Radio wave absorber

Abstract

To provide a radio wave absorbing sheet capable of achieving both high radio wave absorbing characteristics at a center frequency and radio wave absorbing characteristics of a certain level or higher in a wide frequency band, in a high frequency band from a millimeter wave band to a terahertz band.SOLUTION: In the radio wave absorber 10 in which a first laminated dielectric layer 1, a resistance layer 2, a second laminated dielectric layer 3, and a reflective layer 4 are sequentially laminated from a radio wave 11 incident surface side, both the first laminated dielectric layer and the second laminated dielectric layer are constituted as laminated bodies of a plurality of dielectric layers (1a, 1b, 3a, and 3b), and relative dielectric constants of the plurality of dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer are sequentially increased from the radio wave incident surface side toward the reflective layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a radio wave absorber that absorbs unwanted radio waves, and in particular to a radio wave absorber that can absorb radio waves having high frequencies equal to or higher than the millimeter wave band over a wide frequency band. [Background technology]

[0002] In recent years, the use of high-frequency radio waves, such as centimeter waves with a frequency band of several gigahertz (GHz) and millimeter waves with a frequency band of 30 to 300 gigahertz, has been increasing for mobile communications such as mobile phones, wireless LAN, and electronic toll collection systems (ETC).In addition, with the further evolution of the communications environment and the rise of IoT, there is a demand for the development of devices for Beyond 5G and 6G communications, and research is also progressing on technologies that use radio waves with frequencies in the terahertz (THz (1 THz = 1000 GHz)) band.

[0003] As the frequency of radio waves increases, specific materials compatible with high-frequency waves are required for transmitting and receiving systems such as antenna circuits, testing equipment, and devices, as well as for noise suppression and radio wave leakage prevention within these devices. In particular, for devices that handle terahertz radio waves, the location-dependent nature of all materials used, such as circuit conductors, circuit boards, and exterior components, cannot be ignored, requiring precise length, size, and thickness adjustment and design. Furthermore, radio wave absorbers that absorb unwanted radio waves must have specific characteristics, such as absorption characteristics for vertically incident radio waves, absorption characteristics for obliquely incident radio waves, polarization characteristics, absorption characteristics for radio waves at the desired frequency (= center frequency) (e.g., 90% = -10 dB or better), and radio wave bandwidth. However, while research on materials that effectively absorb millimeter-wave band radio waves has progressed relatively well, research on materials that effectively absorb radio waves in the extremely high-frequency band from 300 GHz to 1 THz is still insufficient.

[0004] Conventionally, a radio wave absorbing sheet that absorbs radio waves in the millimeter wave band has been proposed, in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are stacked in this order from the radio wave incident surface side, and the resistance value of the resistive layer is set to 110 Ω / sq or more and 150 Ω / sq or more, thereby achieving broad radio wave absorption characteristics that absorb radio waves of a wide range of frequencies relative to the center frequency that the radio wave absorbing sheet is intended to absorb (Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-132076 Summary of the Invention [Problem to be solved by the invention]

[0006] In the above-mentioned conventional radio wave absorbing sheet, which is an electromagnetic interference type (also called a reflection type or λ / 4 type) radio wave absorbing sheet, the phase between the radio waves reflected on the surface of the radio wave absorbing sheet and the radio waves that pass through the dielectric layer of the radio wave absorbing layer and are reflected by the reflection layer is inverted, so that the radio waves reflected on the radio wave absorbing sheet appear to be absorbed. In this case, a resistive layer formed between the first dielectric layer and the second dielectric layer reflects part of the radio waves that enter the radio wave absorbing sheet, thereby inverting the phase between the radio waves reflected on the surface of the radio wave absorbing sheet, the radio waves reflected by the resistive layer, and the radio waves that pass through the resistive layer and are reflected by the reflection layer, thereby achieving radio wave absorption characteristics that absorb radio waves over a wide frequency band.

[0007] FIG. 5 shows the return loss characteristics of the conventional radio wave absorbing sheet disclosed in Patent Document 1.

[0008] In the conventional radio wave absorbing sheet disclosed in Patent Document 1, a first dielectric layer with a thickness of 150 μm, a resistive layer with a resistance of 140 Ω / sq, a second dielectric layer with a thickness of 150 μm, and a reflective layer are sequentially laminated, and the radio wave absorbing sheet has a center frequency (target frequency) of 300 GHz. The reflection attenuation characteristics of radio waves incident on the radio wave absorbing sheet are shown as the reflection attenuation characteristic indicated by reference numeral 51, which is the measurement result of an actually produced radio wave absorbing sheet, and the reflection attenuation characteristic indicated by reference numeral 52, which is the simulation result. As shown in Fig. 5, the frequency characteristics of the reflection attenuation of the conventional radio wave absorbing sheet exhibit broad radio wave absorption characteristics with an absorption peak at approximately 325 GHz, and it can be seen that a reflection attenuation of -10 dB, which corresponds to 90% radio wave absorption, is achieved in a wide frequency band from approximately 175 GHz to approximately 470 GHz.

[0009] The conventional radio wave absorbing sheet described above has improved the radio wave absorption characteristics of the radio wave interference type radio wave absorbing sheet, which has a large amount of radio wave absorption only in a narrow frequency band near the center frequency, and has achieved broad radio wave absorption characteristics. However, since the return loss near the center frequency where the amount of radio wave absorption peaks remains at approximately -30 dB, it is expected that the amount of radio wave absorption near the center frequency may not be sufficient.

[0010] The present disclosure aims to solve the above-mentioned conventional problems and to realize a radio wave absorbing sheet that can achieve both high radio wave absorption characteristics at the center frequency and radio wave absorption characteristics above a certain level over a wide frequency band in a high frequency band ranging from the millimeter wave band to the terahertz band. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the radio wave absorber disclosed in the present application is a radio wave absorber in which a first laminated dielectric layer, a resistive layer, a second laminated dielectric layer, and a reflective layer are laminated in this order from the radio wave incident surface side, and the first laminated dielectric layer and the second laminated dielectric layer are both configured as laminates of a plurality of dielectric layers, and the relative dielectric constants of the plurality of dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer are successively larger from the radio wave incident surface side toward the reflective layer. [Effects of the Invention]

