding is equal to or higher than the heat treatment temperature of the mixed powder. Furthermore, since it is difficult to completely suppress oxidation even by coating with graphite, the heat treatment temperature after pressure molding is preferably 250 ° C. or less from the viewpoint of suppressing oxidation. The heat treatment after pressure molding here is heat treatment for the purpose of removing residual stress or distortion, resin curing treatment (curing) when using a thermosetting resin, or the like. The heat treatment temperature is set to the resin curing temperature or more for the purpose of resin curing.

  Next, the dust core according to the present invention will be further described. The dust core according to the present invention is a dust core formed by compacting magnetic powder and a binder, and as described above, the magnetic powder comprises metal fine particles mainly composed of Fe and the metal fine particles. And graphite for coating. Further, the dust core has a composite structure composed of particles having a constriction and particles having no constriction. The particle having a constriction is a particle having an irregular shape when observed on one surface of the dust core and having a portion that is thinner than the other portions other than the end portion of the particle. Ordinary particles have a substantially spherical shape with no constriction. The dust core according to the present invention has a composite structure composed of particles having such a constriction and particles having no constriction. Formability is high when a dust core is composed of particles having a constriction and particles having no constriction. In addition, since the particles having a constriction are easily plastically deformed and the bonding force between the particles is increased, a high-strength powder magnetic core can be obtained. Therefore, a dust core having a graphite coating and having the composite structure does not necessarily require a high molding pressure.

Such particles are magnetic powders obtained by heat-treating the above-mentioned mixed powder of iron oxide powder and carbon-containing powder in a non-oxidizing atmosphere and having an average particle diameter of 2.0 μm or more. It can be realized by using. Such magnetic powder includes particles having the above-mentioned constriction. With a magnetic powder having a small average particle diameter, it is difficult to produce particles having a constriction. By employing the dust core having the composite structure, the space factor can be increased and the saturation magnetic flux density can be improved. However, if this is increased too much, the attenuation rate α and loss will increase. Therefore, the saturation magnetic flux density as measured at an applied magnetic field 1.36MA / m at 25 ° C. and 1.40～1.70T, the initial permeability at 1MHz was initial permeability _{mu 10} in _mu 1, 10 MHz When the attenuation factor α expressed by 100 × (μ ₁ −μ ₁₀ ) / μ ₁ is 20% or less, a dust core having an excellent balance of magnetic characteristics such as frequency characteristics and saturation magnetic flux density is provided. Is done. When the saturation magnetic flux density is less than 1.40 T, the characteristics of the dust core having a high saturation magnetic flux density are lost. On the other hand, when the saturation magnetic flux density exceeds 1.70 T, the attenuation rate α increases and exceeds 20%. Also, the loss increases and exceeds 400 kW / cm ³ .

  Hereinafter, the present invention will be described by way of examples. However, the present invention is not limited to these examples.

Example 1
Α-Fe ₂ O ₃ powder having an average particle diameter of 0.61 μm or 0.76 μm and carbon black powder having an average particle diameter of 0.02 μm were weighed and mixed for 1000 minutes by a ball mill. Incidentally, the proportion of carbon black powder to the total of the combined _α-Fe 2 _{O 3} powder and carbon black powder was varied in the range of 17.5~25wt%. The total of the mixed powder was 50 g. IPA was used as the solvent. The obtained slurry was dried, and the dried mixed powder was heat-treated in the tubular furnace under the following conditions. The atmosphere of the heat treatment was a nitrogen gas stream with a flow rate of 2 l / min. After the temperature was raised from room temperature at a rate of 3 ° C./min, the temperature was maintained at a heat treatment temperature set in the range of 900 ° C. to 1100 ° C. for 2 hours and the furnace was cooled to room temperature at a rate of 3 ° C./min. An operation of subjecting 5 g of the treated sample to ultrasonic irradiation in 50 ml of IPA for 10 minutes and magnetic separation with a magnet was repeated until the supernatant became transparent. The obtained powder was subjected to X-ray diffraction using CuKα rays, and the formation of Fe and graphite was confirmed from the diffraction pattern. The magnetic characteristics of the obtained magnetic powder were evaluated. The magnetic properties were measured with an applied magnetic field of 1.36 MA / m using a VSM (sample vibration type magnetometer). Further, the average particle diameter d50 was measured by a wet laser diffraction method (using LA-920 manufactured by Horiba, Ltd.). Isopropyl alcohol was used as a dispersion solvent. The results are shown in Table 1. Table 1 also shows the evaluation results of the carbonyl iron powder (BASF grades HQ and OM) used for comparison.

