Electromagnetic wave absorber

Abstract

The electromagnetic wave absorber includes hexagonal ferrite particles and a thermoplastic resin filled with the hexagonal ferrite particles. The hexagonal ferrite particles include first particles and second particles having a particle size larger than that of the first particles.

Description

Technical Field

[0001] The present disclosure relates to a radio wave absorber for removing unnecessary radio waves such as electromagnetic noise. Background Art

[0002] A radio wave absorber using magnetoplumbite hexagonal ferrite is disclosed in Patent Document 1. The magnetoplumbite hexagonal ferrite of Patent Document 1 has the property of absorbing and removing radio waves in a specific frequency band, and this property is achieved by replacing a part of the iron atoms forming the hexagonal ferrite with other transition metal atoms.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Patent Laid-Open No. 11-354972 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] However, the radio wave absorber of Patent Document 1 mainly absorbs radio waves in a microwave frequency band of less than 20 GHz. Therefore, in a frequency band of 20 GHz or higher, sufficient radio wave absorption performance may not be obtained.

[0008] The present disclosure solves the above problems, and an object thereof is to provide a radio wave absorber having effective radio wave absorption performance in a frequency band of 20 GHz or higher.

[0009] Means for Solving the Problems

[0010] The radio wave absorber includes hexagonal ferrite particles and a holding material filled with the hexagonal ferrite particles. The hexagonal ferrite particles include first particles and second particles having a larger particle size than the first particles.

[0011] Effects of the Invention

[0012] According to this configuration, a radio wave absorber having effective radio wave absorption performance in a frequency band of 20 GHz or higher can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is an example of a schematic cross-sectional view of a radio wave absorber according to Embodiment 1.

[0014] Figure 2 is Figure 1 a modified example of the radio wave absorber.

[0015] Figure 3 is a schematic diagram showing single-domain particles included in the radio wave absorber according to Embodiment 1.

[0016] Figure 4 It is a schematic diagram showing the multi - domain particles contained in the electromagnetic wave absorber related to Embodiment 1. Detailed implementation mode

[0017] Embodiment 1.

[0018] Use Figure 1 The characteristics and structure of the electromagnetic wave absorber 100 related to Embodiment 1 will be described. Figure 1 It is an example of a schematic cross - sectional view of the electromagnetic wave absorber 100 related to Embodiment 1.

[0019] The electromagnetic wave absorber 100 is a ferrite material containing a magnetic material that absorbs the electromagnetic waves incident on the electromagnetic wave absorber 100. In the electromagnetic wave absorber 100, according to the magnetic loss characteristics of the magnetic material, the wave energy of the electromagnetic wave is converted into heat energy, thereby absorbing the electromagnetic wave. In the electromagnetic wave absorber 100, the frequency band in which the magnetic loss of the magnetic material becomes extremely large is the frequency band of the electromagnetic wave that can be absorbed by the electromagnetic wave absorber 100.

[0020] The thickness of the electromagnetic wave absorber 100 is preferably in the range of 1 mm or more and 5 mm or less. If the thickness of the electromagnetic wave absorber 100 is in the range of 1 mm or more and 5 mm or less, both the processability and the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. It should be noted that the size and shape of the electromagnetic wave absorber 100 are not limited to Figure 1 the size and shape thereof, and can be appropriately changed according to the use of the electromagnetic wave absorber 100 and the like.

[0021] As Figure 1 shown in, the electromagnetic wave absorber 100 has hexagonal ferrite particles 1 and a holding material 2 filled with the hexagonal ferrite particles 1. In the electromagnetic wave absorber 100, the hexagonal ferrite particles 1 are dispersed in the holding material 2.

[0022] The hexagonal ferrite particles 1 are magnetic materials having a hexagonal crystal structure in which the constituent elements are arranged in a hexagonal prism shape. The hexagonal ferrite particles 1 have electromagnetic wave absorption performance in the direction perpendicular to the hexagonal face of the crystal structure. As the hexagonal ferrite particles 1, for example, BaFe 12 O 19 and SrFe 12 O 19 and other magnetoplumbite - type hexagonal ferrites are used. The magnetoplumbite - type hexagonal ferrite is also called M - type hexagonal ferrite.

[0023] In addition, as the hexagonal ferrite particles 1, hexagonal ferrite particles 1 having a hexagonal crystal structure different from that of the M-type hexagonal ferrite may also be used. For example, as the hexagonal ferrite particles 1, W-type hexagonal ferrite, Z-type hexagonal ferrite, and Y-type hexagonal ferrite can be used. As the W-type hexagonal ferrite, BaFe 18 O 27 and SrFe 18 O 27 etc. As the Z-type hexagonal ferrite, Ba3Co2Fe 24 O 41 and Sr3Co2Fe 24 O 41 etc. As the Y-type hexagonal ferrite, BaZnFe 12 O 22 etc. In addition, two or more types of hexagonal ferrites may be used in combination as the hexagonal ferrite particles 1. In addition, as the hexagonal ferrite particles 1, hexagonal ferrites obtained by replacing a part of transition metal elements such as Ba, Sr, and Fe constituting the above-mentioned hexagonal ferrite with other transition metal elements such as Ti, Sn, Zr, Cu, and Ni may also be used.

[0024] It should be noted that in the existing radio wave absorber 100, there is a radio wave absorber that uses spinel-type ferrite particles having a cubic crystal structure as the magnetic material. Spinel-type ferrite particles have radio wave absorption performance in a low-frequency band of less than 1 GHz called the megahertz (MHz) band. However, in spinel-type ferrite particles, the magnetic loss in a high-frequency band of 1 GHz or more becomes smaller than the magnetic loss in a low-frequency band of less than 1 GHz, so the radio wave absorption performance in a high-frequency band of 1 GHz or more becomes smaller. Therefore, the conventional radio wave absorber 100 is not suitable as a radio wave absorber 100 used in a high-frequency band of 1 GHz or more called the gigahertz (GHz) band.

[0025] On the other hand, compared with spinel-type ferrite particles, the hexagonal ferrite particles 1 have large magnetic anisotropy of the crystal, so they are suitable as radio wave absorption materials in a high-frequency band of 20 GHz or more. It should be noted that as the hexagonal ferrite particles 1, it is preferable to use the M-type hexagonal ferrite having large magnetic anisotropy of the crystal among the above-mentioned hexagonal ferrites and capable of having radio wave absorption performance in a frequency band of 20 GHz or more.

[0026] The content of the hexagonal ferrite particles 1 in the electromagnetic wave absorber 100 is preferably 70% by weight or more and 95% by weight or less, more preferably 75% by weight or more and 90% by weight or less. When the content of the hexagonal ferrite particles 1 is 75% by weight or more and 90% by weight or less, the mixing and dispersion of the hexagonal ferrite particles 1 in the holding material 2 are easy, and the workability and moldability in the manufacture of the electromagnetic wave absorber 100 can be improved. In addition, when the content of the hexagonal ferrite particles 1 is 75% by weight or more and 90% by weight or less, the electromagnetic wave absorber 100 can sufficiently maintain the electromagnetic wave absorption performance required for the removal of electromagnetic waves. On the other hand, when the content of the hexagonal ferrite particles 1 is less than 70% by weight, it may not be possible to ensure the electromagnetic wave absorption performance required for the removal of electromagnetic waves. On the other hand, when the content of the hexagonal ferrite particles 1 exceeds 95% by weight, it is sometimes difficult to mix and disperse the hexagonal ferrite particles 1 in the holding material 2, which poses an obstacle to workability or moldability.

