JP7494880B2 - Electromagnetic wave absorber - Google Patents

Description

The present invention relates to an electromagnetic wave absorber.

It is known that industrial materials and electronics components installed in environments that use certain electromagnetic waves, such as for communications, radar, and scanners, reflect electromagnetic waves and become a source of noise. In response to this, electromagnetic wave absorbers can be used to prevent noise generation and maintain the original performance and functionality of the product. For example, in wireless communications, this can prevent a decrease in communication speed, in sensors, it can maintain sensitivity, and in image scanners, it can make images clearer.
More specifically, when using high-speed wireless communication using electromagnetic waves such as 5G and 6G in office buildings or apartment buildings, electromagnetic waves reflected from steel columns, beams, supporting rebars, concrete, and glass windows become noise and cause a decrease in communication speed. In response to this, by using a design industrial material applied to the outermost surface of a room, i.e., a design electromagnetic wave absorber with the design of wallpaper instead of conventional wallpaper, it is possible to suppress noise and maintain the original communication speed.
For example, in indoor environments using millimeter wave radars used in fall detection and seat leaving detection systems in toilets of elderly housing and nursing homes with service, or for monitoring in childcare facilities, electromagnetic waves reflected by floor and wall materials (tiles, reinforced concrete) can create multipaths, causing false detections and malfunctions. By installing design-oriented electromagnetic wave absorbers on floors and walls, it is possible to prevent false detections by monitoring radars while maintaining the design of the interior and exterior.

As such an electromagnetic wave absorber, Patent Document 1 discloses an electromagnetic wave absorber that has advantageous properties for attachment to curved surfaces. Patent Document 2 discloses an electromagnetic wave absorber that is easy to attach to uneven surfaces, and that suppresses an increase in sheet resistance and a decrease in return loss even when exposed to a high-temperature, high-humidity environment for a long period of time.

JP 2017-163141 A JP 2019-71463 A

As the applications and environments in which electromagnetic wave absorbers are installed expand, cases are expected in which a load (compressive stress) is applied from the cross-sectional direction (upper side) to an electromagnetic wave absorber installed on a floor or wall. Furthermore, cases are also expected in which the absorber is exposed to high temperature and high humidity for long periods of time while under such a load. The inventors have found a need for an electromagnetic wave absorber whose reflection attenuation characteristics are resistant to a load from above.
In contrast, Patent Document 1 describes setting the bending rigidity to 300 MPa mm4 or ^less to facilitate attachment to curved surfaces, but does not disclose anything about the case where a force is applied in the cross-sectional direction of the electromagnetic wave absorber. Also, Patent Document 2 describes an electromagnetic wave absorber whose bending rigidity is durable in a high-temperature and high-humidity environment, but does not disclose durability with respect to a load in the cross-sectional direction.
SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide an electromagnetic wave absorber that has good resistance to return loss even when a load is applied in the cross-sectional direction.

In order to solve the above problems, one representative electromagnetic wave absorber of the present invention comprises a resistive layer, a dielectric layer, and a reflective layer, and has a Martens hardness of 2.5 N/ ^mm2 or more and 4.5 N/ ^mm2 or less , and the dielectric layer contains one or more types of resin components and one or more types of inorganic fillers .

According to the present invention, the return loss characteristic can have good resistance even when a load is applied in the cross-sectional direction.
Problems, configurations and effects other than those described above will become apparent from the description of the following embodiments.

FIG. 1 is a cross-sectional view that illustrates a reflective electromagnetic wave absorber according to an embodiment. FIG. 2 is a schematic diagram showing an example in which an electromagnetic wave absorber is installed on a floor.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited to this embodiment. In addition, in the description of the drawings, the same parts are denoted by the same reference numerals.

In the present disclosure, the "cross-sectional direction" of an electromagnetic wave absorber means the normal direction of the surface on the side where the electromagnetic wave is incident.
In addition, in this disclosure, the term "floor" is not particularly limited and means a plane parallel to the ground.

