Electromagnetic wave absorber and paste for forming electromagnetic wave absorber

Abstract

To provide an electromagnetic wave absorber having proper electromagnetic wave absorption characteristics without reference to an incident direction of electromagnetic waves in a high-frequency band, and paste for forming the electromagnetic wave absorber, which is suitably used to manufacture the electromagnetic wave absorber.SOLUTION: An electromagnetic wave absorber, which includes an electromagnetic wave absorption layer including a magnetic material, a carbon nano-tube and organic salt, is used.SELECTED DRAWING: None

Description

[Technical Field]

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

[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 radar and the like that utilize 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 capable of effectively absorbing electromagnetic waves in the high frequency band have been proposed, such as an electromagnetic wave absorbing sheet containing carbon nanocoils and resin (for example, Patent Document 1).

[0004] Among the 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 demands, electromagnetic wave absorbers that can effectively absorb electromagnetic waves over a wide range in the high frequency band have been proposed, such as electromagnetic wave absorbers that have an electromagnetic wave absorbing layer containing magnetic crystals made of ε-Fe2O3-based iron oxide (e.g., Patent Documents 2-3, Non-Patent Documents 1-5). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-060060 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-277726 [Patent Document 3] WO08 / 149785 [Non-patent literature]

[0007] [Non-Patent Document 1] 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). [Non-patent document 2] 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). [Non-patent document 3] S. Ohkoshi, S. Kuroki, S. Sakurai, K. Matsumoto, K. Sato, and S. Sasaki, Angew. Chem. Int. Ed., 46, 8392-8395 (2007). [Non-patent document 4] A. Namai, K. Ogata, M. Yoshikiyo, and S. Ohkoshi, Bull. Chem. Soc. Jpn., 93, 20-25 (2020). [Non-patent document 5] M. Yoshikiyo, Y. Futakawa, R. Shimoharai, Y. Ikeda, J. MacDougall, A. Namai, S. Ohkoshi, Chem. Phys. Lett., 803, 139821 (2022). Summary of the Invention [Problem to be solved by the invention]

[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 has an object 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. [Means for solving the problem]

[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 The electromagnetic wave absorber includes an electromagnetic wave absorbing layer that includes a magnetic material, carbon nanotubes, and an organic salt.

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

[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. DETAILED DESCRIPTION OF THE INVENTION

[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 but 350 GHz or lower, and more preferably 60 GHz or higher but 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, but it is preferably in the form of a sheet or film, and more preferably in the form of a film. When the electromagnetic wave absorber is in the form of a film, the shape of the film may have a curved surface or may be composed of only a flat surface, and is preferably a flat plate shape. 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, from the viewpoint of making the film thinner or smaller without impairing the desired effect. The thickness of the film serving as an electromagnetic wave absorber may be uniform or non-uniform.

[0018] <Electromagnetic wave absorbing layer> The electromagnetic wave absorbing layer includes 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. From the viewpoint of the balance between thinning of the electromagnetic wave absorber and electromagnetic wave absorbing performance, the thickness of the electromagnetic wave absorbing layer is preferably 150 μm or less, and more preferably 100 μm or less. There is no particular lower limit to the thickness of the electromagnetic wave absorbing layer as long as it does not impair the effects of the present invention, and examples thereof include 1 μm or more, 10 μm or more, and the like. The electromagnetic wave absorbing layer may have a uniform or non-uniform thickness.

[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 high-frequency electromagnetic waves in the millimeter wave band or higher.

[0023] When the magnetic material contains epsilon iron oxide, the magnetic material may contain, together with the epsilon iron oxide, a magnetic substance having the ability to absorb electromagnetic waves. Preferred examples of the magnetic substance that can be used together with the epsilon iron oxide include barium ferrite magnetic substance and strontium ferrite magnetic substance. In order to obtain good electromagnetic wave absorption properties of the electromagnetic wave absorber, the ratio of the mass of epsilon iron oxide to the total mass of the epsilon iron oxide and the magnetic material other than epsilon 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 iron oxide) Epsilon iron oxide includes ε-Fe2O3 crystals and those with the same crystal structure and space group as ε-Fe2O3, in which part of the Fe site of the ε-Fe2O3 crystal is replaced by an element M other than Fe, and have the formula ε-M x Fe 2-x O3, 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, they may be referred to as "magnetic crystals" in the present specification.

