Electromagnetic wave absorber

Abstract

To provide an electromagnetic wave absorber having a high electromagnetic wave absorption characteristic, and a wide absorption band width.SOLUTION: An electromagnetic wave absorber includes a structure that a first dielectric body and a second dielectric body are laminated, and in which a complex dielectric constant in a maximum absorption frequency of the first and second dielectric bodies satisfies the following formulas (1) and (2): 8≤ε1'-ε2'≤40...(1), and 5.5≤ε1"-ε2"≤25...(2) (In formula (1), ε1' is a real part of the complex dielectric constant of the first dielectric body, and ε2' is a real part of the complex dielectric constant of the second dielectric body. In formula (2), ε1" is an imaginary part of the complex dielectric constant of the first dielectric body, and ε2" is an imaginary part of the complex dielectric constant of the second dielectric body.)SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a radio wave absorber. [Background technology]

[0002] In modern society, radio waves are used in a variety of fields, including broadcasting, communications, medicine, chemical analysis, positioning, remote control, etc. For example, millimeter wave radar, which uses radio waves in the frequency band of 30 GHz to 300 GHz, is one of the key technologies that support autonomous driving of vehicles.

[0003] Millimeter-wave radar uses radio waves in the above frequency bands to measure the distance, speed, and angle to an object. Taking autonomous driving, one of the technologies in use, as an example, with the spread of ADAS (Advanced Driver Assistance Systems), millimeter-wave radars with long-distance detection of 76GHz to 79GHz are being used as forward monitoring radars. Millimeter-wave radars detect the distance to an object by emitting millimeter waves and receiving the millimeter waves reflected by the object with a receiving antenna.

[0004] In millimeter wave radar devices, a shielding material that blocks radio waves is provided between the antenna and the control circuit, which prevents a decrease in detection accuracy caused by receiving millimeter waves reflected by objects other than the target (such as the road surface). As shielding members, for example, Patent Documents 1 and 2 disclose molded bodies and compositions containing resin and carbon fibers. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-161208 A [Patent Document 2] JP 2020-111730 A Summary of the Invention [Problem to be solved by the invention]

[0006] Sensors and communication devices such as millimeter wave radars are required to be smaller and lighter, so the radio wave absorbers (shielding materials) that are components of them are also required to be thinner and lighter while maintaining high radio wave absorption properties. In addition, many radio wave absorbers use magnetic materials because they provide high radio wave absorption characteristics, but they have a problem in that the absorption width is narrow due to the resonance absorption of magnetic materials. An object of the present invention is to provide a radio wave absorber having high radio wave absorbing characteristics and a wide absorption bandwidth. [Means for solving the problem]

[0007] As a result of intensive investigations, the inventors have found that a radio wave absorber having high radio wave absorption characteristics and a wide absorption bandwidth can be obtained by having a laminated structure of two kinds of dielectrics having different complex dielectric constants, in which a difference between the real parts and the imaginary parts of the complex dielectric constants of the dielectrics satisfy a predetermined relationship, and have completed the present invention.

[0008] According to the present invention, the following radio wave absorber is provided. 1. A structure in which a first dielectric and a second dielectric are laminated, A radio wave absorber, wherein the complex dielectric constants of the first dielectric and the second dielectric at a maximum absorption frequency satisfy the following expressions (1) and (2): 8≦ε 1 '-ε 2 '≦40 (1) 5.5≦ε 1 "-ε 2 ”≦25 (2) (In formula (1), ε 1 ' is the real part of the complex permittivity of the first dielectric, and ε 2 ' is the real part of the complex dielectric constant of the second dielectric. In equation (2), ε 1 ” is the imaginary part of the complex permittivity of the first dielectric, and ε 2 " is the imaginary part of the complex permittivity of the second dielectric.) 2. The radio wave absorber according to 1, wherein a ratio (t1 / t2) of a thickness t1 of the first dielectric to a thickness t2 of the second dielectric is 0.08 to 0.4. 3. The radio wave absorber according to 1 or 2, having a maximum absorption frequency in the range of 75 GHz to 110 GHz. 4. The radio wave absorber according to any one of 1 to 3, wherein the sum (t1+t2) of the thickness t1 of the first dielectric and the thickness t2 of the second dielectric is 0.5 mm or less. 5. The radio wave absorber according to any one of 1 to 4, wherein the first dielectric material contains a resin and a carbon fiber. 6. The radio wave absorber according to any one of 1 to 5, wherein the carbon fiber content is 5 mass % or more. 7. A radio wave absorber according to any one of 1 to 6, A radio wave absorbing structure comprising a radio wave reflector laminated on the second dielectric side of the radio wave absorber. Effect of the Invention

