Electromagnetic wave absorber and paste for forming electromagnetic wave absorber

Abstract

The present invention provides an electromagnetic wave absorber that can achieve both good electromagnetic wave absorption characteristics and good heat dissipation in a high-frequency band, and an electromagnetic wave absorber-forming paste suitable for use in the manufacture of the electromagnetic wave absorber. In the electromagnetic wave absorber, a composite layer composed of an electromagnetic wave absorbing material and a thermally conductive material is provided, wherein the electromagnetic wave absorbing material contains ε-type iron oxide, the ε-type iron oxide being obtained from ε-Fe2O3 crystals and ε-Fe2O3 crystals having the same crystal and space group as ε-Fe2O3 and a portion of the Fe sites of the ε-Fe2O3 crystals being substituted with an element M other than Fe, and having the formula ε-M x Fe 2‑x O3 represents one or more crystals selected from the group consisting of x greater than 0 and less than 2.

Description

Technical Field

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

[0002] The use of high-frequency electromagnetic waves is increasing in various information and communication systems, including mobile phones, wireless LANs, ETC systems, intelligent transportation systems, road-side assistance systems for automobiles, and satellite broadcasting. However, this increased use of high-frequency electromagnetic waves can lead to electronic equipment failures or malfunctions due to interference between electronic components. To address this issue, methods are being adopted to absorb unwanted electromagnetic waves using electromagnetic wave absorbers.

[0003] Therefore, in radars and the like that utilize electromagnetic waves in a high-frequency band, electromagnetic wave absorbers are used to reduce the influence of unnecessary electromagnetic waves that should not be received.

[0004] In order to meet such demands, various electromagnetic wave absorbers capable of effectively absorbing electromagnetic waves in high frequency bands have been proposed. As a specific example, an electromagnetic wave absorbing sheet containing a carbon nanocoil and a resin is known (for example, Patent Document 1).

[0005] Research on the use of high-frequency electromagnetic waves is progressing rapidly in automotive driver assistance systems. In these systems, onboard radars for detecting vehicle-to-vehicle distances, for example, utilize electromagnetic waves in the 76 GHz band. Furthermore, the use of electromagnetic waves in high-frequency bands, such as those above 100 GHz, is expected to expand beyond automotive driver assistance systems and into various other applications. Therefore, there is a desire for electromagnetic wave absorbers that can effectively absorb electromagnetic waves in the 76 GHz band or higher.

[0006] In response to such demands, as an electromagnetic wave absorber that can absorb electromagnetic waves well over a wide range in a high-frequency band, an electromagnetic wave absorber having an electromagnetic wave absorption layer containing magnetic crystals composed of ε-Fe2O3-type iron oxides has been proposed (for example, Patent Document 2, Non-Patent Documents 1 to 3).

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2009-060060

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2008-277726

[0011] Non-patent literature

[0012] Non-patent document 1: A. Namai, S. Sakurai, M. Nakajima, T. Suemoto, K. Matsumoto, M. Goto, S. Sasaki, and S. Ohkoshi, Journal of the American Chemical Society (J. Am. Chem. Soc.), 131, 1170-1173 (2009)

[0013] 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)

[0014] 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)

[0015] Non-patent document 4: A. Namai, K. Ogata, M. Yoshikiyo and S. Ohkoshi, Bull. Chem. Soc. Jpn., 93, 20-25 (2020) Summary of the Invention

[0016] Technical problem to be solved by the invention

[0017] However, depending on the usage environment, electromagnetic wave absorbers such as those described in Patent Document 1 may accumulate heat within the absorber. Heat accumulation within the absorber can cause deformation or degradation of the material that makes up the absorber. Furthermore, heat accumulation within the absorber can adversely affect the entire device. Therefore, there is a strong desire to impart heat dissipation properties to the absorber without excessively compromising its excellent electromagnetic wave absorption properties in the high-frequency band.

[0018] The present invention has been made in view of the above-mentioned problems of the prior art, and its object is to provide an electromagnetic wave absorber that can achieve both good electromagnetic wave absorption characteristics and good heat dissipation in a high frequency band, and an electromagnetic wave absorber-forming paste suitable for use in the production of the electromagnetic wave absorber.

[0019] Solutions for solving the above technical problems

[0020] The present inventors have found that, in an electromagnetic wave absorber, a composite layer composed of an electromagnetic wave absorbing material and a thermally conductive material is provided, wherein the electromagnetic wave absorbing material contains ε-Fe2O3 crystals and a composite layer composed of a composite layer composed of an electromagnetic wave absorbing material and a thermally conductive material having the same crystal and space group as ε-Fe2O3 and a portion of the Fe site of the ε-Fe2O3 crystal being substituted by an element M other than Fe, having the formula ε-M x Fe 2-x O3 represents one or more ε-type iron oxides selected from the crystals where x is greater than 0 and less than 2, which can solve the above-mentioned technical problems, thereby completing the present invention.

[0021] The first embodiment of the present invention is an electromagnetic wave absorber.

[0022] It has a composite layer composed of electromagnetic wave absorbing material and thermal conductive material.

[0023] The electromagnetic wave absorbing material contains ε-type iron oxide,

[0024] ε-type iron oxide is obtained from ε-Fe2O3 crystals and crystals and space groups that are the same 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 is represented by the formula ε-M x Fe 2-x O3 represents one or more crystals selected from the group consisting of x greater than 0 and less than 2.

[0025] A second aspect of the present invention is a paste for forming an electromagnetic wave absorber.

[0026] It contains electromagnetic wave absorbing materials and thermal conductive materials.

[0027] The electromagnetic wave absorbing material contains ε-type iron oxide,

[0028] ε-type iron oxide is obtained from ε-Fe2O3 crystals and crystals and space groups that are the same 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 is represented by the formula ε-M x Fe 2-x O3 represents one or more crystals selected from the group consisting of x greater than 0 and less than 2.

[0029] Effects of the Invention

[0030] According to the present invention, it is possible to provide an electromagnetic wave absorber that can achieve both good electromagnetic wave absorption characteristics and good heat dissipation in a high frequency band, and an electromagnetic wave absorber-forming paste suitable for use in producing the electromagnetic wave absorber. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a graph showing the reflection loss of the thin-film electromagnetic wave absorber of Example 1.

[0032] Figure 2 This is a graph showing the reflection attenuation rate of the thin-film electromagnetic wave absorber of Example 3.

[0033] Figure 3 This is a graph showing the reflection loss of the thin-film electromagnetic wave absorber of Example 4.

[0034] Figure 4 This is a graph showing the reflection loss of the thin-film electromagnetic wave absorber of Example 5.

[0035] Figure 5 This is a graph showing the reflection loss of the thin-film electromagnetic wave absorber of Example 6.

[0036] Figure 6 This is a graph showing the reflection loss of the thin-film electromagnetic wave absorber of Example 7.