[0027] Any ε-Fe2O3 crystal can be used. It has the same crystal structure and space group as ε-Fe2O3, but some of the Fe sites of the ε-Fe2O3 crystal are substituted with an element M other than Fe, and has the formula ε-M xFe 2-x The crystal represented by O3, in which x is 0 or more and 2 or less (preferably 0 or more and less than 2), will be described later. In this specification, ε-M is a crystal in which part of the Fe site of the ε-Fe2O3 crystal is substituted with a substitution element M. x Fe 2-x O3 is also called "M-substituted ε-Fe2O3".

[0028] The particle size of the particles having ε-FeO crystals and / or M-substituted ε-FeO crystals as a 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 as a 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 epsilon-type iron oxide magnetic crystals in the magnetic layer, which are manufactured using the method described below, is in the range of less than 80%, and they are a group of particles that are relatively fine and have a uniform particle size.

[0029] In a preferred electromagnetic wave absorbing layer, a powder of such epsilon-type iron oxide magnetic particles (i.e., particles having ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 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. "Having ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 crystals in the magnetic phase" means that the magnetic phase is made of ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 crystals, and includes cases where the magnetic phase contains impurity magnetic crystals that are unavoidable during manufacturing.

[0030] The magnetic crystals of epsilon-type iron oxide may contain impurity crystals of iron oxides that have a different space group or oxidation state from the ε-Fe2O3 crystals (specifically, α-Fe2O3, γ-Fe2O3, FeO, and Fe3O4, as well as crystals in which part of the Fe in these crystals has been replaced with other elements). When the magnetic crystals of epsilon-type iron oxide contain impurity crystals, it is preferable that the magnetic crystals of ε-Fe2O3 and / or M-substituted ε-Fe2O3 are the main phase. In other words, among the magnetic crystals of epsilon-type iron oxide that constitute the magnetic material, the proportion of the magnetic crystals of ε-Fe2O3 and / or M-substituted ε-Fe2O3 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 Rietveld analysis based on the X-ray diffraction pattern. Non-magnetic compounds such as silica (SiO2) formed during the sol-gel process may adhere to the magnetic phase.

[0032] (M-substituted ε-Fe2O3) The type of element M in M-substituted ε-Fe2O3 is not particularly limited as long as it satisfies the conditions that the crystal and space group are the same as those of ε-Fe2O3 and that part of the Fe sites of the ε-Fe2O3 crystal are substituted with an element M other than Fe. M-substituted ε-Fe2O3 may contain multiple types 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 In the composition represented by O3, 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 maximized can be adjusted by adjusting at least one of the type and substitution amount of element M in M-substituted ε-Fe2O3.

[0035] Such M-substituted ε-FeO magnetic crystals can be synthesized, for example, by a process combining a reverse micelle method and a sol-gel method, followed by a firing process, as described below. Alternatively, as disclosed in JP 2008-174405 A, M-substituted ε-FeO magnetic crystals can be synthesized by a process combining a direct synthesis method and a sol-gel method, followed by a firing process.

[0036] in particular, Jian Jin, Shin-ichi Ohkoshi and Kazuhito Hashimoto, ADVANCED MATERIALS 2004,16,No.1, January 5,pp.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.1946-1949, M-substituted ε-Fe2O3 magnetic crystals can be obtained by a process combining the reverse micelle method and the sol-gel method, and a firing process, as described in, for example, Asuka Namai, Shunsuke Sakurai, Makoto Nakajima, Tohru Suemoto, Kazuyuki Matsumoto, Masahiro Goto, Shinya Sasaki, and Shin-ichi Ohkoshi, Journal of the American Chemical Society, Vol. 131, pp. 1170-1173, 2009.

