WO2012146967A1

WO2012146967A1 - Magnetic core powder, dust core, and manufacturing method for dust core

Info

Publication number: WO2012146967A1
Application number: PCT/IB2012/000792
Authority: WO
Inventors: Masashi OHTSUBO; Masaaki Tani; Takeshi Hattori; Yusuke Oishi; Daisuke Okamoto
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2011-04-25
Filing date: 2012-04-24
Publication date: 2012-11-01
Also published as: JP2012230948A

Abstract

A dust core includes a primary phase including soft magnetic particles, and a grain boundary phase formed between the soft magnetic particles. The grain boundary phase has a composite dispersed microstructure in which fine particles including a high-temperature softening material (nanoparticles of silica or alumina) are dispersed in a matrix of a low-temperature softening material (low-melting glass particles). The low-temperature material is made of a first inorganic oxide with a softening point lower than the annealing temperature of the soft magnetic particles. The high-temperature softening material is made of a second inorganic oxide with a softening point higher than the annealing temperature.

Description

MAGNETIC CORE POWDER, DUST CORE, AND MANUFACTURING METHOD

FOR DUST CORE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a dust core with superior volume resistivity (hereinafter simply referred to as "resistivity") and strength, a powder for a magnetic core (hereinafter referred to as "magnetic core powder") which is used in manufacture of a dust core, and a manufacturing method for a dust core.

2. Description of Related Art

[0002] There are a number of products in our surroundings which utilize electromagnetism, such as transformers, motors, generators, speakers, induction heaters, and various kinds of actuators. Many of these products use alternating magnetic fields, and normally have a magnetic core (soft magnet) provided within such an alternating magnetic field to efficiently create a large localized alternating magnetic field.

|0003] This magnetic core is required to not only exhibit high magnetic properties within the alternating magnetic field but also to have low high-frequency loss (hereinafter, referred to as "iron loss" regardless of the material of the magnetic core used) when used within the alternating magnetic field. Iron loss includes eddy current loss, hysteresis loss, and residual strain loss. It is particularly important to reduce eddy current loss which increases with increasing frequency of the alternating magnetic field.

[0004] Accordingly, dust cores made by compacting insulation-coated soft magnetic particles (the individual particles of a magnetic core powder) have been actively developed and studied. In addition to high resistivity and low iron loss due to the presence of an insulating coating, such dust cores have high flexibility in shaping, allowing for easy adaptability to a diverse range of electromagnetic apparatus. Recently, to extend the applications of dust cores, a high emphasis is placed on not only high resistivity but also improved strength and heat resistance.

[0005] As an example of such dust cores, Japanese Patent Application Publication No. 2004-143554 (JP 2004-143554 A) discloses a dust core made by compacting soft magnetic particles having a coating made of silicone resin, glass, and alumina formed on the surface. Since this dust core uses silicone resin as a binder, a rapid decrease in resistivity can occur in cases such as when the dust core is annealed at elevated temperatures. In JP 2004- 143554 A, there is no description about the form (particle diameter, etc.) of the alumina used.

SUMMARY OF THE INVENTION

[0006] The invention provides a dust core that can exhibit high resistivity and high strength in a stable manner even in high temperature ranges, unlike the dust core according to the related art mentioned above. The invention also provides a magnetic core powder suited for manufacture of the dust core, and a manufacturing method for a dust core.

[0007] The inventors have conducted intensive studies with a view to addressing the above-mentioned problems and, after much trial and error, have discovered that a dust core that demonstrates very high resistivity and radial crushing strength can be obtained by forming a grain boundary phase including a low-temperature softening material such as a low-melting glass and a high- temperature softening material such as fine silica particles, between primary phases each including soft magnetic particles. These results have been further developed to complete the invention as discussed below.

[000S] A dust core according to a first aspect of the invention includes a primary phase including soft magnetic particles, and a grain boundary phase formed between the soft magnetic particles, in which the grain boundary phase is a composite of a low-temperature softening material including a first inorganic oxide having a softening point lower than an annealing temperature of the soft magnetic particles, and a high-temperature softening material including a second inorganic oxide having a softening point higher than the annealing temperature.

[0009] The dust core according to the above-mentioned aspect has high resistivity and high strength, and can exhibit these properties in a stable manner even in high temperature ranges. Although it is not necessarily clear as to why the

above-mentioned configuration demonstrates such advantageous effects, currently, the reasons are speculated as follows.

[0010] I the dust core configured as mentioned above, the grain boundary phase made of inorganic oxides retains or binds the soft magnetic particles that make up the primary phase, and secures the insulation between the soft magnetic particles.

Conversely, the dust core configured as mentioned above does not need to use binder resin or the like which has poor heat resistance. Therefore, the above-mentioned configuration can provide a dust core that exhibits high resistivity and high strength even in high temperature ranges.

[0011] A more detailed explanation in this regard is given below. First, a dust core is obtained by compacting a magnetic core powder into an intended shape. Since the residual strain or the like introduced into the soft magnetic particles at this time increases the coercivity and therefore hysteresis loss of the dust core, such residual strain is usually relieved by performing heat treatment (annealing) after the compaction.

[0012] At the time of this annealing, the low-temperature softening material with a softening point lower than the annealing temperature softens (and further melts) so as to encapsulate the soft magnetic particles. This low-temperature softening material contributes to the insulation between the soft magnetic particles, and also acts as a binder to provide strong binding between the soft magnetic particles. Further, the

low-temperature softening material that has softened or the like flows into the gap (e.g. triple point) between the soft magnetic particles, for example, thus positively contributing to more stable retention of the soft magnetic particles. In this way, first, insulation and binding between the soft magnetic particles are secured by the low-temperature softening material. [0013) However, when there is not enough amount of low-temperature softening material between the soft magnetic particles, the soft magnetic particles partially come into direct contact with each other (see FIG. 1 B), which reduces the insulation between the soft magnetic particles and, therefore, the resistivity of the obtained dust core. Such a state is also considered to occur when the low-temperature softening material that has softened during annealing flows out from the gap between the soft magnetic particles.

