Electromagnetic wave absorption sheet

Abstract

This electromagnetic wave absorption sheet has a first dielectric layer, a first resistance layer, a second dielectric layer, a second resistance layer, a third dielectric layer, and a metal layer sequentially stacked from an electromagnetic wave incident surface side. Provided that the dielectric constant of the first dielectric layer is ε1, the thickness thereof is D1, the dielectric constant of the second dielectric layer is ε2, the thickness thereof is D2, the dielectric constant of the third dielectric layer is ε3, and the thickness thereof is D3, the following conditions are satisfied: D1 = λ / 4√ε1 ± 20%; D2 = nλ / 4√ε2 ± 20%; and D3 = nλ / 4√ε3 ± 20%, where λ is a center wavelength of an electromagnetic wave, and n is an odd number equaling or exceeding 3, the surface resistance value of the first resistance layer being higher than the surface resistance value of the second resistance layer.

Description

Electromagnetic wave absorbing sheet

[0001] The present disclosure relates to an electromagnetic wave absorbing sheet capable of absorbing 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, the frequencies of electromagnetic waves absorbed by the above-mentioned conventional electromagnetic wave absorbing sheet have five peaks: 70 GHz, 90 GHz, 120 GHz, 150 GHz, and 178 GHz. However, 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. Moreover, the width of each frequency band is less than 20 GHz, so it cannot be said to be sufficient as an electromagnetic wave absorber capable of absorbing electromagnetic waves in different frequency bands.

[0008] The present disclosure is intended to solve the above-mentioned problems, and aims to realize an electromagnetic wave absorbing sheet that has good absorption characteristics for electromagnetic waves in different frequency bands in the high frequency band from several hundred GHz to the terahertz band.

[0009] In order to solve the above problems, the electromagnetic wave absorbing sheet disclosed in the present application is an electromagnetic wave absorbing sheet in which a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer, and a metal layer are laminated in this order from the electromagnetic wave incident surface side, wherein, when the dielectric constant of the first dielectric layer is ε1 and the thickness is D1, the dielectric constant of the second dielectric layer is ε2 and the thickness is D2, and the dielectric constant of the third dielectric layer is ε3 and the thickness is D3, the following relationships hold: D1=λ / 4√ε1±20% D2=nλ / 4√ε2±20% D3=nλ / 4√ε3±20% where λ is the central wavelength of the electromagnetic wave, and n is an odd number equal to or greater than 3, and the surface resistance value of the first resistive layer is higher than the surface resistance value of the second resistive layer.

[0010] The electromagnetic wave absorbing sheet disclosed in the present application has a structure in which a first dielectric layer, a first resistive layer, a second dielectric layer, a second dielectric layer, a third dielectric layer, and a metal layer are laminated in this order, and the thickness of each dielectric layer satisfies the relationship (Equation 1), (Equation 2), and (Equation 3) between its dielectric constant and the wavelength of the electromagnetic wave, and the surface resistance value of the first resistive layer is higher than the surface resistance value of the second resistive layer.

[0011] As a result, the electromagnetic waves incident on the electromagnetic wave absorbing sheet are reflected by the first resistance layer, the second resistance layer, and the reflective layer, respectively, and cancel out the incident waves, thereby achieving multi-band electromagnetic wave absorption characteristics with multiple wide-band absorption frequency bands for the electromagnetic waves reflected by the electromagnetic wave absorbing sheet.

[0012] Fig. 1 is a cross-sectional view illustrating the configuration of an electromagnetic wave absorbing sheet according to this embodiment; Fig. 2 is a diagram illustrating the frequency characteristics of the return loss of a first electromagnetic wave absorbing sheet according to this embodiment; Fig. 3 is a diagram illustrating the frequency characteristics of the return loss of a second electromagnetic wave absorbing sheet according to this embodiment; Fig. 4 is a diagram illustrating the frequency characteristics of the return loss of a third electromagnetic wave absorbing sheet as a comparative example; and Fig. 5 is a diagram illustrating the frequency characteristics of the return loss of a fourth electromagnetic wave absorbing sheet as a comparative example.

[0013] The electromagnetic wave absorbing sheet disclosed in the present application is an electromagnetic wave absorbing sheet in which a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer, and a metal layer are laminated in this order from the electromagnetic wave incident surface side, wherein, when the dielectric constant of the first dielectric layer is ε1 and the thickness is D1, the dielectric constant of the second dielectric layer is ε2 and the thickness is D2, and the dielectric constant of the third dielectric layer is ε3 and the thickness is D3, then: D1=λ / 4√ε1±20% D2=nλ / 4√ε2±20% D3=nλ / 4√ε3±20% where λ is the central wavelength of the electromagnetic wave, and n is an odd number equal to or greater than 3, and the surface resistance value of the first resistive layer is higher than the surface resistance value of the second resistive layer.

