Radio wave absorbing sheet

Abstract

Provided is a radio wave absorbing sheet with little variation in radio wave absorption amount associated with temperature changes. A first radio wave absorbing sheet comprises a base material made of an organic substance and a powder supported within the base material. The powder is composed of MnZn ferrite powder, aluminum powder, and silicon carbide powder. When the volume ratio of the MnZn ferrite powder is VM, the volume ratio of the aluminum powder is VA, and the volume ratio of the silicon carbide powder is VS with respect to the entire radio wave absorbing sheet, the silicon carbide occupancy ratio RS1 represented by VS / (VM + VA + VS) is 1.5-41% inclusive.

Description

Radio wave absorbing sheet

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

[0002] As communications become more advanced, devices using radio waves in the 3-80 GHz frequency band are becoming more widespread. For example, the fifth-generation communication system (5G), which began commercial use in Japan in 2020, is increasingly using sub-6 frequencies of 3-5 GHz and millimeter-wave frequencies around 28-40 GHz. Furthermore, with the advancement of autonomous driving systems, the use of quasi-millimeter-wave radars using frequencies around 24 GHz and millimeter-wave radars using frequencies around 76-79 GHz is becoming more widespread. Meanwhile, these electronic devices are also facing issues of electromagnetic interference both internally and externally. Therefore, radio wave absorbing sheets that function at frequencies where radio wave interference is a problem are becoming increasingly useful.

[0003] A radio wave absorbing sheet exhibits its radio wave absorbing function by being attached to a substrate having electrical conductivity such as metal, or by providing a conductive layer such as metal on the back surface of the radio wave absorbing sheet. Specifically, a surface reflected wave reflected on the surface of the radio wave absorbing sheet passes through the radio wave absorbing sheet and is totally reflected by a conductive material on the back surface, and the phase of the secondary reflected wave radiated from the surface of the radio wave absorbing sheet is shifted by half a wavelength, and these surface reflected wave and secondary reflected wave cancel each other out, thereby exhibiting radio wave absorption properties (so-called λ / 4 type radio wave absorbing sheet).

[0004] A radio wave absorbing sheet typically has a structure in which a soft resin and / or rubber substrate is supported with a filler for generating a radio wave absorbing effect. The material constants (dielectric constant, magnetic permeability) and thickness of the sheet are controlled to target a resonance peak of radio wave absorption at a specific frequency. The material constants of the radio wave absorbing sheet are adjusted by the material of the substrate that constitutes the sheet and the amount of filler added. Therefore, it is important to control the amount of filler added within the required range.

[0005] A typical example of the above-mentioned radio wave absorbing sheet is one that uses a flexible resin such as rubber or elastomer as a base material and adds carbonyl iron powder or spinel ferrite powder as a filler.

[0006] In recent years, electronic devices that use radio wave absorbing sheets have become increasingly smaller, which is one of the reasons why heat generated by IC chips during operation is trapped inside the electronic devices, causing them to reach high temperatures. As a result, radio wave absorbing sheets are also exposed to high-temperature environments, with temperatures reportedly exceeding 100°C. Under these circumstances, radio wave absorbing sheets designed to maximize radio wave absorption at room temperature have the problem that the material constant of the filler changes when exposed to high-temperature environments, which in turn reduces the radio wave absorption capacity and reduces the effectiveness of electromagnetic wave countermeasures within the electronic devices.

[0007] As a technique for addressing this problem, for example, Patent Document 1 discloses a magnetic material containing epsilon iron oxide and hexagonal ferrite.

[0008] Patent No. 7105435

[0009] However, the magnetic material disclosed in Patent Document 1 has a difference of 2.0 GHz or more between the frequency at which the absorption characteristics (unit: decibel) are best at 20°C, which corresponds to a room temperature environment, and the frequency at which the absorption characteristics are best at 120°C, which corresponds to a high temperature environment.

[0010] Furthermore, the sheet containing a magnetic material disclosed in Patent Document 1 is a magnetic loss type radio wave absorbing sheet. Compared to a dielectric loss type radio wave absorbing sheet, such a magnetic loss type radio wave absorbing sheet has a high frequency dependency, and the radio wave absorbing effect weakens when the frequency is further away from the target frequency band than a certain frequency. Therefore, such a magnetic loss type radio wave absorbing sheet lacks versatility.

[0011] Therefore, an object of the present invention is to provide a radio wave absorbing sheet in which the amount of radio wave absorption varies little with temperature changes.

[0012] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, it was found that by combining, in a predetermined manner, a filler whose real part of the dielectric constant increases with increasing temperature and a filler whose real part of the dielectric constant decreases with increasing temperature, it is possible to obtain a radio wave absorbing sheet whose radio wave absorption amount varies little with temperature changes. The present invention has been made based on the above findings. That is, the gist of the present invention is as follows.

[0013] [1] A radio wave absorbing sheet comprising a substrate made of an organic material and a powder carried in the substrate, wherein the powder is composed of MnZn ferrite powder, aluminum powder, and silicon carbide powder, and the volume ratio of the MnZn ferrite powder to the entire radio wave absorbing sheet is V Ｍ , the volume ratio of aluminum powder is V Ａ , the volume ratio of silicon carbide powder is V Ｓ When V Ｓ / (V Ｍ +V Ａ +V Ｓ ) silicon carbide occupancy ratio R Ｓ１ is 1.5% or more and 41% or less.

[0014] [2] A radio wave absorbing sheet comprising a substrate made of an organic material and a powder carried in the substrate, wherein the powder is composed of MnZn ferrite powder and silicon carbide powder, and the volume ratio of the MnZn ferrite powder to the entire radio wave absorbing sheet is V Ｍ , the volume ratio of silicon carbide powder is V Ｓ When V Ｓ / (V Ｍ +V Ｓ ) silicon carbide occupancy ratio R Ｓ２ 10. A radio wave absorbing sheet, characterized in that the modulus of elasticity is 0.5% or more and 29% or less.

[0015] [3] V Ｍ +V Ａ +V Ｓ The total filler volume V is expressed as Ｐ１ The radio wave absorbing sheet according to [1], wherein the ratio of the surface area to the surface area is 35% or more and 60% or less.