[0009] According to the present invention, it is possible to provide a radio wave absorber having high radio wave absorbing characteristics and a wide absorption bandwidth. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic side view of a radio wave absorbing structure according to one embodiment of the present invention; [Diagram 2] 1A shows the simulation results of the absorption characteristics of the radio wave absorbing structure of Example 1, and FIG. 1B shows the measurement results of the complex dielectric constant of the dielectric A. [Diagram 3] 4 shows the results of simulation of the absorption characteristics of the radio wave absorbing structures produced in Examples 1 to 4. [Figure 4] 1 shows the results of simulating the absorption characteristics of the radio wave absorbing structures produced in Comparative Examples 1 to 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Radio wave absorber] A wave absorber according to one embodiment of the present invention has a structure in which a first dielectric and a second dielectric are laminated. The complex dielectric constants ε (ε=ε'-jε'', j is an imaginary number) of the two dielectrics at the maximum absorption frequency satisfy the following formulas (1) and (2). 8≦ε 1 '-ε 2 '≦40 (1) 5.5≦ε 1 "-ε 2 ”≦25 (2) (In formula (1), ε 1 ' is the real part of the complex permittivity of the first dielectric, and ε 2 ' is the real part of the complex dielectric constant of the second dielectric. In equation (2), ε 1 ” is the imaginary part of the complex permittivity of the first dielectric, and ε 2 " is the imaginary part of the complex permittivity of the second dielectric.)

[0012] In this embodiment, by laminating two types of dielectrics having different dielectric constants with a specific range of dielectric constant difference for radio waves of a specific frequency, a radio wave absorber with high absorption characteristics and a wide absorption bandwidth can be obtained. The real and imaginary parts of the complex dielectric constant can be adjusted, for example, by blending a filler with a matrix such as a resin, as described later. The complex dielectric constant of the dielectric is measured by the free space method as in the examples. The constituent members of the radio wave absorber will be described below.

[0013] <Dielectric> The first and second dielectrics used in the present embodiment are not particularly limited as long as they satisfy the above formulas (1) and (2), and known dielectrics, i.e., materials used as shielding materials for radio wave absorbers, can be used.

[0014] Specific examples of the dielectric include thermoplastic resins and thermosetting resins. Thermosetting resins are preferred because the dielectric constant can be easily adjusted. Specific examples include silicone resins, epoxy resins, phenolic resins, polyurethane resins, thermosetting polyimides, unsaturated polyester resins, and alkyd resins. These resins can be used alone or in combination of two or more. It is preferable to use an appropriate curing agent depending on the type of resin.

[0015] In one embodiment, in order to control the dielectric constant of the dielectric, a filler may be blended into the base material such as the resin. Examples of the filler include carbon fibers, carbon particles, hollow particles, and the like.

[0016] The imaginary part of the complex dielectric constant (ε") is a quantity that correlates with the electrical conductivity of the sample, based on the relationship ε" = σ / ω (ω is the angular frequency of the AC current [rad / s] (ω = 2πf), and σ is the electrical conductivity of the conductor [S / m]). It is possible to significantly improve the electrical conductivity by actively distributing the filler unevenly, for example, by using conductive fibrous fillers (carbon fibers, etc.) that are easier to form a conductive network, or conductive fillers with a large specific gravity (metal particles, carbon particles, etc.), or by combining and mixing fillers with large differences in specific gravity. This usually makes it possible to increase the value of the imaginary part of the complex dielectric constant of the dielectric. Moreover, by adding a filler having a large real part of the complex dielectric constant, the value of the real part of the complex dielectric constant of the dielectric can be increased.

[0017] As the carbon fiber, either pitch-based carbon fiber or PAN-based carbon fiber can be used. The carbon fiber may be used alone or in combination of two or more types. In one embodiment, the carbon fiber content of the first dielectric is 5% by mass or more. This allows the real part and the imaginary part of the complex dielectric constant of the first dielectric to be sufficiently large. The carbon fiber content of the first dielectric may be 10% by mass or more, or may be 13% by mass or more. In one embodiment, the second dielectric does not contain carbon fibers, which can reduce the values ​​of the real part and the imaginary part of the complex dielectric constant of the second dielectric.

