Electromagnetic wave absorbing material and electromagnetic wave absorbing sheet

Abstract

Provided are an electromagnetic wave absorbing material that is flexible and makes it possible to make a dielectric layer thinner, and an electromagnetic wave absorbing sheet in which said material is used.　The present invention is an electromagnetic wave absorbing material that is characterized by containing graphite and an inorganic filler that comprises a crystal phase expressed by formula (1).　Formula (1): MQxTi1-xO3-yRE In formula (1), M indicates one or more alkaline earth metal elements, Q indicates one or more transition metals, RE indicates one or more rare earth elements, and x and y satisfy the following relationships.　0.0≤x<1.0, 0.00≤y<0.05

Description

Electromagnetic wave absorbing materials and electromagnetic wave absorbing sheets

[0001] The present invention relates to an electromagnetic wave absorbing material, particularly an electromagnetic wave absorbing material that is excellent at absorbing high frequency electromagnetic waves such as those in the GHz band, and an electromagnetic wave absorbing sheet using the same.

[0002] With the spread of high-speed communications and driver assistance technologies, the use of high-frequency radio waves in the GHz band is increasing. For example, next-generation communications such as 5G and 6G are expected to utilize electromagnetic waves in the GHz band, enabling high-speed, high-capacity data transmission. Furthermore, millimeter-wave radars used in automobile collision prevention systems utilize high-frequency electromagnetic waves in the 76-79 GHz range. With the future spread of autonomous driving technology, the number of millimeter-wave radars installed is expected to increase dramatically for purposes such as omnidirectional surveying. With the spread of high-frequency communications and the increasing density of electronic devices, concerns have arisen about increased noise due to the reception of unwanted electromagnetic waves within devices, crosstalk, and auto-poisoning caused by malfunctions due to noise generated by the devices themselves. To combat these unwanted electromagnetic waves, electromagnetic wave shielding materials that can be used in the GHz band are needed.

[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 for the GHz band (see Patent Document 1 below). However, this electromagnetic wave absorber could not achieve sufficient absorption performance unless the filling rate of the barium titanate particles as a dielectric filler in the dielectric layer was high, resulting in a thick electromagnetic wave absorber. Furthermore, since it contains a large amount of dielectric filler, it tends to lose flexibility, and the application locations of the electromagnetic wave absorber are limited.

[0004] In order to solve these problems, the present inventors discovered an inorganic filler having a crystalline phase represented by a specific formula, and developed a new electromagnetic wave absorbing material that can be made thinner than conventional electromagnetic wave absorbing materials and can exhibit the same or greater electromagnetic wave absorbing ability despite having a low filling rate (see Patent Documents 2 or 3 below).

[0005] JP 2019-4003 A JP 2024-811 A JP 2024-812 A

[0006] As a result of extensive research, the inventors have developed an electromagnetic wave absorbing material and an electromagnetic wave absorbing sheet using the same, which can be made thinner than conventional materials while reducing the filling rate of the inorganic filler in the dielectric layer, and which further has flexibility and weather resistance, by using an inorganic filler having a crystalline phase represented by the specific formula described above.

[0007] Therefore, one object of the present invention is to provide an electromagnetic wave absorbing material that allows the dielectric layer to be thin and that is flexible, and an electromagnetic wave absorbing sheet using the same. Another object of the present invention is to provide an electromagnetic wave absorbing material that allows the dielectric layer to be thin and that is weather resistant, and an electromagnetic wave absorbing sheet using the same.

[0008] The present invention has the following aspects [1] to

[21] .

[0009] [1] An electromagnetic wave absorbing material comprising graphite and 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 metals, 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)

[0010] [2] The electromagnetic wave absorber according to [1], wherein the graphite is flat graphite.

[0011] [3] The electromagnetic wave absorbing material according to [1] or [2], wherein the graphite is spheroidized graphite.

[0012] [4] The electromagnetic wave absorber according to any one of [1] to [3], wherein the mass ratio of the inorganic filler to the graphite is within the range of 10:90 to 90:10.

[0013] [5] The electromagnetic wave absorber according to [1], wherein the particle size of the graphite is 100 nm or more and 500 μm or less.

[0014] [6] The electromagnetic wave absorber according to any one of [1] to [5], further comprising a rubber material.

[0015] [7] An electromagnetic wave absorbing material comprising an inorganic filler having a crystalline phase represented by the following formula (1) and a rubber material: 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 metals, 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)

[0016] [8] The electromagnetic wave absorbing material according to [6] or [7], wherein the rubber material is at least one selected from the group consisting of nitrile rubber (NBR rubber), urethane rubber (PU), and ethylene propylene rubber (EPDM).

