Core-shell particles, their manufacturing method, and their uses

Abstract

The objective is to provide a soft magnetic material in powder form that exhibits high magnetization, low coercivity, and low eddy current loss. [Solution] Core-shell particles having a core and a shell made up of two or more crystallites: The core portion has oxide grain boundary layers between crystallites. The crystallite consists of Fe element or Fe element and M element (M element is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi); The shell is porous, made of oxide, and covers the entire surface of the core; and, The oxide in the oxide grain boundary layer within the core and the oxide in the shell are the same oxide and contain either a rare earth element R or a rare earth element R and element T (the rare earth element R is one or more elements selected from the group consisting of Y and lanthanide elements, and element T is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W).

Description

Technical Field

[0001] The present invention relates to core-shell particles (particularly, core-shell particles suitable for high-frequency applications), a method for producing the same, and a soft magnetic powder, a molded body, a soft magnetic member, and an electromagnetic wave filter using the core-shell particles.

Background Art

[0002] Soft magnetic materials are used in various applications such as electromagnetic wave filters in communication devices, in addition to power electronics devices such as power conversion devices and wireless power supply devices. In particular, in recent years, high-frequency applications have been progressing in various fields such as an increase in the switching frequency of semiconductors and an increase in communication frequencies. Therefore, as a soft magnetic material that can be used in high-frequency applications, high-frequency response characteristics are also required.

[0003] When used in power electronics device applications, the characteristics required for soft magnetic materials include having a high magnetization for miniaturization of the device (high magnetization), a low coercive force for reducing hysteresis loss (low coercive force), and a high electrical resistance for suppressing eddy current loss. Having a high electrical resistance for suppressing eddy current loss is a particularly important characteristic when used in high-frequency applications. However, although soft magnetic materials usually have a high magnetization and a low coercive force, they have a low electrical resistance, so the loss due to eddy current becomes large under high frequencies. Therefore, conventionally, ferrite having a high electrical resistance, which is an oxide, has been used as a soft magnetic material for high frequencies. However, since ferrite has a low magnetization, the device becomes large. Therefore, in addition to high magnetization and low coercive force, a soft magnetic material having a high electrical resistance is required.

[0004] Due to these circumstances, much research is being conducted on soft magnetic materials that have high electrical resistance in addition to high magnetization and low coercivity. For example, nanogranular materials having a structure in which metal nanoparticles such as Fe and Co are deposited in a highly electrically resistive matrix such as oxides or fluorides (i.e., a nanogranular structure) have been proposed as soft magnetic materials (see Non-Patent Document 1). In addition, a method has been reported in which a thin film having a nanogranular structure is fabricated on a substrate, and the powder of the soft magnetic material is obtained by peeling (separating) and pulverizing the thin film from the substrate (see Patent Document 1). Furthermore, it has been reported that the electrical resistance of soft magnetic metal particles can be increased by covering the surface of the soft magnetic metal particles with a highly electrically resistive layer, for example, by coating Fe-based soft magnetic metal particles with a particle size of several μm to tens of μm produced by atomization with silicon oxide, or by forming an oxide film on the surface of the Fe-based soft magnetic metal particles by phosphoric acid treatment or heat treatment in an oxygen-containing atmosphere such as air (see Patent Documents 2 and 3). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-179199 [Patent Document 2] Japanese Patent Publication No. 2005-213621 [Patent Document 3] Japanese Patent Publication No. 2008-305823 [Non-patent literature]

[0006] [Non-Patent Document 1] Shigehiro Onuma, Ken Masumoto, Materia, Vol. 41 (2002), pp. 402-405. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, the soft magnetic material having a nanogranular structure proposed in Non-Patent Document 1 can only be fabricated as a thin film using methods such as sputtering, and is less versatile compared to powder. Furthermore, the method for obtaining soft magnetic material powder reported in Patent Document 1 is costly because it requires the fabrication of a thin film, and it is difficult to manufacture large quantities of soft magnetic material powder at once.

[0008] Furthermore, in the methods reported in Patent Documents 2 and 3, while it is possible to suppress eddy currents between particles by covering the surface of the soft magnetic metal particles with a high electrical resistance layer, it is not possible to suppress eddy currents generated within the particles, and sufficient electrical resistance cannot be obtained.

[0009] Furthermore, ferrite has traditionally been used as a soft magnetic material for electromagnetic wave filters, but at high frequencies (especially in the GHz band), the imaginary part μ'' of the complex permeability becomes low due to Snoek's limit.

[0010] For these reasons, there is a current need for the development of novel soft magnetic material powders that exhibit high magnetization, low coercivity, and low eddy current loss (i.e., high electrical resistance). In particular, it is desirable that the soft magnetic material powder be usable as a power electronics device material and even at high frequencies. Specifically, it is desirable that the soft magnetic material powder has a low imaginary term μ″ of permeability up to high frequencies, high saturation magnetization, and a stable high real term μ′ of permeability up to high frequencies. In this application, the real term μ′ of permeability is sometimes referred to as the "real part μ' of complex permeability," and the imaginary term μ″ of permeability is sometimes referred to as the "imaginary part μ'' of complex permeability." Furthermore, when used as an electromagnetic wave filter material, it is desirable that the soft magnetic material powder has high electrical resistance, exhibits magnetic resonance in the GHz band, selectively absorbs high-frequency noise in the GHz band, and can be used at high frequencies. In addition, from the viewpoint of mass production and production cost, it is desirable that the manufacturing method for the soft magnetic material powder be simple.

[0011] Therefore, the present invention aims to provide a novel soft magnetic material in powder form that exhibits high magnetization, low coercivity, and low eddy current loss (i.e., high electrical resistance).

[0012] The present invention aims to provide a soft magnetic material powder that can preferably be used as a power electronics device material, and furthermore, can be used as a power electronics device material even at high frequencies. Specifically, the aim is to provide a soft magnetic material powder that has a low imaginary term μ″ of permeability up to high frequencies, high saturation magnetization, and a stable high real term μ′ of permeability up to high frequencies.

[0013] The present invention aims to provide a soft magnetic material powder that can preferably be used as an electromagnetic wave filter material, and in particular can be used as an electromagnetic wave filter material even at high frequencies, specifically a soft magnetic material powder that has high electrical resistance, magnetic resonance in the GHz band, and selectively absorbs high-frequency noise in the GHz band.

[0014] The present invention preferably aims to provide a simple method for producing the soft magnetic material powder of the present invention.

