Radio wave absorber

Abstract

The purpose of the present invention is to provide a radio wave absorber which is capable of absorbing radio waves of a wide range of frequencies.　Provided is a radio wave absorber which has a multilayer body, wherein: the multilayer body has a first dielectric layer, a first resistance layer, a second dielectric layer, a second resistance layer, a third dielectric layer, and a reflection layer; these layers are arranged in this order from the radio wave incidence side; the first resistance layer has a periodic structure; and the periodic structure is formed so as to have periodicity in the plane of the first resistance layer so that radio waves of a predetermined range of frequencies pass.

Description

Radio wave absorber

[0001] The present invention relates to a radio wave absorber.

[0002] For example, radio wave absorbers that absorb radio waves are used to avoid the effects of leaked radio waves emitted to the outside from electric circuits, etc., and undesirably reflected radio waves. In recent years, research has been progressing on technologies that utilize centimeter waves with a frequency band of several gigahertz (GHz), millimeter waves with frequencies from 30 gigahertz to 100 gigahertz, and radio waves with frequencies of 100 GHz or higher as radio waves in high frequency bands that exceed the millimeter wave band, in mobile communications such as mobile phones, wireless LANs, and electronic toll collection systems (ETC).

[0003] Here, Patent Document 1 proposes a radio wave absorber that absorbs unwanted radio waves by suppressing their reflection. In the radio wave absorber of Patent Document 1, a resistive layer is provided on the surface of the dielectric layer on which the radio waves are incident, and a reflective layer that reflects the radio waves is provided on the back surface on the opposite side. This radio wave absorber is a radio wave interference type that absorbs radio waves by causing radio waves reflected by the resistive layer and radio waves reflected by the reflective layer to interfere with each other and cancel each other out.

[0004] Japanese Patent Application Laid-Open No. 2023-133310

[0005] The wave absorption characteristics of the wave absorber of Patent Document 1 are steep in the vicinity of the peak frequency, and there is a concern that the absorption characteristics will be significantly reduced outside of this frequency range.

[0006] An object of the present invention is to provide a radio wave absorber capable of absorbing radio waves over a wide range of frequencies.

[0007] According to the present invention, there is provided a radio wave absorber having a laminate, wherein the laminate has a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer, and a reflective layer, and these layers are arranged in this order from the radio wave incident side, and the first resistive layer has a periodic structure, and the periodic structure is formed to have periodicity within the plane of the first resistive layer so as to allow radio waves of a predetermined frequency range to pass through.

[0008] According to the present invention, radio waves incident on the first dielectric layer within a predetermined frequency range pass through the first resistive layer and are reflected by the second resistive layer and the reflective layer. Radio waves incident on the first dielectric layer with frequencies outside the predetermined range are reflected by the first resistive layer. Radio waves over a wide range of frequencies can be absorbed by the combined effects of the radio waves reflected by the first resistive layer, the radio waves reflected by the second resistive layer, which has a different frequency band from the first radio waves, and the radio waves reflected by the reflective layer, which has a different frequency band from the first radio waves.

[0009] FIG. 1A is a cross-sectional view schematically illustrating a laminate 10 of a radio wave absorber 100 according to an embodiment. FIG. 1B (a) illustrates an example of a planar structure of a first resistive layer, and (b) illustrates an example of a planar structure of a first resistive layer different from that illustrated in FIG. 2A is a graph illustrating radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 20 Ω / sq to 60 Ω / sq. FIG. 2B is a graph illustrating radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 70 Ω / sq to 110 Ω / sq. FIG. 3A is a graph illustrating radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 120 Ω / sq to 160 Ω / sq. FIG. 3B is a graph illustrating radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 170 Ω / sq to 200 Ω / sq. Fig. 4A is a graph showing the radio wave absorption characteristics when the resistance value of the second resistive layer is changed from 20 Ω / sq to 60 Ω / sq. Fig. 4B is a graph showing the radio wave absorption characteristics when the resistance value of the second resistive layer is changed from 70 Ω / sq to 110 Ω / sq. Fig. 5A is a graph showing the radio wave absorption characteristics when the resistance value of the second resistive layer is changed from 120 Ω / sq to 160 Ω / sq. Fig. 5B is a graph showing the radio wave absorption characteristics when the resistance value of the second resistive layer is changed from 170 Ω / sq to 200 Ω / sq. Fig. 6 shows the radio wave absorption characteristics based on the combination of the resistance values ​​(Ω / sq) of the first resistive layer R1 and the second resistive layer R2.