[0017] [9] The electromagnetic wave absorbing material according to any one of [6] to [8], wherein the mass ratio of the inorganic filler to the rubber material is within a range of 50:50 to 90:10.

[0018]

[10] The electromagnetic wave absorber according to [1] or [7], wherein the particle size of the inorganic filler is 10 nm or more and 500 μm or less.

[0019]

[11] The electromagnetic wave absorber according to any one of [1] to

[10] , further comprising a conductive filler.

[0020]

[12] The electromagnetic wave absorber according to any one of [1] to

[11] , wherein Q contains one or more elements selected from the group consisting of Hf, Mn, Fe, Ni, and Zr.

[0021]

[13] The electromagnetic wave absorber according to any one of [1] to

[12] , wherein Q contains Zr.

[0022]

[14] The electromagnetic wave absorber according to any one of [1] to

[13] , wherein M contains one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba.

[0023]

[15] The electromagnetic wave absorber according to any one of [1] to

[14] , wherein M contains Ba.

[0024]

[16] The electromagnetic wave absorber according to any one of [1] to

[15] , 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.

[0025]

[17] An electromagnetic wave absorbing sheet, in which the electromagnetic wave absorbing material according to any one of [1] to

[16] is formed into a sheet having a thickness of 50 μm or more and 1000 μm or less.

[0026]

[18] The electromagnetic wave absorbing sheet according to

[17] , which has a conductive layer on one side of the sheet.

[0027]

[19] A component for an electronic toll collection system, using the electromagnetic wave absorbing material according to any one of [1] to

[16] .

[0028]

[20] An automobile exterior component using the electromagnetic wave absorbing material according to any one of [1] to

[16] .

[0029]

[21] A satellite communication component using the electromagnetic wave absorbing material according to any one of [1] to

[16] .

[0030] The electromagnetic wave absorbing material of the present invention increases the dielectric constant of the dielectric layer by incorporating graphite together with an inorganic filler having a specific crystalline phase, thereby enabling the dielectric layer to be made thinner. Furthermore, the filling rate of the inorganic filler in the dielectric layer can be reduced, thereby increasing flexibility and expanding the range of applications of the electromagnetic wave absorbing material. Furthermore, by using a rubber material as the matrix of the dielectric layer, the dielectric constant of the dielectric layer can be increased, allowing the dielectric layer to be made thinner, and further, the filling rate of the inorganic filler in the dielectric layer can be reduced, thereby increasing flexibility. Furthermore, by using a combination of a specific rubber material and graphite of a specific shape, flexibility can be further increased and volume expansion under high temperature and high humidity conditions can be suppressed.

[0031] 1 is an X-ray diffraction pattern of the inorganic filler produced in Example 1. 2 is a scanning electron microscope photograph of the inorganic filler produced in Example 1. 3 is a graph showing the electromagnetic wave absorption rates of 5.6 to 8.2 GHz measured by the free space method in the electromagnetic wave absorbing sheets of Example 1 and Comparative Example 1. 4 is a graph showing the electromagnetic wave absorption rates of 12.4 to 45.0 GHz measured by the free space method in the electromagnetic wave absorbing sheets of Example 2 and Comparative Example 2.

[0032] The present invention will be described below based on one embodiment, but the present invention is not limited to this embodiment.

[0033] <Electromagnetic wave absorbing material> An electromagnetic wave absorbing material according to one embodiment of the present invention (hereinafter also referred to as the present electromagnetic wave absorbing material) contains graphite and an inorganic filler having a specific crystalline phase. By using graphite and the inorganic filler, the present electromagnetic wave absorbing material can reduce the thickness of the dielectric layer when made into a sheet or the like.

[0034] <Graphite> The graphite used in the present electromagnetic wave absorber is also called graphite or plumbum, and has a layered structure in which carbon atoms form hexagonal crystals. It is generally in the form of powder. The graphite may be either natural graphite or artificial graphite, or a mixture of these may be used. The shape of the graphite is not particularly limited, but examples include flat and spherical shapes, and mixtures of these may also be used. The graphite preferably has a particle size of 500 μm or less, more preferably 100 μm or less, and particularly preferably 50 μm or less. The lower limit is not particularly limited, but is preferably 100 nm or more, more preferably 500 nm or more, and particularly preferably 1000 nm or more.

[0035] Flat graphite (hereinafter referred to as flat graphite) is thin, plate-shaped graphite, and includes flake graphite. The aspect ratio of flat graphite is preferably 1.7 to 5.0, more preferably 1.8 to 4.0, and particularly preferably 1.9 to 3.0. The aspect ratio can be calculated, for example, by measuring the minor axis and major axis of each particle and dividing the major axis by the minor axis.

