Electromagnetic wave absorbing sheet

Abstract

The present invention achieves an electromagnetic wave absorbing sheet which has a plurality of electromagnetic wave absorption bands in a high frequency band from hundreds of GHz to a terahertz band, and in which each electromagnetic wave absorption band has a large band width. Provided is an electromagnetic wave absorbing sheet in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are layered successively starting from an electromagnetic wave entry surface side, wherein when the permittivity of the first dielectric layer is represented as ε1, the thickness thereof is represented as D1, the permittivity of the second dielectric layer is represented as ε2, and the thickness thereof is represented as D2, the following expression (1) and expression (2) are satisfied with regard to the center wavelength λ of electromagnetic waves.　Expression (1): D1=5λ / 4√ε1±20% Expression (2) D2=5λ / 4√ε2±20%

Description

Electromagnetic wave absorbing sheet

[0001] The present disclosure relates to an electromagnetic wave absorbing sheet that absorbs unwanted electromagnetic waves, and in particular to an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in a plurality of frequency bands in high frequency bands equal to or higher than the millimeter wave band.

[0002] In recent years, the use of high-frequency electromagnetic 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 in mobile communications such as mobile phones, wireless LANs, and electronic toll collection systems (ETC). Furthermore, with the further evolution of the communications environment and the rise of IoT, there is a demand for the development of devices for communications beyond 5G and 6G, and research into technologies that utilize radio waves with frequencies in the terahertz (THz (1 THz = 1000 GHz)) band is also progressing.

[0003] Thus, as the frequency of electromagnetic waves used increases, specific materials that are compatible with high-frequency electromagnetic waves are required for transmitting and receiving systems such as antenna circuits, testing equipment devices, and also for noise countermeasures for these devices, prevention of electromagnetic wave leakage, etc. In particular, in devices that handle electromagnetic waves in the terahertz band, location dependency cannot be ignored for all materials used, such as conductors within the circuit, circuit boards, and exterior materials, and strict adjustment and design of length, size, and thickness is required.

[0004] Conventionally, an electromagnetic wave absorbing sheet that absorbs electromagnetic waves of multiple frequencies in the millimeter wave band has been proposed that includes an electromagnetic wave absorbing layer formed by laminating multiple magnetic layers on the front side of a reflective layer, in which magnetic iron oxide, which generates magnetic resonance with electromagnetic waves in the millimeter wave band and absorbs the electromagnetic waves, is dispersed in a dielectric binder (Patent Document 1).

[0005] JP 2022-84611 A

[0006] The above-mentioned conventional electromagnetic wave absorbing sheet can effectively absorb electromagnetic waves of different frequencies in the millimeter wave band by utilizing the different magnetic resonance frequencies of the magnetic iron oxide contained in the magnetic layer.

[0007] However, according to the description of the examples, although the frequency characteristics of the electromagnetic waves absorbed by the above-mentioned conventional electromagnetic wave absorbing sheet have five peaks at 70 GHz, 90 GHz, 120 GHz, 150 GHz, and 178 GHz, the effective electromagnetic wave absorption amount of 10 dB (90% attenuation) is only achieved in three bands: the 120 GHz band, the 150 GHz band, and the 180 GHz band, and furthermore, the width of each frequency band is less than 20 GHz, so it cannot be said to be sufficient as an electromagnetic wave absorbing sheet that can absorb electromagnetic waves in different frequency bands.

[0008] The present disclosure aims to solve the above-mentioned problems and to realize an electromagnetic wave absorbing sheet that has multiple electromagnetic wave absorption bands in the high frequency band from several hundred GHz to the terahertz band, and each of the electromagnetic wave absorption bands has a wide bandwidth.

[0009] In order to solve the above-mentioned problems, the electromagnetic wave absorbing sheet disclosed in the present application is an electromagnetic wave absorbing sheet in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are laminated in this order from the electromagnetic wave incident surface side, and is characterized in that when the dielectric constant of the first dielectric layer is ε1 and the thickness is D1, and the dielectric constant of the second dielectric layer is ε2 and the thickness is D2, the following (Equation 1) and (Equation 2) hold for the central wavelength λ of the electromagnetic wave: D1=5λ / 4√ε1±20% (Equation 1) D2=5λ / 4√ε2±20% (Equation 2).

[0010] The electromagnetic wave absorbing sheet disclosed in the present application comprises an electromagnetic wave absorbing layer in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are laminated in this order, and thereby a cancellation effect is achieved in which electromagnetic waves of a predetermined frequency are absorbed by interference between electromagnetic waves reflected by the surface of the first dielectric layer, electromagnetic waves transmitted through the first dielectric layer and reflected by the resistive layer, and electromagnetic waves transmitted through the resistive layer and the second dielectric layer and reflected by the reflective layer. As a result, an electromagnetic wave absorbing sheet with a wide absorption band over multiple frequency bands can be realized.

