Electromagnetic wave absorber

Abstract

An electromagnetic wave absorber according to the present application includes an electromagnetic wave absorbing layer, the electromagnetic wave absorbing layer containing an electromagnetic wave absorbing material, which is composed of a carbon material, and a binder. The electromagnetic wave absorbing layer is characterized by including a foaming layer and having an uneven part on at least one of the principal surfaces of the electromagnetic wave absorbing layer. The foaming layer is preferably provided on the surface portion of a convex portion and the surface portion of a concave portion of the uneven part.

Description

[Technical field]

[0001] The present application relates to an electromagnetic wave absorber that absorbs electromagnetic waves in the millimeter wave band. [Background technology]

[0002] With the development of wireless communication technology, typified by mobile phones, various devices and sensors are being connected to networks wirelessly. In the medical field, devices are becoming cordless from the perspective of infection prevention, and medical devices are beginning to be connected wirelessly. These communications require high speed and large capacity over relatively short distances, and the frequencies used are high. With the increase in devices using such high frequencies, the risk of malfunctions in electronic devices and communications due to malfunctions caused by noise generated by the devices and interference with the electromagnetic waves used is increasing. Furthermore, in recent years, millimeter wave radars have also begun to be installed in cars to prevent automobile collision accidents. Malfunctions in these medical and automotive devices can affect human lives, so malfunctions must not occur. Therefore, there is an increasing need to apply electromagnetic wave absorbers to circuit elements and transmission lines that emit and receive millimeter wave electromagnetic waves as a measure to prevent malfunctions caused by noise and interference in devices, or so-called EMC (Electromagnetic Compatibility) measures.

[0003] Conventionally used electromagnetic wave absorbing sheets that utilize magnetic loss in magnetic materials are not very effective in suppressing high frequency noise. For this reason, a radio wave absorber that utilizes carbon materials such as carbon black has been proposed (Patent Document 1). However, the radio wave absorber proposed in Patent Document 1 is excellent in radio wave absorption, particularly in the X-band, which is a frequency band of 8 to 12.5 GHz, but does not aim to improve radio wave absorption in the millimeter wave band of 30 to 300 GHz. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-311586 A Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, when suppressing noise by attaching a sheet-shaped electromagnetic wave absorber to an electronic device that you want to prevent the influence of noise with an adhesive or the like, most of the electromagnetic waves that cause noise are absorbed by the electromagnetic wave absorber, and a certain degree of noise suppression effect can be achieved. Also, a part of the electromagnetic waves is reflected by the surface of the electromagnetic wave absorber, and the reflected electromagnetic waves reach other electronic devices, but there is no problem if the electromagnetic wave absorber is attached to the other electronic devices as well.

[0006] However, it may be difficult to attach an electromagnetic wave absorber to all electronic devices. In such cases, it is possible that electromagnetic waves reflected from an electronic device to which an electromagnetic wave absorber is attached may adversely affect other electronic devices.

[0007] The present application has been made to solve the above problems, and provides an electromagnetic wave absorber capable of suppressing electromagnetic waves reflected on the surface. [Means for solving the problem]

[0008] The electromagnetic wave absorber of the present application includes an electromagnetic wave absorbing layer, the electromagnetic wave absorbing layer including an electromagnetic wave absorbing material made of a carbon material and a binder, the electromagnetic wave absorbing layer including a foam layer, and at least one of the main surfaces of the electromagnetic wave absorbing layer having an uneven portion. Effect of the Invention

[0009] According to the present application, it is possible to provide an electromagnetic wave absorber capable of suppressing reflection of electromagnetic waves incident from the outside and absorbing the incident electromagnetic waves. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a perspective view showing an example of an electromagnetic wave absorber according to an embodiment. [Diagram 2] FIG. 2 is a side view of the electromagnetic wave absorber of FIG. [Diagram 3] FIG. 3 is a side view showing another example of the electromagnetic wave absorber according to the embodiment. [Figure 4] FIG. 4 is a schematic diagram for explaining the free space method for measuring the electromagnetic wave reflection attenuation and the electromagnetic wave transmission attenuation of the electromagnetic wave absorber of the embodiment. [Diagram 5] FIG. 5 is a schematic diagram for explaining the free space method for measuring the electromagnetic wave return loss of the electromagnetic wave absorber of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The electromagnetic wave absorber of the present application includes an electromagnetic wave absorbing layer, the electromagnetic wave absorbing layer including an electromagnetic wave absorbing material made of a carbon material and a binder, the electromagnetic wave absorbing layer including a foam layer, and at least one of the main surfaces of the electromagnetic wave absorbing layer having an uneven portion.