[0036] Specific examples of the flat graphite used in the electromagnetic wave absorber include "Product Name CPB" manufactured by Nippon Graphite Co., Ltd. and "Product Name Graphite Powder" manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0037] The spherical graphite (hereinafter referred to as spherical graphite) is not particularly limited, but preferably has a sphericity of 0.75 or more, more preferably 0.80 or more, and particularly preferably 0.85 or more. The particle size of the spherical graphite is preferably 100 nm to 500 μm, more preferably 500 nm to 100 μm, and particularly preferably 1 μm to 50 μm.

[0038] Specifically, the spherical graphite used in the present electromagnetic wave absorber may be "Product Name CGB" manufactured by Nippon Graphite Co., Ltd. The physical properties of the graphite, such as the maximum length, particle diameter, and sphericity, can be measured by the same methods as those for the inorganic filler described below.

[0039] The electromagnetic wave absorber preferably contains 5 to 80% by mass of graphite, more preferably 7 to 65% by mass, and particularly preferably 10 to 50% by mass.

[0040] <Inorganic Filler> The inorganic filler of the present electromagnetic wave absorbing material has 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: 0.0≦x<1.0, 0.00≦y<0.05

[0041] The value of x in the formula (1) is usually 0.0 or more, preferably 0.05 or more, more preferably 0.08 or more, particularly preferably 0.10 or more, and is usually less than 1.0, preferably 0.80 or less, more preferably 0.50 or less, particularly preferably 0.30 or less. The value of x may be adjusted appropriately depending on the application and the desired crystal phase, and by setting the value of x within the above range, particles with excellent structural stability and a high dielectric constant can be provided. In particular, by setting the value of x to be greater than 0 and containing Zr, particles with excellent electromagnetic wave absorption ability can be obtained.

[0042] The value of y in the formula (1) is usually 0.00 or more, preferably 0.0001 or more, more preferably 0.0003 or more, particularly preferably 0.0005 or more, and is usually less than 0.05, preferably 0.03 or less, more preferably 0.01 or less. The value of y may be adjusted appropriately depending on the application and the desired crystal phase, and when the value of y is within the above range, a material having excellent structural stability and a high dielectric constant can be provided.

[0043] The values ​​of x and y in the formula (1) can be confirmed by a composition analysis method such as energy dispersive X-ray analysis (EDS), X-ray fluorescence analysis (XRF), or inductively coupled plasma (ICP).

[0044] In formula (1), M represents one or more alkaline earth metal elements, and preferably contains one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba, and more preferably contains Ba. From the viewpoint of synthesizing a composition that exhibits a high dielectric constant, the alkaline earth metal element M preferably contains one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba in an amount of 50 to 100 mol % relative to the total amount of M, and more preferably contains Ba in an amount of 70 to 100 mol % relative to the total amount of M. It is particularly preferable that M consists of Ba.

[0045] In formula (1), Q represents one or more transition metals, preferably including one or more elements selected from the group consisting of Hf, Mn, Fe, Ni and Zr, and more preferably including Zr.

[0046] In formula (1), RE represents one or more rare earth elements, and preferably includes 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 includes one or more elements selected from the group consisting of Yb, Sm, Nd, La, Eu, and Dy, and particularly preferably includes Yb. From the viewpoint of synthesizing a composition that exhibits a high dielectric constant, the rare earth element RE preferably 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 in an amount of 50 to 100 mol % relative to the total amount of RE, more preferably one or more elements selected from the group consisting of Yb, Sm, Nd, La, Eu, and Dy in an amount of 50 to 100 mol % relative to the total amount of RE, and particularly preferably 80 to 100 mol % of Yb relative to the total amount of RE.

[0047] The crystalline phase of the inorganic filler used in the present electromagnetic wave absorber is Yb-doped BaTi, where M is Ba, Q is Zr, RE is Yb, x is 0.15, and y is 0 in the formula (1). ０．８５ Zr ０．１５ O ３ is most preferred.

