Electron wave absorbing sheet

Abstract

This invention provides an electromagnetic wave absorbing sheet that, using silicon carbide powder which provides lightweight and high insulation, achieves excellent noise attenuation in the submillimeter-wave to millimeter-wave band. The electromagnetic wave absorbing sheet of this invention is characterized by comprising: a substrate composed of an organic material; and silicon carbide (SiC) powder supported on the substrate. In X-ray diffraction using Cu-Kα as a radiation source on the surface of the electromagnetic wave absorbing sheet, a peak intensity I from 4H-SiC appears at 2θ = 34.4–35°. 4H Divide by the peak intensity I from 6H-SiC that appears at 2θ = 35–36°. 6H The obtained value I 4H / I 6H Above the specified value.

Description

Technical Field

[0001] This invention relates to an electromagnetic wave absorbing sheet. Background Technology

[0002] With the rapid development of communications, devices that effectively utilize radio waves in the 3–80 GHz range are becoming increasingly widespread. For example, in fifth-generation (5G) communication systems, which began commercial use in Japan in 2020, there is a growing effective utilization of frequency bands around 3–5 GHz (sub-6 GHz) and 28–40 GHz (millimeter-wave band). Furthermore, in automobiles, with the rapid development of autonomous driving systems, submillimeter-wave radar utilizing frequencies around 24 GHz and millimeter-wave radar utilizing frequencies around 76–79 GHz are becoming increasingly common. On the other hand, electromagnetic interference problems within and outside such devices are becoming increasingly prominent. Therefore, electromagnetic wave absorbing sheets that effectively utilize frequencies that generate electromagnetic interference are becoming increasingly important.

[0003] Electromagnetic wave absorbing sheets function by being mounted on a conductive substrate such as metal, or by having a conductive layer such as metal on the back of the sheet. Compared to surface-reflected waves, the secondary reflected waves, which pass through the interior of the sheet and are totally reflected by the conductive material on the back side, radiate from the surface of the sheet with a phase shift of half a wavelength. The surface-reflected waves and the secondary reflected waves cancel each other out, thus exhibiting electromagnetic wave absorption. Electromagnetic wave absorbing sheets have a structure with a soft resin substrate and fillers for generating the electromagnetic wave absorption effect supported within the substrate. By controlling the material constants (dielectric constant, permeability) and thickness of the substrate, a resonance peak for electromagnetic wave absorption at a specific frequency is achieved. The material constant of the substrate is adjusted according to the substrate material and the amount of filler added, thus controlling the amount of filler within the required range. As a representative example, there are electromagnetic wave absorbing sheets that use flexible resins such as rubber or elastomers as the substrate and add carbonyl iron powder or spinel-type ferrite powder as fillers.

[0004] In recent years, the miniaturization of electronic devices has been a growing trend. Therefore, electromagnetic wave absorbing sheets are placed in confined spaces, requiring a thickness of less than 1 mm. Furthermore, the demand for lightweight electronic devices necessitates even lighter electromagnetic wave absorbing sheets. While the specific gravity of the resin substrate does not vary significantly depending on the type of material, the specific gravity of the loss material varies considerably. Therefore, although it also depends on the amount of filler added, from a lightweight perspective, carbon-based materials or silicon carbide, which have a lower specific gravity than carbonyl iron or soft magnetic ferrites, are preferred as loss materials. Moreover, considering electromagnetic wave absorbing sheets for devices that effectively utilize 3–80 GHz electromagnetic waves, compared to using magnetic fillers with high permeability, which are difficult to achieve in the microwave–millimeter wave band, as loss materials, the design freedom of electromagnetic wave absorbing sheets increases when non-magnetic conductive materials are carried in the resin substrate to cause electrical losses. Considering the practicality of non-magnetic conductive fillers, including cost, carbon-based materials or silicon carbide are also preferred as loss materials.

