JP7016713B2 - Powder magnetic core and powder for magnetic core - Google Patents

Description

The present invention relates to a dust core or the like made of insulated soft magnetic particles.

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

The magnetic core is required to have not only high magnetic properties in an alternating magnetic field but also 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. The iron loss includes eddy current loss, hysteresis loss, and residual loss. Among them, it is important to reduce the eddy current loss that increases in proportion to the square of the frequency of the alternating magnetic field.

As such a magnetic core, a powder magnetic core composed of insulatingly coated soft magnetic particles (each particle of a magnetic core powder) is used. The dust core has a small eddy current loss and a high degree of freedom in shape, and is therefore used in various electromagnetic devices. However, if the insulating layer between adjacent soft magnetic particles (grain boundaries) is made of a non-magnetic material, the magnetic characteristics (magnetic flux density, magnetic permeability, etc.) of the dust core may decrease by the amount of the insulating layer. Therefore, a dust core having a spinel-type ferrite as an insulating layer as an insulating layer has been proposed, and there is a description related to the following patent documents.

Japanese Unexamined Patent Application Publication No. 2003-1518113 Japanese Unexamined Patent Publication No. 2014-183199

When spinel-type ferrite (MFe ₂ O ₄ / M: a metal element that becomes a divalent cation) is used as an insulating layer as in Patent Documents 1 and 2, a dust core having a high magnetic flux density can be obtained. The resistivity was not always high enough.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a new dust core or the like that can secure not only a high magnetic flux density but also a high specific resistance.

As a result of diligent research to solve this problem, the present inventor has made a dust core capable of achieving both high magnetic flux density and high resistivity at a higher level by providing an insulating layer from garnet-type ferrite at the grain boundaries of soft magnetic particles. Succeeded in getting. By developing this result, the present invention described below was completed.

《Powdered magnetic core》
(1) The dust core of the present invention is a powder magnetic core composed of soft magnetic particles made of pure iron or an iron alloy and a grain boundary layer between adjacent soft magnetic particles, and the grain boundary layer is a dust core. It has an insulating layer having a garnet-type ferrite crystal structure.

(2) According to the dust core of the present invention, it is possible to achieve both high magnetic flux density and high resistivity at a much higher dimension than before. The present invention can be said to be epoch-making for the following reasons.

Both spinel-type ferrite and garnet-type ferrite are a type of iron oxide, and their magnetic and electrical characteristics are known as physical property values. According to it, the garnet type ferrite is significantly inferior in magnetic properties to the spinel type ferrite. For example, the saturation magnetization of spinel-type ferrite is (Mn, Zn) Fe ₂ O ₄ : 80 emu / g, Fe ₃ O ₄ : 92 emu / g, while the saturation magnetization of garnet-type ferrite is Y ₃ Fe ₅ O ₁₂ . : 27 emu / g. That is, the garnet-type ferrite has a saturation magnetization of about 1/3 to 1/4 that of the spinel-type ferrite, and the magnetic characteristics are significantly inferior. For this reason, spinel-type ferrite has been widely used when insulating soft magnetic particles with a magnetic material, but garnet-type ferrite has never been used.

However, the present inventor has produced a dust core using garnet-type ferrite as an insulating material, and its magnetic characteristics (for example, magnetic flux density obtained in a predetermined applied magnetic field / simply referred to as "magnetic flux density") and electrical characteristics. (For example, volume specific resistance / also simply referred to as "specific resistance") was measured, and an unexpected result was obtained. That is, the magnetic flux density was as high as the powder magnetic core using the spinel type ferrite, and the specific resistance was much higher than that of the powder magnetic core using the spinel type ferrite. The reason why the results contrary to the conventional wisdom of technology were obtained is not clear at present, but the discovery is epoch-making.

《Powder for magnetic core》
The present invention can be grasped not only as a powder magnetic core but also as a powder for a magnetic core as a raw material thereof. That is, the present invention comprises particles for a magnetic core having soft magnetic particles made of pure iron or an iron alloy and an insulating material for coating the soft magnetic particles, and the insulating material has a magnetic core having a garnet-type ferrite crystal structure. It may be a powder for use.

