Electromagnetic wave absorber and method for manufacturing the same

Abstract

To provide an electromagnetic wave absorber with high electromagnetic wave absorption properties.SOLUTION: An electromagnetic wave absorber 100 includes a first layer 10 containing carbon nanotubes, a plurality of second layers 20 containing pulp and not containing carbon nanotubes. The first layer 10 is provided between a pair of second layers 20.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electromagnetic wave absorber and a method for manufacturing the same.

Background Art

[0002] Conventionally, an electromagnetic wave absorber having a laminated structure formed by laminating a plurality of sheets has been known. Such an electromagnetic wave absorber is used, for example, in offices, laboratories, hospitals, etc. to absorb electromagnetic waves emitted from electronic components such as wireless LAN (Local Area Network).

[0003] For example, Patent Document 1 describes an electromagnetic wave absorber configured by laminating a plurality of fiber sheets containing carbon fibers and thermally fusing them to each other. In such an electromagnetic wave absorber, it is required to enhance the electromagnetic wave absorption property (electromagnetic wave absorbability).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] One of the objects according to some aspects of the present invention is to provide an electromagnetic wave absorber having high electromagnetic wave absorbability. Another object according to some aspects of the present invention is to provide a method for manufacturing an electromagnetic wave absorber having high electromagnetic wave absorbability.

Means for Solving the Problems

[0006] One aspect of the electromagnetic wave absorber according to the present invention is a first layer containing carbon nanotubes, a plurality of second layers containing pulp and not containing carbon nanotubes, and includes The first layer is provided between the pair of second layers.

[0007] In one aspect of the electromagnetic wave absorber, a plurality of the first layers are provided, the first layer and the second layer may be alternately laminated.

[0008] In any aspect of the electromagnetic wave absorber, the thickness of the first layer may be 1 μm or more and 20 μm or less.

[0009] In any aspect of the electromagnetic wave absorber, the basis weight of the second layer is 10 g / m 2 or more and 2 600 g / m or less.

[0010] In any aspect of the electromagnetic wave absorber, the thickness of the electromagnetic wave absorber may be 0.05 mm or more and 10 mm or less.

[0011] In any aspect of the electromagnetic wave absorber, the first layer may contain carboxymethyl cellulose Sodium and may include.

[0012] In any aspect of the electromagnetic wave absorber, the first layer may not contain pulp.

[0013] In any aspect of the electromagnetic wave absorber, the absorption frequency band of the electromagnetic wave may be 2 GHz or more and 18 GHz or less.

[0014] One aspect of the method for manufacturing an electromagnetic wave absorber according to the present invention is a step of forming a first layer containing carbon nanotubes, a step of forming a plurality of second layers containing pulp and not containing carbon nanotubes, A step of disposing the first layer between a pair of the second layers and laminating the first layer and the second layers; including.

[0015] In one aspect of the method for manufacturing the electromagnetic wave absorber, The step of forming the first layer includes: mixing carbon nanotubes, carboxymethyl cellulose Sodium and water to prepare a mixed solution; dispersing the carbon nanotubes contained in the mixed solution by a water-opposed collision method to prepare a dispersion; drying the dispersion; and may include.

[0016] In any aspect of the method for manufacturing the electromagnetic wave absorber, including a step of determining the number of the first layers to be laminated in the step of laminating the first layer and the second layers based on information on the relationship between the number of the first layers and the electromagnetic wave absorption frequency band, which is acquired in advance, the absorption frequency band is 2 GHz or more and 18 GHz or less, in the step of laminating the first layer and the second layers, the number of the first layers determined in the step of determining the number of the first layers may be laminated.

[0017] In one aspect of the method for manufacturing the electromagnetic wave absorber, the absorption frequency band may be 8 GHz or more and 16 GHz or less.

Advantages of the Invention

[0018] The electromagnetic wave absorber according to the present invention can have high electromagnetic wave absorption. Further, according to the method for manufacturing the electromagnetic wave absorber according to the present invention, an electromagnetic wave absorber having high electromagnetic wave absorption can be manufactured.

Brief Description of the Drawings

[0019]

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Figure 11

Mode for Carrying Out the Invention

[0020] 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.

[0021] 1. Electromagnetic Wave Absorber 1.1. Structure First, the electromagnetic wave absorber according to the present embodiment will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing the electromagnetic wave absorber 100 according to the present embodiment.

[0022] As shown in Fig. 1, the electromagnetic wave absorber 100 includes a first layer 10 containing carbon nanotubes and not containing pulp, and a plurality of second layers 20 containing pulp and not containing carbon nanotubes.

[0023] The electromagnetic wave absorber 100 is provided on the base material 2. The electromagnetic wave incident on the electromagnetic wave absorber 100 is incident on the electromagnetic wave absorber 100 from the side opposite to the base material 2. The material of the base material 2 is, for example, a metal such as aluminum.

[0024] The first layer 10 is provided between a pair of second layers 20. For example, a plurality of first layers 10 are provided. The first layer 10 and the second layer 20 are alternately laminated. In the illustrated example, five first layers 10 are provided and six second layers 20 are provided. The uppermost layer and the lowermost layer of the electromagnetic wave absorber 100 are the second layers 20. The lowermost second layer 20 is in contact with the base material 2.

[0025] Note that the number of the first layers 10 is not particularly limited, but is, for example, 1 or more and 20 or less. Similarly, the number of the second layers 20 is not particularly limited, but is, for example, 2 or more and 20 or less. For example, only one first layer 10 may be provided, and a pair of second layers 20 may sandwich the first layer 10.

