JP2019057730A - Electromagnetic wave absorber - Google Patents

Abstract

To provide an electromagnetic wave absorber which can be used for a radar having high solution, and which can sufficiently cope with the case of using a plurality of radars having different frequencies.SOLUTION: There is provided an electromagnetic wave absorber having a dielectric layer and a conductive layer provided on one surface of the dielectric layer, therein the thickness of the conductive layer is ranging from 20 nm to 100 μm, and the band width of a frequency band in which an electromagnetic wave absorption volume of the electromagnetic wave absorber is 20 dB or more is 2GHz or more in a frequency band of 60 to 90 GHz.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an electromagnetic wave absorber for preventing electromagnetic interference.

  In recent years, the use of electromagnetic waves as information communication media has progressed. Examples of the use of such electromagnetic waves include, for example, in the technical field of automobiles, obstacles are detected by radar and brakes are automatically applied, or the speed and distance between vehicles are measured by measuring the speed and distance between surrounding vehicles. There is a collision prevention system that controls. In order for a collision prevention system or the like to operate normally, it is important to avoid receiving unnecessary electromagnetic waves (noise) as much as possible in order to prevent misidentification. Therefore, an electromagnetic wave absorber that absorbs noise may be used in these systems and the like (see, for example, Patent Documents 1 and 2).

  In order to achieve higher detection performance in the collision prevention system and the like, the performance of the radar itself has been improved, and the use of radars with higher frequencies (76.5 GHz, 79 GHz) than the conventional frequencies (24 GHz). Because of the progress, there is a demand for an electromagnetic wave absorber that absorbs noise in a high frequency band. In addition, in order to improve the resolution of the radar, the use frequency is widened (1 GHz in the case of 76 GHz, 4 GHz in the case of 79 GHz), and the electromagnetic wave absorber is also required to have an absorption performance in a wide bandwidth. However, as shown in Patent Documents 1 and 2, conventional electromagnetic wave absorbers can only exhibit absorptivity only in a limited range near the target frequency, and cannot cover a high frequency band. There is. In addition, if the characteristics of the materials that make up the electromagnetic wave absorber change due to changes in the environment in use or over time, the frequency that can be absorbed (absorption peak) may also fluctuate accordingly. There is a concern that sufficient absorption capacity cannot be exhibited. There is also a problem that if the radar frequency fluctuates even a little, the absorption ability cannot be exhibited.

  Further, in the collision prevention system or the like, it is conceivable that a plurality of radars having different frequencies are used in combination in order to achieve higher accuracy. However, as described above, the absorption capability of the electromagnetic wave absorber can usually be exhibited only in a very limited range near the target frequency, so it is necessary to prepare a different electromagnetic wave absorber for each radar having a different frequency. There is a problem that the cost of the electromagnetic wave absorber is increased and the total weight is increased by using a large number of electromagnetic wave absorbers.

Japanese Patent Laid-Open No. 6-120688 Japanese Patent Laid-Open No. 10-13082

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an electromagnetic wave absorber having excellent absorption capability in a wide bandwidth that can be used for a radar having high resolution. .

In order to achieve the above object, the present invention provides the following [1] and [2].
[1] An electromagnetic wave absorber having a dielectric layer and a conductive layer provided on one surface of the dielectric layer, wherein the conductive layer has a thickness in the range of 20 nm to 100 μm and a frequency of 60 to 90 GHz. An electromagnetic wave absorber characterized in that, in a band, a frequency band having an electromagnetic wave absorption amount of 20 dB or more has a bandwidth of 2 GHz or more.
[2] The electromagnetic wave absorber according to [1], wherein the sheet resistance of the conductive layer is set in a range of 1.0 × 10 ^{−7 to} 100 Ω / □.

  The present inventors have paid attention to the relationship between the frequency of radar with improved resolution and the amplitude of the wave, and have earnestly aimed to obtain an electromagnetic wave absorber having excellent absorption capability that can cope with these radars. I did research. As a result, in the frequency band of 60 to 90 GHz, it is found that the above problem can be solved by using an electromagnetic wave absorber that has an electromagnetic wave absorption amount of 20 dB or more and an electromagnetic wave absorber of 2 GHz or more, and the present invention is achieved. It came to.

