WO2011105469A1

WO2011105469A1 - Electromagnetic wave absorber and method for producing same

Info

Publication number: WO2011105469A1
Application number: PCT/JP2011/054091
Authority: WO
Inventors: 正督藤; 孝白井; 実高橋; 誠治高橋; 恭一藤本; 英樹岸野; 宏三林
Original assignee: 国立大学法人名古屋工業大学
Priority date: 2010-02-24
Filing date: 2011-02-24
Publication date: 2011-09-01
Also published as: JP2011176101A

Abstract

Disclosed is an electromagnetic wave absorber, which is lightweight, easy to carry and apply, nonflammable, and applicable to buildings, while having excellent electromagnetic wave absorption characteristics. Also disclosed is a method for producing the electromagnetic wave absorber. Specifically, an electromagnetic wave absorber (1), which is mainly composed of alumina having a porosity within the range of 60-80%, can be obtained by: introducing an alumina ceramic powder (2), a dispersant (3) and distilled water (4) into an alumina pot mill and carrying out ball mill mixing for 24 hours (S10); adding a monomer (5) and a crosslinking agent (6) thereinto and carrying out additional ball mill mixing for 24 hours (S11); defoaming the thus-prepared secondary mixed slurry (S12); then adding a surfactant (7), a polymerization initiator (8) and a polymerization catalyst (9) thereinto and introducing a large amount of air bubbles into the secondary mixed slurry by stirring the slurry with a bubble generator (S13); filling the resulting slurry into a mold (S14); leaving the mold at rest in order to gelatinize the slurry, thereby forming a wet molded body (S15); removing the wet molded body from the mold (S16); drying the wet molded body at 25˚C (S17); and then introducing the dried molded body into a firing furnace and carrying out reduction firing (S18).

Description

Electromagnetic wave absorber and method for producing the same

The present invention relates to an electromagnetic wave absorber that is lightweight and nonflammable and has excellent electromagnetic wave absorption characteristics, and a method for producing the same.

In recent years, the band of radio waves used on automobile roads has been expanding, and in particular, the use of DSRC (dedicated narrow area communication) used in ETC (5.8 GHz) by the promotion of ITS (Intelligent Transport System). This trend has become more pronounced as the field expands. For this reason, the demand of the electromagnetic wave absorber corresponding to DSRC is expanding. As an example, there is an invention of a radio wave absorber described in Patent Document 1.

This Patent Document 1 discloses an invention of a radio wave acoustic wave absorber formed by laminating a radio wave reflector, a sound absorbing unit, a radio wave absorber, and a plate-shaped radio wave absorber protecting unit made of a dielectric. By using the radio wave absorber having such a configuration, it has a function of absorbing radio waves and sound waves coming from the radio wave absorber protection part side, and can be used in the field, which was difficult with conventional radio wave absorbers. It can be installed on roads.

Further, in Patent Document 2, a radio wave formed by bringing carbon dioxide into contact with an electromagnetic wave absorbing mixture obtained by mixing foam particles of volcanic ejecta (shirasu), a radio wave loss material (conductive carbon), and an aqueous alkali silicate solution, and solidifying the carbon dioxide. The invention discloses a radio wave absorber formed by adhering an absorption layer and a radio wave reflection layer, which is a metal plate or metal foil, via an adhesive layer, and a method for manufacturing the same. Thus, the thickness of the adhesive layer can be made much smaller than before, and excellent electromagnetic wave absorption characteristics can be exhibited.

Further, Patent Document 3 proposes a method for manufacturing a conductive ceramic product in which a conductive path made of a reduced fired product of a polymer compound in a nitrogen gas atmosphere is formed between ceramic particles.

JP 2004-003259 A JP 2003-152381 A JP 2005-289695 A

However, in both Patent Document 1 and Patent Document 2, the weight of the radio wave absorber is increased, so that it is difficult to mount and construct around the road (in a tunnel, a ceiling of a parking lot, etc.) However, there is a problem that isotropic electromagnetic wave absorption characteristics cannot be obtained. On the other hand, various lightweight wave absorbers mainly composed of synthetic resins and synthetic fibers have been proposed. However, these wave absorbers have a problem that they do not satisfy the incombustibility requirement required for buildings. there were.

In addition, according to the method for manufacturing a conductive ceramic product disclosed in Patent Document 3, the conductive ceramic product is lighter in weight and isotropic in electrical properties than a conductive ceramic product using a conductive material having a high density. Although a product can be obtained, in order to use this as an electromagnetic wave absorber, further improvement of the electromagnetic wave absorption characteristics was necessary.

Therefore, the present invention has been made to solve such a problem, is lightweight and easy to transport and construct, is nonflammable and can be applied to buildings, and has excellent electromagnetic wave absorption characteristics. It is an object to provide an electromagnetic wave absorber and a method for producing the same.

