Electromagnetic wave absorber

Abstract

Provided is an electromagnetic wave absorber in which the difference between the absorption characteristics of a TE wave and a TM wave when an electromagnetic wave is incident from an oblique direction is small. The electromagnetic wave absorber according to the present invention has, in this order, a reflective layer, a base layer comprising a dielectric or a magnetic material, a microstructure layer comprising a plurality of microstructures, and an overcoat layer having refractive index anisotropy.

Description

Technical Field

[0001] The invention relates to an electromagnetic wave absorber. Background Art

[0002] Radio waves having a frequency band of several gigahertz (GHz) are used in mobile communications such as mobile phones, wireless LANs, and automatic toll collection systems (ETC).

[0003] In recent years, in order to achieve large-capacity, high-speed communication, and simultaneous connection of multiple locations for transmitted and received data, wireless communication using higher frequency bands has been put into practical use, and devices that can realize the practical use of such wireless communication are being developed. In addition, the use of vehicle-mounted radar equipment that utilizes extremely narrow directivity is being promoted.

[0004] Interference caused by diffuse reflection of electromagnetic waves in the housing of a device may cause malfunction of the device. Therefore, suppressing electromagnetic wave noise is important as one of the electromagnetic wave utilization technologies.

[0005] As one method of suppressing electromagnetic wave noise, the use of an electromagnetic wave absorbing sheet may be considered.

[0006] For example, Patent Document 1 proposes an electromagnetic wave attenuation film comprising: a dielectric substrate having a front side and a back side; a thin film conductive layer arranged on the front side; and a planar inductor arranged on the back side, wherein the thin film conductive layer is used in a specific frequency band of millimeter waves including a plurality of discretely arranged metal plates.

[0007] Since electromagnetic wave absorbers installed in electronic devices, building interiors, etc. are used continuously for a long time, Patent Document 1 proposes to provide a top coating on the surface of the electromagnetic wave attenuation film in order to improve environmental resistance such as weather resistance and heat resistance and to match air with impedance to effectively attenuate radio waves.

[0008] Previous technical literature

[0009] Patent Literature

[0010] Patent Document 1: International Publication No. 2022 / 107637 Summary of the invention

[0011] Technical issues to be solved by the invention

[0012] According to the research conducted by the inventors of the present invention, it is known that when an electromagnetic wave attenuation film is used to absorb electromagnetic waves that become noise, if the electromagnetic waves are incident on the electromagnetic wave attenuation film from an oblique direction, there will be differences in the absorption characteristics between the TE wave (orthogonal polarized wave) having an electric field component that is perpendicular to the incident plane and the TM wave (parallel polarized wave) having an electric field within the incident plane.

[0013] An object of the present invention is to solve the problems of the prior art and to provide an electromagnetic wave absorber having a small difference in absorption characteristics between TE waves and TM waves when electromagnetic waves are incident from an oblique direction.

[0014] Means for solving technical problems

[0015] In order to solve the problem, the present invention has the following configuration.

[0016] [1] An electromagnetic wave absorber comprising:

[0017] Reflective layer;

[0018] A substrate layer comprising at least one of a dielectric and a magnetic body;

[0019] A microstructure layer composed of a plurality of microstructures; and

[0020] External coating with anisotropic refractive index.

[0021] [2] The electromagnetic wave absorber according to [1], wherein

[0022] The outer coating has a refractive index anisotropy at least in the in-plane direction.

[0023] [3] The electromagnetic wave absorber according to [1] or [2], wherein

[0024] The birefringence Δn of the overcoat layer is not less than 0.2.

[0025] [4] The electromagnetic wave absorber according to any one of [1] to [3], wherein

[0026] The overcoat layer includes a liquid crystal compound.

[0027] [5] The electromagnetic wave absorber according to any one of [1] to [4], wherein

[0028] The extinction coefficient of the substrate layer is 0.1 or more.

[0029] [6] The electromagnetic wave absorber according to any one of [1] to [5], wherein

[0030] The wavelength at which the electromagnetic wave absorbed by the electromagnetic wave absorber is most absorbed is within the range of 10 μm to 10 cm.

[0031] Effects of the Invention

[0032] According to the present invention, it is possible to provide an electromagnetic wave absorber having a small difference in absorption characteristics between TE waves and TM waves when electromagnetic waves are incident from an oblique direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1This is a diagram schematically showing an example of the electromagnetic wave absorber of the present invention.

[0034] Figure 2 This is a side view for explaining an example of the structure of a unit cell of the electromagnetic wave absorber of the present invention.

[0035] Figure 3 yes Figure 2 A top view of the unit cell is shown.

[0036] Figure 4 This is a conceptual diagram for explaining the polarization of an incident wave.

[0037] Figure 5 This is a graph showing the relationship between the birefringence Δn when a TE wave is incident and the range of the incident angle at which the absorption rate becomes 97% or more.

[0038] Figure 6 This is a graph showing the relationship between the birefringence Δn and the range of the incident angle at which the absorption rate becomes 97% or more when a TM wave is incident.

[0039] Figure 7 This is a diagram schematically showing the relationship between the TE wave and the refractive index of a liquid crystal compound.

[0040] Figure 8 This is a diagram schematically showing the relationship between the TM wave and the refractive index of a liquid crystal compound.