[0016] [4] V Ｍ+V Ｓ The total filler volume V is expressed as Ｐ２ The radio wave absorbing sheet according to [2], wherein the ratio is 30% or more and 60% or less.

[0017] [5] The radio wave absorbing sheet according to any one of [1] to [4], wherein the average particle size of the MnZn ferrite powder, the average particle size of the silicon carbide powder, and the average particle size of the aluminum powder, if included, are all 1 μm or more and 60 μm or less.

[0018] [6] The radio wave absorbing sheet according to any one of [1] to [5], which is a dielectric loss type.

[0019] [7] The frequency at which the return loss peaks at 25°C is f ２５ , the frequency at which the return loss peaks at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | is 2.0 GHz or less, and the frequency f at 25 ° C. ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０ The radio wave absorbing sheet according to any one of [1] to [6], wherein

[0020] [8] A dielectric filler having a dielectric constant real part with a positive temperature dependency and a dielectric filler having a dielectric constant real part with a negative temperature dependency are mixed, and the frequency at which the return loss peaks at 25°C is defined as f ２５ , the frequency at which the return loss peaks at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | is 2.0 GHz or less, and the frequency f at 25 ° C. ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０are all 10 dB or more.

[0021] According to the present invention, it is possible to provide a radio wave absorbing sheet in which the amount of radio wave absorption varies little with temperature changes.

[0022] 10 shows actual measurement data of the return loss of the radio wave absorbing sheet of Example 5. FIG. 11 shows actual measurement data of the return loss of the radio wave absorbing sheet of Comparative Example 1.

[0023] Hereinafter, embodiments (first embodiment, second embodiment, and third embodiment) of the radio wave absorbing sheet of the present invention will be described. However, such descriptions are intended to exemplify the present invention and do not limit the present invention in any way.

[0024] (First Radio Wave Absorbing Sheet) The radio wave absorbing sheet according to the first embodiment of the present invention (hereinafter, sometimes referred to as "first sheet") is a radio wave absorbing sheet comprising a substrate made of an organic material and powder carried in the substrate, the powder being composed of MnZn ferrite powder, aluminum powder, and silicon carbide powder. That is, the filler contained in the first sheet is composed of three types of filler (powders). The first sheet has a volume ratio of MnZn ferrite powder to the entire radio wave absorbing sheet of V Ｍ , the volume ratio of aluminum powder is V Ａ , the volume ratio of silicon carbide powder is V Ｓ When V Ｓ / (V Ｍ +V Ａ +V Ｓ ) silicon carbide occupancy ratio R Ｓ１ is 1.5% or more and 41% or less.

[0025] According to the inventors' intensive studies, it has been found that the real part of the dielectric constant of a radio wave absorbing sheet containing only, for example, MnZn ferrite and / or aluminum as a filler decreases as the temperature increases. That is, it has been found that, for example, MnZn ferrite and aluminum are fillers whose real part of the dielectric constant decreases as the temperature increases. This is presumably due to the scattering of electrons due to lattice vibration, which increases the electrical resistance of the filler. Therefore, in a radio wave absorbing sheet containing only, for example, MnZn ferrite and / or aluminum as a filler, as the temperature increases, the peak of radio wave absorption shifts to the higher frequency side, and the radio wave absorption at the target frequency decreases.

[0026] Furthermore, according to the inventors' intensive research, it has been found that the real part of the dielectric constant of a radio wave absorbing sheet containing only silicon carbide as a filler increases with increasing temperature. That is, it has been found that silicon carbide, for example, is a filler whose real part of the dielectric constant increases with increasing temperature. This is presumably due to the fact that as the temperature increases, the carrier density increases and the electrical resistance of the filler decreases. Therefore, in a radio wave absorbing sheet containing only silicon carbide as a filler, for example, as the temperature increases, the peak of the radio wave absorption amount shifts to the lower frequency side, and the radio wave absorption amount at the target frequency decreases.

[0027] Based on the above-mentioned findings of the inventors, the first sheet is made by mixing three types of fillers: MnZn ferrite powder and aluminum powder, which are fillers whose real part of the dielectric constant decreases with increasing temperature, and silicon carbide powder, which is a filler whose real part of the dielectric constant increases with increasing temperature. By adjusting the volume ratios of these fillers, it is possible to suppress the phenomenon of the frequency at which the wave absorption (return loss) peaks (hereinafter referred to as the "peak frequency") shifting with temperature. As a result, the first sheet is able to maintain the wave absorption at a target frequency at a certain level or higher, even with temperature changes. Thus, the first sheet is based on a three-type filler configuration (MnZn ferrite powder, aluminum powder, and silicon carbide powder) and has been developed by discovering a correlation between the volume ratios of these fillers and the wave absorption at a target frequency.

[0028] Specifically, as described above, the first sheet has a volume ratio of MnZn ferrite powder to the entire sheet of V Ｍ , the volume ratio of aluminum powder to the entire sheet is V Ａ , the volume ratio of silicon carbide powder to the entire sheet is V Ｓ When V Ｓ / (V Ｍ +V Ａ +V Ｓ ) silicon carbide occupancy ratio R Ｓ１ In the sheet based on the above-mentioned three types of filler configuration, the silicon carbide occupation ratio R Ｓ１ If the silicon carbide occupancy ratio R is less than 1.5%, the peak frequency will be significantly shifted to the high frequency side as the temperature increases. Ｓ１ If is less than 1.5%, the peak frequency at 25°C is f ２５ , the peak frequency at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | (hereinafter, this absolute value may be referred to as "frequency change") exceeds 2.0 GHz. As a result, the fluctuation of the radio wave absorption amount due to temperature change becomes large. Furthermore, in the sheets based on the above-mentioned three types of filler configurations, the silicon carbide occupancy ratio R Ｓ１ If the silicon carbide occupancy ratio R exceeds 41%, the peak frequency significantly shifts to the lower frequency side as the temperature increases. Ｓ１ If the frequency change ratio exceeds 41%, the frequency change will exceed 2.0 GHz, resulting in a large fluctuation in the amount of radio wave absorption due to temperature changes.

[0029] In particular, the silicon carbide occupancy ratio R in the first sheet Ｓ１ From the viewpoint of further reducing fluctuations in the amount of electromagnetic wave absorption due to temperature changes, it is preferable that the ratio is 6% or more and 37% or less, and more preferably 12% or more and less than 32%.

