JP2016157753A - Powder magnetic core and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method by which a powder magnetic core with high specific resistance and a high magnetic flux density can be obtained.SOLUTION: The manufacturing method of the powder magnetic core includes: a molding step for pressure-molding a magnetic core powder in which a surface of a soft magnetic particle formed from a pure iron or an iron alloy containing Fe in 90 mass% or more is formed from an ion-oxide particle of a metal element (M) that is one or more of Mn, Zn, Ni, Cu and Mg and a coated partial that is coated by the iron-oxide particle; and a heat treatment step for heating a mold that is obtained after the molding step, at 370 to 600°C. Thus, the powder magnetic core can be obtained in which a spinel-type ferrite (MFeO) is generated between the soft magnetic particles. It is preferable that the magnetic core powder relating to the present invention is obtained via a granulation step for stirring and mixing a soft magnetic powder formed from soft magnetic particles, an oxide powder formed from oxide particles and an iron-oxide powder formed from iron-oxide particles under strong energy satisfying collision energy 1 to 5 J/s g.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a dust core having a large volume specific resistance value (hereinafter simply referred to as “specific resistance”) and a high magnetic flux density, and a method for manufacturing the same.

  There are many products that use electromagnetism around us, such as transformers, motors, generators, speakers, induction heaters, and various actuators. Many of these products use an alternating magnetic field. In order to efficiently obtain a large alternating magnetic field locally, a magnetic core (soft magnet) is usually provided in the alternating magnetic field.

  This magnetic core is required not only to have high magnetic characteristics in an alternating magnetic field but also to have low high-frequency loss (hereinafter simply referred to as “iron loss” regardless of the material of the magnetic core) when used in an alternating magnetic field. . This iron loss includes eddy current loss, hysteresis loss, and residual loss. In particular, it is important to reduce eddy current loss that increases in proportion to the square of the frequency of the alternating magnetic field.

  As such a magnetic core, there is a powder magnetic core obtained by press-molding soft magnetic particle powder (soft magnetic powder) covered with an insulating film (layer). Since this dust core has a small eddy current loss and a high degree of freedom in shape, it is used in various electromagnetic devices such as motor cores. However, if the insulating film is formed of nonmagnetic silicon-based resin, phosphate, or the like, the (saturated) magnetic flux density of the dust core can be reduced. Therefore, it has been proposed to use a ferrite film as the insulating film, and for example, there is a description related to the following patent document.

JP 2005-64396 A JP 2011-2104026 A

  In Patent Document 1, a powder magnetic core (powder powder) obtained by warm-pressing (150 ° C. × 900 MPa) a powder made of iron-based particles having a ferrite film formed on the surface by an aqueous solution reaction method (a kind of electroless plating). Body). However, it takes a long time to form the ferrite film by the aqueous solution reaction method, and the magnetic core powder and the dust core cannot be efficiently produced. Ferrite is a kind of ceramics and is easily broken. For this reason, when the powder after ferrite plating is pressure-molded, a large number of cracks are generated in the ferrite film on the particle surface, and a powder core having a high specific resistance cannot be obtained. Furthermore, although not described in Patent Document 1, normally, in order to reduce hysteresis loss and improve magnetic flux density, strain relief annealing is performed in which the molded body is heated at a high temperature (600 ° C. or higher) after pressure molding. . At this time, the ferrite film previously formed on the surface of the iron-based particles can be reduced by diffusion of O toward the iron-based particles, the ferrite structure can be collapsed, and the specific resistance of the insulating core and thus the dust core can be reduced. .

Patent Document 2 discloses that a composite powder obtained by coating the surface of permalloy alloy (Fe-47% Ni) particles with MgO fine particles, Fe ₂ O ₃ fine particles, and MnO fine particles is temporarily formed, CIP formed (550 ° C.), SPS sintered, There is a description regarding a high-density sintered body (nanomagnetic composite) subjected to HIP molding (800 ° C.). However, a sintered body made of such a permalloy alloy usually requires a high (saturated) magnetic flux density and is different in the application field or technical field from a dust core used without being sintered.

  This invention is made | formed in view of such a situation, and it aims at providing the new powder magnetic core which can aim at the improvement of a specific resistance and magnetic flux density. Moreover, it aims at also providing the manufacturing method of such a powder magnetic core.