The electromagnetic wave absorber according to the invention of claim 1 is an electromagnetic wave absorber mainly composed of a ceramic sintered body having a porosity in the range of 60% to 80%, wherein the ceramic sintered body is made of a net-like material made of carbon. It has a structure in which conductive paths are stretched and has a radio wave absorption performance at a frequency of 5 to 6 GHz of 20 dB or more.

Here, various types of “ceramics” constituting the ceramic sintered body can be used. Examples of oxide ceramics include silica (SiO ₂ ), alumina (Al ₂ O ₃ ), mullite ( Al ₂ O ₃ —SiO ₂ ), zirconia (ZrO ₂ ), etc., and non-oxide ceramics include silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), boron nitride (BN) Etc. can be used.

The electromagnetic wave absorber according to the invention of claim 2 is the structure of claim 1, wherein the ceramic is alumina (Al ₂ O ₃ ).

According to a third aspect of the present invention, there is provided the electromagnetic wave absorber according to the first or second aspect of the invention, wherein the network-like conductive path made of carbon is a high carbon chain formed by polymerizing a monomer. A network of molecular compounds is formed by reduction firing.

According to a fourth aspect of the present invention, there is provided the electromagnetic wave absorber according to the first or second aspect of the invention, wherein the network-like conductive path made of carbon is a non-nitrogen gas-containing network of a polymer compound having carbon atoms. It is formed by reduction firing in an active gas atmosphere.

The electromagnetic wave absorber according to the invention of claim 5 is the structure of claim 4, wherein the polymer compound is formed by polymerizing a monomer.

The method for producing an electromagnetic wave absorber according to the invention of claim 6 is a method for producing an electromagnetic wave absorber mainly comprising a ceramic sintered body having a porosity in the range of 60% to 80%, comprising a ceramic material for sintering. And a slurry adjustment step in which a polymerizable substance that is polymerized to become a polymer is dispersed in a solvent to form a slurry, a bubble introduction step for introducing bubbles into the slurry, and a slurry into which the bubbles are introduced is filled in a mold A molding step of polymerizing the polymerizable substance to form a wet molded body, and a reduction firing step of drying the wet molded body and firing in a reducing atmosphere. The electromagnetic wave absorber has a radio wave absorption performance at a frequency of 5 to 6 GHz of 20 dB or more.

In the method for manufacturing an electromagnetic wave absorber according to the invention of claim 7, in the structure of claim 6, the reducing atmosphere is an inert gas atmosphere containing no nitrogen gas.

The method for producing an electromagnetic wave absorber according to the invention of claim 8 is a method for producing an electromagnetic wave absorber mainly comprising a ceramic sintered body having a porosity in the range of 60% to 80%, comprising a ceramic material for sintering. And a slurry adjustment step in which a polymerizable substance that is polymerized to become a polymer is dispersed in a solvent to form a slurry, a bubble introduction step for introducing bubbles into the slurry, and a slurry into which the bubbles are introduced is filled in a mold A molding step of polymerizing the polymerizable substance to form a wet molded body, and a reduction of drying the wet molded body and firing in a reducing atmosphere of an inert gas not containing nitrogen gas And a firing step.

According to a ninth aspect of the present invention, there is provided the electromagnetic wave absorber manufacturing method according to any one of the sixth to eighth aspects, wherein the slurry adjusting step includes sintering ceramic powder, a dispersant, and the solvent. Water, a monomer (monomer) as the polymerizable substance, and a crosslinking agent are mixed to form a slurry. In the bubble introduction step, a surfactant, a polymerization initiator and / or a polymerization catalyst are added to the slurry. Mixing and stirring, introducing bubbles into the slurry, and removing the wet molded body from the mold between the molding step and the reduction firing step; and drying the wet molded body And a drying step for forming the generated shape, and the reduction firing step is for firing the generated shape.

According to a tenth aspect of the present invention, there is provided a method for producing an electromagnetic wave absorber according to any one of the sixth to ninth aspects, wherein the ceramic is alumina (Al ₂ O ₃ ).
The method for producing an electromagnetic wave absorber according to the invention of claim 11 is the structure according to any one of claims 6 to 10, wherein the monomer is a methacrylamide.

The electromagnetic wave absorber according to the invention of claim 1 is an electromagnetic wave absorber mainly composed of a ceramic sintered body having a porosity in the range of 60% to 80%, and is a netlike conductive material made of carbon in the ceramic sintered body. It has a structure in which roads are stretched, and the radio wave absorption performance at a frequency of 5 to 6 GHz is 20 dB or more.