[0048] The particle diameter of the inorganic filler is not particularly limited, but is usually 10 nm or more, preferably 50 nm or more, more preferably 100 nm or more, and usually 500 μm or less, preferably 200 μm or less, more preferably 100 μm or less. If the particle diameter is above the above lower limit, primary particle aggregation is easily suppressed, thereby obtaining a sheet in which the particles are more uniformly dispersed. On the other hand, if the particle diameter is below the above upper limit, the frequency of electromagnetic waves entering the particles increases, thereby obtaining an electromagnetic wave absorbing material with higher electromagnetic wave absorption capacity. Here, the particle diameter of the inorganic filler can be expressed as the range between the minimum and maximum values ​​of the major axes of 10 randomly selected inorganic fillers measured under a scanning electron microscope. The major axis of the inorganic filler corresponds to the length of the point where the distance between the plates is greatest when the particle is sandwiched between two parallel plates. Note that when graphite is used in this electromagnetic wave absorbing material, the particle diameter of the graphite is preferably larger than the particle diameter of the inorganic filler. The ratio is not particularly limited, but for example, (graphite particle size / inorganic filler particle size) is preferably 2 or more, more preferably 4 or more, even more preferably 6 or more, still more preferably 15 or more, and particularly preferably 25 or more. In addition, it is generally preferable that the ratio is 500 or less.

[0049] 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. There is no particular upper limit for the sphericity, and the higher the better, but it is usually 1 or less. If the sphericity is equal to or higher than the above lower limit, it becomes easy to manufacture an electromagnetic wave absorbing material having high in-plane uniformity in electromagnetic wave absorbing ability.

[0050] The sphericity can be determined by measuring the perimeter of the inorganic filler by scanning electron microscope observation, with reference to the reference literature (John R. Grace and Ariane Ebneyamini, Particuology Volume 54, February 2021, Pages 1-4), and dividing the circumference of a circle having the same area as the inorganic filler by the perimeter of the inorganic filler.

[0051] The relative dielectric constant (ε ｒ) is usually 3,000 or more, preferably 5,000 or more, more preferably 10,000 or more, and particularly preferably 12,000 or more. ｒ ) can be measured by a conventional method, but in this specification, the value measured by the method described in the Examples below is used.

[0052] 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 below is used.

[0053] The electromagnetic wave absorbing material preferably contains 5% to 80% by mass of inorganic filler, more preferably 7% to 65% by mass, and particularly preferably 10% to 50% by mass. Furthermore, the mass ratio of inorganic filler to graphite in the electromagnetic wave absorbing material is preferably within the range of 10:90 to 90:10, more preferably within the range of 15:85 to 85:15, and particularly preferably within the range of 20:80 to 80:20.

[0054] <Other Components> The electromagnetic wave absorbing material may contain components other than graphite and inorganic filler, for example, a conductive filler. Examples of conductive fillers include fillers made of metal, conductive ceramics, conductive glass, conductive polymer, etc. A plurality of these may be contained. The particle size of the conductive filler is not particularly limited, but is usually 10 nm or more, preferably 50 nm or more, more preferably 100 nm or more, and usually 500 μm or less, preferably 200 μm or less, more preferably 100 μm or less.

[0055] The other components are contained in the electromagnetic wave absorber in an amount of preferably 10% by mass or less, more preferably 7% by mass or less, and particularly preferably 5% by mass or less.

[0056] <Polymer> The electromagnetic wave absorbing material contains a polymer in addition to the graphite, inorganic filler, etc. described above. The polymer serves as the matrix of the electromagnetic wave absorbing material, and examples thereof 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, as well as 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. The rubber material may be either natural rubber or synthetic rubber, or a mixture thereof.

[0057] Among these, rubber materials are preferred, nitrile rubber (NBR rubber), urethane rubber (PU rubber), and ethylene propylene rubber (EPDM) are more preferred, and nitrile rubber (NBR rubber) is particularly preferred.

[0058] The polymer content in the electromagnetic wave absorber is preferably 10% by mass or more, more preferably 20% by mass or more, and particularly preferably 30% by mass or more. There is no particular upper limit, but the polymer content is preferably 90% by mass or less, more preferably 80% by mass or less, and particularly preferably 70% by mass or less. When a rubber material is used as the polymer, the mass ratio of the inorganic filler to the rubber material is preferably within the range of 50:50 to 90:10, more preferably within the range of 52:48 to 85:15, and particularly preferably within the range of 55:45 to 80:20.

[0059] <Method for producing inorganic filler> The inorganic filler can be produced by a particle production method including at least a step of mixing raw materials including an M source, a Zr source, a Ti source, and an RE source, and a heat treatment step. In addition to the above steps, the method preferably includes one or more of a drying step and a pulverization step. By including these steps, agglomerated particles can be appropriately dispersed.

[0060] <Raw materials for inorganic filler> The raw materials for each of the elements, i.e., the M source, Zr source, Ti source, and RE source, are not particularly limited, and examples thereof include 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. Furthermore, the compounds of each of the elements may be hydrates, etc. The method for introducing oxygen (O) is not particularly limited, and oxygen can be introduced by using a compound containing an oxygen atom as a raw material for each element, or by performing heat treatment in an oxygen-containing atmosphere such as air.

