JP2012230948A - Powder for magnetic core, dust core, and method of manufacturing the same - Google Patents

Abstract

A dust core having excellent specific resistance and strength is provided.
The present invention provides a dust core comprising soft magnetic particles and a grain boundary phase formed between the soft magnetic particles, the grain boundary phase being lower than the annealing temperature of the soft magnetic particles. In a matrix made of a low temperature softening material (low melting glass particles) made of a first inorganic oxide having a softening point, a high temperature softening material made of a second inorganic oxide having a softening point higher than the annealing temperature (of silica or alumina). It is a composite dispersed structure in which fine particles of nanoparticles are dispersed. When the grain boundary phase is composed of such a composite dispersed structure, each soft magnetic particle is firmly bonded by the low-temperature softening material, and a predetermined interval is maintained by the high-temperature softening material to ensure insulation. In this way, the dust core of the present invention in which high resistivity and high strength are compatible in a high dimension was obtained.
[Selection] Figure 1A

Description

  The present invention relates to a dust core excellent in volume resistivity (hereinafter simply referred to as “resistivity”) and strength, a magnetic core powder used in the production thereof, and a production method.

  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. . Iron loss includes eddy current loss, hysteresis loss, and residual loss. It is particularly important to reduce eddy current loss that increases with the frequency of the alternating magnetic field.

  Therefore, development and research of a powder magnetic core obtained by press-molding soft magnetic particles (magnetic particles for magnetic core) that have been coated with insulation have been actively conducted. Such a powder magnetic core has a high specific resistance and a low iron loss due to the presence of the insulating coating, and has a high degree of freedom in shape and is easily compatible with various electromagnetic devices. Most recently, in order to expand applications of dust cores, not only specific resistance but also strength and heat resistance are emphasized. The proposal regarding such a powder magnetic core exists in the following patent documents, for example.

JP 2004-143554 A

  Patent Document 1 proposes a powder magnetic core formed by press-molding soft magnetic particles having a coating made of silicone resin, glass and alumina formed on the surface thereof. Since this powder magnetic core uses a silicone resin as a binder, the specific resistance can be drastically reduced when annealed at a high temperature. Incidentally, this Patent Document 1 does not describe the form (particle size or the like) of the alumina at all.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a dust core that can stably exhibit high specific resistance and high strength up to a high temperature range, unlike a conventional dust core. And Moreover, the powder for magnetic cores suitable for manufacture of the powder magnetic core, and those manufacturing methods are also provided.

  As a result of extensive research and trial and error, the present inventor has obtained a grain boundary phase composed of a low-temperature softening material such as low-melting glass and a high-temperature softening material such as fine silica particles from soft magnetic particles. It was newly found out that a dust core exhibiting a very high specific resistance and ring crushing strength can be obtained by forming between the main phases. By developing this result, the present invention described below has been completed.

<Dust core and magnetic core powder>
(1) The powder magnetic core of the present invention is a powder magnetic core comprising a main phase composed of soft magnetic particles and a grain boundary phase formed between the soft magnetic particles. A composite of a low temperature softening material composed of a first inorganic oxide having a softening point lower than the annealing temperature of the soft magnetic particles and a high temperature softening material composed of a second inorganic oxide having a softening point higher than the annealing temperature. It is characterized by that.

(2) The dust core of the present invention has high specific resistance and high strength, and can stably exhibit these characteristics even in a high temperature range. The reason why the present invention exhibits such an excellent effect is not necessarily clear, but at present, it is considered as follows.

  In the dust core of the present invention, the soft magnetic particles as the main phase are held or bonded by the grain boundary phase made of an inorganic oxide, and insulation between the soft magnetic particles is secured. Conversely, in the dust core of the present invention, it is not necessary to use a binder resin or the like that is inferior in heat resistance. Therefore, according to the present invention, a dust core that exhibits high specific resistance and high strength can be obtained even in a high temperature range.

  This point is described in detail as follows. First, a dust core is obtained by pressure-molding a magnetic core powder into a desired shape. At this time, residual strain or the like introduced into the soft magnetic particles is usually removed by heat treatment (annealing) after pressure molding in order to increase the coercive force of the dust core and thus the hysteresis loss.

  During this annealing, the low-temperature softening material having a softening point lower than the annealing temperature softens (and melts) and encapsulates the soft magnetic particles. This low-temperature softening material contributes to the insulation between the soft magnetic particles and also serves as a binder to strengthen the bond between the soft magnetic particles. Further, the softened low-temperature softening material flows into gaps (for example, triple points) between the soft magnetic particles and contributes to more stable holding of the soft magnetic particles. Thus, first, the insulation and bonding of each soft magnetic particle is ensured by the low-temperature softening material.