[0026] The thickness T1 of the first layer 10 is not particularly limited, but is preferably 1 μm or more and 20 μm or less, more preferably 1 μm or more and 10 μm or less, and even more preferably 1 μm or more and 5 μm or less. If the thickness T1 is 1 μm or more, it is possible to prevent the electromagnetic wave absorption property from being lowered due to the electromagnetic wave passing through the first layer 10. If the thickness T1 is 20 μm or less, it is possible to prevent the electromagnetic wave absorption property from being lowered due to the electromagnetic wave being reflected by the first layer 10.

[0027] The basis weight of the second layer 20 is not particularly limited, but is preferably 10 g / m 2 or more and 600 g / m 2 or less, more preferably 20 g / m 2 or more and 500 g / m 2 or less, and even more preferably 30 g / m2 Above 350 g / m 2 The following. The rice paper weight of the second layer 20 is 10 g / m 2 If it is above, the electromagnetic wave absorber 100 can have high electromagnetic wave absorption. If the rice paper weight of the second layer 20 is 600 g / m 2 If it is below, the variation in the rice paper weight of the second layer 20 can be reduced.

[0028] The thickness T of the electromagnetic wave absorber 100 is not particularly limited, but is preferably 0.05 mm or more and 10 mm or less. If the thickness T is 0.05 mm or more and 10 mm or less, the electromagnetic wave absorber 100 can have high electromagnetic wave absorption. The shape of the electromagnetic wave absorber 100 is a sheet shape in which the thickness T is sufficiently small compared to the size in the direction orthogonal to the thickness direction.

[0029] 1.2. Structure of the first layer 1.2.1. Carbon nanotube The first layer 10 is a CNT-containing layer containing carbon nanotubes (hereinafter also referred to as "CNT"). Examples of the CNT contained in the first layer 10 include single-walled carbon nanotubes (SWNT: single-walled carbon nanotube) in which a single six-membered ring network (graphene sheet) made of carbon is wound cylindrically, and multi-walled carbon nanotubes (MWNT: multi-walled carbon nanotube) in which a plurality of graphene sheets are wound concentrically. The first layer 10 may contain only one of SWNT and MWNT, or may contain both, but considering the dispersibility of CNT, it is preferably to contain only MWNT.

[0030] The above-mentioned CNT is produced, for example, by an arc discharge method, a laser ablation method, CVD ( Chemical Vapor Deposition) method or the like to a preferred size. The CNT contained in the first layer 10 may be produced by any method.

[0031] The diameter of the CNT is not particularly limited, but is preferably 1 nm or more and 100 nm or less, more preferably 5 nm or more and 50 nm or less, and even more preferably 8 nm or more and 15 nm or less. If the diameter of the CNT is 1 nm or more and 100 nm or less, a dispersion liquid with good dispersibility of the CNT can be prepared when forming the first layer 10. The diameter of the CNT can be measured by SEM (Scanning Electron Microscope).

[0032] The fiber length of the CNT is not particularly limited, but is preferably 0.5 μm or more and 50 μm or less, more preferably 15 μm or more and 35 μm or less. If the fiber length of the CNT is 0.5 μm or more and 50 μm or less, a dispersion liquid with good dispersibility of the CNT can be prepared. The fiber length of the CNT can be measured by SEM. Note that the "fiber length of the CNT" is the length in the state where the CNTs are bundled (banded) by van der Waals forces, and is the length of the CNTs before being dispersed in the solvent.

[0033] The BET specific surface area of the CNT is not particularly limited, but is preferably 50 m 2 / g or more and 500 m 2 / g or less, more preferably 100 m 2 / g or more and 300 m 2 / g or less. If the BET specific surface area of the CNT is 50 m 2 / g or more and 500 m 2 / g or less, a dispersion liquid with good dispersibility of the CNT can be prepared when forming the first layer 10. Note that the "BET specific surface area" refers to the specific surface area measured by the BET (Brunauer Emmett Teller) method and can be measured by an automatic specific surface area measuring device.

[0034] In the first layer 10, the content of CNT is not particularly limited, but is preferably 0.1% by mass or more and 10.0% by mass or less, more preferably 0.5% by mass or more and 5.0% by mass or less, and even more preferably 1.0% by mass or more and 3.0% by mass or less. If the content of CNT is 0.1% by mass or more, the electromagnetic wave absorber 100 can have high electromagnetic wave absorption. Further, if the content of CNT is 5.0% by mass or less, a dispersion liquid with good dispersibility of CNT can be prepared when forming the first layer 10.

[0035] 1.2.2. Carboxymethyl cellulose Sodium The first layer 10 contains carboxymethyl cellulose Sodium (hereinafter also referred to as "CMC"). CMC functions as a dispersant for dispersing CNT when forming the first layer 10. It is preferable to use only CMC as the dispersant for CNT. By using only CMC as the dispersant for CNT, it is possible to prevent the incorporation of air bubbles, etc., compared to the case where an anionic surfactant or the like is added as a dispersant in addition to CMC, for example.