  The electromagnetic wave absorber of the present invention has a frequency band with an electromagnetic wave absorption amount of 20 dB or more in the frequency band of 60 to 90 GHz and 2 GHz or more, and can eliminate noise in a wide frequency band.

It is sectional drawing of the electromagnetic wave absorber which is one of embodiment of this invention. It is a figure explaining the case where the adhesion layer is provided in the electromagnetic wave absorber shown in FIG. (A), (b) is a figure explaining the manufacturing method of the electromagnetic wave absorber shown in FIG. It is sectional drawing of the electromagnetic wave absorber which is other embodiment of this invention. It is a figure explaining the case where the adhesion layer is provided in the electromagnetic wave absorber shown in FIG. (A)-(f) is the graph which measured the reflection absorption amount of Examples 1-6, respectively, and showed the relationship between a frequency (GHz) and reflection absorption amount (dB). (A)-(f) is the graph which measured the reflective absorption amount of Examples 7-10 and Comparative Examples 1 and 2, respectively, and showed the relationship between a frequency (GHz) and reflective absorption amount (dB).

  Next, embodiments of the present invention will be described in detail. However, the present invention is not limited to this embodiment.

  The electromagnetic wave absorber of the present invention has a frequency band of 2 GHz or more, preferably 5 GHz or more, more preferably 10 GHz or more, in the frequency band of 60 to 90 GHz, in which the electromagnetic wave absorption is 20 dB or more. And the upper limit is usually 30 GHz. More preferably, it has 2 GHz or more in the frequency band of 70 to 85 GHz, more preferably 5 GHz or more, and still more preferably 10 GHz or more, and the upper limit is usually 30 GHz.

  The electromagnetic wave absorption amount and the bandwidth of the frequency band in which the electromagnetic wave absorption amount is 20 dB or more can be measured, for example, by a reflected power method, a waveguide method, or the like. In the present invention, a radio wave absorber (radio wave absorbing material) / reflection loss measuring device LAF-26.5B manufactured by Keycom Corporation is used for JIS R 1679 (radio wave absorption characteristic measuring method in millimeter wave band of radio wave absorber). In conformity, the sample was irradiated with electromagnetic waves at an oblique incidence of 15 °, and the amount of reflected absorption was measured to obtain the amount of electromagnetic wave absorption. Further, from the reflection / absorption curve obtained in the same measurement, the frequency band in which the reflection absorption amount is 20 dB or more is specified, and the bandwidth of the frequency band in which the electromagnetic wave absorption amount is 20 dB or more is set.

  According to this configuration, high-frequency electromagnetic waves, for example, electromagnetic waves having a specific wavelength in the frequency band of 76 to 81 GHz can be surely eliminated. Therefore, as a radar having higher resolution, 76 to 81 GHz Even when a nearby frequency is adopted, the generated noise can be surely eliminated. In addition, even if the characteristics of the materials that make up the electromagnetic wave absorber change due to environmental changes or changes over time, and the frequency (absorption peak) that can be absorbed fluctuates, the radar set as an exclusion target It is possible to exhibit sufficient absorption ability at a frequency of. In addition, even when the radar frequency fluctuates, sufficient absorption capability can be exhibited. Furthermore, even when a plurality of radars having different frequencies in the vicinity of the frequency are used, noise from the plurality of radars can be reliably eliminated. For this reason, it is not necessary to use an electromagnetic wave absorber having different performance for each radar having different frequencies as in the prior art, and low cost can be realized.

  The electromagnetic wave absorber of the present invention is any of a magnetic electromagnetic wave absorber using magnetic loss, a dielectric electromagnetic wave absorber using dielectric loss, a conductive electromagnetic wave absorber using resistance loss, and a λ / 4 type electromagnetic wave absorber. Although a λ / 4 type electromagnetic wave absorber is preferable in terms of durability, light weight, and easy thinning, a magnetic electromagnetic wave absorber and dielectric property are excellent in terms of workability. An electromagnetic wave absorber is preferred.