[0005] Furthermore, since the electromagnetic wave absorbing sheet is positioned around the electronic circuit, if the electromagnetic wave absorbing sheet detaches from the mounting part, and the surface resistance of the electromagnetic wave absorbing sheet is less than 10... 10 When the resistance is Ω / □, there is a risk of short circuit caused by the contact between the electromagnetic wave absorbing sheet and the circuit. However, when using carbon-based materials as the loss material, the surface resistance of the electromagnetic wave absorbing sheet, which has a required amount of carbon-based material added to exhibit excellent electromagnetic wave absorption, is less than 10 Ω / □. 10 Ω / □. To address this issue, an insulating layer such as a PET film is considered. However, depending on the material of the insulating layer, this not only reduces heat resistance and flame retardancy but also increases manufacturing costs. Furthermore, carbon-based material powders have high cohesiveness, making them generally difficult to disperse uniformly in the substrate resin, resulting in significant manufacturing unevenness. In radio wave absorbing sheets using carbon-based materials as the loss material, special equipment and processes are often required to minimize the uneven dispersion of the loss material, which is not preferable from a manufacturing cost perspective.

[0006] On the other hand, when silicon carbide is used as the loss material, the surface resistivity of the electromagnetic absorbing sheet, which has a required amount of silicon carbide added to exhibit excellent electromagnetic wave absorption, exceeds 10 Ω·cm. 10 Ω / □. Furthermore, when the use of expensive silicon carbide fibers is considered an exception, inexpensive granular silicon carbide is used industrially, and it is easily dispersed in the resin substrate. Therefore, when manufacturing a lightweight and highly insulating radio wave absorbing sheet, it is preferable to use silicon carbide powder in the loss material. As a radio wave absorbing sheet using silicon carbide powder in the loss material, there exists a radio wave absorbing sheet described in Patent Document 1.

[0007] However, it is not widely known that in electromagnetic wave absorbing sheets using silicon carbide powder as a lossy material, the crystal structure and impurity content of the silicon carbide powder significantly affect the electromagnetic wave absorption properties of the resulting sheet. It is believed that the conductivity of the silicon carbide powder affects the dielectric constant of the electromagnetic wave absorbing sheet, and that a desired electromagnetic wave absorbing sheet can be obtained by controlling its dielectric constant, i.e., controlling the crystal structure, impurity content, particle size, and amount added of the silicon carbide powder. Patent Document 1 only specifies the particle size and amount of silicon carbide added, without addressing the crystal structure and impurity content. Furthermore, Patent Document 1 discloses an embodiment using high-purity green silicon carbide powder (as a specific example, manufactured by Showa Denko Co., Ltd.: Green Densic). The main crystal structure of silicon carbide is a tetrahedron as its smallest unit, and based on the stacked structure of this tetrahedron, 4H-type and 6H-type crystal structures are mainly present. Depending on the crystal structure, the carrier mobility, a semiconductor characteristic, varies. Compared to the 6H type, the 4H type has higher carrier mobility and a higher dielectric loss tangent tanδ (=ε” / ε’), which is related to the electromagnetic absorption performance and is expressed by the real part ε’ and the imaginary part ε” of the dielectric constant of the electromagnetic absorber. However, GreenDensic's crystal structure is mainly of the 6H type, thus it suffers from a low dielectric loss tangent tanδ and low performance of the electromagnetic absorber.

[0008] Existing technical documents

[0009] Patent documents

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

[0011] The problem the invention aims to solve

[0012] Therefore, in view of the above-mentioned problems, the present invention aims to provide an electromagnetic wave absorbing sheet that, in which silicon carbide powder with lightweight and high insulation is used, can achieve excellent noise attenuation in the submillimeter wave to millimeter wave band.

[0013] Solution for solving the problem

[0014] To address the aforementioned problems, the inventors conducted in-depth research and arrived at the following insights: Specifically, by using silicon carbide (SiC) powder, which provides lightweight and high insulation, as the dielectric loss material supported in the substrate, and by employing silicon carbide (SiC) with X-ray diffraction data within specified conditions, the electromagnetic wave absorbing sheet can be made significantly lighter compared to those using carbonyl iron or soft magnetic ferrite as loss fillers. Furthermore, compared to electromagnetic wave absorbing sheets using carbon-based materials as loss fillers, even without an insulating layer such as a PET film on the sheet surface, the surface resistivity of the electromagnetic wave absorbing sheet exceeds 10 Ω·cm. 10The silicon carbide powder in the substrate is easily and uniformly dispersed in the resin substrate, thus suppressing manufacturing unevenness and stabilizing the electromagnetic wave absorption performance. Furthermore, compared to electromagnetic wave absorbing sheets using conventionally known silicon carbide as a loss filler, thinner profiles can be achieved, further improving electromagnetic wave absorption performance.