[0036] The weight average molecular weight of CMC is not particularly limited, but is preferably 5000 or more and 100000 or less, more preferably 10000 or more and 60000 or less, and even more preferably 10000 or more and 35000 or less. If the weight average molecular weight of CMC is 5000 or more, CMC is likely to entangle with CNT and the dispersibility of CNT is improved. However, if the weight average molecular weight is too large, the dispersibility deteriorates conversely, so it is preferably 100000 or less. In addition, the "weight average molecular weight" in this specification refers to the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

[0037] The degree of etherification of CMC is not particularly limited, but is preferably 0.6 or more and 1.2 or less, more preferably 0.6 or more and 0.8 or less. If the degree of etherification of CMC is 0.6 or more and 1.2 or less, a dispersion liquid with good dispersibility can be prepared.

[0038] In the first layer 10, the content of CMC is not particularly limited, but is preferably 0.1% by mass or more and 10.0% by mass or less, more preferably 0.5% by mass or more and 5.0% by mass or less, and even more preferably 1.0% by mass or more and 3.0% by mass or less.

[0039] In the first layer 10, the mass M of CNT CNT to the mass M of CMC CMC of the ratio M CMC / M CNT is preferably 1 / 6 or more and 1 or less (CNT:CMC = 6:1 to 1:1). When the ratio M CMC / M CNT is 1 / 6 or more, a dispersion liquid with good dispersibility of CNT can be prepared when forming the first layer 10. When the ratio M CMC / M CNT is 1 or less, the electromagnetic wave absorber 100 can have high electromagnetic wave absorption.

[0040] 1.2.3. CNT alternative materials The first layer 10 may contain a CNT alternative material. Examples of the CNT alternative material include carbon materials such as carbon black and graphite, and inorganic pigments such as kaolin, mica, and talc.

[0041] By including the above CNT alternative material, the content of CNT in the first layer 10 can be reduced accordingly. Since CNT is expensive, cost reduction can be achieved by replacing a part of CNT with a CNT alternative material. In the first layer 10, the ratio of the mass of the CNT alternative material to the mass of CNT is preferably 1 / 4 or more and 1 or less (CNT: other carbon material = 4:1 to 1:1). If the mass ratio is 1 / 4 or more and 1 or less, cost reduction can be achieved while ensuring the electromagnetic wave absorption of the electromagnetic wave absorber 100.

[0042] 1.2.4. Other additives The first layer 10 may further contain various additives such as a thickener, a preservative, and a pH adjuster, if necessary.

[0043] 1.3. Composition of the Second Layer 1.3.1. Pulp The second layer 20 is a pulp-containing layer containing pulp. The second layer 20 may be a pulp layer composed only of pulp. Examples of the pulp contained in the second layer 20 include chemical pulps such as LBKP (bleached hardwood kraft pulp) and NBKP (bleached softwood kraft pulp), mechanical pulps such as GP (groundwood pulp), PGW (pressure groundwood pulp), RMP (refiner mechanical pulp), TMP (thermomechanical pulp), CTMP (chemi-thermomechanical pulp), CMP (chemimechanical pulp), CGP (chemiground pulp), etc., wood pulps such as DIP (deinked pulp), or non-wood pulps such as kenaf, bagasse, bamboo, cotton, etc. These may be used alone or in any ratio as a mixture of two or more. Further, synthetic fibers may be used within a range not impairing the quality.

[0044] The second layer 20 mainly contains LBKP and preferably contains 70% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass of LBKP based on the total mass of the second layer 20. By this, an electromagnetic wave absorber 100 with less distortion can be obtained. The basis weight of the second layer 20 is 40 g / m 2 In the following cases, it is preferable to blend NBKP, and preferably 30% by mass or less based on the total mass of the second layer 20. By setting it within this range, the desired smoothness and the strength when forming the second layer 20 can be maintained.

[0045] 1.3.2. Other Additives The second layer 20 may further contain various additives such as fillers, paper strength enhancers, sizing agents, bulking agents, yield improvers, drainage improvers, sulfuric acid bands, wet strength enhancers, coloring dyes, coloring pigments, fluorescent brighteners, pitch control agents, thickeners, preservatives, pH adjusters, etc., if necessary.

[0046] 1.4. Electromagnetic Wave Absorbability The electromagnetic wave absorber 100 has electromagnetic wave absorption properties. The absorption frequency band of the electromagnetic waves of the electromagnetic wave absorber 100 is, for example, 1 GHz or higher and 30 GHz or lower, preferably 2 GHz or higher and 18 GHz or lower. The absorption property of the electromagnetic wave absorber 100 with respect to electromagnetic waves is evaluated, for example, by measuring the reflection attenuation amount by the coaxial tube method.

[0047] 1.5. Function and Effect The electromagnetic wave absorber 100 can have high electromagnetic wave absorption properties. The reason therefor will be described below. FIG. 2 is a cross-sectional view for explaining the electromagnetic wave E incident on the electromagnetic wave absorber 100.

[0048] As shown by the solid line in FIG. 2, the electromagnetic wave E is incident on the electromagnetic wave absorber 100 from the uppermost second layer 20a. The incident electromagnetic wave E passes through the second layer 20a and the first layer 10a and is reflected by the upper surface 12b of the first layer 10b. Then, it is presumed that the electromagnetic wave E is attenuated while being repeatedly reflected (while undergoing multiple reflections) between the lower surface 14a of the first layer 10a and the upper surface 12b of the first layer 10b.