[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 microparticles generated within the micelles using a sol-gel method. After separating the silica-coated iron hydroxide microparticles from the liquid, they are subjected to heat treatment in an air atmosphere at a specified temperature (between 700 and 1300°C). This heat treatment yields microparticles of ε-Fe2O3 crystals.

[0038] More specifically, for example, M-substituted ε-Fe2O3 magnetic crystals are produced as follows.

[0039] First, iron (III) nitrate as the iron source, M nitrate salts as the M element source to partially substitute for 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-Fe2O3 magnetic crystal particles are ultimately obtained. When no shape control agent is present, nearly spherical M-substituted ε-Fe2O3 magnetic crystal particles are obtained.

[0041] The alkaline earth metal added as a shape control agent may remain in the surface layer of the resulting M-substituted ε-Fe2O3 magnetic crystals. The mass of the alkaline earth metal in the M-substituted ε-Fe2O3 magnetic crystals is preferably 20 mass% or less, and more preferably 10 mass% or less, of the total mass of the substituting element M and Fe in the M-substituted ε-Fe2O3 magnetic crystals.

[0042] Aqueous ammonia solution is used as the aqueous phase of micellar solution II, in which n-octane is used 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 micellar solution mixture while stirring, and the reaction to produce iron hydroxide or iron hydroxide containing element M within the micelles proceeds. As a result, the particle surfaces of the fine iron hydroxide precipitates produced within the micelles are coated with silica produced 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 placed in a furnace and heat-treated (fired) in air at a temperature ranging from 700°C to 1300°C, preferably from 900°C to 1200°C, and more preferably from 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 particles of M-substituted ε-Fe2O3.

[0045] During this oxidation reaction, the presence of the silica coating contributes to the formation of M-substituted ε-Fe2O3 crystals, which have the same space group as ε-Fe2O3, rather than α-Fe2O3 or γ-Fe2O3 crystals, and also acts to prevent sintering of particles. Furthermore, when an appropriate amount of alkaline earth metal is present, the particles tend to grow into rod-like shapes.

[0046] Furthermore, as described above, M-substituted ε-Fe2O3 magnetic crystals can be synthesized more economically and advantageously by a process that combines a direct synthesis method with a sol-gel method and a firing process, as disclosed in JP 2008-174405 A.

[0047] To put it simply, 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 the substituting element M (Ga, Al, etc.) are dissolved while stirring.

[0048] The sol-gel process is then applied to form a silica coating on the precursor particle surface. After separating the silica-coated particles from the liquid, they are heat-treated (calcined) at a specified temperature to obtain fine particles of M-substituted ε-Fe2O3 magnetic crystals.

[0049] In the synthesis of M-substituted ε-Fe2O3 as described above, iron oxide crystals (impurity crystals) with different space groups and oxidation states from ε-Fe2O3 crystals may be produced. The most common polymorphs with different crystal structures that have the same composition as Fe2O3 are α-Fe2O3 and γ-Fe2O3. Other iron oxides include FeO and Fe3O4. The inclusion of such impurity crystals is not desirable in terms of maximizing the properties of the M-substituted ε-Fe2O3 crystal, but is permissible within a range that does not impair the effects of the present invention.

[0050] In addition, the coercive force H of M-substituted ε-Fe2O3 magnetic crystals c changes depending on the amount of substitution by the substitution element M. In other words, by adjusting the amount of substitution by the substitution element M in the M-substituted ε-Fe2O3 magnetic crystal, the coercive force H c can be adjusted. Specifically, when Al, Ga, or the like is used as the substitution element M, the coercive force H of the M-substituted ε-Fe2O3 magnetic crystal increases as the substitution amount increases. c On the other hand, when Rh or other elements are used as the substitution element M, the coercive force H of the M-substituted ε-Fe2O3 magnetic crystal decreases as the substitution amount increases. c increases. The coercive force H of M-substituted ε-Fe2O3 magnetic crystals changes depending on the amount of substitution by the element M. c In terms of ease of adjusting the value, the substitution element M is preferably Ga, Al, In, Ti, Co, or Rh.