[0011] It is a cross-sectional view for explaining the configuration of the electromagnetic wave absorbing sheet according to the present embodiment. It is a diagram showing the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet according to the present embodiment. It is a diagram showing the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet of a comparative example.

[0012] The electromagnetic wave absorbing sheet disclosed in the present application is an electromagnetic wave absorbing sheet in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are laminated in this order from the electromagnetic wave incident surface side, and is characterized in that, when the dielectric constant of the first dielectric layer is ε1 and the thickness is D1, and the dielectric constant of the second dielectric layer is ε2 and the thickness is D2, the following (Equation 1) and (Equation 2) hold for the center wavelength λ of the electromagnetic wave: D1=5λ / 4√ε1±20% (Equation 1) D2=5λ / 4√ε2±20% (Equation 2).

[0013] With this configuration, a portion of the electromagnetic waves is reflected by the surface of the first dielectric layer, the remaining portion of the electromagnetic waves passes through the first dielectric layer, and then is further reflected by the resistive layer, and the remaining portion of the electromagnetic waves passes through the second dielectric layer and reaches the reflective layer where they are reflected. As a result, the electromagnetic waves reflected by the surface of the first dielectric layer, the electromagnetic waves passed through the first dielectric layer and reflected by the resistive layer, and the electromagnetic waves passed through the resistive layer and the second dielectric layer and reflected by the reflective layer interfere with each other, making it possible to realize an electromagnetic wave absorbing sheet with absorption bands spanning multiple wide frequency bands.

[0014] In the electromagnetic wave absorbing sheet, the surface resistance of the resistive layer is preferably 150 Ω / □ or more and 250 Ω / □ or less, thereby realizing an electromagnetic wave absorbing sheet that targets absorption of electromagnetic waves in the 300 GHz frequency band.

[0015] It is also preferable that the first dielectric layer, the second dielectric layer, and the reflective layer all have light-transmitting properties, and the resistive layer is a transparent layer containing at least one of an oxide conductive material and a conductive polymer, thereby making it possible to provide light-transmitting properties to the entire electromagnetic wave absorbing sheet.

[0016] Furthermore, it is preferable that the thickness of the resistive layer is 0.01 μm or more and 10 μm or less, so that the influence of the thickness of the resistive layer on the electromagnetic wave absorption characteristics can be reduced.

[0017] Furthermore, it is preferable that the surface resistance of the reflective layer is 30 Ω / □ or less, which can reliably reflect electromagnetic waves transmitted through the second dielectric layer and improve the electromagnetic wave absorption characteristics due to interference with incident electromagnetic waves.

[0018] Furthermore, it is preferable that the first dielectric layer and the second dielectric layer contain an acrylic resin, a silicone resin, or a urethane resin, thereby realizing an electromagnetic wave absorbing sheet that is easy to manufacture and has excellent durability.

[0019] It is also preferable that at least one of the first dielectric layer and the second dielectric layer is configured as a laminate of a plurality of dielectric layers, which makes it possible to easily realize a laminate structure of the first dielectric layer, the resistive layer, the second dielectric layer, and the reflective layer.

[0020] The electromagnetic wave absorbing sheet disclosed in the present application will be described below with reference to the drawings.

[0021] (Embodiment) FIG. 1 is a cross-sectional view showing the configuration of an electromagnetic wave absorbing sheet according to this embodiment.

[0022] It should be noted that FIG. 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet according to this embodiment, and the sizes of the components shown in the diagram, particularly the thickness of each layer, are not necessarily depicted in accordance with reality.

[0023] [Overall structure of the electromagnetic wave absorbing sheet] The electromagnetic wave absorbing sheet 100 illustrated in this embodiment is configured by sequentially stacking a first dielectric layer 10, a resistive layer 20, a second dielectric layer 30, and a reflective layer 40 from the incident surface side of the electromagnetic wave 1 to be absorbed.

[0024] The electromagnetic wave absorbing sheet 100 of this embodiment is an electromagnetic wave absorbing sheet 100 that uses the principle of electromagnetic wave interference, in which electromagnetic waves incident on the electromagnetic wave absorbing sheet are reflected by the surface of the first dielectric layer, the resistive layer, and the reflective layer, and the phases of the incident waves incident on the electromagnetic wave absorbing sheet and the reflected waves reflected from the electromagnetic wave absorbing sheet are inverted, so that the reflected waves from the electromagnetic wave absorbing sheet 100 appear to be absorbed.

[0025] [Details of Each Member] Next, each member constituting the electromagnetic wave absorbing sheet 10 according to this embodiment will be described.

[0026] <Dielectric Layer> The first dielectric layer 10 and the second dielectric layer 30 of the electromagnetic wave absorbing sheet according to this embodiment can both be formed from various dielectric materials such as acrylic resin, silicone resin, urethane resin, titanium oxide, polyvinylidene fluoride, polyester resin, glass, silicone rubber, etc.