[0049] As shown by the broken line in FIG. 2, there are also electromagnetic waves that pass through the first layer 10b without being reflected by the upper surface 12b of the first layer 10b. Such electromagnetic waves further pass through the second layer 20c and are presumed to be attenuated while being repeatedly reflected between the lower surface 14b of the first layer 10b and the upper surface (not shown in FIG. 2) of the first layer 10 located below the first layer 10b. Note that, as shown by the broken line in FIG. 2, there are also electromagnetic waves that pass through the first layer 10a without being reflected by the lower surface 14a of the first layer 10a. Also, as shown by the broken line in FIG. 2, there are electromagnetic waves that are reflected by the upper surface 12a of the first layer 10a.

[0050] As described above, since the electromagnetic wave absorber 100 attenuates the incident electromagnetic wave E while undergoing multiple reflections, it can have high electromagnetic wave absorption. As mentioned above, only one first layer 10 may be provided and may be sandwiched between two second layers 20. In such a case, since the electromagnetic wave E attenuates while undergoing multiple reflections between the lower surface of the first layer 10 and the upper surface of the base material 2, it can have high electromagnetic wave absorption. However, in order to have higher electromagnetic wave absorption, it is appropriate to provide two or more, preferably three or more, more preferably five or more first layers 10. is.

[0051] 2. Manufacturing Method of Electromagnetic Wave Absorber Next, a method for manufacturing the electromagnetic wave absorber 100 according to the present embodiment will be described with reference to the drawings. FIG. 3 is a flowchart for explaining the method for manufacturing the electromagnetic wave absorber 100 according to the present embodiment.

[0052] As shown in FIG. 3, the method for manufacturing the electromagnetic wave absorber 100 includes a first layer forming step (step S10) for forming the first layer 10, a second layer forming step (step S20) for forming the second layer 20, and a lamination step (step S30) for laminating the first layer 10 and the second layer 20. Hereinafter, each step of the method for manufacturing the electromagnetic wave absorber 100 will be described in order.

[0053] 2.1. First Layer Forming Step (Step S10) FIG. 4 is a flowchart for explaining the first layer forming step of the method for manufacturing the electromagnetic wave absorber 100. In the first layer forming step, as shown in FIG. 4, it includes a mixed liquid preparation step (step S12) for preparing a mixed liquid, a dispersion preparation step (step S14) for preparing a dispersion, and a drying step (step S16) for drying the dispersion.

[0054] 2.1.1. Mixed Liquid Preparation Step (Step S12) In the mixed solution preparation process, CNT, CMC, and water are mixed to prepare a mixed solution. The mixing of CNT, CMC, and water is performed, for example, by a homogenizer. In the mixed solution preparation process, water is used as a solvent. Examples of water include pure water such as ion-exchanged water, ultrafiltration water, reverse osmosis permeated water, and distilled water, and those with extremely low ionic impurities like ultrapure water. By using water as a solvent, a mixed solution that is more environmentally friendly can be prepared compared to the case of using an organic solvent as a solvent.

[0055] 2.1.2. Dispersion process (step S14) In the dispersion process, the CNT contained in the mixed solution prepared in the mixing process is dispersed by the in-water counter-collision method to prepare a dispersion. By dispersing the CNT contained in the mixed solution by the in-water counter-collision method, the CNT can be dispersed with good dispersibility even if the mixed solution contains only CMC as a dispersant. Thereby, a dispersion with good dispersibility of CNT can be prepared.

[0056] In the "in-water counter-collision method" in the present embodiment, a mixed solution containing CNT is discharged at high pressure from a pair of oppositely arranged nozzle holes (the first nozzle hole and the second nozzle hole), and the mixed solution discharged from the first nozzle hole and the mixed solution discharged from the second nozzle hole are collided to disperse the CNT. Preferably, in the in-water counter-collision method, the CNT contained in the mixed solution discharged from the first nozzle hole and the CNT contained in the mixed solution discharged from the second nozzle hole are collided to disperse the CNT. In the in-water counter-collision method, if the central axis of the first nozzle hole and the central axis of the second nozzle hole intersect each other, the two central axes may be on a straight line or may be inclined to each other.

[0057] In the underwater counter-collision method in the dispersion process, the mixed liquid is preferably discharged from nozzle holes having a diameter of 50 μm or more and 200 μm or less, more preferably 80 μm or more and 120 μm or less, and even more preferably 100 μm, to cause the mixed liquids to collide with each other. If the diameter of the nozzle holes is 50 μm or more, even a mixed liquid with high viscosity can be discharged from the nozzle holes. Furthermore, if the diameter of the nozzle holes is 200 μm or less, the collision energy between the mixed liquids can be increased.

[0058] In the underwater counter-collision method in the dispersion process, it is preferably at 150 MPa or more and 250 MPa or less 、more preferably at 180 MPa or more and 220 MPa or less, and even more preferably at 200 MPa of pressure, the mixed liquid is discharged to cause the mixed liquids to collide with each other. If the pressure is 150 MPa or more, the collision energy between the mixed liquids can be increased. Furthermore, if the pressure is 250 MPa or less, it is possible to suppress the situation where the fiber of CNT is cut due to excessive collision energy and the viscosity of the dispersion liquid becomes low.

[0059] Specifically, the underwater counter-collision method in the dispersion process is carried out using the wet atomization device "STAR BURST LABO" (model name: HJP-25005) manufactured by Sugino Machine Limited. Compared with, for example, an ultrasonic homogenizer or a ball mill, this wet atomization device has a high energy density and can produce a dispersion liquid with good dispersibility in a short time. Furthermore, this wet atomization device can minimize the mixing of impurities and can produce a dispersion liquid with extremely little mixing of impurities.