[0051] And this coercive force H cAs the M-element substitution decreases, the peak frequency at which the electromagnetic wave absorption of epsilon-type 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 the epsilon iron oxide can be controlled, for example, by adjusting the heat treatment (firing) temperature in the above process. The method combining the reverse micelle method and the sol-gel method described above, or the method combining the direct synthesis method and the sol-gel method disclosed in JP 2008-174405 A, makes it 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 iron oxide particles are rod-shaped, the diameter in the major axis direction of the particles observed in a 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 number that is sufficiently large for calculating the average value, but 300 or more is preferred.

[0054] Furthermore, the silica coating formed on the surface of iron hydroxide particles by the sol-gel method may be present on the surface of the M-substituted ε-Fe2O3 magnetic crystals after heat treatment (calcination). The presence of a non-magnetic compound such as silica on the surface of the crystals is preferable in terms of improving the handleability, durability, and weather resistance of the magnetic crystals. Suitable examples of the non-magnetic compound include silica, as well as heat-resistant compounds such as alumina and zirconia.

[0055] However, if the amount of non-magnetic compound attached is too large, the particles may aggregate violently, which is undesirable. When the non-magnetic compound is silica, the mass of Si in the M-substituted ε-Fe2O3 magnetic crystal is preferably 100 mass % or less of the total mass of the substitution element M and the mass of Fe in the M-substituted ε-Fe2O3 magnetic crystal. A part or most of the silica attached to the M-substituted ε-Fe2O3 magnetic crystals can be removed by immersing them in an alkaline solution. 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 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. There are no particular restrictions on the relative dielectric constant of the electromagnetic wave absorbing layer, but it 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, and may be a single-walled carbon nanotube or a multi-walled carbon nanotube. The shape of the carbon nanotubes is not particularly limited, and various shapes can be mentioned, including a needle shape, a cylindrical tube shape, a fishbone shape (fishbone or cup stack 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 absorbing properties provided by the electromagnetic wave absorbing material layer may be impaired. The amount of carbon nanotubes used is preferably 3% by mass or more and 20% by mass or less, and more preferably 5% by mass or more and 10% by mass or less, based on the mass of the solid content other than the organic salt of the electromagnetic wave absorbing layer.

[0061] [Organic salts] By incorporating an organic salt into the electromagnetic wave absorbing layer, it is possible to suppress anisotropy of the electromagnetic wave absorbing characteristics, which is the difference in electromagnetic wave absorbing performance depending on the incident direction 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; and triethylsulfonium bis(trifluoromethanesulfonyl). bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and other bissulfonylimide salts not corresponding to the imidazolium salts; and carboxylic acid salts such as magnesium acetate, potassium acetate, magnesium propionate, potassium propionate, magnesium 2-ethylbutanoate, potassium 2-ethylbutanoate, magnesium 2-ethylhexanoate, and 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, tetrapotassium ethylenediaminetetraacetate, potassium hydrogen phthalate, dipotassium phthalate, potassium hydrogen oxalate, and dipotassium oxalate. As the organic salt, quaternary ammonium salts and imidazolium salts are preferred.

[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 absorbing properties provided by the electromagnetic wave absorbing layer may be impaired. The amount of organic salt used is preferably 1% by mass or more, more preferably 2.5% by mass or more, based on the mass of the solid content other than the organic salt in 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 permittivity 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 using a dielectric powder, 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, based on the mass of the solid contents other than the organic salt of the electromagnetic wave absorbing layer.

[0068] (non-dielectric materials) The electromagnetic wave absorbing layer may contain a non-dielectric material other than the magnetic material, and 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 ferroelectric or ferromagnetic.