[0012] The wave absorber disclosed in the present application is formed by sequentially laminating a first laminated dielectric layer, a resistive layer, a second laminated dielectric layer, and a reflective layer, and the dielectric constants of the dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer increase in order from the radio wave incident surface side toward the reflective layer. This allows sufficient interference between the radio waves reflected by the resistive layer and the reflective layer and the radio waves reflected by the surface of the wave absorber, thereby achieving both high radio wave absorption characteristics for radio waves at the center frequency and frequencies in the band nearby, and radio wave absorption characteristics at or above a certain level for radio waves in a wide frequency band. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view illustrating a configuration of an electromagnetic wave absorbing sheet according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the wave absorption characteristics of the wave absorbing sheet according to the first embodiment. [Figure 3] FIG. 10 is a diagram showing the radio wave absorption characteristics of the radio wave absorbing sheet according to the second embodiment. [Figure 4] FIG. 10 is a diagram showing the radio wave absorption characteristics of a radio wave absorbing sheet of a comparative example. [Figure 5] FIG. 10 is a diagram showing the radio wave absorption characteristics of a conventional radio wave absorbing sheet. DETAILED DESCRIPTION OF THE INVENTION

[0014] The radio wave absorbing sheet disclosed in the present application is a radio wave absorber in which a first laminated dielectric layer, a resistive layer, a second laminated dielectric layer, and a reflective layer are laminated in this order from the radio wave incident surface side, and the first laminated dielectric layer and the second laminated dielectric layer are both constructed as laminates of multiple dielectric layers, and the relative dielectric constants of the multiple dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer gradually increase from the radio wave incident surface side toward the reflective layer.

[0015] By configuring in this manner, the radio wave absorber disclosed in the present application suppresses undesired reflection and scattering of radio waves incident on the first laminated dielectric layer and the second laminated dielectric layer, which are a laminate of multiple dielectric layers, and due to the interference between the radio waves reflected on the surface of the radio wave absorber, the radio waves reflected on the resistive layer, and the radio waves reflected on the reflective layer, it is possible to realize a radio wave absorber that combines high radio wave absorption characteristics for radio waves in a frequency band near the center frequency and radio wave absorption characteristics at or above a certain level in a wide frequency band.

[0016] In the wave absorber disclosed in the present application, it is preferable that the dielectric layer constituting the second laminated dielectric layer has a relative permittivity of not less than 4. In this way, it becomes easy to gradually increase the relative permittivity of the multiple dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer from the radio wave incident surface side toward the reflective layer, and a resistive layer is disposed in a portion where the difference in relative permittivity between adjacent layers becomes large, making it easy to manufacture a wave absorber that achieves desired wave absorption characteristics.

[0017] It is also preferable that the surface resistance value of the resistive layer is 100 Ω / sq or more and 300 Ω / sq or less, thereby achieving a good balance between the amount of radio waves reflected by the reflective layer and the amount of radio waves transmitted through the resistive layer and reflected by the reflective layer.

[0018] Preferably, the dielectric layers and the reflective layer constituting the first and second laminated dielectric layers are all light-transmitting, and the resistive layer is a transparent layer containing at least one of an oxide conductive material and a conductive polymer. In this way, a radio wave absorber having light-transmitting properties as a whole can be easily realized.

[0019] Furthermore, by forming the electromagnetic wave absorbing sheet into a sheet whose thickness is smaller than the size of the electromagnetic wave incident surface, it is possible to realize an electromagnetic wave absorbing sheet that can be easily placed in a desired location.

[0020] Hereinafter, the radio wave absorber disclosed in the present application will be described with reference to the drawings.

[0021] In the following, we will exemplify a radio wave absorbing sheet, which is a sheet-shaped radio wave absorber whose thickness is small relative to its main surface, which is the radio wave incident surface onto which the radio waves to be absorbed are irradiated. A thin radio wave absorbing sheet can be flexible, and therefore can be easily placed on the surface of a curved member, etc. On the other hand, a block-shaped radio wave absorber whose thickness is relatively large relative to its main surface, which is the radio wave incident surface, has higher rigidity than a sheet-shaped radio wave absorber, and due to its shape characteristic of having a certain thickness or more, it can be used, for example, to be placed between multiple members such as a transmitter and a receiver, or as a free-standing radio wave absorber like a partition.

[0022] (Embodiment) [Overall structure of the radio wave absorbing sheet] FIG. 1 is a cross-sectional view showing the configuration of the radio wave absorbing sheet according to this embodiment.

[0023] Note that Figure 1 is a diagram provided to make it easier to understand the configuration of the radio wave absorbing sheet according to this embodiment, and the sizes of the components shown in the figure, particularly the thickness of each layer, are not necessarily depicted in accordance with reality.

[0024] The radio wave absorbing sheet 10 illustrated in this embodiment is constructed by sequentially stacking a first laminated dielectric layer 1, a resistive layer 2, a second laminated dielectric layer 3, and a reflective layer 4 from the incident side of the radio waves 11 to be absorbed.

[0025] The first laminated dielectric layer 1 and the second laminated dielectric layer 3 of the radio wave absorbing sheet 10 according to this embodiment are both formed as a laminate of multiple dielectric layers. In Fig. 1, the first laminated dielectric layer 1 is configured as a laminate of two dielectric layers, a dielectric layer 1a and a dielectric layer 1b, and the second laminated dielectric layer 2 is configured as a laminate of two dielectric layers, a third dielectric layer 3a and a fourth dielectric layer 3b. Here, the relative dielectric constants ε of the first to fourth four dielectric layers 1a, 1b, 3a, and 3b constituting the first laminated dielectric layer 1 and the second laminated dielectric layer 3 according to this embodiment increase successively from the top in Fig. 1, which is the incident surface side of the radio waves 11, to the bottom in Fig. 1, where the reflective layer 4 is disposed.

[0026] That is, when the relative dielectric constant of the first dielectric layer 1a is ε1a, the relative dielectric constant of the second dielectric layer 1b is ε1b, the relative dielectric constant of the third dielectric layer 3a is ε3a, and the relative dielectric constant of the fourth dielectric layer 3b is ε3b, the relationship ε1a<ε1b<ε3a<ε3b holds.

[0027] In the radio wave absorbing sheet according to this embodiment configured as described above, the relative permittivities of the multiple dielectric layers are set to successively larger values ​​from the radio wave incident surface toward the reflecting surface. Therefore, when radio waves enter the radio wave absorbing sheet from the air, reflection of the radio waves at the interfaces of the multiple dielectric layers is reduced, making it easier for the radio waves to enter the dielectric layers. The phase of the incident radio waves reflected from the surface of the first laminated dielectric layer, which is the radio wave incident surface of the radio wave absorbing sheet, the phase of the reflected wave that passes through the first laminated dielectric layer, is reflected by the resistive layer, passes again through the first laminated dielectric layer, and is radiated from the surface of the radio wave absorbing sheet, and the phase of the reflected wave that passes through the first laminated dielectric layer, the resistive layer, and the second laminated dielectric layer, is reflected by the reflecting layer, passes through the second laminated dielectric layer, the resistive layer, and the first laminated dielectric layer, and is radiated from the surface of the radio wave absorbing sheet interfere with each other, thereby attenuating the reflected wave from the radio wave absorbing sheet, making it appear as if the radio waves are absorbed by the radio wave absorbing sheet. This is a radio wave interference type (reflection type, λ / 4 type) radio wave absorbing sheet.

