Radio wave absorber

Abstract

This radio wave absorber is constituted by a sheet-shaped article including resin and carbon particles without containing carbon fibers. In a region of the sheet-shaped article from a first end surface to a second end surface facing the first end surface, 60-80 vol% of the entirety of the carbon particles are present in a region from the first end surface to half of the total thickness.

Description

Radio wave absorber

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

[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 supports 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 a frequency of 76 GHz to 79 GHz, which can detect long distances, are being used as forward-monitoring radars. Millimeter-wave radars emit millimeter waves and receive the millimeter waves reflected by the object using a receiving antenna to detect the distance to the object, etc.

[0004] In millimeter-wave radar devices, a shielding member that blocks radio waves is provided between the antenna and the control circuit. This prevents a decrease in detection accuracy due to reception of millimeter waves reflected by objects other than the target (such as the road surface). For example, Patent Documents 1 and 2 disclose molded bodies and compositions containing resin and carbon fiber as shielding members.

[0005] JP 2019-161208 A JP 2020-111730 A

[0006] Sensors and communication devices such as millimeter-wave radars are required to be smaller and lighter, and therefore, their constituent radio wave absorbers (shielding members) are also required to be thinner and lighter while maintaining high radio wave absorption characteristics. Furthermore, many radio wave absorbers use magnetic materials because they can achieve high radio wave absorption characteristics. However, there is a problem in that the absorption bandwidth is narrow due to resonance absorption of magnetic materials. An object of the present invention is to provide a radio wave absorber that has high radio wave absorption characteristics and a wide absorption bandwidth.

[0007] As a result of extensive research, the present inventors have found that a radio wave absorber with high radio wave absorption characteristics and a wide absorption bandwidth can be obtained by distributing carbon particles unevenly on one side in the thickness direction of the resin matrix, and have completed the present invention.

[0008] According to the present invention, the following radio wave absorbers and the like are provided. 1. A radio wave absorber which is a sheet-like molding containing a resin and carbon particles but no carbon fibers, wherein, within a region from a first end face to an opposing second end face of the sheet-like molding, 60 to 80 volume % of the total carbon particles are present in a region from the first end face to halfway through the total thickness. 2. The radio wave absorber according to 1, which has a thickness of 0.6 mm or more and 1.5 mm or less. 3. The radio wave absorber according to 1 or 2, wherein the carbon particles are ketjen black. 4. The radio wave absorber according to any one of 1 to 3, wherein the content of the carbon particles is 3 volume % or more and 20 volume % or less of the total volume. 5. The radio wave absorber according to any one of 1 to 4, wherein the resin is a thermosetting resin. 6. The radio wave absorber according to 5, wherein the thermosetting resin is a silicone resin. 7. The radio wave absorber according to 5, wherein the thermosetting resin contains polydimethylsiloxane. 8. 8. The radio wave absorber according to any one of 1 to 7, having a maximum absorption frequency in the range of 75 GHz to 110 GHz. 9. The radio wave absorber according to any one of 1 to 8, having no installation angle dependency in its radio wave absorption characteristics. 10. A radio wave absorbing structure comprising: the radio wave absorber according to any one of 1 to 9; and a radio wave reflector laminated on the first end face side of the radio wave absorber. 11. The radio wave absorbing structure according to 10, having an absorption amount of 90% or more of radio waves in the entire frequency range of 75 GHz to 110 GHz. 12. A method for producing the radio wave absorber according to any one of 1 to 9, comprising preparing two or more layers of resin sheets having different carbon particle concentrations, and laminating the two or more layers of resin sheets to form an uneven distribution of carbon particles.

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

[0010] Fig. 1 is a schematic side view of a radio wave absorber according to one embodiment of the present invention. Fig. 2 is a schematic side view of a radio wave absorbing structure according to one embodiment of the present invention. Fig. 3 shows measurement results of the radio wave absorbing characteristics of the radio wave absorbing structures produced in Examples 1 to 3. Fig. 4 shows measurement results of the radio wave absorbing characteristics of the radio wave absorbing structures produced in Comparative Examples 1 to 3. Fig. 5 shows measurement results of the installation angle dependency of the radio wave absorbing characteristics of the radio wave absorbing structure produced in Example 1. Fig. 6 shows measurement results of the installation angle dependency of the radio wave absorbing characteristics of the radio wave absorbing structure produced in Comparative Example 4.