As described above, since a net-like conductive path made of carbon is stretched around the sintered ceramic body, it has radio wave absorption performance, so that isotropic electromagnetic wave absorption characteristics can be obtained. Further, since the porosity is as large as 60% to 80%, it is extremely lightweight. Furthermore, since it is mainly composed of a ceramic sintered body, it is nonflammable and has a sufficient strength for handling even if the porosity is large. Since the radio wave absorption performance at a frequency of 5 to 6 GHz is 20 dB or more, it is suitable as an electromagnetic wave absorber used in ITS including ETC.

In this way, it is lightweight, easy to transport and construct, nonflammable, can be applied to buildings, and becomes an electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics.

In the electromagnetic wave absorber according to the invention of claim 2, since the ceramic is alumina (Al ₂ O ₃ ), in addition to the effect of the invention of claim 1, it can be fired at a lower temperature, Since the raw material is easily available, the effect that it can be produced at a lower cost is obtained.

In the electromagnetic wave absorber according to the invention of claim 3, since the net-like conductive path made of carbon is formed by reducing and firing a network of polymer compounds made of carbon chains formed by polymerizing monomers, The conductive path can be formed tightly and uniformly, and an effect of obtaining an electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics can be obtained.

In the electromagnetic wave absorber according to the invention of claim 4, the network-like conductive path made of carbon is obtained by reducing and firing a network of polymer compounds having carbon atoms in an inert gas atmosphere not containing nitrogen gas. In addition to the effect of the invention of claim 3, the effect of obtaining an electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics can be obtained as compared with the case of reducing and firing in a nitrogen gas atmosphere.

According to the method for producing an electromagnetic wave absorber according to the inventions of claims 6 to 9, a structure in which a high molecular compound network composed of carbon chains is uniformly stretched throughout the wet molded body and at the same time, bubbles are uniformly distributed. It becomes. Therefore, by drying this wet molded body and firing it in a reducing atmosphere, a network of polymer compounds composed of carbon chains is reduced and fired, and a net-like conductive path composed of carbon is stretched around the ceramic sintered body. A ceramic sintered body having a porosity in the range of 60% to 80% is obtained, and an incombustible and lightweight electromagnetic wave absorber is obtained.

Furthermore, according to the manufacturing method of the electromagnetic wave absorber according to the inventions of

claims

7 and 8, since the reduction baking is performed in an inert gas atmosphere not containing nitrogen gas, than the case of reduction baking in a nitrogen gas atmosphere, The effect of becoming an electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics can be obtained.

Thus, it is a light-weight, easy to transport and construct, non-combustible, applicable to buildings, and a method for producing an electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics.

In the method for producing an electromagnetic wave absorber according to the invention of claim 10, since the ceramic is alumina (Al ₂ O ₃ ), in addition to the effects of the inventions of claims 6 to 9, firing is performed at a lower temperature. In addition, since the raw material is easily available, the effect of being able to be manufactured at a lower cost is obtained.

In the method for producing an electromagnetic wave absorber according to the invention of claim 11, the network-like conductive path made of carbon is obtained by reducing and firing a network of polymer compounds made of carbon chains formed by polymerizing methacrylamide. In addition, the net-like conductive path can be formed tightly and uniformly, and an effect of obtaining an electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics can be obtained.

FIG. 1 is a flowchart showing a method of manufacturing an electromagnetic wave absorber according to Example 1 of the present invention. FIG. 2 is a schematic diagram showing a method for measuring the electromagnetic wave absorption characteristics of the electromagnetic wave absorber according to Example 1 of the present invention. FIG. 3 is a graph showing measurement results of electromagnetic wave absorption characteristics of the electromagnetic wave absorber according to Example 1 of the present invention. 4A is a scanning electron microscope (SEM) photograph showing a cross section of the electromagnetic wave absorber according to Example 1 of the present invention, and FIG. 4B is an SEM photograph in which (a) is further enlarged. FIG. 5 is a graph showing measurement results of electromagnetic wave absorption characteristics of the sintered body according to Comparative Example 1. FIG. 6 is a Raman spectrum of the electromagnetic wave absorber according to Example 1 of the present invention. FIGS. 7A and 7B are views showing the installation direction at the time of radio wave absorption evaluation of the electromagnetic wave absorber according to Example 1 of the present invention. FIG. 8 is a graph showing the isotropic evaluation results of the electromagnetic wave absorber according to Example 1 of the present invention. FIG. 9 is a graph showing measurement results of electromagnetic wave absorption characteristics of the electromagnetic wave absorber according to Example 2 of the present invention. FIG. 10 is a Raman spectrum of the electromagnetic wave absorber according to Example 2 of the present invention.