  The present inventor has intensively studied to solve this problem, and as a result of repeated trial and error, was obtained by press-molding a magnetic core powder made of coated particles in which a raw material of ferrite was adhered to the surface of soft magnetic particles. The idea is to form a ferrite film (layer) effective for improving specific resistance and magnetic flux density on the surface of soft magnetic particles (especially between soft magnetic particles) by heat treating (annealing) the compact. succeeded in. By developing this result, the present invention described below has been completed.

<Production method of dust core>
(1) The method for producing a dust core according to the present invention is a metal in which the surface of soft magnetic particles made of an iron alloy containing 90 mass% or more of pure iron or Fe is one or more of Mn, Zn, Ni, Cu, or Mg. A molding step for pressure-molding a magnetic core powder composed of coated particles coated with oxide particles of element (M) and iron oxide particles, and a molded body obtained after the molding step is heated at 370 to 600 ° C. And a heat treatment step, wherein a dust core in which spinel ferrite (MFe ₂ O ₄ ) is generated between the soft magnetic particles is obtained.

(2) According to the production method of the present invention, a dust core having a high specific resistance and a high magnetic flux density can be efficiently obtained even after heat treatment (annealing). The reason for this is not necessarily clear, but at present it can be considered as follows.

In the production method of the present invention, first, the surface of a soft magnetic particle containing pure iron or Fe as a main component is a specific metal element (hereinafter referred to as “S-type ferrite” or simply “ferrite”). A magnetic core powder made of coated particles coated with particles (oxide particles) of M) and iron oxide particles (particularly Fe ₂ O ₃ particles) is used.

Next, the molded body obtained by pressure molding this magnetic core powder is heated at a specific temperature (370 to 600 ° C.). During this heat treatment, iron oxide particles and oxide particles adhering to the surface of the soft magnetic particles react to generate S-type ferrite (MFe ₂ O ₄ ). In this way, a grain boundary layer (ferrite layer) made of S-type ferrite, which is both an insulating material and a magnetic material, is formed between the soft magnetic particles constituting the compact (dust core). Thus, it is considered that the dust core of the present invention has exhibited excellent electrical characteristics (specific resistance) and magnetic characteristics (magnetic flux density, magnetic permeability, etc.).

  Incidentally, since the ferrite layer (insulating layer) according to the present invention is generated in the heat treatment process after the molding process, naturally, cracks and the like do not occur during the molding process, and the soft magnetic particles deformed plastically in the molding process. It is easy to form homogeneously along the surface or grain boundary. Further, the heat treatment step according to the present invention can substantially serve as an annealing step. For this reason, not only the ferrite layer is generated by the heat treatment process, but also the processing strain introduced into the soft magnetic particles in the molding process is removed, and the magnetic flux density of the dust core is improved and the hysteresis loss is reduced. Thus, according to the production method of the present invention, it is possible to efficiently obtain a dust core having higher characteristics than a conventional dust core made of soft magnetic particles previously coated with ferrite.

<Dust core or powder for magnetic core>
The present invention can be grasped not only as the above-described manufacturing method but also as a dust core obtained by the manufacturing method. Furthermore, the present invention can be grasped as a powder for a magnetic core composed of coated particles that are subjected to a molding process.

<Others>
Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. A range such as “a to b” can be newly established with any numerical value included in various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

2 is an X-ray diffraction (XRD) profile obtained by observing grain boundaries of a magnetic core powder before and after heat treatment of a molded body according to Sample 1. FIG. It is the XRD profile which observed the grain boundary before and behind heat processing of the molded object which consists of the powder for magnetic cores manufactured by changing the apparatus used by a granulation process. Fe particles (soft particles) is an XRD profile was observed grain boundaries before and after the heat treatment of the molded article comprising the powder was changed to ZrO ₂ balls.

  One or two or more components arbitrarily selected from the present specification may be added to the above-described components of the present invention. The contents described in this specification can be appropriately applied not only to the production method of the present invention but also to the dust core or the magnetic core powder. The content related to the manufacturing method can be a component related to an object if understood as a product-by-process. Which embodiment is the best depends on the target, required performance, and the like.