[0027] The holding material 2 is a base material that fills the hexagonal ferrite particles 1. There is no limitation on the holding material 2. For example, an organic polymer resin such as a thermoplastic resin is used. There is no limitation on the thermoplastic resin. For example, it is formed into a polymer or copolymer of one or more monomers selected from ethylene, propylene, butadiene, isoprene, styrene, methacrylic acid, acrylic acid, methacrylate, acrylate, vinyl chloride, tetrafluoroethylene, acrylonitrile, maleic anhydride, and vinyl acetate. In addition, as the thermoplastic resin, polyphenylene ether resin, chlorinated polyethylene resin, silicone resin, polyamide resin, polyimide resin, polycarbonate resin, polyester resin, polyacetal resin, polyphenylene sulfide resin, polyethylene glycol resin, polyetherimide resin, polyketone resin, polyetheretherketone resin, polyethersulfone resin, and polyarylate resin can be used.

[0028] In addition, additives such as a flame retardant can be added to the electromagnetic wave absorber 100. There is no limitation on the flame retardant. For example, an organic flame retardant such as chlorinated paraffin and pentabromodiphenyl ether is used. It should be noted that the addition amount of the flame retardant is adjusted to an amount that does not hinder the electromagnetic wave absorption performance of the electromagnetic wave absorber 100.

[0029] Next, the particle size of the hexagonal ferrite particles 1 will be described.

[0030] As described above, as the hexagonal ferrite particles 1, M-type hexagonal ferrite is preferably used. By using M-type hexagonal ferrite as the hexagonal ferrite particles 1, the electromagnetic wave absorber 100 can theoretically be formed to have electromagnetic wave absorption performance in a high-frequency band of 20 GHz or higher. For example, when using BaFe 12 O 19In the case of being an M-type hexagonal ferrite, the radio wave absorber 100 can theoretically be formed to have radio wave absorption performance in the 48 GHz band.

[0031] On the other hand, the magnetic loss characteristics of the hexagonal ferrite particles 1 have the property of changing according to the particle size of the hexagonal ferrite particles 1. Therefore, the frequency band in which the magnetic loss of the hexagonal ferrite particles 1 becomes maximum changes according to the particle size of the hexagonal ferrite particles 1. In addition, since the radio wave absorber 100 utilizes the radio wave absorption performance associated with the magnetic loss characteristics of the hexagonal ferrite particles 1, it is required that the magnetic loss characteristics of the first particles 1a and the second particles 1b become maximum in the frequency band where radio wave absorption performance is required. Therefore, by adjusting the particle size of the hexagonal ferrite particles 1, a radio wave absorber 100 having radio wave absorption performance in a desired frequency band can be obtained.

[0032] As Figure 1 shown, the hexagonal ferrite particles 1 include first particles 1a and second particles 1b having a particle size larger than that of the first particles 1a. The first particles 1a are formed, for example, as fine particles having a particle size of less than 1 μm. The second particles 1b are formed, for example, as coarse particles having a particle size of 5 μm or more. In the following description, the particle size of the first particles 1a is referred to as the "first particle size", and the particle size of the second particles 1b is referred to as the "second particle size".

[0033] The first particle size of the first particles 1a and the second particle size of the second particles 1b in the radio wave absorber 100 can be obtained, for example, by measuring the particle size distribution using the laser diffraction scattering method. The laser diffraction scattering method is a measurement method that measures the angular pattern of the diffraction light intensity and the scattered light intensity that change according to the particle size and uses the Fraunhofer diffraction theory or the Mie scattering theory to obtain the particle size distribution. In the laser diffraction scattering method, a sample of the hexagonal ferrite particles 1 after ashing the radio wave absorber 100 is used. The sample of the hexagonal ferrite particles 1 for measurement is obtained by ashing the radio wave absorber 100 by heat treatment at a temperature of 500°C to 800°C for 5 to 10 hours in an air atmosphere using an electric furnace.

[0034] By using hexagonal ferrite particles 1 containing first particles 1a and second particles 1b having a particle size larger than that of the first particles 1a, a wave absorber 100 having wave absorption performance in different frequency bands in a frequency band of 20 GHz or higher can be obtained. By making the particle size of the first particles 1a smaller than that of the second particles 1b, the aspect ratio of the first particles 1a is smaller than that of the second particles 1b. In addition, by making the aspect ratio of the first particles 1a smaller than that of the second particles 1b, the intensity of the demagnetizing field in the first particles 1a is smaller than that in the second particles 1b. Therefore, in the wave absorber 100, the resonance frequency of the first particles 1a is higher than that of the second particles 1b, and thus the frequency band in which wave absorption performance is obtained in the first particles 1a is wider than the frequency band in which wave absorption performance is obtained in the second particles 1b.

[0035] For example, by setting the first particle size of the first particles 1a to 1 μm or less, a wave absorber 100 having wave absorption performance in the 79 GHz band can be obtained. In addition, by setting the second particle size of the second particles 1b to 5 μm or more, a wave absorber 100 having wave absorption performance in the 28 GHz band can be obtained.

[0036] Furthermore, the first particle size of the first particles 1a is formed such that the lower limit value is 0.05 μm or more. This is because when the first particle size of the first particles 1a is less than 0.05 μm, the first particles 1a firmly aggregate, and thus it is difficult to uniformly mix and disperse them in the holding material 2. In addition, the second particle size of the second particles 1b is formed such that the upper limit value is 100 μm or less. This is because when the second particle size of the second particles 1b exceeds 100 μm, it becomes difficult to adjust the thickness of the wave absorber 100 so that the wave absorber 100 becomes sheet-like.

[0037] In the field of electronic devices, with the miniaturization and high performance of electronic devices, the high-frequencyization of the operating frequency of electronic devices and the high-density installation of components mounted on electrical devices are developing. Therefore, in the field of electronic devices, countermeasures for suppressing electromagnetic noise, which is the main cause of malfunctions in electronic devices, have become an important issue. In particular, in recent years, radio waves in the 28 GHz band are used in high-speed communication devices of the 5G standard, and radio waves in the 79 GHz band are used in millimeter-wave radars for vehicle collision prevention. Therefore, in the field of electronic devices, countermeasures for suppressing electromagnetic noise corresponding to the 28 GHz band and the 79 GHz band are particularly required.

[0038] As described above, countermeasures for suppressing electromagnetic noise in the 28 GHz band and 79 GHz band generated from an electronic device can be achieved, for example, by using a radio wave absorber 100 in which the first particle size of the first particles 1a is 1 μm or less and the second particle size of the second particles 1b is 5 μm or more. Therefore, by mounting the radio wave absorber 100 having the first particles 1a and the second particles 1b with a particle size larger than that of the first particles 1a on the electronic device, unwanted radio waves generated from the electronic device can be absorbed.

[0039] Next, the crystal orientation of the hexagonal ferrite particles 1 will be described. It should be noted that in the following description, the hexagonal plane in the crystal structure of the hexagonal ferrite particles 1 is referred to as the "C plane". In addition, the degree of convergence of the normal direction of the C plane of the hexagonal ferrite particles 1 in the radio wave absorber 100 is referred to as the "C plane orientation degree" of the hexagonal ferrite particles 1 in the radio wave absorber 100.