In the embodiment of the present invention, various “ceramics” constituting the ceramic sintered body can be used. Examples of the oxide ceramics include silica (SiO ₂ ), alumina (Al ₂ ). O ₃ ), mullite (Al ₂ O ₃ —SiO ₂ ), zirconia (ZrO ₂ ), etc., as non-oxide ceramics, silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN) Boron nitride (BN) or the like can be used. In particular, alumina (Al ₂ O ₃ ) is preferably used because it can be fired at a lower temperature and the raw materials are easily available.

In addition, when preparing a predetermined composition using such a ceramic raw material, generally, a powder or granular material of such a ceramic raw material is used, and its size (average particle diameter) is 0.1. The size is about 10 to 10 μm, preferably about 0.1 to 5 μm, more preferably about 0.1 to 1 μm. However, if the average particle size of the powdery or granular material is too large or too small, a sintered body having sufficient strength may not be obtained.
Further, in the embodiment of the present invention, various methods can be used as a method of stretching a network-like conductive path made of carbon in a ceramic sintered body, but carbon chains formed by polymerizing monomers are used. A method in which a network of polymer compounds made of is formed by reduction firing is preferable. As a monomer to be polymerized, various monomers can be used as long as they can form a polymer compound composed of carbon chains.

Specifically, vinyl unsaturated monomers such as methacrylamide, polyol compounds and isocyanate compounds that form urethane bonds by mixing, phenol compounds that form thermosetting polymers, epoxy compounds, and the like can be used. However, vinyl unsaturated monomers such as methacrylamide are particularly preferable.

Further, when a vinyl unsaturated monomer such as methacrylamide is used as the monomer to be polymerized, it is preferable to use a crosslinkable monomer such as N, N′-methylenebisacrylamide at the same time. This is because a net-like conductive path made of carbon can be stretched three-dimensionally.

Further, when forming a polymer network by polymerizing monomers, it is preferable to use a polymerization initiator, a polymerization catalyst, or the like according to the monomers. As such a polymerization initiator, organic peroxides, hydrogen peroxide compounds, azo compounds, diazo compounds, ammonium persulfate, and the like can be used. As the polymerization catalyst, N, N, N ′, N′-tetramethylethylenediamine or the like can be used.

Note that, as long as a polymer compound composed of carbon chains can be formed, not only the monomer but also a polymer obtained by polymerizing the monomer to some extent may be used. In short, in the present invention, it is only necessary to form a polymer compound composed of carbon chains by polymerizing a polymerizable substance having a carbon atom in the molecule. The polymerizable substance includes the monomer.

In an embodiment of the present invention, a predetermined composition is prepared by blending the polymerizable material with the ceramic raw material, and such a composition is generally contained in a predetermined medium. By adding and mixing the ceramic raw material and the polymerizable substance, the ceramic raw material is prepared in the form of an aqueous or non-aqueous slurry in which the ceramic raw material and the like are uniformly dispersed. As a medium in which the ceramic raw material or the like is dispersed, water (distilled water), an organic solvent, or a mixed solvent thereof can be used. From the viewpoint of easy handling, water is preferably used. (Distilled water) cocoons are used and prepared in the form of a water slurry.

Further, when preparing such a slurry composition, it is preferable to use a dispersant for the purpose of uniformly dispersing the ceramic raw material particles or powders in the medium. As such a dispersing agent, those according to the types of ceramic raw materials and polymerizable substances are appropriately selected from various conventionally known dispersing agents and used, for example, ammonium polycarboxylate-based dispersion An agent (anionic dispersant) or the like is used.

In addition, in order to generate bubbles in the composition, when bubbles are generated by adding a foaming agent or by introducing gas into the slurry composition, the generation of such bubbles is easy. A surfactant, etc., and a thickener and a paste for stably holding the introduced bubbles in the composition can be blended. Here, as the foaming agent, a protein-based foaming agent, a surfactant-based foaming agent, etc., as the surfactant, an alkylbenzene sulfonic acid, a higher alkyl amino acid, etc., and further, a thickener or a paste. Examples of the agent include methyl cellulose, polyvinyl alcohol, saccharose, molasses, xanthan gum and the like.

In the composition thus prepared, it is supplied into a mold according to the shape of the target conductive ceramic product, together with a polymerization initiator and a polymerization catalyst as necessary, and for a predetermined time together with the mold. By allowing the composition to stand at a predetermined temperature, the polymerizable substance in the composition is polymerized in the mold.

When the composition containing the polymerizable substance is allowed to stand at a predetermined temperature for a predetermined time in the mold, the polymerization of the polymerizable substance contained in the composition is effective in the mold. In the molded product obtained by demolding after a lapse of a predetermined time, the polymer compound that is a polymer of the polymerizable substance is uniformly distributed. It presents an existing structure.

