Electromagnetic wave absorbers and communication systems

Abstract

To provide an electromagnetic wave absorber for absorbing millimeter-wave electromagnetic waves which can achieve a large amount of electromagnetic wave absorption, and of which sheet thickness can be easily controlled when used for an electromagnetic wave absorbing sheet.SOLUTION: An electromagnetic absorber includes: a large first electromagnetic wave absorbing substance 10 made of a hollow material with a relatively high dielectric constant; a plurality of small second electromagnetic wave absorbing substances 20 made of granular materials with a relatively low dielectric constant bonded to the surface of the first electromagnetic wave absorbing substance 10; and a binding material 30 that bonds the first electromagnetic wave absorbing material 10 and the second electromagnetic wave absorbing substances 20 together.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electromagnetic wave absorber and a communication system, and particularly to an electromagnetic wave absorber and a communication system suitable for absorbing millimeter-wave electromagnetic waves.

Background Art

[0002] Patent Document 1 discloses a matched electromagnetic wave absorber having a wide absorption bandwidth designed under the first-order reflectionless condition and capable of operating even at millimeter waves. This electromagnetic wave absorber is formed by containing carbon black in a thermoplastic rubber, the real part of the complex relative permittivity at 50 GHz measured by the free space method is 6 or more, the dielectric loss tangent (tan δ) is 0.35 or more, and as a method of making the electromagnetic wave absorbing material porous, it is possible to add glass balloons or shirasu balloons to the electromagnetic wave absorbing material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the invention described in Patent Document 1, since the real part of the complex relative permittivity is 6 or more, it is a relatively large value compared to the permittivity 1 of air. With this, the amount of reflection of radio waves propagating in the air is relatively large, and the amount of electromagnetic wave absorption is limited.

[0005] Regarding this point, first, when electromagnetic waves reach carbon black, which is an electromagnetic wave absorbing substance, there are those in which the surface of the carbon black becomes the incident surface and is absorbed, and those in which the surface of the carbon black becomes the reflection surface and is reflected. The relationship between these absorption amounts and reflection amounts is a trade-off relationship, and the larger the absorption amount, the smaller the reflection amount. And the amount of absorption depends on the level of the permittivity.

[0006] However, the amount of absorption depends not only on the dielectric constant but also on the installation environment of the electromagnetic wave absorber. Typically, electromagnetic wave absorbers are installed in an environment with air around them. While the dielectric constant of air is 1, the carbon black described in Patent Document 1 has a higher dielectric constant than that of air, resulting in a greater amount of reflection.

[0007] Furthermore, the invention described in Patent Document 1 uses carbon black alone as the electromagnetic wave absorbing material. However, when such a single electromagnetic wave absorbing material is made into an electromagnetic wave absorbing sheet, millimeter-wave electromagnetic waves cannot be absorbed unless the sheet thickness is controlled to a very precise level of 0.01 mm. Controlling the sheet thickness in this way is extremely difficult.

[0008] Therefore, the object of the present invention is to provide an electromagnetic wave absorber for absorbing millimeter-wave electromagnetic waves that can achieve a large amount of electromagnetic wave absorption and allows for easy control of the sheet thickness when used in an electromagnetic wave absorbing sheet. [Means for solving the problem]

[0009] To solve the above problems, the electromagnetic wave absorber for absorbing millimeter-wave electromagnetic waves of the present invention is A large first electromagnetic wave absorbing material consisting of a hollow material with a relatively high dielectric constant, A plurality of small second electromagnetic wave absorbing materials, each made of a granular material with a relatively low dielectric constant, are bonded to the surface of the first electromagnetic wave absorbing material. It is equipped with.

[0010] The first electromagnetic wave absorbing material has a dielectric constant of 4 or more, and the second electromagnetic wave absorbing material has a dielectric constant of less than 4.

[0011] The weight ratio of the first electromagnetic wave absorbing material to the second electromagnetic wave absorbing material can be set to 7:3 to 8:2.

[0012] The first electromagnetic wave absorbing material may have glass, silica, alumina, zirconia, titania, or ceria as its main component.

[0013] The second electromagnetic wave absorbing material is preferably given a particle size of 30 nm to 40 nm.

[0014] The second electromagnetic wave absorbing material can have a bulk density of 0.3 to 0.5.

