Electromagnetic wave absorber and paste for forming electromagnetic wave absorber

Abstract

Provided are: an electromagnetic wave absorber having good electromagnetic wave absorption characteristics regardless of an incident direction of an electromagnetic wave in a high frequency band; and a paste for forming an electromagnetic wave absorber, which is suitably used for manufacturing the electromagnetic wave absorber.　An electromagnetic wave absorber including an electromagnetic wave absorption layer containing a magnetic material, carbon nanotubes, and an organic salt is used.

Description

Electromagnetic wave absorber and paste for forming electromagnetic wave absorber

[0001] The present invention relates to an electromagnetic wave absorber and a paste for forming an electromagnetic wave absorber.

[0002] The use of high-frequency electromagnetic waves is becoming widespread in various information and communication systems, such as mobile phones, wireless LANs, ETC systems, intelligent road transport systems, driving assistance systems, and satellite broadcasting. However, the expanded use of high-frequency electromagnetic waves raises concerns that interference between electronic components may cause breakdowns or malfunctions in electronic devices. To address this issue, a method has been adopted in which unwanted electromagnetic waves are absorbed by electromagnetic wave absorbers.

[0003] For this reason, even in radars and the like that use electromagnetic waves in the high frequency band, electromagnetic wave absorbers are used to reduce the effects of unnecessary electromagnetic waves that should not be received. To meet such demands, various electromagnetic wave absorbers that can effectively absorb electromagnetic waves in the high frequency band have been proposed. A specific example is an electromagnetic wave absorbing sheet containing carbon nanocoils and a resin (for example, Patent Document 1).

[0004] Among applications of electromagnetic waves in the high frequency band, research has progressed in the field of automobile driving assistance systems. In such automobile driving assistance systems, electromagnetic waves in the 76 GHz band are used in on-board radar for detecting inter-vehicle distances, etc. It is predicted that the use of electromagnetic waves in high frequency bands, for example, 100 GHz or higher, will expand not only to automobile driving assistance systems but also to various other applications. For this reason, there is a demand for electromagnetic wave absorbers that can effectively absorb electromagnetic waves in the 76 GHz band and higher frequency bands.

[0005] In order to meet such a demand, as an electromagnetic wave absorber that can effectively absorb electromagnetic waves over a wide range in the high frequency band, for example, ε-Fe ２ O ３ There have been proposed electromagnetic wave absorbers having an electromagnetic wave absorbing layer containing magnetic crystals made of iron oxides of this type (for example, Patent Documents 2 and 3, and Non-Patent Documents 1 to 5).

[0006] JP 2009-060060 A JP 2008-277726 A WO08 / 149785

[0007] A. Namai, S. Sakurai, M. Nakajima, T. Suemoto, K. Matsumoto, M. Goto, S. Sasaki, and S. Ohkoshi, J. Am. Chem. Soc., 131, 1170-1173 (2009). A. Namai, M. Yoshikiyo, K. Yamada, S. Sakurai, T. Goto, T. Yoshida, T. Miyazaki, M. Nakajima, T. Suemoto, H. Tokoro, and S. Ohkoshi, Nature Communications, 3, 1035 / 1-6 (2012). S. Ohkoshi, S. Kuroki, S. Sakurai, K. Matsumoto, K. Sato, and S. Sasaki, Angew. Chem. Int. Ed. , 46, 8392-8395 (2007). A. Namai, K. Ogata, M. Yoshikiyo, and S. Ohkoshi, Bull. Chem. Soc. Jpn. , 93, 20-25 (2020). M. Yoshikiyo, Y. Futakawa, R. Shimoharai, Y. Ikeda, J. MacDougall, A. Namai, S. Ohkoshi,Chem. Phys. Lett. , 803, 139821 (2022).

[0008] However, when an electromagnetic wave absorber such as that described in Patent Document 2 absorbs electromagnetic waves in a high frequency band such as millimeter waves, the electromagnetic wave absorption characteristics may differ depending on the direction of incidence of the electromagnetic waves onto the electromagnetic wave absorber. For this reason, there is a strong demand for an electromagnetic wave absorber that has good electromagnetic wave absorption characteristics in the high frequency band, regardless of the direction of incidence of the electromagnetic waves.

[0009] The present invention has been made in consideration of the above-mentioned problems of the conventional technology, and aims to provide an electromagnetic wave absorber that has good electromagnetic wave absorption characteristics in the high frequency band, regardless of the direction of incidence of the electromagnetic waves, and an electromagnetic wave absorber-forming paste that is suitably used for producing the electromagnetic wave absorber.

[0010] The present inventors discovered that the above problems can be solved by using an electromagnetic wave absorber including an electromagnetic wave absorbing layer containing a magnetic material, carbon nanotubes, and an organic salt, and thus completed the present invention.

[0011] A first aspect of the present invention is an electromagnetic wave absorber comprising an electromagnetic wave absorbing layer containing a magnetic material, carbon nanotubes, and an organic salt.

[0012] A second aspect of the present invention is a paste for forming an electromagnetic wave absorber, which contains a magnetic material, carbon nanotubes, and an organic salt.

[0013] According to the present invention, it is possible to provide an electromagnetic wave absorber that has good electromagnetic wave absorption characteristics in the high frequency band regardless of the direction of incidence of the electromagnetic waves, and a paste for forming an electromagnetic wave absorber that is suitably used for producing the electromagnetic wave absorber.

[0014] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments and can be implemented with appropriate modifications within the scope of the object of the present invention.

[0015] <Electromagnetic Wave Absorber> The electromagnetic wave absorber includes an electromagnetic wave absorbing layer made of a magnetic material, carbon nanotubes, and an organic salt. The electromagnetic wave absorber may consist of only the electromagnetic wave absorbing layer, or may include a substrate layer that supports the electromagnetic wave absorbing layer.

[0016] From the viewpoint of more reliably absorbing high-frequency electromagnetic waves of the millimeter wave band or higher, the electromagnetic wave absorber preferably absorbs electromagnetic waves in a frequency band of 30 gigahertz (GHz) or higher, preferably 30 GHz or higher and 350 GHz or lower, and more preferably 60 GHz or higher and 350 GHz or lower. The electromagnetic wave absorption of the electromagnetic wave absorber is a value measured on the surface where the electromagnetic wave absorbing layer is exposed.

[0017] The form of the electromagnetic wave absorber is not particularly limited. The form of the electromagnetic wave absorber is preferably a sheet or film, and more preferably a film. When the electromagnetic wave absorber is in the form of a film, the film may have a curved surface or may be composed of only a flat surface, and a flat plate shape is preferred. From the viewpoint of making the film thinner or smaller without impairing the desired effect, the thickness of the film as an electromagnetic wave absorber is preferably 1000 μm or less, more preferably 900 μm or less, even more preferably 450 μm or less, and particularly preferably 300 μm or less. The thickness of the film as an electromagnetic wave absorber may be uniform or non-uniform.

[0018] <Electromagnetic Wave Absorbing Layer> The electromagnetic wave absorbing layer contains a magnetic material, carbon nanotubes, and an organic salt.

[0019] The electromagnetic wave absorbing layer has the property of absorbing electromagnetic waves in the frequency range of 60 GHz or more and 350 GHz or less.

