JP2022105907A - Hard magnetic powder - Google Patents

Abstract

To provide a powder for a magnetic member capable of absorbing high electromagnetic waves having extremely high frequencies.SOLUTION: A hard magnetic powder is made of many flat particles 6. The material of the particles 6 is a rare earth metal-transition metal- B system alloy. The maximum aspect ratio of this powder is 5.0 or more. The preferable rare earth metal is Nd, and the preferable transition element is Fe. According to a preferable embodiment, the alloy comprises 10.8 atom% or more and 16.0 atom% or less of Nd, 5.4 atom% or more and 9.0 atom% or less of B, 0.3 atom% or more and 1.0% atom% of C, 15 atom% or less of O, and the balance Fe with inevitable impurities.SELECTED DRAWING: Figure 2

Description

The present invention relates to a powder having a flat particle shape and having hard magnetism.

Electronic devices such as personal computers and mobile phones have circuits. Due to the radio wave noise radiated from the electronic component mounted on this circuit, radio wave interference between the electronic component and other electronic components and radio wave interference between the electronic circuit and other electronic circuits occur. Radio wave interference causes malfunction of electronic devices. An electromagnetic wave absorption sheet is inserted into an electronic device for the purpose of suppressing malfunction.

In information communication in recent years, the communication speed has been increased. High-frequency radio waves are used for this high-speed communication. Therefore, an electromagnetic wave absorbing sheet suitable for use in a high frequency range is desired.

Japanese Unexamined Patent Publication No. 2020-152979 describes a magnetic sheet suitable for use in a high frequency range. This magnetic sheet contains powder. This powder consists of a large number of particles having a flat shape. The material of this powder is a Fe—Ni—Al—Co alloy.

Japanese Unexamined Patent Publication No. 2020-152979

The magnetic sheet absorbs electromagnetic waves by a natural resonance phenomenon. Natural resonance depends on the precession of the magnetic moment. Natural resonance at high frequencies can contribute to the absorption of high frequency electromagnetic waves. The natural resonance frequency correlates with the coercive force iHc. The coercive force of the powder described in JP-A-2020-152979 is 31 kA / m or less. This powder is not suitable for absorbing electromagnetic waves with extremely high frequencies.

An object of the present invention is to provide a powder for a magnetic member capable of absorbing electromagnetic waves having an extremely high frequency.

The hard magnetic powder according to the present invention is composed of a large number of flat particles. The material of these particles is a rare earth metal-transition metal-B based alloy. The maximum aspect ratio of this powder is 5.0 or more.

Preferably, the alloy contains Nd as a rare earth metal and Fe as a transition element.

Preferably, this alloy is
Nd: 10.8 atomic% or more and 16.0 atomic% or less B: 5.4 atomic% or more and 9.0 atomic% or less C: 0.3 atomic% or more and 1.0 atomic% or less and O: 15 atomic% or less include. The balance is Fe and unavoidable impurities.

Preferably, the coercive force iHc of the powder is 90 kA / m or more. Preferably, the median diameter D50 of the powder is 5 μm or more and 60 μm or less. Preferably, the tap density TD of the powder is 0.50 g / cm ³ or more and 3.50 g / cm ³ or less.

According to another aspect, the magnetic member according to the present invention has a matrix in which the polymer is a base material and a hard magnetic powder dispersed in the matrix. This hard magnetic powder consists of a large number of flat particles. The material of these particles is a rare earth metal-transition metal-B based alloy. The maximum aspect ratio of this hard magnetic powder is 5.0 or more.

The magnetic member using the hard magnetic powder according to the present invention can absorb extremely high frequencies.

FIG. 1 is a schematic cross-sectional view showing a part of a magnetic member according to an embodiment of the present invention. FIG. 2 is an enlarged view showing the flat particles contained in the magnetic member of FIG. 1. FIG. 3 is a backscattered electron image showing a cross section of powder particles according to the first embodiment of the present invention.

Hereinafter, the present invention will be described in detail based on a preferred embodiment with reference to the drawings as appropriate.

[Magnetic sheet]
FIG. 1 shows a magnetic sheet 2 (magnetic member). The magnetic sheet 2 is made of a polymer composition. The magnetic sheet 2 has a matrix 4 of a base polymer and a powder dispersed in the matrix 4. The base polymer is rubber or resin. A powder is an assembly of a large number of particles 6.

[Particle shape]
FIG. 2 shows a cross section of one particle 6. In FIG. 1, reference numeral L1 indicates the length of the major axis of the particle 6, and reference numeral T1 indicates the thickness of the particle 6. The length L1 is larger than the thickness T1. In other words, the particles 6 are flat.