[0037] Figure 7 This is a graph showing the reflection attenuation of the thin-film electromagnetic wave absorber of Example 8.

[0038] Figure 8 This is a graph showing the reflection loss of the thin-film electromagnetic wave absorber of Example 9.

[0039] Figure 9 This is a graph showing the reflection attenuation of the thin-film electromagnetic wave absorber of Example 10. DETAILED DESCRIPTION

[0040] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments at all and can be implemented with appropriate modifications within the scope of the purpose of the present invention.

[0041] Electromagnetic Wave Absorber

[0042] The electromagnetic wave absorber includes a composite layer composed of an electromagnetic wave absorbing material and a thermally conductive material. The electromagnetic wave absorber may be composed solely of the composite layer or may include a base material layer supporting the composite layer.

[0043] Regarding the electromagnetic wave absorber, from the perspective of being able to more reliably absorb high-frequency electromagnetic waves above the millimeter wave band, it is preferred to absorb electromagnetic waves in the frequency band of 30 gigahertz (GHz) or higher, preferably 30 GHz to 300 GHz, and more preferably 40 GHz to 200 GHz. Furthermore, it is preferred that the reflection loss of the electromagnetic wave absorber have a peak with an absolute value of 15 dB or higher. The reflection loss is measured on the exposed surface of the composite layer.

[0044] The form of the electromagnetic wave absorber is not particularly limited, but is preferably a sheet form or a film form, and is preferably a film form.

[0045] When the electromagnetic wave absorber is in the shape of a thin film, the film may have a curved surface or may consist of only a flat surface, but is preferably in the shape of a flat plate.

[0046] From the viewpoint of thinning or miniaturizing the film without impairing the effects of the present invention, the thickness of the film as an electromagnetic wave absorber is preferably 1000 μm or less, more preferably 900 μm or less, further preferably 450 μm or less, and particularly preferably 300 μm or less.

[0047] The thickness of the thin film serving as the electromagnetic wave absorber may be uniform or non-uniform.

[0048] <Compound layer>

[0049] As described above, the composite layer contains both an electromagnetic wave absorbing material and a thermally conductive material. The composite layer's form is not particularly limited. The composite layer may be a laminated composite layer comprising at least one layer of the electromagnetic wave absorbing material and at least one layer of the thermally conductive material, or a single-layer composite layer comprising both the electromagnetic wave absorbing material and the thermally conductive material.

[0050] From the perspective of easily dissipating heat accumulated in the electromagnetic wave absorber uniformly and efficiently, the composite layer is preferably a single-layer composite layer including an electromagnetic wave absorbing material and a thermally conductive material.

[0051] The thickness of the composite layer is not particularly limited within a range that does not hinder the purpose of the present invention. From the perspective of balancing thin film thickness and electromagnetic wave absorption performance of the electromagnetic wave absorber, the thickness of the composite layer is preferably 100 μm or less, more preferably 50 μm or less.

[0052] The lower limit of the thickness of the composite layer is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include 1 μm or more and 10 μm or more.

[0053] The thickness of the composite layer can be uniform or non-uniform.

[0054] Hereinafter, the essential or optional configuration of the composite layer will be described.

[0055] 〔Electromagnetic wave absorbing materials〕

[0056] The electromagnetic wave absorber including the composite layer containing the electromagnetic wave absorber ε-type iron oxide absorbs high-frequency electromagnetic waves in the millimeter wave band and above well, and its electromagnetic wave absorption characteristics are not easily impaired even when used in combination with the thermal conductive material described below.

[0057] The electromagnetic wave absorbing material may contain a magnetic material having the ability to absorb electromagnetic waves in addition to ε-type iron oxide. Preferred examples of magnetic materials that can be used together with ε-type iron oxide include barium ferrite and strontium ferrite.

[0058] From the perspective of achieving good electromagnetic wave absorption characteristics of the electromagnetic wave absorber, the ratio of the mass of ε-type iron oxide to the total mass of the ε-type iron oxide and the mass of the magnetic material other than the ε-type iron oxide is preferably 70 mass% or more, more preferably 80 mass% or more, further preferably 90 mass% or more, further preferably 95 mass% or more, and particularly preferably 100 mass%.

[0059] The electromagnetic wave absorbing material preferably typically contains a binder resin in order to uniformly disperse the ε-type iron oxide in the electromagnetic wave absorbing material and to facilitate forming into a composite layer.

[0060] Hereinafter, ε-type iron oxide and a binder resin, which are main materials constituting the electromagnetic wave absorbing material, will be described.

[0061] (ε-type iron oxide)

[0062] As ε-type iron oxide, it is preferred to use ε-Fe2O3 crystals and ε-Fe2O3 crystals with the same crystal structure and space group as ε-Fe2O3 and a part of the Fe site of the ε-Fe2O3 crystal is replaced by an element M other than Fe, which is obtained by the formula ε-M x Fe 2-x O3 represents at least one selected from the group consisting of crystals in which x is 0 or more and 2 or less (preferably 0 or more and less than 2). Such ε-type iron oxide crystals are magnetic crystals, and therefore, in the present specification, such crystals may be referred to as "magnetic crystals."

[0063] Any ε-Fe2O3 crystal can be used. x Fe 2-x O3 represents a crystal in which x is 0 or more and 2 or less (preferably 0 or more and less than 2), which will be described later.

[0064] In addition, in this specification, the ε-Fe2O3 crystal in which a part of the Fe site is substituted by the substituent element M is also referred to as the ε-M x Fe 2-x O3 is called "M substituted ε-Fe2O3".

[0065] The particle size of particles containing ε-Fe₂O₃ crystals and / or M-substituted ε-Fe₂O₃ crystals in a magnetic phase is not particularly limited within a range that does not hinder the objectives of the present invention. For example, particles containing magnetic crystals of ε-type iron oxide in a magnetic phase, produced by the method described below, have an average particle size within a range of 5 nm to 200 nm as measured by TEM (transmission electron microscopy) photographs.

[0066] Furthermore, the coefficient of variation (standard deviation of particle size / average particle size) of the particles having magnetic crystals of ε-type iron oxide in the magnetic layer produced by the method described below is less than 80%, indicating a relatively fine and uniform particle size group.

[0067] A preferred composite layer uses a powder of magnetic particles of ε-type iron oxide (i.e., particles having ε-Fe₂O₃ crystals and / or M-substituted ε-Fe₂O₃ crystals in the magnetic phase) as the electromagnetic wave absorbing material, or magnetic substance, in the composite layer. The "magnetic phase" here refers to the portion of the powder responsible for its magnetic properties.

[0068] "Presenting 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, including the case where impurity magnetic crystals that are unavoidable in manufacturing are mixed in the magnetic phase.

[0069] The magnetic crystals of ε-type iron oxide may also contain impurity crystals of iron oxides having a space group or oxidation state different from that of 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 is replaced by other elements).

