Laminate including ferrite sintered plate

Abstract

A laminate according to the present invention has electromagnetic wave absorption characteristics in a frequency range of a GHz band and comprises a sintered plate that is formed of magnetoplumbite-type ferrite and a coating layer that has at least one of electrical conductivity and dielectric properties. Thus, the present invention provides a radio wave absorber having excellent electromagnetic wave absorption characteristics.

Description

Laminate containing ferrite sintered plate

[0001] The present invention relates to a laminate including a sintered ferrite plate, and the laminate can be used as a radio wave absorber.

[0002] Electromagnetic waves are used in a variety of electronic devices, including communication devices such as smartphones. However, as electronic devices become smaller and more powerful, electromagnetic interference caused by unwanted electromagnetic waves emitted from electronic devices and other devices has become a major problem. For this reason, radio wave absorbers that absorb unwanted electromagnetic waves are widely used as one of the countermeasures against such electromagnetic interference.

[0003] In recent years, the frequencies used in electronic devices have been shifting from microwaves to millimeter waves, and electromagnetic waves in higher frequency bands are being used. Along with this trend, there is a demand for radio wave absorbers to be able to absorb electromagnetic waves in the GHz band.

[0004] In Patent Document 1, BaFe is used as a radio wave absorber having excellent electromagnetic wave absorption performance in the gigahertz band (20 GHz band). １２－Ｘ (Ti ０．５ Cu ０．５ ) Ｘ O １９ (wherein X is 2.5 or more and 3.5 or less) is disclosed. This sintered body is obtained by spray-drying a slurry obtained by mixing a raw material powder mixture, a dispersant, polyvinyl alcohol, and water, and then press-molding the resulting granules, degreasing them by heating, and then firing them.

[0005] Japanese Patent Application Laid-Open No. 2018-154541

[0006] The inventors have found that when a sintered ferrite body is used as a radio wave absorber, most of the incident electromagnetic waves are reflected by the surface of the ferrite. This is thought to be because the sintering process increases the density of the ferrite, widening the difference between the input impedance of the sintered ferrite body and the wave impedance of air.

[0007] In response to this, it is conceivable to reduce the density of the ferrite, for example, by reducing the amount of ferrite in a radio wave absorber in which ferrite powder is dispersed in a resin, thereby bringing the input impedance of the radio wave absorber closer to the wave impedance of air and suppressing the reflection of electromagnetic waves. However, since reducing the amount of ferrite also reduces the electromagnetic wave absorption characteristics, it becomes necessary to increase the thickness of the radio wave absorber in order to obtain high electromagnetic wave absorption characteristics. This does not meet the demand for reducing the thickness of radio wave absorbers that accompanies the miniaturization of electronic devices.

[0008] An object of the present invention is to provide a radio wave absorber having excellent electromagnetic wave absorbing properties.

[0009] In order to solve the above problems, the present invention provides the following technologies. [1] A laminate having electromagnetic wave absorption properties in the GHz frequency range, the laminate comprising a sintered plate formed of magnetoplumbite ferrite and a coating layer having at least one of electrical conductivity and dielectric properties. [2] The laminate according to [1] above, further comprising an adhesive layer bonding the sintered plate and the coating layer. [3] The laminate according to [1] or [2] above, wherein the crystallite size of the magnetoplumbite ferrite is 200 nm or more. [4] The laminate according to any one of [1] to [3] above, wherein the density of the sintered plate is 3.0 g / cc or more. [5] The laminate according to any one of [1] to [4] above, wherein the magnetoplumbite ferrite is barium ferrite or strontium ferrite. [6] The laminate according to any one of [1] to [5] above, wherein the sintered plate is divided into small pieces.

[0010] The laminate of the present invention comprises a sintered plate and a coating layer having at least one of electrical conductivity and dielectric properties, and therefore the input impedance of the radio wave absorber can be made closer to the wave impedance of air without increasing the thickness, and the laminate can be used as a radio wave absorber with excellent electromagnetic wave absorption performance.

[0011] 1 is a cross-sectional view of a laminate according to an embodiment of the present invention.

[0012] <Laminate> As shown in Fig. 1, the laminate 1 of this embodiment includes a sintered plate 2 formed of magnetoplumbite ferrite (hereinafter, sometimes simply referred to as "ferrite") and a coating layer 3 having at least one of electrical conductivity and dielectric properties. By including the sintered plate 2 and the coating layer 3, it is possible to achieve both thinness and high electromagnetic wave absorption.

[0013] In FIG. 1, the laminate 1 has one sintered plate 2 and one coating layer 3, but the laminate 1 only needs to have at least one sintered plate 2 and one coating layer 3, and may have a plurality of sintered plates 2 and / or a plurality of coating layers 3.

[0014] The thickness of the entire laminate 1 is not limited to a specific numerical range, but is, for example, 10 mm or less, 5 mm or less, or 1 mm or less, and 0.05 mm or more, 0.1 mm or more, or 0.3 mm or more.

[0015] (Sintered Plate) The sintered plate 2 is obtained by forming magnetoplumbite ferrite powder into a plate shape and sintering it. In the sintered plate, the ferrite can be made denser than in a compact obtained by kneading a binder and ferrite powder. Magnetoplumbite ferrite has high magnetocrystalline anisotropy and can exhibit magnetic resonance in the GHz frequency range. Therefore, by using such a sintered plate, the laminate can have excellent electromagnetic wave absorption properties in the GHz frequency range.