Figure 1 is a cross-sectional view that shows a schematic diagram of a reflective electromagnetic wave absorber 10 according to an embodiment. The electromagnetic wave absorber 10 shown in this figure is in the form of a film or sheet, and has a laminated structure including a resistive layer 1, a dielectric layer 2, and a reflective layer 3, in that order. The film-shaped electromagnetic wave absorber 10 has an overall thickness of, for example, 24 to 250 μm. On the other hand, the sheet-shaped electromagnetic wave absorber 10 has an overall thickness of, for example, 0.25 to 7.1 mm.

(Resistance layer)
The resistive layer 1 is a layer for allowing electromagnetic waves incident from the outside to reach the dielectric layer 2. In other words, the resistive layer 1 is a layer for impedance matching according to the environment in which the electromagnetic wave absorber 10 is used. When the electromagnetic wave absorber 10 is used in air (impedance: 377 Ω/□), the sheet resistance of the resistive layer 1 is set to, for example, in the range of 350 to 400 Ω/□.

The resistive layer 1 contains an inorganic material or an organic material having electrical conductivity. Examples of the inorganic material having electrical conductivity include indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), carbon nanotubes, graphene, nanoparticles containing one or more selected from the group consisting of Ag, Al, Au, Pt, Pd, Cu, Co, Cr, In, Ag-Cu, Cu-Au, and Ni nanoparticles, and nanowires. Examples of the organic material having electrical conductivity include polythiophene derivatives, polyacetylene derivatives, polyaniline derivatives, and polypyrrole derivatives. In particular, from the viewpoints of flexibility, film-forming properties, stability, and sheet resistance of 377 Ω/□, a conductive polymer containing polyethylenedioxythiophene (PEDOT) is preferable. The resistive layer 1 may contain a mixture (PEDOT/PSS) of polyethylenedioxythiophene (PEDOT) and polystyrene sulfonic acid (PPS).

The sheet resistance of the resistive layer 1 can be appropriately set, for example, by selecting an organic material having electrical conductivity and adjusting the film thickness. The thickness (film thickness) of the resistive layer 1 is preferably in the range of 0.1 to 2.0 μm, and more preferably in the range of 0.1 to 0.4 μm. If the film thickness is 0.1 μm or more, a uniform film is easily formed, and the film tends to function more satisfactorily as the resistive layer 1. On the other hand, if the film thickness is 2.0 μm or less, sufficient flexibility can be maintained, and cracks in the thin film due to external factors such as bending and pulling after film formation tend to be more reliably prevented. The sheet resistance of the resistive layer 1 can be measured, for example, using Loresta GP MCP-T610 (product name, manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

(Dielectric Layer)
The dielectric layer 2 is a layer for causing interference between an incident electromagnetic wave and a reflected electromagnetic wave. The thickness and other factors of the dielectric layer 2 are set so as to satisfy the condition expressed by the following formula.
d=λ/(4(εr) ^1/2 )
In the formula, λ is the wavelength (unit: m) of the electromagnetic wave to be suppressed, εr is the relative dielectric constant of the material that constitutes the dielectric layer, and d is the thickness (unit: m) of the dielectric layer. Return attenuation is achieved when the phase of the incident electromagnetic wave and the phase of the reflected electromagnetic wave are shifted by π.

