JPH1022676A - Radio wave absorber - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a lightweight radio wave absorber having excellent radio wave absorption characteristics. SOLUTION: The radio wave absorber having a resistor layer, a dielectric layer and a reflection layer, wherein the resistor layer has a volume specific electric resistance of 10 ⁻² to 10 ² Ωcm and a carbon content of 85%. ~
95%, a carbon content of 0.3 to 2.5%, and a carbon long fiber having a lattice constant Lc of graphite of 25 ° or less and a plane spacing d002 of 3.45 ° or more determined by X-ray diffraction. A radio wave absorber characterized by the following.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio wave absorber. More specifically, the present invention relates to a radio wave absorber useful as an outer wall material of a high-rise building.

[0002]

2. Description of the Related Art In recent years, television interference caused by high-rise buildings has become a major social problem. High-rise buildings use a large amount of steel and the outer wall is often made of concrete, but concrete has extremely poor radio wave absorption characteristics, so radio waves reflected by steel may cause radio interference. There are many.

[0003] As one of the methods for preventing the radio wave interference, a concrete wall surface is coated with a material having a good radio wave absorption property such as a ferrite tile.

[0004]

Ferrite tiles have good radio wave absorption, but have a drawback of high specific gravity. Therefore, when a ferrite tile is used, the strength of the structural member must be increased, which increases the cost. Accordingly, an object of the present invention is to provide a radio wave absorber that is lightweight and has excellent radio wave absorption characteristics. The present invention also provides a method for manufacturing such a radio wave absorber.

[0005]

Wave absorber according to the present invention According to an aspect of the carbon filament showing the volume electrical resistivity of the resistance film layer 10 ^-2 ~10 ² Ω · cm was used in the resistance film layer, the resistance It has a dielectric layer behind the film layer and a reflective layer behind the dielectric layer. The radio wave can be efficiently absorbed by installing the radio wave absorber so that the resistive film layer faces the direction of arrival of the radio wave and the carbon fiber bundle is parallel to the electric field direction of the radio wave.

Namely, the gist of the present invention, the resistor layer, a radio wave absorber having a dielectric layer and a reflective layer, resistive element antibodies layer, a volume electrical resistivity value of 10 ^-2 to 10 ² [Omega] cm Yes, with a carbon content of 85-95% and a hydrogen content of 0.3-
A radio wave absorber characterized by using a carbon long fiber having a lattice constant Lc of 25% or less and an interplanar spacing d002 of 3.45 ° or more as determined by X-ray diffraction. Exists.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail. The radio wave absorber according to the present invention consists essentially of three layers: a resistive layer, a dielectric layer and a reflective layer. In the resistive film layer, carbon fiber bundles produced by combining long carbon fibers arranged in the longitudinal direction with a binder such as resin are arranged in parallel at intervals. The thickness (cross-sectional area) of the carbon fiber bundle is
The total cross-sectional area of the carbon fiber is usually 0.2 to 80 mm ² , preferably 0.8 to ²⁰ mm ² . As is well known, carbon long fibers are usually produced in a state where a number of single fibers having a diameter of several μm to several tens μm are bundled, and the carbon fiber bundle used in the present invention is this or a larger number of the same. They are aligned and joined with a binder such as resin or asphalt to be integrated. As the resin as a binder, for example, epoxy resin, phenolic resin, saturated or unsaturated polyester, polyphenylene sulfide, polyphenylene ether, polycarbonate, polyoxymethylene, polystyrene, polyolefin, polyurethane resin, acrylic resin, vinyl acetate resin, polyamide A homopolymer such as a resin, or a copolymer may be used. Of these, epoxy resins, phenol resins, water-soluble polyamide resins, and polyurethane resins are particularly preferred. A method of attaching a binder to carbon fibers can also be performed by a known method. For example, there is a method in which 5,000 to 40,000 long fibrous carbon fiber tows are impregnated with a binder and then dried.
The form of the binder at the time of impregnation may be dissolved in an appropriate solvent or dispersed in water as an emulsion using a surfactant. Examples of the solvent used include 2-butanone, tetrahydrofuran, N, N-dimethylformamide, acetone, chloroform, dichloromethane and the like.