  However, if the amount of the low-temperature softening material interposed between the soft magnetic particles becomes insufficient, the soft magnetic particles partially come into direct contact (refer to FIG. 1B), and the insulation between the soft magnetic particles is lowered, and consequently The specific resistance of the dust core can be reduced. It is considered that such a state can also occur when the low-temperature softening material softened during annealing flows out between the soft magnetic particles.

  However, in the present invention, not only the low-temperature softening material but also the high-temperature softening material having a higher softening point than the annealing temperature exists between the soft magnetic particles (grain boundaries) (see FIG. 1A). This high-temperature softening material does not soften much during annealing, retains the original form as much as possible, and keeps the distance between the soft magnetic particles (inter-particle distance) above a certain level. As a result, the situation where the soft magnetic particles are in direct contact with each other is greatly reduced, and the insulation between the soft magnetic particles is more reliably ensured.

  Here, it is also known that the surface portion of the high-temperature softened material in contact with the low-temperature softened material is softened or melted together with the low-temperature softened material during high-temperature annealing. In this case, a grain boundary phase in which the high-temperature softening material and the low-temperature softening material are fused at least at the interface portion is formed. Specifically, if the low-temperature softening material is a low-melting glass and the high-temperature softening material is silica, the grain boundary phase is a composite in which a silica-concentrated phase made of silica is dispersed in a matrix made of low-melting glass. A dispersed tissue can be formed. In such a case, the high-temperature softening material is not simply embedded in the low-temperature softening material, but both are fused (solidified after melting) at least at the interface portion, and are integrally bonded. The dust core is considered to exhibit particularly high strength.

  In this way, the low-temperature softening material and high-temperature softening material composed of inorganic oxides act synergistically, and it is considered that a powder magnetic core that exhibits stable high specific resistance and high strength can be obtained up to high temperature range. It is done.

<Magnetic core powder>
The present invention can be understood not only as the above-described powder magnetic core but also as a magnetic core powder suitable for its production. That is, the present invention provides a low-temperature softening material comprising soft magnetic particles, a first inorganic oxide having a softening point lower than the annealing temperature of the soft magnetic particles, and a second inorganic oxide having a softening point higher than the annealing temperature. It may be a powder for a magnetic core used for a powder magnetic core, characterized in that the high-temperature softening material comprising: an adhesion layer formed by adhering to the surface of the soft magnetic particles.

<Production method of dust core>
The present invention can also be grasped as a method for manufacturing a dust core suitable for manufacturing the above-described dust core. That is, the present invention provides a filling step of filling the above-described magnetic core powder into a mold, a molding step of press-molding the magnetic core powder in the mold, and a molded body obtained after the molding step, the low temperature And a annealing step of annealing at a temperature equal to or higher than the softening point of the softening material and lower than the softening point of the high-temperature softening material.

<Others>
(1) The “softening point” as used in the present invention is a temperature at which the viscosity of the heated inorganic oxide becomes 10 ^7.5 dPa · s in the course of temperature increase. For this reason, the softening point referred to in the present invention does not necessarily coincide with the generally referred glass transition point (Tg). Incidentally, the softening point is specified by the viscosity of JIS R3103-1 glass and the fixed point of viscosity-Part 1: Method for measuring softening point.

(2) The “annealing temperature” in the present invention is a heating temperature in an annealing process performed on a molded body of soft magnetic particles for the purpose of removing residual strain and residual stress after pressure molding.

(3) It is preferable that the high-temperature softening material according to the present invention partially softens or melts during annealing to form a grain boundary phase integrated with the low-temperature softening material. However, a dust core in which the high-temperature softening material is not softened or melted is also included in the present invention as long as it can exhibit high specific resistance and high strength.

(4) Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. Moreover, a new numerical value range such as “ab” can be configured by arbitrarily combining various numerical values and numerical values included in the numerical value range described in this specification.

It is explanatory drawing which showed typically the main phase and grain boundary phase which concern on the powder magnetic core of this invention. It is explanatory drawing which showed typically the main phase and grain boundary phase which concern on the conventional dust core. It is the electron micrograph which observed the surface vicinity of the soft-magnetic particle which comprises the powder magnetic core which is one Example. It is a dispersion | distribution figure which shows the relationship between the specific resistance of various powder magnetic cores, and crumbling strength. It is a graph which shows the DTA curve obtained by thermally analyzing the low temperature softening material which concerns on this invention.

  The present invention will be described in more detail with reference to embodiments of the invention. One or two or more configurations arbitrarily selected from the present specification may be added to the configuration of the present invention described above. The contents described in the present specification can be applied not only to the powder magnetic core according to the present invention but also to the magnetic core powder and the manufacturing method used for the production thereof. A configuration related to a manufacturing method can be a configuration related to an object if understood as a product-by-process. Note that which embodiment is the best depends on the target, required performance, and the like.

《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 from the viewpoint of handling, availability, and cost. The iron alloy is preferably a Si-containing iron alloy (Fe—Si alloy). This is because Si increases the electrical resistivity of the soft magnetic particles, improves the specific resistance of the dust core, and reduces eddy current loss.