[0015] As a result of various studies and investigations, the inventors have found that the above objective can be achieved by using predetermined core-shell particles as soft magnetic material powder, and have completed the present invention. Specifically, the present invention has the following embodiments [1] to

[13] . [Means for solving the problem]

[0016] [1] Core-shell particles having the following core portion and the following shell portion, consisting of two or more crystallites: The core portion has an oxide grain boundary layer between the crystallites, The crystallite consists of Fe or Fe and M (where M is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi); The shell part is porous, made of an oxide, and covers the entire surface of the core part; and, The oxide in the oxide grain boundary layer in the core part and the oxide in the shell part are the same oxide, and contain a rare earth element R or a rare earth element R and a T element (where the rare earth element R is one or more elements selected from the group consisting of Y and lanthanoid elements, and the T element is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W). [2] The core-shell particles according to [1], wherein the content of the rare earth element R in the oxide is 2 atomic% or more and 16 atomic% or less in the metal element composition ratio in the core-shell particles. [3] The core-shell particles according to [1] or [2], wherein the crystallite diameter of the crystallites is 10 nm or more and 100 nm or less. [4] The core-shell particles according to any one of [1] to [3], wherein the oxide grain boundary layer has a thickness of 1 nm or more and 20 nm or less. [5] The core-shell particles according to any one of [1] to [4], wherein the weight of the oxide in the core-shell particles is 10% by weight or more and 55% by weight or less based on the weight of the core-shell particles. [6] A soft magnetic powder containing the core-shell particles according to any one of [1] to [5]. [7] A molded body which is a compacted body or a sintered body made of the soft magnetic powder according to [6]. [8] A molded body which is a compacted body or a sintered body containing the soft magnetic powder according to [6] and a binder. [9] A soft magnetic member containing the molded body according to [7] or [8].

[10] The soft magnetic member according to [9], wherein the frequency at which the value of the imaginary part (μ”) of the complex magnetic permeability becomes the maximum value is 10 MHz or more.

[11] The soft magnetic member according to [9], wherein the frequency at which the value of the imaginary part (μ”) of the complex magnetic permeability becomes the maximum value is 3 GHz or more.

[12] An electromagnetic wave filter containing the soft magnetic member according to any one of [9] to

[11] .

[13] A method for producing the core-shell particles according to any one of [1] to [5], including the following steps: (1) A step of synthesizing an amorphous powder or a microcrystalline powder of an oxide composed of Fe element or Fe element, M element, rare earth element R or rare earth element R and T element from a mixed solution containing Fe element or Fe element, M element, rare earth element R or rare earth element R and T element by spray pyrolysis; and (2) A step of producing core-shell particles by reducing the amorphous powder or microcrystalline powder of the oxide, However, the M element is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi; The rare earth element R is one or more elements selected from the group consisting of Y and lanthanoid elements; and The T element is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W.

[14] The method according to

[13] , wherein the amount of the rare earth element R contained in the amorphous powder or microcrystalline powder of the oxide is 2 atomic% or more and 16 atomic% or less. [Advantages of the Invention]

[0017] According to the present invention, novel core-shell particles can be provided as soft magnetic powder particles, whereby a novel soft magnetic material having high magnetization, low coercive force, and low eddy current loss (i.e., high electrical resistance) can be provided in powder form.

[0018] According to an aspect of the present invention, a soft magnetic material powder that can be used as a power electronics device material and can further be used as a power electronics device material even at high frequencies can be provided. Specifically, a soft magnetic material powder having a low imaginary part μ″ of magnetic permeability up to high frequencies, having a high saturation magnetization, and having a stable high real part μ′ of magnetic permeability up to high frequencies can be provided.

[0019] According to yet another aspect of the present invention, a soft magnetic material powder can be used as an electromagnetic wave filter material, and in particular can be used as an electromagnetic wave filter material even at high frequencies. Specifically, a soft magnetic material powder is provided that has high electrical resistance, magnetic resonance in the GHz band, and selectively absorbs high-frequency noise in the GHz band.

[0020] According to yet another aspect of the present invention, a method for easily producing the soft magnetic material powder of the present invention can be provided. [Brief explanation of the drawing]

[0021] [Figure 1] This is a schematic diagram of core-shell particles, which are soft magnetic powder particles of the present invention. [Figure 2] This figure shows the FE-SEM observation image of the powder (hydrogen-reduced powder) from Example 3. [Figure 3] This figure shows TEM images of the particle cross-sections of each particle constituting the powder (hydrogen-reduced powder) of Example 3 embedded in resin. [Figure 4] The images show the EDX mapping of the particles constituting the powder (hydrogen-reduced powder) of Example 3 (left figure), the EDX mapping of the core portion of the particles (middle figure), and the line analysis results of the arrows in the middle figure (right figure). [Figure 5] The left figure shows an EDX mapping image of the particle cross-section of the particles constituting the powder (hydrogen-reduced powder) of Example 3, and the right figure shows a high-resolution TEM observation image of the shell portion of the particles. [Figure 6] This figure shows the XRD pattern of the powder (hydrogen-reduced powder) from Example 3. [Figure 7] This figure shows the XRD patterns of the oxide powders (precursors) for Examples 1, 2, and 6, and Comparative Examples 1 to 3. [Figure 8] This figure shows the relationship between the relative density and volume resistivity of each molded article obtained in Example 3 and Comparative Example 1. [Figure 9]This figure shows the impedance measurement results of a molded body obtained by mixing hydrogen-reduced powder (specifically, the hydrogen-reduced powders from Comparative Example 1 and Examples 1 to 5) with epoxy resin and forming it into a ring-shaped compact (the left figure shows the frequency characteristics of the real part μ' of the complex permeability, and the right figure shows the frequency characteristics of the imaginary part μ'' of the complex permeability). [Figure 10] This figure shows TEM observation images (left) and EDX mapping images (right) of the particle cross-section of the hydrogen-reduced powder from Example 7. [Figure 11] This figure shows the impedance measurement results (i.e., the frequency characteristics of the real part μ' and imaginary part μ'' of the complex permeability) of a molded body formed by mixing the hydrogen-reduced powder of Example 7 with epoxy resin and molding it into a ring-shaped compact. [Modes for carrying out the invention]

[0022] The present invention will now be described in detail, with particular emphasis on its preferred embodiments. It should be noted that the present invention is not limited to the following embodiments, and can be implemented with various modifications within the scope of its essence.

[0023] The core-shell particle of the present invention has a core portion and a shell portion, each consisting of two or more crystallites. The core portion has an oxide grain boundary layer between the crystallites, and the crystallites consist of Fe or Fe and M, and always contain Fe. Here, M is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi, and may hereinafter simply be referred to as "M element". The shell portion is porous, consists of an oxide, and covers the entire surface of the core portion. Furthermore, the oxide of the oxide grain boundary layer in the core portion and the oxide in the shell portion are the same oxide and contain a rare earth element R or a rare earth element R and T element. In other words, the shell portion always contains a rare earth element R. Here, the rare earth element R is one or more elements selected from the group consisting of Y and lanthanide elements, and may hereinafter simply be referred to as "rare earth element R". Furthermore, element T is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W, and may hereinafter simply be referred to as "element T". A schematic outline of the specific structure of the core-shell particles of the present invention is shown in Figure 1. The core-shell particles of the present invention will be described below based on this drawing.

[0024] As shown in Figure 1, the core-shell particle 1 of the present invention has a structure consisting of a core portion and a shell portion (also referred to in this application as a "core-shell structure"). The core portion is composed of two or more soft magnetic metal crystallites 11a, and an oxide grain boundary layer 11b is present between the crystallites. Therefore, as shown in Figure 1, the oxide grain boundary layer 11b appears on the surface of the core portion covered by the shell portion. Specifically, the soft magnetic metal crystallites 11a are crystallites composed of Fe element or Fe element and M element. The shell portion covers the entire surface of the core portion and is a porous oxide layer consisting of fine crystal grains (in other words, a layer consisting of fine crystal grains of oxide). This oxide layer is also referred to in this application as an "oxide shell," and corresponds to the oxide shell 10 in Figure 1. The oxide of the oxide grain boundary layer 11b and the oxide in the oxide shell 10 are the same oxide, and are an oxide containing the rare earth element R or the rare earth element R and the T element. The core-shell particle 1 of the present invention is a soft magnetic metal particle.