[0010] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently.

[0011] 1. Description of the Configuration of the Embodiment The configuration of a radio wave absorber 100 according to the embodiment will be described with reference to Fig. 1A and Fig. 1B. Note that Fig. 1 is a diagram provided to facilitate understanding of the configuration of the radio wave absorber 100, and the sizes of the members shown in the diagram, particularly the thickness of each layer, are not necessarily expressed in accordance with reality.

[0012] The radio wave absorber 100 is a sheet-like member configured to absorb radio waves. The radio wave absorber 100 has a laminate 10. The radio wave absorber 100 may have a configuration including only the laminate 10, or the radio wave absorber 100 may have, in addition to the laminate 10, a layer member different from the laminate 10 (for example, a member for reinforcing the radio wave absorber 100). The laminate 10 is formed in a flexible sheet shape as a whole.

[0013] The thickness (mm) of the laminate 10 is specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0 mm, and may be within a range between any two of the values ​​exemplified here.

[0014] The radio wave absorber 100 can be configured, for example, by a radio wave interference type (also called a λ / 4 type or a reflection type) radio wave absorption sheet. Based on the principle of radio wave absorption, a radio wave interference type absorption sheet can be configured such that the thickness d of a dielectric layer having a dielectric constant ε is d=λ / 4=πc / (2ω√ε) according to the wavelength λ (=1 / frequency) of the radio wave to be absorbed when propagating within the dielectric.

[0015] As shown in FIG. 1A , the laminate 10 includes a first dielectric layer D1, a first resistive layer R1, a second dielectric layer D2, a second resistive layer R2, a third dielectric layer D3, and a reflective layer Rf, arranged in this order from the radio wave incident side. The first resistive layer R1, as described below, has a so-called frequency-selective surface, allowing radio waves to pass through at a predetermined frequency. Of the radio waves incident on the first dielectric layer D1, which is the surface dielectric layer, those with the predetermined frequency pass through the first resistive layer R1, while the rest are reflected by the first resistive layer R1. The radio waves reflected by the first resistive layer R1 are referred to as the first radio waves. A portion of the radio waves that pass through the first resistive layer R1 are reflected by the second resistive layer R2. The radio waves reflected by the second resistive layer R2 are referred to as the second radio waves. The radio waves that pass through the first resistive layer R1 pass through the second resistive layer R2 and are reflected by the reflective layer Rf. The radio wave reflected by the reflective layer Rf is referred to as a third radio wave.

[0016] In the radio wave absorber 100 according to the embodiment, the incident radio waves interfere with the first radio wave Rw1, the second radio wave Rw2, and the third radio wave Rw3 described above, and the radio waves are absorbed by the radio wave absorber 100. In other words, because the phases of the radio waves are different (ideally, completely reversed), the radio waves are attenuated, and the radio waves are apparently absorbed by the radio wave absorber 100.

[0017] The radio wave absorption characteristics of the radio wave absorber 100 are such that the frequency range in which the radio wave absorption rate is 80% or more is 60% or more in the frequency range of 50 (GHz) to 300 (GHz). For example, if the radio wave absorption rate of the radio wave absorber 100 is 80% or more in the range of 50 GHz to 280 GHz, then this absorption characteristic is satisfied. Preferably, the absorption characteristics are such that the range in which the radio wave absorption rate is 80% or more is 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more, or 100% in the frequency range of 50 (GHz) to 300 (GHz). Preferably, the absorption characteristics are such that the range in which the radio wave absorption rate is 90% or more is 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, 95% or more, or 100% in the frequency range of 50 (GHz) to 300 (GHz).

[0018] The radio wave absorption characteristics of the radio wave absorber 100 are such that the radio wave absorption rate has multiple absorption peaks in the frequency range of 50 (GHz) to 300 (GHz). The number of the multiple peaks is specifically, for example, 2, 3, 4, or 5, and may be within a range between any two of the numerical values ​​exemplified here. Furthermore, it is preferable that the multiple absorption peaks are peaks with a radio wave absorption rate of 90% or more.