[0012] In the electromagnetic wave absorber of the present application, the electromagnetic wave absorbing layer contains an electromagnetic wave absorbing material made of a carbon material, and therefore, compared to conventional electromagnetic wave absorbing materials made of magnetic materials, it is possible to absorb electromagnetic waves of a higher frequency.

[0013] In addition, since the electromagnetic wave absorbing layer includes a foam layer, the electromagnetic wave absorber can be made lighter and the reflection of electromagnetic waves can be suppressed. The reason why the electromagnetic wave absorbing layer includes a foam layer and can suppress the reflection of electromagnetic waves is considered to be as follows.

[0014] That is, it is generally known that the reflectance of an electromagnetic wave absorber increases when the difference between the dielectric constant of the air layer into which the electromagnetic waves are incident and the dielectric constant of the electromagnetic wave absorber is large. Here, by including a foam layer in the electromagnetic wave absorbing layer, the dielectric constant of the electromagnetic wave absorbing layer can be made closer to the dielectric constant of air, and the difference between the dielectric constant of the air layer and the dielectric constant of the electromagnetic wave absorbing layer becomes smaller. Therefore, it is considered that the reflection of the electromagnetic waves incident on the electromagnetic wave absorbing layer can be suppressed.

[0015] In addition, since the electromagnetic wave absorber of the present application has an uneven portion on at least one main surface of the electromagnetic wave absorbing layer, by making an electromagnetic wave incident on the uneven portion side of the electromagnetic wave absorbing layer, even if the electromagnetic wave is reflected by the uneven portion, the reflected electromagnetic wave is repeatedly reflected and absorbed on the surface portion of the uneven portion, and the proportion of the reflected electromagnetic wave that is absorbed again by the electromagnetic wave absorbing layer increases, so that reflection of the electromagnetic wave can be further suppressed. Therefore, when the electromagnetic wave absorber of the present application has an uneven portion on only one surface of the electromagnetic wave absorbing layer, it is preferable to arrange the uneven portion side of the electromagnetic wave absorbing layer on the surface side on which the electromagnetic wave is incident.

[0016] The electromagnetic wave absorbing layer preferably has a foam layer on the surface of the convex portion of the concave portion of the concave portion, and more preferably has a foam layer on the surface of both the convex portion and the concave portion of the concave portion of the concave portion. This makes it possible to make the dielectric constant of the surface of the concave portion of the electromagnetic wave absorbing layer closer to the dielectric constant of air, and the difference between the dielectric constant of the air layer and the dielectric constant of the surface of the electromagnetic wave absorbing layer becomes smaller. Therefore, the reflection of the electromagnetic wave incident on the electromagnetic wave absorber can be further suppressed.

[0017] Furthermore, if the uneven portion has a gradient structure in which the expansion ratio of the foam layer decreases from the surface toward the inside, the effect of suppressing the reflection of electromagnetic waves becomes greater.

[0018] The cross-sectional shape of the uneven portion is not particularly limited as long as it can improve the electromagnetic wave absorption performance, but is preferably rectangular or trapezoidal in particular. This allows the reflected electromagnetic wave to be repeatedly reflected and absorbed by the rectangular or trapezoidal surface portion even if the electromagnetic wave is reflected by the uneven portion, and the proportion of the reflected electromagnetic wave that is absorbed again by the electromagnetic wave absorbing layer to be further increased, thereby further suppressing the reflection of the electromagnetic wave.

[0019] In addition, the cross-sectional shape of the uneven portion is preferably configured as a repeating pattern of rectangular or trapezoidal shapes of approximately the same shape. This makes the height of each protrusion and the width of each recess in the entire uneven portion approximately uniform, and makes the electromagnetic wave absorbing characteristics approximately uniform over the entire surface of the electromagnetic wave absorbing layer. Here, "approximately the same shape" means that the height of each protrusion and the width of each recess of the rectangular or trapezoidal shape constituting the uneven portion are within a range of ±5% of the arithmetic mean value of each value.

[0020] In the electromagnetic wave absorber of the present application, an electromagnetic wave reflecting layer made of a metal can be further disposed on the surface of the electromagnetic wave absorbing layer that does not have the concave-convex portion according to its application, thereby providing an electromagnetic wave absorber that completely prevents transmission of electromagnetic waves.

