Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B3/00—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form
  • B32B3/26—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a particular shape of the outline of the cross-section of a continuous layer; characterised by a layer with cavities or internal voids ; characterised by an apertured layer
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B7/00—Layered products characterised by the relation between layers; Layered products characterised by the relative orientation of features between layers, or by the relative values of a measurable parameter between layers, i.e. products comprising layers having different physical, chemical or physicochemical properties; Layered products characterised by the interconnection of layers
  • B32B7/02—Physical, chemical or physicochemical properties
  • B32B7/025—Electric or magnetic properties
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q17/00—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K9/00—Screening of apparatus or components against electric or magnetic fields

Definitions

• This disclosure relates to an electromagnetic wave absorber.
• radio waves using the gigahertz frequency band have been used in mobile communications such as mobile phones, electronic toll collection systems (ETC), wireless LANs, and other systems.
• wireless communication using frequency bands of, for example, 30 GHz and above is becoming more practical, and the use of in-vehicle radar is also advancing. Radio waves using such frequency bands cause interference due to diffuse reflection within the housings of various devices, resulting in electromagnetic noise that can cause the devices to malfunction. For this reason, there is a demand to suppress electromagnetic noise.
• an electromagnetic wave absorbing material that absorbs the above-mentioned radio waves As an electromagnetic wave absorbing material that absorbs the above-mentioned radio waves, a laminate sheet has been proposed in which a rubber-like electromagnetic wave absorbing sheet is laminated with a paper-like sheet material such as cardboard (see, for example, Patent Document 1).
• an electromagnetic wave attenuation film for use in the 35 GHz to 50 GHz frequency band and an electromagnetic wave absorbing sheet that uses flat soft magnetic particles to absorb radio waves in the frequency band of 20 GHz or higher have been proposed (see, for example, Patent Documents 2 and 3).
• Patent Document 1 JP 2011-233834 A
• Patent Document 2 JP 2022-80845 A
• Patent Document 3 JP 2015-198163 A
• electromagnetic wave absorbing materials for use in the gigahertz range are known, but absorbing radio waves in high frequency bands such as the gigahertz range is considered difficult, and thick films are usually used. Specifically, it is generally considered difficult to make the thickness of the material significantly thinner than 1/4 of the wavelength.
• An object of one embodiment of the present disclosure is to provide an electromagnetic wave absorber that is thin and has high absorption capacity for radio waves in the gigahertz frequency band (several GHz to 100 GHz).
• An electromagnetic wave absorber having, in the order from the incident direction of the radio wave, a dielectric layer, an air layer, and a metal layer.
• An electromagnetic wave absorber having a dielectric layer, the electromagnetic wave absorber having an incident surface on which radio waves are incident and a back surface which is the surface opposite to the incident surface, and an air layer on the back surface side of the dielectric layer for forming a gap between the electromagnetic wave absorber and a substrate on which the electromagnetic wave absorber is disposed.
• ⁇ 3> The electromagnetic wave absorber according to ⁇ 1> or ⁇ 2>, wherein a total thickness of the dielectric layer and the air layer is 20% or less of a quarter of a wavelength of a frequency at a maximum absorption peak value of the electromagnetic wave absorber.
• ⁇ 4> The electromagnetic wave absorber according to any one of ⁇ 1> to ⁇ 3>, wherein the air layer has a thickness of 0.5 mm or less.
• ⁇ 5> The electromagnetic wave absorber according to any one of ⁇ 1> to ⁇ 4>, wherein the surface resistivity of the dielectric layer is 1.0 ⁇ 10 2 ⁇ / ⁇ to 1.0 ⁇ 10 8 ⁇ / ⁇ .
• an electromagnetic wave absorber is provided that is thin and has high absorption capacity for radio waves in the gigahertz frequency band (several GHz to 100 GHz).
• FIG. 1A is a schematic cross-sectional view showing an example of an electromagnetic wave absorber according to the present disclosure.
