Radio wave absorbent

Abstract

To provide a radio wave absorbent that has high radio wave absorbing properties and can be made thinner than conventional products.SOLUTION: A radio wave absorbent has a laminated structure of a dielectric and a conductor having a plurality of regularly arranged openings, the length of the longest part of the opening is 0.1% to 2% of the wavelength to be absorbed.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 of an object. Taking autonomous driving, one of the technologies that uses this technology, as an example, with the spread of ADAS (Advanced Driver Assistance Systems), millimeter-wave radars with a frequency of 76 GHz to 79 GHz that can detect long distances 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 and returning with a receiving antenna. In addition, radio waves in the range of 24 GHz to 30 GHz, known as quasi-millimeter waves, are used for side monitoring radars, etc. Radio waves of around 28 GHz are also used in communication devices (5G).

[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).

[0005] Patent Document 1 discloses a radio wave interference-type radio wave absorber in which a resistive film, a dielectric layer, and a radio wave shielding layer are laminated in this order. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2023-25502 A Summary of the Invention [Problem to be solved by the invention]

[0007] Sensors such as millimeter wave radar are required to be compact and lightweight, so their constituent radio wave absorbers (shielding materials) also need to be thinner and lighter while maintaining high radio wave absorbing properties. An object of the present invention is to provide a radio wave absorber that has high radio wave absorbing properties and can be made thinner than conventional ones. [Means for solving the problem]

[0008] As a result of extensive research, the inventors have discovered that by using a conductor having a plurality of regularly arranged openings (non-conductor portions) instead of the thin metal plate currently used as a radio wave reflector, a radio wave absorber having higher radio wave absorbing characteristics than conventional ones can be obtained, and have completed the present invention.

[0009] According to the present invention, the following radio wave absorber is provided. 1. A laminated structure of a dielectric and a conductor having a plurality of regularly arranged openings; A radio wave absorber, wherein the length of the longest part of the opening is 0.1% to 2% of the wavelength to be absorbed. 2. The radio wave absorber according to 1, wherein the thickness of the dielectric is less than 10% of the wavelength to be absorbed. 3. The radio wave absorber according to 1 or 2, wherein the imaginary part is larger than the real part of the complex dielectric constant of the dielectric at the wavelength to be absorbed. 4. The radio wave absorber according to any one of 1 to 3, which has a total light transmittance of 29% or less. 5. The radio wave absorber according to any one of 1 to 4, wherein the openings are in a lattice pattern and the shortest distance between adjacent openings is 0.05 mm or less. 6. The radio wave absorber according to any one of 1 to 5, having an electromagnetic wave absorption amount of 30% or more in the entire frequency range from 6.28 GHz to 44 GHz. Effect of the Invention

[0010] According to the present invention, it is possible to provide a radio wave absorber that has high radio wave absorbing properties and can be made thinner than conventional ones. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic side view of a radio wave absorber according to an embodiment of the present invention. [Diagram 2] 1 is a plan view of an example of a conductor used in the present invention. [Diagram 3] 1 is a plan view of an example of a conductor used in the present invention. [Figure 4] 1 is a plan view of an example of a conductor used in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [Radio wave absorber] A radio wave absorber according to one embodiment of the present invention has a laminated structure of a dielectric and a conductor having a plurality of regularly arranged openings, and is characterized in that the length of the longest part of the conductor opening is 0.1% to 2% of the wavelength of the radio wave absorbed by the radio wave absorber (hereinafter referred to as the target absorption wavelength).

[0013] Fig. 1 is a schematic side view of a radio wave absorber according to one embodiment of the present invention. The radio wave absorber 10 has a structure in which a layer made of a mesh-shaped conductor 2 is laminated on one side of a dielectric layer 1. The radio wave absorber 10 absorbs radio waves incident from the dielectric layer 1 side. In this embodiment, the length of the longest part of the opening of the mesh-shaped conductor 2 is set to 0.1% to 2% of the wavelength to be absorbed. This makes it possible to improve the radio wave absorbing performance while making the absorber thinner and lighter than conventional radio wave absorbers.

[0014] The configuration of the radio wave absorber of the present embodiment is not limited to the radio wave absorber 10. For example, a mesh-shaped conductor may be embedded in a dielectric layer. In other words, any structure may be used as long as radio waves pass through the dielectric before coming into contact with the mesh-shaped conductor. Furthermore, the conductor having a number of regularly arranged openings is not limited to a mesh shape (network shape) and may be, for example, a sheet (punched metal) obtained by punching a metal sheet or the like into circles, ellipses, polygons, and the like. Also, a conductor having a number of regularly arranged openings may be formed on one side of the dielectric layer by printing, deposition, etching, etc. The openings may be empty spaces (air layers) or may be made of a non-conductive material. The constituent members of the radio wave absorber will be described below.