[0040] The hexagonal ferrite particles 1 have the property that the crystal easily grows in the direction parallel to the C plane. Since the aspect ratio of the first particles 1a is smaller than that of the second particles 1b, the first particles 1a can be mixed and dispersed in the holding material 2 in a state where the C planes of the first particles 1a are randomly oriented, as compared with the second particles 1b. Therefore, the radio wave absorption performance of the first particles 1a in the radio wave absorber 100 does not have anisotropy, and the first particles 1a can absorb radio waves incident from multiple directions.

[0041] On the other hand, with respect to the second particles 1b, for example, through dispersion or molding during the manufacture of the radio wave absorber 100, the particles are more likely to be oriented in the direction parallel to the C plane than the first particles 1a, and have the property of being easily formed into plate-like particles.

[0042] The C plane of the second particles 1b that have become plate-like crystal particles is more likely to be mixed and dispersed in the holding material 2 in a state where it is oriented in a specific direction, as compared with the C plane of the first particles 1a, and the C plane orientation degree of the second particles 1b is greater than that of the first particles 1a. Therefore, anisotropy is sometimes likely to occur in the radio wave absorption performance of the second particles 1b in the radio wave absorber 100, and the second particles 1b have the property of absorbing only radio waves incident from a specific direction and not absorbing radio waves incident from other directions.

[0043] However, when the electromagnetic wave absorber 100 contains the first particles 1a and the second particles 1b, as the content of the first particles 1a is more than that of the second particles 1b, the C-plane orientation degree of the hexagonal ferrite particles 1 becomes smaller, and the anisotropy of the hexagonal ferrite particles 1 becomes smaller. On the contrary, as the content of the second particles 1b in the electromagnetic wave absorber 100 is more than that of the first particles 1a, the C-plane orientation degree of the hexagonal ferrite particles 1 becomes larger, and the anisotropy of the hexagonal ferrite particles 1 becomes larger. Therefore, the electromagnetic wave absorber 100 is formed of the hexagonal ferrite particles 1 containing the first particles 1a and the second particles 1b, whereby the anisotropy of the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be adjusted.

[0044] It should be noted that the C-plane orientation degree of the hexagonal ferrite particles 1 in the electromagnetic wave absorber 100 can be quantitatively evaluated by the Lotgering method (Lotgering factor) using X-ray diffraction. Hereinafter, the Lotgering method will be described.

[0045] Regarding the C-plane orientation degree f of the hexagonal ferrite particles 1 in the Lotgering method, it is calculated according to Equation (1) using the reference peak intensity P0 obtained from the non-oriented specimen of the hexagonal ferrite particles 1 and the peak intensity P obtained from the oriented specimen of the hexagonal ferrite particles 1.

[0046] f = (P - P0) / (1 - P0)… (1)

[0047] The measured value of the diffraction intensity I0 of the X-ray obtained from the non-oriented specimen of the hexagonal ferrite particles 1 is used to calculate the reference peak intensity P0. The reference peak intensity P0 is expressed as the ratio of the sum of the diffraction intensities I0(xyz) obtained from the crystal orientation plane (xyz) of the non-oriented specimen of the hexagonal ferrite particles 1 to the sum of the diffraction intensities I0(hkl) obtained from the non-oriented specimen of the hexagonal ferrite particles 1, as shown in Equation (2). It should be noted that the variables h, k, l, x, y, and z are integers.

[0048] P0 = ΣI0(xyz) / ΣI0(hkl)… (2)

[0049] The reference peak intensity P0 is used as the correction value of the peak intensity P in Equation (1) to correct the C-plane orientation degree f in the non-oriented specimen of the hexagonal ferrite particles 1 to be 0. It should be noted that the reference peak intensity P0 in the calculation of the C-plane orientation degree f of the hexagonal ferrite particles 1 can be set to P0 = 0.05.

[0050] The peak intensity P is calculated using the measured value of the diffraction intensity I of X-rays obtained from an oriented sample of the hexagonal ferrite particles 1 such as the electromagnetic wave absorber 100. When the crystal orientation plane of the electromagnetic wave absorber 100 is the (00l) plane, the peak intensity P is expressed as shown in Equation (3), which is the ratio of the sum of the diffraction intensities I(00l) obtained from the crystal orientation plane (00l) of the oriented sample of the hexagonal ferrite particles 1 to the sum of the diffraction intensities I(hkl) obtained from the oriented sample of the hexagonal ferrite particles 1.

[0051] P = ΣI(00l) / ΣI(hkl)… (3)

[0052] If the C-plane orientation degree f of the hexagonal ferrite particles 1 in the electromagnetic wave absorber 100 is 0.4 or less, the C-plane of the hexagonal ferrite particles 1 is randomly oriented. Therefore, if the C-plane orientation degree f of the hexagonal ferrite particles 1 is 0.4 or less, the electromagnetic wave absorber 100 has electromagnetic wave absorption performance for electromagnetic waves incident from multiple directions.

[0053] On the other hand, when the C-plane orientation degree f of the hexagonal ferrite particles 1 exceeds 0.4, as the C-plane orientation degree f approaches 1, the anisotropy of the electromagnetic wave absorption performance becomes smaller, and the characteristic of only absorbing electromagnetic waves incident from a specific direction and not absorbing electromagnetic waves incident from other directions becomes stronger. Therefore, in applications where electromagnetic waves are incident from multiple directions, the electromagnetic wave absorber 100 with a C-plane orientation degree f of the hexagonal ferrite particles 1 exceeding 0.4 may not be able to obtain effective electromagnetic wave absorption performance.

[0054] In addition, when the C-plane of the hexagonal ferrite particles 1 is completely randomly oriented, the C-plane orientation degree f becomes 0. However, in the manufacture of the electromagnetic wave absorber 100, it is actually difficult to completely randomly orient the C-plane of the hexagonal ferrite particles 1.

[0055] Therefore, the C-plane orientation degree f of the hexagonal ferrite particles 1 is preferably 0.04 or more and 0.35 or less. If the C-plane orientation degree f of the hexagonal ferrite particles 1 is set to 0.04 or more and 0.35 or less, it is possible to improve both the ease of manufacturing the electromagnetic wave absorber 100 and the electromagnetic wave absorption performance.

[0056] It should be noted that in the above, the electromagnetic wave absorber 100 formed of a single layer has been described, but the same effect is obtained for a multilayer electromagnetic wave absorber 100 formed of two or more layers. Next, Figure 2 An example of a multilayer electromagnetic wave absorber 100 formed of two or more layers will be described.

[0057] Figure 2 is Figure 1A modified example of the electromagnetic wave absorber 100. The electromagnetic wave absorber 100 has a first electromagnetic wave absorption layer 100a and a second electromagnetic wave absorption layer 100b. To explain, in Figure 2 the electromagnetic wave absorber 100, a structure in which the second electromagnetic wave absorption layer 100b and the first electromagnetic wave absorption layer 100a are directly laminated to form a two-layer structure is illustrated, but it is not limited thereto. For example, the electromagnetic wave absorber 100 may also be formed as an electromagnetic wave absorber 100 having a multi-layer structure of three or more layers in which one or more other electromagnetic wave absorption layers are interposed between the second electromagnetic wave absorption layer 100b and the first electromagnetic wave absorption layer 100a and laminated.