[0020] The thickness of the electromagnetic wave absorbing layer is not particularly limited as long as it does not impair the object of the present invention. The thickness of the electromagnetic wave absorbing layer is preferably 150 μm or less, more preferably 100 μm or less, from the viewpoint of the balance between thinning of the electromagnetic wave absorber and electromagnetic wave absorption performance. There is no particular limit to the lower limit of the thickness of the electromagnetic wave absorbing layer as long as it does not impair the effects of the present invention. The thickness of the electromagnetic wave absorbing layer may be, for example, 1 μm or more, or 10 μm or more. The thickness of the electromagnetic wave absorbing layer may be uniform or non-uniform.

[0021] The essential and optional components of the electromagnetic wave absorbing layer will be described below.

[0022] [Magnetic Material] The type of magnetic material is not particularly limited as long as the electromagnetic wave absorber exhibits the desired electromagnetic wave absorption characteristics. The magnetic material preferably contains epsilon-type iron oxide. An electromagnetic wave absorber having an electromagnetic wave absorbing layer containing epsilon-type iron oxide as the magnetic material particularly effectively absorbs electromagnetic waves in high frequency bands above the millimeter wave band.

[0023] When the magnetic material contains epsilon-type iron oxide, the magnetic material may contain, together with the epsilon-type iron oxide, a magnetic material having the ability to absorb electromagnetic waves. Preferred examples of magnetic materials that can be used together with the epsilon-type iron oxide include barium ferrite magnetic materials and strontium ferrite magnetic materials. In order to obtain good electromagnetic wave absorption properties of the electromagnetic wave absorber, the ratio of the mass of the epsilon-type iron oxide to the total mass of the epsilon-type iron oxide and the magnetic materials other than the epsilon-type iron oxide is preferably 70 mass% or more, more preferably 80 mass% or more, even more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 100 mass%.

[0024] The electromagnetic wave absorbing layer typically preferably contains a binder resin for the purposes of uniformly dispersing the magnetic material in the electromagnetic wave absorbing layer and making the electromagnetic wave absorbing layer easily moldable, i.e., the magnetic material is preferably dispersed in the binder resin in the electromagnetic wave absorbing layer.

[0025] Hereinafter, epsilon iron oxide, which is a suitable example of a magnetic material, will be described.

[0026] (Epsilon-type iron oxide) As the epsilon-type iron oxide, ε-Fe ２ O ３ Crystal, and crystal structure and space group are ε-Fe ２ O ３ and ε-Fe ２ O ３ A part of the Fe site of the crystal is substituted with an element M other than Fe, and has the formula ε-M ｘ Fe ２－ｘ O ３ where x is 0 or more and 2 or less (preferably 0 or more and less than 2). Since such epsilon-type iron oxide crystals are magnetic crystals, the crystals may be referred to as "magnetic crystals" in the present specification.

[0027] ε-Fe ２ O ３ Any crystal can be used. The crystal structure and space group are ε-Fe ２ O３ and ε-Fe ２ O ３ A part of the Fe site of the crystal is substituted with an element M other than Fe, and has the formula ε-M ｘ Fe ２－ｘ O ３ In this specification, the crystal represented by the formula (1) where x is 0 or more and 2 or less (preferably 0 or more and less than 2) will be described later. ２ O ３ ε-M in which part of the Fe site of the crystal is replaced by a substitution element M ｘ Fe ２－ｘ O ３ "M-substituted ε-Fe ２ O ３ " is also called.

[0028] ε-Fe ２ O ３ Crystalline and / or M-substituted ε-Fe ２ O ３ The particle size of the particles having magnetic crystals in the magnetic phase is not particularly limited as long as it does not impair the object of the present invention. For example, particles having magnetic crystals of epsilon-type iron oxide in the magnetic phase, produced by the method described below, have an average particle size measured from a TEM (transmission electron microscope) photograph in the range of 5 nm to 200 nm. Furthermore, the coefficient of variation (standard deviation of particle size / average particle size) of particles having magnetic crystals of epsilon-type iron oxide in the magnetic layer, produced by the method described below, is in the range of less than 80%, and they are a group of relatively fine particles with a uniform particle size.

[0029] In a preferred electromagnetic wave absorbing layer, magnetic particles of such epsilon iron oxide (i.e., ε-Fe ２ O ３ Crystalline and / or M-substituted ε-Fe ２ O ３ The powder of ε-Fe (particles with crystals in the magnetic phase) is used as the magnetic material in the electromagnetic wave absorbing layer. The "magnetic phase" here refers to the part of the powder that is responsible for its magnetism. ２ O ３ Crystalline and / or M-substituted ε-Fe ２ O ３ "Having crystals in the magnetic phase" means that the magnetic phase is ε-Fe ２ O ３ Crystalline and / or M-substituted ε-Fe２ O ３ This means that the magnetic phase is made up of crystals, and includes the case where the magnetic phase contains impurity magnetic crystals that are unavoidable during manufacturing.

[0030] The magnetic crystal of epsilon-type iron oxide is ε-Fe ２ O ３ Impurity crystals of iron oxides that have different space groups and oxidation states from the crystals (specifically, α-Fe ２ O ３ , γ-Fe ２ O ３ , FeO, and Fe ３ O ４ , and in these crystals, a part of the Fe may be substituted with other elements). When the magnetic crystals of epsilon-type iron oxide contain impurity crystals, ε-Fe ２ O ３ and / or M-substituted ε-Fe ２ O ３ That is, among the magnetic crystals of the epsilon iron oxide constituting the magnetic material, ε-Fe is preferably the main phase. ２ O ３ and / or M-substituted ε-Fe ２ O ３ The proportion of the magnetic crystals is preferably 50 mol % or more in terms of molar ratio as a compound.

[0031] The abundance ratio of the crystals can be determined by analysis using the Rietveld method based on the X-ray diffraction pattern. The silica (SiO ２ ) and other non-magnetic compounds may adhere to the surface.

[0032] (M-substituted ε-Fe ２ O ３ ) The crystal and space group are ε-Fe ２ O ３ and ε-Fe ２ O ３ As long as the condition that a part of the Fe site of the crystal is substituted with an element M other than Fe is satisfied, M-substituted ε-Fe ２ O ３ The type of element M in the formula is not particularly limited. ２ O ３may contain a plurality of elements M other than Fe.

[0033] Suitable examples of the element M include In, Ga, Al, Sc, Cr, Sm, Yb, Ce, Ru, Rh, Ti, Co, Ni, Mn, Zn, Zr, and Y. Among these, In, Ga, Al, Ti, Co, and Rh are preferred. When M is Al, ε-M ｘ Fe ２－ｘ O ３ In the composition represented by the formula (I), x is preferably, for example, in the range of 0 or more and less than 0.8. When M is Ga, x is preferably, for example, in the range of 0 or more and less than 0.8. When M is In, x is preferably, for example, in the range of 0 or more and less than 0.3. When M is Rh, x is preferably, for example, in the range of 0 or more and less than 0.3. When M is Ti and Co, x is preferably, for example, in the range of 0 or more and less than 1.

[0034] The frequency at which the amount of electromagnetic wave absorption is maximum is M-substituted ε-Fe ２ O ３ The amount of substitution of element M can be adjusted by adjusting at least one of the type and the amount of substitution of element M in the above.

