Electromagnetic wave absorption sheet

Abstract

Provided is an electromagnetic wave absorption sheet that has high electromagnetic wave absorption characteristics with respect to a wide frequency band of electromagnetic waves, and that has a small overall sheet thickness. This electromagnetic wave absorption sheet comprises: a magnetic layer 1 including a magnetic iron oxide 1a, a dielectric constant adjustment agent 1b, and a binder 1c; and an electromagnetic wave reflection layer 2. The magnetic iron oxide is hexagonal ferrite. The input impedance Z of the entire electromagnetic wave absorption sheet is 260 Ω or more and 610 Ω or less. A reflection attenuation curve representing the reflection attenuation characteristics of the sheet has a maximum absorption peak and at least one of a shoulder peak and a second absorption peak smaller than the maximum absorption peak in a frequency band in which the frequency of incident electromagnetic waves is 60 GHz or more and 90 GHz or less. A reflection attenuation amount at the maximum absorption peak is -15 dB or less, a reflection attenuation amount at the shoulder peak or the second peak is -10 dB or less, and a total film thickness of the electromagnetic wave absorption sheet is 500 μm or less.

Description

Electromagnetic wave absorbing sheet

[0001] The present disclosure relates to an electromagnetic wave absorbing sheet that absorbs unwanted electromagnetic waves, and in particular to an electromagnetic wave absorbing sheet that can effectively absorb electromagnetic waves in a wide frequency band above the millimeter wave band.

[0002] Electromagnetic wave absorbers that absorb electromagnetic waves are used to avoid the effects of leakage electromagnetic waves emitted to the outside from electric circuits, etc., and undesirably reflected electromagnetic waves. In recent years, the frequency of the electromagnetic waves used has shifted to higher frequency bands in order to accommodate larger capacity and faster communication speeds, and therefore there is a growing demand for electromagnetic wave absorbers that can absorb electromagnetic waves in higher frequency bands as well as those that absorb unwanted electromagnetic waves.

[0003] As an electromagnetic wave absorber that absorbs and suppresses the reflection of unnecessary electromagnetic waves, a so-called electromagnetic wave interference type (also called λ / 4 type or reflection type) is known, in which an electromagnetic wave reflective layer that reflects electromagnetic waves is provided on the back surface of a dielectric layer, and the phase of the electromagnetic waves reflected by the electromagnetic wave reflective layer and radiated to the outside is shifted by ½ wavelength from the phase of the electromagnetic waves reflected by the surface of the electromagnetic wave absorber, so that the electromagnetic waves reflected from the electromagnetic wave absorber cancel each other out and appear to be absorbed. As such an electromagnetic wave interference type electromagnetic wave absorber, an electromagnetic wave absorbing sheet has been proposed that has high flexibility and translucency and is capable of effectively absorbing electromagnetic waves in a desired frequency band (see Patent Document 1).

[0004] International Publication No. WO2018 / 163584

[0005] The above-mentioned conventional electromagnetic wave absorbing sheet is highly flexible due to the use of a conductive organic polymer film for the resistive film formed on the surface of the dielectric layer, and the resistive film and dielectric layer are constructed using translucent materials, and translucency is ensured by using a conductive mesh or the like with an opening ratio of 35% to 85% in the electromagnetic wave shielding layer, so that the electromagnetic wave absorbing sheet as a whole has a translucency with a total light transmittance of 30% or more while effectively absorbing electromagnetic waves in high frequency bands above the millimeter wave band.

[0006] However, electromagnetic interference-type electromagnetic wave absorbing sheets have the problem that, due to the principle of electromagnetic wave absorption, they only absorb a narrow frequency band of electromagnetic waves, and their absorption capacity drops sharply when the frequency of the incident electromagnetic waves changes slightly. Similarly, due to the principle of electromagnetic wave absorption, a dielectric layer with a thickness corresponding to the frequency of the electromagnetic waves to be absorbed is required. While the thickness of the dielectric layer used in electromagnetic interference-type electromagnetic wave absorbing sheets tends to decrease as the frequency of the electromagnetic waves to be absorbed increases, this has not been sufficient in light of the recent progress in miniaturization and weight reduction of electronic devices.

[0007] The present disclosure aims to solve the problems of the above-mentioned conventional electromagnetic wave absorbers and to provide an electromagnetic wave absorbing sheet that has high electromagnetic wave absorption properties for electromagnetic waves in a wide frequency band and has a thin overall sheet thickness.

[0008] In order to solve the above-mentioned problems, the present application discloses an electromagnetic wave absorbing sheet comprising: a magnetic layer containing magnetic iron oxide, a dielectric constant adjuster, and a binder; and an electromagnetic wave reflecting layer, wherein the magnetic iron oxide is hexagonal ferrite; the input impedance Z of the entire electromagnetic wave absorbing sheet is 260 Ω or more and 610 Ω or less; a reflection loss curve representing the reflection loss characteristics of the electromagnetic wave absorbing sheet has at least one of a maximum absorption peak, a shoulder peak, and a second absorption peak smaller than the maximum absorption peak in a frequency band of 60 GHz or more and 90 GHz or less in which the frequency of the incident electromagnetic wave is 60 GHz or more and 90 GHz or less; the reflection loss at the maximum absorption peak is −15 dB or less; and the reflection loss at the shoulder peak or the second peak is −10 dB or less; and the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less.

[0009] The electromagnetic wave absorbing sheet disclosed in the present application contains hexagonal ferrite as magnetic iron oxide in the magnetic layer, and has an input impedance Z of 260 Ω or more and 610 Ω or less, and a reflection attenuation curve having either a maximum absorption peak and a shoulder peak or a second absorption peak in the frequency band of 60 GHz or more and 90 GHz or less, thereby being able to effectively absorb electromagnetic waves in a wide frequency band.

[0010] FIG. 1 is a schematic diagram illustrating a general configuration of an electromagnetic wave absorbing sheet according to an embodiment of the present invention. FIG. 1 is a diagram illustrating a method for determining the half-width of a peak in a curve showing the change in the value of the imaginary part of the relative permeability of a magnetic layer depending on frequency. FIG. 2 is a diagram showing the change in the value of the imaginary part of the relative permeability of the magnetic layers of the prepared sheets 1 to 5 depending on frequency. FIG. 3 is a diagram showing the reflection attenuation curve of the prepared sheet 1. FIG. 4 is a diagram showing the first-order differential curve, second-order differential curve, and third-order differential curve of the reflection attenuation curve of the sheet 1. FIG. 5 is a diagram showing the reflection attenuation curve of the prepared sheet 2. FIG. 6 is a diagram showing the first-order differential curve, second-order differential curve, and third-order differential curve of the reflection attenuation curve of the sheet 2. FIG. 7 is a diagram showing the reflection attenuation curve of the prepared sheet 3. FIG. 8 is a diagram showing the first-order differential curve, second-order differential curve, and third-order differential curve of the reflection attenuation curve of the sheet 3. FIG. 9 is a diagram showing the reflection attenuation curve of the prepared sheet 4. FIG. 10 is a diagram showing the reflection attenuation curve of the prepared sheet 5. FIG. 11 is a diagram showing the first-order differential curve, second-order differential curve, and third-order differential curve of the reflection attenuation curve of the sheet 5.

[0011] The electromagnetic wave absorbing sheet disclosed in the present application is an electromagnetic wave absorbing sheet comprising: a magnetic layer containing magnetic iron oxide, a dielectric constant adjuster, and a binder; and an electromagnetic wave reflecting layer, wherein the magnetic iron oxide is hexagonal ferrite; the input impedance Z of the entire electromagnetic wave absorbing sheet is 260 Ω or more and 610 Ω or less; a reflection loss curve representing the reflection loss characteristics of the electromagnetic wave has at least one of a maximum absorption peak, a shoulder peak, and a second absorption peak smaller than the maximum absorption peak in a frequency band of 60 GHz or more and 90 GHz or less in which the frequency of the incident electromagnetic wave is 60 GHz or more and 90 GHz or less; the reflection loss at the maximum absorption peak is −15 dB or less; and the reflection loss at the shoulder peak or the second peak is −10 dB or less; and the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less.

[0012] By doing so, the electromagnetic wave absorbing sheet disclosed in the present application can have electromagnetic wave absorbing properties that allow it to effectively absorb electromagnetic waves in a wide frequency band.

[0013] In the electromagnetic wave absorbing sheet, it is preferable that the volume content of the magnetic iron oxide contained in the magnetic layer is 10.0% or more and 45.0% or less, thereby making it possible to obtain a favorable electromagnetic wave absorption effect by the magnetic iron oxide and a favorable value of the input impedance Z of the entire electromagnetic wave absorbing sheet due to the inclusion of the magnetic iron oxide.

[0014] Furthermore, it is preferable that the bandwidth of the frequency where the return loss is −15 dB or less in the return loss curve is 5 GHz or more, thereby achieving high electromagnetic wave absorption characteristics equivalent to an electromagnetic wave absorption rate of 90% over a wide frequency band.

[0015] Furthermore, it is preferable that the return loss of the incident electromagnetic wave in the frequency band of 76 GHz or more and 81 GHz or less is −15 dB or less. By doing so, it is possible to obtain an electromagnetic wave absorbing sheet that effectively absorbs electromagnetic waves in a practical frequency band.

[0016] Furthermore, the magnetic iron oxide is preferably hexagonal ferrite obtained by pulverization, which has a wide particle size distribution and can provide the effect of broadening the width of the return loss peak in the electromagnetic wave absorbing sheet.

[0017] Furthermore, in a curve showing the change in the value of the imaginary part of the relative permeability versus the frequency of the incident electromagnetic wave, the half-width of the peak in the incident electromagnetic wave frequency range of 60 GHz to 90 GHz is preferably 8 GHz or more. A wide half-width in the curve showing the change in the value of the imaginary part of the relative permeability results in a wider frequency band where the reactance component approaches 0 Ω, as in the particle size distribution, and the resistance component approaches 377 Ω, and the frequency band where the impedance of the electromagnetic wave absorbing sheet approaches 377 Ω, which is the characteristic impedance of air, is wider, and the reflection attenuation curve has a shoulder peak or multiple peaks.

[0018] Furthermore, it is preferable that the value of the imaginary part of the relative permeability at 79 GHz is 0.08 or more and 0.25 or less. By doing so, in the electromagnetic interference type radio wave absorbing sheet, the thickness of the layer when reflected waves that cancel each other out is generated becomes thin, and the thickness of the electromagnetic wave absorbing sheet can also be reduced.

[0019] Furthermore, it is preferable that the content of the dielectric constant adjuster in the magnetic layer is 2.0% by volume or more and 4.0% by volume or less, whereby the total thickness of the electromagnetic wave absorbing sheet is 500 μm or less and the impedance Z of the electromagnetic wave absorbing sheet can be set in the range of 260 Ω to 610 Ω.

