JPH08181480A - Electromagnetic wave absorber - Google Patents

Abstract

PURPOSE: To provide an electromagnetic wave absorber that is provided with sound absorbing capability as well, lightweight and excellent in handleability. CONSTITUTION: An electromagnetic wave absorber consists of a composite 3 of mixed fiber 1 and binder 2. At least part of the fiber 1 is metal fiber 1a. The action of the metal fiber 1a to absorb electromagnetic energy obtains high electromagnetic wave absorbing capability. Further, since the electromagnetic wave absorber is formed mainly out of fiber 1, it is lightweight and excellent in handleability. Voids 5 formed among fibers 1 provides high sound absorbing capability.

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 used for absorbing and shielding electromagnetic waves.

[0002]

2. Description of the Related Art In recent years, problems concerning the electromagnetic wave environment have been taken up in earnest, and electromagnetic wave interference has become a major social problem. The following are generally mentioned as the so-called electromagnetic interference. (1) Radar radio waves and TV radio interference caused by buildings Electromagnetic interference caused by unnecessary reflection electromagnetic waves caused by high-rise buildings and large-scale buildings (large bridges, noise barriers on elevated roads, steel towers, etc.), such as wall surfaces The electromagnetic waves reflected by the radio wave generate a ghost image on the TV screen or radar screen. (2) Communication failure in electronic communication During data communication on a wireless LAN in a living space or office, this is a communication failure caused by multipath due to electromagnetic wave reflection or unnecessary electromagnetic wave reflection in the living space or office. However, it is expected that more sophisticated social issues such as housing and offices will become more important social issues. (3) Generation of electromagnetic interference waves from electronic circuits This is a failure that causes electromagnetic interference waves (noise) generated from OA devices, personal computers, etc. to cause a reception failure on televisions and radios, or cause malfunctions on other electronic devices. .

Regarding the above (3), various measures such as suppression of electromagnetic wave radiation from electric and electronic devices, noise reduction action, and enhancement of electromagnetic wave shielding ability of damaged electronic devices are taken. Has been. As one of the effective countermeasures against the electromagnetic wave reception obstacles such as (1) and (2), there is a method of installing an electromagnetic wave absorber using an electromagnetic wave absorbing material on the electromagnetic wave reflecting surface to eliminate unnecessary electromagnetic wave reflection. Has been.

The electromagnetic wave absorbing material is a material which converts electromagnetic wave energy into heat energy, and the electromagnetic wave loss can be broadly classified into three types: conductive loss, dielectric loss and magnetic loss. Materials can be roughly classified into the following three types according to this (Yasutaka Shimizu, "Absorption and shielding of electromagnetic waves", page 107, etc., 1989.
Published Nikkei technical books). (A) Conductive Electromagnetic Wave Absorbing Material This is a material that absorbs electromagnetic waves by a conducting current flowing through a resistor and a resistance wire, such as a woven fabric made of conductive fibers (for example, JP-A-3-226000). (B) Dielectric Electromagnetic Wave Absorbing Material There are carbon rubber, carbon foam polyurethane, and the like, in which foamed polystyrene having a small relative permittivity is mixed with carbon or graphite which is a dielectric loss material. (C) Magnetic Electromagnetic Wave Absorption Material A typical magnetic loss material, which is obtained by sintering ferrite, which is magnetic powder having electromagnetic wave absorption,
There are ferrite plates and rubber ferrite obtained by kneading ferrite powder into rubber.

These electromagnetic wave absorbing materials are designed as an electromagnetic wave absorber by designing their thickness, structure, etc., and are used as electromagnetic wave absorbers, including the outer wall of a building to prevent electromagnetic interference from the above (1) and (2). Is used in various fields. However, most of the electromagnetic wave absorbers made from these electromagnetic wave absorbing materials are sheet-shaped, plate-shaped, or block-shaped, and have problems in handling. For example, in a ferrite sintered body, the weight becomes heavy, which causes a problem in handling, and since rubber ferrite or a conductive woven fabric has a sheet shape, it cannot hold the structure independently,
In addition to the handling problems, increasing the thickness also causes problems due to the increased weight.