[0058] The first electromagnetic wave absorption layer 100a forms a first electromagnetic wave absorption surface 100a1 of the electromagnetic wave absorber 100. In addition, the second electromagnetic wave absorption layer 100b forms a second electromagnetic wave absorption surface 100b1 of the electromagnetic wave absorber 100 at a position opposite to the first electromagnetic wave absorption surface 100a1. In the following description, the C-plane orientation degree f of the hexagonal ferrite particles 1 in the first electromagnetic wave absorption surface 100a1 is referred to as the first orientation degree f1, and the C-plane orientation degree f of the hexagonal ferrite particles 1 in the second electromagnetic wave absorption surface 100b1 is referred to as the second orientation degree f2.

[0059] In Figure 2 the first electromagnetic wave absorption layer 100a contains only the first particles 1a as the hexagonal ferrite particles 1, and the second electromagnetic wave absorption layer 100b contains only the second particles 1b as the hexagonal ferrite particles 1. Others are the same as the electromagnetic wave absorber 100 in Figure 1 and thus the description is omitted.

[0060] As described above, the aspect ratio of the first particles 1a is smaller than that of the second particles 1b. Therefore, in the first electromagnetic wave absorption layer 100a, the C-planes of the first particles 1a are mixed and dispersed in the holding material 2 in a state where they are randomly oriented. On the other hand, in the second electromagnetic wave absorption layer 100b, the C-planes of the second particles 1b are mixed and dispersed in the holding material 2 in a state where they are oriented in a specific direction. That is, in Figure 2 the electromagnetic wave absorber 100, the first orientation degree f1 of the hexagonal ferrite particles 1 in the first electromagnetic wave absorption surface 100a1 is smaller than the second orientation degree f2 of the hexagonal ferrite particles 1 in the second electromagnetic wave absorption surface 100b1.

[0061] Therefore, in Figure 2 the electromagnetic wave absorber 100, the electromagnetic wave absorption performance of the electromagnetic waves incident from multiple directions can be improved by the first electromagnetic wave absorption layer 100a. In addition, in Figure 2In the electromagnetic wave absorber 100, the electromagnetic wave absorption performance of the electromagnetic wave incident in a specific direction can be improved by the second electromagnetic wave absorption layer 100b. In addition, by providing the first electromagnetic wave absorption layer 100a and the second electromagnetic wave absorption layer 100b, the electromagnetic wave absorption performance of each layer can be specialized in the electromagnetic wave absorption performance in a specific frequency band, and thus the electromagnetic wave absorption performance can be further improved.

[0062] It should be noted that the configuration in which the first orientation degree f1 of the hexagonal ferrite particles 1 in the first electromagnetic wave absorption surface 100a1 is less than the second orientation degree f2 of the hexagonal ferrite particles 1 in the second electromagnetic wave absorption surface 100b1 is not limited to Figure 2 the configuration shown. For example, by forming the first electromagnetic wave absorption layer 100a in such a manner that the content of the first particles 1a is greater than the content of the second particles 1b, and forming the second electromagnetic wave absorption layer 100b in such a manner that the content of the second particles 1b is greater than the content of the first particles 1a, the same configuration can also be obtained. In addition, the electromagnetic wave absorber 100 may also interpose other electromagnetic wave absorption layers having orientation degrees different from the first orientation degree f1 and the second orientation degree f2 between the second electromagnetic wave absorption layer 100b and the first electromagnetic wave absorption layer 100a to form a multi-layer electromagnetic wave absorber 100 having three or more layers.

[0063] Next, Figure 3 and Figure 4 will be used to explain the magnetic domain structure of the hexagonal ferrite particles 1. Figure 3 is a schematic diagram showing the single-domain particles 10 contained in the electromagnetic wave absorber 100 according to Embodiment 1. Figure 4 is a schematic diagram showing the multi-domain particles 20 contained in the electromagnetic wave absorber 100 according to Embodiment 1. It should be noted that Figure 3 the shape of the single-domain particles 10 in Figure 4 and the shape of the multi-domain particles 20 in

[0064] are schematically shown and may be different from the actual shape of the particles of the hexagonal ferrite particles 1. The magnetic loss characteristics of the hexagonal ferrite particles 1 have the property of changing according to the magnetic domain structure of the hexagonal ferrite particles 1, and the electromagnetic wave absorption performance in the frequency band where the magnetic loss of the hexagonal ferrite particles 1 becomes maximum changes according to the magnetic domain structure of the hexagonal ferrite particles 1. The hexagonal ferrite particles 1 are roughly divided into single-domain particles 10 and multi-domain particles 20 which are particles other than the single-domain particles 10 according to the magnetic domain structure.

[0065] As Figure 3 shown, the single-domain particles 10 are magnetic particles in which the whole particle is formed by a single magnetic domain 10b and a magnetic moment 10a in a single direction is formed inside the magnetic domain 10b. As Figure 4As shown, the multi-domain particle 20 is a magnetic particle in which one or more magnetic walls 20c are formed inside the particle, and magnetic moments 20a are formed in each magnetic domain 20b divided by the one or more magnetic walls 20c.

[0066] In the multi-domain particle 20, multiple magnetic domains 20b are formed by the magnetic walls 20c. Therefore, as Figure 4 shown, the direction-reversed magnetic moments 20a are easily formed in each magnetic domain 20b. On the other hand, in the single-domain particle 10, different from the multi-domain particle 20, no magnetic wall 20c is formed. Therefore, only a single-direction magnetic moment 10a is formed inside the particle. Therefore, the coercive force of the single-domain particle 10 is larger than that of the multi-domain particle 20.

[0067] As Figure 1 shown, in the hexagonal ferrite particle 1 of the radio wave absorber 100, the first particle 1a has the first single-domain particle 1a1, and the second particle 1b has the second single-domain particle 1b1. By having the first single-domain particle 1a1 and the second single-domain particle 1b1, the hexagonal ferrite particle 1 can increase the coercive force of the radio wave absorber 100. Therefore, the radio wave absorption performance in the frequency band having magnetic loss characteristics can be improved.

[0068] In the following description, when there is no need to specifically distinguish, the single-domain particle 10 refers to both the first single-domain particle 1a1 and the second single-domain particle 1b1. In addition, the multi-domain particle 20 refers to both the first particle 1a other than the first single-domain particle 1a1 and the second particle 1b other than the second single-domain particle 1b1.

[0069] The content of the single-domain particle 10 in the hexagonal ferrite particle 1 represents the ratio of the single-domain particle 10 in the hexagonal ferrite particle 1 in terms of area ratio and is calculated according to Equation (4).

[0070] Content of single-domain particle 10 (%) = (Area of single-domain particle 10 / Area of hexagonal ferrite particle 1) × 100… (4)

[0071] Here, the area of the single-domain particle 10 is the area of the first single-domain particle 1a1 and the area of the second single-domain particle 1b1 in the cross-section of the radio wave absorber 100. In addition, the area of the hexagonal ferrite particle 1 can be set as the sum of the area of the single-domain particle 10 and the area of the multi-domain particle 20.

[0072] The area of the single-domain particle 10 and the area of the multi-domain particle 20 are calculated, for example, by observing the magnetic domains of the cross-section of the radio wave absorber 100 using a scanning probe microscope and a Kerr effect polarizing microscope, etc. The cross-section of the radio wave absorber 100 is subjected to image processing such as imaging, and the area of the single-domain particle 10 and the area of the multi-domain particle 20 are measured respectively.

[0073] When the content of the single-domain particles 10 is increased, the radio wave absorption performance of the radio wave absorber 100 in the frequency band having magnetic loss characteristics can be improved. Therefore, the content of the single-domain particles 10 is preferably 30% or more. From the viewpoint of improving the radio wave absorption performance of the radio wave absorber 100, the content of the single-domain particles 10 is preferably 50% or more, and more preferably 70% or more.