As the electromagnetic wave absorber of the present invention which is the λ / 4 type electromagnetic wave absorber, for example, as shown in FIG. 1, it has a resistance layer A, a dielectric layer B, and a conductive layer C in this order, Examples include resin layers D ₁ and D ₂ provided on the outside of the resistance layer A and the outside of the conductive layer C, respectively. In addition, in FIG. 1, each part is shown typically and is different from the actual thickness, size, etc. (the same applies to the following figures). Further, since the resistance layer A, the dielectric layer B, and the conductive layer C can be sufficiently effective, the resin layers D ₁ and D ₂ are arbitrarily provided.

Since the resistance layer A is required to transmit electromagnetic waves to the inside of the electromagnetic wave absorber, it preferably has a relative dielectric constant close to air. Usually, indium tin oxide (hereinafter referred to as “ITO”) is used. Used. Among these, from 20 to 40% by weight of SnO ₂ , more preferably from 25 to 40% by weight, since the amorphous structure is extremely stable and fluctuations in sheet resistance of the resistance layer A can be suppressed even in a high-temperature and high-humidity environment. Those mainly composed of ITO containing 35% by weight of SnO ₂ are preferably used. In the present invention, “main component” means a component that affects the characteristics of the material, and the content of the component is usually 50% by mass or more of the whole material, and naturally What consists only of an ingredient is also included.

  The sheet resistance of the resistance layer A is preferably set in the range of 320 to 500 Ω / □, more preferably in the range of 360 to 450 Ω / □. This is because when the sheet resistance of the resistance layer A is within the above range, it is easy to selectively absorb electromagnetic waves having a wavelength (frequency) that is widely used as a millimeter wave radar or a quasi-millimeter wave radar.

  The thickness of the resistance layer A is preferably in the range of 15 to 100 nm, and more preferably in the range of 25 to 50 nm. This is because, when the thickness is too thick or conversely too thin, there is a tendency that the reliability of the sheet resistance value is lowered when a change with time or an environmental change is applied.

  The dielectric layer B is obtained by forming a resin composition having a predetermined relative dielectric constant so as to have a predetermined thickness after curing in accordance with the wavelength of the electromagnetic wave to be absorbed, and curing the resin composition. is there. Examples of the resin composition include ethylene vinyl acetate copolymer (EVA), vinyl chloride, urethane, acrylic, acrylic urethane, polyolefin, polyethylene, polypropylene, silicone, polyethylene terephthalate, polyester, polystyrene, polyimide, polycarbonate, polyamide, polysulfone, Synthetic rubber materials such as polyethersulfone and epoxy, and polyisoprene rubber, polystyrene / butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile / butadiene rubber, butyl rubber, acrylic rubber, ethylene / propylene rubber and silicone rubber It is preferable to use it as a resin component. In particular, EVA or acrylic resin is preferably used in terms of moldability and relative dielectric constant. These may be used alone or in combination of two or more, and the dielectric layer B may be a single layer or multiple layers.

  Further, since the dielectric layer B has a wider band as the relative dielectric constant is smaller, a foam obtained by foaming the above material may be used. Moreover, as such a foam, a highly flexible foam is preferably used.

  The relative dielectric constant of the dielectric layer B is preferably in the range of 1 to 10, more preferably in the range of 1 to 5, and still more preferably in the range of 1 to 3. When the relative dielectric constant is within the above range, the dielectric layer can be set to a thickness that can be easily controlled, and the frequency band with an electromagnetic wave absorption of 20 dB or more can be set to a wider bandwidth. Thus, an electromagnetic wave absorber having a more uniform absorption ability can be obtained.

  The relative dielectric constant of the dielectric layer B is measured by a cavity resonator perturbation method using a network analyzer N5230C manufactured by Agilent Technologies, a cavity resonator CP531 manufactured by Kanto Electronics Application Development Co., Ltd., and the like. be able to.