On the other hand, in recent years, in the fields of soundproof walls of elevated roads, partitions and partitions such as houses and offices that handle OA equipment, demands for electromagnetic wave absorbing performance and soundproofing performance have become severe. There are the following two measures to improve the soundproofing performance.
One is a method of blocking the propagation of the sound wave by installing a material such as a metal plate having a high surface density at a position where the propagation path of the sound wave is blocked, that is, a wall or a partition portion. However, with this method, although it is possible to obtain soundproofness and high electromagnetic wave blocking performance, there is a possibility that ghost interference and communication failure of television propagation due to reflection of electromagnetic waves may occur, and soundproofing performance is Not enough.

Another method is to use a sound absorbing material having a high sound absorbing performance as a soundproof wall, an interior material of a living space, or a filling material inside a partition wall. As this sound absorbing material, inorganic fibers such as glass wool and rock wool,
Organic porous materials such as foamed polyurethane are generally widely used. However, although these inorganic fibers and porous materials can be expected to have the effect of improving the soundproofing performance by the sound absorbing effect, almost no effect of absorbing electromagnetic waves can be obtained. That is, it is not possible to obtain electromagnetic wave absorption performance with a sound absorbing material that is widely used for improving sound insulation.

[0008]

As described above, most of the conventional electromagnetic wave absorbing materials have a sheet-like, plate-like, or block-like appearance, so that it is expected that the sound absorbing performance will be improved by the sound absorbing characteristics. Can't
Since the sound absorbing material cannot obtain electromagnetic wave absorbing performance, the present situation is that an electromagnetic wave absorbing body having both electromagnetic wave absorbing performance and sound absorbing performance, and being lightweight and excellent in handleability has not yet been provided.

Therefore, an object of the present invention is to provide an electromagnetic wave absorber having a sound absorbing property, a light weight and an excellent handleability.

[0010]

It is difficult for an electromagnetic wave absorber using a conventional electromagnetic wave absorbing material to have excellent handleability at the same time. Most of the electromagnetic wave absorbers are sheet-like or plate-like or block-like. This is due to the fact that the electromagnetic wave absorber does not have a void structure inside, so that the weight at a predetermined thickness becomes large, and the structure cannot be retained in a shape such as a rubber sheet or a woven fabric.

Further, it is difficult to provide the electromagnetic wave absorber with sound absorbing characteristics because most of the conventional electromagnetic wave absorbers are non-porous materials. In other words, foamed polyurethane and glausur, which have been conventionally used as sound absorbing materials, take advantage of the porosity of the material and, for example, have a complicated cross-sectional shape when sound waves are injected into continuous bubbles, holes, or voids. The sound pressure is reduced due to viscous friction with the wall surface of the bubble or the like while the sound wave is propagated by the bubble or the like, and as a result, the sound wave energy can be absorbed and absorbed in the material. On the other hand, sheet-like,
Since the plate-shaped and block-shaped materials have no bubbles, holes, or voids, they cannot absorb sound wave energy, and there is a limit in improving sound absorption characteristics.

On the other hand, the present inventors have developed a sound absorbing material composed of fibers and a resin binder, and a sound absorbing material composed of powder, fibers and a resin binder. That is, the sound absorbing material is formed by a structure in which fibers and a binder are mixed, or the sound absorbing material is formed by adhering and holding powder inside the voids of a structure in which fibers and a binder are mixed. , It has been found that the sound absorbing material of this structure can obtain excellent sound absorbing performance in the low frequency range.

