JP2002094282A - Radio wave absorbent and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio wave absorbent where conductive carbon is generated in a void of a noncombustible or fire-retardant porous body, and the manufacturing method of the radio wave absorbent. SOLUTION: The void of the noncombustible or fire-retardant porous body carries a catalyst containing metal such as nickel and cobalt by the impregnation or ion exchange method, or the porous body containing the metal elements is prepared for giving a catalyst function to the void and the contact of hydrocarbon is made under heating for generating the conductive carbon in the void inside the porous body, thus manufacturing the radio wave absorbent where the conductive carbon is uniformly distributed.

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 in which conductive carbon is formed in pores of a non-combustible or non-combustible porous body and a method for producing the same.

[0002]

2. Description of the Related Art Normally, a radio wave absorber is obtained by dispersing a radio wave absorbing material such as conductive carbon or ferrite in a base material such as polyurethane foam. As the base material, porous materials such as foamed polyurethane and thermoplastic resins such as styrene resin, silicone resin and phenol resin are often used.
These organic base materials have advantages such as being able to be formed without using a mold, being easily cut into various sizes and shapes by cutting, laminating, etc., and being light,
There is a disadvantage that heat resistance is low. Absorption of a radio wave is performed by the radio wave absorbing material absorbing the radio wave and converting the electromagnetic energy of the electromagnetic wave into heat energy. At this time, the radio wave absorbing material generates heat. Irradiation of high-energy radio waves increases the amount of heat generated by the radio wave absorber, melting the base material,
It causes thermal deformation and thermal decomposition, and may generate and burn harmful gases.

In order to solve such problems, it has been proposed to use an inorganic material as a base material. For example, Japanese Patent Application Laid-Open No. H3-131091 discloses that a powder of a substance having radio wave absorption is contained in continuous pores of a porous sintered material having continuous pores obtained by sintering aluminum or aluminum alloy powder. A radio wave absorbing material is disclosed. JP-A-3-217081 describes a non-combustible radio wave absorber in which carbon fibers are mixed with a silica-alumina-based inorganic foamed binder. Other inorganic materials include foamed cement (JP-A-2-27798), calcium silicate molding material, ACL, foamed concrete, lightweight concrete (JP-A-64-82600), and foamed glass. (JP-A-2-49497)
Etc. are known.

[0004] The method of dispersing the radio wave absorbing material in the base material differs depending on the base material. When the base material is a porous material such as urethane foam that can be compressed and reduced in volume, a radio wave absorber is manufactured by impregnating the base material with a dispersion of a radio wave absorbing material in various dispersion media. If the base material is a resin such as polystyrene resin or silicone resin, add a radio wave absorbing substance to the resin component, add additives such as solvent, plasticizer, anti-aging agent, etc. if necessary, mix and mold. I have.

[0005]

If an inorganic material is used for the base material, the problem of low heat resistance of the conventional radio wave absorber can be solved. However, when dispersing a radio-absorbing substance in an inorganic porous material, it is difficult to uniformly disperse the radio-absorbing substance in a dispersion medium, and it is difficult to manufacture a radio-absorbing substance in which the radio-absorbing substance is uniformly dispersed. Has become. As described above, if the distribution of the radio-absorbing material is not uniform even if it is intended, it is difficult to obtain a radio-absorbing material having good reproducibility with a high yield. This is because radio waves are reflected in areas where the density of the radio-absorbing substance is too high, and radio waves are transmitted in areas where the density of the radio-absorbing substance is too low. This is because the radio wave absorption characteristics are not uniform in the body. In addition, when the radio wave absorber produced using the hydraulic inorganic binder is insufficiently dried, the radio wave absorbing ability of the remaining water itself also contributes, so that the initial characteristics become unstable. A method completely different from the conventional method of mixing a radio-absorbing substance with a hydraulic inorganic binder, placing the mixture in a mold, and solidifying it at room temperature has recently been proposed. In other words, from the New Carbon Forum, Vol. 5, No. 1, p2 states that carbon obtained by catalytic decomposition of methane can be applied to carbon-based electromagnetic wave absorbers, and that conductive carbon is deposited inside a molded inorganic porous material. It describes that a heat-resistant lightweight electromagnetic wave absorber capable of coping with high-energy electromagnetic waves can be manufactured. According to this method, since the radio wave absorber is manufactured under heating using a preformed base material, the problem of residual water in the radio wave absorber manufactured using the hydraulic inorganic binder is solved. However, there is no description in this document regarding a technique for uniformly filling carbon to a predetermined depth and a technique for controlling the packing density to ensure an appropriate gas phase ratio. In addition, when the catalyst component is added to the porous body by the impregnation method, once the catalyst component solution that has entered inside moves to the surface during the drying process, the catalyst concentration on the surface increases, so that methane decomposition easily occurs and the conductive carbon becomes There is also a problem that it is easily generated.
Unless a technique for solving these problems is established, it is impossible to achieve the carbon distribution, carbon density, gas phase ratio, etc. of molded noncombustible or flame-retardant porous bodies of various sizes and shapes as designed. Also, only methane is disclosed as a carbon source,
No other hydrocarbons are described.