[0020] Furthermore, it is preferable that a dielectric layer be formed between the magnetic layer and the reflective layer. This reduces the thickness of the magnetic layer when matching impedance with air, allowing for a lighter electromagnetic wave absorbing sheet. Furthermore, when the adhesion between the magnetic layer and the reflective layer is low, forming a dielectric layer between them is expected to improve the adhesion between the layers.

[0021] The electromagnetic wave absorbing sheet disclosed in the present application will be described below with reference to the drawings.

[0022] (Embodiment) FIG. 1 is a schematic diagram showing the configuration of an electromagnetic wave absorbing sheet according to this embodiment.

[0023] FIG. 1 is a diagram provided to facilitate understanding of the layered structure of the electromagnetic wave absorbing sheet according to this embodiment, and the thickness of each layer shown in the diagram is not based on reality.

[0024] The electromagnetic wave absorbing sheet exemplified in this embodiment comprises a magnetic layer 1 containing magnetic iron oxide 1a, a dielectric constant adjuster 1b, and a binder 1c, and an electromagnetic wave reflecting layer 2 formed on the back side of this magnetic layer 1, i.e., the side opposite to the side on which electromagnetic waves are incident.

[0025] The electromagnetic wave absorbing sheet according to this embodiment is of an electromagnetic wave interference type (also called a λ / 4 type or a reflection type), in which electromagnetic waves reflected by the surface of the magnetic layer 1 and electromagnetic waves transmitted through the magnetic layer 1 and reflected by the surface of the reflective layer 2 arranged on the back surface of the magnetic layer 1 cancel each other out, thereby attenuating the electromagnetic waves reflected from the electromagnetic wave absorbing sheet, thereby apparently absorbing the electromagnetic waves. The electromagnetic wave absorbing sheet according to this embodiment contains magnetic iron oxide 1a and a dielectric constant adjuster 1b in the binder, which is a dielectric, and thereby realizes the electromagnetic wave absorption effect of the magnetic iron oxide 1a, and the inclusion of magnetic iron oxide 1a and the dielectric constant adjuster 1b changes the relative permittivity and relative permeability of the magnetic layer 1, thereby realizing the effects of reducing the thickness of the electromagnetic wave absorbing sheet as a whole and widening the frequency bandwidth of the electromagnetic waves to be absorbed.

[0026] [Magnetic Layer] The magnetic layer 1 of the electromagnetic wave absorbing sheet according to this embodiment contains magnetic iron oxide 1a, a permittivity adjuster 1b, and a binder 1c.

[0027] Hexagonal ferrite can be used as the magnetic iron oxide 1a, and strontium ferrite and barium ferrite can be preferably used. In particular, lanthanum-substituted strontium ferrite is preferably used because its electromagnetic wave absorption properties change little with temperature changes.

[0028] The hexagonal ferrite used as the magnetic iron oxide 1a is preferably obtained by pulverization.

[0029] Hexagonal ferrite obtained by the pulverization method has irregular particle shapes and a large particle size distribution, while hexagonal ferrite obtained by the synthesis method has a substantially uniform spherical shape and a small particle size distribution. As a result, the magnetic resonance frequency of the hexagonal ferrite is dispersed, resulting in an effect of broadening the peak width of the return loss, which represents the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet. Note that by using hexagonal ferrite particles obtained by the pulverization method that have one or more protrusions on the surface, shape anisotropy is increased, resulting in dispersion of the resonance frequency, thereby further enhancing the effect of broadening the peak width of the return loss.

[0030] Here, "having one or more protrusions on the surface" refers to a shape that has sharp parts like the edge of a cross section that results from being crushed, or a part that is missing, as opposed to an approximately spherical body that has no sharp parts on the surface.

[0031] As described above, the hexagonal ferrite obtained by the pulverization method has a large particle size distribution, and therefore the change in the value of the imaginary part of the relative permeability with respect to frequency is gradual. Therefore, by analyzing the frequency characteristics of the value of the imaginary part of the relative permeability of the magnetic layer 1, the magnitude of the particle size distribution of the hexagonal ferrite can be grasped. In addition, the effect of widening the peak width of the return loss curve, which shows the frequency characteristics of the return loss, can be considered to be caused by the widening of the frequency characteristics of the value of the imaginary part of the complex relative permeability.

[0032] Furthermore, particularly with hexagonal ferrite obtained by pulverization, the value of the imaginary part of the relative permeability changes gradually, which also results in a gradual change in impedance that depends on changes in the value of the imaginary part of the relative permeability. As a result, with hexagonal ferrite obtained by pulverization, the reactance component approaches 0 Ω due to changes in impedance when the frequency changes, and the frequency band in which the resistance component approaches 377 Ω is widened. Therefore, the frequency band in which the impedance of the electromagnetic wave absorbing sheet approaches 377 Ω, the characteristic impedance of air, is widened, and the return loss of the electromagnetic wave absorbing sheet is reduced, resulting in a shoulder peak or multiple peaks in the return loss curve.

[0033] As the magnetic iron oxide 1a, in addition to the strontium ferrite and barium ferrite exemplified above, magnetoplumbite-type hexagonal ferrites such as calcium ferrite and red ferrite can be used. 3+ A part of (TiMn) 3+ and Al 3+ and the like can be used, and the anisotropic magnetic field (H A By adjusting the value of ), the magnetic resonance frequency f can be controlled between 5 and 150 GHz.

[0034] The hexagonal ferrite, which is the magnetic iron oxide 1a contained in the magnetic layer 1 of the electromagnetic wave absorbing sheet shown in this embodiment, can be produced by a conventionally known method, such as by going through a raw material mixing step in which raw material powders are mixed to obtain a raw material mixture, a firing step in which this raw material mixture is fired to obtain a fired product, and a grinding step in which the fired product is ground to obtain hexagonal ferrite magnetic powder.

[0035] In addition, epsilon iron oxide is known as a magnetic iron oxide that, like hexagonal ferrite, generates magnetic resonance when exposed to electromagnetic waves with a frequency of several tens of GHz. However, epsilon iron oxide is produced by a method combining the reverse micelle method and the sol-gel method, so the particle size variation is small, and the average particle size is on the order of several tens of nanometers. 50 Since the particle diameter of epsilon iron oxide is smaller than that of hexagonal ferrite, which is on the order of μm, the magnetic anisotropy and shape anisotropy are inherently small. Therefore, it is difficult to obtain the effects obtained when using hexagonal ferrite as magnetic iron oxide, such as the broadening of the return loss peak width as described above, the widening of the frequency band in which the reactance component approaches 0 Ω, and the resistance component approaches 377 Ω. For this reason, it is not preferable to use epsilon iron oxide as the magnetic iron oxide 1a used in the magnetic layer 1 of the electromagnetic wave absorbing sheet disclosed in the present application.

[0036] The dielectric constant adjuster 1b may be a carbon material such as carbon black, carbon nanotubes, or carbon nanostructures, or barium titanate, a conductive polymer, or a metal wire. If the dielectric constant adjuster 1b has an anisotropic shape, the anisotropy occurs, which changes the electromagnetic wave absorption characteristics depending on the direction of incidence on the electromagnetic wave absorbing sheet. Therefore, the dielectric constant adjuster 1b is preferably granular.

[0037] The binder 1c is preferably made of rubber or a thermoplastic elastomer.

[0038] Examples of rubber that can be used include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), urethane rubber (U), silicone rubber (Q), fluororubber (FKM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), and polysulfide rubber (T). Among these, acrylic rubber is preferred for its heat resistance. Silicone rubber (e.g., highly polymerized dimethyl silicone) is also preferred for its excellent heat and cold resistance, minimal temperature dependence of physical properties, solvent resistance, ozone resistance, and weather resistance. It also has excellent electrical insulation and is physically stable over a wide temperature range and frequency band.

[0039] As the thermoplastic elastomer, polystyrene-based TPSS, olefin / alkene-based TPO, polyvinyl chloride-based TPVC, polyester-based TPEE or TPC, polyurethane-based TPU, polyamide-based TPAE, etc. can be used.

[0040] In addition to the magnetic iron oxide 1a, the dielectric constant modifier 1b, and the binder 1c, the magnetic layer 1 may contain a dispersant. The dispersant may be a compound having a polar group such as a phosphate group, a sulfonic acid group, or a carboxy group.

[0041] Examples of phosphoric acid compounds having a phosphoric acid group include arylphosphonic acids such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, and polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrotrismethylenephosphonic acid. Examples of compounds having a polar group such as a carboxy group include aliphatic carboxylic acids having 12 to 18 carbon atoms, such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, and stearic acid.

[0042] The content of magnetic iron oxide 1a in the magnetic layer 1 is preferably 10.0% by volume or more and 45.0% by volume or less, and more preferably 17.5% by volume or more and 30.0% by volume or less.

[0043] If the content of magnetic iron oxide 1a is less than 10.0% by volume, the value of the imaginary part of the relative permeability (μr'') of the magnetic layer 1 becomes smaller than 0.08. As a result, it becomes impossible to achieve a return loss of RL≦−10 dB for the electromagnetic wave absorbing sheet at an electromagnetic wave frequency of 76 GHz to 81 GHz. Furthermore, if the content of magnetic iron oxide 1a is 17.5% by volume or more, it is possible to obtain an electromagnetic wave absorbing sheet with a return loss of RL≦−10 dB for the electromagnetic wave frequency of 76 GHz to 81 GHz.

[0044] If the content of magnetic iron oxide 1a is more than 45.0% by volume, the amount of binder and the amount of dielectric constant adjuster will be relatively small, which may make it impossible to form magnetic layer 1; and, as a result, the value of the imaginary part of the relative permeability (μr″) will be greater than 0.25, making it difficult to adjust the impedance of the electromagnetic wave absorbing sheet to around 377 Ω, and increasing reflection on the surface of the electromagnetic wave absorbing sheet, making it impossible to achieve a return loss of RL≦−10 dB for electromagnetic waves with frequencies of 76 GHz to 81 GHz.

[0045] The content of the dielectric constant modifier 1b in the magnetic layer 1 is preferably 2.0 to 4.0% by volume, and more preferably 2.0 to 3.5% by volume, in the magnetic layer 1. By setting the content of the dielectric constant modifier 1b in the magnetic layer 1 to be in the range of 2.0 to 4.0% by volume, the impedance Z of the electromagnetic wave absorbing sheet can be set in the range of 260 Ω to 610 Ω with a total thickness of the electromagnetic wave absorbing sheet being 500 μm or less.

[0046] The content of the binder 1c in the magnetic layer 1 can be set to a volume % obtained by excluding the above-mentioned magnetic iron oxide 1a and the dielectric constant adjuster 1b from the entire magnetic layer 1.