[0070] When the magnetic crystals of ε-iron oxide contain impurity crystals, it is preferred that ε-Fe₂O₃ and / or M-substituted ε-Fe₂O₃ form the main phase. Specifically, the proportion of ε-Fe₂O₃ and / or M-substituted ε-Fe₂O₃ in the magnetic crystals of ε-iron oxide constituting the electromagnetic wave absorber is preferably 50 mol% or greater, as measured by the molar ratio of the compound.

[0071] The abundance ratio of crystals can be determined by analysis using the Rietveld method based on X-ray diffraction patterns. Non-magnetic compounds such as silicon dioxide (SiO 2 ) formed during the sol-gel process may adhere to the magnetic phase.

[0072] (M replaces ε-Fe2O3)

[0073] The type of element M in M-substituted ε-Fe2O3 is not particularly limited, as long as the crystal and space group are the same as those of ε-Fe2O3 and a portion of the Fe sites in the ε-Fe2O3 crystal are substituted by an element M other than Fe. M-substituted ε-Fe2O3 may contain multiple elements M other than Fe.

[0074] Preferred 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 them, In, Ga, Al, Ti, Co, and Rh are preferred. x Fe 2-x In the composition represented by O3, x is preferably in the range of 0 or more and less than 0.8, for example. When M is Ga, x is preferably in the range of 0 or more and less than 0.8, for example. When M is In, x is preferably in the range of 0 or more and less than 0.3, for example. When M is Rh, x is preferably in the range of 0 or more and less than 0.3, for example. When M is Ti or Co, x is preferably in the range of 0 or more and less than 1, for example.

[0075] The frequency at which the electromagnetic wave absorption amount becomes maximum can be adjusted by adjusting at least one of the type and the amount of the element M substituted in ε-Fe 2 O 3 .

[0076] Such M-substituted ε-Fe2O3 magnetic crystals can be synthesized, for example, by a combination of a reverse micelle method and a sol-gel method, and a calcination step, as described later. Alternatively, M-substituted ε-Fe2O3 magnetic crystals can be synthesized by a combination of a direct synthesis method and a sol-gel method, and a calcination step, as disclosed in Japanese Patent Application Laid-Open No. 2008-174405.

[0077] Specifically, it can be obtained through Jian Jin, Shin-ichi Ohkoshi and Kazuhito Hashimoto, "Advanced Materials (ADVANCED MATERIALS)" 2004, 16, No. 1, January 5, p. 48-51, Shin-ichi Ohkoshi, Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto, "JOURNAL OFAPPLIED" PHYSICS)", 97, 10K312 (2005), Shunsuke Sakurai, Jian Jin, KazuhitoHashimoto and Shin-ichi Ohkoshi, "JOURNAL OF THE PHYSICALSOCIETY 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., combined the reverse micelle method and the sol-gel method and the firing step to obtain M-substituted ε-Fe2O3 magnetic crystals.

[0078] In the reverse micelle method, two micellar solutions containing surfactants, micellar solution I (raw material micelles) and micellar solution II (neutralizer micelles), are mixed to promote the precipitation reaction of iron hydroxide within the micelles. Next, a silica coating is applied to the surface of the iron hydroxide particles generated within the micelles using a sol-gel method. After separation from the liquid, the silica-coated iron hydroxide particles are heat-treated in an atmosphere at a predetermined temperature (within the range of 700-1300°C). This heat treatment yields microparticles of ε-Fe2O3 crystals.

[0079] More specifically, M-substituted ε-Fe2O3 magnetic crystals are produced, for example, in the following manner.

[0080] First, iron (III) nitrate as an iron source, M nitrate as a source of the M element that replaces a part of the iron (aluminum (III) nitrate 9-hydrate in the case of Al, gallium (III) nitrate hydrate in the case of Ga, indium (III) nitrate 3-hydrate in the case of In, titanium (IV) sulfate hydrate and cobalt (II) nitrate 6-hydrate in the case of Ti and Co), and a surfactant (for example, hexadecyltrimethylammonium bromide) are dissolved in the aqueous phase of micellar solution I with n-octane as the oil phase.

[0081] An appropriate amount of alkaline earth metal nitrate (such as Ba, Sr, and Ca) can be pre-dissolved in the aqueous phase of micellar solution I. This nitrate functions as a shape-controlling agent. If the alkaline earth metal is present in the liquid, rod-shaped particles of M-substituted ε-Fe2O3 magnetic crystals are ultimately obtained. In the absence of a shape-controlling agent, nearly spherical particles of M-substituted ε-Fe2O3 magnetic crystals are obtained.

[0082] The alkaline earth metal added as a shape-controlling agent may remain on the surface 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, and more preferably 10% by mass or less, relative to the total mass of the substituting element M and the mass of Fe in the M-substituted ε-Fe2O3 magnetic crystals.

[0083] An aqueous ammonia solution was used as the aqueous phase of micellar solution II containing n-octane as the oil phase.

[0084] After mixing micellar solutions I and II, a sol-gel method is applied. Specifically, silane (e.g., tetraethylorthosilane) is added dropwise to the mixed solution of micellar solutions while continuously stirring, thereby promoting the formation reaction of iron hydroxide or iron hydroxide containing element M within the micelles. As a result, the surfaces of fine iron hydroxide precipitates formed within the micelles are coated with silicon dioxide generated by the hydrolysis of the silane.

[0085] Next, the silica-coated iron hydroxide particles containing the M element are separated from the liquid, washed, and dried. The resulting particle powder is placed in a furnace and heat-treated (fired) in air at a temperature range of 700°C to 1300°C, preferably 900°C to 1200°C, and more preferably 950°C to 1150°C.

[0086] By this heat treatment, an oxidation reaction is promoted in the silica coating, and fine particles of iron hydroxide containing the M element are converted into fine particles of M-substituted ε-Fe2O3.

[0087] During this oxidation reaction, the presence of the silica coating promotes the formation of M-substituted ε-Fe2O3 crystals with the same space group as ε-Fe2O3, rather than α-Fe2O3 or γ-Fe2O3 crystals, and prevents sintering of the particles. Furthermore, the presence of an appropriate amount of alkaline earth metal facilitates the growth of the particles into rod-like shapes.

[0088] Furthermore, as described above, M-substituted ε-Fe 2 O 3 magnetic crystals can be synthesized in an economically more advantageous manner by combining a direct synthesis method with a sol-gel method and a calcination step as disclosed in Japanese Patent Application Laid-Open No. 2008-174405.

[0089] To explain briefly, first, a neutralizing agent such as aqueous ammonia is added to an aqueous solvent in which a trivalent iron salt and a salt of a substituting element M (Ga, Al, etc.) are dissolved while stirring, thereby forming a precursor composed of iron hydroxide (sometimes partially substituted with other elements).

[0090] Then, a silica coating is formed on the surface of the precursor particles using a sol-gel method. After the silica-coated particles are separated from the liquid, they are heat-treated (fired) at a predetermined temperature to obtain fine particles of M-substituted ε-Fe2O3 magnetic crystals.