[0041] Fig. 9 This is a diagram schematically showing another example of the electromagnetic wave absorber of the present invention.

[0042] Fig.10 This is a diagram schematically showing the relationship between the TE wave and the refractive index of a discotic liquid crystal compound.

[0043] Fig.11 This is a diagram schematically showing the relationship between the TM wave and the refractive index of a discotic liquid crystal compound. DETAILED DESCRIPTION

[0044] Hereinafter, the electromagnetic wave absorber of the present invention will be described in detail based on the preferred embodiments shown in the accompanying drawings.

[0045] In the present specification, a numerical range expressed using "to" means a range including the numerical values ​​described before and after "to" as the lower limit and the upper limit.

[0046] In this specification, terms such as “same” include an error range generally allowed in the technical field.

[0047] The following figures are all conceptual diagrams for explaining the electromagnetic wave absorber of the present invention, and therefore, the shape, size, thickness, positional relationship, etc. of each component may not necessarily correspond to the actual one.

[0048] [Electromagnetic wave absorber]

[0049] Figure 1 An example of the electromagnetic wave absorber of the present invention is schematically shown in FIG.

[0050] Figure 1 The electromagnetic wave absorber 10 shown includes a substrate layer 12, a fine structure layer 14 disposed on one main surface of the substrate layer 12, a reflective layer 16 disposed on the other main surface of the substrate layer 12, and an overcoat layer 18 disposed on the main surface of the fine structure layer 14 on the opposite side of the substrate layer 12. That is, the electromagnetic wave absorber 10 includes the reflective layer 16, the substrate layer 12, the fine structure layer 14, and the overcoat layer 18 in this order.

[0051] In the electromagnetic wave absorber 10 , the base layer 12 and the reflective layer 16 , the base layer 12 and the fine-structure layer 14 , and the fine-structure layer 14 and the overcoat layer 18 are attached using an adhesive (binder, adhesive) as necessary.

[0052] The pasting method is not limited, and various known methods that can transmit electromagnetic waves as the target of the electromagnetic wave absorber 10 , such as a method using an OCA (Optical Clear Adhesive) that can transmit electromagnetic waves as the target of the electromagnetic wave absorber 10 , can be used.

[0053] [Base material layer]

[0054] The base layer 12 supports the reflective layer 16 and the fine-structure layer 14 and includes a dielectric and / or a magnetic body. The base layer 12 includes a dielectric and / or a magnetic body and absorbs electromagnetic waves by converting electromagnetic waves into heat using dielectric loss of the dielectric or magnetic loss of the magnetic body.

[0055] Here, a dielectric refers to a substance having a dielectric property superior to an electrical conductivity, and a magnetic substance refers to a substance showing strong magnetism and / or ferromagnetism.

[0056] The substrate layer 12 may be formed of a dielectric or a magnetic body. Furthermore, the substrate layer 12 may be a layer in which two or more dielectrics and / or magnetic bodies are mixed. Alternatively, the substrate layer 12 may be a layer in which dielectric particles and / or magnetic particles are mixed with an adhesive such as a resin.

[0057] Examples of dielectrics include polyesters such as polyethylene terephthalate (PET); polyarylene sulfides such as polyphenylene sulfide; polyolefins such as polyethylene and polypropylene; and resin materials such as polyamide, polyimide, polyamideimide, polyether sulfone, polyether ether ketone, polycarbonate, acrylic resin, and polystyrene.

[0058] Furthermore, as the magnetic body, for example, ferrite and metal magnetic materials can be cited.

[0059] The thickness of the base material layer 12 is not particularly limited as long as it is set to a thickness that can achieve a desired absorptivity due to loss caused by the base material layer.

[0060] 〔Reflective layer〕

[0061] The reflective layer 16 reflects incident electromagnetic waves and also functions as an electrode layer.

[0062] The reflective layer 16 is not limited, and any of various known plate-like materials can be used as long as it can reflect the electromagnetic waves that the electromagnetic wave absorber 10 is intended to receive.

[0063] For example, when the electromagnetic wave targeted by the electromagnetic wave absorber 10 has a wavelength of 10 μm to 10 cm, the reflective layer 16 may include metal layers such as copper, aluminum, gold and silver, inorganic conductive materials such as ITO (tin-doped indium oxide), organic conductive materials such as polythiophene, and graphene.

[0064] Furthermore, the reflective layer 16 of the electromagnetic wave absorber 10 does not need to be a uniform layer (solid layer) over the entire surface, and may have a structure having a uniform reflection distribution in the plane, similar to a uniform layer. For example, the reflective layer 16 may have a metal mesh structure.

[0065] The thickness of the reflective layer 16 is not limited either, and may be appropriately set according to the material forming the reflective layer 16 so as to reflect the target electromagnetic wave at a desired reflectivity.

[0066] 〔Microstructure layer〕

[0067] The microstructure layer 14 is a so-called metasurface structure in which a plurality of microstructures 20 as resonators are arranged on the surface of the base layer 12. Each of the microstructures 20 functions not only as a reflector but also as an electrode. In the present invention, the microstructure layer 14 is basically a known microstructure layer (metamaterial).

[0068] In the example shown in the figure, the fine-structure layer 14 has a structure in which the fine-structures 20 are two-dimensionally arranged at equal intervals in the x direction and the y direction which are orthogonal to each other.

