JP2003198179A - Electromagnetic wave absorber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a matching-type electromagnetic wave absorber which functions in a broad region of incident angle and can be easily designed and manufactured especially in a millimetric-wave region. <P>SOLUTION: An electromagnetic wave reflecting layer 4 is formed on the rear side of an electromagnetic absorbing layer. The electromagnetic wave absorbing layer has a multilayer structure which is provided with a first dielectric layer 1 exhibiting a relative permittivity of 1.2-1.8, a resistor thin film layer 2 exhibiting a sheet resistance of 220 Ω/cm<SP>2</SP>-380 Ω/cm<SP>2</SP>and a second dielectric layer 3 in order from its surface. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic wave absorber for preventing electromagnetic interference, and more particularly to an electromagnetic wave absorber that functions effectively even with obliquely incident electromagnetic waves.

[0002]

2. Description of the Related Art In recent years, electromagnetic waves in the microwave to millimeter wave region have been rapidly used as an information communication medium.
In ITS (Intelligent Transport System), it is about to be used for an automatic toll collection system (ETC) for toll roads, an automatic collision prevention radar (ARPA), and the like. In order for such a system to operate normally, it is necessary to prevent unnecessary electromagnetic waves from being radiated into the space as much as possible, and electromagnetic wave absorbers are used for that purpose.

The electromagnetic wave absorber for such a purpose is of a type that absorbs only electromagnetic waves of a specific frequency, and has an electromagnetic wave absorption layer having a predetermined dielectric constant and dielectric loss tangent and having a predetermined thickness in accordance with the target frequency. A so-called matching type electromagnetic wave absorber having an electromagnetic wave reflection layer provided on one surface and a type that absorbs electromagnetic waves over a wide band of frequencies are known.

The former is a dielectric property and a thickness of the electromagnetic wave absorbing layer so that the electromagnetic wave reflected on the surface of the electromagnetic wave absorbing body and the electromagnetic wave reflected at the interface between the electromagnetic wave reflecting layer on the back side and the electromagnetic wave absorbing layer cancel each other out. Is designed. As this type of electromagnetic wave absorber, an electromagnetic wave absorbing layer in which carbon particles, ferrite particles, titanium oxide particles, silicon carbide fibers and the like having electromagnetic wave absorbing ability are dispersed in various resins and an electromagnetic wave reflecting layer made of metal are laminated. The ones that made them known are known.

For example, JP-A-4-340299 discloses an example in which carbon black is dispersed in epoxy-modified urethane rubber as an electromagnetic wave absorbing layer. This type of electromagnetic wave absorber has a drawback that a sufficient return loss cannot be obtained when the incident angle of the electromagnetic wave exceeds 30 °.

As another matching type electromagnetic wave absorber, a type in which a resistive thin film layer is provided through an electromagnetic wave reflection layer and a spacer of λ / 4 (λ is the wavelength of the electromagnetic wave of interest) is disclosed in Japanese Patent Laid-Open No. Hei 5 (1999) -58. It is disclosed in Japanese Patent No. 114813. However, even with this type of electromagnetic wave absorber, when the incident angle of the electromagnetic wave exceeds 30 °, a sufficient return loss cannot be obtained.

Examples of the latter type having an absorption performance for a wide range of frequencies include, for example, a pyramid-shaped surface, and the dielectric constant is gradually changed from the air layer toward the inside of the electromagnetic wave absorption layer so that the surface of the electromagnetic wave is changed. It is known to absorb while suppressing reflection, and is used in an anechoic chamber. This type has absorption performance even for obliquely incident electromagnetic waves, but has a drawback that the electromagnetic wave absorber becomes too thick, and in the millimeter wave region, one for practical use has not yet been realized.

However, in a matching type electromagnetic wave absorber comprising a laminate of an electromagnetic wave absorbing layer and an electromagnetic wave reflecting layer, an electromagnetic wave absorber having a sufficiently large return loss even for an electromagnetic wave having an incident angle from an oblique direction. For manufacturing, it is known to make the electromagnetic wave absorption layer multilayer.

[0009]

However, when the electromagnetic wave absorbing layer is formed in multiple layers, variations in the thickness and dielectric properties of each layer have a complicated influence on the electromagnetic wave absorbing property, and therefore the manufacturing is extremely difficult. The manufacturing yield will be inferior. Especially in the millimeter wave region, the frequency dependence of the dielectric characteristics is large, so if the target frequency is different,
Although it is necessary to evaluate the dielectric property at that frequency, a special device is required to accurately measure the dielectric property in the millimeter wave region, and an accurate measured value cannot be easily obtained.