[0011] [Radio wave absorber] A radio wave absorber according to one embodiment of the present invention is a sheet-like molded body that contains a resin matrix and carbon particles mixed in the resin, but does not contain carbon fibers. 60 to 80% by volume of the total carbon particles are present in a region from a first end face to halfway through the entire thickness of the sheet-like molded body.

[0012] FIG. 1 is a schematic cross-sectional view of a radio wave absorber according to one embodiment of the present invention. The radio wave absorber 1 is a sheet with a thickness T that contains resin and carbon particles but no carbon fiber. The distance between the first end face 11 and the opposing second end face 12, i.e., the thickness, is T. In this embodiment, 60 to 80 volume percent of all carbon particles are present in a region (I) (FIG. 1, A) extending from the first end face 11 in contact with the radio wave reflector to half of the total thickness T (thickness t = 0.5T). That is, the carbon particles are more concentrated in the region (I) on the first end face 11 side than in the region (II) (FIG. 1, B: thickness t = 0.5T) on the second end face 12 side, which is the radiated radio wave side. Furthermore, the carbon particles are uniformly dispersed in the horizontal direction (the direction of the surface where the radio waves strike). By distributing the carbon particles closer to the first end face 11, a radio wave absorber with high absorption characteristics and a wide absorption bandwidth can be obtained. Furthermore, radio wave absorption can be improved compared to when the same amount of carbon particles is uniformly dispersed. The wave absorber of this embodiment does not contain carbon fiber, which reduces the dependency of the wave absorption characteristics on the installation angle relative to the incident surface.

[0013] In one embodiment, the wave absorption characteristics of a radio wave absorber are not dependent on the installation angle. For example, when measuring the wave absorption characteristics, the radio wave absorber is installed and the first measured wave absorption characteristics are defined as the wave absorption characteristics at 0°, and the wave absorption characteristics measured after rotating the radio wave absorber 90° around the irradiation line axis from the measurement at 0° are defined as the wave absorption characteristics at 90°. "Not dependent on the installation angle" means that the wave absorption characteristics at 0° and the wave absorption characteristics at 90° are substantially the same. For example, it is preferable that the amount of wave absorption at both 0° and 90° is 90% or more over the entire frequency range from 75 GHz to 110 GHz. The same applies to the radio wave absorbing structure described below.

[0014] The proportion (volume %) of carbon particles present in region A can be determined, for example, by measuring the sample with a microfocus X-ray CT device, reconstructing a 3D image with image analysis software, and extracting the carbon particles three-dimensionally based on the contrast of the image data due to the difference in the X-ray absorption coefficients of the resin and the carbon particles.

[0015] A more specific method for determining the proportion (volume %) of carbon particles will now be described. Regions (I) and (II) of the radio wave absorber are each cut into 8 mm x 20 mm squares to serve as samples. The samples are measured using a microfocus X-ray CT device (SkyScan 2211, manufactured by Bruker), and then a 3D image is reconstructed using image analysis software (TRI / 3D-VIE, manufactured by Ratoc Systems Engineering Co., Ltd.). Based on the contrast of the image data due to the difference in the X-ray absorption coefficients of the resin and the carbon particles, the proportion of carbon particles relative to the total carbon particles can be calculated by comparing the distribution of carbon particles in regions (I) and (II).

[0016] Furthermore, when the radio wave absorber is a laminate of resin layers, the ratio (volume %) of carbon particles to all carbon particles present in region (I) can be calculated from the amount of carbon particles added and the volume of the resin layer.

[0017] The ratio of the carbon particles present in the region (I) to the total volume of the carbon particles is 60 to 80% by volume, and preferably 61.5 to 78% by volume. The constituent members of the radio wave absorber will now be described.

[0018] <Resin> Examples of resins used in this embodiment include thermoplastic resins and thermosetting resins. Thermosetting resins are preferred because their dielectric constants 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.

[0019] The resin may contain known resin additives to the extent that the problem can be solved. Examples of known resin additives include stabilizers against heat, light, ultraviolet rays, etc., lubricants, nucleating agents, plasticizers, antistatic agents, mold release agents, flame retardants, softeners, dispersants, antioxidants, colorants, etc. The total content of the known resin additives is preferably 30% by mass or less, more preferably 10% by mass or less, based on the total of the resin and the resin additives.

[0020] <Carbon Particles> Examples of carbon particles include furnace black, channel black, ketjen black, and oil furnace carbon. Ketjen black is preferred because of its high conductivity. Because of its high conductivity, the amount of carbon particles added can be reduced.