[0069] Figure 2 2 is a diagram schematically showing a unit cell composed of one fine structure body 20 of the fine structure body layer 14 and the surrounding area of ​​the fine structure body 20 . Figure 3 yes Figure 2 The top view (top view) of the unit cell is shown. Figure 3 In the figure, the outer coating layer 18 is omitted.

[0070] It can be said that the electromagnetic wave absorber 10 has the following Figure 2 and Figure 3 The unit cells 11 shown are two-dimensionally arranged at equal intervals in the x-direction and the y-direction.

[0071] In the unit cell 11 of the illustrated example, the planar shape of the microstructure 20 is a square cuboid. Figure 1 In the electromagnetic wave absorber 10 shown, the planar shape of the fine structures 20 is a square.

[0072] The shape and forming material of the fine-structure bodies 20 in the fine-structure body layer 14 , the arrangement of the fine-structure bodies 20 , the intervals (pitch) between the fine-structure bodies 20 , etc. are not limited and may be set in the same manner as in a known fine-structure body layer.

[0073] The fine-structure layer 14 may be designed by a known method according to target optical characteristics. For example, commercially available simulation software may be used to set the arrangement of the fine-structures 20 so as to absorb at a target frequency.

[0074] As the material of the fine structure 20 forming the fine structure layer 14, metals and dielectrics can be exemplified. In the case of metals, copper, gold, and silver can be preferably exemplified from the viewpoint of small optical loss, etc. On the other hand, as dielectrics, silicon, titanium oxide, and germanium can be preferably exemplified.

[0075] As the shape of the microstructure 20, there can be exemplified a rectangular shape, a cylindrical shape, a polygonal prism such as a triangular prism as described above, a solid with a V-shaped bottom surface connecting rectangular prisms at the ends as shown in Japanese Patent Gazette No. 2018-46395, a solid with a cross-shaped bottom surface as if rectangular prisms were crossed, a solid with a roughly H-shaped bottom surface like H-steel, and a solid with a roughly C-shaped bottom surface like a C-channel, etc.

[0076] Furthermore, as shown in Japanese Patent Application Laid-Open No. 2018-46395, a solid having a V-shaped bottom surface and a solid having a cross-shaped bottom surface can utilize various shapes obtained by adjusting the angle formed by two rectangular parallelepipeds.

[0077] In addition, a solid having a bottom shape as shown in Figure 5 of "Appl. Sci. 2018, 8(9), 1689; https: / / doi.org / 10.3390 / app8091689" can also be used.

[0078] In the fine-structure layer 14 , only one of the fine-structures 20 having these shapes may be used, or a plurality of them may be used in combination.

[0079] Furthermore, the orientations of the same fine structures 20 may be the same or different, or the same orientation and different orientations may be mixed.

[0080] As described above, the arrangement and spacing (pitch) of the fine structures 20 are not limited to a structure in which the fine structures 20 are arranged at equal intervals in two directions (x direction and y direction) orthogonal to each other. For example, there may be a structure in which the spacing of the fine structures 20 is different, or a structure in which the spacing in the in-plane direction gradually changes. Furthermore, the fine structures 20 may be arranged in one dimension.

[0081] The thickness of the microstructure 20 is not limited and can be appropriately set to reflect the target electromagnetic wave at a desired reflectivity according to the material forming the microstructure 20. The thickness of the microstructure 20 is preferably sufficiently greater than the skin depth d represented by the following formula.

[0082] d=(1 / (π·f·μ·σ))^(1 / 2)

[0083] Here, f represents the frequency [Hz] of the electromagnetic wave, μ represents the magnetic permeability [H / m] of the microstructure, and σ represents the electrical conductivity [S / m] of the microstructure.

[0084] The thickness of the fine structure 20 is preferably 2 to 3 times the depth of the epidermis.

[0085] The size of the fine structure 20 in a plan view is not limited either, and may be set to a size that absorbs the target electromagnetic wave according to the material of which the fine structure 20 is formed, etc.

[0086] 〔Outer coating〕

[0087] The overcoat layer 18 is a layer having anisotropy of refractive index, and is disposed on the side of the fine-structure layer 14 opposite to the base material layer 12 .

[0088] Although described in detail later, by having an outer coating 18 with refractive index anisotropy, the TE wave and TM wave of the electromagnetic wave incident on the electromagnetic wave absorber 10 from an oblique direction act as layers with different refractive indices, thereby reducing the difference in absorption characteristics between the TE wave (orthogonal polarization wave) and the TM wave (parallel polarization wave).

[0089] exist Figure 1 In the example shown, the overcoat layer 18 is a liquid crystal layer containing rod-shaped liquid crystal compounds 30 and oriented and fixed so that the long axis of the liquid crystal compounds 30 is aligned in one direction parallel to the main surface of the overcoat layer 18. Thus, the overcoat layer 18 is a liquid crystal layer having a long axis direction ( Figure 1 The refractive index of the short axis direction ( Figure 1In the direction perpendicular to the paper surface, the refractive index of the layer is different from that of the layer having anisotropy. That is, Figure 1 The overcoat layer 18 of the illustrated example has a refractive index anisotropy in the in-plane direction.