Therefore, an object of the present invention is to provide a matching type electromagnetic wave absorber which functions over a wide range of incident angles, and particularly to provide an electromagnetic wave absorber which can be easily designed and manufactured even in the millimeter wave region. It is in.

[0011]

The above objects can be achieved by the inventions described in the claims. That is, the characteristic configuration of the electromagnetic wave absorber according to the present invention is that the electromagnetic wave reflection layer is laminated on the back surface side of the electromagnetic wave absorption layer, and the electromagnetic wave absorption layer has a relative dielectric constant of 1.2 to 1. 8 is a multilayer structure in which a first dielectric layer having a thickness of 8, a resistance thin film layer having a sheet resistance of 220 Ω / □ to 380 Ω / □, and a second dielectric layer are stacked.

According to this structure, since the resistance thin film is used as the electromagnetic wave absorbing material, unlike the dielectric loss material, the resistance value of the resistance thin film hardly changes from DC to the millimeter wave region.

In the conventional matching type electromagnetic wave absorber (FIG. 2) having the resistive thin film layer 6 as a constituent member, the reflectance of the electromagnetic wave on the surface of the protective film 5 is TE mode when the incident angle of the electromagnetic wave exceeds 30 °. In contrast, while it increases monotonically as the incident angle increases, in the TM mode, it decreases conversely, once becomes zero at the brewster angle, and then increases. As described above, since the conventional matching type electromagnetic wave absorber exhibits different behaviors depending on the mode, the electromagnetic wave reflected on the surface of the electromagnetic wave absorber and the electromagnetic wave reflection layer 8 are reflected in both TE mode and TM mode. It cannot be designed to effectively cancel incoming electromagnetic waves. In addition, in FIG.
Is a dielectric layer, and reference numeral 8 is an electromagnetic wave reflection layer.

In the electromagnetic wave absorber of the present invention, by using a low dielectric constant dielectric layer having a relative dielectric constant in the range of 1.2 to 1.8 in the portion corresponding to the conventional protective film, the electromagnetic wave on the outermost surface can be obtained. It is possible to realize the electromagnetic wave absorber having a reflection attenuation amount of 20 dB or more for both TE mode and TM mode electromagnetic waves even at an incident angle of 50 °, while suppressing the reflectance of the electromagnetic wave. Relative permittivity is 1. When it is less than 2, the permittivity is close to that of air and the effect of providing a dielectric layer on the surface is poor.
If it exceeds 8, the dielectric constant becomes too high, and sufficient return loss cannot be obtained in the portion where the incident angle is large.

The sheet resistance of the resistive thin film layer is 220 Ω /
An appropriate value is determined depending on the relative permittivity of the first dielectric layer within the range from the mouth to 380 Ω / □. When the sheet resistance value of the resistance thin film layer is less than the above lower limit value, the reflection of electromagnetic waves in the resistance thin film layer becomes too large, and sufficient return loss cannot be obtained. No loss is obtained, and the return loss is also small.

As a result, according to the electromagnetic wave absorber of the present invention, since the low dielectric layer made of a porous material is disposed on the surface layer, the reflectance of the outermost surface can be reduced, and both TE and TM modes can be obtained. On the other hand, even if the incident angle becomes large, the reflected wave from the inside of the electromagnetic wave absorber cancels the reflected wave on the outermost surface, and as a result, an electromagnetic wave absorber that functions for a wide incident angle can be realized.

The first dielectric layer is preferably made of porous ultra high molecular weight polyethylene.

According to this structure, the relative dielectric constant is 1.2 to
A dielectric layer of 1.8 can be easily obtained, and a porous body with relatively high thickness accuracy can be obtained by adopting a method such as sintering resin particles of ultra high molecular weight polyethylene. It is convenient.

The resistive thin film layer comprises a polymer film base material, a metal oxide mainly composed of In, Sn, Zn or an alloy thereof, NiCr, FeNi, FeNiCr,
It is preferable to form a thin film of a material selected from CuNi.

According to this structure, the sheet resistance value is 220Ω.
A thin film resistance layer of / mouth to 380 Ω / □ can be easily obtained.

The second dielectric layer is made of polyethylene, polyester, polystyrene, polyimide, polycarbonate, polyamide, polysulfone, polyether sulfone, vinyl chloride, epoxy, polyisoprene rubber,
Polystyrene / butadiene rubber, polybutadiene rubber,
It is preferably made of one or more materials selected from chloroprene rubber, acrylonitrile / butadiene rubber, butyl rubber, acrylic rubber, ethylene / propylene rubber, and silicone rubber.