[0025] As described above, the crystallite constituting the core portion of the core-shell particle 1 of the present invention is a soft magnetic metal crystallite 11a, which is an element that can be reduced by hydrogen reduction. Specifically, it is an element of Fe or a crystallite composed of an element of Fe and an element of M. As described above, the element of M is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi, but preferably it is the ferromagnetic elements Co and Ni. When the crystallite constituting the core portion (soft magnetic metal crystallite 11a) consists only of the element of Fe, the core may be referred to as an "iron particle core" and the crystallite as an "Fe crystallite" in this application.

[0026] The crystallite diameter of the crystallites (soft magnetic metal crystallites 11a) constituting the core portion of the core-shell particle 1 of the present invention is not particularly limited as long as the objective of the present invention is achieved, but the upper limit is preferably 100 nm or less, and more preferably 80 nm or less. When within the above range, in the configuration of the core-shell particle 1 of the present invention, the substantial crystalline magnetic anisotropy is reduced by the random magnetic anisotropy model between multiple (i.e., two or more) Fe elements or crystallites (soft magnetic metal crystallites 11a) composed of Fe and M elements constituting the core portion, and the coercivity of the powder made up of the core-shell particles is more effectively reduced. As a result, it becomes possible to obtain a coercivity of 10 Oe or less at room temperature, which is required for soft magnetic materials. Here, the "random magnetic anisotropy model" is a model widely used to explain the particle size dependence of coercivity in nanocrystalline magnetic materials. Specifically, this mechanism involves making the crystal grain size smaller than the ferromagnetic exchange length (also called the exchange coupling length), and the randomly oriented ferromagnetic phase of the main phase ferromagnetically coupling through the amorphous interface phase, thereby averaging out the crystal magnetic anisotropy and resulting in low coercivity (see, for example, "G. Herzer, IEEE Transactions on Magnetics, (1990), 26, pp. 1397-1402"). The finer the crystallite size, the smaller the coercivity according to the random magnetic anisotropy model, but the more high-resistivity layers required to reduce eddy current losses are needed, which leads to a decrease in magnetization. Therefore, the lower limit of the crystallite size is preferably 10 nm or more, and more preferably 20 nm or more.

[0027] As described above, the oxide grain boundary layer 11b in the core portion of the core-shell particle 1 of the present invention is an oxide grain boundary layer that exists between the crystallites (soft magnetic metal crystallites 11a) constituting the core portion (in other words, around each individual soft magnetic metal crystallite 11a), and exhibits high electrical resistance. Therefore, with the core-shell particle 1 of the present invention having a core portion having this oxide grain boundary layer, the generation of eddy currents is suppressed more effectively, and even in the high frequency band from 10 MHz to several GHz, the imaginary part μ'' of the complex permeability is a low value, and the real part μ' of the complex permeability is a stable value. Due to having magnetic resonance at high frequencies exceeding several GHz (for example, high frequencies of 3 GHz or higher), the value of the imaginary part μ'' of the complex permeability rises sharply.

[0028] The thickness of the oxide grain boundary layer 11b between the soft magnetic metal crystallites 11a in the core (in other words, around each individual soft magnetic metal crystallite 11a) is not particularly limited as long as the objective of the present invention can be achieved, but the lower limit is preferably 1 nm or more, and more preferably 2 nm or more. Within this range, the oxide grain boundary layer insulates between the crystallites (for example, if the crystallites constituting the core consist only of Fe elements, then Fe crystallites) thereby suppressing the generation of eddy currents and more effectively reducing losses up to high frequencies. The upper limit is preferably 20 nm or less, and more preferably 15 nm or less. Within this range, the magnetic interaction between the crystallites constituting the core (for example, if the crystallites constituting the core consist only of Fe elements, then Fe crystallites) is broken, which can more effectively prevent an increase in coercivity.

[0029] In the case where the crystallite (soft magnetic metal crystallite 11a) constituting the core portion of the core-shell particle 1 of the present invention is composed of Fe and M elements, the composition ratio of Fe and M elements is not particularly limited as long as the objective of the present invention is achieved.

[0030] In the core-shell particles 1 of the present invention, the average diameter of the core portion, which is composed of crystallites (soft magnetic metal crystallites 11a) having an oxide grain boundary layer 11b, is not particularly limited as long as the objective of the present invention is achieved. However, from the viewpoint of molding, if the average diameter of the core portion is fine, high pressure is required to increase the molding density. Therefore, the lower limit of the average diameter of the core portion is preferably 100 nm or more, and more preferably 300 nm or more. From the viewpoint of stably producing oxide powder used as a precursor, the upper limit is preferably 50 μm or less, and more preferably 10 μm or less.

[0031] The oxide of the oxide grain boundary layer 11b in the core portion of the core-shell particle 1 of the present invention is the same as the oxide in the oxide shell 10 which is the shell portion of the core-shell particle 1 of the present invention. Specifically, it contains a rare earth element R or a rare earth element R and an element T. As described above, the rare earth element R is one or more elements selected from the group consisting of Y and lanthanide elements, and the element T is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W.

[0032] The shell portion of the core-shell particle 1 of the present invention is an oxide shell 10 made of an oxide layer consisting of fine crystalline grains, and covers the entire surface of the core portion of the core-shell particle 1 of the present invention. Covering means that the entire surface of the core portion is covered with the oxide shell 10, and there are no particular limitations as long as the objective of the present invention is achieved even if the coverage is not uniform.

[0033] Furthermore, the shell portion of the core-shell particle 1 of the present invention is porous. Because the shell portion of the core-shell particle 1 of the present invention is porous, inter-particle insulation can be reliably achieved, and the generation of eddy currents can be effectively and desirablely suppressed.

[0034] The oxide shell 10, which is the shell portion of the core-shell particle 1 of the present invention, is an oxide layer made up of fine crystalline grains, and as described above, the oxide is the same as the oxide of the oxide grain boundary layer 11b in the core portion of the core-shell particle 1 of the present invention. In other words, the oxide in the oxide shell 10 is the same oxide as the oxide of the oxide grain boundary layer 11b.

[0035] The weight ratio of oxide in the core-shell particles 1 of the present invention is not particularly limited as long as the objective of the present invention is achieved. However, the lower limit is preferably 6% by weight or more, and more preferably 10% by weight or more, relative to the weight of the core-shell particles 1. The upper limit is preferably 55% by weight or less, and more preferably 40% by weight or less, relative to the weight of the core-shell particles 1. Within this range, the soft magnetic powder containing the core-shell particles 1 can more effectively obtain low coercivity and high saturation magnetization. Such high saturation magnetization is particularly effective when applied to applications requiring high magnetization, such as power electronics devices.

[0036] The amount of oxide in the core-shell particles 1 of the present invention can be adjusted by adjusting the amount of rare earth element R or the amount of rare earth element R and element T, which are the raw material composition of the oxide. In addition, it can also be adjusted by a physical method, for example, by crushing the shell oxide after synthesizing the core-shell particles and recovering only the core portion magnetically, or by a chemical method, by dissolving the oxide of the shell portion with a solution that preferentially dissolves oxides.

