Radio wave absorber

Abstract

To provide a radio wave absorber excellent in convenience.SOLUTION: Disclosed is a radio wave absorber having a laminate. The laminate has a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer. The layers are arranged in a lamination direction of the laminate in the above-mentioned order. The first reflective layer is adhered to the first dielectric layer via the first adhesive layer in a manner removable from the first dielectric layer. The second reflective layer is adhered to the second dielectric layer via the second adhesive layer in a manner removable from the second dielectric layer.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] Radio wave absorbers do not necessarily have high absorption characteristics over the entire range of a particular frequency, and it may be necessary to use different types depending on the frequency band. In such cases, it may be necessary to prepare different radio wave absorbers, which may reduce convenience.

[0006] An object of the present invention is to provide a radio wave absorber that is highly convenient. [Means for solving the problem]

[0007] According to the present invention, there is provided a radio wave absorber having a laminate, the laminate having a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer, which are arranged in this order in the stacking direction of the laminate, the first reflective layer being adhered to the first dielectric layer via the first adhesive layer so as to be peelable from the first dielectric layer, and the second reflective layer being adhered to the second dielectric layer via the second adhesive layer so as to be peelable from the second dielectric layer.

[0008] According to the present invention, both the first reflective layer and the second reflective layer can be peeled off, and when the layer structure of the dielectric layer or the resistive layer is devised to give different absorption characteristics to both sides, it is possible to selectively peel off the first reflective layer or the second reflective layer depending on the situation, which is highly convenient. [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] FIG. 2A schematically shows the case where the first reflective layer Rf1 of the radio wave absorber 100 shown in FIG. 1A is peeled off for use, and FIG. 2B schematically shows the case where the second reflective layer Rf2 of the radio wave absorber 100 shown in FIG. 1A is peeled off for use. [Figure 3]3A and 3B show the absorption characteristics when radio waves are incident from the first reflective layer side. Fig. 3A is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 20 Ω / sq to 60 Ω / sq. Fig. 3B is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 70 Ω / sq to 110 Ω / sq. [Figure 4] 4A and 4B show the absorption characteristics when radio waves are incident from the first reflective layer side. Fig. 4A is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 120 Ω / sq to 160 Ω / sq. Fig. 4B is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 170 Ω / sq to 200 Ω / sq. [Figure 5] 5A and 5B show the absorption characteristics when radio waves are incident from the first reflective layer side. Fig. 5A is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 20 Ω / sq to 60 Ω / sq. Fig. 5B is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 70 Ω / sq to 110 Ω / sq. [Figure 6] 6A and 6B show the absorption characteristics when radio waves are incident from the first reflective layer side. Fig. 6A is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 120 Ω / sq to 160 Ω / sq. Fig. 6B is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 170 Ω / sq to 200 Ω / sq. [Figure 7] 7A and 7B show the absorption characteristics when radio waves are incident from the second reflective layer side. Fig. 7A is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 20 Ω / sq to 60 Ω / sq. Fig. 7B is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 70 Ω / sq to 110 Ω / sq. [Figure 8]8A and 8B show the absorption characteristics when radio waves are incident from the second reflective layer side. Fig. 8A is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 120 Ω / sq to 160 Ω / sq. Fig. 8B is a graph showing the absorption characteristics of radio waves when the resistance value of the first resistive layer is changed from 170 Ω / sq to 200 Ω / sq. [Figure 9] 9A and 9B show the absorption characteristics when radio waves are incident from the second reflective layer side. Fig. 9A is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 20 Ω / sq to 60 Ω / sq. Fig. 9B is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 70 Ω / sq to 110 Ω / sq. [Figure 10] 10A and 10B show the absorption characteristics when radio waves are incident from the second reflective layer side. Fig. 10A is a graph showing the absorption characteristics of radio waves when the resistance value of the second resistive layer is changed from 120 Ω / sq to 160 Ω / sq. Fig. 10B is a graph showing the absorption characteristics of radio waves 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] As shown in FIG. 1A, the laminate 10 has a first reflective layer Rf1, a first adhesive layer, a first dielectric layer D1, a first resistive layer R1, a third dielectric layer D3, a second resistive layer R2, a second dielectric layer D2, a second adhesive layer, and a second reflective layer Rf2, which are arranged in this order in the stacking direction of the laminate 10. As will be described later, the first resistive layer R1 has a structure in which a so-called frequency selective surface is formed, and the frequency of radio waves that pass through it is a predetermined frequency.

