Electromagnetic wave absorber

Abstract

To provide an electromagnetic wave absorber that has an excellent performance in absorbing electromagnetic waves of high frequencies from 60 GHz to 120 GHz on incidence, and is flexible and can be set on various curved surfaces.SOLUTION: An electromagnetic wave absorber is a structure formed by a sheet 1 containing a conducting material and having a surface resistance of 10 Ω / sq to 104 Ω / sq, which is folded in a pleated shape. A slit 2 is perforated non-parallel to a folded ridge line R.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electromagnetic wave absorber, and more particularly to an electromagnetic wave absorber that is flexible and can be installed on a curved surface. [Background technology]

[0002] In recent years, electromagnetic waves in the quasi-millimeter and millimeter wave ranges with frequencies between 30 and 100 GHz have been used for information and communication. In particular, millimeter waves between 60 and 90 GHz are increasingly being used as radar frequencies for automobile collision prevention systems. This millimeter wave radar is installed inside the bumper of an automobile and is used to measure the distance to surrounding vehicles by emitting millimeter waves to adjust the distance between the vehicles and the speed of the automobile. When these millimeter waves are diffusely reflected around the inside of the bumper, there is a possibility that devices installed in the vicinity may malfunction, so it has become necessary to install electromagnetic wave absorbers inside the body to absorb unnecessary electromagnetic waves. For example, Patent Document 1 discloses a λ / 4 type electromagnetic wave absorber having a structure in which a metal layer is placed on the back surface of a sheet that has magnetic, dielectric and conductive properties, and which cancels out incident millimeter waves by utilizing the phase difference between the incident and reflected millimeter waves.

[0003] Patent Document 2 discloses an electromagnetic wave absorber having a structure in which a conductive sheet is folded in a pleated shape in order to improve the oblique incidence characteristics, which is a problem with the λ / 4 type. The electromagnetic wave absorber of Patent Document 2 is characterized by a mountain-shaped structure obtained by pleating a sheet having a conductive material, and has excellent absorption performance even at oblique incidence because the electromagnetic waves incident on the electromagnetic wave absorber are diffusely reflected inside the absorber. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6522262 [Patent Document 2] Patent Publication 2008-141190 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the electromagnetic wave absorber disclosed in Patent Document 1 has a problem in that it is difficult for the λ / 4 type to obtain a certain electromagnetic wave absorbing effect for vertically incident millimeter waves at a specific frequency.

[0006] Furthermore, in the electromagnetic wave absorber disclosed in Patent Document 2, since the structure is fixed, it can only be bent in one specific direction, such as up and down, and there is a problem that it is difficult to install it on a structure that is curved in multiple directions.

[0007] Therefore, an object of the present invention is to provide an electromagnetic wave absorber that has excellent performance in terms of absorption characteristics when high-frequency electromagnetic waves of 60 GHz to 120 GHz are incident, and further has flexibility so that it can be installed on various curved surfaces. [Means for solving the problem]

[0008] [1] That is, it contains a conductive material and has a surface resistance of 10Ω / □ to 10 4 A structure in which a sheet having a thickness of Ω / □ is folded into a pleated shape, and slits are formed non-parallel to the folded ridges. multiple Drilled The slits are formed as creases. It is possible to bend it The electromagnetic wave absorber is characterized by the above.

[0009] [2] The electromagnetic wave absorber according to [1] above, wherein the slits are formed in a direction perpendicular or oblique to the ridge line.

[0010] [3] The electromagnetic wave absorber according to the above [1] or [2], wherein the sheet has a thickness of 100 μm to 500 μm.

[0011] [4] The electromagnetic wave absorber according to any one of [1] to [3], characterized in that the height of the ridge line formed on one surface of the sheet and the ridge line formed on the other surface opposite the sheet is 0.5 mm to 15 mm, and the folding angle of the sheet is 10° to 45°.

[0012] [5] The electromagnetic wave absorber according to any one of [1] to [4], characterized in that the fiber contained in the sheet is at least one type selected from vinylon fiber, polyester fiber, olefin fiber, and polyphenylene sulfide fiber.

[0013] [6] The electromagnetic wave absorber according to any one of [1] to [5], characterized in that the conductive material is at least one selected from graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and carbon fibers.

