Electromagnetic wave absorber

Abstract

To provide a lightweight electromagnetic wave absorber excellent in electromagnetic wave absorption properties.SOLUTION: The electromagnetic wave absorber 1 includes: a flat base 2; and multiple protrusions 3 that are formed on one surface 1a of the base 2 in a thickness direction. The base 2 and each of the protrusions 3 include a matrix and carbon nanotubes dispersed in the matrix, in which the bulk density is 1.23 kg / m3 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic wave absorber, and more particularly to an electromagnetic wave absorber that is preferably used for absorbing electromagnetic waves, i.e., suppressing reflection. [Background technology]

[0002] Patent Document 1 discloses an electromagnetic wave absorber having a laminated structure in which a first layer made of a dielectric, a second layer having conductivity, a third layer made of a dielectric, and a fourth layer having conductivity are laminated in this order, wherein the sheet resistance of the second layer is 100 Ω / □ or more and 300 Ω / □ or less, and the fourth layer is an electromagnetic wave reflector. Note that □ means a unit cross-sectional area, and will hereinafter also be referred to as sq. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-102665 Summary of the Invention [Problem to be solved by the invention]

[0004] In the field of information and communications using electronic devices, weight reduction of carriers carrying electronic devices used in the aviation, space and terrestrial fields is an essential issue from the viewpoint of reducing fuel consumption.

[0005] In the above applications, absorbing electromagnetic waves generated by electronic devices is important for preventing malfunction of the electronic devices due to noise, controlling the path of electromagnetic waves when communicating between electronic devices using electromagnetic waves, and the like.

[0006] An object of the present disclosure is to provide an electromagnetic wave absorber that is lightweight yet has excellent electromagnetic wave absorption performance. [Means for solving the problem]

[0007] An electromagnetic wave absorber according to one aspect of the present disclosure includes a flat base and a plurality of protrusions formed on one surface of the base in a thickness direction, each of the base and the protrusions including a matrix and carbon nanotubes dispersed in the matrix, and having a bulk density of 1.23 kg / m 3 The following is the result. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, it is possible to provide an electromagnetic wave absorber that is lightweight yet has excellent electromagnetic wave absorption performance. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of an electromagnetic wave absorber according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a graph showing the measurement results of the electromagnetic wave absorption performance for the experimental example. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1. Embodiment An embodiment will be described. Note that the following embodiment is merely a part of various embodiments of the present disclosure. Furthermore, the following embodiment can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved. The figures referred to below are schematic diagrams, and the dimensional ratios of the components in the figures do not necessarily reflect the actual dimensional ratios.

[0011] The electromagnetic wave absorber 1 according to this embodiment comprises a flat base 2 and a plurality of protrusions 3 formed on one surface 1a in the thickness direction of the base 2. Each of the base 2 and the protrusions 3 has a matrix and carbon nanotubes dispersed in the matrix, and has a bulk density of 1.23 kg / m 3 The following is the result.

[0012] According to this embodiment, the bulk density of the electromagnetic wave absorber 1 is 1.23 kg / m 3or less, it is possible to reduce the weight of the electromagnetic wave absorber 1. Furthermore, by providing the protrusions 3 on the electromagnetic wave absorber 1, it is possible to improve the dielectric loss characteristics and the electromagnetic wave absorbing performance.

[0013] The configuration of the electromagnetic wave absorber 1 will now be described more specifically.

[0014] The electromagnetic wave absorber 1 comprises a flat base 2 and a plurality of protrusions 3 formed on one surface 1a of the base 2 in the thickness direction.

[0015] As described above, the electromagnetic wave absorber 1 has a bulk density of 1.23 kg / m 3 The weight of the electromagnetic wave absorber 1 can be reduced by satisfying the following. Furthermore, the electromagnetic wave absorber 1 can attenuate electromagnetic waves by having a matrix and carbon nanotubes dispersed in the matrix as described above. Note that the matrix refers to a matrix in the field of composite materials, and the matrix and carbon nanotubes are composited by dispersing the carbon nanotubes in the matrix.

[0016] The electromagnetic wave absorber 1 preferably has a porous structure. In this case, the bulk density of the electromagnetic wave absorber 1 is 1.23 kg / m 3 The following can be easily achieved. A porous structure is a structure having voids inside. An example of a porous structure is a structure in which long voids are arranged in a honeycomb pattern in the thickness direction of the electromagnetic wave absorber 1. In this case, the electromagnetic wave absorber 1 is porous but is less likely to break when a load is applied in the thickness direction. However, the porous structure of the electromagnetic wave absorber 1 is not limited to this.

