Radio wave absorber and method for manufacturing the same

Abstract

Provided are a so-called radio interference-type radio wave absorber that, even when a protection layer for protecting a resistor film is formed on a surface of the resistor film, can suitably absorb radio waves of a desired frequency, and a method for manufacturing the radio wave absorber. A radio wave interference-type radio wave absorber formed by successively stacking a resistor film 1, a dielectric layer 2, and a radio wave shielding layer 3, wherein absorbed radio waves to be absorbed by the radio wave absorber are radio waves of a high frequency band of a millimeter band or above. The resistor film has a protection layer 4 thereon. The thickness of the dielectric layer is less than a reference thickness dst, which is determined in accordance with the frequency of the absorbed radio waves and the permittivity of the dielectric layer.

Description

[Technical Field]

[0001] The present disclosure relates to a radio wave absorber that absorbs unwanted radio waves, and in particular to a so-called radio wave interference type radio wave absorber that has a protective layer that protects a resistive film on the surface onto which radio waves are incident, and to a method for manufacturing the same. [Background technology]

[0002] 2. Description of the Related Art In order to avoid the influence of leaked radio waves emitted to the outside from electric circuits and the like, and undesired reflected electromagnetic waves, radio wave absorbers that absorb radio waves are used.

[0003] In recent years, research has been progressing into technologies that utilize centimeter waves with a frequency band of several gigahertz (GHz), millimeter waves with frequencies of 30 to 300 gigahertz, and radio waves with frequencies in the terahertz (THz) band, which are higher frequency bands than the millimeter wave band, for mobile communications such as mobile phones, wireless LAN, and electronic toll collection systems (ETC).

[0004] In response to the technological trend of utilizing such high frequency radio waves, it is expected that there will be an increasing demand for radio wave absorbers that can absorb unwanted radio waves from the millimeter wave band to higher frequency bands.

[0005] A known wave absorber that absorbs and suppresses the reflection of unwanted radio waves is the so-called radio wave interference type (also called λ / 4 type or reflection type), in which a resistive film is provided on the surface of the dielectric layer on the side where the radio waves enter, and a radio wave shielding layer that reflects radio waves is provided on the back surface on the opposite side, and the phase of the radio waves reflected by the radio wave shielding layer and radiated to the outside is shifted by 1 / 2 wavelength from the phase of the radio waves reflected by the resistive film surface, thereby canceling out and absorbing the radio waves reflected from the radio wave absorber. Radio wave interference type radio wave absorbers have the advantage of being lighter in weight than radio wave absorbers that magnetically absorb radio waves using magnetic particles, and being easier to manufacture, allowing for lower costs.

[0006] The inventors have proposed a radio wave absorbing sheet, which is a thin, radio wave interference-type radio wave absorber, that uses a conductive organic polymer film as a resistive film formed on the surface of a dielectric layer, thereby achieving good absorption of radio waves in a desired frequency band while also being highly flexible and easy to handle (see Patent Document 1). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. WO2018 / 088492 Summary of the Invention [Problem to be solved by the invention]

[0008] In general, the frequency of the radio waves (hereinafter referred to as "absorbed radio waves") to be absorbed by a radio wave absorber or a radio wave absorbing sheet is set according to the environment and purpose of use. In the case of the radio wave interference-type radio wave absorbing sheet described above, the peak of the radio wave absorption characteristics of the radio wave absorbing sheet can be made to coincide with the frequency of the absorbed radio waves by setting the thickness of the dielectric layer to a predetermined reference thickness (dst) calculated based on the frequency of the absorbed radio waves. Furthermore, since the conventional radio wave absorbing sheet described above has a resistive film made of a conductive organic polymer film, it is effective to provide a protective layer to protect the surface of the resistive film in order to prevent the surface of the resistive film from being scratched and causing a change in the surface resistance value.

[0009] However, as a result of investigations by the inventors, it was confirmed that even when the thickness of the dielectric layer is set to the reference thickness dst calculated from the frequency of the radio waves to be absorbed, when a protective layer is provided on the surface of the resistive film, the peak frequency of the radio wave absorption characteristics absorbed by the radio wave absorbing sheet differs from the set frequency of the absorbed radio waves. Therefore, if the peak value of the frequency of the radio wave absorption characteristics of the radio wave absorbing sheet shifts, a situation arises in which the absorbed radio waves, which are radio waves of the desired frequency, cannot be sufficiently absorbed.

[0010] The present disclosure aims to solve the problems of the conventional radio wave absorbing sheets described above, and to provide a radio wave absorber that can effectively absorb radio waves even when a protective layer that protects a resistive film is formed on the surface of a so-called radio wave interference type radio wave absorber, and a method for manufacturing this radio wave absorber. [Means for solving the problem]

[0011] In order to solve the above problems, the radio wave absorber disclosed in the present application is a radio wave interference type radio wave absorber formed by sequentially laminating a resistive film, a dielectric layer, and a radio wave shielding layer, wherein the radio waves to be absorbed by the radio wave absorber are radio waves in a high frequency band equal to or higher than the millimeter wave band, and wherein a protective layer is provided on the resistive film, and the protective layer has a dielectric constant of 2 or more and 20 or less and a thickness of 10 μm or more and 150 μm or less, the dielectric layer has a dielectric constant D of 2 or more and 8 or less, and the thickness of the dielectric layer is thinner than a reference thickness dst determined according to the frequency of the absorbed radio waves and the dielectric constant of the dielectric layer.

[0012] The method for manufacturing a radio wave absorber disclosed in the present application is a method for manufacturing a radio wave interference type radio wave absorber in which a resistive film, a dielectric layer, and a radio wave shielding layer are sequentially laminated, and which absorbs radio waves in a high frequency band equal to or higher than the millimeter wave band, and is characterized in that a protective layer having a dielectric constant of 2 or more and 20 or less and a thickness of 10 μm or more and 150 μm or less is provided on the resistive film, the dielectric layer has a dielectric constant D of 2 or more and 8 or less, and the thickness of the dielectric layer is set to be thinner than a reference thickness dst determined in accordance with the frequency of the absorbed radio waves and the dielectric constant of the dielectric layer. [Effects of the Invention]

[0013] The radio wave absorbing sheet disclosed in the present application has a protective layer on the surface of a resistive film, the protective layer having a dielectric constant of 2 to 20 and a thickness of 10 μm to 150 μm, and the thickness of the dielectric layer having a dielectric constant of 2 to 8 is thinner than the standard thickness dst. Therefore, the resistive film can be protected by the protective layer, while the desired radio waves can be well absorbed.

[0014] Furthermore, the method of manufacturing a radio wave absorbing sheet disclosed in the present application is fabricated by setting the thickness of the dielectric layer thinner than the reference thickness dst determined according to the frequency of the absorbed radio wave and the dielectric constant of the dielectric layer, thereby making it possible to manufacture a radio wave absorber that has a protective layer that protects the resistive film and that effectively absorbs the desired radio wave. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view illustrating a configuration of an electromagnetic wave absorbing sheet according to an embodiment of the present invention. [Figure 2] 10A and 10B are diagrams illustrating the principle of a simulation for determining the thickness reduction rate of an electromagnetic wave absorbing sheet. [Figure 3] FIG. 10 is a diagram showing the relationship between the frequency of absorbed radio waves and the thickness reduction rate of a dielectric layer when the frequency is 28.5 GHz or more and 140 GHz or less. [Figure 4] FIG. 10 is a diagram showing the relationship between the frequency of absorbed radio waves and the thickness reduction rate of a dielectric layer when the frequency is 200 GHz or more and 414 GHz or less. [Figure 5] FIG. 1 is a diagram showing the relationship between the dielectric constant D of a dielectric layer and the thickness reduction rate T of the dielectric layer when the frequency is 28.5 GHz or more and 60 GHz or less. [Figure 6] FIG. 1 is a diagram showing the relationship between the dielectric constant D of a dielectric layer and the thickness reduction rate T of the dielectric layer when the frequency is greater than 60 GHz and equal to or less than 90 GHz. [Figure 7] FIG. 1 is a diagram showing the relationship between the dielectric constant D of a dielectric layer and the thickness reduction rate T of the dielectric layer when the frequency is greater than 90 GHz and equal to or less than 140 GHz. [Figure 8] FIG. 1 is a diagram showing the relationship between the dielectric constant D of a dielectric layer and the thickness reduction rate T of the dielectric layer when the frequency is 200 GHz or more and 414 GHz or less. DETAILED DESCRIPTION OF THE INVENTION