[0014] With this configuration, the electromagnetic wave absorbing sheet disclosed in the present application can be realized as a multi-band electromagnetic wave absorbing sheet having multiple broadband electromagnetic wave absorption bands due to the canceling interaction between the electromagnetic waves reflected by the two resistive layers and the metal layer and the electromagnetic waves incident on the electromagnetic wave absorbing sheet.

[0015] In the electromagnetic wave absorbing sheet, n in the formulas 2 and 3 is preferably 5. In this way, an electromagnetic wave absorbing sheet having three electromagnetic wave absorption frequency bands can be realized.

[0016] In the electromagnetic wave absorbing sheet, n in the formulas 2 and 3 is preferably 3. In this way, an electromagnetic wave absorbing sheet having two electromagnetic wave absorption frequency bands can be realized.

[0017] Furthermore, it is preferable that the first resistive layer and the second resistive layer are layers containing at least one of an inorganic material having electrical conductivity and a conductive polymer, thereby making it possible to form resistive layers having a predetermined surface resistance value.

[0018] It is also preferable that the thickness of the first resistive layer and the second resistive layer is 0.01 to 10 μm, which eliminates the need to consider the thickness of the resistive layer when designing the electromagnetic wave absorbing sheet, and allows for design taking into account the contribution of only the dielectric layer surface to interference.

[0019] Furthermore, the surface resistance of the metal layer is preferably 30 Ω / □ or less. By setting the surface resistance of the metal layer to a low value of 30 Ω / □ or less, electromagnetic waves that have passed through the third dielectric layer and reached the metal layer can be effectively reflected, resulting in an electromagnetic wave absorbing sheet with high electromagnetic wave absorption properties.

[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 Figure 1 is a diagram drawn to make it easier to understand the configuration of the electromagnetic 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.

[0023] [Overall Structure of Electromagnetic Wave Absorbing Sheet] The electromagnetic wave absorbing sheet 10 illustrated in this embodiment is configured by sequentially stacking a first dielectric layer 11, a first resistive layer 12, a second dielectric layer 13, a second resistive layer 14, a third dielectric layer 15, and a metal layer 16 from the incident surface side of the electromagnetic wave 1 indicated by the white arrow in the figure.

[0024] In the electromagnetic wave absorbing sheet 10 shown in FIG. 1 , the film thickness D1 of the first dielectric layer 11 satisfies the requirement of D1=λ / 4√ε1±20% (Equation 1), where ε1 is the dielectric constant of the dielectric that constitutes the first dielectric layer 11 and λ is the wavelength of the electromagnetic wave that is incident on the electromagnetic wave absorbing sheet 10 according to this embodiment.

[0025] Furthermore, the film thickness D2 of the second dielectric layer 13 satisfies the requirement of D2=nλ / 4√ε2±20% (Equation 2), where ε2 is the dielectric constant of the dielectric that constitutes the second dielectric layer 13 and λ is the wavelength of the electromagnetic wave incident on the electromagnetic wave absorbing sheet 10 of this embodiment.

[0026] Furthermore, the film thickness D3 of the third dielectric layer 15 satisfies the requirement of D3=nλ / 4√ε3±20% (Equation 3), where ε3 is the dielectric constant of the dielectric constituting the third dielectric layer and λ is the wavelength of the electromagnetic wave incident on the electromagnetic wave absorbing sheet 10 according to this embodiment.

[0027] Furthermore, n in the above (Equation 2) and (Equation 3) is a positive odd number equal to or greater than 3.

[0028] The electromagnetic wave absorbing sheet 10 according to this embodiment exhibits electromagnetic wave absorption characteristics having absorption periodicities at the above-set wavelength λ of the electromagnetic wave 1 incident on the electromagnetic wave absorbing sheet 10 as the shortest wavelength, i.e., the highest frequency electromagnetic wave. For this reason, the wavelength λ of the electromagnetic wave to be input into the above formula for determining the thicknesses D1, D2, D3 of the dielectric layers is selected to be the central wavelength of the electromagnetic wave with the highest frequency among the electromagnetic waves absorbed by the electromagnetic wave absorbing sheet 10 described in this embodiment.

[0029] Furthermore, in the electromagnetic wave absorbing sheet 10 according to this embodiment, the first resistive layer 12 and the second resistive layer 14 are both resistive layers having predetermined surface resistance values, and the surface resistance value of the first resistive layer 12 is set to a value higher than the surface resistance value of the second resistive layer 14.

[0030] The metal layer 16 is a layer that reflects the electromagnetic wave 1 incident on the electromagnetic wave absorbing sheet 10 of this embodiment, and it is preferable that its surface resistance value is extremely small compared to the surface resistance values ​​of the first resistance layer 12 and the second resistance layer 14 described above.