[0014] [7] The electromagnetic wave absorber according to any one of [1] to [6], characterized in that an adhesive layer made of an adhesive is provided on the other surface of the sheet. Effect of the Invention

[0015] The electromagnetic wave absorber of the present invention has excellent performance in terms of absorption characteristics when high-frequency electromagnetic waves of 60 GHz to 120 GHz are incident, and further has flexibility so that it can be placed on various curved surfaces. [Brief description of the drawings]

[0016] [Figure 1] 1(a) is a plan view of an electromagnetic wave absorber showing a first embodiment of the present invention, and (b) is a partially enlarged view of FIG. [Diagram 2] 2(a) is a cross-sectional view taken along line AA in an electromagnetic wave absorber showing a first embodiment of the present invention, and (b) is a partially enlarged view of FIG. [Diagram 3] 1 is a reference diagram showing a usage state of an electromagnetic wave absorber according to a first embodiment of the present invention. FIG. [Figure 4] FIG. 11 is a reference diagram showing a usage state of an electromagnetic wave absorber according to a second embodiment of the present invention. [Diagram 5] 1 is a graph showing the electromagnetic wave absorption characteristics of a sheet. [Figure 6] 1 is a graph showing the electromagnetic wave absorption characteristics of the electromagnetic wave absorber showing the first embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the embodiment of the electromagnetic wave absorber according to the present invention will be described in detail with reference to the attached drawings. Note that the embodiment described below is a specific example preferred for carrying out the present invention, and therefore various technical limitations are imposed thereon, but the present invention is not limited to this embodiment unless otherwise specified in the following description to limit the invention. Note that the notation "~" indicating a numerical range includes the upper and lower limits.

[0018] As shown in Figs. 1 to 4, the electromagnetic wave absorber is a structure formed by folding a sheet 1 into pleats, that is, into thin folds by regularly repeating mountain folds and valley folds.

[0019] The sheet 1 contains a conductive material and has a surface resistance of 10 Ω / □ to 10 4 It is a thin member with a resistance of Ω / □. As a result, the sheet 1 has a certain level of absorption characteristics for electromagnetic waves of 60 GHz to 120 GHz.

[0020] The conductive material contained in the sheet 1 can be carbon materials such as carbon powders such as graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene, and carbon fibers. Metal particles or metal foil pieces such as copper, silver, nickel, aluminum, and tin, and metal oxides such as tin oxide, indium oxide, and zinc oxide can also be used. The sheet 1 can also be used by depositing the above-mentioned metal particles, metal foil pieces, and metal oxides on at least one surface of the sheet 1. Among these, at least one selected from carbon powders such as graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene, and carbon fibers is preferable. These carbon materials are lightweight and less susceptible to deterioration due to oxidation, etc., and can be used stably for a long period of time.

[0021] The surface resistance of sheet 1 is 10Ω / □~10 4 Ω / □ is preferable, and 10 2 Ω / □~10 3 It is more preferable that the surface resistance of the sheet 1 is within this range. When the surface resistance of the sheet 1 is within this range, the sheet 1 does not significantly reflect or transmit incident electromagnetic waves of 60 GHz to 120 GHz, and has a certain level of absorption characteristics.

[0022] Various materials other than conductive materials can be used as materials constituting the sheet 1. Materials contained in the sheet 1 are preferably fibers, resin films, etc. Among these, as fibers, vinylon fibers, polyester fibers, olefin fibers, polyphenylene sulfide fibers, etc. are preferable, and these can be used alone or in combination. When the base material constituting the sheet 1 is such fibers, the conductive material can be uniformly dispersed, so that the absorption characteristics are less likely to vary depending on the location of the sheet 1, and electromagnetic waves of 60 GHz to 120 GHz can be stably absorbed.

[0023] In addition, various other materials can be contained as necessary in the sheet 1. For example, various materials such as a dispersant such as a surfactant to disperse the conductive material more uniformly in the sheet 1, a flame retardant to make the sheet 1 less flammable, an antibacterial agent such as a quaternary ammonium salt to prevent the growth of bacteria, and a colorant such as a pigment to color the sheet to a predetermined color can be added in advance to a fiber suspension used in wet papermaking and contained in the sheet 1, or can be impregnated by spraying or the like after the sheet 1 is produced and contained in the sheet 1.

[0024] The sheet 1 is produced by a method such as wet papermaking in which the above-mentioned fibers, conductive material, and other additives as necessary are mixed and uniformly dispersed to prepare a fiber suspension, and then dehydrated and dried in a papermaking machine equipped with a wire such as a cylinder or a long rope to form a sheet of paper.

[0025] The thickness of the produced sheet 1 is preferably 100 μm to 500 μm, and more preferably 200 μm to 400 μm. When the thickness of the sheet 1 is in this range, not only does the sheet 1 have a certain level of absorption characteristics without significantly reflecting or transmitting incident electromagnetic waves of 60 GHz to 120 GHz, but it also has the effect of being lightweight and easy to process in the folding step described below.