[0061] <Method for Identifying the Composition of Inorganic Filler> The method for identifying the composition of the produced inorganic filler is not particularly limited, but examples include a method of identifying the composition by referring to data obtained by X-ray diffraction analysis in a database such as Pearson's Crystal Data.

[0062] <Step of Mixing Raw Materials> In the step of mixing the raw materials, the raw materials are usually used in powder form and mixed. The method of mixing the raw materials is not particularly limited, and they can be mixed wet or dry using ordinary equipment such as a mortar, a ball mill, or a jet mill, but wet mixing is preferred. The medium used in wet mixing can be one or more organic solvents such as water, ethanol, isopropyl alcohol, and methyl ethyl ketone, preferably water, and the amount of the medium used is preferably 30 to 60% by volume, particularly 40 to 50% by volume, based on the volume of the container.

[0063] In mixing the raw materials, the ratio of the sum of Ti and Zr elements to the M element in the raw materials used 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, more preferably 2.0x or less, where x is the M / (Ti+Zr) molar ratio of the target particles. By ensuring that the M / (Ti+Zr) molar ratio of the raw materials used is within the above range, a crystal phase of the target composition can be efficiently obtained.

[0064] <Heat Treatment Step> After the step of mixing the raw materials, heat treatment is performed to obtain particles containing a crystalline phase consisting 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 heat treatment in an oxygen-containing atmosphere such as air. The heat treatment temperature is preferably 1000°C or higher from the viewpoint of increasing reactivity. The upper limit of the heat treatment temperature is preferably below the melting point, glass transition temperature, or other phase transition temperature of the target compound, and is usually 1450°C or lower. The heating time for the heat treatment varies depending on the heating temperature, but is usually 1 hour or more and 20 hours or less.

[0065] <Pulverization Step> After the heat treatment step, the obtained particles may be pulverized. By pulverization, aggregated particles can be separated.

[0066] <Drying step> After the pulverization step, the particles may be dried to remove various solvents or adsorbed water. When the raw materials are wet-mixed, it is preferable to dry them after mixing and before the heat treatment step. The drying method is not particularly limited, but drying can be performed using a dryer, vacuum dryer, freeze dryer, or the like. The temperature atmosphere during drying is preferably 50°C or higher but lower than 300°C, which is a temperature sufficient to evaporate water.

[0067] <Method for producing the present electromagnetic wave absorbing material> The present electromagnetic wave absorbing material can be produced, for example, by mixing the above-mentioned graphite, inorganic filler, and polymer. A liquid solvent may be used during mixing, and the mixture may be dried to remove the solvent, followed by curing to form the present electromagnetic wave absorbing material. Examples of the solvent include water, organic solvents such as ethanol, isopropyl alcohol, and methyl ethyl ketone, and mixtures of these may also be used.

[0068] The mass ratio when mixing is preferably 10 to 90 parts by mass of polymer, 5 to 45 parts by mass of graphite, and 10 to 85 parts by mass of inorganic filler, and more preferably 10 to 40 parts by mass of graphite and 15 to 80 parts by mass of inorganic filler. The present electromagnetic wave absorber preferably uses nitrile rubber (NBR rubber) as the polymer, which allows the dielectric layer to be made thinner.

[0069] The electromagnetic wave absorbing material can be made into a shape and size suited to the intended use, for example, powder, granules, spheres, films, sheets, plates, panels, or other shapes. Among these, it is preferable to make an electromagnetic wave absorbing sheet using the electromagnetic wave absorbing material as a sheet-shaped dielectric layer. The sheet-shaped dielectric layer preferably has a specific gravity of 5 or less, more preferably 4 or less, and particularly preferably 3 or less. To obtain each shape, molding can be performed by, for example, extrusion molding, injection molding, press molding, spraying, coating, sintering and cutting, or other conventional methods. During or after molding, the material may be cured according to the curing method of the polymer used depending on the intended use.

[0070] When the present electromagnetic wave absorbing material is made into an electromagnetic wave absorbing sheet, it can be formed by forming the present electromagnetic wave absorbing material into a sheet and providing a conductive layer on one side thereof. Examples of the conductive layer include a metal-deposited sheet or film, a metal plate, a metal film, a metal mesh, a graphite sheet, a conductive polymer, and a glass substrate with a conductive film. These can be used alone or in combination of two or more. The conductive layer can be formed by coating, pasting, vapor deposition, or the like. Alternatively, the present electromagnetic wave absorbing material-forming mixture can be applied to the metal-deposited surface of a metal-deposited sheet or film that constitutes the conductive layer, and then cured to form an electromagnetic wave absorbing sheet.