"Production method"
(1) The present invention can also be grasped as the above-mentioned method for manufacturing a dust core. That is, the present invention may be a method for producing a dust core including a molding step of pressure-molding a magnetic core powder made of soft magnetic particles coated with an insulating material having a garnet-type ferrite crystal structure into a desired shape. An annealing step (for example, 400 to 750 ° C. in a non-oxidizing atmosphere) of annealing the molded body after the molding step in order to remove the processing strain introduced into the soft magnetic particles and reduce the hysteresis loss of the dust core. (Heating in) may be done.

(2) The present invention can also be grasped as the above-mentioned method for producing a magnetic core powder. That is, the present invention may be a method for producing a powder for a magnetic core, which comprises a coating step of coating the surface of soft magnetic particles with an insulating material having a garnet-type ferrite crystal structure.

"others"
(1) The garnet-type ferrite is basically an oxide of a rare earth element (R) and Fe, and is represented by a general formula of R ₃ Fe ₅ O ₁₂ . R is usually one or more of Y, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Er, Yb or Lu. A typical garnet-type ferrite is Y ₃ Fe ₅ O ₁₂ .

The insulating layer / insulating material referred to in the present invention may consist of only the garnet-type ferrite represented by R ₃ Fe ₅ O ₁₂ , but a part of R may be another element as long as the crystal structure is maintained. It may be substituted with (particularly a metal element) or a mixture thereof. Examples of the substitution element of R include Ca, Nd, Pr and the like (also simply referred to as "R'"). More specifically, for example, the insulating layer / insulating material according to the present invention may consist of only the representative example Y ₃ Fe ₅ O ₁₂ , or Y of Y ₃ Fe ₅ O ₁₂ is Ca, Nd. Alternatively, it may be _R'3 Fe ₅ O ₁₂ substituted with a substitution element (R') consisting of one or more of Pr, or a mixture of Y ₃ Fe ₅ O ₁₂ and _R'3 Fe ₅ O ₁₂ .

The grain boundary layer of the dust core may be made of only the above-mentioned insulating material, or may contain other kinds of insulating materials, binders, and the like. Further, in the present specification, regardless of whether the rare earth element constituting the garnet-type ferrite is a single species or a plurality of species, it is abbreviated as "R" or "R'" for convenience. When there are a plurality of rare earth elements, the composition ratio (atomic ratio) of the garnet-type ferrite crystal structure depends on the total amount of each rare earth element contained.

(2) Unless otherwise specified, "x to y" in the present specification includes a lower limit value x and an upper limit value y. A range such as "a to b" may be newly established with any numerical value included in the various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is a graph which shows the relationship between the density and the magnetic flux density (B _5k ) which concerns on each sample (the dust core). It is a graph which shows the relationship between the density and the resistivity about each sample. It is a graph which shows the relationship between the specific resistance and the magnetic flux density (B _5k ) which concerns on each sample. It is an SEM image which observed the vicinity of the grain boundary of a dust core.

One or more components arbitrarily selected from the present specification may be added to the components of the present invention described above. The contents described in the present specification may appropriately apply not only to the powder magnetic core and the powder for magnetic core of the present invention, but also to the method for producing them. Content about methods can also be a component of things. Which embodiment is the best depends on the target, required performance, and the like.

<< Soft magnetic particles (soft magnetic powder) >>
The soft magnetic particles according to the present invention are made of pure iron or an iron alloy, and it is preferable to use pure iron powder or iron alloy powder as the soft magnetic powder. When pure iron powder is used, the magnetic characteristics (magnetic flux density) of the dust core can be improved. When, for example, an iron alloy (Fe-Si alloy, Fe-Al-Si alloy) powder containing Si or Al is used as the iron alloy powder, the specific resistance of the dust core can be improved (reduction of eddy current loss). .. The iron alloy powder may contain Ni and / or Co, which are elements of the same family (genus 8) as Fe. Further, Fe-49Co-2V (permendur) powder, sendust (Fe-9Si-6Al) powder and the like may be used. The soft magnetic powder may be a mixture of two or more kinds of powder, and may be, for example, a mixed powder of pure iron powder and iron alloy powder.