[0006]

SUMMARY OF THE INVENTION The present invention is an extension of this method, in which a radio wave absorbing substance is deposited in a non-combustible or non-combustible porous body in situ to produce a radio wave absorbing body. In the method, a catalytic function is imparted to pores at any position from the surface to the center of the porous body, and a suitable hydrocarbon is brought into contact with the porous body to secure a constant gas phase rate in the pores of the porous body. By producing conductive carbon while producing conductive carbon, it is possible to produce a light-wave absorber that is excellent in heat resistance and flame retardancy, does not require a careful drying step, and is lightweight and uniformly disperses and supports a wave-absorbing substance.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a catalyst containing a metal such as nickel or cobalt is supported on pores of a nonflammable or flame-retardant porous body by an impregnation method or an ion exchange method, A radio wave absorber is produced by preparing a porous body containing an element, imparting a catalytic function to the pores, and contacting a hydrocarbon under heating to generate conductive carbon in the pores inside the porous body. .

[0008] The feature of the present invention is to impart a catalytic function of generating conductive carbon from hydrocarbons to the pores of a non-combustible or flame-retardant porous material which is a base material of a radio wave absorber, and to provide the hydrocarbons with heating. And to form conductive carbon in the pores in situ. That is, the base material serves as a support for the catalyst, and conductive carbon is generated in situ from the hydrocarbons contacted by the action of the active metal supported on the support, deposited on the support, that is, the pores of the base material, and absorbs radio waves. A body is formed.

The non-combustible or flame-retardant porous material used as the base material includes silica gel, alumina, silica-alumina, carbon-based porous material, zeolite, interlayer-crosslinked clay-based porous material, porous ceramics, porous concrete and the like. Can be mentioned. In addition, glass fiber, silica
An aggregate formed body of inorganic fibers such as alumina fibers and rock wool is also included. The shape of the radio wave absorber can be a sheet shape, a cross shape, a flat plate shape, a pyramid shape, a wedge shape, a conical shape, a quadrangular pyramid shape, or the like. In order to manufacture radio wave absorbers having various shapes, a method of generating conductive carbon in a porous body previously formed into such a shape, cutting the porous body having generated conductive carbon, a hydrostatic pressure method, There is a method of molding into these shapes by a pressure suction filtration method, a hot press method, or the like.

The hydrocarbon used in the present invention may be any hydrocarbon or a mixture containing the hydrocarbon as long as it is a gas when the hydrocarbon is brought into contact with the catalyst supported on the pores of the non-combustible or flame-retardant porous material. May be. As such hydrocarbons,
Alkanes, alkenes, alkyne chain hydrocarbons, alicyclic hydrocarbons and aromatic hydrocarbons can be mentioned, among which chain hydrocarbons are preferred, alkanes are more preferred, and alkanes having 3 or less carbon atoms are most preferred. preferable. Hydrocarbons can be used alone or as mixtures containing them. The mixture containing hydrocarbons includes those containing substances other than hydrocarbons, such as natural gas and petroleum gas. Further, a mixture of a hydrocarbon and an inert gas such as nitrogen or argon can be used.