[0060] The number of passes of the mixed liquid in the wet atomization device is preferably 1 or more and 40 or less, more preferably 2 or more and 10 or less, and even more preferably 2 or 3. If the number of passes is 40 or less, it is possible to suppress the breakage of the CNT fibers due to the collision of the mixed liquids and the decrease in the viscosity of the dispersion liquid. Further, if the number of passes is 2 or more, the CNTs can be dispersed with good uniformity. Furthermore, if the number of passes is 2 or more, no significant difference is confirmed in the dispersibility of the CNTs. Therefore, if the number of passes is 2 or more and 10 or less, it is possible to shorten the processing time by the wet atomization device while maintaining good dispersibility.

[0061] Here, the "number of passes of the mixed liquid in the wet atomization device" refers to the number of circulation times of the mixed liquid in the wet atomization device. For example, "the number of passes is 2" means that the mixed liquid is circulated twice so that the CNTs that have collided once collide again. In this way, the number of passes corresponds to the number of collision times of the CNTs contained in the mixed liquid. Further, the number of passes is proportional to the processing time in the wet atomization device. When the processing time in the wet atomization device is long, the number of circulation times of the mixed liquid increases.

[0062] In addition, if a dispersion liquid with good dispersibility can be produced and an electromagnetic wave absorber with high electromagnetic wave absorption can be manufactured, the device used in the underwater counter-collision method in the dispersion step is not limited to the above wet atomization device "Starburst Turbo". Further, if a dispersion liquid with good dispersibility can be produced and an electromagnetic wave absorber with high electromagnetic wave absorption can be manufactured, it is not necessary to use the underwater counter-collision method in the dispersion step.

[0063] Further, when forming the first layer 10 containing a CNT alternative material such as carbon black, the CNT alternative material may be mixed into the dispersion liquid produced in the dispersion step, or a mixed liquid containing the CNT alternative material may be produced in the mixed liquid production step before performing the dispersion step.

[0064] 2.1.3. Drying process (step S16) In the drying process, the dispersion prepared in the dispersion process is dried. Thereby, the moisture of the dispersion can be evaporated to form the first layer 10. The method for drying the dispersion is not particularly limited, and it may be dried by a hot plate, a heater, etc., or natural drying may be used. As described above, the first layer 10 can be formed.

[0065] 2.2. Second layer forming process (step S20) In the second layer forming process, a pulp-containing slurry without CNT is formed into paper by a paper machine to form the second layer 20. The slurry for forming the second layer 20 has a Canadian Standard Freeness (CSF) of, for example, 200 ml or more and 550 ml or less, preferably 250 ml or more and 500 ml or less. The CSF is determined by the method described in "JIS P 81821-2". In the present invention, the papermaking method is not particularly limited, and it is manufactured by various devices such as a Fourdrinier paper machine, a Fourdrinier multi-layer paper machine, a cylinder paper machine, a cylinder multi-layer paper machine, a Fourdrinier cylinder combined multi-layer paper machine, and a twin-wire paper machine. The papermaking method can be selected from either acid papermaking or neutral papermaking.

[0066] In addition, in the drying process (step S16) of the first layer 10, the dispersion prepared in the dispersion process may be applied to the second layer 20, and the applied dispersion may be dried to form the first layer 10. The method for applying the dispersion to the second layer 20 is not particularly limited. For example, a method of directly applying it to the second layer 20 using a wire bar coater, a knife coater, an air coater, a blade coater, a reverse roll coater, a die coater, etc., or a method of attaching the dispersion to a roller and transferring the dispersion attached to the roller to the second layer 20, so-called roll coater, etc. can be mentioned.

[0067] In the second layer forming step, when applying the dispersion liquid, in order to suppress excessive penetration of the dispersion liquid into the second layer 20 or to improve the strength of the surface of the second layer 20, a sizing liquid containing a known water-soluble polymer such as starch, polyvinyl alcohol, polyacrylamide, etc. may be applied using a size press or the like. As an auxiliary agent in the sizing liquid, for example, surface sizing agents such as styrene-based sizing agents, styrene-acrylate-based sizing agents, olefin-based sizing agents, certain kilkedaimer-based sizing agents, alkenyl succinic anhydride sizing agents, coloring pigments, coloring dyes, fluorescent dyes, defoaming agents, etc. may be used in combination. Examples of the application method of the sizing liquid include a size press, a gate roll coater, a metering sizer, a rod coater, a bar coater, etc.

[0068] Also, in the second layer forming step, when applying the dispersion liquid, in order to suppress excessive penetration of the dispersion liquid into the second layer 20, a paint containing a pigment and an adhesive may be applied. There is no particular limitation on the pigment to be blended in the paint, and inorganic pigments such as kaolin, light calcium carbonate, titanium oxide, plastic pigments, and organic pigments such as plastic pigments can be used. There is no particular limitation on the adhesive, and known adhesives can be used. For example, various copolymer latexes such as styrene-butadiene-based, styrene-acrylic-based, vinyl acetate-acrylic-based, butadiene-methyl methacrylate-based, etc. can be mentioned. Also, in the paint, auxiliary agents such as a pH adjuster, a defoaming agent, a dispersant, a lubricant, a printing suitability improver, a thickener, a water retention agent, a fluorescent dye, a coloring pigment, a coloring dye, etc. can be used within a range that does not impair the effects intended by the present invention.