[0035] Such M-substituted ε-Fe ２ O ３ The magnetic crystals can be synthesized, for example, by a process combining a reverse micelle method and a sol-gel method, and a firing process, as described later. Furthermore, as disclosed in Japanese Patent Laid-Open Publication No. 2008-174405, M-substituted ε-Fe can be synthesized by a process combining a direct synthesis method and a sol-gel method, and a firing process. ２ O ３ Magnetic crystals can be synthesized.

[0036] Specifically, Jian Jin, Shin-ichi Ohkoshi and Kazuki Hashimoto, ADVANCED MATERIALS 2004, 16, No. 1, January 5, p. 48-51, Shin-ichi Ohkoshi, Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto, JOURNAL OF APPLIED PHYSICS, 97, 10K312 (2005), Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto and Shin-ichi Ohkoshi, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, Vol. 74, No. 7, July, 2005, p. As described in "1946-1949," Asuka Namai, Shunsuke Sakurai, Makoto Nakajima, Tohru Suemoto, Kazuki Matsumoto, Masahiro Goto, Shinya Sasaki, and Shin-ichi Ohkoshi, Journal of the American Chemical Society, Vol. 131, pp. 1170-1173, 2009, etc., M-substituted ε-Fe was synthesized by a process combining a reverse micelle method and a sol-gel method and a baking process. ２ O ３ Magnetic crystals can be obtained.

[0037] In the reverse micelle method, two types of micellar solutions containing surfactants, namely micellar solution I (raw micelles) and micellar solution II (neutralizer micelles), are mixed together to allow the precipitation reaction of iron hydroxide to proceed within the micelles. Next, a silica coating is applied to the surface of the iron hydroxide fine particles produced within the micelles by the sol-gel method. After being separated from the liquid, the iron hydroxide fine particles with the silica coating layer are subjected to heat treatment in an air atmosphere at a predetermined temperature (within the range of 700 to 1300°C). This heat treatment results in the formation of ε-Fe ２ O ３ Fine particles of crystals are obtained.

[0038] More specifically, for example, M-substituted ε-Fe can be prepared as follows: ２O ３ Magnetic crystals are produced.

[0039] First, iron (III) nitrate as an iron source, M nitrate salts as M element sources for substituting part of the iron (aluminum (III) nitrate nonahydrate in the case of Al, gallium (III) nitrate hydrate in the case of Ga, indium (III) nitrate trihydrate in the case of In, titanium (IV) sulfate hydrate and cobalt (II) nitrate hexahydrate in the cases of Ti and Co), and a surfactant (e.g., cetyltrimethylammonium bromide) are dissolved in the aqueous phase of micellar solution I, which has n-octane as the oil phase.

[0040] An appropriate amount of alkaline earth metal (Ba, Sr, Ca, etc.) nitrate can be dissolved in the aqueous phase of micellar solution I. This nitrate acts as a shape control agent. When alkaline earth metals are present in the solution, rod-shaped M-substituted ε-Fe ions are eventually formed. ２ O ３ In the absence of a shape control agent, particles of magnetic crystals are obtained. ２ O ３ Magnetic crystal grains are obtained.

[0041] The alkaline earth metal added as a shape control agent is used to form M-substituted ε-Fe. ２ O ３ It may remain in the surface layer of the magnetic crystal. M-substituted ε-Fe ２ O ３ The mass of alkaline earth metal in magnetic crystal is M-substituted ε-Fe ２ O ３ The amount of the substitution element M is preferably 20% by mass or less, and more preferably 10% by mass or less, of the total mass of the substitution element M and the mass of Fe in the magnetic crystal.

[0042] Aqueous ammonia solution is used as the aqueous phase of micellar solution II, which uses n-octane as the oil phase.

[0043] After mixing micellar solutions I and II, the sol-gel method is applied. Specifically, silane (e.g., tetraethylorthosilane) is added dropwise to the mixed micellar solution while stirring is continued, and a reaction to form iron hydroxide or iron hydroxide containing element M is allowed to proceed within the micelles. As a result, the particle surfaces of the fine iron hydroxide precipitate formed within the micelles are coated with silica formed by hydrolysis of the silane.

[0044] Next, the silica-coated M element-containing iron hydroxide particles are separated from the liquid, washed, and dried to obtain a particle powder, which is then charged into a furnace and heat-treated (fired) in air at a temperature range of 700°C to 1300°C, preferably 900°C to 1200°C, and more preferably 950°C to 1150°C. This heat treatment causes an oxidation reaction to proceed within the silica coating, converting the fine particles of M element-containing iron hydroxide into fine M-substituted ε-Fe. ２ O ３ particles.

[0045] During this oxidation reaction, the presence of the silica coating ２ O ３ and γ-Fe ２ O ３ ε-Fe crystals, not ２ O ３ M-substituted ε-Fe with the same space group as ２ O ３ It contributes to the formation of crystals and also prevents sintering of particles. Furthermore, when an appropriate amount of alkaline earth metal is present, the particles tend to grow into rod-like shapes.

[0046] As described above, as disclosed in Japanese Patent Laid-Open No. 2008-174405, M-substituted ε-Fe can be obtained by a process combining a direct synthesis method and a sol-gel method and a firing process. ２ O ３ Magnetic crystals can be synthesized more economically.

[0047] Briefly, a precursor consisting of iron hydroxide (which may be partially substituted with another element) is formed by first adding a neutralizing agent such as aqueous ammonia to an aqueous solvent in which a trivalent iron salt and a salt of a substitution element M (Ga, Al, etc.) are dissolved, while stirring.

[0048] Then, a sol-gel method is applied to form a silica coating layer on the surface of the precursor particles. After separating the silica-coated particles from the liquid, they are heat-treated (fired) at a predetermined temperature to form M-substituted ε-Fe ２ O ３ Fine particles of magnetic crystals are obtained.

[0049] M-substituted ε-Fe as described above ２ O ３ In the synthesis of ε-Fe ２ O ３ Iron oxide crystals (impurity crystals) with different space groups and oxidation states from the crystals may be generated. ２ O ３ The most common polymorphism that has a different crystal structure while having the same composition is α-Fe. ２ O ３ and γ-Fe ２ O ３ Other iron oxides include FeO and Fe ３ O ４ The inclusion of such impurity crystals is due to the M-substituted ε-Fe ２ O ３ Although this is not desirable in terms of maximizing the properties of the crystal, it is acceptable within the range that does not impair the effects of the present invention.

[0050] Also, M-substituted ε-Fe ２ O ３ Coercive force H of magnetic crystals ｃ changes depending on the amount of substitution by the substitution element M. That is, M-substituted ε-Fe ２ O ３ By adjusting the amount of substitution by the substitution element M in the magnetic crystal, M-substituted ε-Fe ２ O ３ Coercive force H of magnetic crystals ｃ Specifically, when Al, Ga, or the like is used as the substitution element M, the more the substitution amount increases, the more the M-substituted ε-Fe ２ O ３ Coercive force H of magnetic crystals ｃ On the other hand, when Rh or the like is used as the substitution element M, the more the substitution amount increases, the more the M-substituted ε-Fe ２ O３ Coercive force H of magnetic crystals ｃ The M-substituted ε-Fe increases depending on the amount of substitution by the substitution element M. ２ O ３ Coercive force H of magnetic crystals ｃ In terms of ease of adjusting the value of the substitution element M, Ga, Al, In, Ti, Co and Rh are preferred.