<Raw material powder>
(1) Soft magnetic particles (soft magnetic powder)
The soft magnetic particles may be composed mainly of a ferromagnetic element such as a Group 8 transition element (Fe, Co, Ni, etc.), but are preferably made of pure iron or an iron alloy in view of characteristics, availability, cost, and the like. When pure iron powder is used as the soft magnetic powder, a high saturation magnetic flux density can be obtained, and the magnetic characteristics of the dust core can be improved. From this viewpoint, when iron alloy powder is used as the soft magnetic powder, the larger the Fe content, the better. For example, when the whole is 100% by mass, iron alloy powder having Fe of 90% by mass or more, further 95% by mass or more may be used. Examples of alloy elements constituting the iron alloy include Si and Al. By including about 1 to 3% by mass of these alloy elements, the specific resistance of the dust core can be further improved. The soft magnetic powder according to the present invention may be a mixed powder obtained by mixing two or more kinds of powders having different compositions or forms (particle diameter, shape).

  The particle size of the soft magnetic particles can be adjusted according to the specifications of the dust core, but is preferably 50 to 500 μm, more preferably 106 to 250 μm. If the particle size is too large, it is difficult to increase the density of the dust core and reduce the eddy current loss. If the particle size is too small, it is difficult to improve the magnetic flux density of the dust core and reduce the hysteresis loss.

  Incidentally, the “particle size” in the present specification is a value indicating the diameter of the soft magnetic particles, and is specified by sieving. Specifically, the median [(d1 + d2) / 2] of the upper limit (d1) and the lower limit (d2) of the mesh size used for sieving the powder was defined as the particle size (D). The particle size is displayed in units of μm, and the decimal part is rounded off. The same applies not only to soft magnetic particles but also to oxide particles and iron oxide particles.

  The soft magnetic powder is manufactured by, for example, an atomizing method, a mechanical pulverizing method, a reducing method, or the like. The atomized powder may be any of water atomized powder, gas atomized powder, and gas water atomized powder. As the soft magnetic particles are substantially spherical, a uniform ferrite layer is formed between the adjacent soft magnetic particles, and the insulation between the soft magnetic particles and thus the specific resistance of the dust core can be improved.

(2) Iron oxide particles (iron oxide powder)
Although there are various types of iron oxides, the iron oxide according to the present invention is mainly preferably Fe ₂ O _{3 and} more preferably α-Fe ₂ O ₃ (hematite). As long as S-type ferrite is formed by the heat treatment step, the iron oxide according to the present invention may contain a small amount of FeO (wustite), Fe ₃ O ₄ (magnetite), or the like.

  The particle size of the iron oxide particles can be appropriately adjusted according to the particle size of the soft magnetic particles, but may be 5 μm or less, and the finer the particle size, the better. If it dares to say, the particle size should just be 0.1 micrometer or more.

(3) Oxide particles (oxide powder)
The oxide particles according to the present invention are made of an oxide of one or more metal elements (M) selected from a specific metal group consisting of Mn, Zn, Ni, Cu and Mg. M is a metal element constituting S-type ferrite together with Fe. M may be one type or two or more types, and the oxide according to the present invention may be a complex oxide (for example, Mn _x Zn _1-x O, 0 <x <1). The valence of M may be 2 (for example, MnO), 3 (for example, Mn ₂ O ₃ ), or 4 (for example, MnO ₂ ). Furthermore, the oxide is not limited to one type, and may be two or more types. That is, the iron oxide particles according to the present invention may be a mixture of a plurality of types of oxide particles (for example, ZnO and MnO ₂ or Mn ₂ O ₃ ).

Incidentally, it is preferable that M contains Mn. Ferrite containing Mn can improve both the specific resistance and magnetic flux density of the dust core. The reason is considered as follows. In the case of a solid solution of a normal spinel and a reverse spinel, the ease of entry of M into the A site or B site in the spinel crystal structure varies depending on the type of M. Due to the difference in M, the magnetic moment generated in each crystal structure also changes. In the case of ferrite having a crystal structure in which Mn (and Zn) is dissolved, a larger magnetic moment is generated and saturation magnetization is larger than in the case where other M is dissolved. In particular, MnFe ₂ O ₄ has the largest saturation magnetization and high specific resistance among various unit ferrites. For these reasons, one type of M is preferably Mn.

  The particle size of the oxide particles can be appropriately adjusted according to the particle size of the soft magnetic particles, but as with the iron oxide particles, for example, it is preferably as fine as 5 μm or less. The iron oxide particles and the oxide particles are preferably mixed in advance before being attached to the surface of the soft magnetic particles.

