EP0997915A2

EP0997915A2 - Flat-paticle iron powder, method for making the same and powder magnetic core using the same

Info

Publication number: EP0997915A2
Application number: EP99119376A
Authority: EP
Inventors: Yukiko c/o Tech. Res. Laboratories Ozaki; Kuniaki c/o Tokyo Head Office Ogura; Tsutomu Yashiro; Tsuneo Murai; Hideo Hishiki
Original assignee: Victor Company of Japan Ltd; Kawasaki Steel Corp
Current assignee: JFE Steel Corp; Victor Company of Japan Ltd
Priority date: 1998-10-30
Filing date: 1999-09-29
Publication date: 2000-05-03
Also published as: EP0997915A3; CA2284139A1

Abstract

A flat-particle iron powder, composed of reduced iron powder obtained by reducing iron oxide, has an average aspect ratio of at least 5 and an average ferrite grain size of 2 to 20 µm, is preferably used for a powder magnetic core, has high permeability over a higher frequency region, and is covered with a compound having an amine-quinone-repeating unit to further improve its magnetic characteristics.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an iron powder, to a method for making the iron powder, and to a powder magnetic core using the iron powder, which core has high permeability over a high-frequency range and which may be used as a reactor core, or a noise filter core or a substitute for a ferrite compact.