In the case of hydrocarbons having the same carbon number, the reactivity of decomposition generally decreases in the order of alkyne>alkene> alkane. However, in the case of alkyne or alkene, the catalyst tends to be deactivated. Alkanes react more as the number of carbon atoms increases, and in the case of alkanes other than methane, hydrocarbons such as methane are also produced. The carbon produced from hydrocarbons is a mixture of amorphous carbon and crystalline carbon. Of these, amorphous carbon is more likely to be produced with alkenes and alkynes than with alkanes when the hydrocarbon has the same carbon number, and is more easily produced with higher carbon numbers when the hydrocarbon is an alkane. When the hydrocarbon is methane, there is no carbon-carbon bond, and if all the carbon-hydrogen bonds are cut, the carbon atom becomes a precursor unit of crystalline carbon. At this time, if there is nickel or the like having a high ability to arrange carbon atoms regularly, crystalline carbon is easily generated.
Accordingly, from the viewpoint of the selectivity of hydrocarbons to carbon, methane is the most preferable, but ethane and propane are second, although the reactivity is low. However, when the carbon generation rate and the reaction temperature are more important than the selectivity to carbon, propane is most preferable, and ethane and methane are used in this order.

In general, as the crystalline carbon obtained from hydrocarbons, hollow filament-like carbon, onion-like carbon, herringbone-like carbon, nanotube, fullerene, etc. are known. , The size of the active metal particles, the reaction temperature and the like. For example, when pure methane is brought into contact with a porous body supporting a nickel-based catalyst and decomposed at 500 ° C. to 800 ° C., at least hollow filamentous carbon (approximately: several tens of nm, several μm in length). As the reaction temperature increases, the yield of carbon increases, but the amount of filamentary carbon tends to decrease and the amount of onion-like carbon tends to increase. When carbon is produced from ethylene, a filament having a helical structure is also obtained. Since none of the various crystalline hydrocarbons described above can take a macrolayer-developed six-membered ring carbon laminated continuous structure like graphite, a gas phase exists between carbon particles. It is considered that when a radio wave absorber is produced by using a material in which a gas phase exists around crystalline carbon as a radio wave absorbing material, the gas phase rate is likely to be high, so that radio waves are hardly reflected.

In order to obtain a large amount of crystalline carbon from a hydrocarbon at a lower reaction temperature, the cleavage of the carbon-carbon bond and the cleavage of the carbon-hydrogen of the hydrocarbon are promoted by reasonably promoting the precursor of the crystalline carbon. It is indispensable to have a catalyst having a function of efficiently producing units and a function of regularly arranging the precursor units. All the catalysts comprising the catalyst component having such a catalytic function can be used in the present invention. Such a catalytic function can be expressed not only by a single active metal component but also by two or more active metal components. The two or more active metal components may form an alloy, but need not be an alloy. As the single active metal component, for example, nickel, cobalt, iron, copper, molybdenum and the like can be used, of which nickel is particularly preferred. Also,
As the second active metal component, a noble metal such as platinum, rhodium, silver or the like can be used. The size of the catalytic metal particles is closely related to the structure and shape of the crystalline carbon. For example, if the metal particles are too large in the methane decomposition, at least no hollow filamentous carbon is generated. In order to keep the metal particles fine, it is necessary to use a suitable carrier, for example, a non-combustible or flame-retardant porous body in the present invention. Further, the use of a suitable co-catalyst has the effect of increasing the yield of crystalline carbon and of favorably affecting its structure and shape. For example, compounds of alkaline earth elements such as calcium and magnesium and alkali metal elements such as sodium and potassium can be used.
Note that the use of multiple single active metal components affects the structure and shape of crystalline carbon.

A method for imparting a catalytic function to the pores of the porous body is to impregnate the porous body with a solution of a compound of a catalytically active metal such as nickel or cobalt, and then to dry the catalyst containing these metals. There are a method of supporting these metal ions in the pores, a method of supporting these metal ions in the pores of the porous body by ion exchange, and a method of preparing a porous body containing these metal elements in advance such as a metallosilicate.