[0030] The first sheet is V Ｍ +VＡ +V Ｓ The total filler volume V is expressed as Ｐ１ In the first sheet, the total filler volume V Ｐ１ When the total volume of the filler V is 35% or more and 60% or less, the resonance frequency of the radio wave absorption amount can be adjusted in the quasi-millimeter wave band to the millimeter wave band. Ｐ１ If the filler total volume V is 60% or less, when a sheet is produced by a wet method such as a doctor blade molding method, shrinkage increases in the heat treatment step of the sheet after molding, and cracks tend to easily occur on the surface of the sheet. Ｐ１ However, if there are circumstances where there is no problem in the manufacturing process of the radio wave absorbing sheet and flexibility of the radio wave absorbing sheet to be obtained is not required, the total volume of filler V Ｐ１ It is acceptable for the ratio to be more than 60%.

[0031] In this specification, the volume ratio of each filler to the entire sheet (in the case of the first sheet, the volume ratio V of the MnZn ferrite powder) is Ｍ , volume ratio V of aluminum powder Ａ , volume ratio V of silicon carbide powder Ｓ ) is determined by the following procedure. First, the radio wave absorbing sheet is cut in a direction perpendicular to the in-plane direction of the sheet, and the cross section is polished by ion milling, after which a backscattered electron image is taken with a scanning electron microscope. Next, using the taken backscattered electron image, the colors of the images of the organic substrate and each filler (in the case of the first sheet, MnZn ferrite powder, aluminum powder, and silicon carbide powder) are multi-valued to separate each material. After the separation is complete, the area ratio of each filler is determined, and this is defined as the volume ratio (sometimes referred to as the added amount) of that filler to the entire sheet. Note that when performing multi-valued processing, appropriate settings can be made so that the boundaries between each material are clear.

[0032] Next, various materials that may be included in the sheet will be described.

[0033] <Substrate> The organic material constituting the substrate is not particularly limited. However, since flexibility and cuttability are generally required for radio wave absorbing sheets, the organic material constituting the substrate is preferably one or more selected from rubber, elastomer, and resin. Among these, the organic material constituting the substrate is preferably one or more selected from silicone resin and acrylic resin, since these have excellent heat and cold resistance and insulation resistance and have a long history of use in radio wave absorbing sheets. Such resin may be a millable resin or a liquid resin.

[0034] Below, silicone resin will be described as an example. Liquid silicone resins available commercially for general industrial use can be used. Silicone resins are available in one-component and two-component liquid types, and curing types include condensation reaction and addition reaction types. Either type of silicone resin can be used. Condensation reaction silicone resins require time to cure, while addition reaction silicone resins can be expected to cure in a relatively short time by incorporating a heating step. Therefore, from the perspective of productivity, it is preferable to use an addition reaction silicone resin. Liquid silicones with too low a viscosity will have poor shape retention during sheet molding, while liquid silicones with too high a viscosity will have difficulty in uniformly dispersing the filler. Therefore, a viscosity of 1 to 10 Pa·s is preferred.

[0035] <Powder> The first sheet includes a powder carried in the substrate, and the powder is composed of three types of fillers: MnZn ferrite powder, aluminum powder, and silicon carbide powder.

[0036] The MnZn ferrite powder contained in the sheet preferably has an average particle size of 1 μm or more and 60 μm or less. If the average particle size of the MnZn ferrite powder is 1 μm or more, the fluidity and dispersibility of the filler are well maintained, and deterioration of manufacturability can be suppressed. Furthermore, if the average particle size of the MnZn ferrite powder is 60 μm or less, the smoothness of the molded sheet surface can be well maintained. From the same viewpoint, the average particle size of the MnZn ferrite powder is more preferably 2 μm or more, even more preferably 3 μm or more, and more preferably 50 μm or less, even more preferably 40 μm or less.

[0037] The aluminum powder (i.e., powder of aluminum alone) contained in the first sheet preferably has an average particle size of 1 μm or more and 60 μm or less. If the average particle size of the aluminum powder is 1 μm or more, the fluidity and dispersibility of the filler are well maintained, and deterioration of manufacturability can be suppressed. Furthermore, if the average particle size of the aluminum powder is 60 μm or less, the smoothness of the molded sheet surface can be well maintained. From the same viewpoint, the average particle size of the aluminum powder is more preferably 2 μm or more, even more preferably 3 μm or more, and more preferably 50 μm or less, even more preferably 40 μm or less.

[0038] The silicon carbide powder contained in the sheet preferably has an average particle size of 1 μm or more and 60 μm or less. If the average particle size of the silicon carbide powder is 1 μm or more, the fluidity and dispersibility of the filler are well maintained, and deterioration of manufacturability can be suppressed. Furthermore, if the average particle size of the silicon carbide powder is 60 μm or less, the smoothness of the molded sheet surface can be well maintained. From the same viewpoint, the average particle size of the silicon carbide powder is more preferably 2 μm or more, even more preferably 3 μm or more, and more preferably 50 μm or less, even more preferably 40 μm or less.

[0039] In this specification, the average particle size of each filler can be measured using a particle size distribution analyzer. The average particle size of each filler can also be measured from the radio wave absorbing sheet as a manufactured product. Specifically, first, the cross section of the radio wave absorbing sheet is polished by ion milling, and then a backscattered electron image is captured using a scanning electron microscope. To improve measurement accuracy, the magnification is set to 500x to 5000x according to the filler particle size. When the filler particle size is mainly small, such as 1 μm, 5000x is preferred, and when the filler particle size is mainly large, such as 60 μm, 500x is preferred. Next, in the captured backscattered electron image, the colors of the images of each filler (in the first sheet, MnZn ferrite powder, aluminum powder, and silicon carbide powder) are multi-valued to separate the fillers. After separation is complete, the circle-equivalent diameter of each filler is defined as the particle size of that filler, and the average value of the particle sizes of all the fillers in the field of view is defined as the average particle size of the filler contained in the radio wave absorbing sheet. Fillers that are located at the edge of the captured backscattered electron image and fillers with particle sizes of less than 0.1 μm on the captured backscattered electron image are removed from the measurement target.