[0031] 1, the electromagnetic wave absorbing sheet 10 may have, on its outermost surface on the side where the electromagnetic waves 1 are incident, a surface resistive layer having a surface resistance value (approximately 377 Ω / □) that matches the impedance in air to prevent surface reflection of the electromagnetic waves incident on the electromagnetic wave absorbing sheet 10. Furthermore, the outermost surface of the electromagnetic wave absorbing sheet 10, including this surface resistive layer, may have a protective layer that protects the electromagnetic wave absorbing sheet 10 from the external environment.

[0032] Furthermore, the back side of the metal layer 16 of the electromagnetic wave absorbing sheet 10 may have an adhesive layer (not shown) so that the electromagnetic wave absorbing sheet can be easily placed in a predetermined location.

[0033] [Details of Each Member] Next, each member constituting the electromagnetic wave absorbing sheet 10 according to this embodiment will be described.

[0034] <Dielectric Layer> The first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15 of the electromagnetic wave absorbing sheet 10 according to this embodiment can all be formed from various dielectric materials such as acrylic resin, silicone resin, urethane resin, etc. In addition to these, other dielectric materials such as silicone rubber, titanium oxide, polyvinylidene fluoride, polyester resin, glass, and silicone rubber can also be used.

[0035] The first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15 can each be formed as a single layer made of a single material. They can also be configured as two or more layers of the same or different materials. The first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15 can all be made of the same dielectric material, or they can be made of different dielectric materials, including the number of layers.

[0036] Furthermore, since the first resistive layer 12 and the second resistive layer 14 described later are preferably formed as thin films, the first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15 can be used as resin substrates when coating and preparing these resistive layers 12 and 14.

[0037] 1 , the first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15 are all made of a translucent, adhesive acrylic-based optical clear adhesive (OCA). By forming the first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15 from an adhesive resin material such as OCA, it is possible to easily manufacture the electromagnetic wave absorbing sheet 10 having a laminated structure in which three dielectric layers, two resistive layers, and a metal layer are laminated, and it is also possible to reduce the amount of material used compared to when adhesive materials or bonding agents are separately used when laminating each layer, so the electromagnetic wave absorbing sheet 10 can be realized at low cost.

[0038] <Resistance Layer> The first resistance layer 12 and the second resistance layer 14 of the electromagnetic wave absorbing sheet 10 shown in this embodiment are layers formed of, for example, a conductive organic compound, a conductive oxide material, or the like, and have a predetermined surface resistance value.

[0039] In the electromagnetic wave absorbing sheet 10 according to this embodiment, a portion of the electromagnetic wave 1 that has passed through the first dielectric layer 11 is reflected by the first resistive layer 12 and then passes through the first resistive layer 12. The electromagnetic wave 1 that has passed through the first resistive layer 12 passes through the second dielectric layer 13, and a portion of the wave is further reflected by the second resistive layer 14 and then passes through the second resistive layer 14. The electromagnetic wave 1 that has passed through the second resistive layer 14 passes through the third dielectric layer 15 and is then reflected by the metal layer 16. As a result, an electromagnetic wave absorber that absorbs electromagnetic waves in a plurality of frequency bands can be obtained due to the interference action of the electromagnetic wave reflected by the first resistive layer 12, the electromagnetic wave reflected by the second resistive layer 14, and the electromagnetic wave reflected by the metal layer 16.

[0040] Therefore, the predetermined surface resistance values ​​of the first resistance layer 12 and the second resistance layer 14 are larger than the surface resistance value of the metal layer 16 described below, and more specifically, are approximately several hundred Ω / □ or more. On the other hand, if the surface resistance values ​​of the first resistance layer 12 and the second resistance layer 14 become too large, the characteristics become similar to those of a dielectric layer, and it becomes impossible to reflect the electromagnetic wave 1 incident on the electromagnetic wave absorbing sheet 10, so it is preferable that the upper limit of the surface resistance values ​​of the first resistance layer 12 and the second resistance layer 14 is approximately 1000 Ω / □.

[0041] Furthermore, as described above, the first resistive layer 12 must transmit a portion of the electromagnetic wave 1 that has passed through the first dielectric layer 11, and this portion of the transmitted electromagnetic wave 1 must be reflected by the second resistive layer 14. Therefore, the surface resistance of the first resistive layer 12 must be greater than the surface resistance of the second resistive layer 14. As a result, the reflection of the electromagnetic wave by the second resistive layer 14, which has a low surface resistance, is greater than the reflection of the electromagnetic wave by the first resistive layer 12, which has a high surface resistance, and the reflection of the electromagnetic wave by the metal layer 16, which has an even lower surface electrical resistance, is greater. By sequentially varying the reflection strength of the electromagnetic wave in each layer in this way, good electromagnetic wave absorption characteristics can be obtained in different frequency bands, thereby realizing a multi-band electromagnetic wave absorbing sheet.