[0074] Next, a method for manufacturing the radio wave absorber 100 will be described.

[0075] The hexagonal ferrite particles 1 are obtained by pulverizing a powder of hexagonal ferrite having a particle size of about 0.2 to 1 mm using a grinder, a ball mill, a bead mill, or the like. The powder of hexagonal ferrite is produced from raw material compounds such as metal oxides and carbonates constituting the ferrite by methods such as a solid-phase reaction method, a coprecipitation method, a reverse micelle method, a hydrothermal synthesis method, or a glass crystallization method. It should be noted that commercially available products can also be used for the powder of hexagonal ferrite and the hexagonal ferrite particles 1.

[0076] The hexagonal ferrite particles 1 are mixed and dispersed in the heat-melted holding material 2 to prepare a resin composition. The mixing of the hexagonal ferrite particles 1 in the heat-melted holding material 2 is not limited. For example, it is carried out by intermittently or continuously operating an extruder having rolls such as a single-screw extruder, a twin-screw extruder, or a multi-screw extruder, a Banbury mixer, a roll, a co-kneader, a Blast mill, or a Brabender Plastograph.

[0077] The resin composition is cooled, and after cooling, it is magnetized by a magnetization device. The magnetization of the resin composition is carried out, for example, by applying a magnetic field having an intensity reaching the saturation point of the maximum magnetic flux density of the resin composition. The magnetization of the resin composition is not limited, and for example, magnetization methods such as a static magnetic field generation method based on a DC electromagnet or a pulsed magnetic field generation method based on a capacitor-type magnetizer are used.

[0078] The magnetized resin composition is heated and melted at the temperature at which the holding material 2 is melted and formed into a specified shape. The shape of the resin composition only needs to be a shape suitable for use as the radio wave absorber 100. For example, the shape of the resin composition is preferably sheet-like.

[0079] It should be noted that in the above molding process, a crushing process of the resin composition can also be carried out before the heat-melting process of the resin composition. By carrying out the crushing process of the resin composition, the heat melting of the resin composition becomes easy, and thus the operability of the heat-melting process can be improved.

[0080] In addition, in the manufacturingFigure 2 In the case of the two-layer structure electromagnetic wave absorber 100 as described above, a sheet in which hexagonal ferrite particles 1 having a particle size of less than 1 μm are dispersed and a sheet in which hexagonal ferrite particles 1 having a particle size of 5 μm or more are dispersed are respectively produced by the above-described process. The electromagnetic wave absorber 100 can be manufactured by laminating the two produced sheets. As a method for laminating the two sheets, there is no particular limitation, and examples thereof include hot press bonding, or bonding using an adhesive or a pressure-sensitive adhesive tape.

[0081] It should be noted that the above-described manufacturing method of the electromagnetic wave absorber 100 is an example and is not limited to the above method, and other manufacturing methods known in the art can be used. For example, in the resin composition molding process, a resin mixture obtained by mixing the particulate holding material 2 and the hexagonal ferrite particles 1 can also be injected into a mold such as a casting mold, and the casting mold can be heated by a heating machine such as a heating cylinder of a molding machine to melt the resin mixture and mold it into a resin composition having a predetermined shape. According to this molding process, the preparation process of the resin composition for melting the holding material 2 can be omitted, and thus the manufacturing man-hour of the electromagnetic wave absorber 100 can be reduced.

[0082] Examples

[0083] In Embodiment 1, through the following examples, it was found that by adjusting the particle size and magnetic domain structure of the hexagonal ferrite particles 1, an electromagnetic wave absorber 100 having effective electromagnetic wave absorption performance in a frequency band of 20 GHz or higher, particularly in a millimeter wave band of 30 to 300 GHz, can be provided. In addition, through the following examples, it was found that by adjusting the degree of orientation of the hexagonal ferrite particles 1 when the hexagonal ferrite particles 1 are filled in the holding material 2, an electromagnetic wave absorber 100 that exhibits effective electromagnetic wave absorption performance for electromagnetic waves incident from multiple directions can be provided. Hereinafter, the examples will be specifically described, but they are not limited to these examples.

[0084] In the examples, as the hexagonal ferrite, an M-type hexagonal ferrite having a composition of BaFe 12 O 19 was used. Using a powder of M-type hexagonal ferrite having a particle size of 0.3 mm, a plurality of ferrite specimens A to G including first particles 1a having a particle size of less than 1 μm and second particles 1b having a particle size of 5 μm or more were produced, where the contents of the first particles 1a and the second particles 1b were different. The plurality of ferrite specimens A to G were produced as follows: The powder of the above-described M-type hexagonal ferrite was pulverized using a ball mill, and then further finely pulverized using a bead mill. By changing the pulverization time using the bead mill, the contents of the first particles 1a having a particle size of less than 1 μm and the second particles 1b having a particle size of 5 μm or more in the plurality of ferrite specimens A to G were respectively adjusted.

[0085] The contents of the first particles 1a with a particle size of less than 1 μm and the second particles 1b with a particle size of 5 μm or more contained in the ferrite samples A to G were calculated by measuring the particle size distribution using the laser diffraction scattering method and normalized in weight percentage units. The contents of the first particles 1a with a particle size of less than 1 μm and the second particles 1b with a particle size of 5 μm or more contained in the ferrite samples A to G are shown in Table 1.

[0086] [Table 1]

[0087]

[0088] As shown in Table 1, in the ferrite sample A, the content of the first particles 1a with a particle size of less than 1 μm was set to 63 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 0 wt%. In the ferrite sample B, the content of the first particles 1a with a particle size of less than 1 μm was set to 55 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 10 wt%. In the ferrite sample C, the content of the first particles 1a with a particle size of less than 1 μm was set to 46 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 13 wt%. In the ferrite sample D, the content of the first particles 1a with a particle size of less than 1 μm was set to 32 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 30 wt%. In the ferrite sample E, the content of the first particles 1a with a particle size of less than 1 μm was set to 23 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 47 wt%. In the ferrite sample F, the content of the first particles 1a with a particle size of less than 1 μm was set to 14 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 52 wt%. In the ferrite sample G, the content of the first particles 1a with a particle size of less than 1 μm was set to 0 wt%, and the content of the second particles 1b with a particle size of 5 μm or more was set to 71 wt%.

[0089] As described below, in order to examine the effects caused by the particle size of the hexagonal ferrite particles 1, radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2 were produced. As shown below, in the radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2, the contents of the first particles 1a with a particle size of less than 1 μm and the second particles 1b with a particle size of 5 μm or more were different, and the production conditions of the other radio wave absorber samples were the same.

[0090] [Example 1]

[0091] To 100 parts by mass of chlorinated polyethylene resin, 900 parts by mass of ferrite sample D was added and mixed at a temperature of 180 °C to prepare a resin composition. A magnetic field of 1.2 T was applied to the prepared resin composition using a magnetization device to perform magnetization treatment. The magnetized resin composition was crushed into pieces about several cm in size using a crusher, and then formed into a sheet at a temperature of 180 °C using a twin-screw extruder equipped with rollers to produce the electromagnetic wave absorber sample of Example 1.

[0092] [Example 2]

[0093] In Example 2, 900 parts by mass of ferrite sample B was added to 100 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180 °C to prepare a resin composition. Through the same magnetization treatment and forming treatment as in Example 1, the electromagnetic wave absorber sample of Example 2 was produced.