The flat particles 6 have shape anisotropy. This anisotropy enhances the real magnetic permeability μ'of the magnetic member. Moreover, in the magnetic sheet 2 containing the flat particles 6 having a small thickness T1, the eddy current loss is suppressed, so that the real part magnetic permeability μ'is less likely to be relaxed. The magnetic sheet 2 containing the flat particles 6 can absorb electromagnetic waves having a high frequency.

[Maximum aspect ratio]
The maximum aspect ratio of this powder is 5.0 or more. A powder having a maximum aspect ratio of 5.0 or more gives a large strain to the particles during flattening. Therefore, the coercive force of this powder is large. In the powder having a maximum aspect ratio of 5.0 or more, the shape anisotropy of the particles 6 is large. Further, in the magnetic sheet 2 containing powder having a maximum aspect ratio of 5.0 or more and a small particle thickness, eddy current loss is suppressed. Therefore, the magnetic sheet 2 containing the powder having a maximum aspect ratio of 5.0 or more can absorb electromagnetic waves having a high frequency.

From the viewpoint of electromagnetic wave absorption in the high frequency region, the maximum aspect ratio is more preferably 8.0 or more, and particularly preferably 10.0 or more. The maximum aspect ratio is preferably 100 or less. In the magnetic member using the powder having the maximum aspect ratio of 100 or less, the portion where the particle 6 comes into contact with the other particles 6 is suppressed, and the loss due to the eddy current is suppressed. From this point of view, the maximum aspect ratio is more preferably 50 or less, and particularly preferably 30 or less.

A resin-filled sample in which the thickness direction of the flat particles 6 can be observed is used for measuring the aspect ratio. This sample is polished and the polished surface is observed by a scanning electron microscope (SEM). The magnification of the image at the time of observation is 1000 times. The obtained backscattered electron image data is used for image analysis. The data of this image is binarized by the image analysis software. When the binarized image of the particle 6 is approximated to an ellipse, the ratio of the length of the major axis to the length of the minor axis of the ellipse is the aspect ratio of the particle 6. The largest aspect ratio (maximum aspect ratio) is selected from the aspect ratios of a large number of binarized particles obtained in the four fields of view.

[Material]
The material of the flat particles 6 is a rare earth metal-transition metal-B-based alloy. This alloy contains a rare earth metal M1, a transition metal M2 and boron B. The metallographic structure of this alloy contains an intermetallic compound (M12 _{M2 14} B). This intermetallic compound contributes to the high coercive force of the powder.

Preferred rare earth metals M1 are exemplified by Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Ho, Er, Tm and Y. Two or more rare earth metals M1 may be used in combination. Preferred transition metals M2 are exemplified by Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo and W. Two or more transition metals M2 may be used in combination.

[Preferable alloy]
A particularly preferred rare earth metal M1 is Nd. A particularly preferred transition metal M2 is Fe. In other words, Nd—Fe—B based alloys are particularly preferable. The metallographic structure of this alloy contains an intermetallic compound (Nd ₂ Fe ₁₄ B). This intermetallic compound contributes to the high coercive force of the powder.

Preferred alloys are Fe-based alloys. This Fe-based alloy is
Nd: 10.8 atomic% or more and 16.0 atomic% or less B: 5.4 atomic% or more and 9.0 atomic% or less C: 0.3 atomic% or more and 1.0 atomic% or less and O: 15 atomic% or less include. Preferably, the balance is Fe and unavoidable impurities.

[Neodymium (Nd)]
Although this alloy contains a large amount of B, the main phases constituting the metal structure are Nd ₂ Fe ₁₄ B, an Nd rich phase, and a small amount of αFe phase. These phases are crystalline phases, not amorphous phases. Nd is an essential element for the formation of an intermetallic compound (Nd ₂ Fe ₁₄ B). The powder having this intermetallic compound has a high saturation magnetization and a high coercive force. From this point of view, in the present invention, the content of Nd is defined as 10.8 atomic% or more. This content is more preferably 10.9 atomic% or more, and particularly preferably 11.1 atomic% or more. Excess Nd precipitates an excess intermetallic compound (Nd ₂ Fe ₁₄ B). Excessive precipitation impairs the ductility of the alloy and hinders the processing into flat grains with a large aspect ratio. From the viewpoint of ductility, the content of Nd is defined as 16.0 atomic% or less in the present invention. This content is more preferably 15.0 atomic% or less, and particularly preferably 14.0 atomic% or less.