[0037] The oxide in the oxide grain boundary layer 11b in the core portion of the core-shell particle 1 of the present invention and the oxide in the oxide shell 10 may contain a rare earth element R or a rare earth element R and a T element. Therefore, the oxide in the oxide grain boundary layer 11b in the core portion of the core-shell particle 1 of the invention and the oxide in the oxide shell 10 may be an oxide composed of a rare earth element R and an Fe element, or an oxide composed of a rare earth element R, an T element and an Fe element, or an oxide composed of a rare earth element R, an T element and an M element, or an oxide composed of a rare earth element R, an Fe element and an M element.

[0038] The core-shell particles 1 of the present invention can be produced by heating and reducing an amorphous or microcrystalline powder of an oxide composed of Fe element, Fe element and M element, and rare earth element R, or rare earth element R and T element in a hydrogen atmosphere. In other words, the oxide powder used for heating and reduction in a hydrogen atmosphere (also referred to as the "precursor" in this application) is a powder composed of amorphous or microcrystalline particles of oxide (i.e., amorphous or microcrystalline powder of oxide). Here, "microcrystalline" in this application means not highly crystalline, and specifically means a level of crystallinity in which small, broad diffraction peaks, as shown in the X-ray diffraction patterns observed in the embodiments of this application, can be observed. When the crystallinity of the oxide fine powder used for heating and reduction in a hydrogen atmosphere is amorphous or microcrystalline, the core-shell structure of the core-shell particles 1 of the present invention can be obtained. However, when it is crystalline (i.e., when the crystallinity is higher than microcrystalline), the core-shell structure of the core-shell particles 1 of the present invention cannot be obtained.

[0039] The crystallinity of oxide powders (amorphous or microcrystalline oxide powders) used as precursors for heating and reduction in a hydrogen atmosphere can be controlled by adjusting the amount of rare earth element R in the amorphous or microcrystalline oxide powder and the temperature used to produce the oxide powder. In particular, the amount of rare earth element R has a significant effect on the crystallinity of the oxide, and adding rare earth element R makes it easier to produce oxides that are amorphous or microcrystalline. The lower limit of the content of rare earth element R in an oxide powder (amorphous or microcrystalline oxide powder) used as a precursor is preferably 2 atomic percent or more, and more preferably 5 atomic percent or more, relative to the total amount of each metal element in the oxide powder (specifically, if the metal elements in the oxide powder consist of rare earth element R and Fe, it is the total amount of the rare earth element R and Fe; if the metal elements in the oxide powder consist of rare earth element R, T element and Fe element, it is the total amount of the rare earth element R, T element and Fe element; if the metal elements in the oxide powder consist of rare earth element R, T element and Fe element and M element, it is the total amount of the rare earth element R, T element and Fe element and M element; and if the metal elements in the oxide powder consist of rare earth element R, Fe element and M element, it is the total amount of the rare earth element R, Fe element and M element). The upper limit is preferably 16 atomic percent or less, and more preferably 10 atomic percent or less, relative to the total amount of each metal element in the oxide powder. In this application, the composition ratio (atomic percent) of the metal elements constituting the powder particles of the oxide powder used as a precursor is also referred to as the "composition ratio (atomic percent) of the metal elements used in the preparation."

[0040] The present invention relates to a powder consisting of core-shell particles 1 produced by heating and reducing an oxide powder (microcrystalline oxide powder) used as a precursor in a hydrogen atmosphere. The lower limit of the content of rare earth elements R in the core-shell particles 1 is preferably 2 atomic percent or more, and more preferably 5 atomic percent or more, in terms of the metal element composition ratio in the core-shell particles 1. The upper limit is preferably 16 atomic percent or less, and more preferably 10 atomic percent or less, in terms of the metal element composition ratio in the core-shell particles 1. Here, the metal element composition ratio in the core-shell particle 1 specifically refers to the composition ratio (atomic %) of each metal element with respect to the total amount of each metal element (specifically, if the metal elements in the oxide powder consist of rare earth element R and Fe, it is the total amount of the rare earth element R and Fe; if the metal elements in the oxide powder consist of rare earth element R, T element and Fe element, it is the total amount of the rare earth element R, T element and Fe element; if the metal elements in the oxide powder consist of rare earth element R, T element and Fe element and M element, it is the total amount of the rare earth element R, T element and Fe element and M element; and if the metal elements in the oxide powder consist of rare earth element R, Fe element and M element, it is the total amount of the rare earth element R, Fe element and M element). In this application, the composition ratio (atomic %) of the metal elements constituting the core-shell particle 1 of the present invention, which is the powder particle of the hydrogen-reduced powder described above, is also referred to as the "metal element composition ratio (atomic %)".

[0041] The average particle size of the core-shell particles 1 of the present invention is not particularly limited as long as the objective of the present invention is achieved, but when the oxide is synthesized by methods such as spray pyrolysis or spray drying, it is preferably 100 nm or more and 50 μm or less.

[0042] The core-shell particles 1 of the present invention are soft magnetic metal particles, and as described above, they are manufactured as a powder containing the core-shell particles 1 (i.e., a soft magnetic powder). In other words, in one aspect of the present invention, it may be a soft magnetic powder containing the core-shell particles 1. The soft magnetic powder should contain a sufficient amount of core-shell particles 1 to achieve the objectives of the present invention, but it is preferable to contain as much as possible, and most preferably it consists solely of the core-shell particles of the present invention.

[0043] Furthermore, in another aspect of the present invention, the compacted or sintered body may be made of soft magnetic powder containing core-shell particles 1. In this case, the soft magnetic powder containing core-shell particles 1 may be compacted or sintered by a known method to form a compacted or sintered body to form a molded body. In other words, the compacted or sintered body is a molded body. The compacted or sintered body (molded body) formed in this way may be used as a soft magnetic member. The compacted or sintered body, which is a molded body used as a soft magnetic member, may be used alone or in combination with other materials, depending on the application.

[0044] In another aspect of the present invention, the material may be a compacted or sintered body containing soft magnetic powder containing core-shell particles 1 and other materials (e.g., a binder). In this case, the mixture of soft magnetic powder containing core-shell particles 1 and other materials (e.g., a binder) can be compacted or sintered by a known method to form a compacted or sintered body to form a molded body. As the binder, for example, a binder made of resin, ceramics, and / or metal can be used, and it is generally preferable to use a resin. The compacted or sintered body (molded body) thus formed may be used as a soft magnetic member. The compacted or sintered body, which is a molded body used as a soft magnetic member, may be used alone or in combination with other materials, depending on the application.

[0045] In another aspect of the present invention, a soft magnetic member comprising a compacted or sintered body (molded body) made of soft magnetic powder containing core-shell particles 1, or a compacted or sintered body (molded body) comprising soft magnetic powder containing core-shell particles 1 and other materials (e.g., a binder), can have a frequency at which the value of the imaginary part μ'' of the complex permeability reaches its maximum value of 10 MHz or higher, and can also be 3 GHz or higher.