• FIG. 1B is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 1C is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 2A is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 2B is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 2C is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 3A is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 3B is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 4 is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• FIG. 5 is a schematic cross-sectional view showing another example of the electromagnetic wave absorber of the present disclosure.
• a numerical range indicated using “ ⁇ ” indicates a range including the numerical value described before “ ⁇ ” as the lower limit and the numerical value described before “ ⁇ ” as the upper limit.
• the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages.
• the upper or lower limit described in a certain numerical range may be replaced with a value (shown in the examples).
• the amount of each component in the composition means the total amount of the multiple corresponding substances present in the composition, unless otherwise specified.
• process includes not only independent processes, but also processes that cannot be clearly distinguished from other processes when the intended purpose is achieved.
• mass % and weight % are synonymous, and “parts by mass” and “parts by weight” are synonymous.
• the electromagnetic wave absorber according to the first embodiment of the present disclosure has a dielectric layer, an air layer, and a metal layer in this order from the incident direction of the radio wave.
• An electromagnetic wave absorber according to a second embodiment of the present disclosure has a dielectric layer, an incident surface on which radio waves are incident and a back surface which is the surface opposite to the incident surface, and an air layer on the back surface side of the dielectric layer to form a gap between the electromagnetic wave absorber and a substrate on which the electromagnetic wave absorber is disposed.
• the electromagnetic wave absorber according to the first embodiment and the electromagnetic wave absorber according to the second embodiment will be collectively referred to simply as the "electromagnetic wave absorber of the present disclosure.”
• the electromagnetic wave absorber of the present disclosure has an air layer on one side of the dielectric layer, and has a layered structure in which the air layer is disposed between the dielectric layer and the metal layer or substrate. This allows for a thinner structure compared to conventional structures in which a single dielectric layer is disposed directly on the surface of the target object (metal layer or substrate (e.g., a metallic or resin housing of a product)), and provides excellent absorption of radio waves in the gigahertz frequency band (several GHz to 100 GHz).
• a thin electromagnetic wave absorber which has been considered difficult to obtain in the past, that is, an electromagnetic wave absorber that is thinner than 1/4 the wavelength of the frequency at the maximum absorption peak of the electromagnetic wave absorber. It is also possible to reduce the weight of the electromagnetic wave absorber.
• the electromagnetic wave absorber of the present disclosure can absorb at least a part of electromagnetic waves in the gigahertz frequency band.
• the electromagnetic wave absorption amount of the electromagnetic wave absorber of the present disclosure is not particularly limited, and is preferably 5 dB or more, more preferably 10 dB or more, and even more preferably 15 dB or more at the absorption peak maximum value.
• the upper limit of the electromagnetic wave absorption amount may be any value, and may be, for example, 40 dB or less at the absorption peak maximum value.
• the amount of electromagnetic wave absorption is represented by the absolute value of the absorption peak intensity, and the absorption peak intensity can be measured according to the method of the examples described later.
• the electromagnetic waves absorbed by the electromagnetic wave absorber of the present disclosure are not particularly limited, but are preferably electromagnetic waves in the gigahertz band, and the absorption peak frequency of the electromagnetic wave absorber (i.e., the frequency of the maximum absorption peak) is preferably 1 GHz to 100 GHz, more preferably 1 GHz to 60 GHz, and particularly preferably 1 GHz to 40 GHz.
• the absorption peak frequency can be measured according to the examples described below.
• the electromagnetic wave absorber according to the first embodiment of the present disclosure has a dielectric layer, an air layer, and a metal layer in this order from the incident direction of the radio wave.
• the electromagnetic wave absorber according to the first embodiment may further have other layers such as a resin layer.
• the incident direction of the electromagnetic wave in the electromagnetic wave absorber is preferably the dielectric layer side.
• the dielectric layer is a layer containing a component exhibiting dielectric properties and having the function of absorbing electromagnetic waves.
• the dielectric layer is preferably a layer containing an electrically insulating component.
• An example of the electrically insulating component is a resin.