The method for producing a radio wave absorber of the present invention will be described below by taking an impregnation method as an example. First, the metal of the catalyst is supported on a substrate, which is a carrier, according to a usual catalyst preparation method.
First, a substrate is impregnated with a solution of a compound containing a catalytically active component. In this case, care is taken so that the catalytically active component reaches the inside of the substrate. For example, a competitive adsorbent such as nitric acid, hydrochloric acid, and organic acid is used. In addition, the solvent of the compound may be selected from the group consisting of water or a non-aqueous solvent or a mixture of both in consideration of the hydrophilicity / hydrophobicity of the substrate in addition to the vapor pressure, the solubility of the solute, the surface tension of the solution, and the like. Choose the right one. For example, for a hydrophilic substrate, a lower alcohol such as methanol or ethanol or a mixture of a lower alcohol and water is preferable as a solvent for nickel acetate. In this case, it is also possible to impregnate simply by dropping the catalyst component solution onto the substrate without removing the air in the substrate. As a method for preventing catalyst component segregation due to the catalyst component solution impregnated in the base material moving to the surface in the drying step described below, for example, a catalyst component solution to which a precipitant such as urea (or the same precursor) is added is used. It is extremely effective that the catalyst component is precipitated in the pores by heating to about 90 ° C. after impregnating the material. Subsequently, the solvent is vaporized and removed by an appropriate method such as heat drying or freeze drying. Drying conditions are determined according to the size of the substrate, the number of substrates to be dried at one time, and the like. In the drying process, redistribution of the impregnated catalytically active component occurs, and care is taken to prevent uneven distribution. After the drying is completed, the active metal compound is heated at a temperature not lower than the decomposition temperature, and then heated in hydrogen to be reduced to a metal. These series of steps can also be performed in a flow-through tube reactor so that equipment for drying, pyrolysis and reduction and hydrogen for reducing the catalyst precursor need not be separately prepared. That is, the base material impregnated with the catalyst is placed in a flow-type tubular reactor that has been set to a reaction temperature, and dried, thermally decomposed, while being moved from the entrance to the center under hydrocarbon flow.
End the reduction process. In this case, hydrogen generated by the decomposition of hydrocarbons is mainly used for the reduction. As the metal compound, for example, in the case of nickel, nitrate, chloride,
Organic salts such as acetates and complex salts can be used. Most preferred are acetates. In particular, when the reduction of the catalyst precursor is carried out in a flow tube reactor using hydrocarbons instead of hydrogen, the substrate containing the catalyst prepared from nickel acetate exhibits the highest carbon yield. Next, conductive carbon is generated in the pores of the substrate. According to a conventional method, a substrate containing the activated catalyst is placed in a flow-type tubular reactor, and the hydrocarbon is flowed. At the time when the carbon density was determined to have reached the target value, the substrate was moved to near the reaction tube outlet and gradually cooled,
Take out of the reaction tube.

Since the reaction for producing carbon from hydrocarbon is an endothermic reaction, the higher the reaction temperature, the higher the conversion of hydrocarbon. Also, the reaction temperature is better as the reaction temperature is higher. When the carbon generation rate is important, the reaction temperature is selected according to the type of hydrocarbon. For example, in the case of methane, 800 ° C. or higher is required in the absence of a catalyst, but 500 ° C. or higher is sufficient in the presence of a catalyst. The reaction time required to produce the desired amount of carbon is inversely proportional to the reaction temperature and proportional to the size of the substrate. Eventually, the reaction time and reaction temperature depend on the crystallinity of the carbon, the size and shape of the
10 minutes to 5 minutes, taking into account the required power
0 min, choose between 300 ° C and 900 ° C.

[0017]

EXAMPLES Example 1 Diatomaceous earth brick (Isolite C
Test piece (30mm × 60mm × 5)
mm) was used. This is impregnated with a 0.1 M aqueous solution of nickel acetate in vacuo at room temperature for 20 minutes, dried at 120 ° C for 12 hours, pyrolyzed at 500 ° C for 4 hours, and then reduced in a tubular electric furnace at 400 ° C in a stream of hydrogen for 0.5 hour in a hydrogen stream. To activate the catalyst.
Subsequently, the atmosphere in the tubular electric furnace was switched to methane, and 5
Methane decomposition was performed at 00 ° C. for 0.5 hour. The flow rate of methane gas was 120 mL / min. In this case, the content of the conductive carbon was 3% by mass. FIG. 1 shows the result of measuring the dielectric constant of this radio wave absorber. FIG. 1 shows that the present radio wave absorber has an excellent radio wave absorption effect.