[0015] <Dielectric> The dielectric material used in this embodiment includes thermoplastic resin, thermosetting resin, etc. Thermosetting resin is preferable because the dielectric constant can be easily adjusted. Specific examples include silicone resin, epoxy resin, phenol resin, polyurethane resin, thermosetting polyimide, unsaturated polyester resin, alkyd resin, etc. 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.

[0016] 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, metal particles, hollow particles, and the like. 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.

[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 content of the carbon fiber relative to the dielectric may be 1% by mass or more, or 2% by mass or more, and may be 5% by mass or less, 4% by mass or less, or 3% by mass or less.

[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. The content of the carbon particles relative to the dielectric may be 0.1% by mass or more, or 0.5% by mass or more, and may be 5% by mass or less, 4% by mass or less, or 3% by mass or less.

[0020] The metal particles are not particularly limited as long as they are particles made of a conductive metal. The metal is preferably iron. Examples of the iron powder particles include spherical iron powder particles, flat iron powder particles, and soft magnetic iron alloy particles. The content of the iron-based particles relative to the dielectric may be 2% by mass or more, or 4% by mass or more, and may be 10% by mass or less, 8% by mass or less, or 6% by mass or less.

[0021] 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.

[0022] 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.

[0023] The content of the hollow particles in the entire wave absorber may be 0.1 mass % or more, or 0.5 mass % or more, and may be 5 mass % or less, or 3 mass % or less.

[0024] 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.

[0025] In one embodiment, the thickness of the dielectric is less than 10% of the wavelength to be absorbed. In this embodiment, even if the dielectric is made thinner than that of a conventional wave absorber, high wave absorption characteristics can be achieved. The thickness of the dielectric may be 8% or less of the wavelength to be absorbed, and can also be 6% or less.

[0026] For example, when the radio wave frequency is 28 GHz, the wavelength is 10.7 mm, so the suitable thickness of the dielectric is less than 1.07 mm. When the radio wave frequency is 44 GHz, the wavelength is 6.8 mm, so the suitable thickness of the dielectric is less than 0.68 mm. From the viewpoint of weight reduction, a thinner dielectric is preferable, but the thickness of the dielectric is usually 0.2 mm or more.

[0027] In one embodiment, the value of the imaginary part of the complex dielectric constant of the dielectric at the wavelength to be absorbed is larger than the value of the real part. This can improve the radio wave absorbing ability of the radio wave absorber. The magnitude relationship between the real part and the imaginary part of the complex dielectric constant can be adjusted by blending a filler into the dielectric as described above. Note that, with only a resin or the like that is the matrix of the dielectric, it is rare for the imaginary part of the complex dielectric constant of the dielectric to be larger than the real part. The complex dielectric constant of the dielectric can be measured by a free space method which will be described later in the Examples.

[0028] 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.

[0029] 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.

[0030] <Conductor> The conductor used in this embodiment is a sheet-like body having a plurality of regularly arranged openings, and the length of the longest part of the openings is 0.1% to 2% of the wavelength to be absorbed. In this embodiment, the absorption ability of a specific wavelength can be improved by adjusting the length of the openings.

[0031] For example, since the wavelength of a radio wave with a frequency of 28 GHz is 10.7 mm, the longest part of the opening is adjusted to 0.0107 mm to 0.214 mm. Since the wavelength of a radio wave with a frequency of 44 GHz is 6.8 mm, the longest part of the opening is adjusted to 0.0068 mm to 0.136 mm. For example, when the frequency of the radio waves to be absorbed is between 28 GHz and 44 GHz, the longest length of the opening can be adjusted to between 0.0068 mm and 0.214 mm. The length of the longest part of the opening is preferably 0.1% to 1.5% of the wavelength to be absorbed, more preferably 0.2% to 1.25%, and even more preferably 0.5% to 1.1%.

[0032] The shape of the opening is not particularly limited. For example, it may be a lattice shape, a circle shape, an ellipse shape, a polygonal shape, or a star shape. The shape of the opening is preferably a lattice shape (square, rectangle, rhombus). The longest part of the opening means, for example, the longer diagonal length in the lattice shape, and means the major axis in the ellipse shape.

[0033] When the conductor is in a lattice shape, the shortest distance between adjacent openings is preferably 0.05 mm or less. This allows the ratio of the openings in the conductor to be increased, and the weight of the radio wave absorber to be further reduced. The lower limit of the spacing between the lines of the conductor is usually 0.01 mm or more.