[0021] In one embodiment, the content of carbon particles in the entire wave absorber is 3% by volume or more and 20% by volume or less. By unevenly distributing the carbon particles, the wave absorber of this embodiment can efficiently exhibit wave absorbing performance with a smaller amount of carbon particles than conventional wave absorbers. The content of carbon fibers may be 5% by volume or more and 15% by volume or less, or may be 5% by volume or more and 10% by volume or less.

[0022] <Others> In one embodiment, in order to control the dielectric constant of the resin, a filler such as hollow particles may be blended into the base material such as the resin. Examples of hollow particles include ceramic balloons, glass balloons, and shirasu balloons.

[0023] 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 pressure resistance is measured by the glycerol method in accordance with ASTM D3102-78.

[0024] <Production of Wave Absorber> The wave absorber of this embodiment can be produced, for example, by mixing the above-mentioned resin, carbon particles, and optional components as required, and molding the mixture into a sheet. For mixing, 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 as needed. When mixing, the various components can be mixed all at once or in portions to obtain predetermined amounts. There are no particular restrictions on the order in which the components are mixed during mixing.

[0025] An example of a method for producing the radio wave absorber of the present embodiment includes a method in which the above-mentioned components are mixed with a thermosetting resin (liquid before hardening) 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, or the like, and then heated.

[0026] In the above method, the uneven distribution state of the carbon particles can be controlled by the standing time of the mixed liquid before heating. The standing time and 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. The solid content concentration of the mixed liquid is determined so as to obtain an appropriate film thickness for the frequency at which the maximum absorption capacity of the radio wave absorber is obtained, and is preferably, for example, 5% by mass or more and 80% by mass or less. The solid content concentration is the content rate of components (solid content) excluding volatile components such as the solvent from the mixed liquid.

[0027] Furthermore, in order to achieve an uneven distribution of carbon particles, the radio wave absorber may be a laminate of two or more resin layers. For example, by producing each of the regions (I) and (II) in FIG. 1 as a single resin sheet having a different carbon particle concentration, and laminating the two layers, the radio wave absorber as a whole has a structure in which carbon particles are unevenly distributed. When the radio wave absorber is formed as a laminate, the resin used in each layer may be the same or different. In one embodiment, the resin used in each layer is the same. Furthermore, one of the layers constituting the laminate does not contain carbon particles.

[0028] In this embodiment, the thickness of the wave absorber (T in FIG. 1) is preferably 0.6 mm or more and 1.5 mm. In this embodiment, by unevenly distributing the carbon particles, it is possible to efficiently achieve wave absorbing performance with a smaller amount of carbon particles than in the past. Therefore, the thickness can be made thinner than in the past. The thickness of the wave absorber may be 1.4 mm or less, or may be 1.3 mm or less.

[0029] In one embodiment, the radio wave absorber is a laminate including a resin layer containing carbon particles and a resin layer not containing carbon particles. In this embodiment, the sum (t1 + t2) of the thickness t1 of the resin layer not containing carbon particles and the thickness t2 of the resin layer containing carbon particles is T in FIG. 1 . The thicknesses t1 and t2 are not limited. For example, the thickness t1 is 0.01 mm or more, 0.05 mm or more, or 0.1 mm or more. Furthermore, the thickness t2 is 0.1 mm or more, 0.2 mm or more, or 0.3 mm or more. In one embodiment, the thickness t2 is thicker than the thickness t1.

[0030] In one embodiment, the resin layer containing no carbon particles is made of only a resin containing no filler. This allows the real and imaginary parts of the complex dielectric constant of the resin layer containing no carbon particles to be reduced. Furthermore, the difference in complex dielectric constant between the resin layer containing carbon particles and the resin layer containing no carbon particles can be controlled simply by adjusting the filler in the resin layer containing carbon particles, thereby reducing manufacturing costs.

[0031] 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 of thickness t1 to thickness t2 (t1 / t2). In this embodiment, high radio wave absorption characteristics are obtained due to dielectric loss and conductive loss, so there is no need to use a magnetic material as a filler.

[0032] [Radio wave absorbing structure] A radio wave absorbing structure according to one embodiment of the present invention includes the radio wave absorber of the present invention described above and a radio wave reflector laminated on the side of the radio wave absorber where the carbon particles are unevenly distributed. Fig. 2 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 2 has a structure formed by laminating the first end face 11 of the radio wave absorber 1 on one side of the radio wave reflector 21 so that the first end face 11 is in contact with the side of the radio wave reflector 21. That is, the region (I) where the carbon particles are unevenly distributed is formed on the radio wave reflector 21. This can improve radio wave absorption.