[0071] 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 usually 100 μm or less, preferably 10 μm or less, more preferably 1 μm or less. The total thickness of the electromagnetic wave absorbing sheet is usually 10 μm or more, preferably 20 μm or more, more preferably 50 μm or more. Also, it is usually 10,000 μm or less, preferably 8,000 μm or less, more preferably 6,000 μm or less, and even more preferably 5,000 μm or less.

[0072] <Applications> The present electromagnetic wave absorbing material can be used to obtain, for example, an electromagnetic wave absorbing sheet that exhibits high electromagnetic wave absorption capacity and is thinner than conventional materials. This electromagnetic wave absorbing sheet can be used, for example, as a ceiling material to prevent malfunctions in ETC (Electronic Toll Collection) systems used at highway toll gates. ETC systems primarily use electromagnetic waves in the 5.8 GHz frequency band, which can cause errors in charging nearby vehicles due to reflections from the gate ceiling or other surfaces. However, the present electromagnetic wave absorbing material can be made into a thin and lightweight ETC electromagnetic wave absorbing sheet, making it easy to install on ceiling surfaces. The above-mentioned electromagnetic wave absorbing sheet can be used as a component for electronic toll collection systems to prevent malfunctions in such systems.

[0073] The electromagnetic wave absorbing sheet can also be used as an electromagnetic wave absorbing sheet for a shield case to prevent errors in IC chips caused by 38 to 76 GHz collision prevention radar used in automobile collision prevention systems, and can also be used as an automobile exterior component to be attached to the exterior of a four-wheeled automobile, etc. Furthermore, the electromagnetic wave absorbing sheet can also be used as an absorbing panel for an aiming test that is installed to prevent erroneous detection of the radar of an adjacent automobile in an aiming test to inspect collision prevention radar at a vehicle inspection center, etc.

[0074] Other uses of the electromagnetic wave absorbing sheet include satellite communication components for covering terminals that communicate with satellites.

[0075] An embodiment of the present invention will be described below, but the present invention is not limited to this embodiment.

[0076] Electromagnetic wave absorbers of Examples 1 to 3 and Comparative Example 1 shown below were prepared.

[0077] [Example 1] <Preparation of inorganic filler> The composition of the feed was BaTi ０．８５ Zr ０．１５ O ３－０．００１ 24.7 kg of barium carbonate (Sakai Chemical Industry Co., Ltd. "BW-KT"), 8.49 kg of zirconium oxide (Nippon Denko Corporation "PCS"), 2.31 kg of titanium oxide (Ishihara Sangyo Kaisha, Ltd. "PT-501R"), and 0.0246 kg of ytterbium oxide (Fujifilm Wako Pure Chemical Industries, Ltd., 98.0%+%) were mixed in a V-type mixer and then calcined in a cordierite crucible at 1300 °C under atmospheric conditions for 4 hours. The resulting powder was analyzed by X-ray diffraction (copper target, tube voltage 45 kV, tube current 30 mA, measurement angle (2Θ): 10-115, step angle: 0.017°, average time / step: 10.15 s). Identification using Pearson's Crystal Data revealed that the resulting phase was a single phase with a barium titanate phase peak shifted by zirconium. The X-ray diffraction chart is shown in Figure 1. Furthermore, observation with a scanning electron microscope confirmed that the particles had a particle diameter of 800 nm to 4 μm. A scanning electron microscope photograph is shown in Figure 2.

[0078] <Preparation of Electromagnetic Wave Absorbing Sheet> 38.1 parts by mass of acrylonitrile-butadiene rubber (ENEOS Materials, NBR N230S), 0.4 parts by mass of stearic acid (Kanto Chemical, purity 95.0+%), 1.9 parts by mass of zinc oxide (Kanto Chemical, purity 99.0+%), 0.2 parts by mass of sulfur (Kanto Chemical, purity 98.0+%), 0.4 parts by mass of N,N'-diethylthiourea (Tokyo Chemical Industry, purity 98.0+%), 44 parts by mass of the inorganic filler, and 15 parts by mass of graphite powder (Fujifilm Wako Pure Chemical Industries, Ltd., purity 98.0+%) were mixed using a two-roll mill (DY6-15, Daihan Co., Ltd.) to obtain a rubber compound. The graphite powder had a particle diameter of 24.5 μm and an aspect ratio of 1.3. Several sheets of this rubber compound were stacked and pressed using a 70 ton press at 10 MPa, 180°C, and 25 minutes to form a dielectric layer with a thickness of 1.96 mm. The specific gravity of the dielectric layer was 1.85 g / cm. ３ It was.

