Electromagnetic wave absorber and manufacturing method thereof

Abstract

To provide an electromagnetic wave absorber capable of effectively absorbing a plurality of types of electromagnetic waves with different frequencies in a high frequency band equal to or higher than the millimeter wave band.SOLUTION: In an electromagnetic wave absorber in which an electromagnetic wave absorbing layer is formed by stacking multiple magnetic layers containing a magnetic iron oxide that magnetically resonates at high frequencies equal to or higher than the millimeter wave band, the magnetic layers include a particulate magnetic iron oxide and a resin binder, the difference in absolute impedance between two adjacent layers of the multiple stacked magnetic layers is 350 Ω or less, and the input impedance of the magnetic layers is matched to the input impedance of the other adjacent magnetic layers.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an electromagnetic wave absorber that absorbs electromagnetic waves, and in particular to an electromagnetic wave absorber that can absorb electromagnetic waves of multiple different frequencies in a frequency band from several tens of gigahertz (GHz) to several hundred gigahertz (GHz), which is called the millimeter wave band, and further in a high frequency band up to 3 terahertz (THz). [Background technology]

[0002] Mobile communications such as mobile phones, wireless LAN, and electronic toll collection systems (ETC) use electromagnetic waves called centimeter waves, which have a frequency band of several gigahertz (GHz).

[0003] As an electromagnetic wave absorbing sheet that absorbs such centimeter waves, a laminate sheet has been proposed in which a rubber-like electromagnetic wave absorbing sheet and a paper-like sheet material such as cardboard are laminated together (see Patent Document 1).In addition, an electromagnetic wave absorbing sheet has been proposed in which thin sheets containing anisotropic graphite and a binder are alternately laminated to adjust the thickness, thereby stabilizing the electromagnetic wave absorption characteristics regardless of the electromagnetic wave incidence direction (see Patent Document 2).

[0004] Furthermore, with the aim of being able to absorb electromagnetic waves in higher frequency bands, an electromagnetic wave absorbing sheet has been proposed that can absorb electromagnetic waves in frequency bands of 20 gigahertz or higher by aligning the longitudinal direction of flat soft magnetic particles with the surface direction of the sheet (see Patent Document 3).

[0005] It is also known that an electromagnetic wave absorber having a packed structure of particles having epsilon iron oxide (ε-Fe2O3) crystals in the magnetic phase exhibits electromagnetic wave absorbing performance in the range of 25 to 100 gigahertz (see Patent Document 4). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2011-233834 A [Patent Document 2] JP 2006-80352 A [Patent Document 3] JP 2015-198163 A [Patent Document 4] JP 2008-60484 A Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, in order to enable even larger volumes of data to be transmitted, wireless communication using frequencies of 60 gigahertz has been planned, and the use of millimeter-wave lasers with frequencies of several tens of gigahertz or more, the so-called millimeter-wave band (30-300 gigahertz), is being promoted as on-board radar equipment that utilizes extremely narrow directivity. Furthermore, research is also being conducted into technology that uses electromagnetic waves with frequencies on the order of terahertz (THz) as electromagnetic waves in a high frequency band beyond the millimeter-wave band.

[0008] However, as an electromagnetic wave absorber, which is one of the electromagnetic wave utilization technologies and is essential for preventing electromagnetic wave leakage, an electromagnetic wave absorber that can absorb electromagnetic waves in the entire millimeter wave band from 30 GHz to 300 GHz, or in the higher frequency band up to 3 terahertz, has not yet been realized. In particular, conventional electromagnetic wave absorbers are limited in the frequency range of electromagnetic waves that they can absorb, and no electromagnetic wave absorber that can absorb electromagnetic waves of multiple different frequencies has been realized.

[0009] An object of the present disclosure is to solve the above-mentioned problems in the conventional technology and to provide an electromagnetic wave absorber that can effectively absorb a plurality of types of electromagnetic waves with different frequencies in a high frequency band equal to or higher than the millimeter wave band. [Means for solving the problem]

[0010] In order to solve the above problems, the present application discloses an electromagnetic wave absorber in which an electromagnetic wave absorbing layer is formed by laminating a plurality of magnetic layers containing magnetic iron oxide that magnetically resonates at a high frequency equal to or higher than the millimeter wave band, the magnetic layers containing the magnetic iron oxide in particulate form and a resin binder, the difference in absolute impedance between two adjacent layers among the plurality of laminated magnetic layers is 350 Ω or less, and the anisotropic magnetic field (H A ) is different from the value of the anisotropic magnetic field of the magnetic iron oxide contained in another one of the magnetic layers, and each of the magnetic layers functions as a filter that absorbs electromagnetic waves of a predetermined frequency and transmits electromagnetic waves of frequencies other than the predetermined frequency.

[0011] Further, the method for producing an electromagnetic wave absorber disclosed in the present application is any one of the methods for producing an electromagnetic wave absorber disclosed in the present application, characterized in that the thickness of the electromagnetic wave absorbing layer is adjusted based on the frequency of the electromagnetic waves to be absorbed. Effect of the Invention

[0012] The electromagnetic wave absorber disclosed in the present application has an anisotropic magnetic field (H A ) in at least one magnetic layer is different from the other magnetic layers. This allows the absorption of electromagnetic waves of multiple frequencies in a high frequency band of several tens of gigahertz or more. [Brief description of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view illustrating a configuration of a sheet-shaped electromagnetic wave absorber according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a diagram showing magnetic properties versus external magnetization to explain the value of the anisotropy field (HA) of magnetic iron oxide. [Diagram 3] FIG. 1 is a diagram illustrating the electromagnetic wave absorption characteristics of epsilon iron oxide in which a portion of the Fe site is substituted. [Figure 4] 4A and 4B are diagrams illustrating impedance matching in the electromagnetic wave absorber according to the present embodiment. [Diagram 5] FIG. 1 is a model diagram for explaining a free space method for measuring the electromagnetic wave absorption characteristics of an electromagnetic wave absorber. [Figure 6] 6A and 6B are diagrams showing the frequency characteristics of the magnetic permeability of magnetic iron oxide caused by gyromagnetic resonance. Fig. 6(a) shows the frequency characteristics of the real part of the magnetic permeability. Fig. 6(b) shows the frequency characteristics of the imaginary part of the magnetic permeability. [Figure 7] FIG. 1 is a diagram showing a model of magnetic particles used in a simulation for determining the change in magnetic permeability depending on the volume content of magnetic iron oxide. [Figure 8] 1A to 1C are diagrams illustrating electromagnetic wave absorption and impedance matching in stacked magnetic layers in an electromagnetic wave absorbing layer of the electromagnetic wave absorber according to the present embodiment. [Figure 9] 3 is an equivalent circuit diagram of stacked magnetic layers in an electromagnetic wave absorbing layer of the electromagnetic wave absorber according to the present embodiment. FIG. [Figure 10] FIG. 11 is a diagram showing a simulation result regarding the electromagnetic wave absorbing characteristics of the electromagnetic wave absorber according to the present embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The electromagnetic wave absorber disclosed in the present application is an electromagnetic wave absorber in which a plurality of magnetic layers containing magnetic iron oxide that magnetically resonates in a high frequency band equal to or higher than the millimeter wave band are laminated to form an electromagnetic wave absorbing layer, and the anisotropic magnetic field (H A ) is different from the value of the anisotropic magnetic field of the magnetic iron oxide contained in the other one of the magnetic layers.