The molded body obtained as described above contains a large amount of water or an organic solvent, particularly when a slurry-like composition is used. Therefore, it is generally dried before reduction firing. The Rukoto. Regarding the drying method and various conditions (drying temperature, drying time, etc.) for drying the molded body, those according to the components contained in the molded body, the medium to be volatilized (water, organic solvent, etc.), etc. Will be selected and adopted.

A sintered ceramic body is obtained by firing the molded body obtained as described above. In addition, when using the shaping | molding die which consists of paper etc. which lose | disappear by combustion, you may carry out reduction | restoration baking without performing mold removal. Further, the drying and firing steps may be performed separately, or the drying and firing steps may be performed continuously in the same firing furnace.

Further, in the firing step for obtaining the ceramic sintered body, it is necessary to carry out reduction firing in which firing is performed in a reducing air flow atmosphere in order to prevent the formed carbon chains from being burned out. The reducing airflow atmosphere is an inert gas atmosphere that does not contain nitrogen gas. Examples of the inert gas not containing nitrogen gas include rare gases such as argon and helium.

The reason for reduction firing in an inert gas atmosphere not containing nitrogen gas is that, as shown in Comparative Example 1 described later, when the reduction firing is performed in a nitrogen gas atmosphere, the obtained ceramic sintered body has excellent electromagnetic wave absorption. This is because the effect of the present invention of having characteristics is not achieved.

Here, when the content of the polymerizable substance in the molded body is the same, the case where the reduction firing is performed under an inert gas atmosphere not containing nitrogen gas, compared to the case where the reduction firing is performed under a nitrogen gas atmosphere, The amount of conductive carbon generated in the ceramic sintered body increases. This is because, when reducing firing in a nitrogen gas atmosphere, the ceramic particles, the polymerizable substance, and the nitrogen gas react to produce a compound.

For example, the ratio of the total carbon amount (mass) of the polymerizable material in the green body before firing is 0.1 to 6 parts by mass with respect to 100 parts by mass of the ceramic raw material, and in an inert gas atmosphere not containing nitrogen gas When the reduction firing was performed, the carbon component content of the ceramic sintered body was in the range of 0.3 mass% to 1.7 mass%. This carbon component content is calculated from the measured value of the amount of components thermally decomposed and burned by thermal analysis, and is a mass ratio with respect to the mass of the ceramic sintered body. On the other hand, when the same compact was subjected to reduction firing in a nitrogen gas atmosphere, the conductive carbon component content in the ceramic sintered body was 0.2% by mass or less.

The carbon component content of the ceramic sintered body is preferably 1.7% by mass or less. This is because if the carbon in the ceramic sintered body increases, the strength of the ceramic sintered body will decrease, and the present inventors have confirmed that sufficient strength can be obtained if it is 1.7% by mass or less. Because it is.

By the above manufacturing method, an electromagnetic wave absorber 1 mainly composed of alumina ceramics having a porosity of 60% to 80%, and having an electromagnetic wave absorption performance at a frequency of 5 to 6 GHz of 20 dB or more is obtained. can get.

Hereinafter, although the electromagnetic wave absorber of this invention and its manufacturing method are demonstrated in reference to drawings according to a specific Example, this invention does not receive any limitation by these Examples.
Example 1
FIG. 1 is a flowchart showing a method of manufacturing an electromagnetic wave absorber according to Example 1 of the present invention. FIG. 2 is a schematic diagram showing a method for measuring the electromagnetic wave absorption characteristics of the electromagnetic wave absorber according to Example 1 of the present invention. FIG. 3 is a graph showing measurement results of electromagnetic wave absorption characteristics of the electromagnetic wave absorber according to Example 1 of the present invention. 4A is a scanning electron microscope (SEM) photograph showing a cross section of the electromagnetic wave absorber according to Example 1 of the present invention, and FIG. 4B is an SEM photograph in which (a) is further enlarged.
Initially, the manufacturing method of the electromagnetic wave absorber which concerns on a present Example is demonstrated with reference to the flowchart of FIG. As shown in FIG. 1, first, primary mixing is performed in which alumina ceramic powder is dispersed in water as a solvent (step S10). That is, alumina ceramic powder 2 (Product No. Al-160SG-4), Dispersant 3 (Product No. D305) and distilled water 4 are put in an alumina pot mill at a compounding ratio shown in Table 1 below, and zirconia is used as a medium. Ball milling is performed for 18 hours using a ball.

