Electromagnetic wave absorbing materials and electromagnetic wave absorbing sheets

Abstract

The present invention provides an electromagnetic wave absorbing material comprising a dielectric layer and a reflective layer (conductive layer), wherein the conductive layer, which is made of metal vapor deposition or the like, is less prone to deterioration. [Solution] The electromagnetic wave absorbing material of the present invention is an electromagnetic wave absorbing material having a conductive layer on one surface of a dielectric layer, characterized in that the difference (E1-E2) between the standard electrode potential (E1) of the metal forming the conductive layer and the standard electrode potential (E2) of the metal element in the dielectric layer is -0.25V or more.

Description

[Technical Field]

[0001] This invention relates to an electromagnetic wave absorbing material, particularly an electromagnetic wave absorbing material that excels at absorbing high-frequency electromagnetic waves such as those in the GHz band, and an electromagnetic wave absorbing sheet using the same. [Background technology]

[0002] With the spread of high-speed communication and driver assistance technologies, the applications of high-frequency radio waves in the GHz band are increasing. For example, next-generation communications such as 5G and 6G utilize electromagnetic waves in the GHz band for communication, enabling high-speed and high-capacity information transmission. Furthermore, millimeter-wave radar used in automotive collision avoidance systems utilizes high-frequency electromagnetic waves in the 76-79 GHz band, and with the future spread of autonomous driving technology, the number of millimeter-wave radars installed for omnidirectional surveying and other applications is expected to increase dramatically. With the proliferation of such high-frequency communications and the increasing density of electronic devices, concerns have been raised about increased noise due to the reception of unwanted electromagnetic waves within devices, interference, and self-inflicted malfunctions caused by self-generated noise. Therefore, electromagnetic shielding materials usable in the GHz band are needed to address these unwanted electromagnetic waves.

[0003] Therefore, the use of a λ / 4 type electromagnetic wave absorber, in which a reflective layer and a resistive layer are bonded to a polymer dielectric layer containing barium titanate particles, has been proposed as an electromagnetic wave shielding material in the GHz band (see Patent Document 1 below). However, this electromagnetic wave absorber could not achieve sufficient absorption performance unless the packing density of barium titanate particles as dielectric fillers in the dielectric layer was high, resulting in a thicker electromagnetic wave absorber. Furthermore, because it contained a large amount of dielectric filler, it tended to lose flexibility, limiting the applications of the electromagnetic wave absorber.

[0004] To address these problems, the inventors have discovered an inorganic filler having a crystalline phase represented by a specific formula, and have developed a new electromagnetic wave absorbing material that can be made thinner than conventional electromagnetic wave absorbing materials and exhibits equivalent or superior electromagnetic wave absorbing ability despite a low packing density (see Patent Documents 2 or 3 below). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-4003 [Patent Document 2] Japanese Patent Publication No. 2024-811 [Patent Document 3] Japanese Patent Publication No. 2024-812 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The inventors have developed an inorganic filler having a crystalline phase represented by a specific formula, as disclosed in Patent Document 2 or 3, and have developed a new electromagnetic wave absorbing material. However, electromagnetic wave absorbing materials using inorganic fillers have been found to have problems where the metal used in the reflective layer (conductive layer) dissolves over time, causing the conductive layer to deteriorate. The inventors of this invention have investigated the cause and achieved the present invention.

[0007] Therefore, the object of the present invention is to provide an electromagnetic wave absorbing material having a dielectric layer and a reflective layer (conductive layer) in which deterioration of the conductive layer, such as that made by metal deposition, is less likely to occur, and an electromagnetic wave absorbing sheet using the same. [Means for solving the problem]

[0008] The present invention has embodiments described in [1] to

[10] below.

[0009] [1]. An electromagnetic wave absorber having a conductive layer on one surface of a dielectric layer, wherein the difference (E1 - E2) between the standard electrode potential (E1) of the metal forming the conductive layer and the standard electrode potential (E2) of the metal element in the dielectric layer is -0.25 V or more.