[0094] [Example 3]

[0095] In Example 3, 900 parts by mass of ferrite sample C was added to 100 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180 °C to prepare a resin composition. Through the same magnetization treatment and forming treatment as in Example 1, the electromagnetic wave absorber sample of Example 3 was produced.

[0096] [Example 4]

[0097] In Example 4, 900 parts by mass of ferrite sample E was added to 100 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180 °C to prepare a resin composition. Through the same magnetization treatment and forming treatment as in Example 1, the electromagnetic wave absorber sample of Example 4 was produced.

[0098] [Example 5]

[0099] In Example 5, 900 parts by mass of ferrite sample F was added to 100 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180 °C to prepare a resin composition. Through the same magnetization treatment and forming treatment as in Example 1, the electromagnetic wave absorber sample of Example 5 was produced.

[0100] [Comparative Example 1]

[0101] In Comparative Example 1, 900 parts by mass of ferrite sample A was added to 100 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180 °C to prepare a resin composition. Through the same magnetization treatment and forming treatment as in Example 1, the electromagnetic wave absorber sample of Comparative Example 1 was produced.

[0102] [Comparative Example 2]

[0103] In Comparative Example 2, 900 parts by mass of ferrite sample G was added to 100 parts by mass of chlorinated polyethylene resin, and the mixture was carried out at a temperature of 180 °C to prepare a resin composition. Through the same magnetization treatment and molding treatment as in Example 1, a radio wave absorber sample of Comparative Example 2 was produced.

[0104] For the radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2, according to the above formula (4), the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 was calculated as a percentage, and the calculated value was used as the content of the single-domain particles 10 for evaluation.

[0105] In addition, for the sheet-shaped radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2, the C-plane orientation degree f of the hexagonal ferrite particles 1 in two surfaces of the radio wave absorber sample was calculated respectively by the above Lotgering method, and the C-plane orientation degree f was quantitatively evaluated.

[0106] In addition, for the radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2, the radio wave absorption performance of electromagnetic waves at 28 GHz and 79 GHz was quantitatively evaluated respectively. The radio wave absorption performance was quantitatively evaluated by calculating the radio wave attenuation amount of electromagnetic waves at 28 GHz and 79 GHz in the radio wave absorber sample. In the measurement of the radio wave attenuation amount, the following free space method was used: the radio wave absorber sample was set in free space, a plane wave was focused on the radio wave absorber sample, and the reflection coefficient of the radio wave absorber sample was measured. The radio wave attenuation amount of the radio wave absorber sample was calculated from the difference between the reflection coefficient measured with the radio wave absorber sample and the reference reflection coefficient measured with an aluminum plate. The radio wave attenuation amount of the radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2 was divided by the radio wave attenuation amount of the radio wave absorber sample of Example 1 and normalized to the relative value with respect to the radio wave attenuation amount of the radio wave absorber sample of Example 1.

[0107] The data of the ratio of the area of the single-domain particles 10, the C-plane orientation degree f of the hexagonal ferrite particles 1, and the radio wave attenuation amount in the radio wave absorber samples of Examples 1 to 5, Comparative Example 1, and Comparative Example 2 are shown in Table 2. It should be noted that in Table 2, one surface of the sheet-shaped radio wave absorber sample is referred to as the "one-sided surface", and the other surface is referred to as the "opposite surface", and the data of the C-plane orientation degree f of the hexagonal ferrite particles 1 in each surface are shown.

[0108] [Table 2]

[0109]

[0110] In the electromagnetic wave absorber sample of Example 1, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 87%. In the electromagnetic wave absorber sample of Example 2, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 93%. In the electromagnetic wave absorber sample of Example 3, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 86%. In the electromagnetic wave absorber sample of Example 4, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 81%. In the electromagnetic wave absorber sample of Example 5, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 78%.

[0111] In the electromagnetic wave absorber samples of Examples 1 to 5, as the content of the first particles 1a with a particle size less than 1 μm is greater than the content of the second particles 1b with a particle size of 5 μm or more, the content of the magnetized single-domain particles 10 increases.

[0112] In addition, in the electromagnetic wave absorber sample of Example 1, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.35, and the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.36. In the electromagnetic wave absorber sample of Example 2, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.28, and the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.29. In the electromagnetic wave absorber sample of Example 3, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.3, and the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.31. In the electromagnetic wave absorber sample of Example 4, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.36, and the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.36. In the electromagnetic wave absorber sample of Example 5, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.4, and the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.39. The C-plane orientation degree f of the hexagonal ferrite particles 1 on both sides of the electromagnetic wave absorber sample is in a substantially the same range.

[0113] In the electromagnetic wave absorber samples of Examples 1 to 5, as the content of the first particles 1a with a particle size less than 1 μm is greater than the content of the second particles 1b with a particle size of 5 μm or more, the C-plane orientation degree f of the hexagonal ferrite particles 1 on both sides of the electromagnetic wave absorber sample decreases.

[0114] In the electromagnetic wave absorber samples of Examples 1 to 5, the electromagnetic wave attenuation amount of the 28 GHz electromagnetic wave is in the range of 1.0 to 1.1, and the electromagnetic wave attenuation amount of the 79 GHz electromagnetic wave is in the range of 0.9 to 1.1, obtaining effective electromagnetic wave absorption performance.

[0115] In contrast, in the electromagnetic wave absorber sample of Comparative Example 1 containing only the first particles 1a with a particle size of less than 1 μm, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 84%, obtaining a value comparable to that of the electromagnetic wave absorber samples of Examples 1 to 5. However, the C-plane orientation degree f of the electromagnetic wave absorber sample of Comparative Example 1 is a lower value compared to the C-plane orientation degree f of the electromagnetic wave absorber samples of Examples 1 to 5. Specifically, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.15. In addition, the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.14. In addition, in the electromagnetic wave absorber sample of Comparative Example 1, the electromagnetic wave attenuation amount of the 79 GHz electromagnetic wave is 1.2, but the electromagnetic wave attenuation amount of the 28 GHz electromagnetic wave is 0.4, and effective electromagnetic wave absorption performance for the 28 GHz electromagnetic wave is not obtained.

[0116] In addition, in the electromagnetic wave absorber sample of Comparative Example 2 containing only the second particles 1b with a particle size of 5 μm or more, the ratio of the area of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 83%, obtaining a value comparable to that of the electromagnetic wave absorber samples of Examples 1 to 5. However, the C-plane orientation degree f of the electromagnetic wave absorber sample of Comparative Example 2 is a higher value compared to the C-plane orientation degree f of the electromagnetic wave absorber samples of Examples 1 to 5. Specifically, the C-plane orientation degree f of the hexagonal ferrite particles 1 on one side surface is 0.65. In addition, the C-plane orientation degree f of the hexagonal ferrite particles 1 on the opposite side surface is 0.66. In addition, in the electromagnetic wave absorber sample of Comparative Example 2, the electromagnetic wave attenuation amount of the 28 GHz electromagnetic wave is 1.3, but the electromagnetic wave attenuation amount of the 79 GHz electromagnetic wave is 0.3, and effective electromagnetic wave absorption performance for the 79 GHz electromagnetic wave is not obtained.

[0117] From the results of Examples 1 to 5, it is shown that in the electromagnetic wave absorber 100 having hexagonal ferrite particles 1 including the first particles 1a and the second particles 1b with a particle size larger than that of the first particles 1a, it is possible to form effective electromagnetic wave absorption performance in a frequency band of 20 GHz or higher. In particular, from the results of Examples 1 to 5, it is shown that an electromagnetic wave absorber 100 having effective electromagnetic wave absorption performance in the millimeter wave band of 30 to 300 GHz can be provided.

