Radio wave absorbing sheet

Abstract

Provided is a radio wave absorbing sheet which uses a silicon carbide powder that imparts light weight and high insulating properties, and which is capable of achieving an excellent noise attenuation effect in a sub-millimeter wave to millimeter wave band. A radio wave absorbing sheet according to the present invention is characterized by comprising: a base material comprising an organic material; and a powder mainly composed of silicon carbide (SiC) supported on the base material, in which the value I4H / I6H obtained by dividing the intensity I4H of a peak derived from 4H-SiC occurring at 2 [theta] = 34.4-35 DEG by the intensity I6H of a peak derived from 6H-SiC occurring at 2 [theta] = 35-36 DEG in X-ray diffraction with Cu-K [alpha] as a radiation source on the surface of the radio wave absorbing sheet is at least a prescribed value.

Description

Technical Field

[0001] The present invention relates to an electromagnetic wave absorbing sheet. Background Art

[0002] With the rapid development of communication, devices that effectively utilize electromagnetic waves in the range of 3 to 80 GHz are becoming widely popular. For example, in the fifth-generation communication system (5G) that has been used as a commercial service in Japan since 2020, the effective utilization of the frequency bands of 3 to 5 GHz as sub6 and around 28 to 40 GHz in the millimeter-wave band is gradually increasing. In addition, in automobiles, with the rapid development of autonomous driving systems, sub-millimeter-wave radars that effectively utilize frequencies around 24 GHz and millimeter-wave radars that effectively utilize frequencies around 76 to 79 GHz are becoming widely popular. On the other hand, the problem of electromagnetic wave interference inside and outside such devices is becoming increasingly prominent. Therefore, an electromagnetic wave absorbing sheet that functions at the frequencies where the problem of electromagnetic wave interference occurs is effectively utilized.

[0003] The electromagnetic wave absorbing sheet functions by being mounted on a conductive substrate such as metal or by providing a conductive layer such as metal on the back surface of the electromagnetic wave absorbing sheet. With respect to the surface reflection wave reflected from the surface of the electromagnetic wave absorbing sheet, the phase of the secondary reflection wave that passes through the inside of the electromagnetic wave absorbing sheet, is totally reflected by the conductive object on the back side, and is radiated from the surface of the electromagnetic wave absorbing sheet is shifted by half a wavelength, and the surface reflection wave and the secondary reflection cancel each other out, thereby exhibiting electromagnetic wave absorption. The electromagnetic wave absorbing sheet has a structure in which a soft resin is used as a substrate and a filler for generating an electromagnetic wave absorption effect is supported in the substrate. By controlling the material constants (dielectric constant, magnetic permeability) of the substrate and the thickness of the substrate, a resonance peak of electromagnetic wave absorption at a specific frequency is achieved. The material constants of the substrate are adjusted according to the substrate material and the filler addition amount, etc., and therefore the filler addition amount is controlled within the required range. As a representative example, there is an electromagnetic wave absorbing sheet in which a flexible resin such as rubber or an elastomer is used as a substrate, and carbonyl iron powder or spinel-type ferrite powder is added as a filler.

[0004] In recent years, the miniaturization of electronic devices has been gradually developing. Therefore, the radio wave absorption sheet is disposed in a narrow space, and the thickness of the radio wave absorption sheet is required to be 1 mm or less. In addition, in recent years, the weight reduction of electronic devices has been required, and the radio wave absorption sheet is also preferably lighter. The specific gravity of the resin base material does not vary significantly depending on the type of material, but the specific gravity of the loss material varies significantly depending on the material. Therefore, although it also depends on the addition amount of the filler, from the viewpoint of weight reduction, it is preferable to select a carbon-based material or silicon carbide having a specific gravity lower than that of carbonyl iron and soft magnetic ferrite as the loss material. In addition, in the case of considering a radio wave absorption sheet corresponding to a device that effectively utilizes radio waves in the 3 to 80 GHz band, compared with the case where a magnetic filler that is difficult to achieve high magnetic permeability in the microwave to millimeter wave band is used as the loss material, when an electric loss is caused by supporting a non-magnetic conductor in the resin base material, the design freedom of the radio wave absorption sheet becomes higher. Considering the practicality of the non-magnetic conductive filler including cost, it is also preferable to select a carbon-based material or silicon carbide as the loss material.