  The soft magnetic particles preferably contain 0.2 to 4% by mass and further 0.8 to 3.5% by mass of Si when the total is 100% by mass. If there is too little Si, there is no effect, and if there is too much Si, the magnetic properties and moldability of the dust core can be reduced. The iron alloy may further contain Co or Al.

  Further, the soft magnetic powder may be a mixed powder obtained by mixing a plurality of powders. For example, a mixed powder such as pure iron powder and Fe-49Co-2V (permendur) powder, pure iron powder and Fe-3Si powder, Sendust (Fe-9Si-6Al) powder and pure iron powder may be used.

  The optimum particle size of the soft magnetic particles varies depending on the type of the dust core to be used. Usually, the particle diameter of the soft magnetic particles is preferably 5 to 500 μm, 20 to 300 μm, and more preferably 40 to 200 μm. If the particle size is too large, it is difficult to increase the density and reduce the eddy current loss. If the particle size is too small, it is difficult to reduce the hysteresis loss. The classification of soft magnetic particles can be easily performed by a sieving method or the like. The particle diameter of the soft magnetic particles referred to in this specification is a particle diameter determined when classified by a sieve having a predetermined mesh size.

  Regardless of the method of producing the soft magnetic particles, the soft magnetic particles may be pulverized powder or atomized powder. The atomized powder may be any of water atomized powder, gas atomized powder, and gas water atomized powder. When gas (water) atomized powder is used, destruction of the insulating coating or the like is suppressed, and a dust core having a high specific resistance is easily obtained stably. This is because each constituent particle of the gas (water) atomized powder has a pseudo-spherical shape, and the aggressiveness between the particles is low.

<Low temperature softener>
The low-temperature softening material is made of a (first) inorganic oxide having a softening point lower than the annealing temperature (annealing temperature of soft magnetic particles) of a molded body obtained by pressure-molding the magnetic core powder. The inorganic oxide may be of any type as long as it has an annealing temperature or higher and a viscosity of 10 ^7.5 dPa · s or lower.

  As such a low-temperature softening material, there is a so-called low melting point glass used for tableware, tiles and the like. The low-temperature softening material may be lead borosilicate glass, but low-melting glass having a composition system with a small environmental load, for example, low-temperature softening material is borosilicate glass, silicate glass, phosphate glass, It is more preferable to use one or more of borate glass and vanadium oxide glass.

As a main component of the borosilicate glass, for _{example, SiO 2 -B 2 O 3 -Li 2} O, SiO 2 -B 2 O 3 -Na 2 O, and _SiO 2 _-B 2 O 3 -CaO. As a main component of the silicate-based glass, for _{example, SiO 2 -Li 2 O, SiO} 2 -Na 2 O, SiO 2 -CaO, SiO 2 -MgO, and the like SiO 2 _-Al 2 _{O 3.} As the main component of the phosphate type glass, for _{example, P 2 O 5 -Li 2 O} , P 2 O 5 -Na 2 O, P 2 O 5 -CaO, P 2 O 5 -MgO, P 2 O 5 -Al ₂ O ₃ and the like. As a main component of the borate-based glass, for _{example, B 2 O 3 -Li 2 O} , B 2 O 3 -Na 2 O, B 2 O 3 -CaO, B 2 O 3 -MgO, B 2 O 3 -Al 2 O ₃ etc. Examples of the main component of the vanadium oxide glass include V ₂ O ₅ —B ₂ O ₃ , V ₂ O ₅ —B ₂ O ₃ —SiO ₂ , V ₂ O ₅ —P ₂ O ₅ , V ₂ O ₅ —B. ₂ O ₃ —P ₂ O ₅ and the like.

These low-melting _{glass, SiO 2, Na 2 O,} ZnO, B 2 O 3, Li 2 O, SnO, BaO, CaO, by adjusting the composition such as _Al 2 _{O 3,} suitable for softening in the annealing temperature The temperature can be adjusted.

  The low-temperature softening material (particularly low-melting glass) is 0.3 to 4% by mass, 0.5 to 3.5% by mass, and further preferably 0.00% when the entire magnetic core powder or the entire powder magnetic core is 100% by mass. It is preferable in it being 7-3 mass%. If the low-temperature softening material is too small, the strength of the dust core will not be sufficiently improved, and if it is excessive, the magnetic properties of the dust core may be reduced.

  Note that the low-temperature softening material only needs to form a grain boundary phase after annealing of the powder magnetic core. That is, in the stage of the magnetic core powder or the stage before the annealing of the powder core, the particles attached to the surface of the soft magnetic particles are sufficient. The particles of the low-temperature softening material (particularly low-melting glass particles) preferably have a particle size smaller than that of the soft magnetic particles, for example, about 0.5 to 90 μm and further about 1 to 50 μm. If the particle size is too small, it becomes difficult to produce and handle, and if the particle size is too large, it is difficult to form a grain boundary phase that is dense and has excellent adhesion.

