Electromagnetic wave absorber

Abstract

Provided is an electromagnetic wave absorber with which the electromagnetic wave absorbing performance can be improved.　This electromagnetic wave absorber comprises a glass fiber and a carbon fiber, wherein the density of the carbon fiber in the electromagnetic wave absorber is 0.01 mg / cm3 to 2.0 mg / cm3, and the thickness of the electromagnetic wave absorber is at least 3 mm.

Description

Electromagnetic wave absorber

[0001] This invention relates to an electromagnetic wave absorber.

[0002] Electromagnetic wave absorbers are known to absorb electromagnetic waves. These absorbers are used, for example, in offices, laboratories, and hospitals to absorb electromagnetic waves emitted from electronic components such as wireless LANs (Local Area Networks).

[0003] For example, Patent Document 1 describes the production of a mixed paper for electromagnetic wave shielding sheets by making paper from a slurry containing LBKP as cellulose fiber and carbon fiber using an oriented paper machine.

[0004] Japanese Patent Publication No. 2022-180792

[0005] However, the electromagnetic wave suppression sheet described in Patent Document 1 is for use in the near field. That is, it absorbs conducted noise in the transmission line by being attached to the vicinity of the transmission line to counter unwanted electromagnetic waves (conducted noise) propagating along the transmission line. Therefore, it does not absorb electromagnetic waves propagating in space away from the source. Furthermore, as described in Patent Document 1, the electromagnetic wave suppression sheet exhibits electromagnetic noise suppression performance by arranging the sheet so that the orientation of the carbon fibers is perpendicular to the transmission line, meaning that its effect is strictly limited to the transmission line, and the orientation of the sheet is also restricted.

[0006] In contrast, the electromagnetic wave absorber of the present invention absorbs electromagnetic waves propagating through space far from the source of electromagnetic waves, regardless of the orientation of the electromagnetic wave absorber. Because it is used in so-called far-field applications, it is suitable for applications such as improving the walls of anechoic chambers and antenna characteristics, regardless of the distance from the source of electromagnetic waves or the orientation of the absorber.

[0007] In this context, the "near-field" refers to the region closer to the source of the electromagnetic wave in question than λ / 2π, where λ is the wavelength of the electromagnetic wave (π is the ratio of a circle's circumference to its diameter).

[0008] One of the objectives of several aspects of the present invention is to provide an electromagnetic wave absorber that can improve electromagnetic wave absorption performance.

[0009] One embodiment of the electromagnetic wave absorber according to the present invention is an electromagnetic wave absorber comprising glass fibers and carbon fibers, wherein the density of the carbon fibers in the electromagnetic wave absorber is 0.01 mg / cm³. ３ 2.0mg / cm or more ３ The following conditions apply, and the thickness of the electromagnetic wave absorber is 3 mm or more.

[0010] In one embodiment of the electromagnetic wave absorber according to the present invention, the density of the carbon fibers in the electromagnetic wave absorber is 0.03 mg / cm³. ３ That's fine too.

[0011] In one embodiment of the electromagnetic wave absorber according to the present invention, the density of the carbon fibers in the electromagnetic wave absorber is 0.60 mg / cm³. ３ The following is also acceptable.

[0012] In one embodiment of the electromagnetic wave absorber according to the present invention, the density of the carbon fibers in the electromagnetic wave absorber is 0.20 mg / cm³. ３ The following is also acceptable.

[0013] In one embodiment of the electromagnetic wave absorber according to the present invention, when the amount of glass fiber contained in the electromagnetic wave absorber is 100 parts by mass, the amount of carbon fiber may be 0.01 parts by mass or more and 2.0 parts by mass or less.

[0014] In one embodiment of the electromagnetic wave absorber according to the present invention, when the amount of glass fibers contained in the electromagnetic wave absorber is 100 parts by mass, the amount of carbon fibers may be 0.03 parts by mass or more.

[0015] In one embodiment of the electromagnetic wave absorber according to the present invention, when the amount of glass fiber contained in the electromagnetic wave absorber is 100 parts by mass, the amount of carbon fiber may be 0.50 parts by mass or less.

[0016] In one embodiment of the electromagnetic wave absorber according to the present invention, when the amount of glass fibers contained in the electromagnetic wave absorber is 100 parts by mass, the amount of carbon fibers may be 0.10 parts by mass or less.