[0021] Hereinafter, an embodiment of the electromagnetic wave absorber of the present application will be described with reference to the drawings. Fig. 1 is a perspective view showing an example of the electromagnetic wave absorber of this embodiment. In Fig. 1, the electromagnetic wave absorber 10 of this embodiment has an uneven portion consisting of recesses 11 and protrusions 12 on the incident surface side of the electromagnetic wave 13. The electromagnetic wave absorber 10 can be produced by producing an electromagnetic wave absorbing sheet containing an electromagnetic wave absorbing material made of a carbon material and a binder, and then forming the uneven portion on the surface using a molding die.

[0022] FIG. 2 is a side view of the electromagnetic wave absorber of FIG. 1. In FIG. 2, the width of the concave portion 11 of the concave-convex portion is A, and the width of the convex portion 12 is B. In this case, A is preferably 6 to 30 mm, B is preferably 6 to 30 mm, and A+B is preferably 12 to 60 mm. Furthermore, A is preferably 0.5 to 5 times the length of the maximum absorption wavelength of the electromagnetic wave, B is preferably 0.5 to 5 times the length, and A+B is preferably 1 to 10 times the length. If the width A and width B of the concave-convex portion are within the above ranges, the radio wave absorption property in the sub-millimeter wave to millimeter wave bands can be further improved. Here, the maximum absorption wavelength of the electromagnetic wave can be appropriately selected depending on the application of the electromagnetic wave absorber, and can usually be appropriately selected from the wavelengths of the electromagnetic wave in the frequency band of 28 to 80 GHz.

[0023] In addition, in FIG. 2, if the height of the convex portion is C and the thickness of the substrate portion is D, the height C of the convex portion is preferably 2.7 to 43 mm. The height C of the convex portion is preferably 0.25 to 4 times the length of the maximum absorption wavelength of the electromagnetic wave. In this range, the proportion of the electromagnetic wave reflected on the bottom surface or wall surface of the uneven portion returning to the incident direction decreases, and the proportion of the reflected electromagnetic wave repeatedly reflected and then re-entering the electromagnetic wave absorbing layer and being absorbed increases. Here, the wall surface refers to the flat portion connecting the convex surface and the concave surface of the uneven portion.

[0024] As described above, it is preferable that the height C of the convex portion is set in a range of 0.25 to 4 as a multiple of the length of the maximum absorption wavelength of the electromagnetic wave, and further that the thickness D of the substrate portion is set in a range of 0.1 to 4 as a multiple of the length of the maximum absorption wavelength of the electromagnetic wave. Within these ranges, the effect of suppressing the reflection of the electromagnetic wave is large, and the strength and flexibility of the electromagnetic wave absorber can be maintained at a practical level.

[0025] In addition, when the convex and concave portions of the uneven portion are formed by curved surfaces, the length from the highest point of the convex portion to the lowest point of the concave portion is defined as the height C of the convex portion, and the length from the lowest point of the concave portion to the lowest point of the substrate portion is defined as the thickness D of the substrate portion.

[0026] Fig. 3 is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of this embodiment. In Fig. 3, the electromagnetic wave absorber 20 of this embodiment includes an electromagnetic wave absorbing layer 20a and an electromagnetic wave reflecting layer 20b. The electromagnetic wave absorbing layer 20a includes an uneven portion consisting of a recess 21 and a protrusion 22 on the incident surface side of the electromagnetic wave 23. The electromagnetic wave absorbing layer 20a can be formed by preparing an electromagnetic wave absorbing sheet containing an electromagnetic wave absorbing material made of a carbon material and a binder, processing it into two sheets with different thicknesses, cutting each sheet into long rectangular electromagnetic wave absorbing layers with widths A and B, and alternately arranging the long rectangular materials (cut electromagnetic wave absorbing layers) with different heights on a metallic electromagnetic wave reflecting layer 20b, thereby forming the electromagnetic wave absorbing layer 20a having the uneven portion shown in Fig. 3.

[0027] In FIG. 3, if the width of recess 21 of the uneven portion is A and the width of protrusion 22 is B, the same length relationship between width A and width B as that described in FIG.

[0028] The electromagnetic wave reflecting layer 20b can be formed from a metal plate, a metal foil, a metal vapor deposition film, or the like.

[0029] Next, each component constituting the electromagnetic wave absorbing layer of the electromagnetic wave absorber of this embodiment will be described.