[0079] This dielectric layer was 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 produce an electromagnetic wave absorbing sheet. For the electromagnetic wave absorbing sheet thus obtained, the electromagnetic wave absorptivity, reflectivity, and transmittance of the dielectric layer in the range of 5.6 to 8.2 GHz were measured using the free space method. The absorption at 5.8 GHz was −25 dB, and −44 dB at the absorption peak of 5.7 GHz. The absorption measurement results are shown in Figure 3.

[0080] [Example 2] <Preparation of electromagnetic wave absorbing sheet> A rubber compound was obtained in the same manner as in Example 1, except that the graphite powder in Example 1 was changed to spherical graphite (CGB-20 manufactured by Nippon Graphite Co., Ltd.). The spherical graphite had a particle diameter of 22.0 μm and a sphericity of 0.9. This rubber compound was molded in the same manner as in Example 1 to form a dielectric layer with a thickness of 1.89 mm. The specific gravity of the dielectric layer was 1.83 g / cm ３The dielectric layer was laminated with an aluminum-deposited PET film in the same manner as in Example 1 to produce an electromagnetic wave absorbing sheet. For the electromagnetic wave absorbing sheet thus obtained, the electromagnetic wave absorptivity, reflectivity, and transmittance of the dielectric layer were measured in the range of 12.4 to 45.0 GHz using the free space method. The absorption amount and frequency at the absorption peak were -39.8 dB at 20.3 GHz and -13.7 dB at 34.0 GHz. When the sheet was rotated 90 degrees, the absorption amount shifted to -33.1 dB at a frequency of 20.6 GHz and -14.7 dB at a frequency of 34.5 GHz. The absorption rate measurement results are shown in Figure 4.

[0081] [Example 3] <Preparation of Electromagnetic Wave Absorbing Sheet> An electromagnetic wave absorbing sheet having a dielectric layer with a thickness of 1.93 mm was prepared in the same manner as in Example 1, except that the graphite powder in Example 1 was changed to flat graphite (manufactured by Nippon Graphite Co., Ltd., product name CPB: aspect ratio 2.1). The specific gravity of the sheet was 1.85 g / cm ３ The electromagnetic wave absorption rate, reflectance, and transmittance of the dielectric layer of the electromagnetic wave absorbing sheet obtained in this manner were measured using the free space method in the frequency range of 12.4 to 45.0 GHz. The absorption amounts were -11.8 dB at 16.4 GHz, -7.3 dB at 27.3 GHz, and -5.4 dB at 38.6 GHz. When the sheet was rotated 90 degrees, the absorption amounts shifted to -12.0 dB at 17.4 GHz, -8.3 dB at 29.3 GHz, and -6.9 dB at 41.2 GHz. The absorption rate measurement results are shown in Figure 4.

[0082] Comparative Example 1 Preparation of Electromagnetic Wave Absorbing Sheet 23 parts by mass of a urethane resin solution ("Takelac TE-5899" manufactured by Mitsui Chemicals, Inc., solid content concentration 30 parts by mass), 92.07 parts by mass of the inorganic filler, and 0.93 parts by mass of conductive fiber ("WK-500" manufactured by Otsuka Chemical Co., Ltd.) were mixed to obtain a mixed slurry. Dentol ("WK-500" manufactured by Otsuka Chemical Co., Ltd.) is a conductive fibrous particle (average fiber diameter 350 nm, average fiber length 5 to 15 μm, resistivity 10) in which conductivity is imparted to potassium titanate fiber. １－２This mixed slurry was applied using a 500 μm thick film applicator onto a release film (Mitsui Chemicals Tohcello Inc.'s "SP-PET (registered trademark) O3-BU" (100 μm) consisting of a polyester film and a silicone-based release layer, and then dried and cured at 80°C for 3 hours or more to obtain a rubber compound. This rubber compound was molded in the same manner as in Example 1-2 to form a dielectric layer with a thickness of 1.85 mm. The specific gravity of the dielectric layer was 4.32 g / cm ３ This dielectric layer was attached to an aluminum-deposited PET film in the same manner as in Example 1-2 to prepare an electromagnetic wave absorbing sheet.

[0083] For the electromagnetic wave absorbing sheet thus obtained, the electromagnetic wave absorptivity, reflectivity, and transmittance of the dielectric layer in the range of 5.6 to 8.2 GHz were measured by the free space method. The absorption at the absorption peak of 5.8 GHz was -25 dB. The measurement results of the absorptivity are shown in Figure 3.