[0005] In addition, since the radio wave absorption sheet is disposed around the electronic circuit, when the radio wave absorption sheet comes off from the mounting portion, when the surface resistance of the radio wave absorption sheet is less than 10 10 Ω / □, there is a risk of short circuit of the circuit caused by contact between the radio wave absorption sheet and the circuit. However, in the case of using a carbon-based material as the loss material, the surface resistance of the radio wave absorption sheet to which the required amount of the carbon-based material is added to exhibit excellent radio wave absorption amount is less than 10 10 Ω / □. To solve this problem, it is considered to mount an insulating layer such as a PET film on the sheet surface, but depending on the material of the insulating layer, not only the heat resistance and flame retardancy are reduced, but also the manufacturing cost is increased. In addition, the cohesion of the powder of the carbon-based material is high, so it is generally difficult to uniformly disperse it in the base resin, and the manufacturing unevenness becomes large. In the radio wave absorption sheet using a carbon-based material as the loss material, special devices and processes are mostly required to reduce the dispersion unevenness of the loss material, which is not preferable from the viewpoint of manufacturing cost.

[0006] On the other hand, in the case of using silicon carbide as the loss material, the surface resistance of the radio wave absorption sheet to which the required amount of silicon carbide is added to exhibit excellent radio wave absorption amount exceeds 10 10 Ω / □. In addition, when the case of using expensive silicon carbide fibers is regarded as an exception, industrially used is granular and inexpensive silicon carbide, which is easily dispersed in the resin base material. Therefore, when manufacturing a light and highly insulating radio wave absorption sheet, it is preferable to use silicon carbide powder as the loss material. As a radio wave absorption sheet using silicon carbide powder as the loss material, there is a radio wave absorption sheet described in Patent Document 1.

[0007] However, in a radio wave absorption sheet using silicon carbide powder in a lossy material, it is not known that the crystal structure and impurity content of the silicon carbide powder significantly affect the radio wave absorption properties of the obtained radio wave absorption sheet. It is considered that the conductivity of the silicon carbide powder affects the dielectric constant of the radio wave absorption sheet, and by controlling the dielectric constant, that is, by controlling the crystal structure, impurity content, particle size, and addition amount of the silicon carbide powder, a desired radio wave absorption sheet can be obtained. In Patent Document 1, only the particle size and addition amount of silicon carbide are limited, and the crystal structure and impurity content are not involved. In addition, in Patent Document 1, an example of using high-purity green silicon carbide powder (as a specific example, manufactured by Showa Denko K.K.: Green Densic) is disclosed. The main crystal structure of silicon carbide has a regular tetrahedron as the minimum structure, and according to the stacking structure of the regular tetrahedron, there are mainly 4H-type and 6H-type crystal structures. Depending on this crystal structure, there are differences in the carrier mobility as a semiconductor characteristic. Compared with the 6H type, the 4H type has a higher carrier mobility, and the dielectric loss tangent tanδ (= ε” / ε’), which is related to the radio wave absorption performance and is represented by the real part ε’ and the imaginary part ε” of the dielectric constant of the radio wave absorption sheet, becomes higher. However, since the crystal structure of Green Densic is mainly of the 6H type, there is a problem that the dielectric loss tangent tanδ is low and the performance of the radio wave absorption sheet is low.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent No. 4113812 Gazette. Summary of the Invention

[0011] Problems to be Solved by the Invention

[0012] Therefore, in view of the above problems, an object of the present invention is to provide a radio wave absorption sheet that can obtain an excellent noise attenuation effect in the sub-millimeter wave to millimeter wave band in a radio wave absorption sheet using silicon carbide powder that provides light weight and high insulation.

[0013] Solutions to the Problems

[0014] The inventors of the present invention conducted in-depth research to solve the above problems and obtained the following insights. That is, a powder mainly composed of silicon carbide (SiC) that provides light weight and high insulation is used as a dielectric loss material supported on a substrate, and silicon carbide (SiC) with X-ray diffraction data within a specified condition is used. As a result, compared with a radio wave absorption sheet using carbonyl iron or soft magnetic ferrite as a loss filler, it can be made lighter. In addition, compared with a radio wave absorption sheet using a carbon-based material as a loss filler, even if an insulating layer such as a PET film is not installed on the surface of the radio wave absorption sheet, the surface resistance of the radio wave absorption sheet exceeds 10 10Ω / sq, the silicon carbide powder in the base material is also easily and uniformly dispersed in the resin base material, thus suppressing manufacturing unevenness and enabling stabilization of the radio wave absorption property. In addition, compared with a radio wave absorption sheet using conventionally known silicon carbide as a loss filler, it is possible to achieve thinning and improve the radio wave absorption property.

[0015] The gist configuration of the present invention completed based on the above-mentioned insights is as follows.

