Electromagnetic wave absorber and method for electromagnetic wave absorption

Abstract

Provided is an electromagnetic wave absorber that has a three-dimensional network structure of conductive fibers and is for absorbing terahertz waves.

Description

Electromagnetic wave absorber and electromagnetic wave absorbing method

[0001] The present invention relates to an electromagnetic wave absorber and an electromagnetic wave absorbing method for absorbing terahertz waves using the electromagnetic wave absorber.

[0002] Anticipating the arrival of a data-driven "ultra-smart society" in the 2030s, a promotion strategy is underway to realize "Beyond 5G." Taking into account the expected data communication volume and number of communication devices in the 2030s, "Beyond 5G" aims to achieve communication speeds 10 times faster than 5G in access networks and 100 times faster than current speeds in core networks. To achieve this goal, development is underway on communication technologies that efficiently and effectively utilize terahertz waves, which are higher-frequency electromagnetic waves.

[0003] Electromagnetic wave absorbers are used to prevent deterioration of reception levels due to multiple interference of electromagnetic waves and deterioration of signal readability due to noise. Currently, electromagnetic wave absorbers using a magnetic material with excellent electromagnetic wave absorption properties as a filler are used to prevent noise and other issues caused by 5G microwaves (3 to 30 GHz). For example, Patent Document 1 discloses a noise absorbing sheet having at least two laminated layers, including a magnetic layer containing a magnetic material and a layer of noise absorbing fabric having metal attached to the constituent fibers, and in which the common logarithm of the surface resistivity of at least one side of the noise absorbing fabric is in the range of 0 to 6.

[0004] International Publication No. 2015 / 076387

[0005] With the increasing use of high-frequency electromagnetic waves, such as terahertz waves, in communication technologies, there is an urgent need to develop new materials capable of efficiently absorbing high-frequency electromagnetic waves (i.e., materials with excellent electromagnetic wave absorption properties). One of the reasons for this is believed to be that magnetic materials, which have excellent electromagnetic wave absorption properties for conventional microwaves, such as those used in 5G, do not exhibit electromagnetic wave absorption properties for high-frequency electromagnetic waves, such as terahertz waves. Therefore, expectations are growing for electromagnetic wave absorbers using metamaterials, artificial optical materials, as a new electromagnetic wave absorber that can also absorb terahertz waves. However, metamaterials are typically manufactured using semiconductor microfabrication technology. These techniques are costly, difficult to fabricate on large areas, and are limited to flat substrates, resulting in poor flexibility and limitations on shape and processing. Furthermore, resonant metamaterials only cover a narrow frequency band.

[0006] An object of the present invention is to provide an electromagnetic wave absorber that is inexpensive, can be made large, has flexibility so that it can be installed anywhere, and is compatible with a wide range of frequencies, and an electromagnetic wave absorption method for absorbing terahertz waves using the electromagnetic wave absorber.

[0007] The above-mentioned problems of the present invention have been solved by the following means. [1] An electromagnetic wave absorber for absorbing terahertz waves, having a three-dimensional mesh structure of conductive fibers. [2] The electromagnetic wave absorber according to [1], wherein the frequency of the terahertz waves is in the range of 0.1 to 10 THz. [3] The three-dimensional mesh structure of conductive fibers has a porosity of 40 to 99.9% and a basis weight of 10 to 3000 g / m ２[4] The electromagnetic wave absorber according to any one of [1] to [3] above, wherein the three-dimensional mesh structure of the conductive fibers is a nonwoven fabric structure. [5] The electromagnetic wave absorber according to any one of [1] to [4] above, wherein the three-dimensional mesh structure of the conductive fibers is such that at least a portion of the non-conductive fibers of the three-dimensional mesh structure are coated with a conductive material. [6] The electromagnetic wave absorber according to [5] above, wherein the coating is a coating formed by a sputtering method. [7] The electromagnetic wave absorber according to [5] or [6] above, wherein the conductive material contains at least one metal selected from gold, silver, copper, aluminum, iron, nickel, chromium, titanium, tungsten, stainless steel, indium, tin, and zinc. [8] The electromagnetic wave absorber according to [5] or [6] above, wherein the conductive material contains a conductive metal oxide. [9] A method for absorbing electromagnetic waves, comprising absorbing terahertz waves using the electromagnetic wave absorber according to any one of [1] to [8].