[0014] However, in the case of the above-mentioned configuration, not only the low-temperature softening material but also the high-temperature softening material having a softening point higher than the annealing temperature is also present between the soft magnetic particles (at the grain boundary) (see FIG. 1 A). This high-temperature softening material does not undergo much softening or the like but more or less retains its original form even during annealing, thus keeping a certain distance or more between the soft magnetic particles (inter-particle distance). As a result, direct contact between the soft magnetic particles is significantly reduced, making it possible to secure the insulation between the soft magnetic particles more reliably.

[0015] It is also known that the surface portion of the high-temperature softening material that is in direct contact with the low-temperature softening material softens or melts together with the low- temperature softening material during annealing at elevated temperatures. A grain boundary phase is formed in this case, in which the high-temperature softening material and the low-temperature softening material fuse together at least in the interface portion. Specifically, in a case where the

low-temperature softening material is a low-melting glass, and the high-temperature softening material is a silica, the resulting grain boundary phase can be formed as a composite dispersed microstructure having a silica-rich phase including the silica dispersed in a matrix of the low-melting glass. In such a case, rather than the high-temperature softening material being simply embedded in the low-temperature softening material, the two softening materials fuse (melt and then solidify) and bind together in at least the interface portion. This is considered to be the reason for the particularly high strength exhibited by the dust core according to the above-mentioned aspect

[0016] It is considered that this synergic action of the low-temperature softening material and the high-temperature softening material each made of an inorganic oxide makes it possible to obtain a dust core that demonstrates high resistivity and high strength in a stable manner even in high temperature ranges.

1001 ] That is, in the dust core according to the above-mentioned aspect, the grain boundary phase may have a composite dispersed microstructure in which fine particles including the high-temperature softening material and having a particle diameter smaller than the soft magnetic particles are dispersed in a matrix of the low-temperature softening material.

[0018] Also, in the dust core according to the above-mentioned configuration, the fine particles may be nanoparticles with an average particle diameter of 5 nm to 500 run.

[0019] Also, in the dust core according to the above-mentioned configuration, the fine particles may be silica particles or alumina particles.

[0020] Also, in the dust core according to the above-mentioned aspect, the low-temperature softening material may be a low-melting glass.

[0021] Also, in the dust core according to the above-mentioned configuration, the low-melting glass may be at least one selected from the group consisting of a borosilicate-based glass, a silicate-based glass, and a phosphate-based glass.

[0022] Also, in the dust core according to the above-mentioned aspect, the soft magnetic particles may be made of pure iron or iron alloy containing silicon.

[0023] Also, in the dust core according to the above-mentioned aspect, the soft magnetic particles may have a particle diameter of 5 urn to 500 μηι.

[0024] Also, in the dust core according to the above-mentioned aspect, the high-temperature softening material and the low-temperature softening material may fuse together at least at an interface portion that is between the high-temperature softening material and the low-temperature softening material. (0025] Also, in the dust core according to the above-mentioned configuration, the low-temperature softening material may be a low-melting glass, the nigh-temperature softening material may be a silica, and the grain boundary phase may have a composite dispersed microstructure in which a silica-rich phase including the silica is dispersed in a matrix of the low-melting glass.

[0026] The invention can be grasped not only as the above-mentioned dust core but also as a magnetic core powder suited for manufacture of the dust core. That is, a magnetic core powder according to a second aspect of the invention is a magnetic core powder used in manufacture of a dust core, including a soft magnetic particle, and an adhesion layer on the soft magnetic particle, the adhesion layer having a low-temperature softening material and a high-temperature softening material that are adhered to a surface of the soft magnetic particle, the low-temperature softening material including a first inorganic oxide having a softening point lower than an annealing temperature of the soft magnetic particle, the high-temperature softening material including a second inorganic oxide having a softening point higher than the annealing temperature.

[0027] Further, the invention can be also grasped as a manufacturing method for a dust core suited for manufacture of the above-mentioned dust core. That is, a manufacturing method for a dust core according to a third aspect of the invention includes filling a die with a magnetic core powder that includes a soft magnetic particle and an adhesion layer on the soft magnetic particle, the adhesion layer having a low-temperature softening material and a high-temperature softening material that are adhered to a surface of the soft magnetic particle, the low-temperature softening material including a first inorganic oxide having a softening point lower than an annealing temperature of the soft magnetic particle, the high-temperature softening material including a second inorganic oxide having a softening point higher than the annealing temperature (filling step), compacting the magnetic core powder in the die (compaction step), and annealing a compact obtained after the compacting, wherein the annealing is performed at a temperature equal to or higher than the softening point of the low-temperature softening material and below the softening point of the high-temperature softening material (annealing step).

[0028] The "softening point" as referred to in the invention means the temperature at which the viscosity of a heated inorganic oxide becomes 10⁷⁵ dPa-s during the course of temperature rise. In this sense, the softening point as referred to in the invention does not necessarily coincide with the glass transition point (Tg) which is a commonly used term. This softening point is determined by, for example, the Japan Industrial Standard (J1SR3103-1 Glass viscosity and viscometric fixed points - Part 1 : Method of determining softening point -).

[0029] Also, the "annealing temperature" as referred to in the invention means the heating temperature in the annealing step that is performed on a compact of soft magnetic particles for the purpose of relieving residual strain or residual stress after compaction.

