Electromagnetic wave absorber and paste for forming electromagnetic wave absorber

Abstract

The purpose of the present invention is to provide an electromagnetic wave absorber with which it is possible to obtain both excellent electromagnetic wave absorption characteristics in a high-frequency band and excellent heat dissipation characteristics, and an electromagnetic wave absorber formation paste suitable for use in producing the electromagnetic wave absorber.　An electromagnetic wave absorber is provided with a composite layer made of an electromagnetic wave absorption material and a thermally conductive material, the electromagnetic wave absorption material being made to contain one or more types of epsilon-type iron oxide selected from: ε-Fe2O3 crystal; and a crystal in which the crystal and the space group are identical to those of ε-Fe2O3 and a part of an Fe site of the ε-Fe2O3 is substituted with an element M other than Fe, and that is represented by the formula ε-MxFe2-xO3, x being above 0 and below 2.

Description

Technical Field

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

Background Art

[0002] In various information communication systems such as mobile phones, wireless LANs, ETC systems, advanced road traffic systems, vehicle driving support road systems, and satellite broadcasts, the use of electromagnetic waves in the high-frequency band is expanding. However, the expansion of the use of electromagnetic waves in the high-frequency band raises concerns about malfunctions and incorrect operations of electronic devices due to interference between electronic components. As a countermeasure against such problems, a method of absorbing unnecessary electromagnetic waves with an electromagnetic wave absorber has been adopted.

[0003]

[0004] Among the applications of electromagnetic waves in the high-frequency band, research on vehicle driving support systems is underway. In such a vehicle driving support system, electromagnetic waves in the 76 GHz band are used in in-vehicle radars for detecting the inter-vehicle distance and the like. And not limited to vehicle driving support systems, it is predicted that the use of electromagnetic waves in high-frequency bands of 100 GHz or higher will expand in various applications. For this reason, an electromagnetic wave absorber that can favorably absorb electromagnetic waves in the 76 GHz band and higher-frequency bands is desired.

[0005] In order to meet such requirements, as an electromagnetic wave absorber that can favorably absorb electromagnetic waves over a wide range in a high-frequency band, for example, an electromagnetic wave absorber including an electromagnetic wave absorption layer containing a magnetic crystal composed of ε-Fe2O3-based iron oxide has been proposed (for example, Patent Document 2, Non-Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, for an electromagnetic wave absorber as described in Patent Document 1, heat may be accumulated in the electromagnetic wave absorber depending on the usage environment. When heat is accumulated in the electromagnetic wave absorber, the materials constituting the electromagnetic wave absorber may be deformed or deteriorated. In addition, when the electromagnetic wave absorber accumulates heat, it may have an adverse effect on the entire device. Therefore, it is strongly desired to impart heat dissipation properties to the electromagnetic wave absorber without excessively degrading the good electromagnetic wave absorption characteristics in the high-frequency band.

[0009] The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide an electromagnetic wave absorber capable of achieving both good electromagnetic wave absorption characteristics in a high-frequency band and good heat dissipation properties, and a paste for forming an electromagnetic wave absorber suitably used for manufacturing the electromagnetic wave absorber.

Means for Solving the Problems

[0010] The inventors of the present invention provided a composite layer composed of an electromagnetic wave absorbing material and a heat conductive material in the electromagnetic wave absorber, and in the electromagnetic wave absorbing material, ε-Fe2O3 crystals, and crystals having the same space group as ε-Fe2O3 and having a part of the Fe sites of the ε-Fe2O3 crystals substituted with an element M other than Fe, and represented by the formula ε-M x Fe 2-x O3, and found that the above problems can be solved by containing one or more types of epsilon-type iron oxides selected from crystals in which x is greater than 0 and less than 2, and thus completed the present invention.

[0011] The first aspect of the present invention is comprising a composite layer composed of an electromagnetic wave absorbing material and a heat conductive material, the electromagnetic wave absorbing material contains epsilon-type iron oxide, Epsilon-type iron oxide is ε-Fe2O3 crystal, and a crystal having the same space group as ε-Fe2O3 and having a part of Fe sites of ε-Fe2O3 crystal substituted with an element M other than Fe, and is represented by the formula ε-M x Fe 2-x O3, and is one or more selected from crystals where x is greater than 0 and less than 2, and is an electromagnetic wave absorber.

[0012] The second aspect of the present invention is comprising an electromagnetic wave absorbing material and a heat conductive material, the electromagnetic wave absorbing material contains epsilon-type iron oxide, epsilon-type iron oxide is ε-Fe2O3 crystal, and a crystal having the same space group as ε-Fe2O3 and having a part of Fe sites of ε-Fe2O3 crystal substituted with an element M other than Fe, and is represented by the formula ε-M x Fe 2-x O3, and is one or more selected from crystals where x is greater than 0 and less than 2, and is a paste for forming an electromagnetic wave absorber.

Effect of the Invention

[0013] According to the present invention, it is possible to provide an electromagnetic wave absorber capable of achieving both good electromagnetic wave absorption characteristics in a high frequency band and good heat dissipation, and a paste for forming an electromagnetic wave absorber suitably used for manufacturing the electromagnetic wave absorber.

Brief Description of the Drawings

[0014]

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Mode for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described in detail. However, 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.

[0016] ≪Electromagnetic Wave Absorber≫ The electromagnetic wave absorber includes a composite layer composed of an electromagnetic wave absorbing material and a heat conductive material. The electromagnetic wave absorber may consist only of such a composite layer, or may include a base material layer that supports the composite layer.

[0017] Regarding the electromagnetic wave absorber, from the viewpoint of being able to more reliably absorb high-frequency electromagnetic waves in the millimeter wave band and above, it is preferable to absorb electromagnetic waves in a frequency band of 30 gigahertz (GHz) or more, preferably 30 GHz or more and 300 GHz or less, and more preferably 40 GHz or more and 200 GHz or less. Further, in the reflection attenuation amount of the electromagnetic wave absorber, it is preferable that there is a peak with an absolute value of 15 dB or more. The reflection attenuation amount is a value measured with respect to the surface where the composite layer is exposed.