2. Description of the Related Art

Miniaturization of electronic components is rapidly progressing with reduction in size of electronic devices. Although many electronic components use ferrite compacts having excellent high-frequency characteristics and low iron loss (core loss), inexpensive powder magnetic cores having high thermal stability of magnetic characteristics have been examined as substitutes for ferrite compacts. The powder magnetic cores are formed by mixing iron powder with a resin binder, compaction-molding the mixture and then curing the resin binder, and are used for reactor cores and noise filter cores. The powder magnetic cores not requiring a sintering step do not have cracks and chips due to shrinkage during sintering and enable the formation of articles having thin articles or complicated shapes. Thus, it is anticipated that electronic components will be miniaturized and novel electronic components, which cannot be produced with conventional materials, will be produced to satisfy design requirements.
Characteristics required for powder magnetic cores are high permeability and low iron loss in the high-frequency region. When the dependence of the initial permeability, at room temperature and 50 Hz to 1 MHz, on the frequency is measured and the frequency at which the initial permeability is 80% of the direct-current initial permeability is defined as a critical frequency, a high direct-current initial permeability and a high critical frequency are required. Alternatively, high effective permeability and a low iron loss are required. The density of the magnetic core and the presence of an effective demagnetizing field affect the permeability, and thus, a higher density of the magnetic core and a lower effective demagnetizing field will result in a higher permeability. The critical frequency increases as the eddy current loss decreases, and thus, a higher insulating capability between particles as constituents of the magnetic core, and a lower iron loss, will result in a higher critical frequency. The insulating property between particles is evaluated by the direct-current resistivity of the magnetic core, and the direct-current resistivity increases as the insulating property becomes higher.
A proposed method for decreasing the effective demagnetizing field is to flatten the iron powder particles used as a raw material for powder iron cores, as disclosed in, for example, Japanese Laid-Open Patent Nos. 62-72102, 63-233508 and 61-223101. Although the direct-current initial permeability is improved by this method, a flattening results in increasing of contact areas between powder particles. Thus, the insulating property between the particles tends to decrease, and the iron loss tends to increase.
Techniques for producing magnetic cores having a high direct-current initial permeability and a high critical frequency are examined by considering improved insulating layers, as disclosed in, for example, Japanese Laid-Open Patent No. 8-260114. Since most insulating layers form hard coatings on the surfaces of particles, the compactness of the powder magnetic core and thus the density are decreased during compaction. Improvement of permeability due to such flattening is, therefore, decreased.
Since the powder magnetic cores using conventional iron powder have low densities, the iron powder is readily corroded not only at the surface of the magnetic core but also in the interstices. Thus, this powder iron core has less reliability when used as an electronic component, as compared with ferrite compacts.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an iron powder for a powder magnetic core which has high corrosion resistance, can be used as a substitute for ferrite compacts, and has a high initial permeability over a high-frequency region; to provide a method for making the iron powder; and to provide a powder magnetic core using the iron powder.
We have discovered that flat reduced iron powder, obtained by flattening porous reduced iron powder and then annealing the flattened powder, improves the magnetic characteristics of the iron powder.
An important feature of the present invention is to provide a flat-particle iron powder comprising reduced iron powder obtained by reducing iron oxide, wherein the flat-particle iron powder has an average aspect ratio of at least about 5 and an average ferrite grain size of about 2 to 20 µm.
Preferably, the flat-particle iron powder is covered with an insulating agent, and the insulating agent comprises an amine-quinone compound having an amine-quinone repeating unit.
It is further important in the present invention to provide a powder magnetic core, prepared by compaction molding of a mixture of a ferromagnetic material, comprising a flat-particle iron powder and a binder, wherein the flat-particle iron powder comprises reduced iron powder obtained by reducing iron oxide, and has an average aspect ratio of at least about 5 and an average ferrite grain size of about 2 to 20 µm.
Preferably, the flat-particle iron powder is covered with an insulating agent, and the insulating agent comprises an amine-quinone compound having an amine-quinone repeating unit.
The binder referred to above is preferably a thermosetting resin, and the thermosetting resin is preferably at least one resin selected from the group consisting of polymeric resins having a quinone-amine repeating unit, epoxy resins, phenolic resins, and polyamide resins.
Another feature of the present invention is to provide a method of making a flat-particle iron powder for a powder magnetic core, which method comprises the steps of flattening a reduced ion powder obtained by reduction of iron oxide, annealing and disintegrating the flat-particle iron powder, classifying the size of disintegrated flat-particle iron powder particles by screening, and treating the surfaces of the resulting flat-particle iron powders with an insulating material. The reduced iron powder is mixed with metallic soap or wax, and then flattened by using a mill.
Preferably, the step of treating the surface of the insulating material comprises dissolving a compound having an amine-quinone repeating unit in a solvent, adding the solution dropwise into the flat-particle iron powder, mixing the flat-particle iron powder, and then removing the solvent. Preferably, the mill is selected from the group consisting of a vibrating ball mill, a vibrating rod mill, a disk mill and a rotating ball mill.
These and other important features are shown by way of illustration in the drawings, which are not intended to define or to limit the scope of the invention, which is defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of a typical flat-particle iron powder in accordance with this invention; and
Fig. 2 is a graph showing the relationship between the effective permeability and the average aspect ratio of a flat-particle iron powder in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Flat-particle iron powders in accordance with the present invention are made of reduced iron powder prepared by reduction of iron oxide. Pure iron powder is generally classified into two groups, reduced iron powder and atomized iron powder, depending upon the manufacturing processes. The reduced iron powder has the general format of a sponge or net, and comprises a multiplicity of long entangled arms. Thus, the reduced iron powder has a larger effective aspect ratio than that of atomized iron powder, even when they have the same average aspect ratio. As a result, the reduced iron powder has a greatly decreased demagnetizing field and is useful for forming magnetic materials.
The average aspect ratio D/t of a flat-particle iron powder in accordance with the present invention is at least about 5. The expression "average aspect ratio D/t" means the average particle diameter D, based upon sieve classification, divided by the average thickness t of the particle.
Fig. 1 is a schematic view of a typical flat-particle iron powder. The average particle diameter D is determined by sieve classification so that the total weight of the particles having diameters larger than the average particle diameter D is equal to the total weight of the particles having diameters smaller than the average particle diameter D. The average thickness t is determined by visual observation of 50 particles using a scanning electron microscope (SEM). Preferably, the average aspect ratio D/t is at least about 10. When the flat-particle iron powder is mixed with a predetermined amount of binder and the mixture is molded at a given pressure, a large average aspect ratio creates a decreased effective demagnetization field and thus an increased direct-current initial permeability. On the other hand, an average aspect ratio of less than about 5 does not substantially contribute to a decrease in the demagnetization field.
Since a flat-particle iron powder has a large contact area between particles when it is molded, insulation between particles decreases even when the type and amount of the binder is optimized. As a result, eddy currents flowing between particles increase and iron loss decreases.
Particles have defects on their surfaces, such as distortion and cracks, which are formed during pulverization and working. Such defects cause a decrease in permeability. Since the effects of these defects are suppressed as the particle size increases, the permeability increases as the particle size increases. A large particle size, however, causes an increase in passage of the eddy currents on the particle surface, and thus an increase in eddy current loss. As a result, the dependence of permeability on frequency is deteriorated, that is, the critical frequency decreases. Accordingly, iron powder having a larger diameter is advantageous for making components which require high permeability in a low-frequency region, in addition to a high-frequency region. When the iron powder is used in a high-frequency region, a smaller particle size is preferred. That is, the particle size is preferably about 2 mm or less and more preferably about 500 µm or less. The average particle size is preferably about 50 to 180 µm, and more preferably about 75 to 120 µm.