[0046] In another aspect of the present invention, a soft magnetic member that can set the frequency at which the value of the imaginary part μ'' of the complex permeability reaches its maximum to 10 MHz or higher, and also to 3 GHz or higher, may be used in an electromagnetic wave filter. The soft magnetic member used in the electromagnetic wave filter may be used alone or in combination with other materials.

[0047] With regard to conditions not specified in this application, there are no particular restrictions as long as the objectives of the present invention are achieved. [Examples]

[0048] The present invention will be described in more detail below with reference to examples and comparative examples of the present invention. The following examples are provided for illustrative purposes only and do not limit the present invention in any way.

[0049] (Example 1) [Synthesis of oxide microcrystalline powder (precursor) by spray pyrolysis method] A metal salt aqueous solution was prepared by mixing 161.6 g of iron nitrate nonahydrate, 5.7 g of samarium nitrate hexahydrate, 3.4 g of zirconyl nitrate dihydrate, 2 L of pure water, and 8.4 ml of nitric acid (1.38 N) to obtain a metal element ratio (atomic %) of Fe:Sm:Zr = 94:3:3. This solution was atomized in a sealed container using an ultrasonic transducer and introduced into an alumina furnace tube with an inner diameter of 42 mm, equipped with an external heating mechanism using an electric furnace and air (flow rate 20 L / min) as the carrier gas. The solution was thermally decomposed at an electric furnace temperature of 900 °C to synthesize oxide microcrystalline powder (sometimes simply referred to as "oxide powder" in this application) as a precursor, and the synthesized oxide microcrystalline powder (oxide powder) was recovered by a subsequent filter.

[0050] [Hydrogen reduction of oxide microcrystalline powder (precursor)] The recovered precursor oxide microcrystalline powder (oxide powder) was placed on a boat and set in a tubular furnace equipped with an external heating mechanism using an electric furnace. Hydrogen reduction was performed at 500°C for 6 hours under a hydrogen atmosphere. The powder obtained by this hydrogen reduction (sometimes simply referred to as "hydrogen-reduced powder" in this application) was used as the powder (sample) obtained in this example (Example 1). It was confirmed that this powder was a soft magnetic powder mainly consisting of the core-shell particles of the present invention by the following evaluation methods. Here, "mainly" means that the core-shell particles of the present invention, which are the soft magnetic powder material, are contained in the soft magnetic powder to an extent that the objectives of the present invention can be achieved.

[0051] [Various evaluations] Regarding the oxide powder (precursor) and the powder after hydrogen reduction obtained in this example, the crystallinity of the oxide powder (precursor) and the crystalline phase of the oxide in the powder after hydrogen reduction were evaluated by X-ray diffraction (XRD) using a Co source, and the crystallite size of Fe crystallites in the powder particles after hydrogen reduction (Fe crystallite size) was calculated using the following Scherrer equation. An Empyrean from Malvern Panalytical was used for the X-ray diffraction (XRD) measurements. D=Kλ / Bcosθ (Scherrer equation) Here, D is the crystallite size (nm), K is the Scherrer constant, λ is the wavelength of the X-ray (nm), B is the diffraction line width broadening (rad), and θ is the Bragg angle (rad).

[0052] The particle size and appearance of the particles constituting the powder obtained in this embodiment were evaluated using a scanning electron microscope (FE-SEM). A JEOL JSM-7800 was used as the scanning electron microscope (FE-SEM).

[0053] The particle cross-sections of the powder particles obtained in this embodiment were observed using a transmission electron microscope (TEM) equipped with an energy-dispersive X-ray analyzer (EDX), with thin sections prepared from a sample of the powder embedded in epoxy resin using a focused ion beam (FIB) system. A JEM-ARM200 microscope manufactured by JEOL was used for the observations.

[0054] The magnetic properties of the powder obtained in this embodiment were determined using a vibrating sample magnetometer (VSM). Coercivity was measured using a Helmholtz coil at a maximum applied magnetic field of 250 Oe, and saturation magnetization was measured using a superconducting magnet at a maximum applied magnetic field of 9 T. A BHV-55 manufactured by RIKEN Electronics was used for measuring coercivity, and a DynaCool manufactured by Quantum Design was used for measuring saturation magnetization. Electrical resistance was determined by preparing a molded body by compacting or electrically pressurizing the powder obtained in this embodiment, and measuring the volume resistivity of the molded body using the four-probe method (Loresta manufactured by Mitsubishi Chemical Analytec). In this case, bulk density was used as the density. For measuring magnetic permeability, a ring-shaped sample with an outer diameter (φ) of 8.0 mm and an inner diameter (φ) of 3.1 mm was prepared by mixing core-shell powder with 8% by mass of epoxy resin and compacting it at 1 GPa. This ring-shaped sample was used as the molded body, and the frequency range from 1 MHz to 3 GHz was measured using an impedance analyzer (Agilent E4991A). Table 1 shows the synthesis conditions (composition ratio of metal elements (atomic %)) and crystallinity of the oxide powder (precursor) in Example 1, as well as the evaluation results of the powder after hydrogen reduction in the same example. In Table 1, "core-shell structure" refers to a structure consisting of an iron particle core having an oxide grain boundary layer and an oxide shell covering the entire surface of the core. ○ indicates the presence of this structure, and × indicates the absence of this structure.

[0055] (Examples 2 to 6 and Comparative Examples 1 to 3) In Examples 2 to 6 and Comparative Examples 1 to 3, each powder (sample) was prepared in the same manner as in Example 1, except that iron nitrate nonahydrate, samarium nitrate hecahydrate, and zirconyl nitrate dihydrate were weighed to achieve the metal element composition ratios (atomic %) shown in Table 1. Their respective properties were also evaluated and confirmed in the same manner as in Example 1. For Examples 2 to 6 and Comparative Examples 1 to 3, the synthesis conditions (metal element composition ratios (atomic %)), crystallinity, and evaluation results of the hydrogen-reduced powders are shown in Table 1, in the same manner as in Example 1.

[0056] [Table 1]

[0057] Figure 2 shows an FE-SEM image of the powder (hydrogen-reduced powder) obtained in Example 3, which is one embodiment of the present invention; Figure 3 shows a TEM observation image of the particle cross-section of the particles constituting the powder; and Figure 4 shows EDX mapping images (left and middle) of the particle cross-section of the particles constituting the powder, and the line analysis results of the arrow lines in the EDX mapping image (middle) (right). From Figures 2 and 3, it was confirmed that the individual particles constituting the powder (hydrogen-reduced powder) obtained in Example 3 have a core-shell structure, i.e., they are core-shell particles. Furthermore, from Figure 4, it was confirmed that the core portion consists of Fe crystallites, and that an oxide grain boundary layer of several nanometers to several tens of nanometers (specifically, with an average thickness of 9.4 nm, as shown in Table 1) exists around the Fe crystallites (in other words, between the Fe crystallites constituting the core portion). Although not shown in the figures, using an evaluation method similar to that of Example 3, it was confirmed that the powders obtained in the other examples of this application (hydrogen-reduced powders) were core-shell particles having a core structure similar to that of the powder obtained in Example 3, while the powders obtained in Comparative Examples 1 to 3 (hydrogen-reduced powders) were particles that did not have a core-shell structure.