[0015] The main structure of the present invention, based on the above insights, is as follows.

[0016] [1] An electromagnetic wave absorbing sheet, characterized in that it comprises: a substrate made of organic matter; and a silicon carbide (SiC) powder supported on the substrate, wherein the peak intensity I of 4H-SiC appears at 2θ = 34.4-35° in X-ray diffraction on the surface of the electromagnetic wave absorbing sheet using Cu-Kα as the radiation source. 4H Divide by the peak intensity I from 6H-SiC that appears at 2θ = 35–36°. 6H The obtained value I 4H / I 6H It is above 0.1.

[0017] [2] According to the electromagnetic wave absorbing sheet described in [1] above, the surface resistance of the electromagnetic wave absorbing sheet is 10 Ω·cm. 10 Ω / □ and above.

[0018] [3] According to the electromagnetic wave absorbing sheet described in [1] or [2] above, the average particle size of the powder mainly composed of silicon carbide (SiC) is 2 μm or more and 40 μm or less.

[0019] [4] The radio wave absorbing sheet according to any one of [1] to [3] above is characterized in that 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.

[0020] [5] The radio wave absorbing sheet according to any one of [1] to [4] above is 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] Invention Effects

[0022] In radio wave absorbing sheets that use silicon carbide powder, which provides lightweight and high insulation, excellent noise attenuation can be achieved in the submillimeter wave to millimeter wave bands. Attached Figure Description

[0023] Figure 1 This is an example showing an X-ray diffraction chart of an electromagnetic absorption sheet. Detailed Implementation

[0024] The embodiments of the present invention will be described below.

[0025] (Electromagnetic wave absorbing sheet)

[0026] The electromagnetic wave absorbing sheet of the present invention is characterized by comprising: a substrate composed of an organic material; and silicon carbide (SiC) powder supported on the substrate, wherein, in X-ray diffraction of the surface of the electromagnetic wave absorbing sheet using Cu-Kα as the radiation source, a peak intensity I from 4H-SiC appears at 2θ = 34.4–35°. 4H Divide by the peak intensity I from 6H-SiC that appears at 2θ = 35–36°. 6H The obtained value I 4H / I 6H It is above 0.1.

[0027] [Substrate]

[0028] The organic material constituting the substrate is not specifically specified. Flexibleness and processability are generally required in electromagnetic wave absorbing sheets, therefore rubber and elastomers are preferred. Among these, silicone resins and acrylic resins are particularly preferred as substrate resins due to their excellent resistance to cold and heat and their strong insulation properties, and their proven use in electromagnetic wave absorbing sheets. There are no particular limitations on the form of the resin raw material; 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 silicones suitable for general industrial applications can be used. Liquid silicones exist in both single-component and two-component forms, and in condensation-reaction and addition-reaction forms; any of these can be used in this invention. Condensation-reaction types require curing time, while addition-reaction types can be expected to cure in a shorter time by incorporating a heating process; therefore, addition-reaction types are preferred if productivity is a consideration. If the viscosity of the liquid silicone is too low, the shape retention during sheet forming deteriorates; if the viscosity is too high, the powder filler is difficult to disperse uniformly. Therefore, liquid silicones with a viscosity of 1 Pa·s or higher and 10 Pa·s or lower are preferred.