[0047] As a method for kneading the material of the magnetic layer 1, for example, known methods using a batch kneader, roll mill, continuous kneader, etc. can be used as appropriate. Furthermore, if the material of the magnetic layer 1 requires vulcanization, known methods using, for example, a hot press or a rotocure (continuous vulcanizer) can be used as appropriate. Furthermore, if the material of the magnetic layer 1 requires secondary vulcanization, known methods using, for example, a far-infrared vulcanization furnace (continuous) can be used as appropriate. If no vulcanizing agent is used, crosslinking can be performed using electron beam crosslinking or gamma ray crosslinking as appropriate. Furthermore, other methods such as injection molding, extrusion molding, and rolling can be used as appropriate to form the magnetic layer 1.

[0048] [Electromagnetic Wave Reflecting Layer] Metal foil is preferred as the electromagnetic wave reflecting layer 2, and various metal foils such as copper foil, aluminum foil, and gold foil can be used. Among these, considering the cost and the effect of oxidation in air, it is preferable to use aluminum foil as the electromagnetic wave reflecting layer 2. When the electromagnetic wave reflecting layer 2 is formed from metal foil, this can be achieved by rolling the metal material. Furthermore, when the electromagnetic wave reflecting layer 2 is formed from a vapor deposition film, it is preferable to appropriately select a vapor deposition method that has conventionally been used to form various vapor deposition films, depending on the metal material to be vapor deposited.

[0049] When the electromagnetic wave reflecting layer 2 is made of aluminum foil, for example, the thickness of the electromagnetic wave reflecting layer 2 is preferably 7 μm to 50 μm.

[0050] Furthermore, in the radio wave absorbing sheet according to this embodiment, the electromagnetic wave reflecting layer 2 can be formed of a vapor-deposited film of a conductive material such as a metal. In particular, by forming a vapor-deposited film of a metal material on one surface of the magnetic layer 1, there is no risk of a gap occurring between the magnetic layer 1 and the electromagnetic wave reflecting layer 2, compared to when the magnetic layer 1 and the electromagnetic wave reflecting layer 2 are formed separately and then arranged in close contact with each other, and therefore the electromagnetic waves that have penetrated the magnetic layer 1 can be reflected at the rear surface thereof.

[0051] On the other hand, when a vapor-deposited film is used for the electromagnetic wave reflecting layer 2, the density of the conductive material in the vapor-deposited film needs to be uniform and sufficient compared to when a metal foil is used for the electromagnetic wave reflecting layer 2. In this case, the surface resistance of the electromagnetic wave reflecting layer 2 is 1×10 -1It is preferable to make the surface resistance Ω / sq or less, and it is preferable to sufficiently control the thickness of the metal vapor deposition film so that the surface resistance is a predetermined value or less.

[0052] In addition to being realized as a metal vapor deposition film deposited on the back side of the magnetic layer 1, the electromagnetic wave reflecting layer 2 can also be realized by vapor depositing a metal material on the surface of a non-metallic flexible sheet-like member and positioning this vapor deposition surface in contact with the magnetic layer 1.

[0053] In addition, methods for laminating the magnetic layer 1 and the electromagnetic wave reflecting layer 2 include forming the magnetic layer 1 by applying a paint for forming the magnetic layer 1 to the electromagnetic wave reflecting layer 2 or a non-metallic flexible sheet associated with the electromagnetic wave reflecting layer 2, forming an adhesive layer between the magnetic layer 1 and the electromagnetic wave reflecting layer 2 which have been prepared separately and bonding them together, or bonding the magnetic layer 1 and the electromagnetic wave reflecting layer 2 together using an adhesive member such as adhesive tape.

[0054] When adopting a method of applying a magnetic layer paint to the electromagnetic wave reflective layer 2 or a non-metallic flexible sheet associated with the electromagnetic wave reflective layer 2, known devices such as a ball mill, bead mill, or disperser can be used as appropriate as a disperser for the magnetic layer paint. In addition, known methods such as comma coating, gravure coating, and die coating can be used as appropriate as the coating method.

[0055] [Dielectric Layer] Although not shown in Fig. 1, the electromagnetic wave absorbing sheet according to this embodiment can employ a configuration in which a dielectric layer is provided between the magnetic layer 1 and the electromagnetic wave reflecting layer 2. By providing a dielectric layer between the magnetic layer 1 and the electromagnetic wave reflecting layer 2, the thickness of the magnetic layer 1 can be reduced when matching impedance with air, allowing the electromagnetic wave absorbing sheet to be made lighter. Furthermore, when the adhesion between the magnetic layer 1 and the electromagnetic wave reflecting layer 2 is low, forming a dielectric layer between them can be expected to improve the adhesion between the layers.

[0056] In particular, when joining the magnetic layer 1 and the electromagnetic wave reflecting layer 2 described above, a non-metallic, flexible sheet is used as the substrate, the electromagnetic wave reflecting layer 2 is formed on the substrate, and with this substrate in place, the magnetic layer 1 is attached to the surface of the substrate opposite to the surface on which the electromagnetic wave reflecting layer 2 is formed, thereby making it possible to more easily and firmly bond the magnetic layer 1 and the electromagnetic wave reflecting layer 2. Also, as described above, even when the magnetic layer 1 and the electromagnetic wave reflecting layer 2 are bonded with double-sided tape or the like, the double-sided tape, which is a dielectric layer, is formed between the magnetic layer 1 and the electromagnetic wave reflecting layer 2.

[0057] In addition to joining the magnetic layer 1 and the electromagnetic wave reflecting layer 2, a dielectric layer can be disposed between the magnetic layer 1 and the electromagnetic wave reflecting layer 2. Even when a dielectric layer is disposed between the magnetic layer 1 and the electromagnetic wave reflecting layer 2, by providing the magnetic layer 1 containing the magnetic iron oxide 1a and the dielectric constant adjuster 1b in the binder, which is a dielectric, as described above, it is possible to achieve the effects of reducing the thickness of the electromagnetic wave absorbing sheet as a whole and of widening the frequency bandwidth of the electromagnetic waves to be absorbed.

[0058] The dielectric layer can be formed from various dielectric materials such as polyester resins (e.g., polyethylene terephthalate, polyethylene naphthalate, etc.), polycarbonate resins, polyacrylic ester resins (e.g., polymethyl methacrylate, etc.), alicyclic polyolefin resins, polystyrene resins (e.g., polystyrene, acrylonitrile-styrene copolymers, etc.), polyvinyl chloride resins, polyvinyl acetate resins, polyethersulfone resins, cellulose resins (e.g., diacetyl cellulose, triacetyl cellulose, etc.), norbornene resins, resins such as polyvinylidene fluoride, rubbers such as silicone rubber, OCA, OCR, etc. The dielectric constant of the dielectric layer is preferably 2 to 5, and the thickness is preferably 50 μm or less, more preferably 25 μm or less.

[0059] By setting the relative permittivity and thickness of the dielectric layer within the above ranges, the thickness of the magnetic layer can be reduced when matching impedance with air, resulting in a lighter electromagnetic wave absorbing sheet with good absorption performance and improved processability.

[0060] [Other Configurations] The electromagnetic wave absorbing sheet described in this embodiment may have a resistive film on the surface side of the magnetic layer 1. The resistive film is made of a conductive organic polymer.

[0061] As the conductive organic polymer used as the resistive film, a conjugated conductive organic polymer is used, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0062] 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.

[0063] The conductive organic polymer used in the resistive film 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. Examples of such anionic groups include groups represented by the general formulas -O-SO3X, -O-PO(OX)2, -COOX, and -SO3X (wherein 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.

[0064] Specific examples of such polyanions include polymers having sulfonic acid groups, such as polystyrene sulfonic acid, polyvinyl sulfonic acid, polyallyl sulfonic acid, polyacrylic sulfonic acid, polymethacrylic sulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, polysulfoethyl methacrylate, poly(4-sulfobutyl methacrylate), and polymethacryloxybenzenesulfonic acid, and polymers having carboxylic acid groups, such as polyvinyl carboxylic acid, polystyrene carboxylic acid, polyallyl carboxylic acid, polyacrylic carboxylic acid, polymethacrylic carboxylic acid, poly(2-acrylamido-2-methylpropanecarboxylic acid), polyisoprene carboxylic acid, and polyacrylic acid. These may be homopolymers or copolymers of two or more types. One type of polyanion may be used alone, or two or more types may be used in combination. Among these polyanions, polymers having sulfonic acid groups are preferred, and polystyrene sulfonic acid is more preferred.

[0065] The conductive organic polymer 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 light transmittance and conductivity.

[0066] 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.

[0067] Furthermore, in the resistive film according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and match the input impedance of the electromagnetic wave absorbing sheet to the impedance value in air. Examples of dopants that can be used include halogens such as iodine and chlorine, Lewis acids such as BF3 and PF5, protonic acids such as nitric acid and sulfuric acid, transition metals, alkali metals, amino acids, nucleic acids, surfactants, dyes, chloranil, tetracyanoethylene, and TCNQ. More specifically, the surface resistance of the resistive film is preferably set to a value within a range of plus or minus a few percent of 377 Ω. In this case, the blending ratio of the conductive organic polymer to the dopant can be, for example, a conductive polymer:dopant ratio of 1:2 to 1:4 by mass.

[0068] Furthermore, the material for forming the resistive film preferably also contains polyvinylidene fluoride.

[0069] When polyvinylidene fluoride is added to a composition used to coat a conductive organic polymer, it functions as a binder in the conductive organic polymer film, improving film-forming properties and adhesion to the substrate.

[0070] Furthermore, it is preferable that the resistive film further contains a water-soluble polyester. Because water-soluble polyesters have high compatibility with conductive polymers, adding a water-soluble polyester to the coating composition of the conductive organic polymer that forms the resistive film can fix the conductive polymer within the resistive film, forming a more homogeneous film. As a result, the use of a water-soluble polyester reduces changes in surface resistance even under harsher high-temperature, high-humidity environments, and allows the impedance to be maintained in a state where it matches the impedance value in air.

[0071] By including polyvinylidene fluoride and water-soluble polyester in the resistive film, the weather resistance of the resistive film is improved, thereby suppressing changes in the surface resistance value of the resistive film over time and realizing a highly reliable electromagnetic wave absorbing sheet that can maintain stable electromagnetic wave absorption characteristics.

[0072] The content of the conductive organic polymer in the resistive film is preferably 10% by mass or more and 35% by mass or less, based on the total mass of the solids contained in the resistive film composition. If the content is less than 10% by mass, the conductivity of the resistive film tends to decrease. Therefore, if the surface electrical resistance of the resistive film is set within a predetermined range to achieve impedance matching, the thickness of the resistive film increases, which tends to thicken the entire electromagnetic wave absorbing sheet and degrade optical properties such as translucency. On the other hand, if the content exceeds 35% by mass, the structure of the conductive organic polymer reduces the applicability when coating the resistive film, making it difficult to form a good resistive film. The haze of the resistive film also increases, which also tends to degrade optical properties.