[0042] Furthermore, it is important to design the surface resistance values ​​of the first resistive layer 12 and the second resistive layer 14 according to the desired loss and bandwidth in each absorption band, taking into consideration the frequency of the electromagnetic waves to be absorbed by the electromagnetic wave absorbing sheet 10 and the set value of n. In particular, it is important to achieve a wide bandwidth of approximately 50 GHz or more in each absorption band, and design is performed using analysis software such as HFSS (product name: manufactured by ANSYS, Inc.) described below so that this can be achieved.

[0043] The first resistive layer 12 and the second resistive layer 14 can be formed as a thin film of a conductive inorganic material or a conductive organic polymer film, and the first resistive layer 12 and the second resistive layer 14 can be formed from the same material or from different materials.

[0044] The conductive inorganic material constituting the first resistive layer 12 and the second resistive layer 14 may be indium tin oxide (ITO), indium oxide, tin oxide, zinc oxide, copper oxide, titanium oxide, vanadium oxide, or a metal oxide, metal nitride, or mixture thereof, in which one or more elements are substituted or combined with any of these. A resistive film can be produced by coating the material on a resin substrate such as polyethylene terephthalate (PET) using a thin film formation method such as ion plating, vapor deposition, sputtering, or various coating methods.

[0045] The first resistive layer 12 and the second resistive layer 14 can be made of a conductive material such as a conductive organic polymer material, a semiconductor material, or a metal oxide.

[0046] The conductive organic polymer material constituting the first resistance layer 12 and the second resistance layer 14 may be a conjugated conductive organic polymer, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0047] In addition, organic polymers whose main chains are composed of a π-conjugated system can be used as the resistive film, such as polyacetylene-based conductive polymers, polyphenylene-based conductive polymers, polyphenylene vinylene-based conductive polymers, polyaniline-based conductive polymers, polyacene-based conductive polymers, polythiophene vinylene-based conductive polymers, and copolymers thereof.

[0048] The conductive organic polymer used in the first resistive layer 12 and the second resistive layer 14 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 generating chemical oxidation doping in the conjugated conductive organic polymer used in the resistive film 1 described above. 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.

[0049] The conductive organic polymers may be used alone or in combination of two or more. Among the materials exemplified above, a polymer consisting of one or two selected from polypyrrole, poly(3-methoxythiophene), poly(3,4-ethylenedioxythiophene), poly(2-anilinesulfonic acid), and poly(3-anilinesulfonic acid) is preferred because it has higher transparency and conductivity.

[0050] In particular, it is preferable to use poly(3,4-ethylenedioxythiophene: PEDOT) and polystyrene sulfonic acid (PSS) as a combination of a conjugated conductive organic polymer and a polyanion.

[0051] The first resistance layer 12 and the second resistance layer 14 may also be configured to contain a carbon material such as a carbon microcoil, a carbon nanotube, or graphene.

[0052] 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.

[0053] 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 film of the electromagnetic wave absorbing sheet according to this embodiment may be either single-walled or multi-walled.

[0054] 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 first resistance layer 12 and the second resistance layer 14 of the electromagnetic 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 electromagnetic wave absorbing sheet 10.

[0055] The resin in which the carbon material is dispersed may be a water-soluble polyester resin.

[0056] The first resistive layer 12 and the second resistive layer 14 using a conductive organic polymer can be formed by applying a coating composition as a paint for forming the resistive layer onto a substrate and drying it.

[0057] Examples of methods that can be used to apply the resistive coating 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 resistive coating paint to evaporate. Residual solvent in the first resistive layer 12 and the second resistive layer 14 tends to reduce the strength of the layers. Examples of drying methods include hot air drying, heat drying, vacuum drying, and natural drying. If necessary, the resistive coating may be formed by irradiating the coating with UV light (ultraviolet rays) or EB (electron beam) to cure the coating.

[0058] The base material used to form the first resistance layer 12 and the second resistance layer 14 is not particularly limited, but various materials such as resin, rubber, glass, and ceramics can be used.

[0059] In order to make the surface resistance value of the first resistance layer 12 greater than the surface resistance value of the second resistance layer 14, the content of the conductive material contained in the second resistance layer 14 can be adjusted to be greater than the content of the conductive material contained in the first resistance layer 12, or the thickness of the second resistance layer 14 can be adjusted to be greater than the thickness of the first resistance layer 12.

[0060] <Metal Layer> As described above, the metal layer 16 of the electromagnetic wave absorbing sheet 10 shown in this embodiment is a layer that reflects the electromagnetic wave 1 that has passed through the first dielectric layer 11, the first resistive layer 12, the second dielectric layer 13, the second resistive layer 14, and the third dielectric layer 15. To reflect as much electromagnetic wave 1 as possible, the lower the surface resistance value of the metal layer 16, the more preferable, and ideally the lower limit is 0 Ω / □. However, to avoid manufacturing issues in realizing the pattern shape of the thin film and to keep the transmitted electromagnetic wave as small as possible, the surface resistance value is preferably 0.01 Ω / □ or more and 30 Ω / □ or less.