[0030] [powder]

[0031] The electromagnetic wave absorbing sheet of the present invention contains powder as a dielectric loss material in a state supported in a substrate. The powder as the dielectric loss material comprises a powder primarily composed of silicon carbide (SiC) (hereinafter also referred to as "silicon carbide powder"). The present invention uses a 4H-type to 6H-type content ratio of 1. 4H / I 6HHigh-purity silicon carbide powder is used as a loss filler. The crystal structure of silicon carbide is primarily tetrahedral, and based on the layered structure of this tetrahedron, it mainly exists in 4H and 6H forms. In high-purity green silicon carbide, the 6H structure is dominant, while the 4H form is less common. In this invention, low-purity black silicon carbide is used; in this case, the 4H and 6H forms coexist, with the 4H form being more prevalent. The silicon carbide powder contained in the radio wave absorbing sheet can be identified by X-ray diffraction on the surface of the radio wave absorbing sheet using Cu-Kα as the radiation source. The X-ray diffraction measurement conditions in this invention are as follows.

[0032] <device>

[0033] Manufacturer: Rigaku Corporation

[0034] Device Name: SmartLab (9kW) Fully Automated Horizontal Multifunctional High-Power X-ray Diffraction System

[0035] X-ray tube: Cu

[0036] <Optical System Conditions>

[0037] CBO selects a narrow slot: BB

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

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

[0040] Photosensitive optical element (PSA): None

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

[0042] <Measurement Conditions>

[0043] Scan axis: 2θ / θ, Mode: Continuous, Range specification: Absolute

[0044] Speed ​​counting time: 3.0 degrees / min

[0045] Scan: 20 degrees ~ 80 degrees

[0046] Data collection interval: 0.01 degrees

[0047] IS: 1 / 2 degree

[0048] Photosensitive slit RS1: 8.0mm

[0049] Photosensitive slit RS2: 13.0mm

[0050] Attenuator: Open

[0051] The data analysis steps are as follows. Using Rigaku Corporation's PDXL powder X-ray analysis software, to eliminate the influence of background in the obtained data, the value yobs-bkg was obtained by subtracting bkg from the diffraction intensity data yobs. Within the range of 2θ = 34.4–35°, the value obtained by subtracting the minimum value from the maximum value of yobs-bkg was set as the peak intensity I from 4H-SiC. 4H Similarly, within the range of 2θ = 35–36°, the value obtained by subtracting the minimum value from the maximum value of yobs-bkg is set as the peak intensity I from 6H-SiC. 6H In this invention, I 4H Divide by I 6H The obtained value I 4H / I 6H The value should be 0.1 or higher, preferably 0.2 or higher. Furthermore, there is no particular limitation on the upper limit, but it is preferably 0.5 or lower.

[0052] The average particle size of the silicon carbide powder is preferably 2 μm or more, more preferably 3 μm or more, more preferably 40 μm or less, and even more preferably 20 μm or less. When the average particle size is less than 2 μm, the flowability and dispersibility of the powder deteriorate, and the manufacturability worsens. On the other hand, when the average particle size is greater than 40 μm, the surface roughness of the film after deposition also increases.

[0053] In this specification, the "average particle size" of silicon carbide powder is determined according to the following steps. First, the cross-section of the radio wave absorbing sheet is ion-polished, and then a backscattered electron image is captured using a scanning electron microscope. To improve measurement accuracy, the magnification is set to 500x to 4000x depending on the powder particle size. 4000x is used when the majority of the powder particles are as small as 2μm, and 500x is used when the majority of the powder particles are as large as 40μm. Next, in the captured image, the maximum diameter of each silicon carbide powder is taken as the particle size of that powder, and the average particle size of all silicon carbide powders within the field of view is defined as the "average particle size" of the silicon carbide powder contained in the radio wave absorbing sheet. Additionally, powder near the edge of the captured image and powder with a particle size (maximum diameter) less than 0.1μm in the captured image are excluded from the measurement.

[0054] The amount of silicon carbide powder added varies depending on the average particle size of the powder, the resonant frequency of the designed electromagnetic absorbing sheet, and the thickness of the sheet. However, when the amount relative to the volume of the electromagnetic absorbing sheet is 20% to 50%, the resonant frequency of electromagnetic absorption can be adjusted in the submillimeter-wave to millimeter-wave bands. Furthermore, when the addition amount exceeds 50%, in the case of wet manufacturing using a scraper molding method, shrinkage during the heat treatment process of the formed sheet increases, making it prone to cracking on the sheet surface. Additionally, the resulting electromagnetic absorbing sheet lacks flexibility. However, if there are no problems in the manufacturing process of the electromagnetic absorbing sheet and flexibility is not required, the amount of silicon carbide powder added can exceed 50%.