[0016] The radio wave absorber disclosed in the present application is a radio wave interference type radio wave absorber formed by sequentially laminating a resistive film, a dielectric layer, and a radio wave shielding layer, wherein the radio waves to be absorbed by the radio wave absorber are radio waves in a high frequency band equal to or higher than the millimeter wave band, and a protective layer is provided on the resistive film, and the protective layer has a dielectric constant of 2 or more and 20 or less and a thickness of 10 μm or more and 150 μm or less, and the dielectric layer has a dielectric constant D of 2 or more and 8 or less, and the thickness of the dielectric layer is thinner than a reference thickness dst determined in accordance with the frequency of the absorbed radio waves and the dielectric constant of the dielectric layer, and satisfies any one of the following conditions 1) to 4): 1) When the frequency of the absorbed radio waves is 28.5 GHz or more and 60 GHz or less, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between the curve expressed by the following (Equation 1) and the curve expressed by the following (Equation 2) for the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Formula 1) Tmax=-0.021×D 2 +0.26×D-1.35 (Formula 2) Tmin=-0.0005×D 2 +0.0056×D-0.017 2) When the frequency of the absorbed radio waves is greater than 60 GHz and equal to or less than 90 GHz, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between the curve expressed by the following (Equation 3) and the curve expressed by the following (Equation 4) for the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Formula 3) Tmax=-0.014×D 2 +0.20×D-1.33 (Formula 4) Tmin=-0.0002×D 2 -0.0029×D+0.008 3) When the frequency of the absorbed radio waves is greater than 90 GHz and equal to or less than 140 GHz, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between the curves expressed by the following (Equation 5) and (Equation 6) for the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Formula 5) Tmax=-0.013×D 2 +0.19×D-1.32 (Formula 6) Tmin=-0.0003×D 2 +0.0017×D-0.0024 4) When the frequency of the absorbed radio waves is 200 GHz or more and 414 GHz or less, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between the curve expressed by the following (Equation 7) and the curve expressed by the following (Equation 8) for the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Formula 7) Tmax=-0.0060×D 2 +0.099×D-1.13 (Formula 8) Tmin=-0.0001×D 2 +8×10 -5 ×D-0.017.

[0017] In this way, the radio wave absorber disclosed in the present application can improve the weather resistance and resistance to physical impact of the resistive film by the protective layer, and can also suppress the effect of the protective layer shifting the frequency of the radio waves to be absorbed, thereby achieving good radio wave absorption characteristics for the absorbed radio waves.

[0018] It is preferable that the return loss for radio waves at the absorption frequency is 10 dB or more.

[0019] It is also preferable that the resistive film is formed of a conductive organic polymer film. It is also preferable that the resistive film contains at least one of carbon microcoils, carbon nanotubes, and graphene. In this way, the resistive film can be easily formed, and the surface resistance value can be maintained even if the wave absorber is deformed.

[0020] Furthermore, it is preferable that the protective layer, resistive film, and dielectric layer are all formed of translucent materials, the radio wave shielding layer is made of a metal mesh, and the total light transmittance of the entire radio wave absorber is 30% or more. In this way, it is possible to see through the radio wave absorber and easily observe the radio wave source that is shielded from radiated radio waves and the state of the shielded device that is protected from external radio waves.

[0021] Furthermore, it is preferable that an adhesive layer is further provided on the back surface of the radio wave shielding layer, which makes it possible to easily place the radio wave absorber at a desired position.

[0022] It is also preferable that the protective layer, the resistive film, the dielectric layer, and the radio wave shielding layer are all formed as thin films, and are formed as a flexible sheet as a whole. In this way, a radio wave absorber that is easy to handle can be realized.

[0023] The method for manufacturing a radio wave absorber disclosed in the present application is a method for manufacturing a radio wave interference-type radio wave absorber in which a resistive film, a dielectric layer, and a radio wave shielding layer are laminated in this order, and which absorbs radio waves in a high frequency band equal to or higher than the millimeter wave band, and in which a protective layer having a dielectric constant of 2 or more and 20 or less and a thickness of 10 μm or more and 150 μm or less is provided on the resistive film, and the dielectric layer has a dielectric constant D of 2 or more and 8 or less, and the thickness of the dielectric layer is set to be thinner than a reference thickness dst obtained in accordance with the frequency of the absorbed radio waves and the dielectric constant of the dielectric layer.

[0024] In this way, the method for manufacturing a radio wave absorber disclosed in the present application can easily manufacture a radio wave absorber that exhibits good radio wave absorption characteristics for the absorbed radio waves by suppressing the influence of the frequency shift of the absorbed radio waves that occurs due to the influence of the protective layer formed by the dielectric layer.

[0025] Hereinafter, the radio wave absorber disclosed in the present application will be described with reference to the drawings.

[0026] Here, the radio wave absorber disclosed in the present application will be described by taking as an example a radio wave absorbing sheet that has a thickness sufficiently small relative to its main area and can be understood as a sheet. In this way, the radio wave absorber disclosed in the present application is a concept that includes both a radio wave absorbing sheet that can be understood as a sheet in terms of the relationship between its surface area and thickness, and a radio wave absorbing block that is relatively thick and can be understood as a block shape overall.

[0027] As will be described later, since the thickness of the dielectric layer of a radio wave interference-type radio wave absorber is proportional to the inverse of the frequency of the radio waves to be absorbed, the thickness of the dielectric layer does not become so thick in a radio wave absorber that absorbs radio waves at high frequencies above the millimeter wave band. Furthermore, for example, in applications such as when the radio wave absorber is placed without gaps on the surface of an electronic device that is a noise source, or when protecting an electronic device placed inside a housing that forms an outer shell from unwanted external radio waves, it is practically effective for the radio wave absorber to have a surface area of ​​at least a certain level. For this reason, it is considered more common for the radio wave absorber disclosed in the present application to be in the form of a sheet that has a certain surface area and a small thickness.

[0028] (Embodiment) FIG. 1 is a cross-sectional view showing the configuration of a radio wave absorbing sheet (radio wave absorber) according to this embodiment.

[0029] Note that Figure 1 is a diagram that is provided to make it easier to understand the configuration of the radio wave absorbing sheet according to this embodiment, and the sizes of the components shown in the figure, particularly their thicknesses, are not shown in accordance with reality.

[0030] [Overall structure of the radio wave absorbing sheet] The radio wave absorbing sheet exemplified in this embodiment has a resistive film 1, a dielectric layer 2, and a radio wave shielding layer 3 laminated in this order, with a protective layer 4 formed on the surface of the resistive film 1 on which radio waves are incident, i.e., on the surface of the resistive film 1 opposite to the side on which the dielectric layer 2 is disposed. In the radio wave absorbing sheet exemplified in Fig. 1, an adhesive layer 5 is laminated on the back side of the radio wave shielding layer 3, i.e., on the surface of the radio wave shielding layer 3 opposite to the side on which the dielectric layer 2 is disposed.

[0031] The radio wave absorbing sheet according to this embodiment is a radio wave interference type (also referred to as a λ / 4 type or a reflection type), in which radio waves 11 incident on the dielectric layer 2 from the resistive film 1 side are reflected at the interface with the radio wave shielding layer 3 disposed on the back side of the dielectric layer 2 and are then emitted to the outside again as reflected waves 13. By setting the thickness d of the dielectric layer 2 to ¼ of the wavelength λ of the incident radio waves (d = λ / 4), the primary reflected wave 12 reflected on the surface of the radio wave absorbing sheet and the secondary reflected wave 13 reflected on the surface of the radio wave shielding layer 3 are shifted in phase by half the wavelength (180°), canceling each other out, making it appear as if the radio waves are absorbed by the radio wave absorbing sheet. Note that the tertiary reflected wave, quaternary reflected wave, and higher-order reflected waves, indicated by dotted lines in FIG. 1 , are superimposed on the primary reflected wave and secondary reflected wave, respectively, because they are in phase with each other. However, their small energy has little effect.

[0032] Here, d=λ / 4 occurs when a material with a dielectric constant ε=1 is used as the dielectric layer 2, and when the dielectric constant of the dielectric material used for the dielectric layer 2 is ε r If , then d=λ / (4(ε r ) -1 / 2 ) and the thickness d of the dielectric layer 2 is calculated as 1 / (ε r ) -1 / 2 By forming the dielectric layer 2 to be thin, the entire electromagnetic wave absorbing sheet can be made thinner, the cost of the electromagnetic wave absorbing sheet can be reduced, and furthermore, a flexible electromagnetic wave absorbing sheet or an elastic electromagnetic wave absorbing sheet can be more easily realized.

[0033] On the other hand, from the principle of radio wave absorption in the radio wave interference type radio wave absorber described above, the thickness d of the dielectric layer 2 in the radio wave interference type radio wave absorber is set to the dielectric constant ε r Taking this into consideration, it is important to set the reference thickness dst as the ideal thickness determined from the frequency of the electromagnetic waves to be absorbed by the electromagnetic wave absorber.

[0034] The radio wave shielding layer 3 formed by laminating it on the back side of the dielectric layer 2 is a layer that reflects incident radio waves on its surface on the dielectric layer 2 side, which is the boundary surface with the dielectric layer 2. For this reason, the radio wave shielding layer 3 is sometimes called a reflective layer.

[0035] According to the principle of radio wave absorption in the radio wave interference-type radio wave absorbing sheet of this embodiment, the radio wave shielding layer 3 only needs to function as a reflective layer that reflects radio waves, and can be easily realized as a metal layer formed of a metal plate. Note that, in order to make the radio wave absorber flexible as a sheet for easy handling, it is more preferable to use, as the radio wave shielding layer 3, a metal foil or a metal thin film formed by depositing a metal material on a thin film made of a non-metallic material such as resin.