Although the shape of the carbon fiber bundle used in the present invention is arbitrary, a rod shape such as a round bar is preferable from the viewpoint of handleability and the like. However, if desired, it may be in the form of a string, a plate or the like. In addition, the bonding state of the resin may be any bonding state, from a weak bonding state sufficient to maintain the integrity of the carbon fiber bundle to a strong bonding state called a so-called carbon fiber rod. be able to. Therefore, in the present invention, if desired, by using a carbon fiber rod as the carbon fiber bundle, the function as a reinforcing member of the mechanical strength of the radio wave absorber can be borne on the resistive film layer.

The arrangement interval of the carbon fiber bundles in the resistive film layer varies depending on the volume specific electric resistance of the long carbon fibers used, the cross-sectional area of the carbon fiber bundles and the like, but is usually 5 to 1000 mm, preferably 10 to 500 mm. For example, in the case of using a rod-shaped carbon fiber bundle having a diameter of 3 mm formed of long carbon fibers having a volume specific electric resistance of about 3 Ω · cm, 50
What is necessary is just to arrange at an interval of about 100 mm. As shown in FIG. 1, a single carbon fiber bundle is usually disposed between both ends of the resistive film layer. If desired, a plurality of short carbon fiber bundles may be disposed to form a resistive film as a whole. The carbon fiber bundle may be arranged in the layer between both ends thereof.
However, even in this case, it is preferable to use a carbon fiber bundle having a length of 500 mm or more from the viewpoint of workability and the like.

[0010] The length of the carbon fiber aggregate can be selected arbitrarily depending on the application. The dielectric layer can be composed of various materials such as glass, polyurethane foam, expanded polystyrene, concrete, cement mortar, calcium silicate, and the like. Preferably, concrete or cement mortar (both are sometimes referred to as concrete in this specification) is used. Concrete can include reinforcing materials such as carbon fiber, steel fiber, glass fiber, and synthetic fiber, lightweight aggregates, and the like. The preferred thickness (d) of the dielectric layer is calculated from the following equation from the wavelength (λ) of the radio wave to be absorbed and the relative permittivity (ε) of the dielectric.

[0011]

D = λ / (4√ε) Therefore, when the relative dielectric constant is increased by including carbon fibers in the concrete, the thickness of the dielectric layer can be reduced. In addition, since the relative permittivity of concrete differs depending on the water content, when the radio wave absorber of the present invention is used for the outer wall of a high-rise building or the like, so that the water content of the dielectric layer does not significantly change due to rainwater penetration or the like. It is preferable to mix a water repellent when the concrete of the dielectric layer is cast. This is because concrete poured with a water repellent has a significantly reduced water absorption, so that the change in water content due to rainwater infiltration and the like, and hence the change in relative dielectric constant, are small.

The reflection layer preferably reflects substantially all the incident radio waves, and is formed of a metal plate, a wire net, a metal rod, or the like. In particular, in the present invention, it is preferable that the dielectric layer is formed of a reinforcing bar also for reinforcing the concrete forming the dielectric layer. That is, it is preferable that the reflection layer is configured by arranging reinforcing bars at intervals so as to be parallel to the carbon fiber bundle forming the resistance film layer. The reinforcing bar may be of a usual thickness used for reinforcing concrete. The interval between the reinforcing bars is usually 25 to 200 mm, preferably 30 to 150 mm. The reflection layer can be composed of a plurality of types of reinforcing bars having different thicknesses, if desired. For example, it is possible to increase the reflectance by arranging thin reinforcing bars between reinforcing thick reinforcing bars arranged at wide intervals.

The radio wave absorber according to the present invention consists essentially of the above-mentioned resistive layer, dielectric layer and reflective layer, but may further have additional layers. For example, in a preferred embodiment of the present invention, a surface layer is formed on the front surface of the resistive film layer. The surface layer may be a water-impermeable decorative material such as a ceramic plate, or may be a radio wave absorbing material such as a ferrite tile.
When the dielectric layer is made of concrete and the reflective layer is made of reinforcing steel, concrete is cast behind the reinforcing bar so that the reinforcing bar is completely buried in the concrete extending from the dielectric layer. That is, it is preferable to form a dielectric layer behind the reflective layer.