The dielectric layer 2 contains one or more types of inorganic fillers and one or more types of resin components. The dielectric layer 2 containing the inorganic fillers tends to be harder than a dielectric layer composed only of a resin component, and is less susceptible to distortion or shrinkage in the cross-sectional direction, making it possible to prevent deformation, so that a constant return loss can be obtained at a selected frequency even when a load is applied.
The density of the dielectric layer 2 can be adjusted depending on the selection of the inorganic filler in the dielectric layer 2 and its content. The content of the inorganic filler (hereinafter also referred to as the "mixing ratio" of the volume ratio) is preferably 10 to 80 parts by volume (%), more preferably 25 to 80 parts by volume (%), relative to 100 parts by volume of the total of the resin and the inorganic filler. By being equal to or more than the lower limit, the compression deformation ratio of the dielectric layer 2 tends to be small, while by being equal to or less than the upper limit, the film shape can be maintained, and the dielectric layer 2 tends to be efficiently manufactured by wet coating (e.g., gravure coating, microcoating, rod coating, comma coating) or extrusion molding. The thickness is preferably 50 to 1000 μm, more preferably 50 to 800 μm, and is preferably selected according to the dielectric constant and the frequency to be attenuated by reflection. The thinner the thickness, the smaller the compression deformation ratio of the dielectric layer 2 tends to be, and the dielectric layer 2 tends to be efficiently manufactured by wet coating or extrusion molding. The content of the inorganic filler in the dielectric layer 2 is determined by cutting the electromagnetic wave absorber to expose a cross section and analyzing the abundance ratio of elements specific to the inorganic filler in the cross section by energy dispersive X-ray spectroscopy (EDX).

The inorganic filler is selected from the group consisting of metal oxides, metal hydroxides, metal carbonates, metal sulfates, metal silicates, metal nitrides, metal phosphates, metal borates, metal titanates, metal sulfides, and carbons. For example, the metal oxides include silica, diatomaceous earth, alumina, zinc oxide, titania, zirconia, calcium oxide, magnesium oxide, iron oxide, tin oxide, chromium oxide, antimony oxide, yttrium oxide, cerium oxide, samarium oxide, lanthanum oxide, tantalum oxide, terbium oxide, copper oxide, europium oxide, neodymium oxide, and ferrites. For example, the metal hydroxides include calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and basic magnesium carbonate. For example, the metal carbonates include heavy calcium carbonate, light calcium carbonate, zinc carbonate, barium carbonate, dawsonite, and hydrotaluisite. Examples of metal silicates include metal sulfates such as barium sulfate and gypsum fiber, calcium silicate (wollastonite, xonotlite), calcium sulfate, kaolin, clay, talc, mica, montmorillonite, bentonite, activated clay, sepiolite, imogolite, sericite, glass fiber, glass beads, glass flakes, etc. Examples of metal nitrides include aluminum nitride, boron nitride, silicon nitride, etc. Examples of metal titanates include potassium titanate, calcium titanate, magnesium titanate, barium titanate, lead aluminum zirconate titanate borate, etc. Examples of metal borates include zinc borate, aluminum borate, etc.
Examples of the filler include metal phosphates such as tricalcium phosphate, metal sulfides such as molybdenum sulfide, metal carbides such as silicon carbide, carbons such as carbon black, graphite, and carbon fiber, and other fillers.

Metal compounds are preferred as inorganic _fillers , such as barium titanate ( _BaTiO3 ), barium _neodymium titanate ( _{Ba3Nd9.3Ti18O54} ), titanium oxide ( _TiO2 ), zinc oxide (ZnO), forsterite ( _Mg2SiO4 ), aluminum oxide ( _Al2O3 ), barium magnesium niobate (Ba( _{Mg1/3Nb2/ 3 ) O3 ) ,} and silicon dioxide ( _SiO2 ).

The inorganic filler is preferably in the form of a powder (e.g., nanoparticles). In particular, a high dielectric filler is preferred because it can be made thin and has a small amount of deformation.

The density of the metal compound is preferably higher than the density of the resin component. The density of the metal compound is preferably 2.0 to 20 g/ ^cm3 or less, more preferably 4.0 to 8.0 g/ ^cm3 , and even more preferably 4.0 to 6.5 g/ ^cm3 .

