Electromagnetic wave absorber

Abstract

To provide a transmission type electromagnetic wave absorber capable of satisfactorily absorbing electromagnetic waves of a high frequency of a milli-meter wave band or higher band, and arranged in consideration of electromagnetic wave reflection on a surface.SOLUTION: The electromagnetic wave absorber is a transmission type electromagnetic wave absorber. The electromagnetic wave absorber comprises an electromagnetic wave absorber layer containing a magnetic iron oxide 1a operable to resonate magnetically and a binder 1b of an organic material. As the magnetic iron oxide, a hexagonal crystal ferrite is included, and the volume-based content of the magnetic iron oxide is 23% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an electromagnetic wave absorber having the property of absorbing electromagnetic waves, and in particular to a transmission-type electromagnetic wave absorber that has magnetic iron oxide as an electromagnetic wave absorbing material and is capable of absorbing high-frequency electromagnetic waves at or above the millimeter wave band. [Background technology]

[0002] In order to avoid the influence of leakage electromagnetic waves emitted to the outside from electric circuits, etc., and undesirably reflected electromagnetic waves, electromagnetic wave absorbing compositions that absorb electromagnetic waves are used. The electromagnetic wave absorbing compositions are molded into a predetermined shape corresponding to the form in which they are used as electromagnetic wave absorbing members, such as block-shaped electromagnetic wave absorbers and sheet-shaped electromagnetic wave absorbing sheets.

[0003] Meanwhile, in recent years, research has been progressing on technologies that utilize centimeter waves having a frequency band of several gigahertz (GHz), millimeter waves having frequencies of 30 gigahertz to 300 gigahertz, and electromagnetic waves having a frequency of 1 terahertz (THz) as electromagnetic waves with a high frequency exceeding the millimeter wave band, in mobile communications such as mobile phones, wireless LANs, automatic toll collection systems (ETC), etc. In response to the technological trend of utilizing such higher frequency electromagnetic waves, there is an increasing demand for electromagnetic wave absorbing compositions and electromagnetic wave absorbers that can absorb electromagnetic waves in the gigahertz to terahertz bands, which absorb unnecessary electromagnetic waves.

[0004] So far, epsilon magnetic iron oxide (ε-FeO) has been used as an electromagnetic wave absorber that absorbs electromagnetic waves at high frequencies above the millimeter wave band, and exhibits electromagnetic wave absorption performance in the range of 25 to 100 GHz. 2 O 3 An electromagnetic wave absorber having a packed structure of particles having epsilon magnetic iron oxide crystals in the magnetic phase has been proposed (see Patent Document 1). Also, a sheet-shaped oriented body has been proposed in which fine particles of epsilon magnetic iron oxide are kneaded with a binder, and an external magnetic field is applied when the binder is dried and hardened to enhance the magnetic field orientation of the epsilon magnetic iron oxide particles (see Patent Document 2).

[0005] Furthermore, as an elastic electromagnetic wave absorbing sheet, an electromagnetic wave absorbing sheet capable of absorbing centimeter waves, which is made of silicone rubber with carbon nanotubes dispersed therein, has been proposed (see Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2008-60484 A [Patent Document 2] JP 2016-135737 A [Patent Document 3] JP 2011-233834 A Summary of the Invention [Problem to be solved by the invention]

[0007] As an electromagnetic wave absorber that absorbs electromagnetic waves of high frequencies above the millimeter wave band, an electromagnetic wave absorber containing an electromagnetic wave absorbing material in an electromagnetic wave absorbing layer that absorbs incident electromagnetic waves by magnetically resonating with them can be realized as a transmission type electromagnetic wave absorber, and can function as, for example, an electromagnetic wave filter that absorbs only electromagnetic waves of a predetermined frequency. In addition, unlike electromagnetic wave absorbers called reflection type or λ / 4 type, which absorb electromagnetic waves by canceling out incident waves and reflected waves in a dielectric layer, it does not have an electromagnetic wave shielding layer that reflects electromagnetic waves, so it has the advantage that the electromagnetic wave absorber as a whole is easily provided with flexibility and elasticity. Furthermore, since the shape of the electromagnetic wave absorber does not affect the electromagnetic wave absorption characteristics, it can be made into an electromagnetic wave absorber of a sheet shape, a block shape, or any other shape desired depending on the use.

[0008] By taking advantage of this characteristic that the electromagnetic wave absorption characteristics are not affected by shape, a wide range of applications can be envisioned for electromagnetic wave absorbers that contain electromagnetic wave absorbing materials in their electromagnetic wave absorbing layers that magnetically resonate with and absorb incident electromagnetic waves. However, no studies have been conducted on electromagnetic wave absorbers that reduce the reflection of electromagnetic waves on their surfaces.

[0009] In order to solve the above-mentioned problems of the conventional art, an object of the present disclosure is to obtain a transmission type electromagnetic wave absorber that effectively absorbs electromagnetic waves at high frequencies above the millimeter wave band and that takes into account the reflection of electromagnetic waves on the surface. [Means for solving the problem]

[0010] In order to solve the above problems, the electromagnetic wave absorber disclosed in the present application is a transmission type electromagnetic wave absorber having an electromagnetic wave absorbing layer containing magnetic iron oxide that magnetically resonates at frequencies equal to or higher than the millimeter wave band and an organic binder, and is characterized in that the imaginary part of the complex relative dielectric constant at 1 GHz is 0.016 or more and the real part of the complex relative dielectric constant at 1 GHz is 5.5 or less. Effect of the Invention