[0040] As the silicon carbide powder, industrially usable silicon carbide powder can be used. Examples of industrially usable silicon carbide powder include silicon carbide powder for grinding and polishing applications, which include 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. Generally, black type silicon carbide powder contains 0.1 to 0.2 mass% Al and 0.1 to 0.3 mass% Fe, while green type silicon carbide powder contains 0.01 to 0.1 mass% Al and 0.02 to 0.03 mass% Fe.

[0041] In particular, it is preferable to use low-purity black silicon carbide powder as the loss filler. Here, the contents of Al and Fe, which are impurity elements in silicon carbide, can be detected by surface analysis of the silicon carbide powder using EPMA (WDS: wavelength dispersive X-ray spectroscopy) on the polished surface (e.g., ion-milled surface) of the radio wave absorbing sheet, for example.

[0042] The crystal structure of silicon carbide powder is mainly a regular tetrahedron with the smallest structure, and the 4H type and 6H type are mainly present due to the stacking structure of the regular tetrahedron. In this regard, high-purity green-type silicon carbide is dominated by the 6H structure, with a small proportion of 4H type. On the other hand, low-purity black-type silicon carbide is a mixture of 4H type and 6H type, with a large proportion of 4H type.

[0043] The silicon carbide powder has a 4H type to 6H type content ratio of I ４Ｈ / I ６Ｈ It is preferable that I ４Ｈ / I ６Ｈ is preferably 0.1 or more, and more preferably 0.2 or more. ４Ｈ / I ６Ｈ When I is 0.1 or more, the dielectric loss tangent tanδ (=ε" / ε'), which is an index showing the degree of energy loss expressed using the real part ε' of the dielectric constant and the imaginary part ε", is high (for example, exceeds 0.2), and a more practical radio wave absorbing sheet is obtained. ４Ｈ / I ６Ｈ The upper limit of is not particularly limited, but is preferably 0.5 or less.

[0044] The characteristics of the silicon carbide powder contained in the radio wave absorbing sheet, especially I ４Ｈ / I ６Ｈ The radio wave absorbing sheet surface can be identified by X-ray diffraction using Cu—Kα as a radiation source. The X-ray diffraction measurement conditions are as follows:

[0045] <Apparatus> Manufacturer: Rigaku Instrument name: Fully automated horizontal multipurpose high-power X-ray diffraction instrument SmartLab (9 kW) Tube: Cu <Optical system conditions> CBO selection slit: BB Incident parallel slit (Soller / PSC): 5.0 deg Length limiting slit (IS length): 10.0 mm Receiving optical element (PSA): None Receiving parallel slit (Soller): 5.0 deg <Measurement conditions> Scan axis: 2θ / θ, Mode: Continuous, Range specification: Absolute Speed ​​counting time: 3.0 deg / min Scan: 20 deg to 80 deg Data collection interval: 0.01 deg IS: 1 / 2 deg, Receiving slit RS1: 8.0 mm Receiving slit RS2: 13.0 mm Attenuator: Open

[0046] Next, the procedure for analyzing the measurement data is as follows. Using Rigaku's integrated powder X-ray analysis software PDXL, the value yobs-bkg is obtained by subtracting bkg from the diffraction intensity data yobs in order to eliminate the background effect of the obtained data. The value obtained by subtracting the minimum value of yobs-bkg from the maximum value in the range of 2θ = 34.4 to 35° is used as the peak intensity I derived 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 the peak intensity I derived from 6H—SiC. ６Ｈ And these I ４Ｈ and I ６Ｈ Using ４Ｈ / I ６Ｈ can be obtained.

[0047] <Other Additives> In addition to the substrate and powder described above, the sheet may contain other additives as long as they do not reduce the radio wave absorption properties or affect the curing characteristics of the substrate. Examples of such additives include flame retardants, flame retardant aids, thermal conductivity improvers, bulking agents, plasticizers, dispersants, antioxidants, etc. These additives may be used alone or in combination of two or more. For example, flame retardants that do not impose a burden on the environment are preferred, and examples include hydroxyl compounds such as aluminum hydroxide and magnesium hydroxide; nitrogen-based compounds such as melamine cyanurate; and the like. Examples of flame retardant aids include red phosphorus. Note that such additives may be in powder form, but in this case, the powdered additive is not included in the powder composed of MnZn ferrite powder, silicon carbide powder, etc., in the present invention.

[0048] <Sheet characteristics> The thickness of the sheet is preferably 0.10 mm or more and 1 mm or less. If the sheet thickness is 0.10 mm or more, it can obtain strength that allows it to be handled. Furthermore, if the thickness is 1 mm or less, it can accommodate the trend toward thinner and smaller electronic devices. Furthermore, depending on the relative dielectric constant of the substrate, if the sheet thickness is 0.10 mm or more and 1 mm or less, it can be a sheet that has a resonance of radio wave absorption in the frequency range of 18 to 90 GHz.

[0049] In the high frequency band above 18 GHz, MnZn ferrite does not have magnetic loss due to the Snoke limit. Therefore, MnZn ferrite acts as a dielectric loss type filler in the high frequency band above 18 GHz. Furthermore, silicon carbide does not have magnetic loss, so it also acts as a dielectric loss type filler. Therefore, since the sheet contains silicon carbide and MnZn ferrite, it is a dielectric loss type radio wave absorbing sheet. In other words, it is a radio wave absorbing sheet used in the frequency range (band) of 18 to 90 GHz.

[0050] In the sheet, the ratio of silicon carbide powder to the entire filler (silicon carbide ratio) is optimized, so the fluctuation of the radio wave absorption amount due to temperature change is small, and the radio wave absorption amount itself is large. Specifically, the frequency at which the return loss of the sheet peaks at 25°C is defined as f ２５ , the frequency at which the return loss peaks at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | (frequency change amount) is preferably 2.0 GHz or less, and the frequency f ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０ In addition, the sheet |f １２０ -f ２５ | (frequency change amount) is more preferably 1.5 GHz or less, and even more preferably 1.0 GHz or less. ２５ and A １２０ is more preferably 15 dB or more, and even more preferably 18 dB or more. In this specification, the absolute value of the return loss is sometimes referred to as the "wave absorption amount." The return loss of the radio wave absorbing sheet can be measured by applying heat to an aluminum plate to which the radio wave absorbing sheet is attached, and scanning the frequency using a return loss measuring device. Specifically, it can be measured by the method described in the Examples.