[0016] [1] A radio wave absorption sheet, characterized by comprising: a base material composed of an organic substance; and powder mainly composed of silicon carbide (SiC) supported in the above-mentioned base material, in X-ray diffraction using Cu-Kα as a radiation source on the surface of the radio wave absorption sheet, the peak intensity I of 4H-SiC appearing at 2θ = 34.4 to 35° 4H divided by the peak intensity I of 6H-SiC appearing at 2θ = 35 to 36° 6H to obtain a value I 4H / I 6H is 0.1 or more.

[0017] [2] The radio wave absorption sheet according to the above [1], characterized in that the surface resistance of the radio wave absorption sheet is 10 10 Ω / sq or more.

[0018] [3] The radio wave absorption sheet according to the above [1] or [2], characterized in that the average particle diameter of the powder mainly composed of silicon carbide (SiC) is 2 μm or more and 40 μm or less.

[0019] [4] The radio wave absorption sheet according to any one of the above [1] to [3], characterized in that the volume ratio of the powder mainly composed of silicon carbide (SiC) to the radio wave absorption sheet is 20% or more and 50% or less.

[0020] [5] The radio wave absorption sheet according to any one of the above [1] to [4], characterized in that the content of Al in the powder mainly composed of silicon carbide (SiC) is 0.1 mass% or more and 0.2 mass% or less, and the content of Fe is 0.1 mass% or more and 0.3 mass% or less.

[0021] Advantages of the Invention

[0022] In a radio wave absorption sheet using silicon carbide powder that brings light weight and high insulation, excellent noise attenuation effects can be obtained in the sub-millimeter wave to millimeter wave bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is an example showing an X-ray diffraction measurement chart of a radio wave absorption sheet. Detailed Embodiments

[0024] Hereinafter, embodiments of the present invention will be described.

[0025] (Radio Wave Absorbing Sheet)

[0026] The radio wave absorbing sheet of the present invention is characterized by comprising: a base material made of an organic substance; and powder mainly composed of silicon carbide (SiC) supported in the above base material. In the X-ray diffraction with Cu-Kα as the radiation source on the surface of the radio wave absorbing sheet, the peak intensity I of 4H-SiC appearing at 2θ = 34.4 - 35° 4H divided by the peak intensity I of 6H-SiC appearing at 2θ = 35 - 36° 6H to obtain a value I 4H / I 6H is 0.1 or more.

[0027] [Base Material]

[0028] The organic substance constituting the base material is not particularly specified. Generally, flexibility and cuttability are required in the radio wave absorbing sheet, so rubber and elastomers are preferred. Among them, silicone resin and acrylic resin have excellent heat and cold resistance and insulation resistance, and there are many actual uses in the radio wave absorbing sheet, so they are particularly preferably used as the base resin. There is no particular limitation on the form of the resin raw material, and either a millable state or a liquid state can be used. Here, an example of an embodiment using liquid silicone resin as a raw material will be described.

[0029] Commercially available liquid silicone for general industrial use can be used as the liquid silicone. The liquid type of silicone has a one-component type and a two-component type, and the curing type has a condensation reaction type and an addition reaction type. Any of them can be applied in the present invention. The curing of the condensation reaction type takes time, while the addition reaction type can be expected to cure in a relatively short time by setting a heating process. Therefore, if productivity is considered, the addition reaction type is preferably used. If the viscosity of the liquid silicone is too low, the shape retention during sheet molding becomes poor, and if the viscosity is too high, it is difficult for the powder filler to be uniformly dispersed. Therefore, a liquid silicone with a viscosity of 1 Pa·s or more and 10 Pa·s or less is preferably used.

[0030] [Powder]

[0031] The radio wave absorbing sheet of the present invention contains powder as a dielectric loss material in a state supported in the base material. The powder as a dielectric loss material contains powder mainly composed of silicon carbide (SiC) (hereinafter, also referred to as "silicon carbide powder"). In the present invention, the content ratio I of the 4H type relative to the 6H type 4H / I 6HHigh-purity silicon carbide powder is used as a loss filler. The crystal structure of silicon carbide mainly has a regular tetrahedron as the smallest structure. According to the stacked structure of the regular tetrahedron, there are mainly 4H type and 6H type. In high-purity green silicon carbide, the 6H structure dominates, and the proportion of the 4H type is small. In the present invention, low-purity black silicon carbide is used. In this case, the 4H type and the 6H type coexist, and the proportion of the 4H type increases. The silicon carbide powder contained in the radio wave absorption sheet can be identified by X-ray diffraction using Cu-Kα as the radiation source on the surface of the radio wave absorption sheet. The measurement conditions of the X-ray diffraction in the present invention are as follows.