[10] An article for absorbing electromagnetic waves, comprising the electromagnetic wave absorber according to any one of [1] to [8].

[0008] The electromagnetic wave absorber of the present invention can be produced inexpensively in a large area, has flexibility so that it can be installed anywhere, and can be used as an absorber for a wide band of terahertz waves. Furthermore, according to the electromagnetic wave absorption method of the present invention, by using the electromagnetic wave absorber of the present invention, terahertz waves can be efficiently absorbed.

[0009] FIG. 1 is an exemplary diagram schematically illustrating the cross section of each electromagnetic wave absorber of the present invention used in the examples. FIG. 2 is a graph showing an example of the results of measuring the terahertz wave absorption rate of each electromagnetic wave absorber having an aluminum film formed on the surface of BEMCOT. FIG. 3 is a graph showing an example of the results of measuring the terahertz wave absorption rate of each electromagnetic wave absorber having an aluminum film formed on the surface of BEMCOT. FIG. 4 is a graph showing an example of the results of measuring the terahertz wave absorption rate of each electromagnetic wave absorber having an aluminum film formed on the surface of BEMCOT. FIG. 5 is a graph showing an example of the results of measuring the terahertz wave reflectance of each electromagnetic wave absorber having an aluminum film formed on the surface of BEMCOT. FIG. 6 is a graph showing an example of the results of measuring the terahertz wave absorption rate of each electromagnetic wave absorber having an aluminum film formed on the surface of BEMCOT. FIG. 7 is a graph showing an example of the results of measuring the terahertz wave absorption rate of each electromagnetic wave absorber having an aluminum film formed on the surface of paper.

[0010] A preferred embodiment of the present invention will be described below, but the present invention is not limited to the following embodiment except as defined by the present invention.

[0011] [Electromagnetic Wave Absorber] The electromagnetic wave absorber of the present invention has a three-dimensional mesh structure of conductive fibers. The electromagnetic wave absorber of the present invention is an electromagnetic wave absorber for absorbing terahertz waves. In the present invention and this specification, "terahertz waves" means electromagnetic waves having a frequency in the range of 0.1 to 10 THz. In the electromagnetic wave absorber of the present invention, the frequency of the terahertz waves is preferably in the range of 0.1 to 5 THz, more preferably in the range of 0.1 to 4 THz, even more preferably in the range of 0.1 to 3 THz, even more preferably in the range of 0.2 to 3 THz, and even more preferably in the range of 0.3 to 3 THz. Furthermore, in the present invention and this specification, "a three-dimensional mesh structure of conductive fibers" means that at least a portion of the fibers constituting the three-dimensional mesh structure are conductive fibers, or that the three-dimensional mesh structure is a mixed composite (homogeneous mixture, heterogeneous mixture, layered mixture) in which a three-dimensional mesh structure is formed by combining non-conductive fibers constituting the three-dimensional mesh structure with conductive fibers constituting the three-dimensional mesh structure.

[0012] The electromagnetic wave absorber of the present invention can have an absorption rate (electromagnetic wave absorption rate) of 70% or more, 80% or more, or even 90% or more for electromagnetic waves in the frequency range of 0.1 to 10 THz (preferably 0.1 to 5 THz, more preferably 0.1 to 4 THz, even more preferably 0.1 to 3 THz, even more preferably 0.2 to 3 THz, and even more preferably 0.3 to 3 THz). For example, in the examples ( FIGS. 2 to 4 and 6 ) described later, it is shown that when the electromagnetic wave absorber of the present invention was irradiated with electromagnetic waves in the frequency range of 0.3 to 3.0 THz, the absorption rate (electromagnetic wave absorption rate) of the electromagnetic waves was 70% or more in all cases. Furthermore, the reflectance (electromagnetic wave reflectance) for electromagnetic waves in this frequency range can be 30% or less, or 20% or less, or even 10% or less. For example, in Example 5 described later, it is shown that when the electromagnetic waves were irradiated to the electromagnetic wave absorber of the present invention, the reflectance (electromagnetic wave reflectance) of the electromagnetic waves was 10% or less in all cases. The "reflectance" of the electromagnetic wave absorber can be determined by irradiating the electromagnetic wave absorber of the present invention with electromagnetic waves having each frequency in 0.01 THz increments for electromagnetic waves in the above frequency range, measuring the reflectance at that frequency, and calculating the arithmetic mean value of each measured value for the above frequency range. The "absorption" of the electromagnetic wave absorber can be determined by irradiating the electromagnetic wave absorber of the present invention with electromagnetic waves having each frequency in 0.01 THz increments for electromagnetic waves in the above frequency range, measuring the transmittance (electromagnetic wave transmittance) and reflectance at that frequency, calculating the absorbance at that frequency using the following (Equation 1), and determining the arithmetic mean value of each calculated value for the above frequency range. Absorption (%) = 100 - {Transmittance (%) + Reflectance (%)} ... (Equation 1)

[0013] The components of the electromagnetic wave absorber of the present invention will be described below.