[0058] Figure 5 shows the EDX mapping image (left) of the cross-section of the particles (core-shell particles) constituting the powder (hydrogen-reduced powder) obtained in Example 3, and the high-resolution TEM analysis image of the shell portion of the particles (core-shell particles). It was confirmed that the shell portion is an oxide layer consisting of fine crystalline grains on the nanometer scale (specifically, 1 to 100 nm) (in this application, the shell (or shell portion) having this structure is also referred to as the "oxide shell (or oxide shell portion)"). Furthermore, from Figures 4 and 5, it was confirmed that the shell portion of the powder (hydrogen-reduced powder) obtained in Example 3 is porous. Although not shown in the figures, the shell portions of the core-shell particles constituting the powder (hydrogen-reduced powder) obtained in other examples of this application were also confirmed to have the same structure as the shell portion of the core-shell particles in Example 3, using the same evaluation method as in Example 3.

[0059] From the results in Figures 2 to 5, it was confirmed that the powder obtained in Example 3 (hydrogen-reduced powder) consists of particles with a core-shell structure (core-shell particles), and that an oxide grain boundary layer of several nanometers to several tens of nanometers (specifically, an average thickness of 9.4 nm, as shown in Table 1) exists around each Fe crystallite constituting the core. From these results, it was confirmed that the individual particles (core-shell particles) constituting the powder obtained in Example 3 (hydrogen-reduced powder) have the structure shown in Figure 1. Although not shown in the figures, as mentioned above, the powders (hydrogen-reduced powders) obtained in other examples of this application yielded similar results to the powder obtained in Example 3 (hydrogen-reduced powder), so it was confirmed that the individual particles (core-shell particles) constituting those powders also have the structure shown in Figure 1, similar to Example 3.

[0060] Figure 6 shows the XRD pattern of the powder (hydrogen-reduced powder) obtained in Example 3. In this figure, only the diffraction peaks of α-Fe and Sm2Zr2O7 were observed. Therefore, it was confirmed that the oxide crystalline phase of the core-shell particles constituting the powder (hydrogen-reduced powder) obtained in Example 3 is Sm2Zr2O7. Thus, it was found that the oxide of the oxide grain boundary layer between crystallites in the core portion of the core-shell particles obtained in Example 3 and the oxide layer consisting of fine crystalline grains that constitute the porous shell portion covering the entire surface of the core portion are the same Sm2Zr2O7. The broad diffraction peaks in Figure 6 are due to the fact that the oxide layer consists of nanometer-scale fine crystalline grains, as also confirmed from the TEM image in Figure 5.

[0061] Table 1 shows the α-Fe crystallite size calculated from the Scherrer equation. It was confirmed that when the metal element composition ratio of Sm and Zr in the initial metal element composition consisting of Fe, Sm, and Zr during the synthesis of oxide powder (precursor) reached 3 atomic percent, core-shell particles (particles having a core-shell structure in which the core portion is an iron particle core with an oxide grain boundary layer and the shell portion is an oxide layer consisting of fine crystalline grains) as shown in Figure 1 were generated. Furthermore, when comparing Examples 1 to 5, in particles having a core-shell structure consisting of an iron particle core with an oxide grain boundary layer on its surface and an oxide shell, where the oxide of the oxide grain boundary layer in the core portion and the oxide of the oxide layer consisting of fine crystalline grains in the shell portion are the same Sm2Zr2O7, with Comparative Example 2, which has the same Sm2Zr2O7 oxide crystalline phase as Examples 1 to 5 but does not have the core-shell structure as Examples 1 to 5, it was confirmed that the Fe crystallite size of Examples 1 to 5 was significantly smaller and the coercivity was also significantly reduced. The decrease in crystallite size is understood to be caused by the fragmentation of Fe crystallites by the Sm2Zr2O7 oxide grain boundary layer.

[0062] The relationship between Fe crystallite size and coercivity has been reported in non-patent literature (Katsuhiko Yoshizawa, Materia (2017), pp. 186). The values ​​of Fe crystallite size and coercivity for Comparative Examples 1 and 2 and Examples 1 to 5 in Table 1 generally agree with the relationship between Fe crystallite size and coercivity reported in the said non-patent literature. In this regard, it is understood that in the powders prepared in Examples 1 to 5, magnetic interactions act between Fe crystallites separated by the Sm2Zr2O7 oxide grain boundary layer, and the low coercivity is due to a decrease in crystal magnetic anisotropy according to the random magnetic anisotropy model.

[0063] As shown in Table 1, comparing the core-shell particles of Examples 4 and 5 with those of Example 3, a slight increase in coercivity is observed due to the increase in the oxide layer (i.e., an increase in the weight (%) of the oxide). Therefore, in the core-shell particles of Examples 4 and 5, although the magnetic interaction between Fe crystallites may be weakened due to the increased thickness of the oxide grain boundary layer, it was found that the low coercivity desirable for the present invention is still maintained.

[0064] The saturation magnetization value was found to decrease with increasing Sm and Zr composition ratio (atomic %), as seen in Examples 1 to 5 in Table 1, when the elemental composition of the base metal, in which elements other than oxygen that form the oxide layer of the core-shell particles, are occupied in the same proportion (i.e., Sm (atomic %) / Zr (atomic %) = 1). For example, the base metal composition (Fe) in Example 5 70 Sm 15 Zr 15 In the case where Sm and Zr all become Sm2Zr2O7, the weight ratio of Sm2Fe2O7 is calculated to be approximately 53%, and it was found that the saturation magnetization value of Example 5 is roughly in agreement with the decrease from the saturation magnetization value of Comparative Example 1.

[0065] Figure 7 shows the XRD patterns of the oxide powders obtained by spray pyrolysis in Comparative Examples 1 to 3, and Examples 1, 2, and 6. By comparing the results of Comparative Example 1, which did not contain Sm or Zr, with those of Example 6, which contained Sm but not Zr (see Figure 7 and Table 1), it was found that the addition of Sm significantly reduced the crystallinity of the oxide. On the other hand, from the results of Comparative Example 3, which contained Zr but not Sm (see Figure 7 and Table 1), it was found that the addition of Zr did not affect the crystallinity of the oxide. By comparing the results of Example 2, which contained Sm and Zr, with those of Example 6, which contained Zr but not Sm (see Table 1), it was found that the addition of Zr, when used together with rare earth elements such as Sm, had the effect of reducing the Fe crystallite size in each particle constituting the powder after hydrogen reduction. By comparing the results of Comparative Example 2, where the composition ratio of Sm element was 1 atom%, with those of Example 1, where the composition ratio of Sm element was 3 atom% (see Figure 7 and Table 1), it was found that when the composition ratio of Sm element was 1 atom%, the crystallinity of the oxide was high, and core-shell particles with the structure shown in Figure 1 could not be obtained after hydrogen reduction. On the other hand, by setting the composition ratio of Sm element to 3 atom%, the crystallinity of the oxide was significantly reduced, resulting in a microcrystalline state where a slight hematite peak could be observed, and thus core-shell particles with the structure shown in Figure 1 could be obtained after hydrogen reduction. By comparing the results of Comparative Examples 1 to 3 with those of Examples 1 to 6 (see Table 1), it was found that the coercivity was significantly reduced because the Fe crystallites constituting the core of the core-shell particles had a structure covered with an oxide grain boundary layer. In Example 2, where the composition ratio of Sm element was 5 atomic%, the hematite peak was hardly observed in the oxide powder, resulting in a state that was almost amorphous. Furthermore, it was found that by hydrogen reduction of the oxide powder, core-shell particles having the structure shown in Figure 1 could be obtained (see Figure 7 and Table 1).