[0019] The above-mentioned radio wave absorption rate corresponds to the ratio of the amount of radio waves absorbed to the amount of radio waves incident on the radio wave absorber 100. The amount of radio waves can be expressed, for example, by converting the radio waves into power. Furthermore, an 80% radio wave absorption rate is approximately 7 dB when converted into decibels (dB), and a 90% radio wave absorption rate is 10 dB.

[0020] In the embodiment, the radio wave absorber 100 is described assuming utilization of the above-mentioned frequencies (GHz frequency band), but is not limited thereto, and the radio wave absorber 100 having the layer structure of the embodiment can also be applied to absorption of radio waves in the MHz band or below.

[0021] 1-1. Dielectric Layers Various dielectric materials can be used for the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3). The dielectric layers can be configured to include a polymer material. Examples of the polymer material include synthetic resins (including thermoplastic elastomers) such as polyvinyl chloride, polyvinylidene fluoride, acrylic resin, ethylene-vinyl acetate copolymer, polyurethane, acrylic urethane resin, ionomer, polyolefin, polypropylene, polyethylene, silicone resin, polyester, polystyrene, polyimide, polyamide, polysulfone, polyethersulfone, and epoxy resin, or synthetic rubbers such as polyisoprene rubber, polystyrene-butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile butadiene rubber, butyl rubber, acrylic rubber, ethylene-propylene rubber, and silicone rubber. These materials can be used alone or in combination to form the polymer material. The dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) may be made of materials such as glass, titanium oxide, alumina, and barium titanate.

[0022] The first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3 may be made of the same dielectric material or different dielectric materials. Each of the dielectric layers (the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3) may be made by stacking multiple dielectric layers. The thicknesses of the dielectric layers (the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3) may be the same or different.

[0023] The relative dielectric constants of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) can be set appropriately, and specifically, for example, are 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10, and may be within a range between any two of the numerical values ​​exemplified here. The relative dielectric constants of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) may be the same or different.

[0024] The thickness (μm) of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) can be, for example, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, or 950, and may be within a range between any two of the numerical values ​​exemplified here.

[0025] The thickness of the first dielectric layer D1 is preferably greater than the thickness of the second dielectric layer D2. Specifically, the thickness of the first dielectric layer D1 is, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, or 6.0 times the thickness of the second dielectric layer D2, or may be within a range between any two of the values ​​exemplified here. The thickness of the first dielectric layer D1 is also preferably greater than the thickness of the third dielectric layer D3. Specifically, the thickness of the first dielectric layer D1 is, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, or 6.0 times the thickness of the third dielectric layer D3, and may be within a range between any two of the numerical values ​​exemplified here.

[0026] 1-2. Resistive Layer The resistive layers (first resistive layer R1 and second resistive layer R2) have the function of reflecting and transmitting radio waves. The first resistive layer R1 also has the function of selecting the frequency of the radio waves that pass through. Because the radio wave absorber 100 has at least two resistive layers in this way, radio wave interference becomes more complex and complicated, making it possible to absorb radio waves in a wide frequency band (for example, 50 GHz to 300 GHz).

[0027] 1B, the first resistive layer R1 has a periodic structure R1S, which is formed to have periodicity within the plane of the first resistive layer R1 so as to pass a predetermined range of frequencies. In other words, the first resistive layer R1 has a structure (a patch-type resonator structure) having a frequency-selective surface.

[0028] Here, the predetermined range of frequencies that the first resistance layer R1 passes through specifically includes, for example, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 400, and 500, and may also include a range between any two of the numerical values ​​exemplified here. Note that the predetermined range of frequencies that the first resistance layer R1 passes through may be divided into multiple frequency ranges. For example, when specified using the numerical values ​​listed above, there are frequencies of 10 GHz or more and 40 GHz or less, and frequencies of 80 GHz or more and 120 GHz or less.

[0029] The shape of the periodic structure R1S is not particularly limited. For example, as shown in FIG. 1B(a), the periodic structure R1S can be composed of a plurality of resistor element portions R1a arranged to have a periodicity within the surface of the first resistor layer R1. Note that in the example of FIG. 1B(a), the resistor element portions R1a are shown as having a circular ring shape, but are not limited thereto and may have other shapes, such as a rectangular shape.