[0014] <Three-dimensional mesh structure of conductive fibers> In the present invention and this specification, the term "three-dimensional fiber mesh structure" refers to an assembly of fibers entangled three-dimensionally. For example, it refers to an assembly of fibers entangled three-dimensionally, like the "three-dimensional fiber mesh structure" of a commercially available "nonwoven fabric." On the other hand, the term "three-dimensional mesh structure of conductive fibers" refers to, as described above, a structure in which at least some of the fibers constituting the three-dimensional mesh structure are conductive fibers, or a mixed composite (homogeneous or heterogeneous mixture) in which a three-dimensional mesh structure is formed by combining non-conductive fibers constituting the three-dimensional mesh structure with conductive fibers constituting the three-dimensional mesh structure. The aforementioned "fibers at least some of which are conductive" may, for example, be a structure in which part or all of the surface of the fibers constituting the nonwoven fabric is coated with a conductive material. Nonwoven fabrics preferably used in the present invention include nonwoven fabrics made of long fibers and / or short fibers. These fibers preferably have a certain degree of softness and strength. From the viewpoints of mass production and low cost, nonwoven fabrics containing long fibers are more preferably applicable. The fiber diameter (equivalent circle diameter of the fiber cross section) of the fibers constituting the three-dimensional network structure is not particularly limited as long as the intended effect is achieved. For example, the three-dimensional network structure can be formed by combining fibers having a fiber diameter of 0.01 μm to 100 μm as needed.

[0015] The three-dimensional mesh structure of the conductive fibers preferably has a wide range of spatial frequency components. By having the three-dimensional mesh structure of the conductive fibers have a wide range of spatial frequency components, it is possible to absorb electromagnetic waves over a wider range. From the viewpoint of achieving a three-dimensional mesh structure having a wider range of spatial frequency components, it is preferable that the three-dimensional mesh structure of the conductive fibers has, for example, a high-order fractal structure, a structure in which thin fibers branch from thick fibers, and / or a structure in which fibers are randomly arranged.

[0016] As will be described later, the three-dimensional mesh structure of the conductive fibers preferably has a gradient of conductivity in the thickness direction (perpendicular to the installation surface of the electromagnetic wave absorber). In order to provide such a gradient, the three-dimensional mesh structure preferably has a small apparent density and an appropriate void ratio and basis weight (basis weight). Furthermore, it is more preferable that the three-dimensional mesh structure of the conductive fibers is not compacted. The density of the three-dimensional mesh structure of the conductive fibers (apparent density (bulk density), synonymous with the apparent density of the electromagnetic wave absorber) is 0.01 to 1 g / cm. ３ is preferably 0.02 to 0.5 g / cm ３ More preferably, it is 0.03 to 0.3 g / cm ３ More preferably, it is 0.05 to 0.2 g / cm ３ It is more preferable that the apparent density is 1000 g / m2. The apparent density can be calculated by dividing the mass of the electromagnetic wave absorber by the volume of the electromagnetic wave absorber (volume including voids). The porosity of the three-dimensional mesh structure of the conductive fiber (synonymous with the porosity of the electromagnetic wave absorber) is preferably 40 to 99.9%, more preferably 60 to 99.9%, even more preferably 80 to 97%, and even more preferably 90 to 97%. The porosity can be determined by the following (Equation 2): Porosity (%) = {1 - (Apparent density of the three-dimensional mesh structure of the conductive fiber / True density of the three-dimensional mesh structure of the conductive fiber)} × 100 (Equation 2) The basis weight of the three-dimensional mesh structure of the conductive fiber (synonymous with the basis weight of the electromagnetic wave absorber) is 10 to 3000 g / m2. ２ It is preferable that the density is 15 to 1000 g / m ２ More preferably, it is 20 to 200 g / m ２ More preferably, it is 25 to 100 g / m ２ Furthermore, the basis weight of the three-dimensional mesh structure of non-conductive fibers in the "three-dimensional mesh structure of conductive fibers" made of fibers at least partially having conductivity is usually 1 to 1000 g / m ２ , preferably 10 to 100 g / m ２ , more preferably 10 to 30 g / m ２ The range is used.