[0036] Furthermore, when the resistive film, the dielectric layer, and the protective layer described below are made of a material that transmits light so that the radio wave absorbing sheet as a whole has a predetermined translucency (for example, a total light transmittance of 30% or more, which is considered to be effective in practical use), it is preferable to form the radio wave shielding layer 3 from a conductive mesh made of metal wires or conductive fibers coated with a conductive material, in order to provide the radio wave shielding layer 3 with translucency as well.

[0037] In the radio wave interference type radio wave absorbing sheet, the resistive film 1 matches impedance between the radio wave absorbing sheet and the air on the front side of the dielectric layer 2, that is, the side on which the radio waves to be absorbed are incident.

[0038] When radio waves propagating through the air enter the radio wave absorbing sheet, if the input impedance value of the radio wave absorbing sheet is different from the impedance value of the air, this impedance difference will increase the amount of reflection of the radio waves entering the radio wave absorbing sheet, degrading the radio wave absorption characteristics of the radio wave absorbing sheet.For this reason, it is important to set the input impedance of the radio wave absorbing sheet to 377 Ω, which is equal to the impedance of air (more precisely, the impedance value of a vacuum), in order to prevent the radio waves from being reflected and scattered at the interface between the dielectric layer and the resistive film when they enter the radio wave absorbing sheet, thereby degrading the radio wave absorption characteristics.

[0039] In the radio wave absorbing sheet of this embodiment, the resistive film 1 is formed as a film of a conductive organic polymer, which ensures flexibility as a radio wave absorbing sheet, and even when the radio wave absorbing sheet is bent sharply, cracks do not occur in the resistive film 1, and good impedance matching can be maintained without changing the surface resistance value. Note that it is not essential for the radio wave absorbing sheet disclosed in the present application to use the resistive film 1 as a conductive organic polymer film. When flexibility is not required for the radio wave absorbing sheet, such as when the radio wave absorbing sheet is thick or when the radio wave absorbing sheet is used on a flat surface, a conventional hard resistive film 1 can be used which is formed by ion plating, vapor deposition, sputtering, or the like using a metal oxide such as indium tin oxide (ITO), indium oxide, tin oxide, or zinc oxide, a metal nitride, or a mixture thereof.

[0040] The protective layer 4 is formed on the surface of the resistive film 1, i.e., on the outermost surface of the radio wave absorbing sheet on the side where radio waves are incident, and is a member that protects the resistive film 1. The surface resistance of the resistive film 1 of the radio wave absorbing sheet may change if moisture adheres to the surface. Furthermore, particularly when a conductive organic polymer film is used as the resistive film 1, the surface may be scratched if it comes into contact with a sharp object or is rubbed with a hard material, which may also change the surface resistance of the resistive film 1. For this reason, it is important to protect the resistive film 1 and maintain impedance matching by covering the surface of the resistive film 1 with the protective layer 4.

[0041] On the other hand, as a result of studies by the inventors, it was confirmed that the peak wavelength of the radio waves absorbed by the radio wave absorber changes depending on whether the protective layer is provided or not. As described above, in order to obtain good radio wave absorption characteristics in a radio wave interference-type radio wave absorber, it is important to match the thickness of the dielectric layer 2 to a predetermined reference thickness dst determined according to the frequency of the radio waves to be absorbed. However, even if the manufacturing conditions can be adjusted to make the thickness of the dielectric layer 2 match the reference thickness dst, if the peak wavelength of the radio waves to be absorbed is shifted due to the provision of the protective layer 4, it is not possible to obtain a radio wave absorber with good radio wave absorption characteristics.

[0042] Therefore, in the wave absorber disclosed in the present application, the thickness of the dielectric layer 2 is reduced by a predetermined amount from the reference thickness dst (thinned), thereby canceling (negating) the change in the peak frequency of the wave absorbed by the wave absorber caused by the protective layer 4, thereby enabling good absorption of waves of a predetermined frequency. The preferred thickness of the dielectric layer 2 when the protective layer 4 is provided will be described in detail later.

[0043] The adhesive layer 5 is a layer formed on the back surface of the radio wave shielding layer 3 so that the radio wave absorbing sheet can be easily attached to a predetermined location. The adhesive layer 5 can be easily formed by applying an adhesive resin paste.

[0044] The adhesive layer 5 is not an essential component of the radio wave absorbing sheet according to this embodiment. When placing the radio wave absorbing sheet at a predetermined location, an adhesive member may be placed on the side of the member to which the radio wave absorbing sheet is attached, and when placing the radio wave absorbing sheet at a predetermined location, an adhesive may be supplied between the radio wave absorbing sheet and the location, or a double-sided tape may be used for adhesion.

[0045] [Details of each component] Next, the individual components constituting the radio wave absorbing sheet according to this embodiment, in which the resistive film is made of a conductive organic polymer and the sheet is flexible as a whole, will be described in detail.

[0046] <Resistance film> In the radio wave absorbing sheet according to this embodiment, the resistive film is made of a conductive organic polymer.

[0047] As the conductive organic polymer, a conjugated conductive organic polymer is used, and it is preferable to use polythiophene or its derivatives, or polypyrrole or its derivatives.

[0048] Specific examples of polythiophene-based conductive polymers suitable for use in the resistive film of the radio wave absorbing sheet according to this embodiment include poly(thiophene), poly(3-methylthiophene), poly(3-ethylthiophene), poly(3-propylthiophene), poly(3-butylthiophene), poly(3-hexylthiophene), poly(3-heptylthiophene), poly(3-octylthiophene), poly(3-decylthiophene), poly(3-dodecylthiophene), poly(3-octadecylthiophene), and poly(3-bromothiophene). thiophene), poly(3-chlorothiophene), poly(3-iodothiophene), poly(3-cyanothiophene), poly(3-phenylthiophene), poly(3,4-dimethylthiophene), poly(3,4-dibutylthiophene), poly(3-hydroxythiophene), poly(3-methoxythiophene), poly(3-ethoxythiophene), poly(3-butoxythiophene), poly(3-hexyloxythiophene), poly(3-heptyloxythiophene), poly(3-octyloxythiophene), poly(3-decyloxythiophene), peroxythiophene), poly(3-dodecyloxythiophene), poly(3-octadecyloxythiophene), poly(3,4-dihydroxythiophene), poly(3,4-dimethoxythiophene), poly(3,4-diethoxythiophene), poly(3,4-dipropoxythiophene), poly(3,4-dibutoxythiophene), poly(3,4-dihexyloxythiophene), poly(3,4-diheptyloxythiophene), poly(3,4-dioctyloxythiophene), poly(3,4-didecyloxythiophene) ), poly(3,4-didodecyloxythiophene), poly(3,4-ethylenedioxythiophene), poly(3,4-propylenedioxythiophene), poly(3,4-butenedioxythiophene), poly(3-methyl-4-methoxythiophene), poly(3-methyl-4-ethoxythiophene), poly(3-carboxythiophene), poly(3-methyl-4-carboxythiophene), poly(3-methyl-4-carboxyethylthiophene), poly(3-methyl-4-carboxybutylthiophene), and the like.

[0049] Specific examples of polypyrrole-based conductive polymers suitable for use in the resistive film include polypyrrole, poly(N-methylpyrrole), poly(3-methylpyrrole), poly(3-ethylpyrrole), poly(3-n-propylpyrrole), poly(3-butylpyrrole), poly(3-octylpyrrole), poly(3-decylpyrrole), poly(3-dodecylpyrrole), poly(3,4-dimethylpyrrole), poly(3,4-dibutylpyrrole), and poly(3-carboxypyrrole). Examples of suitable olefin copolymers include poly(3-methyl-4-carboxypyrrole), poly(3-methyl-4-carboxyethylpyrrole), poly(3-methyl-4-carboxybutylpyrrole), poly(3-hydroxypyrrole), poly(3-methoxypyrrole), poly(3-ethoxypyrrole), poly(3-butoxypyrrole), poly(3-hexyloxypyrrole), poly(3-methyl-4-hexyloxypyrrole), and poly(3-methyl-4-hexyloxypyrrole).

[0050] In addition, organic polymers whose main chains are composed of a π-conjugated system can be used as the resistive film, such as polyacetylene-based conductive polymers, polyphenylene-based conductive polymers, polyphenylene vinylene-based conductive polymers, polyaniline-based conductive polymers, polyacene-based conductive polymers, polythiophene vinylene-based conductive polymers, and copolymers thereof.

[0051] The conductive organic polymer used in the resistive film can use a polyanion as a counter anion. While there are no particular limitations on the polyanion, it is preferable for the polyanion to contain an anionic group capable of generating chemical oxidation doping in the conjugated conductive organic polymer used in the resistive film 1. Examples of such anionic groups include groups represented by the general formulas -O-SO3X, -O-PO(OX)2, -COOX, and -SO3X (where X represents a hydrogen atom or an alkali metal atom). Among these, groups represented by -SO3X and -O-SO3X are particularly preferred due to their excellent doping effect on the conjugated conductive organic polymer.

[0052] The conductive organic polymers may be used alone or in combination of two or more. Among the materials exemplified above, a polymer consisting of one or two selected from polypyrrole, poly(3-methoxythiophene), poly(3,4-ethylenedioxythiophene), poly(2-anilinesulfonic acid), and poly(3-anilinesulfonic acid) is preferred because it has higher transparency and conductivity.