[0016] Although the first adhesive layer and the second adhesive layer are not shown, the first adhesive layer is interposed between the first reflective layer Rf1 and the first dielectric layer D1, and the second adhesive layer is interposed between the second reflective layer Rf2 and the second dielectric layer D2. The first reflective layer Rf1 is adhered to the first dielectric layer D1 via the first adhesive layer so as to be peelable from the first dielectric layer D1, and the second reflective layer Rf2 is adhered to the second dielectric layer D2 via the second adhesive layer so as to be peelable from the second dielectric layer D2.

[0017] The radio wave absorber 100 according to the embodiment is configured so that when the layer configurations of the dielectric layers and the resistive layers are devised to have different absorption characteristics on both sides, the wave absorber 100 can be used by peeling off the first reflective layer Rf1 or by peeling off the second reflective layer Rf2 depending on the situation. The configurations of the dielectric layers and the resistive layers can be selected as appropriate, but the embodiment employs a configuration in which the first dielectric layer D1, the first resistive layer R1, the third dielectric layer D3, the second resistive layer R2, and the second dielectric layer D2 are sequentially stacked, as described above. Here, the radio wave absorption characteristic of the radio wave absorber 100 when the first reflective layer Rf1 is peeled off from the first dielectric layer D1 so that the first dielectric layer D1 becomes the radio wave incident side is defined as the first characteristic, and the radio wave absorption characteristic of the radio wave absorber 100 when the second reflective layer Rf2 is peeled off from the second dielectric layer D2 so that the second dielectric layer D2 becomes the radio wave incident side is defined as the second characteristic. As described above, the embodiment employs a configuration in which the first dielectric layer D1, the first resistive layer R1, the third dielectric layer D3, the second resistive layer R2, and the second dielectric layer D2 are sequentially stacked, but the first characteristic and the second characteristic are different. Note that the first characteristic and the second characteristic are characteristics that represent the relationship between the frequency of the radio wave and the absorption rate of the radio wave (the return loss of the radio wave).

[0018] [When the first reflective layer Rf1 is peeled off (when the first characteristic is exhibited)] 2A, among the radio waves incident on the first dielectric layer D1, which is the surface dielectric layer, radio waves of a predetermined frequency pass through the first resistive layer R1, and 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 second reflective layer Rf2. The radio wave reflected by the second reflective layer Rf2 is referred to as a third radio wave.

[0019] 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, since the phases of the radio waves are different, the radio waves are attenuated and appear to be absorbed by the radio wave absorber 100.

[0020] The first characteristic of the radio wave absorber 100 is 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 60% or more in the range from 50 GHz to 280 GHz, this first characteristic is satisfied. Preferably, the first characteristic is a characteristic in which 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 first characteristic is a characteristic in which 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).

[0021] The first characteristic of the radio wave absorber 100 is that the radio wave absorption rate has an absorption peak in a frequency range of 50 (GHz) to 300 (GHz). The number of absorption peaks is specifically, for example, 1, 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 this absorption peak is a peak where the radio wave absorption rate is 90% or more.

[0022] 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.

[0023] [When using with the second reflective layer Rf2 removed (when the second characteristic is exhibited)] 2B, a part of the radio wave incident on the second dielectric layer D2, which is the surface dielectric layer, is reflected by the second resistive layer R2. The radio wave reflected by the second resistive layer R2 is referred to as the fourth radio wave. Of the radio waves that pass through the second resistive layer R2, those with a predetermined frequency pass through the first resistive layer R1, and the rest are reflected by the first resistive layer R1. The radio waves reflected by the first resistive layer R1 are referred to as fifth radio waves. The radio wave that passes through the first resistive layer R1 is reflected by the first reflective layer Rf1. The radio wave reflected by the first reflective layer Rf1 is referred to as a sixth radio wave.

[0024] In the radio wave absorber 100 according to the embodiment, the incident radio waves interfere with the fourth radio wave Rw4, the fifth radio wave Rw5, and the sixth radio wave Rw6 described above, and the radio waves are absorbed by the radio wave absorber 100. That is, since the phases of the radio waves are different, the radio waves are attenuated and appear to be absorbed by the radio wave absorber 100.

[0025] The second characteristic of the radio wave absorber 100 is that the frequency range in which the radio wave absorption rate is 80% or more occupies 50% or more in the frequency range of 50 (GHz) to 300 (GHz). Preferably, the second characteristic is such that the range in which the radio wave absorption rate is 80% or more is 55% or more, 60% or more, or 65% or more in the frequency range of 50 (GHz) to 300 (GHz). Preferably, the second characteristic is such that the range in which the radio wave absorption rate is 90% or more is 55% or more, 60% or more, or 65% or more in the frequency range of 50 (GHz) to 300 (GHz).