[0010] [2]. The electromagnetic wave absorber according to [1], wherein the dielectric layer contains an inorganic filler having a crystal phase represented by the following formula [1]. MQ , Ti 1-x O 3-y RE …(1)

[0011] In formula (1), M represents one or more alkaline earth metal elements, Q represents one or more transition metal elements, RE represents one or more rare earth elements, and x and y satisfy the following. 0.0 ≦ x < 1.0 0.00 ≦ y < 0.05

[0012] [3]. The electromagnetic wave absorber according to [1] or [2], wherein the metal forming the conductive layer is composed of one or more selected from any of Al, Cu, Sn, In, Ni, Ag, and Fe.

[0013] [4]. The electromagnetic wave absorber according to any one of [1] to [3], wherein the dielectric layer further contains a conductive filler.

[0014] [5]. The electromagnetic wave absorber according to any one of [2] to [4], wherein Q contains one or more elements selected from the group consisting of Hf, Mn, Fe, Ni, and Zr.

[0015] [6]. The electromagnetic wave absorber according to any one of [2] to [4], wherein Q contains Zr.

[0016] [7]. The electromagnetic wave absorber according to any one of [2] to [6], wherein M contains one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba.

[0017] [8]. The electromagnetic wave absorber according to any one of [2] to [6], wherein M contains Ba.

[0018] [9]. The electromagnetic wave absorber according to any one of [2] to [8], wherein the RE contains one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0019]

[10] . An electromagnetic wave absorption sheet obtained by forming the electromagnetic wave absorber according to any one of [1] to [9] into a sheet shape with a thickness of 50 μm or more and 10,000 μm or less. [Advantages of the Invention]

[0020] The electromagnetic wave absorber of the present invention is less likely to deteriorate the conductive layer composed of metal vapor deposition or the like, and thus can be used for a long period of time. [Brief Description of the Drawings]

[0021] [Figure 1] It is an X-ray diffraction pattern of the inorganic filler produced in Example 1. [Figure 2] It is a scanning electron microscope photograph of the inorganic filler produced in Example 1. [Figure 3] It is a graph showing the electromagnetic wave absorption rate of 5.6 to 8.2 GHz of the electromagnetic wave absorption layer measured by the free space method in the electromagnetic wave absorption sheets of Example 1 and Comparative Example 1. [Modes for Carrying Out the Invention]

[0022] Hereinafter, the present invention will be described based on one embodiment. However, the present invention is not limited to this embodiment.

[0023] ​​​​The standard electrode potential refers to the electrode potential under standard conditions in an electrode reaction, with the potential of a standard hydrogen electrode as the reference (0 volts). Furthermore, in this invention, "the standard electrode potential of the metal forming the conductive layer" refers to the standard electrode potential of the element with the highest standard electrode potential among the metal elements contained in the conductive layer, and "the standard electrode potential of the dielectric layer" refers to the standard electrode potential of the element with the lowest standard electrode potential among the metal elements contained in the dielectric layer.

[0024] The dielectric layer of this electromagnetic wave absorbing material is a layer that can change the phase of electromagnetic waves incident from the surface. This electromagnetic wave absorbing material causes a phase difference between the electromagnetic waves reflected from the surface of the dielectric layer and the electromagnetic waves reflected from the conductive layer provided on one side of the dielectric layer. By interfering the electromagnetic waves reflected from the surface and the electromagnetic waves reflected from the conductive layer, the electromagnetic waves cancel each other out and absorb electromagnetic waves. This electromagnetic wave absorbing material excels at absorbing high-frequency electromagnetic waves in the 5.0GHz to 8.0GHz range, is even more effective at absorbing waves in the 5.4GHz to 7.5GHz range, and is particularly effective at absorbing waves in the 5.6GHz to 7.0GHz range.

[0025] <Dielectric layer> The dielectric layer of this electromagnetic wave absorbing material consists of a polymer matrix containing inorganic fillers, and may also contain conductive fillers.