The particle size of the soft magnetic particles can be adjusted according to the specifications of the dust core, but the particle size of the soft magnetic powder is preferably 50 to 500 μm, 75 to 250 μm, and more preferably 106 to 212 μm. If the particle size is too large, the density of the dust core tends to be low and the eddy current loss tends to increase, and if the particle size is too small, the magnetic flux density of the dust core tends to decrease and the hysteresis loss tends to increase.

The "particle size" referred to in the present specification is an index of the size of soft magnetic particles and is specified by sieving. Specifically, the median value [(d1 + d2) / 2] of the upper limit value (d1) and the lower limit value (d2) of the mesh size used for sieving is defined as the particle size (D). In addition, it is displayed in μm units, and the numbers after the decimal point are rounded off.

The method for producing the soft magnetic powder is not limited, and examples thereof include an atomizing method, a mechanical pulverization method, and a reduction method. The atomizing powder may be any of water atomizing powder, gas atomizing powder, and gas water atomizing powder. The atomized powder having substantially spherical particles does not easily break the insulating layer (material) during molding of the dust core, and contributes to increasing the specific resistance of the dust core.

《Insulation layer / insulation material》
The insulating layer / insulating material according to the present invention contains at least a part of a substance having the same crystal structure as the garnet-type ferrite. The insulating layer constituting the grain boundary layer of the dust core is preferably in the form of a film covering the surface of the soft magnetic particles. The insulating material constituting the coating layer of the magnetic core powder may be a continuous film or granular. The insulating material made of garnet-type ferrite can be produced, for example, by a solid-phase reaction method or a coprecipitation method.

The magnetic core powder is obtained, for example, by adhering nanoparticles made of garnet-type ferrite to the surface of soft magnetic particles. The nanoparticles are particles having a particle size of less than 1 μm, and are preferably particles having a particle size of, for example, 25 to 500 nm, 50 to 350 nm, and more preferably 100 to 200 nm. The particle size here means the maximum length (diameter) of each particle. The particle size of the plurality of nanoparticles shall be the arithmetic mean value of the maximum length of each particle. The particle size is specified by observing the nanoparticles with an electron microscope (SEM, TEM, etc.), and the average value of the particle size is obtained by image analysis of the obtained observation image with the attached software. Desired.

"Production method"
(1) Magnetic core powder The magnetic core powder is obtained, for example, through a coating step of coating the surface of each soft magnetic particle with an insulating material. There are various methods for the coating process. For example, a powder for a magnetic core can be produced by a powder mixing method in which a soft magnetic powder and a garnet type ferrite powder (particularly nanopowder) are mixed by a ball mill or the like. A garnet-type ferrite film can also be obtained by heat-treating the garnet ferrite precursor coated by the chemical reaction method at an arbitrary temperature. For example, an aqueous solution method in which a soft magnetic powder is immersed in a reaction solution (product liquid) (Reference: Japanese Patent Application Laid-Open No. 2013-191839), and a spraying method in which a reaction solution is sprayed on a soft magnetic powder (Reference: Japanese Patent Application Laid-Open No. 2014-183199). No.), a one-component method using a reaction solution containing urea (Reference: JP-A-2016-127042) and the like can be adopted.

(2) Powder magnetic core The powder magnetic core is obtained through a molding step of pressure-molding a powder for a magnetic core. The molding step may be cold molding or warm molding. The forming pressure depends on the desired density of the dust core (and thus the magnetic flux density), but may be, for example, 800 to 1600 MPa or even 1000 to 1500 MPa. When the mold lubrication warm high-pressure molding method (reference documents: Japanese Patent No. 3309970, Japanese Patent No. 4024705) is used, molding can be performed at ultra-high pressure without using an internal lubricant, and high-density and high-strength pressure can be obtained. A powder core can be obtained.

A compact obtained by further subjecting an annealing step to a molded body obtained in the molding step may be used as a dust core. In the annealing step, for example, the molded product may be heated at 400 to 750 ° C. and further at 500 to 650 ° C. in a non-oxidizing atmosphere. The annealing step removes the strain introduced into the soft magnetic particles in the molding step and reduces the hysteresis loss due to the strain. The non-oxidizing atmosphere is an inert gas atmosphere, a nitrogen gas atmosphere, a vacuum atmosphere, or the like.