[0091] In the synthesis of M-substituted ε-Fe₂O₃ as described above, iron oxide crystals (impurity crystals) may be produced that have a different space group or oxidation state than the ε-Fe₂O₃ crystals. Among polymorphs that have the same composition as Fe₂O₃ but different crystal structures, α-Fe₂O₃ and γ-Fe₂O₃ are the most common. Other iron oxides include FeO and Fe₃O₄.

[0092] The inclusion of such impurity crystals is not preferred in terms of maximizing the properties of M-substituted ε-Fe2O3 crystals, but is permitted within a range that does not hinder the effects of the present invention.

[0093] In addition, the coercivity H of the ε-Fe2O3 magnetic crystal is replaced by M. c The coercive force H of the M-substituted ε-Fe2O3 magnetic crystal can be adjusted by adjusting the amount of the M-substituted ε-Fe2O3 magnetic crystal. c .

[0094] Specifically, for example, when Al, Ga, etc. are used as the substituent element M, the coercive force H of the M-substituted ε-Fe2O3 magnetic crystal increases as the substitution amount increases. cOn the other hand, when Rh or the like is used as a substituent element M, the coercive force H of the M-substituted ε-Fe2O3 magnetic crystal increases. c The bigger it gets.

[0095] The coercivity H of the M-substituted ε-Fe2O3 magnetic crystal can be easily adjusted by adjusting the substitution amount of the substitution element M. c From the aspect of , as the substituent element M, Ga, Al, In, Ti, Co and Rh are preferred.

[0096] Moreover, as the coercive force H c As the M element is reduced, the frequency of the peak at which the electromagnetic wave absorption of the ε-type iron oxide reaches its maximum value also shifts to the low-frequency side or the high-frequency side. In other words, the frequency of the peak of the electromagnetic wave absorption can be controlled by the amount of substitution of the M element.

[0097] With commonly used electromagnetic wave absorbers, absorption becomes nearly zero if the incident angle or frequency of the electromagnetic wave deviates from the designed values. In contrast, when using ε-type iron oxide, electromagnetic wave absorption is exhibited over a wide frequency range and incident angle, even with slight deviations. This allows for the provision of a composite layer capable of absorbing electromagnetic waves across a wide frequency band.

[0098] The particle size of the ε-type iron oxide can be controlled by, for example, adjusting the heat treatment (firing) temperature in the above-mentioned step.

[0099] According to the above-mentioned method combining the reverse micelle method and the sol-gel method, or the method combining the direct synthesis method and the sol-gel method disclosed in Japanese Patent Application Laid-Open No. 2008-174405, it is possible to synthesize ε-type iron oxide particles having an average particle size ranging from 5 nm to 200 nm as measured by TEM (transmission electron microscope) images. The average particle size of the ε-type iron oxide is more preferably 10 nm or more, and even more preferably 20 nm or more.

[0100] When determining the average particle size as the number average particle size, if the ε-type iron oxide particles are rod-shaped, the diameter of the particles in the major axis direction observed in the TEM image is used as the particle diameter to calculate the average particle size. The number of particles measured when determining the average particle size is not particularly limited as long as it is a sufficient number for calculating the average value, but is preferably 300 or more.

[0101] Furthermore, a silica coating applied to the surface of the iron hydroxide fine particles by the sol-gel method may be present on the surface of the M-substituted ε-Fe2O3 magnetic crystals after heat treatment (firing). The presence of a non-magnetic compound such as silica on the surface of the crystals is preferred for improving the handleability, durability, and weather resistance of the magnetic crystals.

[0102] Preferred examples of the non-magnetic compound include, in addition to silica, heat-resistant compounds such as alumina and zirconia.

[0103] However, if the amount of the non-magnetic compound attached is too large, particles may be rapidly aggregated, which is not preferable.

[0104] When the non-magnetic compound is silicon dioxide, the mass of Si in the M-substituted ε-Fe2O3 magnetic crystal is preferably less than 100 mass % relative to the total mass of the substituting element M and the mass of Fe in the M-substituted ε-Fe2O3 magnetic crystal.

[0105] A portion or most of the silicon dioxide attached to the M-substituted ε-Fe2O3 magnetic crystals can be removed by immersing them in an alkaline solution. The amount of silicon dioxide attached can be adjusted to any desired amount by this method.

[0106] The relative magnetic permeability of the composite layer is not particularly limited, but is preferably not less than 1.0 and not more than 1.5. The method for adjusting the relative magnetic permeability of the composite layer is not particularly limited. Examples of methods for adjusting the relative magnetic permeability of the composite layer include adjusting the amount of substitution element M in the ε-type iron oxide, adjusting the content of the ε-type iron oxide and other magnetic materials in the composite layer, and the like.

[0107] The content of ε-type iron oxide in the composite layer is not particularly limited within a range that does not hinder the purpose 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, relative to the solid content of the composite layer.

[0108] (Binder resin)

[0109] The electromagnetic wave absorbing material typically contains a binder resin. Using a binder resin allows the epsilon-type iron oxide and other magnetic materials to be well dispersed within the binder resin. Furthermore, the inclusion of a binder resin in the electromagnetic wave absorbing material facilitates the formation of a composite layer in a desired shape.

[0110] The binder resin may be, for example, an elastic material such as an elastomer or rubber. Furthermore, 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.

[0111] Preferred examples of the binder resin when it is a thermoplastic resin include polyacetal resin, polyamide resin, polycarbonate resin, polyester resin (polybutylene terephthalate, polyethylene terephthalate, polyarylate, etc.), FR-AS resin, FR-ABS resin, AS resin, ABS resin, polyphenylene ether resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, polyetheretherketone resin, fluorine-based resin, polyimide resin, polyamideimide resin, polyamide bismaleimide resin, polyetherimide resin, polybenzoxazole resin, polybenzothiazole resin, polybenzimidazole resin, BT resin, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resin (e.g., methyl cellulose, ethyl cellulose), (meth)acrylic resin (polymethyl methacrylate, etc.), and polystyrene.

[0112] Preferred examples of the binder resin when it is a thermosetting resin include phenolic resins, melamine resins, epoxy resins, and alkyd resins. As the photocurable resin, resins obtained by photocuring monomers having unsaturated bonds, such as various vinyl monomers and various (meth)acrylates, can be used.

[0113] Preferred examples of the binder resin when it is an elastic material include olefin elastomers, styrene elastomers, polyamide elastomers, polyester elastomers and polyurethane elastomers.

[0114] Furthermore, an aromatic ester-urethane copolymer is preferably used as the binder resin. By using an aromatic ester-urethane copolymer as the binder resin, it is possible to form a thin film-shaped electromagnetic wave absorber that exhibits excellent electromagnetic wave absorption properties even when thin, while having ε-type iron oxide or other magnetic materials well dispersed in the binder resin.