[0073] The resistive film can be formed by applying a coating composition as a paint for forming a resistive film onto a substrate and drying it, as described above.

[0074] Examples of methods that can be used to apply the resistive coating paint to a 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 resistive coating tends to reduce its strength. 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.

[0075] The substrate used to form the resistive film is not particularly limited, but a transparent substrate having light transmissivity is preferred. Examples of materials that can be used for such a transparent substrate include resin, rubber, glass, and ceramics.

[0076] In the electromagnetic wave absorbing sheet of this embodiment, the above-mentioned conductive organic polymer is used to form a resistive film with a surface resistance value of 377 Ω / sq, which makes it possible to match the impedance of the electromagnetic waves incident on the electromagnetic wave absorbing sheet with the impedance in the air, thereby reducing the reflection and scattering of electromagnetic waves on the surface of the electromagnetic wave absorbing sheet and achieving better electromagnetic wave absorption characteristics.

[0077] Furthermore, when a resistive film is formed on an electromagnetic wave absorbing sheet, it is preferable to provide a protective layer as a member for protecting the resistive film on the outermost surface (surface of the resistive film) on the side where the electromagnetic waves are incident. By providing a protective layer, it is possible to suppress changes in the surface resistance value due to the influence of humidity in the air, etc. It is also possible to prevent the resistive film from being damaged by external forces.

[0078] A resin sheet such as polyethylene terephthalate can be used as the protective layer. The resin sheet used as the protective layer has a certain resistance value, but by setting the thickness of the protective layer thin, the effect of the presence or absence of the protective layer on the surface resistance value of the electromagnetic wave absorbing sheet can be reduced to a level that does not cause practical problems. The thickness of the protective layer is preferably 10 μm or more and 100 μm or less. If the protective layer is thinner than 10 μm, the effect of protecting the resistive film is reduced. On the other hand, if it is thicker than 100 μm, the thickness of the entire electromagnetic wave absorbing sheet becomes thick. Furthermore, the effect on the surface resistance value of the electromagnetic wave absorbing sheet becomes non-negligible, which may affect the electromagnetic wave absorption characteristics.

[0079] In the electromagnetic wave absorbing sheet according to this embodiment, an adhesive layer can be provided on the surface of the electromagnetic wave reflecting layer 2 opposite to the magnetic layer 1 .

[0080] By providing an adhesive layer, the electromagnetic wave absorbing sheet can be easily attached to a predetermined location. The adhesive layer can be easily formed by applying an adhesive resin paste. Materials for the adhesive layer include acrylic adhesives, rubber adhesives, and silicone adhesives. The adhesive layer can also be made of adhesive tape such as acrylic adhesive tape or silicone adhesive tape.

[0081] [Characteristics of the Electromagnetic Wave Absorbing Sheet] The overall thickness of the electromagnetic wave absorbing sheet according to this embodiment, i.e., the total film thickness, is 500 μm or less. If the total film thickness of the electromagnetic wave absorbing sheet is greater than 500 μm, the mass of the electromagnetic wave absorbing sheet increases and its flexibility decreases, making it difficult to arrange the electromagnetic wave absorbing sheet according to the shape of the desired location. There is no particular lower limit for the total film thickness of the electromagnetic wave absorbing sheet, but from the viewpoint of the durability of the sheet, i.e., from the viewpoint of preventing it from easily breaking during handling, it is preferable that the total film thickness be 50 μm or more.

[0082] The relative permittivity of the magnetic layer 1 of the electromagnetic wave absorbing sheet according to this embodiment at room temperature is such that the real part (ε′) at a frequency of 79 GHz is preferably 8.5 or more and 18.0 or less, and more preferably 9.4 or more and 13.0 or less. The imaginary part (ε″) of the relative permittivity is preferably 1.0 or more and 3.5 or less, and more preferably 1.2 or more and 3.5 or less.

[0083] The relative permeability of the magnetic layer 1 of the electromagnetic wave absorbing sheet at room temperature is preferably such that the real part (μr') is 0.70 or more and 0.90 or less, and more preferably 0.75 or more and 0.90 or less, at a frequency of 79 GHz. The imaginary part (μr'') of the relative permeability is preferably 0.08 or more and 0.25 or less, and more preferably 0.10 or more and 0.25 or less.

[0084] When the relative permittivity or relative permeability in a medium is greater than 1, the electromagnetic wave propagation speed in the medium is slower than the propagation speed in a vacuum, resulting in a shortening of the electromagnetic wave wavelength in the medium. Therefore, by adjusting the relative permittivity and relative permeability values ​​within the above-mentioned ranges, it is possible to reduce the thickness difference between the layers required as magnetic layers when reflected waves that cancel each other out are generated, and it is possible to reduce the thickness of the electromagnetic wave absorbing sheet.

[0085] The values ​​of the real part of the relative permittivity (ε'), the imaginary part of the relative permittivity (ε"), the real part of the relative magnetic permeability (μ'), and the imaginary part of the relative magnetic permeability (μ") of the magnetic layer 1 of the electromagnetic wave absorbing sheet described above all vary mainly depending on the amount of binder 1c, the content of magnetic powder 1a, and the content and type of dielectric constant modifier 1b contained in the magnetic layer 1. When the values ​​of the real part of the relative permittivity (ε'), the imaginary part of the relative permittivity (ε"), the real part of the relative magnetic permeability (μ'), and the imaginary part of the relative magnetic permeability (μ") are changed by changing the amount of binder 1c, the content of magnetic powder 1a, and the content of dielectric constant modifier 1b, for example, changing the amount of binder 1c also changes the amount of magnetic powder 1a other than binder 1c and the amount of dielectric constant modifier 1b in terms of their content in the magnetic layer 1, making it difficult to determine which parameter in the electromagnetic wave absorbing sheet is affected by the content of binder 1c alone.

[0086] As described above, the real part (μr') and imaginary part (μr'') of the relative permeability also depend on the particle size distribution of the magnetic iron oxide 1a, and the larger the particle size distribution of the magnetic iron oxide 1a, the wider the width of the peak of the relative permeability versus frequency.

[0087] From this, by analyzing the curve that shows the change in the value of the imaginary part of the relative permeability (μr'') with respect to frequency, it is possible to obtain information about the particle size distribution of the magnetic iron oxide contained in the magnetic layer of the electromagnetic wave absorbing sheet. As a result of the inventors' studies, it was found that by taking the half-width of the peak shape from the graph that shows the change in the value of the imaginary part of the relative permeability (μr'') with respect to frequency, it is possible to grasp the magnitude (width) of the distribution of the particle sizes of the hexagonal ferrite and the resulting width of the frequency bandwidth that causes magnetic resonance.

[0088] FIG. 2 shows a method for determining the half-width of the peak from a curve showing the relationship between the value of the imaginary part of the relative permeability (μr″) of the magnetic layer and frequency.

[0089] As shown in FIG. 2, a curve 21 showing the change in the value of the imaginary part of relative permeability (μr″) with respect to frequency has a relatively gentle upward convex peak.

[0090] First, the apex of this peak is defined as P. Next, the baseline 22 of the peak is defined as a bitangent line that touches two points below the curve 21.

[0091] If curve 21 has downwardly convex portions on both sides of the peak, baseline 22 is the common tangent near these two downwardly convex portions. On the other hand, if no downwardly convex portions appear on curve 21 in the predetermined frequency range (60 GHz to 90 GHz in the case of Figure 2) for which the half-width is to be considered, the baseline 22 is the value at the upper end (90 GHz in the case of Figure 2) or lower end (60 GHz) of the frequency range of curve 21. In the case of Figure 2, in the frequency range from 60 GHz to 90 GHz, a downwardly convex portion appears on the lower frequency side, but no downwardly convex portion appears on the higher frequency side. Therefore, baseline 22 is the straight line that passes through the value of curve 21 at 90 GHz, the upper end of the frequency range, and is tangent to curve 21 at the downwardly convex portion formed in the lower frequency portion. If no downwardly convex portions appear on curve 21 in the predetermined frequency range, baseline 22 is the line connecting the upper and lower end values ​​of the frequency range.

[0092] Next, let S be the point where a perpendicular line 23 drawn from the apex P of the peak to the x-axis intersects with the baseline 22. The frequency values ​​T1 and T2 are found at the point where a line 24 passing through the midpoint M between P and S and parallel to the x-axis intersects with the curve 21, and the value of T2 - T1 becomes the half-width of the peak on the curve showing the relationship between the value of the imaginary part of the relative permeability (μr'') in this magnetic layer and frequency.

[0093] It is preferable that the half-width of the peak in the curve showing the relationship between the value of the imaginary part of the relative permeability (μr″) in the magnetic layer and frequency is 8 (GHz) or more, and more preferably 9.8 (GHz) or more. If the half-width is 8 GHz or more, the frequency band in which the reactance component approaches 0Ω and the resistance component approaches 377Ω due to the change in impedance when the frequency of the electromagnetic waves incident on the electromagnetic wave absorbing sheet changes will be wider. As a result, the frequency band in which the impedance of the electromagnetic wave absorbing sheet approaches 377Ω, which is the characteristic impedance of air, will be wider, and it will be possible to obtain an electromagnetic wave absorbing sheet with good electromagnetic wave absorption properties over a wide frequency band.

[0094] The predetermined frequency range in examining the curve showing the relationship between the value of the imaginary part of the relative permeability (μr″) in the magnetic layer and frequency is preferably a frequency range (for example, ±10 to 20 GHz around the center frequency) that includes the frequency range of electromagnetic waves that can become noise (used in millimeter-wave devices) and in which absorption performance can be sufficiently confirmed, from the perspective of having good electromagnetic wave absorption characteristics over a wide range of frequencies around the center frequency. In the electromagnetic wave absorbing sheet of this embodiment, the center frequency of the electromagnetic waves absorbed by the electromagnetic wave absorbing sheet is set to 79 GHz, particularly assuming use in applications such as automotive radar, and therefore the frequency range in which the half-width is examined is set to 60 GHz or more and 90 GHz or less, as described above. Furthermore, when the frequency range in which the half-width is examined is changed, the preferred value of the half-width is preferably set so that the frequency bandwidth in which the return loss is −15 dB or less in the frequency range of ±15 to 20 GHz around the center frequency is 5 GHz or more.