[0017] The matrix of the electromagnetic wave absorber 1 preferably contains a water-soluble polymer. In this case, by producing the electromagnetic wave absorber 1 by a freeze-drying method as described below, the weight of the electromagnetic wave absorber 1 can be easily reduced, and the bulk density of the electromagnetic wave absorber 1 can be reduced to 1.23 kg / m 3 The bulk density of the electromagnetic wave absorber 1 is 1 kg / m or less. 3More preferably, it is:

[0018] The solubility of the water-soluble polymer in water at 25°C is preferably 1 mg / 1 g-H2O or more. The water-soluble polymer may be any of natural polymers, semi-synthetic polymers, and synthetic polymers. The water-soluble polymer contains at least one selected from the group consisting of, for example, guar gum, carrageenan, sodium alginate, corn starch, xanthan gum, sodium chondrile sulfate, sodium hyaluronate, carboxymethylcellulose, sodium carboxymethylcellulose, methylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, cationized guar gum, carboxyvinyl polymer, polyacrylic acid, polyvinylpyrrolidone, and polyvinyl alcohol. The water-soluble polymer preferably contains at least one of carboxymethylcellulose and hydroxypropylmethylcellulose.

[0019] The matrix preferably further contains a cured product of a thermosetting resin. In this case, when the electromagnetic wave absorber 1 comes into contact with water, the water-soluble polymer in the matrix is ​​less likely to be eluted. The thermosetting resin preferably has the property of forming an aqueous emulsion when dispersed in water. In this case, when the electromagnetic wave absorber 1 is produced by a freeze-drying method, the thermosetting resin can be easily dispersed in the matrix of the electromagnetic wave absorber 1, which makes it even more difficult for the water-soluble polymer in the matrix to be eluted in water.

[0020] The thermosetting resin preferably contains an epoxy resin. When the thermosetting resin contains an epoxy resin, the thermosetting resin preferably further contains a curing agent for the epoxy resin, as necessary. In this case, when the electromagnetic wave absorber 1 comes into contact with water, the water-soluble polymer in the matrix is ​​further prevented from eluting. The epoxy resin, or the epoxy resin and curing agent, preferably have the property of forming an aqueous emulsion when dispersed in water. In this case, when the electromagnetic wave absorber 1 is produced by a freeze-drying method, the epoxy resin can be easily dispersed in the matrix of the electromagnetic wave absorber 1, thereby making it further difficult for the water-soluble polymer in the matrix to be eluted in water.

[0021] The epoxy resin is preferably an emulsion-type epoxy resin, commonly referred to as a water-based epoxy resin. For example, the epoxy resin preferably contains at least one selected from the group consisting of bisphenol A epoxy resins and bisphenol F epoxy resins. The curing agent may be a general curing agent for epoxy resins, and may contain at least one selected from the group consisting of phenolic curing agents, amine curing agents, and acid anhydride curing agents.

[0022] The ratio of the thermosetting resin to the electromagnetic wave absorber 1 is preferably 5% by mass or more and 90% by mass or less. Furthermore, when the thermosetting resin contains an epoxy resin, or an epoxy resin and a curing agent, the ratio of the epoxy resin, or the epoxy resin and a curing agent, to the electromagnetic wave absorber 1 is preferably 5% by mass or more and 90% by mass or less. When these ratios are 5% by mass or more, the water-soluble polymer in the matrix can be made even less likely to dissolve in water. Furthermore, when these ratios are 90% by mass or less, the thermosetting resin is less likely to hinder the weight reduction of the electromagnetic wave absorber 1.

[0023] The carbon nanotubes in the electromagnetic wave absorber 1 will now be described.

[0024] When the electromagnetic wave absorber 1 contains carbon nanotubes, electromagnetic waves can be attenuated well within the electromagnetic wave absorber 1, and electromagnetic waves in the millimeter wave band and microwave band in particular can be attenuated efficiently.

[0025] Carbon nanotubes (hereinafter also referred to as CNTs) include, for example, at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0026] The sheet resistance of the electromagnetic wave absorber 1 is preferably 0.01 Ω / sq. or more and 100 Ω / sq. or less. In this case, the electromagnetic waves are particularly likely to attenuate within the electromagnetic wave absorber 1.