• the dielectric layer may also contain other components such as conductive particles, insulating particles, and magnetic particles.
• the dielectric layer may be a layer containing a combination of an insulating resin and a conductive material from the viewpoint of having a certain degree of conductivity and being able to absorb electromagnetic waves.
• the dielectric layer may be, for example, a resin molded product (sheet or film) or a coating of a coating liquid containing a resin.
• thermoplastic resins such as polyesters (polyethylene terephthalate (PET) and the like), epoxy resins, phenoxy resins, polyarylene sulfides (polyphenylene sulfide and the like), (meth)acrylic resins, polyolefins (polyethylene, polypropylene and the like), polystyrene, acrylic resins, polyamides, polyimides, polyamideimides, polyethersulfones, polyetheretherketones, and polycarbonates.
• PET polyethylene terephthalate
• epoxy resins e.g., phenoxy resins, and (meth)acrylic resins
• phenoxy resins polyarylene sulfides
• polyarylene sulfides polyphenylene sulfide and the like
• (meth)acrylic resins polyolefins (polyethylene, polypropylene and the like)
• polystyrene acrylic resins
• polyamides polyimides
• the resins may be used alone or in combination of two or more kinds.
• the resin content is preferably in the range of 10% by mass to 90% by mass, and more preferably in the range of 50% by mass to 80% by mass, based on the total mass of the dielectric layer.
• the conductive particles may be any known filler having electrical conductivity, such as carbon materials (carbon nanotubes (CNT), carbon black (CB), graphite, carbon fibers, etc.), metal materials (metal particles or fibers (silver, copper, nickel, aluminum, etc.)), and metal oxide particles (titanium oxide, tin oxide, indium oxide, ITO, etc.).
• carbon materials carbon nanotubes (CNT), carbon black (CB), graphite, carbon fibers, etc.
• metal materials metal particles or fibers (silver, copper, nickel, aluminum, etc.)
• metal oxide particles titanium oxide, tin oxide, indium oxide, ITO, etc.
• Conductive refers to a property in which the volume resistivity is 10 ⁇ ⁇ cm or less.
• the volume resistivity is a value measured by a four-terminal method, and can be measured, for example, using a low resistance meter Loresta GP (MCP-T610) manufactured by Mitsubishi Chemical Analytech Co., Ltd.
• the particle diameter of the conductive particles is preferably 10 nm to 10,000 nm, more preferably 10 nm to 100 nm, in terms of average primary particle diameter.
• the average primary particle diameter can be measured, for example, using a scanning electron microscope (SEM).
• the conductive particles may be used alone or in combination of two or more kinds.
• the content of the conductive particles is preferably in the range of 10% by mass to 60% by mass, and more preferably in the range of 20% by mass to 50% by mass, based on the total mass of the dielectric layer, from the viewpoint of electromagnetic wave absorption.
• the thickness of the dielectric layer is preferably in the range of 30 ⁇ m to 400 ⁇ m, more preferably in the range of 50 ⁇ m to 300 ⁇ m, and even more preferably in the range of 50 ⁇ m to 150 ⁇ m.
• the thickness of the dielectric layer is the arithmetic average value of measurements taken at any three points on a cross-sectional photograph taken with an optical microscope.
• the surface resistivity of the dielectric layer is preferably 1.0 ⁇ 10 2 ⁇ / ⁇ to 1.0 ⁇ 10 8 ⁇ / ⁇ , and more preferably 1.0 ⁇ 10 3 ⁇ / ⁇ to 5.0 ⁇ 10 4 ⁇ / ⁇ .
• the surface resistivity (unit: ⁇ / ⁇ , ⁇ /sq.) is a value measured by the four-terminal method specified in JIS K7194:1994, and can be measured, for example, using a low resistance meter Loresta GP (MCP-T610; manufactured by Mitsubishi Chemical Analytech Co., Ltd.).
• the volume resistivity of the dielectric layer is preferably 1.0 ⁇ cm to 5.0 ⁇ 10 5 ⁇ cm, more preferably 10 ⁇ cm to 30 ⁇ cm, and even more preferably 20 ⁇ cm to 30 ⁇ cm.