[0030] AJ so, preferably, the high-temperature softening material according to the invention partially softens or melts during annealing to form a boundary phase integrated with the low-temperatUTe softening material. However, dust cores in which the high-temperature softening material has not softened or molten also fall within the scope of the invention as long as the dust cores can demonstrate high resistivity and high strength.

[0031] Unless otherwise specified, the expression "x to y" as used in this specification includes the lower limit x and the upper limit y. Also, various numerical values, and numerical values included in various numerical ranges described in this specification may be combined in an arbitrary manner to define new numerical ranges such as "a to b". BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1A schematically illustrates a primary phase and a grain boundary phase in a dust core according to the invention;

FLO. 1 B schematically illustrates a primary phase and a grain boundary phase in a dust core according to the related art;

FIG 2 is an electron micrograph obtained by observing the vicinity of the surface of a soft magnetic particle that forms a dust core according to an embodiment of the invention;

FIG 3 is a dispersion diagram that illustrates the relationship between the resistivities and radial crushing strengths of various dust cores; and

FIG 4 is a graph that illustrates a DTA curve obtained by thermal analysis of each low-temperature softening material according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[00331 The invention is described in more detail by way of its embodiments. One or two or more configurations arbitrarily selected from within this specification can be added to the configuration of the invention as mentioned above. The description of this specification is applicable to not only the dust core according to the invention but also a magnetic core powder and a manufacturing method that are used for manufacture of the dust core. A configuration related to the manufacturing method can be also interpreted as a configuration related to a product if understood as a product by process. Which one of the embodiments is best depends on the object, required performance, and so on.

(Soft magnetic particles (soft magnetic powder))

[0034] While any ferromagnetic element such as Group-VlII transition elements (Fe, Co, Ni, etc.) suffices as a main component of the soft magnetic particles, the soft magnetic particles are preferably made of pure iron or iron alloy from the viewpoints of ease of handling, availability, cost, and so on. Among iron alloys, a Si-containing iron alloy (Fe-Si alloy) is preferable. This is because Si increases the electric resistivity of the soft magnetic particles, improves the resistivity of the dust core, and reduces eddy current loss. (0035) Preferably, the soft magnetic particle contains 0.2 mass% to 4 mass%, or more preferably 0.8 mass% to 3.5 mass% of Si when the entire soft magnetic particle is defined as 100 mass %. An exceedingly low content of Si does not provide the intended effects, whereas an exceedingly high content of Si can reduce the magnetic properties or compactability of the dust core. Other than Si, the iron alloy may also contain Co or Al.

[0036] Further, the soft magnetic powder may be a powder mixture of a plurality of powders. For example, the soft magnetic powder may be a powder mixture of pure iron powder and Fe-49Co-2V (permendur) powder, of pure iron powder and Fe-3Si powder, or of sendust (Fe-9Si-6Al) powder and pure iron powder.

[0037] The optimal particle diameter of the soft magnetic particles depends on the kind of the target dust core. Normally, the soft magnetic particles preferably have a particle diameter of 5 μπι to 500 μπτ, more preferably 20 um to 300 μτη, or still more preferably 40 ujn to 200 μτη. An exceedingly large particle diameter makes it difficult to achieve increased density and reduced eddy current loss, whereas an exceedingly small particle diameter makes it difficult to achieve reduced hysteresis loss. The soft magnetic particles can be easily classified by sieving or the like. The particle diameter of the soft magnetic particles as referred to in this specification is the particle diameter as determined by classifying the soft magnetic particles using sieves with predetermined mesh sizes.

[0038] Irrespective of the nianufacturing method for the soft magnetic particles, the soft magnetic particles may be in the form of either one of pulverized powder and atomized powder. The atomized powder may be any one selected from the group consisting of water atomized powder, gas atomized powder, and gas- water atomized powder. Use of gas (water) atomized powder suppresses breakage of the insulation coating so that a dust core with high resistivity can be easily obtained. This is because the constituent particles of the gas (water) atomized water are pseudo-spherical, and hence the mutual aggressiveness between the particles is low.

(Low-temperature softening material) (0039] The low-temperature softening material is made of an inorganic oxide (first inorganic oxide) having a softening point lower than the annealing temperature (the annealing temperature of the soft magnetic particles) of a compact obtained by compacting the soft magnetic particles. This inorganic oxide may be of any kind as long as its viscosity becomes 10^{7 3} dPa-s or less at temperatures equal to or higher than the annealing temperature.

[004OJ An example of such a low-temperature softening material is a so-called low-melting glass used for tableware, tiles, and the like. While the low-temperature softening material may be a lead borosilicate-based glass, more preferably, the low-temperature softening material is low-melting glass with a composition type that presents low environmental load, for example, at least one selected from the group consisting of a borosilicate-based glass, a silicate-based glass, a phosphate-based glass, a borate-based glass, and a vanadium oxide-based glass.

(00 1 J Examples of the main component included in the borosilicate-based glass are

and Si0₂-B₂0₃-CaO. Examples of the main component included in the silicate-based glass are Si02-Li₂0, 8ί0₂-Ν¾0,

Si0₂-CaO, Si(¾- gO, and Si h-A Ch. Examples of-the-main component-included in the phosphate-based glass are P₂0₅-Li₂0, Ρ₂0₅-Ν¾0, P₂O_s-CaO, P₂0₃-MgO, and P2O5-AI2O3. Examples of the main component included in the borate-based glass are B₂0₃-Li₂0, Β₂(¼-Ν¾0, B₂C -CaO, B₂0₃-MgO, and B₂0₃-A1₂0₃. Examples of the main component included in the vanadium oxide-based glass are V2OS-B2O3,

V₂0₅-B₂0₃-Si0_2> V₂05-P₂0₅, and ν₂0₅-Β 0₃-Ρ₂0₅.