[0066] As shown in Table 1, Example 6 differs from Examples 1 to 5, which contain Zr, in that Zr is not added. In Example 6, because Zr is not added, the Fe crystallite diameter of each particle constituting the hydrogen-reduced powder (core-shell particles having the structure shown in Figure 1) is larger compared to Examples 1 to 5, but the coercivity is found to be equally low. This is understood to be because the core portion of the core-shell particles (specifically, the Fe crystallites constituting the core portion) has a particle structure separated by an oxide grain boundary layer (i.e., it is a core-shell particle having the structure shown in Figure 1, and the core portion having the core structure shown in the same figure), which greatly contributes to the reduction of coercivity.

[0067] Figure 8 shows the relationship between the relative density and volume resistivity of the compacted or sintered powder obtained in Example 3 and Comparative Example 1. Regarding relative density, in Comparative Example 1, since it contains only Fe, the true density of Fe is 7.87 g / cm³. 3 Using the value of Fe, in Example 3, 84 From the composition of Sm8Zr8, all of Sm and Zr are found in Sm2Zr2O7 (true density 6.91 g / cm³). 3 Fe in the case of ) 84 By calculating the true density of Sm8Zr8, we obtain 7.41 g / cm³. 3 It was determined that the value was [value]. In Example 3, the powder after hydrogen reduction becomes core-shell particles having the structure shown in Figure 1, and includes a core portion and an oxide shell having an oxide grain boundary layer with high electrical resistance. Therefore, it is understood that the powder after hydrogen reduction exhibits a volume resistivity (electrical resistance) that is two orders of magnitude higher than that of Comparative Example 1, in which the powder after hydrogen reduction does not become such core-shell particles.

[0068] Figure 9 shows the frequency characteristics of the real part μ' and imaginary part μ'' of the complex permeability obtained by impedance measurement of molded bodies formed from the hydrogen-reduced powders of each sample (specifically, the hydrogen-reduced powders of Comparative Example 1 and Examples 1 to 5). Table 2 shows the characteristic values ​​of the real part μ' and imaginary part μ'' of the complex permeability for each sample. By forming core-shell particles with the structure shown in Figure 1 (i.e., core-shell particles in which the Fe crystallites constituting the core are covered with an oxide grain boundary layer with high electrical resistance), the generation of eddy currents is suppressed, and as shown in Figure 9 and Table 2, the imaginary part μ'' of the complex permeability is kept low even at high frequencies of 10 MHz or higher, while the real part μ' of the complex permeability remains high even at high frequencies. In particular, it was found that significantly large values ​​can be obtained at high frequencies of 100 MHz or higher.

[0069] Furthermore, the results shown in Figure 9 and Table 2 indicate that, in particular, the hydrogen-reduced powders of Examples 3 to 5 all exhibit magnetic resonance at frequencies above 3 GHz (3000 MHz). This suggests that the core-shell particles of the present invention are promising not only for amplification applications such as power electronics devices, but also as magnetic filter materials in high-frequency ranges exceeding GHz, where conventional soft magnetic metal materials cannot be used.

[0070] [Table 2]

[0071] (Examples 7 to 13) As described above, Sm was used as the rare earth element R in Examples 1 to 6, and Zr was used as the T element in Examples 1 to 5. Furthermore, powders were prepared in which Sm, used as the rare earth element R, and Zr, used as the T element in Examples 1 to 5, were replaced with other rare earth elements R (specifically Nd, Gd) and T elements (specifically Mn, Cr, Al) as samples, and these were designated as Examples 7 to 11. In addition, samples were prepared in which the core was changed from Fe to a two-component system of Fe-Co and Fe-Ni, and these were designated as Examples 12 and 13, respectively.

[0072] Specifically, in Example 7, iron nitrate nonahydrate, samarium nitrate hexahydrate, and manganese nitrate hexahydrate were weighed to achieve the elemental composition ratio (atomic %) shown in Table 3; in Example 8, iron nitrate nonahydrate, samarium nitrate hexahydrate, and chromium nitrate nonahydrate were weighed to achieve the elemental composition ratio (atomic %) shown in Table 3; and in Example 9, iron nitrate nonahydrate, samarium nitrate hexahydrate, and aluminum nitrate were weighed to achieve the elemental composition ratio (atomic %) shown in Table 3. In Example 10, iron nitrate nonahydrate, neodymium nitrate hexahydrate, and zirconyl nitrate dihydrate were weighed to achieve the metal element composition ratio (atomic %) shown in Table 3, and in Example 11, iron nitrate nonahydrate, gadolinium nitrate hexahydrate, and zirconyl nitrate dihydrate were weighed to achieve the metal element composition ratio (atomic %) shown in Table 3. Except for these differences, each powder (sample) was prepared in the same manner as in Example 1, and its properties were evaluated and confirmed in the same manner as in Example 1. Furthermore, in Example 12, iron nitrate nonahydrate, cobalt nitrate hexahydrate, samarium nitrate hexahydrate, and zirconyl nitrate dihydrate were weighed to achieve the metal element composition ratio (atomic %) shown in Table 3. In Example 13, iron nitrate nonahydrate, nickel nitrate hexahydrate, samarium nitrate hexahydrate, and zirconyl nitrate dihydrate were weighed to achieve the metal element composition ratio (atomic %) shown in Table 3. Except for these differences, each powder (sample) was prepared in the same manner as in Example 1, and its properties were evaluated and confirmed in the same manner as in Example 1. For Examples 7 to 13, the synthesis conditions (metal element composition ratio (atomic %)) and crystallinity of the oxide powder (precursor), as well as the evaluation results of the powder after hydrogen reduction, are shown in Table 3. Note that the meaning of "core-shell structure" in Table 3 is the same as in Table 1.

[0073] [Table 3]

[0074] As shown in Table 3, the results from Examples 7 to 9 indicate that if element T is a metallic element such as Mn, Cr, or Al that is not reduced by hydrogen reduction at 500°C, then particles with the core-shell structure shown in Figure 1 (core-shell particles) can be obtained after hydrogen reduction, even if it is not Zr. Furthermore, it was confirmed that the powder consisting of these core-shell particles exhibits desirable soft magnetic properties, with a coercivity of less than 10 Oe.

[0075] As shown in Table 3, the results from Examples 10 and 11 indicate that the rare earth element R is not limited to Sm, and that particles with the core-shell structure shown in Figure 1 (core-shell particles) can be obtained after hydrogen reduction with other rare earth elements as well. Furthermore, it was confirmed that the powder consisting of these core-shell particles exhibits desirable soft magnetic properties, with a coercivity of less than 10 Oe.

[0076] As shown in Table 3, the results from Examples 12 and 13 show that even if the soft magnetic metal constituting the core is not Fe, if it is an Fe-containing alloy such as Fe-Ni or Fe-Co, particles with the core-shell structure shown in Figure 1 (core-shell particles) can be obtained after hydrogen reduction. Furthermore, it was confirmed that the powder consisting of these core-shell particles exhibits desirable soft magnetic properties, with a coercivity of less than 10 Oe.

