JP5986010B2 - Powder magnetic core and magnetic core powder used therefor - Google Patents

Description

  The present invention relates to a dust core having a large volume resistivity value (hereinafter simply referred to as “resistivity”) and a high magnetic flux density, and a powder for a magnetic core used therefor.

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

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

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

WO2003 / 015109 JP 2006-97097 A JP 2005-64396 A

In Patent Document 1, for example, a powder comprising particles having a NiZn ferrite coating having an average thickness of 0.5 μm (500 nm) on the surface of very fine particles (carbonyl iron powder particles) having an average particle diameter of about 4 μm and The molded body is proposed. Patent Document 2 proposes, for example, a powder made of particles in which a ferrite film made of magnetite (Fe ₃ O ₄ ) is provided on the surface of fine particles (Sendust powder particles) having an average particle diameter of about 8 μm and a molded body thereof. Yes.

  In these magnetic core particles, the particle diameter of the soft magnetic particles as the core is very fine, while the ferrite film formed on the surface thereof is very thick. For this reason, in the powder magnetic core which is a molded object described in Patent Document 1 or Patent Document 2, even if the specific resistance is high, the (saturated) magnetic flux density and the magnetic permeability are very low.

In Patent Document 3, for example, particles having an average particle diameter of about 100 μm (gas atomized pure iron powder particles) on a surface provided with a NiZn ferrite coating or Fe ₃ O ₄ ferrite coating with a film thickness of 50 to 150 nm and the particles are used. Has been proposed. Although the details will be described later, the present inventor conducted research and research, and found that a powder magnetic core having a ferrite coating as in Patent Document 3 cannot achieve both high specific resistance and high magnetic flux density.

  The present invention has been made in view of such circumstances, and a dust core in which electrical characteristics such as specific resistance and magnetic characteristics such as magnetic flux density and permeability are compatible at a high level, and its manufacture An object of the present invention is to provide a magnetic core powder suitable for the above.

  As a result of intensive studies to solve this problem and repeated trial and error, the present inventor uses a magnetic core powder made of soft magnetic particles of a specific size coated with a thin film made of spinel ferrite containing Mn. In general, we have succeeded in obtaining a dust core that has a high-level balance between specific resistance and magnetic flux density, which are in a contradictory relationship. By developing this result, the present invention described below has been completed.

<Magnetic core powder>
(1) The magnetic core powder of the present invention is a compound represented by MFe ₂ O ₄ by soft magnetic particles, a metal element (M) that becomes a divalent cation, iron (Fe), and oxygen (O). A magnetic core powder comprising spinel ferrite and a ferrite film covering the surface of the soft magnetic particles, wherein the soft magnetic particles have a particle size of 50 to 250 μm, , Ri average thickness. 10 to 100 nm der, the spinel ferrite is characterized in that said M is a MnZn ferrite is manganese (Mn) and zinc (Zn).

(2) In the magnetic core powder of the present invention, since the soft magnetic particles serving as the core have a relatively large particle size, it is easy to increase the magnetic flux density and the magnetic permeability of the powder magnetic core. In addition, since the surface of the soft magnetic particles is coated with a thin film (ferrite coating) made of a spinel ferrite, which is a magnetic material, the magnetic flux density of the dust core is reduced and the permeability due to the demagnetizing field generated on the surface of the soft magnetic particles. Decrease in magnetic susceptibility is greatly suppressed.

Of course, since the coating film is made of a ceramic mainly composed of iron oxide (Fe ₂ O ₃ ), it exhibits excellent insulation even if it is very thin. Therefore, when the magnetic core powder comprising soft magnetic particles coated with the ferrite coating according to the present invention is used, not only the magnetic characteristics such as magnetic flux density and magnetic permeability but also the electrical characteristics such as specific resistance are excellent. A powder magnetic core can be obtained easily.

(3) Here, the spinel type ferrite constituting the ferrite film according to the present invention is a cubic soft ferrite represented by MFe ₂ O ₄ , where M is Fe, Mn, Ni, Zn, Cu, Mg, It is a metal element that becomes a divalent cation such as Sr.