[0030] <Electromagnetic wave absorbing materials> The electromagnetic wave absorbing material constituting the electromagnetic wave absorbing layer is a carbon material capable of absorbing electromagnetic waves in a frequency band equal to or higher than the millimeter wave band, and specific examples thereof include graphite; carbon black (acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc.); graphene; carbon nanotubes (single-wall carbon nanotubes, multi-wall carbon nanotubes, branched carbon nanotubes, branched multi-wall carbon nanotubes); carbon fibers, etc.

[0031] The carbon material preferably contains at least one of carbon nanotubes and carbon black. These can exert the electromagnetic wave absorbing function of the electromagnetic wave absorbing layer even when used in small amounts. Among carbon nanotubes, branched ones are preferred, and in particular, branched multi-wall carbon nanotubes (carbon nanostructures) are more preferred because they can exert the electromagnetic wave absorbing function of the electromagnetic wave absorbing layer even when used in small amounts.

[0032] The content of the carbon material is preferably 0.05 to 0.10 parts by mass relative to 100 parts by mass of the binder described below. If the content of the carbon material is too low, the amount of electromagnetic waves absorbed by the electromagnetic wave absorbing layer decreases, and if the content is too high, the reflection of electromagnetic waves in the electromagnetic wave absorbing layer increases.

[0033] <Binder> The binder is used as a matrix material for dispersing and fixing the electromagnetic wave absorbing material described above to form a foam layer of the electromagnetic wave absorbing layer. As the binder constituting the electromagnetic wave absorbing layer, a thermoplastic resin binder, a thermosetting resin binder, a rubber-based binder, etc. can be used, but the use of a rubber-based binder can impart flexibility to the entire electromagnetic wave absorber, making it easier to make the electromagnetic wave absorber conform to the surface shape of the electronic device to which it is attached, so it is preferable to use a rubber-based binder as the binder.

[0034] As the rubber-based binder, for example, at least one selected from the group consisting of natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), urethane rubber (PUR), silicone rubber (Q), fluororubber (FKM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), and polysulfide rubber (T) can be used.

[0035] In order to improve the strength of the electromagnetic wave absorber, among the above rubber-based binders, silicone rubber (Q) and fluororubber (FKM) are particularly preferred. As the silicone rubber, any of addition reaction type, condensation reaction type, and peroxide curing type can be used. However, peroxide curing type silicone rubber is preferred because it can control the crosslink density and obtain any hardness.

[0036] Next, an example of a method for manufacturing the electromagnetic wave absorber of this embodiment will be described. First, the electromagnetic wave absorbing material, binder, and electromagnetic wave absorbing layer forming components such as a foaming agent, thickener, vulcanizing agent, and release agent are kneaded in a kneader, and then the kneaded mixture is separated by a calendar molding method to prepare an electromagnetic wave absorbing layer precursor. Next, the prepared electromagnetic wave absorbing layer precursor is cut into a sheet through a crosslinking process and a vulcanization process, and an uneven portion is formed on one side surface through a molding roll to prepare an electromagnetic wave absorbing layer, and for example, the electromagnetic wave absorber shown in Figures 1 and 2 can be obtained.

[0037] Furthermore, two types of electromagnetic wave absorbing layer precursors with different thicknesses can be prepared as described above, and each of the prepared electromagnetic wave absorbing layer precursors can be subjected to a crosslinking process and a vulcanization process to produce electromagnetic wave absorbing layers with different thicknesses. Each electromagnetic wave absorbing layer can be cut into long rectangular electromagnetic wave absorbing layers, and the long rectangular bars (cut electromagnetic wave absorbing layers) with different heights can be arranged alternately on a metallic electromagnetic wave reflecting layer, thereby obtaining, for example, the electromagnetic wave absorber shown in FIG. 3.

[0038] Next, the characteristics of the electromagnetic wave absorber of this embodiment will be described.

[0039] <Electromagnetic wave reflection attenuation and electromagnetic wave transmission attenuation> The electromagnetic wave absorber of this embodiment can have an electromagnetic wave return loss of -10 dB or less and an electromagnetic wave transmission loss of -10 dB or less in a frequency band of 28 to 80 GHz. The attenuation of -10 dB or less means that 10% or less of the incident electromagnetic wave is reflected or transmitted.

[0040] The electromagnetic wave reflection attenuation and electromagnetic wave transmission attenuation of the electromagnetic wave absorber of this embodiment can be measured by the following free space method.