[0053] In particular, it is preferable to use poly(3,4-ethylenedioxythiophene: PEDOT) and polystyrene sulfonic acid (PSS) as a combination of a conjugated conductive organic polymer and a polyanion.

[0054] Furthermore, in the resistive film of the electromagnetic wave absorbing sheet according to this embodiment, a dopant can be used in combination to control the electrical conductivity of the conductive organic polymer and match the input impedance of the electromagnetic wave absorbing sheet to the impedance value in air. Examples of dopants that can be used include halogens such as iodine and chlorine, Lewis acids such as BF3 and PF5, protonic acids such as nitric acid and sulfuric acid, transition metals, alkali metals, amino acids, nucleic acids, surfactants, dyes, chloranil, tetracyanoethylene, and TCNQ. The surface resistance of the resistive film can be adjusted by the blending ratio of the conductive organic polymer and the dopant. A preferred blending ratio of the conductive organic polymer and the dopant is, for example, a mass ratio of conductive polymer:dopant of 1:2 to 1:4.

[0055] Furthermore, the material for forming the resistive film preferably also contains polyvinylidene fluoride and water-soluble polyester, which improves the weather resistance of the resistive film, suppresses changes in the surface resistance of the resistive film over time, and widens the design latitude for the material and thickness of the protective layer that protects the resistive film.

[0056] When polyvinylidene fluoride is added to a composition used to coat a conductive organic polymer, it functions as a binder in the conductive organic polymer film, improving film-forming properties and increasing adhesion to the substrate.

[0057] Furthermore, because water-soluble polyesters have high compatibility with conductive polymers, adding water-soluble polyesters to the coating composition of conductive organic polymers that form the resistive film fixes the conductive polymers within the resistive film 1, making it possible to form a more uniform film. As a result, by using water-soluble polyesters, it is possible to reduce changes in surface resistance even when the film is placed in a harsher environment of high temperature and humidity.

[0058] The content of the conductive organic polymer in the resistive film is preferably 10% by mass or more and 35% by mass or less, based on the total mass of the solids contained in the resistive film composition. If the content is less than 10% by mass, the conductivity of the resistive film tends to decrease. Therefore, if the surface electrical resistance of the resistive film is set within a predetermined range to achieve impedance matching, the resistive film tends to become thicker, which tends to result in an increase in the overall thickness of the radio wave absorbing sheet and a decrease in optical properties. On the other hand, if the content exceeds 35% by mass, the structure of the conductive organic polymer reduces the applicability of the resistive film coating, making it difficult to form a good resistive film. The resistive film also tends to have increased haze, which also tends to result in a decrease in optical properties.

[0059] The resistive film may also be configured to contain a carbon material such as a carbon microcoil, a carbon nanotube, or graphene.

[0060] Carbon microcoils are a type of vapor-grown carbon fiber obtained primarily by catalytically activated pyrolysis of acetylene, and are materials with a 3D helical / spiral structure with coil diameters on the order of microns. The coil diameter is preferably 1 to 10 μm, the carbon fiber forming the coil has a diameter of 0.1 to 1 μm, and the coil length is preferably 1 to 10 mm.

[0061] Specifically, carbon nanotubes can be obtained by vapor phase growth methods such as arc discharge, laser evaporation, pyrolysis, etc. The carbon nanotubes used as the resistive film of the radio wave absorbing sheet according to this embodiment may be either single-walled or multi-walled.

[0062] Graphene can be obtained by, for example, peeling and transfer, SiC pyrolysis, chemical vapor deposition, cutting carbon nanotubes, etc. As the graphene used as the resistive film of the electromagnetic wave absorbing sheet according to this embodiment, it is preferable to use powdered graphene in the form of a scale, from the viewpoint of easily obtaining a desired aspect ratio and of orientation in the electromagnetic wave absorbing sheet.

[0063] The resin in which the carbon material is dispersed may be a water-soluble polyester resin.

[0064] The resistive film can be formed by applying a coating composition as a paint for forming a resistive film onto a substrate and drying it, as described above.

[0065] Examples of methods that can be used to apply the resistive coating paint to a substrate include bar coating, reverse coating, gravure coating, microgravure coating, die coating, dipping, spin coating, slit coating, and spray coating. Drying after application is preferably performed at 100 to 150°C for 5 to 60 minutes under conditions that allow the solvent component of the resistive coating paint to evaporate. Residual solvent in the resistive coating tends to reduce its strength. Examples of drying methods include hot air drying, heat drying, vacuum drying, and natural drying. If necessary, the resistive coating may be formed by irradiating the coating with UV light (ultraviolet rays) or EB (electron beams) to cure the coating.

[0066] The substrate used to form the resistive film is not particularly limited, but a transparent substrate having transparency is preferred. Examples of materials that can be used for such a transparent substrate include resin, rubber, glass, and ceramics.

[0067] <Dielectric layer> The dielectric layer of the radio wave absorbing sheet according to this embodiment can be formed from a dielectric material such as titanium oxide, polyvinylidene fluoride, polyester resin, glass, or silicone rubber. The dielectric layer can be formed as a single layer using one type of material, or can be formed by laminating two or more layers of the same or different materials. The dielectric layer can be formed by coating, press molding, extrusion molding, or the like.

[0068] As described above, the radio wave absorbing sheet according to this embodiment is a radio wave interference type (λ / 4 type) radio wave absorbing sheet that absorbs radio waves by shifting the phase of the radio waves incident on the radio wave absorbing sheet and the reflected waves reflected by the radio wave shielding layer by half a wavelength, so that the incident waves and the reflected waves cancel each other out. For this reason, the thickness of the dielectric layer (d in FIG. 1) is determined according to the wavelength of the radio waves to be absorbed.

[0069] The value of d is d=λ / 4 when there is space between the resistive film and the radio wave shielding layer, i.e., when the dielectric layer is made of air, but when the dielectric layer is made of a material with a dielectric constant of εr, d=λ / 4(εr) -1 / 2 This is preferable because, when the radio wave absorbing sheet according to this embodiment is flexible, the dielectric layer becomes thinner, the thickness of the entire radio wave absorbing sheet becomes thinner, and the sheet can be easily curved. Furthermore, considering that the radio wave absorbing sheet according to this embodiment is often used by being attached to a member from which radio wave leakage is to be prevented using an adhesive layer or the like, which will be described later, it is preferable that the radio wave absorbing sheet is thin and can easily conform to the shape of the attachment portion, and that the sheet is lightweight.

[0070] The thickness of the dielectric layer of the radio wave absorbing sheet according to this embodiment is a design thickness (reference thickness) dst (=λ / 4(εr)) calculated based on the frequency λ of the radio wave absorbed by the dielectric constant εr of the dielectric layer. -1 / 2 ) The range of the reduction rate of the thickness from this standard thickness is determined depending on the frequency of the radio waves to be absorbed by the radio wave absorbing sheet. The reduction rate of the thickness of the dielectric layer will be described in detail later.

[0071] <Radio wave shielding layer> The radio wave shielding layer of the radio wave absorbing sheet according to this embodiment is a member that is arranged on the opposite side of the radio wave absorbing sheet via a dielectric layer and that reflects radio waves that are incident from the resistive film side.

[0072] In order to provide flexibility as a radio wave absorbing sheet, metal foil is preferred as the material for the radio wave shielding layer, and various metal foils such as copper foil, aluminum foil, and gold foil can be used. Among these, aluminum foil is preferred as the radio wave shielding layer, considering cost and the effect of oxidation in air. Metal foils such as aluminum foil that form the radio wave shielding layer can be easily produced by rolling a metal material. Furthermore, when the radio wave shielding layer is formed as a vapor-deposited film in which a metal is vapor-deposited on the surface of a non-metallic material, it is preferred to appropriately select a vapor deposition method that has been conventionally used to form various vapor-deposited films, taking into consideration the heat resistance temperature of the metal material to be vapor-deposited and the non-metallic material, such as a resin, that serves as the base material.

[0073] When aluminum foil is used as a flexible radio wave absorbing sheet, the thickness of the radio wave shielding layer is preferably 1 μm to 20 μm.

[0074] Furthermore, in the radio wave absorbing sheet according to this embodiment, by forming a vapor-deposited film of a metal material directly on the surface of the dielectric layer opposite the side on which the resistive film is formed, the radio wave shielding layer can be formed solely from a vapor-deposited film of a conductive material such as metal. When a vapor-deposited film of metal is formed on the back side of the dielectric layer, no gap is created between the dielectric layer and the radio wave shielding layer, as compared to when the dielectric layer and the radio wave shielding layer are formed separately and then closely attached to each other. Therefore, radio waves that penetrate the dielectric layer can be reflected at the back surface of the dielectric layer, and the frequency of the radio waves absorbed can be accurately controlled by the thickness d of the dielectric layer.