[0026] The second characteristic of the radio wave absorber 100 is that the radio wave absorption rate has an absorption peak in a frequency range of 50 (GHz) to 300 (GHz). The number of absorption peaks is specifically, for example, 1, 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 this absorption peak is a peak where the radio wave absorption rate is 90% or more. Furthermore, it is preferable that the frequency of the absorption peak of the first characteristic is different from the frequency of the absorption peak of the second characteristic.

[0027] 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.

[0028] 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.

[0029] 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. The first dielectric layer D1 and the second dielectric layer D2 may have adhesive layers (first adhesive layer and second adhesive layer) formed on their surfaces on the radio wave incident side. In other words, the first dielectric layer D1 and the second dielectric layer D2 may include adhesive layers, and the dielectric layers and adhesive layers do not necessarily have to be considered as separate components.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 1-2.Resistance layer The resistive layers (first resistive layer R1 and second resistive layer R2) have a function related to the reflection and passage of radio waves. The first resistive layer R1 also has a function related to the selection of the frequency of the radio waves that pass through. The radio wave absorber 100 thus has at least two resistive layers, which makes radio wave interference more complex and enables it to absorb radio waves in a wide frequency band (for example, 50 GHz to 300 GHz). Furthermore, the radio wave absorber 100 is configured such that the transmission path of radio waves differs when the first reflective layer Rf1 is peeled off and when the second reflective layer Rf2 is peeled off, and therefore the first characteristic and the second characteristic can be made different.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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).

[0041] 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.

[0042] 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.

[0043] 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 pass through the first resistive layer R1 and allow the remainder to pass. The second resistive layer R2 does not have the function of selecting the frequency of the radio waves to be passed, 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 1-3.Reflective layer The reflective layers (first reflective layer Rf1 and second reflective layer Rf2) are 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 layers are layers that reflect radio waves that have passed through all of the dielectric layers and resistive layers.

[0059] Unlike resistive layers, the reflective layers (first reflective layer Rf1 and second reflective layer Rf2) do not need to transmit radio waves, and therefore preferably have as low a resistance as possible. Metal foil or metal plate can be used as the reflective layer. To ensure flexibility in the radio wave absorber 100, metal foil is more preferable as the material for the reflective layer, 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. Metal foils such as aluminum foil that form the reflective layer can be easily produced by rolling a metal material. Furthermore, when forming the reflective layer 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.

[0060] The thickness (μm) of the reflective layer (each of the first reflective layer Rf1 and the second reflective layer Rf2) 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, or may be within a range between any two of the numerical values ​​exemplified here. The thicknesses of the first reflective layer Rf1 and the second reflective layer Rf2 may be different.

[0061] 1-4.Adhesive layer The adhesive layers (first adhesive layer and second adhesive layer) can be made of known materials used as adhesive layers in adhesive tapes, etc., such as acrylic adhesives, rubber adhesives, and silicone adhesives. A tackifier or crosslinking agent can be used to adjust the adhesive strength and reduce adhesive residue. The adhesive strength can be, for example, 5 N / 10 mm to 12 N / 10 mm. The thickness (μm) of the adhesive layer (each of the first adhesive layer and the second adhesive layer) is, for example, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 16 0, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 450, and may be within a range between any two of the values ​​exemplified here.

[0062] 2. Description of the Effects of the Embodiments In the radio wave absorber 100 according to the embodiment, both the first reflective layer Rf1 and the second reflective layer Rf2 are peelable. When the layer configurations of the dielectric layers and resistive layers are devised to provide different absorption characteristics on both sides, the radio wave absorber 100 can be selectively used by peeling off the first reflective layer Rf1 or the second reflective layer Rf2 depending on the situation, providing excellent convenience. In the embodiment, the first resistive layer R1 and the second resistive layer R2 have different configurations, and the propagation path of the radio wave is different when the radio wave is incident from the first dielectric layer D1 and when the radio wave is incident from the second dielectric layer D2. In other words, when the radio wave is incident from the first dielectric layer D1, the radio wave first reaches the first resistive layer R1, whereas when the radio wave is incident from the second dielectric layer D2, the radio wave first reaches the second resistive layer R2. As a result, the radio wave absorber 100 has different radio wave absorption characteristics depending on the surface from which the radio wave is incident. Furthermore, the radio wave absorber 100 according to the embodiment can be used by peeling off both reflective layers (the first reflective layer Rf1 and the second reflective layer Rf2). For example, when the radio wave absorber 100 is attached to a metal layer that functions as a reflective layer, both reflective layers can be peeled off to expose both adhesive layers, and either one of the adhesive layers can be attached to the metal layer to which the radio wave absorber 100 is attached.