[0026] <Polymer> The polymer of this electromagnetic wave absorbing material can be any polymer that forms the matrix of the electromagnetic wave absorbing material, and examples include synthetic resins (including thermoplastic elastomers) such as acrylic resin, ethylene vinyl acetate copolymer (EVA), polyvinyl chloride, polyurethane, acrylic urethane resin, ionomer, polyolefin, polypropylene, polyethylene, silicone resin, polyester, polystyrene, polyimide, polyamide, polysulfone, polyethersulfone, and epoxy resin, or rubber materials such as nitrile rubber, urethane rubber, ethylene propylene rubber, polyisoprene rubber, polystyrene-butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile-butadiene rubber, butyl rubber, acrylic rubber, and silicone rubber. Mixtures of these may also be used. Furthermore, the rubber material may be natural rubber, synthetic rubber, or a mixture thereof.

[0027] Among these, rubber materials are preferred, with nitrile rubber (NBR rubber), urethane rubber (PU rubber), ethylene propylene rubber (EPDM), and acrylonitrile-butadiene rubber being more preferred, and nitrile rubber (NBR rubber) being particularly preferred.

[0028] The polymer is preferably present in the dielectric layer at a concentration of 1% by mass or more, more preferably at 3% by mass or more, and particularly preferably at 5% by mass or more. There is no upper limit, but it is preferably at 90% by mass or less, more preferably at 80% by mass or less, and particularly preferably at 70% by mass or less.

[0029] <Inorganic filler> The inorganic filler of this electromagnetic wave absorbing material comprises a crystalline phase represented by the following formula (1). MQ x Ti 1-x O 3-y RE …(1) In equation (1), M represents one or more alkaline earth metal elements, Q represents one or more transition metal elements, RE represents one or more rare earth elements, and x and y satisfy the following conditions. 0.0 ≤ x < 1.0 0.00 ≤ y < 0.05

[0030] The value of x in formula (1) is usually 0.0 or greater, preferably 0.05 or greater, more preferably 0.08 or greater, and particularly preferably 0.10 or greater. It is also usually less than 1.0, preferably 0.80 or less, more preferably 0.50 or less, and particularly preferably 0.30 or less. The value of x can be adjusted as appropriate depending on the application and the desired crystal phase. When x is within the above range, particles with excellent structural stability and high dielectric constant can be provided. In particular, when x is greater than 0 and contains Zr, particles with excellent electromagnetic wave absorption ability can be obtained.

[0031] The value of y in formula (1) is usually 0.00 or greater, preferably 0.0001 or greater, more preferably 0.0003 or greater, and particularly preferably 0.0005 or greater. It is also usually less than 0.05, preferably 0.03 or less, and more preferably 0.01 or less. The aforementioned y can be adjusted as appropriate depending on the application and the desired crystal phase. When y is within the above range, it is possible to provide a material with excellent structural stability and a high dielectric constant.

[0032] The values ​​of x and y in equation (1) can be confirmed by compositional analysis methods such as energy-dispersive X-ray spectroscopy (EDS), X-ray fluorescence spectroscopy (XRF), and radio frequency inductively coupled plasma (ICP).

[0033] In formula (1), M represents one or more alkaline earth metal elements, preferably one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba, and more preferably Ba. From the viewpoint of synthesizing a composition that exhibits a high dielectric constant, the alkaline earth metal element M preferably contains 50 to 100 mol% of one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba relative to the total amount of M, and more preferably contains 70 to 100 mol% of Ba relative to the total amount of M. M is particularly preferably composed of Ba.

[0034] In formula (1), Q represents one or more transition metal elements, preferably one or more elements selected from the group consisting of Hf, Mn, Fe, Ni, and Zr, and more preferably Zr.

[0035] In formula (1), RE represents one or more rare earth elements, preferably comprising one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, more preferably comprising one or more elements selected from the group consisting of Yb, Sm, Nd, La, Eu, and Dy, and particularly preferably comprising Yb. From the viewpoint of synthesizing a composition that exhibits a high dielectric constant, the rare earth element RE preferably contains 50 to 100 mol% of one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu relative to the total amount of RE, more preferably 50 to 100 mol% of one or more elements selected from the group consisting of Yb, Sm, Nd, La, Eu, and Dy relative to the total amount of RE, and particularly preferably 80 to 100 mol% of Yb relative to the total amount of RE.

[0036] The crystalline phase of the inorganic filler used in this electromagnetic wave absorbing material is Yb-doped BaTi, where in equation (1) M is Ba, Q is Zr, RE is Yb, x is 0.15, and y is 0. 0.85 Zr 0.15 O3 is the most preferred.