  The thickness of the dielectric layer B is preferably 50 to 2000 μm, more preferably 100 to 1500 μm, and still more preferably 100 to 1000 μm. If the thickness is too thin, it is difficult to ensure the thickness dimensional accuracy, and the accuracy of the absorption performance may be lowered. If the thickness is too thick, the weight may increase and it may be difficult to handle, and the material cost tends to increase.

The conductive layer C is disposed to reflect the target electromagnetic wave in the vicinity of the back surface of the electromagnetic wave absorber, and the sheet resistance is set sufficiently lower than the sheet resistance of the resistance layer A. For these reasons, examples of the material of the conductive layer C include ITO, aluminum (Al), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), and alloys of these metals. It is done. In particular, by using ITO for the conductive layer C, it is possible to provide a transparent electromagnetic wave absorber, and not only can be applied to parts where transparency is required, but also improve workability. In particular, ITO containing 5 to 15% by weight of SnO ₂ is preferably used. The thickness when ITO is used for the conductive layer C is preferably 20 to 200 nm, and more preferably 50 to 150 nm. This is because if the thickness is too thick, the conductive layer C tends to crack due to stress, and if it is too thin, a desired low resistance value tends to be difficult to obtain. On the other hand, Al or an alloy thereof is preferably used from the viewpoint that the sheet resistance value can be lowered more easily and noise can be further reduced. The thickness of the conductive layer C when Al or a metal alloy thereof is used is preferably 20 nm to 100 μm, and more preferably 50 nm to 50 μm. If the thickness is too thick, the electromagnetic wave absorber tends to be rigid and difficult to handle, and if it is too thin, a desired low resistance value tends to be difficult to obtain. The sheet resistance of the conductive layer C is preferably 1.0 × 10 ^-7 Ω~100Ω, is preferably 1.0 × 10 ^-7 Ω~20Ω.

The resin layers D ₁ and D ₂ serve as substrates when the resistance layer A or the conductive layer C is formed by sputtering or the like. After the resin layers D ₁ and D ₂ are formed on the electromagnetic wave absorber, the resistance layer A and the conductive layer C are externally connected. It plays a role of protecting from impacts from the like. The material of the resin layers D ₁ and D ₂ is preferably one that can withstand high temperatures such as vapor deposition and sputtering used to form the resistance layer A or the conductive layer C. For example, polyethylene terephthalate (PET), Examples thereof include polyethylene naphthalate (PEN), acrylic (PMMA), polycarbonate (PC), and cycloolefin polymer (COP). Of these, PET is preferably used because of its excellent heat resistance and good balance between dimensional stability and cost. The resin layers D ₁ and D ₂ may be made of the same material, or may be made of different materials. Moreover, it may be not only a single layer but a multiple layer, and it is not necessary to provide the resin layers D ₁ and D ₂ .

The thicknesses of the resin layers D ₁ and D ₂ are each preferably 10 to 125 μm, and more preferably 20 to 50 μm. This is because if the thickness is too thin, wrinkles and deformation tend to occur when the resistance layer A is formed. If the thickness is too thick, the flexibility as an electromagnetic wave absorber tends to be reduced. Further, the resin layers D ₁ and D ₂ may have the same thickness or different thicknesses.

In the above embodiment, the electromagnetic wave absorber is composed of a laminate of the resistance layer A, the dielectric layer B, the conductive layer C, and the resin layers D ₁ and D _2. Other layers may be provided. That is, other layers may be provided outside the resin layer D ₁ , between the resistance layer A and the dielectric layer B, between the dielectric layer B and the conductive layer C, outside the resin layer D ₂ , and the like. For example, if a coat layer (not shown) is provided between the resistance layer A and the dielectric layer B, the components in the dielectric layer B can be prevented from diffusing into the resistance layer A. Protection can be achieved. Similarly, when a coating layer (not shown) is provided between the conductive layer C and the dielectric layer B, components in the dielectric layer B can be prevented from diffusing into the conductive layer C. Can be protected. Further, as shown in FIG. 2, when providing an adhesive layer G on the outer side of the resin layer D ₂ is attached to the other member (the attachment member) is facilitated.