[0069] In the manufacturing method of the electromagnetic wave absorber 100, a plurality of coated papers in which the first layer 10 as described above is coated on the second layer 20 may be formed to form a plurality of first layers 10 and a plurality of second layers 20.

[0070] 2.3. Laminating step (step S30) In the lamination process, the first layer 10 is disposed between a pair of second layers 20, and the first layer 10 and the second layer 20 are laminated. As described above, when a plurality of coated papers are formed, adjacent coated papers are adhered with, for example, glue so that the first layer 10 and the second layer 20 alternate, and the first layer 10 and the second layer 20 are laminated. Then, the second layer 20 is adhered to the laminate of the coated papers so that the uppermost layer and the lowermost layer are the second layer 20. Note that the bonding method is not limited to glue.

[0071] Through the above steps, the electromagnetic wave absorber 100 can be manufactured.

[0072] 2.4. Modification In a modification of the method for manufacturing the electromagnetic wave absorber 100, before the lamination process, based on information on the relationship between another first layer 10 and the electromagnetic wave absorption frequency band, which has been acquired in advance, a determination step of determining the number of the first layers 10 to be laminated in the lamination process is included. Then, in the lamination process, the number of the first layers 10 determined in the determination step is laminated. The determination step is performed before the first layer formation step, and in the first layer formation step, the number of the first layers 10 determined in the determination step may be formed. The "information on the relationship between another first layer 10 and the electromagnetic wave absorption frequency band, which has been acquired in advance" used in the determination step is, for example, a graph showing the relationship between the frequency and "S11", as shown in FIGS. 5 and 6 described later. The electromagnetic wave absorption frequency band is preferably 2 GHz or more and 18 GHz or less, and more preferably 8 GHz or more and 16 GHz or less.

[0073] As shown in FIGS. 5 and 6 described later, as the number of laminated first layers 10 increases, the peak of "S11" shifts to the low frequency side. Therefore, based on such a graph, by determining the number of laminated first layers 10, an electromagnetic wave absorber 100 that absorbs a desired frequency can be manufactured.

[0074]

[0075] 3. Experimental Example Experimental examples are shown below to more specifically explain the present invention. Note that the present invention is not limited by the following experimental examples at all.

[0076] 3.1. Evaluation of Electromagnetic Wave Absorbing Property 3.1.1. First Evaluation (1) Preparation of Electromagnetic Wave Absorber First, CNT, CMC, and water were mixed to prepare a mixed solution. For the mixing, a homogenizer "Bio Mixer BM - 2" manufactured by Nippon Seiki Seisakusho Co., Ltd. was used. The mixing treatment time was set to 5 minutes. The mass ratio of CNT to CMC in the mixed solution was CNT:CMC = 1:1. The total of the CNT content and the CMC content in the mixed solution was set to 5% by mass.

[0077] As the CNT, "K - Nanos - 100P" manufactured by KUMHO PETROCHEMICAL was used. The said CNT is MWNT, with a diameter of 8 nm to 15 nm, a fiber length of 27 μm (bundle), and a BET specific surface area of 220 m 2 / g.

[0078] As the CMC, "Cellogen 5A" manufactured by Dai - Ichi Kogyo Seiyaku Co., Ltd. was used. The said CMC has a weight - average molecular weight of 11000 to 15000 and an etherification degree of 0.7. Only CMC was used as the dispersant. Additives such as thickeners were not added.

[0079] Next, for the above - mentioned mixed solution, the underwater counter - collision method was performed to prepare a dispersion in which CNT was dispersed. The underwater counter - collision method was carried out using a wet atomization device "Starburst Turbo" (model name: HJP - 25005) manufactured by Sugino Machine Limited. The diameter of the nozzle hole from which the mixed solution was discharged was set to 100 μm, and the discharge pressure of the mixed solution was set to 200 MPa. The number of passes of the mixed solution through the wet atomization device was set to 2 times. Thereby, a dispersion containing CNT, CMC, and water was prepared.

[0080] The above - mentioned dispersion was coated on a pulp layer ("My Coat Neo's" (registered trademark) manufactured by Hokuriku Corporation) using a roll coater, and then dried at 120 °C for 3 minutes to evaporate the moisture, thereby preparing a coated paper composed of a CNT - containing layer (corresponding to the first layer) and a pulp layer (corresponding to the second layer). The thickness of the CNT - containing layer was set to 3.5 μm.

[0081] The above-mentioned coated paper was laminated so that the CNT-containing layer and the pulp layer alternated to form a laminate of coated paper. The adjacent coated papers were adhered with glue. Further, a pulp layer was adhered to the laminate of coated paper so that the topmost layer and the bottommost layer were pulp layers.

[0082] As described above, an electromagnetic wave absorber was fabricated.

[0083] (2) Evaluation method A metal substrate was adhered to the bottommost layer of the electromagnetic wave absorber fabricated as described above, and the electromagnetic wave absorption property of the electromagnetic wave absorber was evaluated. The evaluation of the electromagnetic wave absorption property was performed by measuring "S11" by measuring the reflection attenuation amount by the coaxial tube method. The absolute value of "S11" corresponds to the absorption amount of electromagnetic waves, and the larger the absolute value of "S11", the higher the electromagnetic wave absorption property. As the tester, a network analyzer "8720ES" manufactured by Agilent Technologies was used. The measurement frequency was set to 50 MHz to 18 GHz.