[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 the 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, and is preferably from 10% by mass to 80% by mass, more preferably from 15% by mass to 75% by mass, based on the mass of the solid content excluding the organic salt in 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 thermoplastic binder resins include polyacetal resins, polyamide resins, polycarbonate resins, polyester resins (polybutylene terephthalate, polyethylene terephthalate, polyarylate, etc.), FR-AS resins, FR-ABS resins, AS resins, ABS resins, polyphenylene oxide resins, polyphenylene sulfide resins, polysulfone resins, polyethersulfone resins, polyetheretherketone resins, fluorine-based resins (polyvinylidene fluoride, etc.), polyimide resins, polyamideimide resins, polyamide bismaleimide resins, polyetherimide resins, polybenzoxazole resins, polybenzothiazole resins, polybenzimidazole resins, BT resins, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resins (e.g., methyl cellulose, ethyl cellulose), (meth)acrylic resins (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 to the electromagnetic wave absorbing layer resistance to cracking when bending or cutting, and low warpage. In order to obtain 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 also containing an aromatic group in the main chain skeleton. The aromatic group in the main chain skeleton may be either 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 above-mentioned 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. The unbranched divalent hydrocarbon group may be 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, these groups preferably have 1 or more and 8 or less carbon atoms, more preferably 2 or more and 6 or less carbon atoms, and even more preferably 2 or more and 4 or less carbon atoms.

[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 explained 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% by mass or more and 30% by mass or less, and more preferably 3% by mass or more and 25% by mass or less, of the binder resin relative to the mass of the solid content excluding the organic salt of the electromagnetic wave absorbing layer.

[0091] [Other ingredients] The electromagnetic wave absorbing layer may contain various additives other than the above components, as long as they do not impair the object of the present invention. Examples of additives that the electromagnetic wave absorbing layer may contain include dispersants, colorants, ultraviolet absorbers, flame retardants, flame retardant assistants, plasticizers, surfactants, etc. These additives are used in amounts that are conventionally used, as long as they do not impair the object of the present invention. By forming a film from the magnetic material described above, carbon nanotubes, organic salts, and optionally a dielectric, a non-magnetic material, a binder resin, and other components, for example, by a method using a paste for forming an electromagnetic wave absorber described below, an electromagnetic wave absorbing layer can be obtained that can be used as an electromagnetic wave absorber with good electromagnetic absorption properties in the high frequency band.

[0092] <Base material 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 of the substrate layer include a layer containing a resin. 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 flat surfaces, and is preferably a flat plate shape. 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 or smaller without impairing the effects of the present invention. There is no particular lower limit to the thickness of the substrate layer as long as it does not impair the effects of the present invention, 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 base layer, a metal layer may be provided on the surface of the base layer opposite to the surface on which the electromagnetic wave absorbing layer is provided. When a metal layer is provided, electromagnetic waves reflected by the metal layer can be attenuated. Preferred metals constituting the metal layer include, for example, 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 lower limit to the thickness of the metal layer as long as it does not impair the effects of the present invention, and examples 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> As a method for forming the electromagnetic wave absorber, a method using a paste for forming an electromagnetic wave absorber is preferred, since it is possible to form an electromagnetic wave absorbing layer with high efficiency without any particular thickness restrictions and to form the electromagnetic wave absorbing layer directly on the base layer. The electromagnetic wave absorber forming paste contains the magnetic material, carbon nanotubes, and an organic salt. The electromagnetic wave absorber forming paste preferably further contains the binder resin. 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. When the binder resin contains a curable resin, the electromagnetic wave absorber forming paste contains a compound that is a precursor of the curable resin. In this case, the electromagnetic wave absorber forming paste contains 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 by taking the total mass of the solid content as 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 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 kinds.

[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 a steric hindrance effect or the like. Surfactants stabilize the dispersion of 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 exerting a steric hindrance effect when the surfactant is adsorbed to the inorganic metal compound particles or when the surfactant is 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 producing the electromagnetic wave absorber is not particularly limited as long as it is possible to produce an electromagnetic wave absorber having a predetermined structure. A preferred method includes a method including an electromagnetic wave absorbing layer forming step in which the above-mentioned paste containing a magnetic material, carbon nanotubes, and an organic salt is applied onto a substrate layer to form a coating film, and then the coating film is dried to form an electromagnetic wave absorbing layer.