[0075] On the other hand, when a vapor-deposited film is used as the radio wave shielding layer, the density of the conductive material in the vapor-deposited film needs to be uniform and sufficient compared to when a metal foil is used. According to the results of the inventors' investigations, the surface resistance of the radio wave shielding layer is 1×10 -1 It is preferable to make the surface resistance Ω / sq or less, and it is preferable to control the thickness of the metal vapor deposition film sufficiently to make the surface resistance value a desired value or less.

[0076] Furthermore, in order to provide a radio wave absorbing sheet with both flexibility and light transmission, a conductive mesh made of conductive fibers can be used as the radio wave shielding layer. For example, the conductive mesh can be made by attaching a metal to a mesh woven from polyester monofilament to make it conductive. Highly conductive metals such as copper and silver can be used as the metal. Furthermore, in order to reduce reflections from the metal film covering the surface of the mesh, products have also been developed that have a black anti-reflection layer applied to the outside of the metal film.

[0077] Another example of a radio wave shielding layer that can be used is a conductive grid in which thin metal wires, such as copper wires, with diameters of several tens to several hundreds of micrometers are arranged vertically and horizontally.

[0078] In addition, when the radio wave shielding layer is formed using the mesh or conductive grid described above, in order to ensure flexibility and translucency, it will be formed to have the minimum thickness possible as long as the surface resistance value required for the radio wave shielding layer can be achieved.

[0079] The aperture ratio of the radio wave shielding layer formed as a mesh or conductive grid is preferably larger from the viewpoint of ensuring light transmittance, but is preferably smaller from the viewpoint of reliably reflecting radio waves on the surface of the radio wave shielding layer and improving the radio wave absorption characteristics of the radio wave absorbing sheet. According to the studies of the inventors, the aperture ratio is preferably 35% or more and 85% or less, and more preferably 35% or more and 75% or less.

[0080] <Protective layer> In the radio wave absorbing sheet according to this embodiment, a protective layer is provided on the surface of the resistive film, which is the radio wave incident surface side.

[0081] By providing a protective layer, it is possible to effectively prevent the conductive organic polymer or carbon material used as the resistive film from changing in surface resistance due to the influence of humidity in the air, and to prevent the conductive organic polymer film, which is the resistive film, from being physically damaged.

[0082] As an example of the protective layer of the radio wave absorbing sheet of this embodiment, a film made of polyethylene terephthalate (PET), which is a polyester film, can be used, and for example, a protective layer formed on the surface coating can be obtained by applying a PET film to a predetermined thickness on a conductive organic polymer film, which is the surface coating, and drying it.Alternatively, a protective layer can be laminated on the surface coating by drying a PET film on another predetermined base film to form a thin film of polyethylene terephthalate, peeling it off from the base film, and attaching it to the surface of the resistive film.

[0083] In addition to the polyethylene terephthalate mentioned above, polyester films such as polybutylene terephthalate, polyethylene-2,6-naphthalate, and polytrimethylene terephthalate can be used as protective layers for the radio wave absorbing sheet according to this embodiment. Other examples include polyolefins such as polyethylene, polypropylene, polybutene-1, and poly-4-methylpentene, as well as copolymers thereof, and polyvinyl chloride. Polyethylene terephthalate is preferred from the viewpoints of film transparency, high protective function for protecting the resistive coating, and flexibility.

[0084] Furthermore, the thickness of the protective layer formed is preferably 10 μm or more and 150 μm or less. If the protective layer is thinner than 10 μm, it may not be able to fully perform its protective function of protecting the surface of the resistive film. A protective layer thickness of 150 μm is sufficient, and if the protective layer is thicker than 150 μm, the flexibility of the radio wave absorbing sheet may be adversely limited by the protective layer. Furthermore, if the protective layer is thicker than necessary, there is a concern that the process of laminating and adhering the protective layer to the surface of the resistive film may become extensive, and from the viewpoints of ease of manufacturing and manufacturing costs, a protective layer that is thicker than necessary is considered undesirable.

[0085] <Adhesive layer> To facilitate placement of the radio wave absorbing sheet according to this embodiment at a predetermined position, the adhesive layer formed on the back surface of the radio wave shielding layer can be made of known materials used for adhesive layers such as adhesive tapes, such as acrylic adhesives, rubber adhesives, and silicone adhesives. A tackifier or crosslinking agent can be used to adjust the adhesive strength to the adherend and reduce adhesive residue. The adhesive strength to the adherend is preferably 5 N / 10 mm to 12 N / 10 mm. If the adhesive strength is less than 5 N / 10 mm, the radio wave absorbing sheet may easily peel off or slip off from the adherend. If the adhesive strength is greater than 12 N / 10 mm, the radio wave absorbing sheet may be difficult to peel off from the adherend.

[0086] The thickness of the adhesive layer is preferably 20 μm to 100 μm. If the thickness of the adhesive layer is thinner than 20 μm, the adhesive strength will be weak, and the radio wave absorbing sheet may easily peel off or slip off from the adherend. If the thickness of the adhesive layer is thicker than 100 μm, the radio wave absorbing sheet will be difficult to peel off from the adherend. Furthermore, if the cohesive strength of the adhesive layer is weak, adhesive residue may remain on the adherend when the radio wave absorbing sheet is peeled off. This may also be a factor in reducing the flexibility of the radio wave absorbing sheet as a whole.

[0087] The adhesive layer used in the radio wave absorbing sheet according to this embodiment can be an adhesive layer that attaches the radio wave absorbing sheet to the object in an unremovable manner, or an adhesive layer that attaches the sheet in a removably manner. As mentioned above, it is not an essential requirement for the radio wave absorbing sheet according to this embodiment to have an adhesive layer, and the radio wave absorbing sheet can be attached to a desired member using any of various conventional adhesive methods.

[0088] (Example) The results of various studies conducted on the radio wave absorbing sheet according to this embodiment will be described below.

[0089] [Reduction in thickness of dielectric layer due to the formation of protective layer] As described above, in the radio wave absorbing sheet according to this embodiment, the protective layer 4 is formed on the surface of the resistive film 1. Then, in response to the peak frequency shift of the radio waves absorbed by the radio wave absorbing sheet caused by the formation of the protective layer 4, the thickness of the dielectric layer 2 is reduced from the reference thickness dst determined from the frequency of the radio waves to be absorbed by the radio wave absorbing sheet, thereby canceling (negating) the change in the peak frequency of the radio waves absorbed by the radio wave absorber due to the protective layer 4, and the radio wave absorbing sheet on which the protective layer 4 is formed can effectively absorb radio waves of the desired frequency. Below, the thickness reduction rate, which is the degree to which the thickness of the dielectric layer 2 is reduced, will be explained. In the following, in this specification, the thickness reduction rate, which is the degree to which the thickness of the dielectric layer 2 is reduced, will be represented as T. The thickness reduction rate T is the degree to which the thickness of the dielectric layer 2 is reduced from the reference thickness dst. is expressed as a negative percentage Therefore, the value of T Absolute value of A larger value of T results in a thinner thickness of the dielectric layer 2. Absolute value of A smaller value indicates that the thickness of the dielectric layer 2 is greater.

[0090] The inventors considered that the frequency peak shift of the absorbed radio waves caused by providing the protective layer 4 is caused by the fact that the protective layer 4, made of a dielectric, is laminated on the resistive film 3, and the incident radio waves are affected by the dielectric constant and thickness of the protective layer 4 when passing through it. Therefore, in a radio wave absorbing sheet having two dielectric layers, the protective layer 4 and the dielectric layer 2, the absorption rate of radio waves of frequencies above the millimeter wave band was calculated, and a simulation was performed to find an appropriate thickness of the dielectric layer 2 for absorbing radio waves of a predetermined frequency, and a thickness reduction rate T was calculated, which is the degree of reduction from a reference thickness dst, which is the preferable thickness of the dielectric layer 2 when the protective layer 4 is not taken into consideration.

[0091] <Simulation details> The simulation considered radio waves to be plane waves propagating in the positive and negative Z (axis) directions, with the propagation constant when this plane wave propagates in a uniform medium with permittivity ε and magnetic permeability μ set to γ ​​and the wave impedance to Z. It was assumed that both waves propagating in the positive Z direction and waves propagating in the negative Z direction exist simultaneously, and the electromagnetic field at each position in the Z direction is a combination of electric field and magnetic field components.

[0092] In this case, if the electric field component of the plane wave at Z=d is E1, the magnetic field component is H1, and the electric field component of the plane wave at Z=0 is E2, and the magnetic field component is H2, the fundamental determinant (1) becomes as follows:

[0093]

number

[0094] Figure 2 is a diagram explaining the concept of the simulation used in this embodiment. Figure 2(a) shows a state in which a plane wave is incident on a medium with a thickness d, and Figure 2(b) shows an equivalent circuit of an electromagnetic wave absorbing sheet in which a protective layer made of a dielectric is formed on a resistive film.

[0095] As shown in Figure 2(a), the simulation used in this embodiment deals with a state in which a radio wave (incident wave) enters a medium with a thickness of d from the left side of the figure, and a part of the wave passes through the medium (transmitted wave). In addition, the reflected wave reflected at the incident surface of the medium is also taken into consideration.