[0051] And this coercive force H ｃ As the M element substitution rate decreases, the peak frequency at which the electromagnetic wave absorption of epsilon iron oxide is maximized also shifts to the lower or higher frequency side. In other words, the peak frequency of the electromagnetic wave absorption can be controlled by the amount of M element substitution.

[0052] In the case of commonly used electromagnetic wave absorbers, if the incident angle or frequency of the electromagnetic wave deviates from the designed values, the absorption amount becomes almost zero. In contrast, when epsilon-type iron oxide is used, electromagnetic wave absorption is exhibited over a wide frequency range and electromagnetic wave incident angle even if the values ​​deviate slightly. Therefore, it is possible to provide an electromagnetic wave absorbing layer that can absorb electromagnetic waves over a wide frequency band.

[0053] The particle size of epsilon-type iron oxide can be controlled, for example, by adjusting the heat treatment (calcination) temperature in the above-mentioned process. According to the aforementioned method combining the reverse micelle method and the sol-gel method, or the method combining the direct synthesis method and the sol-gel method disclosed in JP 2008-174405 A, it is possible to synthesize epsilon-type iron oxide particles having an average particle size measured from a TEM (transmission electron microscope) photograph in the range of 5 nm to 200 nm. The average particle size of epsilon-type iron oxide is preferably 10 nm or more, and more preferably 20 nm or more. When calculating the average particle size, which is the number-average particle size, if the epsilon-type iron oxide particles are rod-shaped, the diameter in the major axis direction of the particles observed in the TEM image is used as the diameter of the particles. The number of particles to be measured when calculating the average particle size is not particularly limited as long as it is a sufficiently large number for calculating the average value, but 300 or more is preferred.

[0054] In addition, the silica coating on the surface of iron hydroxide particles by the sol-gel method is ２ O ３ It may be present on the surface of the magnetic crystal. The presence of a non-magnetic compound such as silica on the surface of the crystal is preferable in terms of improving the handleability, durability, weather resistance, etc. of the magnetic crystal. Suitable examples of non-magnetic compounds include silica, as well as heat-resistant compounds such as alumina and zirconia.

[0055] However, if the amount of the non-magnetic compound attached is too large, the particles may aggregate violently, which is not preferable. ２ O ３ The mass of Si in the magnetic crystal is M-substituted ε-Fe ２ O ３ It is preferable that the amount of M substituted ε-Fe is 100% by mass or less relative to the total mass of the substitution element M and the mass of Fe in the magnetic crystal. ２ O ３ A part or most of the silica attached to the magnetic crystals can be removed by immersing them in an alkaline solution, and the amount of silica attached can be adjusted to any desired amount by this method.

[0056] The relative permeability of the electromagnetic wave absorbing layer is not particularly limited, but is preferably 1.0 or more and 1.5 or less. The method for adjusting the relative permeability of the electromagnetic wave absorbing layer is not particularly limited. Examples of methods for adjusting the relative permeability of the electromagnetic wave absorbing layer include a method for adjusting the amount of substitution by the substitution element M in the epsilon-type iron oxide, and a method for adjusting the content of epsilon-type iron oxide and other magnetic materials other than epsilon-type iron oxide in the electromagnetic wave absorbing layer.

[0057] The content of the epsilon iron oxide in the electromagnetic wave absorbing layer is not particularly limited as long as it does not impair the object of the present invention. The content of the magnetic material is preferably 3% by mass to 50% by mass, more preferably 5% by mass to 30% by mass, particularly preferably 7% by mass to 20% by mass, and most preferably 10% by mass to 15% by mass, based on the mass of the solid content excluding the organic salt in the electromagnetic wave absorbing layer.

[0058] [Carbon nanotubes] The relative dielectric constant of the electromagnetic wave absorbing layer can be adjusted by incorporating carbon nanotubes into the electromagnetic wave absorbing layer. The relative dielectric constant of the electromagnetic wave absorbing layer is not particularly limited, but is preferably 6.5 or more and 65 or less, more preferably 10 or more and 50 or less, and even more preferably 15 or more and 30 or less.

[0059] The type of carbon nanotube is not particularly limited. The carbon nanotube may be a single-walled carbon nanotube or a multi-walled carbon nanotube. The shape of the carbon nanotube is not particularly limited. Examples of such shapes include a needle shape, a cylindrical tube shape, a fishbone shape (fishbone or cup stacked type), a playing card shape (platelet), and a coil shape.

[0060] The amount of carbon nanotubes blended into the electromagnetic wave absorbing layer is not particularly limited as long as the desired effect is not impaired. However, since carbon nanotubes are also conductive materials, if the amount of carbon nanotubes used is excessive, the electromagnetic wave absorption properties provided by the electromagnetic wave absorbing layer may be impaired. The amount of carbon nanotubes used is preferably 3% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 10% by mass or less, based on the mass of the solid content excluding the organic salt of the electromagnetic wave absorbing layer.

[0061] [Organic Salt] By incorporating an organic salt into the electromagnetic wave absorbing layer, the anisotropy of the electromagnetic wave absorption characteristics can be suppressed. Here, anisotropy means that the electromagnetic wave absorption performance differs depending on the direction of incidence of high-frequency electromagnetic waves on the electromagnetic wave absorber.

[0062] The organic salt is not particularly limited and may be a salt formed from an organic anion and an inorganic cation, a salt formed from an inorganic anion and an organic cation, or a salt formed from an organic anion and an organic cation. Examples of organic salts include quaternary ammonium salts such as tetrabutylammonium bromide, tetrabutylammonium hexafluorophosphate, tetraoctylammonium bromide, tributylmethylammonium bis(trifluoromethanesulfonyl)imide, and methyltri-n-octylammonium bis(trifluoromethanesulfonyl)imide; imidazolium salts such as 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-allyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-decyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium triflate, 1-allyl-3-methylimidazolium dicyanamide, and 1-butyl-3-methylimidazolium acetate; hydrochlorides such as dodecylamine hydrochloride; triethylsulfonium bis(trifluoromethanesulfonyl)imide; and carboxylic acid salts such as magnesium acetate, potassium acetate, magnesium propionate, potassium propionate, magnesium 2-ethylbutanoate, potassium 2-ethylbutanoate, magnesium 2-ethylhexanoate, potassium 2-ethylhexanoate, potassium acetate, magnesium acetate, potassium propionate, magnesium propionate, monopotassium citrate, dipotassium citrate, tripotassium citrate, monopotassium trihydrogen ethylenediaminetetraacetate, dipotassium dihydrogen ethylenediaminetetraacetate, tripotassium monohydrogen ethylenediaminetetraacetate, potassium hydrogen phthalate, dipotassium phthalate, potassium hydrogen oxalate, and dipotassium oxalate. Examples of the organic salt include quaternary ammonium salts and imidazolium salts.

[0063] The amount of organic salt to be blended into the electromagnetic wave absorbing layer is not particularly limited as long as the desired effect is not impaired. However, since carbon nanotubes are also conductive materials, if the amount of carbon nanotubes used is excessive, the electromagnetic wave absorption properties provided by the electromagnetic wave absorbing layer may be impaired. The amount of organic salt used is preferably 1 mass % or more, more preferably 2.5 mass % or more, based on the mass of the solid content excluding the organic salt of the electromagnetic wave absorbing layer.