The flat-particle iron powder in accordance with the present invention is composed of substantially pure iron of an α-Fe or ferritic single phase, and the ferritic phase has an average crystal grain size of about 2 to 20 µm. Impurities are concentrated in the grain boundary of the ferritic phase, and are often precipitated as inclusions. The precipitate inhibits migration of the magnetic wall and causes pinning of the magnetic wall. The pinning causes a decrease in permeability and an increase in hysteresis loss. Thus, it is preferable that the crystal grain size in the ferritic phase of the iron powder used as a magnetic material favor being larger. When the average grain size of the ferritic phase is less than about 2 µm, the number of the crystal grain boundaries increases in the iron powder. Thus, the permeability decreases significantly and the hysteresis loss increases. Since the reduced iron powder is a sponge or net composed of long entangled arms, the grain size of the crystal is limited to the width of the arms, that is, about 20 µm. Preferably, the average grain size is about 5 to 10 µm.
The average ferrite grain size is determined as follows. The iron powder is embedded in a resin, and the hardened resin is polished and etched. An optical photograph at a magnification of 400 is taken from the etched cross section. The photographic image is analyzed using a computer. The grain sizes of 50 ferrite crystals are determined, and the average grain size is calculated.
The flat-particle iron powder in accordance with the present invention is prepared by mixing reduced iron with a metallic soap or a lubricant such as wax and then shaping the reduced iron to a flat shape, as herein defined, in a vibrating ball mill, a vibrating rod mill, a disk mill, or a rotating ball mill.
Reduced iron powder used in the present invention is prepared by conventional reduction of iron oxide. The iron oxide used as a raw material is mill-scale powder and/or iron ore powder.
The flat-particle iron powder in accordance with the present invention is suitable for a powder magnetic material using a thermosetting resin as a binder. The flat-particle iron powder is mixed with the thermosetting resin and the mixture is shaped by compaction molding. The thermosetting resin is then hardened to form a powder magnetic core. The compaction molding is performed by a typical powder metallurgical process.
The thermosetting resin used as the binder is at least one resin selected from the group consisting of polymeric resins having an amine-quinone-repeating unit, epoxy resins, phenolic resins, and polyamide resins. Among these, the epoxy resins are preferably used. The binder is preferably added in an amount of about 0.1 to 10 percent by weight and more preferably about 0.5 to 5 percent by weight with respect to the flat-particle iron powder (100 percent by weight).
Prior to compression molding of the flat-particle iron powder in accordance with the present invention, the flat-particle iron powder is preferably covered with an amine-quinone compound having an amine-quinone-repeating unit. In the present invention, pores in the porous structure of the flat-particle iron powder may be filled with the amine-quinone compound. Thus, the covering in the present invention also includes such an embodiment. The thickness of the covering or coating layer is about 5 nm to 2 µm. The thickness is determined by the carbon and nitrogen contents in depth profile analysis by Auger-electron spectroscopy. The insulating layer on the iron powder increases insulation between the iron powder particles in the powder magnetic core and thus significantly decreases the iron loss of the magnetic core. Furthermore, the insulating layer improves corrosion resistance of the iron particles. It is believed that hydrogen bonds formed between oxygen atoms of quinone groups in the amine-quinone compound and the iron powder surfaces formed by oxidation facilitate adsorption and bonding of the coating layer on the iron powder. Preferable amine-quinone compounds are amine-quinone monomers having at least one hydroxyl group in each molecule. An amine-quinone-polyurethane resin prepared by polymerization of such a compound is particularly preferred. The amine-quinone monomer having at least one hydroxyl group has high affinity for iron powder and facilitates the formation of a uniform insulating layer. The amine-quinone-polyurethane resin has high affinity for iron powder and high thermal resistance, and is suitable for use in a high-temperature atmosphere.
Preferably, the content of the amine-quinone compound added to the iron powder is in the range of about 0. 01 to 0.3 percent by weight and more preferably about 0.05 to 0.2 percent by weight. A content of less than about 0.01 percent by weight does not produce insulating effects, whereas a content exceeding about 0.3 percent by weight causes agglomeration of the iron powder due to crosslinking of the resin lying over the adjacent iron particles during the removal of the organic solvent. The agglomerated iron powder inhibits homogeneous mixing with a binder. The coating is performed as follows. A solution of an amine-quinone compound in an organic solvent is added dropwise to the flat-particle iron powder while the flat-particle iron powder is mixed, and then the organic solvent is removed. The concentration of the amine-quinone compound in the solution is preferably about 5 to 80 percent by weight, and more preferably about 20 to 60 percent by weight. Any organic solvent which can dissolve the amine-quinone compound may be used. Examples of such organic solvents include cyclohexanone, tetrahydrofuran (THF), dimethylformamide (DMF), and mixtures thereof with ketonic solvents. Mixing is preferably performed using, for example, an attritor or an explosion-proof high-speed mixer until the iron powder agglomerates formed by segregation of the resin solution in the powder disappear. The solvent is removed and then the iron powder covered with the resin is dried, for example, in a vacuum drier.
The preferable amine-quinone-polyurethane resins in the present invention are polyurethane resins modified with diols having amino and quinone groups represented by the chemical formula (1):
wherein R₁ is hydrogen, branched, linear or cyclic C₁-C₆ alkyl, aralkyl or phenyl which may be substituted by linear, branched or cyclic alkyl; and R₂ is a linear, branched or cyclic C₁-C₁₆ alkylene chain, phenylene, aralkylene, alkarylene, or a polyethylene oxide represented by the chemical formula (2): -(CH₂CH₂O)_nCH₂CH₂- wherein n is an integer of 0 to 50. For example, R₁ may be an ethyl, n-propyl, iso-propyl, benzyl or phenyl group, and R₂ may be a methylene, ethylene, propylene or iso-propylene.
The amine-quinone-polyurethane resin is prepared by reaction of a diol having amine-quinone represented by the chemical formula (1), a linear diol not having amine-quinone, and a diisocyanate. Preferred diols have a molecular weight of approximately 500 to 5,000. Examples of such diols include polycaprolactone diol (PCL), poly (hexamethylene carbonate) diol(PHC), poly(butylene adipate)diol(PBA), poly(hexamethylene adipate)diol(PHA), and 1,4-butane diol (BD). Examples of preferable diisocyanates include tolylene diisocyanate (TDI) and 4,4'-diphenylmethane diisocyanate or methylene di(4-phenylisocyanate) (MDI). Predetermined amounts of raw materials are mixed to form a desired amine-quinone-polyurethane resin. For example, 5 to 40 percent by weight of diol oligomer containing amine-quinone groups is added with respect to the total weight of the PCL and the TDI and the mixture is heated at approximately 60°C for 1 hour for polymerization.
Terminal groups of the urethane molecules in the amine-quinone-polyurethane resin, coated on the flat-particle iron powder and epoxy groups in the epoxy resin as a binder, form crosslinks by condensation. The epoxy resin is tightly bonded to the iron powder, resulting in increased mechanical strength of the powder magnetic core. In compression molding, an amine-quinone-polyurethane resin may be compounded in the binding resin, if necessary. Also, in this case, crosslinks between the epoxy groups and the amine-quinone-polyurethane molecules cause increased mechanical strength of the hardened powder magnetic core. At least about 1 percent by weight of amine-quinone-polyurethane resin must be added to 100 percent by weight of epoxy resin to ensure satisfactory mechanical strength, and such a content of amine-quinone-polyurethane resin is preferable. The upper limit of the content depends on the type of the binder used together. When an excessive amount of amine-quinone-polyurethane resin is added, the mechanical strength decreases. The resin component of the binder may be composed of only the amine-quinone-polyurethane resin. When iron powder not covered with a binder containing an amine-quinone-polyurethane resin is mixed with a binder containing an amine-quinone-polyurethane resin and is subjected to compaction molding, the resulting powder magnetic core also has substantially the same advantages as those of the above-described powder magnetic core.