[0016] In magnetoplumbite ferrite, it is preferable that a portion of Fe is substituted with one or more elements selected from Ti, Mn, Zn, Co, Ni, Cu, Sn, Zr, Al, etc. When a portion of Fe is substituted with these elements, the magnetocrystalline anisotropy and saturation magnetization change, making it possible to control the resonance frequency.

[0017] Specifically, the magnetoplumbite ferrite is preferably barium ferrite or strontium ferrite. Barium ferrite has a resonance frequency in the band of 40 GHz or less, and strontium ferrite has a resonance frequency in the band of 60 GHz or more. In addition, by substituting some of the constituent elements of these ferrites with various elements as described above, the frequency band showing the resonance frequency can be shifted.

[0018] The crystallite size of the magnetoplumbite ferrite is preferably 200 nm or more. When the crystallite size is 200 nm or more, excellent electromagnetic wave absorption performance is obtained. The upper limit of the crystallite size is not particularly limited, but is, for example, about 1500 nm.

[0019] The density of the sintered plate 2 is preferably 3.0 g / cc or more. A density of 3.0 g / cc or more can provide excellent electromagnetic wave absorption performance. The density of the sintered plate 2 is preferably 3.5 g / cc or more, more preferably 4.0 g / cc or more, and even more preferably 4.5 g / cc. Furthermore, a density within this range can provide high mechanical strength. The upper limit of the density is not particularly limited, but is, for example, about 5.50 g / cc.

[0020] The thickness of the sintered plate 2 is not limited to a specific numerical range, but is, for example, 0.03 mm to 5 mm, 0.1 to 3 mm, or 0.1 to 1 mm.

[0021] Grooves may be provided on one or both sides of the sintered plate 2. The grooves are formed so that the sintered plate 2 is divided mainly from the grooves. In other words, the laminate 1 may have grooves formed on the surface of the sintered plate 2 so as to divide the sintered plate 2 into multiple regions. The laminate 1 can be arranged along the curved or uneven surface of an electronic device or the like by bending from the grooves provided in the sintered plate 2. The shapes of the regions and the grooves may be changed as appropriate depending on the intended use, the size of the regions, the thickness of the sintered plate 2, and the like. For the grooves, for example, the form described in JP 2005-015293 A can be adopted. The dividing grooves may be continuous or discontinuous, and may be substituted by forming a large number of minute recesses.

[0022] The sintered plate 2 of the laminate 1 may be divided into small pieces in advance. The sintered plate 2 may be divided starting from the grooves. The sintered plate 2 may also be divided into irregular shapes without forming grooves.

[0023] (Coating Layer) The coating layer 3 is a layer having at least one of electrical conductivity and dielectric properties. Hereinafter, the coating layer 3 may be referred to as a conductive layer when it has electrical conductivity, and as a dielectric layer when it has dielectric properties. (Conductive Layer) The surface electrical resistance value of the conductive layer is 10,000 Ω / cm ２ or less, and preferably 5000 Ω / cm ２ or less, and more preferably 1000 Ω / cm ２ The following are examples of possible applications. A layer made of a resin composition in which a conductive filler is dispersed in a resin, a molded body using a conductive powder, a metal sheet, etc. are possible. A layer made of a resin composition in which a conductive filler is dispersed in a resin is particularly preferred. A conductive layer in which a conductive filler is dispersed in a resin can easily control the input impedance of the conductive layer to obtain desired electromagnetic wave absorption characteristics by changing the content of the conductive filler.

[0024] The material constituting the conductive layer may be any material as long as the conductive layer as a whole has the above-mentioned electrical resistance. Examples of the material constituting the conductive layer include carbon-based materials and metal-based materials. Examples of carbon-based materials include conductive polymers, carbon, graphite, carbon nanofibers, and carbon nanotubes. Examples of metal-based materials include metals such as nickel, copper, gold, silver, aluminum, zinc, tin, lead, chromium, platinum, palladium, tungsten, indium, and molybdenum; alloys containing two or more of these metals; and compounds of these metals. The conductive layer may also contain two or more materials with different compositions.

[0025] When the conductive layer is a layer made of a resin composition in which a conductive filler is dispersed in a resin, the conductive filler may be a powder of a carbon-based material or a metal-based material.

[0026] The conductive filler may have various shapes such as granular, spherical, amorphous, needle-like, flat, and fibrous shapes.

[0027] The average particle size of the conductive filler is not particularly limited, but it is preferable that the average particle size or average fiber length is 20 nm to 1 mm. Having the average particle size or average fiber length within this range ensures excellent dispersibility in resin and good electromagnetic wave absorption. The average particle size of the conductive filler can be measured, for example, by removing the conductive filler contained in a medium from the medium as needed and observing it, or by embedding the removed conductive filler in a resin or the like, cross-sectioning it, and then observing it with a scanning electron microscope. For example, in the case of a particulate conductive filler, the average particle size can be determined from the circle-equivalent diameter of 100 particles. Furthermore, in the case of a needle-shaped or fibrous conductive filler, the average fiber length can be determined from the fiber length.

[0028] As the carbonaceous material that is the conductive filler used in the conductive layer, conductive carbon black, fibrous carbon processed from carbon fiber, and carbon nanotubes are suitable.