[0064] (Dielectric) The electromagnetic wave absorbing material may contain a dielectric for the purpose of adjusting the relative dielectric constant of the electromagnetic wave absorbing layer. The relative dielectric constant of the electromagnetic wave absorbing layer can be adjusted by adjusting the content of the dielectric in the electromagnetic wave absorbing layer.

[0065] Suitable examples of the dielectric include barium titanate, strontium titanate, calcium titanate, magnesium titanate, bismuth titanate, zirconium titanate, zinc titanate, and titanium dioxide. The electromagnetic wave absorbing material may contain a combination of powders of multiple types of dielectrics.

[0066] The particle size of the dielectric powder used to adjust the relative dielectric constant of the electromagnetic wave absorbing layer is not particularly limited as long as it does not impair the object of the present invention. The average particle size of the dielectric powder is preferably 1 nm or more and 100 nm or less, and more preferably 5 nm or more and 50 nm or less. Here, the average particle size of the dielectric powder is the number average size of the primary particles of the dielectric powder observed with an electron microscope.

[0067] When a dielectric powder is used, the amount of the dielectric powder used is not particularly limited as long as the desired effect is not impaired. The amount of the dielectric powder used is preferably 0% by mass or more and 20% by mass or less, and more preferably 0% by mass or more and 10% by mass or less, relative to the mass of the solid contents other than the organic salt of the electromagnetic wave absorbing layer.

[0068] (Non-Dielectric Material) The electromagnetic wave absorbing layer may contain a non-dielectric material other than a magnetic material. The non-dielectric material is not particularly limited as long as it is a material other than materials recognized by those skilled in the art as ferroelectrics or ferromagnetic materials.

[0069] Examples of non-dielectric materials include inorganic fillers. Suitable examples of such inorganic fillers include barium sulfate, aluminum oxide (alumina), aluminum nitride, boron nitride, silicon carbide, silicon dioxide (silica), calcium carbonate, and talc. Among these, barium sulfate, aluminum oxide, and silicon dioxide are more preferred, and barium sulfate is particularly preferred.

[0070] The particle size of the non-dielectric material powder is not particularly limited as long as it does not impair the object of the present invention. The average particle size of the non-dielectric material powder is preferably 1 nm or more and 20 μm or less, and more preferably 5 nm or more and 10 μm or less. Here, the average particle size of the non-dielectric material powder is the number average size of primary particles of the non-dielectric material powder observed with an electron microscope.

[0071] The content of the non-dielectric material in the electromagnetic wave absorbing layer is not particularly limited as long as it does not impair the object of the present invention. The content of the non-dielectric material is preferably 10% by mass or more and 80% by mass or less, more preferably 15% by mass or more and 75% by mass or less, based on the mass of the solid content excluding the organic salt of the electromagnetic wave absorbing layer.

[0072] (Binder Resin) The electromagnetic wave absorbing layer typically contains a binder resin. By using a binder resin, the magnetic material is dispersed well in the binder resin. Furthermore, by including a binder resin in the electromagnetic wave absorbing layer, it is easy to form the electromagnetic wave absorbing layer in a desired shape.

[0073] The binder resin may be, for example, an elastic material such as an elastomer or rubber. The binder resin may be a thermoplastic resin or a curable resin. When the binder resin is a curable resin, the curable resin may be a photocurable resin or a thermosetting resin.

[0074] Suitable examples of the binder resin when it is a thermoplastic resin include polyacetal resin, polyamide resin, polycarbonate resin, polyester resin (polybutylene terephthalate, polyethylene terephthalate, polyarylate, etc.), FR-AS resin, FR-ABS resin, AS resin, ABS resin, polyphenylene oxide resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, polyetheretherketone resin, fluorine-based resin (polyvinylidene fluoride, etc.), polyimide resin, polyamideimide resin, polyamide bismaleimide resin, polyetherimide resin, polybenzoxazole resin, polybenzothiazole resin, polybenzimidazole resin, BT resin, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resin (for example, methyl cellulose, ethyl cellulose), (meth)acrylic resin (polymethyl methacrylate, etc.), and polystyrene.

[0075] Suitable examples of the binder resin when it is a thermosetting resin include phenol resin, melamine resin, epoxy resin, alkyd resin, etc. As the photocurable resin, resins obtained by photocuring various vinyl monomers and monomers having unsaturated bonds such as various (meth)acrylic acid esters can be used.

[0076] Suitable examples of the binder resin that is an elastic material include olefin-based elastomers, styrene-based elastomers, polyamide-based elastomers, polyester-based elastomers, and polyurethane-based elastomers.

[0077] Furthermore, an aromatic ester-urethane copolymer can be used as the binder resin. By using an aromatic ester-urethane copolymer as the binder resin, it is possible to form a film-like electromagnetic wave absorber that exhibits excellent electromagnetic wave absorption properties even when thin, while allowing epsilon iron oxide and other magnetic materials to be well dispersed in the binder resin.

[0078] Furthermore, when an aromatic ester-urethane copolymer is used as the binder resin, it is possible to impart crack resistance and low warpage to the electromagnetic wave absorbing layer when bending or cutting. From the viewpoint of achieving good crack resistance and low warpage of the electromagnetic wave absorbing layer, the glass transition temperature of the binder resin is preferably 100°C or lower, more preferably 0°C or lower. Therefore, the glass transition temperature of the aromatic ester-urethane copolymer is also preferably 100°C or lower, more preferably 0°C or lower.

[0079] The aromatic ester-urethane copolymer is a copolymer containing an ester bond (-CO-O-) and a urethane bond (-NH-CO-O-), and containing an aromatic group in the main chain skeleton. The aromatic group in the main chain skeleton may be an aromatic hydrocarbon group or a heterocyclic aromatic group, with an aromatic hydrocarbon group being preferred. The aromatic ester-urethane copolymer may be a random copolymer in which ester bonds and urethane bonds are randomly introduced into the molecular chain, or may be a block copolymer consisting of one or more ester blocks and one or more urethane blocks.

[0080] The method for producing the aromatic ester-urethane copolymer is not particularly limited. The aromatic ester-urethane copolymer can typically be produced by polymerizing one or more monomers selected from the group consisting of a diol component (a1), a dicarboxylic acid (a2), a hydroxycarboxylic acid component (a3), and a diisocyanate component (a4) in one step or multiple steps.

[0081] The dicarboxylic acid component (a2) and the hydroxycarboxylic acid component (a3) ​​may be used as ester derivatives such as methyl esters and ethyl esters, carboxylic acid halides such as carboxylic acid chlorides, or urethane-forming derivatives.

[0082] The monomer used in the production of the aromatic ester-urethane copolymer is preferably a compound in which two functional groups selected from the group consisting of a hydroxyl group, a carboxyl group, and an isocyanate group are bonded to an unbranched divalent hydrocarbon group. Examples of the unbranched divalent hydrocarbon group include an alkylene group, an alkenylene group, an alkynylene group, an arylene group, or a combination of these groups. The alkylene group, alkenylene group, and alkynylene group preferably have a linear structure.

[0083] When the unbranched divalent hydrocarbon group is an alkylene group, an alkenylene group, or an alkynylene group, the number of carbon atoms in these groups is preferably 1 or more and 8 or less, more preferably 2 or more and 6 or less, and even more preferably 2 or more and 4 or less.