[0095] As explained above, the amounts of binder 1c, magnetic powder 1a, and permittivity adjuster 1b contained in magnetic layer 1, as well as the particle size and particle size distribution of magnetic iron oxide 1a, are intricately related to each other, and these determine the values ​​of the real part of the relative permittivity (εr'), the imaginary part of the relative permittivity (εr''), the real part of the relative permeability (μr'), and the imaginary part of the relative permeability (μr'') of the electromagnetic wave absorbing sheet. For this reason, in the electromagnetic wave absorbing sheet disclosed in the present application, an electromagnetic wave absorbing sheet that can achieve desired electromagnetic wave absorption characteristics based on the shape of the return loss curve that indicates the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet is defined as having at least one of a maximum absorption peak and a shoulder peak or a second absorption peak that is smaller than the maximum absorption peak in the frequency band of 60 GHz or more and 90 GHz or less of the electromagnetic waves to be absorbed, and having a return loss at the maximum absorption peak of −15 dB or less and a return loss at the shoulder peak or the second peak of −10 dB or less.

[0096] Furthermore, as described above, the electromagnetic wave absorbing sheet disclosed in the present application has a content of magnetic iron oxide 1a in the magnetic layer 1 of 10.0 vol % or more and 45.0 vol % or less, a content of dielectric constant adjuster 1b of 2.0 vol % or more and 4.0 vol % or less, with the remainder being binder 1c, and further has values ​​of the real part of the dielectric constant (εr'), the imaginary part of the dielectric constant (εr''), the real part of the relative magnetic permeability (μr'), and the imaginary part of the relative magnetic permeability (μr'') within the above-mentioned numerical ranges, and by making the total thickness of the electromagnetic wave absorbing sheet 500 μm or less, the impedance Z of the electromagnetic wave absorbing sheet is in the range of 260 Ω to 610 Ω.

[0097] Since the impedance Z of the entire electromagnetic wave absorbing sheet including the magnetic layer 1 and the electromagnetic wave reflecting layer 2 changes depending on the frequency of the incident electromagnetic waves, impedance data for frequencies in the frequency range of 76 to 81 GHz is obtained, and based on that data, the impedance value Z of the electromagnetic wave absorbing sheet is selected to be 260 Ω or more and 610 Ω or less.

[0098] Furthermore, the above conditions also apply to a configuration in which a dielectric layer is formed between the magnetic layer 1 and the electromagnetic wave reflecting layer 2 .

[0099] [Electromagnetic Wave Absorption Characteristics] Hereinafter, the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet according to this embodiment will be described.

[0100] The electromagnetic wave absorbing sheet according to this embodiment is understood as a return loss RL, which indicates how much the energy of the electromagnetic wave reflected by the electromagnetic wave absorbing sheet is reduced relative to the energy of the electromagnetic wave incident on the front magnetic layer 1. Furthermore, the change in the return loss with the change in the frequency of the input electromagnetic wave is understood as a return loss curve.

[0101] The electromagnetic wave absorbing sheet disclosed in the present application has a maximum absorption peak in a reflection attenuation curve in a frequency band of 60 GHz or more and 90 GHz or less, and also has at least one of a shoulder peak and a second absorption peak having an absorption amount smaller than that of the maximum absorption peak.

[0102] First, the maximum absorption peak in the reflection attenuation curve of the electromagnetic wave absorbing sheet in this application means the frequency at which the first-order differential curve of the reflection attenuation curve changes from negative to positive as the frequency increases, within the frequency range of the incident electromagnetic wave from 60 GHz to 90 GHz, at which the absolute value of the reflection attenuation is greatest.

[0103] Furthermore, a shoulder peak refers to a peak at which the third-order derivative of the reflection attenuation curve changes from positive to negative as the frequency increases, and which does not correspond to a frequency at which the first-order derivative of the reflection attenuation curve changes from negative to positive, among frequencies at which the second-order derivative of the reflection attenuation curve reaches a positive maximum value.

[0104] Furthermore, the second absorption peak smaller than the maximum absorption peak refers to a portion of the frequency at which the first-order differential curve of the return loss curve changes from negative to positive with increasing frequency, and at which the absolute value of the return loss is smaller than that of the maximum absorption peak.

[0105] The electromagnetic wave absorbing sheet disclosed in the present application has a reflection loss curve in a frequency band of 60 GHz or more and 90 GHz or less, which has at least one of a second absorption peak and a shoulder peak in addition to the maximum absorption peak defined above, and further has a reflection loss at the maximum absorption peak of -15 dB or less and a reflection loss at the shoulder peak or the second peak of -10 dB or less.

[0106] The electromagnetic wave absorbing sheet disclosed in the present application has a reflection attenuation curve that has either a maximum absorption peak and a shoulder peak or a second peak in the frequency band of 60 GHz or more and 90 GHz or less, thereby achieving electromagnetic wave absorption performance that effectively absorbs electromagnetic waves over a wide bandwidth.

[0107] The third-order differential curve used in defining the electromagnetic-wave-absorbing sheet of the present invention is particularly an effective means for understanding the presence of a shoulder peak that appears at a position different from the maximum absorption peak in a reflection attenuation curve. The third-order differential curve makes it possible to reliably detect a shoulder peak, which is a small peak within an absorption peak that can be detected by first-order differentiation.

[0108] The above-mentioned attenuation of -15 dB at the maximum absorption peak corresponds to an attenuation rate of 97%, and can be evaluated as extremely good absorption of electromagnetic waves. Furthermore, the attenuation of -10 dB at the shoulder peak and second absorption peak corresponds to an attenuation rate of 90%, and can be judged to be sufficient numerical values ​​in terms of electromagnetic wave absorption characteristics. In the electromagnetic wave absorbing sheet disclosed in the present application, by specifying the attenuation at the maximum peak to be -15 dB or less and the attenuation at the shoulder peak and second absorption peak to be -10 dB or less, it is possible to obtain an electromagnetic wave absorbing sheet that can well attenuate (absorb) electromagnetic waves of the relevant frequencies.

[0109] In this specification, the magnitude of the return loss is expressed based on the magnitude of the actual numerical value of the return loss expressed in dB. That is, a return loss of -15 dB is smaller than a return loss of -10 dB, and the maximum absorption peak is the smallest return loss expressed in dB. min Similarly, a return loss of −15 dB or less means that the return loss value is less than −15, for example, −20.

[0110] The electromagnetic wave absorbing sheet disclosed in the present application aims to effectively absorb electromagnetic waves in the millimeter wave band over a wide frequency band. Here, the criteria for determining whether or not a sheet "effectively absorbs electromagnetic waves over a wide frequency band" will be described.

[0111] The inventors have used the minimum return loss (RL) as an indicator that the electromagnetic wave absorbing sheet disclosed in the present application effectively absorbs electromagnetic waves in a wide frequency band. min They found that this can be expressed as the absolute value of "minimum value of return loss / bandwidth where return loss is -15 dB or less" using the frequency band width (Freq. range: unit: dB) where the return loss is -15 dB or less (Freq. range: unit: GHz), and they determined that an electromagnetic wave absorbing sheet that has an absolute value of this "minimum value of return loss / bandwidth where return loss is -15 dB or less" of 4.00 or less in the frequency range of 60 GHz or more and 90 GHz or less can be evaluated as being able to effectively absorb electromagnetic waves in a wide frequency band.

[0112] More specifically, in the frequency range of 60 GHz or more and 90 GHz or less, the minimum value of the return loss (RL min When the minimum return loss (RL) is smaller than 20 dB, an electromagnetic wave absorbing sheet having an absolute value of 4.00 or less of "minimum return loss / bandwidth where return loss is -15 dB or less" can be evaluated as an extremely good electromagnetic wave absorbing sheet that can exhibit excellent electromagnetic wave absorption characteristics with a return loss of -15 dB or less over a wide frequency band. min ) is greater than −20 dB, if the absolute value of “minimum value of return loss / bandwidth where return loss is −15 dB or less” is 4.00 or less, the electromagnetic wave absorbing sheet can be evaluated as exhibiting sufficient electromagnetic wave absorption performance over a wide bandwidth.

[0113] Furthermore, when the absolute value of "minimum value of return loss / bandwidth where return loss is -15 dB or less" is greater than 4.00, the width of the frequency band where the return loss of the electromagnetic wave absorbing sheet is -15 dB or less becomes narrower, the return loss curve showing the frequency characteristics of the return loss has a sharp absorption peak, and the absorption characteristics for frequencies shifted from the top frequency of the absorption peak become worse. For this reason, when the frequency of the electromagnetic waves incident on the electromagnetic wave absorbing sheet changes, the amount of electromagnetic waves absorbed will be significantly reduced.

[0114] From these verification results, it can be evaluated that the index of the absolute value of "minimum return loss / bandwidth where return loss is -15 dB or less" is a parameter that represents the balance between the spread of the electromagnetic wave absorption peak by the electromagnetic wave absorbing sheet and the depth of the absorption peak.

[0115] From a practical standpoint, it is preferable that the frequency bandwidth over which the electromagnetic wave absorbing sheet has an electromagnetic wave return loss of -15 dB or less is 5 GHz or more. By having a bandwidth of 5 GHz or more in the range of -15 dB or less, which is a good electromagnetic wave absorption amount (97% attenuation), there is no significant change in the electromagnetic wave absorption amount even when the frequency of the electromagnetic waves incident on the electromagnetic wave absorbing sheet changes, and good electromagnetic wave absorption characteristics can be maintained.

[0116] Furthermore, the value of "bandwidth where return loss is -15 dB or less" is preferably wider, but the upper limit is about 30 GHz, and may be about 20 GHz. Also, the value of "minimum return loss" is preferably smaller, but the lower limit is about -50 dB, so the lower limit of the absolute value of "minimum return loss / bandwidth where return loss is -15 dB or less" is about 0.66.

[0117] Furthermore, when an electromagnetic wave absorbing sheet is used for the purpose of absorbing undesired leakage and reflection of electromagnetic waves in an in-vehicle millimeter-wave radar, the frequency band of the electromagnetic waves used is 76 GHz or more and 81 GHz or less, and therefore, a return loss of -15 dB or less in this frequency band can be determined to be preferable for an electromagnetic wave absorbing sheet to be used in equipment related to in-vehicle radar.

[0118] The absolute value of the "minimum value of return loss / bandwidth where return loss is -15 dB or less" used as the index is largely influenced mainly by the value of the impedance Z of the entire electromagnetic wave absorbing sheet. As described above, the impedance Z of the electromagnetic wave absorbing sheet is determined by the values ​​of the real part of the relative permittivity (εr'), the imaginary part of the relative permittivity (εr''), the real part of the relative permeability (μr'), and the imaginary part of the relative permeability (μr'') of the electromagnetic wave absorbing sheet, as well as the thickness of the electromagnetic wave absorbing sheet. However, what has the greatest effect on the "minimum value of return loss / bandwidth where return loss is -15 dB or less" is the value of the imaginary part of the relative permeability (μr'') of the magnetic layer 1 of the electromagnetic wave absorbing sheet. Therefore, it is preferable to design the electromagnetic wave absorbing sheet so that the value of the imaginary part of the relative permeability (μr'') is appropriate, taking into consideration the material and content ratio of the magnetic iron oxide 1a used for the magnetic layer 1a.