<Magnetic core powder>
(1) Granulation step There are various methods for granulating coated particles (coated composite particles) in which the surface of soft magnetic particles is coated with oxide particles and iron oxide particles or for preparing magnetic core powders composed of the coated particles. . For example, the magnetic core powder is composed of a soft magnetic powder composed of soft magnetic particles, an oxide powder composed of oxide particles, and an iron oxide powder composed of iron oxide particles under strong energy satisfying a collision energy of 1 to 5 J / s · g. It is suitable if it is obtained through a granulation step of stirring and mixing. The coated particles to which such a strong (mechanical) energy is applied have a high surface activity, and further, a mechanochemical reaction easily occurs. For this reason, when the magnetic core powder obtained through the granulation process according to the present invention is used, even when heated (annealed, etc.) at a temperature significantly lower than the general ferrite formation temperature (900 ° C. or higher), soft magnetic An S-type ferrite layer is easily generated stably on the surface of the particle.

  Such a granulation process is performed by using, for example, various (dry) mechanical particle compositing apparatuses, strong impact force, compression force or shear force on raw material particles (soft magnetic particles, iron oxide particles and oxide particles). It can be done by acting. Specifically, coated particles are produced by mechanical complexation using a planetary ball mill, mechano-fusion (made by Hosokawa Micron Corporation), sheeter composer (Tokuju Kogakusho Co., Ltd.), hybridization (Nara Machinery Co., Ltd.), etc. It is preferable to granulate.

  Furthermore, for example, when using mechano-fusion, parameters such as the rotor rotational speed (eg, 1000 rpm), rotor clearance (eg, 2 mm), powder input amount (eg, 200 g), operating time (eg, 15 minutes) are appropriately set. It is preferable to perform the granulation step by operating under the adjusted conditions.

(2) Formulation The S-type ferrite (MFe ₂ O ₄ ) according to the present invention is basically generated by oxide particles and iron oxide particles attached to the surface of soft magnetic particles. Therefore, the oxide particles (powder) and the iron oxide particles (powder) are in the vicinity where the atomic ratio of the metal element (M) and iron (Fe) is 1: 2 (for example, M / Fe = 0.35 in atomic ratio). ~ 0.65) is preferred.

  However, from the viewpoint of achieving both the specific resistance of the dust core and the magnetic flux density, the powder for the magnetic core is oxide particles: 0.04 to 0.8% by mass, and further 0.2 if the total is 100% by mass. It is suitable that it is -0.4 mass%, iron oxide particles: 0.2-4.0 mass%, further 0.8-1.6 mass%, and the balance is soft magnetic particles.

<Molding process>
The molding process according to the present invention is usually performed by pressure molding a magnetic core powder filled in a mold having a cavity having a desired shape. The molding pressure can be adjusted as appropriate, but the higher the density, the higher the density and the higher magnetic flux density of the dust core. As such a high-pressure molding method, it is preferable to use a mold lubrication warm high-pressure molding method. The mold lubrication warm high-pressure molding method consists of a filling process in which a high-grade fatty acid-based lubricant is coated on the inner surface and the magnetic core powder is filled into a mold, and a higher fatty acid-based lubricant between the magnetic core powder and the inner surface of the mold. And a warm high pressure forming step in which pressure forming is performed at a forming temperature and a forming pressure at which another metal soap film is produced. Note that “warm” means that, for example, the molding temperature is set to 70 ° C. to 200 ° C. or even 100 to 180 ° C. in consideration of the influence on the surface coating (or insulating coating) or the alteration of the higher fatty acid lubricant. Good. Details of the mold lubrication warm high pressure molding method are described in many publications such as Japanese Patent Publication No. 3309970 and Japanese Patent No. 4024705.

《Heat treatment process》
The heat treatment process according to the present invention is performed by heating the molded body obtained in the molding process. Through this heat treatment step, the oxide particles coated with the soft magnetic particles and the iron oxide particles react to generate S-type ferrite on the surface of the soft magnetic particles or between the soft magnetic particles. In addition, it is thought that the soft magnetic particle which has Fe as a main component acts catalytically in this ferrite production | generation reaction.

  The heating temperature of the molded body is preferably 370 to 700 ° C, more preferably 500 to 600 ° C. If the heating temperature is too low, ferrite is not stably generated. If the heating temperature is excessive, the soft magnetic particles are oxidized, pre-sintered, etc., and the specific resistance or magnetic flux density of the dust core can be lowered.