[0018] The form of the electromagnetic wave absorber is not particularly limited, but it is preferably in a sheet shape or a film shape, and more preferably in a film shape. When the shape of the electromagnetic wave absorber is a film shape, the shape of the film may have a curved surface, may be composed only of a flat surface, and is preferably flat plate-shaped. The thickness of the film as the electromagnetic wave absorber is preferably 1000 μm or less, more preferably 900 μm or less, still more preferably 450 μm or less, and particularly preferably 300 μm or less, from the viewpoints of thinning or miniaturizing the film without impairing the effects of the present invention. The thickness of the film as the electromagnetic wave absorber may be uniform or non-uniform.

[0019] <Composite layer> As described above, the composite layer includes a heat conductive material together with the electromagnetic wave absorbing material. The form of the composite layer is not particularly limited. The composite layer may be a laminated composite layer including at least one layer made of an electromagnetic wave absorbing material and at least one layer containing a heat conductive material, or may be a single-layer composite layer containing an electromagnetic wave absorbing material and a heat conductive material. Since the heat accumulated in the electromagnetic wave absorber can be dissipated evenly and efficiently, the composite layer is preferably a single-layer composite layer containing an electromagnetic wave absorbing material and a heat conductive material.

[0020] The thickness of the composite layer is not particularly limited as long as it does not inhibit the object of the present invention. From the viewpoint of the balance between the thinning of the electromagnetic wave absorber and the electromagnetic wave absorption performance, the thickness of the composite layer is preferably 100 μm or less, and more preferably 50 μm or less. The lower limit value of the thickness of the composite layer is not particularly limited as long as it does not impair the effects of the present invention, and examples thereof include 1 μm or more and 10 μm or more. The thickness of the composite layer may be uniform or non-uniform.

[0021] Hereinafter, the essential or optional configurations of the composite layer will be described.

[0022] 〔Electromagnetic wave absorbing material〕 The electromagnetic wave absorbing material contains epsilon-type iron oxide. An electromagnetic wave absorber including a composite layer containing epsilon-type iron oxide as the electromagnetic wave absorbing material can favorably absorb electromagnetic waves with high frequencies in the millimeter wave band or higher, and the electromagnetic wave absorption characteristics are hardly inhibited even when used in combination with the heat conductive material described later.

[0023] The electromagnetic wave absorbing material may contain a magnetic material having the performance of absorbing electromagnetic waves together with epsilon-type iron oxide. Preferred examples of the magnetic material that can be used together with epsilon-type iron oxide include barium ferrite magnetic material and strontium ferrite magnetic material. In terms of the good electromagnetic wave absorption characteristics of the electromagnetic wave absorber, the ratio of the mass of epsilon-type iron oxide to the total mass of the mass of epsilon-type iron oxide and the magnetic material other than epsilon-type iron oxide is preferably 70% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass.

[0024] The electromagnetic wave absorbing material preferably contains a binder resin typically for the purpose of uniformly dispersing epsilon-type iron oxide in the electromagnetic wave absorbing material and for the purpose of making it possible to easily form a composite layer.

[0025] Hereinafter, epsilon-type iron oxide and binder resin, which are the main materials constituting the electromagnetic wave absorbing material, will be described.

[0026] (Epsilon-Type Iron Oxide) As the epsilon-type iron oxide, it is at least one selected from the group consisting of ε-Fe2O3 crystals and crystals having the same crystal structure and space group as ε-Fe2O3, and in which a part of the Fe sites of the ε-Fe2O3 crystals is substituted with an element M other than Fe, and is represented by the formula ε-M x Fe 2-x O3, and the x is preferably at least one selected from the group consisting of crystals where x is 0 or more and 2 or less (preferably 0 or more and less than 2). Since such crystals of epsilon-type iron oxide are magnetic crystals, in the specification of the present application, the crystals may be referred to as "magnetic crystals".

[0027] Regarding the ε-Fe2O3 crystal, any one can be used. It has the same crystal structure and space group as ε-Fe2O3, and a part of the Fe sites of the ε-Fe2O3 crystal is substituted with an element M other than Fe, and is represented by the formula ε-M x Fe 2-xCrystals represented by O3, where x is 0 or more and 2 or less (preferably 0 or more and less than 2), will be described later. In the present specification, ε-M in which a part of the Fe sites of the ε-Fe2O3 crystal is substituted with the substitution element M x Fe 2-x FeO3 is also referred to as "M-substituted ε-Fe2O3".

[0028] The particle diameter of the particles having ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 crystals in the magnetic phase is not particularly limited as long as it does not inhibit the object of the present invention. For example, the particles having magnetic crystals of epsilon-type iron oxide manufactured by the method described later and having the magnetic phase have an average particle diameter measured from a TEM (transmission electron microscope) photograph in the range of 5 nm or more and 200 nm or less. In addition, the coefficient of variation (standard deviation of particle diameter / average particle diameter) of the particles having magnetic crystals of epsilon-type iron oxide manufactured by the method described later and having the magnetic layer is in the range of less than 80%, and it is a relatively fine particle group with a uniform particle diameter.

[0029] In a preferred composite layer, such powder of magnetic particles of epsilon-type iron oxide (that is, particles having ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 crystals in the magnetic phase) is used as a magnetic material which is an electromagnetic wave absorbing material in the composite layer. The "magnetic phase" referred to here is the part responsible for the magnetism of the powder. "Having ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 crystals in the magnetic phase" means that the magnetic phase is composed of ε-Fe2O3 crystals and / or M-substituted ε-Fe2O3 crystals, and includes the case where impurity magnetic crystals that are inevitable in production are mixed in the magnetic phase.

[0030] The magnetic crystals of epsilon-type iron oxide may contain impurity crystals of iron oxides having different space groups and oxidation states from ε-Fe2O3 crystals (specifically, α-Fe2O3, γ-Fe2O3, FeO, and Fe3O4, and crystals in which a part of Fe in these crystals is substituted with other elements). When the magnetic crystal of epsilon-type iron oxide contains impurity crystals, it is preferable that the magnetic crystal of ε-Fe2O3 and / or M-substituted ε-Fe2O3 is the main phase. That is, among the magnetic crystals of epsilon iron oxide constituting the electromagnetic wave absorption material, it is preferable that the ratio of the magnetic crystals of ε-Fe2O3 and / or M-substituted ε-Fe2O3 is 50 mol% or more in terms of molar ratio as a compound.