  Incidentally, the particle size of the low melting point glass particles can be specified by various methods, but the particle size referred to in the present specification was specified by image analysis by a scanning electron microscope (SEM) or laser diffraction method.

《High temperature softening material》
The high-temperature softening material is made of a (second) inorganic oxide having a softening point higher than the annealing temperature. The inorganic oxide may be of any type as long as the viscosity is higher than 10 ^7.5 dPa · s at the annealing temperature.

  As such a high-temperature softening material, there are high melting point ceramic particles. Specifically, silica particles, alumina particles, titania particles, zirconia particles, and the like. Such particles are preferably fine particles suitable for avoiding direct contact between soft magnetic particles, for example, nanoparticles having an average particle size of 5 to 500 nm, more preferably 10 to 400 nm. If this particle size is too large, the dust core will have a low density, and if the particle size is too small, direct contact between soft magnetic particles deformed by pressure molding cannot be sufficiently avoided.

  The high-temperature softening material is 0.1 to 2.7% by mass, 0.2 to 2.5% by mass, or further 0.4 to 2.10% with respect to 100% by mass of the entire magnetic core powder or the entire powder magnetic core. It is preferable that it is 3 mass%. If the high-temperature softening material is too small, the specific resistance of the dust core is not sufficiently improved, and if it is too large, the strength of the dust core is reduced.

  When the high-temperature softening material is composed of fine particles (or nanoparticles), the grain boundary phase according to the present invention is the particles of the high-temperature softening material (fine particles or particles) having a smaller particle size than the soft magnetic particles in the matrix composed of the low-temperature softening material. It becomes a composite dispersed structure in which (nanoparticles) are dispersed. In this case, the dust core of the present invention is surrounded by the grain boundary phase (or film) in which the surface of the soft magnetic particles as the main phase is distributed in an integrated or discontinuous state of two or more inorganic oxides. (Or covered) (see FIG. 1A). This is different from the conventional dust core in which the surface of the soft magnetic particles is coated with a single continuous insulating layer or the like (see FIG. 1B).

<Manufacture of magnetic core powder>
The magnetic core powder is obtained by attaching a low-temperature softening material and a high-temperature softening material to the surface of the soft magnetic particles (attachment step). The order of attaching the low-temperature softening material and the high-temperature softening material to the surface of the soft magnetic particles is not limited. That is, the high temperature softening material may be attached to the surface of the soft magnetic particles after the soft magnetic particles are attached first, or vice versa, or they may be attached simultaneously.

  This adhesion step may be performed wet or dry. For example, in a dispersion (slurry) in which a low-temperature softening material (particularly a low-melting glass powder) is dispersed in a dispersion medium, the powder of soft magnetic particles to which the high-temperature softening material is attached is stirred and mixed. May be evaporated and dried (wet deposition process). Moreover, you may mix the powder of the soft magnetic particle to which the high temperature softening material was made to adhere, and a low temperature softening material without using a dispersion medium (dry adhesion process). Furthermore, the soft magnetic particles, the low-temperature softening material, and the high-temperature softening material may be mixed at the same time without using a dispersion medium (simultaneous dry adhesion step). If wet, more uniform adhesion is possible, and if dry, the drying step can be omitted, which is efficient.

<Manufacture of dust core>
The dust core of the present invention includes a filling step of filling a mold having a cavity of a desired shape with a magnetic core powder, a molding step of pressing the magnetic core powder into a molded body, and annealing the molded body. Obtained through an annealing step. Here, the forming process and the annealing process will be described.

(1) Molding process The molding pressure applied to the soft magnetic particles in the molding process is not limited, 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, there is 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-fatty acid lubricant is applied to the inner surface and the magnetic core powder is filled into the mold, and a higher fatty acid-based lubrication between the magnetic core powder and the inner surface of the mold. It comprises a molding temperature at which a metal soap film different from the agent is formed and a warm high-pressure molding process in which molding is performed at a molding pressure.

  Here, “warm” means, for example, a molding temperature of 70 ° C. to 200 ° C., further 100 to 180 ° C., taking into consideration the influence on the surface coating (or insulating coating) and alteration of the higher fatty acid lubricant. To do. 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. According to this mold lubrication warm high-pressure molding method, ultra-high pressure molding is possible while prolonging the mold life, and a high-density powder magnetic core can be easily obtained.

(2) Annealing step The annealing step is performed for the purpose of removing residual strain and residual stress in the molded body, thereby reducing the coercive force and hysteresis loss of the dust core. At this time, the low-temperature softening material softens and flows into the gaps between the soft magnetic particles to hold the soft magnetic particles more stably.