[0061] From the viewpoint of low surface resistivity, it is most preferable to use various metal foils such as aluminum foil, copper foil, and stainless steel (SUS) foil for the metal layer 16. Alternatively, any metal film, such as a coated film of metal paste, can also be used. When forming the metal layer 16 using a metal paste, a method can be used in which a metal paste or metal paint containing metal particles such as copper (Cu) or silver (Ag) is applied to a resin substrate such as PET. Furthermore, the metal layer 16 can also be formed using a metal plating film such as an electroless plating film or an electrolytic plating film.

[0062] In this case, if a coating material using a transparent metal material such as silver nanowires is applied, it is possible to impart translucency while maintaining the required surface resistance value. Note that when translucency is to be imparted, the other constituent elements, the resistive layer and the dielectric layer, are also required to have a certain degree of translucency, and by making these layers translucent collectively, it is possible to impart translucency to the electromagnetic wave absorbing sheet.

[0063] When imparting translucency to the electromagnetic wave absorbing sheet, it is preferable to use the above-mentioned silicone OCA or acrylic OCA for the first dielectric layer, second dielectric layer, and third dielectric layer, for example. Furthermore, it is preferable to use a conductive organic polymer for the first resistive layer and second resistive layer. The total light transmittance of the electromagnetic wave absorbing sheet is preferably 30% or more, and more preferably 50% or more. Having translucency allows the opposite side of the electromagnetic wave absorbing sheet to be seen, so that when used in an anechoic chamber or the like, it becomes possible to see the internal measuring device.

[0064] In addition to the PET mentioned above, various resins, rubber materials, and even various dielectrics (insulators) such as paper and wood can be used as the substrate when forming the metal layer 16.

[0065] If high flexibility is not required for the electromagnetic wave absorbing sheet 10, a metal plate of aluminum, copper, stainless steel (SUS), or the like having a thickness of about 1 to 2 mm or less can be used for the metal layer 16.

[0066] <Adhesive Layer> Although not shown in Fig. 1 , in the electromagnetic wave absorbing sheet 10 according to this embodiment, an adhesive layer can be provided as appropriate on the back side of the metal layer 16. By providing the adhesive layer, the electromagnetic wave absorbing sheet 10 can be easily attached to a predetermined location. The adhesive layer can be easily formed by applying a sticky resin paste.

[0067] The adhesive layer is not an essential component of the electromagnetic wave absorbing sheet 10 according to this embodiment, and when placing the electromagnetic wave absorbing sheet 10 at a predetermined location, an adhesive component may be placed on the component to which the electromagnetic wave absorbing sheet 10 is attached. When placing the electromagnetic wave absorbing sheet 10 at a predetermined location, an adhesive method such as supplying an adhesive between the electromagnetic wave absorbing sheet 10 and the placement location or using double-sided tape can be used.

[0068] <Surface Resistive Layer> A surface resistive layer (not shown) may be provided on the side of the first dielectric layer 11 on which the electromagnetic wave 1 is incident.

[0069] When a surface resistive layer is provided, it can be fabricated in the same manner using the materials and fabrication methods that are suitable for use as the first resistive layer 12 and the second resistive layer 14. However, when a surface resistive layer is provided, the surface resistivity must be adjusted to a value that does not deviate significantly from 377 Ω / □ for impedance matching.

[0070] <Protective Layer> Although not shown in FIG. 1, a protective layer can be formed on the surface of the first dielectric layer 11, that is, on the outermost surface of the electromagnetic wave absorbing sheet 10 on the side where the electromagnetic wave 1 is incident.

[0071] The protective layer is a film that protects the first dielectric layer 11 from being damaged by external forces or from changes in its dielectric constant due to the effects of ultraviolet rays or moisture. The protective layer is not an essential component of the electromagnetic wave absorbing sheet 10 according to this embodiment, and depending on the material of the first dielectric layer 11 and the conditions under which the electromagnetic wave absorbing sheet 10 is used, it is possible to select a configuration of the electromagnetic wave absorbing sheet 10 without a protective layer if there is little concern about changes in the dielectric constant ε1 of the first dielectric layer 11 or damage to the surface due to adhesion of moisture to the surface.

[0072] 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 10 can be kept at a practically negligible level.

[0073] 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.

[0074] 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.

[0075] (Electromagnetic wave absorbing sheet with n=5) FIG. 2 is a diagram showing the electromagnetic wave absorption characteristics of the first electromagnetic wave absorbing sheet designed with n=5 in (Equation 2), which is an equation for calculating the thickness D2 of the second resistance layer 13, and (Equation 3), which is an equation for calculating the thickness D3 of the third dielectric layer 15.