One preferred embodiment of the radio wave absorber according to the present invention is a concrete in which a surface layer is provided, a reflective layer is formed of reinforcing steel also for reinforcement, and extends to the back of the reflective layer. By this, the whole is integrated. Such a radio wave absorber can be easily manufactured by a common method of manufacturing a concrete panel. That is, first, a surface material such as a porcelain plate or a ferrite tile is arranged in a shallow dish-shaped formwork, and carbon fiber bundles are arranged thereon at a predetermined interval. Preferably,
Carbon fiber bundles are mounted horizontally on a number of longitudinal members at predetermined intervals according to the size of the formwork, or carbon fiber bundles are woven in cords at predetermined intervals with cords Is prepared in advance and this is to be placed on the surface material. When such a method is used, work efficiency is higher and dispersion of the intervals between the carbon fiber bundles can be prevented as compared with the case where carbon fiber bundles are arranged one by one. Once the carbon fiber bundle has been placed, the rebar is then placed above it. The reinforcing bars are arranged in parallel with the carbon fiber bundle and such that the distance between the carbon fiber bundle and the reinforcing bar is substantially equal to the thickness of the dielectric layer calculated by the above-described formula. In addition, a reinforcing bar perpendicular to these reinforcing bars is also usually arranged for reinforcement. Therefore, preferably, a reinforcing bar assembly is preferably prepared in advance by arranging reinforcing bars in parallel at a predetermined interval and combining them with reinforcing bars in a direction perpendicular to the reinforcing bars in advance. Install it. By doing so, the work efficiency is high, and the rebar can be accurately arranged at a predetermined position. When the above preparation work is completed, concrete is poured into the formwork so that the reinforcing bar is completely buried and hardened firmly, and the whole is united and cured, so that the radio wave absorber according to the present invention is obtained. can get.

The long carbon fiber used for the resistor layer of the radio wave absorber according to the present invention has a volume specific electric resistance of 10 ⁻² to 10 ^2.
Ω · cm, preferably 10 ⁻¹ to 10 Ω · cm. When the volume specific electric resistance is smaller than 10 ^-2 Ω · cm, a radio wave absorber having good characteristics cannot be formed. In addition, 10 ² Ω
When the diameter is larger than cm, the carbon fiber becomes extremely unstable and fragile, which is not preferable.

The carbon fiber of the present invention has a carbon content of 85 to 85.
95%, preferably 90-95%, and the hydrogen content is 0.3-2.5%, preferably 0.8-2.0%. If the carbon content is less than 85% and the hydrogen content is more than 2.5%, the carbon fiber becomes extremely unstable and fragile, which is not preferable. If the content is less than 0.3%, it is not preferable because a radio wave absorber having good characteristics cannot be formed.

The carbon fiber of the present invention has a lattice constant Lc of graphite of 25 ° or less, preferably 10 to 10%, determined by X-ray diffraction.
20 °, and the plane interval d002 is 3.45 ° or more, preferably 3.45 to 3.48 °. If Lc is larger than 25 ° and d002 is smaller than 3.445 °, it is not preferable because a radio wave absorber having good characteristics cannot be formed.

Next, the manufacturing method of the present invention will be described. As the raw material pitch used in the present invention, for example,
Carbonaceous raw materials such as coal-based coal tar, coal tar pitch, coal liquefaction, petroleum-based heavy oil, pitch, petroleum resin and its thermal polycondensation reaction products, and polymerization reaction products by the catalytic reaction of naphthalene and anthracene Is mentioned. Also,
These carbonaceous raw materials may be used after being subjected to a preliminary treatment such as a heating treatment followed by extraction of a soluble component with a solvent or a hydrogenation treatment in the presence of a hydrogen-donating solvent or hydrogen gas.

The raw material pitch contains ash (Ash component) as an insoluble substance. This is due to an optically anisotropic material which becomes a carbon fiber precursor by heating the raw material pitch. In the case of the liquid crystal pitch, it causes non-uniformity and gives a precursor of disordered structure. Further, after spinning, the fiber obtained by infusibilizing and firing produces physical defects, which adversely affects strength and elastic modulus.