EXAMPLES 1 to 4 and COMPARATIVE EXAMPLES 1 to 4

Mill-scale reduced iron powders A and atomized iron powders B shown in Table 1 were dry-pulverized in a vibrating ball mill and sieved using a sieve having openings of 106 µm or 2 mm. Each powder was placed into a tube furnace and annealed at 800°C for 1 hour in a hydrogen atmosphere of a dew point of 60°C to prepare a sintered cake. The sintered cake was disintegrated and sieved by a sieve having openings of 106 µm or 2 mm to prepare a flat-particle iron powder. The average aspect ratio, the average particle size, and the average ferritic grain size of the resulting powder are shown in Table 2. The flat-particle iron powder was mixed with 1 percent by weight of epoxy resin and the mixture was molded at a molding pressure of 686 MPa to form a ring with an outer diameter of 38 mm, an inner diameter of 25 mm and a thickness of 6.5 mm. The ring was cured at 180 °C for 30 minutes in air to prepare a test piece.
A coil was wound around the test piece and the dependence of the initial permeability of the test piece on the frequency was measured by an impedance analyzer Model 4824A made by the Hewlett-Packard Company to determine the direct-current initial permeability and the critical frequency. The results are shown in Table 2. The average ferritic grain size was determined as follows. The iron powder was embedded in a resin, polished and etched to form a cross section including iron powder. An optical photograph at a magnification of 400 of the cross-section was taken and the images of 50 iron powder particles were stored in a personal computer to measure ferritic grain sizes in these powder particles and to calculate the average of grain sizes in the 50 powder particles.
Comparison between EXAMPLE 1 and COMPARATIVE EXAMPLE 1 shows that a larger average aspect ratio causes a decreased effective magnetic field and thus an increased direct-current initial permeability.
Comparison between EXAMPLE 3 and COMPARATIVE EXAMPLE 3 and between EXAMPLE 4 and COMPARATIVE EXAMPLE 4 shows that the powder magnetic core formed of reduced iron powder has a larger direct-current initial permeability and a higher critical frequency than those of the powder magnetic core formed of atomized iron powder, even when the average aspect ratio is the same.
Accordingly, the magnetic cores in accordance with the present invention had superior magnetic characteristics.