[0076] The electromagnetic wave absorbing sheet 10 disclosed in the present application is a so-called reflective type electromagnetic wave absorbing sheet having a metal layer 16 on the rearmost side of the electromagnetic wave absorbing sheet 10, and therefore the electromagnetic wave absorption characteristics are determined by simulation as the frequency characteristics of the return loss, which is the amount of attenuation in dB of the energy of the reflected wave reflected from the electromagnetic wave absorbing sheet relative to the energy of the incident wave.

[0077] The first electromagnetic wave absorbing sheet was assumed to have an incident electromagnetic wave 1 with a frequency of 300 GHz (center wavelength λ=1 mm), and to use an acrylic OCA with a dielectric constant ε (ε1=ε2=ε3) of approximately 2.5 for each of the first dielectric layer 11, second dielectric layer 13, and third dielectric layer 15. Note that the thicknesses of the dielectric layers were not calculated using the numerical values ​​obtained from (Equation 1), (Equation 2), and (Equation 3), but were set to D1=150 μm and D2=D3=750 μm, which are thicknesses available for acrylic OCA, taking into consideration the actual production of an electromagnetic wave absorbing sheet.

[0078] Assuming that the first resistance layer 12 and the second resistance layer 14 are both made of PEDOT, a conductive organic polymer, the surface resistance value of the first resistance layer 12 was set to 750 Ω / □ and the surface resistance value of the second resistance layer 14 was set to 180 Ω / □.

[0079] The surface resistance value of the resistive layer was optimized so that the bandwidth in each absorption band where the return loss was −10 dB or more was a wide band of approximately 50 GHz or more.

[0080] Assuming that the metal layer 16 is made of aluminum foil, the surface resistance value was set to 0 Ω / □.

[0081] The frequency characteristics of the return loss of the first electromagnetic wave absorbing sheet shown in Figure 2 confirmed that the sheet absorbs electromagnetic waves over a wide range in three frequency bands: a first frequency band (22 to 85 GHz) centered at 50 GHz, a second frequency band (135 to 215 GHz) centered at 150 GHz, and a third frequency band (240 GHz or higher) formed as a higher frequency band including 300 GHz, all of which exhibit a return loss of -10 dB or more, which indicates an attenuation of 90%.

[0082] 2, the return loss is large (downward convex) in the first, second, and third frequency bands. This means that the energy of the electromagnetic waves reflected from the electromagnetic wave absorbing sheet is small in these bands, and indicates that the electromagnetic waves in these bands are absorbed.

[0083] (Electromagnetic wave absorbing sheet with n=3) Next, the electromagnetic wave absorption characteristics when n=3 were confirmed using (Equation 2), which is an equation for calculating the thickness D2 of the second resistive layer 13, and (Equation 3), which is an equation for calculating the thickness D3 of the third dielectric layer 15.

[0084] FIG. 3 is a diagram showing the electromagnetic wave absorption characteristics of the second electromagnetic wave absorbing sheet in which the thicknesses of the second and third dielectric layers are designed with n=3 in (Equation 2) and (Equation 3).

[0085] The second electromagnetic wave absorbing sheet was assumed to have an incident electromagnetic wave 1 with a frequency of 300 GHz (center wavelength λ=1 mm) as in the first electromagnetic wave absorbing sheet, and to use an acrylic OCA with a dielectric constant ε (ε1=ε2=ε3) of approximately 2.5 for each of the first dielectric layer 11, the second dielectric layer 13, and the third dielectric layer 15. Note that the thicknesses of the dielectric layers were not calculated using the numerical values ​​obtained from (Equation 1), (Equation 2), and (Equation 3), but were set to D1=150 μm and D2=D3=450 μm, which are thicknesses available for acrylic OCA, taking into consideration the actual production of an electromagnetic wave absorbing sheet.

[0086] Assuming that the first resistance layer 12 and the second resistance layer 14 are both made of PEDOT, a conductive organic polymer, the surface resistance value of the first resistance layer 12 was set to 628 Ω / □ and the surface resistance value of the second resistance layer 14 was set to 189 Ω / □.

[0087] In the second electromagnetic wave absorbing sheet, the surface resistance value of the resistive layer was optimized so that the bandwidth at which the return loss in each absorption band was −10 dB or more was a wide bandwidth of approximately 50 GHz or more.

[0088] Assuming that the metal layer 16 is made of aluminum foil like the first electromagnetic wave absorbing sheet, the surface resistance value was set to 0 Ω / □.

[0089] The frequency characteristics of the return loss of the second electromagnetic wave absorbing sheet shown in Figure 3 confirmed that the electromagnetic wave absorbing sheet absorbs electromagnetic waves over a wide band in two frequency bands: a first frequency band (35 to 140 GHz) centered on a frequency of 50 GHz, and a second frequency band (215 GHz or higher) formed as a higher frequency band including 300 GHz, both of which exhibit a return loss of -10 dB or more, which indicates an attenuation of 90%.

[0090] Next, as a comparative example, the electromagnetic wave absorbing characteristics of a third electromagnetic wave absorbing sheet, which was composed of two dielectric layers without the second resistive layer and the third dielectric layer, were confirmed.