[0029] The metal-based material used as the conductive filler in the conductive layer is a metal powder, and in particular, iron powder, iron-based alloy powder, and Al flakes are suitable.

[0030] If necessary, the conductive filler may be surface-treated with a titanate-based, silane-based, or other coupling agent to enhance the powder's affinity for the resin. For the surface treatment, 0.1 to 1.0 wt% of the coupling agent is preferably used relative to the conductive filler. By using a coupling agent amount of 0.1 wt% or more, the affinity for the resin can be sufficiently enhanced, and as a result, oxidation stability can be sufficiently maintained. By using a coupling agent amount of 1.0 wt% or less, an increase in the resistance value of the conductive layer can be suppressed. The amount of coupling agent is more preferably 0.1 to 0.5 wt%.

[0031] Among the coupling agents, examples of titanate coupling agents include isopropyl tristearoyl titanate, isopropyl tris(dioctyl pyrophosphate) titanate, isopropyl tri(N-aminoethyl aminoethyl) titanate, tetraoctyl bis(ditridecyl phosphate) titanate, tetra(2-2-diallyloxymethyl-1-butyl)bis(ditridecyl)phosphate titanate, bis(dioctyl pyrophosphate)oxyacetate titanate, and bis(dioctyl pyrophosphate)ethylene titanate.

[0032] Examples of silane coupling agents include vinyltrichlorosilane, vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldieth ...ethoxysilane, and 3-methacryloxypropylmethyldiethoxysilane. Examples of such compounds include acryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2(aminoethyl)3-aminopropylmethyldimethoxysilane, N-2(aminoethyl)3-aminopropyltrimethoxysilane, N-2(aminoethyl)3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilane, and bis(triethoxysilylpropyl)tetrasulfide.

[0033] Furthermore, the conductive filler made of a metal material may be surface-treated with a phosphate compound to prevent oxidation of the powder. The conductive filler made of a metal material is preferably surface-treated with 0.01 to 0.5 wt% of a phosphate compound based on phosphoric acid. Furthermore, the conductive filler made of a metal material that has been surface-treated with a phosphate compound may be surface-treated with 0.01 to 1.0 wt% of a silane coupling agent. When the amount of phosphoric acid is 0.01 wt% or more, oxidation stability can be maintained. When the amount of phosphoric acid is 0.5 wt% or less, an increase in the resistance value of the conductive layer can be suppressed. The amount of phosphoric acid is more preferably 0.01 to 0.4 wt%.

[0034] The conductive filler content in the conductive layer is preferably 1 to 80 vol%, more preferably 5 to 70 vol%. A conductive filler content of 1 vol% or more can achieve sufficiently low surface resistance. Furthermore, a conductive filler content of 80 vol% or less can maintain good sheet strength and flexibility.

[0035] The resin content in the conductive layer is preferably 20 to 99 vol%. A resin content of 20 vol% or more provides good flexibility. A resin content of 99 vol% or less can reduce surface electrical resistance. The resin content in the conductive layer is more preferably 26 to 80 vol%, and even more preferably 28 to 40 vol%.

[0036] The resin is not particularly limited, but examples thereof include styrene-based elastomers, olefin-based elastomers, polyester-based elastomers, polyamide-based elastomers, urethane-based elastomers, and silicone-based elastomers. Examples of styrene-based elastomers include SEBS (styrene-ethylene-butylene-styrene block copolymer). Other examples include PVB resin, EVA resin, PA resin, PPS resin, acrylic resin, epoxy resin, phenolic resin, and polyolefin resin. The elastomers can also be mixed with acrylic resin, epoxy resin, phenolic resin, and polyolefin resin.

[0037] The content of the flame retardant in the conductive layer is preferably 0 to 20 vol%. More preferably, a flame retardant content of 5 vol% or more will provide sufficient flame retardancy. A flame retardant content of 20 vol% or less will provide both good electromagnetic wave absorption and flame retardancy. Suitable flame retardants include melamine polyphosphate, magnesium hydroxide, and hydrotalcite. Magnesium hydroxide and melamine polyphosphate are preferred.

[0038] The content of the antioxidant in the conductive layer is preferably 0 to 3 vol%. More preferably, the content of the antioxidant is 0.5 vol% or more, which provides good oxidation resistance. The content of the antioxidant is 3 vol% or less, which provides both good electromagnetic wave absorption and good oxidation resistance. As the antioxidant, 2',3-bis[[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionyl]]propionohydrazide (IRGANOX MD1024, manufactured by Ciba Specialty Chemicals) or the like is preferably used. Resin antioxidants are selected from tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate], tris-(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, and N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxyhydrocinnamide) that are compatible with the resin. A commercially available antioxidant for rubber-based resins is preferably CTPI N-cyclohexylthiophthalimide (manufactured by Toray Industries, Inc.).

[0039] The conductive layer of the present invention may contain magnetic powder such as magnetite, carbonyl iron, or ferrite to control the input impedance of the conductive layer. The content of the magnetic powder is preferably 0 to 80 vol%, and more preferably 15 to 74 vol%. In a conductive layer containing magnetic powder, changing the content of the magnetic powder makes it easier to control the input impedance of the conductive layer to obtain the desired electromagnetic wave absorption characteristics.