The present inventors have paid attention to the structure in which the fiber and the binder are combined, or the structure in which the powder, the fiber and the binder are combined, and are excellent in handleability and sound absorption performance and have high electromagnetic wave absorption performance. The present invention has led to the provision of an electromagnetic wave absorber having That is, claim 1 of the present invention
The electromagnetic wave absorber according to (1) is composed of a composite body 3 in which a fiber 1 and a binder 2 are mixed, and at least a part of the fiber 1 is a metal fiber 1a.

An electromagnetic wave absorber according to a second aspect of the present invention comprises a composite body 3 in which a powder 4, a fiber 1 and a binder 2 are mixed, and at least a part of the fiber 1 is a metal fiber 1a. The powder 4 is held in the internal voids 5 of the composite by the fibers 1 and the binder 2. Further, the invention of claim 3 is characterized in that a conductive layer 6 is formed on one surface of the composite body 3.

The present invention will be described in detail below. FIG. 1 shows an example of a specific form of an electromagnetic wave absorber according to the invention of claim 1, and a large number of fibers 1 mainly constituting a structural skeleton.
Are bonded to each other at their contact points with a binder 2 to form a composite body 3 in which the fibers 1 and the binder 2 are mixed, and a large number of voids 5 are formed between the fibers 1 as pores. ing. At least a part of the fiber 1 is composed of the metal fiber 1a.

Here, as the fiber 1 forming the structural skeleton of the composite 3, synthetic fiber such as polyester fiber, nylon (polyamide) fiber, polyacrylonitrile fiber, polypropylene fiber, polyethylene fiber, polyvinyl chloride fiber, or wood is used. Fiber, cotton, hemp fiber, bamboo,
Natural fibers such as linter (forehead of cotton), silk and wool, organic fibers such as regenerated cellulose fibers such as rayon, rock fibers, glass fibers, alumina fibers, and inorganic fibers such as SiC fibers can be used. It is not limited to these.

As the fibers 1, the individual single fibers are generally used in a non-sheet form in which they are separated, but they do not necessarily have to be in a non-sheet form in which they are separated, and may be in a sheet form. Examples thereof include organic fiber structures such as non-woven fabric, artificial pulp and filter paper, and inorganic fiber structures such as granular cotton. Further, in the present invention, as the metal fiber 1a, a steel fiber obtained by cutting a thin steel plate can be used, but in addition to this, a brass fiber, a tungsten fiber, and the like can be cited, which have conductivity. However, it is not particularly limited. Further, the fiber diameter and the fiber length are not particularly limited, but the fiber diameter is 50 to 500 μm and the fiber length is 2 to 10 mm.
Something is preferable.

As the binder 2 used in the present invention, polyethylene resin, polystyrene resin, methacrylic resin,
Resin binders such as polyurethane resin, phenol resin and urea resin, or natural starch (rice starch, corn starch, tapioca starch, etc.), starch paste made of modified starch and the like can be used. Of course, it is not limited to these. Further, the method of mixing the binder 2 in the fiber 1 is not particularly limited, but for example, a method of mixing in the form of fine particles or a method of applying the liquid binder 2 can be adopted.

Here, by using a foamable resin binder as the binder 2, it is possible to improve the adhesiveness between the fiber 1 and the metal fiber 1a (and the powder 4 which will be described later), and it has a high structure retention property. However, it is possible to obtain the composite body 3 which is less likely to cause performance deterioration. The method for foaming the resin binder is not particularly limited, but basically there are the following methods (1) utilizing a reaction product gas and (2) using a foaming agent.

The method using the reaction product gas of (1) is
This is a method in which a product released when a liquid raw material is resinified, such as carbon dioxide, formaldehyde, or steam, is used as a foaming agent, and this product is vaporized by reaction heat or heating to obtain a foam. . Further, the method of using the foaming agent (2) is further divided into a method of using a volatile foaming agent and a method of using a decomposable foaming agent. The method using a volatile foaming agent is a method in which a gas such as carbon dioxide gas, propane, butane, ether, acetone, benzene or the like or a volatile liquid is absorbed in a resin or a raw material, and this is vaporized to foam. The method of utilizing a decomposable foaming agent is a method of dispersing or dissolving the foaming agent in the resin raw material and then decomposing the foaming agent by heating to foam the resin. Ammonium carbonate, as the foaming agent used at this time,
There are inorganic compounds such as sodium bicarbonate, azo compounds, sulfonyl hydrazide compounds, nitroso compounds, azide compounds and the like, which can be appropriately selected and used according to use conditions and resin raw materials.