[0084] When the unbranched divalent hydrocarbon group is an arylene group, the arylene group is preferably a phenylene group or a naphthylene group, more preferably a phenylene group, and even more preferably a p-phenylene group.

[0085] Of the divalent hydrocarbon groups having an unbranched structure described above, alkylene groups, arylene groups, and combinations of alkylene groups and arylene groups are preferred.

[0086] Specific preferred examples of the diol component (a1) include ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, and 1,5-pentanediol. Specific preferred examples of the dicarboxylic acid (a2) include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, succinic acid, glutaric acid, adipic acid, oxalic acid, and malonic acid. Specific preferred examples of the hydroxycarboxylic acid component (a3) ​​include 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 6-hydroxynaphthalene-2-carboxylic acid, glycolic acid, lactic acid, and γ-hydroxybutyric acid. Specific preferred examples of the diisocyanate component (a4) include ethylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, m-xylylene diisocyanate, p-phenylene diisocyanate, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and 1,5-naphthalene diisocyanate.

[0087] The weight average molecular weight (Mw) of the aromatic ester-urethane copolymer is preferably from 5,000 to 500,000, and more preferably from 10,000 to 200,000. In the specification of the present application, the weight average molecular weight (Mw) is the weight average molecular weight measured by GPC and converted into polystyrene.

[0088] Commercially available aromatic ester-urethane copolymers include the Vylon series (product name) (manufactured by Toyobo Co., Ltd.) etc. More specifically, Vylon UR-1400, Vylon UR-1410, Vylon UR-1700, Vylon UR-2300, Vylon UR-3200, Vylon UR-3210, Vylon UR-3500, Vylon UR-6100, Vylon UR-8300, and Vylon UR-8700 etc. can be preferably used.

[0089] As the binder resin, it is preferable to use a fluorine-based resin, and it is more preferable to use polyvinylidene fluoride, since the influence of the angle of incidence of the electromagnetic wave on the electromagnetic wave absorption value is small.

[0090] The content of the binder resin in the electromagnetic wave absorbing material is not particularly limited as long as it does not impair the object of the present invention. The electromagnetic wave absorbing layer preferably contains 1 mass % or more and 30 mass % or less, and more preferably 3 mass % or more and 25 mass % or less, of the binder resin, based on the mass of the solid content excluding the organic salt of the electromagnetic wave absorbing layer.

[0091] [Other Components] The electromagnetic wave absorbing layer may contain various additives other than the above-mentioned components, as long as they do not impair the objectives of the present invention. Examples of additives that the electromagnetic wave absorbing layer may contain include dispersants, colorants, UV absorbers, flame retardants, flame retardant assistants, plasticizers, and surfactants. These additives are used in amounts that are conventionally used, as long as they do not impair the objectives of the present invention. The magnetic material described above, carbon nanotubes, organic salts, and optionally dielectrics, non-magnetic materials, binder resins, and other components are formed into a film by, for example, a method using an electromagnetic wave absorber-forming paste described below, to obtain an electromagnetic wave absorbing layer that can be used as an electromagnetic wave absorber with good electromagnetic wave absorption characteristics in the high frequency band.

[0092] <Substrate Layer> The electromagnetic wave absorbing layer may be laminated on a substrate layer. The substrate layer may be a layer containing any substrate as long as it does not impair the effects of the present invention, and examples thereof include layers containing resins. Examples of the resin include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acrylic (PMMA), polycarbonate (PC), cycloolefin polymer (COP), polyethersulfone, polyimide, polyamideimide, etc. Among these, PET is preferred because of its excellent heat resistance and good balance between dimensional stability and cost.

[0093] The shape of the substrate layer may have a curved surface or may be composed of only a flat surface, and a flat plate shape is preferred. The thickness of the substrate layer is preferably 800 μm or less, more preferably 500 μm or less, even more preferably 300 μm or less, and particularly preferably 150 μm or less, from the viewpoint of making the film thinner and smaller without impairing the effects of the present invention. The lower limit of the thickness of the substrate layer is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include 1 μm or more, 10 μm or more, and 50 μm or more.

[0094] <Metal Layer> When the electromagnetic wave absorber includes a substrate layer, a metal layer may be provided on the surface of the electromagnetic wave absorber opposite to the surface on which the substrate layer is provided. When a metal layer is provided, electromagnetic waves reflected by the metal layer can be attenuated. Examples of metals constituting the metal layer include aluminum, titanium, SUS, copper, brass, silver, gold, and platinum. The thickness of the metal layer is not particularly limited, and from the viewpoint of making the electromagnetic wave absorber thin, it is preferably 600 μm or less, more preferably 400 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less. There is no particular limit to the lower limit of the thickness of the metal layer as long as it does not impair the effects of the present invention, and examples thereof include 0.1 μm or more, 1 μm or more, 5 μm or more, and 10 μm or more.

[0095] By combining the electromagnetic wave absorbing layer containing the predetermined components described above with a substrate layer, or a substrate layer and a metal layer as needed, an electromagnetic wave absorber having good electromagnetic wave absorption properties in the high frequency band can be obtained.

[0096] The electromagnetic wave absorber described above can be preferably used as an electromagnetic wave absorbing film for use in various elements (including in-vehicle elements, high-frequency antenna elements, etc.) in various information and communication systems such as mobile phones, wireless LANs, ETC systems, intelligent road transport systems, automobile driving assistance road systems, and satellite broadcasting.

[0097] <Electromagnetic Wave Absorber Forming Paste> A preferred method for forming an electromagnetic wave absorber is to use an electromagnetic wave absorber forming paste, since it allows for highly efficient formation of an electromagnetic wave absorbing layer without any particular thickness restrictions and allows for the electromagnetic wave absorbing layer to be formed directly on a substrate layer. The electromagnetic wave absorber forming paste contains the magnetic material described above, carbon nanotubes, and an organic salt. The electromagnetic wave absorber forming paste preferably further contains the binder resin described above. The electromagnetic wave absorber forming paste may contain substances added to adjust the relative permittivity, relative permeability, etc., as described above for the magnetic material, as well as other components. Note that when the binder resin contains a curable resin, the electromagnetic wave absorber forming paste contains a compound that is a precursor to the curable resin. In this case, the electromagnetic wave absorber forming paste may contain a curing agent, a curing accelerator, a polymerization initiator, etc., as necessary.

[0098] Furthermore, when the paste for forming an electromagnetic wave absorber contains a photopolymerizable or thermally polymerizable compound, the coated film may be exposed to light or heated as necessary to form an electromagnetic wave absorbing layer.

[0099] The paste for forming an electromagnetic wave absorber preferably further contains a dispersion medium. As the dispersion medium, water, an organic solvent, or an aqueous solution of an organic solvent can be used. As the dispersion medium, an organic solvent is preferred because it is easy to dissolve organic components and has a low latent heat of vaporization and is easily removed by drying.