[0011] The electromagnetic wave absorber disclosed in the present application has an electromagnetic wave absorbing layer that contains magnetic iron oxide that resonates at high frequencies above the millimeter wave band as an electromagnetic wave absorbing material, and can convert high-frequency electromagnetic waves of several tens of gigahertz or more into heat and absorb them. In addition, since the real part of the complex relative dielectric constant at 1 GHz is 5.5 or less, reflection on the surface of the electromagnetic wave absorber is suppressed, and electromagnetic waves in the area where the electromagnetic wave absorber is disposed can be efficiently absorbed. [Brief description of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view illustrating a configuration of an electromagnetic wave absorber according to an embodiment of the present invention. [Diagram 2] 2A and 2B are diagrams showing the relationship between the magnetic powder content and the relative dielectric constant in an electromagnetic wave absorbing layer containing strontium ferrite and silicone rubber. FIG. 2(a) shows the relationship between the magnetic powder volume content and the real part of the complex relative dielectric constant. FIG. 2(b) shows the relationship between the magnetic powder volume content and the imaginary part of the complex relative dielectric constant. [Diagram 3] FIG. 1 is a model diagram for explaining a free space method for measuring the reflection of an electromagnetic wave on the surface of an electromagnetic wave absorber. [Figure 4]FIG. 2 is a diagram showing the relationship between the magnetic powder volume content and the relative dielectric constant (real part) in an electromagnetic wave absorbing layer containing strontium ferrite and acrylic rubber. [Diagram 5] FIG. 1 is a diagram showing the relationship between the magnetic powder volume content and the relative dielectric constant (real part) in an electromagnetic wave absorber layer containing epsilon magnetic iron oxide and silicone rubber. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The electromagnetic wave absorber disclosed in the present application is a transmission-type electromagnetic wave absorber having an electromagnetic wave absorbing layer containing magnetic iron oxide that magnetically resonates at frequencies equal to or higher than the millimeter wave band and an organic binder, and has a real part of the complex dielectric constant of 5.5 or less at 1 GHz.

[0014] In this way, the electromagnetic wave absorber disclosed in the present application can absorb electromagnetic waves in a high frequency band of 30 gigahertz or more, which is the millimeter wave band, by magnetic resonance of the magnetic iron oxide contained in the electromagnetic wave absorbing layer together with the organic binder. In addition, since the real part of the complex relative dielectric constant of the electromagnetic wave absorber at 1 GHz is 5.5 or less, the impedance value of the electromagnetic wave absorber becomes close to the impedance value in air, and reflection of the electromagnetic wave incident on the incident surface of the electromagnetic wave absorber is suppressed. Therefore, it is possible to realize an electromagnetic wave absorber that can effectively absorb the electromagnetic waves that pass through the area where it is placed and can be appropriately formed into a preferred shape depending on the members used and the placement location.

[0015] It is more preferable that the real part of the complex relative dielectric constant of the electromagnetic wave absorber at 1 GHz is 4.2 or less.

[0016] In addition, in the electromagnetic wave absorber disclosed in the present application, it is preferable that the magnetic iron oxide contains either hexagonal ferrite or epsilon magnetic iron oxide. By containing hexagonal ferrite or epsilon magnetic iron oxide, which has magnetic resonance at a high frequency equal to or higher than the millimeter wave band and has a large coercive force, as the magnetic iron oxide contained in the electromagnetic wave absorbing layer as the electromagnetic wave absorbing material, it is possible to obtain an electromagnetic wave absorber with high electromagnetic wave absorbing properties.

[0017] In this case, it is preferable that the hexagonal ferrite is strontium ferrite or barium ferrite, and that a part of the Fe site is substituted with a trivalent metal atom. It is also preferable that a part of the Fe site of the epsilon magnetic iron oxide is substituted with a trivalent metal atom. In this way, the frequency of the electromagnetic wave absorbed by the electromagnetic wave absorber can be appropriately adjusted, and the electromagnetic wave absorbing property of absorbing the electromagnetic wave of the desired frequency can be provided.

[0018] Furthermore, it is preferable that the binder contains any one of a thermosetting rubber, a thermoplastic elastomer, and a thermoplastic resin. In this way, it is possible to appropriately adjust the flexibility, elasticity, and the like of the electromagnetic wave absorber, thereby realizing an electromagnetic wave absorber having desired physical properties.

[0019] In addition, the electromagnetic wave absorbing layer is preferably formed in a sheet shape, whereby the entire electromagnetic wave absorbing sheet can be in a sheet shape, and an electromagnetic wave absorber that can be easily disposed in a desired location and handled can be obtained.

[0020] In this way, the electromagnetic wave absorber described in the present application can be configured in various shapes such as a sheet shape, a block shape, etc. Furthermore, the electromagnetic wave absorber disclosed in the present application may be formed by applying an electromagnetic wave absorbing material or by injection molding, and the electromagnetic wave absorber disclosed in the present application includes those formed by various methods without any particular limitation on the formation method.

[0021] Hereinafter, the electromagnetic wave absorber disclosed in the present application will be described with reference to the drawings, taking as an example an electromagnetic wave absorbing sheet formed in a sheet shape as a whole.

[0022] (Embodiment) [Composition of electromagnetic wave absorbing sheet] FIG. 1 is a cross-sectional view showing the configuration of an electromagnetic wave absorbing sheet as an electromagnetic wave absorber according to this embodiment.

[0023] Note that FIG. 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet described in this embodiment, and the sizes and thicknesses of the components shown in the diagram are not depicted based on reality.

[0024] The electromagnetic wave absorbing sheet exemplified in this embodiment includes an electromagnetic wave absorbing layer 1 containing particulate magnetic iron oxide 1a, which is an electromagnetic wave absorbing material, and a rubber binder 1b, and an adhesive layer 2 laminated on the back side (lower side in FIG. 1) of the electromagnetic wave absorbing layer 1. As shown in FIG. 1, by providing the adhesive layer 2 on the back side of the electromagnetic wave absorbing layer 1, the electromagnetic wave absorbing sheet can be easily attached and arranged at a desired position, such as the surface of the housing of an electronic device.