[0034] In one embodiment, the thickness (or wire diameter) of the conductor is 0.5 mm or less, may be 0.3 mm or less, or may be 0.1 mm or less. From the viewpoint of weight reduction, the thinner the conductor, the better, but the thickness is usually 0.01 mm or more.

[0035] In one embodiment, the aperture ratio of the conductor (the ratio of the total area of ​​the openings to the total area of ​​the conductor) is 1% or more, 7% or more, or 9% or more, and is 40% or less, or 35% or less.

[0036] Examples of the conductor include general metal materials such as metals such as iron, aluminum, copper, nickel, gold, and silver, and alloys such as SUS, etc. SUS, aluminum, and copper are preferred because they are inexpensive. It is not preferable to use conductive polymers such as polyaniline and polythiophene as the conductor.

[0037] The conductor used in this embodiment does not need to be entirely conductive, but only needs to be present in the surface area that mainly acts on radio waves. For example, it may be a plated resin sheet in which a metal plating is formed on the surface of a resin sheet having a number of regularly arranged openings. It may also be a plated body in which a mesh sheet (woven fabric) is made from various fibers and the sheet is plated with a metal. From the viewpoint of weight reduction, in one embodiment, the conductor is preferably a metal-plated resin or fiber.

[0038] There are no limitations on the resin and fiber that will become the substrate to be plated, and examples thereof include polyethylene terephthalate (PET), polyester, (meth)acrylate, and the like. An example of the plating is nickel (Ni) plating.

[0039] When using a metal-plated body, the plating thickness should be thicker than the skin depth d expressed by the following formula (1).

number

[0040] The conductor having a number of regularly arranged openings can be formed in various forms, such as by weaving metal wires (plain weave, twill weave, etc.), by printing or vapor-depositing a conductor on a non-conductive substrate, by metal plating on a resin fabric, etc. The type can be selected in consideration of weight, cost, etc. according to the application of the radio wave absorber.

[0041] Figures 2 to 4 are planar photographs of examples of conductors that can be used in this embodiment. Figure 2 shows a sheet of plain woven SUS. Figure 3 shows a sheet of polyester mesh base material (twill weave) that has been subjected to Ni electroless plating. Figure 4 shows a sheet of polyester mesh base material (plain weave) that has been subjected to Ni electroless plating.

[0042] <Manufacture of radio wave absorbers> The radio wave absorber according to this embodiment can be manufactured, for example, by laminating the above-mentioned dielectric and conductor (FIG. 1). For example, it can be manufactured by a method of directly depositing the dielectric 1 on the mesh-shaped conductor 2, or by attaching the mesh-shaped conductor 2 to the dielectric 1. A resin layer such as a general adhesive layer may be formed between the dielectric 1 and the mesh-shaped conductor 2. Although FIG. 1 illustrates a sheet-like radio wave absorber as an example, the radio wave absorber of this embodiment is not limited to a planar body such as a sheet, and can be appropriately shaped and manufactured according to the area where it is to be used.

[0043] Moreover, the radio wave absorber according to an embodiment can also be manufactured by dipping the conductor into a solution of the thermosetting resin before hardening as the dielectric material described above, followed by hardening and drying.

[0044] In one embodiment, the radio wave absorber does not have a resistive coating made of a conductive organic polymer or a conductor on the surface of the dielectric. The radio wave absorber of this embodiment does not need to have a resistive coating because electromagnetic waves are converted into heat inside the radio wave absorber due to dielectric loss of the dielectric.

[0045] In one embodiment, the total light transmittance of the radio wave absorber is 29% or less. In the radio wave absorber of this embodiment, the opening of the dielectric is small, so the total light transmittance is low. Therefore, it is clearly different from a metal mesh device (MMD) that has a high total light transmittance. The total light transmittance is measured by a light transmittance measurement method.

[0046] The radio wave absorber of the present embodiment has high radio wave absorbing performance particularly in the entire frequency range from 28 GHz to 44 GHz. Specifically, the amount of radio waves absorbed in the entire frequency range from 28 GHz to 44 GHz can be 30% or more, and can also be 50% or more or 60% or more. EXAMPLES

[0047] 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).

[0048] Manufacturing Example 1 92.8% by mass (after curing) of silicone resin (polydimethylsiloxane: main material of Silpot184 (registered trademark) manufactured by Dow-Toray Industries, Inc.), 2.2% by mass of carbon fiber (middle fiber, manufactured by Nippon Graphite Fiber Co., Ltd., XN-100-15M, average length 150 μm), and 5% by mass of soft magnetic flat powder (RD145AS manufactured by Sanyo Special Steel Co., Ltd.) were stirred and mixed 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 dielectric.