However, M according to the present invention contains at least Mn. A dust core having a ferrite coating containing Mn is superior to other dust cores in both electrical characteristics such as specific resistance and magnetic characteristics such as magnetic flux density. The detailed mechanism by which such characteristics are expressed is not necessarily clear. The current situation is considered as follows. In the case of a solid solution of normal spinel and reverse spinel, the ease of entry of M element into the A site or B site in the spinel crystal structure varies depending on the type of M element. Along with this, the magnetic moment generated in each crystal structure also changes. Here, in the case of a crystal structure in which Mn (and Zn) is in solid solution, a larger magnetic moment is generated and saturation magnetization is larger than in the case where other M elements are in solution. In particular, MnFe ₂ O ₄ has the largest saturation magnetization and high specific resistance among various unit ferrites. For these reasons, it is considered that the powder magnetic core having the ferrite film containing Mn exhibited excellent characteristics as described above.

<Dust core>
The present invention can be grasped not only as the above-described powder for magnetic cores but also as a powder magnetic core obtained by pressure molding. In addition, the ferrite film according to the present invention hardly breaks or peels off from the surface of the soft magnetic particles during the pressure molding.

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

It is a SEM image (photograph) which observed the surface of particles for magnetic cores (sample No. A1). It is the SEM image which expanded a part (□ section of Drawing 1A). It is the SEM image which expanded the cross section of the particle for magnetic cores. It is the TEM image which expanded a part (□ section of Drawing 1C). It is a dispersion | distribution figure which shows the relationship between the film thickness of the ferrite film which coat | covers the particle for magnetic cores, and the specific resistance of the powder magnetic core which consists of the particles for magnetic cores. It is a dispersion _| distribution figure which shows the relationship between the film thickness of the ferrite film, and the magnetic flux density B5k of the dust core. It is a dispersion | distribution figure which shows the relationship between the film thickness of the ferrite film, and the magnetic permeability of the dust core. It is a dispersion _| distribution figure which shows the relationship between the specific resistance of a powder magnetic core, and magnetic flux density _B5k .

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

<Magnetic core powder>
(1) Soft magnetic particles (soft magnetic powder)
The soft magnetic particles constituting the soft magnetic powder may be composed mainly of a ferromagnetic element such as a group 8 transition element (Fe, Co, Ni, etc.), but pure iron or an iron alloy from the viewpoint of characteristics, availability, cost, etc. Preferably it consists of. In particular, pure iron powder is preferable in terms of obtaining a high saturation magnetic flux density and improving the magnetic properties of the dust core. Further, for example, when Si-containing iron alloy (Fe—Si alloy) powder is used as the iron alloy powder, the electrical resistivity is increased by Si, so that the specific resistance of the powder magnetic core can be improved and eddy current loss can be reduced.

  In addition, the soft magnetic powder may be Fe-49Co-2V (permendur) powder, Sendust (Fe-9Si-6Al) powder, or the like. The soft magnetic powder may be a mixture of two or more kinds of powders, for example, a mixed powder of pure iron powder and Fe—Si alloy powder.

  The particle size of the soft magnetic particles can be changed according to the specifications of the dust core, but the particle size of the soft magnetic powder according to the present invention is preferably 50 to 250 μm, more preferably 106 to 212 μm. If the particle size is too large, it is difficult to increase the density of the dust core and reduce the eddy current loss. If the particle size is too small, it is difficult to improve the magnetic flux density of the dust core and reduce the hysteresis loss.

  The “particle size” in the present specification is a value indicating the diameter of the soft magnetic particles, and is specified by sieving. Specifically, the median [(d1 + d2) / 2] of the upper limit (d1) and lower limit (d2) of the mesh size used for sieving was defined as the particle size (D). Displayed in μm units, rounding off after the decimal point.