[0119] EXAMPLES Hereinafter, a specific example will be described in which five electromagnetic wave absorbing sheets, Sheet 1 to Sheet 5, were actually produced for the electromagnetic wave absorbing sheet described in this embodiment, and their electromagnetic wave absorption characteristics were measured.

[0120] Table 1 shows the structure of each sheet and a list of the measurement results.

[0121]

[0122] <Preparation of Electromagnetic Wave Absorbing Sheet> (Magnetic Layer) In Sheets 1 to 5, the magnetic iron oxide contained in the magnetic layer has a particle diameter D 50 Strontium ferrite (Sr ferrite) having a diameter of 3.1 μm was used, and the content was 25.3 vol % for Sheet 1, 27.5 vol % for Sheet 2, 20.2 vol % for Sheet 3, 30.0 vol % for Sheet 4, and 27.3 vol % for Sheet 5.

[0123] The dielectric constant adjuster used in all cases was conductive carbon black (CB: Lionite (registered trademark) CB (product name) manufactured by Lion Specialty Chemicals Co., Ltd.), with a carbon black content of 3.5 vol % for Sheet 1, 4.0 vol % for Sheet 2, 3.5 vol % for Sheet 3, 3.5 vol % for Sheet 4, and 3.0 vol % for Sheet 5.

[0124] In all of Sheets 1 to 5, silicone rubber (product name "KE-951KU" manufactured by Shin-Etsu Chemical Co., Ltd.) was used as the binder.

[0125] These materials were mixed with 2.5-dimethyl-2.5-di(t-butylperoxy)hexane (product name: C-8A, manufactured by Shin-Etsu Chemical Co., Ltd.) as a vulcanizing agent and stearic acid (product name: Lunac S-50V, manufactured by Kao Corporation) and fatty acid amide (product name: Plastron S, manufactured by Fuso Chemical Co., Ltd.) as processing aids, and then kneaded in a batch kneader at 35 rpm while running cold water to prevent excessive temperature rise to produce a compound. The compound was dissolved in a planetary mixer using toluene as a solvent to produce a coating material, which was then applied to the substrate using a comma coater to form a magnetic layer. Substrates that can be used include aluminum foil, aluminum foil and PET composite films, and aluminum-evaporated films. Specifically, aluminum foil was used as the substrate for Sheets 1, 2, and 4, and aluminum foil and PET composite films for Sheets 3 and 5. The resulting sheets were annealed at 200°C for 4 hours to form the magnetic layer.

[0126] The thickness of the magnetic layer of Sheet 1 was 270 μm, that of Sheet 2 was 300 μm, that of Sheet 3 was 232 μm, that of Sheet 4 was 328 μm, and that of Sheet 5 was 262 μm. In order to measure the relative permittivity and relative permeability, a compound prepared in the same manner as above was vulcanized in a hot press at 165° C. for 7 minutes to prepare a sheet with a thickness of 1 mm.

[0127] The results of measurements of the relative permeability of sheets 1 to 5 thus constructed are shown below.

[0128] FIG. 3 is a diagram showing the relationship between the value of the imaginary part of relative permeability (μr″) and frequency for each sheet produced as an example.

[0129] In Figure 3, the change in the value of the imaginary part of the relative permeability (μr'') of the magnetic layer of sheet 1 is indicated by reference numeral 31, the change in the value of the imaginary part of the relative permeability (μr'') of the magnetic layer of sheet 2 is indicated by reference numeral 32, the change in the value of the imaginary part of the relative permeability (μr'') of the magnetic layer of sheet 3 is indicated by reference numeral 33, the change in the value of the imaginary part of the relative permeability (μr'') of the magnetic layer of sheet 4 is indicated by reference numeral 34, and the change in the value of the imaginary part of the relative permeability (μr'') of the magnetic layer of sheet 5 is indicated by reference numeral 35.

[0130] The imaginary part of magnetic permeability (μr″) at 79 GHz of the magnetic layer of each sheet was 0.120 for sheet 1, 0.114 for sheet 2, 0.111 for sheet 3, 0.132 for sheet 4, and 0.187 for sheet 5.

[0131] In addition, the half-value widths (GHz) of the imaginary part of the relative permeability (μr″) of the magnetic layer of each sheet, as determined by the method using FIG. 2 above, were 10.9 for Sheet 1, 10.5 for Sheet 2, 13.5 for Sheet 3, 10.2 for Sheet 4, and 15.1 for Sheet 5, as shown in FIG. 3.

[0132] (Electromagnetic wave reflecting layer and electromagnetic wave absorbing sheet) In both sheets, the aluminum substrate served as the electromagnetic wave reflecting layer, and its thickness was 10 μm. The thickness of the dielectric layer of sheet 3 was 12 μm, and the thickness of the dielectric layer of sheet 5 was 25 μm.

[0133] The total thickness of the electromagnetic wave absorbing sheets was 280 μm for Sheet 1, 310 μm for Sheet 2, 254 μm for Sheet 3, 338 μm for Sheet 4, and 297 μm for Sheet 5.

[0134] <Evaluation of Characteristics of Electromagnetic Wave Absorbing Sheet> (Measurement Method) The return loss (RL) as the amount of electromagnetic wave absorption was measured for the five experimentally produced electromagnetic wave absorbing sheets using the free space method.

[0135] The free space method measurements were performed using a free space measurement device DPS24-01 (product name) manufactured by Keycom Corporation and a vector network analyzer MS46522B (product name) manufactured by Anritsu Corporation. Using two horn antennas, an input wave (millimeter wave) of a predetermined frequency was irradiated perpendicularly over a 100 mmφ area onto a sample electromagnetic wave absorbing sheet fabricated from a transmitting and receiving antenna via a dielectric lens. The reflected wave from the sample at this time was measured, and the intensity of the input wave was compared with the intensity of the reflected wave to determine the return loss RL, which is the degree of attenuation, in dB. More specifically, the frequency range was 60 GHz to 90 GHz, and 801 measurement points were set up to measure the electromagnetic wave return loss (S11) at normal incidence.

[0136] The relative permittivity, relative permeability, and impedance values ​​of the sample were also measured by irradiating the sample with electromagnetic waves perpendicularly using the free space method as described above, and the relative permittivity and relative permeability were calculated from the transmission characteristics (phase and amplitude) of the electromagnetic waves in a single magnetic layer, and the impedance value Z was calculated from the reflection characteristics (S parameters) of the electromagnetic waves in an electromagnetic wave absorbing sheet including an electromagnetic wave reflecting layer. The impedance value was measured at a frequency of 79 GHz.

[0137] The relative permittivity was measured by measuring the amplitude and phase of the transmitted electromagnetic wave (S21) at normal incidence in the frequency band from 55 GHz to 65 GHz, and calculating the average relative permittivity (real part, imaginary part) using frequency variation method software manufactured by Keycom Corporation.

[0138] The relative permeability was measured by measuring the amplitude and phase of the transmitted electromagnetic wave (S21) at normal incidence in the frequency band from 60 GHz to 90 GHz, and similarly to the relative permittivity, the relative permeability (real part, imaginary part) was calculated from the phase, amplitude, and average relative permittivity from 55 GHz to 65 GHz using permeability calculation software manufactured by Keycom Corporation.

[0139] The impedance was measured in a measurement frequency range of 60 GHz to 90 GHz with normal incidence.

[0140] (Measurement Results) Hereinafter, the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheets 1 to 5 produced as described above will be explained, showing the reflection attenuation curves, which are the frequency characteristics of the reflection attenuation amount for each sheet, and the first-order differential curves, second-order differential curves, and third-order differential curves of these reflection attenuation curves.

[0141] FIG. 4 shows the reflection loss curve of sheet 1.

[0142] In the return loss curve of the sheet 1 shown by reference numeral 41 in FIG. 4, the return loss RL is equal to or less than -19.20 dB in the range of 76 GHz to 81 GHz, and it was confirmed that the sheet 1 can absorb electromagnetic waves in a practical frequency range well.

[0143] In the electromagnetic wave absorbing sheet 1 shown in FIG. 4, the broad index indicating the width of the electromagnetic wave absorption characteristics is as follows: the bandwidth where the return loss is −15 dB or less is 14.66, the minimum value RL of the return loss in the frequency band of 60 to 90 GHz is 14.66, min is −26.32 dB, and the absolute value of the broad index value, “minimum value of return loss / bandwidth where return loss is −15 dB or less”, is 1.80.

[0144] In the return loss curve indicated by reference numeral 41 in FIG. 4, a peak value of the return loss exists at a frequency of 74.70 GHz, and a shoulder peak exists at a frequency of 79.50 GHz.

[0145] FIG. 5 is a diagram showing the first, second and third derivative curves of the reflection attenuation curve of the sheet 1 shown in FIG.

[0146] 5, reference numeral 51 denotes the reflection attenuation curve of the sheet 1, reference numeral 52 denotes the first-order differential curve of the reflection attenuation curve of reference numeral 51, reference numeral 53 denotes the second-order differential curve of the reflection attenuation curve of reference numeral 51, and reference numeral 54 denotes the third-order differential curve of the reflection attenuation curve of reference numeral 51. The first-order differential value, second-order differential value, and third-order differential value are indicated by indicators on the right side of the graph.

[0147] As described above, the maximum absorption peak in the return loss curve of an electromagnetic wave absorbing sheet refers to the frequency at which the absolute value of the return loss is greatest among the frequencies at which the first-order differential curve of the return loss curve changes from negative to positive as the frequency increases, within the frequency range of 60 GHz or more and 90 GHz or less of the incident electromagnetic wave. Since the only frequency at which the first-order differential curve 52 changes from negative to positive is 74.70 GHz, the return loss at the frequency of 74.70 GHz is the maximum absorption peak for sheet 1.

[0148] Furthermore, in the first derivative curve 52 in FIG. 5, the value changes from negative to positive only at the frequency of 74.70 GHz as described above, and therefore, the sheet 1 does not have a second absorption peak.

[0149] 5, at the frequency where the third-order differential curve 54 of the return loss curve 51 goes from positive to negative as the frequency increases, the frequency at which the second-order differential curve 53 of the return loss curve 51 reaches a positive maximum is 79.50 GHz, and therefore it can be seen that the return loss curve 51 of the sheet 1 has a shoulder peak at a frequency of 79.50 GHz. This can also be confirmed from the shape of the return loss curve 41 of the sheet 1 shown in FIG. 4, and the return loss at this shoulder peak is approximately -22 dB. Furthermore, as shown in Table 1, the input impedance value Z of the sheet 1 at a frequency of 79 GHz is 413.25 Ω.

