JPH08340191A - Electromagnetic wave absorber - Google Patents

Abstract

(57) [Summary] [Object] To provide a lightweight and thin resonant electromagnetic wave absorber capable of supporting a wide range of oblique incident angles in the quasi-millimeter wave band and the millimeter wave band represented by 19 GHz and 60 GHz. An electromagnetic wave absorber including an electromagnetic wave absorption layer having a resonator that resonates in a millimeter wave band or a quasi-millimeter wave band in a binder.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an electromagnetic wave absorber,
In particular, the present invention relates to a resonant electromagnetic wave absorber that efficiently absorbs electromagnetic waves in the millimeter wave and quasi-millimeter wave bands and that also exhibits good absorption of obliquely incident electromagnetic waves.

[0002]

2. Description of the Related Art With the progress of advanced information technology, information communication technology has been steadily shifting from wired information transmission to wireless information transmission. With the development of multimedia capable of transmitting a large amount of information at high speed, the frequency band used for communication systems has expanded to the high frequency side.

At present, there are 1.9 GHz band and 2.45 GHz band quasi-microwave band and 19 GHz band quasi-millimeter wave band as frequency bands being used in communication systems. The quasi-microwave band is for personal handy
Phone systems (PHS) and medium-speed wireless LAN local wireless devices, and the quasi-millimeter wave band is applied to high-speed wireless LAN local wireless devices.

In the near future, the use of the millimeter wave band of 60 GHz as an ultrahigh-speed wireless LAN is expected to make a great leap in information communication technology.

As the demand in these frequency bands expands, problems such as mutual interference of electromagnetic waves, interference due to delay dispersion, malfunctions and eavesdropping may occur.

As an electromagnetic wave absorber, a ferrite sintered body tile is generally well known, but the weight per unit area is enormous, and the reflection amount becomes large at a frequency of 1 GHz or more, and the absorption capacity is high. Hardly showed.

There are also known ones in which a resistance film is placed on a conductor plate at a certain distance, and ones in which a composite of ferrite and resin is laminated, but this electromagnetic wave absorber has a basic concept of layer thickness. The wavelength is adjusted to 1/4 of the wavelength of the electromagnetic wave, and there are the drawback that good absorption characteristics cannot be obtained for obliquely incident waves, the narrow absorption band, and the weight per unit area. It had a big drawback.

An electromagnetic wave absorber having a large number of pyramid-shaped particles in which a powder of carbon black or the like is dispersed in polyurethane foam is also known, but the thickness of the absorber is very large, and its shape makes it inconvenient to handle. Therefore, it was difficult to use it except in a special environment such as an anechoic chamber.

[0009]

DISCLOSURE OF THE INVENTION The present invention provides 19 GHz
(EN) Provided is a lightweight and thin resonant electromagnetic wave absorber capable of accommodating a wide range of oblique incident angles in the quasi-millimeter wave band and the millimeter wave band represented by the band and 60 GHz band.

[0010]

[Technical Description] A resonator that resonates in the millimeter wave and quasi-millimeter wave bands is formed of a metal magnetic material or a metal oxide magnetic material, or particles of a metal magnetic material or a metal oxide magnetic material in a binder having a dielectric loss. It was possible to obtain a highly efficient electromagnetic wave absorber by using the resonator dispersed therein as the absorber. In particular, the resonator in which the metal oxide magnetic material is dispersed in the binder exhibits high absorption characteristics in a wide frequency band. In addition, this absorber uses the scattering effect of a granular (spherical or rectangular) resonator and the resonance action of the resonator, so it is good even for obliquely incident electromagnetic waves. It has the characteristic of showing excellent absorption capacity.