EXAMPLES 5 to 8 and COMPARATIVE EXAMPLES 5 and 6

The mill-scale reduced iron powder A shown in Table 1 was dry-pulverized in a vibrating ball mill and sieved using a sieve having openings of 180 µm. The powder was annealed as in Example 1, but the annealing temperature was changed to 700 to 850°C. The sintered cake was disintegrated and sieved by a sieve having openings of 180 µm to prepare a flat-particle iron powder. The flat-particle iron powder was molded and cured as in EXAMPLE 1. A coil was wound around the test piece and the dependence of the initial permeability of the test piece on the frequency was measured to determine the direct-current initial permeability and the critical frequency, as in EXAMPLE 1. The results are shown in Table 3. The average ferritic grain size was determined as follows. The average particle size of the crystal grains in the ferritic phase was determined as in EXAMPLE 1.
Comparison between EXAMPLES 5 and 6 and COMPARATIVE EXAMPLE 5 shows that a larger average aspect ratio caused a decreased effective magnetic field and thus an increased direct-current initial permeability. Comparison between EXAMPLES 7 and 8 and COMPARATIVE EXAMPLE 6 shows that a larger average diameter of the crystal grains in the ferritic phase caused a larger direct-current initial permeability and a higher critical frequency, even when the average aspect ratio was the same. These results suggest that the pinning effect of the magnetic wall due to the crystal grain boundaries was suppressed.

EXAMPLE 9

The mill-scale reduced iron powder A shown in Table 1 was dry-pulverized in a vibrating ball mill for a predetermined time and sieved using a sieve having openings of 180 µm. The powder was annealed at 800°C for 1 hour in a hydrogen atmosphere having a dew point of 60°C. The resulting sintered cake was disintegrated and sieved by a sieve having openings of 180 µm. The average aspect ratio of the resulting flat-particle iron powder was measured. The flat-particle iron powder was mixed with 1 percent by weight of epoxy resin, and the mixture was molded at a pressure of 686 MPa to form a ring having an outer diameter of 38 mm, an inner diameter of 25 mm and a thickness of 6.5 mm. The ring was cured at 180°C for 30 minutes in air to prepare a test piece.
A 40-turn primary coil and a 40-turn secondary coil were wound around the test piece and the effective permeability of the test piece was measured at 100 kHz and a maximum magnetic flux density of 0.05 T using a BH analyzer Model E5060A made by the Hewlett-Packard Company. The relationship between the average aspect ratio and the effective permeability is shown in Fig. 2, in which the error range is represented by error bars.
The graph in Fig. 2 demonstrates that the effective permeability increases as the average aspect ratio increased for the same opening size of the sieve and significantly increased when the average aspect ratio was 5 or more.