[0055] In this specification, the "amount of silicon carbide powder" is determined according to the following steps. Using the backscattered electron image used to determine the "average particle size" of the silicon carbide powder, the colors of the images of the silicon carbide powder and the substrate composed of organic matter are binarized to distinguish the silicon carbide powder and the substrate. After differentiation, the area ratio of the silicon carbide powder is determined and defined as the "amount of silicon carbide powder (volume ratio relative to the electromagnetic absorbing sheet)". Furthermore, during binarization, appropriate settings can be made to clearly define the boundary between the silicon carbide powder and the substrate. In addition, when more than one additive other than silicon carbide powder is present, multi-valued processing can also be used to differentiate them, determine the area ratio of the silicon carbide powder and the additive, and thus determine the "amount of each".

[0056] As silicon carbide powder, industrially usable silicon carbide powder can be used. Industrially usable silicon carbide powder, specifically for grinding and polishing applications, exists in a high-purity green type and a lower-purity black type. The black type is characterized by a higher content of impurity elements Al and Fe compared to the green type. Typically, the black type contains 0.1–0.2 mass% Al and 0.1–0.3 mass% Fe, while the green type contains 0.01–0.1 mass% Al and 0.02–0.03 mass% Fe. In this invention, the low-purity black type silicon carbide powder is preferably used as a loss filler. The content of impurity elements Al and Fe in the silicon carbide can also be identified. In this case, for example, on the polished surface (ion-polished surface, etc.) of an electromagnetic absorbing sheet, the Al and Fe content can be detected by surface analysis using EPMA (WDS: Wavelength Dispersive X-ray Spectroscopy) of the silicon carbide powder.

[0057] Furthermore, in addition to silicon carbide powder, the electromagnetic wave absorbing sheet of the present invention may further include powders other than silicon carbide powder as dielectric loss materials in the substrate. Moreover, in addition to dielectric loss materials, the electromagnetic wave absorbing sheet of the present invention may also include, as needed, additives such as flame retardants, material constant modifiers, thermal conductivity improvers, extenders, plasticizers, dispersants, and antioxidants in the substrate, within a range that does not affect electromagnetic wave absorption, surface resistivity, or the curing characteristics of the substrate resin. There are no particular limitations on the types of these additives; only one type may be added, or two or more may be added. As an example of an additive, conductive powders such as aluminum powder, copper powder, and silver powder can be cited as material constant modifiers. Furthermore, if a flame retardant is used, it is preferably a flame retardant that will not become an environmentally hazardous substance; examples include hydroxyl compounds such as aluminum hydroxide and magnesium hydroxide, and nitrogen compounds such as melamine cyanurate. In addition, red phosphorus can also be added as a flame retardant accelerant as needed. To adjust material constants or to function as an additive or thermal conductivity improver, one or more of the following powders can be added to the substrate: silicon oxide, aluminum oxide, magnesium oxide, zirconium oxide, boron nitride, aluminum nitride, spherical graphite powder, spinel ferrite, hexagonal ferrite, iron-based magnetic alloys (carbonyl iron, FeSiAl, FeSi, FeSiCr, Fe-based amorphous alloys, Fe-based nanocrystalline alloys), aluminum alloys, and calcium carbonate.

[0058] When the additive is in powder form, the average particle size of the additive is not particularly limited, but 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 flowability and dispersibility of the powder deteriorate, and the manufacturability worsens. On the other hand, when the average particle size is greater than 40 μm, the surface roughness of the film after film formation also increases.

[0059] When the aforementioned additive is in powder form, the total amount of powder components in the substrate varies depending on the average particle size of the powder, the resonant frequency of the designed electromagnetic absorbing sheet, and the thickness of the electromagnetic absorbing sheet. However, when the volume ratio relative to the electromagnetic absorbing sheet is 20% or more and 50% or less, the resonant frequency of electromagnetic absorption can be adjusted in the submillimeter-wave to millimeter-wave bands. Furthermore, when the addition amount exceeds 50%, in the case of wet manufacturing using a doctor blade forming method, shrinkage during the heat treatment process of the formed sheet increases, making it prone to cracking on the sheet surface. Additionally, the resulting electromagnetic absorbing sheet lacks flexibility. However, if there are no problems in the manufacturing process of the electromagnetic absorbing sheet and the resulting electromagnetic absorbing sheet does not require flexibility, the addition amount of silicon carbide powder can also exceed 50%.