[0011]

[Means for Solving the Problems]

1. An electromagnetic wave absorber having an electromagnetic wave absorbing layer containing a resonator that resonates in the millimeter wave band or the quasi-millimeter wave band in a binder. The thickness of the electromagnetic wave absorbing layer is not particularly limited. Usually, the thickness of the electromagnetic wave absorbing layer is equal to or larger than the size of the contained resonator. It is expected that the absorption band will be broadened or the absorption capability will be improved by stacking the electromagnetic wave absorption layers or providing the electromagnetic wave absorption layers with irregularities. The thickness of the entire absorber including the electromagnetic wave absorbing layer is preferably 50 mm or less from the viewpoints of ease of handling and fit during construction. Further, in order to develop a desired electromagnetic wave absorption ability and to maintain the film strength,
It is preferably 5 mm or more.

2. In the electromagnetic wave absorbing layer, the volume ratio of the resonator (described in 1) contained in the binder is 2% by volume to
It is 74% by volume. 10% to 60% by volume is particularly preferred. If it is less than 2% by volume, sufficient absorption properties cannot be obtained, and if it exceeds 74% by volume, the rigidity, weight and durability of the absorbent layer are inferior.

The shape of the resonator described in 3.1 is preferably a spheroidal shape (including a spherical shape), a part of the spheroidal shape, a disk shape or a rectangular shape. Preferably,
The sphere is good.

The dimensions of the resonator described in 4.1 are preferably designed so that the resonant vibration mode of the resonator matches the frequency of interest.

When the resonator described in 1 has a spheroidal shape (including a spherical shape), its minor axis diameter and major axis diameter are 0.3 mm to 20 mm. 1 mm for the electromagnetic wave absorption layer in the quasi-millimeter wave band and millimeter wave band
To 10 mm is preferred. If it is less than 0.3 mm, it has a drawback that it does not work as a resonator in the quasi-millimeter wave band. Also, 20
When it exceeds mm, it does not work as a resonator in the millimeter wave band,
Further, it has a drawback that the absorber becomes thick.

When the shape of the resonator described in 1 is a rectangle, the length of one side forming the rectangle is 0.3 mm to 20 mm. For the electromagnetic wave absorbing layer in the quasi-millimeter wave band and the millimeter wave band, 1 mm to 10 mm is preferable. If it is less than 0.3 mm, it has a drawback that it does not work as a resonator in the quasi-millimeter wave band. If it exceeds 20 mm, it does not work as a resonator in the millimeter wave band and the absorber becomes thick.

When the resonator described in 1 has a disk shape or a cylindrical shape, the disk shape has a diameter of 0.3 mm to 20 mm. 1 mm to 1 for the electromagnetic absorption layer in the quasi-millimeter wave band and millimeter wave band
0 mm is preferred. If it is less than 0.3 mm, it has a drawback that it does not work as a resonator in the quasi-millimeter wave band. If it exceeds 20 mm, it has a drawback that it does not work as a resonator in the millimeter wave band.
The thickness of the disc shape is 0.1 mm to 10 mm, preferably 0.3 mm to 10 mm. If it is less than 0.1 mm, it has a drawback that it does not work as a resonator in the quasi-millimeter wave band, and it is 10 mm.
Above the range, it does not work as a resonator in the millimeter wave band, and the absorber becomes thick.

The material of the resonator described in 5.1 is an oxide dielectric containing an alkaline earth element or a dielectric containing carbon. Magnetic substance made of pure metal, alloy or metal oxide. A composite magnetic body composed of powders of pure metals, alloys or metal oxides dispersed in a binder.

The oxide dielectric containing an alkaline earth element described in 6.5 is, for example, Be, Mg, Ca, S.
A perovskite type oxide containing r, Ba or the like.

The pure metals described in 7.5 and are aluminum, iron, cobalt, nickel, copper, zinc, and other almost pure metals.

The alloys described in and of 8.5 refer to metals containing two or more elements represented by silicon steel, sendust, permalloy, alnico and the like.

The metal oxides described in 9.5 and are oxides containing one or more metal elements. For example, it refers to an iron oxide containing elements such as Mn, Zn, Ni, Mg and Cu, and a garnet type metal oxide containing elements such as Al, Mg, Si and Y.