[0027] The proportion of carbon nanotubes in the electromagnetic wave absorber 1, i.e., the content of carbon nanotubes in each of the base portion 2 and the protrusion portion 3, is set appropriately taking into consideration the resistivity of the electromagnetic wave absorber 1 and the like, and is, for example, 60% by mass or less for each of the base portion 2 and the protrusion portion 3. Furthermore, this proportion is more preferably 30% by mass or less, even more preferably 20% by mass or less, and particularly preferably 10% by mass or less.

[0028] The electromagnetic wave absorber 1 may contain amorphous carbon such as carbon black or acetylene black. In this case, the amorphous carbon can adjust the sheet resistance of the electromagnetic wave absorber 1. The proportion of amorphous carbon in the electromagnetic wave absorber 1 is, for example, 50 mass % or less.

[0029] The electromagnetic wave absorber 1 may contain cellulose nanofibers. In this case, the sheet resistance of the electromagnetic wave absorber 1 can be adjusted without impairing the structure or strength of the electromagnetic wave absorber 1. The proportion of cellulose nanofibers is 60% by mass or less relative to the electromagnetic wave absorber 1. If this proportion is 5% by mass or more, the sheet resistance of the electromagnetic wave absorber 1 can be adjusted, and if it is 60% by mass or less, it is unlikely to hinder the weight reduction of the electromagnetic wave absorber 1.

[0030] The thickness of the flat-plate-shaped base 2 of the electromagnetic wave absorber 1 is set appropriately depending on the wavelength of the electromagnetic waves to be shielded, etc. If the thickness of the base 2 is about 1 / 4 of the wavelength of the electromagnetic waves to be shielded, the electromagnetic wave absorber 1 can efficiently shield the electromagnetic waves. Even if the thickness of the base 2 is restricted in this way depending on the wavelength of the electromagnetic waves, if the bulk density of the electromagnetic wave absorber 1 is 1.23 kg / m 3 or less, it is easy to achieve a reduction in the weight of the electromagnetic wave absorber 1. The thickness of the base portion 2 is preferably 0.1 mm or more and 100 mm or less, more preferably 0.5 mm or more and 10 mm or less, and even more preferably 1 mm or more and 5 mm or less.

[0031] The shape of the convex portions 3 of the electromagnetic wave absorber 1 will be described. The shape of the multiple convex portions 3 of the electromagnetic wave absorber 1 is preferably a continuous pyramidal shape. That is, it is preferable that each of the convex portions 3 is a cone shape having a base that contacts one surface 1a of the electromagnetic wave absorber 1, and that the multiple convex portions 3 are continuous on the surface 1a. "Continuous" means that the multiple convex portions 3 are arranged in a tiled pattern at equal (regular) or unequal (irregular) intervals on the one surface 1a of the electromagnetic wave absorber 1. The multiple convex portions 3 may have the same shape or different shapes. The multiple convex portions 3 each include multiple groups consisting of multiple convex portions 3, and the convex portions 3 in the same group may have the same shape, while the convex portions 3 in different groups may have different shapes. Each of the multiple convex portions 3 may have a random cone shape. That is, each of the multiple convex portions 3 may be configured in any shape as long as it is a convex shape that absorbs electromagnetic waves. One surface 1a of the base 2 may have an exposed portion that is not covered by the protrusion 3.

[0032] The height of the protrusions 3 and the shape of the base that contacts the base 2 are set appropriately depending on the wavelength of the electromagnetic waves to be shielded, etc. If the height of the protrusions 3 is about 1 / 4 of the wavelength of the electromagnetic waves to be shielded, the electromagnetic wave absorber 1 can efficiently shield the electromagnetic waves. The height of the protrusions 3 is preferably 0.1 mm or more and 100 mm or less, more preferably 0.5 mm or more and 10 mm or less, and even more preferably 1 mm or more and 5 mm or less.

[0033] The cone shape may include a pyramidal shape. That is, the shape of the bottom of the protrusion 3 may be set arbitrarily, but is preferably polygonal, and more preferably square. The length of one side of the bottom is preferably 0.1 mm to 100 mm, more preferably 0.5 mm to 10 mm, and even more preferably 1 mm to 5 mm.

[0034] The electromagnetic wave absorber 1 is produced, for example, by freeze-drying an aqueous dispersion containing the raw materials of the electromagnetic wave absorber 1. In this case, it is easy to make the electromagnetic wave absorber 1 porous and reduce the bulk density.