[0025] In the electromagnetic wave absorbing sheet according to this embodiment, the magnetic iron oxide 1a contained in the electromagnetic wave absorbing layer 1 causes magnetic resonance, and as a result, the electromagnetic waves are converted into thermal energy by magnetic loss and absorbed. Therefore, unlike a reflective type electromagnetic wave absorbing sheet, there is no need to provide a reflective layer on the surface opposite to the electromagnetic wave incident side of the electromagnetic wave absorbing layer 1, and the electromagnetic wave absorbing sheet can be used as a so-called transmission type electromagnetic wave absorbing sheet that absorbs electromagnetic waves that transmit through the electromagnetic wave absorbing layer 1. Therefore, the electromagnetic wave absorbing sheet can also be used as an electromagnetic wave filter that absorbs electromagnetic waves of a predetermined frequency among the electromagnetic waves that transmit through the electromagnetic wave absorbing sheet.

[0026] In addition, there are no shape restrictions as with reflective electromagnetic wave absorbing sheets in which the frequency of the electromagnetic waves absorbed is determined by the thickness of the electromagnetic wave absorbing layer 1, and stable electromagnetic wave absorption characteristics can be obtained even when the thickness of the electromagnetic wave absorbing layer 1 changes.Furthermore, there is an advantage in that there is a high degree of freedom in terms of shape, such as the ability to make the electromagnetic wave absorbing sheet have different thicknesses in the main surface direction.

[0027] In the electromagnetic wave absorbing sheet shown in this embodiment, various rubber materials are used as the binder 1b constituting the electromagnetic wave absorbing layer 1. Therefore, it is possible to obtain an electromagnetic wave absorbing sheet having elasticity that easily expands and contracts, particularly in the in-plane direction of the electromagnetic wave absorbing sheet. Note that the electromagnetic wave absorbing sheet according to this embodiment has the electromagnetic wave absorbing layer 1 formed by containing magnetic iron oxide 1a in the rubber binder 1b, and therefore has elasticity and large flexibility, so that the electromagnetic wave absorbing sheet can be rolled up when handling it, and can be easily arranged along a curved surface.

[0028] Furthermore, as described above, the electromagnetic wave absorbing sheet of this embodiment has the adhesive layer 2 laminated on the electromagnetic wave absorbing layer 1, so that the electromagnetic wave absorbing sheet can be easily attached and disposed in a desired location, such as the surface of a member surrounding a source that generates high-frequency electromagnetic waves. Incidentally, the presence of the adhesive layer 2 is not an essential requirement for the electromagnetic wave absorbing sheet of this embodiment.

[0029] The electromagnetic wave absorbing sheet according to this embodiment has a real part of the complex relative dielectric constant of 5.5 or less at 1 GHz. Therefore, the impedance value of the electromagnetic wave absorbing sheet becomes about 377Ω, which is the impedance in air, and approaches a so-called impedance matching state, and it is possible to suppress the situation where the electromagnetic wave incident on the electromagnetic wave absorbing sheet is reflected by the surface of the electromagnetic wave absorbing sheet. As a result, it is possible to more effectively absorb the electromagnetic wave passing through the area where the electromagnetic wave absorbing sheet is arranged.

[0030] [Electromagnetic wave absorbing materials] In the electromagnetic wave absorbing sheet according to this embodiment, fine particles (powder) of hexagonal ferrite or epsilon magnetic iron oxide are used as the magnetic iron oxide 1a contained in the electromagnetic wave absorbing layer 1 as the electromagnetic wave absorbing material.

[0031] Hexagonal ferrite has a larger magnetic anisotropy and a larger coercive force than ferrite materials with other structures such as spinel ferrite, and therefore can be used as an electromagnetic wave absorber having high electromagnetic wave absorbing properties.

[0032] As the hexagonal ferrite, magnetic powders of barium ferrite and strontium ferrite can be used. In addition, in the hexagonal ferrite, as in the above-mentioned epsilon magnetic iron oxide, the magnetic resonance frequency can be changed by substituting a part of the Fe site with a trivalent metal atom, and the frequency of the electromagnetic wave absorbed as an electromagnetic wave absorbing material can be adjusted.

[0033] In addition, epsilon magnetic iron oxide (ε-Fe 2 O 3 ) is ferric oxide (Fe 2 O 3 ) in the alpha phase (α-Fe 2 O 3 ) and gamma phase (γ-Fe 2 O 3 ) and is a magnetic material that can be obtained in a single phase state by a nanoparticle synthesis method that combines the reverse micelle method and the sol-gel method.

[0034] Epsilon magnetic iron oxide is a fine particle measuring a few to tens of nanometers, yet has the largest coercive force of any metal oxide, approximately 20 kOe at room temperature.Furthermore, natural magnetic resonance due to the gyromagnetic effect based on precession occurs at frequencies above tens of gigahertz, which is the so-called millimeter wave band, and therefore absorbs high-frequency electromagnetic waves in the millimeter wave band of 30 to 300 gigahertz or higher.

[0035] Furthermore, by making epsilon magnetic iron oxide into a crystal in which some of the Fe sites of the crystal are replaced with trivalent metal elements such as aluminum (Al), gallium (Ga), rhodium (Rh), or indium (In), it is possible to change the magnetic resonance frequency, i.e., the frequency of the electromagnetic waves that it absorbs when used as an electromagnetic wave absorbing material.