[0027] In particular, it is preferable to use acrylic resin, silicone resin, or urethane resin, which are readily available as a sheet of the desired thickness as the material for the dielectric layer and are excellent in durability.

[0028] The first dielectric layer 10 and the second dielectric layer 30 can each be formed as a single layer made of one type of material, or can be formed as a laminate of two or more layers of the same or different materials. The first dielectric layer 10 and the second dielectric layer 30 can be formed using the same dielectric material, or can be formed using different dielectric materials, including the number of layers.

[0029] The thicknesses D1 and D2 of the first dielectric layer 10 and the second dielectric layer 30 are set in accordance with the wavelength λ, which is the reciprocal of the frequency of the electromagnetic wave 1 to be absorbed by the electromagnetic wave absorbing sheet 100, taking into consideration the dielectric constants ε (ε1, ε2) of the materials constituting the respective dielectric layers 10 and 30, so that the following conditional expressions (Expression 1) and (Expression 2) hold.

[0030] When the dielectric constant of the first dielectric layer 10 is ε1 and its thickness is D1, and the dielectric constant of the second dielectric layer 30 is ε2 and its thickness is D2, the following equations are satisfied, relative to the central wavelength λ of the electromagnetic wave: D1 = 5λ / 4√ε1 ± 20% (Equation 1) D2 = 5λ / 4√ε2 ± 20% (Equation 2).

[0031] In addition, when the first dielectric layer 10 and / or the second dielectric layer 30 is formed as a laminate of multiple dielectric layers, the thicknesses D1 and D2 of the laminate are set to satisfy the above (Equation 1) and (Equation 2) using the overall dielectric constants ε1 and ε2 of the multiple dielectric layers that make up each dielectric layer.

[0032] In the electromagnetic wave absorbing sheet according to this embodiment, by satisfying the conditions of (Equation 1) and (Equation 2) above, the phases of the reflected wave that is incident on the electromagnetic wave absorbing sheet 100, passes through the first dielectric layer 10, and is reflected by the resistive layer 20, and the reflected wave that is transmitted without being reflected by the resistive layer 20, passes further through the second dielectric layer 30, and is reflected by the reflective layer 40, can be adjusted with respect to the surface reflected wave that is reflected on the surface of the electromagnetic wave absorbing sheet 100, thereby realizing a return loss characteristic that absorbs electromagnetic waves in multiple wide frequency bands for the electromagnetic waves reflected from the electromagnetic wave absorbing sheet 100.

[0033] As an example, if the center frequency of the electromagnetic waves 1 absorbed by the electromagnetic wave absorbing sheet 100 is set to 300 GHz (wavelength λ ≈ 1), assuming the 300 GHz band, which is the frequency band used in next-generation information communications known as Beyond 5G, in order to create a multi-band electromagnetic wave absorber having electromagnetic wave absorption bands in the 300 GHz band and the 180 GHz band and 60 GHz band, which are frequencies equivalent to 3 / 5 and 1 / 5 of that, it is preferable to use a general dielectric material with a relative permittivity of about 2.5 for the first dielectric layer 10 and the second dielectric layer 30, and to set the thickness to 630 μm to 950 μm.

[0034] In the case of a dielectric layer (10, 30) formed from a dielectric material with a relative dielectric constant of about 2.5, if the thickness is thinner than 630 μm or thicker than 950 μm, the center frequency of the electromagnetic waves to be absorbed will be shifted higher or lower than 300 GHz, and at the same time, there will also be a shift in frequency between the 180 GHz band, which is 3 / 5 of that, and the 60 GHz band, which is 1 / 5 of that, making it impossible to obtain an electromagnetic wave absorbing sheet 100 that absorbs electromagnetic waves in the desired frequency band.

[0035] Although not shown in the drawings, in the electromagnetic wave absorbing sheet 100 according to this embodiment, a polyethylene terephthalate (PET) sheet is used as a resin base material when forming the resistive layer 20 described later, and the first dielectric layer 10 is formed by laminating this PET sheet as the resin base material and an acrylic OCA (Optical Clear Adhesive).

[0036] In this way, by using an OCA having adhesive properties as the material for the first dielectric layer 10, it is possible to easily form a laminated structure of the resistive layer 20 and the first dielectric layer 10. Furthermore, by making the first dielectric layer 10 a two-layer structure of a PET sheet and an OCA, it is possible to select the material and thickness of the resin sheet used as the base material (the PET sheet in the above example) from those preferable for forming the resistive layer 20, and it is possible to obtain an appropriate dielectric constant ε1 and thickness D1 for the first dielectric layer 10 by adjusting the dielectric constant and thickness of the other dielectric material sheet (the acrylic OCA sheet in the above example) used in combination, and it is possible to easily form the resistive layer 20 while ensuring the electromagnetic wave absorbing properties of the electromagnetic wave absorbing sheet, thereby realizing the electromagnetic wave absorbing sheet 100 at low cost.