[0090] In addition, Figure 1 In the example shown, the outer coating layer 18 is configured such that the liquid crystal compound 30 is oriented in one direction parallel to the main surface and has a refractive index anisotropy in the in-plane direction, but the invention is not limited thereto. The outer coating layer may be a layer oriented in one direction in which the liquid crystal compound 30 (the long axis) is tilted relative to the main surface, or a layer oriented in a direction perpendicular to the main surface along which the liquid crystal compound 30 is oriented.

[0091] From the viewpoint of further reducing the difference in absorption characteristics between TE waves (orthogonal polarization waves) and TM waves (parallel polarization waves) in the electromagnetic wave absorber 10 , the overcoat layer 18 preferably has refractive index anisotropy at least in the in-plane direction.

[0092] And, in Figure 1 In the example shown, the outer coating layer 18 includes a rod-shaped liquid crystal compound 30, but the present invention is not limited thereto. Fig. 9 The example shown may also be a layer containing a discotic liquid crystal compound.

[0093] And, in Figure 1 In the example shown, the overcoat layer 18 contains the liquid crystal compound 30, but the present invention is not limited thereto and may be a stretched film such as PET (polyethylene terephthalate) or PEN (polyethylene naphthalate).

[0094] Although described in detail later, from the viewpoint of further reducing the difference in absorption characteristics between TE waves (orthogonal polarization waves) and TM waves (parallel polarization waves), the birefringence Δn of the outer coating layer 18 is preferably 0.2 or more, and more preferably 0.3 or more. The larger Δn can be, the easier it is to adjust the refractive index that effectively acts on each polarization wave, so it is preferred. From the viewpoint of being able to increase the birefringence Δn, the outer coating layer 18 preferably contains the liquid crystal compound 30.

[0095] -Rod-like liquid crystal compound-

[0096] As the rod-like liquid crystal compound contained in the outer coating layer 18, it is preferred to use azo, azomethine, azoxy, cyanobiphenyl, cyanophenyl ester, benzoate, cyclohexanecarboxylic acid phenyl ester, cyanophenylcyclohexane, cyano-substituted phenylpyrimidine, alkoxy-substituted phenylpyrimidine, phenyldioxane, diphenylacetylene and alkenylcyclohexylbenzonitrile.

[0097] As the rod-like liquid crystal compound, not only the low-molecular liquid crystal molecules as described above but also high-molecular liquid crystal molecules can be used.

[0098] In the overcoat layer 18, it is more preferable to fix the alignment by polymerizing the rod-like liquid crystal compound. That is, the overcoat layer 18 is preferably a layer in which a polymerizable rod-like liquid crystal compound is polymerized and fixed.

[0099] As the polymerizable rod-like liquid crystal compound, there can be used Makromol. Chem., Vol. 190, p. 2255 (1989), Advanced Materials, Vol. 5, p. 107 (1993), Advanced Photonics, Vol. Compounds described in Volume 2, Item 036002 (2020), U.S. Patent No. 4,683,327, U.S. Patent No. 5,622,648, U.S. Patent No. 5,770,107, International Publication No. 95 / 022,586, International Publication No. 95 / 024455, International Publication No. 97 / 000600, International Publication No. 98 / 023580, International Publication No. 98 / 052905, Japanese Patent Application No. 1-272551, Japanese Patent Application No. 6-016616, Japanese Patent Application No. 7-110469, Japanese Patent Application No. 11-080081 and Japanese Patent Application No. 2001-064627.

[0100] Furthermore, as the rod-like liquid crystal compound, for example, compounds described in JP-A-11-513019 and JP-A-2007-279688 can also be preferably used.

[0101] -Disc-like liquid crystal compounds-

[0102] As the discotic liquid crystal compound, for example, the discotic liquid crystal compounds described in JP-A-2007-108732 and JP-A-2010-244038 can be preferably used.

[0103] The liquid crystal layer serving as the overcoat layer 18 can be formed by applying a composition containing a liquid crystal compound on an alignment film having a desired alignment pattern, drying the composition, and polymerizing the liquid crystal compound as necessary.

[0104] In addition, in the present invention, an orientation film for orienting the liquid crystal compound 30 may be provided between the outer coating layer 18 and the fine structure layer 14. That is, an orientation film may be provided on the surface of the fine structure layer 14, and the outer coating layer 18 may be formed thereon. Alternatively, a plate-like object having an orientation film on a support may be used, the outer coating layer 18 may be formed on the orientation film of the plate-like object, and then the outer coating layer 18 may be peeled off and transferred to the fine structure layer 14.

[0105] -Oriented film-

[0106] In the present invention, various known alignment films can be used as the alignment film for forming the alignment of the liquid crystal compound 30 constituting the overcoat layer 18 in the thickness direction.

[0107] As the oriented film, for example, there can be exemplified a friction-treated film composed of an organic compound such as a polymer, an oblique vapor-deposited film of an inorganic compound, a film having microgrooves, and a film formed by accumulating an LB (Langmuir-Blodgett) film based on the Langmuir-Blodgett method of organic compounds such as ω-tricosanoic acid, dioctadecylmethylammonium chloride and methyl stearate.

[0108] The alignment film based on the rubbing treatment can be formed by rubbing the surface of the polymer layer several times in a prescribed direction with paper or cloth.

[0109] Preferred examples of materials used for the alignment film include polyimide, polyvinyl alcohol, polymers having polymerizable groups described in JP-A-9-152509, and materials used for forming alignment films described in JP-A-2005-97377, JP-A-2005-99228, and JP-A-2005-128503.