Examples of the material of the coating layer include silicon dioxide (SiO ₂ ), silicon nitride (SiN), aluminum oxide (Al ₂ O ₃ ), aluminum nitride (AlN), niobium oxide (Nb ₂ O ₅ ), and tin / silicon. An oxide (STO), aluminum-containing zinc oxide (AZO), silicon nitride (SiN), or the like can be used.

  As the material of the adhesive layer G, for example, an adhesive such as a rubber adhesive, an acrylic adhesive, a silicone adhesive, and a urethane adhesive can be used. It is also possible to use adhesives such as emulsion adhesives, rubber adhesives, epoxy adhesives, cyanoacrylic adhesives, vinyl adhesives, silicone adhesives, etc., depending on the material and shape of the mounted member It can be selected appropriately. Among them, an acrylic pressure-sensitive adhesive is preferably used from the viewpoint of exhibiting adhesive force over a long period of time and having high attachment reliability.

  Such an electromagnetic wave absorber (see FIG. 1) can be manufactured, for example, as follows.

First, as shown in FIG. 3 (a), to form the resistive layer A on the film resin layer formed into a shape D ₁ (lower in the figure). Further, a conductive layer C on top of the film to the molded resin layer D _2. The resistance layer A and the conductive layer C can be formed by sputtering, vapor deposition, or the like. Of these, it is preferable to use sputtering because the resistance value and thickness can be strictly controlled.

Next, as shown in FIG.3 (b), the resin composition which forms the dielectric material layer B is press-molded in a sheet form. Then, the resistance layer A formed on the resin layer D ₁ is overlaid on _one surface of the dielectric layer B, and the conductive layer C formed on the resin layer D ₂ is overlaid on the other surface. . Thereby, the electromagnetic wave absorber in which the resin layer D ₁ , the resistance layer A, the dielectric layer B, the conductive layer C, and the resin layer D ₂ shown in FIG. 1 are laminated in this order can be obtained.

According to this, since it is easy to control the thickness of the dielectric layer B, an electromagnetic wave absorber that effectively absorbs an electromagnetic wave having a target wavelength (frequency) can be obtained. Moreover, since the resistance layer A and the conductive layer C can be formed separately, the time required for the production of the electromagnetic wave absorber can be shortened, and the production can be carried out at a low cost. In the case where the resin layers D ₁ and D ₂ are not provided, for example, the electromagnetic wave absorber is manufactured by directly sputtering or vapor-depositing the material of the resistance layer A and the conductive layer C on the dielectric layer B. be able to.

  Next, examples of the electromagnetic wave absorber of the present invention which is the magnetic electromagnetic wave absorber or the dielectric electromagnetic wave absorber include those having a dielectric layer E and a conductive layer F as shown in FIG. It is done. The magnetic electromagnetic wave absorber is an electromagnetic wave absorber that absorbs an electromagnetic wave irradiated from the outside of the dielectric layer E by a magnetic loss using a tracking delay of the magnetic moment of the added magnetic body. On the other hand, the dielectric electromagnetic wave absorber is an electromagnetic wave absorber that absorbs by heat loss using the follow-up delay of polarization of the added dielectric. In addition, it is good also as an electromagnetic wave absorber which added combining the magnetic body and the dielectric material.

  In the case of a magnetic electromagnetic wave absorber, the dielectric layer E is formed by adding a magnetic material to a resin composition made of the same material as the dielectric layer B so as to have a predetermined thickness after curing. Can be obtained by curing. Examples of the magnetic material include those that absorb electromagnetic waves by an applied electric field. For example, conductive powder such as ketjen black, acetylene black, furnace black, graphite, and expanded graphite, magnetic powder such as iron, nickel, and ferrite. Etc. can be used. Of these, from the viewpoint of excellent dispersibility in the resin composition, a complex carbonyl metal is preferably used, and carbonyl iron powder is particularly preferably used.