[0028] 1 shows a case where the first laminated dielectric layer 1 and the second laminated dielectric layer are both laminates of two dielectric layers 1a and 1b, and 3a and 3b, respectively, but the number of dielectric layers constituting the first laminated dielectric layer 1 and / or the second laminated dielectric layer 3 may be three or more. In the radio wave absorber disclosed in the present application, when a plurality of dielectric layers made of the same material and having the same relative dielectric constant are laminated as the dielectric layers constituting the first laminated dielectric layer 1 and / or the second laminated dielectric layer 3, the dielectric layers are understood to be a single dielectric layer having a thickness equal to the sum of the thicknesses of the laminated dielectric layers. Therefore, a laminate in which only dielectric layers having the same relative dielectric constant are laminated does not correspond to the first laminated dielectric layer and the second laminated dielectric layer of the radio wave absorber disclosed in the present application.

[0029] [Details of each component] Next, each member constituting the radio wave absorbing sheet 10 according to this embodiment will be described.

[0030] <Dielectric layer> The dielectric layers 1a, 1b, 3a, and 3b constituting the first laminated dielectric layer 1 and the second laminated dielectric layer 3 of the radio wave absorbing sheet 10 according to this embodiment can all be formed from various dielectric materials such as acrylic resin, silicone resin, urethane resin, titanium oxide, polyvinylidene fluoride, polyester resin, glass, silicone rubber, ceramics, and alumina.

[0031] In the electromagnetic wave absorbing sheet according to this embodiment, the dielectric constants of the dielectric layers constituting the first laminated dielectric layer 1 and the second laminated dielectric layer 3 increase sequentially from the radio wave incident surface toward the reflective layer 4. Among the various dielectric materials mentioned above, acrylic resins, silicone resins, urethane resins, and the like are preferred as materials for the dielectric layers of the radio wave absorbing sheet because they are readily available in sheets of the desired thickness and offer excellent durability. Because these dielectric sheets have a dielectric constant of approximately 2.5 to 3.5, they are suitable for use as the first dielectric layer 1a and the second dielectric layer 1b constituting the first laminated dielectric layer 1, which have a low dielectric constant. For example, acrylic resins have various relative dielectric constants ranging from approximately 2.5 to 4.5 depending on the type. However, the conventionally used acrylic OCA (Optical Clear Adhesive) has a relative dielectric constant of approximately 2.5, making it a preferred material for dielectric layers located closer to the radio wave incident surface, where a low relative dielectric constant is preferred.

[0032] Other materials with low dielectric constants include cycloolefin polymers (COP) and fluorine-based resins. For example, fluorinated polyimide (PI) and polyether (PE) resins have a low dielectric constant of 2.2 to 2.5.

[0033] Materials with a dielectric constant of around 3 include polyimide (PI: dielectric constant of approximately 3.5), polyethylene terephthalate (PET: dielectric constant of approximately 3.0), and polycarbonate (PC: dielectric constant of approximately 2.9), which are often used as resin substrates, and these can be selected as materials for the dielectric layers that make up the first laminated dielectric layer 1, which has a low dielectric constant.

[0034] On the other hand, materials for the dielectric layers constituting the second laminated dielectric layer 3, which have a higher dielectric constant than these resin-based dielectric layers, are preferably glass fiber-containing epoxy resins or inorganic dielectric materials such as silica glass, ceramics, and alumina, which have a high dielectric constant of generally 4.0 or higher, and are used as the third and fourth dielectric layers 3a and 3b constituting the second laminated dielectric layer 3. However, as described below, the thickness of the dielectric layer in a radio wave interference-type radio wave absorber is inversely proportional to the dielectric constant. Therefore, if the dielectric constant of the dielectric layer material is too high, the thickness of the dielectric layer becomes too thin, which reduces the feasibility of the radio wave absorbing sheet and the ease of handling during lamination. For example, barium titanate (dielectric constant 1500), a typical high-dielectric-constant ceramic material, has such a high dielectric constant that it is necessary to fabricate an extremely thin ceramic sheet, for example, 3 μm thick, making it less feasible.

[0035] Thus, taking into consideration the constraints in actually producing a radio wave absorbing sheet, the upper limit of the relative permittivity of the dielectric layer used in the radio wave absorbing sheet shown in this embodiment is preferably about 10 or less. Ceramics with a relative permittivity of 10 or less include alumina (relative permittivity of 8.5 to 10) and zirconia (relative permittivity of about 6).

[0036] In selecting the multiple dielectric materials constituting the first laminated dielectric layer 1 and the second laminated dielectric layer 3, it is preferable to use dielectric materials of the same type for the dielectric layers constituting each laminated dielectric layer. Because the difference in the dielectric constants between dielectric materials of the same type is small, even when adjacently arranged, undesired reflection or scattering of radio waves 11 at the boundary due to the difference in dielectric constants is unlikely to occur. For this reason, if the first laminated dielectric layer 1 is formed using a dielectric material with a relatively low dielectric constant, such as an acrylic resin material, a silicone resin material, or PET, and the second laminated dielectric layer 3 is formed using a material with a relatively high dielectric constant, such as glass (glass fiber) or ceramic, the resistive layer 2 is disposed in a portion where the difference in the dielectric constants of the dielectric materials is large. This suppresses reflection and scattering of radio waves 11 incident on the radio wave absorbing sheet 10, thereby forming a radio wave absorbing sheet 10 that effectively absorbs the incident radio waves 11. From this perspective, it is preferable to use a material with a dielectric constant of 4 or more for the dielectric layer constituting the second laminated dielectric layer 3 located closer to the reflective layer 4 than the resistive layer 2.