[0037] The particle size of the inorganic filler is not particularly limited, but is usually 10 nm or larger, preferably 50 nm or larger, more preferably 100 nm or larger, and usually 500 μm or smaller, preferably 200 μm or smaller, more preferably 100 μm or smaller. If the particle size is above the lower limit, primary particle aggregation is easily suppressed, so a sheet with more uniformly dispersed particles can be obtained. On the other hand, if the particle size is below the upper limit, the frequency of electromagnetic wave incidence into the particles increases, so an electromagnetic wave absorbing material with higher electromagnetic wave absorption capacity can be obtained. Here, the particle diameter of the inorganic filler can be expressed as the range of the minimum value and the maximum value by measuring the major axis lengths of 10 randomly selected inorganic fillers in the observation by a scanning electron microscope. The major axis length of the inorganic filler corresponds to the length at the location where the distance between the two plates becomes the largest when the particle is sandwiched between two parallel plates.

[0038] The sphericity of the inorganic filler is usually 0.1 or more, preferably 0.3 or more, more preferably 0.5 or more, and particularly preferably 0.7 or more. The upper limit of the sphericity is not particularly limited, and the higher it is, the better, but it is usually 1 or less. If the sphericity is not less than the above lower limit, it becomes easy to produce electromagnetic wave absorption with high in-plane uniformity in the electromagnetic wave absorption ability.

[0039] The sphericity can be obtained by measuring the perimeter of the inorganic filler in the scanning electron microscope observation and dividing the circumference of a circle having the same area as the inorganic filler by the perimeter of the inorganic filler, referring to the reference document (John R.Grace and Arian Ebneyamini, Particuology Volume54, February 2021, Pages1-4).

[0040] The relative permittivity (ε r ) of the inorganic filler is usually 3000 or more, preferably 5000 or more, more preferably 10000 or more, and particularly preferably 12,000 or more. The relative permittivity (ε r ) can be measured by a conventional method, but in this specification, the value measured by the method described in the examples described later is adopted.

[0041] The dielectric loss tangent (tanδ) of the inorganic filler is usually 0.01 or more, preferably 0.03 or more, more preferably 0.05 or more, and particularly preferably 0.08 or more. The dielectric loss tangent (tanδ) can be measured by a conventional method, but in this specification, the value measured by the method described in the examples described later is adopted.

[0042] The inorganic filler is preferably present in the dielectric layer in an amount of 5% to 99% by mass, more preferably in an amount of 7% to 97% by mass, and particularly preferably in an amount of 10% to 95% by mass.

[0043] <Method for manufacturing inorganic fillers> Inorganic fillers can be produced by a particle manufacturing method that includes at least a step of mixing raw materials containing an M source, a Zr source, a Ti source, and an RE source, and a heat treatment step. Preferably, in addition to the above steps, one or more steps of drying and grinding are included. Including these steps allows for proper dispersion of aggregated particles.

[0044] <Inorganic filler raw materials> The raw materials for each of the aforementioned elements, namely the M source, Zr source, Ti source, and RE source, are not particularly limited. For example, oxides, hydroxides, and halides of each element, as well as inorganic salts such as sulfates, nitrates, and carbonates, organic acid salts such as acetates and citrates, and organic complexes such as alkoxides can be used. Furthermore, the compounds of each of the aforementioned elements may be hydrates or the like. There are no particular restrictions on the method of introducing oxygen (O). It can be introduced by using compounds containing oxygen atoms in the raw materials of each element, or by performing heat treatment in an oxygen-containing atmosphere such as air.

[0045] <Process of mixing raw materials> In the process of mixing the raw materials, powdered raw materials are usually used and mixed. The method of mixing the raw materials is not particularly limited, and they can be mixed wet or dry using conventional equipment such as mortars, ball mills, and jet mills, but wet mixing is preferred. The media used in wet mixing may include one or more organic solvents such as water, ethanol, isopropyl alcohol, and methyl ethyl ketone, preferably water. The amount of media used is preferably 30 to 60% by volume, particularly 40 to 50% by volume, relative to the container volume.