[0035] For example, the electromagnetic wave absorber 1 is a freeze-dried body formed by freeze-drying an aqueous dispersion containing carbon nanotubes and a water-soluble polymer. The aqueous dispersion is prepared, for example, by mixing a water-soluble polymer, carbon nanofibers, and water. The proportion of water in the aqueous dispersion is appropriately set depending on the strength, bulk density, and other factors required of the electromagnetic wave absorber 1, and is, for example, 80% by weight or more and 99.9% by weight or less. The freeze-drying conditions are appropriately set, and the electromagnetic wave absorber 1 can be produced, for example, by freezing the aqueous dispersion at a temperature of -80°C to -40°C, and then drying it under conditions of a temperature of -50°C to -10°C and an absolute pressure of 1 Pa to 50 Pa.

[0036] When freezing the aqueous dispersion, the aqueous dispersion may be placed in a container and frozen with the sides and bottom of the container covered with insulating material. In this case, the aqueous dispersion in the container can be frozen sequentially from top to bottom. This makes it easier for the water-soluble resin molecules to be oriented in the vertical direction, and the electromagnetic wave absorber 1 is more likely to have a porous structure in which long voids are arranged in a honeycomb pattern in the thickness direction.

[0037] Furthermore, the container (i.e., the mold) used for freezing has a recess on its bottom surface. By using a container having such a shape, the convex portions 3 of the electromagnetic wave absorber 1 can be formed. The shape of the recess is designed appropriately to match the shape of the convex portions 3 of the electromagnetic wave absorber 1.

[0038] There are no restrictions on the method for producing the container to be frozen, but it can be easily produced using 3D printing. In particular, it is recommended to use a stereolithography 3D printer, which has high modeling accuracy.

[0039] Furthermore, if a vent hole is drilled at the bottom of the recess, at the location that will be the tip of the electromagnetic wave absorber 1 after molding, the aqueous dispersion liquid will be more easily distributed up to the tip of the recess, improving the yield in forming the tip portion of the electromagnetic wave absorber 1.

[0040] When the matrix of the electromagnetic wave absorber 1 contains a cured product of a thermosetting resin, the electromagnetic wave absorber 1 is a freeze-dried body formed by freeze-drying an aqueous dispersion containing, for example, carbon nanotubes, a water-soluble polymer, and a thermosetting resin. The aqueous dispersion is prepared by mixing, for example, a water-soluble polymer, carbon nanofibers, an aqueous emulsion of a thermosetting resin, and water. The freeze-dried electromagnetic wave absorber 1 can be produced by freeze-drying this aqueous dispersion in the same manner as above.

[0041] When the matrix of the electromagnetic wave absorber 1 contains a cured thermosetting resin, the electromagnetic wave absorber 1 may be formed by, for example, freeze-drying an aqueous dispersion containing carbon nanotubes and a water-soluble polymer, impregnating the freeze-dried body with a thermosetting resin, and then curing the thermosetting resin. In this case, the freeze-dried body can be produced in the same manner as when producing an electromagnetic wave absorber 1 that does not contain a thermosetting resin. To impregnate the freeze-dried body with a thermosetting resin, for example, a varnish containing the thermosetting resin is prepared by impregnating the freeze-dried body with an organic solvent such as methyl ethyl ketone. The freeze-dried body can be impregnated with the thermosetting resin by impregnating the freeze-dried body with this varnish. Next, the varnish-impregnated freeze-dried body is dried to volatilize the organic solvent in the varnish. Next, the freeze-dried body is heated to cure the thermosetting resin. The heating conditions for the freeze-dried body are set appropriately depending on the composition of the thermosetting resin, so that the curing reaction of the thermosetting resin proceeds. In this manner, the electromagnetic wave absorber 1 can be produced.

[0042] The electromagnetic wave absorber 1 according to this embodiment, with its configuration described above, is lightweight yet can suppress the transmission and reflection of electromagnetic waves, particularly efficiently suppressing the transmission and reflection of electromagnetic waves in the microwave to millimeter wave bands. Therefore, the electromagnetic wave absorber 1 can be used to suppress the transmission and reflection of, for example, electromagnetic waves in the 3 GHz to 10 GHz range, which are communication frequencies for ultra-wideband (UWB) wireless communications within artificial satellites, and electromagnetic waves in the 28 GHz and 39 GHz bands, which are frequencies surrounding the communication frequencies for 5G communications. In these cases, the attenuation of electromagnetic waves when they pass through the electromagnetic wave absorber 1 and when they are reflected by the electromagnetic wave absorber 1 can be reduced to -10 dB or less, and even -20 dB or less.