[0015] Furthermore, the communication system of the present invention has a housing into which the above-mentioned electromagnetic wave absorber is mixed. Embodiment of the invention

[0016] <Explanation of the structure> Figure 1 is an SEM image of an electromagnetic wave absorber 100 according to an embodiment of the present invention. Figure 1(a) shows a 200x SEM image, Figure 1(b) shows a 500x SEM image including region A in Figure 1(a), Figure 1(c) shows a 1000x SEM image including region A in Figure 1(a), and Figure 1(d) shows a 2000x SEM image of region A itself in Figure 1(a).

[0017] The electromagnetic wave absorber 100 comprises a first electromagnetic wave absorbing material 10 made of a relatively large hollow material, a second electromagnetic wave absorbing material 20 made of a plurality of relatively small granular materials bonded to the surface of the first electromagnetic wave absorbing material 10, and a binder 30 that bonds the first electromagnetic wave absorbing material 10 and the second electromagnetic wave absorbing material 20.

[0018] The conditions for the first electromagnetic wave absorbing material 10, the second electromagnetic wave absorbing material 20, and the binder 30 that constitute the electromagnetic wave absorber 100 are as follows.

[0019] First, the first electromagnetic wave absorbing material 10 can be a material with a dielectric constant of, for example, 4 or higher. The first electromagnetic wave absorbing material 10 can be one in which glass, silica, alumina, zirconia, titania, or ceria are the main components.

[0020] In this embodiment, as the first electromagnetic wave absorbing material 10, for example, a shirasu balloon mainly composed of glass is used. The shirasu balloon has, for example, an average hollow ratio of 85% to 93%, a primary particle size of 50.0 μm to 200.0 μm, a bulk specific gravity of 0.1 g / cm 3 ~0.8 g / cm 3 , a specific heat of 0.75 J / g·K to 0.85 J / g·K, and a composition ratio of about 73 wt% to 77 wt% of SiO2, about 10 wt% to 15 wt% of Al2O3, about 3 wt% to 5 wt% of Na2O, about 2 wt% to 5 wt% of K2O, and others.

[0021] As the second electromagnetic wave absorbing material 20, those having a primary particle size of 30 nm to 40 nm can be used. The second electromagnetic wave absorbing material 20 preferably has a bulk specific gravity of 0.3 to 0.5, and examples of those corresponding to this include carbon. The type of carbon is not limited, but water-soluble carbon and conductive carbon are preferred.

[0022] [[ID=***]] As the binder 30, for example, water glass mainly composed of SiO2 and Na2O can be used. In this embodiment, water glass with 28 wt% to 38 wt% of SiO2 and 9 wt% to 19 wt% of Na2O is used.

[0023] The mixing ratio of the first electromagnetic wave absorbing material 10, the second electromagnetic wave absorbing material 20, and the binder 30, in terms of weight percentage, with respect to the entire precursor of the electromagnetic wave absorber 100, for example, the weight ratio of the first electromagnetic wave absorbing material 10 is 60 wt% to 82 wt%, the weight ratio of the second electromagnetic wave absorbing material 20 is 8 wt% to 27 wt%, and the weight ratio of the binder 30 is 8 wt% to 12 wt%.

[0024] <Explanation of the principle> The principle by which the electromagnetic wave absorber 100 of this embodiment effectively absorbs electromagnetic waves is as follows.

[0025] First, the second electromagnetic wave absorbing material 20 is a material with a relatively low dielectric constant. Therefore, when electromagnetic waves propagating through the air reach the second electromagnetic wave absorbing material 20, (1-1) some are reflected by the surface of the second electromagnetic wave absorbing material 20 which acts as a reflective surface, but (1-2) the majority of the remaining electromagnetic waves are absorbed by the surface of the second electromagnetic wave absorbing material 20 which acts as an incident surface.

[0026] Next, the electromagnetic waves absorbed by the second electromagnetic wave absorbing material 20 are (2-1) partially reflected toward the incident surface, (2-2) partially consumed within the second electromagnetic wave absorbing material 20, and (2-3) the majority of the remainder is emitted from the exit surface opposite to the incident surface, passing through the coupling body and heading toward the first electromagnetic wave absorbing material 10.

[0027] The first electromagnetic wave absorbing material 10 is a material with a relatively high dielectric constant. However, the difference in dielectric constant between the first electromagnetic wave absorbing material 10 and the second electromagnetic wave absorbing material 20 is small compared to the difference in dielectric constant between the first electromagnetic wave absorbing material 10 and air. Therefore, of the electromagnetic waves that reach the first electromagnetic wave absorbing material 10, (3-1) a portion is reflected by the surface of the first electromagnetic wave absorbing material 10 acting as the reflective surface, but (3-2) the majority of the remaining electromagnetic waves are absorbed by the surface of the first electromagnetic wave absorbing material 10 acting as the incident surface.