[0017] The thickness of the three-dimensional mesh structure of the conductive fiber is not particularly limited, and from the viewpoint of exhibiting higher electromagnetic wave absorption properties and further increasing the flexibility of the conductive fiber, it is preferably 0.1 to 10 mm, more preferably 0.5 to 5 mm, and even more preferably 1 to 3 mm.

[0018] The conductive fibers used in the present invention may be made of a known conductive polymer or the like, in which the fibers constituting the three-dimensional mesh structure of the conductive fibers themselves are formed, or may be made of a non-conductive fiber three-dimensional mesh structure as a base material, with a conductive material coated on the fibers of the three-dimensional mesh structure. Providing a conductivity gradient (higher or lower conductivity) in the thickness direction tends to improve the electromagnetic wave absorption rate and reduce the electromagnetic wave reflectance. Therefore, the three-dimensional mesh structure of the conductive fibers is preferably made of a non-conductive fiber three-dimensional mesh structure as a base material, with a conductive material coated on one side of the three-dimensional mesh structure using a sputtering method, as described below. The "coated" state means that at least a portion of the fibers constituting the three-dimensional mesh structure are coated.

[0019] As described above, the three-dimensional mesh structure of conductive fibers preferably has a conductivity gradient (high conductivity or low conductivity) in the thickness direction of the electromagnetic wave absorber. Due to the appropriate conductivity gradient, electromagnetic waves entering the three-dimensional mesh structure of conductive fibers are captured by the conductive material and converted into electric current. Furthermore, if the conductivity decreases toward the inside (the surface opposite the electromagnetic wave irradiated surface), the electrical resistance increases toward the inside, making it easier to convert the electrical energy into thermal energy through the electrical resistance, thereby enabling efficient absorption of electromagnetic waves. Furthermore, by having a conductivity gradient in the thickness direction, impedance is matched from the outside to the inside, effectively suppressing electromagnetic wave reflection. It is also preferable to provide a conductivity gradient in the thickness direction of the electromagnetic wave absorber so that the conductivity increases toward the inside, the advantages of which will be described later. The conductivity gradient can be formed, for example, by coating one side of the three-dimensional mesh structure with a conductive material using the method described below.

[0020] Examples of fibers that form a three-dimensional mesh structure include non-conductive fibers such as chemical fibers (synthetic fibers made from petroleum-based materials such as polyester, nylon, and acrylic, and regenerated fibers such as cellulose fibers made by refining natural wood, etc.) and natural fibers (plant fibers such as cotton and hemp, and animal fibers such as raw silk and wool). Intrinsically conductive fibers include fibers containing elemental carbon, such as fibers made from carbon nanotubes and graphite. When a fiber is at least partially conductive, the fiber itself can be formed into a three-dimensional mesh structure. When a fiber is not conductive (is a non-conductive fiber), it can be coated with a conductive material as described above. Examples of three-dimensional mesh structures include nonwoven fabric structures, woven fabric structures (woven fabric structures), knitted fabric structures, and paper structures (pulp fiber mesh sheet structures), with nonwoven fabric structures being preferred. Among these, nonwoven fabric structures with randomly shaped and sized voids are preferred. Nonwoven fabric structures with randomly branched fibers are also preferred.

[0021] Examples of conductive substances include metals, conductive metal oxides, conductive polymer materials, and nanocarbon materials (e.g., graphene, carbon nanotubes, etc.), with materials with high conductor loss being preferred. Examples of metals or conductive metal oxides include gold, silver, copper, aluminum, iron, nickel, chromium, titanium, tungsten, stainless steel, and the like, as well as oxides of these metals (e.g., conductive titanium oxide, tungsten oxide, etc.), and other conductive metal oxides (indium tin oxide (ITO), indium zinc oxide (IZO), etc.). That is, the conductive substance preferably contains at least one metal selected from gold, silver, copper, aluminum, iron, nickel, chromium, titanium, tungsten, stainless steel, indium, tin, and zinc. This metal may be in the form of a compound (e.g., oxide) as long as it is conductive. On the other hand, examples of conductive polymer materials include known conductive polymers. In the electromagnetic wave absorber of the present invention, one type of conductive substance may be used alone, or two or more types may be used in combination. In particular, from the viewpoint of raw material costs and process costs, the conductive material preferably contains a conductive material selected from aluminum, gold, and copper, and more preferably contains aluminum and / or gold.