  The method for producing the soft magnetic powder is not limited, and examples thereof include an atomizing method, a mechanical pulverization method, and a reduction method. When atomized powder is used, the shape of soft magnetic particles is almost spherical and the aggression between particles is low, so the destruction of the ferrite coating is suppressed during molding of the dust core, and the high specific resistance of the dust core is stable. Easy to do. The atomized powder may be any of water atomized powder, gas atomized powder, and gas water atomized powder.

(2) Ferrite coating The ferrite coating according to the present invention is made of spinel ferrite (MFe ₂ O ₄ ) and contains Mn as its metal element (M). As long as Mn is contained, M may contain one or more metal elements that are other divalent cations. Further, the ferrite film may contain a modifying element or an inevitable impurity in addition to the elements constituting the spinel type ferrite.

As described above, M constituting the spinel type ferrite includes Fe, Cu and the like in addition to Mn. However, when the present inventors have intensively studied, it is preferable that M contains Zn in addition to Mn. In particular, the ferrite coating is preferably made of MnZn ferrite in which M is Mn and Zn. A dust core made of soft magnetic particles coated with such a spinel ferrite coating has a high specific resistance and a high magnetic flux density. The properties can be balanced in a very high dimension .

  Even if the ferrite coating according to the present invention is very thin, the specific resistance of the powder magnetic core can be sufficiently secured more than the conventional ferrite coating, and the magnetic flux density can be improved. Specifically, the ferrite film according to the present invention exhibits a stable high specific resistance even when the average film thickness is 10 to 200 nm, or even 30 to 100 nm. Within this range, the effect of the ferrite coating on the magnetic flux density and density of the dust core is very small.

  The “average film thickness” referred to in the present specification is a value indicating the thickness of the ferrite coating formed on the surface of the soft magnetic particles, and was determined as follows. First, utilizing the fact that ferrite is an oxide, the distribution of oxygen content on the surface of the coated particles is measured by Auger electron spectroscopy (AES). Then, the maximum value and the minimum value of the oxygen amount are determined, and the depth at the position that becomes the median value is defined as the film thickness of the ferrite coating at the measurement position. This measurement is performed at two arbitrarily selected measurement positions (positions rotated by 90 °) for each particle. Next, the same operation is performed on a total of three particles arbitrarily extracted from the powder. An arithmetic average value of a total of six film thicknesses obtained in this manner was obtained, and this was defined as “average film thickness” in the present specification.

<Dust core>
The dust core of the present invention is formed of a molded body obtained by pressure-molding the above-described magnetic core powder, and is appropriately subjected to heat treatment (annealing or the like) for removing processing strain or the like that causes hysteresis.

(1) Magnetic characteristics The dust core of the present invention thus obtained has a high saturation magnetic flux density. For example, the magnetic flux density (B _5k ) generated in a magnetic field of 5 kA / m is 1.5 T or more, 1.55 T or more. Can exhibit a high magnetic flux density of 1.58 T or more. Further, the magnetic flux density (B _20k ) generated in a magnetic field of 20 kA / m can be 1.9 T or more, 1.93 T or more, or 1.96 T or more.

  The dust core of the present invention has a high magnetic permeability of, for example, 500 or more, 600 or more, and 700 or more.

(2) Electrical characteristics The powder magnetic core of the present invention has a high specific resistance of, for example, 50 μΩm or more, 100 μΩm or more, further 300 μΩm or more, and greatly reduces eddy current loss and the like even when used in a high frequency alternating magnetic field. it can.

(3) Density In the dust core of the present invention, for example, the density ratio (ρ / ρ0), which is the ratio of the bulk density (ρ) of the dust core to the true density (ρ0) of the soft magnetic particles, is 96% or more, If it is 98% or more, more preferably 99% or more, the magnetic properties are improved, which is preferable.

(4) Applications The dust core of the present invention can be used in electromagnetic devices such as motors, actuators, transformers, induction heaters (IH), speakers, and reactors. In particular, it is preferably used for an iron core constituting an armature (rotor or stator) of an electric motor or generator. Among these, the dust core of the present invention is suitable as an iron core for a drive motor that requires low loss and high output (high magnetic flux density). Specifically, the dust core of the present invention is suitable as an iron core for a drive motor of an electric vehicle or a hybrid vehicle.