[0030] 1B(a), when the periodic structure R1S is composed of a plurality of resistor element portions R1a arranged to have a periodicity within the plane of the first resistor layer R1, the size of the resistor element portion R1a in plan view is preferably such that the vertical width W1 and horizontal width W2 are 0.4 mm or more and 1.0 mm or less. That is, the vertical width W1 (mm) and horizontal width W2 (mm) are specifically, for example, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 mm, and may be within a range between any two of the values ​​exemplified here. The vertical width W1 (mm) and horizontal width W2 (mm) may be the same or different.

[0031] 1B(a), when the first resistance layer R1 is composed of a plurality of resistance element portions R1a arranged periodically within the plane of the first resistance layer R1, the distance between a pair of adjacent resistance element portions R1a is preferably 0.15 mm or more and 0.45 mm or less. That is, the distance d1 (mm) and the distance d2 (mm) are, for example, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, or 0.45, and may be within a range between any two of the values ​​exemplified here. The distances d1 (mm) and d2 (mm) may be the same or different.

[0032] 1B(b), the periodic structure R1S may be configured as a network structure (mesh structure) having periodicity and extending over the entire area of ​​the first resistance layer R1. In this way, the shape of the periodic structure R1S is not particularly limited.

[0033] Furthermore, the periodicity of the periodic structure R1S may exist in one direction (for example, the vertical or horizontal direction in the figure) within the plane of the first resistance layer R1, or may exist in two directions (for example, the vertical and horizontal directions in the figure).

[0034] The resistance value (Ω / sq) of the first resistance layer R1 is, for example, 20, 30, 40, 50, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200, or may be within a range between any two of the values ​​exemplified here. For example, the resistance value (Ω / sq) of the first resistance layer R1 is, for example, 20 or more and 200 or less. The thickness (μm) of the first resistance layer R1 is specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, or 40, and may be within a range between any two of the numerical values ​​exemplified here.

[0035] The entire formation area of ​​the first resistive layer R1 should be inside the outer edge of the second resistive layer R2 when the radio wave absorber 100 is viewed from a direction parallel to the thickness direction of the radio wave absorber 100. In other words, the entire first resistive layer R1 should overlap with a part of the second resistive layer R2 when the radio wave absorber 100 is viewed from a direction parallel to the thickness direction of the radio wave absorber 100.

[0036] 1-2-2. Structure of the Second Resistive Layer R2 The second resistive layer R2 is disposed between the first dielectric layer D1 and the second dielectric layer D2, and functions to reflect a portion of the radio waves that pass through the first resistive layer R1. The second resistive layer R2 does not have the function of selecting the frequency of the radio waves that it passes through, and in this embodiment, the second resistive layer R2 is a layer formed as a so-called solid layer. In other words, the second resistive layer R2 has a sheet-like structure in which the entire area inside the outer edge of the second resistive layer R2 is filled with the constituent material of the second resistive layer R2.

[0037] The resistance value (Ω / sq) of the second resistive layer R2 is, for example, 20, 30, 40, 50, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200, or may be within a range between any two of the values ​​exemplified here. For example, the resistance value (Ω / sq) of the second resistive layer R2 is, for example, 20 or more and 200 or less. The thickness (μm) of the second resistive layer R2 is specifically, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, or 40, and may be within a range between any two of the values ​​exemplified here.

[0038] 1-2-3. Materials for Constituting the First Resistive Layer R1 and the Second Resistive Layer R2 The materials for the resistive layers (first resistive layer R1 and second resistive layer R2) may be, for example, conductive organic polymer films, sputtered films, vapor-deposited films, etc. Furthermore, the resistance value of the above-mentioned conductive organic polymer films, sputtered films, and vapor-deposited films can be controlled by the film thickness and formation density, making it easy to form resistive layers with desired resistance values. The materials for the first resistive layer R1 and the second resistive layer R2 may be the same or different.

[0039] The conductive organic polymer used as the resistive layers (first resistive layer R1 and second resistive layer R2) is a conjugated conductive organic polymer, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0040] Furthermore, as the resistive layers (first resistive layer R1 and second resistive layer R2), an organic polymer whose main chain is composed of a π-conjugated system can be used, such as a polyacetylene-based conductive polymer, a polyphenylene-based conductive polymer, a polyphenylene vinylene-based conductive polymer, a polyaniline-based conductive polymer, a polyacene-based conductive polymer, a polythiophene vinylene-based conductive polymer, or a copolymer thereof.