According to this structure, since the workability is excellent, it is possible to form a layer having a good thickness accuracy and to firmly bond the layer with the electromagnetic wave reflection layer, which is convenient.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a schematic sectional structure of an electromagnetic wave absorber according to this embodiment. This electromagnetic wave absorber is
The first dielectric layer 1, the resistive thin film layer 2, the second dielectric layer 3, and the electromagnetic wave reflection layer 4 are laminated in this order from the front surface side.

The first dielectric layer 1 has a relative dielectric constant of 1.
2 to 1.8 are used, and it is particularly preferable to use a porous material. The material of the porous material is not particularly limited, but it is preferable to use a polymer resin material having excellent processability. Examples of the porous form include woven fabrics, nonwoven fabrics, foams, and granular materials sintered in a gas-containing state. Particularly, porous materials produced by sintering ultra high molecular weight polyethylene particles. The body is preferred. The reason is that the resin itself has a small permittivity, which is advantageous for realizing a relative permittivity of 1.8 or less, and the required thickness varies depending on the target frequency. For example, even in a millimeter wave of 76 GHz, a thickness of about 1 mm is required, and it is also necessary to control the thickness to some extent. Therefore, when a porous material is manufactured by sintering resin particles, the thickness is This is because the control is easy and the predetermined thickness can be obtained with relatively high accuracy. The porosity of the porous material is preferably 30 to 85%, more preferably 50 to 80%.

The resistive thin film layer 2 is made of In, Sn, Zn or a metal oxide mainly composed of an alloy thereof, or NiCr.
A material in which a thin film of an alloy such as FeNi, FeNiCr, or CuNi is formed on the base material is used. The sheet resistance of the resistive thin film layer is preferably 220Ω / □ to 380Ω / □, and more preferably 270Ω / □ to 340Ω / □. The resistive thin film layer 2 is formed on a polymer film substrate such as a polyester film or polycarbonate film by a vacuum deposition method,
It can be produced by a method such as sputtering, ion plating, or wet plating.

The material used for the second dielectric layer 3 is not particularly limited, but an organic polymer material is preferable from the viewpoint of workability. Examples of organic polymer materials include synthetic resins such as polyethylene, polyester, polystyrene, polyimide, polycarbonate, polyamide, polysulfone, polyether sulfone, polyvinyl chloride and epoxy, and polyisoprene rubber, polystyrene / butadiene rubber, polybutadiene rubber, chloroprene. Typical examples are various synthetic rubber materials such as rubber, acrylonitrile / butadiene rubber (NBR), butyl rubber, acrylic rubber, ethylene / propylene rubber, and silicone rubber. An inorganic material such as carbon, titanium oxide, alumina, or barium titanate may be added to these resins to adjust the dielectric constant. Further, the second dielectric layer 3 may also serve as the base film that forms the resistive thin film layer 2 described above.

As a material for forming the electromagnetic wave reflection layer 4,
Metal plates or foils of aluminum, copper, iron, stainless steel, etc., polymer films on which a thin film of the above metals has been formed by vacuum deposition or plating, those reinforced with a conductive material such as carbon fiber, etc. Can be used.

In order to fabricate an electromagnetic wave absorber as shown in FIG. 1 by laminating the first dielectric layer 1, the resistive thin film layer 2, the second dielectric layer 3 and the electromagnetic wave reflecting layer 4, the respective layers are prepared. The method of adhering the members with an adhesive is simple. In this case, in order to determine the thickness of the second dielectric layer 3, the base film of the resistance thin film layer 2 and the adhesive layer, and the adhesive layer between the second dielectric layer 3 and the electromagnetic wave reflection layer 4 are used. It can be designed in consideration of the influence of.

The adhesive for adhering the first dielectric layer 1 and the resistive thin film layer 2 should also be thin enough to have no effect.
Alternatively, if the thickness of the first dielectric layer 1 is designed in consideration of the influence, the electromagnetic wave absorber of the present invention can be produced without any problem.

[0030]

EXAMPLES Hereinafter, the electromagnetic wave absorber of the present invention will be specifically described by showing Examples.

(Example 1) Sheet resistance 290 was applied to a polyester film having a thickness of 100 μm by a scatterer method.
A thin film of an oxide of indium and tin (10 wt%) of Ω / □ was formed. Next, ultra high molecular weight polyethylene powder (average molecular weight 4.5 million, true specific gravity 0.935, average particle size 150
(μm) was filled in a cylindrical mold, heated at 130 ° C., and pressed to prepare a preform having a porosity of 55%. This preform was heated to 160 ° C., sintered, and then cooled to obtain a porous sintered body. The obtained porous sintered body was cut into a sheet having a thickness of 1 mm by a lathe to obtain a first dielectric layer. The relative permittivity of this product in the millimeter wave region was 1.5. A 0.5 mm-thick polycarbonate sheet was used as the second dielectric layer. By laminating the first dielectric layer, the film on which the resistive thin film is formed, the second dielectric layer, and the aluminum sheet as the electromagnetic wave reflection layer in the order described above with a double-sided adhesive sheet, a design frequency of 76 GHz is obtained.
The electromagnetic wave absorber of was obtained.