[0040] The conductive layer may be composed of only a conductor, specifically a metal foil obtained by rolling, a thin film formed by vapor deposition or coating of a metal, a compact of a metal powder, etc. Also usable are a thin film coated with a conductive polymer and a sheet of conductive fiber.

[0041] The thickness of the conductive layer is not limited to a specific numerical range, but is, for example, 0.001 mm to 5 mm, 0.005 to 3 mm, or 0.01 to 1 mm.

[0042] In addition, the thickness ratio of the sintered plate 2 to the conductive layer (thickness of the sintered plate: thickness of the conductive layer) can be changed depending on the content of the conductive filler in the conductive layer, etc., and is, for example, 1:3 to 1:0.01.

[0043] (Dielectric Layer) The dielectric layer is not particularly limited as long as it satisfies the following complex dielectric constant. That is, in a predetermined frequency band, the real part ε' of the complex dielectric constant of the dielectric layer is 7 or more, preferably 10 or more, and more preferably 19 or more. Furthermore, in a predetermined frequency band, the imaginary part ε" of the complex dielectric constant is preferably 0.001 or more, more preferably 0.01 or more, and even more preferably 0.1 or more.

[0044] Furthermore, particularly when the sintered plate is made of barium ferrite, the dielectric layer preferably has a complex dielectric constant that satisfies the range expressed by the following formula, where x is the real part ε' of the complex dielectric constant and y is the imaginary part ε" of the complex dielectric constant. When the complex dielectric constant of the dielectric layer is within the range of formula (1), the radio wave absorption performance of the laminate exceeds that of a single-layer ferrite sintered plate of the same thickness. 0.56x + y ≧ 12.0 (where x > 7, y > 0.001) ... (1) Furthermore, it is preferable that the complex dielectric constant of the dielectric layer has a complex dielectric constant that satisfies the range expressed by formula (2). When the complex dielectric constant of the dielectric layer is within the range of formula (2), the radio wave absorption performance of the laminate becomes sufficiently higher than that of a single-layer ferrite sintered plate of the same thickness. 0.56x + y ≧ 14.0 (where x > 10, y > 0.01) ... (2)

[0045] The dielectric layer may be made of any material as long as the dielectric layer as a whole has the above-mentioned complex dielectric constant. Examples of materials constituting the dielectric layer include dielectric inorganic oxide materials, carbonaceous materials, and metal-based materials. Examples of dielectric inorganic oxide materials include barium titanate and strontium titanate. Examples of carbonaceous materials include dielectric polymers, carbon, graphite, carbon nanofibers, and carbon nanotubes. Examples of metal-based materials include metals such as nickel, copper, gold, silver, aluminum, zinc, tin, lead, chromium, platinum, palladium, tungsten, indium, and molybdenum; alloys containing two or more of these metals; and compounds of these metals. The dielectric layer may also contain two or more materials with different compositions.

[0046] When the dielectric layer is a layer made of a resin composition in which a filler is dispersed in a resin, the filler can be a powder of a dielectric inorganic oxide material, a carbon-based material, or a metal-based material.

[0047] The descriptions regarding the shape and average particle size of the conductive filler also apply to the filler in the dielectric layer.

[0048] As the dielectric inorganic oxide filler used in the dielectric layer, powders of barium titanate and strontium titanate are suitable.

[0049] Carbon black, fibrous carbon processed from carbon fiber, and carbon nanotubes are suitable as the carbon-based filler used in the dielectric layer.

[0050] As the metallic filler used in the dielectric layer, iron powder, iron alloy powder, and Al flakes are suitable.

[0051] The description of the treatment of the filler with a coupling agent in the covering layer 3 also applies to the dielectric layer. When the amount of the coupling agent in the dielectric layer is 1.0 wt % or less, a decrease in the complex dielectric constant of the dielectric layer can be suppressed.

[0052] The description of the treatment of the filler with a phosphoric acid compound in the coating layer 3 also applies to the dielectric layer. In the dielectric layer, when the amount of phosphoric acid is 0.5 wt % or less, a decrease in the complex dielectric constant of the dielectric layer can be suppressed.

[0053] The description of the filler content in the covering layer 3 also applies to the dielectric layer. In the dielectric layer, a sufficient complex dielectric constant can be obtained when the filler content is 1 vol % or more.

[0054] The resin content in the dielectric layer is the same as that in the covering layer 3, and by being 20 vol% or more, good flexibility can be obtained. By having the resin content be 99 vol% or less, a decrease in the complex dielectric constant can be suppressed. The resin content in the dielectric layer is more preferably 26 to 80 vol%, and even more preferably 28 to 40 vol%.

[0055] The descriptions regarding the resin, flame retardant, and antioxidant in the coating layer 3 also apply to the dielectric layer.

[0056] The dielectric layer may contain magnetic powder such as magnetite or ferrite to control the input impedance of the dielectric layer. The content of the magnetic powder is preferably 0 to 80 vol%, and more preferably 15 to 74 vol%. In a dielectric layer containing magnetic powder, changing the content of the magnetic powder makes it easier to control the input impedance of the dielectric layer to obtain the desired electromagnetic wave absorption characteristics.

[0057] When the material constituting the dielectric layer is magnetic, such as carbonyl iron, the configuration of the dielectric layer may be designed taking into consideration the change in input impedance due to magnetism, if necessary.