  The annealing temperature can be appropriately selected according to the types of the soft magnetic particles, the low-temperature softening material, and the high-temperature softening material. The annealing temperature is usually about 400 to 900 ° C, further about 600 to 780 ° C. The heating time is preferably about 0.1 to 5 hours, more preferably about 0.5 to 2 hours, and the heating atmosphere is preferably an inert atmosphere.

  In the present invention, since the grain boundary phase is composed of an inorganic oxide and does not contain a silicone resin or the like, annealing can be performed at a higher temperature than in the past as long as it does not exceed the softening point of the high-temperature softening material. This high-temperature annealing can further reduce hysteresis loss. In addition, there is no deterioration of the grain boundary phase or reduction of the specific resistance.

<Dust core>
(1) Characteristics As for the density of the dust core, for example, the density ratio (ρ / ρ ₀ ), which is the ratio of the bulk density (ρ) of the dust core to the true density (ρ ₀ ) of the soft magnetic particles, is 83% or more. 84% or more, 85% or more and 86% or more is preferable because high magnetic properties can be obtained.

The specific resistance of the dust core is an eigenvalue for each dust core that does not depend on the shape. The larger the specific resistance, the more the eddy current loss can be reduced. For example, the specific resistance is preferably 10 μΩ · m or more, 10 ² μΩ · m or more, and more preferably 10 ³ μΩ · m or more.

  The higher the strength of the powder magnetic core, the more preferable the use is. For example, the crushing strength, which is a representative index of strength, may be 20 MPa or more, 40 MPa or more, 60 MPa or more, or 80 MPa or more. In the dust core of the present invention, unlike the conventional dust core, the soft magnetic particles are not only mechanically bonded by plastic deformation but also firmly bonded by a low-temperature softening material. For this reason, the dust core of the present invention has higher strength than the conventional dust core.

(2) Applications The dust core of the present invention can be used for various electromagnetic devices such as motors, actuators, transformers, induction heaters (IH), speakers, reactors, etc., regardless of the form. Specifically, it is preferably used for an iron core constituting a field or armature of an electric motor or generator. Among these, the dust core of the present invention is suitable for an iron core for a drive motor that requires low loss and high output (high magnetic flux density). Incidentally, the drive motor is used in automobiles and the like.

The present invention will be described more specifically with reference to examples.
<Production of sample>
(material)
(1) Soft magnetic particles Gas water atomized powder made of an iron alloy containing Si was prepared as soft magnetic particles (raw material powder). The compositions and particle sizes of the prepared soft magnetic particles are shown in Table 1A and Table 2B (both are simply referred to as “Table 1”). The particle sizes listed in Table 1 are obtained by classification using a sieve having a predetermined mesh size. In addition, even in the case of soft magnetic particles described as “˜” in the particle size column in Table 1, it was confirmed by SEM that no soft magnetic particles of less than 5 μm were contained.

(2) Low melting point glass particles (low temperature softening material)
Low melting point glass particles adhered to the surface of the soft magnetic particles were obtained by the following wet pulverization. Glass beads made of various compositions (first inorganic oxide) shown in Table 2 were prepared as raw materials. In addition, the low melting glass particles A, B, D, and E shown in Table 2 are manufactured by Nippon Glaze Co., Ltd., and the low melting glass particles C and F are manufactured by Toago Material Technology Co., Ltd.

  The roughly pulverized glass beads were put into a chamber of a wet pulverizer (Dynomill: manufactured by Shinmaru Enterprise Co., Ltd.) and finely pulverized by operating a propeller for stirring. This finely pulverized product was collected and dried. Thus, powders composed of various low-melting glass particles were obtained. Each particle size of the obtained low melting point glass particles is shown together in Table 2. The particle size was measured by image analysis using a scanning electron microscope (SEM).

(3) Nano particles (high temperature softening material)
Ceramic nanoparticles (high-temperature softening material) to be attached to the surface of the soft magnetic particles were prepared. The types and average particle diameters of the prepared nanoparticles are shown in Table 1. Incidentally, silica nanoparticles, shown in Table 1 are both SiO ₂ in composition, the average particle diameter 15nm those manufactured by Tokuyama Corp. (Part: QS-10) is. Those having an average particle diameter of 300 nm are manufactured by Admatechs Co., Ltd. (product number: SO-E1). The alumina nanoparticles have a composition of Al ₂ O ₃ and are manufactured by Buehler (Micro Polish II).

  Examples of the method for specifying the average particle size of the nanoparticles include a laser diffraction scattering method, a sedimentation method, and an image analysis method. In the present invention, the average particle size of the nanoparticles was specified by the laser diffraction scattering method.

(Manufacture of magnetic core powder)
Low melting glass particles and nanoparticles were adhered to the surface of the soft magnetic particles by the following methods (adhesion layer forming step).

(1) Wet adhesion process First, the chamber containing the above-mentioned soft magnetic particle and nano particle powders is attached to a container rotating and shaking type powder mixer (a rocking mixer manufactured by Aichi Electric Co., Ltd.), and each powder is stirred. Mixed. This mixing was performed for 30 minutes at 80 rpm. In this way, soft magnetic particles (hereinafter referred to as “primary composite particles”) having nanoparticles adhered to the surface were obtained.