[0037] In the electromagnetic wave absorbing sheet 100 according to this embodiment, the second dielectric layer 30 is also made of an acrylic OCA (Optical Clear Adhesive) with emphasis on translucency and self-adhesive properties.

[0038] In this way, the resistance layer 20, the second dielectric layer 30, and the reflective layer 40 can be bonded together by the adhesive strength of the second dielectric layer 30, which simplifies the configuration of the electromagnetic wave absorbing sheet 100, improving workability during manufacturing and reducing the amount of material used, thereby reducing the cost of producing the electromagnetic wave absorbing sheet 100. Note that, as the OCA used for the first dielectric layer 10 and the second dielectric layer 30, in addition to the acrylic OCA described above, a silicone OCA, a urethane OCA, or the like can also be suitably used.

[0039] Of course, an adhesive material such as a double-sided adhesive sheet can be used to adhere the first dielectric layer 10, the resistive layer 20, the second dielectric layer 30, and the reflective layer 40, and an adhesive can be applied to the surfaces where each layer is adhered to form a laminate as the electromagnetic wave absorbing sheet 100.

[0040] Furthermore, by using a transparent OCA as the first dielectric layer 10 and the second dielectric layer 30, and configuring the resistive layer 20 and the reflective layer 40 described below to be light-transmittable, it is possible to realize an electromagnetic wave absorbing sheet 100 that has translucency and a total light transmittance of a certain level or more as a whole.

[0041] The total light transmittance of the electromagnetic wave absorbing sheet 100 is preferably 30% or more, and more preferably 40% or more. When the electromagnetic wave absorbing sheet 100 has a total light transmittance of 30% or more, the opposite side of the electromagnetic wave absorbing sheet 100 can be seen, and therefore, when used in an anechoic chamber or the like, measurements can be performed while checking the internal measuring device, and practical light transmittance can be ensured. Furthermore, when used in applications requiring design or as an electromagnetic wave absorbing sheet covering a large area such as an entire window, the total light transmittance of the electromagnetic wave absorbing sheet 100 is preferably 60% or more, and more preferably 70% or more.

[0042] <Resistance Layer> The resistance layer 20 of the electromagnetic wave absorbing sheet 100 shown in this embodiment is disposed between the first dielectric layer 10 and the second dielectric layer 30, and functions to reflect a portion of the electromagnetic wave 1 that has passed through the first dielectric layer 10 and transmit the remainder.

[0043] The reflection rate of the electromagnetic wave 1 in the resistive layer 20 is affected by the surface resistance value of the resistive layer 20. The greater the difference between the surface resistance value of the resistive layer 20 and the impedance value of the acrylic OCA or PET that is the insulator constituting the first dielectric layer 10 or the second dielectric layer 30, the greater the reflection rate of the electromagnetic wave 1 in the resistive layer 20 and the smaller the transmission rate of the electromagnetic wave 1. In other words, the impedance of the insulator is 10 ８ This is a very high value compared to the resistance value of the resistive layer 20, which is Ω or more, and therefore the lower the resistance value of the resistive layer 20, the greater the reflection rate.

[0044] For example, in the electromagnetic wave absorbing sheet 100 shown in this embodiment, when the center frequency of the electromagnetic waves 1 to be absorbed is targeted to be 284 GHz, which is called the 300 GHz band, it is desirable to set the surface resistance value of the resistance layer 20 to be 150 Ω / □ or more and 250 Ω / □ or less. In this case, sub-electromagnetic wave absorption bands are generated in the 50 to 80 GHz band, which is one-fifth of the target frequency, and the 150 to 240 GHz band, which is three-fifths of the target frequency.

[0045] If the surface resistance value of the resistive layer 20 is less than 150 Ω / □, more electromagnetic waves are reflected by the resistive layer 20 and less electromagnetic waves are transmitted through it. On the other hand, if the surface resistance value of the resistive layer 20 is greater than 250 Ω / □, less electromagnetic waves are reflected by the resistive layer 20 and more electromagnetic waves are transmitted through it. As a result, the balance in interference between the electromagnetic waves reflected by the resistive layer 20 and the reflective layer 40 is lost, making it difficult to provide a wide absorption band in three frequency bands. In the electromagnetic wave absorbing sheet 100 of this embodiment, the surface resistance value of the resistive layer 20 is designed to achieve an optimal balance between the electromagnetic wave 1 reflected by the resistive layer 20 and the electromagnetic wave 1 that passes through the resistive layer 20 and is reflected by the reflective layer 40. This allows the interference between the electromagnetic wave 1 that is surface-reflected at the surface of the first dielectric layer 10, the electromagnetic wave 1 that is reflected by the resistive layer 20, and the electromagnetic wave 1 that passes through the resistive layer 20 and is reflected by the reflective layer 40 to be appropriately combined, thereby enabling the electromagnetic wave 1 in a wide frequency band to be effectively absorbed in multiple bands. Therefore, it is important to set the surface resistance value of the resistive layer 20 to a value within a predetermined range that corresponds to the center frequency of the electromagnetic wave 1 to be absorbed.

