Electromagnetic wave absorber

Abstract

To provide an electromagnetic wave absorber that can improve electromagnetic wave absorption performance. [Solution] The electromagnetic wave absorber according to the present invention includes a plurality of sheets comprising glass fibers and a conductive carbon material, the plurality of sheets being laminated, the first sheet of the plurality of sheets being provided on the incident surface side to which electromagnetic waves are incident, and the surface resistivity of the first sheet is higher than that of the second sheet.

Description

Technical Field

[0001] The present invention relates to an electromagnetic wave absorber.

Background Art

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

[0003] For example, Patent Document 1 describes an electromagnetic wave absorber provided with a surface material on the surface side intended for the incidence of radio waves. In Patent Document 1, the surface material is made of a polymer material containing glass fibers.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the electromagnetic wave absorber as described above, it is desired to improve the electromagnetic wave absorption performance.

[0006] One of the objects according to some aspects of the present invention is to provide an electromagnetic wave absorber capable of improving the electromagnetic wave absorption performance.

Means for Solving the Problems

[0007] One aspect of the electromagnetic wave absorber according to the present invention is including a plurality of sheets containing glass fibers and a conductive carbon material, the plurality of sheets being laminated, Among the plurality of sheets, the first sheet is provided on the incident surface side where electromagnetic waves are incident, rather than the second sheet among the plurality of sheets. The surface resistivity of the first sheet is higher than that of the second sheet.

[0008] In one aspect of the electromagnetic wave absorber according to the present invention, The difference between the surface resistivity of the first sheet and the surface resistivity of the second sheet may be 100 Ω / sq or more.

[0009] [[ID=...]] In one aspect of the electromagnetic wave absorber according to the present invention, The surface resistivity of the plurality of sheets may decrease as they move away from the incident surface.

[0010] In one aspect of the electromagnetic wave absorber according to the present invention, Among the plurality of sheets, the third sheet is adjacent to the first sheet. Among the plurality of sheets, the fourth sheet is adjacent to the second sheet. The surface resistivity of the third sheet is the same as that of the first sheet. The surface resistivity of the fourth sheet may be the same as that of the second sheet.

[0011] In one aspect of the electromagnetic wave absorber according to the present invention, The surface resistivity of each of the plurality of sheets may be 100 Ω / sq or more and 20000 Ω / sq or less. ... and may be.

[0012] In one aspect of the electromagnetic wave absorber according to the present invention, The average fiber diameter of the glass fiber is 0.05 μm or more and 20 μm or less. The glass fiber includes submicron fibers having an average fiber diameter of 0.05 μm or more and 1.0 μm or less. When the glass fiber contained in the electromagnetic wave absorber is 100 parts by mass, the content of the submicron fiber may be 1.0 part by mass or more.

[0013] In one embodiment of the electromagnetic wave absorber according to the present invention, The thickness may be 3.0 mm or more.

[0014] In one embodiment of the electromagnetic wave absorber according to the present invention, The carbon material may be carbon nanotubes. [Effects of the Invention]

[0015] The electromagnetic wave absorber according to the present invention includes a plurality of sheets containing glass fibers and a conductive carbon material, the plurality of sheets being laminated, the first sheet of the plurality of sheets being located on the incident surface side to which electromagnetic waves are incident, and the surface resistivity of the first sheet is higher than that of the second sheet. Therefore, the electromagnetic wave absorber according to the present invention can improve electromagnetic wave absorption performance. [Brief explanation of the drawing]

[0016] [Figure 1] A schematic cross-sectional view showing an electromagnetic wave absorber according to this embodiment. [Figure 2] A flowchart illustrating the manufacturing method of the electromagnetic wave absorber according to this embodiment. [Figure 3] A diagram illustrating the free-space method used to evaluate electromagnetic wave absorption performance. [Figure 4] A table showing the evaluation results for Examples 1-4 and Comparative Examples 1-4. [Modes for carrying out the invention]

[0017] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. The embodiments described below are not intended to unduly limit the scope of the present invention as described in the claims. Furthermore, not all of the configurations described below are necessarily essential components of the present invention.

[0018] 1. Electromagnetic wave absorber 1.1. Ingredients First, the electromagnetic wave absorber according to this embodiment will be described with reference to the drawings. Figure 1 is a schematic cross-sectional view showing the electromagnetic wave absorber 100 according to this embodiment.

[0019] The electromagnetic wave absorber 100 includes a plurality of sheets 10, as shown in Figure 1. Each sheet 10 includes, for example, glass fibers, a carbon material, and a dispersant.

[0020] 1.1.1. Fiberglass The average fiber diameter of the glass fibers contained in the electromagnetic wave absorber 100 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 fibers is S total It is calculated from the following formula (1), where ρ is the specific surface area of ​​the raw cotton made of glass fibers and ρ is the density of the raw cotton.

[0021] d = 4 / (S total ×ρ)···(1)

[0022] Note that the specific surface area S total (m 2 The density ρ ( / g) is the specific surface area measured by the Brunauer Emmett Teller (BET) method using nitrogen adsorption, and is measured, for example, by an automated specific surface area analyzer (Micromeritics' "TriStarII3020"). The density ρ is, for example, 2.49 g / cm³. 3 That is the case.

[0023] Glass fibers include, for example, submicron fibers. The electromagnetic wave absorber 100 includes a plurality of glass fibers, some of which are submicron fibers. Submicron fibers are glass fibers with an average fiber diameter of 0.05 μm or more and 1.0 μm or less. The average fiber diameter of the submicron fibers 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.