(Embodiment 2) A polyester film having a thickness of 100 μm and a sheet resistance of 250 by sputtering
A thin film of Ω / neck stainless steel (SUS304) was formed. Next, ultra high molecular weight polyethylene powder (average molecular weight 4
A cylindrical mold was filled with 500,000, true specific gravity 0.935, average particle size 150 μm), heated at 130 ° C., and pressed to prepare a preform having a porosity of 42%. 16 of this preform
After heating to 0 ° C. for sintering and cooling, a porous sintered body was obtained. The thickness of the obtained porous sintered body was 0.98 with a lathe.
The sheet was cut into a sheet of mm to obtain a first dielectric layer. The relative permittivity of this product in the millimeter wave region was 1.7. Second
As a dielectric layer of, a thermoplastic rubber (NIPOL 1203JNS (trade name) manufactured by Nippon Zeon Co., Ltd.) having a thickness of 0.42 mm and made of a blend (NBR) of polyvinyl chloride and 15 parts of flake graphite Was used. By laminating the first dielectric layer, the film on which the resistive thin film layer is formed, the second dielectric layer, and the aluminum sheet as the electromagnetic wave reflection layer in the order described above with a double-sided adhesive sheet, an electromagnetic wave with a design frequency of 76 GHz is obtained. An absorber was obtained.

(Comparative Example 1) As the first dielectric layer, a porous ultra-high molecular weight polyethylene having a thickness of 0.85 mm and a relative permittivity of 2 was used, and the resistance thin film layer had a sheet resistance of 210Ω.
An electromagnetic wave absorber was produced in the same manner as in Example 1 except that the one having a / mouth was used.

Table 1 shows the results of measuring the incident angle dependence of the return loss with respect to the millimeter wave of 76 GHz in the electromagnetic wave absorbers of Examples 1 and 2 and Comparative Example 1. Example 1,
The electromagnetic wave absorber of 2 has 20d even at an incident angle of 50 °.
The return loss is B or more, which is clearly superior to that of Comparative Example 1.

[0035]

[Table 1] [Other Embodiments] In the above embodiment, the result was shown when the design frequency of the electromagnetic wave absorber was set to 76 GHz, but the present invention is not limited to this frequency and a wide range from microwaves to millimeter waves. It goes without saying that it can be designed for frequency.

[Brief description of drawings]

FIG. 1 is a sectional view showing an electromagnetic wave absorber of the present invention.

FIG. 2 is a sectional view showing an electromagnetic wave absorber using a conventional resistive thin film layer.

[Explanation of symbols]

5 First dielectric layer 6 Resistive thin film layer 7 Second dielectric layer 8 Electromagnetic wave reflection layer

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Claims (4)

[Claims] 1. An electromagnetic wave absorber having an electromagnetic wave reflection layer laminated on the back surface side of the electromagnetic wave absorption layer, wherein the electromagnetic wave absorption layer comprises:
The first with a relative permittivity of 1.2 to 1.8 in order from the surface
2. An electromagnetic wave absorber having a multilayer structure in which a dielectric layer, a resistive thin film layer having a sheet resistance of 220 Ω / □ to 380 Ω / □, and a second dielectric layer are laminated. 2. The electromagnetic wave absorber according to claim 1, wherein the first dielectric layer is made of porous ultra high molecular weight polyethylene. 3. The resistive thin film layer comprises a polymer film substrate, a metal oxide containing In, Sn, Zn or an alloy thereof as a main component, NiCr, FeNi, FeNiCr,
The electromagnetic wave absorber according to claim 1 or 2, which is formed by forming a thin film of a material selected from CuNi. 4. The second dielectric layer is polyethylene,
Polyester, polystyrene, polyimide, polycarbonate, polyamide, polysulfone, polyether sulfone, vinyl chloride, epoxy, polyisoprene rubber, polystyrene / butadiene rubber, polybutadiene rubber, chloroprene rubber, acrylonitrile / butadiene rubber, butyl rubber, acrylic rubber, ethylene / propylene The electromagnetic wave absorber according to any one of claims 1 to 3, which is made of one or more materials selected from rubber and silicone rubber.