[0046] Furthermore, if the target frequency absorbed by the electromagnetic wave absorption sheet 100 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 resistance layer 20 that will effectively absorb electromagnetic waves of the absorption target frequency.

[0047] The material for the resistive layer 20 used in the electromagnetic wave absorbing sheet 100 according to this embodiment is not particularly limited as long as it can realize the surface resistance value required for the resistive layer 20 set in accordance with the absorption target frequency, such as a surface resistance value of 150 Ω / □ or more and 250 Ω / □ 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 20 having a desired surface resistance value.

[0048] In the electromagnetic wave absorbing sheet 100 shown in this embodiment, if the thickness of the resistive layer 20 is large, the electromagnetic waves 1 passing through the resistive layer 20 will be attenuated, so the resistive layer 20 is preferably a thin film. Furthermore, even if the electromagnetic wave absorbing sheet 100 as a whole is translucent, a thick resistive layer 20 is not preferable because it reduces the light transmittance. From the above perspectives, the resistive layer 20 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.

[0049] The conductive organic polymer used as the resistive layer 20 is a conjugated conductive organic polymer, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0050] Furthermore, the resistive layer 20 can be made of an organic polymer whose main chain is composed of a π-conjugated system, and examples of such an organic polymer include 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.

[0051] The conductive organic polymer used in the resistance layer 20 can use a polyanion as a counter anion. There are no particular limitations on the polyanion, but it is preferable that the polyanion contains an anionic group that can generate chemical oxidation doping in the conjugated conductive organic polymer. Examples of such an anionic group include those represented by the general formula -O-SO ３ X, -O-PO (OX) ２ , -COOX, -SO ３ X (in each formula, X represents a hydrogen atom or an alkali metal atom), and among them, —SO ３ X and -O-SO ３ The group represented by X is particularly preferred.

[0052] 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.

[0053] 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.

[0054] Furthermore, in the resistance layer 20 of the electromagnetic wave absorbing sheet 100 according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and obtain a predetermined resistance value. Examples of 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.

[0055] The content of the conductive organic polymer in the resistive layer 20 is preferably 10% by mass or more and 35% by mass or less, based on the total mass of the solids contained in the resistive layer 20 composition. If the content is less than 10% by mass, the conductivity of the resistive layer 20 tends to decrease. Therefore, if the surface electrical resistance of the resistive layer 20 is set within a predetermined range to achieve impedance matching, the thickness of the resistive layer 20 increases, which tends to thicken the entire electromagnetic wave absorbing sheet, or, if the electromagnetic 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 electromagnetic 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.

[0056] The resistive layer 20 may also be configured to include a carbon material such as a carbon microcoil, a carbon nanotube, or graphene.

[0057] 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.

[0058] Specifically, the carbon nanotubes can be obtained by vapor phase growth methods such as arc discharge, laser evaporation, pyrolysis, etc. The carbon nanotubes used as the resistance layer 20 of the electromagnetic wave absorbing sheet 100 according to this embodiment may be either single-walled or multi-walled.

[0059] 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 the graphene used for the resistance layer 20 of the electromagnetic wave absorbing sheet 100 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 electromagnetic wave absorbing sheet 10.

[0060] The resin in which the carbon material is dispersed may be a water-soluble polyester resin, which improves the weather resistance of the resistance layer 20 and realizes a highly reliable electromagnetic wave absorbing sheet 100 with a stable surface resistance value.

[0061] The resistance layer 20 can be formed by applying a coating composition as a coating material for forming the resistance layer 20 onto a resin substrate and drying it, as described above.

[0062] Examples of methods that can be used to apply the resistance layer-forming paint to the substrate include bar coating, reverse coating, gravure coating, microgravure coating, die coating, dipping, spin coating, slit coating, and spray coating. Drying after application is preferably performed at 100 to 150°C for 5 to 60 minutes under conditions that allow the solvent component of the resistance layer-forming paint to evaporate. Furthermore, if necessary, the resistance layer 20 may be formed by irradiating the coating film with UV light (ultraviolet rays) or EB (electron beam) to cure the coating film.

[0063] The substrate used to form the resistance layer 20 is not particularly limited, but a transparent substrate having transparency is preferred when an electromagnetic wave absorbing sheet having visible light transmittance is to be formed. As the material of such a transparent substrate, various materials such as resin, rubber, glass, and ceramics can be used, and as described above, a PET film having a thickness of 50 μm is used in the electromagnetic wave absorbing sheet 100 exemplified in this embodiment.