[Boron (B)]
B is an essential element for the formation of an intermetallic compound (Nd ₂ Fe ₁₄ B). The powder having this intermetallic compound has a high saturation magnetization and a high coercive force. From this point of view, in the present invention, the content of B is defined as 5.4 atomic% or more. This content is more preferably 5.6 atomic% or more, and particularly preferably 6.3 atomic% or more. The content of B is preferably 9.0 atomic% or less, more preferably 8.5 atomic% or less, and particularly preferably 7.9 atomic% or less.

[Carbon (C)]
C is a particularly important element in the present invention. C is derived from the organic solvent used for flattening and is mixed in the alloy. C is mainly present in the vicinity of the surface of the flat particles 6. According to the knowledge obtained by the present inventor, the high coercive force of the powder is achieved by the mixing of C. It is presumed that the reason is that the compound obtained by binding C to Nd contributes to the coercive force. From the viewpoint of coercive force, the content of C is defined to be 0.3 atomic% or more in the present invention. This content is particularly preferably 0.4 atomic% or more. The contamination of C is accompanied by the contamination of O. As will be described later, excessive mixing of O is not preferable. From this point of view, in the present invention, the content of C is defined as 1.0 atomic% or less. This content is more preferably 0.9 atomic% or less, and particularly preferably 0.8 atomic% or less.

[Oxygen (O)]
O is inevitably mixed with the alloy during atomization and flattening. O reacts with Nd to form an oxide represented by Nd ₂ O ₃ . The formation of this oxide causes a decrease in the amount of Nd ₂ Fe ₁₄ B formed, and inhibits the coercive force of the powder. From the viewpoint of coercive force, the content of O is defined to be 15 atomic% or less in the present invention. This content is more preferably 13 atomic% or less, and particularly preferably 11 atomic% or less.

[Coercive force iHc]
The coercive force iHc is the strength of the external magnetic field required to return the magnetized magnetic material to the unmagnetized state. The magnetic sheet 2 containing a powder having a large coercive force iHc can absorb electromagnetic waves having a high frequency. From this viewpoint, the coercive force iHc of the powder is preferably 90 kA / m or more, more preferably 100 kA / m or more, and particularly preferably 150 kA / m or more. This powder is hard magnetic.

The coercive force is measured with a vibrating sample magnetometer (VSM). The measurement conditions are as follows.
Maximum applied magnetic field: 1204 kA / m
Powder mass: Approximately 70 mg
The direction of the applied magnetic field is the longitudinal direction of the flat particles 6.

[Median diameter D50]
From the viewpoint that a magnetic sheet 2 having a uniform surface and a smooth surface can be obtained, the median diameter D50 of the powder is preferably 60 μm or less, more preferably 50 μm or less, and particularly preferably 40 μm or less. From the viewpoint of coercive force, the median diameter D50 is preferably 5 μm or more, more preferably 6.3 μm or more, and particularly preferably 10 μm or more.

The median diameter D50 is the diameter of the particles 6 at the point where the cumulative curve is 50% when the cumulative curve is obtained with the total volume of the powder as 100%. The median diameter D50 is measured by, for example, the laser diffraction / scattering type particle size distribution measuring device “Microtrack MT3000” manufactured by Nikkiso Co., Ltd. The powder is poured into the cell of this device together with pure water, and the median diameter D50 is detected based on the light scattering information of the particles 6.

[Tap Density TD]
From the viewpoint that a magnetic sheet 2 having a uniform surface and a smooth surface can be obtained, the tap density TD of the powder is preferably 3.50 g / cm ³ or less, more preferably 3.20 g / cm ³ or less, and 2.91 g / cm. Cm ³ or less is particularly preferable. The tap density TD is preferably 0.50 g / cm ³ or more.

In the measurement of tap density TD, about 20 g of powder is filled into a cylinder having a volume of 100 cm ³ . The measurement conditions are as follows.
Drop height: 10 mm
Number of taps: 200

[Manufacturing of powder]
The powder according to the present invention is obtained by subjecting the raw material powder to flattening. The raw material powder can be obtained by a gas atomizing method, a water atomizing method, a disc atomizing method, a pulverizing method or the like. The gas atomizing method is preferred. In the gas atomizing method, the cooling rate is relatively slow. The gas atomizing method has less oxidation.

In the gas atomizing method, the raw metal is heated and melted to obtain a molten metal. This molten metal flows out of the nozzle. Gas (typically argon gas) is sprayed onto this molten metal. Due to the energy of this gas, the molten metal is pulverized into droplets, which are cooled while being dropped. The droplets solidify to form particles. In this gas atomizing method, the molten metal is instantaneously dropleted and cooled at the same time, so that a uniform fine structure can be obtained. Moreover, since droplets are continuously formed, the composition difference between the particles is extremely small.