The type of the binder 2 must be selected in consideration of the adhesiveness and wettability between the fiber 1 and the metal fiber 1a (and the powder 4 described later). Also binder 2
The amount is preferably in the range of about 5 to 20% by weight with respect to the fiber 1. When forming the composite body 3 in which the fibers 1 and the binder 2 are mixed as described above, high sound absorbing performance can be obtained due to the structure of the complicated voids 5 formed between the fibers 1. In the composite body 3 of FIG. 1 in which the metal fiber 1a is used as at least a part of the above, a high electromagnetic wave absorption performance can be obtained by the absorption action of the high frequency energy by the high frequency current flowing in the metal fiber 1a. That is, the metal fiber 1a functions as a resistor that absorbs electromagnetic waves, and the electromagnetic wave absorbing mechanism will be briefly described below.

When considering a resistor, the real number term μr ′ and the imaginary term μr ″ of the magnetic permeability among the electromagnetic material constants, the complex relative permittivity, and the complex relative permeability, and the complex relative permittivity are as follows. expressed.

[0023]

[Equation 1]

Further, using the resistivity ρ [Ωm] of the material,
The complex relative permittivity in the material is expressed as follows.

[0025]

[Equation 2]

The radio wave constant γ of the electromagnetic wave flowing through the material is

[0027]

(Equation 3)

## EQU3 ## According to the above equations (1) and (3),

[0029]

[Equation 4]

Is obtained. Assuming that the resistivity is small, the following approximate expression is obtained.

[0031]

(Equation 5)

Here, μ ₀ is the vacuum transmittance, and ω is the angular frequency. One item, which is the real part of the above equation, relates to the attenuation of electromagnetic waves and is represented by the attenuation constant α.

[0033]

(Equation 6)

From the above equation (8), it can be seen that the smaller the resistance ρ of the material, the greater the attenuation of electromagnetic waves. In the performance of the electromagnetic wave absorbing material, it is required that the reflection coefficient of the electromagnetic wave on the surface of the material is small at the same time as the damping effect of the electromagnetic wave in the material. Therefore, when the electric field reflection coefficient S of the front surface of the material is expressed in decibels, the following formula is obtained.

[0035]

(Equation 7)

The numerator of the formula (9) is obtained by the above (1),
From equations (2) and (3),

[0037]

(Equation 8)

From the above, it can be seen that the reflection coefficient can be reduced by bringing expression (10) closer to zero. For that purpose, 1 / (ωε ₀ ρ) of the imaginary number is made small, that is, the resistivity ρ
Needs to be increased. In other words, if the resistivity ρ is decreased, the electromagnetic wave in the material is more effectively attenuated, but the phenomenon that the reflection of the electromagnetic wave on the surface of the material is increased occurs.

From the above, it can be seen that the electromagnetic wave attenuation (absorption) action inside the material and the electromagnetic wave reflection action on the material surface are greatly affected by the magnitude of the resistivity ρ. Therefore, it is necessary to control the resistivity ρ in an electromagnetic wave absorbing material as a resistor, which is a composite of the metal fibers 1a, to have an electromagnetic wave attenuating action, and to minimize the reflection coefficient of the electromagnetic wave. In general, the damping effect becomes smaller as the resistivity ρ becomes larger than 10 ^-4 Ωm, and the amount of reflection increases if the resistivity ρ is 10 ^-3 Ωm or less. Therefore, as an electromagnetic wave absorbing material, the resistivity ρ
Is preferably within the range of 10 ⁻⁵ Ωm to 10 ⁻² Ωm including the above range.