[0100] Suitable examples of organic solvents used as dispersion media include nitrogen-containing polar solvents such as N,N,N',N'-tetramethylurea (TMU), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylisobutyramide, N,N-diethylacetamide, N,N-dimethylformamide (DMF), N,N-diethylformamide, N-methylcaprolactam, 1,3-dimethyl-2-imidazolidinone (DMI), and pyridine; ketones such as diethyl ketone, methyl butyl ketone, dipropyl ketone, and cyclohexanone; alcohols such as n-pentanol, 4-methyl-2-pentanol, cyclohexanol, and diacetone alcohol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monomethyl ether. Ether alcohols such as ethylene glycol dimethyl ether and diethylene glycol diethyl ether; saturated aliphatic monocarboxylic acid alkyl esters such as n-butyl acetate and amyl acetate; lactate esters such as ethyl lactate and n-butyl lactate; ketones such as acetone, methyl ethyl ketone, cyclohexanone, acetophenone, and benzophenone; methyl cellosolve acetate, ethyl cellosolve acetate, propylene glycol monomethyl ether acetate, and propylene glycol Examples of such ether esters include monoethyl ether acetate, ethyl 3-ethoxypropionate, 2-methoxybutyl acetate, 3-methoxybutyl acetate, 4-methoxybutyl acetate, 2-methyl-3-methoxybutyl acetate, 3-methyl-3-methoxybutyl acetate, 3-ethyl-3-methoxybutyl acetate, 2-ethoxybutyl acetate, 4-ethoxybutyl acetate, 4-propoxybutyl acetate, and 2-methoxypentyl acetate. These may be used alone or in combination of two or more.

[0101] The solid content concentration of the paste for forming an electromagnetic wave absorber is adjusted as appropriate depending on the method for applying the paste for forming an electromagnetic wave absorber, the thickness of the electromagnetic wave absorbing layer, etc. Typically, the solid content concentration of the paste for forming an electromagnetic wave absorber is preferably 3% by mass or more and 60% by mass or less, and more preferably 10% by mass or more and 50% by mass or less. The solid content concentration of the paste is a value calculated as the mass of the solid content, which is the sum of the mass of the components not dissolved in the dispersion medium and the mass of the components dissolved in the dispersion medium.

[0102] (Dispersant) The paste for forming an electromagnetic wave absorber may contain a dispersant for the purpose of dispersing the epsilon-type iron oxide or a substance used to adjust the relative permittivity and relative permeability of the electromagnetic wave absorbing layer well in the electromagnetic wave absorbing layer. The dispersant may be uniformly mixed together with the epsilon-type iron oxide and the binder resin. The dispersant may be blended into the binder resin. Furthermore, the epsilon-type iron oxide or a substance added to adjust the relative permittivity and relative permeability, which has been pre-treated with a dispersant, may be blended into the material constituting the electromagnetic wave absorbing layer.

[0103] The type of dispersant is not particularly limited as long as it does not impair the object of the present invention. The dispersant can be selected from various dispersants that have been used conventionally for dispersing various inorganic and organic fine particles. As the dispersant, a coupling agent, a polymeric dispersant, and a surfactant may be used alone or in combination of two or three.

[0104] The coupling agent binds to the surface of the inorganic metal compound particles and stabilizes the dispersion of the inorganic metal compound particles by steric hindrance effect, etc. The surfactant stabilizes the dispersion of the inorganic metal compound particles by improving the wettability between the inorganic metal compound particles and the solvent by reducing the surface tension of the solvent, and by steric hindrance effect when adsorbed to or free from the inorganic metal compound particles.

[0105] Examples of the coupling agent include a silane coupling agent (for example, phenyltrimethoxysilane), a titanate coupling agent, a zirconate coupling agent, and an aluminate coupling agent.

[0106] Examples of polymeric dispersants include polyamidoamines, polyamidoamine salts, polycarboxylic acids, polycarboxylic acid salts, high molecular weight unsaturated acid esters, modified polyurethanes, modified polyesters, modified poly(meth)acrylates, (meth)acrylic copolymers, naphthalenesulfonic acid-formalin condensates, polyoxyethylene alkyl phosphate esters, polyoxyethylene alkylamines, and pigment derivatives.

[0107] Examples of the surfactant include anionic surfactants, cationic surfactants (for example, tetrabutylammonium bromide), nonionic surfactants, and amphoteric surfactants.

[0108] As the dispersant, it is preferable to use a coupling agent or a surfactant, and it is more preferable to use a combination of a coupling agent and a surfactant.

[0109] The content of the dispersant is not particularly limited as long as it does not impair the object of the present invention. The content of the dispersant is preferably 0% by mass or more and 30% by mass or less, more preferably 0% by mass or more and 15% by mass or less, and particularly preferably 0% by mass or more and 10% by mass or less, relative to the solid content mass of the paste for forming an electromagnetic wave absorber.

[0110] <<Method for manufacturing electromagnetic wave absorber>> The method for manufacturing the aforementioned electromagnetic wave absorber is not particularly limited as long as it is possible to manufacture an electromagnetic wave absorber having a predetermined structure. A preferred method includes an electromagnetic wave absorbing layer forming step of applying the aforementioned paste containing a magnetic material, carbon nanotubes, and an organic salt onto a base layer to form a coating film, and then drying the coating film to form an electromagnetic wave absorbing layer.

[0111] The method for applying the paste for forming an electromagnetic wave absorber onto the substrate layer is not particularly limited as long as it can form an electromagnetic wave absorber of the desired thickness. Examples of application methods include spray coating, dip coating, roll coating, curtain coating, spin coating, screen printing, doctor blade coating, and applicator coating. The coating film formed by the above method is dried to remove the dispersion medium, forming an electromagnetic wave absorbing film on the substrate layer, thereby obtaining an electromagnetic wave absorber. The thickness of the coating film is appropriately adjusted so that the thickness of the electromagnetic wave absorbing film obtained after drying is the desired thickness. The drying method is not particularly limited, and examples include: (1) drying on a hot plate at a temperature of 80°C to 180°C, preferably 90°C to 160°C, for 1 minute to 30 minutes; (2) leaving the paste at room temperature for several hours to several days; and (3) removing the solvent by placing the paste in a hot air heater or infrared heater for several tens of minutes to several hours.

[0112] The method for manufacturing an electromagnetic wave absorber may include a cutting step of cutting the electromagnetic wave absorbing layer or the laminate comprising a base layer and an electromagnetic wave absorbing layer obtained in the electromagnetic wave absorbing layer forming step to obtain electromagnetic wave absorbers of a predetermined size. As described above, the electromagnetic wave absorber has an electromagnetic wave absorbing layer made of a magnetic material, carbon nanotubes, and an organic salt, and therefore has good electromagnetic wave absorption characteristics in the high frequency band.

[0113] As described above, the present inventors have provided the following (1) to (8): (1) an electromagnetic wave absorber comprising an electromagnetic wave absorbing layer containing a magnetic material, carbon nanotubes, and an organic salt; (2) the magnetic material contains epsilon iron oxide, and the epsilon iron oxide is ε-Fe ２ O ３ Crystal and crystal space group ε-Fe ２ O ３ and ε-Fe ２ O ３ A part of the Fe site of the crystal is substituted with an element M other than Fe, and has the formula ε-M ｘ Fe ２－ｘ O ３and x is greater than 0 and less than 2. (3) The electromagnetic wave absorber according to (1) or (2), wherein the organic salt comprises an organic base salt. (4) The electromagnetic wave absorber according to any one of (1) to (3), wherein the organic salt comprises a quaternary ammonium salt and / or an imidazole salt. (5) The electromagnetic wave absorber according to any one of (1) to (4), wherein the ratio of the mass of the organic salt to the mass of solids other than the organic salt in the electromagnetic wave absorbing layer is 1.0 mass % or more. (6) The electromagnetic wave absorber according to any one of (1) to (5), which is in the form of a film. (7) A paste for forming an electromagnetic wave absorber, comprising a magnetic material, carbon nanotubes, and an organic salt. (8) The magnetic material comprises epsilon iron oxide, wherein the epsilon iron oxide is ε-Fe ２ O ３ Crystal and crystal space group ε-Fe ２ O ３ and ε-Fe ２ O ３ A part of the Fe site of the crystal is substituted with an element M other than Fe, and has the formula ε-M ｘ Fe ２－ｘ O ３ wherein x is greater than 0 and less than 2.