[0115] Furthermore, when an aromatic ester-urethane copolymer is used as the binder resin, crack resistance and low warpage during bending or cutting can be imparted to the composite layer.

[0116] From the perspective of achieving 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.

[0117] The aromatic ester-urethane copolymer is a copolymer containing an ester bond (—CO—O—) and a urethane bond (—NH—CO—O—) and containing an aromatic group in the main chain skeleton.

[0118] The aromatic group in the main chain skeleton may be an aromatic hydrocarbon group or a heterocyclic aromatic group, preferably an aromatic hydrocarbon group. 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 a block copolymer composed of one or more ester blocks and one or more urethane blocks.

[0119] 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 or more stages.

[0120] The dicarboxylic acid component (a2) and the hydroxycarboxylic acid component (a3) can be used as ester derivatives such as methyl ester and ethyl ester, esters such as carboxylic acid halides such as carboxylic acid chloride, or carbamate-forming derivatives.

[0121] The monomer used for producing 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 a divalent hydrocarbon group having an unbranched structure.

[0122] Examples of the non-branched divalent hydrocarbon group include an alkylene group, an alkenylene group, an alkynylene group, an arylene group, or a combination thereof. The alkylene group, the alkenylene group, and the alkynylene group preferably have a linear structure.

[0123] When the unbranched divalent hydrocarbon group is an alkylene group, an alkenylene group, or an alkynylene group, the number of carbon atoms in these groups is preferably 1 to 8, more preferably 2 to 6, and even more preferably 2 to 4.

[0124] When the unbranched divalent hydrocarbon group is an arylene group, the arylene group is preferably a phenylene group and a naphthylene group, more preferably a phenylene group, and still more preferably a p-phenylene group.

[0125] Among the non-branched divalent hydrocarbon groups described above, an alkylene group, an arylene group, and a combination of an alkylene group and an arylene group are preferred.

[0126] Preferred specific examples of the diol component (a1) include ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol and 1,5-pentanediol.

[0127] Preferred specific examples of the dicarboxylic acid (a2) include terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid, succinic acid, glutaric acid, adipic acid, oxalic acid and malonic acid.

[0128] Preferred 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.

[0129] Preferred specific examples of the diisocyanate component (a4) include ethylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, m-xylylene diisocyanate, p-phenylene diisocyanate, toluene diisocyanate, 4,4′-diphenylmethane diisocyanate, and 1,5-naphthalene diisocyanate.

[0130] The weight average molecular weight (Mw) of the aromatic ester-urethane copolymer is preferably 5000 to 500000, more preferably 10000 to 200000. In the present specification, the weight average molecular weight (Mw) refers to the weight average molecular weight based on polystyrene conversion measured by GPC.

[0131] Commercially available products of aromatic ester-urethane copolymers include Vylon series (trade name) (manufactured by Toyobo Co., Ltd.). 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 can be preferably used.

[0132] The content of the binder resin in the electromagnetic wave absorbing material is not particularly limited within a range that does not hinder the purpose of the present invention. The electromagnetic wave absorbing material preferably contains 5% by mass to 30% by mass of the binder resin relative to the solid content of the composite layer, and more preferably contains 5% by mass to 25% by mass of the binder resin.

[0133] (Dielectric)

[0134] In order to adjust the relative dielectric constant of the composite layer, the electromagnetic wave absorbing material may contain a dielectric. By adjusting the content of the dielectric in the composite layer, the relative dielectric constant of the composite layer can be adjusted.

[0135] The relative dielectric constant 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.

[0136] Preferred examples of dielectrics include barium titanate, strontium titanate, calcium titanate, magnesium titanate, bismuth titanate, zirconium titanate, zinc titanate, and titanium dioxide. The electromagnetic wave absorbing material may also contain powders of a plurality of dielectrics in combination.

[0137] The particle size of the dielectric powder used to adjust the relative dielectric constant of the composite layer is not particularly limited, as long as it does not hinder the objectives of the present invention. The average particle size of the dielectric powder is preferably from 1 nm to 100 nm, and more preferably from 5 nm to 50 nm. The average particle size of the dielectric powder is the number average diameter of the primary particles of the dielectric powder as observed by electron microscopy.

[0138] When dielectric powder is used to adjust the relative dielectric constant of the composite layer, the amount of dielectric powder used is not particularly limited as long as the relative dielectric constant of each composite layer is within a specified range. The amount of dielectric powder used is preferably 0% by mass to 20% by mass, and more preferably 5% by mass to 10% by mass, relative to the solid content of the composite layer.

[0139] (Carbon Nanotubes)

[0140] By including carbon nanotubes in the electromagnetic wave absorbing material, the relative dielectric constant of the composite layer can be adjusted. Carbon nanotubes can also be used in combination with the above-mentioned dielectric powder.

[0141] The amount of carbon nanotubes blended into the electromagnetic wave absorbing material is not particularly limited as long as it is an amount that maintains the relative dielectric constant of the composite layer within the above-specified range. However, since carbon nanotubes are also conductive materials, excessive amounts of carbon nanotubes may impair the electromagnetic wave absorbing properties of the composite layer.

[0142] The amount of the carbon nanotubes used is preferably from 0 mass % to 20 mass %, and more preferably from 1 mass % to 10 mass %, relative to the solid content mass of the composite layer.

[0143] [Thermal conductive materials]

[0144] 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.

[0145] For example, the thermal conductivity of the thermally conductive material is preferably 15 W / m·K or higher, more preferably 20 W / m·K or higher, further preferably 50 W / m·K or higher, and particularly preferably 200 W / m·K or higher.

[0146] For example, materials exhibiting a thermal conductivity of 15 W / m·K or greater include aluminum oxide, aluminum nitride, silicon carbide, and boron nitride. Two or more of these thermally conductive materials may be used in combination. Among these, aluminum oxide and silicon carbide are preferred because they are readily available and readily provide an electromagnetic wave absorber with excellent heat dissipation and electromagnetic wave absorption properties.

[0147] The shape of the thermally conductive material is not particularly limited as long as it does not hinder the objectives of the present invention. The thermally conductive material is preferably a granular or flaky powder. Thermally conductive materials in such a shape have a small aspect ratio and are difficult to orient within the composite layer. Therefore, using a granular or flaky powder thermally conductive material can easily suppress breakage, cracking, and other problems in the composite layer caused by the orientation of the thermally conductive material.

[0148] The aspect ratio (average major axis length / average minor axis length) of the thermal 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 average minor axis length of the thermal conductive material can be determined as number average lengths by, for example, microscopic observation or SEM observation.

[0149] Furthermore, by using a granular thermally conductive material and a scaly thermally conductive material in combination, the thermal diffusivity of the electromagnetic wave absorber can be easily increased.

[0150] In particular, when granular aluminum oxide and scaly boron nitride are used in combination, there is a tendency for not only the thermal diffusivity but also the thermal conductivity to be significantly improved.