Next, using the magnetic powders No. 1 and No. 3 to No. 9 obtained above, dust cores were produced as follows. An epoxy resin (EPX-6136 manufactured by Somaru Corporation) was used as a binder. The ratio of the amount of the epoxy resin was 4.0 parts by weight with respect to 100 parts by weight of the magnetic powder. The weighed resin was dissolved in acetone and mixed with magnetic powder. The mixture was dried and acetone was volatilized. After classifying the mixture, it was pressure molded under the condition of 600 MPa to obtain a molded body. The obtained molded body was heat-treated in the atmosphere at a temperature of 200 ° C. for 1 hour and then cooled in a furnace to obtain a dust core. The magnetic properties and the crushing strength at room temperature (25 ° C.) of the obtained dust core were evaluated. The space factor Ro was calculated from Rc / Rp × 100. Here, the volume ratio Rp of the metal fine particles mainly composed of Fe was calculated from the saturation magnetization of the magnetic powder shown in Table 1. On the other hand, the saturation magnetization of the dust core was measured, and the volume ratio Rc of the metal fine particles mainly composed of Fe in the dust core was calculated. The initial permeability μi was evaluated using an impedance / gain phase analyzer (manufactured by Agilent, 4194A) under the condition of 100 kHz. Moreover, the maximum magnetic flux density Bm and the coercive force Hc were evaluated under the condition of 1.36 MA / m using VSM (manufactured by Toei Industry Co., Ltd.). Loss was evaluated using a BH analyzer (SY-8232, manufactured by Iwasaki Tsushinki Co., Ltd.) under conditions of 1 MHz and 10 mT. An autograph (manufactured by Shimadzu Corporation, AG-50NG) was used for evaluation of the crushing strength, and a toroidal sample having an outer diameter of 7.2 mm and an inner diameter of 4.0 mm was used. Measure the test force when the core was cracked by applying a load to the sample in the diametric direction, crushing strength = test force x [outer diameter-(outer diameter-inner diameter) / 2] / [thickness x {(outer diameter-inner diameter ) / 2} ² ], the crushing strength was calculated. The results are shown in Table 2.

  Next, using a magnetic powder having an average particle size of 3.3 μm shown in Table 1 and changing the molding pressure to 300 MPa, 600 MPa, and 1000 MPa, a dust core was prepared, and the magnetic characteristics were evaluated. The results are shown in Table 3.

As shown in Table 1, the powder magnetic cores A to D using magnetic powder having an average particle diameter of 3.0 to 15.0 μm and a space factor Ro of 70 to 98 vol% are attenuation factors. It can be seen that the frequency dependency of the magnetic permeability is excellent as compared with E and F dust cores having an average particle size of more than 15.0 μm. Further, the powder magnetic cores A to D have a loss of 400 kW / cm ³ or less, and it is understood that the loss is excellent. In particular, the powder magnetic cores A to C using magnetic powder having an average particle size in the range of 3.0 to 8.0 μm exhibited an excellent attenuation rate of 2% or less. Each of the dust cores A to D exhibited a maximum magnetic flux density in the range of 1.40 to 1.70 T and an initial permeability of 20 or more. Each of the dust cores A to D has a crushing strength of 60 MPa or more. In particular, a dust core using a magnetic powder having an average particle size of 7.0 μm or more showed a high crushing strength of 100 MPa or more. On the other hand, in E and F dust cores using carbonyl iron, the compact density does not increase, so the space factor is as low as 57%, and the maximum magnetic flux density, initial permeability, and pressure ring strength are all low. It was.