• the volume resistivity (unit: ⁇ cm) is a value measured by the four-terminal method specified in JIS K7194:1994, and can be measured, for example, using a low resistance meter Loresta GP (MCP-T610; manufactured by Mitsubishi Chemical Analytech Co., Ltd.).
• the air layer is a layer containing a gas component, and may contain a solid material as long as the gas is present.
• Gaseous components in the air layer include air and inert gases (nitrogen gas, rare gases, etc.).
• the air layer may be, for example, a bag material such as a resin in which a gas component is sealed, or may be a layer formed by laminating a dielectric layer and a metal layer. Only one air layer may be provided, or two or more air layers may be provided.
• the method for providing the air layer is not particularly limited, and examples thereof include the following methods.
• the porosity of the air layer is not particularly limited, but may be, for example, 30% or more, preferably 50% to 100%, and more preferably 80% to 100%.
• the porosity is the ratio (%) of the volume of the void portion where gas components exist to the total volume of the air layer.
• the thickness of the air layer is preferably 500 ⁇ m or less, more preferably in the range of 10 ⁇ m to 400 ⁇ m, even more preferably in the range of 30 ⁇ m to 200 ⁇ m, and particularly preferably in the range of 50 ⁇ m to 100 ⁇ m.
• the thickness of the air layer is the arithmetic average value of measurements taken at any three points on a cross-sectional photograph taken with an optical microscope.
• the total thickness of the dielectric layer and air layer is preferably 1 mm or less, and in terms of thinning while maintaining good electromagnetic wave absorption capacity, a range of 10 ⁇ m to 500 ⁇ m is more preferable, a range of 50 ⁇ m to 300 ⁇ m is even more preferable, and a range of 50 ⁇ m to 100 ⁇ m is particularly preferable.
• the total thickness of the dielectric layer and the air layer is preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less of the length of 1/4 of the wavelength of the frequency at the maximum absorption peak value of the electromagnetic wave absorber (hereinafter also referred to as "peak absorption wavelength ⁇ /4"). Since the total thickness of the dielectric layer and the air layer is never zero, the lower limit may be in a range exceeding 0%.
• the metal layer is not particularly limited, and examples thereof include a metal plate, a molded product, etc.
• the metal layer may be, for example, a metal molded product such as a metal plate or a metal foil.
• Metals for the metal layer include iron, copper, silver, gold, aluminum, or alloys of these, stainless steel, etc.
• the thickness of the metal layer is not particularly limited and may be appropriately selected depending on the purpose, application, etc. Only one metal layer may be provided, or two or more metal layers may be provided.
• the electromagnetic wave absorber of the present disclosure may further have layers other than the dielectric layer, air layer, and metal layer.
• the other layers include a resin layer, a ceramic layer, an extremely thin metal layer, a liquid layer, etc.
• the resin layer is a layer containing a resin component, and unlike the dielectric layer, does not have the function of absorbing electromagnetic waves.
• the resin layer may be provided to impart various functions to the electromagnetic wave absorber other than absorbing electromagnetic waves.
• the resin layer can improve the rigidity of the electromagnetic wave absorber. Only one resin layer may be provided, or two or more resin layers may be provided.
• the resin layer may be, for example, a resin molded product (sheet or film) or a coating of a coating liquid containing a resin.
• the resin component contained in the resin layer is preferably an electrically insulating resin, such as polyester (polyethylene terephthalate (PET) etc.), epoxy resin, phenoxy resin, polyarylene sulfide (polyphenylene sulfide etc.), (meth)acrylic resin, polyolefin (polyethylene, polypropylene etc.), polystyrene, acrylic resin, polyamide, polyimide, polyamideimide, polyethersulfone, polyetheretherketone, and polycarbonate.
• the resin contained in the resin layer may be the same as or different from the resin contained in the dielectric layer.