The monomer 5 (methacrylamide) and the cross-linking agent 6 (N, N′-methylenebisacrylamide), which are materials for forming a polymer compound network, are shown in Table 1 above for this primary mixed slurry. In addition to the blending ratio, ball mill mixing is further performed as secondary mixing for 30 minutes (step S11). The secondary mixed slurry thus adjusted in the slurry adjustment step is taken out from the alumina pot mill and defoamed (step S12), and then the bubble introduction step is performed.
That is, surfactant 7 (trade name “Emulgen 104P” manufactured by Kao Corporation), polymerization initiator 8 (ammonium peroxodisulfate) and polymerization catalyst 9 (N, N, N ′, N′-tetramethylethylenediamine) A large amount of bubbles are introduced into the secondary mixed slurry by adding to the secondary mixed slurry at the blending ratio shown in Table 2 below and stirring with the bubble generator (step S13). The bubble introduction rate is represented by ((1-introduced slurry density) / secondary mixed slurry density) × 100 (%). For example, when 400 ml of secondary mixed slurry is put in a container having a volume of 1000 ml or more and foamed until reaching 1000 ml, the bubble introduction rate is 60 (%).

The bubble-introduced slurry is filled in a mold (step S14) and left for 90 to 120 minutes, whereby the monomer (methacrylamide) is cross-linked and gelled to form a wet molded body (step S15). . The wet molded body is removed from the mold (step S16) and then dried at 25 ° C. (step S17). And it puts into a baking furnace and carries out reduction baking in argon gas atmosphere. The firing temperature is 1800 ° C. and the firing time is 2 hours (step S18).
The electromagnetic wave absorption characteristics of the electromagnetic wave absorber 1 produced in this way were measured in an anechoic chamber as shown in FIG. The measurement method is a combination of the reflected power method and the time domain method, and the measurement conditions are a frequency of 4 GHz to 8 GHz, a normal incidence (incident angle θ = 0 degree), and a measurement body (electromagnetic wave absorber 1) dimensions: 150 mm × 150 mm × 5 mm, the back reflective layer 10 is an aluminum tape having a thickness of 0.3 mm. The measurement results of the electromagnetic wave absorption characteristics are shown in FIG.

As shown in FIG. 3, the radio wave absorption rate is 20 dB or more in the frequency range of 5.5 GHz to 6.0 GHz within the frequency range of 4 GHz to 8 GHz, and further, the radio wave absorption rate of 25 dB at the frequency of 5.8 GHz. was gotten.

The internal structure of the electromagnetic wave absorber 1 was confirmed with a scanning electron microscope (SEM). Fig.4 (a) is a SEM photograph which shows the cross section of the electromagnetic wave absorber 1 which concerns on a present Example. As shown in FIG. 4 (a), it can be seen that there is a structure in which a network of carbon (shown whitish) is stretched around the alumina sintered particles (shown blackish). As shown in the SEM photograph of FIG. 4B, which is a further enlarged view of FIG. 4A, it was confirmed that the carbon network was formed of nanocarbon having a thickness of nanometer size.

Here, FIG. 5 shows the measurement results of the electromagnetic wave absorption characteristics of the sintered body according to Comparative Example 1. FIG. In Comparative Example 1, compared to the present example, after forming a molded body by the same process up to Step S17 in the flowchart shown in FIG. 1, reduction firing was performed in a nitrogen gas atmosphere instead of an argon gas atmosphere. It is. The firing temperature is 1800 ° C. and the firing time is 2 hours. As a result, a sintered body mainly composed of alumina ceramics having a porosity of 60% was obtained. The electromagnetic wave absorption characteristics of the obtained sintered body were measured in the same manner as in Example 1.

As a result, as shown in FIG. 5, the radio wave absorption within the frequency range of 5.5 GHz to 6.0 GHz found in the electromagnetic wave absorber 1 of Example 1 was observed within the frequency range of 4 GHz to 8 GHz. The return loss also decreased. This shows that reduction firing in an inert gas atmosphere containing no nitrogen gas is more important than reduction firing in a nitrogen atmosphere.

Moreover, the analysis of the carbon component contained in the electromagnetic wave absorber 1 of the present example was performed by a Raman spectrum analysis and a solid-in-carbon analyzer.

In FIG. 6, the Raman spectrum of the carbon component contained in the electromagnetic wave absorber 1 is shown. As shown in FIG. 6, the electromagnetic wave absorber 1 contains a carbon component because there are peaks at 1350 cm ⁻¹ (D-band) and 1580 cm ⁻¹ (G-band) derived from the graphite structure. Was confirmed.

The sample was burned at a high temperature, and the carbon component content of the electromagnetic wave absorber 1 was 0.60% by weight as a result of calculation from the measured value of the solid-in-carbon analyzer that analyzes the generated gas with an infrared analyzer.