  1 to 8 show scanning electron microscope (SEM) photographs of the surfaces of the A to H dust cores. As shown in FIG. 1, in a powder magnetic core using magnetic powder having an average particle diameter of 3.0 μm, a particle 1 having a constriction is observed, and it is composed of a particle 1 having a constriction and a particle 2 having no constriction. It can be seen that this is a complex organization. It can be seen that other particles have entered the constricted portion. As is clear from FIGS. 1 to 6, the proportion of particles having a constriction increases as the particle size of the magnetic powder used increases. In each of the dust cores A to H, the number of particles having a constriction is larger in the ratio of the number of coarse particles having a maximum diameter exceeding 6 μm. In the dust cores A to H in Table 2, the space factor did not change much, but the crushing strength changed greatly. This is considered to be due to the fact that the proportion of particles having a constriction increases as the average particle size of the magnetic powder increases. On the other hand, in the structure of the powder magnetic cores G and H using the carbonyl iron of the comparative example, particles having a constriction were not observed.

Moreover, as shown in Table 2, the space factor increased as the molding pressure increased. In the powder magnetic core of K with a space factor exceeding 98%, the maximum magnetic flux density Bm exceeded 1.70T, and the damping rate greatly exceeded 20% and became nearly 40%. Also, the loss was a high value close to 600 kW / cm ³ . In the I and J powder magnetic cores with a molding pressure of 600 MPa or less, the damping rate was suppressed, and a damping rate of 20% or less was obtained. In the powder magnetic cores of I and J having a molding pressure of 600 MPa or less, the maximum magnetic flux density Bm was in the range of 1.40 to 1.70 T, and the initial permeability was 20 or more. When the magnetic powder according to the present invention was used, sufficient powder magnetic core characteristics were exhibited even at a molding pressure as low as 300 MPa. Further, it can be seen that even when the molding pressure is changed from 300 to 1000 MPa, the coercive force does not change and the distortion due to molding is small despite the low heat treatment temperature.

  FIG. 9 shows a scanning electron microscope (SEM) photograph of the magnetic powder used in the above examples. The particle at the center of the photograph is a graphite partially coated particle in which a portion 3 (black portion in the figure) covered with graphite and a portion 4 (white portion in the drawing) not covered with graphite are formed on the surface. The graphite partially coated particles shown in FIG. 9 can be obtained by lowering the temperature of the heat treatment. Using the magnetic powder containing such graphite partially coated particles, the space factor of the magnetic powder in the powder magnetic core after pressure molding can be controlled. When a portion is coated with graphite, it is excellent in that the friction of particles can be reduced and a high-density molded body can be produced at a low pressure.

(Example 2)
An α-Fe ₂ O ₃ powder having an average particle diameter of 0.61 μm, a carbon black powder having an average particle diameter of 0.02 μm, and a Cr powder having an average particle diameter of 0.05 μm, each having a Cr content of 1.5% by mass, Magnetic powder was obtained in the same manner as in Example 1 by weighing to 0.0 mass%, 4.5 mass%, 6.0 mass%, and 7.5 mass%. However, the heat treatment temperature was 1100 ° C. The obtained powder was subjected to X-ray diffraction using CuKα rays, and the formation of Fe and graphite was confirmed from the diffraction pattern. The magnetic characteristics of the obtained magnetic powder were evaluated. The magnetic properties were measured with an applied magnetic field of 1.36 MA / m using a VSM (sample vibration type magnetometer). The average particle diameter d50 measured by a wet laser diffraction method (using LA-920 manufactured by Horiba, Ltd.) was 2 μm. Further, SiC having an average particle size of 0.01 μm is used instead of Cr powder so that the Si content becomes 2.0 mass%, 4.0 mass%, 6.0 mass%, and 10.0 mass%, respectively. Weighed and similarly obtained magnetic powder.

Next, a dust core was produced using the magnetic powder obtained above as follows. Epoxy resin (E-530, manufactured by Somaru Co., Ltd.) was used as a binder. The ratio of the amount of epoxy resin to 100 parts by weight of magnetic powder was 4.0 parts by weight. The weighed resin was dissolved in acetone and mixed with magnetic powder. The mixture was dried and acetone was volatilized. After classifying the mixture, it was pressure molded under the condition of 300 MPa to obtain a molded body. The obtained molded body was heat-treated in the atmosphere at a temperature of 150 ° C. for 1 hour and then cooled in a furnace to obtain a dust core. The frequency characteristics and direct current superposition characteristics at room temperature (25 ° C.) of the obtained dust core were evaluated. The space factor Ro was calculated in the same manner as in Example 1. The initial permeability μi was evaluated under the condition of 10 MHz using an impedance / material analyzer (manufactured by Agilent, 4291B). The frequency characteristic was evaluated by the frequency f _(μi−10%) when the magnetic permeability μi was reduced by 10% with respect to the value evaluated under the condition of 10 MHz. DC superimposition characteristics were measured using a precision LCR meter (Agilent, 4284A) at 1 MHz with a superimposed current value of 0 to 20 A, and the change in inductance with respect to the superimposed current was measured. Each value was converted into an applied magnetic field (A / m) and evaluated. The magnitude H _(μ−1%) of the DC applied magnetic field when the permeability decreased by 1% from the value when no DC magnetic field application was applied due to the increase of the DC applied magnetic field was used for the evaluation of the DC superposition characteristics. The evaluation results are shown in Table 4.