Example 2 The same operation as in Example 1 was performed except that methane was used as a hydrocarbon, drying, thermal decomposition, and reduction were performed in a reaction tube, and methane was used as a reducing agent. The radio wave absorber thus obtained exhibited the same radio wave absorption effect as in Example 1.

Example 3 The same operation as in Example 1 was carried out except that ethylene was used as the hydrocarbon and the flow rate was 40 mL / min. The radio wave absorber thus obtained exhibited the same radio wave absorption effect as in Example 1.

Example 4 The same operation as in Example 1 was performed except that ethane was used as the hydrocarbon and the flow rate was 60 mL / min. The radio wave absorber thus obtained exhibited the same radio wave absorption effect as in Example 1.

Example 5 The same operation as in Example 1 was performed except that benzene was used instead of methane as a hydrocarbon, and the reaction temperature in the furnace was set at 1000 ° C. The radio wave absorber thus obtained has a radio wave absorption effect equivalent to that of the first embodiment.

Example 6 A diatomaceous earth brick (Isolite C
Test piece (15mm × 50mm × 1)
5 mm). This is impregnated with a 0.1 M aqueous solution of nickel acetate in vacuo at room temperature for 20 minutes, dried at 120 ° C for 12 hours, pyrolyzed at 500 ° C for 4 hours, and then reduced in a tubular electric furnace at 400 ° C in a stream of hydrogen for 0.5 hour in a hydrogen stream. To activate the catalyst. Subsequently, the atmosphere in the tubular electric furnace was switched to methane, and methane was decomposed at 500 ° C. for 0.5 hour. The flow rate of methane was 30 mL / min. The conductive carbon content in this case was 1.2% by mass. When the obtained radio wave absorber was cut, it was confirmed that carbon was completely filled into the inside. When the reaction time was set to 12 hours in this experiment, the radio wave absorber after the reaction was powdered. This fact indicates that the method makes it possible to completely fill the pores with carbon.

Reference Example 1 A flat plate having a thickness of 65 mm was formed from the radio wave absorber of Example 1.
This was arranged on the front surface of the metal plate to determine the reflection coefficient. The results are shown in FIG. From this figure, it can be seen that as the frequency increases, the absorption performance increases, that is, the reflection coefficient decreases and 900M
It can be seen that an absorption performance exceeding 16 dB can be obtained at Hz.

Reference Example 2 A flow-type reaction tube was filled with a supported nickel catalyst, and a methane decomposition reaction was performed at 500 ° C. for 0.5 hour to produce carbon around the catalyst. This carbon was pulverized, and 0.5 mL was placed in a glass container and irradiated with radio waves for 3 minutes in a microwave oven (rated output: 600 W, transmission frequency: 2450 MHz). As a result, the temperature of the carbon rose to 150 ° C. This fact indicates that carbon produced by this method has a radio wave absorption effect. Moreover, the effect was found to be equivalent to that of graphite (see Comparative Example 1). No sparks or red heat were generated during the radio wave irradiation test.

Reference Example 3 The radio wave absorber of Example 1 was placed in a commercial electric furnace (heating element was a nichrome wire) whose furnace temperature was adjusted to 750 (± 5) ° C. for 1 hour. When the weight was measured after the heat treatment, the weight reduction rate was 1% or less.

Reference Example 4 The electromagnetic wave absorber of Example 1 was replaced with a microwave oven (rated output: 600
W, transmission frequency: 2450 MHz) and irradiated with radio waves for 1 minute. As a result, the temperature of the radio wave absorber increased to 200 ° C., but the weight reduction rate was 1% or less.

Comparative Example 1 A commercially available high-purity graphite powder (0.5 mL) was placed in a glass container.
Microwave oven (rated output: 600W, transmission frequency: 245
(0 MHz) for 3 minutes. As a result, the temperature of the radio wave absorber increased to 155 ° C. Sparks were emitted 9 seconds after radio wave irradiation, and glowed red 20 seconds after.

Comparative Example 2 A radio wave absorbing material obtained by impregnating a urethane base material with carbon graphite (a general-purpose product for an anechoic chamber and a commercially available product coated with a flame-retardant paint) and a sponge-like urethane not impregnated with carbon graphite The microwave oven (rated output 600
W, transmission frequency 2450 MHz). The radio wave absorbing material impregnated with carbon graphite started emitting smoke about 1 second after the switch was turned on, and ignited about 3 seconds later. Along with this, an unpleasant smell stood in the room. The sponge-like urethane not impregnated with carbon graphite did not generate heat because it did not absorb radio waves, and there was no change even after 5 minutes.