[0033] The radio wave reflector is not particularly limited as long as it has the property of reflecting radio waves without transmitting them, and may be a metal plate, metal foil, metal film, or the like. A resin layer such as a general adhesive layer may be formed between the radio wave absorber and the radio wave reflector. The radio wave absorbing structure can be manufactured, for example, by directly depositing the radio wave absorber 1 on the radio wave reflector 21, or by attaching the radio wave reflector 21 to the radio wave absorber 1.

[0034] Although a sheet-like radio wave absorbing structure is shown as an example in FIG. 2, the radio wave absorbing structure of this embodiment is not limited to a planar body such as a sheet, and can be appropriately shaped to suit the area where it is used.

[0035] The radio wave absorber and radio wave absorbing structure of the present invention have high radio wave absorbing performance over the entire frequency range of 75 GHz to 110 GHz, specifically, they can absorb 90% or more of radio waves over the entire frequency range of 75 GHz to 110 GHz.

[0036] The present invention will be described in more detail below with reference to examples, but is not limited to these examples. In the following, operations for which no particular temperature or pressure conditions are specified were carried out at a temperature of around room temperature (usually 25°C) and at normal pressure (usually 0.1013 MPa).

[0037] Example 1 (1) Preparation of Resin Layer A Containing Carbon Particles 92.5% by mass of silicone resin (polydimethylsiloxane: manufactured by Dow Toray Co., Ltd., a mixture of the main material and curing agent of Silpot 184 (registered trademark)) and 7.5% by mass of carbon particles (Ketjen Black, manufactured by Lion Specialty Chemicals Co., Ltd., EC600JD) were mixed and stirred at 2000 rpm for 2 minutes using a planetary centrifugal mixer (Awatori Rentaro ARE-310). 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 and cured. The cured product (cast film) was peeled off from the aluminum container to prepare a resin layer A. The thickness was 0.52 mm.

[0038] (2) Preparation of Resin Layer B Containing No Carbon Particles Resin layer B was prepared in the same manner as in (1) above, except that the silicone resin (polydimethylsiloxane: a mixture of the main material and curing agent of Silpot 184 (registered trademark) manufactured by Dow-Toray Industries, Inc.) was used as 100% by mass. The thickness of the resin layer B was 0.12 mm.

[0039] (3) Radio wave absorbing structure The resin layers A and B prepared in (1) and (2) above were laminated on an aluminum plate serving as a radio wave reflector (resin layer B / resin layer A / aluminum plate) to prepare a radio wave absorbing structure.

[0040] Examples 2 and 3, Comparative Examples 1 to 3 Radio wave absorbing structures were produced in the same manner as in Example 1, except that the thicknesses of the resin layer A and the resin layer B were changed as shown in Table 1. The thicknesses of the resin layers A and B were controlled by the amounts of the respective mixed liquids poured into the aluminum containers.

[0041] Table 1 shows the thickness of resin layer A, the thickness of resin layer B, the total thickness (T) of resin layer A and resin layer B, half of the total thickness T (T / 2), the carbon particle content (C particle content: volume %) in the region from the aluminum plate side to half the thickness (T / 2), and the maximum absorption frequency.

[0042]

[0043] <Method for measuring the proportion of carbon particles in radio wave absorber> Since the radio wave absorbers produced in the examples and comparative examples are laminates of resin layers, the carbon particle content (C particle content: volume %) in the region from the aluminum plate side to the half value (T / 2) was calculated from the amount of carbon particles added and the volume of the resin layer.

[0044] <Radio wave absorption characteristics> The radio wave absorption characteristics were measured using the free space method. ・Equipment used: PNA microwave network analyzer N5227 (Keysight Technologies) ・Test conditions Atmosphere: in air Temperature: room temperature Frequency: 75 GHz to 110 GHz Irradiation angle: vertical

[0045] Fig. 3 shows the measurement results of the radio wave absorption characteristics of the radio wave absorbing structures produced in Examples 1 to 3. It can be seen from Fig. 3 that the radio wave absorbing structures of the examples have an absorption capacity of 90% or more of radio waves over the entire frequency range of 75 GHz to 110 GHz. From the results of the experiment, it is considered that the uneven distribution rate of carbon particles in the radio wave absorber is reflected in the radio wave absorption characteristics and the width of the absorption bandwidth of the radio wave absorbing structure.

[0046] 4 shows the measurement results of the radio wave absorbing characteristics of the radio wave absorbing structures produced in Comparative Examples 1 to 3. Compared with the radio wave absorbing structures of the Examples, it can be confirmed that the radio wave absorbing structures of the Comparative Examples have lower absorption characteristics.