As is apparent from Table 4, it is understood that the frequency characteristics are improved by adding 7.5% by mass or less of Cr to the magnetic powder. In each of the powder magnetic cores M to Q, f _(μi−10%) was 500 MHz or more, and an excellent frequency characteristic was exhibited as compared with the powder magnetic core L. The dust cores M to Q each have an applied magnetic field H _(μ-1%) of 2000 A / m or more when the magnetic permeability decreases by _1% , which is higher than the dust core L, and has direct current superposition characteristics. Is also excellent. Further, when XPS analysis was performed on the magnetic powder substituted with Cr, it was confirmed that Cr was present in the vicinity of the particle surface in a form bonded to oxygen rather than inside the particle. Such a configuration is considered to contribute to the improvement of frequency characteristics and DC superposition characteristics.

Moreover, in Table 4, it turns out that a frequency characteristic improves also by making Si contain magnetic powder. In particular, in dust cores T and U in which Si is substituted by 6.0 to 10.0% by mass, f _(μi−10%) is 700 MHz or more, and excellent frequency characteristics are exhibited. Moreover, in the powder magnetic cores R to U using powder in which 2 to 10% by mass of Si is substituted, H _(μ-1%) is 2000 A / m or more, which is higher than the powder magnetic core L, and is DC. Excellent superimposition characteristics. In particular, 3000 A / m or more is obtained in the dust cores T and U in which Si is substituted by 6.0 to 10.0 mass%. However, if a large amount of Si is substituted, Ro decreases, and it becomes less than 70%. Therefore, when replacing Si, it is preferable to use it when high Ro and high saturation magnetic flux density are not required.

1: Particles with constriction 2: Particles without constriction 3: Part covered with graphite 4: Part not covered with graphite

Claims (5)

A method for producing a powder magnetic core comprising a step of pressing a mixture of magnetic powder and a binder to obtain a molded body,
The magnetic powder is obtained by heat-treating a mixed powder of iron oxide powder and carbon-containing powder in a non-oxidizing atmosphere, and coats the metal fine particles mainly composed of Fe and the metal fine particles. With graphite to
The magnetic powder has an average particle size of 2.0 to 15.0 μm,
A method for manufacturing a dust core, wherein a space factor of the magnetic powder in the dust core after the pressure molding is in a range of 70 to 98 vol%.   The mixed powder is further mixed with a powder containing Cr, and a part of Fe is Cr so that the proportion of Cr in Fe and Cr as a whole is 7.5 mass% or less (excluding 0). 2. The method of manufacturing a dust core according to claim 1, wherein the dust core is replaced.   The method for producing a dust core according to claim 1 or 2, wherein a molding pressure of the pressure molding is 600 MPa or less. A step of heat-treating the molded body,
The method for producing a dust core according to any one of claims 1 to 3, wherein a temperature at which the compact is heat-treated is 250 ° C or lower. A dust core formed by compacting magnetic powder and a binder,
The magnetic powder includes fine metal particles mainly composed of Fe, and graphite covering the fine metal particles,
The dust core has a composite structure composed of particles having a constriction and particles having no constriction,
The saturation magnetic flux density when measured with an applied magnetic field of 1.36 MA / m at 25 ° C. is 1.40 to 1.70 T,
When the initial permeability at ₁ MHz is μ ₁ and the initial permeability at 10 MHz μ ₁₀ , the attenuation rate represented by 100 × (μ ₁ −μ ₁₀ ) / μ ₁ is 20% or less. Powder magnetic core to do.