[0037] The thicknesses D1 and D2 of the first laminated dielectric layer 1 and the second laminated dielectric layer 3 are determined according to the wavelength λ, which is the reciprocal of the frequency (center frequency: target frequency) of the radio wave 11 to be absorbed by the radio wave absorber sheet 10, by taking into consideration the dielectric constant ε1 of the first laminated dielectric layer 1 (composite dielectric constant of the plurality of laminated dielectric layers) and the dielectric constant ε2 of the second laminated dielectric layer 3 (composite dielectric constant of the plurality of laminated dielectric layers): D1=λ / 4√ε1 (Equation 1) D2=λ / 4√ε2 (Equation 2) Find the value for which the following holds true.

[0038] Incidentally, since an error of about ±10% can be tolerated for the values ​​of D1 and D2, when actually producing the radio wave absorbing sheet 10, the relative permittivity and thickness of the material used for each dielectric layer constituting each laminated dielectric layer can be appropriately selected.

[0039] As described above, in the radio wave absorbing sheet 10 according to this embodiment, a polyethylene terephthalate (PET) sheet, for example, is used as the resin substrate when forming the resistive layer 2 described later, and this PET sheet as the resin substrate can be used as the dielectric layer located closest to the resistive layer among the dielectric layers constituting the laminate of the first laminated dielectric layer 1 (the second dielectric layer 1b in the example of Figure 1).

[0040] Furthermore, by using an adhesive material for each of the dielectric layers constituting the first laminated dielectric layer 1 and the second laminated dielectric layer 3, it is possible to easily form a laminate of dielectric layers and bond the dielectric layer to the resistive layer, or the dielectric layer to the reflective layer, without using a separate material such as an adhesive or double-sided tape for bonding.

[0041] The above-mentioned OCA can be widely used as such an adhesive dielectric sheet material. OCA is composed of various materials, such as acrylic-based OCA, silicone-based OCA, and urethane-based OCA, so an OCA with the required dielectric constant can be selected as the dielectric layer that constitutes the laminated dielectric layer. Furthermore, because it has high translucency in addition to adhesiveness, it is particularly suitable for realizing a radio wave absorbing sheet that is translucent as a whole by constructing other dielectric layer materials, resistive layers, and reflective layers using light-transmitting materials.

[0042] By setting the total light transmittance of the radio wave absorbing sheet 10 to 30% or more, it becomes possible to see the opposite side of the radio wave absorbing sheet 10, and therefore when the radio wave absorbing sheet is used on the wall surface of an anechoic chamber, it becomes possible to perform measurements while checking the measuring device inside. Furthermore, when the radio wave absorbing sheet is used for applications that require design or as a radio wave absorbing sheet that covers a wide area such as an entire window, the total light transmittance of the radio wave absorbing sheet 10 is preferably 60% or more, and more preferably 70% or more.

[0043] <Resistance layer> The resistive layer 2 of the radio wave absorbing sheet 10 shown in this embodiment is disposed between the first laminated dielectric layer 1 and the second laminated dielectric layer 3, and serves to reflect a portion of the radio waves 11 that have passed through the first laminated dielectric layer 1 and transmit the remainder.

[0044] The rate at which the resistive layer 2 reflects the radio waves 11 is affected by the surface resistance value of the resistive layer 2. The greater the difference between the surface resistance value of the resistive layer 2 and the impedance value of the dielectric layers that make up the first and second laminated dielectric layers, the greater the rate at which the radio waves 11 are reflected by the resistive layer 2 and the less the rate at which the radio waves 11 are transmitted. The impedance of the insulator that becomes the dielectric layer is generally 10 8 Ω or more, which is a very high value compared to the resistance value of approximately 100 Ω to several hundred Ω assumed for the resistive layer 2, so the lower the resistance value of the resistive layer 2, the greater the proportion of radio waves 11 that are reflected by the resistive layer.

[0045] In addition, the proportion of radio waves 11 that pass through the resistive layer 2 also changes depending on the frequency of the radio waves 11 incident on the radio wave absorbing sheet 10. For a resistive layer 2 with the same surface resistance value, the proportion of radio waves 11 that pass through the resistive layer 2 increases as the frequency of the radio waves 11 increases.

[0046] In the radio wave absorbing sheet 10 shown in this embodiment, for example, when the center frequency of the radio waves 11 to be absorbed is targeted to be 284 GHz, which is called the 300 GHz band, it is desirable that the surface resistance value of the resistive layer 2 be 100 Ω / sq or more and 300 Ω / sq or less.

[0047] If the surface resistance of the resistive layer 2 is less than 100 Ω / sq, the amount of radio waves 11 reflected by the resistive layer 2 increases while the amount of radio waves 11 transmitted through the resistive layer 2 decreases. On the other hand, if the surface resistance of the resistive layer 20 is greater than 300 Ω / sq, the amount of radio waves 11 reflected by the resistive layer 2 decreases while the amount of radio waves 11 transmitted through the resistive layer 2 increases. As a result, the balance of interference between the radio waves 11 reflected by the resistive layer 2, the radio waves reflected by the reflective layer 4, and the radio waves 11 reflected on the surface of the radio wave absorber 10 is lost, making it difficult to achieve the characteristic radio wave absorption characteristics of the radio wave absorbing sheet 10 according to this embodiment, namely, high radio wave absorption characteristics near the center frequency and good radio wave absorption characteristics over a wide band. Thus, in the radio wave absorbing sheet 10 of this embodiment, it is important to set the surface resistance of the resistive layer 2 to a value within a predetermined range corresponding to the center frequency of the radio waves 11 to be absorbed.

[0048] Furthermore, when the target frequency absorbed by the radio wave absorbing sheet 10 is different from the 300 GHz band described above, it is preferable to use the simulation described below to determine the surface resistance value of the resistive layer 2 that will effectively absorb radio waves of the frequency that is the absorption target.

[0049] The material for the resistive layer 2 used in the radio wave absorbing sheet 10 according to this embodiment is not particularly limited as long as it can realize the surface resistance value required for the resistive layer 2 set according to the absorption target frequency, such as various materials having a surface resistance value of 100 Ω / sq to 300 Ω / sq as described above when the absorption target frequency is in the 300 GHz band. Specifically, conductive organic polymer films, sputtered films, vapor deposition films, etc. can be used effectively. Furthermore, the above-mentioned conductive organic polymer films, sputtered films, and vapor deposition films are preferable in that the surface resistance value can be controlled by the film thickness and formation density, making it possible to easily form a resistive layer 2 having a desired surface resistance value.