[0096] The incident surface side of the medium (the left side in Figure 2(a)) can be considered as Z = 0, and the exit surface side of the radio waves that have passed through (the right side in Figure 2(a)) as Z = d. If the electric field on the incident surface side (the Z = 0 side) is E2 and the magnetic field is H2, and the electric field on the exit surface side (the Z = d side) is E1 and the magnetic field is H1, the medium region can be expressed as the matrix shown in equation (2) below, based on the fundamental determinant (1) above. As shown in Figure 2(a), the electric field and magnetic field on the incident surface side are the sum of the incident wave and the reflected wave, and the electric field and magnetic field on the exit surface side are those of the transmitted wave.

[0097]

number

[0098] At this time, the input impedance Z seen from the incident surface of the medium in can be expressed by the following equation (3) using E1 = Z0 * H1. Also, the vacuum impedance Z0 (which is considered to be the same in air) is expressed by the following equation (4) using the permittivity ε0 and magnetic permeability μ0 in vacuum.

[0099]

number

[0100] The radio wave absorbing sheet according to this embodiment is configured such that, from the radio wave incident side, a protective layer 4 made of a dielectric, a resistive film 1 which is a resistive layer, a dielectric layer 2, and a radio wave shielding layer 3 which can be understood as a metal layer are laminated in this order, as shown in Fig. 1. Fig. 2(b) shows the configuration of this radio wave absorbing sheet as an equivalent circuit.

[0101] In this case, the input impedance of the entire radio wave absorbing sheet is expressed by the formula (5). Note that the protective layer 4 is expressed by a determinant using Ah, Bh, Ch, and Dh.

[0102]

number

[0103] Therefore, the input impedance of the entire radio wave absorbing sheet from the radio wave incident side can be expressed by the following formula (6).

[0104]

number

[0105] Here, when the return loss, which is the degree of reduction in the reflected wave relative to the incident wave, is expressed as Γ, and equation (6) is converted, the following equation (7) is obtained.

[0106]

number

[0107] By modifying this, E(1+Γ)=BH1 and E(1-Γ)=Z0DH1 are obtained, and the return loss Γ is calculated as in the following equation (9).

[0108]

number

[0109] In this embodiment, the return loss RL as an evaluation value of the radio wave absorption characteristics of the radio wave absorbing sheet is expressed in decibels using the return loss ratio Γ as shown in the following formula (10).

[0110]

number

[0111] <Dielectric layer thickness reduction rate> Using the above simulation, the reflection attenuation of incident radio waves in a radio wave absorbing sheet having a protective layer formed on a resistive film was determined, and the thickness of the dielectric layer that can effectively absorb radio waves when radio waves of a specified frequency are incident on the radio wave absorbing sheet with the protective layer formed thereon was calculated.

[0112] The "range in which radio waves are effectively absorbed" is defined as a range in which the return loss is 10 dB or more, that is, the ratio of reflected waves to incident waves is 1 / 10 or less. This is because it is believed that a radio wave absorber or sheet can be effective if it can reduce unwanted radio waves to 1 / 10 or less.

[0113] The dielectric constant of the protective layer was set to a range of 2 or more and 20 or less, taking into consideration that the dielectric constant of the polyethylene terephthalate exemplified above is 2.7, and the dielectric constants of various other materials that can be used as the protective layer. On the other hand, the film thickness of the protective layer was set to a range of 10 μm or more and 150 μm or less, taking into consideration the functional aspect of a thickness that can protect the resistive film, whether or not it significantly impairs the flexibility of the entire sheet, which is particularly required in the case of an electromagnetic wave absorbing sheet, and further, manufacturing conditions such as ease of manufacturing.

[0114] The dielectric constant of the dielectric layer is considered to be in the range of 2 or more and 8 or less, taking into consideration the above-mentioned dielectric layers containing dielectric materials such as titanium oxide, polyvinylidene fluoride, polyester resin, polyolefin resin, glass, and silicone rubber.

[0115] In the simulation, the minimum and maximum values ​​of the dielectric layer thickness dcal at which the return loss is 10 dB or more were determined for the return loss when radio waves of a specific frequency are incident on a radio wave absorbing sheet with a protective layer, by varying the dielectric constant of the protective layer within the ranges of 2 to 20, the thickness of the protective layer within the ranges of 10 μm to 150 μm, and the dielectric constant of the dielectric layer within the ranges of 2 to 8. Then, the thickness of the dielectric layer determined from the frequency of the incident radio waves in a state without a protective layer, that is, the calculation formula d=λ / 4(εr) when the dielectric constant of the dielectric layer is εr, was used. -1 / 2 The thickness reduction rate T (%) relative to the reference thickness dst obtained by the above calculation (T=((dst-dcal) / dst)×100) was calculated.

[0116] Furthermore, when actually manufactured as a radio wave absorbing sheet, the peak frequency of the radio waves actually absorbed by the radio wave absorbing sheet was set to be ±5% lower than the frequency of the incident radio waves that was originally set, in order to ensure sufficient radio wave absorption characteristics.

[0117] <Changes in the rate of reduction in thickness of the dielectric layer depending on the frequency of the absorbed radio waves> Using the above simulation, the inventors confirmed the change in the thickness reduction rate T of the dielectric layer while changing the frequency of the radio waves to be absorbed. Figures 3 and 4 show the change in the range of the thickness reduction rate T of the dielectric layer versus the frequency of the absorbed radio waves.

[0118] FIG. 3 shows the range of the thickness reduction rate of the dielectric layer for radio waves with frequencies between 28.5 GHz and 140 GHz.

[0119] In FIG. 3, the black circle 31 indicates the thickness reduction rate T Absolute value of t is the maximum value, that is, the value of the thickness reduction rate when the calculated thickness dcal of the dielectric layer is the thinnest. Also, the black triangles indicated by the reference numeral 33 indicate the thickness reduction rate T from the reference thickness dst set without considering the protective layer when the calculated thickness dcal is the thickest. Absolute value of This shows the case where is a small value.

[0120] As shown in Figure 3, the thickness reduction rate T Absolute value of The dashed line 31 showing the change in the maximum value of T varies in the ranges of 28.5 GHz to 60 GHz, 60 GHz to 90 GHz, and 90 GHz to 140 GHz. Specifically, when the frequency of the absorbed radio wave is in the range of 28.5 GHz to 60 GHz, the thickness reduction rate T Absolute value of The thickness reduction rate T Absolute value of The value of becomes larger, but the slope of the dashed line 31 showing the degree of change becomes smaller. Furthermore, when the frequency of the absorbed radio wave is in the range of 90 GHz to 140 GHz, the thickness reduction rate T Absolute value of The value of is approximately 92% and remains almost constant. In other words, as the frequency range increases, the thickness reduction rate T Absolute value of The change in is getting smaller.

[0121] On the other hand, the black triangle 33 indicates the thickness dcal value obtained by calculation, which is the thickest, and the thickness reduction rate T from the reference thickness dst set without considering the protective layer. Absolute value of This shows the case where the thickness reduction rate T Absolute value of The value of is approximately 0.1% to approximately 1.5% over the entire frequency range from 28.5 GHz to 90 GHz.

[0122] Next, the inventors confirmed the change in the thickness reduction rate T of the dielectric layer when the radio waves incident on the radio wave absorbing sheet are in the higher frequency range of 200 GHz or higher and 414 GHz or lower. Note that, for the range of incident radio waves exceeding 140 GHz but lower than 200 GHz, it is extremely rare for such frequencies to be actually used in view of standby attenuation of radio waves, and there is thought to be little demand for radio wave absorbing sheets that absorb radio waves in this frequency range, so the specific thickness reduction rate T of the dielectric layer in this embodiment was not examined.

[0123] FIG. 4 shows the range of the thickness reduction rate of the dielectric layer for radio waves with frequencies between 200 GHz and 414 GHz.

[0124] In FIG. 4, the thickness reduction rate T of the dielectric layer indicated by the black circle (reference numeral 41) Absolute value of The maximum value of increases from about 60% to about 91% as the frequency of the radio wave increases (line 42). On the other hand, the thickness reduction rate T of the dielectric layer, indicated by the black triangle (symbol 43), Absolute value of At the minimum value, the change is small even when the radio frequency increases, ranging from about 1.5% to about 3.2%.

[0125] In this way, when the frequency of the radio waves that are incident on and absorbed by the radio wave absorbing sheet is between 200 GHz and 414 GHz, the thickness reduction rate of the dielectric layer T Absolute value of The maximum value of increases as the frequency of the incident radio wave increases. Absolute value of The minimum value of is compared with the value when the frequency is 28.5 GHz or more and 140 GHz or less, as shown in Figure 3. Absolute value of It can be seen that the thickness is slightly larger, that is, the degree of reduction from the reference thickness is larger.

[0126] As shown in Figures 3 and 4, it has been confirmed that the tendency of change in the thickness reduction rate T of the dielectric layer corresponding to the frequency of the incident radio waves differs for each of the four frequency ranges of the radio waves to be absorbed: (1) 28.5 GHz or higher and 60 GHz or lower, (2) over 60 GHz and lower, (3) over 90 GHz and lower, and (4) 200 GHz or higher and 414 GHz or lower. Therefore, in order to obtain a more preferable thickness of the dielectric layer that takes into account the influence of the protective layer, it is effective to first divide the frequency of the radio waves to be absorbed into the four frequency ranges (1) to (4) above, and then determine the preferable thickness reduction rate T corresponding to the dielectric constant of the material used in the dielectric layer for each frequency range. The details of this method are explained below.