Examples of the resin component include acrylic resin, methacrylic resin, silicone resin, polycarbonate, epoxy resin, cryptal resin, polyvinyl chloride, polyvinyl formal, phenol resin, urea resin, and polychloroprene resin. The density of the resin component is preferably 0.7 to 3.0 g/ ^cm3 , and more preferably 0.85 to 2.5 g/ ^cm3 .

(Reflective Layer)
The reflective layer 3 is a layer for reflecting the electromagnetic wave incident from the dielectric layer 2 and directing the electromagnetic wave to the dielectric layer 2. The thickness of the reflective layer 3 is, for example, 4 to 250 μm, and may be 4 to 12 μm or 50 to 100 μm.

The reflective layer 3 is made of a material having electrical conductivity, for example, a sheet resistance value of 100 Ω/□ or less. Such a material may be an inorganic material or an organic material. Examples of inorganic materials having electrical conductivity include indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), carbon nanotubes, graphene, nanoparticles containing one or more selected from the group consisting of Ag, Al, Au, Pt, Pd, Cu, Co, Cr, In, Ag-Cu, Cu-Au, and Ni nanoparticles, and/or nanowires. Examples of organic materials having electrical conductivity include polythiophene derivatives, polyacetylene derivatives, polyaniline derivatives, and polypyrrole derivatives. A film of an inorganic or organic material having electrical conductivity may be formed on a substrate. From the viewpoints of flexibility, film formability, stability, sheet resistance, and low cost, it is preferable to use a laminated film (Al-deposited PET film) having a PET film and an aluminum layer deposited on the surface of the PET film as the reflective layer 3.

(Production method)
The dielectric layer is manufactured, for example, through the following steps. First, the resin and inorganic filler that are to be the dielectric layer materials are weighed according to a designed recipe, put into a mixer, and mixed to make it uniform. The mixture is put into an extruder, where it is melted and kneaded. A thin rod-shaped resin is extruded from the tip of the extruder, cooled in a water tank, and cut to an arbitrary size to prepare resin pellets that are the raw material for the dielectric layer. Next, the prepared resin is melted by applying heat, and a dielectric layer adjusted to a predetermined thickness is manufactured using a device such as an extruder, a calendar film-forming machine, or a heat press machine. The above method is one example, and it is also possible to directly form a film using an extruder or a calendar film-forming machine without including a resin pellet manufacturing process.

In addition to the above methods, the dielectric layer can also be formed by wet coating. Wet coating is performed by applying a dispersion liquid in which the materials constituting the dielectric layer 2 are dispersed onto a substrate to form a coating film, and then drying the coating film. Wet coating methods include gravure coating, roll coating, die coating, comma coating, and knife coating, and comma coating, knife coating, and roll coating are preferred because they tend to form a dielectric layer 2 of sufficient thickness even if the operation of applying the dispersion liquid and drying the coating film is performed only once.

The electromagnetic wave absorber is manufactured, for example, through the following process. First, a roll-shaped dielectric layer is produced by extrusion molding, and then a roll-shaped laminate including the dielectric layer is produced by roll-to-roll lamination. The laminate is cut to a predetermined size to obtain an electromagnetic wave absorber. The roll-shaped dielectric layer is manufactured through (A) a process of preparing a resin composition including the inorganic filler and a resin, and (B) a process of forming a dielectric layer made of this resin composition by extrusion molding. When the frequency band when used as an electromagnetic wave absorber is high (e.g., 60 GHz or more), the thickness of the dielectric layer is sufficiently thin, so that the electromagnetic wave absorber can be manufactured by wet coating. On the other hand, when the frequency band to be applied is low, it is necessary to form it thick, so that manufacturing by wet coating tends to be difficult (e.g., less than 28 GHz). In this case, for example, the dielectric layer can be formed by hot press molding. In other words, the manufacturing method may be appropriately selected according to the frequency band to be used (in other words, the thickness of the dielectric layer).