[0064] <Reflective Layer> The reflective layer 40 is a layer that reflects the electromagnetic wave 1 that has passed through the second dielectric layer 30. Unlike the resistive layer 20, the reflective layer 40 does not need to transmit the electromagnetic wave 1. For this reason, it is preferable that the surface resistance be as low as possible, and a surface resistance value of 0 Ω / □ is most preferable, but when considering manufacturing, the achievable lower limit is thought to be approximately 0.01 Ω / □. Furthermore, the upper limit of the surface resistance value of the reflective layer 40 is preferably 30 Ω / □. If the surface resistance value of the reflective layer 40 is higher than 30 Ω / □, it becomes difficult to obtain selective, high electromagnetic wave absorption characteristics for electromagnetic waves of the desired frequency range. A metal foil or metal plate can be suitably used as such a reflective layer 40.

[0065] To provide flexibility to the radio wave absorbing sheet 10, the reflective layer 40 can be made of a metal foil or a film coated with a conductive metal paste such as silver or copper. 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 40, considering cost and the effect of oxidation in air. A metal foil such as aluminum foil for forming the reflective layer 40 can be easily produced by rolling a metal material. Furthermore, when the reflective layer 40 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.

[0066] The thickness of the reflective layer 40 is not particularly limited, but when a metal foil is used to form the flexible radio wave absorbing sheet 100, if the reflective layer 40 is too thick it will be meaningless in terms of electromagnetic wave absorption properties and will result in unnecessary consumption of material, and if the reflective layer is too thin it may not be strong enough to cause problems such as 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 with 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.

[0067] In the radio wave absorbing sheet 100 according to this embodiment shown in Fig. 1, a vapor-deposited film of a metal material may be formed directly on the surface of the second dielectric layer 30 opposite to the side on which the resistive layer 20 is formed, so that the reflective layer 40 can be formed solely from a vapor-deposited film of a conductive material such as metal. When a vapor-deposited metal film is formed on the back surface of the second dielectric layer 30, no gap is formed between the second dielectric layer 30 and the reflective layer 40, as compared to when the second dielectric layer 30 and the reflective layer 40 are formed separately and then closely arranged. Therefore, the electromagnetic wave 1 transmitted through the second dielectric layer 30 can be reflected at the rear surface of the second dielectric layer 30, making it easy to realize a radio wave absorbing sheet 100 having the frequency characteristics of the desired amount of radio wave absorption.

[0068] On the other hand, when a vapor-deposited film is used for the reflective layer 14, 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 value of the reflective layer to 30 Ω / □ or less, and it is preferable to sufficiently control the thickness of the metal vapor-deposited film to set the surface resistance value to a desired value or less.

[0069] Furthermore, in order to provide the radio wave absorbing sheet 100 with both flexibility and translucency, a conductive mesh made of conductive fibers can be used as the reflective layer 40. 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.

[0070] Alternatively, the reflective layer 40 may be a conductive metal grid or a metal mesh in which thin metal wires, such as copper wires, each having a diameter of several tens to several hundreds of μm are arranged lengthwise and widthwise.

[0071] In addition, when the reflective layer 40 is constructed using the above-mentioned conductive mesh or conductive metal lattice, in order to ensure flexibility and translucency, it will be constructed to have the minimum thickness possible as long as the surface resistance value required for the reflective layer 40 can be realized.

[0072] The aperture ratio of the reflective layer 40 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 electromagnetic waves on the surface of the reflective layer 40 and enhancing the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet 100. 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.

[0073] Incidentally, even when the conductive mesh or conductive grid is used as the reflective layer 40, the surface resistance of the reflective layer 40 is preferably 30 Ω / □ or less.

[0074] <Adhesive Layer> Although not shown in FIG. 1, an adhesive layer can be formed on the back surface of the reflective layer 40 so that the electromagnetic wave absorbing sheet 100 according to this embodiment can be easily placed at a predetermined position.

[0075] The adhesive layer may be made of known materials used as adhesive layers for adhesive tapes and the like, such as acrylic adhesives, rubber adhesives, and silicone adhesives. A tackifier or 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 electromagnetic wave absorbing sheet 100 may easily peel off or become displaced from the adherend. If the adhesive strength is greater than 12 N / 10 mm, the electromagnetic wave absorbing sheet 100 may be difficult to peel off from the adherend.

[0076] 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 electromagnetic wave absorbing sheet 100 may easily peel off or shift from the adherend. If the thickness of the adhesive layer is thicker than 100 μm, the electromagnetic wave absorbing sheet 100 will be difficult to peel off from the adherend. Furthermore, if the cohesive strength of the adhesive layer is weak, adhesive residue may remain on the adherend when the electromagnetic wave absorbing sheet 100 is peeled off. This may also be a factor in reducing the flexibility of the electromagnetic wave absorbing sheet 100 as a whole.