[0118] In existing ferrite materials having electromagnetic wave absorption performance, only ferrite particles of a single particle size are used, and ferrite particles of multiple particle sizes are not used. As shown in Comparative Example 1 and Comparative Example 2, in the case of using only ferrite particles of a single particle size, it is impossible to expect the suppression effect of electromagnetic noise in both the 28 GHz band used in high-frequency communication applications and the millimeter wave band such as the 79 GHz band used in anti-collision radar applications.

[0119] In addition, when the C-plane orientation degree f of the ferrite particles in the ferrite material is large, the anisotropy of the ferrite particles becomes large. When the anisotropy of the ferrite particles is large, an absorption effect on electromagnetic noise from a certain direction is exhibited, but an absorption effect on electromagnetic noise from multiple directions cannot be exhibited. As shown in Comparative Example 2, in the case of a ferrite material containing only large-sized ferrite particles, the C-plane orientation degree f of the ferrite particles becomes large. Therefore, for example, when a high-frequency device such as a high-frequency communication device and a millimeter-wave radar to which the ferrite material of Comparative Example 2 is attached is provided on an outer wall or the like to prevent interference, electromagnetic noise is incident from multiple directions, and thus sufficient radio wave absorption performance cannot be expected.

[0120] However, in the examples of Examples 1 to 5, it is shown that by using the electromagnetic wave absorber 100 containing the first particles 1a and the second particles 1b having a particle size larger than that of the first particles 1a, electromagnetic noise can be suppressed in both the 28 GHz band and the 79 GHz band. In particular, when the first particle size of the first particles 1a is less than 1 μm and the second particle size of the second particles 1b is 5 μm or more, effective radio wave absorption performance can be obtained in both the 28 GHz band and the 79 GHz band, and electromagnetic noise can be suppressed. In addition, in the examples of Examples 1 to 5, it is shown that by using the electromagnetic wave absorber 100 containing the first particles 1a and the second particles 1b having a particle size larger than that of the first particles 1a, compared with the ferrite material containing only the second particles 1b, the C-plane orientation degree can be reduced, and thus an absorption effect on electromagnetic noise from multiple directions can be obtained. In addition, it is shown that as the content ratio of the first particles 1a is larger than the content of the second particles 1b, the content of the single-domain particles 10 after magnetization increases, and the C-plane orientation degree of the hexagonal ferrite particles 1 becomes smaller, exhibiting an absorption effect on electromagnetic noise from multiple directions.

[0121] Next, in order to examine the effects generated by the magnetic domain structure of the hexagonal ferrite particles 1, electromagnetic wave absorber specimens of Examples 6 to 9, Comparative Example 3, and Comparative Example 4 were fabricated. As follows, in the electromagnetic wave absorber specimens of Examples 6 to 9, Comparative Example 3, and Comparative Example 4, the magnetic field strength applied during the magnetization process was different from that in Example 1, and the other electromagnetic wave absorber specimen fabrication conditions were the same as those in Example 1. It should be noted that the magnetic field strength applied during the magnetization process in Example 1 was 1.2 T.

[0122] [Example 6]

[0123] A magnetic field of 0.8 T was applied to the resin composition prepared in the same manner as in Example 1 to perform a magnetization process. An electromagnetic wave absorber specimen of Example 6 was fabricated through the same molding process as in Example 1.

[0124] [Example 7]

[0125] A magnetic field of 0.7 T was applied to the resin composition prepared in the same manner as in Example 1 to perform magnetization treatment. An electromagnetic wave absorber sample of Example 7 was produced by the same molding treatment as in Example 1.

[0126] [Example 8]

[0127] A magnetic field of 0.6 T was applied to the resin composition prepared in the same manner as in Example 1 to perform magnetization treatment. An electromagnetic wave absorber sample of Example 8 was produced by the same molding treatment as in Example 1.

[0128] [Example 9]

[0129] A magnetic field of 0.5 T was applied to the resin composition prepared in the same manner as in Example 1 to perform magnetization treatment. An electromagnetic wave absorber sample of Example 9 was produced by the same molding treatment as in Example 1.

[0130] [Comparative Example 3]

[0131] A magnetic field of 0.35 T was applied to the resin composition prepared in the same manner as in Example 1 to perform magnetization treatment. An electromagnetic wave absorber sample of Comparative Example 3 was produced by the same molding treatment as in Example 1.

[0132] [Comparative Example 4]

[0133] The resin composition prepared in the same manner as in Example 1 was not subjected to magnetization treatment. An electromagnetic wave absorber sample of Comparative Example 4 was produced by the same molding treatment as in Example 1.

[0134] The data of the proportion of the area of single-domain particles 10, the C-plane orientation degree f of hexagonal ferrite particles 1, and the electromagnetic wave attenuation amount in the electromagnetic wave absorber samples of Example 1, Examples 6 to 9, Comparative Example 3, and Comparative Example 4 are shown in Table 3. The evaluation methods for each data are the same as those in Examples 1 to 5 above, so the description is omitted. It should be noted that the data of Example 1 in Table 3 are the same as the data of Example 1 in Table 2.

[0135] [Table 3]

[0136]

[0137] The proportion of the area of single-domain particles 10 in the electromagnetic wave absorber sample of Example 6 with respect to the total area of hexagonal ferrite particles 1 was 62%. The proportion of the area of single-domain particles 10 in the electromagnetic wave absorber sample of Example 7 with respect to the total area of hexagonal ferrite particles 1 was 48%. The proportion of the area of single-domain particles 10 in the electromagnetic wave absorber sample of Example 8 with respect to the total area of hexagonal ferrite particles 1 was 34%. The proportion of the area of single-domain particles 10 in the electromagnetic wave absorber sample of Example 9 with respect to the total area of hexagonal ferrite particles 1 was 30%.

[0138] In addition, the radio wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz in the radio wave absorber sample of Example 6 are both 0.9. The radio wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz in the radio wave absorber sample of Example 7 are both 0.8. The radio wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz in the radio wave absorber sample of Example 8 are both 0.8. The radio wave attenuation amount of the electromagnetic wave at 28 GHz in the radio wave absorber sample of Example 9 is 0.6, and the radio wave attenuation amount of the electromagnetic wave at 79 GHz is 0.7.

[0139] In contrast, in the radio wave absorber sample of Comparative Example 3 where the area ratio of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 29%, the radio wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz are both 0.4. In the radio wave absorber sample of Comparative Example 4 where the area ratio of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 5%, the radio wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz are both 0.2.

[0140] It should be noted that the C-plane orientation degree f of the radio wave absorber samples of Examples 6 to 9 is in the range of 0.32 to 0.35 on both the single-side surface and the opposite-side surface. On the other hand, the C-plane orientation degree f of the single-side surface in the radio wave absorber sample of Comparative Example 3 is 0.36, and the C-plane orientation degree f of the opposite-side surface is 0.35. In addition, the C-plane orientation degree f of the single-side surface in the radio wave absorber sample of Comparative Example 4 is 0.37, and the C-plane orientation degree f of the opposite-side surface is 0.35.