[0051] <Additional Components> The radio wave absorbing sheet is typically used by being attached to some kind of object. Therefore, the sheet may have an adhesive layer on one side (back side). Furthermore, when the object to which the radio wave absorbing sheet is attached is not conductive, the sheet may have a conductive layer on one side (back side). The material of the conductive layer is not particularly limited, but metals are generally used. Examples of metals include brass, copper, iron, nickel, stainless steel, and aluminum. The conductive layer may be made of a single metal, but it may also be a laminate formed by vapor-depositing a metal such as aluminum onto a film. The thickness of the conductive layer may be set taking into consideration the ability to reflect radio waves incident on the radio wave absorbing sheet and excellent flexibility. Specifically, the thickness is preferably 10 nm to 300 μm, and more preferably 50 nm to 100 μm. If the thickness of the conductive layer is 10 nm or less, radio waves incident on the radio wave absorbing sheet may pass through the conductive layer, resulting in a reduced reflection amount at the conductive layer. Furthermore, if the thickness of the conductive layer exceeds 300 μm, the total thickness of the radio wave absorbing sheet becomes undesirably thick, and there is also a risk that the flexibility of the sheet may be lost.

[0052] The sheet may have a protective layer made of a resin material such as PET resin, PEN resin, or polyvinyl chloride resin on one side (surface) for the purpose of improving weather resistance or ensuring high surface resistance. The thickness of the protective layer is not particularly limited as long as it does not reduce radio wave absorption, but specifically, it is preferably 10 μm to 200 μm, and more preferably 30 μm to 130 μm. However, from the viewpoint of flexibility, it is preferable that the sheet does not have the above-mentioned protective layer.

[0053] <Production of Sheet> The method for producing a sheet is not particularly limited, but an example of the method for producing a sheet includes a kneading step of mixing kneading materials including an organic substance (base material) and various fillers to prepare a kneaded mixture, and a molding step of forming the kneaded mixture into a sheet.

[0054] Regarding the kneading process, for example, when a liquid silicone resin is used as the organic material, it can be mixed and kneaded with a kneading material containing various fillers using a planetary agitator. Furthermore, when a millable silicone resin is used as the organic material, it can be mixed and kneaded with a kneading material containing various fillers using a pressure kneader or open roll. In this case, it is preferable to knead while cooling the material to 100°C or below so that vulcanization does not proceed due to heat generated during kneading. Furthermore, when using a doctor blade molding method as the molding process described below, a solvent may be added to the kneaded material to adjust the viscosity of the kneaded material so that it can be molded using a doctor blade. Examples of the solvent include organic solvents that have good solubility for the organic material (substrate), such as toluene and methyl ethyl ketone.

[0055] Examples of the technique for the molding step (technique for forming a sheet) include compression molding, extrusion molding, rolling molding, calendar roll molding, doctor blade molding, etc. In the case of compression molding, for example, the kneaded material is placed in a mold having an indentation so that the radio wave absorbing sheet after molding has a predetermined thickness, and compression molding can be carried out at a temperature of 120 to 200°C at which vulcanization proceeds for 5 to 30 minutes.

[0056] After the molding step, a heat treatment may be carried out to remove the solvent and harden the organic material.

[0057] The first sheet has been described above, but the present disclosure is not limited to the above description and can be modified as appropriate.

[0058] (Second Wave Absorbing Sheet) The wave absorbing sheet according to the second embodiment of the present invention (hereinafter sometimes referred to as "second sheet") is a wave absorbing sheet comprising a substrate made of an organic material and a powder carried in the substrate, the powder being composed of MnZn ferrite powder and silicon carbide powder. That is, the filler contained in the second sheet is composed of two types of filler (powders). That is, the second sheet differs from the first sheet in that it does not contain aluminum powder. And, the second sheet has a volume ratio of MnZn ferrite powder to the entire wave absorbing sheet of VＭ , the volume ratio of silicon carbide powder is V Ｓ When V Ｓ / (V Ｍ +V Ｓ ) silicon carbide occupancy ratio R Ｓ２ is 0.5% or more and 29% or less.

[0059] Based on the findings of the inventors described above regarding the first sheet, the second sheet is made by mixing two types of fillers: MnZn ferrite powder, a filler whose real part of the dielectric constant decreases with increasing temperature, and silicon carbide powder, a filler whose real part of the dielectric constant increases with increasing temperature, and adjusting the volume ratio between them to suppress the phenomenon of peak frequency shift with temperature changes. As a result, the second sheet is able to maintain a certain level or more of electromagnetic wave absorption at a target frequency even with temperature changes. In this way, the second sheet is based on a two-type filler configuration (MnZn ferrite powder and silicon carbide powder) and was developed by discovering a correlation between the volume ratio of these fillers and the electromagnetic wave absorption at a target frequency.

[0060] Specifically, as described above, the second sheet has a volume ratio of MnZn ferrite powder to the entire sheet of V Ｍ , the volume ratio of silicon carbide powder to the entire sheet is V Ｓ When V Ｓ / (V Ｍ +V Ｓ ) silicon carbide occupancy ratio R Ｓ２ In the sheet based on the above-mentioned two types of filler configuration, the silicon carbide occupation ratio R Ｓ２ If the silicon carbide occupancy ratio R is less than 0.5%, the peak frequency will be significantly shifted to the high frequency side as the temperature increases. Ｓ２ If is less than 0.5%, the peak frequency at 25°C is f ２５ , the peak frequency at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f２５ | (frequency change amount) exceeds 2.0 GHz. As a result, the fluctuation of the radio wave absorption amount due to temperature change becomes large. Furthermore, in the sheet based on the above-mentioned two types of filler configuration, the silicon carbide occupation ratio R Ｓ２ If the silicon carbide occupancy ratio R exceeds 29%, the peak frequency significantly shifts to the lower frequency side as the temperature increases. Ｓ２ If the frequency change ratio exceeds 29%, the frequency change will exceed 2.0 GHz, resulting in a large fluctuation in the amount of radio wave absorption due to temperature changes.

[0061] In particular, the silicon carbide occupancy ratio R in the second sheet Ｓ２ From the viewpoint of further reducing fluctuations in the amount of electromagnetic wave absorption due to temperature changes, it is preferable that the ratio is 4% or more and 25% or less, and more preferably 8% or more and 22% or less.