The resistivity ρ of the composite body 3 which is the electromagnetic wave absorbing material according to the present invention is greatly influenced by the kind and the mixing amount of the metal fiber 1a to be composited. Therefore, in order to improve the electromagnetic wave absorbing performance, the target resistance is required. It is necessary to set the type and mixing amount of the metal fiber 1a so that the ratio ρ falls within the range. The most effective method of controlling the resistivity ρ is to adjust the mixing ratio of the metal fibers 1a. For example, the resistivity ρ can be greatly reduced by increasing the mixing ratio. Similarly, the resistivity ρ can be lowered by increasing the fiber diameter or the fiber length of the metal fiber 1a.

In addition to controlling the resistivity ρ, controlling the structure of the voids 5 of the composite 3 composed of the fiber 1 and the binder 2 and the three-dimensional structure (arrangement structure) of the metal fiber 1a can improve the electromagnetic wave absorption performance. It will be greatly involved in improvement. That is, by optimizing the three-dimensional structure and void structure of the metal fiber 1a, the electromagnetic wave absorption / scattering action inside the complex 3 can be enhanced, the high-frequency current flowing in the metal fiber 1a can be increased, and the electromagnetic wave absorbing action can be enhanced. It will be possible. As a specific example, by forming a three-dimensional network structure with a metal fiber, it is possible to expect absorption of electromagnetic waves by a loop current and an increase in scattering action of electromagnetic waves.

From the above, the optimum mixing ratio of the fiber 1 of the composite 3 used as the electromagnetic wave absorber and the metal fiber 1a depends on the kind of the metal fiber 1a, the fiber diameter, the fiber length, the composite structure thereof and the like. Therefore, it cannot be said unequivocally that the metal fiber 1a is 20 in terms of the mixing volume ratio.
% To 80% is desirable. When it is more than this range, the resistivity of the material becomes extremely small, and the electromagnetic wave absorption effect may not be sufficiently obtained due to an increase in the amount of reflection of electromagnetic waves.

FIG. 2 shows an example of an electromagnetic wave absorber according to a second aspect of the present invention, in which a fiber 1 (at least a part of which is a metal fiber 1a) and a binder 2 are mixed. 3 are formed and the powder 4 is adhered and held inside the large number of voids 5 between the fibers 1,
Further, it is designed to provide excellent sound absorbing performance. That is, by using the conductive metal fiber 1a as at least a part of the fiber 1, it is possible to obtain the electromagnetic wave absorbing performance by the absorbing action of the electromagnetic wave energy by the high frequency current generated in the metal fiber 1a inside the composite body 3. In addition, by compounding the powder 4, the structure of the voids 5 of the composite 3 can be further complicated by the powder 4, and at the same time, the vibration action of the powder 4 also works to absorb sound in a low frequency range. The effect can be enhanced.

A specific form of the electromagnetic wave absorber according to the invention of claim 2 is that, as shown in FIG. 2, a large number of fibers 1 mainly constituting a structural skeleton are bonded at their contact intersections with a binder 2. , The fiber 1 (at least a part of which is made of the metal fiber 1a) and the binder 2 are mixed to form a composite body 3,
A large number of voids 5 are formed as pores between the fibers 1.
Then, the powder 4 is adhered to the fibers 1 by the binder 2 and held inside the large number of voids 5 between the fibers 1.

The powder 4 has a particle size of 0.1 μm
It is common to use a material having a bulk density of about 1000 μm and a bulk density in the range of about 0.1 g / cm ³ to about 1.5 g / cm ^3, specifically, gold mica powder, Silica powder, acrylic ultrafine powder, talc powder, calcium silicate powder, fluororesin powder, vermiculite, perlite, bentonite, shirasu balloon, Ishimatsuko, pullulan, fused silica powder, graphite, crystalline cellulose powder , Silicon carbide powder, diatomaceous earth, nylon powder, rock wool shot, and the like. The compounding amount of the powder 4 is not particularly limited, but is preferably about 20% to 80% by volume ratio with respect to the total amount of the fiber 1 including the metal fiber 1a.