[0022] Methods for imparting conductivity to non-conductive fibers or their three-dimensional mesh structures include methods of coating the fibers (or fibrous bodies) with a conductive substance by sputtering, electron beam (EB) deposition, plating, spray coating, thermal deposition, chemical treatment, etc. When coating the three-dimensional mesh structure of non-conductive fibers with a conductive substance by the above methods, for example, the conductive substance can be coated on at least one side of the three-dimensional mesh structure of the non-conductive fibers. It is also preferable to coat both sides of the three-dimensional mesh structure of the non-conductive fibers with a conductive substance.

[0023] When a conductive material is coated on the three-dimensional mesh structure of non-conductive fibers, the amount of conductive material to be applied can be appropriately set depending on the type of conductive material. For example, when the conductive material is aluminum, the amount of application is set to 1 cm of the three-dimensional mesh structure of non-conductive fibers. ３ Preferably, the amount is 30 mg to 10 g, more preferably 45 mg to 3 g, and even more preferably 60 mg to 0.6 g per 1 cm of the three-dimensional mesh structure of non-conductive fibers. ３ Preferably, the weight is 210 mg to 75 g, more preferably 320 mg to 22 g, and even more preferably 430 mg to 4.4 g per unit area. For example, when forming an aluminum or gold film on the surface of a three-dimensional mesh structure of non-conductive fibers by sputtering, the amount of aluminum or gold deposited can be kept within the above-mentioned preferred range by coating the aluminum or gold under the conditions described in the examples.

[0024] The electromagnetic wave absorber of the present invention may be, for example, an electromagnetic wave absorber consisting of a single layer of a three-dimensional mesh structure of conductive fibers, or may be an electromagnetic wave absorber consisting of a laminate (two or more layers, more preferably three or more layers) of independent three-dimensional mesh structures of conductive fibers. Examples of the electromagnetic wave absorber consisting of a single layer of the three-dimensional mesh structure include FIG. 1(a) "one-sheet front," FIG. 1(b) "one-sheet back," FIG. 1(e) "two-sheet front," and FIG. 1(g) "two-sheet back." Examples of the laminate (two layers) include FIG. 1(c) "one-sheet both," FIG. 1(d) "one-sheet front, two-layer," and FIG. 1(f) "one-sheet back, two-layer." By laminating the three-dimensional mesh structure of conductive fibers, the absorption rate of terahertz waves can be further improved (see FIGS. 2 to 4). That is, in the electromagnetic wave absorbing method for absorbing terahertz waves using the electromagnetic wave absorber of the present invention, a method of laminating independent three-dimensional mesh structures of conductive fibers is preferably used. Furthermore, when the electromagnetic wave absorber of the present invention is formed by laminating three-dimensional mesh structures of conductive fibers that are independent of each other as described above, the laminated three-dimensional mesh structures may be made of different materials, but it is preferable that they are made of the same material (the same material and substantially the same structure).

[0025] Since the electromagnetic wave absorber of the present invention is used for absorbing terahertz waves, it can be in a form that does not contain a magnetic material (for example, a material with a relative magnetic permeability of more than 50, preferably a material with a relative magnetic permeability of more than 100, and more preferably a material with a relative magnetic permeability of more than 200). This is because, as the frequency increases, the relative magnetic permeability decreases and the impedance decreases, so that even if a magnetic material is contained, terahertz waves cannot be absorbed.