[0077] As shown in Table 3, in Example 7, despite having a large Fe crystallite size, similar in size to that of Comparative Example 1, the coercivity was found to be very low. Furthermore, Figure 10 shows the TEM observation image and EDX mapping results of the hydrogen-reduced powder of Example 7, and from these results, it was confirmed that the particles had the core-shell structure shown in Figure 1 (core-shell particles). Therefore, it was found that the particle structure of the core-shell particles shown in Figure 1 (specifically, the particle structure in which the Fe crystallites constituting the core of the core-shell particles are separated by oxide grain boundary layers, as shown in Figure 1) greatly contributed to achieving low coercivity.

[0078] Figure 11 shows the frequency characteristics of the real part μ' and imaginary part μ'' of the complex permeability measured by an impedance analyzer on a molded body formed from the hydrogen-reduced powder of Example 7. From this figure, it was found that the hydrogen-reduced powder of Example 7, like the other examples in this application, has core-shell particles with the structure shown in Figure 1 (i.e., core-shell particles in which the Fe crystallites constituting the core are covered with an oxide grain boundary layer with high electrical resistance), thereby suppressing the generation of eddy currents, and the imaginary part μ'' of the complex permeability is very low up to high frequencies exceeding 200 MHz, while the real part μ' of the complex permeability remains stable and high up to about 1 GHz. Furthermore, it was found that the magnetic resonance frequency of the hydrogen-reduced powder of Example 7 occurs at frequencies of 3 GHz (3000 MHz) or higher, similar to Examples 3 to 5, indicating that it is promising not only for amplification applications such as power electronics devices, but also as a magnetic filter material in high-frequency ranges exceeding the GHz band, where conventional soft magnetic metal materials cannot be used.

[0079] From the above results, it was found that the present invention can provide core-shell particles having the following core portion and the following shell portion, which consist of two or more crystallites. The core portion has an oxide grain boundary layer between the crystallites, The crystallite consists of Fe or Fe and M (where M is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi); The shell portion is porous, made of oxide, and covers the entire surface of the core portion; and, The oxide in the oxide grain boundary layer in the core and the oxide in the shell are the same oxide and contain a rare earth element R or a rare earth element R and element T (where the rare earth element R is one or more elements selected from the group consisting of Y and lanthanide elements, and the element T is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W).

[0080] Furthermore, it was confirmed that the core-shell particles of the present invention can provide a novel soft magnetic material in powder form with high magnetization, low coercivity, and low eddy current loss (i.e., high electrical resistance). It was also confirmed that the imaginary part μ'' of the complex permeability is low even under magnetic fields of 10 MHz or higher, and the real part μ' of the complex permeability remains stably high up to high frequencies, thus providing a soft magnetic material powder that can be used even at high frequencies. Therefore, it can be used as a power electronics device material, and it was found that it can be used as a power electronics device material even at high frequencies. In addition, it was confirmed that the core-shell particles of the present invention can provide a soft magnetic material powder with high electrical resistance and magnetic resonance even at frequencies of 3 GHz or higher, so it can also be used as a magnetic filter material in the high-frequency range exceeding the GHz band. Furthermore, it was confirmed that the core-shell particles of the present invention can be manufactured by a process of synthesizing an amorphous oxide powder or microcrystalline powder by spray pyrolysis and a process of hydrogen reduction of the amorphous oxide powder or microcrystalline powder, thus allowing for the simple manufacture of the soft magnetic material powder of the present invention. [Industrial applicability]

[0081] The present invention provides a novel soft magnetic material with high magnetization, low coercivity, and low eddy current loss (i.e., high electrical resistance), making it applicable to any field requiring soft magnetic materials. Therefore, it has potential applications in a wide variety of industries (e.g., the automotive industry, electronics and electrical industry, telecommunications industry, information equipment industry, aerospace industry, military industry, etc.). The present invention also provides a soft magnetic material suitable for high-frequency applications, and therefore, it is particularly promising for use in power electronics devices and electromagnetic wave filters where high frequencies are required. [Explanation of Symbols]

[0082] 1: Core-shell particles 10: Oxide shell 11a: Soft magnetic metal crystallite 11b: Oxide grain boundary layer

Claims

1. A core-shell particle having the following core portion consisting of two or more crystallites and the following shell portion: The core portion has an oxide grain boundary layer between the crystallites, The crystallite consists of Fe or Fe and M (where M is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi); The shell portion is porous, made of oxide, and covers the entire surface of the core portion; and, The oxide in the oxide grain boundary layer in the core and the oxide in the shell are the same oxide and contain a rare earth element R or a rare earth element R and element T (where the rare earth element R is one or more elements selected from the group consisting of Y and lanthanide elements, and element T is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W).

2. The core-shell particle according to claim 1, wherein the content of rare earth element R in the oxide is 2 atomic percent or more and 16 atomic percent or less in terms of the metal element composition ratio in the core-shell particle.

3. The core-shell particle according to claim 1 or 2, wherein the crystallite diameter of the crystallite is 10 nm or more and 100 nm or less.

4. The core-shell particle according to claim 1 or 2, wherein the oxide grain boundary layer has a thickness of 1 nm to 20 nm.

5. The core-shell particle according to claim 1 or 2, wherein the weight of the oxide in the core-shell particle is 10% by weight or more and 55% by weight or less relative to the weight of the core-shell particle.

6. A soft magnetic powder comprising the core-shell particles described in claim 1.

7. A molded body which is a compacted or sintered body made of the soft magnetic powder described in claim 6.

8. A molded body which is a compacted powder or sintered body containing the soft magnetic powder and binder described in claim 6.

9. A soft magnetic member comprising the molded body described in claim 7 or 8.

10. The soft magnetic member according to claim 9, wherein the frequency at which the value of the imaginary part (μ'') of the complex permeability reaches its maximum value is 10 MHz or higher.

11. The soft magnetic member according to claim 9, wherein the frequency at which the value of the imaginary part (μ'') of the complex permeability reaches its maximum value is 3 GHz or higher.

12. An electromagnetic wave filter comprising the soft magnetic member described in claim 10.

13. A method for producing the core-shell particles described in claim 1, comprising the following steps: (1) A step of synthesizing amorphous or microcrystalline powder of an oxide composed of Fe element, Fe element and M element, and rare earth element R or rare earth element R and T element by spray pyrolysis of a mixed solution containing Fe element or Fe element and M element and rare earth element R or rare earth element R and T element; and (2) A step of producing core-shell particles by reducing the amorphous or microcrystalline powder of the oxide, However, the aforementioned M element is one or more elements selected from the group consisting of Co, Ni, Cu, Zn, Sn, Ga, Sn, Pb, and Bi; The rare earth element R is one or more elements selected from the group consisting of Y and lanthanide elements; and, The aforementioned element T is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Zr, Nb, Mo, Mn, Hf, Ta, and W.

14. The method according to claim 13, wherein the amount of rare earth element R contained in the amorphous or microcrystalline powder of the oxide is 2 atomic percent or more and 16 atomic percent or less.

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