[0017] In one embodiment of the electromagnetic wave absorber according to the present invention, the average fiber diameter of the glass fibers is 0.05 μm or more and 20 μm or less, the glass fibers include submicron fibers with an average fiber diameter of 0.05 μm or more and 1.0 μm or less, and when the amount of glass fibers contained in the electromagnetic wave absorber is 100 parts by mass, the content of the submicron fibers may be 1.0 part by mass or more.

[0018] In one embodiment of the electromagnetic wave absorber according to the present invention, the content of the submicron fibers may be 10 parts by mass or more and 90 parts by mass or less.

[0019] In one embodiment of the electromagnetic wave absorber according to the present invention, it may be manufactured using a batch-type wet paper machine.

[0020] In one embodiment of the electromagnetic wave absorber according to the present invention, it may be an electromagnetic wave absorber for reducing multipath interference.

[0021] In one embodiment of the electromagnetic wave absorber according to the present invention, when data is transmitted from a transmitter to a receiver at a frequency of 2.4 GHz via a wireless LAN in a sealed state, the data communication speed may be increased by more than twice.

[0022] The electromagnetic wave absorber according to the present invention comprises glass fibers and carbon fibers, and the density of the carbon fibers in the electromagnetic wave absorber is 0.01 mg / cm³. ３ 2.0mg / cm or more ３ As described below, the thickness of the electromagnetic wave absorber is 3 mm or more, thus improving the electromagnetic wave absorption performance.

[0023] Figure 1 is a diagram illustrating the free-space method used to evaluate electromagnetic wave absorption performance. Figure 2 is a table showing the evaluation results of electromagnetic wave absorption performance in Examples 1 to 8 and Comparative Examples 1 to 3. Figure 3 is a schematic plan view of the evaluation apparatus used to evaluate multipath reduction performance. Figure 4 is a schematic cross-sectional view of the evaluation apparatus used to evaluate multipath reduction performance. Figure 5 is a table showing the evaluation results of multipath reduction performance in Example 1 and Comparative Example 4.

[0024] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the embodiments described below do not unduly limit the content of the present invention described in the claims. Also, not all of the configurations described below are essential constituent elements of the present invention.

[0025] 1. Electromagnetic wave absorber 1.1. Components First, the electromagnetic wave absorber according to the present embodiment will be described. The electromagnetic wave absorber according to the present embodiment contains glass fiber and carbon fiber.

[0026] 1.1.1. Glass fiber The average fiber diameter of the glass fiber contained in the electromagnetic wave absorber according to the present embodiment is, for example, 0.05 μm or more and 20 μm or less, preferably 0.1 μm or more and 10 μm or less, and more preferably 0.5 μm or more and 5.0 μm or less. The average fiber diameter d (μm) of the glass fiber is calculated from the following formula (1), where S is the specific surface area of the raw cotton made of glass fiber and ρ is the density of the raw cotton. ｔｏｔａｌ It is calculated from the following formula (1).

[0027] d = 4 / (S ｔｏｔａｌ × ρ) ··· (1)

[0028] Note that the specific surface area S ｔｏｔａｌ (m ２ / g) is the specific surface area measured by the BET (Brunauer Emmett Teller) method by nitrogen adsorption, and is measured, for example, by an automatic specific surface area measuring device ("TriStar II 3020" manufactured by Micromeritics). The density ρ is, for example, 2.49 g / cm ３ .

[0029] The glass fiber contains, for example, submicron fibers. The electromagnetic wave absorber contains a plurality of glass fibers, and some of the plurality of glass fibers are submicron fibers. The submicron fiber is a glass fiber having an average fiber diameter of 0.05 μm or more and 1.0 μm or less. The average fiber diameter of the submicron fiber is, for example, 0.1 μm or more and 0.9 μm or less, preferably 0.3 μm or more and 0.8 μm or less.

[0030] When the glass fiber contained in the electromagnetic wave absorber is 100 parts by mass, the content of the submicron fiber is, for example, 1.0 part by mass or more and 95 parts by mass or less, preferably 10 parts by mass or more and 90 parts by mass or less, more preferably 30 parts by mass or more and 85 parts by mass or less, and even more preferably 50 parts by mass or more and 80 parts by mass or less. If the content of the submicron fiber is 1.0 part by mass or more, the texture of the electromagnetic wave absorber can be improved. If the content of the submicron fiber is 95 parts by mass or less, the strength of the electromagnetic wave absorber can be ensured.