[0091] FIG. 4 shows the frequency characteristics of the return loss of the electromagnetic wave of the third electromagnetic wave absorbing sheet.

[0092] The third electromagnetic wave absorbing sheet was assumed to have an incident electromagnetic wave 1 with a frequency of 300 GHz (center wavelength λ=1 mm), and to use an acrylic OCA with a dielectric constant ε (ε1=ε2) of approximately 2.5 for both the first and second dielectric layers. The thickness of each dielectric layer was 150 μm.

[0093] It was assumed that the resistive layer between the first and second dielectric layers was made of PEDOT with a surface resistance of 189 Ω / □.

[0094] The metal layer was assumed to be formed by applying silver paste onto a PET substrate, and had a surface resistance of 20 Ω / □.

[0095] The frequency characteristics of the return loss of the third electromagnetic wave absorbing sheet shown in Figure 4 differ from the frequency characteristics of the first and second electromagnetic wave absorbing sheets of this embodiment in that the region of -10 dB or more, which is 90% attenuation, extends to the frequency band of 155 GHz or more, and it can be seen that this is a single-band electromagnetic wave absorbing sheet that does not have different electromagnetic wave absorption bands in the 50 GHz band or the 150 GHz band.

[0096] Next, in a configuration having three dielectric layers, the electromagnetic wave absorption characteristics were examined when the second and third dielectric layers had different thicknesses.

[0097] FIG. 5 is a diagram showing the electromagnetic wave absorption characteristics of a fourth electromagnetic wave absorbing sheet obtained by changing only the thickness of the third dielectric layer from the first electromagnetic wave absorbing sheet when n=5 described above, while leaving the other configurations unchanged.

[0098] The fourth electromagnetic wave absorbing sheet had the dielectric constants ε (ε1 = ε2 = ε3) of the first dielectric layer, second dielectric layer, and third dielectric layer set to approximately 2.5, the thickness of the first dielectric layer set to 150 μm, the thickness of the second dielectric layer set to 750 μm, the surface resistance value of the first resistive layer set to 750 Ω / □, the surface resistance value of the second resistive layer set to 180 Ω / □, and the surface resistance value of the metal layer set to 0 Ω / □.

[0099] In this state, the simulation results are shown for cases where the thickness of the third dielectric layer is set to 600 μm (reference number 51 in FIG. 5), 650 μm (reference number 52 in FIG. 5), 700 μm (reference number 53 in FIG. 5), and the same conditions as the first electromagnetic wave absorbing sheet, 750 μm (reference number 54 in FIG. 5), 800 μm (reference number 55 in FIG. 5), 850 μm (reference number 56 in FIG. 5), and 900 μm (reference number 57 in FIG. 5).

[0100] 5, when only the thickness of the third dielectric layer is changed, three bands are formed in which the return loss is greater than −10 dB (90% attenuation): a frequency band of 50 GHz, a frequency band of 150 GHz, and a frequency band of 250 GHz or more. It is also clear that as the thickness of the third dielectric layer becomes thinner (reference numerals 51 to 53), the peak value of the electromagnetic wave absorption characteristics increases while the width of the absorption frequency band tends to narrow.

[0101] For this reason, in order to construct an electromagnetic wave absorbing sheet having an absorption frequency band of a predetermined width in a plurality of frequency bands, it is considered preferable that the thicknesses of the second dielectric layer and the third dielectric layer be values ​​calculated from (Equation 2) and (Equation 3), respectively. Furthermore, from the frequency characteristics of electromagnetic wave attenuation shown in Fig. 5, if the errors in the thickness D1 of the first dielectric layer, the thickness D2 of the second dielectric layer, and the thickness D3 of the third dielectric layer calculated from (Equation 1), (Equation 2), and (Equation 3) are within ±20%, an electromagnetic wave absorbing sheet having desired multi-band electromagnetic wave absorption characteristics can be realized.

[0102] In this way, in the electromagnetic wave absorbing sheet disclosed in the present application, by setting the thicknesses of the three laminated dielectric layers, the first, second, and third, to values ​​calculated from the formulas (Formula 1), (Formula 2), and (Formula 3), a wide frequency band exhibiting electromagnetic wave absorption characteristics appears at frequencies corresponding to the wavelength λ of the electromagnetic waves used in the formulas, and a multi-band electromagnetic wave absorbing sheet having two absorption frequency bands when n=3 and three absorption frequency bands when n=5 can be realized.

[0103] This is thought to be the result of the electromagnetic waves incident on the electromagnetic wave absorbing sheet being reflected by three layers, the first resistance layer, the second resistance layer, and the metal layer, and the phase of each reflected wave being inverted from the phase of the incident wave, resulting in an overlapping reflective electromagnetic wave absorption effect in which the reflected waves from the electromagnetic wave absorbing sheet are absorbed by electromagnetic wave interference.