[0058] The dielectric layer may be composed of only a dielectric material, such as a metal foil obtained by rolling, a thin film formed by vapor deposition or coating of a metal, a compact of a metal powder, etc. A thin film formed by coating a dielectric polymer may also be used.

[0059] The dielectric layer may have the above complex dielectric constant and may also be conductive. That is, the dielectric layer may be either a conductive dielectric layer or a non-conductive dielectric layer. In other words, the covering layer preferably has at least dielectric properties, and may have both dielectric properties and conductivity.

[0060] Any mention of the thickness of the conductive layer also applies to the thickness of the dielectric layer.

[0061] In addition, the ratio of the thickness of the sintered plate to the thickness of the dielectric layer (thickness of the sintered plate:thickness of the dielectric layer) can be changed depending on the complex dielectric constant of the dielectric layer, and is, for example, 1:3 to 1:0.01, similar to the ratio for the conductive layer.

[0062] The laminate may have a single coating layer 3 as shown in FIG. 1 or may have multiple coating layers. When the laminate has multiple conductive layers, it is preferable that the layer with the lower surface resistance be provided on the side farther from the sintered plate. When the laminate has multiple dielectric layers, it is preferable that the layer with the higher dielectric constant be provided on the side farther from the sintered plate. By stacking the layers in this order, it is expected that the electromagnetic wave absorption characteristics will be further improved.

[0063] (Other Layers) The covering layer 3 may be provided directly on the sintered plate 2, or the laminate 1 may further include an adhesive layer that bonds the sintered plate 2 and the covering layer 3 together. An example of the adhesive layer is a double-sided adhesive tape. The double-sided adhesive tape is not particularly limited, and any known double-sided adhesive tape may be used.

[0064] The laminate 1 may further include a protective layer. In particular, when the laminate 1 is divided by the above-mentioned grooves, the protective layer prevents powder from falling off the sintered plate 2. The protective layer is not particularly limited as long as it is a resin that can stretch without breaking when the sintered plate 2 is folded along the grooves, particularly when folded back against the protective layer. Thermoplastic resins or rubbers with a tensile elongation at break of typically 350% or more at room temperature and excellent adhesive properties can be used. Examples of resins that can be used to form the protective layer include polyurethane resins, styrene-butadiene elastomers, and butadiene-based elastomers.

[0065] The thickness of the protective layer is usually 0.005 to 0.2 mm, preferably 0.01 to 0.1 mm. If the thickness of the protective layer is less than 0.005 mm, it is prone to breakage and it is difficult to prevent powder falling. If the thickness exceeds 0.2 mm, the effect of preventing powder falling saturates, so there is no need to make it thicker than 0.2 mm.

[0066] The laminate 1 may also have an adhesive layer provided on at least one surface thereof. The adhesive layer is used to fix the laminate 1 inside an electronic device or the like, and is therefore disposed on the outermost surface.

[0067] When a protective layer is not provided, the adhesive layer is preferably provided on the opposite side of the sintered plate 2 to the coating layer 3. When an adhesive layer is not provided, the protective layer is preferably provided on the opposite side of the sintered plate 2 to the coating layer 3. When a protective layer is provided, it is preferably provided that at least one of the adhesive layer and the protective layer is provided on the opposite side of the sintered plate 2 to the coating layer 3. For example, the protective layer, sintered plate 2, coating layer 3, and adhesive layer may be arranged in this order, or the adhesive layer, sintered plate 2, coating layer 3, and protective layer may be arranged in this order.

[0068] <Method for manufacturing laminate> The method for manufacturing the laminate 1 includes a step of arranging a coating layer 3 on at least one surface of the sintered plate 2. This step may include bonding the sintered plate 2 and a separately formed coating layer 3 together, or applying a liquid composition containing a material for the coating layer 3 to the sintered plate 2.

[0069] The sintered plate 2 can be manufactured by a known method. For example, known manufacturing methods such as those disclosed in Japanese Patent Application Laid-Open No. 2005-015293 can be referred to as a manufacturing method for the sintered plate 2. The manufacturing process for the sintered plate 2 includes, for example, mixing ferrite powder with a binder resin (and optionally a solvent and other additives); molding the mixture thus obtained by powder compression molding, injection molding, calendaring, extrusion, coating, or the like; and sintering the molded body after degreasing it as necessary.

[0070] For example, in the coating method, a green sheet can be obtained by mixing ferrite powder, a binder resin, and a solvent, applying the mixture to a film using a doctor blade or the like, and then drying the mixture. The resulting green sheet can be used alone or in a stack of multiple green sheets. The stacked multiple green sheets are integrated by pressure, degreased, and sintered.

[0071] The degreasing treatment is usually carried out at a temperature of 150 to 500° C. for 0.5 to 100 hours.

[0072] The sintering temperature is usually 950 to 1400° C., or 1000 to 1350° C. The sintering time is usually 30 to 300 minutes, or 45 to 240 minutes.

[0073] The grooves can be formed in the ferrite sintered plate during or after forming of the ferrite green sheet or after the sintering process.

