Electromagnetic wave absorbing material

Abstract

To provide an electromagnetic wave absorbing material that can be used as an electromagnetic wave absorbing material for an electronic component used in millimeter wave radar and exhibits absorption ability in a high frequency band.SOLUTION: An electromagnetic wave absorbing material includes an inorganic filler and having a crystalline phase expressed by the following formula [1]. MQxTi1-xO3-yRE [1]. (in the formula [1], M indicates one or more alkaline earth metal elements, RE indicates one or more rare earth elements, and x and y satisfy 0.0≤x<1.0 and 0.00≤y<0.05).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic wave absorbing material having a crystalline phase containing rare earth-added titanate (zirconate) and alkaline earth metals. [Background technology]

[0002] Next-generation communications such as 5G and 6G are expected to enable high-speed and high-capacity information communication by utilizing electromagnetic waves in the GHz band, which have higher frequencies than conventionally used electromagnetic waves. Furthermore, millimeter-wave radar used in automobile 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 purposes is expected to increase dramatically. With the spread of such high-frequency communications and the increasing density of electronic devices, there are concerns about increased noise, interference, and self-poisoning due to the reception of unwanted electromagnetic waves within devices. As a countermeasure against these unwanted electromagnetic waves, there is a need for electromagnetic shielding materials that can be used in the GHz band.

[0003] For applications such as shielding unwanted electromagnetic waves in increasingly miniaturized and high-density electronic devices and millimeter-wave radar, absorbing shielding materials that convert the energy of incident electromagnetic waves into thermal energy and attenuate them through resistive loss, dielectric loss, or magnetic loss are more suitable than reflective shielding materials such as metal plates that reflect incident electromagnetic waves.

[0004] In conventional applications up to the MHz band, absorbent shielding materials have typically used composite materials containing ferrite-based magnetic materials and resins. However, these conventional MHz-band absorbent shielding materials have low absorption capacity in the GHz band. Therefore, to obtain sufficient absorption performance, the thickness of the molded body must be increased, making it difficult to implement in devices. Therefore, there is a need to develop new electromagnetic wave absorbers that can be made into thin films even in high-frequency ranges such as the GHz band.

[0005] As a new electromagnetic wave absorber for the GHz band, high dielectric inorganic materials with large dielectric loss are being investigated. For example, ferroelectric materials such as barium titanate have a high relative permittivity and tend to absorb a large amount of electromagnetic wave energy as dielectric loss. Therefore, when evaluated as a sintered body, it has been confirmed that they exhibit radio wave attenuation of -20 dB or more in certain GHz bands (Patent Document 1). Furthermore, instead of using a sintered body, which is difficult to process after post-processing, it has also been proposed to use it as 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 (Patent Document 2).

[0006] When imparting high dielectric properties to a matrix by adding dielectric fillers, it is desirable that the filling density of the high dielectric fillers be 65% by volume or more (Patent Document 3). Below this level, the dielectric constant shifts towards the resin side, which may result in insufficient absorption capacity. This is thought to be because, assuming the composite is an equivalent circuit of a resin with low capacitance and a filler with high capacitance, the dielectric constant is higher in a parallel circuit where the fillers form a continuous layer than in a series circuit where the fillers are isolated and dispersed. However, even when a large amount of dielectric filler is added, the continuity of the filler may decrease due to factors such as particle shape, impurities, and dispersibility. Therefore, in order to obtain high absorption capacity, it is necessary to devise a method for reliable continuous layer formation between fillers. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-306698 [Patent Document 2] Japanese Patent Publication No. 2019-4003 [Patent Document 3] Japanese Patent Publication No. 2001-237507 [Overview of the project] [Problems that the invention aims to solve]

[0008] The object of this invention is to provide an electromagnetic wave absorbing material that can be used as an electromagnetic wave absorbing material for electronic components used in millimeter-wave radar and that exhibits absorption capabilities in the high-frequency band. [Means for solving the problem]

[0009] As a result of diligent research, the inventors of the present invention have discovered that an electromagnetic wave absorbing material containing an inorganic filler and having a crystalline phase represented by a specific formula exhibits equivalent or superior electromagnetic wave absorption capabilities compared to an electromagnetic wave absorbing material of the same structure containing a barium titanate-based composite metal compound, despite being thinner and having a lower inorganic dielectric filler filling rate. This has led to the achievement of the present invention.

