CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/JP2018/002663 filed on Jan. 29, 2018, which claims the benefit of priority from Japanese Patent Application No. 2017-073957 filed on Apr. 3, 2017. The entire disclosures of all of the above applications are incorporated herein by reference.

FIELD

The present invention relates to a dust core comprising soft magnetic particles insulated by a spinel-type ferrite and relates also to relevant techniques.

BACKGROUND

There are a considerable number of products utilizing electromagnetism, such as transformers, motors, generators, speakers, inductive heaters, and various actuators. Many of them utilize an alternating magnetic field and are usually provided with a magnetic core (soft magnet) in the alternating magnetic field in order to obtain a large alternating magnetic field locally and efficiently.

Magnetic cores are required not only to have high magnetic properties in an alternating magnetic field but also to have a less high-frequency loss (referred to as an “iron loss,” hereinafter, regardless of the material of magnetic core) when used in an alternating magnetic field. Examples of the iron loss include an eddy-current loss, a hysteresis loss, and a residual loss, among which the eddy-current loss is important and should be reduced because it increases with the square of the frequency of an alternating magnetic field.

Such magnetic cores in practical use may be compressed powder magnetic cores (referred herein to as “dust cores”) comprising soft magnetic particles (particles of powder for magnetic cores) with insulating coating. The dust cores are used in various electromagnetic devices because of a low eddy-current loss and a high degree of freedom in the shape. However, when the insulating layer present between adjacent soft magnetic particles (at the grain boundary) comprises nonmagnetic silicon particles, resin, compound, or the like, the magnetic properties (such as saturation magnetic flux density and permeability) may deteriorate depending on the nonmagnetic insulating layer. In this context, there are proposed dust cores with insulating layers of a spinel-type ferrite (also simply referred to as “ferrite”) that is a magnetic material, and relevant descriptions are found in the following patent documents.

PATENT DOCUMENTS

Patent Document 1 JP2003-151813APatent Document 2 JP2005-340368APatent Document 3 JP2005-142241APatent Document 4 JP2006-97097APatent Document 5 JP2016-127042APatent Document 6 JP2016-86124A

SUMMARY

Ferrites proposed in Patent Documents 1 to 5 all comprise a metal element (M) such as Mn or Zn, Fe, and O. The insulating layer proposed in Patent Document 6 comprises a mixture layer of silicon particles and specific ferrite particles comprising Ni—Zn—Cu that is free from Fe.

The present invention has been made in view of such circumstances and an object of the present invention is to provide a dust core in which the grain boundary of soft magnetic particles is provided with a novel insulating layer different from conventional ones. Another object of the present invention is to provide relevant techniques thereto.

Solution to the Problem

As a result of intensive studies to achieve the above objects, the present inventors have successfully obtained a dust core that develops a high volume specific resistance (simply referred to as a “specific resistance”) even after heat treatment (annealing), by precipitating Cu and the like in an insulating layer comprising a ferrite. Developing this achievement, the present inventors have accomplished the present invention as will be described hereinafter.

Dust Core

(1) An aspect of the present invention provides a dust core comprising: soft magnetic particles comprising pure iron or an iron alloy; and a grain boundary layer present between adjacent the soft magnetic particles. The grain boundary layer has a main phase and a barrier phase. The main phase comprises a spinel-type ferrite (MxFe3-xO4, 0<x≤1) of a metal element (M), Fe, and O. The metal element (M) serves as a divalent cation. The barrier phase comprises one or more of Cu, Sn, or Co.

(2) The dust core of the present invention can stably exhibit a high specific resistance even when exposed to a high-temperature environment and/or used for a long period of time. For example, even when heat treatment (annealing) is performed for the purpose of removing the strain introduced into the soft magnetic particles during compression molding, the grain boundary layer according to the present invention is less likely to deteriorate in the insulating property, and the dust core of the present invention can stably maintain the high specific resistance. As a result, the dust core of the present invention can have both the reduced eddy-current loss due to the high insulating property of the grain boundary layer and the reduced hysteresis loss due to the lowered coercivity of the soft magnetic particles at high levels.

The reason that the dust core of the present invention can stably maintain the high specific resistance is considered under the present circumstances as follows. Unlike the conventional insulating layers, the grain boundary layer according to the present invention has the barrier phase, which comprises one or more of Cu, Sn, or Co (also simply referred to as a “first metal element” or “M1), in addition to the main phase comprising a ferrite that is a magnetic material having a high insulating property. Cu or the like that constitutes the barrier phase has a small solid solubility limit to Fe (i.e., the solid-solution range is narrow) and can block the Fe diffusion from the soft magnetic particles to the ferrite. This consequently suppresses the phenomenon that Fe diffusing from the soft magnetic particles reduces Fe in the main phase (Fe3++e−→Fe2+) so that the ferrite having a high insulating property changes to FeO or the like having a low insulating property.