(0042 J The softening point of each of these kinds of low-melting glass can be adjusted to a temperature suited for the annealing temperature by adjusting the composition such as SiC^, Na^O, ZnO, B^, Li₂0, SnO, BaO, CaO, or A1₂0₃.

(0043J The content of the low-temperature softening material (particularl the low-melting glass) is preferably 0.3 mass% to 4 mass%, more preferably 0.5 mass% to 3.5 mass%, or still more preferably 0.7 mass% to 3 mass%, when the entire magnetic core powder or the entire dust core is defined as 100 mass %. This is because if the content of the low-temperature softening material is too low, the improvement in the strength of the dust core is not satisfactory, whereas if the content of the low-temperature softening material is too high, the magnetic properties of the dust core may decrease.

[0044] The low-temperature softening material just has to form the grain boundary phase after annealing of the dust core. That is, the low-temperature softening material just has to be adhered to a surface of the soft magnetic particles in the state when the dust core is still in the form of the magnetic core powder or has not been annealed yet The particles (particularly low-melting glass particles) of this low-temperature softening material preferably have a particle diameter smaller than the soft magnetic particles, for example, about 0.5 um to 90 um, or more preferably about 1 um to 50 um. An exceedingly small particle diameter makes manufacture or handling difficult, whereas an exceedingly large particle diameter makes it difficult to form a grain boundary phase that is closely packed with excellent adhesion property.

[0045] While the particle diameter of the low-melting glass particles can be determined by various methods, the particle diameter as referred to in this specification is determined by image analysis with a scanning electron microscope (SEM) or laser diffraction.

(High-temperature softening material)

[0046] The high-temperature softening material is made of an inorganic oxide (second inorganic oxide) having a softening point higher than the annealing temperature. This inorganic oxide may be of any kind as long as its viscosity is more than 10^{7 5} dPa-s at the annealing temperature.

[0047] High-melting ceramic particles exist as an example of such a

high- temperature softening material. Specific examples of the high-melting ceramic particles include silica particles, alumina particles, titanium particles, and zirconia particles. Such particles suitable for avoiding direct contact between the soft magnetic particles are fine particles, for example, nanoparticles preferably with an average particle diameter of 5 run to 500 nm or more preferably with an average particle diameter of 10 nm to 400 nm. An exceedingly large particle diameter leads to low density of the resulting dust core, whereas an exceedingly small particle diameter makes it difficult to sufficiently prevent direct contact between the soft magnetic particles deformed by compacting.

[0048] The content of the high-temperature softening material is preferably 0.1 mass% to 2.7 mass%, more preferably 0.2 mass% to 2.5 mass%, or still more preferably 0.4 mass% to 2.3 mass%, when the entire magnetic core powder or the entire dust core is defined as 100 mass %. This is because if the content of the high-temperature softening material is too low, the improvement in the resistivity of the dust core is not satisfactory, whereas if the content of the high-temperature softening material is too high, the strength of the dust core decreases.

[0049] In a case where the high-temperature softening material is made of fine particles (more specifically nanoparticles), the resulting grain boundary phase according to this embodiment forms a composite dispersed rnicrostructure in which the particles (fine particles or nanoparticles) of the high-temperature softening material with a particle diameter smaller than the soft magnetic particles are dispersed in a matrix of the low-temperature softening material. In this case, the state of the dust core according to this embodiment is such that the surface of the soft magnetic particles that make up the primary phase is surrounded (or coated) by a grain boundary phase (or coating) having two or more kinds of inorganic oxides distributed in an integrated or discontinuous state (see FIG. 1 A). In this respect, the dust core according to this embodiment differs from typical dust cores in which the surface of the soft magnetic particles is coated with a single, continuous insulation layer or the like (see FIG. IB).

(Manufacture of magnetic core powder)

[0050] The magnetic core powder is obtained by adhering the low-temperature softening material and the high-temperature softening material to the surface of the soft magnetic particles (adhesion step). The order in which the low-temperature softening material and the high-temperature softening material are adhered to the surface of the soft magnetic particles is not particularly limited. That is, the low-temperature softening material may be adhered to the surface of the soft magnetic particles first before adhering the high-temperature softening material, and vice versa. Further, the high-temperature softening material and the low-temperature softening material may be adhered concurrently.

10051] This adhesion step may be performed by a wet or dry process. For example, a powder of the soft magnetic particles with the high-temperature softening material adhered may be put in a fluid dispersion (slurry) having the low-temperature softening material (particularly low-melting glass powder) dispersed in a dispersion medium, followed by stirring and mixing, and then the resulting dispersion medium may be evaporated and dried (wet adhesion step). Alternatively, a powder of the soft magnetic particles with the high-temperature softening material adhered may be mixed with the low-temperature softening material without the intermediation of a dispersion medium (dry adhesion step). Further, the soft magnetic particles, the low-temperature softening material, and the high-temperature softening material may be mixed concurrently without the intermediation of a dispersion medium (concurrent dry adhesion v

step). Wet adhesion enables more uniform adhesion, and dry adhesion can obviate the drying step and is therefore efficient.

(Manufacture of dust core)

[0052) The dust core according to this embodiment is obtained through a filling step of filling a die having a cavity of a desired shape with a magnetic core powder, a compaction step of compacting the magnetic core powder into a compact, and an annealing step of annealing the compact The compaction step and the annealing step are described below.

(1. Compaction step)

[0053J While the compacting pressure applied to the soft magnetic particles in the compaction step is not particularly limited, the obtained dust core becomes denser and higher in flux density with increasing compacting pressure. An example of such high-pressure compaction method is die-wall lubricated warm compaction. The die- wall lubricated warm compaction includes a filling step and a warm compaction step. In the filling step, a die having an inner surface coated with a higher fatty acid-based lubricant is filled with the above-mentioned magnetic core powder. In the warm compaction step, compaction is performed at a compacting temperature and compacting pressure such that a metal soap coating separate from the higher fatty acid-based lubricant is produced between the magnetic core powder and the inner surface of the die.