Therefore, if a pitch refined to an ash content of usually 30 ppm or less, preferably 20 ppm or less is used at the stage of spinning, carbon fibers having high tensile strength can be obtained, and the carbon fiber bundle also serves as a reinforcing material. This is useful when used. The ash may be removed at any time before spinning, for example, at the raw material pitch stage or the spinning pitch stage. The ash may be removed by a known method. For example, there are a sedimentation method, a centrifugal separation method, a filtration method, an adsorption method, an acid, an alkali, a washing method with a solvent, and the like. May be repeated. It is also effective to add a porous inorganic substance (such as a filter aid) to increase the efficiency of removal. Industrially, it is preferable to use a sedimentation method, a centrifugal separation method, or a filtration method since continuous or large-scale treatment can be performed.

The pitch purified as described above is converted into a liquid crystal pitch having optical anisotropy according to a conventional method. The optical anisotropy ratio of the pitch used for spinning needs to be 80% or more, preferably 90% or more, and more preferably 95% or more. 80% optical anisotropy
If it is less than, the strength of the carbon fiber is remarkably reduced, and if the firing temperature is increased to increase the tensile strength, the electric resistance is inevitably reduced, and a carbon fiber having a desired high electric resistance cannot be obtained.

The ratio of the optical anisotropy of the pitch is a value obtained by determining, as an area ratio, a portion showing the optical anisotropy in the pitch sample under a polarizing microscope at normal temperature. Specifically, for example, a sample obtained by crushing a pitch sample into a few mm square is embedded in a substantially whole surface of a resin having a diameter of about 2 cm according to a conventional method, and after polishing the surface, the entire surface is polarized. It is determined by observing under a microscope (100 times magnification) and measuring the area ratio of the optically anisotropic portion to the total surface area of the sample.

A method for producing an optically anisotropic liquid crystal pitch can also be performed by a known method. For example, the refined pitch is brought to 350-500 ° C., preferably 380-45 ° C.
At 0 ° C. for 2 minutes to 50 hours, preferably for 5 minutes to 5 hours, under an atmosphere of an inert gas such as nitrogen, argon, or steam,
Alternatively, there is a method of performing heat treatment under blowing or under reduced pressure. As another example, a method of polymerizing a condensed polycyclic hydrocarbon such as naphthalene in the presence of a catalyst such as HF / BF3, or performing a solvent separation using a solvent having a specific solubility parameter on a raw material pitch, There is a method for obtaining a desired pitch.

The carbon content of the spinning pitch is at least 93%, particularly preferably at least 95%. If the carbon content is less than 93%, as in the case of the above-mentioned ash, nitrogen, sulfur, oxygen, etc., which are foreign elements, cause a reduction in strength, and lower the tensile strength of carbon fibers. Using the above-described spinning pitch, melt spinning is performed to obtain pitch fibers. The obtained pitch fiber has a low breaking strength as a single fiber.
0000 pitch fibers are bundled with a sizing agent to form a pitch fiber tow. As the sizing agent, one that dissolves a part of the pitch fibers or adheres or fuses the fibers during the infusibilization treatment is required, and for example, a water emulsion of silicone oil is preferable. Further, in order to more effectively avoid fusion, inorganic fine particles such as carbon black and SiC may be added to the sizing agent.

The pitch fiber tow is heated to 160 to 400 ° C. in an oxidizing gas atmosphere to make it infusible. The obtained infusible fiber tow is calcined in an atmosphere of an inert gas such as nitrogen or argon to obtain carbon fibers. In the present invention, first, in a state where tension is not applied to the infusible fiber tow,
The first carbonization treatment is performed at a temperature of 450 to 600C, preferably 520 to 580C. The temperature of the first carbonization treatment is 450 ° C
If it is lower, the fiber strength is not enough to withstand the tension of the second carbonization treatment, and if it is higher than 600 ° C., the elongation of the fiber decreases, and it is also possible to withstand the tension of the second carbonization treatment. It is not preferable because it cannot be done.

The basis weight of the carbonized yarn obtained by performing the first carbonization treatment is 1.6 to 6.0 g / m, preferably 1.8 to 4.5.
g / m. If the basis weight is less than 1.6 g / m, a large number of carbon fibers must be used when manufacturing a composite material having a predetermined shape. Therefore, if the basis weight is greater than 6.0 g / m, tension is applied. When the second carbonization is performed, the alignment of each fiber in the fiber axis direction is undesirably reduced.