[0074] The conductive layer forming process includes, for example, mixing a conductive filler, a resin, and a solvent; applying the resulting mixture directly to the sintered plate or onto a film (made of PET); and drying the mixture. When the mixture is applied onto a film, a conductive sheet is obtained by drying, so the conductive layer forming process further includes attaching this conductive sheet to the sintered plate with an adhesive. This adhesive may be double-sided tape or a liquid adhesive. Furthermore, the conductive layer may consist of a single conductive sheet, or multiple conductive sheets may be stacked on top of each other. Multiple conductive sheets may be stacked, integrated by pressure, and then attached to the sintered plate.

[0075] The dielectric layer forming step includes, for example, mixing the materials for the dielectric layer (filler, resin, solvent, etc.) described above. In forming the dielectric layer, the operations after obtaining the mixture are the same as those for the conductive layer.

[0076] The protective layer is formed, for example, by applying a resin onto the sintered plate or the covering layer, or by forming a separate sheet and then attaching it to the sintered plate or the covering layer with an adhesive. The adhesive may be either a double-sided tape or a liquid adhesive. The adhesive layer is provided on the outermost surface of the laminate, for example, by applying an adhesive resin or by attaching a double-sided tape.

[0077] <Use of Laminate> The laminate can be used as a radio wave absorber or an electromagnetic wave shield in electronic devices.

[0078] A. Laminate with Conductive Layer <Sample Preparation> [Examples 1 to 3] (1) Formation of Sintered Plate Various powder raw materials (α-Fe ２ O ３ , BaCO ３ , SrCO ３ , TiO ２ , Mn ３ O ４ ) were weighed out in the prescribed amounts shown in Table 1 below, mixed in a wet attritor, filtered, and dried. BaCl was added as a flux to the resulting raw material powder mixture. ２ ・2H ２ O was added and mixed thoroughly. The mixture was extruded to obtain granules, which were then fired at 1280°C in air. The fired product was coarsely pulverized, then pulverized in a wet attritor, washed with water, filtered, dried, and then finely pulverized. The pulverized product was then annealed at 900°C in air to obtain barium ferrite or strontium ferrite powder.

[0079] To the obtained barium ferrite or strontium ferrite powder, polyvinyl butyral (PVB) resin and a plasticizer (benzyl n-butyl phthalate) were added and thoroughly mixed in 3-methyl-3-methoxy-1-butanol to obtain a slurry. This slurry was applied to a PET film using a doctor blade coater to form a coating, which was then dried to obtain a green sheet with a thickness of 60 μm.

[0080] Six green sheets were stacked and laminated for waterproofing, and then pressed together in a warm isostatic press at 34 MPa for 30 minutes to form a single sheet. A metal blade was used to create dividing grooves on the surface at 2.4 mm intervals to obtain a molded sheet. The molded sheet was degreased at 500°C for 40 hours and then fired at 1250°C to obtain a 0.3 mm thick sintered plate of magnetoplumbite-type barium ferrite or strontium ferrite.

[0081] (2) Formation of Conductive Layer A conductive coating material was obtained by mixing 13.6 parts by mass of fibrous conductive carbon, 56.2 parts by mass of granular magnetite, 15.3 parts by mass of aluminum hydroxide, 14.9 parts by mass of acrylic resin, and 100 parts by mass of 3-methyl-3-methoxy-1-butanol.

[0082] The obtained conductive paint was applied to the release-treated surface of a release-treated film (made of PET) so that the thickness after drying was 60 μm, and then heated at 120°C in a hot air circulation dryer to obtain a conductive sheet.

[0083] Six conductive sheets were stacked and further laminated for waterproofing, and then pressed at 34 MPa for 30 minutes in a warm isostatic press to form an integral structure, thereby obtaining a conductive layer with a thickness of 0.3 mm.

[0084] (3) Preparation of Laminate: A conductive layer was attached to one side of a sintered plate using 30 μm thick acrylic double-sided adhesive tape. A 50 μm thick PET film was attached to the other side of the sintered plate, and a 50 μm thick acrylic double-sided adhesive tape was attached to the surface of the conductive layer. The laminate thus obtained was divided along the grooves formed in the sintered plate to obtain samples to be used for the measurements described below.

[0085] [Comparative Example 1] A sample of Comparative Example 1 was obtained by the same procedure as in Example 1, except that no conductive layer was attached to the sintered plate of Example 1. That is, a double-sided tape having a thickness of 50 μm was attached to one side of the sintered plate, and a PET film having a thickness of 50 μm was attached to the other side, and then the sintered plate was divided.

[0086] Comparative Example 2 A sample was obtained in the same manner as in Comparative Example 1, except that the number of stacked green sheets was increased to 12, thereby making the thickness of the sintered plate 0.6 mm.

[0087] Comparative Example 3 was the same as Example 1, except that no sintered plate was provided. That is, a 50 μm thick PET film was attached to one side of the conductive layer of Example 1, and a 50 μm thick double-sided tape was attached to the other side.

[0088] Comparative Example 4 A sample of Comparative Example 4 was obtained in the same manner as in Example 2, except that no conductive layer was attached to the sintered plate of Example 2.

[0089] Comparative Example 5 A sample of Comparative Example 5 was obtained in the same manner as in Example 3, except that no conductive layer was attached to the sintered plate of Example 3.

[0090] <Evaluation> [Crystallite size] The crystallite size of ferrite in the ferrite sintered plates of the examples and comparative examples was determined by Rietveld analysis using an X-ray diffractometer D8 ADVANCE (manufactured by BRUKER) and TOPAS software Ver. 4. The 2θ (θ: Bragg angle) used in the analysis was in the range of 15° to 100°.