[0026] Because the electromagnetic wave absorber of the present invention is an aggregate of conductive fibers, it is lightweight, highly flexible, and can be processed into various shapes. For example, if the three-dimensional mesh structure of the conductive fibers is foldable like paper, it can be easily formed into a three-dimensional shape by folding paper. For example, forming it into a pyramidal or box shape increases the surface area of ​​the electromagnetic wave absorber per unit area, thereby improving its electromagnetic wave absorption properties. Forming it into a pyramidal shape with a subwavelength period increases the average conductivity gradient in the depth direction, thereby improving its electromagnetic wave absorption properties. Furthermore, folding it into an accordion shape improves its elasticity, allowing the electromagnetic wave absorber to be installed in an elastic part. Furthermore, controlling the pitch of the accordion-like irregularities allows for wavelength-selective strong absorption of electromagnetic waves with wavelengths equal to the pitch, thereby enabling tuning of wavelength selectivity and selected wavelength. Furthermore, if the three-dimensional mesh structure of the conductive fibers is flexible like a fabric (woven fabric, nonwoven fabric, etc.), it can be formed into a shape that wraps around an object by cutting, sewing, etc. Therefore, the electromagnetic wave absorber of the present invention can be used in a variety of applications. More specific examples of the shape of the article include the electromagnetic wave absorber of the present invention in the form of a bag, such as clothing, an electromagnetic wave absorbing curtain, wallpaper, a fibrous heat insulating material, a tapestry, an electromagnetic wave absorber for an anechoic chamber, or a drawstring bag. Furthermore, by having the electromagnetic wave absorber of the present invention on (or overlaying) an article that requires electromagnetic wave absorption, such as clothing, an electromagnetic wave absorbing curtain, wallpaper, a fibrous heat insulating material, a tapestry, or a pyramidal electromagnetic wave absorber for an anechoic chamber, it is also possible to efficiently absorb electromagnetic waves over a wider band.

[0027] [Electromagnetic Wave Absorption Method] Another embodiment of the present invention is an electromagnetic wave absorption method in which terahertz waves are absorbed using the electromagnetic wave absorber of the present invention. The preferred frequency range of terahertz waves to be absorbed is as described for the electromagnetic wave absorber of the present invention. From the viewpoint of more efficiently absorbing terahertz waves using the electromagnetic wave absorber of the present invention, when the electromagnetic wave absorber of the present invention has a three-dimensional mesh structure and one surface thereof is coated with a conductive material, the surface coated with the conductive material can be used as the surface facing opposite to the surface irradiated with terahertz waves. The academic reason why terahertz waves can be efficiently absorbed in this manner is not clear, but it is presumed to be due to the following mechanism. A sudden change in impedance (refractive index) increases Fresnel reflection. The gradual change in impedance (refractive index) in the depth direction at the interface between the air in the incident layer and the electromagnetic wave absorber forms a non-reflective surface, and the electromagnetic waves incident on the electromagnetic wave absorber are absorbed and ultimately dissipated as thermal energy. Therefore, it is believed that by irradiating electromagnetic waves from the side of the conductive material where the density is low, opposite to the coated surface of the conductive material, it is possible to suppress reflection and increase the absorption rate.

[0028] The present invention will be described in more detail based on examples. The present invention is not to be construed as being limited to the following examples except as defined in the present invention.

[0029] Example 1 <Preparation of electromagnetic wave absorber having a three-dimensional mesh structure of conductive fibers> A nonwoven fabric, BEMCOT (registered trademark, model number: M-1, thickness: 0.26 mm, manufactured by Asahi Kasei Corporation), was cut into a length of 30 mm and a width of 30 mm, and two sheets were stacked as necessary to form a three-dimensional mesh structure of non-conductive fibers. The apparent density of the BEMCOT used in this example was 0.083 g / cm. ３ , porosity 94%, basis weight 27.5g / m ２An Al film was formed on one or both sides of the surface of the three-dimensional mesh structure of non-conductive fibers using a sputtering device (sputtering deposition). The sputtering conditions were a voltage of 400 V, argon gas (flow rate 60 to 70 sccm), and deposition time of 30 minutes. When the measurement surface (the surface on which electromagnetic waves are incident) is defined as the front surface, an electromagnetic wave absorber in which aluminum (conductive material) is sputtered from the front surface side is referred to as "single-sheet front" ( FIG. 1( a)), an electromagnetic wave absorber in which a conductive material is sputtered from the back surface side is referred to as "single-sheet back" ( FIG. 1( b)), an electromagnetic wave absorber in which a conductive material is sputtered from both the front surface and back surface side is referred to as "single-sheet double" ( FIG. 1( c)), two layers of electromagnetic wave absorbers in which a conductive material is sputtered from the front surface side are superimposed as "single-sheet front double-layer" ( FIG. 1( d)), an electromagnetic wave absorber in which BEMCOT is doubled and a conductive material is sputtered from the front surface side is referred to as "two-sheet front" ( FIG. 1( e)), two layers of electromagnetic wave absorbers in which a conductive material is sputtered from the back surface side are superimposed as "single-sheet back double-layer" ( FIG. 1( f)), and an electromagnetic wave absorber in which BEMCOT is doubled and a conductive material is sputtered from the back surface side is referred to as "two-sheet back" ( FIG. 1( g)). Furthermore, in each electromagnetic wave absorber 1 shown in Figure 1, the fibrous body (a three-dimensional mesh structure of non-conductive fibers) 2 and the conductive material 3 are shown to form independent layers, but in this embodiment, the conductive material 3 has a conductivity gradient formed such that the amount (proportion) of the conductive material 3 decreases in the depth direction of the fibrous body 2 (downward in Figure 1(a)).