[0060] Next, the mixing process of the organic material (raw material resin) that will become the substrate 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 mixer. If the raw material resin is compounded silicone rubber, it can be mixed using a pressure kneader or open rollers. In this case, it is preferable to cool to below 100°C while mixing to prevent vulcanization due to the exothermic reaction of the materials during mixing. Furthermore, when using doctor blade molding as the coating method in the molding method described later, organic solvents with good solubility in the raw material resin, such as toluene or methyl ethyl ketone, can be added to the mixture as a solvent to adjust the viscosity of the mixture so that doctor blade molding can be performed.

[0061] The sheeting of the obtained compound can be achieved by any of the following methods: compression molding, extrusion molding, calendering, roll forming, and scraper forming. As an example of compression molding, the compound is fed into a mold with an engraving, so that the formed radio-absorbing sheet reaches a specified thickness, and compression molding is performed for 5 to 30 minutes at a temperature of 120–200°C, where organosilicon vulcanization is carried out.

[0062] The surface resistivity of the electromagnetic wave absorbing sheet is preferably 10. 10 Ω / □ or higher, more preferably 10 11 Ω / □ or higher. If the surface resistance of the electromagnetic wave absorbing sheet is 10... 10 With an Ω / □ or higher, sufficiently high insulation can be obtained. For example, even if the electromagnetic wave absorbing sheet detaches from the mounting part, the risk of short circuit caused by the electromagnetic wave absorbing sheet coming into contact with the circuit can be reduced.

[0063] The thickness of the electromagnetic wave absorbing sheet is preferably 0.15 mm or more and 1 mm or less. When the sheet thickness is less than 0.15 mm, sufficient operational strength cannot be obtained. Furthermore, when the thickness exceeds 1 mm, it cannot cope with the thinning and miniaturization of electronic devices. In the case of electromagnetic wave absorbing sheets, although it also depends on the relative permittivity of the substrate, when the thickness is in the range of 0.15 mm or more and 1 mm or less, the resonant frequency of electromagnetic wave absorption can be adjusted in the submillimeter wave zone to the millimeter wave zone.

[0064] Electromagnetic wave absorbing sheets are used to attach to any object, so an adhesive layer can be installed on the back of the sheet. Furthermore, when the object to which the electromagnetic wave absorbing sheet is attached is not conductive, a conductive layer such as a metal can be installed on the back of the sheet. The material of the conductive layer is not particularly limited, and metallic materials are commonly used. Examples of metals include brass, copper, iron, nickel, stainless steel, and aluminum. In addition to being formed from a single metallic substance, conductive layers such as aluminum deposited on a film can also be used. The thickness of the conductive layer should be set to ensure that incident electromagnetic waves are reflected and that the sheet has good 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, incident electromagnetic waves will pass through the conductive layer, reducing the amount of reflection at the conductive layer. On the other hand, when the thickness of the conductive layer is greater than 300 μm, in addition to increasing the overall thickness of the electromagnetic wave absorbing sheet, the sheet loses its flexibility.

[0065] Although the sheet of the present invention is conceived as an electromagnetic wave absorbing sheet for far-field applications, it can also be used as a noise suppression sheet for near-field applications. The sheet of the present invention functions as a near-field noise suppression sheet that uses silicon carbide powder as a loss material to suppress electric field noise through electrical and dielectric losses. If a sheet with the substrate structure of the present invention is used, a sheet with a dielectric loss tangent tanδ (=ε” / ε’) greater than 0.2 can be obtained, making it a practical noise suppression sheet. This dielectric loss tangent tanδ (=ε” / ε’) is an indicator of the degree of energy loss when the real part of the dielectric constant is ε’ and the imaginary part is ε”. Furthermore, silicon carbide powder has a high thermal conductivity of 100–350 W / mK; therefore, if a sheet with the substrate structure of the present invention is used, a sheet with a thermal conductivity greater than 0.5 W / mK can be obtained, making it suitable for use as a sheet that combines noise suppression and thermal conductivity.