[0028] The electromagnetic waves absorbed by the first electromagnetic wave absorbing material 10 are (4-1) partially reflected outwards, i.e., toward the second electromagnetic wave absorbing material 20, but (4-2) because the first electromagnetic wave absorbing material 10 has a high dielectric constant, energy is consumed within the first electromagnetic wave absorbing material 10.

[0029] Thus, the electromagnetic wave absorber 100 of this embodiment efficiently absorbs electromagnetic waves propagating through the air by reducing the amount of reflection from the second electromagnetic wave absorbing material 20, which has a relatively low dielectric constant, and efficiently consumes energy in the first electromagnetic wave absorbing material 10 when electromagnetic waves are directed from the second electromagnetic wave absorbing material 20 to the first electromagnetic wave absorbing material 10. This is the principle by which the electromagnetic wave absorber 100 of this embodiment can effectively absorb electromagnetic waves.

[0030] <Manufacturing Method Explanation> The electromagnetic wave absorber 100 of this embodiment has slightly different manufacturing conditions depending on whether the manufacturing method mainly involves manual processes (Example 1) or mainly does not involve manual processes (Examples 2 to 6).

[0031] Furthermore, for the latter case, the manufacturing conditions differ slightly depending on whether the electromagnetic wave absorber 100 is produced in relatively small quantities (Examples 2 to 5) or in relatively large quantities (Example 6). The following describes each manufacturing method and the electromagnetic wave absorber 100 produced by them.

[0032] (Example 1) Step S1: Mixing process 1.4 kg of shirasu balloon (for example, Igawa Sangyo's "Shirafine ISM-M120") and 0.6 kg of dry carbon (for example, Tokai Carbon's "TOKABLACK7270SB") are placed in a V-type mixer (VM-2) manufactured by Tsutsui Rikagaku Kikai Co., Ltd., and mixed for approximately 10 minutes.

[0033] Step S2: Mixing process To the mixture obtained in step S1, add 1 liter of water glass diluted to approximately 10% (for example, "Shika Grade 1" from Kanto Chemical Co., Ltd.), and knead by hand for about 10 minutes. In this case, the solid content of the water glass is only 8 g.

[0034] Step S3: Extrusion process The mixture obtained in step S2 is manually extruded using a 100-mesh sieve.

[0035] Step S4: Drying preparation process The extruded material obtained in step S3 is spread manually onto a tray.

[0036] Step S5: Drying process The precursor of the electromagnetic wave absorber 100 obtained in step S4 is placed in, for example, a dryer (DN-64) of Yamato Scientific Co., Ltd. and dried at a temperature of approximately 120°C for approximately 4 hours.

[0037] Step S6: Cooling process The dried material obtained in step S5 is allowed to cool naturally at room temperature for about 60 minutes to obtain an electromagnetic wave absorber 100 at a temperature of about 70°C or lower.

[0038] (Example 2) Step S11: Mixing process 1.4 kg of shirasu balloon (for example, Igawa Sangyo's "Shirafine ISM-M120") and 0.6 kg of dry carbon (for example, Tokai Carbon's "TOKABLACK7270SB") are placed in a tabletop kneader (SNV-1H) manufactured by Irie Shoji Co., Ltd., and mixed for about 5 minutes.

[0039] Step S12: Mixing process To the mixture obtained in step S11, 1 L of water glass diluted to approximately 10% (for example, "Shika Grade 1" from Kanto Chemical Co., Ltd.) is added and kneaded for approximately 5 minutes under reduced pressure, for example, -77.3 kPa. In this case, the solid content of the water glass is only 8 g. Furthermore, since the kneading process is not performed manually, the mixture obtained in step S11 is sufficiently kneaded, and therefore the extrusion process, as in the manufacturing method of Example 1, is unnecessary.

[0040] Step S13: Drying preparation process The mixture obtained in step S12 is spread out by hand onto a tray.

[0041] Step S14: Drying process The precursor of the electromagnetic wave absorber 100 obtained in step S13 is placed in, for example, a dryer (DN-64) of Yamato Scientific Co., Ltd. and dried at a temperature of approximately 120°C for approximately 4 hours.