[0036] For example, gallium-substituted epsilon magnetic iron oxide, i.e., ε-Ga x Fe 2-x O 3 In the case of ε-Al, the absorption peak is in the frequency range from 30 GHz to 150 GHz by adjusting the substitution amount “x”. x Fe 2-x O 3 In the case of ε-Rh, the absorption peak is in the frequency range of 100 GHz to 190 GHz by adjusting the amount of substitution "x". x Fe 2-x O 3 In the case of , it is possible to shift the frequency band of the electromagnetic waves to be absorbed in the higher direction, from 180 GHz or higher. Therefore, by determining the type of element to be substituted for the Fe site of the epsilon magnetic iron oxide so that the natural resonance frequency of the frequency to be absorbed by the electromagnetic wave absorbing sheet is reached, and further by adjusting the amount of substitution with Fe, the frequency of the electromagnetic waves to be absorbed can be set to the desired value.

[0037] The hexagonal ferrite and epsilon magnetic iron oxide used as the magnetic iron oxide are commercially available, including those in which some of the Fe sites are substituted with metal, and are therefore easily available.

[0038] [binder] The rubber binder 1b constituting the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet according to this embodiment can be made of various rubber materials such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), urethane rubber (PUR), silicone rubber (Q), fluororubber (FKM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), polysulfide rubber (T), and urethane rubber (U).

[0039] Furthermore, from the definition of rubber as a material that has rubber elasticity at room temperature, thermoplastic elastomers such as styrene-based thermoplastic elastomers (SIS, styrene-isoprene copolymer, SBS, styrene-butadiene copolymer), olefin-based thermoplastic elastomers, urethane-based thermoplastic elastomers, and polyester-based thermoplastic elastomers have fluidity at high temperatures but have rubber elasticity at room temperature, and can therefore be used as the rubber binder 1b in the electromagnetic wave absorbing sheet described in this embodiment, and these materials are also broadly included in the rubber material.

[0040] Among these rubber materials, acrylic rubber and silicone rubber are preferably used due to their high heat resistance. Acrylic rubber has excellent oil resistance even in high temperature environments, is relatively inexpensive, and offers excellent cost performance. Silicone rubber is not only heat resistant, but also has high cold resistance. Furthermore, among synthetic rubbers, its physical properties have the least dependency on temperature, and it also has excellent solvent resistance, ozone resistance, and weather resistance. Furthermore, it has excellent electrical insulation properties and is physically stable over a wide temperature range and frequency range.

[0041] In this embodiment, an example has been shown in which a rubber binder is used as the binder for the electromagnetic wave absorbing layer 1, but a resin material other than rubber, such as a thermoplastic resin, can also be used as the binder.

[0042] As the resin binder used in the electromagnetic wave absorbing layer, an epoxy resin, a polyester resin, a polyurethane resin, an acrylic resin, a phenol resin, a melamine resin, or the like can be used. More specifically, as the epoxy resin, a compound in which the hydroxyl groups at both ends of bisphenol A are epoxidized can be used. As the polyurethane resin, a polyester urethane resin, a polyether urethane resin, a polycarbonate urethane resin, an epoxy urethane resin, or the like can be used. As the acrylic resin, a methacrylic resin, a functional group-containing methacrylic polymer obtained by copolymerizing an acrylic acid alkyl ester and / or a methacrylic acid alkyl ester having an alkyl group carbon number in the range of 2 to 18, a functional group-containing monomer, and, if necessary, another modifying monomer copolymerizable therewith, can be used.

[0043] When such a thermoplastic resin is used as a binder, it is possible to obtain an electromagnetic wave absorbing sheet that has a certain degree of flexibility, although it does not have the elasticity as in the case of using the above-mentioned rubber binder. Note that an electromagnetic wave absorbing sheet having flexibility means that, for example, even if the electromagnetic wave absorbing sheet is curved so that both ends of the sheet overlap, or the electromagnetic wave absorbing sheet is wrapped around a metal rod, the sheet does not break or tear, and can return to its original flat shape when an external force is removed from the electromagnetic wave absorbing sheet.

[0044] Furthermore, when a heat-resistant thermoplastic resin with a high melting point is used as the thermoplastic resin for forming the molded body, aromatic polyamides such as 6T nylon (6TPA), 9T nylon (9TPA), 10T nylon (10TPA), 12T nylon (12TPA), MXD6 nylon (MXDPA), and alloy materials thereof, polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether ether ketone (PEEK), polyetherimide (PEI), polyphenylsulfone (PPSU), and the like can be used.

[0045] The binder in the invention disclosed in this application serves as a binding agent for dispersing the electromagnetic wave absorbing material, which is magnetic iron oxide, to prepare a magnetic compound and mold it into a sheet, or for producing a molded body using a mold and molding it into any shape.

[0046] [Dispersant] In order to disperse the magnetic iron oxide, which is an electromagnetic wave absorbing material, well within the rubber binder, it is more preferable to use a dispersant.

[0047] As the dispersant, a compound having a polar group such as a phosphoric acid group, a sulfonic acid group, a carboxy group, etc., can be used. Among these, it is preferable to use a phosphoric acid compound having a phosphoric acid group in the molecule as the dispersant.

[0048] The phosphoric acid compounds include arylsulfonic acids such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, and polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrotrismethylenephosphonic acid. These phosphoric acid compounds have flame retardancy and function as dispersants for fine magnetic iron oxide powder, so that the epsilon magnetic iron oxide particles in the binder can be dispersed well.

[0049] Specifically, phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industries, Ltd., phosphoric acid oxide ester "JP-502" (product name) manufactured by Johoku Chemical Industry Co., Ltd., and the like can be used as a dispersant.