[0031] The average fiber length of the glass fiber is, for example, 0.01 mm or more and 50 mm or less, preferably 0.03 mm or more and 30 mm or less, more preferably 0.05 mm or more and 10 mm or less. If the average fiber length of the glass fiber is 0.01 mm or more, the strength of the electromagnetic wave absorber can be improved. If the average fiber length of the glass fiber is 50 mm or less, the uniformity of the density of the electromagnetic wave absorber can be improved. The average fiber length of the glass fiber is measured by, for example, Diamscope or Diamlength manufactured by Cottonscorp.

[0032] The content of the glass fiber in the electromagnetic wave absorber is 80% by mass or more, preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. If the content of the glass fiber is 80% by mass or more, the flame retardancy of the electromagnetic wave absorber can be improved. The electromagnetic wave absorber may be used outdoors, and it is desired to have flame retardancy. The content of the glass fiber in the electromagnetic wave absorber is, for example, 99.99% by mass or less. The content of the glass fiber is measured by, for example, thermogravimetric analysis (TGA: thermal gravimetric analysis).

[0033] 1.1.2. Carbon fiber The carbon fiber contained in the electromagnetic wave absorber according to this embodiment is a fiber produced by carbonizing an organic fiber such as polyacrylonitrile (PAN) fiber or pitch (by-products such as petroleum, coal, and coal tar) fiber at a high temperature. The carbon fiber is a conductive fiber having conductivity.

[0034] The carbon fibers may be chopped fibers or milled fibers. However, considering the variation in the fiber length of the carbon fibers, chopped fibers are preferred. Chopped fibers are carbon fibers that have been bundled with a sizing agent and cut to a certain length. Milled fibers are produced by crushing chopped fibers.

[0035] The average fiber length of the carbon fibers is, for example, 0.05 mm to 50 mm, preferably 1 mm to 25 mm. If the average fiber length of the carbon fibers is 0.05 mm or more, the electromagnetic wave absorption performance of the electromagnetic wave absorber can be improved. If the average fiber length of the carbon fibers is 50 mm or less, a highly uniform slurry can be prepared. The electromagnetic wave absorber is manufactured by dispersing carbon fibers and glass fibers in water to prepare a slurry, and then forming the slurry into a paper mold. The average fiber length of the carbon fibers can be determined, for example, by reading the fiber lengths of 100 carbon fibers using a digital microscope and taking the number average of the lengths of these 100 fibers. As a digital microscope, for example, a "VHX-1000" manufactured by Keyence Corporation can be used.

[0036] The average fiber diameter of the carbon fibers is, for example, 1 μm to 50 μm, preferably 2 μm to 20 μm, and more preferably 4 μm to 10 μm. The fiber diameter can be determined, for example, by the same method as the fiber length.

[0037] The degree of orientation of the carbon fibers in the electromagnetic wave absorber is, for example, 1.1 or less. The carbon fibers may also be unoriented. Due to the low degree of orientation of the carbon fibers, electromagnetic waves can be absorbed regardless of the orientation of the electromagnetic wave absorber. The degree of orientation of the carbon fibers can be calculated, for example, by photographing the surface of the electromagnetic wave absorber with a digital microscope, performing dynamic binarization on the photographed image to obtain a binarized image, and then performing a Fourier transform on the obtained binarized image.

[0038] The density of carbon fibers in the electromagnetic wave absorber is 0.01 mg / cm³. ３ 2.0mg / cm or more ３ The following is preferred, preferably 0.015 mg / cm³ ３1.0mg / cm or more ３ The following, and more preferably 0.02 mg / cm³ ３ 0.60mg / cm or more ３ The following, and more preferably 0.025 mg / cm³ ３ 0.20mg / cm or more ３ The following, and more preferably 0.030 mg / cm³ ３ 0.15mg / cm or more ３ The following applies:

[0039] The density of carbon fibers in the electromagnetic wave absorber is 0.01 mg / cm³. ３ If the above conditions are met, the electromagnetic wave absorption performance of the electromagnetic wave absorber can be improved. The density of carbon fibers in the electromagnetic wave absorber is 2.0 mg / cm³. ３ The following conditions can suppress the reduction in electromagnetic wave absorption performance caused by reflection of electromagnetic waves from the surface of the electromagnetic wave absorber: If there is too much carbon fiber, electromagnetic waves will be reflected from the surface (incident surface) of the electromagnetic wave absorber, making it difficult for the absorber to absorb them.