[0062] The second sheet is V Ｍ +V Ｓ The total filler volume V is expressed as Ｐ２ In the second sheet, the total filler volume V Ｐ２ When the total volume of the filler V is 30% or more and 60% or less, the resonance frequency of the radio wave absorption amount can be adjusted in the quasi-millimeter wave band to the millimeter wave band. Ｐ２ If the filler total volume V is 60% or less, when a sheet is produced by a wet method such as a doctor blade molding method, shrinkage increases in the heat treatment step of the sheet after molding, and cracks tend to easily occur on the surface of the sheet. Ｐ２ However, if there are circumstances where there is no problem in the manufacturing process of the radio wave absorbing sheet and flexibility of the radio wave absorbing sheet to be obtained is not required, the total volume of filler V Ｐ２ It is acceptable for the ratio to be more than 60%.

[0063] In the second sheet, the details or preferred embodiments other than those described above, specifically the substrate, powders (including MnZn ferrite powder and silicon carbide powder), other additives, sheet properties, additional components, and details or preferred embodiments regarding the manufacture of the sheet are the same as those in the first sheet. Therefore, repeated explanations will be omitted and the explanations in the first sheet will be incorporated by reference.

[0064] The second sheet has been described above, but the present disclosure is not limited to the above description and can be modified as appropriate.

[0065] (Third Radio Wave Absorbing Sheet) The radio wave absorbing sheet according to the third embodiment of the present invention (hereinafter, sometimes referred to as the "third sheet") contains a dielectric filler whose real part of the dielectric constant has a positive temperature dependency and a dielectric filler whose real part of the dielectric constant has a negative temperature dependency. These dielectric fillers are mixed and contained in the third sheet. The third sheet has a frequency at which the return loss at 25°C is at a peak, which is referred to as f ２５ , the frequency at which the return loss peaks at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | is 2.0 GHz or less, and the frequency f at 25 ° C. ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０ are each 10 dB or more.

[0066] In this specification, a "dielectric filler whose real part of the dielectric constant has a positive temperature dependency" refers to a filler whose real part of the dielectric constant increases with increasing temperature (25°C to 120°C), and may be simply referred to as a "positive dielectric filler" hereinafter. Specifically, a filler whose real part of the dielectric constant measured at four points, 25°C, 60°C, 90°C, and 120°C, satisfies (real part of the dielectric constant at 25°C) < (real part of the dielectric constant at 60°C) < (real part of the dielectric constant at 90°C) < (real part of the dielectric constant at 120°C) is defined as a "dielectric filler whose real part of the dielectric constant has a positive temperature dependency." In this specification, a "dielectric filler whose real part of the dielectric constant has a negative temperature dependency" refers to a filler whose real part of the dielectric constant decreases with increasing temperature (25°C to 120°C), and may be simply referred to as a "negative dielectric filler" hereinafter. Specifically, a filler that satisfies the following relationship regarding the real part of the dielectric constant measured at four points of 25°C, 60°C, 90°C, and 120°C is defined as a "dielectric filler whose real part of the dielectric constant has negative temperature dependency." The real part of the dielectric constant is measured at four points of 25°C, 60°C, 90°C, and 120°C. The real part of the dielectric constant at 25°C > the real part of the dielectric constant at 60°C > the real part of the dielectric constant at 90°C > the real part of the dielectric constant at 120°C.

[0067] Based on the findings of the inventors described above regarding the first sheet, in the third sheet, a negative dielectric filler and a positive dielectric filler are mixed, and |f １２０ -f ２５ | is 2.0 GHz or less, and the absolute value of the return loss A ２５ and A １２０ By ensuring that both |f are 10 dB or more, it is possible to suppress the phenomenon of the peak frequency shifting with temperature changes. As a result, in the third sheet, the amount of radio wave absorption at the target frequency can be maintained at a certain level or more even with temperature changes. １２０ -f ２５ |(frequency change amount) is preferably 1.5 GHz or less, and more preferably 1.0 GHz or less. ２５ and A １２０ is preferably 15 dB or more, and more preferably 18 dB or more.

[0068] It should be noted that the third sheet may overlap with the first sheet and / or the second sheet in scope as an invention.

[0069] Examples of the positive dielectric filler include silicon carbide, boron carbide, gallium nitride, etc. The positive dielectric filler contained in the third sheet may be one type alone or a combination of two or more types. In particular, it is preferable to use silicon carbide powder as the positive dielectric filler, and I ４Ｈ / I ６Ｈ It is more preferable to use silicon carbide powder having a value of 0.1 or more (above). In this case, fluctuations in the amount of radio wave absorption due to temperature changes can be further suppressed, and radio wave absorption performance can be further improved.

[0070] Examples of the negative dielectric filler include MnZn ferrite, aluminum, graphite, copper, silver, etc. The negative dielectric filler contained in the third sheet may be one type alone or a combination of two or more types.

[0071] The positive dielectric filler tends to have a larger rate of change in the real part of the dielectric constant with temperature change than the negative dielectric filler. Based on this, the third sheet is designed so that the volume ratio of the positive dielectric filler to the entire sheet is V ＋ , the volume ratio of the negative dielectric filler to the entire sheet is V － When V ＋ / (V ＋ +V － ) Positive dielectric filler occupancy ratio R ＋ is preferably 50% or less.

[0072] The third sheet is V ＋ +V － The total filler volume V is expressed as Ｐ３ In the third sheet, the total filler volume V Ｐ３ When the total volume of the filler V is 30% or more and 60% or less, the resonance frequency of the radio wave absorption amount can be adjusted in the quasi-millimeter wave band to the millimeter wave band. Ｐ３If the filler total volume V is 60% or less, when a sheet is produced by a wet method such as a doctor blade molding method, shrinkage increases in the heat treatment step of the sheet after molding, and cracks tend to easily occur on the surface of the sheet. Ｐ３ However, if there are circumstances where there is no problem in the manufacturing process of the radio wave absorbing sheet and flexibility of the radio wave absorbing sheet to be obtained is not required, the total volume of filler V Ｐ３ It is acceptable for the ratio to be more than 60%.