《Powdered magnetic core》
The dust core preferably has a specific resistance of 500 μΩm or more, 750 μΩm or more, and more preferably 1000 μΩm or more. The powder magnetic core of the present invention has a significantly increased specific resistance with respect to the conventional powder magnetic core whose insulating layer is made of spinel-type ferrite.

Further, the dust core of the present invention secures a magnetic flux density equivalent to that of the conventional dust core made of spinel-type ferrite. For example, when a magnetic field of 5 kA / m is applied, the dust core of the present invention exhibits a high magnetic flux density (B _5K ) of 1.45 T or more, 1.48 T or more, 1.5 T or more, and even 1.51 T or more. Can be.

The dust core of the present invention has a very thin insulating layer and a high density. The (bulk) density is, for example, 7.65 to 7.8 g / cm ² and further 7.7 to 7.7.78 g / cm ² .

The dust core can be used in electromagnetic devices such as motors, actuators, transformers, induction heaters (IHs), speakers, and reactors. In particular, it is preferably used for an iron core constituting an armature (rotor or stator) of an electric machine or a generator.

Multiple powder magnetic cores with different grain boundary layers were produced, their characteristics were measured, and the structure of the grain boundary layer was also observed. The present invention will be described more specifically based on such examples.

<< Manufacturing of powder for magnetic core >>
(1) Soft magnetic powder (raw material powder)
Gas atomized powder made of pure iron was used as the soft magnetic powder. The particle size was 212 to 106 μm → 159 μm (median). The particle size was specified by the method described above.

(2) Coating step As a garnet-type ferrite powder, a commercially available _{Y3 Fe 5 O 12} powder (manufactured by Sigma-Aldrich) was prepared. The Y ₃ Fe ₅ O ₁₂ powder is composed of nanoparticles having a particle size of 100 to 200 nm. This particle size was also specified by the method described above.

The mixed powder of the soft magnetic powder and the Y ₃ Fe ₅ O ₁₂ powder was mixed in a rotary ball mill for 100 hours. The mixing ratio (weight ratio) of both powders was adjusted in the range of Y ₃ Fe ₅ O ₁₂ powder: 0.1 to 3 g with respect to 100 g of soft magnetic powder. In this way, a plurality of magnetic core powders having the surface of the soft magnetic particles coated with _{Y3 Fe 5 O 12} nanoparticles were obtained.

(3) Comparative Example 1
As Comparative Example 1, a magnetic core in which the above-mentioned garnet-type ferrite powder is replaced with a commercially available spinel-type ferrite powder (Mn _0.5 Zn _0.5 Fe _{2 O4} powder / particle size 1 μm or less / manufactured by Sigma-Aldrich). We also prepared multiple powders for use. The soft magnetic powder used and the coating process (mixing ratio and mixing method) were the same as those described above.

(4) Comparative Example 2
We also prepared a plurality of powders for magnetic cores in which the surface of each particle of the above-mentioned soft magnetic powder was coated with phosphate, which is a non-magnetic material. Specifically, first, a phosphate solution was prepared by dissolving a raw material weighed so that the molar ratio was SrO: H ₃ BO ₃ : H ₃ PO ₄ = 1.5: 1: 4 in ion-exchanged water. Prepared. Next, the phosphate solution was put into a beaker containing 100 g of soft magnetic powder and stirred. The mixture was placed in a drying oven in an air atmosphere and heated at 523 K (about 250 ° C.) for 30 minutes. The dried coated powder obtained after volatilization of the solution was used as the powder for the magnetic core. In addition, in the production of the powder for the magnetic core of Comparative Example 2, the description of Japanese Patent No. 4060101 was referred to.

《Manufacturing of dust core》
Various powders for magnetic cores were molded at 12 to 16 t / cm ² (1176 to 1598 MPa) by a mold lubrication warm high-pressure molding method (reference documents: Japanese Patent No. 3309970, Japanese Patent No. 4024705) (molding step). In this way, a powder magnetic core (sample) having a ring shape (40 × 30 × 4 mm) was obtained.

"measurement"
(1) Specific resistance (electrical characteristics)
The specific resistance of each dust core was measured by a 4-terminal method (JIS K7194) using a digital multimeter (R6581 manufactured by ADC Co., Ltd.).