  In the case of a dielectric electromagnetic wave absorber, the dielectric layer E is formed so that a resin composition made of the same material as the dielectric layer B contains a dielectric and has a predetermined thickness after curing. And can be obtained by curing. Examples of the dielectric include those that absorb electromagnetic waves by an applied magnetic field, such as carbon such as ketjen black, acetylene black, furnace black, graphite, expanded graphite, barium titanate, lead zirconate titanate, etc. Ferroelectric materials can be used. Among these, carbon powder is preferably used from the viewpoint of excellent material cost.

  The thickness of the dielectric layer E is preferably 50 to 2000 μm, and more preferably 100 to 1500 μm. This is because if it is too thin, it tends to be difficult to ensure thickness dimensional accuracy, and if it is too thick, not only the material cost increases, but also the weight tends to increase excessively.

  The relative dielectric constant of the dielectric layer E is preferably in the range of 1 to 10, and more preferably in the range of 1 to 5. When the relative dielectric constant is within the above range, the dielectric layer can be set to a thickness that can be easily controlled, and the frequency band with an electromagnetic wave absorption of 20 dB or more can be set to a wider bandwidth. In addition, it is possible to obtain an electromagnetic wave absorber having a more uniform absorption ability. And since setting and manufacture are easy, a low-cost electromagnetic wave absorber is realizable.

  Since the conductive layer F is disposed to reflect an electromagnetic wave having a target wavelength (frequency) in the vicinity of the back surface of the electromagnetic wave absorber, examples of the material of the conductive layer F include ITO, aluminum ( Al), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), and alloys of these metals.

In addition, the thickness of the conductive layer F is preferably 20 nm to 100 μm, and more preferably 50 nm to 50 μm. This is because when the thickness is too thick, stress and cracks tend to easily enter the conductive layer F, and when it is too thin, a desired low resistance value tends to be difficult to obtain. The sheet resistance of the conductive layer F is preferably 1.0 × 10 ^-7 Ω~100Ω, and more preferably 1.0 × 10 ^-7 Ω~20Ω.

  Such an electromagnetic wave absorber (see FIG. 4) can be manufactured by, for example, sputtering or vapor-depositing the material of the conductive layer F on the dielectric layer E formed into a sheet shape by press molding or the like.

  In the above embodiment, the electromagnetic wave absorber is formed of a laminate of the dielectric layer E and the conductive layer F. However, other layers may be provided on the electromagnetic wave absorber. That is, other layers may be provided outside the dielectric layer E, between the dielectric layer E and the conductive layer F, outside the conductive layer F, and the like. For example, when a coat layer (not shown) is provided between the dielectric layer E and the conductive layer F, components in the dielectric layer E can be prevented from diffusing into the conductive layer F. Protection can be achieved. Moreover, as shown in FIG. 5, when the adhesion layer G is provided on the outer side of the resin layer F, attachment to another member (attached member) becomes easy. As the material for the coating layer and the adhesive layer G, the same materials as those in the embodiment shown in FIG. 1 can be used.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to the following examples unless it exceeds the gist.

  As shown below, the electromagnetic wave absorbers of Examples 1 to 10 and Comparative Examples 1 and 2 were prepared, and for these, a radio wave absorber (radio wave absorbing material) / reflection loss measuring apparatus LAF-26.5B manufactured by Keycom Corporation was used. In accordance with JIS R 1679 (a method for measuring radio wave absorption characteristics of a radio wave absorber in the millimeter wave band), an electromagnetic wave was irradiated at an oblique incidence of 15 °, and the amount of reflected absorption was measured. The results are shown in Table 1 below and FIG. 6 and FIG.