Since the ferrite according to the present invention is basically generated using oxide particles and iron oxide particles as raw materials, the atmosphere of the heat treatment step can be performed in an inert gas (Ar, N _2, etc.) atmosphere or a vacuum atmosphere. However, oxygen may be slightly contained in the atmosphere. That is, the heat treatment step may be performed in an atmosphere having an oxygen concentration of 10% or less, 1% or less, or 0.1% or less. Furthermore, ferrite may be easily generated by performing the heat treatment step in an atmosphere containing a very small amount of oxygen. Therefore, the oxygen concentration is preferably 0.05% or more. In particular, when one kind of oxide particles is MnO ₂ particles, ferrite is easily generated stably by performing the heat treatment step in an atmosphere containing a slight amount of oxygen. Conversely, if one kind of oxide particles is Mn ₂ O ₃ particles, ferrite can be stably generated even if the oxygen concentration in the atmosphere is 0.1% or less, further 0.01% or less. In addition, the oxygen concentration as used in this specification is the volume% calculated | required with the oxygen concentration meter in the state of normal temperature and 1 atmosphere.

  In addition, the heat treatment process according to the present invention can also serve as an annealing process for reducing the coercive force or hysteresis loss of the dust core by removing residual strain and residual stress introduced into the soft magnetic particles during the molding process. it can. Of course, the annealing step may be performed separately under conditions (temperature, time, atmosphere) different from the heat treatment step. In any case, the heating time of the heat treatment step may be, for example, 0.1 to 5 hours, and further 0.5 to 2 hours.

<< S-type ferrite >>
When the S-type ferrite generated on the surface or grain boundary of soft magnetic particles is made of MnZn ferrite in which M is Mn and Zn, or NiMn ferrite in which M is Mn and Ni, the high resistivity and high magnetic flux of the dust core It is preferable to achieve both density. Furthermore, M preferably contains Mg in addition to Mn (optionally Zn, Ni). Even if such a ferrite layer is very thin, it exhibits excellent insulation, and there is little change in insulation before and after heat treatment.

<Dust core>
(1) Magnetic characteristics The powder magnetic core according to the present invention has a high saturation magnetic flux density. For example, the magnetic flux density (B _10k ) generated in a magnetic field of 10 kA / m is 1.6 T or higher, 1.7 T or higher, or 1 Can be over 8T. Moreover, the magnetic permeability can be 600 or more, further 700 or more, for example. Further, the specific resistance value can be, for example, 50 μΩm or more, further 100 μΩm or more. The density ratio (ρ / ρ0), which is the ratio of the bulk density (ρ) of the dust core to the true density (ρ0) of the soft magnetic particles, is 95% or more and further 98% or more, for example. The magnetic characteristics are improved, which is preferable.

(2) Applications The dust core according to the present invention can be used for electromagnetic devices such as motors, actuators, transformers, induction heaters (IH), speakers, reactors, and the like. In particular, it is preferably used for an iron core constituting an armature (rotor or stator) of an electric motor or generator. Among them, it is suitable as an iron core for a drive motor that requires low loss and high output (high magnetic flux density), specifically, as an iron core for a drive motor of an electric vehicle or a hybrid vehicle. In addition, even if it is used in any electromagnetic device, the dust core according to the present invention is preferably used in an alternating magnetic field of about 100 to 30000 Hz, and further about 200 to 20000 Hz.

<Preparation of powder for magnetic core>
(1) Raw material powder As a soft magnetic powder, gas water atomized powder (simply referred to as “Fe powder”) made of pure iron was prepared. The particle size of each powder used is 212-106 μm → 159 μm in the order of upper limit value to lower limit value → particle size. In addition, this particle size is the median value of the upper limit value and the lower limit value of the mesh size used when classification (sieving) is performed using an electromagnetic sieve shaker (manufactured by Lecce). It has been confirmed by SEM that this soft magnetic powder does not contain soft magnetic particles of less than 30 μm.

Commercially available Fe ₂ O ₃ powder (particle size: 1 μm) was prepared as the iron oxide powder. As oxide powders, MnO ₂ powder (particle size: 10 μm), Mn ₂ O ₃ powder (particle size: 10 μm), and ZnO powder (particle size: 1 μm) were prepared. The particle size of these powders was also specified in the same manner as the soft magnetic powder.