[0046] In the mixing of raw materials, the ratio of the sum of Ti and Zr elements in the raw materials used to M element is usually 0.5x or more, preferably 0.8x or more, more preferably 1.0x or more, and usually 3.0x or less, preferably 2.5x or less, and more preferably 2.0x or less, with x being the molar ratio of M / (Ti+Zr) of the target particles. By ensuring that the M / (Ti+Zr) molar ratio of the raw materials used is within the above range, a crystalline phase with the desired composition can be efficiently obtained.

[0047] <Heat treatment process> After the mixing of raw materials, heat treatment can be performed to obtain particles containing a crystalline phase composed of the target compound. The atmosphere for the heat treatment is not particularly limited, but from the viewpoint of preventing oxygen deficiency, it is preferable to perform the treatment in an oxygen-containing atmosphere such as air. From the viewpoint of increasing reactivity, the heat treatment temperature is preferably 1000°C or higher. The upper limit of the heat treatment temperature is preferably below the phase transition temperature, such as the melting point or glass transition temperature, of the target compound, and is usually 1450°C or lower. The heating time for heat treatment varies depending on the heating temperature, but it is usually between 1 hour and 20 hours.

[0048] <Grinding process> The resulting particles may be pulverized after the heat treatment process. Pulverization allows for the separation of particles that have undergone aggregation.

[0049] <Drying process> The particles after the grinding process may be dried to remove various solvents or adsorbed water. Furthermore, if the raw materials are wet-mixed, it is preferable to dry them after mixing and before the heat treatment process. There are no particular restrictions on the drying method, but drying can be done using a dryer, vacuum dryer, freeze dryer, etc. The temperature and atmosphere during drying should preferably be between 50°C and 300°C, which is sufficient to evaporate the water.

[0050] <Conductive filler> The dielectric layer may contain conductive fillers, such as metals, graphite powder, conductive ceramics, conductive glass, and conductive polymers. Multiple such fillers may be included. The particle size of the conductive filler is not particularly limited, but is usually 10 nm or larger, preferably 50 nm or larger, more preferably 100 nm or larger, and usually 500 μm or smaller, preferably 200 μm or smaller, more preferably 100 μm or smaller. The particle size of conductive fillers can be measured, for example, in the same way as inorganic fillers.

[0051] Specific examples of conductive fillers include conductive fibrous particles (such as "Dentol WK-500" manufactured by Otsuka Chemical Co., Ltd.) which are metal particles, carbon, or potassium titanate fibers to which conductivity has been imparted.

[0052] The conductive filler is preferably present in the dielectric layer in an amount of 0.1% to 45% by mass, more preferably in an amount of 0.2% to 40% by mass, and particularly preferably in an amount of 0.5% to 35% by mass.

[0053] <Standard electrode potential (E2) of the dielectric layer> The standard electrode potential (E2) of a dielectric layer can be expressed as the value of the metal element with the highest standard electrode potential among those contained in the dielectric layer. In other words, if the dielectric layer contains only inorganic fillers, the standard electrode potential (E2) of the dielectric layer will be the value of the metal element with the highest standard electrode potential among those contained as inorganic fillers. The standard electrode potential (E2) of the dielectric layer is preferably -1.30V or less, more preferably -1.35V or less, and particularly preferably -1.40V or less. The standard electrode potential can be measured by known methods, such as cyclic voltammetry.

[0054] <Conductive layer> The conductive layer is made of a conductive material such as a metal and is a layer that reflects electromagnetic waves transmitted through the dielectric layer. Examples include a metal vapor deposition sheet or film, a metal plate, a metal film, a metal mesh, a graphite sheet, a conductive polymer, a glass substrate with a conductive film, etc. These can be used alone or in combination of two or more.

[0055] The metal forming the conductive layer is preferably composed of one or more selected from any of Al, Cu, Sn, In, Ni, Ag, and Fe.

[0056] The conductive layer can be formed by coating, pasting, vapor deposition, etc., and it is preferably formed by vapor deposition on a film. Also, by applying and curing the mixture for forming the electromagnetic wave absorber on the metal vapor deposition surface of the metal vapor deposition sheet or film constituting the conductive layer, it can be made easier to form an electromagnetic wave absorption sheet.