[0029]

According to the present invention, the following effects can be obtained. 1. Since the catalytically active component is in contact with the substrate in the form of a solution and the hydrocarbon is in the form of a gas, the conductive carbon produced can be uniformly dispersed in the substrate. 2. 3 when catalytic cracking of hydrocarbons with 3 or less carbon atoms
Since conductive carbon is generated at 00 to 900 ° C, a substrate such as glass fiber whose material strength is significantly reduced at 900 ° C or higher can also be used. 3. Radio wave absorbers manufactured at room temperature using hydraulic hardening agents such as cement, radio wave absorbing materials, etc., may crack under high-power radio wave irradiation.The water itself must be dried sufficiently to remove water. There is a problem that the radio wave absorption characteristics in the initial stage of use become unstable because the radio wave absorption ability also contributes, but the radio wave absorber of the present invention is made of non-combustible or flame-retardant porous Since the molded article is used as a base material, the thermal shock resistance and heat resistance are remarkably excellent, and the initial electromagnetic wave absorption characteristics are stable.

[Brief description of the drawings]

FIG. 1 is a graph showing the relative dielectric constant of a radio wave absorber according to one embodiment of the present invention.

FIG. 2 is a graph showing the radio wave absorption performance of a radio wave absorber according to one embodiment of the present invention.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kenji Kubo 1-98 Asahidai, Kuriyama-cho, Yubari-gun, Hokkaido (72) Inventor Takashi Tabata 3-16-1 Shin-Yokohama, Kohoku-ku, Yokohama, Kanagawa Prefecture KC Building E & C F-term (reference) in Engineering Co., Ltd. 4G028 DA01 DB00 DC01 5E321 BB31 BB51 BB60 GG11

Claims (11)

[Claims] 1. A group consisting of crystalline carbon and amorphous carbon in which pores of a porous material are brought into contact with a non-combustible or flame-retardant porous material having a catalytic function given to the pores with hydrocarbons or a mixture containing the hydrocarbons. A radio wave absorber produced from conductive carbon selected from the following. 2. A non-combustible or flame-retardant porous body having a pore provided with a catalytic function, and a hydrocarbon or a mixture containing the hydrocarbon being mixed with 30 parts by mass.
A radio wave absorber in which conductive carbon selected from the group consisting of crystalline carbon and amorphous carbon is generated in pores of a porous body by contacting at 0 to 900 ° C. 3. A group consisting of crystalline carbon and amorphous carbon in the pores of a porous body by contacting a hydrocarbon or a mixture containing the same with a non-combustible or flame-retardant porous body provided with a catalytic function in the pores. Producing a conductive carbon selected from the group consisting of: 4. A non-combustible or flame-retardant porous material having pores provided with a catalytic function is provided with 30% of a hydrocarbon or a mixture containing the hydrocarbon.
A method for producing a radio wave absorber, comprising contacting at 0 to 900 ° C. to generate conductive carbon selected from the group consisting of crystalline carbon and amorphous carbon in pores of a porous body. 5. The method according to claim 3, wherein a solution of the compound containing a catalytically active component is impregnated into a nonflammable or flame-retardant porous body to impart a catalytic function to the pores of the porous body. 6. The radio wave absorber according to claim 1, wherein the hydrocarbon is an alkane having 3 or less carbon atoms. 7. The radio wave absorber according to claim 1, wherein the hydrocarbon is methane. 8. The radio wave absorber according to claim 1, wherein the mixture of hydrocarbons is natural gas. 9. The radio wave absorber according to claim 1, wherein the porous body is any one of an inorganic fiber aggregate forming body, an interlayer crosslinked clay-based porous body, and a carbon-based porous body. 10. The radio wave absorber according to claim 1, wherein the porous body is formed in advance into a sheet, cloth, flat plate, pyramid, wedge, cone, or pyramid. 11. A sheet, cloth, flat plate,
3. The radio wave absorber according to claim 1, wherein the radio wave absorber is formed into a pyramid shape, a wedge shape, a conical shape, or a quadrangular pyramid shape.