[0024] When the glass fibers contained in the electromagnetic wave absorber 100 are 100 parts by mass, the submicron fiber content 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 submicron fiber content is 1.0 part by mass or more, the texture of the electromagnetic wave absorber 100 can be improved. If the submicron fiber content is 95 parts by mass or less, the strength of the electromagnetic wave absorber 100 can be ensured.

[0025] The number-average fiber length of the glass fibers is, for example, 0.01 mm to 50 mm, preferably 0.03 mm to 5 mm, and more preferably 0.05 mm to 1 mm. If the number-average fiber length of the glass fibers is 0.01 mm or more, the strength of the electromagnetic wave absorber can be improved. If the number-average fiber length of the glass fibers is 50 mm or less, the density uniformity of the electromagnetic wave absorber can be improved. The number-average fiber length of the glass fibers is measured, for example, by a Diamscope or Diamlength manufactured by Cottonscorpe.

[0026] The glass fiber content in the electromagnetic wave absorber 100 is, for example, 80% by mass or more and 99.9% by mass or less. If the glass fiber content is 80% by mass or more, the flame retardancy of the electromagnetic wave absorber 100 can be improved. The glass fiber content is measured, for example, by thermogravimetric analysis (TGA).

[0027] Glass fibers constitute a sheet-like substrate. The substrate has a shape in which the length in the in-plane direction (the direction perpendicular to the thickness direction) is sufficiently longer than the thickness direction. The substrate has a porous structure. Therefore, when the substrate is impregnated with a dispersion liquid containing dispersed carbon material, the dispersion liquid can be uniformly absorbed into the substrate in the thickness direction.

[0028] The glass fiber sheet as the base material can be manufactured by a general wet papermaking method. Examples of the wet papermaking method include a method in which a certain amount of a slurry in which glass fibers are dispersed in an aqueous dispersion medium is further diluted with the aqueous dispersion medium and then lifted onto a wire mesh such as a wire gauze, and this wet paper sheet is dried by a dryer to form a sheet. As an industrially mass production method, there is a method in which the dispersion slurry is continuously lifted by a fourdrinier paper machine, a cylinder paper machine, or an inclined paper machine, and this wet paper sheet is dried by a hot air dryer, an infrared dryer, a drum dryer, etc., and the dried sheet is wound up. The aqueous dispersion medium is, for example, water or an aqueous sulfuric acid solution. Further, the aqueous dispersion medium may contain various auxiliaries such as a pH adjuster and an antifoaming agent.

[0029] Further, when dispersing glass fibers in water, a slurry added with a conductive carbon material or its dispersion may be wet-papered. The timing of adding the conductive carbon material or its dispersion may be a method of impregnating the glass fibers before water dispersion, or the glass fibers and the conductive carbon material or its dispersion may be simultaneously put into water and dispersed, and the order is not particularly limited. Therein.

[0030] The thickness of one sheet of the base material is, for example, 0.1 mm or more and 20 mm or less, preferably 0.3 mm or more and 10 mm or less, and more preferably 0.5 mm or more and 5.0 mm or less. If the thickness of the base material is 0.1 mm or more, the strength of the base material can be improved. If the thickness of the base material is 20 mm or less, when the base material is impregnated with the dispersion liquid, the dispersion liquid can penetrate uniformly in the thickness direction of the base material. The thickness of the base material is measured, for example, by an air-type off-line thickness measuring device. ⇒ The upper limit of the thickness has been changed.

[0031] The mass of the base material is, for example, 20 g / m 2 or more and 4000 g / m 2 or less, preferably 60 g / m 2 or more and 3000 g / m 2 or less, and more preferably 100 g / m<​​​​If the mass of the base material is 4000 g / m², the strength of the base material can be improved. 2 The following conditions can be met to reduce weight:

[0032] The base material may contain inorganic fibers other than organic fibers and glass fibers. Examples of organic fibers include polypropylene fibers, acrylic fibers, vinylon fibers, cellulose fibers, polyester fibers, and aramid fibers. Examples of inorganic fibers include rock fibers, metal fibers, alumina fibers, ceramic fibers, and silica fibers.

[0033] 1.1.2. Carbon Materials The carbon material contained in the electromagnetic wave absorber 100 is conductive. The carbon material imparts conductivity to the electromagnetic wave absorber 100. Examples of carbon materials include carbon nanotubes (hereinafter also referred to as "CNTs").

[0034] Examples of carbon nanotubes (CNTs) include single-walled carbon nanotubes (SWNTs), which are formed by winding a single six-membered ring network (graphene sheet) made of carbon into a cylindrical shape, and multi-walled carbon nanotubes (MWNTs), which are formed by winding multiple graphene sheets concentrically. The electromagnetic wave absorber 100 may contain only SWNTs or MWNTs, or it may contain both. Both ends of the CNTs may be closed or open.

[0035] CNTs are fabricated by methods such as arc discharge, laser ablation, and CVD (Chemical Vapor Deposition). These methods allow CNTs to be fabricated to a predetermined size.

[0036] The diameter of the CNTs is, for example, between 1 nm and 100 nm, preferably between 5 nm and 50 nm, and more preferably between 7 nm and 20 nm. A CNT diameter between 1 nm and 100 nm improves the dispersibility of the CNTs in the dispersion. The diameter of the CNTs is measured, for example, by a Scanning Electron Microscope (SEM).