The raw material powder is classified and heat-treated as necessary. The raw material powder is flattened. Typical flattening is done by an attritor. A flattening aid such as stearic acid may be used for flattening. By flattening using a flattening aid, a flat powder having a large median diameter D50 can be obtained. If necessary, the flattened powder is classified. It is preferable that the flattened powder is not heat-treated. This is because the heat treatment reduces the coercive force.

[Molding of magnetic members]
In order to obtain a magnetic member (electromagnetic wave absorber) from this powder, the powder is first kneaded with a base polymer such as a resin and rubber to obtain a polymer composition. Known methods can be adopted for kneading. For example, kneading can be performed by a closed kneader, an open roll, or the like.

Next, a magnetic member is molded from this polymer composition. A known method can be adopted for molding. Molding can be performed by a compression molding method, an injection molding method, an extrusion molding method, a rolling method, or the like. A typical magnetic member is the magnetic sheet 2 (electromagnetic wave absorbing sheet) shown in FIG. Shapes such as a ring, a cube, a rectangular cuboid, and a cylinder can be adopted for the magnetic member.

Various chemicals can be kneaded into the base polymer together with the powder. Examples of chemicals include processing aids such as lubricants and binders. The polymer composition may contain a flame retardant.

Hereinafter, the effects of the present invention will be clarified by Examples, but the present invention should not be construed in a limited manner based on the description of these Examples.

[Example 1]
Raw material powder was obtained by gas atomization and classification. 500 g of this raw material powder was put into the attritor. 1.4 kg of naphthenic solvent was also added to this attritor. As the powder media, 4.8 mm SUJ2 was used. The powder was flattened by this attritor to obtain a powder composed of flat particles. The composition of this powder, median diameter D50, tap density TD and maximum aspect ratio are shown in Table 1 below. The contents of Nd, Fe and B were measured with the powder after atomization and before flattening. An ICP (Inductively Coupled Plasma) emission spectroscopic analyzer was used for this measurement. The contents of C and O were measured in the powder after the flattening process. The content of C was measured by the combustion-infrared absorption method. The content of O was measured by the Infrared Absorption Method for Moisturizing and Melting Inactive Gas. The backscattered electron image of this powder is shown in FIG. FIG. 3 shows a cross section of the particles.

[Example 2-6 and Comparative Example 1-7]
The powders of Example 2-6 and Comparative Example 1-7 were produced in the same manner as in Example 1 except that the composition was as shown in Table 1 below. A flattening aid was used for the flattening of the powders of Example 6 and Comparative Example 3. The powder of Comparative Example 5-7 was heat-treated. In this heat treatment, the powder was held for 1 hour in a high temperature argon gas atmosphere and slowly cooled. Atmospheric temperatures are shown in Table 1 below. The contents of C and O in the powder of Comparative Example 5-7 were measured after the heat treatment.

[Measurement of coercive force]
The coercive force of the powder was measured by the method described above. The results are shown in Table 1 below.

As shown in Table 1, the coercive force of the powder of each embodiment is large. From this evaluation result, the superiority of the present invention is clear.

The powder according to the present invention is suitable for various magnetic members.

2 ... Magnetic sheet 4 ... Matrix 6 ... Flat particles

Claims (7)

Consisting of a large number of flat particles
The material of these particles is a rare earth metal-transition metal-B alloy.
Hard magnetic powder with a maximum aspect ratio of 5.0 or more. The hard magnetic powder according to claim 1, wherein the alloy contains Nd as the rare earth metal and Fe as the transition element. The above alloy is
Nd: 10.8 atomic% or more and 16.0 atomic% or less B: 5.4 atomic% or more and 9.0 atomic% or less C: 0.3 atomic% or more and 1.0 atomic% or less and O: 15 atomic% or less Includes
The hard magnetic powder according to claim 2, wherein the balance is Fe and unavoidable impurities. The hard magnetic powder according to any one of claims 1 to 3, wherein the coercive force iHc is 90 kA / m or more. The hard magnetic powder according to any one of claims 1 to 4, wherein the median diameter D50 on a volume basis is 5 μm or more and 60 μm or less. The hard magnetic powder according to any one of claims 1 to 5, wherein the tap density TD is 0.50 g / cm ³ or more and 3.50 g / cm ³ or less. It comprises a matrix in which the polymer is the substrate and a hard magnetic powder dispersed in this matrix.
The hard magnetic powder consists of a large number of flat particles.
The material of these particles is a rare earth metal-transition metal-B alloy.
A magnetic member having a maximum aspect ratio of 5.0 or more for the hard magnetic powder.

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