[0032] <Apparatus>

[0033] Manufacturer: Rigaku Corporation

[0034] Apparatus name: Fully automatic horizontal multi-functional high-power X-ray diffractometer SmartLab (9kW)

[0035] X-ray tube: Cu

[0036] <Optical system conditions>

[0037] CBO selection slit: BB

[0038] Incident parallel slit (Soller / PSC): 5.0 deg

[0039] Length limiting slit (IS length): 10.0 mm

[0040] Photosensitive optical element (PSA): None

[0041] Photosensitive parallel slit (Soller): 5.0 deg

[0042] <Measurement conditions>

[0043] Scan axis: 2θ / θ, mode: continuous, range specification: absolute

[0044] Speed counting time: 3.0 degree / min

[0045] Scan: 20 degree to 80 degree

[0046] Data acquisition interval: 0.01 degree

[0047] IS: 1 / 2 degree,

[0048] Photosensitive slit RS1: 8.0 mm

[0049] Photosensitive slit RS2: 13.0 mm

[0050] Attenuator: Open

[0051] The analysis procedure of the measurement data is as follows. Using the powder X-ray comprehensive analysis software PDXL manufactured by Rigaku Corporation, in order to eliminate the influence of the background of the obtained data, the value obtained by subtracting bkg from the diffraction intensity data yobs, i.e., yobs - bkg, is used. In the range of 2θ = 34.4 to 35°, the value obtained by subtracting the minimum value from the maximum value of yobs - bkg is defined as the peak intensity I from 4H-SiC. 4H Similarly, in the range of 2θ = 35 to 36°, the value obtained by subtracting the minimum value from the maximum value of yobs - bkg is defined as the peak intensity I from 6H-SiC. 6H In the present invention, I 4H divided by I 6H and the resulting value I 4H / I 6H is 0.1 or more, preferably 0.2 or more. In addition, the upper limit value is not particularly limited, and preferably 0.5 or less.

[0052] The average particle diameter of the silicon carbide powder is preferably 2 μm or more, more preferably 3 μm or more, preferably 40 μm or less, and more preferably 20 μm or less. When the average particle diameter is less than 2 μm, the fluidity and dispersibility of the powder deteriorate, and the manufacturability deteriorates. On the other hand, when the average particle diameter is greater than 40 μm, the unevenness on the surface of the film-formed sheet also becomes larger.

[0053] In this specification, the "average particle diameter" of the silicon carbide powder is determined according to the following procedure. First, the cross-section of the radio wave absorption sheet is ion-polished, and then a backscattered electron image is taken with a scanning electron microscope. In order to improve the measurement accuracy, the magnification is set to 500 times to 4000 times according to the powder particle diameter. In the case where the powder with a small particle diameter such as 2 μm is the main body, it can be 4000 times, and in the case where the powder with a large particle diameter such as 40 μm is the main body, it can be 500 times. Next, in the taken image, the maximum diameter of each silicon carbide powder is taken as the particle diameter of the powder, and the average value of the particle diameters of all the silicon carbide powders within the imaging field of view is defined as the "average particle diameter" of the silicon carbide powder contained in the radio wave absorption sheet. In addition, powders close to the edge of the taken image and powders with a particle diameter (maximum diameter) less than 0.1 μm on the taken image are excluded from the measurement object.

[0054] The addition amount of the silicon carbide powder varies according to the average particle size of the powder, the resonance frequency of the designed electromagnetic wave absorber sheet, and the thickness of the electromagnetic wave absorber sheet. However, when the volume ratio with respect to the electromagnetic wave absorber sheet is 20% or more and 50% or less, the resonance frequency of the electromagnetic wave absorption amount can be adjusted in the sub-millimeter wave band to the millimeter wave band. In addition, when the addition amount is more than 50%, in the case of manufacturing a sheet by a wet method such as doctor blading, the shrinkage in the heat treatment process of the formed sheet becomes large, and cracks are likely to occur on the surface of the sheet. In addition, the obtained electromagnetic wave absorber sheet also lacks flexibility. However, when there is no problem in the manufacturing process of the electromagnetic wave absorber sheet and the obtained electromagnetic wave absorber sheet does not require flexibility, the addition amount of the silicon carbide powder can also be more than 50%.