[0010] In other words, the present invention includes at least the following:

[0011] <1> An electromagnetic wave absorbing material containing inorganic fillers and having a crystalline phase represented by the following formula [1]. MQ x Ti 1-x O3-yRE …[1] (In formula [1], M represents one or more alkaline earth 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) <2> The inorganic filler includes a conductive filler. <1> The electromagnetic wave absorbing material described above. <3> The mass ratio of conductive filler in the inorganic filler is 5% or more and 95% or less. <2> The electromagnetic wave absorbing material described above. <4> The aforementioned Q includes one or more elements selected from the group consisting of Hf, Mn, Fe, Ni, and Zr. <1> ~ <3> An electromagnetic wave absorbing material as described in any of the following. <5> The aforementioned Q includes Zr, <4> The electromagnetic wave absorbing material described above. <6> The aforementioned M comprises one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba. <1> ~ <5> An electromagnetic wave absorbing material as described in any of the following. <7> The above M includes Ba, <6> The electromagnetic wave absorbing material described above. <8> The electromagnetic wave absorber according to any one of <1> to <7>, 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. <9> The electromagnetic wave absorber according to any one of <1> to <8>, comprising a composite material containing the inorganic filler and the composite of the crystal phase and the polymer. <10> The electromagnetic wave absorber according to <9>, wherein the mass ratio of the inorganic filler in the composite material is 5% or more and 95% or less. <11> The electromagnetic wave absorber according to <9> or <10>, wherein the composite material is in a sheet shape with a thickness of 50 μm or more and 1000 μm or less. <12> The electromagnetic wave absorber according to <11>, having a conductive layer on one side of the sheet-shaped composite material.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide an electromagnetic wave absorber that can be used as an electromagnetic wave absorber for electronic components used in millimeter-wave radars and exhibits absorption ability in a high-frequency band.

Brief Description of the Drawings

[0013] [Figure 1] It is an X-ray diffraction pattern of the powder produced in Example 1. [Figure 2] It is a scanning electron micrograph of the powder produced in Example 1. [Figure 3] It is a scanning electron micrograph of the cross section of the electromagnetic wave absorption layer formed in Example 1. [Figure 4] It is a graph showing the electromagnetic wave absorption rates of the electromagnetic wave absorption layers formed in Example 1, Comparative Example 1, and Comparative Example 2. [Figure 5] It is a scanning electron micrograph of the cross section of the electromagnetic wave absorption layer formed in Comparative Example 1.

Embodiments for Carrying Out the Invention

[0014] The embodiments for carrying out the present invention will be described in detail below. However, the present invention is not limited to the embodiments described below, and can be implemented in various ways within the scope of its gist.

[0015] [Electromagnetic wave absorbing material] The electromagnetic wave absorbing material of the present invention is an electromagnetic wave absorbing material that contains an inorganic filler and has a crystalline phase represented by the following formula [1]. MQ x Ti 1-x O3-yRE …[1] (In formula [1], M represents one or more alkaline earth 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) In the above formula [1], Zr, Ti, and O represent zirconium, titanium, and oxygen, respectively.

[0016] In one embodiment of the present invention, the electromagnetic wave absorbing material of the present invention includes, as an inorganic filler, particles having a crystalline phase represented by the above formula [1] (hereinafter sometimes referred to as "particles of the present invention"), and preferably includes the particles of the present invention and a conductive filler such as conductive fibrous particles or conductive spherical particles, preferably conductive fibrous particles.