[0040] The density of carbon fibers in an electromagnetic wave absorber is determined by calculating the weight per unit volume of the electromagnetic wave absorber from its basis weight and weight in square meters, and then multiplying this weight per unit volume by the carbon fiber content (mass %) in the electromagnetic wave absorber. Thus, the density of carbon fibers in an electromagnetic wave absorber is the amount (weight) of carbon fibers per unit volume of the electromagnetic wave absorber. The weight (density) per unit volume of the electromagnetic wave absorber may also be measured according to "JIS P 8118".

[0041] When the amount of glass fibers contained in the electromagnetic wave absorber is 100 parts by mass, the amount of carbon fibers in the electromagnetic wave absorber is, for example, 0.01 parts by mass or more and 2.0 parts by mass or less, preferably 0.02 parts by mass or more and 1.5 parts by mass or less, more preferably 0.025 parts by mass or more and 0.50 parts by mass or less, and even more preferably 0.03 parts by mass or more and 0.10 parts by mass or less.

[0042] If the carbon fiber content in the electromagnetic wave absorber is 0.01 parts by mass or more, the electromagnetic wave absorption performance of the electromagnetic wave absorber can be improved. If the carbon fiber content in the electromagnetic wave absorber is 2.0 parts by mass or less, the reduction in electromagnetic wave absorption performance due to reflection of electromagnetic waves from the surface of the electromagnetic wave absorber can be suppressed. The carbon fiber content in the electromagnetic wave absorber can be measured, for example, by the sulfuric acid decomposition method based on "JIS K 7075".

[0043] 1.1.3. Other Components The electromagnetic wave absorber according to this embodiment may contain fibers other than glass fibers and carbon fibers. Examples of such fibers include rock fibers, metal fibers, alumina fibers, ceramic fibers, silica fibers, aramid fibers, polyamide fibers, polyester fibers, polyethylene fibers, acrylic fibers, rayon fibers, cellulose fibers, and natural fibers made of cotton, linen, and silk. The electromagnetic wave absorber may also contain carbon nanotubes and carbon black.

[0044] Furthermore, the electromagnetic wave absorber may contain additives such as binder resin, water repellent, water-soluble adhesives such as starches, cationic fixatives, surfactants, defoamers, pH adjusters, fine fibers, and fine particles. These additives may be added internally or externally during the wet papermaking process. For example, the strength of the electromagnetic wave absorber can be increased by adding binder resin or water-soluble adhesives. The additive content in the electromagnetic wave absorber is, for example, 0.01% by mass or more and 20% by mass or less.

[0045] 1.2. Shape The electromagnetic wave absorber according to this embodiment has a web-like shape with a higher porosity than ordinary paper. The electromagnetic wave absorber has a porous structure. The electromagnetic wave absorber may also have a sponge shape. The electromagnetic wave absorber may have, for example, a substantially rectangular parallelepiped shape.

[0046] The thickness of the electromagnetic wave absorber is 3 mm to 50 mm, preferably 4 mm to 40 mm, and more preferably 5 mm to 30 mm. If the thickness of the electromagnetic wave absorber is 3 mm or more, the electromagnetic wave absorption performance of the electromagnetic wave absorber can be improved, making it an electromagnetic wave absorber that can be used in the far field. If the thickness of the electromagnetic wave absorber is 50 mm or less, the electromagnetic wave absorber can be easily manufactured by wet papermaking. The thickness of the electromagnetic wave absorber is measured, for example, using an air-type offline thickness measuring device with a measurement pressure of 0.4 kPa.

[0047] The electromagnetic wave absorber may be constructed by laminating fiber-containing layers containing glass fibers and carbon fibers. In this case, adjacent fiber-containing layers may be bonded to each other via an adhesive layer. The material of the adhesive layer is, for example, an adhesive such as a water-based adhesive, a solvent-based adhesive, a chemical reaction-based adhesive, a hot melt adhesive, or a wood glue. The thickness of the adhesive layer is less than the thickness of the fiber-containing layer. The number of fiber-containing layers is, for example, two to ten.

[0048] In an electromagnetic wave absorber, a metal layer may be provided on the surface opposite to the surface to which the electromagnetic waves are incident. Examples of the metal layer include a metal plate and a film with a metal vapor deposition coating. However, if the object to which the electromagnetic wave absorber is attached is made of a metal material, the metal layer may not be provided.