[0054] Here, the term "warm" means, for example, setting the compacting temperature to 70°C to 20O°C, or more specifically 100 °C to 180°C by taking factors such as influence on the surface coating (or insulation coating) and alteration of the higher fatty acid-based lubricant into consideration. The details of this die-wall lubricated warm compaction are described in a number of publications, for example, Japanese Patent No. 3309970 and Japanese Patent No. 4024705. This die-wall lubricated warm compaction enables ultrahigh-pressure compaction while extending the life of the die, making it possible to easily obtain a high-density dust core.

(2. Annealing step)

[0055] The annealing step is performed for the purpose of relieving residual strain and residual stress in the compact, thereby reducing the coercivity and hysteresis loss of the dust core. At this time, the low-temperature softening material softens, and flows into the gap or the like between the soft magnetic particles to provide more stable retention of the soft magnetic particles.

[0056] The annealing temperature can be selected as appropriate in accordance with the kinds of the soft magnetic particles, low-temperature softening material, and high-temperature softening material used. The annealing temperature is normally in the range of about 400°C to 900°C or more specifically 600°C to 780°C. The preferred heating time is 0.1 to 5 hours or more preferably 0.5 to 2 hours, and the heating atmosphere is preferably an inert atmosphere.

[0057] In this embodiment, the grain boundary phase is made of inorganic oxides and does not contain silicone resin or the like. Hence, annealing can be performed at temperatures higher than are commonly used, as long as the softening point of the high-temperature softening material is not exceeded. This high temperature annealing allows a further reduction in hysteresis loss, or the like. Moreover, no degradation of the grain boundary phase and decrease in resistivity take place at that time.

(Dust core)

(1. Properties)

[0058] The density of the dust core, for example, the density ratio p/po representing the ratio of the bulk density (p) of the dust core to the true density (po) of the soft magnetic particles, is preferably 83% or more, more preferably 84% or more, still more preferably 85% or more, or even still more preferably 86% or more to obtain high magnetic properties.

[0059) The resistivity of the dust core is a value unique to each individual dust core which does not depend on the shape of the dust core. The larger the resistivity, the greater the reduction in eddy current loss. Preferably, this resistivity is, for example, 10 μΩ-m or more, more preferably 10² uii-m or more, or still more preferably 10³ μΩ-m or more.

[0060| As for the strength of the dust core, the higher the more preferable because high strength can extend the range of applications of the dust core. For example, the radial crushing strength, which is a representative indicator of the strength of the dust core, is preferably 20 MPa or more, more preferably 40 MPa or more, still more preferably 60 MPa or more, or even still more preferably 80 MPa or more. In the dust core according to this embodiment, unlike in typical dust cores, the soft magnetic particles are not only mechanically bonded together by plastic deformation but are strongly bonded together by the low-temperature softening material. For this reason, the dust core according to this embodiment has higher strength than typical dust cores.

(2. Applications)

[0061 j The dust core according to this embodiment can be applied to various kinds of electromagnetic apparatus irrespective of its form, for example, a motor, an actuator, a transformer, an induction heater (IH), a speaker, or a reactor. Specifically, the dust core is preferably applied to an iron core forming the field magnet or armature of a motor or generator. In particular, the dust core according to this embodiment is suitably applied to an iron core for use in a drive motor which is required to have low loss and high output (high flux density). The drive motor is used in an automobile or the like.

(Examples)

[0062] The invention is described more specifically by way of examples.

(Manufacture of samples)

(Raw materials)

(1. Soft magnetic particles)

[0063] As the soft magnetic particles (raw powder), gas-water atomized powders made from a Si-containing iron alloy were prepared. The composition and particle sizes of the individual kinds of the prepared soft magnetic particles are shown in Table 1 A and Table 1 B (hereinafter, the two tables are collectively referred to as simply "Table 1 ")· The particle sizes illustrated in Table 1 are obtained by classification using sieves with predetermined mesh sizes. It has been confirmed by SEM that even in the case of those soft magnetic particles with particle sizes indicated as "... or less" in the Particle size section of Table 1, soft magnetic particles with particles sizes below 5 um were not contained.

(2. Low-melting glass particles (low-temperature softening material))

[0064] The low-melting glass particles to be adhered to the surface of the soft magnetic particles were obtained by the following wet grinding. As the raw materials, glass beads of the compositions (first inorganic oxide) illustrated in Table 2 were prepared. The low-melting glass particles A, B, D, and E illustrated in Table 2 are those made by Japan Enamel Glaze Co., Ltd., and the low-melting glass particles C and F are those made by Tokan Material Technology Co., Ltd..

[0065] Coarsely ground glass beads were loaded into the chamber of a wet grinder (Dyno Mill: made by Shinmaru Enterprise Co.,), and pulverized by activating a stirring propeller. The pulverized glass beads were collected and dried. In this way, powders made from various kinds of low-melting glass particles were obtained. The particle diameters of the individual kinds of obtained low-melting glass particles are also illustrated in Table 2. These particle diameters were measured by image analysis using a scanning electron microscope (SEM).

(3. Nanoparticles (high-temperature softening material))

[0066] The ceramic nanoparticles (high-temperature softening material) to be adhered to the surface of the soft magnetic particles were prepared. The kinds and average particle diameters of the prepared nanoparticles are illustrated in Table 1. The nanoparticles made of silica illustrated in Table 1 all have the composition of Si(¾. The nanoparticles with an average particle diameter of 15 nm are those made by Tokuyama Corporation (Part No.: QS-10). The nanoparticles with an average particle diameter of 300 nm are those made by Admatechs Company Limited (Part No.: SO-E1 ). Also, the nanoparticles made of alumina have the composition of AI2O3, and are those made by Buhler Ltd. (MicroPolish II).