[0031] The abundance ratio of the crystal can be determined by analysis using the Rietveld method based on the X-ray diffraction pattern. There may be non-magnetic compounds such as silica (SiO2) formed in the sol-gel process adhering to the periphery of the magnetic phase.

[0032] (M-substituted ε-Fe2O3) As long as the crystal satisfies the condition that the space group is the same as that of ε-Fe2O3 and a part of the Fe sites of the ε-Fe2O3 crystal is substituted with an element M other than Fe, the type of element M in M-substituted ε-Fe2O3 is not particularly limited. M-substituted ε-Fe2O3 may contain a plurality of types of elements M other than Fe.

[0033] Preferable 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 preferable. When M is Al, in the composition represented by ε-M x Fe 2-x O3, x is preferably in the range of, for example, 0 or more and less than 0.8. When M is Ga, x is preferably in the range of, for example, 0 or more and less than 0.8. When M is In, x is preferably in the range of, for example, 0 or more and less than 0.3. When M is Rh, x is preferably in the range of, for example, 0 or more and less than 0.3. When M is Ti and Co, x is preferably in the range of, for example, 0 or more and less than 1.

[0034] The frequency at which the electromagnetic wave absorption amount is maximized can be adjusted by adjusting at least one of the type and substitution amount of the element M in M-substituted ε-Fe2O3.

[0035] Such M-substituted ε-Fe2O3 magnetic crystals can be synthesized, for example, by a process combining the reverse micelle method and the sol-gel method and a firing process, which will be described later. Also, M-substituted ε-Fe2O3 magnetic crystals can be synthesized by a process combining the direct synthesis method and the sol-gel method and a firing process, as disclosed in Japanese Patent Application Laid-Open No. 2008-174405.

[0036] Specifically, Jian Jin, Shin-ichi Ohkoshi and Kazuhito 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.1946-1949, 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, p.1170-1173, 2009. etc., by a process combining the reverse micelle method and the sol-gel method and a firing process, M-substituted ε-Fe2O3 magnetic crystals can be obtained.

[0037] In the reverse micelle method, by mixing two types of micelle solutions containing a surfactant, namely micelle solution I (raw material micelle) and micelle solution II (neutralizing agent micelle), a precipitation reaction of iron hydroxide proceeds inside the micelles. Next, by the sol-gel method, a silica coat is applied to the surface of the iron hydroxide fine particles generated inside the micelles. The iron hydroxide fine particles provided with the silica coat layer are, after being separated from the liquid, subjected to heat treatment in an air atmosphere at a predetermined temperature (within the range of 700 to 1300 °C). By this heat treatment, fine particles of ε-Fe2O3 crystals are obtained.

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

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

[0040] An appropriate amount of nitrate of alkaline earth metals (such as Ba, Sr, Ca, etc.) can be dissolved in the aqueous phase of micelle solution I. This nitrate functions as a shape control agent. When an alkaline earth metal is present in the liquid, finally, rod-shaped particles of M-substituted ε-Fe2O3 magnetic crystals are obtained. In the absence of a shape control agent, particles of M-substituted ε-Fe2O3 magnetic crystals close to a spherical shape are obtained.

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

[0042] An aqueous ammonia solution is used for the aqueous phase of micellar solution II with n-octane as the oil phase.

[0043] After mixing micellar solutions I and II, the sol-gel method is applied. That is, while dropping silane (for example, tetraethyl orthosilicate) into the mixed solution of the micellar solution and continuing stirring, the formation reaction of 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 precipitates generated within the micelles are coated with silica generated by the hydrolysis of silane.

[0044] Next, the particle powder obtained by separating, washing, and drying the silica-coated M element-containing iron hydroxide particles from the liquid is charged into a furnace and heat-treated (fired) in air within a temperature range of 700 °C or higher and 1300 °C or lower, preferably 900 °C or higher and 1200 °C or lower, and more preferably 950 °C or higher and 1150 °C or lower. By this heat treatment, an oxidation reaction proceeds within the silica coating, and the fine particles of fine M element-containing iron hydroxide are changed into fine particles of fine M-substituted ε-Fe2O3.

[0045] During this oxidation reaction, the presence of the silica coat contributes to the formation of M-substituted ε-Fe2O3 crystals having the same space group as ε-Fe2O3 rather than α-Fe2O3 or γ-Fe2O3 crystals, and also serves to prevent sintering of the particles. Also, when an appropriate amount of alkaline earth metal coexists, the particle shape tends to grow into a rod shape.

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

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

[0048] The sol-gel method is then 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 (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 generated. The most common polymorphisms with different crystal structures that have the composition of 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 bringing out the properties of the M-substituted ε-Fe2O3 crystal as highly as possible, 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 increases as the substitution amount increases. c increases. The coercive force H of M-substituted ε-Fe2O3 magnetic crystals depends on the amount of substitution by the element M. c From the viewpoint of ease of adjustment, the substitution element M is preferably Ga, Al, In, Ti, Co or Rh.

[0051] And this coercive force H cAs the [parameter] decreases, the frequency of the peak at which the electromagnetic wave absorption amount of epsilon-type iron oxide becomes maximum also shifts to the low-frequency side or the high-frequency side. That is, the frequency of the peak of the electromagnetic wave absorption amount can be controlled by the substitution amount of the M element.

[0052] In the case of a generally used electromagnetic wave absorber, when the incident angle or frequency of the electromagnetic wave deviates from the designed value, the absorption amount almost becomes zero. On the other hand, when epsilon-type iron oxide is used, even if the value deviates slightly, electromagnetic wave absorption is exhibited in a wide frequency range and electromagnetic wave incident angles. Therefore, a composite layer capable of absorbing electromagnetic waves in a wide frequency band can be provided.