[0057] The thickness of the conductive layer is not particularly limited, but is usually 5 nm or more, preferably 8 nm or more, more preferably 10 nm or more, and is usually 100 μm or less, preferably 10 μm or less, more preferably 1 μm or less.

[0058] <Standard electrode potential (E "1"> of the conductive layer) The standard electrode potential (E1) of the conductive layer can be expressed as the standard electrode potential of the metal forming the conductive layer. That is, when the conductive layer is formed of any of Al, Cu, Sn, In, Ni, Ag, and Fe, the standard electrode potential of each metal becomes the standard electrode potential (E1) of the conductive layer. The standard electrode potential (E1) of the conductive layer is preferably +0.5 V or more, more preferably +0.6 V or more, and particularly preferably +0.7 V or more.

[0059] <Difference between E1 and E2> This electromagnetic wave absorber is such that the difference (E1 - E2) between the standard electrode potential (E1) of the metal forming the conductive layer and the standard electrode potential (E2) of the dielectric layer is -0.25 V or more. Thereby, the deterioration of the conductive layer is less likely to progress. From this perspective, the difference between E1 and E2 (E1-E2) is more preferably -0.23V or greater, and particularly preferably -0.22V or greater. The difference between the standard electrode potential (E1) and the standard electrode potential (E2) of the dielectric layer can be adjusted by selecting the metal that forms the conductive layer, selecting the inorganic filler to be included in the dielectric layer, and whether or not conductive fillers are present in the dielectric layer.

[0060] <Manufacturing method for this electromagnetic wave absorbing material> This electromagnetic wave absorbing material can be manufactured, for example, by mixing a polymer with an inorganic filler to form a coating solution, creating a mixed slurry, and then applying the mixed slurry to a metal vapor-deposited film or the like, which will serve as a conductive layer. When preparing the mixed slurry, a liquid solvent may be used for mixing, and the solvent may be removed by drying and curing to form the electromagnetic wave absorbing material. Examples of solvents include organic solvents such as water, ethanol, isopropyl alcohol, and methyl ethyl ketone, and mixtures thereof may also be used.

[0061] When preparing the mixed slurry, the preferred mass ratio is 3 to 80 parts by mass of polymer to 20 to 97 parts by mass of inorganic filler, with 30 to 95 parts by mass of inorganic filler being more preferable. If conductive filler is included, the preferred ratio is 3 to 80 parts by mass of polymer to 0.2 to 40 parts by mass of conductive filler, with 0.5 to 35 parts by mass of conductive filler being more preferable.

[0062] This electromagnetic wave absorbing material can be made into shapes and sizes to suit the application, for example, powder, granules, spheres, films, sheets, plates, panels, and other forms, and among these, it is preferable to make an electromagnetic wave absorbing sheet by making this electromagnetic wave absorbing material into a sheet. To achieve each shape, the product can be molded by methods such as extrusion molding, injection molding, press molding, spraying, coating, sintering and machining, or other conventional methods. Furthermore, during or after molding, the product may be cured according to the polymer curing method used, depending on the application.

[0063] To produce an electromagnetic wave absorbing sheet, for example, the above mixed slurry can be applied to a metal vapor-deposited film such as Al, Cu, Sn, In, Ni, Ag, or Fe and then dried. The total thickness of the electromagnetic wave absorbing sheet is preferably 10 μm to 10,000 μm, more preferably 20 μm to 8,000 μm, and particularly preferably 50 μm to 5,000 μm.

[0064] <Application> This electromagnetic wave absorbing material is designed to prevent degradation of the conductive layer by, for example, setting the difference (E1-E2) between the standard electrode potential (E1) of the metal forming the conductive layer and the standard electrode potential (E2) of the dielectric layer to -0.25V or higher. This electromagnetic wave absorbing material can be used, for example, as a ceiling material to prevent malfunctions in ETC (Electronic Toll Collection) systems used at toll gates on expressways.