[0091] [Density of Sintered Plate] The dimensions and masses of the ferrite sintered plates of the Examples and Comparative Examples were measured, and the densities were calculated based on these values.

[0092] [Surface Electrical Resistivity of Conductive Layer] The surface electrical resistance of the conductive layer was measured in accordance with JIS K 7194. Specifically, a four-point probe was pressed onto the center of the conductive layer before it was attached to the sintered plate, and the surface electrical resistance of the conductive layer of each of the examples and comparative examples was measured using a surface resistance meter LorestaAX MCP-T370 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.). ２ It was.

[0093] [Electromagnetic Wave Absorption Characteristics] For each sample, the S parameters S11 and S21 were measured using a network analyzer ME7838A (manufactured by Anritsu Corporation) and a free space method measuring device (manufactured by Keycom Corporation) while changing the frequency of the incident wave. S11 at the frequency where S11 was minimum (absorption peak frequency) was taken as the reflection loss and S21 as the transmission loss.

[0094]

[0095]

[0096] The samples of Examples 1 to 3 exhibited excellent electromagnetic wave absorption properties, ie, large reflection loss and transmission loss, in the range of 27 to 65 GHz.

[0097] On the other hand, Comparative Examples 1, 4, and 5, which have the configuration of Examples 1 to 3 without the conductive layer, were significantly inferior to Examples 1 to 3 in both return loss and transmission loss.

[0098] Comparative Example 2 had a greater thickness than Comparative Example 1 and had slightly better return loss and transmission loss than Comparative Example 1, but was significantly inferior to Examples 1-3.

[0099] Comparative Example 3 was composed of only a conductive layer, and showed a transmission attenuation value close to that of Example 2, but was even worse than Comparative Example 1 in terms of return attenuation.

[0100] As described above, by laminating the sintered ferrite plate and the conductive layer, a laminate having excellent electromagnetic wave absorption properties was obtained.

[0101] B. Laminate Having Dielectric Layer <Sample Preparation> [Example 4] (1) Formation of Sintered Plate A sintered plate was obtained by the same procedure as in Example 1.

[0102] (2) Formation of Dielectric Layer A solution of 9.6 parts by weight of styrene-based elastomer dissolved in 38.6 parts by weight of cyclohexanone, 81.4 parts by weight of flat Sendust, 3.9 parts by weight of melamine polyphosphate, and 5.1 parts by weight of magnesium hydroxide was mixed, and the resulting slurry was vacuum degassed to obtain a coating material. The resulting coating material was applied to the release-treated surface of a release-treated PET film so that the dried thickness was 60 μm. After heating and drying at 120°C in a hot air circulation dryer, the film was pressure-molded at 120°C and 60 MPa for 2 minutes to obtain a dielectric sheet with a thickness of 60 μm.

[0103] Six dielectric sheets were stacked and laminated in the same manner as the conductive layer, and then integrally molded by pressing to obtain a dielectric layer with a thickness of 0.3 mm. (3) Preparation of Laminate As in Example 1, a dielectric layer was attached to one side of a sintered plate using acrylic double-sided adhesive tape. A PET film and acrylic double-sided adhesive tape were also attached in the same manner as in Example 1, and the obtained laminate was divided along the grooves formed in the sintered plate.

[0104] [Example 5] 100 parts by mass of carbonyl iron powder was mixed with 8 parts by mass of polyvinyl butyral (PVB) resin and 3 parts by mass of plasticizer (benzyl n-butyl phthalate) in 3-methyl-3-methoxy-1-butanol to obtain a slurry. This slurry was applied to a PET film using a doctor blade coater to form a coating, which was then dried to obtain a dielectric sheet with a thickness of 60 μm. A dielectric layer and a laminate were obtained in the same manner as in Example 4 except for the above.

[0105] [Example 6] 8 parts by mass of polyvinyl butyral (PVB) resin and 3 parts by mass of plasticizer (benzyl n-butyl phthalate) were added to 100 parts by mass of Al flakes that had been surface-insulated with silica, and the mixture was thoroughly mixed in 3-methyl-3-methoxy-1-butanol to obtain a slurry. This slurry was applied to a PET film using a doctor blade coater to form a coating, which was then dried to obtain a 60 μm-thick dielectric sheet. A dielectric layer and a laminate were obtained in the same manner as in Example 4, except for the above.

[0106] [Example 7] 43 parts by weight of EVA resin was added to 100 parts by weight of barium titanate powder, mixed thoroughly, and then charged into a Plastomill and kneaded at 90°C for 25 revolutions to obtain a resin composition. The resin composition was stretched multiple times using a biaxial roll heated to 90°C while gradually changing the thickness, to obtain a dielectric layer with a thickness of 0.3 mm. The laminate was produced in the same manner as in Example 4.

[0107] Comparative Example 6 A sample obtained in the same manner as in Comparative Example 1 was used as Comparative Example 6.

[0108] Comparative Example 7 A sample obtained in the same manner as in Comparative Example 2 was used as Comparative Example 7.

[0109] Comparative Example 8 was the same as Example 4, except that no sintered plate was provided. That is, a 50 μm thick PET film was attached to one side of the dielectric layer of Example 4, and a 50 μm thick double-sided tape was attached to the other side.