[0150] Thus, the electromagnetic wave absorbing sheet of sheet 1 satisfies the conditions for exhibiting good electromagnetic wave absorption characteristics over a wide frequency band shown in this embodiment, namely, that the impedance value Z of the entire sheet at a frequency of 79 GHz is 260Ω or more and 610Ω or less, that the return loss curve representing its return loss characteristics has a maximum absorption peak and a shoulder peak in the frequency band of incident electromagnetic waves from 60 GHz to 90 GHz, that the return loss at the maximum absorption peak is −15 dB or less and that at the shoulder peak is −10 dB or less, and that the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less. Therefore, a sheet has been obtained that satisfies the conditions that the absolute values ​​of “minimum return loss value / bandwidth where return loss value is −15 dB or less” are 1.80 and 4.00 or less, which indicate good absorption of electromagnetic waves over a wide frequency band.

[0151] FIG. 6 shows the reflection loss curve of sheet 2.

[0152] In the return loss curve of the sheet 2 indicated by reference numeral 61 in FIG. 6, the return loss RL is −14.03 dB or less in the range of 76 GHz to 81 GHz, and it was confirmed that the sheet 2 can effectively absorb electromagnetic waves in a practical frequency range.

[0153] In the electromagnetic wave absorbing sheet of Sheet 2, whose return loss curve 61 is shown in FIG. 6, as a broad index showing the width of the electromagnetic wave absorption characteristics, the bandwidth where the return loss is −15 dB or less is 7.99 GHz, and the minimum value RL of the return loss in the frequency band of 60 to 90 GHz is min is -30.98 GHz, and the absolute value of the broad index value "minimum value of return loss / bandwidth where return loss is -15 dB or less" is 3.88.

[0154] In the return loss curve indicated by reference numeral 61 in FIG. 6, a peak value of the return loss exists at a frequency of 69.60 GHz, and a second peak exists at a frequency of 81.10 GHz.

[0155] FIG. 7 is a diagram showing the first-order differential curve, the second-order differential curve, and the third-order differential polarity of the reflection attenuation curve of the sheet 2 shown in FIG.

[0156] 7, reference numeral 71 denotes the reflection attenuation curve of the sheet 2, reference numeral 72 denotes the first-order differential curve of the reflection attenuation curve of reference numeral 71, reference numeral 73 denotes the second-order differential curve of the reflection attenuation curve of reference numeral 71, and reference numeral 74 denotes the third-order differential curve of the reflection attenuation curve of reference numeral 71. As in FIG. 5, the first-order differential value, second-order differential value, and third-order differential value are indicated by indicators on the right side of the graph.

[0157] As described above, the maximum absorption peak in the return loss curve of the electromagnetic wave absorbing sheet refers to the frequency at which the first-order differential curve of the return loss curve changes from negative to positive as the frequency increases, within the frequency range of 60 GHz or higher and 90 GHz or lower, at which the numerical value of the return loss RL is smallest, i.e., the frequency at which the absolute value is largest. This definition applies to two locations where the first-order differential curve 72 changes from negative to positive: at frequencies of 69.60 GHz and 81.10 GHz. Of these, the return loss at the frequency of 69.60 GHz indicated by reference numeral 71 is large, at a return loss of -30.98 dB, and therefore the maximum absorption peak of the return loss of the sheet 2 is at a frequency of 69.60 GHz.

[0158] As described above, there are two portions in the first-order differential curve 72 in FIG. 7 where the value changes from negative to positive, and the return loss at the frequency of 81.10 GHz is smaller (greater) than the maximum absorption peak described above, so the electromagnetic wave absorbing sheet of sheet 2 has a second absorption peak at a frequency of 81.10 GHz.

[0159] On the other hand, in FIG. 7 , among the frequencies at which the third-order differential curve 74 of the reflection attenuation curve 71 changes from positive to negative as the frequency increases, there is no frequency at which the second-order differential curve 73 of the reflection attenuation curve 71 reaches a positive maximum value, and therefore it can be confirmed that the reflection attenuation curve 71 of the sheet 2 does not have a shoulder peak.

[0160] It can also be confirmed from the reflection loss curve 61 of sheet 2 shown in Figure 6 that the reflection loss curve of the second sheet has a maximum absorption peak and a second absorption peak, but does not have a shoulder peak. Furthermore, as shown in Table 1, the input impedance value Z of sheet 2 at a frequency of 79 GHz is 265.31 Ω.

[0161] Thus, the electromagnetic wave absorbing sheet of sheet 2 satisfies the conditions for exhibiting good electromagnetic wave absorption characteristics over a wide frequency band shown in this embodiment, namely, that the impedance value Z of the entire sheet at a frequency of 79 GHz is 260Ω or more and 610Ω or less, that the return loss curve representing its return loss characteristics has a maximum absorption peak and a second absorption peak in the frequency band of incident electromagnetic waves from 60 GHz to 90 GHz, that the return loss at the maximum absorption peak is −15 dB or less and that at the second absorption peak is −10 dB or less, and that the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less. Therefore, a sheet has been obtained that satisfies the conditions that the absolute values ​​of “minimum return loss value / bandwidth where return loss value is −15 dB or less” are 3.88 and 4.00 or less, which indicate good absorption of electromagnetic waves over a wide frequency band.

[0162] The reason why the return loss at the maximum absorption peak of the second sheet, whose return loss curve 61 is shown in FIG. 6, is larger than that of the first sheet shown in FIG. 4 is thought to be due to better impedance matching at that frequency.

[0163] FIG. 8 shows the reflection loss curve of sheet 3.

[0164] In the return loss curve of the sheet 3 indicated by reference numeral 81 in FIG. 8, the return loss RL is equal to or less than -20.48 dB in the range of 76 GHz to 81 GHz, and it was confirmed that the sheet 3 can effectively absorb electromagnetic waves in a practical frequency range.

[0165] In the electromagnetic wave absorbing sheet of Sheet 3, whose return loss curve 81 is shown in FIG. 8, as a broad index showing the width of the electromagnetic wave absorption characteristics, the bandwidth where the return loss is −15 dB or less is 17.63 GHz, and the minimum value RL of the maximum return loss in the frequency band of 60 to 90 GHz is minis −33.77 dB, and the absolute value of the broad index value, “minimum value of return loss / bandwidth where return loss is −15 dB or less”, is 1.92.

[0166] In the return loss curve indicated by reference numeral 81 in FIG. 8, a peak value of the return loss exists at a frequency of 75.53 GHz, and a shoulder peak exists at a frequency of 78.71 GHz.

[0167] FIG. 9 is a diagram showing the polarities of the first-order differential curve, second-order differential curve, and third-order differential curve of the reflection attenuation curve of the sheet 3 shown in FIG.

[0168] 9, reference numeral 91 denotes the reflection attenuation curve of the sheet 3, reference numeral 92 denotes the first-order differential curve of the reflection attenuation curve of reference numeral 91, reference numeral 93 denotes the second-order differential curve of the reflection attenuation curve of reference numeral 91, and reference numeral 94 denotes the third-order differential curve of the reflection attenuation curve of reference numeral 91. As in FIGS. 5 and 7, the first-order differential value, second-order differential value, and third-order differential value are indicated by indicators on the right side of the graph.

[0169] As shown in FIG. 9 , when the frequency of the incident electromagnetic wave is in the range of 60 GHz or more and 90 GHz or less, the first-order differential curve of the return loss curve changes from negative to positive as the frequency increases at a frequency of 75.53 GHz, and therefore, the return loss at a frequency of 75.53 GHz for sheet 3 reaches the maximum absorption peak.

[0170] In the first-order differential curve 92 in FIG. 9 , the value changes from negative to positive only at the aforementioned frequency of 75.53 GHz, and therefore, no second absorption peak is present in sheet 3. Meanwhile, in FIG. 9 , among the frequencies at which the third-order differential curve 94 of the reflection attenuation curve 91 changes from positive to negative as the frequency increases, the frequency at which the second-order differential curve 93 of the reflection attenuation curve 91 reaches a positive maximum is 78.71 GHz, and therefore, the reflection attenuation curve 91 of sheet 3 has a shoulder peak at a frequency of 79.50 GHz. This can also be confirmed from the shape of the reflection attenuation curve 81 of sheet 3 shown in FIG. 8 , and the return loss at this shoulder peak is approximately −28 dB. Furthermore, as shown in Table 1, the input impedance value Z of sheet 3 at a frequency of 79 GHz is 406.27 Ω.

[0171] Thus, the electromagnetic wave absorbing sheet of sheet 3 satisfies the conditions for exhibiting good electromagnetic wave absorption characteristics over a wide frequency band shown in this embodiment, namely, that the impedance value Z of the entire sheet at a frequency of 79 GHz is 260Ω or more and 610Ω or less, that the return loss curve representing its return loss characteristics has a maximum absorption peak and a shoulder peak in the frequency band of incident electromagnetic waves from 60 GHz to 90 GHz, that the return loss at the maximum absorption peak is −15 dB or less and that at the shoulder peak is −10 dB or less, and that the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less. Therefore, a sheet has been obtained that satisfies the conditions that the absolute values ​​of “minimum return loss value / bandwidth where return loss value is −15 dB or less” are 1.92 and 4.00 or less, which indicate good absorption of electromagnetic waves over a wide frequency band.

[0172] FIG. 10 shows the reflection loss curve of sheet 4.

[0173] In the return loss curve of sheet 4 designated by reference numeral 101 in FIG. 10, the minimum return loss in the range of 76 GHz to 81 GHz is −6.58 dB, which indicates that the sheet is unable to effectively absorb electromagnetic waves in a practical frequency range.

[0174] Furthermore, the return loss curve 101 of the sheet 4 shown in Fig. 10 shows a tendency for the return loss to gradually decrease as the frequency increases, i.e., for the return loss RL to increase. Although not shown, in the frequency range of 60 GHz or more and 90 GHz or less, the first-order differential curve of the return loss curve shown in Fig. 5 does not have a portion where the value changes from negative to positive, and it was confirmed that there is no absorption peak in the return loss curve shown in Fig. 5.

[0175] Sheet 4 exhibiting such a return loss curve has an impedance value Z of 219.21 at a frequency of 79 GHz for the entire sheet, which does not satisfy the requirement of 260 Ω or more and 610 Ω or less, and the return loss curve representing its return loss characteristics does not show a maximum absorption peak, but only has a portion that meets the condition of a shoulder peak at a frequency of 79.60 GHz. Since the total thickness of the electromagnetic wave absorbing sheet of Sheet 4 is 338 μm, it satisfies the requirement that the layer thickness be 500 μm or less, but the absolute value of the index showing broad electromagnetic wave absorption characteristics, "minimum return loss value / bandwidth where return loss is -15 dB or less," is 9.12, which is larger than 4.00.