The powders of pure metals, alloys and metal oxides described in 10.5 have an average particle size of 0.2 to 100 μm.
m, preferably 0.5-40 μm. If it is less than 0.2 μm, it has a drawback that the absorption efficiency in the quasi-millimeter wave band is low and that it is difficult to prepare ultrafine powder. If it exceeds 100 μm, the absorption efficiency in the millimeter wave band is low and the dispersion in the binder occurs. It has the drawback of becoming difficult.

As the binder described in 11.1 and 5, inorganic and ceramic materials such as thermoplastic and thermosetting organic polymer materials, cement type, calcium silicate type and gypsum type can be used.

Binders preferably used in the present invention are epoxy resin, polyvinyl chloride, ethylene vinyl acetate copolymer,
Organic polymer materials such as polyacrylic resins, fluorine-containing polymers, polyamides, polyesters, silicone resins, polyurethane resins, synthetic rubbers, phosphagen resins and expanded polystyrene. Specific examples of the inorganic ceramic materials include calcium sulfate, calcium silicate, water glass, Portland cement, alumina cement, alkyl silicate, calcium oxide, clay and the like.

In the composite magnetic material described in 12.5, the amount of powder of pure metal, alloy, metal oxide, etc. in the binder is 50 to 94% by weight, preferably 76 to 94% by weight.
It is 92% by weight. If it is less than 50% by weight, the absorption capacity is insufficient, and if it exceeds 94% by weight, the electromagnetic wave absorption capacity is good, but the rigidity, weight and durability are poor.

13. An electromagnetic wave absorber in which an electromagnetic wave absorbing layer containing a resonator is arranged on a conductive material. In the present invention,
The conductive material functions to prevent leakage of electromagnetic waves to the back surface of the electromagnetic wave absorber and to enhance electromagnetic wave absorption capability. However, the conductive material is not always necessary for the present invention.

The conductive material may also serve as a support. Specifically, copper, aluminum, steel, iron, nickel,
Examples thereof include metal plates such as stainless steel and Shinchu, wire mesh, and metal cloth. Such a metal material may be a surface-treated or primer-treated material such as a precoated steel sheet for improving interlayer adhesion.

Further, a conductive coating film containing the above-exemplified metal and a binder and a liquid phase or vapor phase plating layer of the above metal can also be used as the conductive material. For example, a metallized material in which a conductive coating film of the above metal is provided on a non-conductive material such as a plastic material, or an electroless plating layer of copper or Ni, or a vapor deposition layer such as aluminum is formed is also used in the present invention. obtain.

[0030]

[Function] Loss of complex relative permittivity of a material used as a binder of a resonator which is resonated in a target frequency band by forming a spherical, rectangular or other shape resonator in an electromagnetic wave absorber. Term and the loss of the resonator itself, and efficiently absorb the electromagnetic wave energy by using the scattering effect from the resonator.

When the resonator is arranged on the conductive material, the resonance is further enhanced by the image effect, and the conductive material also serves as a shield.

The resonators need not be perfectly densely arranged on the conductive material. The absorption layer can exhibit a large absorption power in a wide band only by scattering the resonators. Therefore, the weight can be reduced as compared with the conventional electromagnetic wave absorber.

Since there are many resonance modes of the resonator, a resonator having a certain size resonates at a plurality of frequencies, and in particular, a resonator formed by dispersing magnetic particles in a binder having a dielectric loss. Can be a broadband electromagnetic wave absorber.

When the shape of the resonator has a high symmetry (spherical symmetry, etc.), the number of resonance points is small. On the contrary, when the resonator has a relatively low symmetry (rectangular shapes with different sides). And spheroids with different axial ratios) have many resonance points. As an example, Table 1 shows the relationship between the radius of the sphere of the spherical resonator and the resonance frequency in each mode.

[0035]

[Table 1]

In conventional electromagnetic wave absorption, since the absorption capacity is expressed by controlling the matching thickness, the matching thickness apparently changes with respect to obliquely incident electromagnetic waves, which is sufficient. It did not exhibit a good absorption capacity.