[0050] In the radio wave absorbing sheet 10 shown in this embodiment, if the thickness of the resistive layer 2 is large, the surface resistance of the resistive layer 2 decreases, increasing the amount of reflected radio waves 11 and attenuating the radio waves 11 that pass through the resistive layer, and therefore the resistive layer 2 is preferably a thin film. Even if the radio wave absorbing sheet 10 as a whole is translucent, a thick resistive layer 2 is not preferable because it reduces the light transmittance. From the above perspectives, the resistive layer 2 is preferably as thin as possible as long as it can achieve a predetermined surface resistance value, and specifically, it is preferably 0.01 μm to 10 μm thick.

[0051] The conductive organic polymer used as the resistive layer 2 is a conjugated conductive organic polymer, and it is preferable to use polythiophene or a derivative thereof, or polypyrrole or a derivative thereof.

[0052] Furthermore, the resistive layer 2 can be made of an organic polymer whose main chain is composed of a π-conjugated system, such as a polyacetylene-based conductive polymer, a polyphenylene-based conductive polymer, a polyphenylene vinylene-based conductive polymer, a polyaniline-based conductive polymer, a polyacene-based conductive polymer, a polythiophene vinylene-based conductive polymer, or a copolymer thereof.

[0053] The conductive organic polymer used in the resistance layer 2 can use a polyanion as a counter anion. While there are no particular limitations on the polyanion, it is preferable for the polyanion to contain an anionic group capable of causing chemical oxidation doping in the conjugated conductive organic polymer. Examples of such anionic groups include groups represented by the general formulas -O-SO3X, -O-PO(OX)2, -COOX, and -SO3X (where X represents a hydrogen atom or an alkali metal atom). Among these, groups represented by -SO3X and -O-SO3X are particularly preferred due to their excellent doping effect on the conjugated conductive organic polymer.

[0054] 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.

[0055] 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.

[0056] Furthermore, in the resistive layer 2 of the radio wave absorbing sheet 10 according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and obtain a predetermined resistance value. Examples of dopants that can be used include 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, and TCNQ.

[0057] The content of the conductive organic polymer in the resistive layer 2 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 2 composition. If the content is less than 10% by mass, the conductivity of the resistive layer 2 tends to decrease. Therefore, if the surface electrical resistance of the resistive layer 2 is set within a predetermined range to achieve impedance matching, the thickness of the resistive layer 2 increases, which tends to thicken the entire radio wave absorbing sheet, or, if the radio wave absorbing sheet 100 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 coating of the resistive layer 20, making it difficult to form a good resistive layer 20. If the radio wave absorbing sheet 100 as a whole is translucent, the haze of the resistive layer 20 tends to increase, which also tends to deteriorate its optical properties.

[0058] The resistive layer 2 may also be configured to contain a carbon material such as a carbon microcoil, a carbon nanotube, or graphene.

[0059] 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 to 10 μm, the carbon fiber forming the coil has a diameter of 0.1 to 1 μm, and the coil length is preferably 1 to 10 mm.

[0060] Specifically, carbon nanotubes can be obtained by vapor phase growth methods such as arc discharge, laser evaporation, pyrolysis, etc. The carbon nanotubes used as the resistive layer 2 of the radio wave absorbing sheet 10 according to this embodiment may be either single-walled or multi-walled.

[0061] Graphene can be obtained by, for example, peeling and transfer, SiC pyrolysis, chemical vapor deposition, cutting carbon nanotubes, etc. As the graphene used for the resistive layer 2 of the radio wave absorbing sheet 10 according to this embodiment, it is preferable to use powdered graphene in a scale shape, from the viewpoint of easily obtaining a desired aspect ratio and of orientation in the radio wave absorbing sheet 10.

[0062] The resin in which the carbon material is dispersed may be a water-soluble polyester resin.

[0063] The resistance layer 2 can be formed by applying a coating composition as a paint for forming the resistance layer 2 onto a resin substrate and drying it, as described above.

[0064] The method for applying the resistance layer-forming paint to the substrate can be, for example, a bar coating method, a reverse coating method, a gravure coating method, a microgravure coating method, a die coating method, a dipping method, a spin coating method, a slit coating method, a spray coating method, or the like. Drying after application may be performed under conditions that allow the solvent component of the resistance layer-forming paint to evaporate, and is preferably performed at 100 to 150°C for 5 to 60 minutes. If necessary, the resistance layer 2 may be formed by irradiating the coating film with UV light (ultraviolet rays) or EB (electron beams) to cure the coating film.

[0065] Although there are no particular limitations on the substrate used to form the resistive layer 2, a transparent substrate is preferred when a radio wave absorbing sheet having visible light transmittance is to be formed. Materials for such a transparent substrate include various materials such as resin, rubber, glass, and ceramics.

[0066] <Reflective layer> The reflective layer 4 is a layer that reflects radio waves 11 that have passed through the second laminated dielectric layer 3. Unlike the resistive layer 2, the reflective layer 4 does not need to transmit radio waves 11. For this reason, it is preferable that the surface resistance be as low as possible, and a surface resistance of 0 Ω / sq is most preferable, but considering manufacturing, the achievable lower limit is thought to be approximately 0.01 Ω / sq. Furthermore, the upper limit of the surface resistance of the reflective layer 40 is preferably 30 Ω / sq. If the surface resistance of the reflective layer 40 is higher than 30 Ω / sq, it becomes difficult to obtain selective, high radio wave absorption characteristics for radio waves of the desired frequency. Metal foil or metal plate can be suitably used as such a reflective layer 4.

[0067] To provide flexibility to the radio wave absorbing sheet 10, the reflective layer 4 can be made of a metal foil or a film coated with a conductive metal paste containing silver, copper, metal nanowires, or the like. Among these, metal foil is preferable due to its low surface resistance, and various metal foils such as copper foil, aluminum foil, and gold foil can be used. Among these, aluminum foil is particularly preferable for the reflective layer 4, considering the cost and the effect of oxidation in air. A metal foil such as aluminum foil for forming the reflective layer 4 can be easily produced by rolling a metal material. Furthermore, when the reflective layer 4 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 for forming various vapor-deposited films, taking into consideration 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.

[0068] The thickness of the reflective layer 4 is not particularly limited, but when a metal foil is used to form the flexible radio wave absorbing sheet 10, if the thickness is too thick it will be meaningless in terms of radio wave absorbing properties and will result in unnecessary consumption of material, while if the thickness is too thin it may cause problems such as insufficient strength and tearing during production or use, so a thickness of 1 μm to 20 μm is preferable. Also, in the case of a film coated with a metal paste, it is sufficient that the film is coated on a substrate such as PET to a thickness that allows it to maintain a sufficiently low resistance value, and a thickness of about 0.01 μm to 10 μm is generally sufficient.