[0066] Example

[0067] The present invention will now be described based on an embodiment of a radio wave absorbing sheet that has the maximum reflection attenuation at 76.5 GHz, but the present invention is not limited thereto.

[0068] <Fabrication of Radio Wave Absorbing Sheets>

[0069] The radio wave absorbing sheet is fabricated using a doctor blade molding method. As the coating liquid, a coating liquid obtained by mixing silicone resin, silicon carbide powder, and toluene as a solvent using a planetary stirrer is used. After the coating liquid is formed on a PET film using a doctor blade molding method, heat treatment is performed to remove the toluene and cure the silicone resin, thus fabricating the radio wave absorbing sheet to a specified thickness. Appropriate use of silicon carbide powder (I...) 4H / I6H Different powders were used as Examples 1-14 and Comparative Examples 1-13. Furthermore, the average particle size, amount added, Al content, and Fe content of the aforementioned silicon carbide powders are described in Table 1. Additionally, the film thickness was set to the film thickness that provides the best electromagnetic wave absorption at the same level of average particle size and amount added.

[0070] <Characteristic Evaluation>

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

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

[0073] "The real part of the dielectric constant ε', the imaginary part of the dielectric constant ε, and the dielectric loss tangent tanδ"

[0074] Vector Network Analyzer

[0075] Manufacturer: Keysight Technologies

[0076] Device Name: Vector Network Analyzer M9374A (60–90 GHz)

[0077] [Device Fixture]

[0078] Manufacturer: Kecomo Co., Ltd.

[0079] Apparatus Name: S-parameter Method Free Space Relative Permittivity Measurement System DPS24

[0080] [software]

[0081] Manufacturer: Keysight Technologies

[0082] Software Name: VNA Soft Front Panel

[0083] [software]

[0084] Manufacturer: Kecomo Co., Ltd.

[0085] Software Name: S-parameter Method for Determining Dielectric Constant and Permeability

[0086] [Measurement Conditions]

[0087] Frequency range: 60–90 GHz

[0088] Data count: 1601

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

[0090] Bandwidth: 1kHz

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

[0092] Time gate span: 0.5 ns

[0093] [Determination Method]

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

[0095] "Radio wave absorption"

[0096] [Device]

[0097] Manufacturer: Keysight Technologies

[0098] Device Name: Vector Network Analyzer M9374A (60–90 GHz)

[0099] as well as

[0100] Manufacturer: Kecomo Co., Ltd.

[0101] Device Name: Lens Antenna Type Oblique Incident Reflection Attenuation Measurement Device LAF-26.5A

[0102] [software]

[0103] Manufacturer: Keysight Technologies

[0104] Software Name: VNA Soft Front Panel

[0105] as well as

[0106] Manufacturer: Kecomo Co., Ltd.

[0107] Software Name: Free Space Type Radio Wave Absorption Measurement Program

[0108] [Measurement Conditions]

[0109] Frequency range: 60–90 GHz

[0110] Data count: 1601

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

[0112] Bandwidth: 1kHz

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

[0114] Time gate span: 0.5ns

[0115] [Determination Method]

[0116] The back of the fabricated sheet was attached to the metal stage of the reflection attenuation measuring device using double-sided tape as an adhesive layer. The electromagnetic wave absorption was measured in this state, and the results at a frequency of 76.5 GHz are shown in Table 1. Furthermore, in the judgment results in Table 1, cases with electromagnetic wave absorption of 10 dB or more are considered OK, and cases with absorption less than 10 dB are considered NG.

[0117] Surface resistivity

[0118] [Device]

[0119] Manufacturer: Simco Japan

[0120] Device Name: ST-4 Surface Resistance Meter

[0121] [Determination Method]

[0122] The surface resistance was measured with the ST-4 surface resistivity meter placed on the surface of the fabricated sheet. The measured surface resistance was expressed in terms of 10... x Ω / □ indicates that the results are shown in Table 1. Additionally, in the judgment results of Table 1, 10 10 Ω / □ and above is OK, less than 10 10 Ω / □ means NG.