[0084] (3) Evaluation results The electromagnetic wave absorption property was evaluated by varying the number of laminated CNT-containing layers. Fig. 5 is a graph showing the electromagnetic wave absorption property of the electromagnetic wave absorber when the basis weight of the pulp layer is 157 g / m 2 and the number of laminated CNT-containing layers was varied from 1 layer to 5 layers and 7 layers. Fig. 6 is a graph showing the electromagnetic wave absorption property of the electromagnetic wave absorber when the basis weight of the second layer is 319 g / m 2 and the number of laminated CNT-containing layers was varied from 1 layer to 5 layers.

[0085] As shown in Fig. 5 and Fig. 6, all the electromagnetic wave absorbers showed electromagnetic wave absorption property. For the electromagnetic wave absorbers with the number of laminated CNT-containing layers other than 1 layer, a peak of "S11" existed at 8 GHz to 16 GHz. As the number of laminated CNT-containing layers increased, the peak shifted to the low-frequency side.

[0086] Also, as shown in FIGS. 5 and 6, when the number of stacked CNT-containing layers is 4 or more, the absolute value of the peak in the range of 8 GHz to 16 GHz exceeds 20 dB, indicating very high electromagnetic wave absorption. The basis weight of the pulp layer is 319 g / m 2 is higher in electromagnetic wave absorption than in the case of a basis weight of 157 g / m 2 .

[0087] 3.1.2. Second Evaluation An electromagnetic wave absorber was fabricated in the same manner as in "3.1.1. First Evaluation", except that the number of stacked CNT-containing layers was set to 3, and the mass ratio M CMC / M CNT of CNT and CMC in the mixed liquid was varied at three levels of 1 / 6, 1, and 6 (CNT:CMC = 6:1, 1:1, 1:6). The thicknesses of the CNT-containing layers with ratios M CMC / M CNT of 1 / 6, 1, and 6 were 3.7 μm, 3.5 μm, and 4.8 μm, respectively. The basis weight of the pulp layer was 319 g / m 2 .

[0088] The electromagnetic wave absorption of the above electromagnetic wave absorber was evaluated by the same evaluation method as in "3.1.1. First Evaluation". FIG. 7 is a graph showing the electromagnetic wave absorption of the electromagnetic wave absorber when the mass ratio of CNT and CMC was varied. As shown in FIG. 7, the electromagnetic wave absorbers with ratios M CMC / M CNT of 1 / 6 and 1 showed higher electromagnetic wave absorption than the electromagnetic wave absorber with a ratio M CMC / M CNT of 6.

[0089] 3.1.3. Third Evaluation An electromagnetic wave absorber was fabricated in the same manner as in "3.1.1. First Evaluation", except that the number of stacked CNT-containing layers was set to 3, and the thickness of the CNT-containing layer was varied at two levels of 1.4 μm and 3.5 μm. The basis weight of the pulp layer was 319 g / m 2 .

[0090] The electromagnetic wave absorber described above was evaluated for its electromagnetic wave absorption properties using the same evaluation method as in "3.1.1. First Evaluation". Figure 8 is a graph showing the electromagnetic wave absorption properties of the electromagnetic wave absorber when the thickness of the CNT-containing layer is varied. As shown in Figure 8, the electromagnetic wave absorber with a CNT-containing layer thickness of 3.5 μm exhibited higher electromagnetic wave absorption properties than the electromagnetic wave absorber with a CNT-containing layer thickness of 1.4 μm.

[0091] 3.1.4. Fourth Evaluation With the number of laminated CNT-containing layers set to 5 layers, an electromagnetic wave absorber was fabricated in the same manner as in "3.1.1. First Evaluation". The size of the pulp layer was 157 g / m 2 was used.

[0092] The dielectric constant of the above electromagnetic wave absorber was measured, and from the measured dielectric constant, the reflection loss (RL) for thicknesses of 2.2 mm, 4.0 mm, 5.2 mm, and 8.0 mm of the electromagnetic wave absorber was calculated by simulation. The greater the absolute value of RL, the higher the electromagnetic wave absorption property. The dielectric constant was measured using a network analyzer "8720ES" manufactured by Agilent Technologies. The simulation used the FDTD method (finite-difference time-domain method).

[0093] Figure 9 is a graph showing the electromagnetic wave absorption properties of the electromagnetic wave absorber when the thickness of the electromagnetic wave absorber is varied. As shown in Figure 9, all of the electromagnetic wave absorbers exhibited high electromagnetic wave absorption properties. In particular, the electromagnetic wave absorbers with thicknesses of 2.2 mm and 4.0 mm exhibited high electromagnetic wave absorption properties.

[0094] 3.2. Evaluation of the dispersibility of CNTs 3.2.1. Preparation of the dispersion liquid Dispersion liquids 1 to 10 were prepared in the same manner as the dispersion liquid in "3.1.1. First Evaluation", except that the mass ratio of CNT to CMC was varied from CNT:CMC = 1:9 to 9:1. Figure 10 is a table showing the preparation conditions of dispersion liquids 1 to 10 and dispersion liquids 11 and 12 described later.

[0095] Dispersion liquid 11 was prepared in the same manner as dispersion liquid 1 described above, except that CMC was not mixed in when preparing the mixed liquid.

[0096] Dispersion liquid 12 was prepared in the same manner as dispersion liquid 4 described above, except that the underwater counter-collision method was not performed on the mixed liquid.