(2) Blending of raw material powder Soft magnetic powder, iron oxide powder and oxide powder were blended in the following two proportions. In addition, the ratio of each powder showed the whole compounding powder as 100 mass%.
(Sample 1) Fe ₂ O ₃ powder: 0.8% by mass, MnO ₂ powder: 0.2% by mass, ZnO powder: 0.1% by mass, Fe powder: remainder (Sample 2) Fe ₂ O ₃ powder: 1 .6 mass%, Mn ₂ O ₃ powder: 0.35 mass%, ZnO powder: 0.2 mass%, Fe powder: balance

(3) Granulation step Each blended powder was stirred and mixed with a planetary ball mill (LP-4 manufactured by Ito Seisakusho). This treatment was performed under the following conditions. ZrO ₂ balls (particle size: 3 mm): 800 g, soft magnetic powder: 100 g, predetermined amounts of iron oxide powder and oxide powder were put into a 250 cc bot made of SKD. It processed in normal temperature and air | atmosphere as rotation speed: 500rpm and processing time: 0.5h. The index value of energy applied at this time is 3 J / s · g. Thus, magnetic core powders according to Sample 1 and Sample 2 were prepared.

<Manufacture of dust core>
(1) Molding Step Using each magnetic core powder, a ring-shaped (outside diameter: φ39 mm × φ30 mm × thickness 5 mm) shaped body was obtained by a mold lubrication warm high pressure molding method. At this time, no internal lubricant or resin binder was used. Specifically, it was molded as follows.

  A cemented carbide mold having a cavity corresponding to a desired shape was prepared. This mold was previously heated to 130 ° C. with a band heater. Further, the inner peripheral surface of this mold was previously subjected to TiN coating treatment, and the surface roughness was set to 0.4Z.

An aqueous dispersion of lithium stearate (1%) was uniformly applied to the inner peripheral surface of the heated mold with a spray gun at a rate of about 10 cm ³ / min. This aqueous dispersion is obtained by adding a surfactant and an antifoaming agent to water. Other details were made in accordance with the descriptions in Japanese Patent Publication No. 3309970, Japanese Patent No. 4024705, and the like.

  Each magnetic core powder was filled into a mold having lithium stearate coated on the inner surface (filling process), and warm-molded at 1568 MPa while maintaining the mold at 130 ° C. (molding process). In this warm molding, none of the molded bodies generated galling or the like with the mold, and the mold could be taken out from the mold with a low pressure.

(2) Heat treatment process (annealing process)
Each obtained compact was put into a heating furnace and heated in an inert gas (Ar) atmosphere having an oxygen concentration of 0.1% for 1 hour. The heating temperature at this time was 400 degreeC. Each molded body was separately heated even in an oxygen-containing atmosphere in which the oxygen concentration in the atmosphere was 0%, 0.01%, or 20%. Thus, various molded bodies (dust cores) after the heat treatment were obtained. The oxygen concentration “%” shown here was determined by measuring with an oxygen concentration meter at room temperature and 1 atm.

<Measurement / Observation>
(1) Specific resistance and magnetic flux density About each molded object before and behind heat processing, each specific resistance ((rho)) and magnetic flux density _B10k were calculated _| required. The specific resistance was calculated from the electrical resistance measured by a four-terminal method using a digital multimeter (manufacturer: ADC Corporation, model number: R6581) and the volume obtained by actually measuring each sample.

The magnetic flux density B _10k was measured with a DC self-recording magnetometer (manufacturer: Toei Kogyo, model number: MODEL-TRF). The magnetic flux density B _10k is a magnetic flux density generated when the magnetic field strength is 10 kA / m.

(2) X-ray diffraction Each compact before and after heat treatment was cut, and the grain boundaries of Fe particles were analyzed by X-ray diffraction (XRD). XRD is performed using an X-ray diffractometer (D8 ADVANCE: Bruker AXS Co., Ltd.), tube: Fe-Kα, 2θ: 30 to 50 deg, measurement conditions: 0.021 deg / step, 9 step / sec. went.

<Evaluation>
(1) Magnetic flux density and specific resistance The molded body before heat treatment according to Sample 1 was ρ = 150 μΩm and B _10k = 1.55T. When this molded body was heat-treated in an inert gas containing 0.1% oxygen, ρ = 90 μΩm and B _10k = 1.7T were obtained. From this, it was found that by implementing the manufacturing method described above, it is possible to improve the magnetic flux density while suppressing a reduction in specific resistance (while ensuring heat resistance).