[0084] <Reliability Test> A reliability test was conducted on the sheet of Example 1 and the sheet of Comparative Example 1 using a thermo-hygrostat (PR-2KP manufactured by Espec). The test conditions were a temperature of 85°C and a relative humidity of 85% RH for 100 hours, and the change in thickness after the test was confirmed. In addition, the sheet of Example 1 and the sheet of Comparative Example 1, each measuring 2.5 cm x 5.0 cm, were subjected to a bending test using a test method (winding method, inner radius 10 mm) in accordance with JIS Z 2248 to check for changes in appearance. While there was no change in appearance in Example 1, fine cracks were observed on the surface of the comparative example.

[0085] <Results> From Figure 3, it was confirmed that the electromagnetic wave absorbing sheet of Example 1, which used the electromagnetic wave absorbing material of the present invention, had a very high electromagnetic wave absorption rate in the range of 5.6 to 8.2 GHz. In addition, it was lightweight because it contained graphite, and was flexible because the filler loading amount was less than that of the sheet containing Dentol. In contrast, it was confirmed that the electromagnetic wave absorbing sheet of Comparative Example 1, which did not contain graphite, had a low electromagnetic wave absorption rate, and because the filler mass portion was large, it had a high specific gravity and was less flexible than Example 1.

[0086] 4, it was found that the electromagnetic wave absorbing sheet of Example 2, which used the electromagnetic wave absorbing material of the present invention, had a small frequency peak shift relative to the sheet arrangement direction. In contrast, it was confirmed that the electromagnetic wave absorbing sheet of Example 3, which contained flake graphite, had a large frequency peak shift when the sheet arrangement direction was changed.

[0087] When the thickness of the electromagnetic wave absorbing sheet was evaluated after the reliability test, the sheet described in Example 1 showed a thickness change of 0.1% or less, while the sheet described in Comparative Example 1 showed a thickness increase of 4.9%.

Claims

1. An electromagnetic wave absorbing material comprising graphite and 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: 0.0≦x<1.0 0.00≦y<0.05) 2. The electromagnetic wave absorbing material according to claim 1, wherein the graphite is flake graphite.

3. The electromagnetic wave absorbing material according to claim 1, wherein the graphite is spheroidized graphite.

4. An electromagnetic wave absorbing material according to any one of claims 1 to 3, wherein the mass ratio of the inorganic filler to the graphite is within the range of 10:90 to 90:

10.

5. The electromagnetic wave absorber according to claim 1, wherein the particle size of the graphite is 100 nm or more and 500 μm or less.

6. The electromagnetic wave absorber according to claim 1, further comprising a rubber material.

7. An electromagnetic wave absorbing material comprising an inorganic filler having a crystalline phase represented by the following formula [1] and a rubber material. 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 metals, 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) 8. The electromagnetic wave absorbing material according to claim 6 or 7, wherein the rubber material is one or more selected from the group consisting of nitrile rubber (NBR rubber), urethane rubber (PU), and ethylene propylene rubber (EPDM).

9. The electromagnetic wave absorbing material according to claim 6 or 7, wherein the mass ratio of said inorganic filler to said rubber material is within the range of 50:50 to 90:

10.

10. The electromagnetic wave absorber according to claim 1 or 7, wherein the particle diameter of the inorganic filler is 10 nm or more and 500 μm or less.

11. The electromagnetic wave absorber according to claim 1 or 6, further comprising a conductive filler.

12. The electromagnetic wave absorber according to claim 1 or 7, wherein Q comprises one or more elements selected from the group consisting of Hf, Mn, Fe, Ni and Zr.

13. The electromagnetic wave absorber according to claim 1 or 7, wherein Q comprises Zr.

14. The electromagnetic wave absorber according to claim 1 or 7, wherein M comprises one or more elements selected from the group consisting of Mg, Ca, Sr and Ba.

15. The electromagnetic wave absorber according to claim 1 or 7, wherein M includes Ba.

16. The electromagnetic wave absorber according to claim 1 or 7, wherein said RE comprises 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.

17. An electromagnetic wave absorbing sheet, which is the electromagnetic wave absorbing material according to claim 1 or 7, formed into a sheet having a thickness of 50 μm or more and 10,000 μm or less.

18. The electromagnetic wave absorbing sheet according to claim 17, which has a conductive layer on one side of the sheet.

19. A component for an electronic toll collection system using the electromagnetic wave absorbing material according to claim 1 or 7.

20. An automobile exterior component using the electromagnetic wave absorbing material according to claim 1 or 7.

21. A satellite communication component using the electromagnetic wave absorbing material according to claim 1 or 7.

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