  Next, a low-melting-point glass dispersion in which the above-described low-melting-point glass particles were dispersed in a dispersion medium (ethanol) 5 to 20 times as large as the above-mentioned low-melting glass particles was prepared. An ultrasonic stirrer was used for dispersion at this time.

  The powder of the primary composite particles described above was put into this low-melting glass dispersion and stirred with an ultrasonic stirrer until the dispersion medium volatilized. At this time, the temperature of the low-melting glass dispersion was 60 to 70 ° C. Further, the powder after the treatment was placed in a constant temperature bath at 130 ° C. and dried in the air atmosphere for 30 minutes. The powder that had solidified after drying was crushed in a mortar. Thus, a powder (magnetic core powder) composed of composite particles in which nanoparticles and low-melting glass particles were adhered to the surface of soft magnetic particles was obtained.

(2) Dry adhesion process A polycontainer containing the primary composite particles and low melting point glass particles described above was attached to a rotating ball mill (manufactured by Tsutsui Rika Kikai Co., Ltd.) and mixed with stirring. This mixing was performed for 30 minutes at 60 rpm. The powder that had solidified after mixing was crushed in a mortar. Thus, a magnetic core powder comprising composite particles in which nanoparticles and low-melting glass particles were adhered to the surface of soft magnetic particles was obtained.

(3) Simultaneous dry adhesion process A polycontainer in which each powder of soft magnetic particles, nanoparticles, and low-melting-point glass particles was simultaneously placed was attached to the container rotating and oscillating powder mixer and mixed with stirring. This mixing was performed for 30 minutes at 80 rpm. The powder that had solidified after mixing was crushed in a mortar. Thus, a magnetic core powder comprising composite particles in which nanoparticles and low-melting glass particles were adhered to the surface of soft magnetic particles was obtained.

  In either case, the total amount of the magnetic core powder was 100% by mass, and the addition amount (m1) of the low melting point glass particles (low temperature softening material) and the addition amount (m2) of the nanoparticles (high temperature softening material) are shown in Table 1, respectively. . In addition, the “ratio to the total amount of adhered particles” shown in Table 1 is a percentage of the ratio of the added amount of nanoparticles (m2 / (m1 + m2)) to the sum of both added amounts (m1 + m2). In Table 1A, the wet deposition process is represented as “wet”, the dry deposition process as “dry”, and the simultaneous dry deposition process as “simultaneous dry”.

(4) Manufacture of Comparative Sample Magnetic core powder made of composite particles in which only one of low melting point glass particles or nanoparticles was attached to the surface of soft magnetic particles was also manufactured. A sample having only low melting point glass particles was produced in the same manner as in the above-described dry adhesion process. A sample containing only nanoparticles was produced in the same manner as the primary composite particles described above. These samples are collectively shown as sample No. 1 in Table 1B. Shown in C1-C7.

  In addition, magnetic core powders composed of composite particles in which nanoparticles and silicone resin were adhered to the surface of soft magnetic particles were also produced (sample Nos. D1 and D2 shown in Table 3). The adhesion at this time was performed as follows. First, a silicone resin was dissolved in 50 to 80 times its dispersion medium (ethanol) to prepare a coating liquid in which this solution and nanoparticles were mixed. Next, soft magnetic particles were mixed into this coating solution, and stirred with an ultrasonic stirring device until the dispersion medium volatilized. The temperature of the coating liquid at this time was 60 to 70 ° C. Further, the powder after the treatment was put in a thermostatic bath and dried for 30 minutes in an air atmosphere. The temperature of the thermostatic chamber at this time is the sample No. In the case of D1, the sample No. The case of D2 was 100 ° C. The powder that had solidified after drying was crushed in a mortar. In addition, the thermosetting silicone resin (Shin-Etsu Chemical Co., Ltd. KR-242A) was used for the silicone resin. As nanoparticles, silica nanoparticles having an average particle diameter of 50 nm and silica sol (IPA-ST, Nissan Chemical Industries, Ltd., nanoparticle concentration: 30%) were used.

(Manufacture of dust core)
(1) Filling Step and Molding Step A ring-shaped (outside diameter: φ39 mm × inside diameter φ30 mm × thickness 5 mm) shaped body was manufactured by using a mold lubrication warm high pressure molding method using each sample (magnetic core powder). No internal lubricant or resin binder was used at the time of molding. Specifically, the die lubrication warm high pressure molding method was performed 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.