[0151] When a granular thermally conductive material and a flaky thermally conductive material are used in combination, from the perspective of achieving both the improved thermal conductivity and the film-forming properties of the paste for forming an electromagnetic wave absorber, the ratio of the mass of the flaky thermally conductive material to the total mass of the granular thermally conductive material and the mass of the flaky thermally conductive material is preferably 7 mass % to 50 mass %, more preferably 10 mass % to 50 mass %, and even more preferably 15 mass % to 40 mass %.

[0152] From the viewpoint of achieving good heat dissipation properties of the electromagnetic wave absorber, the thermally conductive material in the form of granular or flaky powder is preferably dispersed in a matrix composed of the electromagnetic wave absorber.

[0153] The amount of thermally conductive material used is not particularly limited and can be appropriately adjusted to suit the desired level of heat dissipation performance of the electromagnetic wave absorber. Typically, the composite layer preferably contains 30 to 300 parts by mass, and more preferably 40 to 200 parts by mass, of the thermally conductive material per 100 parts by mass of the electromagnetic wave absorber.

[0154] [Other ingredients]

[0155] The composite layer may contain various additives other than the above-mentioned components within the scope that does not hinder the purpose of the present invention. Examples of additives that may be contained in the composite layer include dispersants, colorants, antioxidants, ultraviolet absorbers, flame retardants, flame retardant aids, plasticizers, and surfactants. These additives may be used in amounts that are conventionally used, within the scope that does not hinder the purpose of the present invention.

[0156] For example, by using the method of forming an electromagnetic wave absorber paste described later, the electromagnetic wave absorbing material, thermal conductive material and other components described above are compounded and formed into a film, thereby obtaining a composite layer that can be used as an electromagnetic wave absorber that can achieve both good electromagnetic wave absorption characteristics in the high frequency band and good heat dissipation properties.

[0157] <Base layer>

[0158] The composite layer may be laminated on a substrate layer. The substrate layer may be any substrate layer as long as the effects of the present invention are not impaired, and examples thereof include a layer containing a resin.

[0159] Examples of the resin include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acrylic acid (PMMA), polycarbonate (PC), cycloolefin polymer (COP), polyethersulfone, polyimide, and polyamide-imide. Among these, PET is preferred due to its excellent heat resistance and good balance between dimensional stability and cost.

[0160] The shape of the base material layer may have a curved surface or may consist of only a flat surface, but is preferably a flat plate.

[0161] The thickness of the substrate layer is preferably 800 μm or less, more preferably 500 μm or less, further preferably 300 μm or less, and particularly preferably 150 μm or less, from the viewpoint of thinning or miniaturizing the film without impairing the effects of the present invention.

[0162] The lower limit of the thickness of the substrate layer is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include 1 μm or more, 10 μm or more, and 50 μm or more.

[0163] <Metal layer>

[0164] When the electromagnetic wave absorber includes a base material layer, a metal layer may be provided on the surface of the base material layer opposite to the surface on which the composite layer is provided. Providing a metal layer can attenuate electromagnetic waves reflected by the metal layer. Preferred metals constituting the metal layer include, for example, aluminum, titanium, SUS, copper, brass, silver, gold, and platinum.

[0165] The thickness of the metal layer is not particularly limited, but is preferably 600 μm or less, more preferably 400 μm or less, further preferably 100 μm or less, and particularly preferably 50 μm or less from the viewpoint of thinning the electromagnetic wave absorber.

[0166] The lower limit of the thickness of the metal layer is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include 0.1 μm or more, 1 μm or more, 5 μm or more, and 10 μm or more.

[0167] By combining the composite layer containing the predetermined components described above with a base layer or a base layer and a metal layer as needed, an electromagnetic wave absorber having both good electromagnetic wave absorption characteristics and good heat dissipation properties in a high frequency band can be obtained.

[0168] The electromagnetic wave absorber described above can be preferably used as a film for electromagnetic wave absorption used in various components (including vehicle-mounted components, high-frequency antenna components, etc.) in various information and communication systems such as mobile phones, wireless LANs, ETC systems, intelligent transportation systems, automobile driving assistance road systems, satellite broadcasting, etc.

[0169] 《Paste for forming electromagnetic wave absorber》

[0170] In particular, from the perspectives of being able to efficiently form a composite layer without thickness restrictions and being able to directly form a composite layer on a substrate layer, a method of forming an electromagnetic wave absorber using an electromagnetic wave absorber-forming paste is preferred as a method of forming an electromagnetic wave absorber.

[0171] The electromagnetic wave absorber-forming paste contains the aforementioned electromagnetic wave absorber material and a thermally conductive material. The electromagnetic wave absorber-forming paste preferably also contains the aforementioned binder resin. The electromagnetic wave absorber-forming paste may also contain substances added to adjust the relative dielectric constant, relative magnetic permeability, and other components of the electromagnetic wave absorber material. Furthermore, when the binder resin contains a curable resin, the electromagnetic wave absorber-forming paste contains a compound that serves as a precursor to the curable resin. In this case, the electromagnetic wave absorber-forming paste may contain a curing agent, a curing accelerator, a polymerization initiator, and the like, as needed.

[0172] Furthermore, when the electromagnetic wave absorber-forming paste contains a photopolymerizable or thermopolymerizable compound, the coating film can be exposed or heated as needed to form a composite layer.

[0173] The electromagnetic wave absorber-forming paste preferably further includes a dispersion medium. Water, organic solvents, and aqueous solutions of organic solvents can be used as the dispersion medium. Organic solvents are preferred as the dispersion medium because they easily dissolve organic components and have a low latent heat of evaporation, making them easily removed by drying.

[0174] Preferred 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), 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, Ether alcohols such as diethylene glycol dimethyl ether and diethylene glycol diethyl ether; saturated aliphatic monocarboxylic acid alkyl esters such as n-butyl acetate and amyl acetate; lactic acid esters such as ethyl lactate and n-butyl lactate; ketones such as acetone, methyl ethyl ketone, cyclohexanone, acetophenone, and 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, and 2-methoxypentyl acetate. These can be used alone or in combination of two or more.

[0175] The solid content concentration of the electromagnetic wave absorber-forming paste can be appropriately adjusted depending on the method of applying the electromagnetic wave absorber-forming paste, the thickness of the composite layer, and other factors. Typically, the solid content concentration of the electromagnetic wave absorber-forming paste is preferably from 3% to 60% by mass, and more preferably from 10% to 50% by mass. The solid content concentration of the paste is calculated as the mass of the solid content, the sum of the mass of the components undissolved in the dispersion medium and the mass of the components dissolved in the dispersion medium.