[0030] <Method for Measuring Electromagnetic Wave Absorption Rate and Electromagnetic Wave Reflectance> The electromagnetic wave absorbers produced as described above were measured for their electromagnetic wave absorbance and electromagnetic wave reflectance when irradiated with terahertz waves in the range of 0.3 to 3.0 THz by the following method. The electromagnetic wave transmittance of each of the obtained electromagnetic wave absorbers for terahertz waves was measured using a terahertz spectrometer (TeraProspector, manufactured by Nippou Precision Co., Ltd.). A continuous holder (aperture diameter: 10 mm) was used as the sample holder. The electromagnetic wave reflectance of the electromagnetic wave absorbers for terahertz waves was measured by attaching a unit for an automatic reflection sample stage for terahertz spectrometers (TP-RFL-1) to the terahertz spectrometer. From the transmittance and reflectance measured as described above, the electromagnetic wave absorbance of terahertz waves was calculated using the following (Equation 1). Absorption rate (%) = 100 - {Transmittance (%) + Reflectance (%)} (Equation 1) Note that, using BEMCOT that was not formed by sputtering as a control, the electromagnetic wave absorption rate was measured in the same manner as above. The results of the electromagnetic wave absorption rate measured by the above method are shown in Figures 2 to 4, and the results of the electromagnetic wave reflectance are shown in Figure 5.

[0031] As is clear from Figures 2 to 4, each of the electromagnetic wave absorbers in which aluminum was sputtered onto BEMCOT exhibited an extremely high electromagnetic wave absorption rate of around 90% or more for electromagnetic waves in the frequency range of 0.3 to 3.0 THz. Furthermore, it was revealed that the electromagnetic wave absorption properties were higher when electromagnetic waves were irradiated from the side opposite to the side on which aluminum was sputtered (one sheet back) than when electromagnetic waves were irradiated from the side on which aluminum was sputtered (one sheet front), and further that the electromagnetic wave absorption properties were higher when aluminum was sputtered onto both sides (front and back) of a single sheet of BEMCOT and then irradiated with electromagnetic waves (one sheet both) (Figure 2).

[0032] Furthermore, a comparison of "1 sheet front" and "1 sheet front, 2 layers" in Figure 3 and "1 sheet back" and "1 sheet back, 2 layers" in Figure 4 revealed that an electromagnetic wave absorber consisting of two layers of a three-dimensional mesh structure of conductive fibers has superior electromagnetic wave absorption properties compared to one consisting of a single layer of a three-dimensional mesh structure of conductive fibers. Also, a comparison of "1 sheet front" and "2 sheets front" in Figure 3 and "1 sheet back" and "2 sheets back" in Figure 4 showed that thicker BEMCOT tends to have superior electromagnetic wave absorption properties. Note that the BEMCOT itself used as a control had an absorption rate of less than 60% for electromagnetic waves in any of the above frequency ranges.

[0033] As is clear from FIG. 5, the reflectance of each of the electromagnetic wave absorbers (one sheet front, one sheet both (front and back)) was kept low in the range of 0.3 to 3.0 THz.

[0034] (Example 2) Except for using gold instead of aluminum for sputtering, each electromagnetic wave absorber was obtained by sputtering the front or back surface of BEMCOT in the same manner as in Example 1. The electromagnetic wave absorption rate of each obtained electromagnetic wave absorber was measured when irradiated with terahertz waves in the range of 0.3 to 3.0 THz in the same manner as in Example 1. The results are shown in Figure 6.