[0041] The conductive organic polymer used in the resistance layers (first resistance layer R1 and second resistance layer R2) can use a polyanion as a counter anion. There are no particular limitations on the polyanion, but it is possible to use a conjugated conductive organic polymer containing an anionic group that can generate chemical oxidation doping. Examples of such an anionic group include those represented by the general formula -O-SO ３ X, -O-PO (OX) ２ , -COOX, -SO ３ X (in each formula, X represents a hydrogen atom or an alkali metal atom), and among them, —SO ３ X and -O-SO ３ A group represented by X can be employed.

[0042] The conductive organic polymers may be used alone or in combination of two or more. Among the materials exemplified above, a polymer consisting of one or two selected from polypyrrole, poly(3-methoxythiophene), poly(3,4-ethylenedioxythiophene), poly(2-anilinesulfonic acid), and poly(3-anilinesulfonic acid) is preferred because it has higher transparency and conductivity.

[0043] In particular, it is preferable to use poly(3,4-ethylenedioxythiophene: PEDOT) and polystyrene sulfonic acid (PSS) as a combination of a conjugated conductive organic polymer and a polyanion.

[0044] In addition, in the resistive layers (first resistive layer R1 and second resistive layer R2) of the radio wave absorber 100 according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and obtain a predetermined resistance value. Examples of the dopant include halogens such as iodine and chlorine, and BF ３　 , P.F. ５　 Lewis acids such as these, protonic acids such as nitric acid and sulfuric acid, transition metals, alkali metals, amino acids, nucleic acids, surfactants, dyes, chloranil, tetracyanoethylene, TCNQ, and the like can be used.

[0045] The content of the conductive organic polymer in the resistive layers (the first resistive layer R1 and the second resistive layer R2) is preferably 10% by mass or more and 35% by mass or less, based on the total mass of the solid content contained in the resistive layer composition. If the content is less than 10% by mass, the conductivity of the resistive layer tends to decrease. Therefore, if the surface electrical resistance value of the resistive layer is set within a predetermined range to achieve impedance matching, the film thickness of the resistive layer increases, which tends to thicken the entire radio wave absorber 100 (laminate 10) or, if the resistive layer is translucent, to deteriorate its optical properties. On the other hand, if the content exceeds 35% by mass, the structure of the conductive organic polymer reduces the applicability of the resistive layer when coating it, making it difficult to form a good resistive layer. If the resistive layer is translucent, the haze of the resistive layer increases, which also tends to deteriorate its optical properties.

[0046] Furthermore, the resistive layers (first resistive layer R1 and second resistive layer R2) may be configured to include a carbon material such as carbon microcoils, carbon nanotubes, or graphene.

[0047] Carbon microcoils are a type of vapor-grown carbon fiber obtained primarily by catalytically activated pyrolysis of acetylene, and are materials with a 3D helical / spiral structure with coil diameters on the order of microns. The coil diameter is preferably 1 μm to 10 μm, the carbon fiber forming the coil has a diameter of 0.1 μm to 1 μm, and the coil length is preferably 1 mm to 10 mm.

[0048] Specifically, the carbon nanotubes can be obtained by various methods, such as vapor phase growth methods including arc discharge, laser evaporation, and pyrolysis. The carbon nanotubes used as the resistive layers (first resistive layer R1 and second resistive layer R2) of the radio wave absorber 100 may be either single-walled or multi-walled.

[0049] Graphene can be obtained by, for example, a peeling and transfer method, a SiC pyrolysis method, a chemical vapor deposition method, a method of cutting carbon nanotubes, etc. As graphene to be used as the resistive layer of the wave absorber 100, flake-shaped powder graphene can be used from the viewpoint of easily obtaining a desired aspect ratio and of orientation in the wave absorber 100. As the resin in which the above-mentioned carbon material is dispersed, a water-soluble polyester resin can be used.

[0050] The resistance layers (first resistance layer R1 and second resistance layer R2) can be formed by applying a coating composition serving as a resistance layer-forming paint to a resin substrate and drying it. Examples of methods for applying the resistance layer-forming paint to the substrate include bar coating, reverse coating, gravure coating, microgravure coating, die coating, dipping, spin coating, slit coating, and spray coating. Drying after application is preferably performed at 100 to 150°C for 5 to 60 minutes under conditions that evaporate the solvent components of the resistance layer-forming paint. If necessary, the resistance layer may be formed by irradiating the coating film with UV light (ultraviolet rays) or EB (electron beam) to cure the coating film. While the substrate used to form the resistance layer is not particularly limited, a transparent substrate having transparency is preferred. Examples of materials that can be used for such transparent substrates include various resins such as PET, rubber, glass, and ceramics.