Also, the installation direction of the sample when the isotropic property of the electromagnetic wave absorption characteristic is evaluated is shown in FIGS. 7A and 7B, and the evaluation result is shown in FIG. Specifically, two types of samples (Example 1-1 and Example 1-2) are prepared, and the installation direction at the time of the radio wave absorption evaluation is vertical as shown in FIGS. 7 (a) and 7 (b). The isotropic orientation of the electromagnetic wave absorption characteristics was evaluated. The vertical direction and the horizontal direction are directions different from each other by 90 degrees. Both samples are the electromagnetic wave absorbers 1 obtained in this example. Further, Example 1-1 is a case where only the electromagnetic wave absorber 1 is used, and Example 1-2 is a case where a surface hard vinyl chloride plate (thickness 1 mm) is added to the surface of the electromagnetic wave absorber 1. Further, the measurement method and conditions of the electromagnetic wave absorption characteristics are the same as the measurement in FIG.

From FIG. 8, in the case of Example 1-1, the electromagnetic wave absorption characteristics of 20 dB or more can be confirmed in the vicinity of the frequency of 5.6 GHz in both the vertical and horizontal directions, and there is almost no difference in characteristics depending on the sample installation direction. In the case of Example 1-2, in both the vertical and horizontal directions, the peak of electromagnetic wave absorption characteristics shifts to a frequency near 5.3 GHz, but an electromagnetic wave absorption characteristic of about 20 dB can be confirmed. There is almost no difference.

Moreover, the electromagnetic wave absorber 1 of Example 1 having a porosity of 60% had a bulk density of 1.60 g / cm ³ . On the other hand, among general ceramic-based electromagnetic wave absorbers, the ferrite type is 5.2 to 5.3 g / cm ³ , and the iron / titanium oxide is. 3 to 3.9 g / cm ³ , and the ferrite mortar type (ferrite / cement) is 3.5 to 3.6 g / cm ³ . From this, it can be seen that the electromagnetic wave absorber 1 of Example 1 is lighter than a conventional general ceramic-based electromagnetic wave absorber.

Thus, in the electromagnetic wave absorber 1 according to the present embodiment, a net-like conductive path made of carbon is stretched between ceramic particles in the ceramic sintered body by being manufactured according to the flowchart shown in FIG. It is light in weight, easy to transport and construct, non-combustible, can be applied to buildings, and has excellent electromagnetic wave absorption characteristics.
(Example 2)
The electromagnetic wave absorber according to Example 2 is manufactured according to the flowchart shown in FIG. 1 by changing the mixing ratio of the polymerizable substance and the slurry to Example 1. Table 3 shows the blending ratio (weight ratio) of the bubble-introducing slurry according to Example 2. Hereinafter, changes to the first embodiment will be mainly described.

Specifically, in step S10, ball mill mixing is performed at 60 rpm for 12 hours. In the secondary mixing in step S11, an epoxy resin (product number Q-265) is used as the monomer 5, and an epoxy catalyst (type A manufactured by Chukyo Yushi Co., Ltd.) is used as the crosslinking agent 6. This epoxy catalyst also serves as the polymerization catalyst 9. At this time, after adding an epoxy catalyst and performing ball mill mixing for 1 hour, an epoxy catalyst is added and ball mill mixing is performed for 30 minutes.

In the bubble introduction step of Step S13, Triethylenetetramine (TETA) as the polymerization initiator 8 and the same surfactant 7 as in Example 1 were added to the secondary mixed slurry. Introduce a large amount of air bubbles. At this time, the addition amount of the surfactant 7 was set to 2 wt% as an outer amount with respect to the slurry weight.

In steps S14 and S15, the bubble-introducing slurry was allowed to stand for 12 hours in a molding die to form a wet molded body. In the drying step of Step S17, the wet molded body was dried with a dryer, and at this time, the wet molded body was reduced at a rate of 5% per day until the relative humidity in the dryer became 90% to 50%.

In the reduction firing process of step S18, the reduction firing was performed in an argon gas atmosphere, the firing temperature at this time was 1700 degrees, and the firing time was 2 hours. Thus, the electromagnetic wave absorber 1 having a bubble introduction rate of 70% was produced.

FIG. 9 shows the measurement results of the electromagnetic wave absorption characteristics of the electromagnetic wave absorber 1 produced in this way. The measurement method and measurement conditions are the same as in Example 1.

As shown in FIG. 9, the radio wave absorption rate is 20 dB or more within the frequency range of 5.5 GHz to 6.0 GHz within the frequency range of 4 GHz to 8 GHz, and further, the radio wave absorption rate of 25 dB at the frequency of 5.8 GHz. was gotten.

FIG. 10 shows the Raman spectrum of the carbon component contained in the electromagnetic wave absorber 1 of Example 2. As shown in FIG. 6, the electromagnetic wave absorber 1 contains a carbon component because there are peaks at 1350 cm ⁻¹ (D-band) and 1580 cm ⁻¹ (G-band) derived from the graphite structure. Was confirmed.