[0141] As shown in Examples 6 to 9, when the area ratio of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 30% or more, the radio wave attenuation amount of the electromagnetic wave at 28 GHz is maintained at 0.6 or more, and the radio wave attenuation amount of the electromagnetic wave at 79 GHz is maintained at 0.7 or more. On the other hand, as shown in Comparative Example 3 and Comparative Example 4, when the area ratio of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is less than 29%, the radio wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz are reduced to 0.4 or less. Therefore, from the results of Examples 6 to 9, it is shown that if the area ratio of the single-domain particles 10 to the total area of the hexagonal ferrite particles 1 is 30% or more, the radio wave attenuation amounts of the electromagnetic waves in the 28 GHz band and the 79 GHz band are maintained, and effective radio wave absorption performance is obtained. It should be noted that the results of Examples 6 to 9 were obtained for the first time by focusing on the magnetic domain structure of the hexagonal ferrite particles 1.

[0142] Next, in order to compare the effects in the case where the electromagnetic wave absorber 100 is a multilayer structure with the single-layer electromagnetic wave absorber 100 of Example 1 and conduct an investigation, an electromagnetic wave absorber specimen of Example 10 was fabricated.

[0143] [Example 10]

[0144] 450 parts by mass of ferrite specimen A was added to 50 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180°C to prepare a resin composition. Subsequently, 450 parts by mass of ferrite specimen G was added to 50 parts by mass of chlorinated polyethylene resin and mixed at a temperature of 180°C to prepare a resin composition. A magnetic field of 1.2 T was applied to each of the two prepared resin compositions using a magnetization device to conduct magnetization treatment. The two resin compositions after magnetization treatment were each crushed into pieces of about several centimeters in size using a crusher, and were formed into sheets at a temperature of 180°C using a twin-screw extruder equipped with rollers. The formed sheet-shaped resin compositions were bonded together with a resin adhesive to fabricate the electromagnetic wave absorber specimen of Example 10.

[0145] It should be noted that in Example 10, the sheet-shaped resin composition containing ferrite specimen A corresponds to the above-mentioned first electromagnetic wave absorption layer 100a, and the sheet-shaped resin composition containing ferrite specimen G corresponds to the above-mentioned second electromagnetic wave absorption layer 100b.

[0146] The data of the proportion of the area of single-domain particles 10, the C-plane orientation degree f of hexagonal ferrite particles 1, and the electromagnetic wave attenuation amount in the electromagnetic wave absorber specimens of Example 1 and Example 10 are shown in Table 4. The evaluation methods for each data are the same as those in the above-mentioned Examples 1 to 9, so the description is omitted. It should be noted that the data of Example 1 in Table 4 are the same as the data of Example 1 in Tables 2 and 3. In addition, the single-sided surface in Table 4 corresponds to the above-mentioned first electromagnetic wave absorption surface 100a1, and the C-plane orientation degree f of the single-sided surface corresponds to the above-mentioned first orientation degree f1. In addition, the opposite surface in Table 4 corresponds to the above-mentioned second electromagnetic wave absorption surface 100b1, and the C-plane orientation degree f of the opposite surface corresponds to the above-mentioned second orientation degree f2.

[0147] [Table 4]

[0148]

[0149] In the electromagnetic wave absorber specimen of Example 10, the proportion of the area of single-domain particles 10 relative to the total area of hexagonal ferrite particles 1 was 86%. In addition, the C-plane orientation degree f of the single-sided surface in the electromagnetic wave absorber specimen of Example 10 was 0.15, and the C-plane orientation degree f of the opposite surface was 0.83. In addition, the electromagnetic wave attenuation amounts of the electromagnetic waves at 28 GHz and 79 GHz in the electromagnetic wave absorber specimen of Example 10 were both 1.3.

[0150] In the electromagnetic wave absorber sample of Example 10, the C-plane orientation degree f of one side surface is less than that of the opposite side surface. Further, in the electromagnetic wave absorber sample of Example 10, the C-plane orientation degree f of one side surface is less than the C-plane orientation degree f of one side surface of the electromagnetic wave absorber sample of Example 1. Further, in the electromagnetic wave absorber sample of Example 10, the C-plane orientation degree f of the opposite side surface is greater than the C-plane orientation degree f of the opposite side surface in the electromagnetic wave absorber sample of Example 1. On the other hand, in the electromagnetic wave absorber sample of Example 10, compared with Example 1, it is shown that the electromagnetic wave attenuation amounts of electromagnetic waves at 28 GHz and 79 GHz both increase, and the electromagnetic wave absorption performance is improved.

[0151] Therefore, in Example 10, it is shown that the electromagnetic wave absorber 100 having two particles with different sizes has a multilayer structure of two or more layers, and when the C-plane orientation degree f of one side surface is less than that of the opposite side surface, the electromagnetic wave absorber 100 has effective electromagnetic wave absorption performance.

[0152] As described above, in Examples 1 to 10, it is shown that by adjusting the grain size and the magnetic domain structure, it is possible to provide the electromagnetic wave absorber 100 having electromagnetic wave absorption performance in a frequency band of 20 GHz or higher and capable of absorbing electromagnetic waves incident from multiple directions with little anisotropy. In particular, in Examples 1 to 10, it is shown that it is possible to provide the electromagnetic wave absorber 100 having effective electromagnetic wave absorption performance in a target frequency band, for example, the 28 GHz band and the 79 GHz band.

[0153] Description of reference numerals

[0154] 1 Hexagonal ferrite particle, 1a First particle, 1a1 First single magnetic domain particle, 1b Second particle, 1b1 Second single magnetic domain particle, 2 Holding material, 10 Single magnetic domain particle, 10a Magnetic moment, 10b Magnetic domain, 20 Multi-magnetic domain particle, 20a Magnetic moment, 20b Magnetic domain, 20c Magnetic wall, 100 Electromagnetic wave absorber, 100a First electromagnetic wave absorption layer, 100a1 First electromagnetic wave absorption surface, 100b Second electromagnetic wave absorption layer, 100b1 Second electromagnetic wave absorption surface.

Claims

1. An electromagnetic wave absorber, comprising: A first electromagnetic wave absorption layer that forms a first electromagnetic wave absorption surface of the electromagnetic wave absorber, including first hexagonal ferrite particles and a holding material filled with the first hexagonal ferrite particles; and A second electromagnetic wave absorption layer that forms a second electromagnetic wave absorption surface of the electromagnetic wave absorber at a position opposite to the first electromagnetic wave absorption surface, including second hexagonal ferrite particles and a holding material filled with the second hexagonal ferrite particles, The second electromagnetic wave absorption layer is directly laminated on the first electromagnetic wave absorption layer, The particle size of the second hexagonal ferrite particles is larger than the particle size of the first hexagonal ferrite particles, The particle size of each of the first hexagonal ferrite particles is 0.05 μm or more and less than 1 μm, The particle size of each of the second hexagonal ferrite particles is 5 μm or more and 100 μm or less, The first hexagonal ferrite particles include first single-domain particles, The second hexagonal ferrite particles include second single-domain particles, The ratio of the total area of the first single-domain particles and the second single-domain particles to the total area of the first hexagonal ferrite particles and the first hexagonal ferrite particles is 30% or more, The first orientation degree of the hexagonal ferrite particles in the first electromagnetic wave absorption surface is less than the second orientation degree of the hexagonal ferrite particles in the second electromagnetic wave absorption surface.

2. The radio wave absorber according to claim 1, wherein: The total content of the first hexagonal ferrite particles and the second hexagonal ferrite particles in the electromagnetic wave absorber is 70% by weight or more and 95% by weight or less.

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