  The powder magnetic core of the present invention is preferably used in an alternating magnetic field of about 100 to 30000 Hz, more preferably about 200 to 20000 Hz, regardless of which electromagnetic core is used. This is because the ferrite coating according to the present invention improves the permeability of the powder magnetic core, reduces the drive current required to develop the same magnetic flux density, and is advantageous for reducing copper loss.

The present invention will be described more specifically with reference to examples.
<Production of sample>
(Manufacture of magnetic core powder)
(1) Soft magnetic particles First, gas water atomized powder made of pure iron was prepared as soft magnetic powder. The particle size of each powder used is shown in Table 1. In addition, the particle size shown in Table 1 is the median value of the upper limit value and the lower limit value of the mesh size used when classification (sieving) with an electromagnetic sieve shaker (manufactured by Lecce) as described above. Specifically, the upper limit value to the lower limit value are described in the order of particle size: 250-150 μm → 200 μm, 212-106 μm → 159 μm, 150-53 μm → 102 μm, 106-20 μm → 63 μm, 75-20 μm → 48 μm, 45-45 20 μm → 33 μm. It was confirmed by SEM that none of the powders contained soft magnetic particles of less than 30 μm.

(2) Pretreatment Next, the soft magnetic particles were put into ion-exchanged water. To this ion-exchanged water, a chlorinated aqueous solution (or a sulfidized aqueous solution) containing metal ions shown in Table 1 was added to prepare a treatment liquid that was brought into contact with soft magnetic particles. At this time, the treatment liquid was adjusted to pH 3 to 6, and the mixture was stirred at 80 to 90 ° C. (pretreatment step). This pretreatment makes it easy for metal ions, which will be described later, to adhere uniformly to the surface of the soft magnetic particles, and as a result, a uniform film can be formed.

(3) Main treatment Ammonia (NH3) or sodium hydroxide (NaOH) is added to the treatment solution after the pretreatment step, the treatment solution is adjusted to pH 8 to 10, and it is stirred at 80 to 90 ° C (this treatment). Process). This was done for 1 hour.

(4) Post-treatment Further, the powder filtered off after the coating step was washed with water, and further washed with ethanol to remove Cl and the residue (washing step). The washed powder was dried by heating to 80 to 200 ° C. in an air atmosphere (drying step). By this drying step, the hydroxyl group (—OH) attached to and bonded to the particle surface is removed.

  The powder after the drying step was screened through a sieve (mesh size: −30 μm) (screening step). By this sorting step, fine particles (ferrite fine particles and the like generated without contributing to the coating of the soft magnetic particles in this treatment step) that were adhered to the particles after washing were removed. In this way, a magnetic core powder composed of soft magnetic particles (hereinafter referred to as “coated particles”) coated was obtained.

(Manufacture of dust core)
A ring-shaped molded body (outer diameter: φ39 mm × inner diameter φ30 mm × thickness 5 mm) was manufactured by using the above-described powders for magnetic cores by a mold lubrication warm high-pressure molding method. No internal lubricant or resin binder was used at the time of molding. Details of the mold lubrication warm high-pressure molding method are described in Japanese Patent Publication No. 3309970, Japanese Patent No. 4024705, and the like, and were specifically performed as follows.

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

Lithium stearate (1%) dispersed in an aqueous solution was uniformly applied to the inner peripheral surface of the heated mold at a rate of about 10 cm ³ / min with a spray gun. The aqueous solution used here is obtained by adding a surfactant and an antifoaming agent to water. As the surfactant, polyoxyethylene nonylphenyl ether (EO) 6, (EO) 10 and borate ester Emulbon T-80 were used, and each was added by 1% by volume with respect to the entire aqueous solution (100% by volume). did. As the antifoaming agent, FS Antifoam 80 was used and 0.2% by volume was added to the entire aqueous solution (100% by volume). Further, lithium stearate having a melting point of about 225 ° C. and a particle size of 20 μm was used. The dispersion amount was 25 g with respect to 100 cm ^{3 of the} aqueous solution. Then, this is further refined with a ball mill type pulverizer (Teflon (registered trademark) coated steel balls: 100 hours), and the obtained stock solution is diluted 20 times to obtain an aqueous solution having a final concentration of 1%. It was used for.