[0176] (Dispersant)

[0177] The electromagnetic wave absorber paste may also contain a dispersant to ensure that the ε-type iron oxide and the substance used to adjust the relative permittivity and relative permeability of the composite layer are well dispersed within the composite layer. The dispersant can be uniformly mixed with the ε-type iron oxide or the binder resin. The dispersant can also be blended into the binder resin. Furthermore, the ε-type iron oxide or the substance added to adjust the relative permittivity and relative permeability, pre-treated with a dispersant, can also be blended into the materials constituting the composite layer.

[0178] The type of dispersant is not particularly limited within the scope not hindering the purpose of the present invention. The dispersant can be selected from various dispersants conventionally used for dispersing various inorganic fine particles or organic fine particles.

[0179] Preferred examples of the dispersant include silane coupling agents (eg, phenyltrimethoxysilane), titanate coupling agents, zirconate coupling agents, and aluminate coupling agents.

[0180] The content of the dispersant is not particularly limited within a range that does not hinder the purpose of the present invention. The content of the dispersant is preferably from 0.1% by mass to 30% by mass, more preferably from 1% by mass to 15% by mass, and particularly preferably from 1% by mass to 10% by mass, relative to the solid content of the electromagnetic wave absorber-forming paste.

[0181] 《Method for manufacturing electromagnetic wave absorber》

[0182] The method for producing the electromagnetic wave absorber is not particularly limited as long as an electromagnetic wave absorber having a predetermined structure can be produced.

[0183] A preferred method includes a composite layer forming step of applying the paste containing the electromagnetic wave absorbing material and the thermal conductive material onto the base layer to form a coating film, and then drying the coating film to form the composite layer.

[0184] The method for coating the electromagnetic wave absorber-forming paste on the substrate layer is not particularly limited as long as an electromagnetic wave absorber of the desired thickness can be formed. Examples of coating methods include spray coating, dip coating, roll coating, curtain coating, spin coating, screen printing, doctor blade coating, and laminating.

[0185] The coating film formed by the above method is dried to remove the dispersion medium to form a composite film on the substrate layer, thereby obtaining an electromagnetic wave absorber. The thickness of the coating film is appropriately adjusted so that the composite film obtained after drying has a desired thickness.

[0186] The drying method is not particularly limited, and examples thereof include: (1) a method of drying at a temperature of 80°C to 180°C, preferably 90°C to 160°C, on a hot plate for 1 minute to 30 minutes; (2) a method of leaving the mixture at room temperature for several hours to several days; (3) a method of removing the solvent by placing the mixture in a hot air heater or an infrared heater for tens of minutes to several hours, etc.

[0187] The method for producing an electromagnetic wave absorber may include a cutting step of cutting the composite layer obtained in the composite layer forming step or a laminate including a base layer and a composite layer to obtain an electromagnetic wave absorber of a predetermined size.

[0188] As described above, the electromagnetic wave absorber includes a composite layer composed of the electromagnetic wave absorbing material and the thermally conductive material, and thus can achieve both good electromagnetic wave absorption characteristics in a high frequency band and good heat dissipation properties.

[0189] Example

[0190] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

[0191] [Example 1]

[0192] (Preparation of Paste for Forming Electromagnetic Wave Absorber)

[0193] To 25.1 parts by mass of TMU as a dispersion medium, 39 parts by mass of the following ε-type iron oxide (e.g., iron oxide) as an electromagnetic wave absorber, 2.4 parts by mass of the following carbon nanotubes (CNTs), 8.6 parts by mass of a binder resin, and 21.4 parts by mass of granular alumina powder as a thermal conductive material were added. The binder resin was added as a binder resin solution. The mixture was stirred using a rotary mixer to uniformly dissolve or disperse the components, yielding a paste for forming an electromagnetic wave absorber.

[0194] Use ε-Ga 0.45 Fe 1.55 O3 is ε-type iron oxide. The average particle size of the ε-type iron oxide is 20 nm or more and 30 nm or less.

[0195] As CNTs, multi-walled carbon nanotubes with a major diameter of 150 nm (trade name: VGCF-H; manufactured by Showa Denko K.K.) were used.

[0196] As the dispersant, phenyltrimethoxysilane was used.

[0197] As the binder resin solution, an aromatic ester-urethane copolymer (Vylon UR-3210 manufactured by Toyobo Co., Ltd., glass transition temperature -3°C, weight average molecular weight 40,000, composed of 5 parts by mass of resin and 15 parts by mass of methyl ethyl ketone) was used.

[0198] (Manufacturing of Electromagnetic Wave Absorber Film)

[0199] The above-mentioned electromagnetic wave absorber-forming paste was applied to a PET film (125 μm thick) using an applicator. The coated film was then dried at 90°C for 10 minutes and 130°C for 10 minutes to form a composite layer with a thickness of 35 μm, resulting in a thin film-shaped electromagnetic wave absorber. The resulting film-shaped electromagnetic wave absorber, immediately after drying, was cut into 5 cm squares to prepare test pieces for the following evaluations.

[0200] Reflection attenuation

[0201] A 5 cm square film-shaped electromagnetic wave absorber sample was attached to an aluminum plate. Electromagnetic waves of 40 to 120 GHz were incident on the measurement sample on the aluminum plate, and the reflection loss was measured using a terahertz time-domain spectrometer (manufactured by Advantest).

[0202] The return loss RL(f) at the frequency f is obtained by RL(f)=-10Log(R(f) / 100). Here, R(f) is the reflectivity (%).

[0203] exist Figure 1 3 shows the reflection loss (reflectance (dB)) of the thin film electromagnetic wave absorber of Example 1 in the frequency range of 40 to 120 GHz.

[0204] Thermal conductivity and thermal diffusivity

[0205] The thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured by the following methods. These measurement results are shown in Table 1.

[0206] The thermal diffusivity was measured by a cyclic heating method. Specifically, the thermal diffusivity was measured using a cyclic heating method thermal diffusivity measuring apparatus (FTC-1 model) manufactured by ULVAC RIKEN CO., LTD.

[0207] Based on the thermal diffusivity α measured by the above method, the thermal conductivity λ is calculated by the following formula. In addition, the specific heat C is applied to the following formula ρ The measurement was performed by the DSC method at a measurement temperature of 25° C. using a product manufactured by Hitachi High-Technologies Corporation (X-DSC 7000 model).

[0208] λ=α×C ρ ×ρ×100

[0209] λ: Thermal conductivity (W / (m·K))

[0210] α: Thermal diffusivity (cm2 / s)

[0211] C ρ : Specific heat (J / (g·K))

[0212] ρ: density (g / cm 3 )

[0213] [Example 2]

[0214] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 1, except that the amount of TMU added was changed to 35 parts by mass and the amount of granular alumina powder used was changed to 50 parts by mass. The thermal conductivity and thermal diffusivity of the obtained thin film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are reported in Table 1.