[0015] In this way, the electromagnetic wave absorber disclosed in the present application can absorb electromagnetic waves in the high frequency band of 30 GHz or more, which is the millimeter wave band, by the magnetic resonance of the magnetic iron oxide contained in each magnetic layer. The magnetic resonance frequency of the magnetic iron oxide contained in each magnetic layer is determined by the anisotropic magnetic field (H A), electromagnetic waves of different frequencies can be converted into heat and absorbed by magnetic layers containing magnetic iron oxides with different values ​​of anisotropic magnetic field. This makes it possible to provide an electromagnetic wave absorber that can absorb multiple electromagnetic waves of different frequencies, which is suitable for future uses of high-frequency electromagnetic waves, such as millimeter-wave radar and communications at high frequencies of tens of gigahertz or more.

[0016] In addition, the anisotropic magnetic field (H A When multiple magnetic layers containing magnetic iron oxides with the same value of ) are laminated, the multiple layers can be considered as one magnetic layer.

[0017] In the electromagnetic wave absorber disclosed in the present application, it is preferable that the frequency characteristics of the electromagnetic waves absorbed by the electromagnetic wave absorber have a plurality of peaks, and the number of the peaks of the frequency characteristics is equal to the number of the laminated magnetic layers. In this way, an electromagnetic wave absorber capable of absorbing electromagnetic waves of a plurality of frequencies can be realized by minimizing the number of laminated magnetic layers.

[0018] In the electromagnetic wave absorber disclosed in the present application, it is preferable that the input impedance of the magnetic layer is matched to the input impedance of the other adjacent magnetic layers, and the input impedance of the magnetic layer arranged on the outermost layer on the electromagnetic wave incident side is matched to the impedance in a vacuum. In this way, the electromagnetic wave is not undesirably reflected and scattered when the electromagnetic wave is incident on the electromagnetic wave absorber or when the electromagnetic wave travels through the boundary between different magnetic layers, and the electromagnetic wave absorption characteristics of each magnetic layer can be maximized.

[0019] Furthermore, it is preferable that the input impedance values ​​of the magnetic layers increase in sequence from the electromagnetic wave incident surface side along the electromagnetic wave traveling direction, whereby the electromagnetic wave can be incident on the magnetic layers stacked in sequence from the magnetic layer on the incident surface side, thereby realizing an electromagnetic wave absorber with high electromagnetic wave absorption characteristics.

[0020] It is also preferable that the magnetic layer contains the magnetic iron oxide particles and a resin binder, and is formed into a flexible sheet, thereby realizing an electromagnetic wave absorber in a sheet form that is easy to handle.

[0021] Furthermore, the magnetic iron oxide is preferably epsilon iron oxide, which makes it possible to realize an electromagnetic wave absorber that absorbs high-frequency electromagnetic waves using epsilon iron oxide having a high magnetic resonance frequency.

[0022] It is also preferable that a reflective layer made of a metal plate, metal foil, or metal deposition film is laminated on the surface of the electromagnetic wave absorbing layer other than the electromagnetic wave incident surface. In this way, it is possible to realize an electromagnetic wave absorber that can reliably block and absorb electromagnetic waves in the millimeter wave band or higher frequency band.

[0023] In this case, it is preferable that the reflective layer and the electromagnetic wave absorbing layer are laminated in this order on a resin base material, and an adhesive layer is formed on the surface of the base material opposite to the side on which the electromagnetic wave absorbing layer is disposed. In this way, an electromagnetic wave absorber that is easy to handle can be realized.

[0024] Hereinafter, the electromagnetic wave absorber disclosed in the present application will be described with reference to the drawings.

[0025] In addition, since "radio waves" can be understood in a broader sense as a type of electromagnetic wave, in this specification, the term "electromagnetic waves" will be used, for example, when referring to a radio wave absorber as an electromagnetic wave absorber.

[0026] (First embodiment) As a first embodiment of the electromagnetic wave absorber disclosed in the present application, an electromagnetic wave absorber formed as an electromagnetic wave absorbing sheet having a magnetic layer containing particulate magnetic iron oxide and a resin binder and having flexibility as a whole will be described as an example.

[0027] [Seat configuration] FIG. 1 is a cross-sectional view showing the structure of an electromagnetic wave absorbing sheet as an electromagnetic wave absorber described in this embodiment.

[0028] Note that FIG. 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet according to this embodiment, and the sizes and thicknesses of the components shown in the diagram are not depicted based on reality.

[0029] The electromagnetic wave absorbing sheet exemplified in this embodiment includes an electromagnetic wave absorbing layer 1 formed by laminating five magnetic layers 1a, 1b, 1c, 1d, and 1e containing magnetic iron oxide. In each of the magnetic layers 1a, 1b, 1c, 1d, and 1e constituting the electromagnetic wave absorbing layer 1, particulate magnetic iron oxide 1a1 and 1b1 are arranged in a dispersed state in resin binders 1a2 and 1b2, as shown in a partially enlarged view of the magnetic layers 1a and 1b in FIG.

[0030] In the electromagnetic wave absorbing sheet according to the present embodiment shown in FIG. 1, the anisotropic magnetic field (H A ) are all different values. In this way, in the electromagnetic wave absorbing sheet of this embodiment, each of the five magnetic layers can absorb electromagnetic waves of a predetermined frequency, and the electromagnetic wave absorbing characteristics of the electromagnetic wave absorbing sheet as a whole can form electromagnetic wave absorption peaks for five different frequencies. In the electromagnetic wave absorbing sheet of this embodiment, the anisotropic magnetic field (H A It is not an essential condition for the electromagnetic wave absorber disclosed in the present application that all the values ​​of the anisotropy magnetic field (H A ) is the anisotropy field (H A ), i.e., there are at least two different anisotropy fields (H A By stacking magnetic layers containing magnetic iron oxide having a value of 0.1 Mpa (a), it is possible to realize an electromagnetic wave absorbing sheet having electromagnetic wave absorption characteristics with peaks at multiple frequencies.

[0031] In addition, as for the electromagnetic wave absorbing layer having multiple layers laminated, each layer may be laminated in direct contact with each other, or each layer may be laminated with an adhesive layer between them to bond each layer. When an adhesive layer is interposed between each magnetic layer, the thickness of the adhesive layer is preferably about 20 to 100 μm. If the thickness of the adhesive layer is thinner than 20 μm, the adhesive strength of each layer is small, and the layers may easily peel off or shift. If the thickness of the adhesive layer is larger than 100 μm, the total thickness of the multiple magnetic layers becomes large, and the flexibility of the entire electromagnetic wave absorbing sheet tends to decrease. By setting the thickness of the adhesive layer in the range of 20 to 100 μm, the multiple magnetic layers are well bonded, and an electromagnetic wave absorbing sheet having flexibility can be realized.

[0032] Here, as the adhesive layer, known materials used as adhesive layers of adhesive tapes, acrylic adhesives, rubber adhesives, silicone adhesives, etc. can be used. The adhesive strength is preferably 5N / 10mm to 12N / 10mm. If the adhesive strength is less than 5N / 10mm, each layer may easily peel off or become displaced. Also, if the adhesive strength is more than 12N / 10mm, each layer is difficult to peel off.