[0051] 1-3. Reflective Layer The reflective layer Rf is configured to reflect radio waves that have passed through the resistive layers (first resistive layer R1 and second resistive layer R2). In other words, the reflective layer Rf is a layer that reflects radio waves that have passed through all of the dielectric layers and resistive layers.

[0052] Unlike a resistive layer, the reflective layer Rf does not need to pass radio waves, and therefore preferably has as low a resistance value as possible. Metal foil or metal plate can be used as the reflective layer Rf. To ensure flexibility in the radio wave absorber 100, metal foil is more preferable as the material for the reflective layer Rf, and various metal foils such as silver foil, copper foil, aluminum foil, and gold foil can be used. Considering cost and the effects of oxidation in air, aluminum foil can be used as the reflective layer Rf. Metal foils such as aluminum foil for forming the reflective layer Rf can be easily produced by rolling a metal material. Furthermore, when forming the reflective layer Rf as a vapor-deposited film in which a metal is vapor-deposited on the surface of a non-metallic material, it is preferable to appropriately select a vapor deposition method conventionally used to form various vapor-deposited films, taking into account the heat resistance temperature of the metal material to be vapor-deposited and the non-metallic material, such as a resin, that serves as the base material.

[0053] The thickness (μm) of the reflective layer Rf can be, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, or 500, and may be within a range between any two of the numerical values ​​exemplified here.

[0054] 2. Description of Effects of the Embodiments The radio wave absorber 100 according to the embodiment absorbs radio waves in a wide frequency band by the first radio wave Rw1, second radio wave Rw2, and third radio wave Rw3 described in the above embodiment. The first resistive layer R1 passes radio waves in a predetermined frequency band and reflects radio waves in other frequency bands. The first resistive layer R1 not only passes and reflects radio waves, but also selects the frequencies to be passed. The second resistive layer R2 does not select the frequencies of the radio waves to be passed, but passes some radio waves and reflects others. Radio waves that have entered the radio wave absorber 100 according to the embodiment pass, are reflected, and the selected frequencies to be passed are selected, and the radio wave absorption in the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3 are combined to achieve high absorption characteristics (return attenuation) over a wide frequency band. In particular, the embodiment is effective in absorbing radio waves in the high frequency band (50 GHz to 300 GHz).

[0055] The radio wave absorber 100 according to the embodiment has the first resistive layer R1 having the periodic structure R1S formed thereon, and therefore has effectively improved impedance matching (like a tapered structure), and has good absorption characteristics in a wide frequency band such as 50 GHz to 300 GHz.

[0056] Various embodiments are exemplified below. The embodiments shown below can be combined with each other. [Supplementary Note 1] A radio wave absorber having a laminate, wherein the laminate has a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer, and a reflective layer, arranged in this order from the radio wave incident side, and the first resistive layer has a periodic structure, and the periodic structure is formed to have periodicity within the plane of the first resistive layer so as to pass radio waves of a predetermined frequency range. [Supplementary Note 2] The radio wave absorber according to Supplementary Note 1, wherein the radio wave absorption characteristics of the radio wave absorber are such that the radio wave absorption rate has multiple absorption peaks in a frequency range of 50 (GHz) to 300 (GHz). [Supplementary Note 3] The radio wave absorber according to Supplementary Note 2, wherein the multiple absorption peaks are peaks with an absorption rate of 90% or more. [Supplementary Note 4] The radio wave absorber according to any one of Supplementary Notes 1 to 3, wherein the radio wave absorption characteristics of the radio wave absorber are such that the frequency range in which the radio wave absorption rate is 80% or more is 60% or more in the frequency range of 50 (GHz) to 300 (GHz). [Supplementary Note 5] The radio wave absorber according to any one of Supplementary Notes 1 to 4, wherein the resistance value of the first resistive layer is 20 Ω / sq or more and 200 Ω / sq or less. [Supplementary Note 6] The radio wave absorber according to any one of Supplementary Notes 1 to 5, wherein the second resistive layer has a sheet-like structure in which the entire region inside the outer edge of the second resistive layer is filled with the constituent material of the second resistive layer. [Supplementary Note 7] The radio wave absorber according to any one of Supplementary Notes 1 to 6, wherein the periodic structure is composed of a plurality of resistor element portions arranged to have periodicity in the plane of the first resistor layer, or a periodic network structure extending over the entire area of ​​the first resistor layer. [Supplementary Note 8] The radio wave absorber according to Supplementary Note 7, wherein, when the periodic structure is composed of the plurality of resistor element portions arranged to have periodicity in the plane of the first resistor layer, the resistor element portions have a vertical width and a horizontal width of 0.4 mm or more and 1.0 mm or less in plan view.[Supplementary Note 9] The radio wave absorber according to Supplementary Note 7, wherein, when the first resistive layer is made up of the plurality of resistive element portions arranged so as to have a periodicity in a plane of the first resistive layer, the distance between a pair of adjacent resistive element portions is 0.15 mm or more and 0.45 mm or less. [Supplementary Note 10] The radio wave absorber according to any one of Supplementary Notes 1 to 9, wherein the laminate is formed in a flexible sheet shape as a whole.