[0063] When the radio wave absorber 100 according to the embodiment is used with the first reflective layer Rf1 peeled off, the radio waves in a wide range of frequency bands are absorbed by the radio wave absorber 100 due to the actions of the first radio wave Rw1, the second radio wave Rw2, and the third radio wave Rw3 (first characteristic). Similarly, when the radio wave absorber 100 according to the embodiment is used with the second reflective layer Rf2 peeled off, the radio waves in a wide range of frequency bands are absorbed by the radio wave absorber 100 due to the actions of the fourth radio wave Rw4, the fifth radio wave Rw5, and the sixth radio wave Rw6 (second characteristic). That is, the radio waves that have entered the radio wave absorber 100 according to the embodiment pass through, are reflected by, and select the passing frequency from the multiple resistive layers, and are then absorbed by the first dielectric layer D1, the second dielectric layer D2, and the third dielectric layer D3 in a combined manner, thereby achieving high absorption characteristics (return attenuation) over a wide frequency band. In particular, the embodiment is effective for absorbing radio waves in the high frequency band (50 GHz to 300 GHz).

[0064] 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.

[0065] 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 includes a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer, which are arranged in this order in a stacking direction of the laminate; the first reflective layer is adhered to the first dielectric layer via the first adhesive layer so as to be peelable from the first dielectric layer; The second reflective layer is adhered to the second dielectric layer via the second adhesive layer so as to be peelable from the second dielectric layer. [Appendix 2] 2. The radio wave absorber according to claim 1, a first characteristic is a radio wave absorption characteristic of the radio wave absorber when the first reflective layer is peeled off from the first dielectric layer and the first dielectric layer becomes a radio wave incident side; When the second reflective layer is peeled off from the second dielectric layer and the second dielectric layer becomes the radio wave incident side, the radio wave absorption characteristic of the radio wave absorber is defined as a second characteristic, the first characteristic and the second characteristic are characteristics that represent a relationship between a frequency of a radio wave and an absorption rate, The first characteristic and the second characteristic are different from each other. [Appendix 3] 3. The radio wave absorber according to claim 2, The radio wave absorber has the first characteristic and the second characteristic such that the radio wave absorption rate has an absorption peak in a frequency range of 50 (GHz) to 300 (GHz). [Appendix 4] 4. The radio wave absorber according to claim 3, The absorption peak is a peak where the absorptance is 90% or more. [Appendix 5] A radio wave absorber according to Supplementary Note 3 or Supplementary Note 4, A radio wave absorber in which the frequency of the absorption peak of the first characteristic is different from the frequency of the absorption peak of the second characteristic. [Appendix 6] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 5, the laminate further includes a third dielectric layer, and the resistive layer includes a first resistive layer and a second resistive layer; The laminate is a radio wave absorber in which the first reflective layer, the first adhesive layer, the first dielectric layer, the first resistive layer, the third dielectric layer, the second resistive layer, the second dielectric layer, the second adhesive layer, and the second reflective layer are arranged in this order in the stacking direction. [Appendix 7] 7. The radio wave absorber according to claim 6, 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. [Appendix 8] A radio wave absorber according to any one of Supplementary Note 1 to Supplementary Note 7, The laminate is formed into a flexible sheet shape as a whole.

[0066] 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 graphs shown in Fig. 3A to Fig. 10B show the results of the absorption characteristics of an object having the configuration of the radio wave absorber 100 explained 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.

[0067] The conditions are as follows: In the up-down direction, the first reflective layer Rf1 side is defined as the top, and the second reflective layer Rf2 side is defined as the bottom. The first dielectric layer D1 is an acrylic OCA layer (relative permittivity: 3, dielectric loss 0.05, the same applies below) with a thickness of 425 μm. 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) made of a PET layer (relative permittivity 3.2, dielectric loss 0.01, same below), and an upper layer made of a resistive element portion, with a resistance value of 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 first reflective layer Rf1 and the second reflective layer Rf2 are made of aluminum foil and have a sheet resistance of 10 Ω / sq.