[0042] Step S15: Cooling process The dried material obtained in step S14 is allowed to cool naturally at room temperature for about 60 minutes to obtain an electromagnetic wave absorber 100 at a temperature of about 70°C or lower.

[0043] (Example 3) In step S12 of Example 2, "JIS No. 1" water glass from Osaka Keiso Co., Ltd. was used as the water glass to be added. All other manufacturing conditions were the same as in Example 2.

[0044] (Example 4) In step S12 of Example 2, "Special No. 1" water glass from Osaka Keiso Co., Ltd. was used as the water glass to be added. All other manufacturing conditions were the same as in Example 2.

[0045] (Example 5) In Example 2, "No. 3" water glass from Osaka Keiso Co., Ltd. was used as the water glass to be added in step S12. All other manufacturing conditions were the same as in Example 2.

[0046] (Example 6) Step S21: Mixing and kneading process 11.9 kg of shirasu balloons (for example, Igawa Sangyo's "Shirafine ISM-M120") and 5.1 kg of dry carbon (Tokai Carbon's "TOKABLACK7270SB") are placed into the drum of, for example, Matsubo's Redigge mixer (FM130D), and the shovel that moves the materials within the drum is rotated at 120 minutes. -1 The mixture is added while rotating, mixed for about 1 minute, and then 8.5L of water glass (Osaka Keiso Co., Ltd.'s "No. 3") diluted to about 10% concentration is added uniformly by spraying, and kneaded for about 12 minutes. In this case, the solid content of the water glass is only 340g.

[0047] Step S22: Drying process With the precursor of the electromagnetic wave absorber 100 obtained in step S21 still inside the drum of the Redigee mixer, the shovel is kept rotating for approximately 60 minutes while steam at a temperature of approximately 150°C is introduced into the heating jacket inside the drum, and hot air at approximately 80°C is injected into the drum at a rate of approximately 1000 L / min to dry the precursor.

[0048] Step S23: Cooling process While the dried material obtained in step S22 is inside the drum of the Redigge mixer, the shovel is kept rotating and room temperature water is introduced into the cooling jacket inside the drum for about 8 minutes to cool the dried material and obtain an electromagnetic wave absorber 100 at a temperature of about 70°C or lower.

[0049] <Measurement Results> Next, various measurement results for the electromagnetic wave absorber 100 of this embodiment will be described. Here, each of the electromagnetic wave absorbers 100 from Examples 1 to 7 was selected and, as described below, was kneaded with a base resin such as a thermosetting resin, thermoplastic resin, or elastomer together with a crosslinking agent such as mineral oil containing the same components as the resin, or selectively with additives such as dispersants, to produce an electromagnetic wave absorbing sheet.

[0050] Examples of resins include polypropylene, polyphenylene sulfide, polyamide, polyimide, polyamide-imide, polyasdo, polycarbonate, polyacetal, polyethersulfone, polybutylene, polyetherimide, polyetherketone, polyetherimide, polyalkylene terephthalate, polysulfone, polyphenylene sulfide, polyolefin, polystyrene, syndiotactic polystyrene, acrylonitrile butadiene styrene, polyphenylene oxide, liquid crystal polymer resin, and any combination of any of these. Furthermore, the resin may be in a millable form or in a liquid state.

[0051] The crosslinking agent should be compatible with the resin, and therefore, one containing the same components as the resin is preferable. Peroxide crosslinking agents and addition reaction crosslinking agents can be suitably used, but condensation crosslinking agents are unsuitable for molded products. In this embodiment, a peroxide crosslinking agent containing the same silicone rubber as the resin was used (Shin-Etsu Chemical Co., Ltd.'s "C-15").

[0052] (Electromagnetic wave absorbing sheet) First, an electromagnetic wave absorber 100, a silicone rubber resin, and a peroxide crosslinking agent were prepared. Next, 15 to 100 phr (for example, 70 phr) of the electromagnetic wave absorber 100 and 1 to 5 phr (for example, 2 phr) of the crosslinking agent were mixed with the resin. Then, these were kneaded using a mixing machine (for example, a mixing roll (R-14, 14-inch diameter) manufactured by Kansai Roll Co., Ltd.) or a pressurized kneader at a temperature of room temperature to approximately 40°C for 10 to 45 minutes (for example, 30 minutes) until the desired dispersion was achieved.