[0114] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0115] Examples 1 to 26 Preparation of Paste for Forming Electromagnetic Wave Absorber To 75 parts by mass of 1,3-dimethyl-2-imidazolidinone (manufactured by Mitsui Chemicals, Inc.) as a dispersion medium, epsilon-type iron oxide (product name DEP-24; manufactured by Dowa Electronics Co., Ltd.) as a magnetic material in the amount shown in Table 1, 18 parts by mass of barium sulfate (product name BARIACE B-30; manufactured by Sakai Chemical Industry Co., Ltd.) as a non-magnetic material, 3 parts by mass of polyvinylidene fluoride (product name KF Polymer #7200; manufactured by Kureha Corporation) as a binder resin, and 0.3 parts by mass of trimethoxyphenylsilane (manufactured by Shin-Etsu Silicon Co., Ltd.) were added. The resulting mixture was mixed using a homodisper, and then the components in the mixture were uniformly dispersed using a bead mill.

[0116] To the slurry after dispersion, carbon nanotubes (CNT) (VGCF-H; manufactured by Showa Denko K.K.) were added in the amount shown in Table 1. Organic salts were added in the types and proportions (mass %) shown in Table 1 relative to the mass of solids contained in the slurry after addition of the carbon nanotubes. The slurry after addition of the organic salts was mixed using a homodisper to obtain a paste for forming an electromagnetic wave absorber.

[0117] Comparative Examples 1 to 5 (Preparation of Paste for Forming Electromagnetic Wave Absorber) To 75 parts by mass of 1,3-dimethyl-2-imidazolidinone (manufactured by Mitsui Chemicals) as a dispersion medium, epsilon-type iron oxide (product name DEP-24; manufactured by Dowa Electronics) in the amount shown in Table 1 was added as a magnetic material, 18 parts by mass of barium sulfate (product name BARIACE B-30; manufactured by Sakai Chemical Industry Co., Ltd.) as a non-magnetic material, 3 parts by mass of polyvinylidene fluoride (product name KF Polymer #7200; manufactured by Kureha) as a binder resin, and 0.3 parts by mass of trimethoxyphenylsilane (manufactured by Shin-Etsu Silicone Co., Ltd.). The components were mixed using a homodisper, and then uniformly dispersed using a bead mill.

[0118] To the above-described dispersed slurry, carbon nanotubes (CNT) (VGCF-H; manufactured by Showa Denko KK) were added in the amount shown in Table 1. The slurry after adding the carbon nanotubes was mixed with a homodisper to obtain a paste for forming an electromagnetic wave absorber.

[0119]

[0120] (Production of Electromagnetic Wave Absorber Film) The above-mentioned paste for forming an electromagnetic wave absorber was applied to a PET film (thickness 125 μm) using an applicator. The applied film was then dried using a thermostatic oven under conditions of 100°C for 30 minutes and 130°C for 30 minutes to form an electromagnetic wave absorbing layer with a thickness of 70 μm to 80 μm, thereby obtaining a film-like electromagnetic wave absorber. Immediately after drying, the obtained film-like electromagnetic wave absorber was cut into a 3 cm square shape to prepare test pieces for the following evaluations.

[0121] <Absorption Frequency Peak Top> A 3 cm square sample of the film-like electromagnetic wave absorber was attached to an aluminum plate. Electromagnetic waves in a frequency range of 50 GHz to 2,000 GHz were incident on the measurement sample on the aluminum plate at angles of 0° and 90° to the film-like electromagnetic wave absorber, and the peak tops of the absorption frequencies were measured using a terahertz time-domain spectrometer (manufactured by Advantest Corporation). Electromagnetic waves in a frequency range of 0 GHz to 3,000 GHz were incident on the film-like electromagnetic wave absorbers of the Examples and Comparative Examples at angles of 0° and 90°. The peak tops (GHz) of the absorption frequencies of the electromagnetic waves are shown in Table 2 as X and Y, respectively. Based on the detected peak tops of the absorption frequencies, the electromagnetic wave absorption properties were evaluated according to the following criteria: A: The difference between X and Y was 0 GHz or more and less than 5 GHz. B: The difference between X and Y was 5 GHz or more and less than 10 GHz. C: The difference between X and Y was 10 GHz or more.

[0122]

[0123] Table 2 shows that the electromagnetic wave absorbers of Examples 1 to 26, which have an electromagnetic wave absorbing layer made of a magnetic material, carbon nanotubes, and an organic salt, and in which the magnetic material contains a predetermined epsilon-type iron oxide, have good electromagnetic wave absorption characteristics in the high frequency band, regardless of the incident direction of the electromagnetic waves. On the other hand, Comparative Examples 1 to 5 show that when no organic salt is contained, the electromagnetic wave absorption characteristics vary greatly depending on the incident angle of the electromagnetic waves with respect to the electromagnetic wave absorber.

Claims

1. An electromagnetic wave absorber comprising an electromagnetic wave absorbing layer comprising a magnetic material, carbon nanotubes, and an organic salt.

2. The magnetic material contains epsilon iron oxide, and the epsilon iron oxide is ε-Fe ２ O ３ Crystal and crystal space group ε-Fe ２ O ３ and ε-Fe ２ O ３ A part of the Fe site of the crystal is substituted with an element M other than Fe, and has the formula ε-M ｘ Fe ２－ｘ O ３ 2. The electromagnetic wave absorber according to claim 1, wherein the absorber is one or more crystals selected from the group consisting of crystals represented by the formula:

3. The electromagnetic wave absorber according to claim 1, wherein the organic salt comprises an organic base salt.

4. The electromagnetic wave absorber according to claim 1, wherein the organic salt comprises a quaternary ammonium salt and / or an imidazole salt.

5. The electromagnetic wave absorber according to claim 1, wherein the ratio of the mass of said organic salt to the mass of solid contents other than said organic salt in said electromagnetic wave absorbing layer is 1.0 mass % or more.

6. The electromagnetic wave absorber according to any one of claims 1 to 5, which is in the form of a film.

7. A paste for forming an electromagnetic wave absorber, comprising a magnetic material, carbon nanotubes, and an organic salt.

8. The magnetic material contains epsilon iron oxide, and the epsilon iron oxide is ε-Fe ２ O ３ Crystal and crystal space group ε-Fe ２ O ３ and ε-Fe ２ O ３ A part of the Fe site of the crystal is substituted with an element M other than Fe, and has the formula ε-M ｘ Fe ２－ｘ O ３ 8. The paste for forming an electromagnetic wave absorber according to claim 7, wherein the crystal is one or more selected from crystals represented by the following formula:

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