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

[0054] In addition, a silica coat coated on the surface of fine iron hydroxide particles by the sol-gel method may be present on the surface of the M-substituted ε-Fe2O3 magnetic crystal after heat treatment (firing). When a non-magnetic compound such as silica is present on the surface of the crystal, it is preferable in terms of improving the handleability, durability, weather resistance, etc. of the magnetic crystal. Suitable examples of non-magnetic compounds include, in addition to silica, 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. When the non-magnetic compound is silica, the mass of Si in the M-substituted ε-Fe2O3 magnetic crystal is preferably 100% by mass or less with respect to the total of the mass of the substitution element M and the mass of Fe in the M-substituted ε-Fe2O3 magnetic crystal. Part or most of the silica attached to the M-substituted ε-Fe2O3 magnetic crystal can be removed by immersing it in an alkaline solution. The amount of silica attached can be adjusted to an arbitrary amount by such a method.

[0056] The relative permeability of the composite 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 composite layer is not particularly limited. Examples of the method for adjusting the relative permeability of the composite layer include a method of adjusting the substitution amount by the substitution element M in epsilon-type iron oxide, and a method of adjusting the content of epsilon-type iron oxide and other magnetic materials other than epsilon-type iron oxide in the composite layer.

[0057] The content of epsilon-type iron oxide in the composite layer is not particularly limited as long as it does not inhibit the object of the present invention. The content of the magnetic material is preferably 30% by mass or more, more preferably 40% by mass or more, particularly preferably 60% by mass or more, and most preferably 60% by mass or more and 91% by mass or less with respect to the solid content mass of the composite layer.

[0058] (Binder resin) The electromagnetic wave absorbing material typically contains a binder resin. By using the binder resin, epsilon-type iron oxide and other magnetic materials are well dispersed in the binder resin. In addition, since the electromagnetic wave absorbing material contains the binder resin, it is easy to form a composite layer having a desired shape.

[0059] The binder resin may be an elastic material such as an elastomer or rubber. Also, 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.

[0060] Suitable examples of the case where the binder resin 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, fluororesin, polyimide resin, polyamideimide resin, polyamidebismaleimide resin, polyetherimide resin, polybenzoxazole resin, polybenzothiazole resin, polybenzimidazole resin, BT resin, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resin (for example, methylcellulose, ethylcellulose), (meth)acrylic resin (polymethyl methacrylate, etc.), and polystyrene, etc.

[0061] Suitable examples of the case where the binder resin is a thermosetting resin include phenol resin, melamine resin, epoxy resin, and alkyd resin, etc. As the photocurable resin, resins obtained by photocuring various vinyl monomers and monomers having an unsaturated bond such as various (meth)acrylic acid esters can be used.

[0062] Suitable examples of the case where the binder resin is an elastic material include olefin-based elastomers, styrene-based elastomers, polyamide-based elastomers, polyester-based elastomers, and polyurethane-based elastomers, etc.

[0063] Moreover, as the binder resin, it is preferable to use an aromatic ester-urethane copolymer. By using an aromatic ester-urethane copolymer as the binder resin, it is possible to form a film-like electromagnetic wave absorber that can well disperse epsilon-type iron oxide and other magnetic materials in the binder resin and exhibit excellent electromagnetic wave absorption characteristics even when thin.

[0064] In addition, when using an aromatic ester-urethane copolymer as the binder resin, it is possible to impart crack resistance and low warpage during bending or cutting to the composite layer. From the viewpoint of good crack resistance and low warpage of the composite 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.

[0065] The aromatic ester-urethane copolymer is a copolymer containing an ester bond (-CO-O-), a urethane bond (-NH-CO-O-), and 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, and an aromatic hydrocarbon group is preferred. The aromatic ester-urethane copolymer may be a random copolymer in which an ester bond and a urethane bond are randomly introduced into the molecular chain, or may be a block copolymer composed of one or more ester blocks and one or more urethane blocks.

[0066] 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.

[0067] 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.

[0068] The above monomers used in the production of the aromatic ester-urethane copolymer are preferably compounds in which two functional groups selected from the group consisting of a hydroxyl group, a carboxy group, and an isocyanate group are bonded to a divalent hydrocarbon group having an unbranched structure. Examples of the divalent hydrocarbon group having an unbranched structure 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.

[0069] When the divalent hydrocarbon group having an unbranched structure is an alkylene group, an alkenylene group, or an alkynylene group, the number of carbon atoms of these groups is preferably 1 or more and 8 or less, more preferably 2 or more and 6 or less, and still more preferably 2 or more and 4 or less.

[0070] When the divalent hydrocarbon group having an unbranched structure is an arylene group, the arylene group is preferably a phenylene group or a naphthylene group, more preferably a phenylene group, and still more preferably a p-phenylene group.

[0071] Among the divalent hydrocarbon groups having an unbranched structure described above, an alkylene group, an arylene group, and a combination of an alkylene group and an arylene group are preferable.

[0072] Preferable specific 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. Preferable specific 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, etc. Preferable specific 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, etc. Preferable specific 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, etc.

[0073] The weight average molecular weight (Mw) of the aromatic ester-urethane copolymer is preferably 5,000 or more and 500,000 or less, more preferably 10,000 or more and 200,000 or less. In the specification of this application, the weight average molecular weight (Mw) is the weight average molecular weight in terms of polystyrene measured by GPC.

[0074] Commercially available products of the aromatic ester-urethane copolymer include the Baylon series (trade name) (manufactured by Toyobo Co., Ltd.), etc. More specifically, Baylon UR-1400, Baylon UR-1410, Baylon UR-1700, Baylon UR-2300, Baylon UR-3200, Baylon UR-3210, Baylon UR-3500, Baylon UR-6100, Baylon UR-8300, and Baylon UR-8700, etc. can be preferably used.

[0075] The content of the binder resin in the electromagnetic wave absorbing material is not particularly limited as long as it does not inhibit the object of the present invention. The electromagnetic wave absorbing material preferably contains 5% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 25% by mass or less of the binder resin based on the solid content mass of the composite layer.

[0076] (Dielectric) The electromagnetic wave absorbing material may contain a dielectric for the purpose of adjusting the relative permittivity of the composite layer. By adjusting the content of the dielectric in the composite layer, the relative permittivity of the composite layer can be adjusted. The relative permittivity of the composite 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.

[0077] 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 a plurality of types of dielectrics.