[0057] 3. Examples The inventors confirmed the radio wave absorption characteristics of the characteristic structure of the laminate 10 according to the embodiment (a structure in which the first resistive layer R1 has a periodic structure R1S). The graphs shown in FIGS. 2A to 5B show the results of the absorption characteristics of an object having the configuration of the radio wave absorber 100 described in the embodiment. The vertical axis corresponds to the radio wave absorption rate (return loss) in decibels (dB). The smaller the value on the vertical axis, the better the absorption rate (return loss). The horizontal axis corresponds to the radio wave frequency in GHz.

[0058] The conditions were as follows. The first dielectric layer D1 had a lower layer (200 μm thick) of an acrylic OCA layer (relative dielectric constant: 3, dielectric loss 0.05, the same applies below) and an upper layer (50 μm thick) of a PET layer (relative dielectric constant 3.2, dielectric loss 0.01, the same applies below). OCA is an optically transparent adhesive sheet and stands for Optical Clear Adhesive. The first resistive layer R1 had a lower base layer (100 μm thick) of a PET layer and an upper layer of a resistive element portion, the resistance value of which was 20 to 200 Ω / sq. The second dielectric layer D2 was an acrylic OCA layer with a thickness of 100 μm. The second resistive layer R2 had a lower base layer (50 μm thick) of a PET layer and an upper layer of a resistive portion, the resistance value of which was 20 to 200 Ω / sq. The third dielectric layer D3 is an acrylic OCA layer having a thickness of 150 μm. The reflective layer Rf is a silver nanowire ground having a sheet resistance of 10 Ω / sq.

[0059] For all of the radio wave absorption characteristics shown in Figures 2A to 5B, the frequency range in which the radio wave absorption rate is 80% or more (approximately -7 dB or less in the graph) is 90% or more in the frequency range of 50 (GHz) to 300 (GHz), indicating good absorption characteristics over a wide frequency range. Some characteristics reach 100% (see Figure 3A, etc.). Furthermore, the radio wave absorption characteristics have multiple absorption peaks in the frequency range of 50 (GHz) to 300 (GHz). Additionally, these multiple absorption peaks are peaks in which the radio wave absorption rate is 90% or more.

[0060] Fig. 6 shows the radio wave absorption characteristics based on the combination of the resistance values ​​(Ω / sq) of the first resistive layer R1 and the second resistive layer R2 of the radio wave absorber 100. The results shown in Fig. 6 are obtained by simply expanding the range of verification of the first resistive layer R1 and the second resistive layer R2, and the conditions other than the resistance values ​​are the same as those for the results shown in Figs. 2A to 5B (for example, the thickness of the dielectric layer, etc.).