[0037] The fiber length of the CNTs is, for example, 0.5 μm to 80 μm, preferably 1.0 μm to 60 μm, and preferably 10 μm to 50 μm. If the fiber length of the CNTs is 0.5 μm to 80 μm, the dispersibility of the CNTs in the dispersion can be improved. The fiber length of the CNTs is measured, for example, by SEM. Note that "CNT fiber length" refers to the length of the CNTs when they are bundled together by van der Waals forces, and is the length of the CNTs before they are dispersed in the solvent.

[0038] The BET specific surface area of ​​CNT is, for example, 50m². 2 / g or more 500m 2 It is less than or equal to / g, preferably 100m 2 / g or more 300m 2 The BET specific surface area of ​​CNT is 50m². 2 / g or more 500m 2 If the value is less than / g, the dispersibility of CNTs in the dispersion can be improved. .

[0039] The carbon nanotubes (CNTs) are attached to the glass fibers. The CNTs may also be attached to the glass fibers via a dispersant. The amount of CNTs attached to the glass fibers is, for example, 0.1 g / m². 2 More than 10g / m 2 The following, preferably 0.3 g / m 2 More than 7.0g / m 2 The following, and more preferably 0.5 g / m 2 More than 5.0g / m 2 The following, and more preferably 0.7 g / m 2 More than 3.0g / m 2The following is the result: The amount of CNTs attached is 0.1 g / m 2 If the above conditions are met, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. The amount of CNT attached is 10g / m 2 The following conditions can suppress the reduction in electromagnetic wave absorption performance caused by the reflection of electromagnetic waves from the surface of the electromagnetic wave absorber 100. The CNT content is measured, for example, by TGA.

[0040] Furthermore, the conductive carbon material included in the electromagnetic wave absorber 100 is not limited to CNTs, but may be, for example, carbon black or graphite. For example, using carbon black as the carbon material can reduce costs compared to using CNTs. In addition, multiple types of materials may be used as the carbon material. For example, CNTs and carbon black may be used as the carbon material.

[0041] 1.1.3. Dispersant The dispersant contained in the electromagnetic wave absorber 100 disperses the carbon material in the solvent in the dispersion liquid used to manufacture the electromagnetic wave absorber 100. The dispersant is attached to the glass fibers. The dispersant may also be attached to the glass fibers via the carbon material. The amount of dispersant attached to the glass fibers is, for example, 0.1 g / m 2 More than 10g / m 2 The following, preferably 0.3 g / m 2 More than 7.0g / m 2 The following, and more preferably 0.5 g / m 2 More than 5.0g / m 2 The following, and more preferably 0.7 g / m 2 More than 3.0g / m 2 The following applies: The amount of dispersant attached is 0.1 g / m². 2 The above conditions can improve the dispersibility of carbon materials in the dispersion. Dispersant adhesion amount: 10 g / m 2 The viscosity of the dispersion can be reduced if the following conditions are met: The amount of dispersant adhering to the glass fibers may be the same as the amount of carbon material adhering to the glass fibers. The dispersant content is measured, for example, by TGA.

[0042] In the dispersion, the mass M of the carbon material C The mass M of the dispersant relative to the dispersant DISP Ratio M DISP / M C The ratio is 1 / 5 to 5 (carbon material:dispersant = 5:1 to 1:5), preferably 1 / 4 to 4 (carbon material:dispersant = 4:1 to 1:4), and more preferably 1 / 3 to 3 (carbon material:dispersant = 3:1 to 1:3). DISP / M C If the ratio is 1 / 5 or more, it is possible to enhance the properties specific to carbon materials, such as electromagnetic wave absorption performance. DISP / M C If the value is 5 or less, the dispersibility of the carbon material in the dispersion can be improved.

[0043] The weight-average molecular weight of the dispersant is, for example, 5,000 to 1,000,000, preferably 6,000 to 800,000, and more preferably 7,000 to 200,000. If the weight-average molecular weight of the dispersant is 5,000 or more, the dispersant can easily entangle with the carbon material, improving the dispersibility of the carbon material in the dispersion. However, if the weight-average molecular weight is too high, the dispersibility will worsen, so it is preferable that the molecular weight of the dispersant be 1,000,000 or less. Note that "weight-average molecular weight" refers to the weight-average molecular weight in polystyrene terms measured by gel permeation chromatography (GPC).

[0044] Examples of dispersants include cellulose-based polymers and polyacrylic acid. Examples of cellulose-based polymers include carboxymethylcellulose salts such as sodium carboxymethylcellulose.

[0045] 1.1.4. Other Additives The electromagnetic wave absorber 100 may further contain additives such as binder resins, water repellents, surfactants, defoamers, fine fibers, and fine particles, as needed.

[0046] 1.2. Sheet arrangement and physical properties, etc. As shown in Figure 1, the electromagnetic wave absorber 100 is composed of, for example, multiple sheets 10. The multiple sheets 10 are laminated together. Adjacent sheets 10 are bonded to each other, for example, via an adhesive layer (not shown). The material of the adhesive layer is, for example, 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 substrate.

[0047] The number of sheets 10 is, for example, 2 to 30, preferably 3 to 20. In the illustrated example, there are 6 sheets 10, designated as sheets 10a, 10b, 10c, 10d, 10e, and 10f. The shapes of the multiple sheets 10 are, for example, the same. The density of glass fibers in the multiple sheets 10 is, for example, the same.