<Example 1>
In accordance with the method of obtaining the electromagnetic wave absorber shown in FIG. 1, EVA resin (Evaflex EV250, relative dielectric constant 2.45) manufactured by Mitsui DuPont is press-molded at 120 ° C. and molded into a 560 μm thick sheet, and the dielectric layer B Was made. On one surface of the dielectric layer B, a 38 μm-thick PET film (resin layer D ₁ ) in which ITO is sputtered so as to have a surface resistance of 20 Ω / □ as the conductive layer C is used. Lamination was performed so as to face layer B. Then, the resistance layer A is a 38 μm thick PET film (resin layer D ₂ ) on which the ITO is sputter-formed on the other surface of the dielectric layer B so as to have a surface resistance of 380Ω / □ as the resistance layer A. The target electromagnetic wave absorber was obtained by bonding so as to face the dielectric layer B.

<Example 2>
A target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that the dielectric layer B was changed as follows in accordance with the method for obtaining the electromagnetic wave absorber shown in FIG.
(Dielectric layer B)
50 parts by weight of Sakai Chemical Industry's barium titanate (BT-01) is added to 100 parts by weight of Mitsui DuPont EVA resin (Evaflex EV250), kneaded with a mixing roll, and then press-molded at 120 ° C. Dielectric layer B was fabricated by forming into a 458 μm thick sheet. The dielectric constant of this dielectric layer B was 3.90.

<Example 3>
A target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that the dielectric layer B was changed as follows in accordance with the method for obtaining the electromagnetic wave absorber shown in FIG.
(Dielectric layer B)
To 100 parts by weight of Mitsui DuPont EVA resin (Evaflex EV250), 100 parts by weight of barium titanate (BT-01) manufactured by Sakai Chemical Industry Co., Ltd. was added and kneaded with a mixing roll, and then press-molded at 120 ° C. A dielectric layer B was produced by forming into a 397 μm thick sheet. The dielectric constant of this dielectric layer B was 5.19.

<Example 4>
A target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that the dielectric layer B was changed as follows in accordance with the method for obtaining the electromagnetic wave absorber shown in FIG.
(Dielectric layer B)
200 parts by weight of Sakai Chemical Industry's barium titanate (BT-01) was added to 100 parts by weight of Mitsui DuPont EVA resin (Evaflex EV250), kneaded with a mixing roll, and then press-molded at 120 ° C. Dielectric layer B was fabricated by forming into a 336 μm thick sheet. The dielectric constant of this dielectric layer B was 7.25.

<Example 5>
In accordance with the method of obtaining the electromagnetic wave absorber shown in FIG. 1, the dielectric layer B is changed to one obtained by slicing an olefin foam SCF100 (relative permittivity 1.07) manufactured by Nitto Denko Corporation to a thickness of 822 μm. A target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that A and the conductive layer C were bonded to the dielectric layer B via an acrylic adhesive having a thickness of 30 μm.

<Example 6>
In accordance with the method for obtaining the electromagnetic wave absorber shown in FIG. 1, the dielectric layer B is changed to a polyester foam SCF T100 (relative dielectric constant 1.09) sliced to a thickness of 793 μm, and the resistance layer A and the conductive layer A target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that C was bonded to the dielectric layer B via an acrylic adhesive having a thickness of 30 μm.

<Example 7>
According to the method for obtaining the electromagnetic wave absorber shown in FIG. 4, 300 parts by weight of New Metals End Chemicals carbonyl iron powder YW1 is added to 100 parts by weight of Mitsui DuPont EVA resin (Evaflex EV250) and kneaded with a mixing roll. After that, it was press-molded at 120 ° C. and formed into a 1200 μm thick sheet to produce a dielectric layer E. The dielectric constant of this dielectric layer E was 6.60. An ITO film (surface resistance 20Ω / □) was bonded as one conductive layer F to one surface of the dielectric layer E to obtain a target electromagnetic wave absorber.

<Example 8>
In accordance with the method of obtaining the electromagnetic wave absorber shown in FIG. 4, as the conductive layer F, an aluminum foil / PET composite film (aluminum foil 7 μm / PET 9 μm manufactured by UACJ) is bonded with the aluminum foil surface facing the dielectric layer E. A target electromagnetic wave absorber was obtained in the same manner as in Example 7 except that.