  Each metal core powder was filled in a mold having lithium stearate coated on the inner surface (filling step).

  While maintaining the mold at 130 ° C., the core powder filled in the mold was warm-pressed under a molding pressure of 1568 MPa (molding process). In this warm high-pressure molding, none of the magnetic core powder was galling with the mold, and the molded body could be removed from the mold with a low pressure.

<< Observation >>
(1) Sample No. shown in Table 1 A mode that the powder particle of A1 was observed with the scanning electron microscope (SEM) or the transmission electron microscope (TEM) was shown to FIG. 1A-FIG. 1D. 1A and 1B are SEM images of the particle surface, FIG. 1C is an SEM image of the particle cross section, and FIG. 1D is a TEM image of a film covering the particle surface.

(2) First, from FIGS. 1A to 1C, it was confirmed that a relatively smooth film having a substantially uniform thickness was formed on the surface of substantially spherical soft magnetic particles (pure iron particles). In particular, FIG. 1C shows that the film thickness (t) is about 100 nm, which is very thin (for example, t / d = 100 to 1000) with respect to the particle diameter (d) of the soft magnetic particles.

(3) Next, as can be seen from FIG. 1D, it was confirmed that the coating formed on the surface of the soft magnetic particles was crystalline. When this film was analyzed by X-ray diffraction (XRD), AES or X-ray photoelectron spectroscopy (XPS), the composition was Mn: 13-15 atomic%, Fe: 30 atomic%, O: 55-57 atomic%. Met. Since this composition is almost expressed as MFe ₂ O ₄ (M = Mn, Zn), it was confirmed that the coating formed on the surface of the soft magnetic particles was a ferrite coating made of spinel ferrite.

<Measurement>
Various measurements shown below were performed using each of the above test pieces. The obtained measurement results are also shown in Table 1.

(1) Electrical characteristics (specific resistance)
Specific resistance, which is one of the electrical characteristics, was measured by a four-terminal method (JIS K7194) using a digital multimeter (manufacturer: ADC, Inc., model number: R6581).

(2) Magnetic characteristics Magnetic flux densities B _5k and B _20k , which are one of the magnetic characteristics, were measured by a direct current magnetic flux meter (manufacturer: Toei Kogyo, model number: MODEL-TRF). The magnetic flux densities B _5k and B _20k are magnetic flux densities generated in the test piece when the magnetic field strength is ₅ kA / m and 20 kA / m.

  Further, the magnetic permeability μ shown in the table is a value obtained by reading the maximum magnetic permeability (μmax) from the magnetization curve obtained by the DC self-recording magnetometer.

(3) Density The bulk density of each test piece was determined based on the mass determined by its mass and measurement. Table 1 shows the relative density (%), which is the ratio (bulk density / true density) of the bulk density to the true density (7.87 g / cm ³ ) of the soft magnetic particles (pure iron particles).

(4) Average film thickness As described above, the film thickness of the ferrite coating formed on the surface of the magnetic core particles before compression molding is the surface using an Auger electron spectrometer (PHI700 manufactured by ULVAC-PHI Co., Ltd.). It was obtained by analyzing. Based on the film thickness thus obtained, the average film thickness was calculated by the method described above. The results are shown in Table 1.

<Evaluation>
(1) Effect of film thickness Sample No. shown in Table 1 FIG. 2A shows the relationship between the average film thickness (simply referred to as “film thickness”) of the ferrite film and the specific resistance (ρ) of the test pieces according to A1 to A3, A7, B3, B4, C2, and C3. FIG. 2B shows the relationship between (t) and magnetic flux density (B _5k ), and FIG. 2C shows the relationship between the film thickness and magnetic permeability (μ).