[0033] Furthermore, when magnetic layers having the same value of the anisotropic magnetic field (HA) of the magnetic iron oxide contained in the magnetic layer are laminated directly or via an adhesive layer, the laminate absorbs electromagnetic waves of one frequency and can therefore be regarded as a single magnetic layer.

[0034] FIG. 2 shows the magnetization curve of magnetic iron oxide.

[0035] The magnetization curve 21, which indicates the strength of magnetization remaining in magnetic iron oxide when a magnetic field of varying strength is applied from the outside, draws a so-called hysteresis curve as shown in Figure 2. Magnetic iron oxide that causes magnetic resonance in the millimeter wave band, i.e., in the high frequency band of tens to hundreds of gigahertz, which is the frequency of electromagnetic waves with wavelengths in the mm range, and even higher frequency bands up to 3 terahertz, is a gyromagnetic resonance type magnetic material, so the hysteresis curve of magnetic iron oxide has an obliquely inclined shape. At this time, the value of the applied magnetic field, indicated by arrow 22 in Figure 2, at which the magnetization curve in the hard axis direction reaches the saturation magnetic field, is the anisotropic magnetic field (H A ) and this value indicates the strength of the applied magnetic field at which the spins are aligned in one direction.

[0036] Anisotropic magnetic field (H A The relationship shown in the following formula (1) holds between the value of r(t) and the natural magnetic resonance frequency fr of the magnetic material.

[0037] fr=ν / 2π*H A (1) Here, ν is the gyromagnetic constant, and its value is determined depending on the type of magnetic material.

[0038] Thus, in the gyromagnetic resonance type magnetic material, the anisotropic magnetic field (H A ) and the natural magnetic resonance frequency fr, the electromagnetic wave absorbing layer 1 of this embodiment has a proportional relationship. A ) causes magnetic resonance at different frequencies, converting electromagnetic waves of the respective frequencies into heat and attenuating them. As a result, in the electromagnetic wave absorbing sheet according to this embodiment, each magnetic layer can absorb electromagnetic waves of a predetermined frequency, and stacking the magnetic layers can absorb electromagnetic waves of a plurality of frequencies.

[0039] As shown in Fig. 1, the electromagnetic wave absorbing sheet according to this embodiment is a reflective electromagnetic wave absorbing sheet in which a reflective layer 2 made of a metal material is formed on the back side (lower side in Fig. 1) of an electromagnetic wave absorbing layer 1, and a laminate of the electromagnetic wave absorbing layer 1 and the reflective layer 2 is disposed on a base film 3 which is a resin substrate. An adhesive layer 4 is formed on the side of the base film 3 opposite to the side where the electromagnetic wave absorbing layer 1 is disposed (upper side in Fig. 1) (lower side in Fig. 1).

[0040] As described above, the electromagnetic wave absorbing sheet according to this embodiment converts electromagnetic waves, which are electromagnetic waves, into heat energy by magnetic loss due to magnetic resonance caused by the magnetic iron oxide contained in the electromagnetic wave absorbing layer 1, and absorbs the electromagnetic waves. Therefore, it is possible to absorb electromagnetic waves only with the electromagnetic wave absorbing layer 1. For this reason, it is also possible to use an electromagnetic wave absorbing sheet of a transmission type that absorbs electromagnetic waves that pass through as an electromagnetic wave absorbing sheet that includes only the electromagnetic wave absorbing layer 1. On the other hand, the electromagnetic wave absorbing sheet according to this embodiment is such that electromagnetic waves are incident on one side of the electromagnetic wave absorbing layer 1, and the reflection layer 2, which is a metal layer, is provided on the other side of the electromagnetic wave absorbing layer 1, i.e., the back side of the electromagnetic wave incident side, thereby reliably shielding the electromagnetic waves incident on the electromagnetic wave absorbing layer 1 and reducing the intensity of the electromagnetic waves that are absorbed by the electromagnetic wave absorbing layer 1 and emitted as reflected waves.

[0041] As described later, in the electromagnetic wave absorbing sheet of this embodiment, impedance matching is performed to adjust the thickness of the electromagnetic wave absorbing layer 1 based on the frequency of the electromagnetic wave to be absorbed, so that electromagnetic waves of different frequencies are reliably absorbed by the electromagnetic wave absorbing layer 1. For example, in the case of an electromagnetic wave absorbing sheet that absorbs electromagnetic waves of 75 GHz, the magnetic layer is a thin sheet having a thickness of 1 mm or less. Therefore, even if multiple magnetic layers on such thin films are laminated, the thickness of the electromagnetic wave absorbing layer remains thin, and the electromagnetic wave absorbing layer or the laminate of the electromagnetic wave absorbing layer and the reflective layer is not used as an electromagnetic wave absorbing sheet as it is, but is laminated on a base film 3, which is a resin base material of a predetermined thickness, to improve ease of handling as an electromagnetic wave absorbing sheet.

[0042] Furthermore, since the electromagnetic wave absorbing sheet of this embodiment is often used by being attached to the surface of a component surrounding the source of high-frequency electromagnetic waves, the ease of handling of the electromagnetic wave absorbing sheet is further improved by laminating the adhesive layer 4 to the base fill 3.

[0043] [Magnetic iron oxide] In the electromagnetic wave absorbing sheet according to this embodiment, epsilon iron oxide is used as the particulate magnetic iron oxide.

[0044] Epsilon-iron oxide (ε-Fe2O3) is a phase that appears between the alpha phase (α-Fe2O3) and the gamma phase (γ-Fe2O3) in ferric oxide (Fe2O3), 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.

[0045] Epsilon iron oxide is a fine particle measuring a few to tens of nanometers in size, yet has the largest coercive force of any metal oxide, approximately 20 kOe at room temperature.Furthermore, natural resonance due to the gyromagnetic effect based on precession occurs in the so-called millimeter wave frequency band of tens of gigahertz or more, making it suitable for use as an electromagnetic wave absorbing material that absorbs electromagnetic waves in the millimeter wave band.

[0046] Furthermore, by replacing some of the Fe sites of epsilon iron oxide with trivalent metal elements such as aluminum (Al), gallium (Ga), rhodium (Rh), and 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.

[0047] Figure 3 shows the relationship between the coercive force Hc and the natural resonance frequency f of epsilon iron oxide when different metal elements are substituted for the Fe site. The natural resonance frequency f coincides with the frequency of the electromagnetic waves absorbed.

[0048] From Figure 3, we can see that the natural resonance frequency of epsilon iron oxide, in which part of the Fe site is substituted, varies depending on the type and amount of the substituted metal element. We also see that the higher the natural resonance frequency, the greater the coercive force of the epsilon iron oxide.

[0049] More specifically, gallium-substituted epsilon iron oxide, or ε-Ga x Fe 2-x In the case of O3, by adjusting the substitution amount "x", it has an absorption peak in the frequency band from about 30 GHz to about 150 GHz, and aluminum-substituted epsilon iron oxide, i.e., ε-Al x Fe 2-x In the case of O3, by adjusting the amount of substitution "x", it has an absorption peak in the frequency band of about 100 GHz to 190 GHz. Therefore, by determining the type of element to substitute for the Fe site of epsilon iron oxide and adjusting the amount of substitution with Fe so that the natural resonance frequency of the frequency to be absorbed by the electromagnetic wave absorbing sheet is achieved, it is possible to set the frequency of the electromagnetic waves to be absorbed to the desired value. Furthermore, epsilon iron oxide with rhodium as the substituting metal, i.e., ε-Rh x Fe 2-x In the case of O3, it is possible to shift the frequency band of electromagnetic waves absorbed in the higher direction, from 180 GHz and above.