[0110] The alignment film may be a so-called photo-alignment film in which a material having a photo-alignment property is irradiated with polarized light or non-polarized light to form an alignment film.

[0111] Examples of the photo-alignment material that can be used in the photo-alignment film of the present invention include Japanese Patent Application Publication No. 2006-285197, Japanese Patent Application Publication No. 2007-076839, Japanese Patent Application Publication No. 2007-138138, Japanese Patent Application Publication No. 2007-094071, Japanese Patent Application Publication No. 2007-121721, Japanese Patent Application Publication No. 2007-140465, Japanese Patent Application Publication No. 2007-1 Azo compounds described in Japanese Patent Application No. 56439, Japanese Patent Application No. 2007-133184, Japanese Patent Application No. 2009-109831, Japanese Patent No. 3883848 and Japanese Patent No. 4151746, aromatic ester compounds described in Japanese Patent Application No. 2002-229039, Japanese Patent Application No. 2002-265541 and Japanese Patent Application No. 2002-317013 The maleimide and / or alkenyl-substituted nadic imide compound having a photo-alignment unit described in JP-A-4205195 and JP-A-4205198, the photo-crosslinkable silane derivatives described in JP-A-2003-520878, JP-A-2004-529220 and JP-A-4162850, the photo-crosslinkable polyimide, the photo-crosslinkable polyamide and the photo-crosslinkable ester Preferred examples include compounds capable of photodimerization described in JP-A-9-118717, JP-A-10-506420, JP-A-2003-505561, International Publication No. 2010 / 150748, JP-A-2013-177561, and JP-A-2014-012823, in particular cinnamate compounds, chalcone compounds, and coumarin compounds.

[0112] Among them, it is preferable to use an azo compound, a photo-crosslinkable polyimide, a photo-crosslinkable polyamide, a photo-crosslinkable ester, a cinnamate compound, and a chalcone compound.

[0113] The thickness of the alignment film is not limited, and may be appropriately set according to the material forming the alignment film so as to obtain a desired alignment function.

[0114] The thickness of the alignment film is preferably 0.01 to 5 μm, more preferably 0.02 to 2 μm.

[0115] The method for forming the alignment film is not limited, and various known methods corresponding to the materials for forming the alignment film can be used.

[0116] In the present invention, the thickness of the overcoat layer 18 is not limited and may be appropriately set according to the material forming the overcoat layer 18 and the wavelength of the electromagnetic wave to which the absorber is directed.

[0117] The thickness of the overcoat layer 18 is preferably not less than 1 / 10 of the wavelength of the electromagnetic wave that is the target of the absorber.

[0118] [Function of electromagnetic wave absorber]

[0119] Next, the operation of the electromagnetic wave absorber 10 of the present invention having the above-mentioned structure will be described.

[0120] The electromagnetic wave absorber 10 is considered to absorb electromagnetic waves by reducing the effect of reflected waves by controlling the amplitude and phase of reflected waves at the interfaces of each layer and by absorbing the electromagnetic waves due to the loss of the substrate layer 12 while the electromagnetic waves propagate in the substrate layer 12. In order to absorb all incident electromagnetic waves, the input impedance of the reflective layer side observed from the surface of the electromagnetic wave absorber is set to be equal to the characteristic impedance of the plane wave.

[0121] Here, it can be seen that when the electromagnetic wave that becomes noise is absorbed by the conventional electromagnetic wave absorber as described above, if the electromagnetic wave is incident on the electromagnetic wave absorber from an oblique direction, there are TE waves (orthogonal polarization waves) having a component of the electric field perpendicular to the incident plane and TM waves (parallel polarization waves) having an electric field in the incident plane (reference Figure 4 ), there are differences in absorption characteristics.

[0122] This is believed to be because, as described in Dengaku Ryūshū A, Vol. 122, No. 5 (2002) and Introduction to Radio Wave Absorbers (written by Hashimoto Osamu, published by Morikita, 1997), the input impedance of the reflecting layer side observed from the electromagnetic wave incident surface shows different incidence angle dependencies for TE waves and TM waves.

[0123] Therefore, in the conventional electromagnetic wave absorber, the difference in absorption characteristics between the TE wave and the TM wave becomes large.

[0124] On the other hand, the electromagnetic wave absorber of the present invention includes an overcoat layer 18 having anisotropy of refractive index on the electromagnetic wave incident side, that is, on the fine-structure layer 14 , and can reduce the difference in absorption characteristics between TE waves and TM waves.

[0125] use Figure 5 and Figure 6 This point will be described.

[0126] The unit cell 11 with a design frequency of 300 GHz is set to the following structure. When the refractive index n of the outer coating 18 is changed in various ways and a TE wave or TM wave is incident from an oblique direction, the maximum incident angle at which the absorptivity becomes more than 97% is obtained by simulation. Here, it is confirmed that the frequency of the absorption peak shifts forward and backward according to the refractive index n of the outer coating 18, but is within the range of 295 GHz to 315 GHz. COMSOL Multiphysics was used for the simulation. In addition, the absorptivity can be calculated according to the size of the electromagnetic wave incident on the electromagnetic wave absorber, the size of the electromagnetic wave transmitted through the electromagnetic wave absorber, and the size of the electromagnetic wave reflected from the electromagnetic wave absorber, and is calculated by the following formula: Absorptivity = (size of the incident electromagnetic wave - size of the transmitted electromagnetic wave - size of the reflected electromagnetic wave) / size of the incident electromagnetic wave.