[0049] 1.3. Application and Shape The electromagnetic wave absorber according to this embodiment may be used to reduce multipath. The electromagnetic wave absorber according to this embodiment is, for example, an electromagnetic wave absorber for reducing multipath.

[0050] Multipath is a phenomenon in wireless communication and broadcasting where radio waves transmitted from a transmitter are reflected and diffracted by terrain, buildings, obstacles, and the ionosphere in the upper atmosphere, resulting in them being observed by the receiver through multiple paths. Because the arrival time of the radio waves varies depending on the distance of the path, the receiver receives the same signal repeatedly, causing waveform distortion and phase shifts. As a result, communication problems such as a decrease in communication speed occur.

[0051] The electromagnetic wave absorber according to this embodiment can reduce the multipath described above. By using the electromagnetic wave absorber according to this embodiment, communication interference caused by multipath can be suppressed, and communication becomes possible even in spaces with many reflective objects. For example, when investigating the inside of a metal tank using an unmanned aerial vehicle such as a drone, the metal tank is sealed and prone to multipath. In such cases, by attaching the electromagnetic wave absorber according to this embodiment to the inner surface of the tank, multipath can be reduced, and communication interference during the operation of the unmanned aerial vehicle can be suppressed.

[0052] The electromagnetic wave absorber according to this embodiment can increase the data communication speed by a range of 2 to 5 times when transmitting data from a transmitter to a receiver at a frequency of 2.4 GHz via a wireless LAN (Local Area Network) in a sealed state.

[0053] 2. Method for Manufacturing an Electromagnetic Wave Absorber Next, the method for manufacturing an electromagnetic wave absorber according to this embodiment will be described.

[0054] First, a slurry is prepared for manufacturing the electromagnetic wave absorber. The slurry has a Canadian Standard Filtration Saturation (CSF) of, for example, 200 ml to 550 ml, preferably 250 ml to 500 ml. This range ensures appropriate lightness and strength. The CSF is adjusted by the glass fiber composition, dispersion strength, and the beating machine used for pulp beating. The CSF is determined by the method described in "JIS P 81821-2".

[0055] Specifically, glass fibers are dispersed in an aqueous dispersion medium, and carbon fibers are mixed into the slurry containing the dispersed glass fibers to produce a slurry containing both glass and carbon fibers. The carbon fibers may be added before dispersing the glass fibers in the aqueous dispersion medium, or at the same time as the glass fibers are dispersed. Examples of aqueous dispersion mediums include water and aqueous sulfuric acid solutions.

[0056] Next, it is preferable to make paper from the prepared slurry using a batch-type wet paper machine, dewater it, and then dry it. A batch-type machine is a method of making paper one sheet at a time, rather than a continuous paper machine which makes paper continuously and then winds it up on a reel.

[0057] Specifically, a sieve with a wire mesh stretched over a frame is immersed in the slurry, and the fibers in the slurry are attached to the wire mesh by suction, while the sieve is removed from the slurry. This allows for papermaking. The thickness of the electromagnetic wave absorber can be adjusted by the time and strength of fiber absorption and the concentration of fibers in the slurry. When papermaking is performed using a batch-type wet papermaking machine, the degree of orientation of the carbon fibers is reduced. Papermaking may also be performed using a pulp molding method. Alternatively, papermaking may be performed according to JIS P8222 Pulp - Method for preparing test handmade paper - Method using a standard handmade papermaking machine. Modifications such as changing the shape from circular to square, or adding a suction device for filtered water, may be made to the apparatus described here. Drying is performed using, for example, a hot air dryer, an infrared dryer, or a drum dryer.

[0058] By following the above process, an electromagnetic wave absorber can be manufactured.

[0059] Furthermore, in continuous papermaking, where the paper is ultimately wound onto a reel, if the paper thickness is 3 mm or more, only a very small amount of paper can be wound. Therefore, when manufacturing electromagnetic wave absorbers with a thickness of 3 mm or more, the continuous papermaking method is not preferable; the batch method is preferable.

[0060] The electromagnetic wave absorber of the present invention can absorb electromagnetic waves propagating through space far from the source of electromagnetic waves, regardless of the orientation of the electromagnetic wave absorber, and can be used as an electromagnetic wave absorber for use in the so-called far field. A wide range of applications are envisioned, such as preventing reflections from radar, preventing malfunctions of microwave sensors, absorbing unwanted electromagnetic waves from wireless power supply equipment, improving stealth performance, and improving antenna performance. More specific examples of applications include installation on the walls of anechoic chambers, installation on the roofs of toll booths or between lanes to prevent unwanted electromagnetic wave reflections in ETC (Electronic Toll Collection) systems on expressways, and use as an electromagnetic wave absorber installed between the antenna and control circuit of automobile sensors to prevent malfunctions in autonomous driving.