[0048] Sheet 10a constitutes the incident surface 20 of the electromagnetic wave absorber 100. Electromagnetic waves are incident on the incident surface 20 from the outside. Sheet 10f constitutes the back surface 22 of the electromagnetic wave absorber 100. The back surface 22 is the side that is attached to an object (such as a wall) from which electromagnetic wave intrusion is to be reduced. Although not shown in the figures, a metal layer may be provided on the back surface 22. Examples of the metal layer include a metal plate and a metal-deposited film.

[0049] Sheets 10a, 10b, 10c, 10d, 10e, and 10f are arranged in this order from the incident surface 20 toward the back surface 22. Sheet 10a is located closer to the incident surface 20 than sheets 10b, 10c, 10d, 10e, and 10f. Sheets 10a and 10b are adjacent to each other. Sheets 10b and 10c are adjacent to each other. Sheets 10c and 10d are adjacent to each other. Sheets 10d and 10e are adjacent to each other. Sheets 10e and 10f are adjacent to each other.

[0050] The surface resistivity of sheet 10 is, for example, 100 Ω / □ or more and 20,000 Ω / □ or less, preferably 300 Ω / □ or more and 18,000 Ω / □ or less, and more preferably 500 Ω / □ or more and 16,000 Ω / □ or less. Surface resistivity is related to electromagnetic wave absorption performance, and basically, the lower the surface resistivity, the higher the electromagnetic wave absorption performance. However, if the surface resistivity is less than 100 Ω / □, electromagnetic waves are reflected at the surface of the sheet, and the electromagnetic wave absorption performance decreases. If the surface resistivity of sheet 10 is 100 Ω / □ or more and 20,000 Ω / □ or less, the electromagnetic wave absorber 100 can have high electromagnetic wave absorption performance. The surface resistivity of sheet 10 is measured, for example, using a predetermined measuring instrument in accordance with "JIS K 7194:1994".

[0051] The surface resistivity of the multiple sheets 10 increases as they move away from the incident surface 20. That is, the surface resistivity of sheet 10a is higher than that of sheets 10b, 10c, 10d, 10e, and 10f. The surface resistivity of sheet 10b is higher than that of sheets 10c, 10d, 10e, and 10f. The surface resistivity of sheet 10c is higher than that of sheets 10d, 10e, and 10f. The surface resistivity of sheet 10d is higher than that of sheets 10e and 10f. The surface resistivity of sheet 10e is higher than that of sheet 10f.

[0052] The difference in surface resistivity between adjacent sheets 10 is, for example, 100 Ω / □ or more and 20,000 Ω / □ or less, preferably 300 Ω / □ or more and 15,000 Ω / □ or less, and more preferably 500 Ω / □ or more and 10,000 Ω / □ or less. Within that range, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved.

[0053] Furthermore, if the surface resistivity of sheet 10a is higher than that of sheet 10c, and the surface resistivity of sheet 10c is higher than that of sheet e, then the surface resistivity of sheets 10a and 10b may be the same, the surface resistivity of sheets 10c and 10d may be the same, and the surface resistivity of sheets 10e and 10f may be the same.

[0054] The density of sheet 10 is, for example, 0.05 g / cm³. 3 More than 0.5g / cm 3 The following, preferably 0.1 g / cm³ 3 More than 0.3g / cm 3 The following, and more preferably 0.15 g / cm³ 3 More than 0.2g / cm 3 The following applies: The density of sheet 10 is 0.05 g / cm³. 3 If the above conditions are met, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. The density of sheet 10 is 0.5 g / cm³. 3 The possibility of cracks occurring in the electromagnetic wave absorber can be reduced if the following conditions are met. The density of sheet 10 is calculated, for example, from the area in square meters and the mass.

[0055] The thickness of the electromagnetic wave absorber 100 is, for example, 3.0 mm or more and 20 mm or less, preferably 3.2 mm or more and 15 mm or less, and more preferably 3.4 mm or more and 10 mm or less. If the thickness of the electromagnetic wave absorber 100 is 3.0 mm or more, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. If the thickness of the electromagnetic wave absorber 100 is 20 mm or less, the thickness of the electromagnetic wave absorber 100 can be reduced.

[0056] 1.3. Effects The electromagnetic wave absorber 100 includes a plurality of sheets 10 containing glass fibers and a conductive carbon material. The plurality of sheets 10 are laminated, and sheet 10a, which is the first sheet among the plurality of sheets 10, is positioned on the incident surface 20 side to which electromagnetic waves are incident, rather than sheet 10c, which is the second sheet among the plurality of sheets 10. The surface resistivity of sheet 10a is higher than the surface resistivity of sheet 10c. Therefore, the electromagnetic wave absorber 100 can improve its electromagnetic wave absorption performance. For example, if the surface resistivity of the first sheet is lower than that of the second sheet, the incident electromagnetic waves will be reflected by the surface of the first sheet, and the electromagnetic waves will not be absorbed by the second sheet, resulting in a decrease in electromagnetic wave absorption performance. Furthermore, because the electromagnetic wave absorber 100 contains glass fibers, it can be flame-retardant. The electromagnetic wave absorber 100 may be used outdoors, and flame retardancy is desirable.

[0057] In the electromagnetic wave absorber 100, the difference between the surface resistivity of sheet 10a and the surface resistivity of sheet 10c may be 100 Ω / □ or greater. If the difference in the surface resistivity of sheets 10a and 10c is 100 Ω / □ or greater, the difference in surface resistivity of sheets 10a and 10c can be increased, thereby improving the electromagnetic wave absorption performance of the electromagnetic wave absorber 100.