[0073] The positive dielectric filler contained in the third sheet preferably has an average particle size of 1 μm or more and 60 μm or less. If the average particle size of the positive dielectric filler is 1 μm or more, the fluidity and dispersibility of the filler are well maintained, and deterioration of manufacturability can be suppressed. Furthermore, if the average particle size of the positive dielectric filler is 60 μm or less, the smoothness of the molded sheet surface can be well maintained. From the same viewpoint, the average particle size of the positive dielectric filler is more preferably 2 μm or more, even more preferably 3 μm or more, and more preferably 50 μm or less, even more preferably 40 μm or less.

[0074] The negative dielectric filler contained in the third sheet preferably has an average particle size of 1 μm or more and 60 μm or less. If the average particle size of the negative dielectric filler is 1 μm or more, the fluidity and dispersibility of the filler are well maintained, and deterioration of manufacturability can be suppressed. Furthermore, if the average particle size of the negative dielectric filler is 60 μm or less, the smoothness of the molded sheet surface can be well maintained. From the same viewpoint, the average particle size of the negative dielectric filler is more preferably 2 μm or more, even more preferably 3 μm or more, and more preferably 50 μm or less, and even more preferably 40 μm or less.

[0075] The third sheet preferably includes a substrate made of an organic material, and a positive dielectric filler and a negative dielectric filler are supported in the substrate. In this case, the details and preferred embodiments of the organic material constituting the substrate are the same as those in the first sheet. Therefore, repeated explanations will be omitted, and the explanations in the first sheet will be used by reference.

[0076] In the third sheet, details or preferred embodiments other than those described above, specifically other additives, sheet characteristics, additional components, and details or preferred embodiments regarding the manufacture of the sheet, are the same as those in the first sheet, and therefore, repeated explanations will be omitted and the explanations in the first sheet will be incorporated by reference.

[0077] Although the third sheet has been described above, the present disclosure is not limited to the above description and can be modified as appropriate.

[0078] The present invention will be described in more detail below with reference to examples. However, these examples are intended to illustrate the present invention and are not intended to limit the present invention in any way.

[0079] (Preparation of radio wave absorbing sheet) A coating liquid (kneaded material) was prepared by kneading a silicone resin as an organic material constituting the substrate, various fillers (MnZn ferrite powder, aluminum powder, silicon carbide powder), and toluene as a solvent using a planetary mixer. This coating liquid was used to form a sheet on a PET film using a doctor blade molding method, and then heat treatment was performed to remove the toluene and harden the silicone resin, thereby preparing a radio wave absorbing sheet of a predetermined thickness.

[0080] The volume ratios of various fillers to the entire radio wave absorbing sheet in each example were as shown in Tables 1 and 2. Table 1 compares radio wave absorbing sheets (corresponding to the first sheet) using MnZn ferrite powder, aluminum powder, and silicon carbide powder as fillers, while Table 2 compares radio wave absorbing sheets (corresponding to the second sheet) using MnZn ferrite powder and silicon carbide powder as fillers without using aluminum powder. In the example shown in Table 1, MnZn ferrite powder with an average particle size of 20.5 μm, aluminum powder with an average particle size of 25.4 μm, and silicon carbide powder with an average particle size of 13.2 μm were used. In the example shown in Table 2, MnZn ferrite powder with an average particle size of 34.6 μm and silicon carbide powder with an average particle size of 7.9 μm were used. In addition, in each example, the silicon carbide powder was determined by the procedure described above. ４Ｈ / I ６Ｈ The values ​​used were those with a value of more than 0.2.

[0081] Next, the radio wave absorbing sheets produced in each example were subjected to the following measurements and evaluations.

[0082] <Measurement of the thickness of the radio wave absorbing sheet> [Measuring instrument] Manufacturer: Teclock Product name: Digital thickness gauge Model number: SMD-565A-L [Measurement method] Ten points on the radio wave absorbing sheet were randomly selected and their thicknesses were measured, and the average value was taken as the thickness of the radio wave absorbing sheet. The results are shown in Tables 1 and 2.

[0083] <Measurement of return loss (wave absorption amount) of radio wave absorbing sheet> [Device] Manufacturer: KEYSEIGHT TECHNOLOGY Device name: Vector Network Analyzer M9374A (60 to 90 GHz) Device name: Vector Network Analyzer P5008A (18 to 40 GHz) and, Manufacturer: Keycom Device name: Lens antenna method oblique incidence type return loss measurement device LAF-26.5A [Software] Manufacturer: KEYSEIGHT TECHNOLOGY Software name: VNA Soft Front Panel and, Manufacturer: Keycom Software name: Free space type radio wave absorption amount measurement program [Measurement conditions] Conforms to JIS R 1679:2007. However, the conditions described in this specification take precedence. Dielectric lens antenna method - parallel beam method - perpendicular incidence Frequency range: 60 to 90 GHz, 26.5 to 40 GHz, and 18 to 26.5 GHz Number of data: 1601 Port Power (Port 1): 2 dBm Bandwidth: 1 kHz The time domain method is used to remove the effects of reflected waves and scattered waves from areas other than the vicinity of the sample. Time gate span: 0.5 ns [Measurement method] A 2 mm thick aluminum plate is placed on a hot plate placed on the return loss measurement device, and the return loss A Ｍ The return loss A (25°C) of the sample was measured. After that, the prepared radio wave absorbing sheet was attached to the aluminum plate with a 0.1 mm double-sided tape. In the attached state, heat was applied to the sample to reach a predetermined temperature (25°C, 120°C), and the frequency was scanned to measure the return loss A Ｓ The difference A between the return loss of the sample and the return loss of the benchmark is measured. Ｓ -A Ｍ Absolute value of |A Ｓ -A Ｍ | was calculated as the amount of electromagnetic wave absorption. A thermocouple was placed between the double-sided tape and the aluminum plate, and the temperature indicated by the thermocouple was defined as the temperature at the time of measurement. For reference, the return loss A of the electromagnetic wave absorbing sheet of Example 5 is shown in FIG. Ｓ -A Ｍ 2 shows the actual measurement data of the return loss A of the radio wave absorbing sheet of Comparative Example 1. Ｓ -AＭ The actual measurement data is shown below.