EXAMPLES 10 to 13

The mill-scale reduced iron powder A shown in Table 1 were dry-pulverized as in EXAMPLES 1 and sieved using a sieve having openings of 500 µm. The powder was annealed as in Example 1. The sintered cake was disintegrated and sieved by a sieve having openings of 500 µm to prepare a flat-particle iron powder. The average ferrite particle diameter of the sieved flat-particle iron powder was approximately 10 µm. A predetermined amount of solution containing 10 percent by weight of amine-quinone-polyurethane resin in cyclohexanone was added dropwise to the flat-particle iron powder, the mixture was mixed in a high-speed mixer, and cyclohexanone was removed. Flat-particle iron powders covered with different amounts of amine-quinone-polyurethane resin were thereby prepared, as shown in Table 4. As in EXAMPLE 1, 1 percent by weight of epoxy resin was mixed with 100 percent by weight of the covered flat-particle iron powder and the mixture was molded at a pressure of 686 MPa to form a ring having an outer diameter of 38 mm, an inner diameter of 25 mm, and a thickness of 6.5 mm, and a rectangular parallelepiped having a width of 10 mm, a length of 50 mm, and a thickness of 5 mm. These were cured at 140°C for 30 minutes in air to form test pieces. A coil was wound around the ring piece as in EXAMPLE 9, and the effective permeability at 100 kHz and a maximum magnetic flux density of 0.05 T and the iron loss at 100 kHz and maximum magnetic flux density of 0.01 T were measured using the BH analyzer. The direct-current resistivity in the longitudinal direction of the rectangular parallelepiped was measured by a four-probe method, and then the rectangular parallelepiped piece was placed into a thermostat at 70°C and a relative humidity of 95% for 48 hours to measure the corrosion area percentage formed on the rectangular parallelepiped, as follows. A photograph of the largest face of the surface on the rectangular parallelepiped piece was taken and the area of the discolored corroded region and the area of the uncorroded region were determined by image analysis. The corrosion area ratio was defined as the ratio of the corroded area to the overall area of the observed face. The results are shown in Table 4.
The results of EXAMPLES 10 to 13 show that covering with the amine-quinone-polyurethane resin caused a decreased iron loss, an increased direct-current resistivity, improved insulation between particles in the magnetic core, and a decreased corrosion area ratio.

EXAMPLES 14 to 17

The mill-scale reduced iron powder A shown in Table 1 was dry-pulverized as in EXAMPLE 10 so that the average aspect ratio became 10, and was sieved using a sieve having openings of 500 µm. The flat-particle iron powder was annealed, as in EXAMPLE 1. The resulting sintered cake was disintegrated and sieved by a sieve having openings of 500 µm. The average ferrite particle diameter in the sieved flat-particle iron powder was approximately 8 µm. The flat-particle iron powder was mixed with 1 percent by weight of epoxy resin containing an amine-quinone-polyurethane resin in the amounts shown in Table 5, and the mixture was molded at a pressure of 686 MPa to form a ring having an outer diameter of 38 mm, an inner diameter of 25 mm and a thickness of 6.5 mm, and a rectangular parallelepiped having a width of 10 mm, a length of 50 mm, and a thickness of 5 mm. These were cured at 140°C for 30 minutes in air to prepare test pieces. Magnetic characteristics were measured as in EXAMPLE 10. The coil was unwound and the Radial Crushing strength was measured according to ASTM B439-98. The results are shown in Table 5.
The results of EXAMPLES 14 to 17 show that the binder containing the amine-quinone-polyurethane resin caused a decreased iron loss, an increased direct-current resistivity, improved insulation between particles in the magnetic core, a decreased corrosion area ratio, and an increased Radial Crushing strength.

EXAMPLES 18 to 22 and COMPARATIVE EXAMPLE 7

Polyols and diisocyanates shown in Table 6 were mixed with 2,5-bis(N-2-hydroxyethyl-N-methylamino)-1,4-benzoquinone represented by the chemical formula (3) as an amine-quinone monomer (AQM) according to the formulations shown in Table 6, and were allowed to react at 60°C for 1 hour, in which the ratio of the AQM in Table 6 represents percent by weight to the total weight of all the raw materials. Amine-quinone-polyurethane resins were thereby prepared. The molecular weights of the resulting resins were approximately 5,000 to 50,000.
These polyurethane resins were dissolved into 2-butanone to prepare 10%-by-weight solutions. Each solution was added dropwise to the flat-particle iron powder, prepared in EXAMPLE 14, having an average aspect ratio of 10 and an average ferrite particle diameter of 8 µm, and the mixture was mixed in a high-speed mixer. The solvent was removed.
To 100 percent by weight of the flat-particle iron powder, 1 percent by weight of epoxy resin was added, and the mixture was molded at a pressure of 686 MPa to form a ring having an outer diameter of 38 mm, an inner diameter of 25 mm and a thickness of 6 mm, and a rectangular parallelepiped having a width of 10 mm, a length of 50 mm, and a thickness of 5 mm. These were cured at 140°C for 30 minutes in air to prepare test pieces. Magnetic characteristics were measured as in EXAMPLE 10. The coil was unwound and the Radial Crushing strength was measured, as in EXAMPLE 14. The results are shown in Table 6.
The results of EXAMPLES 18 to 22 and COMPARATIVE EXAMPLE 7 show that the iron powder covered with the amine-quinone-polyurethane resin had an increased direct-current resistivity, a decreased iron loss, and an increased Radial Crushing strength. These trends were noticeable when the amine-quinone monomer (AQM) content increased.