[0215] [Example 3]

[0216] With respect to 35 parts by mass of TMU as a dispersion medium, 29.5 parts by mass of ε-type iron oxide, 2.5 parts by mass of carbon nanotubes (CNTs) and 13 parts by mass of binder resin as electromagnetic wave absorbing materials, and 55 parts by mass of granular aluminum oxide powder as a thermal conductive material are added. The binder resin is added as the following binder resin solution. The components are stirred evenly by a rotation-revolution mixer to obtain a paste for forming an electromagnetic wave absorber. Using the obtained paste for forming an electromagnetic wave absorber, a thin film electromagnetic wave absorber is obtained in the same manner as in Example 1. For the obtained thin film electromagnetic wave absorber, the reflection attenuation (Reflectance (dB)) in the frequency (Frequency) range of 40 to 120 GHz is measured in the same manner as in Example 1. The measurement results are shown in Figure 2 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0217] [Example 4]

[0218] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 3 except that 55 parts by mass of granular alumina powder was replaced with 50 parts by mass of granular alumina powder and 5 parts by mass of flaky boron nitride powder. The reflection loss (reflectance (dB)) of the obtained thin film electromagnetic wave absorber 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. Figure 3 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0219] [Example 5]

[0220] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 3 except that 55 parts by mass of granular alumina powder was replaced with 45 parts by mass of granular alumina powder and 10 parts by mass of flaky boron nitride powder. The reflection loss (reflectance (dB)) of the obtained thin film electromagnetic wave absorber 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. Figure 4 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0221] [Example 6]

[0222] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 3 except that 55 parts by mass of granular alumina powder was replaced with 40 parts by mass of granular alumina powder and 15 parts by mass of flaky boron nitride powder. The reflection loss (reflectance (dB)) of the obtained thin film electromagnetic wave absorber 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. Figure 5 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0223] [Example 7]

[0224] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 3 except that 55 parts by mass of granular alumina powder was replaced with 35 parts by mass of granular alumina powder and 20 parts by mass of flaky boron nitride powder. The reflection loss (reflectance (dB)) of the obtained thin film electromagnetic wave absorber 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. Figure 6 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0225] [Example 8]

[0226] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 2 except that the granular aluminum oxide powder was replaced with granular silicon carbide powder. The reflection loss (dB) of the obtained thin film electromagnetic wave absorber was measured in the same manner as in Example 1 within the frequency range of 40 to 120 GHz. The measurement results are shown in FIG. Figure 7Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0227] [Example 9]

[0228] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 2 except that 50 parts by mass of granular aluminum oxide powder was replaced with 40 parts by mass of granular silicon carbide powder and 10 parts by mass of flaky boron nitride powder. The reflection loss (reflectance (dB)) of the obtained thin film electromagnetic wave absorber 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. Figure 8 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0229] [Example 10]

[0230] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 2 except that the granular aluminum oxide powder was replaced with flaky boron nitride powder. The reflection loss (dB) of the obtained thin film electromagnetic wave absorber was measured in the same manner as in Example 1 within the frequency range of 40 to 120 GHz. The measurement results are shown in FIG. Figure 9 Furthermore, the thermal conductivity and thermal diffusivity of the obtained thin-film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0231] [Comparative Example 1]

[0232] A thin film electromagnetic wave absorber was obtained in the same manner as in Example 1, except that powdered aluminum oxide was not used. The thermal conductivity and thermal diffusivity of the obtained thin film electromagnetic wave absorber were measured in the same manner as in Example 1. These measurement results are shown in Table 1.

[0233]

Table 1

[0234]

[0235] According to Table 1 and Figures 1 to 9 It is found that the electromagnetic wave absorbers of Examples 1 to 10, which include a composite layer composed of an electromagnetic wave absorbing material and a thermally conductive material and in which the electromagnetic wave absorbing material contains a predetermined ε-type iron oxide, can achieve both good electromagnetic wave absorption characteristics and good heat dissipation in a high frequency band.

[0236] On the other hand, according to Comparative Example 1, it is understood that when the electromagnetic wave absorbing material does not include a thermally conductive material, the thermal diffusivity and thermal conductivity of the electromagnetic wave absorber are low.

[0237] Furthermore, comparisons between Example 8 and Example 9, and between Example 2 and Example 3 and Examples 4 to 7 show that the combined use of granular thermally conductive material and flaky thermally conductive material improves thermal diffusivity compared to using only granular thermally conductive material.

[0238] In particular, a comparison of Examples 2 and 3 with Examples 4 to 7 shows that the combined use of granular alumina powder and flaky boron nitride powder can significantly improve thermal diffusivity and thermal conductivity compared to the case of using only granular alumina powder.

Claims

1. An electromagnetic wave absorber, characterized in that: It has a composite layer composed of electromagnetic wave absorbing material and thermal conductive material. The electromagnetic wave absorbing material comprises ε-type iron oxide, The ε-type iron oxide is obtained from ε-Fe2O3 crystals and ε-M x Fe 2-x One or more selected from the crystals represented by O3, wherein the crystals of formula ε-M x Fe 2-x The crystal represented by O3 is a crystal having the same crystal and space group as ε-Fe2O3 and a portion of the Fe site of the ε-Fe2O3 crystal is substituted by an element M other than Fe, wherein x is greater than 0 and less than 2, The thermally conductive material comprises a combination of a granular thermally conductive material and a scaly thermally conductive material. The ratio of the mass of the flaky thermally conductive material to the total mass of the granular thermally conductive material and the mass of the flaky thermally conductive material is 7 mass % or more and 50 mass % or less.

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

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

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

5. The electromagnetic wave absorber according to claim 3, wherein The thermally 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 thermally conductive material includes one or more materials selected from the group consisting of aluminum oxide, silicon carbide, and boron nitride.

7. The electromagnetic wave absorber according to claim 1, wherein The thermally conductive material includes a combination of granular aluminum oxide and flaky boron nitride.

8. The electromagnetic wave absorber according to any one of claims 1 to 5, wherein The composite layer includes 30 parts by mass or more and 300 parts by mass or less of the thermally conductive material relative to 100 parts by mass of the electromagnetic wave absorbing material.

9. The electromagnetic wave absorber according to any one of claims 1 to 5, wherein It is in the shape of a thin film.

10. A paste for forming an electromagnetic wave absorber, characterized in that: Contains electromagnetic wave absorbing materials and thermal conductive materials, The electromagnetic wave absorbing material comprises ε-type iron oxide, The ε-type iron oxide is obtained from ε-Fe2O3 crystals and ε-M x Fe 2-x One or more selected from the crystals represented by O3, wherein the crystals of formula ε-M x Fe 2-x The crystal represented by O3 is a crystal having the same crystal and space group as ε-Fe2O3 and a portion of the Fe site of the ε-Fe2O3 crystal is substituted by an element M other than Fe, wherein x is greater than 0 and less than 2, The thermally conductive material comprises a combination of a granular thermally conductive material and a scaly thermally conductive material. The ratio of the mass of the flaky thermally conductive material to the total mass of the granular thermally conductive material and the mass of the flaky thermally conductive material is 7 mass % or more and 50 mass % or less.

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