[0058] In the electromagnetic wave absorber 100, the surface resistivity of the multiple sheets 10 may decrease as they move away from the incident surface 20. If the surface resistivity decreases as it moves away from the incident surface 20, for example, the reflection of electromagnetic waves from the surface of the electromagnetic wave absorber 100 can be suppressed compared to a case where the surface resistivity increases as it moves away from the incident surface, thereby improving the electromagnetic wave absorption performance of the electromagnetic wave absorber 100.

[0059] In the electromagnetic wave absorber 100, sheet 10b, which is the third sheet among the multiple sheets 10, is adjacent to sheet 10a, and sheet 10d, which is the fourth sheet among the multiple sheets 10, is adjacent to sheet 10c. The surface resistivity of sheet 10b may be the same as that of sheet 10a, and the surface resistivity of sheet 10d may be the same as that of sheet 10c. With such a configuration, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved.

[0060] In the electromagnetic wave absorber 100, the surface resistivity of each of the multiple sheets 10 may be between 100 Ω / □ and 20,000 Ω / □. If the surface resistivity of the sheets 10 is within this range, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved.

[0061] In the electromagnetic wave absorber 100, 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. When the amount of glass fibers contained in the electromagnetic wave absorber 100 is 100 parts by mass, the submicron fiber content may be 1.0 part by mass or more. If the submicron fiber content is within this range, the strength of the substrate can be ensured while improving the texture of the substrate. By improving the texture of the substrate, when the substrate is impregnated with a dispersion liquid containing dispersed carbon material, the carbon material can be uniformly attached to the glass fibers.

[0062] The electromagnetic wave absorber 100 may have a thickness of 3 mm or more. A thickness of 3 mm or more can improve the electromagnetic wave absorption performance of the electromagnetic wave absorber 100.

[0063] In the electromagnetic wave absorber 100, the carbon material may be CNTs. If the carbon material is CNTs, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. Furthermore, the strength of the electromagnetic wave absorber 100 can be improved.

[0064] 2. Method for manufacturing electromagnetic wave absorbers 2.1. Overall Flow Next, the method for manufacturing the electromagnetic wave absorber 100 according to this embodiment will be described with reference to the drawings. Figure 2 is a flowchart for explaining the method for manufacturing the electromagnetic wave absorber 100 according to this embodiment.

[0065] As shown in Figure 2, the method for manufacturing the electromagnetic wave absorber 100 includes, for example, a dispersion preparation step (step S1) for preparing a dispersion in which a conductive carbon material is dispersed; an impregnation step (step S2) for impregnating a substrate made of glass fibers into the dispersion; a sheet preparation step (step S3) for drying the substrate impregnated in the dispersion to produce a sheet 10; and a lamination step (step S4) for laminating the sheets 10. The following describes the case in which carbon nanotubes (CNTs) are used as the carbon material.

[0066] 2.2. Dispersion preparation process (Step S1) In the dispersion preparation process, CNTs and a dispersant are added to water as a solvent, and the CNTs, dispersant, and water are mixed to prepare a mixture. The mixing of CNTs, dispersant, and water is performed, for example, by a homogenizer. The homogenizer may be an ultrasonic type that causes cavitation with ultrasound, an agitator type that stirs the mixture, or a pressure type that applies pressure to the mixture. By performing the homogenization process, the aggregation of CNTs can be reduced. This allows the underwater counter-impact method described later to be carried out smoothly.

[0067] In the mixture, the CNT content is, for example, 0.05% by mass or more and 10% by mass or less, preferably 1% by mass or more and 5% by mass or less. If the CNT content is 0.05% by mass or more, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. If the CNT content is 10% by mass or less, the viscosity of the dispersion can be reduced. If the viscosity of the dispersion is too high, the nozzle holes in the wet atomization device described later may become clogged. In the mixture, the dispersant content is, for example, the same as the CNT content.

[0068] The water in the mixture may be pure water such as ion-exchanged water, ultrafiltered water, reverse osmosis water, and distilled water, or ultrapure water from which ionic impurities have been removed as much as possible, or tap water may be used. It may be present. The mixture uses water as a solvent, making it more environmentally friendly than when an organic solvent is used. The mixture may contain only CNTs, a dispersant, and water. In other words, the mixture may consist only of CNTs, a dispersant, and water.

[0069] Next, the CNTs contained in the mixture are dispersed using a wet atomization device, for example, one that has a ball impact chamber, by an underwater counter-impact method. Specifically, the mixture is sprayed from the nozzle of the wet atomization device and impacted against ceramic balls. The CNTs contained in the mixture are dispersed by the shear force as they pass through the nozzle holes, cavitation caused by spraying into the liquid, and the impact force when the CNTs collide with the ceramic balls.

[0070] The diameter of the nozzle hole is, for example, 100 μm or more and 600 μm or less, preferably 120 μm or more and 350 μm or less. If the diameter of the nozzle hole is 100 μm or more, clogging of the nozzle hole with the mixed liquid can be suppressed. If the diameter of the nozzle hole is 600 μm or less, the collision energy of the mixed liquid can be increased.

[0071] The pressure applied to the mixed liquid sprayed from the nozzle is, for example, 100 MPa to 300 MPa, preferably 150 MPa to 250 MPa. If the pressure is 100 MPa or higher, the collision energy of the mixed liquid can be increased. If the pressure is 300 MPa or lower, it is possible to suppress the collision energy from being too high, which would cause the CNT fibers to break and prevent the desired properties from being obtained.