[0110] Comparative Example 9 was the same as Example 5, except that no sintered plate was provided. That is, a 50 μm thick PET film was attached to one side of the dielectric layer of Example 5, and a 50 μm thick double-sided tape was attached to the other side.

[0111] Comparative Example 10 was the same as Example 6, except that no sintered plate was provided. That is, a 50 μm thick PET film was attached to one side of the dielectric layer of Example 6, and a 50 μm thick double-sided tape was attached to the other side.

[0112] Comparative Example 11 A sample was obtained in the same manner as in Comparative Example 8, except that the number of stacked dielectric sheets was increased to 12, thereby making the thickness of the dielectric layer 0.6 mm.

[0113] Comparative Example 12 A sample was obtained in the same manner as in Comparative Example 9, except that the number of stacked dielectric sheets was increased to 12, thereby making the thickness of the dielectric layer 0.6 mm.

[0114] Comparative Example 13 A sample was obtained in the same manner as in Comparative Example 10, except that the number of stacked dielectric sheets was increased to 12, thereby making the thickness of the dielectric layer 0.6 mm.

[0115] <Evaluation> [Crystallite size] The ferrite sintered plates used in Examples 4 to 7 and Comparative Example 6 were the same as those in Example 1, and therefore had a crystallite size of 932 nm. The ferrite sintered plate used in Comparative Example 7 was the same as that in Comparative Example 2, and therefore had a crystallite size of 824 nm.

[0116] [Density of sintered plate] The ferrite sintered plate used in Examples 4 to 7 and Comparative Example 6 was the same as that used in Example 1, so its density was 4.79 g / cc. The ferrite sintered plate used in Comparative Example 7 was the same as that used in Comparative Example 2, so its density was 4.82 g / cc.

[0117] [Complex Permittivity of Dielectric Layer] The complex permittivity of the dielectric layer was measured by a waveguide method using a Keysight Technologies N5224B network analyzer. The complex permittivity was measured on a 0.1 mm thick dielectric layer obtained by stacking two 60 μm thick dielectric sheets of each material used in the Examples and Comparative Examples, laminating them for waterproofing, and then pressing them in a warm isostatic press at 34 MPa for 30 minutes to form an integral structure. The complex permittivity at the absorption peak frequency determined by measuring the electromagnetic wave absorption characteristics described below is shown in Table 3.

[0118]

[0119] [Electromagnetic Wave Absorption Characteristics] For each sample, the S parameters S11 and S21 were measured while changing the frequency of the incident wave using a network analyzer ME7838A (manufactured by Anritsu Corporation) and a free space method measuring device (manufactured by Keycom Corporation). S11 at the frequency where S11 was minimum (absorption peak frequency) was shown as the reflection loss, and S21 as the transmission loss, and are shown in Table 4. The absorption peak frequency was 27 GHz in all Examples and Comparative Examples.

[0120] [Weight of Laminate] The weight of a 120 mm square sample was measured for each sample. The results are shown in Table 4.

[0121]

[0122] The samples of Examples 4 to 6 exhibited excellent electromagnetic wave absorption properties, ie, large reflection loss and transmission loss, at a frequency of 27 GHz.

[0123] On the other hand, Comparative Example 1, which was constructed without the dielectric layer of Examples 4 to 7, was significantly inferior to Examples 4 to 6 in both the return loss and the transmission loss.

[0124] Comparative Example 7 had a sintered plate with a greater thickness than Comparative Example 6, and had slightly better return loss and transmission loss than Comparative Example 6, but were significantly inferior to Examples 4 to 6. Furthermore, Comparative Example 7 had the same overall thickness as Examples 4 to 6, but was heavier.

[0125] Example 7 exhibited larger return loss and transmission attenuation than Comparative Example 6. Although the return loss and transmission attenuation were inferior to those of Comparative Example 7 having the same thickness, the weight was smaller, which contributed to reducing the weight of the radio wave absorber.

[0126] Comparative Examples 8 to 10 were constructed only with a dielectric layer, and had transmission attenuation values ​​similar to those of Examples 4 to 6, but were inferior in terms of return loss.

[0127] In Comparative Examples 11 to 13, the thickness of the dielectric layer was made thicker than in Comparative Examples 8 to 10, but the return loss was not improved significantly.

[0128] As described above, by laminating the sintered ferrite plate and the dielectric layer, a laminate having excellent electromagnetic wave absorption properties was obtained.

[0129] 1 Laminate 2 Sintered plate 3 Coating layer

Claims

1. A laminate having electromagnetic wave absorption properties in the GHz frequency range, comprising a sintered plate made of magnetoplumbite ferrite and a coating layer having at least one of electrical conductivity and dielectric properties.

2. The laminate according to claim 1, further comprising an adhesive layer that bonds the sintered plate and the covering layer together.

3. The laminate according to claim 1, wherein the crystallite size of the magnetoplumbite ferrite is 200 nm or more.

4. The laminate according to claim 1, wherein the density of the sintered plate is 3.0 g / cc or more.

5. The laminate according to claim 1, wherein the magnetoplumbite ferrite is barium ferrite or strontium ferrite.

6. The laminate according to claim 1, wherein the sintered plate is divided into small pieces.

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