[0068] 3A to 6B, the first characteristic is a characteristic in which the radio wave absorption rate has an absorption peak in the frequency range of 50 (GHz) to 300 (GHz). Furthermore, this absorption peak is a peak where the radio wave absorption rate is 90% or more (-10 dB or less in the graph). Furthermore, as shown in Figures 3A to 6B, the first characteristic shows that 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 characteristic results over a wide frequency range.

[0069] 7A to 10B, the second characteristic has an absorption peak in the frequency range of 50 GHz to 300 GHz, where the radio wave absorption rate is 90% or more (-10 dB or less in the graphs). 7A to 10B, the frequency range in which the radio wave absorption rate is 80% or more (approximately -7 dB or less in the graphs) for the second characteristic is 50% or more in the frequency range from 50 (GHz) to 300 (GHz), and good absorption characteristic results are shown over a wide frequency range. It can be seen that the absorption peak of the first characteristic and the absorption peak of the second characteristic are different, and it is clear that the two characteristics are different.

[0070] 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. The resistive layer does not have to be two layers, and may be one layer. In other words, the third dielectric layer D3 is not provided. In other words, the laminate 10 may have a first reflective layer Rf1, a first adhesive layer, a first dielectric layer D1, only one resistive layer, a second dielectric layer D2, a second adhesive layer, and a second reflective layer Rf2, and these layers may be arranged in this order in the stacking direction of the laminate 10. In this case, it is preferable that the first dielectric layer D1 and the second dielectric layer D2 have different thicknesses and relative dielectric constants. Furthermore, the first resistive layer R1 does not have to have the periodic structure R1S, and the first resistive layer R1 may be a layer formed as a so-called solid layer, similar to the second resistive layer R2. The adhesive layer may be replaced by a connecting structure (e.g., fine irregularities) formed on the dielectric layer. In other words, the adhesive layer may be configured without using the chemical materials described in the embodiments. In this case, the portion of the dielectric layer where the connecting structure is formed corresponds to the adhesive layer. [Explanation of symbols]

[0071] 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 Rf1: 1st reflective layer Rf2: 2nd reflective layer Rw1: 1st radio wave Rw2: Second radio wave Rw3: Third radio wave Rw4: 4th radio wave Rw5: 5th radio wave Rw6: 6th radio wave

Claims

1. A radio wave absorber having a laminate, the laminate includes a first reflective layer, a first adhesive layer, a first dielectric layer, a resistive layer, a second dielectric layer, a second adhesive layer, and a second reflective layer, which are arranged in this order in a stacking direction of the laminate; the first reflective layer is adhered to the first dielectric layer via the first adhesive layer so as to be peelable from the first dielectric layer; The second reflective layer is adhered to the second dielectric layer via the second adhesive layer so as to be peelable from the second dielectric layer.

2. The radio wave absorber according to claim 1, a first characteristic is a radio wave absorption characteristic of the radio wave absorber when the first reflective layer is peeled off from the first dielectric layer and the first dielectric layer becomes a radio wave incident side; When the second reflective layer is peeled off from the second dielectric layer and the second dielectric layer becomes the radio wave incident side, the radio wave absorption characteristic of the radio wave absorber is defined as a second characteristic, the first characteristic and the second characteristic are characteristics that represent a relationship between a frequency of a radio wave and an absorption rate, The radio wave absorber, wherein the first characteristic and the second characteristic are different.

3. The radio wave absorber according to claim 2, The radio wave absorber, wherein the first characteristic and the second characteristic are characteristics in which the radio wave absorption rate has an absorption peak in a frequency range of 50 (GHz) to 300 (GHz).

4. The radio wave absorber according to claim 3, The absorption peak is a peak where the absorption rate is 90% or more.

5. The radio wave absorber according to claim 3, A radio wave absorber in which the frequency of the absorption peak of the first characteristic is different from the frequency of the absorption peak of the second characteristic.

6. The radio wave absorber according to any one of claims 1 to 5, the laminate further includes a third dielectric layer, and the resistive layer includes a first resistive layer and a second resistive layer; The laminate is a radio wave absorber in which the first reflective layer, the first adhesive layer, the first dielectric layer, the first resistive layer, the third dielectric layer, the second resistive layer, the second dielectric layer, the second adhesive layer, and the second reflective layer are arranged in this order in the stacking direction.

7. The radio wave absorber according to claim 6, 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.

8. The radio wave absorber according to any one of claims 1 to 5, The laminate is formed into a flexible sheet shape as a whole.

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