[0069] 1 according to this embodiment, the reflective layer 4 can be formed solely from a vapor-deposited film of a conductive material such as metal by directly forming a vapor-deposited film of a metal material on the surface of the second laminated dielectric layer 3 opposite to the side on which the resistive layer 2 is formed. When a vapor-deposited film of metal is formed on the back surface of the second laminated dielectric layer 3, no gap is formed between the second laminated dielectric layer 3 and the reflective layer 4, as compared to when the second laminated dielectric layer 3 and the reflective layer 4 are formed separately and then closely arranged. Therefore, the radio waves 1 transmitted through the second laminated dielectric layer 3 can be reflected at the position of the back surface of the second laminated dielectric layer 3, making it easy to realize a radio wave absorbing sheet 10 having the frequency characteristics of the desired amount of radio wave absorption.

[0070] On the other hand, when a vapor-deposited film is used for the reflective layer 4, it is necessary to form the conductive material in the vapor-deposited film with a uniform and sufficient density, compared to when a metal foil is used. According to the results of the inventors' studies, it is preferable to set the surface resistance of the reflective layer to 30 Ω / sq or less, and it is preferable to sufficiently control the thickness of the metal vapor-deposited film to set the surface resistance to a desired value or less.

[0071] Furthermore, in order to provide the radio wave absorbing sheet 10 with both flexibility and light transmission, a conductive mesh made of conductive fibers can be used as the reflective layer 4. For example, the conductive mesh can be made by attaching a metal to a mesh woven from polyester monofilament to make it conductive. As the metal, copper, silver, or the like, which have high conductivity, can be used. Furthermore, in order to reduce reflection from the metal film covering the surface of the conductive mesh, a product has also been produced in which a black anti-reflection layer is applied further outside the metal film.

[0072] Furthermore, metal nanowires can also be used for the reflective layer 4. Metal nanowires are needle-shaped metal particles with diameters on the order of nanometers, i.e., from a few nanometers to a few hundred nanometers, and their length is generally at least 1,000 times their thickness. A conductive metal paste containing such metal nanowires, a binder, and other dispersants can be used.

[0073] The content of metal nanowires relative to the total solid content in the metal nanowire paste is preferably 0.1% by mass or more. If the content of the metal nanowires is less than 0.1% by mass, the electrical resistance of the reflective layer becomes too high, making it difficult for radio waves to be reflected on the surface of the reflective layer, resulting in poor electromagnetic wave absorption properties. Furthermore, if the content of the metal nanowires is increased, the surface electrical resistance of the reflective layer decreases, which is preferable in terms of reflecting radio waves. For this reason, there is no upper limit, but from the viewpoint of coating suitability, a content of about 1% by mass is preferred.

[0074] Examples of metals that constitute metal nanowires include iron, cobalt, nickel, copper, zinc, ruthenium, rhodium, palladium, silver, cadmium, osmium, iridium, platinum, and gold. From the viewpoints of electrical conductivity and translucency, copper and silver are preferred, and silver is more preferred.

[0075] Alternatively, the reflective layer 4 may be a conductive metal grid or a metal mesh in which thin metal wires such as copper wires having a diameter of several tens to several hundreds of μm are arranged lengthwise and widthwise.

[0076] In addition, when the reflective layer 4 is formed using the above-mentioned conductive mesh or conductive metal grid, in order to ensure flexibility and translucency, it is configured to have the minimum thickness possible as long as the surface resistance value required for the reflective layer 4 can be realized.

[0077] The aperture ratio of the reflective layer 4 formed as a conductive mesh or conductive grid is preferably larger from the viewpoint of ensuring light transmittance, but is preferably smaller from the viewpoint of reliably reflecting radio waves on the surface of the reflective layer 4 and enhancing the radio wave absorption characteristics of the radio wave absorbing sheet 10. According to studies by the inventors, the aperture ratio is preferably 35% or more and 85% or less, and more preferably 35% or more and 75% or less.

[0078] Incidentally, even when the conductive mesh or conductive grid is used as the reflective layer 4, the surface resistance value of the reflective layer 4 is preferably 30 Ω / sq or less.

[0079] <Adhesive layer> Although not shown in FIG. 1, an adhesive layer can be formed on the rear surface of the reflective layer 4 so that the radio wave absorbing sheet 10 according to this embodiment can be easily placed at a predetermined position.

[0080] The adhesive layer may be made of known materials used for adhesive layers of adhesive tapes and the like, such as acrylic adhesives, rubber adhesives, and silicone adhesives. A tackifier or a crosslinking agent may be used to adjust the adhesive strength to the adherend and reduce adhesive residue. The adhesive strength to the adherend is preferably 5 N / 10 mm to 12 N / 10 mm. If the adhesive strength is less than 5 N / 10 mm, the radio wave absorbing sheet 10 may easily peel off or slip off from the adherend. If the adhesive strength is greater than 12 N / 10 mm, the radio wave absorbing sheet 10 may be difficult to peel off from the adherend.

[0081] The thickness of the adhesive layer is preferably 20 μm to 100 μm. If the thickness of the adhesive layer is thinner than 20 μm, the adhesive strength will be weak, and the radio wave absorbing sheet 10 may easily peel off or slip off from the adherend. If the thickness of the adhesive layer is thicker than 100 μm, the radio wave absorbing sheet 10 will be difficult to peel off from the adherend. If the cohesive strength of the adhesive layer is weak, adhesive residue may remain on the adherend when the radio wave absorbing sheet 10 is peeled off. This may also be a factor in reducing the flexibility of the radio wave absorbing sheet 10 as a whole.

[0082] The adhesive layer that can be used in the radio wave absorbing sheet 10 according to this embodiment can be either an adhesive layer that non-removably attaches the radio wave absorbing sheet 10 to an object to which it is attached, or an adhesive layer that removably attaches the sheet. It is not essential that the radio wave absorbing sheet 10 according to this embodiment has an adhesive layer, and the radio wave absorbing sheet 10 can be adhered to a desired member using any of various conventional adhesive methods.

[0083] <Protective layer> In addition, a protective layer for protecting the surface of the radio wave absorbing sheet 10 can be disposed on the first laminated dielectric layer 1 further on the side of the radio wave 11 incident surface.

[0084] The protective layer is a film that protects the first laminated dielectric layer 1 from being damaged by external forces or from changes in its relative permittivity ε1 due to the effects of ultraviolet rays or moisture. Note that the protective layer is not an essential constituent feature of the radio wave absorbing sheet 10 according to this embodiment, and depending on the material of the first laminated dielectric layer 1 and the conditions under which the radio wave absorbing sheet 10 is used, it is possible to select a configuration of the radio wave absorbing sheet 10 without a protective layer when there is little concern about changes in the permittivity ε1 of the first laminated dielectric layer 1 or damage to the surface due to adhesion of moisture to the surface.