[0017] The electromagnetic wave absorbing material of the present invention includes, for example, the particles of the present invention, which are a single phase of barium titanate (zirconate) with rare earth elements added, represented by formula [1], and conductive fibrous particles as inorganic fillers. When an equivalent circuit of a polymer with low capacitance and a filler with high capacitance is assumed, the filler and polymer form a parallel circuit in a continuous layer, thereby improving the dielectric constant of the composite material. This results in an effect that exhibits an electromagnetic wave absorption rate equivalent to or greater than that of a dielectric layer of single phase particles of barium titanate (zirconate) with rare earth elements added or conductive fibrous particles alone, even when the thickness is thinner.

[0018] [Particles of the present invention] The value of x in formula [1], which represents the crystalline phase of the particles of the present invention, is usually 0.0 or greater, preferably 0.05 or greater, more preferably 0.08 or greater, even more preferably 0.10 or greater, and usually less than 1.0, preferably 0.80 or less, more preferably 0.50 or less, and even more preferably 0.30 or less. The value of x may be adjusted as appropriate depending on the application and the desired crystalline phase. By having x within the above range, particles with excellent structural stability and high dielectric constant can be provided. In particular, by having x greater than 0 and including Zr, particles with excellent electromagnetic wave absorption ability can be obtained.

[0019] The value of y in formula [1] is usually 0.00 or greater, preferably 0.0001 or greater, more preferably 0.0003 or greater, even more preferably 0.0005 or greater, and usually less than 0.05, preferably 0.03 or less, and more preferably 0.01 or less. The value of y may be adjusted as appropriate depending on the application and the desired crystal phase, and by having y within the above range, it is possible to provide a material with excellent structural stability and a high dielectric constant.

[0020] 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).

[0021] 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.

[0022] In formula [1], RE represents one or more rare earth elements, preferably 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 one or more elements selected from the group consisting of Yb, Sm, Nd, La, Eu, and Dy, and even more preferably 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 even more preferably 80 to 100 mol% of Yb relative to the total amount of RE.

[0023] The particle size of the particles of the present invention 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 a sheet with higher electromagnetic wave absorption capacity can be obtained. Here, the particle diameter of the particles of the present invention corresponds to the major axis of the particles observed using a scanning electron microscope. Here, the major axis of the particle corresponds to the length at the point where the distance between two parallel plates is greatest when the particle is sandwiched between them.

[0024] The sphericity of the particles of the present invention is usually 0.1 or higher, preferably 0.3 or higher, more preferably 0.5 or higher, and even more preferably 0.7 or higher. There is no particular upper limit to the sphericity; the higher the better, but it is usually 1 or lower. If the sphericity is above the lower limit mentioned above, it becomes easier to produce sheets with high in-plane uniformity in electromagnetic wave absorption capacity.

[0025] In this specification, sphericity is determined by measuring the perimeter of a particle using a scanning electron microscope, based on the reference (John R. Grace and Arian Ebneyamini, Particuology Volume 54, February 2021, Pages 1-4), and dividing the circumference of a circle with the same area as the particle by the perimeter of the particle.

[0026] The relative permittivity (ε) of the particles of the present invention r The number is usually 3,000 or more, preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 12,000 or more. Relative permittivity (ε r Although it can be measured by conventional methods, in this specification, the values ​​measured by the method described in the examples below are adopted.

[0027] The dielectric loss tangent (tanδ) of the particles of the present invention is usually 0.01 or higher, preferably 0.03 or higher, more preferably 0.05 or higher, and even more preferably 0.08 or higher. The dielectric loss tangent (tanδ) can be measured by conventional methods, but in this specification, the value measured by the method described in the examples below will be adopted.

[0028] [Method for manufacturing particles] The particles of the present invention 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. The particle manufacturing method of the present invention preferably includes, in addition to the above-mentioned steps, one or more of a drying step and a grinding step. By including these steps, the aggregated particles can be properly dispersed.