[0053] The additives can be plasticizers and flame retardants, and either one or any combination thereof may be added. However, the amount added should be 5 phr or less relative to the resin. When silicone rubber is used as the resin, silicone oil, silane coupling agents, etc., can be selected, and mineral oils containing similar components are preferred. As for flame retardants, metal hydrates (aluminum hydroxide, magnesium hydroxide, etc.) and platinum compounds can be used.

[0054] Next, using mixing rolls with diameters of 8 inches to 20 inches (for example, the mixing rolls mentioned above), a rolling process is performed at room temperature to approximately 40°C, to a thickness that takes into account the thinning that will occur in the subsequent processing compared to the thickness of the finished product (for example, an increase of 8% to 12%). Then, in order to load an appropriate amount of uncrosslinked material into the sheet-shaped mold to be used in the subsequent processing, a preliminary molding is performed, in which the material is formed to be slightly smaller than the inner size of the mold cavity.

[0055] Next, using a direct-pressure press (for example, a 100t press manufactured by Mie Kogyo Co., Ltd.), a crosslinking process is performed at a temperature of approximately 120°C to 180°C, under a pressure of 5 MPa to 15 MPa (for example, 10 MPa), for 5 to 30 minutes (for example, 10 minutes). However, the pressure and other conditions can be appropriately changed depending on the size of the sheet mold. After that, if necessary, secondary crosslinking is performed at a temperature of, for example, 150°C to 220°C (for example, 200°C) for 30 minutes to 5 hours (for example, 4 hours). In this way, the electromagnetic wave absorbing sheet is completed.

[0056] The manufacturing process for the electromagnetic wave absorbing sheet described here is merely an example; for example, after extrusion molding, a cross-linking treatment may be performed using a hot air furnace or a constant temperature bath. In this case, heat treatment should be performed at a temperature of approximately 120°C to 180°C (for example, 150°C) for approximately 20 to 40 minutes (for example, 30 minutes).

[0057] (Complex permittivity and complex permeability) The complex permittivity and complex permeability were measured several times for multiple electromagnetic wave absorbing sheets. In the first direction, the complex permittivity had a real part ε' of 5.2 to 5.3 and an imaginary part ε'' of -0.5 to -0.6. In the second direction, which is orthogonal to the first direction, the complex permittivity had a real part ε' of 5.1 to 5.5 and an imaginary part ε'' of -0.5 to -0.6. For each electromagnetic wave absorbing sheet, the complex permeability had a real part ε' of 1 and an imaginary part ε'' of 0 in both the first and second directions.

[0058] What is important here is the value of the real part of the complex permittivity. If this value can be kept below 5.5, it becomes easier to control the thickness of the electromagnetic wave absorbing sheet. In this embodiment, when an electromagnetic wave absorbing sheet is manufactured by mixing multiple electromagnetic wave absorbers 100, even if the sheet thickness is controlled at the 0.1 mm level, electromagnetic waves in the desired frequency band can be effectively absorbed. In contrast, the real part of the complex permittivity in Patent Document 1 is 6 or more, and as mentioned above, the sheet thickness must be controlled at the 0.01 mm level.

[0059] Thus, the electromagnetic wave absorber 100 of this embodiment allows for approximately 10 times easier control of the sheet thickness compared to that of Patent Document 1. The electromagnetic wave absorption condition referred to here means that the reflection loss is approximately -15 dB or less.

[0060] (reflectance) Figures 2 to 7 show the simulation results of reflection loss when millimeter-wave electromagnetic waves are irradiated onto electromagnetic wave absorbing sheets (sheet thickness: 1.9 mm and 2.0 mm) according to Examples 1 to 6, respectively.

[0061] The horizontal axis of Figures 2 to 7 shows the frequency [GHz] of the electromagnetic waves irradiated onto the electromagnetic wave absorbing sheet, and the vertical axis of Figures 2 to 7 shows the reflection loss (reflection amount) [dB]. The free-space method was used to measure the reflection loss, and the reflection loss (S11) was measured assuming that the electromagnetic waves were irradiated onto the electromagnetic wave absorbing sheet at an incident angle of 0°.

[0062] Furthermore, in Figures 2 to 7, the solid and coarsely dashed graphs show the reflection loss when electromagnetic waves are irradiated from the first and second directions at an incident angle of 0° for a sheet thickness of 1.9 mm, respectively, while the dashed and thinly dashed graphs show the reflection loss when electromagnetic waves are irradiated from the first and second directions at an incident angle of 0° for a sheet thickness of 2.0 mm, respectively.