[0085] The protective layer can be made of a resin material such as polyethylene terephthalate. Although the resin material used for the protective layer has a certain resistance value, by setting the thickness of the protective layer thin, the influence of the protective layer on the radio waves 11 incident on the radio wave absorbing sheet 10 can be kept at a practically negligible level.

[0086] (Example) The results of a study on the frequency characteristics of radio wave absorption of the radio wave absorbing sheet according to this embodiment will be described below.

[0087] In the following studies, simulations were performed using full-wave 3D electromagnetic field software "Ansys HFSS (product name: manufactured by ANSYS, Inc.)" that uses the finite element method.

[0088] FIG. 2 is a diagram showing the change in return loss with respect to the frequency of incident radio waves, as the radio wave absorption characteristics of the radio wave absorber sheet of the first example according to this embodiment.

[0089] Here, return loss is the strength (energy) of radio waves reflected by the radio wave absorbing sheet, expressed in dB, relative to the strength (energy) of radio waves incident on the sheet. Therefore, when the return loss is -10 dB, the strength of the reflected radio waves is 1 / 10 (10%) of the incident radio waves, indicating that 90% of the radio wave energy has been absorbed by the radio wave absorbing sheet. In this way, the larger the absolute value of the return loss, the more radio waves are absorbed.

[0090] The radio wave absorption sheet of the first example, whose radio wave absorption characteristics are shown in Figure 2, is designed with an incident radio wave frequency of approximately 300 GHz (center wavelength λ = 1 mm), and the first laminated dielectric layer is a three-layer laminate consisting of a first dielectric layer with a relative permittivity of 2.3 and a thickness of 50 μm, assuming that COP is used from the radio wave incident side; a second dielectric layer with a relative permittivity of 2.5 and a thickness of 110 μm, assuming that acrylic OCA is used; and a third dielectric layer with a relative permittivity of 2.7 and a thickness of 20 μm, assuming that a PET sheet is used as the resin substrate for the resistive layer. The relative permittivity of the entire first laminated dielectric layer is approximately 2.5 and the thickness is 180 μm.

[0091] The second dielectric layer is a two-layer laminate consisting of a fourth dielectric layer with a dielectric constant of 4.5 and a thickness of 15 μm, assuming a glass fiber-filled epoxy resin sheet, and a fifth dielectric layer with a dielectric constant of 8.5 and a thickness of 80 μm, assuming an alumina film, stacked from the resistive layer side. The overall dielectric constant of the second dielectric layer is approximately 7.9, and the thickness is 95 μm.

[0092] The resistive layer was assumed to be made of PEDOT, a conductive organic polymer, and the surface resistance was set to 150 Ω / sq.

[0093] Assuming that the reflective layer is made of aluminum foil, the surface resistance was set to 0 Ω / sq.

[0094] 2, the frequency characteristic 21 of the return loss of the radio wave absorber sheet of the first embodiment is centered around a frequency of 284 GHz, which is called the 300 GHz band, and a return loss of -30 dB (99.9% absorption) is achieved in a frequency bandwidth of approximately 75 GHz, and a return loss of -40 dB, which is an absorption of 99.99%, is achieved in a frequency bandwidth of approximately 40 GHz. It was also confirmed that a return loss of -10 dB (90% radio wave absorption), which can be understood as radio wave absorption above a certain level, was achieved in a wide frequency bandwidth of approximately 225 GHz, ranging from approximately 175 GHz to approximately 400 GHz.

[0095] FIG. 3 is a diagram showing the change in return loss with respect to the frequency of incident radio waves, as the radio wave absorption characteristics of the radio wave absorber sheet of the second example according to this embodiment.

[0096] The radio wave absorption sheet of the second embodiment, whose radio wave absorption characteristics are shown in Figure 3, has the same incident radio wave frequency as the first embodiment, approximately 300 GHz (center wavelength λ = 1 mm), and the first laminated dielectric layer has a laminated structure of two dielectric layers.

[0097] The first laminated dielectric layer of the radio wave absorbing sheet of the second example was a two-layer laminate consisting of a first dielectric layer with a relative permittivity of 2.3 and a thickness of 50 μm, assuming that COP was used from the radio wave incident side, and a second dielectric layer with a relative permittivity of 2.7 and a thickness of 120 μm, assuming that a PET sheet was used as the resin substrate of the resistive layer. The relative permittivity of the entire first laminated dielectric layer was approximately 2.6, and the thickness was 170 μm.

[0098] The second laminated dielectric layer of the radio wave absorbing sheet of the second example was a two-layer laminate consisting of a third dielectric layer with a relative permittivity of 4.5 and a thickness of 15 μm, assuming that a glass fiber-filled epoxy resin sheet was used, and a fourth dielectric layer with a relative permittivity of 8.5 and a thickness of 80 μm, assuming that an alumina film was used, stacked from the resistive layer side. The relative permittivity of the entire second laminated dielectric layer was approximately 7.9, and the thickness was 95 μm.

[0099] The resistive layer was assumed to be made of PEDOT, a conductive organic polymer, and had a surface resistance of 145 Ω / sq.

[0100] The reflective layer was assumed to be made of aluminum foil, and the surface resistance was set to 0 Ω / sq.

[0101] 3, the frequency characteristic 31 of the return loss of the radio wave absorber sheet of the second embodiment is shown. The return loss (99.9% absorption) of -30 dB is achieved in a frequency bandwidth of approximately 70 GHz, and the return loss of -40 dB, which is an absorption of 99.99%, is achieved in a frequency bandwidth of approximately 50 GHz, centered around a frequency of 284 GHz, which is known as the 300 GHz band. It was also confirmed that the return loss of -10 dB (90% radio wave absorption), which can be understood as radio wave absorption above a certain level, is achieved in a wide frequency bandwidth of approximately 215 GHz, from approximately 185 GHz to approximately 400 GHz.

[0102] Next, the frequency characteristics of the return loss of a radio wave absorbing sheet as a comparative example that does not satisfy the requirements for radio wave pause treatment disclosed in the present application will be described.

[0103] FIG. 4 is a diagram showing the change in return loss with respect to the frequency of incident radio waves, as the radio wave absorption characteristics of the radio wave absorbing sheet of the comparative example.