<Example>
The following materials were used, and the resistance layer materials were applied to a substrate and dried to prepare resistance layers according to the examples and comparative examples.
Substrate: Design film LOVAL (Toppan Printing)
Resistance layer material: PEDOT-PSS (Nagase Chemtex)

The following materials were used to prepare reflective layers according to the examples and comparative examples.
Aluminum deposition film Tetolite SC (Oike Kogyo)

The dielectric layers according to the examples and comparative examples were prepared using the following materials.
Resin component Acrylic resin: OC-3405 (manufactured by Saiden Chemical Industry Co., Ltd., density 0.96 g/cm ³ )
Urethane resin: Elastollan C60A10WN Clear (manufactured by BASF Japan, density 1.1 g/cm ³ )
Inorganic filler: Barium titanate powder: BT-01 (manufactured by Sakai Chemical Industry Co., Ltd., density 6.01 g/cm ³ )
Titanium oxide powder: CR-60 (manufactured by Ishihara Sangyo Kaisha, Ltd., density 4.02 g/cm ³ )
Silica: SO-C1 (Admatechs)

Example 1
Acrylic resin:titanium oxide was mixed in a volume ratio of 25:75, and a dielectric layer having a thickness of 550 μm was prepared using an applicator.

Example 2
Urethane resin and barium titanate were mixed in a volume ratio of 30:70 to prepare resin pellets, and a dielectric layer having a thickness of 440 μm was formed using a heat press machine.

Example 3
Urethane resin and silica were mixed in a volume ratio of 30:70 to prepare resin pellets, and a dielectric layer having a thickness of 800 μm was formed using a heat press machine.

Example 4
Urethane resin and barium titanate were mixed in a volume ratio of 45:55 to prepare resin pellets, and a dielectric layer having a thickness of 787 μm was formed using a heat press machine.

Example 5
Urethane resin:barium titanate was mixed in a volume ratio of 55:45, and a dielectric layer having a thickness of 310 μm was produced using an applicator.

(Comparative Example 1)
A dielectric layer having a thickness of 170 μm was prepared using acrylic resin and an applicator, and four of the prepared dielectric layers were laminated to prepare a dielectric layer having a thickness of 694 μm.

(Comparative Example 2)
A dielectric layer having a thickness of 800 μm was prepared using polycarbonate resin and a heat press machine.

(Comparative Example 3)
A dielectric layer having a thickness of 448 μm was prepared using urethane resin and a heat press machine.

The above-mentioned resistive layer and reflective layer were attached to the dielectric layer described in Examples 1 to 5 and Comparative Examples 1 to 3, respectively, to complete the electromagnetic wave absorbers according to each of the Examples and Comparative Examples.

<Evaluation>
Regarding the above-mentioned examples and comparative examples, the cross-sectional hardness, dielectric constant and electromagnetic wave absorption characteristics of the electromagnetic wave absorbers were measured using the following evaluation methods. The inventors found that the Martens hardness, which is a value of the hardness in the cross-sectional direction, is an effective index for determining good return loss characteristics.

(Martens hardness)
Martens hardness is a type of index that indicates the hardness of a material, and is defined as the quotient of the indentation force calculated from the load and the surface area of the indentation calculated from the indentation depth, which is calculated by applying a load to an indenter and pressing it into the surface of a sample, and measuring the depth of the indentation (indentation mark) formed during the indentation. By applying a load in the cross-sectional direction, the change in thickness due to compressive stress in the cross-sectional direction can be confirmed.
The measurements were carried out using a Martens hardness measuring device (Fisherscope HM2000; Fisher Instruments, Inc.) conforming to ISO 14577.
The samples were taken from the cross section of the electromagnetic wave absorber to avoid the influence of layers other than the dielectric layer during the measurement. Specifically, the electromagnetic wave absorber was embedded in a resin such as a cold-curing epoxy resin or a UV-curing resin, and then fully cured, and the measurement surface was obtained by cutting the electromagnetic wave absorber so that its cross section appeared and mechanically polishing it. The specific measurement method was to press an indenter into the dielectric layer on the measurement surface of each sample, and calculate the Martens hardness from the indentation depth and the load. The measurement was performed under the following measurement conditions: test force 1 mN, test force load time 10 seconds, and test force retention time 0 seconds. When a value of Martens hardness is shown without any special notice, it means the Martens strength of the electromagnetic wave absorber at the beginning (initial stage) of production.