[0072] The processing rate of the mixed liquid sprayed from the nozzle is, for example, 20 L / hr to 400 L / hr, preferably 25 L / hr to 100 L / hr. If the processing rate is 20 L / hr or more, the processing time in the wet atomization device can be shortened. If the processing rate is 400 L / hr or less, clogging of the nozzle holes with the mixed liquid can be suppressed.

[0073] In a wet atomization apparatus, the number of passes of the mixed liquid is, for example, 1 to 10 times, preferably 2 to 5 times. If the number of passes is 10 or less, it is possible to suppress the breakage of CNT fibers due to collisions with the mixed liquid, which can prevent the acquisition of desired properties. Furthermore, it is possible to shorten the manufacturing time and save energy.

[0074] Note that "number of passes of the mixed liquid in the wet atomization device" refers to the number of times the mixed liquid is circulated in the wet atomization device. For example, "2 passes" means that the mixed liquid is circulated twice so that a CNT that has already collided with a ceramic ball will collide with the ceramic ball again. Thus, the number of passes corresponds to the number of times the mixed liquid sprayed from the nozzle hole collides with the ceramic ball. The number of passes is proportional to the processing time in the wet atomization device. If the processing time in the wet atomization device is long, the number of times the mixed liquid is circulated increases.

[0075] The CNT content in the dispersion can be adjusted by diluting it with water as appropriate. For example, the CNT content in the dispersion is 0.05% by mass or more and 1% by mass or less, preferably 0.1% by mass or more and 0.6% by mass or less. If the CNT content is 0.05% by mass or more, the amount of CNTs adhering to the electromagnetic wave absorber 100 can be increased, thereby improving the electromagnetic wave absorption performance. If the CNT content is 1% by mass or less, the possibility of the amount of CNTs adhering too much, which would lower the electrical resistance of the electromagnetic wave absorber 100 too much and impair the electromagnetic wave absorption performance can be reduced.

[0076] As a result, a dispersion containing carbon material can be prepared.

[0077] Furthermore, if CNTs can be dispersed in water, the form of the wet atomization apparatus is not particularly limited. It is not possible. Alternatively, CNTs may be dispersed in water without using a wet atomization device.

[0078] Furthermore, although the above example described an example where the solvent of the dispersion is water, any solvent can be used instead of water as long as the CNTs can be dispersed in the solvent.

[0079] 2.3. Impregnation process (Step S2) In the impregnation process, the substrate is impregnated with a dispersion. Multiple substrates are formed from raw cotton made of glass fibers, for example, by the flame method or centrifugal method. Methods of impregnation include, for example, using bar coating, curtain coater, die coater, spray coating, etc., to allow the dispersion to soak into the substrate, or impregnating the entire substrate with the dispersion. Through this process, CNTs adhere to the glass fibers of the substrate.

[0080] 2.4. Sheet manufacturing process (Step S3) In the sheet manufacturing process, a substrate impregnated in a dispersion is dried to produce multiple sheets 10. The drying method is not particularly limited as long as the water contained in the dispersion is evaporated, but examples include hot air drying, infrared drying, and natural drying. When using a drying oven, the temperature of the drying oven is, for example, 70°C to 160°C, preferably 100°C to 140°C. If the temperature of the drying oven is 70°C or higher, the water can be sufficiently evaporated. If the temperature of the drying oven is 160°C or lower, the melting of the glass fibers can be suppressed.

[0081] 2.5. Lamination process (Step S4) In the lamination process, multiple sheets 10 are laminated together. Specifically, multiple sheets 10 are laminated together by bonding them together via an adhesive layer.

[0082] By following the above process, the electromagnetic wave absorber 100 can be manufactured.

[0083] Although it is possible to stack multiple substrates and then immerse the stacked substrates in the dispersion before impregnating them with the dispersion, it is preferable to impregnate the substrates with the dispersion before stacking them, considering the need to uniformly adhere the CNTs to the substrates.

[0084] Furthermore, when carbon black is used instead of CNTs, the CB content in the mixture is, for example, 0.5% by mass or more and 10% by mass or less, preferably 1.0% by mass or more and 5.0% by mass or less. If the CB content is 0.5% by mass or more, the electromagnetic wave absorption performance of the electromagnetic wave absorber 100 can be improved. If the CB content is 10% by mass or less, the viscosity of the dispersion can be reduced.

[0085] 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.

[0086] 3.1. Sheet preparation <Sheet 1> A mixture was prepared by mixing carbon material, a dispersant, and water. A homogenizer, "Biomixer BM-2," manufactured by Nippon Seiki Seisakusho Co., Ltd., was used for mixing. The mixing process time was set to 5 minutes.

[0087] Carbon nanotubes (CNTs) were used as the carbon material. Specifically, LG Chem's "LUCAN.BT1003M" was used as the CNT. This CNT is a MWCNT with a diameter of 8 nm to 17 nm, a fiber length of 10 μm to 50 μm (bundle), and a BET specific surface area of ​​165 m². 2 / g~205m 2 The value is / g. The CNT content in the mixture was set to 3.0% by mass.

[0088] As a dispersant, carboxymethylcellulose (hereinafter also referred to as "CMC") was used. Specifically, "Selogen 5A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd. was used as the CMC. Its weight-average molecular weight is 15,000. Its degree of etherification is 0.7. The CMC content in the mixture was set to 3.0% by mass.