As the binder 2, the above-mentioned ones can be used, but the amount of the binder 2 blended is the fiber 1
On the other hand, about 5 to 20% by weight is preferable. If the amount of the binder 2 is larger than this range, the sound absorbing effect due to the vibrating action of the powder becomes small and the low frequency performance is deteriorated, which is not preferable. The electromagnetic wave absorber according to the invention of claim 3 is the composite body 3 according to the invention of claim 1 or 2.
By providing the conductive layer 6 on the back surface of the above, it is possible to obtain higher electromagnetic wave absorption performance. FIG. 3 shows an example of a specific form in which the conductive layer 6 is provided on the composite body 3 according to the invention of claim 2.
The conductive layer 6 is adhered to the back surface of the composite body 3 formed by the powder 4 being adhered and held by the binder 2 in the void 5 formed between the fiber 1 mainly forming the skeletal structure and the metal fiber 1a. Then, they are stacked.

The conductive layer 6 is not particularly limited, but the conductive property is imparted by plating a metal plate such as an iron plate, an aluminum plate, a galvanized steel plate, or a non-conductive material. A conductive plate or the like can be used. Generally, when high electromagnetic wave absorption performance is required, not only the reflection coefficient S but also the transmission coefficient T must be reduced. The transmission coefficient cannot be generally stated because it depends on the thickness of the material and the degree of attenuation of electromagnetic waves, but it is difficult to completely eliminate the transmitted wave with a structure in which the metal fiber 1a and the fiber 1 are simply combined. On the other hand, since the conductive layer 6 is provided on the back surface of the composite body 3 as in the third aspect of the invention, the transmitted wave can be completely eliminated. That is, the electromagnetic wave incident on the composite body 3 is reflected between the conductive layer 6 on the back surface of the composite body 3 and the front surface of the composite body 3, whereby the electromagnetic wave is attenuated and higher electromagnetic wave absorption performance can be obtained. You can do it.
At this time, it is necessary to select the material used for the conductive layer 6 in consideration of the deterioration of the sound absorption characteristics due to the conductive layer 6. For example, the conductive layer is acoustically transparent, that is, its thickness is 5
When the conductive sheet has a thickness of 0 μm or less, the sound absorbing performance of the composite 3 is not impaired while maintaining the electromagnetic wave absorbing performance. A typical example of such an acoustically transparent conductive sheet is an aluminum foil.

The electromagnetic wave absorber according to the present invention produced as described above is required to have electromagnetic wave absorbing performance and soundproofing performance at the same time as a soundproof wall of an elevated road or a partition or partition of a house or office that handles OA equipment. It is used for a part used for a part to be opened.

[0049]

As described above, the electromagnetic wave absorber of claim 1 comprises the composite body 3 in which the fiber 1 and the binder 2 are mixed, and at least a part of the fiber 1 is the metal fiber 1a. It is possible to obtain the action of absorbing electromagnetic wave energy by the high-frequency current generated in the metal fiber 1a held inside, and the electromagnetic wave scattering and absorption due to the complex structure of the void 5 included in the complex 3 mainly composed of the fiber 1. A high electromagnetic wave absorption performance can be obtained by the combined action. Further, since the composite body 3 is a mixture of the fibers 1 and the binder 2, it is lightweight and excellent in handleability, and high sound absorbing performance can be obtained due to the complicated structure of the voids 5.