Since the present invention utilizes the resonance mode peculiar to the resonator, the absorption capacity does not depend on the incident angle.

In particular, when the resonator has a spherical shape, due to its geometrical reason, an obliquely incident electromagnetic wave,
Shows good absorption capacity.

When spherical resonators having different resonance frequencies (having different sizes and different materials) are mixed, the electromagnetic wave absorber including them has a very wide frequency band. Can be possible.

When the resonator is composed of a magnetic material powder dispersed in a binder, it is possible to construct an electromagnetic wave absorber having a particularly large electromagnetic wave absorption capability and a very wide absorption band. The absorption band can be controlled by changing the average particle size of the magnetic material powder in the resonator. The absorption band can also be controlled by changing the content of the magnetic material powder in the resonator.

[0041]

【Example】

Measurement method The characteristics of the electromagnetic wave when it is incident on the front are measured by a network analyzer (HP8510 manufactured by Hewlett-Packard Co., Ltd.)
The S ₁₁ parameter was measured in the same tube using B). The measurement was performed using a horn antenna in the measurement frequency range of 1 to 40 GHz.

At 60 GHz, a klystron (100 mW output) was used in the transmitter, passed through an isolator and an attenuator, and then applied to the object to be measured using a horn antenna. The reflected wave from the DUT is separated from the traveling wave by a directional coupler, and the reflected wave is input to the above network analyzer through a waveguide mixer (WM 780U manufactured by Sony Tektronix, Inc.) and the signal strength is input. Was measured.

Measurement Method The characteristics when the electromagnetic wave was obliquely incident at 45 degrees were measured at 19 GHz by using a near electromagnetic field antenna measuring device (“NFAMS” manufactured by Icom Co., Ltd.). The shape of the measurement sample is 200m long
A square plate measuring m and 200 mm wide was used. The absorptivity was represented by the average value of the values when TE and TM were incident.

The 45-degree oblique incidence characteristic of 60 GHz was measured by utilizing the system of the measuring method and setting the incident angle to the object to be measured at 45 degrees.

A. Ethylene vinyl acetate copolymer (Mitsui DuPont Chemical Co., Ltd., P-190)
7), MnO, ZnO and Fe ₂ in a molar ratio of 32:14:54.
Ferrite particles containing O ₃ and having an average particle size of 15 μm were kneaded in a proportion of 90% by weight.

This kneaded material was used in the following steps: diameter 1 mm, 1.5 mm, 2 mm,
3mm, 5mm sphere, major axis diameter 5mm, minor axis diameter 3mm spheroid, side 4mm cube, diameter 1.5mm, height 1.5mm
80 ° C using a molding frame for a cylindrical resonator.
It was hot pressed.

Further, the above-mentioned resonators prepared above were scattered almost uniformly on an aluminum foil having a thickness of about 50 μm and a size of 200 mm × 200 mm so as to cover 6% of the surface area, and epoxy resin was applied thereon. Then, each resonator was covered with an epoxy resin (EPOFIX manufactured by struers) to obtain an electromagnetic wave absorber.

The electromagnetic wave absorbers including resonators of various shapes and various sizes were measured for absorption of front incident electromagnetic waves at 19, 40 and 60 GHz by the measuring method. Table 2 shows the results
Shown in

For comparison, the above molar ratio is 32: 1.
4:54, average particle size containing MnO, ZnO and Fe ₂ O ₃ 1
5 μm ferrite particles and ethylene vinyl acetate copolymer (Mitsui DuPont Chemical Co., Ltd., P-1907)
The kneaded material (containing 90% by volume of ferrite) was processed into a sheet with a thickness of 3 mm and a size of 200 mm x 200 mm by hot pressing, and an aluminum foil was attached to one side of the sheet to create a matching absorber. , 40, 60GH depending on measurement method
The absorption amount of the front incident electromagnetic wave at z was measured. The results are shown in Table 2 as Comparative Example 1.

