JPH06283356A - Anisotropic dust core and its manufacturing method - Google Patents

Abstract

PURPOSE:To provide an anisotropic dust core with higher permeability in the circumferential direction than that of conventional materials regarding to a toroidal dust core compression molded with soft magnetic metal powder. CONSTITUTION:The feature of an anisotropic dust core is that the ohmic resistance in the direction of radius or height is in the ratio of 2 to 10000 compared with that in the direction of circumference. Since such an anistropic dust core has few voids cutting particle rows in the direction of circumference, the coefficient of demagnetization is small. Therefore, this core has a higher permeability than that of the dust core of similar density and of which the voids are distributed uniformly isotropically. To manufacture such dust cores, D.C. magnetic field is applied in the direction of circumference when molding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a toroidal dust core formed by cold compression molding of a metal soft magnetic powder in a choke coil applied to a noise filter or the like.

[0002]

2. Description of the Related Art Conventionally, dust cores obtained by compression molding soft magnetic powder have been used for applications such as power supply noise filters and smooth choke coils. Since an eddy current does not easily flow in the dust core when a high-frequency magnetic field is applied, the dust core has better high-frequency characteristics such as magnetic permeability than electromagnetic steel sheets. Further, since it is composed of a metal-based magnetic powder, it has a higher saturation magnetic flux density than an oxide-based core such as soft ferrite.

The demand for switching power supplies for supplying stable power to computers and the like is rapidly increasing. There is a great need for miniaturization and high output of power supplies, and higher frequency switching is being advanced to meet the needs. For this reason, magnetic parts used for noise filters and the like are required to have good high-frequency characteristics and high saturation magnetic flux density. Against this backdrop, dust cores are once again attracting attention as materials that meet these needs. The major developments that have been made so far for dust cores are to improve high-frequency characteristics and magnetic permeability.

In the improvement of high frequency characteristics, development has been carried out with a point of increasing the insulating property between powders. The method is to apply an insulating coating on the powder surface or mix a resin as a binder (for example, JP-A-61-2).
96704). The magnetic permeability of the dust core is significantly lower than that of other materials. The magnetic permeability is low because there are more voids between powder particles than full-density materials such as sintered materials and thin plate materials.
This is because the demagnetizing factor is large. Heretofore, by giving shape anisotropy to the powder particles, the diamagnetic field coefficient of the particles has been reduced and the magnetic permeability of the molded body has been improved. For example, in a compact obtained by compression-molding powder having a flaky particle shape, the powder particles are mechanically oriented so that the plate surfaces are laminated, and a high magnetic permeability is obtained in the plate surface orientation direction (for example, JP-A-63). -285912). However, even if these methods are used, it is impossible to completely eliminate the voids between the powders because it is cold forming. Therefore, from the viewpoint of obtaining a high magnetic permeability, the above method alone is not sufficient, and further effective improvement is desired.

The raw material powder is pure iron powder, Fe--Si
-Based and Fe-Si-Al-based iron-based alloy powders, Ni
-Fe-based alloy powder is used. Japanese Patent Laid-Open No. 2-2900
No. 02 discloses that a Fe-Si-based powder can be produced by a water atomizing method to produce a dust core having high electromagnetic characteristics. The characteristic of this Fe-Si-based powder is that it has a high insulating property due to the stable thin oxide film on the powder surface, and the formed dust core has excellent high frequency characteristics.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a toroidal-shaped dust core having an anisotropy toroidal core having a higher magnetic permeability in the circumferential direction than that of a conventional material. In order to obtain a high magnetic permeability, the fragmentation of the powder particle array in the circumferential direction must be reduced.
The degree of fragmentation of the particle sequence is closely related to the electrical resistivity. Therefore, by defining the ratio of the electrical resistivity in the radial direction or in the height direction to the electrical resistivity in the circumferential direction, an anisotropic dust core which has not existed in the past will be defined.

[0007]

The gist of the present invention is as follows. The dust core of the present invention is a toroidal-shaped dust core obtained by compression-molding a metal-based soft magnetic powder, and the ratio of the electrical resistivity in the radial direction or the height direction to the electrical resistivity in the circumferential direction is 2 Above 1
It is an anisotropic dust core characterized by being 0000 or less. In the method for producing a dust core of the present invention, a magnetic field of 10 Oe or more and 10 kOe or less is applied in the circumferential direction during shaping in the method for producing a dust core having a toroidal shape obtained by compression-molding a metal-based soft magnetic powder. It is a method of manufacturing an anisotropic dust core, which is characterized by the above.

On the other hand, in the present invention, the above-mentioned metallic soft magnetic powder is particularly pure iron powder, Fe-Si system and Fe-Si.
-Al-based alloy powder containing Al, Ni-Fe alloy powder,
Amorphous alloy powder containing Fe-Si-B system and Co-Si-B system is used.

[0009]

The dust core according to the method of the present invention is characterized in that the electrical resistivity in the radial direction or the height direction is higher than the electrical resistivity in the toroidal shape in the circumferential direction. This indicates that the contact frequency of the powder particles in the circumferential direction is higher than that in the radial direction or the height direction, and the particle array is not divided by voids or the like. Such a dust core has a smaller demagnetizing factor in the circumferential direction than a core having the same molding density and isotropically dispersed voids. That is, an anisotropic dust core having a high magnetic permeability in the circumferential direction can be obtained. Further, since the electrical resistivity in the radial direction or the height direction is high, it is difficult for eddy currents to flow when a high frequency magnetic field is applied in the circumferential direction. Therefore, excellent high frequency characteristics such as magnetic permeability can be obtained.