[0065] Furthermore, the above-mentioned electromagnetic wave absorbing sheet can also be used as an electromagnetic wave absorbing sheet for a shielding case to prevent errors in IC chips caused by 38-76 GHz collision avoidance radar used in automobile collision avoidance systems. Furthermore, the electromagnetic wave absorbing sheet described above can also be used as an absorbing panel for aiming tests, which are used to inspect collision avoidance radars at vehicle inspection stations and other locations, to prevent false detection of radars from adjacent vehicles. [Examples]

[0066] An embodiment of the present invention will be described below. However, the present invention is not limited to this embodiment.

[0067] The electromagnetic wave absorbing materials of Example 1 and Comparative Example 1, as shown below, were prepared.

[0068] [Example 1] <Preparation of inorganic fillers> The preparation composition is BaTi 0.85 Zr 0.15 O 3-0.001As Yb, 24.7 kg of barium carbonate (BW-KT, manufactured by Sakai Chemical Co., Ltd.), 8.49 kg of zirconium oxide (PCS, manufactured by Shin-Nippon Denko Co., Ltd.), 2.31 kg of titanium oxide (CR-93, manufactured by Ishihara Sangyo Co., Ltd.), and 0.0246 kg of ytterbium oxide (98.0+%), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., were mixed in a V-type mixer and then calcined in a cordierite crucible at 1300°C in an air atmosphere for 4 hours. The calcined powder was then pulverized using a jet mill. The obtained powder was identified by X-ray diffraction analysis, revealing that the resulting phase was a single phase in which the barium titanate phase was peak-shifted by zirconium. The X-ray diffraction chart is shown in Figure 1. Furthermore, observation using a scanning electron microscope confirmed that the particles had a diameter of 800 nm to 4 μm. A scanning electron microscope image is shown in Figure 2.

[0069] <Fabrication of electromagnetic wave absorbing sheets> A rubber compound was obtained by mixing 38.1 parts by mass of acrylonitrile-butadiene rubber (ENEOS Material NBR N230S), 0.4 parts by mass of stearic acid (Kanto Chemical Co., Ltd., purity 95.0+%), 1.9 parts by mass of zinc oxide (Kanto Chemical Co., Ltd., purity 99.0+%), 0.2 parts by mass of sulfur (Kanto Chemical Co., Ltd., purity 98.0+%), 0.4 parts by mass of N,N'diethylthiourea (Tokyo Chemical Industries Co., Ltd., purity 98.0+%), 44 parts by mass of the above inorganic filler, and 15 parts by mass of graphite powder (Fujifilm Wako Pure Chemical Industries, Ltd., purity 98.0+%) using a two-roll mill (Daihan Co., Ltd. DY6-15). Multiple layers of this rubber compound were stacked and pressed using a 70t press at 10MPa, 180°C, and 25 minutes to form an electromagnetic wave absorbing layer with a thickness of 1.96mm. The specific gravity of the electromagnetic wave absorbing layer was 1.85g / cm³. 3 This electromagnetic wave absorbing layer was then attached to the vapor-deposited surface of an aluminum-deposited PET film (substrate thickness 50 μm, vapor-deposited layer thickness 50 nm) using double-sided tape to create an electromagnetic wave absorbing sheet.

[0070] Figure 3 shows a backscattered electron scanning electron microscope image of the dielectric layer of this electromagnetic wave absorbing sheet. From the binarization of the scanning electron microscope image, 10 randomly selected inorganic filler particles within the dielectric layer were measured for sphericity, and the average value was 0.758. The electromagnetic wave absorption sheet obtained in this manner was subjected to free-space measurements of its electromagnetic wave absorption layer at 5.6–8.2 GHz. The transmittance was 0%. The measurement results for the absorption rate are shown in Figure 3.

[0071] In this electromagnetic wave absorbing sheet, the standard electrode potential (E1) of the conductive layer was -1.66V, and the standard electrode potential (E2) of the dielectric layer was a maximum of -1.45V, with a difference (E1-E2) of -0.21V. Furthermore, carbon, a component of graphite, is excluded as a nonmetal.