[0055] In this specification, the "addition amount" of the silicon carbide powder is obtained according to the following steps. Using the backscattered electron image used when obtaining the "average particle size" of the above-mentioned silicon carbide powder, the colors of the images of the silicon carbide powder and the substrate composed of an organic substance are binarized, thereby distinguishing the silicon carbide powder and the substrate. After the distinction, the area ratio of the silicon carbide powder is obtained and defined as the "addition amount (volume ratio with respect to the electromagnetic wave absorber sheet)" of the silicon carbide powder. In addition, when performing binarization, appropriate settings can be made to clarify the boundary between the silicon carbide powder and the substrate. In addition, in the case where there is more than one additive other than the silicon carbide powder, the distinction can also be made by multi-valued processing, and the area ratios of the silicon carbide powder and the additive are obtained, whereby the respective "addition amounts" can be obtained.

[0056] As the silicon carbide powder, a silicon carbide powder that can be used industrially can be used. As the silicon carbide powder that can be used industrially, it is a silicon carbide powder for grinding and polishing uses, and there are a high-purity green type and a lower-purity black type. The black type is characterized in that the contents of Al and Fe as impurity elements are higher than those of the green type. Generally, the black type contains 0.1 to 0.2 mass% of Al and 0.1 to 0.3 mass% of Fe, and the green type contains 0.01 to 0.1 mass% of Al and 0.02 to 0.03 mass% of Fe. In the present invention, as the silicon carbide powder, it is preferable to use a lower-purity black type silicon carbide powder as the loss filler. The contents of Al and Fe as impurity elements in the silicon carbide can also be identified. In this case, for example, on the polished surface (ion milling surface, etc.) of the electromagnetic wave absorber sheet, the Al and Fe contents can be detected by surface analysis using EPMA (WDS: wavelength dispersive X-ray spectrometry) of the silicon carbide powder.

[0057] In addition, the electromagnetic wave absorbing sheet of the present invention may further contain powders other than silicon carbide powder as dielectric loss materials in the base material. Further, in the base material of the electromagnetic wave absorbing sheet of the present invention, in addition to the dielectric loss materials, flame retardants, material constant modifiers, thermal conductivity improvers, extenders, plasticizers, dispersants, antioxidants and other additives may be added as needed within a range that does not reduce the electromagnetic wave absorption property, surface resistance value or affect the curing characteristics of the base resin. The types of these additives are not particularly limited, and only one type may be added, or two or more types may be added. An example of the additive will be described. As the material constant modifier, electrically conductive powders such as aluminum powder, copper powder, and silver powder can be cited. Further, in the case of a flame retardant, a flame retardant that does not become an environmental load substance is preferable, and hydroxyl group-containing compounds such as aluminum hydroxide and magnesium hydroxide, and nitrogen-based compounds such as melamine cyanurate can be cited. Further, red phosphorus can be added as a flame retardant aid as needed. In order to adjust the material constant or to function as an extender or a thermal conductivity improver, any one or more of silicon oxide, aluminum oxide, magnesium oxide, zirconium oxide, boron nitride, aluminum nitride, spherical graphite powder, spinel-type ferrite, hexagonal ferrite, iron-based magnetic alloys (carbonyl iron, FeSiAl, FeSi, FeSiCr, Fe-based amorphous alloy, Fe-based nanocrystalline alloy), aluminum alloy, and calcium carbonate powder can be added to the base material.

[0058] When the above-mentioned additives are in powder form, the average particle size of the additives is not particularly limited. For example, it is preferably 2 μm or more, more preferably 3 μm or more, preferably 40 μm or less, and more preferably 20 μm or less. When the average particle size is less than 2 μm, the fluidity and dispersibility of the powder deteriorate, and the manufacturability deteriorates. On the other hand, when the average particle size is greater than 40 μm, the unevenness on the surface of the film-formed sheet also becomes larger.

[0059] When the above-mentioned additives are in powder form, the addition amount of all powder components contained in the base material varies depending on the average particle size of the powder, the resonance frequency of the designed electromagnetic wave absorbing sheet, and the thickness of the electromagnetic wave absorbing sheet. However, when the volume ratio with respect to the electromagnetic wave absorbing sheet is 20% or more and 50% or less, the resonance frequency of the electromagnetic wave absorption amount can be adjusted in the submillimeter wave band to millimeter wave band. Further, when the addition amount is greater than 50%, in the case of manufacturing a sheet by a wet method such as doctor blade forming, the shrinkage in the heat treatment process of the formed sheet becomes larger, and cracks are likely to occur on the surface of the sheet. Further, the obtained electromagnetic wave absorbing sheet also lacks flexibility. However, when there is no problem in the manufacturing process of the electromagnetic wave absorbing sheet and the obtained electromagnetic wave absorbing sheet does not require flexibility, the addition amount of the silicon carbide powder can also be greater than 50%.