[0078] The particle diameter of the dielectric powder used for adjusting the relative permittivity of the composite layer is not particularly limited as long as it does not inhibit the object of the present invention. The average particle diameter of the dielectric powder is preferably 1 nm or more and 100 nm or less, more preferably 5 nm or more and 50 nm or less. Here, the average particle diameter of the dielectric powder is the number average diameter of the primary particles of the dielectric powder observed by an electron microscope.

[0079] When adjusting the relative permittivity of the composite layer using the dielectric powder, the amount of the dielectric powder used is not particularly limited as long as the relative permittivity of each composite layer is within a predetermined range. The amount of the dielectric powder used is preferably 0% 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 solid content mass of the composite layer.

[0080] (Carbon nanotube) By incorporating carbon nanotubes into the electromagnetic wave absorbing material, the relative permittivity of the composite layer can be adjusted. The carbon nanotubes may be used in combination with the above dielectric powder.

[0081] The blending amount of carbon nanotubes in the electromagnetic wave absorbing material is not particularly limited as long as the relative permittivity of the composite layer is within the above predetermined range. However, since carbon nanotubes are also conductive materials, if the amount of carbon nanotubes used is excessive, the electromagnetic wave absorption characteristics provided by the composite layer may be impaired. The amount of carbon nanotubes used is preferably 0% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 10% by mass or less, based on the solid content mass of the composite layer.

[0082] 〔Thermally conductive material〕 The thermally conductive material is not particularly limited as long as it is a material recognized by those skilled in the art as having high thermal conductivity. For example, the thermal conductivity of the thermally conductive material is preferably 15 W / m·K or more, more preferably 20 W / m·K or more, still more preferably 50 W / m·K or more, and particularly preferably 200 W / m·K or more.

[0083] For example, substances having a thermal conductivity of 15 W / m·K or more include alumina, aluminum nitride, silicon carbide, and boron nitride. These thermally conductive materials may be used in combination of two or more. Among these, alumina and silicon carbide are preferable because they are easily available and it is easy to obtain an electromagnetic wave absorber having excellent heat dissipation and electromagnetic wave absorption characteristics.

[0084] The shape of the thermally conductive material is not particularly limited as long as it does not inhibit the object of the present invention. The thermally conductive material is preferably a granular or flaky powder. The thermally conductive material having such a shape has a small aspect ratio and is difficult to orient in the composite layer. Therefore, when a thermally conductive material in the form of a granular or flaky powder is used, it is easy to suppress folding, cracking, tearing, etc. of the composite layer caused by the orientation of the thermally conductive material. The aspect ratio (average major axis length / average minor axis length) of the heat conductive material is preferably less than 6, more preferably 5 or less, and even more preferably 3 or less. The average major axis length and the average minor axis length of the heat conductive material can be determined as the number average length by, for example, microscopic observation, SEM observation, etc. Also, by using a combination of a granular heat conductive material and a flaky heat conductive material, it is easy to increase the thermal diffusivity of the electromagnetic wave absorber. In particular, when a combination of granular alumina and flaky boron nitride is used, not only the thermal diffusivity but also the thermal conductivity tends to increase significantly. When a granular heat conductive material and a flaky heat conductive material are used in combination, the ratio of the mass of the flaky heat conductive material to the total mass of the granular heat conductive material and the flaky heat conductive material is preferably 7% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 50% by mass or less, and even more preferably 15% by mass or more and 40% by mass or less from the viewpoint of achieving both the improvement effect of thermal conductivity and the film-forming property of the paste for forming an electromagnetic wave absorber.

[0085] The heat conductive material, which is the above-mentioned granular or flaky powder, is preferably dispersed in the matrix made of the above-mentioned electromagnetic wave absorbing material from the viewpoint of good heat dissipation of the electromagnetic wave absorber.

[0086] The amount of the heat conductive material used is not particularly limited and is appropriately adjusted according to the desired heat dissipation performance level of the electromagnetic wave absorber. Typically, the heat conductive material is preferably used such that the composite layer contains preferably 30 parts by mass or more and 300 parts by mass or less, more preferably 40 parts by mass or more and 200 parts by mass or less of the heat conductive material with respect to 100 parts by mass of the electromagnetic wave absorbing material.

[0087] 〔Other Components〕 The composite layer may contain various additives other than the above components as long as the object of the present invention is not inhibited. Examples of additives that the composite layer may contain include dispersants, colorants, antioxidants, ultraviolet absorbers, flame retardants, flame retardant aids, plasticizers, and surfactants. These additives are used in consideration of the amounts conventionally used as long as the object of the present invention is not inhibited.

[0088] By forming a film while compounding the electromagnetic wave absorbing material, the heat conductive material, and other components as necessary, for example, by using a method using a paste for forming an electromagnetic wave absorber described later, a composite layer can be obtained that can be used as an electromagnetic wave absorber capable of achieving both good electromagnetic wave absorption characteristics in a high frequency band and good heat dissipation.

[0089] <Base material layer> The above-described composite layer may be laminated on a base material layer. The base material layer may be a layer containing any base material as long as the effects of the present invention are not impaired. Examples include layers containing resins. Examples of the above resin include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acrylic (PMMA), polycarbonate (PC), cycloolefin polymer (COP), polyethersulfone, polyimide, polyamideimide, and the like. Among them, PET is preferable because of its excellent heat resistance and good balance between dimensional stability and cost.

[0090] The shape of the base material layer may have a curved surface, may be composed only of a plane, and is preferably flat plate-shaped. The thickness of the base material layer is preferably 800 μm or less, more preferably 500 μm or less, still more preferably 300 μm or less, and particularly preferably 150 μm or less from the viewpoints of thinning the film and miniaturizing it without impairing the effects of the present invention. The lower limit value of the thickness of the base material layer is not particularly limited as long as the effects of the present invention are not impaired. Examples include 1 μm or more, 10 μm or more, 50 μm or more, and the like.