[0061] 6 shows not only the results shown in FIGS. 2A to 5B described above, but also other results. The above-mentioned FIGS. 2A to 5B show results in which the first resistance layer R1 is fixed at 70 (Ω / sq) and the second resistance layer R2 is varied from 20 to 200 (Ω / sq) in increments of 10 (Ω / sq), and results in which the second resistance layer R2 is fixed at 90 (Ω / sq) and the first resistance layer R1 is varied from 20 to 200 (Ω / sq) in increments of 10 (Ω / sq). In addition to these, FIG. 6 shows results in which the first resistance layer R1 is varied from 20 to 200 (Ω / sq) and the second resistance layer R2 is varied from 20 to 200 (Ω / sq) in increments of 20 (Ω / sq), thereby verifying the characteristics of the radio wave absorber 100 over a wider range of resistance setting values ​​for the first resistance layer R1 and the second resistance layer R2. Graphs are omitted.

[0062] Here, N0 to N2 shown in Figure 6 are as follows. N0 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) accounts for 90% or more in the frequency range of 50 (GHz) to 300 (GHz). N1 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is 60% or more and less than 90% in the frequency range of 50 (GHz) to 300 (GHz). N2 indicates that the frequency range in which the radio wave absorption rate (return loss) is 80% or more (approximately -7 dB or less in the graph) is 40% or more and less than 60% in the frequency range of 50 (GHz) to 300 (GHz).

[0063] In terms of the wideness of the frequency range in which absorption is favorable, N0 is the most favorable, followed by N1 and N2 in that order. Through the verification shown in Fig. 6, the inventors confirmed that it is possible to provide the radio wave absorber with wide-range radio wave absorption characteristics when the first resistance layer R1 and the second resistance layer R2 are in the range of at least 20 to 200 (Ω / sq), and confirmed that the effects described in section "2. Description of Effects of the Embodiments" can be expected.

[0064] 4. Modifications In the embodiment, the laminate 10 has been described as having two resistive layers, but is not limited to this. The laminate 10 may have three or more resistive layers. In this case, the additional resistive layers are disposed with a dielectric layer sandwiched therebetween.

[0065] 100: Wave absorber 10: Laminate D1: First dielectric layer D2: Second dielectric layer D3: Third dielectric layer R1: First resistive layer R1S: Periodic structure R1a: Resistive element portion R2: Second resistive layer Rf: Reflective layer Rw1: First radio wave Rw2: Second radio wave Rw3: Third radio wave

Claims

1. A radio wave absorber having a laminate, wherein the laminate has a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer, and a reflective layer, and these layers are arranged in this order from the radio wave incident side, and the first resistive layer has a periodic structure, and the periodic structure is formed to have periodicity within the plane of the first resistive layer so that radio waves of a predetermined frequency range can pass through.

2. A radio wave absorber according to claim 1, wherein the radio wave absorption characteristics of said radio wave absorber are such that the radio wave absorption rate has multiple absorption peaks in the frequency range of 50 (GHz) to 300 (GHz).

3. A radio wave absorber according to claim 2, wherein the plurality of absorption peaks are peaks whose absorption rate is 90% or more.

4. A radio wave absorber according to claim 1, wherein the radio wave absorption characteristics of said radio wave absorber are such that the frequency range in which the radio wave absorption rate is 80% or more accounts for 60% or more in the frequency range from 50 (GHz) to 300 (GHz).

5. A radio wave absorber according to any one of claims 1 to 4, wherein the resistance value of the first resistive layer is 20 Ω / sq or more and 200 Ω / sq or less.

6. A radio wave absorber according to any one of claims 1 to 4, wherein the second resistive layer has a sheet-like structure in which the entire area inside the outer edge of the second resistive layer is filled with the constituent material of the second resistive layer.

7. A radio wave absorber according to any one of claims 1 to 4, wherein the periodic structure is composed of a plurality of resistor element portions arranged to have periodicity within the plane of the first resistor layer, or a periodic network structure extending over the entire area of ​​the first resistor layer.

8. A radio wave absorber as set forth in claim 7, wherein the periodic structure is composed of the plurality of resistive element parts arranged to have periodicity within the plane of the first resistive layer, and the size of the resistive element part in a planar view is 0.4 mm or more in vertical width and 1.0 mm or less in horizontal width.

9. A radio wave absorber as set forth in claim 7, wherein, when the first resistive layer is composed of the plurality of resistive element parts arranged so as to have a periodicity within the plane of the first resistive layer, the distance between any pair of adjacent resistive element parts is 0.15 mm or more and 0.45 mm or less.

10. A radio wave absorber according to any one of claims 1 to 4, wherein the laminate is formed in the shape of a flexible sheet as a whole.

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