[0050] Epsilon iron oxide can be purchased, including those with some of the Fe sites substituted with metals. Epsilon iron oxide is available as roughly spherical or short rod-shaped particles with an average particle size of about 30 nm.

[0051] [Magnetic layer] In the electromagnetic wave absorbing sheet according to this embodiment, each magnetic layer constituting the electromagnetic wave absorbing layer 1 has the above-mentioned magnetic iron oxide particles dispersed in a resin binder, and thus has the flexibility of a sheet.

[0052] The resin binder used in the magnetic layer may be a resin material such as an epoxy resin, a polyester resin, a polyurethane resin, an acrylic resin, a phenol resin, a melamine resin, or a rubber resin.

[0053] More specifically, the epoxy resin may be a compound in which the hydroxyl groups at both ends of bisphenol A have been epoxidized. The polyurethane resin may be a polyester urethane resin, a polyether urethane resin, a polycarbonate urethane resin, an epoxy urethane resin, or the like. The acrylic resin may be a methacrylic resin, such as 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 of 2 to 18, a functional group-containing monomer, and, if necessary, another modifying monomer copolymerizable therewith.

[0054] In addition, rubber-based resins such as styrene-based thermoplastic elastomers SIS (styrene-isoprene block copolymer) and SBS (styrene-butadiene block copolymer), petroleum-based synthetic rubber EPDM (ethylene-propylene-diene rubber), and other rubber-based materials such as acrylic rubber and silicone rubber can be used as binders.

[0055] From the viewpoint of environmental consideration, it is preferable to use a halogen-free resin as the binder. These resin materials are commonly used as binder materials for resin sheets and are therefore easily available.

[0056] In this specification, flexibility refers to a state in which the magnetic layer and the electromagnetic wave absorbing layer formed by stacking the magnetic layers can be curved to a certain degree, i.e., when the sheet is rolled up and then returned to its original shape, it returns to a flat sheet without undergoing plastic deformation such as breakage.

[0057] Each magnetic layer of the electromagnetic wave absorbing sheet according to this embodiment uses epsilon iron oxide as an electromagnetic wave absorbing material, but since epsilon iron oxide is a fine nanoparticle with a particle size of several nm to several tens of nm as described above, it is important to disperse it well in the binder when forming the magnetic layer. For this reason, each magnetic layer constituting the electromagnetic wave absorbing layer contains a phosphoric acid compound such as an arylsulfonic acid such as phenylphosphonic acid or phenylphosphonic acid dichloride, an alkylphosphonic acid such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, or propylphosphonic acid, or a polyfunctional phosphonic acid such as hydroxyethanediphosphonic acid or nitrotrimethylenephosphonic acid. These phosphoric acid compounds have flame retardancy and function as a dispersant for the fine magnetic iron oxide powder, so that the epsilon iron oxide particles in the binder can be dispersed well.

[0058] More specifically, examples of dispersants that can be used include phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industries, Ltd., and oxidized phosphoric acid ester "JP-502" (product name) manufactured by Johoku Chemical Industry Co., Ltd.

[0059] As an example of the composition of the magnetic layer, the resin binder may be 2 to 50 parts and the phosphoric acid compound may be 0.1 to 15 parts per 100 parts of epsilon iron oxide powder. If the resin binder is less than 2 parts, the magnetic iron oxide cannot be dispersed well. In addition, the magnetic layer cannot maintain a sheet-like shape. If the resin binder is more than 50 parts, the volume content of the magnetic iron oxide in the magnetic layer is small, and the magnetic permeability is low, resulting in a reduced effect of absorbing electromagnetic waves.

[0060] If the content of the phosphate compound is less than 0.1 parts, the magnetic iron oxide cannot be dispersed well using a resin binder. If it is more than 15 parts, the effect of dispersing the magnetic iron oxide well is saturated. The volume content of the magnetic iron oxide in the magnetic layer becomes small, and the magnetic permeability becomes low, so the effect of absorbing electromagnetic waves becomes small.

[0061] The magnetic layer can be formed, for example, by preparing a magnetic paint containing at least magnetic iron oxide powder and a resin binder, applying the paint to a predetermined thickness, drying, and then calendering. Alternatively, the magnetic paint can be obtained by mixing at least magnetic iron oxide powder, a phosphoric acid compound as a dispersant, and a binder resin at high speed with a high-speed mixer to prepare a mixture, and then dispersing the mixture with a sand mill.

[0062] In this way, for example, the magnetic layers constituting the electromagnetic wave absorbing layer are prepared on a release sheet that has been coated with a silicone coating on its surface to allow easy peeling of the magnetic layers after drying and calendaring, and the magnetic layers are peeled off from the release sheet and laminated, thereby forming an electromagnetic wave absorbing layer in which magnetic layers containing desired magnetic iron oxide are laminated. Also, by sequentially laminating the magnetic layers using a die coater, roll coater, table coater, or the like, and then collectively performing calendaring on the laminate of the magnetic layers, the productivity of the electromagnetic wave absorbing layer in which the magnetic layers are laminated can be increased.

[0063] In the electromagnetic wave absorbing sheet according to this embodiment, the thickness of each magnetic layer constituting the electromagnetic wave absorbing layer is a major factor that determines the electromagnetic wave absorbing characteristics. The thickness of the magnetic layers will be described in detail later.

[0064] [Reflective layer] In the electromagnetic wave absorbing sheet of this embodiment, as shown in FIG. 1, a reflective layer 2 is formed on the back side of an electromagnetic wave absorbing layer 1 formed by stacking magnetic layers, i.e., on the side opposite to the side where electromagnetic waves are incident.

[0065] The reflective layer 2 may be a metal layer formed in close contact with the back surface (the lower surface in FIG. 1) of the electromagnetic wave absorbing layer 1. Specifically, the reflective layer 2 can be configured as a metal plate arranged in close contact with the surface of the magnetic layer formed at the position farthest from the electromagnetic wave incident side among the magnetic layers constituting the electromagnetic wave absorbing layer 1. The reflective layer can also be configured using a metal foil instead of a metal plate. Furthermore, the reflective layer 2 can be realized as a metal deposition film deposited on the surface of the magnetic layer formed at the position farthest from the electromagnetic wave incident side, or as a metal deposition film formed on the surface of the electromagnetic wave absorbing layer 1 side of a nonmetallic sheet or plate-like member arranged on the surface of the magnetic layer formed at the position farthest from the electromagnetic wave incident side.

[0066] There is no particular limitation on the type of metal constituting the reflective layer 2, and various metal materials can be used, including metal materials that are commonly used in electronic components, such as aluminum, copper, and chromium. It is more preferable to use a metal material that has as low an electrical resistance as possible and is highly corrosion resistant.

[0067] In the electromagnetic wave absorbing sheet of this embodiment, by providing a reflective layer 2 on the back surface of the electromagnetic wave absorbing layer 1, it is possible to reliably prevent electromagnetic waves from penetrating the electromagnetic wave absorbing sheet, and in particular to realize an electromagnetic wave absorbing sheet that prevents leakage of electromagnetic waves emitted to the outside from electrical circuit components driven by high frequency waves.