(2) Magnetic flux density (magnetic characteristics)
The magnetic flux density B _5k of each dust core was measured by a DC self-recording magnetic flux meter (manufacturer: Toei Kogyo, model number: MODEL-TRF). The magnetic flux density B _5k is the magnetic flux density generated in the dust core when a magnetic field of 5 kA / m is applied.

(3) Density The density (bulk density) of each dust core was determined based on the measured mass and dimensions (volume). The true density of the soft magnetic particles (pure iron particles) is 7.87 g / cm ³ .

Based on the characteristic values of each dust core thus obtained, the relationship between the density and the magnetic flux density B _5k is shown in FIG. 1A, the relationship between the density and the specific resistance is shown in FIG. 1B, and the relationship between the specific resistance and the magnetic flux density B _5k is shown. 2 is shown respectively.

"observation"
The vicinity of the grain boundary layer of the dust core using Y ₃ Fe ₅ O ₁₂ powder was observed with a scanning electron microscope (SEM) and analyzed by energy dispersive X-ray spectroscopy (EDX). The SEM image and the element mapping image obtained by this are shown together in FIG.

"evaluation"
(1) Grain boundary layer First, as is clear from FIG. 3, an ultrathin insulating layer is formed on the surface (grain boundary) of the soft magnetic particles, and the insulating layer is Y-Fe-O (garnet type ferrite). It turns out that it consists of.

(2) Magnetic flux density As is clear from FIG. 1A, first, in any insulating layer, the magnetic flux density (B _5k ) tends to increase as the density increases. However, when the insulating layer is made of ferrite, which is a magnetic material, the magnetic flux density is significantly increased as compared with the case where the insulating layer is made of phosphate, which is a non-magnetic material.

It was also found that when the insulating layer is made of garnet-type ferrite, a high magnetic flux density equivalent to that when the insulating layer is made of spinel-type ferrite can be obtained.

(3) Specific resistance As is clear from FIG. 1B, first, in any insulating layer, the specific resistance tends to decrease as the density increases. However, the resistivity of the dust core whose insulating layer is made of garnet-type ferrite is higher than the specific resistance of the powder core whose insulating layer is made of spinel-type ferrite or phosphate in the entire density range of the powder core. It turned out to be just an order of magnitude larger.

(4) Specific resistance and magnetic flux density As is clear from FIG. 2, the powder magnetic core in which the insulating layer is made of garnet-type ferrite is more than the powder magnetic core in which the insulating layer is made of spinel-type ferrite or phosphate. It was found that high resistivity and high magnetic flux density are compatible at a very high level over the density range.

Claims (9)

A dust core composed of soft magnetic particles made of pure iron or an iron alloy and a grain boundary layer between adjacent soft magnetic particles.
The grain boundary layer has an insulating layer containing nanoparticles having a garnet-type ferrite crystal structure.
The particle size of the nanoparticles is smaller than that of the soft magnetic particles, and is 25 to 500 nm . The dust core according to claim 1, wherein the nanoparticles are composed of R ₃ Fe ₅ O ₁₂ (R: rare earth element). The dust core according to claim 2, wherein R is one or more of Y, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Er, Yb or Lu. The insulating layer comprises claim 2 or _{3 comprising R'3 Fe 5 O 12 in which R of the R 3 Fe 5 O 12 is substituted with} a substituent (R') consisting of one or more of Ca, Nd or Pr. The described dust core. The dust core according to any one of claims 1 to 4, wherein the insulating layer includes Y ₃ Fe ₅ O ₁₂ . The magnetic flux density (B _5K ) generated when a magnetic field of 5 kA / m is applied is 1.45 T or more.
The dust core according to any one of claims 1 to 5, wherein the specific resistance is 500 μΩm or more. It is composed of particles for a magnetic core having soft magnetic particles made of pure iron or an iron alloy and an insulating material for coating the soft magnetic particles.
The insulating material contains nanoparticles having a garnet-type ferrite crystal structure.
The particle size of the nanoparticles is smaller than that of the soft magnetic particles and is 25 to 500 nm, which is a powder for a magnetic core. The powder for a magnetic core according to claim 7, wherein the soft magnetic particles have a particle size of 50 to 500 μm . The powder for a magnetic core according to claim 7 or 8, wherein the insulating material contains Y ₃ Fe ₅ O ₁₂ .

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