[0061] 3. Examples and Comparative Examples The present invention will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited in any way by the following examples and comparative examples.

[0062] 3.1. Sample Preparation <Example 1> 60 parts by mass of glass fibers with an average fiber diameter of 0.65 μm and 40 parts by mass of glass fibers with an average fiber diameter of 2.44 μm were placed in a disintegrating pulper, and the raw materials were dispersed to obtain a raw material slurry at a concentration of 1.0% by mass. "B-06-F" manufactured by Unifrax Corporation was used as the glass fibers with an average fiber diameter of 0.65 μm. "B-26-R" manufactured by Unifrax Corporation was used as the glass fibers with an average fiber diameter of 2.44 μm.

[0063] Next, when the glass fiber content in the raw material slurry was 100 parts by mass, 0.1 parts by mass of carbon fiber with a fiber length of 6 mm was added. The raw material slurry was then diluted with water to 0.5% by mass and lightly stirred to prepare a mixed slurry of glass fiber and carbon fiber. As the carbon fiber, Teijin Limited's "Tenax Carbon Fiber Chopped Fiber, 6 mm Carbon Fiber" was used.

[0064] Next, 20 L of the mixed slurry was used to make paper using a batch-type wet paper machine, followed by a dewatering process, and then dried in a hot air dryer at 150°C for 30 minutes. The wet paper machine used was a square sheet machine with a 30 cm square section as specified in JIS P8222, with a suction device added.

[0065] Through the above process, the sample will have a thickness of 10 mm and a carbon fiber density of 0.104 mg / cm³. ３ A sample was prepared to be used as an electromagnetic wave absorber. The thickness of the sample was measured using an air-type offline thickness measuring device manufactured by Yamabun Electric Co., Ltd. The measurement pressure was set to 0.4 kPa. The density of the carbon fibers was determined by calculating the weight per unit volume from the basis weight and mass of the sample, and then multiplying the calculated weight per unit volume by the percentage of carbon fiber content.

[0066] <Example 2> A sample for Example 2 was prepared in the same manner as in Example 1 described above, except that 0.03 parts by mass of carbon fiber was added to the raw material slurry when the amount of glass fiber was 100 parts by mass. The thickness of the sample was 10 mm. The density of carbon fiber in the sample was 0.031 mg / cm³. ３ That was the case.

[0067] <Example 3> A sample for Example 3 was prepared in the same manner as in Example 1 described above, except that 0.5 parts by mass of carbon fiber was added to the raw material slurry when the amount of glass fiber was 100 parts by mass. The thickness of the sample was 10 mm. The density of carbon fiber in the sample was 0.521 mg / cm³. ３ That was the case.

[0068] <Example 4> A sample for Example 4 was prepared in the same manner as in Example 1 described above, except that 2.0 parts by mass of carbon fiber were added to the raw material slurry when the amount of glass fiber was 100 parts by mass. The thickness of the sample was 10 mm. The density of carbon fiber in the sample was 1.960 mg / cm³. ３ That was the case.

[0069] <Example 5> A sample for Example 5 was prepared in the same manner as in Example 1 described above, except that a slurry containing carbon fibers and glass fibers was paper-made in 6 L. The thickness of the sample was 3 mm. The density of carbon fibers in the sample was 0.105 mg / cm³. ３ That was the case.

[0070] <Example 6> A sample for Example 6 was prepared in the same manner as in Example 1 described above, except that a slurry containing carbon fibers and glass fibers was paper-made in 10 L. The thickness of the sample was 5 mm. The density of carbon fibers in the sample was 0.108 mg / cm³. ３ That was the case.

[0071] <Example 7> A sample for Example 7 was prepared in the same manner as in Example 1 described above, except that 0.2 parts by mass of cationic starch was added to a slurry containing carbon fibers and glass fibers, with a total of 100 parts by mass of carbon fibers and glass fibers. "Neotac 40T" manufactured by Nippon Shokuhin Kako Co., Ltd. was used as the cationic starch. The thickness of the sample was 10 mm. The density of carbon fibers in the sample was 0.151 mg / cm³. ３ That was the case.