The ratio of the electrical resistivity in the radial direction or the height direction to the electrical resistivity in the circumferential direction is 2 or more and 100 or more.
It was defined as 00 or less. When the ratio is less than 2, the magnetic permeability and the high frequency characteristics are not significantly different from the isotropic core characteristics. On the other hand, if it exceeds 10,000, the density of the molded article cannot be increased, so that the saturation magnetic flux density is significantly reduced.

As a method for producing the above dust core,
A method was devised which was characterized by applying a magnetic field in the circumferential direction in the cavity during molding. The powder particles are arranged in a string in the direction of the magnetic flux by applying a magnetic field.
An anisotropic dust core can be manufactured by molding so as not to disturb this arrangement. The strength of the magnetic field applied during molding was set to 10 Oe or more and 10 kOe or less. If it is less than 10 Oe, the magnetization of the powder is not sufficiently high, and the alignment of the powder particles is divided during molding, so that voids are easily generated. When it exceeds 10 kOe, the effect of suppressing voids is saturated. Since it is difficult to generate a magnetic field of 10 kOe or more in the mold cavity due to the mold structure, the upper limit of the magnetic field strength was set to 10 kOe. On the other hand, the relationship between the molding direction and the magnetic field application direction is preferably vertical because the array does not easily collapse when molding in the direction perpendicular to the particle array. For example, it is effective to apply a current through the core material of the mold to generate a magnetic field in the circumferential direction and perform molding. Here, a non-magnetic core material is used.

As the metal-based soft magnetic powder to be used, pure iron powder, iron-based alloy powder including Fe-Si system and Fe-Si-Al system, Ni-Fe system alloy powder, Fe-Si-B system and Co system. An amorphous alloy powder containing a -Si-B system is included.

[0013]

EXAMPLES Experiments were conducted using 5 wt% Si-Fe powder. The powder was manufactured by the water atomizing method. The amount of oxygen contained in the produced powder is 0.10 wt%. The powder particle size was adjusted to 45 μm or less, and the average particle size was adjusted to 20 μm. In addition, the surface of the powder was subjected to an insulation coating treatment in order to ensure excellent high frequency characteristics. The shape of the dust core to be formed is a toroidal shape having an outer diameter of 18 mm, an inner diameter of 10 mm and a height of 5 mm. The molding method is one-sided pressing in which the powder is compressed only by the upper punch from the height direction of the toroidal shape.
In addition, a current was passed through the non-magnetic mold core so that a DC magnetic field in the circumferential direction could be applied in the mold cavity. In order to remove the strain of the molded body, heat treatment was performed at 780 ° C. for 2 hours in an Ar atmosphere after molding.

Dust cores were molded under different molding conditions, and their electrical resistivity and magnetic permeability were measured. Regarding the electrical resistivity, the ratio of the resistivity in the radial direction and the resistivity in the height direction to the value in the circumferential direction was obtained. Permeability is 100kHz
It is a value when a high frequency magnetic field of (Bm = 50 mT) is applied. The results are summarized in Table 1.

[0015]

[Table 1]

As is apparent from the present inventions 1, 2, 3, 4, the dust core of the present invention is characterized in that the ratio of the electrical resistivity in the radius direction and the electrical resistivity in the height direction is high with respect to the electrical resistivity in the circumferential direction. That is. And such a dust core has a higher magnetic permeability than a dust core having a similar density. For example, in comparison with Comparative Example 1 in which no magnetic field is applied, 500O
The magnetic permeability of Invention 2 to which the magnetic field of e was applied shows a value higher by 37%. Further, in the present invention, as the density of the molded body increases, the ratio of electrical resistivity decreases rapidly. As described above, according to the present invention, a dust core having high magnetic permeability can be obtained.

Experiments were carried out in which the production method of the present invention was applied to pure iron powder, sendust powder and permalloy powder, and similar effects were obtained there.

[0018]

Industrial Applicability The present invention can provide an anisotropic dust core corresponding to miniaturization and high output of a switching power supply by realizing a high magnetic permeability in the circumferential direction of a toroidal dust core.

Claims (3)

[Claims] 1. A toroidal dust core obtained by compression-molding a metal-based soft magnetic powder, wherein the ratio of the electrical resistivity in the radial direction or the height direction to the electrical resistivity in the circumferential direction is 2. An anisotropy dust core characterized by being 10000 or less. 2. The metallic soft magnetic powder is pure iron powder, Fe
-Si-based and Fe-Si-Al-based iron-based alloy powder, Ni-Fe-based alloy powder, Fe-Si-B-based and C-based
The anisotropic dust core according to claim 1, which is an amorphous alloy powder containing an o-Si-B system. 3. A method of manufacturing a toroidal dust core obtained by compression molding a metal-based soft magnetic powder, wherein a magnetic field of 10 Oe or more and 10 kOe or less is applied in the circumferential direction during molding. Method for manufacturing an oriented dust core.