[0091] <Metal layer> When the electromagnetic wave absorber includes a base material layer, in the electromagnetic wave absorber, a metal layer may be provided on the surface opposite to the surface where the composite layer of the base material layer is provided. When the metal layer is provided, the electromagnetic wave reflected by the metal layer can be attenuated. As the metal constituting the metal layer, for example, aluminum, titanium, SUS, copper, brass, silver, gold, platinum, etc. are preferable. The thickness of the metal layer is not particularly limited, and from the viewpoint of making the electromagnetic wave absorber thin, 600 μm or less is preferable, 400 μm or less is more preferable, 100 μm or less is further preferable, and 50 μm or less is particularly preferable. The lower limit value of the thickness of the metal layer is not particularly limited as long as the effects of the present invention are not impaired. For example, 0.1 μm or more, 1 μm or more, 5 μm or more, 10 μm or more, etc. can be mentioned.

[0092] By combining the composite layer containing a predetermined component described above with a base material layer, or a combination of a base material layer and a metal layer as needed, an electromagnetic wave absorber capable of achieving both good electromagnetic wave absorption characteristics in a high-frequency band and good heat dissipation can be obtained.

[0093] The electromagnetic wave absorber described above can be preferably used as a film for electromagnetic wave absorption used in various elements (including in-vehicle elements, high-frequency antenna elements, etc.) in various information communication systems such as mobile phones, wireless LANs, ETC systems, advanced road traffic systems, vehicle driving support road systems, satellite broadcasts, etc.

[0094] ≪Paste for forming electromagnetic wave absorber≫ As a method for forming the electromagnetic wave absorber, a method of forming using a paste for forming an electromagnetic wave absorber is preferable because a composite layer can be formed with high efficiency without particular limitation on the thickness, and the composite layer can be directly formed on the base material layer. The paste for forming an electromagnetic wave absorber contains the above-described electromagnetic wave absorbing material and a heat conductive material. The paste for forming an electromagnetic wave absorber preferably further contains the above binder resin. The paste for forming an electromagnetic wave absorber may contain substances added for adjusting the relative permittivity, relative permeability, etc. of the electromagnetic wave absorbing material, and other components as described above. When the binder resin contains a curable resin, the paste for forming an electromagnetic wave absorber contains a compound that is a precursor of the curable resin. In this case, the paste for forming an electromagnetic wave absorber contains a curing agent, a curing accelerator, a polymerization initiator, etc. as necessary.

[0095] Also, when the paste for forming an electromagnetic wave absorber contains a photopolymerizable or thermopolymerizable compound, exposure or heating may be performed on the coating film as necessary to form a composite layer.

[0096] The paste for forming an electromagnetic wave absorber preferably further contains a dispersion medium. As the dispersion medium, water, an organic solvent, and an aqueous solution of an organic solvent can be used. As the dispersion medium, an organic solvent is preferable because it is easy to dissolve organic components and has a low latent heat of vaporization and is easily removed by drying.

[0097] Suitable examples of the organic solvent used as the dispersion medium 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), pyridine; ketones such as diethyl ketone, methyl butyl ketone, dipropyl ketone, cyclohexanone; alcohols such as n-pentanol, 4-methyl-2-pentanol, cyclohexanol, diacetone alcohol; ether alcohols such as 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, diethylene glycol dimethyl ether, diethylene glycol diethyl ether; saturated aliphatic monocarboxylic acid alkyl esters such as n-butyl acetate, amyl acetate; lactate esters such as ethyl lactate, n-butyl lactate; ketones such as acetone, methyl ethyl ketone, cyclohexanone, acetophenone, benzophenone; ether esters such as methyl cellosolve acetate, ethyl cellosolve acetate, propylene glycol monomethyl ether acetate, propylene glycol 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, 2-methoxypentyl acetate, etc. These can be used alone or in combination of two or more.

[0098] The solid content concentration of the paste for forming an electromagnetic wave absorber is appropriately adjusted according to the method of applying the paste for forming an electromagnetic wave absorber, the thickness of the composite layer, and the like. 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 of the mass of the components not dissolved in the dispersion medium and the mass of the components dissolved in the dispersion medium as the mass of the solid content.

[0099] (Dispersant) For the purpose of favorably dispersing the above epsilon-type iron oxide and the substances used to adjust the relative permittivity and relative permeability of the composite layer in the composite layer, the paste for forming an electromagnetic wave absorber may contain a dispersant. The dispersant may be uniformly mixed with the above epsilon-type iron oxide and the binder resin. The dispersant may be incorporated into the binder resin. Further, the above epsilon-type iron oxide pretreated with a dispersant or the substance added to adjust the relative permittivity and relative permeability may be incorporated into the material constituting the composite layer.

[0100] The type of the dispersant is not particularly limited as long as it does not inhibit the object of the present invention. The dispersant can be selected from various dispersants conventionally used in the dispersion applications of various inorganic fine particles and organic fine particles.

[0101] Preferable examples of the dispersant include silane coupling agents (for example, phenyltrimethoxysilane), titanate coupling agents, zirconate coupling agents, and aluminate coupling agents.

[0102] The content of the dispersant is not particularly limited as long as it does not inhibit the object of the present invention. The content of the dispersant is preferably 0.1% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 15% by mass or less, and particularly preferably 1% by mass or more and 10% by mass or less with respect to the solid content mass of the paste for forming an electromagnetic wave absorber.

[0103] ≪Method for manufacturing an electromagnetic wave absorber≫ The method for manufacturing the electromagnetic wave absorber described above is not particularly limited as long as it can manufacture an electromagnetic wave absorber with a predetermined structure. As a preferred method, there is a method including a composite layer forming step of applying the above-described paste containing an electromagnetic wave absorbing material and a heat conductive material onto a base material layer to form a coating film, and then drying the coating film to form a composite layer.

[0104] The method of applying the paste for forming the electromagnetic wave absorber onto the base material layer is not particularly limited as long as it can form an electromagnetic wave absorber with a desired thickness. Examples of the coating method include a spray coating method, a dip coating method, a roll coating method, a curtain coating method, a spin coating method, a screen printing method, a doctor blade method, and an applicator method. By drying the coating film formed by the above method to remove the dispersion medium, a composite film is formed on the base material layer, and thereby an electromagnetic wave absorber is obtained. The film thickness of the coating film is appropriately adjusted so that the thickness of the composite film obtained after drying becomes the desired thickness. The drying method is not particularly limited. For example, (1) a method of drying on a hot plate at a temperature of 80°C or higher and 180°C or lower, preferably 90°C or higher and 160°C or lower for 1 minute or more and 30 minutes or less, (2) a method of leaving it at room temperature for several hours to several days, (3) a method of putting it in a warm air heater or an infrared heater for several tens of minutes to several hours to remove the solvent, etc.