[0104] The radio wave absorbing sheet of the comparative example, whose radio wave absorption characteristics are shown in Figure 4, has the same frequency of incident radio waves as that of approximately 300 GHz (center wavelength λ = 1 mm), but the configuration of the radio wave absorbing sheet of the second embodiment described above is such that the order of arrangement of the first to fourth dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer is reversed, and the value of the relative dielectric constant of each dielectric layer becomes successively smaller from the radio wave incident surface side toward the reflective layer.

[0105] Specifically, the first laminated dielectric layer was a two-layer laminate, with the first dielectric layer having a relative permittivity of 8.5 and a thickness of 80 μm, assuming that an alumina film was used on the radio wave incident surface side, and the second dielectric layer having a relative permittivity of 4.5 and a thickness of 15 μm, assuming that a glass fiber-filled epoxy resin sheet was used on the resistive layer side.

[0106] In addition, the second laminated dielectric layer was a two-layer laminate consisting of a third dielectric layer with a relative permittivity of 2.7 and a thickness of 120 μm on the resistive layer side, assuming that a PET sheet was used as the resin substrate for the resistive layer, and a fourth dielectric layer with a relative permittivity of 2.3 and a thickness of 50 μm on the reflective layer side, assuming that a COP was used.

[0107] In this case, the first laminated dielectric layer as a whole has a relative dielectric constant of about 7.9 and a thickness of 95 μm, and the second laminated dielectric layer as a whole has a relative dielectric constant of about 2.6 and a thickness of 170 μm.

[0108] The resistive layer and the reflective layer had the same configuration as the radio wave absorbing sheet of the second embodiment, and the surface resistance value of the resistive layer was set to 145 Ω / sq, and the surface resistance value of the reflective layer was set to 0 Ω / sq.

[0109] The frequency characteristics of the return loss of the radio wave absorbing sheet of the comparative example shown in FIG. 4 are significantly different from the radio wave absorption characteristics of the radio wave absorbing sheet of the first embodiment shown in FIG. 2 and the radio wave absorption characteristics of the second embodiment shown in FIG. 3. In the frequency band of approximately 30 GHz to 500 GHz, which is the frequency band above the millimeter wave band, the maximum return loss is -15 dB at around 120 GHz, which is extremely small.

[0110] In this way, in the radio wave absorbing sheet according to this embodiment, the first laminated dielectric layer, the resistive layer, the second laminated dielectric layer, and the reflective layer are laminated in this order from the radio wave incident surface side, and the dielectric constants of the dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer, which are laminates of multiple dielectric layers, are configured to increase in value from the radio wave incident surface side toward the reflective layer side. This makes it possible to realize a radio wave absorber that combines high radio wave absorption characteristics (for example, a return loss of -30 dB or more) for radio waves in a frequency band of a predetermined width (for example, several tens of GHz) around the center frequency (target frequency) at which absorption peaks for radio waves in a high frequency band above the millimeter wave band, and radio wave absorption characteristics of a certain level or more (for example, a return loss of -10 dB) or more in an extremely wide frequency band spanning, for example, approximately 200 GHz.

[0111] In the above embodiment, the center frequency of the radio waves incident on the radio wave absorbing sheet is 285 GHz, which is in the 300 GHz band. However, even if the center frequency is in a higher frequency band, for example, 1 THz (terahertz), by setting the overall relative dielectric constant and thickness (D1, D2) of the first and second laminated dielectric layers to appropriate values, it is possible to similarly realize a radio wave absorbing sheet or radio wave absorber that has both high radio wave absorption characteristics for radio waves in a predetermined frequency band near the center frequency and radio wave absorption characteristics above a certain level for radio waves in a wide frequency band.

[0112] In the above embodiment, the radio wave absorber (radio wave absorbing sheet) disclosed in the present application has been described as having two laminated dielectric layers, i.e., a first laminated dielectric layer and a second laminated dielectric layer. However, it is also possible to fabricate a radio wave absorber (radio wave absorbing sheet) having three or more laminated dielectric layers with a resistive layer interposed therebetween. In this case, by adjusting the surface resistance of the resistive layers located closer to the radio wave incident surface to have a smaller value, a radio wave absorber having a wide range of radio wave absorption characteristics can be realized due to the combined action of the radio waves reflected by each of the multiple resistive layers and the radio waves reflected by the reflective layer. In this case, it goes without saying that the dielectric constants of the dielectric layers constituting the laminated dielectric layer, which may consist of three or more layers, should be increased in order from the radio wave incident surface side to the reflective layer side. [Industrial Applicability]

[0113] The radio wave absorber disclosed in the present application is useful as a radio wave absorber that has a first laminated dielectric layer, a resistive layer, a second laminated dielectric layer, and a reflective layer laminated in that order, and that achieves both high radio wave absorption characteristics in the vicinity of the center frequency of the radio waves to be absorbed and radio wave absorption characteristics above a certain level over a wide frequency band by making the relative permittivity of each of the dielectric layers constituting the first laminated dielectric layer and the second laminated dielectric layer successively larger from the radio wave incident surface side toward the reflective layer side. [Explanation of symbols]

[0114] 1. First laminated dielectric layer 1a First dielectric layer 1b Second dielectric layer 2 resistance layer 3 Second laminated dielectric layer 3 3a Third dielectric layer 3b Fourth dielectric layer 4 reflective layer 10. Radio wave absorbing sheet (radio wave absorber) 11 Radio Waves

Claims

1. A radio wave absorber in which a first laminated dielectric layer, a resistive layer, a second laminated dielectric layer, and a reflective layer are laminated in this order from the radio wave incident surface side, the first laminated dielectric layer and the second laminated dielectric layer are both configured as a laminate of a plurality of dielectric layers, a plurality of dielectric layers constituting the first and second stacked dielectric layers having successively larger dielectric constants from the radio wave incident surface side toward the reflective layer;

2. 2. The radio wave absorber according to claim 1, wherein the dielectric layers constituting the second laminated dielectric layer have a relative dielectric constant of 4 or more.

3. 2. The method according to claim 1, wherein the surface resistance of the resistive layer is 100 Ω / sq or more and 300 Ω / sq or less. Radio wave absorber.

4. 2. The radio wave absorber according to claim 1, wherein the dielectric layer and the reflective layer constituting the first stacked dielectric layer and the second stacked dielectric layer all have light-transmitting properties, and the resistive layer is a transparent layer containing at least one of an oxide conductive material or a conductive polymer.

5. 2. The radio wave absorber according to claim 1, which is formed in a sheet shape having a size in a thickness direction smaller than the size of the radio wave incident surface.

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