EXAMPLES 23 to 27 and COMPARATIVE EXAMPLES 8 and 9

The mill-scale reduced iron powder A shown in Table 1 was dry-pulverized so that the average particle thickness became approximately 2 µm, and sieved using sieves having openings of 106 µm, 180 µm, 500 µm, 1 mm, 2.0 mm, 2.5 mm, and 3.0 mm, respectively. The sieved iron powders were annealed at 800°C for 1 hour in a hydrogen gas atmosphere having a dew point of 60°C in a tube furnace. The resulting sintered cakes were disintegrated and the product sieved using sieves having openings of 106 µm, 180 µm, 500 µm, 1 mm, 2.0 mm, 2.5 mm, and 3.0 mm respectively. The sieved iron powders were mixed with 1 percent by weight of epoxy resin, and the mixture was molded at a pressure of 686 MPa to form rings having an outer diameter of 38 mm, an inner diameter of 25 mm and a thickness of 6.5 mm. These were cured at 180°C for 30 minutes in air to prepare test pieces. Magnetic characteristics of coiled ring pieces, the dependence of the initial permeability on the frequency, the direct-current initial permeability and the critical frequency were measured using an impedance analyzer, as in EXAMPLE 1. The results are shown in Table 7.
The results shown in Table 7 show that when the thickness t of particles was constant, a larger opening of the sieve resulted in an increased direct-current initial permeability due to an increased average aspect ratio. When the maximum particle size was larger than 2 mm, the critical frequency was significantly decreased due to deterioration of insulation.

As described above, the powder magnetic core in accordance with the present invention has high direct-current initial permeability, high critical frequency, high effective permeability, and low iron loss. Thus, this powder magnetic core can well replace powder magnetic cores formed of conventional ferrite sintered compacts. In addition, the powder magnetic core in accordance with the present invention has high mechanical strength and high corrosion resistance compared to conventional powder magnetic cores.

Sample	Iron Powder	Apparent Density (Mg/m³)	Average Particle Diameter of Powder (µm)	Trace Components (percent by weight)
				O	C	N	Si	Mn	P	S
A	Reduced Iron Powder	2.68	78	0.20	0.001	0.0032	0.02	0.20	0.011	0.003
B	Atomized Iron Powder	3.01	81	0.11	0.001	0.0055	0.02	0.04	0.008	0.006

	Type of Iron Powder	Average Particle Diameter of Powder (µm)	Average Aspect Ratio	Opening of Sieve (µm)	Average Grain Diameter of Ferrite (µm)	DC Initial Permeability (µ_i/µ₀)	Critical Frequency (MHz)
EXAMPLE 1	A	53	10	106	5.1	95	1.21
COMPARATIVE EXAMPLE 1	A	58	3	106	5.0	81	1.05
EXAMPLE 3	A	56	11	106	10.3	97	1.20
COMPARATIVE EXAMPLE 3	B	48	11	106	10.7	80	1.00
EXAMPLE 4	A	65	40	106	16.5	104	1.10
COMPARATIVE EXAMPLE 4	B	63	40	106	16.4	93	0.81

	Type of Iron Powder	Average Particle Diameter of Powder (µm)	Average Aspect Ratio	Opening of Sieve (µm)	Average Grain Diameter of Ferrite (µm)	Direct-Current Initial Permeability (µ_i/µ₀)	Critical Frequency (MHz)
Example 5	A	77	8	180	5.3	96	1.15
EXAMPLE 6	A	78	15	180	5.8	102	1.11
COMPARATIVE EXAMPLE 5	A	80	4	180	4.9	81	1.10
EXAMPLE 7	A	75	12	180	5.1	97	1.18
EXAMPLE 8	A	71	12	180	10.2	100	1.21
COMPARATIVE EXAMPLE 6	A	76	12	180	1.6	91	1.05

	Amount of Amine-Quinone-Polyurethane Resin (% by weight)	Effective Permeability (µ_e/µ₀)	Iron Loss (kW/m³)	DC Resistivity (µΩm)	Corrosion Area Ratio (%)
EXAMPLE 10	0.05	175	32.7	655	5
EXAMPLE 11	0.10	171	25.0	831	3
EXAMPLE 12	0.20	176	21.5	945	1
EXAMPLE 13	0	173	78.1	418	18