[0026] Furthermore, an adhesive layer containing an adhesive is preferably provided on the other surface of the sheet 1. Specifically, the adhesive is preferably an acrylic adhesive such as an acrylic acid ester copolymer, or a urethane adhesive such as a urethane resin. By providing the adhesive layer, the electromagnetic wave absorber formed by folding the sheet 1 into pleats can be fixed to a device such as a millimeter wave radar.

[0027] The sheet 1 thus obtained is then folded into pleats, i.e., into thin folds by regularly repeating mountain folds and valley folds, to produce an electromagnetic wave absorber. By repeatedly folding the sheet 1 to form a pleat, electromagnetic waves of 60 GHz to 120 GHz reflected by the sheet 1 are absorbed by the sheet 1 each time as they are repeatedly reflected, and are attenuated and absorbed stepwise. In this specification, the lines formed by mountain folds and valley folds are both referred to as ridge lines.

[0028] The height h between the ridgeline R formed on one surface of the sheet 1 and the ridgeline R formed on the other surface, which is the opposite surface of the sheet 1, is preferably 0.5 to 15 mm, and more preferably 1 to 10 mm. When the height h between the ridgeline R formed on one surface of the sheet 1 and the ridgeline R formed on the other surface, which is the opposite surface, is within this range, electromagnetic waves of 60 GHz to 120 GHz can be absorbed, and the sheet can be easily installed even in places with limited space, such as the inside of a vehicle body.

[0029] The folding angle α when the sheet 1 is folded is preferably 10° to 45°, and more preferably 20° to 40°. When the folding angle α when the sheet 1 is folded is within this range, electromagnetic waves of 60 GHz to 120 GHz can be efficiently absorbed.

[0030] The pitch width between one ridgeline R and the adjacent ridgeline R on the same side when the sheet 1 is repeatedly folded is preferably 0.5 to 5 mm, and more preferably 1 to 3 mm. When the pitch width between the adjacent ridgelines R on the same side is within this range, the ridgeline formed on one side of the sheet 1, coupled with the height of the ridgeline R formed on the opposite side, can absorb electromagnetic waves with a height of 60 GHz to 120 GHz, and the sheet can be easily installed even in places with limited space, such as the inside of a vehicle body.

[0031] In the electromagnetic wave absorber, the ridge R along which the sheet 1 is folded is provided with slits 2 that are not parallel to the ridge R. That is, the slits 2 are preferably formed to extend perpendicularly to the ridge R, or to extend obliquely at a predetermined angle from the ridge R. More specifically, the slits 2 are preferably formed to be inclined at 30° to 150° with respect to the ridge R. In the first embodiment as shown in FIG. 1 to FIG. 3, the slits 2 are formed to be approximately perpendicular to the multiple ridges R formed on one surface of the sheet 1, and in the second embodiment as shown in FIG. 4, the slits 2 are formed to be at an angle of approximately 45° with respect to the multiple ridges R formed on one surface of the sheet 1. In this way, since the slits 2 that are not parallel to the ridge R are provided, the sheet can be folded not only in the front-rear direction in FIG. 1 due to the pleats, but also in the left-right direction in FIG. 1, so that the sheet can be folded to fit a curved surface in multiple directions such as a convex surface or a concave surface. Therefore, the sheet can be easily installed in a place with limited space, such as the housing of a millimeter wave radar or inside a vehicle body. The slits 2 are formed as cuts made by a blade, or as long, narrow gaps. In the first and second embodiments, the slits 2 are provided continuously, one by one, along a plurality of ridgelines R formed on one surface of the sheet 1, but the slits 2 may be provided discontinuously, such as every other ridgeline R, depending on the curvature of the curved surface to which the sheet 1 is attached. Furthermore, the slits 2 are provided linearly in the first and second embodiments, but they may be curved as long as they can be installed in the device or space to which the sheet 1 is attached.

[0032] The first embodiment of the present invention will be specifically described below. Note that the present invention is not limited to the following examples.

[0033] <Reference examples 1~24> First, flat sheets 1 having various surface resistances were prepared. Specifically, a plurality of sheets 1 having a thickness of 300 μm were prepared by wet papermaking using vinylon fiber as the fiber and multi-walled carbon nanotubes (5 to 20% by weight relative to the vinylon fiber) as the conductive material. The surface resistance of each of these prepared sheets 1 was measured using a surface resistance measuring device (manufactured by Nitto Seiko Analytech Co., Ltd., model number: Loresta GX MCP-T700). In addition, the reflection and transmission amounts of the incident electromagnetic waves with a frequency of 79 GHz were measured for each of the sheets 1 using an electromagnetic wave absorption measuring device (free space method, Japan Fine Ceramics Center, a general incorporated foundation), and the reflectance, transmittance, and absorptance were calculated from the data. Specifically, the reflectance was calculated using the formula: reflectance (%) = (10 ( Reflection amount / 20) ) 2 × 100, and for transmittance, transmittance (%) = (10 ( Transmission amount / 20) ) 2 × 100, and the absorptance was calculated from the formula: absorptance (%) = 100 - reflectance (%) - transmittance (%).