[0050] In addition to the phosphoric acid compounds described above, the dispersing agent to be contained in the electromagnetic wave absorbing sheet described in this embodiment may be, for example, a fatty acid having 12 to 18 carbon atoms, such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, or stearic acid [RCOOH (R is an alkyl or alkenyl group having 11 to 17 carbon atoms)], or a metal soap made of an alkali metal or alkaline earth metal of the above fatty acid, a fluorine-containing compound of the above fatty acid ester, an amide of the above fatty acid, a polyalkylene oxide alkyl phosphoric acid ester, lecithin, a trialkyl polyolefinoxy quaternary ammonium salt (alkyl has 1 to 5 carbon atoms, and olefin is ethylene, propylene, or the like), or copper phthalocyanine. Furthermore, a silane or a silane coupling agent may be used as a dispersing agent. These dispersing agents may be used alone or in combination.

[0051] [How to make electromagnetic wave absorbing sheets] Here, a method for producing the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet according to this embodiment will be described.

[0052] The electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet of this embodiment can be produced by preparing a magnetic compound containing magnetic iron oxide powder 1a and a rubber binder 1b, molding this magnetic compound to a predetermined thickness, and cross-linking it.

[0053] First, a magnetic compound is prepared.

[0054] The magnetic compound can be obtained by kneading hexagonal ferrite or epsilon iron oxide powder with a dispersant and a rubber binder. The kneaded product can be obtained, for example, by kneading with a pressurized batch kneader. At this time, a crosslinking agent can be added as necessary.

[0055] The obtained magnetic compound is crosslinked and molded into a sheet at a temperature of 165° C., for example, using a hydraulic press.

[0056] Thereafter, a secondary cross-linking treatment is carried out in a thermostatic chamber at a temperature of 170° C., and the radio wave absorbing layer 1 can be formed.

[0057] As a method for producing the radio wave absorbing sheet disclosed in the present application, in addition to the above-mentioned method of applying a magnetic paint, for example, an extrusion molding method can also be used.

[0058] Specifically, hexagonal ferrite or epsilon iron oxide powder is blended in advance with a dispersant and a binder as necessary, and the blended materials are fed into a plastic cylinder from a resin feed port of an extrusion molding machine. As the extrusion molding machine, a normal extrusion molding machine equipped with a plastic cylinder, a die provided at the tip of the plasticizing cylinder, a screw rotatably arranged in the plastic cylinder, and a drive mechanism for driving the screw can be used. The molten material plasticized by a band heater of the extrusion molding machine is sent forward by the rotation of the screw and extruded into a sheet shape from the tip. The extruded material is dried, pressurized, calendered, etc. to obtain an electromagnetic wave absorbing layer of a desired shape.

[0059] Alternatively, hexagonal ferrite or epsilon iron oxide powder can be blended in advance with a dispersant and binder as necessary, and the blended materials can be fed into a plastic cylinder from the resin supply port of an injection molding machine. After melt-kneading with a screw in the plasticizing cylinder, the molten resin can be injected into a mold connected to the tip of the injection molding machine to form a molded body.

[0060] By forming a molded body using a mold in this way, the electromagnetic wave absorber can be formed into a desired shape. For example, by forming it into a hollow pyramidal or truncated conical shape as an antenna horn, or into the shape of a housing of an electronic device in which various electronic circuit components are arranged (such as a box or cylinder), a structure having electromagnetic wave absorbing ability can be formed, and the effort of attaching a sheet-like electromagnetic wave absorber to the surface of a housing formed of other materials such as resin can be eliminated.

[0061] [Adhesive layer] As shown in FIG. 1, the electromagnetic wave absorbing sheet according to this embodiment has an electromagnetic wave absorbing layer 1 and an adhesive layer 2 formed on the back surface thereof.

[0062] By providing the adhesive layer 2, the electromagnetic wave absorbing layer 1 can be attached to a desired position on the inner surface of a housing that stores an electric circuit, or on the inner or outer surface of an electric device. In particular, since the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet of this embodiment has flexibility, the adhesive layer 2 allows the electromagnetic wave absorbing sheet to be easily attached to a curved surface, improving the ease of handling of the electromagnetic wave absorbing sheet.

[0063] The adhesive layer 2 may be made of a known material used as an adhesive layer for adhesive tapes, etc., such as an acrylic adhesive, a rubber adhesive, or a silicone adhesive. A tackifier or a crosslinking agent may be used to adjust the adhesive strength to the adherend and reduce adhesive residue. The adhesive strength to the adherend is preferably 5N / 10mm to 12N / 10mm. If the adhesive strength is less than 5N / 10mm, the electromagnetic wave absorbing sheet may easily peel off or shift from the adherend. If the adhesive strength is more than 12N / 10mm, the electromagnetic wave absorbing sheet may be difficult to peel off from the adherend.

[0064] The thickness of the adhesive layer 2 is preferably 20 μm to 100 μm. If the thickness of the adhesive layer is thinner than 20 μm, the adhesive strength is weak, and the electromagnetic wave absorbing sheet may easily peel off or shift from the adherend. If the thickness of the adhesive layer is thicker than 100 μm, there is a risk that the flexibility of the entire electromagnetic wave absorbing sheet may be reduced. If the adhesive layer 2 is thick, the electromagnetic wave absorbing sheet is difficult to peel off from the adherend. If the cohesive strength of the adhesive layer 2 is weak, adhesive residue may be left on the adherend when the electromagnetic wave absorbing sheet is peeled off.

[0065] In the present specification, the adhesive layer 2 may be an adhesive layer 2 that adheres in an unremovable manner, or an adhesive layer 2 that adheres in a removably manner.