• the thickness of the resin layer there are no particular limitations on the thickness of the resin layer, and it may be selected appropriately depending on the purpose, application, etc.
• all of the dielectric layer, air layer, and metal layer may be independent layers (e.g., sheets or films). Also, some of the dielectric layer, air layer, and metal layer may be layers formed by other layers, for example, the dielectric layer or the metal layer may have a void portion, and the air layer may be formed by overlapping the dielectric layer and the metal layer. Also, a structure in which the air layer and the dielectric layer are embedded in the metal layer by fitting the dielectric layer into the opening of the hole of the metal layer having a hole and closing it may be used.
• 1A has a laminated structure of a dielectric layer 12, an air layer 14, and a metal layer 16.
• the incident direction of the radio wave is preferably the incident plane A.
• 1B has a laminated structure of a dielectric layer 12, a layer 24 as an air layer partitioned by arranging partition walls 24a in a lattice pattern in the surface direction, and a metal layer 16.
• the electromagnetic wave absorber shown in Fig. 1C has a structure in which a dielectric layer 12, a resin porous layer 34 in which a plurality of through holes 32 are arranged in the planar direction as an air layer, and a metal layer 16 are laminated.
• the cross section of the through holes 32 in the direction perpendicular to the lamination direction may be any of a circle, an ellipse, a rectangle, etc.
• the through holes 32 may be arranged at regular intervals in the planar direction, or may be scattered at any position.
• the electromagnetic wave absorber shown in FIG. 2A has a structure in which a dielectric layer 12, an air layer 14, a resin layer 18, and a metal layer 16 are laminated in this order.
• the electromagnetic wave absorber shown in FIG. 2B has a structure in which a dielectric layer 12, a resin layer 18, an air layer 14, and a metal layer 16 are laminated in this order.
• the electromagnetic wave absorber shown in FIG. 2C has a structure in which a resin layer 18, a dielectric layer 12, an air layer 14, and a metal layer 16 are laminated in this order.
• the electromagnetic wave absorber shown in FIG. 3A has a structure in which a dielectric layer 12, a first air layer 14, a resin layer 18, a second air layer 14, and a metal layer 16 are laminated in this order.
• the electromagnetic wave absorber shown in FIG. 3B has a structure in which a dielectric layer 12, a first resin layer 18, an air layer 14, a second resin layer 18, and a metal layer 16 are laminated in this order.
• FIG. 4 other structural examples of the electromagnetic wave absorber according to the first embodiment are shown in FIG. 4 and FIG.
• the electromagnetic wave absorber shown in Fig. 4 has a structure in which a dielectric layer 12, an air layer 14, and a metal layer 16 are laminated in this order.
• the dielectric layer 12 is disposed on the side of the metal layer 16 having the air layer 14 (the incident surface A side), so that the air layer 14 is provided to form a gap between the metal layer 16 and the dielectric layer 12.
• the dielectric layer 12 is disposed on the side of the metal layer 16 having the air layer 14 (the incident surface A side), so that the air layer 14 is provided to form a gap between the metal layer 16 and the dielectric layer 12.
• the dielectric layer 12 is disposed so as to cover the entire surface of the incident surface A side of the metal layer 16, but as a modified example, the dielectric layer 12 may enter the hole along the inner wall of the hole forming the air layer 14, and a laminated structure of the air layer 14/dielectric layer 12 may be embedded inside the metal layer 16.
• the electromagnetic wave absorber shown in FIG. 5 has a structure in which a dielectric layer 12, a resin layer 18, an air layer 14, and a metal layer 16 are laminated in this order.
• the resin layer 18 and the dielectric layer 12 are disposed on the side of the metal layer 16 that has the air layer 14 (the incident surface A side), so that an air layer 14 is provided between the metal layer 16 and the resin layer 18 and between the metal layer 16 and the dielectric layer 12.
• the electromagnetic wave absorber according to the second embodiment of the present disclosure has a dielectric layer, an incident surface on which radio waves are incident and a back surface which is the surface opposite to the incident surface, and an air layer on the back surface side of the dielectric layer for forming a gap between the substrate on which the electromagnetic wave absorber is disposed.