(Dielectric constant)
The dielectric constant of the dielectric sheet was measured in accordance with JIS-C2138: 2007. Specifically, the dielectric constant was measured using a cavity resonator method dielectric constant measuring device (manufactured by Agilent Technologies, Inc., product name "PNA-L Network Analyzer N5230A") under conditions of a frequency of 1 GHz, a temperature of 25°C, and a relative humidity of 50%.

(Rate of change in frequency at which maximum return loss occurs)
First, the electromagnetic wave absorber was placed on the floor, and the return loss of the electromagnetic wave absorber was measured using the following device before and after applying a load of 637 N, assuming a human load of 65 kg. Specifically, millimeter waves were irradiated from a transmitting antenna to the electromagnetic wave absorber, and the intensity of the millimeter waves reflected by the electromagnetic wave absorber and incident on a receiving antenna was measured to obtain the return loss (dB).
Furthermore, the return loss of the electromagnetic wave suppressor after it was exposed to an environment of a temperature of 85° C. and a relative humidity of 85% for 1000 hours was similarly measured.
Vector network analyzer (Keysight PNA N5222B 10MHz-26.5GHz, Virginia DiodesInc, WR12 55-95GHz)
High frequency network analyzer (Agilent Technologies, E8362C)

Next, the rate of change of frequency at which the return loss is maximum is derived as an index using the following formula.
Y1 (%) = (D1 - D0) / D0 x 100
D1: The frequency (GHz) at which the electromagnetic wave body with a load of 637 N experiences the maximum return loss
D0: The frequency (GHz) at which the electromagnetic wave absorber without a load has the maximum return loss
Y2 (%) = (D2 - D0) / D0 x 100
D2: The frequency (GHz) at which the electromagnetic wave absorber has the maximum return loss when subjected to a load of 637 N after being exposed to an environment of 85° C. temperature and 85% relative humidity for 1000 hours
D0: The frequency (GHz) at which the electromagnetic wave absorber without a load has the maximum return loss
Y1 represents the rate of change in frequency at which the maximum return loss occurs when a specified load is applied in the cross-sectional direction in an electromagnetic wave absorber immediately after manufacture (initial stage), and Y2 represents the rate of change in the electromagnetic wave absorber after being exposed to an environment of a temperature of 85°C and a relative humidity of 85% RH for 1000 hours.

The measurement results of each of the above-mentioned indexes are shown in Table 1. For electromagnetic wave absorbers with a Martens hardness of 2.5 N/ ^mm2 or more and 4.5 N/mm2 or ^less , the rate of change in frequency at which the maximum return loss occurred showed a good value of 20% or less in the initial cases for Examples 1 to 5. Furthermore, in the case of being exposed to an environment of a temperature of 85°C and a relative humidity of 85% RH for 1000 hours, Examples 1 to 4 showed a good value of 20% or less, and it was found that the return loss characteristics had good resistance to the load in the cross-sectional direction for all of them.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned embodiment, and various modifications are possible without departing from the gist of the present invention.

(Case Study)
As an example, the electromagnetic wave absorber according to the present invention may be used as a floor in a building on which loads from people, furniture, office supplies, etc. are applied. Fig. 2 is a schematic diagram showing an example of an electromagnetic wave absorber 10 installed on a floor 20. However, the installation of the electromagnetic wave absorber is not limited to this as long as a load is expected from the normal direction of the installation surface, and it may be installed on a wall or a slope, or outdoors on a sidewalk or a hill. Furthermore, it is not prevented from being installed on a curved surface. It may also be used as a standard specification when manufacturing or selecting an electromagnetic wave absorber to be applied to the above-mentioned application example.