<Example 9>
In accordance with the method of obtaining the electromagnetic wave absorber shown in FIG. 1, the dielectric layer B is press-molded at 150 ° C. with a thermoplastic acrylic elastomer (clarity 2330, relative dielectric constant 2.55) manufactured by Kuraray Co., Ltd., and a sheet having a thickness of 561 μm The target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that the molded product was changed to one formed into the shape.

<Example 10>
In accordance with the method for obtaining the electromagnetic wave absorber shown in FIG. 1, the dielectric layer B is press-molded at 150 ° C. with a thermoplastic acrylic elastomer (clarity 2330, relative dielectric constant 2.55) manufactured by Kuraray Co., Ltd., and a sheet having a thickness of 538 μm Except that the aluminum foil / PET composite film (aluminum foil 7 μm / PET 9 μm manufactured by UACJ) was bonded as the resistance layer A with the aluminum foil surface facing the dielectric layer B. The target electromagnetic wave absorber was obtained in the same manner as in Example 1.

<Comparative Example 1>
A target electromagnetic wave absorber was obtained in the same manner as in Example 1 except that the dielectric layer B was changed as follows in accordance with the method for obtaining the electromagnetic wave absorber shown in FIG.
(Dielectric layer B)
To 100 parts by weight of Mitsui DuPont EVA resin (Evaflex EV250), 300 parts by weight of Sakai Chemical Industry's barium titanate (BT-01) was added and kneaded with a mixing roll, followed by press molding at 120 ° C. Dielectric layer B was fabricated by molding into a 242 μm thick sheet. The dielectric constant of this dielectric layer B was 14.0.

<Comparative example 2>
According to the method for obtaining the electromagnetic wave absorber shown in FIG. 4, the intended electromagnetic wave absorber was obtained in the same manner as in Example 7 except that the dielectric layer E was changed as follows.
(Dielectric layer E)
400 parts by weight of New Metals End Chemicals' carbonyl iron powder YW1 is added to 100 parts by weight of Mitsui DuPont EVA resin (Evaflex EV250), kneaded with a mixing roll, press-molded at 120 ° C., and 1200 μm thick sheet To form a dielectric layer E. The dielectric constant of this dielectric layer E was 10.3.

  From the results of Table 1 and FIGS. 6 and 7, Examples 1 to 10 have a bandwidth of 2 GHz or more in the frequency band where the reflection absorption amount is 20 dB or more in the frequency band of 60 to 90 GHz. -3, 5, 6, 9, and 10 have a wide width of 10.0 GHz or more. Further, the 20 dB bandwidth tends to increase as the relative dielectric constant decreases. On the other hand, Comparative Examples 1 and 2 exhibited some absorption ability in the frequency band of 60 to 90 GHz, but could not realize the absorption ability with a reflection absorption amount of 20 dB or more in any range. .

  INDUSTRIAL APPLICABILITY Since the present invention can exhibit the performance of absorbing unnecessary electromagnetic waves over a long period of time in a wide frequency band, it can be suitably used for an electromagnetic wave absorber of a millimeter wave radar used in an automobile collision prevention system. In addition, in other applications, such as intelligent road traffic systems (ITS) that perform information communication between automobiles, roads, and people, and next-generation mobile communication systems (5G) that use millimeter waves, it is possible to suppress radio interference and reduce noise. Can be used for purposes.

Claims (2)

  An electromagnetic wave absorber having a dielectric layer and a conductive layer provided on one surface of the dielectric layer, wherein the thickness of the conductive layer is in the range of 20 nm to 100 μm, and in a frequency band of 60 to 90 GHz, An electromagnetic wave absorber characterized in that a frequency band having an electromagnetic wave absorption amount of 20 dB or more has a bandwidth of 2 GHz or more. The electromagnetic wave absorber according to claim 1, wherein the sheet resistance of the conductive layer is set in a range of 1.0 × 10 ^{−7 to} 100 Ω / □.

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