  By the way, when a molded body obtained by press-molding a magnetic core powder composed of soft magnetic particles previously coated with ferrite at 400 ° C. is annealed at a specific resistance of ρ = 300 μΩm (before annealing) to ρ = 5 μΩm (after annealing). It dropped sharply. In addition, the powder for magnetic cores used by the comparative example is manufactured based on Unexamined-Japanese-Patent No. 2013-191839. Other manufacturing conditions were the same as those of Sample 1.

(2) Heat treatment atmosphere Grain boundaries of various molded bodies (sample 1) subjected to the heat treatment step in various atmospheres having different oxygen concentrations were observed by XRD. As a result, the grain boundary of the dust core heat-treated in an inert gas (Ar) having an oxygen concentration of 0.1% was almost only S-type ferrite (see FIG. 1).

S-type ferrite was generated even when the oxygen concentration was other than that, but when the oxygen concentration was too low at 0% (Ar only) or 0.01%, an intermetallic compound (MnZn) was also detected. On the other hand, when the oxygen concentration was excessive at 20%, Fe ₂ O ₃ was also detected in addition to S-type ferrite. This is considered to be because the Fe particles as the base particles were oxidized.

  When the powder magnetic core according to Sample 2 was observed in the same manner, when the oxygen concentration in the heat treatment atmosphere was 0% (Ar only) or 0.01%, no intermetallic compound (MnZn) was detected at the grain boundary, Grain boundaries consisted of only S-type ferrite.

(3) Granulation step The granulation step according to Sample 1 was performed using a general ball mill (AV-1 manufactured by Asahi Rika Seisakusho Co., Ltd.) instead of the planetary ball mill. The processing condition at this time is a rotational speed of 150 rpm, and the index value of the applied energy is 0.1 J / s · g.

  Using the magnetic core powder thus obtained, the above-described forming step and heat treatment step were performed. FIG. 2 shows the result of XRD analysis of the molded body before and after the heat treatment. As can be seen from FIG. 2, when the heat treatment is performed at 400 ° C., it is difficult to generate ferrite if the energy applied to each particle during the granulation step is small.

(4) Base particles The Fe powder according to Sample 1 was replaced with ZrO ₂ balls (particle diameter: 3 mm), and the above-described granulation step, molding step, and heat treatment step were performed. FIG. 3 shows the result of analyzing the molded body before and after the heat treatment by XRD. As can be seen from FIG. 3, it was found that when the base particle is ZrO ₂ , ferrite is not generated on the particle surface even if the base particle is treated in the same manner as the Fe particle.

Claims (7)

The surface of soft magnetic particles made of an iron alloy containing 90 mass% or more of pure iron or Fe is composed of metal element (M) oxide particles and iron oxide particles that are one or more of Mn, Zn, Ni, Cu, or Mg. A molding step of pressure-molding the magnetic core powder comprising the coated particles,
A heat treatment step of heating the molded body obtained after the molding step at 370 to 700 ° C,
A method for producing a dust core, wherein a dust core in which spinel ferrite (MFe ₂ O ₄ ) is generated between the soft magnetic particles is obtained.   The magnetic core powder is composed of the soft magnetic powder composed of the soft magnetic particles, the oxide powder composed of the oxide particles, and the iron oxide powder composed of the iron oxide particles. The strong energy satisfying a collision energy of 1 to 5 J / s · g. The manufacturing method of the powder magnetic core of Claim 1 obtained through the granulation process of stirring and mixing below. The magnetic core powder is 100% by mass as a whole,
The oxide particles: 0.04 to 0.8% by mass,
Iron oxide particles: 0.2 to 4.0% by mass,
The method for producing a dust core according to claim 1, wherein the soft magnetic particles are the balance. The soft magnetic particles have a particle size of 50 to 500 μm,
Particle size of the oxide particles: 5 μm or less,
The particle size of the iron oxide particles: 5 µm or less. The method for producing a dust core according to any one of claims 1 to 3.   The method of manufacturing a dust core according to claim 1, wherein the heat treatment step is performed in an inert gas atmosphere having an oxygen concentration of 10% or less. The oxide particles include MnO, MnO ₂ or Mn ₂ O ₃ particles,
The method for producing a dust core according to claim 5, wherein the oxygen concentration in the inert gas atmosphere is 0.1% or less.   A powder magnetic core obtained by the production method according to claim 1.