[0050]

[Table 2]

B. Ferrite with an average particle size of 15 μm containing ethylene vinyl acetate copolymer (SUMITATER B-11 manufactured by Sumitomo Chemical Co., Ltd.) as a binder in a molar ratio of 32:14:54 and MnO, ZnO and Fe ₂ O _3. The powder was kneaded in an amount of 90% by weight. This kneaded material is warmed to about 90 degrees and extruded into a fiber shape having a diameter of 1.5 mm, and the fiber has a length of 1.
A large number of cylindrical resonators were made by cutting into 5 mm.

Further, the above-mentioned resonator prepared was applied to a surface of an aluminum foil having a thickness of about 50 μm and a size of 200 mm × 200 mm with a double-sided adhesive tape (manufactured by Nitto Denko Corporation) on the surface.
%, 10%, and 30% were scattered almost uniformly to obtain three types of electromagnetic wave absorbers having different resonator amounts.

The absorption amount of the electromagnetic wave absorber for obliquely incident electromagnetic waves was measured by a measuring method. Measurement frequency is 19GH
z and 60 GHz, the incident angle was 45 degrees, and the shape of the resonator included in the electromagnetic wave absorbing layer was spherical (diameter 5 mm). The results are shown in Table 3.

For comparison, the above molar ratio is 32: 1.
4:54, average particle size containing MnO, ZnO and Fe ₂ O ₃ 1
5 μm ferrite particles and ethylene vinyl acetate copolymer (Mitsui DuPont Chemical Co., Ltd., P-1907)
The kneaded material (containing 90% by volume of ferrite) was processed into a sheet with a thickness of 3 mm and a size of 200 mm x 200 mm by hot pressing, and an aluminum foil was attached to one side of the sheet to create a matching absorber. , 19 GHz and 60 GHz, the electromagnetic wave absorption at an incident angle of 45 degrees is shown in Table 3 as Comparative Example 2.

[0055]

[Table 3]

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Masato Morimoto 19-17 Ikedanaka-cho, Neyagawa-shi, Osaka Japan Paint Co., Ltd. (72) Inventor Shoichi Iida 19-17 Ikedanaka-cho, Neyagawa-shi, Osaka Japan Into Inc.

Claims (13)

[Claims] 1. An electromagnetic wave absorber including an electromagnetic wave absorbing layer having a resonator that resonates in a millimeter wave band or a quasi-millimeter wave band in a binder. 2. An electromagnetic wave absorber having the electromagnetic wave absorbing layer according to claim 1 disposed on a conductive material. 3. The electromagnetic wave absorber according to claim 1, wherein the resonator has a spheroidal shape including a spherical shape, and its minor axis diameter and major axis diameter are 0.3 mm to 20 mm. 4. The electromagnetic wave absorber according to claim 1 or 2, wherein the resonator has a rectangular shape, and the sides forming the shape have a length of 0.3 mm to 20 mm. 5. The resonator has a disk shape or a cylindrical shape, the diameter thereof is 0.3 mm to 20 mm, and the thickness thereof is 0.1 mm.
The electromagnetic wave absorber according to claim 1 or 2, which has a length of 1 mm to 20 mm. 6. The electromagnetic wave absorber according to claim 1, wherein the resonator is made of a magnetic material or a dielectric material. 7. The electromagnetic wave absorber according to claim 1, wherein the resonator is a metal magnetic material or a metal oxide magnetic material. 8. The electromagnetic wave absorber according to claim 1, wherein the resonator comprises metal magnetic particles or metal oxide magnetic particles dispersed in a binder. 9. The electromagnetic wave absorber according to claim 1, wherein the resonator is a ferrite sintered body. 10. The electromagnetic wave absorber according to claim 1, wherein the resonator contains ferrite particles in a binder. 11. The electromagnetic wave absorber according to claim 1, wherein the resonator contains iron particles in a binder. 12. The electromagnetic wave absorber according to claim 1, which has a laminated structure. 13. The electromagnetic wave absorber according to claim 1, wherein the surface of the electromagnetic wave absorber has an uneven shape, and the height of the unevenness is 50 mm or less.