[0089] The carbon material contained in the above mixture was dispersed using a wet atomization apparatus equipped with a ball impact chamber. The wet atomization apparatus used was the "Starburst Lab" wet atomization apparatus (model name: HJP-25005) manufactured by Sugino Machine Co., Ltd. The diameter of the nozzle hole from which the mixture was sprayed was set to 0.17 mm, and the pressure applied to the mixture was set to 200 MPa, causing the mixture to impact ceramic balls. The wet atomization apparatus passed the mixture four times. As a result, a dispersion containing carbon material, a dispersant, and water was prepared.

[0090] Sixty parts by mass of glass fibers with an average fiber diameter of 0.65 μm, forty parts by mass of glass fibers with an average fiber diameter of 2.44 μm, and an aqueous dispersion medium pre-mixed with dilute sulfuric acid to pH 3 were placed in a disintegrating pulper, and the raw materials were dispersed to a concentration of 1% by mass to prepare a raw material slurry. Next, the raw material concentration was diluted to 0.1% by mass with water adjusted to pH 3 with concentrated sulfuric acid, and continuous papermaking was performed on an inclined paper machine, followed by a drying process to produce a sheet-like substrate. "B-06-F" manufactured by Unifrax Corporation was used as the glass fiber with an average fiber diameter of 0.65 μm. "B-26-R" manufactured by Unifrax Corporation was used as the glass fiber with an average fiber diameter of 2.44 μm.

[0091] The above substrate was impregnated by diluting a 3.0% dispersion (CNT: 3.0% by mass, CM: 3.0% by mass) to 0.1% (CNT: 0.1% by mass, CMC: 0.1% by mass) and immersing it in the diluted dispersion. After that, the excess dispersion was removed by vacuum, and the removed substrate was heat-dried at 130°C to produce Sheet 1. The surface resistivity of Sheet 1 was 12000 Ω / □. The surface resistivity was measured in accordance with "JIS K 7194" using a "Loresta-AX MCP-T370" (4 terminals) manufactured by Mitsubishi Chemical Analytec Corporation.

[0092] <Sheet 2> Sheet 2 was prepared in the same manner as Sheet 1 described above, except that the dispersion was diluted to 0.2% (CNT: 0.2% by mass, CMC: 0.2% by mass). The surface resistivity of Sheet 2 was 4000 Ω / □.

[0093] <Sheet 3> Sheet 3 was prepared in the same manner as Sheet 1 described above, except that the dispersion was diluted to 0.3% (CNT: 0.3% by mass, CMC: 0.3% by mass). The surface resistivity of Sheet 3 was 2000 Ω / □.

[0094] <Sheet 4> Sheet 4 was prepared in the same manner as Sheet 1 described above, except that the dispersion was diluted to 0.4% (CNT: 0.4% by mass, CMC: 0.4% by mass). The surface resistivity of Sheet 4 was 1000 Ω / □.

[0095] <Sheet 5> Sheet 5 was prepared in the same manner as Sheet 1 described above, except that the dispersion was diluted to 0.5% (CNT: 0.5% by mass, CMC: 0.5% by mass). The surface resistivity of Sheet 5 was 600 Ω / □.

[0096] <Sheet 6> Sheet 6 was prepared in the same manner as Sheet 1 described above, except that carbon black (CB) was used as the carbon material, and the CB content in the dispersion was set to 1.0 mass% and the CMC content in the dispersion was also set to 1.0 mass%. The surface resistivity of Sheet 6 was 14000 Ω / □. Asahi Carbon Co., Ltd.'s "Asahi F-200GS" was used as the CB.

[0097] <Sheet 7> Sheet 7 was prepared in the same manner as Sheet 6 described above, except that the CB content in the dispersion was 2.0% by mass and the CMC content in the dispersion was 2.0% by mass. The surface resistivity of Sheet 7 was 5000 Ω / □.

[0098] <Sheet 8> Sheet 8 was prepared in the same manner as Sheet 6 described above, except that the CB content in the dispersion was 3.0% by mass and the CMC content in the dispersion was 3.0% by mass. The surface resistivity of Sheet 8 was 1800 Ω / □.

[0099] <Sheet 9> Sheet 9 was prepared in the same manner as Sheet 6 described above, except that the CB content in the dispersion was 4.0% by mass and the CMC content in the dispersion was 4.0% by mass. The surface resistivity of Sheet 9 was 900 Ω / □.

[0100] 3.2. Sample Preparation <Example 1> A total of four sheets—one sheet 1, one sheet 2, one sheet 3, and one sheet 4—were laminated together in the order of sheet 1, sheet 2, sheet 3, and sheet 4 to prepare the sample for Example 1. Sheet 1 constitutes the incident surface for electromagnetic waves. Franklin's woodworking adhesive "Titebond III Ultimate" was used as the adhesive for bonding.

[0101] <Example 2> A total of 10 sheets—two sheets 1, two sheets 2, two sheets 3, two sheets 4, and two sheets 5—were laminated together in the order of sheet 1, sheet 2, sheet 3, sheet 4, and sheet 5 to prepare the sample for Example 2. Sheet 1 constitutes the incident surface for electromagnetic waves.

[0102] <Example 3> A total of six sheets, three sheets of sheet 2 and three sheets of sheet 4, were laminated together in the order of sheet 2 followed by sheet 4 to prepare the sample for Example 3. Sheet 2 constitutes the incident surface for electromagnetic waves.

[0103] <Example 4> A total of four sheets—one sheet 6, one sheet 7, one sheet 8, and one sheet 9—were laminated together in the order of sheet 6, sheet 7, sheet 8, and sheet 9 to prepare the sample for Example 4. Sheet 6 constitutes the incident surface for electromagnetic waves.