[0060] Next, the kneading process of mixing the organic matter (raw material resin) that will become the base material with powder components such as silicon carbide powder will be described. If the raw material resin is liquid silicone rubber, it can be mixed with silicon carbide powder using a planetary stirrer. If the raw material resin is kneaded silicone rubber, kneading can be performed using a pressure kneader or an open roll. At this time, it is preferable to cool to 100°C or lower while kneading so that vulcanization does not occur due to the heat release of the material during kneading. In addition, when a doctor blade forming is used as the coating method in the molding method described later, an organic solvent with good solubility in the raw material resin, such as toluene or methyl ethyl ketone, can also be added to the kneaded material to adjust the viscosity of the kneaded material so that doctor blade forming can be performed.

[0061] Regarding the molding of slicing the obtained kneaded material, it can be achieved by any one of compression molding, extrusion molding, calendering molding, roll forming, and doctor blade forming. When the case of compression molding is described as an example, the kneaded material is put into a mold with engraving so that the formed radio wave absorbing sheet becomes a specified thickness, and compression molding is performed at a temperature of 120 to 200°C for 5 to 30 minutes during the vulcanization of silicone.

[0062] The surface resistance of the radio wave absorbing sheet is preferably 10 10 Ω / □ or more, and more preferably 10 11 Ω / □ or more. If the surface resistance of the radio wave absorbing sheet is 10 10 Ω / □ or more, a sufficiently high insulation property can be obtained. For example, even when the radio wave absorbing sheet falls off from the mounting portion, the risk of short circuit of the circuit caused by the contact between the radio wave absorbing sheet and the circuit can be reduced.

[0063] The thickness of the radio wave absorbing sheet is preferably 0.15 mm or more and 1 mm or less. When the thickness of the sheet is less than 0.15 mm, the strength for operation cannot be obtained. In addition, when the thickness exceeds 1 mm, it cannot cope with the thinning and miniaturization of electronic devices. In the case of the radio wave absorbing sheet, although it also depends on the relative dielectric constant of the base material, when the thickness is in the range of 0.15 mm or more and 1 mm or less, the resonance frequency of the radio wave absorption amount can be adjusted in the sub-millimeter wave band to millimeter wave band.

[0064] The electromagnetic wave absorbing sheet is used by attaching it to any object, so an adhesive layer or the like can also be installed on the back surface of the sheet. In addition, when the object to which the electromagnetic wave absorbing sheet is attached is not a conductive object, a conductive layer such as metal can be installed on the back surface of the electromagnetic wave absorbing sheet. The material of the conductive layer is not particularly limited, and a metal-based material is usually used. As the types of metals, brass, copper, iron, nickel, stainless steel, aluminum, etc. can be cited. In addition, the conductive layer can be not only formed of a single metal, but also a conductive layer such as a film formed by vapor-depositing aluminum on a film. The thickness of the conductive layer can be set in consideration of reflecting the electromagnetic waves incident on the electromagnetic wave absorbing sheet and having excellent flexibility. Specifically, it is preferably 10 nm or more and 300 μm or less, and particularly preferably 50 nm or more and 100 μm or less. When the thickness of the conductive layer is less than 10 nm, the electromagnetic waves incident on the electromagnetic wave absorbing sheet will pass through the conductive layer, and the reflection amount at the conductive layer will decrease. On the other hand, when the thickness of the conductive layer is greater than 300 μm, in addition to the total thickness of the electromagnetic wave absorbing sheet becoming thicker, the flexibility of the sheet will also be lost.

[0065] Although the sheet of the present invention is described as an electromagnetic wave absorbing sheet for the far field, it can also be used as a near-field noise suppression sheet. The sheet of the present invention can function as the following near-field noise suppression sheet, which uses silicon carbide powder as a loss material and suppresses electric field noise through electric loss and dielectric loss. If a sheet having the substrate structure of the present invention is adopted, a sheet having a dielectric loss tangent tanδ(=ε” / ε’) greater than 0.2 can be obtained, which is a practical noise suppression sheet. The dielectric loss tangent tanδ(=ε” / ε’) is an index indicating the degree of energy loss when the real part of the dielectric constant is ε’ and the imaginary part of the dielectric constant is ε”. In addition, silicon carbide powder has a high thermal conductivity of 100 to 350 W / mK. Therefore, if a sheet having the substrate structure of the present invention is adopted, a sheet having a thermal conductivity greater than 0.5 W / mK can be obtained, and it can be used as a sheet having both noise suppression and thermal conductivity.

[0066] Examples

[0067] Hereinafter, the present invention will be described based on an example of an electromagnetic wave absorbing sheet having the maximum reflection attenuation amount at 76.5 GHz, but the present invention is not limited thereto.