  First, as can be seen from FIG. 2A, the specific resistance of the dust core generally tends to increase as the thickness of the ferrite coating increases, but this tendency is not proportional. That is, when the film thickness was 10 to 250 nm, the specific resistance of the dust core was stable at 100 μΩm or more. Therefore, according to the ferrite film of the present invention, it was found that a sufficient specific resistance can be obtained even when the film thickness is as thin as several tens of nm.

Next, as can be seen from FIG. 2B, the magnetic flux density of the dust core generally tends to decrease as the thickness of the ferrite coating increases, but this tendency is not proportional. That is, when the film thickness was 10 to 250 nm, the magnetic flux density (B _5k ) of the dust core was stable at 1.5 to 1.6 T. Therefore, according to the ferrite coating according to the present invention, it was found that even when the film thickness was slightly increased to about 200 nm, a sufficient magnetic flux density was obtained as in the case where the film thickness was about several tens of nm.

  Further, as can be seen from FIG. 2C, it was also found that when the thickness of the ferrite coating was 10 to 250 nm, the magnetic permeability was 600 or more, and high magnetic permeability was obtained stably.

(2) Relationship between specific resistance and magnetic flux density FIG. 3 shows the relationship between specific resistance and magnetic flux density (B _5k ) for the test pieces according to all samples shown in Table 1. As can be seen from FIG. 3, the specific resistance and the magnetic flux density are generally in a trade-off relationship. However, in the case of a dust core having a spinel type ferrite film in which the metal element (M) does not contain Mn (M = Ni, Zn), the specific resistance is less than 100 μΩm, and the magnetic flux density is reduced. Nevertheless, the resistivity remained low.

  On the other hand, in the case of the dust core having a ferrite film in which M contains Mn, the above-described correlation is almost established between the specific resistance and the magnetic flux density. Moreover, it has been clarified that when M includes Mn, the specific resistance and magnetic flux density are largely shifted in the direction in which the specific resistance and the magnetic flux density increase as a whole (upper right direction in FIG. 3), compared with the case where M does not include Mn. That is, it was found that in the case of a dust core having a spinel-type ferrite film containing Mn, the specific resistance and the magnetic flux density are compatible at a higher level.

  From the above, it can be seen that when a magnetic core powder composed of soft magnetic particles coated with the ferrite coating according to the present invention is used, a powder magnetic core having both higher specific resistance and higher magnetic flux density than conventional ones can be obtained. It was. When such a dust core of the present invention is used for, for example, a core or a yoke of an automobile motor, a motor with high torque and high efficiency can be obtained while reducing the size and weight.

Claims (6)

The surface of the soft magnetic particle comprising a soft magnetic particle and spinel ferrite, which is a compound represented by MFe ₂ O ₄ by a metal element (M), iron (Fe), and oxygen (O) that are divalent cations A magnetic core powder comprising magnetic core particles having a ferrite coating film,
The soft magnetic particles have a particle size of 50 to 250 μm,
The ferrite coating is Ri average thickness. 10 to 100 nm der,
The spinel-type ferrite is a MnZn ferrite wherein M is manganese (Mn) and zinc (Zn) . The magnetic core powder according to claim 1, wherein the soft magnetic particles have a particle size of 50 to 212 μm. 3. The magnetic core powder according to claim 1, wherein an average film thickness of the ferrite coating is 30 to 100 nm. A powder magnetic core obtained by pressure-molding the magnetic core powder according to any one of claims 1 to 3 . The dust core according to claim 4 , which is used in an alternating magnetic field of 100 to 30,000 Hz. Magnetic flux density (B _5k ) generated in a magnetic field of 5 kA / m is 1.5 T or more,
The volume resistivity value (ρ) is 100 μΩm or more,
And dust core according to claim 4 or 5 permeability (mu) is 500 or more.