[0068] In addition to the above-mentioned use form in which the reflective layer 2 is formed on the back surface of the electromagnetic wave absorbing layer 1 to reliably prevent the transmission of electromagnetic waves, the electromagnetic wave absorbing sheet may be used as an isolator that is intended to allow a part of an object that attenuates electromagnetic waves to penetrate through it. In the electromagnetic wave absorbing sheet according to this embodiment, including the case of such a use form, it is not essential to provide the reflective layer 2 made of a metal film on the back surface side of the electromagnetic wave absorbing layer 1.

[0069] [Base film, adhesive layer] As shown in FIG. 1, the electromagnetic wave absorbing sheet according to this embodiment has a laminate of an electromagnetic wave absorbing layer 1 and a reflective layer 2 formed on a base film 3.

[0070] As described above, in the electromagnetic wave absorbing sheet of this embodiment, higher electromagnetic wave absorbing properties can be imparted by adjusting the thickness of the magnetic layer constituting the electromagnetic wave absorbing layer 1. For this reason, the thickness of the electromagnetic wave absorbing layer 1 cannot be determined only from the viewpoint of the strength as an electromagnetic wave absorbing sheet and ease of handling. Even if the reflective layer 2 is provided by laminating it on the electromagnetic wave absorbing layer 1, if the overall thickness is too thin to obtain a predetermined strength as an electromagnetic wave absorbing sheet, it is preferable to further laminate a base film 3, which is a resin substrate, on the back side of the reflective layer 2 as shown in FIG. 1.

[0071] The base film 3 can be made of various resin films such as PET film, rubber, paper materials such as Japanese paper, etc. The material and thickness of the base film 3 do not affect the electromagnetic wave absorbing characteristics of the electromagnetic wave absorbing sheet according to this embodiment, so a base film 3 made of an appropriate material and having an appropriate thickness can be selected from the practical viewpoints of the strength of the electromagnetic wave absorbing sheet, ease of handling, etc.

[0072] Furthermore, in the electromagnetic wave absorbing sheet according to this embodiment shown in FIG. 1, an adhesive layer 4 is formed on the surface of the base film 3 opposite to the side on which the electromagnetic wave absorbing layer 1 is formed.

[0073] By providing the adhesive layer 4, the laminate consisting of the reflective layer 2 and the electromagnetic wave absorbing layer 1 laminated on the base film 3 can be attached to a desired position on the inner surface of a housing that houses 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, it can be easily attached even to a curved surface, improving the ease of handling of the electromagnetic wave absorbing sheet.

[0074] The adhesive layer 4 may be made of a known material used as an adhesive layer of an adhesive tape or the like, 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.

[0075] The thickness of the adhesive layer 4 is preferably 20 μm to 100 μm. If the thickness of the adhesive layer 4 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 4 is thicker than 100 μm, the electromagnetic wave absorbing sheet is difficult to peel off from the adherend. If the cohesive strength of the adhesive layer 4 is weak, adhesive residue may be left on the adherend when the electromagnetic wave absorbing sheet is peeled off.

[0076] In the present specification, the adhesive layer may be an adhesive layer that adheres in an unremovable manner, or an adhesive layer that adheres in a removably manner.

[0077] Needless to say, when adhering the electromagnetic wave absorbing sheet to a predetermined surface, it is not an essential requirement that the electromagnetic wave absorbing sheet has the adhesive layer 4. The electromagnetic wave absorbing sheet can be attached to a predetermined location by providing adhesiveness to the surface of the member on which the electromagnetic wave absorbing sheet is to be placed, or by using a double-sided tape or adhesive. In this respect, the adhesive layer 4 is not an essential component of the electromagnetic wave absorbing sheet shown in this embodiment.

[0078] Furthermore, the electromagnetic wave absorbing sheet according to this embodiment can be realized as only the electromagnetic wave absorbing layer 1, or as only a laminate of the electromagnetic wave absorbing layer 1 and the reflective layer 2, but these electromagnetic wave absorbing sheets can also have an adhesive layer.

[0079] [Impedance matching] The electromagnetic wave absorbing sheet according to the present embodiment absorbs electromagnetic waves of high frequencies from tens of gigahertz to hundreds of gigahertz, and even up to 3 terahertz, which are called the millimeter wave band. In such an electromagnetic wave absorber that absorbs high frequency electromagnetic waves, the impedance of each magnetic layer, which is an electromagnetic wave absorbing layer, is greatly affected. In a state where the impedance is not matched, that is, in a state where the difference in impedance value between two consecutive layers in the electromagnetic wave entry direction is large, reflection and scattering occur at the interface of the magnetic layer when the electromagnetic wave crosses the boundary and enters the next magnetic layer. In a situation where the electromagnetic wave does not enter the magnetic layer, electromagnetic wave absorption by magnetic resonance in the magnetic iron oxide contained in each magnetic layer cannot be performed well. For this reason, in the electromagnetic wave absorbing sheet according to the present embodiment, it is necessary to match the impedance when the electromagnetic wave enters from the air into the magnetic layer located on the surface of the electromagnetic wave absorbing layer on the side where the electromagnetic wave is incident, and further when the electromagnetic wave enters each of the stacked magnetic layers in sequence.

[0080] The impedance matching will be described below.

[0081] First, as the most straightforward example, the concept of impedance matching will be explained assuming that the electromagnetic wave absorbing layer is formed of one magnetic layer.

[0082] FIG. 4 is a diagram illustrating impedance matching of an electromagnetic wave absorbing layer.

[0083] In addition, FIG. 4 shows an electromagnetic wave absorbing sheet having a reflecting layer 2 on the back side of the electromagnetic wave absorbing layer 1. This is because, if the electromagnetic wave absorbing sheet has a reflecting layer 2 on the back side, the electromagnetic waves not absorbed by the electromagnetic wave absorbing layer 1 are reflected by the reflecting layer and emitted to the front side, which is the incident side of the electromagnetic waves, so that it is easy to measure the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet. Therefore, the reflecting layer 2 is not an essential component when considering impedance matching. In addition, as shown in FIG. 1, the electromagnetic wave absorbing sheet of this embodiment has a base film 3 and an adhesive layer 4 on the back side of the reflecting layer 2, but these base film 3 and adhesive layer 4 are not shown in FIG. 4 because they are irrelevant when considering impedance matching.

[0084] As shown in Fig. 4, electromagnetic waves 11 to be absorbed by the electromagnetic wave absorbing sheet propagate through the air and enter the electromagnetic wave absorbing layer 1. The electromagnetic waves that enter the electromagnetic wave absorbing layer 1 are absorbed by the magnetic resonance of epsilon iron oxide, which is an electromagnetic wave absorbing material in the electromagnetic wave absorbing layer 1 (not shown in Fig. 4), and the significantly attenuated electromagnetic waves are reflected by the reflective layer 2 on the back surface and radiated forward as a reflected wave 12. By measuring the intensity of this reflected reflected wave 12 and comparing it with the intensity of the incident electromagnetic wave 11, the degree of absorption of the electromagnetic wave by the electromagnetic wave absorbing sheet can be grasped.

[0085] Here, the impedance Z of the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet in is expressed as the following equation (2).