[0077] The adhesive layer that can be used for the electromagnetic wave absorbing sheet 100 according to this embodiment can be an adhesive layer that non-removably attaches the electromagnetic wave absorbing sheet 100 to an object to which it is attached, or an adhesive layer that removably attaches the sheet. It is not essential that the electromagnetic wave absorbing sheet 100 according to this embodiment has an adhesive layer, and the electromagnetic wave absorbing sheet 100 can be attached to a desired member using any of a variety of conventional adhesive methods.

[0078] <Protective Layer> In addition to this, a protective layer for protecting the surface of the electromagnetic wave absorbing sheet 100 can be arranged on the side of the first dielectric layer 10 facing the electromagnetic wave 1 .

[0079] The protective layer is a film that protects the first dielectric layer 10 from being damaged by external forces or from changes in its relative dielectric constant ε1 due to the effects of ultraviolet rays or moisture. The protective layer is not an essential component of the electromagnetic wave absorbing sheet 100 according to this embodiment, and depending on the material of the first dielectric layer 10 and the conditions under which the electromagnetic wave absorbing sheet 100 is used, it is possible to select a configuration of the electromagnetic wave absorbing sheet 100 without a protective layer if there is little concern about changes in the dielectric constant ε1 of the first dielectric layer 10 or damage to the surface due to adhesion of moisture to the surface.

[0080] 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 electromagnetic wave 1 incident on the electromagnetic wave absorbing sheet 100 can be kept at a practically negligible level.

[0081] EXAMPLES Hereinafter, the results of a study conducted on the frequency characteristics of electromagnetic wave absorption of the electromagnetic wave absorbing sheet according to this embodiment will be described.

[0082] In the following studies, simulations were performed using full-wave three-dimensional electromagnetic field software "Ansys HFSS (product name: manufactured by ANSYS, Inc.)" using the finite element method.

[0083] FIG. 2 is a graph showing the change in return loss with respect to the frequency of incident electromagnetic waves, as the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet of the example according to this embodiment.

[0084] Here, the return loss is the strength (energy) of the electromagnetic wave reflected by the electromagnetic wave absorbing sheet relative to the strength (energy) of the incident electric field wave, expressed in dB. Therefore, when the return loss is -10 dB, the strength of the reflected electromagnetic wave is 1 / 10 (10%) of the incident electromagnetic wave, which indicates that 90% of the energy of the electromagnetic wave is absorbed by the electromagnetic wave absorbing sheet.

[0085] The first electromagnetic wave absorbing sheet, whose electromagnetic wave absorption characteristics are shown in Figure 2, was designed based on the assumption that the frequency of the incident electromagnetic wave 1 was approximately 300 GHz (center wavelength λ = 1 mm), and that the first dielectric layer 10 and the second dielectric layer 20 were both made of acrylic OCA with a dielectric constant ε (ε1 = ε2) = 2.5. The thicknesses D1 and D2 of each dielectric layer were calculated to be 791 μm from (Equation 1) and (Equation 2), respectively, and within a ±20% range, this range was 632 μm to 949 μm. In this example, taking into consideration the actual fabrication of an electromagnetic wave absorbing sheet, D1 = D2 = 850 μm, within the thickness range of 632 μm to 949 μm that is available for acrylic OCA.

[0086] The resistive layer 20 is assumed to be made of PEDOT, a conductive organic polymer, and is optimized to achieve good electromagnetic wave absorption characteristics with an electromagnetic wave attenuation of -10 dB or more in the 300 GHz band, 180 GHz band, and 60 GHz, with a surface resistance of 189 Ω / □.

[0087] Assuming that the reflective layer 40 is made of aluminum foil, the surface resistance value was set to 0 Ω / □.

[0088] In the frequency characteristics of the return loss of the first electromagnetic wave absorbing sheet of the embodiment shown in Figure 2, it was confirmed that the electromagnetic wave absorbing sheet has absorption bands that absorb electromagnetic waves over a wide band in three frequency bands: a first frequency band (23 to 83 GHz) centered at a frequency of 60 GHz, a second frequency band (130 to 200 GHz) centered at 180 GHz, and a third frequency band (240 to 325 GHz) formed as a higher frequency band including 300 GHz.

[0089] Next, as a comparative example, the electromagnetic wave absorption characteristics of an electromagnetic wave absorbing sheet was measured in which the laminate structure of the first dielectric layer, resistive layer, second dielectric layer, and reflective layer was not changed, but the thickness D1 of the first dielectric layer and the thickness D2 of the second dielectric layer were set to values ​​that did not satisfy the conditions of the above-mentioned (Equation 1) and (Equation 2).

[0090] FIG. 3 is a diagram showing the frequency characteristics of the return loss of the second electromagnetic wave absorbing sheet, which is a comparative example.