The time for the first carbonization treatment is usually 1 second to 10 seconds.
Minutes, preferably 10 seconds to 5 minutes. Next, while applying a tension of 50 g or more, preferably 80 g or more per 1000 filaments, to the first carbonization-treated fiber tow, 700 to 1000 ° C., preferably 730 to 90 ° C.
The second carbonization treatment is performed at 0 ° C, more preferably at a temperature of 750 to 850 ° C. If the temperature of the second carbonization treatment is lower than 700 ° C., the strength and the radio wave absorption characteristics are not sufficient, and
If it exceeds 1000 ° C., only carbon fibers having low electric resistance can be obtained.

The time for the second carbonization treatment is usually 1 second to 10 seconds.
Minutes, preferably 10 seconds to 5 minutes. If the time of the second carbonization treatment is shorter than 1 second, the control of the residence time, that is, the control of the degree of carbonization becomes difficult, and the variation of the electric resistance value becomes large. It is not preferable because it becomes large.

[0029]

EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples. Example A spinning pitch having an optical anisotropy of 100%, a Mettler softening point of 302 ° C., a carbon content of 96% by weight, and an ash content of 17 ppm was spun at a die temperature of 337 ° C. while being bundled with a silicon-based oil, and the number of filaments was 11,000. Book length 7000m
Was obtained.

The obtained pitch fiber tow was heated to 380 ° C. at a heating rate of 1 ° C./min in the air and heated to make it infusible. Next, in a nitrogen gas atmosphere, the first carbonization treatment was performed at 560 ° C. and a residence time of 2 minutes without applying tension to the infusibilized fiber tow. The basis weight of the carbonized yarn obtained in the first carbonization treatment was 2.1 g / m. Further, in a nitrogen gas atmosphere, a tension of 1320 g / tow (120 g / 1000 strands) was applied to the first carbonized fiber tow, and
The second carbonization treatment was performed under the conditions of ° C. and a residence time of 2.5 minutes to produce carbon fibers.

The carbon fiber obtained has a carbon content of 93%,
The hydrogen content was 1.3%, the lattice constant Lc of graphite determined by X-ray diffraction was 17 °, the spacing d002 was 3.46 °, and the volume specific electrical resistance was 0.8 Ω · cm. The obtained carbon fiber tow is solidified with epoxy resin and CF cross section is about 1.3m
An m ² string-like carbon fiber bundle was produced.

On the back of a ceramic plate of about 600 × 900 mm and a thickness of about 13 mm, carbon fiber bundles were arranged at intervals of 40 mm in parallel with the short sides, and bonded to the ceramic plate with an adhesive. Put this in a mold and place it about 5 mm thick at a position about 40 mm above the ceramic plate.
Reinforcing bar assembly where the reinforcing bars are arranged vertically and horizontally at 25mm intervals,
One reinforcing bar was set so as to be parallel to the short side of the ceramic plate.
Next, pour concrete into the formwork and harden well.
Cured for about 3 weeks at room temperature and dried to about 600 × 900 × 10
A radio wave absorber of 0 mm was obtained.

When the radio wave absorption characteristics of this radio wave absorber were measured by the large waveguide method, about 25 d at 600 MHz was obtained.
The absorption characteristics of B were shown. Comparative Example 1 A spinning pitch having an optical anisotropy of 98%, a Mettler softening point of 298 ° C., a carbon content of 94% by weight, and an ash content of 50 ppm was spun at a die temperature of 333 ° C. while being bundled with a silicon-based oil agent. 11,000 pitch fiber tows having a length of 7000 m were obtained.

The obtained pitch fiber tow was heated to 380 ° C. at a heating rate of 1 ° C./min in the air and heated to perform infusibility treatment. Next, in a nitrogen gas atmosphere, the first carbonization treatment was performed at 530 ° C. and a residence time of 2 minutes without applying tension to the infusible fiber tow. The basis weight of the carbonized yarn obtained in the first carbonization treatment was 2.1 g / m. Further, in a nitrogen gas atmosphere, a tension of 1320 g / tow (120 g / 1000 strands) was applied to the first carbonized fiber tow, and
The second carbonization treatment was performed at a temperature of 3 ° C. and a residence time of 3 minutes to produce carbon fibers.