[0063] Figure 2 shows the simulation results of the reflection loss of the electromagnetic wave absorbing sheet according to Example 1. For this electromagnetic wave absorbing sheet, the real part of the complex permittivity was 6.3 for a sheet thickness of 1.9 mm, and for a sheet thickness of 2.0 mm, the real part of the complex permittivity was also 6.3.

[0064] An electromagnetic wave absorbing sheet with a thickness of 1.9 mm exceeds -15 dB in the frequency band of approximately 73.0 GHz to approximately 76.0 GHz when considering irradiation from both the first and second directions. Similarly, an electromagnetic wave absorbing sheet with a thickness of 2.0 mm exceeds -15 dB in the frequency band of approximately 70.5 GHz to approximately 71.5 GHz when considering irradiation from both the first and second directions.

[0065] As shown in Figure 2, when the sheet thickness is relatively thin (1.9 mm), electromagnetic waves with a relatively high bandwidth in the millimeter wave range are effectively absorbed (solid line, coarse dashed line), and when the sheet thickness is relatively thick (2.0 mm), electromagnetic waves with a relatively low bandwidth in the millimeter wave range are effectively absorbed (dotted line, fine dashed line). This trend can also be seen in Figures 3 to 7.

[0066] Figure 3 shows the simulation results of the reflection loss of the electromagnetic wave absorbing sheet according to Example 2. For this electromagnetic wave absorbing sheet, the real part of the complex permittivity was 6.4 for a sheet thickness of 1.9 mm, and for a sheet thickness of 2.0 mm, the real part of the complex permittivity was also 6.4.

[0067] An electromagnetic wave absorbing sheet with a thickness of 1.9 mm exceeds -15 dB in the frequency band of approximately 72.0 GHz to approximately 75.0 GHz when considering irradiation from both the first and second directions. Similarly, an electromagnetic wave absorbing sheet with a thickness of 2.0 mm exceeds -15 dB in the frequency band of approximately 69.0 GHz to approximately 71.0 GHz when considering irradiation from both the first and second directions.

[0068] Figure 4 shows the simulation results of the reflection loss of the electromagnetic wave absorbing sheet according to Example 3. For this electromagnetic wave absorbing sheet, the real part of the complex permittivity was 6.6 for the sheet with a thickness of 1.9 mm, and for the sheet with a thickness of 2.0 mm, the real part of the complex permittivity was also 6.6.

[0069] An electromagnetic wave absorbing sheet with a thickness of 1.9 mm exceeds -15 dB in the frequency band of approximately 72.0 GHz to approximately 75.0 GHz when considering irradiation from both the first and second directions. Similarly, an electromagnetic wave absorbing sheet with a thickness of 2.0 mm exceeds -15 dB in the frequency band of approximately 69.0 GHz to approximately 71.0 GHz when considering irradiation from both the first and second directions.

[0070] Figure 5 shows the simulation results of the reflection loss of the electromagnetic wave absorbing sheet according to Example 4. For this electromagnetic wave absorbing sheet, the real part of the complex permittivity was 6.7 for the sheet thickness of 1.9 mm, and for the sheet thickness of 2.0 mm, the real part of the complex permittivity was also 6.7.

[0071] An electromagnetic wave absorbing sheet with a thickness of 1.9 mm exceeds -15 dB in the frequency band of approximately 72.0 GHz to approximately 75.0 GHz when considering irradiation from both the first and second directions. Similarly, an electromagnetic wave absorbing sheet with a thickness of 2.0 mm exceeds -15 dB in the frequency band of approximately 69.0 GHz to approximately 71.0 GHz when considering irradiation from both the first and second directions.

[0072] Figure 6 shows the simulation results of the reflection loss of the electromagnetic wave absorbing sheet according to Example 5. For this electromagnetic wave absorbing sheet, the real part of the complex permittivity was 6.7 for a sheet thickness of 1.9 mm, and for a sheet thickness of 2.0 mm, the real part of the complex permittivity was also 6.7.

[0073] An electromagnetic wave absorbing sheet with a thickness of 1.9 mm exceeds -15 dB in the frequency band of approximately 69.0 GHz to approximately 71.0 GHz when considering irradiation from both the first and second directions. Similarly, an electromagnetic wave absorbing sheet with a thickness of 2.0 mm exceeds -15 dB in the frequency band of approximately 68.0 GHz to approximately 71.0 GHz when considering irradiation from both the first and second directions.