Lithium stearate (1%) dispersed in an aqueous solution was uniformly applied to the inner peripheral surface of the heated mold at a rate of about 10 cm ³ / min with a spray gun. The aqueous solution used here is obtained by adding a surfactant and an antifoaming agent to water. As the surfactant, polyoxyethylene nonylphenyl ether (EO) 6, (EO) 10 and borate ester Emulbon T-80 were used, and each was added by 1% by volume with respect to the entire aqueous solution (100% by volume). did. As the antifoaming agent, FS Antifoam 80 was used and 0.2% by volume was added to the entire aqueous solution (100% by volume).

A lithium stearate having a melting point of about 225 ° C. and a particle size of 20 μm was used. The dispersion amount was 25 g with respect to 100 cm ^{3 of the} aqueous solution. This was further refined with a ball mill type pulverizer (Teflon (registered trademark) coated steel balls: 100 hours), and the obtained stock solution was diluted 20 times to obtain an aqueous solution having a final concentration of 1%, which was used in the coating step. did.

  Each metal core powder was filled in a mold having lithium stearate coated on the inner surface (filling step). With the mold held at 130 ° C., the filled magnetic core powder was warm-pressed under a molding pressure of 1568 MPa (molding process). In this warm high-pressure molding, none of the magnetic core powders was galling with the mold, and the molded body could be taken out from the mold with a low pressure.

  The mold temperature (130 ° C.) described above is the same as the sample No. In the case of D1, the sample No. In the case of D2, the temperature was changed to 70 ° C.

(2) Annealing process Each obtained compact was annealed at a furnace temperature of 750 ° C. for 1 hour in a nitrogen atmosphere with a flow rate of 8 liters / minute. However, sample No. No. 17 was annealed at 900 ° C. Thus, a plurality of dust cores shown in Table 1 were obtained.

<Measurement>
(1) Specific resistance and crumbling strength The crumbling strength and specific resistance were measured using the above ring-shaped dust core. The crushing strength was measured by a method according to JIS Z 2507. The specific resistance was measured by a 4-terminal method using a digital multimeter (manufacturer: ADC, Inc., model number: R6581). Each measurement result was combined with Table 1 and shown. Further, the correlation between the specific resistance of each dust core and the crushing strength is plotted in FIG.

(2) Density The density of the dust core was determined based on the volume determined from the mass and measurement of each sample (test piece).

<< Observation >>
(1) Sample No. shown in Table 1A A reflection electron structure image obtained by observing the grain boundary phase of No. 2 dust core with a scanning electron microscope (SEM) is shown in FIG. In FIG. 2, the white part is soft magnetic particles, the gray part is a low-melting glass phase (low temperature softening material phase), and the black part is a silica (SiO ₂ ) phase (high temperature softening material phase). From FIG. 2, in the powder magnetic core according to the present invention, the grain boundary phase that is in close contact with the surface of the soft magnetic particles and encapsulates the nanoparticles is composed of nanoparticles made of the high-temperature softening material in the matrix made of the low-temperature softening material. It became clear that it became a composite dispersed structure that was integrated and dispersed.

(2) The formation of such a grain boundary phase is about a sample made of only low melting point glass particles (low temperature softening material) and a sample obtained by adding nanoparticles (high temperature softening material) to the low melting point glass particles. This is also supported by the results of the differential thermal analysis (DTA) performed. The DTA curves for both these samples are shown in FIG. The vertical axis of the graph shown in FIG. 4 is a voltage difference corresponding to the temperature difference ΔT between each sample and the reference material.

  The low melting point glass particles used here are borosilicate glass (A) shown in Table 2. The nanoparticles added to the low-melting glass particles were silica particles having an average particle diameter of 15 nm, and the mixing ratio was 30% by mass (corresponding to sample No. 2). The mixed sample was obtained by mixing low melting point glass particles and nanoparticles in an agate mortar. Moreover, DTA was performed in the air | atmosphere atmosphere with the temperature increase rate of 10 degree-C / min using the thermal analyzer (TG8120) by Rigaku Corporation.

  As is clear from FIG. 4, the DTA curve of the sample consisting only of the low melting point glass particles has a stable region near the softening point (590 ° C.), but the DTA curve of the mixed sample (low melting point glass particles + nanoparticles). Did not see such a stable range. This stable region is considered to be caused by an endothermic reaction accompanying softening or melting of the low-melting glass particles. However, in the case of the mixed sample, it is considered that the nanoparticles (silica particles) react with the low-melting glass particles as the temperature rises and gradually dissolve, the endothermic reaction described above becomes unclear, and the stable region does not appear.

  Such a tendency is not limited to the case where the borosilicate glass particles and the silica particles are combined, but is also considered to be the same when the other types of low melting point glass particles and the nanoparticles are combined. This is because the reaction of the nanoparticles due to the melting of the low-melting glass particles occurs when the low-melting glass particles are melted first. Therefore, regardless of the type of low-melting glass particles, low-melting glass particles that are low-temperature softening materials soften or melt before nanoparticles that are high-temperature softening materials, so the endothermic reaction is unclear and stable. This is because it is considered that the area will not appear.