[0066] Furthermore, when adhering the electromagnetic wave absorbing sheet to a predetermined surface, even if the electromagnetic wave absorbing sheet does not have the adhesive layer 2, the electromagnetic wave absorbing sheet can be attached to a predetermined portion by imparting adhesiveness to the surface of the member on which the electromagnetic wave absorbing sheet is to be placed, for example, by applying an adhesive thereto, or by using a double-sided tape or adhesive. In this respect, it is clear that the adhesive layer 2 is not an essential component of the electromagnetic wave absorbing sheet shown in this embodiment.

[0067] [Dielectric constant] In the electromagnetic wave absorbing sheet according to this embodiment, the real part of the complex relative dielectric constant at 1 GHz is 5.5 or less. This makes it possible to make the input impedance value of the electromagnetic wave absorbing sheet approach 377Ω, which is the impedance value in air (vacuum). This makes it possible to reduce reflection of electromagnetic waves propagated through air when they are incident on the electromagnetic wave absorbing sheet, which is caused by the difference in impedance value between air and the electromagnetic wave absorbing sheet, and more electromagnetic waves are incident on the electromagnetic wave absorbing sheet and absorbed by the magnetic iron oxide powder, which is the electromagnetic wave absorbing material.

[0068] The magnetic iron oxide contained in the electromagnetic wave absorbing layer has a real part of complex dielectric constant of about 20 or more, whether it is hexagonal ferrite or epsilon magnetic iron oxide. Therefore, by using a binder of the polymer material contained in the electromagnetic wave absorbing layer or a filler added to the electromagnetic wave absorbing layer that has a small complex dielectric constant, the content (volume content) of the magnetic iron oxide powder in the electromagnetic wave absorbing layer can be adjusted to make the complex dielectric constant of the electromagnetic wave absorbing layer 5.5 or less. As shown in FIG. 1, in the electromagnetic wave absorbing sheet according to this embodiment, an adhesive layer is formed on the back surface of the electromagnetic wave absorbing layer, but by reducing the thickness of the adhesive layer, the effect of the adhesive layer on the dielectric constant of the entire electromagnetic wave absorbing sheet can be suppressed to a negligible level.

[0069] Here, an electromagnetic wave absorbing layer was actually formed, and the complex dielectric constant and electromagnetic wave absorbing characteristics were measured. The results will be described.

[0070] Figure 2 shows the aluminum-substituted strontium ferrite magnetic powder (SrFe 10.56 Al 1.44 O 19 ) and silicone rubber (KE-541-U (product name) manufactured by Shin-Etsu Chemical Co., Ltd.) as a rubber binder, and shows the change in complex dielectric constant when the volume content of magnetic powder is changed. Figure 2(a) shows the change in the real part ε' of the complex dielectric constant, and Figure 2(b) shows the change in the imaginary part ε'' of the complex dielectric constant.

[0071] The complex relative permittivity of the electromagnetic wave absorbing layer was measured by a capacitance method using an impedance measuring instrument 4291B (product name) manufactured by Agilent Technologies Inc., after preparing electromagnetic wave absorbing layers with different volumetric contents of magnetic powder. More specifically, magnetic compounds with different volumetric contents of strontium ferrite magnetic powder were prepared, molded into a square shape with a thickness of 2 mm and a diagonal of 120 mm, and cross-linked to prepare a measurement sample. This sample was sandwiched between measurement electrodes and measured at a measurement frequency of 1 GHz using a test fixture 16453A (product name manufactured by Agilent Technologies Inc.).

[0072] As shown in Figure 2(a), the real part (ε') of the complex relative permittivity of the electromagnetic wave absorbing layer containing aluminum-substituted strontium ferrite magnetic powder increases as the volume content of the magnetic powder increases from a value of 3.25 when no magnetic powder is included, to approximately 9.5 at a volume content of 50%. Also, as shown in Figure 2(b), the imaginary part (ε'') of the complex relative permittivity increases as the volume content of the magnetic powder increases from a value of 0.03 when no magnetic powder is included, to approximately 0.032 at a volume content of 50%.

[0073] Although the complex dielectric constant of magnetic powder, which is a powder, cannot be measured, it is believed to be greater than 20. In addition, the real part (ε') of the complex dielectric constant represents the amount of energy stored in the dielectric from the external electric field, and the imaginary part (ε'') represents the energy loss of the dielectric due to the external electric field, so only the real part of the complex dielectric constant needs to be considered when determining the impedance of the dielectric.

[0074] Based on this, the reflection and absorption of electromagnetic waves in the electromagnetic wave absorbing layer were measured by actually irradiating the above-prepared sample with electromagnetic waves and using a free space method.

[0075] FIG. 3 is an image diagram illustrating the measurement state using the free space method.

[0076] As shown in FIG. 3, a transmitting antenna 11 is connected to one port (port 1) of a millimeter wave vector network analyzer MS4647B (product name: manufactured by Anritsu Corporation) 10, and a receiving antenna 12 is connected to the other port (port 2). An electromagnetic wave 15 with a frequency of 60 GHz is irradiated onto a measurement sample 14 formed with a diameter of 100 mm through a dielectric lens 13, and a reflected wave S reflected by the sample 14 is 11 and the transmitted wave S 21 was measured.

[0077] The reflected wave S for an incident electromagnetic wave with a strength of 0.1 mW 11 From the magnitude of the reflected wave, the reflection loss (the value is 0 dB when the entire incident wave is reflected) was calculated. 11 The ratio of reflected wave S 11 / incident wave). Also, the transmitted wave S 21 From the magnitude of the peak, the degree of absorption of the electromagnetic wave in the electromagnetic wave absorbing layer of sample 14 was calculated as the electromagnetic wave attenuation (the value when all the incident wave is transmitted is 0 dB).

[0078] The measurement results are shown in Table 1 below.