[0072] <Example 8> A sample for Example 8 was prepared in the same manner as in Example 1 described above, except that 55 parts by mass of glass fibers with an average fiber diameter of 0.65 μm, 35 parts by mass of glass fibers with an average fiber diameter of 2.44 μm, and 10 parts by mass of polyester organic binder fibers were placed in a disintegrating pulper, and the raw materials, which were concentrated to 1.0% by mass, were dispersed to obtain a raw material slurry. The thickness of the sample was 10 mm. The density of carbon fibers in the sample was 0.110 mg / cm³. ３ That was the case.

[0073] <Comparative Example 1> A sample for Comparative Example 1 was prepared in the same manner as in Example 1 described above, except that carbon fibers were not added. In other words, the sample for Comparative Example 1 did not contain carbon fibers. The thickness of the sample was 10 mm.

[0074] <Comparative Example 2> A sample for Comparative Example 2 was prepared in the same manner as in Example 1 described above, except that 3.0 parts by mass of carbon fiber were added when the glass fiber in the raw material slurry was 100 parts by mass. The thickness of the sample was 10 mm. The density of carbon fiber in the sample was 3.123 mg / cm³. ３ That was the case.

[0075] <Comparative Example 3> A sample for Comparative Example 3 was prepared in the same manner as in Example 1 described above, except that a slurry containing carbon fibers and glass fibers was paper-made in 2 L. The thickness of the sample was 1 mm. The density of carbon fibers in the sample was 0.145 mg / cm³. ３ That was the case.

[0076] 3.2. Electromagnetic Wave Absorption Performance 3.2.1. Evaluation Method The electromagnetic wave absorption performance was measured by measuring the return loss of the sample based on the free-space method. The measurement devices used were "DPS-10-02" and "DPS-10-03" manufactured by Keycom Co., Ltd. In the free-space method, the sample is placed between two opposing antennas, and the reflection and transmission measurements can be evaluated by injecting electromagnetic waves radiated from one of the antennas into the sample. Figure 1 is a diagram illustrating the free-space method used to evaluate the electromagnetic wave absorption performance.

[0077] As shown in Figure 1, a sample S was placed between antennas A1 and A2 of the measuring device. A metal plate M was set on the back surface of the sample S (the surface opposite to the surface from which the electromagnetic waves were incident). Next, electromagnetic waves were irradiated from antenna A1 toward the sample S. The electromagnetic waves irradiated from antenna A1 did not reach antenna A2 due to the metal plate M. Then, by receiving the electromagnetic waves reflected from the sample S side with antenna A1, the ratio of the amount of electromagnetic waves incident on the sample S to the amount of electromagnetic waves reflected (reflection attenuation) was derived as the amount of electromagnetic wave absorption. The sample S diffusely reflects the electromagnetic waves internally, converting the electromagnetic waves into heat. As a result, the electromagnetic waves are attenuated. The electromagnetic waves irradiated from antenna A1 are plane waves, also called orthogonal polarization.

[0078] Specifically, the amount of electromagnetic wave absorption was calculated using the following formula (2).

[0079] Electromagnetic wave absorption amount = 10 × log(Pm / Pa) ... (2)

[0080] In equation (2), Pm is the amount of incident electromagnetic wave (unit: W). Pa is the amount of reflected electromagnetic wave (unit: W). The amount of electromagnetic wave absorption is expressed in decibels (dB). If the amount of electromagnetic wave absorption is -10 dB, it means that 90% of the electromagnetic wave was absorbed; at -20 dB, it means that 99% was absorbed; and at -30 dB, it means that 99.9% was absorbed.

[0081] The measurement frequency for electromagnetic wave absorption performance was set to 4 GHz to 110 GHz.

[0082] 3.2.2. Evaluation Results Figure 2 is a table showing the evaluation results of electromagnetic wave absorption performance in Examples 1 to 8 and Comparative Examples 1 to 3. Figure 2 shows the electromagnetic wave absorption performance at 5 GHz, 20 GHz, 50 GHz, and 90 GHz within the measurement frequency range of 4 GHz to 110 GHz.

[0083] As shown in Figure 2, Examples 1 to 8 exhibited electromagnetic wave absorption performance of -3 dB or less across the entire measurement frequency range. On the other hand, Comparative Examples 1 to 3 were unable to satisfy the requirement of -3 dB or less across the entire measurement frequency range.