[0086]

number

[0087] In the above formula (2), Z0 is the impedance value in a vacuum state, which is approximately 377Ω, and is almost the same value as the impedance in air. inBy making the value of Z equal to Z0, the impedance between the air and the electromagnetic wave absorbing layer 1 is matched, and the electromagnetic waves transmitted through the air can be incident directly on the electromagnetic wave absorbing layer 1 without being reflected or scattered on the surface of the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet, and the electromagnetic wave absorbing characteristics of the electromagnetic wave absorbing layer 1 itself can be maximized.

[0088] In the above formula (2), Z in It can be seen that in order to make the value of Z equal to Z0, the thickness d of the electromagnetic wave absorbing layer 1 should be set to a predetermined value once the wavelength λ of the electromagnetic wave is determined. In other words, once the frequency of the electromagnetic wave absorbed by the electromagnetic wave absorbing layer 1 is determined, the optimal thickness d of the electromagnetic wave absorbing layer 1 is determined.

[0089] The electromagnetic wave absorption characteristics resulting from impedance matching can be measured by a free space method using an electromagnetic wave absorbing sheet.

[0090] FIG. 5 shows a schematic diagram of the measurement state based on the free space method.

[0091] As shown in Figure 5, an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer 1 and a reflective layer 2 formed on its back surface is created as a measurement target, and using one port of a millimeter-wave network analyzer, an input wave 11 (millimeter wave) of a predetermined frequency is irradiated onto the electromagnetic wave absorbing sheet from a transmitting / receiving antenna 22 via a dielectric lens 23, and the reflected wave 12 from the electromagnetic wave absorbing sheet is measured.

[0092] The intensity of the input wave 11 and the intensity of the reflected wave 12 obtained at this time are compared to determine the degree of attenuation, RL (Reflection Loss), in dB.

[0093] RL can be calculated using the following formula (3).

[0094]

number

[0095] If the degree of attenuation of the reflected wave 12 obtained at this time is, for example, 15 dB, this means that 99% of the incident wave 11 is absorbed by the electromagnetic wave absorbing sheet and the reflected wave 12 is attenuated to 1%, which means that sufficient electromagnetic wave absorption characteristics are obtained.

[0096] In this way, at the stage where the electromagnetic wave is incident on the electromagnetic wave absorbing layer 1 from the air, the thickness d of the magnetic layer is selected according to the frequency of the electromagnetic wave to be absorbed, and the input impedance Z in By setting the impedance Z0 so that it matches the impedance of the air, higher electromagnetic wave absorption characteristics can be obtained.

[0097] [Stacking of magnetic layers] As explained with reference to Fig. 4, when an electromagnetic wave travels through a boundary between members having different impedances, if the impedance matching is insufficient, the electromagnetic wave will be scattered at the boundary, and the electromagnetic wave absorption characteristics of the electromagnetic wave absorbing sheet will be reduced due to the reflection of undesired electromagnetic waves. This phenomenon is not limited to the case where the electromagnetic wave is incident on the electromagnetic wave absorbing layer from a vacuum (air) as shown in Fig. 4, but also becomes a problem at the boundary between each layer of the magnetic body layer when the electromagnetic wave absorbing layer is a laminate in which multiple magnetic body layers are stacked. For this reason, in the case of the electromagnetic wave absorbing layer shown in this embodiment in which the electromagnetic wave absorbing layer is formed as a laminate of five magnetic body layers, for example, it is preferable to match the impedance between each magnetic body layer in addition to the impedance matching between the magnetic body layer arranged on the outermost surface side where the electromagnetic wave is incident and the air.

[0098] Hereinafter, the electromagnetic wave absorbing characteristics of the electromagnetic wave absorber shown in this embodiment, in which the electromagnetic wave absorbing layer is configured as a laminate of five magnetic layers, will be described through a simulation.

[0099] First, the magnetic permeability of magnetic iron oxide that generates gyromagnetic resonance with respect to the frequency of the incident electromagnetic wave was calculated using the LLG (Landau-Lifshitz-Gilbert) equations shown as the following formulas (4) and (5).

[0100]

number

[0101] In the above formulas (4) and (5), χ s0 is the DC magnetic susceptibility, ω s is the gyromagnetic resonance angular frequency (2πf r ) and a are the damping constants. The results obtained by substituting the eigenvalues ​​of epsilon iron oxide into these are shown in Figure 6.

[0102] Figure 6(a) shows the frequency characteristics of the real part of magnetic permeability (μ') obtained by equation (4), and Figure 6(b) shows the frequency characteristics of the imaginary part of magnetic permeability (μ'') obtained by equation (5).

[0103] As shown in FIG. 6, the epsilon iron oxide used in the above simulation has a peak in the imaginary part of magnetic permeability (μ″) at 70 GHz, and it is found to absorb electromagnetic waves with a wavelength of 70 GHz.

[0104] Next, the volume content of magnetic iron oxide in each magnetic layer is obtained, in accordance with the electromagnetic wave absorber of this embodiment being an electromagnetic wave absorbing sheet in which magnetic iron oxide particles are dispersed in a resin binder. As described above, for example, in a magnetic layer obtained by applying and drying a magnetic paint in which fine particles of epsilon iron oxide, a binder material, and a dispersant are kneaded, the magnetic layer is required to perform a simulation by determining the volume content of the magnetic material in the layer, for reasons such as the fact that the layer contains a resin material and that voids are generated in the sheet-like magnetic layer actually applied.

[0105] FIG. 7 shows a model used in a simulation to determine the magnetic permeability of magnetic iron oxide taking into account the volume content.

[0106] In the simulation, the magnetic permeability μ r μ rBMagnetic powder with a diameter D is placed in a resin layer with a thickness of δ / 2 (magnetic permeability μ r =1) was assumed to surround the area.

[0107] At this time, the permeability μ of the entire electromagnetic wave absorbing layer r can be expressed as the following formula (6).

[0108] Here, the magnetic permeability of the magnetic powder can be expressed as equation (7) using the real part and imaginary part of the magnetic permeability, and can be substituted into equation (6) to obtain equation (8).

[0109]

number

[0110] From the above formula (8), the combined input impedance of the electromagnetic wave absorbing layer in which five magnetic layers are stacked according to this embodiment can be expressed by the following formula (9).

[0111]

number

[0112] Here, Z N is the material impedance of the Nth layer, γ N is the propagation constant of the Nth layer of material, d N is the thickness of the Nth layer, and Z0 is the impedance in air (vacuum).

[0113] 8, a reflective electromagnetic wave absorbing sheet is modeled in which a metal plate reflective layer 67 is disposed on the back surface of an electromagnetic wave absorbing layer formed by stacking five magnetic layers 62, 63, 64, 65, and 66. From the side of the reflective layer 67 that reflects electromagnetic waves, a first magnetic layer 66 (N=1), a second magnetic layer 65 (N=2), a third magnetic layer 64 (N=3), a fourth magnetic layer 63 (N=4), and a fifth magnetic layer 62 (N=5) are stacked in this order.

[0114] In addition, in equation (9), since the metal plate 67 has infinite conductivity (resistance 0), its electric field and magnetic field can be expressed as (0, H1), and the electric field and magnetic field of the reflected wave 61 emitted after passing through the electromagnetic wave absorbing layer, which is a laminate of five magnetic layers, are expressed as (E6, H6).

[0115] If we organize the combined electromagnetic wave absorption by the five magnetic layers in terms of the reflection loss "Γ", the reflection loss RL (reflection loss) when electromagnetic waves are absorbed by the electromagnetic wave absorbing layer, which is a stack of five magnetic layers, can be expressed as equation (10) using the reflection loss "Γ".