(0067] Methods for determining the average particle diameter of nanoparticles include a laser diffraction/scattering method, a sedimentation method, and an image analysis method. In this example, the average particle diameter of nanoparticles was determined by the laser cu^racuon/scattering method.

(Manufacture of magnetic core powder)

(0068] 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 step)

10069] First, the chamber loaded with each of the powders of soft magnetic particles and nanoparticles mentioned above was mounted to a rotating and rocking container-type powder mixer (Rocking Mixer made by Aichi Electric Co., Ltd.), and stirred and mixed. This mixing was performed for 30 minutes under the condition of 80 rpm. Thus, soft magnetic particles with nanoparticles adhered to the surface (this is referred to as "first composite particles") were obtained. [0070J Next, the above-mentioned low-melting glass particles were dispersed in 5 to 20 times volume of dispersion medium (ethanol), thereby preparing a low-melting glass slurry. An ultrasonic stirrer was used to perform this dispersion.

[0071] The first composite particles mentioned above were put into this low-melting glass slurry, followed by stirring by the ultrasonic stirrer until the dispersion medium was vaporized. At this time, the temperature of the low-melting glass slurry was set to 60°C to 70°C. Further, after this process, the resulting powder was put into a constant temperature bath at 130°C, and dried for 30 minutes under an atmospheric environment The powder that has solidified after the drying was crushed using a mortar. In this- way, a powder (magnetic core powder) made from the composite particles of nanoparticles and low-melting glass particles adhered to the surface of the soft magnetic particles was obtained.

(2. Dry adhesion step)

[0072] A plastic container loaded with each of the powders of the first composite particles and low-melting glass particles mentioned above was mounted to a tumbling ball mill (made by Tsutsui Scientific Instruments Co.,Ltd.), followed by stirring and mixing. This mixing was performed for 30 minutes under the condition of 60 rpm. The powder that has solidified after the drying was crushed with a mortar. In this way, a magnetic core powder made from the composite particles of nanoparticles and low-melting glass particles adhered to the surface of the soft magnetic particles was obtained.

(3. Concurrent dry adhesion step)

(0073J A plastic container with the powders of the soft magnetic particles, nanoparticles, and low-melting glass particles loaded concurrently was mounted to the above-mentioned rotating and rocking container-type powder mixer, followed by stirring and mixing. This mixing was performed for 30 minutes under the condition of 80 rpm. The powder that has solidified after the drying was crushed with a mortar. In this way, a magnetic core powder made from the composite particles of nanoparticles and low-melting glass particles adhered to the surface of the soft magnetic particles was obtained.

[0074] For each of the above cases, the content (ml) of the low-melting glass particles (low-temperature softening material) added and the content (m2) of the nanoparticles (high-temperature softening material) added are illustrated in Table 1, under the condition that the entire magnetic core powder is defined as 100 mass %. The "ratio to the whole adhesion layer" in Table 1 refers to the percentage content

(m2/(ml+m2) of the nanoparticles added, relative to the sum of both the added contents (m 1 +m2). In Table 1 A, the wet adhesion step is indicated as "wet", and the dry adhesion step is indicated as "dry", and the concurrent dry adhesion step is indicated as "concurrent dry".

(4. Manufacture of comparative samples)

(0075) Magnetic core powders made from composite particles with only one of the low-melting glass particles and nanoparticles adhered to the surface of the soft magnetic particles were also manufactured. The samples with only the low-melting glass particles adhered were manufactured in the same manner as the above-mentioned dry adhesion step. The samples with only the low-melting glass particles adhered were manufactured in the same manner as the first composite particles mentioned above. These samples are collectively indicated as Sample Nos. CI to C7 in Table 1 B.

(0076] Also, magnetic core powders made from composite particles with the nanoparticles and silicone resin adhered to the surface of the soft magnetic particles were also manufactured (Sample Nos. Dl and D2 illustrated in Table 3). This adhesion was performed as follows. First, silicone resin was dissolved in 50 to 80 times volume of dispersion medium (ethanol), and the resulting solution and the nanoparticles were mixed to prepare a coating liquid. Next, this coating liquid was mixed with the soft magnetic particles, followed by stirring with an ultrasonic stirrer until the dispersion medium was vaporized. The temperature of the coating liquid was set to 60°C to 70°C. Further, after this process, the resulting powder was put into a constant temperature bath and dried for 30 minutes under an atmospheric environment The temperature of the constant temperature bath at this time was set to 130°C for Sample No. Dl, and 100°C for Sample No. D2. The powder that has solidified after the drying was crushed using a mortar. As the silicon resin, thermosetting silicone resin (KR-242A made by Shin-Etsu Chemical Co., Ltd.) was used As the nanoparticles, nanoparticles made of silica and silica sol nanoparticles (1PA-ST made by Nissan Chemical Industries. Ltd.; nanoparticles density 30%) each having an average particle diameter of 50 am were used.

(Manufacture of dust core)

(1. Filling step and compaction step)

[0077] By using each sample (magnetic core powder), a ring-shaped compact (outer diameter φ39 mm x inner diameter φ30 mm x thickness: S mm) was fabricated by die-wall lubricated warm compaction. No internal lubricant, resin binder, or the like was used in this compaction. The die-wall lubricated warm compaction was performed as follows.

[0078] A carbide die having a cavity corresponding to a desired shape was prepared. This die was pre-heated to 130°C with a band heater. The inner surface of the die was TiN-coated in advance to a surface roughness of 0.4 Z.