	Amount of Amine-Quinone-Polyurethane Resin (% by weight)	Effective Permeability y (µ_e/µ₀)	Iron Loss (kW/m³)	DC Resistivity (µΩm)	Corrosion Area Ratio (%)	Radial Crushing strength (MPa)
EXAMPLE 14	5	176	34.1	593	8	22.4
EXAMPLE 15	10	173	28.3	776	5	27.5
EXAMPLE 16	20	175	24.3	895	3	20.7
EXAMPLE 17	0	174	80.9	339	16	11.5

	Diol	Diisocyanate	AQM Content (% by weight)	Effective Permeability (µ_e/µ₀)	Iron Loss (kW/m³)	DC Resistivity (µΩm)	Radial Crushing strength (MPa)
EXAMPLE 18	BA	TDI	10	185	32.6	711	20.7
EXAMPLE 19	CL	MDI	20	181	30.2	803	23.5
EXAMPLE 20	BD	MDI	30	181	27.4	966	24.1
EXAMPLE 21	BD	TDI	40	179	22.3	1193	26.6
EXAMPLE 22	CL	TDI	10	187	31.3	694	21.3
COMPARATIVE EXAMPLE 7	CL	MDI	0	185	89.2	581	12.8
AQM: amine-quinone monomer, BA: (butylene adipate)dial, CL: caprolactone dial, BD: butane diol, TDI: tolylene diisocyanate, MDI: 4,4'-diphenylmethane diisocyanate or methylene di (4-phenylisocyanate) (trivial name)

	Type of Iron Powder	Opening of Sieve (µm)	Average Particle Diameter of Ferrite (µm)	DC initial Permeability (µ_i/µ₀)	Critical Frequency (MHz)
EXAMPLE 23	A	106	5.3	105	1.21
EXAMPLE 24	A	180	5.7	106	1.15
EXAMPLE 25	A	500	5.1	110	1.04
EXAMPLE 26	A	1.0 mm	4.9	112	0.98
EXAMPLE 27	A	2.0 mm	5.0	112	0.85
COMPARATIVE EXAMPLE 8	A	2.5 mm	5.8	114	0.48
COMPARATIVE EXAMPLE 9	A	3.0 mm	5.4	114	0.36

Claims

A substantially flat-particle iron powder comprising reduced iron powder, said powder having an average aspect ratio of at least about 5 and an average ferrite grain size of about 2 to 20 µm.
A substantially flat-particle iron powder according to claim 1, covered with an insulating agent.
A substantially flat-particle iron powder according to claim 2, wherein said insulating agent comprises an amine-quinone compound having an amine-quinone-repeating unit.
A substantially flat-particle iron powder according to claim 3, wherein said amine-quinone compound has at least one OH group.
A substantially flat-particle iron powder according to claim 3, wherein said amine-quinone compound is an amine-quinone-polyurethane resin.
A powder magnetic core, prepared by compaction molding of a mixture of a substantially ferromagnetic material, comprising a substantially flat-particle iron powder and a binder, wherein said powder comprises reduced iron powder and has an average aspect ratio of at least about 5 and an average ferrite grain size of about 2 to 20 µm.
A powder magnetic core according to claim 6, wherein said substantially flat-particle iron powder is covered with an insulating material.
A powder magnetic core according to claim 7, wherein said insulating material comprises an amine-quinone compound having an amine-quinone-repeating unit.
A powder magnetic core according to claim 8, wherein said amine-quinone compound has at least one OH group.
A powder magnetic core according to claim 8, wherein said amine-quinone compound is an amine-quinone-polyurethane resin.
A powder magnetic core according to claim 6, wherein said binder is a thermosetting resin.
A powder magnetic core according to claim 6, wherein said thermosetting resin is at least one resin selected from the group consisting of polymeric resins having a quinone-amine-repeating unit, epoxy resins, phenolic resins, and polyamide resins.
A method for making a substantially flat-particle iron powder for a powder magnetic core comprising the steps of:

mixing reduced iron powder obtained by reduction of iron oxide with at least one of metallic soap and wax using a mill;

flattening said reduced iron powder;

annealing and disintegrating said iron powder;

classifying said iron powder by particle size; and

treating the surface of said iron powder with an insulating material.
A method for making a substantially flat-particle iron powder according to claim 13, wherein the step of treating the surface of the insulating material comprises: dissolving a compound having an amine-quinone repeating unit in a solvent; adding dropwise the resulting solution to said substantially flat-particle iron powder; mixing the resulting substantially flat-particle iron powder; and then removing the solvent.
A method for making a substantially flat-particle iron powder according to claim 13, wherein said mill is selected from the group consisting of a vibrating ball mill, a vibrating rod mill, a disk mill and a rotating ball mill.