In addition to the invention of claim 1, the invention of claim 2 is characterized in that the powder 4 is held in the internal void 5 of the composite body 3, so that the voids formed by compounding the powder 4 are formed. The sound absorption performance of the composite body 3 can be further enhanced by the complicated structure of 5 and the vibrating action of the powder 4. Further, in addition to the inventions of claim 1 and claim 2, the invention of claim 3 has a conductive layer 6 formed on one surface of the composite body 3, so that the electromagnetic wave incident on the composite body 3 is electrically conductive. The electromagnetic wave can be attenuated by being reflected between the surface of the composite 3 and the other surface of the composite 3, and the electromagnetic wave absorption performance can be further enhanced.

[0051]

EXAMPLES The present invention will be further described with reference to examples. Needless to say, the present invention is not limited to the examples. (Example 1) Rock fiber (fiber diameter;
Steel fiber (fiber diameter: 50 μm, 10 μm) cut out from a thin steel plate as a metal fiber 1a
Vermiculite (particle size; 180 μm to 250 μm) is used as the powder 4 and phenol foamed resin fine particles are used as the binder 2, and the total fiber 1 containing 50% of the metal fiber 1a in a mixing volume ratio is 45.
% By weight, 45% by weight of powder 4 and 10% by weight of binder 2, and then the mixture 7 has a thickness of 30 m as shown in FIG.
The binder 2 is foamed and hardened to fill the space between the heating plates 9 and 9 of 130 ° C., which are arranged via the distance bar 8 of m. It was used as an absorber.

Example 2 Rock fiber (fiber diameter: 10 μm) was used as the fiber 1, steel fiber (fiber diameter: 50 μm, fiber length 3 mm) cut out from a thin steel plate was used as the metal fiber 1 b, and vermiculite (powder 4) was used as the powder 4. Particle size; 180 μm to 250 μm),
Furthermore, phenol foamed resin fine particles are used as the binder 2, and these materials are mixed at the same mixing ratio as in Example 1, and then molded in the same manner as in Example 1, whereby the binder 2 is foamed and cured to form an electromagnetic wave absorption composite. Body 3 was produced. Next, an aluminum foil having a thickness of 40 μm was adhered and laminated on the back surface of this composite body 3 to form a conductive layer 6, thereby obtaining an electromagnetic wave absorber.

Comparative Example 1 Commercially available JIS A 9505
Glass wool felt with a density of 24 kg / m ³ as specified in (Asahi Glass Fiber Co., Ltd. “Glass Wool 24K”)
Was used as an electromagnetic wave absorber. Table 1 shows the thicknesses and densities of the composites 3 of Examples 1 and 2 and Comparative Example 1. Further, the electromagnetic wave absorbing performance and the sound absorbing performance of the electromagnetic wave absorbers of Examples 1 and 2 and Comparative Example 1 were measured. The electromagnetic wave absorption performance is
Reflection coefficient (d) in the range of television radio frequency (VHF)
B) was measured and evaluated, and the sound absorption performance was evaluated by measuring the sound absorption coefficient and the peak frequency of the sound absorption coefficient. Sound absorption coefficient here is J
It is the vertical incident sound absorption coefficient measured according to IS A 1405 "Method for measuring vertical incident sound absorption coefficient of building material by in-pipe method", and is measured when the thickness of the electromagnetic wave absorber is 30 mm. The sound absorption coefficient was calculated using standing waves.

Graphs showing frequency dependence of the sound absorption coefficient obtained from the sound absorption coefficient measurement are shown in FIGS. 4 and 5. FIG. 4 is for Example 1 and FIG. 5 is for Comparative Example, and the frequency at which the sound absorption coefficient is maximum was taken as the sound absorption peak frequency. As can be seen from the comparison between FIG. 4 and FIG. 5, it is confirmed that the example has excellent sound absorbing performance in the low frequency range.