[0079] Lithium stearate ( 1 %) dispersed in an aqueous solution was uniformly applied by a spray gun to the inner surface of the heated die at a rate of about 10 cm³/minute. The aqueous solution used here was prepared by adding a surfactant and an antifoam to water. Polyoxyethylene nonylphenyl ether (6EO), polyoxyethylene nonylphenyl ether (10EO), and borate ester Emulbon T-80 were used for the surfactant- each added at 1 volurae% relative to the entire aqueous solution (100 volume%). The FS Antifoam 80 was used for the antifoam and was added at 0.2 volume% relative to the entire aqueous solution ( 100 volume%).

[0080] The lithium stearate used had a melting point of approximately 225°C and a particle diameter of 20 urn. The lithium stearate was dispersed at 25 g per 100 cm³ of the above-mentioned aqueous solution. This was further subjected to a refinement process (Teflon(R)-coated steel spheres: 100 hours) using a ball mill grinder, and the resulting undiluted solution was diluted 20 times to make an aqueous solution with a final concentration of 1%, which was used for the coating step mentioned above.

[0081] The die having an inner surface coated with the lithium stearate was filled with each magnetic core powder (filling step). The magnetic core powder filled into the die was warm compacted basically at a compacting pressure of 1568 MPa while keeping the die at 130°C (compaction step). In this warm compaction, the compacts made from all of the magnetic core powders could be released from the die with low ejection pressure, without causing a problem such as galling with the die.

[0082] The above process was performed by changing the die temperature (130°C) mentioned above to 150°C for Sample No. Dl, and to 70°C for Sample No. D2.

(2. Annealing step)

[0083] Each of the obtained compacts was annealed in a nitrogen atmosphere at a flow rate of 8 L/minute by setting the furnace temperature to 750°C for 1 hour.

However, Sample No. 17 was annealed at 900°C. In this way, the plurality of dust cores illustrated in Table 1 were obtained.

(Measurements)

( 1. Resistivity and radial crushing strength)

[0084] Resistivity and radial crushing strength were measured using each ring-shaped dust core mentioned above. The radial crushing strength was measured by the method specified in the Japanese Industrial Standards (J1SZ 2507). The resistivity was measured by the four-terminal method using a digital multimeter (manufacturer: ADC Corporation, Model No.: R6581). These measurements were illustrated in Table 1. The correlation between the resistivity and radial crushing strength of each dust core was plotted in FIG. 3

(2. Density)

[0085] The density of the dust core was determined on the basis of the mass and dimensional measurements of each sample (specimen).

(Observation) [0086] A reflection electron micrograph of the boundary phase of the dust core of Sample No. 2 illustrated in Table 1 A taken by a scanning electron microscope (SEM) is illustrated in FIG. 2. In FIG. 2, the white section represents a soft magnetic particle (primary phase), the gray section represents a low-melting glass phase (low-temperature softening material phase), and the black section represents a silica (SiOj) phase

(high-temperature softening material phase). It was apparent from FIG. 2 that in the dust core according to the present example, the grain boundary phase that tightly adheres to and encapsulates the surface of the soft magnetic particle assumes the form of composite dispersed microstructure in which nanoparticies made of the high-temperature softening material are integrated while being dispersed in a matrix of the low-temperature softening material.

{0087] Formation of such a grain boundary phase is also evidenced by the results of differential thermal analysis (DTA) conducted for each sample with only low-melting glass particles (low-temperature softening material), and each sample with nanoparticies (high-temperature softening material) added to the low-melting glass particles. The DTA curves for these two kinds of samples are illustrated in FIG. 4. The vertical axis in the graph illustrated in FIG. 4 represents a voltage difference corresponding to the temperature difference ΔΤ between each sample and a reference substance.

[0088] The kind of the low-melting glass particles used here was the borosilicate-based glass (A) illustrated in Table 2. The nanoparticies added to the low-melting glass particles were silica particles with an average particle diameter of 15 nrri, which was mixed at a ratio of 30 mass (corresponding to Sample No. 2). The mixed sample was obtained by rnixing the low-melting glass particles and the nanoparticies using an agate mortar. The DTA was performed with a thermal analyzer (TG8120) made by Rigaku Electric Corporation at a rate of temperature rise of 10°C/min under an atmospheric environment.

[0089] As can be appreciated from FIG 4, the DTA curve for the sample with only low-melting glass particles had a stable region in the vicinity of the softening point (590°C); however, no such stable region was observed in the DTA curve for the mixed sample (combination of low-melting glass particles and nanoparticles). This stable region is considered to be due to the endothermic reaction caused by softening or melting of the low-melting glass particles. However, in the case of the mixed sample, it is speculated that as the nanoparticles (silica particles) react with the low-melting glass particles and gradually melt with rising temperature, the above-mentioned endothermic reaction became less clearly developed, and hence the stable region no longer appeared.

|0090] This tendency is considered to be observed not only for the case where borosilicate-based glass particles and silica particles are combined but also for cases where other kinds of low-melting glass particles and nanoparticles are combined. This is because reaction of nanoparticles due to melting of low-melting glass particles is caused by the low-melting glass particles melting first Accordingly, it is speculated that since the low-melting glass particles as the low-temperature softening material soften or melt first before the nanoparticles as the high-temperature softening material irrespective of the kind of the low-melting glass particles soften or melt, the

above-mentioned endothermic reaction became less clearly developed, and hence the stable region no longer appeared.