[0074] Figure 7 shows the simulation results of the reflection loss of the electromagnetic wave absorbing sheet according to Example 6. For this electromagnetic wave absorbing sheet, the real part of the complex permittivity was 5.3 for a sheet thickness of 1.9 mm, and for a sheet thickness of 2.0 mm, the real part of the complex permittivity was also 5.3.

[0075] An electromagnetic wave absorbing sheet with a thickness of 1.9 mm exceeds -15 dB in the frequency band of approximately 71.0 GHz to approximately 81.0 GHz when considering irradiation from both the first and second directions. Similarly, an electromagnetic wave absorbing sheet with a thickness of 2.0 mm exceeds -15 dB in the frequency band of approximately 79.0 GHz to approximately 83.0 GHz when considering irradiation from both the first and second directions.

[0076] Of the electromagnetic wave absorbing sheets according to Examples 1 to 6, the electromagnetic wave absorbing sheet according to Example 6 had the best reflection loss. Therefore, the electromagnetic wave absorber of Example 6 was positioned as the best example, and the electromagnetic wave absorbing sheet using it was used as the measurement target. Further measurements were taken by changing the frequency of the irradiated electromagnetic waves to 38 GHz, 60 GHz, and 94 GHz.

[0077] Furthermore, in order to confirm that there was no influence due to individual differences, four electromagnetic wave absorbing sheets (the first to the fourth electromagnetic wave absorbing sheets) using the electromagnetic wave absorber of Example 6 were randomly selected and measured.

[0078] Figures 8 to 11 show the measurement results of reflection loss when electromagnetic waves with frequencies of 38 GHz, 60 GHz, 75.5 GHz, and 94 GHz were irradiated onto the first to fourth electromagnetic wave absorbing sheets. The horizontal axis of Figures 8 to 11 shows the thickness [mm] of the electromagnetic wave absorbing sheet, and the vertical axis shows the reflection loss [dB].

[0079] Figure 8 shows the measurement results of the reflection loss when the first electromagnetic wave absorbing sheet (real part of complex permittivity is 5.5) is irradiated with electromagnetic waves at a frequency of 38 GHz. Electromagnetic waves at a frequency of 38 GHz are suitably used, for example, in areas where laying optical fiber cables is difficult, such as mountainous regions or peninsulas, and when installing base stations for mobile communication systems at high density.

[0080] Figure 8 shows that the reflection loss is -15 dB or less when the sheet thickness is 2.6 ± 0.1 mm. The reflection loss is -16.32 dB when the sheet thickness is 2.6 mm.

[0081] Figure 9 shows the measurement results of the reflection loss when the second electromagnetic wave absorbing sheet (real part of complex permittivity is 5.3) is irradiated with electromagnetic waves at a frequency of 60 GHz. Electromagnetic waves at a frequency of 60 GHz are suitably used in devices that employ the WiGig standard, such as gesture controllers and vital sensors.

[0082] Figure 9 shows that the reflection loss is -20 dB or less when the sheet thickness is 2.8 ± 0.1 mm. The reflection loss is -16.31 dB when the sheet thickness is 2.8 mm.

[0083] Figure 10 shows the measurement results of the reflection loss when a third electromagnetic wave absorbing sheet (real part of complex permittivity of 5.2) is irradiated with electromagnetic waves at a frequency of 76.5 GHz. Electromagnetic waves at a frequency of 76.5 GHz are suitably used in applications such as automotive radar.

[0084] Figure 10 shows that the reflection loss is -25 dB or less when the sheet thickness is 2.2 ± 0.1 mm. Furthermore, the reflection loss is -27.45 dB when the sheet thickness is 2.2 mm.

[0085] Figure 11 shows the measurement results of the reflection loss when a fourth electromagnetic wave absorbing sheet (real part of complex permittivity of 5.2) is irradiated with electromagnetic waves at a frequency of 94 GHz. Electromagnetic waves at a frequency of 94 GHz are suitably used in security gates at airports in Europe and the United States, as well as in passive millimeter-wave cameras.

[0086] Figure 11 shows that the reflection loss is -20 dB or less when the sheet thickness is 1.8 ± 0.1 mm. The reflection loss is -21.03 dB when the sheet thickness is 1.8 mm.