[0018] The average length of the carbon fibers is preferably 100 μm to 4000 μm. Within this range, high absorption properties can be obtained with a small amount of carbon fibers. The average length of the carbon fibers may be 150 μm or more, or may be 200 μm or more. The average length herein refers to the average value of the lengths of 25 randomly selected carbon fibers measured with a scanning electron microscope (SEM). The average length is the average length of the carbon fibers (raw material) during the production of the radio wave absorber. It is preferable that the average length of the carbon fibers in the radio wave absorber after production is also within the above range.

[0019] Examples of carbon particles include acetylene black, furnace black, channel black, ketjen black, and oil furnace carbon.

[0020] Examples of hollow particles include ceramic balloons, glass balloons, and shirasu balloons. The average diameter of the hollow particles (d50: median diameter) is preferably 1 / 2 or less of the average length of the carbon fibers. More preferably, it is 1 / 3 or less of the average length of the carbon fibers. The average diameter of the hollow particles is a value based on JIS Z 8819-1:1999.

[0021] The pressure resistance of the hollow particles is preferably 8 MPa or more. If the pressure resistance of the hollow particles is low, they are likely to crack when mixed with a resin, etc. The pressure resistance of the hollow particles is more preferably 10 MPa or more. The compressive strength is measured by the glycerol method in accordance with ASTM D3102-78.

[0022] The dielectric may contain known resin additives within the scope of the problem solving. Known resin additives include stabilizers against heat, light, ultraviolet rays, etc., lubricants, nucleating agents, plasticizers, antistatic agents, release agents, flame retardants, softeners, dispersants, antioxidants, coloring materials, etc. The total content of the above known resin additives is preferably 30% by mass or less, and more preferably 10% by mass or less, of the dielectric.

[0023] In one embodiment, the ratio (t1 / t2) of the thickness t1 of the first dielectric to the thickness t2 of the second dielectric is 0.08 to 0.4, thereby obtaining a radio wave absorber with high absorption characteristics and a wide absorption bandwidth.

[0024] In one embodiment, the sum (t1+t2) of the thickness t1 of the first dielectric and the thickness t2 of the second dielectric is 0.5 mm or less. The wave absorber of this embodiment has excellent wave absorption properties, so the thickness can be thin. The sum (t1+t2) of the thicknesses t2 can be 0.4 mm or less, and can also be 0.3 mm or less. On the other hand, in order to obtain sufficient wave absorption properties, it is usually 0.2 mm or more.

[0025] In one embodiment, the maximum absorption frequency of the radio wave absorber is in the range of 75 GHz to 110 GHz. The maximum absorption frequency can be adjusted, for example, by the complex dielectric constant ε at the maximum absorption frequency or the ratio (t1 / t2) of the thickness t1 of the first dielectric to the thickness t2 of the second dielectric. The radio wave absorber of this embodiment has a wide absorption bandwidth centered on the maximum absorption frequency, and can have an absorption bandwidth of 10 GHz or more, where the amount of attenuation (amount of absorption) is 20 dB or more. In this embodiment, since high radio wave absorption characteristics are obtained due to dielectric loss and conductive loss, it is not necessary to use a magnetic material as a filler.

[0026] The dielectric can be produced, for example, by mixing the above-mentioned resin, the filler, and, if necessary, any optional components, and forming the mixture into a sheet. If necessary, a known dispersing device such as a high-speed disperser, a sand grind mill, a basket mill, a three-roll mill, or a ball mill may be used for mixing. When mixing, the various components can be mixed at once or in portions so as to obtain a predetermined amount. The order of mixing the various components is not particularly limited.

[0027] Examples of the method include a method in which each of the above-mentioned components is mixed with a thermosetting resin (liquid before curing) or a solution in which a resin is dissolved in a solvent, and the mixed liquid is formed into a sheet of a desired thickness by cast film formation, applicator film formation, etc., and heated. The heating time of the mixed liquid can be appropriately adjusted taking into consideration the viscosity of the resin or resin solution used, the solvent used, etc.

[0028] In one embodiment, the second dielectric is made of only resin without filler. This allows the real and imaginary parts of the complex dielectric constant of the second dielectric to be small. In addition, the difference in the complex dielectric constant between the first and second dielectrics can be controlled by simply adjusting the filler of the first dielectric, so that the manufacturing cost can be reduced.

[0029] [Radio wave absorbing structure] A radio wave absorbing structure according to one embodiment of the present invention includes the above-mentioned radio wave absorber of the present invention and a radio wave reflector laminated on the second dielectric side of the radio wave absorber. 1 is a schematic cross-sectional view of a radio wave absorbing structure according to one embodiment of the present invention. The radio wave absorbing structure 1 has a structure in which a radio wave absorber 10 consisting of a first dielectric 11 and a second dielectric 12 is laminated on one surface of a radio wave reflector 21. In this case, the second dielectric 12 side is formed on the radio wave reflector 21. This can improve the radio wave absorption properties.