[0127] <Relationship between the dielectric constant of a dielectric layer and the rate of thickness reduction of the dielectric layer according to the frequency range of the incident radio wave> FIG. 5 is a graph showing the relationship between the dielectric constant and the thickness reduction rate of a dielectric layer for radio waves with frequencies of 28.5 GHz or more and 60 GHz or less.

[0128] In FIG. 5, the black circle indicated by the reference numeral 51 represents the thickness reduction rate T Absolute value of The maximum value (Tmax) of the dielectric constant D of the dielectric layer is "-0.021 × D 2 +0.26×D−1.35” (Equation 1: symbol 52). On the other hand, in FIG. 5, the black triangles indicated by symbol 53 represent the thickness reduction rate T Absolute value of The minimum value (Tmin) of the dielectric constant D of the dielectric layer is "-0.0005 × D 2 +0.0056×D-0.017" (Equation 2: Symbol 54).

[0129] These formulas 1 and 2 were calculated by the least squares method based on the points (black circles: symbol 51, black triangles: symbol 53) shown in FIG. 5. Note that R 2 was 0.99 or more. The following formulas 3 to 8 were also calculated in the same manner.

[0130] From the above, when the frequency of the radio waves to be absorbed is between 28.5 GHz and 60 GHz, the thickness of the dielectric layer is determined based on the frequency of the incident radio waves and the dielectric constant of the material used for the dielectric layer, and by setting the thickness reduction rate T to a value in the range between the above formula (1) and formula (2) taking into account the dielectric constant of the material used for the dielectric layer, the radio waves can be absorbed well with a radio wave reflection attenuation rate of 10 dB or more.

[0131] FIG. 6 is a graph showing the relationship between the dielectric constant and the thickness reduction rate of a dielectric layer for radio waves with frequencies exceeding 60 GHz and not exceeding 90 GHz.

[0132] FIG. 7 is a graph showing the relationship between the dielectric constant and the thickness reduction rate of a dielectric layer for radio waves with frequencies exceeding 90 GHz and not exceeding 140 GHz.

[0133] Furthermore, FIG. 8 is a graph showing the relationship between the dielectric constant and the thickness reduction rate of the dielectric layer for radio waves with frequencies of 200 GHz or more and 420 GHz or less.

[0134] In FIG. 6, the black circle indicated by the reference numeral 61 represents the thickness reduction rate T Absolute value of The maximum value (Tmax) of the dielectric constant D of the dielectric layer is "-0.014 × D 2 +0.20×D−1.33” (Equation 3: symbol 62). On the other hand, in FIG. 6, the black triangles indicated by symbol 63 represent the thickness reduction rate T Absolute value of The minimum value (Tmin) of the dielectric constant D of the dielectric layer is "-0.0002 × D 2 -0.0029×D+0.008" (Equation 4: Code 64) 。 As in Figure 5 when the frequency of the radio waves to be absorbed is 28.5 GHz or more and 60 GHz or less, when the frequency of the radio waves is more than 60 GHz and is 90 GHz or less, the thickness of the dielectric layer is determined by the reference thickness dst of the dielectric layer calculated from the frequency of the incident radio waves and the dielectric constant of the material used for the dielectric layer, and by setting the thickness reduction rate T to be in the range between the above formula (3) and formula (4) taking into account the dielectric constant of the material used for the dielectric layer, it is possible to achieve good absorption of the radio waves with a return loss rate of 10 dB or more.

[0135] Similarly, when the frequency of the incident radio wave is more than 90 GHz and less than 140 GHz, as shown in FIG. 7, the black circle indicated by the symbol 71 indicates the thickness reduction rate T Absolute value of The maximum value (Tmax) of the dielectric constant D of the dielectric layer is "-0.013 × D 2 +0.19×D−1.32” (Equation 5: symbol 72). On the other hand, in FIG. 7, the black triangles indicated by symbol 73 represent the thickness reduction rate T Absolute value of The minimum value (Tmin) of the dielectric constant D of the dielectric layer is "-0.0003 × D 2 +0.0017×D-0.0024" (Equation 6: Code 74) 。 Furthermore, in FIG. 8, which shows the case where the frequency of the incident radio wave is 200 GHz or more and 4140 GHz or less, the black circle indicated by the reference numeral 81 indicates the thickness reduction rate T Absolute value ofThe maximum value (Tmax) of the dielectric constant D of the dielectric layer is "-0.0060 × D 2 +0.099×D−1.13” (Equation 7: symbol 82). On the other hand, in FIG. 8, the black triangles indicated by symbol 83 represent the thickness reduction rate T Absolute value of The minimum value (Tmin) of the dielectric constant D of the dielectric layer is "-0.0001 × D 2 +8×10 -5 ×D-0.017" (Equation 8: symbol 84).

[0136] As described above, in the radio wave absorbing sheet shown in this embodiment, taking into consideration that a protective layer having a dielectric constant of 2 to 20 and a thickness of 10 to 150 μm is formed, the thickness of the dielectric layer is set to a thickness dcal obtained by reducing the reference thickness dst of the dielectric layer, which is calculated from the dielectric constant of the material used in the dielectric layer and the frequency of the radio waves to be absorbed, by a predetermined thickness reduction rate T. In this way, a radio wave absorbing sheet that satisfactorily absorbs predetermined radio waves can be realized while taking into consideration the influence of the protective layer.

[0137] As described above, in the radio wave absorbing sheet of this embodiment, the thickness reduction rate T of the dielectric layer taking the protective layer into consideration can be calculated from the frequency of the radio waves to be absorbed and the dielectric constant of the material used in the dielectric layer. Therefore, a more preferable thickness of the dielectric layer can be set using only the same information as in the case of calculating the conventional reference thickness dst, i.e., the frequency of the absorbed radio waves and the dielectric constant of the dielectric layer, and a radio wave absorbing sheet with good radio wave absorption characteristics can be easily obtained.

[0138] <Example> Next, a specific example of actually producing an electromagnetic wave absorbing sheet will be described.

[0139] As described above, the radio wave absorber disclosed in the present application is characterized in that the thickness of the dielectric layer is reduced from the reference thickness dst by a predetermined thickness reduction rate T in consideration of the influence of the protective layer. Similarly, the method for producing a radio wave absorber disclosed in the present application is characterized in that a preferable thickness of the dielectric layer is calculated in consideration of the influence of the protective layer.

[0140] First, in forming the resistor film, the following components were added and mixed to prepare a resistor film solution. (1) 36.7 parts of conductive polymer dispersion Conductive polymer (PEDOT-PSS) manufactured by Heraeus: PH-1000 (product name), Solid content concentration 1.2% by mass (2) 5.6 parts of PVDF dispersion Arkema: LATEX32 (product name), Solid concentration 20% by mass, solvent water (3) 0.6 parts of water-soluble polyester solution Goo Chemical Industry Co., Ltd.: Plus Coat Z561 (product name) Solid content concentration 25% by mass (4) Organic solvent (dimethyl sulfoxide) 9.9 parts (5) Water-soluble solvent (ethanol) 30.0 parts (6) Water 17.2 parts.

[0141] The resistive film solution prepared above was applied by bar coating onto a 50 μm thick polyethylene terephthalate substrate, and then heated at 150° C. for 5 minutes to form a film.

[0142] Thereafter, a 110 μm thick silicone OCA manufactured by Nichiei Shinka Co., Ltd. was attached to the surface of the substrate opposite to the side on which the polyethylene terephthalate resistive film layer was applied. The radio wave shielding layer was formed using a conductive mesh Su-4X-13227 (product name) manufactured by Seiren Co., Ltd.

[0143] As described above, the thickness of the dielectric layer is determined based on the frequency of the radio waves to be absorbed by the radio wave absorbing sheet to be fabricated and the dielectric constant of the polyethylene terephthalate that forms the dielectric layer.

[0144] In this example, the frequency of the radio waves to be absorbed is 414 GHz. The material used for the dielectric layer is silicone OCA, with a dielectric constant D of 2.7. The polyethylene terephthalate used for the protective layer has a dielectric constant of 3.3, and the thickness of the protective layer is 50 μm.

[0145] First, the reference thickness dst of the dielectric layer obtained by the conventional method of determining the thickness of the dielectric layer without considering the protective layer is d=λ / 4(εr) -1 / 2 This gives 110 μm.

[0146] Since the frequency of the incident radio wave is 414 GHz, the thickness reduction rate T of the dielectric layer can be calculated based on Figure 8. Since the dielectric constant of the protective layer is 3.3, the thickness reduction rate is The absolute value of T Maximum value (max) -93% The absolute value of T The minimum value (min) is -1.6%, and -60.9% was selected as an example.

[0147] As a result, the thickness dcal of the dielectric layer taking into account the thickness reduction rate T is dst × (100-60.9) = 43 μm, and the material application thickness, pressing conditions, drying conditions, etc. are determined so that the dielectric layer becomes 43 μm.