[0087] As described above, the electromagnetic wave absorbing sheet according to this embodiment only needs to be a few millimeters (about 2 mm) thick, so it can be used in communication systems that include millimeter-wave radar, imaging radars that combine optical systems such as dielectric lenses with two-dimensional detectors, etc. In this specification, high-speed wireless LAN base stations are also included in the communication system.

[0088] The communication system of this embodiment may be manufactured by kneading all or part of its housing or support with electromagnetic wave absorber 100, or by using electromagnetic wave absorbing sheet according to this embodiment, or by attaching or placing it on or around electronic equipment such as an electronic control unit (ECU) inside the housing. In this case, the hardness should be 80 or higher, preferably 90 or higher, as measured by durometer type A in accordance with JIS K 6253. [Brief explanation of the drawing]

[0089] [Figure 1] This is an SEM image of the electromagnetic wave absorber 100 according to an embodiment of the present invention. [Figure 2] This figure shows the measurement results of the reflection loss when an electromagnetic wave absorbing sheet according to Embodiment 1 of the present invention is irradiated with electromagnetic waves at a frequency of 76.5 GHz. [Figure 3] This figure shows the measurement results of the reflection loss when an electromagnetic wave absorbing sheet according to Embodiment 2 of the present invention is irradiated with electromagnetic waves at a frequency of 76.5 GHz. [Figure 4] This figure shows the measurement results of the reflection loss when an electromagnetic wave absorbing sheet according to Embodiment 3 of the present invention is irradiated with electromagnetic waves at a frequency of 76.5 GHz. [Figure 5] This figure shows the measurement results of the reflection loss when an electromagnetic wave absorbing sheet according to Embodiment 4 of the present invention is irradiated with electromagnetic waves at a frequency of 76.5 GHz. [Figure 6] This figure shows the measurement results of the reflection loss when an electromagnetic wave absorbing sheet according to Embodiment 5 of the present invention is irradiated with electromagnetic waves at a frequency of 76.5 GHz. [Figure 7] This figure shows the measurement results of the reflection loss when an electromagnetic wave absorbing sheet according to Embodiment 6 of the present invention is irradiated with electromagnetic waves at a frequency of 76.5 GHz. [Figure 8] This figure shows the measurement results of the reflection loss when the first electromagnetic wave absorbing sheet is irradiated with electromagnetic waves at a frequency of 38 GHz. [Figure 9]This figure shows the measurement results of the reflection loss when the second electromagnetic wave absorbing sheet is irradiated with electromagnetic waves at a frequency of 60 GHz. [Figure 10] This figure shows the measurement results of the reflection loss when the third electromagnetic wave absorbing sheet is irradiated with electromagnetic waves at a frequency of 75.5 GHz. [Figure 11] This figure shows the measurement results of the reflection loss when the fourth electromagnetic wave absorbing sheet is irradiated with electromagnetic waves at a frequency of 94 GHz. [Explanation of Symbols]

[0090] 10. First electromagnetic wave absorbing material 20. Second electromagnetic wave absorbing material 30 Binding material 100 Electromagnetic wave absorbers

Claims

1. It consists of a hollow material with a relatively high dielectric constant, and has a dielectric constant of 4 or more, and is a large first electromagnetic wave absorbing material, A plurality of small second electromagnetic wave absorbing materials, each having a dielectric constant of less than 4 and being made of a granular material with a relatively low dielectric constant, are bonded to the surface of the first electromagnetic wave absorbing material. An electromagnetic wave absorber equipped with the following features for absorbing millimeter-wave electromagnetic waves.

2. The electromagnetic wave absorber according to claim 1, wherein the weight ratio of the first electromagnetic wave absorbing material to the second electromagnetic wave absorbing material is 7:3 to 8:

2.

3. The electromagnetic wave absorber according to claim 1, wherein the first electromagnetic wave absorbing material mainly consists of glass, silica, alumina, zirconia, titania, or ceria.

4. The electromagnetic wave absorber according to claim 1, wherein the first electromagnetic wave absorbing material is a shirasu balloon.

5. The electromagnetic wave absorber according to claim 1, wherein the second electromagnetic wave absorbing material has a particle size of 30 nm to 40 nm.

6. The electromagnetic wave absorber according to claim 1, wherein the second electromagnetic wave absorbing material has a bulk density of 0.3 to 0.

5.

7. A communication system having a housing into which the electromagnetic wave absorber described in claim 1 is mixed.

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