[0079] [Table 1]

[0080] As shown in Table 1, when the volume content of the strontium ferrite magnetic powder in the electromagnetic wave absorbing layer is greater than 37.7%, the reflectance at the surface of the electromagnetic wave absorbing layer is greater than 25%. In contrast, when the volume content of the strontium ferrite magnetic powder is 23% or less and the real part of the complex relative dielectric constant of the electromagnetic wave absorbing layer is 5.43 or less, it was confirmed that the reflectance at the surface of the electromagnetic wave absorbing layer is suppressed to 15% or less.

[0081] In a transmission-type electromagnetic wave absorbing sheet containing hexagonal ferrite or epsilon magnetic iron oxide as an electromagnetic wave absorbing material, the higher the volume content of the magnetic powder contained in the electromagnetic wave absorbing layer, the greater the electromagnetic wave attenuation of the electromagnetic waves incident on the sheet. However, as shown in the above measurement results, when the volume content of the magnetic powder contained in the electromagnetic wave absorbing sheet exceeds 37%, the real part of the complex relative dielectric constant exceeds 7, and 25% or more of the incident waves are reflected on the surface of the electromagnetic wave absorbing sheet. For this reason, in a usage situation where the reflection of electromagnetic waves on the surface of the electromagnetic wave absorbing sheet is a problem, there are cases where sufficient electromagnetic wave absorption characteristics cannot be achieved by simply increasing the volume content of the magnetic iron oxide contained in the electromagnetic wave absorbing layer.

[0082] In contrast, in the electromagnetic wave absorbing sheet according to this embodiment, by setting the value of the real part of the complex relative dielectric constant of the electromagnetic wave absorbing sheet to 5.5 or less, the reflection on the surface of the electromagnetic wave absorbing sheet can be reduced to 15% or less, and the electromagnetic wave absorbing sheet can be suitably used in applications where it is desired to suppress the reflection on the surface of the electromagnetic wave absorbing sheet, such as when the electromagnetic wave absorbing sheet is disposed near an electromagnetic wave generating source or near an antenna element. Also, as can be seen from Table 1, by setting the value of the real part of the complex relative dielectric constant of the electromagnetic wave absorbing sheet to 4.2 or less, the reflection on the surface of the electromagnetic wave absorbing sheet can be reduced to 10% or less, which is even more suitable for applications where it is desired to suppress the reflection on the surface of the electromagnetic wave absorbing sheet.

[0083] In addition, the inventors have confirmed that when the volume content of the magnetic iron oxide contained in the electromagnetic wave absorbing layer is constant, the electromagnetic wave attenuation, which is the electromagnetic wave absorbing capacity, increases almost in proportion to the thickness of the electromagnetic wave absorbing layer. Therefore, when the electromagnetic wave absorbing sheet is used in a situation where the reflection of electromagnetic waves on the sheet surface is a problem, it is preferable to adjust the volume content of the magnetic iron oxide in the electromagnetic wave absorbing layer and the layer thickness to provide more preferable electromagnetic wave absorbing characteristics.

[0084] Figure 4 shows the results of the magnetic powder test using aluminum-substituted strontium ferrite (SrFe 10.56 Al 1.44 O 19 5 is a graph showing the change in complex relative dielectric constant (real part) when the volume content of the magnetic powder of an electromagnetic wave absorbing sheet using gallium-substituted epsilon magnetic iron oxide (ε-Ga 0.47 Fe 1.52 O 3 1 is a graph showing the change in complex dielectric constant (real part) when the volume content of magnetic powder is changed in an electromagnetic-wave-absorbing sheet using a magnetic powder having a thickness of 100 nm and a silicone rubber (KE-541-U (product name): complex dielectric constant 3.25, manufactured by Shin-Etsu Chemical Co., Ltd.) as a binder.

[0085] 4 and 5 show the results of measurements made by a capacitance method using an impedance measuring instrument 4291B (product name) manufactured by Agilent Technologies, Inc., after preparing electromagnetic wave absorbing layers with different volume contents of magnetic powder, similar to those shown in FIG. 2. The size of the sample and the measuring tool used in the measurements were the same as those used in the measurements of the electromagnetic wave absorbing sheet containing strontium ferrite and silicone rubber shown in FIG. 2.

[0086] In the case of the electromagnetic wave absorbing sheet containing acrylic rubber as a binder shown in Figure 4, the value of the real part of the complex dielectric constant when no magnetic powder is contained is 4.44, which is larger than the value when silicone rubber is used as the binder. Therefore, in order to make the complex dielectric constant of the electromagnetic wave absorbing layer 5.5 or less, it is necessary to make the volume content 14% or less.

[0087] In addition, when the magnetic powder was changed to epsilon magnetic iron oxide as shown in Figure 5, the increase in the complex dielectric constant when the magnetic powder content increased was slightly larger than the case shown in Figure 3 where the magnetic powder was strontium ferrite, and the volume content of magnetic iron oxide powder for the complex dielectric constant to be 5.5 or less was 22% or less.

[0088] Furthermore, if the value of the real part of the complex dielectric constant of the electromagnetic wave absorbing sheet is the same, the impedance value of the electromagnetic wave absorbing sheet will be approximately the same even if the magnetic powder and binder are different. Therefore, by setting the value of the real part of the complex dielectric constant to 5.5 or less, the surface reflection characteristics of the incident electromagnetic waves can be suppressed, as in the case of the electromagnetic wave absorbing layer containing strontium ferrite and silicone rubber described above.

[0089] As described above, the electromagnetic wave absorbing sheet according to this embodiment has an electromagnetic wave absorbing layer containing magnetic iron oxide that magnetically resonates at frequencies equal to or higher than the millimeter wave band and an organic binder, and by setting the value of the real part of the complex dielectric constant at 1 GHz to 5.5 or less, it is possible to suppress reflection of electromagnetic waves on the surface.