[0105] The method for manufacturing the electromagnetic wave absorber may include a cutting step of cutting the composite layer obtained in the composite layer forming step, or a laminate including the base material layer and the composite layer, to obtain an electromagnetic wave absorber having a predetermined size. As described above, since the electromagnetic wave absorber includes a composite layer composed of an electromagnetic wave absorbing material and a heat conductive material, it can achieve both good electromagnetic wave absorption characteristics in the high frequency band and good heat dissipation properties.

Example

[0106] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited by these examples.

[0107] [Example 1] (Preparation of Paste for Forming Electromagnetic Wave Absorber) To 25.1 parts by mass of TMU as a dispersion medium, 39 parts by mass of the following epsilon-type iron oxide as an electromagnetic wave absorbing material, 2.4 parts by mass of the following carbon nanotube (CNT), 8.6 parts by mass of a binder resin, and 21.4 parts by mass of granular alumina powder as a heat conductive material were added. The binder resin was added as the following binder resin solution. Stirring was carried out by a rotation-revolution mixer to uniformly dissolve or disperse each component to obtain a paste for forming an electromagnetic wave absorber.

[0108] As the epsilon-type iron oxide, ε-Ga 0.45 Fe 1.55 O3 was used. The average particle diameter of the epsilon-type iron oxide was 20 nm or more and 30 nm or less. As the CNT, a multi-walled carbon nanotube with a major axis of 150 nm (trade name VGCF-H; manufactured by Showa Denko KK) was used. As the dispersant, phenyltrimethoxysilane was used. As the binder resin solution, an aromatic ester-urethane copolymer (manufactured by Toyobo Co., Ltd., Baylon UR-3210, glass transition temperature -3°C, weight average molecular weight 40000, consisting of 5 parts by mass of resin and 15 parts by mass of methyl ethyl ketone) was used.

[0109] (Manufacture of Electromagnetic Wave Absorber Film) The above paste for forming an electromagnetic wave absorber was applied to a PET film (thickness 125 μm) by an applicator. Thereafter, the coated film was dried under the conditions of 90°C for 10 minutes and 130°C for 10 minutes to form a composite layer with a thickness of 35 μm, and a film-like electromagnetic wave absorber was obtained. The film-like electromagnetic wave absorber obtained immediately after drying was cut into a square shape with a side length of 5 cm to prepare test pieces for the following evaluation.

[0110] <Reflection Attenuation Amount>[Reflection Attenuation Amount] A sample of a film-shaped electromagnetic wave absorber in the form of a 5 cm square was attached onto an aluminum plate. Electromagnetic waves in the range of 40 - 120 GHz were incident on the measurement sample on the aluminum plate, and the reflection attenuation amount was measured using a terahertz time-domain spectrometer (manufactured by Advantest Corporation). The reflection attenuation amount RL(f) at frequency f is obtained by RL(f)=-10Log(R(f) / 100). Here, R(f) is the reflectance (%). The reflection attenuation amount (Reflectance(dB)) of the film-shaped electromagnetic wave absorber of Example 1 in the frequency range of 40 - 120 GHz is shown in Fig. 1.

[0111] <Thermal conductivity, and thermal diffusivity> For the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured by the following method. These measurement results are shown in Table 1. Regarding the thermal diffusivity, the measurement was carried out by the periodic heating method. Specifically, the measurement of the thermal diffusivity was performed using a periodic heating method thermal diffusivity measurement device (FTC-1 type) manufactured by ULVAC-RIKO, Inc. The thermal conductivity λ was calculated from the following formula based on the thermal diffusivity α measured by the above method. Note that the specific heat C ρ applied to the following formula was measured by the DSC method using a device (X-DSC 7000 type) manufactured by Hitachi High-Tech Sciences Corporation at the measurement temperature of 25°C. λ=α×Cρ×ρ×100 λ: Thermal conductivity (W / (m·K)) α: Thermal diffusivity (cm 2 / s) C ρ : Specific heat (J / (g·K)) ρ: Density (g / cm 3 )

[0112] 〔Example 2〕 A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 1, except that the addition amount of TMU was changed to 35 parts by mass and the amount of granular alumina powder used was changed to 50 parts by mass. For the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0113] [Example 3] To 35 parts by mass of TMU as a dispersion medium, 29.5 parts by mass of epsilon-type iron oxide as an electromagnetic wave absorbing material, 2.5 parts by mass of carbon nanotubes (CNT), 13 parts by mass of a binder resin, and 55 parts by mass of granular alumina powder as a thermal conductive material were added. The binder resin was added as the following binder resin solution. Stirring was carried out with a rotary-revolution mixer to uniformly dissolve or disperse each component to obtain a paste for forming an electromagnetic wave absorber. Using the obtained paste for forming an electromagnetic wave absorber, a film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 1. For the obtained film-shaped electromagnetic wave absorber, in the same manner as in Example 1, the reflectance attenuation amount (Reflectance (dB)) in the frequency range of 40 to 120 GHz was measured. The measurement results are shown in Figure 2. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0114] [Example 4] A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 3, except that 55 parts by mass of granular alumina powder was changed to 50 parts by mass of granular alumina powder and 5 parts by mass of flaky boron nitride powder. For the obtained film-shaped electromagnetic wave absorber, in the same manner as in Example 1, the reflectance attenuation amount (Reflectance (dB)) in the frequency range of 40 to 120 GHz was measured. The measurement results are shown in Figure 3. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0115] [Example 5] A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 3, except that 55 parts by mass of granular alumina powder was changed to 45 parts by mass of granular alumina powder and 10 parts by mass of flaky boron nitride powder. For the obtained film-shaped electromagnetic wave absorber, in the same manner as in Example 1, the reflectance attenuation amount (Reflectance(dB)) in the frequency range of 40 to 120 GHz was measured. The measurement results are shown in Fig. 4. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0116] 〔Example 6〕 A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 3, except that 55 parts by mass of granular alumina powder was changed to 40 parts by mass of granular alumina powder and 15 parts by mass of flaky boron nitride powder. For the obtained film-shaped electromagnetic wave absorber, in the same manner as in Example 1, the reflectance attenuation amount (Reflectance(dB)) in the frequency range of 40 to 120 GHz was measured. The measurement results are shown in Fig. 5. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0117] 〔Example 7〕 A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 3, except that 55 parts by mass of granular alumina powder was changed to 35 parts by mass of granular alumina powder and 20 parts by mass of flaky boron nitride powder. For the obtained film-shaped electromagnetic wave absorber, in the same manner as in Example 1, the reflectance attenuation amount (Reflectance(dB)) in the frequency range of 40 to 120 GHz was measured. The measurement results are shown in Fig. 6. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0118] 〔Example 8〕 A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 2, except that the granular alumina powder was changed to granular silicon carbide powder. For the obtained film-shaped electromagnetic wave absorber, the reflectance (dB) in the frequency range of 40 to 120 GHz was measured in the same manner as in Example 1. The measurement results are shown in Fig. 7. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0119] [Example 9] A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 2, except that 50 parts by mass of granular alumina powder was changed to 40 parts by mass of granular silicon carbide powder and 10 parts by mass of flaky boron nitride powder. For the obtained film-shaped electromagnetic wave absorber, the reflectance (dB) in the frequency range of 40 to 120 GHz was measured in the same manner as in Example 1. The measurement results are shown in Fig. 8. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0120] [Example 10] A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 2, except that the granular alumina powder was changed to flaky boron nitride powder. For the obtained film-shaped electromagnetic wave absorber, the reflectance (dB) in the frequency range of 40 to 120 GHz was measured in the same manner as in Example 1. The measurement results are shown in Fig. 9. Also, for the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0121] [Comparative Example 1] A film-shaped electromagnetic wave absorber was obtained in the same manner as in Example 1, except that powdery alumina was not used. For the obtained film-shaped electromagnetic wave absorber, the thermal conductivity and the thermal diffusivity were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0122]