The following are examples of items that may be included in the present invention, but are not limited to these.
(Item 1)
1. An electromagnetic wave absorber comprising a resistive layer, a dielectric layer, and a reflective layer, the Martens hardness of the dielectric layer being 2.5 N/ ^mm2 or more and 4.5 N/ ^mm2 or less.
(Item 2)
2. The electromagnetic wave absorber according to item 1, wherein the dielectric layer comprises one or more resin components and one or more inorganic fillers.
(Item 3)
3. The electromagnetic wave absorber according to item 2, wherein the inorganic filler is mixed in an amount of 10 to 80% by volume.
(Item 4)
Item 4. The electromagnetic wave absorber according to item 2 or 3, wherein the inorganic filler is a metal compound.
(Item 5)
5. The electromagnetic wave absorber according to item 4, wherein the metal compound is at least one of titanium oxide and barium titanate.
(Item 6)
6. The electromagnetic wave absorber according to any one of items 1 to 5, wherein the dielectric layer has a relative dielectric constant of 10 or more.
(Item 7)
7. The electromagnetic wave absorber according to any one of items 1 to 6, wherein when the electromagnetic wave absorber is placed on a floor and a load of 637 N is applied thereto, the rate of change in frequency at which the maximum return loss occurs is 20% or less.
(Item 8)
8. The electromagnetic wave absorber according to any one of items 1 to 7, having a Martens hardness of 2.5 N/mm2 or more and 4.5 N/ ^mm2 or ^less after being exposed to an environment of a temperature of 85° C. and a relative humidity of 85% RH for 1000 hours.
(Item 9)
Item 9. The electromagnetic wave absorber according to any one of items 1 to 8, wherein when the electromagnetic wave absorber is exposed to an environment of a temperature of 85° C. and a relative humidity of 85% RH for 1000 hours, placed on a floor, and a load of 637 N is applied thereto, a rate of change in frequency at which the maximum return loss occurs is 20% or less.

1...resistive layer, 2...dielectric layer, 3...reflective layer,
10...electromagnetic wave absorber 20...floor

Claims (8)

The insulating film has a resistive layer, a dielectric layer, and a reflective layer, and the Martens hardness of the dielectric layer is 2.5 N/ ^mm2 or more and 4.5 N/ ^mm2 or less ;
The dielectric layer contains one or more resin components and one or more inorganic fillers .
Electromagnetic wave absorber. 2. The electromagnetic wave absorber according to claim 1 , wherein the inorganic filler is mixed in an amount of 10 to 80% by volume. 3. The electromagnetic wave absorber according to claim 1 , wherein the inorganic filler is a metal compound. 4. The electromagnetic wave absorber according to claim 3 , wherein the metal compound is at least one of titanium oxide and barium titanate. 3. The electromagnetic wave absorber according to claim 1, wherein the dielectric layer has a relative dielectric constant of 10 or more. 3. The electromagnetic wave absorber according to claim 1 , wherein when the electromagnetic wave absorber is placed on a floor and a load of 637 N is applied thereto, the rate of change in frequency at which the maximum return loss occurs is 20% or less. 3. The electromagnetic wave absorber according to claim 1, having a Martens hardness of 2.5 N/mm2 or more and 4.5 N/mm2 or ^{less after} being exposed to an environment of a temperature of 85°C and a relative humidity of 85% RH for 1000 hours. 3. The electromagnetic wave absorber according to claim 1 or 2, wherein when the electromagnetic wave absorber is exposed to an environment of a temperature of 85°C and a relative humidity of 85% RH for 1000 hours, placed on a floor, and subjected to a load of 637 N, the rate of change in frequency at which the maximum return loss occurs is 20% or less.

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