[0127] In the structure of the unit cell 11, the vertical and horizontal widths L1 (reference Figure 2 and Figure 3 , the same below) was set to 350 μm, the thickness d2 of the reflective layer 16 was set to 1 μm, the thickness d1 of the base layer 12 was set to 40 μm, the vertical and horizontal width L2 of the fine structure 20 was set to 250 μm, the thickness d3 was set to 1 μm, and the thickness d4 of the outer coating layer 18 was set to three types: 170 μm, 200 μm, and 250 μm. In addition, the reflective layer 16 was made of copper, the refractive index of the base layer 12 was set to 1.53, the extinction coefficient was set to 0.3, the fine structure 20 was made of copper, and the extinction coefficient of the outer coating layer 18 was set to 0 for modeling.

[0128] Figure 5 2 is a graph showing the relationship between the maximum incident angle (hereinafter also simply referred to as the maximum incident angle) at which the absorption rate becomes 97% or more when the TE wave is incident, which is obtained by the above simulation, and the refractive index n. Figure 6 2 is a graph showing the relationship between the maximum incident angle at which the absorption rate becomes 97% or more when the TM wave is incident and the refractive index n, which is obtained by the above-mentioned simulation.

[0129] from Figure 5 It can be seen that the relationship between the refractive index n of the overcoat layer and the maximum incident angle in the case of TE waves reaches a local maximum at a certain refractive index n, and the maximum incident angle decreases as the refractive index moves away from the local maximum.

[0130] On the other hand, from Figure 6 It can be seen that the relationship between the refractive index n of the overcoat layer and the maximum incident angle in the case of TM waves is that the maximum incident angle increases as the refractive index n increases.

[0131] So, from Figure 5 and Figure 6It can be seen that the relationship between the refractive index n of the outer coating 18 and the maximum incident angle is different in TE waves and TM waves. Therefore, for example, when the refractive index of the outer coating is isotropic and the refractive index n is 1.57, the maximum incident angle relative to the TE wave is about 61°, but the maximum incident angle relative to the TM wave is about 46°. In addition, when the refractive index n is 1.87, the maximum incident angle relative to the TE wave is about 50°, but the maximum incident angle relative to the TM wave is about 58°. In this way, it can be seen that in the conventional electromagnetic wave absorber having an outer coating with an isotropic refractive index, the difference in the absorption characteristics of the TE wave and the TM wave becomes large.

[0132] In contrast, in the electromagnetic wave absorber of the present invention, since the outer coating layer has refractive index anisotropy, it functions as a layer with different refractive indices for TE waves and TM waves, thereby reducing the difference in absorption characteristics between TE waves and TM waves.

[0133] For example, Figure 1 As shown, in the case where the outer coating layer includes a liquid crystal compound, by aligning the azimuth direction of the incident wave with the long axis direction of the liquid crystal compound, as shown in FIG. Figure 7 As shown, the refractive index no in the short axis direction of the liquid crystal compound 30 acts on the TE wave of the incident wave I0, and, as Figure 8 As shown in FIG. 1 , the refractive index ne of the liquid crystal compound 30 in the long axis direction acts on the TM wave of the incident wave I0. Therefore, in the structure example in which the above simulation was performed, a rod-shaped liquid crystal compound having a refractive index no of 1.57 and a refractive index ne of 1.87 was used as the liquid crystal compound contained in the overcoat layer, thereby Figure 5 In this case, the maximum incident angle with respect to the TE wave can be set to about 61°, and from Figure 6 In the embodiment of the present invention, the maximum incident angle with respect to the TM wave can be set to about 58°. In this way, the electromagnetic wave absorber of the present invention can reduce the difference in absorption characteristics between TE waves and TM waves incident from an oblique direction by having an outer coating layer with anisotropic refractive index. It is speculated that this is because the effective dielectric constants of TE waves and TM waves change due to the anisotropy of the refractive index of the outer coating layer, so as described above, the input impedance of the reflective layer side observed from the surface of the electromagnetic wave absorber can be made close to the characteristic impedance of the plane wave, thereby improving absorption.

[0134] In the above example, the refractive index of the outer coating in the direction perpendicular to the incident surface is Figure 5 In the graph for TE waves shown in FIG. 1 , the refractive index is set to be the refractive index with a maximum value near the maximum incident angle (=1.57), and the refractive index in the direction parallel to the incident plane is set to be Figure 6In the graph for TM waves shown, the refractive index at the maximum incident angle is set to be approximately the same as the maximum value of the maximum incident angle of the TE wave (=1.87), but it is not limited to this. The refractive index of the outer coating in the direction perpendicular to the incident surface and the refractive index in the direction parallel to the incident surface can be appropriately set so that the maximum incident angle of the TE wave and the maximum incident angle of the TM wave are close to each other.

[0135] From the perspective of being able to make the maximum incident angle larger, it is preferred to set the refractive index of the outer coating in the direction perpendicular to the incident surface to the refractive index at which the maximum incident angle of the TE wave reaches a maximum value, and to set the refractive index in the direction parallel to the incident surface to the refractive index at the maximum incident angle that is approximately the same as the maximum value of the maximum incident angle of the TE wave.