[0041] For example, when the electromagnetic wave absorber 10 shown in Fig. 1 and Fig. 2 is used, as shown in Fig. 4, a vector network analyzer ["MS46522B" (76.5 GHz), "MS46122B-043" (28 GHz)] 30 manufactured by Anritsu Corporation is used to irradiate a predetermined input electromagnetic wave (millimeter wave) 34 from an antenna 31 connected to a first port 30a through a dielectric lens 33 to the uneven portion side of the electromagnetic wave absorber 10, and an electromagnetic wave 35 reflected by the uneven portion of the electromagnetic wave absorbing layer is measured by the antenna 31 arranged on the uneven portion side of the electromagnetic wave absorber 10. An electromagnetic wave 36 transmitted through the electromagnetic wave absorber 10 is measured by an antenna 32 arranged on the back side of the electromagnetic wave absorber 10 and connected to a second port 30b. The intensity of the irradiated electromagnetic wave 34, and the intensity of the reflected electromagnetic wave 35 and transmitted electromagnetic wave 36 are each determined as power values, and the electromagnetic wave reflection attenuation and electromagnetic wave transmission attenuation are calculated in dB from the difference in intensity.

[0042] 3 is used, a vector network analyzer ["MS46522B" (76.5 GHz), "MS46122B-043" (28 GHz)] 30 manufactured by Anritsu Corporation is used, as shown in FIG 5, to irradiate a predetermined input electromagnetic wave (millimeter wave) 34 from an antenna 31 connected to a first port 30a to the uneven portion side of the electromagnetic wave absorber 20 through a dielectric lens 33, and the electromagnetic wave 35 reflected by the uneven portion of the electromagnetic wave absorbing layer and the electromagnetic wave reflecting layer 20b is measured by the antenna 31 arranged on the uneven portion side of the electromagnetic wave absorber 20. The intensity of the irradiated electromagnetic wave 34 and the intensity of the reflected electromagnetic wave 35 are grasped as power values, and the electromagnetic wave return attenuation is calculated in dB units from the difference in intensity. EXAMPLES

[0043] The present application will be described in detail below based on examples, but the present application is not limited to the following examples.

[0044] Example 1 <Preparation of electromagnetic wave absorber> The following electromagnetic wave absorbing layer components were kneaded in a kneader, and the kneaded product was separated by a calendar molding method to prepare two types of electromagnetic wave absorbing layer precursors with different thicknesses. Each of the prepared electromagnetic wave absorbing layer precursors was subjected to a crosslinking process at a crosslinking temperature of 170°C for 1 hour and a vulcanization process to prepare electromagnetic wave absorbing layers with different thicknesses. The prepared electromagnetic wave absorbing layer was formed of a foam layer except for the surface part, but a non-foamed layer (skin layer) was formed on the surface part. Thereafter, the skin layers formed on both sides of the prepared electromagnetic wave absorbing layer were removed, and each of the electromagnetic wave absorbing layers with different thicknesses having a foam layer on the surface part was cut into electromagnetic wave absorbing layers in the form of long rectangular bars, and the long rectangular bars (cut electromagnetic wave absorbing layers) with different heights were alternately bonded and arranged so that the foam layer side was on the upper surface, to prepare an electromagnetic wave absorber (1-1) of Example 1 consisting of only an electromagnetic wave absorbing layer and having a foam layer on the surface parts of the convex parts and the surface parts of the concave parts of the concave parts.

[0045] Next, the electromagnetic wave absorber (1-1) was further placed on a metal plate to produce the electromagnetic wave absorber (1-2) of Example 1, which consists of an electromagnetic wave absorbing layer and a metal plate and has a foam layer also on the surfaces of the convex parts and the concave parts of the uneven portion, as shown in Figure 3.

[0046] The electromagnetic wave absorbing layer of the produced electromagnetic wave absorber had a thickness of 20 mm (height of the convex portion: 10 mm, thickness of the substrate portion: 10 mm), and the metal plate had a thickness of 5 mm. In the electromagnetic wave absorber of Example 1, the width A of the concave-convex portion shown in Fig. 3 was set to 2 mm, and the width B was set to 2 mm.