Table 1

[0123] According to Table 1 and FIGS. 1 to 9, the electromagnetic wave absorber of Examples 1 to 10 includes a composite layer composed of an electromagnetic wave absorbing material and a heat conductive material, and the electromagnetic wave absorbing material contains a predetermined epsilon-type iron oxide, and it can be seen that good electromagnetic wave absorption characteristics in the high frequency band and good heat dissipation can be achieved simultaneously. On the other hand, from Comparative Example 1, it can be seen that when the electromagnetic wave absorbing material does not contain a heat conductive material, the thermal diffusivity and thermal conductivity of the electromagnetic wave absorber are low. Also, according to the comparison between Example 8 and Example 9, and the comparison between Examples 2 and 3 and Examples 4 to 7, it can be seen that by combining the use of granular heat conductive materials and flaky heat conductive materials, the thermal diffusivity is higher than that when only granular heat conductive materials are used. In particular, according to the comparison between Examples 2 and 3 and Examples 4 to 7, it can be seen that by combining the use of granular alumina powder and flaky boron nitride powder, the thermal diffusivity and thermal conductivity can be significantly increased compared to the case where only granular alumina powder is used.

Claims

1. Comprising a composite layer composed of an electromagnetic wave absorbing material and a heat conductive material, wherein the electromagnetic wave absorbing material contains epsilon-type iron oxide, The epsilon-type iron oxide is ε-Fe ２ O ３ crystal, and a crystal having the same space group as ε-Fe ２ O ３ and in which a part of the Fe sites of the ε-Fe ２ O ３ crystal is substituted with an element M other than Fe, and is represented by the formula ε-M ｘ Fe ２－ｘ O ３ wherein x is more than 0 and less than 2, and is one or more selected from crystals the composite layer contains 30 parts by mass or more and 300 parts by mass or less of the heat conductive material with respect to 100 parts by mass of the electromagnetic wave absorbing material, the heat conductive material contains a combination of a granular heat conductive material and a flaky heat conductive material, An electromagnetic wave absorber, wherein the ratio of the mass of the flaky heat conductive material to the total of the mass of the granular heat conductive material and the mass of the flaky heat conductive material is 7% by mass or more and 50% by mass or less.

2. The electromagnetic wave absorber according to claim 1, wherein the electromagnetic wave absorbing material contains carbon nanotubes.

3. The electromagnetic wave absorber according to claim 2, wherein the electromagnetic wave absorbing material contains a binder resin.

4. The electromagnetic wave absorber according to claim 3, wherein in the electromagnetic wave absorbing material, the epsilon-type iron oxide, or the epsilon-type iron oxide and the carbon nanotubes are dispersed in the binder resin.

5. The electromagnetic wave absorber according to claim 3 or 4, wherein the heat conductive material is dispersed in a matrix composed of the electromagnetic wave absorbing material.

6. The electromagnetic wave absorber according to any one of claims 1 to 5, wherein the heat conductive material contains one or more selected from the group consisting of alumina, silicon carbide, and boron nitride.

7. The electromagnetic wave absorber according to claim 5 or 6, wherein the heat conductive material contains a combination of granular alumina and flaky boron nitride.

8. The electromagnetic wave absorber according to any one of claims 1 to 7, which is in film form.

9. Containing an electromagnetic wave absorbing material and a heat conductive material, wherein the electromagnetic wave absorbing material contains epsilon-type iron oxide, The epsilon-type iron oxide is ε-Fe ２ O ３ crystal, and a crystal having the same space group as ε-Fe ２ O ３ and in which a part of the Fe sites of the ε-Fe ２ O ３ crystal is substituted with an element M other than Fe, represented by the formula ε-M ｘ Fe ２－ｘ O ３ and is one or more selected from crystals in which x is greater than 0 and less than 2. containing 30 parts by mass or more and 300 parts by mass or less of the heat conductive material with respect to 100 parts by mass of the electromagnetic wave absorbing material, the heat conductive material contains a combination of a granular heat conductive material and a flaky heat conductive material, A paste for forming an electromagnetic wave absorber, wherein the ratio of the mass of the flaky heat conductive material to the total of the mass of the granular heat conductive material and the mass of the flaky heat conductive material is 7% by mass or more and 50% by mass or less.

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