[0136] Here, as described above, the relationship between the refractive index of the outer coating layer and the maximum incident angle when the wave is TE takes a local maximum at a certain refractive index, and the maximum incident angle decreases as the refractive index moves away from the local maximum ( Figure 5 ). On the other hand, regarding the relationship between the refractive index n of the outer coating and the maximum incident angle for TM waves, as the refractive index n increases, the maximum incident angle increases ( Figure 6 ). Therefore, if the birefringence Δn of the outer coating is small, it is difficult to appropriately set the maximum incident angle of the TE wave and the maximum incident angle of the TE wave. From the viewpoint of being able to appropriately set the maximum incident angle of the TE wave and the maximum incident angle of the TE wave, the birefringence Δn of the outer coating is preferably large. Figure 5 and Figure 6 From the viewpoint of setting the refractive index in the direction parallel to the incident plane to be the refractive index at the maximum incident angle substantially the same as the maximum value of the maximum incident angle of the TE wave, the birefringence Δn of the outer coating is preferably greater than 0.2, and more preferably greater than 0.3.

[0137] And, in Figure 1 In the example shown, the overcoat layer is a layer containing a rod-like liquid crystal compound, but the present invention is not limited thereto, and the overcoat layer may be a layer containing a discotic liquid crystal compound.

[0138] Fig. 9 2 is a diagram schematically showing another example of the electromagnetic wave absorber of the present invention.

[0139] Fig. 9 The electromagnetic wave absorber 10b shown has a base layer 12, a fine structure layer 14 disposed on one main surface of the base layer 12, a reflective layer 16 disposed on the other main surface of the base layer 12, and an outer coating layer 18 disposed on the main surface of the fine structure layer 14 on the opposite side of the base layer 12. That is, the electromagnetic wave absorber 10b has the reflective layer 16, the base layer 12, the fine structure layer 14, and the outer coating layer 18b in this order. Fig. 9The electromagnetic wave absorber 10b shown in the figure has an outer coating 18b instead of the outer coating 18, and has the same Figure 1 The electromagnetic wave absorbers 10 shown have the same structure, so in the following description, different parts will be mainly described.

[0140] The outer coating layer 18b is a liquid crystal layer containing a discotic liquid crystal compound 30b and having the molecular axes of the discotic liquid crystal compound 30b aligned and fixed in one direction parallel to the main surface of the outer coating layer 18b. The molecular axis of the discotic liquid crystal compound 30b is an axis perpendicular to the disc surface. Fig. 9 As shown, the disc plane of the disc-like liquid crystal compound 30b is aligned perpendicularly to the major surface of the overcoat layer 18b.

[0141] Thus, the outer coating layer 18b becomes a discotic liquid crystal compound 30b with its molecular axis direction ( Fig. 9 The refractive index in the direction perpendicular to the molecular axis ( Figure 1 A layer with refractive index anisotropy having a different refractive index in a direction (perpendicular to the paper surface).

[0142] like Fig. 9 As shown, in the case where the outer coating layer includes a discotic liquid crystal compound, by aligning the azimuth direction of the incident wave with the direction of the molecular axis of the liquid crystal compound, as shown in FIG. Fig.10 As shown, the refractive index no of the disc-shaped liquid crystal compound 30b in the disc plane direction acts on the TE wave of the incident wave I0, and, as shown in FIG. Fig.11 As shown, the refractive index ne of the disc-shaped liquid crystal compound 30b in the direction perpendicular to the disc surface acts on the TM wave of the incident wave I0. Therefore, by appropriately selecting the disc-shaped liquid crystal compound contained in the outer coating layer, the refractive index relative to the TE wave and the refractive index relative to the TM wave can be appropriately set to appropriately set the maximum incident angle of the TE wave and the maximum incident angle of the TE wave, respectively, and the difference in the absorption characteristics of the TE wave and the TM wave incident from the oblique direction can be reduced. For example, by using a disc-shaped liquid crystal compound with a refractive index ne of 1.57 and a refractive index no of 1.87, the refractive index of the outer coating layer in the direction perpendicular to the incident surface is set to 1.87, and the refractive index in the direction parallel to the incident surface is set to 1.57, thereby the maximum incident angle of the TE wave becomes about 50°, and the maximum incident angle of the TM wave becomes about 48°, and the maximum incident angle of the TE wave and the maximum incident angle of the TM wave can be made close to each other.

[0143] Furthermore, the extinction coefficient of the base material layer is preferably 0.1 or more.

[0144] As described above, the electromagnetic wave absorber of the present invention has the function of absorbing electromagnetic waves by propagating electromagnetic waves in the substrate layer as one of the functions of absorbing electromagnetic waves. The absorption when the electromagnetic waves are propagated and absorbed in the substrate layer depends on the extinction coefficient of the substrate layer. The extinction coefficient is a parameter representing the loss of energy of light in a substance, and the larger the extinction coefficient, the greater the absorption of light when it travels in the substance. Therefore, the extinction coefficient of the substrate layer is preferably 0.1 or more, more preferably 0.2 or more.