[0047] [Electromagnetic wave absorbing layer components] (1) Rubber-based binder: Silicone rubber (manufactured by Shin-Etsu Chemical Co., Ltd., product name “KE-951KU”, silica particle content: 10% by mass): 100 parts by mass (2) Coloring powder: iron trioxide (III) (manufactured by Shin-Etsu Chemical Co., Ltd., product name “KE Color BR-S”): 1 part by weight (3) Foaming agent: Azobisisobutyronitrile (manufactured by Takehara Rubber Processing Co., Ltd., product name “TR Master AIBN50Q”): 5 parts by mass (4) Thickener: Benzoyl peroxide (manufactured by Shin-Etsu Chemical Co., Ltd., product name “C-1A”): 0.2 parts by mass (5) Vulcanizing agent: dicumyl peroxide (manufactured by Shin-Etsu Chemical Co., Ltd., product name “C-3”): 2.5 parts by mass (6) Release agent: Modified organopolysiloxane (manufactured by Shin-Etsu Chemical Co., Ltd., product name “TA-5”): 0.05 parts by mass (7) Electromagnetic wave absorbing material: Carbon nanostructure (manufactured by Cabot Corporation, product name “ATHLOSSR1200”): 0.07 parts by mass

[0048] Example 2 An electromagnetic wave absorber (2-1) consisting of only the electromagnetic wave absorbing layer of Example 2 and an electromagnetic wave absorber (2-2) consisting of the electromagnetic wave absorbing layer and a metal plate were produced in the same manner as in Example 1, except that the width A of the concave-convex portion of the electromagnetic wave absorbing layer was set to 4 mm and the width B of the concave-convex portion of the electromagnetic wave absorbing layer was set to 4 mm.

[0049] Example 3 An electromagnetic wave absorber (3-1) consisting of only the electromagnetic wave absorbing layer of Example 3 and an electromagnetic wave absorber (3-2) consisting of the electromagnetic wave absorbing layer and a metal plate were produced in the same manner as in Example 1, except that the width A of the concave-convex portion of the electromagnetic wave absorbing layer was set to 6 mm and the width B of the concave-convex portion of the electromagnetic wave absorbing layer was set to 6 mm.

[0050] Example 4 An electromagnetic wave absorber (4-1) consisting of only the electromagnetic wave absorbing layer of Example 4 and an electromagnetic wave absorber (4-2) consisting of the electromagnetic wave absorbing layer and a metal plate were produced in the same manner as in Example 1, except that the width A of the concave-convex portion of the electromagnetic wave absorbing layer was set to 10 mm and the width B of the concave-convex portion of the electromagnetic wave absorbing layer was set to 10 mm.

[0051] Example 5 An electromagnetic wave absorber (5-1) consisting of only the electromagnetic wave absorbing layer of Example 5 and an electromagnetic wave absorber (5-2) consisting of the electromagnetic wave absorbing layer and a metal plate were produced in the same manner as in Example 1, except that the width A of the concave-convex portion of the electromagnetic wave absorbing layer was set to 30 mm and the width B of the concave-convex portion of the electromagnetic wave absorbing layer was set to 30 mm.

[0052] Example 6 An electromagnetic wave absorber (6-1) of Example 6 was produced in the same manner as in Example 4, except that it consisted only of an electromagnetic wave absorbing layer, had a foam layer on the surface of the convex part of the uneven part, and did not have a foam layer on the surface of the concave part.

[0053] Moreover, an electromagnetic wave absorber (6-2) of Example 6 was produced in the same manner as in Example 4, except that it consisted of an electromagnetic wave absorbing layer and a metal plate, had a foam layer on the surface of the convex part of the uneven part, and did not have a foam layer on the surface of the concave part.

[0054] Example 7 An electromagnetic wave absorber (7-1) of Example 7 was produced in the same manner as in Example 4, except that it consisted only of an electromagnetic wave absorbing layer and had no foam layer on the surface of either the convex or concave parts of the concave-convex part.

[0055] Moreover, an electromagnetic wave absorber (7-2) of Example 7 was produced in the same manner as in Example 4, except that it consisted of an electromagnetic wave absorbing layer and a metal plate and did not have a foam layer on the surface of either the convex or concave parts of the uneven portion.

[0056] Example 8 An electromagnetic wave absorber (8-1) consisting of only the electromagnetic wave absorbing layer of Example 8 and an electromagnetic wave absorber (8-2) consisting of the electromagnetic wave absorbing layer and a metal plate were prepared in the same manner as in Example 4, except that the content of the electromagnetic wave absorbing material (carbon nanostructure) in the electromagnetic wave absorbing layer component was changed to 0.03 parts by mass.