[0097] 3.2.2. Evaluation method The dispersion liquids 1 to 12 prepared as described above were placed in a petri dish with a diameter of 8.5 cm and dried overnight at 50°C to evaporate the moisture. Then, the dispersibility of CNTs was evaluated by observing the film-forming property of the dried product. The better the dispersibility of CNTs, the more uniform the film formed. The specific evaluation criteria are as follows.

[0098] A: A film without cracks was formed over the entire surface of the petri dish. B: A film was formed over the entire surface of the petri dish, but cracks occurred. C: No film was formed.

[0099] 3.2.3. Evaluation results Figure 10 shows the evaluation results of the dispersibility of dispersion liquids 1 to 12. Also, Figure 11 is a photograph showing the state after the dispersion liquids 1 to 12 were placed in a petri dish and dried overnight at 50°C.

[0100] As shown in Figures 10 and 11, dispersion liquids 1 to 8 had better film-forming properties and better dispersibility of CNTs compared to dispersion liquids 9 to 12.

[0101] Among dispersion liquids 1 to 7, no significant difference in film-forming property was confirmed, and a film without cracks was formed. For dispersion liquid 8, cracks occurred in the film because the content of CMC with respect to CNTs was low. For dispersion liquids 9 and 10, the content of CMC with respect to CNTs was too low, and no film was formed. The mass M CNT of CMC with respect to the mass M CMC of CNTs, the ratio M CMC / M CNT It was found that a dispersion liquid with good dispersibility of CNTs can be prepared by setting it to 1 / 6 or more.

[0102] Since CMC is not added to the mixed solution in Dispersion Liquid 11, the dispersibility of CNTs is poor, and the dispersion liquid Similar to 9 and 10, no film was formed.

[0103] Since the underwater counter-collision method is not performed on the mixed solution in Dispersion Liquid 12, the dispersibility of CNTs is poor and no film was formed. It was found that a dispersion liquid with good dispersibility of CNTs can be prepared by dispersing CNTs by the underwater counter-collision method.

[0104] The present invention is not limited to the above-described embodiments, and various modifications are further possible. For example, the present invention includes a configuration that is substantially the same as the configuration described in the embodiments. Substantially the same configuration means, for example, a configuration having the same functions, methods, and results, or a configuration having the same purposes and effects. Further, the present invention includes a configuration in which a non-essential part of the configuration described in the embodiments is replaced. Further, the present invention includes a configuration that exhibits the same operational effects as the configuration described in the embodiments or a configuration that can achieve the same purpose. Further, the present invention includes a configuration in which a known technique is added to the configuration described in the embodiments.

Explanation of Reference Numerals

[0105] 2... Substrate, 10, 10a, 10b... First layer, 12a, 12b... Upper surface, 14a, 14b... Lower surface, 20, 20a, 20b, 20c... Second layer, 100... Electromagnetic wave absorber

Claims

1. A plurality of first layers including carbon nanotubes, A plurality of second layers including pulp and not including carbon nanotubes, comprising, The first layer is provided between a pair of the second layers, The first layer and the second layer are alternately laminated, An electromagnetic wave absorber in which the number of the first layers is 4 or more and 20 or less.

2. The electromagnetic wave absorber according to claim 1, wherein the thickness of the first layer is 1 μm or more and 20 μm or less.

3. The basis weight of the second layer is 10 g / m ２ More than 600g / m ２ 3. The electromagnetic wave absorber according to claim 1 or 2, wherein:

4. The electromagnetic wave absorber according to any one of claims 1 to 3, wherein the thickness of the electromagnetic wave absorber is 2.2 mm or more and 4.7 mm or less.

5. The electromagnetic wave absorber according to any one of claims 1 to 4, wherein the first layer contains sodium carboxymethyl cellulose.

6. The electromagnetic wave absorber according to any one of claims 1 to 5, wherein the first layer does not contain pulp.

7. The electromagnetic wave absorber according to any one of claims 1 to 6, wherein the absorption frequency band of the electromagnetic wave is 2 GHz or more and 18 GHz or less.

8. The electromagnetic wave absorber according to any one of claims 1 to 7, wherein the second layer is a pulp layer composed of paper.

9. A step of forming a plurality of first layers including carbon nanotubes, A step of forming a plurality of second layers including pulp and not including carbon nanotubes, A step of alternately laminating the first layer and the second layer, comprising, A method for manufacturing an electromagnetic wave absorber, wherein the number of the first layers is 4 or more and 20 or less.

10. The step of forming the first layer is, A step of mixing carbon nanotubes, sodium carboxymethyl cellulose, and water to prepare a mixed solution, A step of dispersing the carbon nanotubes contained in the mixed solution by a water counter-collision method to prepare a dispersion, A step of drying the dispersion, The method for manufacturing an electromagnetic wave absorber according to claim 9, comprising.

11. Including a step of determining the number of the first layers laminated in the step of laminating the first layer and the second layer based on information on the relationship between the number of the first layers and the absorption frequency band of the electromagnetic wave obtained in advance, The absorption frequency band is 2 GHz or more and 18 GHz or less, In the step of laminating the first layer and the second layer, the number of the first layers determined in the step of determining the number of the first layers is laminated. The method for manufacturing an electromagnetic wave absorber according to claim 9 or 10.

12. The method for manufacturing an electromagnetic wave absorber according to claim 11, wherein the absorption frequency band is 8 GHz or more and 16 GHz or less.

13. The method for manufacturing an electromagnetic wave absorber according to any one of claims 9 to 12, wherein the second layer is a pulp layer made of paper.

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