[0145] The electromagnetic wave absorber 10 of the present invention is not limited to the wavelength of electromagnetic waves to be absorbed, and can absorb electromagnetic waves of various wavelengths including visible light. Among them, the electromagnetic wave absorber preferably absorbs the most electromagnetic waves within the range of 10 μm to 10 cm.

[0146] The electromagnetic wave absorber of the present invention has been described in detail above, but the present invention is not limited to the above-mentioned examples, and various improvements or changes can be made without departing from the gist of the present invention.

[0147] Example

[0148] The features of the present invention will be described in more detail below with reference to examples.

[0149] In addition, the embodiment shown below is an example showing an example of the present invention. Therefore, the present invention should not be limitedly interpreted by the specific examples shown below.

[0150] [Example 1]

[0151] [Production of electromagnetic wave absorber]

[0152] The electromagnetic wave absorber shown below was produced by optical simulation. The simulation was performed using COMSOL Multiphysics, a finite element method simulation software from COMSOL.

[0153] A substrate layer including a dielectric having a thickness of 40 μm, a refractive index of 1.53, and an extinction coefficient of 0.3 was prepared. A copper reflector having a thickness of 1 μm was arranged on the back side of the substrate layer. Furthermore, a microstructure layer having a copper microstructure having a thickness of 1 μm and a side of 250 μm was arranged on the front side. Figure 2 and Figure 3 Schematically shown in FIG. 1 , one unit cell is set to a square shape of 350 μm×350 μm, and one microstructure is arranged at the center of the unit cell. The design frequency of resonance is set to 300 GHz (wavelength 1000 μm).

[0154] An electromagnetic wave absorber was prepared by forming an outer coating layer with a thickness of 200 μm and an anisotropic refractive index having an in-plane birefringence Δn of 0.3 (ne=1.87, no=1.57) on the surface of the microstructure layer formed on the substrate layer on the opposite side to the substrate layer. Figure 7 In the xz plane, ne is parallel to the x-axis and no is parallel to the y-axis.

[0155] [Example 2]

[0156] An electromagnetic wave absorber was produced in the same manner as in Example 1 except that the birefringence Δn of the overcoat layer was set to -0.3 (ne=1.57, no=1.87).

[0157] [Comparative Example 1]

[0158] An electromagnetic wave absorber was produced in the same manner as in Example 1 except that the birefringence Δn of the overcoat layer was set to 0 (n=1.87).

[0159] [Comparative Example 2]

[0160] An electromagnetic wave absorber was produced in the same manner as in Example 1 except that the birefringence Δn of the overcoat layer was set to 0 (n=1.57).

[0161] [evaluate]

[0162] The incident angle of the incident wave incident on the electromagnetic wave absorber in the xz plane (incident plane) was changed in 10° increments, and the absorptivity was calculated by simulation. The frequency of the incident wave was in the range of 200GHz to 400GHz, and the absorptivity was calculated for TE waves and TM waves at 5GHz increments. Based on the calculated absorptivity, the maximum value of the incident angle at which the absorptivity became 97% or more at the frequency where the absorption at an incident angle of 0° became the maximum was obtained as the maximum incident angle. In addition, the difference between the maximum incident angle in the TE wave and the maximum incident angle in the TM wave was obtained.

[0163] Table 1 shows the results.

[0164] [Table 1]

[0165]

[0166] As can be seen from Table 1, in the electromagnetic wave absorber of Example 1, the maximum incident angles of TE and TM waves are both large, and the difference is as small as 4°. Also, in Example 2, the maximum incident angles of TE and TM waves are both small, and the difference is as small as 2°.

[0167] On the other hand, it can be seen that in Comparative Examples 1 and 2, the maximum incident angle of one of the TE wave and the TM wave is large, while the other is small, and the differences therebetween are as large as 8° and 15°, respectively.

[0168] Thus, it is found that the electromagnetic wave absorber of the present invention has a smaller difference in absorption characteristics between TE waves and TM waves than the comparative example, and that the electromagnetic wave absorber of the present invention can arbitrarily adjust the absorption angle range.

[0169] From the above results, the effects of the present invention are obvious.

[0170] Explanation of symbols

[0171] 10, 10b-electromagnetic wave absorber, 11-unit cell, 12-base material layer, 14-microstructure layer, 16-reflective layer, 18, 18b-overcoat layer, 20-microstructure, 30-(rod-shaped) liquid crystal compound, 30b-disc-shaped liquid crystal compound.

Claims

1. An electromagnetic wave absorber, comprising: Reflective layer; A substrate layer comprising at least one of a dielectric and a magnetic body; A microstructure layer composed of a plurality of microstructures; and External coating with anisotropic refractive index.

2. The electromagnetic wave absorber according to claim 1, wherein: The outer coating has a refractive index anisotropy at least in an in-plane direction.

3. The electromagnetic wave absorber according to claim 1, wherein: The birefringence Δn of the overcoat layer is greater than 0.

2.

4. The electromagnetic wave absorber according to claim 1, wherein: The overcoat layer includes a liquid crystal compound.

5. The electromagnetic wave absorber according to claim 1, wherein: The extinction coefficient of the substrate layer is greater than 0.

1.

6. The electromagnetic wave absorber according to claim 1, wherein: The wavelength of the electromagnetic wave absorbed by the electromagnetic wave absorber at which the electromagnetic wave is most absorbed is in the range of 10 μm to 10 cm.

Images