[0068] <Fabrication of the electromagnetic wave absorbing sheet>

[0069] The electromagnetic wave absorbing sheet is fabricated by the doctor blade method. As the coating liquid, a coating liquid obtained by mixing a silicone resin, silicon carbide powder, and toluene as a solvent with a planetary stirrer is used. After forming this coating liquid on a PET film by doctor blade forming, heat treatment is performed to remove toluene and cure the silicone resin to fabricate the electromagnetic wave absorbing sheet to have a prescribed thickness. The above-mentioned silicon carbide powder is appropriately used as I 4H / I6H Different powders were used as Examples 1 to 14 and Comparative Examples 1 to 13. In addition, regarding the average particle size, addition amount, Al content, and Fe content of the above silicon carbide powders, they are as described in Table 1. Additionally, the thickness of the sheet was set to the film thickness with the best radio wave absorption within the same level where the average particle size and addition amount were specified.

[0070] <Characteristic Evaluation>

[0071] "I calculated by X-ray diffraction measurement 4H / I 6H "

[0072] The measurement conditions and analysis procedures are as described in the embodiment, and the results are shown in Table 1. Figure 1 Examples of X-ray diffraction measurement charts of the radio wave absorption sheets of Example 1, Comparative Example 8, and Comparative Example 11 are shown.

[0073] "Real part of dielectric constant ε', imaginary part of dielectric constant ε", and tangent of dielectric loss angle tanδ"

[0074] [Vector Network Analyzer]

[0075] Manufacturer: Keysight Technologies, Inc.

[0076] Device name: Vector Network Analyzer M9374A (60 - 90 GHz)

[0077] [Device Fixture]

[0078] Manufacturer: Kekemon Co., Ltd.

[0079] Device name: S-parameter method free space type relative dielectric constant measurement system DPS24

[0080] [Software]

[0081] Manufacturer: Keysight Technologies, Inc.

[0082] Software name: VNA Soft Front Panel

[0083] [Software]

[0084] Manufacturer: Kekemon Co., Ltd.

[0085] Software name: S-parameter method dielectric constant and permeability measurement program

[0086] [Measurement Conditions]

[0087] Frequency range: 60 - 90 GHz

[0088] Number of data: 1601

[0089] Port power (Port 1): 2 dBm

[0090] Bandwidth: 1 kHz

[0091] The time domain method is used to remove the influence of reflected waves and scattered waves outside the vicinity of the specimen.

[0092] Time gate span: 0.5 ns

[0093] [Measurement method]

[0094] With the fabricated chip fixed to the device, the real part of the dielectric constant ε' and the imaginary part of the dielectric constant ε'' are measured, and the value obtained by dividing ε'' by ε' is taken as the tangent of the dielectric loss angle tanδ = ε'' / ε'. The results at a frequency of 76.5 GHz are shown in Table 1.

[0095] "Radio wave absorption amount"

[0096] [Device]

[0097] Manufacturer: Keysight Technologies, Inc.

[0098] Device name: Vector Network Analyzer M9374A (60 - 90 GHz)

[0099] And

[0100] Manufacturer: Kekemon Co., Ltd.

[0101] Device name: Lens antenna type oblique incidence reflection attenuation measurement device LAF - 26.5A

[0102] [Software]

[0103] Manufacturer: Keysight Technologies, Inc.

[0104] Software name: VNA Soft Front Panel

[0105] And

[0106] Manufacturer: Kekemon Co., Ltd.

[0107] Software name: Free space type radio wave absorption amount measurement program

[0108] [Measurement conditions]

[0109] Frequency range: 60 - 90 GHz

[0110] Number of data: 1601

[0111] Port power (Port 1): 2 dBm

[0112] Bandwidth: 1 kHz

[0113] The time domain method is used to remove the influence of reflected waves and scattered waves outside the vicinity of the specimen.

[0114] Time gate span: 0.5 ns

[0115] [Measurement method]

[0116] The back surface of the fabricated sheet is attached to the metal stage of the reflection attenuation measurement device using double-sided tape as the bonding layer. The radio wave absorption amount is measured in this state, and the results at a frequency of 76.5 GHz are shown in Table 1. In addition, in the judgment results of Table 1, a radio wave absorption amount of 10 dB or more is OK, and less than 10 dB is NG.

[0117] "Surface resistance"

[0118] [Device]

[0119] Manufacturer: Simco Japan

[0120] Device name: Surface Resistivity Meter ST-4

[0121] [Measurement method]

[0122] The measurement is carried out with the Surface Resistivity Meter ST-4 placed on the surface of the fabricated sheet. The measured surface resistance is expressed in 10 x Ω / □, and the results are shown in Table 1. In addition, in the judgment results of Table 1, 10 10 Ω / □ or more is OK, and less than 10 10 Ω / □ is NG.