[0111] The method for applying the paste for forming an electromagnetic wave absorber onto the base layer is not particularly limited as long as it can form an electromagnetic wave absorber of a desired thickness. Examples of the application method include spray coating, dip coating, roll coating, curtain coating, spin coating, screen printing, doctor blade method, and applicator method. The coating film formed by the above method is dried to remove the dispersion medium, thereby 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 a 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 it at room temperature for several hours to several days, and (3) placing it in a hot air heater or infrared heater for several tens of minutes to several hours to remove the solvent.

[0112] The method for manufacturing an electromagnetic wave absorber may include a cutting step of cutting the electromagnetic wave absorbing layer or a laminate comprising a base layer and an electromagnetic wave absorbing layer obtained in the electromagnetic wave absorbing layer forming step to obtain an electromagnetic wave absorber 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 absorbing properties in the high frequency band.

[0113] As described above, the present inventors provide 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; Epsilon iron oxide is an ε-Fe2O3 crystal and a crystal with the same space group as ε-Fe2O3, in which part of the Fe site of the ε-Fe2O3 crystal is replaced by an element M other than Fe, and has the formula ε-M x Fe 2-xO3, wherein x is greater than 0 and less than 2. (3) The electromagnetic wave absorber according to (1) or (2), wherein the organic salt includes 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 solid contents 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 includes epsilon iron oxide; Epsilon iron oxide is an ε-Fe2O3 crystal and a crystal with the same space group as ε-Fe2O3, in which part of the Fe site of the ε-Fe2O3 crystal is replaced by an element M other than Fe, and has the formula ε-M x Fe 2-x O3, wherein x is greater than 0 and less than 2. [Example]

[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 (Mitsui Chemicals, Inc.) as a dispersion medium, epsilon-type iron oxide (product name DEP-24; 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; Sakai Chemical Industry Co., Ltd.) as a nonmagnetic material, 3 parts by mass of polyvinylidene fluoride (product name KF Polymer #7200; Kureha Corporation) as a binder resin, and 0.3 parts by mass of trimethoxyphenylsilane (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 (CNTs) (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 ratios (mass %) shown in Table 1 relative to the mass of solid content 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 (Mitsui Chemicals) as a dispersion medium, epsilon-type iron oxide (product name DEP-24; 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; Sakai Chemical Industry Co., Ltd.) as a non-magnetic material, 3 parts by mass of polyvinylidene fluoride (product name KF Polymer #7200; Kureha) as a binder resin, and 0.3 parts by mass of trimethoxyphenylsilane (Shin-Etsu Silicone Co., Ltd.). The mixture was mixed using a homodisper, and the components were uniformly dispersed using a bead mill.

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

[0119] [Table 1]

[0120] (Production of electromagnetic wave absorber film) The 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 to prepare test pieces for the following evaluations.

[0121] <Absorption frequency peak> A 3 cm square sample of the film-like electromagnetic wave absorber was attached to an aluminum plate. Electromagnetic waves in the 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 the frequency range of 0 GHz or more and 3000 GHz or less are incident at 0° and 90° to the film-shaped electromagnetic wave absorbers of the Examples and Comparative Examples. 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 peak top of the detected absorption frequency, 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] [Table 2]

[0123] According to Table 2, it can be seen that the electromagnetic wave absorbers of Examples 1 to 26 above, which have an electromagnetic wave absorbing layer made of a magnetic material, carbon nanotubes, and an organic salt, and 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 wave. On the other hand, it can be seen from Comparative Examples 1 to 5 that when no organic salt is included, the electromagnetic wave absorption characteristics vary greatly depending on the angle of incidence of the electromagnetic wave with respect to the electromagnetic wave absorber.

Claims

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 comprises epsilon iron oxide; 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. 2. The electromagnetic wave absorber according to claim 1, wherein the organic salt comprises a quaternary ammonium salt and / or an imidazole salt.

5. 2. The electromagnetic wave absorber according to claim 1, wherein the ratio of the mass of the organic salt to the mass of solid contents 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 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 comprises epsilon iron oxide; 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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