[0072] [Comparative Example 1] A mixed slurry was obtained by mixing 23 parts by mass of a urethane resin solution (Takelac TE-5899, manufactured by Mitsui Chemicals, Inc., with a solid content concentration of 30 parts by mass) with 92.07 parts by mass of the above inorganic filler and 0.93 parts by mass of conductive fibers (WK-500, manufactured by Otsuka Chemical Co., Ltd.). Dentor (WK-500, manufactured by Otsuka Chemical Co., Ltd.) is a conductive fibrous particle (average fiber diameter 350 nm, fiber length 5-15 μm, resistivity 10) obtained by imparting conductivity to potassium titanate fibers. 12 It is Ω·cm. This mixed slurry was applied using a 500 μm thick film applicator onto a release film (100 μm thick, "SP-PET® O3-BU" manufactured by Mitsui Chemicals Tohcello Co., Ltd.) consisting of a polyester film and a silicone-based release layer, onto an aluminum-deposited PET film (substrate thickness 12 μm, deposition film thickness approximately 30 nm), and dried and cured at 80°C for more than 3 hours to obtain a rubber compound. This rubber compound was molded in the same manner as in Example 1 to form an electromagnetic wave absorbing layer sheet with a thickness of 1.85 mm. The specific gravity of the electromagnetic wave absorbing layer sheet was 4.32 g / cm³. This electromagnetic wave absorbing layer was laminated with an aluminum-deposited PET film in the same manner as in Example 1 to produce an electromagnetic wave absorbing sheet.

[0073] The electromagnetic wave absorption sheet obtained in this manner was subjected to free-space measurements of its electromagnetic wave absorption layer at 5.6–8.2 GHz. The transmittance was 0%. The measurement results for the absorption rate are shown in Figure 3. In this electromagnetic wave absorbing sheet, the standard electrode potential (E1) of the conductive layer was -1.662V, and the standard electrode potential (E2) of the dielectric layer was -0.59V, with a difference of (E1-E2) of -1.172V.

[0074] <Durability Test> The electromagnetic wave absorbing sheets of Example 1 and Comparative Example 1 were each placed in a constant temperature chamber at 85°C and 85% humidity, left in that state for 100 hours, and visually inspected for any deterioration in the electromagnetic wave absorbing sheets.

[0075] <Result> In Example 1, the electromagnetic wave absorbing sheet, which had a standard electrode potential difference (E1-E2) of -0.21V, showed no degradation of the conductive layer. In contrast, the electromagnetic wave absorbing sheet of Comparative Example 1, which had a standard electrode potential difference (E1-E2) of -1.172V, showed deterioration such as the dissolution of a portion of the conductive layer.

Claims

1. An electromagnetic wave absorbing material having a conductive layer on one surface of a dielectric layer, wherein the difference (E1 - E2) between the standard electrode potential (E1) of the metal forming the conductive layer and the standard electrode potential (E2) of the metal element in the dielectric layer is -0.25V or greater.

2. The electromagnetic wave absorbing material according to claim 1, wherein the dielectric layer includes an inorganic filler having a crystalline phase represented by the following formula [1]. MQ ｘ Ti １－ｘ O ３－ｙ RE …(1) (In formula [1], M represents one or more alkaline earth metal elements, Q represents one or more transition metal elements. RE represents one or more rare earth elements, and x and y satisfy the following conditions. 0.0 ≤ x < 1.0 0.00 ≤ y < 0.05)

3. The electromagnetic wave absorbing material according to claim 1, wherein the metal forming the conductive layer is one or more selected from Al, Cu, Sn, In, Ni, Ag, and Fe.

4. The electromagnetic wave absorbing material according to claim 1, further comprising a conductive filler in the dielectric layer.

5. The electromagnetic wave absorbing material according to claim 2, wherein Q comprises one or more elements selected from the group consisting of Hf, Mn, Fe, Ni, and Zr.

6. The electromagnetic wave absorbing material according to claim 2, wherein Q includes Zr.

7. The electromagnetic wave absorbing material according to claim 2, wherein M comprises one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba.

8. The electromagnetic wave absorbing material according to claim 2, wherein M contains Ba.

9. The electromagnetic wave absorbing material according to claim 2, wherein RE contains one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

10. An electromagnetic wave absorbing sheet comprising an electromagnetic wave absorbing material according to any one of claims 1 to 9, in the form of a sheet with a thickness of 50 μm or more and 10,000 μm or less.

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