[0057] Example 9 An electromagnetic wave absorber (9-1) consisting of only the electromagnetic wave absorbing layer of Example 9 and an electromagnetic wave absorber (9-2) consisting of the electromagnetic wave absorbing layer and a metal plate were prepared in the same manner as in Example 4, except that the content of the electromagnetic wave absorbing material (carbon nanostructure) in the electromagnetic wave absorbing layer component was changed to 0.05 parts by mass.

[0058] Example 10 An electromagnetic wave absorber (10-1) consisting of only the electromagnetic wave absorbing layer of Example 10 and an electromagnetic wave absorber (10-2) consisting of the electromagnetic wave absorbing layer and a metal plate were prepared in the same manner as in Example 4, except that the content of the electromagnetic wave absorbing material (carbon nanostructure) in the electromagnetic wave absorbing layer component was changed to 0.10 parts by mass.

[0059] Example 11 An electromagnetic wave absorber (11-1) consisting of only the electromagnetic wave absorbing layer of Example 11 and an electromagnetic wave absorber (11-2) consisting of the electromagnetic wave absorbing layer and a metal plate were prepared in the same manner as in Example 4, except that the content of the electromagnetic wave absorbing material (carbon nanostructure) in the electromagnetic wave absorbing layer component was changed to 0.13 parts by mass.

[0060] Example 12 An electromagnetic wave absorber (12-1) consisting of only the electromagnetic wave absorbing layer of Example 12 and an electromagnetic wave absorber (12-2) consisting of the electromagnetic wave absorbing layer and a metal plate were prepared in the same manner as in Example 4, except that the electromagnetic wave absorbing material and its content in the electromagnetic wave absorbing layer were changed to 1.50 parts by mass of multi-walled carbon nanotubes (manufactured by Hanwha Chemical Corporation, product name "CM-130")

[0061] Example 13 An electromagnetic wave absorber (13-1) consisting of only the electromagnetic wave absorbing layer of Example 13 and an electromagnetic wave absorber (13-2) consisting of the electromagnetic wave absorbing layer and a metal plate were produced in the same manner as in Example 4, except that the electromagnetic wave absorbing material and its content in the electromagnetic wave absorbing layer were changed to 6.50 parts by mass of carbon black (manufactured by Lion Specialty Chemicals, product name "Lionite CB")

[0062] Comparative Example 1 An electromagnetic wave absorber (1-1) of Comparative Example 1 was produced in the same manner as in Example 1, except that it consisted only of an electromagnetic wave absorbing layer, had no uneven portion, and had a foam layer on the surface.

[0063] Moreover, an electromagnetic wave absorber (1-2) of Comparative Example 1 was produced in the same manner as in Example 1, except that it was made of an electromagnetic wave absorbing layer and a metal plate, had no unevenness, and had a foam layer on the surface.

[0064] Comparative Example 2 Except for excluding the foaming agent from the electromagnetic wave absorbing layer components, an electromagnetic wave absorber (2-1) consisting of only the electromagnetic wave absorbing layer of Comparative Example 2 and an electromagnetic wave absorber (2-2) consisting of the electromagnetic wave absorbing layer and a metal plate were produced in the same manner as in Example 4. The electromagnetic wave absorbing layers of the electromagnetic wave absorbers (2-1) and (2-2) have uneven portions but do not have a foam layer over the entire surface.

[0065] Next, the following characteristics of the electromagnetic wave absorbers produced in Examples 1 to 13 and Comparative Examples 1 and 2 were measured.

[0066] <Electromagnetic wave reflection attenuation and electromagnetic wave transmission attenuation> The electromagnetic wave reflection attenuation and the electromagnetic wave transmission attenuation were measured using the above-mentioned free space method. Specifically, an electromagnetic wave of a predetermined frequency was irradiated onto the electromagnetic wave absorber, and the electromagnetic wave absorber without a metal plate was measured as shown in Figure 4, and the electromagnetic wave absorber with a metal plate was measured as shown in Figure 5.

[0067] Table 1 shows the configuration of each electromagnetic wave absorber, and Table 2 shows the frequency of the irradiated electromagnetic wave, the multiples of width A and width B relative to the wavelength of the irradiated electromagnetic wave, the electromagnetic wave reflection attenuation, and the electromagnetic wave transmission attenuation.