As a result of calculating from the measured value of the solid carbon analyzer, the carbon component content of the electromagnetic wave absorber 1 of Example 2 was 0.83% by weight. It was 1.71 ohm * cm as a result of measuring the electroconductivity of the electromagnetic wave absorber 1 of Example 2 by the four probe method. In addition, the electromagnetic wave absorber 1 of Example 2 having a porosity of 70% had a bulk density of 1.19 g / cm ³ .

In the examples, the porosity was calculated using the density of the primary and secondary slurries in the manufacturing process, but the same result can be obtained by measuring the porosity of the ceramic sintered body. In this case, the measured average porosity is 60 to 80%. Examples of the method for measuring the porosity include a method for measuring the sintered ceramic density / open porosity of fine ceramics as defined in JIS R1643.

In carrying out the present invention, the configuration, components, shape, quantity, material, size, manufacturing method, etc. of other parts of the electromagnetic wave absorber, and other steps of the electromagnetic wave absorber manufacturing method are also described above. It is not limited to examples.

In addition, since the numerical value quoted in the embodiment of the present invention does not indicate a critical value but indicates an appropriate value suitable for implementation, even if the numerical value is slightly changed, the implementation is not denied. Absent.

DESCRIPTION OF SYMBOLS 1 Electromagnetic wave absorber 2 Ceramic powder 3 Dispersant 4 Distilled water 5 Monomer 6 Crosslinking agent 7 Surfactant 8 Polymerization initiator 9 Polymerization catalyst

Claims

An electromagnetic wave absorber mainly composed of a ceramic sintered body having a porosity of 60% to 80%,
An electromagnetic wave absorber having a structure in which a net-like conductive path made of carbon is stretched in the ceramic sintered body and having a radio wave absorption performance at a frequency of 5 to 6 GHz of 20 dB or more.
The electromagnetic wave absorber according to claim 1, wherein the ceramic is alumina (Al 2 O 3 ).
3. The electromagnetic wave absorption according to claim 1, wherein the network-like conductive path made of carbon is formed by reducing and firing a network of a polymer compound formed by polymerizing a monomer (monomer). 4. body.
The network-like conductive path made of carbon is formed by reducing and firing a network of a polymer compound having a carbon atom in an inert gas atmosphere not containing nitrogen gas. Electromagnetic wave absorber.
The electromagnetic wave absorber according to claim 4, wherein the polymer compound is formed by polymerizing a monomer.
A method for producing an electromagnetic wave absorber mainly comprising a ceramic sintered body having a porosity of 60% to 80%,
A slurry adjustment step in which a ceramic material for sintering and a polymerizable material that is polymerized to become a polymer are dispersed in a solvent to form a slurry;
A bubble introduction step for introducing bubbles into the slurry;
A molding step of polymerizing the polymerizable substance to form a wet molded body by filling the slurry into which bubbles have been introduced and allowing to stand,
A method for producing an electromagnetic wave absorber having a radio wave absorption performance of 20 dB or more at a frequency of 5 to 6 GHz, comprising: a reduction firing step of drying the wet molded body and firing in a reducing atmosphere.
The method for producing an electromagnetic wave absorber according to claim 6, wherein the reducing atmosphere is an inert gas atmosphere containing no nitrogen gas.
A method for producing an electromagnetic wave absorber mainly comprising a ceramic sintered body having a porosity of 60% to 80%,
A slurry adjustment step in which a ceramic material for sintering and a polymerizable material that is polymerized to become a polymer are dispersed in a solvent to form a slurry;
A bubble introduction step for introducing bubbles into the slurry;
A molding step of polymerizing the polymerizable substance to form a wet molded body by filling the slurry into which bubbles have been introduced and allowing to stand,
A method for producing an electromagnetic wave absorber, comprising: a reduction firing step of drying the wet molded body and firing in a reducing atmosphere of an inert gas not containing nitrogen gas.
In the slurry adjustment step, a ceramic powder for sintering, a dispersant, water as the solvent, a monomer (monomer) as the polymerizable substance, and a crosslinking agent are mixed to form a slurry,
In the bubble introduction step, a surfactant and a polymerization initiator and / or a polymerization catalyst are mixed and stirred in the slurry to introduce bubbles into the slurry.
Between the molding step and the reduction firing step, a demolding step of taking out a wet molded body from the molding die, and a drying step of drying the wet molded body into a generated shape,
The method for producing an electromagnetic wave absorber according to any one of claims 6 to 8, wherein in the reduction firing step, the generated shaped body is fired.
The method for manufacturing an electromagnetic wave absorber according to any one of claims 6 to 9, wherein the ceramic is alumina (Al 2 O 3 ).
The method for producing an electromagnetic wave absorber according to any one of claims 6 to 10, wherein the polymerizable substance is methacrylamide.