[0034] The results are shown in Table 1, and the relationship between surface resistance and absorption rate is shown in Figure 5. From these results, it can be seen that the flat sheet exhibits an electromagnetic wave absorption rate of 35% or more, and therefore the surface resistance is 10Ω / □ to 10 4 A sheet with an electromagnetic wave absorption rate of less than 35% in the flat sheet stage was judged to be poor, whereas a sheet outside the above range was judged to be poor.

[0035] [Table 1]

[0036] The paper was made by wet papermaking using vinylon fiber as the fiber and multi-walled carbon nanotubes (10% by weight relative to the vinylon fiber) as the conductive material. The basis weight was 100 g / m 2An electromagnetic wave absorber was prepared by folding a sheet 1 having a thickness of 300 μm and a surface resistance of 130 Ω / □ into a pleated shape. Specifically, the sheet 1 was folded to a height h of 2 mm and a folding angle of 10 to 15° to prepare a pleated electromagnetic wave absorber as shown in FIG. 1. Then, a slit 2 was cut with a blade so as to extend perpendicularly to the ridge line R formed on the sheet 1. As described above, the slit 2 allows the sheet to be folded not only in the front-rear direction of FIG. 1 due to the pleats, but also in the left-right direction, so that the sheet can be folded to fit a curved surface curved in multiple directions such as a convex surface or a concave surface.

[0037] Then, in the state where the sample was bent at the slits 2 that crossed the ridgeline R as shown in Fig. 3, electromagnetic waves of a predetermined frequency were irradiated around the slits 2, and the electromagnetic wave absorption characteristics in the millimeter wave band were measured by the free space method based on JIS R1679:2007. Specifically, electromagnetic waves of frequencies between 75 and 110 GHz were irradiated using an electromagnetic wave absorption measuring device (free space method, Japan Fine Ceramics Center Foundation) to measure the return loss. These results are shown in Fig. 6.

[0038] The relationship between the absorption rate and return loss of incident electromagnetic waves is Absorption rate (%) = 100 - Reflectance rate (%) = (1 - (10 ( Return Loss / 20) ) 2 )×100, and when the return loss is -30 dB, -40 dB, and -50 dB, the electromagnetic wave absorption rates are 99.9%, 99.99%, and 99.999%, respectively. Therefore, from the results in Fig. 6, it was found that in this embodiment, electromagnetic waves with frequencies from 75 to 110 GHz can be absorbed by 99.86 (-28.5 dB)% or more.

[0039] Therefore, it was found that an electromagnetic wave absorber, which is a structure formed by folding a sheet 1 containing a conductive material and having a predetermined surface resistance into a pleated shape and has slits 2 formed perpendicular to the folded ridge line R, can sufficiently absorb electromagnetic waves with a frequency of 75 to 110 GHz. Furthermore, this electromagnetic wave absorber can be installed on various curved surfaces such as convex and concave surfaces. [Explanation of symbols]

[0040] 1 sheet 2. Slit R ridge line h height α...Folding angle

Claims

1. Contains conductive material and has a surface resistance of 10Ω / □ to 10 4 A structure formed by folding a sheet having a resistivity of Ω / □ into a pleat shape, in which a plurality of slits are formed non-parallel to the folded ridges, An electromagnetic wave absorber capable of being folded along the plurality of slits.

2. 2. The electromagnetic wave absorber according to claim 1, wherein the slits are formed in a direction perpendicular to or oblique to the ridge line.

3. 3. The electromagnetic wave absorber according to claim 1, wherein the sheet has a thickness of 100 μm to 500 μm.

4. The electromagnetic wave absorber according to any one of claims 1 to 3, characterized in that the height of the ridge line formed on one surface of the sheet from the ridge line formed on the other surface opposite the sheet is 0.5 mm to 15 mm, and the folding angle of the sheet is 10° to 45°.

5. 5. The electromagnetic wave absorber according to claim 1, wherein the fiber contained in the sheet is at least one type selected from the group consisting of vinylon fiber, polyester fiber, olefin fiber, and polyphenylene sulfide fiber.

6. 6. The electromagnetic wave absorber according to claim 1, wherein the conductive material is at least one selected from the group consisting of graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, and carbon fibers.

7. 7. The electromagnetic wave absorber according to claim 1, wherein the other surface of the sheet is provided with an adhesive layer made of an adhesive.

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