[0043] For example, the electromagnetic wave absorber 1 can be applied to satellite applications to absorb noise generated from coils for wireless power supply, or to control the transmission path of signal waves when wirelessly communicating between electronic devices in order to reduce the weight of satellites.

[0044] In addition, the electromagnetic wave absorber 1 can also be used to absorb external noise to improve detection accuracy in equipment for remote sensing and astronomical observation that uses even higher frequency electromagnetic waves such as millimeter waves and terahertz waves.

[0045] [Aspect] The electromagnetic wave absorber (1) of the first embodiment comprises a flat base (2) and a plurality of projections (3) formed on one surface (1a) in the thickness direction of the base (2). Each of the base (2) and the projections (3) has a matrix and carbon nanotubes dispersed in the matrix, and has a bulk density of 1.23 kg / m 3 The following is the result.

[0046] According to this embodiment, an electromagnetic wave absorber (1) that is lightweight yet has excellent electromagnetic wave absorption performance can be obtained.

[0047] In the second aspect, each of the protrusions (3) in the first aspect is formed in a cone shape. The protrusions (3) are continuously arranged in a tiled pattern at equal or irregular intervals on the surface (1a) of the base (2).

[0048] In a third embodiment, the pyramidal shape in the second embodiment includes at least a pyramidal shape.

[0049] In a fourth aspect, in any one of the first to third aspects, the content of carbon nanotubes in each of the base portion (2) and the protrusion portion (3) is 20 mass % or less. [Example]

[0050] Specific examples of this embodiment will be presented below, but this embodiment is not limited to the following examples.

[0051] Sodium carboxymethylcellulose and carbon nanotubes (product name ZEONANO (registered trademark) SG101, manufactured by Zeon Corporation) were added to distilled water, and ultrasonic waves were applied to the resulting mixture at 135 W for 3 minutes three times using an ultrasonic horn to prepare an aqueous dispersion with a solid content of 1% by mass. This aqueous dispersion was poured into a mold and frozen by cooling in a freezer at -45°C for 2 hours. The frozen aqueous dispersion (hereinafter referred to as the frozen product) was then freeze-dried using a freeze dryer. The absolute pressure of the atmosphere surrounding the frozen product was adjusted to a range of 1 Pa to 20 Pa, and the temperature was gradually increased from -45°C to -10°C. The cold trap temperature was set to -80°C, and the frozen product was freeze-dried for 24 hours.

[0052] Using the above method, an electromagnetic wave absorber containing 5% by mass of carbon nanotubes and having a 6 mm thick base and multiple 6 mm high pyramidal protrusions was produced. Under the same conditions, an electromagnetic wave absorber consisting of only a 6 mm thick base was also produced.

[0053] The absorption performance of the electromagnetic wave absorber was evaluated using S parameters based on the free space method by measuring the electric field strength of the reflected wave relative to the electric field strength of the incident wave using the electromagnetic wave absorber and a metal plate.

[0054] The results are shown in Figure 2, which shows the actual measurement values ​​and simulation results of the absorption performance of each of the two electromagnetic wave absorbers. It can be confirmed that the electromagnetic wave absorber with base and protrusions has higher absorption performance in the high frequency range above 30 GHz. The simulation results suggest that the absorption performance of the electromagnetic wave absorber with base and protrusions may be mainly due to dielectric loss, rather than scattering due to reflection on the surface of the electromagnetic wave absorber.

[0055] From the above, it was confirmed that the electromagnetic wave absorber of the embodiment has potential as a means for improving the electromagnetic wave absorption performance. [Explanation of symbols]

[0056] 1. Electromagnetic wave absorber 1a side 2 base 3 Convex part

Claims

1. A flat base portion; a plurality of protrusions formed on one surface of the base in a thickness direction, Each of the base portion and the protrusion portion has a matrix and carbon nanotubes dispersed in the matrix, and has a bulk density of 1.23 kg / m 3 Below is the Electromagnetic wave absorber.

2. Each of the protrusions is formed in a cone shape, The protrusions are arranged continuously in a tiled pattern at equal or unequal intervals on the surface of the base. The electromagnetic wave absorber according to claim 1 .

3. The pyramidal shape includes at least a pyramidal shape. The electromagnetic wave absorber according to claim 2.

4. the content of the carbon nanotubes in each of the base portion and the protrusion portion is 20 mass % or less; The electromagnetic wave absorber according to any one of claims 1 to 3.

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