[0029] <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 as raw materials for each element, or by performing heat treatment in an oxygen-containing atmosphere such as air.

[0030] <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 20 to 80% by volume relative to the container volume, and particularly preferably 25 to 75% by volume.

[0031] 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.

[0032] <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.

[0033] <Grinding process> The resulting particles may be pulverized after the heat treatment process. Pulverization allows for the separation of particles that have undergone aggregation.

[0034] <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.

[0035] [Conductive filler] The raw materials for conductive fibrous particles suitable as conductive fillers included as inorganic fillers in the electromagnetic wave absorbing material of the present invention are not particularly limited, and examples include metals, carbon materials (graphite, carbon nanotubes, graphene, etc.), conductive ceramics, conductive glass, and conductive polymers. These can be used individually or in combination of two or more.

[0036] The average fiber diameter of the conductive fibrous particles is usually 50 nm or more, preferably 70 nm or more, more preferably 100 nm or more, and usually 2000 nm or less, preferably 1000 nm or less, more preferably 500 nm or less. If the average fiber diameter of the conductive fibrous particles is at least the above lower limit, reduction of the fiber length due to excessive crushing of the conductive fibrous particles during kneading can be suppressed, so that the contact points with the electromagnetic wave absorber increase, and the probability of forming a conductive path tends to improve. If the average fiber diameter of the conductive fibrous particles is at most the above upper limit, the specific surface area becomes small, so that the contact points with the electromagnetic wave absorber increase, and the probability of forming a conductive path tends to improve.

[0037] Also, the lower limit of the average fiber length of the conductive fibrous particles is usually 1 μm or more, preferably 2 μm or more, more preferably 5 μm or more, and the upper limit is not particularly limited, but is usually 30 μm or less. If the average fiber length of the conductive fibrous particles is at least the above lower limit, the contact points with the electromagnetic wave absorber increase, and the probability of forming a conductive path tends to improve.

[0038] Here, the average fiber diameter and average fiber length of the conductive fibrous particles are values obtained by averaging the major axis and minor axis of the conductive fibrous particles measured on a scanning electron microscope image. However, for commercially available conductive fibrous particles, catalog values can be adopted.

[0039] The resistivity of the conductive fibrous particles used in the present invention is usually 5×10 2 Ω·cm or less, preferably 3×10 2 Ω·cm or more, more preferably 2×10 2 Ω·cm or more, still more preferably 1×10 2 Ω·cm or less. If the resistivity is at most the above upper limit, it is sufficient to form a conductive path between the particles. There is no particular limitation on the lower limit of the resistivity of the conductive fibrous particles, and the lower the better.

[0040] [Content ratio of conductive filler] The content ratio of conductive fillers, such as conductive fibrous particles, contained in the inorganic filler of the electromagnetic wave absorbing material of the present invention is preferably 5 to 95% by mass, particularly 10 to 90% by mass, especially 15 to 70% by mass, and most preferably 20 to 50% by mass, based on 100% by mass of the inorganic filler. Therefore, when the electromagnetic wave absorbing material of the present invention contains the particles of the present invention and conductive fillers such as conductive fibrous particles as inorganic fillers, the content ratio of the particles of the present invention and conductive fillers in the electromagnetic wave absorbing material of the present invention is preferably 5 to 95% by mass, particularly 10 to 90% by mass, especially 15 to 70% by mass, and most preferably 20 to 50% by mass of the conductive filler relative to 100% by mass of the inorganic filler, and 95 to 5% by mass, particularly 90 to 10% by mass, especially 85 to 30% by mass, and most preferably 80 to 50% by mass of the particles of the present invention. If the content ratio of conductive fillers in the inorganic filler is above the lower limit and the content ratio of the particles of the present invention is below the upper limit, an electromagnetic wave absorbing sheet exhibiting high electromagnetic wave absorption capacity can be obtained, and if the content ratio of conductive fillers is below the upper limit and the content ratio of the particles of the present invention is above the lower limit, an electromagnetic wave absorbing sheet with low electromagnetic wave reflectivity can be produced.