[0090] In the above embodiment, an electromagnetic wave absorbing sheet that is rectangular when viewed in a plane has been described as an example, but there is no restriction on the shape of the electromagnetic wave absorbing sheet, and since the electromagnetic waves are absorbed by magnetic resonance of the magnetic powder, which is the electromagnetic wave absorbing material, the thickness of the electromagnetic wave absorbing sheet does not need to be constant.

[0091] Furthermore, the electromagnetic wave absorber disclosed in the present application is not limited to the sheet-like object shown in the above embodiment, but can be realized as a block-like electromagnetic wave absorber having a certain thickness or more. Furthermore, since there is no restriction on the shape and thickness of the electromagnetic wave absorber in terms of electromagnetic wave absorption characteristics, it can be made into an electromagnetic wave absorber of various shapes, such as a hollow cylinder, a cone shape, a bowl shape, etc. In this way, it is easy to make the shape according to the arrangement location of the electromagnetic wave absorber, and it can also be made into a shape required from other functional aspects, such as when it also has a reflecting member for electromagnetic waves of frequencies other than the frequency absorbed by the electromagnetic wave absorber, and it can also be used well when the reflection of electromagnetic waves on the surface of the electromagnetic wave absorbing sheet worsens electromagnetic interference with equipment.

[0092] In both cases where the electromagnetic wave absorber is in a sheet form and in a form having a predetermined thickness, such as a block form, the hardness of the electromagnetic wave absorber is preferably a durometer hardness (type A) of 80 or less. Having a durometer hardness (type A) of 80 or less has the advantage that it is easy to process and that it is possible to realize an electromagnetic wave absorbing sheet or electromagnetic wave absorber that is less likely to break, such as chipping, even when subjected to impact, because it has a certain degree of softness (elasticity).

[0093] In addition, when a configuration other than the sheet-like electromagnetic wave absorber shown in the above embodiment, for example a block-like configuration, is used as the electromagnetic wave absorber, various methods based on the shape can be adopted as a means for adjusting the value of the real part of the relative dielectric constant at 1 GHz. For example, the value of the relative dielectric constant can be reduced by making the electromagnetic wave absorber a foam. In this case, in addition to the configuration in which the entire electromagnetic wave absorber is made of a foam, a method of making only a part of the electromagnetic wave absorber in the thickness direction of the foam or a method of changing the ratio of voids in the thickness direction of the electromagnetic wave absorber can be used. In addition, the value of the relative dielectric constant of the electromagnetic wave absorber can be reduced by reducing the planar view area of ​​the electromagnetic wave absorber on the side where the electromagnetic wave is incident, that is, by making the shape pointed with respect to the direction of incidence of the electromagnetic wave. Therefore, in addition to the method of adjusting the volume content of the magnetic powder, which is the electromagnetic wave absorbing member, and the method of adjusting the thickness of the electromagnetic wave absorber shown in the above embodiment, the value of the real part of the complex relative dielectric constant of the electromagnetic wave absorber can be set to 5.5 or less by adjusting the shape of the electromagnetic wave absorber. In this way, in the case of an electromagnetic wave absorber having a predetermined thickness, the value of the relative dielectric constant does not need to be uniform in the thickness direction.

[0094] In the electromagnetic wave absorber disclosed in the present application, the numerical value of the real part of the complex relative dielectric constant is set as the value for an electromagnetic wave of 1 GHz because 1 GHz is the highest frequency within a range of frequencies that can be measured relatively easily, and because the value shows a similar tendency to the value of the relative dielectric constant for electromagnetic waves in the millimeter wave band or higher frequency bands that the electromagnetic wave absorber disclosed in the present application is intended to absorb. [Industrial Applicability]

[0095] The electromagnetic wave absorber disclosed in the present application is useful as an electromagnetic wave absorber that absorbs electromagnetic waves in a high frequency band equal to or higher than the millimeter wave band and reduces reflection of electromagnetic waves of the absorbed frequency on its surface. [Explanation of symbols]

[0096] 1. Electromagnetic wave absorbing layer 1a Magnetic iron oxide powder (electromagnetic wave absorbing material) 1b Binder

Claims

1. A transmission-type electromagnetic wave absorber having an electromagnetic wave absorbing layer containing magnetic iron oxide that magnetically resonates at a frequency equal to or higher than the millimeter wave band and an organic binder, The imaginary part of the complex dielectric constant at 1 GHz is 0.016 or more, 1. An electromagnetic wave absorber, characterized in that the real part of the complex dielectric constant at 1 GHz is 5.5 or less.

2. 2. The electromagnetic wave absorber according to claim 1, wherein the real part of the complex relative dielectric constant at 1 GHz is 5.43 or less.

3. 3. The electromagnetic wave absorber according to claim 2, wherein the real part of the complex relative dielectric constant at 1 GHz is 4.2 or less.

4. 2. The electromagnetic wave absorber according to claim 1, wherein the real part of the complex dielectric constant at 1 GHz is 3.25 or more.

5. The electromagnetic wave absorber according to any one of claims 1 to 4, wherein the magnetic iron oxide contains hexagonal ferrite.

6. 6. The electromagnetic wave absorber according to claim 5, wherein the hexagonal ferrite is strontium ferrite or barium ferrite, and a part of the Fe sites is substituted with a trivalent metal atom.

7. 7. The electromagnetic wave absorber according to claim 1, wherein the binder comprises any one of a thermosetting rubber, a thermoplastic elastomer, and a thermoplastic resin.

8. 8. The radio wave absorber according to claim 1, wherein the volume content of said magnetic iron oxide is 23% or less.

9. The electromagnetic wave absorber according to any one of claims 1 to 8, wherein the electromagnetic wave absorbing layer is formed in a sheet shape.

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