[0176] Thus, it was confirmed that the electromagnetic wave absorbing sheet of Sheet 4 corresponds to a comparative example that does not meet the preferred requirements of the electromagnetic wave absorbing sheet disclosed in the present application, and that its electromagnetic wave absorption characteristics are not capable of effectively absorbing electromagnetic waves over a wide frequency band.

[0177] FIG. 11 shows the reflection loss curve of the sheet 5.

[0178] In the return loss curve of the sheet 5 indicated by reference numeral 111 in FIG. 11, the return loss RL is −26.67 dB or less in the range of 76 GHz to 81 GHz, and it was confirmed that the sheet 5 can effectively absorb electromagnetic waves in a practical frequency range.

[0179] In the electromagnetic wave absorbing sheet of sheet 5, whose return loss curve 111 is shown in FIG. 11, as a broad index showing the width of the electromagnetic wave absorption characteristics, the bandwidth where the return loss is −15 dB or less is 15.00 GHz, and the minimum value RL of the maximum return loss in the frequency band of 60 to 90 GHz is min is −40.79 dB, and the absolute value of the broad index value, “minimum value of return loss / bandwidth where return loss is −15 dB or less”, is 2.72.

[0180] In the return loss curve indicated by reference numeral 111 in FIG. 11, a peak value of the return loss exists at a frequency of 74.85 GHz, and a second peak exists at a frequency of 80.55 GHz.

[0181] FIG. 12 is a diagram showing the first, second and third derivative curves of the reflection attenuation curve of the sheet 5 shown in FIG.

[0182] 12, reference numeral 121 denotes the reflection attenuation curve of the sheet 5, reference numeral 122 denotes the first-order differential curve of the reflection attenuation curve of reference numeral 121, reference numeral 123 denotes the second-order differential curve of the reflection attenuation curve of reference numeral 121, and reference numeral 124 denotes the third-order differential curve of the reflection attenuation curve of reference numeral 121. Note that, as in FIGS. 5, 7, and 9, the first-order differential value, the second-order differential value, and the third-order differential value are indicated by indicators on the right side of the graph.

[0183] 12, when the frequency of the incident electromagnetic wave is in the range of 60 GHz to 90 GHz, there are two portions where the first-order differential curve of the return loss curve changes from negative to positive as the frequency increases: at frequencies of 74.85 GHz and 80.55 GHz. Of these, the return loss curve indicated by reference numeral 121 has a large return loss of -40.79 dB at a frequency of 74.85 GHz, and therefore the maximum absorption peak of the return loss of sheet 5 occurs at a frequency of 74.85 GHz.

[0184] In the first-order differential curve 122 in FIG. 12, there are two portions where the value changes from negative to positive, and the return loss at the frequency of 80.55 GHz is smaller (larger) than the maximum absorption peak described above, so the electromagnetic wave absorbing sheet of sheet 5 has a second absorption peak at a frequency of 80.55 GHz.

[0185] On the other hand, in FIG. 12 , among the frequencies at which the third-order differential curve 124 of the reflection attenuation curve 121 changes from positive to negative as the frequency increases, there is no frequency at which the second-order differential curve 123 of the reflection attenuation curve 121 reaches a positive maximum value, and therefore it can be confirmed that the reflection attenuation curve 121 of the sheet 5 does not have a shoulder peak.

[0186] The fact that the reflection attenuation curve of the second sheet has a maximum absorption peak and a second absorption peak, but does not have a shoulder peak, can also be confirmed from the reflection attenuation curve 111 of sheet 5 shown in Fig. 11. Furthermore, as shown in Table 1, the input impedance value Z of sheet 5 at a frequency of 79 GHz is 352.48Ω. In this way, the electromagnetic wave absorbing sheet of sheet 5 satisfies the conditions for exhibiting good electromagnetic wave absorption characteristics over a wide frequency band shown in this embodiment, namely, the impedance value Z of the entire sheet at a frequency of 79 GHz is 260Ω or more and 610Ω or less, the reflection attenuation curve representing its reflection attenuation characteristics has a maximum absorption peak and a second absorption peak in the frequency band of incident electromagnetic waves from 60 GHz to 90 GHz, the reflection attenuation at the maximum absorption peak is -15 dB or less and the reflection attenuation at the second absorption peak is -10 dB or less, and the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less. As a result, the absolute value of "minimum return loss / bandwidth where return loss is -15 dB or less" is 2.72, which satisfies the condition of 4.00 or less, indicating good absorption of electromagnetic waves over a wide frequency range.

[0187] Although not shown in the figures, it was confirmed that in the magnetic layer of an electromagnetic wave absorbing sheet produced using epsilon iron oxide instead of hexagonal ferrite as the magnetic iron oxide contained in the magnetic layer, the half-width of the peak in the curve showing the change in the value of the imaginary part of relative permeability (μr″) with respect to frequencies from 60 GHz to 90 GHz was a small value of 5.4. This is thought to be because the particle size distribution of epsilon iron oxide is smaller than that of hexagonal ferrite. Since the half-width of the peak in the graph showing the change in the value of the imaginary part of magnetic permeability (μr″) is thus small, it is thought that the reflection loss curve showing the reflection loss characteristics of the obtained electromagnetic wave absorbing sheet does not have either a shoulder peak or a second absorption peak in addition to the maximum absorption peak. Furthermore, even if either a shoulder peak or a second absorption peak is present, it is thought that the return loss in the frequency band from 76 GHz to 81 GHz will not be −15 dB or less.

[0188] Furthermore, when the thickness, relative permittivity, and real part of the relative permeability of each layer of an electromagnetic wave absorbing sheet are the same, if the half-value width of the imaginary part of the relative permeability is wide, the absorption characteristics of the electromagnetic wave absorbing sheet having that magnetic layer will fluctuate relatively slowly with changes in frequency. In other words, an electromagnetic wave absorbing sheet having a magnetic layer with a wide half-value width of the imaginary part of the relative permeability will have a relatively wide absorption band when adjusted to absorb at a target center frequency, thereby realizing an electromagnetic wave absorbing sheet that is suitable for practical use.

[0189] As described above, the electromagnetic wave absorbing sheet shown in this embodiment has a characteristic configuration in which the magnetic iron oxide contained in the magnetic layer is hexagonal ferrite, the overall input impedance Z is 260 Ω or more and 610 Ω or less, the return loss curve has at least one of a maximum absorption peak, a shoulder peak, and a second absorption peak smaller than the maximum absorption peak in a frequency band in which the frequency of the incident electromagnetic wave is 60 GHz or more and 90 GHz or less, the return loss at the maximum absorption peak is −15 dB or less, and the return loss at the shoulder peak or the second peak is −10 dB or less, and the total film thickness of the electromagnetic wave absorbing sheet is 500 μm or less, and therefore can satisfy the condition that the absolute value of “minimum return loss / bandwidth where return loss is −15 dB or less”, which is an index of an electromagnetic wave absorbing sheet having broad electromagnetic wave absorption characteristics, is 4.00 or less.

[0190] As a result, it is possible to realize an electromagnetic wave absorbing sheet that has sufficient electromagnetic wave absorption characteristics over a wide frequency band, compared to conventional electromagnetic wave interference type electromagnetic wave absorbing sheets, and is thin overall and easy to handle.

[0191] The electromagnetic wave absorbing sheet disclosed in the present application contains hexagonal ferrite as magnetic iron oxide in the magnetic layer, and the reflection attenuation curve showing the electromagnetic wave absorption characteristics in the frequency band of 60 GHz or more and 90 GHz or less includes a maximum peak value and either a shoulder peak or a second peak, making it an electromagnetic wave absorbing sheet that can exhibit good electromagnetic wave absorption characteristics over a wide frequency band, and is particularly useful in practical fields such as on-board radar.

[0192] The electromagnetic wave absorbing sheet disclosed in this application can contribute to achieving Goal 9 (Build resilient infrastructure, promote inclusive and sustainable industrialization, innovate and foster innovation) out of the 17 Sustainable Development Goals (SDGs) established by the United Nations.

[0193] 1 Magnetic layer 1a Hexagonal ferrite (magnetic iron oxide) 1b Carbon black (dielectric constant adjuster) 1c Binder 2 Electromagnetic wave reflective layer

Claims

1. An electromagnetic wave absorbing sheet comprising a magnetic layer containing magnetic iron oxide, a dielectric constant adjuster and a binder, and an electromagnetic wave reflecting layer, wherein the magnetic iron oxide is hexagonal ferrite, the input impedance Z of the entire electromagnetic wave absorbing sheet is 260 Ω or more and 610 Ω or less, a reflection attenuation curve representing the reflection attenuation characteristics has at least one of a maximum absorption peak, a shoulder peak, and a second absorption peak smaller than the maximum absorption peak in a frequency band of incident electromagnetic waves from 60 GHz to 90 GHz, the reflection attenuation at the maximum absorption peak is -15 dB or less, and the reflection attenuation at the shoulder peak or the second peak is -10 dB or less, and the total thickness of the electromagnetic wave absorbing sheet is 500 μm or less.

2. The electromagnetic wave absorbing sheet according to claim 1, wherein the volume content of said magnetic iron oxide contained in said magnetic layer is 10.0% or more and 45.0% or less.

3. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the frequency bandwidth in the return loss curve where the return loss is -15 dB or less is 5 GHz or more.

4. An electromagnetic wave absorbing sheet according to any one of claims 1 to 3, which has a return loss of -15 dB or less in the frequency band of incident electromagnetic waves from 76 GHz to 81 GHz.

5. The electromagnetic wave absorbing sheet according to any one of claims 1 to 4, wherein the magnetic iron oxide is hexagonal ferrite obtained by pulverization.

6. An electromagnetic wave absorbing sheet according to any one of claims 1 to 5, wherein in a curve showing the change in the value of the imaginary part of the relative permeability versus the frequency of the incident electromagnetic wave, the half-value width of a peak in the frequency range of the incident electromagnetic wave from 60 GHz to 90 GHz is 8 GHz or more.

7. An electromagnetic wave absorbing sheet according to any one of claims 1 to 6, wherein the imaginary part of the relative permeability at 79 GHz is 0.08 or more and 0.25 or less.

8. The electromagnetic wave absorbing sheet according to any one of claims 1 to 7, wherein the content of the dielectric constant adjuster in the magnetic layer is 2.0% by volume or more and 4.0% by volume or less.

9. The electromagnetic wave absorbing sheet according to any one of claims 1 to 8, wherein a dielectric layer is formed between said magnetic layer and said reflective layer.

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