[0084] <Measurement and Determination of Frequency Change Amount> In each example, the frequency was scanned at 25°C to measure the return loss A Ｓ (25 ° C.) Ｓ (25℃)-A Ｍ (25°C)] becomes peak (the amount of radio wave absorption becomes peak) ２５ , and the return loss A Ｓ (120 ° C.) Ｓ (120°C)-A Ｍ (25°C)] peaks at frequency f １２０ Also, these f ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | (frequency change) was calculated. A frequency change of 1.0 GHz or less was evaluated as A, a frequency change of more than 1.0 GHz to less than 1.5 GHz was evaluated as B, a frequency change of more than 1.5 GHz to less than 2.0 GHz was evaluated as C, and a frequency change of more than 2.0 GHz was evaluated as NG. The results are shown in Tables 1 and 2. A smaller frequency change indicates a smaller fluctuation in the amount of radio wave absorption due to temperature changes.

[0085] <f ２５ In each example, the amount of radio wave absorption at frequency f ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０ The results are shown in Tables 1 and 2.

[0086]

[0087]

[0088] From Table 1, when MnZn ferrite powder, aluminum powder, and silicon carbide powder were used as fillers, V Ｓ / (V Ｍ +V Ａ +V Ｓ ) silicon carbide occupancy ratio RＳ１ It can be seen that the radio wave absorbing sheets of Examples 1 to 10, in which the coefficient of variation is 1.5% or more and 41% or less, all have a frequency change of 2.0 GHz or less (see also FIG. 1). In other words, it can be seen that the radio wave absorbing sheets of Examples 1 to 10 have small fluctuations in radio wave absorption amount due to temperature changes. The radio wave absorbing sheets of Examples 1 to 10 also correspond to the third sheet.

[0089] In contrast, the silicon carbide occupancy ratio R Ｓ１ The radio wave absorbing sheet of Comparative Example 1, in which the silicon carbide occupancy ratio R was less than 1.5%, had a frequency change of more than 2.0 GHz (see also FIG. 2). Ｓ１ In the radio wave absorbing sheet of Comparative Example 2, in which the coefficient of resistance was more than 41%, the amount of frequency change exceeded 2.0 GHz.

[0090] Furthermore, from Table 2, when MnZn ferrite powder and silicon carbide powder were used as fillers, V Ｓ / (V Ｍ +V Ｓ ) silicon carbide occupancy ratio R Ｓ２ It can be seen that the radio wave absorbing sheets of Examples 11 to 19, in which the coefficient of variation is 0.5% or more and 29% or less, all have a frequency change of 2.0 GHz or less. In other words, it can be seen that the radio wave absorbing sheets of Examples 11 to 19 have small fluctuations in radio wave absorption amount due to temperature changes. The radio wave absorbing sheets of Examples 11 to 19 also correspond to the third sheet.

[0091] In contrast, the silicon carbide occupancy ratio R Ｓ２ In the radio wave absorbing sheet of Comparative Example 3, in which the silicon carbide occupancy ratio R was less than 0.5%, the frequency change amount exceeded 2.0 GHz. Ｓ２ In the radio wave absorbing sheet of Comparative Example 4, in which the value was more than 29%, the amount of frequency change exceeded 2.0 GHz.

[0092] The radio wave absorbing sheet of the present invention is particularly effective as an electromagnetic noise countermeasure component that is attached to electronic devices and absorbs radio waves generated within these electronic devices, and is a radio wave absorbing sheet that exhibits less fluctuation in radio wave absorption with temperature rise than conventional products.

Claims

1. A radio wave absorbing sheet comprising a substrate made of an organic material and a powder carried in the substrate, wherein the powder is composed of MnZn ferrite powder, aluminum powder, and silicon carbide powder, and the volume ratio of the MnZn ferrite powder to the entire radio wave absorbing sheet is V Ｍ , the volume ratio of aluminum powder is V Ａ , the volume ratio of silicon carbide powder is V Ｓ When V Ｓ / (V Ｍ +V Ａ +V Ｓ ) silicon carbide occupancy ratio R Ｓ１ is 1.5% or more and 41% or less.

2. A radio wave absorbing sheet comprising a substrate made of an organic material and a powder carried in the substrate, wherein the powder is composed of MnZn ferrite powder and silicon carbide powder, and the volume ratio of the MnZn ferrite powder to the entire radio wave absorbing sheet is V Ｍ , the volume ratio of silicon carbide powder is V Ｓ When V Ｓ / (V Ｍ +V Ｓ ) silicon carbide occupancy ratio R Ｓ２ 10. A radio wave absorbing sheet, characterized in that the modulus of elasticity is 0.5% or more and 29% or less.

3. V Ｍ +V Ａ +V Ｓ The total filler volume V is expressed as Ｐ１ 2. The radio wave absorbing sheet according to claim 1, wherein the ratio of the surface area to the surface area is 35% or more and 60% or less.

4. V Ｍ +V Ｓ The total filler volume V is expressed as Ｐ２ 3. The radio wave absorbing sheet according to claim 2, wherein the ratio of the radiative transfer coefficient to the radiative transfer coefficient is 30% or more and 60% or less.

5. A radio wave absorbing sheet according to any one of claims 1 to 4, wherein the average particle size of the MnZn ferrite powder, the average particle size of the silicon carbide powder, and the average particle size of the aluminum powder, if included, are all 1 μm or more and 60 μm or less.

6. The radio wave absorbing sheet according to any one of claims 1 to 4, which is of a dielectric loss type.

7. The frequency at which the return loss peaks at 25°C is f ２５ , the frequency at which the return loss peaks at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | is 2.0 GHz or less, and the frequency f at 25 ° C. ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０ 5. The radio wave absorbing sheet according to claim 1, wherein each of the values is 10 dB or more.

8. A dielectric filler whose real part of the dielectric constant has a positive temperature dependency and a dielectric filler whose real part of the dielectric constant has a negative temperature dependency are mixed together, and the frequency at which the return loss peaks at 25°C is defined as f ２５ , the frequency at which the return loss peaks at 120°C is f １２０ When ２５ and f １２０ The absolute value of the difference between |f １２０ -f ２５ | is 2.0 GHz or less, and the frequency f at 25 ° C. ２５ The absolute value A of the return loss ２５ , and frequency f at 120°C ２５ The absolute value A of the return loss １２０ are all 10 dB or more.

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