[0091] In the second electromagnetic wave absorbing sheet, which is a comparative example, the thickness D1 of the first dielectric layer and the thickness D2 of the second dielectric layer were both set to "λ / 4," the thickness of a dielectric layer used in a typical electromagnetic wave interference-type electromagnetic wave absorber. Specifically, the frequency of the incident electromagnetic wave 1 was set to approximately 300 GHz (center wavelength λ = 1 mm), the same as in the first electromagnetic wave absorbing sheet described above, and both the first and second dielectric layers were assumed to be made of acrylic OCA with a dielectric constant ε (ε1 = ε2) = 2.5. The thickness of each dielectric layer was set to 150 μm (D1 = D2 = 150 μm).

[0092] The surface resistance of the resistive layer was set to 189 Ω / □, the same as the first electromagnetic wave absorbing sheet described above, and the surface resistance of the reflective layer 40 was set to 20 Ω / □, assuming a state in which silver paste is applied to a PET substrate.

[0093] The frequency characteristics of the return loss of the second electromagnetic wave absorbing sheet shown in Figure 3 differ from the frequency characteristics of the first electromagnetic wave absorbing sheet according to this embodiment, and although a return loss of -10 dB or more, which corresponds to 90% absorption, was obtained in the frequency band of 155 GHz or more, it did not become an electromagnetic wave absorbing sheet with multiple absorption bands like the first electromagnetic wave absorbing sheet.

[0094] As described above, in order to realize an electromagnetic wave absorbing sheet having a plurality of absorption bands over a wide frequency range in the electromagnetic wave absorbing sheet according to this embodiment, it has been confirmed that the thickness D1 of the first dielectric layer and the thickness D2 of the second dielectric layer need to satisfy the following conditional expressions with respect to the central wavelength λ of the electromagnetic wave, where the dielectric constant of the first dielectric layer is ε1 and the dielectric constant of the second dielectric layer 30 is ε2: D1=5λ / 4√ε1±20% (Equation 1) D2=5λ / 4√ε2±20% (Equation 2)

[0095] In the above embodiment, the center frequency of the electromagnetic waves incident on the electromagnetic wave absorbing sheet is 300 GHz. However, if the center frequency is in a higher frequency band, for example, 1 THz (terahertz), the thicknesses D1 and D2 of the first and second dielectric layers will be smaller, and if a resin material with a normal dielectric constant of around 2.5 is used, a thinner resin sheet will be required. However, for example, at 1 THz, the center wavelength λ will be about one-third, so it can be understood that this is within a fully practical range.

[0096] The electromagnetic wave absorbing sheet disclosed in the present application has a configuration in which a first dielectric layer, a resistive layer, a second dielectric layer, and a reflective layer are stacked in sequence, thereby making it possible to realize an electromagnetic wave absorbing sheet that has good absorption properties for electromagnetic waves in multiple and wide frequency bands.

[0097] REFERENCE SIGNS LIST 1 (incident) electromagnetic wave 10 first dielectric layer 20 resistive layer 30 second dielectric layer 40 reflective layer 100 electromagnetic wave absorbing sheet

Claims

1. An electromagnetic wave absorbing sheet in which a first dielectric layer, a resistive layer, a second dielectric layer and a reflective layer are laminated in this order from the electromagnetic wave incidence side, characterized in that, when the dielectric constant of the first dielectric layer is ε1, the thickness is D1, and the dielectric constant of the second dielectric layer is ε2 and the thickness is D2, the following (Formula 1) and (Formula 2) hold for the central wavelength λ of the electromagnetic wave: D1 = 5λ / 4√ε1 ± 20% (Formula 1) D2 = 5λ / 4√ε2 ± 20% (Formula 2) 2. The electromagnetic wave absorbing sheet according to claim 1, wherein the surface resistance value of the resistive layer is 150 Ω / □ or more and 250 Ω / □ or less.

3. An electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the first dielectric layer, the second dielectric layer and the reflective layer are all light-transmitting, and the resistive layer is a transparent layer containing at least one of an oxide conductive material or a conductive polymer.

4. The electromagnetic wave absorbing sheet according to any one of claims 1 to 3, wherein the resistance layer has a thickness of 0.01 µm or more and 10 µm or less.

5. The electromagnetic wave absorbing sheet according to any one of claims 1 to 4, wherein the surface resistance of the reflective layer is 30 Ω / □ or less.

6. The electromagnetic wave absorbing sheet according to any one of claims 1 to 5, wherein the first dielectric layer and the second dielectric layer contain an acrylic resin, a silicone resin, or a urethane resin.

7. An electromagnetic wave absorbing sheet according to any one of claims 1 to 6, wherein at least one of the first dielectric layer and the second dielectric layer is constructed as a laminate of a plurality of dielectric layers.

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