[0030] The radio wave reflector is not particularly limited as long as it has the property of reflecting radio waves without transmitting them, such as a metal plate, a metal foil, a metal film, etc. Note that a resin layer such as a general adhesive layer may be formed between the first dielectric and the second dielectric, and between the radio wave absorber and the radio wave reflector. The radio wave absorbing structure can be manufactured, for example, by directly depositing a radio wave absorber on a radio wave reflector, or by attaching a radio wave reflector to a radio wave absorber.

[0031] Although a sheet-like radio wave absorbing structure is shown as an example in FIG. 1, the radio wave absorbing structure of this embodiment is not limited to a planar body such as a sheet, and can be appropriately shaped according to the area where it is used. EXAMPLES

[0032] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, unless otherwise specified, the temperature and pressure conditions were approximately at room temperature (usually 25° C.) and normal pressure (usually 0.1013 MPa).

[0033] [Preparation of the first dielectric] Manufacturing Example 1 (1) Dielectric A 95.0% by mass (after curing) of silicone resin (polydimethylsiloxane: manufactured by Dow-Toray Industries, Inc., the main material of Silpot184 (registered trademark)) and 5.0% by mass of carbon fiber (middle fiber, manufactured by Nippon Graphite Fiber Co., Ltd., and XN-100-15M, average length 150 μm) were mixed and stirred at 2000 rpm for 2 minutes using a planetary centrifugal mixer (Awatori Rentaro ARE-310). To the resulting mixture, 1 part (volume ratio) of a hardener, Silpot184 (registered trademark), was added to the main material 10 parts by volume, and the mixture was further stirred and mixed at 2000 rpm for 2 minutes, and then degassed at 2000 rpm for 2 minutes. The degassed mixture was poured into a 10 cm square aluminum container, and then the aluminum container was heated on a hot plate at 100°C for 40 minutes to harden the mixture. The hardened product (cast film) was peeled off from the aluminum container to produce a first dielectric. The thickness was 0.08 mm.

[0034] (2) Dielectrics B to D The dielectric was produced in the same manner as dielectric A, except that the composition of the silicone resin and the carbon fiber was changed as shown in Table 1. The measurements of the formulations, thicknesses and complex dielectric constants at the maximum absorption frequency of the dielectrics A to D are shown in Table 1. In the table, PDMS represents silicone resin (polydimethylsiloxane).

[0035] [Table 1]

[0036] The complex dielectric constant was measured by the free space method using the following device. Equipment used: PNA microwave network analyzer N5227 (Keysight Technologies) Test conditions Atmosphere: Air Temperature: room temperature Frequency: 75GHz~110GHz Irradiation angle: vertical

[0037] (3) Dielectrics E to H The composition of the silicone resin, carbon fiber, and hollow glass beads (Q-CEL 7040S, manufactured by Potters Barotini Co., Ltd.) was changed as shown in Table 2, and the other components were prepared in the same manner as for dielectric A. Table 2 shows the measurement results of the compositions, thicknesses and complex dielectric constants at the maximum absorption frequencies of the dielectrics E to H.

[0038] [Table 2]

[0039] [Preparation of the second dielectric] Manufacturing Example 2 A second dielectric was produced in the same manner as in Production Example 1, except that the silicone resin (polydimethylsiloxane: manufactured by Dow Toray Industries, Inc., the main material of Silpot184 (registered trademark)) was used at 100 mass %. In the examples described later, the thickness was adjusted so that the total thickness of the first dielectric and the second dielectric was 0.3 mm. In the comparative examples, the thickness of the second dielectric was 0.07 to 0.737 mm (Table 3). The complex dielectric constant of the second dielectric material varied slightly depending on the thickness, but the real part was 2.29 to 2.38 and the imaginary part was 0.39 to 0.59 (Table 3).

[0040] Examples 1 to 4 Comparative Examples 1 to 4 The dielectrics produced in Production Examples 1 and 2 were laminated on an aluminum plate serving as a radio wave reflector (first dielectric / second dielectric / aluminum plate) as shown in Table 3 to produce a radio wave absorbing structure. The type of the first dielectric used in the examples and comparative examples, the real part, the imaginary part and the thickness of the complex dielectric constant of each dielectric are shown in Table 3. The unit of thickness is mm. The difference between the real part and the imaginary part of the complex dielectric constant of the first dielectric and the second dielectric, and the ratio of the thickness of the first dielectric to the second dielectric (t1 / t2) are also shown in Table 3.