(Evaluation)

(0091J From Table 1 and FIG. 3, it was found that the dust cores of Sample Nos. 1 to 18 all have high density (6.9 g/cra³ or more) and excellent magnetic properties, and exhibit both excellent radial crushing strength and resistivity. Specifically, some of these dust cores exhibited resistivity in the range of 10 μΠ-m or more, more specifically

•m or more, or even more specifically further 10³ μΩ-m or more, and also had high strength in the range of 20 MPa to 89 MPa.

[0092J These results are also appreciated from the fact that the marks

corresponding to Sample Nos. 1 to 18 plotted on FIG. 3 are generally shifted toward the upper right as compared with the marks corresponding to Sample Nos. C 1 , C2, C4 to C7, Dl, and D2. [0093] It was also found that the higher the content of the low-melting glass particles added, the greater the increase in radial crushing strength, and also the higher the content of the nanoparticles added, the greater the increase in resistivity. This tendency was the same irrespective of the particle size of the soft magnetic particles, the kind of the low-melting glass particles, the particle size and kind of the nanoparticles, the type of the adhesion step performed, and so on.

[0094] As can be appreciated from Sample No. CI and Sample Nos. C4 to C7, in a case where the grain boundary phase includes only the low-temperature softening material, the corresponding samples exhibited high radial crushing strength but a sharp decrease in resistivity. This tendency was the same irrespective of the kind or content of the low-melting glass particles added

[0095] As can be appreciated from Sample No. C2, in a case where the grain boundary phase includes only the high-temperature softening material, the corresponding sample exhibited high resistivity but a sharp decrease in radial crushing strength.

Further, as can be appreciated from Sample No. C3, when the content of the

high-temperature softening material increased, a proper compact could not be obtained despite the ultrahigh-pressure compaction.

[0096] As can be appreciated from Sample Nos. Dl and D2, a dust core with a grain boundary phase including silicon resin and silica particles exhibits a sharp decrease in radial crushing strength. This is because the high-temperature softening material does not become integrated with the silicon resin even though the high-temperature softening material partially softens or melts during annealing. As compared with dust cores having silicon resin included in the grain boundary phase, the samples according to this embodiment retain high resistivity and also sufficiently high radial crushing strength. The dust core of Sample No. 17, in particular, exhibits both high resistivity and high radial crushing strength, even through the dust core was annealed at a very high temperature (900°C) not commonly employed.

[0097] From the foregoing, it was found that the dust core according to the invention can achieve both high resistivity and high strength owing to the grain boundary phase between the soft magnetic particles being made of otn the low-temperature softening material and the high-temperature softening material, and synergetic action between those two softening materials. Further, it was also found the dust core according to the invention can demonstrate its excellent properties in a stable manner even under high temperature environments.

(Table 1A)

(Table IB)

(Table 2)

(Table 3)

Note: Nanoparlicle density in silica sol: 30%

Claims

CLAIMS:

1. A dust core comprising:

a primary phase including soft magnetic particles; and

a grain boundary phase formed between the soft magnetic particles,

characterized in that the grain boundary phase is a composite of a low-temperature softening material including a first inorganic oxide having a softening point lower than an annealing temperature of the soft magnetic particles, and a high-temperature softening material including a second inorganic oxide having a softening point higher than the annealing temperature.

2. The dust core according to claim 1 , wherein the grain boundary phase has a composite dispersed microstructure in which fine particles including the

high-temperature softening material and having a particle diameter smaller than the soft magnetic particles are dispersed in a matrix of the low-temperature softening material.

3. The dust core according to claim 2, wherein the fine particles are nanoparticles with an average particle diameter of 5 nm to 500 nm.

4. The dust core according to claim 2 or 3, wherein the fine particles are silica particles or alumina particles.

5. The dust core according to any one of claims 1 to 4, wherein the

low-temperature softening material is a low-melting glass.

6. The dust core according to claim 5, wherein the low-melting glass is at least one selected from the group consisting of a borosilicate-based glass, a silicate-based glass, and a phosphate-based glass.

7. The dust core according to any one of claims 1 to 6, wherein the soft magnetic particles are made of pure iron or iron alloy containing silicon

8. The dust core according to any one of claims 1 to 7, wherein the soft magnetic particles have a particle diameter of 5 μπι to 500 um.

9. The dust core according to any one of claims I to 8, wherein the

high-temperature softening material and the low-temperature softening material fuse together at least at an interface portion that is between the high-temperature softening material and the low-temperature softening material.

10. The dust core according to claim 9, wherein:

the low-temperature softening material is a low-melting glass;

the high-temperature softening material is a silica; and

the grain boundary phase has a composite dispersed microstructure in which a silica-rich phase including the silica is dispersed in a matrix of the low-melting glass.

11. A magnetic core powder used in manufacture of a dust core, characterized in that the magnetic core powder includes:

a soft magnetic particle; and

an adhesion layer on the soft magnetic particle, the adhesion layer having a low-temperature softening material and a high-temperature softening material that are adhered to a surface of the soft magnetic particle, the low-temperature softening material including a first inorganic oxide having a softening point lower than an annealing temperature of the soft magnetic particle, the high-temperature softening material including a second inorganic oxide having a softening point higher than the annealing temperature.

12. A manufacturing method for a dust core, characterized by comprising: filling a die with a magnetic core powder that includes a soft magnetic particle and an adhesion layer on the soft magnetic particle, the adhesion layer having a low-temperature softening material and a high-temperature softening material that are adhered to a surface of the soft magnetic particle, the low-temperature softening material including a first inorganic oxide having a softening point lower than an annealing temperature of the soft magnetic particle, the high-temperature softening material including a second inorganic oxide having a softening point higher than the annealing temperature;

compacting the magnetic core powder in the die; and

annealing a compact obtained after the compacting, wherein the annealing is performed at a temperature equal to or higher than the softening point of the

low-temperature softening material and below the softening point of the high-temperature softening material.