[0104] According to the principle of electromagnetic wave absorption in the electromagnetic wave absorbing sheet disclosed in the present application described above, when the value of n in (Equation 2) and (Equation 3) is a positive odd number, 7, it is expected that a multiband electromagnetic wave absorbing sheet having four absorption frequency bands will be realized, and as the value of n increases, an electromagnetic wave absorbing sheet having even more absorption frequency bands will be realized. However, since it is expected that the return loss in each absorption band will decrease as the number of n increases, from a practical standpoint, it is considered that n = 5 or 3 is suitable for a multiband electromagnetic wave absorbing sheet that can achieve an attenuation of 90% or more in multiple frequency bands.

[0105] Furthermore, in the above embodiment, the center frequency of the electromagnetic waves incident on the electromagnetic wave absorbing sheet is 300 GHz, but if the center frequency is in a higher frequency band, for example, 1 THz (terahertz), the thicknesses D1, D2, D3 of the first dielectric layer, second dielectric layer, and third dielectric layer will all be smaller values, and if a resin material with a normal dielectric constant of around 2.5 is used, a thinner resin sheet will be required.However, at 1 THz, for example, the center wavelength λ will be about 1 / 3, which is considered to be within a range that is sufficiently practical.

[0106] As described above, the electromagnetic wave absorbing sheet according to this embodiment comprises three dielectric layers, two resistive layers formed between them, and a metal layer as a reflective layer disposed on the rearmost surface, and by defining the thickness of each dielectric layer based on the relationship between the dielectric constant of the dielectric layer and the wavelength of the incident electromagnetic wave, it is possible to realize a multi-band electromagnetic wave absorbing sheet having multiple absorption frequency bands in high frequency bands above the millimeter wave band.

[0107] Therefore, in technologies that utilize electromagnetic waves in two or more frequency bands, including electromagnetic waves in a high frequency band of several hundred GHz or more and lower frequency bands, the multiple electromagnetic waves used can be effectively absorbed, and necessary electromagnetic waves can be effectively prevented from being affected by the influence of undesired reflected waves.

[0108] For example, the usable frequency range of electromagnetic waves in the so-called 5G frequency band and higher frequencies is limited, and they must be used in a manner that prevents electromagnetic wave leakage outside the designated range. Therefore, when data transmission in multiple 5G frequency bands is performed within a specific facility, it is necessary to prevent all electromagnetic waves in those frequency bands from being emitted outdoors. For example, when transmissions around 28 GHz and around 270 GHz are both performed, if absorbers compatible with only a single frequency band are installed alternately, electromagnetic wave leakage of a certain level or more in each frequency band will occur. Therefore, an electromagnetic wave absorbing sheet having absorption bands in multiple frequency bands, such as the electromagnetic wave absorbing sheet disclosed in the present application, can be suitably used.

[0109] The electromagnetic wave absorbing sheet disclosed in the present application is a multi-band electromagnetic wave absorbing sheet having multiple electromagnetic wave absorption bands in high frequency bands above the millimeter wave band, by laminating a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer, and a metal layer, and by adjusting the thickness of the dielectric layer to the conditions shown in (Formula 1), (Formula 2), and (Formula 3) and by making the surface resistance value of the first resistive layer larger than the surface resistance value of the second resistive layer.

[0110] REFERENCE SIGNS LIST 1 (incident) electromagnetic wave 10 electromagnetic wave absorbing sheet 11 first dielectric layer 12 first resistive layer 13 second dielectric layer 14 second resistive layer 15 third dielectric layer 16 metal layer

Claims

1. An electromagnetic wave absorbing sheet comprising a first dielectric layer, a first resistive layer, a second dielectric layer, a second resistive layer, a third dielectric layer and a metal layer laminated in that order from the electromagnetic wave incidence side, wherein the dielectric constant of the first dielectric layer is ε1, the thickness is D1, the dielectric constant of the second dielectric layer is ε2, the thickness is D2, and the dielectric constant of the third dielectric layer is ε3 and the thickness is D3, and the following relationships are satisfied: D1 = λ / 4√ε1 ± 20% (Formula 1) D2 = nλ / 4√ε2 ± 20% (Formula 2) D3 = nλ / 4√ε3 ± 20% (Formula 3) (where λ is the central wavelength of the electromagnetic wave, and n is a positive odd number equal to or greater than 3), and the surface resistance value of the first resistive layer is higher than the surface resistance value of the second resistive layer.

2. The electromagnetic wave absorbing sheet according to claim 1, wherein n in said formulas 2 and 3 is 5.

3. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein n in said formula 2 or formula 3 is 3.

4. An electromagnetic wave absorbing sheet according to any one of claims 1 to 3, wherein the first resistive layer and the second resistive layer are layers containing at least one of a conductive inorganic material and a conductive polymer.

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

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

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