[0041] The electromagnetic wave absorbing material of the present invention may be provided as a mixture in which the particles of the present invention and a conductive filler are pre-mixed, or they may be provided separately and mixed at the time of use.

[0042] [Polymer] The electromagnetic wave absorbing material of the present invention may further include a composite material containing a polymer (hereinafter sometimes referred to as "the composite material of the present invention"), preferably a composite material containing the particles of the present invention, a conductive filler, and a polymer. The polymers used in such composite materials are not particularly limited and include synthetic resins (including thermoplastic elastomers) such as acrylic resins, ethylene vinyl acetate copolymers (EVA), polyvinyl chloride, polyurethane, acrylic urethane resins, ionomers, polyolefins, polypropylene, polyethylene, silicone resins, polyesters, polystyrene, polyimide, polyamide, polysulfone, polyethersulfone, and epoxy resins, or synthetic rubbers such as polyisoprene rubber, polystyrene-butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile butadiene rubber, butyl rubber, acrylic rubber, ethylene propylene rubber, and silicone rubber. These can be used individually or in combination of two or more.

[0043] The method for mixing the aforementioned polymer with the inorganic filler according to the present invention (preferably the particles and conductive filler of the present invention) is not particularly limited, and for example, manual mixing or the use of a mixer can be employed.

[0044] When the electromagnetic wave absorbing material of the present invention is used as a composite material of a polymer and an inorganic filler (preferably the particles and conductive filler of the present invention), the proportion of the inorganic filler (preferably the particles and conductive filler of the present invention) in the composite material is preferably 5 to 95% by mass, particularly 10 to 90% by mass, and especially 20 to 85% by mass. If the content of the inorganic filler (preferably the particles and conductive filler of the present invention) in the composite material is above the lower limit, the expression of electromagnetic wave absorption ability is excellent, and if it is below the upper limit, the application and molding of the slurry after kneading becomes easy.

[0045] The shape of such composite materials is not particularly limited, and their shape and size can be changed according to the application. For example, they can take the form of powder, granules, spheres, rectangles, sheets, or other shapes. The size of the composite material is not particularly limited, as long as it is larger than the inorganic filler. Furthermore, this composite material can be molded by extrusion molding, injection molding, press molding, spraying, coating, sintering and machining, or other conventional methods, depending on the shape, composition, and application of the composite material. Additionally, during or after molding, the material may be cured according to the curing method of the polymer used, depending on the application.

[0046] [Sheet-shaped composite materials] The composite material of the present invention is preferably formed into a sheet shape and used as an electromagnetic wave absorbing layer.

[0047] In this case, the lower limit of the thickness of the sheet-shaped composite material is usually 1 μm or more, preferably 10 μm or more, more preferably 50 μm or more, and the upper limit is not particularly limited, but is usually 1000 μm or less, preferably 800 μm or less, and more preferably 600 μm or less. If the thickness is above the above lower limit, good electromagnetic wave absorption performance will be exhibited.

[0048] [Conductive layer] In another embodiment, the electromagnetic wave absorbing material of the present invention may be a laminate in which a conductive layer is laminated on one side of an electromagnetic wave absorbing layer made of a sheet-shaped composite material (hereinafter sometimes referred to as "the laminate of the present invention").

[0049] The raw materials for the conductive layer used in the laminate of the present invention are not particularly limited and include, for example, metal vapor-deposited sheets (including films), metal plates, metal films, metal meshes, graphite sheets, conductive polymers, and glass substrates with conductive films. These can be used individually or in combination of two or more types. Furthermore, the above-mentioned metal vapor-deposited sheet (including film) and conductive film-coated glass substrate include a sheet (film) or glass substrate as both the conductive layer and the substrate. In other words, the laminate of the present invention may be a laminate of the composite material sheet of the present invention as a dielectric layer, a conductive layer, and a substrate. Other layers, such as a reflective layer, may also be laminated to the laminate of the present invention.