• a laminated structure of a dielectric layer/air layer is formed by arranging a structure in which a dielectric layer and an air layer are laminated in this order from the incident surface side on which radio waves are incident (e.g., the front surface of the electromagnetic wave absorber) on the substrate.
• the electromagnetic wave absorber according to the second embodiment may further have other layers such as a resin layer, and may be, for example, a laminated structure of a dielectric layer/resin layer/air layer.
• the substrate is an object on which the electromagnetic wave absorber is disposed.
• the substrate is preferably a metal substrate, and examples thereof include housings, caps, fixtures, supports, weights, and sheets and tapes for imparting design to electrical products.
• the air layer is provided on the back surface of the dielectric layer in order to form a gap between the electromagnetic wave absorber and the substrate on which the electromagnetic wave absorber is disposed.
• the back surface refers to the surface opposite to the front surface of the electromagnetic wave absorber, where the incident surface on which the radio waves are incident is the front surface of the electromagnetic wave absorber.
• both the dielectric layer and the air layer may be independent layers (e.g., a sheet or a film).
• the air layer may be a layer formed between the substrate and the dielectric layer when the electromagnetic wave absorber is disposed on the substrate.
• the substrate on which the electromagnetic wave absorber is disposed may have a gap such as a hole, so that an air layer is formed between the dielectric layer and the substrate, or the electromagnetic wave absorber may have a convex portion on the back side (the side facing the substrate) to form an air layer between the dielectric layer and the substrate.
• the substrate may have a structure in which the air layer and the dielectric layer, or the air layer is embedded in the substrate by blocking the opening of the hole of the substrate having a hole with a dielectric layer.
• the method for forming a gap between the electromagnetic wave absorber and the base material on which the electromagnetic wave absorber is disposed to provide an air layer is not particularly limited, and examples thereof include the following methods.
• each figure shows a substrate 26 instead of the metal layer 16 of the first embodiment.
• the electromagnetic wave absorber shown in FIG. 1A has a dielectric layer 12, an incident surface A on which radio waves are incident, and a back surface B which is the surface opposite to the incident surface A, and an air layer 14 on the back surface B side of the dielectric layer 12 to form a gap between the substrate 26 on which the electromagnetic wave absorber is disposed.
• the electromagnetic wave absorber shown in Fig. 4 has a dielectric layer 12, an incident surface A on which radio waves are incident and a back surface B which is the surface opposite to the incident surface A, and an air layer 14 on the back surface B side of the dielectric layer 12.
• the electromagnetic wave absorber shown in Fig. 4 has the dielectric layer 12 disposed on the side of the substrate 26 having the air layer 14 (the incident surface A side), thereby having the air layer 14 for forming a gap between the substrate 26 and the dielectric layer 12, and a laminated structure of the dielectric layer 12/air layer 14 is formed.
• the dielectric layer 12 is arranged so as to cover the entire surface on the incident surface A side of the substrate 26.
• the dielectric layer 12 may enter the hole along the inner wall of the hole forming the air layer 14, and a laminated structure of the air layer 14/dielectric layer 12 may be embedded inside the substrate 26.
• the electromagnetic wave absorber shown in FIG. 5 has a dielectric layer 12, an incident surface A on which radio waves are incident, and a back surface B which is the surface opposite to the incident surface A, and has a resin layer 18 and an air layer 14, in this order, on the back surface B side of the dielectric layer 12.
• the electromagnetic wave absorber shown in FIG. 5 has the resin layer 18 and the dielectric layer 12 arranged on the side of the substrate 26 which has the air layer 14 (the incident surface A side), and thus has the air layer 14 for forming a gap between the substrate 26 and the resin layer 18 and the dielectric layer 12, forming a laminated structure of the dielectric layer 12/resin layer 18/air layer 14.