Table 1 shows the measurement results of the electromagnetic wave absorbing performance and the sound absorbing performance. In Examples 1 and 2, -10 dB to -20 d.
It can be seen that the reflection coefficient of about B is obtained and the electromagnetic wave absorption performance is high, whereas the material of Comparative Example 1 has no electromagnetic wave absorption effect. In addition, although the sound absorbing characteristics of the first and second embodiments are substantially the same, it is understood that the electromagnetic wave absorbing performance of the second embodiment in which the conductive layer 6 is laminated on the back surface is superior.

[0056]

[Table 1]

[0057]

As described above, according to the first aspect of the present invention, since the composite is composed of the fiber and the binder, and at least a part of the fiber is a metal fiber, the metal fiber absorbs electromagnetic wave energy. It is possible to obtain high electromagnetic wave absorption performance, and because it is mainly composed of fibers, it is lightweight and easy to handle, and it is also possible to obtain high sound absorption performance due to the voids formed between the fibers. It is possible to provide an electromagnetic wave absorber having both performance, light weight and excellent handleability.

Further, the invention of claim 2 is composed of a composite body in which powder, fibers and a binder are mixed, and at least a part of the fibers is a metal fiber, and the powder is a composite body composed of the fibers and the binder. Since it is characterized in that it is held in the internal space, it is possible to provide an electromagnetic wave absorber which has both electromagnetic wave absorption performance and sound absorption performance as in the case of the invention of claim 1, and which is lightweight and has excellent handleability. Needless to say, the sound absorbing performance of the composite can be further enhanced due to the complicated structure of the void due to the composite of the powder and the vibrating action of the powder.

Furthermore, the invention according to claim 3 is characterized in that a conductive layer is formed on one surface of the composite, so that the electromagnetic wave incident on the composite is transferred between the conductive layer and the other surface of the composite. Electromagnetic waves can be attenuated by reflection between them, and electromagnetic wave absorption performance can be further enhanced.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of an embodiment of the invention of claim 1.

FIG. 2 is a schematic cross-sectional view showing the structure of an embodiment of the invention of claim 2;

FIG. 3 is a schematic cross-sectional view showing the structure of an embodiment of the invention of claim 3;

FIG. 4 is a graph showing frequency dependence of sound absorption coefficient in Example 1.

5 is a graph showing frequency dependence of sound absorption coefficient in Comparative Example 1. FIG.

FIG. 6 is a cross-sectional view showing molding of a composite.

[Explanation of symbols]

 1 Fiber 1a Metal Fiber 2 Binder 3 Composite 4 Powder 5 Void 6 Conductive Layer

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 13, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

On the other hand, in recent years, in the fields of soundproof walls of elevated roads, partitions and partitions such as houses and offices that handle OA equipment, demands for electromagnetic wave absorbing performance and soundproofing performance have become severe. There are the following two measures to improve the soundproofing performance.
One is a method of blocking the propagation of the sound wave by installing a material such as a metal plate having a high surface density at a position where the propagation path of the sound wave is blocked, that is, a wall or a partition portion. However, with this method, although it is possible to obtain soundproofing and high electromagnetic wave blocking performance, there is a possibility of causing ghost interference or communication failure of TV radio waves due to reflection of electromagnetic waves, and soundproofing performance Not enough.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideyuki Ando, 1048, Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Works Co., Ltd. (72) Inventor, Koji Tsuji, 1048, Kadoma, Kadoma City, Osaka Prefecture 72) Inventor Hajime Sugiyama 1048, Kadoma, Kadoma-shi, Osaka Prefecture Matsushita Electric Works Co., Ltd.

Claims (3)

[Claims] 1. An electromagnetic wave absorber comprising a composite in which fibers and a binder are mixed, and at least a part of the fibers is a metal fiber. 2. A composite comprising a mixture of powder, fibers and a binder, wherein at least a part of the fibers is a metal fiber, and the powder is held in the internal voids of the composite by the fibers and the binder. An electromagnetic wave absorber characterized by being present. 3. The electromagnetic wave absorber according to claim 1, wherein a conductive layer is formed on one surface of the composite.