[0047] [Dependence of radio wave absorption characteristics on installation angle relative to the incident surface] Comparative Example 4 A radio wave absorbing structure was produced in the same manner as in Example 1, except that carbon fiber (Middle Fiber XN-100, manufactured by Nippon Graphite Fiber Co., Ltd., fiber length 100 μm, fiber diameter 10 μm) was used instead of carbon particles.

[0048] The radio wave absorbing characteristics of the radio wave absorbing structures of Example 1 and Comparative Example 4 were measured at different installation angles. The measurement results for the radio wave absorbing structure produced in Example 1 are shown in FIG. 5 . The measurement results for the radio wave absorbing structure produced in Comparative Example 4 are shown in FIG. 6 . In FIGS. 5 and 6 , 0° represents the radio wave absorbing characteristics initially measured after the radio wave absorbing structure was installed, and 90° represents the radio wave absorbing characteristics measured after the radio wave absorbing structure was rotated 90° around the irradiation axis from the initial measurement (0°). That is, between the initial measurement (0°) and the measurement at 90°, the position of the radio wave absorbing structure on the plane remains unchanged, only rotation occurs. It was confirmed from FIGS. 5 and 6 that the radio wave absorbing structure of Example 1 exhibited substantially similar radio wave absorbing characteristics even when rotated, whereas the radio wave absorbing characteristics of the radio wave absorbing structure of Comparative Example 4 changed significantly with rotation. When carbon particles are used, no orientation occurs, so the conduction path does not differ between 0° and 90°, and no particular anisotropy occurs in the dielectric constant, and therefore no change in absorption characteristics is likely to be observed.

[0049] The radio wave absorber of the present invention can be used as a member that absorbs 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. Furthermore, it is suitable as a member that absorbs and shields unwanted radio waves in transportation infrastructure environments such as road guardrails, tunnel inner walls, and smart cities.

[0050] Although several embodiments and / or examples of the present invention have been described in detail above, those skilled in the art will readily be able to make numerous modifications to these exemplary embodiments and / or examples without substantially departing from the novel teachings and advantages of the present invention. Accordingly, these numerous modifications are within the scope of the present invention. The contents of all references cited in this specification and the applications from which this application claims priority under the Paris Convention are incorporated by reference in their entirety. In this specification, "x to y" represents a numerical range of "greater than or equal to x and less than or equal to y." The upper and lower limits of numerical ranges can be arbitrarily combined. Furthermore, two or more of the individual embodiments of the present invention that are not mutually exclusive can be combined, and an embodiment combining two or more embodiments is also an embodiment of the present invention.

Claims

1. A radio wave absorber that is a sheet-like molding that contains resin and carbon particles but does not contain carbon fiber, and in the region from a first end face to the opposing second end face of the sheet-like molding, 60 to 80% by volume of the total carbon particles are present in the region from the first end face to halfway through the entire thickness.

2. The radio wave absorber according to claim 1, having a thickness of 0.6 mm or more and 1.5 mm or less.

3. The radio wave absorber according to claim 1 or 2, wherein the carbon particles are Ketjen black.

4. The radio wave absorber according to any one of claims 1 to 3, wherein the content of said carbon particles is 3% by volume or more and 20% by volume or less of the total volume.

5. The radio wave absorber according to any one of claims 1 to 4, wherein the resin is a thermosetting resin.

6. The radio wave absorber according to claim 5, wherein the thermosetting resin is a silicone resin.

7. The radio wave absorber according to claim 5, wherein said thermosetting resin includes polydimethylsiloxane.

8. The radio wave absorber according to any one of claims 1 to 7, wherein the maximum absorption frequency is in the range of 75 GHz to 110 GHz.

9. The radio wave absorber according to any one of claims 1 to 8, the radio wave absorption characteristics of which do not depend on the installation angle.

10. A radio wave absorbing structure comprising: a radio wave absorber according to any one of claims 1 to 9; and a radio wave reflector laminated on the first end face side of the radio wave absorber.

11. The radio wave absorbing structure according to claim 10, which has an absorption rate of 90% or more of radio waves in the entire frequency range from 75 GHz to 110 GHz.

12. A method for producing a radio wave absorber according to any one of claims 1 to 9, comprising the steps of: preparing two or more layers of resin sheets with different carbon particle concentrations; and laminating said two or more layers of resin sheets to form an uneven distribution of carbon particles.

Images