The carbon fiber obtained has a carbon content of 84%,
The hydrogen content was 2.5%, the lattice constant Lc of graphite determined by X-ray diffraction was 20 °, the interplanar spacing d002 was 3.44 °, and the volume specific electrical resistance was 200 Ω · cm. The obtained carbon fiber tow was solidified with an epoxy resin to produce a string-shaped carbon fiber bundle having a CF cross-sectional area of about 1.3 mm ² .

Using the obtained carbon fiber bundle, a radio wave absorber was obtained in exactly the same manner as in the example. When the radio wave absorption characteristics of this radio wave absorber were measured by the large waveguide method, it showed only about 0.2 dB at the maximum. Comparative Example 2 A spinning pitch having an optical anisotropy of 100%, a Mettler softening point of 301 ° C., a carbon content of 96% by weight, and an ash content of 15 ppm was spun at a die temperature of 335 ° C. while being bundled with a silicon-based oil agent. 11,000 length 7000m
Was obtained.

The obtained pitch fiber tow was heated to 380 ° C. at a heating rate of 1 ° C./min in the air and heated to perform infusibility treatment. Next, in a nitrogen gas atmosphere, the first carbonization treatment was performed at 530 ° C. and a residence time of 2 minutes without applying tension to the infusible fiber tow. The basis weight of the carbonized yarn obtained in the first carbonization treatment was 2.1 g / m. Further, in a nitrogen gas atmosphere, a tension of 1320 g / tow (120 g / 1000 strands) was applied to the first carbonized fiber tow to 1200
The second carbonization treatment was performed at a temperature of 3 ° C. and a residence time of 3 minutes to produce carbon fibers.

The carbon fiber obtained has a carbon content of 99%,
The hydrogen content was <0.3%, the lattice constant Lc of graphite determined by X-ray diffraction was 27 °, the interplanar spacing d002 was 3.47 °, and the volume specific electrical resistance was 0.001 Ω · cm.
The obtained carbon fiber tow was solidified with an epoxy resin to produce a string-like carbon fiber bundle having a CF cross-sectional area of about 1.2 mm ² .

Using the obtained carbon fiber bundle, a radio wave absorber was obtained in exactly the same manner as in the example. When the radio wave absorption characteristics of this radio wave absorber were measured by the large waveguide method, it showed only about 0.5 dB at the maximum.

[Brief description of the drawings]

FIG. 1 is an example of a radio wave absorber according to the present invention.

FIG. 2 is a view in which a resistive film layer is exposed by removing a surface material from the radio wave absorber of FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Surface material which consists of a large tile 2 Carbon fiber bundle which forms a resistive film layer 3 Dielectric layer which consists of concrete 4 Rebar which forms a reflective layer

Claims (7)

[Claims] 1. A resistance layer, a radio wave absorber having a dielectric layer and a reflective layer, resistive element antibodies layer is a volume electric resistance 10 ^-2 to 10 ² [Omega] cm, a carbon content 85-
95%, a carbon content of 0.3 to 2.5%, and a carbon long fiber having a lattice constant Lc of graphite of 25 ° or less and a plane spacing d002 of 3.45 ° or more determined by X-ray diffraction. A radio wave absorber characterized by the following. 2. The resistor layer is formed by arranging long carbon fiber bundles formed by arranging long carbon fibers in a length direction and binding them with a binder at intervals. 2. The radio wave absorber according to 1. 3. The radio wave absorber according to claim ^1, wherein the carbon fiber has a volume specific electric resistance of 10 ⁻¹ to 10 Ω · cm. 4. The method according to claim 1, wherein the total cross-sectional area of the carbon fibers in the carbon fiber bundle is 0.
The radio wave absorber according to any one of claims 1 to 3, which has a thickness of ² to 80 mm2. 5. The radio wave absorber according to claim 1, wherein the reflection layer is made of a reinforcing bar arranged at least in parallel with the carbon fiber bundle. 6. The radio wave absorber according to claim 1, wherein the dielectric layer is concrete or cement mortar. 7. The carbon fiber bundle forming the resistor layer is in contact with concrete or cement mortar, and the reinforcing steel forming the reflecting layer is completely buried in the concrete or cement mortar. 7. The radio wave absorber according to claim 6, wherein the cement mortar is integrated with concrete or cement mortar constituting the dielectric layer. 8.