The dust core of the present invention has the grain boundary layer in which the main phase and the barrier phase coexist, and it is thus supposed that the main phase responsible for the insulating property is protected by the barrier phase and the high specific resistance is stably maintained.

Powder for Magnetic Cores

(1) The present invention can be perceived not only as the dust core but also as a powder for magnetic cores that is a raw material of the dust core. That is, the present invention may also be a powder for magnetic cores that comprises particles for magnetic cores. The particles have: soft magnetic particles comprising pure iron or an iron alloy; and a film that coats the soft magnetic particles. The film has a main phase and a barrier phase. The main phase comprises a spinel-type ferrite (MxFe3-xO4, 0<x≤1) comprising a metal element (M), Fe, and O. The metal element (M) serves as a divalent cation. The barrier phase comprises one or more of Cu, Sn, or Co.

In the powder for magnetic cores of the present invention, the above-described barrier phase is preliminarily formed in the film for the soft magnetic particles. The above-described dust core can be obtained by using the powder for magnetic cores.

(2) The present invention can also be perceived as a powder for magnetic cores that comprises particles for magnetic cores. The particles have: soft magnetic particles comprising pure iron or an iron alloy; and a film that coats the soft magnetic particles. The film has: a first spinel-type ferrite (M1yFe3-yO4, 0<y≤1) comprising a first metal element (M1), Fe, and O, the first metal element (M1) comprising one or more of Cu, Sn, or Co; and a second spinel-type ferrite (M2zFe3-zO4, 0<z≤1) comprising one or more second metal elements (M2), Fe, and O, the one or more second metal elements (M2) being different from the first metal element (M1) and serving as divalent cations.

In the powder for magnetic cores of the present invention or the dust core obtained using the powder, the first metal element (such as Cu) in the first spinel-type ferrite (simply referred to as a “first ferrite”) is preferentially reduced to precipitate (Cu2++2e−→Cu) by Fe diffusing from the underlying soft magnetic particles, and the above-described barrier phase is generated. Thus, also when the powder for magnetic cores of the present invention is used, the above-described dust core can be obtained.

Method of Manufacturing Powder for Magnetic Cores

The above-described powder for magnetic cores can be obtained, for example, by the production method of the present invention as below. That is, there is provided a method of manufacturing a powder for magnetic cores. The method comprises a ferrite generation step of generating a spinel-type ferrite on surfaces of soft magnetic particles comprising pure iron or an iron alloy. The ferrite generation step comprises: a first generation step of generating, on the surfaces of the soft magnetic particles, a first spinel-type ferrite (M1yFe3-yO4, 0<y≤1) comprising a first metal element (M1), Fe, and O, the first metal element (M1) comprising one or more of Cu, Sn, or Co; and a second generation step of generating a second spinel-type ferrite (M2zFe3-zO4, 0<z≤1) comprising one or more second metal elements (M2), Fe, and O, the one or more second metal elements (M2) being different from the first metal element (M1) and serving as divalent cations.

The powder after the ferrite generation step may be preliminarily heat-treated before being formed or molded into a dust core (powder heating step). This can promote the densification of the film coating the soft magnetic particles and the generation of the above-described barrier phase (e.g. M1 precipitation).

The densification of the film and the generation of the barrier phase may be separately performed or may also be performed in parallel. For example, the heating temperature may be set within a low-temperature region (e.g. 480° C. or lower in an embodiment and 430° C. or lower in another embodiment) thereby to allow the film to be densified without generation of the barrier phase (M1 precipitation). In this case, the barrier phase may be generated, for example, during the heat treatment for the dust core (annealing).

In contrast, the heating temperature may be set within a high-temperature region (e.g. 520° C. or higher in an embodiment and 570° C. or higher in another embodiment) thereby to allow both the densification of the film and the generation of the barrier phase to occur in parallel.

The densified film is thought to be less likely to cause deformation, cracks, etc. during compression molding of the powder for magnetic cores and is also thought to prevent direct contact between the soft magnetic particles and contribute to a high specific resistance of the dust core. In addition or alternatively, the densification of the film may be applied to a film that already has the barrier phase.

Method of Manufacturing Dust Core

The above-described dust core can be obtained, for example, by a method of manufacturing a dust core. This method comprises a molding step of compression-molding any of the above-described powders for magnetic cores.

Others

(1) In the present description, not only when one type of a metal element is used but also when plural types of metal elements are used, they are abbreviated as “M,” “M1,” or “M2” for descriptive purposes. When plural types of metal elements are used, “x,” “y,” or “z” representing the composition ratio (atomic ratio) indicates the total of respective metal elements. For example, when M comprises Mn and Zn, “Mx” means Mnx1Znx2, where x=x1+x2 and 0<x1·x2.

(2) Unless otherwise stated, a numerical range “x to y” as referred to in the present description includes the lower limit x and the upper limit y. Any numerical value included in various numerical values or numerical ranges described in the present description may be selected or extracted as a new lower or upper limit, and any numerical range such as “a to b” can thereby be newly provided using such a new lower or upper limit.