[0148] After preparing the radio wave absorbing sheet as described above, a 50 μm thick polyethylene terephthalate sheet with an adhesive layer was attached as a protective layer to the surface of the resistive film of the radio wave absorbing sheet to prepare the radio wave absorbing sheet of this example.

[0149] (Measurement of radio wave absorption characteristics) The radio wave absorption characteristics of the radio wave absorbing sheet prepared above were measured using a THZ-TDS TAS7500SP (product name) manufactured by Advanced Test Co., Ltd. As with the results of the simulation described above, the radio wave absorption characteristics were measured by determining the amount of attenuation of the reflected wave relative to the incident wave as the return loss, and expressed in dB.

[0150] In the case of the radio wave absorbing sheet of the above example, the peak amount of radio wave absorption at a frequency of 414 GHz was 20 dB, and a high amount of radio wave absorption of 99% or more was achieved.

[0151] In addition, the radio wave absorbing sheet produced as an example is provided with a 50 μm thick protective layer, which means that even when the surface of the radio wave absorbing sheet is rubbed, the surface of the resistive film is not scraped off and the surface resistance value can be maintained.Furthermore, the protective layer itself is sufficiently able to withstand such friction.This was confirmed by the results of a sliding test conducted on a white flannel cloth set in a HEIDON sliding tester under the conditions of a load of 2000 g, a sliding speed of 4500 mm / min, a sliding width of 25 mm, and 1000 sliding passes (approximately 10 minutes).

[0152] Furthermore, a flexibility test was conducted in which the radio wave absorbing sheet produced as the above example was placed on a horizontally placed aluminum cylindrical rod (mandrel) with a diameter of 6 mm, with the protective layer side facing out, and 300 g weights were attached to both ends of the sheet and maintained for 30 seconds. No change was observed in the appearance or surface resistance of the sheet surface before and after the test, and no change occurred in the radio wave absorption properties either. This confirmed that the radio wave absorbing sheet of the above example has high flexibility.

[0153] As described above, the radio wave absorbing sheet according to this embodiment is a radio wave interference-type radio wave absorbing sheet comprising a resistive film, a dielectric layer, and a radio wave shielding layer laminated together, and includes a protective layer on the surface of the resistive film. Therefore, even when a flexible radio wave absorbing sheet is formed using a conductive polymer film as the resistive film, the protective layer can prevent the surface resistance of the resistive film from changing. Furthermore, to address the frequency shift of the absorbed radio waves caused by the protective layer having a dielectric constant of 2 to 20 and a thickness of 10 to 150 μm, the thickness of the dielectric layer is adjusted to a thickness dcal that takes into account a predetermined thickness reduction rate determined by the frequency of the absorbed radio waves and the dielectric constant of the dielectric layer, rather than a reference thickness dst calculated only from the radio wave frequency and dielectric constant. This allows for a radio wave absorbing sheet with high radio wave absorption characteristics.

[0154] The radio wave absorbing sheet disclosed in the present application can be realized as a radio wave absorbing sheet with stable, high radio wave absorbing properties and flexibility, by constructing the resistive film from a conductive organic polymer, which can maintain its radio wave absorbing properties even when the radio wave absorbing sheet is bent strongly.

[0155] Furthermore, even when the radio wave absorber is realized as one having a predetermined thickness relative to its surface area, if the radio wave absorber as a whole can be made flexible, the handling of the radio wave absorber when placing it at a predetermined position will be improved, and the radio wave absorber will be highly practical.

[0156] Furthermore, in the case of a radio wave absorber arranged in a tile-like pattern, for example, by making the structure as a whole translucent as described above, it is possible to provide a radio wave absorbing block that can be placed on a window, a transparent wall, etc., and through which the opposite side can be seen. Even in this case, the laminate of the resistive film and the protective layer is matched to the impedance in air, thereby achieving high radio wave absorption characteristics. [Industrial Applicability]

[0157] The radio wave absorber disclosed in the present application is useful as a radio wave interference-type radio wave absorber that can stably exhibit high radio wave absorption characteristics by having a protective layer on its surface. Furthermore, the method for producing a radio wave absorber disclosed in the present application is useful for producing a radio wave absorber that has a protective layer and has excellent radio wave absorption characteristics in consideration of the frequency shift of the absorbed radio waves caused by the protective layer. [Explanation of symbols]

[0158] 1 Resistive film 2. Dielectric Layer 3 Radio wave shielding layer 4 protective layer 5 Adhesive layer dst Reference thickness of the dielectric layer T: Dielectric layer thickness reduction rate dcal: Thickness of the dielectric layer taking into account the thickness reduction rate T

Claims

1. A radio wave interference type radio wave absorber formed by sequentially laminating a resistive film, a dielectric layer, and a radio wave shielding layer, The radio waves to be absorbed by the radio wave absorber are radio waves in a high frequency band equal to or higher than the millimeter wave band, a protective layer on the resistive film; the protective layer has a dielectric constant of 2 or more and 20 or less, and a thickness of 10 μm or more and 150 μm or less, and the dielectric constant D of the dielectric layer is 2 or more and 8 or less, A radio wave absorber characterized in that the thickness of the dielectric layer is thinner than a reference thickness dst determined according to the frequency of the absorbed radio wave and the dielectric constant of the dielectric layer, and satisfies any of the following conditions 1) to 4): 1) When the frequency of the absorbed radio waves is 28.5 GHz or more and 60 GHz or less, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between a curve showing the maximum value Tmax of the absolute value of the thickness reduction rate T expressed by the following (Equation 1) and a curve showing the minimum value Tmin of the absolute value of the thickness reduction rate T expressed by (Equation 2) with respect to the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Equation 1) Tmax = -0.021 × D 2 +0.26×D-1.35 (Equation 2) Tmin = -0.0005 × D 2 +0.0056×D-0.017 2) When the frequency of the absorbed radio waves is greater than 60 GHz and equal to or less than 90 GHz, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between a curve showing the maximum value Tmax of the absolute value of the thickness reduction rate T expressed by the following (Equation 3) and a curve showing the minimum value Tmin of the absolute value of the thickness reduction rate T expressed by (Equation 4) for the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Equation 3) Tmax = -0.014 × D 2 +0.20×D-1.33 (Equation 4) Tmin = -0.0002 × D 2 -0.0029 × D + 0.008 3) When the frequency of the absorbed radio waves is greater than 90 GHz and equal to or less than 140 GHz, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between a curve showing the maximum value Tmax of the absolute value of the thickness reduction rate T expressed by the following (Equation 5) and a curve showing the minimum value Tmin of the absolute value of the thickness reduction rate T expressed by (Equation 6) for the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Equation 5) Tmax = -0.013 × D 2 +0.19×D-1.32 (Equation 6) Tmin = -0.0003 × D 2 +0.0017×D-0.0024 4) When the frequency of the absorbed radio waves is 200 GHz or more and 414 GHz or less, the value of the reduction rate T of the thickness of the dielectric layer from the reference thickness dst is a value between a curve showing the maximum value Tmax of the absolute value of the thickness reduction rate T expressed by the following (Equation 7) and a curve showing the minimum value Tmin of the absolute value of the thickness reduction rate T expressed by (Equation 8) with respect to the dielectric constant D of the dielectric layer, and the deviation of the maximum absorption frequency of the radio waves actually absorbed by the radio wave absorber from the frequency of the absorbed radio waves is within ±5%. (Equation 7) Tmax = -0.0060 × D 2 +0.099×D-1.13 (Equation 8) Tmin = -0.0001 × D 2 +8×10 -5 ×D-0.017

2. A radio wave absorber as described in claim 1, having a reflection loss of 10 dB or more at the frequency of the absorbed radio wave.

3. 3. The radio wave absorber according to claim 1, wherein the resistive film is formed of a conductive organic polymer film.

4. 3. The radio wave absorber according to claim 1, wherein the resistive film contains at least one of a carbon microcoil, a carbon nanotube, and graphene.

5. 5. The radio wave absorber according to claim 1, wherein the protective layer, the resistive film, and the dielectric layer are all formed of light-transmitting materials, the radio wave shielding layer is made of a metal mesh, and the total light transmittance of the entire radio wave absorber is 30% or more.

6. 6. The radio wave absorber according to claim 1, further comprising an adhesive layer on the back surface of the radio wave shielding layer.

7. 7. The radio wave absorber according to claim 1, wherein the protective layer, the resistive film, the dielectric layer, and the radio wave shielding layer are all made into thin film shapes, and are formed as a flexible sheet as a whole.

8. A method for manufacturing a radio wave interference type radio wave absorber that absorbs radio waves in a high frequency band equal to or higher than a millimeter wave band, in which a resistive film, a dielectric layer, and a radio wave shielding layer are sequentially laminated, comprising: a protective layer having a dielectric constant of 2 or more and 20 or less and a thickness of 10 μm or more and 150 μm or less is provided on the resistive film; the dielectric constant D of the dielectric layer is 2 or more and 8 or less, a dielectric layer having a thickness smaller than a reference thickness dst determined in accordance with the frequency of the absorbed radio wave and the dielectric constant of the dielectric layer.

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