<Evaluation>
(1) From Table 1 and FIG. It was found that all of the dust cores 1 to 18 had high density (6.9 g / cm ³ or more), excellent magnetic properties, and excellent crushing strength and specific resistance. Specifically, the specific resistance is 10 μΩ · m or more, 10 ² μΩ · m or more, further 10 ³ μΩ · m or more, and the crushing strength is as high as 20 to 89 MPa.

  These are the same as the sample No. plotted on FIG. The marks 1 to 18 indicate the sample numbers. It can also be seen from the fact that the marks are shifted to the upper right as a whole rather than the marks C1, C2, C4 to C7, D1 and D2.

  Incidentally, the crushing strength increased as the amount of low melting point glass particles added increased, and the specific resistance increased as the amount of nanoparticles added increased. This tendency was the same even when the particle size of the soft magnetic particles, the type of the low-melting glass particles, the particle size and type of the nanoparticles, the method of the adhesion process, and the like were different.

(2) Sample No. C1 and Sample No. As can be seen from C4 to C7, when the grain boundary phase is composed only of the low-temperature softening material, the crushing strength is high, but the specific resistance is extremely low. This tendency was the same even when the type and amount of the low melting point glass particles were changed.

  Sample No. As can be seen from C2, when the grain boundary phase is composed only of the high-temperature softening material, the specific resistance is increased, but the crushing strength is very low. And sample no. As can be seen from C3, when the high-temperature softening material increased, a normal molded body could not be obtained despite the ultra-high pressure molding.

(3) Sample No. As can be seen from D 1 and D 2, it can be seen that the dust core having a grain boundary phase composed of silicone resin and silica particles has an extremely reduced crushing strength. This is because even if a part of the high-temperature softening material is softened or melted during annealing, it is not integrated with the silicone resin. In addition, each sample according to this example retains a sufficiently high crushing strength as well as a high specific resistance even when annealed at a high temperature, compared to a powder magnetic core containing a silicone resin in the grain boundary phase. In particular, sample no. Although the powder magnetic core of 17 is annealed at a very high temperature (900 ° C.), which is not conventional, it exhibits a large crushing strength with a high specific resistance.

  From the above, in the dust core of the present invention, the grain boundary phase between the soft magnetic particles is composed of the low-temperature softening material and the high-temperature softening material, and they can act synergistically to achieve both high specific resistance and high strength. I understood it. Furthermore, it has also been found that the powder magnetic core of the present invention can stably exhibit its excellent characteristics even in a high temperature environment.

Claims (12)

A main phase composed of soft magnetic particles;
A grain boundary phase formed between the soft magnetic particles;
A dust core consisting of
The grain boundary phase is
A low-temperature softening material composed of a first inorganic oxide having a softening point lower than the annealing temperature of the soft magnetic particles is combined with a high-temperature softening material composed of a second inorganic oxide having a softening point higher than the annealing temperature. A dust core characterized in that   2. The dust core according to claim 1, wherein the grain boundary phase has a composite dispersed structure in which fine particles made of the high-temperature softening material and having a smaller particle diameter than the soft magnetic particles are dispersed in a matrix made of the low-temperature softening material. .   The dust core according to claim 2, wherein the fine particles are nanoparticles having an average particle diameter of 5 to 500 nm.   The dust core according to claim 2 or 3, wherein the fine particles are silica particles or alumina particles.   The dust core according to any one of claims 1 to 4, wherein the low-temperature softening material is low-melting glass.   The dust core according to claim 5, wherein the low melting glass is one or more of borosilicate glass, silicate glass, or phosphate glass.   The dust core according to claim 1, wherein the soft magnetic particles are made of an iron alloy containing pure iron or silicon (Si).   The powder magnetic core according to claim 1, wherein the soft magnetic particles have a particle size of 5 to 500 μm.   The dust core according to claim 1, wherein the high-temperature softening material and the low-temperature softening material are fused at least at an interface portion. The low-temperature softening material is a low-melting glass,
The high temperature softening material is silica,
The powder magnetic core according to claim 9, wherein the grain boundary phase has a composite dispersed structure in which a silica concentrated phase made of silica is dispersed in a matrix made of the low melting point glass. Soft magnetic particles,
A low temperature softening material comprising a first inorganic oxide having a softening point lower than the annealing temperature of the soft magnetic particles and a high temperature softening material comprising a second inorganic oxide having a softening point higher than the annealing temperature An adhesion layer formed on the surface of the particles;
A powder for a magnetic core used for a dust core characterized by comprising: A filling step of filling a mold with the magnetic core powder according to claim 11;
A molding step of pressure-molding the magnetic core powder in the mold;
An annealing step of annealing the molded body obtained after the molding step at a temperature equal to or higher than the softening point of the low-temperature softening material and lower than the softening point of the high-temperature softening material;
A method for producing a powder magnetic core comprising the steps of: