Radio wave absorber

Abstract

To provide a radio wave absorber which can absorb radio waves in a wide range of frequencies.SOLUTION: Disclosed is a radio wave absorber having a laminate. 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. The layers are arranged from a radio wave incident side in the above-mentioned order. The first resistive layer has a periodic structure. The periodic structure is formed to have periodicity within a plane of the first resistive layer so that radio waves at frequencies in a preset range may pass through the radio wave absorber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] For example, radio wave absorbers are used to absorb radio waves in order to avoid the effects of leaked radio waves emitted to the outside from electric circuits, etc. 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 GHz to 100 GHz, and radio waves with frequencies above 100 GHz as radio waves with high frequency bands that exceed the millimeter wave band, for mobile communications such as mobile phones, wireless LAN, and electronic toll collection systems (ETC).

[0003] Here, Patent Document 1 proposes a radio wave absorber that absorbs and suppresses the reflection of unnecessary radio waves. 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-133310 Summary of the Invention [Problem to be solved by the invention]

[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.

[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. [Means for solving the problem]

[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 range of frequencies 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. [Brief explanation of the drawings]

[0009] [Figure 1] Fig. 1A is a cross-sectional view schematically showing a laminate 10 of a radio wave absorber 100 according to an embodiment. Fig. 1B (a) shows an example of the planar structure of a first resistive layer, and (b) shows an example of the planar structure of a first resistive layer different from (a). [Figure 2] Figure 2A is a graph showing the radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 20 Ω / sq to 60 Ω / sq, and Figure 2B is a graph showing the radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 70 Ω / sq to 110 Ω / sq. [Figure 3]Fig. 3A is a graph showing the radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 120 Ω / sq to 160 Ω / sq, and Fig. 3B is a graph showing the radio wave absorption characteristics when the resistance value of the first resistive layer is changed from 170 Ω / sq to 200 Ω / sq. [Figure 4] 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, and 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. [Figure 5] 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, and 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. DETAILED DESCRIPTION OF THE INVENTION

[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 an embodiment will be described with reference to Fig. 1A and Fig. 1B. 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. A radio wave interference type absorption sheet can be configured, for example, based on the principle of radio wave absorption, 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] 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, which are arranged in this order from the radio wave incident side. As will be described later, the first resistive layer R1 has a configuration in which a so-called frequency selective surface is formed, and the frequency of the radio waves that pass through it is a predetermined frequency. Of the radio waves incident on the first dielectric layer D1, which is the surface dielectric layer, those with a 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 first radio waves. A part of the radio waves that pass through the first resistive layer R1 is reflected by the second resistive layer R2. The radio waves reflected by the second resistive layer R2 are referred to as second radio waves. The radio wave that passes through the first resistive layer R1 passes through the second resistive layer R2 and is reflected by the reflective layer Rf. The radio wave reflected by the reflective layer Rf is called the 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. That is, 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 accounts for 60% or more in the frequency range of 50 (GHz) to 300 (GHz). For example, when the radio wave absorption rate of the radio wave absorber 100 is 80% or more in the range from 50 GHz to 280 GHz, the radio wave absorption characteristics are 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 an absorption rate of 90% or more for radio waves.

[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 equivalent to approximately 7 dB when converted into decibels (dB), and a 90% radio wave absorption rate is equivalent to 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 layer The dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) can be made of various dielectric materials. The dielectric layers can be made to contain a polymeric material. Examples of the polymeric 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, and 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 polymeric material. Furthermore, 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. Furthermore, each of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and third dielectric layer D3) may be configured by laminating a plurality of dielectric layers. Furthermore, the thicknesses of the dielectric layers (first dielectric layer D1, second dielectric layer D2, and 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, specifically, for example, 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 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 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, or may be within a range between any two of the values ​​exemplified here.

[0026] 1-2.Resistance 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. As 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] 1-2-1. Configuration of the first resistor layer R1 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) with 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. The predetermined frequency range that the first resistive layer R1 passes through may be divided into multiple frequency ranges, such as frequencies above 10 GHz and below 40 GHz, and frequencies above 80 GHz and below 120 GHz, as defined by the numerical values ​​listed above.

[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 plane of the first resistor layer R1. In the example shown in FIG. 1B(a), the resistor element portions R1a are shown to have a circular ring shape, but are not limited to this 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, it is preferable that the planar view size of the resistor element portions R1a be 0.4 mm or more in vertical width W1 and horizontal width W2 inclusive. That is, the vertical width W1 (mm) and horizontal width W2 (mm) are, 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 resistor layer R1 is composed of a plurality of resistor element portions R1a arranged periodically within the plane of the first resistor layer R1, the distance between a pair of adjacent resistor 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) that extends over the entire area of ​​the first resistance layer R1 and has periodicity. 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 resistor 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 resistor 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, 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. Configuration of the second resistor layer R2 The second resistive layer R2 is disposed between the first dielectric layer D1 and the second dielectric layer D2 and serves to reflect a portion of the radio waves that have passed 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 resistance 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, 40, and may be within a range between any two of the numerical values ​​exemplified here.

[0038] 1-2-3. Materials of the first resistive layer R1 and the second resistive layer R2 The resistive layers (first resistive layer R1 and second resistive layer R2) may be made of, for example, a conductive organic polymer film, a sputtered film, or a vapor-deposited film. The resistive values ​​of the conductive organic polymer films, sputtered films, and vapor-deposited films can be controlled by adjusting the film thickness and deposition density, making it easy to form resistive layers with desired resistance values. The first resistive layer R1 and the second resistive layer R2 may be made of the same or different materials.

[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), organic polymers whose main chains are composed of a π-conjugated system can be used, such as polyacetylene-based conductive polymers, polyphenylene-based conductive polymers, polyphenylene vinylene-based conductive polymers, polyaniline-based conductive polymers, polyacene-based conductive polymers, polythiophene vinylene-based conductive polymers, and copolymers thereof.

[0041] The conductive organic polymer used in the resistance layers (the first resistance layer R1 and the second resistance layer R2) can have a polyanion as a counter anion. The polyanion is not particularly limited, but can be a conjugated conductive organic polymer containing an anion group capable of generating chemical oxidation doping. Examples of such anionic groups include groups represented by the general formulas -O-SO3X, -O-PO(OX)2, -COOX, and -SO3X (in each formula, X represents a hydrogen atom or an alkali metal atom). Among these, groups represented by -SO3X and -O-SO3X can be used because they have an excellent doping effect on conjugated conductive organic polymers.

[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] Furthermore, in the resistive layers (first resistive layer R1 and second resistive layer R2) of the 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. As the dopant, halogens such as iodine and chlorine, Lewis acids such as BF3 and PF5, protonic acids such as nitric acid and sulfuric acid, transition metals, alkali metals, amino acids, nucleic acids, surfactants, dyes, chloranil, tetracyanoethylene, TCNQ, etc. 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 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 of the resistive layer is set within a predetermined range to achieve impedance matching, the 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 coating, 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] The resistive layers (first resistive layer R1 and second resistive layer R2) may be configured to include a carbon material such as a carbon microcoil, a carbon nanotube, 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 such as 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 as a resistance layer-forming paint onto a resin substrate and drying it. Examples of methods that can be used to apply the resistance layer-forming paint onto 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 allow the solvent components of the resistance layer-forming paint to evaporate. 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 materials such as 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 (the first resistive layer R1 and the 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 the resistive layers.

[0052] Unlike a resistive layer, the reflective layer Rf does not need to transmit radio waves, and therefore, a resistance value as low as possible is preferable. Metal foil or a 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 that form the reflective layer Rf can be easily produced by rolling a metal material. Furthermore, when the reflective layer Rf is formed 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 the effects of the embodiment The radio wave absorber 100 according to the embodiment absorbs radio waves in a wide range of frequency bands by the first radio wave Rw1, the second radio wave Rw2, and the third radio wave Rw3 described in the above embodiment. The first resistive layer R1 allows radio waves in a predetermined frequency band to pass through 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 frequency of the radio waves passing through it, and some of the radio waves pass through it while other parts are reflected. Radio waves that have entered the radio wave absorber 100 according to the embodiment pass through, are reflected, and have their passing frequencies selected, and are then absorbed by the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3 in combination, thereby achieving 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 therein, 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, and the embodiments shown below can be combined with each other. [Appendix 1] A radio wave absorber having a laminate, 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; the first resistive layer has a periodic structure; The periodic structure is formed to have periodicity within the plane of the first resistive layer so as to pass radio waves within a predetermined range of frequencies. [Appendix 2] 2. The radio wave absorber according to claim 1, The radio wave absorption characteristics of the radio wave absorber are such that the radio wave absorption rate has a plurality of absorption peaks in the frequency range of 50 (GHz) to 300 (GHz). [Appendix 3] 3. The radio wave absorber according to claim 2, The radio wave absorber, wherein the plurality of absorption peaks are peaks whose absorption rates are 90% or more. [Appendix 4] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 3, 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 occupies 60% or more in the frequency range of 50 (GHz) to 300 (GHz). [Appendix 5] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 4, A radio wave absorber, wherein the resistance value of the first resistive layer is 20 Ω / sq or more and 200 Ω / sq or less. [Appendix 6] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 5, The radio wave absorber is configured in 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. [Appendix 7] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 6, The periodic structure is a radio wave absorber that is composed of a plurality of resistive element portions arranged to have periodicity within the plane of the first resistive layer, or a periodic network structure that extends across the entire area of ​​the first resistive layer. [Appendix 8] 8. The radio wave absorber according to claim 7, 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 radio wave absorber has a planar view of the resistive element portion with a vertical width and a horizontal width of 0.4 mm or more and 1.0 mm or less. [Appendix 9] 8. The radio wave absorber according to claim 7, When the first resistive layer is composed of the plurality of resistive element portions arranged to have a periodicity in a plane of the first resistive layer, The radio wave absorber, wherein the distance between a pair of adjacent resistor element portions is 0.15 mm or more and 0.45 mm or less. [Appendix 10] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 9, The laminate is formed into a flexible sheet shape as a whole.

[0057] 3. Working Example The inventors have confirmed the radio wave absorption characteristics of the characteristic structure of the laminate 10 according to the embodiment (the structure in which the first resistive layer R1 has the periodic structure R1S). The graphs shown in Fig. 2A to Fig. 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, measured in GHz.

[0058] The conditions are as follows: The first dielectric layer D1 has a lower layer (200 μm thick) of acrylic OCA layer (relative permittivity: 3, dielectric loss 0.05, same below) and an upper layer (50 μm thick) of PET layer (relative permittivity 3.2, dielectric loss 0.01, same below). OCA is an optically transparent adhesive sheet, and is an abbreviation for Optical Clear Adhesive. The first resistive layer R1 has a lower base material (thickness 100 μm) that is a PET layer, and an upper layer that is a resistive element portion, and its resistance value is set to 20 to 200 Ω / sq. The second dielectric layer D2 is an acrylic OCA layer having a thickness of 100 μm. The second resistive layer R2 has a lower base material (thickness: 50 μm) that is a PET layer, and an upper layer that is a resistive portion, and its resistance value is set to 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 and has a sheet resistance of 10 Ω / sq.

[0059] 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 from 50 (GHz) to 300 (GHz), showing good absorption results over a wide frequency range. In some cases, the absorption rate is 100% (see FIG. 3A, etc.). The radio wave absorption characteristics are such that the radio wave absorption rate has multiple absorption peaks in the frequency range of 50 (GHz) to 300 (GHz), and these multiple absorption peaks have a radio wave absorption rate of 90% or more.

[0060] 4. Variations In the embodiment, a configuration having two resistive layers has been described as an example, but the present invention 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. [Explanation of symbols]

[0061] 100: Radio wave absorber 10: Laminate D1: First dielectric layer D2: Second dielectric layer D3: Third dielectric layer R1: First resistor layer R1S: Periodic structure R1a: Resistor element R2: 2nd resistance layer Rf: reflective layer Rw1: 1st radio wave Rw2: Second radio wave Rw3: Third radio wave

Claims

1. A radio wave absorber having a laminate, 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; the first resistive layer has a periodic structure; The periodic structure is formed to have periodicity within the plane of the first resistive layer so that radio waves within a predetermined frequency range pass through the radio wave absorber.

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

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

4. The radio wave absorber according to claim 1, 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).

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

6. The radio wave absorber according to any one of claims 1 to 4, The radio wave absorber is configured in 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. The radio wave absorber according to any one of claims 1 to 4, The periodic structure is a radio wave absorber that 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 that extends across the entire area of ​​the first resistor layer.

8. The radio wave absorber according to claim 7, 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 radio wave absorber, wherein the resistive element portion has a vertical width and a horizontal width in a plan view of 0.4 mm or more and 1.0 mm or less.

9. The radio wave absorber according to claim 7, When the first resistance layer is composed of the plurality of resistance element portions arranged to have a periodicity in a plane of the first resistance layer, A radio wave absorber, wherein the distance between a pair of adjacent resistive element portions is 0.15 mm or more and 0.45 mm or less.

10. The radio wave absorber according to any one of claims 1 to 4, The laminate is formed into a flexible sheet shape as a whole.

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