[0049] Manufacturing Examples 2 to 6 A dielectric was produced in the same manner as in Production Example 1, except that the composition of the mixed liquid was changed as shown in Table 1. The materials used are as follows: (resin) Silicone resin (polydimethylsiloxane: manufactured by Dow Toray Industries, Inc., Silpot184 (registered trademark)) (Carbon Fiber) · Middle Fiber (manufactured by Nippon Graphite Fiber Co., Ltd.) Average length 150μm (XN-100-15M) Average length 250μm (XN-100-25M) (hollow particles) Hollow glass beads (Potters Barotini Co., Ltd. Q-CEL 7040S) (carbon particles) Ketjen Black (Lionite CB, manufactured by Lion Specialty Chemicals Co., Ltd.) (metal particles) Soft magnetic flat powder (Sanyo Special Steel Co., Ltd. RD145AS, iron-based metal particles)

[0050] [Table 1]

[0051] Example 1 A radio wave absorber was produced by laminating a mesh-shaped conductor, SUS plain weave 400 mesh (longest length of mesh opening: 0.059 mm, wire diameter: 0.023 mm, Clever SUS400-023), on the dielectric produced in Production Example 1.

[0052] Examples 2 to 6, Comparative Examples 1 to 5 Aside from changing the dielectric material and the conductor as shown in Table 2, a radio wave absorber was produced in the same manner as in Example 1.

[0053] [Table 2]

[0054] In Table 2, conductor types A to E are as follows. A: SUS plain weave 400 mesh (longest mesh opening length: 0.059 mm, wire diameter: 0.023 mm, Clever SUS400-023) B: A mesh-shaped conductor (twill weave, maximum length of mesh opening: 0.021 mm) made of polyester mesh substrate with Ni electroless plating (plating thickness: 0.4 μm) C: A mesh-shaped conductor (plain weave, longest length of mesh opening: 0.072 mm) made of polyester mesh substrate with Ni electroless plating (plating thickness: 0.4 μm) D: Aluminum plate (2 mm thick) E: SUS plain weave 100 mesh (longest mesh opening length: 0.239 mm, wire diameter: 0.1 mm, manufactured by Hikari: PS100-321)

[0055] The wave absorption characteristics and total light transmittance of the wave absorbers produced in the examples and comparative examples were measured. The measurement results are shown in Tables 3 and 4. <Radio wave absorption characteristics> The measurement of the radio wave absorption characteristics (radio wave absorption amount and complex dielectric constant of the dielectric) was carried out by the free space method. Equipment used: PNA microwave network analyzer N5227 (Keysight Technologies) Test conditions Atmosphere: Air Temperature: room temperature Frequency: 28GHz~44GHz Irradiation angle: vertical

[0056] <Light transmittance measurement> The light transmittance was measured using a turbidity meter NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd., in accordance with JIS K 7361-1, which is a test method for the total light transmittance of plastic transparent materials.

[0057] [Table 3]

[0058] [Table 4]

[0059] It can be seen from Table 3 that high absorption characteristics of 30% or more can be obtained at all frequencies in Examples 1 to 6. On the other hand, in Comparative Examples 1 to 4, in which an aluminum plate having no openings in the conductor is used, the radio wave absorption amount is less than 30% at all frequencies, even though the same dielectric as in Examples 1 to 3 and 5 is used. In Comparative Example 5, the length of the longest part of the opening of the conductor was longer than 2% of the wavelength to be absorbed, and therefore the radio wave absorption characteristics were lower than those of the Examples. [Industrial Applicability]

[0060] 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]

[0061] 10. Radio wave absorber 1 Dielectric layer 2 Layer made of mesh-like conductor

Claims

1. The dielectric layer and the conductor layer have a plurality of regularly arranged openings. A radio wave absorber, wherein the length of the longest part of the opening is 0.1% to 2% of the wavelength to be absorbed.

2. 2. The radio wave absorber according to claim 1, wherein the thickness of said dielectric material is smaller than 10% of the wavelength to be absorbed.

3. 3. The radio wave absorber according to claim 1, wherein the imaginary part of the complex dielectric constant of the dielectric at the wavelength to be absorbed is larger than the real part.

4. 3. The radio wave absorber according to claim 1, which has a total light transmittance of 29% or less.

5. 3. The radio wave absorber according to claim 1, wherein the openings are in a lattice pattern, and the shortest distance between adjacent openings is 0.05 mm or less.

6. 3. The radio wave absorber according to claim 1, wherein the amount of electromagnetic wave absorption in the entire frequency range of 28 GHz to 44 GHz is 30% or more.

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