[0116]

number

[0117] As described above, it is important to match the impedance in each of the magnetic layers.

[0118] In each of the electromagnetic wave absorbing layers made of five magnetic layers, N=1 to N=5, shown in FIG. 8, if the condition of the following formula (11) is satisfied, the impedance Z of the N layers can be calculated as shown in formula (12). n can be made equal to the impedance in air, Z0.

[0119]

number

[0120] FIG. 9 shows the synthetic impedance of the model shown in FIG. 8 as an equivalent circuit.

[0121] As shown in FIG. 9, the sum of the impedances Z1, Z2, Z3, Z4, and Z5 of the magnetic layers is the composite impedance Z of the electromagnetic wave absorbing sheet. in In this case, if the imaginary part of the return loss Γ in the electromagnetic wave absorbing sheet becomes 0, the synthetic impedance Z in is equal to the impedance in air, Z0.

[0122] FIG. 10 shows the results of a simulation of the electromagnetic wave absorption characteristics of an electromagnetic wave absorber in which the impedance of each of the five magnetic layers is matched by setting each layer to satisfy the requirements of formula (11).

[0123] In FIG. 10, the model shown in FIG. 8 is used, and the resonance frequency of the magnetic layer 66 of the first layer (N=1) from the reflection layer 67 side is 180 GHz, the resonance frequency of the magnetic layer 65 of the second layer (N=2) is 150 GHz, the resonance frequency of the magnetic layer 64 of the third layer (N=3) is 120 GHz, the resonance frequency of the magnetic layer 63 of the fourth layer (N=4) is 90 GHz, and the resonance frequency of the magnetic layer 62 of the fifth layer (N=5) is 70 GHz. This fifth layer 62 is the magnetic layer whose frequency characteristics of the real part of magnetic permeability and the imaginary part of magnetic permeability are shown in FIG. 6. The damping constant of each layer is 0.03, and the DC magnetic susceptibility is 0.05. In order to simplify the conditions as a stacked model of multiple magnetic layers, the volume content of the magnetic material in each layer is assumed to be 100%.

[0124] Here, from equation (1), ν is the gyromagnetic constant, which is a fixed value depending on the type of magnetic iron oxide. Therefore, the natural magnetic resonance frequency fr and the anisotropic magnetic field H A From this relationship, the natural magnetic resonance frequency fr is changed from the first layer to the fifth layer in this model, so that the anisotropy magnetic field H A This is equivalent to changing the

[0125] In this case, in order to match the impedance in each layer, by satisfying the condition of equation (11), the thickness d of the first layer 66 is 856 μm, the thickness d of the second layer 65 is 211 μm, the thickness d of the third layer 64 is 317 μm, the thickness d of the fourth layer 63 is 351 μm, and the thickness d of the fifth layer 62 on the electromagnetic wave incident side is 452 μm. Using these values, the frequency characteristics of the electromagnetic wave were calculated as the attenuation rate shown in equation (10).

[0126] FIG. 10 shows the frequency characteristics of the attenuation rate of an electromagnetic wave absorber in which five magnetic layers are stacked.

[0127] As a result of the above simulation, as shown in FIG. 10, the frequency characteristic 80 of the electromagnetic wave absorber in which five magnetic layers are stacked has absorption peaks at 70 GHz (reference number 81), 90 GHz (reference number 82), 120.5 GHz (reference number 83), 150.5 GHz (reference number 84), and 178 GHz (reference number 85) as the absorption peak frequencies of each layer. In addition, since the impedance of each layer is matched, each absorption peak is formed with an attenuation rate exceeding 10 dB, and it is understood that electromagnetic waves of the corresponding frequency can be attenuated well. In addition, each attenuation peak is formed sharply, and it is understood that electromagnetic waves of intermediate frequencies that are not set as attenuation frequencies are transmitted well without being absorbed. In this way, the anisotropic magnetic field (H A )) is calculated by dividing the anisotropy field (H A ) and further by matching the impedance of the laminated magnetic layers, it can be seen that an electromagnetic wave absorber is formed that has the properties of a filter that transmits electromagnetic waves of frequencies other than a specified frequency.

[0128] The electromagnetic wave absorber shown in Fig. 10 is in a so-called ideal state in which the impedance of each magnetic layer is matched, and in reality, it is difficult to strictly match the impedance of each magnetic layer. In this case, it is preferable to set the impedance value of each layer to gradually increase from the electromagnetic wave incident side (the fifth layer 62 in Fig. 8) toward the first layer 66 closest to the reflection layer 67 along the electromagnetic wave traveling direction. This is because the electromagnetic wave is relatively easy to be incident on a layer with a larger impedance value, but the incidence of the electromagnetic wave is restricted at the boundary portion where the impedance value becomes smaller.

[0129] Moreover, it is preferable that the absolute value of the difference in impedance between adjacent magnetic layers is 350 Ω or less, because if the absolute value of the difference in impedance between two magnetic layers is 350 Ω or more, electromagnetic waves will be significantly scattered and reflected at the boundary surface of the two magnetic layers.

[0130] As an example, when the return loss was calculated by simulation with the fourth layer 63 having a thickness of 860 μm and an impedance of 741.7 Ω, the fifth layer 62 having a thickness of 452 μm and an impedance of 376.7 Ω, and the difference in impedance between adjacent magnetic layers of the fourth layer 63 and the fifth layer 62 being 365 Ω, the return loss of the fourth layer 63 was 5.59 dB, which was smaller than 6 dB (1 / 2).

[0131] On the other hand, when the thickness of the fourth layer 63 is 850 μm and the impedance is 701.6 Ω, the thickness of the fifth layer 62 is 452 μm and the impedance is 376.7 Ω, and the difference in impedance between the adjacent magnetic layers of the fourth layer 63 and the fifth layer 62 is 325 Ω, the reflectance of the fourth layer 63 becomes 6.10 dB, and a return loss rate of 50% or more can be obtained, as a result of simulation. In this way, when the impedance between the stacked magnetic layers is larger than 350 Ω, a significant decrease in the electromagnetic wave absorption characteristics occurs only due to impedance mismatch, and the characteristics as an electromagnetic wave absorber that absorbs electromagnetic waves of multiple frequencies cannot be obtained.

[0132] The value of 350Ω is a condition obtained from a simulation under an extreme condition where the return loss falls below 50% due to impedance mismatch, and is not a value that directly indicates the tolerance range of impedance matching. If an electromagnetic wave absorbing layer is considered to be an electromagnetic wave absorber that effectively absorbs electromagnetic waves of a specified frequency by each magnetic layer, it is considered that an absorption rate of at least 90% or more is required in each layer, and in order to maintain the electromagnetic wave absorption rate of each magnetic layer at 90% or more, i.e., the attenuation rate at a value greater than 15 dB, it is considered that the impedance difference between each magnetic layer should be suppressed to about 25Ω as an absolute value.

[0133] In the above embodiment, an electromagnetic wave absorber is assumed to absorb five frequencies by providing five magnetic layers each having a predetermined resonance frequency according to the model of Fig. 8. However, as described above, there is a restriction that the impedance value of each magnetic layer is preferably larger along the direction of electromagnetic wave propagation, and that the difference in absolute value of the impedance of two adjacent layers is preferably 350Ω or less. If the magnetic layer having the desired resonance frequency cannot be formed as a magnetic layer having a certain thickness or more from the viewpoint of impedance matching, the magnetic layer having the predetermined resonance frequency can be laminated in multiple layers with one or more magnetic layers having other resonance frequencies interposed therebetween.