[0123] "Sheet appearance"

[0124] The surface state of the sheet is judged by visual inspection and touch, and the results are shown in Table 1. In addition, in the judgment results of Table 1, a smooth state without cracks or irregularities on the surface is OK, and the presence of cracks or irregularities is NG.

[0125] "Comprehensive judgment"

[0126] When the judgments of the above three items of radio wave absorption amount, surface resistivity, and sheet appearance are all OK, the comprehensive judgment is OK. Otherwise, the comprehensive judgment is NG, which is shown in Table 1.

[0127] [Results]

[0128] The results of Examples 1 to 14 and Comparative Examples 1 to 13 are shown in Table 1.

[0129] · Examples 1 to 12

[0130] A radio wave absorption sheet in which the average particle diameter and the addition amount of silicon carbide powder are within the ranges of the present embodiment. I 4H / I 6H All are greater than 0.2, the radio wave absorption amount is greater than 10 dB. In addition, the surface resistance is also greater than 10 10 Ω / □, and the sheet surface is also smooth.

[0131] · Examples 13 and 14

[0132] A radio wave absorption sheet in which the average particle diameter and the addition amount of silicon carbide powder are within the ranges of the present embodiment. I 4H / I 6H All are 0.1 or more and less than 0.2, the radio wave absorption amount is greater than 10 dB. In addition, the surface resistance is also greater than 10 10 Ω / □, and the sheet surface is also smooth.

[0133] · Comparative Examples 1, 3, 5, and 6

[0134] A radio wave absorption sheet in which the addition amount of silicon carbide powder is less than the range of the present embodiment. I 4H / I 6H All are greater than 0.2, the tangent of the dielectric loss angle is lower than 0.2, and the radio wave absorption amount is less than 10 dB.

[0135] · Comparative Examples 2 and 4

[0136] A radio wave absorption sheet in which the addition amount of silicon carbide powder is more than the range of the present embodiment. The shrinkage in the heat treatment process of the formed sheet becomes larger, and cracks are generated on the sheet surface.

[0137] · Comparative Example 7

[0138] A radio wave absorption sheet in which the average particle diameter of silicon carbide powder is greater than the range of the present embodiment. Concavities and convexities are generated on the surface of the formed sheet.

[0139] · Comparative Examples 8 to 13

[0140] I 4H / I 6H A radio wave absorption sheet below the scope of the present invention. I 4H / I 6H All are below 0.1, and the radio wave absorption amount is less than 10 dB. In addition, the addition amounts of silicon carbide powder in Comparative Example 10 and Comparative Example 13 are excessive compared to the range of the present embodiment, so the shrinkage in the heat treatment process of the formed sheet becomes larger, and cracks are generated on the sheet surface.

[0141] [Table 1]

[0142]

[0143] Industrial Applicability

[0144] The electromagnetic wave absorption sheet of the present invention is installed in an electronic device and is particularly effective as an electromagnetic wave noise countermeasure member for absorbing the electromagnetic waves generated in these electronic devices. Compared with existing products, it is a lightweight, thin, and highly insulating electromagnetic wave absorption sheet, which is industrially useful.

Claims

1. A radio wave absorbing sheet, characterized in that, Comprising: a base material made of an organic substance; and powder mainly composed of silicon carbide (SiC) supported in the base material, in X-ray diffraction with Cu-Kα as the radiation source on the surface of the radio wave absorbing sheet, the peak intensity I from 4H-SiC appearing at 2θ = 34.4 to 35° 4H divided by the peak intensity I from 6H-SiC appearing at 2θ = 35 to 36° 6H to obtain a value I 4H / I 6H is 0.1 or more.

2. The radio wave absorbing sheet according to claim 1, characterized in that: The surface resistance of the radio wave absorbing sheet is 10 10 Ω / □ or more.

3. The electromagnetic wave absorbing sheet according to claim 1 or 2, characterized in that, The average particle size of the powder mainly composed of silicon carbide (SiC) is 2 μm or more and 40 μm or less.

4. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein The volume ratio of the powder mainly composed of silicon carbide (SiC) to the radio wave absorbing sheet is 20% or more and 50% or less.

5. The electromagnetic wave absorbing sheet according to claim 1 or 2, characterized in that, The content of Al in the powder mainly composed of silicon carbide (SiC) is 0.1 mass% or more and 0.2 mass% or less, and the content of Fe is 0.1 mass% or more and 0.3 mass% or less.

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