[0068] [Table 1]

[0069] [Table 2]

[0070] From Table 2, the electromagnetic wave absorbers of Examples 1 to 13 were able to suppress the electromagnetic wave return loss to -10 dB or less. In particular, the electromagnetic wave absorbers equipped with a metal plate were able to suppress the electromagnetic wave return loss to -14 dB or less, except for Example 7. On the other hand, the electromagnetic wave absorbers of Comparative Examples 1 and 2, both with and without a metal plate, had an electromagnetic wave return loss exceeding -10 dB, and the reflection of the electromagnetic wave on the surface of the electromagnetic wave absorber was greater than any of the Examples.

[0071] The present application can be implemented in other forms than those described above. The embodiments disclosed in the present application are merely examples and are not limited thereto. The scope of the present application shall be interpreted in accordance with the appended claims rather than the above description of the specification, and all modifications within the scope of the claims are intended to be included in the scope of the claims. [Industrial Applicability]

[0072] The electromagnetic wave absorber of the present application can provide an electromagnetic wave absorber that can absorb electromagnetic waves in a high frequency band equal to or higher than the millimeter wave band and can suppress the reflection of electromagnetic waves, and is useful for producing electronic components and electronic devices with excellent EMC. [Explanation of symbols]

[0073] 10, 20 Electromagnetic wave absorber 20a Electromagnetic wave absorbing layer 20b Electromagnetic wave reflective layer 11, 21 Recess 12, 22 Convex 13, 23 Electromagnetic waves 30 Vector Network Analyzer 30a 1st port 30b Secondary Port 31, 32 Antenna 33 Dielectric Lens 34 Input electromagnetic wave 35 Reflected electromagnetic waves 36 Transmitted electromagnetic waves

Claims

1. An electromagnetic wave absorber including an electromagnetic wave absorbing layer, The electromagnetic wave absorbing layer includes an electromagnetic wave absorbing material made of a carbon material and a binder, The electromagnetic wave absorbing layer includes a foam layer, At least one of the main surfaces of the electromagnetic wave absorbing layer has an uneven portion, The electromagnetic wave absorber is characterized in that the width of the concave portions of the uneven portion is 0.5 to 5 times the wavelength of an electromagnetic wave having a frequency of 28 GHz, and the width of the convex portions of the uneven portion is 0.5 to 5 times the wavelength of an electromagnetic wave having a frequency of 28 GHz.

2. The electromagnetic wave absorber according to claim 1 , wherein the carbon material comprises at least one of carbon nanotubes and carbon black.

3. The electromagnetic wave absorber according to claim 2 , wherein the carbon nanotube has branches.

4. 2. The electromagnetic wave absorber according to claim 1, wherein the content of the carbon material is 0.05 to 0.10 parts by mass with respect to 100 parts by mass of the binder.

5. 2. The electromagnetic wave absorber according to claim 1, wherein the electromagnetic wave absorbing layer has a foam layer on each of the surface portions of the convex portions and the surface portion of the concave portions of the concave-convex portion.

6. The electromagnetic wave absorber according to claim 1 , wherein the electromagnetic wave absorbing layer has a foam layer on the surface of the protrusions of the concave-convex portion.

7. The electromagnetic wave absorber according to claim 1 , wherein the electromagnetic wave absorbing layer has a foam layer on the surface of the concave portion of the concave-convex portion.

8. 8. The electromagnetic wave absorber according to claim 1, wherein the height of the projections of the concave-convex portion is 0.25 to 4 times the length of the maximum absorption wavelength of the electromagnetic wave.

9. 8. The electromagnetic wave absorber according to claim 1, wherein the height of the projections of the concave-convex portion is 2.7 to 43 mm.

10. 8. The electromagnetic wave absorber according to claim 1, wherein the width of the concave portions of the concave-convex portion is 6 to 30 mm, and the width of the convex portions of the concave-convex portion is 6 to 30 mm.

11. 2. The electromagnetic wave absorber according to claim 1, wherein the cross-sectional shape of the projections and recesses is configured in a repeating pattern of substantially identical rectangles or trapezoids.

12. 2. The electromagnetic wave absorber according to claim 1, wherein the binder is a rubber-based binder.

13. 2. The electromagnetic wave absorber according to claim 1, wherein the cross-sectional shape of the uneven portion is rectangular or trapezoidal.

14. 2. The electromagnetic wave absorber according to claim 1, wherein the electromagnetic wave absorbing layer has the uneven portion only on one of its principal surfaces, and further comprises an electromagnetic wave reflecting layer made of a metal on the principal surface not having the uneven portion.

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