[0134] In this way, it is possible to obtain an electromagnetic wave absorber having a sufficiently high electromagnetic wave absorption characteristic (electromagnetic wave absorption attenuation rate) at a predetermined resonance frequency and a high and sharp peak in the frequency characteristic.

[0135] Second embodiment Next, a second embodiment of the electromagnetic wave absorber disclosed in the present application will be described.

[0136] The electromagnetic wave absorber according to the second embodiment differs from the electromagnetic wave absorber according to the first embodiment, which is in the form of a sheet having magnetic iron oxide particles dispersed in a resin binder, in that the electromagnetic wave absorbing layer is an electromagnetic wave absorber (bulk) in which magnetic iron oxide particles are densely packed.

[0137] Each magnetic layer constituting such an electromagnetic wave absorber densely packed with magnetic iron oxide particles can be constructed, for example, by creating a case corresponding to a desired magnetic layer thickness determined from the standpoint of a specified planar shape and impedance matching, and filling the inside of the opening with magnetic iron oxide particles.

[0138] For this reason, it is possible to adopt a form in which the magnetic iron oxide particles are packed in a case and stacked together with the case to form an electromagnetic wave absorbing layer. It is also possible to adopt a form in which a coagulant that solidifies the particles is injected into the magnetic iron oxide packed in the case to form multiple solidified magnetic layers, and each magnetic layer is removed from the case and stacked.

[0139] It is also possible to employ a form in which the magnetic iron oxide particles and the resin are melted and extruded through a die to form a plate-like molded body, which is then laminated.

[0140] In each magnetic layer formed as a bulk for use in the electromagnetic wave absorber of this embodiment, for example, the Fe element of ε-iron oxide is replaced with a predetermined amount of aluminum or the like in order to provide a desired resonance frequency, and the impedance of each magnetic layer should be matched, which are similar to those in the first embodiment described for the sheet-like electromagnetic wave absorber.

[0141] As described above, the electromagnetic wave absorber disclosed in the present application has an electromagnetic wave absorbing layer formed as a laminate of a plurality of magnetic layers, and the anisotropic magnetic field (H A ) is different between at least one layer and the other layer. By configuring in this way, it is possible to realize an electromagnetic wave absorber having absorption peaks in a plurality of frequency bands.

[0142] For this reason, even when there are multiple frequencies to be absorbed, such as when a millimeter wave radar is used in which the frequency is switched depending on the detection conditions, it is possible to realize an electromagnetic wave absorbing sheet that can reliably absorb unnecessary electromagnetic waves using a single electromagnetic wave absorber.

[0143] In the above embodiment, the magnetic layer is described using epsilon iron oxide as the magnetic iron oxide. As described above, by using epsilon iron oxide, it is possible to form an electromagnetic wave absorber that absorbs electromagnetic waves in the millimeter wave band of 30 GHz to 300 GHz, and further, by using rhodium or the like as a metal material that substitutes for the Fe site, it is possible to realize an electromagnetic wave absorber that absorbs electromagnetic waves up to several terahertz, which is the highest frequency defined as electromagnetic waves.

[0144] However, in the electromagnetic wave absorber disclosed in the present application, the magnetic iron oxide used in the magnetic layer is not limited to epsilon iron oxide.

[0145] Hexagonal ferrite, a ferrite-based electromagnetic absorber, exhibits electromagnetic wave absorbing properties in the 76 gigahertz band, and strontium ferrite also exhibits electromagnetic wave absorbing properties in the tens of gigahertz band. For this reason, by forming sheet-shaped magnetic layers using magnetic iron oxide particles other than epsilon iron oxide that have electromagnetic wave absorbing properties in the millimeter wave band of 30 gigahertz to 300 gigahertz and, for example, a resin binder, and laminating these, it is possible to realize an electromagnetic wave absorbing sheet that absorbs electromagnetic waves of multiple frequencies in the millimeter wave band.

[0146] For example, the particle diameter of the hexagonal ferrite particles is larger than that of the epsilon iron oxide particles exemplified in the above embodiment, being about 10 μm, and the particle shape is not substantially spherical but is a plate-like or needle-like crystal. Therefore, when forming a magnetic paint using a resin binder, it is preferable to use a dispersant and adjust the kneading conditions with the binder so that when applied as a magnetic paint, the magnetic iron oxide powder is dispersed as uniformly as possible in the magnetic layer and the void ratio is adjusted as small as possible.

[0147] In the above embodiment, the electromagnetic wave absorbing sheet is described in which the electromagnetic wave absorbing layer is configured by stacking five magnetic layers, but the number of magnetic layers in the electromagnetic wave absorbing layer is not limited. As described above, since the frequency range of the electromagnetic waves absorbed by each magnetic layer is limited, it is preferable to determine the number of magnetic layers constituting the electromagnetic wave absorbing layer in accordance with the number of frequency bands of the electromagnetic waves to be absorbed. [Industrial Applicability]

[0148] The electromagnetic wave absorber disclosed in the present application is useful as an electromagnetic wave absorber that absorbs electromagnetic waves of two or more frequencies in a high frequency band equal to or higher than the millimeter wave band. [Explanation of symbols]

[0149] 1. Electromagnetic wave absorbing layer 1a~1e Magnetic layer 1a1, 1b1 Magnetic iron oxide particles 1a2, 1b2 Resin binder 2 reflective layer 3 Base film 4 Adhesive layer

Claims

1. An electromagnetic wave absorber in which an electromagnetic wave absorbing layer is formed by stacking a plurality of magnetic layers containing magnetic iron oxide that magnetically resonates at a high frequency equal to or higher than the millimeter wave band, the magnetic layer contains the magnetic iron oxide in particulate form and a resin binder, a difference in absolute impedance between two adjacent layers of the magnetic layers is 350Ω or less; The anisotropic magnetic field (H A ) is different from the value of the anisotropic magnetic field of the magnetic iron oxide contained in another one of the magnetic layers, Each of the magnetic layers is an electromagnetic wave absorber that functions as a filter that absorbs electromagnetic waves of a predetermined frequency and transmits electromagnetic waves of frequencies other than the predetermined frequency.

2. 2. The electromagnetic wave absorber according to claim 1, wherein the frequency characteristics of the electromagnetic waves absorbed by said electromagnetic wave absorber have a plurality of peaks.

3. 3. The electromagnetic wave absorber according to claim 1, wherein the input impedance values ​​of said magnetic layers increase successively from the electromagnetic wave incident surface side along the electromagnetic wave traveling direction.

4. 4. The electromagnetic wave absorber according to claim 1, wherein the input impedance of said magnetic layer arranged on the outermost layer on the electromagnetic wave incident side is matched to the impedance in a vacuum.

5. 5. The electromagnetic wave absorber according to claim 1, wherein the magnetic iron oxide is at least one selected from the group consisting of epsilon iron oxide and strontium ferrite.

6. A method for producing the electromagnetic wave absorber according to any one of claims 1 to 5, comprising the steps of: A method for producing an electromagnetic wave absorber, comprising adjusting the thickness of the electromagnetic wave absorbing layer based on the frequency of the electromagnetic wave to be absorbed.

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