• the dielectric layer was prepared as follows. Using the above-mentioned components used for the dielectric layer, a coating solution with the content ratio shown in Table 1 was prepared by mixing 5% by mass of CNT and 20% by mass of CB as a filler with a toluene solution of epoxy resin as a base material. The prepared coating solution was applied to a release film with a bar coater to form a coating film. The formed coating film was dried at 100°C to prepare a dielectric thin film with a thickness of 25 ⁇ m. Next, a plurality of these dielectric thin films were stacked and hot-pressed at 120°C to prepare a dielectric layer with a thickness of 100 ⁇ m.
• a metal plate (aluminum material, 120 mm x 120 mm, thickness 0.8 mm) was prepared as a metal layer (or substrate), a spacer with a thickness of 10 ⁇ m was sandwiched between the dielectric layer and the metal plate, and the dielectric layer and the metal plate were overlapped to form an air layer with a thickness of 10 ⁇ m.
• the spacer was an endless ring-shaped spacer with a width of 30 mm, cut out from a double-sided tape with a length of 120 mm, a width of 120 mm, and a thickness of 10 ⁇ m, with a central area of 90 mm x 90 mm cut out to form a square opening. In this manner, an electromagnetic wave absorbing sheet having a laminated structure of dielectric layer/air layer/metal plate was produced.
• the electromagnetic wave reflection characteristics (S11) of the electromagnetic wave at each frequency in the 18 GHz to 110 GHz band were measured for the prepared electromagnetic wave absorbing sheet using a free space type S parameter method (Vector Network Analyzer N5290A manufactured by Keysight Technologies, Inc.). Then, the absorption amount was calculated from the following formula 1 to obtain the electromagnetic wave absorption characteristics.
• Absorption (dB) Input signal ⁇ Reflection characteristic (S11) Equation 1
• the reflection characteristic is also called the reflection coefficient, and is the ratio of a reflected wave to an incident wave.
• the electromagnetic wave absorption characteristics are plotted for the amount of absorption at each frequency, and the point where the amount of absorption is highest (also called the absorption peak maximum value) is taken as the absorption peak.
• the absorptance of the absorption peak is the "absorption peak intensity,” and the frequency at which the absorption peak appears is the "absorption peak frequency.”
• the absorption peak frequency was converted into a wavelength to obtain the absorption peak wavelength ( ⁇ ), and further, ⁇ /4 and the ratio of the "total thickness of the dielectric layer and the air layer" to ⁇ /4 were calculated. The calculated values are shown in Table 1.
• Example 2 Except for changing the components used in the dielectric layer as shown in Table 1, an electromagnetic wave absorbing sheet was produced in the same manner as in Example 1, and similar measurements and calculations were performed. The measurement results and calculated values are shown in Table 1. Note that for Example 8, since the measurement range of the network analyzer is 18 GHz to 110 GHz, the frequency of the absorption peak is expressed as less than 18 GHz. When the frequency is less than 18 GHz, the ratio of the "total thickness of the dielectric layer and the air layer" to ⁇ /4 is less than 8.4%.
• the percentage of the total thickness of the dielectric layer and the air layer to the ⁇ /4 wavelength calculated from the absorption peak wavelength ( ⁇ ) converted from the absorption peak frequency was less than 20%. That is, while it is generally considered difficult to make the thickness significantly thinner than the ⁇ /4 wavelength, in the Examples, the total thickness of the dielectric layer and the air layer was significantly less than 20% of the ⁇ /4 wavelength, and showed good electromagnetic wave absorption characteristics of approximately -10 dB or more. In contrast, in Comparative Examples 1 to 3, since no air layer was provided, no absorption peak was observed in the 18 GHz to 110 GHz band, or the total thickness of the dielectric layer and the air layer was only more than 20% of the ⁇ /4 wavelength.
• Dielectric layer 14 Air layer 16: Metal layer 18: Resin layer 24: Air layer 24a partitioned in a lattice pattern in the surface direction: Partition wall 26: Substrate A: Incident surface on which radio waves are incident B: Surface opposite to the incident surface (rear surface)

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