[0050] These conductive layers can be formed by coating, bonding, vapor deposition, etc. Furthermore, a laminate of the present invention can be formed by applying the composite material forming mixture of the present invention to the metal vapor-deposited surface of a metal vapor-deposited sheet (including a film) constituting the conductive layer and curing it.

[0051] The thickness of the conductive layer is not particularly limited, but is usually 1 nm or more, preferably 2 nm or more, more preferably 5 nm or more, and usually 1000 μm or less, preferably 800 μm or less, more preferably 600 μm or less.

[0052] The laminate of the present invention can be used as an electromagnetic wave absorber in millimeter-wave radar and the like used in automobile collision avoidance systems. [Examples]

[0053] The present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0054] [Example 1] <Particle manufacturing> 8.46 g of barium carbonate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., special grade, 99.0+%), 0.79 g of zirconium oxide (manufactured by Shin-Nippon Denko Co., Ltd., "PC90"), 2.91 g of titanium dioxide (manufactured by Ishihara Sangyo Co., Ltd., "CR-93"), and 0.008 g of ytterbium oxide (manufactured by Furuuchi Chemical Co., Ltd., "78211A") were wet-mixed in 50 mL of water for 6 hours using a 250 mL ball mill, and the mixed raw material powder was dried at 120 °C. The obtained mixed raw material powder was calcined in an alumina crucible at 1150 °C in an air atmosphere for 2 hours. 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 13 μm. A scanning electron microscope image is shown in Figure 2. Furthermore, when the dielectric constant of the powder at around 1 GHz was measured using the perturbation cavity resonance method, the relative dielectric constant was found to be 1.2 × 10⁻⁶. 4 The dielectric loss tangent was 0.094.

[0055] <Manufacturing of laminates> A silicone resin (KR-470, manufactured by Shin-Etsu Chemical Co., Ltd.) and an initiator (TA-100, manufactured by Sunapro Co., Ltd.) were mixed in proportions of 98.5% by mass and 1.5% by mass, respectively. Further mixing of the mixed resin, the powder produced above, and Dentol (WK-500, manufactured by Otsuka Chemical Co., Ltd.) in proportions of 30% by mass, 52.5% by mass, and 17.5% by mass, respectively, was performed to obtain a mixed slurry. A mixed slurry was applied to an aluminum-deposited PET film (substrate thickness 12 μm, deposition film thickness approximately 30 nm) using a 500 μm thick film applicator, and dried and cured at 80°C for more than 3 hours to form an electromagnetic wave absorbing layer made of a sheet-shaped composite material, thereby producing a laminate with a thickness of 239 μm. Furthermore, Dentol (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) in which conductivity is imparted to potassium titanate fibers. 1-2 It is Ω·cm. Figure 3 shows a backscattered electron scanning electron microscope image of the cross-section of the electromagnetic wave absorbing layer of this laminate. From the binarization of the scanning electron microscope image, 10 barium zirconate titanate particles were randomly selected from within the electromagnetic wave absorbing layer, and their sphericity was measured. The average value was 0.758. For the laminate obtained in this manner, the electromagnetic wave absorption rate, reflectance, and transmittance of the electromagnetic wave absorption layer at 45-110 GHz were measured using the free-space method. The transmittance was 0%. The measurement results for the absorption rate are shown in Figure 4.