[0104] <Comparative Example 1> Six sheets 1 were laminated together to create the sample for Comparative Example 1.

[0105] <Comparative Example 2> Six sheets 3 were laminated together to create the sample for Comparative Example 2.

[0106] <Comparative Example 3> Six sheets 5 were laminated together to create the sample for Comparative Example 3.

[0107] <Comparative Example 4> A total of six sheets—two sheets 5, two sheets 3, and two sheets 1—were laminated together in the order of sheet 5, sheet 3, and sheet 1 to prepare the sample for Comparative Example 4. Sheet 5 constitutes the incident surface for electromagnetic waves.

[0108] 3.3. Evaluation Method 3.3.1. Thickness 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.

[0109] 3.3.2. Electromagnetic wave absorption performance The electromagnetic wave absorption performance was assessed by measuring the return loss of the sample based on the free-space method. The measurement device used was the "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 inducing electromagnetic waves radiated from one of the antennas onto the sample. Figure 3 is a diagram illustrating the free-space method used to evaluate the electromagnetic wave absorption performance.

[0110] As shown in Figure 3, sample S was placed between antennas A1 and A2 of the measuring device. A metal plate M was set on the back of sample S. Next, electromagnetic waves were irradiated from antenna A1 toward 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 sample S to the amount of electromagnetic waves reflected (reflection attenuation) was derived as the amount of electromagnetic wave absorption. Sample S diffusely reflects electromagnetic waves internally, converting them into heat. As a result, the electromagnetic waves are attenuated. The electromagnetic waves irradiated from antenna A1 are plane waves, also called orthogonal polarization.

[0111] Specifically, the amount of electromagnetic wave absorption was calculated using the following formula (2).

[0112] Electromagnetic wave absorption amount = 10 × log(Pm / Pa) ... (2)

[0113] In equation (2), Pm is the amount of incident electromagnetic wave (in watts). Pa is the amount of reflected electromagnetic wave (in watts). The amount of electromagnetic wave absorption is expressed in decibels (dB). If the amount of electromagnetic wave absorption is -10 dB, 90% of the electromagnetic wave is absorbed; at -20 dB, 99% is absorbed; and at -30 dB, 99.9% is absorbed.

[0114] The measurement frequency bands for electromagnetic wave absorption, as an indicator of electromagnetic wave absorption performance, were set to 18 GHz to 26.5 GHz, 33 GHz to 50 GHz, and 75 GHz to 100 GHz.

[0115] 3.4. Evaluation Results Figure 4 is a table showing the evaluation results for Examples 1-4 and Comparative Examples 1-4. In Figure 4, the largest absolute value of electromagnetic wave absorption in each measurement frequency band is indicated as "MAX," and the smallest is indicated as "MIN."

[0116] As shown in Figure 4, Examples 1-4 showed higher electromagnetic wave absorption performance compared to Comparative Examples 1-4. In particular, in the measurement frequency bands of 18GHz to 26.5GHz and 33GHz to 50GHz, Examples 1 to 4 showed higher electromagnetic wave absorption performance compared to Comparative Examples 1 to 4.

[0117] As shown in Comparative Examples 1-3, even when multiple sheets with the same surface resistivity were laminated, the electromagnetic wave absorption performance was low. Also, as shown in Comparative Example 4, even when multiple sheets with different surface resistivity were laminated, the electromagnetic wave absorption performance was low when the surface resistivity of the sheet on the incident side was low.

[0118] 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.

[0119] 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. [Explanation of symbols]

[0120] 10, 10a, 10b, 10c, 10d, 10e, 10f...sheet, 20...incident surface, 22...back surface, 100...electromagnetic wave absorber

Claims

1. It includes multiple sheets containing glass fibers and conductive carbon material, The aforementioned multiple sheets are stacked, The first sheet among the plurality of sheets is provided on the incident surface side to which electromagnetic waves are incident, compared to the second sheet among the plurality of sheets. An electromagnetic wave absorber in which the surface resistivity of the first sheet is higher than the surface resistivity of the second sheet.

2. The electromagnetic wave absorber according to claim 1, wherein the difference between the surface resistivity of the first sheet and the surface resistivity of the second sheet is 100 Ω / □ or more.

3. The electromagnetic wave absorber according to claim 1 or 2, wherein the surface resistivity of the plurality of sheets decreases as they move away from the incident surface.

4. The third sheet among the aforementioned plurality of sheets is adjacent to the first sheet, The fourth sheet among the aforementioned plurality of sheets is adjacent to the second sheet, The surface resistivity of the third sheet is the same as the surface resistivity of the first sheet. The electromagnetic wave absorber according to claim 1 or 2, wherein the surface resistivity of the fourth sheet is the same as the surface resistivity of the second sheet.

5. The electromagnetic wave absorber according to claim 1 or 2, wherein the surface resistivity of each of the plurality of sheets is 100 Ω / □ or more and 20,000 Ω / □ or less.

6. 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. The electromagnetic wave absorber according to claim 1 or 2, wherein when the amount of glass fibers contained in the electromagnetic wave absorber is 100 parts by mass, the amount of submicron fibers is 1.0 part by mass or more.

7. An electromagnetic wave absorber according to claim 1 or 2, wherein the thickness is 3.0 mm or more.

8. The electromagnetic wave absorber according to claim 1 or 2, wherein the carbon material is a carbon nanotube.

Images