[0123] "Outer View"

[0124] The surface condition of the sheet was judged by visual inspection and touch, and the results are shown in Table 1. In Table 1, a smooth surface without cracks or unevenness is considered OK, while cracks or unevenness are considered NG.

[0125] "Comprehensive judgment"

[0126] If all three criteria mentioned above—electromagnetic absorption, surface resistivity, and wafer appearance—are met, the overall assessment is considered OK. Otherwise, the overall assessment is considered NG, as shown in Table 1.

[0127] <Results>

[0128] Table 1 shows the results of Examples 1-14 and Comparative Examples 1-13.

[0129] Examples 1-12

[0130] The average particle size and amount of silicon carbide powder added are within the range of the electromagnetic wave absorbing sheet described in this embodiment. 4H / I 6H All values ​​are greater than 0.2, and the electromagnetic wave absorption is greater than 10 dB. Furthermore, the surface resistivity is also greater than 10 dB. 10 Ω / □, and the surface of the sheet is also smooth.

[0131] • Examples 13 and 14

[0132] The average particle size and amount of silicon carbide powder added are within the range of the electromagnetic wave absorbing sheet described in this embodiment. 4H / I 6H All values ​​are above 0.1 and below 0.2, with an electromagnetic wave absorption greater than 10 dB. Furthermore, the surface resistivity is also greater than 10 Ω. 10 Ω / □, and the surface of the sheet is also smooth.

[0133] • Compare Examples 1, 3, 5, and 6

[0134] The amount of silicon carbide powder added is less than that of the electromagnetic wave absorbing sheet described in this embodiment. 4H / I 6H All values ​​are greater than 0.2, the dielectric loss tangent is less than 0.2, and the electromagnetic wave absorption is less than 10dB.

[0135] Comparative Examples 2 and 4

[0136] The electromagnetic wave absorbing sheet contains more silicon carbide powder than is permitted in this embodiment. During the heat treatment process after molding, shrinkage increases, causing cracks to form on the sheet surface.

[0137] Comparative Example 7

[0138] The silicon carbide powder has an average particle size larger than that of the electromagnetic wave absorbing sheet described in this embodiment. This results in an uneven surface on the formed sheet.

[0139] Comparative Examples 8-13

[0140] I 4H / I 6H Electromagnetic absorbing sheets that fall outside the scope of this invention. 4H / I 6H All values ​​were below 0.1, and the electromagnetic wave absorption was less than 10 dB. Furthermore, the amount of silicon carbide powder added in Comparative Examples 10 and 13 was excessive compared to the range of this embodiment, resulting in increased shrinkage during the heat treatment process of the formed sheet and the formation of cracks on the sheet surface.

[0141] [Table 1]

[0142]

[0143] Industrial availability

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

Claims

1. An electromagnetic wave absorbing sheet, characterized by comprising: comprises: a base material composed of an organic substance; and a powder mainly composed of silicon carbide (SiC) supported in the base material, in X-ray diffraction of a surface of the electric wave absorbing sheet with Cu-Ka as a radiation source, a peak intensity I from 4H-SiC appearing at 2θ = 34.4 to 35° 4H is 0.1 or more, an average particle diameter of the powder mainly composed of silicon carbide (SiC) is 2 μm or more and 40 μm or less, and a volume ratio of the powder mainly composed of silicon carbide (SiC) with respect to the electric wave absorbing sheet is 20% or more and 50% or less. 6H 4H 6H is 0.1 or more, an average particle diameter of the powder mainly composed of silicon carbide (SiC) is 2 μm or more and 40 μm or less, and a volume ratio of the powder mainly composed of silicon carbide (SiC) with respect to the electric wave absorbing sheet is 20% or more and 50% or less.​​ 2. The electromagnetic wave absorber sheet according to claim 1, wherein The surface resistance of the electromagnetic wave absorbing sheet is 10 10 Ω / □ or more.

3. The electromagnetic wave absorber sheet according to claim 1 or 2, characterized by The content of Al in the silicon carbide (SiC)-based powder 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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