[0041] [Table 3]

[0042] [Theoretical calculation and simulation of absorption characteristics] The absorption characteristics of the radio wave absorbing structures produced in the examples and comparative examples were obtained by theoretical calculation from a transmission line model using the following theoretical formula, and by numerical calculation by the propagation matrix method.

number

[0043] (In the above equation, Γ is the reflection coefficient, Z in is the input impedance of each layer, Z 0 is the wave impedance of vacuum, Z is the wave impedance of each layer, γ is the propagation constant, and d is the film thickness of each layer.

[0044] Fig. 2(a) shows the simulation results of the absorption characteristics of the radio wave absorbing structure of Example 1, and (b) shows the measurement results of the complex dielectric constant of the dielectric A. Fig. 2(a) shows that the maximum absorption frequency is 97 GHz. Fig. 2(b) also shows that the real part ε of the complex dielectric constant of the first dielectric at the maximum absorption frequency 1 ' is 10.6, and the imaginary part ε 1 " turns out to be 6.53.

[0045] Fig. 3 shows the results of simulating the absorption characteristics of the radio wave absorbing structures produced in Examples 1 to 4 (Ex. 1 to 4). Fig. 3 confirms that the radio wave absorbing structures of the Examples have a wide frequency range of 10 GHz or more, where absorption of 20 dB or more is shown, despite the total dielectric thickness being as thin as 0.3 mm. The maximum absorption frequency was 93 GHz in Example 2, 89 GHz in Example 3, and 84 GHz in Example 4.

[0046] Fig. 4 shows the results of simulating the absorption characteristics of the radio wave absorbing structures produced in Comparative Examples 1 to 4. Compared with the radio wave absorbing structures of the Examples, it can be seen that the radio wave absorbing structures of the Comparative Examples have poor absorption characteristics. The maximum absorption frequency was 93.59 GHz for Comparative Example 1, 91.1 GHz for Comparative Example 2, 110 GHz for Comparative Example 3, and 110 GHz for Comparative Example 4. [Industrial Applicability]

[0047] The radio wave absorber of the present invention can be used as a member for absorbing radio waves in the millimeter wave region in vehicles, civil engineering structures, buildings, port facilities, ship facilities, bridges, power facilities, communication facilities, mechanical facilities, etc. Specifically, it is suitable as a radio wave absorbing and shielding member used in millimeter wave radar devices, electric and electronic devices, etc. used in ships, aircraft, vehicles, etc. In addition, it is suitable as an unwanted radio wave absorbing and shielding member in traffic infrastructure environments such as road guardrails, tunnel inner walls, and smart cities. [Explanation of symbols]

[0048] 1. Radio wave absorbing structure 10. Radio wave absorber 11 First Dielectric 12 Second Dielectric 21 Radio wave reflector

Claims

1. It has a structure in which a first dielectric and a second dielectric are stacked, A radio wave absorber wherein the complex dielectric constants of the first dielectric and the second dielectric at the maximum absorption frequency satisfy the following equations (1) and (2). 8≦e １ '-e ２ '≦40 (1) 5.5≦e １ "-e" ２ ”≦25 (2) (In formula (1), ε １ ' is the real part of the complex permittivity of the first dielectric, and ε ２ ' is the real part of the complex permittivity of the second dielectric. In equation (2), ε １ " is the imaginary part of the complex permittivity of the first dielectric, and ε ２ (where is the imaginary part of the complex permittivity of the second dielectric.)

2. The radio wave absorber according to claim 1, wherein the ratio (t1 / t2) of the thickness t1 of the first dielectric to the thickness t2 of the second dielectric is 0.08 to 0.

4.

3. The radio wave absorber according to claim 1, wherein the maximum absorption frequency is in the range of 75 GHz to 110 GHz.

4. The radio wave absorber according to claim 1, wherein the sum of the thickness t1 of the first dielectric and the thickness t2 of the second dielectric (t1 + t2) is 0.5 mm or less.

5. The radio wave absorber according to claim 1, wherein the first dielectric comprises a resin and carbon fibers.

6. The radio wave absorber according to claim 1, wherein the carbon fiber content is 5% by mass or more.

7. A radio wave absorber according to any one of claims 1 to 6, A radio wave absorbing structure comprising a radio wave reflector laminated on the second dielectric side of the aforementioned radio wave absorber.

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