[0056] [Comparative Example 1] The dielectric constant of barium titanate powder (Sigma-Aldrich, particle size <100 nm, purity ≥99%) at around 1 GHz was measured using the same method as in Example 1, and the relative dielectric constant was found to be 1.2 × 10⁻⁶. 3 The dielectric loss tangent was 0.059. An electromagnetic wave absorbing layer was formed on an aluminum-deposited PET film in the same manner as in Example 1, except that this barium titanate powder was used instead of the particles produced in Example 1, to produce a laminate with a thickness of 357 μm. Figure 5 shows a backscattered electron scanning electron microscope image of the cross-section of this electromagnetic wave absorption layer. For the laminate obtained in this manner, the electromagnetic wave absorption rate, reflectance, and transmittance of the electromagnetic wave absorption layer at 45-110 GHz were measured using the free-space method. The transmittance was 0%. The measurement results for the absorption rate are shown in Figure 4.

[0057] [Comparative Example 2] In Example 1, an electromagnetic wave absorbing layer was formed on an aluminum-deposited PET film in the same manner as in Example 1, except that Dentol (WK-500, manufactured by Otsuka Chemical Co., Ltd.) was not used, and a silicone resin (KR-470, manufactured by Shin-Etsu Chemical Co., Ltd.) and an initiator (TA-100, manufactured by Sunapro Co., Ltd.) were mixed in a ratio of 98.5% by mass and 1.5% by mass, respectively, and then the mixed resin and the powder produced in Example 1 were mixed in a ratio of 30% by mass and 70% by mass, respectively, to produce a laminate with a thickness of 325 μm. For the laminate obtained in this manner, the electromagnetic wave absorption rate, reflectance, and transmittance of the electromagnetic wave absorption layer at 45-110 GHz were measured using the free-space method. The transmittance was 0%. The measurement results for the absorption rate are shown in Figure 4.

[0058] Figure 4 shows that the laminate of Example 1 using the electromagnetic wave absorbing material of the present invention has a very high electromagnetic wave absorption rate in the 70-80 GHz range. In contrast, Comparative Example 1, which does not use the particles of the present invention, and Comparative Example 2, which does not use conductive fibrous particles, exhibit low electromagnetic wave absorption rates.

Claims

1. An electromagnetic wave absorbing material comprising, as an inorganic filler, particles having a crystalline phase represented by the following formula [1]. MQ ｘ Till １－ｘ Oh ３ -]RE …[1] (In formula [1], M represents one or more alkaline earth metal elements, RE represents one or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Q contains one or more elements selected from the group consisting of Hf, Mn, Fe, Ni, and Zr. x and y satisfy the following conditions. 0.0 ≤ x < 1.0 (0.00 ≤ y < 0.05)

2. The electromagnetic wave absorbing material according to claim 1, further comprising a conductive filler as the inorganic filler.

3. The electromagnetic wave absorbing material according to claim 2, wherein the mass ratio of conductive filler in the inorganic filler is 5% or more and 95% or less.

4. The electromagnetic wave absorbing material according to claim 2, wherein the conductive filler is conductive fibrous particles.

5. The electromagnetic wave absorbing material according to claim 1, wherein Q includes Zr.

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

7. The electromagnetic wave absorbing material according to claim 6, wherein M contains Ba.

8. The electromagnetic wave absorbing material according to Claim 1, wherein the RE contains 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 the RE.

9. The electromagnetic wave absorbing material according to claim 1, wherein the relative permittivity (εr) of the particle at a frequency of 1 GHz is 3,000 or more.

10. The electromagnetic wave absorbing material according to claim 1, wherein the dielectric loss tangent (tanδ) of the particle at a frequency of 1 GHz is 0.01 or more.

11. An electromagnetic wave absorbing material according to any one of claims 1 to 10, comprising a composite material containing the inorganic filler and a polymer.

12. The electromagnetic wave absorbing material according to claim 11, wherein the mass ratio of the inorganic filler in the composite material is 5% or more and 95% or less.

13. The electromagnetic wave absorbing material according to claim 11, wherein the composite material is in the form of a sheet with a thickness of 50 μm or more and 1000 μm or less.

14. The electromagnetic wave absorbing material according to claim 13, wherein the sheet-shaped composite material has a conductive layer on one side.

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