[0084] Comparative Example 1 did not have electromagnetic wave absorption performance because it did not contain carbon fibers. In Comparative Example 2, the amount of carbon fibers was too high, causing electromagnetic waves to be reflected off the surface of the sample, resulting in low electromagnetic wave absorption performance. In Comparative Example 3, the sample thickness was too small, resulting in low electromagnetic wave absorption performance.

[0085] Based on the above, the density of carbon fibers in the sample is 0.01 mg / cm³. ３ 2.0mg / cm or more ３ It was found that if the following conditions are met and the sample thickness is 3 mm or more, the electromagnetic wave absorption performance can be improved.

[0086] Examples 7 and 8, which included a binder as an additive, had electromagnetic wave absorption performance equivalent to that of Example 1. Because Examples 7 and 8 contained a binder, the sample intensity was greater than that of Example 1.

[0087] 3.3. Multipath Reduction Performance 3.3.1. Evaluation Method To evaluate the multipath reduction performance, the sample was placed in a sealed space and the communication speed was measured. The samples used were the sample from Example 1 described above and the sample from Comparative Example 4. Comparative Example 4 used foamed polyethylene instead of glass fiber and carbon black instead of carbon fiber. The thickness of the sample was 10 mm.

[0088] Figure 3 is a schematic plan view of the evaluation apparatus 10 used to evaluate the multipath reduction performance. Figure 4 is a schematic plan view of the evaluation apparatus 10 used to evaluate the multipath reduction performance. For convenience, the lid 13 of the box 12 is omitted from Figure 3. Also, the dimensions of each component are shown in Figures 3 and 4.

[0089] As shown in Figure 3, a wireless LAN access point (Wi-Fi) 14 and a terminal (tablet) 16 are arranged inside a sealed aluminum box 12. Specifically, the Wi-Fi 14 is attached to the first inner surface 12a of the box 12 via a spacer 15. The tablet 16 is placed on a base 17 made of expanded polystyrene. Then, the sample S is attached to the second inner surface 12b of the box 12 near the tablet 16. The inside of the box 12 is divided by a middle plate 18 with a slit 18a, with the Wi-Fi 14 placed in one space and the tablet 16 and sample S placed in the other space. The Wi-Fi 14 and tablet 16 are electrically connected to a control unit 19 and are controlled by the control unit 19.

[0090] In the sealed space described above, the communication speed was measured when data was transmitted from the tablet 16 to the Wi-Fi 14 using Sample S from Example 1 and Comparative Example 4. iPerf was used to measure the communication speed. The data transmission frequency was set to 2.4 GHz.

[0091] 3.3.2. Evaluation Results Figure 5 is a table showing the evaluation results of the multipath reduction performance in Example 1 and Comparative Example 4. In Figure 5, "sealed state" means the state in which the lid 13 of the box 12 is closed. "Open state" means the state in which the lid 13 of the box 12 is open. "No sample" means the state in which the sample S is not attached to the inner surface of the box 12.

[0092] As shown in Figure 5, in Example 1, the communication speed was more than twice as high as in the "no sample" case. On the other hand, in Comparative Example 4, no significant difference in communication speed was observed compared to the "no sample" case.

[0093] Based on the above findings, it was found that samples using glass fibers and carbon fibers exhibited significantly higher multipath reduction performance compared to samples using foamed polyethylene and carbon black. Furthermore, it was found that when data was transmitted from a transmitter to a receiver via a wireless LAN at a frequency of 2.4 GHz in a sealed state, the data communication speed of the samples using glass fibers and carbon fibers was more than doubled.

[0094] The embodiments and variations described above are examples only and are not limited thereto. For example, each embodiment and each variation can be combined as appropriate.

[0095] The present invention is not limited to the embodiments described above, and various further modifications are possible. For example, the present invention includes configurations that are substantially identical to those described in the embodiments. A substantially identical configuration is, for example, a configuration that has the same function, method, and result, or a configuration that has the same purpose and effect. The present invention also includes configurations in which non-essential parts of the configuration described in the embodiments are replaced. Furthermore, the present invention includes configurations that produce the same effects or achieve the same purpose as the configuration described in the embodiments. Furthermore, the present invention includes configurations that add known technology to the configuration described in the embodiments.

[0096] 10...Evaluation device, 12...Box, 12a...First inner surface, 12b...Second inner surface, 12c...Third inner surface, 13...Lid, 14...Wi-Fi, 15...Spacer, 16...Tablet, 17...Base, 18...Middle plate, 18a...Slit, 19...Control unit

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