[0035] As is clear from Figure 6, each of the electromagnetic wave absorbers (one sheet front, one sheet back) in which gold was sputtered onto BEMCOT exhibited a high absorption rate for electromagnetic waves in the frequency range of 0.3 to 3.0 THz. It was also revealed that the electromagnetic wave absorption characteristics were higher when electromagnetic waves were irradiated from the side opposite to the side on which gold was not sputtered (one sheet back) than when electromagnetic waves were irradiated from the side on which gold was sputtered (one sheet front). The BEMCOT itself used as a control had an absorption rate of less than 60% for electromagnetic waves in any of the above frequency ranges.

[0036] (Example 3) Each electromagnetic wave absorber having an aluminum film formed by sputtering on one or both sides was obtained in the same manner as in Example 1, except that paper (Multi Paper Super White+, thickness: 0.09 mm, manufactured by Askul Corporation) cut into a length of 25 mm and a width of 25 mm was used instead of BEMCOT. The basis weight of the paper used was 68 g / m ２ The electromagnetic wave absorbency of each of the obtained electromagnetic wave absorbers was measured when irradiated with terahertz waves in the range of 0.3 to 3.0 THz in the same manner as in Example 1. The results are shown in FIG.

[0037] As is clear from FIG. 7, the electromagnetic wave absorber using paper as the fibrous body had excellent absorption characteristics for terahertz waves from the surface (rear surface) opposite to the side on which the aluminum film was formed by sputtering.

[0038] (Example 4) Except for using iron instead of aluminum for sputtering, each electromagnetic wave absorber was obtained by sputtering on the front or back surface of BEMCOT in the same manner as in Example 1. For each of the obtained electromagnetic wave absorbers, the electromagnetic wave absorption rate was measured when irradiated with terahertz waves in the range of 0.3 to 3.0 THz in the same manner as in Example 1. The results showed that all of the electromagnetic wave absorbers had excellent terahertz wave absorption characteristics (figure omitted).

[0039] As described above, the electromagnetic wave absorber of the present invention can be produced inexpensively in a large area, has flexibility so that it can be installed anywhere, and exhibits excellent properties as an absorber for a wide band of terahertz waves. In particular, the electromagnetic wave absorber of the present invention, which has a three-dimensional mesh structure of conductive fibers, has a highly reliable electromagnetic wave absorbing performance and flexibility by adding a conductive material to at least a part of the non-conductive nonwoven fabric or the like.

[0040] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0041] This application claims priority based on Japanese Patent Application No. 2023-140295, filed in Japan on August 30, 2023, the contents of which are incorporated herein by reference.

[0042] 1 Electromagnetic wave absorber 2 Fiber body (three-dimensional mesh structure of non-conductive fibers) 3 Conductive material

Claims

1. An electromagnetic wave absorber for absorbing terahertz waves, having a three-dimensional mesh structure of conductive fibers.

2. The electromagnetic wave absorber according to claim 1, wherein the frequency of the terahertz waves is within the range of 0.1 to 10 THz.

3. The three-dimensional mesh structure of the conductive fiber has a void ratio of 40 to 99.9% and a basis weight of 10 to 3000 g / m ２ 3. The electromagnetic wave absorber according to claim 1 or 2, 4. An electromagnetic wave absorber according to any one of claims 1 to 3, wherein the three-dimensional mesh structure of the conductive fibers is a nonwoven fabric structure.

5. An electromagnetic wave absorber according to any one of claims 1 to 4, wherein the three-dimensional mesh structure of the conductive fibers is such that at least a portion of the non-conductive fibers of the three-dimensional mesh structure are coated with a conductive substance.

6. The electromagnetic wave absorber according to claim 5, wherein the coating is a coating formed by a sputtering method.

7. The electromagnetic wave absorber according to claim 5 or 6, wherein the conductive material contains at least one metal selected from the group consisting of gold, silver, copper, aluminum, iron, nickel, chromium, titanium, tungsten, stainless steel, indium, tin, and zinc.

8. The electromagnetic wave absorber according to claim 5 or 6, wherein the conductive material comprises a conductive metal oxide.

9. A method for absorbing electromagnetic waves, comprising absorbing terahertz waves using the electromagnetic wave absorber according to any one of claims 1 to 8.

10. An article for absorbing electromagnetic waves, comprising the electromagnetic wave absorber according to any one of claims 1 to 8.

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